CA1149572A - High fiber throughput screening system for separating aggregated fiber masses from individualized fibers and soft fiber flocs and a system for forming an air- laid web of dry fibers - Google Patents
High fiber throughput screening system for separating aggregated fiber masses from individualized fibers and soft fiber flocs and a system for forming an air- laid web of dry fibersInfo
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- CA1149572A CA1149572A CA000367216A CA367216A CA1149572A CA 1149572 A CA1149572 A CA 1149572A CA 000367216 A CA000367216 A CA 000367216A CA 367216 A CA367216 A CA 367216A CA 1149572 A CA1149572 A CA 1149572A
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Abstract
ABSTRACT
Methods and apparatus for improving fiber throughput in a high speed production system for forming an air-laid web of dry fibers and wherein individual fibers are separated from aggregated fiber masses in an enclosed, pressurized rotor chamber comprising forming a segment of the chamber wall with a plurality of closely spaced, elongated, narrow slots oriented parallel to the axis of the rotor cnamber.
Methods and apparatus for improving fiber throughput in a high speed production system for forming an air-laid web of dry fibers and wherein individual fibers are separated from aggregated fiber masses in an enclosed, pressurized rotor chamber comprising forming a segment of the chamber wall with a plurality of closely spaced, elongated, narrow slots oriented parallel to the axis of the rotor cnamber.
Description
BACKGROUND OF THE INVENTION
The prescnt invention relates in general to methods and apparatus for forming non-woven fabrics; and, more particularly, to methods and apparatus for improving the fiber throughput capacity of 2-dimensional systems for forming air-laid webs of dry fibers on a high-speed production basis; yet, wherein the web being formed is characterized by a random dispersion of essentially undamaged, uncurled, individualized fibers disposed in a controlled cross-directional profile and is substantially devoid of nits, pills, rice and other aggregated fiber masses so as to result in a web of aesthetically pleasing appearance and increased tensile strenqth irrespective of the basis weight of the web which can range from at least as low as 13 lbs./2880 ft.2 suitable for bath tissue or the lS like to heavier webs suitable for facial tissues, components for feminine napkins, diaper fillers, toweling, wipes, non-woven fabrics, saturating paper, paper webs, paperboard, et cetera.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on paper-making equipment by water-laying a wood pulp fibrous sheet.
Conceptionally, such equipment has been designed so that the configuration of the resulting sheet approaches a planar structure. This allows continuous operation at high speeds;
and, such sheets may be formed at speeds of 3,000 to 4,000 feet per minute. Indeed, recent developments have allowed sustained production at speeds of up to 5,000 feet per minute.
Following formation of the sheet, the water is removed either by drying or by a combination of pressing and drying.
As water is removed during formation, surface tension forces of very great magnitude develop which press the fibers into contact with one another, resulting in overall hydrogen bonding at substantially all fiber intersections; and a ~hin, essentially planar sheet is formed. It is the hydrogen bonds between fibers which provide sheet strength and, such bonds are produced even in the absence of extensive additional pressing. Due to this overall bonding phenomenon, cellulosic sheets prepared by water-laid methods inherently possess very unfavorable tactile properties (e ~., harshness, stiffness, low bulk, and poor overall softness) and, additionally, possess poor absorbency characteristics rendering such sheets generally unsuitable for use as sanitary wipes, bath and facial tissues, and toweling.
To improve these unfavorable properties, water-laid sheets are typically creped from the dryer roll--i.e., the paper is scraped from a dryer roll with a doctor blade.
Creping reforms the flat sheet into a corrugated-like structure, thereby increasing its bulk and simultaneously breaking a significant portion of the fiber bonds, thus artifically ~0 improving the tactile and absorbenGy properties of the material. But creping raises several problems. Conventional creping is only effective on low basis weight webs (e.g., webs having basis weights less than about 15 lbs./2800 ft.2), and higher basis weight webs, after creping, remain quite stiff and are generally unsatisfactory for uses such as quality facial tissues. Because of this, it is conventional practice to employ at least two plies of creped low basis weight paper sheets for such uses. Only by doing this can a suficiently bulky product with acceptable softness be prepared. However, even this process does not completely overcome the detrimental effects of the initial overbondin~
'7~ ~
in a water-laid paper sheet.
Sanford _ al. U.S. Pat. No. 3,301,246 proposes improving the tactile properties of water-laid sheets by thermally predrying a sheet to a fiber consistency substantially in excess of that normally applied to the dryer surface of a paper machine and then imprinting the partially dried sheet with a knuckle pattern of an imprinting fabric. The sheet is thereafter dried without disturbing the imprinted knuckle-pattern bonds. ~hile this method may somewhat improve the softness, bulk and absorbency of the resulting sheet, the spaces between the knuckle bonds are still appreciably compacted by the surface-tension forces developed during water removal, and considerable fiber bonding occurs.
Creping is still essential in order to realize the maximum advantage of the proposed process; and, for many uses, two plies are still necessary.
As will be apparent from the foregoing discussion, conventional paper-making methods utilizing water are geared towards the high speed formation of essentially planar sheets; yet, such methods inherently possess the inefficient attribute of initial "overbonding," which then necessitates a creping step to partially "debond" the sheet to enhance the tactile properties. Also, the extreme water requirements limit the locations where paper-making operations may be carried out. Such operations require removing a large quantity of the water used as the carrier, and the used process water can create an associated water pollution problem. Still further, the essential drying procedures consume tremendous amounts of enexgy.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been 7~ ( used for applications where either li~tle strength is required, such as for absorbent products--i.e., pads~-or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant--i.e., various specialty papers. ~.S. Pat. No. 2,447,161 to Coghill, V.S. Pat. No. 2,810,940 to Mills, and British Pat.
No. 1,088,991 illustrate various air-forming techniques for such applications.
In the late 1940's and early 1950's, work by James D'A.
Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber "disintegrating and dispersing chamber"
wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire--viz., J.D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633 and 2,931,076. However, Clark and his associates encountered serious problems with these types of forming systems as a result of disintegration of the fibers by mechanical co-action of the rotor blades with the chamber wall and/or the screen mounted therein which caused fibers to be "rolled and formed into balls or rice which resist separation"--a phenomenon more commonly referred to today as "pilling". These problems, inter alia, and proposed solutions thereto, are described in, for example. J.D'A. Clark U.S. Pat. No. 2,827,668, J.D'A. Clark et al. U.S. Pat. Nos. 2,714,749 and 2,720,005;
Anderson U.S. Pat. No. 2,738,556; and, Anderson et al. U.S.
Pat. No. 2,738,557. However, prior to the advent of the present invention, it is not believed that systems of the type disclosed by J.D'A. Clark and his associates which employed cylindrical fiber disintegrating and dispersing mechanisms with and/or without rotors, have been suitable for use in production type, air-laid, dry fiber, web forming systems, principally because problems of pilling have not been resolved, and because of severe fiber damage due t~ the disintegrating action of the rotor in Clark's cylindrical chamber.
It should be noted that the aforesaid Clark et al. U.S.
Pat. No. 2,720,005 discloses an air scrabbler system having a foraminous separating wall wherein slots may be formed in the wall rather than relatively small openings such as are employed with conventional woven square-mesh screens. The Clark et al. patent is silent as to the orientation of the slots. However, in the aforesaid Clark U.S. Pat. No. 2,748,429 which also contemplates the use of a slotted separating wall, the slots are shown and described as "circumferentially extending laterally spaced slots~ (See, Col. 3, lines 22-23). Such slot orientation has been found to be substantially inoperable when utilizing 2-dimensional formers of the type employing a horizontally disposed rotor assembly.
A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark.
Certain of these systems are described in: Kroyer U.S. Pat.
Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706 ~5 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115;
Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat.
No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393.
In general, these systems employ a fiber sifting chamber or head having a planar sifting screen which is mounted over a forming wire. Fibers are fed into the sifting chamber where they are mechanically agitated by means of a plurality of mechanically driven rotors mounted for rotation about vertical axes. Each rotor has an array of symmetrical blades which rotate in close proximity to the surface of the sifting screen. The systems described in the aforesaid Kroyer and related patents generally employ two, three, or more side-by-side rotors mounted in a suitable for~ing head.
This type of sifting equipment suffers from poor productivity and other inherent disadvantages, especially when making tissue-weight webs. For example, the rotor action concentrates most of the incoming material at the periphery of the blades where the velocity is at a maximum. Most of the sifting action is believed to take place in these peripheral zones, while other reglons of the sifting screen are either covered with more slowly moving material or are bare. Thus, a large percentage of the sifting screen area is poorly utilized and the system productivity is low. Moreover, fibers and agglomerates tend to remain in the forming head for extended periods of time, especially in the lower velocity, inner regions beneath the rotor blades. This accentuates the tendency of fibers to roll up into pills. Conse~uently, if the forming head is to be cleared of agglomerated material, it is necessary to remove 10~ or more by weight of the incoming material from the forming head for subsequent reprocessing or for use in less critical end products. The separating method used (See, e.g., the aforesaid Kroyer ~.S. Pat. No. 4,014,635) entrains a large number of good fibers with the agglomerates leaving the forming head. The severe mechanical action of the hammermills in the secondary processing system damagès and shortens such otherwise good fibers, while breaking up the agglomerates. Another inherent shortcoming of these systems is a tendency to form webs having a non-uniform f~ 7~
weight profile across their width. (See, e.g., the aforesaid Tapp U.S. Pat. No. 4,060,360~. This is a condition which is very difficult to overcome. It is especially troublesome when making webs in the towelling and lightweight tissue ranges.
~ he inventors have found that, when using high quality fibers--i.e., long, straight fibers, in a sifting type system--the above difficulties were aggravat2d. The rate of pill rormation increased and it was necessary to remove and l~ recycle more than 50% by weight of the incoming fibrous material to produce good guality tissue-weight webs. Pro-ductivity was unacceptably low and excessive damage was done to otherwise qood fibers duxing the secondary hammermilling step. The tensile strength of the webs produced was decreased.
Moreover, the circular movement of the rotors above the screen causes corresponding air and fiber movement in the formin~
region below the screen. Strong, unstable cross-flow forces are present and contribute to non-uniform ~ormation of the web.
Efforts to compensate for the low throughput of si ting type syste~s involve increasing the area of the screens and the forming surface. Thus, fiber is more thinly distributed over the forming surface and is not held in place as rirmly by the suction box. The fibers are easily disturbed at higher speeds and wave patterns are formed~
Fibers are also disturbed by the seal rollers which are required to maintain the forming region at sub-atmospheric pressure. The difficulties described above compound each other and are especially troublesome when forming lightwei~ht webs at acceptable production speeds.
In an effort to overcome the productivity problem, 5~2 complex production sys~ems have been devised utilizing multiple forming heads--for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds--that is, the syst~ms employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.
Moreover, it has been found that the foregoing sifting systems are also deficient in that there is only limited control of cross-directional uniformity of the web being produced--see, e.g., the aforesaid Tapp U.S. Pat. No. 4,060,360--thereby imposing severe constraints when attempting to scale the equipment up to make webs of 96 inches, 120 inches, 200 inches, or moret in width. The tensile properties of the web may suffer as a result of excessive mechanical action in the forming heads and non-uniformities in web weight and formation. The aesthetic appearance of the webs is often less than optimum as a result of wave patterns on the web surface resulting from the closely spaced rotor blades which are rotating in a horizontal plane just above the forming wire` and the other factors described above. To date/ the foregoing problems have been so significant that this type of sifting system has been found totally unsuitable for making relativel~ light weight webs at acceptable production speeds--e.g., webs having basis weights of from 13 lbs./2880 ft.2 to 18 lbs./2880 ft. suitable for use as bath or facial tissues--although such equipment can produce low basis _g_ 5~7~ `
weight webs at low forming wire speeds. Rather, the equipment has generally found application in forming heavier basis weight webs suitable for use in making towels or paperboard where the web imperfections inherently produced can be either tolerated or masked because of the bulk and thickness of the web.
During the 1970's a series of patents were issued ~o C.E. Dunning and his associates which have been assigned to the assignee of the present invention; such patents describing yet another approach to the formation of air-laid dry fiber webs. Such patents include, for example: Dunning U.S. Pat.
Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al.
U.S. Pat. Nos. 3,776,807 and 3,825,381. This development has been found to resolve a number of the problems that have heretofore plagued the industry. For example, high productivity rates have been achieved and fiber webs can easily be formed at high machine speeds. However, the system requires preparation of pre-formed rolls o~ fibers having high cross-directional uniformity and is not suitable for use with bulk or baled fibrous materials. Because of this, problems are experienced when attempting to scale the equipment up to produce wide webs--i.e., webs on the order of 120 inches in width or greater--and the requirement for pre-formed special web rolls having the requisite uniformity in cross-directional profile has bee~ such that, to date, the system has found only limited commercial application.
Indeed, heretofore it has not been believed that air-forming techniques can be advantageously used in high speed production operations to prepare cellulosic sheet material that is sufficiently thin, and yet has adequate strength, together with softness and absorbency, to serve in applications such as bath tissues, facial tissues and light weight toweling.
SUMMARY OF THE INVENTION
It is a general aim of the present invention to provide methods and apparatus which overcome all of the foregoing disadvantages which are characteristic of the prior art, yet which enable significant improvements in terms of productivity.
In one of its principal aspects, it is an object of the invention to provide improved methods and apparatus for air deposition of dry fibers to form webs having any selected one of a wide range of basis weights and wherein the speed of web formation is no longer limited by low screening efficiency or productivity.
It is a further general objective of the invention to provide dry air-laid web forming methods and apparatus characterized by their simplicity, yet which permit of high capacity operation with high fiber throughput and wherein the product produced is characterized by improved properties in terms of strength, tactile properties, freedom from nits, uniformity, and general aesthetis appearance. It is a more specific object to provide methods and apparatus capable of producing high-quality webs at speeds in the range of 300 to
The prescnt invention relates in general to methods and apparatus for forming non-woven fabrics; and, more particularly, to methods and apparatus for improving the fiber throughput capacity of 2-dimensional systems for forming air-laid webs of dry fibers on a high-speed production basis; yet, wherein the web being formed is characterized by a random dispersion of essentially undamaged, uncurled, individualized fibers disposed in a controlled cross-directional profile and is substantially devoid of nits, pills, rice and other aggregated fiber masses so as to result in a web of aesthetically pleasing appearance and increased tensile strenqth irrespective of the basis weight of the web which can range from at least as low as 13 lbs./2880 ft.2 suitable for bath tissue or the lS like to heavier webs suitable for facial tissues, components for feminine napkins, diaper fillers, toweling, wipes, non-woven fabrics, saturating paper, paper webs, paperboard, et cetera.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on paper-making equipment by water-laying a wood pulp fibrous sheet.
Conceptionally, such equipment has been designed so that the configuration of the resulting sheet approaches a planar structure. This allows continuous operation at high speeds;
and, such sheets may be formed at speeds of 3,000 to 4,000 feet per minute. Indeed, recent developments have allowed sustained production at speeds of up to 5,000 feet per minute.
Following formation of the sheet, the water is removed either by drying or by a combination of pressing and drying.
As water is removed during formation, surface tension forces of very great magnitude develop which press the fibers into contact with one another, resulting in overall hydrogen bonding at substantially all fiber intersections; and a ~hin, essentially planar sheet is formed. It is the hydrogen bonds between fibers which provide sheet strength and, such bonds are produced even in the absence of extensive additional pressing. Due to this overall bonding phenomenon, cellulosic sheets prepared by water-laid methods inherently possess very unfavorable tactile properties (e ~., harshness, stiffness, low bulk, and poor overall softness) and, additionally, possess poor absorbency characteristics rendering such sheets generally unsuitable for use as sanitary wipes, bath and facial tissues, and toweling.
To improve these unfavorable properties, water-laid sheets are typically creped from the dryer roll--i.e., the paper is scraped from a dryer roll with a doctor blade.
Creping reforms the flat sheet into a corrugated-like structure, thereby increasing its bulk and simultaneously breaking a significant portion of the fiber bonds, thus artifically ~0 improving the tactile and absorbenGy properties of the material. But creping raises several problems. Conventional creping is only effective on low basis weight webs (e.g., webs having basis weights less than about 15 lbs./2800 ft.2), and higher basis weight webs, after creping, remain quite stiff and are generally unsatisfactory for uses such as quality facial tissues. Because of this, it is conventional practice to employ at least two plies of creped low basis weight paper sheets for such uses. Only by doing this can a suficiently bulky product with acceptable softness be prepared. However, even this process does not completely overcome the detrimental effects of the initial overbondin~
'7~ ~
in a water-laid paper sheet.
Sanford _ al. U.S. Pat. No. 3,301,246 proposes improving the tactile properties of water-laid sheets by thermally predrying a sheet to a fiber consistency substantially in excess of that normally applied to the dryer surface of a paper machine and then imprinting the partially dried sheet with a knuckle pattern of an imprinting fabric. The sheet is thereafter dried without disturbing the imprinted knuckle-pattern bonds. ~hile this method may somewhat improve the softness, bulk and absorbency of the resulting sheet, the spaces between the knuckle bonds are still appreciably compacted by the surface-tension forces developed during water removal, and considerable fiber bonding occurs.
Creping is still essential in order to realize the maximum advantage of the proposed process; and, for many uses, two plies are still necessary.
As will be apparent from the foregoing discussion, conventional paper-making methods utilizing water are geared towards the high speed formation of essentially planar sheets; yet, such methods inherently possess the inefficient attribute of initial "overbonding," which then necessitates a creping step to partially "debond" the sheet to enhance the tactile properties. Also, the extreme water requirements limit the locations where paper-making operations may be carried out. Such operations require removing a large quantity of the water used as the carrier, and the used process water can create an associated water pollution problem. Still further, the essential drying procedures consume tremendous amounts of enexgy.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been 7~ ( used for applications where either li~tle strength is required, such as for absorbent products--i.e., pads~-or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant--i.e., various specialty papers. ~.S. Pat. No. 2,447,161 to Coghill, V.S. Pat. No. 2,810,940 to Mills, and British Pat.
No. 1,088,991 illustrate various air-forming techniques for such applications.
In the late 1940's and early 1950's, work by James D'A.
Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber "disintegrating and dispersing chamber"
wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire--viz., J.D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633 and 2,931,076. However, Clark and his associates encountered serious problems with these types of forming systems as a result of disintegration of the fibers by mechanical co-action of the rotor blades with the chamber wall and/or the screen mounted therein which caused fibers to be "rolled and formed into balls or rice which resist separation"--a phenomenon more commonly referred to today as "pilling". These problems, inter alia, and proposed solutions thereto, are described in, for example. J.D'A. Clark U.S. Pat. No. 2,827,668, J.D'A. Clark et al. U.S. Pat. Nos. 2,714,749 and 2,720,005;
Anderson U.S. Pat. No. 2,738,556; and, Anderson et al. U.S.
Pat. No. 2,738,557. However, prior to the advent of the present invention, it is not believed that systems of the type disclosed by J.D'A. Clark and his associates which employed cylindrical fiber disintegrating and dispersing mechanisms with and/or without rotors, have been suitable for use in production type, air-laid, dry fiber, web forming systems, principally because problems of pilling have not been resolved, and because of severe fiber damage due t~ the disintegrating action of the rotor in Clark's cylindrical chamber.
It should be noted that the aforesaid Clark et al. U.S.
Pat. No. 2,720,005 discloses an air scrabbler system having a foraminous separating wall wherein slots may be formed in the wall rather than relatively small openings such as are employed with conventional woven square-mesh screens. The Clark et al. patent is silent as to the orientation of the slots. However, in the aforesaid Clark U.S. Pat. No. 2,748,429 which also contemplates the use of a slotted separating wall, the slots are shown and described as "circumferentially extending laterally spaced slots~ (See, Col. 3, lines 22-23). Such slot orientation has been found to be substantially inoperable when utilizing 2-dimensional formers of the type employing a horizontally disposed rotor assembly.
A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark.
Certain of these systems are described in: Kroyer U.S. Pat.
Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706 ~5 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115;
Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat.
No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393.
In general, these systems employ a fiber sifting chamber or head having a planar sifting screen which is mounted over a forming wire. Fibers are fed into the sifting chamber where they are mechanically agitated by means of a plurality of mechanically driven rotors mounted for rotation about vertical axes. Each rotor has an array of symmetrical blades which rotate in close proximity to the surface of the sifting screen. The systems described in the aforesaid Kroyer and related patents generally employ two, three, or more side-by-side rotors mounted in a suitable for~ing head.
This type of sifting equipment suffers from poor productivity and other inherent disadvantages, especially when making tissue-weight webs. For example, the rotor action concentrates most of the incoming material at the periphery of the blades where the velocity is at a maximum. Most of the sifting action is believed to take place in these peripheral zones, while other reglons of the sifting screen are either covered with more slowly moving material or are bare. Thus, a large percentage of the sifting screen area is poorly utilized and the system productivity is low. Moreover, fibers and agglomerates tend to remain in the forming head for extended periods of time, especially in the lower velocity, inner regions beneath the rotor blades. This accentuates the tendency of fibers to roll up into pills. Conse~uently, if the forming head is to be cleared of agglomerated material, it is necessary to remove 10~ or more by weight of the incoming material from the forming head for subsequent reprocessing or for use in less critical end products. The separating method used (See, e.g., the aforesaid Kroyer ~.S. Pat. No. 4,014,635) entrains a large number of good fibers with the agglomerates leaving the forming head. The severe mechanical action of the hammermills in the secondary processing system damagès and shortens such otherwise good fibers, while breaking up the agglomerates. Another inherent shortcoming of these systems is a tendency to form webs having a non-uniform f~ 7~
weight profile across their width. (See, e.g., the aforesaid Tapp U.S. Pat. No. 4,060,360~. This is a condition which is very difficult to overcome. It is especially troublesome when making webs in the towelling and lightweight tissue ranges.
~ he inventors have found that, when using high quality fibers--i.e., long, straight fibers, in a sifting type system--the above difficulties were aggravat2d. The rate of pill rormation increased and it was necessary to remove and l~ recycle more than 50% by weight of the incoming fibrous material to produce good guality tissue-weight webs. Pro-ductivity was unacceptably low and excessive damage was done to otherwise qood fibers duxing the secondary hammermilling step. The tensile strength of the webs produced was decreased.
Moreover, the circular movement of the rotors above the screen causes corresponding air and fiber movement in the formin~
region below the screen. Strong, unstable cross-flow forces are present and contribute to non-uniform ~ormation of the web.
Efforts to compensate for the low throughput of si ting type syste~s involve increasing the area of the screens and the forming surface. Thus, fiber is more thinly distributed over the forming surface and is not held in place as rirmly by the suction box. The fibers are easily disturbed at higher speeds and wave patterns are formed~
Fibers are also disturbed by the seal rollers which are required to maintain the forming region at sub-atmospheric pressure. The difficulties described above compound each other and are especially troublesome when forming lightwei~ht webs at acceptable production speeds.
In an effort to overcome the productivity problem, 5~2 complex production sys~ems have been devised utilizing multiple forming heads--for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds--that is, the syst~ms employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.
Moreover, it has been found that the foregoing sifting systems are also deficient in that there is only limited control of cross-directional uniformity of the web being produced--see, e.g., the aforesaid Tapp U.S. Pat. No. 4,060,360--thereby imposing severe constraints when attempting to scale the equipment up to make webs of 96 inches, 120 inches, 200 inches, or moret in width. The tensile properties of the web may suffer as a result of excessive mechanical action in the forming heads and non-uniformities in web weight and formation. The aesthetic appearance of the webs is often less than optimum as a result of wave patterns on the web surface resulting from the closely spaced rotor blades which are rotating in a horizontal plane just above the forming wire` and the other factors described above. To date/ the foregoing problems have been so significant that this type of sifting system has been found totally unsuitable for making relativel~ light weight webs at acceptable production speeds--e.g., webs having basis weights of from 13 lbs./2880 ft.2 to 18 lbs./2880 ft. suitable for use as bath or facial tissues--although such equipment can produce low basis _g_ 5~7~ `
weight webs at low forming wire speeds. Rather, the equipment has generally found application in forming heavier basis weight webs suitable for use in making towels or paperboard where the web imperfections inherently produced can be either tolerated or masked because of the bulk and thickness of the web.
During the 1970's a series of patents were issued ~o C.E. Dunning and his associates which have been assigned to the assignee of the present invention; such patents describing yet another approach to the formation of air-laid dry fiber webs. Such patents include, for example: Dunning U.S. Pat.
Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al.
U.S. Pat. Nos. 3,776,807 and 3,825,381. This development has been found to resolve a number of the problems that have heretofore plagued the industry. For example, high productivity rates have been achieved and fiber webs can easily be formed at high machine speeds. However, the system requires preparation of pre-formed rolls o~ fibers having high cross-directional uniformity and is not suitable for use with bulk or baled fibrous materials. Because of this, problems are experienced when attempting to scale the equipment up to produce wide webs--i.e., webs on the order of 120 inches in width or greater--and the requirement for pre-formed special web rolls having the requisite uniformity in cross-directional profile has bee~ such that, to date, the system has found only limited commercial application.
Indeed, heretofore it has not been believed that air-forming techniques can be advantageously used in high speed production operations to prepare cellulosic sheet material that is sufficiently thin, and yet has adequate strength, together with softness and absorbency, to serve in applications such as bath tissues, facial tissues and light weight toweling.
SUMMARY OF THE INVENTION
It is a general aim of the present invention to provide methods and apparatus which overcome all of the foregoing disadvantages which are characteristic of the prior art, yet which enable significant improvements in terms of productivity.
In one of its principal aspects, it is an object of the invention to provide improved methods and apparatus for air deposition of dry fibers to form webs having any selected one of a wide range of basis weights and wherein the speed of web formation is no longer limited by low screening efficiency or productivity.
It is a further general objective of the invention to provide dry air-laid web forming methods and apparatus characterized by their simplicity, yet which permit of high capacity operation with high fiber throughput and wherein the product produced is characterized by improved properties in terms of strength, tactile properties, freedom from nits, uniformity, and general aesthetis appearance. It is a more specific object to provide methods and apparatus capable of producing high-quality webs at speeds in the range of 300 to
2,000 feet per minute and, even at speeds in excess of 2,000 feet per minute;
It is an object of thc present invention to provide methods and apparatus which are e~ually suitable for mass production on a high-speed basis of webs such as those used as bath or facial tissues, diaper fillers, feminine napkin components, towels, wipes, non-woven fabrics, appliques on non-woven substrates, smooth paper webs, laminated paper webs, paperboard, and similar products, all of which have 7~
physical properties at least equal to, and in some cases better than, those obtained by known dry forming systems.
In this connection, it is an object of the invention to provide improved methods and apparatus for the air deposition S of dry fibers in the manufacture of both relatively thin webs--e.~., webs having basis weights on the order of 13 lbs./2880 ft.2 to 18 lbs./2880 ft.2 suitable for bath and facial tissues--and relatively thick webs--e.g., webs having basis weights on the order of 19 lbs./2880 ft.2 to 40 lbs./28~0 ft.2, and even heavier, suitable for toweling and other uses--yet wherein the resulting product, irrespective of its basis weight, is characterized by its uniformity, tensile strength, freedom from nits, and generally pleasing aesthetic appearance despite having been formed at speeds in the range of 300 to 2000 feet per minute or higher.
In another of its important aspects, it is an object of the present invention to provide a versatile and highly tolerant system for the air deposition of dry fibers which is characterized by its ability to handle wide ranges o pulp and other fibers to form both thin and thick webs or batts, and which is capable of handling fibers having lengths in the 1-5 mm. range--e.g., wood, cotton linters, rayon or synthetic fibers, leather, hemp, thermo-mechanical, secondary and, perhaps, inorganic fibers such as glass microfibers and asbestos--as well as synthetic fibers of considerably greater length and, blends of the foregoing fiber types; yet, wherein the fibers are subjected to only minimal mechanical disintegrating forces and, consequently, are not shortened or otherwise damaged.
In another of its aspects, it is an object of the invention to provide improved methods and apparatus for permitting hlgh throughput of fibers at relatively high speeds, yet wherein there is only a minimal tendency to form pills, nits or the like and, consequently~ where the amount of undesired materials separated and/or recycled can be substantially reduced.
In another of its important aspects~ it is an object of the invention to provide an improved screening arrangement for permitting high fiber throughput with effective screening of undesirable materials such as nits or the like and without subjecting the system to undesired screen plugging. It is an important object of the invention to provide a rotational screening system for a dry forming fiber deposition process wherein provision is made for maintaining a proper balance between rotor speed and both air supply and velocity so as to maintain acceptable cross-dimensional uniformity in the mass flow rate of air-suspended fibers being delivered to the forming wire so as to form an air-laid web characterized by its cross-directional basis weight uniformity which equals or exceeds the cross-directional uniformity of the fibers entering the former.
Therefore, in accordance with one aspect of the present invention there is provided the method of forming a quality web of air-laid dry fibers on a high speed production basis comprising the steps of: a) delivering dry fibrous materials to a forming head positioned over a forming surface;
b) conveying the dry fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs and aggregated fiber masses and in an environment maintained substantially free of fiber grinding and disintegrating forces; c) continuously separating f~om 1% to 10~ of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those haviny a bulk density in excess of .2g/cc.
so as to maximize the separation of aggregated fiber masses from the aerated bed; d) discharging such separated fibrous materials including the aggregated fiber masses contained therein from the forming head; e) discharging the individualized fibers and soft fiber flocs through a high capacity slotted screen; f) conveying the individualized fibers and soft fiber flocs discharged through the slotted screen at a fiber throughput rate anywhere in the range o .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 through an enclosed forming zone towards the moving foraminous forming surface in a rapidly moving air stream; g) air-laying the individualized fibers and soft fiber flocs on the moving foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on the forming surface with such web having a~nit level of from "0" to "3"; and, h) moving the foraminous forming surface at a controlled and selected speed so as to produce an air-laid web having a nit level of from "0" to "3" and any specific desired basis weight in lbs./2880 ft~2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./2880 ft.2.
In accordance with a further aspect there is provided an apparatus for producing a quality web of air-laid dry fibers on a high speed production basis comprising, in combination: a movable foraminous forming surface; a forming head mounted over and forming surface; means for delivering dry fibrous materials to the forminy head; means for conveying the dxy fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs, and aggregated fiber masses while maintaining the forming head substantially free of fiber grinding and disintegrating forces; means for continuously separating from - 13a -1% to 10% of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g./cc.
so as to maximize the separation of aggregated fiber masses from the aerated bed and discharging such separated fibrous materials from the forming head; a discharge opening formed in the forming head; a slotted screen mounted in the discharge opening; means defining an enclosed forming zone mounted between the discharge opening and the forming surface;
means for conveying the individualized fibers and soft fiber flocs from~the forming head through the slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.~ to at least 1.50 lbsO/hr./in.2 and through the forming zone towards the movable foraminous forming surface in a rapidly moving air stream and for air-laying the individualized fibers and soft fiber flocs on the movable foraminous forming surface so as to form an air-laid web of ~andomly oriented dry individualized fibers and soft fiber flocs on the surface during movement thereof with such web having a nit level of from "0" to "3", and, means for controllably moving the foraminous forming surface at a selectable speed so as to produce an air-laid web having a nit level of from "0" to "3" and any specific desired basis weight in lbs./2880 ft 2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./ 2880 ft~2.
DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention will become more readily apparent upon reading the following detailed description and upon reference to the attached drawings, in which:
FIGURE 1 is a schematic view, in side elevation, of one form of apparatus which may be employed for the air - 13 ~
deposition of dry fibers to form a web continuum in accordance with the present invention;
FIG. 2 is a schematic view here illustrating an exemplary - 13c -t~ ~
air-laid, dry fiber, web forming system utilizing two substantially identical cylindrical flow cvntrol and forming heads disposed in side-by-side relationship above the foraminous for~ing wire;
FIG. 3 is an oblique view, partially cut away, here schematically illustrating details of an exemplary novel fiber feed, eductor, flow control, screening, and fiber forming arrangement embodying features of the present invention;
FIG. 4 is a fragmentary front elevational view, partly in section, of the rotor assembly shown in FIG. 3;
FIG. 5 is an end view of a modi~ied rotor assembly similar to that shown in FIG. 3, but here depicting a rotor employing only four rotor ~ars;
FIG. 6 is a diagramatic plan view indicating in schematic, idealized fashion fiber movement through a conventional woven square-mesh screen under the influence OL
air movement and rotor action;
FIG. 7 is a view similar to FIG. 6 but here depicting movement of fibers through a high capacity slotted screen in which the slots are oriented parallel to the axis of the rotor in accordance with the invention;
FIG. 8 is a view similar to FIG. 7, but here illustrating the undesirable plugging action that occurs when the slots of à slotted screen are oriented in a direction generally perpendicular to a plane passing through the axis of the rotor;
FIG. 9 is a photograph illustrating the plugging of a slotted screen that occurs when the slots are oriented at an angle of approximately 45 to a plane passing through the axis of the rotor;
FIG. 10 is an enlarged, fragmentary side elevational view here depicting in diagramatic form the air/fiber stream as it moves through the rotor housing where an annular moving aerated bed of fibers is created and maintained and, thereafter, as it moves through the screening means and forming zone an~ is air-laid on the forming wire to form an air-laid web of fibers and, further, depicting the pressure relationships and the air velocity and rotor bar velocity relationships that are believed to exist when operating the system of the present invention at a desired one of several selectable sets of adjustablP parameters in terms of ratio of air-to-fiber supply, rotor speed, and recycle balance;
FIG. 11 is a highly enlarged view of a portion of the system shown diagramatically in FIG. 10, here depicting how the differential relative velocities of the rotor bars and air stream serve to gencrate a rapidly moving full-width zone of negative pressure in the wake of each rotor bar, thereby lifting fibrous materials off the screen in the region beneath th~ moving negative pressure zone, while permitting individual fibers to dive axially or end-wise through the openings in the screen in those regions of positive pressure drop across the screen between successive negative pressure zones;
FIG. 12 is a graphic representation of a typical set of curves indicative of the functional relationships existing with air-laid web forming systems e~bodying features of the present invention between fiber throughput for specific representative screen designs and rotor assembly operating parameters--viz., rotor RPM and the number of rotor bars -employed;
FIG. 13 is a graphic representation of the functional relationships existing between nit levels in a finished air-laid web made in accordance with the present invention, fiber throughput, and the percentage of fibrous materials separated and/or recycled prior to deposition on a moving forming wire, FIGS. 14 through 20 are photographs of exemplary air-laid fiber webs having increasing nit levels suitable for subjectively evaluating and rating web guality in accordance with subjective visual standards as to product acceptability established by the assignee of the present invention; and, FIG. 21 is a graphic representation depicting the relationship between fiber delivery rates expressea as fiber throughout in pounds per square inch per hour ~lbs./in.2/hr.) and both woven square-mesh screens and slotted screens having screen openings ranging from about 0.03`' in at least one direction to about 0.08" in at least one direction.
While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, eyuivalents and alternatives falling within the spirit and scope of the in-vention as expressed in the appended claims.
DETAILED DESCRIPTION
A. Definitions To facilitate an understanding of the ensuing description and the appended claims, definitions of certain selectedterms and phrases as used throughout the specification and clai~s are set forth below.
The phrase "pulp lum?" is herein used to describe a dense, bonded clump of fibers in the incoming fiberized supply which is most conventionally caused by hard pressing, non-uniform application of debonding agents, and/or inadequate opening or hammermilling. Pulp lumps are present in ordinary commercial grades of pulp.
The words "pill" and~or "rice" are herein each used to describe a dense, rolled up bundle of fibers, often including bonded fibers, which are generally formed by mechanical action during fiber transport or in a rotor chamber where the fibers are commonly, and often intentionally, subjected to mechanical disintegrating action.
The word "nit" is herein used to generically refer to pulp lumps, pills and/or rice. Nits are considered to be an unacceptable defect in light-weight tissues such as bath and facial tissues having basis weights of from 13 lbs./2880 ft 2 to 18 lbs./2880 ft. , and generally result in decreased tensile strength in webs of these, or even of heavier, basis weights.
The terms "floc" and "soft floc" are h~rein used to describe soft, cloud-like accumulations of fibers which behave like individualized fibers in air; i.e., they exhibit relatively high co-efficients of drag in air.
The phrase "aggregated fiber masses" is herein used to generically embrace pulp lumps, pills, rice and/or nits, and to describe aggregations of bonded and/or mechanically entangled fibers generally having a bulk density on the order of greater than .2 grams per cubic centimeter (g./cc.).
Aggregated fiber masses are to be distinguished from flocs and/or soft flocs whose bulk density is generally less than .2 g./cc. Moreover, aggregated fiber masses have a relatively low coefficient of drag in air.
"Bulk density" in the weight in grams of an uncompressed sample divided by its volume in cubic centimeters.
The phrase "semi-cylindrical" is used herein to describe a portion of the rotor chamber wall and/or forming screen, and is intended to mean that wall portion from the upstream leading edge of the screen to and including the full~width separator slot. In the various exemplary embodiments herein described, the phrase "semi-cylindrical" embraces a peripheral wall portion having an included angle of less than 180.
However, such phrase is used herein in a descriptive sense and is not intended to be construed in a limiting sense since those skilled in the art will appreciate as the ensuing description proceeds that the rotor chamber could be cylindrical, or substantially cylindrical, in which event the phrase "semi-cylindrical" would be intended to embrace peripheral wall portions having an included angle of greater than 180.
The phrase "2-dimensional" is used to describe a system for forming a web wherein: i) the cross-section of the syste~
and the flows of air and fiber therein are the same at all sections across the width of the system; and ii), where each increment of system width behaves essentially the same as every other increment of system width; thereby permitting the system to be scaled up or down to produce high quality webs of any suitable and commercially useful widths on a hi~h-speed production basis and wherein a web's cross-directional profile in terms of basis weight can be controlled and, preferably, can be maintained uniform.
The phrase "coefficient of variation" is used herein to describe variations in the cross-directional basis weight profile of both the web being formed and the fibrous materials input to the system, and comprises the standard deviation (u) expressed as a percent of the mean. The coefficient of variation should not vary more than 5% and, preferably, should vary less than 3~ in the cross-machine direction. The basis weight profile in the cross-machine direction of the web being formed may, for example, be determined by weighing strips of the web which are three inches in width (3" C.D.) by seven inches in length (7" M.D.).
The phrases "uniform cross-directional profile", "uniform mass quantum of fibers in the cross-machine direction", and similar phrases, are herein used to describe a condition in the web being formed, as well as in the fiber delivered to the forming apparatus, wherein the coefficient of variation does not vary more than 5% and, preferably, varies less than 3% in the cross-machine direction.
~he phrases "controlled cross-directional deposition", "controlled cross-directional profile", "controlled mass quantùm of fibers in the cross-machine direction", and similar phrases, are herein used to describe a condition wherein the cross-directional profiles of the fiber feed and the web being formed are not necessarily uniform but, rather, may intentionally be non-uniform; and, because the system is substantially devoid of cross-directional flows, the cross-directional profile of the finished web is controlled so as to be similar in profile to the cross-directional profile of the fiber feed --e.g., if the fiber feed has twice the mass quantum of fibers at its center as it does along its marginal edges, the basis-weight of the web produced will also be _ ~9 _ csm/~,~
approximately twice as great a-t its center than at i-ts marginal edges when viewed in cross-directional profile.
One convenient way of delivering fibers to the system is to form a feed mat having a controlled and/or uniform cross directional weight profile. Such systems are described in detail in subsequent portions of this specification. However, other means of delivering fibers to the system at a controlled weight rate in the cross direc-tion may be devised and are within the scope of this invention. While the invention will herein be described in large part in terms of a fibrous material input to the system wherein the coefficient of variation is not more than 5% and, preferably, is less than
It is an object of thc present invention to provide methods and apparatus which are e~ually suitable for mass production on a high-speed basis of webs such as those used as bath or facial tissues, diaper fillers, feminine napkin components, towels, wipes, non-woven fabrics, appliques on non-woven substrates, smooth paper webs, laminated paper webs, paperboard, and similar products, all of which have 7~
physical properties at least equal to, and in some cases better than, those obtained by known dry forming systems.
In this connection, it is an object of the invention to provide improved methods and apparatus for the air deposition S of dry fibers in the manufacture of both relatively thin webs--e.~., webs having basis weights on the order of 13 lbs./2880 ft.2 to 18 lbs./2880 ft.2 suitable for bath and facial tissues--and relatively thick webs--e.g., webs having basis weights on the order of 19 lbs./2880 ft.2 to 40 lbs./28~0 ft.2, and even heavier, suitable for toweling and other uses--yet wherein the resulting product, irrespective of its basis weight, is characterized by its uniformity, tensile strength, freedom from nits, and generally pleasing aesthetic appearance despite having been formed at speeds in the range of 300 to 2000 feet per minute or higher.
In another of its important aspects, it is an object of the present invention to provide a versatile and highly tolerant system for the air deposition of dry fibers which is characterized by its ability to handle wide ranges o pulp and other fibers to form both thin and thick webs or batts, and which is capable of handling fibers having lengths in the 1-5 mm. range--e.g., wood, cotton linters, rayon or synthetic fibers, leather, hemp, thermo-mechanical, secondary and, perhaps, inorganic fibers such as glass microfibers and asbestos--as well as synthetic fibers of considerably greater length and, blends of the foregoing fiber types; yet, wherein the fibers are subjected to only minimal mechanical disintegrating forces and, consequently, are not shortened or otherwise damaged.
In another of its aspects, it is an object of the invention to provide improved methods and apparatus for permitting hlgh throughput of fibers at relatively high speeds, yet wherein there is only a minimal tendency to form pills, nits or the like and, consequently~ where the amount of undesired materials separated and/or recycled can be substantially reduced.
In another of its important aspects~ it is an object of the invention to provide an improved screening arrangement for permitting high fiber throughput with effective screening of undesirable materials such as nits or the like and without subjecting the system to undesired screen plugging. It is an important object of the invention to provide a rotational screening system for a dry forming fiber deposition process wherein provision is made for maintaining a proper balance between rotor speed and both air supply and velocity so as to maintain acceptable cross-dimensional uniformity in the mass flow rate of air-suspended fibers being delivered to the forming wire so as to form an air-laid web characterized by its cross-directional basis weight uniformity which equals or exceeds the cross-directional uniformity of the fibers entering the former.
Therefore, in accordance with one aspect of the present invention there is provided the method of forming a quality web of air-laid dry fibers on a high speed production basis comprising the steps of: a) delivering dry fibrous materials to a forming head positioned over a forming surface;
b) conveying the dry fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs and aggregated fiber masses and in an environment maintained substantially free of fiber grinding and disintegrating forces; c) continuously separating f~om 1% to 10~ of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those haviny a bulk density in excess of .2g/cc.
so as to maximize the separation of aggregated fiber masses from the aerated bed; d) discharging such separated fibrous materials including the aggregated fiber masses contained therein from the forming head; e) discharging the individualized fibers and soft fiber flocs through a high capacity slotted screen; f) conveying the individualized fibers and soft fiber flocs discharged through the slotted screen at a fiber throughput rate anywhere in the range o .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 through an enclosed forming zone towards the moving foraminous forming surface in a rapidly moving air stream; g) air-laying the individualized fibers and soft fiber flocs on the moving foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on the forming surface with such web having a~nit level of from "0" to "3"; and, h) moving the foraminous forming surface at a controlled and selected speed so as to produce an air-laid web having a nit level of from "0" to "3" and any specific desired basis weight in lbs./2880 ft~2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./2880 ft.2.
In accordance with a further aspect there is provided an apparatus for producing a quality web of air-laid dry fibers on a high speed production basis comprising, in combination: a movable foraminous forming surface; a forming head mounted over and forming surface; means for delivering dry fibrous materials to the forminy head; means for conveying the dxy fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs, and aggregated fiber masses while maintaining the forming head substantially free of fiber grinding and disintegrating forces; means for continuously separating from - 13a -1% to 10% of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g./cc.
so as to maximize the separation of aggregated fiber masses from the aerated bed and discharging such separated fibrous materials from the forming head; a discharge opening formed in the forming head; a slotted screen mounted in the discharge opening; means defining an enclosed forming zone mounted between the discharge opening and the forming surface;
means for conveying the individualized fibers and soft fiber flocs from~the forming head through the slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.~ to at least 1.50 lbsO/hr./in.2 and through the forming zone towards the movable foraminous forming surface in a rapidly moving air stream and for air-laying the individualized fibers and soft fiber flocs on the movable foraminous forming surface so as to form an air-laid web of ~andomly oriented dry individualized fibers and soft fiber flocs on the surface during movement thereof with such web having a nit level of from "0" to "3", and, means for controllably moving the foraminous forming surface at a selectable speed so as to produce an air-laid web having a nit level of from "0" to "3" and any specific desired basis weight in lbs./2880 ft 2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./ 2880 ft~2.
DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention will become more readily apparent upon reading the following detailed description and upon reference to the attached drawings, in which:
FIGURE 1 is a schematic view, in side elevation, of one form of apparatus which may be employed for the air - 13 ~
deposition of dry fibers to form a web continuum in accordance with the present invention;
FIG. 2 is a schematic view here illustrating an exemplary - 13c -t~ ~
air-laid, dry fiber, web forming system utilizing two substantially identical cylindrical flow cvntrol and forming heads disposed in side-by-side relationship above the foraminous for~ing wire;
FIG. 3 is an oblique view, partially cut away, here schematically illustrating details of an exemplary novel fiber feed, eductor, flow control, screening, and fiber forming arrangement embodying features of the present invention;
FIG. 4 is a fragmentary front elevational view, partly in section, of the rotor assembly shown in FIG. 3;
FIG. 5 is an end view of a modi~ied rotor assembly similar to that shown in FIG. 3, but here depicting a rotor employing only four rotor ~ars;
FIG. 6 is a diagramatic plan view indicating in schematic, idealized fashion fiber movement through a conventional woven square-mesh screen under the influence OL
air movement and rotor action;
FIG. 7 is a view similar to FIG. 6 but here depicting movement of fibers through a high capacity slotted screen in which the slots are oriented parallel to the axis of the rotor in accordance with the invention;
FIG. 8 is a view similar to FIG. 7, but here illustrating the undesirable plugging action that occurs when the slots of à slotted screen are oriented in a direction generally perpendicular to a plane passing through the axis of the rotor;
FIG. 9 is a photograph illustrating the plugging of a slotted screen that occurs when the slots are oriented at an angle of approximately 45 to a plane passing through the axis of the rotor;
FIG. 10 is an enlarged, fragmentary side elevational view here depicting in diagramatic form the air/fiber stream as it moves through the rotor housing where an annular moving aerated bed of fibers is created and maintained and, thereafter, as it moves through the screening means and forming zone an~ is air-laid on the forming wire to form an air-laid web of fibers and, further, depicting the pressure relationships and the air velocity and rotor bar velocity relationships that are believed to exist when operating the system of the present invention at a desired one of several selectable sets of adjustablP parameters in terms of ratio of air-to-fiber supply, rotor speed, and recycle balance;
FIG. 11 is a highly enlarged view of a portion of the system shown diagramatically in FIG. 10, here depicting how the differential relative velocities of the rotor bars and air stream serve to gencrate a rapidly moving full-width zone of negative pressure in the wake of each rotor bar, thereby lifting fibrous materials off the screen in the region beneath th~ moving negative pressure zone, while permitting individual fibers to dive axially or end-wise through the openings in the screen in those regions of positive pressure drop across the screen between successive negative pressure zones;
FIG. 12 is a graphic representation of a typical set of curves indicative of the functional relationships existing with air-laid web forming systems e~bodying features of the present invention between fiber throughput for specific representative screen designs and rotor assembly operating parameters--viz., rotor RPM and the number of rotor bars -employed;
FIG. 13 is a graphic representation of the functional relationships existing between nit levels in a finished air-laid web made in accordance with the present invention, fiber throughput, and the percentage of fibrous materials separated and/or recycled prior to deposition on a moving forming wire, FIGS. 14 through 20 are photographs of exemplary air-laid fiber webs having increasing nit levels suitable for subjectively evaluating and rating web guality in accordance with subjective visual standards as to product acceptability established by the assignee of the present invention; and, FIG. 21 is a graphic representation depicting the relationship between fiber delivery rates expressea as fiber throughout in pounds per square inch per hour ~lbs./in.2/hr.) and both woven square-mesh screens and slotted screens having screen openings ranging from about 0.03`' in at least one direction to about 0.08" in at least one direction.
While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, eyuivalents and alternatives falling within the spirit and scope of the in-vention as expressed in the appended claims.
DETAILED DESCRIPTION
A. Definitions To facilitate an understanding of the ensuing description and the appended claims, definitions of certain selectedterms and phrases as used throughout the specification and clai~s are set forth below.
The phrase "pulp lum?" is herein used to describe a dense, bonded clump of fibers in the incoming fiberized supply which is most conventionally caused by hard pressing, non-uniform application of debonding agents, and/or inadequate opening or hammermilling. Pulp lumps are present in ordinary commercial grades of pulp.
The words "pill" and~or "rice" are herein each used to describe a dense, rolled up bundle of fibers, often including bonded fibers, which are generally formed by mechanical action during fiber transport or in a rotor chamber where the fibers are commonly, and often intentionally, subjected to mechanical disintegrating action.
The word "nit" is herein used to generically refer to pulp lumps, pills and/or rice. Nits are considered to be an unacceptable defect in light-weight tissues such as bath and facial tissues having basis weights of from 13 lbs./2880 ft 2 to 18 lbs./2880 ft. , and generally result in decreased tensile strength in webs of these, or even of heavier, basis weights.
The terms "floc" and "soft floc" are h~rein used to describe soft, cloud-like accumulations of fibers which behave like individualized fibers in air; i.e., they exhibit relatively high co-efficients of drag in air.
The phrase "aggregated fiber masses" is herein used to generically embrace pulp lumps, pills, rice and/or nits, and to describe aggregations of bonded and/or mechanically entangled fibers generally having a bulk density on the order of greater than .2 grams per cubic centimeter (g./cc.).
Aggregated fiber masses are to be distinguished from flocs and/or soft flocs whose bulk density is generally less than .2 g./cc. Moreover, aggregated fiber masses have a relatively low coefficient of drag in air.
"Bulk density" in the weight in grams of an uncompressed sample divided by its volume in cubic centimeters.
The phrase "semi-cylindrical" is used herein to describe a portion of the rotor chamber wall and/or forming screen, and is intended to mean that wall portion from the upstream leading edge of the screen to and including the full~width separator slot. In the various exemplary embodiments herein described, the phrase "semi-cylindrical" embraces a peripheral wall portion having an included angle of less than 180.
However, such phrase is used herein in a descriptive sense and is not intended to be construed in a limiting sense since those skilled in the art will appreciate as the ensuing description proceeds that the rotor chamber could be cylindrical, or substantially cylindrical, in which event the phrase "semi-cylindrical" would be intended to embrace peripheral wall portions having an included angle of greater than 180.
The phrase "2-dimensional" is used to describe a system for forming a web wherein: i) the cross-section of the syste~
and the flows of air and fiber therein are the same at all sections across the width of the system; and ii), where each increment of system width behaves essentially the same as every other increment of system width; thereby permitting the system to be scaled up or down to produce high quality webs of any suitable and commercially useful widths on a hi~h-speed production basis and wherein a web's cross-directional profile in terms of basis weight can be controlled and, preferably, can be maintained uniform.
The phrase "coefficient of variation" is used herein to describe variations in the cross-directional basis weight profile of both the web being formed and the fibrous materials input to the system, and comprises the standard deviation (u) expressed as a percent of the mean. The coefficient of variation should not vary more than 5% and, preferably, should vary less than 3~ in the cross-machine direction. The basis weight profile in the cross-machine direction of the web being formed may, for example, be determined by weighing strips of the web which are three inches in width (3" C.D.) by seven inches in length (7" M.D.).
The phrases "uniform cross-directional profile", "uniform mass quantum of fibers in the cross-machine direction", and similar phrases, are herein used to describe a condition in the web being formed, as well as in the fiber delivered to the forming apparatus, wherein the coefficient of variation does not vary more than 5% and, preferably, varies less than 3% in the cross-machine direction.
~he phrases "controlled cross-directional deposition", "controlled cross-directional profile", "controlled mass quantùm of fibers in the cross-machine direction", and similar phrases, are herein used to describe a condition wherein the cross-directional profiles of the fiber feed and the web being formed are not necessarily uniform but, rather, may intentionally be non-uniform; and, because the system is substantially devoid of cross-directional flows, the cross-directional profile of the finished web is controlled so as to be similar in profile to the cross-directional profile of the fiber feed --e.g., if the fiber feed has twice the mass quantum of fibers at its center as it does along its marginal edges, the basis-weight of the web produced will also be _ ~9 _ csm/~,~
approximately twice as great a-t its center than at i-ts marginal edges when viewed in cross-directional profile.
One convenient way of delivering fibers to the system is to form a feed mat having a controlled and/or uniform cross directional weight profile. Such systems are described in detail in subsequent portions of this specification. However, other means of delivering fibers to the system at a controlled weight rate in the cross direc-tion may be devised and are within the scope of this invention. While the invention will herein be described in large part in terms of a fibrous material input to the system wherein the coefficient of variation is not more than 5% and, preferably, is less than
3~, and the formation of air-laid webs having a uniform cross-directional profile with a coefficient of variation of not more than 5% and, preferably, of less than 3~, since there is presently a significant demand for such products--particularly in the case of relatively low basis weight webs on the order of 13 lbs./2880 ft. 2 to 18 lbsu/2880 ft. 2 --it should be understood that the invention is not limited to the formation of webs having uniform cross-directional profiles but, rather, is equally useful in the manufacture of webs having controlled cross-directional profiles, both uniform and non-uniform.
The term "throughput" and the phrase "rate of web formation" are herein used generally interchangeably and are to be distinguished from the phrase "ratè of fiber delivery".
Thus, the phrase "rate of fiber delivery" is intended to mean the mass quantum or weight rate of feed of fibrous materials delivered to the forming head, and may be expressed, for example, in units of pounds per hour per inch of former width (lbs/hr./in.), pounds per minute per foot of former width - 2 ~-csm/~
'7~
(lbs./min.Jft.), or in any other suitable units. "Throughput", on the other hand, is intended to describe the screening rate for fibrous materials discharged from the forming head--i.e., the mass quantum or weight rate of fiber delivery through the former screen per unit area of screen surface--and may be expressed, for example, in units of pounds per hour per square inch of effective screen surface - 20a -csm/~
5~
area (lbs./hr./in.2), pounds per minute per s~uare foot of effective screen surface area (lbs./min./ft.2), or any other suitable units. The fiber "throughput" achieved is refdected directly in the "rate of web formation" and may be calculated by multiplication of fiber throughput by the effective length of ihe former screen. "Rate of web formation"--i.e., the rate at which the air-laid web is formed on the moving forming wire or other formlng surface--may be expressed, for example, in units of pounds per hour per inch of former width (lbs./hr./in.), pounds per minute per foot of former width (lbs./min./ft.), or in any other suitable units.
The words "up", "down", "above" and/or "below" are used in a relative, non-limiting sense to describe, merely by way _ example, a relationship of one structural element to a forming wire or to another structural element.
B. Overall System Descriptlon Briefly, and referring first to FIG. 1, there is illustrated an exemplary system for forming an air-laid web 60 of dry fibers and comprising: a fiber metering section, generally indicated at 65; a fiber transport or eductor section, generally indicated at 70; a forming head, generally indicated at 75, where provision is made for controlling air and iber flow, and where individual fibers are screened rom undesirable aggregated fiber masses and, thereafter, are air-laid on a foraminous forming wire 80; a suitable bonding station, generally indicated at 85, where the web is bonded to provide strength and integrity; a drying station, generally indicated at 87, where the bonded csm/~
. .
5~
web 60 is dried prior to storage; and, a take-up or reel-type storage station, generally indicated at 90, where the air-laid web 60 of dry fibers is, after bonding and drying 7 formed into suitable rolls 95 for storage prior to delivery to some subsequent processing operation (not shown) where the web 60 can be formed into specifically desired consumer products.
In order to permit continuous removal of aggregated fiber masses, the forming head 75 includes a separator system, generally indicated at 76. Such separated aggregated fiber masses and individualized fibers entrained therewith are preferably removed from the forming area by means of a suitable conduit 77 maintained at a pressure level lower than the pressure within the forming head 75 by means of a suction fan (not shown). The conduit 77 may convey the masses to some other area (not shown) for use in inferior products, for scrap, or, alternatively, the undesirable aggregated fiber masses may be recycled via conduit 78 to a hammermill, generally indicated at 100, where the masses are subjected to secondary mechanical disintegration prior to reintroduction into fiber meter 65. Finally, the forming head 75 also includes a forming chamber, generally indicated at 79, positioned immediately above the foraminous forming wire 80. Thus, the arrangement is such that individual fibers and soft fiber flocs pass through the forming chamber 79 and are deposited or air-laid on the forming wire 80 to form a web 60 characterized by its controlled cross-directional profile and basis weight.
C. Fiber Metering Section -30While various types of commercially available fiber metering systems can, with suitable modifications, be employed with equipment embodying the features of the present invention, one system which has been found suitable and which permits of the necessary modifyiny adaptations is a RANDO-~EEDER~ (a registered trademark of the manufacturer, Rando Machine Corporation, Macedon, New York). The fiber metering section 65 shown by way of example in FIG. 1 is such a system. Indeed, a RANDO-FEEDER~ is ideally suited for us~ with the present invention when attempting to work with synthetic fibers.
As here shown, the fiber metering section 65 is mounted on the mezzanine floor level lO1 of a suitable paper mill. Fibers may be fed to the fiber separator hopper 102 in any of a variety of conventional ways. For example, pre-opened fibers may be ~anually introduced in bulk through inlet chute 103 which is provided with a closure member 104 so as to maintain an enclosed chamber. Alternatively, batts or other compacted fibers may be introduced through inlet 105 of hammermill lO0 (which is here shown only in diagrammatic block-and-line form and may take any well known conventional form). The compacted batts are fiberized within the hammermill and, after fiberization, the individualized fibers are delivered to the fiber separator hopper 102 via inlet 106.
A fan 107 is provided for removing excess air fro~ the fiber separator hopper 102, thereby permitting the fibers to form a loose fiber bed 108 at the bottom of the hopper 102. Thus, the fan 107 functions to withdraw excess air from the hopper 102 and such excess air, together with some escaping fibrous materials, are thereafter discharged into a suitable waste air filter or cyclone separator (not shown~. If desired, a conventional pre-feeder and opener-blender (not shown) can 0 be used to feed individualized fibers to the fiber meter 65.
In operation, fibers fall from the fiber separator ~ ( ~ 57;Z
hopper 10~ and form a loose bed 108 of open fibers carried by a floor apron conveyor 109. ~n anti-static spray system llO may be provided to minimize adherence of the fibers to portions of the system. The fibers are conveyed by the 5 floor apron conveyor 109 to an elevating apron conveyor 111 havin~ conventional pins and slats (not shown). Fibers are carried upwardly by the elevator apron conveyor to a rotating stripper apron 112 which serves to remove excess fiber stock and return such excess stock to the bed 108. The arrangement is such that a controlled, metered quantity of small opened tufts of fiber remains on the pins of elevator apron conveyor 111 and is carried over the top thereof uniformly across the entire width of pron 111 into an area 113 known as an air bridge.
Fibers delivered to the air bridge 113 are doffed from the pins on apron 111 by means of air flow under the control of a suitable air volume controller 114-. As a result of the flow rate of air movement, a controllable quantity of fibers--uniform throughout the full width of air bridge 113- are deposited on a rotating condenser screen llS, thus forming a full-width uniform feed mat 116 conveyed by roller CQnVeyor 118 to a feed plate 119. The arran~ement is such that as the feed mat 116 takes shape, the resistance of the mat on condensor screen 115 serves to reduce air flow through the screen and, conseguently, proportionally less doffing occurs at apron 111 until a condition of equilibrium is reached.
At the equilibrium point, a sufficient guantity of fibers are doffed to form a continuous uniform feed mat 116, with the balance of unused fibers being returned by the pins on elevator conveyor 111 to the fiber bed 108.
The full-width uniform feed mat 116 is then conveyed over feed plate ll9 by means of feed roller 120 and into the path of teeth formed on an opening roll or lickerin 121.
The lickerin 121 serves to comb individual fibers from the feed mat 116 with the individualized fibers being picked up and carried by a full-width air strea~ passing under feed plate 119 and generated by fan 124 and eductor 70. From this point, the entrained stream of individualized air~suspended fibers is introduced into the main air supply strea~ generated by fan 124 and carried through eductor 70 and the forming head 75, with the fibers exiting the forming head 75 passing through the forming chamber 79 and being uniformly deposited across the full-width of forming wire 80 in a uniform, but completely random, fiber pattern, thereby forming web 60.
D. ~eb Forming, Com~acting, Bonding, Dryin~ & Storage Section As heretofore indicated, fibers are air-laid on the foraminous forming wire 80 at the forming station by means of an air stream generated primarily by fan 124. In addition, a vacuum box 126 positioned immediately below the forming wire 80 and the web forming section 79 serves to maintain a positive downwardly moving stream of air which assists in collecting the web 60 on the moving wire 80. If desired, a second supplementary vacuum box 128 may be provided beneath the forming wire at the point where the web 60 exits from beneath the forming cha~ber 79, thereby insuring that the web is maintained flat against the forming wire.
After for~ation, the web 60 is passed through calender rolls 129 to lightly compact the web and give it sufficient integrity to permit ease of transportation to conveyor belt 130. A light water spray can be applied from nozzle 131 in order to counteract static attraction between th~ web and the wire. An air shower 132 and vacuum box ~34 serve to 7~2 ~
clean loose fibers from the wire 80 and thus prevent fiber build-up.
After transfer to the belt 130, the web 60 may be bonded in any known conventional manner such, merely by way of example, as i) spraying with adhesives such as latex, ii) overall calendering to make a saturating base papex--i.e., a bulky web with a controlled degree of hydrogen bonding-~
adhesive print pattern bonding, or other suitable process.
Such bonding processes do not form part of the present invention and, therefore, are neither shown nor described in detail herein, but, such processes are well known to those skilled in the art of non-woven fabric manufacture. For example, the web 60 may be pattern bonded in the manner described in greater detail in the aforesaid Dunning U.S.
Pat. No. 3,692,622 assigned to the assignee of the present invention. Briefly, in this bonding process, the moisture content of the web is adjusted to 6~ to 3596 by a water spray 135 and, thereafter, the web is bonded by passing it through the nip between a small hard roll 136 and a patterned steel roll 138. Subsequently, the bonded wf~b 60 is transferred to conveyor belt 139 and transported thereby through the drying station 87 to the storage station 90 where the web 60 is taken up on a driven reel 140 to form roll 95 which may thereafter be either stored for subseguent use or unwound at a sùbsequent web processing station (not shown) ~o form any desired end product. The drying station 87 may take any suitable conventional form such, for example, as a pair of closely spaced heated plates 88, 89, or an oven or heated roll (not shown).
Referring to FIG. 2, there has been diagrammatically illustrated a typical system employing multiple forming 57~
heads for increasing overall productivity of the air-laid dry fiber web forming system. As here shown, multiple forming heads 75A - 75N are positioned over the foraminous forming wire 80, with each forming head being supplied with a full-width uniform supply of air-suspended fibers fed from respective ones of a multiplicity of hammermills and fiber mèters (not shown in FIG. 2, but respectively similar to the hammermill 100 and fiber meter 65 shown in FIG. 1). Of course, while only two forming heads 75A and 75N have been shown for illustrative purposes in FIG. 2, those skilled in the art will appreciate that any desired number of forming heads could be used dependent upon the productivity desired in terms of the web's basis weight, forming wire speed, and the speed at which the bonding station can be effectively operated. Thus, it will be appreciated that the air-laid web 60 is formed by a first layer of fibers 60A deposited by forming head 75Aj and _ (where n = any whole integer) successive layer(s) 60N deposited by _ downstream forming head(s) 75~J.
As a consequence of this construction, the speed of the forming wire ~ay be increased by a multiple of the number of forming heads employed.to form a composite web 60 of a selected basis weight for a given forming wire speed.
E. Full-Width Metered Fiber Feed In carrying out the present invention, provision is madè for forming a full-width feed mat of fibers having a controlled cross-directional profile in terms of the mass quantum of fibers constituting the mat. To this end, and as best illustrated in FIG. 3, feed mat 116 may be formed in the manner previously described in connection with the fiber metering section 65 shown by way of example in FIG. 1. Such feed mat 116 has been found to meet the preferred conditions s~ ~
of full-width uniformity in terms of the mass quantum of fibers forming the mat and the coefficient of variation of the fibrous materials input to the system. The mat thus formed--e.g., mat 116--is then fed across feed plate 119 by means of a feed roller 120 into the teeth on lickerin 121 ~hich serves to disaggregate the fibers defining the mat by combing such fibers (along with any pulp lumps, nits and other aggregated fiber masses which are present) out of the mat and feeding such materials directly into a high volume air stream generated by fan 124 (FIG. 1~ and eductor 70 (FIGS. 1 and 3).
In order to permit attainment of the objectives of the invention, the air-to-fiber ratio preferably employed when working with cellulosic wood fibers is on the order of 200-600 cubic feet of air (at standard temperature and atmospheric pressure conditions) per pound of fiber--viz., 200-600 ft.3/lb. Moreover, when employing the exemplary equipment herein described such air is supplied at relatively high volumes which vary dependent upon the operational speed of the rotor assembly and the types of fibers being worked with--i.e., ~olumes ranging from 1,000 to 1,800 ft.3/min./ft. of former width are conventional when working with cellulosic wood fibers. For example, when employing an 8-bar rotor operating at 1432 RPM, the volume of air supplied is preferably on the order of 1500-1650 ft. /min./ft. of former width. On the other hand, when working with synthetic f ibers or cotton linters, f example, considerably higher volumes of air per pound of fiber may be employed--e.g., the air-to-fiber ratio may range from 1,000 to 3,000 ft.3/lb., or even higher.
In operation, the air-suspended fiber stream is conveyed through a suitable fiber transport duct 170 (FIG. 3) from .1.'~.'`1~7~
the full-width eductor 70 to a full-~idth inlet slot 171 formed in the upper surface of, and extending fully across, a generally cylindrical housing 172 which here defines the 2-dimensional flow control, screening and separating zone 75. To insure that full-width mass quantum fiber control is maintained, the exemplary duct 170 is preferably subdivided into a plurality of side-by-side flow channels separated by partitions 174 extending the full length of the duct. It has been found that the desired coefficient of variation constraint in the web being formed can be obtained by spacing the partitions 174 apart by approximately four inches so as to form a plurality of adjacent flow channels extending across the full axial length of housing 172. It has also been found that a partitioned duct arrangement of the type shown in FIG. 3 can be advanta~eously used to accommodate width differences between the feed mat 116 formed in the fiber metering section 65 and the final air-laid web 60 deposited on the foraminous forming wire 80.
For example, excellent results have been obtained when attempting to form a web 60 forty-eight inches in width, utilizing a feed mat 116 only forty inches in width. Thus, in such a system the duct 170 may diverge from a full-width at its upper end of forty inches to a full-width at its lower end of forty-eight inches, with the individual flow channels defined by partitions 174 diverging from approximately three and one-third inches in width to approximately foux inches in width. This insures that cross-flow forces are substantially eliminated, and the mass quantum of air-borne fibrous materials delivered from the lower end of duct 170 to the flow control, screening and separating zone 75 remains substantially unchanged across the full width of the o29--3~7;2 system.
F. Flow Control, Screening and Separat.ion In carrying out the invention, a 2-dimensional cylindrical rotor former is provided which serves to control flow of the air-suspended fi~er stream through a separation zone while minimizing mechanical disintegration of fibrous materials, and which is designed to provide an acceptable level of fine scale air turbulence while insuring that the system is substantially devoid of eddy currents and other undesired cross-flow forces so as to maintain a controlled mass quantum of fibers across the full width of the forming head 75. To accomplish this, the exemplary forming head 75 includes a rotor assembly, generally indicated at 175 in FIGS. 3 and 4, mounted for rotation within housing 172 about a horizontal axis defined by shaft 176. The arrangement is such that the air-suspended fibrous materials introduced radially into housing 172 through inlet slot 171 are conveyed by co-action of the air stream and the rotor assembly 175 through the housing 172 for controlled and selective discharge either a) through a full-width discharge opening, generally, indicated at 178 in FIG. 3, and into forming zone 7~ for ultimate, air-laid deposition on forming wire 80 or, alternatively, b) through a full-width tangential separator slot ~79 formed in housing 172 downstream of the discharge opening 178. The separator slot 179, which here forms part of the separation and/or recycle zone 76 (FIGS. 1 and 3), is preferably on the order of from 3/16" to 3/8" in circumferential width when working with wood ~ibers and, if desired, may he adjustable in any conventional manner ~not shown) so as to permit circumferential widening or narrowing of the slot 179 to optimize separation conditions~
35~;~
~ o per~it controlled, selective discharge of individualized fibers and soft fiber ~locs through opening 178 and into forming zone 79~ while at the same ti~e precluding discharge of nits and other undesired aggregated fiber masses there-through, suitable screening ~eans, generally indicated at180 in FIG. 3, is mounted within discharge opening 178.
Such screening means 180 may, simply take the form of a conventional woven square-mesh wire screen of the ty~e shown at 180A in ~IG. 6 and having openings sized to preclude passage of aggregated fiber masses~ 2 ~ the screen may take the form o~ an 8x8 mesh screen having 64 openings per square inch, a lOxlO mesh screen, a 12x12 mesh screen, or other commonly available woven mesh screens; provided only tha~ the screen openings do not exceed 0.1" open space from wire-to-wire in at least one direction and ha~e between 30~ and 55~ open area and, preferably, between 38~ and 46~ open area. As bes~ shown in FIG. 3, screening means 180 is formed with the same radius of curvature as the se~i-cylindrical portion of housing 172 ~ithin which discharge opening 178 is formed~
As best illustrated by reference to ~IGS. 3 and 4 conjointly, rotor assembly 175 comprises a plurality of transversely extending rotor bars 181, each fixedly mounted on the outer periphery of a plurality o~ closely spaced spiders 182. ~he spiders 182 are, in turn, fixedly ~ounted on shaft 176 which is journalled for rotation in outboard bearing housings 183, 184 (FIG. 4) and which is coupled ~o drive shaft 185 driven by any suitable means ~not shown~.
The arrangement is such that the high volume air-suspended 5'7~
stream of fibrous materials passing through duct 170 is introduced radially into housing 172 through inlet slot 171 and such stream tends to pass across the rotationally driven rotor bars 181--viz., the bars 181 move through the radially entering stream of air-suspended fibers. As a result of rotor bar movement and the high velocity movement of the air stxeam, the air and fibers tend to move outwardly towards th~ wall of housing 172, thus forming an annular, rotating, aerated bed of fibrous materials, best illustrated at 186 in FIG. 10. Such annular-aerated bed 186 of fibrous materials is believed to be on the order of one-half inch to one and one-half inches thick (dependent upon actual operating parameters), and is believed to be moving rotationally at about half the spe~d of the rotor bars 181. For example, in a cylindrical 'ormer having an inside housing diameter of 24" where the rotor assembly 175 is being driven at 1432 RPM, the tip velocity of the rotor bars 181 is on the order of 150 f.p.s. (feet/second) and, consequently, it is believed that the velocity of the aerated bed 186 is on the order of 80 f.p.s. ~hus, since the rotor bars 181 are moving at 150 f.p.s. through an aerated bed of fibers moving in the same direction at approximately 80 f.p.s., the relative velocity between the aerated bed 186 of fibers and the rotor bars 181 is on the order of 70 f.p.s.
In keeping with the invention, the rotor assembly 175 is preferably designed a) to minimize pumping action which tends to reduce the relative speed differential between the rotor bars 181 and the aerated bed 186, thus causing the fibers to move over and beyond the screening means 180, and b) so as to minimize mechanical action between the rotor bars 181 and both the housing 172 and screening means 180, which action tends to disintegrate fibers and aggregated fiber masses carried in the air stream and to generate pills. To this end, the rotor bars 181 are generally of relatively small cross-section--e.g., in the case of the exemplary rectangular bars shown in FIG. 10, such bars are on the order 3/4" in radial height by 3/8" in thickness, such thickness dimension being desired only for purposes of structural integrity--and are moun~ed so as to provide a clearance between the outer edges of the bars 181 and the inner wall surface of the housing 172 and screening means 180 of from 0.10 inches to 0.25 inches and, preferably, from 0.18 inches to 0.20 inches, at least during transit of the rotor bars from the upstream edge 188 of screening means 180 through separator slot 179. In terms of "pumping action", therefore, the signficant bar area is only 3/4" times the width in inches of the forming head 75. To avoid generation of cross-flow forcest it is important that the rotor bars 181 are continuous, extend the full width of the rotor chamber, and are oriented parallel to the axis of the rotor assembly 175.
In carrying out the present invention, the rotor housing 172 is preferably semi-cylindrical in cros~-section throughout at least the arcuate span ranging from the upstream edge 188 of screening means 180 through the tangential separator slot 179, thereby insuring proper clearance between the rotor bars 181 and the inner periphery of both the screening means 180 and housing 172 as the rotor assembly 175 is driven rotationally. The remaining upper segment of the housing 172 may be of any desired shape, including substantially semi-cylindrical, but is preferably relieved immediately adjacent the downstream edge of the inlet slot 171 as indicated at 189, thereby preventing the tendency of those fibers passing the separator slot 179 from impinging against the vertical edge 190 of inlet slot 171 and causing consequent blockage, or partial blockage, of the inlet slot.
Referring again to FIG. 3, it will be apparent from the description as thus far set forth, that as air-suspended fibers are introduced radially into the rotor housing 172 through inlet slot 171, they are moved rapidly through the housing under the influence of the air stream and movement of the rotor bars 181, thus forming the moving annular aerated bed 186 of fibers (FIG. 10) about the lnner periphery of the housing wall. As the aerated bed--which contains individualized fibers, soft fiber flocs, nits and other aggregated fiber masses--passes over the screening means 180, some, but not all, of the individualized fibers and soft fiber flocs pass through the screening means into the forming æone 79, while the balance of the individualized fibers and soft fiber flocs, together with nits and other aggr~gated fiber masses, pass over the screen without exiting from the rotor housing 172. The undesired pills, rice and nits--i.e., aggregated fiber masses--have a bulk density generally in excess of .2 g./cc. and tend to be separated along with some individualized fibers and soft fiber flocs from the aerated bed 186 at the tangential separator slot 179, Wit]l those separate~ materials being centrifugally expelled through the slot 179 where they are entrained in a recycle or separating air stream generated by any suitable means Inot shown) coupled to manifold 191 with the air=
suspended separated particles moving outward through a full-width discharge passage 192 coupled to separator slot 179 and, ultimately, to conduit 77 (FIG. 1). Such separation is aided by a positive air outflow from housing 172 through separator slot 179.
In keeping with the invention, provision is made for insuring positive separation of undesired nits and aggregated fiber masses from individualized fibers and soft fiber flocs, and for preventing movement of the latter through separator slot 179 to the full extent possible, thereby insuring that individualized fibers and soft fiber flocs are retained within rotor housing 172 and move with the aerated bed 186 back to the area of screening means 180 where such desirable materials have successive opportunities to pass through the screening means 180 into the forming zone 79.
To accomplish this, a full-width classifying air jet 194 is provided upstream of the separator slot 179 and downstrear:~
of screening means 180; such air jet being positioned to introduce a full-width air stream generated by any conventional source (not shown) radially into rotor housing 172 just ahead of the separator slot 179. As a consequence, the positive classifying air stream introduced radially into housinq 172 through air jet 194 tends to divert individualized fibers and soft fiber flocs within the aerated bed 186 radially inward as a result of tlle relatively high drag coefficients of such materials and their relatively low bulk density (which is generally on the order of less than .2 g./cc.). Since the nits and aggregated fiber masses have a relatively high bulk density in excess of .2 g./cc. and relatively low drag coefficients, the classifying air stream introduced through the full-width air jet 194 does not divert such materials to any significant extent and, therefore, such undesired materials tend to be centrifugally expelled through the tangential separator slot 179. It has been "~ t~;~
found that the introduction of classifying air through the full-width classifying air jet 194 into housing 172 at pressures on the order of from 50" to 100" H2O and at volumes ran~ing from 1.5 to 2.5 ft.3/min./in. provides an energy level adequate for deflecting a significant portion of the indi~idualized ~ibers and soft fiber flocs. The energy level of the classifying air jet is most conveniently controlled by adjusting its pressure.
In operation, it has been ound that excellent results io are obtained by li~iting the amount of fibrous material removed from the system through separator slot 17g to less than 10~ by weight and, preferably, to between 1% and s% by weight, o~ the fibrous material introduced into the housing 172 tllrough inlet slot 171. Stated differently, at least 90~ of the ~ibrous materials introduced and, preferably between 95% and 99% thereof, ultimately pass through screening means 180 into the forming zone 79 and are air-laid on the foraminous forming wire 80 without requiring any secondary hammermilling operations and without being subjected to any significant mechanical disintegrating forces~ The quantity o~ material separated may be controlled by the operator by varying the voiume of recycle air supplied through manifold 191 and/or by adjusting the circumferential extent of full-width se~arator slot 179 in any suitable manner tnot shown).
AIR-LAID DRY FIBER WEB FO~IATION IN
` ACCORDANCE WITH THE PRESENT INVENTION
Thu~ far, the environment of the invention has been described in connection with methods and apparatus wherein .. i .1 ~,,f1 ~t 7~
a conventional woven square-mesh screen of the type shown at 180A in FIG. 6 is mounted in the discharge opening 178 in forming head 75. While such a web ~orming system has provided significant advantages in terms of fiber throughput capacity for a 2-dimensional forming system, particularly when contrasted with conventional sifting systems of the type disclosed in the aforesaid Kroyer patents, the present inventors have discovered that even yreater improvements can be achieved in fiber throughput capacities when using 2-dimensional web forming systems.
_ High Throughput Screening In accordance with one of the important aspects of the present invention, provision is made for substantially increasing the fiber throughput capacity of a 2-dimensional fiber forming system, yet wherein aggregated fiber masses present within the forming head are effectively precluded from entering the forming zone. Rather, such aggregated fiber masses are separated from the aerated fiber bed 186 in the forming head 75 (FIG. 3) and are discharged through full-width separator slot 179. To accomplish this, a high-capacity slotted screen 180B of the type shown in FIG. 7 is mounted within discharge opening 178 with the screen slots oriented with their long dimensions parallel to the axis of rotor assembly 175.
When utilizing a slotted type screen 180B with a 2-dimensional rotor assembly 175 mounted for rotation about a horizontal axis, it has been found essential that the screen slots be oriented with their long dimensions parallel to the axis of the rotor assembly. When so oriented, individualized fibers tend to move through the screen slots while nits and aggregated fiber masses--e.g., the aggregated fiber masses .
3~
195 shown in FIG. 7--are precluded from passing through the screen since they are generally larger in size then the narrow dimensions of the slots which may range between .02"
and 0.1" open space froln wire-to-wire in at least one direction and, preferably, ranges between .045" and .085"
open space from wire-to-~ire in at least one direction.
Such wire-to-wire dimensions are particularly critical when the system is being used to make high quality, lightweight tissue webs--e.g., webs having low nit levels and basis weights ranging from 13 lbs./2880 ft.2 to 18 lbs./2880 ft.2 and, in some instances, up to 22-25 lbs~/2880 ft.2. However, when the slots of a slotted screen 180B are oriented with their long dimensions perpendicular to a plane passing through the rotor axis as shown in FIG. 8, it has been found that the screen tends to rapidly plug--indeed, when operating under commercial production conditions, it has been found that the screen tends to become completely plugged almost instantaneously. It is believed that such plugging action results from the tendency of individual fibers to "staple"
or "hair-pin" and otherwise hang up or collect within the narrow confines at the end of each slot as best indicated at 196 in the lower right-hand corner of FIG. 8; and, as soon as a few fibers have collected, other fibers and aggregated fiber masses 195 almost instantaneously agglomerate on the screen as depicted in the balance of FIG. 8. This plugging phenomenon is more clearly visible upon reference to the photograph reproduced as FIG. 9--such photograph illustrating a slotted screen 180B wherein the slots are oriented at an angle of 45 to a plane passing through the rotor axis--and, under these conditions, the screen 180B plugged almost completely and instantaneously.
7~ ~
On the other hand, it has been found that a conventional woven square-mesh screen of the type shown at 180A in FIG. 6, and a slotted screen 180B with the slots oriented as shown in FIG. 7, exhibit little or no tendency to plug undex normal operating conditions. Rather, while individualized fibers still have a tendency to "staple" or "hair-pin", as indicated at 197 in FIGS. 6 and 7, there seems to be adequate time and room for the suspended fibers to disengage themselves from the screen; whereas in the arrangements shown in FIGS.
8 and 9, the suspended fibers ~end to catch and congregate in the closely proximate confined corners of the screen slot and, as a result, other fibers and aggregated fiber masse~
195 rapidly accumulate, thus plugging the screen and rendering the system inoperative.
The exemplary system herein described has been depicted in FIGS. 3 and 4 as including a rotor assembly 175 having eight rotor bars 181. However, the number and/or shape of the rotor bars may be varied, provided that such ~odifica-tions are consistent with mechanical stability and low rotor "pumping" action. That is, the rotor assembly 175 must be a dynamically balanced assembly and, therefore, it must include at least two rotor bars. However, it will be appreciated that it can include fewer or more than the eight bars illustrated in FIGS. 3 and 4--for example, excellent results have been achieved with a 4-bar rotor assembly of the type indicated at 175' in FIG. 5. On the other hand, care must be taken to insure that the number of rotor bars employed--e.g., _ rotor bars where n equals any whole integer greater than "1"--and the shape of the rotor bars are such that pumping action is minimized. Otherwise, the rotor assembly 175 will tend to sweep the aerated fiber bed 186 9~;~
over and beyond the screening means 180 rather than permitting and, indeed, assisting fiber movement through the screening means.
In the illustrative form of the invention, the rotor bars 181 have a rectangular cross-section, and pumping action is minimized by keeping the effective rotor bar area relatively small~ ~ 3/4" times the length of the bars which extend across the full width of the rotor housing 172--and by spacing the bars apart circumferentially by 45 (there being eight equally spaced bars) and from the housing 172 by on the order of 0.18" to 0.20". However, the rotor bars 181 need not be rectangular in cross-section. Rather, they can be circular, vane-shaped, or of virtually any other desired cross-sectional configuration not inconsistent with the objective of minimizing rotor pumping action. For example, rotor bars having a circular cross-section would, because of their shape, be even more effective than rectangular bars in terms of minimizing rotor pumping action. However, thè primary function of the rotor assembly as employed in the present invention is, as more fully described in Section I, page 45 _ seq., infra, of this specification, to lift individualized fibers, soft fiber flocs, and aggregated fiber masses off the surface OL the former screening means and, thereby, to prevent plugging of the screen, to prevent 2~ layering of fibers on the screen, and to reopen apertures in the screen so as to permit passage of the air-suspended fiber stream therethrough. This desirable result is achieved by the negative pressure 20nes created in the wakes of the moving rotor bars; and, the negative pressure zones in the wakes of rotor bars having a rectangular cross-section have been found to be as effective for this purpose as those .
,t~57~ ~
created by rotor bars of circular cross-section.
It is significant to a complete understanding of the present invention that one understand the difference between the primary function of the rotor assembly here provided--viæ., to lift fibrous materials upwardly and off the screenor, stated differently, to momentarily disrupt passage of the air-suspended fiber stream through the screen--and that stated for conventional cylindrical rotor systems of the type disclosed, for example, in the aforesaid 3.D'A. Clark patents where the rotor chamber functions as a "disintegrating and dispersing chamber" (See, e.~., col. 4, line 53, J.D'A. Clark ~.S. Pat. No. 2,931,076)--~iz., where the rotor blades mechanically act upon the fibrous materials to "disintegrate"
such materials and propel them through the screen.
H. Forming Zone In keeping with another important aspect of the present invention, provision is made for insuring that individualized fibers passing through the screening means 180 shown in FIG. 3 are permitted to move directly to the foraminous forming wire 80 without being subjected to cross-flow forces, eddy currents or the like, thereby maintaining cross-directional control of the mass quantum of fibers delivered to the forming wire through the full-width of forming zone 79. To accomplish this, provision is made for insuring ~hat the upstream, downstream and side edges of the forming zone--i.e., the boundaries of the zone 79-~are formed so as to define an enclosed forming zone and to thereby preclude intermixing of ambient air with the air/fiber stream exiting housing 172 through screening means 180. It has been found that the air/fiber stream exiting from housing 172 through screening means 180 does not exit radially but, rather, at an acute angle or along chordal lines or vectors which, on average, tend to intersect a line tangent to the mid~point of the screening means 180 at an includ~d angle cr. In the exemplary form of the invention where the screening means 180 covers an arc of approximately 86--i.e., an arc extending clockwise as viewed in FIG. 3 from a point (indicated at 198 in FIG. 3) a~proximately 15~ from the center of inlet slot 171 to a point 188 approximately 245 from the center of inlet slot 171--and, where an 8-bar rotor is being operated at a rotor speed on the order of 1400-1450 RPM, it has been found that the angle cr is generally on the order of 11.
Consequently, the forming zone 79 is preferably provided with sidewalls (a portion of one such sidewall is shown at 199 in FIG. 3), a full-width downstrea~ forming wall 200, and a generally parallel full-width upstream forming wall 201, which are respectively connected to rotor housing 172 at the downstream and upstream edges of screening means 180, and which respectively lie in parallel planes which intersect a linP tangent to the mid-point of the screening means 180 at included angles on the order of 11. The upstream end of forming wall 201 is bent as indicated at 201A, 201B so as to form a shaped portion which generally accommodatas the air/fiber flow pattern exiting the upstream portion of screening means 180. The walls 199, ~00 and 201 serve to enclose the forMing zone 79 and to thereby preclude disruption of the air/fiber stream as a result of mixing between ambient air and the airtfiber stream. The enclosed forming zone 79 is preferably maintained at or near atmospheric pressure so as to prevent inrush and outrush of air and to thereby assist in precluding generation of cross-flow forces within the forming zone. Those skilled in the art will appreciate that angle a can vary with changes in operating parameters such, for example, as changes in rotor RPM. ~owever, for operation at or near optimum conditions, it is believed that the angle a will generally lie within the range of 5 to 20 and, preferably, will lie within the range of 8 to 15.
The lower edges of forming walls 200, 201 terminate slightly above the surface of foraminous forming wire 80--generally terminating on the order of from one-quarter inch to one and one-quarter inches above the wire.
In the exemplary form of the invention shown in FIG. 3, when the angle a is on the order of 11 and when the forming zone 79 is positioned over a horizontal forming surface 80, the upstream and downstream forming walls lie in planes which intersect the horizontall~ disposed forming surface 80 at included acute angles ~ ~here ~ is on the order of 33. Howevcr, those skilled in the art will appreciate that the angular value of ~ is not critical and can vary over a wide range dependant only upon the orientation of the forming surface 80 relative to the forming ~.one 79. For example, one advantage to positioning the forming surface 80 in a horizontal planè as shown in FIG. 3 is that an acute angle ~? of approximately 33 tends to optimize the fiber deposition surface area of the forming surface 80. That is, assuming the forming walls 200, 201 to be parallel and spaced apart by approximately 9" as measured in a direction normal to the walls, and assuming an angle ~ on the order of 33, the lower edges of the forming walls will he on the order of 16" apart in a horizontal plane just above the forming surface 80, thereby providing a total fiber deposition area equal to 16" times the width of the forming æone 79.
Moreover, fiber deposition is optimized by virtue of the fact that the fibers approach the forming surface 80 at an acute angle ~ of about 33 while moving in the direction of forming surface movement.
As the angle ~ is increased--e.g., towards an angle of 90--the area of fiber deposition is reduced, approaching a total deposition surface area equal to only 9" times the width of the for~ing zone 79 under the assumed conditions;
- and, at the same time, the vector component of fiber movement in the direction of movement of the forming surface 80 is also reduced until at an angle ~ of 90, the fibers have no component of movement in the direction of forming surface movement. Such an increase in the angle ~ can be readily achieved by the simple expedient of mounting the forming surface 80 in an inclined plane--viz., inclined upwardly and towards the right as viewed in FIG. 3. Conversly, reduction in the angle ~ below 33 tends to further increase the total area of fiber deposition on the forming surface 80.
However, it is believed that optimum results are attained where angle ~ is on the order of 33 when angle ~ is on the order of 11.
The foregoing arrangement insures that the upstream and downstream boundaries of forming zone 79 generally coincide with the upstream and downstream boundaries of the air/fiber stream exiting the rotor housing 172 through screening means 180, consequently preventing mixing of ambient or room air with the moving air/fiber stream, minimizing impingement of the air/fiber stream on the walls of the forming zone and, thereby preventing the setting up of eddy currents or other gross cross-flow forces which would interfere with the cross-directional mass quantum dispersion of fibers being conveyed through the forming zone 79 in the air stream 5~
across the full-width of the system. Moreover, since constraining walls 200, 201 are parallel, there is no te~dency to decelerate the flow las would be the case where the walls diverge). This fact again aids in preventing eddy currents and other unwanted cross-flow forces. There is, of course, some deceleration of the air/fiber stream as it exits tne housing 172 through screening means 180;
but, such deceleration occurs immediately upon exit from the screening means and produces only a fine scale turbulence effect which does not induce gross eddy currents or cross-flow forces.
The foregoing zone is preferably dimensioned so that under normal adjustment of variable system operating parameters, the velocity of the fiber/air stream through the forming zone is at least 20 f.p.s. and the fibers are capable of traversing the entire length of the forming zone 79 from screen 180 to forming wire 80 in not more than .1 second.
While the forming zone 79 in the exemplary form of the invention has been depicted as including physical walls 199, 200 and 201, those skilled in the art will appreciate that the boundary layer confining means could take other forms if desired without departing from the scope of the invention--for example, the confining boundary walls could take the form of air curtains tnot shown). Moreover, in some cases it might be desirable to have the walls 200, 201 converge slightly so as to accelerate and, therefore, stabilize the flow.
I. Overall System Operation -Numerous system parameters may be varied in the operation of a forming system embodying the features of the presen~
invention in order to form an air-laid web of dry fibers having specific desired characteristics. Selected repre sentative and optimum parameter settings are set forth in greater detail in Section K, page 65 et seq., infra, of this specification where specific examples have been delineated.
Such variable parameters include, for example: air-to-fiber ratio (which is, preferably 200-600 ft.3/lb. when working with cellulosic wood fibers, and preferably 1000 to 3000 ft.3/lb., and perhaps higher, when working with cotton linters and relatively long synthetic fibers); air pressure within housing 172 (which preferably varies from +0.5" to ~3.0"
H2O); rotor speed (which preferably varies from 800 to 1800 RPM); the number, orientation and shape of rotor bars employed;
the quantity of air supplied per foot of former width (which is, preferably, on the order of 1500 to 1650 ft.3/min. with an 8-bar rotor operating at 1432 RPM); the energy level of classifying air supplied (which preferably ranges from 1.5 to 2.5 ft.3/min.tin. or, stated in terms of pressure, preferably ranges from 50" to 100" H2O); recycle or separation balance (which is less than 10% by weight of the fiber supplied and, preferably, fxom 1% to 5% by weight of the fiber supplied);
screen design--viz., whether the screen is a woven square-mesh screen or a slotted screen, the size of the screen openings (which ranges between .02" and 0.1" wire-to-wire open space in at least one direction and, preferably, ranges between .045" and .085" open space from wire-to-wire in at least one direction), the wire diameter used (which preferably varies from on the order of .023" to .064") and, the percentage of open screen area (which is between 30% and 55% and, preferably, varies from 38% to 46%); air pressure within the enclosed forming zone 79 (which is preferably atmospheric);
as well as the physical dimensions of the forming head 75 7~ ~
(which, in the exemplary form of the invention, comprises a generally cylindrical housing 172 having an inside diameter of 24").
Moreover, the rate of production of the web being formed can also be varied by altering numerous other system parameters such, merely by way of example, as the number of forming heads 75 used, the position of the forming head relative to the forming wire--i.e., whether the forming head is mounted in the cross-direction, the machine-direction, or at some angle therebetween--forming wire speed, and the type of fibers used. Still other variable parameters under the control of the operator include the cross-directional profile of the feed mat delivered to the forming head 75. Thus, where it is desired to produce a web having a uniform cross-directional profile with an acceptable coefficient of variation,the feed mat~ ~ feed mat 116 in FIG. 3--preferably will have a uniform cross-directional profile in terms of the mass quantum of fibers present. On the other hand, if one desires to produce an air-laid web having a specific non-uniform cross-directional profile--e.g., an absorbent filler web having a central portion with a relatively high basis weight and marginal edges of relatively low basis weights--it is merely necessary to form either a single feed mat or multiple side-by-side feed mats having the requisite cross-directional profile and, since the present system is substantially devoid of cross-directional forces, the cross-directional profile of the input feed mat(s) will control the cross-directional profile o~ the air-laid web.
Reco~nizing the foregoing, let it be assumed that the operator wishes to form an air-laid web 60 one foot (1'~ in width (all ensuing assumptions are per one foot of width of -~7-5~
the forming head 75) having a controlled uniform cross-directional profile and a basis weight of 17 lbs./2880 ft.2.
Assume further:
a) Air-to-fiber ratio supplied through inlet slot 171 equals 350 ft.3~1b.
b) Inlet slot 171 is 51 in circumferential width--i.e., the dimension from edge 190 ~FIG. 3) to edge 202.
c) Rotor housing 172 is 24" I.D.
d) Rotor asse~bly 175 employs eight egually spaced rectangular rotor bars 181, each 3~4"
in radial hsight by 3/8" in circumferential thickness and extending parallel to the axis of the rotor assembly continuously throughout the full width of rotor housing 172 and, each spaced from the rotor housing 172 by 0.18".
e) Rotor assembly 175 is driven at 1432 RPM.
f) Rotor bar 181 tip velocity equals 150 f.p.s.
g) Relative velocity between the rotor bars 181 and the aerated bed 186 is approximately 70 ~ .p.s .
h) Screening means 180 defines an arc o~ 86, and has 40% open area.
i) Separation and/or recycle through separator slot 179 comprises 5% by weight of fibrous materials supplied through inlet slot 171.
j) The quantity of classifying air introduced through air jet 194 is between 1.5 and 2.5 ft.3/min./in. at pressures between 50" and 100" H2O.
k) Forming walls 200, 201 are parallel and spaced ~. :
~$~57~:
9" apart in a direction normal to the parallel walls 200, 201 and 16" apart in a horizontal plane passing through their lower extremities just above the plane of the forming wire 80.
l) Forming wire speed equals 750 f.p.m.
All of the foregoing operating parameters are either fixed and known, or can be pre-set by the operator, except for the relative velocity between the xotor bars 181 and the aerated bed 186 of fibers within the rotor housing 172. The actual speed of the aerated bed 186 is not known with certainty;
but, it is believed to be substantially less than the rotor bar tip velocity of 150 f.p.s.; and, more particularly, it is believed to be on the order of half the tip velocity of the rotor bars 181. For convenience, it is here assumed to - 15 be approximately 80 f.p.s., an assumption believed to be reasonably accurate based upon observation of overall system behavior, thereby resulting in a relative velocity between the rotor bars 181 and the aerated bed 186 of approximately 70 f.p.s. (see assumption "gn, supra).
Accordingly, supply and velocity relationships within the foregoing ;exemplary system can be readily calculated as ollows; and, such relationships have been illustrated in FIG. 10:
17 x 750 = 4.43 lbs./min.--Rate of formation [I]
2880 of web 60.
The term "throughput" and the phrase "rate of web formation" are herein used generally interchangeably and are to be distinguished from the phrase "ratè of fiber delivery".
Thus, the phrase "rate of fiber delivery" is intended to mean the mass quantum or weight rate of feed of fibrous materials delivered to the forming head, and may be expressed, for example, in units of pounds per hour per inch of former width (lbs/hr./in.), pounds per minute per foot of former width - 2 ~-csm/~
'7~
(lbs./min.Jft.), or in any other suitable units. "Throughput", on the other hand, is intended to describe the screening rate for fibrous materials discharged from the forming head--i.e., the mass quantum or weight rate of fiber delivery through the former screen per unit area of screen surface--and may be expressed, for example, in units of pounds per hour per square inch of effective screen surface - 20a -csm/~
5~
area (lbs./hr./in.2), pounds per minute per s~uare foot of effective screen surface area (lbs./min./ft.2), or any other suitable units. The fiber "throughput" achieved is refdected directly in the "rate of web formation" and may be calculated by multiplication of fiber throughput by the effective length of ihe former screen. "Rate of web formation"--i.e., the rate at which the air-laid web is formed on the moving forming wire or other formlng surface--may be expressed, for example, in units of pounds per hour per inch of former width (lbs./hr./in.), pounds per minute per foot of former width (lbs./min./ft.), or in any other suitable units.
The words "up", "down", "above" and/or "below" are used in a relative, non-limiting sense to describe, merely by way _ example, a relationship of one structural element to a forming wire or to another structural element.
B. Overall System Descriptlon Briefly, and referring first to FIG. 1, there is illustrated an exemplary system for forming an air-laid web 60 of dry fibers and comprising: a fiber metering section, generally indicated at 65; a fiber transport or eductor section, generally indicated at 70; a forming head, generally indicated at 75, where provision is made for controlling air and iber flow, and where individual fibers are screened rom undesirable aggregated fiber masses and, thereafter, are air-laid on a foraminous forming wire 80; a suitable bonding station, generally indicated at 85, where the web is bonded to provide strength and integrity; a drying station, generally indicated at 87, where the bonded csm/~
. .
5~
web 60 is dried prior to storage; and, a take-up or reel-type storage station, generally indicated at 90, where the air-laid web 60 of dry fibers is, after bonding and drying 7 formed into suitable rolls 95 for storage prior to delivery to some subsequent processing operation (not shown) where the web 60 can be formed into specifically desired consumer products.
In order to permit continuous removal of aggregated fiber masses, the forming head 75 includes a separator system, generally indicated at 76. Such separated aggregated fiber masses and individualized fibers entrained therewith are preferably removed from the forming area by means of a suitable conduit 77 maintained at a pressure level lower than the pressure within the forming head 75 by means of a suction fan (not shown). The conduit 77 may convey the masses to some other area (not shown) for use in inferior products, for scrap, or, alternatively, the undesirable aggregated fiber masses may be recycled via conduit 78 to a hammermill, generally indicated at 100, where the masses are subjected to secondary mechanical disintegration prior to reintroduction into fiber meter 65. Finally, the forming head 75 also includes a forming chamber, generally indicated at 79, positioned immediately above the foraminous forming wire 80. Thus, the arrangement is such that individual fibers and soft fiber flocs pass through the forming chamber 79 and are deposited or air-laid on the forming wire 80 to form a web 60 characterized by its controlled cross-directional profile and basis weight.
C. Fiber Metering Section -30While various types of commercially available fiber metering systems can, with suitable modifications, be employed with equipment embodying the features of the present invention, one system which has been found suitable and which permits of the necessary modifyiny adaptations is a RANDO-~EEDER~ (a registered trademark of the manufacturer, Rando Machine Corporation, Macedon, New York). The fiber metering section 65 shown by way of example in FIG. 1 is such a system. Indeed, a RANDO-FEEDER~ is ideally suited for us~ with the present invention when attempting to work with synthetic fibers.
As here shown, the fiber metering section 65 is mounted on the mezzanine floor level lO1 of a suitable paper mill. Fibers may be fed to the fiber separator hopper 102 in any of a variety of conventional ways. For example, pre-opened fibers may be ~anually introduced in bulk through inlet chute 103 which is provided with a closure member 104 so as to maintain an enclosed chamber. Alternatively, batts or other compacted fibers may be introduced through inlet 105 of hammermill lO0 (which is here shown only in diagrammatic block-and-line form and may take any well known conventional form). The compacted batts are fiberized within the hammermill and, after fiberization, the individualized fibers are delivered to the fiber separator hopper 102 via inlet 106.
A fan 107 is provided for removing excess air fro~ the fiber separator hopper 102, thereby permitting the fibers to form a loose fiber bed 108 at the bottom of the hopper 102. Thus, the fan 107 functions to withdraw excess air from the hopper 102 and such excess air, together with some escaping fibrous materials, are thereafter discharged into a suitable waste air filter or cyclone separator (not shown~. If desired, a conventional pre-feeder and opener-blender (not shown) can 0 be used to feed individualized fibers to the fiber meter 65.
In operation, fibers fall from the fiber separator ~ ( ~ 57;Z
hopper 10~ and form a loose bed 108 of open fibers carried by a floor apron conveyor 109. ~n anti-static spray system llO may be provided to minimize adherence of the fibers to portions of the system. The fibers are conveyed by the 5 floor apron conveyor 109 to an elevating apron conveyor 111 havin~ conventional pins and slats (not shown). Fibers are carried upwardly by the elevator apron conveyor to a rotating stripper apron 112 which serves to remove excess fiber stock and return such excess stock to the bed 108. The arrangement is such that a controlled, metered quantity of small opened tufts of fiber remains on the pins of elevator apron conveyor 111 and is carried over the top thereof uniformly across the entire width of pron 111 into an area 113 known as an air bridge.
Fibers delivered to the air bridge 113 are doffed from the pins on apron 111 by means of air flow under the control of a suitable air volume controller 114-. As a result of the flow rate of air movement, a controllable quantity of fibers--uniform throughout the full width of air bridge 113- are deposited on a rotating condenser screen llS, thus forming a full-width uniform feed mat 116 conveyed by roller CQnVeyor 118 to a feed plate 119. The arran~ement is such that as the feed mat 116 takes shape, the resistance of the mat on condensor screen 115 serves to reduce air flow through the screen and, conseguently, proportionally less doffing occurs at apron 111 until a condition of equilibrium is reached.
At the equilibrium point, a sufficient guantity of fibers are doffed to form a continuous uniform feed mat 116, with the balance of unused fibers being returned by the pins on elevator conveyor 111 to the fiber bed 108.
The full-width uniform feed mat 116 is then conveyed over feed plate ll9 by means of feed roller 120 and into the path of teeth formed on an opening roll or lickerin 121.
The lickerin 121 serves to comb individual fibers from the feed mat 116 with the individualized fibers being picked up and carried by a full-width air strea~ passing under feed plate 119 and generated by fan 124 and eductor 70. From this point, the entrained stream of individualized air~suspended fibers is introduced into the main air supply strea~ generated by fan 124 and carried through eductor 70 and the forming head 75, with the fibers exiting the forming head 75 passing through the forming chamber 79 and being uniformly deposited across the full-width of forming wire 80 in a uniform, but completely random, fiber pattern, thereby forming web 60.
D. ~eb Forming, Com~acting, Bonding, Dryin~ & Storage Section As heretofore indicated, fibers are air-laid on the foraminous forming wire 80 at the forming station by means of an air stream generated primarily by fan 124. In addition, a vacuum box 126 positioned immediately below the forming wire 80 and the web forming section 79 serves to maintain a positive downwardly moving stream of air which assists in collecting the web 60 on the moving wire 80. If desired, a second supplementary vacuum box 128 may be provided beneath the forming wire at the point where the web 60 exits from beneath the forming cha~ber 79, thereby insuring that the web is maintained flat against the forming wire.
After for~ation, the web 60 is passed through calender rolls 129 to lightly compact the web and give it sufficient integrity to permit ease of transportation to conveyor belt 130. A light water spray can be applied from nozzle 131 in order to counteract static attraction between th~ web and the wire. An air shower 132 and vacuum box ~34 serve to 7~2 ~
clean loose fibers from the wire 80 and thus prevent fiber build-up.
After transfer to the belt 130, the web 60 may be bonded in any known conventional manner such, merely by way of example, as i) spraying with adhesives such as latex, ii) overall calendering to make a saturating base papex--i.e., a bulky web with a controlled degree of hydrogen bonding-~
adhesive print pattern bonding, or other suitable process.
Such bonding processes do not form part of the present invention and, therefore, are neither shown nor described in detail herein, but, such processes are well known to those skilled in the art of non-woven fabric manufacture. For example, the web 60 may be pattern bonded in the manner described in greater detail in the aforesaid Dunning U.S.
Pat. No. 3,692,622 assigned to the assignee of the present invention. Briefly, in this bonding process, the moisture content of the web is adjusted to 6~ to 3596 by a water spray 135 and, thereafter, the web is bonded by passing it through the nip between a small hard roll 136 and a patterned steel roll 138. Subsequently, the bonded wf~b 60 is transferred to conveyor belt 139 and transported thereby through the drying station 87 to the storage station 90 where the web 60 is taken up on a driven reel 140 to form roll 95 which may thereafter be either stored for subseguent use or unwound at a sùbsequent web processing station (not shown) ~o form any desired end product. The drying station 87 may take any suitable conventional form such, for example, as a pair of closely spaced heated plates 88, 89, or an oven or heated roll (not shown).
Referring to FIG. 2, there has been diagrammatically illustrated a typical system employing multiple forming 57~
heads for increasing overall productivity of the air-laid dry fiber web forming system. As here shown, multiple forming heads 75A - 75N are positioned over the foraminous forming wire 80, with each forming head being supplied with a full-width uniform supply of air-suspended fibers fed from respective ones of a multiplicity of hammermills and fiber mèters (not shown in FIG. 2, but respectively similar to the hammermill 100 and fiber meter 65 shown in FIG. 1). Of course, while only two forming heads 75A and 75N have been shown for illustrative purposes in FIG. 2, those skilled in the art will appreciate that any desired number of forming heads could be used dependent upon the productivity desired in terms of the web's basis weight, forming wire speed, and the speed at which the bonding station can be effectively operated. Thus, it will be appreciated that the air-laid web 60 is formed by a first layer of fibers 60A deposited by forming head 75Aj and _ (where n = any whole integer) successive layer(s) 60N deposited by _ downstream forming head(s) 75~J.
As a consequence of this construction, the speed of the forming wire ~ay be increased by a multiple of the number of forming heads employed.to form a composite web 60 of a selected basis weight for a given forming wire speed.
E. Full-Width Metered Fiber Feed In carrying out the present invention, provision is madè for forming a full-width feed mat of fibers having a controlled cross-directional profile in terms of the mass quantum of fibers constituting the mat. To this end, and as best illustrated in FIG. 3, feed mat 116 may be formed in the manner previously described in connection with the fiber metering section 65 shown by way of example in FIG. 1. Such feed mat 116 has been found to meet the preferred conditions s~ ~
of full-width uniformity in terms of the mass quantum of fibers forming the mat and the coefficient of variation of the fibrous materials input to the system. The mat thus formed--e.g., mat 116--is then fed across feed plate 119 by means of a feed roller 120 into the teeth on lickerin 121 ~hich serves to disaggregate the fibers defining the mat by combing such fibers (along with any pulp lumps, nits and other aggregated fiber masses which are present) out of the mat and feeding such materials directly into a high volume air stream generated by fan 124 (FIG. 1~ and eductor 70 (FIGS. 1 and 3).
In order to permit attainment of the objectives of the invention, the air-to-fiber ratio preferably employed when working with cellulosic wood fibers is on the order of 200-600 cubic feet of air (at standard temperature and atmospheric pressure conditions) per pound of fiber--viz., 200-600 ft.3/lb. Moreover, when employing the exemplary equipment herein described such air is supplied at relatively high volumes which vary dependent upon the operational speed of the rotor assembly and the types of fibers being worked with--i.e., ~olumes ranging from 1,000 to 1,800 ft.3/min./ft. of former width are conventional when working with cellulosic wood fibers. For example, when employing an 8-bar rotor operating at 1432 RPM, the volume of air supplied is preferably on the order of 1500-1650 ft. /min./ft. of former width. On the other hand, when working with synthetic f ibers or cotton linters, f example, considerably higher volumes of air per pound of fiber may be employed--e.g., the air-to-fiber ratio may range from 1,000 to 3,000 ft.3/lb., or even higher.
In operation, the air-suspended fiber stream is conveyed through a suitable fiber transport duct 170 (FIG. 3) from .1.'~.'`1~7~
the full-width eductor 70 to a full-~idth inlet slot 171 formed in the upper surface of, and extending fully across, a generally cylindrical housing 172 which here defines the 2-dimensional flow control, screening and separating zone 75. To insure that full-width mass quantum fiber control is maintained, the exemplary duct 170 is preferably subdivided into a plurality of side-by-side flow channels separated by partitions 174 extending the full length of the duct. It has been found that the desired coefficient of variation constraint in the web being formed can be obtained by spacing the partitions 174 apart by approximately four inches so as to form a plurality of adjacent flow channels extending across the full axial length of housing 172. It has also been found that a partitioned duct arrangement of the type shown in FIG. 3 can be advanta~eously used to accommodate width differences between the feed mat 116 formed in the fiber metering section 65 and the final air-laid web 60 deposited on the foraminous forming wire 80.
For example, excellent results have been obtained when attempting to form a web 60 forty-eight inches in width, utilizing a feed mat 116 only forty inches in width. Thus, in such a system the duct 170 may diverge from a full-width at its upper end of forty inches to a full-width at its lower end of forty-eight inches, with the individual flow channels defined by partitions 174 diverging from approximately three and one-third inches in width to approximately foux inches in width. This insures that cross-flow forces are substantially eliminated, and the mass quantum of air-borne fibrous materials delivered from the lower end of duct 170 to the flow control, screening and separating zone 75 remains substantially unchanged across the full width of the o29--3~7;2 system.
F. Flow Control, Screening and Separat.ion In carrying out the invention, a 2-dimensional cylindrical rotor former is provided which serves to control flow of the air-suspended fi~er stream through a separation zone while minimizing mechanical disintegration of fibrous materials, and which is designed to provide an acceptable level of fine scale air turbulence while insuring that the system is substantially devoid of eddy currents and other undesired cross-flow forces so as to maintain a controlled mass quantum of fibers across the full width of the forming head 75. To accomplish this, the exemplary forming head 75 includes a rotor assembly, generally indicated at 175 in FIGS. 3 and 4, mounted for rotation within housing 172 about a horizontal axis defined by shaft 176. The arrangement is such that the air-suspended fibrous materials introduced radially into housing 172 through inlet slot 171 are conveyed by co-action of the air stream and the rotor assembly 175 through the housing 172 for controlled and selective discharge either a) through a full-width discharge opening, generally, indicated at 178 in FIG. 3, and into forming zone 7~ for ultimate, air-laid deposition on forming wire 80 or, alternatively, b) through a full-width tangential separator slot ~79 formed in housing 172 downstream of the discharge opening 178. The separator slot 179, which here forms part of the separation and/or recycle zone 76 (FIGS. 1 and 3), is preferably on the order of from 3/16" to 3/8" in circumferential width when working with wood ~ibers and, if desired, may he adjustable in any conventional manner ~not shown) so as to permit circumferential widening or narrowing of the slot 179 to optimize separation conditions~
35~;~
~ o per~it controlled, selective discharge of individualized fibers and soft fiber ~locs through opening 178 and into forming zone 79~ while at the same ti~e precluding discharge of nits and other undesired aggregated fiber masses there-through, suitable screening ~eans, generally indicated at180 in FIG. 3, is mounted within discharge opening 178.
Such screening means 180 may, simply take the form of a conventional woven square-mesh wire screen of the ty~e shown at 180A in ~IG. 6 and having openings sized to preclude passage of aggregated fiber masses~ 2 ~ the screen may take the form o~ an 8x8 mesh screen having 64 openings per square inch, a lOxlO mesh screen, a 12x12 mesh screen, or other commonly available woven mesh screens; provided only tha~ the screen openings do not exceed 0.1" open space from wire-to-wire in at least one direction and ha~e between 30~ and 55~ open area and, preferably, between 38~ and 46~ open area. As bes~ shown in FIG. 3, screening means 180 is formed with the same radius of curvature as the se~i-cylindrical portion of housing 172 ~ithin which discharge opening 178 is formed~
As best illustrated by reference to ~IGS. 3 and 4 conjointly, rotor assembly 175 comprises a plurality of transversely extending rotor bars 181, each fixedly mounted on the outer periphery of a plurality o~ closely spaced spiders 182. ~he spiders 182 are, in turn, fixedly ~ounted on shaft 176 which is journalled for rotation in outboard bearing housings 183, 184 (FIG. 4) and which is coupled ~o drive shaft 185 driven by any suitable means ~not shown~.
The arrangement is such that the high volume air-suspended 5'7~
stream of fibrous materials passing through duct 170 is introduced radially into housing 172 through inlet slot 171 and such stream tends to pass across the rotationally driven rotor bars 181--viz., the bars 181 move through the radially entering stream of air-suspended fibers. As a result of rotor bar movement and the high velocity movement of the air stxeam, the air and fibers tend to move outwardly towards th~ wall of housing 172, thus forming an annular, rotating, aerated bed of fibrous materials, best illustrated at 186 in FIG. 10. Such annular-aerated bed 186 of fibrous materials is believed to be on the order of one-half inch to one and one-half inches thick (dependent upon actual operating parameters), and is believed to be moving rotationally at about half the spe~d of the rotor bars 181. For example, in a cylindrical 'ormer having an inside housing diameter of 24" where the rotor assembly 175 is being driven at 1432 RPM, the tip velocity of the rotor bars 181 is on the order of 150 f.p.s. (feet/second) and, consequently, it is believed that the velocity of the aerated bed 186 is on the order of 80 f.p.s. ~hus, since the rotor bars 181 are moving at 150 f.p.s. through an aerated bed of fibers moving in the same direction at approximately 80 f.p.s., the relative velocity between the aerated bed 186 of fibers and the rotor bars 181 is on the order of 70 f.p.s.
In keeping with the invention, the rotor assembly 175 is preferably designed a) to minimize pumping action which tends to reduce the relative speed differential between the rotor bars 181 and the aerated bed 186, thus causing the fibers to move over and beyond the screening means 180, and b) so as to minimize mechanical action between the rotor bars 181 and both the housing 172 and screening means 180, which action tends to disintegrate fibers and aggregated fiber masses carried in the air stream and to generate pills. To this end, the rotor bars 181 are generally of relatively small cross-section--e.g., in the case of the exemplary rectangular bars shown in FIG. 10, such bars are on the order 3/4" in radial height by 3/8" in thickness, such thickness dimension being desired only for purposes of structural integrity--and are moun~ed so as to provide a clearance between the outer edges of the bars 181 and the inner wall surface of the housing 172 and screening means 180 of from 0.10 inches to 0.25 inches and, preferably, from 0.18 inches to 0.20 inches, at least during transit of the rotor bars from the upstream edge 188 of screening means 180 through separator slot 179. In terms of "pumping action", therefore, the signficant bar area is only 3/4" times the width in inches of the forming head 75. To avoid generation of cross-flow forcest it is important that the rotor bars 181 are continuous, extend the full width of the rotor chamber, and are oriented parallel to the axis of the rotor assembly 175.
In carrying out the present invention, the rotor housing 172 is preferably semi-cylindrical in cros~-section throughout at least the arcuate span ranging from the upstream edge 188 of screening means 180 through the tangential separator slot 179, thereby insuring proper clearance between the rotor bars 181 and the inner periphery of both the screening means 180 and housing 172 as the rotor assembly 175 is driven rotationally. The remaining upper segment of the housing 172 may be of any desired shape, including substantially semi-cylindrical, but is preferably relieved immediately adjacent the downstream edge of the inlet slot 171 as indicated at 189, thereby preventing the tendency of those fibers passing the separator slot 179 from impinging against the vertical edge 190 of inlet slot 171 and causing consequent blockage, or partial blockage, of the inlet slot.
Referring again to FIG. 3, it will be apparent from the description as thus far set forth, that as air-suspended fibers are introduced radially into the rotor housing 172 through inlet slot 171, they are moved rapidly through the housing under the influence of the air stream and movement of the rotor bars 181, thus forming the moving annular aerated bed 186 of fibers (FIG. 10) about the lnner periphery of the housing wall. As the aerated bed--which contains individualized fibers, soft fiber flocs, nits and other aggregated fiber masses--passes over the screening means 180, some, but not all, of the individualized fibers and soft fiber flocs pass through the screening means into the forming æone 79, while the balance of the individualized fibers and soft fiber flocs, together with nits and other aggr~gated fiber masses, pass over the screen without exiting from the rotor housing 172. The undesired pills, rice and nits--i.e., aggregated fiber masses--have a bulk density generally in excess of .2 g./cc. and tend to be separated along with some individualized fibers and soft fiber flocs from the aerated bed 186 at the tangential separator slot 179, Wit]l those separate~ materials being centrifugally expelled through the slot 179 where they are entrained in a recycle or separating air stream generated by any suitable means Inot shown) coupled to manifold 191 with the air=
suspended separated particles moving outward through a full-width discharge passage 192 coupled to separator slot 179 and, ultimately, to conduit 77 (FIG. 1). Such separation is aided by a positive air outflow from housing 172 through separator slot 179.
In keeping with the invention, provision is made for insuring positive separation of undesired nits and aggregated fiber masses from individualized fibers and soft fiber flocs, and for preventing movement of the latter through separator slot 179 to the full extent possible, thereby insuring that individualized fibers and soft fiber flocs are retained within rotor housing 172 and move with the aerated bed 186 back to the area of screening means 180 where such desirable materials have successive opportunities to pass through the screening means 180 into the forming zone 79.
To accomplish this, a full-width classifying air jet 194 is provided upstream of the separator slot 179 and downstrear:~
of screening means 180; such air jet being positioned to introduce a full-width air stream generated by any conventional source (not shown) radially into rotor housing 172 just ahead of the separator slot 179. As a consequence, the positive classifying air stream introduced radially into housinq 172 through air jet 194 tends to divert individualized fibers and soft fiber flocs within the aerated bed 186 radially inward as a result of tlle relatively high drag coefficients of such materials and their relatively low bulk density (which is generally on the order of less than .2 g./cc.). Since the nits and aggregated fiber masses have a relatively high bulk density in excess of .2 g./cc. and relatively low drag coefficients, the classifying air stream introduced through the full-width air jet 194 does not divert such materials to any significant extent and, therefore, such undesired materials tend to be centrifugally expelled through the tangential separator slot 179. It has been "~ t~;~
found that the introduction of classifying air through the full-width classifying air jet 194 into housing 172 at pressures on the order of from 50" to 100" H2O and at volumes ran~ing from 1.5 to 2.5 ft.3/min./in. provides an energy level adequate for deflecting a significant portion of the indi~idualized ~ibers and soft fiber flocs. The energy level of the classifying air jet is most conveniently controlled by adjusting its pressure.
In operation, it has been ound that excellent results io are obtained by li~iting the amount of fibrous material removed from the system through separator slot 17g to less than 10~ by weight and, preferably, to between 1% and s% by weight, o~ the fibrous material introduced into the housing 172 tllrough inlet slot 171. Stated differently, at least 90~ of the ~ibrous materials introduced and, preferably between 95% and 99% thereof, ultimately pass through screening means 180 into the forming zone 79 and are air-laid on the foraminous forming wire 80 without requiring any secondary hammermilling operations and without being subjected to any significant mechanical disintegrating forces~ The quantity o~ material separated may be controlled by the operator by varying the voiume of recycle air supplied through manifold 191 and/or by adjusting the circumferential extent of full-width se~arator slot 179 in any suitable manner tnot shown).
AIR-LAID DRY FIBER WEB FO~IATION IN
` ACCORDANCE WITH THE PRESENT INVENTION
Thu~ far, the environment of the invention has been described in connection with methods and apparatus wherein .. i .1 ~,,f1 ~t 7~
a conventional woven square-mesh screen of the type shown at 180A in FIG. 6 is mounted in the discharge opening 178 in forming head 75. While such a web ~orming system has provided significant advantages in terms of fiber throughput capacity for a 2-dimensional forming system, particularly when contrasted with conventional sifting systems of the type disclosed in the aforesaid Kroyer patents, the present inventors have discovered that even yreater improvements can be achieved in fiber throughput capacities when using 2-dimensional web forming systems.
_ High Throughput Screening In accordance with one of the important aspects of the present invention, provision is made for substantially increasing the fiber throughput capacity of a 2-dimensional fiber forming system, yet wherein aggregated fiber masses present within the forming head are effectively precluded from entering the forming zone. Rather, such aggregated fiber masses are separated from the aerated fiber bed 186 in the forming head 75 (FIG. 3) and are discharged through full-width separator slot 179. To accomplish this, a high-capacity slotted screen 180B of the type shown in FIG. 7 is mounted within discharge opening 178 with the screen slots oriented with their long dimensions parallel to the axis of rotor assembly 175.
When utilizing a slotted type screen 180B with a 2-dimensional rotor assembly 175 mounted for rotation about a horizontal axis, it has been found essential that the screen slots be oriented with their long dimensions parallel to the axis of the rotor assembly. When so oriented, individualized fibers tend to move through the screen slots while nits and aggregated fiber masses--e.g., the aggregated fiber masses .
3~
195 shown in FIG. 7--are precluded from passing through the screen since they are generally larger in size then the narrow dimensions of the slots which may range between .02"
and 0.1" open space froln wire-to-wire in at least one direction and, preferably, ranges between .045" and .085"
open space from wire-to-~ire in at least one direction.
Such wire-to-wire dimensions are particularly critical when the system is being used to make high quality, lightweight tissue webs--e.g., webs having low nit levels and basis weights ranging from 13 lbs./2880 ft.2 to 18 lbs./2880 ft.2 and, in some instances, up to 22-25 lbs~/2880 ft.2. However, when the slots of a slotted screen 180B are oriented with their long dimensions perpendicular to a plane passing through the rotor axis as shown in FIG. 8, it has been found that the screen tends to rapidly plug--indeed, when operating under commercial production conditions, it has been found that the screen tends to become completely plugged almost instantaneously. It is believed that such plugging action results from the tendency of individual fibers to "staple"
or "hair-pin" and otherwise hang up or collect within the narrow confines at the end of each slot as best indicated at 196 in the lower right-hand corner of FIG. 8; and, as soon as a few fibers have collected, other fibers and aggregated fiber masses 195 almost instantaneously agglomerate on the screen as depicted in the balance of FIG. 8. This plugging phenomenon is more clearly visible upon reference to the photograph reproduced as FIG. 9--such photograph illustrating a slotted screen 180B wherein the slots are oriented at an angle of 45 to a plane passing through the rotor axis--and, under these conditions, the screen 180B plugged almost completely and instantaneously.
7~ ~
On the other hand, it has been found that a conventional woven square-mesh screen of the type shown at 180A in FIG. 6, and a slotted screen 180B with the slots oriented as shown in FIG. 7, exhibit little or no tendency to plug undex normal operating conditions. Rather, while individualized fibers still have a tendency to "staple" or "hair-pin", as indicated at 197 in FIGS. 6 and 7, there seems to be adequate time and room for the suspended fibers to disengage themselves from the screen; whereas in the arrangements shown in FIGS.
8 and 9, the suspended fibers ~end to catch and congregate in the closely proximate confined corners of the screen slot and, as a result, other fibers and aggregated fiber masse~
195 rapidly accumulate, thus plugging the screen and rendering the system inoperative.
The exemplary system herein described has been depicted in FIGS. 3 and 4 as including a rotor assembly 175 having eight rotor bars 181. However, the number and/or shape of the rotor bars may be varied, provided that such ~odifica-tions are consistent with mechanical stability and low rotor "pumping" action. That is, the rotor assembly 175 must be a dynamically balanced assembly and, therefore, it must include at least two rotor bars. However, it will be appreciated that it can include fewer or more than the eight bars illustrated in FIGS. 3 and 4--for example, excellent results have been achieved with a 4-bar rotor assembly of the type indicated at 175' in FIG. 5. On the other hand, care must be taken to insure that the number of rotor bars employed--e.g., _ rotor bars where n equals any whole integer greater than "1"--and the shape of the rotor bars are such that pumping action is minimized. Otherwise, the rotor assembly 175 will tend to sweep the aerated fiber bed 186 9~;~
over and beyond the screening means 180 rather than permitting and, indeed, assisting fiber movement through the screening means.
In the illustrative form of the invention, the rotor bars 181 have a rectangular cross-section, and pumping action is minimized by keeping the effective rotor bar area relatively small~ ~ 3/4" times the length of the bars which extend across the full width of the rotor housing 172--and by spacing the bars apart circumferentially by 45 (there being eight equally spaced bars) and from the housing 172 by on the order of 0.18" to 0.20". However, the rotor bars 181 need not be rectangular in cross-section. Rather, they can be circular, vane-shaped, or of virtually any other desired cross-sectional configuration not inconsistent with the objective of minimizing rotor pumping action. For example, rotor bars having a circular cross-section would, because of their shape, be even more effective than rectangular bars in terms of minimizing rotor pumping action. However, thè primary function of the rotor assembly as employed in the present invention is, as more fully described in Section I, page 45 _ seq., infra, of this specification, to lift individualized fibers, soft fiber flocs, and aggregated fiber masses off the surface OL the former screening means and, thereby, to prevent plugging of the screen, to prevent 2~ layering of fibers on the screen, and to reopen apertures in the screen so as to permit passage of the air-suspended fiber stream therethrough. This desirable result is achieved by the negative pressure 20nes created in the wakes of the moving rotor bars; and, the negative pressure zones in the wakes of rotor bars having a rectangular cross-section have been found to be as effective for this purpose as those .
,t~57~ ~
created by rotor bars of circular cross-section.
It is significant to a complete understanding of the present invention that one understand the difference between the primary function of the rotor assembly here provided--viæ., to lift fibrous materials upwardly and off the screenor, stated differently, to momentarily disrupt passage of the air-suspended fiber stream through the screen--and that stated for conventional cylindrical rotor systems of the type disclosed, for example, in the aforesaid 3.D'A. Clark patents where the rotor chamber functions as a "disintegrating and dispersing chamber" (See, e.~., col. 4, line 53, J.D'A. Clark ~.S. Pat. No. 2,931,076)--~iz., where the rotor blades mechanically act upon the fibrous materials to "disintegrate"
such materials and propel them through the screen.
H. Forming Zone In keeping with another important aspect of the present invention, provision is made for insuring that individualized fibers passing through the screening means 180 shown in FIG. 3 are permitted to move directly to the foraminous forming wire 80 without being subjected to cross-flow forces, eddy currents or the like, thereby maintaining cross-directional control of the mass quantum of fibers delivered to the forming wire through the full-width of forming zone 79. To accomplish this, provision is made for insuring ~hat the upstream, downstream and side edges of the forming zone--i.e., the boundaries of the zone 79-~are formed so as to define an enclosed forming zone and to thereby preclude intermixing of ambient air with the air/fiber stream exiting housing 172 through screening means 180. It has been found that the air/fiber stream exiting from housing 172 through screening means 180 does not exit radially but, rather, at an acute angle or along chordal lines or vectors which, on average, tend to intersect a line tangent to the mid~point of the screening means 180 at an includ~d angle cr. In the exemplary form of the invention where the screening means 180 covers an arc of approximately 86--i.e., an arc extending clockwise as viewed in FIG. 3 from a point (indicated at 198 in FIG. 3) a~proximately 15~ from the center of inlet slot 171 to a point 188 approximately 245 from the center of inlet slot 171--and, where an 8-bar rotor is being operated at a rotor speed on the order of 1400-1450 RPM, it has been found that the angle cr is generally on the order of 11.
Consequently, the forming zone 79 is preferably provided with sidewalls (a portion of one such sidewall is shown at 199 in FIG. 3), a full-width downstrea~ forming wall 200, and a generally parallel full-width upstream forming wall 201, which are respectively connected to rotor housing 172 at the downstream and upstream edges of screening means 180, and which respectively lie in parallel planes which intersect a linP tangent to the mid-point of the screening means 180 at included angles on the order of 11. The upstream end of forming wall 201 is bent as indicated at 201A, 201B so as to form a shaped portion which generally accommodatas the air/fiber flow pattern exiting the upstream portion of screening means 180. The walls 199, ~00 and 201 serve to enclose the forMing zone 79 and to thereby preclude disruption of the air/fiber stream as a result of mixing between ambient air and the airtfiber stream. The enclosed forming zone 79 is preferably maintained at or near atmospheric pressure so as to prevent inrush and outrush of air and to thereby assist in precluding generation of cross-flow forces within the forming zone. Those skilled in the art will appreciate that angle a can vary with changes in operating parameters such, for example, as changes in rotor RPM. ~owever, for operation at or near optimum conditions, it is believed that the angle a will generally lie within the range of 5 to 20 and, preferably, will lie within the range of 8 to 15.
The lower edges of forming walls 200, 201 terminate slightly above the surface of foraminous forming wire 80--generally terminating on the order of from one-quarter inch to one and one-quarter inches above the wire.
In the exemplary form of the invention shown in FIG. 3, when the angle a is on the order of 11 and when the forming zone 79 is positioned over a horizontal forming surface 80, the upstream and downstream forming walls lie in planes which intersect the horizontall~ disposed forming surface 80 at included acute angles ~ ~here ~ is on the order of 33. Howevcr, those skilled in the art will appreciate that the angular value of ~ is not critical and can vary over a wide range dependant only upon the orientation of the forming surface 80 relative to the forming ~.one 79. For example, one advantage to positioning the forming surface 80 in a horizontal planè as shown in FIG. 3 is that an acute angle ~? of approximately 33 tends to optimize the fiber deposition surface area of the forming surface 80. That is, assuming the forming walls 200, 201 to be parallel and spaced apart by approximately 9" as measured in a direction normal to the walls, and assuming an angle ~ on the order of 33, the lower edges of the forming walls will he on the order of 16" apart in a horizontal plane just above the forming surface 80, thereby providing a total fiber deposition area equal to 16" times the width of the forming æone 79.
Moreover, fiber deposition is optimized by virtue of the fact that the fibers approach the forming surface 80 at an acute angle ~ of about 33 while moving in the direction of forming surface movement.
As the angle ~ is increased--e.g., towards an angle of 90--the area of fiber deposition is reduced, approaching a total deposition surface area equal to only 9" times the width of the for~ing zone 79 under the assumed conditions;
- and, at the same time, the vector component of fiber movement in the direction of movement of the forming surface 80 is also reduced until at an angle ~ of 90, the fibers have no component of movement in the direction of forming surface movement. Such an increase in the angle ~ can be readily achieved by the simple expedient of mounting the forming surface 80 in an inclined plane--viz., inclined upwardly and towards the right as viewed in FIG. 3. Conversly, reduction in the angle ~ below 33 tends to further increase the total area of fiber deposition on the forming surface 80.
However, it is believed that optimum results are attained where angle ~ is on the order of 33 when angle ~ is on the order of 11.
The foregoing arrangement insures that the upstream and downstream boundaries of forming zone 79 generally coincide with the upstream and downstream boundaries of the air/fiber stream exiting the rotor housing 172 through screening means 180, consequently preventing mixing of ambient or room air with the moving air/fiber stream, minimizing impingement of the air/fiber stream on the walls of the forming zone and, thereby preventing the setting up of eddy currents or other gross cross-flow forces which would interfere with the cross-directional mass quantum dispersion of fibers being conveyed through the forming zone 79 in the air stream 5~
across the full-width of the system. Moreover, since constraining walls 200, 201 are parallel, there is no te~dency to decelerate the flow las would be the case where the walls diverge). This fact again aids in preventing eddy currents and other unwanted cross-flow forces. There is, of course, some deceleration of the air/fiber stream as it exits tne housing 172 through screening means 180;
but, such deceleration occurs immediately upon exit from the screening means and produces only a fine scale turbulence effect which does not induce gross eddy currents or cross-flow forces.
The foregoing zone is preferably dimensioned so that under normal adjustment of variable system operating parameters, the velocity of the fiber/air stream through the forming zone is at least 20 f.p.s. and the fibers are capable of traversing the entire length of the forming zone 79 from screen 180 to forming wire 80 in not more than .1 second.
While the forming zone 79 in the exemplary form of the invention has been depicted as including physical walls 199, 200 and 201, those skilled in the art will appreciate that the boundary layer confining means could take other forms if desired without departing from the scope of the invention--for example, the confining boundary walls could take the form of air curtains tnot shown). Moreover, in some cases it might be desirable to have the walls 200, 201 converge slightly so as to accelerate and, therefore, stabilize the flow.
I. Overall System Operation -Numerous system parameters may be varied in the operation of a forming system embodying the features of the presen~
invention in order to form an air-laid web of dry fibers having specific desired characteristics. Selected repre sentative and optimum parameter settings are set forth in greater detail in Section K, page 65 et seq., infra, of this specification where specific examples have been delineated.
Such variable parameters include, for example: air-to-fiber ratio (which is, preferably 200-600 ft.3/lb. when working with cellulosic wood fibers, and preferably 1000 to 3000 ft.3/lb., and perhaps higher, when working with cotton linters and relatively long synthetic fibers); air pressure within housing 172 (which preferably varies from +0.5" to ~3.0"
H2O); rotor speed (which preferably varies from 800 to 1800 RPM); the number, orientation and shape of rotor bars employed;
the quantity of air supplied per foot of former width (which is, preferably, on the order of 1500 to 1650 ft.3/min. with an 8-bar rotor operating at 1432 RPM); the energy level of classifying air supplied (which preferably ranges from 1.5 to 2.5 ft.3/min.tin. or, stated in terms of pressure, preferably ranges from 50" to 100" H2O); recycle or separation balance (which is less than 10% by weight of the fiber supplied and, preferably, fxom 1% to 5% by weight of the fiber supplied);
screen design--viz., whether the screen is a woven square-mesh screen or a slotted screen, the size of the screen openings (which ranges between .02" and 0.1" wire-to-wire open space in at least one direction and, preferably, ranges between .045" and .085" open space from wire-to-wire in at least one direction), the wire diameter used (which preferably varies from on the order of .023" to .064") and, the percentage of open screen area (which is between 30% and 55% and, preferably, varies from 38% to 46%); air pressure within the enclosed forming zone 79 (which is preferably atmospheric);
as well as the physical dimensions of the forming head 75 7~ ~
(which, in the exemplary form of the invention, comprises a generally cylindrical housing 172 having an inside diameter of 24").
Moreover, the rate of production of the web being formed can also be varied by altering numerous other system parameters such, merely by way of example, as the number of forming heads 75 used, the position of the forming head relative to the forming wire--i.e., whether the forming head is mounted in the cross-direction, the machine-direction, or at some angle therebetween--forming wire speed, and the type of fibers used. Still other variable parameters under the control of the operator include the cross-directional profile of the feed mat delivered to the forming head 75. Thus, where it is desired to produce a web having a uniform cross-directional profile with an acceptable coefficient of variation,the feed mat~ ~ feed mat 116 in FIG. 3--preferably will have a uniform cross-directional profile in terms of the mass quantum of fibers present. On the other hand, if one desires to produce an air-laid web having a specific non-uniform cross-directional profile--e.g., an absorbent filler web having a central portion with a relatively high basis weight and marginal edges of relatively low basis weights--it is merely necessary to form either a single feed mat or multiple side-by-side feed mats having the requisite cross-directional profile and, since the present system is substantially devoid of cross-directional forces, the cross-directional profile of the input feed mat(s) will control the cross-directional profile o~ the air-laid web.
Reco~nizing the foregoing, let it be assumed that the operator wishes to form an air-laid web 60 one foot (1'~ in width (all ensuing assumptions are per one foot of width of -~7-5~
the forming head 75) having a controlled uniform cross-directional profile and a basis weight of 17 lbs./2880 ft.2.
Assume further:
a) Air-to-fiber ratio supplied through inlet slot 171 equals 350 ft.3~1b.
b) Inlet slot 171 is 51 in circumferential width--i.e., the dimension from edge 190 ~FIG. 3) to edge 202.
c) Rotor housing 172 is 24" I.D.
d) Rotor asse~bly 175 employs eight egually spaced rectangular rotor bars 181, each 3~4"
in radial hsight by 3/8" in circumferential thickness and extending parallel to the axis of the rotor assembly continuously throughout the full width of rotor housing 172 and, each spaced from the rotor housing 172 by 0.18".
e) Rotor assembly 175 is driven at 1432 RPM.
f) Rotor bar 181 tip velocity equals 150 f.p.s.
g) Relative velocity between the rotor bars 181 and the aerated bed 186 is approximately 70 ~ .p.s .
h) Screening means 180 defines an arc o~ 86, and has 40% open area.
i) Separation and/or recycle through separator slot 179 comprises 5% by weight of fibrous materials supplied through inlet slot 171.
j) The quantity of classifying air introduced through air jet 194 is between 1.5 and 2.5 ft.3/min./in. at pressures between 50" and 100" H2O.
k) Forming walls 200, 201 are parallel and spaced ~. :
~$~57~:
9" apart in a direction normal to the parallel walls 200, 201 and 16" apart in a horizontal plane passing through their lower extremities just above the plane of the forming wire 80.
l) Forming wire speed equals 750 f.p.m.
All of the foregoing operating parameters are either fixed and known, or can be pre-set by the operator, except for the relative velocity between the xotor bars 181 and the aerated bed 186 of fibers within the rotor housing 172. The actual speed of the aerated bed 186 is not known with certainty;
but, it is believed to be substantially less than the rotor bar tip velocity of 150 f.p.s.; and, more particularly, it is believed to be on the order of half the tip velocity of the rotor bars 181. For convenience, it is here assumed to - 15 be approximately 80 f.p.s., an assumption believed to be reasonably accurate based upon observation of overall system behavior, thereby resulting in a relative velocity between the rotor bars 181 and the aerated bed 186 of approximately 70 f.p.s. (see assumption "gn, supra).
Accordingly, supply and velocity relationships within the foregoing ;exemplary system can be readily calculated as ollows; and, such relationships have been illustrated in FIG. 10:
17 x 750 = 4.43 lbs./min.--Rate of formation [I]
2880 of web 60.
4.43 x 1.05 = 4.65 lbs./min.--Rate of fiber [II]
supply through inlet slot 171.
4.65 x 350 = 1627 ft.3/~in.--Vol. of air sup- [III]
plied through inlet slot 171.
2~r x 86 = 1.5 ft.--Screen circumference. lIV]
1.5' x 1' x = 216 in.2--Screen area. ~V]
144 in.2/ft.2 , ~ .
i7;~ ' 4.43 x 60 min.= 1.23 lbs./hr./in.2--Fiber [VI]
216 in.2 through~ut of former screen 180.
1.5ft.2 x 40% = 0.6 ft.2--Amount of open area in [VII~
screen 180.
1627 = 65 f.p.s.--Velocity of air and [VIII]
55/12 x 60 fiber stream entering rotor hous-ing 172 through inlet slot 171.
1627 = 18 f.p.s.~-Velbcity approaching [IX]
1.5 x 60 the screen 180 (i.e., normal to the screen).
1627 = 45 f.p.s.--Velocity throug~ screen [~]
- 0.6 x 60 openings.
1627 = 36 f.p.s.--Velocity in forming [XI]
9/12 x 60 zone 79.
1627 _ = 20 f.p.s.--Velocity normal to [XII]
16~12 x 60 forming wire 80.
150 - 70 = 80 f.p.s.--Velocity vector [XIII]
parallel to the screen 180.
~ = 82 f.p.s.--Air velocity vector [XIV]
composite within housing 172.
4.65 - 4.43 = .22 lbs./min.--Amount of fiber [XV]
removed through separator slot 17g .
Keeping the foregoing supply and velocity relationships in mind, and upon consideration of FIGS. 3 and 10 conjointly, it will be appreciated that the individualized fibers, soft fiber flocs, and any aggregated fi~er masses present in the feed mat 116 (FIG. 3) will be disaggregated and dispersed within the air stream passing through fiber transport duct 170 with essentially the same cross-directional mass quantum relàtionship as they occupied in feed mat 116. Under the assumed conditions, the air/fiber stream enters rotor housing 172 (FIG. 3) at approximately 65 f.p.s. [Eq. VIII~ and at a fiber feed rate of 4.65 lbs./min. [EqO II]. The volume of air supplied to rotor housing 172--vlz., 1,627 ft.3/min.
[Eq. III]--is such that a positive pressure of approximately 1.5" H2O is maintained within the housing 172. Since the 7;2 forming zone 79 is maintained at atmospheric pressure, there exists a pressure drop on the order of 1.5" H2O across the screening means 180 through which the air-suspended fibers pass.
Although the air/fiber stream entering rotor housing 172 through inlet slot 171 is moving radially initially, rotation of the rotor assembly 175 (counterclockwise as viewed in FIGS. 3 and 10) tends to divert the fibers outwardly towards the periphery of housing 172 so as to form an annular aerated bed of fibers, as best illustrated at 186 in FIG. 10. Movement of the rotor bars 181 through the annular aerated bed 186 of fibers at a rotor bar tip velocity of 150 f.p.s. tends to accelerate the air-fiber stream from its entry velocity of 65 f.p.s. [Eg. VIII] to approximately 80 f.p.s., thus resulting in a relative velocity of 70 f.p.s between the rotor bars 181 and the aerated bed 185 of fibers However, because of the clearance of 0.18" between the rotor bars 181 and housing 172, and the relatively small effective area of the rotor bars, only minimal pumping action occurs and there is little or no tendency to roll fibers between the rotor bars 181 and either housing 172 or screening means 180. Therefore, there is little or no tendency to form pills; and, since only minimal mechanical disintegrating action occurs, curlin~ or shortening of individualized fibers is essentially precluded. Rather, the rotor bars 181 sweep through the aerated bed 186 and across screening means 180, thus causing at least certain of the individualized fibers and soft fiber flocs within the aerated bed 186 to move through the screening means--such air-suspended fibers have a velocity vector normal to the screening means 180 of approximately 18 f.p.s. [Eq. IX] and a composite velocity
supply through inlet slot 171.
4.65 x 350 = 1627 ft.3/~in.--Vol. of air sup- [III]
plied through inlet slot 171.
2~r x 86 = 1.5 ft.--Screen circumference. lIV]
1.5' x 1' x = 216 in.2--Screen area. ~V]
144 in.2/ft.2 , ~ .
i7;~ ' 4.43 x 60 min.= 1.23 lbs./hr./in.2--Fiber [VI]
216 in.2 through~ut of former screen 180.
1.5ft.2 x 40% = 0.6 ft.2--Amount of open area in [VII~
screen 180.
1627 = 65 f.p.s.--Velocity of air and [VIII]
55/12 x 60 fiber stream entering rotor hous-ing 172 through inlet slot 171.
1627 = 18 f.p.s.~-Velbcity approaching [IX]
1.5 x 60 the screen 180 (i.e., normal to the screen).
1627 = 45 f.p.s.--Velocity throug~ screen [~]
- 0.6 x 60 openings.
1627 = 36 f.p.s.--Velocity in forming [XI]
9/12 x 60 zone 79.
1627 _ = 20 f.p.s.--Velocity normal to [XII]
16~12 x 60 forming wire 80.
150 - 70 = 80 f.p.s.--Velocity vector [XIII]
parallel to the screen 180.
~ = 82 f.p.s.--Air velocity vector [XIV]
composite within housing 172.
4.65 - 4.43 = .22 lbs./min.--Amount of fiber [XV]
removed through separator slot 17g .
Keeping the foregoing supply and velocity relationships in mind, and upon consideration of FIGS. 3 and 10 conjointly, it will be appreciated that the individualized fibers, soft fiber flocs, and any aggregated fi~er masses present in the feed mat 116 (FIG. 3) will be disaggregated and dispersed within the air stream passing through fiber transport duct 170 with essentially the same cross-directional mass quantum relàtionship as they occupied in feed mat 116. Under the assumed conditions, the air/fiber stream enters rotor housing 172 (FIG. 3) at approximately 65 f.p.s. [Eq. VIII~ and at a fiber feed rate of 4.65 lbs./min. [EqO II]. The volume of air supplied to rotor housing 172--vlz., 1,627 ft.3/min.
[Eq. III]--is such that a positive pressure of approximately 1.5" H2O is maintained within the housing 172. Since the 7;2 forming zone 79 is maintained at atmospheric pressure, there exists a pressure drop on the order of 1.5" H2O across the screening means 180 through which the air-suspended fibers pass.
Although the air/fiber stream entering rotor housing 172 through inlet slot 171 is moving radially initially, rotation of the rotor assembly 175 (counterclockwise as viewed in FIGS. 3 and 10) tends to divert the fibers outwardly towards the periphery of housing 172 so as to form an annular aerated bed of fibers, as best illustrated at 186 in FIG. 10. Movement of the rotor bars 181 through the annular aerated bed 186 of fibers at a rotor bar tip velocity of 150 f.p.s. tends to accelerate the air-fiber stream from its entry velocity of 65 f.p.s. [Eg. VIII] to approximately 80 f.p.s., thus resulting in a relative velocity of 70 f.p.s between the rotor bars 181 and the aerated bed 185 of fibers However, because of the clearance of 0.18" between the rotor bars 181 and housing 172, and the relatively small effective area of the rotor bars, only minimal pumping action occurs and there is little or no tendency to roll fibers between the rotor bars 181 and either housing 172 or screening means 180. Therefore, there is little or no tendency to form pills; and, since only minimal mechanical disintegrating action occurs, curlin~ or shortening of individualized fibers is essentially precluded. Rather, the rotor bars 181 sweep through the aerated bed 186 and across screening means 180, thus causing at least certain of the individualized fibers and soft fiber flocs within the aerated bed 186 to move through the screening means--such air-suspended fibers have a velocity vector normal to the screening means 180 of approximately 18 f.p.s. [Eq. IX] and a composite velocity
5~72 vector of approximately 82 f.p.s. lEq. XIV] directed towards scr ening means 180 at an acute angle--while, at the same time, sweeping nits and aggregated fiber masses over and beyond the screening means 180.
Since the rotor bars 181 are moving throuyh the aerated bed 18~ of fibers at a relative speed 70 f.p.s. faster than movement of the aerated bed, a negative suction zone of 1.7"
H2O is generated in the wake of each rotor bar 181, as best illustrated at 204 in FIG. 10. Each such negative suction zone extends the full-width of the rotor housing 172 and is parallel to the axis of the rotor assembly 175. In the case of rotor bars having a circular cross-section (not shown), the negative suction generated would be on the order of 3.0" H2O. In either case, the negative suction generated is sufficient to momentarily overcome the pressure drop of approximately 1.5" H2O across the screening means 180 and, as a consequence, normal flow of the air/fiber stream through screening means 180 ceases momentarily in the region of the screen beneath the negative suction zone 204. The full-width negative suction zones 204 are, of course, also sweeping across the screening means 180 at the same velocity as the rotor bars 181--viz., 150 f.p.s.--and, as a consequence, the rapidly moving spaced, full-width negative suction zones 204 tend to establish spaced full-width lifting forces which serve two i~portant functions--viz., the generated lifting forces i) tend to lift individualized fibers and soft fiber flocs off screening means 180 in the wakes of the rotor bars across the full-width of rotor housing 172, thus preventing layering of fibers on the screen which tends to plug the screen openings and thus inhibits free movement of fibers through the screen; and ii), tend to lift nits and other Z
aggregated fiber masses off the screening means 180 so as to facilitate their peripheral mov~ment over and beyond the screening means and towards the full-width separator slot 179. Such peripheral movemen~ results from the movement of the annular aerated bed 186 and the sweeping action of the rotor bars 181.
It will be apparent that in the exemplary case employing an 8-bar rotor assembly 175 moving at 1432 RPM with a rotor bar tip velocity of approximately 150 f.p.s., approximately one hundred and ninety-one spaced full-width negative pressure zones 204 or negative impulses are generated per second, each of which sweep across the surface of the screening means 180 throughout the full-width thereof at a velocity equal to that of the rotor bars which generated such zones--viz., 150 f.p.s. Moreover, each full-width negative pressure zone 204 persists over an arcuate span on the order of several times the height of the rotor bars 181; although the actual distance through which the negative pressure zones 204 persist will vary dependent upon the specific operating parameters selected such, for example, as rotor speed, the relative speea differential between tha rotor bars 181 and the aerated bed 186 of fibers, an~ the actual pressure conditions established. However, assuming a ull-width negative pressure zone 204 persisting on the order of three times the height of the rotor bars 181 (which are here 3/4`' in height), then the arcuate extent x (FIG. 10) of the negative pressure zone 204 will be on the order of 2.25".
That is, each negative pressure zone 204 will span approximately 24% of the circumferential space of approximately 9.3"
between two adjacent rotor bars 181 (assuming an 8-bar rotor assembly 175 with an outside diameter of 23.64"--i.e., 24"
.
7~
I.D. for housing 172 minus 2 x .18" clearance). Therefore, the balance of the circumferential region from the trailing edge of each zone 204 to the next succeeding rotor bar 181--viz., a circumferential arc ~ (FIG. 10~ spanning approximately 76% of the circumferential distance between two adjacent rotor bars 181, or a distance of approximately 7"--constitutes a region of positive pressure drop of approximately 1.5" H2O.
Thus, it will be appreciated that in operation, approximately one hundred and ninety-one full-width negative pressure zones 204 alternating with an equal number of full-width zones of positive pressure drop which are each on the order of three times as extensive in duration ~i.e., ~ - 3x), will sweep across screening means 180 each second. As indicated above, in those rapidly sweeping regions beneath the negative pressure zones 204, flow of the air/fioer stream through screening means 180 at 45 f.p.s. [Eq. X] ceases momentarily and, the fibrous material in those regions tends to be lifted off the screening means. Considering any given fixed area of the screening means 180, immediately upon passage of each negative pressure zone 204, the positive pressure drop conditions of approximately 1.5" H2O are restored until the next rotor bar 181 passes thereover; thus permitting the individualized fibers and soft fiber flocs to again move toward the screening means 180 at a velocity of 18 f.p.s.
[Eq. IX~ normal to the screen and at a composite velocity vector of 82 f.p.s. [Eq. XIV] directed towards the screen at an acute angle and, ultimately, through the screen openings at approximately 45 f.p.s. [Eq. X]. Thus, the arrangement is such that plugging of the screen is effectively precluded, while individualized fibers tend to dive end-wise through the screen openings in the regions of positive pressure ~L~ 7~
drop, as best illustrated in FIG. 11--i.e., in those regions between the trailing end of each full-width negative pressure zone 204 and the next succeeding rotor bar 181.
Those individualized fibers, soft fiber flocs, and aggregated fiber masses within the aerated bed 186 of fibers which do not pass through the screening means 18U the first time they are presented thereabove are swept over and beyond the screening means 180 and, thereafter, past classifying air jet 194 (FIG. 3). I~nder the assumed conditions, the individualized fibers and soft fiber flocs tend to be diverted radially inward by the classifying air jet 194, while the undesired aggregated fiber masses are centrifugally and tangentially separated from the aerated bed 186 through full-width separator slot 179 at the rate of .22 lbs./min.
[Eq. XV]. Those individualized fibers and soft fiber flocs remaining in the aerated bed 186 after transit of separator slot 179 are then returned to the region overlying screening means 180, where they are successively acted upon by the rapid succession of pressure reversal conditions from full-width negative pressure zones 204 alternating with full-width zones of positive !?ressure drops until all such materials pass through t~e screening means 180 into forming zone 79.
The airtfiber stream tends to exit from housing 172 non-radially through screening means 180--indeed, as previously indLcated, under the assumed operating conditions the air/fiber stream tends to exit at an acute angle which, on average over the full extent of discharge opening 178, intersects a line tangent to the midpoint of screening means :180 at an included angle Q' of from 5 to 20 and, preferably, on the order of 11. 'lne exiting air/fiber stream decelerates almost immediately to approximately 36 f.p.s. [Eq. XI]
7~
within forming zone 79 and moves through the forming zone toward the foraminous forming wire 80 which is here moving at 750 ft./min. The fibers are air-laid or deposited on forming wire 80 at the rate of 4.43 lbs./min. [Eq. I]--i.e., the difference between the rate of fiber supplied [Eq. II]
and the 5% of fibrous ~aterials supplied which are separated and removed through separating slot 179--to form web 60.
The fibers deposited on the forming wire 80 are held firmly in position thereon as a result of suction box 126 (FIG. 3) and its associated suction fan and ducting which serve to accomodate and remove the high volume of air supplied.
Following formation of web 60 on foraminous forming wire 8Q in the manner above described, the web, carried by forming wire 80 at a speed of 750 ft./~in., exits from beneath forming zone 79. An auxiliary suction box 128 may, if desired, be provided to insure that the web remains flat on the forming wire as it exits from beneath forming zone 79 where the web has bèen subjected to the holding action provided by suction box 126 which accomodates the main air stream. The thus formed web 60 may then be further processed in the manner previously described in detail in Section D, pages 25-27 supra, of this specification. That is, and as best illustrated in FIG. l, the web 60 is preferably passed through calender rolls 129 where it is compacted lightly, and is then transferred to a conveyor belt 130.
The web is thereafter bonded in any suitable bonding station 85, dried at a drying station 87, and formed into a storage roll 95 at a suitable storage station 90.
Those skilled in the art will appreciate that there has herein been described a typical set of operating para~eters for forming an air-laid web 60 of dry fibers at a relatively , 3~
high production speed-- _z , 750 ft./min.--utilizing only a single forming head 75 (FIGS. 1 and 3). The exemplary web thus formed has a basis weight of 17 lbs./2880 ft.2 and is essentially devoid of nits and other aggregated fiber masses which have been removed through separator slot 179. Because the forming head 75 and forming zone 79 have been designed so as to essentially preclude induced cross-flow forces and/or eddy currents therein, the controlled mass quantum dispersion of fibers remains substantially unchanged throug'nout the system, thereby permitting the system to be scaled up or down to form air-laid webs of virtually any desired width and with a controlled coefficient of variation.
The web 60 deposited on forming wire 80 has more than adequate integrity to permit rapid movement of the forming wire. Indeed, if one desires to further increase productivity, n additional forming heads 75A-75N (FIG. 2--where n equals any whole integer) may be utilized and the speed of foraminous forming wire 80 may be increased by a factor equal to the number of separate forming heads used--e.g., under the assu~ed operating conditions, two heads would permit operation at 1,500 f.p.m.; three heads would permit operation at 2,250 f.p.m.; et cetera. Indeed, with the present invention, _ forming wire speed is no lonyer limited by the speed of web ~ormation but, rather, by the speed of such subsequent processing steps as bonding in the web bonding station 85 (FIG. 1).
Experimentation with air-laid, dry fiber, web forming systems embodying the features of the present invention has indicated that a wide range of results are attainable dependent 30` upon the particular operating parameters selected, as reference to the representative experimental data set forth 57~2 in Section K, page 65 et seq., infra, of this specification indicates. Particularly importan~ are such design and operating parameters as: i) rotor design; ii) rotor speed; iiiJ recycle or separation percentage; iv) screen design; and v), air-to-fiber ratio. of the foregoing, rotor design and screen design represent fixed parameters which, once selected, are not normally subject to operator control; whereas the remaining parameters may be varied over wide ranges to provide virtually an infinite range of possible permutations and combinations which can, and will, affect the characteristics o~ both the web produced and the rate of web productivity.
For example, as indicated in Section G at pages 39-40, supra, of this specification, the rotor assembly 175 may be formed with n rotor bars 181 where n equals any whole integer greater than ~ln. However, it has been ascertained that fiber throughput--a limiting constraint when attempting to maximize productivity--is a function of rotor speed multiplied by the square root of the number of rotor bars employed--i.e., fiber throughput: f (RPM x ~No. of rotor bars 181). This relationship wili, of course, vary with the particular screen employed; and, has been graphically illustrated in FIG. 12 wherein fiber throughput in lbs./in./hr. (the ordinate) has been plotted at various rotor speeds for each of a 2-bar, 4-bar, and 8-bar rotor assembly (the abscissa) when using both a coarse wire screen (lOx2.75; .047" wire dia.;
.059" screen opening; and 46.4~ open screen area) and a fine wire screen (16x4; .035" wire dia.; .032" screen opening;
and 38.8% open screen area). As here shown, the circular points 205 are each representative of f iber throughput at a given rotor speed multiplied by the square root of "2" and are, therefore, indicative of throughput for a 2-bar rotor.
7~ `
Similarly, the triangular points 206 are each indicative of fiber throughput for a 4-bar rotor, while the square points 208 are indicativ.e of fiber throughput for an 8-bar rotor.
Thus, the line 209 (FIG. 12) represents the Regressor, or "line-of-best-fit", from which functional relationships between throughput and rotor speed can be determined when using a coarse wire screen of the type described aboveO
`Similarly, the line 210 represents the same functional relationships when using a fine wire screen of the type described above. The data thus corroborates experimental findings that rotor RPM can be reduced while fiber throughput is maintained, or even increased, by going, for example, from a 4-bar rotor assembly 175' (FIG. 5) to an 8-bar rotor assembly 175 (FIG. 3). However, when using an 8-bar rotor assembly 175, the forming system seems to be less tolerant of mismatches between forming air and rotor speed; and, where such mismatches occur, fibers tend to accumulate on the sidewalls 199 of the forming zone 79. This is readily corrected by reducing rotor speed, normally by less than 10~, while maintaining forming air constant.
It has further been discovered that both nit levels in the air-laid web 60, and fiber throughput in lbs./hr./in.2, are a function of the percentage of fibrous materials removed from the aerated bed 186 ~FIG. 10) through the full-width separator slot 179 (FIG. 3). Thus, referring toFIG. 13, line 211 graphically portrays the decreasing relationship of nit level (the ordinate~ with increasing separation/recycle percentages (the abscissa); while, at the same time, increasing separation/recycle percentages are accompanied by increased fiber throughput in lbs./hr./in.2 The graph is here representative of a system in which tile s~ ' rotor assembly 175'--a 4-bar rotor assembly--was driven at 1700 RPM and fibers were introduced into the rotor housing 172 (FI~. 3) in an air stream supplying air at approximately 106 ft.3/min./in. When the percentage of fibrous material separated through separator slot 179 was 1~, fiber throughput was 0.62 lbs./hr./in.2, and the air-laid web 60 exhibited a nit level of "3"--a level deemed to be "poor", or border-line between acceptable and non-acceptable.
As described in more detail in Section J, pages 64 and 65 infra, of this specification, numerical nit levels range from "0" ("excellentn), to "1" ("good"), to "2" ("adequate"), to "3~ ~"poor"), to 4" through ~6" ("inadequate" to "non-acceptable"). Such numerical ratings are subjective ratings based upon visual inspection of the formed web 60 and subjective comparison thereof with pre-established standards.
As the pressure of the recycle air supplied through manifold 191 (FIG. 3) is decreased and/or as separator slot 179 is widened, thereby modulating the pressure conditions within discharge conduits 192 (FIG. 3) and 77 (FIG. 1) which are maintained at a pressure level below that within the forming head 75 by means of a suction fan (not shown), the amount of fibrous material removed from rotor housing 172 through separator slot 179 is increased. Other means such, for example, as venturi passageways (not shown) could also be used to insure a controlled outflow of materials through separator slot 179.
As the percentage of fibrous materials separated and/or recycled incxeases, nit level in the formed web 60 decreases.
At the operating conditions under which FIG. 13 was prepared, when the separation percentage was increased to approximately 2.5~, a web having a nit level of "2" (i.e., an "adequate"
~60-. .
nit level rating) was produced; at a separation percentage of 3~, the web's nit level decreased to approximately "1.6"
(i.e., approximately midway between "adequate" and "good");
at a separation percentage of approximately 3.8%, nit level dropped to "1" ("good"); and, at a separation percentage of 5%, nit level dropped to approximately "0" ("excellentn).
FIG. 19 also shows that the throughput of the forming system was increased from .6~ lbs./hr./in.2 to .96 lbs./hr./in.2 while at the same time improving web quality from ~poorn to "excellentn. The total amount of fiber delivered to the forming system was increased by an even greater percentage to compensate for the increased remov~l of fiber and aggregate for recycling. Productivity of the forming system was thus increased about 55% even though the screen was more heavily loaded with fiber; a very significant improvement. These CQmpariSOnS were made while running good quality pulp (Northern Softwood Kraft) having a low content of pulp lumps and being well fiberized in the hammermill. Poorer quality pulps or less effective fiberization would require higher recycle rates of up to 10~ to maintain an acceptable nit level in the web being formed. When making less critical webs or thick batts, nit level and recycle rate become less critical.
Those skilled in the art will, of course, appreciate that the experimental data set forth in FIG. 13 is only representative for one given set of operating parameters;
and, such data will vary with changes in, e.g., air-to-fiber ratio, type of fiber used, rotor speed, rotor design, air supply, and/or screen characteristics. However, experiments have indicated that recycle percentage is critical and, for cellulosic fibers, should exceed 1%, is preferably between about 1% and 5~, and should be less than on the order of 10%.
' 9S~Z
It has been found that a 2-dimensional air-laid web forming system embodying features of the present invention will, when operating at a proper balance of fiber supply, forming air supply, and rotor speed, not only deliver maximum fiber throughput with minimum recycle, but, moreover, will exert a "healing effect" on basis weight non-uniformities entering the forming head 75 (FIG. 3). That is, the screen 180, when properly loaded with a moving or transient aerated bed 186 of fibers (FIG. 10), acts as a membrane which tends to equalize or even out the passage of fibers through adjacent incremental widths of the screen. Such "healing effect" is only operative over distances of six inches (6!') or less.
However, the "healing effect" will tend to reduce the coefficient of variation within a forming head 75 supplied with an air/fiber stream delivered through a partitioned duct 170 of the type shown in FIG. 3--viz., the effect of non-uniformities present within each four inch wide segment of the air stream exiting the paxtitioned duct 170 will tend to be minimized. The "healing effectl' will not function to even out gross irregularities in fiber basis weight over a wide expanse of former widths. Stated differently, a forming head 75 embodying the features of the present invention can even out either low or high non~
uniformities of up to approximately three inches in width, but it cannot even out gross non-uniformities of eight, twelve, or more, inches in width. It has further been found that if insufficient fiber loading occurs--i.e., if the air-to-fiber ratio increases to substantially above 600 ft.3/lb.
when working, for example, with cellulosic wood fibers--then, i) the aerated bed 186 tends to be starved of fibers;
ii) the "healing effect" is reduced because of an inadequate transient membrane over the screen 180; and ~ input non-, .
uniformities tend to be replicated in the finished web 60, thus deleteriously affecting the coefficient of variation of the finished web.
J. Forminq Capacities and Web Characteristics With 2-Dimensional Systems Embodying Features of the Present Invention Referring to Table I, it will be observed that a single forming head 75 embodying the features of the present invention--e.g., the type shown in FIGS o 1 and 3--and having a ~emi-cylindrical screen 18" in circumferential length, is capable of producing webs having basis weights ranging from 14-40 lbs./2880 ft.2 at forMing wire speeds ranging from about 911 f.p.m. to about 319 f.p.m.
IN ACCORDANCE WITH THE INVENTION
- _ _ Forming Wire Speed-- t./min Basis Weight Product No. of Formin~ Heads lbs./2880 ft.2 Type ~ -- ~ 2 ~ 3 14 Bath Tissue 911 1821 2737 17 Facial Tissue 750 1500 2250 26 Towel 490 981 1471 34 Towel 375 750 1125 _ Towel 319 638 _956 1. The data set forth in this Table I is based up~n a fiber throughput capacity of 1.23 lbs./hr./in. for a single forming head of the type shown at 75 in FIG. 1 and 9, and which uses a relatively fine screen 180 18" in circumferential length and having a screen openinq of 0.050".
TABLE I
These realistically attainable for~ing wire speeds may be doubled, tripled, or even further multiplied by using two, three or more forming heads 75A-75N in the manner shown in FIG. 2. Consequently, the formation of air-laid webs of dry fibers is no longer limited to low forming wire speeds; and, ~ 34~ 57 ~
this is believed to be a direct result of the fiber throughput capacity of each forming head 75 which is capable of delivering in the order of ten times the mass quantum of fibers per square inch of former screen as can be delivered by known prlor art forming systems.
Not only does the present invention permit significant increased rates of productivity at considerable savings in terms of energy consumption and space requirements, but, moreover, the webs produced are not constrained in terms of width limitations, can be formed in an essentially nit-free condition, and are comprised of individualized fibers which have not been disintegrated, shortened, curled, rolled into pills, or otherwise seriously damaged by excessive mechanical action within either the rotor housing 172 (FIG. 3) or in excessive secondary hammermilling operations.
Standards have been established by the assignee of the present invention for subjectively classifying the nit levels in air-laid webs formed of dry fibers. Such subjective standards are based upon visual inspection of the webs and comparison thereof with existing webs having differing nit levels which have been subjectively rated as ~0", "1", "2", "3", "4", "5" and "6". Photographs representative of webs having nit levels of "0", "1", "2", "3", "4", "5" and "6"
are here reproduced as FIGS. 14-20, respectively. FIG. 14 portrays a web having a nit level of "0" which is indicative of a web rated "excellent" and which is essentially free of nits and can, therefore, be used for the highest quality tissue products. FIG. 15 portrays a web having a nit level of "1" which is indicative of a high quality web having only a minimal level of nits and which is classified as "good".
Again, such a web is suitable for use in premium grade, 7~
quality bath and/or facial tissues. FIG. 16 photographically depicts a web having a nit level of "2" which is indicative of a web having a higher percentage of nits; yet which is "acceptable" for usage in quality bath and/or facial tissues.
FIG. 17 comprises a photograph o~ a web having a nit level of "3" which is considered "poor", but which is suitable for occasional usage in quality tissues or for usage in medium grade tissue products. FIGS. 18-20 photographically portray webs having nit levels of "4~, "5" and "6", respectively, and are indicative of webs of inferior quality which are generally not suitable for usage in bath and/or facial tissues. As the ensuing descxiption proceeds, the reader may find it convenient to refer to FIGS~ 14-20 so that the nit levels given in connection with the descriptions of Examples I-X will have greater meaning and significance.
K. Examples--Comparative Representative and/or Optimum Operating Parameters for Air-Laid Dry Fiber Web Forming Systems and Woven Square-Mesh Sceens The ensuing portion of the present specification includes a discussion of the effects of varying various system parameters when utilizing slotted screens in accordance with tho present invention, as well as when utilizing woven square-mesh screens. The Examples given are of actual experimental runs made with the equipment and have been randomly selected solely for the purpose of illustrating the effect of varying one or more of the operating parameters. No effort has been made to optimize operating conditions for each different given Example; although, certain of the Examples do reflect sets of operating parameters which either approach optimized conditions, are at or about optimized conditions, or somewhat exceed optimized conditions. Data for the various parameters for each of the Examples given are set forth in tabular form in Tables II and III, inclusive. Examples I-III, represent operating parameters when utilizing woven square-mesh screens;
whereas Examples IV-X represent operating parameters for a web forming system utilizing slotted screens in accordance with the present invention.
Referring first to Examples I, II and III (Table II, page 67), it will be noted that in the forming systems used to generate the wabs of such Examples, the screens employed were woven square-mesh screens, respectively lOxlO, 12x12, and 8x8, and respectively having 42.3~, 51.8% and 38.9~ open screen area. In all three cases only a single-forming head was used, here having an 8-bar rotor assembly 175 (FIG. 3).
In the case of Examples I, II and III, recycle percentages were 10.2%, 7.5% and 7.9%, respectively. The coefficients of variation for Examples I, II and III were 3.1%, 1.8% and 2.2~, respectively, while the nit levels were "1" ("good") for Examples I and III, and "0" ("excellent~) for Example II. Thus, the webs produced were suitable for use in high quality lightweight tissue products.
It should be noted that in Table II under the category "Product Made", Examples I, II and III have been designated as "Exp."--i.e., "Experimental". This designation has been used simply because the system parameters were not set with any specific product or end use in mind; rather, the web being formed was considered to be an "experimental" web.
However, reference to the data for web basis weight reveals that the experimental webs of Examples I and II are suitable for facial tissue, while the experimental web of Example III
is suitable for toweling.
Forming wire speeds and fiber throughput--the principal -- ' ' . .
357~ ' . _ , Example No. I II III IV V
~ ::
Run No. 2899 2940 2942 1035 1025 _ . .
Fiber Type~ ) N~ N~ N5~ N~ NS~
_ _ Fiber Feed Rate--lbs /in /hr ~2) 9.~ 4.6 20.3 17.1 17.0 53. . . _ Top Air Supply--ft. /min./in. 112 115 115 107 107 _ . _ Air-to-Fiber Ratio -ft.3/lb. 689 1500 331 375 377 _ _ _ _ . ~
No. of Rotors 1 1 1 1 .
No. of Rotor Bars/Rotor 8 8 8 8 8 . _ . _ _ Rotor Speed--RP~I 1200 1550 1600 1400 1800 .' . _ . .
Screen Type 10x10 12x12 8x8 11x2.5 11x2.5 Screen Openin~--Inches .065 .060 .078 .050 .050 _ .
% Open Screen Area 42.3 51.8 38.9 43.6 43.6 _ _ Former Pressure--Inches H~O 1.85 1.5 3.0 1.1 1.6 %Fiber Recycled 10.2 7.5 7.9 5.8 5.3 _ . _ _ .
Amount Fiber Recycled--lbs./in./hr.1.0 0.35 1.6 1.0 0.9 _ _ _ _ ~ .
Fiber Throughput--lbs./hr./in.2 .49 .24 1.04 .89 .89 _ _ . ~.-Classifying Air--ft.~/min./in. 1.3 1.4 2.1 2.2 2.2 Forming Wire Speed--ft./min. 300 150 500 525 500 _ ~ Facial ~acial Product Made Exp. Exp. Exp. Tissue Tissue . _ Basis Weight--lbs./2880 ft.c 16.9 17.6 22.7 17.7 18.6 . ~ .
Coefficient of Variation--C.D.% 3.1 1.8 2.2 2.1_ 7.1 _ _ _ _ _ _ _ Tensile--Gms./3" C.D. Width 505 357 763 335 371 _ _ . _ ~ _ Nit Level 1.0 -0- 1.0 1.1 1.6 _ _ - =
1. NSWK is Northern Softwood Xraft.
2. Fiber feed rates as stated represent maximum former capacity for the operating parameters established.
TABLE II
indicators of productivity--are of particular interest when evaluating the forming process used to form the webs of Examples I, II and III. In the case of Example I, for example, fiber throughput of 0.49 lbs./hr./in. and forming S wire speed of 300 f.p.m. were achieved utilizing a single forming head 75. Both parameters are approximately 40% of the anticipated average maximum production capacity set forth in Table I, page 63, suPra. In the case of the web formed in Example II, fiber throughput of 0.24 lbs./hr./in.2 and forming wire speed of 150 f.p.m. represent approximately ~0% of the anticipated average maximum production capacity set forth in Table I. In the case of Example III, the web produced was substantially heavier than the webs of Examples I and II discussed above, having a basis weight of 22.7 lS lbs./2880 ft.2. Forming wire speed of 500 and thxoughput of 1.04 lbs./hr./in.2 are significantly improved over the comparable parameters for Examples I and II. While the throughput and forming wire speed data set forth in Example III is for a web having a basis weight of 22.7 lbs.~2880 ft.2, such data is equivalent to forming a web of 17 lbs./2880 ft.2 at approximately 668 ft./min.
Thus, it is apparent that the operating parameters used in formation of the web of Example III are such that the system is approaching the anticipated average maximum production capacity set forth in Table I, page 63, ~
That is, according to Table I it is anticipated that a web having a basis weight of 26 lbs./2880 ft. can be formed by a single head 75 at an average maximum forming wire speed of 490 f.p.m.; while a 17 lb./ 2880 ft.2 basis weight web can be formed at an average maximum speed of 750 f.p.m. Consequently, the average maximum forming capacity for forming a web _, 6 ~
having a basis weight of 22.7 lbs./2880 ft.2--i.e., a web identical to that of Example III--would be on the order of 562 f p.m. Therefore, since the web of Example III was formed at 500 f.p.m., it is evident that the actual rate of productivity was approximately 89% of the anticipated average maximum production capacity. In short, the operating parameters used in forming the web of Example III approach optimum settings for forming an air-laid web of dry Northern Softwood Kraft (NSWK) fibers when utilizing an 8x8 woven square-mesh screen and a single forming head 75 having an 8-bar rotor assembly such as that shown in FIG. 3. Production rate may, of course, be further increased by the simple expedient of utilizing two, three or more tandem forming heads 75A-75N in the manner suggested in FIG. 2; an arrangement which would, under the operating parameters set forth for Example III, permit the formation of a web having a basis weight of 22.7 lbs./2880 ft.2 suitable for toweling at forming wire speeds of 1124 f.p.m. (two heads), 1686 f.p.m.
(three heads), et cetera. Alternatively, and assuming all other operating parameters remain unchanged, a web having a basis weight of 17 lbs./2880 ft.2 suitable for use as a facial tissue could be formed at 668 f.p.m. (one head), 1336 f.p.m. (two heads), 2004 f.p.m. (three heads), et cetera.
25 ` When employing a slotted screen in accordance with the present invention such, for example! as that shown in FIG.
7, the results in terms of increased productivity are dramatic. This may be readily demonstrated by reference to Examples IV a~d V (Table II, page 67), and Examples VI
through X (Table III, page 70), and comparing the data there given with that set forth in connection with Examples I-III
'. .
3~
Example No. VII ~II IX X
.___ " . . _ Run No 2717 2861 2908 2909 2946 . __ _ Fiber Type( ) ~WK _ NSWK N~ N5WX
=.=~ , _ ~
Fiber Feed Rate--lbs./in./hr.(2) 26.3 28.9 18.4 18.3 26.0 ~
Top Air Supply--ft.~/min./in. 133 131 129 12~ 119 . ,~ ........ . .
Air-to-Fiber Ratio--~t.J/lb. 312 271 420 423 275 - _ , No. of Rotors 1 1 1 1 No. of Rotor Bars/Rotor _ 4 8 8 8 8 Rotor Speed--RPM 1700 1600 1000 1000 1550 _, .
Screen Type 1~2.75 9x2.5 1~2.5 11x2.5 11x2.5 10 Screen Opening--Inches 059 .063 050 050 050 % Open Screen Area 46.4 45.5 43.6 43.5 43 6 _ . _ Former Pressure--Inches ~ O 1.6 2.0 0.95 0.95 1.7 _ ~ . ___ ~Fiber Recycled 2.7 3.1 5.4 4.9 4.6 Amount Fiber Recycled--lbs./in /hr. 0.7 _ 0.9 1.0 0.9 1.2 Fiber Throughput--lbs./hr.!in. __ 1.a2 _1.55 97 97 1 37 Classifying Air--ft.3/min./in. 2.6 1.6 1.6 1.4 1.8 15 Forming Wire Speed--ft./min. 800 590 375 225 640 - _ _ H.D.
Product Made _ ~ _ Exp. Tbwel Tbwel Ex~.
Basis ~eight--lbs./2880 ft. 17.0 27.3 26.7 44.5 22.3 Coefficient of Variation--C.D.% 4.8 3.5 3.9 4.4 1.1 Tensile--Gms./3" C.D. Width 521 1045 265 559 705 Nit Level 2.0 0.3 1.0 -0- 2~.0 . . - . ~ ., _ _ _ 1. NSWK is Northern Softwood Kraft.
2. Fiber feed rates as stated represent maximum former capacity for the operating parameters established.
TABLE III
i~ 43~2 (Table II). Thus, in Examples IV-X the recycle percentages range from a high of 5.8% (Example IV) to a low of 2.7%
(Example VI). In Examples IV through VI, facial tissue grade webs were produced in accordance with the invention having basis weights ranging from 17.0 lbs.l2880 ft.2 (Example VI) to 18.6 lbs./2880 ft 2 (Example V); while in Examples VII through X, toweling grade webs were produced having basis weights ranging from 22.3 lbs./2880 ft.2 (Example X) to 44.5 lbs./2880 ft.2 (Example IX~. Fiber throughput for the webs of Examples IV through X ranged from .89 lbs./hr./in.2 (Examples IV and V) to 1.55 lbs./hr./in.2 (Example VII).
In terms of formed web characteristics~ the nit levels of "0" ("excellent") "0.3" ("excellent"), "1.0" and "1.1"
("good") for Examples IX, VII, VIII and IV, respectively, compare favorably to the nit levels for Examples I-III. Nit levels for Exa~ples V, VI and X were "1.6", "2.0" and "2.0n, respectively; and, as such, those webs were rated "adequate", although nit level was not qùite as good as in the case of Examples I-III. Coefficients of variation for Examples IV
through X were 2.1~, 7.1~, 4.8%~ 3.5~, 3.9~, 4.4%, and 1.1%, respectively, as compared with Examples I-III where the coefficients of variation were 3.1~, 1.8% and 2.2%. The coefficient of variation for Example V of 7.1~ is relatively ~5 poor and would not generally be acceptable for premium grade facial tissues.
Comparisons of the results attained at the parameter settings for Examples VI and VII (Table III, page 70) with the anticipated average maximum forming capacities set forth in Table I, page 63, supra, reveals that in both cases the rate of productivity attained substantially exceeded the anticipated average maximum capacity for the forming system of the present invention. Thus, while it would normally be anticipated that a single forming head 75 could produce a web having a basis weight of 17 lbs./2880 ft.2 at a forming wire speed of 750 f.p.m. (See, Table I, page 63) in the case of Example VI a 17 lb./2880 ft.2 basis weight web was produced at a forming wire speed of 800 f.p.m.--l.e, approximately 6.6% faster than the average maximum productivity rate anticipated. Nevertheless, the resulting air-laid web was entirely satisfactory for use as a premium grade quality facial tissue. Similarly, the web of Example VII, which has a basis weight of 27.3 lbs./2880 ft.2 suitable for toweling, was actually produced at 590 f.p.m. on a single forming head 75, whereas the anticipated average maximum forming speed for such a web would normally be on the order of 467 f.p.m.
(Cf., Table I, page 63)--i.e., the actual rate of productivity acheived exceeded the anticipated average maximum capacity by approximately 26.3~6. In the case of Examples VI and VII, the fact that productivity rates actually achieved somewhat exceed the average anticipated maximum ratès set forth in Table I is believed to be attributable in large part to the fact that relatively coarse screens were used in making the webs of such Examples--viz., relatively coarse screens having .059" (Example VI) and .063" (Example VII) openings, rather than fine screens having .050" openings and which formed the basis for the data set forth in Table I. Experimental data such as that set forth in Table III suggests that for heavyweight towel products, relatively coarse screens will tend to improve productivity rates without giving rise to any serious problems in terms of operation or web characteristics.
The characteristics of the Example VII web in terms of nit level, coefficient of variation and basis weight are again such that the web produced was of excellent quality suitable for use in premium grade toweling.
It is apparent that the particular parameters used in connection with Examples VI and VII exceed, or at the very least, closely approximate optimum settings, although so~e fine tuning might be required in an effort to further reduce the coefficient of variation and nit level for Example VI.
For example, a reduction in screen opening size--e.g., from the .059" opening used in Example VI to a screen opening on the order of .050n--might well result in optimizing the membrane characteristics of the transient aerated bed 186 of fibers (FIG. lO) so as to produce an increased "healing effect" of the type described in Section I at pages 62-63, supra, of this specification, thereby reducing the coefficient of variation. Similarly, rPduction of rotor speed might produce the same result. And, an increase in the recycle percentage of 2.7% is likely to further reduce the nit level as heretofore described in connection with FIG. 13 (Section I, pages 60-61, supra). For example, comparison of Examples IV
and V (Table II, page 67) reveals that the operating parameters established for both Examples were, with the exception of rotor speed, essentially the same. Rotor speed, however, was only 1400 RPM in the case of Example IV, whereas in Example V it was 1800 RPM. Thus, a decrease in rotor speed of 400 RPM was accompanied by and, presumably, at least in part resulted in, reduction of the coefficient of variation in the formed web from 7.1% (Example V) to 2.i% (Example IV), and a reduction in nit level from "l.6" (Example V) to "l.l"
(Example IV).
As in the case of the woven sguare-mesh screen comparisons (Examples I, II and III, Table II, page 67); where the best 7~
result in terms of productivity was achieved with the coarsest screen--viz., an 8x8 woven square-mesh screen having screen openings .078" in width (Example III)--in the slotted screen comparisons the best result in terms of productivity was also achieved when using a relatively coarse slotted screen--viz., a 9x2.5 screen having screen openings of .063" in width (Example V).
Examples III lTable II, page 67), and VII-X (Table III, page 70), are of interest principally for their showing of typical operating parameters suitable for forming relatively heavy basis weight webs which can be used for toweling products. Considering Example III, it will be noted that when utilizing utilizing an 8x8 woven square-mesh screen, a web having a basis weight of 22.7 lbs./2880 ft.2 was produced at a for~ing wire speed of 500 f.p.m. Considering Examples VII-X (Table III, page 70, supra), it will be noted that the webs there formed in accordance with the invention had basis weights ranging from 22.3 lbs./2880 ft 2 (Example X) to 44.5 lbs./2880 ft.2 (Example XI), coefficients of variation ranging from 1.1~ (Example X) to 4.4~ (Example IX), and nit levels of "0", "0.3n, "1.0" and "2.0" for Examples IX, VII, VIII and X, respectively; all of such basis weights, coefficients of variation and nit levels being entirely suitable for commercial grade, high quality toweling products. The webs of Examples VIII and IX were formed at productivity rates of approximately 78.5% of the average maximum productivity rates anticipated (Cf., Table I, page 63). The web of Example VII (as previously described) was formed at a speed approximately 26.3% in excess of the anticipated average maximum capacity; and, the web of Example X was formed at a speed approximately 12% in excess of the anticipated average maximum capacity.
It is believed that the numerical data set forth in this Section K in connection with Examples I through X
clearly evidences the significant improvement obtained in fiber throughput--i.e., productivity rate--when utilizing slotted screens in accordance with the present invention as contrasted with using conventional woven square-mesh screens of the type shown in FIG. 6. However, the dramatic improvement in throughput is made even more evident upon inspection of that data as reproduced in graphic form in FIG. 21. Thus, as here shown fiber throughput for each of Examples I through X in lbs./hr.~in.2 (the ordinate in FIG. 21) has been plotted versus the screen opening size in inches used with each Example (the abscissa in FIG. 21). The line 216 is thus representative of fiber throughput when using woven square-mesh screens in a 2-dimensional web forming system and has been generated from the throughput data given in Table II
for Examples I, II and III.
As heretofore indicated, remarkably improved throughput rates are attained when utilizing a slotted screen with a 2-dimensional former in accordance with the present invention.
Such results are reflected by the line 218 which has here been generated using the throughput data recorded for Examples IV and V (Table II) and VI-X (Table III).
It will be appreciated by those skilled in the art upon consideration of the data in this Section K and in the preceeding Section J of this specification, that the present invention is uniquely suited for forming high quality webs having virtually any desired basis weight in lbs./2880 ft.2 at relatively high forming wire speeds. Indeed, such extremely high productivity rates may be readily set forth as follows:
5~Z
A web having a basis weight of (x) (17 lbs./2880 ft.2) where "x" is equal to any desired whole or fractional value, can be produced at a forming wire 80 speed of 750 f.p.m.
divided by "x"; or, (x) (17 lbs./2880 ft.2) = forming wire speed (750 f.p.m.) [XVI]
Similarly, where N forming heads 75A-75N are used (See, e.g., FIG. 2), the foregoing relationship of web basis weight to forming wire 80 speed may be expressed as follows:
(x) (17 lbs./2880 ft.2) = forming wire speed (-)~750xf P ~ XVII]
Based on the experimental data reported herein, it is evident that the present invention provides a dramatic improvement in fiber throughput capacity for the forming head. Thus, the data reflects fiber throughputs ranging from 15 somewhat in excess of .5 lbs./hr.in.2 (Example IV) to in excess of 1.50 lbs./hr./in.2 (Example VII) when working with cellulosic wood fibers and a former 75 24" in diameter. Moreover, it should be noted that the foregoing range of from .5 lbs.~hr./in.2 to at least 1.50 lbs./hr./in.2 reflects efforts made to form high quality, lightweight tissue and/or towel grade products.
Where product quality in terms of, for exam~le, nit level can be accepted at lower quality levels, it can be expected that fiber throughput will exceed and, may substantially exceed, the level of 1.50 lbs./hr./in.2. Similarly, when actual production experience has been acquired, it can be expected that fiber throughputs will be regularly achieved which do exceed the level of 1.50 lbs./hr./in.2, and such improved results may also be achieved when the system is scaled up in size~ ~ to rotor assemblies on the order of 36 in diameter. Therefore, the phrase "to at least 1.50 lbs./hr./in.2~ as used herein and in the appended claims is not intended to place an upper limit on throughput capacity.
- Those skilled in the art will appreciate that there has herein been described a novel web forming system characterized by its simplicity and lack of complex, space-consuming, fiber handling equipment; yet, which is effective in forming air-laid webs of dry fibers at commercially acceptable production speeds irrespective of the basis weight of the web being formed. At the same time, the absence of cross-flow forces insures that the finished web possesses the desired controlled C.D. profile which may be either uniform or non-uniform.
Since the rotor bars 181 are moving throuyh the aerated bed 18~ of fibers at a relative speed 70 f.p.s. faster than movement of the aerated bed, a negative suction zone of 1.7"
H2O is generated in the wake of each rotor bar 181, as best illustrated at 204 in FIG. 10. Each such negative suction zone extends the full-width of the rotor housing 172 and is parallel to the axis of the rotor assembly 175. In the case of rotor bars having a circular cross-section (not shown), the negative suction generated would be on the order of 3.0" H2O. In either case, the negative suction generated is sufficient to momentarily overcome the pressure drop of approximately 1.5" H2O across the screening means 180 and, as a consequence, normal flow of the air/fiber stream through screening means 180 ceases momentarily in the region of the screen beneath the negative suction zone 204. The full-width negative suction zones 204 are, of course, also sweeping across the screening means 180 at the same velocity as the rotor bars 181--viz., 150 f.p.s.--and, as a consequence, the rapidly moving spaced, full-width negative suction zones 204 tend to establish spaced full-width lifting forces which serve two i~portant functions--viz., the generated lifting forces i) tend to lift individualized fibers and soft fiber flocs off screening means 180 in the wakes of the rotor bars across the full-width of rotor housing 172, thus preventing layering of fibers on the screen which tends to plug the screen openings and thus inhibits free movement of fibers through the screen; and ii), tend to lift nits and other Z
aggregated fiber masses off the screening means 180 so as to facilitate their peripheral mov~ment over and beyond the screening means and towards the full-width separator slot 179. Such peripheral movemen~ results from the movement of the annular aerated bed 186 and the sweeping action of the rotor bars 181.
It will be apparent that in the exemplary case employing an 8-bar rotor assembly 175 moving at 1432 RPM with a rotor bar tip velocity of approximately 150 f.p.s., approximately one hundred and ninety-one spaced full-width negative pressure zones 204 or negative impulses are generated per second, each of which sweep across the surface of the screening means 180 throughout the full-width thereof at a velocity equal to that of the rotor bars which generated such zones--viz., 150 f.p.s. Moreover, each full-width negative pressure zone 204 persists over an arcuate span on the order of several times the height of the rotor bars 181; although the actual distance through which the negative pressure zones 204 persist will vary dependent upon the specific operating parameters selected such, for example, as rotor speed, the relative speea differential between tha rotor bars 181 and the aerated bed 186 of fibers, an~ the actual pressure conditions established. However, assuming a ull-width negative pressure zone 204 persisting on the order of three times the height of the rotor bars 181 (which are here 3/4`' in height), then the arcuate extent x (FIG. 10) of the negative pressure zone 204 will be on the order of 2.25".
That is, each negative pressure zone 204 will span approximately 24% of the circumferential space of approximately 9.3"
between two adjacent rotor bars 181 (assuming an 8-bar rotor assembly 175 with an outside diameter of 23.64"--i.e., 24"
.
7~
I.D. for housing 172 minus 2 x .18" clearance). Therefore, the balance of the circumferential region from the trailing edge of each zone 204 to the next succeeding rotor bar 181--viz., a circumferential arc ~ (FIG. 10~ spanning approximately 76% of the circumferential distance between two adjacent rotor bars 181, or a distance of approximately 7"--constitutes a region of positive pressure drop of approximately 1.5" H2O.
Thus, it will be appreciated that in operation, approximately one hundred and ninety-one full-width negative pressure zones 204 alternating with an equal number of full-width zones of positive pressure drop which are each on the order of three times as extensive in duration ~i.e., ~ - 3x), will sweep across screening means 180 each second. As indicated above, in those rapidly sweeping regions beneath the negative pressure zones 204, flow of the air/fioer stream through screening means 180 at 45 f.p.s. [Eq. X] ceases momentarily and, the fibrous material in those regions tends to be lifted off the screening means. Considering any given fixed area of the screening means 180, immediately upon passage of each negative pressure zone 204, the positive pressure drop conditions of approximately 1.5" H2O are restored until the next rotor bar 181 passes thereover; thus permitting the individualized fibers and soft fiber flocs to again move toward the screening means 180 at a velocity of 18 f.p.s.
[Eq. IX~ normal to the screen and at a composite velocity vector of 82 f.p.s. [Eq. XIV] directed towards the screen at an acute angle and, ultimately, through the screen openings at approximately 45 f.p.s. [Eq. X]. Thus, the arrangement is such that plugging of the screen is effectively precluded, while individualized fibers tend to dive end-wise through the screen openings in the regions of positive pressure ~L~ 7~
drop, as best illustrated in FIG. 11--i.e., in those regions between the trailing end of each full-width negative pressure zone 204 and the next succeeding rotor bar 181.
Those individualized fibers, soft fiber flocs, and aggregated fiber masses within the aerated bed 186 of fibers which do not pass through the screening means 18U the first time they are presented thereabove are swept over and beyond the screening means 180 and, thereafter, past classifying air jet 194 (FIG. 3). I~nder the assumed conditions, the individualized fibers and soft fiber flocs tend to be diverted radially inward by the classifying air jet 194, while the undesired aggregated fiber masses are centrifugally and tangentially separated from the aerated bed 186 through full-width separator slot 179 at the rate of .22 lbs./min.
[Eq. XV]. Those individualized fibers and soft fiber flocs remaining in the aerated bed 186 after transit of separator slot 179 are then returned to the region overlying screening means 180, where they are successively acted upon by the rapid succession of pressure reversal conditions from full-width negative pressure zones 204 alternating with full-width zones of positive !?ressure drops until all such materials pass through t~e screening means 180 into forming zone 79.
The airtfiber stream tends to exit from housing 172 non-radially through screening means 180--indeed, as previously indLcated, under the assumed operating conditions the air/fiber stream tends to exit at an acute angle which, on average over the full extent of discharge opening 178, intersects a line tangent to the midpoint of screening means :180 at an included angle Q' of from 5 to 20 and, preferably, on the order of 11. 'lne exiting air/fiber stream decelerates almost immediately to approximately 36 f.p.s. [Eq. XI]
7~
within forming zone 79 and moves through the forming zone toward the foraminous forming wire 80 which is here moving at 750 ft./min. The fibers are air-laid or deposited on forming wire 80 at the rate of 4.43 lbs./min. [Eq. I]--i.e., the difference between the rate of fiber supplied [Eq. II]
and the 5% of fibrous ~aterials supplied which are separated and removed through separating slot 179--to form web 60.
The fibers deposited on the forming wire 80 are held firmly in position thereon as a result of suction box 126 (FIG. 3) and its associated suction fan and ducting which serve to accomodate and remove the high volume of air supplied.
Following formation of web 60 on foraminous forming wire 8Q in the manner above described, the web, carried by forming wire 80 at a speed of 750 ft./~in., exits from beneath forming zone 79. An auxiliary suction box 128 may, if desired, be provided to insure that the web remains flat on the forming wire as it exits from beneath forming zone 79 where the web has bèen subjected to the holding action provided by suction box 126 which accomodates the main air stream. The thus formed web 60 may then be further processed in the manner previously described in detail in Section D, pages 25-27 supra, of this specification. That is, and as best illustrated in FIG. l, the web 60 is preferably passed through calender rolls 129 where it is compacted lightly, and is then transferred to a conveyor belt 130.
The web is thereafter bonded in any suitable bonding station 85, dried at a drying station 87, and formed into a storage roll 95 at a suitable storage station 90.
Those skilled in the art will appreciate that there has herein been described a typical set of operating para~eters for forming an air-laid web 60 of dry fibers at a relatively , 3~
high production speed-- _z , 750 ft./min.--utilizing only a single forming head 75 (FIGS. 1 and 3). The exemplary web thus formed has a basis weight of 17 lbs./2880 ft.2 and is essentially devoid of nits and other aggregated fiber masses which have been removed through separator slot 179. Because the forming head 75 and forming zone 79 have been designed so as to essentially preclude induced cross-flow forces and/or eddy currents therein, the controlled mass quantum dispersion of fibers remains substantially unchanged throug'nout the system, thereby permitting the system to be scaled up or down to form air-laid webs of virtually any desired width and with a controlled coefficient of variation.
The web 60 deposited on forming wire 80 has more than adequate integrity to permit rapid movement of the forming wire. Indeed, if one desires to further increase productivity, n additional forming heads 75A-75N (FIG. 2--where n equals any whole integer) may be utilized and the speed of foraminous forming wire 80 may be increased by a factor equal to the number of separate forming heads used--e.g., under the assu~ed operating conditions, two heads would permit operation at 1,500 f.p.m.; three heads would permit operation at 2,250 f.p.m.; et cetera. Indeed, with the present invention, _ forming wire speed is no lonyer limited by the speed of web ~ormation but, rather, by the speed of such subsequent processing steps as bonding in the web bonding station 85 (FIG. 1).
Experimentation with air-laid, dry fiber, web forming systems embodying the features of the present invention has indicated that a wide range of results are attainable dependent 30` upon the particular operating parameters selected, as reference to the representative experimental data set forth 57~2 in Section K, page 65 et seq., infra, of this specification indicates. Particularly importan~ are such design and operating parameters as: i) rotor design; ii) rotor speed; iiiJ recycle or separation percentage; iv) screen design; and v), air-to-fiber ratio. of the foregoing, rotor design and screen design represent fixed parameters which, once selected, are not normally subject to operator control; whereas the remaining parameters may be varied over wide ranges to provide virtually an infinite range of possible permutations and combinations which can, and will, affect the characteristics o~ both the web produced and the rate of web productivity.
For example, as indicated in Section G at pages 39-40, supra, of this specification, the rotor assembly 175 may be formed with n rotor bars 181 where n equals any whole integer greater than ~ln. However, it has been ascertained that fiber throughput--a limiting constraint when attempting to maximize productivity--is a function of rotor speed multiplied by the square root of the number of rotor bars employed--i.e., fiber throughput: f (RPM x ~No. of rotor bars 181). This relationship wili, of course, vary with the particular screen employed; and, has been graphically illustrated in FIG. 12 wherein fiber throughput in lbs./in./hr. (the ordinate) has been plotted at various rotor speeds for each of a 2-bar, 4-bar, and 8-bar rotor assembly (the abscissa) when using both a coarse wire screen (lOx2.75; .047" wire dia.;
.059" screen opening; and 46.4~ open screen area) and a fine wire screen (16x4; .035" wire dia.; .032" screen opening;
and 38.8% open screen area). As here shown, the circular points 205 are each representative of f iber throughput at a given rotor speed multiplied by the square root of "2" and are, therefore, indicative of throughput for a 2-bar rotor.
7~ `
Similarly, the triangular points 206 are each indicative of fiber throughput for a 4-bar rotor, while the square points 208 are indicativ.e of fiber throughput for an 8-bar rotor.
Thus, the line 209 (FIG. 12) represents the Regressor, or "line-of-best-fit", from which functional relationships between throughput and rotor speed can be determined when using a coarse wire screen of the type described aboveO
`Similarly, the line 210 represents the same functional relationships when using a fine wire screen of the type described above. The data thus corroborates experimental findings that rotor RPM can be reduced while fiber throughput is maintained, or even increased, by going, for example, from a 4-bar rotor assembly 175' (FIG. 5) to an 8-bar rotor assembly 175 (FIG. 3). However, when using an 8-bar rotor assembly 175, the forming system seems to be less tolerant of mismatches between forming air and rotor speed; and, where such mismatches occur, fibers tend to accumulate on the sidewalls 199 of the forming zone 79. This is readily corrected by reducing rotor speed, normally by less than 10~, while maintaining forming air constant.
It has further been discovered that both nit levels in the air-laid web 60, and fiber throughput in lbs./hr./in.2, are a function of the percentage of fibrous materials removed from the aerated bed 186 ~FIG. 10) through the full-width separator slot 179 (FIG. 3). Thus, referring toFIG. 13, line 211 graphically portrays the decreasing relationship of nit level (the ordinate~ with increasing separation/recycle percentages (the abscissa); while, at the same time, increasing separation/recycle percentages are accompanied by increased fiber throughput in lbs./hr./in.2 The graph is here representative of a system in which tile s~ ' rotor assembly 175'--a 4-bar rotor assembly--was driven at 1700 RPM and fibers were introduced into the rotor housing 172 (FI~. 3) in an air stream supplying air at approximately 106 ft.3/min./in. When the percentage of fibrous material separated through separator slot 179 was 1~, fiber throughput was 0.62 lbs./hr./in.2, and the air-laid web 60 exhibited a nit level of "3"--a level deemed to be "poor", or border-line between acceptable and non-acceptable.
As described in more detail in Section J, pages 64 and 65 infra, of this specification, numerical nit levels range from "0" ("excellentn), to "1" ("good"), to "2" ("adequate"), to "3~ ~"poor"), to 4" through ~6" ("inadequate" to "non-acceptable"). Such numerical ratings are subjective ratings based upon visual inspection of the formed web 60 and subjective comparison thereof with pre-established standards.
As the pressure of the recycle air supplied through manifold 191 (FIG. 3) is decreased and/or as separator slot 179 is widened, thereby modulating the pressure conditions within discharge conduits 192 (FIG. 3) and 77 (FIG. 1) which are maintained at a pressure level below that within the forming head 75 by means of a suction fan (not shown), the amount of fibrous material removed from rotor housing 172 through separator slot 179 is increased. Other means such, for example, as venturi passageways (not shown) could also be used to insure a controlled outflow of materials through separator slot 179.
As the percentage of fibrous materials separated and/or recycled incxeases, nit level in the formed web 60 decreases.
At the operating conditions under which FIG. 13 was prepared, when the separation percentage was increased to approximately 2.5~, a web having a nit level of "2" (i.e., an "adequate"
~60-. .
nit level rating) was produced; at a separation percentage of 3~, the web's nit level decreased to approximately "1.6"
(i.e., approximately midway between "adequate" and "good");
at a separation percentage of approximately 3.8%, nit level dropped to "1" ("good"); and, at a separation percentage of 5%, nit level dropped to approximately "0" ("excellentn).
FIG. 19 also shows that the throughput of the forming system was increased from .6~ lbs./hr./in.2 to .96 lbs./hr./in.2 while at the same time improving web quality from ~poorn to "excellentn. The total amount of fiber delivered to the forming system was increased by an even greater percentage to compensate for the increased remov~l of fiber and aggregate for recycling. Productivity of the forming system was thus increased about 55% even though the screen was more heavily loaded with fiber; a very significant improvement. These CQmpariSOnS were made while running good quality pulp (Northern Softwood Kraft) having a low content of pulp lumps and being well fiberized in the hammermill. Poorer quality pulps or less effective fiberization would require higher recycle rates of up to 10~ to maintain an acceptable nit level in the web being formed. When making less critical webs or thick batts, nit level and recycle rate become less critical.
Those skilled in the art will, of course, appreciate that the experimental data set forth in FIG. 13 is only representative for one given set of operating parameters;
and, such data will vary with changes in, e.g., air-to-fiber ratio, type of fiber used, rotor speed, rotor design, air supply, and/or screen characteristics. However, experiments have indicated that recycle percentage is critical and, for cellulosic fibers, should exceed 1%, is preferably between about 1% and 5~, and should be less than on the order of 10%.
' 9S~Z
It has been found that a 2-dimensional air-laid web forming system embodying features of the present invention will, when operating at a proper balance of fiber supply, forming air supply, and rotor speed, not only deliver maximum fiber throughput with minimum recycle, but, moreover, will exert a "healing effect" on basis weight non-uniformities entering the forming head 75 (FIG. 3). That is, the screen 180, when properly loaded with a moving or transient aerated bed 186 of fibers (FIG. 10), acts as a membrane which tends to equalize or even out the passage of fibers through adjacent incremental widths of the screen. Such "healing effect" is only operative over distances of six inches (6!') or less.
However, the "healing effect" will tend to reduce the coefficient of variation within a forming head 75 supplied with an air/fiber stream delivered through a partitioned duct 170 of the type shown in FIG. 3--viz., the effect of non-uniformities present within each four inch wide segment of the air stream exiting the paxtitioned duct 170 will tend to be minimized. The "healing effectl' will not function to even out gross irregularities in fiber basis weight over a wide expanse of former widths. Stated differently, a forming head 75 embodying the features of the present invention can even out either low or high non~
uniformities of up to approximately three inches in width, but it cannot even out gross non-uniformities of eight, twelve, or more, inches in width. It has further been found that if insufficient fiber loading occurs--i.e., if the air-to-fiber ratio increases to substantially above 600 ft.3/lb.
when working, for example, with cellulosic wood fibers--then, i) the aerated bed 186 tends to be starved of fibers;
ii) the "healing effect" is reduced because of an inadequate transient membrane over the screen 180; and ~ input non-, .
uniformities tend to be replicated in the finished web 60, thus deleteriously affecting the coefficient of variation of the finished web.
J. Forminq Capacities and Web Characteristics With 2-Dimensional Systems Embodying Features of the Present Invention Referring to Table I, it will be observed that a single forming head 75 embodying the features of the present invention--e.g., the type shown in FIGS o 1 and 3--and having a ~emi-cylindrical screen 18" in circumferential length, is capable of producing webs having basis weights ranging from 14-40 lbs./2880 ft.2 at forMing wire speeds ranging from about 911 f.p.m. to about 319 f.p.m.
IN ACCORDANCE WITH THE INVENTION
- _ _ Forming Wire Speed-- t./min Basis Weight Product No. of Formin~ Heads lbs./2880 ft.2 Type ~ -- ~ 2 ~ 3 14 Bath Tissue 911 1821 2737 17 Facial Tissue 750 1500 2250 26 Towel 490 981 1471 34 Towel 375 750 1125 _ Towel 319 638 _956 1. The data set forth in this Table I is based up~n a fiber throughput capacity of 1.23 lbs./hr./in. for a single forming head of the type shown at 75 in FIG. 1 and 9, and which uses a relatively fine screen 180 18" in circumferential length and having a screen openinq of 0.050".
TABLE I
These realistically attainable for~ing wire speeds may be doubled, tripled, or even further multiplied by using two, three or more forming heads 75A-75N in the manner shown in FIG. 2. Consequently, the formation of air-laid webs of dry fibers is no longer limited to low forming wire speeds; and, ~ 34~ 57 ~
this is believed to be a direct result of the fiber throughput capacity of each forming head 75 which is capable of delivering in the order of ten times the mass quantum of fibers per square inch of former screen as can be delivered by known prlor art forming systems.
Not only does the present invention permit significant increased rates of productivity at considerable savings in terms of energy consumption and space requirements, but, moreover, the webs produced are not constrained in terms of width limitations, can be formed in an essentially nit-free condition, and are comprised of individualized fibers which have not been disintegrated, shortened, curled, rolled into pills, or otherwise seriously damaged by excessive mechanical action within either the rotor housing 172 (FIG. 3) or in excessive secondary hammermilling operations.
Standards have been established by the assignee of the present invention for subjectively classifying the nit levels in air-laid webs formed of dry fibers. Such subjective standards are based upon visual inspection of the webs and comparison thereof with existing webs having differing nit levels which have been subjectively rated as ~0", "1", "2", "3", "4", "5" and "6". Photographs representative of webs having nit levels of "0", "1", "2", "3", "4", "5" and "6"
are here reproduced as FIGS. 14-20, respectively. FIG. 14 portrays a web having a nit level of "0" which is indicative of a web rated "excellent" and which is essentially free of nits and can, therefore, be used for the highest quality tissue products. FIG. 15 portrays a web having a nit level of "1" which is indicative of a high quality web having only a minimal level of nits and which is classified as "good".
Again, such a web is suitable for use in premium grade, 7~
quality bath and/or facial tissues. FIG. 16 photographically depicts a web having a nit level of "2" which is indicative of a web having a higher percentage of nits; yet which is "acceptable" for usage in quality bath and/or facial tissues.
FIG. 17 comprises a photograph o~ a web having a nit level of "3" which is considered "poor", but which is suitable for occasional usage in quality tissues or for usage in medium grade tissue products. FIGS. 18-20 photographically portray webs having nit levels of "4~, "5" and "6", respectively, and are indicative of webs of inferior quality which are generally not suitable for usage in bath and/or facial tissues. As the ensuing descxiption proceeds, the reader may find it convenient to refer to FIGS~ 14-20 so that the nit levels given in connection with the descriptions of Examples I-X will have greater meaning and significance.
K. Examples--Comparative Representative and/or Optimum Operating Parameters for Air-Laid Dry Fiber Web Forming Systems and Woven Square-Mesh Sceens The ensuing portion of the present specification includes a discussion of the effects of varying various system parameters when utilizing slotted screens in accordance with tho present invention, as well as when utilizing woven square-mesh screens. The Examples given are of actual experimental runs made with the equipment and have been randomly selected solely for the purpose of illustrating the effect of varying one or more of the operating parameters. No effort has been made to optimize operating conditions for each different given Example; although, certain of the Examples do reflect sets of operating parameters which either approach optimized conditions, are at or about optimized conditions, or somewhat exceed optimized conditions. Data for the various parameters for each of the Examples given are set forth in tabular form in Tables II and III, inclusive. Examples I-III, represent operating parameters when utilizing woven square-mesh screens;
whereas Examples IV-X represent operating parameters for a web forming system utilizing slotted screens in accordance with the present invention.
Referring first to Examples I, II and III (Table II, page 67), it will be noted that in the forming systems used to generate the wabs of such Examples, the screens employed were woven square-mesh screens, respectively lOxlO, 12x12, and 8x8, and respectively having 42.3~, 51.8% and 38.9~ open screen area. In all three cases only a single-forming head was used, here having an 8-bar rotor assembly 175 (FIG. 3).
In the case of Examples I, II and III, recycle percentages were 10.2%, 7.5% and 7.9%, respectively. The coefficients of variation for Examples I, II and III were 3.1%, 1.8% and 2.2~, respectively, while the nit levels were "1" ("good") for Examples I and III, and "0" ("excellent~) for Example II. Thus, the webs produced were suitable for use in high quality lightweight tissue products.
It should be noted that in Table II under the category "Product Made", Examples I, II and III have been designated as "Exp."--i.e., "Experimental". This designation has been used simply because the system parameters were not set with any specific product or end use in mind; rather, the web being formed was considered to be an "experimental" web.
However, reference to the data for web basis weight reveals that the experimental webs of Examples I and II are suitable for facial tissue, while the experimental web of Example III
is suitable for toweling.
Forming wire speeds and fiber throughput--the principal -- ' ' . .
357~ ' . _ , Example No. I II III IV V
~ ::
Run No. 2899 2940 2942 1035 1025 _ . .
Fiber Type~ ) N~ N~ N5~ N~ NS~
_ _ Fiber Feed Rate--lbs /in /hr ~2) 9.~ 4.6 20.3 17.1 17.0 53. . . _ Top Air Supply--ft. /min./in. 112 115 115 107 107 _ . _ Air-to-Fiber Ratio -ft.3/lb. 689 1500 331 375 377 _ _ _ _ . ~
No. of Rotors 1 1 1 1 .
No. of Rotor Bars/Rotor 8 8 8 8 8 . _ . _ _ Rotor Speed--RP~I 1200 1550 1600 1400 1800 .' . _ . .
Screen Type 10x10 12x12 8x8 11x2.5 11x2.5 Screen Openin~--Inches .065 .060 .078 .050 .050 _ .
% Open Screen Area 42.3 51.8 38.9 43.6 43.6 _ _ Former Pressure--Inches H~O 1.85 1.5 3.0 1.1 1.6 %Fiber Recycled 10.2 7.5 7.9 5.8 5.3 _ . _ _ .
Amount Fiber Recycled--lbs./in./hr.1.0 0.35 1.6 1.0 0.9 _ _ _ _ ~ .
Fiber Throughput--lbs./hr./in.2 .49 .24 1.04 .89 .89 _ _ . ~.-Classifying Air--ft.~/min./in. 1.3 1.4 2.1 2.2 2.2 Forming Wire Speed--ft./min. 300 150 500 525 500 _ ~ Facial ~acial Product Made Exp. Exp. Exp. Tissue Tissue . _ Basis Weight--lbs./2880 ft.c 16.9 17.6 22.7 17.7 18.6 . ~ .
Coefficient of Variation--C.D.% 3.1 1.8 2.2 2.1_ 7.1 _ _ _ _ _ _ _ Tensile--Gms./3" C.D. Width 505 357 763 335 371 _ _ . _ ~ _ Nit Level 1.0 -0- 1.0 1.1 1.6 _ _ - =
1. NSWK is Northern Softwood Xraft.
2. Fiber feed rates as stated represent maximum former capacity for the operating parameters established.
TABLE II
indicators of productivity--are of particular interest when evaluating the forming process used to form the webs of Examples I, II and III. In the case of Example I, for example, fiber throughput of 0.49 lbs./hr./in. and forming S wire speed of 300 f.p.m. were achieved utilizing a single forming head 75. Both parameters are approximately 40% of the anticipated average maximum production capacity set forth in Table I, page 63, suPra. In the case of the web formed in Example II, fiber throughput of 0.24 lbs./hr./in.2 and forming wire speed of 150 f.p.m. represent approximately ~0% of the anticipated average maximum production capacity set forth in Table I. In the case of Example III, the web produced was substantially heavier than the webs of Examples I and II discussed above, having a basis weight of 22.7 lS lbs./2880 ft.2. Forming wire speed of 500 and thxoughput of 1.04 lbs./hr./in.2 are significantly improved over the comparable parameters for Examples I and II. While the throughput and forming wire speed data set forth in Example III is for a web having a basis weight of 22.7 lbs.~2880 ft.2, such data is equivalent to forming a web of 17 lbs./2880 ft.2 at approximately 668 ft./min.
Thus, it is apparent that the operating parameters used in formation of the web of Example III are such that the system is approaching the anticipated average maximum production capacity set forth in Table I, page 63, ~
That is, according to Table I it is anticipated that a web having a basis weight of 26 lbs./2880 ft. can be formed by a single head 75 at an average maximum forming wire speed of 490 f.p.m.; while a 17 lb./ 2880 ft.2 basis weight web can be formed at an average maximum speed of 750 f.p.m. Consequently, the average maximum forming capacity for forming a web _, 6 ~
having a basis weight of 22.7 lbs./2880 ft.2--i.e., a web identical to that of Example III--would be on the order of 562 f p.m. Therefore, since the web of Example III was formed at 500 f.p.m., it is evident that the actual rate of productivity was approximately 89% of the anticipated average maximum production capacity. In short, the operating parameters used in forming the web of Example III approach optimum settings for forming an air-laid web of dry Northern Softwood Kraft (NSWK) fibers when utilizing an 8x8 woven square-mesh screen and a single forming head 75 having an 8-bar rotor assembly such as that shown in FIG. 3. Production rate may, of course, be further increased by the simple expedient of utilizing two, three or more tandem forming heads 75A-75N in the manner suggested in FIG. 2; an arrangement which would, under the operating parameters set forth for Example III, permit the formation of a web having a basis weight of 22.7 lbs./2880 ft.2 suitable for toweling at forming wire speeds of 1124 f.p.m. (two heads), 1686 f.p.m.
(three heads), et cetera. Alternatively, and assuming all other operating parameters remain unchanged, a web having a basis weight of 17 lbs./2880 ft.2 suitable for use as a facial tissue could be formed at 668 f.p.m. (one head), 1336 f.p.m. (two heads), 2004 f.p.m. (three heads), et cetera.
25 ` When employing a slotted screen in accordance with the present invention such, for example! as that shown in FIG.
7, the results in terms of increased productivity are dramatic. This may be readily demonstrated by reference to Examples IV a~d V (Table II, page 67), and Examples VI
through X (Table III, page 70), and comparing the data there given with that set forth in connection with Examples I-III
'. .
3~
Example No. VII ~II IX X
.___ " . . _ Run No 2717 2861 2908 2909 2946 . __ _ Fiber Type( ) ~WK _ NSWK N~ N5WX
=.=~ , _ ~
Fiber Feed Rate--lbs./in./hr.(2) 26.3 28.9 18.4 18.3 26.0 ~
Top Air Supply--ft.~/min./in. 133 131 129 12~ 119 . ,~ ........ . .
Air-to-Fiber Ratio--~t.J/lb. 312 271 420 423 275 - _ , No. of Rotors 1 1 1 1 No. of Rotor Bars/Rotor _ 4 8 8 8 8 Rotor Speed--RPM 1700 1600 1000 1000 1550 _, .
Screen Type 1~2.75 9x2.5 1~2.5 11x2.5 11x2.5 10 Screen Opening--Inches 059 .063 050 050 050 % Open Screen Area 46.4 45.5 43.6 43.5 43 6 _ . _ Former Pressure--Inches ~ O 1.6 2.0 0.95 0.95 1.7 _ ~ . ___ ~Fiber Recycled 2.7 3.1 5.4 4.9 4.6 Amount Fiber Recycled--lbs./in /hr. 0.7 _ 0.9 1.0 0.9 1.2 Fiber Throughput--lbs./hr.!in. __ 1.a2 _1.55 97 97 1 37 Classifying Air--ft.3/min./in. 2.6 1.6 1.6 1.4 1.8 15 Forming Wire Speed--ft./min. 800 590 375 225 640 - _ _ H.D.
Product Made _ ~ _ Exp. Tbwel Tbwel Ex~.
Basis ~eight--lbs./2880 ft. 17.0 27.3 26.7 44.5 22.3 Coefficient of Variation--C.D.% 4.8 3.5 3.9 4.4 1.1 Tensile--Gms./3" C.D. Width 521 1045 265 559 705 Nit Level 2.0 0.3 1.0 -0- 2~.0 . . - . ~ ., _ _ _ 1. NSWK is Northern Softwood Kraft.
2. Fiber feed rates as stated represent maximum former capacity for the operating parameters established.
TABLE III
i~ 43~2 (Table II). Thus, in Examples IV-X the recycle percentages range from a high of 5.8% (Example IV) to a low of 2.7%
(Example VI). In Examples IV through VI, facial tissue grade webs were produced in accordance with the invention having basis weights ranging from 17.0 lbs.l2880 ft.2 (Example VI) to 18.6 lbs./2880 ft 2 (Example V); while in Examples VII through X, toweling grade webs were produced having basis weights ranging from 22.3 lbs./2880 ft.2 (Example X) to 44.5 lbs./2880 ft.2 (Example IX~. Fiber throughput for the webs of Examples IV through X ranged from .89 lbs./hr./in.2 (Examples IV and V) to 1.55 lbs./hr./in.2 (Example VII).
In terms of formed web characteristics~ the nit levels of "0" ("excellent") "0.3" ("excellent"), "1.0" and "1.1"
("good") for Examples IX, VII, VIII and IV, respectively, compare favorably to the nit levels for Examples I-III. Nit levels for Exa~ples V, VI and X were "1.6", "2.0" and "2.0n, respectively; and, as such, those webs were rated "adequate", although nit level was not qùite as good as in the case of Examples I-III. Coefficients of variation for Examples IV
through X were 2.1~, 7.1~, 4.8%~ 3.5~, 3.9~, 4.4%, and 1.1%, respectively, as compared with Examples I-III where the coefficients of variation were 3.1~, 1.8% and 2.2%. The coefficient of variation for Example V of 7.1~ is relatively ~5 poor and would not generally be acceptable for premium grade facial tissues.
Comparisons of the results attained at the parameter settings for Examples VI and VII (Table III, page 70) with the anticipated average maximum forming capacities set forth in Table I, page 63, supra, reveals that in both cases the rate of productivity attained substantially exceeded the anticipated average maximum capacity for the forming system of the present invention. Thus, while it would normally be anticipated that a single forming head 75 could produce a web having a basis weight of 17 lbs./2880 ft.2 at a forming wire speed of 750 f.p.m. (See, Table I, page 63) in the case of Example VI a 17 lb./2880 ft.2 basis weight web was produced at a forming wire speed of 800 f.p.m.--l.e, approximately 6.6% faster than the average maximum productivity rate anticipated. Nevertheless, the resulting air-laid web was entirely satisfactory for use as a premium grade quality facial tissue. Similarly, the web of Example VII, which has a basis weight of 27.3 lbs./2880 ft.2 suitable for toweling, was actually produced at 590 f.p.m. on a single forming head 75, whereas the anticipated average maximum forming speed for such a web would normally be on the order of 467 f.p.m.
(Cf., Table I, page 63)--i.e., the actual rate of productivity acheived exceeded the anticipated average maximum capacity by approximately 26.3~6. In the case of Examples VI and VII, the fact that productivity rates actually achieved somewhat exceed the average anticipated maximum ratès set forth in Table I is believed to be attributable in large part to the fact that relatively coarse screens were used in making the webs of such Examples--viz., relatively coarse screens having .059" (Example VI) and .063" (Example VII) openings, rather than fine screens having .050" openings and which formed the basis for the data set forth in Table I. Experimental data such as that set forth in Table III suggests that for heavyweight towel products, relatively coarse screens will tend to improve productivity rates without giving rise to any serious problems in terms of operation or web characteristics.
The characteristics of the Example VII web in terms of nit level, coefficient of variation and basis weight are again such that the web produced was of excellent quality suitable for use in premium grade toweling.
It is apparent that the particular parameters used in connection with Examples VI and VII exceed, or at the very least, closely approximate optimum settings, although so~e fine tuning might be required in an effort to further reduce the coefficient of variation and nit level for Example VI.
For example, a reduction in screen opening size--e.g., from the .059" opening used in Example VI to a screen opening on the order of .050n--might well result in optimizing the membrane characteristics of the transient aerated bed 186 of fibers (FIG. lO) so as to produce an increased "healing effect" of the type described in Section I at pages 62-63, supra, of this specification, thereby reducing the coefficient of variation. Similarly, rPduction of rotor speed might produce the same result. And, an increase in the recycle percentage of 2.7% is likely to further reduce the nit level as heretofore described in connection with FIG. 13 (Section I, pages 60-61, supra). For example, comparison of Examples IV
and V (Table II, page 67) reveals that the operating parameters established for both Examples were, with the exception of rotor speed, essentially the same. Rotor speed, however, was only 1400 RPM in the case of Example IV, whereas in Example V it was 1800 RPM. Thus, a decrease in rotor speed of 400 RPM was accompanied by and, presumably, at least in part resulted in, reduction of the coefficient of variation in the formed web from 7.1% (Example V) to 2.i% (Example IV), and a reduction in nit level from "l.6" (Example V) to "l.l"
(Example IV).
As in the case of the woven sguare-mesh screen comparisons (Examples I, II and III, Table II, page 67); where the best 7~
result in terms of productivity was achieved with the coarsest screen--viz., an 8x8 woven square-mesh screen having screen openings .078" in width (Example III)--in the slotted screen comparisons the best result in terms of productivity was also achieved when using a relatively coarse slotted screen--viz., a 9x2.5 screen having screen openings of .063" in width (Example V).
Examples III lTable II, page 67), and VII-X (Table III, page 70), are of interest principally for their showing of typical operating parameters suitable for forming relatively heavy basis weight webs which can be used for toweling products. Considering Example III, it will be noted that when utilizing utilizing an 8x8 woven square-mesh screen, a web having a basis weight of 22.7 lbs./2880 ft.2 was produced at a for~ing wire speed of 500 f.p.m. Considering Examples VII-X (Table III, page 70, supra), it will be noted that the webs there formed in accordance with the invention had basis weights ranging from 22.3 lbs./2880 ft 2 (Example X) to 44.5 lbs./2880 ft.2 (Example XI), coefficients of variation ranging from 1.1~ (Example X) to 4.4~ (Example IX), and nit levels of "0", "0.3n, "1.0" and "2.0" for Examples IX, VII, VIII and X, respectively; all of such basis weights, coefficients of variation and nit levels being entirely suitable for commercial grade, high quality toweling products. The webs of Examples VIII and IX were formed at productivity rates of approximately 78.5% of the average maximum productivity rates anticipated (Cf., Table I, page 63). The web of Example VII (as previously described) was formed at a speed approximately 26.3% in excess of the anticipated average maximum capacity; and, the web of Example X was formed at a speed approximately 12% in excess of the anticipated average maximum capacity.
It is believed that the numerical data set forth in this Section K in connection with Examples I through X
clearly evidences the significant improvement obtained in fiber throughput--i.e., productivity rate--when utilizing slotted screens in accordance with the present invention as contrasted with using conventional woven square-mesh screens of the type shown in FIG. 6. However, the dramatic improvement in throughput is made even more evident upon inspection of that data as reproduced in graphic form in FIG. 21. Thus, as here shown fiber throughput for each of Examples I through X in lbs./hr.~in.2 (the ordinate in FIG. 21) has been plotted versus the screen opening size in inches used with each Example (the abscissa in FIG. 21). The line 216 is thus representative of fiber throughput when using woven square-mesh screens in a 2-dimensional web forming system and has been generated from the throughput data given in Table II
for Examples I, II and III.
As heretofore indicated, remarkably improved throughput rates are attained when utilizing a slotted screen with a 2-dimensional former in accordance with the present invention.
Such results are reflected by the line 218 which has here been generated using the throughput data recorded for Examples IV and V (Table II) and VI-X (Table III).
It will be appreciated by those skilled in the art upon consideration of the data in this Section K and in the preceeding Section J of this specification, that the present invention is uniquely suited for forming high quality webs having virtually any desired basis weight in lbs./2880 ft.2 at relatively high forming wire speeds. Indeed, such extremely high productivity rates may be readily set forth as follows:
5~Z
A web having a basis weight of (x) (17 lbs./2880 ft.2) where "x" is equal to any desired whole or fractional value, can be produced at a forming wire 80 speed of 750 f.p.m.
divided by "x"; or, (x) (17 lbs./2880 ft.2) = forming wire speed (750 f.p.m.) [XVI]
Similarly, where N forming heads 75A-75N are used (See, e.g., FIG. 2), the foregoing relationship of web basis weight to forming wire 80 speed may be expressed as follows:
(x) (17 lbs./2880 ft.2) = forming wire speed (-)~750xf P ~ XVII]
Based on the experimental data reported herein, it is evident that the present invention provides a dramatic improvement in fiber throughput capacity for the forming head. Thus, the data reflects fiber throughputs ranging from 15 somewhat in excess of .5 lbs./hr.in.2 (Example IV) to in excess of 1.50 lbs./hr./in.2 (Example VII) when working with cellulosic wood fibers and a former 75 24" in diameter. Moreover, it should be noted that the foregoing range of from .5 lbs.~hr./in.2 to at least 1.50 lbs./hr./in.2 reflects efforts made to form high quality, lightweight tissue and/or towel grade products.
Where product quality in terms of, for exam~le, nit level can be accepted at lower quality levels, it can be expected that fiber throughput will exceed and, may substantially exceed, the level of 1.50 lbs./hr./in.2. Similarly, when actual production experience has been acquired, it can be expected that fiber throughputs will be regularly achieved which do exceed the level of 1.50 lbs./hr./in.2, and such improved results may also be achieved when the system is scaled up in size~ ~ to rotor assemblies on the order of 36 in diameter. Therefore, the phrase "to at least 1.50 lbs./hr./in.2~ as used herein and in the appended claims is not intended to place an upper limit on throughput capacity.
- Those skilled in the art will appreciate that there has herein been described a novel web forming system characterized by its simplicity and lack of complex, space-consuming, fiber handling equipment; yet, which is effective in forming air-laid webs of dry fibers at commercially acceptable production speeds irrespective of the basis weight of the web being formed. At the same time, the absence of cross-flow forces insures that the finished web possesses the desired controlled C.D. profile which may be either uniform or non-uniform.
Claims (9)
1. The method of forming a quality web of air-laid dry fibers on a high speed production basis comprising the steps of:
a) delivering dry fibrous materials to a forming head positioned over a forming surface;
b) conveying the dry fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs and aggregated fiber masses and in an environment maintained substantially free of fiber grinding and disintegrating forces;
c) continuously separating from 1% to 10% of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g/cc. so as to maximize the separation of aggregated fiber masses from the aerated bed;
d) discharging such separated fibrous materials including the aggregated fiber masses contained therein from the forming head;
e) discharging the individualized fibers and soft fiber flocs through a high capacity slotted screen;
f) conveying the individualized fibers and soft fiber flocs discharged through the slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 through an enclosed forming zone towards the moving foraminous forming surface in a rapidly moving air stream;
g) air-laying the individualized fibers and soft fiber flocs on the moving foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on the forming surface with such web having a nit level of from "0" to "3"; and, h) moving the foraminous forming surface at a controlled and selected speed so as to produce an air-laid web having a nit level of from "0" n to "3" and any specific desired basis weight in lbs./2880 ft.2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./2880 ft.2.
a) delivering dry fibrous materials to a forming head positioned over a forming surface;
b) conveying the dry fibrous materials through the forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs and aggregated fiber masses and in an environment maintained substantially free of fiber grinding and disintegrating forces;
c) continuously separating from 1% to 10% of the fibrous materials delivered to the forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g/cc. so as to maximize the separation of aggregated fiber masses from the aerated bed;
d) discharging such separated fibrous materials including the aggregated fiber masses contained therein from the forming head;
e) discharging the individualized fibers and soft fiber flocs through a high capacity slotted screen;
f) conveying the individualized fibers and soft fiber flocs discharged through the slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 through an enclosed forming zone towards the moving foraminous forming surface in a rapidly moving air stream;
g) air-laying the individualized fibers and soft fiber flocs on the moving foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on the forming surface with such web having a nit level of from "0" to "3"; and, h) moving the foraminous forming surface at a controlled and selected speed so as to produce an air-laid web having a nit level of from "0" n to "3" and any specific desired basis weight in lbs./2880 ft.2 ranging from at least as low as 13 lbs./2880 ft.2 to in excess of 40 lbs./2880 ft.2.
2. The method as set forth in claim 1 further characterized in that the individualized fibers and soft fiber flocs air-laid on the moving forming surface in step (g) are conveyed from the slotted screen in step (f) at a rate on the order of 1.23 lbs./hr./in.2, and the forming surface is moved at a controlled and selected speed in step (h) so as to produce an air-laid web having a specific basis weight in lbs./2880 ft. in accordance with the following set of operating parameters: (x) (17 lbs./2880 ft.2) at a forming surface speed on the order of (where x equals any whole or fractional number).
3. The method as set forth in claim 1 further characterized in that from 1% to 5% of the fibrous materials delivered in step (a) are separated from the aerated bed in step (c) and discharged from the forming head in step (d).
4. The method as set forth in claim 1 further characterized in that steps (a), (b), (c), (e), (f) and (g) are carried out in an environment essentially devoid of cross-flow forces so as to maintain cross-directional control of the mass quantum of fibers being processed and of the cross-directional profile of the air-laid web produced.
5. The method as set forth in claim 2 further characterized in that steps (a), (b), (c), (e), (f) and (g) are carried out in an environment essentially devoid of cross-flow forces so as to maintain cross-directional control of the mass quantum of fibers being processed and of the cross-directional profile of the air-laid web produced.
6. Apparatus for forming a quality web of air-laid dry fibers on a high speed production basis comprising, in combination: a movable foraminous forming surface; a forming head mounted over said forming surface; means for delivering dry fibrous materials to said forming head; means for conveying the dry fibrous materials through said forming head in a rapidly moving aerated bed of individualized fibers, soft fiber flocs, and aggregated fiber masses while maintaining said forming head substantially free of fiber grinding and disintegrating forces;
means for continuously separating from 1% to 10% of the fibrous materials delivered to said forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g./cc. so as to maximize the separation of aggregated fiber masses from the aerated bed and discharging such separated fibrous materials from said forming head; a discharge opening formed in said forming head; a slotted screen mounted in said discharge opening; means defining an enclosed forming zone mounted between said discharge opening and said forming surface; means for conveying the individualized fibers and soft fiber flocs from said forming head through said slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 and through said forming zone towards said movable foraminous forming surface in a rapidly moving air stream and for air-laying the indivi-dualized fibers and soft fiber flocs on said movable foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on said surface during movement thereof with such web having a nit level of from "0" to "3"; and, means for controllably moving said foraminous forming surface at a selectable speed so as to pr?luce an air-laid web having a nit level of from "0" to "3"
and any specific desired basis weight in lbs./2880 ft.2 ranging from at least as low as 13 lbs/2880 ft.2 to in excess of 40 lbs./2880 ft.2.
means for continuously separating from 1% to 10% of the fibrous materials delivered to said forming head from the aerated bed with the materials being separated including those having a bulk density in excess of .2g./cc. so as to maximize the separation of aggregated fiber masses from the aerated bed and discharging such separated fibrous materials from said forming head; a discharge opening formed in said forming head; a slotted screen mounted in said discharge opening; means defining an enclosed forming zone mounted between said discharge opening and said forming surface; means for conveying the individualized fibers and soft fiber flocs from said forming head through said slotted screen at a fiber throughput rate anywhere in the range of .5 lbs./hr./in.2 to at least 1.50 lbs./hr./in.2 and through said forming zone towards said movable foraminous forming surface in a rapidly moving air stream and for air-laying the indivi-dualized fibers and soft fiber flocs on said movable foraminous forming surface so as to form an air-laid web of randomly oriented dry individualized fibers and soft fiber flocs on said surface during movement thereof with such web having a nit level of from "0" to "3"; and, means for controllably moving said foraminous forming surface at a selectable speed so as to pr?luce an air-laid web having a nit level of from "0" to "3"
and any specific desired basis weight in lbs./2880 ft.2 ranging from at least as low as 13 lbs/2880 ft.2 to in excess of 40 lbs./2880 ft.2.
7. The combination as set forth in claim 6 wherein said conveying means is adapted to convey the individualized fibers and soft fiber flocs from said slotted screen at a rate on the order of 1.23 lbs./hr./in.2, and said means for controllably moving said forming surface at a selectable speed-is adapted to move said surface at any speed required to produce a web having any specific desired basis weight in lbs./2880 ft.2 in accordance with the following set of operating parameters: (x)(17 lbs./2880 ft.2) at a forming surface speed of (where x equals any whole or fractional number).
8. The combination as set forth in claim 6 wherein said forming head comprises an elongate housing having a semi-cylindrical wall portion, said housing having a full-width fiber inlet formed therein, first and second full-width discharge openings formed in said semi-cylindrical wall-portion, said slotted screen being mounted in said first discharge opening and having the same radius of curvature as said semi-cylindrical wall portion, said second discharge opening comprising a full-width tangential separator slot, and means for continuously introducing a high volume air stream into said housing.
9. The combination as set forth in claim 8 wherein said slotted screen is oriented with the long dimensions of said screen extending longitudinally across said elongate housing.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US10614379A | 1979-12-21 | 1979-12-21 | |
| US106,143 | 1979-12-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1149572A true CA1149572A (en) | 1983-07-12 |
Family
ID=22309722
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000367216A Expired CA1149572A (en) | 1979-12-21 | 1980-12-19 | High fiber throughput screening system for separating aggregated fiber masses from individualized fibers and soft fiber flocs and a system for forming an air- laid web of dry fibers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1149572A (en) |
-
1980
- 1980-12-19 CA CA000367216A patent/CA1149572A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |