Patent Grant
7005043
1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to the papermaker's fabrics used on the dryer section of a paper machine, and particularly on a single-run dryer section. Such fabrics are commonly referred to as dryer fabrics.
2. Description of the Prior Art
As is well known to those of ordinary skill in the art, the papermaking process begins with the deposition of a fibrous slurry, that is, an aqueous dispersion of cellulosic fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving a fibrous web on its surface.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere its constituent fibers to one another to turn the fibrous web into a sheet. The water squeezed from the web is accepted by the press fabric or fabrics, and, ideally, does not return to the web.
The web, now a sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The sheet itself is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of at least some of the drums. The heated drums reduce the water content of the sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section at the downstream end of the paper machine.
Referring, now, more specifically to the dryer section, in the dryer section, the dryer cylinders may be arranged in a top and a bottom row or tier. Those in the bottom tier are staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it passes alternately between the top and bottom tiers as it passes first around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section.
The top and bottom tiers of dryer cylinders may each be clothed with a separate dryer fabric. In such a situation, the paper sheet being dried passes unsupported across the space, or “pocket”, between each dryer cylinder and the next dryer cylinder on the other tier.
In a single tier dryer section, a single row of cylinders along with a number of turning cylinders or rolls may be used. The turning rolls may be solid or vented.
In order to increase production rates and to minimize disturbance to the sheet, single-run dryer sections are used to transport the sheet being dried at high speeds. In a single-run dryer section, a paper sheet is transported by use of a single dryer fabric which follows a serpentine path sequentially about the dryer cylinders in the top and bottom tiers.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks. “Drop off” can also reduce machine efficiency if it leads to sheet breaks.
Many paper mills have addressed this problem by machining grooves into the dryer cylinders or rolls or by adding a vacuum source to those dryer rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric, although both are expensive.
In this connection, fabric manufacturers have also employed application of coatings to fabrics to impart additional functionality to the fabric, such as “sheet restraint methods.” The importance of applying coatings as a method for adding this functionality to, for example, dryer fabrics, has been cited by Luciano-Fagerholm (U.S. Pat. No. 5,829,488 (Albany), titled, “Dryer Fabric With Hydrophilic Paper Contacting Surface”).
Luciano and Fagerholm have demonstrated the use of a hydrophilic surface treatment of fabrics to impart sheet-holding properties while maintaining close to the original permeability. However, this method of treating fabric surfaces, while successful in imparting sheet restraint, enhanced hydrophilicity and durability of the coating is desired. WO Patent 97/14846 also recognizes the importance of sheet restraint methods, and relates to using silicone coating materials to completely cover and impregnate a fabric, making it substantially impermeable. However, this significant reduction in permeability is unacceptable for dryer fabric applications. Sheet restraint is also discussed in U.S. Pat. No. 5,397,438, which relates to applying adhesives on lateral areas of fabrics to prevent paper shrinkage. Other related prior art includes U.S. Pat. No. 5,731,059, which reports using silicone sealant only on the fabric edge for high temperature and anti-raveling protection; and U.S. Pat. No. 5,787,602 which relates to applying resins to fabric knuckles. All of the above referenced patents are incorporated herein by reference.
The present invention is another approach toward a solution to this problem in the form of a dryer fabric having backside vents which permit air trapped in a compression wedge to escape without having to pass through the dryer fabric. The present invention also includes a method for manufacturing the dryer fabric.
Accordingly, the present invention relates primarily to a dryer fabric, although it may find application in any of the fabrics used in the forming, pressing and drying sections of a paper machine, and in the industrial fabrics used in the manufacture of nonwoven fabrics. As such, the papermaker's or industrial fabric comprises a base substrate which takes the form of an endless loop having a backside and a paper-contacting side. A plurality of discrete, discontinuous deposits of polymeric resin material are disposed at preselected locations on the backside. These deposits have a height, relative to the backside, of at least 0.5 mm so that they may separate the backside from the surface of a dryer cylinder or turning roll by that amount when passing therearound. The deposits allow air trapped between the backside and the surface of the dryer cylinder to escape in both the lengthwise and crosswise directions parallel to the surface rather than through the fabric to alleviate the problem of “drop off”.
The preselected locations for the discrete, discontinuous deposits of polymeric resin material may be knuckles formed where the yarns in one direction of the fabric pass over the yarns in the other direction. Alternatively, the preselected locations may be “valleys” between knuckles, an alternative which carries the advantage of bonding two intersecting yarns to one another at their crossing point. Alternatively still, the preselected locations may be two or more consecutive knuckles aligned in the machine or cross-machine direction and the valley or valleys in between. When the preselected locations are aligned in the machine direction, this alternative carries the advantage that it allows improved air channeling. Preferably, the deposits reside only on the knuckles or on the backside surfaces of the yarns, where they would not affect the permeability of the fabric. Further, as the deposits form a sort of discontinuous coating on the backside, they have no effect on its bending properties or on the location of its neutral axis of bending. Finally, by improving the ability of the backside of the fabric to manage air in this manner, rather than through the use of elaborate and complicated weave patterns to provide the backside of the fabric with air channels, the base fabric weave structure used for the base substrate may be provided with other characteristics, such as openness, which would give it higher permeability to improve drying rate, and may be simpler and less costly to manufacture and seam.
The present invention is also a method for manufacturing a papermaker's or industrial fabric, such as a dryer fabric. The method comprises a first step of providing a base substrate for the fabric.
Polymeric resin material is deposited onto preselected locations on the base substrate in droplets having an average diameter of 10μ (10 microns) or more to build up discrete, discontinuous deposits of the polymeric resin material to a height of at least 0.5 mm relative to the surface of the base substrate. At least one piezojet may be used to deposit the polymeric resin material onto the base substrate, although other means for depositing droplets of that size may be known to those of ordinary skill in the art or may be developed in the future. The polymeric resin material is then set or fixed by appropriate means.
The preselected locations may, as stated above, be knuckles formed on the surface of the fabric by the interweaving of its yarns.
Subsequently, the deposits of polymeric resin material may optionally be abraded to provide them with a uniform height over the surface plane of the base substrate.
The present invention will now be described in more complete detail, with frequent reference being made to the figures identified below.
The method for fabricating the papermaker's or industrial fabric of the present invention begins with the provision of a base substrate. Typically, the base substrate is a fabric woven from monofilament yarns. More broadly, however, the base substrate may be a woven, nonwoven or knitted fabric comprising yarns of any of the varieties used in the production of paper machine clothing or industrial fabrics used to manufacture nonwoven articles and fabrics, such as monofilament, plied monofilament, multifilament and plied multifilament yarns. These yarns may be obtained by extrusion from any of the polymeric resin materials used for this purpose by those of ordinary skill in the art. Accordingly, resins from the families of polyamide, polyester, polyurethane, polyaramid, polyolefin and other resins may be used.
Alternatively, the base substrate may be composed of mesh fabrics, such as those shown in commonly assigned U.S. Pat. No. 4,427,734 to Johnson, the teachings of which are incorporated herein by reference. The base substrate may further be a spiral-link belt of the variety shown in many U.S. patents, such as U.S. Pat. No. 4,567,077 to Gauthier, the teachings of which are also incorporated herein by reference.
Moreover, the base substrate may be produced by spirally winding a strip of woven, nonwoven, knitted or mesh fabric in accordance with the methods shown in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference. The base substrate may accordingly comprise a spirally wound strip, wherein each spiral turn is joined to the next by a continuous seam making the base substrate endless in a longitudinal direction.
The above should not be considered to be the only possible forms for the base substrate. Any of the varieties of base substrate used by those of ordinary skill in the paper machine clothing and related arts may alternatively be used.
Once the base substrate has been provided, one or more layers of staple fiber batt may optionally be attached to one or both of its two sides by methods well known to those of ordinary skill in the art. Perhaps the best known and most commonly used method is that of needling, wherein the individual staple fibers in the batt are driven into the base substrate by a plurality of reciprocating barbed needles. Alternatively, the individual staple fibers may be attached to the base substrate by hydroentangling, wherein fine high-pressure jets of water perform the same function as the above-mentioned reciprocating barbed needles. It will be recognized that, once staple fiber batt has been attached to the base substrate by either of these or other methods known by those of ordinary skill in the art, one would have a structure identical to that of a press fabric of the variety generally used to dewater a wet paper web in the press section of a paper machine.
Once the base substrate, with or without the addition of staple fiber batt material on one or both of its two sides, has been provided, it is mounted on the apparatus 10 shown schematically in
Furthermore, for some applications, it may be necessary to apply the resin pattern to the sheet contact side. Also, it is envisioned that the resin application for air control should be applied to both sides of the fabric, either with the same or different patterns.
Referring now more specifically to
The stations are identified as follows:
In the first station, the optional polymer deposition station 14, a piezojet array 16 mounted on transverse rails 18,20 and translatable thereon in a direction transverse to that of the motion of the base substrate 12 through the apparatus 10, as well as therebetween in a direction parallel to that of the motion of the base substrate 12, may be used to deposit a polymeric resin material onto or within the base substrate 12 while the base substrate 12 is at rest. Optional polymer deposition station 14 may be used to deposit the polymeric resin material more uniformly over the base substrate than could be accomplished using conventional techniques, such as spraying, if desired.
The piezojet array 16 comprises at least one but preferably a plurality of individual computer-controlled piezojets, each functioning as a pump whose active component is a piezoelectric element. As a practical matter an array of up to 256 piezo jets or more may be utilized if the technology permits. The active component is a crystal or ceramic which is physically deformed by an applied electric signal. This deformation enables the crystal or ceramic to function as a pump, which physically ejects a drop of a liquid material each time an appropriate electric signal is received. As such, this method of using piezojets to supply drops of a desired material repeatedly so as to build up the desired amount of material in the desired shape in response to computer-controlled electric signals is commonly referred to as a “drop-on-demand” method.
The degree of precision of the jet in depositing the material will depend upon the dimensions and shape of the structure being formed. The type of jet used and the viscosity of the material being applied will also impact of the precision the jet selected.
Referring again to
In the present invention, in which a piezojet array is used to deposit a polymeric resin material onto or within the surface of the base substrate 12, the choice of polymeric resin material is limited by the requirement that its viscosity be 100 cps (100 centipoise) or less at the time of delivery, that is, when the polymeric resin material is in the nozzle of a piezojet ready for deposition, so that the individual piezojets can provide the polymeric resin material at a constant drop delivery rate. In this regard, the viscosity of the polymeric resin material at the point of delivery in conjunction with the jet size is important in defining the size and shape of the droplets formed on the base substrate 12 and in time the resolution of the pattern ultimately achieved. Another requirement limiting the choice of polymeric resin material is that it must partially set during its fall, as a drop, from a piezojet to the base substrate 12, or after it lands on the base substrate 12, to prevent the polymeric resin material from flowing and to maintain control over the polymeric resin material to ensure that it remains in the form of a drop where it lands on the base substrate 12. Suitable polymeric resin materials which meet these criteria and which are preferably abrasion resistant are:
It should be understood that the polymeric resin material needs to be fixed on or within the base substrate 12 following its deposition thereon. The means by which the polymeric resin material is set or fixed depends on its own physical and/or chemical requirements. Photopolymers are cured with light, whereas hot-melt materials are set by cooling. Aqueous-based latexes and dispersions are dried and then cured with heat, and reactive systems are cured by heat. Accordingly, the polymeric resin materials may be set by curing, cooling, drying or any combination thereof.
The proper fixing of the polymeric resin material is required to control its penetration into and distribution within the base substrate 12, that is, to control and confine the material within the desired volume of the base substrate 12. Such control is important below the surface plane of the base substrate 12 to prevent wicking and spreading. Such control may be exercised, for example, by maintaining the base substrate 12 at a temperature which will cause the polymeric resin material to set quickly upon contact. Control may also be exercised by using such materials having well-known or well-defined curing or reaction times on base substrates having a degree of openness such that the polymeric resin material will set before it has time to spread beyond the desired volume of the base substrate 12.
One or more passes over the base substrate 12 may be made by piezojet array 16 to deposit the desired amount of material and to create the desired shape. In this regard, the deposits can take any number of shapes as illustrated generally in
When a desired amount of polymeric resin material has been applied per unit area in a band between the transverse rails 18,20 across the base substrate 12, the base substrate 12 is advanced lengthwise an amount equal to the width of the band, and the procedure described above is repeated to apply the polymeric resin material in a new band adjacent to that previously completed. In this repetitive manner, the entire base substrate 12 can be provided with any desired amount of polymeric resin material per unit area.
Alternatively, the piezojet array 16, again starting from an edge of the base substrate 12, or, preferably, from a reference thread extending lengthwise therein, is kept in a fixed position relative to the transverse rails 18,20, while the base substrate 12 moves beneath it, to apply any desired amount of the polymeric resin material per unit area in a lengthwise strip around the base substrate 12. Upon completion of the lengthwise strip, the piezojet array 16 is moved widthwise on transverse rails 18,20 an amount equal to the width of the lengthwise strip, and the procedure described above is repeated to apply the polymeric resin material in a new lengthwise strip adjacent to that previously completed. In this repetitive manner, the entire base substrate 12 can be provided with the desired amount of polymeric resin material per unit area, if desired.
Note the pattern can be random, a repeating random pattern on a base substrate or such patterns that are repeatable from belt to belt for quality control.
At one end of the transverse rails 18,20, a jet check station 22 is provided for testing the flow of polymeric resin material from each piezojet in the piezojet array 16. There, the piezojets can be purged and cleaned to restore operation automatically to any malfunctioning piezojet unit.
In the second station, the imaging/precise polymer deposition station 24, the only station not optional in the present invention, transverse rails 26,28 support a digital-imaging camera 30, which is translatable across the width of base substrate 12, and a piezojet array 32, which is translatable both across the width of the base substrate 12 and lengthwise relative thereto between transverse rails 26,28, while the base substrate 12 is at rest.
The digital-imaging camera 30 views the surface of the base substrate 12 to locate the knuckles formed where the yarns in one direction of the base substrate 12 weave over those in the other direction. In the weaving process these cross-over points, while being located very close to predetermined or regular intervals, depending upon the weave pattern, do, however, vary. Accordingly, merely attempting to deposit the polymeric resin material at discrete intervals will not insure that all, or the desired number of cross-over points will receive the deposit. Accordingly, a comparison between the actual surface and its desired appearance are made by a fast pattern recognizer (FPR) processor operating in conjunction with the digital-imaging camera 30 in real time. The FPR processor signals the piezojet array 32 to deposit polymeric resin material onto the locations requiring it to match the desired appearance. In the present invention, the polymeric resin material is deposited onto the knuckles on the backside of the fabric to build up discrete, discontinuous deposits of the polymeric resin material thereon. Alternatively, it is deposited onto valleys between knuckles, or onto two or more consecutive knuckles aligned in the machine or cross-machine direction and onto the valleys in between. Essentially, the deposits are provided to separate the backside of the fabric from a dryer cylinder or turning roll so that air, carried by the backside of the fabric into a compression wedge, can escape in both the lengthwise and crosswise directions along the surface of the backside instead of being forced through the fabric, where it would cause “drop off”. Ideally, the deposits are built up gradually through the deposition of droplets of polymeric resin material from the piezojets in multiple passes by piezojet array 32 to attain a height above the knuckle in a nominal range from 0.5 mm to 1.0 mm, so as to separate the backside of the fabric from a dryer cylinder or turning roll by that amount. Multiple passes by piezojet array 32 allow the shapes of the deposits to be carefully controlled so as not to affect the permeability of the dryer fabric. That is to say by depositing the droplets in a repeating pattern, that being by layering one droplet on the top of the next, the height or z-direction of the polymer resin material on the base substrate 12 is controlled and may be uniform, varied or otherwise adjusted as desired. Further, some of the individual piezojets in the piezojet array may be used to deposit one polymeric resin material, while others may be used to deposit a different polymeric resin material, to produce a surface having microregions of more than one type of polymeric resin material. Such accuracy in depositing may avoid the step of grinding or abrading to obtain a monoplanar surface across the polymeric resin material deposited. Of course, a grinding or abrading step may also be done, if so desired.
As in optional polymer deposition station 14, a piezojet check station 34 is provided at one end of the transverse rails 26,28 for testing the flow of material from each piezojet. There, each piezojet in the piezojet array 32 can be purged and cleaned to restore operation automatically to any malfunctioning piezojet unit.
In the third station, the optional setting station 36, transverse rails 38,40 support a setting device 42, which may be required to set the polymeric resin material being used. The setting device 42 may be a heat source, for example, an infrared, hot air, microwave or laser source; cold air; or an ultraviolet or visible-light source, the choice being governed by the requirements of the polymeric resin material being used.
Finally, the fourth and last station is the optional grinding station 44, where an appropriate abrasive is used to provide any polymeric resin material above the surface plane of the base substrate 12 with a uniform thickness. The optional grinding station 44 may comprise a roll having an abrasive surface, and another roll or backing surface on the other side of the base substrate 12 to ensure that the grinding will result in a uniform thickness.
As an example, reference is now made to
The backside 56 of the dryer fabric 50 is the underside thereof in the views shown in
To their advantage, the deposits 60, which, in a sense, form a discontinuous coating on the backside 56 of the dryer fabric 50, have no effect on the bending properties of the dryer fabric 50, as, lying discontinuously on the surface, they affect neither the stiffness of the dryer fabric 50, nor the location of its neutral axis of bending.
In an alternate embodiment of the present invention, the optional polymer deposition station 14, the imaging/repair station 24, and the optional setting station 36 may be adapted to produce a fabric from the base substrate 12 according to a spiral technique, rather than by indexing in the cross-machine direction as described above. In a spiral technique, the optional polymer deposition station 14, the imaging/precise polymer deposition station 24, and the optional setting station 36 start at one edge of the base substrate 12, for example, the left-hand edge in
Alternatively, the optional polymer deposition station 14, the imaging/precise polymer deposition station 24 and the optional setting station 36 may all be kept in fixed. positions aligned with one another, while the base substrate 12 moves beneath them, so that the polymeric resin material desired for the finished fabric may be applied to a lengthwise strip around the base substrate 12. Upon completion of the lengthwise strip, the optional polymer deposition station 14, the imaging/precise polymer deposition station 24 and the optional setting station 36 are moved widthwise an amount equal to the width of the lengthwise strip, and the procedure is repeated for a new lengthwise strip adjacent to that previously completed. In this repetitive manner the entire base structure 12 can be completely treated as desired.
It should be noted that the material need not be a full width belt but can be a strip of material such as that disclosed in U.S. Pat. No. 5,360,656 to Rexfelt, the disclosure of which is incorporated herein by reference, and subsequently formed into a full width belt. The strip can be unwound and wound up on a set of rolls after fully processing. These rolls of belting materials can be stored and can then be used to form an endless full width structure using, for example, the teachings of the immediately aforementioned patent.
It should also be understood that, whatever form (e.g. square, rectangle, cylindrical, trapezoid, etc. see
Finally, as stated above, where the base substrate 12 is endless, it may be necessary to invert it, that is, to turn it inside out, to place the discrete, discontinuous deposits of polymeric resin material on the backside thereof, when the apparatus 10 is used to deposit the polymeric resin material on the top run of the base substrate 12 therethrough. Where the base substrate 12 is not endless, the side being given the discrete, discontinuous deposits will ultimately be placed on the inside when the base substrate 12 is seamed into endless form on a dryer section. In either case, as aforesaid, there may be situations where resin is applied to the sheet contact side in addition to the backside. Also, as an alternative, one might consider depositing a sacrificial material in a desired pattern to create in essence a mold for the resin material thereafter deposited. This sacrificial material can be, for example, wax or a water soluble substance which is then removed leaving the resin set in the desired pattern on the fabric.
Also it may be desired to apply different polymeric resin material on the same fabric at different locations by way of different jets in the array.
Modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the scope of the appended claims. In particular, while piezojets are disclosed above as being used to deposit the polymeric resin material in the preselected locations on the base substrate, other means for depositing droplets thereof in the size range desired may be known to those of ordinary skill in the art or may be developed in the future, and such other means may be used in the practice of the present invention. The use of such means would not bring the invention, if practiced therewith, beyond the scope of the appended claims.
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