Abstract:
An apparatus for preparing a printing sleeve ( 10 ) includes a support ( 31 ); a drum rotatably mounted on the support, the drum adapted to receive the printing sleeve; a radiation source; a controller ( 60 ) programmed to operate the radiation source to emit at least one radiation beam to cut the printing sleeve mounted on the drum into a plurality of separate and interlocking printing sleeve portions ( 10 A,  10 B); and an imaging head ( 40 ) comprising a plurality of individually addressable imaging channels, wherein the imaging channels are operated to direct imaging beams to form an image on the printing sleeve mounted on the drum.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of prior U.S. patent application Ser. No. 11/782,111, filed Jul. 24, 2007, now U.S. Publication No. 2009/0025592, which is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to printing sleeves for printing. The invention may be applied to flexographic printing sleeves, for example. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the art of flexographic printing there is a strong desire to use printing sleeves. Printing sleeves offer better register and faster changeovers than plates that are directly mounted onto a press cylinder. Various imaging systems are used to form images on printing sleeves. For example, computer-to-plate systems (also known as CTP systems) are used to form images on printing sleeves. A plurality of imaged printing sleeves is subsequently provided to a printing press to create various printed articles. Each article typically includes a plurality of images. It is important that the plurality of images be accurately aligned with respect to one another to ensure accurate registration. 
         [0004]    One challenge associated with the use of printing sleeves is the need to replace the entire sleeve when one part of the sleeve needs to be changed. Portions of printing sleeves may require replacement for various reasons. For example, various portions of the printable surface of a printing sleeve may require replacement due to wear or damage to those portions. A portion may also be changed because of a desire to change the image content of that portion. When printing plates are directly mounted onto press cylinders, a desired portion can be readily separated and replaced. This is not easily done with printing sleeves, especially with printing sleeves that include continuous printable surfaces. One possible solution is to divide the printing sleeve into a plurality of segments which are mounted onto the press cylinder. However, the registration requirements that are required by a printing operation makes it difficult to replace a given sleeve segment and maintain registration. 
         [0005]    There remains a need for a printing sleeve made up of a plurality of segments that can be mounted on and demounted from a print cylinder while maintaining a required registration of the printing operation. 
         [0006]    There is also a need for effective and practical methods of making a printing sleeve that includes segments that can be replaced on-press without adversely impacting print registration. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly, according to one aspect of the present invention, an apparatus for preparing a printing sleeve includes a support; a drum rotatably mounted on the support, the drum adapted to receive the printing sleeve; a radiation source; a controller programmed to operate the radiation source to emit at least one radiation beam to cut the printing sleeve mounted on the drum into a plurality of separate and interlocking printing sleeve portions; and an imaging head comprising a plurality of individually addressable imaging channels, wherein the imaging channels are operated to direct imaging beams to form an image on the printing sleeve mounted on the drum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Embodiments and applications of the invention are illustrated by the attached non-limiting drawings. The attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
           [0009]      FIG. 1  shows one example embodiment of a printing sleeve according to the present invention; 
           [0010]      FIG. 2A  shows a plan view of the printing sleeve of  FIG. 1 ; 
           [0011]      FIG. 2B  shows an end view of the printing sleeve of  FIG. 1 ; 
           [0012]      FIG. 2C  shows a printing sleeve as per an example embodiment of the invention; 
           [0013]      FIG. 3  is a view of an apparatus for making a printing sleeve according to the invention; and 
           [0014]      FIG. 4  shows a printing sleeve as per an example embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Throughout the following description specific details are presented to provide a more thorough understanding to persons skilled in the art. However, well-known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
         [0016]    Dividing a printing sleeve into various portions requires angular accuracy to maintain print registration. The required degree of accuracy is presently difficult to achieve without the use of time consuming optical registration methods. The present invention replaces this time consuming process with a self registering process, based on the averaging effect of patterns made up of a number of interlocking features. This increased accuracy can be generated by a plurality of mating projections and notches (e.g. teeth and recesses) that are disposed at the mating surfaces of the coupled sleeve portions. This accuracy arises because inaccuracies associated with individual projections or notches are averaged over the entire plurality of mating projections and notches. Such arrangements of projections and notches are sometimes used by Hirth couplings or Curvic couplings in machine tool applications. See, for example, U.S. Pat. No. 4,353,271. The potential accuracy of this form of coupling can be demonstrated by the Moore 1440 Index, which operates on this principle and has better than 0.1 arc second accuracy. 
         [0017]    Printing sleeves such as flexographic printing sleeves typically include polymer or thin metal core layers that are coated with a layer of modifiable material. The modifiable material is changed in an imagewise fashion as part of a process used to produce a printable surface of the printing sleeve. The modifiable layer can include various polymers and can be modified in various ways to produce a printable surface. For example, photopolymers are typically imagewise exposed with radiation (e.g. actinic radiation) and additionally processed via conventional thermal or chemical techniques to produce printable and non-printable areas. Modifiable materials can be ablated to form a printable surface. Various masks can be used to help define printable and non-printable areas. For example, a mask can be part of a separate film that is superimposed onto the modifiable layer during its exposure. Alternatively, the mask may be an integral component of the printing sleeve. A printing sleeve can also include various layers. 
         [0018]      FIG. 1  shows a printing sleeve  10  that includes multiple separate sleeve portions  10 A and  10 B as per an example embodiment of the invention. Printing sleeves typically include a hollow, substantially cylindrical layer. In this example embodiment of the invention, printing sleeve  10  includes a core layer  13  and modifiable layer  12 . Core layer  13  is generally stiffer and more dimensionally stable than modifiable layer  12  and provides a good structure with which to establish the required registration accuracy. Each of core layer  13  and modifiable layer  12  can include one or more layers. For example, core layer  13  can include various cushion layers to achieve a desired print effect or other various layers to establish a desired print size known in the art as a print repeat. Each of core layer  13  and modifiable layer  12  can be made up of different component parts. For example, core layer  13  can include fiberglass. Modifiable layer  12  can include additional mask layers. 
         [0019]    Sleeve portion  10 A is coupled to sleeve portion  10 B by a pattern  15  of interlocking features including projections  16  and notches  18 . In this example embodiment of the invention, projections  16  and notches  18  are arranged along a circumferential direction  20  of printing sleeve  10 . In this example embodiment of the invention, projections  16  and notches  18  are integrally formed into sleeve portions  10 A and  10 B. In some example embodiments, projections  16  and notches  18  are formed on both ends of a sleeve portion to allow it to be interlocked with a plurality of additional sleeve portions. 
         [0020]      FIGS. 2A and 2B  show various views of sleeve portions  10 A and  10 B of  FIG. 1 . In this example embodiment of the invention, projections  16  and notches  18  are “tapered” interlocking features. Tapered interlocking features can include various shapes such as rounded, triangular or trapezoidal shapes. Tapered interlocking features can be used to reduce clearances between the sleeve portions  10 A and  10 B when they are coupled. This is especially relevant when pattern  15  is cut into printing sleeve  10  to form segments corresponding to sleeve portions  10 A and  10 B. Tapered interlocking features can be used to compensate for the kerf of the cut used to form the segments. A “rectangular” or non-tapered feature profile would have angular “play” equal to twice the kerf. Although tapered interlocking features will also cause sleeve portions  10 A and  10 B to move closer to one another after cutting by an amount proportional to the kerf, registration is still maintained. 
         [0021]    Tapered interlocking features can be used to provide increased surface areas between corresponding interlocked projections  16  and notches  18 . Larger surface areas can be used to reduce stresses between the interlocked sleeve portions when they are used in operation. High stress can damage projections  16  and notches  18  which can adversely affect the registration accuracy required between the sleeve portions. Interlocking surfaces can be tapered in one or more directions.  FIGS. 2A and 2B  show that tapered projections  16  and notches  18  include surfaces  22  that are skewed to a cylindrical axis  24 .  FIG. 2B  shows that surfaces  22  extend radially inwards towards cylindrical axis  24 . Surfaces  22  are coincident with a plane (not shown) that includes cylindrical axis  24 . 
         [0022]    Other example embodiments of the invention can use projections  16  and notches  18  that include other shapes. In some embodiments interlocking pattern profiles employed in Hirth couplings or Curvex couplings can be used. The choice of shape of a given projection  16  and a corresponding mating notch  18  can be chosen in accordance with the method that is employed to produce these features. Regardless of the manufacturing method used to produce a given projection  16  or notch  18 , inaccuracies associated with that method are averaged out by forming a plurality of projections  16  or notches  18 . 
         [0023]    The number of projections  16  and notches  18  in pattern  15  can vary in accordance with various factors. Factors can include a size characteristic of sleeve  10  such as its diameter. In some example embodiments of the invention, pattern  15  is arranged over at least half of the circumference of printing sleeve  10  to average out inaccuracies associated with each of the formed projections  16  and notches  18  in pattern  15 . In some example embodiment of the invention, a first projection  16  can be diametrically opposed from at least one of an additional projection  16  and a notch  18 . In some example embodiments of the invention, pattern  15  can comprise a plurality of groups of one or more projections  16  and notches  18 . 
         [0024]    The plurality of groups can be arranged over at least half of the circumference of printing sleeve  10 . Projections  16  and notches  18  can include complimentary shapes. Pattern  15  can include a repeating pattern. Projections  16  and notches  18  can be arranged regularly or irregularly in pattern  15 . An irregular pattern  15  can include various projections  16  and notches  18  that have different characteristics than other projections  16  and notches  18  in pattern  15 . Different characteristics can include different sizes or shapes or different spacings between adjacent projections or adjacent notches. Irregular patterns  15  can be used to couple a sleeve portion  10 A to sleeve portion  10 B in a single orientation which can be used to ensure that the portions are correctly assembled with respect to each other. 
         [0025]      FIG. 4  shows an alignment projection  17  and an alignment notch  19 . When a regular pattern of projections  16  and notches  18  are cut to form a pattern  15 , there is a possibility of radial misalignment. Using a feature such as alignment projection  17  and alignment notch  19  prevents radial misalignment when joining sleeve portions  10 A and  10 B. 
         [0026]    Sleeve portions  10 A and  10 B can be formed in various ways. In this example embodiment of the invention, sleeve portions  10 A and  10 B are made by cutting printing sleeve into various segments. Interlocking pattern  15  is cut into sleeve  10  to form sleeve portions  10 A and  10 B. Various methods can be used to cut printing sleeve  10 . These methods can include laser cutting. 
         [0027]      FIG. 3  shows a partial schematic view of an apparatus  30  used to form sleeve portions  10 A and  10 B as per an example embodiment of the invention. In this example embodiment of the invention, apparatus  30  is also used to form images on printing sleeve  10 . Computer-to-plate imaging systems such as the Kodak ThermoFlex manufactured by Kodak Graphic Communications Canada Company, British Columbia, Canada have been used to form images on flexographic printing sleeves. Apparatus  30  includes a support  31 . Apparatus  30  also includes a headstock  32 , tailstock  34  and a sleeve support  36  rotatably coupled between the two. Sleeve support  36  comprises a cylindrical body (e.g. a drum) that accurately fits sleeve  10 . Sleeve support  36  is pressurized to facilitate the mounting and demounting of sleeve  10  from sleeve support  36 . In this example embodiment of the invention, sleeve support  36  includes ports  38  which allow a pressurize fluid (e.g. air) to expand sleeve  10  to facilitate its mounting or demounting (sleeve portion  10 B shown as a partially broken view to show ports  38 ). Sleeve support  36  can be moved relatively to at least one of headstock  32  and tailstock  34  to assist in the mounting or demounting of printing sleeve  10 . 
         [0028]    Apparatus  30  includes imaging head  40  that is movable with respect to sleeve support  36 . In this example embodiment of the invention, imaging head  40  is mounted on movable carriage  42 . Carriage  42  is moved along guides  44  to cause imaging head  40  to be moved along a path aligned with an axis of the cylindrical sleeve support  36 . In this example embodiment of the invention, imaging head  40  moves along a path aligned with sub-scan axis  46 . Motion system  50  is used to provide relative motion between imaging head  40  and sleeve support  36 . Motion system  50  (which can include one or more motion systems) includes any suitable prime movers and transmission members needed for the required motion. In this example embodiment of the invention, motion system  50  is used to move sleeve support  36  along a path aligned with main-scan axis  48  while moving imaging head  40  along a path aligned with sub-scan axis  46 . Separate motion systems can also be used to operate different systems within apparatus  30 . 
         [0029]    Imaging head  40  includes a radiation source (not shown), such as a laser. Imaging head  40  is controllable to direct one or more imaging beams (not shown) capable of forming image on printing sleeve  10 . The imaging beams generated by imaging head  40  are scanned over printing sleeve  10  while being image-wise modulated according to image data representing the image to be written. One or more imaging channels are driven appropriately to produce imaging beams with active intensity levels wherever it is desired to form an image portion. Imaging channels not corresponding to the image portions are driven so as not to image corresponding areas. Images can be formed on printing sleeve  10  by different methods. For example, a property or characteristic of the modifiable layer  12  can be changed when irradiated by an imaging beam. An imaging beam can be used to ablate a surface of printing sleeve  10  to form an image. An imaging beam can be used to facilitate a transfer of an image forming material to a surface of printing sleeve  10  to form an image. Imaging head  40  can include a plurality of channels that can be arranged in an array. An array of imaging channels can include a one dimensional or two dimensional array of imaging channels. An imaging beam can undergo a direct path from a radiation source to printing sleeve  10  or can be deflected by one or more optical elements towards printing sleeve  10 . 
         [0030]    Controller  60 , which can include one or more controllers is used to control one or more systems of apparatus  30  including, but not limited to, various motion systems  50  used by sleeve support  36  and carriage  42 . Controller  60  can also control sleeve handling mechanisms that can initiate the loading and/or unloading of printing sleeve  10  to and/or from sleeve support  36 . Controller  60  can also provide image data to imaging head  40  and control imaging head  40  to emit imaging beams in accordance with this data. Various systems can be controlled using various control signals and/or implementing various methods. Controller  60  can be configured to execute suitable software and can include one or more data processors, together with suitable hardware, including by way of non-limiting example: accessible memory, logic circuitry, drivers, amplifiers, A/D and D/A converters, input/output ports and the like. Controller  60  can comprise, without limitation, a microprocessor, a computer-on-a-chip, the CPU of a computer or any other suitable microcontroller. 
         [0031]    Apparatus  30  includes a sleeve cutter  70 . Sleeve cutter  70  can include a radiation source operable for emitting a radiation beam for cutting printing sleeve  10 . In this example embodiment of the invention, sleeve cutter  70  includes laser  72 , such as a CO 2  laser that is mounted on carriage  42 . Laser  72  generates spot  74  (exaggerated for the sake of clarity) using a focusing lens (not shown). Imaging head  40  and laser  72  can move towards and away from sleeve support  36  to accommodate different sleeve diameters. Other devices may also be used as sleeve cutter, for example narrow, high pressure water jets. 
         [0032]    As stated above, motion system  50  is used to establish relative motion between printing sleeve  10  and imaging head  40  as images are formed on printing sleeve  10 . Motion system  50  can also be used to establish relative motion between printing sleeve  10  and laser  72  as printing sleeve  10  is cut into sleeve portions  10 A and  10 B. Motion system  50  and laser  72  can be controlled by controller  60 , or the like to cut printing sleeve  10  in accordance with cutting data provided to the controller. 
         [0033]    In this example embodiment of the invention, pattern  15  is cut once printing sleeve  10  is mounted on sleeve support  36 . When the thickness of modifiable layer  12  is large compared to the thickness of core layer  13 , it may be desirable to clear a band  76  (not shown in  FIG. 3 ) in modifiable layer  12  before cutting pattern  15 . This can reduce the requirements for the depth of focus on laser  72 . The cleared band can follow the shape of pattern  15  or can assume another shape such as the “circumferential” band  76  in  FIG. 2C . In some example embodiments of the invention, band  76  is formed with laser  72 . In some embodiments of the invention, band  76  is formed by imaging head  40 . In some systems, imaging head  40  is used to engrave or ablate modifiable layer  12  to form band  76 . 
         [0034]    In some example embodiments of the invention, pattern  15  is cut after imaging head  40  forms an image on print sleeve  10 . In these embodiments, image data can be modified to account for the kerf of the various cuts so as to properly position images on corresponding sleeve portions  10 A and  10 B. In other example embodiments of the invention, pattern  15  is cut before imaging head  40  forms an image on print sleeve  10 . After cutting, the sleeve portions  10 A and  10 B can be moved together to abut one another prior to imaging. In this way, a replacement sleeve portion can be made to interchange with an existing sleeve portion with a high degree of accuracy. 
         [0035]    Controller  60  can be programmed to form pattern  15 . Pattern  15  can be formed in non-image areas. Typically, when printing sleeves are used to print media for packaging applications, bands or “lanes” of images are formed on printing sleeves. Gutters of non-image areas exist between the lanes. A pattern  15  can be formed in these gutters. 
         [0036]    Those skilled in the related art will quickly realize that the present invention can also be incorporated into a dedicated sleeve cutting device. Such devices can have a similar construction to apparatus  30 , with the obvious exception that imaging head  40  would not be present. 
         [0037]    Effective cutting of sleeve  10  can depend on the power and beam quality of the CO 2  laser, as well as the focal length and type of focusing lens. Cutting speed is proportional to the power of the laser as well as the composition and thickness of the sleeve. Small sleeves with diameters on the order of 100 mm can be typically cut efficiently with laser power in the order of 100 W, although acceptable cuts may be achieved with powers as low as 20 W. The laser beam quality should be good, with M 2  less than 2. The focusing lens is preferably aspheric. Since the desired depth of focus is typically a few millimeters, a lens with an f/# ranging from f/5 to f/10 is suitable. Latitude can be taken with the selection of the focal length of the lens, as the present inventor believes that the cutting is primarily dependant on f/# rather than focal length. 
         [0038]    By way of non-limiting example, the following possible configuration is provided. For convenience, a CO 2  laser lens with a focal length of 25.4 mm (part number 10ZAL254 from ULO Optics Ltd (www.ulooptics.com)) was selected to be used with a Synrad Evolution 100 100 W laser (www.synrad.com). This particular laser has an M 2 =1.2 and a 4 mm beam diameter. The effective f/# associated with this combination is about f/6. The theoretical depth of focus is about 1 mm but the actual depth of focus is about 2 mm because of the “self guidance” effect, which is known to those skilled in the art of CO 2  laser cutting. The spot size (and associated kerf) is approximately 0.1 mm. Register accuracy of about 10 microns can be achieved using these parameters to cut a pattern  15  with trapezoidal feature profiles with a pitch of about 10 mm and height of about 3 mm. Different core materials including fiberglass were easily cut. 
         [0039]    While example embodiments of the invention have employed CO 2  lasers, other types of lasers such as laser diodes, or diode pumped YAG lasers can also be used. CO 2  lasers can be advantageous since typical sleeve materials respond well to CO 2  laser cutting. A CO 2  laser can deform or damage a sleeve support onto which the sleeves are mounted during cutting. To minimize potential damage to the sleeve support, the sleeve support can include a heavy polished copper layer (about 1 mm thick). Polished copper reflects most of the CO 2  laser radiation and conducts heat well to help reduce the effect of the laser on the sleeve support. An alternative is to use disposable intermediate sleeves (similar to sleeves used to build up print cylinders to a required diameter during printing). Such disposable sleeves can be discarded after a number of cutting jobs. Their longevity can be increased when subsequent cuts are made in different locations. 
         [0040]    While a primary function of the described methods and systems is to cut toothed or serrated patterns for registration, it is readily apparent that that the same methods and systems can be used to cut sleeves to length, or cut an end of a sleeve portion that does not require a registration pattern. Cutting sleeves to length prior to use can reduce demands on inventory, as only a few specific lengths need to be stocked. 
         [0041]    It is to be understood that the exemplary embodiments of the invention are merely illustrative and that many variations of the described embodiments can be devised by those skilled in the art without departing from the scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10  printing sleeve 
           10 A sleeve portion 
           10 B sleeve portion 
           12  modifiable layer 
           13  core layer 
           15  pattern 
           16  projections 
           17  alignment projection 
           18  notches 
           19  alignment notch 
           20  circumferential direction 
           22  surfaces 
           24  cylindrical axis 
           30  apparatus 
           31  support 
           32  headstock 
           34  tailstock 
           36  sleeve support 
           38  ports 
           40  imaging head 
           42  carriage 
           44  guides 
           46  sub-scan axis 
           48  main-scan axis 
           50  motion system 
           60  controller 
           70  sleeve cutter 
           72  laser 
           74  spot 
           76  band