Patent Publication Number: US-10308046-B2

Title: Liquid ejection apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2017-005568, which was filed on Jan. 17, 2017, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The following disclosure relates to a liquid ejection apparatus including a conveyor configured to convey a recording medium and an ejector including a plurality of ejection openings from which a liquid is ejected to the recording medium. 
     Description of Related Art 
     There is known an image recording apparatus which includes a conveyor including: a conveyance roller pair (upstream roller pair) disposed upstream of a recording head (ejector) in a conveyance path; a discharge roller pair (first downstream roller pair) disposed downstream of the recording head in the conveyance path; and a switchback roller pair (second downstream roller pair) disposed downstream of the discharge roller pair in the conveyance path. An upper roller of each of the discharge roller pair and the switchback roller pair that is to contact a front surface of the recording medium (i.e., a surface of the recording medium on which the liquid is ejected) is a spur roller having at least one protrusion formed on its outer circumferential surface. This configuration prevents a liquid landed on the front surface of the recording medium from being transferred to the upper roller. 
     SUMMARY 
     In the structure in which the recording medium is conveyed using the three roller pairs (i.e., the upstream roller pair, the first downstream roller pair, and the second downstream roller pair), when the recording medium is conveyed by the first downstream roller pair and the second downstream roller pair in a state in which a trailing end of the recording medium passes through the upstream roller pair while it does not yet pass through an ejection region of the ejector located below the recording head, it is required to stabilize conveyance of the recording medium for ensuring a good recording quality. To this end, the first downstream roller pair and the second downstream roller pair are configured to have mutually different rotation speeds or mutually different conveyance forces, especially, mutually different nipping forces for nipping the recording medium by the upper roller and a lower roller, for instance. Specifically, the rotation speed of the second downstream roller pair is made larger than the rotation speed of the first downstream roller pair, and the conveyance force of the first downstream roller pair is made sufficiently larger (e.g., ten times larger) than the conveyance force of the second downstream roller pair. With this configuration, the recording medium slips between the rollers of the second downstream roller pair, so that the first downstream roller pair mainly conveys the recording medium, achieving stable conveyance of the recording medium. 
     The conveyance force of the first downstream roller pair needs to be made sufficiently larger than the conveyance force of the second downstream roller pair from the viewpoint of ensuring a good recording quality while it is desirable not to increase the conveyance force of the first downstream roller pair too much from the viewpoint of preventing a damage of the recording medium due to pressing by the at least one protrusion of the spur roller. On the other hand, if the conveyance force of the second downstream roller pair is made excessively small, the recording medium slips when the recording medium is conveyed only by the second downstream roller pair, causing a risk that the recording medium fails to be conveyed. Thus, it is difficult to make the conveyance force of the first downstream roller pair sufficiently larger than the conveyance force of the second downstream roller pair. Accordingly, when the recording medium is conveyed by the first downstream roller pair and the second downstream roller pair, the recording medium cannot be stably conveyed, causing a risk of skewing of the recording medium. 
     Accordingly, one aspect of the present disclosure relates to a liquid ejection apparatus capable of ensuring stable conveyance of the recording medium and preventing skewing of the recording medium when the recording medium is conveyed by the first downstream roller pair and the second downstream roller pair upper rollers of which are spur rollers. 
     In one aspect of the present disclosure, a liquid ejection apparatus including: a conveyor configured to convey a recording medium along a conveyance path; and an ejector including a plurality of ejection openings from which a liquid is ejected to a front surface of the recording medium conveyed by the conveyor; wherein the conveyor includes an upstream roller pair disposed upstream of the ejector on the conveyance path, a first downstream roller pair disposed downstream of the ejector on the conveyance path, and a second downstream roller pair disposed downstream of the first downstream roller pair on the conveyance path, the conveyor being configured to convey the recording medium such that respective central positions of the upstream roller pair, the first downstream roller pair, and the second downstream roller pair in a first direction coincide with a central position of the recording medium in the first direction, the first direction being parallel to an axial direction of each of rollers of the upstream roller pair, the first downstream roller pair, and the second downstream roller pair, wherein each of the upstream roller pair, the first downstream roller pair, and the second downstream roller pair includes an upper roller which is to contact a front surface of the recording medium and a lower roller which is to contact a back surface of the recording medium opposite to the front surface, at least one of the upper roller and the lower roller being configured to give a conveyance force to the recording medium while the upper roller and the lower roller nip the recording medium therebetween, so as to convey the recording medium, wherein a rotation speed of the second downstream roller pair is higher than a rotation speed of the first downstream roller pair, and the conveyance force of the first downstream roller pair is larger than the conveyance force of the second downstream roller pair, and wherein the conveyance force of the second downstream roller pair is largest at a position of the second downstream roller pair in the first direction which is nearest to the central position of the second downstream roller pair. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of one embodiment, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view taken along a plane parallel to a vertical direction, the view showing an inside of a printer according to one embodiment; 
         FIG. 2  is a fragmentary sectional view of an ejector of the printer according to the embodiment; 
         FIG. 3  is a plan view showing roller pairs of a conveyor and the ejector of the printer according to the embodiment; 
         FIG. 4  is a view seen in a direction indicated by an arrow IV in  FIG. 3 ; and 
         FIG. 5  is a schematic view showing a relationship between conveyance force of each roller pair and moment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Overall Structure 
     As shown in  FIG. 1 , a printer  1  (as one example of “liquid ejection apparatus”) according to one embodiment includes: a sheet supply tray  10  capable of storing a stack of a plurality of sheets  100 ; a conveyor  20  configured to convey, along a conveyance path R, an uppermost one of the sheets  100  stored in the sheet tray  10 ; an ejector  30  including a plurality of ejection openings  30   x  ( FIG. 2 ) from which ink is ejected to a front surface of the sheet  100  that is being conveyed by the conveyor  20 ; a platen  40  opposed to an ejection surface  30   a  of the ejector  30  in which the ejection openings  30   x  are opened; and a sheet discharge tray  50  for receiving the sheet  100  that has been conveyed by the conveyor  20 . 
     Ejector 
     As shown in  FIG. 2 , the ejector  30  includes a flow-passage unit  30   m  and an actuator unit  30   n.    
     A lower surface of the flow-passage unit  30   m  corresponds to the ejection surface  30   a . There are formed, in the flow-passage unit  30   m , a common passage  30   y  communicating with an ink tank (not shown) and individual passages  30   z  provided for the respective ejection openings  30   x . Each individual passage  30   z  is a flow passage extending from an outlet of the common passage  30   y  to a corresponding one of the ejection openings  30   x  via a corresponding one of a plurality of pressure chambers  30   z   1 . The pressure chambers  30   z   1  are open in an upper surface of the flow-passage unit  30   m.    
     The actuator unit  30   n  includes: an oscillating plate  30   n   1  disposed on the upper surface of the flow-passage unit  30   m  so as to cover the plurality of pressure chambers  30   zl ; a piezoelectric layer  30   n   2  disposed on an upper surface of the oscillating plate  30   n   1 ; and a plurality of individual electrodes  30   n   3  disposed on an upper surface of the piezoelectric layer  30   n   2  so as to be opposed to the respective pressure chambers  30   z   1 . In the oscillating plate  30   n   1  and the piezoelectric layer  30   n   2 , a portion sandwiched by and between each individual electrode  30   n   3  and a corresponding one of the pressure chambers  30   z   1  functions as an individual unimorph actuator for the pressure chamber  30   z   1 . Each actuator is deformable independently of other actuators in accordance with a voltage applied to the corresponding individual electrode  30   n   3  by a head driver  30   d . When the actuator is deformed so as to protrude toward the pressure chamber  30   z   1 , the volume of the pressure chamber  30   z   1  is decreased, so that a pressure is applied to the ink in the pressure chamber  30   z   1  and the ink is accordingly ejected from the ejection opening  30   x.    
     The ejector  30  is of a serial type. The ejector  30  is held by a carriage (not shown) and ejects the ink from the ejection openings  30   x  while reciprocating in a scanning direction. 
     Conveyance Path 
     As shown in  FIG. 1 , the conveyance path R includes: a path R 1  extending from the sheet supply tray  10  to the sheet discharge tray  50 ; and a path R 2  connecting a position A located downstream of the ejector  30  on the path R 1  and a position B located upstream of the ejector  30  on the path R 1 . 
     Conveyor 
     The conveyor  20  includes: a sheet supply roller  11  disposed so as to be held in contact with an uppermost one of the sheets  100  stored in the sheet supply tray  10 ; a roller pair  21  (as one example of “upstream roller pair”) disposed downstream of the position B on the path R 1  and upstream of the ejector  30  on the path R 1 ; a roller pair  22  (as one example of “first roller pair”) disposed downstream of the ejector  30  on the path R 1  and upstream of the position A on the path R 1 ; a roller pair  23  (as one example of “second roller pair”) disposed downstream of the position A on the path R 1 ; a roller pair  24  disposed on the path R 2 ; and guide plates  20   g  that define the conveyance path R. The roller pair  23  is disposed most downstream on the conveyance path R. 
     When single-sided recording is performed, the sheet  100  is conveyed from the sheet supply tray  10  along the path R 1 . The ink is ejected to one surface of the sheet (which was facing downward in the sheet supply tray  10 ) for performing recording. After recording is completed, the sheet  100  is received by the sheet discharge tray  50 . 
     When duplex recording is performed, the sheet  100  is conveyed from the sheet supply tray  10  along the path R 1 . The ink is ejected to the one surface of the sheet  100  for performing recording on the one surface. After recording on the one surface is completed, conveyance of the sheet  100  is suspended with a trailing end of the sheet  100  nipped by the roller pair  23 . Subsequently, the conveyance direction of the sheet  100  is reversed by reverse rotation of the roller pair  23 , so that the sheet  100  is conveyed along the path R 2  and is returned to the path R 1  from the position B. Thereafter, the sheet  100  is conveyed again along the path R 1 , and the ink is ejected to another surface (which was facing upward in the sheet supply tray  10 ) for performing recording on another surface. After recording on another surface is completed, the sheet  100  is received by the sheet discharge tray  50 . 
     Each of the roller pairs  21 - 23  includes an upper roller  21   a - 23   a  that is to contact the front surface of the sheet  100  (which is a surface of the sheet  100  that is opposed to the ejector  30  when the sheet  100  passes between the ejector  30  and the platen  40 ) and a lower roller  21   b - 23   b  that is to contact a back surface of the sheet  100  opposite to the front surface. The upper roller  21   a - 23   a  and the lower roller  21   b - 23   b  rotate in opposite directions while nipping the sheet  100  therebetween, whereby a conveyance force is given to the sheet  100  so as to convey the sheet  100 . 
     The rotation speeds of the roller pairs  21 - 23  are set such that the roller pair located more downstream on the path R 1  has a higher rotation speed. That is, the rotation speed of the roller pair  23  is higher than the rotation speed of the roller pair  22 , and the rotation speed of the roller pair  22  is higher than the rotation speed of the roller pair  21 . This configuration stabilizes conveyance of the sheet  100 . 
     The conveyance forces of the roller pairs  21 - 23  are set such that the roller pair located more upstream on the path R 1  has a larger conveyance force. That is, the conveyance force of the roller pair  21  is larger than the conveyance force of the roller pair  22 , and the conveyance force of the roller pair  22  is larger than the conveyance force of the roller pair  23 . The conveyance force is represented by a product of: a friction coefficient of an outer circumferential surface of each of the upper roller  21   a - 23   a  and the lower roller  21   b - 23   b ; and a nipping force for nipping the sheet  100  by the upper roller  21   a - 23   a  and the lower roller  21   b - 23   b.    
     The conveyor  20  is capable of conveying a plurality of sizes of the sheets  100 . As shown in  FIG. 3 , the conveyor  20  is configured to convey the sheet  100  such that each of centers C 0 -C 2  of the respective roller pairs  21 - 23  in an axial direction of each of the roller pairs  21 - 23  (that is parallel to the scanning direction) and a center O of the sheet  100  in its width direction (that is parallel to the axial direction of each of the roller pairs  21 - 23 ) coincide with each other. In other words, the conveyor  20  is configured to convey the sheet  100  such that the centers C 0 -C 2  of the roller pairs  21 - 23  and the center O of the sheet  100  are aligned with each other in the conveyance direction. Here, a direction parallel to the axial direction of each roller pair  21 - 23  is defined as a first direction. The roller pairs  21 - 23  are disposed such that the center C 0  which is a central position of the roller pair  21  in the first direction, the center C 1  which is a central position of the roller pair  22  in the first direction, and the center C 2  which is a central position of the roller pair  23  in the first direction are aligned with one another in the conveyance direction. The conveyor  20  and the conveyance path R are designed such that the sheet  100  is conveyed in a state in which the center O as the central position of the sheet  100  in the first direction coincides with positions in the first direction corresponding to the respective centers C 0 , C 1 , C 2 . In other words, the conveyor  20 , the conveyance path R, and the roller pairs  21 - 23  are designed such that all of the centers C 0 , C 1 , C 2  of the roller pairs  21 - 23  and the center O of the sheet  100  are located on one plane that is perpendicular to the first direction. In  FIG. 3 , shafts that support the respective roller pairs  21 - 23  are not illustrated. 
     A distance L between the roller pair  21  and the roller pair  23  along the conveyance path R is equal to or smaller than a length, along the conveyance path R, of the sheet  100  having a certain size that is most frequently used (e.g., A4 size or a letter size) among a plurality of sizes of the sheets  100  that can be conveyed by the conveyor  20 . 
     Each of the upper roller  21   a  and the lower roller  21   b  of the roller pair  21  is constituted by one long roller extending in the axial direction. Each of the upper roller  22   a ,  23   a  and the lower roller  22   b ,  23   b  of each roller pair  22 ,  23  is constituted by a plurality of partial rollers spaced apart from one another in the axial direction. 
     The roller pair  22  includes eight pairs of partial rollers  22   a   1 ,  22   b   1 . The partial rollers  22   a   1 ,  22   b   1  of each pair are disposed so as to be in contact with each other. (The partial roller  22   a   1  is one example of “first partial roller”.) The roller pair  23  includes six pairs of partial rollers  23   a   1 ,  23   b   1 . The partial rollers  23   a   1 ,  23   b   1  of each pair are disposed so as to be in contact with each other. (The partial roller  23   a   1  is one example of “second partial roller”.) The eight pairs of the partial rollers  22   a   1 ,  22   b   1  are arranged in the scanning direction so as to be equally spaced apart from one another. The six pairs of the partial rollers  23   a   1 ,  23   b   1  are arranged in the scanning direction so as to be equally spaced apart from one another. Positions of the six pairs of the partial rollers  23   a   1 ,  23   b   1  in the scanning direction respectively correspond to positions, in the scanning direction, of six of the eight pairs of the partial rollers  22   a   1 ,  22   b   1  except two outermost pairs of the partial rollers  22   a   1 ,  22   b   1  in the scanning direction. Position P 1   x , P 1   y  at which the two outermost pairs of the partial rollers  22   a   1 ,  22   b   1  of the roller pair  22  in the scanning direction are respectively disposed are distant from the center C 1  in the scanning direction by the same distance D 1 . (Each of the positions P 1   x , P 1   y  is one example of “first outermost position”.) Positions P 2   x , P 2   y  at which two outermost pairs of the partial rollers  23   a   1 ,  23   b   1  of the roller pair  23  in the scanning direction are respectively disposed are distant from the center C 2  in the scanning direction by the same distance D 2 . (Each of the positions P 2   x , P 2   y  is one example of “second outermost position”.) The position P 2   x  is nearer to the centers C 1 , C 2  than the position P 1   x  in the scanning direction, and the position P 2   y  is nearer to the centers C 1 , C 2  than the position P 1   y  in the scanning direction (D 1 &gt;D 2 ). 
     As shown in  FIG. 1 , each of the partial rollers  22   b   1  of the lower roller  22   b  is a rubber roller having no protrusions on its outer circumferential surface, and each of the partial rollers  23   b   1  of the lower roller  23   b  is a rubber roller having no protrusions on its outer circumferential surface. Each of the partial rollers  22   a   1  of the upper roller  22   a  is a spur roller having at least one protrusion  22   ap  on its outer circumferential surface, and each of the partial rollers  23   a   1  of the upper roller  23   a  is a spur roller having at least one protrusion  23   ap  on its outer circumferential surface. 
     Configuration for Forming the Sheet into Corrugated Shape 
     For forming the sheet  100  into a corrugated or wavy shape along the scanning direction, nine corrugating plates  21   c  are provided over the upper roller  21   a  of the roller pair  21 , eight ribs  40   c  are provided on a front surface of the platen  40  (that is opposed to the ejector  30 ), and seven corrugating spurs  23   c  are provided immediately downstream of the roller pair  23  on the path R 1 , as shown in  FIGS. 1 and 3 . By forming the sheet  100  into the corrugated or wavy shape along the scanning direction, resilience is given to the sheet  100 , so that the sheet  100  can be appropriately conveyed. 
     As shown in  FIG. 3 , the nine corrugating plates  21   c  are arranged in the scanning direction so as to be equally spaced apart from one another. As shown in  FIG. 1 , each corrugating plate  21   c  extends from above the upper roller  21   a  toward the downstream side on the path R 1  and is opposed at its distal end portion to the front surface of the platen  40  with a slight clearance interposed therebetween. 
     As shown in  FIG. 3 , the eight ribs  40   c  are arranged so as to be equally spaced apart from one another in the scanning direction. Each of the ribs  40   c  is disposed between corresponding two of the nine corrugating plates  21   c  that are adjacent to each other in the scanning direction. Each rib  40   c  extends in the conveyance direction. Positions of the eight ribs  40   c  in the scanning direction and positions of the eight pairs of the partial rollers  22   a   1 ,  22   b   1  in the scanning direction are the same. 
     A distal end portion of each rib  40   c  is located at a height level higher than the distal end portion of each corrugating plate  21   c . With this positional relationship, the distal end portions of the eight ribs  40   c  support the sheet  100  from below while the distal end portions of the nine corrugating plates  21   c  press the sheet  100  from above, whereby the sheet  100  is formed into the corrugated or wavy shape along the scanning direction. 
     The seven corrugating spurs  23   c  are arranged so as to be equally spaced apart from one another in the scanning direction. Positions of the seven corrugating spurs  23   c  in the scanning direction are the same as positions, in the scanning direction, of seven of the nine corrugating plates  21   c  except two outermost corrugating plates  21   c . Each of the six pairs of the partial rollers  23   a   1 ,  23   b   1  is disposed between corresponding two of the seven corrugating spurs  23   c  that are adjacent to each other in the scanning direction. 
     As shown in  FIG. 4 , a contact point of the partial rollers  23   a   1 ,  23   b   1  of each pair, namely, a point of nipping the sheet  100 , is located at a height level higher than a lower end of each corrugating spur  23   c . With this positional relationship, the six partial rollers  23   b   1  support the sheet  100  from below while the seven corrugating spurs  23   c  press the sheet  100  from above, whereby the sheet  100  is formed into the corrugated or wavy shape along the scanning direction. As described above, each of the seven corrugating spurs  23   c  is disposed between corresponding two of the six pairs of the partial rollers  23   a   1 ,  23   b   1 . Thus, the roller pair  23  has a function of forming the sheet  100  into the corrugated or wavy shape along the scanning direction. 
     A force by which the corrugating spurs  23   c  press the sheet  100  is smaller than a force by which the corrugating plates  21   c  press the sheet  100 . The conveyance force could be generated at portions corresponding to the corrugating plates  21   c  and the corrugating spurs  23   c . Each of the corrugating plates  21   c  and the corrugating spurs  23   c  are, however, not configured to cooperate with another member disposed thereunder to nip the sheet  100  therebetween. Therefore, the conveyance force indicated above may be ignored. 
     Structure for Supporting Each Roller 
     The seven corrugating spurs  23   c  are fixed to one long shaft  23   cx  extending in the scanning direction. The six partial rollers  23   b   1  of the lower roller  23   b  are fixed to one long shaft  23   bx  extending in the scanning direction. The shafts  23   bx ,  23   cx  are rotatably supported by a housing (not shown) of the printer  1 . 
     The six partial rollers  23   a   1  of the upper roller  23   a  are respectively fixed to six short shafts  23   ax  extending in the scanning direction. Each shaft  23   ax  is rotatably supported by a holder  23   ah . A protruding portion  23   ay  is provided on an upper surface of each holder  23   ah  so as to protrude upward from the upper surface. Each protruding portion  23   ay  passes through a corresponding through-hole formed in a plate  1   p . The protruding portion  23   ay  includes a lower part that is located below the plate  1   p  and an upper part that is located above the plate  1   p . The plate  1   p  is fixed to the housing of the printer  1 . A spring  23   as  (as one example of “biasing member”) is wound around the lower part of each protruding portion  23   ay , and each holder  23   ah  is biased downward by a biasing force of the corresponding spring  23   as . That is, the spring  23   as  applies, to the corresponding partial roller  23   al  of the upper roller  23   a , the biasing force in a direction toward the corresponding partial roller  23   b   1  of the lower roller  23   b.    
     The biasing forces of the six springs  23   as  are set such that the spring  23   as  located nearer to the center C 2  in the scanning direction has a larger biasing force. That is, two of the six springs  23   as  located nearest to the center C 2  in the scanning direction (each as one example of “centrally located one of the plurality of biasing members”) have the largest biasing force (biasing force=“large”), two of the six springs  23   as  located farthest from the center C 2  in the scanning direction (i.e., two outermost springs  23   as  in the scanning direction) have the smallest biasing force (biasing force=“small”), and two of the six springs  23   as  each of which is the second outermost in the scanning direction, namely, each of which is interposed between the spring  23   as  whose biasing force is “large” and the spring  23   as  whose biasing force is “small”, have the biasing force “medium”. Further, two of the six partial rollers  23   a   1  which are located nearest to the center C 2  in the scanning direction (each as one example of “centrally located one of the plurality of second partial rollers”) have a friction coefficient on outer circumferential surfaces thereof larger than that of other partial rollers  23   a I. The friction coefficient of the two partial rollers  23   a   1  is made different from that of other partial rollers  23   a   1  by changing a material for forming the outer circumferential surfaces of the two partial rollers  23   a   1  or by changing processing applied to the outer circumferential surfaces of the two partial rollers  23   a   1 . Owing to the difference in the biasing force among the springs  23   as  and the difference in the friction coefficient of the outer circumferential surface among the partial rollers  23   a   1 , the conveyance force of the roller pair  23  is not constant in the axial direction. Specifically, the conveyance force of the roller pair  23  is largest at a position located nearest to the center C 2  and is smallest at positions located farthest from the center C 2 . 
     While not shown, the eight partial rollers  22   b   1  of the lower roller  22   b  of the roller pair  22  are fixed to one long shaft which extends in the scanning direction and which is rotatably supported by the housing of the printer  1 , like the six partial rollers  23   b   1  of the lower roller  23   b  of the roller pair  23 . Like the six partial rollers  23   a   1  of the upper roller  23   a  of the roller pair  23 , the eight partial rollers  22   a   1  of the upper roller  22   a  of the roller pair  22  are respectively fixed to eight short shafts which extend in the scanning direction and which are supported, through respective holders, by a plate that is fixed to the housing of the printer  1 . Each partial roller  22   a   1  is biased downward by a biasing force of a spring, i.e., in a direction toward the corresponding partial roller  22   b   1 . The biasing forces of the springs respectively provided for the eight partial rollers  22   a   1  are substantially the same. 
     The upper roller  21   a  and the lower roller  21   b  of the roller pair  21  are respectively fixed to long shafts which extend in the scanning direction and which are rotatably supported by the housing of the printer  1 . 
     Relationship Between Conveyance Force and Moment 
     The printer  1  having the roller pairs  21 - 23  constructed and disposed as described above is designed so as to satisfy the following expressions (1)-(4). (Refer to  FIGS. 4 and 5 .)
 
| M 1|&gt;| M 2|  (1)
 
| M 1|/ L 1&lt;∫ F 2( I )· dl   (2)
 
| M 0|&gt;| M 1|  (3)
 
| M 0|/ L 2&lt;∫ F 1( I )· dl   (4)
 
In the above expressions, M 1  represents moment about an axis passing the center C 1  and extending in parallel with the vertical direction, which moment is generated by a variation in a conveyance force F 1  of the roller pair  22  in the axial direction of the roller pair  22  (i.e., a first direction parallel to the scanning direction). M 2  represents moment about an axis passing the center C 2  and extending in parallel with the vertical direction, which moment is generated by a variation in a conveyance force F 2  of the roller pair  23  in the axial direction of the roller pair  23  (i.e., the first direction parallel to the scanning direction). L 1  represents a distance between the roller pair  22  and the roller pair  23  along the path R 1 , F 2 ( l ) represents the conveyance force at each of a plurality of positions l in the axial direction of the roller pair  23 . M 0  represents moment about an axis passing the center C 0  and extending in the vertical direction, which moment is generated by a variation in a conveyance force F 0  of the roller pair  21  in the axial direction of the roller pair  21  (i.e., the first direction parallel to the scanning direction). L 2  is a distance between the roller pair  21  and the roller pair  22  along the path R 1 . F 1 ( l ) is the conveyance force at each of a plurality of positions l of the roller pair  22  in the axial direction. In  FIG. 5 , F 0 ( l ) represents the conveyance force at each of a plurality of positions  1  of the roller pair  21  in the axial direction.
 
     In the present embodiment, each of the roller pairs  22 ,  23  is constituted by the plurality of partial rollers spaced apart from one another in the axial direction. Thus, the above expressions (2) and (4) can be respectively replaced with the following expressions (2′) and (4′):
 
| M 1|/ L 1&lt;Σ F 2′  (2′)
 
| M 0|/ L 2&lt;Σ F 1′  (4′)
 
In the above expressions, F 2 ′ represents a conveyance force of each partial roller of the roller pair  23 , and F 1 ′ represents a conveyance force of each partial roller of the roller pair  22 .
 
     As described above, in the present embodiment, the rotation speed of the roller pair  23  is higher than the rotation speed of the roller pair  22 , and the conveyance force of the roller pair  22  is larger than the conveyance force of the roller pair  23 . Under the conditions, the conveyance force F 2  of the roller pair  23  is not made constant in the axial direction but is made largest at the position located nearest to the center C 2 . This configuration enables stable conveyance of the sheet  100  and prevents skewing of the sheet  100  when the sheet  100  is conveyed by the roller pairs  22 ,  23  whose upper rollers are constituted by the spur rollers. Specifically, the conveyance force F 2  of the roller pair  23  is made largest at the position located nearest to the center C 2  in the axial direction, instead of making the conveyance force F 2  constant in the axial direction. With this configuration, the conveyance force F 1  of the roller pair  22  can be made sufficiently larger than the conveyance force F 2  of the roller pair  23  without increasing the conveyance force F 1  of the roller pair  22  too much or without decreasing the conveyance force F 2  of the roller pair  23  too much. Thus, it is possible to avoid a damage of the sheet  100  due to pressing by the protrusions  22   ap  which would be caused if the conveyance force F 1  of the roller pair  22  were excessively increased or it is possible to avoid a failure in conveyance due to slippage of the sheet  100  which would be caused when the sheet  100  is conveyed only by the roller pair  23  if the conveyance force F 2  of the roller pair  23  were excessively decreased. The present embodiment therefore enables the sheet  100  to be conveyed with high stability and prevents the sheet  100  form skewing. 
     The distance L between the roller pair  21  and the roller pair  23  along the conveyance path R is equal to or smaller than a length, along the conveyance path R, of the sheet  100  having a certain size among the plurality of sizes of the sheets  100  that can be conveyed by the conveyor  20  ( FIG. 3 ), e.g., the most frequently used size such as an A4 size or a letter size. In conveying the sheet  100  having such a size, the sheet  100  is nipped by the roller pair  23  before the trailing end of the sheet  100  passes through the roller pair  21 , thereby avoiding the sheet  100  from being conveyed only by the roller pair  22 . Thus, it is possible to obviate the problem caused when the sheet  100  is conveyed only by the roller pair  22 , i.e., the problem of unstable posture of the sheet  100 . 
     The upper roller  22   a  of the roller pair  22  includes the eight partial rollers  22   al  ( FIG. 3 ) spaced apart from one another in the axial direction. In this case, although the ink landed on the front surface of the sheet  100  is prevented from being transferred to the upper roller  22   a  with higher reliability, fluctuations are likely to be generated in the friction coefficients of the outer circumferential surfaces of the eight partial rollers  22   al  and in the pressing forces with respect to the lower roller  22   b  due to manufacturing error of the partial rollers  22   a   1  and provision of the springs for the respective partial rollers  22   a   1 . Accordingly, the conveyance force F 1  of the roller pair  22  tends to vary in the axial direction, and skewing of the sheet  100  may occur due to unstable conveyance when the sheet  100  is conveyed by the roller pairs  22 ,  23 . In the present printer  1  constructed as described above, however, the sheet  100  can be conveyed with high stability so as to prevent skewing of the sheet  100 . 
     The upper roller  23   a  of the roller pair  23  includes the six partial rollers  23   al  ( FIG. 3 ) spaced apart from one another in the axial direction. This configuration makes it possible to easily change the conveyance force F 2  of the roller pair  23  in the axial direction by adjusting the structure of each of the partial rollers  23   al  and the biasing force of each of the springs  23   as  provided for the respective partial rollers  23   al . This configuration thus enables, with ease, the conveyance force F 2  of the roller pair  23  to be the largest at the position located nearest to the center C 2 . 
     The friction coefficient of the outer circumferential surfaces of the two of the six partial rollers  23   a   1  disposed nearest to the center C 2  in the axial direction is larger than the friction coefficients of the outer circumferential surfaces of the other partial rollers  23   al . This configuration enables, with ease, the conveyance force F 2  of the roller pair  23  to be the largest at the position located nearest to the center C 2  of the axial direction, by adjusting the friction coefficients of the outer circumferential surfaces of the partial rollers  23   a   1 . 
     As shown in  FIG. 4 , the biasing force of the two of the six springs  23   as  provided for the respective two partial rollers  23   a   1  disposed nearest to the center C 2  in the axial direction is larger than that of the other springs  23   as . This configuration enables, with ease, the conveyance force F 2  of the roller pair  23  to be the largest at the position located nearest to the center C 2 , by adjusting the biasing forces of the springs  23   as.    
     The printer l is designed to satisfy the expressions (1) and (2) described above. This configuration more reliably prevents skewing of the sheet  100  when the sheet  100  is conveyed by the roller pairs  22 ,  23  whose upper rollers are the spur rollers. 
     The printer  1  is designed to satisfy the expressions (3) and (4) described above. This configuration prevents skewing of the sheet  100  also when the sheet  100  is conveyed by the roller pairs  21 ,  22 . 
     The rotation speed of the roller pair  22  is higher than the rotation speed of the roller pair  21 , and the conveyance force of the roller pair  21  is larger than the conveyance force of the roller pair  22 . This configuration makes it possible to convey the sheet  100  with high stability when the sheet  100  is conveyed by the roller pairs  21 ,  22 . 
     The roller pair  23  is disposed most downstream on the conveyance path R as shown in  FIG. 1 . With this configuration, even if the sheet  100  skews when being conveyed only by the roller pair  23 , it is possible to prevent or reduce a trouble that arises from skewing, such as jamming of the sheet  100 . 
     The roller pair  23  has a function of forming the sheet  100  into the corrugated or wavy shape along the scanning direction. From the viewpoint of forming the sheet  100  into the corrugated or wavy shape over the entire width of the sheet  100 , it is preferable to dispose the roller pair  23  over the entire width of the sheet  100 . In such a configuration, the conveyance force of the roller pair  23  is made largest at the position located nearest to the center C 2  in the axial direction, instead of making the conveyance force constant in the axial direction, whereby the sheet  100  is stably conveyed and is accordingly prevented from skewing when the sheet  100  is conveyed by the roller pairs  22 ,  23  whose upper rollers are the spur rollers. 
     The second outermost positions (the positions P 2   x , P 2   y ) which are the farthest from the center C 2  of the roller pair  23  in the axial direction are nearer to the centers C 1 , C 2  ( FIG. 3 ) than the first outermost positions (the positions P 1   x , P 1   y ) which are the farthest from the center C 1  of the roller pair  22  in the axial direction (D 2 &lt;D 1 ). This configuration is more likely to satisfy conditions for stabilizing conveyance of the sheet  100  and thereby preventing skewing of the sheet  100  when the sheet  100  is conveyed by the roller pairs  22 ,  23  whose upper rollers are the spur rollers. 
     The conveyance force F 2  of the roller pair  23  is smallest at positions of the roller pair  23  in the axial direction which are located farthest from the center C 2 , as shown in  FIGS. 4 and 5 . In this configuration, the conveyance force F 2  of the roller pair  23  is made largest at the position located nearest to the center C 2  and is made smallest at the positions located farthest from the center C 2 , so that it is possible to stabilize conveyance of the sheet  100  and to thereby prevent skewing of the sheet  100  with higher reliability when the sheet  100  is conveyed by the roller pairs  22 ,  23  whose upper rollers are the spur rollers. 
     While the embodiment of the disclosure has been described above, it is to be understood that the disclosure is not limited to the details of the illustrated embodiment, but may be embodied with other various changes and modifications, which may occur to those skilled in the art, without departing from the scope of the disclosure defined in the attached claims. 
     Modifications 
     The upper roller and the lower roller of the first downstream roller pair do not necessarily have to be constituted by the plurality of partial rollers, but may be constituted by one long roller extending in the axial direction. Similarly, the upper roller and the lower roller of the second downstream roller pair do not necessarily have to be constituted by the plurality of partial rollers, but may be constituted by one long roller extending in the axial direction. Also in such cases, the friction coefficients and the biasing forces of the springs fluctuate in the axial direction, and the conveyance force may vary in the axial direction. 
     The conveyance force of the second downstream roller pair in the axial direction does not necessarily have to be adjusted by adjusting both of the biasing forces of the biasing members provided for the respective partial rollers and the friction coefficients of the outer circumferential surfaces of the partial rollers, but may be adjusted by adjusting only one of the biasing forces and the friction coefficients. 
     The second downstream roller pair does not necessarily have to be disposed most downstream on the conveyance path. The second downstream roller pair does not necessarily have to have the function of forming the recording medium into the corrugated or wavy shape along the axial direction. (The corrugating spurs  23   c  in the illustrated embodiment may be omitted.) A position of each first outermost position in the axial direction and a position of each second outermost position in the axial direction may be the same (D 1 =D 2 ). 
     The ejector is not limited to the serial type but may be a line type. The liquid ejected by the ejector is not limited to the ink but may be any liquid (such as a treatment liquid for causing coagulation or precipitation of a component in the ink). The recording medium is not limited to the sheet but may be any recordable medium such as a cloth. The present disclosure is applicable not only to the printer but also to a facsimile, a copying machine, a multi-function peripheral (MFP) and the like.