MULTI-LAYER FORMING FABRIC WITH STITCHING YARN PAIRS INTEGRATED INTO PAPERMAKING SURFACE

Abstract: A multi-layer papermaker's forming fabric has stitching yarns integrated into the papermaking surface. Each of a plurality of repeating units of the fabric comprises: a set of top machine direction yarns; a set of top cross-machine direction yarns interwoven with the top machine direction yarns; a set of bottom machine direction yarns; a set of bottom cross-machine direction yarns interwoven with the bottom machine direction yarns; and pairs of first and second stitching yarns. The stitching yarn pairs are positioned between pairs of top cross-machine direction yarns. The stitching yarns of each pair are interwoven with the top and bottom machine direction yarns such that, as a fiber support portion of the first stitching yarn is interweaving with the top machine direction yarns, a binding portion of the second stitching yarn is positioned below the top machine direction yarns, and such that as a fiber support portion of the second stitching yarn is interweaving with the top machine direction yarns, a binding portion of the first stitching yarn is positioned below the top machine direction yarns. The first and second stitching yarns cross each other as they pass below a transitional top machine direction yarn. Also, each of the binding portions of the first and second stitching yarns passes below at least one of the bottom machine direction yarns. In this configuration, the stitching yarns are completely integrated into the top, or papermaking, surface of the fabric, and therefore do not adversely impact the papermaking qualities of the fabric.

Description: MULTI-LAYER FORMING FABRIC WITH STITCHING YARN PAIRS INTEGRATED INTO PAPERMAKING SURFACE 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 ( 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 yams 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 yams. As used herein, "coplanar" means that the upper extremities of the yams 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 yams on the paper forming surface to facilitate paper formation and coarser-mesh yams 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 yams 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 yams. In a typical triple layer forming fabric, one or more stitching yams are positioned between some of the CMD yams of the top and bottom layers and interwoven with the top and bottom MD yams. In such a construction, portions of the stitching yams 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 yams. 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 - 4 - 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 yams of a triple layer fabric have a different weave pattern than the top CMD yams (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 yams and the top CMD yams. 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. 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-layer forming fabric that addresses the problem of interlayer wear. These and other objects are satisfied by the present invention, which relates to a multi-layer papermaker's forming fabric with stitching yams integrated into the papermaking surface. The fabric is formed as a plurality of repeating units. Each of the repeating units comprises: a set of top machine direction yarns; a set of top cross- machine direction yams interwoven with the set of top machine direction yams; a set of bottom machine direction yams; a set of bottom cross-machine direction yams interwoven with the set of bottom machine direction yams; and pairs of first and second stitching yams. The stitching yam pairs are positioned between pairs of top - 5 - cross-machine direction yams. The stitching yams of each pair are interwoven with the top and bottom machine direction yams such that, as a fiber support portion of the first stitching yam is interweaving with the top machine direction yams, a binding portion of the second stitching yam is positioned below the top machine direction yams, and such that as a fiber support portion of the second stitching yarn is interweaving with the top machine direction yams, a binding portion of the first stitching yarn is positioned below the top machine direction yams. The first and second stitching yams cross each other as they pass below a transitional top machine direction yarn. Also, each of the binding portions of the first and second stitching ya s passes below at least one of the bottom machine direction yams. In this configuration, the stitching yams are completely integrated into the top, or papermaking, surface of the fabric, and therefore do not adversely impact the papermaking qualities of the fabric. Also, the relatively large number of stitching yarns provides reliable binding of the top and bottom layers of the fabric. In two illustrated embodiments of the fabric, the stitching yams are interwoven with the top MD and CMD yams so that they form a plain weave papermaking surface. In this embodiment, it is preferred that the stitching yams be interwoven as "reverse picks" (this term is defined in detail hereinbelow). In another illustrated embodiment, the stitching yams are interwoven with the top MD and CMD yams to form a 1x2 twill top surface. In these embodiments, the integration of the stitching yams into the papermaking surface of the fabric addresses many of the problems associated with prior art triple layer fabrics, such as distortion of the papermaking surface and inadequate binding of the top and bottom layers. Brief Description of the Figures Figure 1 A 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 IB is a plan view of the bottom layer of the fabric of Figure 1. Figures 2 A through 2J are section views of the stitching yams of the fabric of Figures 1A and IB. Figure 3 A is a top view of an embodiment of a 24 harness multi-layer forming fabric of the present invention having a plain weave top surface. - 6 - Figure 3B is a plan view of the bottom layer of the fabric of Figure 3 A. Figures 4 A through 4L are section views of the stitching yams of the fabric of Figures 3A and 3B. Figure 5 A is a top view of a 24 harness multi-layer forming fabric of the present invention having a 1x2 twill top surface. Figure 5B is a plan view of the bottom layer of the fabric of Figure 5A. Figures 6A through 6F are section views of the stitching yams of the fabric of Figures 5A and 5B. Detailed Description of the Preferred Emhodiments 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 embodiments; rather, these embodiments are 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 IB, in which a single repeat unit of the fabric is shown. As seen in Figure 1 A, the repeat unit of the fabric 20 includes a top layer having ten top MD yams 21-30 and ten top CMD yams 31-40. These are interwoven such that each top CMD yam passes over and beneath top MD yams in an alternating fashion, with each top CMD yam passing over and under the same top MD yams. For example, top CMD yam 31 passes under top MD yam 21, over top MD yam 22, under top MD yam 23, over top MD yam 24 and so on until it passes over top MD yam 30. Similarly, top CMD yam 32 passes under top MD yam 21, over top MD yam 22, under top MD yam 23, over top MD yam 24 and so on until it passes over top MD yam 30. Referring now to Figure IB, a repeatunit of the bottom layer of the fabric is shown. The repeat unit includes ten bottom MD yams 41-50 which are interwoven with ten bottom CMD yams 51-60. The bottom MD yams 41-50 are interwoven with the bottom CMD yams 51-60 in a 1x4 twill type pattern, with each bottom CMD yam passing above one bottom MD yam, below four bottom MD yams, above one bottom MD yarn, and below four bottom MD ya s. For example, bottom CMD yarn 51 passes above bottom MD yam 41, below bottom MD yams 42-45, above bottom MD yam 46, and below bottom MD yams 47 through 50. The other bottom CMD yams follow a similar "over 1/under 4" weave pattern, but each is offset from its nearest bottom CMD yam neighbors by two bottom MD yams. Consequently, bottom CMD yam 52 passes below bottom MD yams 41 and 42, above bottom MD yam 43, below bottom MD yam 44 through 47, above bottom MD yam 48, and below bottom MD yams 49 and 50. Thus the "knuckle" formed by bottom MD yam 43 as it passes below bottom CMD yam 52 is offset from the "knuckle" formed by bottom MD ya 41 as it passes over bottom CMD yam 51 by two bottom MD yarns. The top layer (formed by the top MD yams and the top CMD yams) and the bottom layer (formed by the bottom MD yams and the bottom CMD yams) are stitched together with twenty stitching yams, designated herein as pairs 61a, 61b through 70a, 70b. The stitching yams are positioned in pairs between adjacent CMD yams. For example, stitching yams 61a and 61b are positioned between top CMD yams 31 and 32 and between bottom CMD yams 51 and 52. The stitching yams interweave with the top MD yams and bottom MD yams to bind the top and bottom fabric layers together. As can be seen in Figures 2A through 2J, corresponding pairs of stitching yams interweave with the top MD yams and bottom MD yams in the following pattern. Each of the stitching yams of the repeat unit can be subdivided into two portions: a fiber support portion which interweaves with the top MD yams, and a binding portion which interweaves with a bottom MD yam. These are separated at "transitional" top MD yams, below which one stitching yam of a pair crosses the other stitching yam of the pair. The stitching yams of each pair are interwoven relative to one another such that the fiber support portion of one yam of the pair is positioned above the binding portion of the other yam of the pair. The fiber support portion of the stitching yam of each pair designated with an "a" (e.g.. 61a, 62a, 63a) interweaves in an alternating fashion with five top MD yams (alternately passing over three top MD yams and under two top MD yams), and the other stitching yam of the pair (those designated with a "b") passes over two top MD yams while passing below a top MD yam positioned between those two MD yams. In its fiber support portion, each stitching yam passes over top MD yams that the top CMD yams pass beneath, and passes below top MD yams that each top CMD yarn passes over. In this manner, the stitching yams and top CMD form a plain weave pattern with the top MD yams (see Figure 1A). In its binding portion, each stitching yam passes below one bottom MD yam in the repeat unit such that an "over 4/under 1" pattern is established by the pair of stitching yams on the bottom surface of the fabric 20 (see Figure IB). The weaving pattern of the stitching yams is exemplified in Figure 2D, which illustrates stitching yams 64a, 64b interweaving with top and bottom MD yams. In its fiber support portion, stitching yam 64a passes over top MD yams 21, 23 and 25, and below top MD yams 22 and 24. It then passes below transitional top MD yam 26 and above bottom MD yam 46. In its binding portion, stitching yam 64a passes below top MD yams 27 through 29 while passing above bottom MD yams 47 and 49 and below bottom MD yam 48 to stitch the bottom layer of the fabric 20. Stitching yam 64a then passes between top transitional MD yam 30 and bottom MD yam 50. Figure 2D also illustrates that stitching yarn 64b is interwoven such that its binding portion is below that of stitching yam 64a; stitching yam 64b passes below top MD yams 21 through 25 while passing above bottom MD yams 41, 42, 44, 45 and below bottom MD yam 43. In its fiber support portion, stitching yam 64b passes above top MD yam 27, below top MD yam 28 and above top MD yarn 29. As a result, the fiber support portions of stitching yams 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 yam pairs with the top and bottom MD yams), the same pattern described hereinabove for the stitching yams 64a, 64b relative to each other is followed by the other stitching yam pairs. Referring back to Figures 1A and IB, pairs of stitching yams that are positioned adjacent to and on opposite sides of a top or bottom CMD yam are interwoven with the top or bottom MD yams such that there is an offset of two MD yams between such stitching yam pairs. For example, stitching yam 61a passes above top MD yams 25, 27 and 29 and below bottom MD yam 42. Stitching yam 62a passes above top MD yams 27, 29 and 21 (with top MD yam 21 being a continuation of the pattern on the opposite side) and below bottom MD yam 44. Thus, stitching yam 61a is offset from stitching yam 62a by two top and bottom MD yams. This same two MD yam offset is followed for the interweaving of the other stitching yams. It can also be seen in Figures 1 A and IB that the stitching yarns are interwoven with the top and bottom MD yams as "reversed picks." This term can be understood by examination of stitching yam pairs 61a, 61b, 62a, 62b, 63a, 63b. As shown in Figures 1A and 2 A, stitching yam 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 yam 32, stitching yam 62a is positioned nearer to top CMD yam 32 than is stitching yam 62b. As a result, the fiber support portions of stitching yams 61a, 62a are positioned nearer to top CMD yam 32 than are the fiber support portions of stitching yams 61b, 62b. This relative proximity to the top CMD yam between adjacent pairs of stitching yams is reversed with stitching yam pairs 62a, 62b and 63a, 63b. As seen in Figures 1A, 2B, and 2C, stitching yams 62b and 63b are positioned nearer top CMD yam 33 than stitching yams 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 yams in the top surface of the fabric. When a transitional yam passes over the stitching yams of a pair to form a top surface knuckle, that knuckle tends to receive less upwardly-directed support from the stitching yams at that location than other locations on the top MD yam where it passes over a stitching yam or top CMD yam. As a result, that knuckle tends to be positioned slightly lower than the other top MD knuckles. As seen in Figure 1 A, the top MD knuckles of transitional yams 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 rum affect images printed thereon. By including the stitching yams 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 - 10 - formed on fabric 20 has a less distinct diagonal pattem 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 yams (followed by a "b" stitching yam), then a "b" stitching yam immediately follows the next set of top and bottom CMD yams (followed by an "a" stitching yam). This pattem can be repeated throughout weaving. Although it is preferred that all of the stitching yam pairs follow this pattem (j^e ^ , that 50 percent of the stitching yarn 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. Another embodiment of the present invention is illustrated in Figures 3A, 3B and 4A through 4L, wherein a repeat unit of a 24 harness multi-layer forming fabric designated broadly at 100 is shown. The fabric 100 comprises top machine direction yams 101 through 112, top CMD yams 121 through 132, bottom MD yams 141 through 152, bottom CMD yams 161 through 172, and stitching yams 181a, 181b through 192a, 192b. One pair of stitching yams is positioned between each pair of top CMD yams and each pair of bottom CMD yams. Like the fabric 20, the top MD and CMD yams of the fabric 100 are interwoven such that each top CMD yam passes over and under alternate MD yams, and so that every CMD yam passes over and under the same MD yams. These, in combination with the stitching yam pairs, form a top papermaking surface that has a plain weave pattem (Figure 3A). The bottom MD and CMD yams are interwoven so that each bottom CMD yam follows an "over 1/under 5" pattem relative to the bottom MD yarns, and so that the knuckles formed by the bottom MD yarns take a "broken twill" pattem, in which the knuckles formed under adjacent CMD yams are first offset by two MD yams in one direction, then by three MD yams in the opposite direction. Thus, the knuckles form a zig-zag diagonal pattem (see Figure 3B). Each of the stitching yams of the fabric 100 has a fiber support portion, which interweaves with the top MD yams, and a binding portion, which stitches the bottom layer of the fabric. As in the fabric 20, these portions of the stitching yams are separated at transitional top MD yams, under which both stitching yams of a pair pass under and cross. The fiber support portion of each stitching yam is positioned above the binding portion of the other stitching yam of its pair. Each of the stitching yams of the fabric 100 follows the same weave pattem in its fiber support portion as it interweaves with the top MD yams, with each stitching yam passing over three top MD yams and under two top MD yarns in an alternating fashion. The stitching yams pass over the top MD yams passed under by the top CMD yams, then pass over the top MD yams passed under by the top CMD yams, with the result that the top layer of the fabric 100 has a plain weave surface. Pairs of stitching yarns are interwoven with the top MD yams such that each group of four adjacent stitching yarn pairs falls within a pattem in which the fiber support portions of three of the four pairs of stitching yams are not offset from one another in the MD direction at all; Le ^ , the fiber support portions of each pass over the same top MD yarns. The fiber support portion of fourth pair of stitching yams of the group is offset from the others within the group by two top MD yams. For the fiber support portions of the next group of four yarn pairs, the entire group is offset by two top MD yams in the direction opposite of the offset of the individual stitching yam pair. As an example of this pattem, the stitching yams 188a, 189a, 190a, and 191a form a group of four stitching yams in adjacent stitching yam pairs. Of these, stitching yams 188a, 190a, and 191a pass over top MD yams 105, 107, and 109. Stitching yam 189a passes over top MD yams 107, 109, and 111, which represents a two MD yam offset. The next group of four stitching yam pairs would then begin with stitching yam 191a, which passes over top MD yams 103, 105 and 107; this represents a two top MD yam offset in the direction opposite that of the offset of stitching yam 189a. This pattem continues for each group of four stitching yam pairs. In its binding portion, each stitching yam passes below five top MD yams and above four bottom MD yams while passing below one bottom MD yam to stitch the top and bottom layers together. The bottom MD yam stitched by the stitching yam binding portion follows one of three different patterns; it is either the second, third or fourth bottom MD yam reached by the stitching yam after passing below a transitional top MD yam. For example, stitching yam 182a passes below bottom MD yam 144, the second bottom MD yam it approaches after passing below transitional top MD yarn 102. In contrast, stitching yam 181a passes below bottom MD yam 147, the third bottom MD yam it approaches after passing below transitional top yarn 104, and stitching yam 183a passes below bottom MD yam 146, the fourth bottom MD yam it approaches after passing below transitional top MD yam 102. Notably, the stitching yams of each pair follow the same weave pattem in their binding portions as the other stitching yam of that pair (i.e.. like stitching yam 183a, stitching yam 183b also stitches the fourth bottom MD yam it approaches after passing below a transitional top MD yam). Also, it can be seen from Figures 4A through 4L that the stitching yam pairs follow a pattem in which the stitching yams of the first pair stitch the third bottom MD yam they approach, the stitching yams of the second pair stitch the second bottom MD yam they approach, the stitching yams of the third pair stitch the fourth bottom MD yam they approach, and the stitching yams of the fourth pair stitch the third bottom MD yam they approach. This "third/second/fourth/third" pattem is repeated three times within the repeat unit. Like the stitching yams of the fabric 20, the stitching yams of the fabric 100 are also interwoven as "reverse picks." The "reverse picks" nature of the fabric can be see in Figure 3B, where a zig-zagging line indicates the broken twill pattem of knuckles formed on the bottom layer by the bottom MD yams and the stitching yams. This line indicates locations where a bottom side knuckle formed by a bottom MD yam is sandwiched between two stitching yam knuckles, each of which is offset from the bottom MD knuckle by one bottom MD yam. Following this pattem, it can be seen that stitching yams of adjacent pairs within the pattem are both nearer to the bottom CMD yam they flank than are their paired stitching yams. For those stitching yam knuckles on the diagonal line between which there is no bottom MD knuckle, their stitching yams are farther from the bottom CMD yam they flank than are their paired stitching yams. Thus, the reversing of the stitching yams in this embodiment can be identified by the weave pattem in the bottom layer of the fabric 100. As with the fabric 20, reversing of the stitching yams in the fabric 100 disturbs any pattem formed by top transitional MD yam knuckles and, therefore, provides a fabric that produces a higher quality paper for printing. Those skilled in this art will also appreciate that other plain weave pattems in which the stitching yams are divided differently into fiber support portions and binding portions can be constructed. For example, the fabric can include a top layer in which each stitching yam of a pair passes over two or four top MD yarns in its fiber support portion. As illustrated, the stitching yams can pass over different numbers of top MD yams, or can pass over the same number. Of course, appropriate adjustment of the positioning of the bottom knuckles in the binding portions of such stitching yams should be made with changes to the stitching yam pattem on the top surface. Another embodiment of a multi-layer forming fabric of the present invention is illustrated in Figures 5A, 5B and 6A through 6F, in which a repeat unit of a broken twill multi-layer forming fabric, designated broadly at 200, is illustrated. The repeat unit includes 12 top MD yams 201 through 212, 6 top CMD yams 221 through 226, 12 bottom MD yams 241 through 252, 6 bottom CMD yams 261 through 266, and 12 stitching yams 281a, 281b through 286a, 286b. As shown in Figure 5A, the top surface of the fabric 200 has a 1x2 twill pattem formed by the top MD yams, the top CMD yams and the fiber support portions of the stitching yams. More specifically, each top CMD yarn interweaves with the top MD yams in an "over 2/under 1" pattem; this is demonstrated by top CMD yam 221, which passes over top MD yams 201 and 202, under top MD yam 203, over top MD yams 204, 205, under top MD yam 206, over top MD yams 207, 208, under top MD yam 209, over top MD yams 210, 211, and under top MD yam 212. The remaining top CMD yams follow the same "over 2/under 1" pattem, but are laterally offset from their adjacent CMD yams by two MD yams. For example, top CMD yam 222 passes over top MD yam 201, under top MD yam 202, over top MD yam 203 and 204, and under top MD yam 205 before continuing in an over 2/under 1 pattem. Thus, the "over 2" portion of top CMD yam 222 is first seen as it passes over top MD ya s 203 and 204, which are offset from the top MD yams 201, 202 passed over by top CMD yam 221 by two MD yams. Referring now to Figure 5B, the machine side surface of the fabric 200 formed by the bottom MD and cross MD yams takes the pattem of a "broken twill." Each bottom CMD yam has an "under 5/over 1" repeat pattem with the bottom MD yams. For example, bottom CMD yam 261 passes over bottom MD yam 241, under bottom MD yams 242 through 246, over bottom MD yam 247, and under bottom MD yarns 248 through 252. This "under 5/over 1" pattem is repeated by the remaining CMD yams. However, the bottom side knuckles formed by the bottom MD yams as they pass below the bottom CMD yams are arranged in a broken twill pattem, with the bottom side knuckles being formed by bottom MD yams 241, 243, 245, 242, 246, 244 on bottom CMD yams 261 through 266, respectively, and by bottom yams 247, 249, 251, 248, 252, and 250 on bottom CMD yams 261 through 266 respectively. As can be seen in Figure 5B, these knuckles fail to form a clear diagonal as is characteristic of twill fabrics, but instead form a "broken twill" pattem. The top and bottom layers of the fabric 200 are bound together by the stitching yams listed above, each of which has both a fiber support portion and a binding portion. As with the fabrics 20 and 100 described earlier, the fiber support portion and binding portion of each stitching yam are divided by transitional top MD yarns below which stitching yams of a pair cross each other. The fiber support portion of each stitching yam follows an "over 2/under 1/over 2" pattem. In its binding portion, each stitching yam passes between the top and bottom MD yams with the exception of passing below one bottom MD yam to stitch the top and bottom layers together. The bottom MD yam that is stitched is located either two or three MD yams away from the transitional MD yams that separate the fiber support and binding portions of each stitching yam. This pattem is exemplified by stitching yam 285a, the stitching pattem of which is illustrated in Figure 6E. Stitching yam 285a passes over top MD yams 201 and 202, under top MD yam 203, and over top MD yams 204, 205 before passing below transitional top MD yam 206. In its binding portion, stitching yam 285a passes above bottom MD yams 247 and 248, below bottom MD yam 249 and above bottom MD yams 250, 251 before passing below transitional top MD yam 212 and above bottom MD yam 252. Pairs of stitching yams are interwoven with the top MD yams relative to one another such that their fiber support portions, the top MD yarns, and the top CMD yams form a 1x2 twill pattem. Referring again to Figure 6E, and as described above, stitching yam 285a passes above top MD yams 201, 202 under top MD yam 203, and over top MD yams 204, 205. Both stitching yams 285a, 285b pass below transitional top MD yam 206, after which the fiber support portion of stitching yam 285b continues the over 2/under 1 twill pattem first established by stitching yam 285a. In doing so, stitching yam 285b passes above top MD yams 207, 208, below top MD yam 209 and above top MD yams 210, 211 before passing below transitional top MD ya 212. Figure 5A demonstrates that the stitching yams are interwoven with the top and bottom MD yams relative to top CMD yams such that an "over 2" segment of each fiber support portion is offset by one MD yam from an "over 2" segment of the top CMD yams that flank that stitching yam. For example, the stitching yam 281a passes over top MD yams 202 and 203. The nearest top CMD yams, which are 221 and 222, pass over top MD yams 201, 202 and 203, 204 respectively. Thus, the distinctive diagonal of a twill is formed by the top CMD yams and the fiber support portions of the stitching yams. Figure 5B also illustrates how the stitching yams are stitched into the bottom MD yams. It can be seen in Figure 5B that the knuckle formed by each stitching yam as it passes below a bottom MD yam is positioned such that, in one direction, two bottom CMD ya s reside between the stitching yam knuckle and the knuckle formed by that bottom MD yam over a bottom CMD yam, and in the opposite direction, three bottom CMD yams reside between the stitching yam knuckle and the next knuckle formed by that bottom MD yam over a CMD yam. For example, stitching yam 284a forms a knuckle as it passes under bottom MD yam 241. The bottom MD yam 241 forms a knuckle as it passes under bottom CMD yam 261, which is separated from the knuckle formed by stitching yam 284a by three bottom CMD yams (262, 263, 264). Continuing with the pattem in the other direction, bottom CMD yams 265 and 266 are positioned between the knuckle formed by stitching yam 284a and the knuckle that would be formed by bottom MD yam 241 under the next bottom CMD yam after bottom CMD yam 266 (which would have the same weave pattem as bottom CMD yam 261). Thus, the stitching yam knuckle of stitching yam 284a is separated from bottom MD yam knuckles by three bottom CMD yams in one direction and by two CMD yams in the other direction. Those skilled in this art will appreciate that fabrics of the present invention can be constmcted with other twill pattems in the top layer. For example, a fabric can have a 1x3 or 1x4 twill top layer. Any of these twill pattems can be a conventional twill, such as that of the fabric 100, or can take a broken twill pattem, such as those embodied in 4 or 5 harness satin single layer fabrics. Fabrics can also be constructed in which fiber support portions of stitching yam pairs pass over different numbers of top MD yams. In each instance, the skilled artisan should understand the appropriate modifications to the binding portions of the stitching yams to accommodate differences in the fiber support portions. Those skilled in this art will recognize that, although the plain weave and twill fabrics illustrated and described in detail.herein are preferred, other fabric weaves, such as other twill weaves and satins, that employ pairs of stitching yams integrated into the papermaking surface of a fabric with the top CMD ya s can also be made. Also, any number of configurations of the bottom layer in which stitching yam pairs stitch the bottom MD yams can be used. It is also contemplated that, rather than including a pair of stitching yams between each pair of top CMD yams, a fabric in which a pair of stitching yarns is included between every other pair of top CMD yams can be constructed. 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 yams for each bottom CMD yam, can also be employed. The configurations of the individual yams utilized in the fabrics of the present invention can vary, depending upon the desired properties of the final papermakers' fabric. For example, the yams may be multifilament yams, monofilament yarns, twisted multifilament or onofilament yams, spun yams, or any combination thereof. Also, the materials comprising yams employed in the fabric of the present invention may be those commonly used in papermakers' fabric. For example, the yams may be formed of cotton, wool, polypropylene, polyester, aramid, nylon, or the like. The skilled artisan should select a yam material according to the particular application of the final fabric. Regarding yam dimensions, the particular size of the yams is typically governed by the size and spacing of the papermaking surface. Generally, the diameter of the top CMD yams is about 25 to 75 percent of the diameter of the bottom CMD yams, and the diameter of the top MD yams is about equal to or smaller than the diameter of the top CMD yams. In a typical fabric, the diameter of the top CMD yams is between about 0.11 and 0.17mm, the diameter of the top MD yams is between about 0.11 and 0.15mm, the diameter of the bottom CMD yams is between about 0.20 and 0.40mm, and the diameter of the bottom MD yams is between about 0.17 and 0.25mm. The diameter of the stitching yams is typically between about 0.11 and 0.17mm. Yarns may also vary advantageously in modulus of elasticity. For example, stitching yams that interweave with a fewer number of top MD yams than its paired stitching yam (such as the "b" yams of fabric 20) may have a higher modulus of elasticity (typically between about 10 and 50 percent higher) than its paired stitching yam. 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 pattem, 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 yams 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 yams that can distort the papermaking surfaces of other fabric are less critical to the fabrics of the present invention. The density of the stitching yams 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 yam choices to balance paper forming properties, durability and wear. The foregoing embodiments are illustrative of the present invention, and are not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims: THAT WHICH IS CLAIMED: 1. A papermaker's fabric, comprising top machine direction yarns, top cross machine direction yams, bottom machine direction yams, bottom cross machine direction yams, and sets of first and second stitching yams, 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 yarns; a set of bottom machine direction yams; a set of bottom cross machine direction yams interwoven with said set of bottom machine direction yams; wherein a pair of first and second stitching yams is positioned between each adjacent pair of top cross machine direction yams, 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 yam 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 yams, and such that said first and second stitching yams 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. 2. The papermaker's fabric defined in Claim 1, wherein each of said top cross machine direction yams passes over a first group of alternate top machine direction yams and under a second group of alternate top machine direction yams, with each of said yams of said first group being positioned between a pair of yams of said second group, and with each of said yams of said second group being positioned between a pair of yams of said first group, and wherein said fiber support portions of said first and second stitching yams pass under said first set of altemate top machine direction yams and over said second set of altemate top machine direction yams, such that said top machine direction yams, said top cross machine direction yams, and said fiber support portions of said first and second stitching yams form a plain weave pattem. 3. The papermaker's fabric defined in Claim 2, wherein said fiber support portions of said first stitching yams pass over a first number of said machine direction yams, said fiber support portions of said second stitching yams pass over a second number of said machine direction yarns, and said first number differs from said second number. 4. The papermaker's fabric defined in Claim 2, wherein said fiber support portions of said first stitching yams pass over a first number of said machine direction yams, said fiber support portions of said second stitching yams pass over a second number of said machine direction yarns, and said first number is the same as said second number. 5. The papermaker's fabric defined in Claim 3, wherein said first number is larger than said second number, and wherein said second stitching yam has a higher modulus of elasticity than said first stitching yam. 6. The papermaker's fabric defined in Claim 3, wherein said first number is three, and second number is two. 7. The papermaker's fabric defined in Claim 1, wherein said first and second stitching yams are of a smaller diameter than said top machine direction yams. 8. The papermaker's fabric defined in Claim 1, wherein each of said binding portions of said first and second stitching yams passes beneath only one of said bottom machine direction yams. 9. The papermaker's fabric defined in Claim 1, wherein said repeat unit includes 10 top machine direction yams and 10 bottom machine direction yarns. 10. The papermaker's fabric defined in Claim 1, wherein said repeat unit includes 12 top machine direction yams and 12 bottom machine direction yams. 11. The papermaker's fabric defined in Claim 1 , wherein said top machine direction yams and said top cross machine direction yams are interwoven in a repeating pattem such that each of said top cross machine direction yams passes over a first pair of adjacent top machine direction yams, under a third top machine direction yam adjacent to said first pair, and over a second pair of top machine direction yams positioned adjacent said third top machine direction yam, and wherein adjacent top cross machine direction yams pass over top machine direction yarn pairs that are offset by two top machine direction yams, and wherein said stitching yams are interwoven with said top machine direction yams in a repeating pattem such that said fiber support portion of each of said first and second stitching yarns passes over a third pair of adjacent top machine direction yams, under an adjacent top machine direction yam, and over a fourth pair of adjacent machine direction yarns, then passes under said transitional top machine direction yam, said third and fourth pairs of top machine direction yams being offset from said first and second pairs by one machine direction yam, such that said top machine direction yams, said cross machine direction yams, and said fiber support portions of said first and second stitching yams form a 1x2 twill pattem. 12. The papermaker's fabric defined in Claim 1, wherein said stitching yarns are interwoven with said top and bottom machine direction yams such that adjacent pairs of stitching yams cross beneath transitional top machine direction yams that are offset by two top machine direction yams. 13. The papermaker's fabric defined in Claim 12, wherein between 25 and 50 percent of said first and second stitching yams are interwoven with said top machine direction yams as reversed picks. 14. A papermaker's fabric, comprising 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 yams, 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 yarns; a set of bottom machine direction yams; a set of bottom cross machine direction yams interwoven with said set of bottom machine direction yams; wherein pairs of first and second stitching yams are positioned between pairs of top cross machine direction yams, 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 yams, and such that said first and second stitching yams 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, and wherein between 25 and 50 percent of adjacent pairs of first and second stitching yams are interwoven as reversed picks. 15. The papermaker's fabric defined in Claim 14, wherein each of said top cross machine direction yams passes over a first group of altemate top machine direction yarns and under a second group of altemate top machine direction yams, with each of said yarns of said first group being positioned between a pair of yams of said second group, and with each of said yams of said second group being positioned between a pair of yams of said first group, and wherein a pair of first and second stitching yams is positioned between each pair of top cross machine direction yarns, and wherein said fiber support portions of said first and second stitching yams pass under said first set of altemate top machine direction yams and over said second set of altemate top machine direction yams, such that said top machine direction yams, said top cross machine direction yams, and said fiber support portions of said first and second stitching yams form a plain weave pattem. 16. The papermaker's fabric defined in Claim 15, wherein said fiber support portions of said first stitching yams pass over a first number of said machine direction yams, said fiber support portions of said second stitching yams pass over a second number of said machine direction yams, and said first number differs from said second number. 17. The papermaker's fabric defined in Claim 15, wherein said fiber support portions of said first stitching yams pass over a first number of said machine direction yarns, said fiber support portions of said second stitching yarns pass over a second number of said machine direction yams, and said first number is the same as said second number. 18. The papermaker's fabric defined in Claim 16, wherein said first number is larger than said second number, and wherein said second stitching yam has a higher modulus of elasticity than said first stitching yam. 19. The papermaker's fabric defined in Claim 16, wherein said first number is three, and second number is two. 20. The papermaker's fabric defined in Claim 14, wherein each of said binding portions of said first and second stitching yarns passes beneath only one of said bottom machine direction yams. 21. The papermaker's fabric defined in Claim 14, wherein said repeat unit includes 10 top machine direction yams and 10 bottom machine direction yams. 22. The papermaker's fabric defined in Claim 14, wherein said repeat unit includes 12 top machine direction yams and 12 bottom machine direction yarns. 23. A method of making paper, said method comprising the steps of: (a) providing a papermaker's fabric comprising top machine direction yarns, top cross machine direction yams, bottom machine direction yarns, bottom cross machine direction yams, and sets of first and second stitching yams, 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 a pair of first and second stitching yams is positioned between each adjacent pair of top cross machine direction yams, 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 yarns, a binding portion of said second stitching yarn is positioned below said top machine direction yams, and such that as a fiber support p μouirtuiuonii o υfi s aacuidu s aescυounudu yaumu i lsa i unntecriwwceavviinugg w wiithi s aaαiidu t iojpμ dd uiiirrneeccttiicon yams, a binding portion of said first stitching yam is positioned below said ttoopp machine direction yams, and such that said first and second stitching yyaarmnss ccrroosi,s.. e ~a~c ^ h. 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; (b) applying paper stock to said papermaker's fabric; and (c) removing moisture from said paper stock. 24. The method defined in Claim 23, wherein each of said top cross machine direction yams passes over a first group of altemate top machine direction yarns and under a second group of altemate top machine direction yarns, with each of said yams of said first group being positioned between a pair of yams of said second group, and with each of said yams of said second group being positioned between a pair of yams of said first group, and wherein said fiber support portions of said first and second stitching yams pass under said first set of altemate top machine direction yams and over said second set of altemate top machine direction yams, such that said top machine direction yams, said top cross machine direction yams, and said fiber support portions of said first and second stitching yams form a plain weave pattem. 25. The papermaker's fabric defined in Claim 23, wherein said top machine direction yams and said top cross machine direction yams are interwoven in a repeating pattem such that each of said top cross machine direction yams passes over a first pair of adjacent top machine direction yarns, under a third top machine direction yarn adjacent to said first pair, and over a second pair of top machine direction yams positioned adjacent said third top machine direction yam, and wherein adjacent top cross machine direction yams pass over top machine direction yarn pairs that are offset by two top machine direction yams, and wherein said stitching yams are interwoven with said top machine direction yams in a repeating pattem such that said fiber support portion of each of said first and second stitching yams passes over a third pair of adjacent top machine direction yams, under an adjacent top machine direction yam, and over a fourth pair of adjacent machine direction yams, then passes under said transitional top machine direction yam, said third and fourth pairs of top machine direction yams being offset from said first and second pairs by one machine direction yam, such that said top machine direction yams, said cross machine direction yarns, and said fiber support portions of said first and second stitching yams form a 1x2 twill pattem. 26. A method of making paper, said method comprising the steps of: (a) providing a papermaker's fabric comprising top machine direction ya s, top cross machine direction yams, bottom machine direction yams, bottom cross machine direction yams, and sets of first and second stitching yams, 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 top cross-machine direction yams, said first and second stitching yarns 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 yarn 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 yams 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, and wherein between 25 and 50 percent of adjacent pairs of first and second stitching yams are interwoven as reversed picks; (b) applying paper stock to said papermaker's fabric; and (c) removing moisture from said paper stock. 27. The method defined in Claim 26, wherein each of said top cross machine direction yams passes over a first group of altemate top machine direction yams and under a second group of altemate top machine direction yams, with each of said yams of said first group being positioned between a pair of yams of said second group, and with each of said yams of said second group being positioned between a pair of yarns of said first group, and wherein a pair of first and second stitching yams is positioned between each pair of top cross machine direction yams, and wherein said fiber support portions of said first and second stitching yams pass under said first set of altemate top machine direction yams and over said second set of altemate top machine direction yams, such that said top machine direction yams, said top cross machine direction yams, and said fiber support portions of said first and second stitching yams form a plain weave pattem.

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