Patent Publication Number: US-10308052-B2

Title: Inkjet printer which stiffens sheet

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2017-036436, filed on Feb. 28, 2017, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an inkjet printer configured to perform printing by ejecting ink from an inkjet head. 
     2. Related Art 
     There is known an inkjet printer configured to print an image by ejecting ink from an inkjet head to a sheet. 
     In an inkjet printer described in Japanese Patent Application Publication No. 2004-345791, a sheet is conveyed below an inkjet head by a conveyance roller and the like. Then, an image is printed on the sheet below the inkjet head by ejecting ink from the inkjet head. 
     SUMMARY 
     In the inkjet printer described above, for example, when the sheet is deformed to curl, the sheet conveyed below the inkjet head sometimes comes into contact with the inkjet head. When the sheet comes into contact with the inkjet head, the inkjet head is sometimes damaged. 
     The disclosure is directed to an inkjet printer which can reduce contact of a sheet with an inkjet head. 
     An inkjet printer in accordance with some embodiments includes at least one inkjet head, at least one stiffening conveyor, and at least one platen. The at least one inkjet head is configured to eject ink to a sheet and print an image. The at least one stiffening conveyor is configured to convey the sheet toward the at least one inkjet head in a conveyance direction while stiffening the sheet by deforming the sheet into a wave shape in a width direction orthogonal to the conveyance direction. The at least one platen is arranged to face the at least one inkjet head and has a protrusion. The protrusion is provided on a surface of the at least one platen facing the at least one inkjet head and is configured to support a convex portion in the wave shape of the deformed sheet. The at least one platen is configured to support the sheet while keeping the sheet deformed in the wave shape. 
     In the configuration described above, when the sheet is curled, the sheet is conveyed below the inkjet head with the curl being suppressed by the stiffening. As a result, contact of the sheet with the inkjet head can be reduced. 
     The at least one inkjet head may include: a first inkjet head configured to eject ink to the sheet and print a first image; and a second inkjet head arranged downstream of the first inkjet head in the conveyance direction and configured to eject ink to the sheet and print a second image. The at least one stiffening conveyor may include: a first stiffening conveyor configured to convey the sheet toward the first inkjet head in the conveyance direction while stiffening the sheet by deforming the sheet into a first wave shape in the width direction; and a second stiffening conveyor configured to convey the sheet toward the second inkjet in the conveyance direction while stiffening the sheet by deforming the sheet into a second wave shape in the width direction. The at least one platen may include: a first platen arranged to face the first inkjet head and having a first protrusion, the first protrusion provided on a surface of the first platen facing the first inkjet head and configured to support a first convex portion in the first wave shape of the sheet, the first platen configured to support the sheet while keeping the sheet deformed in the first wave shape; and a second platen arranged to face the second inkjet head and having a second protrusion, the second protrusion provided on a surface of the second platen facing the second inkjet head and configured to support a second convex portion in the second wave shape of the sheet, the second platen configured to support the sheet while keeping the sheet deformed in the second wave shape. A relationship of positions of the first convex portion and a first concave portion in the first wave shape of the sheet may be opposite in the width direction to a relationship of positions of the second convex portion and a second concave portion in the second wave shape of the sheet. The first inkjet head may be configured to perform printing on the first convex portion in the first wave shape of the sheet. The second inkjet head may be configured to perform printing on the second convex portion in the second wave shape of the sheet. 
     In the configuration described above, an increase of a head gap can be suppressed by avoiding printing on a concave portion of the sheet. Accordingly, it is possible to reduce the contact of the sheet with the inkjet head while suppressing a decrease of print quality. 
     The at least one stiffening conveyor may include: a conveyance roller elongated in the width direction; and stiffening rollers arranged along the width direction in such a zigzag pattern that the stiffening rollers are in contact with the conveyance roller at alternately upstream and downstream positions in the conveyance direction, the stiffening rollers configured to convey the sheet by nipping the sheet together with the conveyance roller while deforming the sheet into the wave shape. 
     The first stiffening conveyor may include: a first conveyance roller elongated in the width direction; and first stiffening rollers arranged along the width direction in such a zigzag pattern that the first stiffening rollers are in contact with the first conveyance roller at alternately upstream and downstream positions in the conveyance direction, the first stiffening rollers configured to convey the sheet by nipping the sheet together with the first conveyance roller while deforming the sheet into the first wave shape. The second stiffening conveyor may include: a second conveyance roller elongated in the width direction; and second stiffening rollers arranged along the width direction in such a zigzag pattern that the second stiffening rollers are in contact with the second conveyance roller at alternately upstream and downstream positions in the conveyance direction, the second stiffening rollers configured to convey the sheet by nipping the sheet together with the second conveyance roller while deforming the sheet into the second wave shape. 
     In the configuration described above, the sheet can be easily stiffened while being conveyed by a simple configuration. 
     The at least one stiffening conveyor may include: a conveyance roller with elastic portions formed of elastic members and non-elastic portions formed of non-elastic members alternately arranged along the width direction; first stiffening rollers arranged to be in contact with the elastic portions of the conveyance roller; and second stiffening rollers arranged to be in contact with the non-elastic portions of the conveyance roller. The at least one stiffening conveyor may be configured to convey the sheet by nipping the sheet with the conveyance roller, the first stiffening rollers, and the second stiffening rollers while deforming the sheet into the wave shape by pushing the sheet into the elastic portions with the first stiffening rollers. 
     The first stiffening conveyor may include: a first conveyance roller with elastic portions formed of elastic members and non-elastic portions formed of non-elastic members alternately arranged along the width direction; first stiffening rollers arranged to be in contact with the elastic portions of the first conveyance roller; and second stiffening rollers arranged to be in contact with the non-elastic portions of the first conveyance roller. The first stiffening conveyor may be configured to convey the sheet by nipping the sheet with the first conveyance roller, the first stiffening rollers, and the second stiffening rollers while deforming the sheet into the first wave shape by pushing the sheet into the elastic portions with the first stiffening rollers. The second stiffening conveyor may include: a second conveyance roller with elastic portions formed of elastic members and non-elastic portions formed of non-elastic members alternately arranged along the width direction; third stiffening rollers arranged to be in contact with the elastic portions of the second conveyance roller; and fourth stiffening rollers arranged to be in contact with the non-elastic portions of the second conveyance roller. The second stiffening conveyor may be configured to convey the sheet by nipping the sheet with the second conveyance roller, the third stiffening rollers, and the fourth stiffening rollers while deforming the sheet into the second wave shape by pushing the sheet into the elastic portions with the third stiffening rollers. 
     In the configuration described above, the sheet can be easily stiffened while being conveyed by a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an inkjet printer according to a first embodiment. 
         FIG. 2  is a plan view of the inkjet printer illustrated in  FIG. 1 . 
         FIG. 3  is a front view of a stiffening conveyor in the inkjet printer illustrated in  FIG. 1 . 
         FIG. 4  is a partially enlarged diagram of a platen in the inkjet printer illustrated in  FIG. 1 . 
         FIG. 5  is a control block diagram of the inkjet printer illustrated in  FIG. 1 . 
         FIG. 6  is a diagram illustrating how a sheet is stiffened by the stiffening conveyor. 
         FIG. 7  is a schematic configuration diagram of an inkjet printer according to a second embodiment. 
         FIG. 8  is a plan view of an upstream portion of the inkjet printer illustrated in  FIG. 7 . 
         FIG. 9  is a plan view of a downstream portion of the inkjet printer illustrated in  FIG. 7 . 
         FIG. 10  is a diagram explaining print regions of inkjet heads in the inkjet printer illustrated in  FIG. 7 . 
         FIG. 11  is a schematic configuration diagram of an inkjet printer according to a third embodiment. 
         FIG. 12  is a plan view of the inkjet printer illustrated in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     Description will be hereinbelow provided for embodiments of the present invention by referring to the drawings. It should be noted that the same or similar parts and components throughout the drawings will be denoted by the same or similar reference signs, and that descriptions for such parts and components will be omitted or simplified. In addition, it should be noted that the drawings are schematic and therefore different from the actual ones. 
     First Embodiment 
       FIG. 1  is a schematic configuration diagram of an inkjet printer according to a first embodiment.  FIG. 2  is a plan view of the inkjet printer illustrated in  FIG. 1 .  FIG. 3  is a front view of a stiffening conveyor in the inkjet printer illustrated in  FIG. 1 .  FIG. 4  is a partially enlarged diagram of a platen in the inkjet printer illustrated in  FIG. 1 .  FIG. 5  is a control block diagram of the inkjet printer illustrated in  FIG. 1 . 
     Note that a direction orthogonal to a sheet surface of  FIG. 1  is referred to as front-rear direction and a direction toward a viewer is referred to as front direction. Moreover, up, down, left, and right in the sheet surface of  FIG. 1  are referred to as up, down, left, and right directions. Furthermore, a direction from left to right in  FIG. 1  is a conveyance direction of a sheet and upstream and downstream in the following description mean upstream and downstream in the conveyance direction of the sheet. In  FIGS. 1 to 4, 7 to 9, 11, and 12 , right, left, up, down, front, and rear directions are denoted by RT, LT, UP, DN, FR, and RR, respectively. 
     As illustrated in  FIGS. 1 and 5 , the inkjet printer  1  in the first embodiment includes a conveyor  2 , a printing unit  3 , and a controller  4 . 
     The conveyor  2  conveys a sheet P which is a print medium. The conveyor  2  includes stiffening conveyors  11 A,  11 B, a conveyance roller pair  12 , a conveyance motor  13 , and platens  14 A,  14 B. Note that the stiffening conveyors  11 A,  11 B and the like are sometimes collectively referred to by omitting the alphabets attached to the reference numerals. 
     The stiffening conveyor  11 A is arranged upstream of the inkjet head  41 A. The stiffening conveyor  11 B is arranged downstream of the inkjet head  41 A and upstream of the inkjet head  41 B. Each of the stiffening conveyors  11  includes a conveyance roller  21  and multiple stiffening rollers  22 A,  22 B. 
     The conveyance roller  21  conveys the sheet P by nipping the sheet P together with the stiffening rollers  22 . The conveyance roller  21  is formed in a columnar shape elongating in the front-rear direction (width direction of the sheet P). The conveyance roller  21  is rotationally driven by drive force of the conveyance motor  13 . 
     The stiffening rollers  22  convey the sheet P by nipping the sheet P together with the conveyance roller  21  and stiffen the sheet P by deforming the sheet P into the wave shape. The stiffening rollers  22  are arranged to be in contact with an upper portion of the conveyance roller  21  and follow the conveyance roller  21  to rotate. 
     The stiffening rollers  22  are arranged to be offset downstream relative to the conveyance roller  21 . Specifically, as illustrated in  FIG. 3 , the stiffening rollers  22  are in contact with the conveyance roller  21 , downstream of a center line  26  of the conveyance roller  21  which is parallel to an up-down direction. 
     Moreover, the multiple stiffening rollers  22  are arranged in such a zigzag pattern that the stiffening rollers  22  are in contact with the conveyance roller  21  at alternately upstream and downstream positions in the conveyance direction. Specifically, as illustrated in  FIG. 3 , there are stiffening rollers  22 A whose offset amount is D 1  and stiffening rollers  22 B whose offset amount is D 2  greater than D 1 . Moreover, as illustrated in  FIG. 2 , the stiffening rollers  22 A and the stiffening rollers  22 B are alternately arranged along the front-rear direction. 
     The offset amount D 1  is a distance between the center line  26  of the conveyance roller  21  and a center line  27 A of each stiffening roller  22 A which is parallel to the up-down direction. The offset amount D 2  is a distance between the center line  26  of the conveyance roller  21  and a center line  27 B of each stiffening roller  22 B which is parallel to the up-down direction. 
     As described above, since the stiffening rollers  22  are arranged to be offset downstream relative to the conveyance roller  21 , common tangents of the conveyance roller  21  and the stiffening rollers  22  at nipping points are straight lines titled downward toward the downstream side. Specifically, directions in which the conveyance roller  21  and the stiffening rollers  22  send out the sheet P are tilted downward relative to the horizontal direction as illustrated by the arrows A 1 , A 2  in  FIG. 3 . In  FIG. 3 , the arrow A 1  illustrates the send-out direction of the sheet P at the positions of the stiffening rollers  22 A. The arrow A 2  illustrates the send-out direction of the sheet P at the positions of the stiffening rollers  22 B. 
     The conveyance roller  21  and the stiffening rollers  22  send out the sheet P downward and the sheet P is thereby conveyed toward a position below the inkjet head  41  to be pushed against the platen  14 . This suppresses lifting of the sheet P from the platen  14 . 
     Moreover, the send-out direction (arrow A 1 ) of the sheet P at the positions of the stiffening rollers  22 A is different from the send-out direction (arrow A 2 ) of the sheet P at the positions of the stiffening rollers  22 B, and the sheet P is thereby deformed into the wave shape in the width direction to be stiffened. 
     The conveyance roller pair  12  conveys the sheet P subjected to printing by the printing unit  3  downstream by nipping the sheet P. The conveyance roller pair  12  is arranged downstream of the platen  14 B. The conveyance roller pair  12  includes a conveyance roller  31  and a following roller  32 . 
     The conveyance roller  31  is formed in a cylindrical shape elongating in the front-rear direction. The conveyance roller  31  is rotationally driven by the drive force of the conveyance motor  13 . The following roller  32  is formed in a cylindrical shape elongated to have the same length as the conveyance roller  31 . The following roller  32  is in contact with the conveyance roller  31  and follows the conveyance roller  31  to rotate. 
     The platens  14 A,  14 B support the sheet P conveyed below the inkjet heads  41 A,  41 B, respectively. The platens  14 A,  14 B are arranged below the inkjet heads  41 A,  41 B to face nozzle surfaces  41   a  of the inkjet heads  41 A,  41 B, respectively. The platen  14 B is arranged at a position below the platen  14 A. 
     Each of the platens  14  is formed in a plate shape. Multiple ribs (protrusions)  36  are formed on an upper surface  14   a  of the platen  14  facing the nozzle surface  41   a  of the inkjet head  41 . 
     As illustrated in  FIG. 4 , the ribs  36  are formed to protrude upward from the upper surface  14   a  of the platen  14 . The ribs  36  are portions which support mountain portions (convex portions) Pa of the sheet P stiffened by being deformed into the wave shape by the stiffening conveyor  11 . Here, the mountain portions Pa of the sheet P stiffened by being deformed into the wave shape are portions curving to protrude upward in the sheet P. Portions curving to protrude downward in the sheet P stiffened by being deformed into the wave shape are referred to as valley portions (concave portions) Pb. The cross section of the sheet P along the width direction has the wave shape due to the mountain portions Pa and the valley portions Pb of the sheet P. 
     The multiple ribs  36  are arranged at certain intervals in the front-rear direction. Moreover, in the platens  14 A,  14 B, the ribs  36  are arranged downstream of the respective stiffening rollers  22 A whose offset amount is D 1  in the stiffening conveyors  11 A,  11 B. In the platens  14 A,  14 B, the ribs  36  thus support the respective mountain portions Pa of the sheet P stiffened by being deformed into the wave shape by the stiffening conveyors  11 A,  11 B. 
     Since the ribs  36  protrude from the upper surface  14   a , portions around the ribs  36  in the platen  14  are lower than the ribs  36 . When the sheet P stiffened by being deformed into the wave shape by the stiffening conveyor  11  is conveyed to the platen  14 , the mountain portions Pa of the sheet P pass the ribs  36  and the valley portions Pb pass the portions lower than the ribs  36 . The sheet P is thus kept in a wave shape state. In other words, the platen  14  supports the sheet P while keeping the sheet P in the wave shape state. 
     The printing unit  3  prints an image on the sheet P conveyed by the conveyor  2 . The printing unit  3  includes the inkjet heads  41 A,  41 B. 
     Each of the inkjet heads  41  prints the image by ejecting ink to the conveyed sheet P. The inkjet head  41  has multiple nozzles (not illustrated) which are opened on the nozzle surface  41   a  being a lower surface of the inkjet head  41  and which are arranged along a main scanning direction (front-rear direction), and eject the ink from the nozzles. 
     The inkjet heads  41 A,  41 B are arranged at a certain interval in a sub-scanning direction (left-right direction) which is the conveyance direction of the sheet P, and the inkjet head  41 A is arranged on the upstream side while the inkjet head  41 B is arranged on the downstream side. In the main scanning direction (front-rear direction), the inkjet head  41 B is shifted to the rear side relative to the inkjet head  41 A. In other words, the inkjet heads  41 A,  41 B are arranged in zigzag. Thus, the inkjet head  41 A performs printing on a front region of the sheet P while the inkjet head  41 B performs printing on a rear region of the sheet P. 
     The controller  4  controls operations of units in the inkjet printer  1 . The controller  4  includes a CPU, a RAM, a ROM, a hard disk drive, and the like. 
     Next, operations of the inkjet printer  1  are described. 
     When printing is to be performed in the inkjet printer  1 , first, the controller  4  starts the drive of the stiffening conveyors  11 A,  11 B and the conveyance roller pair  12  by using the conveyance motor  13 . 
     The sheet P is conveyed to the conveyor  2  from the upstream side and is then conveyed in the conveyor  2  while being nipped by the stiffening conveyors  11 A,  11 B and the conveyance roller pair  12 . 
     In this case, in each of the stiffening conveyors  11 , the mountain portions Pa and the valley portions Pb are formed in the sheet Pas illustrated in  FIG. 6  due to the difference between the send-out direction of the sheet P by the stiffening rollers  22 A and that by the stiffening rollers  22 B. Specifically, the mountain portions Pa are formed at the positions where the sheet P is sent out by the stiffening rollers  22 A and the valley portions Pb are formed at the positions where the sheet P is sent out by the stiffening rollers  22 B. The sheet P is thereby stiffened by being deformed into the wave shape in which the mountain portions Pa and the valley portions Pb are alternately formed in the width direction. 
     As illustrated in  FIG. 4 , the sheet P stiffened by being deformed into the wave shape travels downstream below the inkjet head  41  with the mountain portions Pa being supported by the ribs  36  and the valley portions Pb being supported by the upper surface  14   a  of the platen  14 . The sheet P is thereby conveyed below the inkjet head  41  while being kept in a state stiffened by being deformed into the wave shape. 
     The controller  4  drives the inkjet heads  41 A,  41 B based on image data of a print target and causes the inkjet heads  41 A,  41 B to eject the ink to the sheet P conveyed by the conveyor  2 . The image is thereby printed on the sheet P. The sheet P subjected to printing by the inkjet heads  41 A,  41 B is conveyed to a sheet discharger (not illustrated) by the conveyance roller pair  12 . 
     As described above, in the inkjet printer  1 , the stiffening conveyors  11  convey the sheet P to the inkjet heads  41  while stiffening the sheet P by deforming it into the wave shape. The platens  14  support the sheet P below the inkjet heads while keeping the sheet P in the wave shape state. Accordingly, when the sheet P is curled, the sheet P is conveyed below the inkjet heads  41  with the curl being suppressed by the stiffening. As a result, the inkjet printer  1  can reduce the contact of the sheet P with the inkjet heads  41 . 
     In addition, in the inkjet printer  1 , the multiple stiffening rollers  22  are arranged in such a zigzag pattern that the stiffening rollers  22  are in contact with the conveyance roller  21  at alternately the upstream and downstream portions in the conveyance direction. Moreover, the conveyance roller  21  and the stiffening rollers  22  stiffen the sheet P while conveying the sheet P by nipping it. The sheet P can be thereby easily stiffened while being conveyed by a simple configuration. 
     Second Embodiment 
       FIG. 7  is a schematic configuration diagram of an inkjet printer  1 A according to a second embodiment.  FIG. 8  is a plan view of an upstream portion of the inkjet printer  1 A illustrated in  FIG. 7 .  FIG. 9  is a plan view of a downstream portion of the inkjet printer  1 A illustrated in  FIG. 7 . 
     As illustrated in  FIG. 7 , the inkjet printer  1 A according to the second embodiment has a configuration in which the conveyor  2  and the printing unit  3  in the inkjet printer  1  of the first embodiment are replaced with a conveyor  2 A and a printing unit  3 A, respectively. 
     The conveyor  2 A has a configuration in which stiffening conveyors  11 C,  11 D and platens  14 C,  14 D are added to the conveyor  2  of the first embodiment. 
     The stiffening conveyors  11 C,  11 D convey the sheet P to inkjet heads  41 C,  41 D of the printing unit  3  to be described later while stiffening the sheet P by deforming it into the wave shape as in the stiffening conveyors  11 A,  11 B. The stiffening conveyor  11 C is arranged downstream of the inkjet head  41 A and upstream of the inkjet head  41 C. The stiffening conveyor  11 D is arranged downstream of the inkjet head  41 B and upstream of the inkjet head  41 D. 
     The stiffening conveyors  11 C,  11 D have a configuration in which the relationship of the positions of the stiffening rollers  22 A,  22 B in the front-rear direction (width direction of the sheet P) is reversed from that in the stiffening conveyors  11 A,  11 B. 
     Specifically, the stiffening rollers  22 B of the stiffening conveyor  11 C are arranged downstream of the stiffening rollers  22 A of the stiffening conveyor  11 A, respectively, and the stiffening rollers  22 A of the stiffening conveyors  11 C are arranged downstream of the stiffening rollers  22 B of the stiffening conveyor  11 A, respectively. The stiffening conveyors  11 A,  11 C thereby stiffen the sheet P by deforming it into wave shapes opposite in the relationship of the positions of the mountain portions Pa and the valley portions Pb in the width direction. 
     Moreover, the stiffening rollers  22 B of the stiffening conveyor  11 D are arranged downstream of the stiffening rollers  22 A of the stiffening conveyor  11 B, respectively, and the stiffening rollers  22 A of the stiffening conveyors  11 D are arranged downstream of the stiffening rollers  22 B of the stiffening conveyor  11 B, respectively. The stiffening conveyors  11 B,  11 D thereby stiffen the sheet P by deforming it into wave shapes opposite in the relationship of the positions of the mountain portions Pa and the valley portions Pb in the width direction. 
     Platens  14 C,  14 D support the sheet P conveyed below the inkjet heads  41 C,  41 D to be described later. The platens  14 C,  14 D are arranged below the inkjet heads  41 C,  41 D to face nozzle surfaces  41   a  of the inkjet heads  41 C,  41 D, respectively. The platen  14 C is arranged at a position below the platen  14 A and above the platen  14 B. The platen  14 D is arranged at a position below the platen  14 B. 
     The platens  14 C,  14 D have a configuration in which the positions of the ribs  36  are changed from those in the platens  14 A,  14 B. Specifically, the ribs  36  of the platen  14 C are arranged downstream of the stiffening rollers  22 A of the stiffening conveyor  11 C, respectively. The ribs  36  of the platen  14 C thereby support the mountain portions Pa of the sheet P stiffened by being deformed into the wave shape by the stiffening conveyor  11 C. The ribs  36  of the platen  14 D are arranged downstream of the stiffening rollers  22 A of the stiffening conveyor  11 D. The ribs  36  of the platen  14 D thereby support the mountain portions Pa of the sheet P stiffened by being deformed into the wave shape by the stiffening conveyor  11 D. 
     The printing unit  3 A has a configuration in which the inkjet heads  41 C,  41 D are added to the printing unit  3  of the first embodiment. 
     The inkjet heads  41 C,  41 D each have multiple nozzles which are opened on the nozzle surface  41   a  and which are arranged along the main scanning direction (front-rear direction) as in the inkjet heads  41 A,  41 B, and eject the ink from the nozzles. 
     The inkjet head  41 C is paired with the inkjet head  41 A and prints an image in a front region of the sheet P. The inkjet head  41 C is arranged downstream of and away from the inkjet head  41 A. The inkjet head  41 C is arranged at the same position as the inkjet head  41 A in the front-rear direction. 
     The inkjet head  41 D is paired with the inkjet head  41 B and prints an image in a rear region of the sheet P. The inkjet head  41 D is arranged downstream of and away from the inkjet head  41 B. The inkjet head  41 D is arranged at the same position as the inkjet head  41 B in the front-rear direction. 
     The inkjet heads  41 A to  41 D of the printing unit  3 A perform printing on the mountain portions Pa of the sheet P stiffened by being deformed into the wave shapes by the stiffening conveyors  11 A to  11 D corresponding respectively to the inkjet heads  41 A to  41 D. 
     Next, operations of the inkjet printer  1 A are described. 
     When printing is to be performed in the inkjet printer  1 A, first, the controller  4  starts the drive of the stiffening conveyors  11 A to  11 D and the conveyance roller pair  12  by using the conveyance motor  13 . 
     The sheet P is conveyed to the conveyor  2 A from the upstream side and is then conveyed in the conveyor  2 A while being nipped by the stiffening conveyors  11 A to  11 D and the conveyance roller pair  12 . 
     The sheet P conveyed in the conveyor  2 A is stiffened by the stiffening conveyors  11 A to  11 D and is conveyed below the inkjet heads  41 A to  41 D while being kept in the stiffened state by the platens  14 A to  14 D. 
     In this case, as described above, the stiffening conveyors  11 A,  11 C stiffen the sheet P by deforming it into the wave shapes opposite in the relationship of the positions of the mountain portions Pa and the valley portions Pb. Moreover, the controller  4  controls the inkjet heads  41 A,  41 C to cause the inkjet heads  41 A,  41 C to perform printing on the mountain portions Pa of the sheet P. 
     As illustrated in  FIG. 10 , regions Ea corresponding to the mountain portions Pa formed when the sheet P passes below the inkjet head  41 A are subjected to printing by the ink ejection from the inkjet head  41 A. Regions Eb corresponding to the mountain portions Pa formed when the sheet P passes below the inkjet head  41 C are subjected to printing by the ink ejection from the inkjet head  41 C. Here, since the stiffening conveyor  11 A,  11 C stiffen the sheet P such that the relationship of the positions of the mountain portions Pa and the valley portions Pb in the stiffening conveyor  11 A is opposite to that in the stiffening conveyor  11 C, the regions Ea, Eb are regions alternately provided in the width direction of the sheet P. 
     As described above, the inkjet head  41 A,  41 C perform, as a pair, printing on the regions Ea, Eb assigned thereto and thereby print the image in the front region of the sheet P. 
     Furthermore, as described above, the stiffening conveyors  11 B,  11 D also stiffen the sheet P by deforming it into the wave shapes opposite in the relationship of the positions of the mountain portions Pa and the valley portions Pb. Moreover, the controller  4  controls the inkjet heads  41 B,  41 D to cause the inkjet heads  41 B,  41 D to perform printing on the mountain portions Pa of the sheet P. 
     The inkjet heads  41 B,  41 D thereby perform, as a pair, printing on the regions corresponding to the mountain portions Pa formed when the sheet P passes below the inkjet heads  41 B,  41 D as in the inkjet heads  41 A,  14 C, and thereby print the image in the rear region of the sheet P. 
     The sheet P subjected to the printing by the inkjet heads  41 A to  41 D are conveyed to the sheet discharger (not illustrated) by the conveyance roller pair  12 . 
     As described above, in the inkjet printer  1 A, the stiffening conveyor  11 A corresponding to the inkjet head  41 A which is an upstream one of the paired inkjet heads  41 A,  41 C and the stiffening conveyor  11 C corresponding to the inkjet head  41 C which a downstream one of the paired inkjet heads  41 A,  41 C stiffen the sheet P such that the relationship of the positions of the mountain portions Pa and the valley portions Pb in the stiffening conveyor  11 A is opposite to that in the stiffening conveyor  11 C. Similarly, the stiffening conveyor  11 B corresponding to the inkjet head  41 B which is an upstream one of the paired inkjet heads  41 B,  41 D and the stiffening conveyor  11 D corresponding to the inkjet head  41 D which a downstream one of the paired inkjet heads  41 B,  41 D stiffen the sheet P such that the relationship of the positions of the mountain portions Pa and the valley portions Pb in the stiffening conveyor  11 B is opposite to that in the stiffening conveyor  11 D. Moreover, the inkjet heads  41 A to  41 D perform printing on the mountain portions Pa of the sheet P stiffened by being deformed into the wave shapes by the stiffening conveyors  11 A to  11 D corresponding respectively to the inkjet heads  41 A to  41 D. The inkjet heads  41 A to  41 D perform printing on the entire sheet P by performing printing on the mountain portions Pa of the sheet P. 
     In an inkjet method, the larger the head gap being a distance between the inkjet head  41  and the sheet P is, the more likely that landing positions of ink will deviate. The deviation of landing positions of ink causes a decrease of print quality. In view of this, in the inkjet printer  1 A, the inkjet heads  41 A to  41 D perform printing on the mountain portions Pa of the sheet P and avoid printing on the valley portions Pb, and an increase of the head gap is thus suppressed. Accordingly, the inkjet printer  1 A can reduce contact of the sheet P with the inkjet heads  41  while suppressing the decrease of print quality. 
     Third Embodiment 
       FIG. 11  is a schematic configuration diagram of an inkjet printer  1 B according to a third embodiment.  FIG. 12  is a plan view of the inkjet printer  1 B illustrated in  FIG. 11 . 
     As illustrated in  FIG. 11 , the inkjet printer  1 B according to the third embodiment has a configuration different from the inkjet printer  1  of the first embodiment in that the conveyor  2  is replaced with a conveyor  2 B. 
     The conveyor  2 B has a configuration in which the stiffening conveyors  11 A,  11 B of the conveyor  2  in the first embodiment are replaced with stiffening conveyors  51 A,  51 B. 
     The stiffening conveyors  51  each include a conveyance roller  56  and multiple stiffening rollers  57 . 
     The conveyance roller  56  conveys the sheet P by nipping the sheet P together with the stiffening rollers  57 . The conveyance roller  56  is formed in a cylindrical shape elongating in the front-rear direction (width direction of the sheet P). The conveyance roller  56  includes multiple elastic portions  56   a  and multiple non-elastic portions  56   b.    
     The elastic portions  56   a  are portions formed of elastic members such as rubber members. The non-elastic portions  56   b  are portions formed of non-elastic members such as metal members. The elastic portions  56   a  and the non-elastic portions  56   b  are alternately arranged along the front-rear direction (width direction of the sheet P). 
     The stiffening rollers  57  convey the sheet P by nipping the sheet P together with the conveyance roller  56  and stiffen the sheet P by deforming it into a wave shape. The stiffening rollers  57  are arranged to be in contact with an upper portion of the conveyance roller  56  and follow the conveyance roller  56  to rotate. 
     The stiffening rollers  57  are arranged to be in contact respectively with the elastic portions  56   a  and the non-elastic portions  56   b  of the conveyance roller  56 . The stiffening rollers  57  arranged on the elastic portions  56   a  are in contact with the elastic portions  56   a  at such nipping pressure that surfaces of the elastic portions  56   a  are deformed and recessed. At the positions of the elastic portions  56   a , the stiffening rollers  57  thus push the sheet P into the elastic portions  56   a  when the sheet P is conveyed while being nipped by the conveyance roller  56  and the stiffening rollers  57 . Meanwhile, at the positions of the non-elastic portions  56   b , since the non-elastic portions  56   b  do not deform, the sheet P is not pushed into the non-elastic portions  56   b . Accordingly, the sheet P is stiffened by being deformed into the wave shape such that the valley portions Pb are formed at the positions of the elastic portions  56   a  and the mountain portions Pa are formed at the positions of the non-elastic portions  56   b.    
     In the platens  14 A,  14 B, the ribs  36  are arranged downstream of the non-elastic portions  56   b  of the stiffening conveyors  51 A,  51 B. The ribs  36  thereby support the mountain portions Pa of the sheet P stiffened by being deformed into the wave shape by the stiffening conveyors  51 A,  51 B, in the platens  14 A,  14 B. 
     As described above, in the third embodiment, the stiffening conveyors  51  each include the conveyance roller  56  having the elastic portions  56   a  and the non-elastic portions  56   b  which are arranged alternately and the multiple stiffening rollers  57  arranged to be in contact respectively with the elastic portions  56   a  and the non-elastic portions  56   b  of the conveyance roller  56 . Moreover, the sheet P is conveyed while being nipped by the conveyance roller  56  and the multiple stiffening rollers  57 , and the stiffening rollers  57  in contact with the elastic portions  56   a  of the conveyance roller  56  push the sheet P into the elastic portions  56   a  to stiffen the sheet P by deforming it into the wave shape. The sheet P can be thereby easily stiffened while being conveyed by a simple configuration. 
     Other Embodiments 
     The stiffening conveyors  11  in the second embodiment may be replaced with the stiffening conveyors  51  in the third embodiment. In this case, it is only necessary to set the relationship of the positions of the elastic portions  56   a  and the non-elastic portions  56   b  in the stiffening conveyor  51  corresponding to the inkjet head  41 A opposite to that in the stiffening conveyor  51  corresponding to the inkjet head  41 C. Similarly, it is only necessary to set the relationship of the positions of the elastic portions  56   a  and the non-elastic portions  56   b  in the stiffening conveyor  51  corresponding to the inkjet head  41 B opposite to that in the stiffening conveyor  51  corresponding to the inkjet head  41 D. 
     In an inkjet printer, a sheet subjected to printing is more likely to curl and come into contact with an inkjet head while a sheet not subjected to printing is less likely to curl. Accordingly, an inkjet printer in which multiple inkjet heads are arranged in the sub-scanning direction parallel to one another may be configured such that no stiffening of the sheet in the aforementioned embodiments is performed for the most-upstream inkjet head which performs printing on a sheet not subjected to printing and the stiffening of the sheet by the stiffening conveyors  11  or the stiffening conveyors  51  and the keeping of the sheet in the stiffened state by the platens  14  in the aforementioned embodiments are applied to the inkjet heads other than the most-upstream inkjet head. This can reduce the contact of the sheet with the inkjet heads other than the most-upstream inkjet head in which the contact with the sheet due to curling thereof is more likely to occur. 
     Moreover, the inkjet printer may be configured such that no stiffening of the sheet is performed for the most-upstream inkjet head which performs printing on a sheet not subjected to printing and the second embodiment is applied to the inkjet heads other than the most-upstream inkjet head. Specifically, in this case, the inkjet heads other than the most-upstream inkjet head are configured as pairs of two inkjet heads. Moreover, for each pair of inkjet heads, the stiffening conveyors  11  or the stiffening conveyors  51  in the aforementioned embodiments stiffen the sheet such that the relationship of the positions of the mountain portions and the valley portions in one stiffening conveyor is opposite to that in the other stiffening conveyor, and the platens  14  keep the sheet in the stiffened state. Furthermore, the pair of inkjet heads performs printing on the mountain portions of the sheet stiffened by being deformed into the wave shapes. 
     Embodiments of the present invention have been described above. However, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     Moreover, the effects described in the embodiments of the present invention are only a list of optimum effects achieved by the present invention. Hence, the effects of the present invention are not limited to those described in the embodiment of the present invention.