Title of Invention

IMPROVED WOVEN FORMING FABRIC SCREEN FOR USE IN THE FORMING ZONE OF A PAPER MAKING MACHINE

Abstract

An improved woven forming fabric screen for use in the forming zone of a paper making machine, said fabric screen comprising a 16-shaft weave structure, wherein sixteen pairs (A-P) of weft threads extend in a cross-machine direction, each pair of weft threads having a top weft (WT) and a bottom weft (MB) lying one above the other; and warp threads {WP1-WP16} interweave each pair of weft threads in a definite repeating pattern, and extend in a machine direction, each repeating pattern comprising a said warp thread (WP1) having a path passing successively - over two pairs (A, B) of weft threads; then between the top and bottom wefts of two adjacent pairs (C, D) of weft threads; then over the next two adjacent pairs (B, F) of weft threads; then between the top and bottom wefts of the next four pairs (G-J) of weft threads; then under one pair (K) of weft threads; and finally between the top and bottom wefts of the next five pairs (L-P) of weft threads, each warp thread in the pattern following the same path, but with a different pair of weft threads.

Full Text

The present invention relates to an improved woven forming fabric screen for use in a paper making machine. In the paper sheet forming zone of a paper making machine, water suspension of paper pulp is spread on a moving endless belt made of fabric screen woven from synthetic filaments, This forming fabric screen in endless belt form runs over and around a set of components like back roll, couch roll, hydro foils, table rollers, suction box etc. Aqueous suspension of paper pulp is supplied from a head box and evenly distributed over the travelling endless forming fabric screen. Water drains out through the pores provided in the forming fabric screen, with the help of dewatering elements to form a self-supporting paper pulp mat on the forming fabric screen. This is called paper web. When the paper web reaches the end of the forming zone, it is picked up and transferred to the press section of the machine, where the paper web is passed through the nips of a series of press rollers to squeeze out any remaining water from the paper web. Then, the paper web is transferred to a drier section where it is passed over a series of heated cylinders to evaporate the remaining moisture in the paper web and to form a final paper sheet. Forming fabric screen for use in paper making machines is usually in the form of a fine-mesh cloth which is woven in endless manner,or otherwise joined to form an endless web. Quality of paper depends on the web formation of the mesh cloth, i.e. the structure of the forming fabric is of vital -2- and decisive importance. Until the middle of 1950s, all forming fabrics were made of metal wires. Around the fifties, metal wire cloths were replaced by synthetic wire cloths since these synthetic wire cloths have better wear resistance as compared to metal wire cloths. After the introduction of synthetic wire cloths, there have been experimentations to develop various woven fabrics for use in paper manufacturing machines, such improved fabrics have helped in improving productivity, reducing down time and also in reducing costs. But, there is need for improving the fabric structure which would offer longer running time, save power consumption and also reduce consumption of costly chemicals. The forming fabric should have the following characteristics, namely: (a) high resistance to vibration? (b) ability to withstand tension applied to it in a paper making machine, so that it is dimensionally stable? (c) uniform drainage of water through the fabric; (d) good retention of fibres and fillers forming the paper web; (e) should be a good paper sheet supporting surface without excessive markings on the paper sheet; (f) should be a fabric sheet from which the paper sheet can be released while passing to the press section. Production of forming fabric consists of the following main steps: -3- (a) weaving with synthetic wires as warps and wefts on a weaving machine such as a loom; (b) Heat setting the woven fabric under controlled temperature and tension to make it dimensionaliy stable; (c) Seaming after heat setting and cutting the fabric to required length, to join both ends to form an Endless fabric; and (d) Dressing the seamed endless fabric under controlled temperature and tension. In prior art, forming fabrics having 2-shaft weave, 3-shaft weave, 4-shaft weave and 5-shaft weave which are single layer fabrics are known. Single layer fabrics have the following drawbacks, namely, low resistance to abrasion, higher elongation, lower stability and poor fibre retention and have a tendency to leave marking on the paper sheet. To overcome these drawbacks, a double layer forming fabric was evolved. In this double layer fabric, pairs of weft yarns are placed one above the other and warp yarns interweave these two layers of weft yarns. In this case, the warp threads are so woven with the weft threads that the pattern repeats after specific intervals. The pattern repeat of the warp threads recurs after a given number of threads. This warp repeat is called 'Shaft' or 'Harness*. In the double layer fabric, 7-shaft and 8-shaft structures are known. -4- A double layer fabric has two distinct surfaces, namely top surface and bottom surface. The top surface supports paper making fibres, retains them alongwith other ingredients and allows smooth transfer of the paper web from paper web forming part (also called wire part) of the paper making machine to the press part of the machine. The bottom surface of the fabric comes in contact with the components of the paper making machine. So, the bottom surface should have all the characteristics necessary for withstanding wear and tear, and must offer higher running time. In order to achieve this, there must be more material at the bottom surface to withstand abrasion without disturbing other characteristics of the fabric and at the same time must have higher running time. This cannot be achieved in conventional double layer fabrics, as with increase in the diameter and count of bottom weft, air permeability through the fabric reduces. Also rate of drainage of water will be poor, which directly affects the paper making characteristics and runnability of the paper machine. The object of the present invention is to provide an improved woven forming fabric screen which overcomes the drawbacks of conventional forming fabric screens, so that greater number of weft threads can be inserted without sacrificing air permeability, fibre support and other basic requirements of paper making. The fabric of the present invention also helps in drastically cutting down power consumption. To achieve these objects, the present invention provides an improved woven forming fabric screen for use in the forming zone of a paper making machine, said fabric screen comprising a 16-shaft weave structure, wherein sixteen pairs of weft threads extend in a cross-machine direction, each pair of weft threads having a top weft and a bottom weft lying one above the other; and warp threads interweave each said pair of weft threads in a definite repeating pattern, and extend in a machine direction to form said 16-shaft double layer fabric forming screen, each said repeating pattern comprising a said warp thread having a path passing successively -over two said pairs of weft threads; then between the top and bottom wefts of two adjacent pairs of weft threads; then over the next two adjacent pairs of threads; then between the top and bottom wefts of the next four pairs of weft threads; then under one pair of weft threads; and finally between the top and bottom wefts of the next five pairs of weft threads, each warp thread in said pattern following the said path, but with a different pair of weft threads. The improved screen has the following advantages over prior- art: -6- (a) More weft threads per unit with higher diameter and also the longer float in the bottom surface offers more material for abrasion which gives more running time; (b) More weft threads per unit in the top surface of the fabric offer better fibre support, better formation of paper sheet, better paper sheet release from wire part to press part and less marking on the paper sheet; (c) The weave structure of this fabric ensures higher permeability through the fabric, though the weft count and diameter are higher; (d) The higher permeability offers better and smooth drainage of water through the fabric (as its drainage Index is higher than conventional double layer fabric) and this leads to lower drag load for de-watering in a paper making machine and so power consumption is reduced considerably; (e) As this structure gives greater weft count than conventional structure, its T-Count (Warp Count + Weft Count) is higher and this gives the fabric more rigidity and dimensional stability. (f) The fabric has greater number of supporting point per sq. cm. so that it can provide better support to fibres, better sheet transfer and less marking on the paper. The present invention will hereafter be described with reference to the accompanying drawings, wherein -Fig. 1A shows a 2-shaft weave forming fabric; -7- Pigs. 1B and 1C show front view and side view of the weave shown in Fig. 1A; Fig. 2A shows a 3-shaft weave forming fabric; Figs. 2B and 2C show front view and side view of the weave shown in Fig. 2A; Fig. 3A shows a 4-shaft weave forming fabric; Figs,. 3B and 3C show front and side views of the weave shown in Fig. 3A; Fig. 4A shows a 5-shaft weave forming fabric-Figs. 4B and 4C show front and side views of the weave shown in Fig. 4A; Figs, 5A and SB show top surface and bottom surface respectively of a 8-shaft weave double layer forming fabric; Fig. 5C shows the top and bottom surfaces of the fabric of Figs. 5A and 5B combined; Figs, 5D and 5E show respectively the path of a top weft in the top surface of the fabric and the path of the bottom weft in the bottom surface of the fabric of Fig. 5C; Fig. 5F shows the path of a warp thread of Fig, 5C; Fig, 6A shows the top surface of a double layer forming fabric according to the present invention; Fig. SB shows the bottom surface of a double layer forming fabric according to the present invention; Fig. 6C shows the top and bottom surfaces of the fabric shown in Figs. 6A and €B combined; Fig, 6D shows the path of a top weft of Fig, 6C; -8- Fig. 6E shows the preferred path of a bottom weft of Fig. 6C; and Fig. 6F shows the path of a warp thread in the fabric screen of the present invention. Figs. lA-1C show a 2-shaft plain weave, in which a warp thread WP1 passes over a weft thread WP1 and under a weft thread WF2. The vertical column in Fig. 1A shows the warp path and the horizontal column shows the weft path in a weaving pattern. ' + ' sign denotes a warp thread in up position over a weft thread. The blank portions each indicate a weft thread in up position over a warp thread. (' + ' sign and blank portion have the same meaning in all the rest of the Figures). In this fabric, the weft repeat is 2. Figs. 2A-2C shows the weaving pattern of a 3-shaft single layer forming fabric. Here, a warp WP goes over one weft thread WP and under two weft threads WF. The weft repeat is 3. In Figs. 3A-3C, the 4-shaft single layer fabric has a weaving pattern, in which a warp thread WP goes over one weft thread WF and under three weft threads WF. The weft repeat in this structure is 4. 5-shaft single layer fabric shown in Figs. 4A-4C has a structure in which a warp thread WP goes over one weft thread WF and under four weft threads WF. Here, the weft repeat is 5. Figs. 5A-5F show a conventional 8-shaft double layer forming fabric. Fig. 5A shows the top surface of the fabric, -9- while Pig. 5B shows the bottom surface of the fabric. The two surfaces are shown as combined in Fig, 5C to form a double layer fabric, The path of the warp thread is shown in Pig. 5F, Each pair of weft threads comprises a top weft WT and a bottom weft WB arranged one above the other, A warp thread WPl passes over two adjacent pairs {1-2 and 3-4} of weft threads, then between the top and bottom wefts of the next three pairs (5-6, 7-8 and 9-10) of weft threads, under one pair (11-12) of weft threads and again between the top and bottom wefts of the next pair (13-14 & 15-16) of weft threads. Fig. 5D shows the path of top weft WT lying on the top surface of the fabric. The top weft WT passes under the first warp WPl, then under the next four warps {WP2-WP5} , then under the warp WP6 and then again over two warps WP7 and TOPS. Fig. 5E shows the path of a bottom weft WB. The bottom weft passes under the first warp, then over the second warp and thereafter under the remaining six warps. So, here the bottom weft goes over one warp thread and below the other seven warp threads. All the top wefts and bottom wefts follow the same path as shown in Figs. 5D and 5E respectively, but with different warp threads. There are eight top wefts and eight bottom wefts. So, the weft repeat in this weave is 16. The forming fabric of the present invention is shown in Figs. 6A-6F. This is a 16-shaft weave forming fabric screen. Pig. 6A shows the top surface of the fabric, while Fig. 6B -10- shows the bottom surface of the fabric. Fig. 6C shows these two surfaces combined together. In this case, the weft threads extend in a cross-machine direction of the fabric while the warp threads run in the machine direction, i.e. in the longitudinal direction of the fabric. Fig. 6D shows a preferred embodiment of the path of a top weft thread of Fig. 6C. The top weft WT passes under a warp thread, then next over three warp threads WP, under a warp thread, over the next four warp threads, under a warp thread, over the next three warp threads, again under a warp thread and finally over the next two warp threads. From Fig. 6E, it can be seen that, in a preferred embodiment, the bottom weft WB goes under warp threads WPl to WP10, then over one warp thread WP11 and then again under the remaining warp threads WP12 to WP16. So, the bottom weft WB is passing over only one warp thread WPll and is below the remaining warp threads. In this structure, the bottom weft has a float which is much higher than that in the conventional 8-shaft fabric screen of Figs. 5A-5F. Thus, more material is available in the bottom surface of the forming fabric of the present invention to withstand abrasion in a paper making machine. All the top and bottom wefts follow the same path as shown in Figs. 6D and 6E, but with different warp threads as shown in Fig. 6C. Fig. 6F shows the path of the first warp thread of Fig. 6C. Each pair of weft threads comprises a top weft WT and a bottom weft WB. The warp thread WPl passes successively over -11- two pairs (A, B) of weft threads, then between the top and bottom wefts of the next two pairs (C, D) of weft threads, over two pairs (E, F) of weft threads, then between top and bottom wefts of the next four pairs (G-J) of weft threads, then under one pair (K) of weft threads, and finally between top and bottom wefts of the next five pairs (L-P) of weft threads. Each warp thread follows the same path, but with different pairs of weft threads as shown in Fig. 6C. The warp thread pattern repeats after every 16 warp threads. There are 16 top wefts and 16 bottom wefts, so that there are 32 wefts in the fabric per pattern. Here, the weft repeat is 32. As would be clear to a person skilled in the art, the forming fabric of the present invention can be woven on a weaving machine such as a loom in a manner known per se. -12- A comparison between the conventional 8-shaft double layer fabric and the present invention is given below: Conventional 8-Shaft Present Invention: Double Layer Fabric 16-Shaft Double Layer Fabric 1. Warp Dia (mm) 0.20 0.20 2. Warp Count/cm 56 56 3. Top Weft Dia (mm) 0.22 0.20 4. Top Weft Count/cm . 18 21 5. Bottom Weft Dia 0.30 0.35 6. Bottom Weft Count 18 21 7. T-Count 92 98 8. Permeability-CFM 386 423 (eft per min. per sq.ft.) 9. Supporting Points/sq.cm 252 294 10.Drainage Index 15.44 18.33 The above comparison clearly shows the following advantages over the prior art: la) More weft yarns per unit in the top surface of the fabric offers better fibre support; (b) More weft yarns per unit with higher diameter offers more material for abrasion which gives more running time; (c) Higher permeability, although the diameter and count is higher,- -13- (d) Since the T-Count is higher, the fabric is more rigid and dimensionally stable; (e) With higher drainage index the fabric will provide smooth drainage and reduce the load on the fabric and also save energy. -14- WE CLAIM: 1. An improved woven forming fabric screen for use in the forming zone of a paper making machine, said fabric screen comprising a 16-shaft weave structure, wherein sixteen pairs (A-P) of weft threads extend in a cross-machine direction, each pair of weft threads having a top weft (WT) and a bottom weft {WB) lying one above the other; and warp threads (WP1-WP16) interweave each said pair of weft threads in a definite repeating pattern, and extend " in a machine direction to form said 16-shaft double layer fabric forming screen, each said repeating pattern comprising a said warp thread (WP1) having a path passing successively -over two said pairs (A, B) of weft threads; then between the top and bottom wefts of two adjacent pairs (C, D) of weft threads; then over the next two adjacent pairs (E, F) of weft threads; then between the top and bottom wefts of the next four pairs (G-J) of weft threads; then under one pair (K) of weft threads; and finally between the top and bottom wefts of the next five pairs (L-P) of weft threads, each warp thread in said pattern following the said path, but with a different pair of weft threads. -15- 2. An improved woven forming fabric screen as claimed in claim 1, wherein - said top wefts (WT) and bottom wefts (WB) constitute respectively the top surface and the bottom surface of said screen ; a top weft (WT) in each repeating pattern has a path which passes under the first warp thread, then over the next three warp threads, then under the adjacent warp thread and then again over four adjacent warp threads, then under a warp thread and over three adjacent warp threads, then under a warp thread, and finally over the next two warp threads, all the top wefts following the same path, but with different warp threads ; and a bottom weft (WB) in each repeating pattern has a path which passes under the first ten warp threads (WP1-WP10), over the next warp thread (WP11) and then under the rest of the warp threads (WP12-WP16), all the bottom wefts following the same path, but with different warp threads. 3. An improved woven forming fabric screen, substantially as herein described particularly with reference to and as illustrated in the accompanying drawings. 4. A paper making machine incorporating an improved woven forming fabric screen as claimed in claim 1 or 2. An improved woven forming fabric screen for use in the forming zone of a paper making machine, said fabric screen comprising a 16-shaft weave structure, wherein sixteen pairs (A-P) of weft threads extend in a cross-machine direction, each pair of weft threads having a top weft (WT) and a bottom weft (MB) lying one above the other; and warp threads {WP1-WP16} interweave each pair of weft threads in a definite repeating pattern, and extend in a machine direction, each repeating pattern comprising a said warp thread (WP1) having a path passing successively - over two pairs (A, B) of weft threads; then between the top and bottom wefts of two adjacent pairs (C, D) of weft threads; then over the next two adjacent pairs (B, F) of weft threads; then between the top and bottom wefts of the next four pairs (G-J) of weft threads; then under one pair (K) of weft threads; and finally between the top and bottom wefts of the next five pairs (L-P) of weft threads, each warp thread in the pattern following the same path, but with a different pair of weft threads.

Documents:

02191-cal-1998-abstract.pdf

02191-cal-1998-claims.pdf

02191-cal-1998-correspondence.pdf

02191-cal-1998-description(complete).pdf

02191-cal-1998-drawings.pdf

02191-cal-1998-form-1.pdf

02191-cal-1998-form-2.pdf

02191-cal-1998-form-3.pdf

02191-cal-1998-g.p.a.pdf

2191-cal-1998-granted-abstract.pdf

2191-cal-1998-granted-acceptance publication.pdf

2191-cal-1998-granted-claims.pdf

2191-cal-1998-granted-correspondence.pdf

2191-cal-1998-granted-description (complete).pdf

2191-cal-1998-granted-drawings.pdf

2191-cal-1998-granted-examination report.pdf

2191-cal-1998-granted-form 1.pdf

2191-cal-1998-granted-form 2.pdf

2191-cal-1998-granted-gpa.pdf

2191-cal-1998-granted-letter patent.pdf

2191-cal-1998-granted-reply to examination report.pdf

2191-cal-1998-granted-specification.pdf