Patent Publication Number: US-8528898-B2

Title: Sheet conveying apparatus, sheet processing apparatus, and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying apparatus capable of causing a sheet to wait in a conveying route and to meet a following sheet and conveying them in a sheet bundle, a sheet processing apparatus including the sheet conveying apparatus, and an image forming apparatus including the sheet conveying apparatus. 
     2. Description of the Related Art 
     There is a known sheet processing apparatus for performing various kinds of sheet processes such as a binding process, a punching process, and a sorting process for sheets on which images are formed by an image forming apparatus such as a copying machine, a laser beam printer, and a facsimile. Conventionally, this type of sheet processing apparatus is provided with a portion (hereafter referred to as a buffering processing portion) in which a sheet is made to stay temporarily to avoid reduction in sheet processing productivity in a case of a process (e.g., the binding process) requiring a relatively long sheet processing time. 
     In such a sheet processing apparatus, the sheet is made to stay temporarily in the buffering processing portion and then conveyed downstream, with a following sheet conveyed from an upstream side in a sheet bundle of the plurality of sheets. In this way, the processing time for the sheet processing portion provided downstream is secured. In this manner, by causing the sheet to stay in the buffering processing portion provided to a conveying portion between the image forming apparatus and the sheet processing portion while the sheets are processed in the downstream portion and by not stopping discharge operation from the image forming apparatus, production capacity of a system in total is enhanced. 
     As disclosed in U.S. Patent Application Publication No. 2007/0075479 A1 and U.S. Patent Application Publication No. 2004/0175217 A1, for example, there is a sheet processing apparatus including a reversing-type buffering mechanism in which rear ends of the sheets are sorted by a switch member and the sheets are superimposed on each other a bundle while collision with a front end of a following sheet is prevented. 
     As illustrated in  FIG. 18 , a sheet processing apparatus in U.S. Patent Application Publication No. 2007/0075479 A1 has what is called a sheet buffering processing function for causing a plurality of superimposed sheets P to wait in a conveying route on an upstream side of a stapler  1132  and a saddle unit  1135 . An operation flow of the buffering process is as follows. 
     When a rear end portion of a sheet P 1  conveyed from a pair of inlet rollers  1102  is guided by and passes a lower face of a switch member  1114 , a solenoid (not illustrated) is actuated and the switch member  1114  is turned down to guide the sheet P 1  to a buffer path  1113 . In this state, a pair of buffer rollers  1115  and  1116  are rotated reversely. As a result, the rear end of the sheet P 1  enters the buffer path  1113  and then the sheet P 1  is conveyed reversely until a front end of the sheet P 1  arrives at a position B. 
     Then, the switch member  1114  is turned up so that a second sheet P 2  can be conveyed toward the pair of buffer rollers  1116 . At this time, a pair of reversing rollers  1112  is rotated at the time when a front end of the sheet P 2  arrives at the position B to start conveying the sheet P 1  toward the pair of buffer rollers  1116 . With the front ends of the sheet P 1  and the sheet P 2  aligned with each other on a downstream side of the position B, the bundle of the two sheets is conveyed. 
     When a rear end portion of the sheet bundle of the sheets P 1  and P 2  passes the switch member  1114  similarly to the sheet P 1 , the switch member  1114  is turned down and the sheet bundle of the sheets P 1  and P 2  is conveyed to the buffer path  1113 . In this way, the switch member  1114  is reciprocated every time the sheet passes to sequentially superimpose the sheets on each other to thereby bundle the sheets in the conveying route. 
     U.S. Patent Application Publication No. 2004/0175217 A1 is the sheet processing apparatus having a different buffering mechanism from U.S. Patent Application Publication No. 2007/0075479 A1. 
     As illustrated in  FIG. 19 , when a rear end of a sheet P 1  passes a rear end holding member  2135  and arrives at a reversing point provided before a buffer roller  2124 , the sheet P 1  is returned to an upstream side (toward the rear end holding member  2135 ) by reverse rotation of the buffer roller  2124 . At about the same time, the rear end holding member  2135  is separated from a lower conveying guide plate  2123   b  to open a rear end receiving portion  2136 . 
     Then, the rear end holding member  2135  returns to an original position to press the sheet P 1  against the lower conveying guide plate  2123   b  with a friction member of the rear end holding member  2135 . Then, a second sheet P 2  is sent in by a pair of inlet rollers  2121 , passes over the rear end holding member  2135 , and is conveyed by the buffer roller  2124  as well. 
     At this time, the sheet P 1  is pressed against the lower conveying guide plate  2123   b  together with the sheet P 2  by the buffer roller  2124  and tries to move toward a downstream side following the conveyed sheet P 2 . However, the sheet P 1  does not move due to the friction member of the rear end holding member  2135 . 
     Similarly to the sheet P 1 , when a rear end of the sheet P 2  arrives at the reversing point, the sheet P 2  is returned toward the upstream side (toward the rear end holding member  2135 ). Then, the sheet P 2  is superimposed on the sheet P 1  and pressed against the lower conveying guide plate  2123   b  by the friction member of the rear end holding member  2135 . 
     Then, a third sheet (not illustrated) is sent and a rear end of the sheet passes the pair of inlet rollers  2121 . The three sheets P 1 , P 2 , and P 3  are nipped between a pair of upper first discharge rollers  2126   a  and a pair of lower first discharge rollers  2126   b  and the sheet bundle of these three sheets P 1 , P 2 , and P 3  is conveyed to a downstream processing tray. 
     As described above, the prior-art sheet processing apparatus is of the buffering type in which the sheets are superimposed on each other in the bundle while the switch member for sorting the rear ends is reciprocated during intervals of the conveyed sheets. In this way, the apparatus provides satisfactory conveying performance and processing productivity. 
     However, in the buffering type in which the sheets are superimposed on each other in the bundle while the switch member is moved during the sheet interval for each sheet as in U.S. Patent Application Publication No. 2007/0075479 A1, the sheet interval when the sheet passes the switch member needs to be equal to or longer than a reciprocating time of the switch member. Therefore, it is difficult to adapt to a highly productive apparatus with short sheet conveying intervals. 
     Especially, because a thin sheet of 38 gsm to 52 gsm has a low mass per sheet and has low rigidity, discharge of the thin sheet to a stack tray is liable to be unstable. Therefore, to discharge such a thin sheet to the stack tray, a plurality of sheets needs to be bundled in the buffering processing portion before discharge so as to obtain a higher mass and rigidity to be discharged. However, as described above, the highly productive apparatus in which the sheets are conveyed at short intervals is difficult to adapt to the thin sheets of 38 gsm to 52 gsm. 
     Furthermore, in U.S. Patent Application Publication No. 2004/0175217 A1, because the reversing point is provided downstream of the rear end holding member  2135  for sorting of the rear ends, an upward opening operation control is performed so that the rear end portion is reliably conveyed to a lower of the rear end holding member  2135  at the time of reversal. In this way, the reciprocation for opening and closing the rear end holding member  2135  needs to be performed during the sheet interval for each sheet and therefore the apparatus is difficult to adapt to the highly productive apparatus similarly to U.S. Patent Application Publication No. 2007/0075479 A1. 
     As described above, in the prior-art sheet processing apparatus, the sheets are bundled while the switch member for sorting the rear ends is reciprocated during the sheet interval for each sheet and therefore there is a limit to processing velocity and it is difficult to increase productivity. Moreover, in the highly productive apparatus, it is difficult to buffer the sheets in a bundle. Therefore, it is difficult to discharge the plurality of thin sheets of 38 gsm to 52 gsm as a unit to improve stacking performance. As a result, the conveying performance and the stacking performance are difficult to improve. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention increases processing velocity for bundling and conveying sheets to increase productivity to be able to adapt to a highly productive apparatus. 
     Moreover, the invention enables the highly productive apparatus to bundle and convey thin sheets to thereby further improve the conveying performance and the stacking performance. 
     The present invention provides a sheet conveying apparatus which superimposes and conveys a plurality of sheets, including a common conveying route configured to convey a sheet in a first direction, a reverse conveying route which branches off from the common conveying route and conveys the sheet in a second direction opposite to the first direction, to make the sheet waiting in the reverse conveying route, a conveying portion which conveys the sheet in the first direction or in the second direction along the common conveying route or the reverse conveying route, a switch member which is provided at a branch portion between the common conveying route and the reverse conveying route, which can be switched between a first position for guiding the sheet in the first direction from an upstream side of the branch portion and a second position for guiding the sheet in the second direction from a downstream side of the branch portion, and which has a first guide face and a second guide face, located on a back of the first guide face, to guide the sheet, and a controlling portion which controls operations of the conveying portion and the switch member so that a preceding sheet, conveyed in the second direction to wait in the reverse conveying route, is guided by the first guide face of the switch member at the first position, and a following sheet, conveyed in the first direction to be superimposed with the preceding sheet waiting in the reverse conveying route, is guided by the second guide face of the switch member at the second position. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view illustrating structures of a sheet processing apparatus and an image forming apparatus; 
         FIGS. 2A and 2B  are block diagrams of controlling portions of the image forming apparatus and the sheet processing apparatus; 
         FIG. 3A  is a schematic sectional view of the structure of the sheet processing apparatus and  FIG. 3B  is a sectional view of a driving structure of a buffer conveying portion in the sheet processing apparatus; 
         FIG. 4  is a front view of a shift unit; 
         FIGS. 5A ,  5 B, and  5 C are sectional views of an essential portion and for describing operation of the sheet processing apparatus according to a first embodiment; 
         FIGS. 6A ,  6 B, and  6 C are sectional views of the essential portion and for describing operation of the sheet processing apparatus according to the first embodiment; 
         FIG. 7  is a flowchart for describing the operation of the sheet processing apparatus according to the first embodiment; 
         FIG. 8  is a flowchart for describing the operation of the sheet processing apparatus according to the first embodiment; 
         FIG. 9  is an explanatory view for describing control timing of the sheet processing apparatus according to the first embodiment; 
         FIG. 10  is a schematic sectional view of a structure of a sheet processing apparatus according to a second embodiment; 
         FIGS. 11A ,  11 B, and  11 C are sectional views of an essential portion and for describing operation of the sheet processing apparatus according to the second embodiment; 
         FIGS. 12A ,  12 B, and  12 C are sectional views of the essential portion and for describing the operation of the sheet processing apparatus according to the second embodiment; 
         FIGS. 13A and 13B  are sectional views of the essential portion and for describing the operation of the sheet processing apparatus according to the second embodiment; 
         FIG. 14  is a flowchart for describing the operation of the sheet processing apparatus according to the second embodiment; 
         FIG. 15  is a flowchart for describing the operation of the sheet processing apparatus according to the second embodiment; 
         FIG. 16  is a flowchart for describing the operation of the sheet processing apparatus according to the second embodiment; 
         FIG. 17  is an explanatory view for describing control timing of the sheet processing apparatus according to the second embodiment; 
         FIG. 18  is a sectional view for describing a prior-art (U.S. Patent Application Publication No. 2007/0075479 A1) sheet processing apparatus; and 
         FIG. 19  is a sectional view for describing a prior-art (U.S. Patent Application Publication No. 2004/0175217 A1) sheet processing apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     With reference to the drawings, exemplary embodiments of the present invention will be described below in detail as examples. However, dimensions, materials, shapes, and relative positions of component parts described in the following embodiments should be changed properly according to a structure of an apparatus to which the invention is applied and various conditions. Therefore, unless there is a specifying description, a scope of the invention is not limited to them. 
     First Embodiment 
     A sheet processing apparatus and an image forming apparatus having a sheet conveying apparatus according to a first embodiment will be described. 
     (Image Forming Apparatus)  FIG. 1  is a block diagram of the image forming apparatus and the sheet processing apparatus. As illustrated in  FIG. 1 , the image forming apparatus includes an image forming apparatus main body  600  for performing monochrome/color image forming and a sheet stacker  100  as a sheet processing apparatus connected to the image forming apparatus main body  600  in a detachably attachable manner. Therefore, a large number of sheets discharged from the image forming apparatus main body  600  can be stacked in the sheet stacker  100  connected online. 
     The image forming apparatus main body  600  can also be used alone without the sheet stacker  100  connected to its discharge port. Besides the sheet stacker  100 , a sheet processing apparatus having a processing portion such as a stapler to be able to perform side binding and saddle binding processes may be connected to the image forming apparatus to perform processes from image forming to various binding processes in a sequential flow. The image forming apparatus main body  600  may integrally include the sheet stacker  100  as the sheet processing apparatus. 
     Here, a position where a user faces an operation portion  601  for performing various inputs/settings for the image forming apparatus main body  600  is referred to as a front forward side (hereafter, forward side) of the image forming apparatus and a back side of the apparatus which is on an opposite side from the forward side is referred to as a back side.  FIG. 1  is the schematic sectional view illustrating the structure of the image forming apparatus seen from the apparatus forward side. The sheet stacker  100  is connected to a side portion of a discharge port side of the image forming apparatus main body  600 . 
     A four-color toner image is transferred onto a sheet S fed from a cassette  909   a  or  909   b  in the image forming apparatus main body  600  by photosensitive drums  914   a  to  914   d  of yellow, magenta, cyan, and black respectively forming image forming portions. Then, the sheet S is conveyed to a fixing device  904 , where the toner image is fixed, and is discharged as it is to an outside of the apparatus main body by a pair of discharge rollers  907  in a single-side image forming mode. 
     In a both-side image forming mode, the sheet S is handed from the fixing device  904  to a reversing roller  905 , where front and back sides are reversed, and is conveyed toward both-side conveying rollers  906   a  to  906   f . Then, onto the back side, a four-color toner image is transferred again by the photosensitive drums  914   a  to  914   d  of yellow, magenta, cyan, and black. The sheet S on both sides of which the images are transferred is transferred to the fixing device  904  again, where the toner image is fixed, and is discharged to the outside of the apparatus main body by the pair of discharge rollers  907 . 
       FIG. 2A  is a block diagram of an image forming apparatus controlling portion for controlling the image forming apparatus. As illustrated in  FIG. 2A , a CPU circuit portion  630  includes a CPU  629 , a ROM  631 , and RAM  628 . The CPU circuit portion  630  controls a document feeding apparatus controlling portion  632 , an image reader controlling portion  633 , an image signal controlling portion  634 , a printer controlling portion  635 , a sheet stacker controlling portion  636 , and an external interface  637 . The CPU circuit portion  630  controls operation of the entire apparatus according to programs stored in the ROM  631  and the setting of the operation portion  601 . 
     The document feeding apparatus controlling portion  632  controls a document feeding apparatus  650  (see  FIG. 1 ) for feeding a document to an image reader provided on an upper portion of the image forming apparatus main body  600 . The image reader controlling portion  633  controls the image reader. The printer controlling portion  635  controls the image forming apparatus main body  600 . The sheet stacker controlling portion  636  controls the sheet stacker  100 . 
     Here, a structure in which the sheet stacker controlling portion (controlling portion)  636  is mounted to the sheet stacker  100  will be described. However, the invention is not limited to this structure. Instead, the sheet stacker controlling portion  636  may be provided to the image forming apparatus main body  600  integrally with the CPU circuit portion  630  and control the sheet stacker  100  from the image forming apparatus main body  600 . 
     The RAM  628  is used as an area for temporarily storing control data and as a work area for computation involved with control. The external interface  637  is an interface from a computer (PC)  620  and develops printing data into an image and outputs it to the image signal controlling portion  634 . The image reader controlling portion  633  outputs an image read by the image reader to the image signal controlling portion  634  and the image output from the image signal controlling portion  634  to the printer controlling portion  635  is input to an exposure controlling portion. 
     (Sheet Processing Apparatus)  FIG. 3A  is a schematic sectional view of the structure of the sheet stacker  100  as the sheet processing apparatus. As illustrated in  FIG. 3A , the sheet stacker  100  includes a shift unit  108  as a sheet sorting portion, a buffering processing portion as a sheet conveying apparatus (sheet buffering processing portion) and a stack tray (discharge tray) as a stacking portion. The sheet S discharged from the image forming apparatus main body  600  is handed to a pair of inlet rollers  102  of the sheet stacker  100 . At this time, handing timing of the sheet is sensed at the same time by an inlet sensor  101 . While the sheet S conveyed by the pair of inlet rollers  102  passes through a conveying path  103 , an end portion position of the sheet S in a width direction orthogonal to a conveying direction of the sheet is sensed by an end portion sensor  104 . In this way, an amount of a width-direction error (positional displacement) with respect to a conveying center position of the sheet stacker in the width direction is sensed. 
     After the error in the sheet width direction is sensed, shifting operation of the sheet S is performed by moving the shift unit  108  in the forward/backward direction (the sheet width direction) by a predetermined amount while the sheet S is conveyed by pairs of shift rollers  105  and  106 . In other words, by moving the shift unit  108  in the sheet width direction by the predetermined amount, the positional displacement of the sheet in the width direction is corrected. 
     Then, the sheet S conveyed by a pair of pre-buffer conveying rollers  110  and  111  is conveyed by a pair of buffer conveying rollers  115  which is a conveying portion and can be rotated normally and reversely. Then, if the sheet S is discharged to an upper discharge tray  136  which is a stacking portion, an upper path switch member  118  is brought into a state illustrated in a broken line in the drawing by a drive portion (not illustrated) such as a solenoid to guide the sheet S into an upper path conveying path  117  and the sheet S is discharged to the upper discharge tray  136  by a pair of upper discharge rollers  120 . 
     If the sheet S is not discharged to the upper discharge tray  136 , the sheet S conveyed by the pair of buffer conveying rollers  115  is guided into a lower conveying path  121  by the upper path switch member  118 . Then, the sheet S is discharged to a lower discharge tray  137  which is a stacking portion by a pair of lower discharge rollers  141  via a pair of first lower conveying rollers  122  and a pair of second lower conveying rollers  124 . An upper discharge sensor  119 , a lower conveying sensor  123 , and a lower discharge sensor  140  form detecting portions for detecting a front end and a rear end of the sheet in the conveying paths and detecting conveying timing. If these detecting portions do not detect the sheet end portions after prescribed timing, the operation portion  601  displays a signal indicating that the sheet is staying in the apparatus. 
     &lt;Description of the Shift Unit&gt; Next, a structure and operation of the shift unit  108  will be described by using a front view in  FIG. 4 . The conveyed sheet S is conveyed in a direction of an arrow C in the drawing when drive of a shift conveying motor M 208  is transmitted through a drive belt  209  to drive the pair of shift rollers  106  and a drive belt  213  drives the pair of shift rollers  105 . At this time, the end portion sensor  104  is moved in a direction of arrows E by a drive portion (not illustrated) to thereby detect a conveying position of the sheet S (a lateral registration error X which is a positional displacement amount in the width direction). The sheet is moved by a shift amount Z (Z=X+α) of the sheet which is the sum of the lateral registration error X and the shift amount α of the sheet while the sheet is conveyed. By performing this operation in the forward/backward direction (directions of arrows) when the sheet S is nipped between the pairs of shift rollers  105  and  106 , it is possible to move (shift) the sheet S in the width direction (directions of the arrows D) by the predetermined amount while conveying the sheet S in the conveying direction (direction of the arrow C). 
     &lt;Description of Buffering Processing Operation&gt; Next, by using  FIGS. 2B ,  3 B and  5 A to  8 , a structure related to the buffering processing portion and sheet conveying operation will be described. FIG.  2 B is a block diagram illustrating a detail of a conveyance controlling portion  708  of the sheet stacker controlling portion  636 .  FIG. 3B  is a drive block diagram of the buffering processing portion.  FIGS. 5A to 6C  are operation drawings illustrating the buffering processing operation.  FIGS. 7 and 8  are flowcharts illustrating the buffering processing operation. 
     The buffering process is a process for sequentially superimposing the sheets S, which have been conveyed one by one, into a sheet bundle including a plurality of sheets in the conveying path in the apparatus. By performing the buffering process, there are roughly two effects. 
     The first effect is to temporarily delay conveying arrival time of the sheet S at the downstream apparatus without reducing productivity to thereby secure time required for various processes such as alignment of the sheets and a binding process. 
     The second effect is to increase rigidity by conveying the plurality of sheets in a bundle, e.g., by superimposing the plurality of sheets on each other and conveying them in a case of thin sheets to thereby improve conveying performance of the sheets, stacking performance to the stack tray, and aligning performance. 
     As illustrated in  FIG. 2B , the sheet stacker controlling portion  636  includes a CPU  701 , a ROM  702 , RAM  703 , a network interface  704 , an I/O  705 , and a communication interface  706 . The sheet stacker controlling portion  636  controls the conveyance controlling portion  708  through the I/O  705 . The conveyance controlling portion  708  includes respective motors M 110 , M 112 , M 114 , and M 208  and respective sensors  101 ,  104 ,  109 ,  116 ,  119 ,  123 , and  140 . The conveyance controlling portion  708  controls operations of the respective motors according to signals of the respective sensors. 
     As illustrated in  FIG. 3B , the buffering processing portion includes the pair of pre-buffer conveying rollers  110  driven for rotation by the pre-buffer conveying motor M 110 . The buffering processing portion also includes a pair of first buffer conveying rollers  112  and a pair of second buffer conveying rollers  115  as the conveying portion driven for normal and reverse rotations by the buffer conveying motor M 112 . The buffering processing portion also includes the conveying path  103  as a common conveying route for guiding the sheet and a buffer path  113  as a reverse conveying route branching off from a midway point of the conveying path  103  to guide the sheet. Furthermore, in order to guide the sheet S into the buffer path  113 , the buffering processing portion also includes a switch member  114  to be driven by the switch member motor M 114  and the first buffer sensor  109  and the second buffer sensor  116  as the sheet detectors for sensing the end portion of the conveyed sheet S. The buffering processing portion is made up of these members. 
     The conveying path  103  is the common conveying route for guiding the sheet toward the discharge tray (stacking portion) to be stacked with the sheets. The buffer path  113  is the reverse conveying route branching off from the midpoint of the conveying path  103  to guide the sheet in an opposite direction from the discharge tray. 
     The pairs of conveying rollers  112  and  115  are the conveying portion for conveying the sheet S toward the discharge tray or in the opposite direction from the discharge tray in the conveying path  103  and the buffer path  113 . 
     The switch member  114  is provided at a branch portion between the conveying path  103  and the buffer path  113 . The switch member  114  can be switched between a first position and a second position. The switch member  114  guides the sheet from an upstream side of the branch portion toward the discharge tray at the first position (upper position) and guides the sheet from the downstream side of the branch portion toward the buffer path  113  at the second position (lower position). The switch member  114  has a first guide face α for guiding the sheet and a second guide face β opposed to the first guide face α. 
     By using operation sectional views in FIGS.  5 A to  6 C and flowcharts in  FIGS. 7 and 8 , flows of the sheets at the time of the buffering operation will be described below. 
     The conveyed preceding sheet S 1  is subjected to a switchback process (S 801 ) for buffering. First, the switch member  114  waits at the upper position which is a home position illustrated in  FIG. 5A  until the sheet S 1  arrives (S 802 , S 803 ). The preceding sheet S 1  is guided by the first guide face α of the switch member  114  at the upper position and conveyed in a first direction toward the lower conveying path  121  by the pair of pre-buffer conveying rollers  110  and the pair of second buffer conveying rollers  115 . At this time, a front end (downstream end in the conveying direction) of the sheet S 1  is sensed by the second buffer sensor  116  (S 804 , S 805 ). Then, a stop control of the pair of second buffer conveying rollers  115  is performed so that the sheet S 1  stops when a rear end (upstream end in the conveying direction) of the sheet S 1  arrives at a position A as illustrated in  FIG. 5B  based on information of a size of the sheet in the conveying direction and recognized in advance (S 806  to S 808 ). Here, when a clock count has reached 300 since the front end of the sheet S 1  was sensed, the rear end of the sheet S 1  arrives at the position A and the buffer conveying motor M 112  is stopped. 
     When the rear end of the sheet S 1  arrives at the position A, the switch member motor M 114  moves the switch member  114  to the lower position illustrated in  FIG. 5B  for guiding the sheet toward the buffer path  113 . Furthermore, the pair of second buffer conveying rollers  115  starts reverse conveyance in a second direction opposed to the first direction. As a result, the sheet S 1  is guided by the first guide face α of the switch member  114  at the lower position and the rear end portion of the sheet S 1  is guided into the buffer path  113  as illustrated in  FIG. 5C . Then, the sheet S 1  is reversely conveyed by the pair of second buffer conveying rollers  115  until the front end of the sheet S 1  arrives at the position B and the stop control is performed (S 809  to S 817 ). In this way, the sheet S 1  is made to wait in the buffer path  113 . 
     In other words, in the switchback process for the buffering, the preceding sheet S 1  conveyed toward the discharge tray is guided by the first guide face α of the switch member  114  at the upper position. After the rear end of the preceding sheet S 1  passes through the branch portion, the switch member  114  is switched to the lower position. Then, the preceding sheet S 1  conveyed from the downstream side of the branch portion toward the buffer path  113  is guided by the first guide face α of the switch member  114  at the lower position and is made to wait in the buffer path  113 . 
     Next, the operation goes to a buffering superimposing process (S 818 ). First, a front end of a conveyed following sheet S 2  is sensed by the first buffer sensor  109  (S 819  to S 820 ). Then, when the front end of the sheet S 2  arrives at a position C illustrated in  FIG. 6A , the pair of first buffer conveying rollers  112  is activated to start acceleration at a predetermined time so that the front ends of the sheet S 1  stopped in the buffer path  113  and the sheet S 2  are aligned with each other (S 821  to S 823 ). 
     The switch member  114  waits for arrival of the following sheet S 2  conveyed by the pair of pre-buffer conveying rollers  110  while maintained at the lower position for guiding the preceding sheet S 1  toward the buffer path  113 . As illustrated in  FIG. 6B , the sheet S 2  is conveyed toward the pair of second buffer conveying rollers  115  while guided by the second guide face β of the switch member  114  at the lower position. Then, the following sheet S 2  is conveyed side by side with the preceding sheet S which has started accelerating at a predetermined time in advance and meets the sheet S 1  at a downstream portion of the switch member  114  so that front ends of the sheets are aligned with each other (S 824 ). 
     Because the preceding sheet S 1  starts to be conveyed toward the downstream side of a front end position B before meeting the following sheet S 2 , a meeting position of the front ends of the following sheet S 2  and the waiting preceding sheet S 1  is positioned downstream of the front end position B of the preceding sheet S waiting in the buffer path  113 . In the conveying route between the switch member  114  and the meeting position of the sheets, the pair of conveying rollers is not disposed. 
     After the sheet S 1  and the sheet S 2  meet at the downstream portion of the switch member  114 , acceleration of the sheet S 1  ends and the sheet S 1  is conveyed at the same velocity as the sheet S 2 . Then, as illustrated in  FIG. 6C , the sheet S 1  and the sheet S 2  are nipped by the pair of buffer conveying rollers  115  with their front ends aligned with each other, the front ends pass the second buffer sensor  116 , and the sheets are discharged to the upper discharge tray  136  or the lower discharge tray  137  on the downstream side (S 827 ). 
     In other words, in the buffering superimposing process, while the switch member  114  is maintained at the lower position, the following sheet S 2  conveyed from the upstream side of the branch portion toward the discharge tray is guided by the second guide face β of the switch member  114  at the lower position. Then, the sheet S 2  meets the waiting sheet on the downstream side of the switch member  114  and the sheets are conveyed in the sheet bundle toward the discharge tray. 
     Because the sheet S 1  and the sheet S 2  respectively pass the first guide face α and the second guide face β of the switch member  114  maintained at the lower position, the switch member  114  need not be switched and buffering is possible even if a sheet interval between the sheet S 1  and the sheet S 2  is small. Moreover, the sheets are put together into the bundle of two sheets in the buffering processing portion and are discharged to the upper discharge tray  136  or the lower discharge tray  137 . As a result, as described above, even thin sheets having insufficient stiffness can be put together into a bundle of sheets to obtain certain or more stiffness and can be discharged stably without impairing the stacking performance. 
     In a case of a sheet not requiring the buffering process, the switch member  114  is moved to and maintained at the upper position for guiding the sheet from the pre-buffer conveying rollers  110  toward the lower conveying path  121 . Then, after the sheet S discharged from the image forming apparatus main body  600  is moved in advance to a predetermined sorting position in the shift unit  108 , the sheet is discharged one by one to the upper discharge tray  136  or the lower discharge tray  137  without being subjected to the buffering process. 
       FIG. 9  is a drawing illustrating a timing chart of the buffering processing portion. A sheet conveying interval t of the sheets in the sheet stacker  100  is determined by the production number of sheets P per unit time from the image forming apparatus main body  600  which is the apparatus connected on the upstream, a conveying velocity V, and a sheet size L (length in the conveying direction). A conveying interval to of the sheets at the inlet portion of the sheet stacker  100  is the same as a conveying interval tf of the sheets at the pair of pre-buffer conveying rollers  110  before the switch member  114 , if the conveying velocity V is constant. If the buffering process for two sheets at a time is performed in the sheet stacker  100  in the embodiment, the switch member motor M 114  may be driven and either one of upward and downward moving operations F of the switch member  114  may be completed during the conveying interval tf as illustrated in  FIG. 9 . Therefore, even if the conveying interval tf is short because the production number of sheets P increases or the conveying velocity V is set to be low, there is no issue, if the conveying interval tf is not shorter than a moving operation time F of the switch member motor M 114 . As a result, it is possible to adapt to increase in the production number of sheets P and low-velocity conveyance. 
     As described above, according to the embodiment, it is possible to make the following sheet to meet the waiting sheet while maintaining the position of the switch member. Therefore, as compared with the prior-art apparatus in which the switch member is reciprocated, a processing velocity for bundling and conveying the sheets increases. Therefore, it is possible to further increase productivity and the apparatus in the embodiment can adapt to the highly productive apparatus. 
     Even in the case of the highly productive apparatus, it is possible to bundle and convey thin sheets with relatively low stiffness to thereby further improve the conveying performance and the stacking performance. Moreover, it is possible to reduce operating time of the apparatus and power consumption. 
     Second Embodiment 
     Next, a second embodiment will be described.  FIG. 10  is a sectional view of a finisher  200  having a buffering processing portion in a conveying route. Similarly to the sheet stacker  100 , the finisher  200  is a sheet processing apparatus which can be connected to the discharge portion of the image forming apparatus main body  600  and which performs processes for the sheets from the image forming apparatus main body  600 . 
     As illustrated in  FIG. 10 , the finisher  200  includes a stapler (binding portion)  132  for binding the sheets as a processing portion for processing the sheets. Furthermore, the finisher  200  includes a pair of lower discharge rollers  128  as a discharge portion, a draw-in paddle  131  as a butting portion, and a processing tray  138  as a stacking portion. A structure of the apparatus is obtained by adding a sheet processing portion for binding and saddle binding processes to the lower discharge portion of the above-described sheet stacker  100 . Because a sheet conveying operation and a control in discharging the sheets to the upper discharge tray  136  are similar to those of the sheet stacker  100 , they will not be described. 
     If not discharged to an upper discharge tray  136 , the sheets S conveyed by the pair of second buffer conveying rollers  115  are guided into a lower conveying path  121  by an upper path switch member  118 . Then, the sheets S are sequentially conveyed through the conveying path by pairs of lower conveying rollers  122  and  124 . To subject the sheets to a saddle (saddle binding) process, a drive portion (not illustrated) such as a solenoid brings a saddle switch member  125  into a state illustrated in a broke line. Then, the sheets are conveyed into a saddle path  133 , guided into a saddle unit  135  by a pair of saddle inlet rollers  134 , and subjected to the saddle process (saddle binding). Detailed description of a method of the saddle process will not be repeated. 
     To discharge the conveyed sheets S to the lower discharge tray  137 , the sheets S conveyed by the pair of lower conveying rollers  124  are conveyed into a lower path  126  by the saddle switch member  125 . Then, the sheets S are discharged to the processing tray  138  by the pair of lower discharge rollers  128 , aligned in a bundle on the processing tray  138  or subjected to the side binding process by the stapler  132 , and are discharged to the lower discharge tray  137  by the pair of discharge rollers  130  in a state of the sheet bundle. 
     It is known that the side binding process and the saddle (saddle binding) process usually require a certain processing time. The processing time partially depends on an image forming velocity of the image forming apparatus. However, it is usually difficult to complete the process during a sheet discharge interval and the processing time normally exceeds the sheet discharge interval. Therefore, in order to process the sheets without stopping the image forming by the image forming apparatus, a sheet buffering process is performed. 
       FIGS. 11A to 13B  are operation sectional views illustrating a flow of sheets in performing the buffering process for three sheets S 1  to S 3 .  FIGS. 14 to 16  are flowcharts illustrating buffering processing operation of the three sheets S 1  to S 3 . By using  FIGS. 11A to 16 , the flow of the sheets during the buffering process will be described. The three sheets S 1 , S 2 , and S 3  are the first sheet S 1 , the second sheet S 2 , and the third sheet S 3  in an order of conveyance. Here, the first sheet S 1  and the second sheet S 2  correspond to the first preceding sheet and the second preceding sheet sequentially waiting in a reverse conveying route and the third sheet S 3  following them corresponds to the following sheet to be conveyed in the first direction toward the stacking portion together with the preceding sheets. A relationship between the preceding sheets and the following sheets is set properly according to the number of sheets to be made to wait in the reverse conveying route and is not limited to the above-described composition. 
     The conveyed preceding sheet S 1  is subjected to a switchback process for buffering (S 831 ) (see  FIG. 14 ). First, the switch member  114  waits at an upper position which is a home position illustrated in  FIG. 11A  until the sheet S 1  arrives (S 832 , S 833 ). The preceding sheet S 1  is guided by a first guide face α of the switch member  114  at the upper position and conveyed in the first direction toward the lower conveying path  121  by a pair of pre-buffer conveying rollers  110  and the pair of second buffer conveying rollers  115 . At this time, a front end (downstream end in the conveying direction (the first direction)) of the sheet S 1  is sensed by a second buffer sensor  116  (S 834 , S 835 ). Then, a stop control of the pair of second buffer conveying rollers  115  is performed so that the sheet S 1  stops when a rear end (upstream end in the conveying direction (the first direction)) of the sheet S 1  arrives at a position A as illustrated in  FIG. 11B  based on information of a size of the sheet in the conveying direction and recognized in advance (S 836  to S 838 ). Here, when a clock count has reached 300 since the front end of the sheet S 1  was sensed, the rear end of the sheet S 1  arrives at the position A and the buffer conveying motor M 112  is stopped. 
     When the rear end of the sheet S 1  has arrived at the position A, a switch member motor M 114  moves the switch member  114  to a lower position illustrated in  FIG. 11B  for guiding the sheet toward a buffer path  113  (S 839  to S 841 ). Furthermore, the pair of second buffer conveying rollers  115  starts reverse conveyance in the second direction (S 842 ). As a result, the sheet S 1  is guided by the first guide face α of the switch member  114  at the lower position and the rear end portion of the sheet S 1  is guided into the buffer path  113  as illustrated in  FIG. 11C . Then, the sheet S 1  is reversely conveyed by the pair of second buffer conveying rollers  115  until the front end of the sheet S 1  arrives at the position B and the stop control is performed (S 843  to S 847 ). In this way, the sheet S 1  is made to wait in the buffer path  113 . 
     Next, the operation goes to a primary buffering superimposing process (S 848 ) (see  FIG. 15 ) to perform the superimposing process of the sheet S 1  and the sheet S 2  on each other. In other words, the operation goes to the process for sequentially causing the sheets to wait in the buffer path  113 . 
     First, as illustrated in  FIG. 12A , the switch member  114  is returned to the upper position which is the home position. At a time when arrival of the switch member  114  at the upper position is completed, the following sheet S 2  is conveyed while guided by the first guide face α of the switch member  114  at the upper position. For the reversing process of the sheet S 1 , time for reciprocating the switch member  114  from the home position (upper position) to the position (lower position) for guiding toward the buffer path  113  and then to the home position is required between arrival of the sheet S 1  and arrival of the sheet S 2 . 
     Therefore, in order to delay arrival time of the second sheet S 2  at the switch member  114  from the first sheet S 1 , the second sheet S 2  is conveyed at a lower velocity than the first sheet S 1 . To put it concretely, in the present embodiment, as illustrated in  FIG. 17 , regarding a conveyance control of the sheet S 2 , the sheet S 2  is conveyed at the lower velocity than the sheet S 1  until it arrives at the switch member  114  to increase a conveying interval tf 1  (sheet interval) between the sheet S 1  and the sheet S 2 . Therefore, the control is performed so that all of a moving-down time F of the switching member  114  to the lower position, a stop time at the lower position, and a moving-up time F to return to the upper position end during the conveying interval tf 1  between the sheet S 1  and the sheet S 2 . 
     Next, a front end of the following sheet S 2  arrives at a position C illustrated in  FIG. 12A  (S 849  to  852 ). Then, the pair of first buffer conveying rollers  112  is activated to start acceleration at a predetermined time so that the front ends of the sheet S 1  stopped in the buffer path  113  and the sheet S 2  are aligned with each other (S 853 , S 854 ). 
     Furthermore, when the rear ends of the sheet S 1  and the sheet S 2  arrive at the position A (S 855 , S 856 ), the switch member motor M 114  moves the switch member  114  to the lower position illustrated in  FIG. 12B  for guiding the sheets S 1  and S 2  toward the buffer path  113  (S 859 ). Moreover, the pair of second buffer conveying rollers  115  starts reverse conveyance (S 860 ). As a result, the sheets S 1  and S 2  are guided by the first guide face α of the switch member  114  at the lower position and the rear end portions of the sheets S 1  and S 2  are guided into the buffer path  113 . Then, the sheets S 1  and S 2  are reversely conveyed by the pair of second buffer conveying rollers  115  until their front ends arrive at the position B and then a stop control is performed (S 861  to S 865 ). As a result, the sheets S 1  and S 2  are sequentially made to wait in the buffer path  113 . 
     Next, the operation goes to a secondary buffering superimposing process (S 866 ) (see  FIG. 16 ). The switch member  114  waits for arrival of the following sheet S 3  conveyed by the pair of pre-buffer conveying rollers  110  while maintained at the lower position for guiding the preceding sheets toward the buffer path  113 . 
     At this time, the following sheet S 3  is controlled to be conveyed at an equal conveying velocity to the sheet S 1 . Therefore, a conveying interval tf 2  between the sheet S 2  and the sheet S 3  is shorter than the above-described conveying interval tf 1  between the sheet S 1  and the sheet S 2 . The secondary buffering superimposing process does not include a moving-up operation control for returning the switch member  114  to the home position. Therefore, even if the conveying interval tf 2  between the sheet S 2  and the sheet S 3  is shorter than the conveying interval tf 1  between the sheet S 1  and the sheet S 2 , it does not cause a jam in which the sheet collides with the switch member  114 . 
     Therefore, the sheet S 3  is conveyed toward the pair of second buffer conveying rollers  115  while guided by the second guide face β of the switch member  114  at the upper position. Then, the following sheet S 3  is conveyed side by side with the sheets S 1  and S 2  which have started accelerating (S 817 ) at a time when the front end of the sheet S 3  arrives at the position C and meets the sheets S 1  and S 2  at a downstream portion of the switch member  114  so that front ends of the sheets are aligned with each other (see  FIG. 13A ). 
     After the sheets S 1 , S 2 , and the sheet S 3  meet each other at the downstream portion of the switch member  114 , acceleration of the sheets S 1  and S 2  ends and the sheets S 1  and S 2  are conveyed at the equal velocity to the sheet S 3 . As illustrated in  FIG. 13B , the sheets S 1  to S 3  are nipped by the pair of second buffer conveying rollers  115  with their front ends aligned with each other and the front ends pass the second buffer sensor  116  as a sheet bundle (S 874 ) to complete the buffering process (S 875 ). The sheet bundle which has been subjected to the buffering process is discharged to the downstream upper discharge tray  136 , the processing tray  138 , or the saddle unit  135 . 
     Because the similar control to the above-described operation processes ( FIGS. 5A to 5C  and  6 A to  6 C) of the sheet stacker  100  is performed in a case of the buffering process for two sheets, description will not be repeated. In the embodiment, as described above, the three sheets are put together into the bundle in the buffering processing portion and discharged to the upper discharge tray  136 , the processing tray  138 , or the saddle unit  135 . In this way, in performing the side binding process or the saddle (saddle binding) process, it is possible to process the sheets in the sheet processing apparatus without stopping the image forming operation of the image forming apparatus main body  600  to thereby further increase productivity of the entire system. Moreover, by reducing operations of the switch member  114  in superimposing the sheets on each other as in the embodiment, it is possible to widely adapt to increase in the production number of sheets P with small sheet intervals, low-speed conveyance, and the like. 
     Other Embodiments 
     Although the four image forming portions for forming the image on the sheet are used in the embodiments described above, the number and colors of the image forming portions to be used are not limited to those in the embodiments but may be changed properly as needed. 
     The image forming apparatus may be an image forming apparatus such as a printer, a copying machine, and a facsimile or other image forming apparatuses such as a combined machine combining their functions. By applying the invention to the sheet conveying apparatus in each of the image forming apparatuses, similar effects can be obtained. 
     Although the sheet processing apparatus which is detachably attached to the image forming apparatus has been shown as an example in each of the above-described embodiments, the invention is not limited to it. For example, the sheet processing apparatus may be the sheet processing apparatus that the image forming apparatus integrally has. By applying the invention to the sheet conveying apparatus in the sheet processing apparatus, similar effects can be obtained. 
     With the invention, the processing velocity at which the sheets are conveyed in a bundle increases to thereby further increase productivity and the invention can adapt to the highly-productive apparatus. 
     Moreover, even in the case of the highly-productive apparatus, it is possible to convey thin sheets in a bundle to thereby further improve the conveying performance and the stacking performance. 
     Furthermore, it is possible to reduce the operating time of the apparatus to reduce the power consumption. 
     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 modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-276697, filed Dec. 13, 2010, and No. 2011-249455, filed Nov. 15, 2011, which are hereby incorporated by reference herein in their entirety.