Abstract:
An apparatus for coating a web of indeterminate length has a coating element comprising a liquid deflector member for diverting liquid away from a coating surface. The liquid deflector member is arranged beneath a blade member that removes excess coating liquid from the coating surface. Excess coating liquid follows a path away from the coating surface and down the liquid deflector member thereby avoiding contamination of the coating surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is related to U.S. application Ser. No. (Docket 78832), filed ______, by Ramasubramaniam Hanumanthu, et al., and entitled, “Element For Deflecting Excess Liquid From A Coating Surface.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to the field of roller/gravure coating. More particularly, the invention concerns a coating apparatus that meters a film of liquid coating solution from a coating surface or roller and then diverts it away, thereby preventing contamination of the coating surface.  
         BACKGROUND OF THE INVENTION  
         [0003]    In conventional roller/gravure coating processes (as described, for example, in U.S. Pat. No. 4,373,443, Feb. 15, 1983, by Matalia et al., entitled, “Method Of High Viscosity Inking In Rotary Newspaper Presses” where a gravure cylinder provides ink in newspaper presses), a liquid coating composition is directed to the surface of a coating applicator roll  1  by one of several suitable means including rotating (denoted by arrow) the applicator roll  1  through a reservoir  2  of liquid  3 , as illustrated in FIG. 1. The surface of the coating applicator roll  1  may have a smooth finish or it may be engraved with cells/grooves  5  of prescribed volume. Often, the layer of liquid  3  picked up by the applicator roll  1  from the reservoir  2  is subsequently metered to a thinner film using a doctor blade  4 . In gravure coating, for example, the doctor blade  4  removes all the applied liquid except that which is present in the engraved cells  5  formed in the gravure cylinder  1 . Alternatively, the steps of wetting (filling) and doctoring may also be combined as described in U.S. Pat. No. 4,158,333, Jun. 19, 1979, by Navi, titled, “Inking Baffle For Rotary Newspaper Presses.” After the doctoring step, the liquid remaining on the surface of a smooth coating applicator roll or that remaining in the cells  5  of an engraved coating applicator roll is transferred to a moving web  6  by impressing the moving web  6  between the applicator roll  1  and a soft backer or impression roll  7 . In FIG. 1, the web  6  is shown to be moving in the same direction as the surface of the coating applicator roll  1  at the point of contact between the two, but in roller/gravure coating practice, the web may be conveyed in the opposite direction as well. The thickness of coating transferred to the moving web  6  is generally a known fraction of the thickness of liquid film retained on the surface of a smooth coating applicator roll downstream of the doctoring step or, alternatively, it is a known fraction of the volume of the engraved cells  5  per unit surface area of an engraved coating applicator roll  1 .  
           [0004]    Depicted in FIGS. 2 a  and  2   b  , a shortcoming of existing roller/gravure coating processes is that when excess liquid  8  removed by the doctor blade  4  falls back on the surface of the coating applicator roll  1 , it is carried back up to the “bank” of coating liquid  9  that is accumulated between the moving coating applicator roll  1  surface and the stationary doctor blade  4 . Since the excess liquid  8  falls back on and contacts the surface of the coating applicator roll  1  in a turbulent and random manner, this renders the bank of coating liquid  9  uneven in the cross-web direction. The unevenness of the bank of coating liquid  9  in turn causes a coating defect in the form of streaks and bands  10 , as exemplified in FIG. 3. The defect is especially prominent in particulate coating dispersions (as opposed to solutions).  
           [0005]    An analysis of the nature of the flow of metered liquid  3  behind the doctor blade  4  reveals that at low coating applicator roll  1  surface speeds the liquid  3  simply runs back down the surface of the coating applicator roll  1  in a laminar fashion (see flow lines  11  in FIG. 4 a ). However, as speed of the coating applicator roll  1  is raised, a point is reached when the metered liquid  3  separates from the surface of the coating applicator roll  1  and flows (see flow lines  12  in FIG. 4 b ) generally along the underside  13  of doctor blade  4  and away from the surface of the applicator roll  1 .  
           [0006]    Moreover, at some point further downstream of the contact point  14  between the doctor blade  4  and the coating applicator roll  1 , the deflected liquid loses its momentum and therefore separates from the underside surface  13  of the doctor blade  4  and falls or flows vertically downwards under the influence of gravity (refer to FIG. 4 b ).  
           [0007]    Presently the defect can be avoided in one of several ways. One way known to avoid this defect is to maintain the coating speed below the speed of transition from “runback” flow to “deflected” flow. Experimental observations indicate that the speed of transition between runback flow (FIG. 4 a ) and deflected flow (FIG. 4 b ) depends on operating parameters—viscosity and surface tension of liquid; tangent angle between doctor blade  4  and surface of the coating applicator roll  1 ; thickness of the incoming film of liquid; radius of coating applicator roll  1 ; etc. Here, runback flow is defined as the case where liquid removed by the doctor blade  4  runs back down the surface of the coating applicator roll  1 . Deflected flow is where the excess liquid  8  metered by the doctor blade  4  travels away from the surface of the coating applicator roll  1 , along the underside  13  of the doctor blade  4 , up to a point where it loses its momentum, and then further separates from the underside  13  of the doctor blade  4  surface, and drops vertically under the influence of gravity.  
           [0008]    Unfortunately, under normal operating/manufacturing conditions, the speed of transition from runback to deflected flow is too low for it to be a practicable production speed.  
           [0009]    Referring to FIGS. 5 a  and  5   b  , another known way to avoid the defect is to locate the contact point or tip  14  of the doctor blade  4  at application points on the cylindrical coating applicator roll  1  surface that are far from top-dead-center  19 . Then, especially in the case of small diameter cylinders, i.e., typically diameters less than about 5 inches, the deflected excess liquid  8  in all likelihood will not flow back to the cylindrical coating applicator roll  1  surface on its way down (refer to FIG. 5 b ). But at application points close to top-dead-center  19 , and with large diameter coating applicator rolls  1 , the excess liquid  8  will tend to flow back to the surface of the coating applicator roll (FIG. 5 a ).  
           [0010]    Unfortunately, the location of the contact point or tip  14  of the doctor blade  4 , relative to top-dead-center  19  cannot be changed arbitrarily. For instance, to minimize evaporation of coating liquid  3  from the surface of the coating applicator roll  1  in the region between the contact point or tip  14  of the doctor blade  4  and top-dead-center  19 , it may be necessary to narrowly fix the distance of the contact point or tip  14  of the doctor blade  4  from top-dead-center  19 . Similarly, the diameter of the coating applicator roll  1  may also have to be narrowly fixed. This is true, for instance, in the coating of discrete patches or patterns using gravure coating, wherein the ratio of gravure cylinder circumference to engraved patch/pattern length has to be maintained constant.  
           [0011]    While there are no known prior art attempts to solve Applicants&#39; specific problem of diverting coating liquid from the surface of a coating applicator roll having an excess quantity of liquid thereon, U.S. Pat. No. 5,755,883, May 26, 1998, by Kinose et al., titled, “Roll Coating Device For Forming A Thin Film Of Uniform Thickness” discloses a roll coater having a blade scraper for scraping coating liquid from a metal roll and a tray positioned beneath the nip for catching the scraped liquid. This device provides only for preventing fluid from contacting coating elements beneath the nip and does not protect the roll from which the liquid was deposited from receiving excess liquid.  
           [0012]    An attempt to use a similar tray in a location between the underside  13  of the doctor blade  4  and the surface of the coating applicator roll  1  (refer to FIG. 6) was not successful because there is very little room available there. Indeed the deflected excess liquid  8  separates from the underside  13  of the doctor blade  4  so quickly that the lip  20  of the tray  21  would have to be within 0.32 cm (0.125 in) from the underside surface  13  of the doctor blade  4 , and the applicator roll  1  surface. Such tight gaps are not favored in manufacturing environments.  
           [0013]    Yet another scheme to prevent the defect involves the creation of a narrow passageway  22  between the coating applicator roll  1  surface and an element  23 . The coating liquid  3  effectively “floods” the passageway  22  and in this manner defects that persist far upstream of the contact point or tip  14  of doctor blade  4  are forced to damp out before they reach the contact point or tip  14  of doctor blade  14 . In other words, the pressure in the “bank” of coating liquid  9  accumulated between the moving coating applicator roll  1  surface and the stationary doctor blade  4  stays even across the width of the web  6 , at least in the vicinity of the doctor blade tip  14 . However, the drawback of this approach was that to effectively flood the passageway  22  under all operating conditions, the element  23  had to be maintained at gaps less than 0.2 cm (0.08 in) from the coating applicator roll  1  surface. Again, such narrow gaps are not favored in the manufacturing environment.  
           [0014]    Finally, the problem may be inherently solved by using combined feed/blading units, such as the reverse doctor pond feed (U.S. Pat. No. 4,158,333). There, the trailing blade at the exit of the reservoir keeps the excess fluid within the reservoir, and hence there is no occasion for deflection (“deflection” is illustrated in FIG. 4 b ). However, in the present application, reverse doctor pond feed is not practicable.  
           [0015]    Therefore, there persists a need for a roller/gravure coating process in which excess coating liquid material removed by a doctor blade is diverted away from the surface of the coating applicator roll thereby avoiding contamination of the applicator roll surface.  
         SUMMARY OF THE INVENTION  
         [0016]    It is, therefore, an object of the invention to provide a roller/gravure coating apparatus having a liquid metering/diverting element for metering a film of liquid material from the surface of a coating applicator roll and then diverting excess liquid material away from the surface of the coating applicator roll.  
           [0017]    An important feature of the invention is a liquid deflector member arranged proximate to the surface of the coating applicator roll and a metering member for diverting excess liquid away from the coating applicator roll surface.  
           [0018]    To solve this and other objects of the invention, there is provided an apparatus for coating a web of indeterminate length, comprising a source of coating composition; an engraved cylinder at least partially in fluid contact with the source of coating composition. The engraved cylinder includes a plurality of cells for collecting coating composition therein and then transfers the coating composition to the web of indeterminate length. An impression cylinder is in rotating contact with the engraved cylinder, which thereby forms a web transfer path therebetween. The web of indeterminate length is advanced through the web transfer path so that coating composition in plurality of cells transfers to the web of indeterminate length forming an applied coat of coating composition on the web of indeterminate length. The apparatus also comprises a coating element for doctoring the applied coat of coating composition on the web of indeterminate length to a finished coat and then diverting any excess coating composition away from said engraved cylinder.  
           [0019]    It is an advantageous effect of the invention that the liquid deflector member is versatile, cost effective to manufacture, simple to install and operate and can function with minimum variability of settings over a wide range of manufacturing operating conditions 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:  
         [0021]    [0021]FIG. 1 is a front elevation view of a prior art roller/gravure coating process;  
         [0022]    [0022]FIG. 2 a  is a schematic illustration of a partial cross-sectional side view of a prior art roller/gravure coating process illustrating doctored sheet of coating liquid flowing downwardly onto the surface of a coating applicator roll;  
         [0023]    [0023]FIG. 2 b  is a scanned photographic image of a partial cross-sectional side view of a prior art roller/gravure coating process illustrating doctored sheet of coating liquid flowing downwardly onto the surface of a coating applicator roll;  
         [0024]    [0024]FIG. 3 is a scanned image of a coating sample illustrating defects in the form of streaks and bands of a prior art roller/gravure coating process;  
         [0025]    [0025]FIG. 4 a  is a schematic of a roller/gravure coating process illustrating flow of excess coating liquid running back down the surface of a coating applicator roll;  
         [0026]    [0026]FIG. 4 b  is a schematic of a roller/gravure coating process illustrating deflected flow of excess coating liquid along the underside of a doctor blade member;  
         [0027]    [0027]FIG. 4 c  is a scanned image of photographic snapshots depicting the transition of flow behind the blade from “runback” to “deflected” modes. The top and bottom pictures images in this column are the counterparts of the schematic illustrations in FIGS. 4 a  and  4   b,  respectively;  
         [0028]    [0028]FIG. 5 a  is a schematic of a prior art roller/gravure coating process illustrating deflected sheet of coating liquid separating from underside of doctor blade and flowing downwardly onto the surface of a coating applicator roll;  
         [0029]    [0029]FIG. 5 b  is a schematic of a prior art roller/gravure coating process illustrating deflected sheet of coating liquid separated from the doctor blade and the surface of a coating applicator roll;  
         [0030]    [0030]FIG. 6 is a schematic of a prior art element to catch the deflected sheet of liquid after separation from the doctor blade;  
         [0031]    [0031]FIG. 7 is a schematic of another prior art element to flood the passageway between the surface of a coating applicator roll and said element in an attempt to maintain an even bank of coating liquid at the tip of the blade;  
         [0032]    [0032]FIG. 8 a  is a schematic of the element of the invention illustrating orientation with respect to the surface of the coating applicator roll and metering doctor blade;  
         [0033]    [0033]FIG. 8 b  is a scanned image of an application of the invention;  
         [0034]    [0034]FIG. 9 is a schematic of the element of the invention illustrating an unfavorable orientation of liquid deflector member; and, FIGS. 10 a  ,  10   b , and  11  are schematics of the element of the invention illustrating alternative embodiments. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    Turning now to the drawings, and in particular to FIGS. 8 a - 10 , there is illustrated the coating element  25  according to the principles of the invention. According to FIGS. 8 a  and  8   b  , coating element  25  removes excess liquid (l) from the surface  27  of a coating applicator, such as a roll  24 , and then diverts the excess liquid (l) away from the surface  27 . Importantly, coating element  25  has a doctor blade member  26  and a liquid deflector member  28  structurally disposed in a support member  30 .  
         [0036]    Referring to FIGS. 8 a - 11 , doctor blade member  26 , generally has an active end  32  extending from the support member  30  for engaging and removing excess liquid (l) from the surface  27  of coating applicator or roll  24 . Support member  30  is used principally to manipulate and fix the orientation of the active end  32  relative to the surface  27  of the coating applicator or roll  24 . Thus, for most efficient operation, active end  32  of doctor blade member  26 , and more particularly, underside  34 , is arranged preferably at a predetermined angle θ t  with the surface  27  of the coating applicator or roll  24 . The inventors have determined that a preferred range of predetermined angle θ t  is between about 50-60 degrees. Skilled artisans will appreciate that the active end  32  of the doctor blade member  26  contacts the surface  27  of the coating applicator or roll  24  at some well defined point P so that excess coating liquid (l) can be effectively removed from the surface  27 .  
         [0037]    Referring to FIGS. 8 a - 11 , liquid deflector member  28  has an active face  36  (if properly oriented) that diverts excess coating liquid (l) away from the surface  27  of the coating applicator or roll  24 . Thus, excess coating liquid (l) doctored from the surface  27  of coating applicator or roll  24  flows along the underside  34  of active doctor blade member  26  and then along active face  36  of liquid deflector member  28  away from surface  27 . Active face  36  is positioned proximate to both the active end  32  of the doctor blade member  26  and the surface  27  of the coating applicator or roll  24 . The underside  34  of doctor blade member  26  extends from the contact point P to apex  38  by a predetermined clearance (d), described further below. Apex  38  is a point on the underside  34  of blade member  26  that intersects the active face  36  of the liquid deflector member  28 . Further, active face  36  of liquid deflector member  28  is arranged at a predetermined angle θ s  to the underside  34  of the active end  32  of doctor blade member  26 . In the preferred embodiment, active face  36  of liquid deflector member  28  is generally planar (FIG. 8 a ). Alternately, active face  36  may be generally contoured from a point near apex  38  either away (FIG. 10 a ) from the surface  27  of coating applicator or roll  24  or towards (FIG. 10 b ) the surface  27  of coating applicator or roll  24 . Each of these configurations has proven effective in diverting excess liquid (l) away from surface  27 .  
         [0038]    Referring again to FIG. 8 a  , the underside  34  of doctor blade member  26  preferably makes a generally obtuse angle with the adjoining active face  36  of the liquid deflector member  28 . Thus, excess liquid (l) will follow a generally obtuse angular path from the underside  34  of the doctor blade member  26  along the active face  36  of the liquid deflector member  28 .  
         [0039]    Referring now to FIG. 11, alternatively, the underside  34  of doctor blade member  26  may form a generally arcuate path with the active face  36  of the liquid deflector member  28  along which excess liquid (l) flows.  
         [0040]    Referring again to FIGS. 8 a  and  8   b  , liquid deflector member  28  is adjustably fixed to support member  30  with active face  36  positioned close enough to the contact point P that it “captures” the deflected liquid (l) flowing on the underside  34  of doctor blade member  26 . The positioning is important because the deflected liquid (l) could very well lose its momentum and then divert downwardly under the influence of gravity towards surface  27  of the coating applicator or roll  24 .  
         [0041]    Liquid deflector member  28 , preferably made of a rigid metal or plastic, may be structurally affixed to support member  30  in several ways with virtually the same results, including bolting, screwing, riveting, welding, or clamping.  
         [0042]    Referring again to FIGS. 8 a  and  8   b  , there are several important operating constraints on the design of the liquid deflector member  28 . According to FIG. 8 a  , the angle θ s  that the liquid deflector member  28  makes with the underside  34  of the doctor blade member  26  is optimum when the active face  36  of the deflector member  28  is near normal to the doctor blade member  26 . However, in this configuration, there is a high risk that a liquid deflector member  28  having a rather long length might interfere with the rotating surface  27  of coating applicator or roll  24 . Consequently, our experience indicates that a preferred angle θ s  is one that is equal to the tangent angle θ t . When θ s  is less than θ t , full advantage is not taken of the assist that gravity provides to the flow of deflected liquid (l) down the active face  36  of deflector member  28  away from the surface  27  of coating applicator or roll  24 . On the other hand, if  05  is much larger than θ t , there is a rather high risk that the bottom edge  40  of the liquid deflector member  30  might interfere with the surface  27  of the coating applicator or roll  24  further upstream of the doctor blade member  26  (refer to FIG. 9).  
         [0043]    Referring to again FIG. 8 a  , as indicated, it is also important that the underside  34  of doctor blade member  26  have a predetermined clearance (d), i.e., distance between the apex  38  and the contact P. For a given inclination, θ h  of blade member  26  above the horizontal plane, this optimum predetermined clearance (d) depends on the flow rate of deflected liquid (l) (per unit width of coating), q; viscosity of coating liquid, μ; density of coating liquid, ρ; and gravitational acceleration, g: clearance ∝ 
             (       q   2     g     )       1   /   3       ·   f     ,                         
 
         [0044]    f, where f is a monotonically increasing function of the Reynolds&#39; Number (Re), given by  
       Re   ≡         q                 ρ                  μ       .                           
 
         [0045]    In the preferred embodiment, an effective clearance (d) is one in the range of about 0.64 cm (0.25 in) to about 1.9 cm (0.75 in).  
         [0046]    The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
                                         PARTS LIST:                                l   excess coating liquid       P   point of contact       1   coating applicator roll       2   reservoir or pan       3   liquid or coating liquid       4   doctor blade       5   .engraved cells/grooves       6   web       7   soft backer or impression roll       8   excess liquid       9   bank of coating liquid       10   streaks and bands       11   flow line       12   flow line       13   underside of doctor blade 4       14   contact point or tip of doctor blade 4       19   top-dead-center of cylindrical surface of coating applicator roll 1       20   lip of tray 21       21   tray       22   narrow passageway       23   element       24   coating applicator or roll       25   coating element       26   doctor blade member       27   surface of coating applicator       28   liquid deflector member       30   support member of coating element 25       32   active end of doctor blade member 26       34   underside of doctor blade member 26       36   active face of liquid deflector member 28       38   apex       40   bottom edge of liquid deflector member 28