Patent Publication Number: US-8974360-B2

Title: Creasing device, image forming system, and creasing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2010-166369 filed in Japan on Jul. 23, 2010 and Japanese Patent Application No. 2011-015419 filed in Japan on Jan. 27, 2011. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to creasing devices, image forming systems, and creasing methods. More specifically, the invention relates to a creasing device that makes a crease (a fold) on a sheet member (hereinafter, “sheet”) delivered from a preceding stage before the sheet is folded in half, an image forming system including the creasing device, an image forming apparatus, and a sheet finisher that processes a sheet delivered from the image forming apparatus, and a creasing method for use by the creasing device or the image forming system. 
     2. Description of the Related Art 
     What is called saddle-stitched or center-folded booklet production has been conventionally performed. The saddle-stitched booklet production is performed by saddle stitching a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause outer side sheets of the sheet batch to be stretched at a fold line by a greater amount than inner side sheets. Image portions at the fold line on outer side sheets can thus be stretched, resulting in damage, such as coming off of toner, to the image portions in some cases. A similar phenomenon can occur when other folds, such as a z-fold or a tri-fold, are performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch. 
     A creasing device, called a creaser, that forms a crease in a sheet batch before the sheet batch undergoes half fold or the like folding operation so that even outer side sheets can be readily folded, thereby preventing coming off of toner has already been known. 
     An example of such a creasing device is disclosed in Japanese Patent Application Laid-open No. 2008-081258. The creasing device disclosed in Japanese Patent Application Laid-open No. 2008-081258 includes an annular protrusion provided along a perimeter of one roller for forming a crease and an annular concavity created along a perimeter of the other roller so that a pair of the rollers form a crease, having a precise and favorable shape according to a type of the sheet, that extends in a sheet-conveying direction on a sheet when the sheet passes through meshing between the annular protrusion and the annular concavity of the rollers. In the creasing device, the rollers are interchangeable with optimum rollers for a sheet to be creased. 
     Meanwhile, the creasing device disclosed in Japanese Patent Application Laid-open No. 2008-081258 includes the annular protrusion and the annular concavity provided on the perimeters of the paired rollers and forms a crease extending in a sheet conveying direction by causing the sheet to pass the meshing between the rollers. In this technique, in every sheet to be folded, a crease is formed in a to-be-folded portion from an outward side, which is to become an outer side when the sheet is folded, and then the crease is pushed by a push-out member from an inward side, which is to become an inner side when the sheet is folded, to prevent colorant from coming off the sheet. With this configuration, a folding position is likely to deviate from an intended position because the sheet is pushed out to an outward side. In terms of accuracy of folding position, a crease is preferably formed in a to-be-folded portion on an inward side, which allows accurate positioning of a fold. Put another way, importance has conventionally been placed on preventing coming off of colorant and no particular attention has been paid to accuracy in the positioning of the fold. 
     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 an aspect of the present invention, there is provided a creasing device that forms a crease in a to-be-folded portion of a sheet. The creasing device includes a sheet-information reading unit that reads any one of sheet information and binding information; a determining unit that determines a surface, on which the crease is to be formed, of the sheet according to the one of the sheet information and the binding information read by the sheet-information reading unit; and a creasing unit that forms the crease on the surface determined by the determining unit. 
     According to another aspect of the present invention, there is provided an image forming system including a creasing device that forms a crease in a to-be-folded portion of a sheet; a sheet-information reading unit that reads any one of sheet information and binding information; a determining unit that determines a surface, on which the crease is to be formed, of the sheet according to the one of the sheet information and the binding information read by the sheet-information reading unit; and a creasing unit that forms the crease on the surface determined by the determining unit. 
     According to still another aspect of the present invention, there is provided a creasing method for forming a crease in a to-be-folded portion of a sheet. The creasing method includes reading any one of sheet information and binding information; determining a sheet surface, on which the crease is to be formed, according to the one of the sheet information and the binding information read at the reading; and forming the crease in the surface determined at the determining. 
     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. 
     In the following embodiments, a sheet corresponds to a reference numeral P, a creasing device to a reference numeral  100 , a sheet-information reading unit and a sheet-information reading process to step S 201  (a process of a central processing unit), a determining unit and a determining process to steps S 202  to S 207  (processes of the CPU), a creasing unit to each elements and each units defined by a third unit or a fifth unit, respectively. A first rotary member corresponds to a reference numeral  121   b , a second rotary member to a reference numeral  122   a , a creasing member to creasing blades  121   c  and  122   b , creasing grooves to  121   d  and  122   c , rotary drive units to a second motor  135 , a gear speed reduction mechanism  136 , a third motor  139 , a gear speed reduction mechanism  140 , reciprocating drive unit to a first motor  131 , a pulley speed-reduction mechanism  132  and a cam  134 , respectively. A receiving member  122  corresponds to a reference numeral  122 , a reversing mechanism to a sheet reversing mechanism  130 , each branch of twofold forked sheet-conveying path to a first branch of sheet-conveying path  113   a  and a second branch of sheet-conveying path  113   b , respectively. A rotary member that is arranged in a middle portion of the twofold forked sheet-conveying path corresponds to a reference numeral  121   e , a pair of receiving members to reference numerals  122   ca  and  122   cb , creasing processes to step  107 , step S 107   a , step S 107   b , and step S 210 , respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating how the image forming system performs operations, including creasing and folding, the diagram depicting a state in which a sheet is conveyed into a creasing device; 
         FIG. 3  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which a to-be-creased position of the sheet has reached a position where a creasing member is arranged; 
         FIG. 4  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which creasing is being performed; 
         FIG. 5  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which a first sheet has been conveyed into a folding device and a second sheet has been conveyed into the creasing device; 
         FIG. 6  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which the first sheet is immediately before being delivered onto a center-folding tray and the second sheet is being creased; 
         FIG. 7  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which the second sheet and a third sheet are processed as are the sheets illustrated in  FIG. 6 ; 
         FIG. 8  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which all sheets belonging to one sheet batch have been delivered onto the center-folding tray; 
         FIG. 9  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which the sheet batch on the center-folding tray is located at a center-folding position; 
         FIG. 10  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which center folding is started; 
         FIG. 11  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which the center-folded sheet batch is being delivered onto the stacking tray; 
         FIG. 12  is a schematic diagram illustrating how the image forming system performs the operations including creasing and folding, the diagram depicting a state in which the center-folded sheet batch has been delivered onto the stacking tray; 
         FIG. 13  is a diagram illustrating the configuration of a creasing device according to a first embodiment of the present invention, the diagram being an elevation view illustrating a standby state as viewed from a sheet conveying direction; 
         FIG. 14  is a diagram illustrating the configuration of the creasing device according to the first embodiment, the diagram being an elevation view illustrating a state in which the creasing device is performing creasing; 
         FIG. 15  is a simplified side view illustrating the states presented in  FIGS. 13 and 14 ; 
         FIG. 16  is a schematic diagram illustrating a state in which sheet guiding has not been started yet according to the first embodiment; 
         FIG. 17  is a schematic diagram illustrating a state in which sheet guiding is performed according to the first embodiment; 
         FIG. 18  is a schematic diagram illustrating a state immediately before the sheet is creased according to the first embodiment; 
         FIG. 19  is a schematic diagram illustrating a state in which the sheet is being creased according to the first embodiment; 
         FIG. 20  is a flowchart of a process sequence according to the first embodiment; 
         FIG. 21  is an elevation view illustrating the configuration of a creasing device according to a second embodiment; 
         FIGS. 22A and 22B  are simplified side views each illustrating positions of a rotary member, a receiving member, and a sheet according to the second embodiment; 
         FIG. 23  is a schematic diagram illustrating a state in which sheet guiding has not been started yet according to the second embodiment; 
         FIG. 24  is a schematic diagram illustrating a state in which sheet guiding is performed in the second embodiment; 
         FIG. 25  is a schematic diagram illustrating a state immediately before the sheet is creased according to the second embodiment; 
         FIG. 26  is a schematic diagram illustrating a state in which the sheet is being creased in the second embodiment; 
         FIG. 27  is a schematic diagram illustrating a state in which, after creasing the sheet, the creasing device returns to the standby state according to the second embodiment; 
         FIG. 28  is a schematic diagram illustrating a state immediately before the sheet is creased on another side according to the second embodiment; 
         FIG. 29  is a schematic diagram illustrating a state in which the sheet is being creased on the other side according to the second embodiment; 
         FIG. 30  is a flowchart illustrating a process sequence for creasing in the second embodiment; 
         FIG. 31  is a diagram illustrating a schematic configuration of a creasing device according to a third embodiment; 
         FIG. 32  is a schematic diagram illustrating a state in which the sheet is conveyed into a reverse conveying path according to the third embodiment; 
         FIG. 33  is a schematic diagram illustrating a state in which the sheet has been conveyed into the reverse conveying path and immediately before reversing a conveying direction according to the third embodiment; 
         FIG. 34  is a schematic diagram illustrating a state in which the sheet has been conveyed out of the reverse conveying path and into the creasing mechanism by reversing the conveying direction according to the third embodiment; 
         FIG. 35  is a diagram illustrating a schematic configuration of a creasing device according to a fourth embodiment; 
         FIG. 36  is a schematic diagram illustrating how a crease is formed in the sheet according to the fourth embodiment; 
         FIG. 37  is a block diagram illustrating a schematic configuration of the image forming system according to the present embodiment including the first to fourth embodiments; 
         FIGS. 38A and 38B  are explanatory diagrams illustrating a specific example of a magazine-making layout; and 
         FIG. 39  is a flowchart, according to the embodiment, of a process sequence for determining a surface on which a crease is to be formed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Unlike typical creasing that is performed by forming a crease on an outward side of a to-be-folded portion of a sheet and then pushing the to-be-folded portion with a push-out member from an inward side toward rollers to prevent colorant from coming off an image, according to an aspect of the present invention, a surface, on which a crease is to be formed, is selectable. For a sheet, from which coming off of colorant should preferably be prevented, a crease is formed on an outward side of a to-be-folded portion to maintain image quality, while for a sheet, in which less importance is placed on image quality, a crease is formed on an inward side of the to-be-formed portion and the crease is pushed out from the inward side so that the sheet can be folded readily. This allows image quality to be maintained and reduces deviation of a folding position. 
     Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. Identical or substantially identical elements are denoted by same reference numerals and symbols, and repeated descriptions are omitted. 
       FIG. 1  is a diagram illustrating a schematic configuration of an image forming system according to an embodiment of the present invention. The image forming system includes an image forming apparatus PR that forms an image on a sheet, a creasing device  100  that performs creasing, and a folding device  200  that performs folding. 
     The image forming apparatus PR forms a visible image pertaining to image data fed from a scanner, a personal computer (PC), or the like on a sheet of paper. The image forming apparatus PR uses a known print engine of electrophotography, droplet ejection printing, or the like. 
     The creasing device  100  includes a conveying mechanism  110  and a creasing mechanism  120 . The creasing mechanism  120  includes a creasing member  121  and a receiving member  122 , and forms a linear crease by pinching a sheet between the creasing member  121  and the receiving member  122 . The creasing member  121  includes, on an end surface facing the receiving member  122 , a creasing blade unit  121   a  for use in forming a crease. The creasing blade unit  121   a  extends linearly in a direction perpendicular to a sheet conveying direction and includes a pointed end, of which edge lies perpendicular to the sheet conveying direction. A creasing groove  122   c  is cut on a surface, which faces the creasing blade unit  121   a , of the receiving member  122 . The creasing groove  122   c  receives the creasing blade unit  121   a  that fits thereinto. The creasing member  121  and the receiving member  122  are shaped as described above; accordingly, when a sheet is pinched therebetween, the shape of the end of the blade and the shape of the groove leave a crease on the sheet. 
     In this example, the conveying mechanism includes a first pair of conveying rollers  111  and a second pair of conveying rollers  112  and conveys the sheet conveyed from the image forming apparatus PR to a subsequent stage. 
     The folding device  200  includes a center-folding device  250  that performs folding. The sheet creased by the creasing device  100  is delivered to the folding device  200 , in which the sheet is conveyed by conveying rollers  211 , conveying rollers  212 , and conveying rollers  213  to the center-folding device  250 . 
     The center-folding device  250  includes a center-folding tray  251 , a trailing-edge fence  252  provided at a lower end (an upstream edge in the conveying direction) of the center-folding tray  251 , a folding plate  253  and folding rollers  254  for folding the sheet along the crease, and a stacking tray  255 . The trailing-edge fence  252  causes a return roller (not shown) to forcibly press trailing edges of sheets delivered onto the center-folding tray  251  against the trailing-edge fence  252 , thereby aligning the sheets in the sheet conveying direction. A jogger fence (not shown) also aligns sheet edges in the direction perpendicular to the conveying direction. 
     The folding plate  253  presses its pointed end against and along the crease on the aligned sheet batch and pushes the crease into a nip of the folding rollers  254 . The sheet batch pushed into the nip of the folding rollers  254  is creased in the nip. When the sheet batch is to undergo saddle-stitching, after the sheet batch is stitched by a stitching device (not shown) at a portion to be creased, the sheet batch is subjected to the folding operation, what is called as half fold, described above. The half-folded sheet batch is delivered onto and stacked on the stacking tray  255 . 
       FIG. 2  to  FIG. 12  are schematic diagrams illustrating a series of operations, including the folding operation described above, to be performed by the image forming system. In the image forming system, a sheet P 1 , on which an image has been formed by the image forming apparatus PR, is conveyed into the creasing device  100  and stopped at a position where a crease (a fold) is to be formed ( FIG. 2  and  FIG. 3 ). The first sheet P 1  stopped at this position is pinched between the creasing member  121  and the receiving member  122 ; this forms a crease on the first sheet P 1  ( FIG. 4 ). Thereafter, the thus-creased sheet P 1  is conveyed to the folding device  200  ( FIG. 5 ) and temporarily stored in the center-folding tray  251  ( FIG. 6 ). 
     The operations mentioned above with reference to  FIG. 2  to  FIG. 6  are repeatedly performed for a predetermined number of sheets ( FIG. 7 ). When a sheet batch (P 1  to Pn) containing a predetermined number of sheets (P 1  to Pn) has been stored in the center-folding tray  251  ( FIG. 8 ), the trailing-edge fence  252  is moved (upward) to place the crease in the sheet batch at a folding position ( FIG. 9 ). Thereafter, the folding plate  253  is pressed against the crease in the sheet batch to push the crease into the nip of a folding rollers  254 , thereby performing folding ( FIG. 10 ). The sheets folded into a booklet form are sequentially stacked on the stacking tray  255  ( FIGS. 11 and 12 ). 
     Configurations and control operations of the creasing device according to each embodiment of the present invention are described below. 
     First Embodiment 
       FIG. 13  to  FIG. 15  are schematic diagrams illustrating the configuration of the creasing device  100  according to a first embodiment.  FIG. 13  is an elevation view illustrating a standby state as viewed from the sheet conveying direction.  FIG. 14  is an elevation view illustrating a state in which the creasing device  100  is performing creasing.  FIG. 15  is a simplified side view illustrating the creasing device  100  in the states presented in  FIGS. 13 and 14 . 
     Referring to  FIG. 13  to  FIG. 15 , the creasing device  100  includes the creasing blade unit  121   a , which further includes a cylindrical first rotary member  121   b  and a creasing blade  121   c . The first rotary member  121   b  and the creasing blade  121   c  are driven by a driving mechanism to rotate and reciprocate in one piece. 
     A reciprocating driving mechanism that drives the first rotary member  121   b  to reciprocate includes a first motor  131 , a pulley speed-reduction mechanism  132 , a driving belt  133 , and a pair of cams  134 . A rotational driving mechanism includes a second motor  135 , a gear speed reduction mechanism  136 , a pair of sliding members  137 , and a pair of elastic urging members  138 . The pulley speed-reduction mechanism  132  transmits driving power of the first motor  131  to the cams  134 . The driving belt  133  transmits the driving power, which has been transmitted via the pulley speed-reduction mechanism  132  to one of the cams  134 , to the other cam  134  so that the cams  134  arranged on two ends of the first rotary member  121   b  to rotate in one piece. The gear speed reduction mechanism  136  transmits driving power of the second motor  135  to the first rotary member  121   b , thereby rotating the first rotary member  121   b . The pair of disk-like sliding members  137  are coaxially arranged on the two ends of the first rotary member  121   b . The elastic urging members  138 , which are, for instance, compression springs, constantly urge the sliding members  137  elastically toward the cams  134 . 
       FIG. 13  illustrates a state in which the first rotary member  121   b  is most distant from the receiving member  122 , or, put another way, the distance between a rotation center of the cams  134  and surfaces of the sliding members  137  is at its minimum.  FIG. 14  illustrates a state in which the creasing blade  121   c  of the first rotary member  121   b  is fitted into the creasing groove  122   c  of the receiving member  122  to some extent, or, put another way, the distance between the rotation center of the cams  134  and the surfaces of the sliding members  137  is close to its maximum. 
     The first rotary member  121   b , the creasing blade  121   c , the second motor  135 , and the gear speed reduction mechanism  136  are movable in one piece up and down in  FIGS. 13 and 14 . The first rotary member  121   b  and the sliding members  137  rotate, in one piece, around an axis of rotation of the first rotary member  121   b . The elastic urging members  138  bring the sliding members  137  into sliding contact with the cams  134 . A path of contact between the cams  134  and the sliding members  137  limits a range of reciprocating motion of the first rotary member  121   b.    
     The cams  134  are driven by the driving power of the first motor  131  transmitted via the pulley speed-reduction mechanism  132  and the driving belt  133 . The cams  134  are configured such that rotation of the cams  134  causes the sliding members  137 , the first rotary member  121   b , the creasing blade  121   c , the second motor  135 , and the gear speed reduction mechanism  136  to move in one piece. 
       FIG. 15  is a diagram schematically illustrating how the first rotary member  121   b  and the receiving member  122  move toward and away from each other as illustrated in  FIGS. 13 and 14 . The receiving member  122  described above is positioned to face the creasing blade  121   c . A sheet is creased by being pinched between the creasing blade  121   c  and the creasing groove  122   c  of the receiving member  122 . 
     Generally, a sheet P is conveyed by being fed into a nip between guide members (guide plates)  141  and  142  that pinch and guide the sheet P and then receiving a conveying force from the first pair of conveying rollers  111  and the second pair of conveying rollers  112 , as illustrated in  FIG. 16 . A notch  143  that allows passage of the creasing blade  121   c  should preferably be defined in the guide members  141  and  142  to crease the sheet by pinching the sheet between the creasing blade  121   c  and the creasing groove  122   c . The first rotary member  121   b  should preferably be moved away from the notch  143  in the guide members  141  and  142  as illustrated in  FIG. 16 . 
     However, a leading edge of a sheet can be caught by the notch  143  during conveyance of the sheet. To prevent such a situation, there is employed a configuration where a portion of the first rotary member  121   b  covers the notch  143  in the guide members  141  and  142  and, after the leading edge of the sheet passes over the notch  143 , both the first rotary member  121   b  and the creasing blade  121   c  are retracted (in a direction indicated by an arrow D 2 ) and further rotated (in a direction indicated by an arrow R 1 ) as illustrated in  FIG. 18 , causing the creasing blade  121   c  to a point at the sheet P. When a to-be-creased position, at which the sheet P is to be creased, has reached immediately below the creasing blade  121   c , the creasing blade  121   c  is lowered in a direction indicated by an arrow D 1 , as illustrated in  FIG. 19 , thereby pinching the sheet P between the creasing blade  121   c  and the receiving member  122  to form a crease P 1 . 
       FIG. 20  is a flowchart of a process sequence for these operations, or, put another way, a process sequence of the first embodiment. These operations are performed by a central processing unit (CPU)  100   a  of the creasing device  100 , which will be described later with reference to an illustration in  FIG. 37 . The creasing device  100  carries out communications with the image forming apparatus PR and the folding device  200  to receive data about folding, data about sheet types, and the like and performs folding according to the data. 
     When the creasing device  100  is not arranged between the image forming apparatus PR and the folding device  200 , the folding device  200  aligns edges of sheets conveyed from the image forming apparatus PR and folds the sheets without performing creasing, whereas when the creasing device  100  is arranged between the image forming apparatus PR and the folding device  200 , the folding device  200  aligns edges of sheets that have been creased at a predetermined position by the creasing device  100  and folds the sheets. 
     Referring to the flowchart presented in  FIG. 20 , when the creasing device  100  and the folding device  200  are ready to receive a sheet (YES at step S 101 ), the first rotary member  121   b  is moved from a preset home position (step S 102 ), where the creasing blade  121   c  is not facing a sheet and therefore not performing creasing and away from the guide members  141  and  142 , to the notch  143  in the guide members  141  and  142 , thereby covering the notch  143  with a cylindrical side surface of the first rotary member  121   b  (step S 103 ). The creasing blade  121   c , the second motor  135 ; the gear speed reduction mechanism  136 , and the sliding members  137  are also lowered in one piece with the first rotary member  121   b . Meanwhile, when the first rotary member  121   b  is moved down or up, the creasing blade  121   c , the second motor  135 , the gear speed reduction mechanism  136 , and the sliding members  137  (which are called, hereinafter, “accessory mechanism”) are also moved down or up in one piece. In the present embodiment, for convenience, reference symbol D denotes vertical linear motion, while R denotes rotation and, furthermore, D 1  is used to denote upward motion, while D 2  is used to denote downward motion. 
     When the leading edge of the sheet has passed over the notch  143 , there is no longer a possibility that the sheet leading edge is caught by the notch  143 . Accordingly, the first rotary member  121   b  is moved to a standby (retracted) position (step S 105 ). This motion to the standby position is performed by driving the first motor  131  to rotate the cams  134 , thereby moving the first rotary member  121   b  and the accessory mechanism upward. Thereafter, the first rotary member  121   b  is rotated (spun) by the second motor  135  and the gear speed reduction mechanism  136  to cause the creasing blade  121   c  to face a top surface of the receiving member  122  or the creasing groove  122   c  that is formed on the top surface of the receiving member  122  (step S 106 ). From the position of step S 106 , the first motor  131  drives to move the first rotary member  121   b  and the accessory mechanism downward and press the creasing blade  121   c  against the creasing groove  122   c  with the sheet P therebetween at a predetermined pressure (step S 107 ). The predetermined pressure depends on a driving torque of the first motor  131  and a distance between the rotation center of the cams  134  and a contact position of the cams. After the crease P 1  has been formed by this pressing motion, rotation of the first motor  131  is reversed to move the first rotary member  121   b  back to the standby position (step S 108 ). Thereafter, the sheet is conveyed to the folding device  200 . Hence, the sheet, in which the crease P 1  has been formed at the position corresponding to the to-be-folded position, is delivered onto the center-folding tray  251  of the folding device  200  where the sheet undergoes folding. 
     Second Embodiment 
     In the first embodiment, a crease can be formed only from one side of a sheet. A second embodiment that allows a sheet to be creased from two sides of the sheet rather than only from one side is described below. 
       FIG. 21  is a schematic diagram of an elevation view illustrating the configuration of the creasing device  100  according to the second embodiment as viewed from the sheet conveying direction. The creasing device  100  according to the second embodiment differs from the creasing device  100  according to the first embodiment illustrated in  FIG. 13  in not including the receiving member  122  but including a second rotary member  122   a  that is rotatable as is the first rotary member  121   b . The second rotary member  122   a  includes a creasing blade  122   b  and a creasing groove  122   c . A creasing groove  121   d  is additionally cut in the first rotary member  121   b . Each of the creasing blade  122   b  and the creasing groove  122   c  can be configured as a unit to be mounted on an outer boundary of a body of the second rotary member as will be described later. 
     As is the first rotary member  121   b , the second rotary member  122   a  is driven to rotate by driving power of a third motor  139  transmitted via a gear speed reduction mechanism  140  and controlled by the CPU  100   a  of the creasing device  100 . As illustrated in  FIGS. 22A and 22B , this allows for changing a relative position between the first rotary member  121   b  and the second rotary member  122   a  by being rotated separately. The first rotary member  121   b  reciprocates toward and away from the second rotary member  122   a  by actions of the first motor  131  and the cams  134 . Hence, a crease can be formed between the first rotary member  121   b  and the second rotary member  122   a.    
     A method of creasing according to the second embodiment is described below with additional reference to the flowchart presented in  FIG. 30 . 
     Referring to  FIG. 30 , operations to be performed from step S 101  to step S 105  are similar to those of the first embodiment illustrated in  FIG. 20 . However, unlike the first embodiment, as illustrated in  FIG. 23 , the outer boundary of the second rotary member  122   a  is positioned at the notch  143  in the guide members  141  and  142  at step S 103  and, in this state, the first rotary member  121   b  in the standby state is moved down to cover the notch  143  as illustrated in  FIG. 24 . This position, to which the first rotary member  121   b  is lowered to cover the notch  143 , is set so as to leave a clearance above the notch  143  that allows passage of the sheet P. 
     After the leading edge of the sheet P passes over the notch  143  in the guide members  141  and  142  (YES at step S 104 ), the first rotary member  121   b  is moved up to retract (step S 105 ). Subsequently, determination as to which one of the two sides of the sheet a crease is to be formed on is made according to an instruction fed from the image forming apparatus PR side (step S 106   x ). When it is determined that a crease is to be formed on an upper side (YES at step S 106   x ), the first rotary member  121   b  and the second rotary member  122   a  are rotated concurrently (in a direction indicated by arrow R 2  in  FIG. 25 ), causing the creasing groove  122   c  to face the first rotary member  121   b  above to become ready for receiving the creasing blade  121   c  as illustrated in  FIG. 25  (step S 106   a ). In this state, the first rotary member  121   b  is moved down, causing the sheet P to be pinched between the creasing blade  121   c  and the creasing groove  122   c  of the second rotary member  122   a , thereby forming the crease P 1  (step S 107   a ). After the crease P 1  is formed, the first rotary member  121   b  returns to the standby position (step S 108 ) ( FIG. 27 ). 
     In contrast, when it is determined that a crease is to be formed on the lower side of the sheet (NO at step S 106   x ), the first rotary member  121   b  is moved up (in the direction indicated by arrow D 2 ) from the state illustrated in  FIG. 24 , and the first and second rotary members  121   b  and  122   a  are rotated concurrently (in the direction indicated by arrow R 1  in  FIG. 28 ), causing the creasing groove  121   d  to face the second rotary member  122   a  below to be ready for receiving the creasing blade  121   c  as illustrated in  FIG. 28  (step S 106   b ). In this state, the first rotary member  121   b  is moved down, causing the sheet P to be pinched between the creasing groove  121   d  and the creasing blade  122   b  of the second rotary member  122   a , thereby forming the crease P 1  (step S 107   b ). 
     This allows creases to be formed at different positions in different directions. After the crease P 1  has been formed, the first rotary member  121   b  returns to the standby position (step S 108 ) ( FIG. 27 ). This series of operations is repeatedly performed (NO at step S 109 ) until the job ends. On completion of the job (YES at step S 109 ), the process sequence ends. 
     Third Embodiment 
     In the second embodiment, the two creasing blades, or, more specifically, the first creasing blade and the second creasing blade, are provided so that a crease can be formed on any one of the upper side and the lower side. A third embodiment is configured to form a crease on any one of the two sides of a sheet with a single creasing blade. 
       FIG. 31  is a diagram illustrating a schematic configuration of the creasing device  100  according to the third embodiment. Referring to  FIG. 31 , the creasing device  100  according to the third embodiment differs from the creasing device  100  according to the first embodiment in additionally including a sheet reversing mechanism  130 . The first pair of conveying rollers  111  is arranged in an upstream side of the creasing device  100  in the conveying direction, and the sheet reversing mechanism  130  is arranged in a further upstream side to the first pair of conveying rollers  111  in the conveying direction. The sheet reversing mechanism  130  includes a branch conveying path  114  bifurcated from an entrance conveying path  113  at a position between an entrance of the entrance conveying path  113  and the first pair of conveying rollers  111 , a merging conveying path  115  for conveying a sheet, which has been turned over via the branch conveying path  114 , back onto the entrance conveying path  113 , a path-switching flap  113   c  provided at a bifurcation unit where bifurcation into the entrance conveying path  113  and the branch conveying path  114  is made, and conveying rollers  145  for conveying a sheet in a switchback manner on the branch conveying path  114 . 
     By using the sheet reversing mechanism  130 , a crease can be formed in a sheet that has been turned over.  FIG. 32  to  FIG. 34  are schematic diagrams illustrating sheet reversing. As illustrated in  FIG. 32 , when the path-switching flap  113   c  rotates counterclockwise (in a direction indicated by arrow E) to connect a path to the branch conveying path  114  and shuts off a path for direct conveyance from the entrance conveying path  113  to the first pair of conveying rollers  111 , the sheet P conveyed on the entrance conveying path  113  is guided to the branch conveying path  114  and conveyed downward by the conveying rollers  145  to a reverse conveying path  116 . As illustrated in  FIG. 33 , when a trailing edge of the sheet has passed through a bifurcation unit  131   a  where bifurcation into the branch conveying path  114  of the reverse conveying path  116  and the merging conveying path  115  is made, the conveying rollers are rotated in a reversal direction, thereby conveying the sheet P upward. This causes, as illustrated in  FIG. 34 , the sheet to be delivered along a branch shape of the bifurcation unit  131   a  onto the merging conveying path  115  to return to the entrance conveying path  113 , on which the sheet is delivered to the first pair of conveying rollers  111 . 
     The first pair of conveying rollers  111  receives the sheet P, which has been turned over in passing through the reverse conveying path  116 , and delivers the sheet to the creasing mechanism  120 . The creasing mechanism  120  creases the sheet P as described above with reference to  FIGS. 15 to 19 . 
     This configuration allows, even when the creasing mechanism  120  is capable of forming a crease only from one side of a sheet, a crease to be formed on any one of the two sides of the sheet by turning over the sheet. 
     Meanwhile, elements that are not specifically described in the third embodiment have similar configurations and functions to those of the first embodiment. 
     Fourth Embodiment 
     A crease can be formed on one side of a sheet in a selective manner; this can be attained by, for instance, providing conveying paths above and below a creasing mechanism and conveying a sheet to be creased to one of the conveying paths. A fourth embodiment is configured as such. In the fourth embodiment, a conveying path, in which a bottom surface of the sheet faces a creasing blade, and a conveying path, of which a top surface of the sheet faces a creasing blade, are provided. Bifurcation into the two conveying paths is made at a bifurcation point in an upstream side along the sheet conveying direction. A path-switching flap for selecting one of the conveying paths, at which creasing is to be performed, is provided at the bifurcation point. 
       FIG. 35  is a schematic diagram illustrating the configuration of the creasing device  100  according to the fourth embodiment. Referring to  FIG. 35 , the entrance conveying path  113  is vertically bifurcated by a path-switching flap  113   c  into a first-branch conveying path  113   a  and a second-branch conveying path  113   b , which are merged together at a merging point in a downstream side along the sheet conveying direction. The creasing mechanism  120  is provided between a bifurcation point and the merging point of the first- and second-branch conveying paths  113   a  and  113   b . First pairs of conveying rollers  111   a  and  111   b  are provided in an upstream side of the creasing mechanism  120  on the first and second-branch conveying paths  113   a  and  113   b  along the sheet conveying direction while second pairs of conveying rollers  112   a  and  112   b  are provided in a downstream side of the creasing mechanism  120  along the sheet conveying direction. 
     The creasing mechanism  120  includes a first creasing blade  121   ea  on a top side of a creasing member  121   e  and a second creasing blade  121   eb  on a bottom side of the creasing member  121   e . The creasing mechanism  120  further includes a first receiving member  122   ca  in which a first creasing groove  122   ca   1  is cut and a second receiving member  122   cb  in which a second creasing groove  122   cb   1  is cut. The first creasing blade  121   ea  faces the first receiving member  122   ca  by interposing the first-branch conveying path  113   a  in between, and the second creasing blade  121   eb  faces the second creasing groove  122   cb   1  by interposing the second-branch conveying path  113   b . The first and second creasing grooves  122   ca   1  and  122   cb   1  and the first and second creasing blades  121   ea  and  121   eb  are arranged on a line and configured to move vertically from a standby position illustrated in  FIG. 35  as indicated by arrows. It is therefore possible either that the first creasing blade  121   ea  is fitted into the first creasing groove  122   ca   1  by interposing a sheet in between or that the second creasing blade  121   eb  is fitted into the second creasing groove  122   cb   1  by interposing a sheet in between. 
     A driving mechanism for the creasing member  121   e  is not specifically described. For instance, such a mechanism as that mentioned in the first embodiment that allows vertical movement can be employed. 
     When the entrance conveying path  113  and the creasing mechanism  120  are configured as described above, a crease can be formed on a lower side of the sheet as follows. As presented in  FIG. 36 , which is the schematic diagram illustrating the operations, the path-switching flap  113   c  is directed downward to guide the sheet P to the second-branch conveying path  113   b , which is an upper branch of the vertically bifurcated conveying path. The sheet P is conveyed by the first pair of conveying rollers  111   b  in the second-branch conveying path  113   b  to a creasing position. When the sheet P has reached the creasing position, the creasing member  121   e  is moved up, causing the second creasing blade  121   eb  to be fitted into the second creasing groove  122   cb   1  with the sheet P therebetween. Hence, a crease is formed on the lower side of the sheet P. 
     A crease can be formed on an upper side of the sheet P as follows. The path-switching flap  113   c  is switched to direct upward to guide the sheet P to the first-branch conveying path  113   a , which is a lower branch of the vertically bifurcated conveying path. The creasing member  121   e  is moved down at the creasing position to form a crease on the upper side of the sheet P. 
     The configuration described above allows a crease to be formed on any one of the two sides of the sheet only by switching between the first- and second-branch conveying paths  113   a  and  113   b  that are arranged next to the entrance conveying paths  113 . 
     Meanwhile, elements that are not specifically described in the fourth embodiment have similar configurations and functions to those of the first embodiment. 
       FIG. 37  is a block diagram illustrating an electrical configuration (control configuration) of the image forming system according to the present embodiment including the first to fourth embodiments. 
     Referring to  FIG. 37 , the image forming system according to the present embodiment includes the creasing device  100 , the folding device  200  that performs folding, and the image forming apparatus PR. The creasing device  100  and the image forming apparatus PR are connected via a communication interface  100 - 1 , via which information about sheets, a post-processing mode, an anomaly, and the like are notified. Similarly, the creasing device  100  and the folding device  200  that performs folding are connected via a communication interface  100 - 2 . 
     The creasing device  100  includes the CPU  100   a  that controls the entire creasing device and its various units and an input-output (I/O) unit  100   b  that manages inputs and outputs between the CPU  100   a , and various sensors and drivers that drive solenoids, motors, and the like. The CPU  100   a  performs control operations by reading program codes stored in a read only memory (ROM) (not shown), storing the program codes into a random access memory (RAM) (not shown), and executing program instructions defined in the program codes by using the RAM as a working area and a data buffer. 
     In the present embodiment, a crease can be formed in a selected side of the two surfaces of the sheet P.  FIG. 39  is a flowchart of a process sequence for determining a surface, in which a crease is to be formed. 
     Referring to  FIG. 39 , when sheet information or binding information is notified from the image forming apparatus PR to the creasing device  100  via the communication interface  100 - 1 , the CPU  100   a  of the creasing device  100  reads the sheet information or the binding information that are notified from the image forming apparatus PR (step S 201 ) and determines whether or not monochrome printing has been performed (step S 202 ). In a case that monochrome printing has been performed, because coming off of colorant does not occur, it is determined that a crease is to be formed on an inward side of the sheet P to keep high accuracy in determining a folding position (step S 208 ). Creasing is performed accordingly (step S 210 ). 
     If it is determined that monochrome printing has not been performed (NO at step S 202 ), a determination is made as to whether or not a specific type of sheet is used to prevent colorant from coming off (step S 203 ). If it is determined that the specific type of sheet is used to prevent the colorant from coming off, process control proceeds to step S 208 , and operations pertaining to step S 208  and step S 210  are performed. 
     If it is determined that the specific type of sheet is not used to prevent the colorant from coming off, a determination is made as to whether or not the number of stacked sheets to be folded at once is equal to or larger than a predetermined number (step S 204 ). If the number of stacked sheets to be folded at once is equal to or larger than the predetermined number, or, put another way, when the number of the stacked sheets is equal to or larger than the predetermined number that makes an angle of a fold of the stacked sheets large enough not to cause coming off of colorant, process control proceeds to step S 208 , and operations pertaining to step S 208  and step S 210  are performed. 
     If it is determined that the number of sheets is fewer than the predetermined number, a determination is made as to whether or not the sheet has been printed in a magazine-making layout in any one of a saddle-stitching mode and a center-folding mode (step S 205 ). Coming off of colorant does not occur from a sheet that is printed in the magazine-making layout in the saddle-stitching mode or the center-folding mode because no image is formed at a to-be-center-folded portion of the sheet. Accordingly, if it is determined that the sheet has been printed in the magazine-making layout, process control proceeds to step S 208  and operations pertaining to step S 208  and step S 210  are performed. The magazine-making layout is described below with reference to  FIG. 38A  by way of an example of making a 12-page booklet by using three sheets. On one side of a first sheet of the three sheets, 12P′ (P′ denotes a page number) and 1P′ are printed, while 2P′ and 11P′ are printed on the other side of the first sheet; on one side of a second sheet, 10P′ and 3P′ are printed, while 4P′ and 9P′ are printed on the other side of the second sheet; on one side of a third sheet, 8P′ and 5P′ are printed, while 6P′ and 7P′ are printed on the other side of the third sheet. The three sheets are overlaid one after another, saddle stitched, and folded (center-folded) as illustrated in  FIG. 38B . 
     If it is determined that the sheet has not been printed in the magazine-making layout (NO at step S 205 ), a determination is made as to whether or not the saddle-stitching mode has been selected (step S 206 ). If it is determined that saddle-stitching mode has been selected (YES at step S 206 ), a determination is made as to whether or not the sheet to be creased is for a cover (step S 207 ). If it is determined that the sheet is not for the cover, process control proceeds to step S 208 , and operations pertaining to step S 208  and step S 210  are performed. When the sheet not for the cover (i.e., the sheet, for which a result of determination made at step S 207  is YES) is saddle-stitched, a to-be-folded portion of the sheet is hidden; therefore, coming off of colorant at the to-be-folded portion does not pose a problem, and accordingly, the crease is to be formed on the inward side of the sheet. In the example illustrated in  FIGS. 38A and 38B , a sheet to be a cover corresponds to the first sheet having pages numbered 1P′, 2P′, 11P′ and 12P′. 
     In contrast, if it is determined that saddle-stitching mode has not been selected (NO at step S 206 ) and it is determined that the sheet is for a cover (YES at step S 207 ), colorant may come off. In such a case, it is determined that the crease is to be formed on the outward side (step S 209 ) and creasing is performed accordingly (step S 210 ). 
     A side, from which a crease is to be formed, is selected in this way. Accordingly, for a sheet, on which image quality should preferably be maintained, an outward side is selected as the side where a crease is to be formed at step S 209 , while for a sheet, on which higher importance should preferably be placed on accuracy in a folding position rather than on image quality, an inward side is selected as the side on which a crease is to be formed at step S 208 . By selecting any one of the outward side and the inward side in this way, both maintaining image quality and high accuracy in a folding position can be satisfied. 
     According to an aspect of the present invention, a surface, on which a crease is to be formed, of a sheet is determined based on sheet information or binding information, and creasing is performed according to a result of the determination. This allows both maintaining image quality and high accuracy in a folding position to be satisfied while paying attention to both preventing colorant from coming off and keeping accuracy in positioning. 
     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.