Patent Publication Number: US-6709517-B1

Title: Method and device for coating a running material web

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US national phase of PCT application PCT/EP00/11170 filed Nov. 11, 2000 with a claim to the priority of German application 199 62 844.0 filed Dec. 23, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method of coating a running material web wherein the coating material is applied as a free-falling curtain to the web and the edges of the curtain are split off by splitters at both sides for setting the coating width before contact with the material web. In addition the invention concerns a coating device that is particularly suited for carrying out the method according to the invention and to a splitter particularly usable in the coating device. 
     BACKGROUND OF THE INVENTION 
     In order to coat a running material web, for instance of paper, cardboard, plastic, or metal, so-called curtain coaters are used where the coating material (pigment, plastic dispersion, etc.) is applied from a slot nozzle in a free-falling curtain to the material web. In order to produce a stable and uniform curtain across the coating width, WO 98/47630 and DE 197 35 588 describe how the two curtain edges are guided by respective guides extending downward from the slot nozzle to near the material web. In order to set the desired curtain width, the curtain edges are split off before the curtain actually contacts the material web by splitters that extend transverse to the web-travel direction underneath the guides. The two splitters each have an upwardly directed splitting edge and an outwardly directed inclined upper catching surface on which the material split from the edge of the curtain can flow outward. 
     If the curtain coater is used to apply dispersion adhesives to a web-shaped splitter in the production of self-sticking labels, release strips, etc. or to apply a barrier layer in the production of a water-impermeable packing material, it is necessary to be able to set the coating width over a wide range since the web width varies considerably depending on the material being coated. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the invention to improve on a method of and apparatus of the described type such that widely varying coating widths can be set without diminishing the coating quality. 
     A further object of the invention is to provide a splitter that is particularly suited for use in the coating apparatus according to the invention. 
     SUMMARY OF THE INVENTION 
     This object is attained in that first a first outer splitter at each edge creates a narrow curtain and then a second inner splitter at each edge creates the desired coating width. 
     The provision of two splitters on each side makes it possible to set a very narrow coating width by splitting off a substantial lateral region of the curtain without coating errors on the longitudinal edges. In order to avoid coating errors (variations in the application density or the coating width, gaps in the curtain, etc.) inner splitters are used whose splitter edges are very short. The bulk of the flow of coating material is split off by the outer splitters whose edges are sufficiently high that they avoid an overflow of the split-off coating material. The inner splitters take care of the rest of the curtain edge. They can thus have a lower height without the split-off coating material flowing inward over the splitting edge as is not wanted. A shorter height of the splitting edge of the inner element is advantageous since its inner edge extending parallel to the flow direction of the curtain also serves to guide the curtain edge. Too great a height allows separation of the curtain edge from the inner face too far above the web being coated. In this case its position moves and a ridge is formed of the coating material on the material web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawings serve to describe the invention with reference to simplified illustrated embodiments. Therein: 
     FIG. 1 is a longitudinal section through a curtain coater according to the invention; 
     FIG. 2 is the edge region in a cross section through the web; 
     FIG. 3 is a top view of an edge region; 
     FIG. 4 is a cross section through the adjuster for splitting off the edge; 
     FIG. 5 is a longitudinal section through a splitter; 
     FIG. 6 is the corresponding top view; and 
     FIG. 7 is the corresponding cross section 
    
    
     SPECIFIC DESCRIPTION 
     The coating apparatus shown in the figures serves for applying a dispersion adhesive (e.g. an acrylate) in a water solution to a siliconized paper web  1 . As a result of its advantageous features the apparatus also serves for applying other dispersions to paper, plastic foils, or thin strips or to coat paper or cardboard webs with paint. 
     It has a slot nozzle  2  that is hung above the path of the web  1  in the frame of the applicator. The slot nozzle  2  is connected to an unillustrated supply of coating material and has on its underside a slot-shaped opening from which the coating material flows out and forms a free-falling curtain  3  that drops onto the upper web surface. The width of the slot of the nozzle  2  is greater than the maximum width of the web  1 . 
     Underneath the slot nozzle  2  the web  1  is supported over its entire width by a support roller  4 . The support roller  4  engages the uncoated web  1  from below. After being coated, the web  1  leaves the support roller  4  generally horizontally. 
     The slot nozzle  2  is vertically adjustable above the top centerline of the support roller  4  in order to be able to set the spacing of the outlet slot from the web  1  and thus the distance through which the coating material will fall. In addition the slot nozzle  2  is movable horizontally in and against a web-travel direction so that it can be moved upstream from the region directly above the center of the support roller  4  to a region upstream of the support roller  4 . Here the slot nozzle  3  is above a catch trough  5  that is arranged immediately upstream of the support roller  4  above the web  1 . In this position when the coating apparatus is started at first a stable curtain  3  is produced that falls into the catch trough  5 . Thereafter the slot nozzle  2  with the attached elements is moved in the web-travel direction—to the right in FIG.  1 —into its working position above the support roller  4 . The region between the downstream side of the catch trough  5  and the top centerline of the counter roller  4  is covered by a catch plate  6  extending over the entire working width. The end of the catch plate  6  is provided with a scraper  7  that limits the surface air entrained by the web  1  before engaging the curtain  3 . 
     Since the free-falling curtain  3  tends as a result of surface tension to deflect inward on its edges, on both sides of the slot nozzle  2  there are fixed bar-shaped guides  8  that extend down to immediately adjacent the web  1 . Preferably the two guides  8  are each fixed immediately next to the outlet slot of the slot nozzle  2  and their inner faces turned toward the curtain  3  are smooth. As described in German 197 35 558 the inner face of each guide  8  is coated with a supplemental liquid that reduces surface tension, preferably water. The supplemental liquid flowing downward on the guides  8  serves to hold the curtain  3  better on the guides  8 . Alternatively the guides  8  can be inclined inward. 
     Both longitudinal sides of the coating apparatus have means underneath the guides  8  for limiting the curtain  3  to the desired coating width. The limiting means are each mounted on the frame of the nozzle  2  so that when the nozzle  2  is shifted in or against the web-travel direction they move synchronously. In addition they are movable transverse to the web-travel direction in order to be able to set the width of the curtain  3  falling on the web  1  to widely different web widths. The edge-limiting means on both sides are mounted and constructed symmetrically to each other. They can be transversely adjustable independently of each other. 
     The edge-limiting means are each comprised of two blade-shaped generally upright and spaced splitters  9  and  10  that are mounted on the longitudinal sides of a floor plate  11 . Each of the splitters  9  and  10  ends in a sharp splitting edge and the splitting edge of the inner splitter  9  is set so low that the curtain  3  does not separate from its inner face  12  before it contacts the web  1 . Each inner splitter  9  forms with the respective outer splitter  10  and the base plate  11  an edge trough  13  that conducts away the coating material falling between the two respective splitters  9  and  10 . As visible in FIG. 3 the splitters  9  and  10  are extended upstream to form the trough  13  so that its end is above the catch plate  6 . The floor plate  11  is inclined downward so that the caught coating material drops down onto the catch plate  6  and thence runs off toward the catch trough  5 . 
     On the outside face of the splitter  10  is a transverse trough  14  positioned to catch the outer curtain edge. The length of each transverse trough  14  measured transverse to the web-travel direction is such that when a minimal coating width is set the trough outer ends will extend out to below the respective guide  8 , that is past the lateral edge of the outlet slot of the nozzle  2 . Each transverse trough  14  has side walls  15  (FIG. 4) that extend up to the splitter edge of the respective splitter  10 . The splitters  10  close the inner ends of the respective troughs  14 . Trough depth is sufficient to catch and carry away the split-off coating liquid with minimal coating width without any overflow. Preferably the transverse troughs  14  have floors that are inclined inward and the troughs  14  each drain at their inner ends near the splitter  10  into a respective outlet trough  16 . The outlet troughs  16 , which project against the web-travel direction also to the region above the catch plate  6 , conduct the coating material into the catch trough  5 . The inwardly inclined transverse trough  14  with the outlet trough  16  arranged against the respective splitter  10  has two advantages: First, the catch trough  5  only need extend over the axial length of the support roller  4  since, even with maximum coating width, the outlet trough  16  is near the counter roller. Second, the outer end of the transverse trough  14  will not touch the support roller  4  when the blades  9  and  10  are shifted with the transverse troughs  14  inward to a minimal coating width as shown in dashed lines in FIG.  4 . 
     In operation the two splitters  9  and  10  with the respective transverse troughs  14  are moved inward until the inner splitters  9  are set at the location of the desired coating edges. The outer splitter  10  that is taller splits off the outer portion of the curtain edge and diverts it to the transverse trough  14  so it can flow via the outlet trough  16  into the catch trough  5 . This leaves a narrower inner curtain whose edges are split off by the inner splitters  9  and diverted out via the edge troughs  13 . The inner faces  12  of the splitters  9  thus serve as guides for the edges of the curtain portion that is applied to the web  1 . The shorter height of the splitters  9  ensures that the curtain edges do not separate from them before contacting the web  1 . 
     In order to avoid forming a longitudinal ridge of coating material along an edge of the web  1 , the lower Bide of each edge trough  13  is provided with a suction line  17  that opens via a suction conduit  18  in the floor plate  11  into a suction slot  19  in the inner face  12  of the respective inner splitter  9 . The suction slot  19  extends at a spacing of 0.1 mm-1.0 mm from the web  1  on the lower side of the splitter  9  near the curtain  3 . It has a length in the web-travel direction of 2 mm-15 mm, preferably 3 mm-5 mm, so that the suction is maintained even with some movement of the curtain  3 . The suction conduit  17  extends out below the transverse trough  14  and is connected to a pump with a separator for the coating material. In order to prevent caking in the suction slot  19 , it is flushed with water. To this end there is parallel to the suction line  17  a pair of water-supply lines  20  that are immediately adjacent and open into the suction conduit  18 . The flushing water is carried away together with the suctioned-off coating material through the suction line  17 . 
     The length of the suction conduit  18  in the base plate  11  is the same as the spacing of the splitters  9  and  10  from each other. It should be as small as possible in order that too much pressure is not lost and there is no caking in the passage  18 . A short suction conduit  18  is made possible by a narrow edge trough  13  that must only carry away a small stream of split-off coating material. 
     In order to maintain the curtain edges against the inner faces  12  of the splitters  9 , these faces  12  are sprayed with a supplemental liquid. This supplemental liquid is such that it flows from the splitter edge of the splitter  9  down both sides of the splitter  9 . This downwardly flowing supplemental liquid thus forms a supplemental liquid layer immediately beneath the splitter edge of the splitter  9  so as to prevent caking of coating material at the splitter edge. 
     As shown in FIG. 7, the splitter  9  is made of two parts, a side wall  21  having a sharp edge is mounted on the floor plate and is formed in its upper part on the outside face with an upwardly open groove  22 . On the outside of the wall  21  is a thin blade-shaped cover plate  23  that extends over the groove  22  and thus forms a passage open at the splitter edge. A water-supply line  24  opens into this passage. The water fed in from the supply line  24  exits from the top of the split edge and runs down the inner face of the plate  21  as a supplemental liquid stabilizing the curtain and also along the wall  21  into the edge trough  13 . The entire splitter  9  is thus coated with water so as to prevent any caking of the coating material on the splitter  9 . 
     Alternatively to the provision of an additional supplemental liquid, caking on the splitter  9  can be prevented by cooling. Cooling forms condensate on the outer surfaces that form a liquid layer for the coating liquid. Cooling of the splitter  9  can be done by pumping cold water through cavities inside the splitter  9 . Alternately the cooling can be done by means of one or more Peltier elements that are mounted for example inside the edge groove  13  on the side wall  21 . Such cooling elements have the advantage that they need no external equipment.