Patent Publication Number: US-10308060-B2

Title: Sheet post-processing apparatus and image forming system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet post-processing apparatus for applying binding processing to sheets, and an image forming system including the sheet post-processing apparatus. 
     2. Description of the Related Art 
     A sheet post-processing apparatus including a sheet binding mechanism that receives sheets, on which images are formed by an image forming apparatus, on a processing tray and applies binding processing to the sheet bundle on the processing tray has heretofore been known. Among methods for binding a sheet bundle by the sheet post-processing apparatus is a method for sandwiching the sheets between a pair of crimping members having recesses and protrusions of tooth shape, and pressing the sheets to form crimps so that fibers of the sheets are entangled with and fastened to each other. 
     According to the method for binding a sheet bundle by crimping, if the pressing force between the pair of crimping members is small, sheets of less entangleable fibers, such as sheets of thick paper and sheets of special paper, are likely to exfoliate from the bound sheet bundle. If the pressing force between the pair of crimping members is small, sheets are also likely to exfoliate from a thick sheet bundle. On the other hand, if the pressing force of the crimping members is large, thin paper, plain paper, and thin sheet bundles can be broken or damaged. In other words, there has been a problem that the method cannot accommodate various sheet types since a binding failure or sheet damage occurs depending on the sheet type. 
     SUMMARY OF THE INVENTION 
     There is provided a setting unit that sets the number of binding points at which to bind sheets, and binding is applied along a predetermined direction as many times as the number of binding points set by the setting unit. 
     This facilitates changing fastening force of the sheets, so that appropriate fastening force can be secured according to conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram showing an image forming system including a sheet post-processing apparatus according to the present invention. 
         FIG. 2  is a plan view showing a configuration of a binding part of the sheet post-processing apparatus and a state in which a sheet is discharged onto a processing tray. 
         FIG. 3  is a plan view showing a state in which a trailing edge of the sheet is aligned on the processing tray. 
         FIG. 4  is a plan view showing a state in which side ends of the sheet are aligned on the processing tray. 
         FIGS. 5A and 5B  are configuration diagrams showing a driving mechanism of a binding tool in a binding unit of the sheet post-processing apparatus. 
         FIG. 6  is a block diagram showing a control configuration of the image forming system including the sheet post-processing apparatus. 
         FIG. 7  is an operation flowchart showing sheet stacking, alignment, binding, and discharge processing in the sheet post-processing apparatus. 
         FIG. 8  is an operation flowchart showing an operation of the binding processing of the sheet post-processing apparatus. 
         FIGS. 9A, 9B, and 9C  are plan views showing binding positions according to the number of binding points in the case of corner binding. 
         FIGS. 10A, 10B, and 10C  are plan views showing binding positions according to the number of binding points in the case of two-point binding. 
         FIGS. 11A and 11B  are plan views showing states in which the binding processing is stopped. 
         FIG. 12  is a flowchart showing a binding operation in corner binding mode processing of the sheet post-processing apparatus. 
         FIGS. 13A and 13B  are schematic diagrams showing an operation of the binding unit in the corner binding mode processing. 
         FIG. 14  is a plan view showing binding traces when binding is applied to a plurality of binding positions in the corner binding mode processing. 
         FIG. 15  is a flowchart showing a binding operation in two-point binding mode processing of the sheet post-processing apparatus. 
         FIGS. 16A and 16B  are schematic diagrams showing an operation of the binding unit in the two-point binding mode processing. 
         FIG. 17  is a plan view showing a plurality of binding positions in the two-point binding mode. 
         FIG. 18  is a plan view showing a modification of the plurality of binding positions in the corner binding mode. 
         FIGS. 19A, 19B, and 19C  are plan views showing patterns of a binding direction of a plurality of binding positions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an overall configuration of an image forming system including a sheet post-processing apparatus according to the present invention. The image forming apparatus shown in the diagram includes an image forming apparatus  1 , a document reading apparatus  2 , a document feed apparatus  3 , and a sheet post-processing apparatus  4 . The image forming apparatus  1  includes an image forming unit that prints an image on a sheet. The document reading apparatus  2  reads an image to be printed on a sheet from a document. The document feed apparatus  3  conveys the document to a reading unit of the document reading apparatus  2 . The sheet post-processing apparatus  4  is connected to a sheet discharge port of the image forming apparatus  1  and applies post-processing to a sheet discharged from the image forming apparatus  1 . 
     The image forming apparatus  1  includes a cassette  5  which can store about 100 sheets, and a storage  6  which can store about 1000 sheets. The sheets in the cassette  5  or the storage  6  are taken out and sent to an image forming unit  1 A one by one. 
     The image forming unit  1 A performs printing on a sheet. The image forming unit  1 A includes a beam projector  12 , a developing device  13 , and a transfer charger  14 . The beam projector  12  forms an electrostatic latent image on a photosensitive drum  11 . The developing device  13  applies toner ink to the electrostatic latent image. The transfer charger  14  transfers the image ink formed on the photosensitive drum  11  to a sheet. A fixing roller  15  arranged downstream heats and fixes the image on the sheet. The sheet is conveyed to the sheet post-processing apparatus  4 . 
     The document reading apparatus  2  includes a first platen  16  and a second platen  17 . The first and second platens  16  and  17  are made of transparent glass and horizontally juxtaposed on an upper part of the apparatus. The first platen  16  is used to read a document that is manually set. The second platen  17  is used to read a document that moves at a predetermined speed. 
     The document reading apparatus  2  includes a reading carriage  18  and a photoelectric conversion unit inside. The photoelectric conversion unit includes a condenser lens  20  and a photoelectric conversion element  21  such as a CCD image sensor. The reading carriage  18  includes a lamp which emits light to a document, and two mirrors which guide the light reflected from the document to the condenser lens  20  and the photoelectric conversion element  21 . 
     The reading carriage  18  reciprocates in a sub scanning direction (horizontal direction) under the first platen  16 . The reading carriage  18  irradiates a document with light, and reads the document by photoelectrically converting the reflected light from the document by the photoelectric conversion element  21 . Image data on the document read by the photoelectric conversion element  21  is transmitted as an image signal to the beam projector  12 . 
     The document feed apparatus  3  passes a document set on a sheet feed tray  22  over the second platen  17  by a sheet conveyance mechanism  24 , and discharges the document to a sheet discharge tray  23 . The reading carriage  18  stops at a position under the second platen  17  and reads the document passing over the second platen  17 . 
     The sheet post-processing apparatus  4  applies binding processing to sheets on which images are formed (printed) by the image forming unit of the image forming apparatus  1 . The sheet post-processing apparatus  4  includes a binding part  50 , first and second stacking trays  56  and  57 , an insert tray  52 , a first conveyance path  53 , a second conveyance path  54 , and a third conveyance path  55 . The binding part  50  performs binding processing on sheets. Sheets are discharged to the first and second stacking trays  56  and  57 . Sheets such as a document, color paper, and a slip sheet are set on the insert tray  52 . The first conveyance path  53  guides a sheet from the image forming apparatus  1  to the binding part  50 . The second conveyance path  54  guides a sheet from the insert tray  52  to the binding part  50 . The third conveyance path  55  branches off the first conveyance path  53  and guides the sheet from the image forming apparatus  1  to the first stacking tray  56 . 
     The sheet post-processing apparatus  4  further includes a sheet feed mechanism and a plurality of conveyance roller pairs. The sheet feed mechanism separates and feeds the sheets on the insert tray  52  one by one. The plurality of conveyance roller pairs is arranged at appropriate intervals to convey sheets along the first to third conveyance paths  53 ,  54 , and  55 . A downstream portion of the first conveyance path  53  and a downstream portion of the second conveyance path  54  constitute a common conveyance path (third conveyance path)  56   a . A discharge roller pair  60  for discharging a sheet from a discharge port  58  is arranged on the common conveyance path  56   a . A carry-out sensor  59  detects the sheet conveyed to the discharge port  58 . 
       FIG. 2  is a plan view showing a configuration of the binding part  50  of the sheet post-processing apparatus and a state of a sheet in the binding part  50 .  FIGS. 3 and 4  are plan views showing the state of the sheet in the binding part  50 . As shown in  FIG. 2 , the binding part  50  includes a processing tray  61 , trailing edge regulation plates  62 , a pair of side regulation plates (first and second side regulation plates)  63   a  and  63   b , a binding unit  64 , a paddle  65 , and a discharge belt  66  (see  FIG. 1 ). Sheets  30  are stacked on the processing tray  61 . The trailing edge regulation plates  62  align the trailing edges of the sheets on the processing tray  61 . The side regulation plates  63   a  and  63   b  align both ends of the sheets  30  in the width direction. The binding unit  64  binds the sheets  30  aligned on the processing tray  61 . The paddle  65  is arranged above the processing tray  61 . The discharge belt  66  discharges the sheets on the processing tray  61  to the second stacking tray  57 . 
     The paddle  65  is rotatably supported by a rotating shaft  65   a . The paddle  65  is driven by a paddle driving motor M 1  to rotate in the direction of the arrow in the diagram, and thereby pushes out the sheets  30  toward the trailing edge regulation plates  62 . The discharge belt  66  is stretched between a pair of pulleys. The discharge belt  66  is driven by a discharge motor M 5  to discharge the sheets on the processing tray  61  to the second stacking tray  57 . 
     The side regulation plate  63   a  and  63   b  are provided on the left and right side ends of the processing tray  61 , with the sheets  30  therebetween. The side regulation plate  63   a  is connected to a first shift motor M 2  via a rack mechanism (not shown). The side regulation plate  63   b  is connected to a second shift motor M 3  via a rack mechanism. The side regulation plates  63   a  and  63   b  are thus moved to reciprocate in the width direction of the sheets  30  by forward and backward rotations of the first and second shift motors M 2  and M 3 . 
     The sheets sequentially discharged from the first and second conveyance paths  53  and  54  onto the processing tray  61  are aligned one by one by the trailing edge regulation plates  62 , the side regulation plates  63   a  and  63   b , and the paddle  65 . The aligned sheets are positioned to a binding position of the binding unit  64  which is provided at a corner on one side of the processing tray  61 . 
     The positioning will be described in detail. A sheet  30  carried out to the processing tray  61  is moved toward the trailing edge regulation plates  62  by the rotation of the paddle  65  as shown in  FIG. 2 . The trailing edge of the sheet is abutted against the trailing edge regulation plates  62 , whereby the trailing edge of the sheet  30  is aligned. With the trailing edge of the sheet aligned, the first shift motor M 2  is driven to move the side regulation plate  63   a  toward the other side regulation plate  63   b  as shown in  FIG. 3 . The side regulation plate  63   a  makes contact with one end of the sheet  30  and moves the sheet  30  toward the side regulation plate  63   b . The other end of the sheet  30  then comes into contact with the side regulation plate  63   b . As shown in  FIG. 4 , both sides of the sheet are thereby aligned and the sheet  30  is positioned. That is, in the present embodiment, the sheet is aligned by driving the first shift motor M 2 . The first shift motor M 2  stops if the one side regulation plate  63   a  is moved to a position at which the other end of the sheet  30  comes into contact with the side regulation plate  63   b.    
       FIGS. 5A and 5B  are configuration diagrams showing a driving mechanism for operating a binding tool  70  in the binding unit  64 . The binding unit  64  includes the binding tool  70  serving as a binding section for applying binding to sheets. As shown in  FIGS. 5A and 5B , the binding tool  70  includes a lower tooth member  71  and an upper tooth member  72 . The lower tooth member  71  is arranged on a sheet placing surface side of the processing tray  61 . The upper tooth member  72  is opposed to the lower tooth member  71 . The pair of tooth molds sandwiches and presses a sheet bundle therebetween. The sheets are thereby deformed and fibers of the sheets are entangled with each other to apply binding (crimp binding) to the sheets. 
     The lower tooth member  71  of the binding tool  70  is provided so that its upper surface  71   a  is flush with the sheet placing surface of the processing tray  61 . Binding teeth  71   c  of recessed and protruded shape are formed on an inner bottom of a groove portion  71   b . Binding teeth  72   c  of recessed and protruded shape to mesh with the binding teeth  71   c  of the lower tooth member  71  are formed on the bottom of the upper tooth member  72 . 
     An eccentric cam  73  is in contact with a top surface  72   a  of the upper tooth member  72 . Rotation of the eccentric cam  73  moves the upper tooth member  72  downward so that the binding teeth  72   c  of the upper tooth member  72  mesh with the binding teeth  71   c  of the lower tooth member  71 . The sheets between the binding teeth  72   c  of the upper tooth member  72  and the binding teeth  71   c  of the lower tooth member  71  are thereby pressed to expose fibers on the surfaces of the sheets, and the fibers of the sheets are entangled with and fastened to each other. 
     The eccentric cam  73  is connected to a binding motor M 4  via drive transmission gears  74 ,  75 , and  76 . The binding motor M 4  is driven to rotate the eccentric cam  73 . The eccentric cam  73  is configured so that if the eccentric cam  73  is rotated by a half turn, the upper tooth member  72  moves from a standby position to a meshing position and moves to the standby position again. That is, the binding tool  70  presses and binds the sheets as the eccentric cam  73  is rotated by a half turn. 
     A tension spring  78  is provided to move the upper tooth member  72  to the standby position. One end of the tension spring  78  is attached to an attachment tab  72   b  formed on the upper surface  72   a  of the upper tooth member  72 . The other end of the tension spring  78  is attached to an attachment pin  79  which is provided on a side plate (not shown) of the apparatus. As shown in  FIG. 5A , the tension spring  78  thus pulls up and moves the upper tooth member  72  to the standby position according to the rotation of the eccentric cam  73 . As shown in  FIG. 5B , if the eccentric cam  73  is rotated further, the eccentric cam  73  moves the upper tooth member  72  to the meshing position against the pulling force of the tension spring  78 . 
     As described above, the binding unit  64  integrally includes the binding tool  70 , the eccentric cam  73 , the drive transmission gears  74 ,  75 , and  76 , the driving motor M 4 , and the tension spring  78 . The binding unit  64  is configured to move along one side of the sheets on the processing tray  61  according to a guide groove  80  provided near the end portion of the processing tray  61 . 
     Specifically, a pair of slide pins  67   a  and  67   b  is provided in parallel on the end portion of the binding unit  64 . The slide pins  67   a  and  67   b  are engaged with the guide groove  80  which is formed in a unit frame along the width direction of the sheets. A unit moving mechanism  69  shown in  FIG. 6 , including a rack mechanism or a belt mechanism, enables the binding unit  64  to reciprocate along the guide groove  80 . The slide pin  67   a  serves as a rotation fulcrum about which the binding unit  64  swings. The guide groove  80  is formed to branch out at an end, and configured so that the slide pin  67   b  is guided into a branch part  80   a . If the slide pin  67   b  is guided into the branch part  80   a , the binding unit  64  swings with the slide pin  67   a  as the fulcrum, and changes its orientation to be oblique to the one side of the sheets. 
     The binding tool  70  is configured to be capable of reciprocation by itself in a direction orthogonal to the width direction, aside from movement by the binding unit  64  in the width direction of the sheets. The binding tool  70  includes a binding tool moving mechanism  49  (see  FIG. 6 ) for reciprocating the upper tooth member  72  and the lower tooth member  71  in that direction. Although a specific configuration of the binding tool moving mechanism  49  is not shown in the drawings, the binding tool moving mechanism  49  includes a publicly-known endless belt or rack mechanism. 
       FIG. 6  is a block diagram showing a control system of the image forming system. A configuration of a control apparatus  11   a  of the image forming system will be described with reference to  FIG. 6 . The control apparatus  11   a  includes a main body control unit  12   a  and a sheet post-processing control unit  13   a . The main body control unit  12   a  controls operations of the image forming apparatus  1 , the document reading apparatus  2 , and the document feed apparatus  3 . The sheet post-processing control unit  13   a  controls the sheet post-processing apparatus  4 . 
     The image forming apparatus  1  includes an input unit  14   a  which includes an operation panel and is arranged on a front side where the user of the image forming system is positioned. The user of the image forming system inputs various types of information, including image finishing specifications, size information about sheets to be printed, binding mode information, and bonding force information, from the operation panel of the input unit  14   a.    
     The main body control unit  12   a  controls the document reading apparatus  2  and the document feed apparatus  3  according to the contents input to the input unit  14   a . The main body control unit  12   a  then controls the image forming apparatus  1  to print images of documents read by the document reading apparatus  2  on sheets and send the sheets to the sheet post-processing apparatus  4  in succession. In addition, the main body control unit  12   a  outputs sheet information indicating the size, type, and the number of sheets to be printed, the binding mode information, and the bonding force information to the sheet post-processing control unit  13   a  based on the contents input to the input unit  14   a . The sheet post-processing control unit  13   a  outputs a binding unavailability signal and other signals indicating the state of the sheet post-processing apparatus  4  to the main body control unit  12   a.    
     The sheet post-processing control unit  13   a  controls a post-processing operation which is performed on the image-formed sheets delivered from the image forming apparatus  1 . The sheet post-processing control unit  13   a  includes a CPU, and controls an operation of the entire sheet post-processing apparatus  4  by executing a control program stored in a ROM  15   a . The sheet post-processing control unit  13   a  is therefore connected with the binding motor M 4 , the paddle driving motor M 1 , the first and second shift motors M 2  and M 3 , the binding tool moving mechanism  49 , the unit moving mechanism  69 , and a conveyance driving device  35 . The binding motor M 4  drives the eccentric cam  73  of the binding tool  70 . The paddle driving motor M 1  drives the paddle  65  to rotate. The conveyance driving device  35  includes a plurality of driving motors for driving the conveyance roller pairs arranged on the conveyance paths  53 ,  54 , and  55 , and the discharge roller pair  60 . 
       FIG. 7  is an operation flowchart showing an operation for sheet stacking, alignment, binding, and discharge processing. The sheet post-processing control unit  13   a  executes the sheet stacking, alignment, binding, and discharge processing as shown in  FIG. 7 . The sheet post-processing control unit  13   a  initially determines by using the carry-out sensor  59  whether a sheet is carried out onto the processing tray  61  (step S 1 ). The carry-out sensor  59  turns on if a sheet carried out to the discharge port  58  is detected. If the sheet is discharged from the discharge port  58  to the processing tray  61 , the trailing edge of the sheet passes the detection position and the carry-out sensor  59  turns off. If the carry-out sensor  59  switches from on to off, then the sheet post-processing control unit  13   a  determines that a sheet is carried out onto the processing tray  61 . 
     The sheet post-processing control unit  13   a  then controls the driving of the paddle driving motor M 1  to rotate the paddle  65  and perform trailing edge alignment processing on the sheet discharged to the processing tray  61  (step S 2 ). As shown in  FIG. 2 , the sheet  30  is moved by the paddle  65  in a direction opposite to the sheet carry-out direction. As shown in  FIG. 3 , the trailing edge of the sheet  30  is thereby abutted against and aligned by the trailing edge regulation plates  62 . 
     After the alignment of the trailing edge of the sheet  30  ends, the sheet post-processing control unit  13   a  performs alignment processing in the width direction (step S 3 ). In this alignment processing, the sheet post-processing control unit  13   a  moves the first side regulation plate  63   a  toward the second side regulation plate  63   b . One end of the sheet  30  in the width direction is thus pushed by the first side regulation plate  63   a , and the sheet  30  moves toward the second side regulation plate  63   b . The other end of the sheet  30  in the width direction then comes into contact with the second side regulation plate  63   b . Both ends of the sheet  30  are thereby aligned in the width direction as shown in  FIG. 4 . 
     The distance by which the sheet post-processing control unit  13   a  moves the first side regulation plate  63   a  toward the second side regulation plate  63   b  is determined according to the sheet size. More specifically, the sheet post-processing control unit  13   a  moves the first side regulation plate  63   a  to a position at which the distance between the side regulation plates  63   a  and  63   b  is slightly shorter than the length of the sheet  30  in the width direction. The sheet size is included in the sheet information from the main body control unit  12   a.    
     Next, the sheet post-processing control unit  13   a  determines whether the sheet  30  positioned by the alignment of the trailing edge and the width direction on the processing tray  61  is the last sheet (step S 4 ). If the sheet  30  is not determined to be the last one, the sheet post-processing control unit  13   a  returns to step S 1  and repeats the processing of step S 2  and subsequent steps. The sheets  30  delivered from the discharge port  58  in succession are thus accumulated on the processing tray  61 . Information about the last sheet is included in the sheet information transmitted from the main body control unit  12   a  sheet by sheet. Whether the sheet  30  is the last one is determined based on the sheet information. If the sheet  30  is determined to be the last one, the binding unit  64  performs binding processing on the sheets  30  (step S 5 ). The sheets  30  binding-processed on the processing tray  61  are then discharged to the second stacking tray  57  (step S 6 ). 
       FIG. 8  is an operation flowchart showing details of the binding processing (step S 5 ) by the sheet post-processing apparatus  4 . In the binding processing, the sheet post-processing control unit  13   a  obtains various types of information, including the sheet size information, the binding mode information, and the bonding force information, from the main body control unit  12   a  (steps S 11  to S 13 ). The binding mode information indicates the binding position(s) of a sheet bundle. The binding mode information is input to the input unit  14   a  by the user of the image forming system. 
     Binding modes in the present embodiment include a “corner binding mode” in which the sheets  30  are bound at a corner and a “two-point binding mode” in which the sheets  30  are bound at two points along a side. The binding modes are specified by way of operation buttons provided on the input unit  14   a . The bonding force information is also an input item to be input to the input unit  14   a  by the user. Binding forces for the crimp binding, “high”, “normal”, and “low”, are specified by way of operation buttons provided on the input unit  14   a . If the user intends to perform temporary binding by which bound sheets can be easily exfoliated afterward, “low” is selected. In the “corner binding mode” according to the present embodiment, “oblique binding” for applying binding in a direction crossing two sides of a corner of the sheets is performed on the corner of the sheets. In the “two-point binding mode”, “parallel binding” for applying binding in a direction parallel to one side of the sheets  30  is performed on two points along the one side of the sheets  30 . 
     Obtaining such pieces of information, the sheet post-processing control unit  13   a  makes various settings about binding. Initially, the sheet post-processing control unit  13   a  sets the number of binding points based on the bonding force information (binding point number setting unit/first setting unit) (step S 14 ). For example, if the user selects “high” on the operation panel of the input unit  14   a , the number of binding points is set to “3”. If “normal” is selected, the number of binding points is set to “2”. If “low” is selected, the number of binding points is set to “1”. The sheet post-processing control unit  13   a  thus has a function as a setting unit for setting the number of binding points of the sheets  30 . 
     The sheet post-processing control unit  13   a  then sets a region or regions available for binding (effective binding region(s)) based on the sheet size information and the binding mode information (step S 15 ). In the case of corner binding, an effective binding region is set in a normally-set margin portion at a corner of the sheets  30 . In the case of two-point binding, effective binding regions are set in a normally-set margin portion on one side of the sheets  30 . 
     Next, the sheet post-processing control unit  13   a  sets a binding position(s) based on the sheet size information, the binding mode information, and the number of binding points (binding position setting unit/second setting unit) (step S 16 ). To set the binding position(s), a data table is previously set in the program. The sheet post-processing control unit  13   a  extracts and sets suitable data from the table based on the sheet size, the binding mode, and the number of binding points. It will be understood that the post-processing control unit  13   a  may calculate the binding position(s) from the sheet size, the binding mode, and the number of binding points each time without using the previously-stored data. 
     In the present embodiment, the binding position(s) is/are evenly set within each effective binding region. More specifically, if the number of binding points is one, the binding position is set at the center position in the effective binding region. If the number of binding points is two, the effective binding region is divided into two equal blocks, and the binding positions are set at the centers of the respective blocks. If the number of binding points is three, the binding positions are set at the centers of three equal blocks. 
       FIGS. 9A to 9C  show binding positions according to the number of binding points in the case of corner binding. In corner binding, an effective binding region A is a substantially square range falling within a triangular area T at the corner of the sheets  30 . In such an effective binding area A of substantially square shape, if the number of binding points is one, as shown in  FIG. 9A , a binding position b 11  is set on a line L 11  that connects the centers of a pair of opposite sides toward the corner of the sheets in the effective binding region A. If the number of binding points is two, as shown in  FIG. 9B , the square is divided into two equal rectangular blocks by the line L 11 , and binding positions b 12  and b 13  are set on center lines L 12  and L 13  of the respective blocks parallel to the line L 11 . If the number of binding points is three, as shown in  FIG. 9C , the effective binding region A is divided into three equal rectangular blocks, and binding positions b 14 , b 15 , and b 16  are set on center lines L 14 , L 15 , and L 16  of the respective blocks. The first position to apply binding to thus varies with the number of binding points on the sheets  30 . The pitch between the binding positions also varies with the number of binding points. 
       FIGS. 10A to 10C  show binding positions according to the number of binding points in the case of two-point binding. In two-point binding, effective binding regions A 1  and A 2  of the same rectangular shape are set in a margin portion of one side of the sheets  30 . The effective binding regions A 1  and A 2  are located at equal distances d from the top and bottom sides of the sheets  30 , respectively. Binding positions are set on a line L 21  that connects the centers of the opposed sides orthogonal to the one side of the sheets  30  in the respective effective binding regions A 1  and A 2 . If the number of binding points is one, binding positions b 21  and b 22  are set at the centers of respective line segments parallel to the one side of the sheets  30  in the rectangular effective binding regions A 1  and A 2 . If the number of binding points is two, the effective binding regions A 1  and A 2  are each divided into two blocks adjoining along the side of the sheets  30 , and binding positions b 23 , b 24 , b 25 , and b 26  are set at the centers of respective line segments in the divided blocks. If the number of binding points is three, the effective binding regions A 1  and A 2  are each divided into three blocks adjoining along the side of the sheets  30 , and binding positions b 27 , b 28 , b 29 , b 30 , b 31 , and b 32  are set at the centers of respective line segments in the divided blocks. In the case of two-point binding, like corner binding, the first position to apply binding to thus varies with the number of binding points on the sheets  30 . A pitch h between the binding positions also varies with the number of binding points. 
     In setting the binding position(s), the sheet post-processing control unit  13   a  determines whether the set position(s) is/are available for binding (step S 17 ). If any of the set binding position(s) is unavailable for binding, the sheet post-processing control unit  13   a  resets all the settings of the binding position(s) (step S 21 ). The sheet post-processing control unit  13   a  then outputs an error signal to the main body control unit  12   a  to notify the main body control unit  12   a  that the binding processing is not possible (step S 22 ). The main body control unit  12   a  then informs the user that the binding mode and the number of binding points input from the input unit  14   a  are not executable. The state in which binding is not possible refers to when crimps overlap. Crimps can overlap if the sheet size is small or if the number of binding points is large. 
     If the binding processing is possible, the sheet post-processing control unit  13   a  performs either corner binding mode processing or two-point binding mode processing according to the selected binding mode (steps S 18 , S 19  and S 20 ). 
       FIG. 12  is an operation flowchart showing the corner binding mode processing.  FIGS. 13A and 13B  are schematic diagrams showing an operation of the binding unit in the corner binding mode processing. If the corner binding mode is selected, the sheet post-processing control unit  13   a  controls the unit moving mechanism  69  (see  FIG. 6 ) to move the binding unit  64  located at a home position HP in a direction parallel to one side of the sheets  30  (step S 111 ) (see  FIG. 13A ). The sheet post-processing control unit  13   a  then stops the binding unit  64  at a predetermined stop position K 1  at the end of the one side of the sheets  30  (see  FIG. 13B ). Here, the binding unit  64  at the stop position K 1  swings and is located at the corner of the sheets  30 , with the slide pin  67   b  in the branch part  80   a.    
     The sheet post-processing control unit  13   a  then controls the driving of the binding motor M 4  to actuate the binding tool  70  and apply binding to a set corner binding position (step S 112 ). If the binding at the set binding position ends, the sheet post-processing control unit  13   a  determines whether binding has been completed as many times as the set number of binding points (step S 113 ). If binding has not been completed as many time as the set number of binding points (set number of times of binding), the sheet post-processing control unit  13   a  moves the binding tool  70  by a pitch h toward the diagonal center of the sheets  30 , with the binding unit  64  stopped at the predetermined stop position K 1  (step S 114 ). The sheet post-processing control unit  13   a  thereby moves the binding tool  70  to the next binding position, and actuates the binding tool  70  to apply binding (step S 115 ). If the set number of binding points is not reached, the sheet post-processing control unit  13   a  further moves the binding tool  70  to the next set binding position and actuates the binding tool  70 . In such a manner, as shown in  FIG. 14 , crimps K 1   a , K 1   b , and K 1   c  as many as the set number of binding points are formed on the sheets  30 . 
     As described above, in the case of the corner binding mode, the sheet post-processing control unit  13   a  controls the binding tool  70  to apply the binding processing to a plurality of binding points along the diagonal direction toward the center of the sheets  30 . If the binding positions are at the corner of the sheets  30 , the sheet post-processing control unit  13   a  thus applies the binding processing along the diagonal direction of the sheets  30 . 
     If the set number of times of binding is reached (“YES” in step S 113 ), the sheet post-processing control unit  13   a  returns the binding tool  70  to K 1  which is the initial position (step S 116 ). The sheet post-processing control unit  13   a  then controls the unit moving mechanism  69  to return the binding unit  64  to the home position HP, and ends the corner binding mode processing (step S 117 ). 
       FIG. 15  is an operation flowchart of the two-point binding mode processing. If the two-point binding mode is selected, the sheet post-processing control unit  13   a  controls the unit moving mechanism  69  (see  FIG. 2 ) to move the binding unit  64  located at the home position HP in the direction parallel to the one side of the sheets  30  as shown in  FIG. 16A  (step S 211 ). In the present example, the number of binding points is three. As shown in  FIG. 17 , three binding positions Q 1 , Q 2 , and Q 3  are set in a first effective binding region, and three binding positions Q 4 , Q 5 , and Q 6  are set in a second effective binding region. 
     As shown in  FIG. 16B , if the binding unit  64  reaches the first binding position Q 1 , the sheet post-processing control unit  13   a  stops the binding unit  64  (step S 212 ), and drives the binding motor M 4  to apply binding to the binding position Q 1  of the sheets  30 . Like the corner binding mode, the sheet post-processing control unit  13   a  then determines whether binding has been completed as many times as the set number of binding points (step S 213 ). If binding has not been completed as many times as the set number of binding points, the sheet post-processing control unit  13   a  controls the unit moving mechanism  69  to move the binding unit  64  in the direction parallel to the one side of the sheets  30  (step S 214 ). The sheet post-processing control unit  13   a  then drives the binding motor M 4  to apply binding to the next binding position Q 2  (step S 215 ). 
     After the application of the binding to the binding position Q 2 , the sheet post-processing control unit  13   a  checks again whether binding has been completed as many times as the set number of binding points (step S 213 ). If binding has not been completed as many times as the set number of binding points, the sheet post-processing control unit  13   a  moves the binding unit  64  to the set next binding position Q 3  and applies binding. The sheet post-processing control unit  13   a  then repeats the operation of steps S 213  to S 215  until binding is completed as many times as the set number of binding points. More specifically, the sheet post-processing control unit  13   a  moves the binding unit  64  to the binding positions Q 1  to Q 6  of the sheets  30  and repeats the operation for applying binding. 
     In the two-point binding mode, the binding points are grouped in twos. The sheet post-processing apparatus according to the present embodiment has three levels of operation of fastening force, “high”, “normal”, and “low”. If the fastening force is set to “high”, the sheet post-processing apparatus performs binding at six points. If the fastening force is set to “normal”, the sheet post-processing apparatus performs binding at four points. If the fastening force is set to “low”, the sheet post-processing apparatus performs binding at two points. 
     As described above, in the case of the two-point binding mode, the sheet post-processing control unit  13   a  controls the binding tool  70  to apply the binding processing to a plurality of binding points along the direction of the one side of the sheets  30 . If the binding positions lie in positions parallel to the side of the sheets  30 , then the sheet post-processing control unit applies the binding processing along the direction of the side. 
     After binding has been applied to the sheets  30  as many times as the set number of binding points, the sheet post-processing control unit  13   a  returns the binding unit  64  to the home position HP and ends the two-point binding mode processing (step S 216 ). 
     In the foregoing embodiment, the effective binding region(s) is/are divided into equal blocks according to the number of binding points, and binding positions are set at the centers of the respective blocks. However, as shown in  FIG. 18 , binding positions may be set at a constant pitch H from one end side to the other end side of an effective binding region regardless of the number of binding points. If binding positions are set at a constant pitch H, a binding position can overlap with a printed character or the like as shown in  FIG. 11A . Binding positions can exceed the effective binding regions as shown in  FIG. 11B . In such cases, the sheet post-processing control unit  13   a  determines that binding positions are unable to be set. The sheet post-processing control unit  13   a  then informs the user that the binding mode and the number of binding points input from the input unit  14   a  are not executable. 
     In the foregoing embodiment, the corner binding uses oblique binding in which the sheets are bound in a direction crossing the two sides of the corner as shown in  FIG. 19A . However, parallel binding may be used in which the crimps are directly opposed to the opposite sides of the sheets  30 . If there is a plurality of binding points, binding positions are set toward the center along a side of the sheets as shown in  FIG. 19B . Oblique binding and parallel binding both may be made available so that the user can arbitrarily select one. A mode in which parallel binding is applied to one location at arbitrary position on the one side of the sheets  30  may be provided aside from the corner binding mode and the two-point binding mode. If the number of binding points is more than one, binding positions may be set along one side of the sheets  30  and parallel binding may be applied as shown in  FIG. 19C . 
     According to the foregoing embodiment, the binding force is manually set by using the operation buttons “high”, “normal”, and “low” provided on the input unit  14   a . However, the main body control unit  12   a  or the sheet post-processing control unit  13   a  may automatically determine and set an appropriate number of binding points and appropriate binding positions based on information about sheet types, such as thin paper, plain paper, thick paper, and special paper, and the binding mode information input from the input unit  14   a.    
     The sheet post-processing apparatus  4  described above is configured to be able to set the number of binding points, and can thereby freely adjust the fastening force of a sheet bundle. This allows appropriate binding according to the number of sheets in the sheet bundle, the type of the sheets, and the like. Since the fastening force of the sheet bundle can be freely adjusted, binding desired by the user can be easily performed. 
     In the corner binding mode, the sheet post-processing apparatus  4  performs binding on a plurality of points toward the inner side of the sheets  30 . In the two-point binding mode, the sheet post-processing apparatus  4  performs binding on a plurality of points along the one side of the sheets  30 . This can prevent interference with handling of the bound sheet bundle. In other words, if binding is applied to a plurality of points, the binding positions are set in a direction according to the binding mode. This offers excellent handling of the bound sheet bundle. 
     This application claims priority based on Japanese Patent Application No. 2016-179235, filed Sep. 14, 2016, all the contents of which are incorporated herein.