Patent Publication Number: US-10781066-B2

Title: Sheet postprocessing device and image forming system therewith

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2018-102700 filed on May 29, 2018, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to a sheet postprocessing device for performing postprocessing such as binding and folding process on a sheet on which an image has been formed by image forming apparatuses such as copiers and printers, and also relates to image forming systems provided with such a sheet postprocessing device. 
     Conventionally, sheet postprocessing devices are used which can perform postprocessing including binding process in which a plurality of sheets on which images have been formed by an image forming apparatus such as a copier or printer are stacked and the stacked bundle of sheets is bound with staples, as well as folding process in which twofold or threefold folding is performed on a bundle of sheets. 
     In such sheet postprocessing devices, sheets on which images have been formed are conveyed in a sheet conveying passage and stacked on a sheet tray. When a predetermined number of sheets are stacked on the sheet tray, binding or folding process is applied on a bundle of sheet on the sheet tray. 
     For example, there are known sheet postprocessing devices in which a predetermined number of sheets are stacked on the sheet tray and binding and folding process is applied on a bundle of sheets on the sheet tray. 
     Inconveniently, however, in the conventional sheet postprocessing devices mentioned above, when a sheet is stacked on a sheet tray from a sheet conveying passage, the conveyed sheet may make close contact with the stacked sheets to cause a jam (sheet jam). 
     SUMMARY 
     According to one aspect of the present disclosure, a sheet postprocessing device includes a sheet conveying passage, a sheet tray, an inlet guide, a processing device, and a blowing device. Though the sheet conveying passage, a sheet on which an image has been formed is conveyed. The sheet tray is arranged downstream of the sheet conveying passage and has sheets stacked on it. The inlet guide is arranged at the downstream end of the sheet conveying passage and guides a sheet to the sheet tray. The processing device performs predetermined processing on a sheet stacked on the sheet tray. The blowing device blows air, from the upstream side in the sheet conveying direction, between the top face of the top-most sheet in the sheets stacked on the sheet tray and the bottom face of the subsequent sheet sent from the inlet guide into the sheet tray. 
     This and other objects of the present disclosure, and the specific benefits obtained according to the present disclosure, will become apparent from the description of embodiments which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the structure of an image forming system composed of a sheet postprocessing device according to an embodiment of the present disclosure and an image forming apparatus with which the sheet postprocessing device is coupled; 
         FIG. 2  is a sectional view showing the structure of the image forming apparatus with which the sheet postprocessing device according to the embodiment of the present disclosure is coupled; 
         FIG. 3  is a sectional view showing the structure of the sheet postprocessing device according to the embodiment of the present disclosure; 
         FIG. 4  is a sectional view showing the structure of and around a sheet folding unit of the sheet postprocessing device according to the embodiment of the present disclosure; 
         FIG. 5  is a diagram showing the structure of and around a first folding roller pair and a second folding roller pair in the sheet postprocessing device according to the embodiment of the present disclosure; 
         FIG. 6  is a diagram showing the structure of and around the first folding roller pair and the second folding roller pair in the sheet postprocessing device according to the embodiment of the present disclosure, illustrating a state where a bend is formed in a sheet on which first folding process has been applied; 
         FIG. 7  is a diagram showing the structure of and around the first folding roller pair and the second folding roller pair in the sheet postprocessing device according to the embodiment of the present disclosure, illustrating a state where a sheet on which first folding process has been applied is being discharged from a lower discharge port; 
         FIG. 8  is a diagram showing the structure of and around an inlet guide of the sheet postprocessing device according to the embodiment of the present disclosure; 
         FIG. 9  is a diagram showing the structure of an inlet roller pair in the sheet postprocessing device according to the embodiment of the present disclosure; 
         FIG. 10  is a diagram showing a state where the sheet folding unit of the sheet postprocessing device according to the embodiment of the present disclosure is drawn out from the postprocessing device main body; and 
         FIG. 11  is a block diagram showing control paths in the sheet postprocessing device according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the accompanying drawings, embodiments of the present disclosure will be described. 
     With reference to  FIGS. 1 to 11 , an image forming system composed of a sheet postprocessing device  30  according to one embodiment of the present disclosure and an image forming apparatus  10  with which the sheet postprocessing device  30  is coupled will be described. Although this embodiment deals with a multifunction peripheral as one example of the image forming apparatus  10 , the sheet postprocessing device  30  of the present disclosure can also be similarly coupled with any image forming apparatuses other than the digital multifunction peripheral, such as a laser printer, an ink-jet printer, and a facsimile machine. 
     As shown in  FIG. 1 , the image forming apparatus  10  is used in a form coupled with the sheet postprocessing device  30 . Based on image data input from outside via an unillustrated network communication portion or based on image data read by an image reading portion  11  arranged in an upper part of the image forming apparatus  10 , the image forming apparatus  10  prints an image on a sheet. As shown in  FIG. 2 , the image forming apparatus  10  includes a sheet feeding portion  15 , an image forming portion  18 , a fixing portion  19 , discharge roller pairs  23  and  24 , and a main body controller  100 . The sheet feeding portion  15  feeds sheets. The image forming portion  18  forms a toner image on a sheet. The fixing portion  19  fixes a toner image on a sheet. The discharge roller pairs  23  and  24  convey a fixed sheet and discharge it to a sheet discharge portion  21  and a sheet postprocessing device  30  respectively. The main body controller  100  controls the operation of the image forming apparatus  10 , and is configured to be communicable with a postprocessing controller  101 , which will be described later, of the sheet postprocessing device  30  so that the main body controller  100  also controls the postprocessing controller  101 . 
     The sheet postprocessing device  30  performs on the sheets conveyed from the image forming apparatus  10  postprocessing such as punch hole-forming, binding, and folding. The sheet postprocessing device  30  is not limited to one which performs postprocessing on the sheets conveyed automatically from the image forming apparatus  10 . Instead, it may be one which conveys on its own a sheet set on an unillustrated tray by a user to a position where postprocessing is possible to perform postprocessing on the sheet. 
     As shown in  FIG. 3 , the sheet postprocessing device  30  includes a punch hole forming device  33 , a stapling unit  35 , and a sheet folding unit  60 . The punch hole forming device  33  applies predetermined punching process on a sheet. The stapling unit  35  binds a plurality of stacked sheets with a staple. The sheet folding unit  60  applies folding process on a sheet. The punch hole forming device  33  and the stapling unit  35  are provided in a postprocessing device main body  31 , and the sheet folding unit  60  is removably fitted to the postprocessing device main body  31 . 
     The sheet postprocessing device  30  includes a sheet feeding port  36 , a main discharge tray  38 , a sub discharge tray  40 , a retraction drum  41 , a postprocessing controller  101 , various switching members, various rollers, and the like. Through the sheet feeding port  36 , the sheets discharged from a discharge portion  7  (see  FIG. 2 ) of the image forming apparatus  10  are conveyed in. The main discharge tray  38  receives the sheets discharged from a main discharge port  37 . The sub discharge tray  40  receives the sheets discharged from a sub discharge port  39 . The retraction drum  41  temporarily retracts the sheets in a predetermined conveying passage. The postprocessing controller  101  controls the sheet postprocessing device  30  comprehensively. The postprocessing controller  101  is one example of “a controller” according to the present disclosure. 
     The sheet feeding port  36  communicates with the main discharge port  37  with each other through a first conveying passage  42 . A second conveying passage  43  which is connected to the first conveying passage  42  to branch off from it is connected to the sub discharge port  39 . A third conveying passage  44  which is connected to the first conveying passage  42  to branch off from it is connected to the sheet folding unit  60 . A fourth conveying passage  45  connected to the third conveying passage  44  to branch off from it curves along the circumference of the retraction drum  41  and joins the first conveying passage  42 . The third conveying passage  44  is one example of “a sheet conveying passage” according to the present disclosure. 
     Sheets conveyed in through the sheet feeding port  36  are sent on to the downstream side by the registration roller pair  46 . At the downstream end of the first conveying passage  42 , a main discharge roller pair  47  for sending sheets out to the main discharge tray  38  is provided. The main discharge roller pair  47  is configured such that, when sending sheets on to the stapling unit  35 , the rollers separate from each other to release the nip. The main discharge tray  38  mainly receives a bundle of sheets which has been bound in the stapling unit  35 . It is also possible to receive sheets which are not post-processed or only punched on the main discharge tray  38 . 
     At the downstream end of the second conveying passage  43 , a sub discharge roller pair  48  for sending sheets out to the sub discharge tray  40  is provided. The sub discharge tray  40  mainly receives sheets which are discharged without being post-processed or which have only been punched at the postprocessing device  30 . 
     The punch hole forming device  33  is arranged between the sheet feeding port  36  and the registration roller pair  46  so as to face the first conveying passage  42  from above. The punch hole forming device  33  applies punching process on a sheet conveyed through the first conveying passage  42  with predetermined timing. 
     The stapling unit  35  is arranged on the downstream side of the first conveying passage  42  to face it from below. The stapling unit  35  applies stacking process by stacking a plurality of sheets to form a bundle of sheets, as wells as applies binding process by binding a bundle of stacked sheets with a staple. 
     The retraction drum  41 , when binding a plurality of bundles of sheets successively, if a previous bundle is being bound, retracts the first sheet of the subsequent bundle on the outer circumferential face of the retraction drum  41  temporarily, and then conveys it to the stapling unit  35  with the second sheet overlapping the first one. 
     Next, the sheet folding unit  60  in the sheet postprocessing device  30  will be described. In the following description, for convenience, “a sheet S” can be a single sheet S or a bundle of a plurality of sheets S. 
     As shown in  FIG. 4 , the sheet folding unit  60  is provided in a lower part of the sheet postprocessing device  30  and on the downstream side of the third conveying passage  44 . When a user selects folding process, the sheet folding unit  60  performs, for example, twofold folding or threefold folding process to a sheet S. 
     The sheet folding unit  60  includes a sheet inlet passage  61 , a sheet tray  63 , and an aligning member  65 . The sheet inlet passage  61  leads to the downstream end of the third conveying passage  44 . The sheet tray  63  is composed of an upstream-side sheet stacking portion  63   a  and a downstream-side sheet stacking portion  63   b  on which sheets S conveyed in from the sheet inlet passage  61  are stacked. The aligning member  65  aligns the position of sheets S stacked on the sheet tray  63 . 
     The sheet folding unit  60  is provided with a first folding device (folding device)  70 , a sheet entrance path  81 , and a second folding device (folding device)  90 . The first folding device (folding device)  70  performs first folding process on a sheet S. The sheet S on which first folding process has been applied by the first folding device  70  can enter a sheet entrance path  81 . The second folding device (folding device)  90  performs second folding process on the sheet S on which first folding process has been applied by the first folding device  70 . The first folding device (folding device)  70 , the second folding device (folding device)  90 , and a stapling device  67  are examples of “a processing device” according to the present invention. The processing device performs predetermined processing on a sheet S stacked on the sheet tray  63 . 
     The sheet folding unit  60  further includes a conveyance destination switching member  83  and a lower discharge tray (discharge tray)  87 . The conveyance destination switching member  83  switches the conveyance destination of the sheet S on which first folding process has been applied by the first folding device  70 . The lower discharge tray  87  receives a sheet S discharged from a lower discharge port (sheet discharge port)  85 . 
     The sheet inlet passage  61  is a passage for conveying into the sheet folding unit  60  the sheet S which has been conveyed through the third conveying passage  44 . The sheet inlet passage  61  is composed of an inlet guide  611  which guides a sheet S. At a downstream end of the inlet guide  611 , an inlet roller pair  612  for sending the sheet S into the sheet folding unit  60  is provided. 
     The upstream-side sheet stacking portion  63   a  and the downstream-side sheet stacking portion  63   b  are composed of, for example, a plate-like member, and they are provided so as to describe a straight line extending obliquely from upper right to lower left inside the sheet folding unit  60  (that is, so as to incline downward toward the downstream side). Specifically, the upstream-side sheet stacking portion  63   a  is arranged on the upstream side of a push-out mechanism  71 , which will be described later, in the sheet conveying direction. On the other hand, the downstream-side sheet stacking portion  63   b  is arranged apart from the upstream-side sheet stacking portion  63   a , on the downstream side of the push-out mechanism  71  in the sheet conveying direction. Arranged above the upstream-side sheet stacking portion  63   a  is the stapling device  67  which applies binding process on a bundle of sheets which is folded at the first folding device  70 . 
     The aligning member  65  includes an upper moving member  651 , a lower moving member  652 , and width-adjusting members  653   a  and  653   b . The upper and lower moving members  651  and  652  align the leading edge and the trailing edge of sheets S stacked on the upstream-side and downstream-side sheet stacking portions  63   a  and  63   b . The width-adjusting members  653   a  and  653   b  align the side ends of sheets S in the sheet width direction perpendicular to the sheet conveying direction. 
     The upper moving member  651  is fitted to an upstream-side belt  655  stretched between an upstream-side driving pulley  654   a  and an upstream-side driven pulley  654   b  which are arranged under the upstream-side sheet stacking portion  63   a . The lower moving member  652  is fitted to a downstream-side belt  657  stretched between a downstream-side driving pulley  656   a  and a downstream-side driven pulley  656   b  which are arranged under the downstream-side sheet stacking portion  63   b . The lower moving member  652  sustains the leading edge of a sheet S. Moving the upper and lower moving members  651  and  652  according to the size of sheets S (length in the sheet conveying direction) allows the position of the sheets S stacked on the upstream-side and downstream-side sheet stacking portions  63   a  and  63   b  to be aligned in the sheet conveying direction (that is, in the longitudinal direction of sheets S). 
     A pair of width-adjusting member  653   a  is provided on the upstream-side sheet stacking portion  63   a  at an interval in the sheet width direction (in the direction perpendicular to the plane of  FIG. 4 ). A pair of the width-adjusting member  653   b  is provided on the downstream-side sheet stacking portion  63   b  at an interval in the sheet width direction. A pair of width-adjusting member  653   a  moves, by a rack and pinion mechanism (unillustrated), according to the size of a sheet S (its length in the sheet width direction). A pair of width-adjusting member  653   b  moves, by a rack and pinion mechanism (unillustrated), according to the size of a sheet S (its length in the sheet width direction). This enables the width-adjusting members  653   a  and  653   b  to perform width alignment and skew correction for sheets S. 
     Alignment by the aligning member  65  is performed every time a sheet S is stacked on the sheet tray  63 . When sheets S reach a predetermined number, they are aligned by the aligning member  65 , and are then moved to the position for binding process or folding process. 
     Above the upstream-side and downstream-side sheet stacking portions  63   a  and  63   b , elastic upstream-side and downstream-side paddles  66   a  and  66   b  are provided respectively. The upstream-side and downstream-side paddles  66   a  and  66   b  rotate in the clockwise direction in  FIG. 4 , and thereby thrust the leading edge of a sheet S sent out from the sheet inlet passage  61  to the sheet tray  63  onto the lower moving member  652  of the aligning member  65 . When a sheet S is moved by the aligning member  65  to the position for binding process or folding process, the upstream-side and downstream-side paddles  66   a  and  66   b  are held at a rotational position retracted from the sheet S. 
     The first folding device  70  includes a push-out mechanism  71  and a first folding roller pair  75 . The push-out mechanism  71  pushes out a sheet S. The first folding roller pair  75  performs folding process on a sheet S pushed out by the push-out mechanism  71 . 
     The push-out mechanism  71  is arranged between the upstream-side and downstream-side sheet stacking portions  63   a  and  63   b , under the first folding roller pair  75 . The push-out mechanism  71  has a folding blade  72  made of sheet metal which makes contact with the bottom face of the sheet S. The push-out mechanism  71  has a motor and a transmission mechanism (neither of them is illustrated) which makes the folding blade  72  move perpendicularly to the bottom face of a sheet S. The folding blade  72  pushes out a sheet S and feeds it to a first nip portion N 1 , which will be described later, of the first folding roller pair  75 . 
     As shown in  FIG. 5 , the first folding roller pair  75  is composed of a first roller  76  and a second roller  77  which is located downstream of the first roller  76  in the sheet conveying direction. The first and second rollers  76  and  77  are driven to rotate by a motor via a driving force transmission mechanism (neither of these are illustrated). 
     Between the first and second rollers  76  and  77 , the first nip portion N 1  is formed to which a sheet S is fed in by the folding blade  72  of the push-out mechanism  71  (see  FIG. 4 ). When a sheet S passes through the first nip portion N 1  while being nipped, a first fold is formed on the sheet S. 
     On the downstream side of the first nip portion N 1  of the first folding roller pair  75 , there is provided a first discharge conveying passage  88  which leads to the lower discharge port  85  (see  FIG. 4 ). At the downstream end of the first discharge conveying passage  88 , there is provided a discharge roller pair  86 . The first discharge conveying passage  88  is a conveying passage for conveying a sheet S on which first folding process has been applied to the lower discharge port  85  without performing second folding process. 
     The sheet entrance path  81  is connected to the first discharge conveying passage  88  to branch off from it. The conveyance destination switching member  83  is provided at the branch portion between the sheet entrance path  81  and the first discharge conveying passage  88 , and by pivoting, it switches the conveyance destination of a sheet S on which first folding process has been applied between the first discharge conveying passage  88  and the sheet entrance path  81 . 
     The sheet entrance path  81  is provided so that a sheet S on which first folding process has been applied by the first folding device  70  can enter it to retract while being bent. The sheet entrance path  81  is arranged opposite the first roller  76  across the conveyance destination switching member  83 . The sheet entrance path  81  is curved in the direction along the circumferential face of the second roller  77 . 
     The sheet entrance path  81  is formed so as to correspond to the thickness of the maximum number of sheets S allowing folding process by the sheet folding unit  60 . For example, when folding process can handle one to five sheets, the sheet entrance path  81  is structured to have a space big enough to allow entry of sheets S with the thickness of five folded sheets S (the thickness after first folding process, that is, the thickness corresponding to ten sheets). 
     At the downstream end of the sheet entrance path  81 , a stopper  81   a  is provided. The first fold of a sheet S which has entered (retracted to) the sheet entrance path  81  strikes the stopper  81   a.    
     The second folding device  90  performs second folding process on a sheet S on which first folding process has been applied and which has struck the stopper  81   a.    
     Specifically, the second folding device  90  has a second folding roller pair  91 . The second folding roller pair  91  performs second folding process on a sheet S on which first folding process has been applied. The second folding roller pair  91  is composed of the first roller  76  mentioned above and a third roller  92  located above the first roller  76 . The first roller  76  is a common roller shared between the first and second folding roller pairs  75  and  91 . The third roller  92  is, via a driving force transmission mechanism, driven to rotate by a motor (neither of these are illustrated). 
     Between the first and third rollers  76  and  92 , there is formed a second nip portion N 2 . As shown in  FIG. 6 , with the leading edge of a sheet S on which first folding process has been applied in contact with the stopper  81   a , sheet conveyance by the first folding roller pair  75  is continued. This causes a bend S 1  to be formed in the sheet S, and the bend S 1  passes while being nipped by the second nip portion N 2  of the second folding roller pair  91 . This causes a second fold to be formed in the sheet S. 
     As shown in  FIG. 5 , on the downstream side of the second nip portion N 2  of the second folding roller pair  91 , a second discharge conveying passage  89  which joins the first discharge conveying passage  88  is provided. The second discharge conveying passage  89  is a conveying passage for conveying a sheet S on which second folding process has been applied to the lower discharge port  85  via the first discharge conveying passage  88 . 
     Next, with reference to  FIGS. 4 to 7 , folding process (operation) applied on a sheet S by the sheet folding unit  60  will be described. Folding process on a sheet S is applied by the postprocessing controller  101  (see  FIG. 3 ) included in the sheet postprocessing device  30 . 
     First, twofold folding process will be described. Twofold folding process is applied when a user selects a twofold mode using the operation panel  12  (see  FIG. 2 ) of the image forming apparatus  10 . The conveyance destination switching member  83  pivots to a position indicated by solid lines in  FIG. 5 , and thereby the conveyance destination of a sheet S on which first folding process has been applied by the first folding device  70  is directed to the first discharge conveying passage  88 . 
     A sheet S conveyed in through the sheet inlet passage  61  is stacked on the upstream-side and downstream-side sheet stacking portions  63   a  and  63   b  and is aligned by the aligning member  65 . The aligning member  65  arranges the sheet S at a predetermined position so that the folding position of the sheet S faces the tip end of the folding blade  72 . Next, the folding blade  72  of the push-out mechanism  71  is thrust out to raise the sheet S upward (in the direction perpendicular to the sheet S). Here, the folding blade  72  makes contact with the folding position of the sheet S. The sheet S raised by the folding blade  72  enters, while being bent, the first nip portion N 1  of the first folding roller pair  75 . In the sheet S which has passed through the first nip portion N 1 , the first fold is formed. The sheet S in which the first fold has been formed is, via the first discharge conveying passage  88 , discharged from the lower discharge port  85  to the lower discharge tray  87  (see  FIG. 7 ). The push-out mechanism  71  retracts the folding blade  72  to its original standby position. Thereafter, similar folding process is applied repeatedly. 
     Next, threefold folding process will be described. Threefold folding process is applied when a user selects a threefold mode using the operation panel  12  (see  FIG. 2 ) of the image forming apparatus  10 . The process up to first folding process on a sheet S by the first folding device  70  is similar to that in the twofold folding process described above, and thus no overlapping description will be repeated. The conveyance destination switching member  83  pivots to a position indicated by dash-dot-dot-lines in  FIG. 5 , and thereby the conveyance destination of a sheet S on which first folding process has been applied by the first folding device  70  is directed to the sheet entrance path  81 . Thus, a sheet S on which first folding process has been applied is conveyed toward the sheet entrance path  81 . The sheet S enters the sheet entrance path  81  and the first fold (folded end) in the sheet S strikes the stopper  81   a  of the sheet entrance path  81 . 
     After the first fold of the sheet S strikes the stopper  81   a , the first folding roller pair  75  continues to be driven to rotate. Thus, as shown in  FIG. 6 , the sheet S, while in contact with the inner face of the curved sheet entrance path  81 , the conveyance destination switching member  83 , and the like, bends so as to bulge toward the second nip portion N 2  of the second folding roller pair  91 . 
     The bend S 1  formed in the sheet S enters the second nip portion N 2  of the second folding roller pair  91 . On the sheet S which has passed the second nip portion N 2 , the second fold is formed. The sheet S on which the second fold has been formed is conveyed through the second discharge conveying passage  89  while winding around the circumferential face of the third roller  92  and is discharged by the discharge roller pair  86  from the lower discharge port  85  to the lower discharge tray  87 . 
     Next, the structure of and around the sheet inlet passage  61  will be described in detail. 
     As shown in  FIG. 8 , on the upstream side of the inlet roller pair  612  in the sheet conveying direction, a blowing device  110  is provided. The blowing device  110  blows air between the top face of the top-most sheet S in the sheets S stacked on the sheet tray  63  and the bottom face of the sheet S sent out from the inlet guide  611  to the sheet tray  63 . 
     Specifically, the blowing device  110  has a blowing fan  111  and a duct  112 . The blowing fan  111  generates a flow of air. The duct  112  is connected to the blowing fan  111  and discharges air from the blowing fan  111 . The duct  112  is provided so as to discharge air parallel to the sheet tray  63 . 
     In a part, facing the duct  112 , of a curved guide face  611   b  on the right side of the inlet guide  611  in  FIG. 8 , an air inflow port  611   a  for taking in air discharged from the duct  112  is formed. A plurality of inflow ports  611   a  are provided at predetermined intervals in the sheet width direction (in the direction perpendicular to the plane of  FIG. 8 ). That is, between the air inflow ports  611   a  themselves, partitions (unillustrated) extending along the sheet conveying direction are provided for preventing a sheet S from going outside through the air inflow port  611   a.    
     Below the inlet guide  611 , a guide member  615  for guiding a sheet S to the inlet roller pair  612  is provided. The inlet guide  611  and the guide member  615  form the sheet inlet passage  61 . The inlet guide  611  and the guide member  615  are curved in such directions that their respective upstream parts in the sheet conveying direction recede from each other. The guide member  615  has also a function of guiding air which has been discharged from the duct  112  and has not flowed into the air inflow port  611   a  to the bottom face of a sheet S. 
     The inlet roller pair  612  is composed of a driving roller  613  and a driven roller  614  which is in pressed contact with the driving roller  613 . The driving roller  613 , as shown in  FIG. 9 , has a rotary shaft  613   a  and a plurality of roller bodies  613   b  which are arranged at predetermined intervals in the axial direction and are fixed to the rotary shaft  613   a . The driven roller  614  has a rotary shaft  614   a  and a plurality of roller bodies  614   b  which are arranged at predetermined intervals in the axial direction and are fixed to the rotary shaft  614   a . Thus, air discharged from the blowing device  110  strikes the bottom face of a sheet S which passes through the inside of the inlet guide  611  in a curved state, then flows along the bottom face of the sheet S, then passes between the bottom face of the sheet S and the rotary shaft  614   a  of the driven roller  614 , and then flows between the top face of the top-most sheet S stacked on the sheet tray  63  and the bottom face of the sheet S discharged from the inlet guide  611  to the sheet tray  63 . 
     To the rotary shaft  613   a  of the driving roller  613 , a plurality of resin pressing members  616  are fitted which can rotate idly about the rotary shaft  613   a . When the pressing members  616  make contact with a sheet S being conveyed by the inlet roller pair  612 , they pivot to a position indicated by solid lines in  FIG. 8 . On the other hand, when the trailing edge of a sheet S has passed the inlet roller pair  612 , the pressing members  616  pivot, by its own weight, to a position indicated by dash-dot-dot-lines in  FIG. 8  and presses the upstream-side end part (trailing edge) of the sheet S against the sheet tray  63 . 
     As shown in  FIG. 4 , in the third conveying passage  44 , a sheet detection sensor  120  for sensing a sheet S is provided. 
     The third conveying passage  44 , the blowing device  110 , and the sheet detection sensor  120  are provided in the postprocessing device main body  31 . On the other hand, as mentioned above, the inlet guide  611 , the sheet tray  63 , the stapling device  67 , the first folding device  70 , the second folding device  90 , the lower discharge port  85 , and the lower discharge tray  87  are provided in the sheet folding unit  60 . Thus, when a sheet jam occurs in the sheet tray  63 , the stapling device  67 , the first folding device  70 , the second folding device  90 , or the like, the sheet folding unit  60  can be drawn out, as shown in  FIG. 10 , with the third conveying passage  44  and the blowing device  110  left behind in the postprocessing device main body  31 . 
       FIG. 11  is a block diagram showing the control paths in the sheet postprocessing device  30 . When the sheet postprocessing device  30  is used, different parts of the device are controlled in different manners, and thus the control paths in the whole sheet postprocessing device  30  are complex. Thus, the following description focuses on those control paths which are essential for the implementation of the present disclosure. 
     As shown in  FIG. 11 , the postprocessing controller  101  is composed of a CPU (central processing unit), a ROM (read-only memory), a RAM (random access memory), and the like, and is configured to be communicable with the main body controller  100  of the image forming apparatus  10 . Also, the postprocessing controller  101  can control the punch hole forming device  33 , the stapling unit  35 , the blowing device  110 , the sheet folding unit  60 , various rollers, and the like, and is configured to be communicable with the sheet detection sensor  120 . 
     The ROM stores data and the like that are not changed during the use of the sheet postprocessing device  30 , such as a control program for the sheet postprocessing device  30  and values needed for control. The RAM stores necessary data generated while the sheet postprocessing device  30  is controlled, data temporarily needed to control the sheet postprocessing device  30 , and the like. 
     The postprocessing controller  101  can control the stapling device  67 , the first folding device  70 , the second folding device  90 , the upstream-side driving pulley  654   a , the downstream-side driving pulley  656   a , the conveyance destination switching member  83 , and the like. The postprocessing controller  101  controls the whole sheet postprocessing device  30 . 
     Here, in this embodiment, when performing postprocessing on a sheet S by the sheet folding unit  60 , the postprocessing controller  101  makes the blowing device  110  start to blow air when the sheet detection sensor  120  senses the first sheet S. 
     The postprocessing controller  101 , when applying binding process on a sheet S by the stapling device  67 , makes the aligning member  65  align a predetermined number of sheets S and then move them to the binding process position. Here, the postprocessing controller  101  makes the aligning member  65  move the sheets S to the binding process position and, substantially at the same time, makes the blowing device  110  stop blowing air. 
     The postprocessing controller  101 , when applying folding process on a sheet S by the first folding device  70 , makes the aligning member  65  align a predetermined number of sheets S and then move them to the folding process position. Here, the postprocessing controller  101  makes the aligning member  65  move the sheets S to the folding process position and, substantially at the same time, makes the blowing device  110  stop blowing air. When further applying folding process on the sheets S on which binding process has been applied, the postprocessing controller  101 , while keeping the blowing device  110  from blowing air, has the sheets S moved from the binding process position to the folding process position. 
     In this embodiment, as described above, the blowing device  110  is provided which blows air between the top face of the top-most sheet S in the sheets S stacked on the sheet tray  63  and the bottom face of a sheet S sent out from the inlet guide  611  to the sheet tray  63 . This permits an air layer to be formed between the bottom face of a sheet S sent from the inlet guide  611  into the sheet tray  63  (hereinafter also called a subsequent sheet S) and the top face of the top-most sheet S in the sheets which have already been stacked on the sheet tray  63  (hereinafter also called a top-most sheet S on the sheet tray  63 ), and thereby it is possible to prevent the subsequent sheet S from making close contact with the top-most sheet S on the sheet tray  63 . This helps prevent the occurrence of a sheet jam. 
     As mentioned above, the blowing device  110  is arranged upstream of the inlet roller pair  612 . This allows air to be easily blown between the top face of the top-most sheet S on the sheet tray  63  and the bottom face of the subsequent sheet S. 
     Also, as mentioned above, the pressing member  616  is provided which is arranged downstream of the inlet guide  611  and presses the upstream-side end part (trailing edge) of a sheet S which has passed the inlet roller pair  612 . This helps suppress a rise (curl) at the trailing edge of a sheet S on the sheet tray  63 , and thus it is possible to prevent the downstream-side end part (leading edge) of the subsequent sheet S from being caught by the trailing edge of the sheet S on the sheet tray  63 . This helps prevent the occurrence of a sheet jam. 
     As mentioned above, the sheet tray  63  inclines downward toward the downstream side. This helps reduce the angle at which the subsequent sheet S approaches the sheet S on the sheet tray  63 . That is, the subsequent sheet S can be conveyed nearly parallel to the sheet S on the sheet tray  63 . This make it easier for the air to flow along the top face of the top-most sheet on the sheet tray  63  and the bottom face of the subsequent sheet, and thereby it is possible to prevent the subsequent sheet S from making close contact with the top-most sheet S on the sheet tray  63 . 
     As mentioned above, the duct  112  discharges air substantially parallel to the sheet tray  63 , and thus it is possible to pass (send) air smoothly between the top-most sheet S on the sheet tray  63  and the subsequent sheet S. 
     Also, as mentioned above, the postprocessing controller  101  starts the air blow by the blowing device  110  when the sheet detection sensor  120  senses the sheet S, stops the air blow by the blowing device  110  when, during binding process, making the aligning member  65  move a bundle of sheets on the sheet tray  63  to the binding process position, and stops the air blow when, during folding process, making the aligning member  65  move the sheet S on the sheet tray  63  to the folding process position. This prevents, during binding or folding process, a sheet S from being flipped by the airflow. 
     Also, as mentioned above, the postprocessing device main body  31  which includes the third conveying passage  44  and the blowing device  110 , as well as the sheet folding unit  60  which includes the inlet guide  611 , the sheet tray  63 , the stapling device  67 , the first folding device  70 , the second folding device  90 , the lower discharge port  85  and the lower discharge tray  87  and which is removable from the postprocessing device main body  31  are provided. Thus, when a sheet jam occurs in the sheet tray  63 , stapling device  67 , the first folding device  70 , or the second folding device  90  and jam handling is performed, the sheet folding unit  60  can be drawn out with the blowing device  110  and the third conveying passage  44  left behind in the postprocessing device main body  31 . That is, the sheet tray  63 , stapling device  67 , the first folding device  70 , and the second folding device  90  can separate from the blowing device  110  and the third conveying passage  44 . This facilitates jam handling. 
     The embodiments disclosed above should be understood to be in every aspect illustrative and not restrictive. The scope of the present disclosure is defined not by the description of the embodiments given above but by the appended claims, and should be understood to encompass any modifications made in the sense and scope equivalent to those of the claims. 
     For example, although the above embodiments deal with an example where the sheet folding unit  60  is provided with the first folding device  70  and the second folding device  90 , this is in no way meant to limit the present disclosure. Instead, the sheet folding unit  60  may not be provided with the second folding device  90 . 
     Also, although the above embodiments deal with an example where the pressing member  616  is provided which presses the upstream-side end part (trailing edge) of the sheet S which has passed the inlet roller pair  612 , this is in no way meant to limit the present disclosure. Instead, the pressing member  616  may not be provided. In this case, for example, every time a sheet S is stacked on the sheet tray  63 , the lower moving member  652  can be moved to the upstream side and, before the subsequent sheet S is conveyed, the lower moving member  652  can be retracted to the original position. In this way, moving the lower moving member  652  upstream allows the upstream-side end part (trailing edge) of the sheet S on the sheet tray  63  to be pressed by the driven roller  614  of the inlet roller pair  612  or by the guide member  615 . This helps suppress a rise (curl) at the trailing edge of a sheet S on the sheet tray  63 , and thus it is possible to prevent the subsequent sheet S from being caught by the trailing edge of the sheet S on the sheet tray  63 .