Patent Publication Number: US-7584948-B2

Title: Sheet processing apparatus and control method therefor

Description:
This is a continuation of U.S. patent application Ser. No. 10/953,396 filed Sep. 29, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet processing apparatus that sequentially receives sheets discharged from an image forming apparatus and carries out post-processing on the sheets, and a control method therefor. 
   2. Description of the Related Art 
   Conventionally, there has been known an image forming system including an image forming apparatus, such as a copying machine or a laser beam printer, in which the image forming apparatus has connected thereto a sheet processing apparatus, such as a finisher, which is capable of performing various types of post processing needed by a user, such as bundle discharge processing and stapling processing, on sheets discharged from the image forming apparatus. 
   The sheet processing apparatus incorporated in the image forming system includes an intermediate tray where sheets with images formed thereon discharged one by one from the image forming apparatus are sequentially received and superimposed into a sheet bundle, and then post-processing, such as stapling processing, is performed on the sheet bundle, and a loading tray that receives each sheet bundle subjected to the post-processing, discharged from the intermediate tray. 
   The sheet processing apparatus carries out an aligning operation for aligning sheets whenever a sheet is discharged onto the intermediate tray. Further, when sheets for one bundle are discharged onto the intermediate tray, the sheet processing apparatus carries out not only the aligning operation but also other types of processing including stapling processing on the discharged sheet bundle, followed by carrying out a bundle discharge operation for discharging the sheet bundle onto the loading tray. Only after the discharge of the sheet bundle from the intermediate tray, the discharge of succeeding sheets onto the intermediate tray can be carried out. Therefore, sheets have to be discharged from the image forming apparatus at intervals of space corresponding to a time period needed for completing the post-processing of sheets on the intermediate tray. 
   To enable such discharge and post-processing of sheets, a first method has been known (e.g. in U.S. Pat. No. 6,199,850) in which the timing of forming an image on each sheet is adjusted to a time period needed for the post-processing of a sheet bundle to thereby adjust time intervals at which sheets are discharged from the image forming apparatus into the sheet processing apparatus, on a sheet-by-sheet basis. According to the first method, however, time intervals at which images are formed on sheets are increased, which results in reduced productivity of the image formation. 
   Further, a second method (buffering method) has been known in which the sheet processing apparatus keeps each sheet received from the image forming apparatus on standby in a conveying path leading to the intermediate tray, and sequentially superimposes succeeding sheets on the standby sheets (preceding sheets), one upon another, then discharging the stacked sheets which are on standby onto the intermediate tray at a time after the immediately preceding sheet bundle has been discharged from the intermediate tray onto the loading tray. The second method enables the image forming apparatus to form images on sheets and discharge the sheets having images formed thereon into the sheet processing apparatus at predetermined time intervals, regardless of a time period needed for post-processing. Therefore, it is possible to prevent reduction of productivity of the image formation. 
   However, when sheets to be kept on standby are superimposed one upon another by the above conventional buffering method, a preceding one has to be conveyed to a predetermined position and caused to wait at the position for arrival of the succeeding one. If the succeeding one arrives before the preceding one is brought into the standby position, ends of the sheets cannot be accurately aligned with each other when they are superimposed on upon the other. As a result, when a plurality of sheets, which are thus superimposed, are discharged onto the intermediate tray, sheet alignment in a conveyance direction (i.e. longitudinal alignment) becomes insufficient. Particularly when the sheet conveying speed is increased for enhancement of productivity of the image formation, not only the time intervals at which images are formed on sheets but also the space intervals between sheets being conveyed are shortened, which makes it difficult to accurately superimpose or align the sheets when they are placed on standby, i.e. during buffering (temporary storage at an intermediate location) thereof. 
   SUMMARY OF THE INVENTION 
   It is a first object of the present invention to provide a sheet processing apparatus and a control method therefor, which are capable of accurately superimposing or aligning sheets discharged from an image forming apparatus during buffering even when the space intervals between the sheets are reduced. 
   It is a second object of the present invention to provide a sheet processing apparatus and a control method therefor, which are capable of carrying out post-processing, such as sheet alignment and stapling processing, without necessitating adjustment of time intervals at which sheets are processed by an image forming apparatus and discharged therefrom, thereby ensuring high productivity. 
   To attain the above objects, in a first aspect of the present invention, there is provided a sheet processing apparatus that sequentially receives sheets discharged from an image forming apparatus and carries out post-processing on the sheets, comprising a sheet bundle stacking section that stacks sheets for one bundle, for carrying out the post-processing thereon, a sheet retaining section provided at a location upstream of the sheet bundle stacking section, for retaining a plurality of sheets discharged from the image forming apparatus, such that each preceding sheet and at least one succeeding sheet are superimposed one upon another, a sheet conveying section that receives the sheets discharged from the image forming apparatus and conveys the sheets to the sheet retaining section, and conveys the sheets superimposed one upon another by the sheet retaining section to the sheet bundle stacking section, and a conveyance control section that controls the sheet conveying section such that a conveying time period over which a sheet preceding a final one of the sheets to be superimposed one upon another by the sheet retaining section is conveyed over a predetermined path section by the sheet conveying section is shorter than a conveying time period over which the final one is conveyed over the predetermined path section. 
   Preferably, the conveyance control section controls the sheet conveying section such that a conveying speed at which the sheet preceding the final one is conveyed is higher than a conveying speed at which the final one is conveyed. 
   Preferably, the conveyance control section has a first conveying mode in which conveying time periods over which all the sheets to be superimposed one upon another by the sheet retaining section are conveyed over the predetermined path section are set to an equal conveying time period, and a second conveying mode in which the conveying time period over which the sheet preceding the final one is conveyed over the predetermined path section is shorter than the conveying time period over which the final one is conveyed over the predetermined path section, and the conveyance control section switches between the first conveying mode and the second conveying mode, depending on time intervals at which the sheets are discharged from the image forming apparatus. 
   More preferably, the conveyance control section controls the sheet conveying section such that in the second conveying mode, a conveying speed at which the sheet preceding the final one is conveyed is higher than a conveying speed at which the final one is conveyed. 
   More preferably, the conveyance control section controls the sheet conveying section such that the conveying time period in the second conveying mode over which the sheet preceding the final one is conveyed is shorter than the conveying time period in the first conveying mode. 
   To attain the above objects, in a second aspect of the present invention, there is provided a method of controlling a sheet processing apparatus that sequentially receives sheets discharged from an image forming apparatus and carries out post-processing on the sheets, the sheet processing apparatus including a sheet bundle stacking section that stacks sheets for one bundle, for carrying out the post-processing thereon, a sheet retaining section provided at a location upstream of the sheet bundle stacking section, for retaining a plurality of sheets discharged from the image forming apparatus, such that each preceding sheet and at least one succeeding sheet are superimposed one upon another, the method comprising a sheet conveying step of conveying the sheets discharged from the image forming apparatus to the sheet retaining section, and a conveyance control step of controlling sheet conveyance in the sheet conveying step such that a conveying time period over which a sheet preceding a final one of the sheets to be conveyed in the sheet conveying step and superimposed one upon another by the sheet retaining section is conveyed over a predetermined path section is shorter than a conveying time period over which the final sheet is conveyed over the predetermined path section. 
   The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal cross-sectional view showing the construction of an image forming apparatus equipped with a sheet processing apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram showing the arrangement of a controller that controls the overall operation of the image forming apparatus; 
       FIG. 3  is a view showing the appearance of an operating/display unit; 
       FIGS. 4A to 4C  are views showing soft keys arranged on a display section of the operating/display unit; 
       FIG. 5  is a longitudinal cross-sectional view showing the construction of a finisher; 
       FIG. 6  is a block diagram showing the arrangement of a finisher control section; 
       FIG. 7  is a view useful in explaining an aligning operation carried out on a processing tray; 
       FIG. 8  is a view useful in explaining the aligning operation carried out on the processing tray; 
       FIG. 9  is a view useful in explaining the aligning operation carried out on the processing tray; 
       FIG. 10  is a view showing a plurality of sheet bundles stacked on a stack tray when an alignment position is changed alternately on a sheet bundle-by-sheet bundle basis; 
       FIG. 11  is a view useful in explaining a two-point binding mode; 
       FIG. 12  is a view useful in explaining an inner-side oblique binding mode; 
       FIG. 13  is a view useful in explaining an outer-side oblique binding mode; 
       FIG. 14  is a view useful in explaining sheet conveyance in a non-sorting mode; 
       FIG. 15  is a view useful in explaining sheet conveyance in a stapling sorting mode; 
       FIG. 16  is a view useful in explaining sheet conveyance in a sorting operation for a second bundle (i.e. a bundle for a second copy); 
       FIG. 17  is a view useful in explaining the sheet conveyance in the sorting operation for the second bundle; 
       FIG. 18  is a view useful in explaining an operation of discharging a sheet bundle; 
       FIG. 19  is a view useful in explaining an operation of returning a sheet bundle; 
       FIGS. 20A and 20B  are views useful in explaining the operation of returning the sheet bundle; 
       FIG. 21  is a view useful in explaining sheet conveyance in a sorting operation for a second bundle in the case where three sheets are superimposed one upon another; 
       FIG. 22  is a view useful in explaining the sheet conveyance in the sorting operation for the second bundle in the case where three sheets are superimposed one upon another; 
       FIGS. 23A and 23B  are timing charts useful in explaining two modes of motor speed control executed for a winding operation by a buffer roller; 
       FIG. 24  is a view useful in explaining sheet conveyance in a sorting mode; 
       FIG. 25  is a view useful in explaining the sheet conveyance in the sorting mode; 
       FIG. 26  is a flowchart showing an operation mode-determining process; 
       FIG. 27  is a flowchart showing a non-sorting process; 
       FIG. 28  is a flowchart showing a sorting process; 
       FIG. 29  is a flowchart showing a stapling sorting process; 
       FIGS. 30A and 30B  are flowchart showing a sorted paper sequence process; 
       FIG. 31  is a flowchart showing a bundle discharge operation-determining process; 
       FIG. 32  is a flowchart showing a stapling process; and 
       FIG. 33  is a longitudinal cross-sectional view showing another arrangement of the finisher. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described in detail with reference to the drawings showing a preferred embodiment thereof. 
     FIG. 1  is a longitudinal cross-sectional view showing the construction of an image forming apparatus equipped with a sheet processing apparatus according to an embodiment of the present invention. The image forming apparatus is comprised of an image forming apparatus main unit  10 , and a finisher  500  implementing the sheet processing apparatus according to the embodiment. Further, the image forming apparatus main unit  10  includes an image reader  200  that reads original images, a printer  300 , and an operating/display unit  400 . 
   The image reader  200  is equipped with an original feeder  100 . The original feeder  100  sequentially feeds originals set on an original tray with their front surfaces facing upward, one by one from the leading page in a leftward direction as viewed in  FIG. 1 , such that the originals are guided along a curved path and conveyed from the left onto a platen glass  102  and then through a moving original reading position to the right, followed by being discharged to an external discharge tray  112 . 
   As each original passes the moving original reading position on the platen glass  102  from left to right, an image of the original is read by a scanner unit  104  held in a position corresponding to the moving original reading position. This reading method is generally called the “moving original reading method”. More specifically, as the original passes the moving original reading position, a surface of the original to be scanned is irradiated with light from a lamp  103  of the scanner unit  104 , and reflected light from the original is guided to a lens  108  via mirrors  105 ,  106 , and  107 . The light having passed through the lens  108  forms an image on an imaging surface of an image sensor  109 . 
   Each original is thus conveyed so as to pass the moving original reading position from left to right, whereby scanning is performed to read the original with a direction orthogonal to the conveying direction of the original as the main scanning direction and the conveying direction of the original as the sub scanning direction. More specifically, as the original passes the moving original reading position, the image of the original is read line by line in the main scanning direction by the image sensor  109  while the original is being fed in the sub scanning direction, whereby the whole original image is read. 
   The original image optically read by the image sensor  109  is converted into image data by the same for output to an image signal control section  202 , referred to hereinafter. The image data output from the image sensor  109  is subjected to predetermined processing by the image signal control section  202 , and then discharged as a video signal to an exposure control section  110  of the printer  300 . 
   Alternatively, it is also possible to convey the original to a predetermined position on the platen glass  102  and temporarily stop the same, and cause the scanner unit  104  to scan the original from left to right to thereby read the original. This reading method is the so-called “stationary original reading method”. 
   In the case of reading an original without using the original feeder  100 , first, a user lifts the original feeder  100  and places an original on the platen glass  102 , whereafter the scanner unit  104  is caused to scan the original from left to right to read the same. In short, when the original feeder  100  is not used for reading an original, stationary original reading is performed. 
   The exposure control section  110  of the printer  300  modulates a laser beam based on the video signal output from the image reader  200  and then outputs the modulated laser beam. The laser beam is irradiated onto a photosensitive drum  111  while being scanned by a polygon mirror  110   a . On the photosensitive drum  111 , an electrostatic latent image is formed according to the scanned laser beam. When stationary original reading is performed, the exposure control section  110  outputs the laser beam such that a proper image (non-mirror image) is formed. 
   The electrostatic latent image formed on the photosensitive drum  111  is visualized as a developer image by a developer supplied from a developing device  113 . On the other hand, a sheet is fed from a cassette  114  or  115 , a manual sheet feeder  125 , or a double-sided conveying path  124  and conveyed in between the photosensitive drum  111  and a transfer section  116  in timing synchronized with the start of irradiation of the laser beam. The developer image formed on the photosensitive drum  111  is transferred onto the fed sheet by the transfer section  116 . 
   The sheet having the developer image transferred thereon is conveyed to a fixing section  117 , and the fixing section  117  fixes the developer image on the sheet by heating and pressing the sheet. The sheet having passed through the fixing section  117  passes through a flapper  121  and discharge rollers  118  so as to be discharged from the printer  300  to the finisher  500  outside the image forming apparatus main unit  10 . 
   When the sheet is to be discharged face-down, i.e. with an image-formed surface thereof facing downward, the sheet having passed through the fixing section  117  is temporarily guided into an inverting path  122  by switching operation of the flapper  121 , and then, after the trailing edge of the sheet has passed through the flapper  121 , the sheet is switched back and discharged from the printer  300  by the discharge rollers  118 . This sheet discharge mode will be hereinafter referred to as “inverted discharge”. The inverted discharge is carried out when images are sequentially formed starting with the leading page, e.g. when images read using the original feeder  100  are formed or when images output from a computer are formed. The sheets thus discharged by the inverted discharge are stacked in the correct page order. 
   When a hard sheet, such as an OHP sheet, is supplied from the manual sheet feeder  125 , and an image is formed on this sheet, the sheet is not guided into the inverting path  122 , and hence discharged by the discharge rollers  118 , face-up, i.e. with an image-formed surface thereof facing upward. Further, when a double-sided printing mode for forming images on both sides of a sheet is set, the sheet is guided into the inverting path  122  by switching operation of the flapper  121 , and then conveyed to a double-sided conveying path  124 , followed by being fed in again between the photosensitive drum  111  and the transfer section  116  in the timing in synchronism with the start of irradiation of the laser beam. On the other hand, the sheet discharged from the printer  300  is sent to the finisher  500 . The finisher  500  performs post-processing on the sheet discharged from the printer  300 . 
     FIG. 2  is a block diagram showing the arrangement of a controller that controls the overall operation of the image forming apparatus. The controller is comprised of a CPU circuit section  150 , an original feeder control section  101 , an image reader control section  201 , the image signal control section  202 , a printer control section  301 , an operation display control section  401 , and a finisher control section  501 . An external computer  210  is connected to the image signal control section  202  via an external interface (I/F)  209 . 
   The CPU circuit section  150  incorporates a CPU  151 , a ROM  152 , and a RAM  153 , and performs centralized control of the above-mentioned sections, by the CPU  151  executing control programs stored in the ROM  152 . The RAM  153  temporarily stores control data, and is also used as a work area for carrying out arithmetic operations when the CPU  151  executes the control programs. 
   The original feeder control section  101  controls the original feeder  100  according to instructions from the CPU circuit section  150 . The image reader control section  201  controls the scanner unit  104 , the image sensor  109 , and so forth, and transfers an analog image signal output from the image sensor  109  to the image signal control section  202 . 
   The image signal control section  202  converts the analog image signal from the image sensor  109  into a digital signal, then performs various kinds of processing on the digital signal, and converts the processed digital signal into a video signal, followed by delivering the video signal to the printer control section  301 . Further, the image signal control section  202  performs various kinds of processing on a digital image signal input from the computer  210  via the external I/F  209 , and converts the processed digital image signal into a video signal, followed by delivering the video signal to the printer control section  301 . The operations executed by the image signal control section  202  are controlled by the CPU circuit section  150 . The printer control section  301  drives the exposure control section  110  based on the received video signal. 
   The finisher control section  501  is incorporated in the finisher  500 , and exchanges information with the CPU circuit section  150  to thereby control the overall operation of the finisher  500 . Details of this control will be described hereinafter. 
   The operation display control section  401  controls exchange of information with the operating/display unit  400  and the CPU circuit section  150 . The operating/display unit  400  includes a plurality of keys for configuring various functions for image formation, and a display section for displaying information indicative of the configurations, as described in detail hereinafter. The operation display control section  401  outputs key signals corresponding to respective operations of keys to the CPU circuit section  150 , and displays the corresponding pieces of information on the display section based on signals from the CPU circuit section  150 . 
     FIG. 3  is a view showing the appearance of the operating/display unit  400 . On the operating/display unit  400 , there are arranged a start key  402  for starting image forming operation, a stop key  403  for interrupting the image forming operation, a ten-key numeric keypad including keys  404  to  412  and  414  for setting input numbers, an ID key  413 , a clear key  415 , a reset key  416 , and so forth. 
   Further, the operating/display unit  400  includes a liquid crystal display  420  formed with a touch panel, on which soft keys are provided. For example, the image forming apparatus of the present invention has a non-sorting mode (group mode), a sorting mode, a stapling sorting mode (binding mode), and so forth, as post-processing modes of the finisher  500 . These modes are set or configured by input operations through the operating/display unit  400 . 
     FIGS. 4A to 4C  are views illustrating soft keys arranged on the display  420  of the operating/display unit  400 . When a “sorter” key  421  as a soft key is selected on an initial screen of the display  420  shown in  FIG. 4A , a menu option-selecting screen shown in  FIG. 4B  is displayed, and the processing mode is configured on the menu option-selecting screen. When a “stapling” key  422  as a soft key is selected on the menu option-selecting screen, a stapling position-setting screen shown in  FIG. 4C  is displayed. 
     FIG. 5  is a longitudinal cross-sectional view showing the construction of the finisher  500 . The finisher  500  performs various types of post-processing including processing for sequentially taking in sheets discharged from the image forming apparatus main unit  10  and aligning the sheets taken in into a bundle, a stapling process for stapling the trailing end of the sheet bundle by a stapler, a punching process for punching holes in the trailing end of the sheet bundle, a sorting process for sorting sheets taken in, and a non-sorting process. 
   In the state of the finisher  500  being connected to the image forming apparatus main unit  10 , if a sheet is discharged from the image forming apparatus main unit  10  by inverted discharge, with an image-formed surface thereof facing downward, the finisher  500  performs stapling and other types of processing on the fed sheet with the image-formed surface thereof facing downward. 
   The finisher  500  takes in a sheet discharged from the image forming apparatus main unit  10  by an inlet roller pair  502 , and conveys the sheet taken in to a buffer roller  505  via a conveying roller pair  503 . An inlet sensor  531  is disposed in a conveying path between the inlet roller pair  502  and the conveying roller pair  503 . In a conveying path between the conveying roller pair  503  and the buffer roller  505 , there is disposed a punching unit  550  which is operated, as required, to punch holes in a sheet conveyed thereto in the vicinity of the trailing edge thereof. 
   The buffer roller  505  is capable of winding a predetermined number of sheets conveyed thereto around the outer periphery thereof, and has pressing rollers  512 ,  513 , and  514  provided around the outer periphery thereof, for winding sheets therearound. The sheets wound around the outer periphery of the buffer roller  505  are conveyed in the direction of rotation (counterclockwise rotation, as viewed in  FIG. 5 ) of the buffer roller  505 . 
   A switching flapper  511  is disposed between the pressing rollers  513  and  514 , while a switching flapper  510  is disposed at a location downstream of the pressing roller  514 . The switching flapper  511  peels off the sheets wound around the buffer roller  505  to guide the same into a non-sorting path  521  or keeps the same in the state wound around the buffer roller  505 . On the other hand, the switching flapper  510  peels off the sheets wound around the buffer roller  505  to guide the same into the sorting path  522 , or simply guides the same into a buffer path  523 , in the state wound around the buffer roller  505 . 
   When the sheets wound around the buffer roller  505  are to be guided into the non-sorting path  521 , the switching flapper  511  is operated to peel off the sheets wound around the buffer roller  505  to guide the same into the non-sorting path  521 . The sheets guided into the non-sorting path  521  are discharged onto a sample tray  701  via a discharge roller pair  509 . In an intermediate portion of the non-sorting path  521 , there is disposed a sheet discharge sensor  533 . 
   When the sheets wound around the buffer roller  505  are to be guided into the buffer path  523 , neither the switching flapper  510  nor the switching flapper  511  is operated, and the sheets are sent into the buffer path  523  in the state wound around the buffer roller  505 . In an intermediate portion of the buffer path  523 , there is disposed a buffer path sensor  532  that detects a sheet on the buffer path  523 . 
   Further, when the sheets wound around the buffer roller  505  are to be guided into the sorting path  522 , not the switching flapper  511  but the switching flapper  510  is operated to peel off the sheets wound around the buffer roller  505  to guide the sheets into the sorting path  522 . The sheets guided into the sorting path  522  are stacked onto an intermediate tray (hereinafter referred to as the processing tray)  630  via conveying roller pairs  506  and  507 . The sheets stacked on the processing tray  630  as a bundle are subjected to the aligning processing, the stapling process, and so forth, as required, followed by being discharged onto a stack tray  700  by discharge rollers  680   a  and  680   b.    
   The discharge roller  680   b  is supported by a swinging guide  650 . The swinging guide  650  is swung by a swinging motor  660  to bring the discharge roller  680   b  into contact with a top sheet of a sheet bundle on the processing tray  630 . The discharge roller  680   b  in contact with the top sheet on the processing tray  630  cooperates with the discharge roller  680   a  to discharge the sheet bundle on the processing tray  630  toward the stack tray  700 . 
   The stapling process is performed by a stapler  601 . The stapler  601  is movably disposed along the rear end of the processing tray  630  such that the stapler  601  can staple the trailing end (right-side end, as viewed in  FIG. 5 ) of a sheet bundle stacked on the processing tray  630 . 
     FIG. 6  is a block diagram showing the arrangement of the finisher control section  501 . The finisher control section  501  includes a CPU  550 , a ROM  551 , and a RAM  552 . The finisher control section  501  communicates with the CPU circuit section  150  provided in the image forming apparatus main unit  10  via a communication IC, not shown, for data exchange, and executes various programs stored in the ROM  551  to control the driving of the finisher  500  according to instructions from the CPU circuit section  150 . Connected to the CPU  550  are an inlet motor M 1 , a buffer motor M 2 , a sheet discharge motor M 3 , a swinging guide motor M 150 , a paddle motor M 160 , a bundle discharge motor M 180 , the inlet sensor  531 , the buffer path sensor  532 , the sheet discharge sensor  533 , and so forth. 
     FIGS. 7 ,  8 , and  9  are views useful in explaining an aligning operation carried out on the processing tray  630 . When a first sheet is discharged from the image forming apparatus main unit  10  onto the processing tray  630 , laterally inner-side and outer-side aligning members  641  and  642  having been on standby at respective home positions (indicated by one-dot-chain lines) are moved to respective positions PS 11  and PS 21  to define space therebetween which is slightly larger in width than the width of sheets sequentially discharged (see  FIG. 7 ). 
   When the sheet discharged onto the processing tray  630  falls in between the aligning members  641  and  642  with a trailing edge thereof supported by a stopper  631 , the aligning member  641  is moved to a position PS 12  in timing in which the underside surface of the discharged sheet comes into contact with a support surface of the processing tray  630  (see  FIG. 8 ). As a result, the sheet is moved by the aligning member  641  to be aligned with a first alignment position  690 . Here, a central position on a sheet in the transverse direction thereof is referred to as an alignment position for convenience of description. After aligning the first sheet, the aligning member  641  returns to the position PS 11 , and is placed on standby there until a second sheet is discharged onto the processing tray  630 . 
   When the second sheet is discharged onto the processing tray  630 , the aligning member  641  is moved again to the position PS 12  to align the sheet with the first alignment position  690 . At this time, the inner-side aligning member  642  is held in the state standing in a position PS 22  to play the role of an alignment reference. The operation described above is continuously carried out until alignment of a final sheet of one sheet bundle is completed. After completion of the discharge and alignment of the sheets that form the first sheet bundle (i.e. the first copy), the sheets are discharged onto the stack tray  700 . 
   After the first sheet bundle is discharged onto the stack tray  700 , the aligning member  641  is moved from the position PS 12  to a position PS 13 , and the aligning member  642  is moved from the position PS 22  to a position PS 23  (see  FIG. 9 ). 
   Subsequently, when a first sheet (leading sheet) for a second sheet bundle (i.e. a second copy) is discharged onto the processing tray  630 , as in the case of the first sheet bundle, the sheet falls in between the aligning members  641  and  642  with a trailing edge thereof supported by the stopper  631 . Then, the aligning member  642  is moved from the position PS 23  to a position PS 24  in timing in which the underside surface of the discharged sheet comes into contact with the support surface of the processing tray  630 . 
   As a result, the sheet is moved by the aligning member  642  to be aligned with a second alignment position  691 . Before each of second and following sheets is discharged, the aligning member  642  is moved to the position PS 23 , and is placed on standby there until a succeeding sheet is discharged onto the processing tray  630 . 
   When the succeeding sheet is discharged onto the processing tray  630 , the aligning member  642  is moved again to the position PS 24  to align the sheet with the second alignment position  691 . At this time, the outer-side aligning member  641  is held in the state standing in the position PS 13  to play the role of an alignment reference. The operation described above is continuously carried out until alignment of a final sheet of one sheet bundle is completed. After completion of the discharge and alignment of the sheets that form the second sheet bundle (i.e. the second copy), the second sheet bundle is discharged onto the stack tray  700 . In this case, the sheet bundle aligned with the first alignment position  690  is at a location offset laterally inward of the sheet bundle aligned with the second alignment position  691 , by a predetermined distance (offset distance L). 
   Thus, the aligning operation is performed for changing the alignment position alternately on a sheet bundle-by-sheet bundle basis.  FIG. 10  is a view showing a plurality of sheet bundles stacked on the stack tray  700  by changing the alignment position alternately on a sheet bundle-by-sheet bundle basis. The alternate changes in the alignment position sort the sheet bundles such that they are alternately offset from each other by the offset distance L. 
   It should be noted that the offset distance L is set to respective different values for the sorting mode and the stapling sorting mode. For example, the offset distance L for the stapling sorting mode is set to a distance L 1  that prevents staples that bind respective adjacent sheet bundles from being overlapped one upon another in the stacked state of the sheet bundles. On the other hand, the offset distance L for the sorting mode is set to a distance L 2  that enables the sheet bundles to be reliably distinguished from each other. These offset distances L 1  and L 2  are set such that L 1 &lt;L 2  holds, so that the sorting process in the stapling sorting mode can be sped up. 
   Now, a description will be given of a stapling operation. In the present embodiment, the binding mode of the stapling operation by the stapler  601  includes a plurality of stapling modes, such as a one-point binding mode for outer-side oblique binding and inner-side oblique binding, and a two-point binding mode.  FIG. 11  is a view useful in explaining the two-point binding mode.  FIG. 12  is a view useful in explaining an inner-side oblique binding mode, and  FIG. 13  is a view useful in explaining an outer-side oblique binding mode. It should be noted that in  FIGS. 11 to 13 , solid lines represent sheets aligned with the first alignment position  690 , and two-dot chain lines represent sheets aligned with the second alignment position  691 . 
   In the stapling mode, the stapler  601  is on standby at a predetermined clinch position during alignment of each sheet, and then when the discharge and alignment of a final sheet of one sheet bundle is completed, the stapler  601  is moved by the offset distance L 1  set for the sheet bundle, and carries out the stapling operation. The stapler  601  is moved with its orientation changed according to the binding mode (the outer-side oblique binding mode, the inner-side oblique binding mode, or the two-point binding mode). 
   In the two-point binding mode, a stapling operation for stapling the trailing end of a sheet bundle aligned with the alignment position  690  or  691  at two points is performed (see  FIG. 11 ). In the inner-side oblique binding mode, a stapling operation for stapling the trailing end of a sheet bundle aligned with the alignment position  690  or  691  with the stapler  601  in a laterally inner oblique position is performed (see  FIG. 12 ). Further, in the outer-side oblique binding mode, a stapling operation for stapling the trailing end of a sheet bundle aligned with the alignment position  690  or  691  with the stapler  601  in a laterally outer oblique position is performed (see  FIG. 13 ). 
   When the alignment position of sheets to be discharged is located toward the outer-side of the processing tray  630 , the inner-side aligning member  642  makes reciprocating movement to shift each sheet toward the outer-side aligning member  641  as the alignment reference. On the other hand, when the alignment position of the sheets to be discharged is located toward the inner side of the processing tray  630 , the outer-side aligning member  641  makes reciprocating movement to shift each sheet toward the inner-side aligning member  642  as the alignment reference. 
   Next, a description will be given of a bundle discharge operation in the stapling mode. In a one-point stapling sorting mode, when the aligning operation described hereinbefore is completed, a stapling operation by the stapler  601  is started. Further, speed control of the swinging guide motor M 150  is performed such that lowering of the swinging guide  650  is started during the aligning operation or the stapling operation and the discharge roller  680   b  reaches a sheet bundle immediately before or after completion of the stapling operation. 
   The timing in which the lowering of the swinging guide  650  is started is changed according to the number of the sheets of a sheet bundle stacked on the processing tray  630 . More specifically, when the number of the sheets of a sheet bundle is small, the distance the discharge roller  680   b  travels to reach the sheet bundle is long whereas the operation time of the stapler  601  is short, and hence the lowering of the swinging guide  650  is started at an early stage of the aligning operation. On the other hand, when the number of the sheets of a sheet bundle is large, the distance the discharge roller  680   b  travels to reach the sheet bundle is short whereas the operation time of the stapler  601  is long, and hence the lowering of the swinging guide  650  is started almost simultaneously with the start of the stapling operation. 
   When a predetermined time period has elapsed, which is required for the discharge roller  680   b  to stop bounding after the discharge roller  680   b  has reached the sheet bundle, it is determined whether or not the stapling operation is completed. If the stapling operation is completed, the sheet bundle is discharged onto the stack tray  700  by the discharge rollers  680   a  and  680   b . On the other hand, if the stapling operation is not completed, the completion of the stapling operation is awaited. 
   In discharging a sheet bundle onto the stack tray  700  after completion of the stapling operation, control of the discharge speed of the sheet bundle is carried out. That is, the discharge speed is controlled such that the sheet bundle is conveyed at a relatively high speed after the start of the sheet conveyance, and the discharge speed of the sheet bundle is reduced immediately before the trailing end of the sheet bundle leaves the discharge rollers  680   a  and  680   b , to a suitable discharge speed for stacking onto the stack tray  700 . 
   On the other hand, in a two-point stapling sorting mode, when the stapler  601  is moved to a stapling position where a sheet bundle is to be stapled at a second point after completion of a stapling operation for stapling the sheet bundle at a first point, lowering of the swinging guide  650  is started. While the sheet bundle is stapled at the second point, the swinging guide  650  is on standby with the discharge roller  680   b  in contact with the sheet bundle. Then, the discharge roller  680   b  starts the bundle discharge operation upon completion of the stapling operation. Thereafter, the same operation as in the one-point binding mode is carried out. 
   Next, a description will be given of sheet conveyance in the finisher  500  in each of the non-sorting mode, the stapling sorting mode, and the sorting mode. 
     FIG. 14  is a view useful in explaining sheet conveyance in the non-sorting mode. When the non-sorting mode is designated by the user, the inlet roller pair  502 , the conveying roller pair  503 , and the buffer roller  505  are driven for rotation, whereby a sheet P discharged from the image forming apparatus main unit  10  is taken into the finisher  500  and conveyed. The switching flapper  511  is driven by a solenoid, not shown, so that the sheet P is conveyed from the buffer roller  505  to the non-sorting path  521  without being wound around the buffer roller  505 . Then, when the trailing edge of the sheet P is detected by the sheet discharge sensor  533 , the discharge roller pair  509  is rotated at a suitable conveyance speed for stacking sheets on the sample tray  701 , whereby the sheet P is discharged onto the sample tray  701 . 
     FIG. 15  is a view useful in explaining sheet conveyance in the stapling sorting mode. When the stapling sorting mode is designated by the user, the inlet roller pair  502 , the conveying roller pair  503 , and the buffer roller  505  are driven for rotation, whereby a sheet P discharged from the image forming apparatus main unit  10  is taken into the finisher  500  and conveyed. 
   The switching flappers  510  and  511  are held in stoppage at respective positions shown in  FIG. 15 , so that the sheet P is guided into the sorting path  522 . The sheet P guided into the sorting path  522  is discharged onto the processing tray  630  via the conveying roller pair  507 . When the sheet P is discharged onto the processing tray  630 , an auxiliary tray  670  projecting from the finisher  500  at a location slightly below the front end of the processing tray  630  prevents the sheet P discharged via the conveying roller pair  507 , from hanging downward or failing to return, and facilitates alignment of sheets on the processing tray  630 . 
   The sheet P discharged onto the processing tray  630  starts moving on the processing tray  630  toward the stopper  631  due to its own weight. This movement of the sheet P is assisted by an assisting member, such as a paddle, not shown. When the trailing edge of the sheet P is brought into contact with the stopper  631  and stopped, alignment of discharged sheets is performed by the aligning members  641  and  642 . When a predetermined number of sheets P is aligned, the stapling and bundle discharge operations described above are carried out, followed by the bundle of the sheets P being discharged onto the stack tray  700 . Since each sheet is discharged from the image forming apparatus main unit  10  with its image-formed surface facing downward, the bundle of the predetermined number of aligned sheets P has a leading page placed at the bottom thereof, with its image-formed surface facing downward and the following pages sequentially stacked on the leading page in page order. 
   Now, a description will be given of how sheets for forming a second sheet bundle (second copy) are conveyed during a time period from the start of taking-in of the sheets P for forming the first sheet bundle (first copy) to the discharge of the same as a sheet bundle.  FIGS. 16 and 17  are views useful in explaining sheet conveyance during operation of sorting the second sheet bundle. 
   When discharged from the image forming apparatus main unit  10 , a sheet P 1  as a first page for forming the second sheet bundle is wound around the buffer roller  505  by the switching operation of the switching flapper  510  (see  FIG. 16 ). The buffer roller  505  is stopped when the sheet P 1  is conveyed by a predetermined distance from the buffer path sensor  532 . 
   When the leading edge of a sheet P 2  as a second page is advanced from the inlet sensor  531  by a predetermined distance (50 mm in the present embodiment, as described hereinafter), the buffer roller  505  starts to be rotated, whereby the sheet P 2  is laid over the sheet P 1  wound around the buffer roller  505  such that the sheet P 2  is advanced from the sheet P 1  by a predetermined distance (see  FIG. 17 ). In other words, the sheet P 2  is laid over the sheet P 1  in a manner offset therefrom by the predetermined distance. The sheets P 1  and P 2  wound around the buffer roller  505  are peeled off the buffer roller  505  by the switching flapper  510  and conveyed as a two-sheet bundle P into the sorting path  522 . 
   At a time point the two-sheet bundle P is conveyed into the sorting path  522 , a bundle discharge operation for discharging the preceding sheet bundle P stacked on the processing tray  630  has been completed, and the swinging guide  650  is in a lowered state in contact with the sheet bundle P.  FIG. 18  is a view useful in explaining the operation of discharging a sheet bundle. The two-sheet bundle P discharged via the conveying roller pair  507  is brought in between the discharge rollers  680   a  and  680   b.    
     FIGS. 19 ,  20 A and  20 B are views useful in explaining the operation of returning a sheet bundle. At a time point the trailing end of a sheet bundle P passes through the conveying roller pair  507  and reaches the processing tray  630 , the discharge rollers  680   a  and  680   b  are driven for reverse rotation, whereby the sheet bundle P is moved toward the stopper  631  (see  FIG. 19 ). Before the trailing end of the sheet bundle P comes into abutment with the stopper  631 , the swinging guide  650  starts to be moved upward (see  FIG. 20A ), and the discharge roller  680   b  is moved away from the sheet surface (see  FIG. 20B ). 
   The sheet bundle P is conveyed in a state where the sheets thereof are offset from each other in the conveying direction. More specifically, the sheet P 2  is offset from the sheet P 1  in a direction away from the stopper  631 . The third and following sheets are discharged onto the processing tray  630  through the sorting path  522  as in the discharge operation for the first sheet bundle. After discharge of the second sheet bundle onto the stack tray  700 , the same operation is repeatedly carried out, whereby a predetermined number of sheet bundles (copies) are stacked on the stack tray  700 . 
   On the stack tray  700 , the sheet bundles are stacked in a manner alternately offset from each other (see  FIG. 10 ). Further, each of the sheet bundles has a leading page placed at the bottom thereof, with its image-formed surface facing downward and the following pages sequentially stacked on the leading page in page order. 
   It should be noted that although in the present embodiment, two sheets are superimposed by the buffer roller  505 , this is not limitative, but three or more sheets may be superimposed on upon another. For example, in the case of superimposing three sheets, sheet conveyance for a second sheet bundle is performed similarly to the sheet conveyance described hereinbefore with reference to  FIGS. 16 and 17  until a sheet P 2  for a second page of the second sheet bundle is superimposed on the a sheet P 1  for a first page of same. 
     FIGS. 21 and 22  are views useful in explaining sheet conveyance during operation of sorting the second sheet bundle in the case where three sheets are superimposed. The sheet P 2  is wound around the buffer roller  505  in a state superimposed on the sheet P 1 , and conveyed into the buffer path  523  in the state wound around the buffer roller  505 . Then, the buffer roller  505  is stopped again when the sheet P 2  is conveyed by a predetermined distance from the buffer path sensor  532  (see  FIG. 21 ). 
   Then, when the leading edge of a sheet P 3  as a third page is advanced from the inlet sensor  531  by a predetermined distance, the buffer roller  505  starts to be rotated again, whereby the sheet P 3  is superimposed on the sheet P 2  in a manner offset from the sheet P 2  such that the sheet P 3  is advanced from the sheet P 2  by a predetermined distance. The sheets P 1 , P 2  and P 3  wound around the buffer roller  505  are peeled off the buffer roller  505  by the switching flapper  510  and conveyed as a three-sheet bundle P into the sorting path  522  (see  FIG. 22 ). 
   In the present embodiment, there are provided two modes, i.e. a normal mode and a small sheet interval mode, for carrying out an operation of winding sheets around the buffer roller  505 . In the following, a description will be given of a motor speed control pattern in each of the modes.  FIGS. 23A and 23B  are timing charts useful in explaining the two modes of the motor speed control executed for the winding operation by the buffer roller  505 . In these figures, the position of a sheet described as “buffer motor reached” corresponds to the position of the sheet having reached the location of the pressing roller  512  (i.e. the position indicated by a symbol A in  FIG. 17 ). The position of a sheet described as “sheet meeting position reached” corresponds to the position of the sheet having reached a location (indicated by a symbol B in  FIG. 17 ) where a sheet wound around the buffer roller  505  and a sheet discharged from the image forming apparatus main unit  10  meet each other. 
     FIG. 23A  illustrates the normal mode of the motor speed control. When the sheet P 1  for the first page of the second bundle is discharged from the image forming apparatus main unit  10 , the inlet roller pair  502  and the conveying roller pair  503  are driven by the inlet motor M 1  at the same speed as a discharge speed V 1  of the image forming apparatus main unit  10  so as to ensure smooth sheet passage between the image forming apparatus main unit  10  and the finisher  500 . The sheet P 1  is then conveyed by the buffer roller  505 , which is driven by the buffer motor M 2  at the conveying speed V 1 , and the pressing rollers  512 ,  513 , and  514 , to be guided into the buffer path  523 . When the sheet P 1  is conveyed by a predetermined distance after the leading edge of the sheet P 1  is detected by the buffer path sensor  532 , the driving of the buffer motor M 2  is stopped to stop conveyance of the sheet P 1 . 
   Then, at a time point the leading edge of the sheet P 2  discharged from the image forming apparatus main unit  10  reaches a predetermined position in the finisher  500  (i.e. when the sheet P 2  is conveyed by 50 mm after having been detected by the inlet sensor  531 ), the buffer motor M 2  is started, and the conveying speed of the buffer roller  505  driven by the buffer motor M 2  is increased to the conveying speed V 1 . As a result, the sheets P 1  and P 2  superimposed one upon the other are guided into the sorting path  522  by being conveyed at the constant speed. When three or more sheets are superimposed, the superimposing operation is performed under the same motor speed control, followed by the sheets being guided into the sorting path  522 . 
     FIG. 23B  shows the small sheet interval mode of the motor speed control. When the sheet P 1  for the first page of the second bundle is discharged from the image forming apparatus main unit  10 , the inlet roller pair  502  and the conveying roller pair  503  are driven by the inlet motor M 1  at the same speed as the discharge speed V 1  of the image forming apparatus main unit  10  so as to ensure smooth sheet passage between the image forming apparatus main unit  10  and the finisher  500 . It should be noted that the small sheet interval mode may be set by a service man when the image forming apparatus main unit  10  with the discharge speed thereof being faster than a predetermined speed and the finisher  500  are installed, or may be set according to an instruction from the image forming apparatus main unit  10  depending upon an image forming mode then selected. 
   When the trailing end of the sheet P 1  leaves the image forming apparatus main unit  10 , the inlet motor M 1  is accelerated to a conveying speed V 2  which is faster than the discharge speed V 1  of the image forming apparatus main unit  10 . This makes it possible to increase the space interval between the sheet P 1  and the succeeding sheet P 2 . In doing this, if the trailing end of the final sheet for the first sheet bundle has passed between the buffer roller  505  and the pressing roller  514 , the conveying speed of the buffer roller  505 , which is driven by the buffer motor M 2 , and the pressing roller  512  is increased simultaneously. On the other hand, if the trailing end of the final sheet for the first sheet bundle has not passed between the buffer roller  505  and the pressing roller  514 , the buffer motor M 2  is accelerated to the conveying speed V 2  after the first sheet bundle has passed between the buffer roller  505  and the pressing roller  514 . 
   Thereafter, the sheet P 1  is conveyed at the conveying speed V 2  by the buffer roller  505  and the pressing rollers  512 ,  513  and  514  to be guided into the buffer path  523 . When the sheet P 1  is conveyed by a predetermined distance after the leading edge of the sheet P 1  is detected by the buffer path sensor  532 , the driving of the buffer motor M 2  is stopped to stop conveyance of the sheet P 1 . In the meantime, when the trailing end of the sheet P 1  has passed through the conveying roller pair  503  driven by the inlet motor M 1 , the inlet motor M 1  is decelerated to the conveying speed V 1  at which the following sheet P 2  is to be discharged from the image forming apparatus main unit  10 . 
   Then, at a time point the leading edge of the sheet P 2  discharged from the image forming apparatus main unit  10  reaches a predetermined position in the finisher  500 , the buffer motor M 2  is started, and the conveying speed of the buffer roller  505  driven by the buffer motor M 2  is increased to the conveying speed V 1 . As a result, the sheets P 1  and P 2  superimposed one upon the other are guided into the sorting path  522  by being conveyed at the constant speed. 
   The sheet P 2  is guided into the sorting path  522  after being superimposed on the sheet P 1 , and hence, differently from the case of conveying the sheet P 1 , when the trailing end of the sheet P 2  leaves the image forming apparatus main unit  10 , the inlet motor M 1  is not accelerated to the conveying speed V 2 . This is because if the conveying speed of the sheet P 2  were increased to the conveying speed V 2 , the space interval between the sheet P 2  and the final sheet for the first sheet bundle would be reduced. 
   However, when a time period between discharge of the final sheet for the first sheet bundle onto the processing tray  630  and discharge of the sheet bundle of the sheets P 1  and P 2  superimposed one upon the other onto the processing tray  630  is longer than a time period needed for post-processing, such as the aligning processing and the stapling process, executed on the processing tray  630 , the inlet motor M 1  may be accelerated to a conveying speed V 3  which is higher than the conveying speed V 1  and lower than the conveying speed V 2  so as to convey the sheet P 2  at the accelerated speed V 3 . 
   Although the present embodiment relates to the conveyance control executed in the case where two sheets are superimposed, similar conveyance control may be executed in the case where three or more sheets are superimposed. In this case, the conveying speed for the final one of the sheets to be superimposed is not increased to prevent reduction of the space interval between the final one of the superimposed sheets and the final sheet for the first bundle sheet, but the other sheets preceding the final one are conveyed at an increased conveying speed, whereby the sheet superimposing operation is carried out with an increased inter-sheet distance. The motor speed control is performed in parallel with a sorted sheet sequence process, described in detail hereinafter, which is executed by the CPU  550 . 
     FIGS. 24 and 25  are views useful in explaining sheet conveyance in the sorting mode. When the sorting mode is set, the inlet roller pair  502  and the conveying roller pair  503  are driven for rotation, as in the case of the stapling sorting mode, whereby sheets discharged from the image forming apparatus main unit  10  are sequentially stacked onto the processing tray  630  (see  FIG. 24 ). Thereafter, a bundle discharge operation described hereinbefore is carried out to discharge a sheet bundle P onto the stack tray  700 . 
   In the meantime, a sheet P 1  discharged from the image forming apparatus main unit  10  is wound around the buffer roller  505  by the operation of the switching flapper  510 , and the buffer roller  505  is stopped when the sheet P 1  is advanced by a predetermined distance from the buffer path sensor  532  (see  FIG. 25 ). Then, when the leading edge of a succeeding sheet P 2  is advanced from the inlet sensor  531  by a predetermined distance, the buffer roller  505  starts to be rotated, whereby the succeeding sheet P 2  is superimposed on the sheet P 1  such that it is advanced from the sheet P 1  by a predetermined distance. 
   The conveying operation is thus carried out, in the same manner as in the stapling sorting mode described above, and a predetermined number of sheet bundles are stacked on the stack tray  700  in a manner alternately offset from each other. The sheet bundles are stacked one upon another in such a manner that the leading page of each sheet bundle is placed at the bottom of the sheet bundle, with its image-formed surface facing downward and the following pages sequentially stacked on the leading page in page order. 
   Next, a description will be given of the bundle discharge operation of the sheet processing apparatus (finisher) configured as above. The finisher control section  501  provides control to carry out sheet conveyance in each of the non-sorting mode, the stapling sorting mode, and the sorting mode. In the finisher control section  501 , processing depending on a designated mode is carried out by related parts of the section  501  according to instructions from the CPU circuit section  150 . 
     FIG. 26  is a flowchart of an operation mode-determining process. A program for executing the operation mode-determining process is stored in the ROM  551  of the finisher control section  501  and executed by the CPU  550 . First, turn-on of a finisher start signal is awaited (step S 1 ). The finisher start signal is turned on when the start key  402  is depressed which is provided on the operating/display unit  400  of the image forming apparatus main unit  10 , for starting copying, and then a signal for starting a finisher operation is input from the image forming apparatus main unit  10  to the CPU  550  of the finisher control section  501  via the communication IC. When the finisher start signal is turned on, the CPU  550  starts driving the inlet motor M 1 , the buffer motor M 2 , the sheet discharge motor M 3 , and so forth (step S 2 ). On the other hand, when the finisher start signal is not turned on, the finisher  500  enters a standby state. 
   Then, the operating mode that is set is determined (step S 3 ). If the set operating mode is the non-sorting mode, the non-sorting process is executed (step S 4 ). If the set operating mode is the sorting mode, the sorting process is executed (step S 5 ). Further, if the set operating mode is the stapling sorting mode, a stapling sorting process is executed (step S 6 ). The non-sorting process, the sorting process, and the stapling sorting process will be described in detail hereinafter. 
   When the process in the determined set mode, i.e. the process in a corresponding one of the steps S 4  to S 6  is completed, the driving of the inlet motor M 1 , the buffer motor M 2 , the sheet discharge motor M 3 , and so forth is stopped (step S 7 ). Then, the process returns to the step S 1 , wherein the finisher  500  enters the standby state. 
     FIG. 27  is a flowchart of the non-sorting process executed in the step S 4  in  FIG. 26 . In the non-sorting process, a sheet P is guided onto the sample tray  701 , and therefore the flapper  511  is driven to select the non-sorting path  521  as a path into which the sheet P is to be conveyed (step S 11 ). Then, it is determined whether or not the finisher start signal is on (step S 12 ). If the finisher start signal is on, the sheet P discharged from the image forming apparatus main unit  10  is guided into a sheet path through the finisher  500 . 
   When the sheet P guided into the sheet path is conveyed via the inlet roller pair  502  driven by the inlet motor M 1 , turn-on of a signal from the inlet (path) sensor  531  in response to detection of the leading edge of the sheet P thereby is awaited (step S 13 ). When the signal from the path sensor  531  is turned on, turn-off of the signal from the path sensor  531  in response to leaving of the trailing end of the conveyed sheet P therefrom is awaited (step S 14 ). When the signal from the path sensor  531  is turned off, the process returns to the step S 12 . If it is determined again in the step S 12  that the finisher start signal is on, the steps S 13  and S 14  are repeatedly executed. 
   On the other hand, when the finisher start signal is turned off, discharge of all sheets onto the sample tray  701  is awaited (step S 15 ). Then, when the discharge of all the sheets is completed, the switching operation of the flapper  511  is cancelled (step S 16 ) to terminate the non-sorting process, followed by the process returning to the main process. 
     FIG. 28  is a flowchart of the sorting process executed in the step S 5  in  FIG. 26 . In the sorting process, the sheet P is guided onto the processing tray  630 , and therefore the flapper  511  is driven to select the sorting path  522  as a path into which the sheet P is to be conveyed (step S 21 ). Then, it is determined whether or not the finisher start signal is on (step S 22 ). If the finisher start signal is on, the sheet P discharged from the image forming apparatus main unit  10  is guided into a sheet path through the finisher  500 . When the sheet P guided into the sheet path is conveyed via the inlet roller pair  502  driven by the inlet motor M 1 , turn-on of the signal from the path sensor  531  in response to detection of the leading edge of the sheet P thereby is awaited (step S 23 ). 
   When the signal from the path sensor  531  is turned on, the sorted sheet sequence process is started (step S 24 ), and turn-off of the signal from the path sensor  531  in response to leaving of the trailing end of the conveyed sheet P therefrom is awaited (step S 25 ). When the signal from the path sensor  531  is turned off, the process returns to the step S 22 . If it is determined again in the step S 22  that the finisher start signal is on, the steps S 23 , S 24 , and S 25  are repeatedly executed. On the other hand, when the finisher start signal is turned off, discharge of all the sheets onto the processing tray  630  is awaited (step S 26 ). Then, when the discharge of all the sheets is completed, the switching operation of the flapper  511  is cancelled (step S 27 ) to terminate the sorting process, followed by the process returning to the main process. 
     FIG. 29  is a flowchart of the stapling sorting process executed in the step S 6  in  FIG. 26 . In the stapling sorting process, the sheet P is guided into the processing tray  630 , and therefore, first, the flapper  511  is driven to select the sorting path  522  as a path into which the sheet P is to be conveyed (step S 31 ). 
   Then, it is determined whether or not the finisher start signal is on (step S 32 ). If the finisher start signal is on, the sheet P discharged from the image forming apparatus main unit  10  is guided into the sheet path in the finisher  500 . When the sheet P guided into the sheet path is conveyed via the inlet roller pair  502  driven by the inlet motor Ml, turn-on of the signal from the path sensor  531  in response to detection of the leading edge of the sheet P thereby is awaited (step S 33 ). Then, when the signal from the path sensor  531  is turned on, the sorted sheet sequence process is started (step S 34 ). 
   Further, turn-off of the signal from the path sensor  531  in response to leaving of the trailing end of the conveyed sheet P therefrom is awaited (step S 35 ). When the signal from the path sensor  531  is turned off, the process returns to the step S 32 . If it is determined again in the step S 32  that the finisher start signal is on, the steps S 33 , S 34 , and S 35  are repeatedly executed. On the other hand, when the finisher start signal is turned off, discharge of all the sheets onto the processing tray  630  is awaited (step S 36 ). Then, when the discharge of all the sheets is completed, the switching operation of the flapper  511  is cancelled (step S 37 ), followed by terminating the stapling sorting process. 
     FIGS. 30A and 30B  are flowchart of the sorted sheet sequence process executed in the step S 24  in  FIG. 28  and the step S 34  in  FIG. 29 . The sorted sheet sequence process is carried out on each conveyed sheet, and a program for executing this process is stored in the ROM  551  and executed by the CPU  550  by multi-task processing (parallel processing). 
   First, it is determined whether or not a sheet to be conveyed is a sheet to be wound around the buffer roller  505  (hereinafter referred to as “the wound sheet”) (step S 41 ). If the sheet is designated as the wound sheet, it is determined whether the conveyance control mode for carrying out a winding operation is set to the normal mode or the small sheet interval mode (step S 42 ). This determination is carried out by comparing a time interval (sheet discharge interval) between discharge of a preceding sheet discharged from the image forming apparatus main unit  10  into the finisher  500  and discharge of a following sheet discharged from the same, with a time interval (sheet winding interval) required for conveying the preceding sheet on standby at a predetermined position on the buffer path  523  to the sheet meeting position and superimposing the preceding sheet and the following sheet assuming that the winding operation is carried out at a conveying speed at which each sheet discharged from the image forming apparatus main unit  10  into the finisher  500  is conveyed. When the sheet discharge interval is longer than the sheet winding interval, the conveyance control mode is set to the normal mode (step S 43 ). 
   On the other hand, when the sheet winding interval is longer than the sheet discharge interval, the conveyance control mode is set to the small sheet interval mode (step S 44 ). Information on the sheet discharge interval has already been notified by the CPU circuit section  150  to the finisher control section  501  at a time point the preceding sheet was conveyed into the finisher  500 . Thereafter, the process proceeds to a step S 45 . 
   If the sheet is not designated as the wound sheet in the step S 41 , the setting of the conveyance control mode is not executed. Then, when the sheet is conveyed from the path sensor  531  by a predetermined distance (50 mm in the present embodiment) (step S 45 ), the buffer motor M 2  is started to drive the buffer roller  505  (step S 46 ). At this time, the sorted sheet sequence process is started in response to turn-on of the signal from the path sensor  531 , so that the buffer motor M 2  is started when the leading edge of the sheet is advanced 50 mm downstream of a position where it was when the signal from the path sensor  531  was turned on. The start of the buffer motor M 2  is intended to further convey the sheet and to restart the “wound sheet” which is in stoppage in a state wound around the buffer roller  505 . By starting the buffer motor M 2  in this timing, the sheet in the state superimposed on the wound sheet can be conveyed together therewith. 
   It should be noted that although in the present embodiment, as a condition for defining the timing in which the buffer motor M 2  is started, the distance over which the leading edge of the sheet is advanced downstream of the position where it was when the signal from the path sensor  531  was turned on is set to 50 mm, the distance can be set to another value as desired. 
   Then, the sheet is conveyed by a predetermined distance (150 mm in the present embodiment) (step S 47 ), and it is determined whether or not the sheet is a wound sheet (step S 48 ). If the sheet has been designated as a wound sheet, the flapper  510  is driven to select the buffer path  523  as a path into which the sheet is to be conveyed (step S 49 ). Sheet conveyance is continued in this state, whereby the sheet can be guided into the buffer path  523  for winding the sheet around the buffer roller  505 . 
   Then, turn-on of the signal from the path sensor  532  disposed in the buffer path  523  is awaited (step S 50 ), and when the signal from the path sensor  532  is turned on, stop control for stopping the buffer motor M 2  is started to wind the sheet around the buffer roller  505  (step S 51 ). When the leading edge of the sheet passes by the path sensor  532 , winding control for stopping the buffer roller  505  is carried out. In this case, the buffer roller  505  is stopped with an overrun amount of the buffer roller  505  taken into consideration. 
   After the buffer roller  505  is stopped, the sheet wound around the buffer roller  505  is placed on standby as it is until the buffer roller  505  is restarted for another succeeding sheet. Then, after restart of the buffer roller  505 , completion of discharge of the sheet onto the processing tray  630  is awaited (step S 52 ), and upon completion of the discharge, the count of a discharge counter indicative of the number of sheets discharged onto the processing tray  630  is incremented by one (step S 53 ), followed by terminating the present process to return to the main process. 
   On the other hand, if it is determined in the step S 48  that the sheet is not a wound sheet, the flapper  510  is driven to select the sorting path  522  as a path into which the sheet is to be conveyed (step S 54 ). By driving the flapper  510 , the sheet is guided not into the buffer path  523 , but into the sorting path  522  as a discharge path leading to the processing tray  630 . Then, completion of discharge of the sheet onto the processing tray  630  is awaited (step S 55 ), and after completion of the discharge is confirmed, the count of the discharge counter is incremented by one (step S 56 ). Thereafter, the sheet is aligned with a sheet aligning position set on a sheet-by-sheet basis, using the two aligning members  641  and  642  (step S 57 ). In this step, the aligning operation for the sheet is performed in a direction approximately orthogonal to the sheet conveying direction upon discharge of the sheet onto the processing tray  630 , and by turning the paddle, not shown, the sheet is also aligned in the sheet conveying direction. Thereafter, a bundle discharge operation-determining process, described in detail hereinafter, is executed (step S 58 ), followed by terminating the present process to return to the main process. 
     FIG. 31  is a flowchart of the bundle discharge operation-determining process executed in the step S 58  in  FIG. 30B . First, it is determined whether or not the operating mode is set to the stapling sorting mode (step S 61 ). If it is determined that the operating mode is set to the stapling sorting mode, it is determined whether or not sheets discharged onto the processing tray  630  are to be discharged in a bundle (step S 63 ). If the sheets are not to be discharged in a bundle, the present process is terminated to return to the sorted sheet sequence process as the higher-order process. On the other hand, if the sheets discharged onto the processing tray  630  are to be discharged in a bundle, the stapling process is executed (step S 64 ). The stapling process will be described in detail hereinafter. 
   After completion of stapling process for the sheet bundle on the processing tray  630 , the swinging guide  650  is lowered to bring the sheet discharge roller  680   b  into contact with the sheet bundle on the processing tray  630  (step S 65 ). 
   Then, after the sheet discharge roller  680   b  stops bounding, the sheet discharge roller  680   b  is driven by a predetermined amount to discharge the sheet bundle from the processing tray  630  onto the stack tray  700  while controlling the speed of the bundle discharge motor M 180  (step S 66 ). 
   Then, the stack tray  700  is lifted up and down, whereby the operation of stacking the sheet bundle onto the stack tray  700  is completed (step S 67 ). Thereafter, the count of the discharge counter is set to a value of 0 (step S 68 ), followed by terminating the present process to return to the sorted sheet sequence process as the higher-order process. 
   On the other hand, if it is determined in the step S 61  that the operating mode is not set to the stapling sorting mode, it is determined in a step S 62  whether or not the sheets discharged onto the processing tray  630  are to be discharged in a bundle. If the sheets are not to be discharged in a bundle, the present process is terminated to return to the sorted sheet sequence process as the higher-order process. 
   On the other hand, if it is determined in the step S 62  that the sheets discharged onto the processing tray  630  are to be discharged in a bundle, the swinging guide is operated in the step S 65  to bring the sheet discharge roller  680   b  into contact with the sheet bundle on the processing tray  630 . Then, after the sheet discharge roller  680   b  stops bounding, the sheet discharge roller  680   b  is driven by a predetermined amount in the step S 66  to discharge the sheet bundle from the processing tray  630  onto the stack tray  700  while controlling the speed of the bundle discharge motor M 180 . 
   Then, the stack tray  700  is lifted up and down in the step S 67 , whereby the operation of stacking the sheet bundle onto the stack tray  700  is completed. Thereafter, the count of the discharge counter is set to the value of 0 in the step S 68 , followed by terminating the present process to return to the sorted sheet sequence process as the higher-order process. 
     FIG. 32  is a flowchart of the stapling process executed in the step S 64  in  FIG. 31 . First, the stapler  601  is moved by a predetermined amount to a stapling position (step S 71 ), where the sheet bundle on the processing tray  630  is aligned by the outer-side aligning member  641  and the inner-side aligning member  642  (step S 72 ), and then a stapling operation is carried out (step S 73 ). 
   Thereafter, it is determined whether or not the two-point binding mode of the stapling operation has been designated (step S 74 ). If the two-point binding mode has not been designated, the alignment of the sheet bundle by the outer-side aligning member  641  and the inner-side aligning member  642  is cancelled (step S 77 ), followed by terminating the present process to return to the bundle discharge operation-determining process as the higher-order process. 
   On the other hand, if the two-point binding mode has been designated in the step S 74 , the stapler  601  is moved by a predetermined amount to a second stapling position (step S 75 ), where a stapling operation for stapling the sheet bundle at a second point is carried out (step S 76 ), and the alignment of the sheet bundle by the outer-side aligning member  641  and the inner-side aligning member  642  is cancelled (step S 77 ), followed by terminating the present process to return to the bundle discharge operation-determining process as the higher-order process. 
   As described above, according to the sheet processing apparatus of the present embodiment, even when the interval between sheets discharged from the image forming apparatus main unit  10  is short, a plurality of sheets can be accurately superimposed by the buffer roller  505 . 
   Although in the present embodiment, the buffer mechanism of a type in which sheets are wound around a roller is employed, this is not limitative, but a buffer mechanism with another configuration may be employed. For example, as shown in  FIG. 33 , a buffer path  801  into and from which sheets are conveyed in a switch-back manner may be formed in a conveying path in a finisher  800  to superimpose sheets one upon another in the buffer path  801 . 
   With this configuration, when two sheets are to be superimposed in the small sheet interval mode, a first sheet is conveyed to a path  802 , and then the sheet is switched back and conveyed into the buffer path  801  to be kept on standby therein. In this case, when the trailing end of the first sheet leaves the image forming apparatus, the conveying speed is changed from V 1  to V 2  so as to convey the first sheet into the buffer path  801  with an increased space interval from a second sheet. Then, the second sheet is conveyed at the speed V 1 , and the first sheet is conveyed from the buffer path  801  such that the second sheet conveyed from the image forming apparatus can be superimposed on the first sheet, in a state offset therefrom by a predetermined distance or offset. Thereafter, the two sheets may be conveyed to the post-processing section. 
   When three sheets are to be superimposed, first and second sheets are conveyed at the speed V 2 , and a third one is conveyed at the speed V 1 . 
   It is further understood by those skilled in the art that the foregoing is a preferred embodiment of the invention, and that various changes and modification may be made without departing from the spirit and scope thereof. 
   CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2003-341906 filed Sep. 30, 2003, which is hereby incorporated by reference herein.