Patent Publication Number: US-8534660-B2

Title: Sheet processing apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus that performs a folding operation such as a Z-shape folding operation on a sheet ejected from an image forming apparatus like a printing apparatus. In addition, the present invention relates to the image forming apparatus equipped with the sheet processing apparatus. 
     2. Description of the Related Art 
     Up until now, a known sheet processing apparatus is included in an image forming apparatus such as a copier and a printer and performs a folding operation such as a Z-shape folding operation on a sheet on which an image has been formed by an image forming unit and the like. In such a sheet processing apparatus, a member with which the tip end or the rear end of a sheet comes into contact is provided, for example, on a conveyance path for conveying the sheet on which an image has been formed. When the sheet comes into contact with the member, a deflection is formed at the middle part of the sheet. Then, the middle part of the sheet is conveyed between rollers serving as folding units, so that folding lines are formed on the sheet. 
     Further, for example, Patent Document 1 discloses another method for forming folding lines on a sheet. In other words, there is provided a space in which the sheet is deflected so as to form a loop on a conveyance path, and a pressing claw is put in the loop to make the tip end of the loop mesh with a folding unit. Thus, folding lines are formed on the sheet. 
     However, the above methods have following drawbacks. Specifically, if there is a large distance between conveyance guide plates constituting the conveyance path provided between the folding unit and the contact member with which the tip end or the rear end of the sheet comes into contact, the deflection formed at the middle part of the sheet has a high degree of freedom. As a result, the deflection formed at the middle part of the sheet fluctuates. Thus, due to the fluctuation of the deflection formed at the middle part of the sheet, the position of the middle part of the sheet, on which folding lines have been formed by the folding unit, is shifted from an expected folding position. As a result, the folding accuracy of the sheet also fluctuates. 
     Further, when a folding operation is performed on the same sheet plural times, a part on which folding lines have been formed on an upstream side comes into contact with the contact member as the tip end part or the rear end part of the sheet so that the sheet has a deflection at its middle part. However, if the distance between the conveyance guide plates is large as described above, an unnecessary deflection is formed at the middle part of the sheet. As a result, the position of the folding lines formed on the upstream side is shifted from the folding position of the sheet conveyed between the folding units, which results in the occurrence of a so-called box folding phenomenon. 
     Patent Document 1: JP-A-2006-62810 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the above points and may provide a sheet processing apparatus to improve the folding accuracy of a sheet and an image forming apparatus. 
     According to an aspect of the present invention, there is provided a sheet processing apparatus that forms folding lines at a predetermined position of a sheet after image formation and outputs the sheet. The apparatus includes a contact unit with which a tip end or a rear end of the conveyed sheet comes into contact and by which a deflection is formed at a middle part of the sheet; folding units that form the folding lines by conveying the middle part of the sheet, at which the deflection is formed by the contact unit, between the folding units; and a deflection controlling unit that controls the deflection at the middle part of the sheet formed by the contact unit. 
     According to another aspect of the present invention, there is provided an image forming apparatus including the sheet processing apparatus described above. 
     According to embodiments of the present invention, the folding accuracy of a sheet can be improved. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing the schematic configuration of an image forming apparatus equipped with a sheet processing apparatus according to embodiments; 
         FIG. 2  is a side view showing the schematic configuration of the sheet processing apparatus according to the embodiments; 
         FIG. 3  is a diagram showing a state immediately before a first folding operation is performed on a sheet; 
         FIG. 4  is a diagram showing a state after the first folding operation is performed on the sheet; 
         FIG. 5  is a diagram showing a state immediately before a second folding operation is performed on the sheet; 
         FIG. 6  is a diagram showing a state when a Z-shape folding operation is completed; 
         FIG. 7  is a diagram showing a state when the sheet just comes into contact with a stopper at its tip end; 
         FIG. 8  is a diagram showing a state when the sheet has a deflection at its middle part with the tip end having come into contact with the stopper on a conveyance path shown in  FIG. 7 ; 
         FIG. 9  is a diagram showing a state when the sheet just comes into contact with the stopper at its tip end; 
         FIG. 10  is a diagram showing a state when the sheet has a deflection formed at its middle part with the tip end having come into contact with the stopper on the conveyance path shown in  FIG. 9 ; 
         FIGS. 11A through 11D  are diagrams showing a process in which a box folding phenomenon occurs; 
         FIG. 12  is a diagram showing the finished Z-shape sheet in which the box folding phenomenon occurs; 
         FIG. 13  is a diagram showing the conveyance path, which is provided between the stopper serving as a contact unit with which the tip end or the rear end of the conveyed sheet comes into contact and folding rollers serving as folding units; 
         FIG. 14  is a diagram showing the stopper guide plate having through-holes as seen from the side of mylars; 
         FIG. 15  is a diagram showing a state immediately after the sheet comes into contact with the stopper at its tip end on the conveyance path where the mylars are provided; 
         FIG. 16  is a diagram showing a state immediately before a Z-shape folding part is formed in the sheet for the second time; 
         FIG. 17  is a plan view showing a state when the sheet enters the conveyance path where the mylars are provided; 
         FIG. 18  is a diagram showing a state immediately before the sheet comes into contact with the stopper on the conveyance path where the mylars are provided; 
         FIG. 19  is a diagram showing the state when the sheet moves to the upstream side of the mylars on the conveyance path; and 
         FIG. 20  is a diagram in which a driving mechanism is provided on one of the conveyance guide plates constituting the conveyance path. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described in detail. 
     (Schematic Configuration of Image Forming Apparatus Equipped with Sheet Processing Apparatus) 
       FIG. 1  is a side view showing the schematic configuration of an image forming apparatus equipped with a sheet processing apparatus. As shown in  FIG. 1 , the sheet processing apparatus  100  according to the embodiments is externally attached to the main body of the image forming apparatus  101  such as a copier and a printer in a detachable manner and provided next to the image forming apparatus  101 . 
     The sheet processing apparatus  100  performs a predetermined folding operation such as a Z-shape folding operation and a double folding operation on a sheet on which an image has been formed by an image forming unit and the like of the image forming apparatus  101 , and then ejects the sheet to a prescribed position. 
     Next, the schematic configuration of the sheet processing apparatus according to the embodiments are described. 
     (Schematic Configuration of Sheet Processing Apparatus) 
       FIG. 2  is a side view showing the schematic configuration of the sheet processing apparatus according to the embodiments. As shown in  FIG. 2 , the sheet processing apparatus  100  receives a sheet, on which an image has been formed by the image forming unit and the like of the image forming apparatus  101 , at a sheet receiving slot “a.” Then, the sheet processing apparatus  100  performs a folding operation on the predetermined sheet. After that, the sheet processing apparatus  100  ejects the folded sheet from a sheet ejecting slot “b” to a post-processing apparatus when performing a post processing operation such as a punching operation and a stapling operation on the sheet involved. Otherwise, the sheet processing apparatus  100  ejects the sheet to a sheet folding tray “c” inside the sheet processing apparatus  100 . 
     Further, the sheet processing apparatus  100  has, for example, conveyance paths  1  through  9  serving as conveyance paths for conveying the sheet to positions corresponding to places at which the folding operation is performed. Note that the conveyance paths  7  and  8  are also used for straight conveyance in which the folding operation is not performed. The conveyance paths are constructed by conveyance guide plates that guide the conveyance of the sheet, and the conveyance guide plates are provided on both sides in the thickness direction of the sheet so as to have a predetermined distance (space). Further, the conveyance paths are provided so to be connected to one another in such a manner as to be held by pairs of rollers, i.e., pairs of two folding rollers. 
     The conveyance paths  2 ,  3 , and  4  have stoppers  21 ,  31 , and  42 , respectively, serving as contact units with which the tip end or the rear end of the conveyed sheet comes into contact, thereby forming a deflection in the sheet. Note that the stoppers  21 ,  31 , and  41  are configured to be movable so as to correspond to the folding position of the sheet. 
     Further, the conveyance path  2  is constructed by stopper guide plates  22  and  23  serving as the conveyance guide plates that guide the conveyance of the sheet up to the stopper  21 . The conveyance path  3  is constructed by stopper guide plates  32  and  33  serving as the conveyance guide plates that guide the conveyance of the sheet up to the stopper  31 . The conveyance path  4  is constructed by stopper guide plates  42  and  43  serving as the conveyance guide plates that guide the conveyance of the sheet up to the stopper  41 . 
     At connection parts of the conveyance paths  1  through  9  shown in  FIG. 2 , switching claws  50  through  53  that introduce the sheet into the corresponding conveyance path are provided, and folding rollers  61  through  64  serving as folding units for forming folding lines on the conveyed sheet are provided. 
     With the above configuration, the sheet processing apparatus  100  receives the sheet, on which an image has been formed by the image forming unit and the like of the image forming apparatus  101  on an upstream side, at the sheet receiving slot a. Then, when performing the folding operation on the sheet, the sheet processing apparatus  100  introduces the sheet onto the respective conveyance paths inside the sheet processing apparatus  100  with the switching claw  50  for the folding operation. Note that an additional-folding roller unit  70  provided on the conveyance path  6  additionally performs the folding operation on the folded sheet. 
     Further, with the configuration of the sheet processing apparatus  100  described above, it becomes possible to perform various folding operations, such as a Z-shape folding operation, a double folding operation, an outward triple folding operation, an inward triple folding operation, a simple quarto folding operation, and a gate folding operation, on the sheet. Note that, when not performing the folding operation on the sheet, the sheet processing apparatus  100  directly ejects the sheet conveyed via the sheet receiving slot a to the sheet ejecting slot b with the switching claw  50  so as to be conveyed into the post processing apparatus on a downstream side. 
     Next, specific examples of the folding operations executable by the sheet processing apparatus  100  are described below. 
     (Z-shape Folding Operation) 
     Referring first to  FIGS. 3 through 6 , the Z-shape folding operation according to the embodiments is described.  FIG. 3  is a diagram showing a state immediately before a first folding operation is performed on the sheet.  FIG. 4  is a diagram showing a state after the first folding operation is performed on the sheet.  FIG. 5  is a diagram showing a state immediately before a second folding operation is performed on the sheet.  FIG. 6  is a diagram showing a state when the Z-shape folding operation is completed. Note that  FIGS. 3 through 6  are diagrams in which a part of the sheet processing apparatus  100  is enlarged and schematically shown. 
     As shown in  FIG. 2 , the sheet, which has been conveyed from the sheet receiving slot a as indicated by an arrow into the sheet processing apparatus  100 , is guided onto the conveyance path  1  by the switching claw  50 . As shown in  FIG. 3 , the sheet guided onto the conveyance path  1  is caused to pass through the nip of folding rollers  60  and  61  by a guide member, not shown, or the like. Here, the conveyed sheet S comes into contact with the stopper  31 , which is provided on the conveyance path  3  and movable so as to correspond to a folding position, at its tip end. As a result, a deflection is formed at the middle part of the sheet S. 
     As shown in  FIG. 4 , the deflection of the sheet S formed by the processing shown in  FIG. 3  moves into the nip of the folding rollers  61  and  62 . Thus, the first folding operation is performed on the sheet S. 
     Here, the sheet S is conveyed onto the conveyance path  4  by the switching claw  51  shown in  FIG. 2 . Note that when moving into the nip of the folding rollers  61  and  62 , the deflection formed at the middle part of the sheet S is guided to the side of the folding rollers  61  and  62  by a first turning-up prevention claw, not shown, or the like that prevents turning-up of the sheet S occurring, for example, when it is being conveyed. 
     Then, as shown in  FIG. 5 , the sheet S comes into contact with the stopper  41 , which is provided on the conveyance path  4  and movable so as to correspond to a folding position, at its tip end. Thus, the sheet S has a deflection formed at its middle part. The deflection of the sheet S formed by the processing shown in  FIG. 5  moves into the nip of the folding rollers  63  and  64 . In this manner, the second folding operation is performed on the sheet S. As a result, the sheet S has a Z-shape folding part formed therein as shown in  FIG. 6 . 
     In the manner described above, the Z-shape folding operation is completed. Note that when moving into the nip of the folding rollers  63  and  64 , the deflection formed at the middle part of the sheet S is guided to the side of the folding rollers  63  and  64  by a second turning-up prevention claw, not shown, or the like that prevents turning-up of the sheet S, for example, when it is being conveyed. 
     Then, the sheet S, on which the folding operation has been performed, passes through the conveyance path  6  shown in  FIG. 2 , is guided onto the conveyance path  7  by the switching claw  52 , and is stacked on the sheet folding tray c. Note that when the sheet S is conveyed into the post-processing apparatus, it is directed by the switching claw  52 . 
     (Double Folding Operation) 
     Next, the double folding operation according to the embodiments is described. The sheet, which has been conveyed from the sheet receiving slot a as indicated by the arrow in  FIG. 2  into the sheet processing apparatus  100 , is guided onto the conveyance path  1  by the switching claw  50 . Then, when the conveyed sheet S comes into contact with the stopper  21 , which is provided on the conveyance path  2  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. When the deflection moves into the nip of the folding rollers  60  and  61 , folding lines are formed on the sheet S. 
     In the manner described above, the double folding operation is completed. Note that when moving into the nip of the folding rollers  60  and  61 , the deflection formed at the middle part of the sheet S is guided into the nip of the folding rollers  60  and  61  by a folding blade, not shown, or the like that presses the deflection of the sheet S into the nip of the folding rollers  60  and  61 . Note that it is also possible to make the deflection of the sheet S move into the nip of the folding rollers  60  and  61  without using the folding blade. 
     The sheet S, on which the double folding operation has been performed, is caused to pass through the nip of the folding rollers  61  and  62  by a guide member, not shown, or the like without entering the conveyance path  3 , and is then guided onto the conveyance path  5  by the switching claw  51 . Further, the sheet S passes through the conveyance path  6 , is guided onto the conveyance path  7  by the switching claw  51 , and is stacked on the sheet folding tray c. Note that when the sheet S is conveyed into the post-processing apparatus, it is directed by the switching claw  52 . 
     (Outward Triple Folding Operation, Inward Triple Folding Operation, and Simple Quarto Folding Operation) 
     Next, the outward triple folding operation, the inward triple folding operation, and the simple quarto folding operation according to the embodiments are described. The sheet, which has been conveyed from the sheet receiving slot a as indicated by the arrow in  FIG. 2  into the sheet processing apparatus  100 , is guided onto the conveyance path  1  by the switching claw  50 . Then, when the conveyed sheet S comes into contact with the stopper  21 , which is provided on the conveyance path  2  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. 
     When the deflection moves into the nip of the folding rollers  60  and  61 , the first folding operation is performed on the sheet S. Then, the sheet S is conveyed onto the conveyance path  3 . When moving into the nip of the folding rollers  60  and  61 , the deflection formed at the middle part of the sheet S is guided into the nip of the folding rollers  60  and  61  by the folding blade, not shown, or the like that presses the deflection of the sheet S into the nip of the folding rollers  60  and  61 . Note that it is also possible to make the deflection of the sheet S move into the nip of the folding rollers  60  and  61  without using the folding blade. 
     Then, when the sheet S comes into contact with the stopper  31 , which is provided on the conveyance path  3  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. The deflection of the sheet S moves into the nip of the folding rollers  61  and  62 . Thus, the second folding operation is performed on the sheet S. Note that when moving into the nip of the folding rollers  61  and  62 , the deflection formed at the middle part of the sheet S is guided to the side of the folding rollers  61  and  62  by the first turning-up prevention claw, not shown, or the like. Thus, the folding operation on the sheet S is completed. 
     Note that the outward triple folding operation, the inward triple folding operation, and the simple quarto folding operation are almost the same except for a position at which the sheet S is folded for the first time. The folding position of the sheet S is adjusted by changing the position of the stopper  21 . The folding position of the sheet S for the second time is determined based on the position of the sheet S folded for the first time. In accordance with the folding positions thus determined, the first and second folding operations are performed on the sheet S. 
     Particularly, in the case of the inward triple folding operation, when the deflection formed in the sheet S moves into the nip of the folding rollers  61  and  62 , the tip end of the sheet S folded for the first time is guided by, for example, the first turning-up prevention claw, not shown, or the like so as to move into the nip of the folding rollers  60  and  61 . 
     Then, the sheet S, on which the above folding operation has been performed, is guided onto the conveyance path  5  by the switching claw  51 , passes through the conveyance path  6 , is guided onto the conveyance path  7  by the switching claw  52 , and is stacked on the sheet folding tray c. Note that when the sheet S is conveyed into the post-processing apparatus, it is directed by the switching claw  52 . 
     (Gate Folding Operation) 
     Next, the gate folding operation according to the embodiments is described. The sheet, which has been conveyed from the sheet receiving slot a as indicated by the arrow in  FIG. 2  into the sheet processing apparatus  100 , is guided onto the conveyance path  1  by the switching claw  50 . Then, when the conveyed sheet S comes into contact with the stopper  21 , which is provided on the conveyance path  2  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. 
     When the deflection moves into the nip of the folding rollers  60  and  61 , the first folding operation is performed on the sheet S. Then, the sheet S is conveyed onto the conveyance path  3 . When moving into the nip of the folding rollers  60  and  61 , the deflection formed at the middle part of the sheet S is guided into the nip of the folding rollers  60  and  61  by the folding blade, not shown, or the like. Note that it is also possible to make the deflection of the sheet S move into the nip of the folding rollers  60  and  61  without using the folding blade. 
     Then, when the sheet S comes into contact with the stopper  31 , which is provided on the conveyance path  3  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. The deflection formed at the sheet S moves into the nip of the folding rollers  61  and  62 . Thus, the second folding operation is performed on the sheet S. After this, the sheet S is conveyed onto the conveyance path  4  by the switching claw  51 . 
     At this time, the deflection of the sheet S is guided to the folding rollers  61  and  62  by, for example, the first turning-up prevention claw, not shown, or the like, and the tip end of the sheet S folded for the first time is guided into the nip of the folding rollers  61  and  62  by the first turning-up prevention claw, and the like. Then, when the conveyed sheet S comes into contact with the stopper  41 , which is provided on the conveyance path  4  and movable so as to correspond to a folding position, at its tip end, a deflection is formed at the middle part of the sheet S. 
     When the deflection formed at the middle part of the sheet S moves into the nip of the folding rollers  63  and  64 , a third folding operation is performed on the sheet S. Thus, the gate folding operation is completed. At this time, when the deflection of the sheet S is guided into the nip of the folding rollers  63  and  64  by the second turning-up prevention claw, not shown, or the like, and the tip ends of the sheet S folded for the first and second times are guided so as to move into the nip of the folding rollers  63  and  64 . 
     Then, the sheet S, on which the above folding operation has been performed, passes through the conveyance path  6 , is guided onto the conveyance path  7  by the switching claw  52 , and is stacked on the sheet folding tray c. Note that when the sheet S is conveyed into the post-processing apparatus, it is directed by the switching claw  52 . 
     (Deflection of Sheet Formed on General Conveyance Path) 
     Here, a description is made of a deflection formed when the sheet comes into contact with the stopper at its tip end on a general conveyance path.  FIG. 7  is a diagram showing a state when the sheet just comes into contact with the stopper at its tip end. 
     As shown in  FIG. 7 , the conveyance path  2  is provided between the stopper  21  and the folding rollers  60  and  61 . Further, the conveyance path  2  is constructed by stopper guide plates  22  and  23  serving as the conveyance guide plates and configured to have a predetermined space where the sheet S is conveyed. Further,  FIG. 7  shows an example in which the sheet S is conveyed onto the conveyance path  2  and just comes into contact with the stopper  21  at its tip end. 
       FIG. 8  is a diagram showing a state when the sheet S has a deflection at its middle part with the tip end come into contact with the stopper on the conveyance path  2  shown in  FIG. 7 . As shown in  FIG. 8 , if there is a large distance between the stopper guide plates  22  and  23  constructing the conveyance path  2  provided between the stopper  21  and the folding rollers  60  and  61 , the deflection formed at the middle part of the sheet has a high degree of freedom. 
     If the deflection formed at the middle part of the sheet S has a high degree of freedom, the sheet S is allowed to be deflected in a manner as shown in  FIGS. 7 and 8 . As a result, the deflection formed at the middle part of the sheet S fluctuates. Thus, due to the fluctuation of the deflection formed at the middle part of the sheet S, the position of the middle part of the sheet S, which moves into the nip of the folding rollers  60  and  61 , is shifted from an expected folding position. As a result, the folding accuracy of the sheet S also fluctuates. 
     Next, a description is made of a deflection formed when the sheet S is folded plural times in the sheet processing apparatus  100  as in the case of the Z-shape folding operation. That is, the deflection formed when the sheet S is folded for the second or subsequent times is described.  FIG. 9  is a diagram showing a state when the sheet S just comes into contact with the stopper at its tip end. 
     As shown in  FIG. 9 , the conveyance path  4  is provided between the stopper  41  and the folding rollers  63  and  64 . Further, the conveyance path  4  is constructed by the stopper guide plates  42  and  43  and configured to have a predetermined space where the sheet S, on which the folding lines have been formed on the upstream side, is conveyed.  FIG. 9  shows a state in which the sheet S just comes into contact with the stopper  41  at the part of the folding lines serving as its tip end. 
       FIG. 10  is a diagram showing a state when the sheet S has a deflection formed at its middle part with the tip end come into contact with the stopper  41  on the conveyance path  4  shown in  FIG. 9 . As shown in  FIG. 10 , if there is a large distance between the stopper guide plates  42  and  43  when the sheet S comes into contact with the stopper  41  at the part of the folding lines serving as its tip end, an unnecessary deflection is formed at the middle part of the sheet S between the stopper guide plates  42  and  43 . 
     When the sheet S is conveyed between the folding rollers  63  and  64  with the unnecessary deflection formed at the middle part of the sheet S between the stopper guide  42  and  43 , the folding position of the sheet S folded for the first time is moved in the direction as indicated by arrows A. As a result, a box folding phenomenon, in which new folding lines are formed at a position shifted from the folding position of the sheet S folded for the first time, occurs. 
     Here, the box folding phenomenon is specifically described.  FIGS. 11A through 11D  are diagrams showing a process in which the box folding phenomenon occurs. That is, the box folding phenomenon occurs in the sheet S in the order from  FIGS. 11A through 11D . 
     As shown in  FIG. 11A , the sheet S comes into contact with the stopper  41  at the part, on which the folding lines have been formed on the upstream side, serving as its tip end. Here, the deflection formed at the sheet S is conveyed between the folding rollers  63  and  64 . At this time, the unnecessary deflection of the sheet S surrounded by a circle in  FIG. 11B  is moved in the direction as indicated by an arrow P when the sheet S is conveyed between the folding rollers  63  and  64 . Further, when the sheet S is folded for the second time, the tip end x of the sheet S is conveyed between the folding rollers  63  and  64 . 
     Then, as shown in  FIG. 11C , when the sheet S is further conveyed between the folding rollers  63  and  64 , the unnecessary deflection of the sheet S surrounded by the circle is also further conveyed in the direction as indicated by the arrow P. In addition, when the sheet S is conveyed between the folding rollers  63  and  64 , new folding lines are formed at the middle part of the unnecessary deflection as indicated by an arrow y shown in  FIG. 11D . At the same time, the folding position at which the folding-lines have been formed for the first time is moved in the direction as indicated by an arrow A. As a result, the second folding lines are formed around the folding position at which the folding lines have been formed for the first time. 
       FIG. 12  is a diagram showing the finished Z-shape sheet S in which the box folding phenomenon occurs. As shown in  FIG. 12 , the new folding lines are formed around the position at which the folding lines have been formed on the sheet S for the first time, whereby the box folding phenomenon occurs. 
     (Deflection Controlling Unit) 
     First Embodiment 
     Next, a deflection controlling unit according to the embodiments is described.  FIG. 13  is a diagram showing the conveyance path, which is provided between the stopper serving as the contact unit with which the tip end or the rear end of the conveyed sheet comes into contact and the folding rollers serving as the folding units. 
     As shown in  FIG. 13 , the conveyance path  2 , which is provided between the stopper serving as the contact unit with which the tip end or the rear end of the conveyed sheet comes into contact and the folding rollers serving as the folding units, is configured to have a predetermined space with the stopper guide plates  22  and  23  serving as the conveyance guide plates. 
     At the sheet conveyance surface of the stopper guide plate  22  constituting the conveyance path, mylars  24  that are elastic substances having a smooth front surface and made of, for example, a plate-like member are attached as the deflection controlling units. For example, the mylars  24  are plate members formed of resin or the like. 
     On the other hand, at the parts of the sheet conveyance surface of the stopper guide plate  23  corresponding to the mylars, grooves or through-holes are formed. 
       FIG. 14  is a diagram showing the stopper guide plate  23  having the through-holes as seen from the side of the mylars  24 . As shown in  FIG. 14 , when the stopper guide plate  23  of  FIG. 13  is seen from the side of the mylars  24  in the direction as indicated by an arrow z, stopper claws  21 - 1  serving as the stoppers, with which the tip end of the sheet comes into contact, are provided in the stopper guide plate  23 . Further, the through-holes  25 , in which the mylars  24  of the stopper guide plate  22  are inserted, are formed in the stopper guide plate  23 . The mylars  24  are inserted in the through-holes  25  so as not to interfere with the stopper claws  21 - 1 . 
     Thus, when the mylars  24  are inserted in the through-holes  25 , grooves, or the like of the stopper guide plate  23 , they are provided so as to overlap the opposing stopper guide plate  23 . Note that since the mylars  24  are, for example, thin, and poor in strength due to their low elasticity, the sheet on the conveyance path can enter a space between the mylars  24  and the stopper guide plate  23 . 
     Further, as shown in  FIG. 13 , the mylars  24  are composed of, for example, two mylars  24 - 1  and  24 - 2 , and provided in a partial range on the stopper guide plate  22  along the sheet conveyance direction. Here, the mylars  24 - 1  and  24 - 2  are provided symmetrically with respect to the center of the conveyed sheet. 
     Further, as described below, when the stopper guide plate  22  is seen in plan view, the mylars  24 - 1  and  24 - 2  may be provided in an inverted V-shape such that an interval between the mylars gets gradually smaller from the upstream side to the downstream side of the conveyance path where the sheet is conveyed. Moreover, the number of the mylars  24  is not limited to two, but the mylars  24  may be composed of three or more mylars. For example, another mylar may be provided at the center of the conveyance path other than the two mylars described above. In addition, plural mylars, i.e., two mylars, may be each provided symmetrically with respect to the center of the conveyance path. 
     Further, if the mylars  24  are made of conductive resin such as polyester elastomer, they may also serve as a static electricity removing unit that removes static electricity at the closed surface of the sheet just before the sheet is folded and conveyed by the folding rollers. 
     Further, the tip ends of the maylars  24  on the upstream side of the conveyance path where the sheet is conveyed are shaped such that the tip ends are distant from the opposing stopper guide plate  23  at the most upstream position of the tip ends, and come closer to the stopper guide plate  23  and finally make contact therewith on the downstream side of the conveyance path. 
     Note that the conveyance path  3 ,  4 , or the like may have the same configuration as that of the conveyance path  2 . 
     Next, a description is made of the deflection, folding accuracy, or the like of the sheet S conveyed through the conveyance path according to the first embodiment shown in  FIG. 13 . 
     (Improvement in Folding Accuracy) 
       FIG. 15  is a diagram showing a state immediately after the sheet S comes into contact with the stopper  21  at its tip end on the conveyance path where the mylars are provided. As shown in  FIG. 15 , when the sheet S, which is placed between the stopper guide plates  22  and  23 , comes into contact with the stopper  21  at its tip end, it is pressed against the stopper guide plate  23 . 
     Thus, a deflection is not formed in the sheet S within a sheet conveyance region where the mylars  24  are provided, but is formed only at a position right above the nip of folding rollers  60  and  61 . Accordingly, the fluctuation of the deflection formed in the sheet S between the stopper guide plates  22  and  23  is reduced, and the sheet S is conveyed between the folding units at an expected folding position to have folding lines formed thereon. Therefore, the folding position of the sheet S is not shifted. As a result, it is possible to improve the folding accuracy of the sheet S ejected from the sheet processing apparatus  100 . The folding accuracy of the sheet S is also improved on the conveyance paths  3  and  4 . 
     Note that in the above embodiment, the deflection is formed in the sheet S when the sheet comes into contact with the stopper  21  at its tip end. However, the embodiment of the present invention is not limited to this. For example, a deflection can be formed in the sheet S when the rear end of the sheet S comes into contact with a stopper that is movable in accordance with a folding position. 
     (Prevention of Box Folding Phenomenon) 
       FIG. 16  is a diagram showing a state immediately before a Z-shape folding part is formed in the sheet S for the second time. As shown in  FIG. 16 , immediately after the tip end of the folding part of the sheet S formed for the first time comes into contact with the stopper  41 , the sheet S placed between the stopper guide plates  42  and  43  constituting the conveyance path  4  is pressed against the stopper guide plate  43  by the mylars  24 . 
     Thus, an unnecessary deflection is not formed in the sheet S between the stopper guide plates  42  and  43  within a sheet conveyance region where the mylars  24  are provided, but it is formed only at a position right above the nip of folding rollers  63  and  64 . 
     Accordingly, even after the deflection of the sheet S is conveyed between the nip of the folding rollers  63  and  64 , the position of the folding part of the sheet S formed for the first time, which is indicated by an arrow A in  FIG. 10 , is not shifted. As a result, it is possible to prevent the occurrence of the box folding phenomenon in which the position of folding lines formed on the sheet S for the first time is shifted from the folding position of the sheet S conveyed between the folding units. 
     (Prevention of Wrinkles) 
     Next, a description is made of a state until the sheet S comes into contact with the stopper  21  on the conveyance path where the mylars  24  are provided.  FIG. 17  is a plan view showing a state when the sheet S enters the conveyance path where the mylars are provided.  FIG. 18  is a diagram showing a state immediately before the sheet S comes into contact with the stopper on the conveyance path where the mylars are provided. 
     As shown in  FIG. 17 , with the provision of the mylars  24  between the conveyance guide plates (for example, between the stopper guide plates  22  and  23 ), a space between the conveyance guide plates is narrowed to eliminate a gap. Therefore, in this case, when the sheet S poor in strength such as a thin paper moves to the mylars  24 , a dilation  81  may be generated at the front surface of the sheet S. 
     When the sheet S poor in strength moves into the space between the conveyance guide plates in contact with the mylars  24  in the direction as indicated by an arrow in  FIG. 17 , there is a concern that the dilation  81  is generated at the front surface of the sheet S to cause wrinkles on the sheet S. 
     However, as shown in  FIG. 17 , the two mylars  24  composed of the mylars  24 - 1  and  24 - 2  are provided in an inverted V-shape such that an interval between the mylars gets gradually smaller from the upstream side to the downstream side of the conveyance path where the sheet S is conveyed. 
     Thus, even if the dilation  81  is generated at the front surface of the sheet S, it is stretched to the side of the rear end of the sheet S or the sides of the left and right ends of the sheet S as the sheet S is conveyed toward the stopper  21  in the direction as indicated by an arrow in  FIG. 18 . Accordingly, wrinkles occurring at the front surface of the sheet S caused by the dilation of the sheet S are prevented. As a result, even if a thin paper is used as described above, it is possible to prevent the occurrence of wrinkles on the paper. 
     Note that the same effect as the above can be obtained if the conveyance paths  3  and  4  are provided with the same configuration. 
     (Prevention of Jam) 
     Next, a description is made of a state when the sheet S moves to the upstream side of the mylars  24 .  FIG. 19  is a diagram showing the state when the sheet S moves to the upstream side of the mylars  24  on the conveyance path. As shown in  FIG. 19 , the mylars  24  provided on the conveyance path have an inclination on the upstream side of the conveyance path when seen from the side of the mylars  24 . 
     In other words, the tip ends of the maylars  24  on the upstream side of the conveyance path where the sheet is conveyed are shaped such that the tip ends are distant from the opposing stopper guide plate  23  at the most upstream position of the tip ends, and come closer to the stopper guide plate  23  and finally make contact therewith on the downstream side of the conveyance path. Accordingly, the mylars  24  are shaped in such a manner as to guide the conveyed sheet S in the direction as indicated by an arrow B shown in  FIG. 19 . 
     Thus, even when the conveyed sheet S comes into contact with the mylars  24  at its tip end, the tip end of the sheet S is guided in the direction as indicated by the arrow B shown in  FIG. 19 . Accordingly, even where the sheet S poor in strength such as a thin paper comes into contact with the mylars  24  at its tip end, it is possible to prevent the occurrence of jam due to the buckled sheet S. Note that the same effect as the above can be obtained if the conveyance paths  3  and  4  are provided with the same configuration as the above. 
     As described above, according to the first embodiment, the elastic substances (elastic members) such as the mylars  24  are provided on the conveyance guide plate between the folding units and the contact unit with which the sheet S comes into contact. In other words, with the simpler configuration and at low cost, the distance between the conveyance guide plates is narrowed so as to control the deflection of the sheet S formed when the conveyed sheet S comes into contact with the contact unit at its tip end or rear end. Thus, the sheet S is conveyed between the folding units at an expected folding position, whereby folding lines are formed on the sheet S without being shifted. As a result, it is possible to improve the folding accuracy of the sheet S to be ejected. 
     Further, even when the sheet S is folded for the second or subsequent times, the folding position of the sheet S on which folding lines have been formed on the upstream side is not shifted from the folding position of the sheet S conveyed between the folding units, thus making it possible to appropriately reduce the box folding phenomenon. 
     Further, if the elastic substance such as the mylar  24  is provided in one of the pair of conveyance guide plates, one of the conveyance guide plates, which is provided between the folding units and the contact unit with which the sheet S comes into contact, is constituted by a resin member or a metal member having rigidity. Thus, it is possible to narrow the distance between the conveyance guide plates without degrading conveyance quality. 
     Further, as in the case described above, when the elastic substances such as the mylars  24  are inserted in the grooves formed in the conveyance guide plates to eliminate a gap between the conveyance guide plates and the elastic substances, it is possible to prevent an unnecessary deflection from being formed in the sheet S between the conveyance guide plates, reduce the fluctuation of the deflection of the sheet S, and improve the folding accuracy of the sheet S to be ejected. 
     Further, since the elastic substances such as the mylars  24  are symmetrically provided on the conveyance guide plate with respect to the center of the conveyed sheet S, it is possible to prevent a skew (inclination of the sheet S) occurring when the sheet S moves to the conveyance guide plates. 
     Further, the elastic substances such as the mylars  24  provided on the conveyance guide plate are arranged in an inverted V-shape from the upstream side to the downstream side of the conveyance path where the sheet is conveyed. Therefore, even if a dilation is generated at the tip end of the sheet S when the tip end of the conveyed sheet S moves to the mylars  24 , it is stretched to the side of the rear end of the sheet S or the sides of the left and right ends of the sheet S as the sheet S moves. Thus, it is possible to prevent the occurrence of wrinkles in the sheet S moving to the milars  24 . 
     Further, static electricity at the closed surface of the sheet S is removed immediately before the sheet S is folded and conveyed by the folding rollers. Therefore, when a user opens the closed surface of the sheet S, no electrostatic force is generated at the closed surface of the sheet S. As a result, the user can easily open the closed surface of the sheet S. 
     Further, the elastic substances such as the mylars  24  provided on the conveyance guide plate are shaped in such a manner as to guide the sheet S to the mylars  24 . Therefore, even if the thin paper poor in strength comes into contact with the elastic substance such as the mylars  24 , it is possible to reduce a load at this time and prevent the occurrence of jam. 
     Second Embodiment 
     Next, a second embodiment of the present invention is described.  FIG. 20  is a diagram in which a driving mechanism is provided on one of the conveyance guide plates constituting the conveyance path. As shown in  FIG. 20 , the driving mechanism serving as a conveyance path space-controlling unit is connected to the stopper guide plate  22  constituting the conveyance path  2 . The driving mechanism is constructed by, for example, an arm  26 , an arm  27 , and a cam  28 . The cam  28  is connected to a motor, not shown. Further, the stopper guide plate  23  is attached to a sheet surface sensor  29 . Note that the conveyance paths  3 ,  4 , and the like can have the same configuration as the conveyance path  2 . 
     The stopper guide plate  22  is capable of moving in the directions as indicated by an arrow C with the driving mechanism described above. When the stopper guide plate  22  moves to the stopper guide plate  23 , a distance between the stopper guide plates  22  and  23  can be made smaller. 
     The stopper guide plate  22  moves to the stopper guide plate  23  at the following timing. That is, the distance between the stopper guide plates  22  and  23  is held to be normal until the sheet is conveyed onto the conveyance path  2  shown in  FIG. 20 . However, when the sheet passes through the stopper guide  22  and comes into contact with the stopper  21  at its tip end on the conveyance path  2 , the stopper guide plate  22  immediately moves to the stopper guide plate  23  to reduce the distance between the stopper guide plates  22  and  23 . 
     Specifically, when the sheet detection sensor  29  attached to the stopper guide plate  23  detects the tip end of the conveyed sheet, the motor is driven and the stopper guide plate  22  starts moving in the direction of the opposing stopper guide plate  23  driven through the cam  28 , the arm  27 , and the arm  26 . 
     Further, until the deflection formed in the middle part of the sheet is folded and conveyed between the folding rollers  60  and  61 , the stopper guide plate  22  is on standby and then returns to an initial position. 
     By making the distance between the stopper guide plates  22  and  23  smaller, it becomes possible to form the deflection of the sheet, which is formed when the sheet comes into contact with the stopper  21  at its tip end, at the position right above the nip of folding rollers  60  and  61 . 
     As described above, according to the second embodiment, the distance between the conveyance guide plates provided between the folding units and the contact unit with which the sheet comes into contact is made smaller when the sheet comes into contact with the contact unit at its tip end or rear end to form the deflection at the middle part of the sheet. As a result, the sheet has a lesser degree of freedom to form the deflection between the conveyance guide plates, thereby controlling the deflection of the sheet. 
     Thus, it is possible to reduce the fluctuation of the deflection formed in the sheet and make the sheet be conveyed between the folding units at an expected folding position. As a result, the folding accuracy of the sheet ejected from the sheet processing apparatus can be improved. 
     Further, in the case of folding the sheet for the second or subsequent time, the distance between the conveyance guide plates provided between the folding units and the contact unit with which the sheet comes into contact is made smaller when the sheet comes into contact with the contact unit to form the deflection at its tip end or rear end on which folding lines have been formed on the upstream side. 
     As described above, the sheet has a lesser degree of freedom to form the deflection between the conveyance guide plates, thereby controlling the deflection of the sheet. Thus, it is possible to make the sheet be conveyed between the folding units at an expected folding position. As a result, the position of the middle part of the sheet conveyed between the folding units is not shifted. In addition, the position of the folding lines of the sheet formed on the upstream side is not shifted from the folding position of the sheet conveyed between the folding units. Therefore, the box folding phenomenon can be prevented. 
     Moreover, the driving mechanism for moving the conveyance guide plate, which is provided between the folding units and the contact unit with which the sheet comes into contact so as to make the distance between the conveyance guide plates smaller, is connected to only one of the conveyance guide plates. As a result, it is possible to simply configure the mechanism of the sheet processing apparatus at low cost. 
     As described above, according to the second embodiment of the present invention, the folding accuracy of the sheet can be improved. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese Priority Application Nos. 2009-108104 filed on Apr. 27, 2009, and 2010-022391 filed on Feb. 3, 2010, the entire contents of which are hereby incorporated herein by reference.