Abstract:
Affords sheet stacking device that, in stacking sheets onto a loading tray, enables stacking of the sheets always in the correct posture regardless of the nature of the sheets&#39; surface material. The sheet stacking device includes a tray unit located downstream of a sheet discharging port, a sheet-end regulating member provided on the tray unit, a conveying unit for transporting sheets carried out onto the tray unit though the sheet discharging port, toward the sheet-end regulating member, and a conveyance controller for controlling the conveying unit. In conveying toward the sheet-end regulating member a sheet having been carried onto the tray unit, the conveyance controller varies the running time during which the conveying unit applies conveyance force to the sheet, in accordance with the material nature of the sheet surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention—involving sheet stacking devices, and finishers furnished with the devices, that stack/store on a tray sheets carried out from, typically, an image-forming apparatus—relates to improvements in sheet storing mechanisms that orderly store sheets against a regulating stop on the tray. 
     2. Description of the Related Art 
     In general, sheet stacking devices of this sort are known broadly as devices that are provided with a loading tray forming a path break downstream of the sheet discharging port, and that stack/store onto the loading tray sheets turned out from the sheet discharging port by a sheet discharging roller. The loading tray may be configured as a stack tray that simply houses the sheets, or may be configured as a processing tray that subjects the sheets to final-stage processing. 
     In particular, if the loading tray is configured as a processing tray, a sheet aligning mechanism is required to load the sheets (bundle) on the tray and subject the sheets to final-stage processing such as stapling, punching, or stamping. The sheet aligning mechanism, which is thus for positioning/stacking the sheets on the tray in a predetermined position (final-stage processing position), is constituted by a regulating stop on the loading tray, against which one end (leading end or trailing end) of the sheets abuts and is thereby regulated, and conveying means for conveying the sheets toward the stop. 
     For example, Japanese Unexamined Pat. App. Pub. No. 2003-267622 (cf. FIG. 2 in particular) discloses a device that stacks and collates on a processing tray sheets carried out from an image forming apparatus through a sheet discharging port, and staple-binds sheet bundles stacked on the processing tray. Therein, the processing tray is provided with a stop member against which sheet ends abut and are thereby regulated, as well as a conveying member (a “paddle member” in Japanese Unexamined Pat. App. Pub. No. 2003-267622) above the processing tray, for transporting toward the stop member sheets carried in through the sheet discharging port. 
     Similarly, Japanese Unexamined Pat. App. Pub. No. 2006-248684 (cf. FIG. 2 in particular) discloses a sheet storing mechanism that carries out onto the processing tray sheets from the sheet discharging port, and that, with a belt member disposed above the tray to let it elevate/lower, aligns the sheets by their abutting against a regulating stop. 
     In either of the conventional sheet storing mechanisms disclosed in Japanese Unexamined Pat. App. Pub. Nos. 2003-267622 and 2006-248684, to align against the regulating stop sheets carried in onto the tray, the conveying means (paddle or belt member), which applies a conveyance force on the sheets, aligns them using a preset conveyance force and running time (design parameters). 
     In implementations in which, as described above, sheets are carried in onto a loading tray disposed downstream of the sheet discharging port and aligned in the predetermined position (final-stage processing position, etc.), a conveying rotor (roller, belt, or the like) that moves up and down in accordance with the amount of sheets loaded is provided on the tray, and by means of the rotor, sheets are abutted against the regulating stop and are thereby aligned. Any of various and diverse mechanisms, such as a roller member, a belt member, or a paddle member, is conventionally employed as the conveying mechanism, which transports sheets on the tray toward the regulating stop for positioning. 
     Therein, a sheet-feeding rotor that in this manner conveys a sheet on the tray toward the regulating stop halts it immediately after the sheet leading end has abutted against the regulating stop, to prevent the sheet from being damaged by being over-conveyed. 
     The conveyance force that the sheet-feeding rotor imparts to the sheets by is conventionally set at a defined value (design parameter). Consequently, when the material nature of the sheet surface differs the conveyance force imparted to the sheet varies, giving rise to registration or skewing problems, in which the sheets fail to reach the regulating stop reliably, or to problems of damage, such as leading-end crumpling, affecting sheets having abutted against the regulating stop. 
     In particular, for sheets subjected to final-stage processing, a recent trend is to use sheets differing significantly in the material nature of the surface, such as color copy sheets and monochromatic copy sheets. In such cases, mixing for example gloss paper, whose surface coefficient of friction is low, with plain paper, whose coefficient of friction is relatively large, may lead to the former not reaching the regulating stop (registration problem) or to skewing, and may lead to sheet damage such as leading end crumpling in the latter. 
     To solve such problems, for example, the engaging force between the sheet-feeding rotor and the sheets on the tray could be more/less adjusted. However, in a situation, for example, in which color images alternate with monochromatic images on sheet by sequentially conveyed sheet, the above-described problem cannot be solved unless the pressure-contact force of the sheet-feeding rotor is adjusted for each sheet. Accordingly, adjustment of the pressure-contact force of the sheet-feeding rotor requires complicated mechanisms and controls. 
     BRIEF SUMMARY OF THE INVENTION 
     Under these circumstances, the present inventors arrived at the concept of varying the running time of the sheet-feeding rotor depending on the nature of the paper material of the sheets conveyed, or detecting the time that the sheets are conveyed until they reach the loading tray, and varying the running time of the sheet-feeding rotor in accordance with how long/short the conveyance period is. 
     A main object of the present invention is to provide a sheet stacking device that, when sheets are aligningly stacked at the regulation stop located on the loading tray, can always stack the sheets in the correct posture regardless of the material nature of the sheet surface. 
     Another object of the present invention is to provide a sheet stacking device and a finisher which, when the sheets are loaded and stored, can always stack the sheets in the correct posture without being affected by the material nature of the sheets or environmental conditions such as temperature and humidity. 
     To accomplish the above-described objects, the present invention adopts the following configuration. A sheet stacking device comprises a sheet discharging port through which sheets are sequentially carried out, tray means (for example, a processing tray  20  described below) located downstream of the sheet discharging port, a sheet-end regulating member ( 20   a  described below) provided on the tray means, conveying means (for example, belt transporting means  17  described below) for transporting the sheet carried out onto the tray means though the sheet discharging port, toward the sheet end regulating means, and conveyance control means for controlling the conveying means. In conveying the sheet carried onto the tray means toward the sheet-end regulating member, the conveyance control means varies the running time during which the conveying means applies a conveyance force to the sheet, depending on a material nature of the sheet surface. 
     A sheet discharging path is connected to the sheet discharging port to feed the sheet. The sheet discharging port includes sensor means for detecting a moving time of the sheet being conveyed. The conveyance control means sets a time during which the conveying means applies the conveyance force to the sheet on the basis of a pass time of the sheet determined on the basis of an output signal from the sensor means. In this case, the sensor means comprises (1) a sensor that senses a leading end and a trailing end of the moving sheet or (2) a sensor that senses the sheet moving over a predetermined distance. 
     The conveyance control means comprises comparing means for comparing the pass time of the sheet determined on the basis of the output signal from the sensor means with a preset reference sheet pass time. The conveyance control means sets the running time during which the conveying means applies the conveyance force to the sheet, on the basis of a result of the comparison from the comparing means. 
     The conveyance control means comprises input means for inputting a material nature of the sheet surface to the device and sets the running time during which the conveying means applies the conveyance force to the sheet, on the basis of information input by the input means. 
     The conveying means comprises a belt member located between the sheet discharging port and an uppermost sheet on the tray means. 
     A finisher according to the present invention comprises a processing tray on which sheets carried out from an image forming apparatus are set in a bundle, final-stage processing means for subjecting the sheet bundle on the processing tray to final-stage processing, a sheet-end regulating member provided on the processing tray, conveying means for transporting the sheet carried out onto the processing tray through the sheet discharging port, toward the sheet-end regulating member, and conveyance control means for controlling the conveying means. In conveying the sheet carried onto the tray means toward the sheet-end regulating member, the conveyance control means varies the running time during which the conveying means applies a conveyance force to the sheet, depending on a material nature of the sheet surface. 
     According to the present invention, when the sheet carried out onto the tray means through the sheet discharging port is allowed to abut against the regulating stop for alignment, the running time during which the conveying means applies the conveyance force to the sheet is varied depending on the material nature of the sheet surface. The present invention thus exerts the following effects. 
     Even when the sheet stacked on the tray means differs in the nature of its surface material, the sheet is transported toward the regulating stop using the running time corresponding to the nature of the material. The sheet can thus be orderly stacked on the tray at a predetermined position. 
     In particular, the material nature of the sheet surface is detected on the basis of the time required to convey the sheet to the sheet discharging port. The sheet on the tray is transported toward the regulating stop according to the sheet conveying time. Thus, even a sheet that differs significantly in the nature of its surface material can be loaded and stored in a predetermined reference position. This enables, for example, a sheet comprising coating paper (gloss paper or the like) on which a color image is printed and a sheet comprising plain paper on which a monochromatic image is printed to be orderly stacked and stacked at the reference position. 
     The remaining part of the configuration requires only the adjustment of the length of the running time of the conveying means; the conveying means may be a roller, a belt, or a paddle located on the tray means. Therefore, the apparatus can be inexpensively configured to have a very simple structure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram generally illustrating an image forming system composed of a finisher comprising a sheet stacking device according to the present invention, and an image forming apparatus that carries out sheets to the finisher; 
         FIG. 2  is an enlarged diagram of an essential part of the finisher in the system shown in  FIG. 1 ; 
         FIG. 3  is diagrams illustrating how a sheet is stacked on a processing tray in the apparatus shown in  FIG. 2 , wherein  FIG. 3A  shows that a leading end of the sheet has been carried into the apparatus, and  FIG. 3B  shows that the leading end of the sheet has been carried onto the processing tray; 
         FIG. 4  is diagrams illustrating states in stacking a sheet onto a processing tray in the apparatus shown in  FIG. 2 , wherein  FIG. 4A  represents a state in which the trailing end of a sheet has advanced in above the tray, and  FIG. 4B  represents a state in which the sheet aligns with a regulating member on the tray; 
         FIG. 5  is a diagram illustrating a state in stacking a sheet onto the processing tray in the apparatus shown in  FIG. 2 , and represents a state in which the sheet has been aligned with a regulating member on the tray; 
         FIG. 6  is a timing chart for explaining states in stacking a sheet onto the processing tray in the apparatus shown in  FIG. 2 ; 
         FIG. 7  is a diagram illustrating the arrangement of sensor means different from that in the apparatus shown in  FIG. 2 ; and 
         FIG. 8  is a diagram illustrating a sheet detecting mechanism on a storing tray in the apparatus shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described below on the basis of an illustrated embodiment.  FIG. 1  is a diagram generally illustrating a system comprising a finisher comprising a sheet stacking device according to the present invention, and an image forming apparatus that carries out sheets to the finisher.  FIG. 2  is an enlarged diagram of an essential part of the finisher. The “image forming system” and the “finisher” will be described below in this order. 
     Image Forming System 
     The image forming system shown in  FIG. 1  is composed of an image forming apparatus A and a finisher B. A sheet stacking device C is incorporated in the finisher B. The image forming apparatus A is composed of a sheet feeding stacker  1 , an image forming section  2  that forms an image on a sheet from the sheet feeding stacker  1 , a scanner section  3 , and a document feeding section  4 . The image forming section  2  is composed of an electrostatic printing mechanism, an ink jet printing mechanism, an offset printing mechanism, or the like. The image forming section  2  is configured to copy and print image data optically read by the scanner section  3 , on a sheet from the sheet feeding stacker  1 . The illustrated image forming section  2  is an electrostatic printing mechanism including a developing member  5   a , a charger  5   b , and a print head  6  arranged around a photosensitive drum  5 . The print head  6  forms an electrostatic latent image on the photosensitive drum  5 . The developing member  5   a  applies toner ink to the electrostatic latent image. The charger  5   b  transports the image to the sheet for printing. A fixer  7  fixes the image to the sheet to which the toner ink has been transported. The resulting sheets are sequentially carried out through the sheet discharging port  8 . Illustrated at reference numeral  9  is a circulating path along which a sheet with a printed front surface is fed again to the photosensitive drum  5  and an image is then formed on a back surface of the sheet. 
     The scanner section  3  is composed of a platen  3   a  on which a document is placed, a reading carriage  3   b  that line-sequentially scans a document image along the platen  3   a , and a photoelectric converting element  3   c . The document feeding section  4  is mounted above the scanner section  3   a  to separately feed documents placed and set on a document tray  4   a  to the platen  3   a  and to accommodate the documents on a sheet discharging tray  4   b . The system also functions as a network printer in such a manner that image data from the external image forming apparatus, for example, a computer, is transported to the print head  6 , which forms an image on a sheet on the basis of the data. 
     Finisher 
     The finisher B according to the present invention is coupled to the sheet discharging port  8  in the image forming apparatus A. The finisher B sequentially receives sheets with images formed thereon to carry out a “bookbinding process,” a “jog process,” and a “sheet carry-out (storing) process” on the sheets. Thus, the image forming system shown in  FIG. 1  is composed of the image forming apparatus main body comprising a copier, a print function, a facsimile function, and the like, and the finisher coupled to the image forming apparatus main body. The finisher B comprises, as processing operation modes, a series of final-stage processing operations such as a binding process of setting and binding sheets with images formed thereon according to page number and the jog process of sorting and storing the set sheets before discharging. To control each operation mode, an operator sets, on the image forming apparatus A, a print mode such as the number of sheets to be printed and a printing function and simultaneously sets a final-stage processing mode such as the “binding process,” the “jog process,” or the “sheet carry-out (storing) process.” In accordance with a command signal from the image forming apparatus A, the finisher B carries out a process according to the operation mode. 
     The finisher B shown in  FIG. 2  is composed of a sheet discharging path  11  which receive sheets sequentially discharged by the above-described image forming apparatus A and which carry out the sheets downstream, a processing tray  20  (“tray means”; this also applies to the description below) located below a sheet discharging port (sheet discharging port in the finisher B)  13   a  of the sheet discharging path  11 , and a storing tray  30  located downstream of the processing tray  20 . A carry-in roller  14  is provided on the sheet discharging path  11  to convey a sheet fed toward a carry-in port  12 . The carry-in roller  14  is composed of a pair of rollers that are in pressure contact with each other. The sheet discharging path  11  also includes an inlet sensor S 1  that detects a leading end and a trailing end of a conveyed sheet. 
     The sheet discharging path  11  diverges into a first sheet discharging path  11   a  and a second sheet discharging path  11   b . The processing tray  20  (tray means; this also applies to the description below), described below, is located downstream of the first sheet discharging path  11   a . A first storing tray  30   a  is located downstream of the processing tray  20 . A second storing tray  30   b  is located on the second sheet discharging path  11   b.    
     That is, a sheet from the image forming apparatus A is guided to the carry-in path  11  by the carry-in roller  14  and selectively fed to the first sheet discharging path  11   a  or the second sheet discharging path  11   b  via a path switching piece  15 . The processing tray  20  is located downstream of a sheet discharging port (hereinafter referred to as a “first sheet discharging port”)  13   a  of the first sheet discharging path  11   a . The second storing tray  30   b  is located at a sheet discharging port (hereinafter referred to as a “second sheet discharging port”)  13   b  of the second sheet discharging path  11   b . Thus, sheets from the image forming apparatus A are selectively guided toward the processing tray  20  via the path switching piece  15  or to the second storing tray  30   b . The sheets guided to the processing tray  20  are set in a bundle, subjected to final-stage processing such as stapling, and then stored on the first storing tray  30   a , provided downstream of the processing tray  20 . On the other hand, the sheets guided toward the second storing tray  30   b  are stacked on the second storing tray  30   b  without being post-processed. 
     Configuration of Processing Tray 
     The processing tray  20  is located below the first sheet discharging port  13   a  so as to form a step. Sheets are temporarily placed and supported on the processing tray  20  and post-processed in this condition. A mechanism corresponding to the functions of the final-stage processing carried out on the sheets is incorporated in the processing tray  20 . The illustrated processing tray  20  comprises the “bookbinding function,” the “jog function,” and the “sheet carry-out function” of carrying out sheets from the first sheet discharging port  13   a  directly (without final-stage processing) to the downstream first storing tray  30   a.    
     The “bookbinding function” stacks and staples a series of sheets carried out from the image forming apparatus A, on the processing tray  20  according to page number, and carries out the resulting sheet bundle to the first storing tray  30   a  for storing. The “jog function” sorts, stores, and sets the series of sheets carried out from the image forming apparatus A, on the first storing tray  30   a . Thus, the processing tray  20  comprises a jog shift mechanism that shifts each of the sheets in a direction orthogonal to a conveying direction by a predetermined amount. The illustrated jog shift mechanism, biasing aligning means (side regulating means  21  described below) provides this function. Forward/reverse roller means  26  is located on the processing tray  20 . 
     A staple device (final-stage processing means, shown in  FIG. 2 )  24 , the sheet-end regulating member  20   a , the conveying means (“forward/reverse roller means  26 ” and “belt transporting means  17 ” described below), and the side regulating means  21  are arranged on the processing tray  20 ; the sheet end regulating means  20   a  positions and aligns each of the sheets with the final-stage processing position, and the conveying means transports the sheet to the sheet-end regulating member. The sheet-end regulating member  20   a  is formed to project upward from the processing tray  20  so that the leading or trailing end of the sheet in the conveying direction abuts against the sheet end regulating means  20   a  for regulation. Similarly, the side regulating member  21  is formed to project upward from the processing tray  20  so that a side edge of the sheet which is orthogonal to the sheet conveying direction abuts against the side regulating member  21 . 
     Configuration of Conveying Means 
     A sheet discharging roller  29  is located at the sheet discharging port  13   a . The illustrated sheet discharging roller  29  is in pressure contact with a driving end of a caterpillar belt  18  described below to carry out the sheet upward from the tray through the sheet discharging port  13   a . The sheet discharging roller  29  may be composed of a pair of rollers that is not in pressure contact with the caterpillar belt  18  as shown in the figure. 
     The forward/reverse roller means  26  and the belt transporting means  17  are arranged on the processing tray  20 ; the forward/reverse roller means  26  conveys the sheet carried onto the tray, toward a downstream side and then switches the sheet back to an upstream side, and the belt transporting means  17  transports the sheet fed by the roller means, to the regulating member  20   a . The forward/reverse roller means  26  and the belt transporting means  17  constitute the conveying means for “transporting the sheet carried out through the sheet discharging port, toward the sheet-end regulating member.” Thus, the forward/reverse roller means  26  is supported by a swinging bracket  26   a  so as to be movable up and down with respect to the processing tray  20 . The forward/reverse roller means  26  is coupled to a shift motor (not shown in the drawings). A forward reverse motor (not shown in the drawings) is coupled to the forward/reverse roller  26 . 
     The belt transporting means  17  is composed of a pair of pulleys  16   a  and  16   b  between which the caterpillar belt  18  extends as shown in  FIG. 2 . The belt transporting means  17  is located between the sheet discharging port  13   a  and the processing tray  20  so as to be movable up and down along the sheet conveying direction (to and from the sheet-end regulating member  20   a ). That is, a driving motor M 1  is coupled to the driving pulley  16   a . The driven pulley  16   a  moves up and down according to the amount of sheets loaded on the tray. The caterpillar belt  18  is wound between the pulleys. Thus, the illustrated apparatus is configured so that the sheet carried onto the tray is transported to the sheet-end regulating member  20   a  by the forward/reverse roller means  26  and the belt transporting means  17 . Of course, the “conveying means,” composed of the forward/reverse roller means  26  and the belt transporting means  17 , may be composed of only the forward/reverse roller or the belt transporting means. 
     The final-stage processing means, in the figure, the staple device  24 , is located on the processing tray  20  configured as described above. The sheet from the sheet discharging port  13   a  is carried onto the processing tray  20  by the forward/reverse roller means  26  and the belt transporting means  17 . The trailing end of the sheet advances onto the processing tray  20 . The forward/reverse roller means  26  is reversed to switch back the sheet, which thus advances to the lower half portion of the belt of the belt transporting means  17 . Subsequently, the belt transporting means  17  allows the trailing end of the sheet to abut against the sheet-end regulating member  20   a  for regulation. 
     Sheet push-out means  25  (sheet discharging means; this also applies to the description below) is located on the processing tray  20  as described below in order to transport the post-processed sheet to the downstream first storing tray  30   a . A guide groove (not shown in the drawings) along which a push-out pawl  25   a  moves is provided in the center of the processing tray  20  in a sheet width direction. The push-out pawl  25   a  transports the sheet positioned on the downstream sheet-end regulating member  20   a  to the sheet discharging port  13   c  (hereinafter referred to as a “third sheet discharging port”) along the guide groove. To achieve this, a belt member  48  is extended between a pair of pulleys  46  and  47  provided on a rear surface of the processing tray  20 , and the push-out pawl  25   a  integrally fixed to the belt member  48 . A push-out pawl driving motor M 5  is coupled to the pulley  46 . Consequently, the sheet push-out pawl driving motor M 5  swings the push-out pawl  25   a  so as to longitudinally cross the periphery of the processing tray  20 . In addition to the push-out pawl  25   a , the forward/reverse roller means  26 , configured as described above, is provided at the third sheet discharging port  13   c.    
     Sheet Stack Device 
     A sheet stack device D described below is located downstream of the processing tray  20 , described above. As shown in  FIG. 2 , the first storing tray  30   a  is located at the third sheet discharging port  13   c  so as to form a step. As shown in  FIG. 2 , the first storing tray  30   a  is located at the third sheet discharging port  13   c  so as form a step. As shown in  FIG. 2 , the first storing tray  30   a  is composed of a tray member attached to an apparatus frame so that the sheet from the third sheet discharging port  13   c  is loaded and stored on the first storing tray  30   a . The sensor lever  19  is located above the first storing tray  30   a . The sensor lever  19  is configured to detect whether or not the sheets loaded on the first storing tray  30   a  have reached a maximum allowable amount (sheet full) and/or whether or not any sheet is loaded on the tray (sheet remaining). 
     Thus, as shown in  FIG. 8 , the sensor lever  19  is supported on the apparatus frame so as to be pivotable around a support shaft  19   a . The sensor lever  19  further includes a paper contact piece  19   b  at a leading end thereof and a flag  19   c  at a base end thereof. The paper contact piece  19   b  is configured to swing around a support shaft  19   a  so as to come into contact with the uppermost sheet on the first storing tray  30   a . The flag  19   c  is configured such that the position thereof is detected by a photosensor. The sensor lever  19  is biased so as to always lie at a retracted portion by a bias spring  19 S. The sensor lever  19  is coupled to an electromagnetic solenoid  19 M so as to move to a sensing portion against the force of the spring. 
     The flag  19   c  of the sensor lever  19  comprises a first flag fr 1  and a second flag fr 2  which, when the paper contact piece  19   b  moves in conjunction with the sheets on the tray, allow “sheet full sensing,” “sheet empty sensing,” and “lever standby position sensing” to be performed at the corresponding positions. To sense the flags fr 1  and fr 2 , a first sensor Sa and a second sensor Sb are arranged at positions shown in  FIG. 8 . 
     Thus, the positional relationship between the first and second flags fr 1  and fr 2  and the first and second sensors Sa and Sb is set such that when the paper contact piece  19   b  is at the sheet full sensing position, “Sa=ON &amp; Sb=ON,” and when the paper contact piece  19   b  is at the sheet empty sensing position on the first storing tray  30   a , “Sa=OFF &amp; Sb=OFF,” and such that when the paper contact piece  19   b  senses a different condition of the paper surface, “Sa=ON &amp; Sb=OFF,” and when the paper contact piece  19   b  is at the retracted position (state shown in  FIG. 8 ), “Sa=OFF &amp; Sb=ON.” 
     The present invention is thus characterized by controlling the forward/reverse roller means  26  and the belt transporting means  17  as follows. First, when the leading end of the sheet advances onto the tray, the forward/reverse roller  26  stands by above the tray (condition shown in  FIG. 2 ) so as not to obstruct the sheet. Then, after the leading end of the sheet advances onto the tray, the forward/reverse roller  26  lowers to a position where the forward/reverse roller  26  engages with the sheet on the tray. At the same time, the roller  26  rotates clockwise to convey the sheet downstream. Upon elapse of an expected time required for the trailing end of the sheet to be carried onto the tray, the forward/reverse roller  26  starts rotating counterclockwise. Thus, the forward/reverse roller  26  switches back and conveys the sheet carried onto the tray toward the sheet-end regulating member  20   a . Then, upon elapse of an expected time required for the trailing end of the sheet to be fed to the belt transporting means  17 , the forward/reverse roller  26  retracts upward from the tray. The belt transporting means  17  transports the sheet so that the sheet abuts against the sheet-end regulating member  20   a.    
     The driving of the forward/reverse roller means  26  and the belt transporting means  17  is controlled by, for example, a control CPU for the final-stage processing device B. The present invention is thus characterized in that this control CPU (“control means  40 ”; this also applies to the description below) “varies a driving stop timing for the belt transporting means depending on the material nature of the sheet surface.” The control means  40  is configured so as to (1) detect the conveying condition of the sheet to stop the belt transporting means  17  in accordance with the detection result or to (2) stop the belt transporting means  17  in accordance with the sheet-surface material nature input by the operator. 
     The case in which the control means  40  detects the conveying condition of the sheet to control the belt transporting means  17  will be described.  FIG. 6  shows a timing chart for this case, and  FIGS. 3 to 5  show operating conditions. Description will be given with reference to  FIG. 6 . The control CPU (not shown in the drawings) receives a sheet discharging instruction signal from the image forming apparatus A and then rotationally controls the carry-in roller  14  and the sheet discharging roller  29  (the illustrated sheet discharging roller  29  is a driving pulley). The rotational driving is performed by the driving motor M 1 . 
     Then, the sheet fed to the carry-in port  12  is fed downstream by the carry-in roller  14 . The inlet sensor S 1  then senses the leading end of the sheet (condition shown in  FIG. 3A ). The sheet advances onto the processing tray through the sheet discharging port  13   a . At this time, a counter provided in the control CPU starts measurement with a reference clock. Furthermore, a forward reverse control timer is actuated on the basis of a sheet leading end sensing signal. 
     Then, when the inlet sensor S 1  senses the sheet trailing end, the control means  40  issues a trailing end sensing signal to stop the counter from the clock measurement (condition shown in  FIG. 3B ). Almost simultaneously with the stop, the control means  40  reads a reference value. The reference value is prepared in, for example, a RAM as a sheet conveying time corresponding to a sheet size. The control means  40  then reads a reference conveying time required to convey the sheet on the basis of the sheet size information pre-received from the image forming apparatus A. The control means  40  then compares the measurement time in the counter with the reference conveying time stored in the RAM. 
     Then, in response to the leading end sensing signal from the inlet sensor S 1 , the control means  40  actuates a forward reverse control timer. Upon elapse of an expected time (timer  1 ) required for the sheet leading end to reach the position of the forward/reverse roller  26 , the control means  40  lowers the forward/reverse roller means  26  onto the sheet placed on the tray. Simultaneously with the lowering, the control means  40  rotates the forward/reverse roller means  26  clockwise. Then, the sheet is carried out downstream as shown in  FIG. 4A . The sheet trailing end is carried onto the processing tray though the sheet discharging port. 
     Then, upon elapse of an expected time (timer  2 ) required for the sheet trailing end to advance onto the processing tray, the control means  40  starts rotating the forward/reverse roller  26  in the reverse (counterclockwise) direction (state shown in  FIG. 4B ). Then, the sheet trailing end is fed toward the sheet end regulating means  20   a  along the processing tray  20  by means of the caterpillar belt  18  of the belt transporting means. Upon elapse of an expected time (timer  3 ) required for the sheet trailing end to be fed to the belt transporting means  17 , the control means  40  moves the forward/reverse roller means  26  upward so that the forward/reverse roller means  26  stands by above the tray. 
     Then, the control means  40  compares the “sheet conveying time” determined from the sensing signal from the inlet sensor S 1  with the prepared “reference conveying time.” The control means  40  sets a belt stop time (belt running time) on the basis of the comparison result. As the belt stop time, the time when the caterpillar belt  18  is stopped is determined on the basis of the time when the sheet trailing end is detected by the inlet sensor S 1 . For example, when the “sheet conveying time” is equal to the “reference conveying time,” the sheet is considered to be conveyed without slippage. Then, the belt stop time determined by the control means  40  is set on the basis of a conveying distance over which the sheet trailing end carried out through the sheet discharging port  13   a  travels until the end reaches the sheet-end regulating member  20   a.    
     When the “sheet conveying time”&gt;“reference conveying time,” the sheet is considered to slip while being fed by the forward/reverse roller  26  and the belt transporting means  17 . The belt stop time is set equal to the “reference conveying time+slippage amount.” For example, in the figure, the “sheet conveying time−reference conveying time” is calculated, and the belt stop time is set in stages according to the calculated value. Alternatively, the “belt stop time” can be set by the calculating means so as to correct the sheet slippage amount on the basis of the “sheet conveying time−reference conveying time.” 
     When the “belt stop time” has passed, the control means  40  stops the driving motor M 1  (condition shown in  FIG. 5 ). 
     In the present invention, the sensor sensing the “moving time” of the sheet traveling along the sheet discharging path  11  is the single sensor S 1  located on the sheet discharging path to detect the sheet leading and trailing ends to determine the moving time, as described above. However, the sensor may be configured as described below. 
     As shown in  FIG. 7 , a first sensor S 1  and a second sensor S 2  are arranged on the sheet discharging path  11  at a distance L from each other. The first sensor S 1  senses the leading or trailing end of the sheet, and the sheet end is then sensed by the second sensor. This enables detection of the “moving time” of the sheet moving over the distance L. The sheet slippage amount can be determined by configuring the remaining part of the apparatus as described above. 
     Now, a case where the control means  40  is configured so as to stop the belt transporting means  17  according to the “sheet-surface material nature input by the operator” will be described. 
     The case where the sheet conveying condition is detected to control the belt transporting means  17  will be described. Although not shown in the drawings, a control panel is provided on the image forming apparatus A or the finisher B and includes an input key via which the operator inputs information. Then, for example, the panel display section displays choices such as “coating paper” and “plain paper.” The “belt stop time” is set on the basis of the material-nature and sheet-size information selected by the operator. The control means  40  sets the “belt stop time” on the basis of the material and sheet size information specified via the input key. The other operations are similar to those described above and will thus not be described below. 
     The present application claims priority from Japanese Patent App. No. 2007-203767, which is herein incorporated by reference.