Patent Publication Number: US-9415960-B2

Title: Printing apparatus and printing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus and a printing method in which an image is printed on a sheet using a plurality of print sections positioned so as to be deviated from each other in a sheet conveying direction. 
     2. Description of the Related Art 
     In general, a conveying path along which a sheet is conveyed through a position opposite to a print section includes an upstream side conveying roller positioned on an upstream side of the print section in a sheet conveying direction and a downstream side conveying roller positioned on a downstream side of the print section in the sheet conveying direction. A speed at which the sheet is conveyed by the downstream side conveying roller is set higher than a speed at which the sheet is conveyed by the upstream side conveying roller. Furthermore, a sheet sandwiching force exerted by the downstream side conveying roller and a pinch roller opposite to the downstream side conveying roller is set weaker than a sheet sandwiching force exerted by the upstream side conveying roller and a pinch roller opposite to the upstream side conveying roller. Thus, the downstream side conveying roller conveys the sheet while causing slippage between the downstream side conveying roller and the sheet. As a result, the sheet can be adequately conveyed with no slack. 
     Japanese Patent Laid-Open No. H08-337011(1996) describes a printing apparatus including a first print section configured to print an image on one surface of a sheet and a second print section configured to print an image on the other surface of the sheet, the first and second print sections being positioned so as to be deviated from each other in the sheet conveying direction. A first conveying path along which the sheet is conveyed to the first print section includes an upstream side conveying roller and a downstream side conveying roller. Similarly, a second conveying path along which the sheet is conveyed to the second print section includes an upstream side conveying roller and a downstream side conveying roller. In the first print section, the sheet is conveyed with no slack by the upstream side conveying roller and downstream side conveying roller in the first conveying path. Similarly, in the second print section, the sheet is conveyed with no slack by the upstream side conveying roller and downstream side conveying roller in the second conveying path. 
     However, when the sheet is conveyed from the first conveying path to the second conveying path, the sheet is conveyed by the downstream side conveying roller in the first conveying path and the upstream side conveying roller in the second conveying path. In this case, the former downstream side conveying roller otherwise positioned on the downstream side in the conveying direction is positioned on the upstream side in the conveying direction. The latter upstream side conveying roller otherwise positioned on the upstream side in the conveying direction is positioned on the downstream side in the conveying direction. Thus, the slippage otherwise caused between the former downstream side conveying roller and the sheet does not occur, and the sheet may be slack between the former downstream side conveying roller and the latter upstream side conveying roller. Such slack of the sheet particularly causes disturbance when a high-quality image is printed and also causes a sheet jam. 
     SUMMARY OF THE INVENTION 
     The present invention provides a printing apparatus and a printing method which enable a high-quality image to be printed by conveying a sheet with no slack between a first conveying path and a second conveying path and which also allow suppression of a possible sheet jam. 
     In the first aspect of the present invention, there is provided a printing apparatus comprising: a first conveying unit configured to convey a sheet through a first conveying path; a first print unit configured to print an image on the sheet in the first conveying path; a second conveying unit configured to convey the sheet through the second conveying path; a second print unit configured to print an image on the sheet in the second conveying path; and a guide unit configured to guide the sheet conveyed through the first conveying path to the second conveying path through a third conveying path with a changeable length. 
     In the second aspect of the present invention, there is provided a printing method comprising: a first conveying step of conveying a sheet through a first conveying path; a first print step of printing an image on the sheet in the first conveying path; a second conveying step of conveying the sheet through the second conveying path; a second print step of printing an image on the sheet in the second conveying path; a step of guiding the sheet conveyed through the first conveying path to the second conveying path through a third conveying path; and a step of changing a length of the third conveying path so as to absorb slack in the sheet between the first conveying path and the second conveying path. 
     According to the present invention, the sheet conveyed along the first conveying path is guided to the second conveying path through the third conveying path with the changeable length. Thus, the sheet can be conveyed with no slack between the first conveying path and the second conveying path. As a result, a high-quality image can be printed, and a possible sheet jam can be suppressed. Furthermore, a decrease in print speed can be suppressed by setting the length of the third conveying path to an optimum value. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an important part of a printing apparatus according to a first embodiment of the present invention; 
         FIG. 2A ,  FIG. 2B , and  FIG. 2C  are each a diagram illustrating a sheet conveying operation in the printing apparatus in  FIG. 1 ; 
         FIG. 3A ,  FIG. 3B , and  FIG. 3C  are each a diagram illustrating the sheet conveying operation in the printing apparatus in  FIG. 1 ; 
         FIG. 4A ,  FIG. 4B , and  FIG. 4C  are each a diagram illustrating the sheet conveying operation in the printing apparatus in  FIG. 1 ; 
         FIG. 5  is a diagram illustrating the sheet conveying operation in the printing apparatus in  FIG. 1 ; 
         FIG. 6  is a configuration diagram of an important part of a printing apparatus according to a second embodiment of the present invention; 
         FIG. 7  is a configuration diagram of an important part of a printing apparatus according to a third embodiment of the present invention; 
         FIG. 8  is a diagram illustrating that a U-turn section in the printing apparatus in  FIG. 7  moves to a different position; 
         FIG. 9  is a diagram illustrating a print head in the printing apparatus in  FIG. 7 ; 
         FIG. 10  is a flowchart illustrating a conveying operation and a printing operation in the printing apparatus in  FIG. 7 ; 
         FIG. 11A ,  FIG. 11B , and  FIG. 11C  are each a diagram illustrating a sheet conveying operation in the printing apparatus in  FIG. 7 ; 
         FIG. 12A  and  FIG. 12B  are each a diagram illustrating the sheet conveying operation in the printing apparatus in  FIG. 7 ; 
         FIG. 13  is a configuration diagram of an important part of a printing apparatus according to a fourth embodiment of the present invention; 
         FIG. 14  is a diagram illustrating that a guide flapper in the printing apparatus in  FIG. 13  rotates to a different position; 
         FIG. 15  is a configuration diagram of an important part of a printing apparatus according to a fifth embodiment of the present invention; 
         FIG. 16  is a configuration diagram of an important part of a printing apparatus according to a sixth embodiment of the present invention; 
         FIG. 17  is a configuration diagram of an important part of a printing apparatus according to a seventh embodiment of the present invention; and 
         FIG. 18  is a configuration diagram of an important part of a printing apparatus according to an eighth embodiment of the present invention; 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below based on the drawings. The embodiments described below are applied examples of an ink jet printing apparatus of what is called a full line type configured to enable an image to be printed on a print medium (sheet) using an ink jet print head. The printing apparatus according to the present invention is applicable to liquid ejecting apparatuses configured to execute various processes (printing, processing, coating, irradiation, reading, inspection, and the like) on various media (sheets) using a liquid ejecting head that enables a liquid to be ejected. The media (including print media) include various media such as paper, plastic, film, textiles, metal, and flexible substrates to which a liquid containing ink is applied and the material of which is not limited. A method for applying the liquid containing ink is not limited to a method for ejecting the liquid. 
     First Embodiment 
       FIG. 1  is a schematic configuration diagram of an ink jet printing apparatus according to the present embodiment. The ink jet printing apparatus prints an image on a front surface and a back surface of a print medium such as a sheet using ink jet print heads  1  and  2 . 
     The print heads  1  and  2  enable ink to be ejected through ejection ports at tips of nozzles. The plurality of nozzles are arranged to form a nozzle array extending all over the assumed maximum print width of a print medium  3 . The print heads  1  and  2  are long-line-shaped ink jet print heads which may each be configured, for example, such that a plurality of unit nozzle chips with a plurality of nozzles arranged in a staggered manner is combined together or such that a plurality of nozzles is arranged in a line. The print heads  1  and  2  eject ink through ejection ports at the tips of the nozzles using ejection energy generating elements. The ejection energy generating elements may be, for example, electrothermal conversion elements (heaters), piezo elements, electrostatic elements, or MEMS elements. Each of the print heads  1  and  2  includes a total of three nozzle arrays, that is, a nozzle array for ejection of a cyan ink, a nozzle array for ejection of a magenta ink, and a nozzle array for ejection of a yellow ink. The number of ink colors and the number of nozzle arrays formed are each not limited to three but are optional. The print heads  1  and  2  are supplied with ink from corresponding ink tanks (not depicted in the drawings) through ink tubes. The print heads  1  and  2  may each be a unit integrated with ink tanks that store corresponding inks. The print heads  1  and  2  are held in a head holder (not depicted in the drawings). 
     In a first print section, the print medium  3  is conveyed in a direction of arrow A through a first conveying path, and in a second print section, conveyed in a direction of arrow B through a second conveying path. The print head  1  prints an image on the print medium  3  in the first conveying path. The print head  2  prints an image on the print medium  3  in the second conveying path. 
     In the first conveying path, a pair of conveying rollers (first pair of upstream side conveying rollers) is provided on a conveying-direction (direction of arrow A) upstream side of the print head  1 , the pair including a main conveying roller  4  and a main pinch roller  5  that rotates in conjunction with the main conveying roller  4 . Furthermore, a pair of conveying rollers (first pair of downstream side conveying rollers) is provided on a conveying-direction downstream side of the print head  1 , the pair including a sub conveying roller  6  and a sub pinch roller  7  that rotates in conjunction with the sub conveying roller  6 . Similarly, in the second conveying path, a pair of conveying rollers (second pair of upstream side conveying rollers) is provided on the conveying-direction (direction of arrow B) upstream side of the print head  2 , the pair including a main conveying roller  8  and a main pinch roller  9  that rotates in conjunction with the main conveying roller  8 . Furthermore, a pair of conveying rollers (second pair of downstream side conveying rollers) is provided on the conveying-direction downstream side of the print head  2 , the pair including a sub conveying roller  10  and a sub pinch roller  11  that rotates in conjunction with the sub conveying roller  10 . The main pinch rollers  5  and  9  that rotate in conjunction with the main conveying rollers and the sub pinch rollers  7  and  11  that rotate in conjunction with the sub conveying rollers are biased toward the corresponding main conveying rollers  4  and  8  and sub conveying rollers  6  and  10 . 
     A position where an image is printed by the print head  1  is denoted by Ph 1 . The position of a nip portion between the main conveying roller  4  and the main pinch roller  5  is denoted by Pr 1 . The position of a nip portion between the sub conveying roller  6  and the sub pinch roller  7  is denoted by Pr 2 . Moreover, a position where an image is printed by the print head  2  is denoted by Ph 2 . The position of a nip portion between the main conveying roller  8  and the main pinch roller  9  is denoted by Pr 3 . The position of a nip portion between the sub conveying roller  10  and the sub pinch roller  11  is denoted by Pr 4 . 
     The speed at which the print medium  3  is conveyed by the pair of conveying rollers  6  and  7  provided on the conveying-direction downstream side of the print head  1  is set higher than the speed at which the print medium  3  is conveyed by the pair of conveying rollers  4  and  5  provided on the conveying-direction upstream side of the print head  1 . Furthermore, a sandwiching force exerted on the print medium  3  by the pair of conveying-direction downstream side conveying rollers  6  and  7  is set weaker than a sandwiching force exerted on the print medium  3  by the pair of conveying-direction upstream side conveying rollers  4  and  5 . Thus, when the print medium  3  is conveyed by the pair of conveying rollers  4  and  5  and the pair of conveying rollers  6  and  7  while being sandwiched between the conveying rollers, slippage occurs between the print medium  3  and the pair of conveying-direction downstream side conveying rollers  6  and  7 . Similarly, the speed at which the print medium  3  is conveyed by the pair of conveying rollers  10  and  11  provided on the conveying-direction downstream side of the print head  2  is set higher than the speed at which the print medium  3  is conveyed by the pair of conveying rollers  8  and  9  provided on the conveying-direction upstream side of the print head  2 . Furthermore, a sandwiching force exerted on the print medium  3  by the pair of conveying-direction downstream side conveying rollers  10  and  11  is set weaker than a sandwiching force exerted on the print medium  3  by the pair of conveying-direction upstream side conveying rollers  8  and  9 . Thus, when the print medium  3  is conveyed by the pair of conveying rollers  8  and  9  and the pair of conveying rollers  10  and  11  while being sandwiched between the conveying rollers, slippage occurs between the print medium  3  and the pair of conveying-direction downstream side conveying rollers  10  and  11 . 
     The conveying speed of the pair of conveying rollers  4  and  5  is denoted by V 1 . The conveying speed of the pair of conveying rollers  6  and  7  is denoted by V 2 . The conveying speed of the pair of conveying rollers  8  and  9  is denoted by V 3 . The conveying speed of the pair of conveying rollers  10  and  11  is denoted by V 4 . Then, the conveying speeds are in relations represented by:
 
 V 2&gt; V 1  Expression (1)
 
 V 4&gt; V 3  Expression (2)
 
     When the same conveying section is shared by the first print section and the second print section, the speeds V 1  and V 3  are in a relation represented by:
 
 V 1= V 3  Expression (3)
 
     Furthermore, the sandwiching force of the pair of conveying rollers  4  and  5  is denoted by P 1 . The sandwiching force of the pair of conveying rollers  6  and  7  is denoted by P 2 . The sandwiching force of the pair of conveying rollers  8  and  9  is denoted by P 3 . The sandwiching force of the pair of conveying rollers  10  and  11  is denoted by P 4 . Then, the sandwiching forces are in relations represented by:
 
 P 1&gt; P 2  Expression (4)
 
 P 3&gt; P 4  Expression (5)
 
     When the same conveying section is shared by the first print section and the second print section, the sandwiching forces P 1  and P 3  are in a relation represented by:
 
 P 1= P 3  Expression (6)
 
     Between the first print section and the second print section, a third conveying path with a U-turn conveying path  19   a  corresponding to a curved portion is formed in order to convey the print medium  3  from the first print section to the second print section. The U-turn conveying path  19   a  is formed of a guide element. The guide element  19  includes a U-turn outer peripheral guide  12  forming an outer peripheral guide surface and a U-turn inner peripheral guide  18  forming an inner peripheral guide surface. The print medium  3  is conveyed from the first print section to the second print section along an inner periphery of the outer peripheral guide and an outer periphery of the inner peripheral guide  18 . 
     The guide element  19  forming the U-turn conveying path  19   a  is guided by guides  14  and  15  so as to be movable in the directions of arrows C 1  and C 2 . The position of the U-turn conveying path  19   a  is displaced in the direction of arrow C 1  or C 2  to change a conveying path distance L from the position Pr 2  of the nip portion between the sub conveying roller  6  and the sub pinch roller  7  to the position Pr 3  of the nip portion between the main conveying roller  8  and the main pinch roller  9 . The guide element  19  is biased in the direction of arrow C 1  by the force W of a U-turn portion spring  13  positioned between the guide element  19  and a fixed block  17  to keep the stopper portion  12   a  of the outer peripheral guide  12  in abutting contact with a U-turn portion stopper  16 . This regulates a movement limit position of the guide element  19  in the direction of arrow C 1 . 
     Now, a conveying operation and a printing operation performed by thus configured printing apparatus will be described based on  FIGS. 2A to 5 . 
     First, as depicted in  FIG. 2A , the print medium  3  fed to the first print section is conveyed in the direction of arrow A while being held at the nip portion between the main conveying roller  4  and the main pinch roller  5 . The conveying speed for the print medium  3  at the position Pr 1  of the nip portion of the pair of conveying rollers  4  and  5  is denoted by Vpa. When a leading end  3   a  of the print medium  3  is conveyed to a print position Ph 1  in the first print section, printing of an image using ink ejected by the print head  1  starts to be performed on a front surface (one surface) of the print medium  3 . 
     Subsequently, as depicted in  FIG. 2B , the leading end  3   a  of the print medium  3  reaches the position Pr 2  of the nip portion between the sub conveying roller  6  and the sub pinch roller  7 . Then, the print medium  3  is conveyed by the pair of conveying rollers  4  and  5  and the pair of conveying rollers  6  and  7 . The print medium  3  is then conveyed while slipping on the pair of conveying rollers  6  and  7  as described above. At this time, the conveying speed for the print medium  3  remains at Vpa. 
     Subsequently, as depicted in  FIG. 2C , the print medium  3  is conveyed in the direction of arrow D along the inner periphery of the outer peripheral guide  12  and the outer periphery of the inner peripheral guide  18 , which form the U-turn conveying path  19   a . At this time, the print medium  3  is bent to generate a force PA that causes the print medium  3  to push the guide element in the direction of arrow C 2 . The force W of the spring  13  which biases the guide element in the direction of arrow C 1  is set stronger than the force PA as indicated by:
 
 W&gt;PA   (7)
 
     Thus, the guide element  19  does not move, and at this time, the conveying speed for the print medium  3  remains at Vpa. 
     Subsequently, as depicted in  FIG. 3A , the leading end  3   a  of the print medium  3  reaches the position Pr 3  of the nip portion between the main conveying roller  8  and the main pinch roller  9 . Then, the print medium  3  is conveyed in the direction of arrow B by the pair of conveying rollers  8  and  9 . At this time, the print medium  3  is bent to generate a force PB that causes the print medium  3  to push the guide element in the direction of arrow C 2 . The force W of the spring  13  which biases the guide element in the direction of arrow C 1  is set stronger than the force PB as indicated by:
 
 W&gt;PB   (8)
 
     Thus, the guide element  19  does not move, and at this time, the conveying speed for the print medium  3  remains at VPa. 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 3B  to allow a trailing end  3   b  of the print medium  3  to leave the position Pr 1  of the nip portion of the pair of conveying rollers  4  and  5  in the first print section. Thus, a trailing end  3   b  side portion of the print medium  3  is conveyed by the pair of conveying rollers  6  and  7 , whereas a leading end  3   a  side portion of the print medium  3  is conveyed by the pair of conveying rollers  8  and  9 . 
     The conveying speed V 2  of the pair of conveying rollers  6  and  7  and the conveying speed V 3  of the pair of conveying rollers  8  and  9  are in a relation represented by:
 
 V 2&gt; V 3  Equation (9)
 
     A relation represented by Expression (10) is present between a conveying speed Vpb for the print medium  3  at the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  and a conveying speed Vpa for the print medium  3  at the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 .
 
 Vpa&lt;Vpb   Expression (10)
 
     The difference between the conveying speeds Vpa and Vpb acts to make a portion of the print medium  3  between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9  slack to generate a force PC that presses the guide element  19  hard in the direction of arrow C 2 . The force W of the spring  13  which biases the guide element in the direction of arrow C 1  is set weaker than the force PC as indicated by:
 
 W&lt;PC   Expression (11)
 
     Therefore, the guide element  19  moves in the direction of arrow C 2 . Thus, the position of the U-turn conveying path  19   a  is displaced in the direction of arrow C 2  to increase the conveying path distance L (see  FIG. 1 ) between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9 . As a result, the slack is absorbed by a portion of the print medium  3  located at the U-turn conveying path  19   a , allowing the print medium  3  to be conveyed with no slack. 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 3C , and the trailing end  3   b  of the print medium  3  passes through the print position Ph 1 . Thus, the printing of the front surface of the print medium  3  using the print head  1  ends. When the leading end  3   a  of the print medium  3  reaches the print position Ph 2  in the second print section, printing of the back surface (the other surface) of the print medium  3  using the print head  2  is started. As is the case with  FIG. 3B , the difference between the conveying speeds Vpa and Vpb acts to make the portion of the print medium  3  between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9  slack to generate the force PC that presses the guide element hard in the direction of arrow C 2 . Thus, as is the case with  FIG. 3B , the guide element  19  is moved in the direction of arrow C 2 . 
     The amount of slack in the portion of the print medium  3  between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9  depends on the difference in conveying speed between the pair of conveying-direction upstream side conveying rollers and the pair of conveying-direction downstream side conveying rollers in each of the first and second print sections. When, in each of the first and second print sections, the conveying speed of the pair of conveying-direction downstream side conveying rollers is increased by 3% with respect to the pair of conveying-direction upstream side conveying rollers, the speeds V 1 , V 2 , V 3 , and V 4  are in a relation represented by:
 
 V 1: V 2: V 3: V 4=1:1.03:1:1.03   Expression (12)
 
     For example, when the print medium  3  has an A4 size and is 297 mm in length in the conveying direction, slack of up to 8.91 mm in length is to be created. Given that the print medium  3  is conveyed with the slack uncontrolled, the print medium  3  may be jammed in the U-turn conveying path  19   a . Furthermore, in accordance with Expressions (4), (5), and (6) illustrated above, the sandwiching force of the pair of conveying rollers  6  and  7  and the sandwiching force P 3  of the pair of conveying rollers  8  and  9  are in a relation represented by:
 
 P 3&gt; P 2  Expression (13)
 
     Thus, the sandwiching force P 3  of the pair of conveying rollers  8  and  9  is stronger than the sandwiching force P 2  of the pair of conveying rollers  6  and  7 . Consequently, when the print medium  3  has high rigidity, the print medium  3  may be conveyed in a direction opposite to the direction of arrow D. If the print medium  3  thus has high rigidity, the slack created in the print medium  3  may acts as a disturbance when a high-quality photograph image is printed, degrading the quality of a print image. Furthermore, if the print medium  3  has low rigidity, the slack may lead to a jam. 
     In the present embodiment, the position of the U-turn conveying path  19   a  is displaced in the direction of arrow C 2  to allow the portion of the print medium  3  between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9  to absorb the slack. This allows the print medium  3  to be conveyed with no slack. 
     The maximum distance the guide element  19  moves in the direction of arrow C 2 , in other words, the maximum amount of displacement of the U-turn conveying path  19   a  in the direction of arrow C 2 , is half the amount of slack in the portion of the print medium  3  between the pair of conveying rollers  6  and  7  and the pair of conveying rollers  8  and  9 . For example, if, when the conveying speed of the pair of conveying-direction downstream side conveying rollers is increased by 3% with respect to the pair of conveying-direction upstream side conveying roller, slack of up to 8.91 mm is created in the print medium  3  of A4 size, then the maximum amount of displacement of the U-turn conveying path  19   a  in the direction of arrow C 2  is 4.455 mm. The maximum amount of displacement in the directions of arrow C 2  is set to a value at which slack created in a print medium  3  that is longest in the conveying direction can be absorbed. 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 4A , and the leading end  3   a  of the print medium  3  reaches the position Pr 4  of the nip portion of the pair of conveying rollers  10  and  11 . As is the case with  FIG. 3B , the force PC that pushes the guide element  19  hard in the direction of arrow C 2  is exerted due to the difference between the conveying speed Vpb at the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  and the conveying speed Vpa at the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . Thus, as is the case with  FIG. 3B , the guide element is moved in the direction of arrow C 2 . 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 4B , and the trailing end  3   b  of the print medium  3  leaves the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7 . This allows the slack of the print medium  3  to be taken up. The force that pushes the guide element  19  in the direction of arrow C 2  returns to the force PA exerted by bending of the print medium  3  as is the case with  FIG. 2C . As indicated by Expression (7) described above, the force W of the spring  13  is set stronger than the force PA, and thus, the guide element  19  gradually moves in the direction of arrow C 1  due to the bias force of the spring  13 . At this time, the conveying speed for the print medium  3  is at Vpa. 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 4C , and the trailing end  3   b  of the print medium  3  leaves the U-turn conveying path  19   a . At this time, the conveying speed for the print medium  3  remains at Vpa. 
     Subsequently, the print medium  3  is conveyed as depicted in  FIG. 5 , and the trailing end  3   b  of the print medium  3  leaves the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9  and then passes through the print portion Ph 2  of the print head  2 . Thus, printing of the back surface of the print medium  3  using the print head  2  ends. At this time, the conveying speed for the print medium  3  is at Vpb. Subsequently, the print medium  3  leaves the position Pr 4  of the nip portion of the pair of conveying rollers  10  and  11  and is discharged. 
     The force W of the U-turn portion spring  13  is determined by the relation between the forces PA and PB exerted by bending of the print medium  3  and the force PC resulting from the difference in the conveying speed for the print medium  3 . Furthermore, forces PA and PB exerted by bending of the print medium  3  vary in accordance with the curvature of the U-turn conveying path  19   a . Thus, the relation between the forces PA and PB and the curvature of the U-turn conveying path  19   a  is set based on measurement results for the forces PA and PB exerted when various print media  3  are used. Additionally, the force W of the U-turn portion spring  13  may be adjustable in accordance with the rigidity of the print medium  3  used. 
     As described above, the position of the U-turn conveying path is displaced to change the length of the third conveying path, allowing absorption of the slack of the print medium resulting from the difference between the conveying speed in the first print section and the conveying speed in the second print section. This eliminates the cause of a disturbance associated with the conveying operation for the print medium, allowing a high-quality image to be printed on the front surface and back surface of the print medium. Furthermore, the print medium can be reliably conveyed without being jammed. 
     Second Embodiment 
       FIG. 6  is a diagram illustrating an important part of a printing apparatus according to a second embodiment of the present invention. 
     The guide element  19  forming a U-turn conveying path  19   a  along which the print medium  3  is conveyed from the first print section to the second print section is provided between the first print section and the second print section. The guide element  19  includes the U-turn outer peripheral guide  12  and the U-turn inner peripheral guide  18 . The print medium  3  conveyed to first print section is conveyed along the inner periphery of the outer peripheral guide  12  and the outer periphery of the inner peripheral guide  18 . The guide element  19  is provided so as to be able to pivot around a shaft  20  in the directions of arrows E 1  and E 2 . It is possible to change, in accordance with the pivoting of the guide element  19 , the conveying path distance L between the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  in the first print section and the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9  in the second print section. The guide element  19  is biased in the direction of arrow E 1  by the weight of the guide element  19 . The stopper portion  12   a  of the outer peripheral guide  12  is in abutting contact with the stopper  16  to regulate a pivot limit position in the direction of arrow E 1 . The behavior of the print medium  3  and the movement of the guide element  19  are similar to those in the above-described first embodiment. 
     Third Embodiment 
       FIG. 7  is a schematic configuration diagram of an ink jet printing apparatus according to the present embodiment. An image is printed on the front surface and back surface of the print medium  3  such as a sheet using ink jet print heads  1  and  2 . 
     First, since the print heads  1  and  2  are similarly configured, the configurations will be described based on  FIG. 9 , using the print head  1  as a representative.  FIG. 9  is a bottom view of the print head  1  as seen from an ink ejection port side. An ink jet printing apparatus uses ejection energy generating elements such as electrothermal conversion elements (heaters), piezo elements, electrostatic elements, or MEMS (Micro Electro Mechanical Systems) to eject ink through ejection ports at nozzle tips. The printing apparatus in the present example is a printing apparatus of what is called a full line type, and the print head  1  is a long-line-shaped ink jet print head extending over the maximum print width of the print medium  3 . In the print head  1  in the present example, nozzle arrays are formed to extend all over the print medium  3  in a width direction thereof. 
     The print head  1  includes nozzles Y through which a yellow ink is ejected, nozzles M through which a magenta ink is ejected, and nozzles C through which a cyan ink is ejected. For each color ink, three arrays of nozzles are formed. Furthermore, nozzles Y, M, and C are positioned in this order along the conveying direction (arrow A) for the print medium  3 . A plurality of the nozzles Y is arranged to form nozzle arrays Y 1 - 1 , Y 2 - 1 , and Y 3 - 1  in the print head  1  and to form nozzle arrays Y 1 - 2 , Y 2 - 2 , and Y 3 - 2  in the print head  2 . A plurality of the nozzles M is arranged to form nozzle arrays M 1 - 1 , M 2 - 1 , and M 3 - 1  in the print head  1  and to form nozzle arrays M 1 - 2 , M 2 - 2 , and M 3 - 2  in the print head  2 . A plurality of the nozzles C is arranged to form nozzle arrays C 1 - 1 , C 2 - 1 , and C 3 - 1  in the print head  1  and to form nozzle arrays CM 1 - 2 , C 2 - 2 , and C 3 - 2  in the print head  2 . These nozzle arrays are formed to extend along a direction intersecting (in the present example, orthogonal to) the conveying direction for the print medium  3 . 
     The number of ink colors and the number of print heads are each not limited to three but is optional. The print heads  1  and  2  are supplied with ink from corresponding ink tanks (not depicted in the drawings) via ink tubes. The print head  1  may provide a unit integrated with the corresponding ink tanks, and the print head  2  may provide a unit integrated with the corresponding ink tanks. The print heads  1  and  2  are held in corresponding head holders (not depicted in the drawings). 
     In the printing apparatus in  FIG. 7 , the conveying section (first conveying section) configured to convey the print medium  3  is provided in the first print section provided with the print head  1 . As is the case with the above-described embodiments, the first conveying section in the present example includes, on the upstream side of the print head  1  in the conveying direction (direction of arrow A), the pair of conveying rollers with the main conveying roller  4  serving as a driving roller and the main pinch roller  5  serving as a driven roller. The first conveying section also includes, on the downstream side of the print head  1  in the conveying direction, the pair of conveying rollers with the sub conveying roller  6  serving as a driving roller and the sub pinch roller  7  serving as a driven roller. The conveying section (second conveying section) configured to convey the print medium  3  is provided in the second print section provided with the print head  2 . As is the case with the above-described embodiments, the second conveying section in the present example includes, on the upstream side of the print head  2  in the conveying direction (direction of arrow B), the pair of conveying rollers with the main conveying roller  8  serving as a driving roller and the main pinch roller  9  serving as a driven roller. The second conveying section also includes, on the downstream side of the print head  2  in the conveying direction, the pair of conveying rollers with the sub conveying roller  10  serving as a driving roller and the sub pinch roller  11  serving as a driven roller. 
     The main pinch rollers  5  and  9  and sub pinch rollers  7  and  11 , rotated in conjunction with the corresponding conveying rollers, are biased by pinch roller springs (not depicted in the drawings) with respect to the corresponding main conveying rollers  4  and  8  and sub conveying rollers  6  and  10 , respectively. The print medium  3  is conveyed in the directions of arrows A and B by the pairs of conveying rollers. 
     In the print head  1  in the first print section, the nozzle array Y 1 - 1  for the yellow ink is positioned on the most upstream side in the conveying direction (direction of arrow A). The nozzle array C 3 - 1  for the cyan ink is positioned on the most downstream side in the conveying direction. As is the case with the above-described embodiments, the position of the nip portion of the pair of conveying rollers  4  and  5  is denoted by Pr 1 . The position of the nip portion of the pair of conveying rollers  6  and  7  is denoted by Pr 2 . In the print head  2  in the second print section, the nozzle array Y 1 - 2  for the yellow ink is positioned on the most upstream side in the conveying direction (direction of arrow B). The nozzle array C 3 - 2  for the cyan ink is positioned on the most downstream side in the conveying direction. As is the case with the above-described embodiments, the position of the nip portion of the pair of conveying rollers  8  and  9  is denoted by Pr 3 . The position of the nip portion of the pair of conveying rollers  10  and  11  is denoted by Pr 4 . 
     A U-turn section  112  is installed between the first print section and the second print section as a conveying section (third conveying section) configured to convey the print medium  3  from the first print section to the second print section. The print medium  3  is conveyed along an inner side surface of the U-turn section  112  to the second print section. 
     The U-turn section  112  is moved in the directions of arrows F 1  and F 2  by a motor gear  118  rotated by a motor  119  and a rack  117  integrated with the U-turn section  112  and meshed with the motor gear  118 . The U-turn section  112  is moved in the directions of arrows F 1  and F 2  by the motor  119  to change the conveying path distance L (see  FIG. 7 ) between the position Pr 2  and the position Pr 3 . Movement of the U-turn section  112  in the direction of arrow F 1  reduces the conveying path distance L. Movement of the U-turn section  112  in the direction of arrow F 2  increases the conveying path distance L. 
     A sensor  116  configured to detect the leading end  3   a  and trailing end  3   b  of the print medium  3  is set on the upstream side of the first print section in the conveying direction. Based on a detection signal from the sensor  116  and the conveying speed for the print medium  3 , the conveying-direction length PL of the print medium  3  is measured. When the length PL of the print medium  3  is detected, a moving operation of the U-turn section  112  in the direction of arrow F 1  or F 2  is immediately started in accordance with the length PL. The moving operation ends before the leading end  3   a  of the print medium  3  reaches a position opposite to the nozzle array Y 1 - 1  in the print head  1 . 
     Now, the movement position of the U-turn section  112  will be described. 
     First, a distance LL is set based on the conveying-direction length PL of the print medium  3  as indicated by:
 
 LL=PL   Expression (20)
 
     Moreover, when a correction margin for a conveying distance for the print medium  3  is 10%, the distance LL is set taking the correction margin into account as indicated by Expression (21) illustrated below. The correction margin is calculated using a tolerance for the conveying-direction length of the print medium  3 , an error in the movement position of the U-turn section  112 , an error in the conveying path length of the U-turn section  112 , and the like. 
     Moreover, when a margin-less printing is performed on the print medium  3 , the distance LL is set taking into account the length of an ink ejection area (print area) spreading out from the print medium  3 . For the margin-less printing, ink is ejected into the ejection area spreading out from the print medium  3  forward and backward in the conveying direction and spreading out from the print medium  3  in a lateral width direction. For example, when the ink ejection area spreads out from the print medium  3  by 10 mm forward and backward in the conveying direction, the total of the spreading length of the ejection area in the conveying direction is 20 mm. In this case, the distance LL is set taking into account the spreading length (10 mm) of the ink ejection area forward in the conveying direction as indicated by:
 
 LL =( PL+ 10 mm)×1.1  Expression (21)
 
     As depicted in  FIG. 8 , the distance between the nozzle array C 3 - 1 , positioned on the most downstream side of the print head  1  in the conveying direction, and the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9  is denoted by L 1 . Furthermore, the distance between the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  and the nozzle array Y 1 - 2  positioned on the most upstream side of the print head  2  in the conveying direction is denoted by L 2 . In a case A where the distances L 1  and L 2  are in a relation represented by Expression (23) illustrated below, the above-described conveying path distance L in  FIG. 7  is set so as to make the distance L 1  equal to the distance LL as indicated by Expression (24) illustrated below. 
     Case A
 
 L 1≧ L 2  Expression (23)
 
 L 1= LL   Expression (24)
 
     On the other hand, in a case B where the distances L 1  and L 2  are as represented by Expression (25) illustrated below, the conveying path distance L in  FIG. 7  is set so as to make the distance L 2  equal to the distance LL as indicated by Expression (26) illustrated below. 
     Case B
 
 L 1&gt; L 2  Expression (25)
 
 L 2= LL   Expression (26)
 
     Thus, when the distance L 1  is equal to or shorter than the distance L 2 , the conveying path distance L is changed so as to make the distance L 1  equal to the distance LL. When the distance L 1  is longer than the distance L 2 , the conveying path distance L is changed so as to make the distance L 2  equal to the distance LL. Therefore, the conveying path distance L is equal to or longer than the distances L 1  and L 2 . 
     The relation between the distances L 1  and L 2  is fixed by the positions of the print head  1 , the print head  2 , the pair of conveying rollers  6  and  7 , and the pair of conveying rollers  8  and  9 . As described below, the U-turn section  112  is moved so as to set the conveying path distance L in the case A or the conveying path distance L in the case B. 
     When the conveying-direction maximum length of the print medium  3  is denoted by PLmax, the corresponding distance LLmax is set as follows in accordance with the relation between the distances L 1  and L 2 . That is, in a case C where the distances L 1  and L 2  are in a relation represented by Expression (27) illustrated below, the conveying path distance L in  FIG. 7  is set so as to make the distance L 1  longer than the distance LLmax as indicated by Expression (28) illustrated below. 
     Case C
 
 L 1≦ L 2  Expression (27)
 
 L 1&gt; LL max  Expression (28)
 
     On the other hand, in a case D where the distances L 1  and L 2  are in a relation represented by Expression (29) illustrated below, the conveying path distance L in  FIG. 7  is set so as to make the distance L 2  longer than the distance LLmax as indicated by Expression (30) illustrated below. 
     Case D
 
 L 1&gt; L 2  Expression (29)
 
 L 2&gt; LL max  Expression (30)
 
     Now, operations of the printing apparatus configured as described above will be described based on a flowchart in  FIG. 10  and schematic diagrams of an important part in  FIGS. 11A, 11B, 11C, 12A, and 12B . 
     First, a printing operation is started to feed the print medium  3  to the print section (sheet feeding) (step S 1 ). The leading end  3   a  and trailing end  3   b  of the print medium  3  are detected by the sensor  116  to allow the conveying-direction length PL of the print medium  3  to be detected (step S 2 ). When the length PL of the print medium  3  is detected, the operation shifts to step S 3 . When the length PL of the print medium  3  fails to be detected, the operation shifts to step S 12  to provide an error display, while stopping the conveying operation of the print medium  3 . 
     In step S 3 , based on the detected length PL of the print medium  3 , the distance LL is determined in accordance with Expression (21) illustrated above. In the case A where the distances L 1  and L 2  are in the relation indicated by Expression (23) illustrated above, step S 4  shifts to step S 5 . In the case B where the distances L 1  and L 2  are in the relation indicated by Expression (25) illustrated above, step S 4  shifts to step S 6 . In step S 5 , the U-turn section  112  is moved so as to make the distance L 1  equal to LL. In step S 6 , the U-turn section  112  is moved so as to make the distance L 2  equal to LL. 
     Subsequently, the leading end  3   a  of the print medium  3  reaches the position opposite to the nozzle array Y 1 - 1  located on the most upstream side of the print head  1  in the conveying direction, and then, the first print section starts a printing operation (step S 7 ). For the margin-less printing, the first print section starts a printing operation at a position short of the position opposite to the nozzle array Y 1 - 1  (for example, a position 10 mm short of the position opposite to the nozzle array Y 1 - 1 ). While the print medium  3  is being conveyed in the direction of arrow A as depicted in  FIG. 11A , an image is printed on the print medium  3  by the print head  1  in the first print section. The print medium  3  is conveyed in the direction of arrow D along the U-turn section  112  as depicted in  FIG. 11B . Printing of the print medium  3  is continued even when the leading end  3   a  of the print medium  3  does not reach the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . Then, when the trailing end  3   b  of the print medium  3  passes through a position opposite to the nozzle array C 3 - 1  located on the most downstream side of the print head  1  in the conveying direction, the printing operation by the first print section is ended (step S 8 ). For the margin-less printing, the printing operation is ended when the trailing end  3   b  of the print medium  3  has moves away, for example 10 mm, from the position opposite to the nozzle array C 3 - 1 . 
     After the printing operation using the print head  1  thus ends, the print medium  3  is conveyed in the direction of arrow D as depicted in  FIG. 11C . The leading end  3   a  of the print medium  3  reaches the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . Subsequently, the trailing end  3   b  of the print medium  3  leaves the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7 . In other words, after the printing operation using the print head  1  ends, the print medium  3  shifts from a state where the print medium  3  is conveyed only by the pair of conveying rollers  6  and  7  as depicted in  FIG. 11B  to a state where the print medium  3  is conveyed also by the pair of conveying rollers  8  and  9  as depicted in  FIG. 11C . Subsequently, the trailing end  3   b  of the print medium  3  leaves the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7 . Thus, the print medium  3  is conveyed only by the pair of conveying rollers  8  and  9  as depicted in  FIG. 12A . 
     When the leading end  3   a  of the print medium  3  reaches the position opposite to the nozzle array Y 1 - 2  located on the most upstream side of the print head  2  in the conveying direction, the second print section starts a printing operation as depicted in  FIG. 12A  (step S 9 ). For the margin-less printing, the second print section starts the printing operation at a position short of the position opposite to the nozzle array Y 1 - 2  (for example, a position 10 mm short of the position opposite to the nozzle array Y 1 - 2 ). The trailing end  3   b  of the print medium  3 , which is separated from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7 , moves along the U-turn section  112 . 
     Subsequently, the print medium  3  is conveyed in the direction of arrow B by the pair of conveying rollers  8  and  9  and the pair of conveying rollers  10  and  11  as depicted in  FIG. 12B . The print head  2  continues to perform the printing operation on the print medium  3  conveyed as described above. When the trailing end  3   b  of the print medium  3  passes through the nozzle array C 3 - 2  located on the most downstream side of the print head  2  in the conveying direction, the printing operation by the second print section is ended (step S 10 ). For the margin-less printing, the printing operation is ended when the trailing end  3   b  of the print medium  3  has moves away, for example 10 mm, from the position opposite to the nozzle array C 3 - 2 . 
     The print medium  3  with the image printed thereon is discharged from the print section (step S 11 ). Thus, the conveying operation and printing operation on the first print medium  3  are ended. When an image is to be printed on the second or subsequent print medium, a similar conveying operation and a similar printing operation are repeated. 
     Thus, in the present embodiment, the leading end  3   a  of the print medium  3  is prevented from thrusting into the nip portion of the pair of conveying rollers  8  and  9  during printing using the print head  1 . Furthermore, the print head  2  starts printing after the trailing end  3   b  of the print medium  3  separates from the nip portion of the pair of conveying rollers  6  and  7  in the first print section. Therefore, the printing using the first print section is not affected by the conveying rollers in the second print section. Additionally, the printing using the second print section is not affected by the conveying rollers in the first print section. As a result, the cause of a disturbance associated with the conveying operation for the print medium is eliminated to allow a high-quality image to be printed. Furthermore, the U-turn section  112  is moved in accordance with the conveying-direction length PL of the print medium  3  to allow the distance between the first print section and the second print section to be adjusted. Consequently, a high printing speed can be maintained without an overly long distance between the first print section and the second print section. The U-turn section  112  is not limited to the configuration in which U-turn section  112  is moved by the motor  119 . For example, the movement position of the U-turn section  112  corresponding to the conveying-direction length PL of the print medium  3  may be preset so that the user can manually move the U-turn section  112  to the movement position. 
     Fourth Embodiment 
       FIGS. 13 and 14  are diagrams illustrating an important part of a printing apparatus according to a fourth embodiment of the present invention. 
     Two U-turn sections  112  and  120  are installed between the first print section including the print head  1  and the second print section including the print head  2 , as a conveying mechanism that conveys the print medium  3  conveyed from the first print section to the second print section. The print medium  3  conveyed to the first print section is conveyed through a first conveying path along an inner side surface of the U-turn section  112  or through a second conveying path along an inner side surface of U-turn section  120 . One of the first and second conveying paths is selected by rotating a guide flapper  121 . In other words, the first and second conveying paths can be used as a conveying section (third conveying section) that conveys the print medium  3  from the first print section to the second print section. As depicted in  FIG. 13 , the conveying path distance of the first conveying path between the position Pr 2  and the position Pr 3  is denoted by L 30 . The conveying path distance of the second conveying path between the position Pr 2  and the position Pr 3  is denoted by L 31 . 
     As is the case with the third embodiment, the distance LL is determined based on the conveying-direction length PL of the print medium  3 . Furthermore, a distance from the nozzle array C 3 - 1  located on the most downstream side of the print head  1  in the conveying direction, through the first conveying path to the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9  is denoted by L 11 . A distance from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  through the first conveying path to a nozzle array Y 1 - 2  located on the most upstream side of the print head  2  is denoted by L 12 . Additionally, a distance from the nozzle array C 3 - 1  located on the most downstream side of the print head  1  in the conveying direction, through the second conveying path to the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9  is denoted by L 21 . A distance from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  through the second conveying path to the nozzle array Y 1 - 2  located on the most upstream side of the print head  2  is denoted by L 22 . 
     When a conveying path distance L 30  is shorter than the distance L 21  and the distance L 22 , the guide flapper  121  is rotated in the direction of arrow G 1  as depicted in  FIG. 14  to convey the print medium  3  along the inner side surface of U-turn section  120 . On the other hand, when the conveying path distance L 30  is longer than the distance L 21  and the distance L 22 , the guide flapper  121  is rotated in the direction of arrow G 2  as depicted in  FIG. 13  to convey the print medium  3  along the inner side surface of U-turn section  112 . 
     When the conveying-direction maximum length of the print medium  3  is denoted by PLmax, the corresponding distance LLmax is set as follows in accordance with the relation between the distances L 11  and L 12 . That is, in a case E where the distances L 11  and L 12  are in a relation indicated by Expression (31) illustrated below, the conveying path distance L 30  is set to make the distance L 11  longer than the distance LLmax as indicated by Expression (32) illustrated below. 
     Case E
 
 L 11≦ L 12  Expression (31)
 
 L 11&gt; LL max  Expression (32)
 
     On the other hand, in a case F where the distances L 11  and L 12  are in a relation indicated by Expression (33) illustrated below, the conveying path distance L 30  is set to make the distance L 11  longer than the distance LLmax as indicated by Expression (34) illustrated below. 
     Case F
 
 L 11&gt; L 12  Expression (33)
 
 L 11&gt; LL max  Expression (34)
 
     Fifth Embodiment 
       FIG. 15  is a diagram of an important part of a printing apparatus according to a fifth embodiment of the present invention. 
     In the present embodiment, the print head  2 , the pair of conveying rollers  8  and  9 , and the pair of conveying rollers  10  and  11  provide a print section unit  113 . The print section unit  113  is moved in the directions of arrows H 1  and H 2  by a driving mechanism including a motor  119 , a motor gear  118  rotated by the motor  119 , and a rack  117  provided on the print section unit  113  to mesh with the motor gear  118 . The motor  119  drives and moves the print section unit  113  in the directions of arrows H 1  and H 2 . Such movement of the print section unit  113  increases and reduces a conveying path direction L from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  to the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . The print section unit  113  moves in the direction of arrow H 1  to reduce the conveying path distance L. The print section unit  113  moves in the direction of arrow H 2  to increase the conveying path distance L. 
     As is the case with the above-described third embodiment, the distance LL is determined based on the conveying-direction length PL of the print medium  3 . Then, the print section unit  113  is moved so as to establish a state equivalent to the case A or case B according to the third embodiment. Furthermore, when the conveying path distance calculated from the conveying-direction maximum length PLmax of the print medium  3  is denoted by LLmax, the movement position of the print section unit  113  is set to establish a state equivalent to the case C or case D according to the third embodiment. When the conveying-direction length of the print medium  3  is shorter than the maximum length PLmax, the print section unit  113  is moved in the direction of arrow H 1 . 
     Sixth Embodiment 
       FIG. 16  is a diagram illustrating an important part of a printing apparatus according to a sixth embodiment of the present invention. In the present embodiment, the first print section and the second print section are disposed on a substantially straight line. The conveying section configured to convey the print medium  3  from the first print section to the second print section is provided between these two print sections. The conveying section has a curved portion  140 . 
     The curved portion  140  is moved in the directions of arrows J 1  and J 2  by a driving mechanism including a motor  119 , a motor gear  118  driven by the motor  119  to rotate, and a rack  117  integrated with the curved portion  140  to mesh with the motor gear  118 . The motor  119  drives and moves the curved portion  140  in the directions of arrows J 1  and J 2 . This increases and reduces a conveying path direction L from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  to the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . The curved portion  140  moves in the direction of arrow J 1  to reduce the conveying path distance L. The curved portion  140  moves in the direction of arrow J 2  to increase the conveying path distance L. 
     As is the case with the above-described third embodiment, the distance LL is determined based on the conveying-direction length PL of the print medium  3 . Then, the curved portion  140  is moved so as to establish a state equivalent to the case A or case B according to the third embodiment. Furthermore, when the distance calculated from the conveying-direction maximum length PLmax of the print medium  3  is denoted by LLmax, the movement position of the curved portion  140  is set to establish a state equivalent to the case C or case D according to the third embodiment. When the conveying-direction length of the print medium  3  is shorter than the maximum length PLmax, the curved portion  140  is moved in the direction of arrow J 1 . 
     Seventh Embodiment 
       FIG. 17  is a diagram illustrating an important part of a printing apparatus according to a seventh embodiment of the present invention. 
     The first print section with the print head  1  and the second print section with the print head  2  are disposed on a substantially straight line. The print head  2 , the pair of conveying rollers  8  and  9 , and the pair of conveying rollers  10  and  11  provide a print section unit  113 . The print section unit  113  is moved in the directions of arrows K 1  and K 2  by a driving mechanism including a motor  119 , a motor gear  118  driven and rotated by the motor  119 , and a rack  117  integrated with the print section unit  113  to mesh with the motor gear  118 . The motor  119  drives and moves the print section unit  113  to increase or reduce the conveying path direction L from the position Pr 2  of the nip portion of the pair of conveying rollers  6  and  7  to the position Pr 3  of the nip portion of the pair of conveying rollers  8  and  9 . The print section unit  113  moves in the direction of arrow K 1  to reduce the conveying path distance L. The print section unit  113  moves in the direction of arrow K 2  to increase the conveying path distance L. 
     As is the case with the above-described third embodiment, the distance LL is determined based on the conveying-direction length PL of the print medium  3 . Then, the print section unit  113  is moved so as to establish a state equivalent to the case A or case B according to the third embodiment. Furthermore, when the conveying path distance calculated from the conveying-direction maximum length PLmax of the print medium  3  is denoted by LLmax, the movement position of the print section unit  113  is set to establish a state equivalent to the case C or case D according to the third embodiment. When the conveying-direction length of the print medium  3  is shorter than the maximum length PLmax, the print section unit  113  is moved in the direction of arrow K 1 . 
     Eighth Embodiment 
       FIG. 18  is a diagram illustrating an important part of a printing apparatus according to an eighth embodiment of the present invention. 
     The first print section with the print head  1  and the second print section with the print head  2  are disposed on a substantially straight line. Between the print sections, a conveying section is provided which is configured to convey the print medium  3  from the first print section to the second print section. The conveying section includes two U-turn sections  112  and  120 . The print medium  3  is conveyed to the second print section through a first conveying path along an inner side surface of the U-turn section  112  or through a second conveying path along an inner side surface of the U-turn section  120 . One of the first and second conveying paths is selected by rotating a guide flapper  123  in the direction of arrow M 1  or M 2 . 
     As is the case with the above-described fourth embodiment, the distance L 30  is determined based on the conveying-direction length PL of the print medium  3 . As is the case with the fourth embodiment, when the conveying path distance L 30  is shorter than the distance L 21  and the distance L 22 , the guide flapper  121  rotates in the direction of arrow M 2  as depicted by a dotted line in  FIG. 18  to convey the print medium  3  along the inner side surface of the U-turn section  120 . On the other hand, when the conveying path distance L 30  is longer than the distance L 21  and the distance L 22 , the guide flapper  121  rotates in the direction of arrow M 1  as depicted by a solid line in  FIG. 18  to convey the print medium  3  along the inner side surface of the U-turn section  112 . Furthermore, when the conveying pathe distance calculated from the conveying-direction maximum length PLmax of the print medium  3  is denoted by LLmax, one of the first and second conveying paths is selected to establish a state equivalent to the case E or case F according to the fourth embodiment. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2014-077466, filed Apr. 4, 2014 which is hereby incorporated by reference wherein in its entirety.