Patent Publication Number: US-9415560-B2

Title: Sheet processing apparatus, image forming system, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2013-138173 filed in Japan on Jul. 1, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus that processes a sheet, such as a sheet of paper, to an image forming apparatus that includes the sheet processing apparatus, and to an image forming system. 
     2. Description of the Related Art 
     A conventionally-known image forming system includes a sheet processing apparatus in which a sheet fastening apparatus is provided to perform an operation to fasten a bundle of sheets that have images formed by an image forming apparatus. 
     In a sheet processing apparatus disclosed in Japanese Patent Application Laid-open No. 2010-274623, a sheet fastening apparatus that has a pressing fastener system is provided to fasten a bundle of sheets, without using metallic staples, by strongly nipping the bundle of sheets by using a pair of pressing fastener teeth that are pressing fastener members that have concavity and convexity so that the fibers of the sheets are tangled and the sheets are pressed to be fastened. Pressure fastening, i.e., fastening a bundle of sheets without using metallic staples, can eliminate the trouble in removing the metallic staples from the bundle of sheets when the bundle of sheets is disposed of or is put into a shredder. 
     In the sheet processing apparatus disclosed in Japanese Patent Application Laid-open No. 2010-274623, after an image is formed on a bundle of sheets, the bundle of sheets is conveyed into a gap between one of the pressing fastener teeth and the other one of the pressing fastener teeth in the sheet fastening apparatus. Next, the one of the pressing fastener teeth is moved toward the bundle of sheets so that the bundle of sheets is pressed by the pair of pressing fastener teeth and the bundle of sheets is fastened. Then, after the one of the pressing fastener teeth is moved in a direction away from the bundle of sheets, the bundle of sheets is conveyed toward a discharge tray and is discharged onto the discharge tray. 
     However, a large pressing force is required to strongly nip a bundle of sheets by using a pair of pressing fastener teeth that has concavity and convexity. Therefore, a problem occurs in that, after a bundle of sheets is fastened by pressure, the bundle of sheets adheres to the one of the pair of pressing fastener teeth that does not move in a direction away from the bundle of sheets after the fastening operation is performed. If the bundle of sheets is conveyed toward the discharge tray while the sheet adheres to the pressing fastener tooth, there is a possibility that a problem occurs, such as a conveyance failure or damage to the sheet. 
     In consideration of the above, there is a need to provide a sheet processing apparatus, an image forming system that includes the sheet processing apparatus, and an image forming apparatus that make it possible to prevent a conveyance failure or damage to a sheet. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     A sheet processing apparatus includes: a sheet fastening unit of a pressing fastener method in which a bundle of sheets is fastened by using a pair of pressing fastener members; a conveying unit that conveys the bundle of sheets that are fastened by the sheet fastening unit; a separating unit that, after the sheet fastening unit performs a fastening operation on the bundle of sheets, moves bothe one and the other pressing fastener members, between which the bundle of sheets is interposed, so as to separate a sheet that adheres to the pressing fastener member; and a control unit that, after the separating unit finishes an operation to separate the sheet, controls the conveying unit so as to convey the bundle of sheets that are fastened by the sheet fastening unit. 
     An image forming system includes: an image forming apparatus that forms an image on a sheet; and a sheet processing unit that processes a sheet that has an image formed by the image forming apparatus. The sheet processing unit is as described above. 
     An image forming apparatus forms an image on a sheet. The image forming apparatus comprising a sheet processing apparatus as described above. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic configuration diagrams that illustrate an example of the overall configuration of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is a schematic configuration diagram that illustrates an exemplary configuration of an image forming apparatus of the image forming system according to the present embodiment; 
         FIG. 3  is a plan view that illustrates an exemplary configuration of a sheet post-processing apparatus of the image forming system according to the present embodiment; 
         FIG. 4  is a front view of the sheet post-processing apparatus; 
         FIG. 5  is an explanatory diagram that illustrates the home position of a bifurcating claw that switches a path of a sheet that is received by the sheet post-processing apparatus; 
         FIG. 6  is an explanatory diagram that illustrates the position of the bifurcating claw when the sheet received by the sheet post-processing apparatus is switch to a bifurcating path; 
         FIG. 7  is an explanatory diagram that illustrates an example of a fastener with teeth opened and a driving mechanism thereof; 
         FIG. 8  is an explanatory diagram that illustrates an example of the fastener with the teeth closed and the driving mechanism thereof; 
         FIGS. 9A and 9B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus after an initialization operation is completed; 
         FIGS. 10A and 10B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when a sheet is received; 
         FIGS. 11A and 11B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when the position of a sheet is set in a width direction; 
         FIGS. 12A and 12B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when the position of the trailing edge of the sheet is set; 
         FIGS. 13A and 13B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when the subsequent sheet is received; 
         FIGS. 14A and 14B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when another subsequent sheet is received; 
         FIGS. 15A and 15B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus after an operation to align a bundle of sheets is completed and before a fastening operation is started; 
         FIGS. 16A and 16B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when the bundle of sheets starts to be discharged after the fastening operation is completed; 
         FIGS. 17A and 17B  are a plan view and a front view that illustrate the internal state of the sheet post-processing apparatus when the bundle of sheets is being discharged after the fastening operation is completed; 
         FIG. 18  is a diagram that illustrates a modified example of the sheet post-processing apparatus; 
         FIGS. 19A and 19B  are perspective views that illustrate a moving mechanism that moves the fastener; 
         FIGS. 20A to 20D  are cross-sectional views that illustrate pressure fastening according to the present embodiment; 
         FIGS. 21A to 21C  are plan views that illustrate pressure fastening according to the present embodiment; 
         FIG. 22  is a flowchart of a pressure fastening operation; 
         FIGS. 23A to 23G  are diagram that illustrate the steps of a sheet post-processing operation by using a provided pressing mechanism; 
         FIG. 24  is a block diagram that illustrates an exemplary configuration of the relevant part of a control system for performing a fastening operation in the sheet post-processing apparatus; 
         FIGS. 25A to 25D  are diagrams that illustrate an operation when a fastening operation is performed on two areas of the bundle of sheets; 
         FIGS. 26A and 26B  are diagrams that illustrate a modified example when a fastening operation is performed on two areas of the bundle of sheets; 
         FIGS. 27A and 27B  are diagrams that illustrate an operation when a fastening operation is performed on three areas of the bundle of sheets; 
         FIG. 28  is a diagram that illustrates a second modified example of the sheet post-processing apparatus; and 
         FIGS. 29A to 29D  are diagrams that illustrate an operation to process the second and subsequent sets. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An explanation is given below of an embodiment of the present invention with reference to the drawings. 
       FIGS. 1A and 1B  are schematic configuration diagrams that illustrate an example of the overall configuration of an image forming system according to an embodiment of the present invention. An image forming system  100  of  FIG. 1A  has an exemplary configuration where a sheet processing apparatus (referred to as the “sheet post-processing apparatus” below)  201  that is a sheet processing apparatus is installed in an image forming apparatus  101  that is an image forming unit that forms an image on a sheet that is a sheet on the basis of an input image. Furthermore, the image forming system  100  of  FIG. 1B  has an exemplary configuration where the sheet post-processing apparatus  201  is connected to the image forming apparatus  101 . 
     The image forming apparatus  101  of the present embodiment forms, on a sheet, images including toner images by using an electrophotographic system; however, it may form images by using other systems, such as an ink-jet system. Furthermore, according to the present embodiment, an explanation is given of the image forming system in which the image forming apparatus  101  and the sheet post-processing apparatus  201  are combined; however, the present invention can be applied to the image forming apparatus  101  that has the built-in sheet post-processing apparatus  201 . 
     Furthermore, the present invention is applicable even if the sheet post-processing apparatus  201  is configured as a sheet processing apparatus that is separated from the image forming apparatus  101 . In this case, the sheet processing apparatus may be provided with a cassette or a tray on which a sheet to be fastened is placed, a tray to which a bundle of sheets is output after a fastening operation is performed, or the like. 
       FIG. 2  is a schematic configuration diagram that illustrates an exemplary configuration of the image forming apparatus  101  of the image forming system  100  according to the present embodiment. 
     In  FIG. 2 , the image forming apparatus  101  is a tandem-type color image forming apparatus that has an indirect transfer system using an intermediate transfer member. An image forming unit  110  that is a toner-image forming unit is provided in substantially the middle section of the image forming apparatus  101 . The image forming unit  110  includes image forming stations  111 Y,  111 M,  111 C, and  111 K of four colors (Y: yellow, M: magenta, C: cyan, and K: black) (the attached characters Y, M, C, and K are omitted below as appropriate) that are provided such that they are arranged in a predetermined direction. 
     The image forming apparatus  101  further includes a feeding tray  120  that is a plurality of sheet feeding units that is a recording-medium feeding unit and that is provided under the image forming unit  110 . It further includes a sheet-feeding conveyance path (vertical conveyance path)  130  that conveys a sheet to a secondary transfer unit  140  and a fixing unit  150 , the sheet being a recoding medium picked up by the feeding tray  120 . The image forming apparatus  101  further includes a bifurcating sheet-discharge path  160  and a two-sided conveyance path  170 , the bifurcating sheet-discharge path  160  conveys, toward the sheet post-processing apparatus  201 , a sheet on which an image (toner image) is fixed, and the two-sided conveyance path  170  turns over the sheet that has an image formed on a first surface (front surface) thereof so that an image is formed on a second surface (back surface) thereof. 
     The image forming apparatus  101  further includes a scanner unit  180  that is an image read unit and includes an automatic document feeder (ADF)  185  that is original-document feed unit. The scanner unit  180  reads an image of an original document and converts it into an electric signal, the original document being an image read object that is placed on a glass surface that is a platen. Furthermore, one or more original documents are placed on the automatic document feeder (ADF)  185  so that an image thereof is read by the scanner unit  180 , and each of the original documents is conveyed to the glass surface at the read position of the scanner unit  180 . 
     The image forming unit  110  includes photosensitive drums that are the image carriers for the colors Y, M, C, and K of the image forming stations  111 . A charge unit that is a charging unit, a development unit that is a developing unit, a primary transfer unit, a cleaning unit, a neutralization unit that is a neutralizing unit are provided around each of the photosensitive drums along its outer periphery. Furthermore, the image forming unit  110  includes an undepicted optical writing unit that is a light irradiation unit and includes an intermediate transfer belt  112  that is an intermediate transfer member. The optical writing unit is provided under each of the image forming stations  111 , and it forms an electrostatic latent image by emitting light to each of the photosensitive drums on the basis of image data that is generated by using the reading result of the scanner unit  180  for each of the colors. The intermediate transfer belt  112  is provided above the image forming stations  111 , and the image (toner image) formed on each of the photosensitive drums is transferred by the primary transfer unit. 
     The intermediate transfer belt  112  is rotatably supported by a plurality of supporting rollers. A supporting roller  114 , which is one of the supporting rollers, is opposed to a secondary transfer roller  115  through the intermediate transfer belt  112  in the secondary transfer unit  140 . In the secondary transfer unit  140 , the image (toner image) on the intermediate transfer belt  112  is secondarily transferred onto a sheet. A replaceable toner container  116  is provided above the intermediate transfer belt  112 . 
     The image forming process of the image forming apparatus (tandem-type color image forming apparatus that has an indirect transfer system) that has the above-described configuration is well-known, and it is not directly related to the scope of the present invention; therefore, a detailed explanation is omitted. 
     The fixing-finished sheet on which a fixing operation has been performed by the fixing unit  150  is conveyed by a conveyance roller  162 , and its conveying direction is switched by a conveyance-path switch member  161 . Thus, the fixing-finished sheet is conveyed to the bifurcating sheet-discharge path  160  or the two-sided conveyance path  170 . 
     To perform post processing on a plurality of sheets including a sheet on which an image has been formed, the sheet post-processing apparatus  201  of the present embodiment includes a conveyance-path fastening mechanism that is a sheet fastening unit that fastens a bundle of sheets that includes a plurality of sheets, i.e., a bundle of sheets. The conveyance-path fastening mechanism includes a configuration for stacking and aligning sheets within a sheet conveyance path and includes a fastener that is a fastening unit that fastens the stacked sheets. 
       FIG. 3  and  FIG. 4  are a plan view and a front view that illustrate an exemplary configuration of the sheet post-processing apparatus  201  that includes the conveyance-path fastening mechanism that is included in the image forming system  100  according to the present embodiment. 
     The sheet post-processing apparatus  201  includes an entry sensor  202 , entry rollers  203 , a bifurcating claw (switch claw)  204 , sheet discharge rollers  205 , a shift link  206 , a shift cam  207 , a shift cam stud  208 , a shift home-position sensor  209 , and a fastener  210 . 
     The entry sensor  202  detects the presence or absence of the leading edge and the trailing edge of a sheet that is delivered to the sheet post-processing apparatus  201  through a sheet discharge roller  102  of the image forming apparatus  101 . 
     The entry rollers  203  are located on the entry of the sheet post-processing apparatus  201 , and it has the capability to convey a sheet to the sheet post-processing apparatus  201 . By using the roller nip of the entry rollers  203 , it is possible to bring a sheet into contact with it for skew correction. The entry rollers  203  are driven by an undepicted driving source that can be controlled. The driving source is controlled by a control unit that is described later, whereby the entry rollers  203  are controlled so as to be driven to rotate and be stopped by the driving source, and the distance over which a sheet is conveyed by the entry rollers  203  is controlled. The control unit may be provided in the image forming apparatus  101 . 
     The bifurcating claw  204  is a rotatable claw that switches a conveyance path that is provided to guide the trailing edge of a sheet to a bifurcating path  241 . Furthermore, the bifurcating claw  204  is configured to press the sheet against the conveyance surface of the bifurcating path, and the sheet can be fixed by using the pressure. 
     The sheet discharge rollers  205  are located on the outlet of the sheet post-processing apparatus  201 , and it has the capability to convey, shift, and discharge a sheet. Furthermore, the sheet discharge rollers  205  are driven by an undepicted driving source that can be controlled. The driving source is controlled by a control unit that is described later, whereby the sheet discharge rollers  205  are controlled so as to be driven to rotate and be stopped by the driving source, and the distance over which a sheet is conveyed by the sheet discharge rollers  205  is controlled. 
     A conveying unit that conveys a sheet in the sheet post-processing apparatus  201  according to the present embodiment is constituted by, for example, the entry rollers  203 , the sheet discharge rollers  205 , and the driving sources that drive them. 
     The shift link  206  is provided on the end of the shaft of the sheet discharge rollers  205 , and it is a section that receives the moving force for shifting. 
     The shift cam  207  includes the shift cam stud  208 , and it is a disk-shaped component that rotates. The rotation of the component shifts the sheet discharge rollers  205  that are connected to an elongated hole section of the shift link  206  via the shift cam stud  208 . 
     The shift cam stud  208  operates in conjunction with the elongate hole section of the shift link  206  to change the rotary movement of the shift cam  207  into a linear movement in the axial direction of the sheet discharge rollers  205 . 
     The shift home-position sensor  209  detects the position of the shift link  206  and determines that the detected position is the home position (stand-by position). 
     The fastener  210  is a tool or device that fastens a bundle of sheets by squeezing, pressing, and fastening processing without using metallic staples. According to the present embodiment, the fastener  210  is used to nip a bundle of sheets by using a single pair of teeth that have concavity and convexity on their surfaces so that the sheets are deformed and the fibers thereof are tangled. For example, the well-known fastener that is disclosed in Japanese Examined Utility Model Application Publication No. 36-013206 can be used as the above type of fastener  210 . Furthermore, a U-shaped cut is formed in a bundle of sheets and the portion is bent, a slit is simultaneously formed near the bent portion, and the end of the cut and bent portion is passed through the slit so as to be prevented from being released from it, whereby it is possible to use a fastener that fastens a bundle of sheets without using metallic staples (e.g., see Japanese Examined Utility Model Application Publication No. 37-007208). A fastening unit that fastens a bundle of sheets is not limited to the fastener of the present embodiment, and it may be appropriate if it has a capability to fasten sheets by squeezing, pressing, and fastening processing, i.e., fastening sheets by applying pressure so that the fibers of the sheets are tangled. 
     A sheet edge sensor  220  is a sheet-edge detection unit and is a sensor that detects the side edge of a sheet. When a sheet is aligned, the detection position that is detected by the sensor is used as a reference for aligning sheets. 
     A fastener home-position sensor  221  is a sensor that detects the position of the fastener  210  that is movable in a width direction that intersects with the conveying direction of a sheet. The home position (stand-by position) is where the fastener  210  is located at a position so as not to interfere with a conveyed sheet of the maximum size, and the position is detected by the fastener home-position sensor  221 . 
     A fastener-movement guide rail  230  is a rail that guides a movement of the fastener  210  so that the fastener  210  can move in a width direction of a sheet in a stable manner. 
     A conveyance path  240  is a normal pathway for conveying and discharging the received sheet. The bifurcating path  241  is provided to stack and align sheets, and it is the conveyance path to which a sheet is conveyed starting from the trailing edge side thereof due to a switchback of the sheet. 
     A contact surface  242  is a reference surface for bringing the trailing edges of sheets into contact with it and aligning the sheets on a fastening-operation tray (staple tray)  243  that is a sheet containing section that contains sheets that are to be fastened. For example, according to the present embodiment, teeth  261  are the teeth that are configured such that a single pair of concavity and convexity is engaged, and sheets are nipped so that the sheets are deformed and the fibers thereof are tangled. 
       FIGS. 5 and 6  are explanatory diagrams that illustrate a detailed exemplary configuration of the bifurcating claw  204  that switches a path of a sheet received by the sheet post-processing apparatus  201  and the periphery thereof.  FIG. 5  is an explanatory diagram that illustrates the home position of the bifurcating claw  204 . Furthermore,  FIG. 6  is an explanatory diagram that illustrates the position of the bifurcating claw when the path of a sheet received by the sheet post-processing apparatus  201  is switched to the bifurcating path  241 . 
     The bifurcating claw  204  is configured to rotate so as to switch the conveyance path  240  and the bifurcating path  241 . As illustrated in  FIG. 5 , the home position of the bifurcating claw  204  is the rotation position where the sheet received from the right side in the drawing can be conveyed without any resistance. The bifurcating claw  204  is always pressed by a spring  251  as illustrated in  FIG. 5 . The spring  251  is engaged with a bifurcating-claw movable lever section  204   a . The bifurcating-claw movable lever section  204   a  is also connected to a bifurcation solenoid  250  via a link. Furthermore, the conveyance surface of the bifurcating path  241  and the bifurcating claw  204  are configured to nip a sheet within the conveyance path. As for switching of the conveyance path, when the bifurcation solenoid  250  is turned on, the bifurcating claw  204  is rotated in the direction of the arrow A1 in  FIG. 6  so that the conveyance path  240  is closed, and a sheet is guided into the bifurcating path  241 . 
     According to the present embodiment, a unit that stacks a plurality of sheets, which are the objects to be fastened, to produce a bundle of sheets is constituted by the entry roller  203 , the sheet discharge rollers  205 , the bifurcating claw  204 , the fastening-operation tray  243  including the contact surface  242 , the driving sources for driving them, and the like. 
       FIGS. 7 and 8  are explanatory diagrams that illustrate an example of the configuration and operation of the fastener  210 .  FIG. 7  is an explanatory diagram that illustrates an example of the fastener  210  with the teeth  261  opened and the driving mechanism thereof, and  FIG. 8  is an explanatory diagram that illustrates an example of the fastener  210  with the teeth  261  closed and the driving mechanism thereof. The configuration of the fastener  210  is not limited to the configuration of  FIGS. 7 and 8 . 
     In  FIG. 7 , the teeth  261  are the pair of upper and lower teeth and are configured to engage with each other. The teeth  261  are provided on the end of a group of combined links and are configured to be in contact with or be away from each other due to the rotation of a pressing lever  262 . The pressing lever  262  is rotated in the direction of the arrow A3 of  FIG. 8  by a cam  266  that rotates in the direction of the arrow A2 of  FIG. 8 . The cam  266  is rotated due to the driving force applied from a drive motor  265 , and it is controlled so as to be located in the detection position on the basis of the detection information of a cam home-position sensor  267 . The detection position of the cam home-position sensor  267  is the home position (stand-by position) of the cam  266 , and the teeth  261  are opened while in this position. 
     An operation is performed to fasten sheets as illustrated in  FIG. 8 . While the pair of the teeth  261  are opened, a sheet P is inserted into the gap therebetween, and the cam  266  is rotated in the direction of the arrow A2 of  FIG. 8  in accordance with the rotation of the drive motor  265 . Due to the displacement of the cam surface, the pressing lever  262  is rotated in the direction of the arrow A3 in the drawing. The rotative force is increased through the group of links by using the lever, and it is transmitted to the teeth  261  in the end. When the cam  266  is rotated for a certain degree, the teeth  261  are engaged with each other to nip the sheet P. Due to the nip, the sheet P is pressed and deformed, and the fibers of the adjacent sheets are tangled and joined. Afterward, the drive motor  265  is rotated in reverse and is stopped at the detection position of the cam home-position sensor  267 . Furthermore, the pressing lever  262  has spring characteristics; therefore, it is bent when an overload is applied, whereby the overload is released. 
     In the fastener  210  that is configured as described above in  FIGS. 7 and 8 , the fastening force, i.e., the force with which the teeth  261  are engaged to nip the sheet P so as to press and deform it, is changed, and the fastening strength for fastening a bundle of sheets whose fibers are tangled is changed. The fastening force with which the teeth  261  are engaged is changed according to the rotative force (torque) during the rotation of the pressing lever  262  via the cam  266 , i.e., the torque (moment of force) generated by the drive motor  265 . The torque generated by the drive motor  265  is changed according to the motor current supplied to the drive motor  265 . Therefore, by controlling the motor current supplied to the drive motor  265 , the fastening force of the fastener  210  is changed according to a fastening mode, such as a proper fastening mode or a temporary fastening mode, whereby the fastening strength for a bundle of sheets can be changed. 
     Next, an explanation is given of an example of a fastening operation of the sheet post-processing apparatus  201 . 
       FIGS. 9A to 17B  are plan views and front views of the sheet post-processing apparatus  201  when the fastening operation is performed according to the present embodiment. Out of  FIGS. 9A to 17B ,  FIGS. 9A, 10A, 11A, 12A, 13A, 14A ,  15 A,  16 A, and  17 A are plan views of the sheet post-processing apparatus  201 , and  FIGS. 9B, 10B, 11B, 12B, 13B, 14B, 15B, 16B, and 17B  are front views of the sheet post-processing apparatus  201 . 
     First, in  FIGS. 9A and 9B , when the image forming apparatus  101  starts to output a sheet, each unit is moved to its home position, whereby an initialization operation is completed. 
     Next, in  FIGS. 10A and 10B , before the sheet P output from the image forming apparatus  101  is conveyed to the sheet post-processing apparatus  201 , the sheet post-processing apparatus  201  receives information on an operating mode and information on the sheet P and, in accordance with the pieces of information, it enters a receiving standby state. The operating modes according to the present embodiment include a straight mode, a shift mode, and a fastening mode; however, this is not a limitation. 
     While in a receiving standby state during a straight mode, the sheet discharge roller  102  of the image forming apparatus  101  is rotated in the direction of the arrow A4 in the drawing so that the sheet P discharged from the image forming apparatus  101  is sent to the sheet post-processing apparatus  201 . In the sheet post-processing apparatus  201 , each of the entry roller  203  and the sheet discharge roller  205  starts to rotate in a predetermined rotation direction (the direction A5 and the direction A6 in the drawing) so that the received sheet P is conveyed in a predetermined conveying direction (to the left in the drawing). The plurality of sheets P are sequentially conveyed and discharged and, when the final sheet is discharged, each of the rollers  203  and  205  is stopped. 
     While in a receiving standby state during a shift mode, first, each of the entry roller  203  and the sheet discharge roller  205  starts to rotate in a predetermined rotation direction so that the received sheet P is conveyed in a predetermined conveying direction (to the left in the drawing) in the same manner as in the straight mode. During a shift sheet discharge operation, the sheet P is received and conveyed and, when the trailing edge of the sheet P passes through the entry roller  203 , the shift cam  207  is rotated for a certain degree so that the sheet discharge roller  205  is moved in the axial direction thereof. At this time, the sheet P is also moved in accordance with the movement of the sheet discharge roller  205 . Furthermore, after the sheet P is discharged, the shift cam  207  is rotated to return to the home position and stands by for the subsequent sheet. This operation of the sheet discharge roller  205  is repeated until the sheets in the same “set” are discharged. If a sheet in the subsequent “set” is conveyed, the shift cam  207  is rotated in the rotation direction opposite to the previous direction so that the sheet is moved to the opposite side and is discharged. 
     While in a receiving standby state during a fastening mode, the entry roller  203  is stopped, and the sheet discharge roller  205  starts to rotate in the direction of the arrow A6 in the drawing so that the received sheet P is conveyed in a predetermined conveying direction (to the left in the drawing). Furthermore, the fastener  210  is moved to stand by at the stand-by position (home position) that is retracted from the end of the sheet P in a width direction by a certain distance. 
     Afterward, when the sheet P is conveyed to the sheet post-processing apparatus  201 , the leading edge of the sheet P is detected by the entry sensor  202 . After it is detected, the sheet P is conveyed for a certain distance (a distance such that the leading edge of the sheet P is brought into contact with the nip of the entry roller  203  and the sheet P is bent for a certain degree). After it is conveyed, the entry roller  203  starts to rotate. Thus, skew of the sheet P is corrected. 
     Next, in  FIGS. 11A and 11B , the distance over which the sheet P is conveyed is counted by using, as a reference, the detection information of the entry sensor  202  that detects the trailing edge of the sheet P, and the positional information on the sheet P is determined. When the trailing edge of the sheet P passes through the nip of the entry roller  203 , the entry roller  203  stops so as to receive the subsequent sheet. At the same time, the shift cam  207  is rotated in the direction (clockwise direction) of the arrow A7 of  FIG. 11A , and the sheet discharge roller  205  starts to move in the axial direction thereof together with the sheet P. Then, the sheet P is conveyed at a tilt in the direction of the arrow A8 of  FIG. 11A . Afterward, when the sheet edge sensor  220 , which is provided together with or is installed in the fastener  210 , detects the sheet P, the shift cam  207  is stopped and is then rotated in reverse. The reverse rotation of the shift cam  207  is stopped when the sheet edge sensor  220  enters a non-detection state. After the above-described operation is completed, the trailing edge of the sheet P passes through the leading edge of the bifurcating claw  204  and is located at a predetermined position, and the rotation of the sheet discharge roller  205  in the direction of the arrow A9 in the drawing is stopped. 
     Next, in  FIGS. 12A and 12B , the bifurcating claw  204  is rotated in the direction (clockwise direction) of the arrow A10 in  FIG. 12B  so that the conveyance path is switched. Afterward, the sheet discharge roller  205  is rotated in the reverse direction (counterclockwise direction) of the arrow A11 in the drawing, and the sheet P is conveyed in the arrow A12 in the drawing so that the trailing end section of the sheet P is conveyed to the bifurcating path  241 . Due to this conveyance, the sheet P is brought into contact with the contact surface  242  of the fastening-operation tray  243  for alignment, and the sheet discharge roller  205  is stopped. Here, the conveying force of the sheet discharge roller  205  is set to be low so that, when the sheet P is brought into contact, it slips. 
     Next, in  FIGS. 13A and 13B , the bifurcating claw  204  is rotated in the direction (counterclockwise direction) of the arrow A13 in  FIG. 13B , and the trailing edge of the sheet P within the bifurcating path  241  is strongly pressed against the contact surface of the bifurcating claw  204  for standby. When subsequent sheet P′ is output from the image forming apparatus  101 , the entry roller  203  performs an operation to correct the skew of the sheet P′ in the same manner as for the first sheet P. Furthermore, at the same time as the entry roller  203  starts to rotate, the sheet discharge roller  205  also starts to rotate in the rotation direction (the direction A6 in the drawing) to convey the sheet. 
     Next, in  FIGS. 14A and 14B , the above-described operations of  FIGS. 11A to 12B  are performed on the second and subsequent sheets P″, . . . , and the sheets are sequentially moved to a target position and are stacked, whereby the bundle of aligned sheets Ps is stacked on the conveyance path. 
     Next, in  FIGS. 15A and 15B , an operation is completed to stack the final sheet on the bundle of aligned sheets Ps, the sheet discharge roller  205  is rotated in the direction (clockwise direction) of the arrow A14 of  FIG. 15B  so that the bundle of sheets Ps is conveyed for a certain distance, and it is then stopped. Due to this operation of the sheet discharge roller  205 , it is possible to eliminate the bending that occurs when the trailing edge of a sheet is brought into contact with the contact surface  242 . Afterward, the bifurcating claw  204  is rotated in the direction (clockwise direction) of the arrow A15 of  FIG. 15B  so that the direction of the leading edge thereof is changed, and the pressing force applied to the bundle of sheets Ps is released. 
     Next, in  FIGS. 16A and 16B , the sheet discharge roller  205  is rotated in the direction of the arrow A16 in the drawing, the bundle of sheets Ps is conveyed for a distance such that the position of the teeth  261  of the fastener  210  matches the processing position (fastening position) of sheets, and it is then stopped. Thus, the position of the teeth  261  of the fastener  210  is caused to match the processing position (fastening position) of sheets in the sheet conveying direction. Furthermore, the fastener  210  is moved in the direction of the arrow A17 of  FIG. 16A  for a distance such that the position of the teeth  261  of the fastener  210  matches the processing position of the sheets, and it is then stopped. Thus, the position of the teeth  261  of the fastener  210  matches the processing position (fastening position) of the sheets in the sheet width direction. At this time, the bifurcating claw  204  is rotated in the direction (counterclockwise direction) of the arrow A18 of  FIG. 16B  so that the direction of the leading edge thereof is changed, and it enters a state for receiving a sheet. Afterward, the drive motor  265  of the fastener  210  is turned on, and the bundle of sheets Ps is pressed and squeezed by the teeth  261 , whereby the fibers of the sheets P are tangled, the sheets are joined, and the bundle of sheets Ps is fastened. 
     Next, in  FIGS. 17A and 17B , when the sheet discharge roller  205  is further rotated in the direction of the arrow A16 in the drawing, the bundle of fastened sheets Ps is discharged. After the bundle of sheets Ps is discharged, the shift cam  207  is rotated in the direction A19 in the drawing to return to the home position, and the fastener  210  is moved in the direction of the arrow A20 in the drawing to return to the home position. As described above, the operation to fasten the bundle of sheets Ps is completed. 
       FIG. 18  is a diagram that illustrates a modified example of the sheet post-processing apparatus. 
     As illustrated in  FIG. 18 , the fastener  210  may be configured to tilt at 45° relative to the sheet conveying direction and move in a direction tilted at 45° relative to the sheet conveying direction. 
       FIGS. 19A and 19B  are perspective views that illustrate a moving mechanism  280  that moves the fastener  210 .  FIG. 19A  is a perspective view that illustrates a state where the fastener  210  is in the fastening position, and  FIG. 19B  is a perspective view that illustrates a state where the fastener  210  is in the home position. 
     As illustrated in the drawings, the moving mechanism  280  includes an eccentric cam  282  and an arm member  281 . One end of the arm member is secured to the eccentric cam  282 , and the other end of the arm member is secured to the fastener  210 . The fastener  210  is supported such that it can slide and move along the fastener-movement guide rail  230 . 
     When an undepicted moving motor is driven so that the eccentric cam  282  is rotated in the direction of the arrow B in the drawing, the fastener  210  is moved from the fastening position illustrated in  FIG. 19A  to the home position illustrated in  FIG. 19B  along the fastener-movement guide rail  230 . 
     Next, the characteristics of the present embodiment are explained. 
     As explained above with reference to  FIGS. 17A and 17B , when a sheet discharge roller  20  is rotated to discharge the bundle of sheets Ps on which the fastening operation has been performed, the teeth  261  are in the position (the position where the sheets are fastened) opposed to the bundle of sheets Ps. After the bundle of sheets Ps is conveyed, the fastener is moved in the direction of the arrow A20 in the drawing to return to the home position. With this configuration, a problem of a conveyance failure occurs. As a result of the inventors&#39; hard study on the problem, the following is proved. A high pressing force is required to strongly nip a bundle of sheets by using the pair of the teeth  261 ; therefore, the sheet of the bundle, on which pressure fastening has been performed, adheres to the teeth. Thus, it is proved that, while the sheet adheres to a lower tooth  261   b , the bundle of sheets Ps is conveyed by the sheet discharge roller  20  and therefore the sheet sticks to the lower tooth  261   b  and a conveyance failure occurs. 
     Therefore, according to the present embodiment, the pair of the teeth  261  is moved from the position opposed to the bundle of sheets Ps to the retracted position that is not opposed to the bundle of sheets Ps and, after the sheet adhering to the teeth is separated, the sheet discharge roller  20  is rotated so that the bundle of sheets Ps is conveyed. A detailed explanation is given below with reference to the drawings. 
       FIGS. 20A to 20D  are cross-sectional views that illustrate pressure fastening according to the present embodiment, and  FIGS. 21A to 21C  are plan views that illustrate pressure fastening according to the present embodiment. Furthermore,  FIG. 22  is a flowchart of a pressure fastening operation. 
     As illustrated in  FIG. 21A , according to the present embodiment, the home position of the fastener  210  is the retracted position where the teeth  261  are not opposed to the bundle of sheets Ps, and the fastener  210  is located in the retracted position except when a fastening operation is performed on the bundle of sheets Ps. 
     When a fastening operation is performed, the fastener  210  is moved from the retracted position so that the fastener  210  is moved to the fastening position, as illustrated in  FIGS. 20A and 21B . As illustrated in the flow of  FIG. 22 , when the fastener  210  is moved to the fastening position (YES at S 1 ), a fastening operation is performed (S 2 ). Specifically, as illustrated in  FIG. 20B , an upper tooth  261   a  is moved, the upper tooth  261   a  and the lower tooth  261   b  are engaged to nip the bundle of sheets Ps, and pressure is applied to the bundle of sheets Ps, whereby the bundle of sheets Ps is fastened. When the fastening operation is completed, the upper tooth  261   a  is moved in a direction away from the bundle of sheets Ps so that the teeth are opened as illustrated in  FIG. 20C . At this time, a sheet sometimes adheres to the lower tooth  261   b  that is fixed at a predetermined position. As a high pressing force is required to strongly nip the bundle of sheets by using the pair of the teeth  261 , the bundle of sheets on which pressure fastening has been performed adheres to the tooth. Out of the pair of the teeth  261 , the upper tooth  261   a  is moved in a direction away from the bundle of sheets Ps after pressure fastening is performed; therefore, a sheet is separated from the upper tooth  261   a  due to the stiffness of the sheet, or the like. Conversely, the lower tooth  261   b  remains at the position after pressure fastening is performed; therefore, after the teeth are opened, a sheet still adheres to the lower tooth  261   b . If the bundle of sheets Ps is conveyed in the state illustrated in  FIG. 20C , there is a possibility that the bundle of sheets Ps sticks to the lower tooth  261   b  while being conveyed and a conveyance failure or damage to a sheet occurs. 
     Therefore, according to the present embodiment, as illustrated in  FIGS. 20D and 21C , the fastener  210  is moved from the fastening position to the retracted position that is the home position (S 3 ). As the fastener  210  is moved to the retracted position, the sheet adhering to the lower tooth  261   b  is separated. When the fastener  210  is moved to the retracted position (YES at S 3 ), the sheet discharge roller  205  starts to be driven (S 4 ) so that the bundle of sheets Ps is conveyed. 
     Thus, according to the present embodiment, after the fastening operation is performed, an operation is performed to move the pair of teeth to the retracted position so as to separate the sheet that adheres to the teeth and then convey the bundle of sheet. This prevents the bundle of sheets Ps from sticking to the lower tooth  261   b  while being conveyed. Thus, a conveyance failure can be prevented. Furthermore, according to the present embodiment, when the fastener  210  is moved from the fastening position to the retracted position, the bundle of sheets Ps near the fastener  210  is pressed by the pair of the sheet discharge rollers  205 . Therefore, the sheet adhering to the lower tooth  261   b  is separated from the teeth  261  without moving to the retracted position together with the fastener  210 . Therefore, when the fastener  210  is moved from the fastening position to the retracted position, the sheet adhering to the lower tooth  261   b  is prevented from moving to the retracted position together with the fastener  210 , and the occurrence of wrinkles or damages to a sheet can be prevented. Specifically, according to the present embodiment, the pair of the sheet discharge rollers  205  serves as a pressing unit, and the pair of the sheet discharge rollers  205  and the moving mechanism  280  serve as a separating unit that separates a sheet adhering to the lower tooth  261   b.    
     Furthermore, the fastener  210  and the sheet discharge rollers  205  are sometimes located away from each other depending on the configuration of the apparatus, for example, in a case of the configuration where a fastening operation is performed on a bundle of sheets on the staple tray  243  that contains sheets that are to be fastened. In this case, there is a possibility that the pressure of the sheet discharge rollers  205  is not sufficiently applied and the sheet adhering to the teeth  261  is moved to the retracted position together with the fastener  210 . In such a case, a pressing mechanism may be provided to press a bundle of sheets. 
       FIGS. 23A to 23G  are diagram that illustrate the steps of a sheet post-processing operation by using a provided pressing mechanism  270 . 
     As illustrated in  FIGS. 23A to 23G , the pressing mechanism  270  is provided between the fastener  210  and the sheet discharge roller  205 . The pressing mechanism  270  includes a pressing member  271 , a pressing lever  272 , a pressing spring  273 , and a pressing solenoid  274 . 
     A pressing section  271   a  that presses the bundle of sheets Ps is provided on an end of the pressing member  271  on the side of the fastener  210  such that the pressing section  271   a  protrudes toward the bundle of sheets Ps. One end of the pressing lever  272  is secured to an end of the pressing member on the side of the sheet discharge roller  205 . The pressing lever  272  near its middle section is biased toward the fastener  210  by the pressing spring  273 . The other end of the pressing lever  272  is secured to the pressing solenoid  274 . 
     As illustrated in  FIG. 23A , the pressing solenoid  274  is usually on, and the pressing member  271  is located in a pressure released position that is away from the bundle of sheets Ps. Furthermore, with the configuration illustrated in  FIGS. 23A to 23G , the fastener  210  is retracted above a stack guide  275  that includes the contact surface  242  of the staple tray  243 . 
     As illustrated in  FIG. 23B , when a fastening operation is performed on the bundle of sheets Ps, the pressing solenoid  274  is turned off. Then, the pressing lever  272  is rotated in a counterclockwise direction in the drawing due to the biasing force of the pressing spring  273 . Then, the pressing section  271   a  of the pressing member  271  is brought into contact with the bundle of sheets Ps on the staple tray  243  so as to press the bundle of sheets Ps against the staple tray  243 . 
     After the bundle of sheets Ps is pressed by the pressing member  271 , the fastener  210 , which is retracted above the stack guide  275 , is moved to the fastening position that is opposed to the bundle of sheets Ps, as illustrated in  FIG. 23C . After the fastener  210  is moved to the fastening position, the upper tooth  261   a  and the lower tooth  261   b  of the fastener  210  are engaged to nip the bundle of sheets Ps, pressure is applied to the bundle of sheets Ps, and the bundle of sheets Ps is fastened, as illustrated in  FIG. 23D . 
     After the bundle of sheets Ps is fastened, the teeth  261   a  and  261   b  are separated from the bundle of sheets Ps, as illustrated in  FIG. 23E . Next, as illustrated in  FIG. 23F , the fastener  210  is moved to the retracted position above the stack guide  275 . At this time, as an area near the fastened area of the bundle of sheets Ps is pressed by the pressing member  271 , the sheet adhering to the teeth  261  is separated during the movement of the fastener  210 . Thus, the sheet adhering to the teeth  261  is prevented from being moved to the retracted position together with the fastener  210 , and the occurrence of wrinkles or damages to a sheet can be prevented. 
     After the fastener  210  is moved to the retracted position, the pressing solenoid is turned on, and the pressing member  271  is rotated in a clockwise direction in the drawing against the biasing force of the pressing spring  273 , as illustrated in  FIG. 23G . Thus, the pressing member  271  is separated from the bundle of sheets Ps and is moved to the pressure released position, and the pressure to the bundle of sheets Ps is released. After the pressing member  271  is moved to the pressure released position, the sheet discharge roller  205  is rotated in a counterclockwise direction in the drawing to discharge the bundle of sheets Ps that is on the staple tray  243 . 
     In the apparatus illustrated in  FIGS. 23A to 23G , the sheet discharge roller  205  is located at a long distance from the trailing end section of the sheet on which fastening has been performed. Therefore, when the sheet discharge roller  205  is driven to rotate so as to convey the bundle of sheets Ps, there is a possibility that the trailing end section of the sheet moves upward or downward in the drawing due to the oscillation that occurs in the apparatus, or the like. At this time, if the fastener  210  is not retracted to the retracted position, there is a possibility that a sheet sticks to the teeth and a conveyance failure occurs. However, according to the present embodiment, the fastener  210  is retracted to the retracted position; therefore, even if the trailing end section of a sheet is bent while the bundle of sheets is conveyed, the sheet can be conveyed without sticking to the teeth. Thus, it is possible to prevent a conveyance failure due to sticking to the teeth. 
       FIG. 24  is a block diagram that illustrates an exemplary configuration of the relevant part of a control system for performing a fastening operation in the sheet post-processing apparatus. 
     A control unit  300  that is the control unit includes, for example, a CPU, a RAM, a ROM, an I/O interface, or the like. Furthermore, the control unit  300  is connected to, via an undepicted I/O interface, the moving mechanism  280 , the pressing mechanism  270 , the fastener home-position sensor  221 , the fastener  210 , a conveying unit  290  that includes, for example, a driving source for driving the sheet discharge rollers  205 , or the like. The control unit  300  controls the moving mechanism  280 , the pressing mechanism  270 , the fastener  210 , the conveying unit  290 , or the like, on the basis of a program that is stored in the ROM, or the like, so as to perform the above-described fastening operation. 
     Furthermore, in the above explanation, a fastening operation is performed on a single area of the bundle of sheets Ps; however, a fastening operation may be performed on a plurality of areas. 
       FIGS. 25A to 25D  are diagrams that illustrate an operation when a fastening operation is performed on two areas of the bundle of sheets Ps. 
     As illustrated in  FIG. 25A , the fastener  210 , which is located in the retracted position, is moved to a first fastening position illustrated in  FIG. 25B  by the above-described moving mechanism  280 . After a fastening operation is performed to fasten the bundle of sheets Ps, the fastener  210  is moved to a second fastening position and a fastening operation is performed, as illustrated in  FIG. 25C . After the bundle of sheets Ps is fastened at two areas thereof, the fastener  210  is moved to the retracted position, as illustrated in  FIG. 25D . After it is moved to the retracted position, the bundle of sheets Ps is conveyed. 
     As illustrated in  FIGS. 26A and 26B , the first fastening position and the second fastening position may be apart from each other and, as illustrated in  FIGS. 27A and 27B , fastening may be applied to three areas. In any case, after a fastening operation is performed on multiple areas, the fastener  210  is returned to the retracted position and the bundle of sheets Ps is conveyed; thus, the bundle of sheets Ps can be conveyed without sticking to the teeth. Furthermore, the position where the fastener initially stands by does not need to be the same as the retracted position. 
       FIG. 28  is a diagram that illustrates a second modified example of the sheet post-processing apparatus. 
     As illustrated in  FIG. 28 , a sheet output from the image forming apparatus  101  is delivered to a sheet post-processing apparatus  201   b  according to the second modified example. After the sheet is delivered to the sheet post-processing apparatus  201   b  according to the second modified example, the sheet is conveyed by a conveyance roller  4  and a conveyance roller  5 , is passed through a conveyance path that is obtained by rotating a switch claw  9  due to the moving force of the sheet, and is conveyed to an alignment unit  18  by a conveyance roller  7  and a conveyance roller  8 . The conveyed sheet drops due to its own weight in the direction of the arrow B, and it is aligned in a conveying direction by a trailing-edge fence  11 . The trailing edge of a sheet is previously detected by a sensor S 2  and, after the time during which the sheet can be aligned in the conveying direction, it is aligned in a width direction by an alignment fence  10 . This operation is repeatedly performed so that a large number of sheets are aligned one by one. 
     After alignment of the final sheet is completed, a fastener  12  performs pressure fastening on the bundle of aligned sheets, a release belt  14  in the alignment unit  18  is rotated in the direction of the arrow C, and the bundle of sheets is released from the alignment unit  18  in the direction of the arrow D by a release claw  13  that is secured to the release belt  14 . The bundle of sheets is discharged onto and is stacked on a tray  3  by a discharge roller  15  and a driven roller  16 . The tray  3  includes a mechanism that moves up and down in accordance with the number of stacked sheets. 
     The driven roller  16  is attached to a conveyance guide plate  17 , it is configured to rotate around a supporting point  17   a  so that, even if the thickness of a bundle of sheets to be conveyed is changed, the same conveying force can be obtained, and it is configured to press the discharge roller  15  due to the weight of the conveyance guide plate  17 . These are the operation performed in the case of a single bundle. 
     In the case of two or more sets, the image forming apparatus  101  continuously performs copying in a copy interval between the final sheet in the previous set and the first sheet in the subsequent set, which is the same interval as that for the other cases, and sends it to the sheet post-processing apparatus  201   b  according to the second modified example. 
     An explanation is given of an operation to process the second and subsequent sets with reference to  FIGS. 29A, 29B, 29C, and 29D . 
     The conveyance rollers  4  and  5  are rotated in the direction of the arrows in  FIG. 29A  so that the first sheet in the second set is conveyed. If the sensor S 2  detects the trailing edge of the sheet and if the alignment unit  18  is not in a state for receiving the sheet, a conveyance roller  6  and the conveyance rollers  7  and  8  are rotated in reverse in the direction of the arrows in  FIG. 29B . Then, the sheet is conveyed by the switch claw  9  as illustrated in  FIG. 29B  and, when the sensor S 2  detects the edge of the sheet, it is stopped. 
     When the second sheet in the second set is conveyed by the conveyance rollers  4  and  5  as illustrated in  FIG. 29C  and when the sensor S 2  detects the leading edge thereof, the conveyance rollers  6 ,  7 , and  8  are rotated in the direction of the arrows of  FIG. 29D  so that the two sheets are conveyed in a stacked manner. At this time, if the sensor S 2  detects the trailing edge of the sheet and if the alignment unit  18  is in a state for receiving the sheet, the sheet is continuously discharged. Conversely, if the alignment unit  18  is not in a state for receiving the sheet, the same operation as that for the first sheet is repeated. Thus, after the same operation as that for the first sheet is repeated on the second and subsequent sheets in the second set until the alignment unit  18  enters a state for receiving the sheet, the two or more sheets are discharged in a stacked manner. 
     By performing the above operation, post processing can be effectively performed without decreasing the productivity during an operation to staple two or more sets. 
     Furthermore, in the post-processing apparatus  201   b  according to the second modified example, the same configuration as that of the above-described fastener  210  can be used as the configuration of the fastener  12 , whereby the same advantages as that described above can be produced. Moreover, in the post-processing apparatus  201   b  according to the second modified example, after the fastener  12  is moved to the retracted position, a bundle of sheets on which pressure fastening has been performed is conveyed toward the discharge tray  3 . Thus, in the post-processing apparatus  201   b  according to the second modified example, a bundle of sheets can be conveyed toward the discharge tray  3  without sticking to the teeth of the fastener  12 . 
     Furthermore, in the above explanation, the fastener is moved by the moving mechanism  280  so that the fastener  210  is moved to the retracted position that is not opposed to the bundle of sheets Ps; however, the bundle of sheets Ps may be moved to retract the fastener  210 . Furthermore, both the fastener  210  and a bundle of sheets may be moved to retract the fastener  210  or locate it at the fastening position. 
     The above explanation is an example, and the present invention produces a specific advantage with respect to each of the following aspects. 
     Aspect 1 
     A sheet processing apparatus, such as the sheet post-processing apparatus  201 , includes a sheet fastening unit, such as the fastener  210 , of a pressing fastener method tin which a bundle of sheets is fastened by using a pair of pressing fastener members, such as the teeth  261 ; the conveying unit  290  that conveys the bundle of sheets Ps that are fastened by the sheet fastening unit; a separating unit (including the moving mechanism  280 , and the like, in the present embodiment) that, after the sheet fastening unit performs a fastening operation on the bundle of sheets, moves both one pressing and the other fastener members, between which the bundle of sheets is interposed, so as to separate a sheet that adheres to the pressing fastener member; and a control unit, such as the control unit  300 , that, after the separating unit finishes an operation to separate the sheet, controls the conveying unit  290  so as to convey the bundle of sheets Ps that are fastened by the sheet fastening unit. 
     According to aspect 1, after the sheet separating unit separates, from the pressing fastener member, the sheet that adheres to the pressing fastener member, a bundle of sheets, such as the bundle of sheets Ps, which are fastened by the sheet fastening unit, such as the fastener  210 , is conveyed. Thus, a conveyance failure or damage to sheets can be prevented. 
     Aspect 2 
     In (aspect 1), the separating unit includes a moving unit, such as the moving mechanism  280 , that moves a pair of the pressing fastener members, such as the teeth  261 , relative to the bundle of sheets from a fastening position at which the bundle of sheets is fastened, to a retracted position and, after the pair of pressing fastener members is moved to the retracted position, the control unit, such as the control unit  300 , controls the conveying unit  290  so as to convey the bundle of sheets that are fastened by the sheet fastening unit, such as the fastener  210 . 
     According to aspect 2, the sheet that adheres to the pressing fastener member can be separated by moving the pair of pressing fastener members, such as the teeth  261 , to the retracted position. Furthermore, during a conveyance of the bundle of sheets, such as the bundle of sheets Ps, that are fastened by the sheet fastening unit, such as the fastener  210 , the pair of pressing fastener members, such as the teeth  261 , is located in the retracted position. Thus, during a conveyance of the bundle of sheets on which a fastening operation has been performed, the bundle of sheets can be conveyed without sticking to the pressing fastener member. 
     Aspect 3 
     In (aspect 2), the separating unit includes a pressing unit, such as the pressing mechanism  270 , that presses the bundle of sheets, the moving unit, such as the moving mechanism  280 , moves the pair of pressing fastener members, such as the teeth  261 , parallel to a surface of the sheet, and the moving unit moves the pair of pressing fastener members while the pressing unit presses the bundle of sheets. 
     According to aspect 3, when the pair of pressing fastener members, such as the teeth  261 , is moved to the retracted positional relation, the sheet adhering to the pressing fastener member can be separated from the pressing fastener member without being moved together with the pressing fastener member. Thus, sheets can be prevented from being bent or damaged. 
     Aspect 4 
     In (aspect 3), the moving unit, such as the moving mechanism  280 , is configured such that the fastening unit, such as the fastener  210 , is capable of fastening the bundle of sheets, such as the bundle of sheets Ps at multiple locations of the bundle of sheets. 
     With this configuration, a fastening operation can be performed on a bundle of sheets at multiple locations by simply moving the moving unit, such as the fastener. 
     Aspect 5 
     An image forming system includes the image forming apparatus  101  that forms an image on a sheet, such as a sheet of paper; and a sheet processing unit, such as the sheet post-processing apparatus  201 , that processes a sheet that has an image formed by the image forming apparatus  101 , and the sheet processing unit is the sheet processing apparatus according to any one of (aspect 1) to (aspect 4). 
     With this configuration, the occurrence of a conveyance failure can be prevented. 
     Aspect 6 
     An image forming apparatus forms an image on a sheet, such as a sheet of paper, and it includes the sheet processing apparatus according to any one of (aspect 1) to (aspect 4) as a sheet processing unit that processes a sheet with an image formed thereon. 
     With this configuration, the occurrence of a conveyance failure can be prevented. 
     According to an embodiment, after a sheet separating unit separates, from a pressing fastener member, a sheet that adheres to the pressing fastener member, a bundle of sheets that are fastened by a sheet fastening unit is conveyed. Thus, a conveyance failure or damage to a sheet can be prevented. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.