Abstract:
A method of making structured microdots ( 154 ) for printing plate registration includes forming a first plurality of square spots ( 156 ) less than or equal to 11 microns; wherein a first group ( 157 ) of the first plurality of square spots is formed in a first pattern ( 158 ); and wherein the first pattern is less than or equal to 66 microns and comprises a first microdot.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket No. K001281USO1NAB), filed herewith, entitled SYSTEM FOR FORMING STRUCTURED MICRODOTS, by Anderson; U.S. patent application Ser. No. ______ (Attorney Docket No. K001282USO1NAB), filed herewith, entitled METHOD OF FORMING SECURITY MARKINGS, by Anderson; and U.S. patent application Ser. No. ______ (Attorney Docket No. K001283USO1NAB), filed herewith, entitled SYSTEM FOR FORMING SECURITY MARKINGS USING STRUCTURED MICRODOTS; the disclosures of which are incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to flexographic printing and in particular to using microdots for plate registration. 
       BACKGROUND OF THE INVENTION 
       [0003]    The flexographic industry has developed over the years from hand carved rubber plates with no expectations of register control between colors, to use of a photopolymer plate  100 , shown in  FIG. 1 , supported by a polymer sheet  101  to provide dimensional stability and allow registration between colors. 
         [0004]    Flexographic printing was the least of the print processes in terms of capabilities and the lowest cost compared to the traditional offset, letterpress and rotogravure printing processes used for package printing. Since the introduction of the photopolymer plate making systems, the growth in use of flexography has been significant, becoming the largest printing process used in packaging, and in some regions, like North America gaining over 80 percent of market share. 
         [0005]    Throughout the development process, registration systems have steadily improved as the flexographic printing process has improved. The flexographic plate is selectively exposed to ultra violet light and unwanted areas washed away leaving a raised printing surface  103 , shown in  FIG. 2 . Exposed photopolymer  102  with a lower floor level  104  that does not print, is separated by a displacement called the relief height  105 , hence flexography is a relief printing process. The vast majority of the flexographic plates manufactured today are in sheet format, and are attached to a printing cylinder or sleeve  107 , shown in  FIG. 3 , in register, using highly engineered double sided sticky back tape  106  with specific adhesion and compressibility properties for best print quality or productivity or both. 
         [0006]    The plates  110 , shown in  FIG. 4 , all have some form of register mark  111  and  112  on each side of the plate to allow easier plate to plate registration so that minimal time is spent in registration on press setup and optimum image quality is achieved. Over the years the types of register marks have progressed as mounting systems and methods have advanced. Originally the register marks  111  and  112  shown in  FIG. 4 , were large cross hairs for register on press. Placing the plates was a highly skilled process and often resulted in mis-register for at least one of the colors. 
         [0007]    To enable greater productivity and accuracy, video mounting systems were developed, shown in  FIG. 5 . Two or more cameras  121 ,  123 , were positioned on a frame  120 , relative to a cylinder, for mounting the plate cylinder or sleeve  107 , with focal points  122  and  124  on the surface of the cylinder or sleeve. The cameras were adjusted laterally  125  for the first plate and aligned with the plate cylinder or sleeve  107 . Mounting tape is applied, and the plate is positioned above the plate cylinder or sleeve, shown in  FIG. 6 , with minor adjustments  126  to match the register marker  111  and  112 , before fixing the cameras  121  and  123  in place. The plate is then affixed to the mounting tape  106  and the plate is imaged. Additional plates are mounted without adjustment of the camera position, as shown in  FIG. 7 , for accurate plate to plate location. 
         [0008]    As demand for flexographic printing process grows, it is moving to process printing, that is, building images and colors out of four (CMYK), six, or seven process colors. It is important for process printing that the colors are in accurate registration to each other. With the printed dots being as small as 10-20 microns, any shift in registration can cause color shifts, image errors, or interference patterns with a negative impact on the final image. This has driven the industry to smaller register marks, shown in  FIG. 8 , with the large manual cross hairs  130 , moving to smaller cross hairs  131  for video mounting, and then microdots  132 . 
         [0009]    As applications for functional printing develop for very small lines and circuits as small as five microns in width, and the need for accurate layer to layer registration, on printing register, and the mounting of the plates accurately on the print cylinders and sleeves increases, there is a further need for improvement in microdots. The current state of the art is to place 2 or more microdots on each of the plates,  115 ,  116 , shown in  FIG. 9 , and use video mounting systems to locate and position the plate manually or automatically, shown in  FIG. 10 , and then fix the plates in place on engineered double sided mounting tape on the sleeve or cylinder. 
         [0010]    Current microdots are typically 200-250 microns in size. A recent publication, U.S. Publication No. 2011/0265676, describes a registration system employing a scattered microdot pattern with each dot about 200 microns. Such large dots are objectionable when visible in the printed product. Smaller registration features are desired to ensure invisibility. In the printing of functional materials, such as electronic circuits, component sizes of five microns or less are desirable. When printing multiple layers, registration accuracy must be improved. 
         [0011]    Traditionally the size of the microdots in flexo is limited to the size of a separate stand alone dot made of a group of pixels that can be consistently and independently formed on the plate. These are described as the minimum isolated dot size. In the majority of the flexoplate market this is presently between 120 and 250 microns. 
         [0012]    Traditional digital flexo imaging technology uses Gaussian lasers,  140 , shown in  FIG. 11 , versus the proposed technology using SQUAREspot imaging  141 . The traditional Gaussian imaging produces an error in imaging  142 , shown in  FIG. 12 , that limits the capabilities of imaging and image transfer, unlike the SQUAREspot imaging  143 . This invention uses the exact reproduction of the original digital data  150 , shown in  FIG. 13A , on the thermal imaging layer  151 , shown in  FIG. 13B , and to the final plate  152 , shown in  FIG. 13C , as shown in the photograph in  153  in  FIG. 14 . 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly, according to one aspect of the present invention a method of making microdots for printing plate registration includes forming a first plurality of square spots less than or equal to 11 microns,  156 . A first group of the first plurality of square spots  157  is formed in a first pattern and the first pattern is less than or equal to 66 microns and comprises a first microdot. 
         [0014]    The invention provides a shaped dot with an equivalent circular diameter of about 33 to 66 microns on the printing plate surface  161 , which is significantly smaller than traditional microdots  160 . The shaped spot comprises a pattern of six or more square pixels each less than  11  microns edge length arranged in a contiguous manner  170 , shown in  FIG. 17 . In conjunction with a machine vision system, the shaped spot provides registration information in two dimensions  171  for aligning the two or more printing plates for a multicolor or multilayer print. The structure of the patterns allows greater accuracy of register, providing specific points of reference and a smaller scale. This also allow directional confirmation to ensure the plate is correctly aligned  171 ,  172  and not rotated or inverted  173 ,  174 . The combination of pixels allows greater resolution and accuracy going forward, especially with high resolution or automated mounting systems. Alternatively, structured microdots could be applicable, as shown in  FIGS. 18 ,  19 , and  20 , Examples 1-20. 
         [0015]    Pixel for pixel imaging allows the combination of unique patterns of pixels or additional image components to make an identifier for brand owners,  180 , shown in  FIGS. 21 and 181 , shown in  FIG. 22  for security functionality, to identify the file preparer or printer at a micro level on the plate and on the print. 
         [0016]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a plan view of prior art registration marks on a printing plate. 
           [0018]      FIG. 2  is a plan view of exposed and prepared photopolymer plate for printing. 
           [0019]      FIG. 3  is a plan view of mounted photopolymer plate ready for printing. 
           [0020]      FIG. 4  is a plan view of simple cross hair register marks on the two sides of the printing plate for manual register alignment on press. 
           [0021]      FIG. 5  is a plan view of video mounting camera are aligned relative to first print cylinder or sleeve. 
           [0022]      FIG. 6  is a plan view of a plate aligned using video mounting camera for first plate to first print cylinder or sleeve. 
           [0023]      FIG. 7  is a plan view of a plate aligned using fixed video mounting camera positions for additional plates relative to print cylinder or sleeve. 
           [0024]      FIG. 8  is a progression from large pairs of manual mounting marks, to smaller video camera marks and microdots. 
           [0025]      FIG. 9  is a plan view of a plate showing pair of microdot register marks on the plate. 
           [0026]      FIG. 10  is a plan view of a plate aligned using fixed video mounting camera positions for additional plates relative to print cylinder or sleeve. 
           [0027]      FIG. 11  is a schematic of two existing laser imaging technologies. 
           [0028]      FIG. 12  is a schematic of imaging characteristics of two existing laser imaging technologies. 
           [0029]      FIGS. 13A-13C  are representations of pixel for pixel imaging with a square spot imaging system. 
           [0030]      FIG. 14  is a photographic reproduction of a Kodak Flexcel NX plate, retaining pixel for pixel the original digital data in imaging. 
           [0031]      FIG. 15  is a schematic showing forming of a first plurality as a pixel and multiple pluralities as dot. 
           [0032]      FIG. 16  illustrates the scale reduction for traditional microdot to proposed structured microdot size. 
           [0033]      FIG. 17  shows a primary structured microdot. 
           [0034]      FIG. 18  shows an alternative version of the structured microdots built from individual square pixels. 
           [0035]      FIG. 19  shows an alternative versions of the structured microdots built from individual square pixels. 
           [0036]      FIG. 20  shows an alternative versions of the structured microdots built from individual square pixels. 
           [0037]      FIG. 21  shows additional structured microdots around for security or identification marker features. 
           [0038]      FIG. 22  shows a combination of positions for identification or security marker features. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    The present invention will be directed in particular to elements forming part of or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
         [0040]      FIG. 5  shows a video camera mounting for a device consisting of two cameras  121 ,  123  connected to the display. The cameras provide a point of reference in the direction of the cylinder  107  and allow adjustment on the first plate  110 , shown in  FIG. 6 , so the register mark for each side of the plate  111 ,  112 , often still small crosses, are in the camera center. The cameras are locked in position for each following plate to allow register. 
         [0041]    Referring to  FIG. 10 , some brand owners and packaging buyers objected to relatively large register marks in the artwork, especially as the capabilities and expectations for flexographic printing improved, so a less obvious register locator was needed. The new registration system suggested was a “microdot”  115 ,  116 , which used isolated dots in the plate to provide the register mark, for use with video mounting systems. The microdot provided register control and a reduced impact on the final print. 
         [0042]    A limitation of flexographic plate making with the existing plate manufacturers at the time was a minimum size for an isolated dot, which was 200-250 microns in size due to the manufacturing constraints. A smaller dot would not hold on the plate. (The standard industry specification today for the microdot size is 200-250 microns in diameter.) 
         [0043]    The flexographic printing industry is now capable of matching or beating competitive printing processes in terms of print capabilities for resolution, density, and production speeds. It is now normal for flexographic printing to use process printing to build colors out of two or more screens instead of traditional spot colors, raising the flexographic printing capabilities even further. One issue in the transition from single spot colors to process printing, is that the demands on holding register are more critical than ever, with mis-register between two or more colors potentially causing a shift in the color and visible print defects. 
         [0044]    Process printing today uses minimum dot sizes of 10-30 microns depending on application and print capabilities, so the use of a standard 200 micron registration mark is large and causes severe challenges on registration as print capabilities and expectations continue to rise. 
         [0045]    Video mounting equipment has been improving significantly for manual and automatic registration control, yet registration remains controlled by a general 200 micron round dot, limited by the minimum isolated dot capabilities of traditional plate making 
         [0046]    All traditional lasers, see  FIG. 11 ,  140 , used for imaging of digital flexographic plate or film, use a round laser beam of Gaussian format, which has high power in the center and low power at the edges. The result is an inability to image the square pixels of a digital file accurately, with an imaging error in all cases. See  FIG. 12 . 
         [0047]    Square spot imaging systems such as Kodak Flexcel NX, image flexographic plates, as shown in  FIG. 11 , to a thermal imaging layer (TIL), which is laminated to the flexographic plate. A pixel for pixel image transfer  150 ,  151 , and  152  is created as shown in  FIG. 13 . This results in an imaged plate  153  which retains pixel to pixel integrity as shown in  FIG. 14 . 
         [0048]    The structured microdot solution  154  allows a much higher level of process printing to be used because of the benefits of imaging accuracy. See  FIG. 15 . With the improvement in pixel for pixel imaging also came a smaller minimum isolated dot size of 50 microns, further increasing capabilities. 
         [0049]    In March 2012 a panel of industry speakers at the FFTA Forum in San Antonio stated that “flexographic printing can match or better offset and rotogravure printing on all items, except register, and this remains the greatest limitation for process printing in flexography.” The current microdots used in flexographic printing are limited to 200 microns in size by the minimum isolated dots capabilities of the plates, while the structured microdot solution can form 50 microns minimum isolated dots. 
         [0050]    Current microdots are basically round dots of photopolymer without shape or structure to provide data other than basic location. The present invention uses the pixel for pixel imaging capabilities of the structured microdot system to produce a new structured microdot to supply greater accuracy in register, but also to provide additional data in terms of plate position, direction, and inversion. 
         [0051]    The present invention uses structured microdots that are 66 microns or less in dimension, and are a combination or a series of imaged pixels to produce a structured shape allowing straight edges for register along with image components that will indicate direction, rotation, or inversion issues, for use with manual or automatic video mounting systems. The present invention allows use of a single pair of microdots on either side of the plate, as shown in  FIG. 9 . Patterns of microdots in the image area allows for on-press monitoring of register and automatic adjustments, such as U.S. Publication No. 2011/0265676. 
         [0052]    The present invention provides a smaller microdot compared to the prior art, with clear structure. See  FIGS. 16 and 17 . This provides clear benefits in the capabilities to achieve better accuracy in register. 
         [0053]    There are several options for the structured microdot as shown in  FIGS. 18 ,  19 , and  20 . One embodiment of a structured register microdot is shown in  FIG. 17  with 6×6 pixel outer structure to provide large scale register with straight edges for maximum reference. 
         [0054]    The inner section provides the ability to register to an even finer level with suitable equipment, or for more challenging demands. The inner section also provides a clear indication if the plate is rotated or inverted, and can act as a check point to ensure the plate is mounted in the correct orientation. These features combine to enhance the register capabilities of the flexographic plate system in mounting before coming to the printing press, allowing reduced adjustments on press, greater image accuracy, and production efficiencies. 
         [0055]    The benefits of using structured microdots for the flexographic printing industry have enhanced benefits for specific markets. For example, moving from a broad round microdot with no straight edges or corners, to a structure in the present invention, with right angle corners, provides benefits in registration measurement and control. Reducing the size of the microdot from 200 microns to the structure microdot of 66 microns or less also provides a significant opportunity to reduce registration errors and increase accuracy. 
         [0056]    Changing from traditional Gaussian imaging with or without oxygen inhibition in plate making, with its inherent errors in size and variability, to pixel for pixel imaging accuracy, enhances register capabilities and reduces significantly the plate making tolerances on the microdot and register accuracy. A square outer structure with straight edges and a right angle corner also allows greater accuracy in two perpendicular directions. 
         [0057]    Having an inner component allows higher accuracy on registration if the application requires it. Having an inner structure that is not consistent in all directions allows any rotation of the plate to be identified and rejected or corrected, relevant to manual and automatic mounting systems. An inner structure on the structured microdot, allows identification if the plate is inverted with the non-imaged side up instead of down, which is particularly useful when using automatic mounting systems. 
         [0058]    Functional printing applications tend to rely on the printing of lines and circuits more than dots and process on the final substrate. There exists a need to obtain the highest level of accuracy in mounting relates to the need to print circuits and lines of down to five microns in position to the next conductive or resistive material, and achieve the required electronic conductive or resistive properties and circuit function. 
         [0059]    When printing, a 200 micron microdot is too large to truly provide the tolerances required. The reduction in size and increase in accuracy for the structured microdot suits the demands of industry and the inner structure allows the highest level of accuracy, especially with automated mounting systems. The clear and immediate identification of any rotation and inversion of the plate is important where the layers of the circuits and their direction may not be obvious, and costs in the components mean errors are extremely expensive 
         [0060]    Although all of the applications of the structured microdot have focused on registration benefits, in the discussion above there are also opportunities to utilize the structured microdot in new ways when it is printed. One opportunity is in covert security to identify the pre-press provider, printer, or both, which is especially useful for brands that use several prepress and printers for the same product range. 
         [0061]    The use of the microdot for a covert security feature may be feasible depending on the print and resolution capabilities of the flexo printing process. In its simplest form it could be a series of 66 micron full blocks, half blocks, and triangles in a set combination in one or more of the eight possible locations around the structured microdot. See  FIGS. 21 and 22 . 
         [0062]    The ability of the microdot to be used in a covert security manner, with patterns only known or understood by the brand owners and their prepress providers gives a strong tool for identification of illegal or incorrect actions. The structured microdot, when using all security areas as solid blocks, similarly to the areas shown in  FIG. 21 , is no larger in side to side dimension than that the existing standard microdot at 200 microns. With the higher resolution capabilities of some applications and printers, there is no reason why these security markers cannot be more communicative and detailed. See  FIG. 22 . Overall it provides the brand owners, pre-press provider, or printer with a new microscopic identifier opportunity. These security features can be created out of a single color, or combination of the process printing colors (CMYK) for further unique combinations. 
         [0063]    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]    
       
           100  unexposed photopolymer 
           101  supporting polymer 
           102  exposed photopolymer 
           103  plate print surface 
           104  plate floor surface 
           105  plate relief height 
           106  double sided mounting tape 
           107  plate cylinder or sleeve 
           110  imaged printing plate ready for mounting 
           111  operator side cross hair register mark 
           112  machine side cross hair register mark 
           115  operator side microdot register mark 
           116  machine side microdot register mark 
           120  mounting camera frame 
           121  operator side mounting camera 
           122  operator side camera focus point 
           123  machine side mounting camera 
           124  machine side camera focus point 
           125  lateral adjustments for cameras for initial location control 
           126  fine lateral adjustments for cameras for final location control 
           130  original large register cross hairs for manual plate mounting 
           131  smaller register cross hairs for video plate mounting 
           132  microdots for smallest video plate mounting marks 
           140  Gaussian laser with round later beam technology 
           141  SQUAREspot laser with sharp square profile technology 
           142  Gaussian laser imaging with round laser beam technology 
           143  SQUAREspot laser imaging with sharp square profile technology 
           150  digital file ready for imaging showing individual pixels forming a structured microdot 
           151  imaged thermal imaging laser showing retained digital data as pixels forming a structured microdot 
           152  final Flexcel NX flexographic plate showing retained digital data to final plate forming a structured microdot 
           153  plate of structured microdots 
           154  structured microdots 
           156  single plurality to form pixel 
           157  multiple pluralities to form structured dot 
           158  dots build to form required image structured microdot 
           160  scaled illustration of traditional microdot for comparison 
           161  scaled illustration of proposed structured microdot demonstrating size reduction to enhance registration control 
           170  overall proposed structured microdot constructed out on individual pixels 
           171  outer square line provide scale at less than 66 microns, straight edges, right angle corners to give maximum point of reference and register capabilities 
           172  inner block provides secondary reference point for finer register control 
           173  asymmetrical structure for inversion and rotation identification 
           174  further asymmetrical structure for inversion and rotation identification 
           180  eight additional structured microdot locations for potential security of identification functions 
           181  additional various alternative structured microdots in locations 1, 2, and 8 for potential individual security or identification functions