Abstract:
Coating is applied from a coating application chamber to a moving web or backing roll by a rotating premetering rod, a curved or straight wedge, or a premetering blade which is urged against the moving substrate by a plurality of profiling mechanisms which are spaced from one another in the cross machine direction to control the cross machine direction profile of the applied coating to avoid coating streaks. The profiling mechanisms may be hydraulic actuators, machine screws or thermal expansion driven pistons. Cross machine profiling of the premetering device allows control with minimum pressure on the web. Adjustments to the premetering device are only made where necessary to correct nonuniform coating application where detected.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to apparatus for applying coating to moving substrates such as paper, and applicator rolls in general, and to metering apparatus in particular.  
         BACKGROUND OF THE INVENTION  
         [0002]    Paper with high quality finish may be created by applying a thin layer of coating material to one or both sides of the paper. The coating is typically 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 of the organic type or of a synthetic composition. Rosin, gelatins, glues, starches or waxes may also be applied to paper for sizing.  
           [0003]    Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers and other applications requiring good printing characteristics for color photos or other specialized paper qualities.  
           [0004]    Various devices have been employed in the past to apply coatings to paper, either directly, by flooding the web as it passes through a pond, as in a short dwell coater, or by first applying the coating to a roll, as in a size press. Once coating has been applied to the substrate, it is necessary to meter the coating to a desired thickness and uniform level. Uneven coating thickness will produce blemishes and quality variances in the finished paper, and is highly undesirable.  
           [0005]    The pond of coating material employed in the short dwell coater is formed by feeding an excess amount of coating material into a pond housing positioned beneath a backing roll over which a paper web is wrapped. The pond is caused to overflow in the up machine direction, thereby flooding the web and pre-wetting it as it approaches the pond. Downstream from the pond, a metering element, such as a blade, controls the amount of coating material that is applied to the web. The excess coating metered by the metering element is turned downwardly into the pond creating a recirculating zone between the down machine end of the pond and the coating feed at the up machine end of the pond where the excess coating overflows.  
           [0006]    The trend in papermaking is to increase efficiency by increasing the speed of formation of the paper. Coating costs can be minimized by coating the paper while still on the papermaking machine. Increasing the paper web speed is critical to continued increases in papermaking productivity. But, because the paper is made at higher and higher speeds, and because of the advantages of on-machine coating, the coaters in turn must also run at high speeds. The need to produce lightweight coated paper to hold down the weight of the paper, and the costs of the coating material, makes the use of short dwell coaters more desirable, since, by subjecting the paper web to the coating material for a short period of time, the depth of penetration of the coating is limited, resulting in a lower coating weight.  
           [0007]    However, the use of short dwell coaters at high machine speeds can lead to defects in the coating, typically coating streaks. As the speed of the machine increases, the fluid flow in the pond becomes chaotic and the recirculating flow forms a vortex. The result of the turbulent, chaotic flow is that the location where the paper becomes wetted by the coating begins to oscillate so uniformity of coating contact in the machine direction and the cross machine direction is lost. The turbulent flow which causes streaking in the coating is responsive to adjustments in the coater. Nonetheless, as machine speeds increase, greater control of coater parameters is required.  
           [0008]    What is needed is a metering device which can be adjusted to overcome the problem of streaking caused by higher operating speeds.  
         SUMMARY OF THE INVENTION  
         [0009]    The coaters of this invention employ one of three mechanisms. In one embodiment, a metering rod is disposed in front of a final metering blade. In another embodiment a curved or straight wedge is disposed in front of a final metering blade. In a further embodiment, a premetering blade is biased against a backing roll by a pneumatic tube located near the tip of the blade. The pneumatic tube holds the blade such that the flat of the blade is in contact with the film of coating being applied to the paper web. Each device for premetering the coating is mounted for adjustable movement toward and away from the backing roll over which a paper web is wrapped. Three mechanisms for moving the premetering devices are disclosed. Each mechanism is used as a series of like devices arrayed in the cross machine direction. The first mechanism is a hydraulic piston, the second mechanism is a machine screw, and the third mechanism is a thermal expansion driven piston. Cross machine profiling of the premetering device allows coating control with minimum pressure on the web. Adjustments to the premetering device are only made when necessary to correct detected streaking or to preferentially bias the coat weight.  
           [0010]    It is a feature of the present invention to provide a short dwell coater which operates at higher speeds.  
           [0011]    It is another feature of the present invention to provide a short dwell coater which can be profiled in the cross machine direction.  
           [0012]    It is a further feature of the present invention to provide a short dwell coater which improves the hydrodynamics of the coating pond in the coater.  
           [0013]    It is an additional feature of the present invention to provide a short dwell coater with an improved premetering device.  
           [0014]    Further objectives, 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  
       [0015]    [0015]FIG. 1 is a perspective view of the short dwell coater of this invention as used to apply a coating to a web of paper on a backing roll.  
         [0016]    [0016]FIG. 2 is a cross-sectional view of the short dwell coater of FIG. 1.  
         [0017]    [0017]FIG. 3 is a cross-sectional view of an alternative embodiment short dwell coater of this invention.  
         [0018]    [0018]FIG. 4 is a cross-sectional view of a further alternative embodiment short dwell coater of this invention.  
         [0019]    [0019]FIG. 5 is a cross-sectional view of a machine screw mechanism for moving a premetering device.  
         [0020]    [0020]FIG. 6 is a cross-sectional view of a hydraulic mechanism for moving a premetering device.  
         [0021]    [0021]FIG. 7 is a cross-sectional view of a thermal expansion mechanism for moving a premetering device.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring more particularly to FIGS.  1 - 7 , wherein like numbers refer to similar parts a coater  20  is shown in FIGS. 1 and 2. The coater  20  has an applicator head  22  positioned beneath and next to a backing roll  24 . A web of paper  26  is supported on the surface  28  of the roll  24 . The applicator head  22  has a housing  23  with a baffle plate  30  which extends upstream of a premetering wedge  32  which is upstream of a final metering blade  34 . Coating is supplied through a passageway  36  between the baffle plate  30  and the premetering wedge  32  to form a coating application chamber or pond  38 . The coating overflows the upstream baffle plate  30  and is collected in a collector trough  40  from which the overflow coating is collected for cleaning and recycling. The coating overflow serves to remove air from the coating pond  38  and control the fluid dynamics within the pond  38 .  
         [0023]    The final metering blade  34  controls the thickness of the coating on the paper web  26 . An inflatable pneumatic tube  42  is mounted to a support  44  which extends from the housing  23 . The pneumatic tube  42  is engaged against the final metering blade  34 . Inflation of the pneumatic tube  42  controls the amount of pressure with which the final metering blade  34  engages the web  26 . By adjusting the pressure applied by the tube  42  to the final metering blade  34 , the coating thickness may be controlled.  
         [0024]    The premetering wedge  32  has a smooth upper surface  46  which engages the web  26  to define a converging wedge of coating. The premetering wedge  32  serves to control the formation of vortices within the pond  38 . The wedge  32  is positioned between a front support  48  and a rear support  50 . Sealing gaskets  52  prevent coating from passing between the wedge  32  and the front and rear supports  48 ,  50 . A pneumatic tube  54  is positioned between the bottom  56  of the wedge  32  and a support piston  58 . The pneumatic tube  54  provides support for the wedge  32  in the cross machine direction. A plurality of hydraulic actuators  60  provide cross machine profiling of the premetering wedge  32 .  
         [0025]    As machine speed is increased, flow instability can develop resulting in streaking of the coating on the web  26 . To overcome the streaking produced by speed-induced instabilities within the coater  20 , various parameters of the applicator head  22  can be adjusted. Some of the parameters which can be adjusted are the rate of flow of coating to the applicator head  22 , the air pressure in the tube  54  which supports the premetering wedge  32 , and the air pressure in the tube  42  which supports the final metering blade  34 . Moreover, the distance between the upstream baffle plate  30  and the backing roll can be adjusted. However the adjustability of the foregoing parameters is limited by various runnability and maintenance and wear considerations. Placing the upstream baffle plate  30  too close to the backing roll can result in damage to the coater during a paper break. Increasing the pressure with which the premetering wedge  32  and the final metering blade  34  are pressed against the backing roll can result in excessive wear and increased stress on the web.  
         [0026]    Cross machine profiling by adjusting the pressure on the premetering wedge  32  at discrete locations in the cross machine direction can overcome coat weight variation and streaking without affecting runnablity of the coater  20 . A series of actuator mechanisms spaced from one another in the cross machine direction and independently controllable by the controller  41  provide an improved ability to overcome streaks in the coating.  
         [0027]    Three alternative embodiment actuator mechanisms which can be used to perform cross machine direction profiling of the premetering wedge  32  are shown in FIGS.  5 - 7 . A hydraulic system  61 , shown in FIG. 6, has a hydraulic piston  62  which moves in a cylinder  63  which is supplied with hydraulic fluid by a supply passage  64 . The piston  62  supports a pneumatic tube  66  which in turn supports a premetering wedge  65  such as the wedge  32  shown in FIG. 2. The hydraulic piston  62  may also support other premetering devices as discussed below.  
         [0028]    A screw drive support system  67 , shown in FIG. 5, has a machine screw  68  driven by a motor  70 . The machine screw  68  supports a piston  72  on which is mounted a pneumatic tube  74 . The piston tube arrangement can be used to support the premetering wedge  32  as well as other premetering devices as discussed below. The screw drive support system  67  is somewhat more complicated than a hydraulic support system. Nonetheless, the screw support system  67  advantageously requires no power to maintain a given profile and the profile is maintained when the machine is shut down.  
         [0029]    A differential thermal expansion profiling system  76  is shown in FIG. 7. Dissimilar metals expand at different rates when heated. This principal is typically used in thermostats where a lever arm composed of two dissimilar metals bends in response to the dissimilar expansion of the two metals and closes or opens a switch. The metal rod  78  may be composed of a metal with a high coefficient of thermal expansion, such as aluminum or magnesium, positioned in a cylinder  80  of low expansion metal such as molybdenum or chromium. A heater element  82  is positioned to heat the metal rod to cause the high expansion metal rod  78  to push against a piston  84 . The piston  84 , in a manner similar to the pistons of the mechanisms shown in FIGS. 5 and 6, supports a pneumatic tube  86  which in turn can support a premetering wedge  32  or other premetering device as discussed below. The rod  78  is insulated from the cylinder  80  by insulation  88 . Insulation between the rod  78  and the cylinder  80  increases the effect of the differential expansion of the rod  78  and the cylinder  80 . However it may be preferable to place the insulation external to the cylinder  80  so that the motion of the piston  84  is strictly controlled by temperature which is easily monitored and controlled.  
         [0030]    Of course the principle of using dissimilar metals could be employed with various levers and mechanisms to amplify the force or the displacement produced by the differential expansion between two metals.  
         [0031]    The profiling mechanisms shown in FIGS.  5 - 7  can be used to introduce cross-machine profiling into a variety of applicator heads. An alternative embodiment coater  120 , shown in FIG. 3, has an upstream baffle plate  130  which extends from the coater housing ahead of a final metering blade  134 . A premetering blade  132  is positioned between the baffle plate  130  and the final metering blade  134 . The premetering blade  132  is held in a rigid mount  146  and pressed tangentially against a paper web  126  supported by a backing roll  124 . The premetering blade  132  is supported by a pneumatic tube  154  which is mounted to a piston  158  which moves toward and away from the backing roll within a hydraulic cylinder  160 . The other mechanisms shown in FIGS. 5 and 7 could also be used to profile the premetering blade  132 . It should be understood, that although only a single piston and cylinder mechanism is illustrated in FIG. 3, a plurality of profiling mechanisms will be disposed spaced from one another in the cross machine direction, all bearing on the pneumatic tube  154  and being controlled individually or in groups to adjust the pressure on the premetering blade  132  to thereby obtain uniformity of coating application.  
         [0032]    Another alternative embodiment applicator head  222  having a premetering rod  232  is shown in FIG. 4. The rod  232  is mounted in the applicator head  222  between an upstream baffle plate  230  and a final metering blade  234 . The rod  232  is driven to rotate and thus perform the premetering function. The rod  232  rotates within a rod support fixture  258  which is displaced toward the backing roll by a pneumatic tube  254  which is supported by a plurality of hydraulic pistons  260  in hydraulic cylinders  262 . Alternatively, the premetering rod  232  can be profiled in the cross machine direction by any of the mechanisms disclosed in FIGS.  5 - 7 .  
         [0033]    The spacing between profile actuators is typically three to four inches in a modern papermaking machine. However it should be understood that the distance between profiling actuators could be greater or less than three to four inches. FIG. 1 shows a plurality of hydraulic pistons arranged in the cross machine direction. Each piston is independently controlled so that the pressure on the premetering device can be varied in the cross machine direction to respond to the nonuniform coating weight detected in the web as it is formed.  
         [0034]    It should be understood that the although the coaters of this invention have been illustrated for applying coatings directly to a substrate which is a moving web, they may also be employed in a size press for applying coating to a substrate which comprises a roll surface for transfer to a paper web.  
         [0035]    It should also be understood that minimal or no profiling may also be performed if the uniformity of the coating application dictates. The premetering concepts described here improve the uniformity of the coating forces against the web and thus improve the coating application to the web compared to other arrangements.  
         [0036]    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.