Multi-layer forming fabric with stitching yarn pairs integrated into the papermaking surface

Abstract: A papermaker's fabric (20), comprising top machine direction yarns (21-30), top cross machine direction yarns (31-40), bottom machine direction yarns (41-50), bottom cross machine direction yarns (51-60), and sets of first and second stitching yarns (61a-70a and 61b-70b), said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yarns; a set of top cross machine direction yarns interwoven with said set of top machine direction yarns; a set of bottom machine direction yarns; a set of bottom cross machine direction yarns interwoven with said set of bottom machine direction yarns; wherein pairs of first and second stitching yarns are positioned between pairs of top cross machine direction yarns, said first and second stitching yarns of each pair being interwoven with said top and bottom machine direction yarns, and wherein said pair of first and second stitching yarns is positioned between each pair of adjacent top cross machine direction yarns; each of said first stitching yarns passes over a first number of top machine direction yarns, and each of said second stitching yarns passes over a second number of top machine direction yarns; said first number is different than said second number; and wherein at least some of the top cross machine direction yarns are immediately adjacent to either two first stitching yarns or two second stitching yarns.

Description: Field of the Invention The present invention relates generally to papermaking, and relates more specifically to fabrics employed in papermaking. Background of the Invention In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a "forming fabric," provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run ( i.e. , the "machine side") of the fabric. After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer of the press felt. The paper is then transferred to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging. Typically, papermaker's fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. In a flat woven papermaker's fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" (MD) and "cross machine direction" (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers' fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term "endless belt" as used herein refers to belts made by either method. Effective sheet and fiber support and an absence of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Wire marking is particularly problematic in the formation of fine paper grades, as it affects a host of paper properties, such as sheet mark, porosity, see through, and pin holing. Wire marking is the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yarns of the forming fabric. This problem is generally addressed by providing a permeable fabric structure with a coplanar surface that allows paper fibers to bridge adjacent yarns of the fabric rather than penetrate the gaps between yarns. As used herein, "coplanar" means that the upper extremities of the yarns defining the paper-forming surface are at substantially the same elevation, such that at that level there is presented a substantially "planar" surface. Accordingly, fine paper grades intended for use in quality printing, carbonizing, cigarettes, electrical condensers, and like grades of fine paper have typically heretofore been formed on very finely woven or fine wire mesh forming fabrics. Regretably, such finely woven forming fabrics often are delicate and lack dimensional stability in either or both of the machine and cross machine directions (particularly during operation), leading to a short service life for the fabric. In addition, a fine weave may adversely effect drainage properties of the fabric, thus rendering it less suitable as a forming fabric. To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and coarser-mesh yarns on the machine contact side to provide strength and durability. For example, fabrics have been constructed to include one fabric layer having a fine mesh, another fabric layer having a coarser mesh, and stitching yarns that bind the layers together. These fabrics, known as "triple layer" fabrics, are illustrated in U.S. Patent No. 4,501,303 to Osterberg, U.S. Patent No. 5,152,326 to Vohringer, and U.S. Patent No. 5,437,315 to Ward. Although these fabrics have performed successfully, they have some shortcomings that relate to the inclusion of the stitching yarns. In a typical triple layer forming fabric, one or more stitching yams are positioned between some of the CMD yarns of the top and bottom layers and interwoven with the top and bottom MD yarns. In such a construction, portions of the stitching yarns form part of the papermaking surface of the fabric. As a result, the appearance of paper formed with the fabric can be affected (sometimes adversely) by the presence of the stitching yarns. In addition, triple layer fabrics have proven to have problems with interlayer wear. As the fabric is used on a paper machine, the top and bottom layers tend to shift relative to one another, both in the machine direction and the cross machine direction, due to the tension imparted to the fabric by the rolls. This effect is exacerbated on paper machines, such as the so-called "high-wrap" machines, that include multiple rolls, including some which contact the top layer of the fabric. This shifting can cause the fabric to wear and decrease in thickness, which can adversely affect the drainage of the fabric and, accordingly, its performance in papermaking. In many instances, it is this interlayer wear, rather than the wear of the machine side surface of the fabric machine against the paper machine, that determines the longevity of the fabric. Further, because the stitching yarns of a triple layer fabric have a different weave pattern than the top CMD yarns (i.e., they interweave with the bottom CMD yams also, whereas the top CMD yams do not), there can be differences in tension between the stitching yarns and the top CMD yarns. These differences can induce the fabric to distort out-of-plane, which can in turn reduce the quality of paper produced with the fabric. Also, the stitching yarns of a triple layer fabric should be sufficiently strong and durable to bind the top and bottom layers and to resist the wear and abrasion conditions that the bottom layer experiences while in contact with the paper machine, yet should be delicate enough to produce high quality paper. This balance can be quite difficult to strike. US-A-5,152,326 relates to a composite papermaking fabric in which two fabric layers are interconnected in such a way that relative movement between the layers is completely or to a great extent eliminated while keeping the upper fabric from causing marking of the paper. Summary of the Invention In view of the foregoing, it is an object of the present invention to provide a multi-layer forming fabric construction with little distortion in the top fabric layer. It is also an object of the present invention to provide a multi-layer forming fabric construction that produces a high quality paper. It is another object of the present invention to provide a multi-layer forming fabric construction that maintains the top and bottom layers in a tightly bound condition. It is a further object of the present invention to provide a multi-laye forming fabric that addresses the problem of interlayer wear. These and other objects are satisfied by the present invention. The present invention is defined by claims 1 to 6 and relate to a papermakers fabric and method of making thereof. The papermaker's fabric of claims 1 to 5 comprises top machine direction yams, top cross machine direction yams, bottom machine direction yams, bottom cross machine direction yams, and sets of first and second stitching yarns, said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yams; a set of top cross machine direction yams interwoven with said set of top machine direction yams; a set of bottom machine direction yams; a set of bottom cross machine direction yams interwoven with said set of bottom machine direction yarns; wherein pairs of first and second stitching yams are positioned between pairs of adjacent top cross machine direction yarns, said first and second stitching yams of each pair being interwoven with said top and bottom machine direction yams such that, as a fiber support portion of said first stitching yam is interweaving with said top machine direction yams, a binding portion of said second stitching yarn is positioned below said top machine direction yams, and such that as a fiber support portion of said second stitching yam is interweaving with said top machine direction yams, a binding portion of said first stitching yam is positioned below said top machine direction yarns, and such that said first and second stitching yarns cross each other as they pass below a transitional top machine direction yam, and such that each of said binding portions of said first and second stitching yams passes below at least one of said bottom machine direction yams; each of said first stitching yams passing over a first number of top machine direction yams, and each of said second stitching yarns passing over a second number of top machine direction yarns. In the fabric of claims 1 to 5, a pair of first and second stitching yarns is positioned between each pair of adjacent top cross machine direction yams; said first number is different than said second number; and at least some of the top cross machine direction yams are immediately adjacent to either two first stitching yams or two second stitching yams. The method of making paper of claim 6, comprises the steps of: (a) providing a papermaker's fabric according to one or more of the daims 1 to 5; (b) applying paper stock to said papermaker's fabric; and (c) removing moisture from said paper stock. In an illustrated embodiment of the fabric, the stitching yarns are interwoven with the top MD and CMD yarns to that they form a plain weave papermaking surface. In this embodiment, the integration of the stitching yarns into the papermaking surface of the fabric addresses many of the problems associated with prior art triple layer fabrics, such as distortion of the parpermaking surface and inadequate binding of the top and bottom layers. Brief Description of the Figures Figure 1A is a top view of an embodiment of a 20 harness multi-layer forming fabric of the present invention having a plain weave top surface. Figure 1B is a plan view of the bottom layer of the fabric of Figure 1. Figures 2A through 2J are section views of the stitching yarns of the fabric of Figures 1A and 1B. Detailed Description of the Preferred Embodiments The present invention will be described more particularly hereinafter with reference to the accompanying drawings. The invention is not intended to be limited to the illustrated embodiment; rather, this embodiment is intended to fully and completely disclose the invention to those skilled in this art. A 20 harness multi-layer forming fabric, generally designated at 20, is illustrated in Figures 1A and 1B, in which a single repeat unit of the fabric is shown. As seen in Figure 1A, the repeat unit of the fabric 20 includes a top layer having ten top MD yams 21-30 and ten top CMD yarns 31-40. These are interwoven such that each top CMD yam passes over and beneath top MD yarns in an alternating fashion, with each top CMD yarn passing over and under the same top MD yarns. For example, top CMD yarn 31 passes under top MD yarn 21, over top MD yarn 22, under top MD yarn 23, over top MD yarn 24 and so on until it passes over top MD yarn 30. Similarly, top CMD yarn 32 passes under top MD yarn 21, over top MD yarn 22, under top MD yam 23, over top MD yarn 24 and so on until it passes over top MD yarn 30. Referring now to Figure 1B, a repeat.unit of the bottom layer of the fabric is shown. The repeat unit includes ten bottom MD yarns 41-50 which are interwoven with ten bottom CMD yarns 51-60. The bottom MD yarns 41-50 are interwoven with the bottom CMD yarns 51-60 in a 1x4 twill type pattern, with each bottom CMD yarn passing above one bottom MD yarn, below four bottom MD yarns, above one bottom MD yarn, and below four bottom MD yarns. For example, bottom CMD yarn 51 passes above bottom MD yarn 41, below bottom MD yarns 42-45, above bottom MD yarn 46, and below bottom MD yarns 47 through 50. The other bottom CMD yarns follow a similar "over 1/under 4" weave pattern, but each is offset from its nearest bottom CMD yam neighbors by two bottom MD yarns. Consequently, bottom CMD yarn 52 passes below bottom MD yarns 41 and 42, above bottom MD yarn 43, below bottom MD yarn 44 through 47, above bottom MD yarn 48, and below bottom MD yarns 49 and 50. Thus the "knuckle" formed by bottom MD yarn 43 as it passes below bottom CMD yarn 52 is offset from the "knuckle" formed by bottom MD yarn 41 as it passes over bottom CMD yam 51 by two bottom MD yarns. The top layer (formed by the top MD yarns and the top CMD yarns) and the bottom layer (formed by the bottom MD yams and the bottom CMD yarns) are stitched together with twenty stitching yarns, designated herein as pairs 61a , 61b through 70a, 70b. The stitching yarns are positioned in pairs between adjacent CMD yarns. For example, stitching yarns 61a and 61b are positioned between top CMD yarns 31 and 32 and between bottom CMD yarns 51 and 52. The stitching yams interweave with the top MD yarns and bottom MD yarns to bind the top and bottom fabric layers together. As can be seen in Figures 2A through 2J , corresponding pairs of stitching yarns interweave with the top MD yarns and bottom MD yarns in the following pattern. Each of the stitching yarns of the repeat unit can be subdivided into two portions: a fiber support portion which interweaves with the top MD yarns, and a binding portion which interweaves with a bottom MD yarn. These are separated at "transitional" top MD yarns, below which one stitching yarn of a pair crosses the other stitching yam of the pair. The stitching yarns of each pair are interwoven relative to one another such that the fiber support portion of one yarn of the pair is positioned above the binding portion of the other yarn of the pair. The fiber support portion of the stitching yarn of each pair designated with an "a" ( e.g. , 61a , 62a , 63a ) interweaves in an alternating fashion with five top MD yarns (alternately passing over three top MD yarns and under two top MD yarns), and the other stitching yarn of the pair (those designated with a "b" ) passes over two top MD yarns while passing below a top MD yarn positioned between those two MD yarns. In its fiber support portion, each stitching yarn passes over top MD yarns that the top CMD yarns pass beneath, and passes below top MD yarns that each top CMD yarn passes over. In this manner, the stitching yarns and top CMD form a plain weave pattern with the top MD yams ( see Figure 1A ). In its binding portion, each stitching yarn passes below one bottom MD yarn in the repeat unit such that an "over 4/under 1" pattern is established by the pair of stitching yarns on the bottom surface of the fabric 20 ( see Figure 1B). The weaving pattern of the stitching yarns is exemplified in Figure 2D , which illustrates stitching yarns 64a, 64b interweaving with top and bottom MD yarns. In its fiber support portion, stitching yarn 64a passes over top MD yarns 21, 23 and 25, and below top MD yarns 22 and 24. It then passes below transitional top MD yam 26 and above bottom MD yarn 46. In its binding portion, stitching yarn 64a passes below top MD yarns 27 through 29 while passing above bottom MD yarns 47 and 49 and below bottom MD yarn 48 to stitch the bottom layer of the fabric 20 . Stitching yarn 64a then passes between top transitional MD yarn 30 and bottom MD yarn 50 . Figure 2D also illustrates that stitching yarn 64b is interwoven such that its binding portion is below that of stitching yarn 64a ; stitching yarn 64b passes below top MD yarns 21 through 25 while passing above bottom MD yarns 41, 42, 44, 45 and below bottom MD yarn 43 . In its fiber support portion, stitching yarn 64b passes above top MD yam 27, below top MD yarn 28 and above top MD yarn 29. As a result, the fiber support portions of stitching yarns 64a, 64b combine to form the "over 1/under 1" pattern of a plain weave on the top layer, and the binding portions of stitching yams 64a, 64b combine to form the "over 4/under 1" pattern described above. As can be seen in Figures 2A through 2C and Figures 2E through 2J (which depict the interweaving patterns of the other stitching yarn pairs with the top and bottom MD yarns), the same pattern described hereinabove for the stitching yarns 64a, 64b relative to each other is followed by the other stitching yarn pairs. Referring back to Figures 1A and 1B , pairs of stitching yarns that are positioned adjacent to and on opposite sides of a top or bottom CMD yarn are interwoven with the top or bottom MD yams such that there is an offset of two MD yams between such stitching yarn pairs. For example, stitching yarn 61a passes above top MD yarns 25, 27 and 29 and below bottom MD yarn 42 . Stitching yarn 62a passes above top MD yarns 27, 29 and 21 (with top MD yarn 21 being a continuation of the pattern on the opposite side) and below bottom MD yarn 44 . Thus, stitching yarn 61a is offset from stitching yam 62a by two top and bottom MD yarns. This same two MD yarn offset is followed for the interweaving of the other stitching yarns. It can also be seen in Figures 1A and 1B that the stitching yarns are interwoven with the top and bottom MD yarns as "reversed picks." This term can be understood by examination of stitching yarn pairs 61a, 61b, 62a, 62b, 63a, 63b. As shown in Figures 1A and 2A, stitching yarn 61a is positioned nearer to top CMD yam 32 than is stitching yarn 61b . As seen in Figures 1A and 2B, on the other side of top CMD yarn 32, stitching yarn 62a is positioned nearer to top CMD yarn 32 than is stitching yarn 62b. As a result, the fiber support portions of stitching yarns 61a , 62a are positioned nearer to top CMD yarn 32 than are the fiber support portions of stitching yams 61b, 62b. This relative proximity to the top CMD yarn between adjacent pairs of stitching yarns is reversed with stitching yarn pairs 62a, 62b and 63a, 63b. As seen in Figures 1A, 2B, and 2C, stitching yarns 62b and 63b are positioned nearer top CMD yarn 33 than stitching yarns 62a, 63a , with the result again that the fiber support portions of the nearer stitching yams are also positioned nearer to top CMD yarn 33 . It has been discovered that this "reversed picks" configuration is particularly effective in masking the presence of stitching yarns in the top surface of the fabric. When a transitional yarn passes over the stitching yarns of a pair to form a top surface knuckle, that knuckle tends to receive less upwardly-directed support from the stitching yarns at that location than other locations on the top MD yarn where it passes over a stitching yarn or top CMD yarn. As a result, that knuckle tends to be positioned slightly lower than the other top MD knuckles. As seen in Figure 1A , the top MD knuckles of transitional yarns form a diagonal line; because the knuckles of this diagonal may all be positioned somewhat lower than the remaining top MD knuckles, paper formed on such a fabric can show this pattern, which can in turn affect images printed thereon. By including the stitching yarns as reversed picks, such as is illustrated in fabric 20, however, the diagonal formed by the transitional top MD knuckles is disturbed somewhat and is less distinctly defined. As such, paper formed on fabric 20 has a less distinct diagonal pattern due to these knuckles, and printing on the paper is improved. Those skilled in this art will appreciate that the afore-described "reverse picks" configuration is created in the fabric by weaving the stitching yams into the top and bottom MD yarns so that first an "a" stitching yam immediately follows the weaving of top and bottom CMD yarns (followed by a "b" stitching yarn), then a "b" stitching yarn immediately follows the next set of top and bottom CMD yarns (followed by an "a" stitching yarn). This pattern can be repeated throughout weaving. Although it is preferred that all of the stitching yarn pairs follow this pattern ( i.e. , that 50 percent of the stitching yam pairs be "reversed"), some benefit can be obtained by reversing only a smaller percentage (for example 25, 33, or 40 percent) of the stitching yam pairs. Those skilled in this art will recognize that, although the plain weave fabric illustrated and described in detail.herein is preferred, other fabric weaves, such as other twill weaves and satins, that employ pairs of stitching yams integrated into the papermaking swrface of a fabric with the top CMD yams can also be made. In addition, although the illustrated fabrics have equal numbers of top and bottom MD and CMD yams, this need not be the case for the present invention; other ratios, such as two top CMD yarns for each bottom CMD yarn, can also be employed. The configurations of the individual yarns utilized in the fabrics of the present invention can vary, depending upon the desired properties of the final papermakers' fabric. For example, the yarns may be multifilament yarns, monofilament yams, twisted multifilament or monofilament yarns, spun yams, or any combination thereof. Also, the materials comprising yarns employed in the fabric of the present invention may be those commonly used in papermakers' fabric. For example, the yarns may be formed of cotton, wool, polypropylene, polyester, aramid, nylon, or the like. The skilled artisan should select a yarn material according to the particular application of the final fabric. Regarding yarn dimensions, the particular size of the yarns is typically governed by the size and spacing of the papermaking surface. Generally, the diameter of the top CMD yarns is about 25 to 75 percent of the diameter of the bottom CMD yarns, and the diameter of the top MD yarns is about equal to or smaller than the diameter of the top CMD yarns. In a typical fabric, the diameter of the top CMD yarns is between about 0.1 and 0.17mm, the diameter of the top MD yarns is between about 0.11 and 0.15mm, the diameter of the bottom CMD yarns is between about 0.20 and 0.40mm, and the diameter of the bottom MD yarns is between about 0.17 and 0.25mm. The diameter of the stitching yarns is typically between about 0.11 and 0.17mm. Yarns may also vary advantageously in modulus of elasticity. For example, stitching yarns that interweave with a fewer number of top MD yarns than its paired stitching yarn (such as the "b" yarns of fabric 20) may have a higher modulus of elasticity (typically between about 10 and 50 percent higher) than its paired stitching yarn. As the foregoing discussion demonstrates, the fabrics of the present invention address problems encountered with prior art triple layer forming fabrics. The fabrics of the present invention integrate the stitching yams into the top surface of the fabric, whether it be a plain weave, a twill, a satin, or other pattern, and therefore avoid the marring of the papermaking surface that can accompany stitching yams that comprise less of the papermaking surface. The integration of the fabric attributable to the stitching yarns also greatly reduces (if not eliminating) interlayer wear. In addition, because the stitching yams comprise such a large portion of the papermaking surface, the differences in tension between the top CMD yams and the stitching yarns that can distort the papermaking surfaces of other fabric are less critical to the fabrics of the present invention. The density of the stitching yarns also provides a tighter and more reliable binding of the top and bottom layers of the fabric, which can provide the designer with a wider variety of yarn choices to balance paper forming properties, durability and wear.

Claims: A papermaker's fabric (20), comprising top machine direction yams (21-30), top cross machine direction yarns (31-40), bottom machine direction yams (41-50), bottom cross machine direction yams (51-60), and sets of first and second stitching yams (61a-70a and 61b-70b), said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yams; a set of top cross machine direction yarns interwoven with said set of top machine direction yams; a set of bottom machine direction yarns; a set of bottom cross machine direction yams interwoven with said set of bottom machine direction yarns; wherein pairs of first and second stitching yarns are positioned between pairs of adjacent top cross machine direction yarns, said first and second stitching yarns of each pair being interwoven with said top and bottom machine direction yarns such that, as a fiber support portion of said first stitching yam is interweaving with said top machine direction yarns, a binding portion of said second stitching yam is positioned below said top machine direction yams, and such that as a fiber support portion of said second stitching yarn is interweaving with said top machine direction yams, a binding portion of said first stitching yam is positioned below said top machine direction yams, and such that said first and second stitching yarns cross each other as they pass below a transitional top machine direction yam, and such that each of said binding portions of said first and second stitching yarns passes below at least one of said bottom machine direction yarns; each of said first stitching yarns passing over a first number of top machine direction yarns, and each of said second stitching yams passing over a second number of top machine direction yarns; characterized in that a pair of first and second stitching yams is positioned between each pair of adjacent top cross machine direction yams; said first number is different from said second number; and at least some of the top cross machine direction yams are immediately adjacent to either two first stitching yams or two second stitching yarns. The papermaker's fabric defined in Claim 1, wherein, beginning with a selected top cross machine direction yam and moving in one direction parallel to the machine direction for the length of the full repeat unit, between 25 and 50 percent of the first stitching yarns are nearer to said selected top cross machine direction yarn than the second stitching yams of their respective pairs. The papermaker's fabric defined in Claim 2, wherein between 40 and 50 percent of the first stitching yams are nearer to said selected top machine direction yam than the second stitching yams of their respective pairs. The papermaker's fabric defined in Claim 1, wherein each of the top cross machine direction yarns positioned between adjacent pairs at stitching yarns is immediately adjacent to either two first stitching yarns or two second stitching yarns. The papermaker's fabric defined in Claim 1, wherein said repeat unit includes 10 top machine direction yarns and 10 bottom machine direction yams. A method of making paper, said method comprising the steps of: (a) providing a papermaker's fabric according to one or more of the claims 1 to 5; (b) applying paper stock to said papermaker's fabric; and (c) removing moisture from said paper stock.

Inventors: WARD KEVIN J, WARD, KEVIN, J.
Applicants: WEAVEXX CORP, WEAVEXX CORPORATION
Application year: 1997
Main classification: D21F1
IPC codes: D21F1