Patent Publication Number: US-7900912-B2

Title: Sheet processing apparatus and image forming system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus for performing a sheet processing such as a sort process and a staple process, and to an image forming system. 
     2. Description of the Related Art 
     Up to now, known is a sheet processing apparatus for performing a post-treatment such as a sort process and a staple process for a sheet delivered from an image forming apparatus or the like. In the sheet processing apparatus of this kind, the sheet delivered from the image forming apparatus or the like is received on an intermediate process tray to be subjected to the post-treatment such as the sort process and the staple process for a sheet stack on the intermediate process tray. The sheet stack subjected to the post-treatment is nipped by a pair of delivery rollers and is transported to a stack tray to be stacked (see U.S. Pat. No. 6,219,503). 
     In this case, the pair of delivery rollers is controlled to be in an open (i.e., spaced) state during the post-treatment for the sheet stack on the intermediate process tray, and to be in a closed state after the post-treatment is completed. 
     However, in conventional sheet processing apparatuses, a control of an opening amount of the pair of delivery rollers is not performed during the post-treatment for the sheet stack on the intermediate process tray, so the opening amount of the pair of delivery rollers is kept constant irrespective of a thickness of the sheet stack. As a result, even when the sheet stack is thin, there is required a certain period of time for an opening/closing operation of the pair of delivery rollers. 
     On the other hand, with an increase in processing speed of an image forming apparatus in recent years, the high processing speed is also demanded in the sheet processing apparatus. Thus, a waste of time such as a certain period of time required for the opening/closing operation of the pair of delivery rollers cannot be ignored any more, so it is demanded that the opening/closing operation of the pair of delivery rollers is performed swiftly to speed up the delivery process of the sheet stack. 
     To satisfy the demand, there is a possible way in which the opening/closing operation of the pair of delivery rollers itself is speeded up. However, even when the opening and closing operation of the pair of delivery rollers is simply speeded up, there arises a problem in that the pair of delivery rollers is bounced when the pair of delivery rollers is closed to nip the sheet stack, thereby causing vibration and noise. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in the above-mentioned background, and therefore has an object to provide a sheet processing apparatus, a control method, and a program in which a delivery process for a sheet stack delivered from an intermediate process tray may be speeded up without causing any new problems. 
     To attain the above-mentioned object, according to an aspect of the present invention, a sheet processing apparatus connected to an image forming apparatus, includes: an intermediate process tray which receives sheets delivered from the image forming apparatus as a sheet stack; a process unit which applies a predetermined process for the sheet stack received on the intermediate process tray; a pair of rollers which nips and delivers the sheet stack subjected to the predetermined process by the process unit; a stack tray which receives the sheet stack delivered by the pair of rollers; and a controller which moves one roller of the pair of rollers to a standby position corresponding to a thickness of the sheet stack to be delivered onto the stack tray. 
     Other objects and features of the present invention will become apparent as follows in this specification with reference to the accompanying drawings. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a schematic structure of an image forming system using a sheet processing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing a structure of a control system of the image forming system; 
         FIG. 3  is a cross-sectional view showing a schematic structure of the sheet processing apparatus; 
         FIG. 4  is a cross-sectional view showing a schematic structure of an intermediate process tray of a finisher serving as a sheet processing apparatus; 
         FIG. 5  is a flowchart showing a mode discrimination process performed by the finisher; 
         FIG. 6  is a flowchart showing a staple sort process performed by the finisher; 
         FIG. 7  is a flowchart showing a sheet-stack delivery operation discrimination process performed by the finisher; 
         FIG. 8A  is an explanatory diagram of a sheet delivery process to the intermediate process tray performed by the finisher; and  FIG. 8B  is an explanatory diagram of a sheet delivery process to the intermediate process tray performed by the finisher; and 
         FIG. 9  is a flowchart showing a rocking guide control process performed by the finisher. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be described in detail with reference to the drawings.  FIG. 1  is a cross-sectional view showing a schematic structure of an image forming system using a sheet processing apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , in the image forming system, a sheet processing apparatus  2  composed of a folding device  400  and a finisher  500  is connected to an image forming apparatus main body  1 . 
     The image forming apparatus main body  1  includes an image reader  200  and a printer  300 , and the image reader  200  is mounted with an original feeding apparatus  100 . The original feeding apparatus  100  picks up an original to be set one by one from a top page, feeds the picked-up original from left to right on a platen glass plate  102  through a curved path, and then delivers the original to a delivery tray  112 . 
     In this case, a scanner unit  104  is held in a predetermined position, and reads an original image when the original passes on the scanner unit  104  from left to right. A reading method of this case is called an original flow reading. In a process in which the original passes on the scanner unit  104 , the original is irradiated with light emitted from a lamp  103  of the scanner unit  104 . Reflected light from the original which received the irradiated light enters an image sensor  109  through mirrors  105 ,  106 , and  107 , and a lens  108  as image light reflecting the original image. The image sensor  109  photoelectrically converts the entered image light and outputs the converted light as an image signal. 
     It should be noted that the original fed from the original feeding apparatus  100  is allowed to stop once on the platen glass plate  102 , and the scanner unit  104  is moved from left to right, thereby making it possible to read the original image. A reading method of this case is called an original fixed-reading. When the original image is read without using the original feeding apparatus  100 , a user sets the original on the platen glass plate  102  by lifting up the original feeding apparatus  100 . In this case, the original fixed-reading is performed. 
     The image signal outputted from the image sensor  109  is subjected to an image processing by an image processing part  203  shown in  FIG. 4 , and then is sent to an exposure control part  110 . The exposure control part  110  controls a laser (not shown) so that a laser beam modulated based on the image signal is outputted. The exposure control part  110  also performs a rotation control and the like of a polygon mirror for exposing and scanning a photosensitive drum  111  with a laser beam. By the exposing and scanning with the laser beam, an electrostatic latent image corresponding to the original image is formed on photosensitive drum  111 . The electrostatic latent image formed on the photosensitive drum  111  is developed as a toner image by a developing device  113 . The toner image formed on the photosensitive drum  111  is transferred onto a sheet by a transferring part  116 . The sheet is fed from any one of a cassette  114 , a cassette  115 , a manual feeding part  125 , and a duplex transport path  124 . 
     The sheet subjected to the transferring process of the toner image is transported to a fixing part  117 , and the toner image related to the transferring process is fixed on the sheet by the fixing part  117 . The sheet subjected to the fixing process is guided to a path  122  once by a flapper  121 , and is switched back to be guided to a main body delivery roller  118  by the flapper  121  after a rear end of the sheet has passed through the flapper  121 . As a result, the sheet is delivered from the printer  300  by the main body delivery roller  118  in a state where a transfer surface of the toner image faces downward (faces down). A delivery method of this case is called a reverse delivery. 
     In the case where original images and the like of a plurality of pages are printed out from the top page by delivering the sheets while facing down, it is possible to deliver the sheets in such a manner that the sheets are stacked in order from the top page viewed from a print surface thereof. Note that when an image formation is performed on a hard sheet such as an OHP sheet from the manual feeding part  125 , the sheet is delivered by the main body delivery roller  118  in a stare where the printing surface faces upward (faces up) without being guided to the path  122 . When an image is formed on both sides of the sheet, the sheet is guided directly from the fixing part  117  to the main body delivery roller  118 , and is switched back immediately after the rear end of the sheet has passed through the flapper  121 , thereby being guided to the duplex transport path  124  by the flapper  121 . 
     The sheet delivered from the printer  300  is delivered to the folding device  400 . The folding device  400  performs a fold process in which the delivered sheet is folded into a Z-shape, and delivers the sheet to the finisher  500 . Note that the folding device  400  performs the fold process only in a case where a size of the delivered sheet is A3 size or B4 size and the fold process is designated. In the other cases, the sheet is delivered to the finisher  500  without being folded. The finisher  500  performs a bookbinding process, a staple (stitch) process, a punch (piercing) process, and the like for the sheet delivered from the folding device  400 . 
     Next, a structure of a control system of the image forming system shown in  FIG. 1  will be described with reference to the block diagram of  FIG. 2 . The image forming system controls a series of image forming processes and the sheet processing using a CPU circuit part  150  as a core. The CPU circuit part  150  is connected to an operating part la, an original feeding apparatus control part  101 , an image reader control part  201 , an image signal control part  202 , a printer control part  301 , a folding device control part  401 , and a finisher control part  501 , respectively. In addition, the image signal control part  202  is connected to a computer  208  through an external I/F, thereby making it possible to taking in image data processed by the computer  208 . 
     The CPU circuit part  150  includes a ROM  151  and a RAM  152  in addition to a CPU (not shown). The CPU circuit part  150  controls the series of image forming processes and the sheet processing while appropriately outputting control signals to the original feeding apparatus control part  101 , the image reader control part  201 , the image signal control part  202 , the printer control part  301 , the folding device control part  401 , and the finisher control part  501  according to a program stored in the ROM  151  and an operation setting by the operating part  1   a . Note that the RAM  152  is used as an area for temporarily storing the control data or as a work area for the calculation related to the control. 
     The original feeding apparatus control part  101  controls the original feeding process performed by the original feeding apparatus  100 . The image reader control part  201  controls the reading process of the original image performed by the image reader  200 , and the printer control part  301  controls the image forming process performed by the printer  300 . The folding device control part  401  controls the sheet fold process performed by the folding device  400 , and the finisher control part  501  controls a bookbinding process, a punch process, a staple process, and the like performed by the finisher  500 . The image signal read by the image reader  200  is outputted to the printer control part  301  through the image signal control part  202 . 
     It should be noted that the original feeding apparatus control part  101 , the image reader control part  201 , the image signal control part  202 , the printer control part  301 , the folding device control part  401 , and the finisher control part  501  include a CPU, a ROM, and a RAM in a similar manner as the CPU circuit part  150 . The ROM of the finisher control part  501  stores program codes for performing a process according to flowcharts shown in  FIGS. 5 ,  6  and  7 , and  9  to be described later. 
     Next, the sheet processing apparatus  2  will be described with reference to  FIG. 3 . In this embodiment, the image forming apparatus main body  1  includes the sheet processing apparatus  2  which is composed of the folding device  400  and the finisher  500  to be connected to the image forming apparatus main body  1 . 
     The sheet delivered from the image forming apparatus main body  1  to the folding device  400  is transported to a folding and transporting horizontal path  411 . In this case, the sheet is detected by a folding and transporting horizontal path sensor  430 . When the sheet is detected, in a case where the fold process is not performed for the sheet, the folding device control part  401  turns off a folding path selection flapper  412 , thereby transporting the sheet to the finisher  500  without folding thereof. 
     In a case where the fold process is performed for the sheet, the folding device control part  401  turns on the folding path selection flapper  412 , thereby folding the sheet to be transported to a transporting and folding path  413 . After the fold process is completed by a folding roller  414 , the folding device control part  401  turns off the folding path selection flapper  412 , thereby transporting the sheet to the finisher  500 . 
     The sheet transported to the finisher  500  is first transported into a saddle selection flapper part  515 . When a bookbinding process is performed, the finisher control part  501  turns on the saddle selection flapper  515 , thereby transporting the sheet to a saddle transporting path  524 . 
     When a punch process, a staple process, and the like are performed, the finisher control part  501  drives the saddle switching flapper  515  so that the sheet is transported in a direction of a punch unit  550  through a transport path  520 . Alternatively, when the bookbinding process is performed, the finisher control part  501  drives the saddle switching flapper  515  so that the sheet is transported in a direction of a bookbinding unit  525  through the bookbinding path  524 . Note that the other components of the finisher  500  are briefly described herein, and a detailed description thereof will be given later in a process of explaining the punch process, the staple process, and the like. 
     Reference numeral  503  denotes a transport roller, reference numeral  531  denotes an entrance path sensor, and reference numeral  505  denotes a large transport roller. Reference numeral  509  denotes a pair of delivery rollers which delivers the sheet transported through a transport path  521  onto a sample tray  701 . Reference numeral  511  denotes a switching flapper which switches a sheet transport destination to the sample path  521  or a sort path  522 . Reference numeral  510  denotes a switching flapper which switches the sheet transport destination to the sort path  522  or a buffer path  523 . 
     Reference numeral  630  denotes an intermediate tray (hereinafter, referred to as “process tray” for receiving sheets temporarily and performing a sort (alignment) process and a staple process. Reference numeral  507  denotes a pair of delivery rollers for delivering sheets as a sheet stack onto a process tray  630 , and reference numeral  680  denotes a pair of sheet-stack delivery rollers for delivering the sheet stack received on the process tray  630  onto the stack tray  700 . Reference numeral  517  denotes an alignment plate for aligning the sheet stack received on the process tray  630  in a width direction. Reference numeral  603  denotes a drawing paddle for hitting the sheet against a rear end of the process tray  630 . Reference numeral  601  denotes a stapler for stitching the sheet stack received on the process tray  630 . The stapler  601  is capable of moving in a substantially vertical direction with respect to a sheet transport direction, and moves along an end portion of the sheet, thereby making it possible to perform the staple process such as a two position stitch and the like. 
     Next, a structure of the process tray  630  will be described with reference to  FIG. 4 . Between an upper transport roller  507   a  and a lower transport roller  507   b  of a pair of transport rollers  507 , a knurling belt  602  is mounted. The knurling belt  602  is composed of elastic deformable members made of rubber and resin, respectively, and has a larger diameter than that of the lower transport roller  507   b . The sheet is nipped between the knurling belt  602  and the upper transport roller  507   a  to be delivered onto the process tray  630 . As indicated by the alternate long and two short dashes line, a distance L between a surface where the knurling belt  602  is in contact with the upper transport roller  507   a  and a rotation center  507   c  of the lower transport roller  507   b  is set such that a sheet P is thrown at a targeted conveying speed to land on the process tray  630  in a predetermined position. 
     The rear end portion of the process tray  630 , in other words, a portion close to the folding device  400  which is located on a right side of  FIG. 3 , is positioned lower than a leading end portion of the process tray  630 . Thus, the sheet P delivered onto the process tray  630  recedes to the rear end side of the process tray  630  as indicated by the solid line, to be received by a rear end stopper  691 . Note that, as described later, the sheet is actually allowed to hit against the rear end stopper  691  forcibly by the drawing paddle  603  or through a reverse rotation of the pair of sheet-stack delivery rollers  680  to thereby perform an alignment of the rear end portion of the sheet stack. 
     While the sheet is received on the process tray  630 , a pair of the alignment plates  517  (one of which is not shown) is repeatedly brought into contact with and spaced apart from the sheet from both sides in a width direction of the sheet, thereby performing the alignment of the sheet stack in the width direction thereof. 
     When the sheet stack on the process tray  630  reaches a predetermined thickness, a lower part of the knurling belt  602  interrupts the sheet receding toward the rear end stopper  691 . As a result, as indicated by the alternate long and two short dashes line of  FIG. 4 , the knurling belt  602  is pulled and deformed to be flattened when the displacement roller  516  is displaced. 
     When the staple process is performed, the rear end stopper  691  is rotated as indicated by the alternate long and two short dashes line of  FIG. 4  so as not to interrupt the staple operation. The stapler  601  approaches an anvil  519 , and the stapler  601  and the anvil  519  nip the sheet stack to thereby stitch the sheet stack by a staple  692 . 
     The sheet stack subjected to a stitch process is delivered onto the stack tray  700  or the sample tray  701  by a rotation of the knurling belt  602  which is restored to its original circular shape by the further displacement of the displacement roller  516 , and a rotation of the pair of sheet-stack delivery rollers  680  which descends to approach the process tray  630 . 
     It should be noted that the upper delivery roller  680 a of the pair of sheet-stack delivery rollers  680  is moved up and down by a rotation of a rocking guide  685  to be brought into contact with and spaced apart from the lower delivery roller  680   b . A nipping force of the sheet stack is controlled by a drive mechanism (not shown) of the rocking guide  685 . To be specific, the rocking guide  685  is driven by a stepping motor, is moved in a direction in which the upper delivery roller  680   a  is opened by a normal rotation of the motor, and is moved in a direction in which the upper delivery roller  680   a  is closed by a reverse rotation of the motor. In this case, the stepping motor is controlled to be driven such that the nipping force of the sheet stack by the pair of sheet-stack delivery rollers  680  is set to be constant irrespective of the thickness of the sheet stack. In other words, the position of the upper delivery roller  680   a  in a case of the sheet-stack containing 3 sheets differs from that in a case of the sheet-stack containing 100 sheets, but the stepping motor is controlled to be driven such that the force of nipping the sheet stack is set to be constant. 
     When the sheet stack is delivered from the process tray  630 , the upper delivery roller  680   a  is returned to a position indicated by the solid line of a direction in which the upper delivery roller  680   a  moves away from the process tray  630 , in preparation for the series of processes for a group of sheets related to a subsequent sheet stack, and the rear end stopper  691  is also returned to the position indicated by the solid line. 
     Next, a puddle operation performed when a first sheet group of sheet groups is delivered onto the process tray  630  will be described. It should be noted that the sheet is referred to as “sheet group”, because the sheet to be delivered onto the process tray  630  is not always one, but there is also a case where a plurality of sheets are overlapped to be delivered. In other words, even when the sheet is referred to as “sheet group”, there is a case where the sheet is only one. 
     In the alignment operation, when the sheet group is delivered onto the process tray  630 , the pair of sheet-stack delivery rollers  680  is normally in an opened state. Accordingly, it is not possible that the sheet group which is thrown onto the process tray  630  by the rotations of the pair of delivery rollers  507  and the knurling belt  602  is pushed back toward the rear end stopper  691  by the pair of sheet-stack delivery rollers  680 . As a result, every time the sheet group is delivered onto the process tray  630 , the drawing paddle  603  shown in  FIG. 3  is rotated, thereby pushing back the sheet group thrown onto the process tray  630  in a direction of the rear end stopper  691 . 
     However, when the first sheet group related to the sheet stack to be stapled is delivered onto the process tray  630 , the pair of sheet-stack delivery rollers  680  is in a closed state. Accordingly, in a case where the first sheet group is delivered onto the process tray  630 , the first sheet group is allowed to hit against the rear end stopper  691  by driving the pair of sheet-stack delivery rollers  680  to be rotated in a reverse direction by a predetermined amount. In this case, the drawing paddle  603  is stopped. After that, the pair of sheet-stack delivery rollers  680  is in a closed state, and the alignment in a width direction of the sheet group is performed by the alignment plate  517 . 
     Further, the pair of sheet-stack delivery rollers  680  is maintained in the opened state until when the sheet group related to the sheet stack to be stapled is completely delivered onto the process tray  630 . Thus, when a second and subsequent sheet groups are delivered onto the process tray  630 , the sheet groups are allowed to hit against the rear end stopper  691  by the drawing paddle  603 . As described above, the drawing paddle  603  operates on the second and the subsequent sheet groups and does not operate on the first sheet group, thereby making it possible to reduce the number of operations and suppress friction. 
     It should be noted that in a case where a plurality of sheets are stored in the buffer path  523 , and the plurality of sheets are simultaneously delivered onto the process tray  630 , the drawing paddle  603  is not allowed to operate when the first sheet group stored in the buffer path  523  is delivered onto the process tray  630 . 
     This is because, in a case where the sheet group is to hit against the rear end stopper  691  only by the drawing paddle  603  when the buffered sheet group is received on the process tray  630 , only an upper sheet of the sheet group is allowed to hit against the rear end stopper  691 , and a lower sheet of the sheet group is not allowed to hit against the rear end stopper  691  when a coefficient of friction between the plurality of sheets is small. To avoid such the failure, the pair of sheet-stack delivery rollers  680  is rotated in a reverse direction in a state where the sheet group is nipped by the pair of sheet-stack delivery rollers  680 , thereby also allowing the lower sheet of the sheet group to hit against the rear end stopper  691 . 
     Next, a standby position of the upper delivery roller  680   a  of the pair of sheet-stack delivery rollers  680  will be briefly described. The standby position of the upper delivery roller  680   a  is set so that a plurality of standby positions may be additionally set between a maximum separation position (corresponding to a position indicated by the dotted line of  FIG. 4 ) and a contact position with the sheet stack (corresponding to a position indicated by the alternate long and two short dashes line of  FIG. 4 ). A selection process of the standby positions will be described later. 
     Next, a control process of the finisher will be described with reference to the flowchart. First, an operation mode discrimination process will be described with reference to the flowchart of  FIG. 5 . 
     When a start signal or the like of the finisher process is transmitted from the CPU circuit part  150  (Step S 1 ), the finisher control part  501  starts driving an entrance motor, a buffer motor, and a delivery motor (not shown) in the finisher  500  (Step S 2 ). 
     It is assumed that the start signal contains various information which is necessary for the operation mode and the staple process such as the number of sheet stacks to be stapled and the number of sheet stacks (i.e., number of copies). However, the various information necessary for the staple process such as the number of sheet stacks (i.e., number of copies) may be transmitted to the finisher control part  501  from the CPU circuit part  150  prior to the start signal of the finisher process when the image forming process is started, for example. 
     Next, the finisher control part  501  discriminates operation mode information included in the various information (Step S 3 ). The finisher control part  501  performs a control of a non-sort process when the operation mode information is a non-sort mode (Step S 4 ), performs a control of a sort process when the operation mode information is a sort mode (Step S 5 ), and performs a control of a staple sort process when the operation mode information is a staple sort mode (Step S 6 ). 
     Upon completing any one of the above-mentioned processes, the finisher control part  501  stops driving the entrance motor, the buffer motor, and the delivery motor (not shown) within the finisher  500  (Step S 7 ), and returns to the standby state of Step S 1 . 
     Next, the staple sort process of Step S 6  shown in  FIG. 5  will be described in detail with reference to the flowchart of  FIG. 6 . 
     Upon starting the staple process, the finisher control part  501  first drives the switching flapper  511  to guide the sheet in a direction of the process tray  630  (Step S 301 ). In this case, the finisher control part  501  switches the switching flapper  515  in advance so as to select the transport path  520  without selecting the bookbinding path  524  because the bookbinding process is not performed therein. 
     Next, the finisher control part  501  discriminates whether or not the start signal of the finisher process sent from the CPU circuit part  150  is turned on (Step S 302 ). As a result, when the start signal is turned on, the finisher control part  501  discriminates whether or not the entrance path sensor  531  is turned on, to thereby discriminate whether or not the sheet is delivered from the folding device  400  (Step S 303 ). 
     When the sheet is delivered from the folding device  400 , the finisher control part  501  starts a sheet sorting sequence process (Step S 304 ). The sheet sorting sequence process is assigned to every sheet, and is performed by a program in multitasking. In the sheet sorting sequence process, a buffering process in which the buffer path  523  is selected to temporarily hold the sheet is performed by switching the switching flapper  510  appropriately, and a delivery process in which the sheet within the buffer path  523  and the subsequent sheets are simultaneously delivered onto the process tray  630  is performed by selecting the sort path  522 . 
     In the sheet sorting sequence process, the staple process for the sheet stack received on the process tray  630 , and a sheet-stack delivery operation discrimination process which is described later are also performed. Note that the buffering process for the sheet is performed to allow a margin in terms of time necessary for a process at a downstream side, and is not performed for the first several sheets. 
     The finisher control part  501  starts the sheet sorting sequence process in Step S 304 , and then discriminates whether or not the entrance path sensor  531  is turned off, to thereby discriminate whether or not the rear end of the sheet has passed through the position of the entrance path sensor  531  (Step S 305 ). 
     In a case where the rear end of the sheet has passed through the position of the entrance path sensor  531 , the finisher control part  501  returns to Step S 302  to discriminate whether or not the start signal is turned on. On the other hand, in a case where the rear end of the sheet has not passed through the position of the entrance path sensor  531 , the finisher control part  501  returns to Step S 304  to continue the sheet sorting sequence process. 
     In Step S 302 , in a case where it is discriminated that the start signal is turned off, the finisher control part  501  completes delivering all the sheets to the process tray  630 , and then switches the switching flapper  511  in another direction (Steps S 306  and S 307 ) to return to the flow of  FIG. 5 . 
     Next, the sheet-stack delivery operation discrimination process performed in the process of the sheet sorting sequence process will be described with reference to the flowchart of  FIG. 7 . 
     In the sheet-stack delivery operation discrimination process, the finisher control part  501  first determines whether or not the operation mode is the staple mode (Step S 601 ). As a result, when it is determined that the operation mode is not the staple mode, the finisher control part  501  discriminates whether or not the sheet delivered onto the process tray  630  is a plurality of sheets, in other words, a sheet group related to the sheet-stack delivery (Step S 602 ). When it is determined that the sheet is the sheet group related to the sheet-stack delivery in Step S 602 , the finisher control part  501  proceeds to Step S 605  to be described later. When it is determined that the sheet is not the sheet group related to the sheet-stack delivery, the finisher control part  501  returns to the flow of the sheet sorting sequence process. 
     Further, when it is determined that the operation mode is the staple mode in Step S 601 , the finisher control part  501  discriminates whether or not the sheet delivered onto the process tray  630  is a plurality of sheets, in other words, the sheet group related to the sheet-stack delivery (Step S 603 ). When it is determined that the sheet is not the sheet group related to the sheet-stack delivery in Step S 603 , the finisher control part  501  returns to the flow of the sheet sorting sequence process. 
     On the other hand, when it is determined that the sheet is the sheet group related to the sheet-stack delivery in Step S 603 , the finisher control part  501  performs the staple process for the sheet group on the process tray  630  to form a sheet stack (Step S 604 ), and proceeds to Step S 605 . 
     In Step S 605 , the finisher control part  501  rotates the rocking guide  685 , thereby performing the rocking guide control process in which the upper delivery roller  680   a  of the pair of sheet-stack delivery rollers  680  is brought into contact with the sheet stack on the process tray  630  to deliver the sheet stack onto the stack tray  700  (Step S 605 ). 
     The rocking guide control process will be described in detail with reference to the flowchart of  FIG. 9 . Note that, in the flowchart of  FIG. 7 , the staple process of Step S 604  and the rocking guide control process of Step S 605  are described as completely different processes for convenience, but a part of the rocking guide control process is executed as a part of the staple process. In addition, a part of the rocking guide control process is executed also as a part of the sheet sorting sequence process of Step S 304  shown in  FIG. 6 . 
     Next, the finisher control part  501  raises and lowers the stack tray  700  to complete the operation of receiving the sheet stack onto the stack tray  700  (Step S 606 ). Then, the finisher control part  501  resets a delivery counter to “0” which has been counted up every time one sheet is delivered onto the process tray  630  in the sheet sorting sequence process (Step S 607 ), and returns to the sheet sorting sequence process. 
     It should be noted that the number of sheets which is counted by the delivery counter is used as data for determining the thickness of the sheet stack as described later. 
     Next, prior to a description as to the rocking guide control process, a description as to a delivery method of the sheet onto the process tray  630  will be given by taking cases where the number of sheets per stack is three and five as examples. 
     As shown in  FIG. 8A , in a case where the number of sheets per stack is three, each sheet of a first set is not buffered in the buffer path  523 , and is delivered onto the process tray  630  one by one. Each sheet of a second set is appropriately subjected to a buffering process to gain time for the staple process for the sheet stack of the first set or for the delivery process onto the stack tray  700 . 
     To be specific, in this embodiment, the number of sheets to be buffered in the buffer path  523  is set to two, and the buffered first sheet and second sheet of the second set and a third sheet of the second set transported thereafter are overlapped to be simultaneously delivered onto the process tray  630  (see reference symbol X of  FIG. 8 ). A control is performed for sheets of a third set and subsequent sets in the same manner as in the sheet of the second set. 
     As shown in  FIG. 8B , also in a case where the number of sheets per stack is five, five sheets of the first set are not buffered in the buffer path  523 , and are delivered onto the process tray  630  one by one for the same reason as in the case where the number of sheets per stack is three. 
     Five sheets of the second and subsequent sets are appropriately subjected to the buffering process to gain time for the staple process for the sheet stack of the first set or for the delivery process onto the stack tray  700 . In other words, as in the process for the second set in the case where the number of sheets per stack is three, the first and second sheets of the second set are buffered in the buffer path  523  and are simultaneously delivered onto the process tray  630  in a state where the two sheets and the third sheet of the second set which is transported thereafter are overlapped with each other. Then, the fourth and fifth sheets of the second set are delivered onto the process tray  630  one by one without being buffered. The sheets of the third and subsequent sets are controlled in the same manner as in the sheet of the second set. 
     Next, the rocking guide control process will be described in detail with reference to a flowchart shown in  FIG. 9  (and with continued reference to  FIG. 4 ). In the rocking guide control process, the finisher  501  first discriminates whether or not the first sheet delivered onto the process tray  630  is the last sheet related to the sheet stack (Step S 901 ). 
     It should be noted that the “first sheet” means a sheet which is to be first delivered onto the process tray  630 . As the first sheet to be delivered, there are a case where the number of sheets per stack is one, and a case where the number of sheets per stack is two or three which are buffered and then overlapped with each other to be transported. 
     For example, the first sheet of the second set in the case where the number of sheets per stack is three is the sheet which is first delivered onto the process tray  630 , because the three sheets are overlapped with each other to be transported to the process tray  630  as described above. The first sheet of the second set in the case where the number of sheets per stack is three is the last sheet related to the sheet stack. As a result, it is discriminated that the first sheet of the second set in the case where the number of sheets per stack is three is the last sheet related to the sheet stack in Step S 901 . 
     In addition, for example, as shown in  FIG. 8B , the second to fourth sheets of the first set are the sheets which are first delivered onto the process tray  630 , that is, the second to fourth sheets are neither the first sheet nor the last sheet related to the sheet stack. Thus, it is discriminated that the first sheet is not the last sheet related to the sheet stack in Step S 901 . 
     In Step S 901 , when it is discriminated that the first sheet is the last sheet related to the sheet stack, the finisher control part  501  sets a standby position of the rocking guide  685 , that is, a standby position of the upper delivery roller  680   a  of the pair of sheet-stack delivery rollers  680 , as X 1  (Step S 902 ). The standby position X 1  is a standby position set between the maximum separation position of the upper delivery roller  680   a  of the pair of sheet-stack delivery rollers  680  and a position where the upper delivery roller  680   a  is brought into contact with the sheet stack. In this embodiment, the standby position X 1  is set to a position which is spaced apart from the position where the upper delivery roller  680   a  is brought into contact with the sheet stack by a small distance. 
     The standby position X 1  is set by assuming the following situation. That is, as described above, the first sheet to be delivered onto the process tray  630  allows the rear end of the sheet to hit against the rear end stopper  691  by rotating the pair of sheet-stack delivery rollers  680  in a reverse direction by a predetermined amount after the rear end of the sheet has fallen onto the process tray  630 . After that, the sheet is aligned in a vertical direction with respect to the sheet transport direction. However, when the sheets are held to be nipped by the pair of sheet-stack delivery rollers  680 , wrinkles of the sheets may be caused in the alignment process. 
     Therefore, it is necessary to open the pair of sheet-stack delivery rollers  680 , and after the alignment process is finished (after the staple process in the staple mode), it is necessary to close the pair of sheet-stack delivery rollers  680  in preparation for the sheets to be delivered onto the stack tray  700 . As a result, the standby position X 1  is set to the position which is spaced apart from the position where the upper delivery roller  680   a  is brought into contact with the sheet stack by a small distance so that the upper delivery roller  680   a  is immediately brought into contact with the sheet stack after being separated from the sheet stack. 
     Next, at a timing of opening the pair of sheet-stack delivery rollers  680  (Step S 903 ), the finisher control part  501  moves the upper delivery roller  680   a  to the standby position X 1  (Step S 904 ). Then, at a timing of closing the pair of sheet-stack delivery rollers  680  (Step S 905 ), the finisher control part  501  allows the upper delivery roller  680   a  to be brought into contact with the sheet stack on the process tray  630  to nip the sheet stack (Step S 906 ). 
     Next, the finisher control part  501  allows the sheet stack on the process tray  630  to be delivered onto the stack tray  700  by rotating the pair of sheet-stack delivery rollers  680  in a forward direction (Step S 907 ). When there is a subsequent sheet to be delivered onto the process tray  630  (Step S 908 ), the finisher control part  501  returns to Step S 901 , and when there is no subsequent sheet to be delivered onto the process tray  630  (Step S 908 ), the finishing control part  501  returns to the flow shown in  FIG. 7 . 
     In Step S 901 , when it is discriminated that the first sheet to be delivered onto the process tray  630  is not the last sheet related to the sheet stack, that is, when the number of sheets per stack is equal to or more than four or the like, the finisher control part  501  sets the standby position of the upper delivery roller  680   a  to an opening position X 2  (Step S 909 , see dotted lines of  FIGS. 8A and 8B ). The second and subsequent sheets to be delivered onto the process tray  630  are allowed to hit against the rear end stopper  691  by the drawing paddle  603 , so the opening position X 2  is set as the maximum opening position so as not to interrupt the rotation of the drawing paddle  603 . 
     Next, at a timing of opening the pair of sheet-stack delivery rollers  680  (Step S 910 ), the finisher control part  501  moves the upper delivery roller  680   a  to the maximum standby position X 2  (Step S 911 ). Then, the finisher control part  501  waits until the last sheet related to the sheet stack is delivered onto the process tray  630  (Step S 912 ) and discriminates a thickness of the sheet stack on the process tray  630  (Step S 913 ). In this embodiment, the sheet-stack thickness discrimination process is performed by using the number of sheets to be delivered which is counted by the delivery counter. 
     Next, the finisher control part  501  moves the upper delivery roller  680   a  to a standby position X 3  according to the calculated thickness of the sheet stack (Step S 914 ) and advances to Step S 905 . The standby position X 3  is set so that the upper delivery roller  680   a  is spaced apart from the sheet stack to perform the alignment process for the sheet stack after the last sheet is delivered onto the process tray  630 , and then the upper delivery roller  680   a  is brought into contact with the sheet stack when the sheet stack is delivered from the process tray  630  onto the stack tray  700 . 
     In this embodiment, the standby position X 3  is set to a position where a distance between the upper surface of the sheet stack and the upper delivery roller  680   a  becomes a predetermined small distance without interrupting the alignment process. As a result, when the sheet stack is delivered from the process tray  630  onto the stack tray  700 , it is possible to bring the upper delivery roller  680   a  into contact with the sheet stack swiftly to speed up the delivery process for the sheet stack from the intermediate process tray  630 . 
     As described above, in the case where the upper surface of the sheet stack is spaced apart from the upper delivery roller  680   a  by a small distance, when the pair of sheet-stack delivery rollers  680  is closed to nip the sheet stack, there is no possibility to raise a problem in that the pair of sheet-stack delivery rollers  680  to cause vibration or noise. Note that, as is assumed from the above description, the distance between the upper surface of the sheet stack and the upper delivery roller  680   a  is constant. However, the standby position X 3  itself changes according to the thickness of the sheet stack. 
     It should be noted that the present invention is not limited to the embodiment as described above. For example, the sheet-stack thickness discrimination process is performed by using only the number of sheets per stack to be received on the process tray  630 . However, the thickness of the sheet stack may be discriminated by other methods described below. 
     For example, the thickness of the sheet stack may be determined based on the number of sheets per stack to be received on the process tray  630  and sheet attribute information inputted from the operating part la of the image forming apparatus main body  1 . The sheet attribution information which can be inputted from the operating part  1   a  may contain a sheet type such as thick paper, extremely thick paper, thin paper, and an OHP sheet, and a thickness of one sheet. 
     In addition, a sensor for measuring a thickness of a single sheet may be provided in the sheet transport path from the image forming apparatus main body  1  to the process tray  630  to perform actual measurement of the thickness of the sheet to be delivered. A detailed description as to the sensor will be omitted because the sensor is well known. The thickness of the sheet may be also discriminated based on a movement of a movable core by allowing the sheet to pass between a magnetic sensor provided in the sheet transport path and the movable core. Alternatively, it is possible to discriminate the thickness of the sheet by allowing the sheet to pass through the predetermined pair of rollers to measure a displacement of the pair of rollers when the sheet passes therebetween. 
     Further, it is possible to measure the thickness of the sheet stack per stack which is received on the process tray  630 . As shown in  FIG. 4 , it is also possible that a distance measurement sensor  693  is provided on the rocking guide  685  or the like, and a distance from the bottom to the upper surface of the sheet stack which is received on the process tray  630  by the distance measurement sensor  693  to calculate the thickness of the sheet stack based on the measured value. 
     Further, the object of the present invention is also achieved by providing a system or an apparatus with a recording medium on which a program code of software for realizing a function of the embodiment is recorded, and by reading and executing the program code stored in the recording medium by the system or a computer of the apparatus (or a CPU, an MPU, or the like). 
     In this case, the program code itself, which is read from the recording medium, realize the function of the embodiment described above. As a result, the program code and the recording medium on which the program code is recorded constitute the present invention. 
     For the recording medium for supplying the program code, a floppy (registered trademark) disk, a hard disk, a magneto-optical disk, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-RAM, a DVD-RW, a DVD+RW, magnetic tape, a nonvolatile memory card, a ROM, and the like may be used. Alternatively, it is possible to download the program code through a network. 
     By executing the program code read by the computer, the function of the embodiment described above is realized, and in addition, an operating system (i.e., OS) or the like running on the computer carries out a part of or all of the actual process based on an instruction of the program code, which also realizes the function of the embodiment described above. 
     Further, the program code read from the recording medium is written in the memory which is provided to a function expansion board inserted into the computer or a function expansion unit connected to the computer, and then the function expansion board or a CPU or the like provided to the function expansion unit performs a part of or all of the actual process based on the instruction of the program code, which also realizes the function of the embodiment described above. 
     According to the embodiment, it is possible to provide the sheet processing apparatus capable of speeding up the delivery process of the sheet stack from the intermediate process tray without causing any new problems such as vibration and noise. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2005-248159, filed Aug. 29, 2005, which is hereby incorporated by reference herein in its entirety.