In the press section of a paper making machine the still very wet paper web is carried between the press rolls supported by at least press felt. Generally, a "traditional"prior art press felt comprises at least two, and often three or more layers. These are a press felt base fabric which forms the core of the press felt, and which amongst other things provides the press felt with adequate mechanical strength and stability. The press felt base fabric is typically a single woven layer, or a multilayer fabric comprising several woven layers. In each case, each layer is the length and width of the finished press felt. The other layer or layers comprise at least one layer of batt which is needle punched, or otherwise mechanically, attached to at least one side of the press felt base fabric; in a two layer fabric the batt is needled to only one side of the press felt base fabric, and in a three, or more, layer fabric the additional layers of batt are needled to both sides of the press felt base fabric. The press felt functions in the press section of the papermaking machine to support the embryonic paper web as it leaves the forming section, and to provide a reservoir into which water that is expressed from the web by mechanical pressing by the press section press rolls may move. The structure of the press section base fabric has an influence on the drainage properties of the press felt of which it forms one part.

Although this inherently simple from of press felt is known, in more modern practise the press section base fabric itself is often more than one layer of obtained by spirally assembling the fabric from a relatively narrow strip of woven material, often about 1 m wide, using the methods broadly described by Best et al, US 5,268, 076, Svensson et al, US 5,360, 656, Fekete, US 5,785, 818. or Baker et al, US 6,030, 908.

In these fabrics, during the spiral assembly step there are two possible ways to assemble the fabric: either the strips in all of the layers are all wound at the same small angle to the machine direction, or although the winding angles are all still the same, the succeeding layers are in a sequence such that in a first layer the warp yarns are located at a first small angle to one side of the machine direction, and in a second immediately adjacent layer the warp yarns are located at a second small angle to the other side of the machine direction. In the context of this invention, the term"machine direction" refers to the direction in which the press felt moves within the press section and the term"cross machine direction"refers to a direction essentially within the plane of the press felt and substantially perpendicular to the machine direction. In most spirally wound multilayer press felt base fabrics all of the angles to each side of the machine direction are the same, and generally are from about 3° to about 10° to the machine direction. It then follows that the warp yarns in each succeeding layer form a shallow V with each other.

Fabrics of this type using two, three or four layers angled relative to each other are known, and have met with significant commercial success. However although these multilayer fabrics are successful, it has been found that some structures used in such fabrics can cause marking of the wet paper web, the cause of. which appears to be uneven drainage of water into and through the press felt from the wet paper web.

The cause for this uneven drainage has now been found to be related at least in part to the structure of each of the layers of the press felt base fabric. These layers are spiral wound from strips woven to an essentially regular square weave pattern, such as a plain weave: the warp yarns are usually more or less in the machine direction and the weft yarns are more or less in the cross machine direction, the deviation in each case being the angle chosen in the spiral winding step. If two layers made from strips each of which has the same essentially square pattern are located at the same angle and in the same direction relative to the machine direction then uneven drainage of the web does not generally occur. But if the two layers are located at the same angle to the machine direction but with each layer differently oriented relative to the machine direction then uneven drainage can occur, which in its turn can result in marking of the wet paper web.

It has now been realised that this marking is caused by the formation of a Moire pattern between the two overlaid fabric webs. When two essentially square patterns, such as those found in woven fabrics, are over laid with coincident axes, so that the two sets of warp yarns and weft yarns are essentially parallel to each other, a regular pattern of spaces through the two layers can be seen. But if the two layers are overlaid with the two sets of warp yarns and weft yarns at a small angle to each other, then a regular pattern of spaces through the two layers cannot be seen, and a so-called Moiré pattern is found. When a Moire pattern is obtained areas are visible with but little open space together with areas of considerable open space. In the areas with low open space, water drainage will be lower than in the areas with considerable open space. It then follows that even though the fabric strips used in the spiral winding process to make the layers have a relatively open weave pattern, the drainage properties of a pair of layers located at a small angle to each other is directly related to the Moire pattern.

Additionally, when the batt is needled onto one, or both, sides of the spirally wound base fabric, the needles tend not to puncture the monofilament yarns from which the individual press felt base fabric layers are woven, but rather penetrate beside them. It then follows that any Moire pattern inherent in the alignment of the layers in the individually spirally wound press felt base fabric layers tends to be reproduced in the batt layer or layers, thus exacerbating the water drainage problems.

This invention seeks to provide a solution to this problem of uneven drainage, and thus to provide a e press fabric base fabric for use in a spirally wound press felt base fabric comprising at least two layers which does not result in the presence of a Moire pattern between the two adjacent layers which may cause both uneven web drainage or web marking.. According to this invention, this improvement in water drainage properties is obtained by the introduction of randomness into the weave pattern of at least some of the layers used in the press felt base fabric. It also then follows that if the Moire pattern is eliminated, then reproduction of this pattern in the batt layers is not possible, thereby reducing or substantially eliminating any mark imparted to. the paper web carried by the press felt.

Thus in a first broad embodiment, this invention seeks to provide a multi layer spirally wound base fabric for use in making a press section fabric for a paper making machine which press section fabric includes the spirally wound base fabric together with at least one layer of batt needled to at least one side of the base fabric and which base fabric has a machine direction and a cross machine direction substantially perpendicular to the machine direction, wherein: - the base fabric includes N fabric layers; - in the base fabric N is an integer and is at least 2; - in the base fabric each layer comprises warp and weft yarns woven to a repeating pattern, the warp yarns in each layer being spirally wound at a small angle to the machine direction; - in the press felt base fabric the warp yarns in succeeding fabric layers are arranged in a sequence such that in a first layer the warp yarns are spirally wound at a first small angle to one side of the machine direction and in a second immediately adjacent layer the warp yarns are spirally wound at a second small angle to the other side of the machine direction so that the direction of the warp yarns in each layer forms a shallow V with the direction of the warp yarns in any immediately adjacent flayers ; in the base fabric at least, N-1 of the layers are woven. to'at least one repeat pattern which introduces at least some randomness into the appearance of the fabric surface.

Preferably, in the base fabric all of the N layers are woven to at least one repeat pattern which introduces at least some randomness into the appearance of the fabric surface. More preferably, in the base fabric all of the N layers are woven to a repeat pattern which introduces at least some randomness into the appearance of the fabric repeat pattern.

The invention will now be described with reference to the attached drawings in which: Figure 1 shows schematically two overlaid identical square patterns in which the axes of the patterns are essentially parallel; Figure 2 shows schematically the same two square patterns in which the axes of the two patterns are at a small angle; Figures 3 and 4 show two views of a first fabric useable according to this invention at different magnifications; Figures 5 and 6 show two views of a second fabric useable according to this invention at different magnifications; and Figure 7 shows a third fabric according to this invention.

Referring first to Figures 1 and 2 the principles of a Moire pattern will be explained. In each of these figures two square grid patterns 1 and 2, which will in practise correspond to the warps and wefts. in two fabrics woven to the same pattern, for example to a plain weave, are shown.

In Figure 1, the two patterns 1 and 2 are shown with their two axes as at, 3 and 4 essentially parallel. It can then be seen that in the overlap portion 5 of the two patterns 1 and 2 although the free space between adjacent elements of each grid is now much smaller, the free space is constant across the overlap area.

But in Figure 2, due to the small angle between the axes of the two grids, it can be seen that the free space is not constant over the overlap area. Areas of relatively higher free space, as at 6 and 7, and areas of lower free space, as at 8 and 9, can both be readily identified. This is a simple example of a Moire pattern obtained by combining two essentially square grid patterns with a small angle, as at 10, between their axes. Similar patterns can be developed for the more complicated weave patterns used in the manufacture of the strips used for spirally wound press felt base fabrics. It can then be seen that, if a pattern of this type exists between two adjacent spirally wound layers within a press felt base fabric, drainage of water through the fabric will not be constant and uniform over the area of the press felt. It is this variation in drainage capabilities which causes marking in the wet paper web as it passes through the paper making machine press section.

In order to overcome the formation of a Moire pattern in the fabrics of this invention a level of randomness is introduced into the weave pattern of fabric strips used in the spiral winding process to create the press felt base fabric. The consequence of this is that the surface of the fabric no longer is the conventional more or less rectangular grid of a conventional woven fabric.

A first example of a fabric useful in this invention is shown in Figures 3 and 4. This is a pattern where the weft yarns, which are in the cross machine direction, do not follow the usual more or less linear path across the fabric. Although the warp yarns 11 in Figures 3 and 4 follow a more or less linear path, the weft yarns 12 and 13 do not follow a linear path; it can be seen that wefts 12 and 13 are not even more or less parallel to each other. This breaks up the grid effect, and reduces the Moire pattern effects. This can be seen in Figure 4, where the possibility of a Moire pattern being formed is decreased by the randomness of the weave pattern used in the production of the strips used in the spiral winding process.

The details of this fabric are as follows. In both the weave pattern given below, and those for later Figures, the weft yarns are across the diagram, and the warp yarns up and down the diagram. In the weave patterns, warp yarns 1 and 2 are used to form a leno weave on the edge of the fabric which does not participate in this invention.

Warp yarn size:. 30mm; warp yarn count: 12.2 yarn/cm.

Weft yarn size:. 40mm; weft yarn count: 11.0 yarn/cm.

Weave pattern. 3 4 5 6 7 8 9 10 4 X X X 3 x x x x 2 x x x x I x x x x 3 4 5 6 7 8 9 A second example of a fabric useful in this invention is shown in Figures 5 and 6. This is a pattern where the weft yarns, which are in the cross machine direction, do not follow the usual more or less linear path across the fabric. Although the warp yarns 14 in Figures 5 and 6 follow a more or less linear path, only the weft yarn 16 follows a linear path; it can be seen that wefts 15 and 16 are not even more or less parallel to each other. It can also be seen from Figure 5 that two different sized weft yarns have been used. This breaks up the grid effect, and reduces the Moire pattern effects.

This can be seen in Figure 6, where the possibility of a Moire pattern being formed is decreased by the randomness of the weave pattern.

The details of this fabric are as follows.

Warp yarn size:. 50mm; warp yarn count: 5.5 yarn/cm..

Weft yarn sizes:. 30mm and. 48mm; weft yarn count: 12.2 yarn/cm.

Weave pattern. 3 4 5 6 7 8 9 10 16 X X X X 15 X X 14 X X X X 13 X X 12 X X X X 11 X X 10 X X X X 9 X X 8 X X X X 7 X X 6 X X X X i 3 4 5 6 7. 8 9 10 5 X X 4 X X X X 3 X X 2 X X X X 1 X X A third example of a fabric useful in this invention is shown in Figure 7. This again is a pattern where the weft yarns, which are in the cross machine direction, do not follow the usual more or less linear path across the fabric. Although the warp yarns 17 in Figure 7 follow a more or less linear path, the weft yarns 18 and 19 do not follow a linear path; it can be seen that wefts 18 and 19 are not even more or less parallel to each other. This again breaks up the grid effect, and reduces the Moire pattern effects. This can be seen in Figure 7, where the possibility of a Moire pattern being formed is decreased by the randomness of the weave pattern.

Weave pattern. 3 4 5 6 7 8 9 10 3 4 5 6 7 8 9 10 8 X X X X 7 X X 6 X X X X 5 X X 4 X X X X 3 X X 2 X X X X 1 X X It is noted that fabrics of this broad type in which at least some of the yarns do not follow a linear path are described by Wright, in US 5,025, 839.

Within the concepts of this invention there are certain flexibilities in the choices that can be made.

For the invention to be useful, the press felt base fabric has to include at least two spirally wound layers which do not have their axes in alignment. Generally, the fabric axes will be at an angle of from about 3° to about 10° to the machine direction of the press section. Although the two spirally wound fabrics are generally at the same angle to the machine direction, this need not be so.

Although the fabrics of this invention include two or more spirally wound layers, it is not necessary for them all to be made from the same fabric. It is only necessary that the spirally wound fabric should include sufficient layers including randomness for the Moire pattern effects to be eliminated. This will generally call for one layer with randomness for every layer without. In fabrics with an odd number of layers it is preferred that there be more layers with randomness than without.