Abstract:
The method and apparatus for registration of a lithographic plate includes a means for ensuring that the plate is in contact with registration pins on imaging and bending devices and a means for finely adjusting an initial position of the plate with respect to registration pins on the bender to bring the plate in registration with the specified plate lock-up position on the press cylinder. Three separate electrical circuits ensure that the plate is in contact with the registration pins if each separate circuit is able to conduct current between the respective pins and the plate. The fine adjustment is accomplished by rotation of eccentric registration pins until the plate is within tolerance.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/808,872 filed May 26, 2006. 
     
    
     FIELD OF TECHNOLOGY  
       [0002]     The technology disclosed in this specification is in the field of registration of an imaged lithographic plate for mounting on a corresponding press cylinder. More particularly, the technology embodies an imaging device and a plate bending device by which the registration method can be practiced.  
       BACKGROUND  
       [0003]      FIG. 1  illustrates a prior art method of registration of a lithographic plate with an imager, plate bender, and press cylinder. The continuity of registration is maintained from the imaging device, to the bender device, and to the emplacement of the plate on a press cylinder.  
         [0004]     The plate bender often performs a punching function, as well as the bending function. But the plate bending function and the plate punching function can be performed by separate devices.  
         [0005]     The terms plate bender, bender, plate punch/bender, or similar nomenclature means a device that performs both the punching and the bending function; a separate device for performing the punching function; and a separate device for performing the bending function.  
         [0006]     The prior art lithographic imaging device uses three points of contact (i.e., three fixed pins) for positioning a lithographic printing plate on the imager. See  FIG. 1  at -A-. The plate is positioned in contact with the locations of each of the three fixed pins prior to imaging, but due to out of tolerance issues and other issues this does not always happen. See  FIG. 1  at -B-. Once the plate is positioned firmly against the fixed pins, the plate is imaged. See  FIG. 1  at -C-. Registration is created by the relationship of the image on the plate to the three fixed pins on the imaging device. This results in an image that is square with respect to the three fixed pins and parallel to the two fixed pins that reside on one edge of the imaged plate.  
         [0007]     After the plate has been imaged, a punch/bender device positions the plate on the punch/bender by bringing the plate into contact with three fixed pins on the punch/bender. The relative locations of the fixed pins on the punch/bender are substantially the same as the relative locations of the fixed pins on the imaging device. See  FIG. 1  at -D-. The imaged plate is positioned against the three fixed pins until the plate is in register (i.e., in contact with the three fixed pins). The image is square to the pins and it is in the same register position as it was in the imaging device, thereby creating continuity of register within  0 . 001  inch. See  FIG. 1  at -E-. After the imaged plate is brought into contact (i.e., in register) with each of the three pins, the punch/bender punches and bends the plate. The result is an image that is square to the bend and is in register with the press manufacturer&#39;s specifications. See  FIG. 1  at -F-.  
         [0008]     Register requires the image to be substantially square to the formed bends and notch locations on the plate. See  FIG. 1  at -G-. The continuity of register becomes the relationship between the image to the formed bends and register notch locations required by the press manufacturer. See  FIG. 1  at -H-. The lead and tail bends must be straight and parallel with each other for maximum plate on press register and performance. See  FIG. 1  at -I-.  
         [0009]     The foregoing registration system provides relatively accurate registration for most printing applications. However an embodiment of a back-up system described in this specification provides (i) verification of image register tolerance, (ii) fine adjustment of the position of the plate if its registration falls outside the tolerance limits of the press manufacturer, and (iii) ensurance that the plate is in contact with all three pins.  
       SUMMARY  
       [0010]     An embodiment of the process of registration of a lithographic plate begins with placing the plate on an imaging device. A first edge of the plate is positioned in contact with one fixed registration pin on the imaging device and a second edge adjacent the first edge of the plate is positioned in contact with two fixed registration pins on the imaging device. The three fixed registration pins on the imaging device are in locations on the imaging device that are substantially the same as the relative locations of the manufacturer&#39;s specified plate lock-up position on a corresponding press cylinder. An operator of the imaging device or a computing device such as a PLC receives an indication that the plate is in contact with all three registration pins and signals the imaging device to image the plate. If on the other hand the plate is not in contact with all three registration pins the PLC, for example, signals the imaging device to move the plate into contact with all three registration pins. Upon sensing full contact between the plate and each of the three pins, the imaging device images the plate. Plate contact with all three fixed registration pins establishes registration between the image and the three fixed registration pins. The imaged plate is transported to a lithographic processor and the image is processed. Another phase of the embodiment of the process of registration begins with transporting the imaged plate to a bending device. The bender extends three eccentric rotatable registration pins above a bed on the bending device. A first edge of the plate is meant to be positioned in contact with one eccentric rotatable registration pin and a second edge adjacent the first edge of the plate is meant to be positioned in contact with two eccentric rotatable registration pins on the bending device. The three eccentric rotatable registration pins are also meant to be in locations that are substantially the same as the relative locations of the fixed registration pins on the imaging device. However, three point contact may not occur and the registration pins may be in locations that are not substantially the same as the relative locations of the fixed registration pins on the imaging device. These conditions may be due to, for example, out of tolerance pin sizes or pin locations. The condition of failure to contact the plate with all three pins is corrected by sensing that there is a full contact, or an out of contact, condition. In an out of contact condition, the plate is moved into full contact. After full contact is achieved, a sensing device senses whether the registration pins may be in locations that are not substantially the same as the relative locations of the fixed registration pins on the imaging device. If this condition exists, the eccentric rotatable registration pins are rotated and the plate is thereby moved from its initial position to another position that brings the image on the plate into registration with the manufacturer&#39;s specified plate lock-up position on the press cylinder. After the plate is in registration, the three eccentric rotatable registration pins are retracted below the bed of the bending device. The bending device then punches and bends the imaged plate and the bent and punched plate is transported to the press cylinder.  
         [0011]     The apparatus for determining whether the lithographic plate is in contact with the pins on an imaging or bending device is a sensor circuit. The circuit has a source of current connected to a registration pin and to a lithographic plate. Current flows through the circuit when the plate is in contact with the pin. And does not flow when the plate is not in contact with the pin. In most cases the sensor is comprised of three pins each pin having a separate circuit. Current flow in each circuit must be detected prior to generation of a signal that triggers the imager or bender to initiate an operation. If current is not detected in each separate circuit, a signal is generated to the imager or bender to move the plate into contact with each of the pins. Two robotic grippers on each side of the plate are often used to move the plate under the direction of a vision system.  
         [0012]     A system for correcting positional errors of the plate on a bender with respect to a target position uses eccentrically rotatable pins that are in contact with a plate on the bed of a bending device. The rotatable pins are in substantially the same relative positions as the pins of an imaging device. Consequently, the eccentrically rotatable pins may rotated to move the plate in register with specified tolerances of a press cylinder. The system has three pins, one of which is for contact with a first edge of the plate and the other two of which are for contact with a second edge of the plate, the first and second edges adjacent one another. Each pin has a shaft offset from a central axis, an upper shaft extension in line with the central axis, a drive means for rotating the shaft and the upper extension of the shaft, whereby each pin is eccentrically rotated. Alignment marks are in registration with fixed pins of the imaging device, the alignment marks are indicative of a targeted plate position relative to an out of register plate position on the bed of the bending device. The targeted plate is positioned in contact with the upper shaft extensions. One or more cameras scan the alignment marks, compare the alignment marks to the targeted position, and indicate an in-register, or out of register, plate position when the target plate position is respectively inside, or outside, of the alignment marks. Separate cameras scan each alignment mark. The plate is moved from an out of register position to an in register position by rotation of the upper shaft extensions until the plate alignment marks are positioned in-line with the targeted plate position. Each pin is moveable in clockwise or counter-clockwise rotation and each pin is rotable independently of the other pins. There is a means for extending and retracting the pins above and below the bed of the bender. The process corrects an out of register position of the plate with respect to the target position of the plate by determining the extent of the out of register plate position based upon camera images of the position of the plate relative to the alignment marks, calculating error corrections to move the plate into register, sending independent error correction signals to each pin, and rotating each pin in accordance with its respective error correction signal. One or more iterations of the process is performed until the error is corrected and the plate is in register. Prior to commencing a new cycle of correction the pins must be rotated from pin correction positions back to home pin positions. The process can use a triangulation algorithm to calculate error corrections. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0013]      FIG. 1  is a description of a prior art registration system.  
         [0014]      FIG. 2  is a plan view of an embodiment of three point plate registration system.  
         [0015]      FIG. 3  is a schematic view of a plate positioned for registration in an embodiment of the three point plate registration system.  
         [0016]      FIG. 4  is a schematic view of a plate after registration in an embodiment of the three point plate registration system.  
         [0017]      FIG. 5  is a schematic view of the movement of pins in the embodiment of  FIG. 3 .  
         [0018]      FIG. 6  is a elevation view of an embodiment of a registration pin assembly in a pin up position.  
         [0019]      FIG. 7  is a elevation view of an embodiment of a registration pin assembly in a pin down position.  
         [0020]      FIG. 8  is a plan view of an embodiment of the three point plate registration system in the context of a punch/bender. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0021]     Alignment, i.e. registration, of the plate on the press cylinder is critical. In fact, high rotational speed printing operations often require a registration tolerance within 0.001 of an inch. In the final analysis, it is the image on the plate relative to the press manufacturer&#39;s specified plate lock-up position on the press cylinder that must be “square.” 
         [0022]     There are two important steps during pre-press preparation of a plate, each of which can cause the image to be out of square when the plate is locked-up on the press. The two steps are shown in  FIG. 2  at A through D, an embodiment of three point plate registration system  22 .  
         [0023]     The first step is positioning the plate against the three, stationary registration pins  401 A, B, and C on imager  18  and then imaging the plate, as shown in  FIG. 2  at A through C. Prior to implementation of the second step, the imaged plate is transported by a belt transporter  8  or by hand from imager  18  to lithographic processor  9 , which processes the image. After processing, the plate is transported to punch/bender  19 . In the second step, punch/bender  19  bends and notches the imaged plate in a manner that will ensure the image on the plate remains in registration with the three registration pins  401 A, B, and C of imager  18 , as shown in  FIG. 2  at A through D. In other words, continuity of location of the image on the plate relative to imager&#39;s three registration pins  401 A, B, and C during the punching/bending operation is imperative.  
         [0024]     Prior to imaging, the plate is positioned on imaging module  18  by placing it in contact with the imager&#39;s three fixed registration pins  401 A, B, and C.  FIG. 2  at A through C. Once firmly in contact with the three fixed registration pins  401 A, B, and C, the plate is imaged. Therefore, registration is established by the relationship between the image and the three pins  401 A, B, and C.  
         [0025]     As shown in  FIG. 2  at D, punch/bender  19  also uses substantially the same three registration pin locations  401 A, B, and C as does imaging module  18 , thereby maintaining continuity of the register relationship between the image and the three pins  401 A, B, and C. Registration of the image on the plate with the punch/bender module  19  is dependent upon the proper location of the plate relative to the three pins  401 A, B, and C of punch/bender module  19 . The imaged plate is positioned in contact with the three pins  401 A, B, and C on punch/bender  19 , as was done with imaging module  18 . Registration is accomplished, when the image is square with the three pins  401 A, B, and C on the punch/bender  19 . Normally, the image will be in registration when the plate is firmly positioned against all three of the registration pins  401 A, B, and C. However, in some cases the initial position of the plate with respect to pins  401 A, B, and C needs fine adjustment to bring the image into registration within the requisite press manufacturer&#39;s tolerance, typically of 0.001 inches. As previously mentioned, the goal is to create and maintain continuity of registration between the image and pins  401 A, B, and C. Achievement of fine adjustment is accomplished by a mechanism for moving pins  401 A, B, and C. After registration on the punch/bender  19 , the imaged plate is notched and bent so that the image is square to the bend and the notch and in registration with the manufacturer&#39;s specified plate lock-up position on the press cylinder. Registration of the imaged plate and the bent and notched imaged plate must be identical to each other, within the foregoing tolerance, and both must be identical to the manufacturer&#39;s specified plate lockup position on the press cylinder.  
         [0026]     In an embodiment, a back-up system is employed to doubly ensure that the plate is in contact with all imager  18  and/or punch/bender  19  pins  401 A, B, and C. The back-up system wires each pin  401 A, B, and C and the plate to a source of current. Each pin-plate combination has its own separate circuit. A current will flow between a pin  401 A, B, or C and the plate, if the plate is in contact with the pin  401  A, B, or C. If, and only if, the plate is in contact with all three pins  401 A, B, and C will a current flow between each pin and the plate. If this does not occur, the plate will not be imaged until it is moved in contact with all of the pins  401 A, B, and C and the imager  18  indicates that current flow exists between each pin  401 A, B, and C and the plate. This same system may also be used on the punch/bender  19 .  
         [0027]      FIGS. 3, 4 , and  5  illustrate an embodiment of the three point pin registration system  22 . This embodiment is comprised of rotatable pins  401  A, B, and C on punch/bender  19 . In contrast, the pins  401 A, B, and C of imager  18  are stationary. The punch/bender&#39;s rotatable pins permit fine adjustment of the plate to obtain repeatable registration within the press manufacturer&#39;s tolerance limits. The moveable pin system is another back-up system to doubly ensure that the image remains in registration. It emulates the three pin system used by imager  18  with the exception that pins  401  of punch/bender  19  rotate eccentrically for making fine adjustments to the plate orientation. Each pin in punch/bender  19  is in the same location relative to the three pins of imager  18 .  FIG. 3  is a diagrammatic illustration of a plate positioned for registration on punch/bender  19 .  FIG. 4  is a diagrammatic illustration of the embodiment of  FIG. 3  after registration.  FIG. 5  is a diagrammatic illustration of the movement of pins  401 A, B, and C of plate registration system  22 .  
         [0028]     The three pins  401 A, B, and C extend above the level of the bed of punch/bender module  19 , when in use. When not in use, pins  401  A, B, and C are retracted below the level of the bed.  
         [0029]     The plus sign (“+ sign”) in  FIGS. 3, 4 , and  5  represents the location of central axis  408 A, B, and C of pin shafts  407 A, B, and C ( FIG. 6  also shows the pin shafts). Pin shafts  407 A, B, and C are driven by drive motors  406 A, B, and C ( FIG. 6 ). Drive motors  406 A, B, and C are mounted below the level of the bed of punch/bender  19 . Eccentric registration pins  401 A, B, and C are an upper extension of pin shafts  407 A, B, and C. Eccentric registration pins  401 A, B, and C are rotated eccentrically by pin shafts  407 A, B, and C. Eccentric registration pins  401 A, B, and C rotate eccentrically due to their off-set from central axis  408 A, B, and C of pin shafts  407 A, B, and C. Pin shafts  407 A, B, and C are in-line with central axis  408 A, B, and C and are, therefore, rotating concentrically with central axis  408 A, B, and C.  
         [0030]     In  FIGS. 3 and 8 , alignment marks  405 A and B (shown as circles on  FIGS. 3-5 ) are used to indicate the desired target position of the plate relative to the initial position of the plate with respect to its contact with each of pins  401 A, B, and C of punch/bender  19 . Alignment marks  405 A and B are placed on the lithographic plate during imaging by imager  18 . Alignment marks  405 A and B are in registration with the imager&#39;s pins  401 A, B, and C. Cameras  403 A and B on punch/bender  19  scan alignment marks  405 A and B on the positioned plate and compare the alignment marks  405 A and B with targets  409 A and B (shown as squares on  FIGS. 3-5 ). An in-registration position is indicated when target squares  409 A and B are inside alignment mark circles  405 A and B. The comparison, reveals that the initial position of the plate (indicated by alignment marks  405 A and B) is some slight distance from pre-determined target positions  409 A and B. This means that the plate is not in registration with pre-determined target positions  409 A and B. Therefore, a correction is made to the plate position shown in  FIG. 4 . The correction is made by slight rotation of eccentric pins  401 A, B, and C by drive motors  406 A, B, and C. Rotation of pins  401 A, B, and C moves the plate&#39;s position until plate alignment marks  405 A and B are positioned in-line with the target positions  409 A and B. The slight rotation of pins  401 A, B, and C can be seen by comparison of the + signs in  FIGS. 3 and 4 . In  FIG. 3 , one of the bars of the + signs is horizontal and the others are vertical. After correction as shown in  FIG. 4 , the + sign for pin  401 A has rotated counterclockwise approximately 30°, the + sign for pin  401 B has rotated clockwise approximately 30°, and the + sign for pin  401 C has rotated clockwise approximately 45°.  
         [0031]      FIG. 5  more clearly illustrates the movement of the plate from an initial out-of-register position in  FIG. 3  to an in-register position in  FIG. 4 . Dotted line circle in  FIG. 5  indicates the initial out-of-register position of pins  401 A, B, and C and the solid line indicates the in-register position. The circular arrow segments indicate the clockwise or counter-clockwise direction of pins  401 A, B, and C, as the case may be. The straight arrow segments indicate the direction of movement of the plate as a result of rotation of eccentric pins  401 A, B, and C.  
         [0032]     Plate registration system  22  rotates each pin  401 A, B, or C independently of the other pins.  
         [0033]     System  22  may use an iterative process wherein the images from cameras  403 A and B of target positions  409 A and B relative to alignment marks  405 A and B on the plate  405 A an B are used to calculate out-of-registration errors and, based upon the out-of-registration errors, calculate error correction signals using a triangulation algorithm. System  22  feeds back independent error corrections to each independent drive motor  406 A, B, and C and each independent drive motor eccentrically rotates its associated pin  401 A, B, and C a number of degrees commensurate to its correction error signal. This sequence is followed by another iteration of the sequence until the error correction approaches -0- at which time the plate is in registration. Error correction signals may be sent to pin motors  406  in the form of a number of pixels needed to correct the error. If the motor  406  is a stepper motor, the number of pixels is translated into the number of rotational steps (or the number of degrees the motor must turn if it is not a step motor) the stepper motor must take to move the plate into an in-registration position. Other forms of signaling may be used to accomplish the same goal as well as other forms of drive motors  FIGS. 6 and 7 , respectively, illustrate registration pin assembly  420  in a “pin-up” position and a “pin-down” position. Registration pin assembly  420  is affixed to punch/bender module  19  by assembly mounting bracket  413 . Pin assembly  420  is also affixed to the punch/bender module  19  by stop block  414 . Dual rod cylinder assembly  412 A is responsible for moving pins  401 A, B, or C upwardly and downwardly. When dual rod cylinder assembly  412 A is signaled by system  1  to move its associated pin  401 A, B, or C into the up position, a pneumatic system on punch/bender  19  pumps air into dual rod cylinder  412 B and raises its associated pin  401 A, B, or C. When dual cylinder  412 B is signaled to return to the down position, a port opens the cylinder to the atmosphere and relieves the air pressure thereby allowing its associated pin  401 A, B, or C to move to the down position. Dual rod cylinder assembly  412 A has both dual cylinders  412 B and dual rods  412 C. Air cylinder  412 B is a moving part and cylinder rods  412 C are stationary parts. Cylinder rods  412 C are affixed to stationary platform  412 D. When air cylinders  412 B are actuated they move upwardly by pushing against stationary cylinder rods  412 C. Air cylinders  412 B are slidably engaged for vertical movement within slide  412 E. Drive motor  406  is attached to motor bracket  417 , which is attached to movable dual rod cylinder  412 B. Motor shaft  421  is fixedly connected within the lower end of coupler  411  and pin shaft  407  is connected within the upper end of coupler  411 . Pin shaft  407  is journaled in bearing housing  410 , the top of which extends into stop block  414  where bearing housing  410  is fixedly engaged, i.e., bearing housing  410  does not rotate. To cause its associated pin  401 A, B, or C to retract, i.e., move down, cylinders  412 B are opened to the atmosphere and it slides down slide  412 E and rods  412 C are retracted into cylinders  412 B and the pins  401 A, B, or C follow.  
         [0034]     If a pin  401 A, B, or C is rotated a number of degrees it must rotate back to its home position before beginning a new cycle of fine adjustment of the previously adjusted plate or another plate. A proximity sensor  419  is used as a homing device for accomplishing this task.  
         [0035]     Grippers  422  may be used to move the plate on imager  18  bed or on the punch/bender  19  bed. Grippers  422  are particularly useful to make positive contact between the plate and each of the three pins  401 A, B, or C on the punch/bender  19 . Grippers  422  are most generally operated by a robotic/vision system. An embodiment of the grippers used in the three point plate registration system  22 , grasps the plate as it moves onto the bed of the punch/bender  19  without assistance from an operator. For example, two grippers  422  may be used on each side of the plate. Grippers  422  close their hands on the plate and position the plate against pins  401 A, B, or C under the direction of the vision system. Gripper  422  may also assist movement of the plate into and out of imager  18  and/or punch/bender  19 .  FIG. 6  illustrates the bed of a punch/bender  19 , such as that described in U.S. Pat. No. 5,970,774.  FIG. 6  illustrates two grippers  422  holding a plate on one side, cameras  403 A and B, alignment marks  405 A and B on the plate, and pins  401  A, B, and C.  
         [0036]     Although the apparatus for registration of an imaged lithographic plate with an imager, plate bender, and press cylinder, and the process thereof, have been described with reference to the embodiments, those skilled in the art will recognize that numerous changes may be made in form and detail without departing from the spirit and scope of the apparatus and process.