Abstract:
Methods and apparatus are provided for improving the accuracy and streamlining the process of preparing a flexographic printing sleeve to be imaged with a printing image to produce a flexographic printing forme. Layout details are determined by a controller and transferred to an automated cutting machine. The controller may also control a printer to print indicia on a sleeve for use in aligning the cut flexographic plate sections.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    The benefit of the filing date of Canadian patent application No. 2,359,259 filed on Oct. 18, 2001 is claimed herein.  
         TECHNICAL FIELD  
         [0002]    This invention relates to flexographic printing, and more specifically to an improved process for preparing flexographic printing sleeves.  
         BACKGROUND  
         [0003]    Flexographic printing is a method of direct rotary printing that uses resilient relief image plates. The plates are typically made of rubber or photopolymer. Flexographic printing has found particular application in packaging where it has displaced rotogravure and offset lithography printing techniques in many cases. While flexographic printing can produce high quality printed products, making flexographic printing formes according to prior art processes can be undesirably time consuming and labour intensive.  
           [0004]    Typical conventional flexographic plates have a flat polyester base coated with a photopolymer layer. The photopolymer layer is sensitive to ultraviolet (UV) radiation, such that it hardens when exposed to UV light. In a first step, a floor is set by exposing the back of the plate to UV light. The floor forms the base of the relief that will be formed in further imaging steps. A film mask, which is imaged in a separate process, is placed over the top of the photopolymer layer, and drawn down by a vacuum frame to ensure good contact. The photopolymer layer is then flood exposed to UV light, thereby hardening or cross-linking the regions of the photopolymer layer not covered by the mask. After the mask is removed, the plate is processed in solvents to form a printing image by removing the unexposed areas of the photopolymer layer which were covered by the mask. After processing with solvents the plate must be dried. Drying may take several hours.  
           [0005]    Digital flexography follows a similar process except that the plate has an integral UV-opaque mask layer coated over the photopolymer layer. The mask layer is selectively ablated by a digital imager with a high-power laser imaging head to form an image mask that is opaque to UV light in non-ablated areas. Once the mask is formed, processing of the plate continues as it would for conventional flexographic plates except that there is no need to use a vacuum frame to ensure good contact between the mask and photopolymer layer since the mask layer is integral with the photopolymer layer. Other flexographic plate formulations, such as Cyrel® Fast made by E. I. Dupont de Nemours and Company, eliminate the use of solvents for the processing step and reduce the combined processing and drying time.  
           [0006]    The processed flexographic plate is then mounted on a cylinder for use on a flexographic press using an adhesive layer such as a double sided adhesive tape or foam. The imaged plate must be mounted in precise registration on the cylinder. This is done using mechanical and/or electronic aids. Accurate registration is key in producing a high quality printed product. Mounting is typically done by skilled operators.  
           [0007]    When a rectangular flexographic plate is mounted on a press cylinder there is a gap or “seam” where the top and bottom of the plate approach one another. On the printing press, the printing stock is backed up by an impression cylinder. The impression cylinder presses the printing stock into contact with the flexographic plate relief mounted on the press cylinder. The impression cylinder sets a contact pressure for the printing operation. As the seam contacts the impression cylinder on each rotation, the discontinuity jolts the impression cylinder slightly, a phenomenon known as “press bounce” or “cylinder bounce”. This jolt puts an upper limit on the impression speed, beyond which registration and other printing errors occur.  
           [0008]    A common method of reducing the effects of cylinder bounce is to stagger the seam around the cylinder. This method is particularly effective when a repeated pattern is imaged across the cylinder; a common situation in flexographic printing. The plates are arranged so that the impression cylinder is always contacting a relief image and does not fall into a seam. A staggered seam can be achieved by laying out the image so that several plate sections are applied to the cylinder in what are known as lanes. In FIG. 1-A a number of plate sections  40  have been cut and imaged. In FIG. 1-B the plate sections  40  are shown wrapped around cylinder  32 . The seam  42  for the flexographic plate in each lane is offset from the seams of other lanes so that the seams are distributed around the circumference of the cylinder. The impression cylinder no longer falls into a seam since it is always riding on the image relief of one or more lanes.  
           [0009]    A staggered seam may also be achieved by cutting the plate seam in a staircase shape. FIG. 1-C shows a photopolymer plate  30  cut with a staircase seam  33 . The seam layout has the same repeat as the image elements  31 . In FIG. 1-D plate  30  is shown wrapped around cylinder  32 . The location of seam  33  is chosen so that the plate completely wraps around the cylinder with the seams precisely lining up.  
           [0010]    While a staggered seam is effective in reducing the effects of cylinder bounce, the manual cutting, mounting, and registration of the plates on the press cylinder is both time consuming and lacking in accuracy for high quality imaging.  
           [0011]    To avoid registration problems, the mask layer may be imaged after mounting the plate on the cylinder. In this way, the registration is provided by the imaging device, which can place an image very accurately. The UV exposure and processing of a plate imaged while on a cylinder in this manner requires specialized equipment, now commonly available, that can operate on round cylinders rather than flat plates.  
           [0012]    In order to make the handling of cylindrical photopolymer plates more convenient, sleeve substrates have been developed. A sleeve substrate typically comprises a cylindrical tube of nickel, polyester or some other material. The sleeve substrate material is chosen to have a certain degree of elasticity so that air pressure can be used to expand the sleeve slightly, thus allowing it to be slid over a cylinder on a cushion of air. Once the air supply is removed, the sleeve substrate shrinks so that it is held tightly in place. A photopolymer plate, referred to herein as a “flexographic printing precursor”, may be mounted on the sleeve substrate using double-sided tape in the same way flat plates are mounted on a cylinder. The cut photopolymer plate is wrapped around the sleeve in approximate registration and then imaged on a digital imager to produce a flexographic printing forme which is ready to be placed on a printing cylinder for use in a flexographic printing operation. This process employing a sleeve substrate as a base for mounting a flat plate is known in the industry as Plate-on-Sleeve (PoS).  
           [0013]    [0013]FIG. 2 shows a flow diagram of a prior art process for making a typical PoS flexographic printing forme. A flexographic printing precursor  1  comprising a photoplymer layer and a UV opaque mask layer is back exposed in step  2  to set a floor for the relief image. In step  3  the flexographic printing precursor is cut into sections so that it can be applied to a sleeve substrate in lanes to form a staggered seam. The flexographic printing precursor sections are then mounted on a sleeve substrate using double-sided tape in step  4  to produce a flexographic printing sleeve. Registration of the flexographic printing precursor sections must be accurate enough to ensure that the image will not run into a seam, but since the flexographic printing sleeve is not yet imaged, the accuracy required is significantly reduced. Alternatively, the flexographic printing precursor could be cut to form a section with a staggered seam as shown in FIG. 1-C and mounted as a single piece to a sleeve substrate in step  4 .  
           [0014]    Referring again to FIG. 2 image data  7  is typically pre-formatted by one or more computer workstations connected to a network to enable file or data transfer. A packaging workflow system  5  and a controller  6  combine to layout an image including the details of how it will be imaged and printed. These workstations provide functionality enabling an operator to take an image file from a customer and arrange the image for optimal printing.  
           [0015]    A digital imager  8  ablates the UV opaque mask layer according to the image data  7 . The flexographic printing sleeve is then exposed to UV light in step  9 , hardening or cross-linking areas where the UV opaque mask layer has been ablated. A processing step  10  follows. Processing may include washing in solvents, drying, and a final UV exposure to fully harden the photopolymer and remove tackiness. The finished photopolymer printing forme  11  is then ready for printing on a flexographic press.  
           [0016]    Digital imaging devices for imaging such flexographic printing sleeves are typically built in the general form of a lathe. Such machines have a mandrel on which a flexographic printing sleeve can be mounted, a fixed headstock for driving the flexographic printing sleeve, a moveable tailstock for supporting the flexographic printing sleeve, and a travelling imaging head. The imaging head typically has a radiation source, such as a laser, capable of imagewise ablating the mask layer.  
           [0017]    There is a need for methods for streamlining the making of flexographic printing sleeves. In high quality printing there is a particular need for methods for cutting, handling and mounting sections of flexographic printing precursor on a sleeve substrate in such a way that registration errors are minimised. It is desirable that the overall time required to make a flexographic printing forme be reduced.  
         SUMMARY OF INVENTION  
         [0018]    A method is provided for automating the cutting of a flexographic printing precursor into sections for application to a sleeve substrate. A controller is operative to provide previously determined layout information to the cutting operation. The controller is also operative to provide layout information to a printing device enabling registration marks to be printed on the sleeve substrate. The printed registration marks aid in the accurate placement of the flexographic printing precursor sections on the sleeve substrate.  
           [0019]    A significant advantage of the photopolymer platemaking process outlined herein is that the accuracy of the operation is improved while the time taken in tedious manual operations is reduced. The application of the methods of the present invention will speed up these processes and improve registration accuracy. The possibility of operator error is also reduced. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]    In drawings which illustrate non-limiting embodiments of the invention:  
         [0021]    [0021]FIG. 1-A is a depiction of a prior art flat flexographic printing plate cut into lanes;  
         [0022]    [0022]FIG. 1-B is a depiction of a prior art flexographic printing plate mounted on a printing cylinder in lanes with staggered seams;  
         [0023]    [0023]FIG. 1-C is a depiction of a prior art flat flexographic printing plate cut with a staircase seam;  
         [0024]    [0024]FIG. 1-D is a depiction of a prior art flexographic printing plate with a staircase seam wrapped around a printing cylinder;  
         [0025]    [0025]FIG. 2 is a flowchart illustrating a prior art process for making a flexographic printing forme;  
         [0026]    [0026]FIG. 3 is a flowchart illustrating an improved method according to this invention;  
         [0027]    [0027]FIG. 4 is a schematic depiction of apparatus according to the invention;  
         [0028]    [0028]FIG. 5-A is a depiction of a sleeve substrate with registration marks corresponding to a staggered seam printed thereon; and,  
         [0029]    [0029]FIG. 5-B is a depiction of a sleeve substrate with registration marks corresponding to a staircase seam printed thereon.  
     
    
     DESCRIPTION  
       [0030]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0031]    [0031]FIG. 3 is a flowchart which illustrates a preferred embodiment of this invention. This invention provides automatic methods and apparatus for producing flexographic printing sleeves. The items drawn in broken lines in FIG. 3 are not directly applicable to the present invention but are included to show the context of the methods of the present invention in the overall process of making a flexographic printing forme.  
         [0032]    Throughout this description and the following claims:  
         [0033]    “flexographic printing precursor” means a layer of material that may be imagewise converted and processed to form a relief surface for flexographic printing (whether or not is has been imaged and/or processed);  
         [0034]    “sleeve substrate” means a cylindrical tube of material that may be used as a base to support a flexographic printing precursor;  
         [0035]    “flexographic printing sleeve” means a sleeve substrate with a flexographic printing precursor on it; and,  
         [0036]    “flexographic printing forme” means a flexographic printing sleeve that has been imaged and is ready to be placed on a printing cylinder for use in a flexographic printing operation.  
         [0037]    [0037]FIG. 3 shows a controller  21  which may comprise a software program running on a computer workstation. Controller  21  is connected via a network or some other data connection to a digital imager  8 . Controller  21  facilitates the interactive arrangement of sections of flexographic printing precursor on a sleeve substrate to produce a desired seam layout for the resulting flexographic printing sleeve. Controller  21  comprises a display such as a computer workstation monitor. An operator is able to view a facsimile of the printing image on the display. Software running in controller  21  allows an operator to define a desired seam layout. The operator can use an input device, such as a mouse, light pen, trackball, touch-sensitive screen or the like to draw in seams to create an arrangement of one or more sections of flexographic printing precursor. Controller  21  may additionally be programmed with functionality to aid the operator by suggesting a seam layout calculated according to an algorithm set to minimise plate wastage or some other optimisation function. The seam layout may comprise, for example, a number of lanes or a staircase seam.  
         [0038]    Once the seam layout has been designated, controller  21  transfers seam location information  22  to a controllable cutting device  23 . Device  23  cuts the flexographic printing precursor into one or more sections according to the seam location information provided to it by controller  21 . The cuts could divide the flexographic printing precursor into simple rectangular sections, could provide a staggered seam, or could provide a more complex seam.  
         [0039]    Controller  21  may implement a packaging workflow system  5 . Controller  21  may comprise any combination of one or more date processors and may be either a stand-alone device or connected together with other devices in a computer network. Information or data transfer can be accomplished in a variety of manners and this application should be understood to cover any means of file or data transfer via any form of data storage or transmission. The term “information” used in reference to seam location includes any form of data or encoding that can be used to transfer seam layout details between process steps.  
         [0040]    [0040]FIG. 4 schematically depicts apparatus  50  according to the invention. Apparatus  50  comprises controller  21 , cutting device  23  and printing device  52 . Controller  21  may comprise an interactive user interface which allows an operator (not shown) to designate an arrangement of sections of flexographic printing precursor on a sleeve substrate. Controller  21  provides seam information  22  to cutting device  23 , and registration information  24  to printing device  52 , based on the arrangement designated by the operator.  
         [0041]    Cutting device  23  is configured to cut a flexographic printing precursor plate  56  into sections  58  according to seam information  22 . A suitable controllable cutting device  23  is produced by Misomex International of Nashua, N.H. Misomex have a range of flatbed x-y plotting machines with cutting heads available. Such machines are capable of accurately and quickly cutting many types of material. The flexographic printing precursor can be cut with a plate protective layer intact or removed depending on the user&#39;s preference. Any cutting device capable of cutting a flexographic printing precursor in accordance with seam information  22  provided by controller  21  could be used in this invention. Cutting device  23  does not have to be a flatbed device; the plate could also be cut on a cylinder. Cutting may optionally be combined with another process such as mounting or imaging by providing a controllable cutting tool in the apparatus used to perform such other process.  
         [0042]    Printing device  52  is configured to print registration marks  62  on sleeve substrate  64  according to registration information  24 . Registration marks  62  may comprise outlines, corner placement marks, crosshair targets or any other format and may correspond to the edges of sections  58 . If flexographic printing precursor plate  56  or sections  58  are imaged prior to application to sleeve substrate  64  then registration marks  62  may correspond to imaged features on sections  58 . Printing device  52  may also print instructions (not shown) on sleeve substrate  64 . Printing device  52  may comprise an inkjet printer head in a stand-alone machine or advantageously an inkjet printer head could be integrated into the digital imaging device  8 .  
         [0043]    Plate  56  may be imaged before it is cut into sections  58 , sections  58  may be imaged after being cut but before being attached to sleeve substrate  64 , or sections  58  may be imaged after being attached to sleeve substrate  64 .  
         [0044]    In the FIG. 4 embodiment, cutting device  23  is shown cutting a staircase seam  60 , and printing device  52  shown printing staircase-shaped registration marks  62 . FIG. 5-A depicts a sleeve substrate  64  with registration marks  62  printed thereon which correspond to an arrangement of sections (not shown) in lanes to produce a staggered seam. FIG. 5-B depicts a sleeve substrate  64  with registration marks  62  printed thereon which correspond to an arrangement of a single staircase-shaped section (not shown) to produce a staircase seam. It is to be understood that other seam configurations are equally within spirit and scope of the invention.  
         [0045]    An adhesive layer may be applied to sleeve substrate  64  before registration marks  62  are printed thereon. Once the adhesive layer, which may comprise double-sided tape, is applied to the sleeve, registration marks  62  are printed onto the adhesive layer to accurately indicate the desired placement of the cut sections  58  of flexographic printing precursor. In this embodiment, printing device  52  may comprise any device capable of making marks on the adhesive layer.  
         [0046]    Alternatively, an adhesive layer may be applied to the back or “inner surface” of the flexographic printing precursor sections  58 . The term “inner surface” is used to describe a surface of section  58  which is mounted on the outer surface of sleeve substrate  64 . In this case, the order of operations may change but the principles of the invention still apply. The sections  58  of flexographic printing precursor  56  can be cut with the adhesive layer on the inner surface thereof and registration marks  62  can be printed directly onto sleeve substrate  64 . Such minor modifications should be understood to come within the spirit and scope of this invention.  
         [0047]    “Adhesive layer” includes any adhesive for attaching the flexographic printing precursor sections to a sleeve substrate. The adhesive could be in tape or in sheet form, or coated, sprayed, or otherwise applied to either the outer surface of the sleeve substrate or the inner surface of the flexographic printing precursor sections.  
         [0048]    The mounting of sections  58  with the aid of the registration marks  62  may be done on a separate mounting jig or the jig functions may be incorporated into imaging device  8 . Advantageously, since the flexographic printing precursor sections  58  have been accurately cut in the automated cutting step and regulation marks  62  are accurately positioned, based on the same data used to cut the flexographic printing precursor sections  58 , sections  58  can be placed properly on the sleeve substrate by aligning features of the flexographic printing precursor sections with corresponding ones of the registration marks  62 .  
         [0049]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.