Abstract:
A method for automatically exchanging a printing plate ( 500 ) in a print unit of a rotary press having a blanket cylinder ( 250 ) and a plate cylinder ( 260 ) carrying the printing plate includes disengaging the plate cylinder from the blanket cylinder; reducing the speed of the plate cylinder (v_P) to a rendezvous speed (v_R) which is greater than 0; automatically clamping the printing plate onto the plate cylinder at the rendezvous speed (v_R); and increasing the speed of the plate cylinder (v_P) to a printing speed (v_DG).

Description:
[0001]    Priority to German Patent Application No. 101 56 378.7, filed Nov. 16, 2001 and hereby incorporated by reference herein, is claimed.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention is directed to a method for automatically exchanging a printing plate in a print unit of a rotary press having a blanket cylinder and a plate cylinder carrying the printing plate, as well as to a rotary press with a plate cylinder and a blanket cylinder.  
           [0003]    German Patent Application No. DE 199 42 617 describes such a method and such a machine, the printing plate being brought into contact with a nip roller in order to perform a plate replacement. The nip roller applies pressure to the printing plate, pressing it toward the plate cylinder during the plate replacement. In this manner, the gap between the blanket cylinder and the plate cylinder can be kept very small when a plate replacement is performed “on the fly”. To perform a plate replacement, the plate cylinder in question is disengaged from the associated blanket cylinder and its rotation is stopped.  
           [0004]    The previously existing design for automatically clamping a plate provides for the printing press to be at a standstill when the plate is inserted. On the one hand, this stoppage constitutes a delay in the execution of the automatic plate exchange; on the other hand, following the stoppage, a build-up time is again needed to start the machine. This contradicts the philosophy of a plate exchange operation that does not necessitate an operator.  
           [0005]    A printing plate exchange without the intervention of an operator is also known, a cassette-type system being used to automate the process of removing and bringing in the plate (H. Kipphahn: Handbook of Print Media, pp. 321 to 323; Springer Verlag, 2000).  
           [0006]    In the context of printing presses, it is also generally known to perform an angular synchronization from print unit to print unit with respect to the main and follower drives.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to devise a method of automatic plate exchange and a corresponding rotary press which will improve the automatic plate exchange process.  
           [0008]    The present invention provides a method for automatically exchanging a printing plate ( 500 ) in a print unit of a rotary press having a blanket cylinder ( 250 ) and a plate cylinder ( 260 ) carrying the printing plate, characterized by the following method steps: disengaging the plate cylinder from the blanket cylinder; reducing the speed of the plate cylinder (v_P) to a rendezvous speed (v_R) which is greater than 0; automatically clamping the printing plate to the plate cylinder at the rendezvous speed (v_R); and increasing the speed of the plate cylinder (v_P) to a printing speed (v_DG).  
           [0009]    A rotary press for printing on a material web, having a print unit including a blanket cylinder ( 250 ) and a plate cylinder ( 260 ), carrying a printing plate ( 500 ), including a plate-conveyor unit ( 410 ) which transports the printing plate to the plate cylinder, wherein a control device ( 600 ) is provided, which, for the automatic plate exchange, adjusts the positions and speeds of the printing plate ( 500 ) and of the plate cylinder ( 260 ) to one another, whereby, given a plate cylinder speed greater than 0, the plate-conveyor unit ( 410 ) brings the printing plate into a clamping device ( 420 ) of the plate cylinder or moves it away from the same.  
           [0010]    The printing press even rotates during the process of clamping a plate, thus, it is not stopped. The entire process of automatically replacing a plate takes less time, and the need is eliminated for additional build-up phases.  
           [0011]    A control device, position-sensing means, as well as drive means are advantageously provided to synchronize the movements of the plate cylinder and of the printing plate, in order to ensure a rapid, reliable and trouble-free clamping and unclamping of the printing plate.  
           [0012]    To be able to ensure a reliable clamping of the printing plate regardless of insignificant positioning and speed errors, when initiating the rendezvous, the speed of the plate cylinder is lower for a short time than that of the printing plate. Therefore, for a brief duration, the printing plate moves at a faster rate than the plate cylinder, so that the printing plate is pressed for a short time against the clamping bar of the plate cylinder for the clamping procedure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    On the basis of schematized representations, an exemplary embodiment of the rotary press according to the present invention, as well as the method according to the present invention are described in the following. The figures show:  
         [0014]    [0014]FIG. 1 a rotary press in a greatly simplified, cutaway view;  
         [0015]    [0015]FIG. 2 a flow chart of the method according to the present invention for automatic plate exchange; as well as  
         [0016]    [0016]FIGS. 3 a, b  illustrative time diagrams of plate cylinder and printing plate speeds during implementation of the automatic plate exchange. 
     
    
     DETAILED DESCRIPTION  
       [0017]    In accordance with FIG. 1, a web  120  to be printed on is fed between two blanket cylinders  240  and  250 . For the sake of simplicity, merely the right print unit is shown and described in greater detail. Printing plates  500  containing the images to be printed are mountable on a plate cylinder  260  assigned to right blanket cylinder  250 . In addition, a damping unit  270 , as well as an inking unit  280  are assigned in a generally known manner to plate cylinder  260 . FIG. 1 shows the positions of the plate cylinder and of the blanket cylinder during implementation of an automatic plate exchange. In this context, plate cylinder  260  is laterally disengaged from its associated blanket cylinder  250 . Printing plate  500  to be clamped is moved by a plate-conveyor unit, shown here simply as a suction device  410 , toward plate cylinder  260  (as shown by the arrow). However, to clamp and unclamp printing plate  500 , cassette-type systems, known from the related art, may also be used for the automatic plate exchange. In this context, a nip roller  300  presses printing plate  500 , bringing it into close proximity of plate cylinder  260 . As a result, a minimal gap between blanket cylinder  250  and plate cylinder  260  suffices to implement the printing plate exchange. At least one clamping bar  420  is provided at the plate cylinder. During the clamping procedure, the clamping bar holds printing plate  500 , conveyed to the plate cylinder, at its front plate edge  510 , viewed in the moving direction (as shown the arrow). Printing plate  500  is subsequently pulled by the rotary motion of plate cylinder  260  out of the printing-plate cassette, for example, and clamped onto the plate cylinder. Upon reaching a trailing edge of printing plate  500 , nip roller  300 , for example, is briefly moved slightly away and is then forced into direct contact again, in order to press the trailing edge of the printing plate into a corresponding groove of plate cylinder  260 .  
         [0018]    The speed during clamping of printing plate  500  corresponds more or less to rendezvous speed v —R  and amounts to about 60 to 100 impressions per hour. A rendezvous duration T_R. in which the speed of the plate cylinder&#39;s drive is more or less the same as that of the printing plate&#39;s drive, amounts to about 3 seconds. On the other hand, the time lag between the time plate edge  510  is introduced into clamping bar  420  and trailing edge of printing plate  500  into a corresponding holding device of the plate cylinder, amounts maximally to about one minute.  
         [0019]    A control device  600  is provided to coordinate the movements and the speeds of plate cylinder  260  and of printing plate  500 . In addition to a signal-processing unit  610 , it includes, in particular, position-sensing means  620  and, in some instances, speed-sensing means. From the recorded data, unit  610  computes the corresponding control variables for the individual drive means of the print unit, the plate cylinder and, respectively, of plate-conveyor unit  410 . In the context of the control, the printing-plate drive and the main drive, i.e., the drive for the plate cylinder, constitute a compound drive whose mechanisms may be used for purposes of synchronization. The speed profile is able to be adapted to the handling speed of the plate, i.e. to plate speed v_P. The printing press rotates during the entire automatic plate-exchange operation. This applies especially to the clamping and, analogously, to the unclamping of printing plate  500 . During the actual clamping of the plate (“rendezvous”), the printing press fundamentally rotates at a speed—rendezvous speed v_R—that is reduced compared to a printing speed v_DG. A corresponding speed profile is taken as a basis for the clamping operation illustrated in FIG. 1. Thus, a synchronization is carried out between the drive for plate insertion and the main drive, i.e., the drive of plate cylinder  260 . Functioning as a type of synchronization unit, control device  600  cooperates with the various detection means and the drives of the plate cylinder and of the plate-conveyor unit.  
         [0020]    An open-loop control instead of the feedback control is also a cost-effective alternative for the entire functional sequence of the automatic plate exchange. However, it is less advantageous in terms of design safety. The open-loop control is implemented, for example, via a slip clutch. The termination or end condition is to be implemented in this context via a time-out mechanism, in order to avoid an endless loop in the event of a failed rendezvous; or it is implemented via the feedback “clamping is accomplished”.  
         [0021]    The flow chart for the automatic plate exchange of FIG. 2 is run through exactly once for each plate to be replaced. Here φ pos  (phi_pos) signifies the machine starting angle: the machine angle starting from which the drive is activated for the plate insertion. φ limit  (phi_limit) is the machine limiting angle: the machine angle at which the clamping bar is no longer able to be reached from the plate edge.  
         [0022]    In this context, the following possible operating-time errors are to be recorded by control device  600  using appropriate detection means, i.e., the automatic plate exchange procedure is to be stopped, and an error signal to this effect is to be output:  
         [0023]    No plate is inserted at a print unit where a printing plate needs to be exchanged.  
         [0024]    A wrong plate is inserted at a print unit where a printing plate needs to be exchanged, or the plate is wrongly inserted.  
         [0025]    The slip at the drive is too great.  
         [0026]    In addition, when unclamping the printing plate, it is important that it be pulled out completely, so that the clamping of the new printing plate is not obstructed.  
         [0027]    In accordance with FIGS. 3 a  and  b,  the following speed profiles are taken as a basis for the main drive. In accordance with FIG. 3 a,  plate cylinder  260  initially runs at speed v_P, which corresponds, for example to printing speed v_DG (typ. 3,000-15,000 impressions per hour). Starting at a specific point in time, the speed of plate cylinder v_P is linearly reduced, for example, to rendezvous speed v_R. Depending on the characteristic of plate-cylinder speed v_P. at the calculated point in time, i.e., at the machine starting angle φ pos , the drive is activated for the plate insertion. In the process, printing-plate speed v_D is raised from zero to approximately rendezvous speed v_R. For the rendezvous duration T_R, both drives, namely the drive for plate cylinder  260 , as well as the drive for printing plate  500  run at the same rate of speed. The printing plate is then securely held at plate edge  510  by clamping bar  420  at plate cylinder  260 . While the drive of the printing plate may be deactivated, the drive of the plate cylinder increases the speed of plate cylinder v_P and, thus, also the clamped printing plate again to printing speed v_DG. Since it holds at the point in time of the rendezvous that v_P=v_D, control device  600  must ensure that plate edge  510  meets with clamping bar  420  at precisely the correct instant and at the proper speed.  
         [0028]    Deviating herefrom, in the context of the rendezvous according to FIG. 3 b , printing plate  500  is brought to a speed that is greater than rendezvous speed v_R. In particular, the speed of printing plate v_D is also selected to be greater than the speed of plate cylinder v_P. Printing plate  500  is thereby reliably pressed for a brief period of time toward clamping bar  420  of plate cylinder  260  to effect the clamping. In this context, rendezvous speed v_R varies over time. Following the rendezvous, at rendezvous speed v_R, the drive of plate-conveyor unit  410  is switched off, and printing plate  500  is pulled by the rotary motion of plate cylinder  260  out of the cassette and clamped onto plate cylinder  260 . The drive of the plate cylinder subsequently increases the speed of the plate cylinder again to printing speed v_D. In the event that, upon reaching machine limiting angle φ limit , control device  600  does not receive the confirmation from a suitable detection means that printing plate  500  is properly fixed to plate cylinder  260 , the automatic plate exchange operation is stopped.  
         [0029]    It is also possible that v_D&lt;v_P is selected for the rendezvous, thus that printing plate  500  is not overtaken by plate cylinder  260 .  
         [0030]    The present invention is applicable to a wide variety of printing presses, such as to sheet-fed printing presses. The term rotary presses refers both to web presses as well as to sheet-fed printing presses.  
                                             Reference Symbol List                                    120   web           240, 250   blanket cylinder           260   plate cylinder           270   damping unit           280   inking unit           300   nip roller           410   suction device           420   clamping bar           500   printing plate           510   plate edge           600   control device           610   signal-processing unit           620   position detector           T_R   rendezvous time           v_D   printing plate speed           v_DG   printing speed           v_P   plate cylinder speed           v_R   rendezvous speed           φ limit     machine limiting angle           φ pos     machine starting angle