Patent Publication Number: US-2012025452-A1

Title: Sheet post-processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/616,076 filed on Dec. 26, 2006, the entire contents of which are incorporated herein by reference. 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-372752 filed on Dec. 26, 2005, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet post-processing apparatus that performs post-processing for a sheet on which an image has been formed in an image forming apparatus such as a copier, a printer, or a composite device. 
     2. Description of the Related Art 
     In recent years, there is developed a sheet post-processing apparatus, which is disposed adjacent to the sheet discharge section of an image forming apparatus main body for the purpose of performing post-processing for a sheet on which an image has been formed in the image forming apparatus. The sheet post-processing apparatus performs post-processing for a sheet, such as sorting of printed sheets discharged from the image forming apparatus or stapling thereof. The sheet post-processing apparatus performs post-processing for a succeeding sheet after completion of the post-processing for a preceding sheet. In order for the sheet post-processing apparatus to wait until the post-processing for a preceding sheet has been completed, an apparatus in which a retreating path is formed in the middle of a path reaching to a stapler is disclosed in Japanese Patent Publication (Kokoku) No. H6-99070. 
     However, in the case where the retreating path is formed in the middle of a path reaching to a stapler as described above, the length of a sheet path extending from the discharge section of the image forming apparatus main body to the stapler is increased. This may prevent size reduction of the entire apparatus. 
     In light of the above, an apparatus in which a processing tray for use in staple processing and a standby tray which retains a sheet to be supplied to the processing tray in a standby mode are vertically provided to reduce the length of the sheet path has been proposed. In this apparatus, a sheet is dropped from the standby tray and is supplied onto the processing tray. Further, such an apparatus has a sheet discharge tray on which a sheet discharged from the standby tray or processing tray is placed and a rear end guide that supports the rear end of the sheet on the discharge tray between the discharge tray. 
     However, if deformation of a sheet occurs when the sheet is discharged from the standby tray or processing tray to the sheet discharge tray, the sheet is caught by the rear side which may result in sheet jamming. Further, if the rear guide contacts a sheet when the sheet is aligned on the processing tray, the rear guide serves as resistance to adversely affecting the sheet alignment performance. 
     Further, in some cases, the sheet discharge tray is slid on the side surface of the post-processing apparatus main body, with the sheet placed thereon, so as to be disposed adjacent to the position of the standby tray or processing tray. If the rear end of the sheet on the discharge tray is slipped from the upper end of the rear guide at this time, the sheet may be fed back to the processing tray or standby tray side. 
     To cope with the above problems, a small-sized sheet post-processing apparatus that prevents sheet jamming from occurring when the sheet is discharged from the standby tray or processing tray to the discharge tray without adversely affecting the sheet alignment performance made in the processing tray has been desired. Further, a sheet post-processing apparatus that prevents the sheet placed on the discharge tray from being fed back to the backside at the time of the slide of the discharge tray has been desired. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a sheet post-processing apparatus in which a sheet is dropped from the standby tray to thereby be supplied to the processing tray and the length of a sheet path is reduced. It is desirable for the sheet post-processing apparatus to prevent sheet jamming from occurring when the sheet is discharged to the discharge tray, to exhibit an excellent sheet alignment performance on the processing tray, and to prevent the sheet placed on the discharge tray from reversely being fed to the processing tray side at the time of the slide of the discharge tray. 
     According to an embodiment of the present invention, a sheet post-processing apparatus is characterized by containing: a standby tray for making a sheet discharged from an image forming apparatus stand by, a processing tray arranged under the standby tray, a processing mechanism for performing post-processing for the sheet, a sheet discharge tray for loading the sheet, the sheet discharge tray being slidably arranged adjacent to the standby tray or the processing tray on the downstream side with respect to the standby tray and processing tray in the sheet discharge direction and tilted such that the discharge direction front end of the sheet is positioned higher than the discharge direction rear end thereof, and a rear end supporting mechanism arranged on the discharge direction rear end side and allows the rear end of the sheet on the sheet discharge tray to be brought into contact therewith, wherein the center area upper end of the rear end supporting mechanism is positioned lower than the peripheral area upper end of the rear end supporting mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the main part of a sheet post-processing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a top view of the main part of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 3A  is a view schematically showing a configuration of the sheet post-processing apparatus according to the embodiment of the present invention in a state where a sheet is discharged onto a first sheet discharge tray; 
         FIG. 3B  is a view schematically showing a configuration of the sheet post-processing apparatus according to the embodiment of the present invention in a state where the sheet discharge tray is slid; 
         FIG. 4  is a perspective view showing a stapler of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 5  is an explanatory view showing vertical alignment rollers according to the embodiment of the present invention; 
         FIG. 6  is a perspective view showing a paddle mechanism according to the embodiment of the present invention; 
         FIG. 7  is a perspective view schematically showing a standby tray and a processing tray according to the embodiment of the present invention; 
         FIG. 8  is a top view of the standby tray and processing tray according to the embodiment of the present invention; 
         FIG. 9  is a perspective view schematically showing horizontal alignment plates and a conveyer belt according to the embodiment of the present invention; 
         FIG. 10  is a perspective view schematically showing a part of a shutter according to the embodiment of the present invention in a state where a sheet is discharged onto the first sheet discharge tray; 
         FIG. 11  is a perspective view schematically showing a part of the shutter according to the embodiment of the present invention in a state where the sheet discharge tray is slid; and 
         FIG. 12  is an explanatory view showing the movement of the standby tray according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is a perspective view showing the main part of a sheet post-processing apparatus  7  according to an embodiment of the present invention.  FIG. 2  is a top view of the main part of the sheet post-processing apparatus  7 .  FIGS. 3A and 3B  are views each schematically showing a configuration of the sheet post-processing apparatus  7  disposed adjacent to an image forming apparatus  5  such as a copier. 
     The sheet post-processing apparatus  7  has a pair of entrance rollers  22  have an upper roller  22   a  and lower roller  22   b  that feed a sheet P, on which an image has been formed by the image forming apparatus  5  and which is discharged by a pair of discharge rollers  6 , into the sheet post-processing apparatus  7 . The entrance rollers  22  are driven by an entrance roller motor  26 . Between the entrance rollers  22  and a standby tray  10 , a sheet path ceiling  36  for leading the sheet P to a pair of sheet feed rollers  24  is installed. Under the standby tray  10 , a processing tray  12  for loading the sheets P dropped and supplied from the standby tray  10  is arranged. 
     While the sheets P are stapled by a stapler  14  which is a processing mechanism for performing post-processing, the processing tray  12 , supports the sheets P to be loaded in an aligned state. As shown in  FIG. 4 , the stapler  14  is slidable by a staple drive section  49  in u direction denoted by the arrow in the illustration. The stapler  14  performs staple processing after being rotated in accordance with the staple direction and positioned. The processing tray  12  has a pair of upper vertical alignment roller  38   a  and lower vertical alignment roller  38   b  shown in  FIG. 5  at the rear end side portion of the sheet P. 
     The upper vertical alignment roller  38   a  and lower vertical alignment roller  38   b  align a plurality of sheets P dropped and supplied from the standby tray  10  in the vertical direction which is a feeding direction. The upper and lower vertical alignment rollers  38   a  and  38   b  serve as bundle feeding rollers for holding a sheet bundle T after stapled and taking out it from the stapler  14 . The upper vertical alignment roller  38   a  is driven by a vertical alignment upper roller motor  40 . The lower vertical matching roller  38   b  is driven by a vertical alignment lower roller motor  42 . 
     At the position opposite to the rear end of each of the sheets P which have been dropped and supplied from the standby tray  10  onto the processing tray  12 , a paddle  44  is arranged. The paddle  44 , which is configured to be rotatable, aligns vertically the uppermost sheet P loaded on the processing tray  12 . The paddle  44 , as shown in  FIG. 6 , has a receiving portion  44   a  for receiving the sheets P dropped and supplied onto the processing tray  12  from the standby tray  10 , a beating portion  44   b  for beating down the sheets P on the processing tray  12 , and a feeding portion  44   c  for aligning the sheets P on the processing tray  12 . The puddle  44  is driven by a paddle motor  46 . The paddle  44  is composed of an elastic rubber material and has elasticity. 
     At the end of the processing tray  12  on the side of the stapler  14 , a stopper  45  for making contact with the rear end of each of the sheets P and vertically aligning the rear end position is arranged. Almost at the center of the processing tray  12 , a conveyor belt  50  serving as a sheet discharge mechanism is installed. The conveyer belt  50  feeds the sheet bundle T, which is stapled and taken out from the stapler  14  by the upper and lower vertical alignment rollers  38   a  and  38   b , to first or second sheet discharge tray  16  or  18 . To the conveyer belt  50 , a feed pawl  50   a  for hooking the rear end of the sheet bundle T is attached. The first or second sheet discharge tray  16  or  18  is configured to be slidable on the side surface of the sheet post-processing apparatus  7  in the vertical direction. 
     At the end of the processing tray  12  on the side of the sheet discharge tray  16  or  18 , pluralities of alignment rollers  61  are arranged. The alignment rollers  61  are supported by a shaft  60  and contact the bottom surface of the sheet P. The alignment rollers  61  have a pair of center alignment rollers  61   a  which are disposed symmetrically with respect to the conveyor belt  50  and a pair of side alignment rollers  61   b  which are disposed on both sides of the center alignment rollers  61   a.    
     The alignment rollers  61  are rotated in g direction (first direction) denoted by the arrow in  FIG. 5  to thereby feed the sheet P to the stopper  45  side so as to allow the rear end of the sheet P to contact with the stopper  45 , thereby vertically aligning the sheet P. On the other hand, when the alignment rollers  61  are rotated in h direction (second direction opposite to the first direction) denoted by the arrow in  FIG. 5 , the sheet P is fed to the sheet discharge tray  16  or  18  sides. The alignment rollers  61  and conveyer belt  50  use the same shaft  60 . The shaft  60  is made rotatable in forward and reverse directions by a belt motor  62 . The conveyer belt  50  is driven through a one-way clutch (not shown) and therefore is rotatable only in t direction denoted by the arrow in  FIG. 5 . 
     The standby tray  10  can drop and supply the sheets P onto the processing tray  12  and also can feed the sheets P toward the first or second sheet discharge tray  16  or  18 . The standby tray  10  is so tilted as to support the sheets P such that the position of the front ends of the sheets P becomes higher than that of the rear ends thereof. As shown in  FIGS. 7 and 8 , the standby tray  10  has a pair of tray members  10   a  and  10   b . The tray members  10   a  and  10   b  receive the sheets in a state that they slide in the width of the sheet P and support both sides of the sheet P. On the tray members  10   a  and  10   b , standby stoppers  10   c  and  10   d  for aligning the rear ends of the sheets P are installed. The standby tray  10  slides and moves by a standby tray motor  34 . 
     The standby tray  10  has pinch rollers  10   e  which hold the sheet P on the standby tray  10  between themselves and standby tray rollers  28  provided for aligning the sheet P and feed it toward the sheet discharge tray  16  or  18 . Note that, without providing the pinch roller  10   e , it is possible to feed the sheet P on the standby tray  10  toward the sheet discharge tray  16  or  18  only by the standby tray rollers  28 . The standby rollers  28  are controlled by a standby tray roller drive source  30  in terms of their vertical movement and rotated by a standby tray roller motor  32 . 
     Between the standby tray  10  and the processing tray  12 , horizontal alignment plates  47   a  and  47   b  shown in  FIG. 9  are installed. When the sheets P are dropped and supplied from the standby tray  10  onto the processing tray  12 , the horizontal alignment plates  47   a  and  47   b  prevent the sheets P from turning away in the horizontal direction perpendicular to the feeding direction and horizontally align them. The horizontal alignment plates  47   a  and  47   b  are formed slidably in v direction denoted by the arrow so as to fit to the width of the sheets P by a horizontal alignment motor  48 . 
     The first or second sheet discharge tray  16  or  18  is so tilted as to support the sheets P such that the position of the front ends of the sheets P becomes higher than that of the rear ends thereof. One of the first and second sheet discharge trays  16  and  18  is selected, and the selected one is moved up and down by a sheet discharge drive section  52 . The first or second sheet discharge tray  16  or  18  is moved up and down up to almost the same height as that of the sheet discharge section of the standby tray  10  or processing tray  12  when loading the sheets P to improve the consistency of the sheets P to be discharged. That is, the sheets P are not disturbed due to a difference in vertical positions. 
     At the position below the sheet discharge section of the processing tray  12  and between the processing tray  12  and sheet discharge trays  16 ,  18 , a shutter  86  which is a rear end supporting mechanism for supporting the rear end of the sheet P discharged onto the sheet discharge trays  16  and is provided. As shown in  FIGS. 10 and 11 , the shutter  86  has stationary portions  87  and movable portions  88 . The movable portions  88  can be extended relative to the stationary portions  87  and can be contracted to overlap the stationary portions  87  for housing. 
     When the sheet P is discharged from the processing tray  12  onto the first or second sheet discharge tray  16  or  18 , the movable portions  88  are contracted to overlap the stationary portions  87 , as shown in  FIG. 10 . At this time, the upper end of the movable portions  88  lies at substantially the same height as the upper end of the stationary portions  87  of the shutter  86 . 
     In a state where the movable portions  88  are contracted, center upper portions  86   a , which are upper ends of the center area that faces center alignment rollers  61   a  of the shutter  86 , are located lower than the peripheral upper portions  86   b  which are upper ends of the peripheral area. This configuration allows the center alignment rollers  61   a  to be completely exposed from the upper end of the shutter  86 . Therefore, the center portion of the sheet P that has run over the processing tray  12  and hung there from, contacts the center alignment rollers  61   a  without fail. 
     When the first or second sheet discharge tray  16  or  18  is slid, the movable portions  88  of the shutter  86  are extended. At this time, the movable portions  88  of the shutter  86  are slid in i direction denoted by the arrow in  FIG. 11  while being guided by the stationary portions  87 . The extension/contraction of the moveable portions  88  of the shutter  86  is controlled by a belt motor  62  for driving the alignment rollers  61  and conveyer belt  50 . The movable members  88  has a rack- and pinion mechanism which is driven by a link mechanism for transferring the drive of the belt motor  62  to the rack. The link mechanism has an electromagnetic clutch and controls ON/OFF of the electromagnetic clutch to thereby transfer the drive of the belt motor  62  to the pinion. 
     When the electromagnetic clutch is turned ON while the belt motor  62  drives the alignment rollers  61  in h direction denoted by the arrow in  FIG. 5 , the movable portions  88  are slidably extended in i direction denoted by the arrow in  FIG. 11 . On the other hand, when the electromagnetic clutch is turned ON while the belt motor  62  drives the alignment rollers  61  in g direction denoted by the arrow in  FIG. 5 , the movable portions  88  are slid in the direction opposite to the i direction to be contracted. The movable portions  88  can be extended up to the front face of the sheet discharge section of the processing tray  12  and, further, up to the front face of the sheet discharge section of the standby tray  10  to thereby close the sheet discharge sections of the processing tray  12  and standby tray  10 . This prevents the sheet P on the sheet discharge tray  16  or  18  from reversely being fed to the processing tray  12  or standby tray  10  when the sheet discharge tray  16  or  18  is slid upward with the sheets P loaded thereon. 
     Next, the operation of the present invention will be described. When the sheet is discharged from the processing tray  12  of the sheet post-processing apparatus  7 , the shutter  86  is located below the sheet discharge section of the processing tray  12 , as shown in  FIG. 3A . In this state, the sheet P which has been discharged onto the sheet discharge tray  16  or  18  is supported by the shutter  86  at the rear end thereof. When the sheet discharge tray  16  or  18  is slid, the shutter  86  closes the front face of the sheet discharge sections of the processing tray  12  and standby tray  10  to thereby prevent the sheet P on the sheet discharge tray  16  or  18  from reversely being fed to the processing tray  12  or standby tray  10 , as shown in  FIG. 3B . 
     After an image has been formed on a sheet P in the image forming apparatus  5 , the sheet P on which an image has been formed is discharged by the sheet discharge roller  6  of the image forming apparatus  5  to the sheet post-processing apparatus  7 . When receiving the sheet P, the sheet post-processing apparatus  7  performs a different operation, depending on a case of performing the post-processing of the sheet P or a case of performing no post-processing. Or the sheet post-processing apparatus  7  performs a different operation, depending on a case of that the preceding sheet P is in execution of the post-processing or that is finished it. 
     When the post-processing is not to be performed, the sheet P which has been fed from the entrance rollers  22  to the sheet feed rollers  24  through the sheet path ceiling  36  is fed to the standby tray  10  by the paper feed rollers  24 . Then, the sheet P is moved down onto the standby tray  10  and is fed while being held between the standby tray rollers  28  rotated in the direction of the arrow f and pinch rollers  10   e  of the standby tray  10  to be discharged onto the first sheet discharge tray  16  from the sheet discharge section of the standby tray  10 . 
     At this time, the sheet discharge tray  16  is slid from substantially the same height position as the sheet discharge section of the processing tray  12  in the direction of the arrow j up to substantially the same height position as the sheet discharge section of the standby tray  10 , as denoted by the dotted line in  FIG. 3B . At the same time, the movable portions  88  are extended in the direction of the arrow i such that the upper portions  86   a  and  86   b  thereof reach the position below the sheet discharge section of the standby tray  10 . 
     As a result, the sheet P on the sheet discharge tray  16  which has been discharged from the standby tray  10  is supported by the extended movable portions  88  of the shutter  86  at the rear end thereof. The movable portions  88  are extended by the ON operation of the electromagnetic clutch while the belt motor  62  rotates the alignment rollers  61  in the direction of the arrow h. At this time, the shutter  86  is extended such that the center upper portions  86   a  become lower than the peripheral upper portions  86   b . Therefore, even if the sheet P is discharged from the standby tray  10  in a state where its center portion in the feeding direction is deformed, the deformed portion is not brought into contact with the shutter  86 , thereby avoiding jamming of the sheet P. 
     In this way, on the first sheet discharge tray  16 , sheets P directly discharged from the standby tray  10  are sequentially loaded. The first sheet discharge tray  16  is arranged in a tilted manner such that the front end of the sheet P is positioned higher than the rear end thereof, so that, the preceding sheet P loaded on the first sheet discharge tray  16  is not pressed out by making contact with the front end of the succeeding sheet P. Namely, the discharged sheet P is sequentially loaded on the first sheet discharge tray  16  unless the order is disturbed. Even if the preceding sheet P is pressed by the succeeding sheet P and is slightly displaced, the sheet P drops by its own weight to the shutter  86  side. Therefore, the rear ends of the sheets P are brought into contact with the shutter  86  on the first sheet discharge tray  16  and thereby the alignment of the sheets P is accomplished. 
     When the first sheet discharge tray  16  is full, the first and second sheet discharge trays  16  and  18  are slid in the direction of the arrow j so that the sheet P is discharged onto the second sheet discharge tray  18 . The movable portions  88  of the shutter  86  are further extended before the slide of the first and second sheet discharge trays  16  and  18  to close the sheet discharge section of the standby tray  10 . 
     Therefore, it is possible to prevent the sheet P placed on the first sheet discharge tray  16  from reversely being fed to the standby tray  10  when the first sheet discharge tray  16  passes the front face of the standby tray  10 . After the second sheet discharge tray  18  is slid up to the position corresponding to the sheet discharge section of the standby tray  10 , the movable portions  88  are contracted so that the upper portions  86   a  and  86   b  of the shutter  86  are positioned below the sheet discharge section of the standby tray  10  to open the sheet discharge section of the standby tray  10 . In this state, the sheet P can be discharged from the standby tray  10  onto the second sheet discharge tray  18 . 
     Next, a case where staple processing which is a post-processing is to be performed and no preceding sheets P in execution of the staple processing remain on the processing tray  12  will be described. At this time, the standby tray  10  slides and moves the tray members  10   a  and  10   b  respectively up to the positions indicated by the dotted lines in  FIG. 12  in the directions of arrows m and n to open the dropping and supplying path of the sheet P. The horizontal alignment plates  47   a  and  47   b , to align the sheet P dropping from the sheet feed rollers  24  in the horizontal direction, are arranged so that the gap between the horizontal alignment plates  47   a  and  47   b  is made almost equal to the width of the sheet P. By doing this, the sheet P fed by the sheet feed rollers  24 , without the feeding being obstructed by the standby tray  10 , is dropped and supplied directly onto the processing tray  12 . 
     At the time of dropping and supplying, the upper vertical alignment roller  38   a  is shifted upward. Both sides of the sheet P drop in contact with the horizontal alignment plates  47   a  and  47   b  and are aligned in the horizontal direction. The rear end of the first sheet P dropped and supplied onto the processing tray  12  is placed on the receiving portion  44   a  of the paddle  44 , and the front end thereof is contact with the alignment rollers  61 . Then, the paddle  44  rotates in the direction of the arrow o, drops the rear end of the sheet P from the receiving portion  44   a , and beats down it onto the processing tray  12  by the beating portion  44   b . Furthermore, the paddle  44  feeds the sheet P in the direction of the arrow q by the feeding portion  44   c . At the same time, the sheet P is fed toward the stopper  45  by the alignment rollers  61  rotated in the direction of the arrow g, and the rear end of the sheet of paper P is brought into contact with the stopper  45 , and the vertical alignment of the sheet P is accomplished. 
     When the sheet P is vertically aligned on the stopper  45  side, the front end of the sheet P hangs outward from the sheet post-processing apparatus  7  as denoted by the dotted line in  FIG. 3A . At this time, the back surface of the sheet P is contact with the alignment rollers  61  and upper portions  86   a  and  86   b  of the shutter  86 . The contact with shutter  86  offers a resistance to the vertical alignment of the sheet P. 
     However, the vertical positions of the center upper portions  86   a  of the shutter  86  are lower than those of the peripheral upper portions  86   b  disposed on both sides of the center upper portions  86   a , so that the central alignment rollers  61   a  are completely projected upward from the center upper portions  86   a  of the shutter  86 . That is, at the center area, the sheet P is contact with the center alignment rollers  61   a  without fail without being affected by a resistance of the shutter  86 . Particularly, in the case of the sheet P having a large size such as A3 (JIS standard), the center portion thereof in the feeding direction is easily deformed. However, even if the center portion of the sheet P has been deformed, the sheet P can surely receive a feeding force from the center alignment rollers  61   a  without being affected by a resistance of the shutter  86 . Thus, the sheet P obtains a satisfactory vertical alignment by the alignment rollers  61 . 
     After that, the second or subsequent sheet P is dropped onto the processing tray  12  with both sides thereof brought into contact with the horizontal alignment plates  47   a  and  47   b  for horizontal alignment. The second or subsequent sheet P is, after the rear end thereof is dropped onto the receiving portion  44   a  of the paddle  44 , fed to the stopper  45  sides only by the rotation of the paddle  44 . The rotation of the paddle  44  brings the rear end of the sheet P into contact with the stopper  45 , thereby accomplishing the vertical alignment of the sheet P. The vertical alignment of the sheet P on the processing tray  12  may be performed by moving up and down the vertical alignment roller  38   a  for each alignment operation and by rotating it in the opposite direction to the arrow r. 
     In this way, the sheet P on which an image has been formed is loaded directly on the processing tray  12  from the sheet feed rollers  24  while sequentially being aligned in the horizontal direction and vertical direction. When the sheets P reach a predetermined number, the stapler  14  staples the sheets P on the processing tray  12  at a desired position and bundles them to form the sheet bundle T. Thereafter, the upper vertical alignment roller  38   a  is moved down onto the sheet bundle T. The sheet bundle T is then held between the upper vertical alignment roller  38   a  rotated in the direction of the arrow r and the lower vertical alignment roller  38   b  rotated in the direction of the arrow s and is fed toward the first sheet discharge tray  16 . 
     When the rear end of the sheet bundle T passes the upper and lower vertical alignment rollers  38   a  and  38   b , it is hooked by the feed pawl  50   a  of the conveyor belt  50  rotated in the direction of the arrow t and is sent to the first sheet discharge tray  16 . At this time, the alignment rollers  61  are rotated in the direction of the arrow h. As a result, the sheet bundle T is fed to the sheet discharge tray  16  or  18  side by feeding forces of both the conveyer belt  50  and alignment rollers  61 . Since the conveyer belt  50  and alignment rollers  61  use the same shaft  60 , they are always driven at the same time. That is, the feeding forces supplied from the conveyer belt  50  and alignment rollers  61  are continued to be applied to the sheet bundle T. As a result, the sheet bundle T is smoothly and stably fed by means of both the conveyer belt  50  and alignment rollers  61  onto the sheet discharge tray  16  or  18 . 
     The first sheet discharge tray  16  is arranged in a tilted manner and therefore the front end of the sheet P is positioned higher than the rear end thereof, so that the preceding sheet P sent to the first sheet discharge tray  16  is not pressed out by making contact with the front end of the succeeding sheet bundle T. Further, even if the preceding sheet bundle T is slightly displaced by the succeeding sheet P, since the first sheet discharge tray  16  is arranged in a tilted manner, the sheet bundle T drops by its own weight on the shutter  86  side and is loaded on the first sheet discharge tray  16  with the rear end thereof aligned at the point of contact with the shutter  86 . 
     Next, a case where the staple processing which is a post-processing is to be performed and preceding sheets P in execution of the staple processing remain on the processing tray  12  will be described. At this time, the standby tray  10  slides and moves the tray members  10   a  and  10   b  from the position indicated by the dashed line in  FIG. 12  respectively in the opposite direction of the direction of the arrow m and in the opposite direction of the direction of the arrow n, and is moved to the position indicated by the solid line shown in  FIG. 12 , and can support the sheet P. The standby tray rollers  28  are shifted above the standby tray  10  so as not to disturb the sheets P. The sheets P discharged from the image forming apparatus  5  and fed by the sheet feed rollers  24  are loaded once on the standby tray  10  to wait for the processing tray  12  to be free. 
     After the sheets P are loaded on the standby tray  10 , the standby tray rollers  28  are moved down onto the standby tray  10  and are rotated in the opposite direction of the direction of the arrow f. As a result, the sheets P are sent toward the standby stoppers  10   c  and  10   d  and are vertically aligned with the rear end of the sheets P brought into contact with the standby stoppers  10   c  and  10   d.    
     Since the standby tray  10  is arranged in a tilted manner, a force in the direction of the standby stoppers  10   c  and  10   d  is applied to the sheets P by the own weight. Thus, even if, for example, the succeeding sheet P is fed from the sheet feed rollers  24  in a state that it is curled convexly and is fed to the standby tray  10 , the preceding sheet P loaded on the standby tray  10  is not pressed out by making contact with the front end of the succeeding sheet P. Namely, the fed sheet P is sequentially loaded on the standby tray  10  unless the order is disturbed. Further, even if the preceding sheet P is pressed by the succeeding sheet P and is slightly displaced, since the standby tray  10  is arranged in a tilted manner, the sheet P drops by its own weight down to the position where the rear end thereof is brought into contact with the standby stoppers  10   c  and  10   d  and is loaded on the standby tray  10  with the rear end aligned. 
     During this period, when the preceding sheet P on the processing tray  12  is discharged on the side of the first sheet discharge tray  16  and the processing tray  12  becomes free, the standby tray  10  slides and moves the tray members  10   a  and  10   b  respectively up to the positions indicated by the dotted lines in  FIG. 12  in the directions of the arrows m and n from the position indicated by the solid line in  FIG. 12  via the position indicated by the alternate long and short dash line in  FIG. 12 . By doing this, for example, two sheets P standing by on the standby tray  10 , when the tray members  10   a  and  10   b  reach the position indicated by the alternate long and short dash line in  FIG. 12 , are dropped and supplied onto the processing tray  12  from between the tray members  10   a  and  10   b . At this time, the horizontal alignment plates  47   a  and  47   b  are arranged so as to make the interval between them almost equal to the width of the sheets P. Therefore, the sheets P dropped from the standby tray  10  are controlled on both sides by the horizontal alignment plates  47   a  and  47   b  and are aligned horizontally. 
     The lower side sheet P of the two sheets P dropped onto the processing tray  12  is sent in the direction of the arrow q by the lower vertical alignment roller  38   b  rotated in the opposite direction of the direction of the arrow s, and the rear end of the sheet P is brought into contact with the stopper  45 , and the vertical alignment of the sheet P is accomplished. The upper side sheet P of the two sheets P dropped onto the processing tray  12  is sent in the direction of the arrow q by the upper vertical alignment roller  38   a  rotated in the opposite direction of the direction of the arrow r, and the rear end of the sheet P is brought into contact with the stopper  45 , and the vertical alignment of the sheet P is accomplished. Thereafter, the upper vertical alignment roller  38   a  is shifted upward. 
     The third and subsequent sheets P discharged from the image forming apparatus  5  are directly dropped and supplied onto the processing tray  12  from between the tray members  10   a  and  10   b  without standing by on the standby tray  10 . Thereafter, the third and subsequent sheets P are sequentially aligned on the sheets P loaded earlier on the processing tray  12  by the paddle  44 . 
     When the sheets P loaded on the processing tray  12  reach a predetermined number, the sheets P are stapled by the stapler  14  to form a sheet bundle T. Thereafter, the sheet bundle T is fed toward the first sheet discharge tray  16  by the upper and lower vertical alignment rollers  38   a  and  38   b . When the rear end of the sheet bundle T passes the upper and lower vertical alignment rollers  38   a  and  38   b , it is hooked by the feed pawl  50   a  of the conveyor belt  50  and is sent to the first sheet discharge tray  16 . At this time, the alignment rollers  61  are rotated in the direction of the arrow h. As a result, the sheet bundle T is fed onto the sheet discharge tray  16 . 
     Since the conveyer belt  50  and alignment rollers  61  use the same shaft  60 , they are always driven at the same time. That is, the feeding forces supplied from the conveyer belt  50  and alignment rollers  61  are continued to be applied to the sheet bundle T. As a result, the sheet bundle T is smoothly and stably fed by means of both the conveyer belt  50  and alignment rollers  61  onto the sheet discharge tray  16  or  18 . 
     When the first sheet discharge tray  16  is full while the sheet bundles T are sequentially discharged, subsequent sheet bundles T are discharged onto the second sheet discharge tray  18  in place of the first sheet discharge tray  16 . In order for the sheet bundle T to be discharged onto the second sheet discharge tray  18 , the first and second sheet discharge trays  16  and  18  are slid in the direction of the arrow j. At this time, the movable portions  88  of the shutter  86  are extended in the i direction denoted by the arrow in  FIG. 11  to close the sheet discharge sections of the processing tray  12  and standby tray  10 , as shown in  FIG. 3B . 
     Therefore, it is possible to prevent the sheet bundle T placed on the first sheet discharge tray  16  from reversely being fed to the processing tray  12  or standby tray  10  when the first sheet discharge tray  16  passes the sheet discharge section of the processing tray  12  or standby tray  10 . After the second sheet discharge tray  18  is slid up to the position corresponding to the sheet discharge section of the processing tray  12 , the movable portions  88  of the shutter  86  are contracted to the position shown in  FIG. 10 . That is, the movable portions are positioned below the sheet discharge section of the processing tray  12  to open the sheet discharge section of the processing tray  12 . In this state, the sheet bundle T can be discharged from the processing tray  12  onto the second sheet discharge tray  18 . 
     In this embodiment having such a configuration, when the staple processing is to be performed after image forming and the preceding staple processing is not finished on the processing tray  12 , the standby tray  10  installed above the processing tray  12  waits for the succeeding sheets P. Thereafter, after the processing tray  12  becomes free, the sheets P standing by on the standby tray  10  are dropped and supplied and then are moved onto the processing tray  12 . Therefore, the practical feeding path from the standby tray  10  to the processing tray  12  in the sheet post-processing apparatus  7  can be shortened and the sheet post-processing apparatus can be miniaturized. 
     Further, the center upper portions  86   a  of the shutter  86 , which support the rear end of the sheet P discharged onto the sheet discharge tray  16  or  18  are located lower than the peripheral upper portions  86   b . Therefore, the sheet P can contact the center alignment rollers  61   a  without fail on the processing tray  12  without being obstructed by the shutter  86 . As a result, the vertical alignment of the sheet P on the processing tray  12  can be achieved. Further, even when the movable members  88  of the shutter  86  are extended up to the position corresponding to the sheet discharge section of the standby tray  10 , the center upper portions  86   a  are located lower than the peripheral upper portions  86   b . Therefore, even if the sheet P is discharged from the standby tray  10  in a state where its center portion in the feeding direction is deformed, the deformed portion is not brought into contact with the shutter  86 , thereby avoiding jamming of the sheet P. 
     Further, in the present embodiment, the alignment rollers  61  and conveyer belt  50  have the same shaft  60 . That is, the feeding forces supplied from the conveyer belt  50  and alignment rollers  61  are continued to be applied to the sheet bundle T. As a result, the sheet bundle T is effectively be discharged onto the sheet discharge tray  16  or  18 . Further, the movable members  88  of the shutter  86  are extended while the sheet discharge trays  16  and  18  are slid to close the sheet discharge sections of the processing tray  12  and/or standby tray  10 . Therefore, it is possible to prevent the sheet P placed on the first sheet discharge tray  16  from reversely being fed to the processing tray  12  or standby tray  10  while the first sheet discharge tray  16  is slid. 
     The present invention is not limited to the above embodiment and various modifications are possible within the scope of the invention. For example, a method of dropping and supplying the sheet P from the standby tray to the processing tray is not limited to one based on the slide movement but may be implemented by a rotational movement. Further, the post-processing is not limited to the staple processing, but may be any processing, such as hole-punching processing in sheets, as far as it is post-processing to be applied to the sheet. 
     The values of the width of the center area of the rear end supporting mechanism and difference in the vertical position between the center area and peripheral area thereof is not limited, and they may be set to any value as far as they fall within a range where the rear end of the sheet on the sheet discharge tray can appropriately be supported, where the alignment rollers can be contact with the back surface of the sheet without fail, and where the deformed portion is not brought into contact with the rear end supporting mechanism when the sheet is discharged onto the sheet discharge tray. Further, a drive means for moving the movable sections of the rear end supporting mechanism may have any configuration.