Patent Publication Number: US-7708398-B2

Title: Printing apparatus and printing method

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to a printing apparatus and a printing method. 
   The printing apparatus is not limited to a printer, copier, and a facsimile which perform a printing process in an ink jet manner, but broadly includes apparatuses used to manufacture a color filter of a liquid crystal display, an organic EL display, a biochip, etc. 
   2. Related Art 
   As an example of such a printing apparatus, an ink jet printer is known. A typical ink jet printer prints images by alternately performing a dot formation process in which a moving head ejects ink to form dots and a transport process in which a medium (paper, fabric, OHP paper, etc) is transported. 
   Such a printer may include a sensor for sensing the end of the medium. In addition, such a sensor may include a lever arranged so as to rotatably move in the way of a transport path of the medium and a photo interrupter for changing a signal level of a detection signal in response to movement of the lever (for example, see JP-A-8-259037). 
   In a sensor using such a lever, paper may be jammed when the paper is transported in a backward direction. 
   SUMMARY 
   An advantage of some aspects of the invention is that it provides a printing apparatus capable of sensing the end of a medium with no danger of medium jam occurring in backward transport. 
   According to an aspect of the invention, there is provided a printing apparatus including: (A) a transport mechanism transporting a medium in a forward direction and a backward direction; (B) a head printing dots on the medium; (C) a sensor sensing existence of the medium in a non-contact manner; (D) a reversion mechanism reversing the medium; and (E) a controller controlling the sensor to sense an end of the medium when the transport mechanism transports the medium in the forward direction so as to allow the head to print the dots on the surface of the medium and controlling the sensor to sense the end of the medium when the transport mechanism transports the medium in the backward direction so as to allow the reversion mechanism to reverse the medium after the dots are printed on the surface thereof. 
   Other features of the invention are apparent from the following description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
       FIG. 1  is a schematic diagram illustrating a lateral end surface of a printer. 
       FIG. 2  is a block diagram illustrating an overall configuration of the printer. 
       FIG. 3  is diagram illustrating a path of rear feeding. 
       FIG. 4  is a diagram illustrating a path of front feeding. 
       FIGS. 5A to 5C  are diagrams explaining transport and ejection. 
       FIG. 6  is a diagram illustrating a path of reversion. 
       FIG. 7  is a diagram illustrating an optical sensor from view of feeding side. 
       FIG. 8A  is a diagram explaining detection of the front end of paper sheet P at the time of forward transport. 
       FIG. 8B  is a diagram explaining detection of the rear end of the paper sheet P at time of forward transport. 
       FIG. 8C  is a diagram explaining detection of the rear end of the paper sheet P at time of backward transport. 
       FIG. 8D  is a diagram explaining detection of the front end of the paper sheet P at time of backward transport. 
       FIGS. 9A and 9B  are diagrams explaining a comparative example. 
       FIG. 9C  is a diagram explaining another comparative example. 
       FIG. 10  is a diagram illustrating a configuration of a lower guide. 
       FIG. 11  is a diagram illustrating the lower guide at the vicinity of the optical sensor. 
       FIG. 12  is a diagram illustrating an upper guide unit having an upper guide. 
       FIG. 13  is a diagram illustrating the vicinity of the upper guide unit from view of a platen side. 
       FIG. 14  is a diagram illustrating the vicinity of the upper guide unit from view of feeding side. 
       FIGS. 15A to 15C  show that a seal member is attached to the upper guide. 
       FIG. 16  shows that an upper protrusion portion is inserted into a concave portion. 
       FIG. 17  is a diagram explaining a relationship between the seal member and the upper protrusion portion. 
       FIG. 18  is a diagram illustrating a platen when viewed obliquely. 
       FIG. 19A  shows that the front end of the paper sheet P is printed in a printing process with no margin. 
       FIG. 19B  shows that the rear end of the paper sheet P is printed in the printing process with no margin. 
       FIG. 19C  shows that the side end of the paper sheet P is printed in the printing process with no margin. 
       FIG. 20  is a flow chart illustrating a printing process of both surfaces according to an embodiment. 
       FIG. 21  is a diagram illustrating a position of the rear end of the paper sheet P at the time of finishing a surface printing process. 
       FIG. 22  is a diagram illustrating a sensor sensing the end of paper in a comparative example. 
       FIGS. 23A and 23B  are diagrams explaining the surface printing process in the comparative example. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The following descriptive features are apparent from the following description and the accompanying drawings. 
   A printing apparatus includes (A) a transport mechanism transporting a medium in a forward direction and a backward direction; (B) a head printing dots on the medium; (C) a sensor sensing existence of the medium in a non-contact manner; (D) a reversion mechanism reversing the medium; and (E) a controller controlling the sensor to sense an end of the medium when the transport mechanism transports the medium in the forward direction so as to allow the head to print the dots on the surface of the medium and controlling the sensor to sense the end of the medium when the transport mechanism transports the medium in the backward direction so as to allow the reversion mechanism to reverse the medium after the dots are printed on the surface thereof. 
   In the printing apparatus with the above-described configuration, it is possible to sense the end of the medium with no danger of medium jam occurring in the backward transport. 
   The sensor may include a light-emitting portion and a light-receiving portion, and the printing apparatus may further includes a lower guide and an upper guide for guiding the medium between the light-emitting portion and the light-receiving portion. In this way, it is possible to shorten a distance between the light-emitting portion and the light-receiving portion and to pass the medium between the light-emitting portion and the light-receiving portion. 
   The sensor may include an upper protrusion portion and a lower protrusion portion, the upper protrusion portion may include one of the light-emitting portion and the light-receiving portion and the lower protrusion portion includes the other thereof, the lower guide may include a base surface, ribs protruding upward from the base surface so as to contact the medium, and an insertion portion for inserting the lower protrusion portion, and an opening may be provided at a position of an optical axis of the sensor in the lower guide and the opening is formed in the base surface between the ribs. In this way, it is possible to prevent the medium from becoming jammed. 
   The top surface of the lower protrusion portion may be positioned above the base surface of the lower guide. In this way, it is possible to improve sensing precision of the sensor. 
   The lower guide may further include an uplift portion covering the lower protrusion portion, and an inclined plane may be formed between the base surface on an upstream side and a downstream side of the uplift portion in the forward direction. In this way, it is possible to prevent the medium from becoming jammed. 
   The printing apparatus may further include a flexible seal member. In the printing apparatus, the sensor may be fixed to the lower guide, the upper guide may be pivotable on the lower guide, and the seal member may seal a boundary between the sensor and the upper guide. In this way, it is possible to prevent the medium from becoming inserted into the boundary between the relatively pivotable upper guide and the sensor. 
   The printing apparatus may further include: a first bulge supporting the medium; a second bulge supporting the medium and being provided on a more downstream side in the forward direction than the first bulge; and a groove formed between the first bulge and the second bulge. In the printing apparatus, the controller may control the transport mechanism to transport the medium so as not to position the rear of the medium on the more downstream side of the forward direction than the first bulge, and simultaneously controls the head to print the dots on one surface of the medium. In addition, the controller may control the transport mechanism to transport the medium so as to position the rear of the medium in the groove, and simultaneously controls the head to print the dots on the rear end of the medium while printing the dots on the other-side of the medium. In this way, when the medium is transported backward after the printing process of the dots on the surface, it is difficult for the rear end of the medium to become jammed in the first bulge, thereby preventing the medium from becoming jammed. 
   A printing method includes: feeding a medium to a transport mechanism; sensing the end of the medium with a non-contact sensor before feeding the medium to the transport mechanism; printing dots on a surface of the medium while the transport mechanism transports the medium in a forward direction; transporting the medium in a backward direction, which is a direction opposite to the forward direction in order that a reversion mechanism reverses the medium after printing the dots on the surface of the medium; and sensing the end of the medium by means of the non-contact sensor when the medium is transported in the backward direction. 
   According to such a printing method, it is possible to sense the end of the medium in a manner in which the medium does not become jammed at the time of the backward transport. 
   Overview of Printer 
     FIG. 1  is a schematic diagram illustrating a lateral end surface of the printer.  FIG. 2  is a block diagram illustrating an overall configuration of the printer. 
   A printer  1  includes a rear feeding unit  2 , a front feeding unit  3 , a carriage unit  4 , a transport unit  5 , an ejection unit  6 , and a controller  10 . 
   The rear feeding unit  2  is a mechanism for feeding paper sheet P, which stocks in a rear surface of the printer  1 , to the transport unit  5 . The front feeding unit  3  is a mechanism for feeding the paper sheet P from a tray  25 , which can be attached to a front surface of the printer  1 , to the transport unit  5 . The carriage unit  4  is a mechanism for allowing a head  48  to eject ink in order to form an image on the paper sheet P while moving to a carriage  46 . The carriage  46  is guided by a shaft  47  so as to move in the vertical direction of  FIG. 1 . The transport unit  5  is a mechanism for transporting the fed paper sheet P and control the position of the paper sheet P relative to the head  48 . The ejection unit  6  is a mechanism for ejecting the paper on which the image is formed to outside of the printer  1 . The controller  10  controls the entirety of the printer  1  so as to control each unit on the basis of print data received from outside apparatuses. 
   The printer  1  can optionally retain a reversion unit  100 . The reversion unit  100  is a mechanism for reversing the paper sheet P when both surfaces of the paper sheet P are to be printed upon. Various sensors (sensor group) are provided in the printer  1 . As described below, an optical sensor  37  is included in the sensor group. The controller  10  controls each unit on the basis of the sensing result of the sensor group. 
   As guide members for guiding the paper during feeding or transport, a guide roller  40 , an upper guide  43 , a lower guide  44 , and a platen  45  are provided in the printer  1 . 
   Path of Paper Sheet P 
   Rear Feeding 
   A path of the paper sheet P during rear feeding will be described with reference to  FIG. 1 . In addition, timing of operation and drive of each member is controlled by the controller  10 . 
   At the time of starting the feeding process, a hopper  12  is raised about a supporting point  12   a  so that the uppermost sheet of the paper sheet P stacked in the hopper  12  comes in contact with a feeding roller  11 . When a rear feeding roller  11  rotates, the paper sheet P is fed to a downstream side. The fed paper sheet P is inserted between the rear feeding roller  11  and a retardation roller  13 . A torque limiter mechanism applies a predetermined rotation resistance to the retardation roller  13  in order to prevent two sheets of the paper sheet P from being transported at once. The sheet of the paper sheet P stopped by the retardation roller  13  is returned to the hopper  12  by a returning lever  14 . 
     FIG. 3  is diagram illustrating a path of rear feeding. The rotation of the rear feeding roller  11  induces the paper sheet P to be fed from hopper  12  to the inside of the printer  1 . Generally, the front end of the fed paper sheet P is guided downward to the right by the rear feeding guide  15 . Moreover, when the rear feeding roller  11  continues to rotate, the front end of the paper sheet P comes in contact with the lower guide  44 , and is then guided rightward by the lower guide  44 . When the paper sheet P is bent upward, the front end of the paper sheet P comes in contact with an upper guide  43  or the guide roller  40  so as to be guided rightward by the upper guide  43 . The front end of the paper sheet P guided by the upper guide  43  or the lower guide  44  passes through the optical sensor  37 , which is described below, and reaches the transport unit  5 . 
   Front Feeding 
     FIG. 4  is a diagram illustrating a path of front feeding. Hereinafter, the path of the paper sheet P at the time of front feeding will be described with reference to  FIG. 1 . 
   In the tray  25 , a pick-up roller  26  comes in contact with the uppermost sheet of the paper sheet P. At this time, when the pick-up roller  26  rotates, the paper sheet P continues to be moved to the front feeding roller  28 . The paper sheet P that continues to be moved from the tray  25  is inserted between the front feeding roller  28  and a separating roller  29 , and then continues to be moved to an assist roller  30  by rotation of the front feeding roller  28  while double sheet feeding is prevented by the separating roller  29 . The continuous rotation of the front feeding roller  28  induces the paper sheet P inserted between the front feeding roller  28  and the assist roller  30  to be fed. At this time, the front end of the paper sheet P raises the hopper  49 , and then a lower surface of the hopper  49  guides the paper sheet P rightward. While the front end of the paper sheet P guided by the hopper  49  is guided by the upper guide  43  or the lower guide  44 , the front end of the paper sheet P passes through the optical sensor  37 , which is described below, and then reaches the transport unit  5 . 
   Transport and Ejection 
     FIGS. 5A to 5C  are diagrams explaining transport and ejection. Hereinafter, transport of the paper sheet P by the transport unit  5  will be described with reference to  FIG. 1 . 
   The paper sheet P fed from the rear feeding unit  2  or the front feeding unit  3  is inserted into a transport roller  41  of the transport unit  5  and a driven roller  42 . While an amount of rotation of the transport roller  41  is controlled by a controller  10  and a position of the paper sheet P relative to the head  48  is controlled, the paper sheet P is transported. That is, the paper sheet P is transported by the transport roller  41 . At this time, the paper sheet P is supported by the platen  45  from below. In addition, as described below, a groove for carrying out the printing process with no margin is provided in the platen  45 . 
   When the paper sheet P is transported to a position opposite the head  48 , an image formed by numerous dots is printed on the paper sheet P by a dot formation process in which dots are formed on the paper sheet P by ejecting ink from the head  48  while the carriage  46  and a transport process in which the paper sheet P is transported by a predetermined amount are alternately performed. When the transport process is carried out several times, the front end of the paper sheet P passes through the auxiliary roller  57 . In addition, when the process is further carried out several times, the front end of the paper sheet P reaches the ejection unit  6  (where the paper sheet P is inserted between the ejection roller  55  and an ejection-side driven roller  56 ). 
   After the front end of the paper sheet P reaches the ejection unit  6 , the paper sheet P is transported by the transport roller  41  and the ejection roller  55 . The transport roller  41  and the ejection roller  55  are controlled so as to be synchronized with each other. At this time, when the transport process is carried out several times, the rear end of the paper sheet P passes through the optical sensor  37 , which is described below, and then passes through the transport roller  41 . 
   The rear end of the paper sheet P passes through the transport roller  41 , and then an amount of rotation of the ejection roller  55  is controlled by the controller  10 . At this time, the paper sheet P is transported while the position of the paper sheet P relative to the head  48  is controlled. That is, the paper sheet P is transported by the ejection roller  55 . When the printing process of the image on the paper sheet P is finished, the controller  10  further rotates the ejection roller  55  to eject the paper sheet P. 
   When the transport roller  41  or the ejection roller  55  is reversely rotated, as described below, the paper sheet P can be transported in a reverse direction (reverse transport). When transporting the paper sheet P to the reversion unit  100  to reverse the paper sheet P, the paper sheet P is reversely transported. When the reverse transport is carried out in a state where the rear end of the paper sheet P passes through the transport roller  41  and is placed on the platen  45 , the rear end of the paper sheet P passes through the transport roller  41 , and afterward passes through the optical sensor  37 . In this case, when the reverse transport continues, the front end of the paper sheet P sequentially passes through ejection roller  55 , the transport roller  41 , and the optical sensor  37 . 
   Reversion 
     FIG. 6  is a diagram explaining a path of reversion. 
   The paper sheet P reversely transported from the transport unit  5  is supplied to the reversion unit  100 . The paper supplied to the reversion unit  100  is guided upward to the left by an uplift mechanism (not shown), and then reaches a first roller  102 . The first roller  102  is configured to rotate in synchronization with the transport roller  41 . An amount of rotation of the first roller  102  is controlled by the controller  10  so that the paper sheet P is supplied to a second roller  104 . The second roller  104  is configured so as to rotate in synchronization with the first roller  102 . That is, an amount of rotation of the second roller  104  is also controlled by the controller  10 . When the second roller  104  rotates, the paper sheet P presses the hopper  49  downward so that the paper sheet P is guided rightward by the upper surface of the hopper  49 . 
   Before the paper sheet P is supplied to the reversion unit  100 , the image is printed on the upper surface of the paper sheet P. When the paper sheet P is supplied to the reversion unit  100 , and then reaches the reversion unit  5 , the printed surface is turned downward and the back surface of the paper sheet P is turned upward. In the following description, the upper surface of the paper sheet P before the reversion is referred to as “a one surface” and the back surface of the paper sheet P after the reversion is “the other surface”. 
   The front end of the paper sheet P at the time of printing the one surface becomes the rear end of the paper sheet P at the time of printing the other surface. In addition, the rear end of the paper sheet P at the time of printing the one surface becomes the front end of the paper sheet P at the time of printing the other surface. In the following description, after the paper sheet P is supplied from the reversion unit  100  to the transport unit  5 , the head end (the rear end of the paper sheet P at the time of printing the one surface) of the paper sheet P is replaced with “a front end” and the tail end (the front end of the paper sheet P at the time of printing the one surface) of the paper sheet P is replaced with “a rear end”. 
   In the following description, the time when the paper sheet P is transported from the feeding unit to the transport unit, time when the paper sheet P is transported from the reversion unit to the transport unit, or time when the paper sheet P is transported from the transport unit to the ejection unit is referred to as “forward transport” irrespective of reversion of the front and rear ends. Conversely, the time when the paper sheet P is transported from the transport unit to the reversion unit or time when the paper sheet P is transported from the ejection unit to the reversion unit is referred to as “backward transport”. 
   Optical Sensor  37   
   Configuration of Optical Sensor  37   
     FIG. 7  is a diagram illustrating the optical sensor  37  when viewed from feeding side. A solid line shown in  FIG. 7  is the optical sensor  37 . A diagonal line shown in  FIG. 7  is a lateral side of the paper sheet P. 
   The optical sensor  37  has a C shape. That is, the optical sensor  37  includes an upper protrusion portion  37   a,  a lower protrusion portion  37   b,  and a support portion  37   c.  A light-receiving portion  371  is provided in the upper protrusion portion  37   a  and a light-emitting portion  372  is provided in the lower protrusion portion  37   b.  The light-receiving portion  371  which is positioned above the light-emitting portion  372  senses light emitted from the light-emitting portion  372 . 
   Since the light-receiving portion  371  is provided in the upper side and a light receiving surface of the light-receiving portion  371  faces downward, dust does not settle in the light receiving surface. Accordingly, it is possible to carry out a stable sensing process. In order to maintain a precision of the position between the light-receiving portion  371  and the light-emitting portion  372 , the upper protrusion portion  37   a  and the lower protrusion portion  37   b  are incorporated with each other through the support portion  37   c  (in this way, the optical sensor  37  has the ⊂ shape). 
   Sensing of Optical Sensor  37   
   When the light-receiving portion  371  senses light emitted from the light-emitting portion  372 , the optical sensor  37  outputs a signal of an H level. Alternatively, when the light-receiving portion  371  does not sense light emitted from the emitting portion  372 , the optical sensor  37  outputs a signal of an L level. When the paper sheet P is placed in the optical sensor  37 , the paper sheet P cuts an optical axis of the optical sensor  37 . Accordingly, the light-receiving portion  371  cannot sense the light emitted from the light-emitting portion  372 , and thus the optical sensor  37  outputs the signal of the L level. In this way, the controller  10  can detect whether there is the paper sheet P placed in the optical sensor  37  on the basis of the output of the optical sensor  37 . 
   As described below, the controller  10  can detect where the end of the paper sheet P is placed, on the basis of timing of change in the output of the optical sensor  37 . 
     FIG. 8A  is a diagram explaining detection of the front end of paper sheet P at the time of the forward transport (when viewed from top). When the front end of the paper sheet P passes through the optical sensor  37 , as shown in  FIG. 8A , the output of the optical sensor  37  is changed from the H level to the L level. In this way, when the output of the optical sensor  37  is changed from the H level to the L level at the time of the forward transport, the controller  10  can detect the front end of the paper sheet P. 
     FIG. 8B  is a diagram explaining detection of the rear end of paper sheet P at the time of the forward transport. When the rear end of the paper sheet P passes through the optical sensor  37 , as shown in  FIG. 8B , the output of the optical sensor  37  is changed from the L level to the H level. In this way, when the output of the optical sensor  37  is changed from the L level to the H level at the time of the forward transport, the controller  10  can detect the rear end of the paper sheet P. 
     FIG. 8C  is a diagram explaining detection of the rear end of paper sheet P at the time of the backward transport. When the rear end of the paper sheet P passes through the optical sensor  37 , as shown in  FIG. 8C , the output of the optical sensor  37  is changed from the H level to the L level. In this way, when the output of the optical sensor  37  is changed from the H level to the L level at the time of the backward transport, the controller  10  can detect the rear end of the paper sheet P. 
     FIG. 8D  is a diagram explaining detection of the front end of paper sheet P at the time of the backward transport. When the rear end of the paper sheet P passes through the optical sensor  37 , as shown in  FIG. 8D , the output of the optical sensor  37  is changed from the L level to the H level. In this way, when the output of the optical sensor  37  is changed from the L level to the H level at the time of the backward transport, the controller  10  can detect the front end of the paper sheet P. 
   Lower Guide  44   
     FIGS. 9A and 9B  are diagrams explaining a comparative example. The lower protrusion portion  37   b  is required to be inserted into the lower guide  44  in order to pass the paper sheet P guided to the lower guide  44  between the upper protrusion portion  37   a  and the lower protrusion portion  37   b  of the optical sensor  37 . Meanwhile, the upper surface of the light-emitting portion  372  is required to be exposed through the lower guide  44  in order to allow the light emitted from the light-emitting portion  372  to be received by the light-receiving portion  371 . In this case, like the comparative example, when the lower protrusion portion  37   b  is inserted into the lower guide  44  during the exposure of the top surface of the lower protrusion portion  37   b,  the front end of the paper sheet P is interrupted by a boundary between the lower guide  44  and the lower protrusion portion  37   b.  Accordingly, the paper sheet P may become jammed. 
     FIG. 9C  is a diagram explaining another comparative example. In this comparative example, since the paper sheet P is not inserted into the boundary between the lower guide  44  and the lower protrusion portion  37   b  at the time of the forward transport of the paper sheet P, a height on the downstream side of the transport direction of the boundaries between the lower guide  44  and the lower protrusion portion  37   b  is lowered. However, in such a configuration, when the paper sheet P is transported backward, the paper sheet P may become inserted into the boundary between the lower guide  44  and the lower protrusion portion  37   b.    
   In this embodiment, the lower protrusion portion  37   b  is inserted into the lower guide  44  in the following manner. Moreover, in the configuration according to this embodiment described below, the paper can be prevented from becoming jammed even when transported backward. 
     FIG. 10  is a diagram illustrating a configuration of lower guide  44 .  FIG. 11  is a diagram illustrating a configuration of the lower guide  44  in the vicinity of the optical sensor  37 . Hereinafter, the lower guide  44  will be described with reference with  FIGS. 10 and 11 . 
   A plurality of ribs  44 B are provided in the lower guide  44 . The ribs protrude upward from a base surface  44 A and extend along a direction of transport of the paper sheet P. When the lower guide  44  guides the paper  4 , the paper sheet P is supported by the ribs  44 B. The ribs  44 B prevent the paper sheet P from becoming adhered to the base surface  44 A due to static electricity and the like. 
   The optical sensor  37  described above is provided in the lateral end of the lower guide  44 . The lower protrusion portion  37   b  of the optical sensor  37  is inserted into the lower guide  44  (where an insertion portion for inserting the lower protrusion portion  37   b  is provided in the lower guide  44 ). Moreover, the optical sensor  37  is fixed on the lower guide  44  in the support portion  37   c.    
   A top surface of the lower protrusion portion  37   b  of the optical sensor  37  (excluding an upper portion of the light-emitting portion  372 ) is covered with the uplift portion  44 C of the lower guide  44 . An inclined plane is configured between the uplift portion  44 C and the base surface  44 A. In this way, by configuring the uplift portion  44 C, it is possible to prevent the front end (the rear end at time of backward transport) of the paper sheet P from becoming jammed in the boundary between the lower guide  44  and the lower protrusion portion  37   b.    
   An opening  44 D is provided in the lower guide  44 . The light-emitting portion  372  of the optical sensor  37  is positioned below the opening  44 D and the optical axis of the optical sensor  37  passes through the opening  44 D. That is, the lower protrusion portion  37   b  of the optical sensor  37  is exposed through the opening  44 D. The opening  44 D is provided in the base surface between the ribs  44 B. Since the opening  44 D is provided in such a position, it is possible to prevent the front end (the rear end at time of the backward transport) of the paper sheet P from becoming jammed in the boundary between the lower guide  44  and the lower protrusion portion  37   b  (in addition, if the opening  44 D is provided in the rib  44 B, the front end (or the rear end) of the paper sheet P may be inserted into the boundary between the rib  44 B and the lower protrusion portion  37   b ). 
   The height of the uplift portions  44 C is configured to be the same as that of the ribs  44 B. Accordingly, it is possible to prevent the front end (the rear end at the time of the backward transport) of the paper sheet P from becoming inserted into the boundary between the ribs  44 B and the uplift portions  44 C. The surface of the uplift portion  44 C is positioned above the top surface of the lower protrusion portion  37   b  of the optical sensor  37 . In this way, it is possible to prevent the front end (the rear end at the time of the backward transport) of the paper sheet P from becoming inserted into the boundary between the lower guide  44  and the lower protrusion portion  37   b.    
   The height of the base surface  44 A may be different from that of the top surface of the lower protrusion portion  37   b  of the optical sensor  37 . In this embodiment, the height of the lower protrusion portion  37   b  is larger than that of the base surface  44 A (see  FIG. 17 ). In this way, it is possible to shorten a distance between the light-emitting portion  372  and the light-receiving portion  371  as much as possible. Accordingly, it is possible to improve a sensing precision of the optical sensor  37 . 
   When the paper sheet P is fed, the lateral side of the paper sheet P is guided to a lateral guide surface  44 E of the lower guide  44  irrespective of a size of the paper sheet P. The lateral guide surface  44 E is formed more closely to the paper sheet P than the inside of the support portion  37   c  of the optical sensor  37 . Such a configuration can prevent the lateral side of the paper sheet P from coming in contact with the inside of the support portion  37   c  of the optical sensor  37  (see the positional relationship between the lateral end (the lateral side) of the paper sheet P and the support portion  37   c ). In this way, it is possible to prevent the front end (the rear end of the backward transport) of the paper sheet P from becoming inserted into the boundary between the inside surface of the support portion  37   c  of the optical sensor  37  and the lower guide  44 . 
   Upper Guide  43   
   Configuration of Peripheral Members of Upper Guide  43   
     FIG. 12  is a diagram illustrating an upper guide unit  9  having an upper guide  43 .  FIG. 13  is a diagram illustrating the vicinity of the upper guide unit from view of a platen side.  FIG. 14  is a diagram illustrating the vicinity of the upper guide unit  9  from view of feeding side. Hereinafter, the upper guide  43  will be described with reference to  FIGS. 12 to 14  in addition to  FIG. 1 . 
   The upper guide unit  9  is a unit for retaining the above-described upper guide  43  and the transport-side driven roller  42  so as to be pivotable on the apparatus main body. The upper guide unit  9  includes a sub frame  8 , a pivotable shaft  31 , a first coil spring  32 , a second coil spring  33 , and roller shaft  34  in addition to the above-described upper guide  43  and the transport-side driven roller  42 . 
   The sub frame  8  is formed in the manner that a metal plate is bent. The sub frame  8  is attached to a main frame  7  of the apparatus main body. Hooks  8   a  and  8   b,  a flange piece  8   c,  and bearing portions  8   f  and  8   g  are formed in the sub frame  8 . The hooks  8   a  and  8   b  are configured to be suspended by a fixation portion  7   b  (see  FIG. 13 ) formed in the main frame  7 . A boss  8   d  and a long hole  8   e  are formed in the flange piece  8   c.  The boss  8   d  of the flange piece  8   c  is inserted into a hole  7   a  of the main frame  7 . The long hole  8   e  is a hole in which the sub frame  8  is fixed by a fixation screw  35  in the hole  7   c  of the main frame  7 . The bearing portions  8   f  and  8   g  re bearings that support the pivotable shaft  31 . 
   The pivotable shaft  31  is a shaft that supports the upper guide  43  so as to be pivotable on the sub frame  8 . The pivotable shaft  31  is supported to the sub frame  8  through the bearing portions  8   f  and  8   g  and supports the upper guide  43  to be pivotable through a shaft hole formed in the upper guide  43 . In addition, the upper guide  43  is pivotable as much as the thickness of paper. 
   The first coil spring  32  grants a rotation force about the pivotable shaft  31  between the sub frame  8  and the upper guide  43 . The rotation force is granted to the upper guide  43  so as to be moved in a direction in which the transport-side driven roller  42  is lowered about the pivotable shaft  31 . In this way, the pivotable shaft  31  is positioned at the coil center of the first coil spring  32 . In addition, one end of the first coil spring  32  is suspended by the sub frame  8  through a hook portion  8   h  and the other end thereof presses the upper guide  43  from the top surface thereof. 
   The second coil spring  33  grants the rotation force about the pivotable shaft  31  between the sub frame  8  and the roller shaft  34 . The rotation force stabilizes the roller shaft  34  on the upper guide  43  and grants a rotation force in the direction, where the transport-side driven roller  42  is moved downward about the pivotable shaft  31 , through the roller shaft  34  in the upper guide  43 . In this way, the pivotable shaft  31  is positioned at the coil center of the second coil spring  33 . In addition, one end of the second coil spring  33  is suspended in the sub frame  8  through the hook portion  8   h  and the other end thereof presses the roller shaft  34  from the an upside. 
   The roller shaft  34  is a shaft that rotatably supports the transport-side driven roller  42 . The upper guide  43  supports the roller shaft  34 . Since a spring force of the second coil spring  33  is applied from the upside, the roller shaft  34  is supported so as not to be deviated from the upper guide  43 . 
   Seal Member 
   The upper guide  43  is required to guide the paper sheet P so as to pass through the optical sensor  37 . Accordingly, a concave portion  43   a  is formed in the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37  is configured so as to be inserted into the concave portion  43   a.  In this way, the paper sheet P guided to the upper guide  43  is configured so as to pass below the upper protrusion portion  37   a  of the optical sensor  37  inserted into the concave  43   a.    
   Meanwhile, like the foregoing description in  FIG. 9B , when the upper guide  43  guides the paper sheet P, it is required to prevent the front end (the rear end at the time of the back transport) of the paper sheet P from becoming inserted into the boundary between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37 . However, since the upper guide  43  is supported so as to be pivotable on the main frame  7  and the optical sensor  37  is fixed to the lower guide  44 , the upper guide  43  is relatively postured relative to the upper protrusion portion  37   a  of the optical sensor  37 . That is, it is required to prevent the front end (the rear end at the time of the backward transport) of the paper sheet P from becoming inserted in the boundary between the upper guide  43  and the upper protrusion portion  37   a  which are relatively postured. 
   In this embodiment, since the seal member  38  covers the boundary between the upper guide  43  and the upper protrusion portion  37   a,  the front end (the rear end of the backward transport) of the paper sheet P is prevented from becoming inserted into the boundary between the upper guide  43  and the upper protrusion portion  37   a.  Hereinafter, the seal member  38  will be described. 
     FIGS. 15A to 15C  show that a seal member  38  is attached to the upper guide  43 . 
   The seal member  38  is a member that has a flexible sheet shape (or a film shape), and specifically is a PET film. Holes  38   a  and  38   b  are formed at the vicinity of both ends of the seal member  38 . A hook  43   d  is formed above the concave portion  43   a  of the upper guide  43 . The seal member  38  is rolled through a through-hole  43   f  in the manner the hole  38   a  is suspended by the hook  43   d  and the seal member  38  is retained in the upper guide  43  in the manner that the hole  38   b  is suspended by the hook  43   d.  A tension-granting portion  43   e,  which is formed in the upper guide  43 , prevents the seal member  38  from being bent. 
     FIG. 16  shows that an upper protrusion portion  37   a  is inserted into a concave portion  43   a.    FIG. 17  is a diagram explaining a relationship between the seal member  38  and the upper protrusion portion  37   a.    
   When the seal member  38  is rolled in the upper guide  43 , a space is formed surrounded by the concave portion  43   a  and the seal member  38 . The upper protrusion portion  37   a  of the optical sensor  37  is inserted into the space. 
   As shown in  FIG. 17 , the seal member  38  seals the boundary between the upper guide  43  and the upper protrusion portion  37   a.  In this way, it is possible to prevent the front end (the rear end of the backward transport) of the paper sheet P from becoming inserted into the boundary between the upper guide  43  and the upper protrusion portion  37   a.  Moreover, even when the upper guide  43  is pivoted, and thus the upper protrusion portion  37   a  comes in contact with the seal member  38 , the seal member  38  is deformed so that the boundary between the upper guide  43  and the upper protrusion portion  37   a  keep to be sealed. That is why the seal member  38  is formed of a flexible material. 
   Since the seal member  38  is rolled in the upper guide  43  so as to be retained, the seal member  38  can seal both boundaries between the upper guide  43  and the upper protrusion portion  37   a  (a boundary between the upper guide  43  of the through-hole  43   f  and the upper protrusion portion  37   a  and the a boundary between the upper guide  43  of the tension-granting portion  43   e  and the upper protrusion portion  37   a ). In this way, it is possible to prevent the front end (the rear end of the backward transport) of the paper sheet P from becoming inserted into the boundary between the upper guide  43  and the upper protrusion portion  37   a  not only in the forward transport, but also in the backward transport. 
   A portion filling the concave portion  43   a  in the seal member  38  is positioned between the upper protrusion portion  37   a  and the lower protrusion portion  37   b  of the optical sensor  37 . The portion of the seal member  38  may formed so as to fill an area of the optical axis of the optical sensor  37  or so as not to fill the area of the optical axis. When the seal member  38  is formed so as to avoid the area of the optical axis of the optical sensor  37 , the seal member  38  may be made of a light-shielding material. Alternatively, when the seal member  38  is formed so as to fill the area of the optical axis of the optical sensor  37 , the seal member  38  is required to be made of a transparent material. 
   Platen  45   
     FIG. 18  is a diagram illustrating a platen  45  when viewed obliquely. In order to simplify a configuration shown in  FIG. 18 , the upper guide unit  9  is not separated. Upstream-side bulges  45   a,  in-between bulges  45   b,  and downstream bulges  45   c  are formed in the platen  45 . A sponge  58  absorbing ink is inserted in the platen  45 . The sponge  58  is an absorber absorbing ink which is not deposited on the paper sheet P in a printing process with no margin. 
     FIG. 19A  shows that the front end of the paper sheet P is printed in the printing process with no margin. As shown in  FIG. 19A , an upstream-side groove  45   d  and a downstream-side groove  45   e  are formed in the platen  45  and the sponge  58  is inserted into the upstream-side groove  45   d  and the downstream-side groove  45   e.  The upstream-side groove  45   d  is formed between the upstream-side bulge  45   a  and the in-between bulge  45   b  and the downstream-side groove  45   e  is formed between the in-between bulge  45   b  and the downstream-side bulge  45   c.    
   When the dots are printed on the front end of the paper sheet P in the printing process with no margin, first, the controller  10  rotates the transport roller  41  to transport the paper sheet P so that the front end of the paper sheet P is placed on the downstream-side groove  45   e.  In addition, the controller  10  controls the head  48  to eject ink so that an image is printed on the front end of the paper sheet P. At this time, the ink which is not deposited on the paper sheet P is absorbed by the downstream-side groove  45   e.    
     FIG. 19B  shows that the rear end of the paper sheet P is printed in the printing process with no margin. 
   When the dots are printed on the rear end of the paper sheet P in the printing process with no margin, the head  48  ejects the ink in the state where the rear end of the paper sheet P is placed on the upstream-side groove  45   d  to print an image on the rear end of the paper sheet P. At this time, the ink which is not deposited on the paper sheet P is absorbed by the upstream-side groove  45   d.    
     FIG. 19C  shows that the side end of the paper sheet P is printed in the printing process with no margin. As shown in  FIG. 19C , a lateral groove  45   f  is provided in the platen  45  and the sponge  58  is inserted into the lateral groove  45   f.    
   When the dots are printed on the paper sheet P with a standard size, the lateral end (the lateral side) of the paper sheet P is placed on the lateral groove  45   f.  The head  48  ejects the ink on an area which is broader than the paper surface, and then the image is formed on the entire width of the paper sheet P. At this time, the ink which is not deposited on the paper sheet P is absorbed by the sponge  58  of the lateral groove  45   f.    
   As shown in  FIGS. 19A and 19B , an inclined plane is configured on the upstream side in the transport direction of the upstream-side bulge  45   a,  the in-between bulge  45   b,  and the downstream-side bulge  45   c.  The reason why the inclined plane is formed in the bulges in this way is that the front end of the paper sheet P is prevented from becoming jammed in the bulges when the paper sheet P is transported. 
   Alternatively, as shown in  FIGS. 19A and 19B , the inclined plane is not configured in the downstream side in the transport direction of the upstream-side bulge  45   a,  the in-between bulge  45   b,  and the downstream-side bulge  45   c.  If the inclined plane is configured in the downstream side in the transport direction of the bulges, the front end of the paper sheet P easily comes in contact with the sponge  58 . Accordingly, the other surface of the paper sheet P may be smeared. 
   Printing Process of Both Sides 
     FIG. 20  is a flow chart illustrating a printing process of both surfaces according to an embodiment. In a printing process of both sides according to the embodiment, one surface and the other surface are assumed to be configured to be “a printing process with a margin” and “the printing process with no margin”, respectively. For example, when a new year&#39;s card is printed, a recipient, an address, etc. are printed on the one surface and a photo is printed on the other surface. At this time, the one surface and the other surface correspond to “the printing process with the margin” and “the printing process with no margin”. 
   First, the controller  10  carries out a determination of a printing surface (S 001 ). Specifically, the controller  10  determines an image about the existence of margin to be printed on the one surface of the paper sheet P and an image about the non-existence of the margin to be printed on the other surface of the paper sheet P. For example, when a new year&#39;s card is printed, the controller  10  determine a recipient, an address, etc. to be printed on the one surface of the paper sheet P and a photo to be printed on the other surface. In addition, in order to carry out the printing process in such an order, the controller  10  controls a direction in which a postcard is set to be displayed on a display unit (a liquid crystal panel, etc.) which is not shown. In the case of the rear feeding process, when the postcard is set so that a surface on which a recipient, an address, etc are printed is faced upward, the recipient, the address, etc. can be printed on the one surface and the photo can be printed on the other surface. 
   Next, the controller carries out a paper feeding process. The controller  10  controls the rear feeding roller  11  to rotate in the case of the rear feeding process and the front feeding roller  28  to rotate in the case of the front feeding process as much as a predetermined amount of rotation (S 002 , see  FIGS. 3 and 4 ). 
   When the feeding process is normally carried out, the front end of the paper sheet P reaches the optical sensor  37 , and then the optical sensor  37  senses the front end of the paper sheet P (YES in S 003 ). Even when the controller  10  rotates the rear feeding roller  11  or the front feeding roller  28  as much as the predetermined amount of rotation, and then the optical sensor  37  does not sense the front end of the paper sheet P (NO in S 003 ), the controller  10  senses that jam (paper jam) occurs in the feeding unit (S 101 ). In this case, the controller  10  displays the jam occurrence and the jam occurrence location (in this case, the inside of the feeding unit) on the display unit (the liquid crystal panel, etc) which is not shown. 
   When the optical sensor  37  senses the front end of the paper sheet P (YES in S 003 ), the controller  10  controls the paper sheet P to be transported as much as a predetermined transport so as to transport the paper sheet P to a print start position (where the process is referred to as “a cuing process”) (S 004 ). Moreover, an amount of transport in the manner that the optical sensor  37  senses the front end of the paper sheet P, and then controller  10  transports the paper sheet P to the print start position is predetermined. In this way, the controller  10  carries out the cuing process on the basis of the result that the optical sensor  37  senses. After the cuing process, the paper sheet P is opposed with the head  48 . 
   Next, the controller  10  controls the printing process with the margin on the one surface of the paper sheet P (S 005 ). At this time, the controller  10  alternately repeats a dot formation process in which dots are formed on the paper sheet P in the manner of ejecting ink from the head  48  in the middle of movement of the carriage  46  and a transport process in which the paper sheet P is transported as much as a predetermined amount of transport. When the transport process continues several times, the rear end of the paper sheet P reaches the optical sensor  37 . 
   Even when predetermined times of the transport process are carried out, but the optical sensor  37  does not sense the rear end of the paper sheet P (NO in S 006 ), the controller  10  senses that jam occurs in the vicinity of the transport roller  41  (S 102 ). In this case, the controller  10  displays the jam occurrence and the jam occurrence location (in this case, the vicinity of the transport roller) on the display unit (the liquid crystal panel, etc) which is not shown. 
   After the optical sensor  37  senses the rear end of the paper sheet P (YES in S 006 ), the printing process with the margin continues for some time, and then the printing process on the one surface ends (S 007 ). 
     FIG. 21  is a diagram illustrating a position of the rear end of the paper sheet P at the time of finishing a surface printing process. As shown in  FIG. 21 , the rear end of the paper sheet P is supported by the upstream-side bulge  45   a.  Alternatively, the rear end of the paper sheet P may be placed in the more upstream side (a left side in  FIG. 21 ) than the upstream-side bulge  45   a.  However, in this embodiment, the rear end of the paper sheet P is prohibited from being placed in a more downstream side (a right side in  FIG. 21 ) than the upstream-side bulge  45   a  at the time of finishing the printing process on the one surface. Namely, in this embodiment, the controller  10  controls the printing process with the margin on the one surface so that the rear end of the paper sheet P is not placed in the more downstream side than the upstream-side bulge  45   a  at the time of finishing the printing process on the one surface. Moreover, the optical sensor  37  senses the rear end of the paper sheet P, and then the controller  10  memorizes the amount of transport until the printing process with the margin ends 
   Next, the controller  10  starts a reversion process (S 008 ). When the backward transport is carried out in the case shown in  FIG. 21 , the rear end of the paper sheet P passes through the transport roller  41 , and afterward passes through the optical sensor  37 . 
   Even when the backward transport is carried out as much as a predetermined amount of transport (S 009 ), but the optical sensor  37  does not sense the rear end of the paper sheet P (NO in S 010 ), the controller  10  senses that jam occurs in the transport roller  41  (S 103 ). In this case, it is assumed that the rear end of the paper sheet P cannot pass through the transport roller  41 . A reference of the predetermined amount of transport in S 009  is set depending on the amount of transport memorized in S 007 . The larger the amount of transport memorized is in S 007 , the larger the reference of the predetermined amount of transport is set. The controller  10  displays the jam occurrence and the jam occurrence location (in this case, the transport roller) on the display unit (the liquid crystal panel, etc) which is not shown. 
   After the optical sensor  37  senses the rear end of the paper sheet P (YES in S 010 ), the controller  10  further carries out the predetermined amount of transport (S 011 ). Even when the backward transport is carried out as much as the predetermined amount of transport, but the optical sensor  37  does not sense the rear end of the paper sheet P (NO in S 012 ), the controller  10  senses that jam occurs in the vicinity of the transport roller  41  (S 104 ). In this case, the controller  10  displays the jam occurrence and the jam occurrence location (in this case, the vicinity of the transport roller) on the display unit (the liquid crystal panel, etc) which is not shown. 
   When the optical sensor  37  senses the front end of the paper sheet P (YES in S 012 ), it is assumed that the paper sheet P is supplied to the reversion unit  100 . Subsequently, the controller  10  carries out a reverse process as much as a predetermined amount of reversion (S 013 ). That is, the controller  10  controls the first roller  102  and the second roller  104  to rotate as much as a predetermined amount of rotation. When the reversion process is carried out as much as the predetermined of reversion, the front end (the rear end in the printing process of the one surface) of the paper sheet P reaches the optical sensor  37 . 
   Even when the reversion process is carried out as much as the predetermined amount of reversion, but the optical sensor  37  does not sense the rear end of the paper sheet P (NO in S 014 ), the controller  10  senses that jam occurs in the reversion unit  100  (S 105 ). Moreover, it is assumed that the paper sheet P is jammed in the inside of the reversion unit  100 . The controller  10  displays the jam occurrence and the jam occurrence location (in this case, the inside of the reversion unit) on the display unit (the liquid crystal panel, etc) which is not shown. 
   After the optical sensor  37  senses the front end of the paper sheet P (YES in S 014 ), the controller  10  carries out the cuing process in which the printing process is carried out on the other surface (S 015 ). Since the printing process with no margin is carried out in the printing process on the other surface, the front end of the paper sheet P after the cuing process is placed on the downstream-side groove  45   e  (see  FIG. 19A ). 
   Next, the controller  10  controls the printing process with no margin on the other surface of the paper sheet P to be started (S 016 ). At this time, the controller  10  alternately repeats the dot formation process in which dots are formed on the paper sheet P in the manner of ejecting ink from the head  48  in the middle of movement of the carriage  46  and the transport process in which the paper sheet P is transported as much as a predetermined amount of transport. Moreover, when the printing process with no margin on the front end of the paper sheet P is carried out, the ink which is not deposited on the paper sheet P is absorbed by the sponge  58  of the downstream-side groove  45   e  (see  FIG. 19A ). When the printing process with no margin on the lateral end of the paper sheet P, the ink which is not deposited on the paper sheet P is absorbed by the sponge  58  of the lateral-side groove  45   f  (see  FIG. 19C ). When the transport process continues several times in the printing process with no margin on the other surface, the rear end of the paper sheet P reaches the optical sensor  37 . 
   Even when a predetermined number of times of the transport process is carried out several times, but the optical sensor  37  does not sense the rear end of the paper sheet P (NO in S 017 ), the controller  10  senses that jam occurs in the vicinity of the transport roller  41  (S 106 ). In this case, the controller  10  displays the jam occurrence and the jam occurrence location (in this case, the vicinity of the transport roller) on the display unit (the liquid crystal panel, etc) which is not shown. 
   After the optical sensor  37  senses the rear end of the paper sheet P (YES in S 107 ), the controller  10  controls the ejection range of ink to be limited in the printing process on the rear end of the paper sheet P on the basis of the result sensed by the optical sensor  37 . When the ink is ejected broadly, the ink is wasted and the sponge of the upstream-side groove  45   d  is smeared. Accordingly, the ink is ejected only in the proper range of ejection corresponding to location of the sensed rear end. Moreover, when the printing process with no margin on the rear end of the paper sheet P is carried out, the ink which is not deposited on the paper sheet P is absorbed by the sponge of the upstream-side groove  45   d  (see  FIG. 19B ). 
   The controller  10  controls an ejection process to be carried out after end of the printing process with no margin on the other-end surface (S 019 ), and then printing process on both surface to be finished. 
   FIRST COMPARATIVE EXAMPLE 
     FIG. 22  is a diagram illustrating a sensor sensing the paper end in a comparative example. A paper-end sensor  70  in the comparative example includes a lever  70   a,  a light-shielding portion  70   b,  and a sensor unit  70   c.  When the front end of the paper sheet P reaches the paper-end sensor  70 , the front end of the paper sheet P raises the lever  70   a.  Subsequently, the light-shielding portion  70   b  rotates and is inserted between a light-emitting portion and a light-receiving portion (not shown) of the sensor unit  70   c  so that the paper-end sensor  70  senses the front end of the paper sheet P. Moreover, when the rear end of the paper sheet P passes through the lever  70   a,  the lever  70   a  returns to the original position and the paper-end sensor  70  senses the rear end of the paper sheet P. 
   In such a comparative example, since the paper sheet P is not required to be guided to the narrow portion between the light-receiving portion  371  and the light-emitting portion  372  like the optical sensor  37  according to the embodiment, the above-described upper guide  43  is not required. 
   However, since the paper-end sensor  70  cannot sense the paper end except for the forward transport of the paper sheet P, the paper-end sensor  70  cannot sense the paper end during the backward transport of the paper sheet P. 
   In such a comparative example, the optical sensor  37  according to the embodiment senses the front end of the rear end of the paper sheet P in the non-contact manner. Accordingly, even during the backward transport of the paper sheet P, the optical sensor  37  can sense the paper end. In addition, in the above-described embodiment, such an optical sensor  37  can sense the rear end of the paper sheet P in the backward transport so as to detect the jam in the transport roller *see S 010  and S 103  shown in  FIG. 20 ). Alternatively, the optical sensor  37  can sense the front end of the paper sheet P in the backward transport so as to detect the jam in the vicinity of the transport roller (see S 012  and S 104  shown in  FIG. 20 ). 
   SECOND COMPARATIVE EXAMPLE 
     FIGS. 23A and 23B  are diagrams explaining the surface printing process in the comparative example. In the comparative example, the printing process with no margin is carried out when an image is printed on the one surface in the printing process of the both surface. 
   Like the foregoing description in  FIG. 19B , the rear end of the paper sheet P is placed on the upstream-side groove  45   d  when an image is printed on the rear end of the paper sheet P in the printing process with no margin. In this way, in the comparative example, when the printing process of the one surface ends, and then the paper sheet P is transported backward so as to be reversed, the backward transport is started in the manner shown in  FIG. 23A . 
   Meanwhile, an inclined plane is configured on the downstream side in the transport direction of the upstream-side bulge  45   a  (in order that the front end of the paper sheet P does not come in contact with the sponge  58 ). In this way, when the backward transport is carried out from a state shown in  FIG. 23A , as shown in  FIG. 23B , the rear end of the paper sheet P is jammed by the upstream-side bulge  45   a.  Accordingly, the paper sheet P may be easily jammed. 
   Unlike such a comparative example, the printing process with no margin is carried out when the image is printed on the other surface and the printing process with the margin is carried out when the image is printed on the one surface. In this way, it is possible to prevent the paper from becoming jammed as shown in  FIG. 23B . 
   OTHER EMBODIMENTS 
   The above-described embodiment is described with reference to the printer, but may be described with reference to a printing apparatus, a recording apparatus, a liquid ejecting apparatus, a printing method, a recoding method, a liquid ejection method, a printing system, a recording system, a computer system, a program, a memory medium memorizing a program, a display screen, a screen displaying method, a prints manufacturing method, etc. 
   Moreover, the printer and the like according to the embodiment is described, but the embodiment is described to easily understand the gist of the invention and the invention is not limited thereto. The invention may be modified and improved without deviation of the gist of the invention and may include the equivalents. 
   About Printer 
   In the above-described embodiment, the printer is described, but the invention is not limited thereto. For example, the technology related to the embodiment may be applied to various printing apparatus such as a color filter manufacturing apparatus, a dye apparatus, a micro treatment apparatus, a semiconductor manufacturing apparatus, a surface treatment apparatus, a three-dimensional modeling apparatus, a liquid vaporization apparatus, an organic EL manufacturing apparatus (in particular, a polymer EL manufacturing apparatus), a display manufacturing apparatus, a film coating apparatus, or a DNA chip manufacturing apparatus to which ink jet technology is applied. 
   About Ink 
   In the above-described embodiment, dye ink or pigment ink is ejected from nozzles with reference to the printer. However, a liquid ejected from the nozzles is not limited to such ink. For example, the liquid (including water) including a metal material, an organic material (in particular, a polymer material), a magnetic material, a conductive material, a wiring material, a film coating material, electronic ink, machining fluid, and gene solution, may be ejected from the nozzles. 
   About Nozzle 
   In the above-described embodiment, the ink is ejected using a piezoelectric element. However, the liquid ejecting method is not limited thereto. For example, another method such as a method of generating bubbles in nozzles by heat may be used. 
   CONCLUSION 
   (1) The above-described printing apparatus  1  (one example of a printing apparatus) includes a transport unit  5 , an ejection unit  6  (one example of a transport mechanism), a head  48 , an optical sensor  37 , a reversion unit  100  (one example of a reversion mechanism), and a controller  10 . In addition, when printing an image on an one surface of a medium, a controller  10  controls paper sheet P (one example of a medium) to be transported to the transport unit  5  and the ejection unit  6  in a manner of a forward transport (to transport in a forward direction) and controls the optical sensor  37  to sense an end of the paper sheet P. 
   In this case, when the sensor shown in  FIG. 22  is used, the sensor can sense the end of the paper which is forward transported. However, the sensor can sense the end of the paper which is backward transported. 
   Accordingly, in the above-described embodiment, the optical sensor  37  capable of sensing existence or non-existence of paper (one example of the medium) in a non-contact manner is used. In this way, when transporting the paper sheet P to the transport unit  5  and the ejection unit  6  in the manner of the backward transport (to transport in a backward direction) in order to reverse the paper sheet P to the reversion unit  100  after a printing process of the one surface, the controller  10  can controls the optical sensor  37  to sense the end of the paper sheet P. 
   (2) According to the above-described embodiment, the optical sensor  37  includes a light-emitting portion  372  and a light-receiving portion  371  (see  FIG. 1 ). When using such a sensor, it is desirable to shorten a distance between the light-emitting portion  372  and the light-receiving portion  371  to improve a sensing precision. Moreover, it is required that the paper sheet P passes between the light-emitting portion  372  and the light-receiving portion  371 . 
   In this way, the above-described printer  1  includes a lower guide  44  and an upper guide  43  guiding the paper sheet P between the light-emitting portion  372  and the light-receiving portion  371 . Accordingly, it is possible to pass the paper sheet P between the light-emitting portion  372  and the light-receiving portion  371  by shortening the distance between the light-emitting portion  372  and the light-receiving portion  371 . 
   (3) The above-described optical sensor  37  includes an upper protrusion portion  37   a  and a lower protrusion portion  37   b  and the upper protrusion portion  37   a  includes the light-receiving portion  371  and the lower protrusion portion  37   b  includes the light-emitting portion  372  (see  FIG. 7 ). In this case, when the lower protrusion portion  37   b  is inserted into the lower guide  44 , as shown in  FIG. 9B , the paper sheet P may be jammed in a boundary between the lower guide  44  and the lower protrusion portion  37   b.  Thus, in the above-described lower guide  44 , an insertion portion into which the lower protrusion portion  37   b  is inserted is formed and an opening  44   b  is formed in a position of an optical axis of the optical sensor  37 . 
   Meanwhile, a base surface  44 A and ribs  44 B are formed in the lower guide  44  (see  FIG. 11 ). In addition, when an opening  44 D is formed in the rib  44 B, as shown in  FIG. 9B , the front end of the paper sheet P may be jammed in the boundary. 
   In this way, in the above-described embodiment, the opening  44 D is formed on the base surface  44 A between the ribs  44 B. Since the paper sheet P comes in contact with the ribs  44 B, but does not come in contact with the base surface  44 A between the ribs  44 B, the paper sheet P is not jammed in the boundary between the base surface  44 A and the lower protrusion portion  37   b.  As a result, it is possible to prevent the paper sheet P from becoming jammed. 
   In the above-described embodiment, the upper protrusion portion  37   a  includes the light-receiving portion  371  and the lower protrusion portion  37   b  includes the light-emitting portion  372 , and vise versa. However, when the light-receiving portion  371  is above the light-emitting portion  372 , dust does not settle and it is possible to carry out a stable sensing process. 
   (4) The optical sensor  37  senses the existence or the non-existence of the paper sheet P in the manner that the light-receiving portion  371  senses light emitted from the light-emitting portion  372 . Accordingly, if the distance between the light-emitting portion  372  and the light-receiving portion  371  becomes shortened, the sensing precision is improved. In this way, in the above-described embodiment, the top surface of the lower protrusion portion  37   b  is configured to be placed above the base surface  44 A of the lower guide  44  (see  FIG. 17 ). In this way, since the distance between the light-emitting portion  372  and the light-receiving portion  371  is shortened as much as possible, it is possible to improve the sensing precision of the optical sensor  37 . 
   (5) An uplift portion  44 C that covers the lower protrusion portion  37   b  is formed in the above-described lower guide  44  (see  FIG. 11 ). Accordingly, it is possible to reduce an exposure portion in the boundary between the lower guide  44  and the lower protrusion portion  37   b.  Moreover, an inclined plane is formed between the uplift portion  44 C and the base surface  44 A on an upstream side in the transport direction and a downstream side in the transport direction of the uplift portion  44 C (see  FIG. 11 ). As a result, in the forward transport and the backward transport, it is possible to prevent the paper sheet P from becoming jammed in the uplift portion  44 C. 
   (6) In the above-described embodiment, the optical sensor  37  is fixed to the lower guide  44  and the upper guide  43  is formed to be pivotable on the lower guide  44 . In this way, a positional relationship between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37  is changed. Specifically, before and after a position in which the paper sheet P is jammed between the transport roller  41  and the transport-side driven roller  41 , the positional relationship between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37  is changed. Meanwhile, it is required to prevent the front end of the paper sheet P from becoming jammed in the boundary between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37 . 
   In this way, in the above-described embodiment, a flexible seal member  38  seals the boundary between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37 . In this way, it is possible to prevent the front end of the paper sheet P from becoming inserted into the boundary between the upper guide  43  and the upper protrusion portion  37   a  of the optical sensor  37  which are relatively postured. 
   (7) In the above-described embodiment, an upstream-side bulge  45   a  (one example of a first bulge), an in-between bulge  45   b  (one example of a second bulge), and an upstream-side groove  45   d  (one example of a groove) are formed in a platen  45 . In addition, in the printing process with no margin on the rear end, the controller  10  controls the paper sheet P to be transported to a transport unit  5  and an ejection unit  6  so as to place the rear end of the paper sheet P on the upstream-side groove  45   d.  Simultaneously, the controller  10  controls a head  48  to print an image on the rear end of the paper sheet P (see  FIG. 19B ). 
   However, when the printing process with no margin on the one surface is carried out, the rear end of the paper sheet P is jammed in the upstream-side bulge  45   a  in the backward transport, and thus the paper sheet P may be jammed. 
   The above-described controller  10  controls the paper sheet P to be transported to the transport unit  5  and the ejection unit  6  so as not to place the rear end of the paper sheet P on the more downstream side in the transport direction than the upstream-side bulge  45   a.  Simultaneously, the controller  10  controls the head  48  to print an image on the one surface. In addition, the controller  10  controls the paper sheet P to be transported to the transport unit  5  and the ejection unit  6  so as to place the rear end of the medium on the upstream-side groove  45   d.  Simultaneously, the controller  10  controls the head  48  to print an image on the rear end of the paper sheet P (see  FIG. 19B ). When an image on which a margin exists and an image on which a margin does not exist are printed on the paper sheet P in the printing process of both surface, the controller  10  controls the printing process with the margin on the one surface and the printing process with no margin on the other surface to be carried out. 
   In this way, in the backward transport after the printing process of the one surface, it is difficult the rear end of the paper sheet P to be jammed in the upstream-side bulge  45   a.  As a result, it is possible to reduce a possibility of paper jam. 
   (8) The above-described printing method (one example of a printing method) is carried out in the manner that the paper sheet P (one example of a medium), first, is fed to the transport unit  5  (one example of a transport mechanism), the optical sensor  37  (one example of a non-contact sensor) senses the front end of the paper sheet P before the paper sheet P is fed to the transport unit  5 , the transport unit  5  and the ejection unit  6  forward transport the paper sheet P during the printing process of the one surface (one example of a printing process), and then the reversion unit  100  (one example of a reversion mechanism) backward transports the paper sheet P in order to reverse the paper sheet P after the printing process of the one surface. The above-described optical sensor  37  is a sensor capable of sensing the existence or the non-existence of the paper sheet P in a non-contact manner. Accordingly, the optical sensor  37  can sense the front end and the rear end of the paper sheet P in the manner of the backward transport.