WO1991002642A1

WO1991002642A1 - Molded paper clothing

Info

Publication number: WO1991002642A1
Application number: PCT/US1990/002624
Authority: WO
Inventors: Robert G. Wilson
Original assignee: Huyck Corporation
Priority date: 1989-08-25
Filing date: 1990-05-10
Publication date: 1991-03-07
Also published as: AU5735390A; CA2016998A1

Abstract

A molded paper clothing of a cast plastic grid with uniform drainage interstices (4) bounded on all four sides by streamline shaped interstitial edges and a method to produce same is presented. In the machine direction (1), reinforcing strands (2) are molded into the gridlike clothing, the strands being of synthetic fiber or metal and are chosen to increase tensile strength and wear resistance. The clothing is made into an endless loop and can be used as the support fabric for all sections of a papermaking machine. The method to produce the molded paper clothing includes introducing a first plastic sheet (P) onto a patterned embossed surface of a forming roll (E), applying heat and pressure with heated pressure roll (B) and applying additional heat with heaters (HTR) and additional pressure with pressure rolls (C).

Description

MOLDED PAPER CLOTHING

BACKGROUND OF THE INVENTION

This invention relates to papermaking and more particularly to a novel permeable paper machine forming fabric screen which maximizes support for the paper fiber, minimizes wire marks on the paper, has a long life, is stretch resistant, is mechanically stable, and is economical to produce. The concepts of^" the present invention also may be applied in other stages of the papermaking process, to form fabric screens serving as dryer fabrics or press felt base fabrics.

The present day papermaking machine is a sophisticated device to remove water from the paper furnish. Water is generally removed sequentially in three stages of the papermaking machine. In the first section or the forming section, the furnish is deposited on a forming wire or fabric and water is drained through the forming fabric to leave a paper web generally having an 18-25% solids content. This paper web is carried to a wet press felt section and passed through one or more nip presses. A paper felt including a base fabric to which fibers are needled is used to carry the paper web in this section. The outgoing paper web has a solids content of approximately 36-44%. In the final stage, the paper web is carried to heated cylinders which dry the paper web resulting in a web with about a 92-96% solids content. A dryer fabric is used to transport the paper web through this final stage.

The fabric or clothing used to support and transport the paper web during these stages must perform a widely diverse range of functions according to its position on the machine. Generally, the clothing or fabric used in each section of the papermaking machine is manufactured to meet the specific requirements of each section. Thus, a different felt or fabric is used in different sections of the papermaking machine.

The fabric in the forming section serves as a filter to separate the cellulosic fibers from the aqueous medium. The fabric separates the fibers by providing drainage of the aqueous medium through the mesh openings, also known as drainage holes. The forming fabric also functions as a drive belt to transport the paper web.

In the forming section, the fabric used to support and transport the web is woven to form a regular array of mesh openings. The web is woven in a variety of manners and in a variety of meshes. Mesh refers to warp strands per inch of width. The fabric is woven from natural or synthetic yarns. The desired characteristics of the fabric in the forming section include stability in the plane of the cloth, flexibility in the machine direction, resistance to stretching, resistance to wear and high drainage capacity. The fabric, in addition to providing effective support for the web, must produce a smooth papermaking surface to reduce or eliminate wire marking on the forming web.

The fabric in the press section serves as support for the paper web while more water is removed. The paper web and fabric, generally called a press felt, are passed through rollers and water is thereby extracted from the web and felt. It is desirable that the felt of the press section have large voids so that water may be conveyed from the web through the felt and removed after passage over a suction box.

Important characteristics of the papermaking felt include maximizing the amount of moisture absorbed by the felt, the ability of the felt to release the collected water to a suction pipe, the air permeability of the felt, and the flow resistance of the felt. In addition to having the above production characteristics, the papermaking felt should have high resistance to compaction, high resistance to heat degradation and long life.

In the dryer section of a papermaking machine arrays of heated cylindrical rolls are arranged and spaced in staggered parallel rows. The paper web is passed to and fro between the array of dryer rolls in a generally serpentine manner to ensure that both sides of the paper web contact the dryer rolls. Dryer fabrics are used to transport the web through the dryer rolls. These fabrics must be fluid permeable to permit the moist air and steam to pass therethrough.

Since the dryer fabric is used to support the paper web as it passes between dryer cylinders, desirable characteristics of the dryer fabric include high permeability, high machine direction strength and high resistance to heat degradation. In addition to these characteristics, it is important for the dryer fabric to have a long life.

To date no fabric has been produced incorporating the desired characteristics for each paper making section to thereby provide a single papermaking fabric for a papermaking machine.

SUMMARY OF THE INVENTION

The fabric of the present invention is a molded, embossed or cast grid with drainage interstices bounded on all sides by streamlined shaped interstitial edges. In the load carrying or machine direction, reinforcing strands are molded or cast into the structure to allow for high tension holding capabilities. The strands, for example, can be synthetic materials such as Kevlar , metal such as bronze or plastic such as polyester.

The fabric is produced by embossing or penetrating a plastic sheet through the use of heat and pressure on a patterned forming roll. The forming roll is manufactured with longitudinal and circumferential grooves so that as the plastic is heated and pressed against the roll it takes the shape of the pattern on the roll surface. In the machine direction, reinforcing strands are fed onto the forming roll surface into the circumferential grooves so that the strands are encased in the resultant molded fabric. The fabric is made into an endless belt by using an appropriate adhesive to precisely attach the lapped ends of the fabric.

Due to the fact that there are no woven fibers exposed from the molded fabric, the fibers of the web cannot get entangled in the finished fabric. This leads to a papermaking fabric that has excellent sheet release, has very clean surface operation, produces an excellent papermaking surface because there can be no picking, and produces high machine efficiency.

The molded fabric sheet is drier off the couch roll because there are no hidden voids in the fabric to hold water and thereby rewet the paper web. Furthermore, better vacuum efficiency is achieved as water is more easily stripped from the surface of the molded fabric than from conventional woven fabrics. In addition, the drainage is uniform as there is no weight variation of the fabric due to the absolute hole uniformity. Finally, due to the straight-through nature of the drainage channels , edge trim and full width knock-off is minimized and high pressure shower cleaning is very efficient. In addition to the superior performance characteristics of the molded fabric of the present invention, the production rate of molded fabric is 50-100 times greater than the production rate of woven fabric. Therefore, a substantial cost advantage exists with the fabric of the present invention.

It is therefore an object of the present invention to produce a papermaking fabric that is useful in all phases of a papermaking machine.

Another object of the present invention is to produce a fabric that has superior stretch resistance when compared to conventional woven fabrics.

Yet another object of the present invention is to produce a papermaking fabric that has a completely coplanar surface.

Still another object of the present invention is to produce a papermaking fabric that produces a superior quality paper.

Another object of the present invention is to produce a papermaking fabric that carries a minimum amount of water.

A further object of the present invention is to produce a papermaking fabric that has a long life.

Still another object of the present invention is to produce a papermaking fabric that is low in cost.

Still another object of the present invention is to produce a papermaking fabric that is easy to clean. These and other features and objects of the present invention will be more fully understood from the following detailed description of the invention taken together with the drawings, in which corresponding reference numerals represent corresponding parts throughout the several views.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional magnified view of the preferred embodiment of the papermaking fabric of the present invention;

FIG. 2 is a process diagram showing how to make the papermaking fabric of the present invention;

FIG. 3 is a close-up view of the surface of the roll used to make the papermaking fabric of the present invention;

FIG. 4 is a cross-sectional view of the roll surface of the roll used to make the papermaking fabric of the present invention;

FIG. 5 is a cross-sectional view of the portion of the papermaking fabric where it is joined to form the endless loop of the papermaking fabric of the present invention; and

FIG. 6 is a cross-sectional view of a simplified embodiment of the papermaking fabric of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will initially be described broadly, with a more detailed description following. The present invention is a unique papermaking fabric and a method to produce it. The papermaking fabric of the present invention is a molded, embossed or cast grid with drainage holes bounded on all sides by streamlined shaped interstitial edges to allow for uniform drainage throughout the fabric. In the load carrying or machine direction, reinforcing strands are molded or cast into the structure to allow for high tension holding capability.

As used herein, the term "mesh" refers to the number of strands per inch with machine direction being the first number and cross machine direction being the second number. Therefore a 70 x 80 mesh fabric would have 70 strands per inch in the machine direction and 80 strands per inch in the cross machine direction. "Fabric" refers to an endless belt used to support the papermaking web through any stage of the papermaking machine. "Load bearing yarns" refer to yarns in the fabric that provide tensile strength in the machine direction. "Wear yarns" refer to yarns that improve wearability of the fabric on the machine contacting side. "Reinforcing yarns" is used to refer to load bearing yarns and wear yarns. Finally, the term "molded" is meant to include casting or embossing or any heat and pressure treatment of the fabric.

FIG. 1 depicts a magnified view of the papermaking fabric of the present invention. The arrow 1 of FIG. 1 shows the machine direction or the direction of travel of the web. Uniform interstices 4 provide the drainage holes, which may be of any shape, for example round, square, oval triangular or rectangular, required for papermaking fabrics. The interstices 4 can be spaced such that meshes of up to 70 x 80 are possible with this fabric. In the machine direction, load bearing yarns 2 are molded into the fabric to provide high tensile strength. These load bearing yarns are preferably made of synthetic fibers such as Kevlar, a trademark of-El Dupont de Nemours Company of Wilmington, Delaware for an aromatic polyamide containing para-oriented linkages (or briefly an aramide) , or metal strands such as bronze. The embodiment of the fabric of the present invention shown in FIG. 1 incorporates wear yarns 3 on the machine side of the fabric. The wear yarns are preferably formed from a plastic such as polyester or nylon.

FIG. 2 discloses a preferred method of manufacture of the molded fabric of the present invention. The method is performed by a patterned forming roll E, generally having a 4-6 foot diameter. Plastic film P is transported to the forming roll E through lead-in roll A and heated pressure roll B. Reinforcing yarn RY, which can be a load bearing yarn or wear yarn, is similarly transported to the patterned forming roll E through lead-in roll A' and heated pressure roll B'. A second layer of plastic P' is transported to the forming roll through lead roll A" and heated pressure roll B". The arrangement of the rolls in effect sandwiches the reinforcing yarn, RY between two layers of plastic P and P'. If desired, additional reinforcing yarns can be transported to the patterned forming roll E. Additional heaters HTR and pressure rolls C around the surface help mold the forming fabric. The final product of molded fabric is transported off the forming roll E by take-off roll D.

The forming fabric is molded by the patterned forming roll E. The patterned forming roll E has an embossed surface as shown in FIGS. 3 and 4. FIG. 3 is a three dimensional magnified view of a part of the roll surface. FIG. 4 shows the dimension of a typical groove on the surface of the roll. Of course, the pattern on the forming roll may be changed to accomodate the requirements of the finished molded fabrics. In the preferred embodiment, each groove is 0.020 inches deep and .007 inches wide. The grooves form a point, shown in FIG. 4 at Point "P", so that when the plastic film is molded through heat and pressure, the surface of the formed fabric takes on the shape of the pattern of the roll. Additional reinforcing yarns in the machine direction can be transported to the patterned forming roll so that increased tensile strength and increased wear resistance of the fabric can be achieved. As shown in FIG. 1 the wear yarns 3 form the surface of the fabric that contacts the papermaking machine. The fabric can be constructed so that the wear yarns embedded in the plastic increase the life of the fabric.

A typical known duplex forming fabric has a volume of material equal to approximately .0005 inches 3/inches2 of fabric to wear away. If the molded fabric of the present invention has a flat bottom and is a mesh of 70 x 80 with machine direction yarns being .18 mm wide and cross machine direction yarns being .16 mm wide, the volume of material available for wear is as follows:

(.18 mm x 25.4 mm x 70) + (.18 x .16 x 70 x 80) = 481.32 mm² inch in

481.32 mm 2 x 1 inch2 x depth = Volume of material for wear in 2 (25.4)2mm2 inch2 of fabric

746 x depth = Volume of material for wear

2 inch of fabric

For a wear depth of .004 inches, the molded fabric has approximately .003 inches 3/inches2 of fabric to wear away.

This is approximately six times the amount of a typical known duplex fabric. Thus, increased life expectancy results with the fabric of the present invention. : ^■

FIG. 5 shows the joining point at which the fabric of the present invention is formed into an endless loop through the use of a lap joint. The basic plastic material to form the fabric is shown by P and P' . The reinforcing yarn is shown by RY. Preferably, the interface between the two ends is approximately 2-4 inches in length as shown by G. The fabric is cut down on either end so that the total depth of the joined fabric at the lap joint is equal to the depth of fabric throughout the rest of its length. Adhesive AD is used to join the interface between the two lap joints. If the adhesive shear strength is 5000 psi and the joint is 2 inches long and the mesh is 70 and the width of each strand is 0.006 inches, the expected tensile strength would be:

.006 x 2.0 x 70 x 5000 = 4200 ^pounds inch This strength is over four times the strength of Kevlar yarns which are generally used alone in known fabrics. Thus, by incorporating the concepts of the present invention, a papermaking fabric at its point of joining to form an endless belt has superior machine direction strength over that of woven fabrics. In addition, with a fabric formed according to the concepts of the present invention, there is no surface discontinuity at the seam joint thereby reducing imperfections in the paper web.

FIG. 6 shows a simplified molded fabric made according to the teachings of the present invention. The plastic grid is strengthened in the machine direction by load bearing yarn 2. The load bearing yarn may be made of Kevlar or bronze strands. No wear yarn is embedded in the plastic grid in this embodiment. Streamlined edges at the interstices allow for uniform drainage across the fabric. The fabric depicted in FIG. 6 is easier to manufacture and less costly than the fabric depicted in FIG. 1. The fabric of the present invention is superior to known woven fabrics for a number of reasons. Because the fabric has strands embedded in the machine direction, the fabric is more resistant to stretching than in woven fabrics. In addition, the web surface of the fabric is coplanar as the web surface matches the embossed surface of the roll. The molded fabric achieves absolute support coplanarity, while woven fabrics contain dips and valleys at the yarn interlacing or crossing points and these are responsible for surface imperfections on the paper web. The absolute support surface coplanarity virtually eliminates wire marks on the paper web, in turn producing superior web as wire marking is minimized while sheet release is improved. Finally, it is expected that the fabric of the present invention will have a longer life than conventional fabrics as there is more material to wear on the machine side of the fabric.

While the foregoing invention has been described with reference to its preferred embodiments, various alterations and modifications will occur to those skilled in the art. For example, the number and kind of strands per inch can be altered in the fabric to achieve different operating characteristics. These and other alterations and modifications are intended to fall within the scope of the appended claims.

What is claimed is:

Claims

1. An endless molded paper fabric for a papermaking machine comprising: a plastic grid with uniform drainage holes, the drainage holes being bounded on all sides by interstitial edges; reinforcing yarns embedded in the plastic grid in the direction of travel of the fabric on the papermaking machine.

2. -The endless fabric of claim 1 wherein the plastic grid is formed from polyester.

3. The endless fabric of claim 1 wherein the plastic grid is formed from nylon.

4. The endless fabric of claim 1 wherein the reinforcing yarns are made of aramide.

5. The endless fabric of claim 1 wherein the reinforcing yarns are made of bronze.

6. The endless fabric of claim 1 wherein the plastic grid contains at least 80 interstices per inch in the machine direction and 70 interstices per inch in the cross machine direction.

7. The endless fabric according to claim 1 wherein the fabric is formed endless by lapping each end of the plastic grid and adhesively bonding the lapped ends together.

8. An endless molded fabric clothing for a papermaking machine comprising: a plastic grid with rectangular drainage holes, having a machine side and a support side, the drainage holes being bounded on all sides; reinforcing yarns embedded in the plastic grid in the direction of travel of the fabric on the papermaking machine; wear yarns embedded in the plastic grid in the machine direction and positioned so that the wear yarns are positioned on the machine side of the plastic grid.

9. The endless fabric of claim 8 wherein the plastic grid is made of polyester.

10. The endless fabric of claim 8 wherein the plastic grid is made of nylon.

11. The endless fabric of claim 8 wherein the reinforcing yarns are made of ara ide.

12. The endless fabric of claim 8 wherein the reinforcing yarns are made of bronze.

13. The endless fabric of claim 8 wherein the wear yarn is made of polyester.

14. The endless fabric of claim 8 wherein the wear yarn is made of polyaramid.

15. The endless fabric of claim 8 wherein the fabric is formed endless by lapping each end of the plastic grid and adhesively bonding the lapped ends together.

16. A process for producing a molded paper clothing comprising: introducing a first plastic sheet onto a patterned embossed surface of a forming roll; introducing reinforcing yarns onto the surface of the forming roll; introducing a second plastic sheet onto the surface of forming roll; applying heat and pressure to surface of the forming roll such that the reinforcing yarns are embedded in a plastic grid formed from the first and second plastic sheets and the plastic grid is formed with uniform interstices.

17. The process of claim 16 wherein the first and second plastic sheets are made of polyester.

18. The process of claim 16 wherein the first and second plastic sheets are made of nylon.

19. The process of claim 16 wherein the first and second plastic sheets are made of polyaramid.

20. The process of claim 16 wherein the reinforcing yarns are made of aramide.

21. The process according to claim 16 wherein the reinforcing yarns are made of metal.

22. The process of claim 16 wherein the formed plastic grid contains at least 80 interstics per inch in the machine direction and 70 interstices per inch in the cross-machine direction.

23. A process for producing a molded fabric clothing comprising: introducing a first plastic sheet onto an embossed surface of a forming roll; introducing reinforcing yarns onto the surface of the forming roll; introducing a second plastic sheet onto the surface of the forming roll; introducing wear yarns onto the surface of the forming roll; applying heat and pressure to surface of the forming roll such that reinforcing yarns and wear yarns are embedded in a plastic grid formed from the first and second plastic sheets and the plastic grid is formed with uniform interstices.

24. The process of claim 23 wherein the first and second plastic sheets are made of polyester.

25. The process of claim 23 wherein the first and second plastic sheets are made of nylon.

26. The process of claim 23 wherein the first and second plastic sheets are made of polyaramid.

27. The process of claim 23 wherein the reinforcing yarns are made of aramide.

28. The process of claim 23 wherein the reinforcing yarns are made of metal.

29. The process of claim 23 wherein the wear yarns are made of nylon.

30. The process of claim 23 wherein the wear yarns are made of polyester.

31. The process of claim 23 wherein the wear yarns are made of polyaramid.

32. The process of claim^"23 wherein the plastic grid contains at least 80 interstices per inch in the machine direction and 70 interstices per inch in the cross-machine direction.