Abstract:
A method of producing an endless belt includes the steps of: securing axial fibers relative to a mandrel, the axial fibers being spaced apart from one another at desired intervals and extending substantially parallel to a longitudinal axis of the mandrel; applying a polymeric base layer to the mandrel in a thickness sufficient to embed the axial fibers; wrapping circumferential fibers onto the polymeric base layer with sufficient tension to partially embed the circumferential fibers in the polymeric base layer; applying a polymeric top stock layer over the polymeric base layer and circumferential fibers; and curing the base layer and the top stock layer. This method can improve productivity and performance of endless belts, particularly if the wrapping and latter applying steps closely follow the first applying step.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application Serial No. 60/378,146, filed May 14, 2002, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to nip presses, and more particularly to shoe presses.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a typical papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper “stock”) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a “forming fabric,” provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the “machine side”) of the fabric.  
           [0004]    After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more presses (often roller presses) covered with another fabric, typically referred to as a “press felt.” Pressure from the presses removes additional moisture from the web; the moisture removal is often enhanced by the presence of a “batt” layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.  
           [0005]    Over the last 25 or 30 years, a “shoe press” has been developed for the press section of the papermaking machine. A shoe press includes a roll or similar structure that mates with a “shoe” of an opposed roll or press structure; the surface of the shoe is somewhat concave and approximates in curvature the convex profile of the mating roll. This arrangement can increase the width of the nip in the direction of paper travel, thereby enabling greater amounts of water to be removed therein.  
           [0006]    Endless belts or blankets have traditionally been used in shoe press operations. The belt overlies and contacts the shoe of the press; in turn, a press felt such as that described above overlies the shoe press belt, and the paper web overlies the press felt. The shoe press belt and press felt travel through the nip and, in doing so, convey the paper web through the nip. The press felt is driven by a set of drive rollers arranged around the shoe or by the press roll itself. In older embodiments, shoe press belts were also driven by sets of drive rollers arranged around the shoe. In some newer configurations, however, the shoe press belt is clamped or otherwise fixed to the edges of circular head plates located on either end of the shoe, such that rotation of the head plates causes the shoe press belt to rotate and travel through the nip.  
           [0007]    Given the performance requirements, a shoe press belt should be sufficiently flexible to pass around the drive rollers or head plates and through the shoe and sufficiently durable to withstand the repeated application of pressure within the nip. Because of these performance parameters, most endless belts are formed entirely or predominantly of a polymeric material (often polyurethane). Many shoe press belts also include reinforcing fibers or a reinforcing fabric between or embedded in polymeric layers. Also, shoe press belts may be configured to encourage water to pass from the paper web. To this end, some shoe press belts have grooves or blind-drilled holes in the surface adjacent the press felt that serve to vent water from the paper that is exiting the press felt.  
           [0008]    Some of the issues that arise with the manufacture of a shoe press belt are the accurate placement of reinforcing fibers within the belt (and the application of material around them). Proposed approaches to the creation of shoe press belts are discussed in, for example, U.S. Pat. No. 5,525,194 to Jermo, U.S. Pat. No. 5,134,010 to Schiel, U.S. Pat. No. 5,320,702 to Matuschczyk, and U.S. Pat. No. 5,118,391 to Matuschczyk. However, there still exists a need for expediting and improving the manufacturing processes for shoe press belts.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention can facilitate the production of shoe press belts, and in particular shoe press belts having axially-extending reinforcing fibers that are positioned radially inwardly of circumferentially-extending fibers. As a first aspect, the present invention is directed to an endless belt for a shoe press, comprising: a polymeric matrix formed into an endless loop; multiple bands of axial fibers, the fibers being embedded in the polymeric matrix, the bands including spacing material at each end that maintains a desired circumferential spacing between the fibers and further including securing structure that is adapted for securing the fibers to a mandrel; and circumferential fibers that circumferentially overlie and are spaced from the axial fibers, the circumferential fibers being embedded in the polymeric matrix. In some embodiments, the polymeric matrix comprises a base layer in which the axial fibers are embedded and a top stock layer that overlies the circumferential fibers. The sheet material and securing structure can maintain the axial fibers in a desired position and spacing during the production of the belt.  
           [0010]    As a second aspect, the present invention is directed to an endless belt for a shoe press comprising: a polymeric base layer formed of a first polymeric material; axially extending fibers embedded in the base layer; circumferential fibers that circumferentially overlie the polymeric base layer; and a polymeric top stock layer that circumferentially overlies the circumferential fibers, the top stock layer being formed of a second polymeric material that differs from the first polymeric material. In this configuration, the belt can include one material that is particularly suited for contact with a shoe press and another material that is particularly suited for contact with a press felt.  
           [0011]    As a third aspect, the present invention is directed to a method of producing an endless belt, comprising the steps of: securing axial fibers relative to a mandrel, the axial fibers being spaced apart from one another at desired intervals and extending substantially parallel to a longitudinal axis of the mandrel; applying a polymeric base layer to the mandrel in a thickness sufficient to embed the axial fibers; wrapping circumferential fibers onto the polymeric base layer with sufficient tension to partially embed the circumferential fibers in the polymeric base layer; applying a polymeric top stock layer over the polymeric base layer and circumferential fibers; and curing the base layer and the top stock layer. This method can improve productivity and performance of endless belts, particularly if the wrapping and latter applying steps closely follow the first applying step. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]    [0012]FIG. 1 is a front section view of a shoe press belt manufactured by the process of the present invention.  
         [0013]    [0013]FIG. 2 is a front view of a mandrel employed in the process of the present invention.  
         [0014]    [0014]FIG. 3 is an enlarged partial front view of an end portion of the mandrel of FIG. 2 with axial fibers mounted thereon.  
         [0015]    [0015]FIG. 4 is a front view of the mandrel of FIG. 2 with axial fibers mounted thereon.  
         [0016]    [0016]FIG. 5A is a top view of a band of axial fibers (including its laminated ends) to be included in a shoe press belt according to the present invention being formed on a fixture.  
         [0017]    [0017]FIG. 5B is a front view of the band of axial fibers and the fixture of FIG. 5A.  
         [0018]    [0018]FIG. 6A is an enlarged top view of one end of the band of axial fibers of FIG. 5A.  
         [0019]    [0019]FIG. 6B is an enlarged top view of one end of an alternative laminated section of a band of axial fibers according to the present invention.  
         [0020]    [0020]FIG. 7 is a perspective view of the mandrel of FIG. 2 with base layer and top stock nozzles and a circumferential fiber applicator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The present invention will now be described more fully hereinafter, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. Thicknesses and dimensions of some components may be exaggerated for clarity.  
         [0022]    Referring now to the drawings, a portion of a shoe press belt, designated broadly at  20 , is illustrated in FIG. 1. The belt  20  has an endless looped polymeric matrix  21  that, in the illustrated embodiment, includes a base layer  22 , axially-extending reinforcing fibers  24 , circumferentially extending reinforcing fibers  26 , and a top stock layer  28 . In the illustrated embodiment, the base layer  22  completely encapsulates the axial fibers  24  (which are typically positioned about 0.025″-0.050″ above the bottom surface of the base layer  22 ) and extends about 0.020″ above the tops of the axial fibers  24 . The circumferential fibers  26  are partially embedded (typically buried about halfway) in the base layer  22 . The top stock layer  28  covers and seals the circumferential fibers  26 ; the top stock layer  28  cross-links with the base layer  22  and provides adequate thickness (typically between about 0.050 and 0.300 inches) for further finishing operations. A typical belt  20  may be between about 40 and 80 inches in diameter, 50 and 400 inches in length, and 0.100 and 0.300 inches in thickness.  
         [0023]    Both the base layer  22  and top stock layer  28  are typically formed of a polyurethane-based material (i.e., one that is primarily formed of polyurethane), preferably one having a hardness of between about 29 and 60 on the Shore D scale, or alternatively may be formed of polyester. The material may have fillers, additives and the like (for exemplary materials, see U.S. Pat. No. 4,859,396 to Krenkel et al., the disclosure of which is hereby incorporated herein by reference in its entirety). It may be preferable to employ two different polyurethane-based materials for the base and top stock layers  22 ,  28 . For example, a slightly harder material (e.g. one with a Shore D hardness of between about 29 and 45) may be used for the base layer  22 , which will be in contact with the shoe of a shoe press, and a slightly softer material (e.g., one with a Shore D hardness of between about 45 and 60) may be used for the top stock layer  28 , which will be in contact with a press felt.  
         [0024]    The reinforcing fibers  24 ,  26  may be formed of any suitable reinforcing material, but will ordinarily be formed of polyester, aramid, liquid crystal polymer, or other high performance fibers between about 0.008 and 0.050 inches in diameter. The fibers  24 ,  26  may be monofilament or multifilament strands. It is also contemplated that the fibers  24 ,  26  make take a flat, ribbonlike form, as this configuration may provide performance and manufacturing advantages.  
         [0025]    Those skilled in this art will appreciate that, although a shoe press belt is described herein, a belt of similar structure may also be employed as a shoe calender belt; reference herein to a belt for a shoe press in intended to also include a belt for a shoe calender.  
         [0026]    Referring now to FIG. 2, the belt  20  may be formed on a mandrel  30 . Ordinarily, the mandrel  30  is supported at either end by bearings  35  on which it is rotatably mounted. The mandrel  30  should have a cylindrical working surface  32  that is long enough to accommodate the largest anticipated paper machine working width (typically 400 inches), the additional length required to reach the shoe press heads (10-20 inches per end), the additional length required to form any belt tabs (10-20 inches per end) (see U.S. Pat. No. Re 33,034 to Schiel for a description of belt tabs), and the space required to start and end the rotational cast process ( 12  inches per end). The length of the working surface  32  should be selected accordingly.  
         [0027]    Preferably, the mandrel  30  includes a slightly undersized inner metallic or composite core  33  and a hard outer layer  34  (formed of rubber or some other easily worked material) that provides the working surface  32 . It is preferred that, if a separate outer layer is used and it is formed of an elastic or polymeric material, the outer layer is “bone-hard” (typically between 0 and 2 on the Pusey and Jones hardness scale), and that it be of sufficient thickness that, through grinding, the diameter can be modified to enable the formation of belts of slightly different diameters.  
         [0028]    Prior to the application of polyurethane or other suitable polymeric material to the mandrel  30 , provisions may be made to the working surface  32  to assist with belt removal. Exemplary surface treatments include coating with mold release, wrapping with sheets of Teflon® or other low friction material, or the like.  
         [0029]    After the mandrel  30  has been prepared, the axial reinforcing fibers  24  are loaded onto the ends of the mandrel  30 . In one embodiment of the invention, the axial fibers  24  are first formed into laminated multifiber bands (one of which is illustrated in FIGS. 3 through 6A and designated therein at  40 ). The band  40  includes a plurality of fibers  24  (for example, 70 at a time) strung in parallel relationship and laminated at each end with lamination sheets  42  or other sheet material. Adhesive on the lamination sheets  42  can adhere the sheets  42  together; alternatively, the lamination sheets  42  can be heat-bonded. Other spacing material, such as a slotted card, may also be used to maintain the axial fibers in a desired spacing.  
         [0030]    In the illustrated embodiment, tails  44  of the fibers  24  extend beyond the lamination sheets  42  and are knotted together. The knotted portions  46  of the band  40  are then secured to the ends of the mandrel  30  with tensioning hooks (not shown) mounted in a ring  36  located on the end of the mandrel  30 ; if desired, the tensioning hooks may include a spring mechanism to maintain relatively consistent tension in the fibers  24 . In other embodiments, a grommet (designated at  48  in FIG. 6B) or other suitable securing structure for attachment to the mandrel  30  may be included in the lamination sheets  42  in place of the knotted portions  46 .  
         [0031]    The lamination sheets  42  may maintain the fibers  24  at a desired uniform spacing between adjacent fibers  24  and at a desired distance from the working surface  32 . Alternatively, a spacer ring or toothed belt or chain (not shown) can be attached to the ends of the mandrel  30  to maintain the fibers  24  in these positions.  
         [0032]    The axial fiber bands  40  can be formed, for example, with a fixture such as that designated at  49  in FIGS. 5A and 5B. Axial fibers  24  are dispensed from individual creels  51  and threaded sequentially through a spacer board  53 , between vertically stacked rollers  55 , through second and third spacer boards  57   a,    57   b  (passing through a tensioning weight  59  between the spacer boards  57   a,    57   b ), and through a narrower spacing card  61  that positions the fibers  24  in a desired regular gapped relationship (typically, the gap between adjacent fibers is between about 0.030 and 0.250 inches). The fibers  24 , while remaining in the gapped relationship, extend to a platform  63  that slides on rails  67  (driven by a screw  65 ) away from the spacing card  61 . The platform  63  includes hooks (not shown) onto which the knotted portions  46  of the band  40  are hooked.  
         [0033]    Referring still to FIGS. 5A and 5B, the band  40  is produced by locking the holding rollers  55  so that the fibers  24  do not slip, creating a desired tension in the fibers  24  by sliding the platform  63  along the rails  67  with the screw  65 , and laminating either one or, preferably and as shown, two sections of the fibers  24  near the spacer card  61  with the lamination sheets  42   a,    42   b.  Doing so completes the production of one band  40 , which now has lamination sheets  42 ,  42   a  on both ends, and begins the production of the next band  40 , which now has one end laminated with lamination sheet  42   b.  The portions of the fibers  24  between the lamination sheets  42   a,    42   b  are cut and knotted, the band  40  is removed and stored, and the lamination sheet  42   b  and its attached fibers are moved to and mounted on the platform  63  to complete the production cycle.  
         [0034]    Referring now to FIG. 7, after the axial fibers  24  have been loaded onto the mandrel  30  and are positioned as desired, the base layer  22  and circumferential fibers  26  are applied. The base layer  22  may be applied by a casting nozzle such as that designated at  50  in FIG. 7. The base layer  22  is preferably applied to a thickness that fully embeds the axial fibers  24  (a thickness that exceeds the top of the axial fibers  24  by about 0.020 inches is preferred. During application, the nozzle  50  begins at one end of the mandrel  30  and moves axially on a track (not shown) as the mandrel  30  rotates about its axis; in this manner, the working surface  32  of the mandrel  30  becomes coated with the base layer  22 .  
         [0035]    Referring still to FIG. 7, the circumferential fibers  26  are applied after application of the base layer  22  (preferably while the base layer  22  is still semi-soft) and before, during, or immediately after the application of the top stock layer  28  (in the illustrated embodiment, the circumferential fibers  26  are applied immediately before the application of the top stock layer  28 ). Individual creels of fibers (not shown) are mounted on a cart (also not shown) that is attached to and moves axially in concert with a nozzle  56  that applies the top stock layer  28 ; as many as six or more fibers  26  may be wound into the base layer  22  at once. In the illustrated embodiment, a rod  54  extends downwardly from the nozzle arm  58 ; the rod  54  has a forked lower end  54   a  that includes a cross-roller  54   b  over which the circumferential fibers  26  are fed prior to application to the base layer  22 . The circumferential fibers  26  are tensioned by means known to those skilled in this art in order to control penetration of the circumferential fibers  26  into the base layer  22 . Preferably, the circumferential fibers  26  are tensioned such that they are buried halfway (i.e. half of the cross-section of the fiber  26  is buried) in the base layer  22  (this tension is typically between about 0.25 and 5 pounds). It is also preferred that the top stock layer  28  be applied shortly after (i.e., within 15 minutes) or almost simultaneous with of the winding of the circumferential fibers  26 , as doing so can encourage cross-linking between the base layer  22  and the top stock layer  28 .  
         [0036]    Those skilled in this art will recognize that a belt can be formed with a single material pass (i.e. formed as a one polymeric layer that embeds both the axial and the circumferential fibers  24 ,  26 ) rather than the two-shot process described above. In that instance the polymeric matrix  21  is a single unitary layer. Other embodiments may include more than two layers. Such embodiments may include one layer the embeds the axial fibers  24 , another layer that embeds the circumferential fibers  26 , and a third layer that provides the contact surface with a press felt.  
         [0037]    After application of the top stock layer  28 , the base layer  22  and top stock layer  28  of the polymer matrix  21  are cured to form the belt  20 . Once the belt  20  has been cured, post-curing operations can be carried out as the belt  20  remains on the mandrel  30 . Such operations may include trimming to the proper length and approximate thickness, grinding to its finished thickness, and venting (typically with the formation of blind drilled holes or grooves). Other operations are described in PCT Application No. US02/06520, filed Mar. 4, 2002, the disclosure of which is hereby incorporated herein in its entirety.  
         [0038]    Once the post-curing processing of the belt  20  has been completed, the belt  20  is removed from the mandrel  30 . Removal can be carried out in any manner known to those skilled in this art.  
         [0039]    The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as recited in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.