Patent Publication Number: US-11377321-B2

Title: Post-processing device and image forming system provided with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of Ser. No. 15/670,394 filed on Aug. 7, 2017, which is a continuation application of Ser. No. 14/694,441 filed on Apr. 23, 2015, which claims priorities of Japanese Patent Applications No. 2014-089467 filed on Apr. 23, 2014 and No. 2014-089468 filed on Apr. 23, 2014, the disclosures of which are incorporated herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a post-processing device that accumulates sheets fed from an image forming device and performs post-processing for the accumulated sheets and to a post-processing mechanism capable of performing the post-processing without delaying an image forming process performed upstream of the post-processing device. 
     Description of the Related Art 
     In general, a system of such a type uses an image forming device provided at an upstream side to form an image on sheets and then uses a post-processing device provided at a downstream side to accumulate the image-formed sheets into a set and to perform bookbinding and other post-processing. Such a system is required, in terms of system operating efficiency, to continuously form an image by means of the upstream side image forming device and to perform the post-processing at the downstream side in accordance with a processing speed of the image forming device. 
     Thus, in order to meet the above requirement, a conveying system is adopted, in which a succeeding sheet that has been subjected to image formation in an upstream side device and fed therefrom during execution of post-processing such as bookbinding performed for preceding sheets accumulated and aligned in the post-processing device is made to temporarily stay in the middle of a conveying path and then fed to the post-processing device after completion of the post-processing operation. 
     For example, Japanese Patent No. 5,248,785 (Patent Document 1) discloses a post-processing device provided with a straight path that guides a sheet fed from an image forming device to a first processing section and a switchback path that is branched from the straight path and guides the sheet to a second processing section. This publication proposes a path configuration where a succeeding sheet fed from an upstream side while the first processing section performs a processing operation is made to stay in the switchback path for guiding a sheet to the second processing section and a succeeding sheet fed from the upstream side while the second processing section performs a processing operation is made to stay in the straight path. 
     An image forming device provided upstream of the above post-processing device is configured to form an image on a sheet at a predetermined timing irrespective of the downstream side post-processing operation and discharges the image-formed sheet from a sheet discharge port. The post-processing device receives the discharged sheet and makes the received sheet stay in the switchback path for guiding a sheet to the second processing section when a post-processing mode specifies the first processing section, while makes the received sheet stay in the straight path for guiding a sheet to the first processing section when the post-processing mode specifies the second processing section. Then, after completion of a current job in the first or second processing section, the staying sheet is fed to the first or second processing for subsequent processing. 
     PATENT DOCUMENT 
     
         
         [Patent Document 1] Japanese Patent Publication No. 5,248,785 
       
    
     As described above, it is known that, in the processing device having the first and second processing sections, a path buffer section in which the succeeding sheet is made to stay while the first processing section performs the processing operation is disposed on a path (switchback path of the above publication) for guiding a sheet to the second processing section, and a path buffer section in which the succeeding sheet is made to stay while the second processing section performs the processing operation is disposed on a path (straight path of the above Patent Document 1) for guiding a sheet to the first processing section. 
     As described above, conventionally, in the configuration in which the post-processing is selectively performed in the first and second processing sections, when an executing mode designates the first processing section, the sheet is made to temporarily stay in an buffer area on a sheet conveying path for the second processing section; while when the executing mode designates the second processing section, the sheet is made to temporarily stay in an buffer area on a sheet conveying path for the first processing section. 
     In the configuration in which the buffer areas are disposed on first and second different areas of the sheet conveying path, it is necessary to provide an area space, a sheet conveying mechanism, a processing mechanism (e.g., path open/close guide) for preventing sheet jamming for each area, resulting in increase in device size and cost. Particularly, the increase in device size goes against a demand for space saving. 
     An object of the present invention is to provide a processing device capable of arranging, while saving a space, a small-sized buffer mechanism for a sheet to temporarily stay before being conveyed to first and second different accumulating positions. 
     SUMMARY OF THE INVENTION 
     In a first aspect, a post-processing device for processing a sheet comprises a post-processing section for post-processing a sheet; a buffer section for temporarily staying a plurality of sheets at an upstream of the post-processing section; and a conveying section for conveying, one by one, the plurality of sheets stayed at the buffer section to the post-processing section. 
     In a second aspect, the post-processing device according to the first aspect further comprises a conveying path for guiding a sheet from a carry-in port to the post-processing section, wherein the buffer section is located in the conveying path. 
     In a third aspect, which is the post-processing device according to the first aspect, the conveying section includes a conveying roller pair for conveying a sheet from the buffer section to the post-processing section, and a transferring roller pair arranged upstream of the conveying roller pair. 
     In a fourth aspect, which is the post-processing device according to the first aspect, the post-processing section includes a binding unit for binding sheets. 
     In a fifth aspect, the post-processing device according to the first aspect further comprises a stopper for stopping a subsequent sheet following a sheet transferred to the post-processing section. 
     In a sixth aspect, which is the post-processing device according to the fifth aspect, a retaining section includes the stopper abutting against a front edge of the subsequent sheet to prevent movement of the subsequent sheet. 
     In the seventh aspect, which is the post-processing device according to the first aspect, the plurality of sheets is disposed on the buffer section to be off-set from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view of an entire configuration of an image forming system according to the present invention; 
         FIG. 2  is an explanatory view of a configuration of a post-processing device in the system illustrated in  FIG. 1 ; 
         FIG. 3  is an explanatory view of a configuration of a first post-processing section in the post-processing device of  FIG. 2 ; 
         FIG. 4  is an explanatory view of a configuration of a second post-processing section in the post-processing device of  FIG. 2 ; 
         FIG. 5  is an explanatory view of a sheet conveying state in the post-processing device of  FIG. 2 ; 
         FIG. 6A  is an explanatory view illustrating a state where sheets made to stay in a path buffer section are conveyed to the post-processing section, and  FIG. 6B  is an explanatory view illustrating a mechanism that conveys buffer sheets to the second post-processing section; 
         FIGS. 7A and 7B  are explanatory views each illustrating an operation state where buffer sheets are conveyed to a first post-processing section, in which  FIG. 7A  illustrates a state where the sheets are made to stand by in the buffer section and  FIG. 7B  illustrates a state where the buffer sheets are conveyed from the buffer section to first post-processing section; 
         FIGS. 8A and 8B  are explanatory views each illustrating an operation state where the buffer sheets are conveyed to the first post-processing section, in which  FIG. 8A  illustrates a state where the buffer sheets are positioned to a processing position on a processing tray and  FIG. 8B  illustrates a state where the buffer sheets are made to abut against a regulating stopper; 
         FIGS. 9A and 9B  are explanatory views each illustrating an operation state where the buffer sheets are conveyed to a second post-processing section, in which  FIG. 9A  illustrates a state where the sheets are made to stand by in the buffer section and  FIG. 9B  illustrates a state where the buffer sheets start being conveyed from the buffer section to second post-processing section; 
         FIG. 10  is an explanatory view illustrating an operation state where the buffer sheets are conveyed to the second post-processing section, in which a first sheet is being carried in the second post-processing section from the buffer section; 
         FIG. 11  is an explanatory view of a state where the buffer sheets are conveyed to the second processing section, which illustrates a state different from that in  FIG. 10  where the first sheet is carried in the second post-processing section from the buffer section; 
         FIG. 12  is an explanatory view illustrating an operation state where the buffer sheets are conveyed to the second post-processing section, in which a last sheet is being carried in the second post-processing section from the buffer section; 
         FIG. 13  is an explanatory view of a control configuration in the image forming system of  FIG. 1 ; 
         FIG. 14  is a flowchart illustrating a procedure of post-processing operation (first post-processing mode) in the image forming system of  FIG. 1 ; and 
         FIG. 15  is a flowchart illustrating a procedure of post-processing operation (second post-processing mode) in the image forming system of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail based on illustrated preferred embodiments. An image forming system illustrated in  FIG. 1  includes an image forming device A that forms an image on a sheet and a post-processing device B that performs post-processing (finishing) for the image-formed sheet. In this system, a host device (computer, etc.) that creates image data is connected upstream of the image forming device A over a network. The following describes the image forming device A and post-processing device B in this order. 
     [Image Forming Device] 
     The image forming device A is disposed upstream of the post-processing device B and is configured to form an image on a sheet and feeds the image-formed sheet to the downstream side post-processing device B through a sheet discharge port  21 . The illustrated image forming device A includes, in a device housing  10 , a sheet supply section  11 , an image forming section  13 , a sheet discharge section  19 , and a data processing section  18 . 
     The sheet supply section  11  stores sheets to be image-formed in sheet supply stackers (sheet cassettes)  12   a ,  12   b , and  12   c  and delivers the sheets of a size specified by an operator one by one to a downstream side image forming section  13 . 
     The image forming section  13  forms an image based on specified image data onto the specified size sheet fed from the sheet supply section  11 . As an image forming mechanism of the image forming section  13 , various types of image forming mechanisms, such as an electrostatic print mechanism as illustrated, an inkjet print mechanism, a transfer ribbon print mechanism, a thermal print mechanism, and an offset print mechanism can be adopted. 
     Operation in the image forming section  13  having the illustrated electrostatic print mechanism is as follows. That is, a latent image is formed on a photosensitive drum by means of an optical beam (light emitter)  15 , and toner ink is adhered to the latent image by means of a developing unit  16  to thereby form an image on a surface of the drum. The image is then transferred, by means of a charger  17 , onto a sheet fed from the sheet supply section  11 . In the sheet discharge section  19 , the sheet fed from the charger  17  is heated to fix the image thereonto. The resultant sheet is conveyed along a sheet discharge path  20  to the sheet discharge port  21 . The sheet discharge port  21  is connected with a duplex path  22 , along which the sheet that has once been carried out from the sheet discharge port  21  is fed back (switchback-conveyed) to the image forming section  13  with a conveying direction thereof reversed. During the conveyance along the duplex path  22 , front and back surfaces of the sheet are reversed. Then, an image is formed on the back surface of the sheet, and the resultant sheet is carried out through the sheet discharge port  21 . 
     A reference numeral  23  in  FIG. 1  denotes a scanner unit. The scanner unit  23  includes a platen  24  on which a document is placed, a reading carriage  25  that scans the document on the platen to read an image on the document, and an image processing section  26  that transfers image data corresponding to the read image to the data processing section  18  of the image forming device A. 
     A reference numeral  27  in  FIG. 1  denotes a feeder unit. The feeder unit  27  feeds sheets set on a sheet supply stacker  28  to the platen  24  one by one while separating them from each other to house the read document sheet in a sheet discharge stacker  29 . 
     [Configuration of Post-Processing Device] 
     The post-processing device B connected to the above-described image forming device A executes a “first post-processing mode” that accumulates image-formed sheets into a set (group) and performs binding (end binding) or a “second post-processing mode” that accumulates the image-formed sheets in a bundle and then performs folding for the sheet bundle and then houses the resultant (bound or folded) sheet bundle in a downstream side housing section. The illustrated system can further execute a “third post-processing mode (printout mode)” that houses image-formed sheets in a downstream side housing section. 
     As illustrated in  FIG. 2 , the post-processing device B includes, in a casing  30 , a first post-processing section (first accumulating section)  35  for executing the first post-processing mode and a second post-processing section (second accumulating section)  49  for executing the second post-processing mode. A sheet conveyed along a conveying path  32  having a carry-in port  31  connected to the main body sheet discharge port  21  is distributed to one of the first post-processing section  35  and the second post-processing section  49 . 
     [Configuration of Conveying Path] 
     The conveying path  32  includes a straight path  33  and a branch path  34 . The branch path  34  is branched from the straight path  33  at a path branch portion  32   a  (hereinafter, referred to as “branch portion”), and the sheet carried in from the carry-in port  31  is delivered in one of a straight path direction and a branch path direction. To realize this, a path switching unit  44  is disposed at the branch portion  32   a . The path switching unit  44  guides a sheet to sheet discharge ports of the respective paths based on a signal from a controller  76  to be described later. 
     The illustrated straight path  33  is constituted by a substantially straight path. The straight path  33  has the carry-in port  31  at one side surface (right side wall) of the casing  30  and a path sheet discharge port  33   a  at the other side surface (left side wall) of the casing  30 . That is, as illustrated in  FIG. 2 , the straight path  33  is formed so as to horizontally cross the casing  30  in a substantially straight line (or in a curved line), and the first post-processing section  35  is disposed downstream of the path sheet discharge port  33   a  (hereinafter, referred to as “first sheet discharge port”). 
     The branch path  34  extends in a direction crossing the straight path  33 . That is, in the casing  30 , the straight path  33  extends substantially horizontally in the casing  30 , while the branch path  34  extends substantially vertically. As illustrated in  FIG. 2 , the branch path  34  is branched from the horizontally-extending straight path  33  so as to guide a sheet in the vertical direction toward the second post-processing section  49  from a path sheet discharge port  34   a  (hereinafter, referred to as “second sheet discharge port”). 
     The path switching unit  44  is disposed at the branch portion  32   a . The path switching unit  44  guides a sheet fed from the carry-in port  31  to the sheet discharge port (first sheet discharge port)  33   a  of the straight path  33  or sheet discharge port (second sheet discharge port)  34   a  of the branch path  34 . To realize this, the path switching unit  44  includes a movable guide member (flapper member) and a not illustrated shift unit (operating solenoid, motor, etc.) that shifts an angular position of the movable guide member. 
     Conveying units for conveying a sheet are disposed in the straight path  33  and branch path  34 . In the example illustrated in  FIG. 3 , a carry-in roller  45  and a sheet discharge roller  46  (first sheet discharge roller) are arranged in a sheet discharge direction in the straight path  33 . Specifically, the carry-in roller  45  is disposed at the carry-in port  31 , the sheet discharge roller  46  is disposed at the sheet discharge port  33   a , and a conveying roller (intermediate roller)  43  is disposed between the rollers  45  and  46 . On the other hand, in the branch path  34 , a sheet discharge roller  47  (second sheet discharge roller) and a third sheet discharge roller are arranged. Specifically, the sheet discharge roller  47  is disposed at the branch path port (branch portion)  32   a , and the third sheet discharge roller is disposed downstream of the roller  47 . 
     The above-described rollers  45 ,  46 ,  47 , and  48  (carry-in roller and first, second, third sheet discharge rollers) receive, from a not illustrated drive motor, conveying force for conveying a sheet in the conveying direction. Specifically, the carry-in roller  45  and intermediate roller  43  receive conveying force for conveying a sheet in the sheet discharge direction. The first sheet discharge roller  46  receives conveying force (normal rotation) for carrying out a sheet toward the first sheet discharge port and conveying force (reverse rotation) for conveying a sheet toward the branch path. 
     The second sheet discharge roller  47  receives conveying force for conveying a sheet in both normal/reverse rotation directions of the drive motor. The third sheet discharge roller  48  receives conveying force for carrying out a sheet from the second sheet discharge port  34   a . In the illustrated device, the conveying force is transmitted from a single drive motor to the carry-in roller  45  and first to third sheet discharge rollers  46  to  48  through a clutch mechanism. 
     A switchback path  60  (hereinafter, referred to as “first switchback path”) is formed between the first sheet discharge port  33   a  and first post-processing section  35 . The first switchback path  60  moves backward a sheet carried out by means of the first sheet discharge roller  46  and whose conveying direction is reversed to a predetermined post-processing position. On the other hand, a switchback path  61  (hereinafter, referred to as “second switchback path”) is formed in the branch path  34 . The second switchback path  61  guides a sheet whose conveying direction is reversed when a rear end of the sheet passes through the branch portion  32   a  to the second sheet discharge port  34   a . The switchback paths  60  and  61  are each constituted by a path (straight path  33 , branch path  34 ) for guiding a sheet and normal/reverse rotation control of the conveying roller (first sheet discharge roller  46 , second sheet discharge roller  47 ). 
     Thus, a sheet carried in from the carry-in port  31  and conveyed along the straight path  33  enters the first post-processing section  35  (processing tray  36  to be described later) in a direction opposite to a direction in which the sheet is carried in, i.e., from the rear end thereof. Further, a sheet conveyed along the branch path  34  enters the second post-processing section  49  (accumulation guide  49   a  to be described later) in the direction opposite to the carry-in direction, i.e., from the rear end thereof. The first switchback path  60  and the second switchback path  61  are disposed spaced apart from each other at respective downstream and upstream sides in the sheet discharge direction of the straight path  33  (see  FIG. 2 ). 
     Sheet sensors Se 1  and Se 2  are disposed in the straight path  33 . The sheet sensor Se 1  detects a sheet passing through the carry-in port  31 . The sheet sensor Se 2  detects a sheet passing through the first sheet discharge port  33   a . Detection signals of the respective sensors Se 1  and Se 2  are used as a reference signal for sheet conveying control, such as control of a guide direction of the path switching unit  44 , control of start/stop of the conveying roller, and determination control of sheet jamming. 
     In the illustrated device, the first sheet discharge roller  46  and the second sheet discharge roller  47  disposed opposite to each other with respect to the branch portion  32   a  are each composed of a pair of rollers that put a sheet therebetween into a pressurization state where the sheet is pressurized at a predetermined pressure when contacting each other and put the sheet into a pressurization release state when separating from each other or while keeping the contacting state. Although a configuration of the roller pair is not illustrated especially, one of the pair of rollers that are brought into press-contact with each other is axially supported so as to be movable in the press-contact direction. The pair of rollers pressurizes each other in the press-contact direction by means of a biasing spring, and the pressurizing force is released or reduced by means of a release lever connected to an actuator such as an operating solenoid. The roller pair is thus put into the pressurization state to pressurize each other when the sheet is conveyed by this roller pair or into the pressurization release state to reduce the pressurizing force therebetween when the sheet is conveyed by a different adjacent roller pair so as not to impede the sheet conveyance. 
     The roller pair is shifted between the pressurization state and pressurization release state by the release lever  67  and an actuator such as an operating solenoid  68 . That is, the roller pair is rotated in a press-contact state when a plurality of sheets are conveyed in an overlapped state in a predetermined direction, while the roller pair is separated from the sheet (or press-contact force between the roller pairs is reduced) when the sheet is conveyed by a different adjacent roller pair so as not to impede the sheet conveyance. 
     [First Post-Processing Section] 
     The first post-processing section  35  disposed downstream of the first sheet discharge port  33   a  will be described using  FIG. 3 . A processing tray  36  having a sheet placement surface  36   a  on which a sheet is placed is disposed downstream of the first sheet discharge port  33   a  so as to form a step from the straight path  33 . A conveying rotating body  62  (paddle rotating body, belt rotating body, etc.) that conveys a sheet toward a predetermined post-processing position is disposed above the processing tray  36 . 
     The sheet placement surface  36   a  has a sheet end regulating unit  38  that stops a sheet at a predetermined position (post-processing position), an aligning unit  39  that positions a width direction of sheets stacked on the sheet placement surface to a reference position, and a post-processing unit (stapler)  37  that performs binding for sheets. 
     As illustrated in  FIG. 3 , the processing tray  36  has a sheet carry-in unit  42  that guides a sheet from the first sheet discharge port  33   a  to the sheet placement surface  36   a  and a sheet carry-out unit  40  that carries out a sheet (bundle) that has been subjected to the post-processing from the sheet placement surface to a first housing stacker  41  on downstream. 
     The sheet carry-in unit  42  is disposed between the first sheet discharge port  33   a  and the processing tray  36  and configured to reverse the conveying direction of the sheet (to switchback the sheet) carried out from the sheet discharge port and feed the sheet toward the sheet placement surface  36   a . The sheet carry-in unit  42  is constituted by a lifting roller, a paddle rotating body, a belt rotating body, or the like. In the illustrated example, the sheet carry-in unit  42  is constituted by a lifting roller configured to be vertically movable between an operating position at which it is engaged with a sheet carried out from the first sheet discharge port  33   a  and a standby position retreated from the operating position. The sheet carry-in unit  42 , i.e., the lifting roller is connected to a not-illustrated lifting motor and a not-illustrated normally and reversely rotatable motor such that it is rotated in a normal direction until a sheet rear end passes through the first sheet discharge port  33   a  and thereafter rotated in a reverse direction. 
     The sheet carry-out unit  40  is constituted by a conveyor mechanism that is reciprocated along the sheet placement surface  36   a  between a processing position and the downstream side housing stacker  41 . The conveyor mechanism includes an engagement member  40   a  engaged with an end edge of a sheet bundle on the sheet placement surface and a belt (not illustrated) that reciprocally moves the engagement member along the sheet placement surface. Further, the sheet placement surface  36   a  has a roller (fixed roller)  63  opposite to the sheet carry-in unit  42  (lifting roller). Sheets are conveyed in the sheet discharge direction by the roller  63  and lifting roller  42  while being nipped between the rollers  63  and  42 . 
     [Second Post-Processing Section] 
     The second post-processing section  49  disposed downstream of the second sheet discharge port  34   a  will be described using  FIG. 4 . There are disposed, in the second post-processing section  49 , an accumulation guide  49   a  that accumulates and aligns sheets fed from the branch path  34 , a binding unit  52  that binds a bundle of the accumulated sheets, and folding units  53  and  54  that center-fold a sheet bundle. 
     The accumulation guide  49   a  is constituted by a guide member having a stacking surface  49   b  on which sheets fed from the second sheet discharge port  34   a  are stacked in a vertically standing state (standing posture). The accumulation guide  49   a  has a leading end regulating unit  50 . The leading end regulating unit  50  is disposed so as to be movable along the accumulation surface and configured to stop an end portion (leading end portion) of a sheet for position regulation. The accumulation guide  49   a  further has an aligning unit  51  that positions a width direction of stacked sheets to a reference position. 
     The binding unit  52  is constituted by a saddle stitching staple unit that saddle-stitches a sheet bundle on the stacking surface  49   b . A configuration of this staple unit is widely known, so description thereof will be omitted. The folding unit is constituted by a folding roll and a folding blade  54 . The folding roll  53  is constituted by a pair of rolls that are brought into press-contact and is connected to a drive motor (not illustrated) that is rotated in a clockwise direction (folded sheet discharge direction) in  FIG. 4 . 
     The folding blade  54  is constituted by a plate-like member that pushes a folding position of a sheet bundle into between the rolls and is connected to a shift motor (not illustrated) so as to be reciprocally moved between a standby position separated from the roll pair and an operating position between the rolls. The aligning unit  51  is constituted by a pair of left and right aligning plates that can move a sheet supported on the stacking surface in the width direction and has a not illustrated aligning motor. 
     A sheet discharge path  65  and a sheet discharge roller (fourth sheet discharge roller) that carry out a sheet bundle that has been subjected to folding are disposed downstream of the folding roll  53 , and a second housing stacker  55  is disposed downstream of the sheet discharge path  65  and the sheet discharge roller  66 . The sheet discharge path  65  extends in a direction substantially perpendicular to the stacking surface  49   b  of the second post-processing section  49  and carries out the sheet bundle in the same direction as that in which the straight path  33  carries out the sheet. The second housing stacker  55  is disposed below the first housing stacker  41 . 
     [Buffer Section] 
     A sheet carried in from the carry-in port  31  and conveyed along the straight path  33  is guided to the first post-processing section  35  through the first sheet discharge port  33   a  or guided to the second post-processing section  49  through the branch path  34 . A bundle of sheets accumulated into a set (group) in the first post-processing section  35  is subjected to the binding and then housed in the first housing stacker  41  (first post-processing mode). On the other hand, a bundle of sheets accumulated into a set in the second post-processing section  49  is subjected to the binding and the folding and then housed in the second housing stacker  55  (second post-processing mode). 
     Thus, sheet conveying control is required, in which a sheet carried in from the carry-in port  31  is made to stay in the middle of the sheet conveying path until completion of post-processing operation of the first post-processing section  35  or the second post-processing section  49 . This control is required for the upstream side image forming device A to perform (during execution of post-processing operation) a print process of forming an image on a sheet without interruption. 
     To this end, a path buffer section Pb (hereinafter, referred to as “buffer section”) is provided in the conveying path (straight path  33  and branch path  34 ), and a sheet fed from the upstream side during execution of the post-processing operation is temporarily stored in the buffer section Pb. One or a plurality of sheets can be made to stay depending on the length of time of the post-processing. As a process speed in the upstream side image forming device A becomes high, the number of sheets to be made to stay in the buffer section Pb of the post-processing device B becomes large. 
     A configuration of the buffer section Pb in the illustrated device will be described. As illustrated in  FIG. 3 , the buffer section Pb is disposed astride over the straight path  33  positioned downstream of the branch portion  32   a  and the branch path  34 . Specifically, the buffer section Pb is disposed between the first sheet discharge roller  46  (first conveying roller) of the straight path  33  and the second sheet discharge roller  47  (second conveying roller) of the branch path  34 , and a sheet is bridged between both the rollers  46  and  47  (bridge support structure). That is, a distance between the first sheet discharge roller  46  and the second sheet discharge roller  47  is set to a value smaller than a conveying direction length of a minimum-sized sheet to be post-processed. 
     The first and second sheet discharge rollers  46  and  47  are connected to a drive motor (not illustrated) so as to be normally/reversely rotatable. A controller  76  to be described later reverses a rotation direction of the first sheet discharge roller  46  from the normal rotation to reverse rotation to thereby guide a sheet on the straight path  33  to the branch path  34  and then fed to the buffer section Pb. At subsequent sheet supply timing, when the first and second sheet discharge rollers  46  and  47  are rotated in a clockwise direction (normal direction), the sheet staying in the buffer section Pb to the first post-processing section  35 ; while when the first and second sheet discharge rollers  46  and  47  are rotated in a counterclockwise direction (reverse direction), the sheet staying in the buffer section Pb to the second post-processing section  49 . 
     In the present invention, the first and second conveying rollers for making a sheet temporarily stay are constituted by the “roller pair  46  disposed at the sheet discharge port (first sheet discharge port) of the straight path  33 ” and “roller pair  47  disposed at an entrance of the branch path  34 ”; however, positions of the first and second conveying rollers are not limited to these positions, but the first and second conveying rollers may be located at any positions in the respective paths as long as they are front and rear roller pairs disposed at an interval smaller than the conveying direction length of the sheet. Further, although the first and second conveying rollers are each preferably a normally reversely rotatable roller pair, a plurality of roller pairs that can convey the sheet in both sheet discharge direction and counter-sheet discharge direction may be provided as rollers for making a sheet temporarily stay. 
     The following describes control of the conveying rollers (sheet discharge rollers) when a sheet is made to stay in the buffer section Pb. The conveying roller  45  and first sheet discharge roller  46  (first conveying roller) are rotated in the sheet discharge direction (normal direction; clockwise direction in  FIG. 3 ) to convey a sheet fed to the carry-in port  31  along the straight path  33 . Then, upon passage of a rear end of the sheet through the branch portion  32   a , the first sheet discharge roller  46  is rotated in the counter-sheet discharge direction (reverse direction; counterclockwise direction in  FIG. 3 ). At the same time, the path switching unit  44  is set in a posture (state indicated by a dashed line in  FIG. 3 ) that guides the sheet to the second sheet discharge port side, and the second sheet discharge roller  47  (second conveying roller) is rotated in the counterclockwise direction in  FIG. 3 , to thereby allows the sheet to enter the branch path  34  from its rear end portion side. 
     Drive of the path switching unit  44  and conveying rollers (sheet discharge rollers)  46  and  47  is controlled based on detection signals from the respective sheet sensors Se 1  and Se 2 . A controller  75  to be described later rotates the first sheet discharge roller  46  and the second sheet discharge roller  47  by a rotation amount previously set in accordance with a sheet size and stops the rollers  46  and  47 . Then, the sheet conveyed from the carry-in port  31  is fed to the buffer section Pb of the branch path  34  and temporarily stays therein. 
     [Control Configuration] 
     The following describes a control configuration in the image forming system of  FIG. 1 .  FIG. 12  is a block diagram of the control configuration. As illustrated in  FIG. 12 , the control configuration in the image forming system includes an image forming control section  70  and a post-processing control section  75 . The image forming control section  70  includes a sheet feeding control section  72  and a mode setting unit  71 . The mode setting unit  71  includes an input section  73  such as a control panel. The image forming control section  70  forms an image on a sheet under an image forming condition set in the mode setting unit  71 . The sheet feeding control section  72  controls sheet feeding operation of feeding a sheet of a size set in the mode setting unit  71  from the sheet supply section  11  to image forming section  13 . The mode setting unit  71  sets image forming conditions such as color/monochrome setting, enlarge/reduction ratio setting, and cover print setting. In addition, the mode setting unit  71  sets a mode of the post-processing performed for an image-formed sheet. 
     The post-processing device B can execute the first post-processing mode (end binding mode), second post-processing mode (bookbinding mode), and third post-processing mode (printout mode). In the first post-processing mode, image-formed sheets are accumulated and aligned on the processing tray  36  (first post-processing section), subjected to binding, and housed in the first housing stacker  41 . In the second post-processing mode, image-formed sheets are accumulated and aligned on the accumulation guide  49   a  (second post-processing section), subjected to saddle stitching and folding, and housed in the second housing stacker  55 . In the third post-processing mode, a sheet fed to the carry-in port  31  is not subjected to the post-processing but directly housed in the first housing stacker  41 . The “end binding” refers to finishing that binds the accumulated and aligned sheet bundle at one or a plurality of positions along an end surface thereof, “saddle stitching” refers to finishing that binds the accumulated and aligned sheet bundle at a plurality positions around a center portion thereof. 
     The post-processing control section  75  includes a control CPU  76  that operates the post-processing device B according to a specified post-processing mode (finishing), a ROM  77  that stores an operation program, and a RAM  78  that stores control data. The control CPU  76  includes a conveying control section  76   a  that controls conveyance of a sheet fed to the carry-in port  31 , an accumulation operation control section  76   b  that controls sheet accumulation operation, a binding operation control section  76   c  that controls sheet bundle binding operation, and a folding operation control section  76   d  that controls sheet folding operation. 
     The above control sections  76   a  to  76   d  select and execute the operation mode (first post-processing mode) in which the post-processing is performed in the first post-processing section  35  or operation mode (second post-processing mode) in which the post-processing is performed in the second post-processing section  49 . 
     The conveying control section  76   a  is connected to a control circuit (driver) of a drive motor (not illustrated) driving the carry-in roller  45  (intermediate roller  43 ) and first sheet discharge roller  46  of the conveying path  32  and receives detection signals from the respective sheet sensors Se 1  and Se 2  disposed along the conveying path  32 . Further, the conveying control section  76   a  is connected to a control circuit (driver) of a drive motor (not illustrated) driving the second and third sheet discharge rollers  47  and  48  and receives a detection signal from a sheet sensor Se 3  disposed in the branch path  34 . 
     The conveying control section  76   a  executes control to make a sheet fed to the carry-in port  31  stay in the buffer section Pb during execution of the post-processing operation (e.g., folding operation) in the first post-processing section  35  or the second post-processing section  49 . To this end, the conveying control section  76   a  incorporates a buffer sheet number calculating unit  79  that calculates the number of sheets to be made to stay during execution of the post-processing operation and a buffer sheet conveying amount setting unit  80  that conveys a buffer sheet (sheet to be made to stay) at the carry-in port  31  to the buffer section Pb. Details of the buffer sheet number calculating unit  79  and buffer sheet conveying amount setting unit  80  will be described later. 
     The conveying rollers in each of the straight path  33  and the branch path  34  may be driven by a single drive motor or a plurality of independent drive motors. When the conveying rollers in each of the paths  33  and  34  may be driven by a single drive motor, drive of the motor is transmitted to the conveying rollers through a clutch unit. The accumulation operation control section  76   b  transmits a control signal to drive circuits of the normally reversely rotatable motor for the sheet carry-in unit  42  (lifting roller) and sheet discharge motor for the sheet carry-out unit  40  in order to accumulate sheets in the first accumulating section  35 . Further, the binding operation control section  76   c  transmits a control signal to drive circuits of drive motors (not illustrated) incorporated respectively in the end binding stapler  37  of the first post-processing section  35  and the saddle stitching stapler  52  of the second accumulating section  49 . 
     The folding operation control section  76   d  is connected to a drive circuit of a roll drive motor that drives the folding roll pair  53 . Further, the folding operation control section  76   d  transmits a control signal to a control circuit of the second and third sheet discharge rollers  47  and  48  of the branch path  34  and a control circuit of the shift unit that controls movement of the leading end regulating unit  50  of the accumulation guide  49   a  to a predetermined position and receives a detection signal from sheet sensors (not illustrated) disposed in the respective conveying paths. 
     [Buffer Sheet Number Calculating Unit] 
     The following describes the buffer sheet number calculating unit  79 . The calculating unit  79  is incorporated in the conveying control section  76   a . The calculating unit  79  calculates the number of sheets to be made to stay in the path from a conveying direction length (size information) of a sheet transmitted from the image forming device A and a post-processing time (e.g., sheet alignment time+binding time+folding time+processed sheet carry-out time) previously set and stored as data in the RAM  78 . Assuming that the post-processing time is Td, image forming process time is Tp, and buffer sheet number is Bs, [Bs=Td/Tp] . . . (expression 1) is satisfied. Thus, the number of sheets to be made to stay in the buffer section Pb during execution of the post-processing operation in the first post-processing section  35  or the second post-processing section  49  is set. 
     [Buffer Sheet Conveying Amount Setting Unit] 
     When the buffer sheets whose number has been calculated by the buffer sheet number calculating unit  79  are conveyed to the buffer section Pb and temporarily stopped (made to stay), the conveying control section  76   a  controls the standby state of the buffer sheets such that the buffer sheets are overlapped while being offset front and rear in the path direction. This is in order to quickly and reliably (without jamming) convey the buffer sheets to the first post-processing section  35  or the second post-processing section  49  after completion of the post-processing operation. 
     In the illustrated device, a first buffer sheet n is conveyed to the buffer section Pb, and a second buffer sheet is offset (displaced) upstream by a previously set offset amount δ in the conveying direction. To realize this, the conveying control section  76   a  sets (varies), in accordance with the number of the buffer sheets, a conveying amount of the first and second sheet discharge rollers  46  and  47  based on a detection signal of the sheet discharge sensor Se 2  that detects the leading end of the sheet. 
     For example, assuming that the conveying amount of the first buffer sheet is (Ln), the conveying amount of the second buffer sheet is set to (Ln−δ), and conveying amount of the third buffer sheet is set to (Ln−2×δ), and thereafter similarly, the conveying amount is gradually reduced. The reason that a plurality of sheets is displaced (offset) from each other in the conveying direction will be described later. 
     [Post-Processing Operation] 
     The thus configured post-processing control section  75  controls the post-processing device B to execute the following processing operations. As described above, the post-processing control section  75  can make the post-processing device B execute the first post-processing mode (end binding in the first post-processing section  35 ), second post-processing mode (bookbinding in the second post-processing  49 ), and third post-processing mode (printed sheet housing processing in the first post-processing section  35 ). Sheets fed from the image forming device A according to a mode set in the mode setting unit  71  are accumulated in the first post-processing section  35  or the second post-processing section  49 , subjected to the post-processing, and housed in the downstream side stacker or  55 . The following describes “conveying order of sheets to be conveyed to post-processing section”, “conveying operation of buffer sheets”, “first post-processing operation”, and “second post-processing operation” in this order. 
     [Conveying Order of Sheets to be Conveyed to Post-Processing Section] 
     As illustrated in  FIG. 5 , sheets fed from the carry-in port  31  are conveyed to the first post-processing section along the first switchback path  60  or to the second post-processing section  49  along the second switchback path  61 . The sheets are fed to the carry-in port  31  in the image forming order of (n), (n+1), (n+2). The controller (control CPU)  76  conveys the sheets to the first post-processing section  35  through the straight path  33  or to the second post-processing section  49  through the branch path  34 . During execution of the post-processing operation in the first post-processing section  35  or the second post-processing section  49 , the sheets are made to temporarily stay in the buffer section Pb. 
     In the first post-processing section  35 , the sheets fed from the carry-in port  31  are stacked on the sheet placement surface  36   a  of the processing tray  36  in the order of (n), (n+1), (n+2) from below. Similarly, the sheets to be made to stay in the buffer section Pb are stacked on the path in the order of (n), (n+1), (n+2) from below. Further, similarly, in the second post-processing section  49 , the sheets are stacked in the order of (n), (n+1), (n+2) from below. In this state, the sheets in the first post-processing section  35  and those in the buffer section Pb have the same vertical posture, while the vertical posture of the sheets in the second post-processing section  49  differs from that of the sheets in the first post-processing section  35  and the buffer section Pb. 
     [Conveying Operation of Buffer Sheets to Post-Processing Section] 
     The plurality of sheets made to stay in the buffer section Pb are collectively fed in a bundle for the first post-processing section  35  while fed one by one in the order that they are fed to the carry-in port  31  for the second post-processing section  49 . The details will be described below with reference to  FIG. 5 . The sheets are fed to the carry-in port  31  in the image forming order of (n), (n+1), (n+2). These sheets are stacked in the buffer section Pb in the same order of (n), (n+1), and (n+2). 
     Upon completion of the post-processing operation of the post-processing section  35  (or  49 ), the controller  76  rotates the first and second sheet discharge (conveying) rollers  46  and  47  disposed along the buffer section Pb in the sheet conveying direction. At this time, the controller collectively conveys the buffer sheets stacked in a bundle for the first post-processing section  35 ; while conveys the buffer sheets stacked in a bundle one by one for the second post-processing section  49 . 
     [First Post-Processing Mode] 
     The following describes an operation procedure of the first post-processing mode (end binding mode) according to a flowchart of  FIG. 14 . Upon power-on of a device power supply, the controller  76  executes initializing operation (descriptions of which will be omitted) (St 01 ). The path switching unit  44  is positioned so as to allow a sheet to be conveyed in the sheet discharge direction along the straight path  33 . The leading and rear ends of the sheet are detected by means of the sensors Se 1  and Se 2 , respectively, and a timer is activated based on a detection signal from the sensor. Which one of the detection signals from the sensors Se 1  and Se 2  is used to determine (monitor) the sheet conveying state may be appropriately set. 
     At a timing at which the sheet leading end is carried out from the first sheet discharge port  33   a , the lifting roller  42  (sheet carry-in unit) is moved down from the standby position to operating position at which it is engaged with the sheet. At the same time, the lifting roller  42  is rotated in the sheet discharge direction to convey the sheet in the sheet discharge direction (St 03 ). At a timing at which the sheet rear end is carried out from the first sheet discharge port  33   a , the lifting roller  42  is reversely rotated (i.e., rotated in the counter-sheet discharge direction) (St 04 ). Then, the conveying direction of the sheet is reversed (i.e., the sheet is switched back). Accordingly, the sheet is carried into the processing tray disposed downstream of the first sheet discharge port  33   a  from the rear end side thereof and stopped by abutting against the sheet end regulating unit  38  on the tray (St 05 ). Then, the controller  76  positions the width direction of the sheet carried in the first post-processing section  35  to a reference position. 
     The controller  76  repeats the operation from step St 02  to step St 05  until it receives a job end signal from the image forming device A. With the above procedure, sheets on which an image has been formed in the image forming device A are accumulated and aligned in the first post-processing section  35 . Upon reception of the job end signal from the image forming device A, the controller  76  controls the post-processing unit  37  (stapler) of the first post-processing section  35  to execute the post-processing operation (St 06 ). Subsequently, upon reception of an operation completion signal from the post-processing unit  37 , the controller  76  carries out the processed sheet bundle toward the downstream side first housing stacker  41  (St 07 ). 
     Simultaneously with execution of the above post-processing operation, the controller  76  acquires, from the image forming device A, setting information (in this case, first post-processing mode) of the post-processing mode, a process speed of the image formation, and sheet size information and calls post-processing operation time data in the first post-processing section  35  stored in the RAM of the post-processing device B. Based on the above information, the controller  76  calculates the number of sheets to be made to stay in the path by means of the buffer sheet number calculating unit  79  and calculates a conveying amount of the buffer sheets by means of the buffer sheet conveying amount setting unit  80  (St 08 ). 
     When a succeeding sheet reaches the carry-in port  31 , the controller  76  detects the succeeding sheet by means of the carry-in sensor Se 1  and acquires, from a timer&#39;s time in the RAM  78 , an estimated time for the sheet rear end to reach the branch portion  32   a . After that, the controller  76  activates a timer in response to the detection signal from the carry-in sensor Se 1 . 
     Then, after timer-up, i.e., after elapse of the estimated time for the sheet rear end to pass through the branch portion  32   a , the controller  76  changes the direction of the path switching unit  44  as indicated by the dashed line in  FIG. 3  and, at the same time, rotates the first sheet discharge roller  46  and the second sheet discharge roller  47  in the counter-sheet discharge direction (St 09 ). Then, the sheet enters the branch path  34  from the rear end side, and the controller  76  rotates the first and second sheet discharge rollers  46  and  47  by an amount set by the conveying amount setting unit  80  and then stops the rotation (St 10 ). 
     Then, when the succeeding (second) sheet (n+1) reaches the carry-in port  31 , the controller  76  carries the sheet (n+1) in the buffer section Pb in the same manner as above. A conveying amount of the sheet (n+1) is set to a value smaller than that of the preceding sheet (n) by the offset amount δ. Further, the succeeding (third) sheet (n+2) is carried in the buffer section Pb in the same manner with the conveying amount set to a value smaller than that of the preceding (second) sheet (n+1) by the offset amount δ (St 11 ). 
     As described above, the sheets fed from the image forming device A during execution of the post-processing operation in the first post-processing section  35  are temporarily retained in a state of being bridged over the branch portion  32   a  between the straight path  33  and the branch path  34  (execution of St 11 ). Then, the controller  76  receives a sheet carry-in permission signal from the first post-processing section  35  (St 12 ). In response to the permission signal, the controller  76  rotates the first and second sheet discharge rollers  46  and  47  in the sheet discharge direction (clockwise direction in  FIG. 3 ). Then, the buffer sheets are conveyed from the buffer section Pb to the first sheet discharge port  33   a  in an overlapped state (St 13 ). The controller  76  rotates the lifting roller  42  and conveying rotating body  62 , with the result that the sheet bundle abuts against the sheet end regulating unit  38  of the first post-processing section  35  of the post-processing device B and stopped. 
     [Second Post-Processing Mode] 
     The second post-processing mode (bookbinding mode) will be described using  FIG. 15 . Upon power-on of a device power supply, the controller  76  executes initializing operation (St 01 ). When the sheet is fed to the carry-in port  31 , the controller  76  detects the leading end thereof and activates a timer (St 02 ). Then, after timer-up, i.e., after elapse of the estimated time for the sheet rear end to pass through the branch portion  32   a , the controller  76  changes the direction of the path switching unit  44  and rotates the first and second sheet discharge rollers  46  and in the counter-sheet discharge direction to thereby guide the sheet from the straight path  33  to the branch path  34 . The sheet is conveyed from the second sheet discharge port  34   a  to the second post-processing section  49  and accumulated on the accumulation guide  49   a  (St 20 ). Upon reception of the job end signal from the image forming device A, the controller  76  moves the sheets accumulated on the accumulation guide  49   a  to a binding position and performs binding by means of the binding unit (St 21 ). 
     Subsequently, the controller  76  moves the sheet bundle to a folding position, rotates the folding roll pair  53  in a folding direction, and moves the folding blade  54  from the standby position to operating position. Then, the sheet bundle is folded at its center (St 22 ). After execution of the folding operation, the folded sheet bundle is fed to and housed in the second housing stacker  55  (St 23 ). 
     Simultaneously with execution of the above post-processing operation, the controller  76  calculates the number of sheets to be made to stay by means of the buffer sheet number calculating unit  79  and calculates the conveying amount of the buffer sheets by means of the buffer sheet conveying amount setting unit  80  same as in the first post-processing mode (St 24 ). 
     When a succeeding sheet reaches the carry-in port  31 , the controller  76  conveys the succeeding sheet to the buffer section Pb and makes it stand by therein as in the first post-processing mode (St 25 ). Also in this case, as in the first post-processing mode, the conveying amount is reduced by the offset amount δ and, thereby, a plurality of sheets is made to stay in an overlapped manner in such a manner that they are displaced from each other. 
     As described above, the sheets fed from the image forming device A during execution of the post-processing operation (binding operation and folding operation) in the second post-processing section  49  are temporarily retained in a state of being bridged over the branch portion  32   a  between the straight path  33  and the branch path  34 . Then, the controller  76  receives a sheet carry-in permission signal from the second post-processing section  49  (St 26 ). In response to the permission signal, the controller  76  rotates the third sheet discharge roller  48  in the sheet discharge direction (counterclockwise direction in  FIG. 3 ). Then, the buffer sheet n that has first been made to stay is conveyed from the second sheet discharge port  34   a  to the accumulation guide  49   a.    
     At this time, a stopper  56  is positioned so as not to allow the succeeding overlapped buffer sheets (n+1, n+2) to be drawn downstream (in a direction toward the accumulation guide  49   a ). The controller  76  conveys sequentially the plurality of sheets from the buffer section Pb to the accumulation guide  49   a  in the order that they are made to stay in the buffer section Pb. A sheet conveying speed (peripheral speed of the third sheet discharge roller  48 ) at this time is set higher than a process speed of the image forming device A. The conveying operation of the buffer sheets to the second post-processing section  49  will be described later. 
     As described above, the sheets fed from the image forming device A during execution of the post-processing operation in the second post-processing section  49  are conveyed to the buffer section Pb disposed over the branch portion  32   a  between the straight path  33  and the branch path  34  and temporarily retained in a state of being bridged between the first and second sheet discharge rollers  46  and  47  arranged at a predetermined interval (execution of St 25 ). Then, when a state is reached in which the sheets can be carried in the second post-processing section  49 , the controller  76  conveys one by one the buffer sheets (n, n+1, n+2) from the second sheet discharge port  34   a  to the accumulation guide  49   a  in the order that they are fed to the carry-in port  31 . After all the buffer sheets are carried in on the accumulation guide, a succeeding sheet is stacked on the accumulated buffer sheets. 
     A sheet fed to the carry-in port  31  during operation of conveying the buffer sheets to the accumulation guide  49   a  is made to stay in the buffer section Pb. This operation flow is repeated until all the sheets are conveyed to the second processing section  49  from the buffer section Pb. To this end, it is necessary to set a speed Vb (in the illustrated example, peripheral speed of the third sheet discharge roller  48 ) of conveying the buffer sheet to the second post-processing section  49  higher than a speed Va (process speed) of forming an image in the image forming device A (Vb&gt;Va). When the speed difference reaches the conveying amount of the buffer sheet, conveyance of all the buffer sheets to the downstream side accumulating section  49  is completed. 
     [Operation State of Buffer Sheet Conveyance] 
     &lt;First Post-Processing Mode&gt; 
     The following describes a buffer sheet conveying state in the first post-processing mode with reference to  FIGS. 7A and 7B .  FIG. 7A  illustrates a state where a plurality of sheets are conveyed to the buffer section Pb and made to temporarily stay. The sheets fed to the buffer section Pb from the carry-in port  31  are retained by the first and second sheet discharge rollers  46  and  47  in a state of being overlapped in the order of the first sheet (n), second sheet (n+1), and third sheet (n+2) from below. More specifically, the plurality of sheets is overlapped in a scale shape with the offset amount δ in the feeding order (in the order that they are fed to the carry-in port  31 ). 
     When carrying the buffer sheets in the first post-processing section  35 , the controller  76  rotates the first and second sheet discharge rollers  46  and  47  in a stopped state in a direction (clockwise direction in  FIG. 7A ) guiding the buffer sheets to the first sheet discharge port  33   a  as illustrated in  FIG. 7A . Then, the plurality of overlapped sheets are fed from the first sheet discharge port  33   a  to the first accumulating section (processing tray)  35  and made to abut against the sheet end regulating unit  38  illustrated in  FIG. 7B  by the normal/reverse rotation of the lifting roller  42 . 
     At this time, conveying forces (f 1 , f 2 , and f 3 ) are applied from the conveying rotating body  62  to the plurality of sheets, respectively. As illustrated in  FIG. 8B , the conveying force f 3  acting on the topmost sheet (n+2) engaged with the conveying rotating body  62  is largest, followed by the conveying force f 2  acting on the sheet (n+1) under the sheet (n+2) and conveying force f 1  acting on the sheet (n) under the sheet (n+1). In this state, the sheets are displaced from each other by the offset amount δ in an approaching/separating direction toward the sheet end regulating unit  38 . 
     Thus, among the sheets displaced from each other by the offset amount δ, the lowermost sheet is closest to the stopper (sheet end regulating unit  38 ), and the topmost sheet is farthest to the stopper. On the contrary, the conveying forces f are applied to the sheets such that the largest conveying force is applied to the topmost sheet. 
     &lt;Second Post-Processing Mode&gt; 
     The following describes a buffer sheet conveying state in the second post-processing mode with reference to  FIGS. 9A and 9B  and  FIG. 10 .  FIG. 9A  illustrates a state where a plurality of sheets are conveyed to the buffer section Pb and made to temporarily stand by. The sheets fed to the buffer section Pb from the carry-in port  31  are retained by the first and second sheet discharge rollers  46  and  47  in a state of being overlapped in the order of the first sheet (n), second sheet (n+1), and third sheet (n+2) from below. More specifically, the plurality of sheets is overlapped in a scale shape with the offset amount δ in the feeding order (in the order that they are fed to the carry-in port  31 ). The offset amount δ in this case may be the same as or different from the offset amount δ set in the first post-processing mode. 
     When a state is reached in which the sheets can be carried in the second post-processing section  49  (when the post-processing operation in the second post-processing section is completed), the controller  76  conveys one by one the sheets made to stay in the buffer section Pb from the second sheet discharge port  33   a  to the accumulation guide  49   a  in the order that they are fed to the carry-in port  31 . Specifically, the controller  76  rotates the second and third sheet discharge rollers  47  and  48  in the sheet conveying direction. At this time, the first sheet discharge roller  46  is rotated in the same direction or separated from the sheets. 
     Subsequently, the controller  76  puts each of the first and second sheet discharge rollers  46  and  47  in a state of being separated from the sheets (in a standby posture). Then, as illustrated in  FIG. 10 , the sheet is nipped by the third sheet discharge roller  48  and fed to the accumulation guide  49   a  by rotation thereof. At this time, the stopper  56  is activated so as to prevent the overlapped upper sheets (n+1, n+2) from being drawn downstream (in the direction toward the accumulation guide  49   a ). 
     In this manner, the plurality of sheets made to stay in the buffer section Pb are fed to the accumulation guide  49   a  in the order that they are stacked (in the order of n (sheet that has first been made to stay), n+1, and n+2). 
     The following describes a conveying mechanism of the branch path  34  illustrated in  FIG. 6B . As described above, along the branch path  34 , the second and third sheet discharge rollers  47  and  48  are disposed separated from each other so as to feed the sheet from the second sheet discharge port  34   a  to second post-processing section (accumulation guide  49   a ). A separating unit that prevents sheet overlap feeding is disposed between the second and third sheet discharge rollers  47  and  48 . When one (lowermost one) of the overlapped sheets is carried out, the separating unit prevents the sheets stacked on the lowermost sheet from being drawn downstream (in the direction toward the accumulation guide  49   a ). 
     The separating unit illustrated in  FIG. 6B  includes a braking lever  56  having a locking portion and an operating solenoid  57  that swings the braking lever  56  between a standby position and an operating position. When the lower sheet (e.g., the first buffer sheet) is conveyed from the second sheet discharge port  34   a  to accumulation guide  49   a , the separating unit locks the upper sheet (second buffer sheet) to prevent it from being drawn downstream (in the direction toward the accumulation guide  49   a ). 
     The following describes another example of the conveying state of the buffer sheets in the second post-processing mode with reference to  FIG. 11 .  FIG. 11  illustrates a state where a plurality of sheets are conveyed to the buffer section Pb and made to temporarily stay therein. The sheets fed to the buffer section Pb from the carry-in port  31  are retained by the first and second sheet discharge rollers  46  and  47  in a state of being overlapped in the order of the first sheet (n), the second sheet (n+1), and the third sheet (n+2) from below. More specifically, the plurality of sheets is overlapped in a scale shape with the offset amount γ in the feeding order (in the order that they are fed to the carry-in port  31 ). The offset amount γ is set larger than a difference between a length (lp) of a sheet to be conveyed and a length (lg) between the first and third sheet discharge rollers  46  and  48  (lp−lg&lt;γ). 
     Then, when a state is reached in which the sheets can be carried in the second post-processing section  49  (when the post-processing operation in the second post-processing section is completed), the controller  76  rotates the first to third sheet discharge rollers  46  to  48  in the sheet conveying direction so as to convey one by one the sheets made to stay in the buffer section Pb from the second sheet discharge port  33   a  to the accumulation guide  49   a  in the order that they are fed to the carry-in port  31 . 
     Then, the controller  76  separates the second sheet discharge roller  47  from the sheets and stops or decelerates the first conveying roller  46  during a time from when a leading end of the first sheet (n) passes through the third sheet discharge roller  48  and a rear end thereof passes through the first conveying roller  46  until a rear end of the second sheet (n+1) passes through the first conveying roller  46  to thereby allow the third sheet discharge roller  48  to convey only a lower sheet (e.g., the first buffer sheet (n)) from the second sheet discharge port  34   a  to accumulation guide  49   a . At the same time, the first sheet discharge roller  46  locks the upper sheets (second (n+1) and succeeding buffer sheets) to thereby prevent them to be drawn downstream (in the direction toward the accumulation guide  49   a ). When the rear end of the first sheet (n) passes through the second sheet discharge roller  47 , the controller  76  sets back the second sheet discharge roller  47  to a press-contact position to thereby allow the sheet staying in the buffer section Pb to be conveyed to the third sheet discharge roller  48  by the first and second sheet discharge rollers  46  and  47 . The above operation is repeatedly performed to thereby carry the sheets made to stay in the buffer section Pb one by one into the accumulation guide  49   a.    
       FIG. 12  illustrates a state where a succeeding sheet is fed to the carry-in port  31  from the image forming device A during conveyance of the sheets (n, n+1, n+2, . . . ) made to stay in the buffer section Pb to the second post-processing section  49 . In this state, the buffer sheet is fed in a direction of an arrow A from the first sheet discharge roller  46 , while the succeeding sheet fed from the carry-in port  31  is fed in a direction of an arrow B. That is, the buffer sheet and the succeeding sheet are moved in opposite directions while passing by each other at vertically overlapping positions. 
     Then, the controller  76  rotates the third sheet discharge roller  48  in a direction of an arrow in  FIG. 12  (second sheet discharge roller  47  is rotated in the same direction or stopped/separated). Then, the sheet staying in the buffer section Pb is fed from the second sheet discharge port  34   a  to the second processing section  49 . At this time, the controller  76  separates the pair of first sheet discharge rollers  46  from each other. This prevents the succeeding sheet to be made to pass through the first sheet discharge roller  46  in the opposite direction from being impeded. 
     Although a configuration of the roller pair is not illustrated especially, one of the pair of rollers is axially supported so as to be movable between a press-contact direction and a separating direction and biased by means of a biasing spring in the press-contact direction. Further, a release lever that shifts the movable side roller in the separating direction against the biasing force of the spring is provided and is moved by a drive unit such as an operating solenoid or an operating cam.