GB2424227A - Woven papermaking fabric - Google Patents

Abstract

A woven papermaking fabric has warps 10,20 and 40,50 stacked perpendicularly to the faces of the fabric in order to provide longitudinally extending grooves 76 for the enhanced removal of trapped air in high speed machines. In the embodiment shown the grooves are formed by single warps 30,60 of a second set located between the stacked pairs of the first set. Two (Fig 7) or three (Fig 16) stacked pairs may be followed by a single warp or one stacked pair may be followed by three single warps (Fig 22). In a second embodiment (Fig 4) both sets comprise stacked pairs (11,21 and 32,41) but warps (41) of the second set are thinner than those (21) of the first set to form the grooves. In another embodiment (Fig 15) all the warps form stacked pairs and the grooves (6) are formed between the narrower warps (23,53) on the surface.

Description

GROOVED PAPERMAKING FABRIC

Field of the invention

The present invention relates to papermakers fabrics including warp yarns arranged in a stacked configuration, and in particular in a configuration which provides longitudinal grooves in at least one of the paper or machine side surfaces of the fabric.

Back2rOund of the invention To increase efficiency and achieve economy in production, there is a constant requirement by paper manufacturers for increased operating speeds for papermaking machines. However, increases in speed create corresponding problems in each section of the machine.

In the dryer section of papermaking machines, increases in operating speeds result in increasingly greater amounts of air becoming entrained in the papermaking fabrics, such that a boundary layer of air moves with the fabrics when they are in use. The result of this phenomenon is that as the fabric carrying the sheet approaches a machine component leading to a change of direction of the fabric, such as a turning roll or dryer cylinder, the entrained and boundary air tend to be pushed through the fabric as it wraps the component. This causes the sheet being supported by the fabric to billow outwards and away from the fabric, and can cause sheet quality problems, and breakages. This phenomenon is discussed further in US Patent No. 4,290,209 to Buchanan et al. The use of a single layer fabric providing continuous support for the paper sheet can achieve a reduction in the entrainment of air; however, the problems of speed differences over the cylinders in the dryer section can result in disadvantageous stretching, and consequent problems of sheet quality.

Attempts to resolve or reduce this problem have thus focused on mechanical solutions to modify components within the machine itself, for example by the arrangement of the dryer drums, as in US Patent 4,835,881. Alternatively, grooves can be machined into selected rolls to provide an air channel, as disclosed in US Patents 5,241,760; 5,887,358 and 5,172, 491. As a further approach, suction openings can be provided in selected rolls, as described in US Patent 5,172,491, or vacuum means can be provided at appropriate locations to provide suction which is applied to the machine side of the fabric, such as described in US Patent No. 6,224,715 and CA Patent No. 1,178,804.

Recently, attempts have been made to design dryer fabrics so as to allow the entrained air to be vented along the machine side of the fabric. For example, US 2004/0126545 to Toney et al. discloses a dryer fabric for single tier dryer sections, whereby a plurality of discrete discontinuous deposits of polymeric resin are applied to the raised portions (knuckles) on the machine side of the fabric, being set and abraded if necessary. The deposits are intended to provide the only contact points between the fabric and the rolls or cylinders, by separating the machine side of the fabric from the roll or cylinder surface to enable trapped air to escape in the machine direction (MD) or cross-machine direction (CD). However, this approach has the disadvantages of requiring an extra processing step, the addition of otherwise superfluous mass to the fabric, and may impede the efficiency of the drying process. Further, in this proposed solution, the open area for air passage may not be sufficient to achieve any significant reduction in the problem of entrapped air.

US 4,813,156 to Ashworth et al., discloses a "supplementary fabric", or alternatively a set of additional MD yarns, to be attached to the machine side of a "base" dryer fabric, to provide for void spaces to receive boundary layer air, and thus reduce the pushing of the air through the fabric. However, as the "base" fabric without the additional yarns or fabric would otherwise meet the requirements for the dryer section, this approach adds undesirable excess mass and an additional step in construction of the fabric.

More recently, Enqvist et al. in US Patent 6,742,548 and in WO 2004/076742 discloses fabrics in which grooves are provided in either the machine side or the paper side of the fabric, or both, by creating an overlap between selected adjacent warp yarns, so that two overlapped yarns are together wider in the plane of the fabric than each single warp yarn by a specified distance, to form the desired grooves. W02004/076742 suggests various methods by which the overlap can be achieved, for example by the selection of high shrinkage materials for the weft (CD) yarns, or the application of high tension to the warp yarns during weaving. Methods of facilitating the required overlap are also disclosed, such as the use of low friction weft yams, by selection of appropriate materials or by coating. However, there are disadvantages due to the imposition of limitations on the specific types of yarn materials which can be used, and any inconsistencies in the movement of the warp yams will lead to inconsistencies in the formation of the grooves, particularly where the grooves are created in both sides of the fabric.

Josef et al. in US 6,837,275 and 6,837,276 discloses fabrics in which a weave pattern provides for three sets of warp (MD) yarns. The pattern provides for long floats of the first two sets in the machine side of the fabric, whereas the third warp yarns act as binder yarns and follow a path in which they interlace in the machine side of the fabric with only one weft in each repeat of the weave pattern. In the MD, between each interlacing point of each third weft yam, a longitudinal groove is defined. The intended effect of the interlacing of a third warp yarn with a single weft yarn between the long floats of the adjacent first or second warp yarns is that the interlacing point will be drawn up into the fabric, i.e. away from the machine side surface, so that there will be reduced or no contact between the machine side knuckles formed by the third warp yarns and the surface of the rolls or cylinders, thus reducing the obstruction of air passage along consecutive sections of the grooves between these interlacing points. However, the presence of these knuckles will inevitably create an intermittent restriction on, and thus alter, the air flow, thus reducing the effectiveness of the grooves, each of which will tend to function more as a sequence of shorter grooves than as a continuous longitudinal groove along the entire length of the fabric.

To overcome these and other disadvantages, it has now been found that weave patterns can be provided for dryer fabrics in which grooves are created by the pattern itself along the full length in the MD, either across the entire CD of the fabric, or at selected portions, such as strips adjacent to the lateral edges, without any superfluous yarns or materials, thus avoiding any undesirable excess mass, and the uncertainties of the approaches which require modifications to be made to the fabric after weaving. This in turn allows for the selection of materials for dryer fabrics to be based on the preferred physical properties for the intended use, without restricting the choices to a more limited range of specialized materials, e.g. those having slippery surfaces, or requiring additional manufacturing steps such as coating or lubrication.

The fabrics of the invention have the further advantages of having a lower weight, and of being more economical to manufacture than the fabrics of the prior art, in requiring less material. Still further, by providing the required path within the fabric for entrapped air removal, expensive (and irreversible) modifications to the machine components can be avoided.

In particular, it has been found that weave patterns can be provided in which the MD yarns comprise stacked groups, being pairs, triples or groups of a greater number of yarns, which can be woven so as to create spaces in the machine side between selected groups, thus providing for continuous longitudinal grooves in the MD, each groove having a substantially constant cross-section along the entire MD length of the fabric, thus providing for substantially consistent, uninterrupted and effective air flow. Thus, the weave patterns of the invention are particularly suitable for use in the construction of fabrics in accordance with US Patent No. 5,103,874 to Lee.

The invention seeks to provide a woven papermaker's fabric having a machine side (MS) surface and a paper side (PS) surface, including at least one grooved portion woven in a first repeating weave pattern which comprises (a) cross-machine direction (CD) weft yarns; and (b) at least a first and second set of machine direction (MD) warp yarns, wherein (i) all the warp yarns of the first set are stacked in substantial alignment in a vertical plane perpendicular to the PS surface with warp yarns selected from the first and second sets of warp yarns; and (ii) the warp yarns of the second set are interwoven with selected weft yarns to provide longitudinal grooves having a substantially constant cross-section along the MD in one of the PS and MS surfaces at groove locations whereby a cross-sectional warp yarn height in the vertical plane through the fabric at each groove location is less than a cross- sectional height of adjacent stacked warp yams.

The invention further seeks to provide a woven papermaker's fabric having a machine side (MS) surface and a paper side (PS) surface, and including at least one grooved portion woven in a repeating weave pattern and comprising (a) cross-machine direction (CD) weft yams; (b) a first set of machine direction (MD) warp yarns interwoven with at least some of the weft yams and comprising a plurality of groups of stacked warp yams wherein each group comprises at least two yams substantially aligned in a vertical plane perpendicular to the PS surface; and (c) a second set of MD warp yams interwoven with at least some of the weft yams to provide longitudinal grooves having a substantially constant cross-section along the MD in at least one of the PS and MS surfaces at groove locations between selected groups of the stacked warp yams of the first set, whereby a cross-sectional warp yam height in the vertical plane through the fabric at each groove location is less than a cross- sectional height of the adjacent stacked warp yams of the first set.

In a first broad embodiment of the invention, the weave pattern provides for all the warp yams of the first and second set to be substantially the same size, but for the lesser warp yarn height in the vertical plane to be attained by providing at least one less warp yam at each groove location than in each adjacent group of warp yarns of the first set. Thus, at each groove location, there can be any suitable number of adjacent warp yams woven to contribute to the fabric layer in the ungrooved surface, which contributes to the horizontal aspect of the configuration of the grooves.

In second and third broad embodiments of the invention, the weave patterns provide for the warp yarns of the second set to have different dimensions from those of the first set. This can be achieved by providing either for the warp yarns of the second set to have a smaller cross- sectional height than the warp yarns of the first set in at least one layer, measured in a direction substantially perpendicular to the fabric surfaces, or for the warp yams of the second set to have a smaller cross- sectional width in the CD than warp yams of the first set with which the warp yams of the second set are stacked.

The suitable ranges of proportionate differences of dimensions of the sets of warp yams in each of these broad embodiments will generally depend on the physical properties required for the fabric, based on the intended end use.

The longitudinal grooves of the fabrics of the invention extend the entire length of the fabric in the MD. However, as noted above, they can be provided for only one or more portions of the fabric, for example at each lateral edge, with an ungrooved section between the grooved edge portions; or the repeating weave pattern can be repeated across the entire CD of the fabric.

Detailed description of the drawin2s

The invention will now be described in greater detail, with reference to the drawings, in which Figure 1 shows a cross-section in the crossmachine direction of a first embodiment of the invention; Figure 2 shows a cross-section in the machine direction of the embodiment of Figure 1, in two repeats of the weave pattern; Figure 3 is a weave diagram of the embodiment of Figure 1; Figure 4 shows a cross-section in the crossmachine direction of a second embodiment of the invention; Figure 5 shows a cross-section in the machine direction of the embodiment of Figure 4, in two repeats of the weave pattern; Figure 6 is a weave diagram of the embodiment of Figure 4; Figure 7 shows a cross-section in the crossmachine direction of a third embodiment of the invention; Figures 8 to 13 show the paths of the machine direction yarns in sequence in the embodiment of Figure 7, in two repeats of the cross-machine direction weave pattern; Figure 14 is a weave diagram of the embodiment of Figure 7; Figure 15 shows a cross-section in the cross-machine direction of a fourth embodiment of the invention; Figure 16 shows a cross-section in the cross-machine direction of a fifth embodiment of the invention; Figures 17 to 20 show the paths of the machine direction yams in sequence in the embodiment of Figure 16, in two repeats of the cross-machine direction weave pattern; Figure 21 is a weave diagram of the embodiment of Figure 16; Figure 22 shows a cross-section in the cross-machine direction of a sixth embodiment of the invention; and Figure 23 is a perspective view of a fabric of the embodiment of Figure 16.

Referring to Figures 1 to 3, a first embodiment of a fabric 1 of the invention is shown, having a paper side surface 2, and a machine side surface 4. The warp yams 10, 20, 30, 40, 50 and interweave with weft yams 200, identified individually as weft yams 201 to 206. In Figure 1, a cross-section in the CD, the arrangement of the six warp yams of this pattern 10 to 60 is shown. The warp yarns 10, 20, and 40, 50 together comprise stacked pairs of a first set of warp yarns. A second set of warp yams is shown as warp yams 20 and 50, between which a groove 6, which extends longitudinally in the MD of the fabric 1, is provided by the weave pattern in the machine side surface 4, below each of warp yarns 30 and 60, which are not exposed in the machine side surface 4.

In this embodiment, the fabric has a warp fill, after heatsetting, of for example 100% in the PS, and 50% in the MS, for a total warp fill of 150%. However, those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

In Figure 2, a cross-section in the MD, two repeats of the weave pattern are shown, indicating the paths of the six warp yarns 10 to 60 as they interweave with selected weft yams 200.

Figure 3 is a weave diagram of the complete pattern, in which the six warp yams 10 to 60 are identified along the lower edge of the diagram, and their respective paths shown in the vertical columns reading upwards, as they interweave with selected weft yarns 200, identified by individual numbers 201 to 206 along the left side of the diagram. As can be seen from Figure 3, the paths of the six warp yams 10 to 60 are such that warp yarns 30 and 60 are not exposed on the machine side surface 4 of the fabric 1.

Referring now to Figures 4 to 6, a second embodiment of the fabric 1 of the invention is shown. As best seen in Figure 4, a cross-section in the CD, all of the four warp yams 11, 21, 31 and 41 comprise a first set of stacked pairs, 11, 21 and a second set of stacked pairs 31, 41. However, warp yams 11, 31, exposed on the paper side surface 2, are smaller in cross- section than warp yarn 21, in the direction substantially perpendicular to the paper side surface 2 and machine side surface 4 of the fabric 1. Warp yam 41 is shown in Figure 4 as being substantially of the same cross- sectional height as warp yams 11 and 31, and smaller than each adjacent warp yam 21. However, it is not necessary for warp yarn 41 to be of the same size as warp yams 11 and 31, which can be of any suitable size for the intended end use of the fabric. Similarly, for ease of understanding, warp yam 21 is shown as being substantially larger than warp yarns 11 and 31. However, it would only be necessary to provide sufficient difference to create a longitudinal groove beneath warp yarn 41, and the selection of appropriate warp yarn shapes and sizes, and the proportionate difference in dimensions, will be dictated by the requirements for the intended end use. Thus, a longitudinal groove 6 is provided between each lower warp yam 21 of the first set, below the stacked pair of warp yarns 31, 41 of the second set.

In this embodiment, the fabric has a warp fill, after heatsetting, for example, of 100% in the PS, and 100% in the MS, for a total warp fill of 200%. However, those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

Figure 5, a cross-section in the CD, shows the paths of the four warp yams 11, 21, 31 and 41 as they interweave with the weft yams 200, identified individually as weft yams 211, 212, 213 and 214, in two repeats of the weave pattern.

Figure 6 is a weave diagram of the complete pattern, in which the four warp yarns 11 to 41 are identified along the lower edge of the diagram, and their respective paths shown in the vertical colunms reading upwards, as they interweave with selected weft yarns 200, identified by individual numbers 211 to 214 along the left side of the diagram. As can be seen from Figure 6, the paths of the four warp yarns 11 to 41 are such that warp yams 41 are not exposed on the machine side surface 4 of the fabric 1.

Referring now to Figures 7 to 14, a third embodiment of the invention is shown. Figure 7 shows a cross-section in the CD of a fabric 1, having a paper side surface 2 and machine side surface 4. Paper side warp yams 12 and 32 and machine side warp yarns 22 and 42 are arranged such that a longitudinal groove 6 is provided between adjacent stacked pairs either of warp yams 12 and 22 or of warp yams 22 and 42. In Figure 7, these yams are identified with the suffixes A, B or C for ease of understanding. Thus, reading from left to right in Figure 7, two warp yam pairs 12A, 22A and 32A, 42A are followed in the pattern by a single warp yarn I 2B exposed in the paper side surface 2. Under warp yarn 1 2B a longitudinal groove 6 is provided. Thereafter in the pattern, warp yarn pairs 32B, 42B, and 12C, 22B, are followed by a second longitudinal groove 6, under warp yarn 32C which is exposed in the paper side surface 2. Thus the stacked warp yams 12A, 22A, 32A, 42A, 32B, 42B, 12C and 22B together comprise a first set of warp yams, and warp yams 12B and 32C together comprise a second set of warp yams.

In this embodiment, the fabric has a warp fill, after heatsetting, for example, of 100% in the PS, and 67% in the MS, for a total warp fill of 167%. However, those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

The individual paths of each of these warp yams 12A, 12B, 12C, 22A, 22B, 32A, 32B, 32C and 42A, 42B, are shown successively in Figures 8 to 13, as they interweave with selected weft yams 200, identified individually as weft yarns 221 to 224. Figures 10 and 13 show the paths of the warp yarns 12B and 32C, each exposed in the paper side surface, and between each of which a longitudinal groove 6 is formed.

Figure 14 is a weave diagram of the complete pattern, in which the ten warp yams 12A to 32C are identified along the lower edge of the diagram, and their respective paths shown in the vertical columns reading upwards, as they interweave with selected weft yams 200, identified by individual numbers 221 to 224 along the left side of the diagram. As can be seen from Figure 14, the paths of the four warp yarns 12A to 32C are such that warp yams 12B and 32C are not exposed on the machine side surface 4 of the fabric 1.

Referring now to Figure 15, a fourth embodiment of the invention is shown, in cross-section in the CD. In this embodiment, fabric 1, having a paper side surface 2 and machine side surface 4, comprises paper side warp yams 13, 33 and 43, and machine side warp yarns 23 and 53, woven in a weave pattern in which all the warp yams comprise stacked pairs. Each of machine side warp yams 23 and 53 is narrower measured in the CD than the corresponding one of paper side warp yams 13, 33 and 43, such that between each pair of adjacent warp yarns 23 and 53, a longitudinal groove 6 is provided by the weave pattern. In this embodiment, warp yarns 13, 33 and 43 together comprise a first set of warp yarns, each of which is stacked in vertical aligimient with one of a second set of warp yarns, 23 or 53.

In this embodiment, the fabric has a warp fill, after heatsetting, for example, of 100% in the PS, and 50% in the MS, for a total warp fill of 150%. However, those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

Referring now to Figures 16 to 21, a fifth embodiment of the invention is shown, which is suitable for use for fabrics where a low warp fill is required. Figure 16 shows a cross-section in the CD of a fabric 1, having a paper side surface 2 and a machine side surface 4, and indicates that the openness of the weave can be increased without affecting the provision of the longitudinal grooves. In this embodiment, the warp yams 14, 24, 34,44, 54 and 64 comprise three stacked pairs of a first set of warp yarns. Between the third stacked pair, 54, 64 and the first pair 14, 24 of the next repeat of the weave pattern, a longitudinal groove 6 is provided by the pattern under a warp yarn of a second set, shown as warp yarn 74.

In this embodiment, the fabric warp fill, selected according to the intended end use, can after heatsetting be as low as 50% in the PS, and 37% in the MS, for a total warp fill of 87%.

However, these values are exemplary only, and those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

Figures 17 to 19 show the successive paths of the three stacked pairs of warp yarns, 14, 24; 34, 44; and 54, 64, of this embodiment, as they interweave with selected weft yarns 200, in two repeats of the weave pattern. Figure 20 shows the path of warp yam 74 as it interweaves with selected weft yams 200 to provide the longitudinal groove 6.

Figure 21 is a weave diagram of the complete pattern, in which the seven warp yarns 14 to 74 are identified along the lower edge of the diagram, and their respective paths shown in the vertical colunms reading upwards, as they interweave with selected wefi yarns 200, identified by individual numbers 241 to 244 along the left side of the diagram. As can be seen from Figure 21, the paths of the seven warp yarns 14 to 74 are such that warp yams 74 are not exposed on the machine side surface 4 of the fabric 1.

In each of the embodiments thus far described, the longitudinal grooves 6 provided by the weave patterns each have a width in the CD equal to or approximately equal to one warp yarn, and spaced apart from each other by one or more warp yams in the machine side surface 4. However, the longitudinal grooves 6 can be made wider to meet the intended end use of the fabrics 1, by the selection of weave patterns in which the grooves are the width of two or more of the warp yams.

For example, referring now to Figure 22, a sixth embodiment of the invention is shown, in which the longitudinal grooves 6 are the width of three warp yams. Thus, in each repeat of the weave pattern there is a single stacked pair of a first set of warp yarns, 15 and 25, followed in each repeat by three warp yams of a second set, shown as 35, 45 and 55, below which a wide longitudinal groove 6 is provided, having the total combined width in the CD of yarns 35, 45 and 55.

In this embodiment, the fabric has a warp fill, after heatsetting, for example, of 100% in the PS, and 25% in the MS, for a total warp fill of 125%. However, those of skill in the art will adjust the weaving and heatsetting parameters to achieve a desired warp fill in accordance with the properties required for the intended end use for the fabric.

Referring now to Figure 23, a perspective view of one of the fabrics 1 of the invention is shown, from the under or machine side surface 4. This figure corresponds to the weave pattern of the embodiment shown in Figure 16. Running from the lower left to the upper right of the figure, groups of three warp yarns, 24, 44 and 64 are separated by longitudinal grooves 6, each interweaving with selected weft yams 200 and being stacked below a corresponding one of warp yams 14, 34 and 54, which each interweave with selected weft yams 200. As can best be seen at the upper left of the figure, the machine side warp yarns 24, 44 and 64 are not exposed on the paper side surface 2 of the fabric 1, and the paper side warp yams 14, 34 and 54 are not exposed on the machine side surface 4 of the fabric 1. Adjacent to each warp yarn 54 and warp yarn 14 of the next repeat of the weave pattern, each warp yarn 74 is exposed on the paper side surface 2 but not on the machine side surface 4, thus providing the longitudinal grooves 6. As described above in relation to Figure 16, the warp yam pairs 14, 24; 34, 44; and 54, 64 together form a first set of warp yarns, and warp yarns 74 comprise a second set.

In addition to the features described above, other properties of the fabrics incorporating the weaves of the invention can be selected according to the intended end use of the fabric. As the longitudinal grooves are provided by the weave pattern during weaving, the provision of the grooves does not itself impose any restriction on the materials which can be used for the yarns, which will be dictated by the desired properties for the end use. Similarly, the dimensions of the yarns used in the fabrics of the invention can be selected according to the properties required for the intended end use, provided that the appropriate relationship between those dimensions is maintained according to the selected embodiment.

Claims (14)

1. Claims 1. A woven papermaker's fabric having a machine side (MS) surface

   and a paper side (PS) surface, including at least one grooved portion woven in a first repeating weave pattern which comprises (a) cross-machine direction (CD) weft yarns; and (b) at least a first and second set of machine direction (MD) warp yarns, wherein (i) all the warp yams of the first set are stacked in substantial alignment in a vertical plane perpendicular to the PS surface with warp yams selected from the first and second sets ofwarpyams;and (ii) the warp yarns of the second set are interwoven with selected weft yams to provide longitudinal grooves having a substantially constant cross-section along the MD in one of the PS and MS surfaces at groove locations whereby a cross-sectional warp yam height in the vertical plane through the fabric at each groove location is less than a cross- sectional height of adjacent stacked warp yams.
2. 2. A woven papermaker's fabric as claimed in Claim 1 wherein all the MD warp yams of the first set are stacked only with other warp yams of the first set and in groups comprising at least two yams.
3. 3. A woven papermaker's fabric having a machine side (MS) surface and a paper side (PS) surface, and including at least one grooved portion woven in a repeating weave pattern and comprising (a) cross-machine direction (CD) wefi yams; (b) a first set of machine direction (MD) warp yams interwoven with at least some of the weft yams and comprising a plurality of groups of stacked warp yams wherein each group comprises at least two yarns substantially aligned in a vertical plane perpendicular to the PS surface; and (c) a second set of MD warp yams interwoven with at least some of the weft yams to provide longitudinal grooves having a substantially constant cross-section along the MD in at least one of the PS and MS surfaces at groove locations between selected groups of the stacked warp yams of the first set, whereby a cross-sectional warp yam height in the vertical plane through the fabric at each groove location is less than a cross- sectional height of the adjacent stacked warp yarns of the first set.
4. 4. A woven papermaker's fabric as claimed in Claim 1 or Claim 3, comprising two grooved portions each being located adjacent to a lateral edge of the fabric.
5. 5. A woven papennaker's fabric as claimed in Claim I or Claim 3, wherein the entire fabric is woven to the first repeating weave pattern.
6. 6. A woven papermaicer's fabric as claimed in any of Claims 2 to 5 wherein each group of stacked warp yarns comprises a pair.
7. 7. A woven papermaker's fabric as claimed in any of Claims 2 to 5 wherein the vertical plane through the fabric at the groove locations is provided by a single layer of warp yarns of the second set interwoven with selected weft yarns.
8. 8. A woven papermaker's fabric as claimed in Claim 7 wherein each groove location comprises between 1 and 4 adjacent warp yarns of the second set.
9. 9. A woven papermaker's fabric as claimed in any of Claims 2 to 5 wherein the warp yarns of the first set include MS warp yarns which are exposed on the MS surface of the fabric and not the PS surface of the fabric, and each warp yarn of the second set has a smaller cross- sectional height than each MS warp yarn of the first set.
10. 10. A woven papermaker's fabric as claimed in Claim 9 wherein each group of stacked warp yarns comprises a pair.
11. 11. A woven papermaker's fabric as claimed in Claim 3, wherein grooves are provided on both the PS and MS surfaces.
12. 12. A woven papermaker's fabric as claimed in Claim I wherein each warp yarn of the second set is stacked with, and has a smaller cross-sectional width in the CD than, a warp yarn of the first set.
13. 13. A woven papermaker's fabric as claimed in any of Claims ito 11 wherein the fabric has a warp fill after heatsetting of at least 80%.
14. 14. A woven papermaker's fabric as claimed in any of Claims 1 to ii wherein the fabric has a warp fill after heatsetting of at least 150%.