Patent Publication Number: US-2023159297-A1

Title: Sheet discharge apparatus and image forming system

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet discharge apparatus that discharges a sheet and an image forming system that forms an image on a sheet. 
     Description of the Related Art 
     A sheet processing device (also referred to as a finisher) that performs processes, such as sorting, binding, and alignment, on sheets having images formed thereon has been developed as an option for image forming apparatuses, such as electrophotographic copying machines and laser beam printers. Some sheet processing devices have the configuration in which when continuously processes a plurality of sheet bundles, the sheet processing device temporarily stops receiving a sheet from an image forming apparatus to wait for the end of the previous sheet bundle process. In this case, the productivity (the throughput) of the image forming system decreases. 
     Therefore, a method is proposed for stacking a sheet received from the image forming apparatus in the sheet processing device to temporarily hold (buffer) the sheet during processing a sheet bundle and after the processing of sheet bundle is finished, placing the sheet on a processing tray as a sheet bundle. Japanese Patent Publication No. 06-099070 describes the configuration in which a sheet received from an image forming apparatus is held using two conveying paths branched inside a finisher, and the two sheets are overlapped and stacked on a processing tray. 
     However, if the sheet conveying speed is increased to further improve the productivity of the image forming system, the sheet is discharged at high speed from the image forming apparatus or the sheet processing device and, thus, it is likely that the placement positions of the sheets discharged to the discharge destination, such as a discharge tray, are not the same. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a sheet discharge apparatus includes a first stacking member provided outside an apparatus main body, a first conveying device disposed in a first conveying path extending toward the first stacking member in the apparatus main body, wherein the first conveying device conveys a sheet to the first stacking member, a discharge device configured to receive the sheet conveyed by the first conveying device and discharge the sheet to the first stacking member, wherein the discharge device discharges a plurality of the sheets each conveyed from the first conveying device in the form of a bundle of sheets, a sheet processing device configured to receive a sheet that is not discharged by the discharge device and processes the received sheet, a second stacking member provided outside the apparatus main body, wherein the sheet processed by the sheet processing device is stacked on the second stacking member, and a controller configured to control the first conveying device and the discharge device to perform a bundle discharge operation for discharging the bundle of sheets from the discharge device onto the first stacking member. The controller sets a discharge speed at which a sheet is discharged by the discharge device to a first discharge speed when the bundle of sheets includes a first number of sheets and sets the discharge speed to a second discharge speed that is lower than the first discharge speed when the bundle of sheets includes a second number of sheets that is greater than the first number of sheets. 
     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 schematic illustration of an image forming system according to an embodiment of the present disclosure. 
         FIG.  2    is a cross-sectional view of a superposition processing unit according to an embodiment. 
         FIG.  3    is a hardware configuration diagram of the image forming system according to the embodiment. 
         FIG.  4    is a functional block diagram of the image forming system according to the embodiment. 
         FIGS.  5 A to  5 G  illustrate the operation performed by the superposition processing unit according to the embodiment. 
         FIGS.  6 A and  6 B  are a flowchart illustrating a control example of the superposition processing unit according to the embodiment. 
         FIGS.  7 A to  7 C  illustrate a method for controlling a protrusion amount between sheets by using the superposition processing unit according to the embodiment. 
         FIG.  8    illustrates the different discharge trajectories of bundles of sheets due to different discharge speeds according to the embodiment. 
         FIG.  9 A  illustrates the discharge trajectories when the discharge speed is set constant regardless of the number of sheets in a bundle; and  FIG.  9 B  illustrates the discharge trajectories when the discharge speed is changed according to the number of sheets in a bundle. 
         FIGS.  10 A and  10 B  illustrate the difference in discharge trajectory caused by the difference in basis weight. 
         FIGS.  11 A and  11 B  illustrate the curl directions of sheets. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. 
       FIG.  1    is a schematic illustration of an image forming system  1 S according to the present embodiment, viewed from the front side. The image forming system  1 S includes an image forming apparatus  1  that forms an image on a sheet, a sheet processing apparatus  4  that processes a sheet having, thereon, an image formed by the image forming apparatus  1 , a relay unit  14  that conveys the sheet from the image forming apparatus  1  to the sheet processing apparatus  4 , and an image reading device  2 . A variety of sheets (print media) of different sizes and materials can be used. Examples of a sheet include paper, such as plain paper and thick paper, surface-treated sheet materials, such as a plastic film, fabric, and coated paper, and specially shaped sheet materials, such as envelopes and index paper. The operation performed by each of devices constituting the image forming system  1 S is briefly described and, subsequently, the operation performed by the sheet processing apparatus  4  is described in detail below. 
     The image forming apparatus  1  includes an electrophotographic image forming unit  8  for forming an image, and a feeding device  6  for feeding sheets to the image forming unit  8  one by one. The image forming unit  8  is a cartridge integrally incorporating a photosensitive drum  9 , which is an image bearing member (an electrophotographic photosensitive member), and a charging device and a developing device for performing an electrophotographic process by acting on the photosensitive drum  9 . A scanner unit  15  serving as an exposure device is disposed above the image forming unit  8 , and a transfer roller  10  serving as a transfer device is disposed at a position facing the photosensitive drum  9 . A fixing device  11 , discharge rollers  12   a,  and reverse rollers  12   b  are disposed above the transfer roller  10 . The fixing device  11  has a heat fixing configuration and includes, for example, a cylindrical film, a heater unit that includes a heater and that is disposed inside the film, and a pressure roller in pressure contact with the heater via the film. 
     A plurality of feeding devices  6  for feeding sheets are disposed below the image forming unit  8 . Each of the feeding devices  6  includes a cassette  6   a  serving as a storage unit (a storage box) for storing a plurality of sheets and a feeding unit  6   b  for feeding sheets one by one from the cassette  6   a.    
     When the image forming apparatus  1  performs an image forming operation, the surface of the photosensitive drum  9  is uniformly charged by the charging device in the image forming unit  8 , and the scanner unit  15  emits a laser beam to the surface of the photosensitive drum  9  on the basis of image information and forms an electrostatic latent image. The electrostatic latent image is developed (visualized) with toner serving as a developer supplied from the developing device, and a toner image is formed on the surface of the photosensitive drum  9 . 
     In parallel with the operation performed by the image forming unit  8 , sheets are fed one by one from the cassette  6   a  by the feeding unit  6   b  in any of the feeding devices  6  and are conveyed toward registration rollers  7 . After correcting the skew of the sheet, the registration rollers  7  feed the sheet to a transfer portion between the photosensitive drum  9  and the transfer roller  10  in synchronization with the formation of the toner image by the image forming unit  8 . Then, the toner image is transferred from the photosensitive drum  9  to the sheet in the transfer portion. 
     The sheet that has passed through the transfer portion is delivered to the fixing device  11 . In the fixing device, when the sheet is nipped by the film and the pressure roller and passes through a fixing nip (a nip between the heater unit and the pressure roller), the toner on the sheet is heated and pressurized. Thus, a toner image is fixed onto the sheet. 
     In the case of single-sided printing, the sheet that has passed through the fixing device  11  is discharged from the image forming apparatus  1  by the discharge rollers  12   a  and is received by the relay unit  14 . In the case of double-sided printing, the sheet having a toner image formed on the first side and having passed through the fixing device  11  is guided by the reverse rollers  12   b,  is switched back by the reverse rollers  12   b,  and is conveyed again to the registration rollers  7  via a re-conveying path  13 . Thereafter, an image is formed on a second side opposite to the first side by passing through the transfer portion and the fixing device  11 , and the sheet is delivered to the relay unit  14  by the discharge rollers  12   a.    
     The image reading device  2  is attached to the top of the image forming apparatus  1 . The image reading device  2  includes a reading sensor  2   s  that reads image information from an original and an original conveying unit that conveys the originals one by one to the reading sensor  2   s.  The image forming apparatus  1  can perform both a copying operation of forming an image based on image information acquired by the image reading device  2  and a printing operation of forming an image based on image information received from the outside of the image forming apparatus  1 . 
     According to the present embodiment, the relay unit  14  is disposed in a space (also referred to as an internal discharge space) between the image forming apparatus  1  and the image reading device  2  in the vertical direction (the vertical direction when the image forming system  1 S is installed on a horizontal plane). The relay unit  14  conveys the sheet discharged from the image forming apparatus  1  in a substantially horizontal direction viewed from the front side, toward the sheet processing apparatus  4  that is installed alongside of the image forming apparatus  1  on a common installation surface with the image forming apparatus  1 . The relay unit  14  includes a sheet sensor  52  serving as a detection unit for detecting passage of a sheet. The sheet sensor  52  is, for example, a reflective photosensor that emits infrared light onto the conveying path and detects light reflected by a sheet passing through the conveying path to determine the presence of the sheet. While the image forming system  15  including the relay unit  14  is used as an example, a sheet may be delivered directly from the image forming apparatus  1  to the sheet processing apparatus  4 . 
     Furthermore, the image forming apparatus  1  includes a display unit  5  (an operation unit, an operation display unit) that serves as a user interface of the image forming system  1 S. The display unit  5  has a function of displaying the operation status of the system, such as paper jamming and malfunction and an instruction for the user to perform an operation, such as replacement of consumables in the apparatus and removal of a jammed sheet. The user can operate the touch panel function of a display of the display unit  5 , numeric keys, and the like to perform various settings and provide instructions to the image forming system  1 S. 
     Note that the configuration of the image forming apparatus is not limited to the direct transfer system illustrated in  FIG.  1   . The configuration may be an intermediate transfer system in which a toner image formed in the image forming unit is transferred onto a sheet via an intermediate transfer member. The image forming apparatus may be a color image forming apparatus using a plurality of image forming units. In addition, the image forming mechanism is not limited to the electrophotographic method, and may employ, for example, an inkjet printing unit or an offset printing mechanism. 
     Sheet Processing Apparatus 
     The sheet processing apparatus  4  includes a sheet processing device  71  that processes a sheet. The sheet processing apparatus  4  has a function of discharging, as a processing result, sheets that are received from the image forming apparatus  1  and are processed by the sheet processing device  71 . Alternatively, the sheet processing apparatus  4  can discharge, as a processing result, sheets received from the image forming apparatus  1  without performing a binding process. 
     The sheet processing apparatus  4  includes a receiving path  81 , an internal discharge path  82 , a first discharge path  83 , and a second discharge path  84  as conveying paths for conveying sheets. Furthermore, the sheet processing apparatus  4  includes, as discharge destinations for discharging sheets, an upper discharge tray  25  and a lower discharge tray  37  each protruding outward from an apparatus main body  4 A (a housing in which the receiving path  81 , the internal discharge path  82 , the first discharge path  83 , and the second discharge path  84  are provided) to the outside of the apparatus main body  4 A. The receiving path  81  is a conveying path for receiving a sheet from the image forming apparatus  1  and conveying the sheet, and the internal discharge path  82  is a conveying path for conveying a sheet toward the sheet processing device  71 . The first discharge path  83  is a conveying path for discharging a sheet to the upper discharge tray  25 , and the second discharge path  84  is a conveying path for discharging a sheet to the lower discharge tray  37 . As described above, according to the present embodiment, the receiving path  81  and the first discharge path  83  form a first conveying path toward the upper discharge tray  25  serving as a first stacking member, and the internal discharge path  82  is provided as a second conveying path that branches from the first conveying path. In addition, the second discharge path  84  is provided as a third conveying path extending from the sheet processing device  71  toward the lower discharge tray  37  serving as a second stacking member. 
     The receiving path  81  has, disposed therein, inlet rollers  21 , pre-branch rollers  22 , and an inlet sensor  27 . The first discharge path  83  has, disposed therein, discharge and reverse rollers  24  as a reversal conveying unit. The internal discharge path  82  has, disposed therein, internal discharge rollers  26 , intermediate conveying rollers  28 , kickout rollers  29 , and a pre-intermediate stacking sensor  38 . The second discharge path  84  has bundle discharge rollers  36  disposed therein. The pre-branch rollers  22  serve as a first conveying device of the present embodiment, the discharge and reverse rollers  24  serve as a second conveying device of the present embodiment, and the internal discharge rollers  26  serve as a third conveying device of the present embodiment. Each of the inlet rollers  21 , the pre-branch rollers  22 , the discharge and reverse rollers  24 , the internal discharge rollers  26 , the intermediate conveying rollers  28 , the kickout rollers  29 , and the bundle discharge rollers  36  is a roller pair in which the rollers are in contact with each other at the circumferential surface to form a nip that nips and conveys a sheet. The discharge and reverse rollers  24  also serve as a discharge device for discharging a sheet. 
     Both the inlet sensor  27  and the pre-intermediate stacking sensor  38  are examples of a sheet detection unit for detecting the passage of a sheet at a predetermined detection position in the conveying path in the sheet processing device. As the inlet sensor  27  and the pre-intermediate stacking sensor  38 , a reflective photosensor, for example, is used that emits infrared light to the inside of the conveying path and detects the light reflected by the sheet passing through the conveying path to determine the presence of the sheet. 
     The sheet conveying path in the sheet processing apparatus  4  is described below. A sheet conveyed from the image forming apparatus  1  via the relay unit  14  is received by the inlet rollers  21  of the sheet processing apparatus  4  and is conveyed to the pre-branch rollers  22  through the receiving path  81 . The inlet sensor  27  detects the sheet at a detection position between the inlet rollers  21  and the pre-branch rollers  22 . The pre-branch rollers  22  conveys the sheet received from the inlet rollers  21  toward the first discharge path  83 . 
     At a predetermined time point after the inlet sensor  27  detects the passage of the trailing edge of the sheet, the pre-branch rollers  22  accelerate the sheet conveying speed to a speed higher than that of the relay unit  14 . Alternatively, the sheet conveying speed of the inlet rollers  21  may be set higher than that of the relay unit  14 , and the sheet conveying speed may be accelerated by the inlet rollers  21  upstream of the pre-branch rollers  22 . In this case, it is desirable to install a one-way clutch between the conveying roller of the relay unit  14  and a motor that drives the conveying roller so that the conveying roller runs idle even if the sheet is pulled by the inlet rollers  21 . 
     When the sheet discharge destination is the upper discharge tray  25 , the discharge and reverse rollers  24  discharge the sheet received from the pre-branch rollers  22  to the upper discharge tray  25 . In this case, the discharge and reverse rollers  24  decelerate the discharge speed to a predetermined discharge speed at a predetermined time point after the trailing edge of the sheet moves past the pre-branch rollers  22 . 
     When the sheet discharge destination is the lower discharge tray  37 , the discharge and reverse rollers  24  performs switchback conveyance of the sheet received from the pre-branch rollers  22  to convey the sheet to the internal discharge path  82 . That is, the discharge and reverse rollers  24  convey the sheet toward the outside of the sheet processing apparatus  4  in the discharge direction and reverse their rotation directions to convey the sheet in the opposite direction before the trailing edge of the sheet in the discharge direction moves past the discharging and reversing rollers  24 . A check valve  23  is provided at a branching portion (between the pre-branch rollers  22  and the discharge and reverse rollers  24 ) where the internal discharge path  82  branches from both the receiving path  81  and the first discharge path  83  upstream of the discharge and reverse rollers  24  in the discharge direction. The check valve  23  functions as a guide (a restricting member) that restricts the backward movement of the sheet switched back by the discharge and reverse rollers  24  to the receiving path  81 . That is, the discharge and reverse rollers  24  reverse the conveying direction of the sheet after the trailing edge of the sheet in the discharge direction moves past the check valve  23  to perform switchback conveyance. 
     The internal discharge rollers  26 , the intermediate conveying rollers  28 , and the kickout rollers  29  disposed in the internal discharge path  82  sequentially pass the sheet received from the discharge and reverse rollers  24  to the next rollers to convey the sheet toward the sheet processing device  71 . The pre-intermediate stacking sensor  38  detects the sheet positioned between the intermediate conveying rollers  28  and the kickout rollers  29 . The pre-intermediate stacking sensor  38  is, for example, a reflective photosensor that emits infrared light to the inside of the conveying path and detects the light reflected by the sheet passing through the conveying path to determine the presence of the sheet. 
     The sheet processing apparatus  4  includes a superposition processing unit  4 B including the discharge and reverse rollers  24  and the internal discharge rollers  26  and can perform an operation to superpose a plurality of sheets conveyed from the image forming apparatus  1  one on top of another by using the superposition processing unit  4 B. According to the present embodiment, the superposition processing unit  4 B holds, in the internal discharge path  82 , a first sheet conveyed through the receiving path  81  by using the discharge and reverse rollers  24  and the internal discharge rollers  26 . Subsequently, the superposition processing unit  4 B superposes a second sheet conveyed through the receiving path  81  on the first sheet. The superposition processing unit  4 B also has a function of outputting the superposed sheets onto the upper discharge tray  25  (superposed discharge) and a function of conveying the superposed sheets to the sheet processing device  71  (a buffer function). The configuration and operation of the superposition processing unit  4 B are described in detail below. 
     After aligning the plurality of sheets received from the internal discharge path  82 , the sheet processing device  71  performs a binding process at a predetermined position of the sheet bundle. The sheet processing device  71  includes a stapler  50  as processing equipment and an upper intermediate stacking guide  31  and a lower intermediate stacking guide  32  that constitute an intermediate stacking member (a processing tray) on which sheets to be processed are stacked. 
     A vertical alignment reference plate  39  serving as a reference member is disposed at the downstream end of the sheet processing device  71  in the conveying direction of the kickout roller  29 . The position of the sheet bundle in the vertical direction (the conveying direction) is aligned by bringing the edges of the sheets in the conveying direction into contact with the vertical alignment reference plate  39 . A half-moon roller  33  rotatably supported by the upper intermediate stacking guide  31  is provided downstream of the pressing guide  56 . 
     The half-moon roller  33  is a moving member (a paddle member, a conveying member) for bringing the sheet that has passed through the kickout rollers  29  into contact with the vertical alignment reference plate  39 . After the trailing edge of the sheet moves past the pre-intermediate stacking sensor  38 , the half-moon roller  33  conveys the sheet toward the vertical alignment reference plate  39  at a predetermined time point. The contact pressure of the half-moon roller  33  against the sheet is adjusted to such an extent that the half-moon roller  33  slips on the sheet when the sheet is in contact with the vertical alignment reference plate  39 . A flexible pressing guide  56  is fixed to the upper intermediate stacking guide  31  and presses the sheet in the sheet processing device  71  downward with a predetermined pressure to prevent the sheet from lifting. Furthermore, a bundle pressing flag  30  is rotatably supported downstream of the kickout rollers  29  to prevent the trailing edge of the sheet from lifting so that the trailing edge of the sheet already stacked in the sheet processing device  71  does not interfere with the leading edge of the succeeding sheet discharged by the kickout rollers  29 . 
     When the alignment of a predetermined number of sheets on the intermediate stacking member is finished, the stapler  50  performs a binding operation. Then, the bundle discharge guide  34  serving as an extrusion member driven by a guide drive unit  35  moves in a direction from the standby position illustrated in  FIG.  1    toward the bundle discharge rollers  36  (a bundle discharge direction). Thus, the sheet bundle is pushed out of the intermediate stacking member. When the leading edge of the sheet bundle in the bundle discharge direction reaches the bundle discharge rollers  36 , the bundle discharge guide  34  stops and returns to the standby position again. The bundle discharge rollers  36  serving as a discharge device (a fourth conveying device) discharges the sheet bundle received from the bundle discharge guide  34  to the lower discharge tray  37 . 
     Both the upper discharge tray  25  and the lower discharge tray  37  are movable vertically relative to the housing of the sheet processing apparatus  4 . Sheet presence sensors  51  and  53  for detecting the presence/absence of a sheet on a tray are disposed on the upper discharge tray  25  and the lower discharge tray  37 , respectively. The sheet presence sensors  51  and  53  are, for example, reflective photosensors that determine the presence/absence of a sheet by emitting infrared light upward from the tray stacking surface and detecting reflected light from the sheets. The sheet processing apparatus  4  further includes a sheet surface detection sensor that detects the upper surface position of the sheets (the sheet stack height) on each of the upper discharge tray  25  and the lower discharge tray  37 . 
     When the sheet surface detection sensor detects a sheet, the corresponding one of the upper discharge tray  25  and the lower discharge tray  37  tray is lowered in an A 2  or B 2  direction. When the sheet presence sensor  51  or  53  detects that the sheet has been removed from the upper discharge tray  25  or the lower discharge tray  37 , the tray is raised in the A 1  or B 1  direction. The upper discharge tray  25  and the lower discharge tray  37  are controlled to move up and down according to the number of stacked sheets so that the upper surfaces of the stacked sheets are positioned below the discharge and reverse rollers  24  and the bundle discharge rollers  36  in the vertical direction, respectively. According to the present embodiment, the upper discharge tray  25  serving as the first stacking member and the lower discharge tray  37  serving as the second stacking member are controlled to be raised and lowered by motor drive. However, the upper discharge tray  25  and the lower discharge tray  37  may be configured so as to be raised and lowered by an urging unit, such as a spring. 
     The stapler  50  is an example of the processing equipment. For example, a sorting mechanism for sorting sheets and a center-folding processing unit for center folding a plurality of sheets and perform saddle stitch book binding may be provided. 
     Superposition Processing Unit 
       FIG.  2    is an enlarged view of the superposition processing unit  4 B. The sheet conveying path between the inlet rollers  21  and the pre-branch rollers  22  (the receiving path  81 ) consists of an upper inlet guide  40  and a lower inlet guide  41 . The sheet conveying path between the internal discharge rollers  26  and the intermediate conveying rollers  28  (the internal discharge path  82 ) consists of an upper internal discharge guide  46  and a lower internal discharge guide  47 . A conveying guide that guides a sheet from the same side as the upper inlet guide  40  between the pre-branch rollers  22  and the discharge and reverse rollers  24  is referred to as an upper reverse guide  42 . A conveying guide that guides a sheet from the same side as the lower internal discharge guide  47  between the discharge and reverse rollers  24  and the internal discharge rollers  26  is referred to as a lower reverse guide  43 . The first discharge path  83  consists of the upper reverse guide  42  and the lower reverse guide  43 . 
     The sheet conveyed by the inlet rollers  21  is guided to the pre-branch rollers  22  by the upper inlet guide  40  and the lower inlet guide  41 . The inlet sensor  27  is disposed on the upper inlet guide  40 . As the inlet sensor  27 , a reflective photosensor can be used that determines the presence of a sheet at the detection position by emitting infrared light to the receiving path  81  and detecting reflected light from the sheet. In this case, a hole having a diameter greater than the diameter of the spotlight of the inlet sensor  27  is formed in a portion of the lower inlet guide  41  facing the inlet sensor  27  so that the infrared light is not reflected when a sheet does not pass through the receiving path  81 . 
     The check valve  23  is disposed downstream of the pre-branch rollers  22  and at a portion where the receiving path  81  and the internal discharge path  82  branch from the first discharge path  83 . The check valve  23  is rotatably supported by the upper internal discharge guide  46  via a rotating shaft  23   a.  In addition, the check valve  23  is always urged by a spring (not illustrated) in a C 2  direction (the clockwise direction in  FIG.  2   ) toward a position (refer to  FIG.  2   ) where the top end of the check valve  23  overlaps the upper reverse guide  42  as viewed from the axial direction of the rotating shaft  23   a  (the width direction of a sheet). The spring constant of the spring is set to such a value that when a sheet delivered from the pre-branch rollers  22  is brought into contact with the check valve  23 , the check valve  23  rotates in the C 1  direction (the counterclockwise direction in  FIG.  2   ) against the biasing force of the spring. Thus, the check valve  23  enables the sheet conveyed from the pre-branch rollers  22  toward the discharge and reverse rollers  24  to pass therethrough. In addition, when the trailing edge of the sheet in the receiving path  81  passes through the check valve  23 , the check valve  23  rotates in the C 2  direction and restricts the sheet from returning from the discharge and reverse rollers  24  to the pre-branch rollers  22 . 
     The discharge and reverse rollers  24  consist of an upper roller  24   a  and a lower roller  24   b.  According to the present embodiment, driving force is input to both the upper roller  24   a  and the lower roller  24   b,  and the rotations of the upper roller  24   a  and the lower roller  24   b  are always synchronized. 
     The discharge and reverse rollers  24  are configured to contact each other (a close operation) and separate from each other (an open operation) by a plunger solenoid  45 . More specifically, one end of a separation lever  44  is connected to the roller shaft of the upper roller  24   a,  and the separation lever  44  is supported by a lever fulcrum shaft  44   a  in a rotatable manner with respect to the upper reverse guide  42 . A solenoid connection shaft  44   b  provided at the other end of the separation lever  44  is connected to a plunger of the plunger solenoid  45 . 
     When the plunger solenoid  45  is powered on, the plunger is attracted in a D 1  direction by a magnetic force. Thus, the separation lever  44  rotates in an E 1  direction, and the discharge and reverse rollers  24  are separated from each other (the nip of the roller pair is released). When the plunger solenoid  45  is powered off, the upper roller  24   a  is brought into contact with the lower roller  24   b  by the biasing force of a pressure spring  48  connected to the roller shaft of the upper roller  24   a,  and the discharge and reverse rollers  24  are in contact with each other (the nip is closed). At this time, the separation lever  44  rotates in the E 2  direction as the upper roller  24   a  moves, and the plunger of the plunger solenoid  45  moves in the D 2  direction. 
     The internal discharge rollers  26  form a roller pair adjacent to the discharge and reverse rollers  24  in the sheet conveying direction in the internal discharge path  82 . The roller pair is capable of forward rotation and reverse rotation. That is, the internal discharge rollers  26  can convey a sheet in both the direction from the discharge and reverse rollers  24  to the sheet processing device  71  (hereinafter referred to as a G 1  direction) and the direction from the sheet processing device  71  to the discharge and reverse rollers  24  (hereinafter referred to as a G 2  direction). 
     Hardware Configuration 
     The hardware configuration of the image forming system  1 S according to the present embodiment is described below with reference to  FIG.  3   .  FIG.  3    mainly illustrates, of the hardware configuration of the image forming system  1 S, a portion related to the configuration of the sheet processing apparatus  4 . A video controller  601  performs overall control of the image forming system  1 S including the image forming apparatus  1  and the sheet processing apparatus  4 . An engine control unit  602  controls the image forming apparatus  1 . 
     A main control unit  603  controls the sheet processing apparatus  4 . A signal line  604  is a signal line for serial command transmission to transmit a command from the video controller  601  to the engine control unit  602  by serial communication, and a signal line  605  is similarly used to transmit a command from the video controller  601  to the main control unit  603 . A signal line  606  is a signal line for serial status transmission to transmit status data from the engine control unit  602  to the video controller  601  by serial communication in response to a command, and a signal line  607  is similarly used to transmit status data from the main control unit  603  to the video controller  601 . To perform an image forming operation, the video controller  601  transmits serial commands to the engine control unit  602  and the main control unit  603  and receives status data from the engine control unit  602  and the main control unit  603 . Thus, the video controller  601  performs control. In this way, when a plurality of apparatuses are connected and the image forming system  1 S operates, the video controller  601  manages the control and status of each of the apparatuses and ensures the consistency of the operations performed by the apparatuses. 
     The main control unit  603  includes a central processing unit (CPU)  608  that controls various operations performed by the sheet processing apparatus  4  and a random access memory (RAM)  609  that temporarily stores control data necessary for the operations performed by the sheet processing apparatus  4 . 
     The main control unit  603  further includes a nonvolatile read only memory (ROM)  610  that stores programs and control tables necessary for the operation performed by the sheet processing apparatus  4 . The main control unit  603  further includes a communication unit  611  for communicating with the video controller  601 , a system timer  612  that generates timings necessary for various controls, and an input and output (I/O) port  613  that inputs and outputs control signals from and to various units of the sheet processing apparatus  4 . The main control unit  603  is a control integrated circuit (IC) to which the above-described elements are connected via a bus  614 . 
     Input signals from the inlet sensor  27  and the sheet presence sensors  51  and  53  of the upper discharge tray  25  and the lower discharge tray  37  are transmitted to the main control unit  603  via input circuits  615 ,  626 , and  628 , respectively. The control signals from the main control unit  603  are transmitted to an inlet motor  641 , a pre-branch motor  642 , a discharge and reverse motor  643 , an internal discharge motor  644 , and the plunger solenoid  45  via drive circuits  618 ,  619 ,  620 ,  621 , and  623 , respectively. Thus, the driving of actuators is controlled. 
     Functional Block 
     The functional blocks of the present embodiment are described below with reference to  FIG.  4   . The main control unit  603  illustrated in  FIG.  4    has a function of performing a sheet conveying operation by using the sheet processing apparatus  4 . The main control unit  603  has at least the functions of the communication unit  611 , a system timer  612 , a sheet conveying controller  701 , a sensor controller  720 , a motor controller  721 , and a solenoid controller  722 . 
     The sensor controller  720  is a unit for inputting signals from the inlet sensor  27  and the sheet presence sensor  51  of the upper discharge tray  25  to the sheet conveying controller  701 . The sheet conveying controller  701  includes a superposed conveying controller  711  and a sheet count controller  712 . The sheet conveying controller  701  controls the motor controller  721  and the solenoid controller  722  on the basis of the input from the sensor controller  720  to achieve the operations performed by the superposition processing unit  4 B, the upper discharge tray  25 , and the lower discharge tray  37 . The superposed conveying controller  711  controls conveyance of a sheet to the superposition processing unit  4 B and the upper discharge tray  25  while managing the position of the sheet on the basis of mainly the input from the sensor controller  720 . The sheet count controller  712  determines the timing of discharging the superposed sheets to the upper discharge tray  25  on the basis of the maximum number of sheets that can be superposed by the superposition processing unit  4 B (the number of superposable sheets) and the current number of superposed sheets. 
     The inlet motor  641  drives the inlet rollers  21 , the pre-branch motor  642  drives the pre-branch rollers  22 , and the discharge and reverse motor  643  drives the discharge and reverse rollers  24 . The internal discharge motor  644  drives the internal discharge rollers  26 , and the plunger solenoid  45  drives the separation lever  44 . The operations performed by these elements to be driven are described in detail below. 
     Superposed Discharge Operation 
     An overview of a bundle discharge operation (a superposed discharge operation) in which the superposed conveying controller  711  superposes and discharges a plurality of sheets by the superposition processing unit  4 B is described with reference to  FIGS.  5 A to  5 G . Hereinafter, among the sheets to be subjected to the superposed discharge operation, a sheet conveyed from the image forming apparatus  1  to the sheet processing apparatus  4  first (a first sheet) is referred to as a “sheet S 1 ”, and a sheet conveyed from the image forming apparatus  1  to the sheet processing apparatus  4  second (a second sheet) is referred to as a “sheet S 2 ”. In addition, the conveying speed of the pre-branch rollers  22 , the discharge and reverse rollers  24 , and the internal discharge rollers  26  before acceleration (the conveying speed in the relay unit  14 ) is defined as V 1 , and the conveying speed after acceleration is defined as V 2 . The conveying speed when a sheet is discharged by the discharge and reverse rollers  24  is defined as V 3 . According to the present embodiment, the conveying speed V 3  can be changed according to the number of sheets in the bundle. 
     Referring to  FIG.  5 A , at the time the trailing edge of the preceding sheet S 1  passes the inlet sensor  27 , the pre-branch rollers  22  and the discharge and reverse rollers  24  are accelerated from the speed V 1  to the speed V 2 . By accelerating the conveying speed of the sheet S 1 , even in the case where the image forming apparatus  1  is a high-performance machine with high throughput, the sheet interval required for switchback can be ensured between the sheet S 1  and the succeeding sheet S 2 . However, if the sheets S 1  and S 2  do not collide with each other, a configuration in which the conveying speed at the inlet sensor  27  is not accelerated may be employed. In this case, the conveying speed in the superposition processing unit  4 B may be set to V 1  at all times. At the time in  FIG.  5 A , the discharge and reverse rollers  24  are conveying the sheet S 1  in an F 2  direction. 
     Referring to  FIG.  5 B , at the time the trailing edge of the sheet S 1  passes the inlet sensor  27 , moves a predetermined distance, and passes through the check valve  23 , the sheet S 1  is temporarily stopped. The “predetermined distance” is the distance at which the trailing edge of the sheet S 1  in the F 2  direction has passed through the check valve  23  and does not reach the nip of the discharge and reverse rollers  24 . 
     Referring to  FIG.  5 C , the discharge and reverse rollers  24  change their rotation directions and convey the sheet S 1  in an F 1  direction at the speed V 2 . The driving of the internal discharge rollers  26  is started before the leading edge of the sheet S 1  in the F 1  direction reaches the internal discharge rollers  26 , and the internal discharge rollers  26  further convey the sheet S 1  in the G 1  direction. 
     Referring to  FIG.  5 D , the sheet S 1  is nipped by the internal discharge rollers  26 . When the leading edge of the sheet S 1  in the G 1  direction (the F 1  direction) moves past the internal discharge rollers  26  and, thereafter, the sheet S 1  is conveyed by a predetermined distance, the conveyance of the sheet S 1  is stopped. The “predetermined distance” is less than the distance at which the leading edge of the sheet S 1  reaches the intermediate conveying rollers  28 . When the sheet S 1  is nipped by the internal discharge rollers  26 , the upper roller  24   a  of the discharge and reverse rollers  24  is moved in the E 1  direction by the separation lever  44  so as to be separated from the lower roller  24   b.  Note that the discharge and reverse rollers  24  are driven so as to separate from each other before the leading edge of the succeeding sheet S 2  reaches the discharge and reverse rollers  24 . 
     Referring to  FIG.  5 E , after the trailing edge of the succeeding sheet S 2  passes the inlet sensor  27 , the pre-branch rollers  22  and the discharge and reverse rollers  24  are accelerated to the speed V 2  in the same manner as the preceding sheet S 1 . When the trailing edge of the sheet S 2  passes the inlet sensor  27  and, thereafter, a predetermined time T_wait elapses, the internal discharge rollers  26  start rotating again toward the discharge and reverse rollers  24 , and the sheet S 1  is conveyed in the G 2  direction. The predetermined time T_wait is described in more detail below. When the relative speeds of the sheets S 1  and S 2  become equal, the upper roller  24   a  of the discharge and reverse rollers  24  is driven in the E 2  direction and comes into contact with the lower roller  24   b,  and the discharge and reverse rollers  24  nip the sheets S 1  and S 2  at the same time. At this time, the leading edge of the sheet S 1  and the leading edge of the sheet S 2  in the F 2  direction are aligned. In addition, before the discharging and reversing rollers  24  nip the sheets S 1  and S 2 , the rotation speed of the discharge and reverse rollers  24  is adjusted so as to be equal to the speed V 2 , which is the conveying speed of the sheets S 1  and S 2 . 
     Referring to  FIG.  5 F , when the trailing edge of the sheet S 2  passes through the check valve  23 , the sheets S 1  and S 2  form a sheet bundle S′ in which both the leading edges and trailing edges of the sheets S 1  and S 2  in the F 2  direction are aligned. 
     Referring to  FIG.  5 G , the speed of the sheet bundle S′ is changed to the speed V 3  before a distance L from the discharge and reverse rollers  24 , and the sheet bundle S′ is discharged to the upper discharge tray  25  by the discharge and reverse rollers  24 . 
     Thus, the operation of superposing and discharging the two sheets S 1  and S 2  while aligning the two sheets S 1  and S 2  in the superposition processing unit  4 B (the superposed discharge operation) is completed. When the image forming operation is continuously performed on a large number of sheets, two-sheet bundles are stacked on the upper discharge tray  25  by repeating the above-described superposed discharge operation. 
     Advantages of the present embodiment are described below, compared with the case where the sheets S 1  and S 2  are discharged one by one without being subjected to the superposed discharge operation. When the sheets S 1  and S 2  are discharged one by one, the positions and postures of the sheets S 1  and S 2  that have passed through the discharge and reverse rollers  24  may be lost before the sheets S 1  and S 2  land on the upper surface of the upper discharge tray  25  or the upper surface of the sheets on the upper discharge tray  25 . This is because while falling down, each of the sheets S 1  and S 2  receives air resistance and, thus, moves in all directions as viewed from above. 
     In contrast, according to the present embodiment, the positions of the sheets S 1  and S 2  are aligned in the sheet conveyance direction and the sheets S 1  and S 2  are superposed in advance and are discharged, so that the positions and postures of the sheets S 1  and S 2  are less likely to be lost. 
     When the sheet bundle discharged through the superposed discharge operation is compared with the sheets discharged one by one, the projected areas of the sheet (bundle) and the single sheet as viewed from above are the same, but the weight of the sheet bundle is two times the weight of the single sheet. For this reason, the sheet bundle is less susceptible to air resistance. As a result, even when the sheet discharge speed of the discharge and reverse rollers  24  is increased to improve the productivity of the image forming system  1 S and the productivity of the sheet processing apparatus  4 , a degradation of the sheet stackability can be avoided. 
     Three or More Sheet Superposed Discharge Operation 
     While the above description has been made with reference to the conveyance of two sheets, the sheet processing apparatus  4  of the present embodiment can perform a superposed discharge operation to align the positions of three or more sheets, superpose the sheets on top of another in the superposition processing unit  4 B, and discharge the sheets to the upper discharge tray  25 . 
     When the superposed discharge operation is performed for three sheets, two sheets S 1  and S 2  are first superposed in the same procedure as described above with reference to  FIGS.  5 A to  5 F . Thereafter, the discharge and reverse rollers  24  illustrated in  FIG.  5 F  are reversed again so that the sheet bundle S′ is conveyed in the G 1  direction. Subsequently, a third sheet S 3  is subjected to operations the same as those performed on the sheet S 2  in  FIGS.  5 C to  5 F  while the sheet bundle S′ is being subjected to operations the same as those performed on the sheet S 1  in  FIGS.  5 C to  5 F . 
     As a result, after the sheet bundle S′ is temporarily stopped while being held by the internal discharge rollers  26  in the internal discharge path  82 , when the predetermined time T_wait elapses since the inlet sensor  27  detected the trailing edge of the third sheet S 3 , the internal discharge rollers  26  convey the sheet bundle S′ in the G 2  direction. Thereafter, the discharge and reverse rollers  24  that have been open is closed, so that the three sheets S 1 , S 2 , and S 3  are nipped by the discharge and reverse rollers  24  at the same time. When the trailing edge of the sheet S 3  passes through the check valve  23 , a sheet bundle is formed in which both the leading edges and the trailing edges of the three sheets S 1 , S 2 , and S 3  are aligned. 
     When the number of sheets in the superposed discharge operation is three, the sheet bundle is directly discharged in the G 2  direction by the discharge and reverse rollers  24  and is stacked on the upper discharge tray  25 . When the number of sheets in the superposed discharge operation is four or more, the discharge and reverse rollers  24  convey the sheet bundle again in the G 1  direction and repeats the same operations as in  FIGS.  5 C to  5 F . In this manner, the number of sheets to be superposed can be increased. 
     The sheet count controller  712  manages the number of sheets to be superposed in the superposition processing unit  4 B on the basis of the number of sheets superposable by the superposition processing unit  4 B and information regarding the sheet to be conveyed. That is, the sheet count controller  712  determines whether the sheet conveyed to the superposition processing unit  4 B is immediately discharged to the upper discharge tray  25  or is superposed on top of another. 
     As an example of the determination technique, let N denote the number of superposable sheets by the superposition processing unit  4 B. Then, the sheet count controller  712  generates a sheet bundle of N−1 sheets and discharges the sheet bundle to the upper discharge tray  25 . Only when the sheet count controller  712  determines that the Nth sheet is the last sheet, the number of sheets to be superposed is set to N. In this way, the sheet count controller  712  prevents discharge of the Nth sheet to the upper discharge tray  25  as one sheet. 
     As a specific example, the number of superposable sheets by the superposition processing unit  4 B is five in the configuration example according to the present embodiment. In this case, the sheet count controller  712  repeatedly performs the superposed discharge operation for four sheets and stacks a sheet bundle of the four sheets on the upper discharge tray  25 . At this time, if the sheet count controller  712  determines that the fifth sheet is the last sheet and the last sheet is discharged as one sheet if the superposed discharge operation for four sheets is repeated to the end, a superposed discharge operation for five sheets including the last sheet is performed, and the sheets are discharged to the upper discharge tray  25 . If the last sheet is superposed on another sheet even after the superposed discharge operation for four sheets is performed to the end, the sheet bundle is discharged to the upper discharge tray  25  when the sheet bundle including the last sheet is formed. 
     That is, when executing a job of discharging a predetermined number of sheets to the upper discharge tray  25 , the sheet count controller  712  changes the number of sheets in a bundle formed through the superposed discharge operation in accordance with a predetermined number of sheets so that each of the predetermined number of sheets is always included in a sheet bundle of two or more sheets formed through the superposed discharge operation and is discharged to the upper discharge tray  25 . As a result, it is possible to prevent a single sheet from being discharged to the upper discharge tray  25  and, thus, prevent a degradation of the sheet stackability. Note that the technique for controlling the number of sheets in the superposed discharge operation is not limited thereto. Any technique that avoids discharge of a single sheet can be employed. For example, in the above-described example, the number of sheets in the successive superposed discharge operations may be four, . . . , four, three, and two. 
     Way to Find T_wait 
     The timing management (a method for obtaining T_wait described above) is described that is performed by the superposed conveyance controller  711  to align the leading edges of the sheets S 1  and S 2  in the superposition processing unit  4 B. 
       FIG.  7 A  illustrates the positional relationship between the sheets S 1  and S 2  at the moment the inlet sensor  27  detects the trailing edge of the sheet S 2 . A distance L 1  is the distance from the detection position of the inlet sensor  27  to the nip position of the discharge and reverse rollers  24  (the length measured along the receiving path  81  and the first discharge path  83 ). A distance L 2  is the distance from the position where after passing the internal discharge rollers  26 , the leading edge of the reversed sheet S 2  moves a predetermined distance d 1  and stops to the nip of the discharge and reverse rollers  24  (the length measured along the first discharge path  83  and the internal discharge path  82 ). 
       FIG.  7 B  illustrates the positional relationship between the sheets S 1  and S 2  when conveyance of the sheet S 1  in  FIG.  7 A  is started in the F 2  direction (the G 2  direction) and the conveying speed of the sheet S 1  becomes equal to the conveying speed of the sheet S 2 . At this time, it is assumed that the trailing edges of the sheets S 1  and S 2  in the F 2  direction are shifted from each other by a protrusion amount Kt. 
       FIG.  7 C  illustrates a change in the speed of each of the sheets S 1  and S 2  during the operations illustrated in  FIGS.  7 A and  7 B . In  FIG.  7 C , “A” represents the moment when the inlet sensor  27  detects the trailing edge of the sheet S 2  as illustrated in  FIG.  7 A  and, thus, the pre-branch rollers  22  start accelerating from the speed V 1  to the speed V 2  with a constant acceleration. In  FIG.  7 C , “B” represents the time the sheet S 2  has completed accelerating to the speed V 2 . 
     In  FIG.  7 C , “C” represents the time a predetermined time T_wait has elapsed since the inlet sensor  27  detected the trailing edge of the sheet S 2 , that is, the time the internal discharge rollers  26  start conveying the sheet S 1  in the G 2  direction. In  FIG.  7 C , “D” represents the time the relative speed between the sheets S 1  and S 2  becomes zero as illustrated in  FIG.  7 B . 
     Let T_merge denote the elapsed time from A to D. Let T 1  denote the time required for the pre-branch rollers  22  to accelerate from the speed V 1  to the speed V 2  (the elapsed time from A to B). Let T 2  denote the time from acceleration of the pre-branch rollers  22  to the speed V 2  until start of the rotation of the internal discharge rollers  26  (the elapsed time from B to C). As can be seen from the definitions of T 1 , T 2  and T_wait, T_wait=T 1 +T 2 . Let T 3  denote the time required for the stopped sheet S 1  to accelerate to the speed V 2  with a constant acceleration (the elapsed time from C to D). 
     Let X 2  denote the distance that the sheet S 1  moves from the position in  FIG.  7 A  to the position in  FIG.  7 B . Then, as can be seen from the above description, X 2  is the distance that the sheet S 1  moves from C to D in  FIG.  7 C  and can be given by the following equation: 
         X 2=( V 2× T 3)/2   (1).
 
     In addition, let X 1  denote the distance that the sheet S 2  moves from the position in  FIG.  7 A  to the position in  FIG.  7 B . Then, X 1  is the distance that the sheet S 2  moves from A to D in  FIG.  7 C  and can be given by the following equation: 
         X 1=( V 1 +V 2)× T 1/2 +V 2×( T 2 +T 3)   (2).
 
     From the positional relationship between the sheets S 1  and S 2  at the time in  FIG.  7 B , the following relationship holds: 
         L 1 −X 1 =L 2 −X 2 −Kt    (3).
 
     Substituting equations (1) and (2) into equation (3) and expanding and rearranging the equation yields the following equation: 
         L 1 −L 2 +Kt =( T ½)× V 1+( T ½ +T 2 +T  3/2)× V 2   (4).
 
     Substituting T_wait=T 1 +T 2  into the above equation (4) and rearranging the equation, the waiting time T_wait from when the trailing edge of the sheet S 2  passes the inlet sensor  27  to when the internal discharge rollers  26  start conveying the sheet S 1  is given for the protrusion amount Kt by the following equation: 
         T _wait=( L 1 −L 2 +Kt )/ V 2−( T ½)× V 1 /V 2+( T 1 −T 3)/2   (5).
 
     To align the leading and trailing edges of the sheets S 1  and S 2  and superpose the sheets S 1  and S 2 , the waiting time T_wait can be calculated by setting Kt=0 in the above equation (5). By starting the conveyance of the sheet S 1  by the internal discharge rollers  26  on the basis of the calculated T_wait, the sheet bundle S′ in which the leading and trailing edges of the sheets S 1  and S 2  are aligned can be formed. In addition, by using the same value of T_wait when three or more sheets are superposed, a sheet bundle in which the leading and trailing edges of the sheets are aligned can be formed. 
     Control Example 
     An example of a method for controlling the sheet processing apparatus  4  that achieves the superposed discharge operation described with reference to  FIGS.  5 A to  5 G  is described below with reference to the flowchart illustrated in  FIGS.  6 A and  6 B . This flow is executed each time the main control unit  603  of the sheet processing apparatus  4  receives a notification from the video controller  601  that one sheet is discharged from the image forming apparatus  1 . The steps of the flowchart are executed by the superposed conveying controller  711  illustrated in  FIG.  4    unless otherwise specified. 
     In the following description, the term “first sheet” refers to a sheet conveyed first to the sheet processing apparatus  4  among the sheets to be superposed in the superposition processing unit  4 B to form a sheet bundle. For example, when four sheets are superposed and discharged to the upper discharge tray  25 , the sheet conveyed to the sheet processing apparatus  4  after the last sheet of the preceding sheet bundle (that is, the (4n+1)th sheet) is the first sheet. The term “last sheet” refers to a sheet that is conveyed to the sheet processing apparatus  4  last among the sheets to be superposed in the superposition processing unit  4 B to form a sheet bundle (that is, the 4nth sheet in the above example). 
     In step S 101 , rotation of the inlet rollers  21  and the pre-branch rollers  22  is started at the speed V 1 . Thereafter, the processing proceeds to step S 102 . If the inlet rollers  21  and the pre-branch rollers  22  are already rotating at the speed V 1 , the rotation of the rollers is continued. 
     In step S 102 , it is determined whether the current sheet is a first sheet. If Yes, then the processing proceeds to step S 103 , and if No, the processing proceeds to step S 106 . 
     In step S 103 , the discharge and reverse rollers  24  are brought into contact with each other, and the rotation of the discharging and reversing rollers  24  is started in a direction in which the first sheet is conveyed toward the upper discharge tray  25  (the G 2  direction) at the speed V 1  (refer to the sheet S 1  in  FIG.  5 A ). The processing proceeds to step S 104 . 
     In step S 104 , it is determined whether the trailing edge of the first sheet has passed the inlet sensor  27 . If Yes, the processing proceeds to step S 105 , and if No, the processing proceeds to step S 104 . 
     In step S 105 , the pre-branch rollers  22  and the discharge and reverse rollers  24  are accelerated to the speed V 2  (refer to the sheet S 1  in  FIG.  5 A ). The processing proceeds to step S 111 . 
     In step S 106 , it is determined whether the trailing edge of the current sheet (one of the second and succeeding sheets) has passed the inlet sensor  27 . If Yes, the processing proceeds to step S 107 , and if No, the processing proceeds to step S 106 . 
     In step S 107 , the pre-branch rollers  22  and the discharge and reverse rollers  24  are accelerated to the speed V 2 . As a result, the conveying speed of the current sheet is accelerated from the speed V 1  to the speed V 2  (refer to the sheet S 2  in  FIG.  5 D ). The processing proceeds to step S 108 . 
     In step S 108 , it is determined whether a predetermined time T_wait has elapsed since the time the trailing edge of the current sheet passed the inlet sensor  27 . If Yes, the processing proceeds to step S 109 , and if No, the processing proceeds to step S 108 . 
     In step S 109 , rotation of the internal discharge rollers  26  is started again in the direction in which the sheet is conveyed toward the discharge and reverse rollers  24  (the F 2  direction) at a speed V 2  (refer to the sheet S 1  in  FIG.  5 D ). The processing proceeds to step S 110 . 
     In step S 110 , at the time the conveying speed of the sheets (a bundle) conveyed by the internal discharge rollers  26  and the conveying speed of the current sheet become equal, the upper roller  24   a  of the discharge and reverse rollers  24  is moved in the E 2  direction so as to come into contact with the lower roller  24   b  (refer to  FIG.  5 E ). As a result, the sheets (the bundle) conveyed by the internal discharge rollers  26  and the current sheet are nipped by the discharge and reverse rollers  24  at the same time (refer to  FIG.  5 E ). The processing proceeds to step S 111 . 
     In step S 111 , it is determined whether the current sheet is the last sheet. If Yes, the processing proceeds to step S 112 , and if No, the processing proceeds to step S 116 . 
     In step S 112 , the sheet bundle including the last sheet is discharged to the upper discharge tray  25  (refer to  FIG.  5 F ). That is, the sheet conveyance started in step S 107  or S 109  by using the discharge and reverse rollers  24  and the internal discharge rollers  26  is continued. 
     In step S 113 , when the trailing edge of the sheet bundle reaches the distance L from the discharge and reverse rollers  24 , the conveying speed of the discharge and reverse rollers is changed to the speed V 3 , and the sheet bundle is discharged to the upper discharge tray  25 . According to the present embodiment, the discharge speed V 3  is set according to the number of sheets in a sheet bundle to be discharged. 
     In step S 114 , it is determined whether the trailing edge of the sheet bundle has passed through the discharge and reverse rollers  24 . If Yes, the processing proceeds to step S 115 , and if No, the processing proceeds to step S 114 . 
     In step S 115 , the pre-branch rollers  22  is decelerated to the speed V 1 , and the discharge and reverse rollers  24  and the internal discharge rollers  26  are stopped. Thus, the flow ends. If the current sheet is the last sheet in the job (if no more sheets are conveyed from the image forming apparatus  1 ), the inlet rollers  21  and pre-branch rollers  22  are also stopped in step S 115 . 
     In step S 116 , it is determined whether the trailing edge of the current sheet (a sheet other than the last sheet) has passed through the check valve  23 . If Yes, the processing proceeds to step S 117 , and if No, the processing proceeds to step S 116 . 
     In step S 117 , the discharge and reverse rollers  24  and the internal discharge rollers  26  are temporarily stopped (refer to the sheet S 1  in  FIG.  5 B ). The processing proceeds to step S 118 . 
     In step S 118 , rotation of the discharge and reverse rollers  24  and rotation of the internal discharge rollers  26  are started in a rotation direction for conveying the sheets (the bundle) in the direction after reversal (the F 1  direction, G 1  direction) at the speed V 2  (refer to the sheet S 1  in  FIG.  5 C ). The processing proceeds to step S 119 . 
     In step S 119 , it is determined whether the leading edge of the sheets (the bundle) has passed through the internal discharge rollers  26 . If Yes, the processing proceeds to step S 120 , and if No, the processing proceeds to step S 119 . 
     In step S 120 , the upper roller  24   a  is separated from the lower roller  24   b  of the discharge and reverse rollers  24 . The processing proceeds to step S 121 . 
     In step S 121 , at the position where the leading edge of the sheets (the bundle) has passed through the internal discharge rollers  26  and has been conveyed by a predetermined distance, the pre-branch rollers  22  is decelerated to the speed V 1 , and the discharge and reverse rollers  24  and the internal discharge rollers  26  are stopped. Thus, the flow ends. As a result, the sheets (the bundle), which are the target of the superposed discharge operation and are still superposed on other sheets, are held while being nipped by the internal discharge rollers  26  (refer to the sheet S 1  in  FIG.  5 D ). 
     As described in step S 113 , according to the present embodiment, the discharge speed V 3  is set according to the sheet bundle to be discharged. The reason for this setting is described below with reference to  FIG.  8   . The flight distance of the sheet bundle discharged by the discharge and reverse rollers  24  varies according to the number of sheets in a bundle. If an air resistance R is not taken into account, the flight trajectory of the sheet bundle is determined by the initial speed V 3  and a discharge angle θ at the time of discharge of a sheet material. Therefore, the flight distance is constant regardless of the number of sheets in a bundle. In reality, the air resistance R acts on the sheet material, and the discharge trajectory of the sheet bundle is changed for each number of sheets in the bundle. 
     For this reason, if the discharge speed V 3  is constant regardless of the number of sheets in a bundle, variations occur in the flight distance of the sheet bundle from the discharge and reverse rollers  24 , as illustrated in  FIG.  8   , leading to a degradation of stackability or a discharge failure. If the flight distance of the discharged sheet bundle is too large, the sheet bundle may jump out the sheets already stacked on the upper discharge tray  25 . If the flight distance of the sheet bundle is insufficient, the trailing edge of the sheet bundle is caught by the discharge and reverse rollers  24 , which causes a discharge failure. 
     Therefore, according to the present embodiment, the discharge speed V 3  is set according to the number of sheets in a bundle to be discharged to avoid the above-described issues.  FIG.  9 A  illustrates the flight trajectories when the discharge speed is set to a constant value regardless of the number of sheets in a bundle, and  FIG.  9 B  illustrates the flight trajectories when the discharge speed is changed according to the number of sheets in a bundle. 
     As illustrated in  FIG.  9 A , when the discharge speed is set to a constant value (a discharge speed V 31 ) regardless of the number of sheets in a bundle, the flight distance of the bundle from the discharge and reverse rollers  24  varies. Accordingly, as illustrated in  FIG.  9 B , different discharge speeds are set. That is, the discharge speed for the 2 sheet bundle is set to V 32 , the discharge speed for the 3 sheet bundle is set to V 33 , and the discharge speed for the 4 sheet bundle is set to V 34 . As a result, as illustrated in  FIG.  9 B , the flight distance of the sheet bundle from the discharge and reverse rollers  24  can be made constant. As the number of sheets in a bundle increases, the mass of the bundle increases. Thus, the bundle of sheets is less likely to be influenced by the air resistance, so the flight distance of the bundle of sheets increases. Therefore, the discharge speed V 3  is set such that the discharge speed V 3  decreases with increasing number of sheets in the bundle. 
     That is, when the number of sheets in the sheet bundle is a first number of sheets (for example, 2 sheets), the discharge speed V 3  is set to a first discharge speed (for example, a discharge speed V 32 ). When the number of sheets in the sheet bundle is a second number of sheets that is greater than the first number of sheets (for example, 3 sheets), the discharge speed V 3  is set to a second discharge speed that is lower than the first discharge speed (for example, a discharge speed V 33 ). 
     As a result, regardless of the number of sheets in the bundle, the sheet bundle can be discharged to the target position, improving the stackability. Note that the discharge speed V 3  can be changed by the time the trailing edge of the sheet bundle passes through the discharge and reverse rollers  24  at the latest. 
     Instead of changing the discharge speed V 3  according to the number of sheets in the bundle, a threshold may be set for the number of sheets in a bundle. For example, when the number of sheets in the bundle is 2 or 3, the discharge speed V 3  may be set to the first discharge speed. When the number of sheets in the bundle is 4 (which is greater than or equal to the threshold), the discharge speed V 3  may be set to the second discharge speed (in this case, the threshold is 4 sheets). The discharge speed V 3  can be appropriately set according to the discharge angle of the discharge and reverse rollers  24 , the positional relationship between the upper discharge tray  25  and the discharge and reverse rollers  24 , and the like. 
     In some cases, the flight distance changes according to the basis weight of the sheet, giving an impact on the stackability. For example,  FIG.  10 A  illustrates the discharge trajectory of a sheet bundle S′w 1  with a basis weight of W 1 . The discharge speed V 3  is set to one of V 32  to V 34  according to the number of sheets in the bundle. In this setting, when a sheet bundle S′w 2  with a basis weight of W 2  is discharged, the flight distance differs from that of the sheet bundle S′w 1  with a basis weight of W 1  ( FIG.  10 B ). More specifically, the influence of air resistance increases with decreasing basis weight of the sheets and, thus, the flight distance of the bundle of the sheets decreases. 
     Accordingly, when a bundle (a first sheet bundle) consisting of a plurality of sheets of a first basis weight is formed, the discharge speed V 3  is set to a third discharge speed. When a bundle (a second sheet bundle) consisting of a plurality of sheets with a second basis weight (the sheets with a basis weight that is less than the first basis weight) is formed, the discharge speed V 3  is set to a fourth discharge speed. The fourth discharge speed is set so as to be greater than the third discharge speed. 
     In addition, in some cases, the flight distance varies according to the size of the sheet, giving an impact on the stackability. In this case, the setting of the discharge speed V 3  may be changed according to the sheet size and the number of sheets in the same manner as described above. Furthermore, in some cases, the curl direction of the sheets varies according to the print mode (for example, double-sided printing or single-sided printing) or the medium type ( FIGS.  11 A and  11 B ). The effect of the air resistance R on a sheet bundle may vary according to the curl direction, resulting in a difference in flight distance. As a result, variations in the flight distance of the discharged sheet bundle may occur, and the stackability may deteriorate. In this case, the setting of the discharge speed V 3  may be changed according to the print mode or the medium type and the number of sheets in the bundle in the same manner as described above. 
     As described above, according to the present embodiment, when a plurality of sheets that are continuously conveyed are discharged onto the first stacking member, the superposition processing unit  4 B can superpose the plurality of sheets while aligning the edges of the sheets and, thereafter, discharge the sheets. As a result, the stackability of the sheets on the first stacking member can be improved while maintaining the productivity. In addition, when the superposition processing unit  4 B superposes a plurality of sheets while aligning the edges of the sheets and, thereafter, discharge the sheets, the discharge speed can be changed according to the number of sheets that form the bundle. As a result, the flight distance of the sheet bundle discharged onto the first stacking member from the discharge and reverse rollers  24  can be made constant and, thus, the sheet stackability can be improved. 
     While the configuration example of the present embodiment has been described with reference to the maximum number of sheets that can be superposed by the superposition processing unit  4 B (the number of superposable sheets) being five, the number of superposable sheets can be appropriately changed according to the particular configuration of the superposition processing unit  4 B and the performance required for the superposition processing unit  4 B. 
     Buffer Operation Performed by Sheet Processing Device 
     The superposition processing unit  4 B of the present embodiment can also operate as a buffer unit that superposes and holds sheets received from the image forming apparatus  1  while the sheet processing device  71  is processing sheets. By performing the buffer operation, collision of sheets in the sheet processing device  71  is avoided without decreasing the productivity of the image forming apparatus  1 . As a result, the productivity of the image forming system  1 S is improved. 
     When the buffer operation is performed, the operation performed by the superposition processing unit  4 B is basically common to the superposed discharge operation, except that the bundle of superposed sheets is conveyed to the sheet processing device  71  via the internal discharge path  82 . That is, in the operations illustrated in  FIGS.  5 A to  5 G , the bundle of superposed sheets illustrated in  FIG.  5 F  is not discharged to the upper discharge tray  25  but is conveyed to the sheet processing device  71  via, for example, the internal discharge rollers  26 . In addition, after the sheet bundle is conveyed to the sheet processing device  71 , the succeeding sheets for which buffering is not needed are switched back one by one by the discharge and reverse rollers  24 , and the sheets are conveyed to the sheet processing device  71 . 
     In the buffer operation, a protrusion amount Kt ( FIG.  7 B ) may be set so that the leading edges of the superposed sheets are shifted from each other by a predetermined distance. In this case, it is desirable to set the protrusion amount Kt so that the lower sheet (the sheet S 1  in  FIG.  7 B ) in the sheet processing device  71  protrudes further downstream in the sheet conveying direction toward the sheet processing device  71 . In this way, the half-moon roller  33  can be brought into contact with each of the sheets in the bundle superposed through the buffer operation, and an aligning operation can be effectively performed. 
     As described above, the superposition processing unit  4 B of the present embodiment has the function of performing a superposed discharge operation when discharging sheets to the outside of the sheet processing apparatus  4  without the processing performed by the sheet processing device  71  and also has the function of buffering sheets to be processed by the sheet processing device  71 . As a result, the size and cost of the apparatus can be reduced as compared with the configuration including two mechanisms for superposing sheets in order to achieve the above-described two functions. 
     Modification 
     According to the present embodiment, the internal discharge path  82  serving as the second conveying path communicates with the sheet processing device  71 . However, a configuration in which the second conveying path communicates with a discharge destination other than the sheet processing device  71  may be employed. For example, the sheet processing device  71  may be removed, and a sheet conveyed via the internal discharge path  82  may be discharged to the lower discharge tray  37  without being processed. Alternatively, an configuration may be employed in which the second conveying path has a dead end and does not communicate with the outside of the sheet processing apparatus  4 . 
     In addition, while the present embodiment has been described with reference to the sheet discharge apparatus of the sheet processing apparatus  4  provided separately from the image forming apparatus  1 , the present technology is applicable to a sheet discharge apparatus that discharges a sheet from the image forming apparatus  1  or other apparatuses that handle sheets. 
     In addition, according to the present embodiment, the bundle discharge rollers  36  for discharging the sheets processed by the sheet processing device  71  to the lower discharge tray  37  can be applied as the discharge device. When the bundle discharge rollers  36  discharge a sheet bundle, the discharge speed may be appropriately set according to the number of sheets that form the sheet bundle. 
     Other Embodiments 
     The present invention can be also implemented by performing the following processing. That is, a program that provides at least one of the functions of the above-described embodiment is supplied to a system or apparatus via a network or a storage medium, and at least one processor of the computer of the system or apparatus reads and executes the program. Alternatively, the present invention is implemented by using a circuit (for example, an application specific integrated circuit (ASIC)) that provides the at least one function. 
     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. 2021-189216 filed Nov. 22, 2021, which is hereby incorporated by reference herein in its entirety.