Patent Publication Number: US-9427985-B2

Title: Printing apparatus, control method for printing apparatus, and storage medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus for printing a sheet by a printhead and, more particularly, to a printing technique of conveying sheets to a printing area facing a printhead while part of a succeeding sheet overlaps part of a preceding sheet. 
     2. Description of the Related Art 
     In recent years, a printing apparatus is expected to increase the speed of printing to improve the productivity. As one of methods of increasing the speed, an interval between printing sheets to be successively fed is shortened. As a technique of shortening the interval between printing sheets, in addition to a method of simply shortening the interval between printing sheets, Japanese Patent Laid-Open No. 2001-324844 proposes a method of conveying sheets by making the marginal area of the leading edge of the succeeding sheet overlap the marginal area of the trailing edge of the preceding sheet, and forming images while the sheets overlap each other. This is a technique of forming images by excluding unnecessary portions (the interval between sheets and the marginal portion of each sheet) except for image forming areas. 
     However, if an inkjet printing apparatus executes high-density printing on an area where sheets overlap each other using a large amount of ink, wavy wrinkles called cockling can occur on the printing sheet due to moisture of the ink. When, therefore, cockling occurs, the printing sheet may unwantedly float, and graze against the printhead, thereby causing a stain on the printing sheet, or disabling conveyance to a discharge unit such as a discharge roller to cause a paper jam. In addition, when the distance between the printhead and the first surface of the sheet becomes unstable, an ink landing position may shift to degrade the image quality. Particularly, at the time of printing the second surface in double-sided printing, ink has been applied to the entire area of the sheet in printing of the first surface, and thus the printing sheet may largely float. 
     The present invention has been made in consideration of the above-described problem, and provides a printing technique capable of increasing the speed of a printing operation by implementing successive overlapped conveyance of sheets without causing a stain on the sheet, a paper jam, or the like, or degrading the image quality at the time of double-sided printing. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a printing apparatus comprising: a feeding unit configured to feed a printing sheet stacked on a stacking unit; a conveying unit configured to convey the printing sheet fed by the feeding unit; a printing unit configured to print the printing sheet conveyed by the conveying unit; and a control unit configured to control the feeding unit and the conveying unit, wherein when continuously printing on first surfaces of a plurality of printing sheets, the feeding unit forms an overlap state in which a trailing edge of a preceding sheet and a leading edge of a succeeding sheet overlap each other, and the conveying unit conveys the printing sheets while keeping the overlap state, and when performing double-sided printing on a first surface and a second surface of the printing sheet, the feeding unit does not form the overlap state. 
     According to another aspect of the present invention, there is provided a printing apparatus comprising: a feeding unit configured to feed a printing sheet stacked on a stacking unit; a conveying unit configured to convey the printing sheet fed by the feeding unit; a printing unit configured to print the printing sheet conveyed by the conveying unit; and a control unit configured to control the feeding unit and the conveying unit, wherein when continuously printing on first surfaces of a plurality of printing sheets, the control unit controls to form an overlap state in which a trailing edge of a preceding sheet and a leading edge of a succeeding sheet overlap each other, and when performing double-sided printing on a first surface and a second surface of the printing sheet, the control unit does not control to form the overlap state. 
     According to still another aspect of the present invention, there is provided a control method for a printing apparatus including a feeding unit configured to feed a printing sheet stacked on a stacking unit, a conveying unit configured to convey the printing sheet fed by the feeding unit, and a printing unit configured to print the printing sheet conveyed by the conveying unit, the method comprising: a control step of controlling the feeding unit and the conveying unit so that the feeding unit forms an overlap state in which a trailing edge of a preceding sheet and a leading edge of a succeeding sheet overlap each other and the conveying unit conveys the printing sheets while keeping the overlap state when continuously printing on first surfaces of a plurality of printing sheets, and the feeding unit does not form the overlap state when performing double-sided printing on a first surface and a second surface of the printing sheet. 
     According to the present invention, it is possible to provide a printing technique capable of increasing the speed of a printing operation by implementing successive overlapped conveyance of printing sheets without causing a stain on the printing sheet, a paper jam, or the like, or degrading the image quality at the time of double-sided printing. 
     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 view for explaining a successive overlapped conveyance operation in a printing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a view for explaining the successive overlapped conveyance operation in the printing apparatus according to the embodiment of the present invention; 
         FIG. 3  is a view for explaining the successive overlapped conveyance operation in the printing apparatus according to the embodiment of the present invention; 
         FIGS. 4A and 4B  are views for explaining the arrangement of a pickup roller; 
         FIG. 5  is a block diagram showing the printing apparatus according to the embodiment; 
         FIGS. 6A and 6B  are flowcharts illustrating a successive overlapped conveyance sequence according to the embodiment; 
         FIG. 7  is a view for explaining an operation of making a succeeding sheet overlap a preceding sheet; 
         FIG. 8  is a view for explaining the operation of making the succeeding sheet overlap the preceding sheet; 
         FIG. 9  is a flowchart for explaining the skew correction operation of the succeeding sheet according to the embodiment; 
         FIG. 10  is a flowchart for explaining an operation of calculating the leading edge position of the succeeding sheet; 
         FIG. 11  is a view for explaining a reversing operation at the time of double-sided printing in the printing apparatus according to the embodiment; 
         FIG. 12  is a view for explaining the reversing operation at the time of double-sided printing in the printing apparatus according to the embodiment; 
         FIG. 13  is a flowchart for explaining feeding sequence selection processing according to the embodiment; 
         FIGS. 14A and 14B  are flowcharts illustrating a double-sided printing sequence according to the embodiment; and 
         FIGS. 15A and 15B  are flowcharts illustrating a normal sequence according to the embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
       FIGS. 1 to 3  are sectional views for explaining a successive overlapped conveyance operation in a printing apparatus according to the embodiment of the present invention. The schematic arrangement of the printing apparatus according to the embodiment will first be described with reference to ST 1  of  FIG. 1 . The printing apparatus according to the embodiment can print on both the first and second surfaces of a printing sheet. 
     In ST 1  of  FIG. 1 , reference numeral  1  denotes printing sheets. The plurality of printing sheets  1  are stacked on a feeding tray  11  (a stacking unit). A pickup roller  2  abuts against the top printing sheet  1  stacked on the feeding tray  11  to pick it up. A feeding roller  3  feeds the printing sheet  1  picked up by the pickup roller  2  toward the downstream side of a sheet conveyance direction. A feeding driven roller  4  is biased against the feeding roller  3  to sandwich the printing sheet  1  with the feeding roller  3 , thereby feeding the printing sheet  1 . 
     A conveyance roller  5  conveys the printing sheet  1  fed by the feeding roller  3  and feeding driven roller  4  to a position facing a printhead  7 . A pinch roller  6  is biased against the conveyance roller  5  to sandwich the printing sheet with the conveyance roller  5 , thereby conveying the printing sheet. 
     The printhead  7  prints the printing sheet  1  conveyed by the conveyance roller  5  and pinch roller  6 . In this embodiment, an inkjet printhead which prints the printing sheet  1  by discharging ink from the printhead will be exemplified. A platen  8  supports the reverse surface of the printing sheet  1  at the position facing the printhead  7 . A carriage  10  mounts the printhead  7  and moves in a direction intersecting the sheet conveyance direction. 
     A discharge roller  9  discharges the printing sheet printed by the printhead  7  to the outside of the apparatus. Spurs  12  and  13  rotate while they are in contact with the printing surface of the printing sheet printed by the printhead  7 . The spur  13  on the downstream side is biased against the discharge roller  9 , and no discharge roller  9  is arranged at a position facing the spur  12  on the upstream side. The spur  12  is used to prevent the floating of the printing sheet  1 , and is also referred to as a pressing spur. 
     A conveyance guide  15  and a flapper  20  guide the printing sheet  1  between a feeding nip portion formed by the feeding roller  3  and feeding driven roller  4  and a conveyance nip portion formed by the conveyance roller  5  and pinch roller  6 . The flapper  20  is pivotable by the reaction force of the printing sheet  1  conveyed by the feeding roller  3 . A sheet detection sensor  16  detects the leading edge and trailing edge of the printing sheet  1 . The sheet detection sensor  16  is provided downstream of the feeding roller  3  in the sheet conveyance direction. A sheet pressing lever  17  makes the leading edge of the succeeding sheet overlap the trailing edge of the preceding sheet. The sheet pressing lever  17  is biased by an elastic member (for example, a spring) around a rotating shaft  17   b  in a counterclockwise direction by setting the state shown in ST 1  of  FIG. 1  as a neutral point. A distal end  17   c  of the sheet pressing lever  17  which is in contact with the printing sheet  1  is biased by the elastic member (for example, the spring) around a point  17   a  in a clockwise direction in  FIG. 1 . A sheet detection sensor  22  (second sheet detection sensor) detects the leading edge and trailing edge of the printing sheet  1 . The sheet detection sensor  22  (second sheet detection sensor) detects the timing at which the leading edge of the printing sheet  1  enters the conveyance nip portion formed by the conveyance roller  5  and pinch roller  6 , and the timing at which the trailing edge of the printing sheet  1  currently undergoing a printing operation passes through the conveyance nip portion. A reverse conveyance guide  21  is used for a reversing unit for reversing the printing sheet  1 , and guides, to the feeding nip portion formed by the feeding roller  3  and feeding driven roller  4 , the printing sheet  1  conveyed in a backward direction by the conveyance roller  5 . 
       FIGS. 4A and 4B  are views for explaining the arrangement of the pickup roller  2 . As described above, the pickup roller  2  abuts against the top printing sheet stacked on the feeding tray  11  to pick it up. A driving shaft  19  transmits driving of a feeding motor (to be described later) to the pickup roller  2 . When picking up the printing sheet, the driving shaft  19  and the pickup roller  2  rotate in a direction indicated by an arrow A in  FIGS. 4A and 4B . A projection  19   a  is formed in the driving shaft  19 . A concave portion  2   c  in which the projection  19   a  fits is formed in the pickup roller  2 . As shown in  FIG. 4A , when the projection  19   a  abuts against a first surface  2   a  of the concave portion  2   c  of the pickup roller  2 , driving of the driving shaft  19  is transmitted to the pickup roller  2 . In this case, when the driving shaft  19  is driven, the pickup roller  2  is also rotated. On the other hand, as shown in  FIG. 4B , when the projection  19   a  abuts against a second surface  2   b  of the concave portion  2   c  of the pickup roller  2 , driving of the driving shaft  19  is not transmitted to the pickup roller  2 . In this case, even if the driving shaft  19  is driven, the pickup roller  2  is not rotated. Also, when the projection  19   a  is formed between the first surface  2   a  and the second surface  2   b  without abutting against the first surface  2   a  or the second surface  2   b , even if the driving shaft  19  is driven, the pickup roller  2  is not rotated. 
       FIG. 5  is a block diagram showing the printing apparatus according to this embodiment. An MPU  201  controls the operation of each unit, data processing, and the like. As will be described later, the MPU  201  also functions as a conveyance control unit capable of controlling conveyance of the printing sheets so that the trailing edge of a preceding sheet and the leading edge of a succeeding sheet overlap each other. A ROM  202  stores data and programs to be executed by the MPU  201 . A RAM  203  temporarily stores processing data to be executed by the MPU  201  and data received from a host computer  214 . When continuously printing on the first surfaces of the plurality of printing sheets, the MPU  201  controls to form an overlap state in which the leading edge of the succeeding sheet overlaps the trailing edge of the preceding sheet. Alternatively, when performing double-sided printing on the first and second surfaces of the printing sheet, the MPU  201  does not control to form the overlap state. 
     A printhead driver  207  controls the printhead  7 . A carriage motor driver  208  controls a carriage motor  204  for driving the carriage  10 . A conveyance motor  205  drives the conveyance roller  5  and discharge roller  9 . A conveyance motor driver  209  controls the conveyance motor  205 . A feeding motor  206  drives the pickup roller  2  and feeding roller  3 . A feeding motor driver  210  controls the feeding motor  206 . 
     In the host computer  214 , a printer driver  2141  is used to communicate with the printing apparatus by collecting printing information such as a printing image and printing image quality when the user instructs the execution of a printing operation. The MPU  201  exchanges the printing image and the like with the host computer  214  via an I/F unit  213 . 
     The successive overlapped conveyance operation in continuous printing of only the first surfaces of the printing sheets  1  will be described in time series with reference to ST 1  of  FIG. 1  to ST 9  of  FIG. 3 . When the host computer  214  transmits printing data via the I/F unit  213 , the printing data is processed by the MPU  201 , and then loaded into the RAM  203 . The MPU  201  starts a printing operation based on the loaded data. 
     A description will be provided with reference to ST 1  of  FIG. 1 . The feeding motor driver  210  drives the feeding motor  206  at low speed. This rotates the pickup roller  2  at 7.6 inches/sec. When the pickup roller  2  rotates, the top printing sheet (a preceding sheet  1 -A) stacked on the feeding tray  11  is picked up. The preceding sheet  1 -A picked up by the pickup roller  2  is conveyed by the feeding roller  3  rotating in the same direction as that of the pickup roller  2 . The feeding motor  206  also drives the feeding roller  3 . This embodiment will be described by using an arrangement including the pickup roller  2  and the feeding roller  3 . However, an arrangement including only a feeding roller for feeding the printing sheet stacked on the stacking unit may be adopted. 
     When the sheet detection sensor  16  provided on the downstream side of the feeding roller  3  detects the leading edge of the preceding sheet  1 -A, the feeding motor  206  is switched to high-speed driving. That is, the pickup roller  2  and feeding roller  3  rotate at 20 inches/sec. 
     A description will be provided with reference to ST 2  of  FIG. 1 . When the feeding roller  3  is continuously rotated, the leading edge of the preceding sheet  1 -A pushes the flapper  20  away against its own weight, and also rotates the sheet pressing lever  17  about the rotating shaft  17   b  in the clockwise direction against the biasing force of the spring. When the feeding roller  3  is further continuously rotated, the leading edge of the preceding sheet  1 -A abuts against the conveyance nip portion formed by the conveyance roller  5  and pinch roller  6 . At this time, the conveyance roller  5  stops. By rotating the feeding roller  3  by a predetermined amount even after the leading edge of the preceding sheet  1 -A abuts against the conveyance nip portion, alignment of the preceding sheet  1 -A is performed to correct the skew while the leading edge of the preceding sheet  1 -A abuts against the conveyance nip portion. The skew correction operation will also be referred to as a registration adjustment operation. 
     A description will be provided with reference to ST 3  of  FIG. 1 . Upon end of the skew correction operation of the preceding sheet  1 -A, the conveyance motor  205  is driven to start rotation of the conveyance roller  5 . The conveyance roller  5  conveys the sheet at 15 inches/sec. After the preceding sheet  1 -A is aligned with the position facing the printhead  7 , a printing operation is performed by discharging ink from the printhead  7  based on the printing data. Note that the alignment operation is performed by making the leading edge of the printing sheet abut against the conveyance nip portion to temporarily position the printing sheet at the position of the conveyance roller  5 , and controlling the rotation amount of the conveyance roller  5  with reference to the position of the conveyance roller  5 . 
     The printing apparatus of this embodiment is a serial type printing apparatus in which the carriage  10  mounts the printhead  7 . A conveyance operation of intermittently conveying the printing sheet by a predetermined amount using the conveyance roller  5  and an image forming operation of discharging ink from the printhead  7  while moving the carriage  10  incorporating the printhead  7  when the conveyance roller  5  stops are repeated. By repeating the conveyance operation and the image forming operation, the printing operation of the printing sheet is performed. 
     When alignment of the preceding sheet  1 -A is performed, the feeding motor  206  is switched to low-speed driving. That is, the pickup roller  2  and feeding roller  3  rotate at 7.6 inches/sec. While the conveyance roller  5  intermittently conveys the printing sheet by the predetermined amount, the feeding motor  206  also intermittently drives the feeding roller  3 . That is, while the conveyance roller  5  rotates, the feeding roller  3  also rotates. While the conveyance roller  5  stops, the feeding roller  3  also stops. The rotation speed of the feeding roller  3  is lower than that of the conveyance roller  5 . Consequently, the sheet is stretched between the conveyance roller  5  and the feeding roller  3 . The feeding roller  3  is rotated together with the printing sheet conveyed by the conveyance roller  5 . 
     Since the feeding motor  206  is intermittently driven, the driving shaft  19  is also driven. As described above, the rotation speed of the pickup roller  2  is lower than that of the conveyance roller  5 . Consequently, the pickup roller  2  is rotated together with the printing sheet conveyed by the conveyance roller  5 . That is, the pickup roller  2  rotates ahead of the driving shaft  19 . More specifically, the projection  19   a  of the driving shaft  19  is spaced apart from the first surface  2   a  and abuts against the second surface  2   b . Therefore, the second printing sheet (a succeeding sheet  1 -B) is not picked up soon after the trailing edge of the preceding sheet  1 -A passes through the pickup roller  2 . After the driving shaft  19  is driven for a predetermined time, the projection  19   a  abuts against the first surface  2   a  and the pickup roller  2  starts to rotate. 
     A description will be provided with reference to ST 4  of  FIG. 2 . In ST 4 , a state in which the pickup roller  2  starts to rotate, and picks up the succeeding sheet  1 -B is shown. Due to a factor such as the responsiveness of the sensor, the sheet detection sensor  16  requires a predetermined interval or more between the printing sheets to detect the edges of the printing sheets. That is, it is necessary to separate the leading edge of the succeeding sheet  1 -B from the trailing edge of the preceding sheet  1 -A by a predetermined distance to provide a predetermined time interval from when the sheet detection sensor  16  detects the trailing edge of the preceding sheet  1 -A until it detects the leading edge of the succeeding sheet  1 -B. To achieve this, the angle of the concave portion  2   c  of the pickup roller  2  is set to about 70°. 
     A description will be provided with reference to ST 5  of  FIG. 2 . The succeeding sheet  1 -B picked up by the pickup roller  2  is conveyed by the feeding roller  3 . At this time, the preceding sheet  1 -A undergoes an image forming operation by the printhead  7  based on the printing data. When the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, the feeding motor  206  is switched to high-speed driving. That is, the pickup roller  2  and feeding roller  3  rotate at 20 inches/sec. 
     A description will be provided with reference to ST 6  of  FIG. 2 . The distal end  17   c  of the sheet pressing layer  17  presses the trailing edge of the preceding sheet  1 -A downward, as shown in ST 5  of  FIG. 2 . It is possible to form a state in which the leading edge of the succeeding sheet  1 -B overlaps the trailing edge of the preceding sheet  1 -A by moving the succeeding sheet  1 -B at a speed higher than that at which the preceding sheet  1 -A moves downstream by the printing operation of the printhead  7  (ST 6  of  FIG. 2 ). Since the preceding sheet  1 -A undergoes the printing operation based on the printing data, it is intermittently conveyed by the conveyance roller  5 . On the other hand, after the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, the succeeding sheet  1 -B can catch up with the preceding sheet  1 -A by continuously rotating the feeding roller  3  at 20 inches/sec. 
     A description will be provided with reference to ST 7  of  FIG. 3 . After forming an overlap state in which the leading edge of the succeeding sheet  1 -B overlaps the trailing edge of the preceding sheet  1 -A, the succeeding sheet  1 -B is conveyed by the feeding roller  3  until the leading edge of the succeeding sheet  1 -B stops at a predetermined position upstream of the conveyance nip portion. The position of the leading edge of the succeeding sheet  1 -B is calculated from the rotation amount of the feeding roller  3  after the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, and controlled based on the calculation result. At this time, the preceding sheet  1 -A undergoes an image forming operation by the printhead  7  based on the printing data. 
     A description will be provided with reference to ST 8  of  FIG. 3 . When the conveyance roller  5  stops to perform the image forming operation (ink discharge operation) of the last row of the preceding sheet  1 -A, the feeding roller  3  is driven to make the leading edge of the succeeding sheet  1 -B abut against the conveyance nip portion, thereby performing the skew correction operation of the succeeding sheet  1 -B. 
     A description will be provided with reference to ST 9  of  FIG. 3 . When the image forming operation of the last row of the preceding sheet  1 -A ends, it is possible to perform alignment of the succeeding sheet  1 -B while keeping the state in which the succeeding sheet  1 -B overlaps the preceding sheet  1 -A by rotating the conveyance roller  5  by a predetermined amount. The succeeding sheet  1 -B undergoes a printing operation by the printhead  7  based on the printing data. When the succeeding sheet  1 -B is intermittently conveyed for the printing operation, the preceding sheet  1 -A is also intermittently conveyed, and is finally discharged outside the printing apparatus by the discharge roller  9 . 
     When alignment of the succeeding sheet  1 -B is performed, the feeding motor  206  is switched to low-speed driving. That is, the pickup roller  2  and feeding roller  3  rotate at 7.6 inches/sec. If there is printing data even after the succeeding sheet  1 -B, the process returns to ST 4  of  FIG. 2  to pick up the third printing sheet. 
     A reversing operation at the time of double-sided printing will be described in time series with reference to ST 11  of  FIG. 11  to ST 16  of  FIG. 12 . Note that an operation of printing the first surface at the time of double-sided printing is the same as that in ST 1  to ST 3  of  FIG. 1  described above and a description thereof will be omitted. 
     When executing double-sided printing, the MPU  201  controls to reverse the printing surface of the printing sheet (sheet  1 -A) from the first surface to the second surface after completion of the printing operation of the first surface of the sheet  1 -A. The MPU  201  conveys the printing sheet from the side of the conveyance roller  5  to the side of the feeding roller  3  in a direction opposite to the conveyance direction at the time of the printing operation via the sheet pressing lever  17  and the reverse conveyance guide  21 . The reverse conveyance guide  21  reverses the sheet  1 -A from the first surface to the second surface, and sets the second surface of the sheet  1 -A as a printing surface. The MPU  201  conveys the reversed sheet  1 -A from the side of the feeding roller  3  to the side of the conveyance roller  5 . 
     Practical processing at the time of the reversing operation will be described below with reference to  FIGS. 11 and 12 . In ST 11  of  FIG. 11 , when the printing operation of the sheet  1 -A ends, the conveyance roller  5  and discharge roller  9  stop rotating. The discharge roller  9  rotates until the trailing edge of the sheet  1 -A is set at a position a distance LA away from the conveyance nip portion formed by the conveyance roller  5  and pinch roller  6 , and the discharge roller  9  and spur  13  hold the sheet  1 -A which has passed through the conveyance nip portion. At this time, the flapper  20  is at a lower position by its own weight, as shown in ST 11  of  FIG. 11 , and guides the sheet  1 -A to the reverse conveyance guide  21 . 
     A description will be provided with reference to ST 12  of  FIG. 11 . The conveyance roller  5  and discharge roller  9  reversely rotate in a direction (the clockwise direction in  FIG. 11 ) opposite to that at the time of the printing operation to cause the sheet  1 -A to re-enter the conveyance nip portion of the conveyance roller  5  and pinch roller  6 , thereby conveying the sheet  1 -A toward the conveyance guide  15  and sheet pressing lever  17 . At this time, the conveyance roller  5  rotates at 8 inches/sec. When the conveyance roller  5  and discharge roller  9  start to reversely rotate, the feeding roller  3  also forwardly rotates (in the counterclockwise direction in ST 12  of  FIG. 11 ). The feeding roller  3  rotates at 8 inches/sec. 
     A description will be provided with reference to ST 13  of  FIG. 11 . When the conveyance roller  5  continuously rotates in the clockwise direction in  FIG. 11 , one edge (the trailing edge at the time of printing the first surface) of the sheet  1 -A rotates the distal end  17   c  of the sheet pressing lever  17  in the counterclockwise direction against the biasing force of the spring. The sheet pressing lever  17  is configured to be rotatable while it is in contact with the sheet  1 -A when the sheet  1 -A is conveyed in the conveyance direction at the time of the reversing operation. Note that the sheet pressing lever  17  can be arranged at a position which does not contact the sheet  1 -A when the printing sheet is conveyed in the conveyance direction at the time of the reversing operation. The sheet pressing lever  17  may be configured so that the one edge of the sheet  1 -A passes through the sheet pressing lever  17  without contacting the lower position of the distal end  17   c  of the sheet pressing lever  17 . When the conveyance roller  5  further continuously rotates in the clockwise direction in  FIG. 11 , one edge of the sheet  1 -A is guided to the reverse conveyance guide  21 . 
     A description will be provided with reference to ST 14  of  FIG. 12 . When the conveyance roller  5  further continuously rotates in the clockwise direction in  FIG. 12 , one edge (the trailing edge at the time of printing the first surface) of the sheet  1 -A is guided to the reverse conveyance guide  21 , and enters the feeding nip portion formed by the feeding roller  3  and feeding driven roller  4 . When the sheet detection sensor  16  detects one edge of the sheet  1 -A, the conveyance roller  5  rotates by a predetermined amount, and then the conveyance roller  5  and feeding roller  3  temporarily stop rotating. In this arrangement, when the sheet  1 -A reaches a position at which the sheet detection sensor  16  detects one edge of sheet  1 -A, the other edge of the sheet  1 -A passes through the distal end  17   c  of the sheet pressing lever  17 . 
     A description will be provided with reference to ST 15  of  FIG. 12 . By continuously rotating (forwardly rotating) the feeding roller  3 , one edge of the sheet  1 -A pushes the flapper  20  away against its own weight and the reaction force of the sheet  1 -A, and meets the conveyance guide  15  again. At this time, since the other edge of the sheet  1 -A has passed through the sheet pressing lever  17 , the reaction force of the sheet  1 -A when one edge of the sheet  1 -A pushes the flapper  20  away can be minimized. When the feeding roller  3  is further continuously rotated, one edge of the sheet  1 -A abuts against the conveyance nip portion formed by the conveyance roller  5  and pinch roller  6  to perform skew correction, as in ST 2 . 
     A description will be provided with reference to ST 16  of  FIG. 12 . Upon end of the skew correction operation of the sheet  1 -A, the conveyance motor  205  is driven to start rotation of the conveyance roller  5 . The conveyance roller  5  conveys the sheet at 15 inches/sec. The sheet  1 -A is aligned with the position facing the printhead  7 . At this time, the surface of the sheet  1 -A facing the printhead  7  is the second surface. The printing operation of the second surface of the aligned sheet  1 -A is performed by discharging ink from the printhead  7  based on the printing data. 
       FIG. 13  is a flowchart for explaining feeding sequence selection processing according to the embodiment, which is executed under the control of the MPU  201 . In step S 301 , when the host computer  214  transmits the sheet information and printing data of the printing sheet  1  via the I/F unit  213 , a printing operation starts. The MPU  201  selects a feeding sequence of the printing sheet  1  based on the sheet information of the printing sheet  1  acquired from the host computer  214 . In step S 302 , the MPU  201  confirms the sheet type of the printing sheet  1 . If it is determined in step S 302  that the sheet type is not plain paper (NO in step S 302 ), the process advances to step S 310 , and a normal sequence is selected as a feeding sequence. 
     If it is determined in step S 302  that the sheet type is plain paper (YES in step S 302 ), the process advances to step S 303 , and the MPU  201  determines based on the printing data whether to perform double-sided printing. If double-sided printing is to be performed (YES in step S 303 ), the process advances to step S 306 , and the MPU  201  confirms a sheet size. In this embodiment, the A4 size (a sheet length of 297 mm) and LTR size (a sheet length of 279.4 mm) are to undergo double-sided printing. Note that the sheet type to undergo double-sided printing is not limited to them, and various settings can be made. 
     If it is determined in step S 306  that the sheet size is the A4 size or LTR size (YES in step S 306 ), the process advances to step S 308 , and a double-sided printing sequence is selected as a feeding sequence. On the other hand, it is determined in step S 306  that the sheet size is neither the A4 size nor the LTR size (NO in step S 306 ), the process advances to step S 307 , and the normal sequence is selected as a feeding sequence. 
     If it is determined in step S 303  not to perform double-sided printing (NO in step S 303 ), the process advances to step S 305 . In step S 305 , the MPU  201  confirms the sheet size. In this embodiment, the A4 size and LTR size are to undergo successive overlapped conveyance. If the sheet size is the A4 size or LETTER size (YES in step S 305 ), the process advances to step S 309 , and a successive overlapped conveyance sequence is selected as a feeding sequence. In the successive overlapped conveyance sequence of continuously printing on the first surfaces of a plurality of printing sheets, the MPU  201  controls to form the overlap state in which the leading edge of the succeeding sheet overlaps the trailing edge of the preceding sheet. 
     On the other hand, it is determined in step S 305  that the sheet size is neither the A4 size nor the LTR size (NO in step S 305 ), the process advances to step S 310 , and the normal sequence is selected as a feeding sequence. In the double-sided printing sequence or normal sequence of performing double-sided printing on the first and second surfaces of the printing sheet, the MPU  201  does not control to form the overlap state. The feeding sequence selection processing then ends. 
       FIGS. 14A and 14B  are flowcharts for explaining the double-sided printing sequence executed under the control of the MPU  201 . In step S 501 , a printing operation by the double-sided printing sequence starts. To print on the first surface of the sheet, the feeding operation of the sheet  1 -A starts in step S 502 . The feeding motor  206  is driven to rotate the pickup roller  2  at 7.6 inches/sec. The pickup roller  2  picks up the sheet  1 -A, and the feeding roller  3  feeds the sheet  1 -A toward the printhead  7 . 
     In step S 503 , the sheet detection sensor  22  (second sheet detection sensor) detects the leading edge of the sheet  1 -A. After the sheet detection sensor  22  detects the leading edge of the sheet  1 -A (YES in step S 503 ), controlling the rotation amount of the feeding roller  3  makes the leading edge of the sheet  1 -A abut against the conveyance nip portion to perform the skew correction operation of the sheet  1 -A in step S 504 . 
     In step S 505 , alignment of the sheet  1 -A is performed based on the printing data. That is, by controlling the rotation amount of the conveyance roller  5 , the sheet  1 -A is conveyed to a printing start position with reference to the position of the conveyance roller  5  based on the printing data. 
     In step S 506 , a printing operation starts when the printhead  7  discharges ink to the first surface of the sheet  1 -A. More specifically, the printing operation of the first surface of the sheet  1 -A is performed by repeating a conveyance operation of intermittently conveying the sheet  1 -A by the conveyance roller  5  and an image forming operation (ink discharge operation) of discharging ink from the printhead  7  by moving the carriage  10 . 
     Upon completion of the printing operation of the first surface of the sheet  1 -A in step S 507  (YES in step S 507 ), the MPU  201  advances the process to step S 508 . In step S 508 , the discharge roller  9  is rotated until the trailing edge of the sheet  1 -A is set at a predetermined position (a position the distance LA away from the conveyance nip portion in ST 11  of  FIG. 11 ). 
     In step S 509 , the MPU  201  confirms the sheet length of the sheet  1 -A, and confirms whether the sheet length corresponds to the A4 size or LTR size. With respect to a sheet set to undergo double-sided printing, L 1  represents the upper limit of the sheet length, and L 2  represents the lower limit of the sheet length. The sheet length of the sheet  1 -A to undergo printing can be calculated based on the driving amount of the conveyance roller  5  from when the sheet detection sensor  22  (second sheet detection sensor) detects the leading edge of the sheet  1 -A until the sheet detection sensor  22  detects the trailing edge of the sheet  1 -A. 
     If it is determined in step S 509  that the sheet length of the sheet  1 -A to undergo printing does not satisfy a relationship “L 1 &lt;sheet length&lt;L 2 ” (NO in step S 509 ), that is, if the sheet length does not correspond to the A4 size or LTR size, the process advances to step S 510 . In step S 510 , the MPU  201  discharges the sheet  1 -A, thereby terminating the printing operation (step S 511 ). 
     If it is determined in step S 509  that the sheet length of the sheet  1 -A to undergo printing satisfies the relationship “L 1 &lt;sheet length&lt;L 2 ”, that is, if the sheet length corresponds to the A4 size or LTR size (YES in step S 509 ), the process advances to step S 513 . In step S 513 , the MPU  201  stops rotation by the conveyance roller  5  and discharge roller  9  and stands by until ink discharged to the first surface of the sheet  1 -A is dried. A standby time (t 1 ) at this time is decided based on the type of ink, the overlapped ejection amount of ink, the ejection amount of ink per unit area, the environmental temperature, and the like. 
     In step S 514 , the MPU  201  reversely rotates the conveyance roller  5  and discharge roller  9  in a direction (the clockwise direction in ST 12  of  FIG. 11 ) opposite to that at the time of the printing operation, thereby causing one edge (the trailing edge at the time of printing the first surface) of the sheet  1 -A to re-enter the conveyance nip portion. Furthermore, simultaneously with the start of the reverse rotation of the conveyance roller  5  and discharge roller  9 , the MPU  201  also forwardly rotates the feeding roller  3  (the counterclockwise direction in ST 12  of  FIG. 11 ). At this time, the conveyance roller  5  rotates at 8 inches/sec, and the feeding roller  3  also rotates at 8 inches/sec. When each roller is continuously rotated, one edge of the sheet  1 -A enters the feeding nip portion of the feeding roller  3  and feeding driven roller  4 . 
     When the sheet detection sensor  16  detects one edge of the sheet  1 -A in step S 515 , the conveyance roller  5 , discharge roller  9 , and feeding roller  3  stop rotating in step S 516 . In step S 517 , a drying standby time (t 2 ) is provided. This operation need not be performed if the MPU  201  has performed the drying standby operation for the standby time (t 1 ) in step S 513 . In this case, the process may transit to the next step by setting t 2 =0. When, for example, a sufficient marginal portion where no ink is discharged exists in the trailing edge of the sheet  1 -A, the drying standby operation for the standby time (t 2 ) is required. In this case, a drying time is allocated before the ink discharged to the first surface of the sheet  1 -A reaches the conveyance nip portion, thereby preventing the ink from being transferred to the pinch roller  6 . However, when the sheet  1 -A is conveyed from the feeding nip portion by pushing the flapper  20  away, a paper jam may occur at high probability depending on the water content of the sheet  1 -A. To cope with this, the drying standby operation for the appropriate standby time (t 2 ) is performed so that drying increases the rigidity of the sheet  1 -A and the sheet  1 -A can pass through the flapper  20  and conveyance guide  15 . 
     In step S 518 , the feeding roller  3  is forwardly rotated (in the counterclockwise direction in  FIG. 12 ) at 8 inches/sec, thereby feeding the sheet  1 -A toward the printhead  7 . In step S 519 , the sheet detection sensor  22  (second sheet detection sensor) detects one edge of the sheet  1 -A. After the sheet detection sensor  22  (second sheet detection sensor) detects one edge of the sheet  1 -A (YES in step S 519 ), the MPU  201  advances the process to step S 520 . By controlling the rotation amount of the feeding roller  3 , one edge of the sheet  1 -A is made to abut against the conveyance nip portion to perform the skew correction operation of the sheet  1 -A in step S 520 . 
     In step S 521 , alignment of the sheet  1 -A is performed based on the printing data. That is, by controlling the rotation amount of the conveyance roller  5 , the sheet  1 -A is conveyed to a printing start position with reference to the position of the conveyance roller  5  based on the printing data. 
     In step S 522 , a printing operation starts when the printhead  7  discharges ink to the second surface of the sheet  1 -A. More specifically, the printing operation of the second surface of the sheet  1 -A is performed by repeating a conveyance operation of intermittently conveying the sheet  1 -A by the conveyance roller  5  and an image forming operation (ink discharge operation) of discharging ink from the printhead  7  by moving the carriage  10 . Upon completion of the printing operation of the second surface of the sheet  1 -A in step S 523  (YES in step S 523 ), the sheet  1 -A is discharged in step S 524 , and the double-sided printing operation ends (step S 525 ). 
       FIGS. 15A and 15B  are flowcharts for explaining the normal sequence executed under the control of the MPU  201 . In step S 601 , a printing operation by the normal sequence starts. In step S 602 , the feeding motor  206  is driven at low speed to rotate the pickup roller  2  at 7.6 inches/sec. The pickup roller  2  picks up the preceding sheet  1 -A, and the feeding roller  3  feeds the preceding sheet  1 -A toward the printhead  7 . In step S 603 , the sheet detection sensor  16  detects the leading edge of the preceding sheet  1 -A. After the sheet detection sensor  16  detects the leading edge of the preceding sheet  1 -A (YES in step S 603 ), controlling the rotation amount of the feeding roller  3  makes the leading edge of the preceding sheet  1 -A abut against the conveyance nip portion to perform the skew correction operation of the preceding sheet  1 -A in step S 604 . 
     In step S 605 , alignment of the preceding sheet  1 -A is performed based on the printing data. That is, the preceding sheet  1 -A is conveyed to a printing start position with reference to the position of the conveyance roller  5  based on the printing data by controlling the rotation amount of the conveyance roller  5 . 
     In step S 606 , a printing operation starts when the printhead  7  discharges ink to the preceding sheet  1 -A. More specifically, the printing operation of the preceding sheet  1 -A is performed by repeating a conveyance operation of intermittently conveying the preceding sheet  1 -A by the conveyance roller  5  and an image forming operation (ink discharge operation) of discharging ink from the printhead  7  by moving the carriage  10 . The feeding motor  206  is intermittently driven at low speed in synchronization with the operation of intermittently conveying the preceding sheet  1 -A by the conveyance roller  5 . That is, the pickup roller  2  and feeding roller  3  intermittently rotate at 7.6 inches/sec. 
     In step S 607 , it is determined whether there is printing data of the next page. If there is no printing data of the next page (NO in step S 607 ), the process advances to step S 613 . Upon completion of the printing operation of the preceding sheet  1 -A in step S 613  (YES in step S 613 ), the preceding sheet  1 -A is discharged in step S 614 , thereby terminating the printing operation (step S 621 ). 
     If the determination processing in step S 607  determines that there is printing data of the next page (YES in step S 607 ), the feeding operation of the succeeding sheet  1 -B starts in step S 608 . More specifically, the feeding motor  206  is driven at low speed to cause the pickup roller  2  to pick up the succeeding sheet  1 -B and cause the feeding roller  3  to feed the succeeding sheet  1 -B toward the printhead  7 . The pickup roller  2  rotates at 7.6 inches/sec. As described above, since the large concave portion  2   c  of the pickup roller  2  is provided with respect to the projection  19   a  of the driving shaft  19 , the succeeding sheet  1 -B is fed while having a predetermined interval with respect to the trailing edge of the preceding sheet  1 -A. 
     In step S 609 , the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B. After the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B (YES in step S 609 ), the MPU  201  advances the process to step S 610 . In step S 610 , by controlling the rotation amount of the feeding roller  3 , the succeeding sheet  1 -B is conveyed so that its leading edge is at a position a predetermined amount after the conveyance nip portion, thereby stopping feeding the succeeding sheet  1 -B. The preceding sheet  1 -A is intermittently conveyed based on the printing data, and undergoes a printing operation. 
     Upon completion of the printing operation of the preceding sheet  1 -A in step S 611  (YES in step S 611 ), the preceding sheet  1 -A is discharged in step S 612 . Furthermore, by controlling the rotation amount of the feeding roller  3 , the leading edge of the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform the skew correction operation of the succeeding sheet  1 -B in step S 615 . 
     In step S 616 , alignment of the succeeding sheet  1 -B is performed based on the printing data. That is, by controlling the rotation amount of the conveyance roller  5 , the succeeding sheet  1 -B is conveyed to a printing start position with reference to the position of the conveyance roller  5  based on the printing data. In step S 617 , a printing operation starts when the printhead  7  discharges ink to the succeeding sheet  1 -B. The printing operation of the succeeding sheet  1 -B is the same as that of the preceding sheet  1 -A, as described in step S 606 . 
     In step S 618 , it is determined whether there is printing data of the next page. If there is printing data of the next page (YES in step S 618 ), the process returns to the processing in step S 608  to repeat the same processing. If the determination processing in step S 618  determines that there is no printing data of the next page (NO in step S 618 ), the process advances to step S 619 . Upon completion of the printing operation of the succeeding sheet  1 -B in step S 619  (YES in step S 619 ), the succeeding sheet  1 -B is discharged in step S 620 , thereby terminating the printing operation (step S 621 ). 
       FIGS. 6A and 6B  are flowcharts illustrating the successive overlapped conveyance sequence in continuous printing of only the first surfaces. In step S 1 , when the host computer  214  transmits printing data via the I/F unit  213 , a printing operation starts. In step S 2 , the feeding operation of the preceding sheet  1 -A starts. More specifically, the feeding motor  206  is driven at low speed. The pickup roller  2  rotates at 7.6 inches/sec. The pickup roller  2  picks up the preceding sheet  1 -A, and the feeding roller  3  feeds the preceding sheet  1 -A toward the printhead  7 . 
     In step S 3 , the sheet detection sensor  16  detects the leading edge of the preceding sheet  1 -A. When the sheet detection sensor  16  detects the leading edge of the preceding sheet  1 -A (YES in step S 3 ), the feeding motor  206  is switched to high-speed driving in step S 4 . That is, the pickup roller  2  and feeding roller  3  rotate at 20 inches/sec. In step S 5 , by controlling the rotation amount of the feeding roller  3  after the sheet detection sensor  16  detects the leading edge of the preceding sheet  1 -A, the leading edge of the preceding sheet  1 -A is made to abut against the conveyance nip portion to perform the skew correction operation of the preceding sheet  1 -A. 
     In step S 6 , alignment of the preceding sheet  1 -A is performed based on the printing data. That is, the preceding sheet  1 -A is conveyed to a printing start position with reference to the position of the conveyance roller  5  based on the printing data by controlling the rotation amount of the conveyance roller  5 . In step S 7 , the feeding motor  206  is switched to low-speed driving. In step S 8 , a printing operation starts when the printhead  7  discharges ink to the preceding sheet  1 -A. More specifically, the printing operation of the preceding sheet  1 -A is performed by repeating a conveyance operation of intermittently conveying the preceding sheet  1 -A by the conveyance roller  5  and an image forming operation (ink discharge operation) of discharging ink from the printhead  7  by moving the carriage  10 . The feeding motor  206  is intermittently driven at low speed in synchronization with the operation of intermittently conveying the preceding sheet  1 -A by the conveyance roller  5 . That is, the pickup roller  2  and feeding roller  3  intermittently rotate at 7.6 inches/sec. 
     In step S 9 , it is determined whether there is printing data of the next page. If there is no printing data of the next page, the process advances to step S 25 . Upon completion of the printing operation of the preceding sheet  1 -A in step S 25 , the preceding sheet  1 -A is discharged in step S 26 , thereby terminating the printing operation. 
     If there is printing data of the next page, the feeding operation of the succeeding sheet  1 -B starts in step S 10 . More specifically, the pickup roller  2  picks up the succeeding sheet  1 -B, and the feeding roller  3  feeds the succeeding sheet  1 -B toward the printhead  7 . The pickup roller  2  rotates at 7.6 inches/sec. As described above, since the large concave portion  2   c  of the pickup roller  2  is provided with respect to the projection  19   a  of the driving shaft  19 , the succeeding sheet  1 -B is fed while having a predetermined interval with respect to the trailing edge of the preceding sheet  1 -A. 
     In step S 11 , the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B. When the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, the feeding motor  206  is switched to high-speed driving in step S 12 . That is, the pickup roller  2  and feeding roller  3  rotate at 20 inches/sec. In step S 13 , by controlling the rotation amount of the feeding roller  3  after the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, the succeeding sheet  1 -B is conveyed so that its leading edge is at a position a predetermined amount before the conveyance nip portion. The preceding sheet  1 -A is intermittently conveyed based on the printing data. Continuously driving the feeding motor  206  at high speed forms the overlap state in which the leading edge of the succeeding sheet  1 -B overlaps the trailing edge of the preceding sheet  1 -A. 
     In step S 14 , it is determined whether predetermined conditions (to be described later) are satisfied. If the predetermined conditions are satisfied, it is determined in step S 15  whether the image forming operation of the preceding sheet  1 -A has started. If it is determined that the image forming operation has started, the process advances to step S 16 ; otherwise, the process stands by until the image forming operation starts. In step S 16 , the leading edge of the succeeding sheet  1 -B is made to abut against the conveyance nip portion while keeping the overlap state, thereby performing the skew correction operation of the succeeding sheet  1 -B. If it is determined in step S 17  that the image forming operation of the last row of the preceding sheet  1 -A has ended, in step S 18  alignment of the succeeding sheet  1 -B is performed while keeping the overlap state. 
     If it is determined in step S 14  that the predetermined conditions are not satisfied, the overlap state is canceled to perform alignment of the succeeding sheet  1 -B. More specifically, if it is determined in step S 27  that the image forming operation of the last row of the preceding sheet  1 -A has ended, the discharge operation of the preceding sheet  1 -A is performed in step S 28 . During this operation, the feeding motor  206  is not driven, and thus the succeeding sheet  1 -B stops while its leading edge is at the position the predetermined amount before the conveyance nip portion. Since the preceding sheet  1 -A is discharged, the overlap state is canceled. In step S 29 , the leading edge of the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform the skew correction operation of the succeeding sheet  1 -B. In step S 18 , alignment of the succeeding sheet  1 -B is performed. 
     In step S 19 , the feeding motor  206  is switched to low-speed driving. In step S 20 , a printing operation starts by discharging ink from the printhead  7  to the succeeding sheet  1 -B. More specifically, the printing operation of the succeeding sheet  1 -B is performed by repeating a conveyance operation of intermittently conveying the succeeding sheet  1 -B by the conveyance roller  5  and an image forming operation (ink discharge operation) of discharging ink from the printhead  7  by moving the carriage  10 . The feeding motor  206  is intermittently driven at low speed in synchronization with the operation of intermittently conveying the succeeding sheet  1 -B by the conveyance roller  5 . That is, the pickup roller  2  and feeding roller  3  intermittently rotate at 7.6 inches/sec. 
     In step S 21 , it is determined whether there is printing data of the next page. If there is printing data of the next page, the process returns to step S 10 . If there is no printing data of the next page, when the image forming operation of the succeeding sheet  1 -B is complete in step S 22 , the discharge operation of the succeeding sheet  1 -B is performed in step S 23  and the printing operation ends in step S 24 . 
       FIGS. 7 and 8  are views for explaining an operation of making the succeeding sheet overlap the preceding sheet according to this embodiment. The operation of forming the overlap state in which the leading edge of the succeeding sheet overlaps the trailing edge of the preceding sheet, which has been explained in steps S 12  and S 13  of  FIG. 6A , will be described. 
       FIGS. 7 and 8  are enlarged views each showing a portion between the feeding nip portion formed by the feeding roller  3  and feeding driven roller  4  and the conveyance nip portion formed by the conveyance roller  5  and pinch roller  6 . 
     Three states in a process of conveying the printing sheets by the conveyance roller  5  and feeding roller  3  will be sequentially described. The first state in which an operation of making the succeeding sheet chase the preceding sheet is performed will be described with reference to ST 1  and ST 2  of  FIG. 7 . The second state in which an operation of making the succeeding sheet overlap the preceding sheet is performed will be described with reference to ST 3  and ST 4  of  FIG. 8 . The third state in which it is determined whether to perform the skew correction operation of the succeeding sheet while keeping the overlap state will be described with reference to ST 5  of  FIG. 8 . 
     In ST 1  of  FIG. 7 , the feeding roller  3  is controlled to convey the succeeding sheet  1 -B, and the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B. A section from the sheet detection sensor  16  to a position P 1  at which the succeeding sheet  1 -B can be made to overlap the preceding sheet  1 -A is defined as a first section A 1 . In the first section A 1 , an operation of making the leading edge of the succeeding sheet  1 -B chase the trailing edge of the preceding sheet  1 -A is performed. The position P 1  is decided based on the arrangement of the mechanism. 
     In the first state, the chasing operation may stop in the first section A 1 . If, as shown in ST 2  of  FIG. 7 , the leading edge of the succeeding sheet  1 -B passes the trailing edge of the preceding sheet  1 -A before the position P 1 , the operation of making the succeeding sheet overlap the preceding sheet is not performed. 
     In ST 3  of  FIG. 8 , a section from the above-described position P 1  to a position P 2  at which the sheet pressing lever  17  is provided is defined as a second section A 2 . In the second section A 2 , the operation of making the succeeding sheet  1 -B overlap the preceding sheet  1 -A is performed. 
     In the second state, the operation of making the succeeding sheet overlap the preceding sheet may stop in the second section A 2 . If, as shown in ST 4  of  FIG. 8 , the leading edge of the succeeding sheet  1 -B cannot catch up with the trailing edge of the preceding sheet  1 -A within the second section A 2 , it is impossible to perform the operation of making the succeeding sheet overlap the preceding sheet. 
     In ST 5  of  FIG. 8 , a section from the above-described position P 2  to a position P 3  is defined as a third section A 3 . The position P 3  is the position of the leading edge of the succeeding sheet when the succeeding sheet stops in step S 13  of  FIG. 6A . While the succeeding sheet  1 -B overlaps the preceding sheet  1 -A, the succeeding sheet  1 -B is conveyed so that its leading edge reaches the position P 3 . In the third section A 3 , it is determined whether to perform alignment of the succeeding sheet  1 -B by making it abut against the conveyance nip portion while keeping the overlap state. That is, it is determined whether to perform alignment of the succeeding sheet by executing a skew correction operation while keeping the overlap state or to perform alignment of the succeeding sheet by canceling the overlap state and performing a skew correction operation. 
       FIG. 9  is a flowchart for explaining the skew correction operation of the succeeding sheet according to this embodiment. The processing of determining whether the predetermined conditions are satisfied, which has been explained in step S 14  of  FIG. 6A , will be described in detail. 
     The operation of determining whether to perform a skew correction operation by making the leading edge of the succeeding sheet  1 -B abut against the conveyance nip portion while keeping the overlap state between the preceding sheet  1 -A and the succeeding sheet  1 -B or to perform a skew correction operation by canceling the overlap state between the preceding sheet  1 -A and the succeeding sheet  1 -B and then making the leading edge of the succeeding sheet  1 -B abut against the conveyance nip portion will be described. 
     In step S 101 , the operation starts. In step S 102 , it is determined whether the leading edge of the succeeding sheet  1 -B has reached the determination position (the position P 3  in ST 5  of  FIG. 8 ). If the leading edge of the succeeding sheet  1 -B has not reached the determination position (NO in step S 102 ), it is uncertain whether the leading edge of the succeeding sheet  1 -B abuts against the conveyance nip portion by conveying the succeeding sheet  1 -B by a predetermined amount, and thus a skew correction operation for only the succeeding sheet is decided (step S 103 ), thereby terminating the determination operation (step S 104 ). That is, after the trailing edge of the preceding sheet  1 -A passes through the conveyance nip portion, only the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform a skew correction operation, and then alignment of only the succeeding sheet  1 -B is performed. 
     On the other hand, if it is determined that the leading edge of the succeeding sheet  1 -B has reached the determination position P 3  (YES in step S 102 ), it is determined whether the trailing edge of the preceding sheet  1 -A has passed through the conveyance nip portion (step S 105 ). If it is determined that the trailing edge of the preceding sheet  1 -A has passed through the conveyance nip portion (YES in step pS 105 ), the succeeding sheet does not overlap the preceding sheet, and thus a skew correction operation for only the succeeding sheet is decided (step S 106 ). That is, only the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform a skew correction operation, and then alignment of only the succeeding sheet  1 -B is performed. 
     On the other hand, if it is determined that the trailing edge of the preceding sheet  1 -A has not passed through the conveyance nip portion (NO in step S 105 ), it is determined whether the overlap amount of the trailing edge of the preceding sheet  1 -A and the leading edge of the succeeding sheet  1 -B is smaller than a threshold (step S 107 ). The position of the trailing edge of the preceding sheet  1 -A is updated along with the printing operation of the preceding sheet  1 -A. The position of the leading edge of the succeeding sheet  1 -B is at the above-described determination position. That is, the overlap amount decreases along with the printing operation of the preceding sheet  1 -A. If it is determined that the overlap amount is smaller than the threshold (YES in step S 107 ), the overlap state is canceled, and a skew correction operation for only the succeeding sheet is decided (step S 108 ). That is, after the image forming operation of the preceding sheet  1 -A ends, the succeeding sheet  1 -B is not conveyed together with the preceding sheet  1 -A. More specifically, the conveyance motor  205  drives the conveyance roller  5  to convey the preceding sheet  1 -A. However, the feeding roller  3  is not driven. Therefore, the overlap state is canceled. Furthermore, only the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform a skew correction operation, and then alignment of only the succeeding sheet  1 -B is performed. 
     If it is determined that the overlap amount is equal to or larger than the threshold (NO in step S 107 ), it is determined whether the succeeding sheet  1 -B reaches the pressing spur  12  when alignment of the succeeding sheet  1 -B is performed (step S 109 ). If it is determined that the succeeding sheet  1 -B does not reach the pressing spur  12  (NO in step S 109 ), the overlap state is canceled and a skew correction operation for only the succeeding sheet is decided (step S 110 ). That is, after the image forming operation of the preceding sheet  1 -A ends, the succeeding sheet  1 -B is not conveyed together with the preceding sheet  1 -A. More specifically, the conveyance motor  205  drives the conveyance roller  5  to convey the preceding sheet  1 -A. However, the feeding roller  3  is not driven. Consequently, the overlap state is canceled. Furthermore, only the succeeding sheet  1 -B is made to abut against the conveyance nip portion to perform a skew correction operation, and then alignment of only the succeeding sheet  1 -B is performed. 
     If it is determined that the succeeding sheet  1 -B reaches the pressing spur  12  (YES in step S 109 ), it is determined whether there is a gap between the last row of the preceding sheet and the row immediately preceding the last row (step S 111 ). If it is determined that there is no gap (NO in step S 111 ), the overlap state is canceled and a skew correction operation for only the succeeding sheet is decided (step S 112 ). If it is determined that there is a gap (YES in step S 111 ), the skew correction operation of the succeeding sheet  1 -B is performed while keeping the overlap state, and alignment of the succeeding sheet  1 -B is performed (Step S 113 ). That is, during a period from when the image forming operation of the preceding sheet  1 -A starts until the image forming operation ends, the succeeding sheet  1 -B is made to abut against the conveyance nip portion while overlapping the preceding sheet  1 -A. More specifically, the conveyance roller  5  and the feeding roller  3  are rotated by driving the feeding motor  206  together with the conveyance motor  205 . After the skew correction operation, alignment of the succeeding sheet  1 -B is performed while the succeeding sheet  1 -B overlaps the preceding sheet  1 -A. 
     As described above, the operation of determining whether to keep or cancel the overlap state between the preceding sheet  1 -A and the succeeding sheet  1 -B is performed. 
       FIG. 10  is a flowchart for explaining an arrangement of calculating the leading edge position of the succeeding sheet after alignment of the succeeding sheet according to this embodiment. 
     In step S 201 , the process starts. In step S 202 , a printable area with a sheet size is loaded. Since the uppermost printable position, that is, the upper end margin is specified, the upper end margin of the printable area is set as a leading edge position (step S 203 ). Note that the leading edge position is defined by the distance from the conveyance nip portion. 
     The first printing data is loaded (step S 204 ). With this processing, the position of the first printing data from the leading edge of the sheet is specified (detection of a non-printing area), and thus it is determined whether the distance between the leading edge of the sheet and the first printing data is larger than the previously set leading edge position (step S 205 ). If the distance between the leading edge of the sheet and the first printing data is larger than the previously set leading edge position (YES in step S 205 ), the leading edge position is updated by the distance between the leading edge of the sheet and the first printing data (step S 206 ). If the distance between the leading edge of the sheet and the first printing data is equal to or smaller than the previously set leading edge position (NO in step S 205 ), the process advances to step S 207 . 
     Next, the first carriage movement instruction is generated (step S 207 ). It is determined whether a sheet conveyance amount for the first carriage movement is larger than the previously set leading edge position (step S 208 ). If the sheet conveyance amount for the first carriage movement is larger than the previously set leading edge position (YES in step S 208 ), the leading edge position is updated by the sheet conveyance amount for the first carriage movement (step S 209 ). If the sheet conveyance amount for the first carriage movement is equal to or smaller than the previously set leading edge position (NO in step S 208 ), the leading edge position is not updated. In this manner, the leading edge position of the succeeding sheet  1 -B is confirmed (step S 210 ), and the process ends (step S 211 ). Based on the confirmed leading edge position, it is possible to determine (step S 109  of  FIG. 9 ) whether the succeeding sheet  1 -B reaches the pressing spur  12  when performing alignment of the succeeding sheet B. 
     According to the above embodiment, when continuously printing on the first surfaces of a plurality of printing sheets, an overlap state in which the leading edge of the succeeding sheet overlaps the trailing edge of the preceding sheet is formed at the time of continuous printing of the first surfaces, and the printing sheets are conveyed while keeping the overlap state. This can increase the speed of a printing operation. Alternatively, when performing double-sided printing on the first and second surfaces of a printing sheet, a control operation of forming the overlap state is not performed. This can suppress a stain on the sheet, a paper jam, and degradation in image quality. 
     When performing the printing operation of the preceding sheet  1 -A by the printhead  7 , the feeding motor  206  is driven in synchronization with the conveyance motor  205  before the sheet detection sensor  16  detects the leading edge of the succeeding sheet  1 -B, and the feeding motor  206  is continuously driven after the sheet detection sensor  16  detects the leading edge of the succeeding sheet, thereby making it possible to perform a chasing operation to make the succeeding sheet overlap the preceding sheet. 
     OTHER EMBODIMENTS 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     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. 2014-116204, filed Jun. 4, 2014, which is hereby incorporated by reference herein in its entirety.