WO2008073301A2

WO2008073301A2 - Machine side layer weave design for composite forming fabrics

Info

Publication number: WO2008073301A2
Application number: PCT/US2007/025083
Authority: WO
Inventors: Rex Barrett
Original assignee: Astenjohnson, Inc.
Priority date: 2006-12-08
Filing date: 2007-12-06
Publication date: 2008-06-19
Also published as: US20080169040A1; WO2008073301A3

Abstract

A machine side layer weave design for use in woven composite papermakers forming fabrics. The composite fabric has two sets of we yarns interwoven with at least one system of warp yams. In each repeat of the machine side layer weave design, at least 50% of the warp yarns pass under two adjacent weft yams on the machine side of the fabric to form a double warp knuckle, and each double war knuckle is bounded on either side by a single warp knuckle formed on each of the first and second weft passed under by the double warp knuckle. Further, in each repeat of the weave, each weft yarn is passed under by two adjacent warp yams to form two adjacent single warp knuckles. The knuckles are evenly distributed across the machine side surface. The machine side layer weave design is effective in minimizing or eliminating fabric edge curl.

Description

[0001] MACHINE SIDE LAYER WEAVE DESIGN

FOR COMPOSITE FORMING FABRICS

[0002] FIELD OF THE INVENTION

[0003] The present invention is directed to woven, composite forming fabrics for use in papermaking machines. It is particularly concerned with such fabrics wherein the machine side fabric weave helps to reduce edge curl. [0004] BACKGROUND

[0005] Composite forming fabrics, such as those described in US

5,826,627 (Seabrook et al.), WO 06/034576 (Danby et al) or US 7,108,020 (Stone) are comprised of two sets of weft yarns interwoven with at least one system of warp yarns. One of the sets of weft yarns is located on the paper side surface (PS) of the fabric to provide support for the papermaking fibers in the sheet forming process; these weft yarns tend to be small and closely woven, typically in a plain weave or 3-shed pattern so as to maximize the number of available support points for the papermaking fibers. The function of the PS weave is to optimize paper sheet formation by providing a very finely woven papermaking surface which will support the fibers suspended in the stock as it drains through the fabric. Consequently, the warp and weft yarns in the PS are generally of relatively small diameter, in the range of from about 0.10mm to about 0.25mm depending on the papermaking application for which the fabric is intended. Generally speaking, "better" quality sheets, which offer improved printing characteristics and uniformity, are formed on fabrics having a larger number of support points; such fabrics have a very high yarn density on the PS and are woven using relatively small yarns. The warp and weft yarns in the PS are thus interwoven a great number of times to provide the desired sheet support characteristics. For example, in known PS weave designs there can be as many as 100 warp yarns per inch (39.4 yarns per cm), or more. In such a woven structure, each weft yarn will pass over and under each of the warp yarns 100 times in each inch of fabric width. Typical warp yarn counts in these fabrics will usually range from about 60 to 100 yarns per inch interwoven with from about 50 to 150 weft yarns per inch, although higher and lower numbers of yarns are known and used. [0006] On their own, these finely woven PS weave designs are not sufficiently rugged or stable to withstand the stresses to which they will be exposed on modern high speed papeπnaking machines and could not be used for this purpose on their own without additional reinforcement. Therefore, a second set of weft yarns is located on the machine side surface (MS) of the fabric and is interwoven with the warp yarns according to a pattern which is intended to provide a measure of wear resistance and stability to the overall fabric. The function of the cooperating MS weave is to provide a rugged, stable platform upon which the PS weave is mounted to maximize fabric wear life and stability. The MS weft yarns are usually larger than those used in the PS, often by an order of magnitude of about 2, and are typically interwoven according to a weave design that maximizes the wear volume presented to the fabric bearing surfaces of the papermaking machine. This is usually accomplished by interweaving the MS warp and second set of weft yarns so that relatively long weft floats are formed. These yarns will generally not be interwoven with the same frequency as those in the PS because the MS weave in a composite forming fabric is a comparatively coarser structure. These MS layer weave designs are typically 4-, 5-, 6- or more shed patterns which cause the MS weft yarns to bow outwardly between the points at which they are interlaced by the warp yarns. Typical yarn counts used in the MS weave of a composite forming fabric will be in the range of from about 100 x 66 yarns/in (warp x weft) (39 x 26 yarns/cm) to about 50 x 25 yarns/in (20 x 10 yarns/cm). Composite forming fabrics in use today will generally be woven at 2:1 or 3:2 weft count ratios (PS:MS) so the number of PS weft will be from 1.5 to 2.0 times the number in the MS.

[0007] The warp and weft yarns are generally monofilaments formed from extruded thermoplastic polymers, with poly amides (PA) and polyethylene terephthalate (PET) being the preferred materials for such applications. [0008] In composite papeπnaker's fabrics, the PS weave and MS weave are woven simultaneously on the loom as a unified structure using either, or both, the warp and/or weft yarns of one or both weaves to tie the two woven structures together. The resulting fabric, known variously in the industry as a triple layer fabric, composite fabric, multilayer fabric, etc., will be referred to henceforth as a "composite" fabric. A composite fabric is one woven using at least two sets of weft yarns (arranged in two layers one over the other, with the individual yarns not necessarily in vertical alignment) and at least one system of warp yarns. For example, in the weave patterns described in WO 06/034576 (Danby et al) or US 7,108,020 (Stone), one set of warp yarns woven in pairs or triplets is used to interconnect the PS and MS weft into a unified fabric. Composite fabrics of this sort are referred to as "warp tie designs" meaning that the warp yarns are used to integrate the two sets of weft yarns together into the resulting fabric. In US 5,826,627 (Seabrook et al.), the disclosed fabric contains two sets of weft yarns and two sets of warp yarns, and it is a portion of the PS weft which are used to integrate the two sets of warp and weft yarns into a unified structure. Fabrics of this sort are referred to as "intrinsic weft tie designs", "weft tie designs" or "sheet support binder" (SSB) fabrics because it is the PS weft yarns, which provide support for the sheet as it is being formed and which are thus "intrinsic" to the PS, that are used to unite the two fabric structures into a unified whole. The present invention is applicable to any composite fabric including both warp tie designs and weft tie designs.

[0009] Although both warp tie and weft tie design forming fabrics provide various benefits to the papermaker, they share a common problem. During and following the manufacturing process, which involves interweaving the warp and weft yarns according to the chosen design using large industrial looms, the resulting fabrics tend to curl at their lateral or side edges. The curl is frequently up and out of the plane of the PS, and may involve from about 2 inches to 2 feet (5 to 60 cm) or more of each of the lateral edges of the fabric, causing the fabric to form a pronounced "U" shape. In extreme cases, the fabric may roll up like a tube when removed from the loom. This is a severe problem for the fabric manufacturer, as the fabric must be flattened in some manner before it is usable on the papeπnaking machine for which it is intended. While some fabric edge curl can be tolerated, it is highly desirable to provide a fabric which, when initially installed, provides a flat, generally planar paper forming surface and which will continue to do so over its life on the papermaking machine.

[0010] At present, manufacturers of forming fabrics for papermaking machines must stabilize (heat set) these textiles under conditions of high heat and tension so as to flatten the fabrics to the greatest degree possible. Even under conditions of extreme heat and tension, which conditions may push the physical limits of these fabrics to the maximum they can tolerate without significant degradation of their beneficial papermaking properties, it has not always been possible to completely eliminate fabric edge curl. Fabrics with edge curl are not generally desirable in the papermaking process, as the curl breaks the vacuum seal between the fabric and suction boxes during operation thus reducing their efficiency and providing inadequate sheet drainage. In addition, paper formed at the lateral edges where the curl is present will not be saleable and must be trimmed from the final product.

[0011] The phenomenon of fabric edge curl has been intensely studied by manufacturers of papermaking machine fabrics in an effort to determine its root causes and a method for their resolution. One cause may lie in the polymeric thermoplastic materials chosen for use in the monofilament yarns of the fabrics. For example, it is well known that polyamides are not dimensionally stable in the wet conditions found in the forming sections of papermaking machines and, when polyamide yarns are mixed in the fabric weave structure with yarns formed from e.g. polyesters (which are comparatively more stable in similar conditions), the different physical behaviors of the two types of yarn materials may contribute to any propensity for the fabric edges to curl. US 6,828,261 to Soelch et al. addresses this possibility by providing a yarn formed from two compatible polymers, one having a slightly lower melting point than the other. Tate et al. in US 5,324,392 discloses a similar method of attacking this problem. Bhatt et al. in US 5,502,120 and US 5,169,711 discloses monofilaments and forming fabrics woven therefrom which are comprised of a blend of thermoplastic polyurethane and a polyester. However, although these solutions have been somewhat effective, they have not on their own entirely eliminated the edge curl problem.

[0012] Another method of addressing the problem has been to abrasively surface, or score, the fabrics and component yarns along the lateral fabric edges to reduce their ability to curl (see for example US 4,941,239 to Fliss or US 5,546,643 to Hawes et al.). However, this solution has not been entirely satisfactory either and has contributed to premature wear along the fabric edges.

[0013] US 4,281,688 to Kelly et al. attempts to solve the problem of edge curl in a single layer forming fabric by providing a plurality of dominating floats on each fabric face. US 4,356,844 and US 4,453,573 both to Thompson, offer similar methods of solving this problem by modifying the fabric weave design without compromising papeπnaking properties. US 6,123,116 to Ward et al. addresses the problem by providing a fabric having a large number of relatively small CD yarns in the MS, rather than a small number of relatively large yarns. Barrett, US 5,544,678, discloses a composite forming fabric having an N x 2N machine side layer weave design in which N is the number of sheds in which the fabric is woven but the disclosed designs do not share features with the present invention. Other solutions have also been proposed. [0014] However, none of these proposals has provided a commercially successful solution that has been entirely satisfactory for the elimination of edge curl in composite forming fabrics. [0015] SUMMARY

[0016] The present invention is based on the understanding that edge curl in composite forming fabrics is due, at least in part, to the differing amount of crimp that is imparted to the relatively small diameter cross- machine direction (CD) oriented weft yarns located on the PS of the fabric as compared to the amount of crimp imparted to the relatively fewer and larger diameter weft yarns located on the MS of the fabric. As used herein, the term "crimp" is defined as the bend in the yarns formed by the interweaving of each warp and weft yarn. In a typical composite fabric, which will have a weft ratio (number of PS weft : number of MS weft) of from 3:2 to 2:1, the number of PS interweaves is greater than the number of MS interweaves (by a magnitude of from about 1.5 to 2) due to the greater number of weft involved. It is believed that the greater amount of crimps in the PS weft yarns as compared to the MS weft yarns causes the lateral edges of a forming fabric to curl upwards. In order to balance or counteract this effect, it is believed that the MS weft yarns must be crimped to a greater degree, which is accomplished according to the present invention.

[0017] The present invention is directed to a machine side layer weave design for use in a composite forming fabric. The MS weave design is constructed and arranged so as to counteract the edge curl effects induced in the overall fabric by the crimp imparted to the yarns in the paper side layer weave. In the fabrics of this invention, in each repeat unit of the MS weave pattern, at least 50% of the warp yarns pass under two weft yarns in succession in the MD to form a double warp knuckle. A double warp knuckle occurs when a warp yarn passes or is crimped under two weft yarns located adjacent to one another in the fabric weave structure. A single warp knuckle occurs when a warp yarn passes or is crimped under a single weft yarn, and is also used herein to refer to each knuckle formation of a double warp knuckle. First and second single warp knuckles are located on the next adjacent warp yarns on each lateral side of the double warp knuckle. The first single warp knuckle is produced by a warp yarn being crimped under a first of the two weft yarns of the double warp knuckle, and the second single warp knuckle is formed under a second of the two weft yarns of the double warp knuckle. [0018] In each repeat of the MS weave pattern, each MS weft yarn is interlaced by two adjacent warp yarns so as to form two adjacent MS warp knuckles. In other words, two adjacent warp yarns pass together under each MS weft yarn at each MS weft interlace point. This serves to pull the weft yarn "up" into the interior of the forming fabric causing the MS weft yarn float to bow outwardly, forming an enhanced weft wear plane that is in contact with the fabric bearing surfaces of the papeπnaking machine on which it is installed. This helps to increase both the wear resistance of the fabric and the "intensity" of crimp imparted to the MS weft. This, in turn, is believed to offset the amount of curl imparted to the fabric by the crimp in the PS weft, thus helping to "pull" the lateral edges of the fabric downward so that it lies flatter and has less curl than that obtained in previous designs. [0019] The double and single warp knuckles occur throughout and are uniformly distributed over the MS surface of the fabric in groups which include two single knuckles and one double knuckle. The distribution of the warp knuckles is reasonably uniform and does not appear to impart a pronounced twill or herringbone knuckle pattern onto the fabric surface as the warp knuckles are offset from and do not form twill lines. Thus, the MS surface of the novel fabrics is not susceptible to guiding or related tracking issues such as sometimes occur in fabrics where the MS yarn knuckles have a pronounced or clearly discernible angular orientation. Overall, benefits of this novel weave pattern include improved fabric guiding/tracking, good resistance to abrasive weave, and fabric stability.

[0020] In fabrics according to the present invention, at least 50% of the warp yarns pass under two consecutive MS layer weft yarns to form a double warp knuckle, and the warp yarns always pass together in pairs under each weft yarn at each MS weft interlace point to each form two single adjacent MS warp knuckles. Each warp yarn adjacent to the double MS warp knuckle passes under one of the two consecutive MS layer weft yarns of the double warp knuckle on either side thereof.

[0021] In a preferred embodiment, a flat woven composite forming fabric for use in the forming section of a papermaking machine is provided with the following: i. a first set of weft yarns located on the paper receiving side (PS) of the fabric, ii. a second set of weft yarns located on the machine contacting surface side (MS) of the fabric, and iii. at least a first system of warp yarns interwoven with the weft yarns of the first and second sets of weft yarns according to an overall weave pattern for the composite fabric which provides for differing weave patterns on each of the PS and MS sides of the fabric, wherein: in each repeat of the weave pattern for the MS of the flat woven composite forming fabric, the warp yarns pass together in adjacent pairs under the weft yarns in the second set of weft yarns to form two adjacent single warp knuckles at each MS weft interlace point.

[0022] Preferably, in each repeat of the MS weave, at least 50% of the warp yarns pass under two adjacent weft yarns in the second set of weft yarns to form a double warp knuckle.

[0023] Alternatively, in each repeat of the MS weave, 100% of the warp yarns include double warp knuckles formed by passing under two adjacent ones of the weft yarns in the second set of weft yarns.

[0024] Preferably, there is a single warp knuckle located in the next adjacent warp yarn on each lateral side of every double warp knuckle. [0025] Preferably, a first one of the single warp knuckles is formed under a first of the two weft yarns passed under by the warp yarn forming the double warp knuckle, while a second one of the single warp knuckles is formed under a second of the two weft yarns passed under by the warp yarn forming the double warp knuckle.

[0026] In the preferred fabrics according to the invention, a repeating pattern of the MS warp knuckles is uniformly distributed over the MS surface of the fabric, offset from and not forming diagonal twill lines. [0027] Preferably, the flat woven composite forming fabric is woven according to a design requiring 24 sheds in the loom. Alternatively, the flat woven composite forming fabric is woven according to a design requiring 20 sheds in the loom. As a further alternative, the flat woven composite forming fabric is woven according to a design requiring 16 sheds in the loom. [0028] In a preferred embodiment, the MS weave is woven according to a design requiring 4, 5 or 6 sheds in the loom. Alternatively, the MS weave is woven according to a design requiring 12 sheds in the loom. [0029] Preferably, the warp and weft yarn materials are selected from the group consisting of PET, PBT, PEN, nylon or a polymer blend of thermoplastic polyurethane and a polyester such as is disclosed in US

5,169,711 or US 5,502,120 (Bhatt et al.). Preferably, the warp and weft materials are extruded monofilaments.

[0030] In the preferred fabrics according to the invention, the cross sectional shape of the monofilaments comprising the warp and weft yarns is one, or a combination of the following: round, oval, trapezoidal, elliptical, hollow, rectangular or square.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be explained in more detail in connection with the drawings in which presently preferred embodiments are shown.

[0033] Figure 1 is a weave diagram of a first embodiment of a fabric according to the present invention.

[0034] Figure 2 is a simplified version of the weave diagram of Figure 1 which has been reversed to show only the MS knuckle pattern of the fabric whose full weave design appears in Figure 1.

[0035] Figure 3 is plan view of one repeat of the weave of the MS surface of the fabric whose design is shown in Figures 1 and 2.

[0036] Figure 4 is a perspective view of the MS surface of the fabric whose design is shown in Figures 1 and 2.

[0037] Figure 5 is a photograph of the MS surface of a fabric woven according to the weave design shown in Figure 1.

[0038] Figure 6 is a photograph of the PS surface of a fabric woven according to the weave design shown in Figure 1.

[0039] Figure 7 is a simplified weave diagram of the MS surface of a fabric which is a further embodiment of the present invention and is woven according to a 10-shed design.

[0040] Figure 8 is a simplified weave diagram of the MS surface of a fabric that is an alternate embodiment of the 10-shed weave design shown in

Figure 7. [0041] Figure 9 is a simplified weave diagram of the MS surface of a fabric which is a further embodiment of the present invention and is woven according to a weave design requiring 8-sheds.

[0042] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0043] Certain terminology is used in the following description for convenience only and is not considered limiting. Words such as "up", "down", "top", and "bottom" designate direction in the drawings to which reference is made. This terminology includes the words specifically noted above, derivatives thereof and words of similar input. Additionally, the terms "a" and "one" are defined as including one or more of the referenced data unless specifically noted. The term "crimp" is defined as the bend in the yarns formed by the interweaving of each warp and weft yarn. A "double warp knuckle" is the adjacent knuckle formations caused by a warp yarn passing (or being crimped) under two adjacent weft yarns in the fabric weave structure. A "single warp knuckle" is the knuckle formation caused by a warp yarn passing or being crimped under a single weft yarn, or also refers to each of the adjacent knuckle formations of a double warp knuckle. "Yarns" refers to monofilaments, multi-filaments, twisted or cabled fibers, stacked or assembled two-part fibers or fiber assemblies, or any other type of elongated filament-like structure used for weaving industrial papermaking fabrics. "Interlace" refers to the bending or crimping of a yarn from one of the wrap or weft systems around another yarn from the other of the warp or weft systems as they pass beneath or around one another.

[0044] A weave diagram of a first embodiment of a fabric 200 according to the present invention is shown in Figure 1. As is customary in these weave diagrams, the warp yarns 212 are numbered horizontally across the top of the pattern as 1-24, and the weft yarns 214 are numbered in sequence vertically down the left side of the pattern as 101-142. This is a 24-shed weave pattern, with the PS layer 202 weave design woven using warp yarns 212 numbered 1- 12 and the MS layer 204 woven using warp yarns 212 numbered as 13-24. The overall weave pattern requires 42 weft yarns 214 for a repeat (18 "primary" weft yarns 214A interwoven on the PS layer 202 only, 12 "intrinsic" weft yarns 214B interwoven as 6 pairs to form the part of the PS layer 202 and "tie" into the MS layer 204 to bind the two fabric layers 202, 204 together in a similar manner to Seabrook et al. US 5,826,627, which is incorporated by reference herein as if fully set forth; and 12 MS weft yarns 214C woven only in the MS layer 204) and is woven at a 2:1 weft ratio, meaning there are twice as many PS weft yarns 214A, 214B than MS weft yarns 214C (18 + 6 intrinsic weft binder pairs in the PS and 12 in the MS). In the diagram, a white square indicates that a warp yarn 212 is beneath a weft yarn 214 as viewed from the PS. MS warp knuckles 220 are formed on the MS surface where the warp yarns pass under the MS weft yarns.

[0045] A simplified version of the weave diagram showing the warp yarns 212 and only the MS weft yarns 214C used in the formation of the MS layer 204 of the fabric and a clearer picture of the warp knuckle pattern, is provided in Figure 2. In Figure 2, the locations where the warp yarns 212 pass under (i.e., are on the MS surface of the fabric) the MS weft yarns 214C to form the single and double warp knuckle arrangement of this invention are clearly shown. The knuckles 220A, 220B, 220C and 220D formed by warp yarns 214 numbered 17, 18 and 19 as they pass under weft yarns 214C numbered 129 and 132 are exemplary. In Figure 2, it can be seen that the warp yarn number 17 passes under one weft yarn 129 to form a single warp knuckle 220A, the immediately adjacent warp yarn 18 passes under weft yarns 129 and 132 respectively to form a double warp knuckle consisting of knuckles 220B and 220C, and the next adjacent warp yarn 19 forms a single warp knuckle 220D as it passes under the weft yarn 132. Another way of describing the arrangement of warp knuckles 220 in Figures 1 and 2 is to say that all of the warp yarns 212 only interlace with the MS weft yarns 214 as adjacent pairs at each MS weft yarn interlace point to form two single knuckles 220A, 220B or 220C, 220D on each MS weft yarn 214 (here specifically explained and illustrated with respect to weft yarns 129 and 132) in the pattern. In the weave diagrams of Figures 1 and 2, these MS weft yarns 214C are identified as 101, 104,108, 111, 115, 118, 122, 125, 129, 132, 136 and 139. [0046] Figures 3 and 4 are line drawings of the MS surface 204 of the novel fabric 200 which is made in accordance with the weave pattern provided in Figures 1 and 2. Figure 3 is a planar view of the MS surface 203 of the fabric, while Figure 4 is a bottom perspective representation of the MS layer 204 of the same fabric as is shown in Figure 3. The single warp knuckles 220A, 220D and the double warp knuckles formed by the knuckles 220B and 220C over (with respect to the MS surface) adjacent MS weft yarns 214 are shown in different shading in Figure 3.

[0047] Figures 5 and 6 are photographs of the MS and PS of an actual fabric 200 woven according to the weave design shown in Figure 1. The warp knuckles 220A, 220B, 220C and 220D are indicated in Figure 5. In these photographs, the weft yarns 214 are oriented horizontally across the page while the warp yarns 212 run vertically up and down the page. The PS surface 202 shown in Figure 6 is a plain weave and is a construction similar to that illustrated in Seabrook et al. in which every 4^th weft yarn is an intrinsic weft yarn pair 214B, and each pair member alternately passes down to the MS and interlaces there with one of the warp yarns 212 to bind the PS and MS layers together into a unified structure.

[0048] Figure 7 is a weave diagram for the MS layer of a first alternate embodiment of a fabric 200' according to the invention. The MS weave diagram of Figure 7 is similar to Figure 2 in that only the MS weft yarns 214C (numbered 301 - 320) are shown interweaving with the warp yarns 212' (numbered 1-10) illustrated in a 10-shed pattern in order to form the warp knuckles 220'. Unlike the weave structure illustrated in Figures 1-6, the weave pattern of Figure 7 is a structure in which every second warp yarn 212' (here numbered 1, 3, 5, 7, and 9) does not form any double warp knuckles - only the even numbered warp yarns 212' weave under two adjacent MS weft yarns 214C to form the double warp knuckles. Therefore 50% of the warp yarns form double warp knuckles. However, as in the pattern shown in Figures 1 and 2, two adjacent warp yarns 212' pass under each MS weft yarn 214C. Other weave patterns would also be possible with at least 50% of the warp yarns having double warp knuckles. [0049] Figure 8 is a weave diagram for the MS layer of a second alternate embodiment of a fabric 200" according to the invention. Like Figure 7, the MS layer in this embodiment is woven according to a 10-shed pattern to form warp knuckles 220"; however, it is more similar to Figures 1 and 2 in that every warp yarn 212" (numbered 1-10) includes double warp knuckles formed under adjacent ones of the MS weft yarns 214C" (here numbered 401- 420). In a similar manner to the first embodiment of Figures 1 and 2, a single warp knuckle is located in the next adjacent warp yarns 212" on each side of the double warp knuckle.

[0050] Figure 9 is a weave diagram for the MS layer of a third alternate embodiment of a fabric 200"' according to the invention. The MS layer is woven in an 8-shed pattern to form warp knuckles 220"'. It has all of the features of the designs shown in Figures 1, 2 and 8 in that every warp yarn 212"' (here numbered 1-10) passes under two adjacent ones of the weft yarns 214C"' (here numbered 501-532) in the repeat to form a double warp knuckle. A single warp knuckle is located in the adjacent warp yarn 212"' on each side of the double warp knuckle, with one of the single warp knuckles being located on each of the weft yarns 214C" used to form the double warp knuckle such that each MS weft yarn 214C" is passed under by two warp yarns 212"' at each MS weft interlace point.

[0051] In each of the embodiments of the fabric according to the invention, a first one of the single warp knuckles 220, 220', 220", 220"' is formed under a first of the two MS weft yarns 214C, 214C, 214C", 214C"' passed under by the warp yarn 212, 212', 212", 212"' forming the double warp knuckle, while a second one of the single warp knuckles 220, 220', 220", 220"' is formed under a second of the two MS weft yarns 214C, 214C, 214C", 214C"' passed under by the warp yarn 212, 212', 212", 212"' forming the double warp knuckle. This results in every group of MS knuckle formations being in a ^αZ" or reverse "Z" pattern formed by four of the MS warp knuckles 220, 220', 220", 220"' (two single knuckles on each side of a double knuckle) that are generally uniformly distributed on the MS surface of the fabrics 200, 200', 200", 200"'. In order to avoid tracking or guiding problems, the knuckles are not aligned along and are generally offset from twill lines. Additionally, the intermixing of the "Z" and reverse "Z" knuckle pattern formations across the MS fabric surface also helps to eliminate such problems. [0052] Test Results

[0053] In order to determine whether the MS weave design of the present invention is effective in reducing or eliminating fabric edge curl, two samples of similar construction composite forming fabrics were woven and then heat-set under identical conditions. Sample 1 was woven using a MS layer weave design which was in accordance with the teachings of the present invention and in which the warp yarns pass together in adjacent pairs under all of the machine side layer weft yarns to form two adjacent single warp knuckles at each MS weft interlace point. Sample 2 was a prior art fabric construction including a machine side layer weave in accordance with the teachings of Barrett, US 5,544,678 and in which the warp yarns are not arranged to pass together in adjacent pairs under all of the MS weft yarns. Both fabrics were intrinsic weft tie type composite forming fabrics similar to those described by Seabrook et al US 5,826,627.

Table 1:

[0054] In Table 1, the terms "PS", "MS", "MD" and "CD" have the meanings previously ascribed. The term "Mesh" refers to the number of MD yarns per inch of fabric width; the term "Count" refers to the number of CD yarns per inch of fabric length. The term "dia." refers to the diameter of the given strand; the term ""mat'l type" means material type and refers to the type of polymer from which the monofilament strand used in the fabrics was extruded. The term "PET" means polyethylene terephthalate" and the term "PAM" means polyamide (also referred to as nylon).

[0055] The wet and dry curl test values were obtained in the following way. Samples from both the inventive and the prior art fabrics were obtained and their MD length and CD width measured. The samples are laid onto a flat surface and the height of their curl relative to that flat surface was measured. The curl test value obtained is a ratio that utilizes the measured height of the sample fabric curl above the flat surface relative to a chord length of that particular curl. Higher curl test values indicate larger amounts of fabric curl. Positive curl test values indicate fabric curl towards the paper side surface, while negative curl test values indicate fabric curl towards the machine side surface. In the dry curl test, the fabric sample is fully dry; in the wet curl test, the fabric sample is soaked in room temperature water for 24 hours prior to this measurement. Curl test values around zero indicate that the fabric sample is essentially flat with no curl at its edge. Curl test values near 2.0 are considered unacceptable and the fabric will require further treatment in order to reduce its edge curl.

[0056] In Table 1 above, the data from the inventive fabric samples show that both the dry curl and wet curl test values are equal to zero, while these values for the prior art fabric are 0.88 and 1.88 respectively. This means that the samples of fabric woven in accordance with the teachings of the invention exhibited no curl either when wet or dry, while the prior art fabric exhibited a nearly unacceptable degree of curl at its lateral side edges. From this, we conclude that the machine side layer weave design employed in the fabrics of this invention is effective to reduce fabric edge curl. [0057] While the preferred embodiments of the invention have been described in detail, the invention is not limited to these specific embodiments described above which should be considered as merely exemplary. For example, while the preferred fabric is flat woven, it could be continuously woven, and the systems of yarns would be first and second systems of cross- direction yarns interwoven with at least one system of machine direction yarns according to the invention as described above. Additionally, while the preferred embodiment of the fabric is for use as a forming fabric on a papeπnaking machine, additional embodiments could prove useful in other sections of a papermaking machine. Further modifications and extensions of the present invention may be developed and all such modifications are deemed to be within the scope of the present invention as defined by the appended claims.

Claims

1. A flat woven composite forming fabric for use in a forming section of a papermaking machine, the fabric comprising: i. a first set of weft yarns located on a paper receiving side (PS) of the fabric, ii. a second set of weft yarns located on a machine contacting surface side (MS) of the fabric, and iii. at least a first system of warp yarns interwoven with the weft yarns of the first and second sets of weft yarns according to an overall weave pattern for the composite fabric which provides for differing weave patterns on each of the PS and MS sides of the fabric, wherein: in each repeat of the weave pattern for the MS of the flat woven composite forming fabric, the warp yarns pass together in adjacent pairs under all of the weft yarns in the second set of weft yarns to form two adjacent single warp knuckles at each MS weft interlace point.

2. Fabric according to Claim 1, wherein at least 50% of the warp yarns include double warp knuckles formed by passing under two adjacent ones of the weft yarns.

3. Fabric according to Claim 1, wherein 100% of the warp yarns include double warp knuckles formed by passing under two adjacent ones of the weft yarns in the second set of weft yarns.

4. Fabric according to Claim 3, wherein one of the single warp knuckles of a next adjacent one of the warp yarns is located on each lateral side of each of the double warp knuckles.

5. Fabric according to Claim 4, wherein a first one of the single warp knuckles is formed under a first of the two weft yarns passed under by the warp yarn forming the double warp knuckle, while a second one of the single warp knuckles is formed under a second of the two weft yarns passed under by the warp yarn forming the double warp knuckle.

6. Fabric according to Claim 1, wherein a repeating pattern of the two adjacent single warp knuckles is uniformly distributed over the MS surface of the fabric, offset from diagonal twill lines.

7. Fabric according to Claim 1, wherein the overall weave pattern is a 24- shed design.

8. Fabric according to Claim 7, wherein the MS weave is woven according to a 4 or 6 shed design.

9. Fabric according to Claim 7, wherein the MS is woven according to a 12- shed design

10. Fabric according to Claim 1, wherein the overall weave pattern is a 20- shed design.

11. Fabric according to Claim 10, wherein the MS weave is woven according to a 5-shed design.

12. Fabric according to Claim 1, wherein the overall weave pattern is a 16- shed design.

13. Fabric according to Claim 11, wherein the MS weave is woven according to a 4-shed design.

14. Fabric according to Claiml2, wherein the MS is woven according to a 12-shed design.

15. Fabric according to claim 1, further comprising a second system of warp yarns interwoven with the first set of weft yarns located on the PS of the fabric.

16. Fabric according to claim 1, wherein the warp knuckles on the MS are all arranged in groups of four knuckles in a Z or reverse Z pattern.

17. Fabric of claim 1, wherein the MS weft yarns include MS floats over at least 4 warp yarns.

18. A flat woven composite forming fabric for use in a forming section of a papermaking machine, the fabric comprising: i. a first set of weft yarns located on a paper receiving side (PS) of the fabric, ii. a second set of weft yarns located on a machine contacting surface side (MS) of the fabric, and iii. at least a first system of warp yarns interwoven with the weft yarns of the first and second sets of weft yarns according to an overall weave pattern for the composite fabric which provides for differing weave patterns on each of the PS and MS sides of the fabric, wherein: the warp yarns weave under the weft yarns in the second set of weft yarns to form MS warp knuckles arranged in groups of four adjacent knuckles in a Z or reverse Z pattern, the groups being generally uniformly spaced over the MS of the fabric.

19. A woven composite fabric for use in a papermaking machine, the fabric comprising: i. a first set of cross-direction yarns located on a paper receiving side (PS) of the fabric, ii. a second set of cross-direction yarns located on a machine contacting surface side (MS) of the fabric, and iii. at least a first system of machine direction yarns interwoven with the cross-direction yarns of the first and second sets of cross-direction yarns according to an overall weave pattern for the composite fabric which provides for differing weave patterns on each of the PS and MS sides of the fabric, wherein: in each repeat of the weave pattern for the MS of the composite fabric, the machine direction yarns pass together in adjacent pairs under all of the cross-direction yarns in the second set of cross-direction yarns to form two adjacent single warp knuckles at each MS cross-direction yarn interlace point.