Abstract:
A method of removing slip sheets from a printing plates stack is disclosed, wherein the slip sheets are interspersed between the printing plates includes moving at least two parallel energy chains. Each of the energy chains carries a plurality of vacuum cups over the stack of printing plates. The vacuum cups are attached to a top slip sheet placed on top of the printing plates stack and peel off sheets while vacuum is maintained and transferring the top slip sheet into a slip sheet bin.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 14/283,306, filed May 21, 2014, entitled VACUUM CONVEYOR FOR REMOVING SLIP SHEETS, by Guttmann et al.; the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     The invention relates to systems for handling flat sheets of media and more particularly to a system for holding and conveying a sheet from a stack of sheets. 
     BACKGROUND OF THE INVENTION 
     Plates, films, and proofing media for imaging systems, such as computer-to-plate (CTP) systems which are used in the graphic arts industry, are commonly stacked in boxes with slip sheet sheets interspersed between adjacent media sheets. The slip sheet primarily functions to protect the media surface from damage and to prevent adjacent media sheets from adhering to each other. The slip sheet is particularly important when the active emulsion on the media is either sticky or particularly sensitive to damage. 
     In the development of media for imaging tasks, particularly plates for lithographic printing operations, a number of often conflicting parameters such print run length, exposure sensitivity, exposure latitude and processing requirements must often be traded off against one another to achieve the best results. Some media have particularly good performance in relation to the abovementioned parameters, but suffer from extreme delicacy of the unexposed media emulsion. In a specific case, some LH-PI lithographic plates have a particularly delicate emulsion and may be scratched even by the action of removing the slip sheet. However, once exposed, the emulsion is durable and the plate images have good on-press performance. 
     Other commonly available media may have similar problems with delicate emulsion surfaces although, depending on the thickness and particular characteristics of the emulsion, scratches may or may not remain visible after subsequent processing. While not all media require special attention to how slip sheets are removed, the problem has been evident in a number of media products in the graphic arts industry. 
     In U.S. Pat. No. 5,655,452 (Blake et. al.) a slip sheet removal mechanism for removing a slip sheet from a plate is described. Briefly, the removal operates by activating peeler airflow to initiate separation between the slip sheet and the plate. A number of suction cups are pivoted into a location above the slip sheet and the stack of plates is moved to bring the slip sheet into engagement with the suction cups. A combination of movement of the plate stack and pivoting of the suction cups is used to separate the slip sheet and move it towards a pair of nip rollers that complete the removal operation. 
     There remains a need for better methods of handling slip sheets and there is a particular need for such a slip sheet removal mechanism that performs the removal without any damage to the media emulsion. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method of removing slip sheets from a printing plates stack is disclosed, wherein the slip sheets are interspersed between the printing plates includes moving at least two parallel energy chains. Each of the energy chains carries a plurality of vacuum cups over the stack of printing plates. The vacuum cups are attached to a top slip sheet placed on top of the printing plates stack and peel off sheets while vacuum is maintained and transferring the top slip sheet into a slip sheet bin. 
     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 
       In order to understand the invention and to see how it may be carried out in practice, examples will now be described, by way of non-limiting descriptions only, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a pair of adjustable lightweight energy chains with plurality of vacuum cups in process of removing slip sheet. 
         FIG. 2   a  shows a schematic illustration of a lightweight energy chains with plurality of vacuum cups attached to a slip sheet; 
         FIG. 2   b  shows a schematic illustration of a lightweight energy chains with plurality of vacuum cups attached to a slip sheet which is already separated from plate stack; 
         FIG. 3  shows a schematic illustration of a lightweight energy chains with plurality of vacuum cups removing a slip sheet from stack of printing plates; 
         FIG. 4  shows a schematic illustration of a lightweight energy chains holding plurality of vacuum cups moving the slip sheets into a bin; and 
         FIG. 5  shows a schematic at which the vacuum cups are brought into upper position and are switched off, readying the plate picker mechanism to pick up the a printing plate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     The invention is described in relation to a novel slip sheet removal system that is able to remove a slip sheet from a media without damaging the delicate emulsion. The method of slip sheet removal specifically avoids causing any relative movement between the slip sheet and the media emulsion. While of particular application in removing slip sheet from a stack of media in a graphic arts imaging system, the method and apparatus of the present invention may be useful in handling other types of media particularly where the media is delicate and susceptible to damage. The term “media” should be read to include all manner of media used in imaging and printing operations including, but not limited to, plates, films, paper and coated paper. 
     In a particular application, lithographic plates are often shipped in boxes of 25 or more plates with thin paper slip sheets interspersed between plates. In an automatic plate handling system the stack of plates are commonly placed in an access position from which they are loaded onto the imaging engine by some manner of automated handler. The slip sheets present a problem for automatic media handling in that they must be removed prior to imaging. The removal is often complicated by electrostatic attraction between the sheet and the media surfaces. Because the slip sheet is in intimate contact with the surface of the media a removal mechanism should also allow for the ingress of air under the slip sheet as it is removed. The slip sheet removal mechanism has the task of reliably separating the sheet from the plate and removing it to a location where it can be disposed of. 
     It has been observed by the inventors of the present invention that emulsion damage may occur during slip sheet removal whenever the edges are dragged or the material is bunched up and/or creased. During the slip sheet removal process, the shearing action between the slip sheet and the plate becomes localized to these creased areas and scuffing or scratching may occur. Further, it has been determined that if the slip sheet can be engaged without forming creases so that it remains flat, and the engaged sheet can be lifted away from the media in substantially this condition, scuffing may be completely avoided. 
     A solution for removing slip sheets  120  from printing plates  124  shown in  FIGS. 1-5 , deploys a vacuum conveyor adapted to attach to a slip sheet  120  covering the top printing plate  124  on a printing plates. The vacuum conveyor contains brackets which are attached to the links of a commercially available energy chain  108 . The brackets can be made for example from metal or plastic material. Those chains are available in various sizes and configurations. The most prominent manufacturer of these chains is IGUS (igus.com). The energy chain  108  are adapted to move back and forth as is shown by numeral  128 , by rotating via the two pairs of sprockets  132  and  136 . The distance between the pair of energy chains  108  is adjusted by altering the position of sprockets  132  on a spline, to comply with the width of printing plate  124 . A series of vacuum cups  116  are mounted on energy chain  108 . The energy chains are used to support the vacuum cups  116 , and as conduits for the air tube which feeds the vacuum cups  116 . 
     In order to prevent relative shearing motion between slip sheet and printing plate the energy chains  108 , vacuum cups  116  and slip sheet separator  104  are moved together in direction  129  after the slip sheet is picked up. 
       FIG. 2   a  shows a slip sheet  120  attached to vacuum cups  116  and energy chains  108 .  FIG. 2   b  shows a slip sheet attached to plurality of vacuum cups which has already been vertically separated from a plate stack. The energy chains advance in direction  128 . The motion is intermittent, i.e. the vacuum cups  116  move to the right (direction  128 ) but once the slip sheet  120  is peeled off the cups the energy chains rotate back to their home position ready to pick another slip sheet. The vacuum is applied to vacuum cups while the slip sheet is peeled off the vacuum cups. 
     Continuous motion of energy chains  108  is not required; motion is applied only when a slip sheet  108  is needed to be removed from printing plate  124 . In  FIG. 3  it is shown how the vacuum cups  116  pick up a slip sheet  108  from the top printing plate  124  of stack  304 . 
       FIG. 4  shows slip sheet  120  moved to the right towards slip sheet bin  404 . Slip sheet  120  is peeled of vacuum cups  116  and directed to the slip sheet bin  404  by slip sheet separator  104 , while the vacuum is constantly maintained during the peeling operation. 
     Once the peeling of a slip sheet is completed, the vacuum cups are brought into the upper position as is shown in  FIG. 5  and the vacuum system for vacuum cups  116  is switched off. The vacuum cups  116  are brought into upper position by rotating the energy chain counter clockwise. At this stage a separate plate picking mechanism (not shown) is used to lift printing plate  124  and to bring it into an imaging device (not shown). 
     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 
     
         
           104  slip sheet separator 
           108  pair of energy chains 
           116  vacuum cups 
           120  slip sheet 
           124  printing plate 
           128  energy chain intermittent movement direction 
           129  mechanism movement direction for plate separation. 
           132  pair of left side sprockets 
           136  pair of right side sprockets 
           304  printing plates stack 
           404  slip sheet bin