Abstract:
A post-handling apparatus having an aligning tray configured to receive a sheet from an image forming apparatus; an ejected sheet tray located adjacent to the aligning tray; a conveyer configured to convey the sheet on the aligning tray to the ejected sheet tray; and a sheet presser configured to press a sheet previously ejected onto the ejected sheet tray against the ejected sheet tray. The sheet presser presses the sheet previously ejected to the ejected sheet tray when a leading edge of a sheet currently conveyed to the ejected sheet tray falls down on the ejected sheet tray, and is in a receding position receding from the ejected sheet tray when a trailing edge of the sheet currently conveyed to the ejected sheet tray falls down on the ejected sheet tray.

Description:
This application claims benefit of priority to Japanese Patent Application No. 2014-190926 filed Sep. 19, 2014, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a post-handling apparatus and an image forming system, and more particularly to a post-handling apparatus configured to handle sheets ejected from an image forming apparatus and an image forming system comprising the post-handling apparatus. 
     As an example of post-handling apparatuses configured to handle sheets ejected from an image forming apparatus, a post-handling apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-145528 is well known. Such a conventional post-handling apparatus handles sheets ejected from an image forming apparatus and ejects the sheets to an ejected sheet tray of the post-handling apparatus. The ejected sheet tray conventionally has a mechanism to move up and down the ejected sheet tray depending on the load on the ejected sheet tray. 
     Meanwhile, it is suggested that the mechanism to move up and down the ejected sheet tray depending on the load on the ejected sheet tray be omitted from the ejected sheet tray in order to downsize the post-handling apparatus. However, omitting the mechanism from the ejected sheet tray may cause a problem that when a sheet is newly ejected onto the ejected sheet tray, sheets previously ejected onto the ejected sheet tray fall therefrom due to impact with the sheet newly coming to the ejected sheet tray. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a post-handling apparatus that prevents sheets previously ejected onto an ejected sheet tray from falling therefrom due to impact with a sheet newly coming to the ejected sheet tray, and an image forming system comprising the post-handling apparatus. 
     According to a first aspect of the present invention relates to a post-handling apparatus for handling sheets ejected from an image forming apparatus, and the post-handling apparatus comprises: an aligning tray configured to receive a sheet from the image forming apparatus; an ejected sheet tray located adjacent to the aligning tray; a conveyer configured to convey the sheet on the aligning tray to the ejected sheet tray in a sheet ejecting direction; and a sheet presser configured to press a sheet previously ejected onto the ejected sheet tray against the ejected sheet tray, wherein the sheet presser presses the sheet previously ejected onto the ejected sheet tray when a leading edge, with respect to the sheet ejecting direction, of a sheet currently conveyed to the ejected sheet tray falls down on the ejected sheet tray, and is in a receding position receding from the ejected sheet tray when a trailing edge, with respect to the sheet ejecting direction, of the sheet currently conveyed to the ejected sheet tray falls down on the ejected sheet tray. 
     According to a second aspect of the present invention relates to an image forming system, and the image forming system comprises: the post-handling apparatus described above; and an image forming apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an image forming system according to an embodiment of the present invention. 
         FIG. 2  is a schematic view of the internal structure of a post-handling apparatus and an image forming apparatus of the image forming system. 
         FIG. 3  is a perspective view of the post-handling apparatus. 
         FIG. 4  is a perspective view of a portion around an ejected sheet tray. 
         FIG. 5  is a perspective view of the portion around the ejected sheet tray. 
         FIG. 6  is a perspective view of a portion around a monitoring system. 
         FIG. 7  is a sectional view illustrating a moment when the leading portion of a sheet coming to the ejected sheet tray falls in the ejected sheet tray. 
         FIG. 8  is a perspective view of the portion around the ejected sheet tray in a state where sheet pressers are in a receding position. 
         FIG. 9  is a timing chart indicating the relation between a point of time when the trailing edge of a sheet being ejected from an aligning tray to the ejected sheet tray comes out of the aligning tray and a point of time when a movement of the sheet pressers to the receding position is started. 
         FIG. 10  is a sectional view illustrating a moment when the trailing edge of a sheet coming to the ejected sheet tray falls in the ejected sheet tray. 
         FIG. 11  is a sectional view illustrating a state where a sheet is stuck between the sheet pressers and the exit of the post-handling apparatus. 
         FIG. 12  is a block diagram indicating the relation among components relating to sheet ejection, the control unit, the image forming apparatus and sensors. 
         FIG. 13  is a flowchart indicating a procedure for controlling sheet ejection from the post-handling apparatus. 
         FIG. 14  is a flowchart indicating the procedure for controlling sheet ejection from the post-handling apparatus. 
         FIG. 15  is a flowchart indicating the procedure for controlling sheet ejection from the post-handling apparatus. 
         FIG. 16  is a schematic view from the vertical direction of the aligning tray, the ejected sheet tray and a rear-side aligning plate. 
         FIG. 17  is a flowchart indicating the procedure for controlling sheet ejection from the post-handling apparatus. 
         FIG. 18  is a flowchart indicating the procedure for controlling sheet ejection from the post-handling apparatus. 
         FIG. 19  is a flowchart indicating the procedure for controlling sheet ejection from the post-handling apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Structure of Image Forming System and Post-Handling Apparatus; See FIGS.  1 - 3   
     An image forming system  1  and a post-handling apparatus  3  according to an embodiment of the present invention are hereinafter described with reference to the drawings. In the following description, the direction in which an image forming apparatus  2  ejects a sheet is referred to as a positive side in x-direction or a positive x-direction, and a direction opposite thereto is referred to as a negative side in x-direction or a negative x-direction. The direction in which a rear-side aligning plate  29  pushes a sheet or a stack of sheets P to an ejected sheet tray  35  is referred to as a positive side in y-direction or a positive y-direction, and a direction opposite thereto is referred to as a negative side in y-direction or a negative y-direction. In this embodiment, the y-direction is orthogonal to the x-direction and the vertical direction. 
     As illustrated in  FIG. 1 , the image forming system  1  comprises an image forming apparatus  2 , a post-handling apparatus  3  and an image reading apparatus  100 . The image forming system  1  further comprises a control unit  4  inside the image forming apparatus  2  as illustrated in  FIG. 2 , and the control unit  4  controls the image forming apparatus  2 , the post-handling apparatus  3  and the image reading apparatus  100 . 
     The image forming apparatus  2  comprises an imaging unit  5  configured to form toner images by a conventional electrophotographic process. Sheets P are fed from a sheet feeder section one by one, and the toner images are transferred to the sheets P at a transfer section  12  one by one. The imaging unit  5  may be configured to form a full-color toner image or to form a monochromatic toner image. Further, if the imaging unit  5  is configured to form a full-color toner image, the imaging unit  5  may be a tandem type or a four-cycle type. The sheets P that received toner images at the transfer section  12  undergo a heating treatment at a fixing unit  14  for fixation of toner thereon, and the sheets P are ejected from the image forming apparatus  2  one by one through a pair of ejection rollers  16 . The ejection of each sheet from the image forming apparatus  2  is detected by a sheet ejection sensor  60  located near an exit of the image forming apparatus  2 . 
     The post-handling apparatus  3  is an apparatus configured to handle, for example, align and/or staple the sheets P ejected from the image forming apparatus  2 . As illustrated in  FIG. 3 , the post-handling apparatus  3  comprises an aligning tray  21 , a leading-edge support member  23 , a trailing-edge support member  25 , a front-side aligning plate  27 , a rear-side aligning plate  29 , a stapler  31 , and an ejected sheet tray  35 . The ejected sheet tray  35  is located adjacent to the aligning tray  21  and more specifically at the positive side in y-direction of the aligning tray  21 . 
     A basic process carried out by the post-handling apparatus  3  is described below. Each printed sheet P ejected from the exit of the image forming apparatus is received on the aligning tray  21  and is conveyed to a predetermined position on the aligning tray  21  while being aligned by use of the leading-edge support member  23 , the trailing-edge support member  25 , the front-side aligning plate  27  and the rear-side aligning plate  29 . The aligned stack of sheets P is stapled by a stapler  31  as needed. The sheet P or the stack of sheets P conveyed to the predetermined position on the aligning tray  21  is pushed out to the ejected sheet tray  35  by the rear-side aligning plate  29 . This is a basic process carried out by the post-handling apparatus  3 . 
     Detailed Structure Around Ejected Sheet Tray; See FIGS.  4 - 6   
     As illustrated in  FIG. 4 , the post-handling apparatus  3  further comprises a sheet ejection sensor  37  configured to detect a sheet or a stack of sheets ejected onto the ejected sheet tray  35 , and sheet pressers  39  configured to hold the ejected sheet or the ejected stack of sheets on the ejected sheet tray  35 . As illustrated in  FIG. 5 , the post-handling apparatus  3  still further comprises a motor  43  serving as a drive source of the sheet pressers  39 , a shaft  45  configured to transmit the drive force from the motor  43  to the sheet pressers  39 , and a monitoring system  47  configured to monitor the rotation of the shaft  45 . 
     The sheet ejection sensor  37  is fitted in a dent made in the upper surface of the ejected sheet tray  35 . When a sheet P or a stack of sheets P ejected from the aligning tray  21  covers the upper surface of the ejected sheet tray  35 , the sheet ejection sensor  37  detects a sheet on the ejected sheet tray  35 . Then, the sheet ejection sensor  37  sends a signal indicating the state to the control unit  4 . 
     The sheet pressers  39  are to prevent one or more sheets P previously ejected onto the ejected sheet tray  35  from falling from the ejected sheet tray  35  due to collision with one or more sheets P newly ejected thereto. The sheet pressers  39  are also to prevent the sheets P already ejected onto the ejected sheet tray  35  from curling, thereby preventing the sheets P from increasing in volume on the ejected sheet tray  35 . As seen in  FIG. 4 , each of the sheet pressers  39  is substantially J-shaped when viewed from the x-direction. Respective one ends of the sheet pressers  39  are fixed to the shaft  45  such that the sheet pressers  39  are capable of rotating along with the shaft  45 . With the rotation of the sheet pressers  39 , the respective other ends of the sheet pressers  39  hit and press the upper surface of the sheet or the stack of sheets P on the ejected sheet tray  35 . In this embodiment, a total of four sheet pressers  39  are fixed to the shaft  45 . 
     The motor  43  is a drive source of the rotation of the sheet pressers  39 . The motor  43  is activated at appropriate times under control of the control circuit  4 . As illustrated in  FIG. 5 , the motor  43  is located around a corner of the ejected sheet tray  35  at the negative end in x-direction and the negative end in y-direction. The motor  43  is enclosed in sheet metal. 
     The shaft  45  is a round bar extending in the x-direction and is located at the negative end in y-direction of the sheet ejected tray  35 . As described above, the sheet pressers  39  are fixed to the shaft  45 . A first end of the shaft  45  is connected to the motor  43  via a gear. Accordingly, the shaft  45  transmits the force supplied from the motor  43  to the sheet pressers  39 . Additionally, near a second end of the shaft  45 , a closure plate  47   a , which is a component of the monitoring system  47 , is provided. 
     The monitoring system  47 , as illustrated in  FIG. 6 , comprises the closure plate  47   a  and a shaft sensor  47   b . As mentioned above, the closure plate  47   a  is fixed to a portion of the shaft  45  near the second end. The closure plate  47   a  is a sector plate, and its main surfaces in a sector form are perpendicular to the x-direction. The shaft sensor  47   b  is an optical sensor having a light emitting element and a light receiving element. The shaft sensor  47   b  is shaped like a right-angled U having an open upper side when viewed from the y-direction. In the monitoring system  47  having this structure, the closure plate  47   a  cuts across the open side of the right-angled U shape of the shaft sensor  47   b  as the shaft  45  rotates. In this moment, the closure plate  47   a  blocks light coming from the light emitting element of the shaft sensor  47   b . In this way, the monitoring system  47  monitors the rotation of the shaft  45 . 
     Sheet Ejection from Post-Handling Apparatus 
     Sheet ejection from the post-handling apparatus  3  is described below. In the following description, a sheet which has not been ejected completely onto the ejected sheet tray  35  is referred to as a sheet P 1 , and a sheet which has been ejected completely onto the ejected sheet tray  35  is referred to as a sheet P 2 . In the following, a case where a sheet P 1  is ejected from the aligning tray  21  to the ejected sheet tray is considered. 
     A sheet P 1  ejected from the image forming apparatus  2  is received on the aligning tray  21  and then ejected therefrom to the ejected sheet tray  35  by the rear-side aligning plate  29 . In this moment, if there are any sheets P 2  on the ejected sheet tray  35 , as illustrated in  FIG. 7 , the sheet pressers  39  press the upper surface of the stack of sheets P 2 . However, while the sheet P 1  is coming from the aligning tray  21  to the ejected sheet tray  35 , the sheet pressers  39  move back in a receding position. The following description is focused on this point. 
     Initially, as illustrated in  FIG. 8 , before a sheet P 1  is ejected from the image forming apparatus  2  to the aligning tray  21 , the sheet pressers  39  are hidden in holes G formed at the negative side in y-direction of the ejected sheet tray  35 , that is, are located in the receding position. When the sheet ejection sensor  37  detects a sheet or a stack of sheets P 2  on the ejected sheet tray  35 , a signal is sent to the control unit  4 . Thereafter, at a time when the sheet P 1  is ejected from the image forming apparatus  2  to the aligning sheet  21 , the motor  43  is activated. Thereby, the sheet pressers  39  rotate along with the shaft  45  and come out of the holes G. Then, as illustrated in  FIG. 7 , the sheet pressers  39  press the upper surface of the sheet or the stack of sheets P 2  on the ejected sheet tray  35 . Accordingly, when the leading edge E 1  of the sheet P 1  falls down on the sheet or the stack of sheets P 2  on the ejected sheet tray  35 , the sheet or the stack of sheets P 2  is already pressed by the sheet pressers  39 . Therefore, the sheet P 1  passes over the sheet pressers  39  and falls down on the sheet or the stack of sheets P 2 . 
     While the sheet P 1  is passing over the sheet pressers  39 , the sheet pressers  39  start receding into the holes G. As illustrated in  FIG. 9 , a point of time T 1  when the sheet pressers  39  start the receding movement is Δt earlier than a reference time T 2 , wherein Δt is a period of time required for a movement of the sheet pressers  39  from the ejected sheet tray  35  into the holes G, and T 2  is a point of time when the trailing edge of the sheet P 1  comes out of the aligning tray  21 . Accordingly, as illustrated in  FIG. 10 , when the trailing edge E 2  of the sheet P 1  comes out of the aligning tray  21 , the receding movement of the sheet pressers  39  has been already completed. The point of time T 2  when the trailing edge E 2  of the sheet P 1  comes out of the aligning tray  21  is approximated from a point of time when the rear-side aligning plate  29  reaches the positive end in y-direction of the aligning tray  21 . The period of time Δt required for a movement of the sheet pressers  39  from the ejected sheet tray  35  into the holes G is determined from a value preliminarily stored in the control unit  4 . 
     After completion of the ejection of the sheet P 1 , the sheet pressers  39  start coming to a position again to press the upper surface of the stack of sheets P 2  on the ejected sheet tray  35 . In this regard, the control unit  4  perceives the travel distance (rotation angle) of the sheet pressers  39  from information sent from the monitoring system  47 . If the travel distance is smaller than a predetermined value, the control unit  4  determines that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity of the ejected sheet tray  35 . 
     When the control unit  4  determines that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity of the ejected sheet tray  35 , the control unit  4  stops the image forming apparatus  2  from starting a next image forming process. Also, an interface IF provided on the image forming apparatus  2  displays information that sheets have been stacked on the ejected sheet tray  35  up to the limit of loadable capacity, that is, information that the ejected sheet tray  35  is full. Further, the control unit  4  keeps the sheet pressers  39  pressing the upper surface of the stack of sheets P 2 . In other words, the control unit  4  prohibits the sheet pressers  39  from receding. 
     However, if there is a sheet P 1  remaining on the aligning tray  21 , the control unit  4  permits ejection of the sheet P 1  from the aligning sheet tray  21  in order to complete the ejection of the sheet P 1  from the post-handling apparatus  3 . In this regard, when the leading edge E 1  of the sheet P 1  comes out of the aligning tray  21  to the ejected sheet tray  35 , the sheet pressers  39  press the upper surface of the stack of sheets P 2  on the ejected sheet tray  35 . While the sheet P 1  is being ejected from the aligning tray  21  to the ejected sheet tray  35 , the sheet pressers  39  move to the receding position. Thus, if there is a sheet P 1  remaining on the aligning tray  21 , the control unit  4  once cancels the prohibition of the receding movement of the pressers  39 . Then, after completion of the ejection of the sheet P 1  from the post-handling apparatus  3 , the sheet pressers  39  come to a position again to press the upper surface of the stack of sheets P 2  on the ejected sheet tray  35 . 
     Thereafter, when the user discharges the stack of sheets P 2  from the ejected sheet tray  35 , the control unit  4  receives information from the sheet ejection sensor  37  that there are no sheets on the ejected sheet tray  35 . In response, the control unit  4  permits the image forming apparatus  2  to start a next image forming process and returns the display screen on the interface IF to a normal screen. Further, the control unit  4  returns the sheet pressers  39  to the receding position. 
     Incidentally, if a receding movement of the sheet pressers  39  from the ejected sheet tray  35  by order of the control unit  4  is not completed within a predetermined period of time, the control unit  4  takes a process depending on whether there is a sheet on the ejected sheet tray  35 . 
     If a receding movement of the sheet pressers  39  from the ejected sheet tray  35  by order of the control unit  4  is not completed within a predetermined period of time and if there is a sheet on the ejected sheet tray  35 , the control unit  4  takes a process similar to the process the control unit  4  carries out when the control unit  4  has determined that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. Specifically, the control unit  4  stops the image forming apparatus  2  from starting a next image forming process and drives the interface IF to display information that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. This permits a user to perceive the problem, for example, that a sheet P 1  is stuck between the sheet pressers  39  in the middle of a receding movement and the exit of the post-handling apparatus  3  as illustrated in  FIG. 11 , and the user can remove the sheet P 1 . 
     If a receding movement of the sheet pressers  39  from the ejected sheet tray  35  by order of the control unit  4  is not completed within a predetermined period of time and if there are no sheets on the ejected sheet tray  35 , the control unit  4  determines that any component relating to the sheet pressers  39  has a defect. Then, the control unit  4  stops the image forming apparatus  2  from staring a next image forming process and drives the interface IF to display information that any component relating the sheet pressers  39  has a defect. 
     Control of Sheet Ejection 
     In a procedure of controlling sheet ejection, first, an MFP controller  401  of the control unit  4  obtains information from the user interface IF provided on the image forming apparatus  2 , a computer terminal PC, and the sensors provided in the image forming apparatus  2  and the post-handling apparatus  3 . In accordance with the information, an engine controller  402  of the control unit  4  controls image formation in the image forming apparatus  2  and ejection of a printed sheet P from the image forming apparatus  2 . A post-handling controller  403  of the control unit  4  activates the motor  43  for the support members, the aligning plates and the pressers  39 , and other members to carry out sheet ejection. In the following, a control flow is described with reference to  FIGS. 13 through 19 . 
     Main Routine 
     First, the main routine of the control procedure illustrated in  FIGS. 13 through 15  is described. The main routine starts at a time of issuance of an image forming command from the control unit  4 , that is, at a time of determination to eject a sheet from the image forming apparatus  2 . 
     At step MS 1  of the main routine, the control unit  4  determines the position of sheet pressers  39  from the signal sent from the monitoring system  47 . If the control unit  4  determines that the sheet pressers  39  are not in the receding position (home position), the control procedure goes to step MS 2 , and if the control unit  4  determines that the sheet pressers  39  are in the receding position, the control procedure goes to step MS 7 . 
     At step MS 2  of the main routine, the control unit  4  activates the motor  43  to start moving the sheet pressers  39  to the receding position. 
     At step MS 3  of the main routine, the control unit  4  starts a timer A 1 . The timer A 1  is to count down a predetermined time T 3  from the time of starting a receding movement of the sheet pressers  39 . The predetermined time T 3 , which is stored in the control unit  4 , is a period of time required for a movement of the sheet pressers  39  from the ejected sheet tray  35  to the receding position plus a predetermined margin. 
     At step MS 4  of the main routine, the control unit  4  determines whether the predetermined time T 3  has passed. When the control unit  4  determines that the predetermined time T 3  has passed, the control procedure goes to step MS 5 . The control procedure stands by at step MS 4  until the predetermined time T 3  has passed. 
     At step MS 5  of the main routine, the control unit  4  stops the motor  43  to stop the receding movement of the sheet pressers  39 . 
     At step MS 6  of the main routine, the control unit  4  determines the position of the sheet pressers  39  from the signal sent from the monitoring system  47 . If the control unit  4  determines that the sheet pressers  39  are in the receding position (home position), the control procedures goes to step MS 7  of the main routine. If the control unit  4  determines that the sheet pressers  39  are not in the receding position, the control procedure goes to step FS 1  of a first subroutine, which will be described later. In a case where the control procedure goes from step MS 6  of the main routine to step FS 1  of the first subroutine, the control procedure thereafter returns from the first subroutine to step MS 2  of the main routine or ends. 
     At step MS 7  of the main routine, the control unit  4  determines from the signal sent from the sheet ejection sensor  37  whether there are any sheets P 2  on the ejected sheet tray  35 . If the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , the control procedure stands by at this step until the control unit  4  determines that there is a sheet P 2  on the ejected sheet tray  35 , that is, until a sheet has been ejected from the post-handling apparatus  3 . When the control unit  4  determines that there are any sheets P 2  on the ejected sheet tray  35 , the control procedure goes to step MS 8  of the main routine. 
     At step MS 8  of the main routine, the control unit  4  determines from the signal sent from the sheet ejection sensor  60  located near the exit of the image forming apparatus  2  whether ejection of a sheet P 1  from the image forming apparatus  2  onto the aligning tray  21  of the post-handling apparatus  3  has been started. When the control unit  4  determines that ejection of a sheet P 1  from the image forming apparatus  2  onto the aligning tray  21  has been started, the control procedure goes to step MS 9 . The control procedure stands by at step MS 8  until sheet ejection onto the aligning tray  21  is started. 
     At step MS 9  of the main routine, the control unit  4  activates the motor  43  to start moving the sheet pressers  39  to the ejected sheet tray  35 . 
     At step MS 10  of the main routine, the control unit  4  starts a timer A 2 . The timer A 2  is to count down a predetermined time T 4  from the time of starting a movement of the sheet pressers  39  to the ejected sheet tray  35 . The predetermined time T 4 , which is stored in the control unit  4 , is a period of time required for a movement of the sheet pressers  39  from the receding position to the ejected sheet tray  35  plus a predetermined margin. 
     At step MS 11  of the main routine, the control unit  4  determines whether the predetermined time T 4  has passed. When the control unit  4  determines that the predetermined time T 4  has passed, the control procedure goes to step MS 12 . The control procedure stands by at step MS 11  until the predetermined time T 4  has passed. 
     At step MS 12  of the main routine, the control unit  4  stops the motor  43  to stop the movement of the sheet pressers  39  to the ejected sheet tray  35 . At the same time, the rear-side aligning plate  29  starts moving toward the ejected sheet tray  35 . 
     At step MS 13  of the main routine, the control unit  4  determines whether the rear-side aligning plate  29  has passed a predetermined position on the aligning tray  21 . As illustrated in  FIG. 16 , the predetermined position is a position at a distance L 1  in the negative y-direction from the positive end in y-direction of the aligning tray  21 , that is, from the exit of the post-handling apparatus  3 . The distance L 1  is obtained by v×Δt, where v denotes the travel speed of the rear-side aligning plate  29 , and Δt denotes the period of time required for a movement of the sheet pressers  39  from the ejected sheet tray  35  to the receding position. When the control unit  4  determines that the rear-side aligning plate  29  has passed the predetermined position on the aligning tray  21 , the control procedure goes to step MS 14 . The control procedure stands by at step MS 13  until the rear-side aligning plate  29  has passed the predetermined position on the aligning tray  21 . 
     At step MS 14  of the main routine, the control unit  4  activates the motor  43  to start a movement of the sheet pressers  39  to the receding position. Therefore, when the trailing edge E 2  of the sheet P 1  being ejected from the aligning tray  21  falls down on the ejected sheet tray  35 , the sheet pressers  39  have already receded from the ejected sheet tray  35 , and there is no possibility that the sheet pressers  39  get stuck between the previously ejected sheet P 2  and the newly ejected sheet P 1 . 
     At step MS 15  of the main routine, the control unit  4  activates a timer A 3 . The timer A 3  is to count down the predetermined time T 3  from the time of starting a movement of the sheet pressers  39  to the receding position. As mentioned above, the predetermined time T 3  is stored in the control unit  4  and is a period of time required for a movement of the sheet pressers  39  from the ejected sheet tray  35  to the receding position plus a predetermined margin. 
     At step MS 16  of the main routine, the control unit  4  determines whether the predetermined time T 3  has passed. When the control unit  4  determines that the predetermined time T 3  has passed, the control procedure goes to step MS 17 . The control procedure stands by at step MS 16  until the predetermined time T 3  has passed. 
     At step MS 17  of the main routine, the control unit  4  stops the motor  43  to stop the movement of the sheet pressers  39  to the receding position. 
     At step MS 18  of the main routine, the control unit  4  determines the position of the sheet pressers  39  from the signal sent from the monitoring system  47 . If the control unit  4  determines that the sheet pressers  39  are in the receding position (home position), the control procedure goes to step MS 19  of the main routine. If the control unit  4  determines that the sheet pressers  39  are not in the receding position, the control procedure goes to step FS 1  of the first subroutine, which will be described later. In a case where the control procedure goes from step MS 18  of the main routine to step FS 1  of the first subroutine, the control procedure thereafter returns from the first subroutine to step MS 14  of the main routine or ends. 
     At step MS 19  of the main routine, the control unit  4  determines whether the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 , that is, whether the sheet P 1  has been completely ejected from the aligning tray  21 . When the control unit  4  determines that the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 , the control procedure goes to step MS 20 . The control procedure stands by at step MS 19  until the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 . It is determined from the travel distance of the rear-side aligning plate  29  whether the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 . 
     At step MS 20  of the main routine, the control unit  4  activates the motor  43  to start a movement of the sheet pressers  39  to the ejected sheet tray  39 . 
     At step MS 21  of the main routine, the control unit  4  starts a timer A 4 . The timer A 4  is to count down the predetermined time T 4  from the time of starting a movement of the sheet pressers  39  to the ejected sheet tray  35 . As mentioned above, the predetermined time T 4  is stored in the control unit  4  and is a period of time required for a movement of the sheet pressers  39  from the receding position to the ejected sheet tray  35  plus a predetermined margin. 
     At step MS 22  of the main routine, the control unit  4  determines whether the predetermined time T 4  has passed. When the control unit  4  determines that the predetermined time T 4  has passed, the control procedure goes to step MS 23 . The control procedure stands by at step MS 22  until the predetermined time T 4  has passed. 
     At step MS 23  of the main routine, the control unit  4  stops the motor  43  to stop the movement of the sheet pressers  39  to the ejected sheet tray  35 . 
     At step MS 24  of the main routine, the control unit  4  determines whether sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. If the control unit  4  determines that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity, the control procedure goes to step SS 1  of a second subroutine, which will be described later. If the control unit  4  determines that sheets have not been stacked on the ejected sheet tray  35  up to the limit of loading capacity, the control procedure goes to step MS 25 . 
     At step MS 25  of the main routine, the control unit  4  determines whether a next sheet is to be ejected from the image forming apparatus  2 . If there is a next sheet to be ejected from the image forming apparatus  2 , the control procedure returns to step MS 2 . If there is no sheet to be ejected from the image forming apparatus  2 , the control procedure ends. 
     First Subroutine 
     The first subroutine illustrated in  FIG. 17  is a routine that is selected in a case where a movement of the sheet pressers  39  to the receding position by order of the control unit  4  is not completed within a predetermined period of time. 
     At step FS 1  of the first subroutine, the control unit  4  determines from a signal sent from the sheet ejection sensor  37  whether there are any sheets P 2  on the ejected sheet tray  35 . If the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , the control procedure goes to step FS 2 . If the control unit determines that there are any sheets P 2  on the ejected sheet tray  35 , the control procedure goes to step FS 3 . 
     At step FS 2  of the first subroutine, the control unit  4  stops the image forming apparatus  2  from starting a next image forming process. Also, the control unit  4  drives the interface IF provided on the image forming apparatus  2  to display information that a component relating to the sheet pressers  39  has a defect. Thereafter, the control procedure ends. 
     At step FS 3  of the first subroutine, the control unit  4  stops the image forming apparatus  2  from starting a next image forming process. Also, the control unit  4  drives the interface IF provided on the image forming apparatus  2  to display information that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity, that is, display information that the ejected sheet tray  35  is full. Further, the control unit  4  prohibits the sheet pressers  39  from moving to the receding position. 
     At step FS 4  of the first subroutine, the control unit  4  determines from the signal sent from the sheet ejection sensor  37  whether there are any sheets P 2  on the ejected sheet tray  35 . If the control unit  4  determines that there are any sheets P 2  on the ejected sheet tray  35 , the control procedure stands by at this step until the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , that is, until a user removes the sheets P 2  from the ejected sheet tray  35 . When the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , the control procedure goes to step FS 5 . 
     At step FS 5  of the first subroutine, the control unit  4  permits the image forming apparatus  2  to start a next image forming process. Also, the control unit  4  returns the display screen of the interface IF provided on the image forming apparatus  2  to a normal screen. Further, the control unit  4  cancels the prohibition of a receding movement of the sheet pressers. Thereafter, the control procedure returns to step MS 2  or step MS 14  of the main routine or returns to step SS 5  of the second subroutine, which will be described below. 
     Second Subroutine 
     The second subroutine illustrated in  FIGS. 18 and 19  is a routine that is selected in a case where the control unit  4  determines that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. 
     At step SS 1  of the second subroutine, the control unit  4  stops the image forming apparatus  2  from starting a next image forming process. Also, the control unit  4  drives the interface IF to display information that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. Further, the control unit  4  prohibits the sheet pressers  39  from moving to the receding position. 
     At step SS 2  of the second subroutine, the control unit  4  determines from signals sent from sensors and other devices provided around the aligning tray  21  whether there is a sheet P 1  on the aligning tray  21 . If the control unit  4  determines that there is a sheet P 1  on the aligning tray  21 , the control procedure goes to step SS 3 . If the control unit  4  determines that there are no sheets P 1  on the aligning tray  21 , the control procedure goes to step SS 16 . 
     At step SS 3 , the control unit  4  cancels the prohibition of a receding movement of the sheet pressers  39 . Substantially at the same time as the cancellation of the prohibition of a receding movement of the sheet pressers  39 , the control unit  4  drives the rear-side aligning plate  29  of the aligning tray  21  to move toward the ejected sheet tray  35 . Thereby, the sheet P 1  on the aligning tray  21  is moved from the aligning tray  21  toward the ejected sheet tray P 1 . 
     At step SS 4  of the second subroutine, the control unit  4  determines whether the rear-side aligning plate  29  has passed the predetermined position on the aligning tray  21 . As mentioned above, the predetermined position is a position at a distance L 1  in the negative y-direction from the positive end in y-direction of the aligning tray  21 , that is, from the exit of the post-handling apparatus  3 . When the control unit  4  determines that the rear-side aligning plate  29  of the aligning tray  21  has passed the predetermined position, the control procedure goes to step SS 5 . The control procedure stands by at step SS 4  until the rear-side aligning plate  29  of the aligning tray  21  has passed the predetermined position. 
     At step SS 5  of the second subroutine, the control unit  4  activates the motor  43  to start a movement of the sheet pressers  39  to the receding position. Accordingly, when the trailing edge E 2  of the sheet P 1  falls down on the ejected sheet tray  35 , the sheet pressers  39  has receded from the ejected sheet tray  35  into the receding position, and there is no possibility that the sheet pressers  39  are stuck between the previously ejected sheet P 2  and the sheet P 1  newly coming to the ejected sheet tray  35 . 
     At step SS 6  of the second subroutine, the control unit  4  starts a timer A 5 . The timer A 5  is to count down the predetermined time T 3  from the start of a movement of the sheet pressers  39  toward the receding position. As mentioned above, the predetermined time T 3  is stored in the control unit  4  and is a period of time required for a movement of the sheet pressers  39  from the ejected sheet tray  35  to the receding position plus a predetermined margin. 
     At step SS 7  of the second subroutine, the control unit  4  determines whether the predetermined time T 3  has passed. When the control unit  4  determines that the predetermined time T 3  has passed, the control procedure goes to step SS 8 . The control procedure stands by at step SS 7  until the predetermined time T 3  has passed. 
     At step SS 9  of the second subroutine, the control unit  4  determines the position of the sheet pressers  39  from the signal sent from the monitoring system  47 . If the control unit  4  determines that the sheet pressers  39  are in the receding position (home position), the control procedure goes to step SS 10  of the second subroutine. If the control unit  4  determines that the sheet pressers  39  are not in the receding position, the control procedure goes to step FS 1  of the first subroutine. In a case where the control procedure goes from the step SS 9  of the second subroutine to the first subroutine, the control procedure thereafter returns from the first subroutine to the second subroutine or ends. 
     At step SS 10  of the second subroutine, the control unit  4  determines whether the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 , that is, whether the sheet P 1  has been completely ejected from the aligning tray  21 . When the control unit  4  determines that the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 , the control procedure goes to step SS 11 . The control procedure stands by at step SS 10  until the rear-side aligning plate  29  has reached the positive end in y-direction of the aligning tray  21 . 
     At step SS 11  of the second subroutine, the control unit  4  activates the motor  43  to start a movement of the sheet pressers  39  toward the ejected sheet tray  35 . 
     At step SS 12  of the second subroutine, the control unit  4  starts a timer A 6 . The timer A 6  is to count down the predetermined time T 4  from the time of starting a movement of the sheet pressers  39  toward the ejected sheet tray  35 . As mentioned above, the predetermined time T 4  is stored in the control unit  4  and is a period of time required for a movement of the sheet pressers  39  from the receding position to the ejected sheet tray  35  plus a predetermined margin. 
     At step SS 13  of the second subroutine, the control unit  4  determines whether the time T 4  has passed. When the control unit  4  determines that the time T 4  has passed, the control procedure goes to step SS 14 . The control procedure stands by at step SS 13  until the time T 4  has passed. 
     At step SS 14  of the second subroutine, the control unit  4  stops the motor  43  to stop the movement of the sheet pressers  39  toward the ejected sheet tray  35 . 
     At step SS 15  of the second subroutine, the control unit  4  prohibits the sheet pressers  39  from receding. 
     At step SS 16  of the second subroutine, the control unit  4  determines from the signal sent from the sheet ejection sensor  37  whether there are any sheets P 2  on the ejected sheet tray  35 . If the control unit  4  determines that there are any sheets P 2  on the ejected sheet tray  35 , the control procedure stands by at this step until the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , that is, until a user has removed the sheets P 2  from the ejected sheet tray  35 . When the control unit  4  determines that there are no sheets P 2  on the ejected sheet tray  35 , the control procedure goes to step SS 17 . 
     At step SS 17  of the second subroutine, the control unit  4  permits the image forming apparatus  2  to start a next image forming process. Also, the control unit  4  returns the display screen of the interface IF to a normal screen. Further, the control unit  4  cancels the prohibition of a receding movement of the sheet pressers  39 . Thereafter, the control procedure goes to step MS 25  of the main routine. 
     Advantageous Effects 
     In the image forming system  1  and in the post-handling apparatus  3 , when the leading edge E 1  of a sheet P 1  or a stack of sheets P 1  newly coming to the ejected sheet tray  35  falls down on the ejected sheet tray  35 , the sheet pressers  39  press the sheets P 2  previously ejected onto the ejected sheet tray  35 . Thereby, in the image forming system  1  and in the post-handling apparatus  3 , it is possible to prevent the sheets P 2  previously ejected onto the ejected sheet tray  35  from falling from the ejected sheet tray  35  due to collision with the sheet or the stack of sheets P 1  newly coming to the ejected sheet tray  35 . Also, it is possible to prevent the sheets P 2  previously ejected onto the ejected sheet tray  35  from becoming out of alignment due to collision with the sheet or the stack of sheets P 1  newly coming onto the ejected sheet tray  35 . 
     In the image forming system  1  and in the post-handling apparatus  3 , further, since the sheet pressers  39  press the sheets P 2  ejected onto the ejected sheet tray  35 , it is possible to prevent the sheets P 2  from curling, thereby preventing the sheets P 2  from increasing in volume on the ejected sheet tray  35 . 
     Moreover, in the image forming system  1  and in the post-handling apparatus  3 , when the trailing edge E 2  of the sheet or the stack of sheets P 1  falls down on the ejected sheet tray  35 , the sheet pressers  39  have already receded from the sheet ejected tray  35 , and therefore, there is no possibility that the sheet pressers  39  are stuck between the sheets P 2  previously ejected onto the ejected sheet tray  35  and the sheet or the stack of sheets P 1  newly coming onto the ejected sheet tray  35 . 
     In the image forming system  1  and in the post-handling apparatus  3 , the amount of sheets P 2  loaded on the ejected sheet tray  35  is derived from the travel distance of the sheet pressers  39  from the receding position to the upper surface of the sheets P 2  on the ejected sheet tray  35 . Therefore, it is not necessary to provide the image forming system  1  or the post-handling apparatus  3  with an additional system configured to figure out the amount of sheets P 2  loaded on the ejected sheet tray  35 . Thus, in the image forming system  1  and the post-handling apparatus  3 , figuring out the amount of sheets P 2  loaded on the ejected sheet tray  35  based on the movement of the sheet pressers  39  results in space saving and a reduction in cost. 
     As illustrated in  FIG. 11 , if the sheet or the stack of sheets P 1  is stuck between the exit of the post-handling apparatus  3  and the sheet pressers  39 , the movement of the sheet pressers  39  cannot be completed. In order to address this trouble, if a movement of the sheet pressers  39  to the receding position by order of the control unit  4  is not completed within the predetermined time and if there are any sheets P 2  on the ejected sheet tray  35 , the control unit  4  drives the interface IF provided on the image forming apparatus  2  to display information that sheets have been stacked on the ejected sheet tray  35  up to the limit of loading capacity. By the display, the user can possibly perceive that a sheet or a stack of sheets P 1  is stuck between the exit of the post-handling apparatus  3  and the sheet pressers  39  and remove the sheet or the stack of sheets P 1  therefrom. 
     In a case where a movement of the sheet pressers  39  to the receding position by order of the control unit  4  is not completed within the predetermined time, the control unit  4  takes different procedures depending on whether there are any sheets on the ejected sheet tray  35 . This control contributes to a decrease in the frequency of calling a serviceman of a maintenance agency for maintenance of the image forming apparatus  2  and the post-handling apparatus  3  or the image forming system  1 , compared to a case of adopting a control procedure in which the control unit  4  drives the interface IF to display information that a component relating to the sheet pressers  39  has a defect regardless of whether there are any sheets on the ejected sheet tray  35 . 
     Modification 
     An image forming system  1 A and a post-handling apparatus  3 A according to a modification are different from the image forming system  1  and the post-handling apparatus  3  according to the above-described embodiment in the condition of starting the control procedure for sheet ejection from the post-handling apparatus. According to the modification, the control procedure is started on the condition that the post-handling apparatus  3 A or the image forming system  1 A is started. 
     By setting a start of the post-handling apparatus  3 A or the image forming system  1 A as the condition for a start of the control procedure, the sheet pressers  39  can be returned to the receding position (home position) before the control unit  4  issues an image forming command. Therefore, at a time of issuance of an image forming command, the main routine starts from step MS 7 , and a series of actions from image formation to sheet ejection from the post-handling apparatus  3  can be performed more smoothly. 
     Other Embodiments 
     Post-handling apparatuses and image forming systems according to the present invention are not limited to the embodiment and the modification described above. For example, the control unit  4  may be provided not in the image forming apparatus  2  but in the post-handling apparatus  3 . 
     Although the present invention has been described in connection with the embodiment and the modification above, it is to be noted that various changes and modifications may be obvious to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.