JP5806750B2 - Post-processing apparatus and image forming system - Google Patents

Description

  The present invention relates to a post-processing apparatus having a buffer function for retaining a subsequent sheet while post-processing is performed on an image-formed sheet.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine, a system has been provided in which a post-processing device (finisher) is connected downstream of the image forming apparatus in the sheet conveying direction to perform post-processing such as stapling or punching. . The sheets received from the image forming apparatus are sequentially stacked on an intermediate tray (hereinafter, processing tray) provided upstream of the stacking tray. After all the sheets constituting the booklet are stacked, the sheets are placed on the processing tray. A post-processing apparatus that performs post-processing such as stapling has been proposed. The sheet bundle that has been post-processed on the processing tray is discharged from the processing tray to the stacking tray.

  In addition, while performing post-processing (staple processing, etc.) on the preceding sheet bundle in the processing tray, the number of succeeding sheets is counted on the upstream side of the processing tray so that the subsequent sheet does not collide with the sheet bundle being post-processed. There is a device for superimposing sheets (hereinafter referred to as buffering) (Patent Document 1). In Patent Document 1, such a configuration prevents a reduction in image formation productivity during post-processing. Specifically, in Patent Document 1, a sheet is wound around a winding roller provided upstream of a processing tray that performs post-processing, the roller is stopped and waited, and the roller is again turned on when a subsequent sheet arrives. By driving, the wound sheet and the succeeding sheet are overlapped. By superimposing a predetermined number of sheets to be the subsequent sheet bundle, the sheets of the subsequent sheet bundle are prevented from being discharged to the processing tray during the post-processing of the preceding sheet bundle on the processing tray. Accordingly, the sheet bundle can be post-processed without increasing the sheet conveyance interval in the image forming apparatus, and the productivity of the image forming apparatus is not reduced.

  In addition, there has been proposed an apparatus that prohibits a buffering operation for a specific material or restricts the number of sheets stacked in the buffering operation (Patent Document 2). In Patent Document 2, by prohibiting or limiting the buffering operation of special sheets such as index paper, thick paper, and thin paper, special sheets are forcibly buffered, causing scratches and wrinkles on the sheets, and jamming. Is prevented.

JP 9-48545 A JP 2001-97631 A

  In a conventional post-processing apparatus, when a sheet for which buffering is prohibited as described in Patent Document 2 is conveyed after a sheet that can be buffered, post-processing of a sheet bundle that precedes the processing tray once is performed. After waiting for completion, the buffered sheet is discharged to the processing tray. Then, the image forming apparatus main body discharges the sheet for which buffering is prohibited to the post-processing apparatus with a wider sheet interval from the preceding sheet than when buffering is performed. Thereafter, the post-processing apparatus performs a buffering canceling process of discharging the succeeding sheet, for which buffering is prohibited, to the processing tray without buffering.

  As a configuration for performing buffering, there is a configuration in which buffering is performed by performing switchback in a conveyance path that has been conventionally proposed, in addition to a configuration in which buffering is performed by winding as in Patent Document 2. In the switchback, after the sheet trailing edge passes through the branching point with the conveying path that performs buffering on the conveying path, the sheet is conveyed in the reverse direction and guided toward the conveying path that performs buffering by a path branching flapper, Wait until the next sheet arrives. In the apparatus having the configuration as in the above two examples, the sheet must be conveyed by a predetermined distance for buffering, and a predetermined time is required for the buffering itself. Therefore, depending on the post-processing time of the preceding sheet bundle and the productivity of the upstream image forming apparatus main body, the sheets are conveyed one by one to the processing tray without performing any buffering. There is a problem that productivity is increased when the sheet is controlled to be discharged from the main body of the image forming apparatus with a sheet interval in advance corresponding to the processing time.

  In recent years, in the POD market, print jobs in which various types of sheets are mixed in such a single bundle are frequently executed by form printing or transaction printing. In the POD market, high productivity is required. However, when performing the above-mentioned operations, the cancellation of the buffering process occurs in the entire image forming apparatus to which the conventional post-processing apparatus is connected. Productivity is reduced due to the factor of the processing equipment.

The object of the present invention has been made in view of the above problems, and has the following configuration in order to achieve the above object. That is, a post-processing apparatus that performs post-processing on a plurality of sheets received from the image forming unit, and a sheet conveying unit that conveys the received sheets, and a sheet on which the plurality of sheets conveyed by the sheet conveying unit are stacked A stacking unit, a post-processing unit that performs post-processing on a sheet bundle including a plurality of sheets stacked by the sheet stacking unit, and is disposed upstream of the sheet stacking unit, and retains one or more sheets. A buffer unit that performs a buffer process of superimposing the staying sheet and a subsequent sheet; an acquisition unit that acquires sheet information for determining whether or not the buffer process by the buffer unit is prohibited; Buffer processing is prohibited for the first sheet of a predetermined number of sheets subsequent to the sheet bundle to be post-processed by the processing means. If a sheet not, and a control means for said predetermined number of based on the second sheet information of the sheet of the sheet, and controls whether to stay with the first sheet.

  When setting whether or not to buffer sheets for the preceding sheet bundle, not only is it possible to buffer subsequent sheets in the sheet bundle being post-processed, but also about subsequent sheets Also determine whether buffering is possible and set buffering. As a result, it is possible to eliminate the cancellation of the buffering process and to prevent a decrease in productivity in a job in which various sheets are mixedly loaded.

1 is an overall configuration diagram of a system. The block diagram of the whole controller of a system. Explanatory drawing of an operation display part. Finisher sectional view. Finisher block diagram. Explanatory drawing of non-sort operation | movement. Explanatory drawing of sort operation | movement. Explanatory drawing of the sort operation | movement after the 2nd copy. Explanatory drawing of a staple sort operation | movement. Explanatory drawing of a staple mode setting. The flowchart of the buffer control operation | movement of CPU952. The flowchart of sheet | seat space | interval control of CPU901. Explanatory drawing of communication data. The flowchart at the time of sheet | seat information notification reception of CPU952. Explanatory drawing of post-processing time acquisition table T1. FIG. 10 is a flowchart for determining a buffer mode of a CPU 952. Explanatory drawing of buffer number counter acquisition table T2. FIG. 10 is a flowchart for determining a buffer mode of a CPU 952. FIG. 9 is a flowchart of buffer availability determination by a CPU 952. The figure for demonstrating the space | interval at the time of buffer control.

<First embodiment>
Embodiments of the present invention will be described below with reference to the drawings.

(Overall configuration / basic operation)
FIG. 1 is an overall configuration diagram showing the main configuration of an embodiment of an image forming apparatus according to the present invention. As shown in FIG. 1, the image forming apparatus includes an image forming apparatus main body 10 that is an image forming unit that performs an image forming process and a finisher 500 that is a post-processing unit. The image forming apparatus main body 10 is an image for reading a document image. A reader 200 and a printer 300 are provided. The finisher 500 is mounted so as to receive a document sent from the image forming apparatus main body 10.

  A document feeder 100 is mounted on the image reader 200. The document feeder 100 feeds documents set upward on the document tray one sheet at a time in order from the first page, and flows on the platen glass 102 from the left through a curved path, passing through the reading position. The paper is conveyed to the right and then discharged toward the external paper discharge tray 112.

  When the original passes through the sink reading position on the platen glass 102 from the left to the right, the original image is read by the scanner unit 104 held at a position corresponding to the sink reading position. This reading method is generally referred to as document scanning. Specifically, when the original passes through the reading position, the original reading surface is irradiated with the light from the lamp 103 of the scanner unit 104, and the reflected light from the original passes through the mirrors 105, 106, and 107 to the lens. To 108. The light that has passed through the lens 108 forms an image on the imaging surface of the image sensor 109.

  By transporting the document so that it passes through the flow reading position from the left to the right in this way, the document reading scan in which the direction orthogonal to the document transport direction is the main scanning direction and the transport direction is the sub-scanning direction is performed. Done. That is, the entire original image is read by conveying the original in the sub-scanning direction while reading the original image by the image sensor 109 line by line in the main scanning direction when the original passes the flow reading position. The optically read image is converted into image data by the image sensor 109 and output. Image data output from the image sensor 109 is input as a video signal to the exposure control unit 110 of the printer 300 after predetermined processing is performed in an image signal control unit 922 described later.

  It is also possible to read the original by conveying the original onto the platen glass 102 by the original feeder 100 and stopping it at a predetermined position, and scanning the scanner unit 104 from left to right in this state. Is omitted.

  The exposure control unit 110 of the printer 300 modulates and outputs a laser beam based on the input video signal, and the laser beam is irradiated onto the photosensitive drum 111 while being scanned by the polygon mirror 110a. An electrostatic latent image corresponding to the scanned laser beam is formed on the photosensitive drum 111.

  The electrostatic latent image on the photosensitive drum 111 is visualized as a developer image by the developer supplied from the developing device 113. In addition, at a timing synchronized with the start of laser light irradiation, a sheet is fed from each of the cassettes 114 and 115, the manual paper feeding unit 125, or the double-sided conveyance path 124.

  The fed sheet is stopped once it reaches the roller 119. At the time of this stop, the downstream apparatus (here, finisher 500) is notified of the sheet information of the stopped sheet via a communication means described later. The sheet information includes paper size, basis weight, sheet material type, post-processing mode, and the like. Although details will be described later, when the CPU 952 of the finisher 500 receives notification of sheet information from the CPU 901 of the image forming apparatus body 10, the CPU 952 compares the paper size and the post-processing mode of the temporarily stopped sheet and the sheet conveyed immediately before. To do. Then, the post-processing time required in the own apparatus is calculated, and the sheet interval from the previous sheet is determined. The sheet interval information is notified to the CPU 901 of the image forming apparatus main body 10. The CPU 901 of the image forming apparatus main body 10 stops the sheet on the roller 119 until the sheet interval received from the CPU 952 of the finisher 500 elapses. When the stop time elapses, the sheet is conveyed between the photosensitive drum 111 and the transfer unit 116. The developer image formed on the photosensitive drum 111 is transferred onto a sheet fed by the transfer unit 116.

  The sheet on which the developer image has been transferred is conveyed to the fixing unit 117, and the fixing unit 117 fixes the developer image on the sheet by heat-pressing the sheet. The sheet that has passed through the fixing unit 117 is discharged from the printer 300 to the outside (finisher 500) through the flapper 121 and the discharge roller 118.

  Here, when the sheet is discharged with its image forming surface facing downward (face-down), the sheet that has passed through the fixing unit 117 is once guided into the reversing path 122 by the switching operation of the flapper 121. Then, after the trailing edge of the sheet passes through the flapper 121, the sheet is switched back and discharged from the printer 300 by the discharge roller 118. Hereinafter, this form of paper discharge is called reverse paper discharge. This reverse paper discharge is performed when forming an image sequentially from the first page, such as when forming an image read using the document feeder 100 or when forming an image output from a computer. The sheet order after discharge is the correct page order.

  Further, when double-sided recording for forming an image on both sides of the sheet is set, the sheet is conveyed to the double-sided conveyance path 124 after being guided to the reverse path 122 by the switching operation of the flapper 121. Thereafter, the sheet guided to the duplex conveyance path 124 is fed again between the photosensitive drum 111 and the transfer unit 116 at the timing described above. The sheet discharged from the printer 300 is sent to the finisher 500. The finisher 500 performs each process such as a binding process.

(System block diagram)
Next, the configuration of a controller that controls the entire image forming apparatus will be described with reference to FIG. FIG. 2 is an overall block diagram showing a configuration of a controller that controls the entire image forming apparatus of FIG.

  As shown in FIG. 2, the controller includes a CPU circuit unit 900. The CPU circuit unit 900 includes a CPU 901, a ROM 902, and a RAM 903, and comprehensively controls each block shown in FIG. 2 by a control program stored in the ROM 902. The RAM 903 temporarily stores control data and is used as a work area for arithmetic processing associated with control.

  The document feeder control unit 911 drives and controls the document feeder 100 based on an instruction from the CPU circuit unit 900. The image reader control unit 921 performs drive control on the scanner unit 104 and the image sensor 109 described above, and transfers an analog image signal output from the image sensor 109 to the image signal control unit 922.

  The image signal control unit 922 performs each process after converting the analog image signal from the image sensor 109 into a digital signal, converts the digital signal into a video signal, and outputs the video signal to the printer control unit 931. In addition, the digital image signal input from the computer 905 via the external I / F 904 is subjected to various processes, and the digital image signal is converted into a video signal and output to the printer control unit 931. The processing operation by the image signal control unit 922 is controlled by the CPU circuit unit 900. The printer control unit 931 drives the above-described exposure control unit 110 based on the input video signal.

  The operation display device controller 941 exchanges information between the operation display device 400 and the CPU circuit unit 900. The operation display device 400 includes a plurality of keys for setting various functions relating to image formation, a display unit for displaying information indicating a setting state, and the like. The operation display device 400 outputs a key signal corresponding to the operation of each key to the CPU circuit unit 900 and displays corresponding information on the display unit based on the signal from the CPU circuit unit 900.

  The finisher control unit 951 is mounted on the finisher 500, and performs drive control of the entire finisher by exchanging information with the CPU circuit unit 900. This control content will be described later.

(Operation display device)
FIG. 3 is a diagram showing an operation display device 400 in the image forming apparatus of FIG. The operation display device 400 includes a start key 402 for starting an image forming operation, a stop key 403 for interrupting the image forming operation, ten keys 404 to 412 and 414 for setting numerical values, an ID key 413, and a clear key. 415, a reset key 416, a user mode key (not shown) for setting various devices, and the like are arranged. In addition, a display unit 420 having a touch panel formed on the surface is arranged, and soft keys can be created on the screen.

  The image forming apparatus has various processing modes such as non-sorting, sorting, stapling sorting (binding mode), and bookbinding mode as post-processing modes. Such setting of the processing mode is performed by an input operation from the operation display device 400. For example, when setting the post-processing mode, when the “finish” soft key is selected on the initial screen shown in FIG. 3, a menu selection screen is displayed on the display unit 420, and the processing mode is selected using this menu selection screen. Is set.

(Finisher)
Next, the configuration of the finisher 500 will be described with reference to FIG. FIG. 4 is a configuration diagram of the finisher 500 of FIG. The finisher 500 sequentially takes in the sheets discharged from the image forming apparatus main body 10, aligns a plurality of fetched sheets and bundles them into one bundle, stapling processing that binds the rear end of the bundled sheet bundle with staples, and sort Each sheet post-processing such as processing and non-sort processing is performed.

  The finisher 500 takes in the sheet discharged from the image forming apparatus main body 10 by an inlet roller pair 502 driven by an inlet motor M1 described later. The sheet taken inside by the inlet roller pair 502 is sent toward the buffer roller 505 via a pair of conveying rollers 503 and 504 that are also driven by an inlet motor M1 described later. A conveyance sensor 531 is provided in the middle of the conveyance path between the entrance roller pair 502 and the conveyance roller pair 503 to detect the passage of the sheet.

  The buffer roller 505 is driven by a buffer motor M2 described later. The buffer roller 505 is a roller capable of laminating a predetermined number of sheets fed via the conveying roller pairs 503 and 504 on the outer periphery thereof and winding the sheet on the outer periphery of the roller. Wound by 512, 513, 514. The wound sheet is conveyed in the rotation direction of the buffer roller 505. A switching flapper 511 driven by a solenoid S1 described later is disposed between the pressing rollers 513 and 514, and a switching flapper 510 driven by a solenoid S2 described later is disposed downstream of the pressing roller 514. ing. The buffer roller 505 is located between the conveyance path from the position where the sheet is received from the image forming apparatus main body 10 to the processing tray 630. That is, it is located upstream of the processing tray 630 from which the sheet is discharged.

  The switching flapper 511 is a flapper for separating the sheet wound around the buffer roller 505 from the buffer roller 505 and guiding it to the non-sort path 521 or the sort path 522. The switching flapper 510 is a flapper for separating the sheet wound around the buffer roller 505 from the buffer roller 505 and guiding the sheet to the sort path 522 or the sheet wound around the buffer roller 505 to the buffer path 523.

  When the sheet wound around the buffer roller 505 is guided to the non-sort path 521, the switching flapper 511 is operated to peel the sheet wound from the buffer roller 505 and guided to the non-sort path 521. The sheet guided to the non-sort path 521 is discharged onto the sample tray 701 via a conveyance roller pair 509 driven by a discharge motor M3 described later. A conveyance sensor 533 is provided in the middle of the non-sort path 521.

  When the sheet wound around the buffer roller 505 is guided to the buffer path 523, the switching flapper 510 and the switching flapper 511 do not operate, and the sheet is sent to the buffer path 523 while being wound around the buffer roller 505. A conveyance sensor 532 for detecting a sheet on the buffer path 523 is provided in the middle of the buffer path 523. When the sheet wound around the buffer roller 505 is guided to the sort path 522, the switching flapper 511 is not operated and the switching flapper 510 operates to peel the sheet wound from the buffer roller 505, and this sheet is guided to the sort path 522. .

  The sheet guided to the sort path 522 is discharged onto a processing tray 630 serving as a sheet stacking unit via a pair of conveyance rollers 507 and 509 driven by a discharge motor M3 described later. The sheets discharged in a bundle on the processing tray 630 are moved to the rear end side in the conveying direction by a knurled belt 661 driven in synchronization with the conveying roller pair 509 and a paddle 660 driven by a paddle motor M7 described later. It is pulled back. The pulled back sheet hits the stopper 631 and stops.

  The alignment members 641 provided on the front side and the back side on the processing tray 630 are respectively moved in a direction perpendicular to the sheet conveying direction by a front alignment motor M5 and a rear alignment motor M6 described later. After the sheets stacked on the processing tray 630 are aligned by the alignment member 641 and subjected to a stapling process as necessary, the sheets are stacked by a discharge roller pair 680 including discharge rollers 680a and 680b. It is discharged onto the tray 700.

  The discharge roller pair 680 is driven by a bundle discharge motor M4 described later, and the discharge roller 680b is supported by the swing guide 650. The swing guide 650 is driven by a swing motor M8, which will be described later, and swings so that the discharge roller 680b contacts the uppermost sheet on the processing tray 630. When the discharge roller 680b is in contact with the uppermost sheet on the processing tray 630, the discharge roller 680b discharges the sheet bundle on the processing tray 630 toward the stack tray 700 in cooperation with the discharge roller 680a. Is possible.

  The appearance tray 670 is driven by an appearance tray motor M11 described later. When stacking sheets on the processing tray 630, the protruding / extracting tray 670 is protruded upward to prevent the sheet P discharged by the conveying roller pair 507 from hanging down or returning, and on the processing tray 630. Sheet alignment is improved.

  The stack tray 700 can be moved up and down by a tray lifting motor M12 described later. A paper surface detection sensor 540 described later can detect the top surface of the tray or the sheet on the tray. By controlling the tray lifting / lowering motor M12 in accordance with the input from the paper surface detection sensor 540, the above-mentioned uppermost surface is always controlled to be a fixed position. The sample tray 701 is not movable up and down like the stack tray 700, and is fixed at the position shown in FIG.

  Stapling processing is performed by the stapler 601. The stapler 601 is driven by a staple motor M9, which will be described later, and performs a binding process. The stapler 601 binds the sheet bundle stacked on the processing tray 630 at the last position (rear end) of the sheet in the sheet conveyance direction.

  The stapler 601 is configured to be movable in a direction perpendicular to the transport direction along the outer periphery of the processing tray 630 by a later-described stapler moving motor M10. The stapler 601 is moved to a position according to the binding position specified by the user. , Move in advance before the sheet arrives.

(Finisher block diagram)
Next, the configuration of the finisher control unit 951 that drives and controls the finisher 500 will be described with reference to FIG. FIG. 5 is a block diagram showing a configuration of the finisher control unit 951 in FIG. As shown in FIG. 5, the finisher control unit 951 includes a CPU 952, a ROM 953, a RAM 954, and the like. It communicates with the CPU circuit unit 900 provided on the image forming apparatus main body 10 side via a communication IC (not shown) to exchange data such as job information and sheet delivery notification. Based on instructions from the CPU circuit unit 900, various programs stored in the ROM 953 are executed to control the drive of the finisher 500.

  Various input / outputs provided in the finisher 500 will be described. The finisher 500 is provided with the inlet motor M1, the buffer motor M2, the paper discharge motor M3, the solenoid S1, the solenoid S2, and the conveyance sensors 531 to 534 for conveying the sheet described above. Further, in order to perform post-processing such as sorting and stapling as described above, the bundle discharge motor M4, the front alignment motor M5, the rear alignment motor M6, the paddle motor M7, the swing motor M8, the staple motor M9, the stapler moving motor M10, An in / out tray motor M11, a tray lifting / lowering motor M12, and a paper surface detection sensor 540 are provided. Note that the sheet buffer device is realized by buffering the sheet in the finisher in the present embodiment.

(Sheet flow)
Next, the sheet flow in the finisher 500 will be described in accordance with the non-sort mode, the sword mode, and the staple mode.

(Non-sort operation)
First, the flow of sheets in the non-sort mode will be described with reference to FIGS. When the user selects “finish” as a soft key on the initial screen shown in FIG. 3 on the operation display device 400 of the image forming apparatus main body 10, a finish menu selection screen 1001 as shown in FIG. Is displayed. Here, when the user selects none of the soft keys in FIG. 10A and finishes the selection of finishing, the non-sort mode is set.

  When the user designates the non-sort mode and submits a job, the CPU 901 of the CPU circuit unit 900 selects the non-sort mode in addition to information such as the sheet size to the CPU 952 of the finisher control unit 951. The information regarding the job is notified in advance.

  When the sheet P is discharged from the image forming apparatus main body 10 to the finisher 500, the CPU 901 of the CPU circuit unit 900 notifies the CPU 952 of the finisher control unit 951 that the sheet delivery is started. Hereinafter, various input / output controls in the finisher 500 of the CPU 952 will be described.

  Upon receiving the notification of the start of sheet delivery, the CPU 952 drives the entrance motor M1, the buffer motor M2, and the paper discharge motor M3, and as shown in FIG. 6, the entrance roller pair 502, the transport roller pairs 503 and 504, and the buffer roller 505. The conveyance roller pair 509 is driven to rotate. The sheet P discharged from the image forming apparatus main body 10 is taken into the finisher 500 and conveyed.

  The switching flapper 511 is rotationally driven to the position illustrated in FIG. 6 by the solenoid S 1, and the sheet P is guided to the non-sort path 521 without being buffered by the buffer roller 505. When the trailing edge of the sheet P is detected by the transport sensor 533, the speed of the paper discharge motor M3 is changed, the transport roller pair 509 is rotated at a speed suitable for stacking, and the sheet P is discharged onto the sample tray 701.

(Sort mode operation)
Next, the flow of sheets in the sort mode will be described with reference to the flowcharts of FIGS. 7, 10, and 11. When finishing selection is completed with the “sort” soft key 1002 selected on the finishing menu selection screen shown in FIG. 10A, the sorting mode is set. When a sort mode is designated by the user and a job is submitted, the CPU 901 of the CPU circuit unit 900 notifies the CPU 952 of the finisher control unit 951 that the sort mode has been selected, as in the non-sort mode. .

  Hereinafter, the operation in the sort mode when the number of sheets constituting one “part” which is one sheet bundle is three will be described. First, a case where the buffer mode of each sheet is set to “pass” by setting a buffering operation mode (hereinafter referred to as buffer mode) by the CPU 952 described later will be described. When the sheet P is discharged from the image forming apparatus main body 10 to the finisher 500, the CPU 901 of the CPU circuit unit 900 notifies the CPU 952 of the finisher control unit 951 that the sheet delivery is started. Hereinafter, various input / output controls in the finisher 500 of the CPU 952 will be described.

  The CPU 952 that has received the notification of the start of sheet delivery drives the inlet motor M1 and the buffer motor M2, thereby moving the inlet roller pair 502, the conveying roller pairs 503 and 504, and the buffer roller 505 as shown in FIG. Rotation drive. The sheet P discharged from the image forming apparatus main body 10 is taken into the finisher 500 and conveyed. At this time, each sheet is conveyed without being buffered by the buffer roller 505.

  FIG. 11 is a flowchart showing the flow of buffering operation (hereinafter referred to as buffer operation) by the CPU 952. After detecting the ON of the conveyance sensor 531 (S101), the CPU 952 conveys the sheet P by the entrance motor M1 for a predetermined distance (S102), and proceeds to S103.

  If the buffer mode is “pass” in S103, the switching flapper 510 is positioned so as to guide the sheet to the sort path 522 side as shown in FIG. 7A in S104. If the job is continued (NO in S113), the process returns to S101, and the sheet is continuously retained until the next sheet arrives, and waits for the sheet to be guided.

  The switching flapper 511 is also positioned at the position shown in FIG. 7A, and the sheet P1 is guided to the sort path 522 side. The sheet P guided to the sort path 522 is discharged to the processing tray 630 by the conveyance roller pair 506 and 507. The CPU 952 detects the trailing edge of the sheet P by the conveyance sensor 534 and then detects that the sheet P has advanced a predetermined distance, whereby the sheet P1 is discharged to the processing tray 630. Is detected.

  The sheet P1 discharged onto the processing tray 630 starts to move on the processing tray 630 toward the stopper 631 by its own weight. The movement of the sheet P is configured to be assisted by an assisting member such as a paddle 660 or a knurled belt 661. When the rear end of the sheet P1 comes into contact with the stopper 631 and the sheet P1 stops, the alignment of the sheets discharged by the alignment member 641 is performed. Similarly, the sheets P2 and P3 are stacked on the processing tray 630.

  Thereafter, as shown in FIG. 7B, the swing motor M8 is driven to lower the swing guide 650, and the sheet bundle P is sandwiched between the discharge rollers 680a and 680b to perform the bundle discharge operation. Is discharged to the stack tray 700. Each sheet bundle is a bundle that is stacked upward in the page order with the first page with the image forming surface facing downward as the bottom, and is sequentially stacked on the stack tray 700 (FIG. 7C).

  Next, the buffer operation when the buffer mode of the sheet P1 and the sheet P2 is set to “buffer” and the buffer mode of the sheet P3, which is the succeeding sheet of the sheet P2, is set to “final paper” by the setting of the buffer mode. 8 and will be described with reference to the flowchart of FIG.

  In FIG. 11, when the CPU 952 detects that the conveyance sensor 531 is turned on for the sheet P1 (S101) and conveys the sheet P1 by a predetermined distance (S102), the process proceeds to S103.

  In S103, the buffer mode of the sheet P1 is determined. Since the buffer mode is “buffer”, the process proceeds to S107. Since sheet P1 is the first buffer (YES in S107), switching flapper 510 is switched to the buffer path 523 side as shown in FIG. 8A (S108). When the conveyance sensor 532 is turned on (S110) and the sheet P1 is conveyed by a predetermined distance by the buffer motor M2 (S111), the buffer motor is stopped (S112). Accordingly, as shown in FIG. 8B, the sheet P1 is stopped in a state where it is wound around the buffer roller 505 in order to perform buffering by overlapping with the subsequent sheet P2, and until the sheet P2 arrives. stand by. That is, the sheet stays on the predetermined conveyance path until one or more subsequent sheets arrive.

  Subsequently, the CPU 952 similarly detects the ON of the conveyance sensor 531 for the sheet P2 (S101), and when the sheet P2 is conveyed by a predetermined distance by the entrance motor M1 (S102), the process proceeds to S103.

  Since the buffer mode of the sheet P2 is “buffer” as well as the sheet P1, the process proceeds to S107. Since the sheet P2 is the second buffer in S107 (NO in S107), the process proceeds to S109. In step S109, the buffer motor M2 is activated to rotate the buffer roller 505, so that the sheet P1 and the sheet P2 are overlapped on the buffer roller 505. When the conveyance sensor 532 is turned on (S110) and the sheet P2 is conveyed by a predetermined distance by the buffer motor M2 (S111), the buffer motor is stopped (S112). As a result, the sheet P1 and the sheet P2 are stopped while being wound around the buffer roller 505, as shown in FIG.

  Next, the flow of the sheet P3 whose buffer mode is “final paper” will be described. When the CPU 952 detects that the conveyance sensor 531 is turned on by the sheet P3 (S101), the sheet P3 is conveyed by a predetermined distance by the entrance motor M1 (S102). Thereafter, as shown in FIG. 8D, the switching flapper 510 is switched so that the sheet is guided to the sort path 522 (S105). Then, by starting the buffer motor M2 in S106, the buffer roller 505 starts rotating, and the next sheet P3 is overlapped with the sheet P1 and the sheet P2 released from the winding of the buffer roller 505. Then, the sheet is conveyed to the sort path 522 as shown in FIG.

  At this time, the operation of discharging the bundle of sheet bundles P stacked on the processing tray 630 has been completed, and the processing tray 630 is ready to accept sheets. Thereafter, the sheet bundle P is discharged onto the processing tray 630. As described above, for each bundle of sheets, the first sheet to the predetermined number of sheets (in this processing flow, the sheet one sheet before the last sheet) are temporarily buffered, and after the last sheet arrives, A discharge operation is performed on a set of sheet bundles including one or more ringed sheets and a final sheet.

  When there are fourth and subsequent sheets, they are discharged onto the processing tray 630 through the sort path 522 in the same manner as the sheet discharging operation for the first set of bundles. For the next and subsequent sheet bundles, after the second sheet bundle is discharged to the stack tray 700, the same operation is repeated, and a predetermined set number of sheet bundles are stacked on the stack tray 700. .

(Stapling mode operation)
Next, the flow of sheets in the staple mode will be described with reference to FIGS. When the “staple” soft key 1003 is pressed on the finishing menu selection screen as shown in FIG. 10A, a stapling setting screen 1010 shown in FIG. 10B is displayed on the display unit 420. In this display, the user can select a binding method such as corner binding or two-point binding.

  When the staple mode is set by the user, as in the sort mode, the CPU 901 of the CPU circuit unit 900 notifies the CPU 952 of the finisher control unit 951 that the staple mode has been selected. The CPU 952 controls various inputs and outputs in the finisher 500 to sequentially stack sheets on the processing tray 630 in the same manner as the sheet flow in the sort mode (FIG. 9A).

  As shown in FIG. 9B, after all the sheets constituting one booklet are stacked on the processing tray 630 and the alignment process by the alignment member 641 for the last stacked sheet is completed, the staple motor M9 is turned on. The sheet bundle is bound by the stapler 601. As shown in FIG. 9C, the sheet bundle P is bound by the needle H on the rear end side with respect to the transport direction.

  After the binding operation by the stapler 601 is completed, the swing guide 650 is lowered by driving the swing motor M8, the sheet bundle P is sandwiched between the discharge rollers 680a and 680b, and the bundle discharge operation is performed. It is discharged to the tray 700. (FIG. 9 (d)). As in the sort mode operation, while the stapling process is performed on the sheet bundle P in the processing tray 630, the subsequent sheet is wound by the buffer roller 505 (FIG. 9D). As described above, by performing buffering on the next sheet bundle during post-processing on the previous sheet bundle, it is possible to perform stapling (post-processing) without reducing productivity.

(Sheet information notification and sheet interval control)
Control of the sheet discharge interval from the image forming apparatus main body 10 by the CPU 901 of the image forming apparatus main body 10 will be described with reference to the flowchart of FIG. 12 and FIG. As described above, when the sheet fed from the cassette 114 or the like reaches the roller 119, it is temporarily stopped by the printer control unit 931 in response to a command from the CPU 901. FIG. 12 is a flowchart showing a flow in which the CPU 901 determines the sheet interval when the roller 119 stops. Hereinafter, unless otherwise specified, processing by the CPU 901 is described. For convenience, the Nth sheet is referred to as a sheet N and the (N + 1) th sheet is referred to as a sheet N + 1 in successive sheets.

  First, in S1001, the CPU 901 of the image forming apparatus main body 10 notifies the CPU 952 of the finisher 500 of the sheet information of the sheet N via a communication IC (not shown). The format of this sheet information notification is shown in FIG. In the sheet information format, sheet information is defined for each sheet. In addition to the information on the sheet N, the format shown here also includes information necessary for determining whether or not a buffer can be described later regarding the sheet N + 1 subsequent to the sheet N. In the present embodiment, the sheet length, sheet width, basis weight, sheet material type, and post-processing mode (post-processing type) of the sheet N + 1 are attached. The “standard paper interval” in the sheet information notification is a time calculated from the productivity in the image forming apparatus main body 10. For example, when images are formed at regular intervals at a rate of 120 sheets per minute, the standard inter-sheet time is 500 [msec]. The standard inter-sheet time is calculated by the CPU 901 and attached to the sheet information notification. This information may be defined in advance according to the specifications of the apparatus.

  In step S <b> 1002, the CPU 901 waits until the sheet interval information of the sheet N is received from the CPU 952 of the finisher 500. The transmission of sheet interval information by the CPU 952 will be described later. If sheet interval information is received from the CPU 952, the process advances to step S1003. Note that the format of the sheet interval information received from the CPU 952 is shown in FIG.

At S1003, CPU 901 substitutes "between required paper time" of the sheet interval information notification received from the CPU952 to the variable _{T D} disposed in RAM 903. At S1004, CPU 901 is a sheet N will determine whether the top sheet of the job, (YES at S1004) if the first sheet of the job, to store the time stamp of the time variable _{T P} on RAM 903 ( S1005). In the case of the first sheet of the job, there is no sheet space from the previous sheet. Instead, the time finisher 500 takes to accept preparation (= _{T D)} only, sets _{T P} to wait.

In S1006, CPU901 is, to save the current time stamp in the time variable _{T N} on the RAM903. Then, the CPU 901 waits until T _N ≧ T _P + T _D (S1007). T _P + _{T D} from the start the time _{T P} conveyance before sheet N-1 by the roller 119, indicating the time that elapses between the required paper time _{T D.} That is, when _{T N} ≧ _T P + _T roller 119 waits until the _D starts conveying a sheet N-1, a sheet interval time of the sheet N is ensured above _{T D.}

Then, _{T N} ≧ _T P + _T When a _D (YES at S1007), after storing the timestamp of that point in time _{T P} (S1008), the processing proceeds to S1009. In step S <b> 1009, the CPU 901 requests the printer control unit 931 to resume conveyance of the sheet N, and the printer control unit 931 resumes conveyance of the sheet N by the roller 119.

(Buffer information setting)
Next, regarding the flow in which the CPU 952 of the finisher 500 notifies the CPU 901 of the sheet interval information notification based on the content of the sheet information notification of the sheet N received from the CPU 901 of the image forming apparatus body 10, FIG. 14 and FIG. Will be described. Hereinafter, unless otherwise specified, processing performed by the CPU 952 is described.

First, the CPU 952 waits until the sheet information of the sheet N is notified from the CPU 901 in S1101, and when receiving the sheet information notification, the CPU 952 stores the sheet information in the RAM 954, and proceeds to S1102. In step S1102, the standard inter-sheet time information of the sheet information of the received sheet _N is substituted into a variable IN arranged on the RAM 954.

In S1103, the CPU 952 clears the necessary inter-paper time D, which is a variable arranged on the RAM 954, to 0, and proceeds to S1104. If it is determined in step S1104 that the sheet N is the first sheet of “copy”, which is a product unit (YES in step S1104), the process advances to step S1105. At S1105, the sheet information of the sheet N and sheet N-1, with reference to the post-processing time table T1 shown in FIG. 15, it substitutes the post-processing time acquired from the table T1 to the variable _{T B} on RAM 954.

The post-processing time table T1 in FIG. 15 is a time required between the sheets N-1 and N, that is, a time required for the post-processing of the sheet N-1, and a preparation for performing the post-processing of the sheet N (for example, This is a table for obtaining the total time of movement of the stapler 601 to the initial position. For example, the sheet N-1, when sheet N both discharge destination is the post-processing mode in "Tray 700" is "1 places binding (forward)", the _{T B,} is substituted is 1200 (in msec) . Also, for example, if the sheet N is the head of the job and there is no sheet N-1, the discharge destination of the sheet N is “tray 700”, and the mode is “bind”, the preparation time for receiving the sheet N is as follows: T _B in 2000 (in units of msec) is assigned. Note that the post-processing time table T1 is defined in advance according to the specifications of the apparatus.

After obtaining a post-processing time in S1105, the CPU 952 proceeds to S1106, performs processing _{F A.} Against the top sheet of the process F _A "part" is a process for performing setting of the buffer mode, a calculation between required paper time, the details will be described later. Here, the necessary inter-sheet time refers to the necessary inter-sheet time between the sheets N-1 and N, and the buffer operation of the sheet N in addition to the post-processing contents of the sheets N-1 and N. The required time varies depending on the presence or absence.

In S1104, the CPU 952 is, if the sheet N is the top sheet of the "parts" (NO in S1104), the process proceeds to S1107, performs processing _{F B.} If the process F _B is other than the leading paper "parts" is a processing to calculate the setting of the buffer mode and inter required paper time, the details will be described later.

After S1106 of the process _{F A} or S1107 of _{F B,} in S1108, the CPU 952, the process _{F A} or process _F between required paper time D calculated in _B is equal to or greater than _{I N.} If inter required paper time D is greater than _{I N} (NO in S1108), the processing proceeds to S1109. If inter required paper time D is smaller than _{I N} (YES in S1108), assigns the _{I N} to D (S1111), the processing proceeds to S1109.

  In step S1109, the CPU 952 sets the value of D to the required sheet interval time for notification of sheet interval information, and transmits the sheet interval information notification to the CPU 901 via a communication IC (not shown). Then, the process proceeds to S1110, and if the job is continued (NO in S1110), the process returns to S1101.

(Buffer mode setting / Calculation of required paper time: First paper)
Next, the flow configuration and the buffer mode, calculation between required paper time at the top sheet of the "parts" in the process F _A by the CPU 952, will be described with reference to a flowchart and Figure 17 of FIG. 16. Hereinafter, processing by the CPU 952 will be described unless otherwise specified.

First, the CPU 952, at S1201, compares the post-processing time _{T B} obtained in S1105 shown in FIG. 14 described above, also the inter-acquired standard paper time S1102 shown in FIG. 14 described above _{I N.} Is larger post-processing time _{T B} is to S1202 (YES at S1201), the process proceeds otherwise to S1214 (NO in S1201).

  In S1202, the value acquired from the buffer number acquisition table T2 shown in FIG. 17 is set in the variable buffer number counter C on the RAM 954 according to the discharge destination and the post-processing mode. For example, C = 0 in the sort mode for discharging to the sample tray 701, and C = 2 in the binding mode for discharging to the stack tray 700. This is information on the maximum number of sheets to be overlapped on the sheet N. When C = 0, no overlap is performed, and when C = 2, at most two sheets are wound around the buffer roller. This means that a maximum of three sheets including “” are overlapped and conveyed. Here, it is assumed that the buffer number acquisition table T2 is defined in advance. Further, the information defined in the buffer number acquisition table T2 is not limited to that shown in FIG. 17, and more values may be defined corresponding to the functions of the apparatus and other post-processing modes.

If the buffer counter C = 0 in S1203 (YES in S1203), the process proceeds to S1214, and if it is any other value (NO in S1203), the process proceeds to S1204. In S1204, the CPU 952 is cleared to zero a variable _{T N} arranged in RAM 954. The inter-bundle time _TN , which is a variable, indicates how long the inter-bundle time between the sheet before the buffer process is performed in the finisher 500 and the buffer-processed sheet bundle when superposition is performed by the buffer. It is a variable for memorizing what was done.

In S1205, the CPU 952 passes the sheet information of the sheet N to process _{F C,} stores the return value to the buffer availability information on RAM 954. Process F _C, the sheet based on the sheet information passed is checked whether the available buffer, a process of returning as a return value whether the buffer (TRUE or FALSE), the details will be described later.

  In S1206, the CPU 952 determines the buffer availability information of the sheet N acquired in S1205. If the buffer availability information is TRUE, the process proceeds to S1207, and if it is FALSE, the process proceeds to S1213. Subsequently, in S1207, the CPU 952 determines whether or not the sheet N is the last sheet of “copy”, and if it is the last sheet (YES in S1207), the process proceeds to S1213, and if not the last sheet (NO in S1207). The process proceeds to S1208.

In S1208, the CPU 952 passes the sheet information of the sheet N + 1 that is attached to the information sheet N to process _{F C,} to obtain the buffer availability information sheet N + 1. To save the return value of the processing _{F C} to the buffer availability information on RAM 954. In S1209, the CPU 952 determines the buffer availability information of the sheet N + 1 acquired in S1208. If the buffer availability information is TRUE, the process proceeds to S1210, and if it is FALSE, the process proceeds to S1213.

In S1210, the CPU 952 stores the buffer mode of the sheet N as “buffer” in the RAM 954, and proceeds to S1211. Then, the CPU 952 adds the standard paper between _{I N} bundles between time _{T N} (S1211). Then, in S1212, the CPU 952 substitutes 0 between required paper time D, and the process ends _{F B.}

When the process proceeds from S1206, S1207, and S1209 to S1213, the CPU 952 clears the buffer counter C to 0 and proceeds to S1214. Then, at S1214, it sets the buffer mode of the sheet N to "pass", by substituting the _{T B} between required paper time D in S1215, the process ends _{F B.}

(Buffer mode setting / calculation of required paper interval: other than the first paper)
Next, the flow of the setting of the buffer mode and the calculation of the necessary inter-sheet time for the sheets other than the head of the process F _B and “part” by the CPU 952 will be described using the flowchart of FIG. Hereinafter, processing by the CPU 952 will be described unless otherwise specified.

  First, in step S1301, the CPU 952 determines the buffer mode of the sheet N-1. If the mode is “buffer” (YES in step S1301), the process proceeds to step S1302. The process proceeds to S1314.

In S1302, the CPU 952 determines whether or not the sheet N is the last sheet of “copy”. If it is not the last sheet (NO in S1302), the process proceeds to S1303, and if it is the last sheet (YES in S1302), the process proceeds to S1307. In the subsequent S1303, the CPU 952 determines a processing time _{T B,} compares the flux between time _{T N} + between the standard paper time _{I N,} whether it is necessary to buffer. If it is determined that _{T B> T N + I N} (YES at S1303), the case where productivity improvement by buffering is expected, the process proceeds to S1304. If it is determined that _{T B ≦ T N + I N} , the case that are not expected to be improved productivity by buffering, the process proceeds to S1307.

In S1304, the CPU 952 passes the sheet information of the sheet N + 1 that is attached to the information sheet N to process _{F C,} to obtain the buffer availability information sheet N + 1. To save the return value of the processing _{F C} to the buffer availability information on RAM 954. In S1305, the CPU 952 determines the buffer availability information of the sheet N + 1 acquired in S1304. If the buffer availability information is TRUE, the process proceeds to S1306, and if it is FALSE, the process proceeds to S1307.

  In S1306, the CPU 952 decrements the buffer counter C by 1, and proceeds to S1308. On the other hand, if the process proceeds from S1302, S1303, and S1305 to S1307, the CPU 952 clears the buffer counter C to 0 and proceeds to S1308. In S1308, if buffer counter C ≠ 0 (NO in S1308), the process proceeds to S1309, and if C = 0 is determined (YES in S1308), the process proceeds to S1312.

In step S <b> 1309, the CPU 952 sets the buffer mode of the sheet N to “buffer” and stores it in the RAM 954. Then, the CPU 952 adds the standard paper between _{I N} bundles between time _{T N} (S1310). Then, the CPU 952 in S1311 is, between the required paper time D substituting between standard paper _{I N,} the process ends _{F B.}

In step S1312, the CPU 952 sets the buffer mode of the sheet N to “final paper” and stores it in the RAM 954. Then, the CPU 952 substitutes the necessary sheet interval time D obtained by subtracting the time (inter-batch time T _N + standard sheet interval I _N ) offset by the buffer operation from the post-processing time TB of the preceding sheet bundle ( S1313), the process ends _{F B.} In step S <b> 1314, the CPU 952 sets the buffer mode of the sheet N to “pass” and stores it in the RAM 954. Then, in S1315, the CPU 952 substitutes the standard paper between _{I N} between required paper time D, and the process ends _{F B.}

(Buffer availability determination)
Next, the flow of determining whether or not the buffer is possible based on the process F _C by the CPU 952 and the passed sheet information will be described with reference to the flowchart of FIG. Hereinafter, the processing by the CPU 952 will be described unless otherwise specified. Here, the determination of whether or not buffering is possible refers to determining whether or not to apply buffer processing to a sheet.

First, in S1401～S1403, the CPU 952 determines, sheet information passed to process _{F C,} the sheet material type information, respectively OHP, coated paper, whether either index sheet. If the sheet is one of the above types, the process proceeds to S1408, and if not, the process proceeds to S1404.

  In S1404, the CPU 952 determines whether the basis weight is in the range of 50 gsm to 200 gsm, whether the paper width is in the range of 182 mm to 297 mm in S1405, or whether the paper length is in the range of 182 to 216 mm in S1406. If it is outside the range in each determination, the process proceeds to S1408, and if all are within the range, the process proceeds to S1407.

In step S1407, the CPU 952 determines that the sheet is a bufferable sheet, and substitutes TRUE for the result. In step S1408, the CPU 952 determines that the sheet cannot be buffered, and substitutes FALSE for the result. Then, the CPU 952 returns the result as a return value, the process ends _{F C.}

  Note that each sheet material type and each sheet specification (size, basis weight, etc.) shown in the determination of FIG. 19 can be changed according to the function and specification of the apparatus to which the present invention is applied. Therefore, it is not limited to the values and conditions shown in FIG.

  As described above, when determining whether or not to perform buffer processing on the sheet N, the present invention determines not only the sheet N but also the buffer availability determination result of the subsequent sheet N + 1. As a result, even when the sheet N + 1 is a non-bufferable sheet, the buffer process is not canceled after the sheet N is buffered, and the post process is performed at an optimal sheet interval in any combination of sheets. Is possible.

  A specific difference between the conventional example and this embodiment will be described with reference to FIG. As shown in the uppermost part of FIG. 20, the paper interval time I is set to 500 ms. Further, it is assumed that the processing time P = 700 ms is required for the final sheet of the sheet bundle. In this case, a shortage of 200 ms occurs with respect to the inter-paper time for processing the final paper. In this case, if the first sheet of the next sheet bundle (X + 1-th copy) can be buffered, the time between the sheet bundle and the sheet bundle (inter-bundle time B) can be made longer than the processing time P. . Thereby, productivity is not affected. In FIG. 20, the time between bundles B is set to 1000 ms. At this time, the paper interval time I remains at 500 ms.

  Next, in the buffer control of the conventional example, consider a case where the first sheet of the next sheet bundle (X + 1th copy) is a sheet that cannot be buffered. In this case, since the first sheet cannot be buffered, the first sheet of the next sheet bundle is conveyed after the processing time P has elapsed. If the inter-sheet time I remains at 500 ms, the paper discharge is delayed by the amount of time (200 ms) that was waited until the processing of the previous sheet bundle is completed.

  However, in the conventional buffer control, the following situation can occur. It is assumed that the first sheet of the next sheet bundle (X + 1 copy) is a sheet that can be buffered, and the second sheet is a sheet that cannot be buffered. In this case, the first sheet is buffered until the processing time P of the last sheet of the previous sheet bundle (Xth section) has elapsed. However, since the second sheet cannot be buffered, the buffer of the first sheet once buffered is canceled and the first sheet is discharged. In this case, the discharge of the first sheet is delayed by the buffer time (300 ms). If the sheet interval time I of the first sheet and the second sheet of the next sheet bundle (X + 1 copy) remains 500 ms, the sheet is discharged more than when the sheet is conveyed after the processing time P has passed. Paper will be delayed. As a result, the system does not buffer the first sheet and waits until the processing of the previous sheet bundle is completed, and then the productivity is further reduced as compared with the case where the sheet is discharged. This is because it is determined whether or not to buffer using only the information of the first sheet of the sheet bundle, and if the second sheet cannot be buffered, the first and second sheets are overlapped. This is because the paper is discharged without being canceled (buffer cancellation).

  On the other hand, in the present embodiment, whether or not to buffer the Yth sheet is determined using information on the Yth sheet and the (Y + 1) th sheet of the next sheet bundle (X + 1th copy). Thereby, it is possible to prevent a decrease in productivity that may occur in the conventional example.

Claims (7)

A post-processing device that performs post-processing on a plurality of sheets received from an image forming unit,
Sheet conveying means for conveying the received sheet;
Sheet stacking means for stacking a plurality of sheets conveyed by the sheet conveying means;
Post-processing means for performing post-processing on a sheet bundle including a plurality of sheets stacked by the sheet stacking means;
Buffer means disposed upstream of the sheet stacking means, for buffering the one or more sheets to be retained and for superposing the retained sheets and subsequent sheets;
Obtaining means for obtaining sheet information for determining whether or not the sheet is prohibited from buffer processing by the buffer means;
When the first sheet among the predetermined number of sheets subsequent to the sheet bundle to be post-processed by the post-processing means is a sheet for which buffer processing is not prohibited, two of the predetermined number of sheets And a control unit that controls whether or not the first sheet is retained based on sheet information of the first sheet. Wherein if the first sheet is a sheet that is not prohibited the buffering, if the predetermined number of the second sheet of the sheet is prohibited buffering, the The post-processing apparatus according to claim 1, wherein the first sheet is not retained. When the first sheet is a sheet that is not prohibited from the buffer processing, the control means, if the second sheet of the predetermined number of sheets is not prohibited from buffer processing, The post-processing apparatus according to claim 2, wherein the first sheet is retained.   If the first sheet is not retained, even if the first sheet is not prohibited from buffer processing, the control unit may transfer the image forming unit to the post-processing device. 3. The control unit according to claim 2, wherein the discharge interval between the first sheet and the preceding preceding sheet is controlled to be wider than the discharge interval when the first sheet is retained. 3. A post-processing apparatus according to 3.   The post-processing apparatus according to claim 1, wherein the sheet information includes at least one of a paper size, a basis weight, a sheet material type, and a post-processing type. The sheet for which the buffer processing is prohibited is
If the paper size of the sheet is outside the predetermined size range, or
Sheet whose basis weight of the sheet information is outside a predetermined basis weight range, or
The sheet information of the sheet material type is index sheet post-processing apparatus according to any one of claims 1 to 5, characterized in that one of the OHP.   An image forming system comprising: the post-processing device according to claim 1; and an image forming device that performs image forming processing on a sheet.

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