Patent Publication Number: US-9890011-B2

Title: Sheet processing apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet processing apparatus performing such processes of aligning and binding a plurality of sheets and to an image forming apparatus employing the same. 
     Description of the Related Art 
     A sheet processing apparatus requires a certain processing time in general in performing a process including a binding process or the like on sheets. This processing time exceeds, in most cases, a sheet discharge interval of the sheets discharged out of an image forming apparatus, and it is difficult to complete the binding process or the like within the sheet discharge interval. 
     Hitherto, in order not to drop productivity of an image forming process by interrupting the image forming process, a method of retaining the first several sheets after the beginning of the process and making them temporarily stand by during the process is widely adopted as ‘buffering’. Japanese Unexamined Patent Application Publication No. 10-181988 discloses a buffer unit configured such that a plurality of sheets fed from an upstream conveyance path is sequentially wound around a roller having a large diameter while shifting leading edges thereof so as to overlap with each other. The buffer unit then discharges the plurality of wound sheets to a downstream conveyance path in response to an output signal. 
     In terms of an amount of the shift among the sheets during the buffering operation, it is difficult to align the both sheets in a conveyance direction if the sheet (lower sheet) closer to a tray surface (supporting surface) of a processing tray is shifted upstream more than the sheet (upper sheet) more distant from the tray surface. Meanwhile, it is difficult to convey a plurality of sheets while precisely superposing edges of the sheets. Thus, a target value of the shift amount is set in general such that the lower sheet proceeds downstream with respect to the upper sheet. The shift amount among the sheets is set to be around 3 to 10 mm in general and to be constant during the buffering operation in the document described above and other known apparatuses. 
     For instance, in a case of a saddle stitching binding process (saddle processing), sheets are released onto a processing tray from a discharge roller pair. A certain sheet processing apparatus is provided with an inertia alignment system in which the sheets or the sheet bundle formed by the buffering operation are abutted against a leading edge regulating member on the processing tray and are aligned by the gravity acting on the sheet or the sheet bundle and by inertia given them in discharging operation. 
     Still further, there is another aligning system for aligning buffered sheets in which the lower sheet is conveyed to abut against the leading edge regulating member at first, and then the upper sheet is aligned by an alignment member such as a paddle. In the case of this aligning system, because it is necessary to align only the upper sheet, the alignment member is preferred to be a sliding member performing aligning operation while sliding on the upper side of the sheets, like the paddle, rather than a nipping member nipping the sheets. Therefore, it is difficult to increase force for aligning the sheet. 
     Next, an inter-stack time during which a processing operation is performed will be described. In a case of not performing buffering operation, the inter-stack time is determined by subtracting a conveyance time equivalent to a sheet length from a time interval from a leading edge of a preceding sheet to a leading edge of a succeeding sheet (referred to as Top-to-Top hereinafter) which is determined by productivity of each system. That is, the inter-stack time is expressed by Top-to-Top−L/V, where L is a sheet conveyance length and V is a sheet conveyance speed. In a case of performing the buffering operation, since the preceding sheet is retained to overlap with the succeeding sheet, the inter-stack time may be expressed as 2×Top-to-Top−L/V. Thus, it can be seen that the inter-stack time increases. Because the sheet length in the case of performing the buffering operation is a length of the overlapping sheets (a length of the sheet bundle), sheet conveyance length equals the sum of the original sheet length L and the buffering shift amount. That is, the larger the buffering shift amount, the shorter the inter-stack time is. Since this is not preferable for the original purpose of assuring a processing time, the buffering shift amount has been preferred to be as small as possible. 
     As illustrated in  FIG. 15A , when sheets S 1  and S 2  are conveyed while overlapping with each other (buffering conveyance), a pasting force Fa is generated between the sheets. When sheets with a fine surface and having a high smoothness such as a coated sheet, a film, and an OHP sheet are buffered, air between the sheets is eliminated by a force bringing the sheets close to each other such as a nip pressure of a conveying roller pair for example. That is, the sheets are put into a vacuum-like condition by which the sheets are stuck on each other. In this case, the pasting force Fa becomes very large. In the case of the coated sheet or the like where pasting force Fa is large, pasting force Fa is overwhelmingly larger than an aligning force Fb (inertia force in the inertia alignment system for example) of the inertia alignment system or the alignment system employing the slide returning member. Therefore, alignment accuracy is partially lost as illustrated in  FIG. 15B  even after performing the alignment operation, leading to degradation of quality of resultant products. 
     It is desirable to perform the buffering operation from aspects of avoiding drop of productivity and of assuring the inter-stack time. However, it has been difficult to align the sheets as described above in buffering the smooth sheets such as the coated sheet, the film, and the OHP sheet. Still further, even in a case of a plain sheet whose surface is not treated, it is conceivable that pasting force Fa between the sheets increases to drop the alignment accuracy when environmental humidity is high or concentration of an output image is high (in a case of a solid coated image for example). Then, many apparatuses have been configured not to perform the buffering operation and while accepting the drop of productivity by pausing the image forming process during the processing operation. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present disclosure, a sheet processing apparatus includes a sheet overlap processing portion configured to form a sheet bundle in which a plurality of sheets to be processed overlap each other while being shifted from each other, a conveyance roller pair configured to convey the sheet bundle formed by the sheet overlap processing portion, a sheet supporting portion supporting the sheet bundle conveyed by the conveyance roller pair, and an end regulation member configured to abut with and regulate a downstream end, in a conveyance direction of the conveyance roller pair, of the sheet bundle supported on the sheet supporting portion. The sheet processing apparatus further includes a control portion configured to control a distance H and/or a shift amount L 1  such that a relationship of L&lt;H≦L+L 1  holds, where L is a length of each sheet among the sheet bundle in the conveyance direction, H is a distance from the conveyance roller pair to the end regulation member in the conveyance direction, and L 1  is a shift amount between a first sheet among the sheet bundle and a second sheet adjacent to the first sheet among the sheet bundle. 
     According to another aspect of the present disclosure, a sheet processing method includes steps of making a first sheet and a second sheet overlap each other such that the first sheet precedes in a sheet conveyance direction with respect to a second sheet adjacent the first sheet, aligning the first sheet and the second sheet by conveying the first sheet and the second sheet overlapping each other along the conveyance direction of the conveyance roller pair such that a downstream end in the conveyance direction abuts against an end regulation member, and processing the first and second sheets aligned by the aligning step. The aligning step includes a separating step of conveying the second sheet toward downstream in the conveyance direction by the conveyance roller pair in a state in which an upstream end in the conveyance direction of the first sheet is discharged from the conveyance roller pair and the downstream end of the first sheet is in contact with the end regulation member, so as to relatively move the second sheet with respect to the first sheet. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view schematically illustrating a copier, i.e., an image forming apparatus, including a sheet processing apparatus of the present disclose. 
         FIG. 2  is a section view schematically illustrating a finisher. 
         FIG. 3  is a section view illustrating a saddle processing portion. 
         FIG. 4  is a section view illustrating a sheet supporting portion (processing tray) of the saddle processing portion. 
         FIG. 5  is an enlarged section view of the saddle processing portion. 
         FIG. 6A  is a schematic diagram illustrating a first stage of a sheet aligning operation. 
         FIG. 6B  is a schematic diagram illustrating a second stage of the sheet aligning operation. 
         FIG. 6C  is a schematic diagram illustrating a third stage of the sheet aligning operation. 
         FIG. 6D  is a schematic diagram illustrating a fourth stage of the sheet aligning operation. 
         FIG. 6E  is a schematic diagram illustrating a fifth stage of the sheet aligning operation. 
         FIG. 7A  is a schematic diagram illustrating a first stage of buffering operation (an overlap processing operation). 
         FIG. 7B  is a schematic diagram illustrating a second stage of the buffering. 
         FIG. 7C  is a schematic diagram illustrating a third stage of the buffering. 
         FIG. 8  is a block diagram illustrating a control system of the image forming apparatus. 
         FIG. 9  is a block diagram illustrating a control structure of the finisher. 
         FIG. 10A  is a diagram illustrating a first buffered sheet aligning operation of the saddle processing portion. 
         FIG. 10B  is a diagram illustrating a state of the saddle processing portion after executing the buffered sheet aligning operation. 
         FIG. 11A  is a diagram illustrating a second buffered sheet aligning operation of the saddle processing portion. 
         FIG. 11B  is a diagram illustrating a state of the saddle processing portion after executing the buffered sheet aligning operation. 
         FIG. 12A  is a diagram illustrating an initial stage of a third buffered sheet aligning operation of the saddle processing portion. 
         FIG. 12B  is a diagram illustrating an intermediate stage of the buffered sheet aligning operation. 
         FIG. 12C  is a diagram illustrating a state of the saddle processing portion after executing the buffered sheet aligning operation. 
         FIG. 13A  is a flowchart illustrating a control process of the finisher. 
         FIG. 13B  is a flowchart illustrating a sheet aligning process in processing a high smoothness sheet. 
         FIG. 14  is a diagram illustrating a buffered sheet aligning operation of a processing tray of a different embodiment. 
         FIG. 15A  is a diagram illustrating a conventional buffered sheet aligning operation. 
         FIG. 15B  is another diagram illustrating the conventional buffered sheet aligning operation. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An image forming apparatus of the present disclosure will be described below with reference to the drawings. This image forming apparatus includes a sheet processing apparatus configured to align and process sheets discharged onto a processing tray in a discharge direction, and may be a copier, a printer, a facsimile, or a multifunction printer for example. In the following description, a monochromatic/color copying machine (refers to as a ‘copier’, hereinafter)  1000  will be exemplified and described as the image forming apparatus. 
       FIG. 1  is a sectional view schematically illustrating the copier  1000  of the present embodiment.  FIG. 2  is a section view schematically illustrating a finisher  100 , i.e., a sheet processing apparatus of the present embodiment. As shown in  FIG. 1 , the copier  1000  includes a copier body  600  and the finisher  100 . The finisher  100  of the present embodiment is detachably attached to the copier body  600  which serves as a copier by itself. In other words, the finisher  100  is an optional attachment of the copier body  600 . It is noted that while the detachable finisher  100  will be described in the present embodiment, the finisher  100  and the copier body  600  may be integrated as a single unit. That is, both the copier body to which the finisher is attached as the optional device and the copier body into which the fisher is integrally assembled are examples of the image forming apparatus. 
     The copier body  600 , i.e., a body of the image forming apparatus, includes a sheet storage portion  602 , a sheet feeding portion  603  feeding a sheet S stored in the sheet storage portion  602 , and an image forming portion  604  forming an image on the sheet S fed by the sheet feeding portion  603 . The copier body  600  also includes an operation portion  607  to be operated by a user in setting the sheet. The operation portion  607  is a discrimination portion discriminating sheet information such as a basis weight, classification of a coated or non-coated sheet, a length in a sheet conveyance direction, and a width in a width direction orthogonal to the sheet conveyance direction. The copier body  600  further includes a document feeder  605  feeding a document and an image reader  606  reading image information from the document fed by the document feeder  605 . The sheet storage portion  602  includes cassettes  909   a  and  909   b  each storing the sheet S. The sheet S stored in the cassettes  909   a  and  909   b  is fed to the image forming portion  604  at a predetermined timing by the sheet feeding portion  603 . The image forming portion  604  includes photosensitive drums  914   a ,  914   b ,  914   c , and  914   d  on which color toner images of yellow, magenta, cyan, and black are formed, respectively, and is configured to transfer the respective color toner images formed on the respective photosensitive drums onto the sheet S. Thus, an unfixed toner image is formed on the sheet S. Then, the unfixed toner image is fixed by a fixing unit  904 , and the sheet S is discharged to the finisher  100  by a discharge roller  907 . It is noted that in a case of duplex printing, the sheet S is reversed by a reverse roller  905 , and the reversed sheet S is conveyed to the image forming portion  604  again by conveyance rollers  906   a ,  906   b ,  906   c ,  906   d ,  906   e , and  906   f  provided along a reverse conveyance path. Then, the abovementioned image forming operation is repeated again. Meanwhile, in a case of forming an image of image information of the document described above, the image information of the document fed from the document feeder  605  is read by the image reader  606 . Then toner images corresponding to the image information are formed on the photosensitive drums  914   a ,  914   b ,  914   c , and  914   d , and are transferred onto the sheet S, and are fixed on the sheet S. 
     A basic operation of the finisher (sheet processing apparatus)  100  will now be described. The finisher  100  is connected downstream of the copier body  600  and is configured to receive a plurality of sheets S fed from the copier body  600  and to perform a stapling process in a stapling processing portion  140 , a saddling process in a saddle processing portion  800 . As illustrated in  FIG. 2 , the sheet S fed from the copier body  600  is passed to an inlet roller pair  102  of the finisher  100 . At this time, an inlet sensor  101  simultaneously detects a passing timing of the sheet S. A lateral registration detection sensor  104  detects an end position of the sheet S conveyed by the inlet roller pair  102  and passing through a conveyance path  103 , i.e., an upstream conveyance path. The lateral registration detection sensor  104  detects a transverse registration error of the sheet S with respect to a center position. 
     In response to the detection of the lateral displacement detected by the lateral registration detection sensor  104 , a shift unit  108  conducts a shift operation (referred to also as a ‘transverse registration detection process’ hereinafter) of the sheet S being conveyed by shift roller pairs  105  and  106  by moving by a predetermined amount in a front/back direction. After finishing the shift operation conducted by the shift unit  108 , the sheet S is conveyed by a conveyance roller pair  110 . The sheet S conveyed by the conveyance roller pair  110  is conveyed downstream further by a buffer roller pair  115 . In a case where the sheet S is to be discharged to an upper stacking tray  136 , an upper path switch member  118  moves toward an upper path conveyance path by a driving member such as a solenoid not illustrated. Thereby, the sheet S is lead to an upper path  117  and is discharged onto the upper stacking tray  136  by an upper discharge roller pair  120 . It is noted that a buffering process (sheet overlap process) conducted by a buffer processing portion  100 C will be described later. 
     Meanwhile, in a case where the sheet S is not discharged to the upper stacking tray  136 , the upper path switch member  118  is moved a position of a solid line indicated in  FIG. 2 . Thereby, the sheet S is lead to and passes through a stack conveyance path  121 , i.e. a downstream conveyance path, by a buffer roller pair  122  and a stack conveyance roller pair  124 . In a case where a saddle stitching binding process (saddle processing) is to be performed on the sheet S, a saddle path switch member  125  is moved to a saddle path  133  side by a driving member such as a solenoid not illustrated. Then, the sheet S is conveyed to the saddle path  133  and is lead to a saddle processing portion  800 , i.e., a saddle unit, by an inlet roller pair  134  to undergo the saddle stitching binding process (saddle processing). It is noted the saddle processing conducted by the saddle processing portion  800  will be described later. 
     In a case where no saddle stitching binding process (saddle processing) is to be conducted, the saddle path switch member  125  is moved to a position of a solid line indicated in  FIG. 2 . In a case where a binding process is to be conducted, the sheet S conveyed by the stack conveyance roller pair  124  is sequentially conveyed onto a processing tray  138  of a stapling processing portion  140  to conduct the binding process by a stapler  132 . In a case where no binding process is to be conducted by the stapling processing portion  140 , the sheet S is discharged onto a lower stacking tray  137  without going through the processing tray  138 . 
     The stapling processing portion  140  includes a sheet detecting sensor  127  and a discharge roller  128  disposed along the stapler path  126 . A knurled belt  158  is nipped by the discharge roller  128  and is disposed so as to be in contact with the processing tray (sheet supporting portion)  138 . The processing tray  138  is inclined moderately with an approximately horizontal angle and is provided with an erectable leading edge stopper  138   a , i.e., an end regulation member, at a lower end side thereof. The processing tray  138  is also provided with a lower roller  130  that can be normally and reversely driven at an upper end thereof and an upper roller  129  driven to open/close with respect to the lower roller  130  by an arm so as to be in contact with/separate from the lower roller  130 . The upper roller  129  and the lower roller  130  compose a conveyance roller pair having a nip in a state when the upper roller  129  is closed. A paddle  131  in which a plurality of flexible members radially extend is disposed behind the discharge roller  128  so as to come into contact with the processing tray  138 . A stapler  132  is disposed on the leading edge stopper  138   a  side (the lower end) of the processing tray  138 . 
     The sheet conveyed through the stapler path  126  is discharged out of the discharge roller  128  onto the processing tray  138 . At this time, the paddle  131  is located at its home position not in contact with the sheet, and the upper roller  129  is located at an open position separated from the lower roller  130 . In a case when the sheet is a long sheet, a leading edge of the sheet projects out of the lower roller  130 , and is discharged such that the sheet hangs on the lower stacking tray  137 . A trailing edge of the sheet is guided by the knurled belt  158  and is lead to the processing tray  138 . When the sheet detecting sensor  127  detects the trailing edge of the sheet, the paddle  131  is driven and the upper roller  129  moves to a close position. Then, the upper roller  129  and the lower roller  130  rotate reversely. Thereby, the sheet on the processing tray  138  is pulled into a direction opposite to the direction in which the sheet is discharged by the discharge roller  128 . That is, the trailing edge of the sheet becomes a leading edge, and the sheet abuts against the leading edge stopper  138   a  erecting on the processing tray  138  by being conveyed by the paddle  131  and the knurled belt  158 , i.e., sliding alignment members. In the same manner, a predetermined number of sheets discharged onto the processing tray  138  are conveyed toward the leading edge stopper  138   a . Then, the respective sheets abut against the leading edge stopper  138   a  and are thus aligned as an aligned sheet stack. In this state, the leading edge stopper  138   a  recedes from the processing tray  138  and the binding process is conducted by the stapler  132 . The bound sheets (sheet bundle) is moved back to the side of the roller pair  129  and  130  by a rear-pushing member not illustrated and is discharged by the rotating roller pair  129  and  130  onto the lower stacking tray  137 . 
     Next, the saddle processing portion  800  will be described with reference to  FIGS. 3 through 6E . As illustrated in  FIGS. 3 and 4 , the saddle processing portion  800  includes a processing tray  15 , i.e., a sheet supporting portion, of a steep angle close to verticality, that is, an approximately vertical angle. A space  831  is formed upstream of and above the processing tray  15 . A conveying-in path  833  is partitioned from the space  831  by a guide  830  and is formed on one side, i.e., on a side of the saddle body  832 , of an upper side (left side in  FIGS. 3 and 4 ) of the space  831 . A saddle inlet sensor  62  and an inlet roller pair  801 , i.e., a conveyance roller pair, are disposed sequentially from upstream along the conveying-in path  833 , and a push member  12  is disposed from a position where the guide  830  is terminated. The push member  12  is configured to be movable in a direction coming into contact with/separating from the processing tray  15  by a link mechanism. The push member  12  stays at a stand-by position erecting on the saddle body  832  side in receiving the sheet and moves to an operating position urging the sheet to another side, i.e., to the processing tray  15  side, when the trailing edge of the sheet passes through the inlet roller pair  801 . 
     A swingable pressure member  11  is disposed on an upstream side of the processing tray  15 . The pressure member  11 , whose tip is hooked, presses a trailing edge (upstream end) of the sheet pushed down to the processing tray  15 . An intermediate part and a downstream part of the processing tray form a passage  835  having a narrow vertical width for guiding a predetermined number of sheets. A stapler  820 , an intermediate roller  804 , an alignment plate  815 , an alignment roller  802 , and a leading edge stopper  805 , i.e., an end regulation member, are sequentially disposed along the passage  835  from upstream to downstream. The intermediate roller  804  and the alignment roller  802  are configured to convey the sheet or the sheet stack on the processing tray  15  and the alignment plate  815  is configured to move in a direction orthogonal to a sheet conveyance direction to align a side edge of the sheet. The leading edge stopper  805  is movable in a vertical direction along the processing tray  15  and abuts against the leading edge (downstream end) of the sheet discharged onto the processing tray  15  to align a position in the conveyance direction (vertical direction) of the sheet. The stapler  820  binds the sheet stack on the processing tray  15  by a plurality of widthwise points at a middle position in a lengthwise direction. 
     A folding plate  803  is disposed beneath the processing tray  15  and on a downstream of the intermediate roller  804 , and a folding roller pair  819  is disposed within the saddle body  832 , i.e., on the side opposite from the processing tray  15 . The folding plate  803  is configured to move across the processing tray  15 , and the folding roller pair  819  is configured to nip the sheet bundle folded in two by the folding plate  803  and to convey while gripping the sheet bundle folded in two (see  FIG. 5 ). The saddle body  832  is provided with a conveyance path  836  extending in a transverse direction and conveying the folded sheet bundle. Conveyance roller pairs  839  and  840  are disposed along the conveyance path  836 . The sheet bundle that has undergone through the saddle stitching binding process (saddling process) is discharged out of the conveyance path  836  on a stacking tray not illustrated. 
     Next, a basic operation of the saddle processing conducted by the saddle processing portion  800  will be described with reference to  FIGS. 6A through 6E . The sheet lead from the saddle path  133  (see  FIG. 2 ) to the saddle processing portion (saddling unit)  800  is released by the inlet roller pair  801  so as to head toward the leading edge regulation member along the passage  835 . At this time, as illustrated in  FIG. 6A , the push member  12  is located at the stand-by position erecting along the saddle body  832  and the pressure member  11  is located at its stand-by position of the closed state in which the tip thereof is in contact with the processing tray  15 . Still further, the leading edge stopper  805  is held at its stand-by position such that a middle part of the sheet corresponds to a binding position of the stapler  820 , and the alignment plate  815  is located at its stand-by position separated from the sheet side edge. In the state when the downstream end (leading edge) of the preceding sheet S 1  passes through the inlet roller pair  801 , the preceding sheet S 1  abuts against the leading edge stopper  805  and is aligned in the conveyance direction of the preceding sheet S 1  due to the gravity of the sheet owing to the steep inclination of the processing tray  15  and to inertia of the sheet caused by the release of the sheet by the inlet roller pair  801 . Still further, the alignment plate  815  moves in the direction orthogonal to the conveyance direction to align the sheet in the width direction. 
     As illustrated in  FIG. 6B , when the saddle inlet sensor  62  detects that the upstream end (trailing edge) of the preceding sheet S 1  has passed through the inlet roller pair  801  and the guide  830 , the pressure member  11  moves from the stand-by position to the open position. Still further, as illustrated in  FIG. 6C , the push member  12  moves from the stand-by position to the processing tray  15  side and moves the upstream end (trailing edge) side of the preceding sheet S 1  onto the processing tray  15 . In this state, as illustrated in  FIG. 6D , the pressure member  11  is moved back again to the close position, i.e., the stand-by position, and presses the upstream end (trailing edge) of the sheet S so as to be placed on the processing tray  15 . The push member  12  is returned back again to the stand-by position, i.e., the erecting position. 
     In this state, as illustrated in  FIG. 6E , a next sheet S 2  is conveyed into the conveying-in path  833  by the inlet roller pair  801 . Then, similarly to the preceding sheet S 1 , the succeeding sheet S 2  abuts a leading edge of the sheet against the leading edge stopper  805  on the processing tray  15 , is aligned in a width direction of the sheet by the alignment plate  815 , and is aligned with the preceding sheet S 1 . An operation of forcibly urging the preceding sheet S 1  so as to press against the processing tray  15  to avoid the leading edge of the succeeding sheet S 2  from hitting against the trailing edge of the preceding sheet S 1  will be referred to as a trailing edge sorting hereinafter (see Step S 204  in  FIG. 13A ). 
     As described above, a predetermined number of sheets is aligned on the processing tray  15  while sorting the trailing edges to form a sheet stack composed of the predetermined number of sheets. The sheet stack abuts against and is aligned by the leading edge stopper  805  (end regulation member) located at an upper position and is bound by the stapler  820  in this condition. The leading edge stopper  805  is lowered by a predetermined amount so as to move to a lower position in this condition and to assist such that the sheet bundle is moved altogether to the leading edge stopper  805  by the intermediate roller  804  and the alignment roller  802 . The sheet bundle is positioned by stopping the leading edge stopper  805  at a position where a binding position at a middle part of the sheet bundle corresponds to the folding plate  803 . In this condition, the folding plate  803  is moved to project as illustrated in  FIG. 5  such that the middle part of the sheet bundle is gripped by the nip of the folding roller pair  819 . The folding roller pair  819  grips and conveys the sheet bundle while folding in two at the binding position, i.e., the middle part of the sheet bundle. The sheet bundle folded in two is conveyed further through the conveyance path  836  by the conveyance roller pairs  839  and  840  and is discharged on to the stacking tray. 
     Succeedingly, a buffer processing portion  100 C, i.e., a sheet overlap processing portion, that functions in the binding process conducted by the stapling processing portion  140  and in the saddling process conducted by the saddle processing portion  800  will be described with reference to  FIGS. 7A through 7C . Because a certain period of time is required in conducting the binding process or the saddle stitching and double folding process in the stapling processing portion  140  and the saddle processing portion  800  described above, the sheet on which an image has been formed is temporarily retained in the buffer processing portion  100 C within the finisher  100  during those process. In order to temporarily retain the preceding sheet on which the image has been formed, the buffer processing portion  100 C, i.e., the sheet overlap processing portion, makes the preceding sheet, i.e., a first sheet, and a succeeding sheet, i.e., a second sheet adjacent to the first sheet, overlap each other so as to form a sheet bundle. This arrangement makes it possible to continuously form images without interrupting the image forming operation of the copier body  600  or to continue the image forming process with a short interrupt period of time at least as compared to a case in which no sheet is buffered. This arrangement makes it also possible to continuously convey the sheets on which the images have been formed to the finisher  100  at processing intervals of the copier body  600  itself or at intervals close to that intervals and thus to prevent productivity of the copier body  600  from dropping. 
     The buffer processing portion  100 C includes a stack conveyance path  121  to which the sheet whose lateral registration and aligning process has been conducted by the shift unit  108  is conveyed by the conveyance roller pair  110  from the conveyance path  103  and a buffer path  113 , i.e., a branched path, branched from a connecting part, i.e., a branch portion, of the conveyance path  103  and the stack conveyance path  121 . A buffer switch member  114  is disposed at the connecting part (A). The buffer switch member  114  switches the sheet fed from the conveyance path  103  to a first position leading to the stack conveyance path  121  and to a second position leading the sheet from the stack conveyance path  121  to the buffer path  113 . The stack conveyance path  121  includes a buffer roller pair  115 , i.e., a first roller pair, capable of rotating normally and reversely and a second buffer sensor  116 . A second buffer roller pair  112 , i.e., a second roller pair, capable of rotating normally and reversely is disposed along the buffer path  113 . It is noted a first buffer sensor  109 , i.e., a sensor for sheet overlap processing portion, is disposed upstream of the conveyance roller pair  110  of the conveyance path  103 . During the buffering process, the upper path switch member  118  is located at a position of a solid line in  FIGS. 2 and 7A  and the sheet will not be lead to the upper path  117  (see  FIG. 2 ). 
     Next, an operation of the buffering process conducted by the buffer processing portion  100 C will be described. The stapling processing portion  140  (binding processing portion) and the saddle processing portion  800  conduct the operation such as the binding process requiring a certain period of time and primarily retain (or buffer) the sheets on which the images have been formed and fed from the copier body  600  at predetermined intervals in the buffer processing portion  100 C. The binding process is actuated when the first buffer sensor  109  detects the leading edge of the preceding sheet S 1  to be retained. The buffer switch member  114  is held at a first position indicated by a solid line in  FIG. 7A , and the preceding sheet S 1  is conveyed by the conveyance roller pair  110  from the conveyance path  103  to the stack conveyance path  121 . The buffer sensor  116  detects the leading edge of the preceding sheet S 1  in the stack conveyance path  121 . Thereby, a position of a trailing edge (downstream end) of the preceding sheet S 1  is detected from a sheet size recognized in advance or a conveyance elapsed time of the first buffer sensor  109  and the buffer sensor  116 . As illustrated in  FIG. 7A , the preceding sheet S 1  is conveyed by the buffer roller pair  115  until when the sheet trailing edge (downstream end) passes over the branch portion A. 
     When the trailing edge of the preceding sheet S 1  passes over the branch portion A, the buffer switch member  114  is changed over to the second position indicated by a broken line. As illustrated in  FIG. 7B , the preceding sheet S 1  is reversely conveyed by when the buffer roller pair  115  reversely rotates. Then, the preceding sheet S 1  reversely conveyed by from the trailing edge as a head by the buffer switch member  114  located at the second position is lead to the buffer path  113 . Then, the preceding sheet S 1  is conveyed to the buffer path  113  by the second buffer roller pair  112 . During the reverse conveyance of the preceding sheet S 1 , the buffer sensor  116  detects timing when the trailing edge of the preceding sheet S 1  passes through and the buffer sensor  116  is turned off. Then, a predetermined reverse conveyance amount is calculated from the timing, and the second buffer roller pair  112  is stopped. In this condition, the preceding sheet S 1  is set such that a predetermined length at the leading edge side during the normal conveyance of the preceding sheet S 1  (the trailing edge side during the reverse conveyance) is left on the stack conveyance path  121 . When the leading edge during the reverse conveyance of the preceding sheet S 1  passes through the branch portion A, the buffer switch member  114  is changed over to the first position. 
     As illustrated in  FIG. 7B , the succeeding sheet S 2  consecutively conveyed from the copier body  600  after the predetermined sheet interval is fed from the conveyance path  103 . Then, as illustrated in  FIG. 7C , a leading edge of the succeeding sheet S 2  enters the branch portion A and is conveyed so as to overlap over the preceding sheet S by the buffer switch member  114  located at the first position. Then, in a condition in which the leading edge parts of the sheets overlap with a predetermined shift amount, the second buffer roller pair  112  normally rotates to normally convey the preceding sheet S 1  toward the stack conveyance path  121 . Thereby, the two sheets are conveyed through the stack conveyance path  121  in a overlapped condition overlapping over the predetermined amount such that the succeeding sheet S 2  conveyed by the conveyance roller pair  110  is on an upper position and the preceding sheet S 1  conveyed by the second buffer roller pair  112  is on a lower position. The sheet bundle (overlapped sheets) is conveyed further through the stack conveyance path  121  toward the stapling processing portion  140  or the saddle processing portion  800 . 
     The shift amount between the leading edges of the preceding and succeeding sheets S 1  and S 2  may be arbitrarily set by controlling the buffer processing portion  100 C such that the second buffer roller pair  112  starts to rotate at a predetermined rotation start timing from when the first buffer sensor  109  detects the leading edge of the succeeding sheet S 2 . For instance, if the conveyance of the preceding sheet S 1  is started again right after when the first buffer sensor  109  is turned ON by the leading edge of the succeeding sheet S 2 , the overlap amount in the sheet bundle is set small (the shift amount is set large), that is, the preceding sheet S 1  largely precedes with respect to the succeeding sheet S 2 . In contrary, if the re-conveyance timing of the preceding sheet S 1  is retarded, the overlap amount in the sheet bundle is set large (the shift amount is set small), that is, the overlap amount of the succeeding sheet S 2  with respect to the preceding sheet S 1  is set large. If the re-conveyance timing of the preceding sheet S 1  with respect to the succeeding sheet S 2  is retarded further, they are conveyed by being shifted such that the leading edge of the succeeding sheet S 2  precedes over the preceding sheet S 1 , i.e., such that the succeeding sheet S 2  precedes the preceding sheet S 1 . 
     It is noted that while the binding process of overlapping two sheets has been described in the embodiment described above, the present disclosure is not limited to such configuration and it is possible to form a sheet bundle in which a plurality of sheets, e.g., three or four sheets, are stuck by retaining two or three sheets in the buffer path  113 . In these cases, it is possible to arbitrarily set a sheet overlap amount between the second and third sheets and between an N−1th sheet and an Nth sheet by adjusting the sheet halting timing, similarly to the first sheet and the second sheet. 
     Next, a control portion of the copier body  600  and the finisher  100  (control portion of the image forming apparatus  1000 ) will be described. As illustrated in  FIG. 8 , the copier body  600  of the present embodiment is connected with a computer  620  (external PC) through an external interface  637 . A CPU circuit portion  630  includes a CPU  629 , a ROM  631  and a RAM  650  and communicates with a document feeder control portion  632 , an image reader control portion  633 , an image signal control portion  634 , and the printer control portion  635 . The CPU circuit portion  630  also communicates with an operation portion  607  and a finisher control portion  636 . 
     As illustrated in  FIG. 9 , the finisher control portion  636  includes a CPU (microcomputer)  701 , a RAM  702 , a ROM  703 , an input/output portion (I/O)  705 , a communication interface  706  and a network interface  704 . Still further, the input/output portion  705  is connected with a conveyance control portion  707 , a processing tray control portion  708 , a biding control portion  709 , a stacking tray control portion  710 , and a saddle control portion  711 . 
     The conveyance control portion  707  receives signals from the inlet sensor  101 , the first buffer sensor  109  and the buffer sensor  116 . The conveyance control portion  707  also outputs signals to respective motors M 120 , M 180 , M 122 , M 112  and M 114 , each serving as actuators, of an upper discharge roller pair  120 , a shift unit  180 , a buffer roller pair  122 , a second buffer roller pair  112 , and a buffer switch member  114 . 
     The saddle control portion  711  receives a signal from the saddle inlet sensor  62 . The saddle control portion  711  also outputs signals to respective motors (actuators) M 801 , M 805 , M 12 , M 11 , M 804 , and M 802  of the inlet roller pair  801 , the leading edge stopper  805 , the push member  12 , the pressure member  11 , the intermediate roller  804  and the alignment roller  802 . The saddle control portion  711  also outputs signals to respective motors (actuators) M 819  and M 803  of the folding roller pair  819  and the folding plate  803 . 
     Next, the present embodiment applied to the saddle processing will be described with reference to  FIGS. 10A through 12C  and to flowcharts in  FIGS. 13A and 13B . According to the present embodiment, the sheet bundle overlapped by the buffering process described above is conveyed to the processing tray  15  while maintaining both productivity of the sheet processing and alignment of the sheets. 
     When a saddle job is inputted, the respective members move to the stand-by positions for accepting the sheet S (see Steps S 201  and S 202  in  FIG. 13 ). At this time, the alignment plate  815  stands by at a position slightly wider than a sheet width, and the leading edge stopper  805  stands by at the position lower than the stapling position by a half of the sheet length as described above. After that, the sheet passed to the finisher  100  is conveyed to the saddle processing portion  800  through the respective conveyance rollers in Step S 203  and undergoes the operations of alignment in the sheet conveyance direction and in the width direction and of the trailing edge sorting in Step S 204 . The abovementioned operations are conducted to a final sheet of the series of aligned sheet stack of each job in Step S 205 . Then, a stapling process is carried out by the stapler  820  in Step S 206 . After the stapling process, the sheet bundle is moved to the position where the middle part of the sheet bundle faces a center of the nip of the folding roller pair  819  and is fed to the nip of the folding roller pair  819  by the thrust action of the folding plate  803  to form a saddle bundle in Steps S 207  and S 208 . Then, in a case when the saddle bundle is a final bundle of the job, i.e., Yes in Step S 209 , the job ends as it is in Step S 215 . In a case when there is a next sheet stack to be processed, i.e., No in Step S 209 , the job enters a ready state to accept a next sheet. 
     Then, it is judged whether or not a head sheet of a next sheet stack is suitable for buffering in Step S 210  based on input information set by the user by operating the operation portion  607 . In a case where the sheet should not be buffered, i.e., No in Step S 210 , a signal to pause the image forming operation is outputted to the copier body  600  (to the image forming apparatus) in Step S 211 . In this case, the copier body  600  restarts the image forming operation by estimating timing when the saddle processing ends and conveys the sheet on which an image has been formed to the saddle processing portion  800  in Step S 203 . The sheet mentioned here as not to be buffered includes a case of a thick sheet whose grammage is 300 g/m 2  for example, i.e., a case when conveyance resistance against each motor of the conveyance rollers exceeds a rated torque in conveying overlapped sheets (sheet bundle) through the conveyance path or a case when a conveyance force of each motor is insufficient. It is noted that while it is confirmed whether or not the head sheet of the next sheet stack is suitable for buffering after completing the saddle bundle processing in  FIG. 13A  for the convenience of the flowchart, actually information may be exchanged between the finisher and the image forming apparatus during the previous sheet bundle processing to determine whether or not the buffering operation is to be carried out, i.e., whether or not the pausing of the image forming operation occurs. 
     In other words, Step S 210  is a step for selecting a first conveyance process of conveying sheets in a form of the sheet bundle to the sheet supporting portion via the buffer processing portion and a second conveyance process of conveying the sheets one by one to the sheet supporting portion without forming the sheet bundle by the buffering process. Then, the conveyance control portion  707  executes the first conveyance process in conveying a sheet having a first thickness and the second conveyance process in conveying a sheet having a second thickness. Because it is not necessary to pause the image forming operation of the copier body  600  in the case of the second conveyance process, it is possible to improve the productivity as compared to the case of the first conveyance process. 
     In a case when the head sheet of the next sheet stack is suitable for buffering, i.e., Yes in Step S 210 , it is confirmed whether or not the sheet is a high smoothness sheet based on the input information such as a type of the sheet set by the user in Step S 212 . The high smoothness sheet is a sheet whose surface is highly smooth such as a coated sheet, a film sheet and an OHP sheet. In the case of the high smoothness sheet, a high pasting force Fa is generated between the buffered sheets as illustrated in  FIG. 15 . Therefore, in the case of the high smoothness sheet, i.e., Yes in Step S 212 , the conveyance control portion  707  sets and controls timing of sending a re-start signal to the second buffer conveyance motor M 112  after when the first buffer sensor  109  detects the leading edge of the succeeding sheet in the buffer processing portion  100 C. Thereby, the conveyance control portion  707  sets such that a buffering shift amount L 1  is larger than a distance L 2  from the trailing edge (other end) of the preceding sheet to the inlet roller when one end of the preceding sheet abuts against the leading edge stopper  805 , i.e., such that L 1 &gt;L 2  in Step S 213 . 
     That is, as illustrated in  FIG. 10A , the shift amount L 1  of the preceding sheet S 1  is set to be larger than the distance L 2  from the trailing edge of the preceding sheet S 1  to the inlet roller pair  801  in a state when the preceding sheet S 1  abuts against the leading edge stopper  805 , i.e., L 1 &gt;L 2 . In this situation, a relationship of L&lt;H&lt;L+L 1  holds, where H is a distance from the inlet roller pair  801  to the leading edge stopper  805  in the sheet conveyance direction, and L is a sheet length. In detail, with reference also to  FIG. 13B , after when the preceding sheet S 1  passes through the inlet roller pair  801  in Step S 204   a  and abuts against the leading edge stopper  805  in Step S 204   b , a separation force Fc 2  is generated as the trailing edge of the succeeding sheet S 2  is conveyed further by the inlet roller pair  801 . It is possible to separate the buffered sheets and to convey in the conveyance direction by making the separation force (alignment force) Fc 2  exceed the pasting force Fc 1  in Step S 204   c . As illustrated in  FIG. 10B , among the buffered sheets S 1  and S 2 , the succeeding sheet S 2  is forcibly conveyed into the path by the inlet roller pair  801  (conveyance roller pair) in a condition in which the preceding sheet S 1  is in contact with the leading edge stopper  805 . Thereby, the succeeding sheet S 2  is released such that the leading edge thereof abuts against the leading edge stopper  805  by inertia, and the both sheets S 1  and S 2  abut against and aligned by the leading edge stopper  805 . After thus completing the alignment in the conveyance direction of the buffered sheets S 1  and S 2 , the alignment in the width direction and the trailing edge sorting are carried out in the same manner with the sheet processing of the first part (first sheet bundle) described above in Step S 204   d . Thus, it is ready to accept a next sheet. The alignment and the trailing edge sorting are carried out on a sheet S 3  (third sheet of the second part) after that and on a sheet SN (final sheet of the second part) in the same manner with that carried out on the first stack, and a saddle bundle is formed by conducting the stapling process and the thrusting and folding operation. This operation is conducted until a final sheet bundle is formed and then the job ends in Step S 215 . 
     Thus, the sheet processing method of the present embodiment includes the overlap processing step of making the sheets overlap each other in Step S 213 , the aligning step of aligning the overlapping sheet bundle in Steps S 203  and S 204 , and the processing step of processing the aligned sheet bundle by the stapling process or the like in Step S 206 . In particular, the aligning step includes a separation step of forcibly conveying the succeeding sheet S 2  by the inlet roller pair  801  in a condition in which the trailing edge of the preceding sheet S 1  is discharged from the inlet roller pair  801  and the leading edge of the preceding sheet S 1  is in contact with the leading edge stopper  805  in Step S 204   c . This separation step enables the succeeding sheet S 2  to move relatively with respect to the preceding sheet S 1  and is smoothly aligned even if the sheets are high smoothness sheets. 
     It is noted that as long as the inlet roller pair  801  forcibly conveys the succeeding sheet S 2  in the condition in which the leading edge of the preceding sheet S 1  is in contact with the leading edge stopper  805 , it is possible to arrange such that shift amount L 1 =distance L 2  (distance H=sheet length L+shift amount L 1 ). However, in a case of taking a tolerance of the sheet length and variation of the conveyance speed into account, it is preferable to set L 1  to be larger than L 2  so that the forcible conveyance of the succeeding sheet S 2  is stably executed. 
     Still further, while the amount of the buffer shift amount L 1  is controlled corresponding to the smoothness of the sheet in the present embodiment, it is also possible to set such that L 1 &gt;L 2  if there is a possibility that the pasting force between the sheets is high. For instance, it is conceivable to control such that the buffer shift amount is L 1 &gt;L 2  in a case when humidity is high, an output image is a uniform image (solid image), or a gloss processing is implemented on an output image by a transparent toner or the like. 
     Meanwhile, in a case when a head sheet of the next sheet stack is not a high smoothness sheet, i.e., No in Step S 212 , the buffer shift amount is set to be about 3 mm as illustrated in  FIG. 11A  in Step S 214 . Because the pasting force Fi 1  is small in the case when the sheet is not the high smoothness sheet, even the aligning force Fi 2 , i.e., the inertia of the succeeding sheet S 2 , exceeds the pasting force Fi 1  and alignment of the sheet in the conveyance direction is achieved. That is, in the state in which the leading edge of the preceding sheet S 1  is in contact with the leading edge stopper  805 , the succeeding sheet S 2  is also released by the inertia so as to abut against the leading edge stopper  805  even if the trailing edge of the succeeding sheet S 2  is discharged from the inlet roller pair  801 . Thereby, the buffer sheets S 1  and S 2  are aligned as illustrated in  FIG. 11B . After that, a saddle bundle is formed until a final sheet of a final bundle is processed similarly to the case of the high smoothness sheet, and the job is finished. 
     It is thus possible to align the both sheets by setting such that L 1 &gt;L 2  in the case of the high smoothness sheet and such that L 1  is about 3 mm in the case when the sheet is not the high smoothness sheet. Because the larger the buffering shift amount L 1 , the shorter the inter-stack time in the case of not the high smoothness sheet, it is desirable to set the shift amount L 1  around a small value of about 3 mm. While the buffer shift amount is desirable to be small of about 3 mm from an aspect of improvement of productivity, such a small shift amount is not suitable in the case of the high smoothness sheet because the pasting force Fc 1  becomes great and the separation force of the inertia falls under the pasting force Fc 1 , which situation may cause alignment failure. In contrast, it is possible to generate a large separation force Fc 2  by a conveyance force of the inlet roller pair  801  by setting the buffering shift amount L 1  such that L 1 &gt;L 2  and to achieve the alignment for the first time by setting as Fc 2 &gt;Fc 1 . 
     That is, in the case of the high smoothness sheet, it is suitable to be L 1 &gt;L 2  from the aspect of alignment. In the case of not the high smoothness sheet, while either cases of L 1 &gt;L 2  and L 1 =3 mm are permissible, it is desirable to set to be L 1 =3 mm from an aspect of assuring the sheet inter-stack time. 
     That is, Step S 212  is a step of recognizing whether or not the sheet is the high smoothness sheet. In other words, the Step S 212  is a step of switching a process of the control portion such that the shift amount L 1  is set to a predetermined amount to hold the relationship of L 1 &gt;L 2  in the case when the sheet is the high smoothness sheet and a shift amount L 1  is set smaller than the predetermined shift amount when the sheet is not the high smoothness sheet. 
     As a method of setting to be L 1 &gt;L 2  as described above, there is also a method of adjusting a moving position of the leading edge stopper  805 , besides the method of adjusting the buffering shift amount L 1 . That is, this is a method of moving the leading edge stopper  805  such that the value of the distance H meets the relationship of L&lt;H≦L+L 1  about the sheet length L and the distance H from the inlet roller pair  801  to the leading edge stopper  805 . This method also realizes the state in which the succeeding sheet S 2  is conveyed by the inlet roller pair  801  when the leading edge stopper  805  rises with respect to the preceding sheet S 1  and the leading edge of the preceding sheet S 1  abuts against the leading edge stopper  805 . In this case, it is necessary to move the leading edge stopper  805  such the stapler  820  corresponds to a position of a half of a sheet length in saddle stitching the sheet on the processing tray at a middle part of the sheet length. Therefore, this operation may become cumbersome, and when a number of sheets of the saddle stitching sheet stack is less, the moving time of the leading edge stopper  805  is added to the binding process of the stapler, thus there is a small chance of dropping of the productivity. 
     While the operation of buffering two sheets has been described above and the shift amount L 1  between the first and second sheets has been set to be larger than L 2  as illustrated in  FIG. 12A . Still further, it is possible to buffer and align three or more sheets by setting a shift amount L 3  between the second and third sheets to be larger than a distance L 4  from the trailing edge of the succeeding sheet S 2  to the inlet roller pair  801  when the succeeding sheet S 2  abuts against the leading edge stopper  805  (see  FIG. 12C ). In this case, because the distances L 2  and L 4  are equal unless the leading edge stopper  805  is moved during the stacking the buffer sheets, the shift amounts L 1  and L 3  are also equal. 
     While the sheet is drawn to move in the vertical direction in  FIGS. 10A through 12C , actually, the processing tray  15 , i.e., the sheet supporting portion, is composed of the inclined surface of a steep gradient as illustrated in  FIGS. 3 through 6E . In  FIGS. 10A through 12C , the right side is a stacking surface (supporting surface) of the processing tray and the sheet is stacked and aligned on the stacking surface. While a processing tray  138  (supporting portion) of a staple processing portion described later with reference to  FIG. 14  is also the same, the preceding sheet S 1  among the sheet bundle formed in the buffer processing portion is conveyed precedingly out of the inlet roller pair  801  and is stacked on the stacking surface of the processing tray. The succeeding sheet is conveyed out of the inlet roller pair  801  behind the preceding sheet and is stacked on the preceding sheet. 
     As illustrated in  FIGS. 12A through 12C , the same applies to the case of three or more sheets, and the second sheet S 2  becomes a succeeding sheet relatively with respect to the first sheet S 1  and becomes a preceding sheet relatively with respect to the third sheet S 3 . Thus, the sheet bundle formed in the buffer processing portion includes two more sheets of the preceding sheet retained in advance and the succeeding sheet relatively succeeding the preceding sheet. The conveyance roller pair conveys the preceding sheet such that the preceding sheet precedes relatively the succeeding sheet. On the sheet supporting portion, the preceding sheet is stacked on the sheet supporting portion at a relatively close position to the supporting surface with respect to the succeeding sheet. 
     As described above, because the leading edge stopper  805  is located at the position lower than the stapling position by a half of a sheet length (Ls=L/2), a distance L 2  between a trailing edge of a sheet and the inlet roller pair  801  varies depending on the length of the sheet. Therefore, the buffer shift amount L 1  needs to be set at a different value depending on the sheet length. The control portion recognizes the sheet size by the setting made by the user through the operation portion  607  or by timing of detection of the first and second buffer sensors  109  and  116 . That is, the operation portion  607  and the first and second buffer sensors  109  and  116  are one example of a sheet size detection portion capable of detecting the sheet size. Based on the recognition of the sheet size, the control portion adjusts the position of the leading edge stopper  805  and relatively changes the distance between L 1  and L 2  based on the sheet size such that the relationship of L 1 &gt;L 2  is held. 
     The inertia alignment method of aligning the sheets by inertia by releasing the sheet by the conveyance roller pair (inlet roller pair) on the processing tray whose angle is steep close to verticality, like the saddle processing, has been applied in the abovementioned description. Then, an embodiment applied to an alignment method realized by a sliding alignment member such as a paddle will be described with reference to  FIGS. 2 and 14 . 
     The present embodiment is applied to the stapling processing portion  140  illustrated in  FIG. 2 . The buffered sheet bundle S 1  and S 2  are discharged out of a discharge roller  128  of the stapler path  126  to the processing tray  138  with a moderate inclination angle close to horizontality. 
     At this time, an upper roller  129  is located upward at an open position, and the sheet bundle is discharged out of the discharge roller  128  onto the processing tray  138  while reversing (replacing) its leading edge and trailing edge. The upper roller  129  and a lower roller  130  receive the sheet bundle discharged out of the discharge roller  128 . After conveying the received sheet bundle by nipping and by rotating normally, the upper roller  129  and the lower roller  130  rotate reversely. When the upper roller  129  and the lower roller  130  rotate reversely, the sheet bundle is conveyed by the upper roller  129  and the lower roller  130  toward a leading edge stopper  138   a , i.e., an end regulation member, by setting the trailing edge of the sheet bundle until then as a head (leading edge). It is noted that as illustrated in  FIG. 2 , the sheet heading onto the processing tray  138  is lead such that the head, i.e., an end on the side of the sheet bundle abutting against the leading edge stopper  138   a , is guided by the knurled belt  158  onto the processing tray  138 . The sheet conveyed toward the leading edge stopper  138   a  is conveyed by the paddle  131  on the processing tray  138  toward the leading edge stopper  138   a  such that the end of the sheet abuts against the leading edge stopper  138   a . After when the end of the sheet abuts against the leading edge stopper  138   a , the paddle  131  slides on an upper surface of the sheet. 
     In this case, the buffered sheets have been made to overlap each other such that the leading edge of the succeeding sheet S 2  precedes the leading edge of the sheet S by a predetermined amount L 1  in the buffer processing portion  100 C. It is noted that because lengths of the preceding sheet S 1  and the succeeding sheet S 2  are equal and the leading and trailing edges of the sheets are reversed on the processing tray  138 , the leading edge of the preceding sheet S 1  (an end to abut against the leading edge stopper  138   a ) precedes by the predetermined amount L 1 . 
     The sheet bundle of sheets S 1  and S 2  discharged out of the discharge roller  128  onto the processing tray  138  are discharged while positioning the preceding sheet S 1  at a lower position and the succeeding sheet S 2  at an upper position on the processing tray  138  inclined moderately. Then, the paddle  131  comes into contact with the succeeding sheet S 2  on the upper side, and the leading edge of the preceding sheet S 1  on the lower side preceding by the predetermined shift amount L 1  abuts against the leading edge stopper  138   a . Because the large pasting force (Fc 1 : see  FIG. 10A ) acts between the sheets S 1  and S 2  of the sheet bundle, there is a case when the sheets is not separated by a separation force of the paddle  131 . 
     In the present embodiment, in contrast, the upper roller  129 , i.e., an opening/closing roller, moves to a close position and forms a conveyance roller pair nipping the sheet with the lower roller  130 . Meanwhile, a distance L 2  between the trailing edge of the preceding sheet S 1  and the nip of the roller pair of the upper roller  129  and the lower roller  130  is set to a value smaller than the predetermined shift amount L 1  in the state in which the leading edge of the preceding sheet S 1  is in contact with the leading edge stopper  138   a . To that end, the predetermined shift amount L 1  and a distance between the roller pair  129  and  130 , i.e., a conveyance roller pair, are set in advance. Accordingly, similarly to the saddle processing described above, a conveyance force of the roller pair  129  and  130  surpasses the pasting force and the succeeding sheet S 2  is forcibly conveyed. In a state in which the trailing edge of the succeeding sheet S 2  is released from the conveyance roller pair  129  and  130 , the succeeding sheet S 2  is pulled back by the paddle  131  and abuts against the leading edge stopper  138   a . Thereby, the buffered sheets are aligned in the conveyance direction and a succeeding sheet succeeding to the buffered sheets is also aligned respectively by the paddle  131  and others. The buffer sheet is aligned widthwise by a side end aligning portion not illustrated, and the aligned sheet stack is stapled by the stapler  132 . The sliding alignment member is not limited to be the paddle and other members such as a knurled belt is applicable in the same manner. 
     It is noted that the mode of conducting the buffering process such that a next sheet does not go to the processing portion during the binding process or the saddle stitching and double folding process of the preceding sheet has been exemplified in the embodiment described above. However, the process of the preceding sheet is not limited only to be binding or folding. For instance, the technique of the present disclosure is also applicable to such a buffer unit as to retain succeeding sheets and hold not to be conveyed to a processing portion during the preceding sheets are aligned in the width direction at the processing portion. 
     It is noted that it is possible to apply the sheet bundle alignment method of the relationship of L 1 &gt;L 2  described above to all alignment methods, not only to the inertia alignment method and the alignment method by means of the slide returning member such as the paddle. Still further, all kinds of binding process such as one adopting a buffer roller capable of winding up a sheet, besides one adopting the buffer path of the present embodiment, is applicable as the binding process. The sheet processing of the present embodiment is applicable also to all kinds of processes, besides the saddle processing and the staple processing described above. 
     OTHER EMBODIMENTS 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiments of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiments. The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-154003, filed on Aug. 4, 2015, which is hereby incorporated by reference herein in its entirety.