Patent Publication Number: US-2010117284-A1

Title: Sheet folding apparatus, image forming apparatus, and sheet folding method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from: U.S. provisional applications 61/114,024 filed on Nov. 12, 2008, and 61/150,259 filed on Feb. 5, 2009, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a sheet folding apparatus, an image forming apparatus, and a sheet folding method, and particularly to a sheet folding apparatus, an image forming apparatus, and a sheet folding method for stitching and folding a printed sheet. 
     BACKGROUND 
     There is a sheet finisher which is installed downstream from an image forming apparatus such as a copier, printer or multi-function peripheral (MFP) and carries out finishing such as punching and stitching of printed sheets. 
     Recently, sheet finishers have more diverse functions. There is a sheet folding apparatus having a folding function to fold a part of a sheet and a saddle stitching and middle-folding function to stitch the middle of a sheet with staples and then fold the sheet in the middle, in addition to the punching and stitching functions (see JP-A-6-266182). 
     The sheet folding apparatus having the saddle stitching and middle-folding function is capable to produce a booklet from plural printed sheets (bookbinding). 
     The sheet folding apparatus stacks printed sheets on a sloped tray called stack tray. After the middle of the sheets is stitched with staples or the like, a pair of rollers called fold rollers forms a fold at the stitched part. 
     A sheet printed by an image forming apparatus passes through a carrying path having an exit facing upward, and is discharged to the stack tray. The sheet discharged from the carrying path moves up in a free state toward the upper side of the slope of the stack tray. Then, the sheet falls by its own weight along the slope of the stack tray. 
     There is a roller called assist roller ahead of the exit of the carrying path. When a sheet is to be sent out to the stack tray, the assist roller is located at a position (first position) where the assist roller does not prevent the sheet from being sent out. When the sheet is completely sent out to the stack tray, the assist roller moves to a position (second position) where the assist roller is abutted against the sent-out sheet. The assist roller rotates in a direction of moving the sheet on the stack tray downward and guides the downward movement of the sheet. 
     A stacker having a stack pawl stands by below the stack tray. The stacker receives the lower end of the sheet pushed down from the upper side of the slope of the stack tray by the assist roller. 
     When a subsequent sheet is to be sent out to the stack tray, the assist roller returns to the position where the assist roller does not prevent the sheet from being sent out. The assist roller reciprocates between the first position and the second position, for example, by the tensile force of a solenoid and the restoring force of a spring coil. The assist roller pushes the sheet down and thereby pushes the rear end of the sheet against the stacker. Thus, the sheet is longitudinally aligned. 
     In order to improve the speed of booklet production, it is necessary to increase the carrying speed of a sheet moving through the carrying path. However, in the conventional sheet folding apparatus, if a sheet is discharged at a high speed from the carrying path, the sheet may fly out significantly upward at the time of discharge and then collide with another unit situated above the stack tray. 
     If the sheet collides with another unit, inconvenience occurs such as damage to the sheet and generation of unwanted collision noise. To avoid such inconvenience, a dedicated carrying path to guide the sheet which flies out of an exit of the carrying path must be provided, or a separate member to protect the end of the sheet must be provided, causing reduction in spatial use efficiency and increase in cost. 
     Also, since a sheet discharged from the exit of the carrying path is in a completely free state until guidance by the assist roller starts, there is a high probability that the direction of the sheet is shifted from an upright position and becomes skewed. 
     SUMMARY 
     In view of the above circumstances, it is an object of the invention to provide a sheet folding apparatus, an image forming apparatus, and a sheet folding method in which unwanted flying-out of a sheet can be restrained when the sheet is discharged from the carrying path to the stack tray, and in which the sheet stacked on the stack tray can be prevented from being skewed. 
     An aspect of the disclosure is a sheet folding apparatus including: a carrying path through which a printed sheet is carried and which has an exit facing upward; a tray on which the sheet carried through the carrying path is stacked while being vertically sloped; an assist roller which rotates in a manner of pushing the sheet against the tray and pushing the sheet upward on the tray before a rear end of the sheet is discharged from the exit after a forward end of the sheet passes the exit of the carrying path, and which rotates in a reverse direction in a manner of pushing the sheet downward on the tray after the rear end of the sheet is discharged from the exit; and a stacker which is provided below the tray and receives and supports the rear end of the sheet pushed down by the assist roller. 
     Another aspect of the disclosure is an image forming apparatus including: an image forming unit which prints image data on a sheet; a carrying path through which the sheet that is printed is carried and which has an exit facing upward; a tray on which the sheet carried through the carrying path is stacked while being vertically sloped; an assist roller which rotates in a manner of pushing the sheet against the tray and pushing the sheet upward on the tray before a rear end of the sheet is discharged from the exit after a forward end of the sheet passes the exit of the carrying path, and which rotates in a reverse direction in a manner of pushing the sheet downward on the tray after the rear end of the sheet is discharged from the exit; and a stacker which is provided below the tray and receives and supports the rear end of the sheet pushed down by the assist roller. 
     Still another aspect of the disclosure is a sheet folding method including: carrying a printed sheet through a carrying path having an exit facing upward; pushing the sheet against a vertically sloped tray by an assist roller and pushing the sheet upward on the tray by rotation of the assist roller before a rear end of the sheet is discharged from the exit after a forward end of the sheet passes the exit of the carrying path; rotating the assist roller in a reverse direction and thus pushing the sheet downward on the tray after the rear end of the sheet is discharged from the exit; receiving and supporting the rear end of the sheet pushed down by the assist roller, by a stacker provided below the tray; and pushing a middle part of a plurality of the sheets stacked in the tray and having their rear ends received by the stacker, into a nip of a folding roller pair by a folding blade, and thus forming a fold thereon. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the attached drawings, 
         FIG. 1  is a perspective view showing an exemplary appearance of an image forming apparatus; 
         FIG. 2  is a sectional view showing an exemplary configuration of the image forming apparatus; 
         FIG. 3  is a sectional view showing an exemplary configuration of a sheet folding apparatus; 
         FIG. 4  is an enlarged sectional view showing a part of the sheet folding apparatus; 
         FIG. 5  is a sectional view particularly showing an assist roller and an exemplary roller driving system in a carrying path, of the configuration of the sheet folding apparatus; 
         FIG. 6  is a perspective view particularly showing the assist roller and the exemplary roller driving system in the carrying path, of the configuration of the sheet folding apparatus; 
         FIG. 7  is a flowchart showing an example of processing until a sheet is stacked on a stacker after exiting the carrying path; 
         FIG. 8A  to  FIG. 8C  and  FIG. 9A  to  FIG. 9C  are conceptual views of operation for explaining the movement of the assist roller; 
         FIG. 10  is a functional block diagram showing an exemplary configuration of a driving system of a solenoid; 
         FIG. 11A  to  FIG. 11C  show a conventional example of a driving control sequence of a solenoid; and 
         FIG. 12A  to  FIG. 12D  show exemplary driving control sequences of a solenoid according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of a sheet folding apparatus, an image forming apparatus, and a sheet folding method will be described with reference to the attached drawings. 
     (1) Configuration of Image Forming Apparatus and Sheet Folding Apparatus 
       FIG. 1  is a perspective view of appearance showing a basic exemplary configuration of an image forming apparatus  10  according to the embodiment. The image forming apparatus  10  has a scanning unit  11  which scans an original, an image forming unit  12  which electrographically prints image data of the scanned original onto a sheet, a sheet finisher  20  which performs finishing such as sorting, punching, folding, and saddle stitching of the printed sheet, and so on. The image forming unit  12  has an operation unit  9  for a user to carry out various operations. 
       FIG. 2  is a sectional view showing an exemplary configuration of details of the image forming apparatus  10 . 
     The image forming unit  12  of the image forming apparatus  10  has a photoconductive drum  1 . Around the photoconductive drum  1 , there is a charging unit  2 , an exposure unit  3 , a developing unit  4 , a transfer unit  5 A, a neutralizing unit  5 B, a separation pawl  5 C, and a cleaning unit  6 . A fixing unit  8  is provided downstream from the neutralizing unit  5 B. These units carry out image formation substantially according to the following procedures. 
     The charging unit  2  uniformly charges the surface of the photoconductive drum  1 . The scanning unit  11  converts a scanned original to image data and outputs the image data to the exposure unit  3 . The exposure unit  3  irradiates the photoconductive drum  1  with a laser beam of an intensity corresponding to the level of the image data, and thus forms an electrostatic latent image on the photoconductive drum  1 . The developing unit  4  develops the electrostatic latent image with toner and forms a toner image on the photoconductive drum  1 . 
     A sheet housing unit  7 A houses sheets. Several carrying rollers carry a sheet from the sheet housing unit  7 A to a transfer position (a gap between the photoconductive drum  1  and the transfer unit  5 A). At the transfer position, the transfer unit  5 A transfers the toner image on the photoconductive drum  1  to a photoconductive sheet. The neutralizing unit  5 B eliminates electric charges on the surface of the sheet to which the toner image is transferred. The separation pawl  5 C separates the sheet from the photoconductive drum  1 . An intermediate transfer unit  7 B carries the separated sheet to the fixing unit  8 . The fixing unit  8  heats and pressurizes the sheet and thus fixes the toner image on the sheet. A discharge unit  7 C outputs the sheet, on which fixing is finished, to the sheet finisher  20 . 
     The cleaning unit  6  located downstream from the separation pawl  5 C removes the developer remaining on the surface of the photoconductive drum  1  and prepares for next image formation. 
     In the case of double-side printing, the sheet having the toner image fixed to its surface is diverted from the ordinary discharge path by a carrying path switching plate  7 D. The sheet is then switched back and reversed in a reverse carrying unit  7 E. Printing similar to one-side printing is carried out on the back side of the reversed sheet. The sheet is then outputted from the discharge unit  7 C to the sheet finisher  20 . 
     The sheet finisher  20  has a finishing unit  21 , a sheet folding apparatus  30 , a sheet bundle placing unit  41  and so on. 
     The finishing unit  21  carries out processing to discharge a printed sheet to a fixed tray  22  or a movable tray  23 , and processing to stitch an edge of a sheet bundle by a staple unit  24 . 
     The sheet folding apparatus  30  is located below the finishing unit  21 . The sheet folding apparatus  30  stitches the middle of plural printed sheets with staples, folds the part stitched with staples in the middle, and thus produces a booklet (saddle stitching). 
     The booklet that is saddled-stitched by the sheet folding apparatus  30  is outputted to the sheet bundle placing unit  41 . The bound booklet is ultimately placed in the sheet bundle placing unit  41 . 
       FIG. 3  is a sectional view showing an exemplary configuration of details of the sheet folding apparatus  30 .  FIG. 4  is an enlarged sectional view showing a part of the sheet folding apparatus  30 . 
     An entrance roller pair  31  provided at the entrance of the sheet finisher  20  receives the printed sheet discharged from the discharge unit  7 C of the image forming unit  12  and delivers the sheet to an intermediate roller pair  32 . The intermediate roller pair  32  delivers the sheet to an exit roller pair  33 . The exit roller pair  33  sends out the sheet to a stack tray  34  having a sloped placing surface. 
     The path of the sheet from the entrance roller pair  31  to the exit roller pair  33  is hereinafter called carrying path  300  for sheet. An exit  300   a  of the carrying path  300  faces upward. The forward end of the sheet discharged from the exit  300   a  heads upward on the slope of the stack tray  34 . 
     An assist roller  332  is provided ahead of the exit roller pair  33 . The operation and action of the assist roller  332  will be described later. 
     A stacker  35  having a stack pawl  211  stands by below the stack tray  34 . The stacker  35  receives the lower end of the sheet sent from the carrying path  300  and the sheet is stacked sequentially. 
     The assist roller  332  pushes down the sheet toward the stacker  35  to align the sheet longitudinally. When the number of sheets reaches to a prescribed number designated by the operation unit  9 , a lateral alignment unit  40  carries out lateral alignment of the sheets. 
     A stapler (saddle stitching unit)  36  is provided at an intermediate part of the stack tray  34 . When the stack tray  34  receives a sheet, the position of the stack tray  34  rises from a standby position S 1  to a sheet receiving position S 2 , as shown in  FIG. 4 . The sheet receiving position S 2  is adjusted so that the position on the sheet bundle to be stapled (the middle in vertical direction of the sheet bundle) faces the stapler  36 . 
     As the sheet bundle is saddle-stitched by the stapler  36 , the stacker  35  moves down until the position on the sheet bundle where a fold should be formed (the position that is the middle in the vertical direction of the sheet bundle and is stapled) comes in front of a folding blade  37  (folding position S 3  in  FIG. 4 ). 
     As the position where a fold should be formed comes in front of the folding blade  37 , a forward end  37   a  of the folding blade  37  pushes in the surface that should be the inner surface after the sheet bundle is folded. 
     A folding roller pair  38  is provided ahead in the direction of movement of the folding blade  37 . The nip of the folding roller pair  38  drags in the sheet bundle that is pushed in by the folding blade  37 . Thus, a fold is formed in the middle of the sheet bundle. 
     After forming the fold on the sheet bundle, the folding roller pair  38  carries the sheet bundle to a fold reinforcing unit  50  (see  FIG. 3 ) on the downstream side. 
     The fold reinforcing unit  50  has a fold reinforcing roller pair  51  (an upper roller (second roller)  51   a  and a lower roller (first roller)  51   b ). When the folding roller pair  38  carries the sheet bundle to the position where the fold on the sheet bundle faces the fold reinforcing roller pair  51 , the folding roller pair  38  stops rotating and temporarily stops carrying the sheet bundle. 
     The fold reinforcing roller pair  51  moves in a direction orthogonal to the carrying direction of the sheet bundle (direction along the fold line) while pressurizing the fold. The fold is thus reinforced. 
     After the fold is reinforced by the fold reinforcing unit  50 , the folding roller pair  38  resumes rotation and carries the sheet bundle toward a discharge roller pair  39 . The discharge roller pair  39  sends out the sheet bundle to the sheet bundle placing unit  41 . The sheet bundle placing unit  41  stacks the saddle-stitched sheet bundle (booklet). 
     (2) Operation of Assist Roller 
       FIG. 5  and  FIG. 6  show an exemplary configuration of a driving system of the assist roller  332 , the intermediate roller pair  32 , the exit roller pair  33  and the like. 
     As shown in  FIG. 6 , a carrying motor  301  transmits a rotational driving force to an intermediate roller gear  304  via a gear  303 . The intermediate roller gear  304  rotates a driving roller  32   b  of the intermediate roller pair  32  via a shaft  352 . The intermediate roller pair  32  is located in an upper part of the carrying path  300  and carries a sheet downward in the carrying path  300  by the rotation of the driving roller  32   b  and a driven roller  32   a . A sheet detection sensor  333  is provided at a halfway part of the carrying path  300 . 
     The intermediate roller gear  304  fits with a gear  305  and transmits the rotational driving force of the carrying motor  301 . The rotation of the gear  305  is transmitted to a exit roller gear  308  via a timing belt  310  laid across a gear  306 , a tension roller  307 , the exit roller gear  308  and a tension roller  309 . 
     The exit roller gear  308  rotates a driving roller  33   b  of the exit roller pair  33  via a shaft  350 . The exit roller pair  33  is located at the exit of the carrying path  300  and pushes out a sheet upward above the carrying path  300  by the rotation of the driving roller  33   b  and a driven roller  33   a.    
     An intermediate gear  311  that is axially fixed to the shaft  350  is provided ahead of the exit roller gear  308 . The intermediate gear  311  fits with an assist roller gear  312 . The assist roller gear  312  rotates the assist roller  332  via a shaft  351  in the opposite direction of the driving roller  33   b  of the exit roller pair  33 . 
     There is a solenoid  334  below the carrying path  300 . A plunger  334   a  of the solenoid  334  engages with a solenoid gear  340  that is axially fixed to a shaft  341 . When the solenoid  334  is on, the electromagnetic force of the solenoid  334  pulls the plunger  334   a  toward the inner side of the solenoid  334 . As the plunger  334   a  is pulled in, the solenoid gear  340  and the shaft  341  rotate clockwise in  FIG. 5  and  FIG. 6 . 
     As the shaft  341  rotates, when the solenoid  334  is on, first levers  342  at both ends of the shaft  341  rotate clockwise as well. 
     Meanwhile, second levers  343  are provided at both ends of the shaft  350  that axially fixes the driving roller  33   b  of the exit roller pair  33 . The shaft  350  supports the central part of the second levers  343  so that the second levers  343  pivotally turn about a pivot P. A pawl  344  engaged with the first lever  342  is provided at the lower ends of the second levers  343 . The upper ends of the second levers  343  pivotally support the shaft  351  of the assist roller  332 . 
     If the first levers  342  rotate clockwise, the engagement of the pawls  344  causes the second levers  343  to rotate counterclockwise about the pivot P, and also the positions of the shaft  351  and the assist roller  332  rotationally move counterclockwise about the pivot P. Consequently, when the solenoid  334  is on, the assist roller  332  moves from a first position A to a second position B shown in  FIG. 5 . 
     An engagement pawl  342   a  is provided at a proximal part of the first levers  342 , as shown in  FIG. 5 . One end of a spring  360  engages with the engagement pawl  342   a . The other end of the spring  360  engages with a frame of the sheet folding apparatus  30 . 
     When the solenoid  334  is turned on, the first levers  342  rotate clockwise as described above and the engagement pawl  342   a  pulls the spring  360 . When the solenoid  334  is turned on from off-state, the assist roller  332  moves from the first position A to the second position B. While the solenoid  334  is on, the assist roller  332  maintains the second position B against the energizing force of the spring  360 . 
     When the solenoid  334  is turned off from on-state, the plunger  334   a  is released from the electromagnetic force. Consequently, the energizing force of the spring  360  causes the first levers  342  to rotate counterclockwise and the second levers  343  to rotate clockwise. The assist roller  332  returns to the first position A from the second position B. 
     Thus, the assist roller  332  reciprocates between the first position A and the second position B according to on and off state of the solenoid  334 . 
       FIG. 7  is a flowchart showing an example of processing until a sheet is stacked on the stacker  35  after exiting the carrying path  300 .  FIG. 8A  to  FIG. 8C  and  FIG. 9A  to  FIG. 9C  are conceptual views of operation for explaining the movement of the assist roller  332  and the like in the processing. 
     The sheet detection sensor  333  is provided in the carrying path  300  and detects the passage of a sheet S. The sheet detection sensor  333  is, for example, an optical sensor. The sheet detection sensor  333  is a sensor which turns on when the forward end of the sheet S passing through the carrying path  300  is detected and which turns off when the rear end of the sheet S passes. 
     In ACT  10  of  FIG. 7 , it is determined whether the sheet S is detected by the sheet detection sensor  333  or not. The part of the sheet S to be detected is not particularly limited at this point. For example, the forward end of the sheet S may be detected. 
     In ACT  11 , the assist roller  332  starts rotating. While the sheet S passes through the carrying path  300 , the exit roller pair  33  (the driving roller  33   b  and the driven roller  33   a ) rotates in the direction of an arrow shown in  FIG. 8A  and carries the sheet S toward the exit  300   a  of the carrying path  300 . The driving roller  33   b  is connected to the intermediate gear  311  at one end of the shaft  350 , as shown in  FIG. 6 . The intermediate gear  311  fits with the assist roller gear  312  which is coaxial with the assist roller  332 . Therefore, the assist roller  332  rotates in the opposite direction of the driving roller  33   b  of the exit roller pair  33  and rotates in the direction of moving the sheet S upward. The diameter of the assist roller  332  is set in such a manner that the upward moving speed of the sheet S becomes the same as the speed of the sheet S moving through the carrying path  300 . 
     In ACT  11 , the assist roller  332  starts rotating by the detection of the sheet S by the sheet detection sensor  333 . The sheet S may be detected further upstream in the carrying path  300 , for example, at the position of the entrance roller pair  31  located at the entrance of the sheet finisher  20 , and the assist roller  332  may start synchronously with the start of rotation of the intermediate roller pair  32  and the exit roller pair  33 . 
     When the assist roller  332  starts rotating, the assist roller  332  is located at a standby position where the assist roller  332  does not prevent the sheet S from being discharged from the exit  300   a  of the carrying path  300 , that is, at the first position A in  FIG. 5 ,  FIG. 8A  and the like. 
     In ACT  12 , it is determined whether or not the forward end of the sheet S reaches an abutment position of the assist roller  332  and the stack tray  34 . The position of the forward end of the sheet S can be found from the elapsed time from the time when the sheet detection sensor  333  detects the forward end of the sheet S, and the carrying speed of the sheet S. 
     Here, the abutment position refers to the position where the outer circumferential surface of the assist roller  332  and the surface of the stack tray  34  are abutted against each other when the assist roller  332  is pivotally turned about the pivot P. When the assist roller  332  is located at the second position shown in  FIG. 5 ,  FIG. 8B  and the like, the outer circumference of the assist roller  332  and the surface of the stack tray  34  are abutted against each other at the abutment position. 
     If it is determined in ACT  12  that the forward end of the sheet S reaches the abutment position, the assist roller  332  moves from the standby position (first position A) to the second position B in ACT  13 . The assist roller  332  moves by turning on of the solenoid  334  as described before. 
     When the assist roller  332  moves to the second position B, the rear end of the sheet S is still in the carrying path  300 . If the movement of the assist roller  332  is too early, the assist roller  332  may push the forward end of the sheet S, and may push the sheet S back to the direction of the carrying path  300 . Thus, with a certain margin period after the forward end of the sheet S reaches to the abutment position, the assist roller  332  may move to the second position. 
     When the assist roller  332  moves to the second position B, the assist roller  332  rotates in a manner of pushing the sheet S against the stack tray  34  and pushing the sheet S upward on the stack tray  34  (see  FIG. 8B ). 
     The assist roller  332  rotates to push the sheet S up along the slope of the stack tray  34  (see  FIG. 8C ). 
     In ACT  14 , it is determined whether or not the rear end of the sheet S passes through the exit  300   a  of the carrying path  300 . This determination, too, is made according to the elapsed time from the time when the sheet detection sensor  333  detects the forward end of the sheet S. 
     If it is determined that the rear end of the sheet S passes through the exit  300   a , the assist roller  332  stops rotating (ACT  15 ). Then, the assist roller  332  rotates in the reverse direction, that is, the direction of moving the sheet S downward (ACT  16 ). 
     In order to reverse the direction of rotation of the assist roller  332 , the direction of rotation of the carrying motor  301  can be reversed and the direction of rotation of the driving roller  33   b  of the exit roller pair  33  can be reversed. After the rear end of the sheet S passes through the exit  300   a , there is no sheet S in the carrying path  300  until the next sheet S is delivered. Therefore, there is no problem with reversing the direction of rotation of the intermediate roller pair  32  and the exit roller pair  33 . 
     As shown in  FIG. 9A , with the reversal of the direction of rotation, the assist roller  332  pushes down the sheet S along the slope of the stack tray  34 . 
     To prevent the sheet S from returning back to the carrying path  300 , with a predetermined margin period after the rear end of the sheet S passes through the exit  300   a  of the carrying path  300 , the assist roller  332  may stop rotating. With the margin period, the entire sheet S is completely laid on the stack tray  34 . Then, the assist roller  332  starts rotating in the reverse direction. 
     In ACT  17 , it is determined whether longitudinal alignment is finished or not, that is, whether or not the rear end of the sheet S slides down to the position where the rear end of the sheet S abuts against the stack pawl  211  of the stacker  35  (see  FIG. 9B ). 
     When longitudinal alignment is finished, the solenoid  334  is turned off and the assist roller  332  returns from the abutment position (second position B) to the standby position (first position A) in ACT  18  (see  FIG. 9C ). Then, the assist roller  332  stops rotating (ACT  19 ) and prepares for the following sheet S sent into the carrying path  300 . 
     As a prescribed number of sheets are stacked in the stacker  35 , the stacked sheet bundle is saddle-stitched or middle-folded to create a booklet. 
     A conventional assist roller only plays the role of causing the sheet S on the stack tray  34  to move downward. Therefore, the sheet S discharged from the carrying path  300  flies out toward the upper side of the stack tray  34  in a free state without being restricted by the assist roller or the like. As the carrying speed of the sheet S increases, the amount of upward flying of the sheet at the time of discharge increases as well, and the sheet may collide with another unit located above the stack tray  34 , for example, the staple unit  24  or the like of the finishing unit  21  shown in  FIG. 2 . If the sheet collides with another unit, inconvenience occurs such as damage to the sheet and unwanted collision noise. 
     Since the sheet discharged from the exit of the carrying path is in a completely free state until downward guidance by the assist roller is started, there is a high probability that the direction of the sheet may be shifted from upright and the sheet may be skewed. 
     In contrast, the sheet folding apparatus  30  and the sheet folding method according to the embodiment employ the following technique. That is, as soon as the forward end of the sheet S exits the exit  300   a  of the carrying path  300 , the assist roller  332  presses the sheet S against the stack tray  34  and pushes up the sheet S toward the upper side of the stack tray  34  by the rotation of the assist roller  332 . Further, as the rear end of the sheet S exits the exit  300   a  of the carrying path  300 , the assist roller  332  reverses the direction of the rotation. Then, the assist roller  332  pushes down the sheet S toward the lower side of the stack tray  34  to align the sheet S longitudinally. In the above mentioned technique, the assist roller  332  constantly restricts the sheet S, and thus prevents the sheet S from flying up from the exit  300   a  of the carrying path  300  and colliding with a unit located above does not occur. Moreover, since the sheet S does not move in the space in a free state, the sheet does not skew, either. 
     To prevent the sheet S from flying upward or being skewed, a technique of reducing the carrying speed of the sheet S may be employed. However, reducing the carrying speed of the sheet S results in lowering a throughput of booklet production. 
     In contrast, the sheet folding apparatus  30  and the sheet folding method according to the embodiment, since the upward flying and skewing of the sheet S do not occur, is capable of increasing the carrying speed of the sheet S and improving the throughput of the booklet production. 
     (3) Driving Control of Solenoid 
     As described above, the electromagnetic force of the solenoid  334  and the restoring force of the spring  360  reciprocate the assist roller  332 . 
       FIG. 10  is a functional block diagram schematically showing the configuration for reciprocation of the assist roller  332 . A solenoid driving circuit  370  supplies a driving current to the solenoid  334  and turns the solenoid  334  on and off according to the on and off state of the driving current. When the solenoid  334  is turned on, the solenoid  334  pulls the plunger  334   a  inward. The pull-in force of the plunger  334   a  causes the assist roller  332  to move from the first position A to the second position B about the pivot P and pulls the spring  360 . When the solenoid  334  is turned off, the restoring force of the spring  360  causes the assist roller  332  to return to the first position A from the second position B and causes the plunger  334   a  to be pulled out of the solenoid  334 . 
       FIG. 11A  to  FIG. 11C  show a conventional example of a driving control sequence of the solenoid  334 .  FIG. 11A  shows on and off state of the driving current of the solenoid  334 .  FIG. 11B  and  FIG. 11C  show the speed and position, respectively, of the plunger  334   a  and mechanical components interlocked with the movement of the plunger  334   a  according to on and off state of the driving current. 
     In the conventional driving control of the solenoid  334 , a shift from off-state to on-state or a shift from on-state to off-state is carried out stepwise. Therefore, the moving speed of the plunger  334   a  or the like is very high as shown in  FIG. 11B  and rough and loud impact noise occurs when the solenoid  334  shifts from off-state to on-state or shifts from on-state to off-state. 
     In the sheet folding apparatus  30  according to the embodiment, in order to restrain the occurrence of such impact noise, a buffer period is provided when the solenoid  334  shifts from off-state to on-state or shifts from on-state to off-state. In the buffer period, the driving current of the solenoid  334  is chopped. 
       FIG. 12A  to  FIG. 12D  show exemplary driving control sequences of the solenoid  334  according to the embodiment. 
       FIG. 12A  shows an example in which a pulse-like driving current with a constant pulse period is used as a driving current in buffer periods and in which the pulse width is gradually increased during a buffer period when a shift from off-state to on-state is made, whereas the pulse width is gradually decreased during a buffer period when a shift from on-state to off-state is made. 
       FIG. 12B  shows an example in which a pulse-like driving current with a constant pulse width is used as a driving current in buffer periods and in which the pulse period is gradually shortened during a buffer period when a shift from off-state to on-state is made, whereas the pulse period is gradually lengthened during a buffer period when a shift from on-state to off-state is made. 
     In either method of  FIG. 12A  or  FIG. 12B , during the buffer period from off-state to on-state, the average driving current slowly increases as the duty factor of the pulse increases. On the other hand, during the buffer period from on-state to off-state, the average driving current slowly decreases as the duty factor decreases. 
     Consequently, the moving speed of the plunger or the like is lower than in the conventional example, as shown in  FIG. 12C , and impact noise can be reduced. 
       FIG. 12A  to  FIG. 12D  show examples in which a buffer period is provided both in shifting from off-state to on-state and in shifting from on-state to off-state is made. However, it is also possible to provide a buffer period in one of the shifting. 
     The above technique enables reduction in impact noise of the solenoid  334  with an extremely simple method. 
     As described above, according to the sheet folding apparatus  30 , the image forming apparatus  10 , and the sheet folding method according to the embodiment, unwanted flying of a sheet can be restrained when the sheet is discharged to the stack tray from the carrying path, and the sheet stacked on the stack tray can be prevented from being skewed. Also, impact noise of the solenoid  334  can be reduced with a simple method. 
     Embodiments of the invention are not limited to the above embodiments themselves and components can be modified in implementation stages without departing from the scope of the invention. Also, various embodiments can be realized by combinations of plural components disclosed in the above embodiments. For example, some components of all the components in each embodiment may be deleted. Components across different embodiments may be combined.