Abstract:
An apparatus and method for cleaning a metering roll having a composite sleeve or a metal core with a ceramic coating involves the use of a laser. The roll&#39;s ceramic coating is covered with a matrix of cells that can get plugged with a polymeric contaminant, such as dried ink. The laser is uniquely focused to provide a beam intensity profile that matches multiple curved surfaces of the cells. The laser applies heat to each cell at a temperature that destroys the contaminant, yet leaves the ceramic coating intact. The heat is rapidly delivered and rapidly removed from the roll to minimize the amount of heat conducted to the roll&#39;s metal core. In addition, a special pneumatic guide bearing makes it possible to clean the metering roll while it is still in the printing press.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to the metering roll of a printing press, and more specifically to cleaning the metering roll. 
     2. Description of Related Art 
     In a printing process, a metering roll (anilox roll) transfers ink to a plate, which in turn applies the ink to the material being printed, such as paper or a consumer product label. Some metering rolls have a ceramic coating covered with a dense matrix of extremely small cells that hold the ink. Over time, the cells get plugged with dried or otherwise cured ink which reduces the effectiveness of the roll. 
     Currently, metering rolls are cleaned of their contaminants (e.g., dyes, ink, binders, plasticizers, etc.) with strong solvents, soda blasting, and ultrasonic processes. These methods, however, have limited effectiveness and serious drawbacks. 
     Many water-based dyes and inks are resistant to common solvents. Some solvents can no longer be used, because of their negative effect on the environment. Since ceramic can be porous, some solvents and/or chemicals penetrate completely through the ceramic coating to attack the roll&#39;s metal core. This can lead to the ceramic coating separating from the roll. Excessive heating can also damage the interface between the roll&#39;s metal core and the ceramic, due to the differences of their thermal expansion properties. Ultrasonic and soda blast cleaning can physically damage the ceramic itself. And today&#39;s conventional methods of cleaning require that the metering roll be removed from the printing press. Moreover, there is a trend toward providing metering rolls with ever smaller hole diameters, which make the cells even more difficult to clean. 
     SUMMARY OF THE INVENTION 
     To avoid the limitations and problems of existing methods of cleaning metering rolls, it is an utmost primary object of the invention to shape the intensity distribution of a laser beam to match the curved geometry of the cells of a ceramic coated metering roll. 
     A second object of the invention is to employ an anilox cell geometry that promotes a smooth pattern of airflow delivered by an air nozzle that provides an angled approach. 
     A third object is to focus a laser beam toward a focal point that is below the bottom of the cell being cleaned. 
     A fourth object is to use heat to destroy the contaminants of a metering roll while minimizing the heat conducted to the roll&#39;s metal core. 
     A fifth object is to provide a non-contact method of removing contaminants from a plugged metering roll, regardless of the hole diameter of the cells. 
     A sixth object is to clean a metering roll without having to remove it from the printing press. 
     A seventh object is to turn the laser beam off as it passes between cells to minimize the heat delivered to the roll. 
     An eighth object is to employ a guide bearing that maintains a constant separation distance between the lens and the ceramic surface of the metering roll, regardless of slight misalignments of the cleaning apparatus and cylindrical discrepancies of the roll. 
     A ninth object is to have the laser beam target travel in a helical pattern around a metering roll, with the pattern being superimposed on a similar helical pattern of cells. 
     A tenth object of the invention is to adjust the focus of the laser beam by test burning the ink off a paper label. 
     An eleventh object of the invention is to rotate a metering roll using a “non-slip” synchronous motor whose speed is substantially constant, regardless of slight variation in torsional load. 
     A twelfth object is to compensate for limited encoder resolution by periodically delaying the firing time of the laser in response to an encoder compensation input. 
     These and other objects of the invention are provided by a novel apparatus and method for cleaning a ceramic coated metering roll. The method uses a laser that is uniquely focused to provide a beam intensity profile that suits the multiple curved surfaces of cells that are plugged with a polymeric contaminant. The laser applies heat to the roll at a temperature that destroys the contaminant yet leaves the ceramic coating intact. The heat is rapidly delivered and rapidly removed from the roll to minimize the amount of heat conducted to the roll&#39;s metal core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a cleaning apparatus cleaning a metering roll. 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of the metering roll of FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of the cells in the roll of FIG.  1 . 
     FIG. 4A illustrates the focusing characteristics of a lens according to one embodiment of the invention. 
     FIG. 4B illustrates the focusing characteristics of a lens according to another embodiment of the invention. 
     FIG. 5 is another schematic view of the invention showing the lens and guide bearing. 
     FIG. 6 shows the setup for adjusting the focus by using a test decal. 
     FIG. 7A shows the power intensity profile of a laser beam superimposed on the geometry of a cell according to one embodiment of the invention. 
     FIG. 7B shows the power intensity profile of a laser beam superimposed on the geometry of a cell according to another embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A metering roll cleaning apparatus  10 , of FIG. 1, is cleaning a metering roll  12 . Details and examples of a metering roll can be found in U.S. Pat. No. 4,566,938, which is specifically incorporated by reference herein. Roll  12 , as also shown in FIG. 2, has a metal core  14  (e.g., steel) with a coating  16  (e.g., ceramic, nickel, copper, chrome and various combinations and layers thereof). Coating  16  has a plurality of cells  18  that are used for holding a dye (e.g., ink) and subsequently transferring the dye onto a plate (not shown) of a printing press or directly to a substrate. It should be noted that in a preferred embodiment of the invention, coating  16  consists of a ceramic material; however, coating  16  actually represents any coating material having thermal properties that are dissimilar to those of metal core  14 . For example, in one embodiment of the invention, coating  16  is chrome plating over a steel roll that has mechanically engraved or chemically etched porosity for holding the dye. In other words, roll  12  encompasses rolls having a metal core base as well as the latest technology of a roll having a composite sleeve base. 
     In time, the dye may dry or cure on roll  12  to produce a polymer contaminant  20  that plugs cells  18 , as shown in FIG.  3 . Polymer contaminant  20  represents any one of a variety of substances including (but not limited to) inks, dyes, binders, plasticizers, ultraviolet cured photo-polymers, and adhesives. 
     Returning to FIG. 1, apparatus  10  serves to remove contaminant  20  from roll  12 . Apparatus  10  includes bearings  22  that rotatably mount roll  12  about a longitudinal axis  24 . Roll  12  is rotatably driven by a synchronous motor  26  through a gear reducer  28 . An encoder  30 , coupled to the rotation of roll  12 , generates a pulsed feedback signal  32  representing the rotational speed of rotor  12 . A first datum  34  represents a generally fixed frame of reference. In one embodiment of the invention, datum  34  represents the frame of a printing press where roll  12  is cleaned without being removed from its press. In such an application, bearings  22  are integral components of the press. In another embodiment of the invention, datum  34  represents an independent frame, separate from the printing press, so roll  12  can be removed from the press and cleaned at a remote location. 
     A guideway  38 , fixed relative to datum  34 , slidingly guides a support frame  36 . Support frame  36  is driven in a direction generally parallel to longitudinal axis  24  by way of a nut  40  coupled to a leadscrew  42 . Leadscrew  42  is driven by a motor  44  through a gear reducer  46 . An encoder  48  provides a feedback signal  50  representing the longitudinal position of frame  36  in relation to guideway  38 . 
     Attached to frame  36 , is a laser  52 , a beam expander  54 , a partial reflector  56 , a beam analyzer  58 , and a lens, such as lens  60  or lens  60 ′. Laser  52  emits a narrow concentrated laser beam  62   a . Beam expander  54 , downstream of laser  52 , widens beam  62   a  to create beam  62   b  having a lower intensity (energy level per unit of area). Partial reflector  56 , downstream of beam expander  54 , passes 1% of beam  62   b  onto beam analyzer  58  for monitoring the intensity distribution of beam  62   b . Reflector  56  reflects 99% of beam  62   b  to project a beam  62   c  onto lens  60 , which is downstream of reflector  56 . 
     Lens  60  focuses beam  62   c  toward roll  12  to destroy (by heat) contaminant  20  in and around cells  18 . Once destroyed, a pressurized fluid, such as air  64 , blows contaminant  20  out of cells  18 . Motor  26  continuously turns roll  12 , while motor  44  continuously feeds frame  36  longitudinally, so that laser beam  62   c  traverses all of cells  18  to clean substantially the entire ceramic coating  16 . 
     It has been found that the cleaning process is most effective when particular attention is given to focusing beam  62   c . Referring to FIG. 4A, lens  60  focuses beam  62   c  to a number of points to define a length of spherical aberration  66 . Within spherical aberration  66 , beam  62   c  converges to a minimum width  68  (i.e., spotsize) referred to as a focal point  70 . Surprisingly, best results are obtained when the position of lens  60  is set to place focal point  70  below a curved bottom surface  72  of cells  18 , as shown in FIG.  5 . This setup contours the profile of the beam intensity over the distance from the center of beam  62   c  to suit the contour of the cell&#39;s curved bottom  72 , the cell&#39;s rounded beveled entryway  74 , and an annular area  76  surrounding each cell  18 . The fit between the intensity distribution  77  of beam  62   c  superimposed on the geometry of cell  18  is shown in FIG.  7 A. The ordinate  79  with reference to distribution  77  is in terms of energy per unit of area (e.g., watts/cm 2 ), while the abscissa  81  is the radial distance from the center  83  of beam  62   c.    
     To maintain the proper focus, a guide bearing  78  holds lens  60  at the desired distance from ceramic coating  16 ; regardless of discrepancy of the cylindricity of roll  12 , and regardless of possible slight out of parallelism between guideway  38  and axis  24 . 
     The distance between lens  60  and roll  12  is adjustable by virtue a threaded coupling  80  that opposes a compression spring  82 . A slide  83  allows spring  82  to urge lens  60  toward ceramic coating  16  (It should be noted that the spring, slide and adjustment features are schematically illustrated). In one embodiment of the invention, bearing  78  is fluid dynamically spaced apart from ceramic coating  16  by a thin cushion of pressurized air  84  supplied through hose  86 . Air cushion  84  minimizes the effects of possible irregularities, such as contaminant buildup  88  and ceramic protrusions  90 . 
     One method of adjusting coupling  80 , to set the proper focus, is done by first applying an adhesive backed test decal  92  to roll  12 , as shown in FIG.  6 . Decal  92  has a base material  94  of paper with a printed dye coating  96 . A conventional consumer product label would be one example of decal  92 . Laser  52  is controlled to pass across decal  92  while coupling  80  is adjusted until beam  62   c  burns ink  96  off decal  92  without doing substantial damage to the decal&#39;s base material  94 . 
     In cleaning ceramic coated metering roll  12 , it is important to take into account the material property dissimilarities of the roll&#39;s metal core  14 , ceramic coating  16 , and polymeric contaminant  20 . In particular it is not unusual for there to be a 20% difference in the coefficient of thermal expansion between steel and ceramic, and steel can have 50% higher thermal conductivity than ceramic. In addition, common polymeric contaminants, steels, and ceramics have a wide range of disassociation temperatures (i.e., temperature at which the material melts, burns, breaks down, or otherwise changes significantly in its state or physical properties). The polymeric disassociation temperature of many dried or cured printing dyes is typically between 300° F. to 600° F. The ceramic disassociation temperature of many ceramics is about 3,000° F. to 4,000° F.; while common steels melt at a temperature of around 2,700° F. to 2,900° F. 
     When using a laser  52  to clean ceramic coated metering roll  12 , excessive heat may lead to thermal cracking and separation between metal core  14  and ceramic coating  16 , due to their differences in thermal properties. Referring back to FIG. 1, it has been found that heating a first region  98  of ceramic coating  16  to a temperature of between 400° F. to 1,000° F. effectively destroys common polymeric contaminants  20  while leaving ceramic  16  substantially intact. Blowing ambient air  64  (at a temperature below the polymeric disassociation temperature) not only clears contaminants from cells  18 , but also serves to cool region  98  and minimize the amount of heat that can penetrate to metal core  14 . By subsequently changing the circumferential and longitudinal position of roll  12  relative to laser beam  62   c  (as indicated by arrows  100  and  102 , respectively), region  98  is allowed to cool further to a level below the polymeric disassociation temperature. Eventually, a second region  104  is cleaned in the same manner as the first. 
     The size of regions  98  and  104  subjected to beam  62   c  are wide enough to not only clean each cell  18 , but to also clean annular area  76  (FIG. 3) surrounding each cell  18 . Preferably, regions  98  and  104  are at least twice as wide as a widest span  106  of cells  12 . 
     To enhance the cleaning process, each cell  18  has a rounded beveled entryway  74  and a curved bottom surface  72  to readily receive, redirect, and exhaust pressurized air  64  in and out of each cell  18 . The cell geometry and the approach angle of air  64  provides a smooth airflow pattern that facilitates expelling contaminant  20  from cells  18 , as shown in FIG.  3 . 
     Another supply of pressurized ambient air is delivered into a housing  108  that holds lens  60  (see FIG.  5 ). The airflow pattern  110  travels generally away from lens  60  and toward cells  18  to protect lens  60  from being struck by fragments of polymeric contaminants  20 . 
     A further enhancement of the cleaning process involves pulsating laser beam  62   a - 62   c  on and off for individually firing at each individual cell  18 . This is done by setting the timing and frequency of the pulses in synchronization with both the circumferential and longitudinal repositioning of cells  18  relative to beam  62   c . In one embodiment of the invention, motor  26  is a synchronous motor that turns at a substantially constant speed to change the rotational position of roll  12  at a substantially constant rate of rpm (revolutions per minute). The longitudinal feed motor  44  is also a synchronous motor having a substantially constant speed. The speed of motor  44  is set as a function of the rotational speed (e.g., rpm.) of motor  26 , a longitudinal spacing  112  between adjacent cells  18 , and, of course, the mechanical characteristics of lead screw  42  and gear reducers  28  and  46 . The speed relationship between motors  26  and  44  is analogous to turning threads on a lathe. With the motor speeds properly set in relation to each other, the region (e.g., regions  98  and  104 ) illuminated by beam  62   c  will inscribe a helical pattern. The pattern is superimposed upon the helical distribution of cells  18  by adjusting the timing and frequency of the on/off pulsating of laser  62   a.    
     The pulsating frequency of laser  52  is set as a function of the rotational speed (rpm) of roll  12 , a diameter  114  of roll  12 , and a circumferential separation distance  116  between two adjacent cells. The pulsating frequency of laser  52  is set equal to the frequency at which cells  18  pass across the path of laser beam  62   c . The timing of the beam pulses serve to align the phase of the frequency at which cells  18  pass across the path of beam  62   c  to the phase of the pulsating frequency of laser  52 . 
     Although controlling the pulsating frequency of laser  52  and the speed of motors  26  and  44  can be carried out by any one of a variety of conventional control means available to those skilled in the art, in one embodiment of the invention, a computer based control  118  having a manual input  120  (e.g., keyboard, monitor with touch-sensitive screen, etc.) is used. 
     Control  118  generates a rotational speed signal  122  and a longitudinal speed signal  124  that establishes the speed of motors  26  and  44 , respectively. Accurate feedback on the rotation of motors  26  and  44  are provided by encoders  30  and  48  which respectively generate the encoder feedback signals  32  and  50 . Manual input  120  provides input regarding the physical distribution of cells  18  for establishing target speeds of motors  26  and  44  and also for establishing a target frequency at which laser  52  is to be pulsating on and off. Control  118  provides an output signal  126  that triggers laser  52  upon control  118  counting a predetermined number of pulses  32  since the last firing. Once operating, the firing of laser  52  is fine tuned manually. This is done by observing the cleaning results of the first few cells and then providing control  118  with an encoder compensation signal  128  via manual input  120 . Signal  128  tells control  118  to add an extra pulse to its count of pulses from encoder  32  periodically after a predetermined number of firings. The duration of each laser beam pulse is set by way of manual input  120  after referencing beam analyzer  58 . 
     In one embodiment of the invention, beam expander  54 , lens  60  (P/N 285767), and partial reflector  56  (P/N 0405-2000) are provided by II-VI Incorporated of Saxonburg, Pa. And beam analyzer  58  is a Model LBA-300 PC provided by Spiricon of Ogden, Utah. 
     In another preferred embodiment, shown in FIG. 4B, lens  60  is replaced by lens  60 ′, which is also known as a transmissive beam integrator and is provided by Laser Power Optics of San Diego, Calif. Lens  60 ′ consists of a diamond turned optical component that slices high power beam  62   c  into segments and overlaps the segments generally at the surface of roll  12 . This produces a generally uniform energy distribution  77 ′, as shown in FIG.  7 B. However, rather than a Gaussian or normal distribution over a circular area, as produced by lens  60 , lens  60 ′ focuses a substantially uniform energy distribution over a generally square area. Such an energy distribution should prove most effective in cleaning rolls that are contaminated with a coating of generally uniform thickness. 
     Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those skilled in the art. Therefore, the scope of the invention is to be determined by reference to the claims which follow.