GB2157328A - Improved multilayer forming fabric - Google Patents

Abstract

A multilayer paper makers felt comprises high density polyester warp monofilaments (20-26) having a flattened profile so that the major axis of the flattened monofilaments lies parallel to the plane of the fabric. The fabric is heat set under tension in the warp direction. <IMAGE>

Description

SPECIFICATION Improved multilayer forming fabric This invention relates to a paper machine forming fabrics and is particularly directed to an improved multilayer forming fabric comprising at least two layers of synthetic weft monofilaments woven with synthetic warp monofilaments at least some of which have a flattened profile.

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.

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 papermakers by providing increased fiber retention and fabric stability. There are, however, some disadvantages inherent with present duplex designs.

One problem relates to obtaining a monoplane upper surface. Monoplane is defined as the condition where the outer peripheries of the warp and weft knuckles lie substantially in the plane of a fabric surface.

For example, the method described to do this in the double layer fabric of U.S. patent 4,071,050 is to interweave the warp strands into the upper layer weft strands, thus causing the weft strands to crimp to produce a monoplane upper surface after the fabric has been heat set. In practice, because of the complex nature of this weave design, diagonal troughs are formed on the upper surface which mark the paper produced on the fabric. These troughs are described in U.S. patent 4,112,982 which proposes a method of achieving a weft dominated upper surface which, theoretically, can be heat set to a monoplane condition.

It is generally accepted that it is desirable to have a fabric with weft-dominated under surface so that the weft knuckles provide protection against wear for the load-bearing warp strands. This condition is attainable in certain weave structure by heat setting as described in U.S. patent 4,041,989. However, we have found that heat setting also causes the weft knuckles on the upper surface to protrude above the warp and these protruding weft knuckles will mark the paper. To attain the most desirable condition we have found it preferable to use multilayer fabric designs in which the warp strands interweave with the weft in the lower layer and the warp knuckles lie above the weft knuckles on the upper surface in the "as woven" state.In these designs, after heat setting, the load bearing warp strands will lie wholly within the plane of the weft on the underside of the fabric where they are protected from wear. However, at the upper surface, the warp knuckles will generally remain above the weft knuckles which is not the most desired condition.

A second problem is encountered with the use of high molecular weight polyester warp.

It is advantageous to use this material because it provides increased tensile strength and modulus of elasticity and is generally more abrasion resistant, so that it reduces the incidence of damage by high pressure needle showers which are needed to keep the fabric clear of mesh clogging material. We have found that when high molecular weight polyester warp is used in conventional double layer (duplex) fabrics, it tends to cause excessive crimp in the weft. Particularly, it induces a lateral component of weft crimp which makes joining the ends of the fabric in a woven seam difficult and more time consuming.

A third problem encountered with conventional multilayer fabrics is related to their having increased void volume as compared to single layer fabrics. This can cause excessive water to be carried within the mesh of the fabric which re-wets the paper just prior to its passing into the press section of the paper machine.

The aim of the present invention is to improve on conventional multilayer forming fabrics of the prior art, to overcome the above mentioned problems and to provide advantages which are outlined in the following description.

According to a broad aspect of the present invention there is provided a multilayer forming fabric having at least two layers of weft strands interwoven with synthetic monofilament warp strands. The warp strands extend in the machine direction and have a flattened cross-section. The major axis of the crosssection lies substantially parallel to the plane of the fabric thereby reducing the void volume of the fabric. The weft strands at the surface of the fabric that is subjected to wear are prominent over the warp strands and the weft strands at the opposed pulp carrying surface lie substantially in the same plane as the warp strands.

Further, in accordance with another aspect of the invention the major axis of the flattened warp strands is substantially equal to the diameter of round monofilaments found in similar conventional duplex or multilayer forming fabrics, and the flattened warp has substantially the same cross-sectionai area as said round warp.

An important aspect of the invention is that it allows the fabric to be woven with high molecular weight polyester warp.

Flattened polyester monofilament warp, woven into dupex or multilayer forming fabric according to the invention, has the following characteristics. Being thinner than the equivalent round warp, it does not project as much above the upper plane of the fabric. It also bends with less applied stress in the weaving process, forming a more stable knuckle which has a tighter crimp. The ease of crimping in the warp causes less crimp to be forced into the weft. Also, during weaving, the weft is crimped against a flat side of the warp and, as a consequence the crimp in the weft is further reduced since it is more difficult to bend or wrap the weft over warp that has a flat surface than it is to bend it over warp that is round.Further, with flattened warp there is a better distribution of forceps during the transfer of the beat-up impact from the weft to the warp in the weaving process.

The preferred fabric structure, as mentioned previously, characteristically provides for dominant weft knuckles at the bottom surface.

Although the vertical component of bottom weft crimps is reduced by the use of flattened warp, the level of the warp is raised by virtue of its being flat with the result that the predominance of the weft knuckles at the bottom surface remains substantially unaffected.

The above characteristics of the flattened warp are particularly helpful when high molecular weight polyester material is used, in which case there is a significant reduction in lateral crimp in the weft which facilitates seaming.

Regarding molecular weight, this is commonly measured by viscosity of a solution of the polyester at 30'C in a Ubbelohde viscometer and related to a viscosity index of l.V.

number. When used in the fabric of the invention the heat set polyester warp material would preferably have a molecular weight equivalent to an l.V. number greater than 0.75.

It is known that in any given fabric structure if the thickness of the warp filaments is reduced then the thickness of the fabric formed by these warp filaments will also be reduced. Further, if the strand volume in any unit area of a fabric remains unchanged, and the thickness of the fabric is reduced, then the void volume of the fabric will be reduced. In the fabric of the invention, therefore, since warp monofilaments are flattened with the cross-sectional area of the flattened monofilaments remaining substantially unchanged, then, regardless of the fabric structure, the void volume will be reduced accordingly and the fabric will therefore have a reduced tendency to re-wet the paper as explained previously.

Important features of the fabric of the invention are now enumerated: 1. The upper surface of the fabric is substantially monoplane, thus improving the papermaking qualities.

2. At the underside of the fabric, the weft knuckles project beyond the plane of the warp knuckles, thus improving the wear resistance of the fabric.

3. The flattened warp makes it practical to use high molecular weight polyester warp material by overcoming chronic seaming difficulties associated with the use of this material in warp having the conventional round crosssection.

4. The void volume of the fabric is reduced by the use of flattened monofilament warp, thus reducing the amount of water carried in the mesh and the consequent rewetting of the paper.

5. The fabric is easier to weave because of lowered stress required to crimp the flattened warp.

6. The fabric is more resistant to distortion in its own plane because of the increased contact area between the weft and the flattened warp.

Preferred embodiments of the present invention will now be described with reference to the examples illustrated in the accompanying drawings in which: Figure 1 is a perspective view of a forming fabric; Figure 2 is an enlarged view of the upper surface of a portion of 7 shaft 14 repeat pattern duplex fabric, woven with flattened polyester monofilament warp according to the present invention; Figures 2A and 2B are cross-sectional views along section lines X-X and Y-Y respectively of Fig. 2; and Figure 3 is a greatly enlarged cross-section view of a flattened warp monofilament as utilized in the forming fabric of the present invention.

In Fig. 1 there is shown a forming fabric, indicated generally at 100 which is in the shape of an endless belt and has opposed side edges 101 and 102. The belt is preferably woven as-a flat fabric with the ends joined at a seam shown at 104 and with the warp extending in the longitudinal direction shown by the arrow A.

Fig. 2 depicts a topside view of an all monofilament 7 shaft 1 4 repeat duplex weave forming fabric, a weave pattern which is known and has been used in the papermaking industry. The numerals 20, 21, 22, 23, 24, 25 and 26 refer to consecutive warp strands.

The weft is paired in two layers and numbered 1 to 14 as shown and repeating as 1', 2', etc. to 14' and so on. In this structure the first warp monofilament, number 20 passes in order between a first pair of weft strands 1 and 2, over a second and third pair of weft strands 3 and 4 and 5 and 6, between a fourth pair 7 and 8, between a fifth pair 9 and 10, under a sixth pair 11 and 1 2 and between a seventh pair 1 3 and 14 and so on.

The other warp monofilaments in the series of seven follow the same general weaving pattern and are positioned with respect to the weft monofilaments as shown.

After the fabric is woven it is heat-set under controlled tension in the warp direction which causes a crimp exchange to occur and the mesh becomes compacted so that the upper and lower layers of weft will lie closer together. It will be seen in the cross-sectional view, Fig. 2B, that at the under side of the fabric, which is the side that is exposed to the most wear, the long bottom weft knuckles are depressed below the level of the knuckles of the warp strands that inter-weave them by an amount indicated by the reference letter P.

In selected conventional mesh patterns, as the weft crimp is enhanced at the expense of the warp crimp the upper weft knuckles rise towards the upper surface and the lower weft knuckles move downward towards the lower surface. This condition is known and is common to many multilayer fabrics. However, when high molecular weight standard round warp is used in attempts to take advantage of the beneficial properties of this harder more abrasive resistance material, the expected crimp exchange does not occur and some lateral crimp is forced into the weft at the expense of vertical crimp. This condition not only reduces the beneficial effects of normal crimp exchange but forces the weft into a complex crimp configuration which makes the seaming operation very difficult as explained previously.

We have found that when the high molecular weight polyester monofilament is flattened void volume is reduced, normal expected crimp exchange does occur, the monoplane characteristic is maintained at the upper surface of the fabric and the weft crimp is predominant at the under surface of the fabric as shown at P in Fig. 2B. In addition to this, lateral crimp in the weft is minimized when flattened high molecular weight polyester warp material is used and the fabric retains all the beneficial characteristics imparted by the use of this material.

While only one mesh pattern is illustrated, other known multi layer single ply or multi-ply mesh patterns may be employed and it is not intended to limit the invention to the pattern shown in Fig. 2.

Although in the preferred embodiment of the invention, as described above, the warp strands are flattened monofilaments, it will be understood that, if the fabric is woven in endless fashion, it will be the monofilament weft strands that are flattened.

Claims (6)

1. A multilayer forming fabric having at least two layers of weft strands interwoven with synthetic monofilament warp strands with said warp strands extending in the machine direction and having a flattened crosssection; the major axis of said cross-section lying substantially parallel to the plane of the fabric thereby reducing the void volume of the fabric, said weft strands at the surface of the fabric that is subjected to wear being prominent over the warp strands and said weft strands at the opposed pulp carrying surface lying substantially in the same plane as the warp strands.

2. A multilayer forming fabric as claimed in claim 1 wherein said warp strands are high molecular weight polyester monofilaments.

3. A multilayer forming fabric is claimed in claim 2 wherein said molecular weight has an I.V. number greater than 0.7 as measured in the Ubbelohde viscometer.

4. A multilayer forming fabric as claimed in claim 1 wherein said cross-section of said flattened warp strands are substantially the same as that of round warp strands in conventional forming fabrics with said major axis of said flattened warp being substantially equal to the diameter of said conventional round warp.

5. A multilayer forming fabric as claimed in claim 1 wherein said warp strands lie wholly within the plane formed by said prominent weft strands at said surface subjected to wear.

6. A multilayer paper-machine forming fabric, substantially as hereinbefore described with reference to the accompanying drawings.