Patent Publication Number: US-7905473-B2

Title: Sheet creaser including a cam guided pressing unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2008-032229 filed in Japan on Feb. 13, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet creaser, a sheet conveyer including a conveying path on which the sheet creaser is provided, a sheet finisher including the sheet creaser, an image forming apparatus including the sheet finisher or the sheet finisher. 
     2. Description of the Related Art 
     In the field of image forming apparatuses such as inkjet printers, electrophotographic copiers, facsimile machines, and multifunction products (MFPs), sheet finishers that receive a set of sheet-like recording mediums (hereinafter, “sheets”) from an image forming apparatus and perform post-processing such as stapling have been widely used. With the development of multi-functional-sheet finishers, sheet finishers with both a side-stitch function and a saddle-stitch function have appeared. In most of the sheet finishers with the saddle-stitch function, a folding unit that folds the set of sheets includes at least one pair of rollers called pressure rollers and a plate member called folding plate. More particularly, the folding plate is aligned with a line to be folded of the set of sheets, and inserts the set of sheets into a nip between the pressure rollers. Thus, a crease is made along the line to be folded on the set of sheets with the nip. 
     Some folding units include a first pair of pressure rollers and a second pair of pressure rollers. The set of sheets is pressed twice with the first pressure rollers and the second pressure rollers, which makes a stronger crease. 
     However, even when the set of sheets is pressed twice, it is difficult to make a crease strong enough due to a short pressing time and a low pressing force. Because a rotation axis of the pressure rollers runs parallel to a direction perpendicular to a sheet conveying direction, a folded side of the set of sheets is pressed in the nip between the pressure rollers only for a short time. Moreover, because the pressure rollers nip the entire folded side at the same time, the pressing force on the set of sheets is distributed, i.e., the pressing force per unit area is low. 
     There has been disclosed a technology for making a stronger crease, in which a slide-pressing unit re-presses the folded side while sliding in a direction perpendicular to the sheet conveying direction. 
     Japanese Patent Application Laid-open No. 2003-341930 discloses a sheet finishing method of accumulating a plurality of sheets received from the image forming apparatus and saddle-stitching/half-folding the sheets. More particularly, after the sheets are saddle-stitched, the stitched sheets are inserted in between a pair of first pressure rollers in such a manner that a center line with respect to the sheet conveying direction is pressed by the folding plate. Thus, a crease is made on the sheets. After that, the crease is re-pressed by a second pressure roller that is sliding in the direction perpendicular to the sheet conveying direction in such a manner that a rotational axis of the second pressure roller is oblique with respect to the crease. Thus, the strong crease is made on the sheets. 
     In Japanese Patent Application Laid-open No. 2003-341930, a guiding member that is swinging upward guides the second pressure roller so that the second pressure roller moves up slantwise and then moves down onto the crease. The guiding member is swung by a driving force of a motor. 
     In a typical sheet creaser that makes the strong crease by re-pressing the folded side of the sheets with a slidable pressure roller, such as the second pressure roller disclosed in Japanese Patent Application Laid-open No. 2003-341930, sliding in the direction perpendicular to the sheet conveying direction, if the folded side of the sheets is thick, a load on the motor steeply increases when the slidable pressure roller slides up on the crease. This may results in a step-out of the motor. 
     In Japanese Patent Application Laid-open No. 2003-341930, the increase in load on the motor when the second pressure roller slides up on the crease is suppressed by the presence of the guiding member. However, if the size of sheets is variable, the guiding member has to move in the sheet-width direction to near the corner of the current sheets. That is, it is necessary to provide a moving space extending in the sheet-width direction. Moreover, it is necessary to provide a driving unit that moves the guiding member. This brings an increase of costs and an increase of necessary space for the driving unit. Because a typical driving unit includes a motor and a driving-force transmission mechanism, it is expected to bring a large increase in the number of parts and a large increase in the necessary space. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to one aspect of the present invention, there is provided a sheet creaser including a pressing unit that presses a folded side of a stack of sheets folded by a folding unit, thereby making a strong crease on the stack of sheets, which includes a pressure roller that slides on the folded side while rotating, an elastic biasing unit that presses the pressure roller in a thickness direction of the stack of sheets, and a driving unit that slides the pressure roller in a direction substantially perpendicular to a conveying direction of the stack of sheets; and a lifting unit that, when the pressure roller slides to a first position, temporarily lifts up the pressure roller, and when lifted-up pressure roller slides to a second position, lifts the lifted-up pressure roller down onto the folded side. The first position and the second position are located before a corner of the folded side, whereby the pressure roller cannot slide up on the folded side. 
     Furthermore, according to another aspect of the present invention, there is provided a method of creasing sheets in a sheet creaser including a pressing unit that presses a folded side of a stack of sheets folded by a folding unit, thereby making a strong crease on the stack of sheets. The pressing unit includes a pressure roller that slides on the folded side while rotating, an elastic biasing unit that presses the pressure roller in a thickness direction of the stack of sheets, and a driving unit that slides the pressure roller in a direction substantially perpendicular to a conveying direction of the stack of sheets. The method includes first lifting including temporarily lifting up, when the pressure roller slides to a first position, the pressure roller; second lifting including lifting down, when lifted-up pressure roller slides to a second position, the lifted-up pressure roller onto the folded side, wherein the first position and the second position are located before a corner of the folded side, whereby the pressure roller cannot slide up on the folded side; sliding, after the pressure roller is lifted down onto the folded side, the pressure roller that is pressed by an elastic force of the elastic biasing unit back and forth along the folded side. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a system including a sheet finisher and an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a side-stitch tray and a saddle-stitch tray shown in  FIG. 1 , viewed from the front side of the sheet finisher; 
         FIGS. 3 to 10  are schematic diagrams for explaining operations in a saddle-stitch mode according to the embodiment; 
         FIG. 11  is a block diagram of the control structure of the system according to the embodiment; 
         FIG. 12  is a schematic diagram for explaining a slide-pressing process in which a slidable pressure roller slide-presses a folded side of a stack of sheets, depicting a state where the rotating slidable pressure roller is sliding on the folded side; 
         FIG. 13  is a schematic diagram for explaining the slide-pressing process, depicting a state where the stack of sheets is ejected at the end of the slide-pressing process; 
         FIGS. 14A and 14B  are schematic diagrams for explaining operations of a slide-pressing mechanism, depicting a state where the slidable pressure roller is at its HP; 
         FIGS. 15A and 15B  are schematic diagrams for explaining operations of the slide-pressing mechanism, depicting a state where a first guiding member that is attached to the slidable pressure roller slides up on a second guiding member; 
         FIGS. 16A and 16B  are schematic diagrams for explaining operations of the slide-pressing mechanism, depicting a state where the first guiding member is at an upmost position on the second guiding member, (stand-by position); 
         FIGS. 17A and 17B  are schematic diagrams for explaining operations of the slide-pressing mechanism, depicting a state where the first guiding member slides from the second guiding member down onto the folded side; 
         FIGS. 18A and 18B  are schematic diagrams of a guide mechanism for explaining its operations, depicting a state where the second guiding member is at its HP; 
         FIG. 19  is a schematic diagram of the guide mechanism for explaining its operations, depicting a state where the second guiding member slides from its HP to a position to guide the first guiding member up and then down onto a corner of the folded side; and 
         FIG. 20  is a flowchart of the slide-pressing process according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of the structure of a system including a sheet finisher PD as a sheet post-processing device and an image forming apparatus PR according to an embodiment of the present invention. 
     The sheet finisher PD is attached to a side of the image forming apparatus PR. A sheet ejected from the image forming apparatus PR is conveyed to the sheet finisher PD. The sheet passes through a conveyer path A for single-sheet processing (e.g., a punching unit  100  is located near the conveyer path A). After that, the sheet is conveyed by the operation of switching claws  15  and  16  to any one of a conveyer path B connecting to an upper tray  201 , a conveyer path C connecting to a shift tray  202 , a conveyer path D connecting to a side-stitch tray F for alignment and stapling. The image forming apparatus PR includes, although not shown in the drawings, an image processing circuit for converting received image data into printable image data, an optical writing device that writes a latent image with a light on a photosensitive element based on an image signal received from the image processing circuit, a developing device that develops the latent image to a toner image, a transferring device that transfers the toner image onto a sheet, and a fixing device that fixes the toner image on the sheet. The image forming apparatus PR sends the sheet with the fixed toner image to the sheet finisher PD. Upon receiving the sheet from the image forming apparatus PR, the sheet finisher PD performs a certain post-processing with the sheet. Although the above explanation is made assuming that the image forming apparatus PR is an electrophotographic machine, the image forming apparatus PR can be any type of image forming apparatus such as an inkjet machine or a thermal-transfer machine. 
     After the alignment and stapling is performed at the side-stitch tray F with the sheet that has been passed through the conveyer paths A and D, the sheet is conveyed by the operation of a guiding member  44  to either the conveyer path C connecting to the shift tray  202  or a saddle-stitch tray G for saddle-stitch and folding. If the sheet is conveyed to the saddle-stitch tray G, the sheet is folded or the like at the saddle-stitch tray G. The folded sheet is conveyed to a conveyer path H and ejected onto a lower tray  203 . The conveyer path D is provided with a switching claw  17  that keeps a position as shown in  FIG. 1  by support of a low load spring (not shown). After the back end of the sheet passes the switching claw  17  while the sheet is conveyed by rotation of a pair of conveyer rollers  7 , the sheet is reversed along a turn guiding member  8  by reverse-rotation of a pair of conveyer rollers  9 , in some cases, together with reverse-rotation of at least one of a pair of conveyer rollers  10  and a pair of stapled-sheet conveyer rollers  11  (brush rollers). Thus, the sheet is conveyed with the back end ahead to a sheet accommodating unit E for pre-stacking. When the next sheet is conveyed to the sheet accommodating unit E, the two sheets are conveyed out of the sheet accommodating unit E overlapped with each other. It is possible to convey three or more sheets overlapped with one another by repeating those operations. 
     An entrance sensor  301  that detects the sheet coming from the image forming apparatus PR, a pair of entrance rollers  1 , the punching unit  100 , a punch-waste hopper  101 , a pair of conveyer rollers  2 , and the switching claws  15  and  16  are arranged near the conveyer path A in this order, with the entrance sensor  301  being closest to the image forming apparatus PR. The switching claws  15  and  16  keep positions as shown in  FIG. 1  by support of springs (not shown). When corresponding solenoids (not shown) are turned ON, the switching claws  15  and  16  switch ON. The sheet is conveyed to one of the conveyer paths B, C, and D depending on a switching pattern of the switching claws  15  and  16 . 
     When the sheet is to be conveyed to the conveyer path B, the solenoids are kept OFF, and thereby the switching claws  15  and  16  are in the positions shown in  FIG. 1 . As a result, the sheet is conveyed to the shift tray  202  though a pair of conveyer rollers  3  and a pair of ejection rollers  4 . When the sheet is to be conveyed to the conveyer path C, the both solenoids are turned ON so that the switching claw  15  turns upward and the switching claw  16  turns downward. Thus, the sheet is conveyed to the shift tray  202  through a pair of ejection rollers  6 . When the sheet is to be conveyed to the conveyer path D, the solenoid for the switching claw  16  is turned OFF and the solenoid for the switching claw  15  is turned ON so that the switching claw  15  turns upward and the switching claw  16  turned downward. 
     The sheet finisher PD can perform various sheet processing including punching using the punching unit  100 , alignment and side stitch using a pair of jogger fences  53  and a side-stitch stapler S 1 , alignment and saddle stitch using an upper saddle-stitch jogger fence  250   a , a lower saddle-stitch jogger fence  250   b , and a saddle-stitch stapler S 2 , sorting using the shift tray  202 , half-folding using a folding plate  74  and a pair of first pressure rollers  81 . Moreover, the sheet finisher PD can perform slide-pressing using a slide-pressing unit  525  (see  FIG. 15 ) as a subsequent process of the half-folding to make a crease on the folded stack of sheets stronger. 
     As show in  FIG. 1 , a sheet ejecting unit that ejects the sheets on the shift tray  202  includes the ejection rollers  6  ( 6   a ,  6   b ), a reverse roller  13 , a sheet sensor  330 , the shift tray  202 , a shifting mechanism that shifts the shift tray  202  back and forth in a direction perpendicular to the sheet conveying direction, and a lifting mechanism that lifts the shift tray  202  up and down. 
     The reverse roller  13  is made of sponge. When the sheet is ejected by the ejection rollers  6 , the reverse roller  13  comes in contact with the sheet so that the back end of the sheet abuts against an end fence, which makes the sheets stacked on the shift tray  202  aligned. The reverse roller  13  rotates by the rotation of the ejection rollers  6 . There is a lift-up stop switch (not shown) near the reverse roller  13 . When the shift tray  202  lifts up and pushes the reverse roller  13  up, the lift-up stop switch turns ON and a shift-tray lifting motor (not shown) stops. Thus, the shift tray  202  cannot move up beyond a predetermined position. 
     The sheet sensor  330  is arranged near the reverse roller  13 . The sheet sensor  330  detects a position of the top one out of sheets stacked on the shift tray  202 . When it is determined using the sheet sensor  330  that the position of the top sheet reaches a predetermined height, the shift tray  202  moves down by a predetermined amount by the action of the shift-tray lifting motor so that the position of the top sheet is always at the same level. 
     The ejection rollers  6  are formed with a driving roller  6   a  and a driven roller  6   b . The driven roller  6   b  is arranged upstream of the driving roller  6   a , and is rotatably attached to a free end of an open/close guiding plate. The open/close guiding plate is attached to the sheet finisher PD rotatably around the other end, arranged with the free end being closer to the shift tray  202 . The driven roller  6   b  comes in contact with the driving roller  6   a  under the weight of the driven roller  6   b  or by a biasing force, and the sheet is ejected through between the driving roller  6   a  and the driven roller  6   b . When stapled sheets are to be ejected, the open/close guiding plate moves up to a predetermined position, and then moves down at predetermined timing decided based on a detection signal from an ejection sensor  303 . The predetermined position is decided based on a detection signal from a guiding-plate open/close sensor (not shown). The open/close guiding plate moves up, driven by a guiding-plate open/close motor (not shown). 
     When the sheet is conveyed to the side-stitch tray F by the rotation of the stapled-sheet conveyer rollers  11 , the sheet is stacked on the side-stitch tray F. More particularly, the sheet goes backward by rotation of a reverse roller  12  in the vertical direction (i.e., the sheet conveying direction), and abut against an end fence  51 , which makes the sheets stacked on the side-stitch tray F aligned. A direction perpendicular to the sheet conveying direction (i.e., the sheet-width direction) is aligned with the jogger fences  53 . When it is determined based on a staple signal from a control circuit  350  that a last one of a set of sheets is stacked on the side-stitch tray F, the side-stitch stapler S 1  stapes the set of sheets. A sheet pressing member  110  presses a side of the set of sheets when the side-stitch stapler S 1  staples the sheets. 
     A home position (HP) of a lifting claw  52   a  is detected with an ejection-belt HP sensor  311 . The ejection-belt HP sensor  311  turns ON/OFF by operation of the lifting claw  52   a  attached to a lifting belt  52 . Two lifting claws  52   a  are attached to an outer surface of the lifting belt  52 , with the lifting claws  52   a  being opposed to each other. The two lifting claws  52   a  alternately lift the set of sheets out of the side-stitch tray F. 
     The lifting belt  52  rotates between a driving pulley and a driven pulley along a center line of the aligned sheet width. A plurality of lifting rollers  56  are attached rotatably to a driving shaft, working as driven rollers. The lifting rollers  56  are arranged symmetric to each other with respect to the lifting belt  52 . 
     The reverse roller  12  swings around a fulcrum  12   a  by a tapping solenoid, which causes the back end of the sheets stacked on the side-stitch tray F to abut against the end fence  51 . The reverse roller  12  rotates counterclockwise. The pair of jogger fences  53  is arranged so that both width-direction sides of the stacked sheets put between them. The jogger fences  53  slide in the sheet-width direction back and forth via a timing belt (not shown) by positive-driving or negative-driving of a jogger motor (not shown). The side-stitch stapler S 1  moves to a target position in the sheet-width direction via a timing belt (not shown) by positive-driving or negative-driving of a stapler moving motor (not shown) to staple the target position of the sheet side. 
     A saddle-stitch mechanism related to the slide-pressing process is explained below. A side-stitch mechanism is not explained, because the side-stitch mechanism is not a feature of the sheet finisher PD. 
       FIG. 2  is a schematic diagram of the side-stitch tray F and the saddle-stitch tray G viewed from the front side of the sheet finisher PD.  FIGS. 3 to 10  are schematic diagrams for explaining operations in a saddle-stitch mode. 
     It is assumed that the sheet is conveyed to the conveyer path D by the operation of the switching claws  15  and  16 , and then is conveyed to the side-stitch tray F by the operation of the conveyer rollers  7 ,  9 , and  10 , and the stapled-sheet conveyer rollers  11 . At the side-stitch tray F, the sheet is aligned with the stapled-sheet conveyer rollers  11  both in the saddle-stitch mode and the side-stitch mode (see  FIG. 3 ). In other words, the operations in the saddle-stitch mode and the stapling mode are same before a set of sheets is stapled in the side-stitch mode. 
     After a set of sheets (hereinafter, “stack of sheets  603 ”) is roughly aligned at the side-stitch tray F, the stack of sheets  603  is lifted up with the lifting claw  52   a . As shown in  FIG. 4 , a front end of the stack of sheets  603  is conveyed to a position between an inner circumference of the guiding member  44  and the lifting rollers  56 , passed between a roller  36  and a driven roller  42  that are in an open position in which a distance between the roller  36  and the driven roller  42  is wider than a thick of the stack of sheets  603 . After that, the roller  36  swings to a close position by a motor M 1  and a cam  40 , and the stack of sheets  603  is nipped by the roller  36  and the driven roller  42  with a predetermined pressure. The stack of sheets  603  is then conveyed to the saddle-stitch tray G by the rotation of the roller  36  and the lifting rollers  56  as shown in  FIG. 5 . The roller  36  rotates by a timing belt  38 . The lifting rollers  56  that are attached to the driving shaft of the lifting belt  52  rotate in synchronization with the lifting belt  52 . 
     In the saddle-stitch tray G, the stack of sheets  603  is conveyed with a pair of upper conveyer rollers  71  and a pair of lower conveyer rollers  72  ( 72   a ,  72   b ) to a position at which the front end of the stack of sheets  603  abuts against a movable backend fence  73  as shown in  FIG. 6 . The position of the movable backend fence  73  depends on a length of the sheets. When the front end of the stack of sheets  603  abuts against the movable backend fence  73 , the lower conveyer rollers  72  apart from each other and a back end of the stack of sheets  603  is tapped with a tapping claw  251  as shown in  FIG. 7 . Thus, the stack of sheets  603  is finely aligned with respect to the sheet conveying direction. In this manner, even when the alignment of the stack of sheets  603  breaks during the travel from the side-stitch tray F to the movable backend fence  73 , the tapping with the tapping claw  251  makes the stack of sheets  603  aligned. 
     The stack of sheets  603 , the movable backend fence  73 , and the relative members shown in  FIG. 7  are in saddle-stitch positions. The stack of sheets  603  is aligned with respect to its width with the upper saddle-stitch jogger fence  250   a  and the lower saddle-stitch jogger fence  250   b . The saddle-stitch stapler S 2  staples a center position of the aligned stack of sheets  603 . It is noted that the position of the movable backend fence  73  is decided based on a pulse from a backend-fence HP sensor  322 , and the position of the tapping claw  251  is decided based on a pulse from a tapping-claw HP sensor  326 . 
     As shown in  FIG. 8 , while the lower conveyer rollers  72  apart from each other, the movable backend fence  73  lifts the stapled stack of sheets  603  up to a position so that the center position, i.e., the stapled position is aligned with the folding plate  74 . After that, the folding plate  74  inserts the center position into between the rotating first pressure rollers  81  by pressing the center position in a direction perpendicular to the surface of the stack of sheets  603 . The rotating first pressure rollers  81  nip the stack of sheets  603 , and convey the stack of sheets  603  with a pressure. Thus, a crease is made on the center of the stack of sheets  603 . In this manner, the stapled stack of sheets  603  is lifted up to the position for folding without fails only by the movement of the movable backend fence  73 . 
     As shown in  FIG. 10 , the crease of the folded stack of sheets  603  is made stronger, re-pressed by a pair of second pressure rollers  82 . The re-pressed stack of sheets  603  is ejected onto the lower tray  203  via a pair of ejection rollers  83 . When it is determined using an upstream sheet sensor  323  that the back end of the stack of sheets  603  has been passed through the upstream sheet sensor  323 , those members of the saddle-stitch tray G prepare for the next saddle stitch, more particularly, the folding plate  74  and the movable backend fence  73  return to the HPs and the lower conveyer rollers  72  return to a nip position for forming the nip. If a sheet size and number of sheets of the next set of sheets are same as the stack of sheets  603 , the movable backend fence  73  may move directly to the position shown in  FIG. 2  instead of the HP. Whether the stack of sheets  603  is stacked on the lower tray  203  is determined based on the position of the back end of the stack of sheets  603  detected using a downstream sheet sensor  324 . The second pressure rollers  82  are not shown in  FIG. 1 . It is possible to design, based on its design conditions, the sheet creaser without provided with the second pressure rollers  82 . 
     A slidable pressure roller  600  and a mechanism for driving the slidable pressure roller  600  are not shown in  FIGS. 9 and 10 . Those units will be described with reference to  FIG. 12  and the subsequent drawings. 
       FIG. 11  is a block diagram of the control structure of the system according to the embodiment. The control circuit  350  that controls the sheet finisher PD can be a micro computer, including a central processing unit (CPU)  360  and an input/output (I/O) interface  370 . The CPU  360  receives via the I/O interface  370  various signals from various switches on an operation panel  380  of the image forming apparatus PR and from various sensors such as the sheet sensor  330 . The CPU  360  controls, based on the received signals, various components including the motor that lifts up/down the shift tray  202 , the motor that opens/closes the open/close guiding plate, the motor that shifts the shift tray  202 , the motor that drives the reverse roller  12 , various solenoids including the tapping solenoid, the motors that drive various conveyer rollers, the motors that drive various ejection rollers, the motor that drives the lifting belt  52 , the motor that moves the side-stitch stapler S 1 , the motor that rotates the side-stitch stapler S 1  to a slant position, the motor that moves the jogger fences  53 , the motor that swings the guiding member  44 , the motor that drives the lifting rollers  56 , the motor that moves the movable backend fence  73 , the motor that moves the folding plate  74 , the motor that drives the first pressure rollers  81 . The motor that drives the stapled-sheet conveyer rollers  11  sends a pulse signal to the CPU  360 . Upon receiving the pulse signal, the CPU  360  counts the received pulse signal and controls a solenoid  170  (not shown) and a jogger motor  158  (not shown) based on a result of count. 
     The CPU  360  controls those components by reading program codes from a read only memory (ROM) (not shown), loading the program codes on a work area of a random access memory (RAM) (not shown), and executing the loaded program codes. 
       FIGS. 12 and 13  are schematic diagrams for explaining a slide-pressing process performed by the slidable pressure roller  600 . The slidable pressure roller  600  is located adjacent to a downstream side of the first pressure rollers  81  in the sheet conveying direction. The slidable pressure roller  600  slides in a direction perpendicular to the sheet conveying direction. As shown in  FIG. 12 , after the stack of sheets  603  is folded by the first pressure rollers  81 , the stack of sheets  603  is conveyed in the sheet conveying direction indicated by an arrow. The stack of sheets  603  is stopped, under constant pulse control, when a predetermined time has passed since the front end of the stack of sheets  603  passes the upstream sheet sensor  323 . Meanwhile, the motor that drives the first pressure rollers  81  is a stepping motor. The stack of sheets  603  is stopped so that the front end is on a sliding area of the slidable pressure roller  600 . After that, a folded side  603   a  (i.e., the front end) is slide-pressed by the sliding slidable pressure roller  600 , and thus the strong crease is made. After the slide-pressing, the stack of sheets  603  is conveyed in the sheet conveying direction indicated by an arrow shown in  FIG. 13 . 
       FIG. 14A  is a schematic diagram of a slide-pressing mechanism viewed along the sheet conveying direction; and  FIG. 14B  is a schematic diagram of the slide-pressing mechanism viewed from the left side of the stack of sheets  603  across the sheet conveying direction.  FIGS. 14 to 17  are schematic diagrams for explaining operations of the slide-pressing mechanism.  FIGS. 14A and 14B  depict a state where the slide-pressing operation starts. As shown in  FIGS. 14A and 14B , the slide-pressing mechanism includes a mechanism for driving the slidable pressure roller  600  (hereinafter, “slide mechanism”) and a mechanism for driving a second guiding member  611  (hereinafter, “guide mechanism”). 
     The slide mechanism includes a holder  601 , a first guiding member  602 , a spring  609 , a first slider  608 , a first sliding shaft  607 , a first stepping motor  612 , a first pulley  605 , and a first timing belt  606 . 
     The slidable pressure roller  600  is fit in the holder  601  in such a manner the slidable pressure roller  600  is rotatably attached to a spindle  601   a  of the holder  601 . Thus, the slidable pressure roller  600  slides while rotating. The first guiding member  602  is attached, as a projection, to a side face of the holder  601  that faces opposite to the sheet conveying direction. The holder  601  is suspended from the first slider  608  via a shaft. Due to an elastic force of the spring  609  between the holder  601  and the first slider  608 , the holder  601  is movable up and down. The spring  609  is a so-called compression spring. The holder  601  and the slidable pressure roller  600  are always pressed against a guiding plate  613  that forms a part of the sheet conveyer path by the elastic force of the spring  609 . 
     The first slider  608  is slidably attached to the first sliding shaft  607  to slide in the direction perpendicular to the sheet conveying direction. The first slider  608  is fixed to the first timing belt  606  that is located above the first sliding shaft  607 . The first timing belt  606  is stretched between a pulley  612   a  and the first pulley  605 . The pulley  612   a  is a driving pulley and the first pulley  605  is a driven pulley. The pulley  612   a  is provided to a driving shaft of the first stepping motor  612 . With this configuration, the first slider  608  slides back and forth along the first sliding shaft  607  by the rotation of the first timing belt  606 . 
     A first light sensor  604  is provided near an end of the first sliding shaft  607 . Assume now that the first light sensor  604  is provided near the end of the first sliding shaft  607  close to the first pulley  605  as shown in  FIG. 14A . A shielding plate  610  is attached to the first slider  608  so that the shielding plate  610  shields the first light sensor  604  when the first slider  608  is in the HP. Thus, the first light sensor  604  detects whether the first slider  608  is in the HP. In other words, the HP of the slidable pressure roller  600  is a position where the shielding plate  610  that is attached to the first slider  608  as a projection shields the first light sensor  604 . Motion of the slidable pressure roller  600  is controlled by a driving pulse of the first stepping motor  612  by referring to a distance from the HP. Therefore, various patterns of motion can be made in consideration of the variable sheet width. 
       FIGS. 18A ,  18 B and  19  are schematic diagrams of the guide mechanism for explaining its operations. As shown in  FIGS. 18A and 18B , the guide mechanism includes a second sliding shaft  616 , a second timing belt  617 , a second pulley  618 , and a second stepping motor  619 . 
     The second sliding shaft  616  runs parallel to the first sliding shaft  607 , i.e., in the direction perpendicular to the sheet conveying direction. The second guiding member  611  is slidably attached to the second sliding shaft  616  to slide in the direction perpendicular to the sheet conveying direction. The second guiding member  611  is fixed to the second timing belt  617  that is located above the second sliding shaft  616 . The second timing belt  617  is stretched between a pulley  619   a  and the second pulley  618 . The pulley  619   a  is a driving pulley and the second pulley  618  is a driven pulley and. The pulley  619   a  is provided to a driving shaft of the second stepping motor  619 . With this configuration, the second guiding member  611  slides back and forth along the second sliding shaft  616  by the rotation of the second timing belt  617 . 
     The second guiding member  611  is located upstream of the sheet with respect to the sliding direction of the first slider  608 . The second guiding member  611  is arranged so that a lower surface  602   a  of the first guiding member  602  slides, accompanied by the sliding of the first slider  608 , on an upper surface  611   a  of the second guiding member  611 . The lower surface  602   a  and the upper surface  611   a  make a cam mechanism. That is, when the lower surface  602   a  slides on the upper surface  611   a , the slidable pressure roller  600  moves up above the sheet surface in the presence of the elastic force of the spring  609  nevertheless, and then moves down onto the sheet surface. More particularly, the slidable pressure roller  600  is moved up before reaching a left side  603   b  of the stack of sheets  603 , and then moved down on the left side  603   b . The positions where the slidable pressure roller  600  is moved up and down depend on shape and position of the second guiding member  611 . 
     With this configuration, the slide-pressing mechanism operates as follows from the initial state shown in  FIGS. 14A and 14B . The first timing belt  606  rotates by the driving force of the first stepping motor  612 , and the first slider  608  slides along the first sliding shaft  607  in the sliding direction indicated by the arrow shown in  FIG. 14A  by the rotation of the first timing belt  606 . The slidable pressure roller  600  also slides in the sliding direction accompanied by the sliding of the first slider  608 . During the sliding of the slidable pressure roller  600 , the curved lower surface  602   a  slides up on the slope upper surface  611   a , and thereby the slidable pressure roller  600  is moved up. At that time, the spring  609  arranged between the holder  601  and the first slider  608  shrinks. This elastic force of the spring  609  works as a part of the pressing force to press the folded side  603   a  of the stack of sheets  603 .  FIGS. 16A and 16B  depict a state where the slidable pressure roller  600  is on an upmost position of the second guiding member  611 . After that, the slidable pressure roller  600  moves gradually down onto the left side  603   b  as shown in  FIGS. 17A and 17B . The slidable pressure roller  600  slides forth along the crease of the stack of sheets  603  to a right side  603   c . Thereafter, the slidable pressure roller  600  returns back to the HP along the sliding path same as but reverse of the forth-sliding. During this slide-pressing operation, the elastic force of the spring  609  is applied onto the crease while the slidable pressure roller  600  is sliding on the crease. Thus, the strong crease is made. 
     The angle of slope of the upper surface  611   a  is relatively small so that the slidable pressure roller  600  moves to a level above the folded side  603   a  of the stack of sheets  603  with a relatively small change in load when the first guiding member  602  slides on the second guiding member  611 . Therefore, no trouble occurs such as the step-out of the first stepping motor  612 . 
     It is necessary to move, based on sheet-size data received from the image forming apparatus, the second guiding member  611  to a position outside of the sheet width, and stand-by the second guiding member  611  at that position. This is because it is necessary to temporarily move up the slidable pressure roller  600  so as to fall the slidable pressure roller  600  down onto the folded side  603   a . The second guiding member  611  is, as described above, fixed to the second timing belt  617  and moved accompanied by the rotation of the second timing belt  617 . The second timing belt  617  is rotated by the driving force of the second stepping motor  619  via the second pulley  618 . A shielding plate  615  is attached to the second guiding member  611  as a projection so that the shielding plate  615  shields a second light sensor  614  when the second guiding member  611  is in the HP. The distance from the HP is measured by using a pulse of the second light sensor  614 . If the sheet width is small, the second guiding member  611  moves from the position as shown in  FIGS. 18A and 18B  to the position corresponding to the sheet width as shown in  FIG. 19 . In this manner, it is possible to smoothly guide the slidable pressure roller  600  to the folded side  603   a  just by adjusting the position of the second guiding member  611  in the sheet width direction. 
       FIG. 20  is a flowchart of the slide-pressing process according to the embodiment. When the stack of sheets  603  is conveyed from the image forming apparatus PR to the sheet finisher PD, i.e., when the slide-pressing process starts, the sheet finisher PD determines whether the saddle-stitch mode is ON (Step S 101 ). If the saddle-stitch mode is ON (Yes at Step S 101 ), the sheet finisher PD acquires the sheet-width data from the image forming apparatus PR (Step S 102 ). The image forming apparatus PR obtains the sheet-width data by referring to a command received via an operation panel (not shown) or the size of original sheet and the size of sheet to be fed. 
     After acquiring the sheet-width data, the second guiding member  611  is moved to the stand-by position by the driving force of the second stepping motor  619  (Step S 103 ). The stand-by position of the second guiding member  611  is set to a position L 1  mm away from the HP shown in  FIG. 18A . In other words, the second guiding member  611  stands-by at that position as shown in  FIG. 19 . The slidable pressure roller  600  is moved from the HP shown in  FIG. 14A  to the stand-by position shown in  FIG. 16A  by the driving force of the first stepping motor  612  (Step S 104 ). The stand-by position of the slidable pressure roller  600  is set to a position L 2  mm away from the HP. When the upstream sheet sensor  323  turns ON, i.e., the folded side  603   a  of the stack of sheets  603  passes through the upstream sheet sensor  323  (Yes at Step S 105 ), the stack of sheets  603  is conveyed by a predetermined distance measured based on pulses and then is stopped at that position (Step S 106 ). The stack of sheets  603  is stopped so that the folded side  603   a  is aligned with the sliding area of the slidable pressure roller  600 . 
     The slidable pressure roller  600  is slid back and forth on the folded side  603   a  by the driving force of the first stepping motor  612  (Step S 107 ). More particularly, the slidable pressure roller  600  moves from the position shown in  FIG. 16A  in the sliding direction indicated by the arrow, and falls down onto the left side  603   b  of the stack of sheets  603  as shown in  FIG. 17A . After that, the slidable pressure roller  600  slides forth to a position X mm before the right side  603   c , where X is just a small distance, and then slides back along the folded side  603   a . The sliding motion of the slidable pressure roller  600  is controlled in an accurate manner by using the number of steps of the first stepping motor  612 . 
     The slidable pressure roller  600  slides back from the position shown in  FIG. 17A  to the stand-by position shown in  FIG. 16A  along the sliding path same as but reverse of the forth-sliding (Step S 108 ). When the downstream sheet sensor  324  turns from ON to OFF, i.e., the downstream sheet sensor  324  detects the back end of the stack of sheets  603  (Yes at Step S 109 ), the sheet finisher PD checks whether the job related to the saddle-stitch mode has been completed (Step S 110 ). If the job has been completed (Yes at Step S 110 ), the second guiding member  611  moves back to the HP (Step S 111 ) and the slidable pressure roller  600  slides back to the HP (Step S 112 ). The process control then goes to end. 
     In this manner, as described with reference to  FIGS. 16A and 17A , the slidable pressure roller  600  moves down onto the left side  603   b  of the stack of sheets  603  instead of sliding up on the left side  603   b . Therefore, a step-out of the first stepping motor  612  due to the excessive load is prevented. 
     The sheet creaser incorporated in the sheet finisher is described in the embodiment. However, the sheet creaser capable of the slide-pressing can be incorporated in a sheet conveyer, an image forming apparatus, an image forming system, or the like from viewpoints of space savings. If the sheet creaser is incorporated in the sheet conveyer, the sheet creaser is, for example, placed upstream of a cutting device that cuts the stack of sheets  603 . 
     The embodiment of the present invention brings various effects as follows. 
     Firstly, the slidable pressure roller  600  gradually moves up and then gradually moves down onto the folded side  603   a  instead of sliding up on the folded side  603   a , which suppresses an amount of increase in the load on the first stepping motor  612  that drives the slidable pressure roller  600 . Therefore, a step-out of the first stepping motor  612  is prevented. 
     Secondly, if the sheet width is variable, the second guiding member  611  moves to the stand-by position corresponding to the current sheet width so that the slidable pressure roller  600  moves down onto the folded side  603   a  without sliding up on the corner of the stack of sheets  603 . In other words, it is possible to deal with the variable sheet size with the simple configuration requiring a relatively small space. 
     Thirdly, the slidable pressure roller  600  gradually moves up and then gradually moves down onto the folded side  603   a  instead of sliding up on the folded side  603   a . Thus, no tear is made on the corner of the stack of sheets  603 . 
     According to an aspect of the present invention, it is possible to provide a small-space low-cost sheet creaser capable of making a strong crease with preventing a step-out of a motor. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.