Patent Publication Number: US-9415964-B2

Title: Post-processing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2014-059391 filed Mar. 24, 2014. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a post-processing device. 
     SUMMARY 
     According to an aspect of the invention, there is provided a post-processing device including: 
     a transport unit that transports a recording material which is sequentially transported from an upstream side toward a post-processing unit that performs post-processing on the recording material; 
     a standby unit that is connected to the transport unit, and allows a recording material which is backhauled by a transport section reversible from the transport unit to temporarily stand by; 
     a transport member that transport the recording material which stands by in the standby unit and the recording material which is transported in the transport unit to the post-processing unit with the recording materials stacked; and 
     an elastic member that is arranged on an upstream side of a connection position of the transport unit where the standby unit is connected, guides the recording material sequentially transported from the upstream side to the post-processing unit, and guides the backhauled recording material to the standby unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic cross-sectional view illustrating an internal configuration of an image forming system; 
         FIG. 2  is a schematic cross-sectional view illustrating a recording material transport path of a post-processing device; 
         FIG. 3A  is a schematic cross-sectional view illustrating a configuration of the vicinity of a compile tray, and  FIG. 3B  is a planar schematic view illustrating the configuration of the vicinity of the compile tray; 
         FIG. 4A  is an enlarged schematic cross-sectional view illustrating the arrangement of a first elastic member and a second elastic member in a second post-processing transport path S 2 , and  FIG. 4B  is a planar schematic view illustrating specific examples of the first elastic member and the second elastic member; 
         FIGS. 5A and 5B  are schematic views illustrating effects of the first elastic member and the second elastic member in the second post-processing transport path S 2 ; 
         FIGS. 6A to 6D  are schematic views illustrating a stacking operation in the post-processing device; 
         FIG. 7  is an example of a timing chart of the stacking operation in the post-processing device; and 
         FIGS. 8A to 8D  are schematic views illustrating a three-sheet stacking operation in the post-processing device. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the invention will be described in detail with reference to the accompanying drawings and by using exemplary embodiments and specific examples. However, the invention is not limited to the exemplary embodiments and the specific examples. 
     In addition, the drawings that are used in the following description are schematic, and it should be noted that the ratio of each dimension and the like are different from actual and members that are not necessary for the description are appropriately omitted for ease of understanding. 
     In the drawings, the front-back direction is referred to as an X-axis direction, the right-to-left direction is referred to as a Y-axis direction, and the up-and-down direction is referred to as a Z-axis direction to facilitate the understanding of the following description. 
     (1) Overall Configuration and Operation of Image Forming System 
       FIG. 1  is a schematic configuration view illustrating an image forming system  1  to which a post-processing device according to this exemplary embodiment is applied. The image forming system  1  that is illustrated in  FIG. 1  includes an image forming apparatus  2  such as a printer and a copying machine that forms an image by an electrophotography method, and a sheet processing apparatus  3  that performs post-processing on sheets P where toner images are formed by the image forming apparatus  2 . Hereinafter, an overall configuration and an operation of the image forming system  1  will be described with reference to the drawing. 
     (1.1) Overall Configuration and Operation of Image Forming Apparatus 
     The image forming apparatus  2  is configured to have a control device  10 , a sheet feeding device  20 , photoconductor units  30 , developing devices  40 , a transfer device  50 , and a fixing device  60 . A transport device  100  is arranged on an upper surface (Z direction) of the image forming apparatus  2 , and the sheet P as a recording material where the image is recorded is guided to a post-processing device  200 . 
     The control device  10  has an image forming apparatus control unit  11  that controls the operation of the image forming apparatus  2 , a controller unit  12  that prepares image data in response to a print processing request, an exposure control unit  13  that controls lighting of an exposure device LH, a power supply device  14 , and the like. The power supply device  14  applies voltage to charging rollers  32 , developing rollers  42 , primary image transfer rollers  52 , a secondary image transfer roller  53  (described later), and the like, and supplies power to the exposure device LH. 
     The controller unit  12  converts printing information that is input from an external information transmission device (for example, a personal computer) to image information for forming a latent image, and outputs a driving signal to the exposure device LH at a predetermined timing. The exposure device LH according to this exemplary embodiment is configured to have an LED head where light emitting diodes (LEDs) are linearly arranged. 
     The sheet feeding device  20  is disposed in a bottom portion of the image forming apparatus  2 . The sheet feeding device  20  has a sheet loading plate  21 , and the multiple sheets P as recording media are loaded on an upper surface of the sheet loading plate  21 . The sheets P that are loaded on the sheet loading plate  21  with width-direction positions determined by a regulating plate (not illustrated) are drawn out forward (−X direction), sheet by sheet, from an upper side by a sheet drawer unit  22 , and then are transported to a nip portion of a registration roller pair  23 . 
     The photoconductor units  30  are disposed in parallel above the sheet feeding device  20  (Z direction), and have photosensitive drums  31  as rotationally driven image holding members. The charging rollers  32 , the exposure device LE, the developing devices  40 , the primary image transfer rollers  52 , and cleaning blades  34  are arranged in a direction of rotation of the photosensitive drums  31 . Cleaning rollers  33  that clean surfaces of the charging rollers  32  are arranged to face and to be in contact with the charging rollers  32 . 
     The developing devices  40  have developing housings  41  in which developers are accommodated. The developing rollers  42  that are arranged to face the photosensitive drums  31 , and a pair of augers  44  and  45  that stir and transport the developer to the developing roller  42  side and arranged diagonally downward from a back surface side of the developing roller  42  are arranged in the developing housings  41 . Layer regulating members  46  that regulate the layer thicknesses of the developers are arranged in proximity to the developing rollers  42 . 
     Each of the developing devices  40  is configured to be substantially similarly to the other developing devices  40  with the exception of the developers accommodated in the developing housings  41 . The developing devices  40  respectively form yellow (Y), magenta (M), cyan (C), and black (K) toner images. 
     Surfaces of the rotating photosensitive drums  31  are charged by the charging rollers  32 , and electrostatic latent images are formed by latent image forming light that is emitted from the exposure device LH. The electrostatic latent images that are formed on the photosensitive drums  31  are developed as the toner images by the developing rollers  42 . 
     The transfer device  50  has an intermediate image transfer belt  51  where the toner images of the respective colors that are formed in the photosensitive drums  31  of the respective photoconductor units  30  are subjected to multi layer transfer, and the primary image transfer rollers  52  that perform sequential transfer (primary image transfer) on the toner images of the respective colors formed in the respective photoconductor units  30  to the intermediate image transfer belt  51 . The transfer device  50  is configured to further have the secondary image transfer roller  53  that performs collective transfer (secondary image transfer) on the toner images of the respective colors, which are superposed on the intermediate image transfer belt  51  and transferred, to the sheet P as the recording medium. 
     The toner images of the respective colors, which are formed on the photosensitive drums  31  of the respective photoconductor units  30 , are subjected to sequential electrostatic transfer (primary image transfer) on the intermediate image transfer belt  51  by the primary image transfer rollers  52  to which a predetermined transfer voltage is applied from the power supply device  14 , which is controlled by the image forming apparatus control unit  11 , and the like so that superposed toner images in which toner of the respective colors are superposed are formed. 
     The superposed toner images on the intermediate image transfer belt  51  are transported to an area (secondary image transfer unit T) where the secondary image transfer roller  53  is arranged as a result of a movement of the intermediate image transfer belt  51 . At the timing when the superposed toner images are transported to the secondary image transfer unit T, the sheet P is supplied from the sheet feeding device  20  to the secondary image transfer unit T. A predetermined transfer voltage is applied to the secondary image transfer roller  53  from the power supply device  14  that is controlled by the image forming apparatus control unit  11 , and a multi-toner image on the intermediate image transfer belt  51  is collectively transferred on the sheet P that is sent out from the registration roller pair  23  and is guided by a transportation guide. 
     The residual toner on the surfaces of the photosensitive drums  31  is removed by the cleaning blades  34 , and is collected in a waste developer accommodating portion. The surfaces of the photosensitive drums  31  are charged again by the charging rollers  32 . The residue that is not removed by the cleaning blades  34  but is attached to the charging rollers  32  is captured and accumulated on surfaces of the cleaning rollers  33  that rotate in contact with the charging rollers  32 . 
     The fixing device  60  has a heating module  61  and a pressure module  62 , and a fixing nip portion N (fixing area) is formed by an urge area between the heating module  61  and the pressure module  62 . 
     The sheet P, to which the toner image is transferred in the transfer device  50 , is transported to the fixing device  60  through the transportation guide in a state where the toner image is not fixed. The toner image is fixed on the sheet P that is transported to the fixing device  60  due to pressure bonding and heating effects by the pair of heating module  61  and pressure module  62 . 
     The sheet P where the fixed toner image is formed is guided by transportation guides  65   a  and  65   b , and is discharged from a discharge roller pair  69  to the transport device  100  that is arranged on the upper surface of the image forming apparatus  2 . 
     (1.2) Configuration and Operation of Sheet Processing Apparatus 
     The sheet processing apparatus  3  has the transport device  100  that transports the sheets P which are output from the image forming apparatus  2  to a further downstream side, and the post-processing device  200  that has a compile tray  210  which aligns the sheets P transported from the transport device  100  into a bundle, an end binding mechanism (stapler)  220  which binds end portions of the sheets P, a saddle folding processing mechanism  230  that performs saddle folding processing to produce a booklet, and the like. 
     The transport device  100  has an inlet roller  110  that receives the sheets P which are output via the discharge roller pair  69  of the image forming apparatus  2 , a first transport roller  120  that transports the sheets P which are received by the inlet roller  110  to the downstream side, and a second transport roller  130  that transports the sheets P toward the post-processing device  200 . 
     The post-processing device  200  has a first post-processing transport path S 1 , a second post-processing transport path S 2 , a third post-processing transport path S 3 , and a fourth post-processing transport path S 4  as recording material transport units on a downstream side of a receiving roller  201  that receives the sheets P via the transport device  100 . The first post-processing transport path S 1 , the second post-processing transport path S 2 , and the fourth post-processing transport path S 4  are selected by a switching gate G 1 . In addition, the first post-processing transport path S 1  and the second post-processing transport path S 2  are selected by a switching gate G 2 . 
     The first post-processing transport path S 1  is connected to a top tray TR 1 , and the sheet P that is not post-processed is discharged from the first post-processing transport path S 1 . 
     The second post-processing transport path S 2  is connected to the compile tray  210  that is disposed on the downstream side, and a sheet bundle PB that is produced by the stapler  220  is discharged on a stacker tray TR 2 . 
     The third post-processing transport path S 3  as a recording material standby unit is disposed to branch from the middle of the second post-processing transport path S 2 , and temporarily holds the sheet P which is reversely transported from the compile tray  210  side. 
     The fourth post-processing transport path S 4  is connected to the saddle folding processing mechanism  230 , and the booklet that is produced by the saddle folding processing mechanism  230  is discharged to a booklet tray TR 3 . 
     (2) Configuration and Operation in Vicinity of Recording Material Transport Path and Compile Tray 
       FIG. 2  is a schematic cross-sectional view illustrating a recording material transport path in the post-processing device  200 .  FIG. 3A  is a schematic cross-sectional view illustrating a configuration of the vicinity of the compile tray  210 .  FIG. 3B  is a planar schematic view illustrating the configuration of the vicinity of the compile tray  210 . FIG.  4 A is an enlarged schematic cross-sectional view illustrating the arrangement of a first elastic member  215  and a second elastic member  216  in the second post-processing transport path S 2 .  FIG. 4B  is a planar schematic view illustrating specific examples of the first elastic member  215  and the second elastic member  216 .  FIGS. 5A and 5B  are schematic views illustrating effects of the first elastic member  215  and the second elastic member  216  in the second post-processing transport path S 2 . 
     (2.1) Recording Material Transport Path 
     The receiving roller  201  that brings in the sheet P as the recording material through a recording material discharge port  100   a  of the transport device  100  is arranged in the post-processing device  200 . 
     The post-processing transport paths as the transport units branch into two on the downstream side of the receiving roller  201 . A transport roller  204  and a top discharge roller  205  are disposed in the first post-processing transport path S 1 , which branches upward in a vertical direction (Z direction in  FIG. 1 ), among the post-processing transport path, and discharge the sheet P where the image is formed as it is or the unnecessary sheet P to the top tray TR 1 . 
     Among the post-processing transport paths, the fourth post-processing transport path S 4 , which has a transport roller  231  and branches downward (−Z direction in  FIG. 1 ), binds the sheet bundle PB in a central portion and then discharges the sheet bundle PB after performing saddle folding processing to fold the sheet bundle PB into two in the bound portion. 
     Among the post-processing transport paths that branch upward, the second post-processing transport path S 2 , which branches further in a substantially horizontal direction (X direction in  FIG. 1 ), performs end binding processing and corner portion binding processing on the sheet bundle PB along one end edge and discharges the plural sheet bundles PB in a state of being offset by set. 
     A reverse roller  211  for a buffer and a compile discharge roller  212  are disposed in the second post-processing transport path S 2 , and the sheets P are sequentially discharged on the compile tray  210  by the compile discharge roller  212 . 
     In addition, if necessary, the sheet bundle PB that is aligned on the compile tray  210  is discharged onto the stacker tray TR 2  by an eject roller  213  after the binding processing by the stapler  220 . 
     (2.2) Configuration and Operation in Vicinity of Compile Tray 
     The compile tray  210  is disposed to be inclined so that the sheet P drops along an upper surface of a bottom portion  210   a  where the sheet P is loaded. As illustrated in  FIGS. 4A and 4B , the compile tray  210 , on one end side, has an end guide  210   b  that is arranged to align trailing edge side end portions of the sheets P which drop along the bottom portion  210   a.    
     A paddle  217  is disposed above the compile tray  210 . The paddle  217  is configured to push the sheet P that is transported in a first traveling direction F 1  (illustrated in  FIGS. 4A and 4B ) in a second traveling direction F 2  (illustrated in  FIGS. 4A and 4B ) on the compile tray  210  by rotating in an arrow direction in  FIGS. 4A and 4B . 
     Tampers  218  that align the sheets P in a width direction are disposed in a middle portion of the compile tray  210  in a direction intersecting with the second traveling direction F 2  so that a distance from each other changes in response to driving of a motor (not illustrated). 
     The eject roller (sheet bundle transport roller)  213  is disposed on a tip end side of the compile tray  210 . The eject roller (sheet bundle transport roller)  213  is configured to have a first eject roller  213   a  and a second eject roller  213   b.    
     The first eject roller  213   a  is configured to be retractable with respect to the second eject roller  213   b  in response to the driving of the motor and the like (not illustrated). The second eject roller  213   b  is arranged to be fixed to a back surface side of a surface of the bottom portion  210   a  of the compile tray  210  where the sheet P is loaded, and is configured to perform only a rotational movement. 
     The second eject roller  213   b  is rotationally driven in a state where the first eject roller  213   a  is in contact with the sheet P, and the sheet bundle PB is raised to be transported to the stacker tray TR 2 . 
     The third post-processing transport path S 3  branches toward a lower side between the reverse roller  211  of the second post-processing transport path S 2  and the switching gate G 2 , and a reversible buffer roller  214  is disposed in the third post-processing transport path S 3 . 
     (2.3) Switching Configuration and Effect of Recording Material Transport Path 
     As illustrated in  FIGS. 4A and 4B , the first elastic member  215  and the second elastic member  216  that guide the sheet P which is sequentially transported from the receiving roller  201  to the compile tray  210  side and guide the sheet P which is reversely transported by the reverse roller  211  to the third post-processing transport path S 3  are disposed on an upstream side of a connection position of the second post-processing transport path S 2  where the third post-processing transport path S 3  is connected. 
     The first elastic member  215  is arranged so that a base end portion  215   a  is fixedly supported by the second post-processing transport path S 2  and a tip end portion  215   b  protrudes to a sheet transport area of the second post-processing transport path S 2 . 
     The second elastic member  216  is arranged so that a base end portion  216   a  is fixedly supported by the third post-processing transport path S 3  and a tip end portion  216   b  protrudes to a downstream side more than the tip end portion  215   b  of the first elastic member  215   
     The first elastic member  215  and the second elastic member  216  are formed of Mylar films having different thicknesses from each other. When the thickness of the first elastic member  215  is t 1  and the thickness of the second elastic member  216  is t 2 , t 1  is larger than t 2 . 
     Specifically, the thickness t 1  of the first elastic member  215  is 0.15 mm to 0.3 mm, and the thickness t 2  of the second elastic member  216  is 0.05 mm to 0.14 mm. 
     The specific examples of the first elastic member  215  and the second elastic member  216  are illustrated in  FIG. 43 . The first elastic member  215  is shaped so that the three tip end portions  215   b  protrude in a rectangular shape in a central portion. The second elastic member  216  protrudes in a rectangular shape at three places to correspond to the tip end portions  215   h  of the first elastic member  215 . In addition, triangular notches are formed on both sides of each of the tip end portions  216   b . Accordingly, the second elastic member  216  is relatively more likely to be bent than the first elastic member  215 . 
     As a result, the tip end portion  216   b  of the second elastic member  216  is bent downward, as illustrated in  FIG. 5A , due to a transporting force in a leading edge portion of the sheet P when the sheet P is transported to a downstream side in the second post-processing transport path S 2 , and a gap is formed between the tip end portion  215   b  of the first elastic member  215  and the tip end portion  216   b  of the second elastic member  216 . As such, the second post-processing transport path S 2  communicates with the downstream side, and the sheet P is transported to the downstream side. 
     In addition, as illustrated in  FIG. 5B , a surface side of the second elastic member  216  is supported by the first elastic member  215  that is thick and is relatively higher in rigidity than the second elastic member  216  when the sheet P, which is temporarily transported to the compile tray  210  side, is transported to an upstream side through reversal between the reverse roller  211  and the compile discharge roller  212 . Then, the second post-processing transport path S 2  is closed, and a leading edge (trailing edge in the case of progressive transport) of the backhauled sheet P is guided in contact with a back surface side of the second elastic member  216  and is transported to the third post-processing transport path S 3 . 
     (3) Stacking Operation During Post-Processing 
       FIGS. 6A to 6D  are schematic views illustrating a stacking operation in the post-processing device  200 . F g.  7  is an example of a timing chart of the stacking operation in the post-processing device  200 .  FIGS. 8A to 8D  are schematic views illustrating a three-sheet stacking operation in the post-processing device  200 . 
     Hereinafter, the stacking operation in the post-processing device  200  will be described with reference to the drawings. 
     (3.1) Stacking Operation During Post-Processing 
     In a case where the binding processing as an example of post-processing is performed in the post-processing device  200  on the sheets P that are sequentially transported via the transport device  100 , the switching gate G 1  is switched to the second post-processing transport path S 2  side, and the sheets P that are transported from the receiving roller  201  are sequentially transported to the compile tray  210  via the second post-processing transport path  82 . Then, the binding processing is performed by the stapler  220  in a state where a predetermined number of the sheets P are aligned as the sheet bundle PB. 
     In this case, a predetermined processing time is required in the post-processing device  200  for the stapler  220  to perform the binding processing on the sheet bundle PB which is aligned on the compile tray  210  and for the sheet bundle PB to be discharged to the stacker tray TR 2 . However, from the high-speed image forming apparatus  2 , next sets of sheets P are transported successively to the post-processing device  200  via the transport device  100 . 
     Accordingly, in the post-processing device  200 , the sheets P are detected by a receiving detection sensor SR 1  that is disposed at a nip outlet of the receiving roller  201  when the first sheet P of the next set is transported to the receiving roller  201 . 
     The first sheet P of the next set is temporarily transported to the second post-processing transport path S 2  with the switching gate G 1  switched (refer to  FIG. 6A ). The reverse roller  211  and the compile discharge roller  212  that are disposed in the second post-processing transport path S 2  are stopped and then reversed when the trailing edge of the sheet P is detected by the receiving detection sensor SR 1  and the trailing edge of the sheet P passes the connection position of the third post-processing transport path S 3  with respect to the second post-processing transport path S 2 . 
     As a result, the sheet P that is temporarily transported to the second post-processing transport path S 2  is backhauled by the reverse roller  211  and the compile discharge roller  212  which are reversed, and the trailing edge of the sheet P is guided in contact with the back surface side of the second elastic member  216  and is transported to the third post-processing transport path S 3  (refer to  FIG. 6B ). 
     Then, the sheet P is transported by the buffer roller  214  that is disposed in the third post-processing transport path S 3 , and comes out of a nip of the reverse roller  211  that is disposed in the second post-processing transport path  32 . Then, the buffer roller  214  stops, and the first sheet P is temporarily held in the third post-processing transport path S 3 . 
     Next, the second sheet P of the next set is transported to the second post-processing transport path S 2 . In synchronization with the transport of the leading edge of the second sheet P of the next set to immediately in front of the reverse roller  211  that is disposed in the second post-processing transport path S 2 , the first sheet P that is temporarily held in the third post-processing transport path S 3  is transported to the reverse roller  211  in a state of being stacked with the leading edge of the second sheet P of the next set with the buffer roller  214  disposed in the third post-processing transport path S 3  reversed (refer to  FIG. 60 ). 
     Then, the first sheet P and the second sheet P of the next set are transported to the compile tray  210 , by the compile discharge roller  212 , in a stacked state (refer to  FIG. 6P ). 
     The second elastic member  216  is disposed in the post-processing device  200  according to this exemplary embodiment to guide the sheet P, which is reversed by the reverse roller  211  and transported, to the third post-processing transport path S 3 . The surface side of the second elastic member  216  is supported by the first elastic member  215 , which is thick and is relatively higher in rigidity than the second elastic member  216 , to close the second post-processing transport path S 2 , and the trailing edge of the backhauled sheet P is guided in contact with the back surface side of the second elastic member  216  and is transported to the third post-processing transport path S 3 . 
     Accordingly, no transport path switching operation is required when the first sheet P of the next set is backhauled by the reverse roller  211  and the compile discharge roller  212  and is transported into the third post-processing transport path S 3 , and the backhauling may be initiated immediately. 
     When the second sheet P of the next set is transported to the second post-processing transport path S 2 , the tip end portion  216   b  of the second elastic member  216  is bent downward due to the transporting force of the leading edge portion of the second sheet P, the gap is formed between the tip end portion  215   b  of the first elastic member  215  and the tip end portion  216   b  of the second elastic member  216 , and the second sheet P is transported to the downstream side. In other words, no transport path switching operation is required for the stacking as well, and the stacking with the first sheet P which is transported out of the third post-processing transport path S 3  is allowed. 
     According to the post-processing device  200  of this exemplary embodiment, no transport path switching operation is required and the backhauling may be initiated immediately in this manner. As such, the post-processing device  200  according to this exemplary embodiment may be combined with the high-speed image forming apparatus  2 . Also, since no transport path switching operation is required, noise following a switching operation may be suppressed, an increase in cost may be suppressed, and reduction in size may be allowed. 
     (3.2) Timing Chart of Stacking Operation 
       FIG. 7  illustrates an example of the timing chart of the operation at a time of stacking between the first sheet P and the second sheet P of the next set in a case where post-processing is performed in the compile tray  210 . 
     In this example, the stacking operation is performed under the following conditions. 
     Sheet P: Letter size (landscape-feeding) 
     Interval of sheet receiving from the transport device  100 : 400 ms 
     Transport speed in the second post-processing transport path S 2 : 1,000 mm/s 
     Transport speed during the backhauling: 1,300 mm/s 
     In  FIG. 7 , t 1  represents a wait time at a time of switching of the reverse roller  211  and the compile discharge roller  212  from a normal rotation to reversal. According to this example, no transport path switching operation is required, and thus t 1  is equal to 5 ms and the switching may be performed substantially immediately. 
     (3.3) Three-Sheet Stacking Operation 
     As illustrated in  FIG. 8A , the first sheet P that is temporarily held in the third post-processing transport path S 3  is transported to the reverse roller  211  in a state of being stacked with the leading edge of the second sheet P of the next set with the buffer roller  214  disposed in the third post-processing transport path S 3  reversed in synchronization with the transport of the leading edge of the second sheet P of the next set to immediately in front of the reverse roller  211  that is disposed in the second post-processing transport path S 2  during the transport of the second sheet P of the next set to the second post-processing transport path S 2 . 
     Then, the trailing edge of the sheet P is transported, in a state where the first sheet P and the second sheet P of the next set are stacked, beyond the connection position of the third post-processing transport path S 3  in the second post-processing transport path S 2 . Then, the reverse roller  211  and the compile discharge roller  212  are reversed, and the trailing edge of the sheet P is guided in contact with the back surface side of the second elastic member  216  and is transported to the third post-processing transport path S 3  in a state where the first sheet P and the second sheet P are stacked (refer to  FIG. 8B ). 
     Then, the third sheet P of the next set is transported to the second post-processing transport path S 2 . In synchronization with the transport of the leading edge of the third sheet P of the next set to immediately in front of the reverse roller  211  that is disposed in the second post-processing transport path S 2 , the first sheet P and the second sheet P of the next set that are temporarily held in the third post-processing transport path S 3  are transported to the reverse roller  211  in a state of being stacked with the buffer roller  214  disposed in the third post-processing transport path S 3  reversed (refer to  FIG. 8C ). 
     The first sheet P, the second sheet P, and the third sheet P of the next set are transported to the compile tray  210  by the compile discharge roller  212  in a state where the three sheets are stacked (refer to  FIG. 8D ). 
     Combination with the higher-speed image forming apparatus  2  is allowed by holding the two sheets P of the next set in the third post-processing transport path S 3  in a stacked state and stacking the two sheets P with the third sheet P of the next set in this manner. Also, since no transport path switching operation is required, noise following a switching operation may be suppressed, an increase in cost may be suppressed, and reduction in size may be allowed. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.