COMPOSITE FORMING FABRIC

Abstract: A composite paper making fabric comprising at least two complete weaves each formed by its own set of warp and weft yarns and interconnected by a binder yarn which is interwoven with the two complete waves. An upper one of the complete weaves constitutes a paper-side weave which is comprised of flattened warp yarns inter­ woven with its weft yarns.

Description: BACKGROUND OF INVENTION Field of Invention The present invention relates to paper machine forming fabrics in accordance with the preamble of claim 1 and is particularly directed to a composite fabric comprised of at least two complete weaves, each having its own set of warp and weft yarns, with a warp or weft binder yarn that interconnects the two layers. The upper weave, that is the paper-side weave, is provided with flattened warp yarns. In the continuous manufacture of paper, the paper machine is comprised essentially of a forming section, a press section, and a dryer section. In the forming section a dilute slurry of fibers and fillers is directed onto the surface of a moving forming fabric by means of a head box. As the forming fabric moves along the forming section, water is removed from the slurry by gravity and various dewatering devices. By the end of the forming section a continuous wet but self-supporting web of fibers and fillers remains on the surface of the forming fabric. The web then passes out of the forming section into the press section where more water is removed by mechanical pressing, after which the web passes into the dryer section where the remaining water is removed by an evaporative process. Description of prior art In recent years forming fabrics have been woven of plastic polymeric filaments in single-layer twill patterns and, although improvements have been made to produce reasonably satisfactory single-layer fabrics, the more recent development of multi-layer forming fabrics has given additional benefits to paper makers by providing increased fiber retention and fabric stability. Typically, the paper side or upper layer of a composite forming fabric of the prior art is a fine mesh plain weave, which provides excellent retention of fibers, good dewatering, and a minimum of mark in the paper produced on its surface. The running side, or bottom layer, of such a composite fabric is usually a coarser mesh, with larger diameter strands than those of the upper layer, in order to provide resistance to stretching, narrowing, and wear. The two layers of a composite fabric are typically interconnected in one of two ways. The first and most common method is to use a weft binder, which is usually a finer diameter yarn than those of the two layers, and is woven so as to interweave the top and bottom warp yarns and thus bind the two layers together. The other method is to interweave the warp yarns of the top layer with the weft yarns of the bottom layer, so as to bind the two layers together. Composite forming fabrics having this description and with various binder yarn configurations are well known, examples of which are described in CA-A-1,115,177 and US-A-4,501,303. The preamble of claim 1 is based on the prior art forming fabric of CA-A-1,115,177. The importance of fabric surface geometry and, in particular, the size of the surface openings (frames) defined by the strands in the top layer, is described in the inventor's paper "Retention and Drainage of Multilayer Fabrics" (Pulp & Paper Canada, May 1986). For optimum fiber retention, it is advantageous to make these openings, particularly their machine direction lengths, as small as possible. In addition, it is often desirable to make the openings in the fabric small so that the dewatering capacity of the fabric is reduced, and thus more controlled. One of the problems suffered by paper machine screens made as composite fabrics is that the plain weave construction of their upper layer, by the very nature of the weave geometry, imposes severe restrictions on the degree to which the size of openings in the fabric can be reduced. Another problem suffered by composite fabrics in some applications arises from their greater thickness, which increases the void volume, resulting in higher volumes of water being carried by the fabric. On some paper machines, the greater thickness of the composite fabric results in unacceptable defects in the formation of the paper web. A further problem suffered by composite fabrics is that the warp or weft binder yarns distort the upper paper-making surface, typically creating a localized surface depression often referred to as a "dimple". If the "dimple" is too deep, or results in blockage of some of the openings in the top layer, an unacceptable wire mark may be produced in the paper sheet formed on the top layer. OBJECTS OF INVENTION An important object of the present invention is to overcome the above-mentioned problems by providing a composite fabric which has substantially smaller surface openings in the upper or paper-side layer by using monofilament warp strands with a flattened profile (cross-section). Another object of the present invention is to provide a composite fabric of reduced thickness. Yet another feature of the present invention is to reduce the severity of the "dimples" in the upper layer created by the warp or weft binder yarns that are used to join the two layers of the composite fabric. The use of flattened, high molecular weight, polyester warp strands in multi-layer fabrics has been described in GB-A-2,157,328. In this case, however, the objectives of using flattened warp strands were to improve wear resistance and to reduce the thickness and hence the void volume of the fabric. In addition, importantly, that invention applied specifically to those double-layer fabrics in which there is only one set of warp yarns. The older German Patent DE-A-680 111 shows the use of flat wire warps in a single layer forming fabric of metallic wires. According to the present invention there is provided a composite paper-making forming fabric, comprising at least two complete weaves, each formed by its own set of warp and weft yarns and being interconnected by binder yarns which are separately interwoven with said two complete weaves, characterised in that an upper one of said complete weaves constituting a paper-side weave comprises flattened warp yarns having an aspect ratio of width to height of between 1.20 and 2.30 and interwoven with said weft yarns, and has a plain weave, and in that a bottom one of said complete weaves constituting the machine-side weave comprises flattened warp yarns having an aspect ratio of width to height of between 1.20 and 2.30, wherein said bottom weave has a mesh count of substantially half that of said upper weave. A forming fabric of this kind will have reduced thickness and improved fiber retention. Moreover, the upper weave will have a machine-direction frame length which is less than that when round warp yarns are used. Further preferred embodiments of the invention are set forth in the subordinate claims. BRIEF DESCRIPTION OF DRAWINGS A preferred embodiment of the present invention will now be described with reference to an example thereof as illustrated in the accompanying drawings, in which: FIGURE 1 is a plan view of the upper layer of a composite fabric of the prior art; FIGURES 1A and 1B are sectional views of the composite fabric along lines A-A and B-B respectively; FIGURE 2 is a plan view of the upper layer of a composite fabric in which the warp yarns of the upper layer have a flattened profile; FIGURES 2A and 2B are sectional views along lines A-A and B-B respectively; FIGURE 3 is a plan view of the upper layer of a composite fabric of the invention; FIGURES 3A and 3B are sectional views, similar to Figures 2A and 2B, but illustrating a modified lower weave with flattened warps; and FIGURE 4 is an enlarged cross-section of one of the flattened warp yarns. DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, Figure 1 depicts, in plan view, the upper layer 10 of a composite fabric of the prior art, in which all of the strands 11 and 12 have a round cross-section. In this upper layer, warp strands 11 and weft strands 12 are interwoven in a plain weave construction. Figures 1A and 1B illustrate the composite nature of the fabric comprising an upper layer 10 of warps 11 and wefts 12 in plain weave construction and a lower layer 13 having a four-harness satin weave with coarser warps 14 and wefts 15 and with half the mesh count of the upper layer. The two layers are tied together in the weft direction by a binder yarn 16. The cross-machine direction width of the surface openings (frames) in the upper layer 10 is illustrated by dimension "x" and the machine direction length of the frames is shown by dimension "y". Figure 2 is a plan view of the upper layer 20 of a composite fabric having the same mesh count as the fabric in Figure 1. However, the warp yarns 21 of the upper plain weave layer have a flattened profile and the weft yarns 22 are of a larger diameter. The shape of the flattened warps 21 is shown in the sectional view of Figure 2A and, in greatly enlarged cross-section, in Figure 4. The lower layer 23 is a four-harness satin weave with coarse warps 24 and wefts 25, with half the mesh count of the upper layer 20. The two layers are tied together in the weft direction by a binder yarn 26. The cross-machine direction width dimension of the frames "x¹" has been reduced due to the use of the flattened warp strands 21 which are wider than the round strands 11 of Figure 1. A reduction in the machine direction dimension "y¹" of the frames has been achieved by the use of larger diameter weft strands 22. Flattened warp makes possible the use of either larger diameter weft at the same weft count or, alternately, unchanged weft diameter at a higher weft count. Either combination achieves the same result of a reduced machine direction frame length. A plain weave upper layer with a warp count of 24.8 strands per cm (63 strands per inch) has been woven with flattened warps having dimensions of 0.0114 cm x 0.0190 cm (.0045" x .0075"), that is, an aspect ratio of 1.67. This enabled 0.0198 cm (.0078") weft to be woven at a weft count of 29.1 strands per cm (74 strands per inch), whereas with round warp of 0.0178 cm (.007") diameter at the same warp count 24.8 strands per cm (63 strands per inch) it was not possible to use a weft size larger than 0.0183 cm (.0072") at a weft count of 29.1 strands per cm (74 strands per inch). A similar result was achieved in the same plain weave upper layer at the same warp count 24.8 strands per cm (63 strands per inch) with flattened warps having dimensions of 0.0112 cm x 0.0196 cm (.0044" x .0077"), that is, an aspect ratio of 1.75. Figures 3, 3A and 3B depict one embodiment of a composite fabric according to the invention. In this embodiment the upper layer 30 is the same as upper layer 20 of Figure 2, with the same reduced frame width x¹ and length y¹. The lower layer 33 is a four-harness satin weave with coarse warps 34 and wefts 35, again with half the mesh count of the upper layer 30, but with the warps 34 having a flattened profile. The two layers are again interconnected in the weft direction by a binder yarn 36. Although the embodiments illustrated in Figures 2 and 3 show a bottom weave with half the mesh count of the upper weave, it is understood that the invention is not limited to composite fabrics having this particular mesh ratio. That is, the mesh ratio of warps and wefts in the upper weave to warps and wefts in the bottom weave may be 3:2, 4:3, 5:4, or any combination, as described in the prior art. Figure 4 is a greatly enlarged cross-section of one of the flattened warps showing the flattening aspect ratio, which is defined herein as the strand width "b" divided by the strand height "a". Increasing the warp flattening aspect ratio, particularly by increasing the strand width "b" at constant strand height "a" enables substantial degrees of reduction in the size of fabric surface openings to be realized. Higher flattening ratios also enable reductions in fabric thickness to be achieved, particularly if flattened warps are also used in the bottom layer 23 of the composite fabric. For example, when the aforementioned 24.8 (63) mesh plain weave upper weave with 0.0114 cm x 0.0190 cm (.0045" x .0075") flattened warps was combined with a bottom weave using 0.0190 cm x 0.0381 cm (.0075" x .015") flattened warps (aspect ratio of 2.0) or 0.0185 cm x 0.0381 cm (.0073" x .015") (aspect ratio of 2.05) at a mesh count of 12.4 strands per cm (31½ strands per inch), reductions of 0.005 cm - 0.008 cm (.002" - .003") in fabric thickness were observed, compared to the same mesh counts woven with round warp strands. Preferably, the flattening aspect ratio of the monofilament warp yarns in either the top or bottom layer will be 1.20 - 2.30. More preferably, an aspect ratio of 1.30 - 2.00 has been found to be desirable for the flattened warps of the upper layer in order to control the machine direction length of surface openings and the dewatering capacity of the fabric. A preferred aspect ratio for the flattened warps of the bottom layer is 1.60 - 2.20 which enhances reductions in fabric thickness without detrimental effects on the resistance of the cloth to stretching and narrowing. The use of flattened warps in the upper layer reduces the severity of the "dimples" associated with weft binder yarns, and thus reduces the tendency for wire mark in the paper sheet. In composite fabrics of the prior art, when round cross-section warps of the upper layer are used as binder yarns the resultant "dimples" in the top surface are deeper and more disruptive to the adjacent mesh than those formed with weft binders. In the composite fabric of the invention, the use of flattened warps makes it practicable to use warp binders, since the mesh distortion and depth of the "dimples" is greatly reduced. Also, in the case of warp binder yarns, the top layer disruption is reduced even further if smaller diameter bottom weft strands are used in the bottom layer at only those positions where the top layer warp binder actually interweaves the bottom weft layer. This smaller diameter bottom weft may also advantageously be a different material than the regular bottom weft yarns; for example, polyamides such as nylon 6 or nylon 66 may be used instead of polyester. The invention applies to composite fabrics with an upper fabric layer woven with warp mesh counts of 14 - 39 strands per cm (36 - 100 strands per inch), which is the normal range for paper machine forming fabrics. More preferably, the warp mesh count of the upper weave will be 16 - 31.5 strands per cm (40 - 80 strands per inch). Typical flat warp dimensions for the preferred ranges of aspect ratio and warp mesh count are: Aspect ratio = 1.3 Aspect ratio = 2.0 16 strands per cm (40 strands per inch) 0.0254 cm x 0.0330 cm (.010" x .013") 0.0206 cm x 0.0412 cm (.0081" x .0162") 31.5 strands per cm (80 strands per inch) 0.0119 cm x 0.0155 cm (.0047" x .0061") 0.0096 cm x 0.0193 cm (.0038" x .0076") This invention is not limited to the weaves illustrated; that is, the upper fabric layer and the lower fabric layer can be woven in any construction and in any mesh count. Accordingly, it is within the ambit of the present invention to cover any obvious modifications, provided such modifications fall within the scope of the appended claims.

Claims: A composite paper-making forming fabric comprising at least two complete weaves (30, 33), each formed by its own set of warp (31, 34) and weft (32, 35) yarns and being interconnected by binder yarns (36) which are separately interwoven with said two complete weaves, characterised in that an upper one (30) of said complete weaves constituting a paper-side weave comprises flattened warp yarns (31) having an aspect ratio of width to height of between 1.20 and 2.30 and interwoven with said weft yarns (32), and has a plain weave, and in that a bottom one (33) of said complete weaves constituting the machine-side weave comprises flattened warp yarns (34) having an aspect ratio of width to height of between 1.20 and 2.30, wherein the mesh ratio of warps (31) and wefts (32) in the upper weave (30) to warps (34) and wefts (35) in the bottom weave (33) may be substantially 2:1, 3:2, 4:3 or 5:4. The composite forming fabric as claimed in claim 1, wherein said upper weave has a warp mesh count of 14 - 39 strands per cm (36 - 100 strands per inch). The composite forming fabric as claimed in claim 2, wherein said upper weave (30) has a warp mesh count of about 16 - 31.5 strands per cm (40 - 80 strands per inch). The composite forming fabric as claimed in claim 1, in which said binder yarns (36) are woven in the weft direction. The composite forming fabric as claimed in claim 1, in which said binder yarns (36) are woven in the warp direction. The composite forming fabric as claimed in claim 1, in which said flattened warps (31) have an aspect ratio of width to height of between 1.30 and 2.00. The composite forming fabric as claimed in claim 1, in which said flattened warps (31) have an aspect ratio of width to height of between 1.67 - 1.75. The composite forming fabric as claimed in claim 1, in which said flattened bottom warps (34) have an aspect ratio of width to height of between 1.60 - 2.20. The composite forming fabric as claimed in claim 1, in which said flattened bottom warps (34) have an aspect ratio of width to height of between 2.00 - 2.05.

Inventors: DALE, BERNARD JOHNSON
Applicants: JWI LTD.
Application year: 1987
Main classification: D21F1
IPC codes: D03D15, D03D11, D21F1