Abstract:
A sheet stacking apparatus includes a sheet discharging device for discharging sheets; a stacking tray for stacking sets of the sheets discharged by the sheet discharging device; a retaining tray for retaining one sheet discharging by the discharging device; a shifting device for shifting, in a direction crossing with a direction of the discharging of the sheet, the one sheet on the retaining tray; a feeding device for feeding the one sheet to the stacking tray; whereby the sets of the sheets are grouped with the sheet shifted by the shifting device.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a sheet accumulation apparatus, for example, a sheet accumulation apparatus for accumulating cut sheets (hereinafter, “sheet”) which are discharged from the main assembly of an image forming apparatus after an image is formed thereon. It also relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, which comprises such a sheet accumulation apparatus. More specifically, the present invention relates to such a sheet accumulation apparatus that accumulates and jogs the discharged sheets to abut them to a reference, with slight displacement of the uppermost or bottom most sheet, and an image forming apparatus comprising such a sheet accumulation apparatus. 
     In a conventional sheet accumulation apparatus, the sheets are sorted into a set of sheets, and each set is delivered into a different tray as disclosed in Japanese Laid-Open Patent Application No. 313,261/1990. 
     However, in the conventional sheet accumulation apparatus, trays must be shifted for each set of sheets. Therefore, in order to increases the number of sheet sets or sheets in each set, which can be handled by a conventional sheet accumulation apparatus, the number of trays must be increased, and in addition, a large power source is necessary to move a large number of trays. As a result, apparatus size becomes larger, increasing the apparatus cost and also requiring a larger space for the apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the aforementioned shortcomings of a conventional sheet accumulation apparatus, and its object is to provide a sheet accumulation apparatus capable of slightly displacing the topmost or bottommost sheet of each sheet set from the rest in order to make it easier to separate the accumulated sheet sets from each other, and also to provide an image forming apparatus comprising such a sheet accumulation apparatus, since such a sheet accumulation apparatus and an image forming apparatus can eliminate the need for size increase and also can afford cost reduction. 
     According to an aspect of the present invention, there is provided a sheet stacking apparatus comprising sheet discharging means for discharging sheets; a stacking tray for stacking sets of the sheets discharged by said sheet discharging means; a retaining tray for retaining one sheet discharging by said discharging means; shifting means for shifting, in a direction crossing with a direction of the discharging of the sheet, the one sheet on said retaining tray; feeding means for feeding the one sheet to said stacking tray; whereby the sets of the sheets are grouped with the sheet shifted by said shifting means. 
     According to another aspect of the present invention, there is provided a sheet stacking apparatus comprising sheet discharging means for discharging sheets; a first tray for stacking the sheets discharged by said discharging means; aligning means for moving the sheets supported on said first tray in a direction crossing with a direction of discharging of the sheet, to align the sheets; first driving means for driving said aligning means; a first sheet regulating member for regulating the sheets moved in a direction crossing with the discharging direction by said aligning means; second driving means for retracting said first sheet regulating member from said first tray; a second sheet regulating member disposed more remote from said aligning means than said first sheet regulating member; and moving means for moving the sheets supported on said first tray to second tray adjacent the first tray. 
     According to a further aspect of the present invention, there is provided a sheet stacking apparatus comprising sheet discharging means for discharging sheets; a first tray for stacking the sheets discharging by said discharging means; aligning means for moving the sheets supported on said first tray in a direction crossing with a direction of discharging of the sheet, to align the sheets; first driving means for driving said aligning means; a first sheet regulating member for regulating the sheets moved in a direction crossing with the discharging direction by said aligning means; second driving means for retracting said first sheet regulating member from said first tray; a second sheet regulating member disposed more remote from said aligning means than said first sheet regulating member; and moving means for moving the sheets supported on said first tray to a second tray adjacent said first tray; control means for controlling said second driving means to retract said first sheet regulating member and for controlling said first driving means to align a first one sheet of the set of the sheets by abutting it to said second sheet regulating member. 
     In this aspect of the present invention, it is preferable that said control means controls said second driving means so as to reset said first sheet regulating member to an upper surface of said first tray, and controls said first driving means so as to abut second and subsequent sheets of the set of the sheets to said first regulating member. 
     According to a further aspect of the present invention, there is provided a sheet stacking apparatus comprising sheet discharging means for discharging sheets; a first tray for stacking the sheets discharged by said discharging means; aligning means for moving the sheets supported on said first tray in a direction crossing with a direction of discharging of the sheet, to align the sheets; first driving means for driving said aligning means; a first sheet regulating member for regulating the sheets moved in a direction crossing with the discharging direction by said aligning means; second driving means for retracting said first sheet regulating member from said first tray; a second sheet regulating member disposed more remote from said aligning means than said first sheet regulating member; and moving means for moving the sheet supported on said first tray to a second tray adjacent said first tray; control means for controlling said second driving means to retract said first sheet regulating member and for controlling said first driving means to align a last one sheet of the set of the sheets by abutting it to said second sheet regulating member. 
     According to a further aspect of the present invention, there is provided a sheet stacking apparatus comprising sheet discharging means for discharging sheets; an alignment tray for stacking the sheets discharged by said discharging means in an aligned state; aligning means for aligning the sheets on said alignment tray by moving them in a direction crossing with a sheet discharging direction; a sheet regulating member disposed opposed to said aligning means; a stacking tray for receiving the sheets on said alignment tray; wherein said sheet regulating member is movable between a first position for receiving the sheet moved by said aligning means and a second position retracted from said first position to increase a movement distance through which the sheets are moved from said first position in a direction crossing with said discharging direction. 
     The present invention is also applicable to an image forming apparatus comprising an image forming section, and a sheet accumulation apparatus in which the sheets on which an image has been formed in the image forming section are discharged and accumulated in the same manner as described above. 
     With the provision of the above described structure, adjacent two sets of sheets are separated by a single sheet slightly displaced from the rest. More specifically, after the first set of sheets is accumulated in a first tray and is conveyed into a second tray, the first sheet regulating member is retracted to a position (second position) below the sheet accumulation surface of the first tray. Then, the first (bottommost) sheet of the following set of sheets is conveyed into the first tray, and this sheet is aligned on the second sheet regulating member by the abutting means. Thereafter, this sheet is conveyed into the second tray by the conveying means. At this point of the operation, the first set of sheets jogged by the first sheet regulating member, and the first sheet of the following set of sheets jogged by the second sheet regulating member, are displaced from each other in the direction perpendicular to the sheet discharge direction. 
     Further, the first sheet regulating member is returned to a position (first position) above the sheet accumulation surface of the first tray by the second driving means, and the second sheet and the sheets thereafter of the following set of sheets are conveyed into the first tray. The second sheet and the sheets thereafter of the following set of sheets conveyed into the first tray are jogged against the returned first regulating means by the abutting means, and then, are conveyed into the second tray by the moving means. 
     When the above operation is repeated, the bottommost sheet of the following set of sheets, which indicates the border between adjacent two sets of sheets, sticks out from between the first set of sheets and the second set of sheets. Therefore, even when multiple sets of sheets are conveyed into a single first tray and accumulated therein, they do not mix. As a result, the subsequent sorting step can be easily and efficiently carried out. 
     As described above, according to the sheet accumulation apparatus in accordance with the present invention, when multiple sets of sheets are accumulated in a single tray, the topmost sheet or the bottommost sheet of each set of sheets is slightly displaced from the rest, which causes the displaced sheets to stick out from the border of adjacent two sets of sheets. Therefore, even when multiple sets of sheets are accumulated in a single tray, they do not mix, making the subsequent sorting step easier to carry out, and thereby improving operational efficiency. Further, it becomes unnecessary for the apparatus to have a large number of trays. Consequently, increase in the apparatus size can be avoided, affording thereby cost reduction. 
     Further, size and cost reduction of a sheet accumulation apparatus can be realized by simplifying the structure which retracts the sheet regulating member. 
     Further, the same effects as those described above will be provided by an image forming apparatus comprising the above described sheet accumulation apparatus. 
     These and other object, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a sheet accumulation apparatus in accordance with the present invention, and a copying machine comprising the sheet accumulation apparatus. 
     FIG. 2 is a more detailed sectional view of the sheet accumulation apparatus illustrated in FIG.  1 . 
     FIG. 3 is a plan view of the stapling tray of the sheet accumulation apparatus. 
     FIG. 4 is a sectional view of the stapling tray. 
     FIG. 5 is a sectional view of the abutting reference plate of the stapling tray and its adjacencies, depicting how the abutting reference plate is rotatively retracted. 
     FIG. 6 is a sectional view of the abutting reference plate, and its adjacencies, depicting how the abutting reference plate is retracted downward. 
     FIG. 7 is a sectional view of the abutting reference plate and its adjacencies, depicting how the abutting reference plate is rotatively retracted into the stapling tray. 
     FIG. 8 is a sectional view of the abutting reference plate and its adjacencies, depicting how the abutting reference plate is retracted in the direction parallel to the surface of the stapling tray. 
     FIG. 9 is a sectional view of the sheet accumulation apparatus in which a sheet is being discharged into the second tray of the sheet accumulation apparatus. 
     FIG. 10 is a sectional view of the sheet accumulation apparatus in which the sheets outputted from a personal computer have been discharged into the second tray. 
     FIG. 11 is a perspective view the oscillating guide of the sheet accumulation apparatus, and its adjacencies, depicting how the oscillation guide is oscillated. 
     FIG. 12 is an enlarged sectional view of one of the essential portions of the sheet accumulation apparatus. 
     FIG. 13 is an enlarged sectional view of the same as the above, depicting how a sheet sags as its trailing end hangs up on the top edge of the slotted bottom guide. 
     FIG. 14 is an enlarged sectional view of the same, depicting the state of a roller guide at the first position. 
     FIG. 15 is an enlarged sectional view of the same, depicting the state of a stopper which is blocking the opening of an F section. 
     FIG. 16 is an enlarged sectional view of the same, depicting the sate of the roller guide at the second position. 
     FIG. 17 is a schematic sectional view of the sheet accumulation apparatus, depicting how a sheet is discharged into the second tray from the top, and the stapling tray which is a first tray. 
     FIG. 18 is a schematic sectional view of the sheet accumulation apparatus, depicting the user selected number of sheets which have been sorted and accumulated into the stapling tray. 
     FIG. 19 is a schematic sectional view of the sheet accumulation apparatus in which a stapled sheet set is being discharged. 
     FIG. 20 is a schematic sectional view of the sheet accumulation apparatus in which the stapled sheet set has been discharged. 
     FIG. 21 is a schematic sectional view of the sheet accumulation apparatus into which a sheet has just begun to enter. 
     FIG. 22 is a schematic sectional view of the sheet accumulation apparatus in which the first sheet has been wrapped around a buffer roller. 
     FIG. 23 is a schematic sectional view of the sheet accumulation apparatus in which the first and second sheets S 1  and S 2  are being conveyed in layers. 
     FIG. 24 is a schematic sectional view of the sheet accumulation apparatus in which two sheets are being discharged in layers. 
     FIG. 25 is a schematic sectional view of the sheet accumulation apparatus, depicting how the second and third trays from the top are shifted upward and downward, respectively. 
     FIG. 26 is an enlarged sectional view of the discharge roller and its adjacencies, depicting a sheet which is sagging with its trailing end hanging up on the discharge roller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1 shows the internal structure of an image forming apparatus  50  to which the present invention is applicable. In this case, the apparatus is a copying machine. In the drawing, a reference numeral  1  designates the main assembly of a sheet accumulation apparatus in accordance with the present invention;  100 , the main assembly, that is, the image forming section, of the copying machine;  200 , a cassette capable of accommodating a plurality of sheets of different sizes; and a reference numeral  300  designates an apparatus for feeding originals (hereinafter, ADF) which automatically feeds a set of originals. 
     On the top surface of the sheet accumulation apparatus  1 , a stopper member  2  is disposed. When the sheet accumulation apparatus  1  is connected to the copying machine main assembly  100 , this stopper member  2  is engaged in a hold portion  133  provided on the lateral surface of the copying machine main assembly  100 , so that the positional relationship between the two apparatuses can be accurately established. Below the sheet accumulation apparatus  1 , a folding unit or an sheet accumulation apparatus placement table  70  is disposed. The table is provided with a caster  80  so that the apparatuses can be moved around. 
     Thus, a paper jam or the like, which happens to occurs adjacent to the sheet discharge portion of the copying machine main assembly  100 , or the junction between the sheet accumulation apparatus  1  and the copying machine main assembly  100 , can be easily handled just by rotating the stopper member  2  in the direction indicated by an arrow X in FIG. 2 to break its engagement with the hold portion  133 , and then, horizontally moving the sheet accumulation apparatus  1  away from the copying machine main assembly  100 . 
     The cut sheets (hereinafter, “sheets”) on which an image has been formed are conveyed to either sheet accumulation apparatus  1 , or an unillustrated folding apparatus disposed within the sheet accumulation apparatus placement table, depending on the processing necessary thereafter. In this embodiment, only a case in which the sheets are processed within the sheet accumulation apparatus  1  will be described. 
     Referring to FIG. 2, when the sheets are to be processed within the sheet accumulation apparatus  1 , the upstream end of the first flapper  3  located closest to the copying machine main assembly  100  is directed forward, and the upstream end of the second flapper  4  located on the downstream side of the first flapper is directed upward. As a result, the sheets are sent to the first conveyance path  6  through a roller pair  5 , and then, conveyed further downstream from the first conveyance path  6 . On the other hand, when the sheets are to be conveyed to the folding apparatus, the upstream end of the first flapper  3  is directed upward as shown in the drawing, whereby the sheets are sent through the third conveyance path  7  to the folding apparatus disposed at a location ahead of a dotted arrow mark in the drawing. 
     Referring to the same drawing, a reference numeral  8  designates the second conveyance path (buffer path);  9 , a buffer roller;  14 ,  15  and  16 , buffer path rollers; and reference numerals  10 ,  11  and  12  designate sheet detection sensors, which detect a passing sheet as well as a staying sheet. A reference numeral  17  designates the first discharge roller, which is apart of discharging means;  18 , a pressure roller, which also is a part of the discharging means;  19 , a jog-discharge belt, which is a part of abutting means. The jug-discharge belt  19  is pinched by the first discharge roller  17  and the pressure roller  18 , and is rotatively driven by the first discharge roller  17 . Further, in order to prevent the belt from from dislodging, the jog-discharge belt  19  is provided with an unillustrated endless rib. This endless rib is disposed on the approximate center line of the internal surface of the belt, and is engaged with the first discharge roller  17 . 
     A reference numeral  25  designated a abutting plate, which is provided on a stapling tray, that is, the first tray (abutting tray), to jog the sheets (FIG.  3 ). A reference numeral  20  designates a trailing end regulating member capable of taking two positions: a home position for sequentially accumulating the sheets, and a retraction position where it is retracted when a stapler  22  is moved forward or backward. More specifically, the trailing end regulating member  20  shares the same space as the stapler  22 ; therefore, the trailing end regulating member  20  is rendered rotatively retractable to a position outlined with a dotted line when it is necessary to change the position of the stapler  22 . 
     Next, referring to FIGS. 3-8, a sheet abutting section  60 , which pertains to the gist of the present invention, will be described in detail. 
     In this embodiment, the widthwise abutting of the sheets is carried out by a widthwise sheet shifting guide  21 , shown in FIGS. 3 and 4, which functions as abutting means. The stapler  22  is rendered movable within a range indicated by arrow marks in FIG. 3, so that it can carry out three binding modes: a two point biding mode, a single point front binding mode, and a single point rear binding mode. The stapler  22  has substantially the same structure as a commercially available ordinary automatic stapling apparatus driven by a solenoid or an electric motor, and binds a set of sheets by striking a staple into a set of sheets. At this time, however, the detailed description of the operation of the stapler  22  will be omitted. Referring again to FIGS. 3 and 4, a reference numeral  63  designates an abutting reference plate which functions as a sheet regulating member (first sheet regulating member). As the abutting reference plate  63  is retracted, the moving rang of the widthwise sheet shifting guide  21  slightly increases to allow one sheet in each set of sheets to be slightly displaced from the others. 
     In a normal abutting movement, the widthwise sheet shifting member  21  moves in the horizontal direction of FIG.  4 . This horizontal movement is caused by a motor  65  as the first driving means. More specifically, as a pinion gear  66  is rotated by the motor  65 , the widthwise sheet shifting guide  21  is horizontally moved through a rack  67  and a linkage member  68 . Then, as a sheet conveyed into the stapling tray  38  is moved to the right in FIG. 4 by the widthwise sheet shifting guide  21 , it is nudged against the abutting reference plate  63  or a sheet regulating fixed member  64  as the second sheet regulating member, being thereby jogged. 
     FIGS. 5-8 illustrate several examples of the method for retracting the abutting reference plate  63 . The abutting reference plate  63  illustrated in FIG. 5 has a fairly large surface. It is rotatively retracted to a position below the sheet accumulation surface of the stapling tray  38  by a stepping motor  63   b  as the second driving means, by way of a rotational axis  63   a.  FIG. 6 shows another method, in which an abutting reference plate  63 ′ is vertically retracted to a position below the sheet accumulation surface of the stapling tray  38 . FIG. 7 shows another method, in which an abutting reference plate  63 ″ is rotatively retracted into the stapling tray  38 . FIG. 8 shows another method, in which a abutting reference plate  63 ″′ is moved in the direction parallel to the direction in which the widthwise sheet shifting member  21  is moved. As the abutting reference plate  62  is retracted, a sheet is moved beyond the home position of the abutting reference plate  62 , and is nudged against the fixed member  64 , by the widthwise sheet shifting guide  21 , being thereby jogged in a manner to be slightly displaced from the other sheets in a sheet set. 
     Referring again to FIG. 2, reference numerals  23 ,  24  and  25  designate the third, second and first trays from the top, each of which constitutes a second tray into which sheets are discharged from a discharge opening  50  formed in the side wall of the sheet accumulation apparatus  1 . A reference numeral  26  designates a tray unit, to which these trays  23 ,  24  and  25  are attached, being tilted toward the sheet accumulation apparatus  1  and vertically aligned. The tray unit  26  is attached to the side wall surface  1   a  of the sheet accumulation apparatus  1  in a manner to allow the tray unit  26  to be vertically moved by an unillustrated driving power source disposed in the bottom portion thereof. 
     Also on the side wall surface of the sheet accumulation apparatus  1 , top and bottom regulating guides (hereinafter, “slotted top guide” and “slotted bottom guide”)  27  and  27   a  are provided, which come in contact with the lower ends (trailing ends) of the sheets discharged and accumulated into the trays  23 ,  24  and  25 , and prevent the sheets from sliding upstream, that is, backward. 
     Next, referring to FIG. 1, the structure of the copying machine main assembly  100  will be described. In the copying machine main assembly  100  as the image forming portion, a reference numeral  101  designates a glass plate on which a sheet of original is placed;  103  and  104 , scanning deflection mirrors (scanning mirrors) for changing the direction of the light reflected by the original;  105 , a focusing lens with variable power;  106 , a first scanning mirror comprising an illumination lamp for reading the original sent from the ADF  300 ;  107 , a registration roller;  108  and  110 , a photosensitive member and a pressure roller, respectively;  111 , a conveyer belt for conveying a recording sheet, on which an image has been recorded, toward a fixing apparatus; and a reference numeral  112  designates a fixing device for thermally fixing the recording sheet. 
     Reference numerals  113  and  117  designate a conveyor roller;  114 , a flapper for switching the direction in which the delivered recording sheet is conveyed;  115 , a conveyer roller for conveying the recording sheet toward the sheet accumulation apparatus  1 ;  116 , an inversion path for inverting the recording sheet;  118 , a conveyer roller for conveying a sheet from the sheet feeder cassette to a photosensitive drum unit;  119 ,  120  and  120 , a conveyer roller for conveying a sheet from a manual sheet feeder unit to the photosensitive drum unit, a manual feeder tray, and a separation pad, correspondingly;  122 ,  123  and  125 , a laser, a polygon mirror, and a light path changing mirror, correspondingly, which form an image on a photosensitive drum; and a reference numeral  124  designates a motor for driving the polygon mirror  123 . 
     A reference numeral  200  designates a sheet feeder cassette capable of accommodating various sheets of a different size, with the provision of dedicated compartments for different sizes, and also capable of sending an appropriate sheet to the copying machine main assembly  100  is response to a signal from the copying machine main assembly  100 ;  201 , a conveyer roller for pulling out a sheet from the cassette  200 ; and a reference numeral  202  designates an intermediate roller for mediating the upward conveyance of a sheet pulled out from the cassette  200 . The surface of the photosensitive drum  103  is covered with a seamless photosensitive material composed of a photosensitive material and an ordinary electrically conductive material. This drum  108  is rotatively supported by an axis, and is rotated in the direction of an arrow mark in the drawing, by a main motor (unillustrated) which rotates in response to the pressing of a copy start key. After the completion of a revolution control process and a voltage control process (pre-image formation processes), the original placed on the original placement glass plate  101  is illuminated by the illumination lump integrated with the first scanning mirror  106 . The light reflected by the original is deflected by the scanning mirrors  103  and  104 , transmitted through the lens  5 , and forms an optical image as it is focussed on a light reception element deposed within the lens unit. 
     The optical image formed on the light reception element by the reflected light from the original is converted into electric signals, and thus generated electric signals are sent to an image processing section (unillustrated). In the image processing section, after the electric signals are subjected to a predetermined data processing procedure in response to the instruction given to the copying machine main assembly  100  by a user, the electric signals are sent to a laser  112 . The electric signals having been subjected to the data processing procedure are converted into light in the laser  112 . Then, the thus generated light is deflected by polygon mirrors  123  and  125 , forming an electrostatic latent image on the photosensitive member. Then, the latent image is visualized as a toner image using toner, and the toner image is transferred onto a sheet of transfer paper which will be described later. 
     As for the sheet of transfer paper placed in the cassette  200  or the manual feeder tray, it is sent into the copying machine main assembly  100  by the sheet feeder rollers  118 ,  119 ,  201  and  202 , and further conveyed toward the registration roller  109  with precise timing so that the leading end of the latent image and the leading end of the transfer paper synchronously arrive in the transfer portion. Thereafter, the toner image on the drum  108  is transferred onto the transfer paper as the transfer paper is passed between the photosensitive member  103  and the roller  110 . 
     After the transfer of the other image, the transfer paper is separated from the photosensitive drum  108 , and is guided to the fixing device  112  by the conveyer belt  111 . In the fixing device  112 , the toner image is fixed to the transfer paper by heat and pressure. 
     In the ADF  300 , a reference numeral  310  designates an original set placement tray on which a set of original is placed. The sheets in a set of original placed on the original placement tray  310  are separated one by one from underneath by a half-moon roller  304  and a separation roller  303 , and are conveyed to the original placement glass plate  101  by a conveyer roller  350  and a full-width belt  306 , through paths I and II. As the first sheet of original is positioned at the exposure position, the ADF  300  is temporarily stopped, being prepared for the starting of a subsequent copying operation. 
     In the case of single sided originals, after the copying operation, the sheet of original is sent back to the original set placement tray  310  by a large conveyer roller  307  through paths IV and VI, and is deposited on the topmost sheet of the set of sheets of original  302  by the discharge roller  308 . A reference numeral  309  designates a recycle lever for detecting a single full circulation of all the sheets in the set of originals. More specifically, the recycle lever  309  is placed on top of the set of originals. As the sheets of original are sequentially circulated through the ADF  300 , and the last sheet of original is pulled away from underneath the recycle lever  309 , the recycle lever  309  falls down due to its own weight, signaling the completion of a single full circulation of all the sheets in the set of originals. 
     On the other hand, in the case of double sided originals, each sheet of original is first introduced into the path III through the paths I and II, and then, the direction of a pivotable sheet path change flapper  310  is switched to introduce the leading end of the sheet of original into the path IV. Then, the sheet of original is passed through the path II by the conveyer roller  305 , and conveyed onto the original placement glass plate  101  to be set thereon. In other words, the sheet of original is inverted by the coordinated functions of the large conveyer roller  307  and an inversion path constituted of the paths III-IV-II. 
     Next, the sheet discharging structure of the sheet accumulation apparatus  1  will be described in detail with reference to the drawings. Referring to FIGS. 12 and 13, a reference numeral  31  designates an oscillation guide as the oscillation member. The oscillation guide  31  rotatively holds a movable discharge roller  33  as a part of moving means, and presses the movable discharge roller  33  onto the discharge roller  32  as another part of the moving means, as shown in FIG.  12 . More specifically, when discharging a sheet, a cam  351  illustrated in FIG. 11 is rotated in the direction of an arrow mark in the same drawing by a driving power source M, so that the oscillation guide  31  is oscillated downward about an oscillation axis  31   a  to apply downward pressure to the movable discharge roller  33 . Further, when in a stapling mode which will be described later, the oscillating guide  31  is oscillated upward to separate the movable discharge roller  33  from the discharge roller  32 , disabling the sheet conveying function provided by the movable discharge roller  33  and the discharge roller  32 . In other words, the oscillation guide  31  plays a role as switching means for switching the relationship between the movable discharge roller  33  and the discharge roller  33 , between a sheet conveyance enabled state and a sheet conveyance disabled state. Further, the sheet nudging surface  31   b  of the oscillation guide  31  is provided with a rib  311  (FIG.  11 ). 
     Referring to FIG. 12, a reference numeral  30  designates a stopper rotatable about a rotational axis  30   a.  When moving the trays, the stopper  30  is rotated to a position outlined with a solid line in FIG. 13, and held there, so that the discharge opening  50  is blocked by a shutter portion  30   b  provided on the top end of the stopper  30 . With the provision of the above structure, when the tray  24  holding the accumulated sheets passes the F section and the discharged opening  50 , the sheets S having been accumulated on the tray  24  are prevented from moving upstream, that is, backward, into the discharge opening  50 . When discharging sheets, the stopper  30  is rotated in the direction of an arrow mark Y in FIG. 12 to unblock the discharge opening  50 . Further, when in a stapling mode which will be described later, it is rotated in the same direction as the oscillation guide  31  as shown in FIG. 16 to unblock the discharge opening  50 . Further, the stopper  30  and slotted top guide  27  are provided with ribs similar to those provided on the oscillation guide  31 . 
     Therefore, the oscillation guide  31  and stopper  30  form a flat surface which bridges between the slotted top and bottom guides  27  and  27   a,  so that the sheets having been accumulated on the tray  24  are allowed to vertically move, and the stopper  30  prevents the rotational movement of the oscillation guide  31  in the opening direction, that is, the stopper  30  also functions as a member for holding the oscillation guide  31 . 
     Also referring to FIG  12 , a reference numeral  34  designates a roller guide, which is a guide member for guiding the discharged sheet S to the tray  24 . As shown in the drawing, the roller guide  34  is a rotatable member comprising a horizontal arm  34   a  and a vertical arm  3   b,  wherein the vertical arm  34   b  is perpendicularly erected from the horizontal arm  34   a.  It is horizontally disposed immediately below the discharge opening  50 . A reference numeral  31   a  designates a driving axis to which the discharge roller  32  is fixed. It is rotatively driven by an unillustrated driving means. A reference numeral  34   c  designates an oscillation axis portion to which the roller guide  34  is rotatively attached, wherein the roller guide  34  is kept under pressure generated in the direction of an arrow mark A in FIG. 12 by a spring  37  attached to the horizontal arm  34   a,  and when in the stapling mode, it is moved to be retained at a position illustrated in FIG. 16 (hereinafter, second position), at which its surface becomes level with that of the slotted bottom guide  27   a,  so that even if the trailing end of a sheet S in a sheet set S accumulated in the tray  24  happens to lean on the side wall of the sheet accumulation apparatus  1 , with the middle portion of the sheet S sagging, it is prevented from hanging up between the slotted bottom guide  27   a  and the discharge roller  32 . 
     Normally, the sheets discharged by the discharge roller  32  and movable discharge roller  33  are sequentially accumulated in the trays  24 . However, if the structure of the slotted guiding surface is as simple as that of a guide member  40  illustrated in FIG. 13, the trailing end of a discharged sheet is liable to hang up at the transfer point between the discharge roller  32  and guide member  40 , preventing the sheet from being completely discharged, when a sheet is still holding the charge resulting from image formation, when the coefficient of inter-sheet friction is too high for desirable sheet discharge, or when the like situations occur. Therefore, the slotted bottom guide  27   a  is provided with ribs. With the provision of the above structure, the friction between the trailing end of a sheet and the slotted bottom guide  27   a  is sufficiently reduced to allow sheets to accumulate in the tray  24  without hanging up by the trailing end. 
     Further, the provision of the above structure eliminates the need for extending or retracting the slotted top guide  27 , or providing a separate slotted guide which can be moved into, or moved away from, the gap below the slotted top guide  27 ; therefore, the structure can be simplified. Further, the above provision makes it possible to use the surface of the rib  311  of the sheet nudging surface of the oscillation guide  31 , as the extension of the slotted top guide  27 , as shown in FIG. 25; therefore, the intervals among the trays can be narrowed. Referring to FIG. 25, as the oscillation guide  31  is rotated in the direction of the arrow mark in the drawing, the discharge opening is widened or narrowed. Therefore, normally, after the maximum number of sheets are accumulated in the tray  23 , the tray  24  must be located above the bottom end point Z of the slotted top guide  27  because of the following reason. For example, a stapling operation, which will be described later, is carried out while the accumulated sheets are still in the tray  24 . Therefore, as the oscillation guide  31  is rotated upward, the sheets having been accumulated below the point Z are liable to slide down from the tray  24  toward the stapling tray  38 . 
     Thus, when carrying out a stapling operation, the interval between the trays  23  and  24  must be at least as large as the distance between the surface of the top sheet of the set of sheets accumulated in the trays  24  to its maximum capacity, and the point Z. However, such a requirement is not mandatory when choice in operational mode is limited. For example, if the operational mode, which is available when the sheets accumulated in the tray  24  remain therein while sheets are discharged into the tray  23 , is limited to a non-stapling operation, the oscillation guide  31  does not need to be rotated upward; therefore, the sheet nudging surface  31   b  of the oscillation guide  31  can be used as an extension of the slotted to guide  27 . With this arrangement, it is possible to allow the tray  24  to descend below the point Z while the sheets remain in the tray  24 . Therefore, the interval beaten the adjacent two trays can be narrowed to reduce the size of the tray unit. 
     Referring to FIG. 16, in this embodiment, the top end of the vertical arm  34   b  is provided with an engagement claw  35 , which projects outward of the apparatus. This engagement claw  35  projects upward of the tray  24  when the roller guide  34  is held at the second position. With the provision of the above arrangement, even when the trailing end of the topmost sheet S of the sheet set Sa having accumulated in the tray  24  happens to lean on the side wall of the sheet accumulation apparatus  1 , with its middle portion sagging, the trailing end can be prevented from reaching beyond a point G (FIG.  16 ); therefore, it is possible to prevent a sheet from hanging up and causing a paper jam as the following sheet is discharged, or from hanging up and increasing the load against the abutting movement of the widthwise sheet shifting guide  21 . 
     The oscillating movement of the roller guide  34  is linked to the movement of the oscillation guide  31  by was of a linkage member  36 . In other words, as the oscillation guide  31  is oscillated downward to push down the upper arm  36   a  of the linkage member  36  as shown in FIG. 12, the linkage member  36  is rotated downward about a rotational axis  36   b.  Consequently, the horizontal arm  34   a  of the roller guide  34  is pushed down by the lower are  36   c  of the linkage member  36 . 
     As the horizontal arm  34   a  is pushed down, stretching the spring  37 , the roller guide  34  is oscillated to be retracted to a position (hereinafter, first position) at which the tip of the engagement claw  35  does not projects beyond the outward facing peripheral surface of the discharge roller  32  (FIG.  12 ). This spring  37  and the linkage member  36  constitute means for rotating the roller guide  34 . 
     With the provision of the above arrangement for retracting the roller guide  34  to the first position, the sheet S is prevented from hanging up between the roller guide  34  and discharge roller  32  when being discharged. In other words, the sheet S is reliably discharged. Further, referring to FIG. 14, the above structure also provides a recess I, which is illustrated by hatches in FIG. 14, between the roller guide  34  and the slotted bottom guide  27   a,  reducing the surface area which creates friction. Therefore, the discharged sheet S is allowed to smoothly fall into the tray  24 . 
     On the other hand, as the oscillation guide  31  is oscillated upward, allowing the linkage member  36  rotating upward, the roller guide  3  is return to the second position illustrated in FIG. 16 by the resiliency of the spring  37 . As the roller guide  34  is returned to the second position, the recess I is eliminated, and the elimination of the recess I prevents the sheet S from hanging up when the apparatus is in the stapling mode. 
     Next, the operation of the sheet accumulation apparatus  1  constructed in the above described manner, and the operation of the image forming apparatus  500  comprising the sheet accumulation apparatus  1 , will be described. 
     When sheets are discharged without stapling, they are directly discharged into the trays  23 ,  24  and  25 , which are the third, second and first trays counting from the top, and are all second order trays in terms of sheet transfer. First, referring to FIG. 9, a case in which sheets are discharged into the tray  24 , the second tray from the top, will be described. As a non-stapling mode is selected by the user, the oscillation guide  31  is oscillated by the cam  351  (FIG. 11) to a position at which the movable discharge roller  33  is caused to press on the discharge roller  33  as shown in FIG.  12 . At this point of time, the stopper will have been rotated in the direction of an arrow mark Y, being held there, and the roller guide  34  will have been held at the first position by the oscillation guide  31  and the linkage member  36 , with the engagement claw  35  having been retracted below the outward facing peripheral surface of the discharge roller  32 . 
     With the sheet accumulation apparatus  1  being in the above described state, the sheets discarded from the copying machine main assembly  100  are conveyed through the path  6 , which is illustrated in FIG. 2, and are relayed to a pair of rollers  17  and  18  to be conveyed further downstream. Thereafter, the sheets are directed toward the tray  24  by the oscillation guide  31 , are passed between the discharge rollers  32  and  33 , are discharged from the discharge opening  50 , and are sequentially deposited in the trays  24  (FIG.  12 ). With the sheet accumulation apparatus  1  being in the above state, even when the sheet S is not discharged with sufficient velocity from the discharge opening  50  due to such conditions of the sheet S that the sheet S is till holding the charge or the inter-sheet friction is rather high, the presence of the roller guide  34  at the first position prevents the sheet S from hanging up on the discharge roller  32 , and the presence of the recess I between the discharge roller  32  and the slotted bottom guide  27   a  allows the sheet S to smoothly descend into the tray  24  without interference. 
     Next, a case in which a large number of copies are made using plain sheets S will be described. First, it is confirmed by sheet-presence sensors  23   a,    24   a  and  25   a  (FIG. 2) provided in the corresponding trays  23 ,  24  and  25  that there is no sheet in the trays  23 ,  24  and  25 . Then, the tray unit  26  is moved to a predetermined position so that the tray  23  is enabled to receive the first sheet. As the number of sheets accumulated in the tray  23  reaches a predetermined value, the tray unit  26  is lowered to a predetermined position so that the top surface of the accumulate sheets is lowered to the level at which the surface of the tray  23  was located when it received the first sheet. This operation is repeated until the sheets are accumulated in the tray  23  to its maximum capacity. As soon as it is detected that the maximum capacity of the tray  23  has been reached, a stop signal is sent to the copying machine main assembly  100  to temporarily hold sheet discharge. 
     Next, in order to accumulate sheets in the tray  26 , the tray unit  26  is lowered to a predetermined position at which the first sheet for the tray  24  is to be deposited in the tray  24 . Then, the copying machine main assembly  100  is instructed to restart the copying operation. After, the copying operation is restarted, the operation described in the foregoing paragraph with reference to the tray  23  is repeated until the tray  24  is filled to its maximum capacity. It should be noted here that when switching the sheet accumulating operation from the tray  24  to the tray  25 , switching is made in the same manner as the switching from the tray  23  to the tray  24 . 
     The coping machine main assembly  100  in this embodiment is of a digital type, and comprises a scanner section which reads the image on a sheet of original, and a printer section which reproduces the image, wherein each section can be independently operated. More specifically, in the scanner section, a sheet of original is illuminated with a lamp. Then, the light reflected from the original is broken down into microscopic dots (picture element), and converted into electric signals corresponding to the optical density of the original, by a light reception element (photoelectric conversion). In the printer section, a copy of the original image is formed through a process in which an electrostatic latent image is formed on a drum by scanning the drum by a laser beam reflecting the electrical signals sent from the scanner section, a process in which the latent image is developed with developer, a process in which the developed image is transferred, and a process in which the transferred image is fixed. 
     Therefore, the imaging signals generated by the scanner section can be transferred to a facsimile machine  502  by connecting an interface  501  to the digital copying machine as shown in FIG. 1, and also, the electric signals received from the facsimile machine  502  can be sent to the printer section through the interface  501  to be reproduced into an image on a sheet of transfer paper. Further, the image signals received from a computer device  503  such as a personal computer may be sent to the printer section to be reproduced into an image on the transfer sheet, or the image read by the scanner section may be fed into a personal computer through the interface  501 . 
     As described above, with the provision of the interface  501 , a latest digital copying machine can be used not only as a copying machine which copies an original by reading the original which is sent from the ADF  300  and is placed on the original placement glass plate  101 , but also as a facsimile machine or a printer for a personal computer. 
     However, in order for the copying machine  100  to be usable in a more productive manner, the copying machine  100  must be capable of sorting sheets into separate trays, or specific trays numbered and selected by the user. Therefore, in this embodiment, the tray  23  is designated to accumulate the sheets (hereinafter, facsimile sheet) related to a facsimile machine; the tray  24  is designated to accumulate the sheets (hereinafter, computer sheet) related to a personal computer; and the tray  25  is designated to accumulate the sheets (hereinafter, copy mode sheet) when the copying machine  100  is in a copying mode. Next, the operation for discharging sheets into specific trays will be described. 
     First, referring to FIG. 10, a case in which the copy mode sheet S is discharged after a certain number of the computer sheets have been received and are remaining in the tray  24 , that is, a case in which the tray  25 , the first tray from the top, is used will be described. When the first tray  25  is used after a certain number of the computer sheets have been received and are remaining in the tray  24 , the tray unit  26  (FIG. 2) is lowered so that the tray  25  is positioned to receives the first sheet for the tray  25 . This operation for lowering the tray unit  26  is the same as the operation for lowering the tray unit  26  during an ordinary copying mode, except that the tray  24  has not been filled to its maximum capacity. 
     Next, a case in which the facsimile sheets are accumulated after a certain number of the computer sheets have been received and are remaining in the tray  24 , the second tray from the top, will be described. In other ways, a case in which the tray  23 , the third tray from the top, is used will be described. In order to accumulate sheets in the third tray  23  while the sheets accumulated in the second tray  24  remain in the second tray  24 , the tray unit  26  is raised. At this point of time, the stopper  30  is rotated to a position indicated by a solid line in FIG. 15 so that the shutter portion  30   b  closes the space F, an area hatched in FIG. 12, to prevent the sheet S from entering the space F. As a result, it becomes possible to move the second tray  24  upward while it is still holding the sheet S. The sheet nudging surface of the shutter portion  30   b  is provided with ribs so that the sheet S can slide on the sheet nudging surface as easily as on the sheet nudging surface  31   b  (FIG. 11) of the oscillation guide  31  and on the surface of the slotted bottom guide  27   a.    
     With the provision of the above described structure, the sheet S can pass the location of the discharge opening  50  without sliding upstream into the discharge opening  50 . Therefore, it becomes possible to move upward the second tray  24  while it is still holding the accumulated sheets. Also, as will be described later, the interval between the adjacent trays can be narrowed by making the sheet nudging surface of the oscillation guide  31  function as the extension of the slotted top guide  27 , so that the size of the tray unit  26  can be reduced. 
     Even though the operation described above is an operation for switching from the second tray  24  to the third tray  23 , the tray switching operation remains the same no matter which tray becomes the starting tray. 
     Next, the stapling operation of the sheet accumulation apparatus  1  will be described. To begin with, when in a stapling-sorting mode in which produced copies are stapled, the sheets are not accumulated directly into the trays  23 ,  24  or  25 , and instead, they are first accumulated into the stapling tray  38  shown in FIG.  2 . As a stapling-sorting mode is selected by the user, the sheet accumulation apparatus  1  operates as will be described next with reference to FIGS. 17-20. 
     As a stapling-sorting mode is selected by the user, the oscillation guide  31  is moved upward by an unillustrated driving power source, and is positioned to widen the discharge opening as illustrated in FIG.  16 . In other words, the discharge roller  32  and the movable discharge roller  33  become separated from each other. When the sheet accumulation apparatus  1  is in this state, the sheets discharged from the copying machine main assembly  100 , that is, the image forming section, are sent through the first conveyance path  6 , and are relayed to the first discharge roller  17  and the pressing roller  18 . Then, the sheets discharged by the first discharge roller  17  and the pressure roller  18  are accumulate in the stapling tray  38  since the oscillation guide  31  is up. At this point of time, the tray  24  is positioned higher than when the sheet accumulation apparatus  1  is in a non-stapling mode, and supports the leading end of a sheet to assist the sheet in moving in the upstream direction, that is, the direction opposite to the sheet discharge direction, as shown in FIG.  17 . 
     Also referring to FIG. 17, the sheet discharged onto the top surface of the stapling tray  38  is not only assisted in sliding down on the stapling tray  38  in the upstream direction due to its own weight, by the inclination of the stapling tray  38  and the raised sheet landing point in the tray  24 , but also is nudged in the upstream direction of the stapling tray  38  by the discharge-jog belt  19  being rotated in the direction of an arrow mark in synchronism with the first discharge roller  17 . As a result, the sheet hits the trailing end regulating member  20 , to be jogged in the sheet discharge direction. As for the widthwise sheet abutting, that is, the sheet abutting in the direction perpendicular to the sheet discharge direction, it is carried out by the widthwise sheet shifting guide  21  illustrated in FIGS. 3 and 4. More specifically, the widthwise sheet shifting guide  21  is activated a predetermined time after the sheet deposited in the stapling tray  38  hits the trailing and regulating member  20 . It is moved in the rear to front direction of FIG. 2 by a predetermined distance. With this movement of the widthwise sheet shifting guide  21 , and the presence of the abutting reference plate  63  or sheet regulating fixed member  64 , the sheet is jogged in the width direction. 
     The above operation is repeated for the second sheet and the following sheets until all the sheets, the number of which is set by the user, are accumulated in the stapling tray  38 . Referring to FIG. 18, as the user selected number of sheets are accumulated and aligned in the stapling tray  38 , the stapler  22  is activated to staple the sheets, on the spots chosen by the user (FIG.  3 ). After stapling, the oscillation guide  31  is lowered, and as the discharged roller  32  is rotated in the direction of an arrow mark in FIG. 19, a set of sheets accumulated on the stapling tray  38  and stapled thereon is discharged into the tray  24  (FIG.  20 ). 
     If the discharge opening is left exposed by the oscillation guide  31 , the sheets accumulated in the trays are liable to slide upstream back into the stapling tray  38  when sheets are selectively discharged into the trays  23 ,  24  and  25 . However, the structure in this embodiment is such that the trays  23 ,  24  and  25  become movable only after the oscillation guide  31  and stopper  30  are positioned to cover the discharge opening, and the roller  34  is rotated. With the provision of the above structure, a sheet is allowed to move across the discharge opening of the stapling tray  38  without sliding into the discharge opening. Therefore, it is possible to move the trays  23 ,  24  and  25  while the accumulated sheets are still in the trays, and such capability makes it possible to take full advantage of the capability of the image forming apparatus  500  comprising the interface  501 . 
     On the other hand, when the apparatus is in a stapling mode, the first sheet discharged in the following set of sheets is retained in the sheet accumulation apparatus  1 , and is discharged together with the second sheet, being laid in layers, since the sheets are continuously discharged from the copying machine main assembly  100 . This operation will be described with reference to FIGS. 21-24. FIG. 21 depicts a state of the sheet accumulation apparatus  1  which the sheet S began to enter. The first sheet S 1  discharged from the copying machine main assembly  100  is sent to a buffer path  88  by directing downward the upstream side ends of the flappers  3  and  4 . The sheet S 1  sent to the buffer path  8  is conveyed in the direction of an arrow mark in the drawing, in a manner to be wrapped around the buffer roller  9 . At this point of time, the flapper  39  is rotated in a manner to send the sheet S 1  toward the roller  15 . Then, as soon as the leading end of the sheet S 1  is detected by a sensor  11 , the buffer roller  9  is stopped in the state as shown in FIG.  22 . 
     Then, as the second sheet S 2  enters the sheet accumulation apparatus  1 , the buffer roller  9  is caused to rotate again, conveying the first and second sheets S 1  and S 2  in layers as shown in FIG.  23 . Next, as the trailing end of the first sheet S 1  passes the location of the flapper  39 , the flapper  39  is rotated in a manner to direct the sheet S toward the discharge rollers  17  and  18 . Then, two sheets are discharged, together in layers, into the stapling tray  38 . This operation is carried out to prevent the sheet S from being discharged by the discharge roller  17  and  18  while the stapler is stapling. In other words, a stapling operation can be carried out without temporarily holding the operation of the copying machine main assembly  100 . It is also possible to wrap the third sheet, the fourth sheet, and so on, around the buffer roller  9  to earn more time for the stapling operation. 
     The above operation is repeated to produce multiple stapled copy sets Sa. Referring to FIGS. 14 and 16, when the copy sets Sa which are excessively bent or bulky are accumulated in the tray  24 , the topmost sheet S of the topmost copy set Sa which has been already accumulated in the tray  24  is liable to come above the point G. If this occurs, the following copy set Sa is liable to hang up as it is discharged, causing thereby a jam, or is liable to increase the load against the abutting movement of the widthwise sheet shifting guide  21 , impeding thereby the abutting performance of the widthwise sheet shifting guide  21 . However, in this embodiment, when the sheet set Sa is discharged, the roller guide  34  is at the second position, and therefore, the engagement claw  35  of the roller guide  34  is projecting into the space above the tray  24 , being enabled to press down the trailing end of the sheet S. Therefore, occurrence of a jam or the like can be prevented. 
     In other words, in this embodiment, normally, when sheets are discharged, the roller guide  34  is placed at the first position to create the recess I (FIG.  14 ); therefore, sheets can be smoothly discharged. Further when the sheet accumulation apparatus  1  is in a stapling mode, the roller guide  34  is placed at the second position (FIG. 16) to form a temporary wall which is level with the slotted bottom guide  27   a  and temporarily abolishes the recess I, and also to cause the engagement claw  35  to protrude into the space above a pertinent tray so that it becomes possible to avoid occurrence of such a situation that a sheet S discharged into the tray hangs up on the discharge roller  32  as shown in FIG.  26 . 
     Next, an operation in which multiple sheet sets which are not stapled are sorted and accumulated into the trays  23 ,  24  and  25  will be described. The first set of sheets are accumulated in the stapling tray  38 , and are transferred onto the tray  24 , for example. Next, a controlling means CPU provided in the image forming apparatus  500  controls the stepping motor  63   b  so that the abutting reference plate  63  is retracted to the second position, that is, the location below the sheet accumulation surface of the stapling tray (abutting tray)  38 . Then, the first sheet for the following sheet set (bottommost sheet of the following sheet set) is conveyed onto the stapling tray  38 , and is jogged against the training end regulating member  20  by the discharge roller  32  and discharge-jog belt  19 , which are being rotated in reverse. Then, the control means CPU controls the motor  65  for driving the widthwise sheet shifting guide  21 , in such a manner that the sheet is butted against the sheet regulating fixed member  64 . This sheet is conveyed into the tray  24  by the discharge roller  32  which, at this moment, is being controlled by the controlling means in a manner to rotate forward, and the movable discharge roller  33  which has been lowered to make it work in coordination with the discharge roller  32 . At this point of time, the first sheet set having been jogged against the abutting reference plate  63  and the first sheet of the following sheet set, which has been jogged against the sheet regulating fixed member  64  are displaced from each other by a distance equal to the distance between the abutting reference plate  63  and the sheet regulating fixed member  64 , in the direction perpendicular to the sheet conveyance direction. 
     Next, the abutting reference plate  63  is returned by the stepping motor  63  to the first position, that is, the location above the sheet accumulation surface of the stapling tray  38 , as illustrated by a solid line in FIG.  5 . Then, the second sheet and the sheets thereafter of the following sheet set are conveyed onto the stapling tray  38 , are abutted to or jogged against the trailing end regulating member  20  by the discharge-jog belt  19 , and are jogged against the returned abutting reference plate  63  by the widthwise sheet shifting guide  21 . Finally, a sheet set made up of the second sheet and the sheet thereafter of the following sheet set is conveyed onto the tray  24  from the tray  38 . 
     As the above operation is repeated, the bottommost sheet of the following sheet set sticks out to indicates the border between the preceding sheet set and the following sheet set. Therefore, even when multiple sheet sets are conveyed into a single tray  24  (or  23  or  25 ) and accumulated therein, the accumulated sheet sets do not mix, making the subsequent sorting step easier and more efficient. In this embodiment, the first (bottommost) sheet of a sheet set is displaced, but the last (topmost) sheet of a sheet set may be displaced to obtain the same effect. 
     While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.