Abstract:
Two coating supply tubes extend parallel to one another and run the full width of a moving substrate in the cross machine direction. Coating is supplied separately to each supply tube from opposite ends. The supply tubes discharge coating through spaced metering holes into an application chamber defined between a sidewall mounted to each supply tube. The counterflow arrangement of the coating supply tubes results in the fall off of coating pressure in one tube being canceled out by the increased pressure in the other tube. The fall off may be further counteracted by varying the spacing between metering holes the greater the distance from the coating inlet, by varying the diameter of the metering holes, or both. The tendency of the heated coating to cause a temperature gradient may be counteracted by cantilevering the applicator head on arms from a support beam through which a temperature-controlling fluid is circulated.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/072,742, filed Jan. 27, 1998, the disclosure of which is incorporated by reference herein. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to coating applicators in general and to apparatus for applying coatings to moving substrates in particular. 
     Paper of specialized performance characteristics may be created by applying a thin layer of coating material to one or both sides of the paper. One type of coating fluid is a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder which may be of the organic type or of a synthetic composition. Another type of fluid is a starch and water solution used in sizing applications. Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers. The coated paper may be formed with a smooth bright surface which improves the readability of the text and the quality of photographic reproductions. Coated papers are divided into a number of grades. The higher value grades, the so-called coated free-sheet, are formed of paper fibers wherein the lignin has been removed by digestion. Less expensive grades of coated paper contain ten percent or more ground-wood pulp which is less expensive than pulp formed by digestion. 
     Coated papers are often used for high-quality printing or in other applications where visible variations in coating weight would significantly detract from the appearance of the paper. It is therefore of key concern to maintain coating thickness consistency across the width of the treated web. Greater efficiency and cost control in papermaking has driven the construction of ever wider papermaking machines, sometimes of 300-400 inches or more. In conventional fountain applicators, a single supply chamber extends the full width of the web in the cross machine direction. This supply chamber may be fed from one or both ends. To minimize fall off of coating ejected from a nozzle which terminates the supply chamber, coating is supplied at a high pressure. Nevertheless, such coaters are prone to heavier coating application at the ends. 
     Furthermore, the heated coatings which are frequently employed can, over the extended cross machine width of the coater head, result in temperature gradients which cause bowing of the head with resultant coat weight variations. 
     What is needed is a papermaking fountain applicator which may be operated at lower pressures while still supplying consistent coating levels to the substrate in the cross machine direction. 
     SUMMARY OF THE INVENTION 
     The coating applicator of this invention has two coating supply tubes which extend parallel to one another and run the full width of the substrate in the cross machine direction. Coating is supplied separately to each supply tube from opposite ends. The supply tubes discharge coating through spaced metering holes into an application chamber defined between a sidewall mounted to each supply tube. The counterflow arrangement of the coating supply tubes results in the fall off of coating pressure in one tube being canceled out by the increased pressure in the other tube at any particular point moving across the coater head in the cross machine direction. The tendency of the pressure to fall as the coating moves through the supply tube may be further counteracted by varying the spacing between metering holes with cross machine position, by varying the diameter of the metering holes, or both. 
     The tendency of the heated coating to cause a temperature gradient in the applicator head may be counteracted by cantilevering the applicator head on arms from a support beam through which a temperature-controlling fluid is circulated. 
     It is a feature of the present invention to provide a coating applicator which supplies a coating to a jet applicator nozzle at a constant pressure. 
     It is another feature of the present invention to provide a coating applicator which is conveniently profile controlled. 
     It is an additional feature of the present invention to provide a papermaking coating applicator which is less susceptible to bowing due to temperature gradients. 
     It is also a feature of the present invention to provide a papermaking coating applicator which operates at reduced coating pressures. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the coating applicator of this invention on a papermaking machine. 
     FIG. 2 is a perspective view, partially broken away in section, of the papermaking machine applicator of the apparatus of FIG.  1 . 
     FIG. 3 is a side elevational view of an alternative embodiment coating applicator of this invention having an offset support beam with temperature maintenance. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to FIGS. 1-3, wherein like numbers refer to similar parts, the coating applicator  20  of this invention is shown in FIGS. 1 and 2. The applicator  20  has two elements which control the quantity and thickness of coating  22  applied to a moving substrate, for example a paper web  24  supported by a backing roll  26 . These two elements are the applicator head  28  and the metering blade assembly  30 . Coating  22  is supplied under pressure to the applicator head  28  and ejected from an applicator head nozzle  32  on to the moving web  24 . The metering blade  34  of the assembly  30  engages the coated web downstream of the applicator head  28  and removes excess coating  22 . Applied coating which is not retained on the web is collected in a coating pan  36  and recirculated. 
     As shown in FIG. 2, the applicator head  28  has two segments  38  which are pivotably connected. The machine direction is defined as the direction of movement of the web  24 . The cross machine direction is the direction parallel to the axis of the backing roll  26 . A first coating supply tube  40  is affixed to a first bracket assembly  42  which has a series of aligned ears  44  which are rotatably mounted on brass bushings to a second bracket assembly  46  which is bolted to a rectangular support beam  52  which extends the length of the applicator head in the cross machine direction. A second coating supply tube  48  is fixed to the second bracket assembly  46 . The second coating supply tube  48  extends parallel to the first coating supply tube  40 . The support beam  52  is a rigid rectangular section member which may be as tall or taller than the coating applicator itself. The support beam  52  and the applicator mounted thereon will preferably be supported on pivoting arms, not shown, which allow the applicator to be withdrawn from the backing roll during start up or in the case of a sheet break. 
     An inflatable air tube  50  is positioned between the support beam  52  and a lower plate  54  of the first bracket assembly  42 . The first coating supply tube  40  has a plurality of metering holes  58  positioned above a first chamber floor segment  56 . The second coating supply tube  48  has a plurality of metering holes  58  positioned above a second chamber floor segment  60 . In the operational configuration, the air tube  50  is inflated to bring the coating supply tubes together such that the first chamber floor segment  56  engages the second chamber floor segment  60 . A liquid tight seal is formed between the adjacent chamber floor segments by a resilient gasket such as a cylindrical neoprene tube  62  which is received within a groove  64  defined along the center of the second chamber floor segment  60 . 
     A nozzle chamber  66  is defined between a first wall  68  which extends upwardly from the first coating supply tube  40  and a second wall  70  which extends upwardly from the second coating supply tube  48 . The first wall  68  and the second wall  70  converge to define a cross machine gap  72  through which coating is ejected from the nozzle  32 . To provide for ready replacement of the terminal segments of the first wall and second wall, the first wall preferably includes a replaceable first terminal segment  74  attached to a lower portion  76  of the first wall  68 ; and the second wall includes a replaceable second terminal segment  78  attached to a lower portion  80  of the second wall  70 . 
     To promote the uniformity of the jet of coating exiting from the nozzle gap  72 , coating  22  is supplied to the nozzle chamber  66  through both the first coating supply tube  40  and the second coating supply tube  48 . The first coating supply tube  40  has an inlet end  82  through which coating under pressure is introduced. The second coating supply tube  48  has an inlet end  84  which is spaced from the first coating supply tube inlet end  82  in the cross machine direction. The two coating supply tube inlet ends  82 ,  84  are spaced on opposite sides of the applicator head  28 . Hence, the coating in one of the coating supply tubes flows in a direction counter to the direction of flow in the other coating supply tube. The end of each coating supply tube opposite its inlet end will preferably have a smaller outlet through which 10-20 percent of the coating leaves the coating supply tube to be recirculated. The coating supply tubes provide a means for introducing coating to the nozzle chamber in opposite but parallel directions. 
     When the high viscosity coating  22  is supplied to the nozzle chamber  66  through one of the coating supply tubes, there will be a pressure drop from the inlet end to the outlet end. This drop in pressure will tend to result in reduced flow velocity of the coating through the metering holes  58  adjacent the outlet end of a coating supply tube. However, because the outlet end of one coating supply tube discharges coating into the nozzle chamber adjacent the inlet end of the other coating supply tube, where the pressure is higher, the effect of the pressure drop is canceled out. Thus the falling pressure moving in the cross machine direction along one coating supply tube coincides with the rising pressure in the opposed coating supply tube moving in the same direction. The result of this arrangement is to equalize the pressure along the entire cross machine direction width of the applicator head  28 . In coating supply tubes with equally spaced metering holes  58 , the metering holes along one tube may be spaced apart approximately 0.5 to 4.2 inches in the cross machine direction, in a preferred embodiment the holes may be spaced from about 1.4 inches to 2.8 inches. The holes in the first coating supply tube are staggered from the holes in the second supply tube, such that a hole in one coating supply tube discharges coating into the chamber across from a land in the opposite coating supply tube. 
     This effect may be emphasized by adjusting the spacing between metering holes or the diameter of the metering holes. Generally, in the center region of each tube, the spacing of the holes, the diameter of the holes, or both would remain constant, with increased spacing, decreased diameter or both toward the ends of the tubes. Generally, the variation in hole diameter or spacing will occur about one meter from the end. For example, the metering holes may be spaced approximately 1.4-2.8 inches apart at the center of a coating supply tube, with the spacing being gradually increased until adjacent metering holes are approximately 2.8 to 4.2 inches apart at an end. As an alternative to varying the spacing between holes, the diameter of the holes could be varied plus or minus 50 percent. This variation would take place over the typically 400 in. width of the coating applicator  20 . As an example, the nominal diameter of the holes might be about ⅜ of an inch, with a variation of plus or minus 50 percent. The coating supply tubes may be about four inches in diameter, with a range of supply tube diameter of from about 2½ inches to 10 inches. It should be noted that although cylindrical coating supply tubes are illustrated, tubes of other profile may be employed. 
     As shown in FIG. 1, the coating applicator  20  is provided with profiling capability by a series of threaded adjustment rods  86  which extend from a profiling bar  88  which is bolted to the first bracket assembly  42  to a series of corresponding threaded holes in the terminal segment  74  on the first nozzle wall  68 . By adjusting the rods  86 , the width of the gap  72  in the machine direction may be controlled as it extends in the cross machine direction. The terminal segment  74  preferably narrows or necks down below the location of attachment of the adjustment rods  86 , facilitating the bending of the upper portion of the terminal segment. As shown in FIG. 2, the adjustment rods  86  in a preferred embodiment may be spaced approximately eight inches apart, but the spacing may range from two to sixteen inches. 
     As shown in FIG. 1, a sheet metal cover  90  extends over the adjustment rods  86 , being received within a groove in the first terminal segment  74  and being screwed to the profiling bar  88 . Another sheet metal cover  92  extends from the second terminal segment  78  and into the coating pan  36 . Another cover  94  descends from the metering blade assembly  30  to direct coating into the coating pan  36 . 
     An alternative embodiment applicator head assembly  96  is shown in FIG.  3 . The assembly  96  thermally isolates the applicator head  98  from the support beam  100 , by cantilevering the applicator head from the support beam on a series of support arms  102 , each spaced from one another in the cross machine direction approximately two feet apart. The applicator head  98  has a first coating supply tube  104  which is pivotably connected to the support arms  102 . The first coating supply tube  104  is also pivotably connected to the bracket  106 . A second coating supply tube  108  is fixed to the bracket  106 . To adjust the angle of the applicator head  98  with respect to the support beam  100 , a screw jack  110  extends between the support beam  100  and the bracket  106 . 
     As in the applicator  20 , coating is supplied to the first coating supply tube  104  at an inlet end  112  from a pressurized coating supply. Coating is simultaneously supplied to the second supply tube at an opposite end. The coating travels through the coating supply tube and enters the applicator nozzle  114 . A fraction of the coating is recirculated through a recirculation outlet  116 . Often coating fluid temperatures are other than the ambient temperature. On applicator heads in which the main support beam is an integral part of the applicator head, the introduction of warm coating into the applicator head can create a thermal gradient between the heated portions of the applicator head and the unheated support beam. 
     The applicator  96  counters this thermal gradient effect by thermally isolating the support beam  100  from the portions of the applicator head through which the heated coating flows. In addition, temperature compensating fluid, preferably water  118 , is pumped through the support beam  100  to keep the support beam within a limited range of temperature and to thereby prevent temperature-gradient-induced bowing of the support beam. In a preferred embodiment, water would be maintained at the desired temperature range within a rig, not shown, and pumped into four corner chambers  120  defined by rectangular plates  122  running the entire cross machine direction length of the support beam and welded in place. Although the key requirement of the temperature compensating water  118  is that its temperature be maintained within a desired range, the water may be maintained at a level slightly above freezing, for example 40 degrees Fahrenheit. Where required by temperature gradients present in the system, temperature compensating water at different temperatures and/or flow may be introduced into one or more of each of the four corner chambers. This variation may extend so far as to discontinue flow through one or more of the chambers. With this control, it is possible to control the position of the beam. 
     The chilled water would tend to cause the metal support beam  100  to condense water vapor from the surrounding air. This “sweating” of the support beam would have the advantageous effect of preventing coating build-up on the support beam. The coating pan  134  is preferably connected directly to the support beam  100 . The temperature compensating water  118  is recirculated to the temperature maintaining rig after having passed through the support beam. 
     The applicator  96  also has an alternative profiling structure, in which an array of screws  124  extend between a terminal wedge  126  and a protrusion  128  extending from a lower portion  130  of the chamber wall  132  connected to the first coating supply tube  104 . The terminal wedge  126  extends from the lower portion  130  of the chamber wall on a narrow segment of material, permitting it to be urged toward the second wall  132  of the chamber to control the variation of the coating jet in the cross machine direction. 
     It should be noted that although the substrate has been illustrated as a paper web supported by a backing roll, the substrate may alternatively be a roll itself, which receives the coating for downstream application to a paper web, for example as in a size press. It should be noted that where coating or coating material is referred to herein, pigmented coatings, sizing solutions, and other fluids which may be applied to a paper web are included. The coating applicator of this invention may also be used in off-machine applications as well as on-machine. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.