MXPA99011636A - Limiting orifice drying medium, apparatus therefor, and cellulosic fibrous structures produced thereby - Google Patents

Abstract

A limiting orifice through-air-drying medium for papermaking or other absorbent embryonic webs. The medium may be used in an apparatus which can be embodied in a cover and a roll. The medium has the unique combination of a relatively high binding fatigue strength and relatively low pressure drop. The medium may comprise a laminate of a plurality of plies. The intermediate plies of the laminate may be woven with a square weave. The medium may also be used for other types of drying.

Description

DRYING MEDIUM OF LIMITING HOLE, APPARATUS FOR THE SAME, AND STRUCTURES OF CELLULOSE FIBERS PRODUCED WITH THEMSELVES FIELD OF XA INVENTION The present invention relates to an apparatus for drying by passage of air, in particular, with an apparatus that limits the flow of air for drying through a structure of cellulosic fibers and with absorbent embryonic wefts that are dried by air passage, on it.

BACKGROUND OF THE INVENTION The absorbent embryonic frames are a raw material of daily life. Absorbent embryo frames include cellulose fiber structures, absorbent foams, etc. The structures of cellulosic fibers have become a raw material of daily life. The cellulosic fiber structures are found in facial tissues, toilet paper and paper towels. In the manufacture of cellulosic fiber structures, a wet embryonic web of cellulosic fibers dispersed in a liquid carrier is deposited on a forming mesh. The humid embryonic web P953 can be dried by any one of the various known means, or by combinations thereof. Each of these known drying media will affect the properties of the resulting cellulosic fiber structure. For example, the drying medium and the drying process can influence the softness, caliber, tensile strength and absorbency of the resulting cellulosic fiber structure. Importantly, the medium and process used to dry the cellulosic fiber structure also affects the speed at which it can be manufactured without being limited in speed by the means and drying processes. An example of a drying medium is the felt strips. Felt drying strips have been used for a long time to drain an embryonic structure of cellulosic fibers through the capillary flow of the liquid carrier towards a permeable felt medium maintained in contact with the embryonic web. However, draining a cellulose fiber structure with a felt band results in a uniform overall compression and compaction of the embryonic web of cellulosic fiber structure to be dried. Felt band drying can be assisted by a vacuum, or can be assisted by opposite press rolls. The press rollers maximize the mechanical compression of the felt against the cellulose fiber structure. Examples of felt strip drying are illustrated in U.S. Patent 4,329,201 issued May 11, 1982 to Bolton and U.S. Patent 4,888,096 issued December 19, 1989 to Cowan et al. It is also known in the art how to dry a cellulose fiber structure via capillary flow, using a porous cylinder having preferential pore sizes. Examples of capillary flow drying techniques are illustrated in commonly assigned U.S. Patent No. 4,556,450 issued December 3, 1985 to Chuang et al., Incorporated herein by reference, in 5,598,643 issued 4. February 1997 in the name of Chuang et al. , and in U.S. Patent 4,973,385 issued November 27, 1990 to Jean et al. Drying the cellulosic fiber structures by vacuum draining, without the aid of felt strips, is known in the art. The vacuum drying of the cellulose fiber structure mechanically removes moisture from the cellulosic fiber structure using vacuum shoes and vacuum boxes. The vacuum deflects discrete regions of the cellulosic fiber structure within the drying band. Preferably, the drying band is an air passage drying band having a resin frame with a pattern with deflection conduits therethrough, as set forth in commonly assigned United States Patent No. 4,637,859 granted to Trokhan and incorporated here as a reference. Vacuum draining over such a web produces a multi-region cellulosic fiber structure having an essentially continuous high density network and discrete low density regions distributed therein. Drainage with a band such as this produces a cellulosic fiber structure having different amounts of moisture in the two regions mentioned above. The different amounts of moisture in the different regions of the cellulosic fiber structure can limit the speed of the papermaking process. The limitation occurs because the two regions will dry at different speeds. The region that has the slowest drying speed will then control the overall speed of the papermaking process. In another drying process, an important success has been obtained by drying the embryonic web of a cellulose fiber structure by passing air. In a typical air-drying process, an air-permeable foraminous band supports the embryonic web to be dried. The air flow passes through the cellulosic fiber structure and through the permeable band. The flow of dry air mainly the embryonic plot by evaporation. The regions which coincide with the foramina of the air-permeable web and which are deflected therein, are preferably dried, and the size of the resulting structure of cellulose fibers increases. The regions that coincide with the knuckles in the air-permeable band are dried to a lesser degree. Various modifications and improvements have been made to the air-permeable bands used in the art for drying by air passage. For example, the air permeable band can be made with a relatively high open area. Or, the band can be made to have reduced air permeability. The reduced air permeability can be achieved by applying a resin mixture to seal the interstices between the woven strands in the band. The drying band can be impregnated with metallic particles to increase its thermal conduction capacity and reduce its emissive capacity. Preferably, the drying band is constructed from a photosensitive resin comprising a continuous network. The drying band can be specially adapted for air currents at high temperature.

Examples of air-pass drying technology are found in U.S. Patent Republic No. 28,459 issued July 1, 1975 to Cole et al .; United States Patent 4,172,910 issued October 30, 1979 to Rotar; U.S. Patent 4,251,928 issued February 24, 1981 to Rotar et al .; United States patent assigned in common form 4,528,239 granted on July 9, 1985 to Trokhan; and United States Patent 4,921,750 issued May 1, 1990 to Todd. Additionally, various attempts have been made in the art to regulate the drying profile of the cellulosic fiber structure while it is still an embryonic web to be dried. These attempts can use either a drying band, or an infrared dryer in combination with a Yankee hood. Examples of profiled drying are illustrated in U.S. Patent 4,583,302 issued April 22, 1986 to Smith, and U.S. Patent 4,942,675 issued July 24, 1990 to Sundovist. The prior art, even that specifically directed to drying by passage of air, does not address the problems encountered when drying a cellulosic fiber structure of multiple regions. As seen in the above, different regions of paper dried by air passage have different moisture contents. But a first region of the cellulosic fiber structure, having a lower density or basis weight than that of a second region, will normally have a relatively greater air current therethrough, than that which will have a second region. This relatively greater air current occurs because the first region of base weight or lower density has, proportionally, less flow resistance to the passage of air through the embryonic web than the second region. The differential air flow does not shift, and may even increase, the differential moisture content of the different regions. This problem is exacerbated when the multi-region cellulosic fiber structure to be dried is transferred to a Yankee drying drum. In a Yankee drying drum, only certain regions of the cellulosic fiber structure are in contact with the circumference of a hot cylinder. Normally, the most intimate contact with the Yankee drying drum occurs in regions of high basis weight or high density. These regions have more moisture than the low-density or low-basis-weight regions. The hot air that comes from a hood can be introduced to the surface of the structure of P953 cellulosic fibers opposite the hot cylinder. Preferential drying of this surface of the cellulosic fiber structure occurs by the convective transfer of heat from the air stream in the hood of the Yankee drying drum. To allow the complete drying of the high-density and high-density regions of the cellulosic fiber structure to occur and to avoid scorching or burning of the low-density or low-density basis regions already dried by the air from the In the hood, the air temperature of the Yankee hood should be decreased and / or the residence time of the cellulosic fiber structure in the Yankee hood should be increased, decreasing the production rate. Consequently, the speed of production of the cellulosic fiber structure must decrease, to compensate for the higher humidity in the region of high basis weight or high density. An improvement in the art, which focuses on this problem, is illustrated by commonly assigned United States Patent No. 5,274,930 issued on January 4, 1994 to Ensign et al. and which discloses drying by limiting orifice of cellulosic fiber structures in conjunction with air-drying, the patent of which is incorporated herein by reference. This patent shows an apparatus using a micropore drying medium having a higher flow resistance than the interstices between the fibers of each region of the cellulosic fiber structure. The micropore medium is the limiting orifice in the process of drying by air passage, so that a more uniform moisture distribution is achieved in the drying process. A further improvement to the apparatus set forth in Ensign et al. "930 is the apparatus disclosed in commonly assigned U.S. Patent No. 5,581,906 issued December 10, 1996 to Ensign et al., And incorporated herein by reference." Ensign et al. '906 discloses an apparatus Microporous drying that has multiple zones and more efficiently dries the cellulosic fiber structure than the types of apparatuses disclosed in the prior art The above micropore drying apparatuses must desirably provide a means which at the same time limits the flow of air through the cellulosic fiber structure and having sufficient fatigue strength of the bend to withstand the cyclic loading inherent to papermaking with the claimed apparatus.For example, the medium can be executed as the roll cover axially While the roller and medium are rotated, any portion of the medium alternatively receives both positive and negative pressure loads. The reversal of the charge from positive to negative subjects a cycle to the medium, with an alternating pressure that must be resisted by the medium. In this way, the medium must have an adequate resistance to bending fatigue, to resist this cyclic loading. One solution to the problem of providing adequate resistance to bending fatigue could simply be to make the medium stronger. However, this solution, without further ado, brings other problems. While the. medium becomes stronger, typically becomes thicker and may have less open area. The medium having a less open area finds a pressure drop larger than a medium having a relatively more open area. The benefits of minimizing the pressure drop are known and discussed in the '906 patent of Ensign et al. before mentioned. In addition, while the medium becomes thicker, it also becomes more difficult to manufacture. Consequently,. It is an object of this invention to provide a means for use with a micropore apparatus, in particular with the apparatus of the aforementioned patents 906 of Ensign et al and 930 of Ensign et al. It is also an object of the present invention to provide a means that can be used with the capillary drainage apparatus, such as the apparatus of the '450 patent of Chuang et al. or the above-mentioned "305 application of Chuang et al." It is also an object of the present invention to provide a means that can be used with air-pass drying and conventional felt drainage.It is further an object of this invention to provide the means providing both adequate resistance to bending fatigue and a relatively small pressure drop In particular, it is an object to provide the means having a relatively small pressure drop.

SUMMARY OF THE INVENTION The invention comprises a generally flat drying medium. The drying means comprises a plurality of folds juxtaposed together in facing relation. The medium has a bending fatigue strength of at least 25 pounds per inch and a pressure drop of less than 70 inches of water in a standard flow of 800 cubic feet per minute per square foot. The medium may comprise a first fine fold. The first thin fold can be a woven wire cloth. The first thin fold can have a woven design in Dutch diagonal lines. The first fold may have a nominal pore size of 20 microns or less. Opposed to the first fold is the coarsest fold in the middle. He P953 The coarser fold of the medium may also comprise a woven fabric or it may be a perforated metal plate. Intermediates between the first fold and the coarser fold are at least intermediate folds. The intermediate folds may comprise a pattern design.

DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side elevation view of an apparatus in accordance with the present invention. Figure 2 is a fragmentary top plan view of a medium according to the present invention, shown partially in sections.

DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1, the present invention comprises a micropore drying medium 40 for a limiting orifice air-drying apparatus 20. The apparatus 20 and the medium 40 may be made in general and operated in accordance with the aforementioned U.S. Patent Nos. 5,274,930 and 5,581,906, the teachings of which are incorporated herein by reference. The apparatus 20 removes moisture from an embryonic web 21. The apparatus 20 may comprise a permeable cylinder 32. The micropore medium P953 40 circumscribes the permeable cylinder 32 and is preferably joined thereto with a hot shrink fit, a forced fit, threaded fasteners, brass welding, etc. It will be recognized that other executions of the apparatus 20 and the medium 40 may be possible. For example, the apparatus 20 may comprise a divided vacuum slot or the medium 40 may comprise an endless band. A support member 28, such as an air passage drying band, wraps the permeable cylinder 32 from an inlet roller 34 or an imitation roller 36 which subtends an arc defining a circular segment. The circular segment can be subdivided into multiple zones having differential pressures mutually different from the ambient atmospheric pressure. The web 21 to be dried is sandwiched between the support member 28 and the medium 40. The micropore medium 40 according to the present invention may comprise a multi-fold web 41-46. A medium 40 having six folds 41-46 will be discussed below, although it will be understood that the invention is not limited thereby. A medium having any plurality of folds 41-46 and meeting the criterion of pressure drop and fatigue resistance of the fold exposed below is suitable for the present invention.

P953 The medium 40 according to the present invention has a bending fatigue strength of at least 25, preferably at least 50, and more preferably at least 75 pounds per inch. The fatigue strength of the bend is measured according to the following procedure: A sample with dimensions of 1 inch wide x 2 inches long is provided. The long address of the sample corresponds to the address of the machine during papermaking. The sample is marked, in the widthwise direction, through the center of the first fold 41. The marking is carried out with a Scratchall with a carbide tip, using hand pressure. The brand line should be approximately at half the thickness of the first fold 41. A three-point bend test apparatus is provided. The apparatus has a mounting frame comprising two supports oriented in a vertical direction on which the sample to be tested is placed. The apparatus further has a movable spreader which is capable of applying a downward load in a position midway between the two supports. The brackets have a width of at least 1 inch and a radius of 1/8 inch. The supports have a free space between them of 0.750 of an inch.

P953 The sample to be tested is placed on the apparatus and oriented so that the first fold 41 is in tension and placed away from the head applying the variable down load. The sample is simply supported on the two supports. The marking line is centered between the supports. A variable down load is applied to the sample, at the midpoint between the supports and directly opposite the marking line. The load is applied in the form of a sine wave at a frequency of 3 Hertz. The load is subjected to a cycle between a maximum load value and a value of 1/10 of the maximum, to provide an R ratio of 0.10. Three different maximum load values are used. The magnitudes of the maximum load values depend on the resistance to bending with a displacement of 0.2 percent, of the sample. The deflection of the sample under the first load cycle is measured in the fatigue strength of the bend being tested. The deflection can be measured by an extensometer and a quadrant gauge, as is known in the art. The appropriate equipment is manufactured by the Mechanical Testing Systems Company of Edon Praine, Minnesota and sold as Model 632 MTS. It is judged that the sample being tested has failed when the P953 deflection in any given cycle is twice the deflection of the first cycle. Resistance to bending resistance with a displacement of 0.2 percent can be found in general, in accordance with ASTM D790-92, Method 1, modified as follows. A 1 x 2 inch sample of medium 40 is provided. The sample (without mark line) is loaded into the aforementioned three-point bending test apparatus and tested once in the bend at a crosshead speed of 0.02. inch per minute until the plastic deformation is present. The resistance to bending with a displacement of 0.2 percent is then found by drawing a straight line parallel to the linear portion of the curve of stresses / strains of the bend, and displaced from the origin, on the abscissa 0.0015 inches (0.2 percent of space). 0.750 inches). The displacement resistance of 0.2 percent displacement of 0.2 percent is found, as the intersection of this line and the bending load against the deflection curve. The three samples are tested in this way, and the results are averaged to give a single reference point of resistance to bending displacement of 0.2 percent. The values corresponding to 60 are found, P953 85 and 110% of the resistance to bending with a displacement of 0.2 percent. In this way, three values are used for the maximum load values in the determination of the fatigue strength of the bend, that is, 0.60, 0.85 and 1.10 of the resistance to bending with a displacement of 0.2%. Three fatigue tests are run to the fault, as described above. Each of the fatigue tests uses one of the three maximum load values mentioned above, each load is a multiple of 0.60, 0.85 and 1.10 of the folding resistance by slip of 0.2 percent. Three samples are run in each of the three specified charges, for a total of nine samples. For each maximum load value, the three reference points are averaged, for a total of 9 samples. For each maximum load value, the three reference points are averaged to give a single reference point. The three resulting reference points are plotted on a semi-logarithmic curve that displays load versus number of cycles, as is known in the art. The fatigue strength of the bend is then asymptote of the curve through the three reference points. The curve takes the general form Y = AX "°" 5 + B, where B is this asymptote. The asymptote of the curve corresponds to the fatigue strength of the bend for the three points of P953 reference under consideration. While a person with ordinary skill in the art will know mathematical technique to solve this equation for B, the fatigue resistance of the bend is found more easily using any regression program common to most engineering software programs. An adequate program is Excel, sold by Microsoft Corporation of Redmond, Washington. The medium 40 according to the present invention also has a dry pressure drop of less than 70, preferably less than 50, and more preferably less than 30 inches of water. The pressure drop is measured as follows. A suitably sized sample of the medium 40 is held in a test chamber so that a four inch diameter section of the medium 40 is exposed to air flow therethrough. The test apparatus comprises a tube length of 7 inches long and has an inner diameter of two nominal inches. The inner diameter of the tube then tapers at an included angle of 7o over a nominal inside diameter of 16 inches long to 4 inches. The sample from the medium 40 is then attached to the 4 inch nominal inside diameter portion of the apparatus. Downstream of the sample 40 the apparatus is tapering again in a P953 included angle of 1 ° from a nominal internal diameter of 4 inches to a nominal internal diameter of 2 inches. This 2-inch inner diameter section of the test apparatus is also straight and at least 7 inches long. The medium 40 is oriented so that the first fold 41 faces the high pressure (upstream) of the side of the air flow. Air flow of eight hundred scfm (cubic feet per standard minute) per square foot is applied through the medium 40 for a total of about 70 scfm for the sample described herein. The static pressure through the sample is measured by means of a manometer, a pair of pressure transducers or other suitable means known in the field. A comparison of several prior art media and one [or more] medium 40 according to the present invention is shown in Table I below.

P953 TABLE I Construction Drop Resistance Pressure to 800 to Fatigue SCFM / sqft of Bending (inches to (pounds / water) inch) Technique 325x2300 Cross fabric 78 10 Previous I Dutch 4 Folds Square 150x150 Square 60x60 Dutch plane 12x64 Technique 325x2300 Cross fabric 100 124 Previous II Dutch 5 Folds Square 150x150 Square 60x60 Dutch plane 12x64 Weight plate perf. gauge 16/23% open area 3/32 in. holes diam. in step 3/16 inch Technique 165x1400 Cross fabric 30 15 Previous Dutch III Square 150x150 4 Folds Square 60x60 Dutch flat 12x64 Present 165x1400 Cross fabric 51 N / A Invention I Dutch 5 Folds Square 150x150 Square 60x60 Dutch flat 12x64 Weight plate perf. Gauge 16/23% open area 3/32 in. Holes diam. in step 3/16 inch Present 165x1400 Cross fabric 30 65 Dutch invention II Square 150x150 6 Folds Square 60x60 Square 30x30 Square 16x16 Weight of plate perf.

P953 Construction Resistance Drop Pressure at 800 to Fatigue SCFM / sqft of Bending (inches (lbs / water) inch) Gauge 24/37% open area and 0.080 in. Holes. Diameter In step of 0.125 in. Present 165x1400 Cross fabric 30 approx. N / A Dutch invention III Square 150x150 6 Folds Square 60x60 Square 30x30 Square 16x16 Weight of plate perf. Caliber 24/32% open area and 0.065 in. Holes Diameter In step of 0.109 in.

If one takes the Previous Technique I, from Table I as a reference point, it might be easy to believe that the problem of low resistance to bending fatigue can be solved by adding a perforated plate as the last fold 45, which results in the Technique Previous II. However, Prior Art II illustrates the relationship between resistance to bending fatigue and pressure drop. As the resistance to fatigue of the fold increases, so does the pressure drop increase. Driving unacceptable operating results. In contrast to the Prior Art III it has an accepted pressure drop but resistance to bending fatigue unacceptable.

P953 In this way, it is only with the present invention that an acceptable combination of bending fatigue strength and pressure drop results. One should, preferably, not attempt to achieve acceptable pressure drop and bending fatigue resistance by using a first very open crease 41 and a relatively thick perforated plate having a low open area for the last crease 46. Such a mode may provide unacceptable sheet or drainage support. Comparison of Prior Art III with the Present Invention I indicates that adding a perforated plate to achieve resistance to bending fatigue also increases the pressure drop by approximately 21 inches of water. It is only with the present invention that it goes from the middle 40 of the Prior 4-fold technique to the 6-fold means 40 of the present invention, that the pressure drop remains constant while the fatigue resistance of the bend increases to an acceptable value. It is expected that the Present Invention I has a bending fatigue strength as great as that shown in Prior Art II. In accordance with the present invention, the combination of folds 42-46 after the first fold 41 adds no more than 5 inches of water to the pressure drop across the medium 40 at 800 scfm per square foot.

P953 As shown above, the means 40 comprises a plurality of folds that go from a first fold 41 to a last fold 46. The folds 41-46 of the medium 40 serve three different functions: support for the frame 21 made on them , resistance and connections between the support folds and the resistance folds. The connecting folds are necessary because the first fold 41 is so thin and deformable that it would deform in the interstices of the resistance folds 45-46 without intermediate folds 42-44 as connectors therebetween. The deformation would break the hydraulic connection between the first fold 41 and the weft 21. The intermediate folds 401 maintain the generally flat configuration of the first fold 41. The folds 41-46 are preferably arranged from the thinnest fold 41 to the fold coarser 46. Thinner fold 41 provides support as discussed above. The coarser fold 46 and possibly one or two folds adjacent to the coarser fold 46 provide strength. The intermediate folds 42-44 between the first fold 41 and the resistance folds 45-56 provide hydraulic connection between them and support for the first fold 41 above them. It is important that each fold 41-45 in the middle 40 above the perforated plate 46 be capable of P953 provide both perpendicular and lateral fluid flow. Preferably, when the folds 40-46 are considered as a unitary assembly for the medium 40, the means 40 exhibits the properties of pressure drop and bending fatigue resistance described herein. The first fold 41 of the medium 40 is in contact with the web 21. The first fold 41 is typically the thinnest fold of the medium 40 and has pores or other interstitial flow channels finer than the median interstices in the web 21 which is going to dry up. Preferably, the pores of the first fold 41 have a nominal size of 20 microns or less, more preferably 15 microns or less and most preferably 10 microns or less. The pore size is derived from SAE ARP Standard 901 published on March 1, 1968, and is incorporated herein by reference. The first fold 41 according to the present invention can have a Dutch woven design in diagonal lines. A Dutch woven design in diagonal lines may be woven with small pores sufficient to provide a boundary hole for fluid flow therethrough while paper made thereon is dried during papermaking. Also, a Dutch design woven in diagonal lines can have a design to provide a P953 small enough pore size for capillary drainage to occur. A Dutch woven design in diagonal lines has both warps and silk wefts that pass alternatively over two and under two meshes in each direction. Alternatively, a pattern design can be used prophetically, although it may not have enough small pores. Also, a wide-mesh diagonal fabric or a ZZ design of wide-mesh diagonal fabric can be used prophetically. The drawings are illustrated in the Haver and Boecker literature and in Patent No. 4,691,744 issued September 8, 1987, to Haver et al., Which is incorporated herein by reference. The coarser fold 46 of the medium 40 can be a perforated plate or a woven metal fabric. This fold 46 is additional to the web 21. A plate having a continuous support network for the load path is preferred in order to withstand the diametrically applied loads and the circumferential tensions encountered when the medium 40 is used for manufacturing paper . The thickness of the coarsest fold 46 is preferably from about 0.020 to 0.030 inch for the embodiments described herein. If the coarsest fold 46 is too thick, fabrication may result P953 more difficult. If a perforated plate is used for the thicker medium 46, and the plate is too thin, it will probably not be able to meet the bending fatigue strength requirements set forth herein. A portion of the fatigue strength of the bend not provided by the coarser fold 46 can be compensated by providing stronger intermediate folds 42-45. This arrangement is generally not desirable since it increases the pressure drop and can interfere with the fluid path for fluid flow through the medium 40. The perforated plate can have an open area ranging from 20 to 40%, and preferably it goes from 30 to 37%. The folds 42-45 between the first crease or the thinnest crease 41 and the coarsest crease 46 are designated as intermediate folds 401. The middle folds 401 are preferably woven. If the intermediate folds 401 are woven, preferably the specific pattern provides an unobstructed flow channel, i.e., a pore, in a direction perpendicular to the plane of the fold 401 through that entire crease 401. A preferred design for this fold 401 It is a design of squares, although a squared diagonal squares design will also be sufficient. A design of diagonal fabric squares has square openings and wefts that go through P953 above two and below one or two warps in a diagonal pattern. A checkered design has the warp and weft meshes woven in a simple pattern one on one or one below. In the degenerate case, the warp and weft meshes have the same diameter. The pattern count of a pattern design is the same in both directions, and the flow path is straight, in a direction perpendicular to the plane of that fold 401. A pattern design for intermediate folds 401 is preferred, because a pattern of frames provides the best balance of the two-phase fluid flow in the perpendicular and lateral directions of that 401 fold. Compared to a design of identical mesh count boxes, the diagonal fabric pattern can use larger diameter meshes to obtain greater density and resistance. A flat Dutch design uses a pattern of drawing squares with warps larger in diameter than the wefts. A reverse plan Dutch design is also feasible, and has a pattern of drawing frames with larger diameter plots than warps. Contrary to the teachings of the prior art, it is preferred that none of the intermediate folds 401 have a Dutch flat design. Designs such as, for example, diagonal weave P953 Dutch, Dutch flat and Dutch reverse plane, when used for intermediate folds 401 tend to restrict too much air flow through the medium 40. In contrast, the flat check drawings provide improved drainage to drain the 21th grid. Improved drainage is due to the projected upper open area of the flat drawing. If desired, other types of drawings can be used, provided that the fold 401 has air flow both perpendicular to the medium 40 and lateral, ie, inside the fold 401. The folds 41-46 can be joined together to form a unitary means 40, as follows: First, the intermediate folds 401 are calendered individually. Optionally, the first fold 41 can also be calendered. The calendering should be sufficient to provide adequate knuckle area but not to bend the fibers or reduce the open area of the pores considerably. The calendering is sufficient to reduce the thickness of the folds 41-45 to about 65 to 80 percent of its original thickness. It will be recognized by a person of ordinary skill that a considerable range of calendering levels can be used to provide the desired knuckle area. The knuckle area is important to provide adequate film strength between the folds.

P953 The folds 41-46 are superimposed then one on top of the other in the desired sequence. As noted above, preferably, but not necessarily, the folds are arranged monotonously in the order from that crease 41 which has the smallest pore size to the crease 46 which has the largest pore size. The folds 41-46 are then sintered to join each fold to the adjacent folds 41-46. The sintering can be carried out in accordance with the processes used by those of ordinary skill to make filter media, as is known in the art. The sintering operation produces a sheet means 40 as described herein.

Present Invention I The following describes the medium 40 listed as Present Invention I, in Table I above. The folds 41-45 of the middle 40 were made of 304L or 316L stainless steel. The last fold 46 was made of 304 stainless steel. The first fold 41 of the medium 40 is very thin, to provide the micropores that limit the flow of air through the medium 40 and the absorbent embryonic web 21. The first fold 41 comprises a woven metal screen that has a Dutch diagonal woven design of 165 x 1400. The screen was made with mesh P953 warp of 0.0028 inches in diameter and weft meshes of 0.0016 inches in diameter. As noted above, a pattern design for the first fold 41 is not preferred, so that the first fold 41 will have pores small enough to provide adequate weft support, adequate hydraulic connections and a limiting orifice for air flow through of the screen 21. The second fold 42 of the medium 40 is underlying the first fold 41. The second fold 42 comprises a woven metal cloth having a 150 x 150 square mesh pattern of 0.0026 inch diameter, to provide suitable support for the first fold 41. The third fold 43 of the medium 40 is underlying the second fold 42. The third fold 443 comprises a woven metal cloth having a pattern of 60 x 60 mesh squares 0.0075 inches in diameter. The fourth fold 44 of the middle 40 is underlying the third fold 43. The fourth fold 44 comprises a woven wire cloth having a 30 x 30 mesh design of 0.016 inch diameter meshes. The fifth fold 45 of the middle 40 is underlying the fourth fold 44. The fifth fold 45 comprises a woven wire cloth having a 16 x 16 mesh design of 0.028 inch diameter meshes.

P953 The coarser fold 46 of the medium 40 provides support for the balance of the medium 40. The coarsest fold 46 is a perforated metal plate. For the embodiment described herein, a sixth fold 46 comprises a 24-gauge steel plate having a thickness of 0.0239 inch, and approximately 37 percent open area was found to work well. The open area of approximately 37 percent was provided by 0.080 inch diameter holes arranged bilaterally at 60 degrees in a 0.125 inch step. The perforation pattern is arranged in a path parallel to the machine direction. As will be recognized by a person with ordinary skill, generally for equivalent open areas, a pattern is preferred which provides a larger number of holes smaller than an orifice pattern comprising a smaller number of relatively larger orifices. The coarsest fold 46 of the medium 40 was the sixth fold 46 in the embodiment described herein. However, it will be recognized that a medium 40 may be made in accordance with the present invention having from three to nine folds. Alternatively, the coarser fold 46 may comprise a woven fabric. If the coarsest fold 46 is a woven fabric, it may comprise a pattern of P953 12 x 12 mesh screens with 0.032 inch diameter. It will be understood that the description of 12 x 12 designates that there are 12 of the meshes per inch of direction taken perpendicular to the greater length of the meshes and the first direction is the warp direction. The aforementioned medium 40 is useful for drying an embryonic web 21 having a pulp filtration resistance (PFR) of 5 to 20, and preferably from 10 to 11. The pulp filtration resistance is measured in accordance with the procedure set forth in commonly assigned U.S. Patent No. 5,228,954 issued July 20, 1993 to Vinson et al. , which is incorporated here as a reference. As used herein, a "weft" or "structure of cellulosic fibers" refers to structures, such as paper, which comprises at least fifty percent cellulosic fibers and a balance of synthetic fibers, organic fillers, materials of inorganic fillers, foams, etc. Structures of cellulosic fibers suitable for use with the present invention can be found in commonly assigned U.S. Patent Nos. 4,191,609 issued March 4, 1980 to Trokhan.; 4,637,859 issued on January 20, 1987 to Trokhan; and 5,245,025 granted on 14 P953 September 1993 to Trokhan et al., Which are incorporated herein by reference. As used herein, a weft is considered "absorbent" if it can contain and retain water, or remove water from a surface. The rate of removal of water for the apparatus 20 according to the present invention is measured in terms of pounds of water removed per pound of fiber divided by the time the fibers are subjected to the process. Mathematically, this can be expressed as: Water removal rate = (pounds of water removed / pounds of fiber) / time in seconds The rate of water removal is determined by measuring the consistencies of the embryonic web 21 before and after the device 20 using gravimetric weighting and convective drying achieve a completely dry reference line. While the medium 40 and the apparatus 20 in accordance with the present invention have been analyzed in conjunction with air-drying of an embryonic web 21, it will be recognized that the invention described and claimed herein is not limited by this analysis. The present invention can also be used in conjunction with felt drying or also with capillary drying device.

P953

Claims (10)

1. CLAIMS: 1. A generally flat drying medium, the drying medium comprises a plurality of folds attached in facing relationship, the medium having a fatigue strength of at least 25 pounds per inch and a pressure drop. of less than 70 inches of water at a flow rate of 800 cubic feet per standard minute, per square foot.
2. 2. A medium according to claim 1, wherein the bending fatigue strength is at least 50 pounds per inch.
3. 3. A medium according to claim 2, wherein the resistance to fatigue of the fold is at least 75 pounds per inch.
4. 4. A medium according to claims 1, 2 or 3, where the pressure drop is less than 50 inches of water.
5. 5. A medium according to claim 4, wherein the pressure drop is less than 30 inches of water.
6. 6. A generally flat drying medium having two opposite faces, the drying means comprises a plurality of folds, a first fold, the first fold is disposed on one side of the medium, a coarser fold, the coarser fold is arranged on the opposite side of the middle, and a plurality of intermediate folds P953 between the first fold and the coarser fold, each of the plurality of intermediate folds comprises a design having an unobstructed flow channel perpendicular to the plane of the intermediate folds.
7. 7. A medium according to claim 6, wherein at least one of the intermediate folds comprises a frame design.
8. 8. A medium according to claims 6 and 7, wherein the first fold comprises a Dutch diagonal weave design.
9. 9. A medium according to claims 6, 7 and 8, wherein the coarsest fold comprises a perforated metal plate, and preferably, the metal plate has an open area of 20 to 40 percent.
10. 10. A medium according to the claims 1,2,3,4,5,6,7,8 and 9, wherein at least one fold of the medium has a pore size of 20 microns or less, and preferably, the fold having the pore size of 20 microns or less is an outer fold of the medium and remains in contact with the weft during papermaking. P953