Abstract:
A method for forming an image on a flexographic plate includes exposing a back of the flexographic plate to form a floor; providing a screened image; locating isolated dots on the screened image; exposing a front of the flexographic plate to form the image, isolated dots and scaffold dots adjacent to the isolated dots; wherein the scaffold dots do not extend to the floor; and applying a solvent to wash away the unexposed material and the scaffold dots from the flexographic plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 14/169,566, filed Jan. 31, 2014, entitled APPARATUS FOR FORMING AN IMAGE ON A FLEXOGRAPHIC MEDIA, by Bielak. 
     FIELD OF THE INVENTION 
     The present invention relates to a method forming an image on a flexographic plate. 
     BACKGROUND OF THE INVENTION 
     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. In a traditional flexographic prepress process with chemical etching there is no possibility of fine control of relief properties other than depth of relief. 
     Specifically, the shape of the cross-section profile directly influences the quality of reproduction of small features such as highlight elements and/or file linework details, process tolerance to changes in pressure applied by plate and/or sleeve to substrate and other vital characteristics. 
     Flexographic printing uses a flexible relief plate to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. The Kodak Flexcel NX plate is one such relief plate. The process used to produce an image on the plate usually comprises the following steps:
         1. Exposing the back of the plate to UV light.   2. Exposing an intermediate film to the desired image.   3. Laminating the film to the top of the plate.   4. Exposing the plate though the film using UV light.   5. Removing the film.   6. Using a solvent to wash away the unexposed plate material.   7. Applying additional exposure to harden the plate.   8. Drying the plate to remove as much of the solvent as possible.       

     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 absorption in 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. 
     For every opening in the image layer, a cone of UV light with an angle of about 40 degrees from the normal propagates through the plate forming cone shaped relief dots. A cross-section of a plate  500  is shown in  FIG. 5 . The following features are depicted in the cross-section  500 : a solid area  504 ; an isolated dot  508 ; and an array of closely spaced dots created by a halftone screen  512 . The height of the plate relief is shown by numeral  516  and plate floor by numeral  520 . 
     Isolated dots, such as isolated dot  508 , can be problematic. There may be insufficient exposure to solidly and anchor the dot to the plate floor  520 . Even if the dot forms properly, excess printing pressure could cause the dot to deform during printing. The dots in the middle of the halftone array  512  fair better since they are supported to either side by nearby dots. However, dots at the edge of the array  512  could suffer from some of the same problems as the isolated dot  508 . Dot deformation can cause a large objectionable blot to form on the printing substrate. This is called a scum dot in the industry. Ensuring good dot formation and eliminating the possibility of scum dot formation is the object of this invention. 
     Large dots can support themselves even in isolation. For the Flexcel NX plate, a minimum dot size of 4×4 pixels is usually sufficient to ensure proper dot formation in all cases. However, a large minimum dot in a halftone makes it difficult to print light grey tones. Bump curves or screening strategies are used to try to mitigate this problem with mixed success. 
     The typical plate relief is 20 mils (0.5 mm). Reducing the relief, improves the dots ability to stand on its own. The disadvantage is that over a long print run, dirt can collect in the wide areas of the floor and if sufficient dirt accumulates then this dirt will transfer to the substrate. 
     A method that has been successfully used in laser ablation mask plates (LAMS) is to deliberately expose dots  608  that are too small to properly form in the areas surrounding dots  604  that need additional support as is shown on a printing plate profile  600  in  FIG. 6 . There is some risk the dots may print despite the lower relief—ink may accumulate on the recessed dots over several print cycles and then transfer to the next substrate all at once. Debris accumulation may also be a problem. 
     Another method suggests small dots which are interspersed with large dots. Halftone screen  700  as is shown in  FIG. 7 . As halftone dots  704  become sparse, rather than remove a dot completely a halftone dot is replaced with small printing dot  708 . This is not ideal but often the tonal value of the resulting halftone screen is less than the sparse array because the remaining dots have additional support. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method for forming an image on a flexographic plate includes exposing a back of the flexographic plate to form a floor; providing a screened image; locating isolated dots on the screened image; exposing a front of the flexographic plate to form the image, isolated dots and scaffold dots adjacent to the isolated dots; wherein the scaffold dots do not extend to the floor; and applying a solvent to wash away the unexposed material and the scaffold dots from the flexographic plate. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents, in diagrammatic form, a prior art digital front end driving an imaging device; 
         FIG. 2  represents, in diagrammatic form, a prior art laser imaging head situated on the imaging carriage imaging on a plate mounted on an imaging cylinder; 
         FIG. 3  shows a prior art halftone rendered image; 
         FIG. 4  shows a prior art rendered image on flexographic plate; 
         FIG. 5  shows a prior art cross-section of an imaged printing plate; 
         FIG. 6  shows prior art engraved dots that are too small to properly form the areas surrounding larger dots; 
         FIG. 7  shows prior art engraved flexographic plate showing small dots which are interspersed with large dots; 
         FIG. 8  shows exposure intensity as a function of distance into the plate; 
         FIG. 9A  shows a cross-section of an engraved isolated printing dot surrounded by non-printing scaffold dots; 
         FIG. 9B  shows a cross-section of the engraved isolated printing dot after the surrounded by non-printing scaffold dots have been washed out by a solvent; 
         FIG. 10  shows a two-dimensional top view of how a typical halftone in the highlights might look like; 
         FIG. 11  shows scaffold dot placements required to ensure that scaffold dots do not become attached to other imaged features on the plate; and 
         FIG. 12  shows another restriction with the placement of scaffold dots. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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. 
       FIG. 1  shows an imaging device  108 . The imaging device is driven by a digital front end (DFE)  104 , which comprises a computer or microprocessor, which 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  104  and imaging device  108  over interface line  112 . 
       FIG. 2  shows an imaging system  200 . The imaging system  200  includes an imaging carriage  232  an imaging head  220 , which comprises a plurality of lasers and which is mounted, imaging head  220  are controlled by controller  228 . The imaging head  220  is configured to image on a substrate  208 , the substrate can be a film to be attached as a mask to a flexographic plate, or alternatively a flexographic plate that will be directly imaged by imaging system  200 . The substrate  208  is mounted on a rotating cylinder  204  for exposure. The carriage  232  is adapted to move substantially in parallel to cylinder  204  guided by an advancement screw  216 . The substrate  208  is imaged by imaging head  220  to form an imaged data  212  on substrate  208 . 
       FIG. 3  shows a halftone rendered image  300 . The rendered image  300  was prepared by DFE  104 , to be further imaged on substrate  208 .  FIG. 4  shows rendered image  300  imaged by imaging head  220  on substrate  208  forming an imaged substrate  400 . 
     The ideal solution to the problem of small dots with insufficient support is to raise the floor of the plate surrounding the dot. Small dots with carefully controlled size and spacing are used to modify the floor height. The method takes advantage of the side affect of back exposure that the plate material above the plate floor is partially exposed. Exposure above a threshold value causes the plate material to solidify. Exposure is a linear integrating process; therefore, front exposure  816  can be combined with back exposure  804  to exceed the threshold  808 . The exposure intensity  820  as a function of distance into the plate is shown in the  FIG. 8 . 
     The figure shows the ultra violet (UV) radiation intensity as a function of distance though the plate thickness. This intensity decays as it penetrates the plate material because of absorbers added to the polymer mix. The intensity changes according to Beer&#39;s Law:
 
 I=I   0   e   −αx  
 
     Plate material that is not exposed above a threshold value is washed away when the plate is processed. The shaded area shows the thickness that remains and where the plate floor  824  is. However, the plate material between the plate floor and the front surface of the plate is partially exposed. The exposure deficit  812 , shown in  FIG. 8 , represents the additional exposure needed to solidify the remainder of the plate. Note that near the plate floor, very little additional energy is required to exceed the threshold value. Therefore a front exposure that does not create a dot can be used to raise the plate floor. 
     The printing plate cross-section  900  shown in  FIG. 9A  shows and isolated printing dot  908  surrounded by non-printing scaffold dots  916 . The scaffold dots  916  are used to raise the floor  912  of the plate surrounding the isolated dot. The cross-section  900  depicts a one-dimensional profile. Printing dots  908  and non-printing scaffold dots  916  are formed on an imaged film mask  904  prior to UV exposure.  FIG. 9B  shows a cross-section of the isolated printing dot  908  with raised floor  912  and plate floor  920  after the scaffold dots  916  have been removed by the solvent. Alternatively substrate  208  is a printing plate which can be imaged directly by imaging system  200  without a need to use intermediate steps of imaging a mask on a film, laminating the film on the plate and to apply UV exposure steps. 
       FIG. 10  shows a two-dimensional top view of how a typical halftone  1000  in the highlights might look like. 
     The intensity of the scaffold dot beam decays with distance from the mask  904  because of absorption in the plate (Beer&#39;s Law) and because the beam expands as it propagates through the plate (Inverse Square Law). The intensity equation is: 
             I   =       I   0     ⁢       ⅇ       -   α     ⁢           ⁢   x         x   2               
The intensity drops rapidly and passes below the threshold for plate solidification. If the dot is small enough (I 0  is small enough) then intensity will drop below threshold before reaching the plate floor. The resulting conical plug (of the scaffold dots)  916  of solid plate material will be washed away by the solvent in the plate processing step.
 
     As seen in the cross-section, the beam continues to the floor of the plate and beyond. Near the floor, the additional exposure needed to solidify the polymer is small and the floor is raised ( 912 ). In addition, the beams from adjacent dots begin to overlap and the added exposure further raises the floor. This places an additional restriction on the scaffold dots—the density of the dots cannot exceed a maximum value else the floor will rise to meet the solidified conical plugs and the plugs will not be washed away. 
     A further restriction on scaffold dot  916  placements is required to ensure that scaffold dots do not become attached to other imaged features on the plate.  FIG. 11  illustrates the problem. With the scaffold dot plug  1104  anchored to an adjoining feature  1108 , the dot will not be washed away by the solvent. The solution is to maintain a minimum distance between scaffold dots and other features. 
     There is one additional restriction with the placement of scaffold dots  916  that is illustrated in  FIG. 12 . The figure shows part of a large plate feature  1208 . The front exposure to create this feature propagates through the plate and is partially scattered by changes in refractive index as the plate material forms cross-links. This scatter spreads out and raises the floor near the edge of the feature  1208 . The effect is proportional to the size of the feature and decays with distance from the edge of the feature. Small features  1204  do not raise the floor significantly as there is insufficient energy in the refractive scatter. For reference, the dotted line in the figure shows the location of the floor had no scatter occurred  1212 .  FIG. 12  shows how a scaffold dot  916  could become attached to this raised floor. Detection of a raised floor and suppression of scaffold dots in the area of that floor is a further aspect of this invention. 
     In summary the main features of this invention are: Use small dots  916  (scaffold dots) that have insufficient exposure to solidify all the way to the plate floor. Control the maximum density scaffold dots to ensure that floor is not raised to meet the bottom of scaffold dot, maintain spacing of the scaffold dots far enough away from other features to ensure that they do not connect and refrain from placing scaffold dots in regions where the floor could be raised by back scatter. 
     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. 
     PARTS LIST 
     
         
           104  digital front end (DFE) 
           108  imaging device 
           112  interface line 
           200  imaging system 
           204  rotating cylinder 
           208  substrate 
           212  imaged data on substrate 
           216  screw 
           220  imaging head 
           228  controller 
           232  carriage 
           300  rendered halftone image to be imaged on substrate 
           400  rendered image imaged on substrate 
           500  flexographic printing plate profile 
           504  solid area 
           508  isolated dot 
           512  array of closely spaced dots created by a halftone screen 
           516  plate relief 
           520  plate floor 
           600  flexographic printing plate profile 
           604  printing dot 
           608  partially printed dots 
           700  halftone screen 
           704  halftone dots 
           708  supporting dots 
           804  back of plate 
           808  threshold 
           812  exposure deficit 
           816  front of plate 
           820  exposure intensity 
           824  plate floor 
           900  printing plate cross-section 
           904  exposure mask (TIL) 
           908  printing dot 
           912  raised floor 
           916  scaffold dot 
           920  plate floor 
           1000  highlights view in a typical halftone 
           1104  scaffold dot plug 
           1108  plate adjoining feature 
           1204  small plate features 
           1208  large plate feature 
           1212  plate floor had no scatter occurred