CA1157302A - Twin-wire web forming section - Google Patents
Twin-wire web forming sectionInfo
- Publication number
- CA1157302A CA1157302A CA000383379A CA383379A CA1157302A CA 1157302 A CA1157302 A CA 1157302A CA 000383379 A CA000383379 A CA 000383379A CA 383379 A CA383379 A CA 383379A CA 1157302 A CA1157302 A CA 1157302A
- Authority
- CA
- Canada
- Prior art keywords
- web
- forming
- roll
- machine
- band
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F9/00—Complete machines for making continuous webs of paper
- D21F9/003—Complete machines for making continuous webs of paper of the twin-wire type
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- Paper (AREA)
Abstract
TWIN-WIRE WEB FORMING SECTION
ABSTRACT OF THE DISCLOSURE
An efficient and highly effective twin-wire web forming section utilizes and initial web forming section providing bidirectional dewatering over at least a portion of its length between a pair of dewatering bands passing over a forming roll, wherein the web in insufficiently dewatered to complete formation. In a subsequent dewater-ing zone over a second forming roll the direction of primary drainage is substantially reversed for at least a portion of the thickness of the web, and dewatering effected to the extent of completing formation of the web.
The reversal of water flow has the effect of reforming the interior of the web, with consequent modification to formation. This aspect of the process of supplemented by the provision of a zone intermediate the initial web formation zone and the completed formation zone wherein relative longitudinal displacement occurs between the two bands so as to assist reforming of the web interior, with enhanced floc modification.
ABSTRACT OF THE DISCLOSURE
An efficient and highly effective twin-wire web forming section utilizes and initial web forming section providing bidirectional dewatering over at least a portion of its length between a pair of dewatering bands passing over a forming roll, wherein the web in insufficiently dewatered to complete formation. In a subsequent dewater-ing zone over a second forming roll the direction of primary drainage is substantially reversed for at least a portion of the thickness of the web, and dewatering effected to the extent of completing formation of the web.
The reversal of water flow has the effect of reforming the interior of the web, with consequent modification to formation. This aspect of the process of supplemented by the provision of a zone intermediate the initial web formation zone and the completed formation zone wherein relative longitudinal displacement occurs between the two bands so as to assist reforming of the web interior, with enhanced floc modification.
Description
`` 1~S7302 TWIN--WIRE WEB FORMING SECTION
This invention is directed to a web forming :
system, and in particular to a system utilizing a twin-wire web former.
The use of twin-wire formers, particularly in the manufacture of paper webs such as newsprint has received an enormous amount of development in recent years, and considerable progress has been made towards improvement of web formation, with consequent enhancement of the product. However, there have remained inherent unsolved problems in the advance of formers.
One approach by the present inventOr to the problems of web formation involves the adaptation of a fourdrinier type machine to a twin wire forming section by the super- :
imposition of a forming roll and subsequent dewatering 15 roll in cooperating relation with the elementsof an existing -fourdrinier machine, as disclosed in Canadian patent application Serial No. 350,147 filed April 18, 1980.
The present invention is directed to an improved forming sectiQn wherein unexpected advantages were found in the control of web formation, with consistent achievement in improved web formation, a capability of operating with a wide spectrum of varying stocks, over a wide speed range, to produce webs over a wide range of basis weight.
In carrying out tests on the referred-to fourdrinier conversion work it was discovered that ,: , -' .
:
: ' ' ' dewatering of the stock in order to form the web should be interrupted, and stock dewatering reversed in direction for at least a portion of the thickness of the web, in order to reform the interior of the web.
A particular and apparently inherent problem of two-wire forming has been the absence of so-called Z-strength. Twin wire formers have, up to the present all tended to produce strong two ply formation with weakness in the central zone, giving a tendency to separate due to the low Z-axis strength.
Referring to the prior art, certain machines come quite close to providing the desired dewatering sequences. However, most instances, presumably because of the use of dewatering means other than forming rolls, such as foils, table rolls and curved shoes, web formation is not satisfactory, there being a severe retention problem, with loss of fines and fibre through the wires. This problem appears to stem from high magnitude forces of short duration. Such machines include the VertiformaTM and machines of the type shown in Canadian Patent No. 981,952 E.J. Justus which issued January 20, 1976. Substantially all of these prior art machines appear to produce webs afflicted with ply separation, that is to say, there is weakness in the centre section of the web, in the Z-direction.
The presently disclosed system thus provides a method of forming a web between a pair of permeable dewatering bands passing about a pair of forming rolls wherein initial dewatering includes bidirectionally forming the outer faces of the web, adjacent the constraining bands, to the extent that web formation is incomplete interiorly of the web, and subsequently continuing dewatering of the web about the second of the forming rolls wherein the direction of dewatering flow through the web is substantially reversed for at least a significant portion of the thickness of the web, and . ' ' ' :-:
:
- ` 1157302 formation is substantially completed, whereby the interior of the web is effectively reformed. The system further includes the provision of relative longitudinal displace-ment between the bands in order to enhance the reformation of the web interior, with consequent floc modification.
The invention thus provides in one embodiment a web forming machine having a pair of endless forming bands to receive stock in dewatering web forming relation therebetween, comprising a first forming roll located within the loop of one of the bands and having an open roll surface to support the one band in water receiving relation therewith; a second forming roll located within the loop of the other one of the bands, the bands passing in tensioned mutually opposed relation from the first roll to the second roll during operation of the mechine; an initial web forming zone for receiving a layer of stock in deposited relation from a suitable source on at least one of the bands, at least a portion of the initial forming zone having both of the bands sandwiching the stock passing about a portion of the periphery of the first forming roll to provide bidirectional dewatering of the stock to the extent to only partially form the web, the bands carrying the partially formed web to a further formation zone located about the periphery of the second 25 - roll and providing direction reversed dewatering to at least a portion of the thickness of the web to substantially complete the formation of the web.
In comparing characteristic web formation provided by a machine operating in accordance with the teachings of the present invention, fines retention is excellent, surpassing that of an equivalent fourdrinier. Furthermore, the machine is particularly stable in sheet formation with respect to stock consistency variations, and the consistency for the entry to the first forming roll is not critical, unlike the case of the "SymformerTM" which employs a forming shoe.
, .
.
.. . . .
The described embodiment permits the arrangement of the forming rolls in predetermined spaced apart relation to provide a stretch therebetween along which the bands travel in generally tangential relation to the rolls, having the partially formed web in sandwiched relation therebetween.
Owing to the differences in the respective instantaneous linear velocities of the bands relative longitudinal band displacement occurs which correspondingly affects the substantially unformed interior of the web, to assist in reforming the web, thus tending to modify floc formation within the web. It has been found that while at present the individual quantitative effects of reversed dewatering and differential longitudinal web displacement may not readily be determined, the two effects appear to be lS complementary and lead to significantly reformed flocking, with enhanced formation.
In one embodiment the provisions of the initial web forming zone include a first stretch of one of the bands, on which stock is deposited from a headbox, the band passing over suitable guide means such as foils or table rolls to facilitate the initial formation of the lower face of the web upon the band, prior to sandwiching by the second band for passage about a portion of the periphery of the first forming roll.
The provision of an initial dewatering stretch akin to that of a fourdrinier machine permits initial dewatering of the stock and commencement of formation, with beneficial accumulation of fines, prior to the commencement of bidirectional dewatering.
To clarify the term, formation is generally accepted as meaning the distribution of fibres and fibreflocks in a finished sheet viewed when illuminated by light passing through the sheet i.e. backlighted. As the web is dewatered, the fibres become fixed in their relative positions, thereby determining their location in the finished sheet and such fixed fibres are deemed as "formed"
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in the instant disclosure with those fibres or fiberflocks still suspended in sufficient water to permit mobility and change in their relationship or orientation being termed as "unformed". I
Thus when practicing the present invention there -will always be as above indicated an "unformed" portion in :
the interior of the web when the.web leaves the first forming roll.
To further clarify the action of the present invention, the second forming roll, as above indicated, causes flow in the reverse direction through a portion of the web i.e. the balance of the water flow towards the two wires at the second roll is different from that of the first forming roll so that at least some of the fibres in the interior of the web are subject to flow in the opposite direction which tends to influence their final disposition in the formed sheet.
Between the two rolls the two wires or web forming bands are substantially parallel and thus there is a section between the two rolls where little drainage occurs and the drainage is in effect interrupted. In this section as above indicated there is also relative machine direction movement between.the.wires to apply shear to the forming web.
A forming machine constructed in accordance with the presently disclosed arrangement using forming rolls including a non-suction open surfaced roll normally makes possible the provision o a substantially suctionless machine wherein the absence of.drag, previously occasioned by the widespread use of suction boxes, permits the driving of one band loop by the other band loop without any increase in power requirement, and even with a diminution in drive power requirements considered in relation to an equivalent fourdrinier machine. The improved machine also results in improved band life.
.Thus, in the case of typical fourdrinier machines ,...... ' ~ ~ ' ' ' , - .. . :' ~ .- .
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using flat boxes, foils and the like, which create significant drag, the parasitic drag can take up to 60%
of the drive power. The drag of such dewatering devices is both speed and moisture content sensitive.
The sealing of a band to a suction box surface is largely influenced by the wetness of the band and web.
~wing to the enhanced dryness of webs upon completion of dewatering over the second forming roll of the present invention, the drag over any suction boxes being used is correspondingly diminished. Additionally, where suction boxes might be used, they can be operated at correspondingly less vacuum.
In comparison with an equivalent fourdrinier machine it appears that for equivalent bone dryness at the couch, the subject forming section offers the potential saving of up to 50% power consumption. Compared with a commercial twin wire machine relying upon a forming shoe, a saving of about 40~ drive power is expected.
Alternatiuely, operational speed-up for no increase in power consumption can be achieYed.
Certain embodiments of the invention are described, reference being made to the accompanying drawings, wherein:
Figure 1 is a schematic side elevation of a first embodiment;
Figure 2 is a like view of a second embodiment;
Figure 3 is a like view of a third embodiment;
and Figures 4A and 4B show photomicrographs of paper made in accordance with the present invention and paper made by a corresponding fourdrinier machine.
Turning to Figures 1 and 2, the respective machines 10, 30 each comprise an upper band loop 14 and a lower band loop 12. The bands 12, 14 are usually so-called fourdrinier wieres, generally known as forming fibrics.
A slice portion 18 of a headbox (not shown) deposits a layer of stock on a first stretch 20 of the :~
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band 12. This stretch 20 is supported in tensioned relation upon a plurality of drainage elements such as foil sections or table rolls, to provide initial landing of the stock upon the band 12, and to promote satisfactory initial stock dewatering downwardly.
The bands 12 and 14 converge about the open faced roll 22 which comprises a grooved roll having a foraminous wire sleeve shrunk thereover. Such a roll 22 would have an open area in the order of 80% of its face area.
The angle of wrap of the bands 12, 14 must be limited in order to preclude total formation on roll 22.
The illustrated approximately 45 wrap angle has been found to be effective. Operation in the range 60 to 25 in order to maintain incomplete formation, with compensation by stock dilution adjustment is anticipated, after which the sand-wiched stock, passes tangentially from the roll 22 to the roll 24. After passage about the roll 24 the dewatering of the web is sufficiently complete that the web is fully formed.
In the stretch between rolls 22 and 24, owing to the difference in linear speeds of the two bands necessitated by their respective reversed inner and outer positions on the two rolls 22, 24, there is relative longitudinal movement of one band relative to the other band, thereby producing a corresponding relative displacement between the two facing formed portions of the web carried by the respective bands.
From the final formation achieved by the machine the longitudinal relative movement appears to play some part in reforming the interior of the web by modifying the floc formation prior to its being dewatered about the roll 24 to the extent that formation is then completed.
The roll 24 in the embodiments illustrated in Figures 1 and 2 is a plain roll, whereby dewatering rill occur substantially solely in an outward direction relative to the roll surface.
Upon passage from the roll 24 in the Figure 1 embodiment the bands 12, 14 pass by way of guide roll 26 to a table roll 28, whereat the band 14 diverges gradually :
by a small angle in the range 2 to 6 from the web and the band 12. The table roll 28 assists in maintaining the web upon the band 12 for passage about a suction couch roll 30, in known fashion. In the Figure 2 embodiment only the table roll 28 is necessary, in view of the bands leaving the underside of the roll 24.
While the second forming roll 24 has been found to operate effectively as a solid roll, it is contemplated that in certain instances for heavier grades, or uslng stocks more difficult to dewater, that a suction forming roll may be used; or an open roll such as roll 22.
Referring to the Figure 2 embodiment, owing to the fact of downward dewatering taking place at the second forming roll, whereby handling of the water is more readily arranged, this embodiment is suited for a pulp forming section or for such as heavyweight low speed kraft grades wherein greater quantities of water are removed.
In the Figure 3 embodiment the pre-drainage band stretch is dispensed with. The machine is sufficiently versatile that it is not very sensitive to stock variation, and the characteristic improved formation can usually be achieved in the shortened version, which can be based upon either the Figure 1 or the Figure 2 embodiment.
The web formation produced by the improved former has demonstrated superior qualities of symmetry, homo-geneous structure and Z-direction bond strength, which particularly suits the requirements for many specialty papers.
The present invention makes possible the formation of webs using substantially suctionless equip-ment, marked by the absence of vacuum forming rolls and the substantial absence of suction forming boxes and the like, or the number of such boxes may be reduced and operated at less vacuum, with reduced drag.
The second wire of the combination does not require to be independently driven, and can be effectively . ..
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, g driven by the first wire, while the wire drive power consumption is effectively diminished, relative to an equivalent fourdrinier machine due to the reduced wire drag, made possible by the reduced use of suction boxes or by operation under loss vacuum. The presently disclosed arrangement also leads to extended life of the woven wires or fabrics, due to the substantially diminished sliding wear across the covers of suction boxes, which formerly applied, in the case of most fourdriniers. This benefit applies equally over those twin wire machines relying upon forming shoes to dewater the web.
The portion of the forming path, wherein the other face of the web is formed by predominant drainage towards the open-face roll, provides an excellent surface formation and good two-sidedness.
The short duration of the passage about the open-faced first forming roll results in a partially formed web, so that it is believed in the succeeding portion of the forming path, where significant relative longitudinal displacement is applied across the thickness of the web breaks up floc formations are broken up in the unformed interior of the web, with a marked reduction in floc formations which contributes to improved formation.
Referring to Figures 4A and 4B, Figure 4A is a photomicro-graph of a cross-section of newsprint made by a twin wire former built and operated in accordance with the present invention, and Figure 4B
is a like photograph for the same grade of paper made from the same stock on a corresponding fourdrinier machine.
It will be noted that Figure 4A shows significantly more long fires crossing the central plane of the sheet, than for the fourdrinier sheet, Figure 4B, indicating the superior interply bonding indicative of enhanced Z-strength, achieved by the present invention.
In order to further demonstrate the capability of a machine constructed in accordance with the present invention, operating in the manner herein taught, the tabulated results below are for a model test section capable also of operation in a fourdrinier mode to provide comparison, over several tests, with sheet made by an equivalent fourdrinier machine.
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In the foregoing test results, under "Test No."
the numeral indicates the sequence number of the test, the initial "F" indicates the "Fourdrinier" mode machine, and the initials T/W indicates the Twin Wire machine constructed according to Figure 1 of the present invention. Also:
"B.W." is basis weight of the sheet.
"M/C Speed FPM" is the respective machine operat-ing speed in feet per minute.
"H'D Box Cons. %" is the percentage of solids in the furnish of the headbox.
"Specific Volume" is volume, in cubic centimetres per gram weight, of the furnish.
"Air Res. Gurley SjmL" is a porosity test of the bone dry paper involving the percolation through the thickness of the web in a time of 'S' seconds of a milli litre of air at standard conditions of~pressure and temperature.
"Break'g Length MD-m" is the length of sheet in metres "m", taken in the machine direction "MD", at which the sheet breaks under its own weight (i.e. a measure of longitudinal tensile strength).
"Break'g Length CD-m" is the transverse tensile strength similarly measured, in metres "m" in the cross machine direction "CD" at which the sheet breaks under its own weight.
"Tens. Ratio MD/CD" is the ratio of tensile strength of the web in the machine direction (MD) compared with the cross machine direction (CD).
"Tear Index MD mN.m2/g" is the tear strength of the web in the machine direction (MD), measured in milli-Newtons of force per unit of basis weight, basis weight being expressed in the numerator, by inversion, as square metre per grams.
"Tear Index CD mN.m2/g" is the corresponding tear strength of the web in the cross machine direction (CD).
"Ratio MD/CD" is the ratio of tear strengths, in , .
` ` 1157302 the machine direction (MD) compared with the cross machine direction (CD).
"Caliper In." is the thickness of the web in inches.
"Scott Int. Bond kJ/m2" is a pick test indicative of web internal bond strength (the Z-direction strength) measured in kilo Jou~les~ per square metre.
Based on the comparative test values in the foregoing table wherein tests are paired on the basis of the same sheet caliper,~ ~t the same machine speed the following comments are made:
In the first and second set of values, tests lF and 3 T/W, both at 2000 ft/min, there is a slight difference in basis weight, although the T/W furnish is less dense.
Web porosity is substantially diminished i.e.
the T/W web is 40% less porous ; the tensile strength of the twin wire sheet is about 34% greater in the machine direction; and about 23% greater in the cross machine direction; the web internal bond strength is about 20%
greater for the twin wire machine web.
It will be seen that most of the tests show consistent similar improvements of the twin wire sheet over that of the fourdrinier, except in the case of fine paper test 7F:15 T/W, where the twin wire sheet was more porous (however, the furnish used was much lighter (46 v-s 60) and even then the bond strength is 8% greater for the twin wire sheet.
It should be remarked that fourdrinier sheet is usually consistently and markedly superior in bond strength to sheets from other known twin wire formers, so that the comparative benefits of the present invention are quite evident.
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This invention is directed to a web forming :
system, and in particular to a system utilizing a twin-wire web former.
The use of twin-wire formers, particularly in the manufacture of paper webs such as newsprint has received an enormous amount of development in recent years, and considerable progress has been made towards improvement of web formation, with consequent enhancement of the product. However, there have remained inherent unsolved problems in the advance of formers.
One approach by the present inventOr to the problems of web formation involves the adaptation of a fourdrinier type machine to a twin wire forming section by the super- :
imposition of a forming roll and subsequent dewatering 15 roll in cooperating relation with the elementsof an existing -fourdrinier machine, as disclosed in Canadian patent application Serial No. 350,147 filed April 18, 1980.
The present invention is directed to an improved forming sectiQn wherein unexpected advantages were found in the control of web formation, with consistent achievement in improved web formation, a capability of operating with a wide spectrum of varying stocks, over a wide speed range, to produce webs over a wide range of basis weight.
In carrying out tests on the referred-to fourdrinier conversion work it was discovered that ,: , -' .
:
: ' ' ' dewatering of the stock in order to form the web should be interrupted, and stock dewatering reversed in direction for at least a portion of the thickness of the web, in order to reform the interior of the web.
A particular and apparently inherent problem of two-wire forming has been the absence of so-called Z-strength. Twin wire formers have, up to the present all tended to produce strong two ply formation with weakness in the central zone, giving a tendency to separate due to the low Z-axis strength.
Referring to the prior art, certain machines come quite close to providing the desired dewatering sequences. However, most instances, presumably because of the use of dewatering means other than forming rolls, such as foils, table rolls and curved shoes, web formation is not satisfactory, there being a severe retention problem, with loss of fines and fibre through the wires. This problem appears to stem from high magnitude forces of short duration. Such machines include the VertiformaTM and machines of the type shown in Canadian Patent No. 981,952 E.J. Justus which issued January 20, 1976. Substantially all of these prior art machines appear to produce webs afflicted with ply separation, that is to say, there is weakness in the centre section of the web, in the Z-direction.
The presently disclosed system thus provides a method of forming a web between a pair of permeable dewatering bands passing about a pair of forming rolls wherein initial dewatering includes bidirectionally forming the outer faces of the web, adjacent the constraining bands, to the extent that web formation is incomplete interiorly of the web, and subsequently continuing dewatering of the web about the second of the forming rolls wherein the direction of dewatering flow through the web is substantially reversed for at least a significant portion of the thickness of the web, and . ' ' ' :-:
:
- ` 1157302 formation is substantially completed, whereby the interior of the web is effectively reformed. The system further includes the provision of relative longitudinal displace-ment between the bands in order to enhance the reformation of the web interior, with consequent floc modification.
The invention thus provides in one embodiment a web forming machine having a pair of endless forming bands to receive stock in dewatering web forming relation therebetween, comprising a first forming roll located within the loop of one of the bands and having an open roll surface to support the one band in water receiving relation therewith; a second forming roll located within the loop of the other one of the bands, the bands passing in tensioned mutually opposed relation from the first roll to the second roll during operation of the mechine; an initial web forming zone for receiving a layer of stock in deposited relation from a suitable source on at least one of the bands, at least a portion of the initial forming zone having both of the bands sandwiching the stock passing about a portion of the periphery of the first forming roll to provide bidirectional dewatering of the stock to the extent to only partially form the web, the bands carrying the partially formed web to a further formation zone located about the periphery of the second 25 - roll and providing direction reversed dewatering to at least a portion of the thickness of the web to substantially complete the formation of the web.
In comparing characteristic web formation provided by a machine operating in accordance with the teachings of the present invention, fines retention is excellent, surpassing that of an equivalent fourdrinier. Furthermore, the machine is particularly stable in sheet formation with respect to stock consistency variations, and the consistency for the entry to the first forming roll is not critical, unlike the case of the "SymformerTM" which employs a forming shoe.
, .
.
.. . . .
The described embodiment permits the arrangement of the forming rolls in predetermined spaced apart relation to provide a stretch therebetween along which the bands travel in generally tangential relation to the rolls, having the partially formed web in sandwiched relation therebetween.
Owing to the differences in the respective instantaneous linear velocities of the bands relative longitudinal band displacement occurs which correspondingly affects the substantially unformed interior of the web, to assist in reforming the web, thus tending to modify floc formation within the web. It has been found that while at present the individual quantitative effects of reversed dewatering and differential longitudinal web displacement may not readily be determined, the two effects appear to be lS complementary and lead to significantly reformed flocking, with enhanced formation.
In one embodiment the provisions of the initial web forming zone include a first stretch of one of the bands, on which stock is deposited from a headbox, the band passing over suitable guide means such as foils or table rolls to facilitate the initial formation of the lower face of the web upon the band, prior to sandwiching by the second band for passage about a portion of the periphery of the first forming roll.
The provision of an initial dewatering stretch akin to that of a fourdrinier machine permits initial dewatering of the stock and commencement of formation, with beneficial accumulation of fines, prior to the commencement of bidirectional dewatering.
To clarify the term, formation is generally accepted as meaning the distribution of fibres and fibreflocks in a finished sheet viewed when illuminated by light passing through the sheet i.e. backlighted. As the web is dewatered, the fibres become fixed in their relative positions, thereby determining their location in the finished sheet and such fixed fibres are deemed as "formed"
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in the instant disclosure with those fibres or fiberflocks still suspended in sufficient water to permit mobility and change in their relationship or orientation being termed as "unformed". I
Thus when practicing the present invention there -will always be as above indicated an "unformed" portion in :
the interior of the web when the.web leaves the first forming roll.
To further clarify the action of the present invention, the second forming roll, as above indicated, causes flow in the reverse direction through a portion of the web i.e. the balance of the water flow towards the two wires at the second roll is different from that of the first forming roll so that at least some of the fibres in the interior of the web are subject to flow in the opposite direction which tends to influence their final disposition in the formed sheet.
Between the two rolls the two wires or web forming bands are substantially parallel and thus there is a section between the two rolls where little drainage occurs and the drainage is in effect interrupted. In this section as above indicated there is also relative machine direction movement between.the.wires to apply shear to the forming web.
A forming machine constructed in accordance with the presently disclosed arrangement using forming rolls including a non-suction open surfaced roll normally makes possible the provision o a substantially suctionless machine wherein the absence of.drag, previously occasioned by the widespread use of suction boxes, permits the driving of one band loop by the other band loop without any increase in power requirement, and even with a diminution in drive power requirements considered in relation to an equivalent fourdrinier machine. The improved machine also results in improved band life.
.Thus, in the case of typical fourdrinier machines ,...... ' ~ ~ ' ' ' , - .. . :' ~ .- .
.
.~. ..
. ~
using flat boxes, foils and the like, which create significant drag, the parasitic drag can take up to 60%
of the drive power. The drag of such dewatering devices is both speed and moisture content sensitive.
The sealing of a band to a suction box surface is largely influenced by the wetness of the band and web.
~wing to the enhanced dryness of webs upon completion of dewatering over the second forming roll of the present invention, the drag over any suction boxes being used is correspondingly diminished. Additionally, where suction boxes might be used, they can be operated at correspondingly less vacuum.
In comparison with an equivalent fourdrinier machine it appears that for equivalent bone dryness at the couch, the subject forming section offers the potential saving of up to 50% power consumption. Compared with a commercial twin wire machine relying upon a forming shoe, a saving of about 40~ drive power is expected.
Alternatiuely, operational speed-up for no increase in power consumption can be achieYed.
Certain embodiments of the invention are described, reference being made to the accompanying drawings, wherein:
Figure 1 is a schematic side elevation of a first embodiment;
Figure 2 is a like view of a second embodiment;
Figure 3 is a like view of a third embodiment;
and Figures 4A and 4B show photomicrographs of paper made in accordance with the present invention and paper made by a corresponding fourdrinier machine.
Turning to Figures 1 and 2, the respective machines 10, 30 each comprise an upper band loop 14 and a lower band loop 12. The bands 12, 14 are usually so-called fourdrinier wieres, generally known as forming fibrics.
A slice portion 18 of a headbox (not shown) deposits a layer of stock on a first stretch 20 of the :~
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. . :
- ::
band 12. This stretch 20 is supported in tensioned relation upon a plurality of drainage elements such as foil sections or table rolls, to provide initial landing of the stock upon the band 12, and to promote satisfactory initial stock dewatering downwardly.
The bands 12 and 14 converge about the open faced roll 22 which comprises a grooved roll having a foraminous wire sleeve shrunk thereover. Such a roll 22 would have an open area in the order of 80% of its face area.
The angle of wrap of the bands 12, 14 must be limited in order to preclude total formation on roll 22.
The illustrated approximately 45 wrap angle has been found to be effective. Operation in the range 60 to 25 in order to maintain incomplete formation, with compensation by stock dilution adjustment is anticipated, after which the sand-wiched stock, passes tangentially from the roll 22 to the roll 24. After passage about the roll 24 the dewatering of the web is sufficiently complete that the web is fully formed.
In the stretch between rolls 22 and 24, owing to the difference in linear speeds of the two bands necessitated by their respective reversed inner and outer positions on the two rolls 22, 24, there is relative longitudinal movement of one band relative to the other band, thereby producing a corresponding relative displacement between the two facing formed portions of the web carried by the respective bands.
From the final formation achieved by the machine the longitudinal relative movement appears to play some part in reforming the interior of the web by modifying the floc formation prior to its being dewatered about the roll 24 to the extent that formation is then completed.
The roll 24 in the embodiments illustrated in Figures 1 and 2 is a plain roll, whereby dewatering rill occur substantially solely in an outward direction relative to the roll surface.
Upon passage from the roll 24 in the Figure 1 embodiment the bands 12, 14 pass by way of guide roll 26 to a table roll 28, whereat the band 14 diverges gradually :
by a small angle in the range 2 to 6 from the web and the band 12. The table roll 28 assists in maintaining the web upon the band 12 for passage about a suction couch roll 30, in known fashion. In the Figure 2 embodiment only the table roll 28 is necessary, in view of the bands leaving the underside of the roll 24.
While the second forming roll 24 has been found to operate effectively as a solid roll, it is contemplated that in certain instances for heavier grades, or uslng stocks more difficult to dewater, that a suction forming roll may be used; or an open roll such as roll 22.
Referring to the Figure 2 embodiment, owing to the fact of downward dewatering taking place at the second forming roll, whereby handling of the water is more readily arranged, this embodiment is suited for a pulp forming section or for such as heavyweight low speed kraft grades wherein greater quantities of water are removed.
In the Figure 3 embodiment the pre-drainage band stretch is dispensed with. The machine is sufficiently versatile that it is not very sensitive to stock variation, and the characteristic improved formation can usually be achieved in the shortened version, which can be based upon either the Figure 1 or the Figure 2 embodiment.
The web formation produced by the improved former has demonstrated superior qualities of symmetry, homo-geneous structure and Z-direction bond strength, which particularly suits the requirements for many specialty papers.
The present invention makes possible the formation of webs using substantially suctionless equip-ment, marked by the absence of vacuum forming rolls and the substantial absence of suction forming boxes and the like, or the number of such boxes may be reduced and operated at less vacuum, with reduced drag.
The second wire of the combination does not require to be independently driven, and can be effectively . ..
, .
, ~
, g driven by the first wire, while the wire drive power consumption is effectively diminished, relative to an equivalent fourdrinier machine due to the reduced wire drag, made possible by the reduced use of suction boxes or by operation under loss vacuum. The presently disclosed arrangement also leads to extended life of the woven wires or fabrics, due to the substantially diminished sliding wear across the covers of suction boxes, which formerly applied, in the case of most fourdriniers. This benefit applies equally over those twin wire machines relying upon forming shoes to dewater the web.
The portion of the forming path, wherein the other face of the web is formed by predominant drainage towards the open-face roll, provides an excellent surface formation and good two-sidedness.
The short duration of the passage about the open-faced first forming roll results in a partially formed web, so that it is believed in the succeeding portion of the forming path, where significant relative longitudinal displacement is applied across the thickness of the web breaks up floc formations are broken up in the unformed interior of the web, with a marked reduction in floc formations which contributes to improved formation.
Referring to Figures 4A and 4B, Figure 4A is a photomicro-graph of a cross-section of newsprint made by a twin wire former built and operated in accordance with the present invention, and Figure 4B
is a like photograph for the same grade of paper made from the same stock on a corresponding fourdrinier machine.
It will be noted that Figure 4A shows significantly more long fires crossing the central plane of the sheet, than for the fourdrinier sheet, Figure 4B, indicating the superior interply bonding indicative of enhanced Z-strength, achieved by the present invention.
In order to further demonstrate the capability of a machine constructed in accordance with the present invention, operating in the manner herein taught, the tabulated results below are for a model test section capable also of operation in a fourdrinier mode to provide comparison, over several tests, with sheet made by an equivalent fourdrinier machine.
, ' ' ',, ' .
:
`` 1157302 Q N u~ N OD 1`It~ r ~ O ~ _I ~
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In the foregoing test results, under "Test No."
the numeral indicates the sequence number of the test, the initial "F" indicates the "Fourdrinier" mode machine, and the initials T/W indicates the Twin Wire machine constructed according to Figure 1 of the present invention. Also:
"B.W." is basis weight of the sheet.
"M/C Speed FPM" is the respective machine operat-ing speed in feet per minute.
"H'D Box Cons. %" is the percentage of solids in the furnish of the headbox.
"Specific Volume" is volume, in cubic centimetres per gram weight, of the furnish.
"Air Res. Gurley SjmL" is a porosity test of the bone dry paper involving the percolation through the thickness of the web in a time of 'S' seconds of a milli litre of air at standard conditions of~pressure and temperature.
"Break'g Length MD-m" is the length of sheet in metres "m", taken in the machine direction "MD", at which the sheet breaks under its own weight (i.e. a measure of longitudinal tensile strength).
"Break'g Length CD-m" is the transverse tensile strength similarly measured, in metres "m" in the cross machine direction "CD" at which the sheet breaks under its own weight.
"Tens. Ratio MD/CD" is the ratio of tensile strength of the web in the machine direction (MD) compared with the cross machine direction (CD).
"Tear Index MD mN.m2/g" is the tear strength of the web in the machine direction (MD), measured in milli-Newtons of force per unit of basis weight, basis weight being expressed in the numerator, by inversion, as square metre per grams.
"Tear Index CD mN.m2/g" is the corresponding tear strength of the web in the cross machine direction (CD).
"Ratio MD/CD" is the ratio of tear strengths, in , .
` ` 1157302 the machine direction (MD) compared with the cross machine direction (CD).
"Caliper In." is the thickness of the web in inches.
"Scott Int. Bond kJ/m2" is a pick test indicative of web internal bond strength (the Z-direction strength) measured in kilo Jou~les~ per square metre.
Based on the comparative test values in the foregoing table wherein tests are paired on the basis of the same sheet caliper,~ ~t the same machine speed the following comments are made:
In the first and second set of values, tests lF and 3 T/W, both at 2000 ft/min, there is a slight difference in basis weight, although the T/W furnish is less dense.
Web porosity is substantially diminished i.e.
the T/W web is 40% less porous ; the tensile strength of the twin wire sheet is about 34% greater in the machine direction; and about 23% greater in the cross machine direction; the web internal bond strength is about 20%
greater for the twin wire machine web.
It will be seen that most of the tests show consistent similar improvements of the twin wire sheet over that of the fourdrinier, except in the case of fine paper test 7F:15 T/W, where the twin wire sheet was more porous (however, the furnish used was much lighter (46 v-s 60) and even then the bond strength is 8% greater for the twin wire sheet.
It should be remarked that fourdrinier sheet is usually consistently and markedly superior in bond strength to sheets from other known twin wire formers, so that the comparative benefits of the present invention are quite evident.
~, :
.. : . ~ ,. . . .
.
: . , :
- : : .
Claims (10)
1. A web forming machine having first and second endless forming bands to receive stock in dewatering web forming relation therebetween comprising:
a source for depositing stock on one of said bands, an open stretch of said one band leading to a first forming roll, said open stretch receiving a layer of stock thereon from said source for downward drainage there-through including band support means located in supporting relation beneath said one of said bands to facilitate drainage of said stock layer and impart turbulence to said stock layer upon passage therepast, said first forming roll located within the loop of said first band, said first and second bands passing over an arcuate portion of said first forming roll in wrap relationship therewith to form in combination with said open stretch a first forming zone, said first forming zone being adapted to separate a significant amount of water from said web but in a manner and amount such that formation of said web is only partially completed when said web leaves said first forming zone, said first forming roll having an impermeable open surface to support said bands in water receiving relation therewith to permit bidirectional dewatering at said first forming roll;
a second forming roll located within the loop of said second band, said first and second bands passing in tensioned, mutually opposing relation from said first roll to said second roll and at least partially around said second roll during operation of the machine to provide a further formation zone at said second forming roll;
said further formation zone located about said second roll effecting reversal of internal water flow within at least a portion of the thickness of the web being formed to complete said formation of said web.
a source for depositing stock on one of said bands, an open stretch of said one band leading to a first forming roll, said open stretch receiving a layer of stock thereon from said source for downward drainage there-through including band support means located in supporting relation beneath said one of said bands to facilitate drainage of said stock layer and impart turbulence to said stock layer upon passage therepast, said first forming roll located within the loop of said first band, said first and second bands passing over an arcuate portion of said first forming roll in wrap relationship therewith to form in combination with said open stretch a first forming zone, said first forming zone being adapted to separate a significant amount of water from said web but in a manner and amount such that formation of said web is only partially completed when said web leaves said first forming zone, said first forming roll having an impermeable open surface to support said bands in water receiving relation therewith to permit bidirectional dewatering at said first forming roll;
a second forming roll located within the loop of said second band, said first and second bands passing in tensioned, mutually opposing relation from said first roll to said second roll and at least partially around said second roll during operation of the machine to provide a further formation zone at said second forming roll;
said further formation zone located about said second roll effecting reversal of internal water flow within at least a portion of the thickness of the web being formed to complete said formation of said web.
2. The forming machine as claimed in claim 1, including a band relative displacement zone located inter-mediate said first forming zone and said further formation zone, wherein one said band is displaced longitudinally relative to the other said band, whereby flocs present within the web are significantly modified.
3. The machine as claimed in claim 1, wherein said open stretch is comprised of said second band.
4. The machine as defined in claim 1 wherein said arcuate portion of said first roll comprises an arc subtending an angle in the range of 25 to 60°.
5. The machine as claimed in claim 4, wherein said subtended arc comprising an angle of about 45°
6. The machine as claimed in claim 1, wherein said first band lies above said second band.
7. The machine as claimed in claim 1, wherein said first band lies beneath said second band.
8. The machine as claimed in claim 2, wherein said second forming roll having an open surface to receive water from said web in inward dewatering relation relative to the surface of said roll.
9. The machine as claimed in claim 6, wherein said second forming roll being a vacuum roll, to assist dewatering of said stock.
10. The machine as claimed in claim 1, wherein said machine is a modified fourdrinier machine and wherein said second band comprises a forming wire of said fourdrinier machine and said open stretch is comprised of said forming wire of said fourdrinier machine on which said stock from said source is initially deposited and wherein said first forming roll is in superposed relation thereover, said second forming roll being located within the loop of the fourdrinier wire.
Priority Applications (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000383379A CA1157302A (en) | 1981-08-07 | 1981-08-07 | Twin-wire web forming section |
| FI822705A FI822705L (en) | 1981-08-07 | 1982-08-03 | BANFORMNINGSMASKIN |
| EP82304098A EP0072188A3 (en) | 1981-08-07 | 1982-08-03 | Web forming machine |
| BR8204635A BR8204635A (en) | 1981-08-07 | 1982-08-06 | CONTINUOUS PAPER FORMING MACHINE AND CONTINUOUS PAPER FORMING PROCESS |
| JP13783782A JPS5841993A (en) | 1981-08-07 | 1982-08-07 | Web forming method and apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000383379A CA1157302A (en) | 1981-08-07 | 1981-08-07 | Twin-wire web forming section |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1157302A true CA1157302A (en) | 1983-11-22 |
Family
ID=4120630
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000383379A Expired CA1157302A (en) | 1981-08-07 | 1981-08-07 | Twin-wire web forming section |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0072188A3 (en) |
| JP (1) | JPS5841993A (en) |
| BR (1) | BR8204635A (en) |
| CA (1) | CA1157302A (en) |
| FI (1) | FI822705L (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE458212B (en) * | 1987-07-17 | 1989-03-06 | Valmet Paper Machinery Inc | FORMERS FORMING A PAPER PATH |
| US7070678B2 (en) * | 2001-11-30 | 2006-07-04 | Kimberly-Clark Worldwide, Inc. | Paper webs having a watermark pattern |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3150037A (en) * | 1962-01-04 | 1964-09-22 | Huyck Corp | Papermaking machine utilizing centrifugal dewatering |
| US3311533A (en) * | 1963-04-29 | 1967-03-28 | Pulp Paper Res Inst | Apparatus for making formed fibrous webs |
| SE7507159L (en) * | 1975-06-23 | 1976-12-24 | Karlstad Mekaniska Ab | DEVICE FOR MANUFACTURE OF A FIBER RAILWAY |
| FI761030A (en) * | 1976-04-14 | 1977-10-15 | Valmet Oy | |
| SE410482B (en) * | 1978-02-15 | 1979-10-15 | Karlstad Mekaniska Ab | PROCEDURE AND DEVICE FOR A DOUBLE WIRE FORMER |
-
1981
- 1981-08-07 CA CA000383379A patent/CA1157302A/en not_active Expired
-
1982
- 1982-08-03 EP EP82304098A patent/EP0072188A3/en not_active Withdrawn
- 1982-08-03 FI FI822705A patent/FI822705L/en not_active Application Discontinuation
- 1982-08-06 BR BR8204635A patent/BR8204635A/en unknown
- 1982-08-07 JP JP13783782A patent/JPS5841993A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0072188A3 (en) | 1984-03-07 |
| BR8204635A (en) | 1983-07-26 |
| JPS5841993A (en) | 1983-03-11 |
| EP0072188A2 (en) | 1983-02-16 |
| FI822705L (en) | 1983-02-08 |
| FI822705A0 (en) | 1982-08-03 |
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