Abstract:
A method for conditioning a metering blade of a paper coating machine. The method includes the steps of providing a paper coating machine having at least one metering blade and passing a movable substrate through the paper coating machine such that the metering blade contacts the substrate to distribute any coatings applied to the substrate. The method further includes the step of applying a solution of abrasive material to the substrate at a location upstream of the metering blade such that the abrasive material engages and conditions the blade.

Description:
[0001]    The present invention is directed to a method for conditioning a metering blade, and more particularly, to a method for conditioning a metering blade using a solution of abrasive particles.  
         BACKGROUND OF THE INVENTION  
         [0002]    In order to manufacture coated papers, a paper substrate is typically provided, and a coating is then deposited, in a liquid state, onto the substrate. The coated substrate is then passed underneath a doctor or blade or series of doctors or blades which remove extraneous coating material from the coated substrate and smooth, spread and distribute the coating on the substrate. The coated substrate is then dried or cured, and may then be wound around a take-up reel and shipped to a customer for further processing.  
           [0003]    When the coated substrate passes underneath the blades, the blades may create a series of visible lines on the coating in the machine direction. In particular, when debris from the paper manufacturing or coating process is captured between the nip of the blade and the coated substrate, the debris can cause such lines (known as “blade lines”) or other defects on the coated substrate. Furthermore, when new blades are used in the coating process, the blades may not be precisely calibrated or shaped which can also cause blade lines to be formed on the coated substrate.  
           [0004]    Blade lines have attempted to be eliminated by changing the formulation of the coating. For example, the coating may be diluted to ensure that the coating is still in a fluid state and has not yet begun to harden when the coated substrate passes underneath the blades. However, diluting the coating may increase manufacturing time (due to the increased time required to cure the coating) and may increase the roughness of the paper coating.  
           [0005]    Accordingly, there is a need for a method for conditioning a metering blade to reduce the presence of coating blade lines.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is a method for conditioning a metering blade through the use of abrasive particles or an abrasive solution to reduce the presence of coating blade lines. In one embodiment, the invention is a method for conditioning a metering blade of a paper coating machine. The method includes the steps of providing a paper coating machine having at least one metering blade and passing a movable substrate through the paper coating machine such that the metering blade contacts the substrate to distribute any coatings applied to the substrate. The method further includes the step of applying a solution of abrasive material to the substrate at a location upstream of the metering blade such that the abrasive material engages and conditions the blade. Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a schematic representation of one embodiment of the method of the present invention, used with a jet coating system; and  
         [0008]    [0008]FIG. 2 is a schematic representation of one embodiment of the method of the present invention, used with a short dwell system. 
     
    
     DETAILED DESCRIPTION  
       [0009]    As shown in FIG. 1, the present invention is a system  11  that may be used to apply a coating to a substrate  10 . The system  11  may be located in or be part of a paper coating machine which includes a feed path  9  for receiving the substrate  10  therethrough. In a typical coating process, the substrate  10  is passed around a backing roll  12 , and a desired coating is applied to the substrate  10  at an application station  15 . The substrate  10  can be any of a variety of materials, such as paper. The application station  15  may include a jet applicator  14  which includes a jet reservoir  16  and a nozzle  18  that can spray the coating from the jet reservoir  16  onto the substrate  10 . After the coating is applied to the substrate  10 , the substrate  10  is passed about the roll  12  and moved in a downstream direction indicated by the arrows of FIG. 1. The substrate  10  is then passed under a metering blade or blades  20 , which removes any extra coating from the substrate  10  and spreads the coating evenly across the substrate  10 . Any coating that is scraped away by the metering blades  20  falls into a catch cavity  22  and is captured in a catch pan  24  located below the blades  20 . The substrate  10  is then passed downstream for further processing.  
         [0010]    The system  11  includes a primary coating tank  26  and a secondary coating tank  28 , each tank  26  and  28  being coupled to the jet reservoir  16 . The primary coating tank  26  is coupled to the jet reservoir by a primary supply line  29  and a common supply line  30 . The primary supply line  29  may include a filter  32  and pump  34  located therein. The secondary coating tank  28  is coupled to the jet reservoir  16  by a secondary supply line  36  and the common supply line  30 . The secondary supply line  36  may include a filter  37  and pump  39  located therein. Although various pumps may be used, the pumps  34 ,  39  are preferably positive displacement pumps.  
         [0011]    The primary coating tank  26  stores the primary coating that is desired to be deposited on the substrate  10 . The primary coating can be nearly any desired material which is desired to be coated on the substrate  10 , such as, but not limited to, pigmented coatings, mineral coatings, and coatings including clay or calcium carbonate, binders, pigments, lubricants, cross linkers, dispersants, and/or other additives to form a glossy, dull, or matte coating.  
         [0012]    During normal operation, the pump  34  is activated to supply the primary coating from the primary coating supply tank  26  and through the filter  34 . The primary coating is then introduced into the jet reservoir  16  via the primary supply line  29  and common supply line  30 . The primary coating is then sprayed from the jet reservoir  16  onto the substrate  10  by the nozzle  18 .  
         [0013]    The coated substrate is then fed downstream and the coating is metered by the blade  20 . The extra coating that is removed by the blades  20  is captured in the catch pan  24  and fed to the return valve  40 . During normal operations, the return valve  40  returns the removed primary coating to the primary coating supply tank  26  via a primary coating return line  42 .  
         [0014]    The system  11  of the present invention may include the secondary coating tank  28  which stores a secondary coating, such as a solution of abrasive material therein, although nearly any solution which provides the desired abrasive properties and conditioning effects described below may be used. For example, in one embodiment the abrasive solution may include coarse ground carbonate particles having an average particle size of about 0.1 to about 10 microns, preferably about 2 to about 6 microns, suspended in a water solution. The abrasive solution may include a percentage of solids of between about 5% to about 80%, preferably between about 50% to about 70%. The solids suspended in the liquid solution may include pigments (such as clay and the abrasive particles), binders (such as latex and starch), and additives (such as lubricants). The total parts of the binder to parts pigment may be about 5-30. The binder may also constitute about 4% to 26% of the total weight of the solids. The parts of the abrasive particles to total pigment may be between about 5 to 100 parts. The abrasive particles may constitute between about 4% to about 100% of the pigment, by weight. The abrasive particles may constitute between about 4% to about 90% of the total weight of the solids. As will be discussed in greater detail below, the amount of abrasive particles in the solution is preferably sufficient to pull any debris from between the nip of the substrate  10  and blade  20  without causing undo wear upon the blades.  
         [0015]    The secondary coating can be supplied to the application station  15  when it is desired to condition the metering blades  20 . For example, when an operator switches to new metering blades or when it is noticed that blade lines are present on the coated substrate  10 ′, the secondary coating may be applied to the substrate  10  to condition the metering blades  20  in the desired manner. It is expected that any portions of the substrate  10  which include the secondary coating thereon may be discarded. Although the conditioning method of the present invention may be most beneficial for smooth substrates, the invention can be used with nearly any substrate or coating process, regardless of the roughness of the substrate.  
         [0016]    In order to supply the secondary coating to the application station  15 , pump  39  is activated to supply the secondary coating from the secondary tank  28  to the jet reservoir  16  via the secondary supply line  36  and common supply line  30 . The nozzle  18  then sprays the secondary coating onto the substrate  10 . In this manner, when the secondary coating is applied to the substrate  10 , the secondary coating helps to clean the blades  20  and remove any debris trapped underneath the blades  20 . In other words, the secondary coating pushes any material or debris that is trapped between the nip of the blades  20  and the substrate  10  through the nip. Furthermore, the secondary coating helps to “wear in” or condition new metering blades. Although the system of FIG. 1 illustrates one system for applying an abrasive solution to the substrate  10 , it should be understood that any of a wide variety of manner of applying the secondary coating to the substrate  10  may be used without departing from the scope of the present invention.  
         [0017]    As noted earlier, during normal operations the primary coating is supplied from the primary supply tank  36  to the jet reservoir  16  via pump  34 , and then applied to the substrate  10  at the application station  15 . When it is desired to condition the blades  20 , the pump  34  is preferably turned off or gradually ramped down while the pump  39  is simultaneously activated or gradually ramped up to pass the secondary coating through the common supply line  30 . The pumps  34 ,  39  are preferably controlled so that the flow rate of fluid through the common supply line  30  and applied at the application station  15  remains constant during the switchover to the secondary coating. Furthermore, as soon as any secondary coating is applied to the substrate  10 , the coating return valve  40  may be switched over such that any fluids caught by the catch pan  40  are routed to the secondary coating supply tank  28  via a secondary coating return line  44 .  
         [0018]    The pumps  34 ,  39  are preferably controlled such that the percentage of flow in the common supply line  30  and applied at the application station  15  contributed from the primary coating supply tank  26  is gradually reduced while the percentage of flow in the common supply line  30  and applied at the application station  15  contributed from the secondary coating supply tank  28  is gradually increased. In this manner, the secondary and primary coating are at least partially co-mingled in the common supply line  30  during the “phasing in” and “phasing out” stages of the secondary coating. The “phase in” step is preferably a linear phase in of the secondary coating and may take place over any desired length of time, such as, for example, 45 seconds. Once the fluids in the common supply line  30  and sprayed by the nozzle  18  consists 100% of the secondary coating, the secondary coating is then applied to the substrate  10  for the desired period of time, for example, from about 0 to about 3 minutes or more. Alternately, if the system  11  is being operated from a start-up condition, the “phase in” step may be bypassed, and the conditioning operations may commence by supplying the entire coating from the secondary coating supply tank  28 .  
         [0019]    Next, the “phase out” of the secondary coating is initiated. The phase out may be accomplished by controlling the pumps  34 ,  39  such that the percentage of secondary coating in the flow in the common supply line  30  and sprayed by the nozzle  18  is gradually decreased (preferably linearly) while the percentage of the primary coating is simultaneously increased (preferably linearly). Once the supply of the secondary coating reaches 0% of the flow in the common supply line  30  and nozzle  18 , the pump  39  may be turned off, and the coating return valve  40  is switched such that any fluids caught by the catch pan  24  are routed to the primary coating supply tank  26  via the primary coating return line  42 . The “phase out” operation may be as long as desired, for example, 45 seconds.  
         [0020]    Both the “phase in” and “phase out” procedures are preferably controlled to maintain the percentage of solids and viscosity of the fluids sprayed onto the substrate  10  at a generally constant level. It is desired to maintain the deposited coat weight at a constant or close to constant level to avoid overloading the systems and to avoid any web breaks. Furthermore, it is not necessary that the secondary coating be the only fluid sprayed onto the substrate  10 . In other words, a solution of the secondary coating and the primary coating may be sufficient to condition the blades  20 .  
         [0021]    [0021]FIG. 2 illustrates the system  11  of the present invention wherein the application station  15  includes a short dwell applicator  50 . In this case, the substrate  10  is passed through or immersed in a bath  52  containing the coating to be applied  54 , and the blades  56  remove any extra coating  54  and evenly distribute the coating  54  on the substrate. The system and method of the present invention may also be used with a combination dwell or jet-type applicators. Furthermore, the system and method of the present invention are not limited to use with dwell or jet-type applicators, and can be used with nearly any method or mechanism for applying a coating to a substrate.  
         [0022]    Having described the invention in detail and by reference to the preferred embodiments, it will be apparent that modifications and variations thereof are possible without departing from the scope of the invention.