Abstract:
Negatives, and laminates containing negatives, for exposing liquid photopolymers in UV flexographic plate making are created by printing opaque images directly onto the protective or release cover films used in plate making processes. Combining negatives and cover films in this way eliminates two layers and two interfaces in the exposure process, reducing cost and improving quality. By bringing the negative images as close as possible to the liquid photopolymer, printed dots of “digital quality” are created.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a Division of U.S. patent application Ser. No. 14/176,110 filed Feb. 2, 2014, which is a Continuation-In-Part of U.S. patent application Ser. No. 14/027,760 filed Sep. 16, 2013. This application claims priority thereto. Patent application Ser. No. 14/027,760 is also a Continuation-In-Part of U.S. patent application Ser. No. 13/618,507 filed Sep. 14, 2012, which in turn is a Continuation-In-Part of U.S. patent application Ser. No. 12/895,561 filed Sep. 30, 2010. A related patent application is U.S. patent application Ser. No. 13/902,301 filed Feb. 19, 2013. These applications are owned in common and have at least one inventor in common, and are incorporated into the instant application in their entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not applicable. 
       REFERENCE TO A BIOLOGICAL SEQUENCE LISTING 
       [0004]    Not applicable. 
       BACKGROUND OF INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    This invention is in the field of printing, more specifically in the field of making flexographic printing plates with digital quality flat top dots using liquid photopolymers, and still more specifically in the field of using solvent-based inks to print the flexographic plate negatives. 
         [0007]    2. Description of the Related Art 
         [0008]    A prior art method of preparation of a simple flexographic (“flexo”) printing plate from cast liquid photopolymer currently involves the steps of (a) printing a black negative image on a white substrate; (b) photographing the negative image; (c) developing the film negative; (d) positioning the film negative on the bottom glass of a UV exposure unit; (e) positioning liquid dams and taping the negative in place with masking tape as necessary; (f) placing a thin plastic vacuum sheet (cover film) over the negative; (g) casting liquid photopolymer over the film to the proper depth; (h) applying a vacuum to the laminate thus formed (negative, vacuum sheet, and photopolymer); (i) exposing this laminate to UV (or other appropriate actinic) radiation through the negative for an amount of time sufficient to create a cross-linked polymerized image in the photopolymer; (j) removing the laminate from the unit and separating the vacuum sheet and negative from the cross-linked photopolymer; and (k) removing the unexposed material to develop a relief image. 
         [0009]    The vacuum step (h) is important. If there are any pockets of air between the negative and the adjacent surface of the photopolymer, the UV light will be refracted by the interfaces between the film, the air, and the photopolymer and the final image after exposure will be distorted. If the pockets of air are large enough, they can even lead to mechanical failure of the plate by creating thin spots in the photopolymer. Moreover, ambient oxygen in any air pockets in contact with surface of the photopolymer inhibits full curing of the photopolymer all the way to the photopolymer surface. (This is thought to be because oxygen in the air reacts with gases formed within the photopolymer layer during UV curing, the products of which slow the curing rate.) Provided the negative and vacuum sheet are skillfully placed, the vacuum system in the special exposure unit will remove a great majority (but not all) such pockets of air. The tops of the half tone dots produced with distortion due to the presence of air are rounded; the printed images of such dots oftentimes appear as tiny squiggles where the dots should be. Even with perfect alignment and application of vacuum, single halftone or pixel dots produced this way suffer from being separated from the photopolymer by the additional thickness of a cover sheet. They do not have steep shoulders and cause dot gain when printed. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    The laminates and methods of the present invention relate to printing, namely creating fine detail inkjet images that can be placed in direct contact with the surfaces of cast liquid photopolymer to produce flexo plates with “digital quality” flat top dots, that is, single pixel dots having a shape like that produced by IR ablation of digital photopolymer sheets. The methods further reduce the aforementioned problems with gas bubbles, eliminate layers of material, eliminate the need for expensive e.g. microporous ink-receptive coatings, reduce the number of steps required and the level of expertise necessary to execute them, and produce final prints of higher quality than achievable by current methods under the best of conditions. 
         [0011]    Not only do dots created using the laminates and methods of the present invention produce a better image, it has also been determined that flexo plates with such digital quality dots last longer (endure more impressions) than prior art liquid dots because they distribute the impact stress more evenly throughout the polymer. All of the above steps except perhaps (c), (h) and (i) require human handling and make the entire process slow, another reason for combining or eliminating some of these steps. 
         [0012]    Until the applicant&#39;s invention described in U.S. patent application Ser. No. 13/902,301 filed Feb. 19, 2013, it was not possible to create images for flexo plate making from solvent-based inks that were sufficiently opaque to UV radiation. Only applicant&#39;s U.S. patent application Ser. No. 14/027,760 filed Sep. 16, 2013, describes and claims laminates to produce digital quality flat top dots using liquid photopolymers and solvent-based inkjet inks. This Continuation-In-Part of that application describes and claims other aspects of the same methods and laminates involving liquid photopolymers. 
         [0013]    The first embodiment of the present invention provides for a relief negative image to be printed with solvent-based ink onto a vinyl-based film. This film is placed on the lower glass of an exposure unit, preferably image side up, and liquid photopolymer is cast over it. The second embodiment provides for a solvent ink negative image to be printed on a cover film made of a layer of solvent ink receptive material superimposed on a layer of adhesive-releasing material. In the third embodiment, the solvent ink printable surface may be on a backing laminate consisting of a flexographic plate backing sheet made of, or coated with, polyvinyl-based material on its upper surface, with a tie-coat applied to the lower surface. The fourth embodiment is a backing laminate consisting of the laminate of U.S. patent application Ser. No. 14/027,760 printed with solvent-based ink and applied to the upper surface of a layer of liquid photopolymer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional illustration of a prior art method of using a liquid photopolymer polymer to make a flexographic plate. 
           [0015]      FIG. 2  is a cross-sectional illustration of a prior art method of using a liquid photopolymer to make a flexographic I-plate. 
           [0016]      FIG. 3  is a highly magnified cross-sectional illustration of a single halftone or pixel dot produced by the prior art methods. 
           [0017]      FIG. 4  is a highly magnified cross-sectional illustration of the prior art dot during printing. 
           [0018]      FIG. 5  is a cross-sectional illustration of a solvent ink negative printed on a protective cover film in accordance with the first embodiment of the present invention. 
           [0019]      FIG. 6  is a cross-sectional illustration of a solvent ink negative printed on a laminate in accordance with the second embodiment of the present invention. 
           [0020]    The third embodiment is not illustrated. 
           [0021]      FIG. 7  is a cross-sectional illustration of the present invention, namely the Jetsetter Genesys™ backing laminate of U.S. patent application Ser. No. 14/027,760, in accordance with the fourth embodiment of the present invention. 
           [0022]      FIG. 8  is a cross-sectional illustration of the method for using the Jetsetter Genesys™ backing laminate for making a flexographic I-plate in accordance with the present invention. 
           [0023]      FIG. 9  is a cross-sectional illustration showing removal of the printed cover film and release liner from the finished Jetsetter Genesys™ flexographic printing plate made using the fourth embodiment of the present invention. 
           [0024]      FIG. 10  is a highly magnified cross-sectional illustration of a single halftone or pixel dot produced by casting liquid photopolymer over the first embodiment printed cover film. 
           [0025]      FIG. 11  is a highly magnified cross-sectional illustration of a single halftone or pixel dot produced by the first embodiment during printing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring now to the drawings, which are not to scale, and in which like reference characters refer to like elements among the drawings,  FIG. 1  is a magnified cross-sectional illustration of a prior art method of using a liquid photopolymer to make a flexographic plate. An arrangement of plate-making layers  40  is assembled to make a traditional flexographic plate using a liquid photopolymer where a prior art substrate  30  is laminated to the cast liquid. The steps to accomplish this are:
   (a) Place the image negative  27  on the bottom glass  24 ;   (b) Cover (protect) the negative  27  with a clear cover film  41 ;   (c) Turn on a vacuum source (not shown) connected to the lower vacuum grooves  34  to remove air from between the bottom glass  24 , the image negative  27 , and the clear cover film  41 ;   (d) Depending on the viscosity of the liquid photopolymer, nonporous damming material  42  may have to be placed around all sides of the negative  27  to control flow of liquid photopolymer over the negative by creating a shallow basin on the negative; (Dams are optional and not used in all instances. The liquid photopolymer is very viscous and does not readily flow after lamination in the process.)   (e) Pour or cast liquid photopolymer  43  over the cover film  41  (within the dams  42  if applicable);   (f) Apply a polyester substrate  30  over the liquid photopolymer  43  with the tie-coat side down (tie-coat not visible in this view);   (g) Lower the top glass  26  over the entire plate-making arrangement  40 ;   (h) Turn on a vacuum source (not shown) connected to the upper vacuum grooves  28  to remove air from between the substrate  30  and the upper glass  26 ;   (i) Turn on the upper UV lights  52  for a prescribed amount of time to shine through the upper glass  26 , causing a uniform floor layer  211  to form in the photopolymer layer  43  below the substrate  30 ;   (j) Close the shutter (not shown) to prevent light reflection from the upper glass during the image exposure step;   (k) Turn on the lower UV lights  25  to shine through the clear areas  29  in the negative  27 , causing vertical regions  210  in the liquid photopolymer  43  above the clear areas  29  to polymerize; and   (l) Remove the plate-making arrangement  40  from the unit for further processing to create the relief image, namely, by turning the plate-making arrangement over, removing the cover film  41  and dams  42 , and draining/collecting the un-polymerized liquid photopolymer for recycle and reuse.   
 
         [0039]      FIG. 2  is a magnified cross-sectional illustration of a prior art method of using a liquid photopolymer to make a flexographic I-plate. A prior art plate-making arrangement  60  is assembled to make a flexographic I-plate using a liquid photopolymer where the polyester substrate  30  is laminated to the cast liquid. The steps involved are:
   (a) Place the relief image negative  27  on the bottom glass  24 ;   (b) Cover (protect) the negative with a clear cover film  41 ;   (c) Turn on a vacuum source (not shown) connected to the lower vacuum grooves  34  to remove air from between the bottom glass  24 , the image negative  27 , and the clear cover film  41 ;   (d) Depending on the viscosity of the liquid photopolymer, nonporous damming material  42  may have to be placed around all sides of the negative  27  to control flow of liquid photopolymer over the negative by creating a shallow basin on the negative;   (e) Pour or cast liquid photopolymer  43  over the cover film  41  (within the dams  42  if applicable);   (f) Apply a polyester substrate  30  over the liquid photopolymer  43  with the tie-coat side down (tie-coat not visible in this view);   (g) Place the masking film  51  on top of the polyester substrate  30  and register it so that the clear areas  53  on the masking film  51  properly surround the clear areas  29  on the image negative  27  below;   (h) Lower the top glass  26  over the entire plate-making arrangement  60 ;   (i) Turn on a vacuum source (not shown) connected to the vacuum grooves  28  to remove air from between the upper glass  26 , the masking film  51 , and the substrate  30 ;   (j) Turn on the upper UV lights  52  to shine through the clear areas  53  in the masking negative  51  for a limited time, causing vertical regions  54  in the liquid photopolymer below the clear areas  53 , adjacent to the polyester substrate  30 , to polymerize a portion of the liquid photopolymer layer  43  to form island floor areas;   (k) Close the shutter (not shown) to prevent light reflection from the upper glass during the image exposure step;   (l) Turn on the lower UV lights  25  to shine through the clear areas  29  in the relief image negative  27 , causing vertical regions  210  in the un-polymerized liquid photopolymer above the clear areas  29  (and below the vertical regions  54 ) to polymerize an additional amount of the liquid photopolymer layer  43  to form the relief image; and   (m) Remove the plate-making arrangement  60  from the unit for further processing to uncover the relief image, namely, by turning the plate-making arrangement over, removing the cover film  41  and dams  42 , and draining/collecting the un-polymerized liquid photopolymer for recycle and reuse.   
 
         [0053]    Note that this plate-making arrangement  60  puts an image-quality-reducing boundary between the masking film  51  and the polyester substrate  30  which must also be evacuated by the vacuum grooves  28 . Regardless of how well the vacuum works, it is not perfect, and the masking film  51  is displaced away from the liquid photopolymer layer  43  by the thickness of the polyester substrate  30 . (The boundaries between the liquid photopolymer layer  43  and the polyester substrate  30  above it and the cover film  41  below it are substantially free of air because they cover the free surfaces of the liquid.) 
         [0054]      FIG. 3  is a more highly magnified cross-sectional view of a portion of a liquid photopolymer layer  43  exposed and developed as described in  FIG. 1 or 2  under a good vacuum as taught in the prior art. It shows a single relief dot  20  formed above a one-pixel-wide transparent area  29  in the film negative  27 . Even under good vacuum there will be pockets of gas  21  (predominantly air) beneath the cover film  41 . These variable gaps, in addition to the thickness of the cover film  41 , cause the shoulders  23  of the dot to be about 35° off the vertical. When this dot is printed, the edges  50  of the dot  20  become distorted as shown in  FIG. 4 . 
         [0055]      FIG. 4  is a highly magnified view of the same dot  20  as it impacts a sheet of paper  44 . As the dot  20  is squeezed upward by the print pressure, the dot edges  50  are crushed against the paper  44 , increasing the diameter of the printed dot (“dot gain”) and roughening its edges (reducing its sharpness). 
         [0056]      FIG. 5  is a cross-sectional illustration of a first embodiment of the present invention, namely a solvent ink negative image  130  printed on an image sheet  131  made entirely of solvent ink receptive material. The solvent inks made in accordance with U.S. patent application Ser. No. 13/902,301 are satisfactorily printed on cover film material comprising polyvinyl-based polymers, particularly polyvinylethylene. This embodiment may be used on the bottom glass of a UV exposure unit in place of a relief image negative covered by a protective cover film, with or without the Jetsetter Genesys™ backing laminate described further below. Using this embodiment, the entire relief image negative  27  of the prior art is eliminated, along with the air-containing interface  21  between the negative and the cover film  41  of the prior art (see  FIG. 3 ). The scope of this invention includes, by way of example and not limitation, mixtures or copolymers containing both polyvinyls and polyethylenes, which would be both solvent ink compatible and adhesive releasing, and polyvinyls and polyesters, which would be both solvent ink compatible and adhesive retaining. 
         [0057]    In this description of the invention, inkjet printers are mentioned because they have been used by the inventor to generate the desired high-definition images with solvent-based inks. However, any printer capable of printing solvent-based inks is within the scope of this invention without limitation. 
         [0058]      FIG. 6  is a cross-sectional illustration of a second embodiment image sheet of the present invention, namely a solvent ink negative image  130  printed on a protective cover film  140  made of a layer of solvent ink receptive material  131  superimposed on a layer of adhesive-releasing material  141 . As in the first embodiment described in  FIG. 13 , the solvent inks made in accordance with U.S. patent application Ser. No. 13/902,301 are satisfactorily printed on cover sheet material comprising polyvinyl-based polymers, particularly polyvinylethylene. The layer of adhesive releasing material  141  may be comprised of a variety of polymer-based films such as polyethylene. The layer of adhesive releasing material  141  allows this embodiment to be used in place of the combination of a masking negative and release liner on top of the backing sheet, and is distinct in this regard from the first embodiment. This cover film/release liner image sheet may be thicker than the cover film used in the first embodiment. 
         [0059]    Alternatively, as a third embodiment of the present invention (not drawn) the layer of adhesive-releasing material may be a backing laminate consisting of a flexographic plate backing sheet made from a solvent ink compatible polymer, with a tie-coat applied to the bottom surface. This would allow a solvent ink image to be printed directly onto the backing sheet to be used in the manner of the backing sheet taught and claimed in the aforementioned U.S. patent application Ser. No. 12/895,561. 
         [0060]      FIG. 7  is a cross-sectional illustration of the fourth embodiment of the present invention. It is a backing laminate called the Jetsetter Genesys™ backing laminate  150 , ready for solvent-based ink printing. The unprinted two-layer image sheet  140  of  FIG. 6  has been placed against the adhesive coating  91  on a polyester backing sheet  31 . A solvent-based image can now be printed on the solvent ink receptive layer  131 . 
         [0061]      FIG. 8  is a cross-sectional illustration of the method for using the Jetsetter Genesys™ backing laminate of the fourth embodiment together with a relief negative of the first embodiment in making a flexographic I-plate in accordance with the present invention. Steps for preparing an I-plate from liquid photopolymer starting with these two laminates are as follows:
   (a) Print the digital representation of the island image  81  on the solvent ink receptive side of the Jetsetter Genesys™ backing laminate  150  (see  FIG. 7 ) using a suitable inkjet printer;   (b) Print the digital representation of the relief image  130  onto the solvent ink receptive image sheet  131  (see  FIG. 5 );   (c) Place the image sheet  131  with the relief image  130  on the bottom glass  24  of the UV exposure unit (preferably, but not essentially, with the image side facing up);   (d) Preferably but optionally, turn on a vacuum source (not shown) connected to the lower vacuum grooves  34  to remove air from between the bottom glass  24 , the image negative  27 , and the clear cover film  41 ;   (e) Cast a layer of liquid photopolymer  43  adjacent to the relief image sheet  131  (employing dams  42  if necessary);   (f) Place the tie-coat  32  of the Jetsetter Genesys™ backing laminate  150  in contact with the upper surface of the liquid photopolymer layer  43  (registering the island image properly with respect to the relief image negative) creating an exposure laminate consisting of, from the bottom up, sheet  131 , photopolymer layer  43 , and backing laminate  150 ;   (g) Lower the top glass  26  onto the laminate  150 ;   (h) Preferably but optionally, turn on a vacuum source (not shown) connected to the upper vacuum grooves  28  to remove air from between the substrate  30  and the upper glass  26 ;   (i) Shine polymerizing light  52  through the Jetsetter Genesys™ backing laminate  150  for an amount of time suitable to form polymerized island floor areas  54  of a desired thickness in the liquid photopolymer in contact with the tie-coat  32 ;   (j) Shine polymerizing light  25  through the relief image  130  for an amount of time suitable to form a polymerized relief image  210  in the liquid photopolymer between the polymerized island floor areas  54  and the relief image sheet  131 ; and   (k) Remove the exposure laminate from the unit for further processing to develop the relief image, namely, by turning the exposure laminate over and draining/collecting the un-polymerized liquid photopolymer for recycle and reuse.   
 
         [0073]    Image quality using these laminates is high because, especially with the relief ink image facing up as shown in  FIG. 8 , both ink-bearing films are in intimate contact with both the upper and lower surfaces of the photopolymer layer. The upper (Jetsetter Genesys™) laminate rests directly on the photopolymer layer, and the liquid photopolymer flows directly onto the image surface of the inked surface of the lower cover film. (The space  160  in  FIG. 8  is a drawing artifact due to the exaggerated vertical dimension of the drawing.) 
         [0074]      FIG. 9  is a cross-sectional illustration showing removal of the printed cover film and release liner from the finished flexographic printing plate produced using the fourth embodiment Jetsetter Genesys™. After exposure is complete, the lower printed cover film  130  is peeled off of the relief image  210  and discarded. The upper printed release liner  111  is peeled off of the Jetsetter Genesys™ laminate  150  and discarded or reused. The polymerized island and relief images  54  and  210  respectively are cleaned and cured, leaving the finished flexographic printing plate  110  with the adhesive layer  91  exposed. 
         [0075]    The present invention also includes performing the above method with the alteration of substituting, in step e, for the fourth embodiment backing laminate  150 , solely a polyvinyl-based backing sheet (or other liquid photopolymer-compatible backing sheet) and tie coat, to produce a relief image only (method not illustrated). 
         [0076]    The present invention further includes performing the above method with the alteration of substituting, in step (f), for backing laminate  150 , the two-layer laminate of  FIG. 6  with layer  141  being a polyester backing sheet with a tie-coat applied to the bottom surface (method not illustrated). 
         [0077]      FIG. 10  is a highly magnified view of a relief dot  20  produced using the first laminate of the invention as shown in  FIG. 5  and prepared in accordance with the above method of the invention. The cover sheet  131  printed with solvent ink image  130  takes the place of the prior art negative  27  (see  FIGS. 1 and 2 ) and places the image  130  directly against the bottom surface  22  of the liquid photopolymer layer  43 . The UV radiation spreads out much less as it enters the photopolymer than it does in the prior art (depicted in  FIG. 3 ). This causes the shoulders  23  of the dot to be extended and substantially vertical. When this dot is printed, the shoulders  23  accommodate print pressure as shown in  FIG. 11 . 
         [0078]      FIG. 11  is a highly magnified view of the dot  20  of  FIG. 10  as it impacts a sheet of paper  44 . As the dot  20  is squeezed upward by the print pressure, the dot edges  50  retain their diameter and roundness.