Patent Publication Number: US-10315880-B2

Title: Sheet processing apparatus and image forming apparatus including the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus for applying processing to sheets and an image forming apparatus, and more particularly to improvement of sheet alignment when discharging a sheet bundle from a placing tray which processes sheets. 
     2. Description of the Related Art 
     Some image forming apparatuses, like a copying machine, a laser beam printer, a facsimile, and combined machines thereof, may conventionally include a sheet processing apparatus for performing sheet processing such as binding processing and sort processing on image-formed sheets. 
     Such an image forming apparatus performs so-called buffer processing in which a subsequent sheet or sheets are once kept on standby to reduce delay of carry-in of subsequent sheets while the binding or sort processing is performed on a preceding sheet bundle on a placing tray. Keeping one or a plurality of subsequent sheets on standby reduces the chances to stop the carry-in of subsequent sheets if the sheet processing on the sheet bundle on the placing tray takes some time. 
     A sheet processing apparatus capable of higher speed and greater capacity has been desired in recent years. To meet such a demand, an apparatus described in Japanese Patent No. 4058374 (corresponding U.S. Pat. No. 7,165,764 B2) has been proposed. In this apparatus, a plurality of subsequent sheets mentioned above and a sheet bundle on a placing tray are nipped by discharge rollers in an overlapping manner, and simultaneously transported to a stacking tray side (see FIG. 26 of the foregoing patent literature). 
     The sheet bundle on the placing tray is then discharged to the stacking tray. At this point, the discharge rollers are rotated backward to store the subsequent sheets into the placing tray (see FIGS. 27 and 28 of the foregoing patent literature). Since the discharge of the sheet bundle from the placing tray and the transportation of the subsequent sheets are simultaneously performed for so-called simultaneous bundle discharge, the discharge time of the sheets can be reduced, compared to when the buffer processing is performed. 
     The sheet processing apparatus according to the foregoing patent literature seldom causes a problem if the sheet bundle placed on the placing tray is bounded by a binding unit such as a stapling unit. However, it has found that the following problem occurs if the simultaneous bundle discharge described above is performed on unbound sheet bundles, like when unbound sheet bundles are discharged to the stacking tray by changing the placing position on the placing tray sheet bundle by sheet bundle. 
     The problem will be described with reference to  FIGS. 29A to 29D  accompanying the present specification.  FIGS. 29A to 29D  show a sheet processing apparatus which performs simultaneous bundle discharge similar to that of FIG. 26 to FIG. 28 of the foregoing patent literature. In the accompanying  FIG. 29A , an unbound sheet bundle TB 2  is placed on a placing tray Tr. A preceding sheet bundle TB 1  is already stacked on a stacking tray TE on the downstream side of the placing tray Tr. The sheet bundle TB 1  is not bound, either, and is shifted from the sheet bundle TB 2  in a sheet width direction for the sake of sorting. In such a state, as shown in  FIG. 29A , two subsequent sheets np 1  and np 2  are conveyed by conveyance rollers HR. 
     Next, as shown in  FIG. 29B , the sheet bundle TB 2  is pushed by a pushing member Ph, which reciprocates on the placing tray Tr, in advance to precede the subsequent sheets np 1  and np 2 . The subsequent sheets np 1  and np 2  conveyed afterward and the sheet bundle on the placing tray Tr are nipped together by discharge rollers ER in an overlapping state (state in which the sheet bundle TB 2  precedes), and transported toward the stacking tray TE. 
     By the transportation by the discharge rollers ER, the sheet bundle TB 2  is discharged to the stacking tray. For example, if the sheets here curl upward, as shown in  FIG. 29C , the topmost sheet of the sheet bundle TB 2  is pushed by the subsequent sheets np 1  and np 2  to deteriorate alignment on the stacking tray TE. 
     If the sheet bundle TB 2  on the placing tray Tr is stacked on the stacking tray TE, the discharge rollers ER then rotate in reverse directions to store the subsequent sheets np 1  and np 2  into the placing tray. Since the simultaneously-discharged sheet bundle is not bound, as shown in  FIG. 29D , the sheets electrostatically adhere to the switched-back subsequent sheets np 1  and np 2  and are conveyed backward with the subsequent sheets np 1  and np 2 . This also deteriorates alignment. 
     The subsequent sheets np 1  and np 2  can be discharged after the sheet bundle TB 2  on the placing tray is discharged to the stacking tray TE. However, simply delaying the discharge of the subsequent sheets np 1  and np 2  increases discharge time. 
     It is thus an object of the present invention to provide an apparatus which discharges subsequent sheets and a bundle on the placing tray without hindrance to each other to stack sheet bundles without deteriorating alignment or impairing rapidity even if the sheet bundle on the placing tray is not bound. 
     SUMMARY OF THE INVENTION 
     To solve the foregoing problem, the present invention includes the following configuration: 
     A sheet processing apparatus including a sheet processing unit that processes a sheet placed on a placing tray, a discharge roller that discharges the sheet processed on the placing tray to a stacking tray, a conveyance roller that discharges a sheet to the placing tray or conveys a subsequent sheet from upstream to downstream of the discharge roller and switchback-conveys the subsequent sheet upstream again, and a standby path that keeps a sheet switched back by the conveyance roller on standby, wherein when the subsequent sheet is conveyed to a downstream side of the discharge roller by the conveyance roller and then returned to an upstream side by switchback conveyance, the discharge roller nips and discharges the sheet on the placing tray to the stacking tray during the switchback conveyance. 
     Consequently, there can be provided an apparatus that discharges a bundle on the placing tray during the switchback conveyance of subsequent sheets to stack sheet bundles without much deteriorating alignment or impairing rapidity even if the sheet bundle on the placing tray is not bound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram showing an overall configuration of a combination of an image forming apparatus and a sheet processing apparatus according to the present invention in combination. 
         FIG. 2  is an overall explanatory diagram showing the sheet processing apparatus according to the present invention. 
         FIG. 3  is an enlarged side explanatory diagram near a processing tray (placing tray) of the sheet processing apparatus. 
         FIG. 4  is a driving explanatory diagram showing conveyance rollers, a branch roller, and discharge rollers. 
         FIG. 5  is an explanatory diagram showing a configuration for moving a binding unit arranged on a reference surface side of the placing tray of  FIG. 3  in a sheet width direction. 
         FIG. 6  is an explanatory diagram showing a configuration for moving alignment members (alignment plates) that are arranged on the placing tray of  FIG. 3  and move in the sheet width direction. 
         FIGS. 7A and 7B  are explanatory diagrams showing a sheet stiffening mechanism in conveying a sheet or sheets to the placing tray.  FIG. 7A  is a perspective view of the sheet stiffening mechanism near the center in the sheet width direction.  FIG. 7B  is a sectional explanatory diagram of the sheet stiffening mechanism. 
         FIGS. 8A to 8C  are explanatory diagrams showing sheets placed and shifted on the placing tray by a shift of the alignment plates of the placing tray shown in  FIG. 6 , and sheets discharged from the placing tray and stacked on a stacking tray.  FIG. 8A  is an explanatory diagram in which four two-sheet bundles are formed.  FIG. 8B  is an explanatory diagram in which four ten-sheet bundles are formed by shifting and discharging sheets in twos.  FIG. 8C  is an explanatory diagram in which four ten-sheet bundles are formed by discharging sheets in tens. 
         FIGS. 9A and 9B  are explanatory diagrams of simultaneous bundle discharge in which subsequent sheets and a sheet bundle on the placing tray are simultaneously nipped and discharged by the discharge rollers.  FIG. 9A  is an explanatory diagram in which a first sheet is conveyed to the placing tray side.  FIG. 9B  is an explanatory diagram in which the first sheet is carried in to the placing tray and a second sheet is conveyed. 
         FIGS. 10A and 10B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 9A and 9B .  FIG. 10A  is an explanatory diagram in which a third sheet (first subsequent sheet) starts being carried in during processing of a two-sheet bundle on the placing tray.  FIG. 10B  is an explanatory diagram in which the third sheet (first subsequent sheet) is continuously conveyed beyond the discharge rollers. 
         FIGS. 11A and 11B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 10A and 10B .  FIG. 11A  is an explanatory diagram in which sheet processing (binding processing) is performed on the sheet bundle on the placing tray, and the subsequent sheet is switched back and carried in to a branch path.  FIG. 11B  is an explanatory diagram in which the sheet processing (binding processing) continues to be performed on the sheet bundle on the placing tray, and a second subsequent sheet is conveyed by the conveyance rollers. 
         FIGS. 12A and 12B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 11A and 11B .  FIG. 12A  is an explanatory diagram in which the sheet processing on the sheet bundle on the placing tray is completed, the sheet bundle starts being pushed out, and the subsequent sheets are conveyed to the position of the discharge rollers.  FIG. 12B  is an explanatory diagram in which the sheet bundle on the placing tray and the two subsequent sheets are nipped together and conveyed to the stacking tray side by the discharge rollers. 
         FIGS. 13A and 13B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 12A and 12B .  FIG. 13A  is an explanatory diagram in which the discharge rollers discharge the sheet bundle on the placing tray to the stacking tray, stop once, and then start to switchback-convey the subsequent sheets.  FIG. 13B  is an explanatory diagram in which the two subsequent sheets finish being carried in to the placing tray and proceed to the sheet processing. 
         FIGS. 14A and 14B  are explanatory diagrams of advance bundle discharge in which a sheet bundle on the placing tray is discharged to the stacking tray while subsequent sheets are switchback-conveyed.  FIG. 14A  is an explanatory diagram in which a first sheet is conveyed to the placing tray side.  FIG. 14B  is an explanatory diagram in which the first sheet is carried in to the placing tray and aligned and shifted while a second sheet is conveyed. 
         FIGS. 15A and 15B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 14A and 14B .  FIG. 15A  is an explanatory diagram in which the second sheet is carried in to the placing tray and aligned and shifted.  FIG. 15B  is an explanatory diagram in which the subsequent sheet is switchback-conveyed, and the sheet bundle on the placing tray starts being pushed out. 
         FIGS. 16A and 16B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 15A and 15B .  FIG. 16A  is an explanatory diagram in which when the subsequent sheet is switched back and positioned on an upstream side of the discharge rollers, the sheet bundle on the placing tray is nipped by the discharge rollers and starts being discharged in advance.  FIG. 16B  is an explanatory diagram in which two subsequent sheets are conveyed to the conveyance rollers, and the sheet bundle finishes being discharged from the placing tray. 
         FIGS. 17A and 17B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 16A and 16B .  FIG. 17A  is an explanatory diagram in which one of the discharge rollers (discharge upper roller) is lifted up in preparation for passage of the subsequent sheets through the position of the discharge rollers.  FIG. 17B  is an explanatory diagram in which the discharge upper roller is lowered, and when the trailing edges of the nipped subsequent sheets pass the conveyance rollers, the subsequent sheets are switched back. 
         FIGS. 18A and 18B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 17A and 17B .  FIG. 18A  is an explanatory diagram in which the two subsequent sheets are carried in to the placing tray.  FIG. 18B  is an explanatory diagram in which a subsequent sheet passes the discharge rollers and starts being switchback-conveyed, and the sheet bundle starts being pushed out. 
         FIGS. 19A and 19B  are explanatory diagrams of stepwise advance bundle discharge, a modification of  FIG. 15A  to  FIG. 18B , in which a sheet bundle on the placing tray is discharged to the stacking tray stepwise while subsequent sheets are switchback-conveyed.  FIG. 19A  is an explanatory diagram subsequent to  FIGS. 14A and 14B , in which a ten-sheet bundle is placed on the placing tray and is aligned and shifted to one side.  FIG. 19B  is an explanatory diagram in which an eleventh sheet starts being switched back as a subsequent sheet, and the sheet bundle starts being pushed out. 
         FIGS. 20A and 20B  are explanatory diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 19A and 19B .  FIG. 20A  is an explanatory diagram in which when the subsequent sheet is switched back and positioned on the upstream side of the discharge rollers, the sheet bundle on the placing tray is nipped by the discharge rollers and starts being discharged in advance.  FIG. 20B  is an explanatory diagram in which the nipping and discharge of the sheet bundle is suspended and the discharge rollers are separated due to carry-in of two subsequent sheets. 
         FIGS. 21A and 21B  are explanatory diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 20A and 20B .  FIG. 21A  is a state explanatory diagram in which the nipping and discharge of the sheet bundle is suspended, and an explanatory diagram in which the subsequent sheets move through the position of the discharge rollers to the downstream side.  FIG. 21B  is a state explanatory diagram in which the nipping and discharge of the sheet bundle is suspended, and an explanatory diagram in which the subsequent sheets move through the position of the discharge rollers to the downstream side and start being switched back. 
         FIGS. 22A and 22B  are explanatory diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 21A and 21B .  FIG. 22A  is an explanatory diagram in which when the subsequent sheets are switched back to the upstream side of the discharge rollers, the discharge rollers nip the sheet bundles again and start the next stage of discharge.  FIG. 22B  is an explanatory diagram in which the sheet bundle is discharged to the stacking tray by the discharge rollers, and three subsequent sheets are carried in. 
         FIGS. 23A and 23B  are explanatory diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 22A and 22B .  FIG. 23A  is an explanatory diagram in which the three subsequent sheets are conveyed to the placing tray side.  FIG. 23B  is an explanatory diagram in which the discharge upper roller starts to descend for nip conveyance after the subsequent sheets pass the discharge rollers. 
         FIGS. 24A and 24B  are explanatory diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 23A and 23B .  FIG. 24A  is an explanatory diagram in which the discharge rollers nip the subsequent sheets and rotate backward to switchback-convey the sheets to the placing tray.  FIG. 24B  is an explanatory diagram in which the subsequent sheets are stored into the placing tray and aligned and shifted to a position different from that of the previous sheet bundle with the discharge rollers separated. 
         FIGS. 25A and 25B  show modifications of  FIGS. 16B and 17A .  FIG. 25A  is an explanatory diagram in which when two subsequent sheets pass the position of the discharge rollers, the subsequent sheets are conveyed in a nipped state without the discharge rollers being separated.  FIG. 25B  is an explanatory diagram in which switchback is started when the trailing edges of the two nipped subsequent sheets pass the conveyance rollers. 
         FIGS. 26A and 26B  are explanatory diagrams of a state similar to that of  FIGS. 17A and 17B .  FIG. 26A  is an explanatory diagram in which one of the discharge rollers (discharge upper roller) is lifted up in preparation for the passage of the three subsequent sheets through the position of the discharge rollers.  FIG. 26B  is an explanatory diagram in which the discharge upper roller is lowered, and when the trailing edges of the nipped subsequent sheets pass the conveyance rollers, the subsequent sheets start being switched back. 
         FIG. 27  is a flowchart showing both the simultaneous bundle discharge of  FIGS. 9A to 13B  and the advance bundle discharge of  FIGS. 14A to 18B . 
         FIG. 28  is a block diagram of a control configuration in the entire configuration of  FIG. 1 . 
         FIGS. 29A to 29D  are explanatory diagrams showing a case in which subsequent sheets and an unbound sheet bundle on a placing tray are simultaneously nipped and discharged by discharge rollers (simultaneous bundle discharge).  FIG. 29A  is an explanatory diagram in which two subsequent sheets are conveyed to the placing tray side.  FIG. 29B  is an explanatory diagram in which the simultaneous bundle discharge of the sheet bundle on the placing tray and the subsequent sheets is performed.  FIG. 29C  is an explanatory diagram showing a stacked state of sheets on the stacking tray by the simultaneous bundle discharge.  FIG. 29D  is an explanatory diagram showing the stacked state of the sheets on the stacking tray when the subsequent sheets discharged by the simultaneous bundle discharge are switchback-conveyed. 
         FIGS. 30A and 30B  are explanatory diagrams showing the positions of conveyance rollers which switchback-convey subsequent sheets.  FIG. 30A  is an explanatory diagram of a state in which two subsequent sheets are conveyed.  FIG. 30B  is an explanatory diagram of a state in which three subsequent sheets are conveyed. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A mode for carrying out the invention will be described below with reference to the drawings.  FIG. 1  is an overall configuration diagram showing an image forming system including an image forming apparatus A and a sheet processing apparatus B according to the present invention.  FIG. 2  is an explanatory diagram showing a detailed configuration of the sheet processing apparatus B. 
     In the accompanying drawings, similar components are designated by the same reference numerals throughout the entire specification. 
     [Image Forming System] 
     The image forming system shown in  FIG. 1  includes the image forming apparatus A and the sheet processing apparatus B. A carry-in port  30  of the sheet processing apparatus B is connected to a main body discharge port  3  of the image forming apparatus A. The image forming system is configured so that sheets on which images are formed by the image forming apparatus A are stapled by the sheet processing apparatus B and stored on a first stacking tray  24  or a second stacking tray  26 . An escape tray  22  for directly storing sheets without stapling processing is arranged above the first stacking tray  24 . 
     [Image Forming Apparatus A] 
     The image forming apparatus A will be described with reference to  FIG. 1 . The image forming apparatus A is configured so that a sheet is fed from a sheet feeding unit  1  to an image forming unit  2 , and the sheet is printed by the image forming unit  2  and then discharged from the main body discharge port  3 . The sheet feeding unit  1  includes sheet feed cassettes  1   a  and  1   b  in which a plurality of sizes of sheets is stored. Designated sheets are separated one by one and fed to the image forming unit  2 . 
     The image forming unit  2  includes, for example, an electrostatic drum  4 , around which a print head (laser emitter)  5 , a developing device  6 , a transfer charger  7 , and a fixing device  8  are arranged. In the image forming unit  2 , the laser emitter  5  forms an electrostatic latent image on the electrostatic drum  4 . The developing device  6  applies toner to the electrostatic latent image. The transfer charger  7  transfers the resulting image to a sheet. The fixing device  8  heats and fixes the image for image formation. Sheets on which images are formed in such a manner are sequentially conveyed out from the main body discharge port  3 . A circulation path  9  is a two-sided printing path through which a sheet printed on the front side is conveyed from the fixing device  8 , turned over via a switchback path  10 , and fed to the image forming unit  2  so that the back side of the sheet is printed. Such a two-sided printed sheet is turned over via the switchback path  10  and then conveyed out from the main body discharge port  3 . 
     An image reading apparatus  11  scans a document sheet set on a platen  12  by a scan unit  13  and electrically reads the document sheet by a photoelectric conversion element (for example, CCD)  13 . The image data is digitally processed by an image processing unit, for example, and transferred to a data storage unit  14 , and an image signal is transmitted to the laser emitter  5 . A document feeding apparatus  15  feeds document sheets accommodated in a document stacker  16  to the platen  12 . 
     The image forming apparatus A having the foregoing configuration includes an image formation control unit  200  shown in  FIG. 28 . Image forming conditions are set from a control panel  18  via an input unit  203 . Examples of the image forming conditions include print conditions such as sheet size, color/monochrome print, the number of copies to print, one-sided/two-sided print, and enlargement/reduction print. The image forming apparatus A stores image data read by the scan unit  13  or image data transferred from an external network into a data storage unit  17 . The image data is transferred from the data storage unit  17  to a buffer memory  19 , and a data signal is sequentially transferred from the buffer memory  19  to the laser emitter  5 . 
     Sheet processing conditions are also input and specified from the control panel  18 , along with the image forming conditions including the one-sided/two-sided print, enlargement/reduction print, and monochrome/color print specifications mentioned above. Examples of the sheet processing conditions include settings such as “printout mode”, “end binding mode (first processing)”, “sort (jog) mode (second processing)”, and “saddle stitch mode”. Such processing conditions will be described later. 
     [Sheet Processing Apparatus B] 
     As shown in  FIGS. 1 and 2 , the sheet carry-in port  30  is arranged on one side of an apparatus frame  20  of the sheet processing apparatus B. The escape tray  22  for stacking single sheets and relatively thick sheets is arranged on the other outer side. The first stacking tray  24  for stacking end-bound sheets and a relatively large amount of sheets is located below the escape tray  22 . The first stacking tray  24  can be lifted up and down. The second stacking tray  26  for stacking saddle-stitched or folded sheets is arranged below the first stacking tray  24 . In this invention, an end refers to surfaces near an end portion of a sheet, i.e., the front and back surfaces of an edge portion of the sheet. 
     [Sheet Conveyance Path] 
     A conveyance path  42  extending substantially straight from a carry-in path  32  to a placing tray outlet  50  is arranged from the carry-in port  30  of the sheet processing apparatus B. A punch unit  31  is arranged on the carry-in path  32 . The punch unit  31  performs punching processing on a sheet end or, if needed, on a midsection in the conveyance direction. A punch waste box  31   b  for accumulating punch wastes occurring during punching processing is detachably attached to the apparatus frame  20  on the lower side of the punch unit  31  across the carry-in path  32 . 
     Carry-in rollers  34  for conveying a sheet are arranged on a downstream side of the punch unit  31 . The carry-in rollers  34  convey the sheet at high speed. Conveyance rollers  44  capable of forward and reverse rotations are arranged on the conveyance path  42  downstream of the carry-in rollers  34 . The conveyance rollers  44  guide the sheet to a placing tray  54 , which is a first processing tray, and the first stacking tray  24  on the downstream side. There is a sheet conveyance path outlet  46  behind the conveyance rollers  44 . 
     Discharge rollers  48  capable of forward and reverse rotations are arranged on the downstream side of the conveyance path output port  46 . The discharge rollers  48  switch back and carry in a sheet to the placing tray  54 , directly discharge a sheet to the first stacking tray  24 , or discharge a bundle of sheets end-bound on the placing tray  54  from the placing tray  54  to the first stacking tray  24 . 
     [Escape Path and Branch Path] 
     The conveyance path  42  is branched into an escape path  38  and a branch path  70  at a branch position  36 . The escape path  38  guides a sheet to the escape tray  22 . The branch path  70  guides a relatively long sheet to a stacker  84  serving as a second processing tray for saddle stitch processing or folding processing. A path switch gate  37  is arranged at the branch position  36 . The switch gate  37  is used to select whether to simply convey a sheet to the conveyance path  42 , convey the sheet to the escape bath  38 , or switch back the sheet on the conveyance path  42  and guide the sheet to the branch path  70 . 
     As shown in  FIGS. 2 and 3 , the branch path  70  is a path curved downward to surround the placing tray  54  beside the placing tray  54 . As will be described later, the branch path  70  also serves as a standby path in which a subsequent sheet or sheets is/are kept on standby as a standby sheet or sheets. Escape rollers  39  for conveying a sheet and escape discharge rollers  40  for discharging the sheet to the escape tray  22  are arranged on the escape path  38 . 
     [End Binding Part] 
     The placing tray  54  is arranged below the conveyance path outlet  46  of the conveyance path  42 . An end binding part  60  for binding the ends of sheets temporarily stacked on the placing tray  54  is located on the lower end of the placing tray  54 . The end binding part  60  will be described later with reference to  FIGS. 3 and 5 . 
     [Saddle Stitching Part] 
     A relatively long sheet is once conveyed through the conveyance path  42  toward the placing tray  54  and to the downstream side of the switch gate  37 . The relatively long sheet is then switchback-conveyed to the branch path  70 , and stacked in the stacker  84  (second processing tray) via a branch outlet  76 . There is arranged a saddle stitching part  80  which binds the midsection of sheets stacked in the stacker  84 . As shown in  FIG. 2 , a change flapper  78  is arranged at the branch outlet  76 . The change flapper  78  biases the sheets to the left in the diagram each time a sheet is carried in to the stacker  84  from branch discharge rollers  74 . The change flapper  78  thereby prevents collision between the trailing edges of the preceding sheets and the leading edge of the next sheet. 
     [Stacker (Second Processing Tray)] 
     A stopper  85  for defining the carry-in position of the sheets is located on the stacker  84 . A moving belt  88  stretched across an upper pulley  86  and a lower pulley  87  beside the stacker  84  is driven by a stopper moving motor  85 M, whereby the stopper  85  is moved in the direction of the arrow in the diagram. The stopper  85  is stopped at each of the following positions: a position in which the trailing edges of the sheets can be changed by the change flapper  78  when a sheet is carried in to the stacker  84 ; a position in which a saddle stitching unit  82  performs saddle stitching on substantially the center of the sheets in the conveyance direction; and a position in which a reciprocating folding blade  94  presses the saddle-stitched position into between a pair of folding rollers  92  to fold the bundle of sheets in two. Saddle stitch alignment plates  81  are arranged above and below the folding rollers  92 . The saddle stitch alignment plates  81  perform an alignment operation by pressing both side edges of the sheets in a sheet width direction each time a sheet is carried in to the stacker  84 . 
     [Saddle Stitching Unit] 
     The saddle stitching part  80  includes an anvil  83 . For example, when a staple is driven into a bundle of sheets by a driver in the saddle stitching unit  82 , the anvil  83  arranged in the opposite position bends the legs of the staple. Since the saddle stitching unit  82  is already widely known, a description thereof will be omitted. The binding means is not limited to only the mechanism of driving a staple through a sheet bundle. A mechanism of applying an adhesive to the midsections of the sheets in the conveyance direction and bonding the sheets into a bundle may be used. 
     [Second Stacking Tray] 
     The sheet bundle bound by the saddle stitching unit  82  is folded in two by the folding rollers  92  and the folding blade  94  which presses the sheet bundle into between the folding rollers  92 . The sheet bundle, as being folded in two, is discharged to the second stacking tray  26  by the folding rollers  92  and bundle discharge rollers  96  located downstream. A pressing roller  102  and a pressing lever  104  are attached to the second stacking tray  26 . The pressing roller  102  is a freely-rotatable roller swingably attached to where the folded sheet bundle is dropped in to the second stacking tray  26  with the folded back side first. The pressing lever  104  presses stacked folded sheet bundles from above to keep them from spreading out. The pressing roller  102  and the pressing lever  104  prevent the folded sheet bundles to spread out with a drop in stackability. 
     [Branch Position and End Binding Part] 
     Referring to  FIG. 3 , the branch position  36  and the end binding part  60  will be described further.  FIG. 3  shows the carry-in path  32 , the conveyance path  42 , the escape path  38 , and the branch path  70 . As has been described, the carry-in path  32  extends from the carry-in port  30 , and the carry-in rollers  34  are arranged thereon. The conveyance path  42  extends straight from the carry-in path  32  toward the placing tray  54 . The escape path  38  extends upward in the diagram from the conveyance path  42 . The branch path  70  curves downward and guides a sheet to the stacker  84 . The switch gate  37  is arranged in the branch position  36 . The switch gate  37  is selectively positioned to guide the sheet from the carry-in path  32  to the escape path  38  or the conveyance path  42 , or the sheet switchback-conveyed from the conveyance path  42  to the branch path  70 . 
     In the present embodiment, for example, as shown in  FIG. 3 , the switch gate  37  in the solid-lined position blocks the escape path  38  to guide the sheet from the carry-in path  32  to the conveyance path  42  (in  FIG. 4 , a path defined by a conveyance upper guide  42   ug  and a conveyance lower guide  42   sg ). The switch gate  37  in the broken-lined position guides the sheet from the carry-in path  32  to the escape path  38 , and the sheet switchback-conveyed from the conveyance path  42  to the branch path  70 . 
     The conveyance rollers  44  are arranged on the foregoing conveyance path  42 , immediately before the conveyance path outlet  46  which is the final end. The conveyance rollers  44  rotate forward and backward, and come into contact with and separate from each other. Specifically, the conveyance rollers  44  in a pressure contact state can rotate in one direction to convey a sheet toward the placing tray  54 , and rotate in the other direction to switchback-convey the sheet. 
     [On Switchback Conveyance] 
     The switchback conveyance is performed by rotating the conveyance rollers  44  in the other direction after a sheet sensor  42 S arranged on the conveyance path  42  immediately after the switch gate  37  detects passage of the trailing edge of the sheet. During the rotation in the other direction, the switch gate  37  is moved to the position where the carry-in path  32  is blocked (broken-lined position in  FIG. 3 ), whereby the sheet is conveyed to the branch path  70  and successively conveyed by the branch rollers  72 . If the trailing edge of the sheet reaches a predetermined position, the branch rollers  72  are stopped, and the sheet is kept on standby in the branch path  70 . 
     The discharge rollers  48  are arranged at the placing tray outlet  50  (outlet of the placing tray  54 ) on the downstream side of the conveyance rollers  44 . The discharge rollers  48  rotate forward and backward, and come into contact with and separate from each other. The discharge rollers  48  include a discharge upper roller  48   a  and a discharge lower roller  48   b . The discharge upper and lower rollers  48   a  and  48   b  in a pressure contact state rotate in one direction to convey a sheet to the first stacking tray  24  in cooperation with the foregoing conveyance rollers  44 . The discharge rollers  48  are also used when discharging a bundle of sheets stacked on the placing tray  54  in cooperation with a reference surface  57  which is a moving member for pressing the bundle of sheets to the first stacking tray  24 . 
     [Carry-in to Placing Tray  54 ] 
     Carrying-in of a sheet to the placing tray  54  will be described. To carry in a sheet to the placing tray  54 , the sheet released from the conveyance rollers  44  is conveyed to the right in  FIG. 3  over the slope of the placing tray  54  by rotating the discharge rollers  48  located downstream in the other direction. A raking roller  56  is rotated counterclockwise in the diagram to transport the conveyed sheet. By the transportation, the leading edge of the sheet in the conveyance direction is abutted against and stopped at the reference surface  57  serving as a reference for end binding. Here, the raking roller  56  slides over the sheet to prevent the leading edge of the sheet from buckling after abutted against the reference surface. In such a manner, the discharge rollers  48  have the function of switchback conveying and sending the sheet discharged from the conveyance rollers  44  to the reference surface  57  of the placing tray  54 . 
     [Movement and Binding Processing of End Binding Unit] 
     Each time a sheet is released from the conveyance rollers  44 , the discharge rollers  48  and the raking roller rotate to send the sheet to the reference surface  57  to stack sheets on the placing tray  54 . Synchronously with the stacking operation, the alignment plates  58  are brought into contact with the sheets from both sides in the sheet width direction, whereby the sheets are aligned to the center of the placing tray  54  in the width direction. Such stacking and alignment are repeated until a specified number of sheets are bundled. If the specified number of sheets are stacked, an end binding unit  62  is moved to a desired binding position. Here, the end binding unit  62  moves over a moving platform  63  in the sheet width direction along the ends of the sheets. Such a movement is made by engaging and guiding a moving pin  62   b  of the end binding unit  62  with the shown groove rail arranged in the moving platform  63  in the sheet width direction. 
     Since the binding processing of the end binding unit  62  performing the first processing of the present invention is already known, a description thereof will be omitted. If the end binding unit  62  stops at a specified binding position, an end binding motor  62 M is driven to rotate. The end binding motor  62 M moves a not-shown driver to drive a staple into the bundle of sheet, and the driven staple is bent by an anvil for stapling processing. Such binding processing is performed in a plurality of positions over the ends of the corners of the sheets and the end in the width direction. 
     [Discharge of End-Bound Sheets] 
     A reference surface moving belt  64  stretched across a right pulley  65  and a left pulley  66  under the placing tray  54  moves counterclockwise in the diagram, whereby the reference surface  57  connected to the reference surface moving belt  64  moves to the left in the diagram. The reference surface  57 , functioning as a moving member, pushes the bound end side of the sheet bundle bound by the end binding unit  62  toward the first stacking tray  24 . Along with the pushing, the discharge rollers  48  arranged at the outlet of the placing tray  54  press the bound sheet bundle from the front and back, and rotate clockwise and counterclockwise, respectively, to discharge the bound sheet bundle to the first stacking tray  24 . 
     [Lifting of First Stacking Tray] 
     The first stacking tray  24  on which sheet bundles are stacked will be described. As shown in  FIG. 3 , the first stacking tray  24  is arranged at substantially the same sloping angle as that of the placing tray  54 . Bound sheet bundles discharged from the placing tray  54  as well as single sheets discharged from the conveyance path  42  by the conveyance rollers  44  and the discharge rollers  48  are stacked on the first stacking tray  24 . 
     A lifting motor  24 M for lifting the first stacking tray  24  up and down is arranged on the bottom side of the first stacking tray  24 . The driving of the lifting motor  24 M is transmitted to a lifting pinion  109 . The lifting pinion  109  is engaged with lifting racks  107  which are vertically fixed to and arranged on both sides of an erected surface  28  of the apparatus frame  20 . Although not shown in particular, a lifting rail arranged on the erected surface  28  vertically guides the first stacking tray  24 . 
     A sheet surface sensor  24 S arranged on the erected surface  28  detects the position of the first stacking tray  24  or the position of the sheets stacked on the first stacking tray  24 . If the sheet surface sensor  24 S detects the position, the lifting motor  24 M is driven to rotate the lifting pinion  109  to descend.  FIG. 3  shows a state in which the sheet surface sensor  24 S detects the top surface of the first stacking tray  24 . The first stacking tray  24  is somewhat lowered to accept a sheet bundle. The top surface of the outlet position from the placing tray  54  and the top surface of the first stacking tray  24  are thus positioned with a difference in height. 
     Next, the rotational driving and the contact and separation of the conveyance rollers  44  and the discharge rollers  48  will be described with reference to  FIG. 4 . 
     [Rotational Driving of Conveyance Upper Rollers] 
     The conveyance rollers  44  including conveyance upper rollers  44   a  and conveyance lower rollers  44   b  are driven by a conveyance roller motor  44 M. The conveyance roller motor  44 M includes a hybrid stepping motor, on which a speed detection sensor  44 S for detecting the rotation speed of the motor shaft is arranged. The driving of the conveyance roller motor  44 M is transmitted to an arm gear  126  via transmission gears  120  and  122  and a transmission belt  124 . The driving from the arm gear  126  is transmitted by a transmission belt  128  to an upper roller shaft  44   uj  of the conveyance upper rollers  44   a  which are supported by a conveyance roller support arm  136 . 
     [Contact and Separation of Conveyance Upper Rollers] 
     The conveyance upper rollers  44   a  are attached to move around the shaft of the arm gear  126  to come into contact with and separate from the conveyance lower rollers  44   b  which are fixed in position. The contact and separation is effected by a conveyance roller moving arm  130  which includes a rear sector gear attached to the shaft of the arm gear  126 . A spring  134  for biasing the conveyance upper rollers  44   a  is attached to the moving arm tip at the end. More specifically, a conveyance roller moving arm motor  130 M engaged with the foregoing rear sector gear is driven to rotate forward and backward. Rotations in one direction move the conveyance upper rollers  44   a  in a releasing direction of the arrow O. Rotations in the other direction move the conveyance upper rollers  44   a  in a pressure contact direction of the arrow C in which the conveyance upper rollers  44   a  come into pressure contact with the conveyance lower rollers  44   b . The conveyance roller moving arm motor  130 M also includes a stepping motor. The position of the conveyance roller moving arm  130  is detected by a conveyance roller moving arm sensor  130 S. 
     [Rotational Driving of Conveyance Lower Roller Etc.] 
     The conveyance lower rollers  44   b  are driven to rotate by transmitting the driving of the conveyance roller motor  44 M to a reception gear  142 , which is fixed to a conveyance lower roller shaft  44   sj , via the transmission gear  120  and a transmission belt  138 . 
     The driving of the reception gar  142  rotates the raking roller  56  via a one-way clutch gear  144  and a belt with projections  146  which also serves as a transmission belt. Since the raking roller  56  is driven via the one-way clutch gear  144 , the raking roller  56  rotates only in the direction of the full-lined arrow in  FIG. 4  even if the reception gear  142  rotates forward and backward as has been described. The raking roller  56  rotates to move a sheet only toward the reference surface  57  of the placing tray  54 . While the foregoing belt with projections  146  is described to rotate the raking roller  56  at the end, the raking roller  56  may be omitted and only a circular raking belt may be rotated. 
     The driving of the conveyance roller motor  44 M is also transmitted via the transmission gear  120  and a transmission belt  148  to a branch lower roller shaft  72   sj  of a branch lower roller  72   b  of the branch rollers  72  which convey a sheet in the branch path  70 . 
     With the foregoing configuration, as the conveyance roller motor  44 M rotates forward and backward, the conveyance rollers  44  and the branch rollers  72  rotate in one direction, or the directions of the solid-lined arrows, and in the other direction (switchback direction), or the directions of the broken-lined arrows. The raking roller  56  rotates in the direction toward the reference surface  57 , or the direction of the solid-lined arrow. The conveyance roller motor  44 M can be set to convey a sheet at a predetermined speed when the sheet is conveyed toward the placing tray  54  or switchback-conveyed toward the branch path  70 . 
     [Rotational Driving of Discharge Upper Roller] 
     The discharge rollers  48  including the discharge upper roller  48   a  and the discharge lower roller  48   b  are driven by a discharge roller motor  48 M. The discharge roller motor  48 M also includes a hybrid stepping motor. A speed detection sensor  48 S for detecting the rotation speed of the motor shaft is similarly arranged. The driving of the discharge roller motor  48 M is transmitted to an arm gear  156  via transmission gears  150  and  152  and a transmission belt  154 . A transmission belt  158  transmits the driving of the arm gear  156  to a discharge upper roller shaft  48   uj  of the discharge upper roller  48   a  supported by a discharge roller support arm  166 . 
     [Contact, Separation, Etc. Of Discharge Upper Roller] 
     The discharge upper roller  48   a  is attached to move around the shaft of the arm gear  156  to come into contact with and separate from the discharge lower roller  48   b  which is fixed in position. The contact and separation is effected by a discharge roller moving arm  160  which includes a rear sector gear attached to the shaft of the arm gear  156 . A spring  164  for biasing the discharge arm roller  48   a  is attached to the moving arm tip at the end. A discharge roller moving arm motor  160 M engaged with the foregoing rear sector gear is driven to rotate forward and backward. Rotations in one direction move the discharge upper roller  48   a  in a releasing direction of the arrow O. Rotations in the other direction move the discharge upper roller  48   a  in a pressure contact direction of the arrow C in which the discharge upper roller  48   a  comes into pressure contact with the discharge lower roller  48   b . The discharge roller moving arm motor  160 M also includes a stepping motor. The position of the discharge roller moving arm  160  is detected by a discharge roller moving arm sensor  160 S. 
     The discharge lower roller  48   b  is driven to rotate by transmitting the driving of the discharge roller motor  48 M to a reception gear  169 , which is fixed to a discharge lower roller shaft  48   sj , via the transmission gear  150  and a transfer belt  168 . 
     [Speed Setting of Discharge Roller Motor] 
     With the foregoing configuration, as the discharge roller motor  48 M rotates forward and backward, the discharge rollers  48  rotate in one direction, or the directions of the solid-lined arrows in the diagram, and in the other direction, or the directions of the broken-lined arrows in the diagram (the switchback direction on the placing tray  54  toward the reference surface  57  after a sheet is released from the conveyance rollers  44 ). The speed setting of the discharge roller motor  48 M can be changed so that the discharge rollers  48  are driven at a predetermined speed. 
     In the present embodiment, when the conveyance rollers  44  are conveying a sheet like during switchback conveyance for standby conveyance, the discharge upper roller  48   a  is located in a separated position off the discharge lower roller  48   b  since the separate driving motors are difficult to operate in a linked manner. 
     [Standby Conveyance and Second Tray Conveyance] 
     Returning to  FIG. 3 , standby conveyance in which a sheet is switchback-conveyed to the branch path  70  for standby for the purpose of the foregoing end binding will be described. If the end binding unit  62  of the placing tray  54  performs the binding processing, the next sheet needs to be prevented from being carried in before the completion of the end binding processing on the preceding sheet bundle. The reason is that the carry-in speed of a sheet image-formed by the image forming apparatus A is high and the sheet intervals are short. The first sheet or up to the second sheet conveyed to the conveyance path  42  through the carry-in path  32  is/are then once switchback-conveyed on the conveyance path  42 , and the switchback-conveyed sheet(s) is/are kept on standby in the branch path  70 . The sheet(s) kept on standby in the branch path  70  is/are then sent out to overlap with the subsequent second or third sheet, whereby an interval time between sheet bundles is ensured (such an operation is disclosed, for example, in FIG. 10 of Japanese Patent No. 5248785). 
     As employed herein, switchback conveying one or more sheets from the conveyance path  42  to the branch path  70 , keeping the sheet(s) on standby in the branch path  70 , and sending out the sheet(s) on standby with the next sheet will be referred to as “standby conveyance”. Sheets having a relatively small length in the conveyance direction, such as A4, B5, and letter-size sheets, are often end-bound by standby conveyance. Such sheets are switchback-conveyed for standby conveyance without much protruding downstream from the placing tray  54 . The sheets are less likely to skew during the conveyance. Since the distance to the placing tray  54  is relatively small, the sheets, if somewhat skewed, can be corrected by the alignment operation of the alignment plates  58 . 
     The completion of the end binding processing includes not only the completion of the discharge operation of the sheet bundle from the placing tray  54  to the first stacking tray  24 , but also an initial setting operation of the alignment plates  58  on the placing tray  54 , recovery of the reference surface moving belt  64  to its initial position, and/or setting of initial positions of various mechanisms for accepting the next sheet. 
     Next, a case of performing saddle stitching by the saddle stitching unit  82  and conveying the sheets to the stacker  84 , or second processing tray, to fold the sheets into a folded sheet bundle by the folding rollers  92  and the folding blade  94  will be described. For conveyance to the stacker  84 , a sheet conveyed through the carry-in path  32  to the conveyance path  42  is once switchback-conveyed on the conveyance path  42 . The switchback-conveyed sheet is then conveyed from the branch path  70  to the stacker  84 . As employed herein, conveying the switchback-conveyed sheet to the stacker  84  via the branch path  70  will be referred to as “second tray conveyance”. 
     [Switchback Conveyance] 
     Suppose that a sheet is “standby-conveyed” by the conveyance rollers  44 . In the present embodiment, if the trailing edge of the sheet is detected by the sheet sensor  42 S arranged at the branch position between the conveyance path  42  and the branch path  70 , the sheet is switchback-conveyed to the branch path  70  and nipped by the branch rollers  72  located in the branch path  70 . The rotation of the branch rollers  72  is then stopped. In the case of performing “second tray conveyance” in which sheets are stacked in the stacker  84  located on the downstream side of the branch path  70  for saddle stitch processing, the sheet switchback-conveyed by the conveyance rollers  44  is similarly sent to the branch rollers  72  of the branch path  70  and to the stacker  84  without being stopped. 
     The discharge rollers  48  can rotate forward and backward. If the trailing edge of a subsequent sheet (s) conveyed by the conveyance rollers  44  (a sheet kept on standby in the branch path  70 , a sheet from the carry-in path, or a stack of such sheets) is released from the conveyance rollers  44 , the discharge rollers  48  nip the sheet(s). The discharge rollers  48  then rotate backward to switchback-convey and store the subsequent sheet(s) into the placing tray  54 . 
     [Discharge of Sheet Bundle] 
     As described above, the discharge rollers  48  are configured so that the discharge upper roller  48   a  is swingable. The discharge upper roller  48   a  descends to the pressure contact position in which to come into pressure contact with the discharge lower roller  48   b  (the broken-line position in  FIG. 4 ), and ascends to the separated position above the discharge lower roller  48   b  (the solid-lined position in  FIG. 4 ). After the sheet processing of a sheet bundle on the placing tray  54 , to discharge the sheet bundle to the first stacking tray  24 , the reference surface  57  is initially moved toward the placing tray outlet  50  and pushed up by the reference surface moving belt  64 . The discharge upper roller  48   a  subsequently descends to the pressure contact position, nips the sheet bundle with the discharge lower roller  48   b , transports the sheet bundle toward the placing tray outlet  50 , and discharges the bundle to the first stacking tray  24 . 
     [Sheet Processing Unit] 
     The discharge rollers  48  discharge a sheet bundle processed by the sheet processing unit of the placing tray. The sheet processing according to the present embodiment includes binding processing and so-called jog processing. In the binding processing, the sheet bundle is bound by the end binding unit  62 . The jog processing refers to sorting unbound sheet bundles on the first stacking tray  24  by changing the positions of the sheet bundles on the placing tray  54  by the alignment plates  58  and discharging the sheet bundles. The sheet processing may include other processing, such as lamination by gluing and punching processing for making holes in the sheets. 
     [Movement of End Binding Unit] 
     The end binding unit  62  for stapling a sheet bundle has been described as the sheet processing unit of the invention. A movement of the end binding unit  62  in the width direction of the sheet bundle will be described with reference to  FIG. 5 .  FIG. 5  shows that the end binding unit  62  for stapling a sheet bundle moves over the moving platform  63 . The moving platform  63  is arranged on the apparatus frame  20  of the sheet processing apparatus B with a front side up and a rear side down in  FIG. 5 . Referring also to  FIG. 3 , a substantially-straight moving groove  63   b  for guiding the moving pin  62   b  protruding from the end binding unit  62  side is formed in the moving platform  63 . A guide pin  62   c  arranged on the tip side of the end binding unit  62  is engaged with an orientation guide  63   e  arranged on the moving platform  63 . 
     The end binding unit  62  is coupled with a moving platform belt  63 Mb which is moved by a unit moving motor  63 M. Moving positions of the end binding unit  62  include a corner binding position Cp 1  on the rear side, a multiple binding range Ma 1  to Ma 2  on the center side, and a corner binding position Cp 2  on the front side. The end binding unit  62  is also controlled to be positioned in a staple loading position and a home position HP. In the staple loading position, the rear part of the end binding unit  62  is directed to outside the apparatus at the front side. The home position HP is the position of the end binding unit  62  before a start of binding. The home position HP also serves as a manual binding position on the front side. The apparatus of the present embodiment thus includes, as a sheet processing unit, the end binding unit  62  which performs binding processing on arbitrary positions of the sheet bundle placed on the placing tray  54 . The sheet processing unit includes the alignment plates  58  which are paired in a sheet width direction. The alignment plates  58  align sheets each time a sheet is carried in to the placing tray  54 . 
     [Alignment Plates] 
     Next, the alignment plates  58  which come into contact with the side edges of sheets to align the sheets or change the placing position of the sheets each time a sheet is carried in to the placing tray  54  will be described with reference to  FIG. 6 .  FIG. 6  is a top view of the placing tray  54 . The alignment plates  58  include a front alignment plate  58   a  on the front side and a rear alignment plate  58   b  on the rear side. The front alignment plate  58   a  and the rear alignment plate  58   b  include a front alignment surface  58   af  and a rear alignment surface  58   bf , respectively, which come into contact with and separate from the side edges of sheets. The contact and separation with/from the side edges of the sheets are effected by moving a front alignment plate rack  59   a R by a front alignment motor  59   a M via a gear  59   a G. The front alignment plate rack  59   a R is arranged on the bottom of the front alignment plate  58   a  and guided by front rack guides  58   a RG. The contact and separation are also effected by moving a rear alignment plate rack  58   b R by a rear alignment motor  59   b M via a gear  59   b G. The rear alignment plate rack  58   b R is arranged on the bottom of the rear alignment plate  58   b  and guided by rear rack guides  58   b RG. 
     For multiple binding, the front alignment plate  58   a  and the rear alignment plate  58   b  can align the sheets with reference to the sheet center. For corner binding, like  FIG. 6 , the front alignment plate  58   a  and the rear alignment plate  58   b  can align the sheets with reference to one side. In such a manner, the reference of alignment of the front and rear alignment plates  58   a  and  58   b  can be changed. Serving as a sheet processing unit, the front and rear alignment plates  58   a  and  58   b  can perform so-called jog processing for sorting sheet bundles by aligning each sheet bundle placed on the placing tray  54  to either one side and discharging the same to the first stacking tray. 
     [ Sort Processing (Second Processing)] 
     To perform the sort processing which is the second processing according to the present invention, for example, maximum sheets shown in  FIG. 6  are carried in to the placing tray  54 . The front alignment plate  58   a  which is located outside in the sheet width direction is moved by Sf 1  in the diagram. The sheets are thereby moved by Sf 2  on the rear side. The side edges of the sheets come into contact with the rear alignment plate  58   b  which is retracted in advance, whereby the sheets are positioned on the placing tray  54  as aligned to the rear side. On the other hand, if the rear alignment plate  58   b  is moved to the front side, the sheets are positions as aligned to the front side. Sheets can be sorted in such a manner. 
     The sort processing will be further described with reference to  FIGS. 8A to 8C .  FIGS. 8A to 8C  are diagrams for describing sheets that are placed on the placing tray  54  and shifted by a shift movement of the alignment plates  58  of  FIG. 6  and the sheets that are then discharged from the placing tray  54  and stacked on the first stacking tray  24 . In the following diagrams, a reference symbol BP followed by a numeral represents a sheet bundle. For example, BP 2  represents the second sheet bundle. A reference symbol P followed by a numeral in a bundle indicates page number from the beginning. For example, P 4  represents the fourth page (fourth sheet) from the beginning. 
       FIG. 8A  is a diagram in which four two-sheet bundles are formed. Here, three bundles of sheets shifted by the placing tray  54  and discharged to the first stacking trays  24  in twos are already stacked. If the fourth sheet bundle is placed on the placing tray  54 , the front alignment plate  58   a  is shifted by Sf 1  to the rear side, and the sheet bundle is shifted by Sf 2  into contact with the rear alignment plate  58   b  which is shifted in advance. The resulting sheet bundle is discharged to the first stacking tray  24  by the discharging rollers  48 , whereby four sorted sheet bundles can be sorted (jogged) and stacked on the first stacking tray  24 . 
     In  FIG. 8B , four ten-sheet bundles are formed by shifting and discharging sheets in twos. Like  FIG. 8A , sheets are sorted and shifted in twos on the placing tray  54  by the alignment plates  58 , and then discharged as a bundle from the placing tray  54  to the first stacking tray  24  by the discharge rollers  48 . In such a manner, four ten-sheet bundles can be sorted (jogged) on the first stacking tray  24 . 
     Unlike  FIG. 8B  in which sheets in a bundle are discharged in twos,  FIG. 8C  is an explanatory diagram in which four ten-sheet bundles are formed by discharging sheets in tens. In this case, as will be described later, standby sheets can be increased to perform slow discharge operations, as compared to the case of discharging standby sheets in twos. 
     [Sheet Stiffening Mechanism] 
     Return to  FIG. 6 . A sheet stiffening mechanism used in conveying a sheet or sheets to the placing tray  54  will be described with reference to  FIGS. 7A and 7B . The sheet stiffening mechanism is intended to prevent a sheet bundle from curling up near the outlet of the placing tray  54  because of low stiffness of sheets when the sheets are carried in to the placing tray  54  from the conveyance path  42  by the conveying rollers  44  or when the leading edges of the sheets are passed between the discharge rollers  48  and the sheets are switchback-conveyed to the upstream side again. 
       FIG. 6  shows the conveyance lower rollers  44   b  of the conveyance rollers  44  and stiffening rollers  45  intended for stiffening. The stiffening rollers  45  are supported above the conveyance lower roller shaft  44   sj  by roller arms  45   am . A sheet is conveyed from the conveyance path  42  to pass the stiffening rollers  45 . 
       FIG. 7A  is a perspective view of the sheet stiffening mechanism near the center in the sheet width direction.  FIG. 7B  is an explanatory sectional view of the sheet stiffening mechanism. As can be seen from  FIGS. 7A and 7B , a bottom portion of a roller arm  45   am  is rotatably supported by an arm shaft  45   aj  axially between the respective pairs of conveyance upper and lower rollers  44   a  and  44   b  constituting the conveyance rollers  44 . A rotatable stiffening roller  45  is attached to the end of the roller arm  45   am  by a roller shaft  45   kj . The stiffening roller  45  rotates according to sheet conveyance, and is thus less likely to damage the surface of the conveyed sheet. 
     A coil spring  45   kb  is wound around the arm shaft  45   aj  at the bottom portion of the roller arm  45   a  supporting the stiffening roller  45 . As shown in  FIG. 7B , the stiffening roller  45  is thereby biased constantly (to an extent of stiffening the conveyed sheet) in the direction of the arrow. As shown in  FIGS. 7A and 7B , the stiffening roller  45  is located somewhat on the side of the conveyance upper rollers  44   a  with respect to the pressure contact position between the conveyance upper rollers  44   a  and the conveyance lower rollers  44   b . The conveyed sheet is thereby corrugated and stiffened in the sheet width direction crossing the conveyance direction. The winding of the coil spring  45   kb  can produce large corrugations for stiffening if a sheet bundle is thin and low in rigidity. If the sheets are thick and high in rigidity, small corrugations can be produced to stiffen the sheets but not too much as hinders conveyance. 
     [Confirmation by Number of Sheets Conveyed] 
       FIGS. 30A and 30B  are diagrams showing an experiment on sheet conveyance by using the stiffening rollers  45 .  FIGS. 30A and 30B  also show the positions of discharge rollers ER for switchback conveying sheets. The experiment was performed by changing the number of sheets of a sheet bundle conveyed by conveyance rollers HR.  FIG. 30A  is an explanatory diagram showing a conveyance state of two sheets. As shown in  FIG. 30A , if two sheets (subsequent sheets np 1  and np 2 ) were conveyed by the conveyance rollers HR, the sheets were somewhat stiffened by the conveyance rollers HR. The sheets were guided downward by a guide GA between the conveyance rollers HR and the discharge rollers ER, and relatively smoothly conveyed to the conveyance rollers ER. 
       FIG. 30B  is an explanatory diagram showing a state in which three sheets are conveyed by the conveyance rollers HR. When three sheets (subsequent sheets np 1 , np 2 , and np 3 ) were conveyed by the conveyance rollers HR, the lowermost-layer sheet np 1  of the sheets guided downward by the guide GA between the conveyance rollers HR and the discharge rollers ER, though somewhat stiffened by the conveyance rollers HR, is curled up to the side of a placing tray Tr. All the subsequent sheets curled up accordingly to cause a jam. 
     In such a manner, it has been confirmed that if the foregoing stiffening rollers  45  are used, one to two sheets can be conveyed without a problem, and three or more sheets often cause a jam. Such a confirmation result later explains what the present invention solves (in the foregoing description of sheet stiffening, the reference numerals of the conveyance rollers and the discharge rollers are different from those of the present embodiment, whereas the members are substantially the same). 
     Now, “simultaneous bundle discharge” and “advance bundle discharge” will be described. The “simultaneous bundle discharge” refers to an operation in which the discharge rollers  48  nip and transport a sheet kept on standby in the branch path  70  and a sheet from the conveyance path (hereinafter, such sheets will be referred to collectively as “subsequent sheets”) together with a sheet bundle placed on the placing tray  54 , discharge the sheet bundle to the foregoing first stacking tray  24 , and switchback-convey the subsequent sheets to the placing tray  54 . The “advance bundle discharge” refers to an operation in which the discharge rollers  48  nip and discharge the sheet bundle processed on the placing tray  54  to the first stacking tray  24  during switchback conveyance of the subsequent sheets, and then the subsequent sheets are carried in to the placing tray  54 . The “simultaneous bundle discharge” of a two-sheet bundle with two subsequent sheets will be described with reference to the sheet conveyance diagrams of  FIGS. 9A to 13A  and the flowchart of  FIG. 27 . The “advance bundle discharge” of a two-sheet bundle with two subsequent sheets will be described with reference to the sheet conveyance diagrams of  FIGS. 14A to 18B  and the flowchart of  FIG. 27 . 
     As shown in  FIG. 27 , in a determination step, whether the sheet processing on the placing tray  54  of the sheet processing apparatus is the binding processing which is the first processing using the end binding unit  62  or the sort (jog) processing which is the second processing using the alignment plates  58  is initially selected. The first processing and the second processing may be determined according to time needed for the processing. The first processing needs a longer processing time. 
     As has been described, the processing time of the binding processing which is the first processing is longer than that of the sort (jog) processing which is the second processing. Suppose that the stapling processing using the end binding unit  62  on the left part of the flowchart of  FIG. 27  is selected. If the stapling processing is selected, carry-in S 11  of sheets to the placing tray  54 , sheet alignment S 12  by the alignment plates  58 , and conveyance S 13  of subsequent sheets are performed. Such a flow of sheets will be described in order from  FIGS. 9A and 9B . 
       FIGS. 9A and 9B  are explanatory diagrams in which the discharge rollers  48  simultaneously nip a subsequent sheet and a sheet bundle on the placing tray  54  to start “simultaneous bundle discharge”. In  FIG. 9A , the conveyance rollers  44  convey a first sheet P 1  from the conveyance path  42  to the side of the placing tray  54 . In such a state, if the trailing edge of the sheet is detected by the sheet sensor  42 S and a not-shown counter counts up to a predetermined number, the first sheet P 1  is discharged from the conveyance rollers  44  to the placing tray  54 . At the same time, the discharge upper roller  48   a  of the discharge rollers  48  starts being moved from the separated position (solid lines in  FIG. 9A ) to the pressure contact position (broken lines in  FIG. 9A ) in which the discharge upper roller  48   a  comes into pressure contact with the discharge lower roller  48   b.    
     As shown in  FIG. 9B , the sheet released from the conveyance rollers  44  is then nipped by the discharge rollers  48 , and switchback-conveyed by the counterclockwise rotation of the discharge upper roller  48   a  and the clockwise rotation of the discharge lower roller  48   b . The sheet is further conveyed toward the reference surface  57  by the raking roller  56  and the belt with projections  146 , and accommodated into and placed on the placing tray  54 . In synchronization with the accommodation, the alignment plates  58  are moved for centering. The next second sheet is carried in. If the leading edge is detected by the sheet sensor  42 S, the discharge upper roller  48   a  starts being moved from the pressure contact position (solid-lined position in  FIG. 9B ) to the separated position (broken-lined position in  FIG. 9B ) to carry in the sheet. The same operation as that of  FIG. 9A  is then repeated on the second sheet. After the formation of a two-sheet bundle BP 1  (P 1  and P 2 ), the processing proceeds to  FIGS. 10A and 10B . 
       FIGS. 10A and 10B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 9A and 9B .  FIG. 10A  shows a state in which a first subsequent sheet (wp 1 ) which is a third sheet (P 3 ) starts being carried in during the processing of the two-sheet bundle BP 1  on the placing tray. The alignment of the sheet bundle BP 1  on the placing tray  54  is complete, and the end binding unit  62  is moved to a binding position, i.e., in preparation for the binding processing. 
     As shown in  FIG. 10B , the leading edge of the third sheet P (first subsequent sheet wp 1 ) is continuously conveyed beyond the discharge rollers  48  by the conveyance rollers  44 . Since the subsequent sheet P 3  is to be switchback-conveyed, the switch gate  37  located in the branch position between the conveyance path  42  and the branch path  70  moves to the shown position for guiding the sheet to the branch path  70 . 
     Next,  FIGS. 11A and 11B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 10A and 10B . In  FIG. 11A , the end binding unit  62  starts to perform the end binding sheet processing on the sheet bundle BP 1  on the placing tray  54 . Since the subsequent sheet P 3  is not able to be carried in to the placing tray  54  during the processing, the conveyance rollers  44  continue switchback conveyance. The sheet is moved to the downstream side of the branch path  70  by the branch rollers  72  which are located on the branch path  70  and rotate in synchronization with the rotation of the conveyance rollers  44 . If the subsequent sheet P 3  is nipped by the branch rollers  72 , the switch gate  37  is lifted up to open the conveyance path  42 . Meanwhile, the end binding unit  62  is performing the corner binding processing on the sheet bundle BP 1 . 
     In  FIG. 11B , the corner binding sheet processing on the sheet bundle BP 1  on the placing tray is continued. In the meantime, a second subsequent sheet P 4  is sent to the conveyance rollers  44  by the carry-in rollers  34 . If the subsequent sheet P 4  is detected by the sheet sensor  42 S, the standby sheet wp 1  (first subsequent sheet P 3 ) kept on standby in the branch path  70  in advance and the subsequent sheet P 4  are both conveyed toward the conveyance rollers  44  with a difference of wp 1  therebetween. Here, the conveyance speed of the standby sheet is 650 mm/sec. At this stage, the binding processing of the sheet bundle BP 1  on the placing tray  54  is completed. In  FIG. 27 , such a state is shown as the binding processing S 14  on the sheet bundle BP 1  on the placing tray  54 . 
     Next,  FIGS. 12A and 12B  will be described.  FIGS. 12A and 12B  are explanatory diagrams of the simultaneous bundle discharge subsequent to  FIGS. 11A and 11B .  FIG. 12A  shows a state in which the binding sheet processing of the sheet bundle BP 1  on the placing tray  54  is complete, and the sheet bundle BP 1  starts being pushed by the reference surface  57 . At the same time, the two subsequent sheets P 3  and P 4  are conveyed to the position of the conveyance rollers  48  to overlap with the sheet bundle BP 1  on the conveyance tray  54 . In  FIG. 27 , such a state is shown as feeding S 15  of the standby sheets as a standby complete bundle. 
     [Execution of Simultaneous Bundle Discharge] 
     Next,  FIG. 12B  is a diagram relating to the simultaneous bundle discharge described so far, in which the sheet bundle BP 1  on the placing tray  54  and the two subsequent sheets P 3  and P 4  are nipped together by the discharge rollers  48  and conveyed to the first stacking tray  24 . As shown in  FIG. 12B , the discharge upper roller  48   a  is lowered to the position in which the discharge upper roller  48   a  comes into pressure contact with the discharge lower roller  48   b . The discharge rollers  48  simultaneously nip the sheet bundle BP 1  on the placing tray  54  and the subsequent sheets P 3  and P 4 , discharges the sheet bundle, and transports the subsequent sheets in the discharge direction. The conveyance speeds of the sheet bundle BP 1  and the conveyance sheets P 3  and P 4  are both reduced to 600 mm/sec. The simultaneous bundle discharge is performed at a speed of 480 mm/sec. In  FIG. 27 , such a state is shown as a bundle discharge step S 16  for simultaneous bundle discharge of the sheet bundle on placing tray  54  and the standby sheets. 
     If the simultaneous bundle discharge is executed, the processing proceeds to the state shown in  FIGS. 13A and 13B  subsequent to  FIGS. 12A and 12B . In  FIG. 13A , the discharge rollers  48  initially discharge the sheet bundle BP 1  placed on the placing tray  54  to the first stacking tray  24 . In such a state, the discharge rollers  48  once stop rotating. In such a state, as shown in the eclipse in  FIG. 13A , the subsequent sheets P 3  and P 4  have a difference as much as a distance of wp 1 . A distance between the subsequent sheet P 3  and the sheet sensor  42   s  is set to be SB 1 . The discharge rollers  48  then start to rotate backward (in  FIG. 13A , the discharge upper roller  48   a  to rotate counterclockwise, and the discharge lower roller  48   b  to rote clockwise). In  FIG. 27 , such a state is shown as a carry-in step S 17  for switching back the standby sheets to the placing tray  54 . The speed of the sheets switchback-conveyed to the placing tray  54  is 600 mm/sec. 
     The reverse rotations of the discharge rollers  48  place the subsequent sheets P 3  and P 4  as a second sheet bundle BP 2  on the placing tray  54  in the state shown in  FIG. 13B . In  FIG. 13B , the simultaneous bundle discharge is completed. 
     In  FIG. 27 , whether to complete the simultaneous bundle discharge is shown as step S 18 . In step S 18 , if there is the next processing (subsequent sheets to be carried in), the processing returns to  FIG. 10A  to continue the simultaneous bundle discharge until a specified number of sheet bundles are processed. If there is no subsequent sheet and the sheet processing is to be ended, then in  FIG. 13B , the binding processing is performed on the sheet bundle on the placing tray  54  without a subsequent sheet. The sheet bundle is discharged to the first stacking tray  24 , and the sheet processing is completed. 
     The execution procedure of the simultaneous bundle discharge has been described above. Since the subsequent sheets and the sheet bundle on the placing tray  54  are overlapped for processing, the processing time can be reduced to improve the processing speed. Such a procedure does not cause a problem if the sheet bundle on the placing tray  54  is bound. On the other hand, in the case of the sort processing without binding, the alignment of sheets stacked on the first stacking tray  24  may deteriorate as has been described as a problem with reference to  FIGS. 29A to 29D . For the sake of discharge of a sheet bundle and transportation of subsequent sheets to improve such a problem without much decreasing the processing speed, the “advance bundle discharge” in which the sheet bundle is discharged in advance during switchback conveyance of the subsequent sheets will be described in order with reference to the sheet conveyance diagrams of  FIGS. 14A to 18B  and the right part of the flowchart of  FIG. 27 . 
     More specifically, in the description of  FIG. 27  so far, the binding processing which is the first processing using the end binding unit  62  is described to be selected as the sheet processing on the placing tray  54  of the sheet processing apparatus. In the following description, the sort (jog) processing which is the second processing using the alignment plates  58  is described to be selected. In such a case, the sort processing (jog) of sheet bundles in the right part of the flowchart of  FIG. 27  is selected. If the sort processing is selected, carry-in S 21  of sheets to the placing tray  54 , sort processing S 22  for changing and shifting a placing position on the placing tray  54  simultaneously with sheet alignment by the alignment plates  58 , and conveyance S 23  of subsequent sheets are performed. Such a flow of sheets will be described in order from  FIGS. 14A to 14B . 
       FIGS. 14A and 14B  are explanatory diagrams showing a case in which a sheet bundle on the placing tray  54  is discharged to the first stacking tray  24  while subsequent sheets are switchback-conveyed by the conveyance rollers  44  (advance bundle discharge).  FIGS. 14A and 14B  are substantially the same as  FIGS. 9A and 9B  describing the foregoing simultaneous bundle discharge, except the operation of the alignment plates  58 . In  FIG. 14A , the conveyance rollers  44  convey a first sheet P 1  from the conveyance path  42  to the side of the placement tray  54 . In such a state, if the trailing edge of the sheet is detected by the sheet sensor  42 S and the not-shown counter counts up to a predetermined number, the first sheet P 1  is discharged from the conveyance rollers  44  to the placing tray  54 . At the same time, the discharge upper roller  48   a  of the discharge rollers  48  starts being moved from the separated position (solid lines in  FIG. 14A ) to the pressure contact position (broken lines in  FIG. 14A ) in which the discharge upper roller  48   a  comes into pressure contact with the discharge lower roller  48   b.    
     Subsequently, as shown in  FIG. 14B , the sheet released from the conveyance rollers  44  is nipped by the discharge rollers  48 , and switchback-conveyed by the counterclockwise rotation of the discharge upper roller  48   a  and the clockwise rotation of the discharge lower roller  48   b . The sheet is further conveyed toward the reference plane  57  by the raking roller  56  and the belt with projections  146 , and accommodated into and placed on the placing tray  54 . Here, the conveyance speed of the sheet toward the reference plane  57  is 650 mm/sec. In synchronization with the center accommodation of the sheet, the alignment plates  58  are moved to align the sheet to one side on the placing tray  54 . If the next second sheet P 2  is carried in and the leading edge is detected by the sheet sensor  42 S, the discharge upper roller  48   a  starts being moved from the pressure contact position (solid-lined position in  FIG. 14B ) to the separated position (broken-lined position in  FIG. 14B ) to carry in the sheet. The same operation as that of  FIG. 14A  is then repeated on the second sheet to form a two-sheet bundle BP 1  (P 1  and P 2 ) aligned to one side. The processing proceeds to  FIGS. 15A and 15B . 
       FIGS. 15A and 15B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 14A and 14B .  FIG. 15A  is a diagram in which the second sheet is carried in to the placing tray  54  and aligned and shifted. Since the sheet processing here includes only changing the position of a sheet bundle to be discharged to the first stacking tray  24  on the first stacking tray  24 , the sheet processing is performed in a shorter processing time than when the binding process is. 
     [Pushing Up 1 of Bundle During Switchback Conveyance] 
     Next, as shown in  FIG. 15B , the leading edge of a third sheet P 3  (first standby sheet wp 1 ) is continuously conveyed beyond the discharge rollers  48  by the conveyance rollers  44 . Since the subsequent sheet P 3  is to be switchback-conveyed, the switch gate  37  located in the branch position between the conveyance path  42  and the branch path  70  moves to the shown position for guiding the sheet to the branch path  70 . If the switchback conveyance of the sheet is started, a reference surface moving motor  64 M is activated to push out the sheet bundle BP 1  to the discharge tray outlet  50  by the reference surface  57 . The pushing timing may be such that the sheet bundle BP 1  immediately starts being pushed when the alignment plates  58  finish aligning the sheet bundle BP 1  on the placing tray  54  to one side. As in the present invention, the sheet bundle BP 1  may start being pushed after the switchback conveyance of the subsequent sheet P 3  is started, in which case the sheet bundle BP 1  is pulled backward by the subsequent sheet P 3  for improved alignment. Here, the setting value of the switchback conveyance speed of the subsequent sheet is 750 mm/sec. The setting value of the pushing speed of the reference surface  57  is 600 mm/sec. 
     [Execution of Advance Bundle Discharge] 
       FIGS. 16A and 16B  are explanatory diagrams of the advance bundle discharge subsequent to  FIGS. 15A and 15B .  FIG. 16A  is a diagram in which when the subsequent sheet P 3  is switchback-conveyed by the conveyance rollers  44  and returned to the upstream side of the discharge rollers  48 , the discharge upper roller  48   a  of the discharge rollers  48  is lowered to nip the sheet bundle BP 1  on the placing tray  54  and start bundle discharge in advance. The subsequent sheet P 3  is forwarded to the branch path  70  by the switch gate  37 , and further forwarded to the downstream side of the branch path  70  by the branch rollers  72 . The trailing edge of the subsequent sheet P 3  is thereby located upstream of the discharge rollers  48  and switchback-conveyed without interfering with the discharge of the sheet bundle BP 1 . As described above, the discharge rollers  48  can thus discharge the sheet bundle BP on the placing tray  54  to the first stacking tray  24  immediately after the subsequent sheet P 3  passes to the upstream side. Here, the reference surface  57  having pushed out the sheet bundle BP 1  to the side of the placing tray outlet  50  returns from the broken-lined position in  FIG. 16A  to the original solid-lined position. In  FIG. 27 , such a state is shown as switchback and return S 24  of the standby (subsequent) sheet to the upstream of the discharge rollers  48 . This stage of switchback is shown as bundle discharge step S 25  for discharging the sheet bundle on the placing tray  54  as described above. 
     [Completion of Advance Bundle Discharge] 
       FIG. 16B  is a diagram in which the discharge rollers  48  continue discharging the sheet bundle BP 1  from the placing tray  54  to the first stacking tray  24 . The sheet bundle discharge speed here is reduced from 600 mm/sec to 350 mm/sec to avoid deterioration of alignment. Immediately after the state of  FIG. 16B , the sheet bundle BP 1  is discharged to the first tacking tray  24 , whereby the advance bundle discharge is completed. Meanwhile, a second subsequent sheet P 4  is conveyed toward the conveyance rollers  44  by the carry-in rollers  34 . If the subsequent sheet P 4  is detected by the sheet sensor  42 S, the standby sheet wp 1  (first subsequent sheet P 3 ) kept on standby in the branch path  70  in advance and the subsequent sheet P 4  are both conveyed toward the conveyance rollers  44  with a difference of wp 1  therebetween. Here, the conveyance speed of the subsequent sheets is 650 mm/sec. 
     [Conveyance of Subsequent Sheets (Discharge Roller Up)] 
     After the discharge of the sheet bundle BP 1  in  FIG. 16B , the subsequent sheets P 3  and P 4  are conveyed by the conveyance rollers  44  to approach the discharge rollers  48  as shown in  FIG. 17A . Here, the discharge upper roller  48   a  is retracted from the pressure contact position shown by the broken lines in  FIG. 17A  to the separated position shown by the solid lines. The subsequent sheets P 3  and P 4  pass the position of the discharge rollers  48 . If the subsequent sheets are three or more in number, the movement of the discharge upper roller  48   a  to the lifted separated position as shown in  FIG. 17A  facilitates the passage of the sheets through the position of the discharge rollers  48 . A description thereof will be given later. 
     [Switchback of Subsequent Sheets (Carry-in to Placing Tray)] 
     If the subsequent sheets P 3  and P 4  in the state of  FIG. 17A  pass the position of the discharge rollers  48 , the subsequent sheets P 3  and P 4  are nipped by the discharge rollers  48  again and transported to the side of the first stacking tray  24  as shown in  FIG. 17B . Then, the discharge rollers  48  once stop rotating. In such a state, as shown in the ellipse in  FIG. 17B , the subsequent sheets P 3  and P 4  have a difference as much as a distance of wp 1 . A distance between the subsequent sheet P 3  and the sheet sensor  42 S is set to be SB 1 . The discharge rollers  48  then start to rotate backward (rotate in directions reverse to the directions of the arrows in  FIG. 17B ). In  FIG. 27 , such a state is shown as carry-in step S 26  for switching back only the standby sheets to the placing tray  54 . The switchback conveyance speed is reduced from 600 mm/sec to 300 mm/sec when the sheets are released to the placing tray  54 . The discharge rollers  48  are then stopped. The sheets are carried in to the placing tray  54  at a setting value of 600 mm/sec. 
     By the reverse rotation of the discharge rollers  48 , the subsequent sheets P 3  and P 4  enter the state shown in  FIG. 18A  as a second sheet bundle BP 2  on the placing tray  54 . In  FIG. 18A , the two subsequent sheets P 3  and P 4  are carried in as the sheet bundle BP 2  to the placing tray  54 . After the carry-in, the discharge upper roller  48   a  is once moved to the separated position. In the meantime, the alignment plates  58  shift the sheets to one side. The carry-in rollers  34  start to convey the next subsequent sheet P 5 . 
     [Pushing Up 2 During Switchback Conveyance] 
       FIG. 18B  is a diagram showing a state in which the subsequent sheet P 5  which is a standby sheet wp 1  passes the conveyance rollers  48  and starts being switchback-conveyed, and the reference plane  57  starts to push the sheet bundle BP 2  on the placing tray  54 . Such a state is substantially the same as that of  FIG. 15B  described for the advance bundle discharge. A description thereof is thus omitted here. The reference surface moving motor  64 M is activated during the switchback conveyance of the subsequent sheet P 5 , so that the reference surface  57  pushes out the sheet bundle BP 2  to the placing tray outlet  50 . Here, the setting value of the switchback conveyance speed of the subsequent sheet is 750 mm/sec. The setting value of the pushing speed by the reference surface  57  is 600 mm/sec. 
     As described above, if there is a next sheet bundle to be processed, the processing returns to  FIG. 15A  and is repeated until a specified number of sheet bundles are formed. If there is no next sheet, no subsequent sheet is carried in in the state of  FIG. 19A , and only the sheet bundle on the placing tray  54  is discharged to complete the processing. In  FIG. 27 , whether to complete the processing is shown as step S 27 . In step S 27 , if there is a next sheet bundle to be processed (subsequent sheet to be carried in), the processing returns to  FIG. 15A  to continue the advance bundle discharge until a specified number of sheet bundles are formed. If there is no subsequent sheet and the processing is to be ended, then in  FIG. 18A , the sheet bundle on the placing tray  54  is shifted to one side without a subsequent sheet. The sheet bundle is discharged to the first stacking tray  24 , and the processing for shifting sheets to one side without binding processing is completed. 
     The execution procedure of the advance bundle discharge during switchback of a subsequent sheet has been described above. Since the sheet bundle on the placing tray  54  is discharged without a subsequent sheet being stacked thereon, the sheets stored on the first stacking tray  24  are less pushed or drawn by subsequent sheets. This reduces deterioration of the alignment of the sheets stored on the first stacking tray  24 . Since the sheet bundle on the placing tray  54  is discharged in advance during the standby operation of a subsequent sheet, the processing can be performed without much reducing the processing speed. 
     As described above, the present invention includes the discharge mode “simultaneous bundle discharge” in which the sheet bundle on the placing tray is discharged with subsequent sheets as described with reference to the sheet conveyance diagrams of  FIGS. 9A to 13B  and the left part of the flowchart of  FIG. 27 , and the discharge mode “advance bundle discharge” in which the sheet bundle on the placing tray  54  is discharged in advance during switchback of a subsequent sheet as described with reference to the sheet conveyance diagrams of  FIGS. 14A to 18B  and the right part of the flowchart of  FIG. 27 . Depending on whether the binding processing (first processing) or the sort (jog) processing (second second processing) using the alignment plates  58  is performed, the discharge modes are changed as described above to avoid deterioration of the alignment of the sheet bundles and a drop in the processing speed of the apparatus during the sort processing in particular. 
     [Modification of Advance Bundle Discharge (Stepwise Advance Bundle Discharge)] 
     Next, a modification of  FIGS. 15A to 18B  will be described with reference to  FIGS. 19A to 26B . This modification is suitably applicable to the case described in  FIG. 10C , in which a ten-sheet bundle is placed on the placing tray  54  and discharged to the first stacking tray  24 . A difference from the operation of  FIGS. 15A to 18B  is that there are three or more subsequent sheets, and the sheet bundle on the placing tray  54  is discharged to the first stacking tray  24  stepwise (stepwise advance bundle discharge) while subsequent sheets to be standby sheets wp are switchback-conveyed. 
     [Start of Bundle Discharge During Switchback of Subsequent Sheets] 
       FIG. 19A  shows a state of sheets subsequent to  FIGS. 14A and 14B . In the state of  FIG. 19A , a ten-sheet bundle BP 1  is placed on the placing tray  54 , and the placed sheets finish being aligned and shifted to one side. A subsequent sheet P 11  (standby sheet wp 1 ) has started to be carried in by the carry-in rollers  34 . Next, in  FIG. 19B , the subsequent sheet P 11  starts being switched back as a standby sheet wp by the conveyance rollers  44 . According to the start of the switchback, the reference surface  57  starts to push the sheet bundle BP 1 . Again, since the sheet bundle BP 1  on the placing tray  54  starts being pushed during the switchback of a subsequent sheet, the sheet bundle is less disturbed even though not bound. 
       FIGS. 20A and 20B  are diagrams showing the stepwise advance bundle discharge subsequent to  FIGS. 19A and 19B .  FIG. 20A  is a diagram in which when the subsequent sheet P 11  is switched back and positioned on the upstream side of the discharge rollers  48 , the discharge upper roller  48   a  is lowered to nip the sheet bundle BP 1  on the placing tray  54  to discharge the bundle in advance. In such a state, the reference surface  57  having pushed the sheet bundle BP 1  returns to its original position, and the subsequent sheet P 11  is further switchback-conveyed to the branch path  70  by the branch rollers  72 . The pushing speed of the reference surface  57  and the bundle discharge speed of the discharge rollers  48  to the first stacking tray  24  here are set to be slower than in the foregoing  FIGS. 15A to 16B  since the sheet bundle BP 1  includes a greater number of sheets. 
     [Suspension of Advance Bundle Discharge] 
     Next, in  FIG. 20B , a second subsequent sheet P 12  is carried in. The nipping and discharge of the sheet bundle by the discharge rollers  48  is suspended, and the discharge upper roller  48   a  is lifted up to the separated position. In such a state, the sheet bundle BP 1  discharged in advance is temporarily stopped near the outlet of the placing tray  54 . Since the sheet bundle is somewhat curved in shape, the sheet bundle will not collapse. A not-shown auxiliary tray for supporting the sheet bundle BP 1  near the discharge lower roller  48   b  or a member for pressing the sheet bundle BP 1  may be provided. 
       FIGS. 21A and 21B  are state diagrams subsequent to  FIGS. 20A and 20B .  FIG. 21A  is a diagram in which the nipping and discharge of the sheet bundle by the discharge rollers  48  is suspended. The subsequent sheet P 11  kept on standby as the standby sheet wp 1  in the branch path  70  and the subsequent sheet P 12  carried in by the carry-in rollers  34  pass between the discharge upper roller  48   a  and the discharge lower roller  48   b  as a bundle. Even in such a case, the sheet bundle BP 1  discharged in advance remains temporarily stopped near the outlet of the placing tray  54 . 
     [Execution of Stepwise Discharge of Preceding Sheet Bundle] 
       FIGS. 22A and 22B  are state diagrams subsequent to  FIGS. 21A and 21B . In  FIGS. 22A and 22B , stepwise discharge of the preceding sheet bundle is executed. In  FIG. 22A , when the two subsequent sheets P 11  and P 12  are switched back to the upstream side of the discharge rollers  48 , the discharge upper roller  48   a  is lowered again. By the lowering, the sheet bundle BP 1  on the placing tray  54  in the process of being discharged by the discharge rollers  48  is nipped again, and the discharge rollers  48  are rotated for the next stage of discharge. Next, in  FIG. 22B , the sheet bundle BP 1  is discharged to the first stacking tray  24  by the discharge rollers  48 . In preparation for the carry-in of the next subsequent sheet, the discharge upper roller  48   a  then moves from the pressure contact position shown by the broken lines in  FIG. 22B  to the separated position shown by the solid lines. Meanwhile, two standby sheets wp 1  and wp 2  (subsequent sheets P 11  and P 12 ) in the branch path  70  serving as the standby path and a subsequent sheet P 13  are set on the upstream side of the conveyance rollers  44  with their leading edges apart from each other. 
       FIGS. 23A and 23B  are diagrams of the stepwise advance bundle discharge subsequent to  FIGS. 22A and 22B .  FIG. 23A  shows a state in which the three subsequent sheets P 11 , P 12 , and P 13  are conveyed to the side of the conveyance tray  54  by the conveyance rollers  44 . Two of the three subsequent sheets are the standby sheets wp 1  and wp 2  kept on standby in the branch path  70  serving as the standby path. In such a state, the discharge upper roller  48   a  is located in the separated position to wait for the leading edges of the subsequent sheets to pass.  FIG. 23B  illustrates the leading edges of the three subsequent sheets P 11 , P 12 , and P 13  positioned past the discharge rollers  48 . In this state explanatory diagram, the discharge upper roller  48   a  starts to move from the separated position shown by the broken lines in  FIG. 23B  to the lowered position shown by the solid lines in preparation for the discharge of the trailing edge of the sheet bundle of the three subsequent sheets from the conveyance rollers  44 . 
       FIGS. 24A and 24B  are diagrams subsequent to  FIGS. 23A and 23B . In  FIG. 24A , the discharge upper roller  48   a  moves in the direction of coming into pressure contact with the discharge lower roller  48   b , and rotates counterclockwise to switchback-convey the leading edges of the three subsequent sheets P 11 , P 12 , and P 13  toward the placing tray  54 . In such a case, as shown in the ellipse in  FIG. 24A , the subsequent sheets P 11 , P 12 , and P 13  have a difference as much as a distance of wp 1  from each other. A distance between the subsequent sheet P 11  which is the standby sheet wp 1  and the sheet sensor  42 S is designed to be SB 1 . 
     The reason for the provision of such differences is that when the subsequent sheets are abutted against and aligned by the reference surface  57  of the placing tray  54 , the topmost sheet is conveyed by the raking roller  56  and the belt with projections  146 . Without such differences or with the differences in reverse order, the topmost sheet would come into contact with the reference plane  57  first and the lower sheets would fail to come into contact. The discharge rollers  48  start to rotate backward (in the direction of the arrow in  FIG. 24A ) in such a state. The speed is reduced from 600 mm/sec to 300 mm/sec when the subsequent sheets are released to the placing tray  54 . Then, the discharge rollers  48  are once stopped. The subsequent sheets are carried in to the placing tray  56  by conveyance at a setting value of 600 mm/sec. 
     Next, in  FIG. 24B , the three subsequent sheets P 11 , P 12 , and P 13  are stored into the placing tray  54 . At the same time, the discharge upper roller  48   a  is lifted up and separated to the separated position. The three subsequent sheets P 11 , P 12 , and P 13  are aligned and shifted to a position different from that of the previous sheet bundle BP 1 . The processing then returns to the state of  FIG. 19A  and is repeated until ten sheets are placed on the placing tray  54 . If there is a next sheet to be processed, the processing up to  FIGS. 24A and 24B  is repeated. If not, the ten sheets placed on the placing tray  54  are discharged to the first stacking tray  24  to complete the processing. Here, the bundle is sorted and shifted to a position different from that of the previous bundle. 
     The execution procedure of the advance bundle discharge for discharging a sheet bundle stepwise during the switchback of standby sheets wp among three subsequent sheets has been described above. Even in such a case, the sheet bundle on the placing tray  54  is discharged without the subsequent sheets being stacked thereon. The sheets placed on the first stacking tray  24  are therefore less pushed or drawn by the subsequent sheets. This reduces deterioration of the alignment of the sheets stored on the first stacking tray  24 . Since the sheet bundle on the placing tray  54  is discharged in advance during the standby operation of the subsequent sheets, the processing can be performed without much reducing the processing speed. Since the sheet bundle can be pushed out and discharged from the placing tray  54  at relatively low speed, the sheet bundle is less likely to collapse. 
     [Conveyance of Two Subsequent Sheets with Discharge Rollers Closed (Modification of  FIGS. 17A and 17B )] 
     Next, a second modification of the present invention will be described with reference to  FIGS. 25A and 25B .  FIGS. 25A and 25B  show modified states of  FIGS. 16B and 17A  among the state diagrams described above for advance bundle discharge in  FIGS. 14A to 18B . In  FIG. 16B , the sheet bundle BP 1  is discharged. Then, as shown in  FIG. 17A , the subsequent sheets P 3  and P 4  are continuously conveyed by the conveyance rollers  44  to approach the discharge rollers  48 . Here, the discharge upper roller  48   a  is retracted from the pressure contact position shown by the broken lines in  FIG. 17A  to the separated position shown by the solid lines. The subsequent sheets P 3  and P 4  pass the position of the discharge rollers  48 . After the passage, the discharge upper roller  48   a  is moved to the pressure contact position again. 
     If the subsequent sheets are two in number, the number of standby sheets wp is one. There is not much room in distance between the sheets, and the discharge upper roller  48   a  needs to be quickly opened and closed. Such an operation needs a discharge roller moving arm motor  160 M of relatively large size (see  FIG. 4 ). As has been described as the test confirmation with reference to  FIGS. 30A and 30B , if the number of subsequent sheets is two, the subsequent sheets can be conveyed without a problem even with the discharge upper roller  48   a  in the pressure contact state, not opened from the pressure contact position to the separated position. 
       FIGS. 25A and 25B  show a state in which the two subsequent sheets pass the discharge rollers  48  and are then switched back and carried in to the placing tray  54 .  FIG. 25A  is an explanatory diagram showing a state in which when the two subsequent sheets P 3  and P 4  pass the position of the discharge rollers, the subsequent sheets are conveyed in a nip state without the discharge upper roller  48   a  being lifted up to the separated position. As shown in  FIG. 25B , the two subsequent sheets P 3  and P 4  are then carried in to the placing tray  54  by the reverse rotation of the discharge rollers  48  in the nip state. 
     As a result, the discharge upper roller  48   a  does not need to be opened and closed if there is not a time margin sufficient for the carry-in to the placing tray  54  between the preceding sheets and the subsequent sheets (in conveying two subsequent sheets including one standby sheet). The discharge roller moving arm motor  160 M therefore does not need to be increased in size. The apparatus can thus be reduced in size and weight. 
     [Conveyance of Three Subsequent Sheets with Discharge Rollers Opened and Closed (Like  FIGS. 17A and 17B )] 
     If the number of subsequent sheets is three or more, as has been described in  FIG. 30B , the lowermost sheet np 1  of the subsequent sheets guided downward by the guide GA between the conveyance rollers HR and the discharge rollers ER curls up to the side of the placing tray Tr even if somewhat stiffened by the conveyance rollers HR. All the subsequent sheets curl accordingly to cause a jam. 
     As shown in  FIGS. 26A and 26B  (a similar state to that of  FIGS. 17A and 17B ), the subsequent sheets are then accepted with the discharge upper roller  48   a  located in the separated position.  FIG. 26A  shows such a state in which the discharge upper roller  48   a  is lifted up to the separated position in preparation for the passage of the three subsequent sheets through the position of the discharge rollers. The state then transitions to  FIG. 26B , in which if the leading edges of the subsequent sheets pass the discharge upper roller  48   a , the discharge upper roller  48   a  is lowered. If the trailing edges of the three subsequent sheets nipped by the discharge rollers  48  pass the conveyance rollers  44 , switchback is started to carry in the subsequent sheets to the placing tray  54 . 
     In such a case, the discharge roller moving arm motor  160 M for moving the discharge upper roller  48   a  up and down does not need to be increased in size. The reason is that if the subsequent sheets are three or more in number, the number of standby sheets wp is two or more and there is a time margin between sheets to be carried in to the placing tray  54 . The subsequent sheets can thus be moved relatively slowly without increasing the discharge roller moving arm motor  160 M in size. 
     As described above, the number of subsequent sheets to be carried in to the placing tray  54  is determined in the determination step. If the number of subsequent sheets is two, a nip acceptance step of conveying the subsequent sheets with the discharge upper roller  48   a  kept closed in the pressure contact position is performed. The subsequent sheets are switchback-conveyed and carried in to the placing tray  54 . On the other hand, if the number of subsequent sheets is three or more, the processing proceeds to an open acceptance step in which the discharge upper roller  48   a  is once lifted up to the separated position. The processing then proceeds to a nipping step of lowering the discharge upper roller  48   a  to nip the subsequent sheets after the leading edges of the subsequent sheets pass the discharge upper roller  48   a , and the subsequent sheets are carried in to the placing tray  54 . Since the discharge upper roller  48   a  is thus opened and closed depending on the number of subsequent sheets, the sheets can be switchback-conveyed to the placing tray  54  without increasing the driving source in size. 
     In the present embodiment, a discharge step of nipping the sheet bundle on the placing tray  54  by the discharge rollers  48  and discharging the sheet bundle from the placing tray  54  to the first stacking tray  24  is performed at a stage when the subsequent sheets are returned to the upstream side, before the foregoing nip acceptance step or open acceptance step. 
     [Description of Control Configuration] 
     A system control configuration of the foregoing image forming apparatus will be described with reference to the block diagram of  FIG. 28 . The system of the image forming apparatus shown in  FIG. 1  includes the image formation control unit  200  of the image forming apparatus A and the sheet processing control unit  204  (control CPU) of the sheet processing apparatus B. The image formation control unit  200  includes a sheet feed control unit  202  and the input unit  203 . As has been described, a “print mode” and a “sheet processing mode” can be set on the control panel  18  arranged on the input unit  203 . 
     The sheet processing control unit  204  is a control CPU for operating the sheet processing apparatus B according to the sheet processing mode specified as described above. The sheet processing control unit  204  includes a ROM  206  which stores an operation program, and a RAM  207  which stores control data. Signals from various sensor input units, including a carry-in sensor  30 S for detecting a sheet in the carry-in path  32 , the sheet sensor  42 S for detecting a sheet in the conveyance path  42 , the branch sensor  70 S for detecting a sheet in the branch path  70 , and the sheet surface sensor  24 S for detecting a sheet surface on the first stacking tray  24 , are input to the sheet processing control unit  204 . 
     The sheet processing control unit  204  includes a sheet conveyance control unit  210 . The sheet conveyance control unit  210  controls a carry-in roller motor  34 M on the carry-in path  32  of a sheet, the conveyance roller motor  44 M on the conveyance path  42  and the branch path, the discharge roller motor  48 M at the outlet of the placing tray  54 , and the discharge roller moving arm motor  160 M for lifting the discharge upper roller  48   a  up and down. The sheet processing control unit  204  further includes a punch driving control unit  211  and a placing tray (processing tray) control unit  212 . The punch driving control unit  211  controls a punch motor  31 M for performing punching processing on sheets in the punch unit  31 . The placing tray control unit  212  controls the alignment plates  58  and the like for performing a sheet stacking operation on the placing tray  54 . The sheet processing control unit  204  further includes an end binding control unit  213  and a first stacking tray lifting control unit  214 . The end binding control unit  213  controls the end binding motor  62 M of the end binding unit  62  which performs the end binding on the sheet bundle on the placing tray  54 . The first stacking tray lifting control unit  214  controls the lifting motor  24 M which lifts up and down according to end-bound sheet bundles and sheet switchback on the first stacking tray  24 . 
     The sheet processing control unit  204  further includes a stacker control unit  216  and a saddle stitch control unit  217 . The stacker control unit  216  controls the saddle stitch alignment plates  81  of sheets stacked in the stacker  84  which is the second processing tray for saddle stitch processing, and the stopper  85  for regulating the leading edges of the sheets. The saddle stitch control unit  217  controls the saddle stitching unit  82  which binds the sheet bundle in the center in the conveyance direction. 
     The sheet processing control unit  204  further includes a folding and discharge control unit  218 . The folding and discharge control unit  218  controls a folding processing unit and bundle discharge rollers  98  which fold the saddle-stitched sheet bundle in two and discharge the sheet bundle to the second stacking tray  26 . Such control units, the sensors for detecting conveyed sheets, and the driving motors are connected as described above in the description of each operation mode. 
     [Description of Sheet Processing Mode] 
     The sheet processing control unit  204  according to the present embodiment configured as described above makes the sheet processing apparatus B perform, for example, a “print out mode”, “end binding mode (first processing)”, “sort (jog) mode”, and “saddle stitching mode”. Such processing modes will be described below. 
     (1) “Print Out Mode” 
     Receive image-formed sheets from the main body discharge port  3  of the image forming apparatus A. Store the sheets into the first stacking tray  24  by using the conveyance rollers  44  and the discharge rollers  48 . 
     (2) “End Binding Mode (First Processing)” 
     Receive image-formed sheets from the main body discharge port  3  by the placing tray  54 . Align the sheets into a bundle, perform the binding processing by the end binding unit  62 , and store the resultant into the first stacking tray  24 . In this end binding processing, “standby conveyance” for switchback-conveying and temporarily keeping a preceding sheet or sheets in the branch path  70  on standby as a standby sheet or sheets wp is performed to prevent the discharging of subsequent sheets from the main body discharge port  3  from being interrupted. 
     (3) “Sort (Jog) Mode (Second Processing)” 
     Receive image-formed sheets from the main body discharge port  3  by the placing tray  54 . Shift the sheets one by one to either the front side or the rear side for one-side alignment, and store the resultant into the first stacking tray  24  without binding. By using the one-side shift members, sheets can be sorted (jogged) on the first stacking tray  24  as described in  FIGS. 8A to 8C . Even with the sorting (jog), the “standby conveyance” for switchback-conveying and temporarily keeping a preceding sheet or sheets in the branch path  70  on standby as a standby sheet or sheets wp is performed to prevent the discharging of subsequent sheets from the main body discharge port  3  from being interrupted. 
     (4) “Saddle Stitching Mode” 
     Receive image-formed sheets from the main body discharge port  3  of the image forming apparatus A by the stacker  84 . Align the sheets into a bundle. Bind the sheets substantially in the center of the accepting conveyance direction by the saddle stitching unit  82 . Fold the bound sheets into a booklet shape and store the resultant into the second stacking tray  26 . In the saddle stitch processing, the “second tray conveyance” for once discharging sheets from the main body discharge port  3  onto the first stacking tray  24 , switchback-conveying the sheets to the branch path  70 , and conveying the sheets to the stacker  84  is performed. 
     As described above, according to the foregoing embodiment, an apparatus that prevents deterioration of sheet alignment on the first stacking tray  24  due to subsequent sheets and thus reduces the occurrence of sheet jams can be provided. An apparatus in which the driving source for moving the discharge upper roller  48   a  to open and close is not increased in size can also be provided. 
     The present invention is not limited to the foregoing exemplary embodiment. Various modifications may be made without departing from the invention. The present invention is directed to all technical matters included in the technical concept set forth in the claims. While the foregoing exemplary embodiment is a suitable example, it is possible for those skilled in the art to make various alternatives, corrections, modifications, and improvements from the contents disclosed in this specification. Such alternatives, corrections, modification, and improvements are within the technical scope set forth in the accompanying claims. 
     This application claims the priority of Japanese Patent Application No. 2016-182626 filed on Sep. 20, 2016, Japanese Patent Application No. 2016-182627 filed on the same date, and Japanese Patent Application No. 2016-182628 filed on the same date, which are incorporated herein by reference.