Abstract:
An image forming apparatus capable of connecting to a sheet post-processing equipment, includes an image forming unit which forms an image on a sheet; a reversing unit which reverses the sheet between front and back sides for forming an image on the back side after an image has been formed on the front side by the image forming unit; a two-sided conveying unit which conveys the sheet having been reversed by the reversing unit to the image forming unit for forming the image on the back side; a retaining unit which temporarily retains the sheet on a conveying path of the two-sided conveying unit by using the reversing unit and the two-sided conveying unit and then discharges the sheet to the sheet post-processing equipment when a post-processing procedure is selected; and a conveyance control unit which controls a conveying speed at a time of discharging the sheet from the two-sided conveying unit to the sheet post-processing equipment to be faster than a conveying speed at a time of conveying the sheet from the reversing unit to the two-sided conveying unit when the post-processing procedure is selected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image forming apparatus such as a copying machine and a laser-beam printer, and more particularly to the control of an image forming apparatus in case of connecting a piece of post-processing equipment for performing stapling processing, punching processing or the like to a sheet after image formation thereon.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventionally, for performing various kinds of processing such as stapling, punching, folding, binding to sheets on which images have been formed, an image forming apparatus capable of connecting a piece of post-processing equipment to the discharge port of the main body of the image forming apparatus has been provided.  
         [0005]     For preventing the sheets to be discharged at even intervals from the main body of an image forming apparatus from hindering the processing of a post-processing unit in case of making a piece of post-processing equipment perform post-processing, it is necessary to hold a large interval between the sheets to be discharged, or to buffer (retain) the sheets to be discharged at the former stage of the post-processing unit. Among inexpensive apparatus having low level productivity, there is one widening the intervals of the sheets to be discharged, but generally a lot of pieces of post-processing equipment has the buffer function for heightening the level of productivity.  
         [0006]     Moreover, an image forming apparatus performing the control of preventing the lowering of the level of productivity at the time of a two-sided copying mode previously has been proposed (for example, U.S. Pat. No. 5,778,300).  
         [0007]     However, the above-mentioned conventional image forming apparatus has a problem of being large in size and increasing in cost of the main body of the image forming apparatus and a piece of post-processing equipment owing to the space for a buffer unit and a driving mechanism in the case where the buffer function is disposed in the inside of the main body of the image forming apparatus or in the inside of the post-processing equipment.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to an image forming apparatus including a buffer function which does not increase the size and the cost of the main body of the image forming apparatus and a post-processing equipment and does not lower the productivity of image formation in case of performing post-processing.  
         [0009]     In one aspect of the present invention, an image forming apparatus operable to form images on sheets and connectable to a sheet post-processing equipment includes: an image forming unit which selectively forms an image on front and back sides of a sheet; a reversing unit configured to reverse the sheet between the front and back sides so as to facilitate forming an image on the back side after an image has been formed on the front side by the image forming unit; a two-sided conveying unit configured to convey the sheet having been reversed by the reversing unit to the image forming unit to form an image on the back side; a retaining unit temporarily retaining the sheet on a conveying path of the two-sided conveying unit with the reversing unit and the two-sided conveying unit, and then discharging the retained sheet to the sheet post-processing equipment when a post-processing procedure is selected; and a conveyance control unit controlling a conveying speed, at a time of discharging the sheet from the two-sided conveying unit to the sheet post-processing equipment, to be faster than a conveying speed at a time of conveying the sheet from the reversing unit to the two-sided conveying unit when the post-processing procedure is selected.  
         [0010]     In another aspect of the present invention, an image processing apparatus installable to a sheet post-processing equipment, including: an image forming unit which forms an image on a sheet; a first conveying portion conveying the sheet having the image formed thereon on a first conveying path; a retaining unit retaining the sheet conveyed by the first conveying portion in a second conveying path upstream with respect to the sheet post-processing equipment, wherein a conveying direction of the retained sheet discharged from the second conveying path is reverse to a conveying direction of the sheet conveyed to the second conveying path; a second conveying portion conveying the sheet from the retaining unit to the sheet post-processing equipment; and a conveyance control unit controlling a conveying speed of the sheet out of the second conveying path to be faster than a conveying speed of the sheet into the second conveying path.  
         [0011]     In yet another aspect of the present invention, an image forming apparatus capable of connecting to a sheet post-processing equipment, including: an image data inputting unit which inputs a plurality of pages of image data to be formed as images on sheets; an image forming unit configured to form the image on the sheet based on the image data inputted with the inputting unit; a retaining unit including a first conveying path branching from a second conveying path of the sheet post-processing equipment, the retaining unit temporarily retaining the sheet having the image formed thereon by the image forming unit, and then while the sheet post-processing equipment is performing post-processing of a sheet bundle, conveying the sheet to the sheet post-processing equipment; and a control unit which differentiates an order of pages on which the image is formed from an order of pages at a time of normally forming the image in the sheet bundle, to which the sheet post-processing equipment performs the post-processing.  
         [0012]     Other features and advantages of the present invention will become apparent to those skilled in the art upon reading of the following detailed description of embodiments thereof when taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0014]      FIG. 1  is a sectional view showing the configuration of an image forming apparatus according to an embodiment of the present invention;  
         [0015]      FIG. 2  is a sectional view showing the configuration of an image forming apparatus according to a first embodiment of the present invention;  
         [0016]      FIG. 3  is a block diagram showing the configuration of a control system of the image forming apparatus according to the first embodiment;  
         [0017]      FIG. 4  is an explanatory view showing a one-sided image forming operation of the image forming apparatus according to the first embodiment;  
         [0018]      FIGS. 5A, 5B ,  5 C,  5 D,  5 E,  5 F and  5 G are explanatory views showing a two-sided image forming operation of the image forming apparatus according to the first embodiment;  
         [0019]      FIG. 6  is an explanatory view showing a state in which a sheet bundle is loaded on an intermediate processing tray at the time of two-sided image formation in the image forming apparatus according to the first embodiment;  
         [0020]      FIG. 7  is an explanatory view showing an operation display unit of the image forming apparatus according to the first embodiment;  
         [0021]      FIGS. 8A and 8B  are explanatory views showing a display unit at the time of the selection of a post-processing mode of the image forming apparatus according to the first embodiment;  
         [0022]      FIGS. 9A, 9B ,  9 C,  9 D,  9 E,  9 F,  9 G and  9 H are explanatory views showing an image formation operation at the time of the selection of the post-processing mode of the image forming apparatus according to the first embodiment;  
         [0023]      FIGS. 10A, 10B ,  10 C,  10 D and  10 E are explanatory views showing the operation of a two-sided conveying path unit at the time of the selection of the post-processing mode of the image forming apparatus according to the first embodiment;  
         [0024]      FIGS. 11A, 11B ,  11 C,  11 D,  11 E,  11 F,  11 G and  11 H are explanatory views showing an image formation operation at the time of the selection of the post-processing mode of an image forming apparatus according to a third embodiment of the present invention;  
         [0025]      FIG. 12  is a sectional view showing the configuration of an image forming apparatus according to a fourth embodiment of the present invention; and  
         [0026]      FIGS. 13A, 13B ,  13 C,  13 D,  13 E and  13 F are explanatory views showing the image formation operation at the selection of the post-processing mode of the image forming apparatus according to the fourth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0027]     Embodiments of the invention will be described in detail below with reference to the drawings.  
         [0028]      FIG. 1  is a sectional view showing the main configuration of an image forming apparatus according to an embodiment of the present invention.  
         [0029]     The image forming apparatus is composed of a printer  100  and an image reader  200 , and is therein provided with an image forming unit and a retaining unit  300  for retaining a sheet on which an image has been formed by the image forming unit. Moreover, a document feeder  400  is installed in the image reader  200 , and a finisher (post-processing unit)  500  for performing desired post-processing to the sheet on which the image has been formed is connected to the printer  100 .  
         [0030]     The finisher  500  is a device for performing the processing of sheets such as an aligning operation, a binding operation, a bundle discharging operation and the like, and the retaining unit  300  is installed on the upstream side (former stage) of the finisher  500 . The retaining unit  300  utilizes a two-sided conveying path for re-conveying a sheet, or an image formation medium, to the image forming unit for forming an image on the other surface of the sheet after an image has been formed on one surface of the sheet by the image forming unit. Moreover, a control unit for controlling the speed of carrying a sheet into the retaining unit  300  to be different from the speed of carrying out the sheet from the retaining unit  300  is provided.  
         [0000]     First Embodiment  
         [0031]      FIG. 2  is a longitudinal sectional view showing the configuration of an image forming apparatus according to a first embodiment of the present invention.  
         [0032]     The document feeder  400  feeds originals set upwards on an original tray one by one from the top page in a left direction, and stops the fed original at a predetermined position on a platen glass  201  through a bent path. The image reader  200  scans the original in this state with a scanner unit  202  from the left side of the drawing to the right direction to read the original.  
         [0033]     At the time of the scanning by the scanner unit  202 , the reading surface of the original is irradiated with light from a lamp of the scanner unit  202 , and the reflected light from the original is guided to a lens through a mirror. The light having passed through the lens is focused on an imaging surface of an image sensor  203 . The image optically read here is converted into image data by the image sensor  203  to be outputted. The image data outputted from the image sensor  203  receives predetermined processing by an image signal control unit  281  shown in  FIG. 3 , which will be described later. After that, the processed image data is inputted into an exposure control unit  120  of the printer  100  as a video signal.  
         [0034]     Incidentally, in  FIG. 2 , reference numerals  102   a  to  102   d  denote pickup rollers, reference numerals  103   a  to  103   d  denote feeding rollers, reference numerals  104  and  107  to  109  denote conveying rollers, a reference numeral  106  denotes a sensor, a reference numeral  116  denotes a charger, and a reference numeral  119  denotes a cleaner.  
         [0035]     Next, the operation of a one-sided mode for performing image formation on one side of a sheet is described with reference to  FIG. 4 .  
         [0036]     The exposure control unit  120  of the printer  100  modulates a laser beam on the basis of an input video signal, and outputs the modulated laser beam. The modulated laser beam is scanned by a polygon mirror (not shown) while being irradiated on a photosensitive drum  115 . Thereby, an electrostatic latent image is formed on the photosensitive drum  115  according to the scanning laser beam.  
         [0037]     The electrostatic latent image formed on the photosensitive drum  115  is visualized as an image of a developer by the developer fed from a developing machine  117 . Moreover, a sheet fed from each of cassettes  101   a  to  101   d  or a manually paper feeding tray  105  by being guided by a conveying path  147  is knocked against resist rollers  113  by the front edge of the sheet, and stops once there. After that, the sheet is conveyed to a position between the photosensitive drum  115  and a transferring unit  118  at the timing synchronizing with the start of the irradiation of the laser beam. Then, the developer image formed on the photosensitive drum  115  is transferred to the sheet fed by the transferring unit  118 . The inclination of the sheet is corrected by the temporary stop of the sheet by being knocked against the resist rollers  113  by the front edge of the sheet.  
         [0038]     The sheet on which the developer image has been transferred is conveyed to a fixing unit  121 , and the fixing unit  121  presses this sheet in the state of contacting with it while heating to fix the developer image on the sheet. The sheet having passed through the fixing unit  121  is guided to a conveying path  142  by a flapper  133 , and is discharged to the outside (finisher  500 ) out of the printer  100  through discharge rollers  111 . At this time, because the sheet is discharged in the state in which the image formation surface thereof faces downwards (facedown), the order of sheets after the discharging is the correct order of pages in the case where image formation is performed in order from the first page.  
         [0039]     Moreover, the operation in the case where a two-sided mode for performing image formation on both sides of a sheet is set is described with reference to  FIGS. 5A  to  5 G.  
         [0040]     Similar to the case of one-sided mode, a sheet fed from each of the cassettes  101   a  to  101   d  or the manual paper feeding tray  105  is knocked against the resist rollers  113  by the front edge of the sheet. After that, the sheet is conveyed to a position between the photosensitive drum  115  and the transferring unit  118 . Then, the sheet passes through the fixing unit  121 , and image formation is performed on one surface of the sheet. The sheet is guided to a conveying path  143  from a conveying path  141  by the switching of the flapper  133 , and stops in the state of being nipped by reverse rollers  112  (see  FIG. 5A ).  
         [0041]     Then, when a flapper  132  is switched and the reverse rollers  112  are driven in reverse, the sheet is guided from the conveying path  143  to a two-sided conveying path  145  (see  FIG. 5B ), and then is guided to a conveying path  146 . Thus, the sheet is again knocked against the resist rollers  113  by the front edge to stop there (see  FIG. 5C ). At this point, the front and the back of the sheet have been reversed.  
         [0042]     After that, the sheet is again fed to a position between the photosensitive drum  115  and the transferring unit  118 , and image formation is performed on the other surface of the sheet through the fixing unit  121  (see  FIG. 5D ). Then, the flapper  133  is switched, and the sheet is guided from the conveying path  141  to the conveying path  142  (see  FIG. 5E ) to be discharged from the printer  100  to the outside (finisher  500 ) through the discharge rollers  111  (see  FIG. 5F ). Image formation is successively performed (see  FIG. 5G ).  
         [0043]     Hereupon, in the case of the two-sided mode, because the sheet is discharged in the state in which the surface of the sheet on which the later image formation is performed faces downwards, the corresponding image formation onto the reverse side of the sheet is previously performed for adjusting the order of page numbers when the sheet is discharged from the printer  100  to the outside, as shown in  FIG. 6 . A reference numeral  503  in  FIG. 6  denotes a discharged paper bundle belt, and a reference numeral  504  denotes a stopper plate.  
         [0044]     Moreover, in the case where image formation is performed on the two sides of a plurality of sheets as shown in  FIGS. 5A  to  5 G, the image formation on the plurality of sheets is performed in parallel in the first embodiment. However, the image formation may be performed in such a way that the image formation of both the front and the back sides of a sheet is performed before the start of the image formation on the next sheet.  
         [0045]     A sheet discharged from the printer  100  is sent to the finisher  500 , and the sent sheet is discharged onto a bundled sheet discharging belt  503  by the discharge rollers  501  of the finisher  500 . In this case, an intermediate processing tray having low friction is provided at a position higher than the bundled sheet discharging belt  503  by several millimeters in parallel with the bundled sheet discharging belt  503 , and more correctly, the sheet is discharged on the intermediate processing tray. Then, the discharged sheet falls towards the lower right direction in  FIG. 2  by its own weight along the intermediate processing tray (bundled sheet discharging belt  503  ), which is obliquely provided. Moreover, a fan-shaped returning roller  502  rotates counterclockwise, and consequently a friction member formed on an arc portion of the returning roller  502  abuts against the sheet. The friction member also works to make the sheet fall into the lower right direction, and makes the edge of the sheet knock against the stopper plate  504 . Thereby, the aligning operation of the sheet in the longitudinal direction (sending direction) is performed.  
         [0046]     Moreover, an aligning plate  506  is formed on each of the front side and the back side of the intermediate processing tray, and the aligning plates  506  are driven at every discharge of a sheet onto the intermediate processing tray to perform the aligning operation of the sheets on the intermediate processing tray in their transverse direction (width direction). Then, when a predetermined number of sheets has been discharged to be loaded on the intermediate processing tray, the bundled sheet discharging belt  503  is driven to discharge the sheets onto a stack tray  507 .  
         [0047]     Next, the configuration of a controller conducting the control of the image forming apparatus according to the first embodiment is described with reference to  FIG. 3 .  FIG. 3  is a block diagram showing the configuration of the controller of the control system conducting the control of the image forming apparatus shown in  FIG. 2 .  
         [0048]     The controller, as shown in  FIG. 3 , includes a CPU circuit unit  150 . The CPU circuit unit  150  includes a CPU (not shown), a ROM  151  and a RAM  152  therein, and collectively controls each of the following blocks in accordance with a control program stored in the ROM  151 , a document feeder control unit  480 , an image reader control unit  280 , the image signal control unit  281 , an external interface (I/F)  282 , a computer  283 , a printer control unit  180 , an operation display control unit  680  and a finisher control unit  580 . The RAM  152  temporarily holds control data, and is used as a work area for arithmetic processing accompanying the control.  
         [0049]     The document feeder control unit  480  performs the driving control of the document feeder  400  on the basis of the instructions from the CPU circuit unit  150 . The image reader control unit  280  performs the driving control of the above-mentioned scanner unit  202 , the image sensor  203  and the like, and transfers an analog image signal output from the image sensor  203  to the image signal control unit  281 .  
         [0050]     The image signal control unit  281  converts the analog image signal from the image sensor  203  to a digital signal before performing various kinds of processing thereof. Then, the image signal control unit  281  converts the processed digital signal to a video signal to output the video signal to the printer control unit  180 . Moreover, the image signal control unit  281  performs various kinds of processing of a digital image signal input from the computer  283  through the external I/F  282 , and converts the digital image signal to a video signal to output the video signal to the printer control unit  180 . The processing operation of the image signal control unit  281  is controlled by the CPU circuit unit  150 . The printer control unit  180  controls the above-mentioned exposure control unit  120 , the photosensitive drum  115 , the developing machine  117  and the transferring unit  118  on the basis of the input video signal to perform image formation, and controls each of the conveying rollers such as the discharge rollers  111 , the reverse rollers  112 , the resist rollers  113  and the like to perform the conveyance control of sheets.  
         [0051]     The operation display control unit  680  performs the exchanges of information between an operation display unit  600  and the CPU circuit unit  150 . The operation display unit  600  includes a plurality of keys for setting various functions pertaining to the image formation, a display unit for displaying the information indicating a set state, and the like. The operation display unit  600  outputs a key signal corresponding to the operation of each of the keys to the CPU circuit unit  150 , and displays the corresponding information on the display unit on the basis of a signal from the CPU circuit unit  150 .  
         [0052]      FIG. 7  is a view showing the operation display unit  600  in the image forming apparatus of  FIG. 2 .  
         [0053]     On the operation display unit  600 , there are arranged a start key  602  for starting an image formation operation, a stop key  603  for suspending an image formation operation, numeric keys  604  to  612  and  614  for setting a number of sheets or the like, an ID key  613 , a clear key  615 , a reset key  616  and the like. Moreover, a liquid crystal display unit  620  forming a touch panel thereon is disposed, and then soft keys can be formed on the screen of the display unit  620 .  
         [0054]     For example, the image forming apparatus of the first embodiment includes each processing mode such as a non-sort mode (group mode), a sort mode, a staple sort mode (binding mode) and the like as the post-processing mode of the finisher  500 . When such a processing mode is set, the setting is performed by an input operation from the operation display unit  600 .  FIGS. 8A and 8B  are views showing display screens of the display unit  620  at the time of the selection of the post-processing mode. When a “sorter” key, one of the soft keys, is selected in an initial screen shown in  FIG. 8A , a menu selection screen shown in  FIG. 8B  is displayed on the display unit  620 , and the setting of the processing mode is performed by using the menu selection screen.  
         [0055]     Next,  FIGS. 9A  to  9 H are used while the image formation operation at the time of selecting the post-processing mode is described. Here, the conveying motor control at the time of a buffer operation using the two-sided conveying path  145  is described.  
         [0056]     As the first embodiment, a description is given to an image forming apparatus which has the sheet conveying speeds of about  100  mm/sec at the photosensitive drum  115  and the fixing unit  121  of the image forming unit and can perform the image formation of 14 sheets in the size of A4 paper per minute. In this case, the discharge interval of sheets is about 428 mm, and an interval between sheets is about 218 mm. In the following, a case where three pages of originals receive stapling processing is described.  
         [0057]     First, in the case where the staple sort mode is selected in the screen of  FIG. 8B , the printer control unit  180  and the finisher control unit  580  synchronize with each other while the whole image forming apparatus performs the stapling processing operation in conformity with the instructions of the CPU circuit unit  150 .  
         [0058]     The printer control unit  180  determines whether the buffer operation, which will be described in the following, is performed or not according to a designated mode. In the first embodiment, the following processing is performed in the case where the designated mode is performing finisher processing, the number of copies is two or more, and the image formation is performed on one face.  
         [0059]     First, as for a fist bundle, the processing similar to the above-mentioned one face operation is performed. That is to say, a developer image formed on the photosensitive drum  115  is transferred to a sheet fed from each of the cassettes  101   a  to  101   d  or the manually paper feeding tray  105  by being guided by the conveying path  147  by the transferring unit  118 . Then, the sheet having passed through the fixing unit  121  is guided to the conveying path  142  by the flapper  133 , and is discharged to the outside (finisher  500 ) out of the printer  100  through the discharge rollers  111  (see  FIG. 9A ). Similarly, image formation is performed also on the sheets of a second page and a third page, and the sheets are discharged into the finisher  500  (see  FIG. 9B ).  
         [0060]     When three pages of sheets to be bundle and stapled have been discharged onto the above-mentioned intermediate processing tray (the bundled sheet discharging belt  503 ), the aligning operation of the sheets is performed by the aligning plates  506 . After that, a stapler  505  is driven to perform a stapling operation, and the stapled bundle of sheets is discharged onto the stack tray  507  by the bundled sheet discharging belt  503 .  
         [0061]     In this case, when a succeeding sheet is discharged into the finisher  500  while the first bundle is adjusted to receive the stapling operation after the third page of the sheets has been discharged into the finisher  500  (see  FIG. 9B ), there is produced a problem such that the stapling operation of the first bundle is not performed normally, or that the succeeding sheet is also stapled at the same time. Consequently, the succeeding sheet is not discharged into the finisher  500  until the stapling processing of the first bundle has been completed to be discharged onto the stack tray  507 .  
         [0062]     Accordingly, as shown in  FIG. 9C , the flapper  133  is switched to convey a fourth page of the sheets into the conveying path  143 . In this case, it is necessary to convey the fourth page of the sheets into the two-sided conveying path  145  after having been switched back in the conveying path  143  by the time when the succeeding fifth page is discharged (in a time between the sheets of the fourth page and the fifth page). Accordingly, the flapper  132  is switched into the direction to convey a sheet into the two-sided conveying path  145  at the timing when the trailing edge of the sheet of the fourth page passes through the flapper  132  after the fourth page having been conveyed into the conveying path  143 , and at the same time the conveying rollers  112  and  114  rotate in reverse to convey the sheet into the two-sided conveying path  145  (see  FIG. 9D ).  
         [0063]     Next, as shown in  FIG. 9E , at a point of time when the fourth page of the sheets has reached a predetermined position in the two-sided conveying path  145 , conveying rollers  110  stop rotating. On the other hand, the succeeding fifth page of the sheets is conveyed to the conveying path  143  similarly to the fourth page of the sheets. Now, it is needed to convey the fifth page of the sheets into the two-sided conveying path  145  to be discharged into the finisher  500  in the state of being put together with the fourth page (being put upon the fourth page) after having been switched back in the conveying path  143  by the time when the succeeding sixth page is discharged (in a time between the sheets of the fifth page and the sixth page).  
         [0064]     Accordingly, similarly to the fourth page of the sheets, the flapper  132  is switched into the direction to convey a sheet into the two-sided conveying path  145  at the timing when the trailing edge of the sheet passes through the flapper  132 , and the conveying rollers  112  and  114  rotate in reverse to convey the sheet into the two-sided conveying path  145  (see  FIG. 9E ).  
         [0065]     Now, the movements of the sheets in the two-sided conveying path  145  are described in detail.  FIGS. 10A  to  10 E are views showing the operation of the two-sided conveying path unit.  
         [0066]      FIG. 10A  shows the operation of a sheet in the two-sided conveying path  145  shown in  FIGS. 9D  to  9 E. After a sheet A has been conveyed into the two-sided conveying path  145 , the flapper  132  is switched for conveying the succeeding sheet B into the conveying path  143 . Conveying rollers  110   a  and  110   b  rotate into a direction to send the sheet A into the inside of the two-sided conveying path  145 . Moreover, a movable stopper  153 , which is housed in the wall of the conveying path at the time of a two-sided operation, moves to a position where the movable stopper  153  supports the front edge of the sheet in the direction of movement according to the size of the sheet A.  
         [0067]      FIG. 10B  shows the operations of sheets in the two-sided conveying path  145  in the state of  FIG. 9F . The sheet A is in a state of being housed in the two-sided conveying path  145 , and is supported by the movable stopper  153 . Consequently, the sheet A is in the state of not falling downward in the two-sided conveying path  145 . The flapper  132  is switched so as to convey the succeeding sheet B into the two-sided conveying path  145 . The conveying rollers  110   a  and  110   b  stop rotating so as not to convey the sheet A, and the conveying roller  110   b  is separated from the conveying roller  110   a  so as to put the sheet B between them.  
         [0068]     Next, at the point of time when the front edge of the sheet B has passed through the position between the conveying rollers  110   a  and  110   b , the conveying roller  110   b  is joined with the conveying roller  110   a . After that, only the conveying roller  110   b  is rotated into the direction to send the sheet B into the inside of the two-sided conveying path  145  while the conveying roller  110   a  remains stopped in order that the sheet A is not be conveyed and only the sheet B is be conveyed in the two-sided conveying path  145  (see  FIG. 10C ). When the front edge of the sheet B is conveyed to a position where the front edge comes in contact with the movable stopper  153 , the conveying roller  110   b  stops rotating (see  FIGS. 9G and 10D ).  
         [0069]     On the other hand, at this point of time, the stapling processing of the first bundle in the finisher  500  is completed, and the first bundle is discharged onto the stack tray  507 . Both the fourth page and the fifth page of the sheets waiting in the two-sided conveying path  145  are simultaneously conveyed from the two-sided conveying path  145  to the conveying path  142  at a point of time when a predetermined time has passed from the start of the stapling of the first bundle, and both the pages are discharged onto the finisher  500 . At this time, both the conveying rollers  110   a  and  110   b  are rotated to convey both pages, put one upon another together, from the two-sided conveying path  145  to the conveying path  142  (see  FIG. 10E ).  
         [0070]     In this case, if the fourth and the fifth pages of the sheets are conveyed at a normal speed, the fourth and the fifth pages of the sheets cannot be discharged into the finisher  500  by the time when the sixth page of the sheets is conveyed into the conveying path  142 . Accordingly, when the fourth and the fifth pages of the sheets are conveyed from the two-sided conveying path  145  to the conveying path  142 , the conveying rollers  110   a  and  110   b  are rotated at a higher speed to discharge the pages. In the first embodiment, the conveying rollers  110   a  and  110   b  are adapted to rotate at a speed being twice as fast as the conveying speed of the fixing unit  121  to convey the pages.  
         [0071]     After the succeeding sixth page of the sheets has passed through the fixing unit  121 , the sixth page is guided to the conveying path  142  by the flapper  133 , and is discharged from the printer  100  to the finisher  500  through the discharge rollers  111  at a uniform speed (see  FIG. 9H ). At the point of time when the sixth page has been discharged, three pages of the sheets to be bundled and stapled have been discharged on the intermediate processing tray. Consequently, after an aligning operation of the sheets has been performed by the aligning plates  506 , the stapler  505  is driven to perform a stapling operation. Then, the stapled sheets are discharged onto the stack tray  507  by the bundled sheet discharging belt  503 .  
         [0072]     On and after that, the movement of every sheet at a seventh, an eighth and a ninth page in a third bundle is similar to that of every sheet at the fourth, the fifth and the sixth page as shown in  FIG. 9B .  
         [0073]     As described above, by discharging the sheets retained in the two-sided conveying path  145  at a higher speed, the sheets can be discharged onto the intermediate processing tray while maintaining the page numbers without colliding with the succeeding sheet.  
         [0000]     Second Embodiment  
         [0074]     In the second embodiment, the description is given to the image forming apparatus which has the sheet conveying speed of the image forming unit of about  110  mm/sec and can perform the image formation of 21 sheets in the size of A4 paper per minute. In this case, the discharge interval of sheets is about 314 mm, and an interval between sheets is about 104 mm. In the following, similarly to the first embodiment, a case where three pages of originals receive stapling processing is described.  
         [0075]     As for a fist bundle, the processing similar to that of the above-mentioned first embodiment is performed. As for the fourth page of the sheets, similarly as shown in  FIG. 9C , the flapper  133  is switched to convey the fourth page into the conveying path  143 . In this case, it is necessary to convey the fourth page of the sheets into the two-sided conveying path  145  after having been switched back in the conveying path  143  by the time when the succeeding fifth page of the sheets is discharged (in a time between the sheets of the fourth page and the fifth page). Accordingly, the conveying rollers  112  and  114  begin to increase their rotation speeds at the timing when the fourth page of the sheets passes through the fixing unit  121  to convey the sheet. In the second embodiment, the conveying rollers  112  and  114  increase their speeds to be twice as fast as the conveying speed of the fixing unit  121 .  
         [0076]     After that, the flapper  132  is switched into the direction of conveying a sheet into the two-sided conveying path  145  at the timing when the front edge of the sheet reaches the flapper  132 , and the conveying rollers  112  and  114  rotate in reverse to convey the sheet into the two-sided conveying path  145  (see  FIG. 9D ). At this time, the conveying rollers  110  rotate at a double-speed similar to the conveying rollers  112  and  114 . On the other hand, as described above with reference to  FIGS. 5A  to  5 G, in the case where the two-sided conveying path  145  is used for two-sided image formation, the conveying rollers  110  perform conveyance at a speed equal to the conveying speed of the fixing unit  121 .  
         [0077]     Next, as shown in  FIG. 9E , at a point of time when the fourth page of the sheets has reached a predetermined position in the two-sided conveying path  145 , the conveying rollers  110  stop rotating. On the other hand, the succeeding fifth page of the sheets is conveyed to the conveying path  143  similar to the fourth page of the sheets. Now, it is needed to convey the fifth page of the sheets into the two-sided conveying path  145  to be discharged into the finisher  500  in the state of being put on together with the fourth page after having been switched back in the conveying path  143  by the time when the succeeding sixth page is conveyed into the conveying path  142  (in a time between the sheets of the fifth page and the sixth page). Accordingly, the conveying rollers  112  and  114  begin to increase their speeds at the timing when the fifth page of the sheets has passed through the fixing unit  121  to convey the sheet. In the second embodiment, the conveying rollers  112  and  114  increase their speeds up to four times as fast as the conveying speed of the fixing unit  121 .  
         [0078]     After that, as for the fifth page of the sheets, similar to the fourth page of the sheets, the flapper  132  is switched into the direction to convey the sheet into the two-sided conveying path  145  at the timing when the front edge of the sheet reaches the flapper  132 , and the conveying rollers  112  and  114  rotate in reverse to convey the sheet into the two-sided conveying path  145  (see  FIG. 9F ). At this time, the conveying rollers  110  are rotating at a quad-speed similar to the conveying rollers  112  and  114 . On the other hand, at this point of time, the stapling processing of the first bundle in the finisher  500  has been completed, and the first bundle is discharged onto the stack tray  507 .  
         [0079]     After that, both the fourth page and the fifth page of the sheets waiting in the two-sided conveying path  145  (see  FIG. 9G ) are conveyed from the two-sided conveying path  145  to the conveying path  142  in the state in which one of them is put upon the other at a point of time when a predetermined time has passed from the start of the stapling of the first bundle, and both the pages are discharged into the finisher  500 . In this case, the conveying rollers  110  and  111  are rotating at a quad-speed.  
         [0080]     After the succeeding sixth page of the sheets has passed through the fixing unit  121 , the sixth page is guided into the conveying path  142  by the flapper  133 , and is discharged from the printer  100  to the finisher  500  through the discharge rollers  111  at a uniform speed (see  FIG. 9H ). At the point of time when the sixth page has been discharged, three pages of the sheets for a bundle to be stapled have been discharged on the intermediate processing tray. Consequently, after an aligning operation of the sheets is performed by the aligning plates  506 , the stapler  505  is driven to perform a stapling operation. Then, the stapled sheets are discharged onto the stack tray  507  by the bundled sheet discharging belt  503 .  
         [0081]     On and after that, the movement of every sheet at a seventh page, at an eighth page and at a ninth page in a third bundle is similar to that of every sheet at the fourth page, at the fifth page and at the sixth page.  
         [0000]     Third Embodiment  
         [0082]     As a third embodiment of the present invention, an image forming apparatus having a narrow sheet interval is described. In this case, the discharge of the fourth page and the fifth page of the sheets, which have been retracted into the two-sided conveying path  145 , to the finisher  500  can be too late for the discharge of the sixth page of the sheets in the above-mentioned operation. A method for performing stapling processing without widening the time between the fifth sheet and the sixth sheet by changing the order of image formation in such a case is described with reference to  FIGS. 11A  to  11 H.  
         [0083]     In the case where the staple sort mode is selected in the screen of  FIG. 8B , the printer control unit  180  and the finisher control unit  580  synchronize with each other while the whole image forming apparatus performs the stapling processing operation in conformity with the instructions of the CPU circuit unit  150 .  
         [0084]     The printer control unit  180  determines whether the buffer operation (retaining processing), which will be described in the following, is performed or not according to a designated mode. In the third embodiment, the following processing is performed in the case where the designated mode is performing the finisher processing, the number of printed copies is two or more, and the image formation is performed on one face of a sheet.  
         [0085]     First, as for a fist bundle, the processing similar to that of the above-mentioned first embodiment and the second embodiment is performed. As for a second bundle, in the case where the printer control unit  180  determines that a buffer operation is needed, fist, the image formation of a fifth page is performed and then the image formation of a sixth page is performed as to the image data which have been sent from the image reader  200  or the computer  283  to be stored in the memory, and the sheets having received the image formation are conveyed (see  FIG. 11B ). Incidentally, the numerals attached to the sheets in  FIG. 11A  to  11 H denote the order of image pages and are different from the order of the paper conveyance.  
         [0086]     When a fifth page of the sheets is conveyed, as shown in  FIG. 11C , the flapper  133  is switched to convey the fifth page of the sheets into the conveying path  143 . In this case, it is necessary to convey the fifth page of the sheets into the two-sided conveying path  145  after having been switched back in the conveying path  143  by the time when the succeeding sixth page is conveyed to the conveying path  141  (to the position of the flapper  132 ) (in a time between the sheets of the fifth page and the sixth page). Accordingly, the conveying rollers  112  and  114  begin to increase their speeds to convey the sheet at the timing when the fourth page has passed through the fixing unit  121 . In the third embodiment, the conveying rollers  112  and  114  increase their speeds to be twice as fast as the conveying speed of the fixing unit  121 .  
         [0087]     After that, at the timing when the front edge of the sheet reaches the flapper  132 , the flapper  132  is switched into the direction to convey the sheet to the two-sided conveying path  145 , and the conveying rollers  112  and  114  rotate in reverse to convey the sheet into the two-sided conveying path  145  (see  FIG. 11D ). At this time, the conveying rollers  110  are rotating at a double-speed similar to the conveying rollers  112  and  114 .  
         [0088]     Next, as shown in  FIG. 11E , at the point of time when the fifth page of the sheets has reached the predetermined position in the two-sided conveying path  145 , the conveying rollers  110  stop rotating. On the other hand, the succeeding sixth page of the sheets is conveyed into the conveying path  143  similar to the fifth page of the sheets. Now, it is necessary to convey the sixth page of the sheets into the two-sided conveying path  145  after having been switched back in the conveying path  143  by the time when the succeeding fourth page of the sheets is conveyed into the conveying path  141  (in a time between the sheets of the sixth page and the fourth page). At this time, the conveying rollers  110  are rotating at a double-speed similarly as described above.  
         [0089]     After that, as for the sixth page of the sheets, similar to the fifth page of the sheets, at the timing when the front edge of the sheet reaches the flapper  132 , the flapper  132  is switched into the direction to convey the sheet into the two-sided conveying path  145 , and the conveying rollers  112  and  114  convey the sheet into the two-sided conveying path  145  by returning the motor into the positive rotation thereof (see  FIG. 11F ). At this time, the conveying rollers  110  are rotating at a double-speed similar to the conveying rollers  112  and  114 . On the other hand, at this point of time, the stapling processing of the first bundle in the finisher  500  is completed, and the first bundle is discharged onto the stack tray  507 .  
         [0090]     The next fourth page of the sheets begins to increase its speed at the timing of having passed through the fixing unit  121 , and is guided into the conveying path  142  by the flapper  133  to be discharged from the printer  100  into the finisher  500  through the discharge rollers  111  (see  FIG. 11G ).  
         [0091]     Then, successive to the fourth page of the sheets, both the fifth page of the sheets and the sixth page of the sheets waiting in the two-sided conveying path  145  are conveyed from the two-sided conveying path  145  to the conveying path  142  in the state of being one above the other (see  FIG. 11H ).  
         [0092]     At the point of time when the fifth page and the sixth page of the sheets have been discharged onto the intermediate processing tray, three pages of sheets to be bundled and stapled have been discharged onto the intermediate processing tray. Consequently, after the aligning operation of the sheets has been performed by the aligning plates  506 , the stapler  505  is driven to perform a stapling operation, and the stapled sheets are discharged onto the stack tray  507  by the discharged paper bundle belt  503 .  
         [0093]     On and after this, as for the movements of a seventh page, an eighth page and a ninth page of the sheets in a third bundle, the movements are similar to those of the fourth page, the fifth page and the sixth page of the sheets shown in the drawings from  FIG. 11B .  
         [0000]     Fourth Embodiment  
         [0094]     Next, as a fourth embodiment of the present invention, an image forming apparatus shown in  FIG. 12  is described. The image forming unit thereof is similar to those of the first and the second embodiments, but the configuration of the sheet discharging unit thereof is different from those of the first and the second embodiments.  
         [0095]     First, the differences of the operation of the present embodiment from the operations of the first and the second embodiments are described. At the time of the one face operation, a sheet on which an image is formed by the photosensitive drum  115 , the developing machine  117 , the transferring unit  118  and the fixing unit  121  is conveyed into the conveying paths  141  and  142 . Then, in the case where no post-processing is designated, the sheet is discharged onto a sheet discharging tray  508  in the state of being facedown through the conveying path  143  and the discharge rollers  111 . Moreover, in the case where post-processing is designated, the sheet is conveyed into the conveying path  143 . After the trailing edge of the sheet has passed the flapper  133 , the flapper  133  is switched, and the sheet is conveyed to a conveying path  144  into the reverse direction. That is to say, the conveying path  143  is used as a reverse conveying path. The sheet sent to the finisher  500  through the conveying path  144  is discharged onto the bundled sheet discharging belt  503  by the discharge rollers  501 . On and after this, similar to the first and the second embodiments, the post-processing is performed and the processed sheets are discharged on the stack tray  507 .  
         [0096]     Moreover, at the time of a two-sided operation, a sheet having received image formation is conveyed into the conveying path  143 . After the trailing edge of the sheet has passed the flapper  132 , the flapper  132  is switched to convey the sheet into the two-sided conveying path  145  in the reverse direction.  
         [0097]     After that, the sheet again passes through the image forming unit. In the case where no post-processing is designated, the sheet is discharged into the sheet discharging tray  508 . In the case where post-processing is performed, the sheet is discharged onto the stack tray  507  after the processing.  
         [0098]     Next, the buffer control of sheets in the case where three pages of originals are copied and the copied sheets are processed to be stapled is described with reference to  FIGS. 13A  to  13 F.  
         [0099]     First, as for the first bundle, the processing similar to that of the above-mentioned one face operation is performed. That is to say, a developer image formed on the photosensitive drum  115  is transferred by the transferring unit  118  onto a sheet fed from each of the cassettes  101   a  to  101   d  or the manually paper feeding tray  105  by being guided by the conveying path  147 . Then, the sheet having passed through the fixing unit  121  is guided into the conveying path  143  by the flapper  133 . After that, the flapper  133  is switched, and the sheet is discharged from the printer  100  to the outside (finisher  500 ) through the conveying path  144  and the discharge rollers  501  (see  FIG. 13A ). Image formation is similarly performed to the second page and the third page of the sheets, and the pages are discharged into the finisher  500  (see  FIGS. 13B and 13C ).  
         [0100]     When three pages of sheets to be bundled and stapled have been discharged onto the intermediate processing tray, the aligning operation of the sheets is performed by the aligning plates  506 . After that, the stapler  505  is driven to perform a stapling operation, and the stapled bundle of sheets is discharged onto the stack tray  507  by the bundled sheet discharging belt  503 .  
         [0101]     As shown in  FIG. 13D , while the stapling processing of the first bundle is being performed, the sheets in the conveying path  143  are not conveyed into the finisher  500  to be retained, and the next fifth page is conveyed into the conveying path  143 .  
         [0102]     At this point of time, the stapling processing of the first bundle in the finisher  500  has been completed, and the first bundle is discharged onto the stack tray  507 . Both the fourth page and the fifth page of the sheets waiting in the conveying path  143  are conveyed in the sate of being one above the other from the conveying path  143  to the conveying path  144  at a point of time when a predetermined time has passed from the start of the stapling of the first bundle, and both the pages are discharged into the finisher  500 . At this time, conveying rollers  124  are rotated into the direction reverse to the direction at the time of carrying in, and thereby both the pages are simultaneously conveyed from the conveying path  143  to the conveying path  144  (see  FIG. 13E ).  
         [0103]     If the fourth and the fifth pages of the sheets are conveyed at a normal speed in the case where the processing time in the finisher  500  is long, the fourth and the fifth pages of the sheets cannot be discharged into the finisher  500  by the time when the sixth page of the sheets is conveyed. Accordingly, when the fourth and the fifth pages of the sheets are conveyed from the conveying path  143  to the conveying path  144 , the conveying rollers  124  are rotated at a higher speed to discharge the pages of the sheets. In the fourth embodiment, the conveyance speed of the pages is adapted to a speed being twice as fast as the conveying speed of the fixing unit.  
         [0104]     After the succeeding sixth page of the sheets has passed through the fixing unit  121 , the sixth page is guided to the conveying path  143  by the flapper  133 , and is switched back to be discharged from the printer  100  into the finisher  500  through the discharge rollers  501  at a uniform speed (see  FIG. 13F ). Then, at the point of time when the sixth page of the sheets has been discharged onto the intermediate processing tray, three pages of the sheets to be bundled and stapled have been discharged on the intermediate processing tray. Consequently, after an aligning operation of the sheets is performed by the aligning plates  506 , the stapler  505  is driven to perform a stapling operation. Then, the stapled sheets are discharged onto the stack tray  507  by the bundled sheet discharging belt  503 .  
         [0105]     On and after that, the movement of every sheet at a seventh page, at an eighth page and at a ninth page in a third bundle is similar to that of every sheet at the fourth page, at the fifth page and at the sixth page shown in the drawings of  FIG. 12C .  
         [0106]     In the first embodiment, the carry-in speed into the two-sided conveying path used also as a sheet retaining unit is adapted to be a uniform speed, and the carry-out speed is adapted to be a double-speed. Moreover, in the second embodiment, similarly the carry-in speed is adapted to be a double-speed, and the carry-out speed is adapted to be a quad-speed. However, the carry in speed and the carry-out speed are not limited to the configurations of those embodiments, and can be obtained as arbitrary speeds by which processing can be performed between pages according to the configuration of an image forming apparatus.  
         [0107]     Moreover, in the first and the second embodiments, when a sheet is retained in the two-sided conveying path  145 , a reverse operation is performed by using the conveying path  143 . However, it is possible to implement the control similar to that in these embodiments by adopting a configuration for performing the reverse operation by using the conveying path  142  for discharge without any dedicated path for reversing such as the conveying path  143 .  
         [0108]     Moreover, in an image forming apparatus adopting a configuration in which a conveying path for conveying a sheet directly from the conveying path  143  into the two-sided conveying path  145  is added to the configurations of the first and the second embodiments, the sheet may be retained in the two-sided conveying path  145  through the added conveying path, and may be discharged by the way similar to those of the first and the second embodiments at the time of discharging the sheet from the two-sided conveying path  145 .  
         [0109]     In the image forming apparatus having such a configuration, for example, it is possible to shorten the conveying interval between the fourth page of the sheets and the fifth page of the sheets, and it is also possible to reduce or eliminate the increasing amount of the conveying speed at the time of carrying in a sheet into the two-sided conveying path  145  to reduce the load of the motor.  
         [0110]     Moreover, in the above-mentioned embodiments, descriptions have been given on the supposition that the number of pages of a bundle is three. If the number of the pages of one bundle increases, only repetitions of an operation similar to that of the sixth page of the sheets in the above-mentioned embodiments are performed for the increased sheets.  
         [0111]     Moreover, the number of sheets to be retained in the two-sided conveying path  145  or the reverse conveying path  143  for awaiting stapling processing has been supposed to be two, but it is also possible to perform a similar operation by increasing the number of sheets to be retracted in the case where the stapling processing takes a long time. Conversely, in the case where the stapling processing takes a short time, similar effects can be obtained even if the number of sheet to be retracted is set to be one.  
         [0112]     Moreover, the processing executed in the finisher  500  is not limited to the stapling processing shown in the embodiments, but the control similar to that in the embodiments can be performed in all of the processing which can be conceived as to the discharged sheets such as adjustment, punching, folding, and binding.  
         [0113]     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 embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 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.  
         [0114]     This application claims priority from Japanese Patent Application No. 2004-082410 filed Mar. 22, 2004 and No. 2005-065360 filed Mar. 9, 2005, which are hereby incorporated by reference herein.