Abstract:
A tandem printing system that can achieve stable web conveyance, regardless of the configuration of a tandem printing system or the type of a web, by preventing malfunctions including a broken web that may occur when slack in a web occurs at the time the printing operation stops and the slack is then eliminated after the printing operation has started. 
     By executing low-speed conveyance of web by a web feeding mechanism before the printing operation begins, slack W 22   a , W 10   a , and W 15   a  in the web located downstream of the guide rollers, and the slacked web is accumulated in the air loop portion, thereby completing the standby for printing. At that time, the amount of web conveyance (accumulation) is saved. Then, high-speed printing operation begins, and when the amount of printing has reached the amount of web conveyance saved, high-speed conveyance by the web feeding mechanism begins.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application serial No. 2007-019329, filed on Jan. 30, 2007, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a web conveyance method and apparatus of a tandem printing system in which a plurality of printers are tandemly arranged. 
       BACKGROUND OF THE INVENTION 
       [0003]      FIG. 2  shows an example of a tandem printing system in which two printers P 1  and P 2  are tandemly arranged in order to create images on both sides of a web which is illustrated as a long, continuous belt-like paper. That is, a web W is fed to the printer P 1  in the first stage and images are formed on its first surface (main surface) by an image-forming apparatus  19 . And, after the web W has been turned over by a turn bar T and then fed into the printer P 2  in the subsequent stage, images are formed by an image-forming apparatus  20  on the second surface (back-side surface) of the web W. Thus, this is a double-side printing system. 
         [0004]      FIG. 3  shows an example of a spot-color tandem printing system in which the printing with a first color is executed by printer P 1  in the first stage and then the printing with a second color is executed by printer P 2  in the subsequent stage. 
         [0005]    The present invention relates to a web conveyance method and apparatus of those tandem printing systems. 
         [0006]      FIG. 4  shows the outline of conveying the web in the tandem printing system. 
         [0007]    A web W which has on its first surface an image created by printer P 1  is discharged from the printer P 1  by a carrier roller  15 . The web W is guided to printer P 2  via a guide roller  8 , a guide roller  9 , and a guide roller  10  which constitute a turn bar T disposed subsequently after the printer P 1 . The web W is then fed into a web feeding mechanism  23  via a guide roller  7  and a guide roller  22 ; and after an air loop  24  has been formed, the web W is conveyed to an image-forming apparatus  20  by a carrier roller  21 . After an image has been formed on the second surface of the web W by the image-forming apparatus  20 , the web W is discharged from the printer P 2  by a carrier roller  25 . 
         [0008]    Herein, the carrier roller  15  of the printer P 1 , web feeding mechanism  23  of the printer P 2 , carrier roller  21 , and the carrier roller  25  are all capable of conveying the web W. On the other hand, the guide rollers  8  to  10  which constitute a turn bar T and the guide rollers  7  and  22  located at the entrance of the printer P 2  are driven rollers having no conveyance capability or unrotatable, immovable rollers. 
         [0009]    In the above-mentioned tandem printing system, the guide roller  22  and guide rollers  7  to  10 , located upstream of the web feeding mechanism  23 , create a load on the web W during the printing operation, which prevents slack in the web along the path of the web as shown as W 22   b  and W 10   b.    
         [0010]    When the printing operation is stopped, the inertia of the web W itself or the inertia of the rollers disposed on the web conveyance path causes the web to be conveyed too much, which creates slack W 22   a  and slack W 10   a  between the two printers P 1  and P 2 . For example, if the printing operation begins in the state where slack W 22   a  is present as shown in the drawing, the portion of the web located downstream of the web feeding mechanism  23  accelerates, while the portion of the web upstream of the guide roller  22  remains stationary. If acceleration of the portion of the web W located downstream of the web feeding mechanism  23  has stopped before slack W 22   a  disappears; at the moment when slack W 22   a  disappears, the portion of the web located upstream of the guide roller  22  is momentarily accelerated at the highest speed. For this reason, a great deal of impulse is to be imposed on the portion of the web between the web feeding mechanism  23  and the guide roller  22 . At this time, if the web feeding mechanism  23  has a weak conveyance force, an error which eliminates the air loop  24  will stop printing, and if the web W is thin, the web W may break. The same phenomena will occur in and around slack W 10   a.    
         [0011]    To avoid such phenomena, for example, as disclosed in Japanese patent laid-open No. 2004-292133, a method in which a web is conveyed early by a web feeding mechanism  23  has been proposed. 
       SUMMARY OF THE INVENTION 
       [0012]    In a tandem printing system, the amount of slack occurring in the web differs depending on the configuration of the tandem printing system and the type of the web. That is, the amount of slack in the web changes according to conditions including friction force occurring between a turn bar and the web, the inertia moment of guide rollers that constitute the turn bar, and the weight of the web. For this reason, it is difficult to detect early the timing at which a web is conveyed by the above-mentioned web feeding mechanism  23 . Therefore, depending on the configuration of the tandem printing system or the type of the web, the slack in the web occurs when the printing operation has stopped, and at the moment when the slack disappears due to the start of the printing operation, the web may break or some malfunction may occur, thereby causing unstable conveyance of the web. 
         [0013]    An objective of the present invention is to provide a web conveyance method or an apparatus of a tandem printing system which inhibits the web from breaking or some kind of malfunction from occurring at the moment when the slack in the web is taken in due to the start of the printing operation, thereby achieving reliably constant conveyance of the web. 
         [0014]    The present invention, in an aspect, is a tandem printing system which comprises first and second printers tandemly arranged to form images on a web, a web feeding mechanism for feeding the web into the second printer, and 
         [0015]    an air loop mechanism disposed subsequently after the web feeding mechanism to create slack in the web and convey the web; wherein 
         [0016]    a tensile force of the web that has been discharged from the first printer is detected, and the resulted tensile force is reported to the control apparatus of the second printer; and 
         [0017]    when the control apparatus of the second printer receives a standby-for-printing instruction and also receives information indicating that the resulted tensile force is less than a prescribed value, the web feeding mechanism starts conveying the web at a low speed; and 
         [0018]    when information indicating that the resulted tensile force has exceeded the prescribed value is received, the low-speed web conveyance is halted. 
         [0019]    In a preferred embodiment of the present invention, at the receipt of the information indicating that the web&#39;s tensile force exceeds a prescribed value, an instruction to start the printing operation is provided, and according to the start-printing instruction, high-speed web conveyance begins. 
         [0020]    Furthermore, in a preferred embodiment of the present invention, after the high-speed web conveyance has started according to the start of printing, and when the amount of the remaining web in the air loop mechanism becomes less than a prescribed value, high-speed web conveyance by the web feeding mechanism begins. 
         [0021]    According to preferred embodiments of the present invention, malfunctions including a broken web are prevented thereby achieving stable web conveyance. 
         [0022]    Other objectives and characteristics of the present invention will be clearly described in the embodiments described hereafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic drawing of a tandem printing system which is an embodiment of the present invention. 
           [0024]      FIG. 2  is an image drawing of the printing operation executed by a double-side tandem printing system. 
           [0025]      FIG. 3  is an image drawing of the printing operation executed by a spot-color tandem printing system. 
           [0026]      FIG. 4  shows an example of the condition of the web in the tandem printing system. 
           [0027]      FIG. 5  is a diagram of the control block of a tandem printing system which is an embodiment of the present invention. 
           [0028]      FIG. 6  is a time chart of the web conveyance procedure in a tandem printing system which is an embodiment of the present invention. 
           [0029]      FIG. 7  shows an example of the condition of the web when standby for printing has been completed in a tandem printing system which is an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Next, embodiments of the present invention will be described with reference to the drawings. The present invention can be applied to both tandem printing systems shown in  FIG. 2  and  FIG. 3 ; however, in the descriptions below, the tandem printing system including a turn bar T, shown in  FIG. 2 , is given as an example. 
         [0031]      FIG. 1  is a schematic drawing of a tandem printing system which is an embodiment of the present invention. Herein, a tandem printing system shown in  FIG. 4  incorporates a mechanism for detecting a web&#39;s tensile force that is imposed on a carrier roller  15  and a communication means I for reporting information about the web&#39;s tensile force to printer P 2 . The carrier roller  15  is designed such that it moves up and down according to the web&#39;s tensile force imposed on the carrier roller  15  of the printer P 1 . By detecting the up and down movement of the roller, it is possible to detect a web&#39;s tensile force imposed on the carrier roller  15 . Furthermore, the information about the web&#39;s tensile force is reported to the control apparatus of the printer P 2  via a communication means I. 
         [0032]      FIG. 5  is a diagram of the control block in this embodiment. 
         [0033]    First of all, printer P 1  comprises a microcomputer  210  for controlling the operation of the printer, a motor M 11  for providing a rotational drive force to a carrier roller  11 , and a motor driver  211  for driving the motor M 11 . The printer P 1  further comprises a motor M 15  for providing a rotational drive force to a carrier roller  15  and a motor driver  215  for driving the motor M 15 . Furthermore, the printer P 1  also includes a position sensor S 15  for detecting the up and down positions of the carrier roller  15 . 
         [0034]    The microcomputer  210  receives a start-printing instruction from an upper-level controller  50 , which controls the tandem printing system, via a communication means I 1 , emits a drive signal to motor drivers  211  and  215 , thereby executing high-speed web conveyance to conduct the printing operation. Furthermore, the microcomputer  210  monitors the signal from a position sensor S 15  and reports the information via a communication means I to a microcomputer  220  for controlling printer P 2 . 
         [0035]    Next, printer P 2  also comprises a microcomputer  220  for controlling the operation of the printer, a motor M 21  for providing a rotational drive force to a carrier roller  21 , and a motor driver  221  for driving the motor M 21 . The printer P 2  further comprises a motor M 23  for providing a rotational drive force to a web feeding mechanism  23 , a motor driver  223  for driving a motor M 23 , a motor M 25  for providing a rotational drive force to the carrier roller  25  and a motor driver  225  for driving the motor M 25 . Furthermore, the printer P 2  also includes a rotational sensor S 23  for detecting the rotation of the web feeding mechanism  23  and a memory  222 . 
         [0036]    In the same manner as printer  1 , the microcomputer  220  receives a start-printing instruction from an upper-level controller  50  via a communication means I 2 , emits a drive signal to motor drivers  221  and  225 , thereby executing high-speed web conveyance to conduct the printing operation. Furthermore, the microcomputer  220  monitors the rotational sensor S 23  while emitting a drive signal to the motor driver  223  and conveying the web, and by measuring the number of revolutions of the web feeding mechanism  23 , the microcomputer  220  calculates the amount of conveyance of the web. Moreover, the microcomputer  220  is designed such that it receives information about the web&#39;s tensile force reported via the communication means I. 
         [0037]      FIG. 6  is a time chart of the web conveyance procedure in a tandem printing system, shown in  FIGS. 1 and 5 , which is an embodiment of the present invention. Hereafter, the operation of the system will be described with reference to the drawings. 
         [0038]    (1) At timing t 1 , a standby-for-printing instruction is supposed to be sent from the upper-level controller  50  to each microcomputer  210  and  220  of the printers P 1  and P 2  thereby activating both printers to be ready for the printing operation. The printer P 1  side microcomputer  210  reports the condition of the position sensor S 15  to the printer P 2  side microcomputer  220 . At this time, when the carrier roller  15  is located at position  15   a  shown in  FIG. 1 , there is no web&#39;s tensile force, which means that the web W is in the state of W 15   a , W 10   a , and W 22   a . On the other hand, when the carrier roller  15  is located at position  15   b  in  FIG. 1 , there is a tensile force on the web, which means that the web W is in the state of W 15   b , W 10   b , and W 22   b.    
         [0039]    When information received by the printer P 2  side microcomputer  220  indicates no web&#39;s tensile force (less than a prescribed value), the microcomputer  220  starts low-speed web conveyance by the web feeding mechanism  23  as shown in  FIG. 6(   b ). At the same time, as shown in  FIG. 6(   c ), the amount of web conveyance by the web feeding mechanism  23 , that is, the amount of web accumulated by the air loop  24  starts to be measured. At this time, the speed of the low-speed web conveyance should be set sufficiently low to prevent damage to the web, including a broken or folded web, from occurring due to the web&#39;s tensile force and also should be set as high as possible to prevent the reduction of printout efficiency (throughput). 
         [0040]    On the other hand, when information received by the printer P 2  side microcomputer  220  indicates the existence of web&#39;s tensile force (more than a prescribed value), it is determined that there is no slack of the web, and low-speed web conveyance by the web feeding mechanism  23  is not executed. 
         [0041]    (2) At timing t 2 , the position sensor S 15  is supposed to detect position  15   b  in  FIG. 1  (existence of web&#39;s tensile force). Then, the printer P 1  side microcomputer  210  reports the information to the printer P 2  side microcomputer  220 . When the printer P 2  side microcomputer  220  receives the information indicating the existence of web&#39;s tensile force, it stops the low-speed web conveyance by the web feeding mechanism  23 , and saves the amount of web conveyance by the web feeding mechanism  23  that has been measured in the memory  222 . 
         [0042]      FIG. 7  shows an example of the condition of the web when standby for printing has been completed in a tandem printing system which is an embodiment of the present invention. The drawing shows the condition in which the web is accumulated in the air loop  24  at the timing t 2 . Thus, slack in the web W which was present at positions W 15   b , W 10   b , and W 22   b  has been eliminated, and excessive portions of web W are collected in the air loop  24 . This is the condition when the printer P 2 &#39;s standby for printing has been completed. 
         [0043]    On the other hand, as stated above, when low-speed web conveyance by the web feeding mechanism  23  is not executed, the amount of web conveyance is indicated as “0” and saved in the memory  222 . Furthermore, even if low-speed web conveyance by the web feeding mechanism  23  has reached the constant amount of conveyance, if the printer P 2  could not receive information indicating the existence of web&#39;s tensile force, an error seems to be occurring. Accordingly, low-speed web conveyance by the web feeding mechanism  23  is halted, and an error is reported, such as no web, broken web, malfunction of the web feeding mechanism  23 , or abnormality in the communication means I. 
         [0044]    (3) The upper-level controller  50  receives information indicating the existence of web&#39;s tensile force and can emit a start-printing instruction when the condition shown in  FIG. 7  has been reached. In  FIG. 6 , the time interval from timing t 2  to timing t 3  seems to be long, however, it is possible to start printing immediately. 
         [0045]    Now, at timing t 3 , a start-printing instruction is supposed to be sent from the controller  50  to each microcomputer  210  and  220  of the printers P 1  and P 2 . In response, the printers P 1  and P 2  drive a carrier roller  21 , carrier roller  25  and a carrier roller  15  simultaneously thereby starting high-speed web conveyance to execute the printing operation. At this time, when the amount of web conveyance saved in the memory  222  is “0,” the web feeding mechanism  23  simultaneously starts high-speed web conveyance. 
         [0046]    On the other hand, when the amount of web conveyance saved in the memory  222  is not “0,” the number of printed pages (the amount of printing) is compared with the amount of web conveyance saved in the memory  222 , and at the time when the difference is close to “0,” high-speed web conveyance by the web feeding mechanism  23  begins. By doing so, slack in the web accumulated in the air loop  24  portion, as shown in  FIG. 7 , as the result of the conveyance by the carrier rollers  21  and  25  is eliminated, and at the moment when the air loop  24  shown in  FIG. 1  is formed, high-speed web conveyance by the web feeding mechanism  23  can begin. Furthermore, moderate slack is also formed in the web located downstream of the carrier roller  15 . 
         [0047]    By starting the printing operation according to the procedure mentioned above, it is possible to accelerate the web while a tensile force is being provided on the web located upstream of the web feeding mechanism  23 , therefore, high-speed web conveyance is possible without providing more impulse than necessary. Furthermore, by executing the above-mentioned low-speed web conveyance by the web feeding mechanism  23  right before executing the printing operation, stable web conveyance becomes possible because a tensile force can be imposed on the web for a short time thereby preventing the web from curling.