Patent Publication Number: US-8523165-B2

Title: Recording media sheet processing system, image forming system including same, and insertion method used therein

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent specification is based on and claims priority from Japanese Patent Application No. 2010-109453, filed on May 11, 2010 in the Japan Patent Office, which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a sheet processing system including an insertion device that inserts recording media sheets in envelopes, an image forming system including same, and a method of inserting sheets in envelopes. 
     2. Description of the Background Art 
     There are post-processing apparatuses that, in addition to aligning, sorting, folding, stapling, and/or punching sheets of recording media, are also capable of automatically enveloping the sheets (hereinafter “enclosure”) in envelopes. Such post-processing apparatuses typically determine whether the envelope can accommodate the enclosure based on the sizes of the envelope and the enclosure, which are either input manually or measured automatically by the apparatus. 
     Accordingly, various approaches have been proposed to handle a mismatch between the size of the envelope and that of the enclosure, for example, by having the apparatus indicate that the envelope cannot accommodate the enclosure. 
     Alternatively, JP-2004-045650-A proposes an image forming apparatus provided with a post-processing unit that makes the processing efficient and reduces the work of the user as follows. The image forming apparatus includes an image forming unit to form images on sheets of recording media according to image data transmitted from an image reading unit; the post-processing unit; a sheet size input unit via which the user inputs the size of the sheet to be inserted in the envelope; an envelope size input unit via which the user inputs the size of the envelope; and a determination unit to determine whether the envelope accommodates the enclosure based on the sizes of the enclosure and the envelope. When the enclosure is larger than the envelope, the post-processing unit folds the enclosure, so that the folded enclosure can be inserted in the envelope. 
     The above-described approach, however, has several drawbacks. For example, this approach does not take account of a case in which multiple folded sheets are further flattened to make the whole bunch thinner, and therefore it is possible that the apparatus mistakenly assumes that the folded sheets cannot be accommodated in the envelopes. Additionally, although the apparatus can determine whether the folded sheets are too thick to fit into the envelope by measuring the thickness of the folded sheets, the folded sheets are wasted if the apparatus makes the determination that the folded sheets do not fit the envelope only after the sheets are folded. 
     SUMMARY OF THE INVENTION 
     One illustrative embodiment of the present invention provides a recording media sheet processing system that includes a folding device, an insertion device to insert in an envelope an enclosure including a folded sheet, and a controller operatively connected to the folding device and the insertion device. The folding device includes a folding unit to fold a sheet of recording media and a squeezing unit to squeeze a folded portion of the folded sheet. The controller includes an envelope selector for selecting an envelope type from a group of selectable predetermined envelope types, a selector for selecting whether to fold the sheet and a folding style of the sheet from a group of selectable predetermined folding styles, a first storage unit, a second storage unit, a calculator to calculate a total converted quantity of the enclosure, a determination unit, and a squeezing setter. 
     The first storage unit stores a first folding-related equivalent quantity into which a quantity of each sheet not to be squeezed by the squeezing unit of the folding device is converted corresponding to the selected folding style, and the second storage unit stores a maximum quantity of sheets insertable in each envelope type. The total converted quantity of the enclosure is calculated using the first folding-related equivalent quantity stored in the first storage unit and the folding style selected by the selector. The determination unit compares the calculated total converted quantity of the enclosure with the maximum quantity of sheets insertable in the selected envelope type, and then determines whether the selected envelope type accommodates the enclosure before the recording media sheet processing system processes the sheet. The squeezing setter sets the number of times the squeezing unit squeezes the sheet and increases that number of times when the determination unit determines that insertion is not feasible. When the determination unit determines that insertion is feasible, the sheet processing is started and the insertion device inserts the enclosure in the envelope. 
     Another illustrative embodiment provides a method of inserting in an envelope an enclosure including a folded sheet. The method includes a step of selecting an envelope type from a group of selectable predetermined envelope types, a step of selecting whether to fold the sheet inserted in the envelope and a folding style of the sheet from a group of selectable predetermined folding styles, a step of obtaining, from a pre-stored table, a first folding-related equivalent quantity for each sheet of the enclosure, into which a quantity of each sheet is converted corresponding to the selected folding style, a step of obtaining, from a pre-stored table, a maximum quantity of sheets insertable in the selected envelope type, a step of calculating a total converted quantity of the enclosure using the first folding-related equivalent quantity and the selected folding style, a step of comparing the calculated total converted quantity of the enclosure with the maximum quantity of sheets insertable in the selected envelope type, determining whether the selected envelope type accommodates the enclosure before the sheet is processed, a step of increasing the number of times the folded sheet is squeezed when the determination unit determines that insertion is not feasible, and a step of starting processing the sheet and inserting the enclosure in the envelope when the determination unit determines that insertion is feasible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a diagram illustrating a configuration of an image forming system according to an illustrative embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a schematic configuration of an online control system of the image forming system shown in  FIG. 1 ; 
         FIG. 3  is a diagram that illustrates a configuration of a post-processing apparatus; 
         FIG. 4  is a front view of a mechanism for performing center folding included in the post-processing apparatus shown in  FIG. 3 ; 
         FIG. 5  illustrates a state in which a bundle of sheets is aligned on an edge-stapling tray; 
         FIG. 6  illustrates a state subsequent to that shown in  FIG. 5 , in which the bundle of sheets is pushed up by a release pawl from the edge-stapling tray; 
         FIG. 7  illustrates a state subsequent to that shown in  FIG. 6 , in which the bundle is being forwarded to a center-folding tray; 
         FIG. 8  illustrates the bundle on the center-folding tray; 
         FIG. 9  illustrates a state subsequent to that shown in  FIG. 8 , in which the bundle is being stapled on the center-folding tray; 
         FIG. 10  illustrates a state subsequent to that shown in  FIG. 9 , in which the bundle is positioned with the center portion of the bundle facing a folding plate; 
         FIG. 11  illustrates a state subsequent to that shown in  FIG. 10 , in which the bundle is additionally squeezed by a squeezing unit; 
         FIG. 12  illustrates the bundle discharged after folded in two and squeezed; 
         FIG. 13  illustrates an interior of an insertion device according to an embodiment; 
         FIG. 14  is a perspective view that illustrates a feed cassette of an image forming apparatus and a size detecting system to detect the size of the envelope or enclosure stored in the feed cassette; 
         FIG. 15  is a perspective view that illustrates a variation of the feed cassette and the size detecting system; 
         FIG. 16  is a cross-sectional view of the feed cassette and the size detecting system shown in  FIG. 15 ; 
         FIG. 17  is a cross-sectional view that illustrates a main portion of an envelope chuck unit in the insertion device; 
         FIG. 18  is a cross-sectional view that illustrates the main portion of the envelope chuck unit, in which an opening of the envelope is positioned beneath a lower end of an unsealing sheet; 
         FIG. 19  is a cross-sectional view that illustrates the main portion of the envelope chuck unit, in which the lower end of the unsealing sheet is in the envelope; 
         FIG. 20  is a perspective view that illustrates a state in which reverse rotation of chuck rollers is stopped, thereby stopping the envelope; 
         FIG. 21  is a front view of a pack unit of the insertion device; 
         FIG. 22  illustrates an envelope; 
         FIG. 23  illustrates an original document; 
         FIG. 24  illustrates an operation panel; 
         FIG. 25  illustrates indications on a display of the operation panel shown in  FIG. 24 ; 
         FIG. 26  is a flowchart illustrating a sequence of insertion processes executed after the user sets the type of folding on the display of the operation panel; 
         FIG. 27  illustrates an indication reporting an error that appears on the display of the operation panel; and 
         FIG. 28  is a flowchart of a procedure to insert enclosures in envelopes when the squeezing unit squeezes folded sheets twice. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to  FIG. 1 , an image forming system according to an illustrative embodiment of the present invention is described. 
       FIG. 1  is a front view illustrating a configuration of an image forming system according to an embodiment of the present invention. 
     In  FIG. 1 , the image forming system according to the present embodiment includes an image forming apparatus  1 , a folding and stapling device  3 , and an insertion device or enclosing device  4 . The image forming apparatus  1  includes a multifunction peripheral (MFP) as a main body. An automatic document feeder (ADF)  2  and an operation panel  1 -A including a display  1 -D (shown in  FIG. 24 ) are provided above the MFP, and multiple feed cassettes  1 -B are provided beneath the MFP. The folding and stapling device  3  is a so-called post-processing apparatus, and hereinafter referred to as the post-processing apparatus  3 . 
     One of the multiple feed cassettes  1 -B can store envelopes, and another feed cassette  1 -B can store sheets of recording media to be inserted in the envelopes (hereinafter “enclosures”). To insert the enclosures in the envelopes in this system, the enclosures and the envelopes are transported to the post-processing apparatus  3  and the insertion device  4 , respectively. The post-processing apparatus  3  folds the enclosures as required, and then the insertion device  4  inserts the enclosures in the respective envelopes, after which the envelopes are discharged onto a stack tray  4 -A. 
       FIG. 2  is a block diagram illustrating a schematic configuration of an online control system of the image forming system shown in  FIG. 1 . 
     In the online image forming system shown in  FIG. 2 , the post-processing apparatus  3  is connected to the image forming apparatus  1 , and the insertion device  4  is connected to post-processing apparatus  3 . The image forming apparatus  1 , the post-processing apparatus  3 , and the insertion device  4  include central processing units (CPUs)  1 U,  3 U, and  4 U, respectively. Additionally, the image forming apparatus  1  includes a communication port  1 P. The post-processing apparatus  3  includes communication ports  3 P 1  and  3 P 2 . The insertion device  4  includes a communication port  4 P. Thus, the image forming apparatus  1  and the post-processing apparatus  3  communicate with each other via the communication ports  1 P and  3 P 1 , and the post-processing apparatus  3  and the insertion device  4  communicate with each other via the communication ports  3 P 2  and  4 P. The operation panel  1 -A is connected to the MFP of the image forming apparatus  1  via an interface (I/F) not shown and displays various indications such as those shown in  FIGS. 24 ,  25 , and  27 , instructed by the CPU  1 U. The CPUs  1 U,  3 U, and  4 U; and the operation panel  1 -A can together form a controller of the image forming system. Users can input instructions or data to the image forming apparatus  1  by pressing keys on the operation panel  1 -A or touching the display  1 -D. 
     Each of the image forming apparatus  1 , the post-processing apparatus  3 , and the insertion device  4  further includes a read-only memory (ROM) and a random-access memory (RAM). Each of the CPUs  1 U,  3 U, and  4 U reads out program codes from the ROM, runs the program codes in the RAM, and then performs operations defined by the program codes using the RAM as a work area and a data buffer. Thus, the CPUs  1 U,  3 U, and  4 U control the indications on the display  1 -D (shown in  FIG. 24 ) of the operation panel  1 -A and operations of the image forming system. 
     These apparatuses and the device are connected in series electrically via the communication ports  1 P,  3 P 1 ,  3 P 2 , and  4 P as well as mechanically via at least a sheet conveyance path. Thus, when the image forming system operates online, all the image forming apparatus  1 , the post-processing apparatus  3 , and the insertion device  4  can be controlled electrically simultaneously. The processes in the flowcharts shown in  FIGS. 26 and 28 , described later, are instructed by the CPU  1 U and executed by the respective apparatuses and the device. 
       FIG. 3  is a diagram that illustrates a configuration of the post-processing apparatus  3 . 
     The post-processing apparatus  3  according to the present embodiment is connected to a side of the image forming apparatus  1  as shown in  FIG. 1 , and sheets discharged from the image forming apparatus  1  are conveyed to the post-processing apparatus  3 . The post-processing apparatus  3  includes a conveyance path A along which a punch unit  100  for punching the sheets one by one is provided, a conveyance path B leading to an upper tray  201 , along which a pair of conveyance rollers  103  is provided, a conveyance path C leading to a shift tray  202 , along which a pair of pressure rollers  5  and a pair of discharge rollers  6  are provided, and a conveyance path D leading to a processing tray F on which the sheets are aligned and stapled. The sheet transported from the image forming apparatus  1  is transported along the conveyance path A and then is sent to the conveyance path B, C, or D by the separation pawls  15  and  16 . The processing tray F is hereinafter also referred to as the edge-stapling tray F. A bundle of sheets aligned or aligned and stapled on the edge-stapling tray F along its side is led by a bifurcation guide  54  and a movable guide  55  to the conveyance path C leading to the shift tray  202  or a processing tray G for folding and stapling the bundle of sheets along its centerline. The processing tray G is hereinafter also referred to as the center-folding tray G. The sheets folded on the center-folding tray G are transported through a conveyance path H and further transported by a pair of discharge rollers  118  to the insertion device  4  provided downstream from the post-processing apparatus  3  in a direction in which the sheets are conveyed through the image forming system (hereinafter “sheet conveyance direction”). A pair of conveyance rollers  257  is provided along the conveyance path H. 
     Additionally, a pair of conveyance rollers  7  and a separation pawl  17  are provided along the conveyance path D. The separation pawl  17  is retained at a position shown in  FIG. 3  by a low-load spring. After a trailing edge of the sheet passes by the separation pawl  17 , at least one of pairs of conveyance rollers  9  and  10 , and a discharge roller  11  is rotated in reverse, thereby leading the trailing edge of the sheet along a guide roller  8  to a stack portion E. By repeating this operation, a subsequent sheet can be stacked on the sheet in the stack portion E. Then, the multiple sheets can be transported from the stack portion E at a time. 
     The image forming apparatus  1  further includes an entry detector  301  for detecting the sheet received by the post-processing apparatus  3 . The entry detector  301  is provided along the conveyance path A, which is a common path for sheets led to the conveyance paths B, C, or D. A pair of entrance rollers  110 , the punch unit  100 , and a punch chad container  101 , a pair of conveyance rollers  102 , and the separation pawls  15  and  16  are provided downstream from the entry detector  301 , in that order, in the sheet conveyance direction. The separation pawls  15  and  16  are retained at the positions shown in  FIG. 3  by springs, respectively. When a solenoid is turned on, the separation pawls  15  and  16  are rotated up and down in  FIG. 3 , respectively. Rotation of the separation pawls  15  and  16  switches the route of the sheet among the conveyance paths B, C, and D. To guide the sheet to the conveyance path B, the separation pawl  15  is at the position shown in  FIG. 3  and the solenoid is off. To guide the sheet to the conveyance path C, the solenoid is turned on, thereby rotating the separation pawls  15  and  16  upward and downward, respectively, from the position shown in  FIG. 3 . To guide the sheet to the conveyance path D, the separation pawl  16  is at the position shown in  FIG. 3  and thus the solenoid is off. The separation pawl  15  is rotated upward from the position shown in  FIG. 3  by turning on the solenoid. 
     The sheet transported through the conveyance path C and that transported through the conveyance path D are sent to a conveyance path I, which is bifurcated into conveyance paths J and K by a separation pawl  116 . Both the sheet that is not stapled and a bundle of stapled sheets can be transported through the conveyance path K. The sheet transported through the conveyance path H and that transported through the conveyance path K are sent to a conveyance path L via a separation pawl  117 . Then, the sheet is transported by the pair of discharge rollers  118  to the downstream apparatus that in the present embodiment is the insertion device  4 . 
     The discharge rollers  6 , a return roller  13 , a sheet detector  330 , the shift tray  202 , an elevation unit for the shift tray  202 , and a shift mechanism for shifting the shift tray  202  together form a sheet stacker of the post-processing apparatus  3 . The sheet stacker has a known configuration, and thus the description thereof omitted. 
     The processing tray F is for edge stapling. The sheets guided by the discharge roller  11  are staked one on another n the edge-stapling tray F. An alignment roller  12  aligns the sheets sent to the stapling tray F one by one in a longitudinal direction of the sheet in parallel to the sheet conveyance direction, and a pair of jogger fences  53  pushes the sheets from both sides to align the sheets in a transverse direction or sheet width direction, perpendicular to the sheet conveyance direction. The CPU  3 U transmits a stapling signal to a side stapler S 1 , thereby causing it to staple the bundle of sheets, in intervals between printing jobs, that is, after the last sheet in a job is stacked on the processing tray F and before the initial sheet of a subsequent job is transported thereto. A release belt  52  provided with a pair of release pawls  52   a  and  52   a ′ forwards the bundle of sheets to the discharge rollers  6  (a driving roller  6   a  and a driven roller  6   b ) immediately after stapling. At this time, the shift tray  202  is at an upper position to receive the sheets (receiving position). 
     A home position (HP) detector  311  detects the positions of the release pawls  52   a  and  52   a ′, that is, whether they are at home positions. The HP detector  311  is turned on and off by the release pawl  52   a  provided at the release belt  52 . The two release pawls  52   a  and  52   a  are provided on an outer circumferential surface of the release belt  52  at positions facing each other. The release pawls  52   a  and  52   a ′ transport the bundle stacked on the processing tray F alternately. Additionally, the release belt  52  may be rotated in reverse as required so that the leading side of the sheets can be aligned on the back of the release pawl  52   a ′ facing the release pawl  52   a  on standby, waiting for the bundle. 
     The release belt  52  is driven by a motor, not shown. The release belt  52  and a driving pulley for it are provided at a driving shaft of the release belt  52 , at a center of alignment in the sheet width direction, and multiple release rollers  56  are positioned at predetermined constant intervals symmetrically. The peripheral velocity of the release rollers  56  is higher than that of the release belt  52 . The alignment roller  12  is caused to swing on a support point by a solenoid. Accordingly, the alignment roller  12  intermittently pushes the sheet on the edge-stapling tray F, thereby causing the sheet to constant a back fence  51 . The alignment roller  12  rotates counterclockwise. A jogger motor capable of rotating in both normal and reverse directions drives the pair of jogger fences  53  via a timing belt, and thus the jogger fences  53  move reciprocally in the sheet width direction perpendicular to the sheet conveyance direction. 
     A stapler motor capable of rotating in both normal and reverse directions drives the side stapler S 1  via a timing belt, and thus the side stapler S 1  moves in the sheet width direction to staple a predetermined position in an edge portion of the sheets. A stapler HP detector is provided in an end portion of the movable range of the side stapler S 1  to detect whether the side stapler S 1  is at its home position. The position in the sheet width direction stapled by the side stapler S 1  is determined by the amount by which the side stapler S 1  moves from the home position. 
     The bifurcation guide  54  and the movable guide  55  guide the bundle of sheets to the center-folding tray G. The bifurcation guide  54  is rotatable vertically in  FIG. 3  around a support point  54   a , and a rotary pressure roller  57  is provided on the downstream side of the bifurcation guide  54 . The bifurcation guide  54  is pressed to the release roller  56  by a spring. The bifurcation guide  54  is driven by a cam, and its rotational position is controlled by the cam. 
     The movable guide  55  is rotatably supported by a rotation shaft of the release roller  56 , and a link arm is rotatably connected to the movable guide  55  so that the movable guide  55  can rotate a predetermined angle range via the link arm. The movable guide  55  is driven similarly by the cam that drives the bifurcation guide  54 , and its rotational position is controlled by the cam. Thus, the bifurcation guide  54  and the movable guide  55  are driven in conjunction with each other by the identical cam. 
     As shown in  FIG. 3 , the center-folding tray G is positioned substantially vertically, downstream from the sheet guide unit constructed of the movable guide  55  and the release roller  56 . A center-folding mechanism is provided in a center portion of the center-folding tray G, and an upper bundle guide  92  and a lower bundle guide  91  are provided above and beneath the center-folding mechanism, respectively. A pair of upper bundle conveyance rollers  71  and a pair of lower bundle conveyance rollers  72  are provided at an upper position and a lower position, respectively, of the upper bundle guide  92 . A pair of jogger fences  250  extending along a side of the lower bundle guide  91  is provided on either side thereof. Additionally, a center stapler unit UNI is provided at the same position as the jogger fences  250 . The jogger fences  250  align the sheets in the sheet width direction perpendicular to the sheet conveyance direction, driven by a driving unit (not shown). The center stapler unit UNI includes two center staplers S 2  each including a clincher unit and a driving unit. The center staplers S 2  are arranged at a predetermined interval in the sheet width direction. It is to be noted that, although two fixed center staplers S 2  each including the clincher unit and the driving unit are provided in the present embodiment, alternatively, a single center stapler may be moved to staple two positions. 
     Each of the pair of upper bundle rollers  71  and the pair of lower bundle rollers  72  includes a driving roller and a driven roller. Additionally, a detector to measure the distance (i.e., nip distance) between the upper bundle rollers  71  is provided. When the upper bundle rollers  71  clamp the bundle of sheets therebetween, the detector detects the nip distance between the upper bundle rollers  71  and transmits the nip distance to the CPU  3 U. Thus, the CPU  3 U can obtain the thickness of the bundle. The CPU  3 U can select one of multiple operational modes, described below, according to the thickness of the bundle thus obtained. 
     The post-processing apparatus  3  further includes a movable back fence  73  disposed crossing the lower bundle guide  91 . The movable back fence  73  can be moved by a driving unit via a timing belt in the sheet conveyance direction, which is vertical in  FIG. 3 . Although not shown, the driving unit to move the movable back fence  73  includes a driving pulley around which the timing belt is wound, a driven pulley, and a stepping motor to drive the driving pulley. Similarly, an aligning pawl  251  and a driving unit to drive it are provided on the side of an upper end of the upper bundle guide  92 . 
     The driving unit moves the aligning pawl  251  via a timing belt  252  reciprocally in a direction away from the bundle guide unit including the lower and upper bundle guides  91  and  92  and the opposite direction to push the trailing end of the bundle (positioned on the upstream side when the bundle is introduced to the bundle guide unit). 
     The center-folding mechanism is positioned at a substantially center of the center-folding tray G and includes a folding plate  74 , a pair of folding rollers  81 , and the conveyance path H through which a bundle of folded sheets is transported. 
     Slots are formed in the folding plate  74  to engage two shafts projecting from front and back plates, respectively, and thus the folding plate  74  is supported by the shafts. Rotation of a driving unit is converted into a reciprocal linear movement by a link arm and a driving cam, and thus the folding plate  74  is moved. The folding plate  74  moves reciprocally between a home position outside a storage area of the center-folding tray G for storing the bundle and a position inside the storage area of the center-folding tray G to push the bundle into the nip between the folding rollers  81 . 
     It is to be noted that, in  FIG. 3 , reference numeral  302  denotes an upper discharge detector to detect the sheet discharged to the upper tray  201 ,  303  denotes a shift discharge detector to detect the sheet discharged to the shift tray  203 ,  304  denote a sheet detector to detect the position of the sheet to be stored in the stack portion E,  305  denotes a sheet detector to detect sheet conveyance to the edge-stapling tray F,  310  denotes a sheet detector to detect whether any sheet is present on the edge-stapling tray F,  321  denotes a sheet detector to detect the sheet transported to the center-folding tray G,  322  denotes a fence HP detector to detect whether the movable back fence  73  is at the home position, and  326  denotes a pawl HP detector to detect whether the aligning pawl  251  is at the home position. 
     In the post-processing apparatus  3  according to the present embodiment, the sheet is discharged to the following destinations according to the post processing performed. 
     Mode  1  (no stapling): The sheets are transported through the conveyance paths A and B and discharged to the upper tray  201  without being stapled. 
     Mode  2  (no stapling): The sheets are transported through the conveyance paths A, C, I, and J, and then discharged to the shift tray  202  without being stapled. 
     Mode  3  (sorting): The sheets are transported through the conveyance paths A, C, I, and J, and then discharged to the shift tray  202 . The shift tray  202  moves in the direction perpendicular to the sheet conveyance direction each time the last sheet in a set of output sheets is discharged thereto, thus sorting the sheets. 
     Mode  4  (stapling): The sheets are transported through the conveyance paths A and D to the processing tray F. After aligned and stapled on the processing tray F, the stapled sheets are transported through the conveyance path C to the shift tray  202 . 
     Mode  5  (center stapling and bookbinding): The sheets are transported through the conveyance paths A and D to the processing tray F. After aligned and stapled along the centerline of the sheets on the processing tray F, the stapled sheets are folded in two along the centerline on the processing tray G, transported through the conveyance paths H and L, and then discharged to the downstream device by the discharge rollers  118 . 
     Mode  6  (inserting sheets into envelopes): The sheets are transported through the conveyance path L, discharged to the insertion device  4 , and inserted in the envelopes. How to process (e.g., whether to fold or not) sheets to be inserted in envelopes can be selected from: A) The sheets are transported to the conveyance path L after transported through the conveyance paths A, C, I, and K without stapling (no stapling); B) The sheets are transported through the conveyance paths A and D, aligned and stapled on the processing tray F, and transported through the conveyance path K; and C) The sheets are transported through the conveyance paths A and D, aligned and stapled along the centerline on the processing tray F, folded along the centerline on the processing tray G, and transported through the conveyance paths H. 
       FIGS. 4 through 12  illustrate processes performed in the mode  5  in which center stapling and bookbinding are executed.  FIG. 4  is a front view illustrating states of the edge-stapling tray F and the center-folding tray G before stapling and folding. 
     Referring to  FIG. 3 , the sheet guided by the separation pawls  15  and  16  from the conveyance path A to the conveyance path D is transported to the edge-stapling tray F by the conveyance rollers  7 ,  9 , and  10  and the discharge roller  11  shown in  FIG. 4 . The bundle of sheets guided to the edge-stapling tray F by the discharge rollers  11  are aligned similarly to the above-described mode  4 . As shown in  FIG. 5 , the back fence  51  aligns the bundle of sheets. 
     After the bundle of sheets is roughly aligned on the edge-stapling tray F, the release pawl  52   a  lifts the bundle as shown in  FIG. 6 . Then, the release roller  56  and the pressure roller  57  clamp a leading-edge portion of the bundle therebetween as shown in  FIG. 7 . Subsequently, the bifurcation guide  54  as well as the movable guide  55  rotates, thus forming the route to the center-folding tray G as described above. The bundle of sheets is transported further by the release pawl  52   a  and the release roller  56  through this route to the center-folding tray G. The release roller  56  is provided at the driving shaft of the release belt  52  and driven in synchronization with the release belt  52 . 
     Subsequently, the release pawls  52   a  transport the bundle until the trailing edge of the bundle passes by the release roller  56 . Further, the upper bundle conveyance rollers  71  and the lower bundle conveyance rollers  72  transport the bundle to the position shown in  FIG. 8 . The position of the back fence  73  is set at one of multiple different positions according to the sheet size in the sheet conveyance direction, and the back fence  73  waits for the bundle at the position corresponding to the sheet size. When the leading edge of the bundle comes into contact with the back fence  73  on standby, as shown in  FIG. 9 , the lower bundle conveyance rollers  72  are disengaged from each other. Then, the aligning pawl  251  pushes the trailing edge of the bundle, and thus the bundle of sheets is fully aligned in the sheet conveyance direction. Further, the sheets are aligned in the sheet width direction by the jogger fences  250  positioned beneath the center stapler unit UNI in  FIG. 9 . Thus, the sheets are aligned in the sheet width direction by the jogger fences  250  and in the sheet conveyance direction (longitudinal direction of sheets) by the back fence  73  and the aligning pawl  251 . 
     At that time, the amounts by which the back fence  73  (stopper) and the pair of jogger fences  250  push the bundle of sheets to align it are set to optimum values according to the sheet size, the number of sheets, and the thickness of the bundle. It is to be noted that, when the bundle of sheets is relatively thick, it occupies a larger area in the conveyance path with the remaining space therein reduced, and accordingly a single alignment operation is often insufficient to align it. In this case, the number of times the alignment operation is repeated is increased to align the sheets neatly. 
     Additionally, as the number of sheets increases, it takes longer to stack multiple sheets one on another on the upstream side, and accordingly it takes longer before the processing tray G receives a subsequent bundle of sheets. Consequently, the increase in the number of times the alignment operation is performed does not cause a loss time in the sheet processing system, and thus streamlined, reliable alignment can be attained. As described above, the sheets can be processed efficiently by adjusting the number of times the alignment operation is performed according to the time required for the processing on the upstream side. 
     Subsequently, the center stapler S 2  staples the bundle of sheets along its centerline as shown in  FIG. 9 . Accordingly, the back fence  73  sets the bundle of sheets such a position that the center stapler S 2  can staple a center portion of the bundle. It is to be noted that the positions of the movable back fence  73  and the aligning pawl  251  are determined by the number of pulses of the fence HP detectors  322  and that of the pawl HP detector  326 , respectively. 
     As shown in  FIG. 10 , after stapled along the centerline, the bundle is not clamped by the lower bundle conveyance rollers  72  but is transported upward as the back fence  73  moves. The bundle is stopped at the position where the center portion of the bundle to be folded faces the edge of the folding plate  74 . Subsequently, as shown in  FIGS. 10 and 11 , the folding plate  74  pushes the portion adjacent to the staple binding the sheets in a direction substantially perpendicular to the surface of the sheets into the nip between the folding rollers  81  positioned facing the folding plate  74 . The folding rollers  81 , which start rotating in advance, transport the bundle while pressing the bundle. Thus, the bundle is folded in two along the centerline. 
       FIGS. 11 and 12  illustrate a squeezing unit  200  to further squeeze the folded sheets, provided along the conveyance path H shown in  FIG. 3 . It is to be noted that, in  FIGS. 11 and 12 , reference numeral  60  represents the bundle of center-folded sheets. If necessary, the bundle  60  is further folded after pressed and transported by the folding rollers  81 . That is, the bundle  60  is further squeezed by a pressure roller  258  (hereinafter “additional squeezing”) to strengthen the folded line of the bundle  60  or to make the bundle  60  thinner as shown in  FIG. 11 . 
     Because the direction in which the bundle of sheets is transported after stapling along centerline is upward, the bundle can be transported reliably by the back fence  73  only. If the device is configured so that the bundle to be folded is transported down, the bundle might fail to follow the downward movement of the back fence  73  because of effects of friction and static electricity. Thus, reliable conveyance of the bundle cannot be secured. Therefore, such a configuration in which the bundle to be folded is transported down requires another conveyance member such as a conveyance roller and becomes more complicated. 
     Referring to  FIGS. 11 and 12 , descriptions are given below of a configuration and operation of the squeezing unit  200  provided along the conveyance path H shown in  FIG. 3 . 
     The squeezing unit  200  additionally squeezes the folded portion of the bundle of center-folded sheets  60  folded in the post-processing apparatus  3  to make it thinner. Referring to  FIGS. 11 and 12 , the squeezing unit  200  may has a known configuration and, in the present embodiment, includes a pressure roller  258 , a driving motor  258   a  to move the pressure roller  258 , a guide  258   b  extending in the direction perpendicular to the sheet conveyance direction, to guide a holder of the pressure roller  258  movably, vertically in  FIGS. 11 and 12 , a support shaft  258   c  to apply pressure to the pressure roller  258  with a compression spring, and a bracket  258   d  to support the pressure roller  258 . The bracket  258   d  is supported slidably on a guide rail extending in the direction perpendicular to the sheet conveyance direction. The support shaft  258   c  connects the holder of the pressure roller  258  to the bracket  258   d  in such a way that the holder can move vertically relative to the bracket  258   d . Additionally, a folded portion detector  323  to detect the leading-edge portion of the bundle  60  is provided downstream from the folding rollers  81  and upstream from the pressure roller  258 . 
       FIG. 11  illustrates a state in which the pressure roller  258  of the squeezing unit  200  squeezes the folded portion of the bundle  60  after the folding rollers  81  fold the sheets in two, and  FIG. 12  illustrates a state in which the bundle  60  is discharged from the squeezing unit  200  after squeezed by it. 
     In  FIGS. 11 and 12 , the pressure roller  258  is positioned adjacent and downstream from the folding rollers  81  in the sheet conveyance direction and rolls in the direction perpendicular to the sheet conveyance direction. As shown in  FIG. 11 , after folded along its centerline by the folding rollers  81 , the bundle  60  is further transported in the direction indicated by arrow X shown in  FIG. 11 . The bundle  60  is transported a predetermined distance (predetermined conveyance distance) after the leading-edge portion of the bundle  60  passes by the folded portion detector  323  and stopped at a position where the leading-edge portion of the bundle  60  is pressed by the pressure roller  258 . When a stepping motor is used as the conveyance motor, this distance can be controlled by the number of pulses of the stepping motor. It is to be noted that the predetermined conveyance distance by which the bundle  60  is transported from the folded portion detector  323  can be controlled in other ways, without using the step number of the stepping motor, and the conveyance motor is not limited to the stepping motor. The pressure roller  258  rolls on and squeezes the folded portion (i.e., leading-edge portion) of the bundle  60  in the direction perpendicular to the sheet conveyance direction. Accordingly, the leading-edge portion of the bundle  60  is stopped at a position on the route of the pressure roller  258 . 
     An initial position of the pressure roller  258  is outside a bundle conveyance area in which the bundle  60  is transported, and the bundle  60  is stopped when the leading-edge portion of the bundle  60  reaches the position shown in  FIG. 11 . With the bundle retained at that position, the driving motor  258   a  rotates, causing via a transmission unit the bracket  258   d  to slide on the guide rail reciprocally in the direction perpendicular to the sheet conveyance direction to squeeze the leading-edge portion of the bundle  60  along the folded lines. Thus, the folded portion of the bundle  60  is further squeezed, strengthening the folded lines and flattening the bundle  60 . After the additional squeezing by the squeezing unit  200  is completed and the pressure roller  258  returns to the initial position, outside the bundle conveyance area, the folding rollers  81  resume transporting the bundle  60 . Then, as sown in  FIG. 12 , the bundle  60  passes by the sheet detector  256 , is further transported in the direction indicated by arrow X shown in  FIG. 12  by the conveyance rollers  257 , and then discharged by the discharge rollers  118  shown in  FIG. 3  to the downstream device, that is, the insertion device  4 . 
     When the folded portion detector  323  detects a trailing-edge portion of the bundle  60 , both the folding plate  74  and the back fence  73  return to the respective home positions. Then, the lower bundle conveyance rollers  72  move to press against each other as a preparation for receiving a subsequent bundle of sheets. Further, if the number and the size of sheets forming the subsequent bundle are similar to those of the previous bundle of sheets, the back fence  73  may move again to the position shown in  FIG. 8  and wait there. 
     It is to be noted that, although the post-processing apparatus  3  shown in  FIGS. 3 through 12  has a capability of center folding (i.e., folding sheets in two), known folding devices capable of folding sheets in two, three, or four; folding sheets into Z-like shape, double door-like shape, accordion-like shape; or at least two of them may be used. Those configurations are disclosed in JP-200967537, which is incorporated by reference herein in its entirety. 
       FIG. 13  illustrates an interior of the insertion device  4  according to the present embodiment. 
     The envelopes stored in the feed cassette  1 -B of the image forming apparatus  1  are fed to an image forming unit inside the image forming apparatus  1 , and the image forming unit prints addresses on the envelopes, after which the envelopes are transported to the post-processing apparatus  3  and further to the insertion device  4 . The envelope enters an entrance path  505  leading from an entrance of the insertion device  4 , and an entry detector  504  detects the envelope. Then, the respective conveyance rollers are driven and start transporting the envelope. A pivotable upper separation pawl  506  is provided at a bifurcation position from which the entrance path  505  bifurcates into an upper conveyance path  507  on the side of an upper discharge tray  525  and a lower conveyance path  509 . 
     In  FIG. 13 , the upper separation pawl  506  pivots to an upper position and guides the envelope to the lower conveyance path  509 . Additionally, a pivotable lower separation pawl  510  is provided at a bifurcation position from the lower conveyance path  509  between a vertical conveyance path  511  and an enclosure conveyance path  512 . To guide the envelope, the lower separation pawl  510  pivots counterclockwise in  FIG. 13  to a position to open the vertical conveyance path  511 . Thus, the envelope is guided to the vertical conveyance path  511 . A pair of chuck rollers, namely, a lower chuck roller  520  and an upper chuck roller  536 , is provided extreme downstream in the vertical conveyance path  511 , and an unsealing sheet  521  is partially in contact with a part of the lower chuck roller  520 . Further, a pair of pivotable rollers  522  is provided downstream from the unsealing sheet  521 . The upper and lower chuck rollers  536  and  520  clamp a gusset portion of the envelope, retaining the envelope there, and wait for the enclosure. At this time, the pivotable rollers  522  are withdrawn from the envelope in the directions indicated by arrows Y 1  and Y 1 ′, respectively, not to contact the envelope. 
     In the image forming apparatus  1 , an image reading unit reads image data of an original document sent by the ADF  2 , and then a sheet sized corresponding to the size of the original document is fed from the feed cassette  1 -B to the MFP. After an image is formed on the sheet, the sheet is transported to the post-processing apparatus  3 . The sheet to be inserted in the envelope (i.e., enclosure) is folded or stapled, or folded and stapled as required in the post-processing apparatus  3 , after which the sheet is transported to the insertion device  4 . When neither folded nor stapled, the enclosure is transported through the conveyance paths A, C, I, K, and L in the post-processing apparatus  3  to the entrance path  505  of the insertion device  4 . After the entry detector  504  detects the enclosure, the conveyance rollers are driven and start transporting the enclosure. 
     The upper separation pawl  506  pivots to the upper position, thus guiding the enclosure to the lower conveyance path  509 . The lower separation pawl  510  pivots to the position shown in  FIG. 13 , thus guiding the enclosure to the enclosure conveyance path  512 . The enclosure passes by an enclosure detector  513  and is stacked on an intermediate tray  515 . Subsequently, a return roller  514  moves to a position in contact with the intermediate tray  515  and transports the enclosure toward a back stopper  518 . Further, a pair of side joggers  517  aligns the enclosure. This operation is repeated until all the enclosures are aligned on the intermediate tray  515 . 
     After a bundle of enclosures are stacked on the intermediate tray  515 , the back stopper  518  is withdrawn in the direction indicated by arrow Y 2 . A front stopper  516  starts moving in the direction indicated by arrow shown in  FIG. 13  to a position indicated by broken lines and transports the bundle of enclosures inside a pack unit  519 . Then, the bundle of enclosures is clamped in nips between upper rollers  542  and lower rollers  543 , arranged vertically (shown in  FIG. 21 ), in the pack unit  519 . After the enclosures are transported therein, the pack unit  519  pivots about a support point  546  in the direction indicated by arrow Y 3  shown in  FIG. 13 . Then, a single enclosure or multiple enclosures to be inserted in a single envelope are transported by the upper rollers  542  and the lower rollers  543  of the pack unit  519  into the envelope retained by the pair of chuck rollers  520  and  536 . After the enclosures are put in the envelope, the pivotable rollers  522  move in the direction opposite to the directions indicated by arrows Y 1  and Y 1 ′, respectively, and start transporting the envelope to a discharge path  523 . The envelope is transported through the discharge path  523 , passes by an envelope detector  524 , and is stacked on an envelope tray  526 . 
     It is to be noted that, when the upper separation pawl  506  pivots clockwise from the position shown in  FIG. 13  to a position to open the upper conveyance path  507 , the envelope or the sheet is discharged from the upper conveyance path  507  to the upper discharge tray  525 . It is to be noted that, in  FIG. 13 , reference numeral  508  denotes a discharge detector to detect the object to be discharged to the upper discharge tray  525 . 
       FIG. 14  is a perspective view that illustrates the feed cassette  1 -B of the image forming apparatus  1  and a size detecting system to detect the size of the envelope or enclosure stored in the feed cassette  1 -B. 
     In  FIG. 14 , a planar size indicator  527  is attached to each feed cassette  1 -B. Each size indicator  527  is sized according to the size of the sheets or envelopes contained therein. The main body of the image forming apparatus  1  includes a size detector  528  corresponding to each size indicator  527 . When the feed cassette  1 -B is set in the main body, the size detector  528  detects the size indicator  527  and thus recognizes the size of the sheets or envelopes (in  FIG. 14 , envelopes Pf) contained in the feed cassette  1 -B. Additionally, a size sticker  529  (i.e., size label) is stuck to side face of the feed cassette  1 -B so that the user can recognize the size or type of the objects contained therein. 
       FIG. 15  is a perspective view that illustrates a variation of the feed cassette  1 -B of the image forming apparatus  1  and the size detecting system to detect the size of the envelope or enclosure stored therein.  FIG. 16  is a cross-sectional view of the feed cassette and the size detecting system shown in  FIG. 15 . 
     A feed cassette  1 -B 1  shown in  FIGS. 15 and 16  includes a bottom plate  530  on which the envelopes Pf are stacked and a pair of side guides  531  and  532  slidable in a direction indicated by arrow M shown in  FIG. 16 , along a guide rod  533 . The envelopes Pf are set in a center portion of the bottom plate  530 , pushed by the side plates  531  and  532 . Additionally, a size detector  534  is provided beneath the bottom plate  534 . The size detector  534  detects the position of the side guide  532  to detect the size of the objects (in  FIGS. 15 and 16 , envelopes Pf) stacked on the bottom plate  530 . More specifically, the size detector  534  compares the detected position of the side guide  532  with size data stored preliminarily therein and thus recognizes the size of the sheets or the envelopes Pf set on the bottom plate  530 . For example, a variable-resistance position detector can be used as the size detector  534 . The CPU  1 U can easily detect the size of the objects contained in the sheet cassette  1 -B 1  based on the resistance output by the variable-resistance type position detector or changes in the resistance. 
       FIG. 17  is a cross-sectional view that illustrates a main portion of an envelope chuck unit in the insertion device  4 . 
     In  FIG. 17 , the lower chuck roller  520  and the upper chuck roller  536 , provided extreme downstream in the vertical conveyance path  511 , together form an envelope chuck unit  538 . The chuck rollers  520  and  536  are arranged substantially vertically in  FIG. 17  and can rotate while pressing against each other, forming a nip portion therebetween. The chuck rollers  520  and  536  may be rollers, cones, or spheres. Envelope guides  535  and  539  to guide the envelope Pf to the nip portion between the chuck rollers  520  and  536  are provided upstream from the chuck rollers  520  and  536  in the vertical conveyance path  511  in the direction in which the envelope is transported (hereinafter “envelope conveyance direction”). An envelope detector  537  is provided on an upstream side of the nip portion in the envelope conveyance direction. The unsealing sheet  521  in contact with the lower chuck roller  520  is formed of a plastic sheet such as Mylar and can deform elastically. The unsealing sheet  521  is provided at such a position that a part of the unsealing sheet  521  can enter an opening Pon (shown in  FIG. 18 ) of the envelope Pf supported by the chuck rollers  520  and  536 , thereby unsealing the envelope Pf. 
     The envelope guides  535  and  539  guide the envelope Pf from the vertical conveyance path  511  to the nip portion between the chuck rollers  520  and  536  and further downward from the nip portion between the chuck rollers  520  and  536  along a circumferential surface of the lower chuck roller  520 . 
     The unsealing sheet  521  may be a thin resin film member and positioned adjacent to the lower chuck roller  520 . An upper side of the unsealing sheet  521  is fixed, and, in an ordinary state, a portion of the unsealing sheet  521  adjacent to a lower end portion  521   a  (shown in  FIG. 18 ) thereof is pressed against the lower chuck roller  520  with a predetermined pressure due to the elasticity of the material of the unsealing sheet  521 . 
       FIG. 18  is a cross-sectional view of the main portion of the envelope chuck unit  538  and illustrates a state in which the opening Pon of the envelope Pf is positioned beneath the lower end portion  521   a  of the unsealing sheet  521 .  FIG. 19  is another cross-sectional view of the main portion of the envelope chuck unit  538 , and the lower end portion  521   a  of the unsealing sheet  521  is in the envelope Pf in  FIG. 19 . 
     In the envelope chuck unit  538 , the envelope guides  535  and  539  guide the envelope Pf to the nip portion between the chuck rollers  520  and  536  when the envelope Pf is transported downward in  FIG. 18 . Subsequently, the chuck rollers  520  and  536  rotate and transport the envelope Pf between the chuck roller  520  and the unsealing sheet  521 . When the sheet or enclosure is guided into the envelope Pf, the envelope Pf is stopped at such a position that a flap Pfc of the envelope Pf is clamped by the chuck rollers  520  and  536  as shown in  FIG. 18 . More specifically, when the envelope detector  537  detects passage of an end of the flap Pfc of the envelope Pf, the CPU  4 U stops a driving motor that drives the chuck rollers  520  and  536 , thus stopping the envelope Pf. At that time, the opening Pon of the envelope Pf is positioned lower than the lower end portion  521   a  of the unsealing sheet  521 . 
     Subsequently, the CPU  4 U rotates the chuck rollers  520  and  536  in reverse, which is the direction indicated by arrow N shown in  FIG. 18 . Thus, the envelope Pf is switchbacked and transported upward in the vertical conveyance path  511 . At that time, because the lower side of the unsealing sheet  521  is in contact with the flap Pfc of the envelope Pf due to its elasticity, the lower end portion  521   a  of it enters the opening Pon of the envelope Pf as shown in  FIG. 19 . The reverse rotation of the chuck rollers  520  and  536  is stopped in this state, and upward conveyance of the envelope Pf is stopped. 
       FIG. 20  is a perspective view illustrating this state, and the envelope Pf is opened by the lower end portion  521   a  of the unsealing sheet  521  that is in the opening Pon of the envelope Pf.  FIG. 21  is a front view of the pack unit  519  of the insertion device  4 . 
     In the configuration shown in  FIG. 21 , the pack unit  519  includes an upper pack portion  540  and a lower pack portion  541 , and the upper rollers  542  and the lower rollers  543  are rotatively attached to the upper pack portion  540  and a lower pack portion  541 , respectively. Additionally, entry guides  544  and  545  are respectively provided on the right end sides of the upper pack portion  540  and the lower pack portion  541  in  FIG. 21 . Base ends (proximal ends) of the entry guides  544  and  545  are rotatively supported by the upper pack portion  540  and the lower pack portion  541 , respectively, and dismal end sides of the entry guides  544  and  545  are biased toward each other by springs with a relatively small pressure, respectively. With this configuration, when a bundle of enclosures passes between the entry guides  544  and  545 , the entry guides  544  and  545  are pushed away from each other. Thus, the resistance that the bundle of enclosures receives can be lower when the bundle is transported. 
     The pack unit  519  pivots about the support point  546  supporting the pack unit  519 , and the entry guides  544  and  545  are inserted between the flap Pfc and the unsealing sheet  521 , which is on standby at the position shown in  FIG. 20 . In this state, the front stopper  516  moves in the direction indicated by arrow shown in  FIG. 13  as described above, and the upper and lower rollers  542  and  543  are driven. Then, the enclosure passes between the entry guides  544  and  545  and is inserted in the envelope Pf. 
       FIG. 22  is a diagram of the envelope Pf. 
     In  FIG. 22 , reference characters L 1  and L 2  represent a length (width) of an opening of the envelope Pf (hereinafter “opening length”) and a depth of the envelope Pf, respectively. The opening length L 1  may be equivalent to the width of the envelope Pf. The envelope size is determined by the opening length L 1  and the depth L 2 . 
       FIG. 23  is a diagram of an original document OD. 
     In  FIG. 23 , reference characters L 3  and L 4  represent a length in a sub-scanning direction and a length in a main scanning direction of the original document OD. The original document size is determined by the lengths L 3  and L 4 . 
       FIG. 24  is a front view of the operation panel  1 -A provided on an upper face of the image forming apparatus  1 . 
     Referring to  FIG. 24 , the operation panel  1 -A includes the display  1 -D, a group of numeric keys b, a STOP key c, a START key d, a POWER button e, and a group of function selection keys f. The display  1 -D displays various messages and input keys in layers. The user can input numbers by pressing the numeric keys b. The user can stop processing by pressing the STOP key c. Pressing the START key d generates a trigger signal to start image formation. The user can turn on and off the image forming system by pressing the POWER button e. The group of function selection keys f includes keys with which the user selects copying, printing, scanning, or the like. 
       FIG. 25  illustrates indications on the display  1 -D of the operation panel  1 -A shown in  FIG. 24 . 
     The indications shown in  FIG. 25  appear when A4 size sheets are stored laterally in the first feed cassette  1 -B Y (hereinafter “A4Y sheets”) and A4 size sheets are stored lengthwise in the second feed cassette  1 -B Y (hereinafter “A4T sheets”). 
     It is to be noted that, in the configuration shown in  FIG. 25 , the size of the original document is A4 size, and the same sized sheets are set laterally in one of the feed cassettes  1 -B and lengthwise in the other. 
     To insert the sheet into the envelope, the user presses an INSERTION button a 1  of an insertion tab on the display  1 -D shown in  FIG. 25 , sets the original document in the ADF  2 , and presses the START key d on the operation panel  1 -A. Then, the envelope is fed from the feed cassette  1 -B. The sheet is fed from the first or second feed cassette  1 -B, and an image is formed on the sheet according to the image data of the original document. Although copying the original document is performed here as an example, alternatively, image data transmitted from, for example, computers, can be printed on the sheet in a manner similar to copying. To fold the sheet, the user presses a FOLDING button a 2  and set the type of folding (i.e., folding style), for example, folding it in two or three. To staple the sheet, the user presses a FINISHER button a 3  and sets the type of stapling, namely, center stapling or side stapling. 
     Descriptions are given below of cases in which an A4Y sheet (or multiple A4 sheet) not folded as well as an A3 sheet (or multiple A3 sheets) folded are inserted in a single envelope. 
       FIG. 26  is a flowchart illustrating a sequence of insertion processes executed after the user presses the INSERTION button a 1  on the display  1 -D of the operation panel  1 -A and sets the folding style. 
     Table 1 illustrates relations among sheet sizes, folding styles, and first and second converted quantities or folding-related equivalent quantities for each sheet to be squeezed by the squeezing unit  200  and for each not to be squeezed by it. The first and second folding-related equivalent quantities increase as the folding number increases. Table 2 illustrates the relation between envelope types and maximum number of sheets insertable in the envelope. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Folding 
                 First equivalent quantity 
                 Second equivalent 
               
               
                   
                 style 
                 for each sheet 
                 quantity for each sheet 
               
               
                 Sheet 
                 (Folding 
                 (default, without 
                 (after additional squeezing 
               
               
                 size 
                 number) 
                 additional squeezing) 
                 is executed once) 
               
               
                   
               
             
            
               
                 A3 
                 Not folded 
                 1 
                 — 
               
               
                   
                 Two 
                 2 
                 1 
               
               
                   
                 Three 
                 3 
                 2 
               
               
                   
                 Four 
                 4 
                 3 
               
               
                 A4 
                 Not folded 
                 1 
                 — 
               
               
                   
                 Two 
                 4 
                 3 
               
               
                   
                 Three 
                 5 
                 4 
               
               
                   
                 Four 
                 6 
                 5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Maximum 
               
               
                   
                 Envelope type 
                 insertable number of sheets 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 A (for A4 size sheets) 
                 5 
               
               
                   
                 B (for A4 size sheets) 
                 10 
               
               
                   
                 C (for A3 size sheets) 
                 5 
               
               
                   
                 D (for A3 size sheets) 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Tables 1 and 2 may be stored in the storage unit such as the RAM of the CPU  1 U in the image forming apparatus  1 , and the CPU  1 U refers to those relations to execute predetermined calculations in the control described below. 
     It is to be noted that the converted quantity for each sheet not to be folded remains “1”, and the unfolded sheet is not squeezed by the squeezing unit  200 . Therefore, the equivalent quantity for each unfolded sheet to be squeezed once by the squeezing unit  200  is not available and shown as “-” in Table 1. 
     In other words, the number of times the squeezing unit  200  squeezes the sheet satisfies a relation:
 
N&lt;M
 
     wherein N is a positive integer representing the number of times the squeezing unit  200  squeezes the sheet and M is a positive integer representing the folding-related equivalent quantity for each sheet. 
     Additionally, although Table 1 illustrates only the relations regarding A3 size and A4 size, the image forming system according to the present can store data of relations between folding styles and the folding-related equivalent quantities regarding all sheet sizes insertable in envelopes. 
     In the flowchart shown in  FIG. 26 , after the user inputs the number of sheets inserted, the folding style, and the like on the display  1 -D, at S 101  the CPU  1 U refers to Table 1 and calculates the converted number of sheets in total using the first folding-related equivalent quantity corresponding to the sheet type (e.g., sheet size and sheet thickness) and the folding style. At S 102  the CPU  1 U refers to Table 2 and calculates the number of sheets insertable in that envelope type (maximum insertable number of sheets). At S 103 , the CPU  1 U compares the converted number of sheets in total with the maximum insertable number of sheets and at S 104  determines whether the envelope can accommodate the enclosure. When the envelope can accommodate the enclosure (Yes at S 104 ), at S 105  the CPU  1 U starts image formation on the sheet, folding the sheet, and inserting the sheet in the envelope. 
     By contrast, when insertion is not feasible (No at S 104 ), at S 106  the CPU  1 U increases by one the number of times the squeezing unit  200  squeezes the folded sheet or multiple folded sheets (the number of times of the additional squeezing). Then, the CPU  1 U refers to Table 1 and recalculates the converted number of sheets in total using the second folding-related equivalent quantity corresponding to the sheet type (i.e., sheet size and sheet thickness) and the folding style. Subsequently, at S 107  the CPU  1 U compares the recalculated converted number of sheets in total with the number of sheets insertable and at S 108  determines whether the envelope can accommodate the enclosure. When insertion is executable (Yes at S 108 ), the process proceeds to S 105 . Then, image formation on the sheet, folding the sheet, and inserting the sheet in the envelope are executed. 
     By contrast, when insertion is not executable (No at S 108 ), folding the sheet, and inserting the sheet in the envelope are not executed. At S 109 , the CPU  1 U causes the display  1 -D to display an error message as shown in  FIG. 27 . At S 110 , the display  1 -D prompts the user to cancel the job or change setting of the job. 
     Specific cases are described below. 
     Case 1 
     In case 1, a unfolded single A4Y sheet as well as three A3 sheets folded in two are inserted in a single envelope of type B (see Table 2). The user inputs “one unfolded A4Y sheet” and “three A3 sheets folded in two” as the enclosure on the display  1 -D. Then, the CPU  1 U calculates the converted number of sheets in total using the first folding-related equivalent quantity corresponding to the sheet type and the folding style (S 101 ). 
     Referring to Table 1, the CPU  1 U retrieves, from the prestored table, the first folding-related equivalent quantity for each sheet corresponding to the sheet type and the folding style and then stores it. The first folding-related equivalent quantity of an unfolded A4 sheet remains “1”. The first folding-related equivalent quantity of A3 size folded in two is “2”, and three A3 sheets folded in two are equivalent to six unfolded sheets (2×3). Accordingly, the number of sheets in total is “7” (1+2×3). 
     Subsequently, the CPU  1 U refers to Table 2 and obtains the converted insertable number of sheets (S 102 ) and stores it. The maximum number of sheets insertable in the envelope of type A is “10”. After these values are thus obtained, the CPU  1 U compares the converted number of sheets in total, “7”, with the maximum insertable number of sheets, “10”. Since the envelope of type A can accommodate the converted number of the enclosures (7&lt;10), the CPU  1 U determines that image formation, folding, and insertion are feasible (S 103  and S 104 ) and starts the processing (S 105 ). 
     Case 2 
     In case 2, a single A4Y sheet (not folded) as well as three A3 sheets (folded in two) are inserted in a single envelope of type A (see Table 2). After the user inputs insertion-related settings, the CPU  1 U refers to Table 1 and retrieves the first folding-related equivalent quantity for each sheet corresponding to the sheet type and the folding style and stores it. Referring to Table 1, the first folding-related equivalent quantity of an unfolded A4Y sheet remains “1”. The first folding-related equivalent quantity of A3 size folded in two is “2”, and three A3 sheets folded in two are equivalent to six sheets (2×3). Accordingly, the converted number of sheets in total is calculated as follows (S 101 ).
 
1+2×3=7
 
     The maximum number of sheets insertable in the envelope of type A is “5” (S 102 ). When these values are compared with each other (S 103 ), the converted number of sheets in total is greater than the maximum number of sheets insertable in the envelope of type A (7&gt;5). Because the envelope cannot accommodate the converted number of the enclosures (S 104 ), the CPU  1 U recalculates the converted total number of sheets for a case in which additional squeezing is executed once. Referring to Table 1, when additional squeezing is executed once, the second folding-related equivalent quantity for a single A3 sheet folded in two is “1”, and the converted total number of sheets is calculated as follows (S 106 ).
 
1+1×3=4
 
     Accordingly, the converted total number of the enclosures is smaller than the maximum insertable number of sheets in the envelope (4&lt;5). Then, the CPU  1 U determines that the enclosures can be inserted in the envelope (S 107  and S 108 ) and starts the processing (S 105 ). 
     It is to be noted that, because the initial position of the pressure roller  258  is outside the sheet conveyance path on the back side of the insertion device  4  as shown in  FIG. 12 , executing the additional squeezing once means that the pressure roller  258  moves from the back side to the front side of the device and returns to the back side once. Additionally, the number of times the additional squeezing is executed depends on the elasticity of the compression spring, and the pressure roller  258  may move only once from the back side to the front side of the device or from the front side to the back side of the device, not reciprocally. 
     Case 3 
     In case 3, two A4Y sheets (not folded) as well as five A3 sheets (folded in two) are inserted in a single envelope of type A (see Table 2), and the additional squeezing is performed once. After the user inputs insertion-related settings, the CPU  1 U refers to Table 1 and retrieves the folding-related equivalent quantity for each sheet corresponding to the sheet type and the folding style and stores it. Referring to Table 1, the folding-related equivalent quantity of an unfolded A4Y sheet remains “1”. The first folding-related equivalent quantity of A3 size folded in two is “2”, and five A3 sheets folded in two are equivalent to 10 sheets (2×5). Accordingly, the converted number of sheets in total is calculated as follows (S 101 ).
 
1×2+2×5=12
 
     The maximum number of sheets insertable in the envelope of type A is “5” (S 102 ). When these values are compared with each other (S 103 ), the converted number of sheets in total is greater than the maximum number of sheets insertable in the envelope of type A (12&gt;5). Because the envelope cannot accommodate the converted number of the enclosures (S 104 ), the CPU  1 U recalculates the converted number of sheets in total for a case in which additional squeezing is executed once. Referring to Table 1, the second folding-related equivalent quantity for a single A3 sheet folded in two is “1” when the sheet is to be squeezed once, and the converted total number of sheets is calculated as follows (S 106 ).
 
1×2+1×5=7
 
     Thus, the converted total number of sheets, “7”, is greater than the maximum insertable number of sheets, “5”, even after the additional squeezing is executed once (S 107 ). The CPU  1 U determines that insertion is not feasible (No at S 108 ) and causes the display to display an error message such as the one shown in  FIG. 27  (S 109 ). Additionally, the CPU  1 U prompts the user to cancel the job or change the setting of the job (S 110 ). Referring to  FIG. 27 , the user can cancel the job by pressing a CANCEL button a 5  or change the setting by pressing a CHANGE SETTING button a 6  on the display  1 -D. It is to be noted that the error message is not limited to the one shown in  FIG. 27 . For example, the display  1 -D may report only cancellation of the job. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Folding style 
                 Equivalent quantity 
                 Deducted value 
               
               
                 Sheet 
                 (Folding 
                 for each sheet (default, 
                 per sheet for each 
               
               
                 size 
                 number) 
                 without additional squeezing) 
                 additional squeezing 
               
               
                   
               
             
            
               
                 A3 
                 Not folded 
                 1 
                 — 
               
               
                   
                 Two 
                 2 
                 −1 
               
               
                   
                 Three 
                 3 
                 −1 
               
               
                   
                 Four 
                 4 
                 −1 
               
               
                 A4 
                 Not folded 
                 1 
                 — 
               
               
                   
                 Two 
                 4 
                 −1 
               
               
                   
                 Three 
                 5 
                 −1 
               
               
                   
                 Four 
                 6 
                 −1 
               
               
                   
               
            
           
         
       
     
     Descriptions are made below of a procedure when the additional squeezing is performed twice or more. 
       FIG. 28  is a flowchart of a procedure when the additional squeezing is performed twice. 
     Steps from S 201  through S 210  shown in  FIG. 28  are identical or similar to those from S 101  through S 110  shown in  FIG. 26 , and thus the descriptions thereof are omitted. The procedure shown in  FIG. 28  are similar to that shown in  FIG. 26  except the steps S 206 A,  207 A, and  208 A added between steps S 108  and S 109  in  FIG. 26 . 
     At S 206 A, the CPU  1 U increases by one the number of times the folded sheet is to be squeezed by the squeezing unit  200 . Then, the CPU  1 U refers to Table 1 and recalculates the converted number of sheets in total using the second folding-related equivalent quantity corresponding to the sheet type (i.e., sheet size) and the folding style. It is to be noted that, at S 206 A, the number of times of the additional squeezing is not increased for folded sheets to be squeezed once, having a second folding-related equivalent quantity of “1”. 
     Subsequently, at S 207 A the CPU  1 U compares the recalculated converted number of sheets in total with the number of sheets insertable and, at  5208 A, determines whether the envelope can accommodate the enclosure. When insertion is feasible (Yes at S 208 A), the process proceeds to S 205 . By contrast, when insertion is not feasible, the process proceeds to S 209  and S 210 . 
     Case 4 
     As another case of the procedure shown in  FIG. 28 , two unfolded A4Y sheets, an A3 sheet folded in two, as well as five A3 sheets folded in three are inserted in an envelope of type B in case 4. 
     After the user inputs, on the display  1 -D, that “two unfolded A4Y sheets”, “one A3 sheet folded in two”, and “five A3 sheets folded in three” are inserted in a B type envelope, the CPU  1 U refers to Table 3 and obtains the folding-related equivalent quantities corresponding to the sheet type (i.e., sheet size) as well as the folding style and stores it. The folding-related equivalent quantity of an unfolded A4 sheet remains “1”. The folding-related equivalent quantities of an A3 sheet folded in two and an A3 sheet folded in three are “2” and “3”, respectively. Thus, the converted number of sheets in total can be calculated as follows.
 
1×2+2×1+3×5=19
 
     The CPU  1 U refers to Table 2 and obtains the maximum number of sheets insertable in the B type envelope, which is “10” (S 202 ), and stores it. When compared with each other (S 203 ), the converted total number of sheets is greater than the maximum insertable number of sheets (19&gt;10). Because the envelope cannot accommodate the converted number of the enclosures (No at S 204 ), the CPU  1 U recalculates the converted number of sheets in total for a case in which additional squeezing is executed once. Referring to Table 3, after the additional squeezing is executed once, the folding-related equivalent quantity of the A3 sheet folded in two can be calculated as 2−1=1 the folding-related equivalent quantity of the A3 sheet folded in three, to be squeezed once, can be calculated as 3−1=2. Accordingly, the converted total number of sheets can be calculated as follows (S 206 ).
 
1×2+1×1+2×5=13
 
     As a result, the converted total number of sheets is “13”, which is still greater than the maximum insertable number of sheets, “10” (S 207 ). Because the envelope cannot accommodate the converted number of the enclosures (No at S 208 ), the CPU  1 U recalculates the converted number of sheets in total for a case in which additional squeezing is executed again, that is, twice. 
     Referring to Table 3, the when additional squeezing is to be executed again, the folding-related equivalent quantity for a single A3 sheet folded in three is “1” (3−1×2). The converted total number of sheets for one A3 sheet folded in two and five A3 sheets folded in three is calculated as 1×2+2×5=12. 
     Because the folding-related equivalent quantity of the A3 sheet folded in two, to be squeezing once is reduced to “1” at S 206 , the second squeezing is not executed on the A3 sheet folded in two at S 206 A. Thus, the folding-related equivalent quantity of the A3 sheet folded in two remains “1”. 
     Accordingly, the converted number of sheets in total is calculated as “8” (1×2+1×1+1×5) at S 206 A. 
     When compared with each other, the converted total number of sheets is smaller than the maximum insertable number of sheets (8&lt;10) at S 207 A. Thus, the envelope can accommodate the enclosures (Yes at S 208 A), and the process proceeds to S 205 . Then, image formation, folding, and insertion can be started. 
     By contrast, when the envelope still cannot accommodate the enclosure, the process proceeds to S 209  and S 210 . The CPU  1 U causes the display  1 -D to display an error message as shown in  FIG. 27  and prompts the user to cancel the job or change setting of the job. 
     It is to be noted that, in the relation among sheet type, folding style, and the folding-related equivalent quantity for each sheet shown Table 3, the folding-related equivalent quantity is deducted by one as the number of times of the additional squeezing is increased by one. 
     Additionally, in this calculation, the number of times of additional squeezing is not increased for the sheet whose folding-related equivalent quantity is “1” because the folding-related equivalent quantity should be 1 or greater. 
     Further, the relation shown in Table 3 can be stored in the RAM of the image forming apparatus  1  as a table. The CPU  1 U refers to the relation in addition to Table 1 in performing the procedure shown in  FIG. 28 . 
     As described above, in the present embodiment, the system can determine whether the envelope can accommodate the enclosure when the user inputs the insertion-related settings including the envelope type, sheet type, and folding style before the post-processing apparatus  3  starts image formation on the sheet and folding the sheet. Further, when the envelope cannot accommodate the enclosure, the number of times folded sheets are squeezed is increased to reduce the thickness of the enclosure. Therefore, sheets are not wasted when the envelope cannot accommodate the enclosure and the productivity can be improved. 
     Additionally, the system can insert folded sheets and unfolded sheets together or multiple sheets folded in different styles in a single envelope. 
     The present embodiment can attain the following effects. 
     1) When insertion is not feasible, the number of times the folded enclosures is squeezed is increased to reduce the thickness of the enclosures. Therefore, the enclosure that is thicker than the capacity of the envelope can be squeezed to be insertable in the envelope. 
     2) The folding-related equivalent quantity for each folded sheet, based on which the CPU  1 U determines whether insertion is feasible, is set separately for the sheet to be squeezed by the squeezing unit  200  and the sheet not to be squeezed. Thus, when the number of times the squeezing unit  200  squeezes the sheet is changed, in particular, the number of times of squeezing is increased, the converted quantity of the squeezed sheet can be smaller. 
     3) The CPU  1 U can recognize that insertion is feasible after the converted quantity of the squeezed sheet is reduced and the envelope can accommodate the enclosure. 
     4) When setting insertion of enclosures including a folded sheet in envelopes, the user need not set whether the additional squeezing is performed or the number of times the additional squeezing is performed. 
     5) The system can automatically set whether the additional squeezing is performed and the number of times the additional squeezing is performed. Thus, functionality as well as usability of the system can be improved. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.