Abstract:
A system for forming a flexographic plate includes a digital front end (DFE) for rendering a halftone image; wherein the DFE identifies pixels in the halftone image as being part of an edge region or interior region based on the pixels proximity to an edge image features; wherein the DFE applies a fine pattern to each of the edge regions; wherein the DFE applies a coarse pattern to each of the interior regions; an interface for transmitting the patterned image to an imaging device; and wherein the imaging device images the flexographic plate with the patterned image.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket No. K001865USO1NAB), filed herewith, entitled FLEXOGRAPHIC SURFACE PATTERNS, by Bielak; the disclosure of which is incorporated herein. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method forming an image on a flexographic plate. 
       BACKGROUND OF THE INVENTION 
       [0003]    In graphic arts technology, a number of well-established printing processes utilize image carriers with three-dimensional (3D) representation of data, the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. The relief is composed of the raised features on the plate such as the features labeled  204 ,  208 , and  212  in  FIG. 2 . It is the relief that accepts and transfers ink to the substrate. In a traditional flexographic prepress process with chemical etching there is no possibility of fine control of relief properties other than depth of relief  216 . 
         [0004]    Flexographic printing uses a flexible relief plate to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. A simplified diagram of a flexographic printing press is shown in  FIG. 1 . Ink  10  in a fountain pan is taken up by a rubber roller  12  and transferred to the surface of the Anilox roller  14 . The surface of the Anilox roller is composed of an array of indented cells that allow careful metering of the ink volume. A doctor blade  16  removes any excess ink from the roller before the ink is transferred to the printing plate cylinder  18 . Mounted on the plate cylinder is a flexographic printing plate  20 . The final step transfers the ink from the plate to the substrate  22  with the impression cylinder  24  supplying support for the substrate. 
         [0005]    The process used to produce an image on flexible relief plate usually comprises the following steps: 
         [0006]    Exposing the back of the plate to UV light; 
         [0007]    Exposing an intermediate film to the desired image; 
         [0008]    Laminating the film to the top of the plate; 
         [0009]    Exposing the plate though the film using UV light; 
         [0010]    Removing the film; 
         [0011]    Using a solvent to wash away the unexposed plate material; 
         [0012]    Applying additional exposure to harden the plate; and 
         [0013]    Drying the plate to remove as much of the solvent as possible. 
         [0014]    The back exposure is used to establish the floor of the plate. The intensity of the exposure decreases as the illumination penetrates the plate because of absorbers added to the plate material. Once the intensity drops below a threshold value, there is insufficient cross linking in the polymer comprising the plate and the remaining under-exposed polymer can be washed away. This is usually the top 0.5 mm of the plate. To form the relief, the front of the plate is exposed, through an image layer with enough intensity that sufficient cross linking occurs all the way down to the plate floor. 
         [0015]    For every opening in the image layer, a cone of UV light with an angle of about 40 degrees from a normal to the plane propagates through the plate forming cone shaped relief dots. A cross-section of a plate  200  is shown in  FIG. 2 . The following features are depicted in the cross-section  200 : a solid area  204 ; an isolated dot  208 ; and an array  212  of closely spaced dots created by a halftone screen. The height of the plate relief is shown by numeral  216  and plate floor by numeral  220 . 
         [0016]    Ink uniformity and density can be improved if a surface pattern or texture is applied to the flat tops of the relief as shown in the  FIG. 3 . The stretched checkerboard pattern  304  is composed of 5×10 micron rectangles and works well for process inks printed on a paper substrate. 
         [0017]    Such a fine pattern has an additional advantage in that it allows the edges of printing features to be well defined. The pattern does have its limits. When printing on plastic substrates, voids can appear in large features due to air entrapment. The pattern also performs poorly if large volumes of ink need to be transferred to the substrate. To eliminate these problems, a coarser pattern is required. However, a coarser pattern will compromise edge definition. 
         [0018]    In flexographic printing, large solid areas of relief can suffer from a number of artifacts. The ink deposits unevenly, resulting in a reduction in color density and in a mottled appearance to the solid. Ink can be squeezed off the relief near edges resulting in low ink density just inside the edge and high density just outside the edge. Air bubbles trapped between the plate and substrate can cause voids to appear at the trailing edge of large features. Prior art exists to mitigate some these problems as described below. 
         [0019]    Early flexography printing relied on a flat, smooth surface for the relief.  FIG. 4  shows a section of halftone with a smooth relief. In large solid regions of image, the ink deposition was uneven resulting in a reduction in measured ink density. With high impression force, ink often squeezed out at relief edges reducing ink density just inside the edge with a ring of high density ink just outside the edge. 
         [0020]    One method of improving the performance of the plate is to apply a very fine pattern  504 , shown in  FIG. 5 , to the surface of the relief. This creates a texture that is smaller than the resolution of the flexographic printing method. The stretched checkerboard of  FIG. 5  is one such example. 
         [0021]    The dimensions of the pixels shown in  FIG. 5  are 5.3 by 10.6 microns. The pixels that form on the plate are slightly smaller, creating small gaps between pixels at the corners. The edges of the pixels fall off at an angle of about 40 degrees to a valley floor, 3 to 4 microns below the relief surface. This shape allows ink to settle in the valleys and to migrate between valleys at the pixel corners. The result is a more even deposition of ink on the substrate. 
         [0022]    Some imaging devices used to make flexographic plates do not have sufficient resolution to image very fine textures. For these devices, another method using coarser textures was developed. The problem with coarser textures is that the dot edge can be compromised. To avoid this, patterning is suppressed a set distance from the dot edge (the keep-away).  FIG. 10  shows one implementation of this method. A coarser pattern  1004  is used in the interior of the relief and a 2 pixel keep-away  1008  is implemented at the edges. This keep-away  1008  conserves the shape of the dot. 
         [0023]    The pattern shown in  FIG. 10  is a regularly spaced pattern but this is not a requirement of the method. Stochastic screening methods have been used to randomly locate voids in the relief areas without violating the keep-away rule. An example of the stochastic method can be seen in the lower left corner of  FIG. 3  identified as ‘Traditional Plate Cell Patterning’  308 . 
         [0024]    To overcome the weakness of these methods, this invention combines a fine pattern at the edges of printing features with a coarser pattern in the interior of the features. 
       SUMMARY OF THE INVENTION 
       [0025]    Briefly, according to one aspect of the present invention a system for forming a flexographic plate includes a digital front end (DFE) for rendering a halftone image; wherein the DFE identifies pixels in the halftone image as being part of an edge region or interior region based on the pixels proximity to an edge image features; wherein the DFE applies a fine pattern to each of the edge regions; wherein the DFE applies a coarse pattern to each of the interior regions; an interface for transmitting the patterned image to an imaging device; and wherein the imaging device images the flexographic plate with the patterned image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows simplified diagram of a flexographic printing press (prior art); 
           [0027]      FIG. 2  imaged plate cross section(prior art); 
           [0028]      FIG. 3  shows texture applied to the flat tops of the printing relief (prior art); 
           [0029]      FIG. 4  shows a section of halftone with a smooth relief (prior art); 
           [0030]      FIG. 5  shows a very fine pattern applied to the surface of the relief; 
           [0031]      FIG. 6  represents in diagrammatic form of a digital front end driving an imaging device (prior art); 
           [0032]      FIG. 7  represents in diagrammatic form the laser imaging head situated on the imaging carriage imaging on a plate mounted on an imaging cylinder (prior art); 
           [0033]      FIG. 8  shows a halftone rendered image (prior art); 
           [0034]      FIG. 9  shows a rendered image on flexographic plate (prior art); 
           [0035]      FIG. 10  shows a regularly spaced pattern; 
           [0036]      FIG. 11  shows a printed scheme adapted to eliminate trailing edge voids by forming of two surface patterns; 
           [0037]      FIG. 12  shows larger valleys in the pattern adapted for white ink; 
           [0038]      FIG. 13  shows a block diagram illustrating the steps of the patterning method; and 
           [0039]      FIG. 14  showing decisions on edge, interior and external pixels according to pixel window on a pixel sample. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
         [0041]    While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as covered by the appended claims. 
         [0042]      FIG. 6  shows an imaging device  608 . The imaging device is driven by a digital front end (DFE)  604 . The DFE receives printing jobs in a digital form from desktop publishing (DTP) systems (not shown), and renders the digital information for imaging. The rendered information and imaging device control data are communicated between DFE  604  and imaging device  608  over interface line  612 . 
         [0043]      FIG. 7  shows an imaging system  700 . The imaging system  700  includes an imaging carriage  732 , on which an imaging head  720  is mounted, and the imaging head  720  is controlled by controller  728 . The imaging head  720  is configured to image on a substrate  708 . The substrate can be a film attached as a mask to a flexographic plate, or alternatively a flexographic plate that will be directly imaged by imaging system  700 . The substrate  708  is mounted on a rotating cylinder  704  for exposure. The carriage  732  is adapted to move substantially in parallel to cylinder  704  guided by an advancement screw  716 . The substrate  708  is imaged by imaging head  720  to form imaged data  712  on substrate  708 . 
         [0044]      FIG. 8  shows a halftone rendered image  800 . The rendered image  800  was prepared by DFE  604 , to be further imaged on substrate  708 .  FIG. 9  shows rendered image  800  imaged by imaging head  720  on substrate  708  forming an imaged substrate  900 . 
         [0045]    When printing on certain plastic substrates, such as Low-density Polyethylene (LDPE), voids appear on the trailing edge of large solid relief areas. These voids are due to entrapment of air bubbles between the plate and the substrate. A solution is to allow slightly deeper valleys in the pattern and slightly larger gaps between pixels in the interior of the relief. This allows ink and air to flow more freely. 
         [0046]    White ink is used on clear plastic material as the base for printing spot and process colors. The volume of white ink required to achieve a good level of opacity is much higher than the volume used for the color inks Consequently, the patterns used for color printing are too fine to work for the white inks Therefore, coarser patterns that scale with ink volume are required. The exposed data is analyzed to find areas which represent the interior of the relief  1104  and the edges areas  1108 . This allows different treatment to areas  1104  and areas  1108  with respect to patterning. 
         [0047]      FIG. 11  shows a printed scheme adapted to eliminate trailing edge voids by forming two surface patterns. The first pattern is optimized for the edges of the relief  1108 . The second is optimized for the interior of the relief  1104  and is adapted to the particular application. 
         [0048]    White ink requires larger valleys in the pattern as is illustrated by  FIG. 12 . Larger interior pattern  1204  allows entrapped air to move more freely. The edge pattern  1208  preserves the dot shape while still allows the trapped air to squeeze through at the corners of the stretched checker board. 
         [0049]      FIG. 13  contains a block diagram that shows the steps of the patterning method. An image  1304  is provided and an edge detection step is executed on image  1304 . The results of the edge detection step are edge pixel mask  1308  and interior pixel mask  1312 . Fine pattern  1316  is applied on edge pixel mask  1308  to create a fine patterned edge. A coarse pattern  1320  is applied on interior pixel mask  1312  to create a coarse patterned interior pixels structure  1328 . The final step is to combine the fine patterned edge  1324  and the coarse patterned interior  1328  into a patterned image  1332 . 
         [0050]    Exposed relief pixels are selected to be part of the edge region or interior region based on the pixels proximity to an edge of the relief. In the preferred embodiment, the method used to achieve this partition is a 5×5 pixel window. The pixels in the window are examined. In the case that all the pixels in the window are exposed pixels then the center pixel is deemed to be an interior pixel. If the center pixel is an exposed pixel and at least one of the other pixels in the window is not an exposed pixel, then the center pixel is deemed to be an edge pixel. The result of this operation is to designate an interior pixel mask and an edge pixel mask. All other pixels in the image are deemed exterior pixels and are ignored.  FIG. 14  illustrates how windowing is applied to a representative sample of pixels  1400 . A decision to designate a pixel as external is decided when the pixel window is placed in position  1404  relative to sample pixels  1400 . Similarly in positions  1408  and  1412  the decision is made to designate an edge pixel. An interior pixel is designated for position  1416 . 
         [0051]    A fine texture pattern is chosen for the edge pixels and the preferred pattern is a 5×10 micron stretched checkerboard. The fine pattern  1112  is repeated in both dimensions to span the width and height of the rendered image forming the fine pattern image, which is used at the edge areas  1108 . A coarser pattern  1116  is chosen for the interior pixels. Some examples are shown in  FIGS. 11 and 12 . The exact choice is based on the thickness of the ink being printed. The coarse pattern is repeated in both dimensions to span the width and height of the rendered image forming coarse pattern image. 
         [0052]    Exposed pixels in the rendered image are replaced by pixels from the fine pattern image and the coarse pattern image. For every pixel in the rendered image, if the corresponding pixel in the fine pattern mask is set then that pixel in the rendered image is replaced by the corresponding pixel in the fine pattern image. Similarly, for every pixel in the rendered image, if the corresponding pixel in the coarse pattern mask is set then that pixel in the rendered image is replaced by the corresponding pixel in the coarse pattern image. 
         [0053]    While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. The principles of the present invention may similarly be applied to other types of electrical storage cells, such as energy-storage capacitors. 
         [0054]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10  ink 
           12  rubber roller 
           14  Anilox roller 
           16  doctor blade 
           18  printing plate cylinder 
           20  flexographic printing plate 
           22  substrate 
           24  impression cylinder 
           200  cross-section of plate 
           204  solid area 
           208  isolated dot 
           212  array of closely spaced dots created by a halftone screen 
           216  plate relief 
           220  plate floor 
           304  stretched checkerboard pattern 
           308  stochastic patterning 
           504  fine pattern 
           604  digital front end (DFE) 
           608  imaging device 
           612  interface line 
           700  imaging system 
           704  rotating cylinder 
           708  substrate 
           712  imaged data on substrate 
           716  screw 
           720  imaging head 
           728  controller 
           732  carriage 
           800  rendered halftone image to be imaged on substrate 
           900  rendered image imaged on substrate 
           1004  coarse pattern 
           1008  2 pixel keep-away 
           1104  pattern for the relief interior 
           1108  pattern for the relief edges 
           1112  fine pattern 
           1116  course pattern 
           1204  white color interior pattern 
           1208  white color edge pattern 
           1304  image 
           1308  edge pixels 
           1312  interior pixels 
           1316  fine pattern 
           1320  coarse pattern 
           1324  fine pattern applied to edge pixels 
           1328  coarse pattern applied to interior pixels 
           1332  patterned image 
           1400  pixels sample 
           1404  external pixel decision 
           1408  edge pixel decision 
           1412  edge pixel decision 
           1416  interior pixel decision