Patent Publication Number: US-11661302-B2

Title: Sheet folding apparatus and image forming apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a sheet folding apparatus for performing folding processing on a sheet, and an image forming apparatus including a sheet folding mechanism for performing the folding processing on a sheet. 
     BACKGROUND ART 
     Conventionally, a sheet folding apparatus for performing folding processing at a predetermined position on a sheet on which an image is formed by an image forming apparatus such as a copying machine or a printer is widely known. The folding processing includes double folding in which a sheet is folded at a center position, triple folding in which a sheet is folded inward at two positions, and so-called Z-folding in which a sheet is alternately folded inward and outward to be folded in three. 
     There has been known a sheet folding apparatus in which a conveyance roller pair is provided on the upstream side of a horizontal upper guide plate, a folding roller pair is provided on the downstream side thereof, and a sheet guiding-deflecting member (i.e., a pushing member) is provided on the upstream side of the folding roller pair (e.g., see Patent Documents 1 and 2). 
     The apparatus described in Patent Document 1 includes a sheet conveyance unit for conveying a sheet, and a sheet folding unit for performing folding operation of the sheet conveyed by the sheet conveyance unit. In this document, the sheet is conveyed by the sheet conveyance unit in a state in which the front end of the sheet is nipped and stopped by the sheet folding unit, and the sheet is bent and slacked between the sheet conveyance unit and the sheet folding unit. There is disclosed a technique for forming Z-folding on the conveyed sheet by pushing a sheet guide unit (pushing member) into a slacked part of the sheet. 
     In the apparatus described in Patent Document 2, similarly to the apparatus described in Patent Document 1, a slacked part is formed in a sheet, and when the sheet guide unit is pushed into the slacked part, a roller arranged at the front end of the sheet guide unit is pressed against a guide for guiding the sheet. This document discloses a technique in which the sheet guide unit moves on the sheet along the guide while maintaining the pressed state and a folding position of the sheet is guided to the sheet folding unit. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2002-68583 
     Patent Document 2: Japanese Patent Application Laid-Open No. 2005-67741 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     However, in the sheet folding apparatus described in the above-mentioned patent document, in a state in which the sheet folding roller pair holds the front end of the sheet and stops, the sheet guiding unit pushes the part which is to be the folding line of the sheet and approaches the sheet folding roller pair. Accordingly, there is a problem that the load applied to the sheet guiding unit is large. 
     Furthermore, when the stiffness of the sheet is high or the thickness of the sheet is large, the load applied to the sheet guide unit is further increased. Therefore, there is a fear that the sheet guide unit is stopped in the middle of pushing and conveying the sheet to cause the part which is to be a fold line of the sheet to approach the folding roller pair and that a conveyance failure (jamming) is caused. 
     To solve such problems in the prior art, an object of the present invention is to provide a sheet folding apparatus capable of reducing the load acting on the sheet guide unit when the sheet is subjected to the folding processing. 
     Means for Solving the Problem 
     The present invention provides a sheet folding apparatus including a conveyance roller arranged on a conveyance path and configured to convey a sheet in a predetermined conveyance direction, a folding roller pair arranged on a downstream side of the conveyance roller in the conveyance direction and configured to nip a predetermined position of the sheet by a nip portion thereof and form a fold line, a pushing member configured to move to a pushing position to push the predetermined position of the sheet for the folding roller pair to nip the predetermined position, and a controller. Here, the controller controls operations of the conveyance roller, the folding roller pair, and the pushing member: so as to perform first folding processing for forming a first fold line owing to that the folding roller pair nips a first position of the sheet, which has a loop to be formed on an upstream side of the folding roller pair due to continuous conveyance of the sheet by the conveyance roller in a stopped state in which a front end of the sheet being conveyed from the conveyance roller is nipped by the folding roller pair at a nip portion thereof, as starting conveyance of the sheet from the stopped state while the pushing member is pushing the sheet and moving toward the pushing position; and so as to perform second folding processing for forming a second fold line, after the first folding processing, owing to that the folding roller pair nips a second position of the sheet with the loop formed. 
     Advantageous Effect of the Invention 
     According to the sheet folding apparatus of the present invention, since the sheet pushed by the pushing member toward the pushing position is conveyed at the same time by the folding roller pair, it is possible to reduce the load applied on the pushing member that pushes and moves the sheet. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an overall configuration view of an image forming system according to a preferred embodiment of the present invention. 
         FIG.  2    is a schematic configuration view of a sheet folding apparatus of  FIG.  1   . 
         FIG.  3    is a block diagram showing the control configuration of the sheet folding apparatus. 
         FIG.  4    is a flowchart for explaining a folding processing operation of the sheet folding apparatus. 
         FIGS.  5 A to  5 C  are views showing folding processing of the sheet folding apparatus in the order of steps. 
         FIGS.  6 A to  6 C  are views showing the folding processing following  FIG.  5 C  in the order of steps. 
         FIGS.  7 A and  7 B  are views showing the folding processing following  FIG.  6 C  in the order of steps. 
         FIG.  8    is a view showing the folding processing according to a modification of the present invention. 
         FIGS.  9 A and  9 B  are views showing the folding processing according to another modification of the present invention. 
         FIGS.  10 A and  10 B  are views showing the folding processing according to another modification of the present invention. 
         FIGS.  11 A and  11 B  are views showing a stopping position of a pushing plate. 
         FIGS.  12 A to  12 F  are views showing the folding processing of the sheet folding apparatus in the order of steps. 
         FIG.  13    is a flowchart for explaining the folding processing operation of the sheet folding apparatus. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, preferred embodiments of a sheet folding apparatus according to the present invention will be described in detail.  FIG.  1    shows the entire configuration of an image forming system including an image forming apparatus A which is a copying machine, a sheet folding apparatus B which is connected to a sheet discharge port of the image forming apparatus A, and a post-processing apparatus C which is connected to the downstream side of the sheet folding apparatus B. 
     [Image Forming Apparatus] 
     The image forming apparatus A of  FIG.  1    is an electrostatic printing apparatus as described below, and various structures such as a copying machine, a printer, and a printing machine can be employed. The image forming apparatus A includes a sheet feeding unit  2 , a printing unit  3 , a sheet discharge unit  4 , and a controller inside a casing. A plurality of cassettes corresponding to the sheet size are prepared in the sheet feeding unit  2 , and the sheet having the size instructed by the controller is fed out to a sheet feeding path  6 . Registration rollers  7  are arranged on the sheet feeding path  6 , and after the sheet is aligned at the front end, the sheet is fed to the printing unit  3  on the downstream side at a predetermined timing. 
     An electrostatic drum  10  is arranged in the printing unit  3 , and a printing head  9 , a developing device  11 , a transfer charger  12 , and the like are arranged around the electrostatic drum  10 . The printing head  9  is configured of, for example, a laser light emitting device, and forms an electrostatic latent image on the electrostatic drum  10 , causes toner ink to adhere to the latent image with the developing device  11 , and performs printing on the sheet with the transfer charger  12 . The printed sheet is fixed by a fixing device  13  and is discharged to a sheet discharge path  17 . A sheet discharge port  14  formed in the casing and sheet discharge rollers  15  are arranged in the sheet discharge unit  4 . In  FIG.  1   , a reference numeral  16  denotes a circulation path, and after the sheet discharged from the sheet discharge path  17  is turned upside down in a switchback path, the sheet is fed to the registration rollers  7  again, and an image is formed on a back surface of the sheet. The printed sheet on which the image has been formed on one side or both sides in this manner is discharged from the sheet discharge port  14  by the sheet discharge rollers  15 . 
     The image forming apparatus A includes, on the casing, a scanner unit  20  for optically reading a document image to be printed by the printing head  9 . The scanner unit  20  includes, as is generally known, a platen  23  on which a document sheet is to be placed, a carriage  21  which scans the document image along the platen  23 , and an optical reading unit (e.g., a CCD device)  22  which photoelectrically converts an optical image sent from the carriage  21 . As shown in  FIG.  1   , a document feeding apparatus  25  which automatically feeds the document sheet to the platen  23  is arranged on the platen  23 . 
     [Sheet Folding Apparatus] 
     The sheet folding apparatus B shown in  FIG.  1    is connected to the sheet discharge port  14  of the image forming apparatus A, and performs folding processing on the sheet on which an image is formed by the image forming apparatus A and discharged. As shown in  FIG.  1   , the sheet folding apparatus B includes a conveyance roller pair  110  arranged on the upstream side along the conveyance path and a folding roller pair  111  arranged on the downstream side thereof. 
     As shown in  FIG.  2   , the conveyance roller pair  110  includes an upper conveyance roller  110   a  and a lower conveyance roller  110   b  each formed of a rubber roller. The lower conveyance roller  110   b  is arranged so as to face the upper conveyance roller  110   a , is pressed against the upper conveyance roller  110   a  by an elastic force of a spring (not shown), and is rotated in a driven manner. The upper conveyance roller  110   a  is connected to a conveyance drive motor of a conveyance drive mechanism (not shown), and is rotated by the rotation of the conveyance drive motor. The conveyance roller pair  110  is rotationally driven in the conveyance direction, and passes the sheet discharged and received from the image forming apparatus A on the upstream side to the downstream along the conveyance path. 
     The folding roller pair  111  includes an upper folding roller  111   a  and a lower folding roller  111   b  each formed of a rubber roller. The lower folding roller  111   b  is arranged so as to face the upper folding roller  111   a , is pressed against the upper folding roller  111   a  by an elastic force of a spring (not shown), and is rotated in a driven manner. The upper folding roller  111   a  is connected to a folding drive motor of a folding drive mechanism (not shown), and is driven and rotated by the rotation of the conveyance drive motor in an appropriate manner for forward/reverse rotation, driving speed, and the like. 
     A sensor  118  is arranged above the upstream side of the folding roller pair  111 . The sensor  118  can detect the conveyance timing of the front end of the sheet, thereby controlling the timing of stopping the sheet being conveyed along the conveyance path. Specifically, when the sensor  118  detects the conveyance timing of the front end of the sheet, and the front end of the sheet is determined to have reached the center of the nip portion of the folding roller pair  111 , the subsequent rotation of the folding roller pair  111  is stopped. 
     An upper conveyance guide  113  is arranged between the conveyance roller pair  110  and the folding roller pair  111 . The upper conveyance guide  113  is formed from a position right after the conveyance roller pair  110  to a position above a pushing plate (pushing member)  115  so as to guide the front end of the sheet from the conveyance roller pair  110  to the pushing plate  115 . The upper conveyance guide  113  is for regulating the flow of the conveyed sheet, is arranged on the upper side of the conveyance face of the conveyance path, and has a shape bent downward toward the downstream side. 
     A lower conveyance guide  114  is arranged below the upper conveyance guide  113 . The lower conveyance guide  114  is for regulating the flow of the conveyed sheet in cooperation with the upper conveyance guide  113 , is arranged on the lower side of the conveyance path, and has a shape bent downward toward the downstream side in correspondence with the upper conveyance guide  113 . A gap is formed on the downstream side of the lower conveyance guide  114 , and the conveyance path is opened downward. 
     On the downstream side of the upper conveyance guide  113 , an upper folding guide  116  is arranged above the conveyance path so as to extend over the upstream and downstream sides of the upper folding roller  111   a . The upper folding guide  116  is formed so as to guide the front end and the sheet folding portion of the sheet conveyed along the conveyance path to the folding roller pair  111 . 
     Below the conveyance path facing the upper folding guide  116 , a lower folding guide  117  is arranged so as to extend over the upstream and downstream sides of the lower folding roller  111   b . The lower folding guide  117  forms, at the upstream side of the folding roller pair  111 , a horizontal surface parallel to the conveyance path in the conveyance direction and an inclined surface so as to guide the front end and the sheet folding part of the sheet conveyed along the conveyance path to the nip portion of the folding roller pair  111  in corporation with the upper folding guide  116 . 
     The pushing plate  115  is arranged between the conveyance roller pair  110  and the folding roller pair  111  so as to be movable in parallel with the conveyance path at the upstream side of the folding roller pair  111 . The pushing plate  115  is connected to a pushing plate drive motor of a pushing plate drive mechanism (not shown), and moves between the upstream side position at the lower side of the lower conveyance guide  114  and the downstream side position in front of the folding roller pair  111  along the conveyance direction by driving the pushing plate drive motor. 
     [Control Unit] 
       FIG.  3    shows the configuration of the controller of the sheet folding apparatus B. The controller includes a folding control unit  301  for controlling folding processing of the sheet in the sheet folding apparatus B. The folding control unit  301  includes a control unit  311 , a sheet thickness recognition unit  312 , and a holding force adjustment unit  313 , and is configured by, for example, a CPU. The folding control unit  301  loads a folding control program  302  stored in a ROM, and performs the folding processing while storing temporary information in a storage unit  303  as necessary. 
     The sensor  304  shown in  FIG.  3    includes a sheet position detection unit (not shown). An input unit  305  includes an input interface such as a switch to enable an operation on the folding control unit  301 . A communication unit  309  includes a serial communication interface for communicating with the image forming apparatus A or the like, such as a UART. 
     The folding control unit  301  can transmit signals to a conveyance motor  306 , a folding motor  307 , and a pushing motor  308  to control driving thereof. According to the control signals from the folding control unit  301 , the conveyance motor  306  can drive the conveyance roller pair  110 , the folding motor  307  can drive the folding roller pair  111 , and the pushing motor  308  can drive the pushing plate  115 , respectively. 
     The folding control unit  301  acquires information relating to the sheet to be conveyed, such as the stiffness and the thickness of the sheet discharged from the image forming apparatus A. Such information is input before the folding processing of the sheet discharged from the image forming apparatus A starts in the sheet folding apparatus B. The information is basically acquired by communication from the image forming apparatus A via the communication unit  309 , but can also be acquired from a sensor or the like arranged in the sheet folding apparatus B. 
     [Folding Processing Operation] 
     The folding processing operation of the sheet by the folding control unit  301  will be described with reference to the flowchart of  FIG.  4   . In the folding processing operation, first folding processing of forming a first fold line of Z-folding on a sheet and second folding processing of forming a second fold line of Z-folding on the sheet are continuously performed in a series of operations. 
     First, the conveyance roller pair  110  is rotationally driven (step St 01 ), and the sheet is conveyed to the folding roller pair  111 . At this time, the pushing plate  115  is arranged so as to fill the gap between the lower conveyance guide  114  and the lower folding guide  117  and to guide the front end of the sheet to the lower folding guide  117 . 
     Next, the folding roller pair  111  is rotationally driven (step St 02 ) to nip the front end of the sheet. When the sensor  118  is turned on (Yes in step St 03 ), after counting a predetermined value with the counter, it is recognized that the front end of the sheet is nipped by the folding roller pair  111 , and folding roller pair  111  is stopped (step St 04 ). 
     In order to form Z-folding on the sheet, the pushing plate  115  is moved in parallel from a position between the lower conveyance guide  114  and the lower folding guide  117  to a retracting position below the lower conveyance guide  114 . Thus, the gap is formed between the lower conveyance guide  114  and the lower folding guide  117 , and a loop space for creating a loop on the sheet is defined between the conveyance roller pair  110  and the folding roller pair  111  below the gap (step St 05 ). 
     When the conveyance of the sheet by the conveyance roller pair  110  reaches a predetermined amount after the pushing plate  115  moves to the retracting position, the pushing plate  115  starts horizontal movement toward the folding roller pair  111  (step St 06 ). The sheet is fed by the conveyance roller pair  110  while the folding roller pair  111  is kept stopped, thereby forming a loop shape that hangs down from the gap to the loop space. The pushing plate  115  is pushed in toward the nip portion of the folding roller pair  111  while pushing the loop-shaped sheet, so that the first fold line of the Z-folding is formed on the sheet. 
     Here, when the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or the thickness of the sheet is larger than a predetermined value (Yes in step St 07 ), the folding roller pair  111  is rotationally driven in the conveyance direction at the time when the pushing plate  115  advances to a predetermined position in front (i.e., at the upstream side) of a stopping position set near the nip portion of the folding roller pair  111  (step St 08 ). As a result, the front end of the sheet nipped by the folding roller pair  111  is conveyed to the downstream side. The pushing plate  115  enters the nip portion of the folding roller pair  111  so as to be guided to the sheet conveyed to the downstream side by the folding roller pair  111 . 
     Thus, by the auxiliary operation in which the sheet pushed by the pushing plate  115  is conveyed by the folding roller pair  111  at the same time, the load applied to the pushing plate  115  to push the sheet is reduced as compared with the case in which the auxiliary operation is not performed. Further, as a criterion for determining that the stiffness or thickness of the sheet is larger than a predetermined value, for example, it is possible to set a case in which the basis weight of the sheet exceeds 91 g/m 2 . 
     When the fold line of the sheet formed by pushing the loop-shaped sheet as described above reaches the stopping position near the nip portion of the folding roller pair  111 , the horizontal movement of the pushing plate  115  toward the nip portion of the folding roller pair  111  is stopped (step St 09 ). At the same time, the rotational drive of the folding roller pair  111  is also stopped (step St 10 ). 
     Thereafter, while the pushing plate  115  is kept stopped, the folding roller pair  111  is rotationally driven in the reverse direction, that is, the direction opposite to the conveyance direction (step St 11 ). As a result, only the front end of the sheet is returned in the direction opposite to the conveyance direction without shifting the position of the fold line formed by the pushing plate  115  in the conveyance direction. 
     Next, when the front end of the sheet reaches a predetermined reverse rotation stopping position, the reverse rotation operation of the folding roller pair  111  is stopped (step St 12 ). Thereafter, the folding roller pair  111  is rotationally driven in the forward direction, that is, in the conveyance direction again and the sheet is conveyed in the conveyance direction, thereby forming the second fold line of the Z-folding on the sheet (step St 13 ). Thereafter, the pushing plate  115  is moved from the stopped state to the retracting position at the upstream side again (step St 14 ), and the series of folding processing operations is completed. 
     In step St 07 , when it is determined that the stiffness or thickness of the sheet is equal to or smaller than the predetermined value (No in Step St 07 ), when the pushing plate  115  reaches the stopping position near the folding roller pair  111 , the folding roller pair  111  is rotatably driven in the conveyance direction to convey the sheet in the conveyance direction (Step St 13 ). Thus, the position of the fold line of the sheet pushed by the pushing plate  115  is nipped by the folding roller pair  111  and the first fold line of the Z-folding is formed on the sheet. In this case, the folding roller pair  111  is rotationally driven in the conveyance direction when the pushing plate  115  has advanced to the predetermined position in front of the stopping position, and the auxiliary operation of conveying the sheet to the downstream side is not performed. Thereafter, the pushing plate  115  is moved from the stopping position to the retracting position at the upstream side again (step St 14 ), and the series of folding processing operations is completed. 
     Next, the folding processing operation in the case in which the stiffness or thickness of the sheet is larger than the predetermined value will be described in detail with reference to  FIGS.  5 A to  7 B .  FIGS.  5 A to  7 B  show a process in which the sheet folding apparatus B introduces the sheet from the image forming apparatus A and performs the folding processing while conveying the sheet in the order of steps. 
     First, as shown in  FIG.  5 A , owing to the rotational drive of the conveyance roller pair  110  in the conveyance direction, the sheet discharged from the image forming apparatus A on the upstream side passes through the conveyance roller pair  110  and is conveyed to the downstream side on the conveyance path between the upper conveyance guide  113  and the lower conveyance guide  114 . 
     As shown in  FIG.  5 B , the sheet is conveyed through the conveyance path between the upper conveyance guide  113  and the pushing plate  115  and the conveyance path between the upper folding guide  116  and the lower folding guide  117 . When the front end of the sheet is detected by the sensor  118  and further conveyed by a predetermined amount after being nipped by the folding roller pair  111 , the rotational drive of the folding roller pair  111  is stopped. 
     Next, as shown in  FIG.  5 C , in a state in which the front end of the sheet is nipped and held by the folding roller pair  111 , the pushing plate  115  is moved to the retracting position to the upstream side, and the loop space for forming a loop on the sheet is defined on the lower side between the lower conveyance guide  114  and the lower folding guide  117 . By continuing the rotational drive of the conveyance roller pair  110  even thereafter, the loop hanging down in the loop space is formed on the sheet. 
     As shown in  FIG.  6 A , the sheet is fed by the conveyance roller pair  110  by a predetermined amount in the conveyance direction to form the loop. Thereafter, the pushing plate  115  is horizontally moved from the retracting position to the downstream side. 
     Next, as shown in  FIG.  6 B , when the pushing plate  115  reaches a predetermined position while moving toward the nip portion of the folding roller pair  111  during pushing the sheet, the folding roller pair  111  is rotationally driven in the conveyance direction. The timing for starting the rotational operation of the folding roller pair  111  is preferably set before the load received from the sheet by the pushing plate  115  which is moving toward the nip portion of the folding roller pair  111  becomes the largest. 
     As a result, the sheet is conveyed such that the front end thereof advances to the downstream side of the nip portion of the folding roller pair  111 . At this time, as shown in  FIG.  6 B , the loop on the sheet is located below the pushing plate  115 , and the lower part of the loop is conveyed to the downstream side by the folding roller pair  111  while the upper part of the loop is pushed by the pushing plate  115  and moves to the downstream side. Thus, since the pushing plate  115  enters the nip portion of the folding roller pair  111  so as to be guided by the sheet being conveyed to the downstream side by the folding roller pair  111  at the same time while pushing the sheet, the load applied to the pushing plate  115  is reduced by the pushing operation on the sheet. It is preferable that the speed at which the pushing plate  115  pushes and moves the sheet and the speed at which the folding roller pair  111  conveys the sheet are set to be the same. 
     The sheet is pushed into a vertical gap between the upper folding guide  116  and the lower folding guide  117  by the pushing plate  115  in a state in which the part of the sheet against which the front end thereof abuts is folded, so that the first fold line of the Z-folding is formed on the sheet. As shown in  FIG.  6 C , when the first fold line of the sheet is pushed by the pushing plate  115  and reaches the vicinity of the nip portion of the folding roller pair  111 , the operations of the pushing plate  115  and the folding roller pair  111  are stopped. 
     Next, while the pushing plate  115  is kept stopped, the folding roller pair  111  is rotationally driven in the reverse direction, that is, the direction opposite to the conveyance direction. As a result, as shown in  FIG.  7 A , only the front end of the sheet can be returned in the direction opposite to the conveyance direction in a state in which the position of the first fold line is kept unchanged. When the front end of the sheet reaches a predetermined position downstream of the nip portion of the folding roller pair  111 , the reverse rotation operation of the folding roller pair  111  is stopped. 
     In  FIG.  7 A , the reverse rotation operation of the folding roller pair  111  is stopped when the front end of the sheet reaches a position right in front of the nip portion at the downstream side of the folding roller pair  111 . However, the timing of stopping the reverse rotation operation of the folding roller pair  111  is not limited thereto, and can be set to a different position as long as a state in which the front end of the sheet is at the downstream side of the nip portion and the nipped state is maintained. 
     Thereafter, as shown in  FIG.  7 B , while moving the pushing plate  115  from the stopped state to the retracting position, the folding roller pair  111  is rotationally driven in the conveyance direction. Thus, the first fold line of the sheet is nipped and folded by the folding roller pair  111 , and the first folding processing is completed. 
     Thereafter, the sheet is further conveyed in the conveyance direction, thereby a part forming a loop at the upstream side of the folding roller pair  111  is squeezed from the above and below between the upper folding guide  116  and the lower folding guide  117 . Finally, the squeezed part of the sheet is overlapped with the rear end side of the sheet, nipped and folded by the folding roller pair  111 , a second fold line is formed, and the second folding processing is completed. 
     [Control of Folding Processing Operation in Accordance with Thickness of Sheet] 
     Since the stiffness of the sheet is increased in accordance with the thickness thereof, even if the timing of starting the drive of the folding roller pair  111  is set early as described in relation to step St 08  of  FIG.  4   , the upper surface of the sheet bent by the pushing of the front end of the pushing plate  115  may warp and come into contact with the lower surface of the upper folding guide  116  to cause a large resistance. As a result, there is a fear that the load applied to the pushing plate  115  cannot be sufficiently reduced. Therefore, it is preferable to change and adjust the starting timing of the drive of the folding roller pair  111  in step St 08  of  FIG.  4    in accordance with the thickness and stiffness of the sheet acquired from the communication unit  309 . 
     Specifically, when the sheet thickness recognition unit  312  of the folding control unit  301  recognizes the thickness of the sheet to be introduced from the image forming apparatus A to be equal to or larger than a second predetermined thickness that is further thicker than the predetermined thickness, the rotational driving of the folding roller pair  111  in the conveyance direction is started when the front end of the pushing plate  115  advances to a second predetermined position (Ps 403 ) shown in  FIG.  8    in the conveyance direction. The predetermined thickness of the sheet is set to basis weight of the sheet of 91 g/m 2  in the embodiment described above, but here, the second predetermined thickness which is thicker than the predetermined thickness can be set to a criterion, for example, as a case of the basis weight of the sheet of 105 g/m 2 . 
       FIG.  8    shows the position of the pushing plate  115  corresponding to the thickness of the sheet and the sheet conveyance amount (conveyance direction length) in the conveyance direction by the folding roller pair  111 . In  FIG.  8   , a stopping position Ps 401  of the pushing plate  115  is set near the nip portion of the folding roller pair  111 . As shown in  FIG.  8   , when the thickness of the sheet is larger than the predetermined thickness and does not exceed the second predetermined thickness, a predetermined position Ps 402  is set to the upstream side of the conveyance direction with respect to the stopping position Ps 401 . The second predetermined position Ps 403  is set further upstream in the conveyance direction with respect to the predetermined position Ps 402 . 
     In the case that the thickness of the sheet is larger than the predetermined thickness, in accordance with the thickness, the rotational drive of the folding roller pair  111  is started when the front end of the pushing plate  115  reaches the predetermined position Ps 402  or the second predetermined position Ps 403 , and the rotational drive of the folding roller pair  111  is stopped when the pushing plate  115  reaches the stopping position Ps 401 . 
       FIG.  9 A  shows a conveyance amount CR 1  of the sheet at the time when the folding roller pair  111  is started to be driven when the front end of the pushing plate  115  reaches the predetermined position Ps 402  in step St 08  of  FIG.  4   .  FIG.  9 B  shows a conveyance amount CR 2  of the sheet at the time when the folding roller pair  111  is started to be driven when the front end of the pushing plate  115  reaches the predetermined position Ps 403 . As can be seen from the comparison between  FIGS.  9 A and  9 B , when the starting timing of the drive of the folding roller pair  111  corresponds to the second predetermined position Ps 403  ( FIG.  9 B ), the conveyance amount CR 2  of the sheet by the folding roller pair  111  is larger than the conveyance amount CR 1  when the starting timing of the drive of the folding roller pair  111  corresponds to the predetermined position Ps 402  ( FIG.  9 A ). 
     When the rotational drive of the folding roller pair  111  is started before the fold line of the sheet formed and pushed out by the pushing of the pushing plate  115  reaches the nip portion of the folding roller pair  111 , the front end of the sheet is conveyed to the downstream side with respect to the fold line of the sheet in the conveyance direction. Therefore, in order to return the front end of the sheet to the position of the fold line of the sheet in the conveyance direction, the folding roller pair  111  may be reversely driven in a state that the pushing plate  115  is stopped at the stopping position Ps 401 . Thus, the front end position of the sheet can be aligned with or brought close to the fold line position of the sheet. 
       FIGS.  10 A and  10 B  show the reverse-sheet conveyance amount in the opposite direction to the conveyance direction when the folding roller pair  111  is reversely driven in step St 11  of  FIG.  4   .  FIG.  10 A  shows a reverse-conveyance amount CB 1  by the folding roller pair  111  when the starting timing of the drive of the folding roller pair  111  corresponds to the predetermined position Ps 402  in step St 08  of  FIG.  4 A .  FIG.  10 B  shows a reverse-conveyance amount CB 2  by the folding roller pair  111  when the starting timing of the drive of the folding roller pair  111  corresponds to the second predetermined position Ps 403 . As can be seen from the comparison between  FIGS.  10 A and  10 B , in accordance with the conveyance amounts CR 1 , CR 2  of the sheet in step St 08 , the reverse-conveyance amount by the folding roller pair  111  is larger when the starting timing of the drive of the folding roller pair  111  corresponds to second predetermined position Ps 403  than that when the starting timing of the drive of the folding roller pair  111  corresponds to the predetermined position Ps 402 . 
     As described above in relation to step St 11  of  FIG.  4   , when the folding roller pair  111  is reversely driven, the pushing plate  115  is held in the stopped state at the stopping position. Specifically, by applying a hold current to the pushing motor  308  by the holding force adjustment unit  313  of the folding control unit  301 , it is possible to reliably hold the pushing plate  115  at the stopping position. The hold current is preferably set in accordance with various external forces acting on the pushing plate  115  in the stopped state at the stopping position. In the present embodiment, the hold current is output by the holding force adjustment unit  313  in accordance with the stiffness or thickness of the sheet acquired from the communication unit  309 , and when the thickness of the sheet is recognized to be equal to or larger than the predetermined thickness or the second predetermined thickness, the holding current is output higher than when it recognized to be smaller than the predetermined thickness. 
     As described above, as a method of reducing the load applied to the pushing plate  115  when the folding processing is performed on the sheet having the stiffness or thickness larger than the predetermined value, the starting timing of the drive of the folding roller pair  111  is changed. As described above, when the folding processing is performed on the sheet having the stiffness or thickness larger than the predetermined value, in order to reduce the load for the pushing plate  115  to push the sheet, the folding roller pair  111  is rotationally driven in the conveyance direction when the pushing plate  115  reaches the predetermined position in the middle of moving toward the nip portion of the folding roller pair  111  while pushing the sheet. Therefore, the front end of the sheet advances to the downstream side of the nip portion of the folding roller pair  111 , and it is necessary to rotationally drive the folding roller pair  111  in the reverse direction, that is, in the direction opposite to the conveyance direction while the pushing plate  115  is kept stopped at the position where the pushing plate  115  reaches the vicinity of the nip portion of the folding roller pair  111 . 
     However, if the pushing plate  115  is stopped at the position being the same as in the case in which the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or thickness of the sheet is equal to or smaller than the predetermined value, when the folding roller pair  111  is reversely rotated, there is no sufficient gap between the front end of the pushing plate  115  and the vicinity of the nip portion of the folding roller pair  111 , a load is applied to the sheet being reversely conveyed, and there is a case in which the reverse conveyance cannot be performed. Therefore, the description will be provided below on the stopping position of the pushing plate  115  and control for stopping the pushing plate  115  when folding processing is performed on the sheet having the stiffness or thickness larger than the predetermined value. 
       FIGS.  11 A and  11 B  show, in the case that the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or thickness of the sheet is equal to or smaller than the predetermined value, the position (first pushing position) of the pushing plate  115  where the pushing plate  115  pushes the fold line of the sheet to the vicinity of the nip portion and stops, and the position (second pushing position) of the pushing plate  115  where the pushing plate  115  pushes the sheet having the stiffness of thickness larger than the predetermined value and stops when the front end of the sheet is conveyed in the direction opposite to the conveyance direction by reverse rotation of the folding roller pair  111 . As described above, below the conveyance path facing the upper folding guide  116 , the lower folding guide  117  is arranged so as to extend over the upstream and downstream sides of the lower folding roller  111   b . The lower folding guide  117  includes, at the upstream side of the folding roller pair  111 , an inclined portion in which the width in the thickness direction of the sheet is gradually narrowed along the conveyance direction and a parallel portion in which the width in the thickness direction of the sheet is constant along the conveyance direction, so as to guide the front end and the sheet folding part of the sheet conveyed along the conveyance path to the nip portion of the folding roller pair  111  in corporation with the upper folding guide  116 . 
     The first pushing position is the position where the front end of the pushing plate  115  is in the region of the parallel portion of the conveyance path and the fold line of the sheet can be delivered to the folding roller pair  111 . When the pushing plate  115  is stopped at the first pushing position, the front end of the pushing plate  115  does not contact the nip portion of the folding roller pair  111 . The second pushing position is the position where the front end of the pushing plate  115  is in the region of the parallel portion of the conveyance path and is located on the upstream side in the conveyance direction with respect to the first pushing position. The second pushing position can be appropriately set between the downstream side of the inclined portion of the conveyance path in the conveyance direction and the first pushing position. However, it is preferable that a gap in which the fold line of the sheet cannot be delivered to the folding roller pair  111  is secured between the front end of the pushing plate  115  and the nip portion of the folding roller pair  111 . For example, as shown in  FIGS.  11 A and  11 B , the front end of the pushing plate  115  may be stopped at the position corresponding to the outer periphery of the folding roller pair  111  when viewed in the conveyance direction, and the position may be set as the second pushing position. By stopping at this position, the sufficient gap is secured between the front end of the pushing plate  115  and the nip portion of the folding roller pair  111 . Thus, it is possible to reduce the load applied when the folding roller pair  111  is reversely rotated and the sheet is conveyed in the direction opposite to the conveyance direction. 
     Next, description will be provided, with reference to  FIGS.  12 A to  12 F , on a folding processing operation to temporarily stop the pushing plate  115  at the second pushing position in the case in which the stiffness or thickness of the sheet is larger than the predetermined value.  FIGS.  12 A to  12 F  show a process in which the sheet folding apparatus B introduces the sheet from the image forming apparatus A and performs the folding processing while conveying the sheet in the order of steps. 
     As shown in  FIG.  12 A , owing to the rotational drive of the conveyance roller pair  110  in the conveyance direction, the sheet discharged from the image forming apparatus A on the upstream side is conveyed to the downstream side on the conveyance path between the upper conveyance guide  113  and the lower conveyance guide  114 . Further, the sheet is conveyed through the conveyance path, located at the downstream side of the above, between the upper conveyance guide  113  and the pushing plate  115  and the conveyance path between the upper folding guide  116  and the lower folding guide  117 . When the front end of the sheet is detected by the sensor  118  and further conveyed by a predetermined amount after being nipped by the folding roller pair  111 , the rotational drive of the folding roller pair  111  is stopped. 
     Next, as shown in  FIG.  12 B , in a state in which the front end of the sheet is nipped and held by the folding roller pair  111 , the pushing plate  115  is moved to the retracting position to the upstream side, and the loop space for forming a loop on the sheet is defined on the lower side between the lower conveyance guide  114  and the lower folding guide  117 . Thereafter, by rotationally driving the conveyance roller pair  110  in a state in which the front end of the sheet is nipped by the folding roller pair  111 , the loop hanging down in the loop space is formed on the sheet. 
     Thereafter, the pushing plate  115  is moved from the retracting position to the downstream side in the same manner as described with reference to  FIGS.  6 A to  6 C . As the pushing plate  115  moves, the front end of the pushing plate  115  moves toward the nip portion of the folding roller pair  111  while pushing the sheet. When the pushing plate  115  reaches a predetermined position in the middle of moving, the folding roller pair  111  is rotationally driven in the conveyance direction. The timing for starting the rotational operation of the folding roller pair  111  is preferably set to the timing before the load received from the sheet by the pushing plate  115  which is moving toward the nip portion of the folding roller pair  111  becomes the largest. 
     As a result, as shown in  FIG.  12 C , the sheet is conveyed such that the front end thereof advances to the downstream side of the nip portion of the folding roller pair  111 . At this time, as shown in  FIG.  12 C , the loop on the sheet is located below the pushing plate  115 , and the lower part of the loop is conveyed to the downstream side by the folding roller pair  111  while the upper part of the loop is pushed by the pushing plate  115  and moves to the downstream side. Thus, since the pushing plate  115  enters the nip portion of the folding roller pair  111  so as to be guided by the sheet being conveyed to the downstream side by the folding roller pair  111  at the same time while pushing the sheet, the load applied to the pushing plate  115  is reduced by the pushing operation of the sheet. It is preferable that the speed at which the pushing plate  115  pushes and moves the sheet and the speed at which the folding roller pair  111  conveys the sheet are set to be the same. 
     The sheet is pushed into the parallel portion between the upper folding guide  116  and the lower folding guide  117  by the pushing plate  115  in a state in which the part of the sheet against which the front end thereof abuts is folded, so that the first fold line of the Z-folding is formed on the sheet. When the first fold line of the sheet is pushed by the pushing plate  115  and reaches the second pushing position, the operations of the pushing plate  115  and the folding roller pair  111  are stopped. 
     Next, while the pushing plate  115  is stopped, the folding roller pair  111  is rotationally driven in the reverse direction, that is, the direction opposite to the conveyance direction. As a result, as shown in  FIG.  12 D , only the front end of the sheet can be returned in the direction opposite to the conveyance direction in a state in which the position of the first fold line is kept unchanged. Moreover, since the pushing plate  115  is stopped in a state of being stopped at the second pushing position, a predetermined distance is secured between the front end of the pushing plate  115  and the nip portion of the folding roller pair  111 , and when returning only the front end of the sheet in the direction opposite to the conveyance direction, the load applied on the sheet can be reduced. When the front end of the sheet reaches the predetermined position downstream of the nip portion of the folding roller pair  111 , the reverse rotation operation of the folding roller pair  111  is stopped. 
     In  FIG.  12 D , the reverse rotation operation of the folding roller pair  111  is stopped when the front end of the sheet reaches the nip portion of the folding roller pair  111 . However, the timing of stopping the reverse rotation operation of the folding roller pair  111  is not limited thereto, and can be set to a different position as long as a state in which the front end of the sheet is at the downstream side of the nip portion and the nipped state is maintained. 
     Thereafter, as shown in  FIG.  12 E , at the same time as driving the conveyance roller pair  110  in the conveyance direction, the pushing plate  115  is moved from the second pushing position to the first pushing position. Accordingly, the first fold line of the sheet approaches the vicinity of the nip portion of the folding roller pair  111 , and it becomes possible to deliver the first fold line of the sheet to the folding roller pair  111 . Thereafter, as shown in  FIG.  12 F , by rotating the folding roller pair  111  in the forward rotation, that is, in the conveyance direction, the first fold line of the sheet is nipped and folded by the folding roller pair  111 , and the first folding processing is completed. 
     Thereafter, the sheet is further conveyed in the conveyance direction, thereby a part forming a loop at the upstream side of the folding roller pair  111  is squeezed from the above and below between the upper folding guide  116  and the lower folding guide  117 . Finally, the squeezed part of the sheet is overlapped with the rear end side of the sheet, nipped and folded by the folding roller pair  111 , the second fold line is formed, and the second folding processing is completed. 
     [Folding Processing Operation] 
     Description will be provided, with reference to the flowchart of  FIG.  13   , on the folding processing operation to temporarily stop the pushing plate  115  at the second pushing position in the case in which the stiffness or thickness of the sheet is larger than the predetermined value. In the folding processing operation, the first folding processing of forming the first fold line of Z-folding on the sheet and the second folding processing of forming the second fold line of Z-folding are continuously performed in a series of operations. 
     First, the conveyance roller pair  110  is rotationally driven (step St 101 ), and the sheet is conveyed to the folding roller pair  111 . At this time, the pushing plate  115  is arranged so as to fill the gap between the lower conveyance guide  114  and the lower folding guide  117  and to guide the front end of the sheet to the lower folding guide  117 . 
     Next, the folding roller pair  111  is rotationally driven in the conveyance direction (step St 102 ). When the sensor  118  is turned on (Yes in step St 103 ), after counting a predetermined value with the counter, it is recognized that the front end of the sheet is nipped by the folding roller pair  111 , and folding roller pair  111  is stopped (step St 104 ). 
     In order to form the Z-folding on the sheet, the pushing plate  115  is moved in parallel from the position between the lower conveyance guide  114  and the lower folding guide  117  to the retracting position below the lower conveyance guide  114 . Thus, the gap is formed between the lower conveyance guide  114  and the lower folding guide  117 , and the loop space for creating a loop on the sheet is defined between the conveyance roller pair  110  and the folding roller pair  111  below the gap (step St 105 ). 
     When the conveyance of the sheet by the conveyance roller pair  110  reaches a predetermined amount after the pushing plate  115  moves to the retracting position, the pushing plate  115  starts horizontal movement toward the folding roller pair  111  (step St 106 ). The sheet is fed by the conveyance roller pair  110  while the folding roller pair  111  is stopped, thereby forming a loop shape that hangs down from the gap to the loop space. The pushing plate  115  is pushed toward the nip portion of the folding roller pair  111  while pushing the loop-shaped sheet, so that the first fold line of the Z-folding is formed on the sheet. 
     Here, when the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or the thickness of the sheet is larger than a predetermined value (Yes in step St 107 ), the folding roller pair  111  is rotationally driven in the conveyance direction when the pushing plate  115  advances to a predetermined position in front (i.e., at the upstream side) of a stopping position set near the nip portion of the folding roller pair  111  (step St 108 ). As a result, the front end of the sheet nipped by the folding roller pair  111  is conveyed to the downstream side. 
     Thus, by the auxiliary operation in which the sheet pushed by the pushing plate  115  is conveyed by the folding roller pair  111  at the same time, the load applied to the pushing plate  115  by the pushing of the sheet is reduced as compared with the case in which the auxiliary operation is not performed. Further, as a criterion for determining that the stiffness or thickness of the sheet is larger than a predetermined value, for example, it is possible to set a case in which the basis weight of the sheet exceeds 91 g/m 2 . 
     When the first fold line of the loop-shaped sheet is pushed by the pushing plate  115  and the front end of the pushing plate  115  reaches the second pushing position, the operation of the pushing plate  115  toward the nip portion of the folding roller pair  111  is stopped. At the same time, the rotational drive of the folding roller pair  111  and the conveyance roller pair  110  is also stopped (step St 109 ). 
     Thereafter, while the pushing plate  115  is stopped, the folding roller pair  111  is rotationally driven in the reverse direction, that is, the direction opposite to the conveyance direction (step St 110 ). As a result, only the front end of the sheet is returned in the direction opposite to the conveyance direction without shifting the position of the first fold line pushed by the pushing plate  115  in the direction opposite to the conveyance direction. 
     Next, when the front end of the sheet reaches a predetermined reverse rotation stopping position, the reverse rotation operation of the folding roller pair  111  is stopped (step St 111 ). Thereafter, the pushing plate  115  starts to move from the second pushing position to the first pushing position. At the same time, the conveyance roller pair  110  is rotationally driven in the conveyance direction (step St 112 ). When the front end of the pushing plate  115  reaches the first pushing position, the rotational drive is stopped (step St 113 ). The folding roller pair  111  is rotationally driven in the forward direction, that is, in the conveyance direction again and the sheet is conveyed in the conveyance direction, thereby forming the second fold line of the Z-folding on the sheet (step St 114 ). Thereafter, the pushing plate  115  is moved from the stopped state to the retracting position at the upstream side again (step St 115 ), and the series of folding processing operations is completed. 
     In step St 107 , when it is determined that the stiffness or thickness of the sheet is equal to or smaller than the predetermined value (No in Step St 107 ), when the pushing plate  115  reaches the first pushing position near the folding roller pair  111 , the folding roller pair  111  is rotatably driven in the conveyance direction to convey the sheet in the conveyance direction (Step St 114 ). Thus, the position of the fold line of the sheet pushed by the pushing plate  115  is nipped by the folding roller pair  111  and the first fold line of the Z-folding is formed on the sheet. In this case, the folding roller pair  111  is rotationally driven in the conveyance direction when the pushing plate  115  has advanced to the predetermined position in front of the first pushing position, and the auxiliary operation of conveying the sheet to the downstream side is not performed. Thereafter, the pushing plate  115  is moved from the first pushing position to the retracting position at the upstream side again (step St 115 ), and the series of folding processing operations is completed. 
     In the series of the folding processing operations described above, the front end of the pushing plate  115  is temporarily stopped at the second pushing position, and then moved to the first pushing position. However, the pushing plate  115  is not limited to moving in this order, and may move to the first pushing position and stop, and move to the second pushing position at a timing at which the folding roller pair  111  is reversely rotated to return the front end of the sheet. Owing to that the front end of the pushing plate  115  moves to the second pushing position, a predetermined distance is secured between the front end of the pushing plate  115  and the nip portion of the folding roller pair  111 , and when returning only the front end of the sheet in the direction opposite to the conveyance direction, the load applied on the sheet can be reduced. 
     Further, description will be provided on a case in which the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or thickness of the sheet is further larger than the predetermined value. In this case, as a criterion for determining that the stiffness or thickness of the sheet is further larger than the predetermined value, for example, it is possible to set a case in which the basis weight of the sheet exceeds 105 g/m 2 . When it is determined that the stiffness or thickness of the sheet is further larger than the predetermined value, the pushing plate  115  is not moved in step St 112 , and only the conveyance roller pair  110  may be started to rotate in the conveyance direction and the first fold line may be conveyed toward the nip portion of the folding roller pair  111 . When the stiffness or thickness of the sheet is further larger than the predetermined value, the stiffness of the sheet is high, and the sheet can be conveyed only by the conveyance roller pair  110  without using the pushing plate  115 , and the first fold line can be conveyed toward the folding roller pair  111  without moving the pushing plate  115 . 
     Further, when the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or thickness of the sheet is further larger than the predetermined value, the conveyance roller pair  110  may be started to rotate in the conveyance direction before the pushing plate  115  starts to move in step St 112 . When the stiffness or thickness of the sheet is further larger than the predetermined value, the stiffness of the sheet is high, and the sheet can be conveyed only by the conveyance roller pair  110 . Accordingly, the load applied at the time of initial movement of the pushing plate  115  temporarily stopped at the parallel portion of the conveyance path pushing the first fold line can be reduced. 
     Further, when the sheet thickness recognition unit  312  of the folding control unit  301  determines that the stiffness or thickness of the sheet is further larger than the predetermined value, while stopping the pushing plate  115  in step St 110 , the hold current for holding the motor for driving the pushing plate  115  may be increased to increase the holding force to stop the pushing plate  115  when the folding roller pair  111  is reversely rotated, that is, rotationally driven in the direction opposite to the conveyance direction. Thus, it is possible to hold the pushing plate  115  at the second pushing position even in a case in which the folding roller pair  111  is reversely rotated and the sheet is conveyed in the direction opposite to the conveyance direction. 
     [Post-Processing Apparatus] 
     The post-processing apparatus C is provided with a post-processing path  38  continuous on the downstream side of the lower folding guide  117  of the sheet folding apparatus B. Post-processing devices such as a staple unit and an alignment unit are arranged on the post-processing path  38 , and sheets from the image forming apparatus A are received from the sheet folding apparatus B via the lower folding guide  117 , subjected to staple processing, alignment processing, and the like, and discharged to a sheet discharge tray  37 . When such post-processing is not required to be performed, the sheet conveyed from the image forming apparatus A via the sheet folding apparatus B passes through the post-processing apparatus C as it is and is stored in the sheet discharge tray  37 . 
     Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and can be appropriately modified within the scope not departing from the technical scope of the present invention. In the above description, the sheet folding apparatus has been described as an independent apparatus separate from the image forming apparatus, but a series of configurations for realizing the sheet folding apparatus of the present invention may be configured as a part of the image forming apparatus or may be configured as a part of the post-processing apparatus. 
     Here, the present application claims priority from Japanese Patent Application No. 2020-214610 and Japanese Patent Application No. 2021-191712 incorporated herein by reference.