Abstract:
A digital printing system employing tandem marking engines for duplex printing utilizing a variable dwell time in the output sheet inverter of the first marking engine to provide correct positioning of the leading edge of the inverted sheet for arrival at the entrance of the second marking to avoid the seam in the photoreceptor.

Description:
BACKGROUND 
     The present disclosure relates to digital printing systems having plural tandem marking or printing engines of the type with seamed endless photoreceptor belts. In such printing systems, it is common practice to invert the sheet after marking on one side thereof in a first of the printing engines and for feeding the inverted sheet into a second printing engine for marking on the opposite side of the sheet to thus facilitate high speed duplex digital printing. However, in printing systems of this type arrangement, problems have been encountered in proper registration of the leading edge of the inverted sheet onto the photoreceptor of the second printing engine for proper placement of the image on the sheet and for avoiding the seam in the photoreceptor of the second marking engine. Where the inverted sheet from the first marking engine is transported by a transporter to the second marking engine, errors in timing, transport speed and positioning of the sheet can accumulate to cause misregistration of the sheet on the second photoreceptor. This is particularly troublesome in view of the requirement that the sheet be placed on the second photoreceptor within a window of plus or minus 30 milliseconds timing with respect to the movement of the photoreceptor. 
     Typically, tandem marking engines employed for duplex printing operate to synchronize the position of the seams by varying the speed of the photoreceptor in the second marking engine and can result in problems with front to back image-to-paper registration due to paper shrinkage from heating in the first marking engine&#39;s fuser and differences in the photoreceptor belt length causing varied photoreceptor speed. 
     Heretofore digital printing systems employing tandem marking engines for duplex printing have operated in accordance with the procedure shown in  FIG. 3  wherein at step  60  the system schedules the arrival times of the sheet stock in the initial and subsequent marking engines; and, proceeds to have the feeder eject the sheet stock at step  62  to meet the scheduled arrival time as determined in step  60 , at step  64  arrives at the entrance of the first marking engine and is registered thereon at step  66  for upper registration for marking. At step  68 , the sheet is registered for image transfer from the photoreceptor belt and arrives at the discharge exit at the first marking engine at step  70 . The system then submits the sheet stock to the inverter at step  72 ; and, at step  74  the inverter discharges the sheet stock after a fixed dwell time. 
     Thus, it has been desired to provide a way of improving the registration of the leading edge of sheets emanating from a first tandem marking engine onto the second marking engine. 
     BRIEF DESCRIPTION 
     The present disclosure describes a digital printing system employing tandem marking engines for duplex printing and utilizes a variable dwell time in the output inverter of the first marking engine to provide for correct positioning of the leading edge of the inverted sheet for arrival at the entrance of the second marking engine. The system presently disclosed avoids the seam in the photoreceptor of the second marking engine and properly positions the leading edge of the sheet for correct front-to-back image registration on the second photoreceptor for image transfer to the sheet. The system of the present disclosure thus eliminates the need to synchronize the seam positions of the photoreceptors in the tandem marking engines and permits the speed of the photoreceptor in the second marking engine to be varied for purposes of controlling the image magnification thereon without regard to seam position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a digital printing system having plural marking engines in tandem in accordance with the present disclosure; 
         FIG. 2  is a flow diagram of the method of sheet transport control in the system of  FIG. 1 ; and, 
         FIG. 3  is a diagram similar to  FIG. 2  of the prior art systems. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a digital printing system according to the present disclosure is indicated generally at  10  and includes a sheet feeder assembly indicated generally at  12 , a first marking engine indicated generally at  14  including a photoreceptor belt  16  of the endless seamed type and a plurality of colorant generators  18  operative for effecting color image formation on the belt  16 . The marking engine  14  includes a fuser indicated generally at  20  and a transporter providing a transport path  22  through the marking engine. The photoreceptor  16  is operative to transfer the image to the sheet stock on path  22  at a transfer station indicated in dashed outline and denoted with reference numeral  24 . 
     From the marking at station  24 , the sheet stock is advanced along path  22  and is discharged from the fuser  20  along path  22  to an inverter  26  which inverts the marked sheet and maintains the sheet for a controlled dwell time before reentry onto the path  22  and movement to the entrance station  28  for the second marking engine indicated generally at  30 . 
     The sheet stock is controlled, as will hereinafter be described, to arrive at the registration point indicated by the arrow and denoted by reference numeral  35  in marking engine  30  at a controlled time. 
     The second marking engine  30  includes a photoreceptor  32  of the seamed belt type and has colorant generators  34  disposed for forming a color image on the photoreceptor  32 . The photoreceptor  32  is operative to transfer the color image to the second side of the sheet at a transfer station indicated in dashed outline and denoted by reference numeral  33 . The marking engine  30  also includes a post-marking fuser  36 , the output from which the sheet is inputted to a second inverter indicated generally at  38  which restores the sheet to its original orientation and discharges the duplex marked sheet to a finisher indicated generally at  40 . 
     The system of  FIG. 1  includes a controller  50  which is operatively connected as indicated by the dashed lines in  FIG. 1  for controlling the marking engines  14 ,  30  and the inverter  26  as will hereinafter be described. 
     The system of the present disclosure allows the two marking engines to have their photoreceptors run at different speeds without the need for synchronizing the location of the photoreceptor belt seams. The controller monitors the seam position by a sensor (not shown) and then schedules the closest available image panel on the photoreceptor  32  of the second marking engine  30  with the marked image printed by the first marking engine  14 . The controller is then operative to determine the time that the leading edge of the sheet stock needs to arrive at the docking plane  28  of the second marking engine  30  in order to synchronize with the time that the image will be transferred from the photoreceptor  32  to the second side of the sheet at station  33 . The controller  50  of the system  10  adjusts the time the sheet stock is parked or dwells in the inverter  26  by increasing the paper path velocity downstream of the fuser, or increasing the deceleration rate of the sheet along the path  22  as it enters the inverter  26  and by increasing the acceleration rate required to eject the sheet from the inverter  26 . It will be understood that the dwell time in the inverter  26  must be of sufficient length to accommodate the timing correction needed to synchronize the sheet with the scheduled arrival time at docking time plane station  28 . 
     The system of the present disclosure thus provides a digital printing system employing tandem marking engines for duplex printing in which the need to synchronize the seams of the photoreceptor belts in the first and subsequent marking engines is eliminated and the speed of the second photoreceptor may be varied only as needed to control image magnification. The system of the present disclosure thus improves the front to back (show through) image to paper registration and improved image-to-paper registration in the process direction by having variable inverter dwell time prior to entry into the second marking engine. The system can thus accommodate variations in paper path velocity and the length of the paper due to shrinkage in the fuser and further provides for decreased cycle in time on the order of one minute as a result of elimination of the need for photoreceptor belt synchronization between the marking engines. 
     Referring to  FIG. 2 , the process is illustrated in flow diagram wherein the controller schedules immediate arrival times at engines  1  and  2  at step  100  and proceeds to eject the sheet stock from the feeder into engine  1  at the scheduled time at step  102 . The sheet stock arrives at the first marking engine entrance at step  104  and is registered at an internal registration station at step  106  and proceeds to the onset of marking at station  24  at step  108 . The sheet stock then arrives at the fuser output at step  110  and is inputted to the inverter  26  at step  112 . At step  114 , the sheet stock is outputted from the inverter at the appropriate speed at step  114  to arrive at the scheduled time for the second marking engine. At step  116 , the sheet stock arrives at the docking station  28  and at the registration station  35  of the second marking engine  30  at station  118  and proceeds at step  120  to begin image transfer at station  33  and then proceeds to output from the fuser  36  at step  122 . 
     The sheet stock then arrives at the entrance to the second marking engine at step  76  and the registration station at  78  and proceeds to be marked by transfer of the image thereon at step  80  and is subsequently moved to the discharge station of the second marking engine at step  82 . 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.