Patent Publication Number: US-2010124424-A1

Title: Duplex printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a duplex printer that allows automatic printing of a sheet of paper on both sides. 
     2. Description of Related Art 
     Recently, duplex printing that allows automatic printing of a sheet of paper on both front and back sides is promotionally utilized in the interest of an effective use of resources or the like. As a device that can achieve duplex printing, for example, known is a duplex stencil printer that has two cylindrical drums with perforated stencil sheets wound therearound. Printing on one side of a paper is done by one of the drums and then the paper printed on its one side is fed to the other of the drums. Subsequently, printing on another side of the paper is done by the other of the drums. 
     In such a duplex printer, when inks printed on the one side does not yet dry, the paper may become tainted accidentally during feeding the paper for printing on the other side or during printing on the other side of the paper. 
     Therefore, proposed is a duplex printer in which papers already printed its one side are temporally stacked on a stacking tray provided within the printer in order to secure time for drying inks and then printing on another side of the stacked papers is done (Patent Document 1: Japanese Patent Application Laid-Open No. 2005-29375). According to this duplex printer, it can be prevented that papers become tainted. 
     SUMMARY OF THE INVENTION 
     Generally with a printer, when a paper jam (sheet jam, misfeed) occurs, printing is automatically stopped. Then, a user opens a cover of the printer and removes a jammed paper. 
     According to the above-mentioned duplex printer disclosed in the Patent Document 1, since papers already printed on its one side are temporally stocked on the stacking tray, a user may accidentally touch the papers stacked on the stacking tray during removing a jammed paper. Thus, the papers may become tainted or get winkled, so that papers are wasted needlessly. 
     The present invention has been achieved in order to solve the above problems and an object of the present invention is to provide a duplex printer that can reduce needlessly-wasted papers. 
     An aspect of the present invention provides a duplex printer that includes a sheet feed unit for feeding sheets; a first printing unit for printing on one side of the sheets fed from the sheet feed unit; an intermediate sheet transport unit for transporting the sheets of which the one side has been printed by the first printing unit; a sheet stacking unit for stacking the sheets transported from the intermediate sheet transport thereon; a sheet stack detector for detecting stacking of the sheets on the sheet stacking unit; an intermediate sheet feed unit for feeding out the sheets stacked on the sheet stacking unit in a stacked order after a predetermined sheet feed-out condition has been met; a second printing unit for printing on another side of the sheets fed out from the sheet stacking unit by the intermediate sheet feed unit; a sheet ejection unit for ejecting the sheets of which the other side has been printed by the second printing unit; a plurality of sheet detectors for detecting the sheets being transported along a first transport path and a second transport path (the first transport path is a path from the sheet feed unit to the sheet stacking unit via the first printing unit and the second transport path is a path from sheet stacking unit to the sheet ejection section via the second printing unit); and a controller. The controller is operable to: (a) determine whether or not a sheet jam occurs in the first transport path or the second transport path based on detection results of the plurality of sheet detectors, (b) stop operations of the sheet feed unit, the first printing unit and the intermediate sheet transport unit when it is determined in (a) that the sheet jam occurs only in the first transport path among the first transport path and the second transport path, (c) feed out all of the sheets stacked on the sheet stacking unit by controlling the intermediate sheet feed unit, (d) print the other side of the sheets fed out from the sheet stacking unit by controlling the second printing unit; and (e) eject the papers of which both sides have been printed by controlling the sheet ejection unit. 
     It is preferable that the predetermined sheet feed-out condition is a condition in which a preset time has elapsed since the sheet stack detector has detected stacking of a first sheet. 
     Alternatively, it is preferable that the predetermined sheet feed-out condition is a condition in which the number of stacked sheets on the sheet stacking unit has reached to a preset value, and the controller is further operable to, if the predetermined sheet feed-out condition has been met at a time when the sheet jam occurs only in the first transport path, start operations of (b), (c), (d) and (e). 
     Here, it is further preferable that the controller is further operable to, if the predetermined sheet feed-out condition has not been met at a time when the sheet jam occurs only in the first transport path, start an operation of (b) first and start operations of (c), (d) and (e) after a preset time has elapsed since the sheet stack detector has detected stacking of a first sheet. 
     In addition, it is preferable that the duplex printer further includes a display for displaying a warning display screen for notifying that the sheet jam occurs, and the controller displays the warning display screen on the display before operating (c), (d) and (e). 
     Further, it is preferable that the duplex printer is a stencil printer. 
     According to the aspect of the present invention, needlessly-wasted papers can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram showing a duplex printer according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing functional configurations of the duplex printer shown in  FIG. 1 ; 
         FIG. 3  is a flowchart showing operations when a paper jam occurs in the duplex printer shown in  FIG. 1 ; 
         FIG. 4  is an example of a warning display screen for notifying a paper jam to a user; 
         FIG. 5  is a partial flowchart explaining a sheet feed-out condition; and 
         FIG. 6  is a partial flowchart explaining another sheet feed-out condition. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of a duplex printer according to the present invention will be explained with reference to the drawings. 
     As shown in  FIG. 1 , the duplex printer  1  according to the present embodiment includes an original scanning unit  2 , a stencil making unit  3 , a sheet feed section (unit)  4 , a first printing unit  5 , an intermediate sheet transport section (unit)  6 , a sheet stacking unit  7 , an intermediate sheet feed section (unit)  8 , a second printing unit  9 , a sheet ejection section (unit)  10 , an operation panel  11  and a controller  12 . 
     The original scanning unit  2  is provided at an upper portion of the duplex printer  1  and can optically scan originals to be copied. 
     The stencil making unit  3  includes a stencil sheet container  32 , feed rollers  33 , a thermal head  34 , a paten roller  35 , stencil feed-out rollers  36  and a cutter  37 . The stencil sheet container  32  contains a rolled stencil sheet  31  therewithin. The feed rollers  33  feed the stencil sheet contained within the stencil sheet container  32 . The thermal head  34  is provided downstream of the feed rollers  33  along a feeding direction. Heater elements are aligned linearly along a primary sweeping direction on the thermal head  34 . The platen roller  35  presses a stencil sheet  31  onto the thermal head  34 . The stencil feed-out rollers  36  feeds a stencil sheet  31  already perforated by the thermal head  34 . The cutter  37  cuts out a stencil sheet  31  at a desired position. 
     The stencil making unit  3  can be moved by a drive unit (not shown) between a first stencil sheet supply position (shown by dashed lines in  FIG. 1 ) and a second stencil sheet supply position (shown by solid lines in  FIG. 1 ). When the stencil making unit  3  is located at the first stencil sheet supply position, a stencil sheet can be supplied to an after-mentioned first drum  51 . When the stencil making unit  3  is located at the second stencil sheet supply position, a stencil sheet can be supplied to an after-mentioned second drum  91 . 
     The sheet feed section  4  includes a sheet feed tray  41 , primary sheet feed rollers  42  and secondary sheet feed rollers  43 . Papers (print media) P are stacked on the sheet feed tray  41 . The primary sheet feed rollers  42  pick up the papers P sheet by sheet to feed them. The secondary sheet feed rollers  43  feed the papers P fed by the primary sheet feed rollers  42  toward the first printing unit  5  at a preset timing. 
     The first printing unit  5  is provided downstream of the sheet feed section  4  along the feeding direction of papers P. The first printing unit  5  includes the first drum  51 , a first pressure roller  52  and a first stencil disposal unit  53 . 
     Outer circumferential wall of the first drum  51  is made of ink-permeable porous material. The first drum  51  is rotated by a main motor  13 . A damper  54  is provided on the outer circumferential wall of the first drum  51 . The damper  54  clamps a perforated stencil sheet made by the stencil making unit  3 , so that the stencil sheet is wound around the outer circumferential wall of the first drum  51 . 
     An ink supply unit  55  is provided within the first drum  51 . The ink supply unit  55  includes a squeegee roller  56 , a doctor roller  57  and an ink supply pump (not shown). Inks are supplied from the ink supply pump to a cleavage between the squeegee roller  56  and the doctor roller  57  that are rotated in synchronization with a rotation of the first roller  51  to form ink deposition and then supplied onto an inner surface of the first drum  51 . 
     The first pressure roller  52  is provided outside the outer circumferential wall of the first drum  51  at a position coincident with the squeegee roller  56 . The first pressure roller  52  presses a paper P fed from the secondary sheet feed rollers  43  onto the outer circumferential wall of the first drum  51 . The first pressure roller  52  can be moved by a drive unit (not shown) between a pressing position and a resting position. When the first pressure roller  52  is located at the pressing position, i.e., when printing is not being operated, the first pressure roller  52  is pressed onto the outer circumferential wall of the first drum  51 . When the first pressure roller  52  is located at the resting position (i.e., while printing is not done), a sufficient distance can be ensured between the first pressure roller  52  and the outer circumferential wall of the first drum  51 . 
     The first stencil disposal unit  53  removes a used stencil sheet  31  from the first drum  51  and then stocks the used stencil sheet  31  therewithin. 
     In addition, the first printing unit  5  further includes a separator  58 , a drum encoder  59  and a reference sensor  60  (shown in  FIG. 2 ). The separator  58  is provided on a downstream side of the first drum  51  along the feeding direction of papers P and peels a printed paper P apart from the first drum  51 . The drum encoder  59  generates a pulse signal in synchronization with a rotational angle of the first drum  51 . The reference sensor  60  detects a rotational reference position of the first drum  51 . 
     According to the above-mentioned configurations, the first printing unit  5  clamps an edge of stencil sheet  31  fed from the stencil making unit  3  and then the first drum  51  is rotated to wind the fed stencil sheet  31  on the outer circumferential wall of the first drum  51 . Papers P fed in synchronization of the rotation of the first drum  51  are pressed onto the stencil sheet  31  wound around the first drum  51 , so that inks are transferred onto one side of the papers P through perforated portion on the stencil sheet  31  to print images on the one side of the papers P. 
     The intermediate sheet transport section  6  is provided downstream of the first printing unit  5  along the feeding direction of papers P. The intermediate sheet transport section  6  includes a sheet transport section (unit)  61  and a sheet reversing section (unit)  62 . 
     The sheet transport section  61  includes a pair of pulleys  63 , a continuously-looped feeding belt  64  and a fan  65 . The pair of pulleys  63  is provided with a preset distance therebetween and rotated by a motor (not shown). The feeding belt  64  is placed around the pair of pulleys  63  and moved along with a rotation of the pair of pulleys  63 . The fan  65  generates a suction force for suctioning papers P onto the feeding belt  64 . The sheet transport section  61  receives papers P whose one side was already printed in the first printing unit  5  and transports them to the reversing section  62  with suctioning another unprinted side of the papers P on the feeding belt  64 . 
     The sheet reversing section  62  includes a pair of pulleys  66 , a curved support plate  67 , a continuously-looped feeding belt  68  and a fan  69 . The pair of pulleys  66  is rotated by a motor (not shown). The feeding belt  68  is placed around the pair of pulleys  66  and the support plate  67  and moved along with a rotation of the pair of pulleys  66 . The fan  69  generates a suction force for suctioning papers P onto the feeding belt  68 . 
     The sheet reversing section  62  moves the feeding belt  68  in a clockwise direction in  FIG. 1  to reverse the papers P fed from the sheet transport section  61  with suctioning the other imprinted side of the papers P on the feeding belt  68 . Then, the papers P are transported to the sheet stacking unit  7  with the printed one side faced down. 
     The sheet stacking unit  7  is provided downstream of the intermediate sheet transport section  6 . At the sheet stacking unit  7 , the papers P reversed at the sheet reversing section  62  are stacked with the printed one side faced down. Time for drying inks is secured by temporally stocking the papers P whose one side has been printed. 
     A sheet stack detection sensor (sheet stack detector)  71  is provided between the intermediate sheet transport section  6  and the sheet stacking unit  7 . The sheet stack detection sensor  71  detects the papers P passing from the intermediate sheet transport section  6  to the sheet stacking unit  7 . 
     The intermediate sheet feed section  8  includes a sheet feed out section (unit)  81 , tertiary sheet feed rollers  82  and quaternary sheet feed rollers  83 . The sheet feed out section  81  feeds out the papers P stacked on the sheet stacking unit  7  sheet by sheet. The tertiary sheet feed rollers  82  feed the papers P fed out by the sheet feed out section  81 . The quaternary sheet feed rollers  83  subsequently feed the papers P fed from the tertiary sheet feed rollers  82  toward the second printing unit  9  at a preset timing. 
     The sheet feed out section  81  has the same configurations as those of the sheet transport section  61 . Namely, the sheet feed out section  81  includes a pair of pulleys  84 , a continuously-looped feeding belt  85  and a fan  86 . The pair of pulleys  84  is provided with a preset distance therebetween and rotated by a motor (not shown). The feeding belt  85  is placed around the pair of pulleys  84  and moved along with a rotation of the pair of pulleys  84 . The fan  86  generates a suction force for suctioning papers P onto the feeding belt  85 . 
     The sheet feed out section  81  is controlled by the controller  12  to start feeding-out of the undermost paper P from the papers P stacked on the sheet stacking unit  7  sheet by sheet after a preset time has elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit  7 . 
     The second printing unit  9  is provided downstream of the intermediate sheet feed section  8  along the feeding direction of papers P and has the same configurations to those of the first printing unit  5 . Namely, the second printing unit  9  includes the second drum  91 , a second pressure roller  92  and a second stencil disposal unit  93 . 
     The second drum has the same configurations as those of the first drum  51 . Namely, a clamper  94  is provided on an outer circumferential wall of the second drum  91 . The second drum  91  is rotated by the main motor  13  in synchronization with the rotation of the first drum  51 . 
     An ink supply unit  95  is provided within the second drum  91 . Inks are supplied from an ink supply pump (not shown) to a cleavage between a squeegee roller  96  and a doctor roller  97  that are rotated in synchronization with a rotation of the second roller  91  to form ink deposition and then supplied onto an inner surface of the second drum  91 . 
     The second pressure roller  92  is provided outside the outer circumferential wall of the second drum  91  at a position coincident with the squeegee roller  96 . The second pressure roller  92  presses a paper P fed from the quaternary sheet feed rollers  83  onto the outer circumferential wall of the second drum  91 . The second pressure roller  92  can be moved by a drive unit (not shown) between a pressing position and a resting position similarly to the first pressure roller  51 . 
     The second stencil disposal unit  93  removes a used stencil sheet  31  from the second drum  91  and then stocks the used stencil sheet  31  therewithin. 
     In addition, the second printing unit  9  further includes a separator  98  and a reference sensor  8  (not shown). The separator  98  is provided on a downstream side of the second drum  91  along the feeding direction of papers P and peels a printed paper P apart from the second drum  91 . The reference sensor detects a rotational reference position of the second drum  91 . 
     According to the above-mentioned configurations, the second printing unit  9  clamps an edge of a stencil sheet  31  fed from the stencil making unit  3  and then the second drum  91  is rotated to wind the fed stencil sheet  31  on the outer circumferential wall of the second drum  91 . The papers P fed in synchronization of the rotation of the second drum  91  are pressed onto the stencil sheet  31  wound around the second drum  51 , so that inks are transferred onto the other side of the papers P through perforated portion on the stencil sheet  31  to print images on the other side of the papers P. 
     The sheet ejection section  10  is provided downstream of the second printing unit  9  along the deeding direction of papers P. The sheet ejection section  10  includes a sheet ejection section (unit)  101  and a sheet ejection tray  102 . 
     The sheet ejection section  101  has the same configurations as those of the sheet transport section  61 . Namely, the sheet ejection section  101  includes a pair of pulleys  103 , a continuously-looped feeding belt  104  and a fan  105 . The pair of pulleys  103  is provided with a preset distance therebetween and rotated by a motor (not shown). The feeding belt  104  is placed around the pair of pulleys  103  and moved along with a rotation of the pair of pulleys  103 . The fan  105  generates a suction force for suctioning papers P onto the feeding belt  104 . 
     The sheet ejection section  101  receives the papers P whose both sides was already printed from the second printing unit  9  and transports them to the sheet ejection tray  102  with suctioning the papers P on the feeding belt  103 . Then, the papers P whose both sides were already printed and that are ejected by the sheet ejection section  101  are stacked on the sheet ejection tray  102 . 
     The operation panel  11  is provided at the upper portion of the duplex printer  1 . The operation panel  11  includes various operational keys (not shown), such as a start key for starting a stencil making operation, a printing operation or the like, a stop key for stopping the above operations, a mode selection key for selecting one of printing modes (e.g., a duplex printing mode) and numerical keys for inputting the number of sheets to be printed or the like. 
     In addition, a touchscreen (not shown) is provided at the operation panel  11 . The touchscreen concurrently equips a function as a display device and a function of an input device. A user can perform operations such as an input of various function settings by touching a surface of the touchscreen with his/her finger or the like. Further, the operation panel  11  displays a warning display screen  120  on the touchscreen to notify a paper jam to a user, as explained later. 
     In the duplex printer  11 , plural sheet detection sensor (sheet detectors) are provided, i.e. a sheet sensor  111 , a first feed sensor  112 , a second feed sensor  113  and a sheet ejection sensor  114 . 
     A detection point of the sheet sensor  111  is set at a position between the primary sheet feed rollers  42  and the secondary sheet feed rollers  43 . The sheet sensor  111  detects whether or not a paper P is fed from the primary sheet feed rollers  42 . A detection point of the first feed sensor  112  is set at a position between the sheet transport section  61  and the sheet reversing section  62 . The first feed sensor  112  detects whether or not a paper P is fed from the sheet transport section  61 . 
     A detection point of the second feed sensor  113  is set at a position between the tertiary sheet feed rollers  82  and the quaternary sheet feed rollers  83 . The second feed sensor  113  detects whether or not a paper P is fed from the tertiary sheet feed rollers  82 . A detection point of the sheet ejection sensor  114  is set at the sheet ejection section  101 . The sheet ejection sensor  114  detects whether or not a paper P is fed from the second printing unit  9 . 
     The above-mentioned sensors  111  to  114  function as sheet detectors for detecting papers P feed along a first transport path  115  and a second transport path  116 . The first transport path  115  is a path from the sheet feed section  4  to the sheet stacking unit  7  via the first printing unit  5 . The second transport path  116  is a path from sheet stacking unit  7  to the sheet ejection section  10  via the second printing unit  9 . Each output of the sensors  111  to  114  is supplied to the controller  12 . 
     Note that each of the sensors  111  to  114  may be a pass-through type sensor composed of a light emitting element and a light receiving element that are located oppositely or a reflective type sensor composed of a light emitting element and a light receiving element that are located sequentially. 
     The controller  12  controls whole operations of the duplex printer  11  by executing processings according to control programs. The controller  12  is composed of a CPU (Central Processing Unit) that executes processings such as various arithmetic processings and input/output of data, and so on. 
     As shown in  FIG. 2 , a ROM (Read Only Memory)  14  and a RAM (Random Access Memory)  15  are connected to the controller  12 . The ROM  14  stores the control programs and so on. The RAM  15  is utilized as a temporary memory storage of data and a work area for computing of the controller  12 . The controller  12  controls, according to the control programs, the original scanning unit  2 , the stencil making unit  3 , the sheet feed section  4 , the first printing unit  5 , the intermediate sheet transport section  6 , the intermediate sheet feed section  8 , the second printing unit  9 , the sheet ejection section  10 , the operation panel  11  and the main motor  13 . 
     In addition, image data is input to the controller  12  from an image processing unit  16 . The image data is digital data for image processing for making a stencil sheet. The digital data to be input to the controller  12  is made through A/D conversion of electrical analog signals generated based on scanning by the original scanning unit  2 . 
     Further, the controller  12  receives each output of the drum encoder  59 , the reference sensor  60 , the sheet sensor  111 , the first feed sensor  112 , the second feed sensor  113  and the sheet ejection sensor  114 . The controller  12  calculates a rotational angle of the first drum  51  based on a reference pulse output from the reference sensor  60  and a pulse continuously output from the drum encoder  59 . The controller  12  determines whether or not a paper jam occurs based on the calculated rotational angle and the outputs of the sheet sensor  111 , the first feed sensor  112 , the second feed sensor  113  and the sheet ejection sensor  114 . 
     Furthermore, the controller  12  also receives an output of the sheet stack detection sensor  71 . The controller  12  controls the sheet feed out section  81  of the intermediate sheet feed section  8  so as to start feeding-out of the papers P stacked on the sheet stacking unit  7  sheet by sheet in a stacked order after the preset time has elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit  7 . 
     Next, operations of the duplex printer  1  will be explained. Main operations from stencil making to printing will be described hereinafter. 
     Upon receiving a command signal for starting stencil making in a duplex printing mode due to a user&#39;s operation on the operation panel  11 , the first and second stencil disposal unit  53  and  93  are operated to remove used stencil sheets wound around the outer circumferential walls of the first and second drums  51  and  91 , respectively. 
     Subsequently, the controller  12  drives the original scanning unit  2  to scan both sides of an original. Analog signals generated based on scanning by the original scanning unit  2  is converted to digital data by an A/D converter (not shown) and the digital data are transmitted to the image processing unit  16 . 
     The image processing unit  16  converts the digital data from the A/D converter to binary data of black-and-white image data and then also generates control data for the thermal head  34  in the stencil making unit  3  based on the binary data. The image processing unit  16  transmits these data to the controller  12  as the image data. The controller  12  stores the image data relating to the both sides of the original in the RAM  15 . 
     Subsequently, the controller  12  drives the stencil making unit  3  to execute stencil making processing for one side of the original. In the stencil making processing, the controller  12  drives the feed rollers  33  and the paten roller  35  in the stencil making unit  3  to feed a stencil sheet  31  with pressing it onto the thermal head  34 . In synchronization with this feeding of the stencil sheet  31 , the controller  12  retrieves the image data relating to the one side of the original from the RAM  15  to perforate the stencil sheet  31  by heats generated by the heater elements of the thermal head  34 . Images corresponding to the one side of the original are transferred on the stencil sheet  31  as a perforated pattern. 
     Subsequently, the controller  12  moves the stencil making unit  3  to the first stencil sheet supply position using the drive unit (not shown) and causes the clamper  54  to clamp and edge of the stencil sheet  31  on which the images corresponding to the one side of the original are transferred. Then, the controller  12  rotates the first drum  51  in a predetermined direction by controlling the main motor  13  to wind the stencil sheet  31  around the outer circumferential surface of the first drum  51 . An end edge of the stencil sheet  31  is cut out by the cutter  37 , so that the stencil sheet  31  is entirely wound around the outer circumferential surface of the first drum  51 . In this manner, a loading process of the stencil sheet  31  on the first drum  51  is completed. 
     Next, the controller  12  operates the stencil making unit  3  to execute stencil making processing for another side of the original similarly to the stencil making processing for the one side of the original. 
     Upon completion of the stencil making processing for the other side of the original, the controller  12  moves the stencil making unit  3  to the second stencil sheet supply position and then performs a loading process of the stencil sheet  31  on the second drum  91  similarly to the loading process of the stencil sheet  31  on the first drum  51 . 
     When a command signal for starting duplex printing is input due to a user&#39;s operation on the operation panel  11  after completion of the loading processes of the stencil sheets  31  on the first and second drums  51  and  91 , the controller  12  rotates the first and second drums  51  and  91  in synchronization by controlling the main motor  13 . Collaterally, the controller  12  rotates the primary and secondary sheet feed rollers  42  and  43  to feed a paper P to the first drum  51 . In synchronization with these operations, the controller  12  also moves the first pressure roller  52  to the pressing position to press the paper P onto the stencil sheet  31  wound around the outer circumferential surface of the first drum  51 . 
     Supplying inks from the ink supply unit  55  to the inner surface of the first drum  51 , the inks are permeated through the perforated portions of the stencil sheet  31 . Then, the images corresponding to the one side of the original are printed on one side of the paper P that is pressed onto the stencil sheet  31  wound around the outer circumferential surface of the first drum  51  by the first pressure roller  52 . 
     The paper P that has been printed on its one side is peeled apart from the first drum  51  by the separator  58  and then transported to the intermediate sheet transport section  6 . 
     The controller  12  operates the sheet transport section  61  of the intermediate sheet transport section  6  to transport the paper P to the sheet reversing section  62 . Then, the controller  12  operates the sheet reversing section  62  to reverse the paper P with suctioning another unprinted side of the paper P on the feeding belt  68 . The paper P whose printed one side was faced up is transported to the sheet stacking unit  7  after the printed one side is faced down. 
     In a case where plural sheets of papers P are printed, the above-described series of processes is repeated. Therefore, the paper P with its printed one side faced down is sequentially stacked on the sheet stacking unit  7 . 
     The controller  12  controls the sheet feed out section  81  of the intermediate sheet feed section  8  so as to start feeding-out of the undermost paper P from the papers P stacked on the sheet stacking unit  7  sheet by sheet after a preset time has elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit  7 . The papers P fed to the sheet stacking unit  7  are stocked on the sheet stacking unit  7  until the preset time has elapsed and thereby time for drying inks is secured. 
     Subsequently, the controller  12  rotates the tertiary and quaternary sheet feed rollers  82  and  83  to transport the paper P fed out from the sheet stacking unit  7  to the second drum  91  using the sheet feed out section  81 . In synchronization with this operation, the controller  12  also moves the second pressure roller  92  to the pressing position to press the paper P onto the stencil sheet  31  wound around the outer circumferential surface of the second drum  91 . 
     Supplying inks from the ink supply unit  95  to the inner surface of the second drum  91 , the inks are permeated through the perforated portions of the stencil sheet  31 . Then, the images corresponding to the other side of the original are printed on the other side of the paper P that is pressed onto the stencil sheet  31  wound around the outer circumferential surface of the second drum  91  by the second pressure roller  92 . 
     The paper P that has been printed on its both sides is peeled apart from the second drum  91  by the separator  98  and then ejected to the sheet ejection tray  102  by the sheet ejection section  101 . By repeating the above-described series of processes as many times as the number of sheets to be printed, the duplex printing process is completed. 
     In the above-described duplex printing operation processes, the controller  12  calculates a rotational angle of the first drum  51  based on a reference pulse output from the reference sensor  60  and a pulse continuously output from the drum encoder  59 . Then, the controller  12  determines whether or not a paper jam occurs based on the calculated rotational angle and the outputs of the sheet sensor  111 , the first feed sensor  112 , the second feed sensor  113  and the sheet ejection sensor  114 . 
     Operations of the duplex printer  1  in a case where a paper jam occurs will be explained with reference to a flowchart shown in  FIG. 3 . 
     In step S 10 , the controller  12  determines whether or not a paper jam(s) occurs based on detection results of the sensors  111  to  114  at each time when the rotational angle of the first drum  51  becomes coincident with each reference angle preset for the sensors  111  to  114 . 
     When at least one of the sensors  111  to  114  doesn&#39;t detect the paper P at the time corresponds with the reference angle, the controller  12  determines that a paper jam occurs. On the other hand, when all four of the sensors  111  to  114  detect the paper P at each time corresponds with the reference angles, the controller  12  determines that no paper jam occurs. 
     When it is determined that no paper jam occurs (NO in step S 10 ), the process flow returns to step S 10 . On the other hand, when it is determined that a paper jam occurs (YES in step S 10 ), the controller  12  determines whether or not the paper jam(s) occurs only in the first transport path  115  in step S 20 . Specifically, it is determined that the paper jam(s) occurs only in the first transport path  115  in a case where the sensor that didn&#39;t detect the paper P at the time corresponds with the reference angle in step S 10  is the sheet sensor  111  or the first feed sensor  112 . 
     When it is determined that the paper jam(s) occurs only in the first transport path  115  (YES in step S 20 ), the process flow proceeds to step S 30 . On the other hand, when it is determined that the paper jam(s) occurs only in the second transport path  116  or both in the first and second transport paths  115  and  116  (NO in step S 20 ), the controller  12  stop operating the units and sections of the duplex printer  1  in step S 40  and then the process flow proceeds to step S 50 . 
     In step S 30 , the controller  12  stops operations of the sheet feed section  4 , the first printing unit  5  and the intermediate sheet transport section  6  that assume an upstream transport process of papers P from the sheet stacking unit  7 . Collaterally, the controller  12  sequentially feeds out the papers P stacked on the sheet stacking unit  7  using the intermediate sheet feed section  8  to print each other side of the fed-out papers P using the second drum  91 . The papers P that have been printed on their both sides are ejected by the sheet ejection section  10 . On completion of ejecting all the papers P that were stacked on the sheet stacking unit  7  and are printed on their both sides, the process flow proceeds to step S 50 . 
     Note that, if the preset time has not yet elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit  7  at a time when the paper jam(s) occurs only in the first transport path  115 , the controller  12  starts feeding out the papers P stacked on the sheet stacking unit  7  using the intermediate sheet feed section  8  after the preset time has elapsed. 
     In step S 50 , the controller  12  displays the warning display screen  120  shown in  FIG. 4  on the touchscreen of the operation panel  11  to notify a paper jam to a user. 
     According to the above-described embodiment, when a paper jam(s) occurs only in an upstream feed path of papers P to the sheet stacking unit  7 , an upstream transport process of papers P from the sheet stacking unit  7  is stopped. Collaterally, the intermediate sheet feed section  8 , the second printing unit  9  and the sheet ejection section  10  that assume a downstream transport process of papers P from the sheet stacking unit  7  are operated to print the other side of all the papers P stacked on the sheet stacking unit  7  at a time when the paper jam(s) occurs and eject them. Since there no paper P is on the sheet stacking unit  7  at a time when a user clears the paper jam(s), it can be prevented for the user to accidentally touch the papers P whose other side is not yet printed. Therefore, the papers P never become tainted or get winkled and thereby papers never be wasted needlessly. 
     In addition, when the paper jam(s) occurs only in the first transport path  115 , the warning display screen  120  is displayed after ejecting all the papers P stacked on the sheet stacking unit  7 . Therefore, it can be prevented for a user to try to clear the paper jam(s) while the papers P remain on the sheet stacking unit  7 . 
     Note that, in the above descriptions, the papers P are fed out from the sheet stacking unit  7  by the sheet feed out section  81  after the preset time has elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit  7  (see  FIG. 5 : step S 22 ). However, feeding-out of the papers P may be started after the number of papers P stacked on the on the sheet stacking unit  7  has reached to a preset value (see  FIG. 6 : YES in step S 21 ). 
     In this case, the controller  12  counts the number of papers P stacked on the on the sheet stacking unit  7  based on the detection results of the sheet stack detection sensor  71  and feeds out the undermost paper P from the papers P stacked on the sheet stacking unit  7  sheet by sheet after the number of stacked papers P has reached to the preset value. 
     According to this operation, time for drying inks can be secured for the papers P whose one side has already been printed and that are stocked on the sheet stacking unit  7 . 
     Here, if the number of stacked sheets on the sheet stacking unit  7  has not reached to a preset value at a time when the paper jam(s) occurs only in the first transport path  115  (see  FIG. 6 : NO in step S 21 ), the controller  12  firstly stops operations of the sheet feed section  4 , the first printing unit  5  and the intermediate sheet transport section  6  (step S 301 ). And then, the controller  12  starts feeding out the papers P stacked on the sheet stacking unit  7  using the intermediate sheet feed section  8  (step S 30 ) after the preset time has elapsed since the sheet stack detection sensor  71  has detected the first paper P to be stacked on the sheet stacking unit (see  FIG. 6 : YES in step S 22 ). Therefore, time for drying inks can be secured for the papers P whose one side has already been printed. Note that, in this case, the upstream transport process of papers P from the sheet stacking unit  7  (the operations at the upstream of the sheet feed section  4 , the first printing unit  5  and the intermediate sheet transport section  6 ) is not stopped in step S 30  but stopped in step S 301  previously. 
     In the above-described embodiment, stencil printing is done using the image data made from scanning of the original by the original scanning unit  2 . However, the present invention can be applied to a case where used is image data transmitted from a information processing device such as a personal computer connected via a communication link or the like. 
     In the above-described embodiment, papers P being transported are detected by the four sensors  111  to  114 . However, the number of sensors for detecting papers P is not limited to the above embodiment. Installation positions of sensors for detecting papers P are not limited to the above embodiment, either.