The present invention relates generally to nip presses, and more particularly to shoe presses.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Referring now to the drawings, a portion of a shoe press belt, designated broadly at 20, is illustrated in
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.
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.
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.
Referring now to
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.
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.
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
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
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.
The axial fiber bands 40 can be formed, for example, with a fixture such as that designated at 49 in
Referring still to
Referring now to
Referring still to
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.
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.
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.
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.
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/378,146, filed May 14, 2002, the disclosure of which is hereby incorporated herein by reference in its entirety.
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