Patent Publication Number: US-2007108690-A1

Title: Sheet processing apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a sheet processing apparatus (finisher) for performing processes such as alignment and stapling with respect to the image formed sheets discharged from an image forming apparatus such as a copying machine, facsimile machine, printer, complex machine and the like, and an image forming apparatus.  
      2. Description of the Related Art  
      Conventionally, various sheet processing apparatuses have been proposed for stacking the sheets performed with image formation in the image forming apparatus while aligning the sheets on a processing tray, and conveying the sheets performed with the stapling process and the like to a stack tray different from the processing tray (see for example, Japanese Patent Application Laid-open No. 10-194582). Japanese Patent Application Laid-open No. 10-194582 discloses superimposing a few sheets at the beginning of the subsequent job discharged from the image forming apparatus and having such sheets temporarily waiting while the stapling process is being performed on the sheet bundle of the previous job in the processing tray to ensure the sheet processing time of the previous job. After discharging the previous processed sheet bundle to the stack tray, the sheets on standby are discharged to the processing tray all at one.  
      That is, the sheets discharged from the image forming apparatus are superimposed in pluralities and wrapped around a buffer roller at the upstream side of the processing tray provided in a tilted manner. The sheet bundle in which a number of sheets are superimposed is then discharged once to the processing tray with a bundle conveyance roller, and then the bundle conveyance roller is reversely rotated to convey the sheet bundle towards a stopper provided at one end of the processing tray. The bundle conveyance roller opens the nip at a timing of immediately before the impact so that the sheet bundle does not bend due to the impact force of when abutting against the stopper, and releases the sheet bundle. Subsequently, the sheet bundle is aligned by being abutted against the stopper using the force of inertia of the sheet bundle. Therefore, during the processing of the previous sheet bundle, the image forming operation of the image forming apparatus does not need to be temporarily interrupted to have the subsequent sheet bundle waiting, thereby achieving high productivity.  
      Conventionally, the sheet discharged from the image forming apparatus to the processing tray is aligned one sheet at a time. However, a sheet processing apparatus is also proposed that creates a temporal margin by aligning the discharged sheets in a stack of two or more sheets superimposed to enhance the response to high speed equipments (see, for example, Japanese Patent Application Laid-open No. 2004-35156).  
      In other words, in the above case, the sheet discharged from the tilted processing tray has the front end in the discharging direction abutted against an abutment member provided at one end side of the processing tray The abutment member moves in a direction of approaching the stopper at the other end of the processing tray, and abuts and positions the back end of the sheet in the discharging direction to the stopper, thereby aligning the sheets in the discharging direction. When a number of sheets are discharged while superimposing, the undermost sheet that slidably abuts the stacking surface of the processing tray, in particular, may not completely move to the stopper. The undermost sheet may also be aligned by pushing the sheet back towards the stopper with the abutment member as described above. 
      Japanese Patent Application Laid-open No. 10-194582     Japanese Patent Application Laid-open No. 2004-35156    

      However, the sheet processing apparatus disclosed in one of the above publications has the following problems.  
      In the case of the sheet processing apparatus disclosed in the publication of Japanese Patent Application Laid-open No. 10-194582, the sheet stack is aligned in the conveying direction using the force of inertia produced by the tilt of the processing tray. Accuracy in the timing of opening the nip of the bundle conveyance roller and releasing the sheet stack immediately before the sheet stack abuts against the stopper is required, and the accuracy in the timing of superimposing a number of sheets is also required in the buffer roller. The sheet stack in which the sheets are superimposed in pluralities is fed to the processing tray while being sandwiched by the nip between the rollers of the bundle conveyance roller, but the series of operations described above can only be applied if the previous sheet bundle is already present in the processing tray, and is only applied to a few sheets at the beginning of the subsequent sheet bundle. Furthermore, if the sheet stack in which the sheets are superimposed in pluralities is fed to the processing tray in the state of a skew feeding, the alignment may not be sufficiently performed as correction of the skew feeding is troublesome.  
      In the sheet processing apparatus disclosed in the publication of Japanese Patent Application Laid-open No. 2004-35156, the other remaining sheets may be superimposed in pluralities and aligned with the few sheets at the beginning of the sheet bundle, thereby resolving one of the problems in Japanese Patent Application Laid-open No. 10-194582. In this case, however, since a movable abutment member for aligning the front ends of the sheets in the conveying direction by abutting is provided, the operation control of the abutment member is further required, thereby complicating the apparatus configuration and increasing the cost.  
      The present invention aims to provide a sheet post-processing apparatus that moves a plurality of sheets in a superimposed manner, and performs collective alignment and stacking with respect to the plurality of sheets to realize reduction in the aligning time at low cost, and an image forming apparatus.  
     SUMMARY OF THE INVENTION  
      In order to achieve the above object, according to the present invention, there is provided a sheet processing apparatus including: a sheet conveying portion which conveys sheets; a sheet moving unit which includes a gripping portion capable of gripping the ends of the plurality of sheets conveyed by the sheet conveying portion, and moves the sheets by moving the gripping portion while gripping the sheets; a sheet abutment member which is abutted with the ends of the sheet moved by the sheet moving unit; and a processing tray which stacks the sheets moved by the sheet moving unit and performs processes thereon; wherein the sheet moving unit releases the plurality of sheets from the gripping portion by abutting the ends of the plurality of gripped sheets to the sheet abutment member, and accommodates the plurality of sheets into the processing tray  
      The image forming apparatus includes an image forming portion for forming an image on the sheet; and the sheet processing apparatus for processing the sheet formed with image by the image forming portion.  
      According to the sheet processing apparatus of the present invention, sheets are divided into a sheet group made up of a plurality of sheets and then moved, and aligning and stacking are performed all at once with respect to each group to form a sheet bundle including a predetermined number of sheets on the processing tray. The temporal margin is thereby produced compared to when aligning and stacking are performing one sheet at a time, and thus is effective in realizing high speed process at low cost and maintaining high productivity. Furthermore, the aligning property is enhanced by allocating the margin time for the increase in the number of aligning operations in the direction orthogonal to the moving direction and the reduction of the aligning speed.  
      According to the image forming apparatus of the present invention, the total productivity is enhanced by aligning a high performance sheet processing apparatus in which temporal aligning process margin and the like are enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view showing an image forming apparatus such as a copying machine equipped with a sheet processing apparatus of the present embodiment;  
       FIG. 2  is a block diagram showing a configuration of a controlling portion responsible for controlling the entire image forming apparatus of the present embodiment;  
       FIG. 3  is a view showing a configuration of the main part of the sheet processing apparatus of the present embodiment;  
       FIG. 4  is a plane cross sectional view taken from line X-X of  FIG. 2 ;  
       FIG. 5  is a view showing a configuration of the present embodiment;  
       FIG. 6  is a view explaining a Z-shape folding operation;  
       FIG. 7  is a view explaining a Z-shape folding operation;  
       FIG. 8  is a view explaining a Z-shape folding operation;  
       FIG. 9  is a view explaining a Z-shape folding operation;  
       FIG. 10  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 11  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 12  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 13  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 14  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 15  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 16  is a view showing the operation of the sheet processing apparatus of the present embodiment;  
       FIG. 17  is a view showing the operation of the sheet processing apparatus of the present embodiment; and  
       FIG. 18  is a flow chart illustrating the operation of the present embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The preferred embodiments of the sheet processing apparatus and the image forming apparatus according to the present invention will now be described in detail with reference to the drawings.  
      (Image Forming Apparatus)  
       FIG. 1  is a cross sectional view of the main parts of an image forming apparatus main body  900  according to the present invention including an automatic original feeding device  950  and a both sides inverting device  901 . The image forming apparatus main body  900  is configured as below. The sheets set in the sheet cassettes  902   a  to  902   e  are conveyed to a registration roller  910  by means of sheet feeding rollers  903   a  to  903   e  and a conveyance roller  904 . The digital original data obtained by reading the original fed from the automatic original feeding device  950  with an image reading device  951  is exposed to a photosensitive drum  906  by an exposure unit  908 . The processes for obtaining the visible image from the electrostatic latent image is performed by a primary charger  907  and a development device  909  configuring an image forming portion with the photosensitive drum  906 , and the toner image is formed on the photosensitive drum  906 . When the sheet is conveyed to the transfer position by the registration roller  910  at a timing the front end of the sheet and the front end of the toner image on the photosensitive drum  906  coincide, the transfer bias is applied to the sheet by a transfer separation charger  905 , and the toner image on the photosensitive drum  906  is transferred to the sheet. Such transferred sheet is then conveyed to a fixing device  912  by a conveyance belt  911 . The toner image is then thermally fixed by being conveyed while being sandwiched between a heating roller and a pressure roller configuring the fixing device  912 . In this case, the foreign materials such as residual toners attached to the photosensitive drum  906  and not transferred to the sheet are scraped off with the blade of a cleaning device  913 , and thus the drum surface is cleaned and prepared for the next image formation. Such fixed sheet is discharged towards the sheet processing apparatus (finisher) of the present embodiment shown below one at a time by a discharge roller  914 , or is conveyed to the both sides inverting device  901  by switching a flapper  915 , and the image formation is again performed.  
      (Controlling Portion)  
      The configuration of a controller serving as the controlling portion responsible for the control of the entire image forming apparatus will now be described with reference to  FIG. 2 .  FIG. 2  is a block diagram showing a configuration of the controller responsible for the control of the entire image forming apparatus shown in  FIG. 1 .  
      As shown in  FIG. 2 , the controller includes a CPU circuit portion  206  incorporating CPU (not shown), ROM  207 , and RAM  208 . Each block  202 ,  209 ,  203 ,  204 ,  201 ,  205 , and  210  are collectively controlled by the control program stored in the ROM  207 . The RAM  208  temporarily holds the control data and is used as the work region of the calculation process involved in the control.  
      A DF (original feeder) controlling portion  202  drives and controls the automatic original feeding device  950  based on the instruction from the CPU circuit portion  206 . The image reader controlling portion  203  performs drive control on the above described image reading device  951  and the like, and transfers the analog image signal output from the image reading device  951  to the image signal controlling portion  204 .  
      The image signal controlling portion  204  performs each process after converting the analog image signal from the image reading device  951  to a digital signal, and converts the digital signal to a video signal and outputs the video signal to a printer controlling portion  205 . Furthermore, the image signal controlling portion  204  performs various processes on the digital image signal input from a computer  200  via an external I/F  201 , and converts the digital image signal to the video signal and outputs the video signal to the printer controlling portion  205 . The processing operation by the image signal controlling portion  204  is controlled by the CPU circuit portion  206 . The printer controlling portion  205  drives the above described exposure unit based on the input video signal.  
      An operation portion  209  includes a plurality of keys for setting various functions related to image formation, and a display portion for displaying the information indicating the set state and the like, and outputs the key signal corresponding to the operation of each key to the CPU circuit portion  206 , and displays the corresponding information on the display portion based on the signal from the CPU circuit portion  206 .  
      In the present embodiment, the finisher controlling portion  210  is mounted on the sheet processing apparatus  1 , and performs the drive control of the sheet processing apparatus  1  by exchanging information with the CPU circuit portion  206 . The finisher controlling portion  210  is provided on the image forming apparatus main body  900  side integrally with the CPU circuit portion  206  so as to directly control the sheet processing apparatus  1  from the apparatus main body  900  side.  
      (Sheet Processing Apparatus)  
       FIG. 3  shows, in an enlarged manner, the main parts of the sheet processing apparatus  1  for receiving the sheet formed with image discharged from the apparatus main body  900  and performing the process.  FIG. 4  is a cross sectional view taken along line X-X of  FIG. 3 . The sheet discharged from the apparatus main body  900  is taken into the conveyance path  4  by an entrance side roller  2  provided on the upstream part of the sheet processing apparatus  1 . The sheet is then fed towards the conveyance roller  3  from the conveyance path  4 . The entrance side roller  2 , the conveyance path  4 , and the conveyance roller  3  configure the conveying portion in the present embodiment along with the conveyance path  10  to be hereinafter described.  
      The conveyance path  10  extends from the conveyance roller  3 , and further, a movement gripper  5  acting as a sheet moving unit of rotating drum shape that grips the received sheet and inverts and moves the sheet over a predetermined distance is provided on the downstream. Two movement grippers  5  are fixed in line on the axis line of the rotation shaft  5   c . The two movement grippers  5  normally rotates or reversely rotates when receiving the driving force from the power source motor at the rotation shaft  5   c  via a driving mechanism (not shown). A gripping portion  5   a  of cut-out form is formed at one part on the circumference of the two movement grippers  5 . The cut-out end of the gripping portion  5   a  is formed as a tapered part  5   d  having an inclined surface, and is devised so as to easily take in the sheet from the cut-out end. The terminating end of the gripping portion  5   a  is formed as an abutment surface  5   b  for abutting and positioning the front ends of the sheets. The gripping portion  5   a  has an elastic member such as a spring for gripping a number of sheets with elastic force, and the movement gripper  5  rotates while holding the number of sheets. The number of sheets moved by the relevant rotation is then inverted with one end gripped by the gripping portion  5   a  at above the processing tray  7  and the other end, which is the free end, placed in the processing tray  7  serving as the stacking portion.  
      The movement gripper  5  is further provided with a flag  5   e  (see  FIG. 4 ) for recognizing the position of the gripping portion  5   a  in  FIG. 3  as the receiving position to easily guide the sheets fed from the conveyance path  10  to the gripping portion  5   a . A flag sensor  12  for detecting the cut-out of the flag  5   e  corresponding to the receiving position is fixed to the apparatus frame. A flapper  6  switched by the drive of the solenoid SL 1  is provided between the terminating end of the conveyance path  10  and the movement gripper  5 , and is rotated so as to take both positions of a position (solenoid SL 1  in OFF state) shown with a solid line and a position (solenoid SL 1  in ON state) shown with a broken line. The flapper  6  opens the conveyance path  10  at the solid line position and closes the path at the broken line position. After the sheets pass the conveyance path  10  and the flapper  6 , the flapper  6  is switched to the state of the broken line position, so that the sheets are moved in the opposite direction and guided to a waiting path  11  provided below the conveyance path  10 . Furthermore, a sheet detection sensor  15  for detecting the sheet and achieving the conveyance timing is provided in the vicinity of the conveyance roller  3 .  
      The processing tray  7  is provided at a predetermined tilt angle under the movement gripper  5 , and a stopper  8  serving as a sheet abutment member is provided at the lower part of the tilt. The stopper  8  overlaps the movement gripper  5  in the rotating direction, and for example, four of the same are provided in a straight line at positions not overlapping each other in the width direction orthogonal to the moving direction. The movement of the sheet is completed when one end of the sheet moved by the movement gripper  5  abuts the stopper  8 , but since the gripping portion  5   a  continues to rotate, the sheet is released from being gripped by the gripping portion  5   a  and dropped in the processing tray  7  and accommodated therein. In other words, the sheet moving passage by the movement gripper  5  is from the flapper  6  to the stopper  8 . The length L of the moving passage is set longer than the maximum length of the sheet such as A3 size conveyed by the sheet processing apparatus.  
      As in the plan view shown in  FIG. 4 , a pair of aligning plates  9   a ,  9   b  configuring the aligning portion for aligning the ends of the sheets accommodated in the processing tray  7  in the sheet width direction by sandwiching the sheets in the sheet width direction orthogonal to the moving direction is aligned. The aligning plates  9   a ,  9   b  are movable in the X 1  direction shown with an arrow when driven by the drive motor.  
      Referring again to  FIG. 3 , a stack conveying projection  13  is provided in the vicinity of the stopper  8  of the processing tray  7 . The stack conveying projection  13  is fixed to the outer peripheral surface of a belt  12   c  winded between the pulleys  12   a ,  12   b  parallel to the upper surface of the processing tray  7  for conveying the sheets on the processing tray  7  to a stack tray  20 . One of the pulleys  12   a ,  12   b  is the driving side and the rotational power from the driving source motor is transmitted thereto. When the belt  12   c  is rotatably traveled in the counterclockwise direction in  FIG. 3 , the stack conveying projection  13  can move from right to left in the figure along the upper surface of the processing tray  7 . The stack conveying projection  13  is retreated with the position on the right from the stopper surface  8  as the home position so as not to inhibit the operation when taking the sheet into the processing tray  7 .  
      The stack tray  20  receives the sheet bundle fed by the stack conveying projection  13  from the processing tray  7  and accommodates the sheet bundle all at once. The stack tray  20  is movable in the up and down direction when driven by the drive source motor. Furthermore, the paper level height position of the sheet bundle stacked on the stack tray  20  is detected by a sheet level sensor  21 , and control is performed such that when the sheet bundle at the uppermost position is discharged, the sheet bundle on the second level lowers by the thickness of the discharged sheet bundle so to be at the predetermined height position.  
      A stapler  30  serving as a post-processing unit is supported at a movement table  31  to selectively staple the sheet bundle stacked on the processing tray  7 . Two pins  32   a ,  32   b  of the movement table  31  are supported at a rail  33  provided in the frame and the movement table  31  moves by being guided by the rail  33  when receiving the rotational power of the driving motor. The stapler  30  thereby moves to a plurality of locations such as front stapling position, two-location stapling positions (front, back), and back stapling position and the like. The stapler  30  can be moved (X 3  direction shown with an arrow of  FIG. 4 ) to the retreating position between the stapler  30  and the movement table  31  in the middle of the movement in the front and back direction of the stapler  30  to avoid interference with the movement gripper  5  and the like.  
      In order to guide the sheets to a folding (Z-form folding) device  50  provided as a post-processing unit different from the stapler  30 , the sheets are fed to the folding path  17  through the flapper  16  operated by the solenoid SL 2  provided in the middle of the conveyance path  4 . The conveyance roller  18  is provided in the folding path  17 , and the sheet is fed to the folding device  50  provided at the downstream thereof. With reference to  FIG. 5 , the folding device  50  has a configuration of deflecting the sheet and folding the deflected sheet by the nip between the folding rollers. The nip is formed when the folding rollers  52 ,  53  are pressure welded to the folding roller  51 . Two sets of conveyance rollers  54 ,  55  are provided on both sides of the folding rollers  51 ,  52  with the conveyance guide in between.  
      Therefore, the folding device  50  operates as sequentially shown in FIGS.  6  to  9 . One conveyance roller  54  conveys the sheet fed from the folding path  17  in the conveying direction, and the other conveyance roller  55  reversely rotates at a predetermined timing after conveying the sheet in the conveying direction along with the conveyance roller  54 . Deflection is produced at the sheet since the rotational conveying force is applied in the opposite direction by the two sets of conveyance rollers  54 ,  55  ( FIG. 6 ). When the deflection grows, the sheet enters the nip between the folding rollers  51 ,  52 , which are facing each other and rotated in the direction of the arrow in advance, the first folding is performed on the sheet. The first folded portion is further conveyed by the folding rollers  51 ,  52  while being inserted to the nip of the conveyance roller  56  that is rotating at the downstream side.  
      Subsequently, the conveyance roller  56  reversely rotates at the predetermined timing and again produces deflection in the sheet, as shown in  FIG. 7 . Such deflected section is inserted to the nip formed between the folding roller  51  and the folding roller  53  and the second folding is performed on the sheet. The sheet is thus folded into a Z-shape by the second folded portion.  
      As shown in  FIG. 8 , the sheet folded into a Z-shape in the above manner is conveyed by the folding rollers  51 ,  53  and the conveyance rollers  57 ,  70 . When the back end of the Z-folded sheet passes through a switch back flapper  58 , the conveyance rollers  57 ,  70  reverse rotate and the Z-folded sheet is conveyed to a switch back path  59 . The switch back flapper  58  guides the sheet while changing the position at a predetermined timing when driven by the solenoid SL 3  ( FIG. 9 ). In  FIG. 5 , the terminating end of the switch back path merges again with the folding path  17 , and the Z-folded sheet is conveyed by the conveyance rollers  54 ,  55  and the conveyance rollers  60 ,  61  on the downstream thereof. The conveyance roller  61  is provided above the processing tray  7  at a nip angle that allows the sheet to be discharged to the stopper  8  of the processing tray  7  without the movement gripper  5  being an obstruction.  
      The conveyance rollers  18 ,  19 ,  54 ,  60 ,  61  and the folding rollers  51 ,  52 ,  53  rotate only in one direction when receiving the rotational power of the driving source motor, but the conveyance rollers  55 ,  56 ,  57  can be switched between forward rotation and reverse rotation when receiving the rotational power of the driving source motor. The switch timing from the forward rotation to the reverse rotation is set with the output of the detection signal from the path sensor (not shown) as the trigger.  
      The operation for each processing mode will now be described.  FIG. 18  shows the flow chart corresponding to the operation, and the operation will be described in line with the flow chart.  
     First Embodiment Staple Sort Mode  
      When the user specifies the staple sort mode (step: S 1 ), the first sheet P 1  formed with image shown with a broken line in the figure is discharged from the apparatus main body  900  of the image forming apparatus as shown in  FIG. 10 . The discharged sheet P 1  is received by the entrance side rollers  2 ,  3  and conveyed to the conveyance paths  4 ,  10 . The flappers  6  are at the position of the figure and convey the sheet P 1  to the movement gripper  5 . The movement gripper  5  is in a stopped state at the receiving position (step: S 2 ). The movement gripper  5  is moved to the receiving position when not in the receiving position (step: S 3 ). As the conveyance proceeds, the front end of the sheet P 1  is guided to the gripper tapered surface  5   d  and enters the gripping portion  5   a , and abuts against the abutment surface  5   b  of one of the two movement grippers  5  lined on the rotating shaft. The movement gripper  5  is still stopped over a predetermined time after the abutment, and the rotation of the conveyance roller  3  is continued. Thus, as shown in  FIG. 11 , the sheet P 1  has the front end of the sheet reliably abutted to the other abutment surface  5   b  while forming a loop in the space near the tapered surface  5   d , and the skew feeding is corrected (step: S 4 ).  
      When the predetermined time has elapsed with the sheet P 1  gripped by the gripping portion  5   a , as shown in  FIG. 12 , the movement gripper  5  that was in the stopped state starts to normally rotate in the counterclockwise direction by the driving motor to move sheet P 1  (step: S 5 ). When the back end of the sheet P 1  passes through the flapper  6  ( FIG. 13 ), the sheet stops once (step: S 7 ), the position of the flapper  6  is switched by the drive (ON) of the solenoid SL 1 , and the drive motor reversely rotates thereby switching and moving the movement gripper  5  in the clockwise direction (step: S 9 ). The back end of the sheet P 1  is then inserted to the waiting path (retreating portion)  11  while being guided by the flapper  6 . The movement gripper  5  moves to the receiving position and stops while gripping the sheet P 1  (step: S 10 ,  11 ).  
      Subsequently, as shown in  FIG. 14 , the next sheet P 2  is also conveyed by the conveyance rollers  2 ,  3  similar to the previous sheet P 1 , and the front end enters the gripping portion  5   a  while the skew feeding is being corrected (step: S 2  to  5 ). When the superimposing number is two as in the present embodiment, the movement of the sheets P 1 , P 2  by the movement gripper  5  starts thereafter. When the number set for the superimposing number is two or more, after the back ends of the sheets P 1 , P 2  pass through the flapper  6  by the movement gripper  5 , the flapper  6  is switched, the back ends of the sheets P 1 , P 2  are guided to the waiting path  11  and the next sheet P 3  to be conveyed is waited, similar to the sheet P 1 . After the switch back conveyance by forward and backward movement is completed for the number of set sheets, and determination is made on whether or not the sheet is the last sheet of the superimposed sheets (step: S 6 ), the flapper  6  again returns to the original position. Subsequently, the movement gripper  5  that was in the stopped state resumes the rotation and moves the sheets P 1 , P 2  held in the superimposed manner. As the rotation advances, the front ends of the sheets P 1  P 2  held together in the gripping portion  5   a  abut against the stopper  8 , and released from being gripped by the gripping portion  5   a . As shown in  FIG. 15 , the front end of the sheet group Pb made up of sheets P 1 , P 2  of the present embodiment is stopped by the stopper  8 , and the gripping portion  5   a  continuously rotates and stops when moved up to the receiving position (step: S 12 ). Simultaneously, the back end side winded to the movement gripper  5  of rotating drum shape separates from the movement gripper  5  and then inverted, and dropped onto the processing tray  17 .  
      If the sheet group Pb is a small size such as A4 size, the sheet group is accommodated within the dimension in the conveying direction of the processing tray  7 , but if the sheet group P 3  is a the large size of A3 size and the like that is long in the conveying direction, the sheet group projects out from the processing tray  7  and is accommodated as if riding on the stack tray  20 . The sheet group Pb superimposed on the movement gripper  5  at this point has the front end abutted against the stopper  8  and aligned, and is thus dropped onto the processing tray  7  with the front ends aligned. Since the processing tray  7  has the stopper  8  provided on the lower side of the tilt, the front end is abutted against the stopper  8  and positioned in the dropping operation, and then accommodated in the processing tray  7 .  
      As apparent from the above, the sheet processing apparatus of the present embodiment has a simple configuration having the moving unit with a sheet superimposing function referred to as the movement gripper  5  as the main part. In this case, the sheet group Pb made up of a plurality of sheets is superimposed while the skew feeding is being corrected, and is conveyed until the front end of the sheet reliably abuts against the stopper  8  while being gripped by the gripping portion  5   a . A high precision control is not required for the timing and the speed at which the sheet P is released from the gripping portion  5   a . Furthermore, the sheet P is stacked on the processing tray  7  with the surface recorded with the image facing down (inverted state) without providing a special inverting mechanism.  
      When accommodating into the processing tray  7 , the aligning plates  9   a ,  9   b  waiting at a slightly widely opened position from the sheet width dimension approach each other in a direction of sandwiching the sheet group Pb, thereby aligning the sheets in the sheet width direction. After the sheet width aligning, the aligning plates  9   a ,  9   b  are again opened and spaced apart from each other to prepare for the accommodation of the next sheet group Pb. The relevant aligning process is repeated with respect to the number of superimposed sheets of the third sheet P 3 , the fourth sheet P 4 , the fifth sheet P 5  and so on, and when the process on the last sheet Pn of the scheduled number N is completed, alignment of one of the sheet bundle, that is, one copy is completed (step: S 13 ). Thus, the sheet bundle including a predetermined number of sheets is formed by repeating the accommodation of the sheet group including a plurality of sheets in the processing tray  7 . As hereinafter described, the number of sheets forming the sheet group may be set to an arbitrary number with respect to each sheet group as long as it is not one.  
      Description has been made with the number of superimposing sheets as two in the present embodiment. However, if one sheet bundle is configured by an odd number of sheets, the last sheet of the sheet bundle is moved to the processing tray  7  alone without being superimposed. In this case, a control different from the other sheets that are moved in a superimposed manner must be performed. For example, the back end of the sheet is not introduced into the waiting path  11  by the reverse rotation. In order to perform the same control, for example, a dummy operation of conveying the second sheet into the movement gripper  5  is performed by reverse rotation. In either method, the processing efficiency lowers since only one sheet is moved and aligned in the processing tray  7 . The following countermeasures are taken to resolve such problem. Suppose the sheet P discharged from the apparatus main body  900  of the image forming apparatus is a page two pages before the last page. In this case, if the sheet P two pages before is gripped by the movement gripper  5  along with the sheet P further before the relevant sheet, the sheet P of the last page and the sheet P one page before the last page can be conveyed together in a superimposed state by the movement gripper  5  after the two sheets P held in the superimposed manner are conveyed.  
      When the sheet P two pages before are not gripped by the movement gripper  5  along with the sheet P of the page further before the relevant page, three sheets are conveyed together while being superimposed on the movement gripper  5 . The three sheets include the sheet P two pages before, the sheet P one page before the last page, and the sheet P of the last page. If the three sheets P are conveyed in this manner, the sheet P conveyed the last will not be conveyed alone. Furthermore, when gripping the three sheets P while being superimposed on the movement gripper  5 , the superimposing operation of the first and the second page is applied with the operation of sheet P 1  and the superimposing operation of the third page is applied with the operation of sheet P 2 .  
      Determination is then made whether the setting of the stapling process is made (step: S 14 ), and if the setting is made, stapling is performed by the stapler  30  waiting at the stapling position (step: S 15 ). In the case of the two-location stapling mode, the stapler  30  first staples the front position of the two locations and then moves back to staple the back position.  
      The sheet bundle performed with the stapling operation stops at the position retreated from the stopper  8 , and is pushed out as the stack conveying projection  13  in waiting moves in the direction of the stack tray  20 , whereby the sheet stack is discharged to the stack tray  20  at the paper level position (step: S 16 ). When the setting of the stapling process is not made, the sheet bundle is discharged to the stack tray  20  as it is. Subsequently, the stack conveying projection  13  retreats to prepare for the subsequent sheet bundle, and at the same time, the stack tray  20  rises or lowers to again align the paper level.  
      As shown in  FIG. 16 , the above described operations are repeated to bundle and obtain a desired number of sheet bundles on the stack tray  20 .  
      In the first embodiment, the two sheet (sheet P 1 , P 2 ) at the beginning of the first sheet bundle, and furthermore, for the subsequent sheets, two or three sheets are also superimposed on the movement gripper  5 . In order not to interrupt the image forming operation of the apparatus main body  900 , the processing time for the stapling process performed at the completion of stacking of the sheet bundle with respect to each bundle must be ensured. That is, the sheet at the beginning of the second or the subsequent sheet bundle must temporarily retain the image-formed sheet sent between the stapling process of the first sheet bundle and the discharge by the stack conveying projection  13 . Therefore, the sheet at the beginning of the second or the subsequent sheet bundle is desirably superimposed in three or more sheets. The number of superimposed sheets is appropriately determined according to the production ability of the copying machine, the processing ability of the sheet processing apparatus, and the stapling mode of one location stapling or two location stapling.  
      Furthermore, description has also been made in superimposing two or three sheets and aligning and superimposing the same all together in the processing tray  7  with respect to the sheets at the middle after the sheet at the beginning of the sheet bundle. However, the present embodiment aims to create a temporal margin compared to when performing the sheet aligning and stacking process one sheet at a time, where the aligning property is further enhanced by allocating the margin time for the increase in the number of aligning operations and the reduction in the aligning speed.  
     Second Embodiment Sort Mode  
      The operation of when the sort mode in which the stapling is not performed is specified by the user will now be described. In this mode as well, two, or three or more sheets are stacked and accommodated in the processing tray  7  while being sequentially superimposed on the movement gripper  5 , similar to the staple and sort mode. After the sheet of the last page is aligned, the stack conveying projection  13  immediately discharges the sheet bundle to the stack tray  20 . Subsequently, the next subsequent sheet bundle is similarly divided and accommodated in the processing tray  7 , but the aligning position in the sheet width direction is performed at a position different from the previous sheet bundle. This is so as to distinguish the previous sheet bundle when the subsequent sheet bundle is stacked in the stack tray  20 . Therefore, the sheet bundle is stacked up to the last stack at the stack tray  20  while changing the aligning position in the sheet width direction on the processing tray  7  for each bundle. Other operations are the same as the staple sort mode.  
      When Z-folded and non-folded sheets are mixed in the sheet bundle, the following is executed. The non-folded sheets (A4 size) other than the Z-folded sheets performed with Z-folding are superimposed by two or three or more sheets by the movement gripper  5 , and accommodated in the processing tray  7 , similar to the first embodiment. The Z-folded sheets (A3 size) are discharged from the copying machine in a non-folded state and handed to the entrance side roller  2 . The flapper  16  positioned at the downstream of the entrance side roller  2  switches the path to the folding path  17 . Thereafter, the Z-folded sheets are fed to the folding device  50 , and folded to a Z-shape by the folding rollers  51 ,  52 ,  53 . The length in the conveying direction of the Z-folded sheet is substantially equal to the A4 size. The Z-folded sheet is fed with the image face facing upward and the folded portion at above the back end, and inverted at the curved path of the conveyance rollers  60 ,  61  and then discharged to the processing tray  7  by the conveyance roller  61 . The discharged Z-folded sheet has the end positioned at the stopper  8  and stationed, and performed with the aligning process in the sheet width direction. The non-folded sheet after the Z-folded sheet is discharged from the copying machine at a constant interval, and guided and conveyed by the flapper  16  which conveying path is again switched to the movement gripper  5  side. Since the sheet from when the previous Z-folded sheet is performed with the folding process until the sheet is stacked on the processing tray  7  must be temporarily retained, the subsequent sheets immediately after the Z-folded sheet are superimposed by four sheets and sandwiched by the gripping portion  5   a  (see  FIG. 17 ). The superimposing number may be appropriately determined depending on the production of the copying machine, and the folding processing ability of the folding device  50 . Such operation is repeatedly performed for the Z-folded sheet and the non-folded sheet to form the aligned bundle at the processing tray  7 . Subsequently, stapling is performed, as necessary, and discharged to the stack tray  20  by the stack conveying projection  13 .  
      As can be understood from the above configuration and operation, each embodiment has the following advantages.  
      (1) By temporarily retaining the sheet subsequent to the previous sheet bundle being performed with the post process at the movement gripper  5 , the sheet conveyance does not need to be stopped even in the post process operation, that is, the image forming operation of the image forming apparatus does not need to be temporarily interrupted, thereby being very effective in maintaining high productivity.  
      (2) The sheets after a desired number of sheets from the beginning of the sheet bundle of the set number of sheet accommodated in the processing tray  7  are also divided into the sheet groups of a few number of sheets, and each divided group is accommodated all together in the processing tray  7  with the movement gripper  5 , so that a plurality of sheets are simultaneously aligned, temporal aligning margin is created, and the aligning ability is enhanced.  
      (3) The movement gripper  5  is a rotating drum that inverts and moves with the sheet wrapped thereon, thereby facilitating the operation control and allowing temporary retention.  
      (4) The waiting path  11  for temporarily accommodating the back end of the sheet while being gripped by the gripping portion  5   a  of the movement gripper  5  is provided. The back end of the previous sheet is retreated so as not to interfere with the front end of the subsequent sheet, which is effective in suppressing the occurrence of a jam in time of the superimposing operation at the movement gripper  5 .  
      (5) The sheet following the sheet processed by the post processing unit is superimposed in pluralities at the movement gripper  5 , and then accommodated all together in the processing tray  7 , so that the conveyance of the subsequent sheet does not need to be stopped even if the post process is performed on the sheet at the middle of the sheets sequentially conveyed to form the sheet bundle.  
      The numerical examples of the embodiment and the configuration, operation and advantages thereof have been explained, but other embodiments, applications, modifications, and a combination thereof are possible without deviating from the scope of the present invention. For example, in each embodiment, the Z-folding device is applied as the unit for performing a process on an arbitrary sheet in the middle of the sheet bundle, but in place thereof, a puncher for opening a hole in the sheet, a fold-line device for forming a fold-line on the sheet, a cutting device for cutting the sheet etc may be applied.  
      This application claims the benefit of Japanese Patent Applications No. 2005-326848 filed on Nov. 11, 2005 and No. 2006-292165 filed on Oct. 27, 2006 which are hereby incorporated by reference herein in their entirety.