Abstract:
A sheet folding apparatus including a sheet supporting member which has an inclined surface to support a stack of sheets, a sheet position adjuster which adjusts a position of the stack of sheets along the inclined surface, a sheet pressing unit which presses the stack of sheets against the inclined surface, a folding unit which folds the stack of sheets pressed by the sheet pressing unit, and a controller which controls the sheet pressing unit to press the stack of sheets on the sheet position adjuster against the inclined surface, and to create a gap between the stack of sheets on the sheet position adjuster and the sheet pressing unit to receive a sheet onto the sheet position adjuster.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Applications No. 60/952,836, filed Jul. 30, 2007; No. 60/968,541, filed Aug. 28, 2007; No. 60/968,853, filed Aug. 29, 2007; No. 60/969,126, filed Aug. 30, 2007; No. 60/969,148, filed Aug. 30, 2007; and No. 60/980,727, filed Oct. 17, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a sheet folding apparatus, a sheet folding method and an image forming apparatus. 
       BACKGROUND 
       [0003]    In an image forming system, an optional sheet post-process apparatus can be connected to an image forming apparatus such as a multifunction peripheral. Recently, a sheet post-process apparatus is proposed which has a function that aligns ends of a stack of sheets printed by the multifunction peripheral are aligned in length (longitudinal) and width (lateral) directions, and performs saddle stitch binding of the stack of sheets to obtain a booklet. 
         [0004]    As the sheet post-process apparatus, US Patent Application Publication No. 2004/0254054A1 discloses a sheet folding device that pushes out a folding plate in the direction perpendicular to a vertical sheet conveying path to insert the sheet or sheet stack between a pair of folding rollers and fold the sheet or sheet stack nipped and fed by the folding rollers. 
         [0005]    In the paper folding device, the rear ends (lower ends) of the sheets stacked between stack conveying guide plates disposed along a sheet conveying path are supported by a movable rear end fence and elevated along the sheet conveying path. 
         [0006]    Since the sheet having no rigidity (firmness) is buckled on the movable rear end fence, accurate positioning of the sheet stack can not be attained by the elevation control of the movable rear end fence. Also with respect to a curled sheet, accurate positioning of the sheet stack can not be attained by the elevation control of the movable rear end fence. In addition, when a sheet is caught by the sheet conveying guide plate, it becomes difficult to align the lower ends of the sheets. 
         [0007]    Further, high-speed conveyance of the sheet stack to the processing position is not attainable in a state where buckling or the like occurs. 
       SUMMARY 
       [0008]    According to an exemplary embodiment, one aspect of the invention relates to a sheet folding apparatus including: a sheet supporting member which has an inclined surface to support a stack of sheets; a sheet position adjuster which adjusts a position of the stack of sheets along the inclined surface; a sheet pressing unit which presses the stack of sheets against the inclined surface; a folding unit which folds the stack of sheets pressed by the sheet pressing unit; and a controller which controls the sheet pressing unit to press the stack of sheets on the sheet position adjuster against the inclined surface, and to create a gap between the stack of sheets on the sheet position adjuster and the sheet pressing unit to receive a sheet onto the sheet position adjuster. Another aspect of the invention relates to a sheet folding method including: supporting a stack of sheets in an inclined position; pressing the stack of sheets against a sheet supporting member; and folding the stack of sheets in a state where the stack of sheets is pressed against the sheet supporting member. 
         [0009]    Another aspect of the invention relates to an image forming apparatus including: a printer which prints an image on a sheet; a sheet supporting member which has an inclined surface to support a stack of sheets including the sheet; a sheet position adjuster which adjusts a position of the stack of sheets along the inclined surface; a sheet pressing unit which presses the stack of sheets against the inclined surface; a folding unit which folds the stack of sheets pressed by the sheet pressing unit; and a controller which controls the sheet pressing unit to press the stack of sheets on the sheet position adjuster against the inclined surface, and to create a gap between the stack of sheets on the sheet position adjuster and the sheet pressing unit to receive a sheet onto the sheet position adjuster. 
         [0010]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is an exemplary view showing an inner structure of a sheet post-process apparatus of an embodiment of the invention. 
           [0013]      FIG. 2  is an exemplary view schematically showing a main unit of the sheet post-process apparatus shown in  FIG. 1 . 
           [0014]      FIG. 3  is an exemplary view showing a detailed structure of a sheet pressing unit shown in  FIG. 2 . 
           [0015]      FIG. 4  is an exemplary view showing an example in which an eccentric cam shown in  FIG. 3  is driven by another driving source. 
           [0016]      FIG. 5  is an exemplary view showing movement of the sheet pressure plate shown in  FIG. 3 . 
           [0017]      FIG. 6  is an exemplary view showing a structure of a stapler shown in  FIG. 2 . 
           [0018]      FIG. 7  is an exemplary view showing a structure of a sheet conveying guide for guiding a sheet stack to the stapler shown in  FIG. 2 . 
           [0019]      FIG. 8  is an exemplary view showing a detailed structure of a sheet folding unit shown in  FIG. 2 . 
           [0020]      FIG. 9  is an exemplary view showing the side of a lateral alignment unit shown in  FIG. 2 . 
           [0021]      FIG. 10  is an exemplary view showing the back of the lateral alignment unit shown in  FIG. 2 . 
           [0022]      FIG. 11  is an exemplary view schematically showing a control circuit of the sheet post-process apparatus shown in  FIG. 2   
           [0023]      FIG. 12  is a flowchart showing a bookbinding process performed by the control circuit shown in  FIG. 11 . 
           [0024]      FIG. 13  is a flowchart showing an example of a sheet pressing process shown in  FIG. 12 . 
           [0025]      FIG. 14  is a flowchart showing a modification of the sheet pressing process shown in  FIG. 13 . 
           [0026]      FIG. 15  is a flowchart showing an example of a sheet folding process shown in  FIG. 12 . 
           [0027]      FIG. 16  is a flowchart showing a modification of the sheet folding process shown in  FIG. 15 . 
           [0028]      FIG. 17  is a flowchart showing a modification in which the sheet pressing process and the sheet folding process shown in  FIG. 12  are made independent as a sheet pressing and folding process. 
           [0029]      FIG. 18  is a flowchart showing a modification of the sheet pressing and folding process shown in  FIG. 17 . 
           [0030]      FIG. 19  is an exemplary view showing a positional relationship between the sheet pressure plate and a sheet stack obtained by the sheet pressing process for stapling shown in  FIG. 12 . 
           [0031]      FIG. 20  is an exemplary view showing a positional relationship between the sheet pressure plate and the sheet stack obtained by the sheet pressing process for sheet folding shown in  FIG. 12 . 
           [0032]      FIG. 21  is an exemplary timing chart of a sheet pressure plate drive motor and a lateral alignment motor. 
           [0033]      FIG. 22  is an exemplary view showing a modification of the lateral alignment unit shown in  FIG. 10 . 
           [0034]      FIG. 23  is an exemplary view showing a modification of the sheet pressing unit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Hereinafter, a sheet post-process apparatus of an embodiment will be described with reference to the accompanying drawings. This sheet post-process apparatus is optionally connected to a multifunction peripheral  1001  as an image forming apparatus, and has a function in which ends of a stack of sheets printed by the multifunction peripheral  1001  are aligned in length (longitudinal) and width (lateral) directions, a longitudinal center of the stack of sheets is stapled, and folding is further performed at the longitudinal center portion, and by this, the stack of sheets is bound as a booklet. In this function, stapling is performed at, for example, two places along a folding axis. 
         [0036]      FIG. 1  shows a front cross-sectional diagram of the sheet post-process apparatus with the multifunction peripheral  1001 , and  FIG. 2  shows a front cross-sectional diagram at left side, and a right side cross-sectional view at right side, of a main structure of the sheet post-process apparatus. The sheet post-process apparatus includes a sheet folding apparatus PS 1  which performs bookbinding of sheets, a finisher device PS 2  which sorts or staples the sheets, and a sheet conveying mechanism DS which conveys the sheets to a selected one of the sheet folding apparatus PS 1  and the finisher device PS 2 . The finisher device PS 2  includes a sorter SR which sorts the sheets from the sheet conveying mechanism DS by selectively driving conveying rollers to discharge the sorted sheets to sheet trays TR 1  and TR 2 , and a stapler ST which staples the sheets stacked on a tray TR 3  by the sorter SR. After stapling, the sorter SR discharges the stapled sheets to the tray TR 2 . 
         [0037]    The sheet folding apparatus PS 1  includes a stack plate  1 , a stapler  2 , a sheet folding unit  3 , a sheet pressing unit  4 , a sheet position adjuster  5 , a lateral alignment unit  6 , and a belt conveying section  7 . The stack plate  1  has a sheet loading surface  101  which is disposed as an inclined surface of a sheet path. The sheet loading surface  101  is inclined to form a large angle with respect to the horizontal plane. The stapler  2  is disposed along the sheet path and above the sheet folding unit  3 . The stapler  2  and the sheet folding unit  3  may constitute a saddle stitch binding process section. The lateral alignment unit  6  is disposed along the sheet path and above the stapler  2 . The sheet pressing unit  4  is disposed at a lower part of the stack plate  1 . The sheet position adjuster  5  is disposed along the sheet path and below the sheet folding unit  3 . The stapler  2  and the sheet folding unit  3  served as the saddle stitch binding process section perform a saddle stitch binding process for the stack of sheets in a state where the stack of sheets is pressed by the sheet pressing unit  4 . 
         [0038]    The belt conveying section  7  includes a sheet conveying belt  7 A to drive rollers to convey sheets sequentially discharged as printed materials from the multifunction peripheral  1001  through a sheet conveying path  107 , and a conveying motor  7 B to drive the sheet conveying belt  7 A. The sheet conveying path  107  ejects the sheets successively to the sheet path on the stack plate  1 . The sheets slide down successively along the stack plate  1 . 
         [0039]    The sheet position adjuster  5  includes a stacker  5 A, a conveying belt  5 B and a conveying motor  5 C. 
         [0040]    The stacker SA may be a pair of hooks. The stacker  5 A supports the sheets sequentially sliding down along the stack plate  1  and stacked on the stack plate  1 . The stacker  5 A regulates the lower end position of the stack of sheets SP. The conveying belt  5 B is coupled to the stacker  5 A. The conveying motor  5 C drives the conveying belt  5 B in order to lift up and down the stacker SA along the sheet path. The stacker  5 A aligns the lower end of the stack of sheets SP, and moves up and down along the stack plate  1  to set a center of the stack of sheets SP to a stapling position and a folding position. The center of the stack of sheets SP at the stapling position faces a staple supported by a driver unit  2 A of the stapler  2 . The center of the stack of sheets SP at the folding position faces a folding blade  3 C of the sheet folding unit  3 . 
         [0041]    The stack plate  1  is partially opened so that the sheet folding unit  3  and the stapler  2  are exposed in the sheet path. 
         [0042]      FIG. 3  shows a structure of the sheet pressing unit  4  in detail. The sheet pressing unit  4  includes a flat sheet pressure plate  4 A which presses the stack of sheets SP supported by the stack plate  1  and the stacker  5 A toward the stack plate  1  side, an eccentric cam  4 B that rotates in contact with the sheet pressure plate  4 A, and a sheet pressure plate drive motor  4 C that drives the eccentric cam  4 B. The sheet pressure plate  4 A swings with the rotation of the eccentric cam  4 B about the base axis on the lower end side to temporarily press the stack of sheets SP. The sheet pressure plate  4 A is at a standby position apart from the stack plate  1  at the time of sheet stacking, and is set to the sheet pressing position after the longitudinal center of the stack of sheets SP is arrived at the stapling position. Further, the sheet pressure plate  4 A is again returned to the standby position after execution of stapling, and is again set to the sheet pressing position after the longitudinal center of the stack of sheets SP is arrived at the folding position. Here, the eccentric cam  4 B and the sheet pressure plate drive motor  4 C constitute a sheet pressure plate drive device. 
         [0043]    Incidentally, for example, as shown in  FIG. 4 , the eccentric cam  4 B may be driven by using the conveying motor  7 B of the belt conveying section  7  and a one-way clutch mechanism  4 C rotated by the conveying belt  7 A. The conveying belt  7 A shown in  FIG. 4  rotates in a counterclockwise direction and conveys the sheet. When conveying of all sheets is completed, it becomes unnecessary to use the conveying belt  7 A. Thus, the one-way clutch mechanism  4 C is in an idle state when the conveying belt  7 A rotates in a counterclockwise direction in  FIG. 4 . At this time, the sheet pressure plate  4 A is kept at the standby position by the force of a spring or the like. The conveying belt  7 A is rotated in the clockwise direction to set the sheet pressure plate  4 A to the sheet pressing position. At this time, the one-way clutch mechanism  4 C is put in a coupling state, the motive power from the drive belt  7 A is transmitted to the eccentric cam  4 B, and the sheet pressure plate  4 A is moved to the sheet pressing position. By this, the sheet pressure plate  4 A presses the stack of sheets SP supported by the stack plate  1  and the stacker  5 A. In this example, the eccentric cam  4 B, the conveying belt  7 A, the conveying motor  7 B, the one-way clutch mechanism  4 C, the spring and the like serve as a drive device of the sheet pressure plate  4 A. 
         [0044]    The upper part of  FIG. 5  shows the movement of the sheet pressure plate  4 A in a case of pressing the stack of sheets SP. 
         [0045]    A snapshot P 1  indicates the sheet pressure plate  4 A at the standby position. The sheet pressure plate  4 A has an angle against the stack plate  1  at P 1 . The sheet pressure plate  4 A may be in a substantially vertical state at the standby position. 
         [0046]    A snapshot P 2  indicates the sheet pressure plate  4 A moving in parallel to approach the stack plate  1  from the standby position after the instant indicated by the snapshot P 1 . The sheet pressure plate  4 A may move in a direction indicated by a broken arrow  502  perpendicular to the stack plate  1 . The sheet pressure plate  4 A may shift horizontally as indicated by a solid arrow  501 . The sheet pressure plate  4 A may shift may move in parallel posture with the posture at the standby position. 
         [0047]    A snapshot P 3  indicates the lower part of the sheet pressure plate  4 A contacting with the stack plate  1  after the instant indicated by the snapshot P 2 . The sheet pressure plate  4 A may rotate in a direction indicated by a rounded solid arrow  503 . The sheet pressure plate  4 A may rotate about the lower part (base axis on the lower end side, for example) so that the upper part moves toward the stack plate  1 . 
         [0048]    A snapshot P 4  indicates the sheet pressure plate  4 A at the sheet pressing position after the instant indicated by the snapshot P 3 . The upper part of the sheet pressure plate  4 A arrives at the stack plate  1  to contact in substantially parallel with the stack plate  1 . By this, the sheet pressure plate  4 A presses the stack of sheets SP. 
         [0049]    On the other hand, the lower part of  FIG. 5  shows the movement of the sheet pressure plate  4 A in a case of releasing the pressure of the stack of sheets SP. 
         [0050]    A snapshot P 5  indicates the upper part of the sheet pressure plate  4 A getting away from the stack plate  1  after the instant indicated by the snapshot P 4 . The sheet pressure plate  4 A may rotate in a direction indicated by a rounded solid arrow  504 . The sheet pressure plate  4 A may rotate about the lower part (base axis on the lower end side, for example) so that the upper part moves against the stack plate  1 . The sheet pressure plate  4 A may rotate about the lower part to take a posture in parallel with the posture at the standby position. A snapshot P 6  indicates the sheet pressure plate  4 A moving in parallel to separate from the stack plate  1  after the instant indicated by the snapshot P 5 . The sheet pressure plate  4 A may move in a direction indicated by a broken arrow  505  perpendicular to the stack plate  1 . The sheet pressure plate  4 A may shift horizontally as indicated by a solid arrow  506 . The sheet pressure plate  4 A may shift may move in parallel posture with the posture at the standby position indicated by a snapshot P 7 . By this, the pressure of the stack of sheets SP is released. 
         [0051]    In the case of pressing the stack of sheets SP, the lower end of the sheet pressure plate  4 A first contacts the stack of sheets SP, and next, the upper end of the sheet pressure plate  4 A contacts the stack of sheets SP. The sheet pressure plate  4 A serves to eliminate buckling and curl of the stack of sheets SP by pressing the stack of sheets SP first from the lower end side. 
         [0052]      FIG. 6  shows a structure of the stapler  2 . The stapler  2  is, for example, of a separation type including the driver unit  2 A and an anvil unit  2 B. The driver unit  2 A ejects a staple from a staple surface by sinking from a normal position indicated by a broken line  61  to a sinking position indicated by a broken line  62 . The anvil unit  2 B operates to sink the driver unit  2 A. The driver unit  2 A sinks together with the stack of sheets SP from the sheet loading surface  101  by the anvil unit  2 B at the time of stapling, and staples the stack of sheets SP. A sheet conveying guide G is provided between the stapler  2  and the sheet folding unit  3  as shown in  FIG. 7 . In the operation of the anvil unit  2 B for stapling, the staple surface of the driver unit  2 A is sunk together with the stack of sheets SP. In view of this, the sheet conveying guide G includes a sheet loading surface offset in the sinking direction from the staple surface of the driver unit  2 A, and a guide surface that extends to the staple surface of the driver unit  2 A from the sheet loading surface and can be depressed. A pair of uprising members GA is provided as a part of the sheet loading surface of the sheet conveying guide G. The uprising members GA are located at a position apart from the sheet folding unit  3  by substantially the same distance as the upper end of the sheet loading surface  101  which is disposed below the sheet folding unit  3  along the sheet path. When the longitudinal center of the stack of sheets SP is moved to the folding position after stapling, the uprising members cause the stack of sheets SP to be symmetric with respect to a pair of folding rollers  3 A and  3 B. In this case, the height conditions of the stack of sheets SP at the upper and lower sides of the sheet folding unit  3  along the stack plate  1  are made substantially equal to each other. This results in that asymmetric distortion of the stack of sheets SP caused by a step  701  between the sheet loading surface of the sheet conveying guide G and the sheet loading surface  101  is made uniform. 
         [0053]      FIG. 8  shows a structure of the sheet folding unit in detail. The sheet folding unit  3  includes the pair of folding rollers  3 A and  3 B made of metal, rubber, resin or the like, and the folding blade  3 C as a protruding plate that can reciprocate with respect to a nip between the folding rollers  3 A and  3 B. By the folding blade  3 C, the longitudinal center of the stack of sheets SP is inserted into the nip between the pair of folding rollers  3 A and  3 B. The stack of sheets SP is folded by the rotation of these folding rollers  3 A and  3 B, and is discharged to the booklet discharge tray TR. 
         [0054]      FIG. 9  shows the side of the lateral alignment unit  6 , and  FIG. 10  shows the back of the lateral alignment unit  6 . The lateral alignment unit  6  includes a pair of lateral alignment plates  6 A and  6 B which are disposed at the upper part of the stack plate  1  and a lateral alignment motor  6 C which drives the lateral alignment plates  6 A and  6 B. The lateral alignment plates  6 A and  6 B include a pair of support base members BM disposed at the back side of the stack plate  1 , and a pair of jogger fences JF coupled to both ends of the support base members through slits provided in the stack plate  1 . The lateral alignment plates  6 A and  6 B are driven by the lateral alignment motor  6 C when the longitudinal center of the stack of sheets SP is set to the stapling position or the folding position. The lateral alignment plates  6 A and  6 B perform a lateral aligning operation of moving in the width direction of the stack of sheets SP and temporarily pinching the SP so that both side ends of the stack of sheets SP are aligned with the jogger fences JF. 
         [0055]      FIG. 11  schematically shows a control circuit of the sheet post-process apparatus. The control circuit includes a CPU  11  which controls the operation of the whole apparatus, a ROM  12  which holds a control program of the CPU  11 , initial data and the like, a RAM  13  which temporarily stores data input to and output from the CPU  11 , and an input and output interface  14  which inputs and outputs various data between the CPU  11  and peripheral circuits, and these components are interconnected by a bus. The stapler  2 , the sheet folding unit  3 , the sheet pressing unit  4 , the sheet position adjuster  5 , the lateral alignment unit  6 , a sensor group  15 , a motor group  16 , and a conveying guide switch group  17  are connected to the input and output interface  14  as the peripheral circuits. The input and output interface  14  is connected also to the multifunction peripheral  1001  to acquire size data, sheet type data and print number data of sheets output as printed materials, a bookbinding command, and the like. The sensor group  15  includes, for example, a sensor which detects that the longitudinal center of the stack of sheets SP is set to the stapling position, a sensor which detects that the longitudinal center of the stack of sheets SP is set to the folding position, and a sensor which detects a sheet passing through the belt conveying section  7 . The motor group  16  includes a conveying motor for the sheet conveying mechanism DS, a drive motor for the sorter SR, a drive motor for the stapler ST, a conveying motor  7 B for the belt conveying section  7 , a conveying motor  5 C for the sheet position adjuster  5 , a drive motor for the sheet pressing unit  4 , a drive motor for the lateral alignment plates  6 A and  6 B of the lateral alignment unit  6 , and the like. The conveying guide switch group  17  includes, for example, branch switches for the sheet conveying mechanism DS. 
         [0056]      FIG. 12  shows a bookbinding process performed by the control circuit shown in  FIG. 11 . The bookbinding process is started in response to a bookbinding command from the multifunction peripheral  1001 . When the bookbinding process is started, it is repeatedly checked at Act  1  whether sheet stacking is completed. When the completion of the sheet stacking is detected from such a fact that the number of sheets ejected to the sheet path by the belt conveying section  7  reaches the number of sheets output from the multifunction peripheral  1001 , at Act  2 , the stack of sheets SP is conveyed to the stapling position. Specifically, the sheet position adjuster  5  is driven to lift up the lower end reference plate  5 . At Act  3 , it is repeatedly checked whether (substantially the longitudinal center of) the stack of sheets SP is present at the stapling position. This is confirmed in a manner that the stacker  5 A is detected, for example, by a sensor disposed according to the sheet size. Upon confirmation, it is checked at Act  4  whether the stack of sheets SP is of large-sized sheets which are large enough to use the sheet pressure plate  4 A. When it is confirmed from the size data that the stack of sheets SP is of the large-sized sheets, a sheet pressing process is performed at Act  5 . In this sheet pressing process, the sheet pressing unit  4  is driven to obtain the movement of the sheet pressure plate  4 A shown in the upper part of  FIG. 5 . When the stack of sheets SP is pressed by the sheet pressure plate  4 A, a stapling process is performed by driving the stapler  2  at Act  6 . After the stapling process, a standby process of the sheet pressure plate  4 A is performed at Act  7 . In this standby process, the sheet pressing unit  4  is driven to obtain the movement of the sheet pressure plate  4 A shown in the lower part of  FIG. 5 . The sheet pressure plate drive device moves the sheet pressure plate  4 A from the sheet pressing position to the standby position in a shorter time than a time of the movement from the standby position to the sheet pressing position of Act  5 . On the other hand, if the size data indicates that a size of stack of sheets SP is short not enough to use the sheet pressure plate  4 A at Act  4 , a stapling process is performed at Act  8  without pressing the stack of sheets SP by the sheet pressing unit  4  and returning the sheet pressure plate  4 A to the standby position. This stapling process is identical to the stapling process performed at Act  6 . 
         [0057]    After Act  7  or Act  8 , the stack of sheets SP is conveyed to the folding position at Act  9 . Specifically, the sheet position adjuster  5  is driven to lift down the stacker  5 A. Act  10 , it is repeatedly checked whether (longitudinal center of) the stack of sheets SP is present at the folding position. This is confirmed in a manner that the stacker  5 A is detected, for example, by a sensor disposed according to the sheet size. Upon confirmation, a sheet pressing process is performed at Act  11 . In this sheet pressing process, the sheet pressing unit  4  is driven to obtain the movement of the sheet pressure plate  4 A shown in the upper part of  FIG. 5 . When the sheet pressure plate  4 A presses the stack of sheets SP, a sheet folding process is performed at Act  12  by driving the sheet folding unit  3 . The stack of sheets SP is put in a state of being folded by the sheet folding process and is discharged to the booklet discharge tray TR. After the sheet folding process, a standby process of the sheet pressure plate  4 A is performed at Act  13 . At this standby process, the sheet pressing unit  4  is driven to obtain the movement of the sheet pressure plate  4 A shown in the lower part of  FIG. 5 . The sheet pressure plate drive device moves the sheet pressure plate  4 A from the sheet pressing position to the standby position in a shorter time than that of the movement from the standby position to the sheet pressing position at Act  12 . After execution of Act  13 , the bookbinding process is ended. 
         [0058]    Incidentally, in the above-mentioned bookbinding process, the sheet size in which the sheet pressure plate  4 A can be used may have such a condition that when the stack of sheets SP is set to the stapling position, the lower end of the stack of sheets SP is below the upper end of the sheet pressure plate  4 A. When the sheet folding apparatus handles only sheets having such a size that the sheet pressure plate  4 A can be used at the stapling position, above mentioned Act  4  and Act  8  are omitted. Further, when the sheet folding apparatus handles only sheets having such a size that the sheet pressure plate  4 A can not be used at the stapling position, above mentioned Act  4  to Act  7  are omitted. 
         [0059]      FIG. 13  shows an example of a process performed at Act  5  and Act  11  shown in  FIG. 12 . When the sheet pressing process is started, the sheet pressure plate drive device is activated at Act  21  to move the sheet pressure plate  4 A to the sheet pressing position. At Act  22 , it is repeatedly checked whether the sheet pressure plate  4 A is arrived at the sheet pressing position. When the arrival at the sheet pressing position is detected, the sheet pressure plate drive device is deactivated at Act  23  to keep the sheet pressure plate  4 A at the sheet pressing position. The process is ended with the execution of Act  23 . 
         [0060]      FIG. 14  shows a modification of the process shown in  FIG. 13 . In this modification, Act  21  shown in  FIG. 13  is replaced by Act  24  to Act  26 . When sheet pressing process is started, it is checked at Act  24  whether sheets are in condition where a problem occurs due to high speed of the sheet pressure plate drive device. In the sheet condition such as a thin type in which curl is liable to occur, the high speed becomes a cause of occurrence of a sheet jam. Further, in the sheet condition such as a large number of sheets to be stapled, the high speed becomes a cause of occurrence of defective stapling. When one of the sheet conditions is detected, the sheet pressure plate  4 A is moved at Act  25  to the sheet pressing position by the low-speed operation of the sheet pressure plate drive device. In this low-speed operation, a portion where a large torque is obtainable in the drive device such as a motor is used for sheet pressing. Incidentally, at this low-speed operation, the sheet pressure plate  4 A is driven at low speed only when the sheet pressure plate drive device starts to operate, and the moving speed of the sheet pressure plate  4 A may be gradually accelerated. 
         [0061]    Further, when any of the above-mentioned sheet conditions is not detected, the sheet pressure plate  4 A is moved at Act  26  to the sheet pressing position in a normal manner by the high-speed operation of the sheet pressure plate drive device. At Act  22  subsequent to Act  25  or Act  26 , it is repeatedly checked whether the sheet pressure plate  4 A is arrived at the sheet pressing position. When the arrival at the sheet pressing position is detected, the sheet pressure plate drive device is deactivated at Act  23  to keep the sheet pressure plate  4 A at the sheet pressing position. The process is ended with the execution of Act  23 . 
         [0062]      FIG. 15  shows an example of the sheet folding process performed at Act  12  shown in  FIG. 12 . When this sheet folding process is started, at Act  31 , the folding blade  3 C starts reciprocating to insert the stack of sheets SP between the pair of folding rollers  3 A and  3 B. It is repeatedly checked at Act  32  whether the folding blade  3 C finishes reciprocating. When the folding blade  3 C finishes reciprocating, the sheet folding process is ended. 
         [0063]      FIG. 16  shows a modification of the sheet folding process shown in  FIG. 15 . In this modification, Act  32  shown in  FIG. 15  is replaced by Act  33 . When this sheet folding process is started, at Act  31 , the folding blade  3 C is driven to perform the reciprocating operation that inserts the stack of sheets SP between the pair of folding rollers  3 A and  3 B. At Act  33 , it is repeatedly checked whether the stack of sheets SP is arrive at a position where it is nipped between the pair of folding rollers  3 A and  3 B. When this arrival is detected, the sheet folding process is ended. 
         [0064]      FIG. 17  shows a modification in which Act  11  and Act  12  shown in  FIG. 12  are made independent as a sheet pressing and folding process. The sheet pressing and folding process is used to shorten the processing time by driving the folding blade  3 C before the sheet pressure plate  4 A is arrived at the sheet pressing position to temporarily operate the sheet pressure plate  4 A and the folding blade  3 C in parallel. When the sheet pressing and folding process is started, the sheet pressure plate drive device is activated at Act  41  to move the sheet pressure plate  4 A moved to the sheet pressing position. At act  42 , it is repeatedly checked whether a state that allows driving of the folding blade  3 C is established. The state that arrows the driving of the folding blade  3 C is regarded as a state in which the sheet pressure plate  4 A can be arrived at the sheet pressing position before the folding blade  3 C contacts the stack of sheets SP. When the state that arrows driving of the folding blade  3 C is detected, at Act  43 , the folding blade  3 C is driven to perform the reciprocating operation of inserting the stack of sheets SP between the pair of folding rollers  3 A and  3 B. At Act  44 , it is repeatedly checked whether the sheet pressure plate  4 A is arrived at the sheet pressing position. When the arrival at the sheet pressing position is detected, at Act  45 , the sheet pressure plate drive device is deactivated to keep the sheet pressure plate  4 A at the sheet pressing position. At Act  46 , it is repeatedly checked whether the reciprocating operation of the folding blade  3 C is completed. When the completion of the reciprocating operation is detected, the sheet pressing and folding process is ended. 
         [0065]      FIG. 18  shows a modification of the sheet pressing and folding process shown in  FIG. 17 . In this modification, Act  46  shown in  FIG. 17  is replaced by Act  47 . That is, at Act  47 , it is repeatedly checked whether the stack of sheets SP is arrived at a position where the stack of sheets SP is nipped between the pair of folding rollers  3 A and  3 B. When this arrival is detected, the sheet pressing and folding process is ended. 
         [0066]    Incidentally, the lateral alignment operation of the lateral alignment plates  6 A and  6 B can be performed by driving the lateral alignment motor  6 C to align the side ends of the stack of sheets SP before the stapling and before the sheet folding. However, in this case, it is preferable to optimize the drive start timing of the sheet pressure plate drive motor  4 C of the sheet pressure plate  4 A with respect to the lateral alignment motor  6 C. 
         [0067]      FIG. 19  shows a positional relationship between the sheet pressure plate  4 A and the stack of sheets SP obtained by pressing the stack of sheets SP for stapling, and  FIG. 20  shows a positional relationship between the sheet pressure plate  4 A and the stack of sheets SP obtained by pressing the stack of sheets SP for sheet folding. Here, the stack of sheets SP has a sheet size which is determined to be pressed by the sheet pressure plate  4 A at each of the stapling position and the sheet folding position. 
         [0068]    When the stack of sheets SP is located at the stapling position shown in  FIG. 19 , the sheet pressure plate  4 A does not contact the stack of sheets SP by merely shifting in parallel. When reaching the sheet pressing position, the sheet pressure plate  4 A contacts the stack of sheets SP. On the other hand, when the stack of sheets SP is located at the sheet folding position shown in  FIG. 20 , the sheet pressure plate  4 A contacts the stack of sheets SP by merely shifting in parallel. Thus, after the sheet pressure plate drive motor  4 C of the sheet pressure plate  4 A is started, a difference occurs in the time required for the sheet pressure plate  4 A to actually contact the stack of sheets SP. 
         [0069]      FIG. 21  shows a timing chart of the sheet pressure plate drive motor  4 C and the lateral alignment motor  6 C. 
         [0070]    For stapling, the lateral alignment motor  6 C drives the lateral alignment plates  6 A and  6 B. The lateral alignment motor  6 C starts to slow at an instant indicated by a broken line  221 . The lateral alignment motor  6 C stops after a predetermined time elapses from an instant indicated by a broken line  221 . 
         [0071]    The sheet pressure plate drive motor  4 C starts and accelerates to drive the sheet pressure plate  4 A from an instant indicated by a broken line  220 . The sheet pressure plate drive motor  4 C drives beyond the instant indicated by the broken lines  224 ,  221  and  222 . The sheet pressure plate drive motor  4 C stops at an instant indicated by a broken line  223  after a period for slowing. 
         [0072]    The sheet pressure plate  4 A may be at the standby position indicated as P 1  in  FIG. 5  before the sheet pressure plate drive motor  4 C starts to drive at the instant indicated by the broken line  220 . 
         [0073]    The sheet pressure plate  4 A may move in parallel to approach the stack plate  1  from the standby position after the sheet pressure plate drive motor  4 C starts to drive. The sheet pressure plate  4 A may move beyond the position indicated as P 2  in  FIG. 5 . 
         [0074]    The lower part of the sheet pressure plate  4 A may contact with the stack plate  1  after an instant indicated by a broken line  224 , but the sheet pressure plate  4 A may still not contact with the stack of sheet until an instant indicated by a broken line  222 . The sheet pressure plate  4 A may rotate beyond the position indicated as P 3  in  FIG. 5 . 
         [0075]    The sheet pressure plate  4 A may contact with the stack of sheet after the instant indicated by a broken line  222 . The sheet pressure plate  4 A may be at the sheet pressing position indicated as P 4  in  FIG. 5  at the instant indicated by a broken line  223 . 
         [0076]    The anvil unit  2 B starts to move toward the driver unit  2 A at the instant indicated by a broken line  223 . 
         [0077]    The sheet pressure plate  4 A does not press the stack of the sheets during a term indicated by an arrow  211 . The sheet pressure plate  4 A may keep off from the stack of the sheets during a term indicated by an arrow  211 . 
         [0078]    The sheet pressure plate  4 A contacts with the stack plate  1  during a term indicated by an arrow  213 . 
         [0079]    The sheet pressure plate  4 A press the stack of the sheets during a term indicated by an arrow  212 . 
         [0080]    On the other hand, for folding, the lateral alignment motor  6 C starts to slow at an instant indicated by a broken line  221  as same as for stapling. 
         [0081]    The sheet pressure plate drive motor  4 C starts and accelerates to drive the sheet pressure plate  4 A from an instant indicated by a broken line  221  at the time as same as the lateral alignment motor  6 C starts to slow. The sheet pressure plate drive motor  4 C drives beyond the instant indicated by the broken lines  222  and  223 . The sheet pressure plate drive motor  4 C stops after a period for slowing. The folding blade  3 c starts proceeding to insert the stack of sheets between the pair of folding rollers  3 A and  3 B after the sheet pressure plate drive motor  4 C stops. 
         [0082]    An arrow  214  in  FIG. 21  indicates a time difference between start of sheet pressure plate drive motor  4 C for stapling and start of actual pressing of stack of sheets by sheet pressure plate  4 A. At stapling, the sheet pressure plate drive motor  4 C can start earlier than the lateral alignment motor  6 C stops. 
         [0083]    An arrow  215  in  FIG. 21  indicates a time difference between start of sheet pressure plate drive motor  4 C for folding and start of actual pressing of stack of sheets by sheet pressure plate  4 A. At folding, there is little time in which the lateral alignment motor  6 C and the sheet pressure plate drive motor  4 C can drive simultaneously. 
         [0084]    That is, the sheet pressure plate  4 A starts proceeding at stapling earlier that at folding by a term indicated by a arrow  219 . 
         [0085]    Although driving of the sheet pressure plate drive motor  4 C of the sheet pressure plate  4 A is started almost at the same time as the stop of the lateral alignment motor  6 C in pressing the stack of sheets SP for sheet folding, it is started before the stop of the lateral alignment motor  6 C in pressing the stack of sheets SP for stapling. By such control, the time required for pressing the stack of sheets SP can be shortened. 
         [0086]      FIG. 22  shows a modification of the lateral alignment unit  6 . In this modification, an arch-shaped conductive member BMX is further provided. The support base member BM of the lateral alignment plates  6 A and  6 B is disposed to be exposed on the sheet loading surface  101 , and the arch-shaped conductive member BMX is disposed to be in parallel to the jogger fence JF and to stride the support base member BM. The arch-shaped conductive member BMX prevents the stack of sheets SP from directly contacting with the support base member BM. By this, sliding of the stack of sheets SP is improved and adhesion by static electricity can be removed. 
         [0087]      FIG. 23  shows a modification of the sheet pressing unit  4  shown in  FIG. 3 . In this modification, the sheet pressure plate  4 A shown in  FIG. 3  is replaced by a sheet pressure film  4 D. The sheet pressure film  4 D pushes a stack of sheets SP supported by the stack plate  1  and the stacker  5 A. The sheet pressure film  4 D pushes the stack of sheets SP toward the stack plate  1  side. A wind-up roll  4 F winds up one end of the sheet pressure film  4 D. An axis of the wind-up roll  4 F may be stationary with the stack plate  1 . The belt  4 E drives the wind-up roll  4 F. The other end of the sheet pressure film  4 D may be stationary with the stack plate  1 . The sheet pressure film  4 D curls to the stack plate  1  side. The wind-up roll  4 F winds up the sheet pressure film  4 D to decrease a contact area of the sheet pressure film  4 D with the stack of sheets SP. The wind-up roll  4 F winds out the sheet pressure film  4 D to increase the contact area of the sheet pressure film  4 D with the stack of sheets SP. 
         [0088]    The axis of the wind-up roll  4 F may set lower than an upper end of the stack of sheets SP supported by the stacker  5 A. The sheet pressure film  4 D may curl upwardly. The wind-up roll  4 F may wind out the sheet pressure film  4 D to raise a top of a curl portion of the sheet pressure film  4 D. The contact area of the sheet pressure film  4 D with the stack of sheets SP may increase according to rising the top of the curl portion of the sheet pressure film  4 D. The wind-up roll  4 F may wind up the sheet pressure film  4 D to lower the top of the curl portion of the sheet pressure film  4 D. The contact area of the sheet pressure film  4 D with the stack of sheets SP may decrease according to lowering the top of the curl portion of the sheet pressure film  4 D. The sheet pressure film  4 D is apart from the stack plate  1  each time the stacker  5 A receives a sheet. The sheet pressure film  4 D presses the sheet after the longitudinal center of the sheet arrives at the stapling position. Further, the sheet pressure plate  4 A is again returned to the standby position after execution of stapling, and is again set to the sheet pressing position after the longitudinal center of the stack of sheets SP is arrived at the folding position. Even when the sheet pressure film  4 D is used as stated above, the stack of sheets SP can be pressed similarly to the sheet pressure plate  4 A. 
         [0089]    Hereinafter, merits obtained in this embodiment will be described. 
         [0090]    Sheets sequentially ejected from the belt conveying section  7  are slid down along the inclined stack plate  1  by their own weight at the time of stacking, and stacked on the stack plate  1  as a stack of sheets SP supported by the stacker  5 A. This stack of sheets SP is lifted up and down by the stacker  5 A at the time of sheet conveying. At the time of sheet stacking or sheet conveying, for example, the sheet pressure plate  4 A is located at the standby position sufficiently apart from the sheet loading surface  101 , thereby securing a wide sheet path between the sheet loading surface  101  and the sheet pressure plate  4 A. This makes defective conveying such as a sheet jam difficult to occur at the time of sheet conveying. As a result of securing the wide sheet path, buckling of the stack of sheets SP at the time of sheet stacking becomes liable to occur. However, since the sheet pressure plate  4 A is set to the sheet pressing position after the sheet stacking, the buckling of the stack of sheets SP can be eliminated. Further, even if sheets which are liable to be curled are stacked as a stack of sheets SP, this curl can be eliminated. When the buckling or curl is eliminated as stated above, the position accuracy of the stack of sheets SP moved to the stapling position or the folding position can be improved. Further, since the stack of sheets SP is pressed by the sheet pressure plate  4 A before the stapling or the sheet folding, these processes can be stably performed. Further, since the sheet pressure plate  4 A starts to press the stack of sheets SP from its lower end side, the buckled or curled stack of sheets SP can be finely extended without generating wrinkles. Moreover, the sheet pressure plate  4 A can be driven by the simple drive device. 
         [0091]    As shown in  FIG. 12 , only when it is confirmed that the stack of sheets SP arrived at the stapling position has a sheet size large enough to use the sheet pressure plate  4 A, the stack of sheets SP is pressed by the sheet pressure plate  4 A, and stapling is performed in this state. That is, since pressing the stack of sheets SP and returning the sheet pressure plate  4 A to the standby position are omitted for the stack of sheets SP having such a small sheet size that the lower end does not contact the sheet pressure plate  4 A, the total processing time for the stack of sheets SP can be shortened. Further, in the sheet condition such as a thin type in which curl is liable to occur or a large number of sheets to be stapled, the high speed of the sheet pressure plate drive device becomes a cause of occurrence of a sheet jam or defective stapling. However, in pressing the stack of sheets SP shown in  FIG. 14 , the low-speed operation of the sheet pressure plate drive device is selected when it is confirmed that the stack of sheets SP is in the sheet condition as stated above. Thus, a portion where a large torque is obtainable in the drive device such as a motor is used for the sheet pressing, so that the foregoing problem can be prevented in pressing the stack of sheets SP. On the other hand, when it is confirmed that the stack of sheets SP is in the sheet condition where the foregoing problem does not occur, the high-speed operation of the sheet pressure plate drive device is selected normally. Accordingly, the total process time for the stack of sheets SP as stated above can be shortened. Further, the sheet pressure plate drive device drives the sheet pressure plate  4 A at low speed only at the start of operation in pressing the stack of sheets SP shown in  FIG. 12 , and gradually accelerates the moving speed of the sheet pressure plate  4 A, or moves the sheet pressure plate from the sheet pressing position to the standby position in a shorter time than that of the case of sheet pressing in the sheet pressure plate standby shown in  FIG. 12 . Accordingly, also with these drive manners, the total processing time can be shortened. 
         [0092]    Even if the sheet size allows the stack of sheets SP to be pressed in any of the stapling position and the sheet folding position by the sheet pressure plate  4 A, after the start of the sheet pressure plate drive motor  4 C of the sheet pressure plate  4 A, there occurs a difference in the time required for the sheet pressure plate  4 A to actually contact the stack of sheets SP. Since this time difference can be previously calculated from the sheet size, in view of the free running time of the sheet pressure plate  4 A corresponding to the position of the stack of sheets SP, control is performed to optimize the drive start timing-of the sheet pressure plate drive motor  4 C of the sheet pressure plate  4 A with respect to the lateral alignment motor  6 C. That is, the drive timing of the sheet pressure plate drive motor  4 C is made early by the free running time of the sheet pressure plate  4 A which is increased when the stack of sheets SP set at the stapling position is pressed, and wasteful time consumption is reduced. Accordingly, the time required for pressing the stack of sheets SP can be shortened. 
         [0093]    The pair of uprising members GA makes the height conditions of the stack of sheets SP at the upper side and the lower side of the sheet folding unit  3  substantially equal to each other, and this uniforms the asymmetric distortion of the stack of sheets SP generated by the step between the sheet loading surface of the sheet conveying guide G and the sheet loading surface  101 . In addition, since the stapler  2  and the sheet folding unit  3  can be disposed to be close to each other, the sheet folding apparatus can be constructed to be very compact. 
         [0094]    The lateral alignment unit  4  has the structure in which the support base members BM of the lateral alignment plates  6 A and  6 B are disposed at the back of the stack plate  1 , or the arch-shaped conductive member BMX is disposed to stride the support base members BM of the lateral alignment plates  6 A and  6 B disposed to be exposed on the sheet loading surface  101 . When the support base members BM are at the back of the stack plate  1 , the support base members BM do not contact the stack of sheets SP in the lateral alignment operation. Further, the arch-shaped conductive members BMX contact only a part of the stack of sheets SP. Accordingly, sliding of the stack of sheets SP is improved and adhesion by static electricity can be removed. 
         [0095]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited-to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalent.