Sheet finisher and image forming system using the same

A folding device of the present invention includes a fold plate and a fold roller pair for folding a sheet or a sheet stack conveyed thereto. A controller causes the fold roller pair to move back and forth while nipping the folded portion of the sheet or that of the sheet stack at its nip for thereby continuously exerting pressure on the folded portion. The fold roller pair is rotated in opposite directions for thereby sharpening the fold of the sheet or that of the sheet stack.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a folding device mounted on or operatively connected to a copier, printer or similar image forming apparatus for folding a sheet or recording medium or a sheet stack carrying images thereon or a sheet finisher for folding, sorting, stacking, stapling, center-stapling or otherwise finishing the sheet or the sheet stack, and an image forming system consisting of the sheet finisher and image forming apparatus.

2. Description of the Background Art

A sheet finisher positioned at the downstream side of an image forming apparatus for stapling or otherwise finishing a sheet stack is well known in the art. To meet the increasing demand for multiple functions, a sheet finisher having a center-stapling capability in addition to the conventional edge-stapling capability has recently been proposed. Further, a sheet finisher with a center-folding capability in addition to the center-stapling capability has been proposed to fold a center-stapled sheet stack at the center for thereby producing a pamphlet.

A sheet finisher with the binding capability mentioned above uses, in many cases, one or more pairs of fold rollers to fold a sheet stack. In this type of sheet finisher, a flat fold plate is caused to contact the stapled position of a sheet stack and push it into the nip of each fold roller pair, thereby folding the sheet stack.

When the fold plate is used to push a sheet stack into the nip of each fold roller, it is necessary to locate the sheet stack at a position where it faces the fold roller. Therefore, the fold roller pair located at the first stage is exposed to a sheet conveyance path, so that the sheet stack must be conveyed via the position where the fold roller pair is exposed. At this instant, if the sheet stack is relatively thick, then it is likely that the leading edge of the sheet stack facing the fold roller pair is caused to abut against the rollers or to be caught by the rollers and bent thereby.

In light of the above, it has been customary to use means for preventing a sheet stack from contacting the rollers, e.g., a shutter. The shutter prevents the leading edge of a sheet stack from contacting the rollers until it reaches a preselected position. However, the shutter or similar movable member must be driven by a mechanism arranged in the vicinity of the conveyance path, making the sheet finisher bulky. Moreover, the shutter slides on the surface of a sheet when operated, lowering the quality of an image printed on the sheet.

On the other hand, when a sheet stack is relatively thick, the folding device of the type described is apt to fail to sharply fold the sheet stack, leaving a swell in the sheet stack. To solve this problem, Japanese Patent Laid-Open Publication No. 9-2735, for example, discloses a folding system configured to pass a relatively thick, center-folded sheet stack through the nip of a told roller pair, reverse the rotation of the fold roller pair to again pass the sheet stack through the above nip, and repeat such a procedure a plurality of times. This system, however, has a drawback that the sheet stack, passed through the nip of the fold roller pair a plurality of times, is smeared around the fold due to sliding contact with the fold roller pair, failing to achieve high quality when implemented as a pamphlet.

To protect a sheet stack from smearing mentioned above, Japanese Patent Laid-Open Publication No. 10-218483, for example, proposes a system that lowers a speed at which a sheet stack is pulled out at the time of reversal of rotation of the fold roller pair, thereby efficiently obviating the swell of the sheet stack. This system, however, cannot fully free a sheet stack from smears although reducing them.

Japanese Patent Laid-Open Publication Nos. 2000-72320 and 2001-146363 each teach a system in which two fold roller pairs are arranged such that the former fold roller pair folds a sheet stack, and then the latter fold roller pair makes the fold of the sheet stack more firm. Although this kind of scheme almost frees a sheet stack form smears, it cannot sharply fold a relatively thick sheet stack and therefore fails to solve the problem of swell. Further, the system is not satisfactory as to productivity and whether or not a desired degree of fold can be formed.

Of course, for a given degree of pressure, the fold of a sheet stack becomes dull as the number or sheets constituting the sheet stack increases. In light of this, Japanese Patent Laid-Open Publication No. 3,254,363, for example, proposes a system including selecting means for selecting either one of a first and a second mode and counting means for counting sheets constituting a single sheet stack. In the first mode, a fold roller pair is rotated only in the forward direction to fold a sheet stack one time while, in the second mode, it is rotated in the forward direction and then in the reverse direction to fold the sheet stack two times. The second mode is selected in accordance with the output of the counting means, thereby sharpening the fold of the sheet stack when the sheet stack has more than a preselected number of sheets.

Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-183568 and 2000-198613.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a sheet finisher capable of preventing the leading edge portion of a sheet stack from bending, insuring high-quality folding and high-quality center folding and binding without resorting to a shutter or similar special member, and an image forming system including the same.

It is a second object of the present invention to provide a folder and a sheet finisher capable of efficiently obviating the swell of a sheet stack without smearing it and therefore insuring a high-quality bound sheet stack, and an image forming system including the same.

A folding device of the present invention includes a fold plate and a fold roller pair for folding a sheet or a sheet stack conveyed thereto. A controller causes the fold roller pair to move back and forth while nipping the folded portion of the sheet or that of the sheet stack at its nip for thereby continuously exerting pressure on the folded portion. The fold roller pair is rotated in opposite directions for thereby sharpening the fold of the sheet or that of the sheet stack.

A sheet finisher including the folding device and an image forming system consisting of the sheet finisher and an image forming apparatus are also disclosed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the sheet finisher and image forming system in accordance with the present invention will be described hereinafter.

First Embodiment

Referring toFIG. 1of the drawings, an image forming system embodying the present invention is shown and directed mainly toward the first object. As shown, the image forming system is generally made up of an image forming apparatus PR and a sheet finisher PD operatively connected to one side of the image forming apparatus PR. A sheet or recording medium driven out of the image forming apparatus PR via an outlet95is introduced in the sheet finisher PD via an inlet18. In the sheet finisher PD, a path A extends from the inlet18and includes finishing means for finishing a single sheet. In the illustrative embodiment, this finishing means is implemented as a punch unit or punching means100. Path selectors15and16steer the sheet coming in through the path A to any one of a path B terminating at an upper tray201, a path C terminating at a shift tray202, and a processing tray F. The processing tray F is used to position, staple or otherwise process a sheet or sheets and, in this sense, will sometimes be referred to as a staple tray hereinafter.

The image forming apparatus PR includes at least an image processor, an optical writing unit, a developing unit, an image transferring unit, and a fixing unit although not shown specifically. The image processor converts an image signal input thereto to image data that can be printed out. The optical writing unit optically scans the surface of a photoconductive element in accordance with the image data output from the image processor, thereby forming a latent image. The developing unit develops the latent image with toner to thereby produce a corresponding toner image. The image transferring unit transfers the toner image to a sheet. The fixing unit fixes the toner image on the sheet. While the image forming apparatus PR is assumed to execute an electrophotographic process, it may alternatively be of the type executing any other conventional image forming process, e.g., an ink-jet or a thermal transfer image forming process. In the illustrative embodiment, the image processor, optical writing unit, developing unit, image transferring unit and fixing unit constitute image forming means in combination.

Sheets sequentially brought to the staple tray F via the paths A and D are positioned one by one, stapled or otherwise processed, and then steered by a guide plate54and a movable guide55to either one of the path C and another processing tray G. The processing tray G folds or otherwise processes the sheets and, in this sense, will sometimes be referred to as a fold tray hereinafter. The sheets folded by the fold tray G are guided to a lower tray203via a path H. The path D includes a path selector17constantly biased to a position shown inFIG. 1by a light-load spring not shown. An arrangement is made such that after the trailing edge of a sheet has moved away from the path selector17, among a prestack roller8, rollers9and10and a staple outlet roller11, at least the prestack roller8and roller9are rotated in the reverse direction to convey the trailing edge of the sheet to a prestacking portion E and cause the sheet to stay there. In this case, the sheet can be conveyed together with the next sheet superposed thereon. Such an operation may be repeated to convey two or more sheets together.

On the path A merging into the paths B, C and D, there are sequentially arranged an inlet sensor301responsive to a sheet coming into the finisher PD, an inlet roller pair1, the punch unit100, a waste hopper101, roller pair2, and the path selectors IS and16. Springs, not shown, constantly bias the path selectors15and16to the positions shown in FIG.1. When solenoids, not shown, are energized, the path selectors15and16rotate upward and downward, respectively, to thereby steer the sheet to desired one of the paths B, C and D.

More specifically, to guide a sheet to the path B, the path selector15is held in the position shown inFIG. 1while the solenoid assigned thereto is deenergized. To guide a sheet to the path C, the solenoids are energized to rotate the path selectors15and16upward and downward, respectively. Further, to guide a sheet to the path D, the path selector16is held in the position shown inFIG. 1while the solenoid assigned thereto is turned off; at the same time, the solenoid assigned to the path selector15is turned on to rotate it upward.

In the illustrative embodiment, the finisher PD is capable of selectively effecting punching (punch unit100), jogging and edge stapling (jogger fence53and edge stapler S1), sorting (shift tray202) or folding (fold plate74and fold rollers81and82), as desired.

A shift tray outlet section I is located at the most downstream position of the sheet finisher PD and includes a shift outlet roller pair6, a return roller13, a sheet surface sensor330, and the shift tray202. The shift tray outlet section I additionally includes a shifting mechanism J shown inFIG. 2 and ashift tray elevating mechanism K shown in FIG.3.

As shown inFIGS. 1 and 3, the return roller13contacts a sheet driven out by the shift outlet roller pair6and causes the trailing edge of the sheet to abut against an end fence32shown inFIG. 2for thereby positioning it. The return roller13is formed of& sponge and is caused to rotate by the shift outlet roller6. A limit switch333is positioned in the vicinity of the return roller13such that when the shift tray202is lifted and raises the return roller13, the limit switch333turns on, causing a tray elevation motor168to stop rotating. This prevents the shift tray202from overrunning. As shown inFIG. 1, the sheet surface sensor330senses the surface of a sheet or that of a sheet stack driven out to the shift tray202.

As shown inFIG. 3specifically, the sheet surface sensor330is made up of a lever30, a sensor330arelating6to stapling, and a sensor330brelating to non-stapling. The lever30is angularly movable about its shaft portion and made up of a contact end30acontacting the top of the trailing edge of a sheet on the shift tray202and a sectorial interrupter30b. The upper sensor330aand lower sensor330bare mainly used for staple discharge control and shift discharge control, respectively.

More specifically, in the illustrative embodiment, the sensors330aand330beach turn on when interrupted by the interrupter30bof the lever30. Therefore, when the shift tray202is lifted with the contact end30aof the lever30moving upward, the sensor330aturns off. As the shift tray202is further lifted, the sensor330bturns off. When the outputs of the sensors330aand330bindicate that sheets are stacked on the shift tray202to a preselected height, the tray elevation motor168is driven to lower the shift tray202by a preselected amount. The top of the sheet stack on the shift tray202is therefore maintained at a substantially constant height.

The shift tray elevating mechanism K will be described in detail with reference to FIG.3. As shown, the mechanism K includes a drive unit L for moving the shift tray202upward or downward via a drive shaft21. Timing belts23are passed over the drive shaft22and a driven shaft22under tension via timing pulleys. A side plate24supports the shift tray202and is affixed to the timing belts23. In this configuration, the entire unit including the shift tray202is supported by the timing belts23in such a manner as to be movable up and down.

The drive unit L includes a worm gear25in addition to the tray elevation motor168, which is a reversible drive source. Torque output from the tray elevation motor168is transmitted to the last gear of a gear train mounted on the drive shaft21to thereby move the shift tray202upward or downward. The worm gear25included in the driveline allows the shift tray202to be held at a preselected position and therefore prevents it from dropping by accident.

An interrupter24ais formed integrally with the side plate24of the shift tray202. A full sensor334responsive to the full condition of the shift tray202and a lower limit sensor335responsive to the lower limit position of the shift tray202are positioned below the interrupter24a. The full sensor334and lower limit sensor335, which are-implemented by photosensors, each turn off when interrupted by the interrupter24a. In FIG.3, the shift outlet roller6is not shown.

As shown inFIG. 2, the shifting mechanism J includes a shift motor169and a cam31. When the shift motor or drive source169causes the cam31to rotate, the can31causes the shift tray202to move back and forth in a direction perpendicular to a direction of sheet discharge. A pin31ais studded on the shift cam31at a position spaced from the axis of the shift cam31by a preselected distance. The tip of the pin31ais movably received in an elongate slot32bformed in an engaging member32a, which is affixed to the back of the end fence32not facing the shift tray202. The engaging member32amoves back and forth in a direction perpendicular to the direction of sheet discharge in accordance with the angular position of the pin31a, entraining the shift tray202in the same direction. The shift tray202stops at a front position and a rear position in the direction perpendicular to the sheet surface ofFIG. 1(corresponding to the positions of the shift cam31shown in FIG.2). A shift sensor336is responsive to a notch formed in the shift cam31. To stop the shift tray at the above two positions, the shift motor169is selectively energized or deenergized on the basis of the output of the shift sensor336.

Guide channels32care formed in the front surface of the end fence32. The rear edge portions of the shift tray202are movably received in the guide channels32c. The shift tray202is therefore movable up and down and movable back and forth in the direction perpendicular to the direction of sheet discharged, as needed. The end fence32guides the trailing edges of sheets stacked on the shift tray202for thereby aligning them.

FIG. 4shows a specific configuration of the arrangement for discharging a sheet to the shift tray202. As shown inFIGS. 1 and 4, the shift roller pair6has a drive roller6aand a driven roller6b. A guide plate33is supported at its upstream side in the direction of sheet discharge and angularly movable in the up-and-down direction. The driven roller6bis supported by the guide plate33and contacts the drive roller6adue to its own weight or by being biased, nipping a sheet between it and the drive roller6a. When a stapled sheet stack is to be driven out to the shift tray202, the guide plate33is lifted and then lowered at a preselected timing, which is determined on the basis of the output of a guide plate sensor331. A guide plate motor167drives the guide plate33in such a manner in accordance with the ON/OFF state of a limit switch332.

FIG. 5shows the staple tray F as seen in a direction perpendicular to the sheet conveyance plane.FIG. 6shows a drive mechanism assigned to the staple tray F while FIG.7shows a sheet stack discharging mechanism. As shown inFIG. 6, sheets sequentially conveyed by the staple outlet roller pair11to the staple tray F are sequentially stacked on the staple tray F. At this instant, a knock roller12knocks every sheet for positioning it in the vertical direction (direction of sheet conveyance) while jogger fences53position the sheet in the horizontal direction perpendicular to the sheet conveyance (sometimes referred to as a direction of sheet width). Between consecutive jobs, i.e., during an interval between the last sheet of a sheet stack and the first sheet of the next sheet stack, a controller350(seeFIG. 17) outputs a staple signal for causing an edge stapler S1to perform a stapling operation. A discharge belt52with a hook52aimmediately conveys the stapled sheet stack to the shift outlet roller pair6, so that the shift outlet roller pair6conveys the sheet stack to the shift tray202held at a receiving position.

As shown inFIG. 7, a belt HP (Home Position) sensor311senses the hook52aof the discharge belt52brought to its home position. More specifically, two hooks52aand52a′ are positioned on the discharge belt52face-to-face at spaced locations in the circumferential direction and alternately convey sheet stacks stapled on the staple tray F one after another. The discharge belt52may be moved in the reverse direction such that one hook52aheld in a stand-by position and the back of the other hook52a′ position the leading edge of the sheet stack stored in the staple tray F in the direction of sheet conveyance, as needed. The hook52atherefore plays the role of positioning means at the same time.

As shown inFIG. 5, a discharge motor157causes the discharge belt52to move via a discharge shaft65. The discharge belt52and a drive pulley62therefor are positioned at the center of the discharge shaft65in the direction of sheet width. Discharge rollers56are mounted on the discharge shaft65in a symmetrical arrangement. The discharge rollers56rotate at a higher peripheral speed than the discharge belt52.

More specifically, torque output from the discharge motor157is transferred to the discharge belt52via a timing belt and the timing pulley62. The timing pulley (drive pulley)62and discharge rollers56are mounted on the same shaft, i.e., the discharge shaft65. An arrangement may be made such that when the relation in speed between the discharge rollers56and the discharge belt52should be varied, the discharge rollers56are freely rotatable on the discharge shaft65and driven by part of the output torque of the discharge motor157. This kind of scheme allows a desired reduction ratio to be set up.

The surface of the discharge roller56is formed of rubber or similar high-friction material. The discharge roller56nips a sheet stack between it and a press roller or driven roller57due to the weight of the driven roller57or a bias, thereby conveying the sheet stack.

A processing mechanism will be described hereinafter. As shown inFIG. 6, a solenoid170causes the knock roller12to move about a fulcrum12ain a pendulum fashion, so that the knock roller12intermittently acts on sheets sequentially driven to the staple tray F and causes their trailing edges to abut against rear fences51. The knock roller12rotates counterclockwise about its axis. A jogger motor158drives the jogger fences53via a timing belt and causes them to move back and forth in the direction of sheet width.

As shown inFIG. 8, a mechanism for moving the edge stapler S1includes a reversible, stapler motor159for driving the edge stapler S via a timing belt. The edge stapler S is movable in the direction of sheet width in order to staple a sheet stack at a desired edge position. A stapler HP sensor312is positioned at one end of the movable range of the edge stapler S1in order to sense the stapler S brought to its home position. The stapling position in the direction of sheet width is controlled in terms of the displacement of the edge stapler S1from the home position.

As shown inFIG. 9, the edge stapler S1is capable of selectively driving a staple into a sheet stack in parallel to or obliquely relative to the edge of the sheet stack. Further, at the home position, only the stapling mechanism portion of the edge stapler S1is rotatable by a preselected angle for the replacement of staples. For this purpose, an oblique motor160causes the above mechanism of the edge stapler S1to rotate until a sensor313senses the mechanism reached a preselected replacement position. After oblique stapling or the replacement of staples, the oblique motor160causes the stapling mechanism portion to return to its original angular position.

As shown inFIGS. 1 and 5, a pair of center staplers S2are affixed to a stay63and are located at a position where the distance between the rear fences51and their stapling positions is equal to or greater than one-half of the length of the maximum sheet size, as measured in the direction of conveyance, that can be stapled. The center staplers S2are symmetrical to each other with respect to the center in the direction of sheet width. The center staplers S2themselves are conventional and will not be described specifically. Briefly, after a sheet stack has been fully positioned by the jogger fences53, rear fences51and knock roller5, the discharge belt52lifts the trailing edge of the sheet stack with its hook52to a position where the center of the sheet stack in the direction of sheet conveyance coincides with the stapling positions of the center staplers S2. The center staplers S2are then driven to staple the sheet stack. The stapled sheet stack is conveyed to the fold tray G and folded at the center, as will be described in detail later.

There are also shown inFIG. 5a front side wall64a, a rear side wall64b, and a sensor responsive to the presence/absence of a sheet stack on the staple tray F.

Reference will be made toFIG. 15as well as toFIG. 1for describing a mechanism for steering a sheet stack. To allow the sheet stack stapled by the center staplers S2to be folded at the center on the fold tray G, sheet stack steering means is located at the most downstream side of the staple tray F in the direction of sheet conveyance in order to steer the stapled sheet stack toward the fold tray G.

As shown inFIG. 15, the steering mechanism includes the guide plate54and movable guide55mentioned earlier. As shown inFIGS. 10 through 12, the guide plate54is angularly movable about a fulcrum54ain the up-and-down direction and supports the press roller57, which is freely rotatable, on its downstream end. A spring58constantly biases the guide plate54toward the discharge roller56. The guide plate54is held in contact with the cam surface61aof a cam61, which is driven by a steer motor161.

The movable guide55is angularly movably mounted on the shaft of the discharge roller56. A link arm60is connected to one end of the movable guide55remote from the guide plate54at a joint60a. A pin studded on the front side wall64a,FIG. 5, is movably received in an elongate slot60bformed in the link arm60, limiting the movable range of the movable guide55. A spring59holds the link arm60in the position shown in FIG.10. When the steer motor161causes the cam61to rotate to a position where its cam surface61bpresses the link arm60, the movable guide55connected to the link arm60angularly moves upward along the surface or the discharge roller56. A guide HP sensor315senses the home position of the cam61on sensing the interrupter portion61cof the cam61. Therefore, the stop position of the cam61is controlled on the basis of the number of drive pulses input to the steer motor161counted from the home position of the cam61, as will be described later in detail.

FIG. 10shows a positional relation to hold between the guide plate54and the movable guide55when the cam61is held at its home position. As shown, the guide surface55aof the movable guide55is curved and spaced from the surface of the discharge roller56by a preselected distance. While part of the guide plate55downstream of the press roller57in the direction of sheet conveyance is curved complementarily to the surface of the discharge roller56, the other part upstream of the same is flat in order to guide a sheet stack toward the shift outlet roller6. In this condition, the mechanism is ready to convey a sheet stack to the path C More specifically, the movable guide55is sufficiently retracted from the route along which a sheet stack is to be conveyed from the staple tray F to the path C. Also, the guide plate54is sufficiently retracted from the surface of the discharge roller56. The guide plate54and movable guide55therefore open the above route sufficiently wide; the opening width is generally dependent on the stapling ability of the edge stapler S1and usually corresponds to the thickness of fifty ordinary sheets or less.

When the leading edge of a sheet stack steered by the guide plate54contacts the guide surface55aof the movable guide55, the guide surface55acauses the leading edge to make a hairpin turn with a small diameter R. When the cam61is in the home position, the movable guide55abuts against a plate, not shown, and biased by the spring59in the counterclockwise direction.

FIG. 11shows a condition wherein the guide plate54is moved about the fulcrum54acounterclockwise (downward) by the cam61with the press roller57pressing the discharge roller57. As shown, when the cam61rotates clockwise, it causes the guide plate54to move from the opening position to the pressing position along the cam surface61aof the cam61. As the cam61further rotates clockwise, its cam surface61braises the link arm60and thereby causes the movable guide55to move.

FIG. 12shows a condition wherein the cam61has further rotated from the above position to move the movable guide55clockwise (upward). In this condition, the guide plate54and movable guide55form the route extending from the staple tray F toward the fold tray G.FIG. 5shows the same relation as seen in the direction of depth.

In the condition shown inFIG. 10, a sheet stack positioned and stapled on the staple tray F can be delivered to the shift tray202while, in the condition shown inFIG. 12, the sheet stack can be delivered to the fold tray G. The guide surface55aof the movable guide55can block the space in which the guide55is movable, allowing a sheet stack to be smoothly delivered to the fold tray G. In this manner, the guide plate and movable plate55are sequentially moved in this order while overlapping each other, forming a smooth path for conveyance.

In the condition shown inFIG. 12, the guide plate54contacts the discharge roller56obliquely relative to the direction of sheet conveyance, compared to the condition shown in FIG.10. The guide plate54therefore guides the leading edge of the sheet stack toward the press roller57while restricting it in a wedge fashion. Although a sheet stack to be delivered to the fold tray G has been stapled at the center with the leading edge remaining free, such a sheet stack is restricted, as stated above, and pressed by the press roller57and then introduced in the gap between the movable guide55and discharge roller66. The leading edge of the sheet stack can therefore enter the above gap without becoming loose. The movable guide55steers, or turns, the sheet stack toward the fold tray G. It follows that the angle of conveyance can be freely selected in terms of the angle θ of the movable guide55, i.e., the circumferential length of the movable guide55. However, the maximum angle of conveyance is limited to 180° in relation to the other mechanisms.

Although the path selectors15and16shown inFIG. 1are capable of switching the conveyance path, they do not exert a conveying force themselves. Therefore, when the selector15or16steers a stack of several sheets or several ten sheets by a large angle, the sheet stack is apt to jam the path due to a difference in friction between the outer surface and the inner surface.

While in the illustrative embodiment the guide plate54and movable guide55share a single drive motor, each of them may be driven by a respective drive motor, so that the timing of movement and stop position can be controlled in accordance with the sheet size and the number of sheets stapled together.

The fold tray G will be described specifically with reference toFIGS. 13 and 14. As shown, the fold tray G includes a fold plate74for folding a sheet stack at the center. The fold plate74is formed with elongate slots74aeach being movably received in one of pins64cstudded on each of the front and rear side walls64aand64b. A pin74bstudded on the fold plate74is movably received in an elongate slot76bformed in a link arm76. The link arm76is angularly movable about a fulcrum76a, causing the fold plate74to move in the right-and-left direction as viewed inFIGS. 13 and 14. More specifically, a pin75bstudded on a fold plate cam75is movably received in an elongate slot76cformed in the link arm76. In this condition, the link arm76angularly moves in accordance with the rotation of the fold plate cam75, causing the fold plate74to move back and forth perpendicularly to a lower guide plate91and an upper guide plate92(see FIG.15).

A fold plate motor166causes the fold plate cam75to rotate in a direction indicated by an arrow in FIG.13. The stop position of the fold plate cam75is determined on the basis of the output of a fold plate HP sensor325responsive to the opposite ends of a semicircular interrupter portion75aincluded in the cam75.

FIG. 13shows the fold plate74in the home position where the fold plate74is fully retracted from the sheet stack storing range of the fold tray G. When the fold plate cam75is rotated in the direction indicated by the arrow, the fold plate74is moved in the direction indicated by an arrow and enters the sheet stack storing range of the fold tray G.FIG. 14shows a position where the fold plate74pushes the center of a sheet stack on the fold tray G into the nip between a pair of fold rollers81. When the fold plate cam75is rotated in a direction indicated by an arrow inFIG. 14, the fold plate74moves in a direction indicated by an arrow out of the sheet stack storing range.

While the illustrative embodiment is assumed to fold a sheet stack at the center, it is capable of folding even a single sheet at the center. In such a case, because a single sheet does not have to be stapled at the center, it is fed to the fold tray G as soon as it is driven out, folded by the fold plate74and fold roller pair81, and then delivered to the lower tray203, FIG.1.

FIG. 16shows a specific arrangement supporting the staple tray F and processing tray G,FIG. 15, such that they can be pulled out together to facilitate jam processing, maintenance or replacement. As shown, the told tray G extends perpendicularly from a bent portion, which is the arc of the discharge roller56, while the staple tray F obliquely extends from the bent portion with an acute angle. WhileFIG. 16shows only the end face of the staple tray F and that of the fold tray G, the trays F and G are accommodated in the direction of depth at least in the width of the tray F shown in FIG.5.

The angle of the staple tray F should preferably be as small as possible in order to reduce the projection area in the vertical direction and therefore the area to be occupied by the sheet finisher PD. However, in the illustrative embodiment, the fold plate74, link arm76, fold plate cam75and fold plate motor166constituting the folding mechanism ofFIGS. 13 and 14are arranged in the space between the fold tray G (guide plates91and92) and the staple tray F. More specifically, the folding mechanism is interposed between the edge stapler S1and the center staplers S2. The angle of the staple tray F relative to the fold tray G is selected such that none of the structural parts of the folding mechanisms interferes with any one of the structural parts of the staple tray F. The folding mechanism is positioned below the staple tray F so inclined. This arrangement allows the staple tray F, fold tray G and folding means to be arranged within the minimum vertical projection area.

To fold a sheet stack at the center, the center of the sheet stack should be coincident with a folding position assigned to the fold plate74, as will be described specifically later. For this purpose, in the illustrative embodiment, a movable rear fence73is included in the lower guide plate91such that the trailing edge of a folded sheet stack (leading edge when the sheet stack is to be conveyed) rests on the fence73. The movable rear fence73is movable upward or downward to bring the center of the sheet stack resting thereon to the folding position.

As shown inFIG. 1, the movable rear fence73is affixed to a drive belt73cpassed over a drive pulley73aand a driven pulley73band caused to move upward or downward by a rear fence motor not shown. Such a mechanism for moving the movable rear fence73, like the folding mechanism, is arranged in the space between the staple tray F and the fold tray G so as not to increase the vertical projection area.

As shown inFIG. 16, a unit U including the staple tray F and fold tray G, which have the relation stated above, is supported by a pair of guide rails66extending inward from an opening67formed in the finisher PD and can be pulled out of the finisher PD along the guide rails66. The guide plates91and92are hinged to the rear end of the unit U with their front ends being openable away from each other. A magnet, for example, may used to lock the openable ends of the guide plates91and92.

The unit U having the above configuration can be pulled out in the event of a jam and allows a jamming sheet to be easily removed. More specifically, when a jam occurs at the fold tray G side, the operator should only pull out the unit U halfway and can rapidly deal with the jam while watching the guide plates91and92opened away from each other. After the jam processing, when the operator pushes the unit U into the finisher PD, the guide plates91and92are automatically closed by the edges of the opening67and locked by the magnet. This obviates an occurrence that the operator fails to close the guide plates91and92and makes the next step impracticable.

While the guide rails66are positioned at the fold tray G side of the opening67, they may, of course, be located at any other position, e.g., a position above the guide plates91and92.

In the illustrative embodiment, the staple tray F is inclined by a large angle in relation to the fold tray G and folding mechanism, i.e., positioned obliquely at as small an angle as possible relative to the told tray G, as stated earlier. In this arrangement, the fold tray G is positioned below the staple tray F, so that the space above the staple tray F is questionable in the aspect of efficient use of space. In light of this, in the illustrative embodiment, the path D and prestacking portion E are positioned in parallel to the staple tray F while a waste receiver101aincluded in the waste unit101is held in an inclined position in the space available in the upper right portion, as seen in FIG.1. This promotes the efficient use of the limited space available in the finisher PD.

In the above configuration, if the sheet size is large, then a sheet stored in the prestacking portion E waits for the next sheet with its trailing edge in the direction of sheet conveyance protruding from the portion E. At this instant, because the sheet prestacking portion E is positioned in the upper right portion of the finisher PD, a sufficient space is available below the portion E and prevents the sheet from jamming the path.

Further, the folding mechanism of the fold tray G is located between the edge stapler S1and the center staplers S2, so that a sufficient space is available below the fold plate74even when the sheet size is large. Therefore, a sufficient space is guaranteed below the leading edge of a sheet despite that the sheet is conveyed vertically along the guide plates91and92.

Reference will be made toFIG. 17for describing a control system included in the illustrative embodiment. As shown, the control system includes a control unit350implemented as a microcomputer including a CPU (Central Processing Unit)360and an I/O (Input/Output) interface370. The outputs of various switches arranged on a control panel, not shown, mounted on the image forming apparatus PR are input to the control unit350via the I/O interface370. Also input to the control unit350via the I/O interface370are the output of the inlet sensor301, the output of an upper outlet sensor302, the output of a shift outlet sensor303, the output of a prestack sensor304, the output of a staple discharge sensor305, the output of a sheet sensor310, the output of the belt HP sensor311, the output of the staple HP sensor312, the output of the stapler oblique HP sensor313, the output of a jogger fence HP sensor314, the output of the guide home position sensor315, the output of a stack arrival sensor321, the output of a movable rear fence HP sensor322, the output of a fold position pass sensor323, the output of a lower outlet sensor324, the output of a fold plate HP sensor325, the output of sheet surface sensors330,330aand330b, and the output of the guide plate sensor331.

The CPU360controls, based on the above various inputs, the tray motor168assigned to the shift tray202, the guide plate motor167assigned to the guide plate, the shift motor169assigned to the shift tray202, a knock roller motor, not shown, assigned to the knock roller12, various solenoids including the knock solenoid (SOL)170, motors for driving the conveyor rollers, outlet motors for driving the outlet rollers, the discharge motor157assigned to the belt52, the stapler motor159assigned to the edge stapler S1, the jogger motor158assigned to the jogger fences53, the steer motor161assigned to the guide plate54and movable guide55, a motor, not shown, assigned to rollers for conveying a sheet stack, a rear fence motor assigned to the movable rear fence73, and a fold roller motor, not shown, assigned to the fold roller81. The pulse signals of a staple conveyance motor, not shown, assigned to the staple discharge rollers are input to the CPU360and counted thereby. The CPU360controls the knock SOL170and jogger motor158in accordance with the number of pulse signals counted. The fold roller motor is implemented by a stepping motor and controlled by the CPU360either directly via a motor driver or indirectly via the I/O370and motor driver.

Further, the CPU360causes the punch unit100to operate by controlling a clutch or a motor. The CPU360controls the finisher PD in accordance with a program stored in a ROM (Read Only Memory), not shown, by using a RAM (Random Access Memory) as a work area.

Specific operations to be executed by the CPU360in various modes available with the illustrative embodiment will be described hereinafter.

First, in a non-staple mode A, a sheet is conveyed via the paths A and B to the upper tray201without being stapled. To implement this mode, the path selector15is moved clockwise, as viewed inFIG. 1, to unblock the path B. The operation of the CPU360in the non-staple mode will be described with reference to FIG.18.

As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU360causes the inlet roller pair1and conveyor roller pair2on the path A to start rotating (step S101). The CPU360then checks the ON/OFF state of the inlet sensor301(steps S102and S103) and the ON/OFF state of the upper outlet sensor302(steps S014and S105) for thereby confirming the passage of sheets. When a preselected period at time elapses since the passage of the last sheet (YES, step S106), the CPU360causes the above rollers to stop rotating (step S107). In this manner, all the sheets handed over from the image forming apparatus PR to the finisher PD are sequentially stacked on the upper tray201without being stapled. It desired, the punch unit100, which intervenes between the inlet roller pair1and conveyor roller pair2, may punch the consecutive sheets.

In a non-staple mode B, the sheets are routed through the paths A and C to the shift tray202. In this mode, the path selectors15and16are respectively moved counterclockwise and clockwise, unblocking the path C. The non-staple mode B will be described with reference toFIGS. 19A and 19B.

As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU360causes the inlet roller pair1and conveyor roller pair2on the path A and the conveyor roller pair S and shift outlet roller pair6on the path C to start rotating (step3201). The CPU360then energizes the solenoids assigned to the path selectors15and16(step S202) to thereby move the path selectors15and16counterclockwise and clockwise, respectively. Subsequently, the CPU360checks the ON/OFF state of the inlet sensor301(steps S203and S204) and the ON/OFF state of the shift outlet sensor303(steps S205and S206) to thereby confirm the passage of the sheets.

On the elapse of a preselected period of time since the passage of the last sheet (YES, step S207), the CPU360causes the various rollers mentioned above to stop rotating (S208) and deenergizes the solenoids (steps5209). In this manner, all the sheets that have entered the finisher PD are sequentially stacked on the shift tray202without being stapled. Again, the punch unit100intervening between the inlet roller pair1and conveyor roller pair2may punch the consecutive sheets, if desired.

In a sort/stack mode, the sheets are also sequentially delivered from the path A to the shift tray202via the path C. A difference is that the shift tray202is shifted perpendicularly to the direction of sheet discharge copy by copy in order to sort the sheets. The path selectors15and16are respectively rotated counterclockwise and clockwise as in the non-staple mode B, thereby unblocking the path C. The sort/stack mode will be described with reference toFIGS. 20A and 20B.

As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU360causes the inlet roller pair1and conveyor roller pair2on the path A and the conveyor roller pair5and shift outlet roller pair6on the path C to start rotating (step S301) The CPU360then energizes the solenoids assigned to the path selectors15and16(step S302) to thereby move the path selectors15and16counterclockwise and clockwise, respectively. Subsequently, the CPU360checks the ON/OFF state of the inlet sensor301(steps S303and S304) and the ON/OFF state of the shift outlet sensor303(step3305)

If the sheet that has passed the shift outlet sensor303is the first sheet of a copy (YES, step S306), then the CPU360turns on the shift motor169(step S307) to thereby move the shift tray202perpendicularly to the direction of sheet conveyance until the shift sensor336senses the tray202(steps S308and S309). When the sheet moves away from the shift outlet sensor303(YES, step S310), the CPU360determines whether or not the sheet is the last sheet (step S311) If the answer of the step S311is NO, meaning that the sheet is not the last sheet of a copy, and if the copy is not a single sheet, then the procedure returns to the step S303. If the copy is a single sheet, then the CPU360executes a step S312.

If the answer of the step S306is NO, meaning that the sheet that has passed the shift outlet sensor303is not the first sheet of a copy, then the CPU360discharges the sheet (step S310) because the shift tray202has already been shifted. The CPU360then determines whether or not the discharged sheet is the last sheet (step S311). If the answer of the step S311is NO, then the CPU360repeats the step S303and successive steps with the next sheet. If the answer of the step S311is YES, then the CPU360causes, on the elapse of a preselected period of time, the inlet roller pair1, conveyor roller pairs2and5and shift outlet roller pair6to stop rotating (step S312) and deenergizes the solenoids assigned to the path selectors15and16(step S313). In this manner, all the sheets that have sequentially entered the finisher PD are sorted and stacked on the shift tray202without being stapled. In this mode, too, the punch unit100may punch the consecutive sheets, if desired.

In a staple mode, the sheets are conveyed from the path A to the staple tray F via the path D, positioned and stapled on the staple tray F, and then discharged to the shift tray202via the path C. In this mode, the path selectors15and16both are rotated counterclockwise to unblock the route extending from the path A to the path D. The staple mode will be described with reference toFIGS. 21A through 21C.

As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU360causes the inlet roller pair1and conveyor roller pair2on the path A and the conveyor roller pairs7,9and10and staple outlet roller11on the path D and knock roller12to start rotating (step S401). The CPU360then energizes the solenoid assigned to the path selector15(step S402) to thereby cause the path selector15to rotate counterclockwise.

After the stapler HP sensor312has sensed the edge stapler S1at the home position, the CPU360drives the stapler motor159to move the edge stapler S1to a preselected stapling position (step S403). Also, after the belt HP sensor311has sensed the belt52at the home position, the CPU360drives the discharge motor157to bring the belt52to a stand-by position (step S404). Further, after the jogger fence motor HP sensor has sensed the jogger fences53at the home position, the CPU360moves the jogger fences53to a stand-by position (step5405). In addition, the CPU360causes the guide plate54and movable guide55to move to their home positions (step S406).

If the inlet sensor301has turned on (YES, step S407) and then turned off (YES, step S408), if the staple discharge sensor305has turned on (YES, step S409) and if the shift outlet sensor303has turned on (YES, step S410), then the CPU360determines that a sheet is present on the staple tray F. In this case, the CPU360energizes the knock solenoid170for a preselected period of time to cause the knock roller12to contact the sheet and force it against the rear fences51, thereby positioning the rear edge of the sheet (step S411). Subsequently, the CPU360drives the jogger motor158to move each jogger fence53inward by a preselected distance for thereby positioning the sheet in the direction of width perpendicular to the direction of sheet conveyance and then returns the jogger fence53to the stand-by position (step S412). The CPU360repeats the step S407and successive steps with every sheet. When the last sheet of a copy arrives at the staple tray F (YES, step S413), the CPU360moves the jogger fences53inward to a position where they prevent the edges of the sheets from being dislocated (step S414). In this condition, the CPU360turns on the stapler S1and causes it to staple the edge of the sheet stack (step S415).

On the other hand, the CPU360lowers the shift tray202by a preselected amount (step S416) in order to produce a space for receiving the stapled sheet stack. The CPU360then drives the shift discharge roller pair6via the shift discharge motor (step S417) and drives the belt52by a preselected amount via the discharge motor157(step S418), so that the stapled sheet stack is raised toward the path C. As a result, the stapled sheet stack is driven out to the shift tray202via the shift outlet roller pair6. After the shift outlet sensor303has turned on (step S419) and then turned off (step S420), meaning that the sheet stack has moved away from the sensor303, the CPU360moves the belt52and jogger fences53to their stand-by positions (steps S421and S422), causes the shift outlet roller pair6to stop rotating on the elapse of a preselected period of time (step S423), and raises the shift tray202to a sheet receiving position (step S424). The rise of the shift tray202is controlled in accordance with the output of the sheet surface sensor330responsive to the top of the sheet stack positioned on the shift tray202.

After the last copy or set of sheets has been driven out to the shift tray202, the CPU360returns the edge stapler S1, belt52and jogger fences53to their home positions (steps S426, S427and S428) and causes the inlet roller pair1, conveyor roller pairs2,7,9and10, staple discharge roller pair11and knock roller12to stop rotating (step S429). Further, the CPU360deenergizes the solenoid assigned to the path selector15(step3430. Consequently, all the structural parts are returned to their initial positions. In this case, too, the punch unit100may punch the consecutive sheets before stapling.

The operation of the staple tray F in the staple mode will be described more specifically hereinafter. As shown inFIG. 6, when the staple mode is selected, the jogger fences53each are moved from the home position to a stand-by position 7 mm short of one end of the width of sheets to be stacked on the staple tray F (step S405). When a sheet being conveyed by the staple discharge roller pair11passes the staple discharge sensor305(step S409), the jogger fence53is moved inward from the stand-by position by 5 mm.

The staple discharge sensor305senses the trailing edge of the sheet and sends its output to the CPU360. In response, the CPU360starts counting drive pulses input to the staple motor, not shown, driving the staple discharge roller pair11. On counting a preselected number of pulses, the CPU360energizes the knock solenoid170(step S412). The knock solenoid170causes the knock roller12to contact the sheet and force it downward when energized, so that the sheet is positioned by the rear fences51. Every time a sheet to be stacked on the staple tray F1passes the inlet sensor301or the staple discharge sensor305, the output of the sensor301or305is sent to the CPU360, causing the CPU360to count the sheet.

On the elapse of a preselected period of time since the knock solenoid170has been turned off, the CPU360causes the jogger motor158to move each jogger fence53further inward by 2.6 mm and then stop it, thereby positioning the sheet in the direction of width. Subsequently, the CPU360moves the jogger fence53outward by 7.6 mm to the stand-by position and then waits for the next sheet (step S412). The CPU360repeats such a procedure up to the last page (step S413). The CPU360again causes the jogger fences53to move inward by 7 mm and then stop, thereby causing the jogger fences53to retain the opposite edges of the sheet stack to be stapled. Subsequently, on the elapse of a preselected period of time, the CPU360drives the edge stapler S1via the staple motor for thereby stapling the sheet stack (step S415). If two or more stapling positions are designated, then the CPU360moves, after stapling at one position, the edge stapler S1to another designated position along the rear edge of the sheet stack via the stapler motor159At this position, the edge stapler S1again staples the sheet stack. This is repeated when three or more stapling positions are designated.

After the stapling operation, the CPU360drives the belt52via the discharge motor157(step S418). At the same time, the CPU360drives the outlet motor to cause the shift outlet roller pair6to start rotating in order to receive the stapled sheet stack lifted by the hook52a(step S417). At this instant, the CPU360controls the jogger fences53in a different manner in accordance with the sheet size and the number of sheets stapled together. For example, when the number of sheets stapled together or the sheet size is smaller than a preselected value, then the CPU360causes the jogger fences53to constantly retain the opposite edges of the sheet stack until the hook52afully lifts the rear edge of the sheet stack. When a preselected number of pulses are output since the turn-on of the sheet sensor310or the belt HP sensor311, the CPU360causes the jogger fences53to retract by 2 mm and release the sheet stack. The preselected number of pulses corresponds to an interval between the time when the hook52acontacts the trailing edge of the sheet stack and the time when it moves away from the upper ends of the jogger fences53.

On the other hand, when the number of sheets stapled together or the sheet size is larger than the preselected value, the CPU360causes the jogger fences53to retract by 2 mm beforehand. In any case, as soon as the stapled sheet stack moves away from the jogger fences53, the CPU360moves the jogger fences53further outward by 5 mm to the stand-by positions (step S422) for thereby preparing it for the next sheet. If desired, the restraint to act on the sheet stack may be controlled on the basis of the distance of each jogger fence from the sheet stack.

In a center staple and bind mode, the sheets are sequentially conveyed from the path A to the staple tray F via the path D, positioned and stapled at the center on the tray F, folded on the fold tray G, and then driven out to the lower tray203via the path H. In this mode, the path selectors15and16both are rotated counterclockwise to unblock the route extending from the path A to the path D. Also, the guide plate54and movable guide plate55are closed, as shown inFIG. 25, guiding the stapled sheet stack to the fold tray G. The center staple and bind mode will be described with reference toFIGS. 22A through 22C.

As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU360causes the inlet roller pair1and conveyor roller pair2on the path A and the conveyor roller pairs7,9and10and staple outlet roller11on the path D and knock roller12to start rotating (step S401). The CPU360then energizes the solenoid assigned to the path selector15(step S402) to thereby cause the path selector15to rotate counterclockwise.

Subsequently, after the belt HP sensor311has sensed the belt52at the home position, the CPU360drives the discharge motor157to move the belt52to the stand-by position (step S503). Also, after the jogger fence HP sensor has sensed each jogger fence53at the home position, the CPU360moves the jogger fence53to the stand-by position (step S504). Further, the CPU360moves the guide plate54and movable guide55to their home positions (steps S505).

If the inlet sensor301has turned on (YES, step S506) and then turned off (YES, step S507), if the staple discharge sensor305has turned on (YES, step S508) and if the shift outlet sensor303has turned on (YES, step5509), then the CPU360determines that a sheet is present on the staple tray F. In this case, the CPU360energizes the knock solenoid170for the preselected period of time to cause the knock roller12to contact the sheet and force it against the rear fences51, thereby positioning the trailing edge of the sheet (step S510). Subsequently, the CPU360drives the jogger motor158to move each jogger fence53inward by the preselected distance for thereby positioning the sheet in the direction of width perpendicular to the direction of sheet conveyance and then returns the jogger fence53to the stand-by position (step S511). The CPU360repeats the step S407and successive steps with every sheet. When the last sheet of a copy arrives at the staple tray F (YES, step S512), the CPU360moves the jogger fences53inward to the position where they prevent the edges of the sheets from being dislocated (step S513).

After the step S513, the CPU360turns on the discharge motor157to thereby move the belt52by a preselected amount (step S514), so that the belt52lifts the sheet stack to a stapling position assigned to the center staplers S2. Subsequently, the CPU360turns on the center staplers S2at the intermediate portion of the sheet stack for thereby stapling the sheet stack at the center (step S515). The CPU360then moves the guides54and55by a preselected amount each in order to form a path directed toward the fold tray G (step S516) and causes the upper and lower roller pairs71and72of the fold tray G to start rotating (step S517). As soon as the movable rear fence73of the fold tray G is sensed at the home position, the CPU360moves the fence73to a stand-by position (step S518). The fold tray G is now ready to receive the stapled sheet stack.

After the step S518, the CPU360further moves the belt52by a preselected amount (step S519) and causes the discharge roller56and press roller57to nip the sheet stack and convey it to the fold tray G. After the leading edge of the stapled sheet stack has arrived at the stack arrival sensor321(step S520), the CPU360causes the fold roller pair81to rotate in the reverse direction (step S521), so that the sheet stack can be conveyed downward without being folded at a portion Q (see FIG.26). Subsequently, on the elapse of a preselected period of time in which the leading edge of the sheet stack is expected to move away from the portion Q, the CPU360causes the fold roller pair81to stop rotating (step S522). As soon as the sheet stack has been conveyed by a preselected distance, the CPU360causes the upper and lower roller pairs71and72to stop rotating (step S523) and then releases the lower rollers72from each other (step S524). Subsequently, the CPU360causes the fold plate74to start folding the sheet stack (step S525) and causes the fold roller pairs81and82and lower outlet roller pair83to start rotating (step S526). The CPU360then determines whether or not the folded sheet stack has moved away from the pass sensor323(steps S527and S528). If the answer of the step S528is YES, then the CPU360brings the lower rollers72into contact (step S529) and moves the fold plate74and guides54and55to their home positions (steps S530and S531).

In the above condition, the CPU360determines whether or not the trailing edge of the folded sheet stack has moved away from the lower outlet sensor324(steps S532and S533). If the answer of the step S533is YES, then the CPU360causes the fold roller pairs81and82and lower outlet roller pair83to further rotate for a preselected period of time and then stop (step S534) and then causes the belt52and jogger fences53to return to the stand-by positions (steps S535and S536). Subsequently, the CPU360determines whether or not the above sheet stack is the last copy of a single job to perform (step S537). If the answer of the step S537is NO, then the procedure returns to the step S506. If the answer of the step S537is YES, then the CPU360returns the belt52and jogger fences53to the home positions (steps S538and S539). At the same time, the CPU360causes the inlet roller pair1, roller pairs2,7,9and10, staple discharge roller pair11and knock roller12to stop rotating (step S540) and turns off the solenoid assigned to the path selector15(step S541). As a result, all the structural parts are returned to their initial positions.

The stapling and folding operations to be performed in the center fold mode will be described in more detail hereinafter. A sheet is steered by the path selectors15and16to the path D and then conveyed by the roller pairs7,9and10and staple discharge roller11to the staple tray F. The staple tray F operates in exactly the same manner as in the staple mode stated earlier before positioning and stapling (see FIG.23). Subsequently, as shown inFIG. 24, the hook52aconveys the sheet stack to the downstream side in the direction of conveyance by a distance matching with the sheet size. After the center staplers S2have stapled the center of the sheet stack, the sheet stack is conveyed by the hook62ato the downstream side by a preselected distance matching with the sheet size and then brought to a stop. The distance of movement of the sheet stack is controlled on the basis of the drive pulses input to the discharge motor157

Subsequently, as shown inFIG. 25, the sheet stack is nipped by the discharge roller56and press roller57and then conveyed by the hook52aand discharge roller56to the downstream side such that it passes through the path formed between the guides54and55and extending to the fold tray G. The discharge roller56is mounted on the drive shaft65associated with the belt52and therefore driven in synchronism with the belt52, as stated earlier. Subsequently, as shown inFIG. 26, the sheet stack is conveyed by the upper and lower roller pairs71and72to the movable rear fence73, which is moved from its home position to a position matching with the sheet size beforehand and held in a stop for guiding the lower edge of the sheet stack. At this instant, as soon as the other hook52′ on the belt52arrives at a position close to the rear fence51, the hook52ais brought to a stop while the guides54and55are returned to the home positions to wait for the next sheet stack.

As shown inFIG. 27, the sheet stack abutted against the movable rear fence73is freed from the pressure of the lower roller pair72. Subsequently, as shown inFIG. 28, the fold plate74pushes part of the sheet stack close to a staple toward the nip of the fold roller pair81substantially perpendicularly to the sheet stack. The fold roller pair81, which is caused to rotate beforehand, conveys the sheet stack reached its nip while pressing it. As a result, the sheet stack is folded at its center.

As shown inFIG. 29, the leading edge of the center-folded sheet stack enters the nip of the second fold roller pair82. At this time, the first and second fold roller pairs81and82are caused to stop rotating and then, on the elapse of a preselected period of time, resume the conveyance of the sheet stack. It is noteworthy that the preselected period of time mentioned above is variable in accordance with the number of sheets and sheet size. For example, when the number of sheets constituting a stack is relatively large, a substantial period of time elapses until the next sheet stack enters the folding section. In such a case, the above period of time may be added to the preselected period of time, so that the fold of the sheet stack can be made sharper, or more firm, without degrading the productivity of the image forming apparatus PR. Further, the fold roller pairs81and82may be repeatedly rotated in opposite directions within the preselected period of time by an amount small enough to prevent the leading edge of the sheets stack from slipping out of the nip of the fold roller pair82, which is about several millimeters wide. This will stroke and thereby sharpen the fold of the sheet stack.

As shown inFIG. 30, the sheet stack with the fold sharpened by the fold roller pair82is driven out to the lower tray203by the lower outlet roller pair83. At this instant, as soon as the pass sensor323senses the trailing edge of the sheet stack, the fold plate74and movable rear fence73are returned to their home positions while the lower roller pair72is released from each other so as to wait for the next sheet stack. Alternatively, the rear fence73may be held at the same position without being returned to the home position if the next job deals with the same sheet size and the same number of sheets.

As stated above, in the illustrative embodiment, the direction of rotation of the fold roller is switched in accordance with whether a sheet should be folded by the fold roller or whether it should be guide to a preselected position on a path before folding. It is therefore possible to guide the leading edge of a sheet stack in the direction of conveyance when the sheet stack should be introduced into the path. The illustrative embodiment therefore protects the leading edge portion of a sheet stack from bending without resorting to a shutter or similar special member, thereby insuring desirable folding and therefore desirable center stapling and folding.

More specifically, in the illustrative embodiment, the prestacking portion E is positioned on the path D, which extends to the staple tray F, and allows two or more sheets to be conveyed to the staple tray F together. Therefore, the entry of the first sheet of the next set of sheets in the stapling section can be delayed without regard to the edge/center staple mode. It follows that high productivity is achievable by the positioning and stapling time being intentionally reduced.

The comparatively short path C allows sheets to be driven out to the same tray (shift tray202) without regard to stapling/non-stapling. Sheets can therefore be driven out in two different modes at the minimum cost.

Further, either one of the edge stapler S1and center staplers S2, which are independent of each other, suitable for stapling is always positioned in the vicinity of the position assigned to the jogger fence53. This successfully reduces the overall positioning and stapling time and thereby guarantees high productivity. In addition, the belt52and hook52acan selectively move a sheet stack to the upstream side or the downstream side in the direction of conveyance, implementing the delicate adjustment of the stapling position, as desired.

Moreover, the stack moving means plays the role of an edge guide for guiding the lower edge of a sheet stack at the same time, simplifying the construction and reducing cost. In addition, the positioning position is variable in accordance with the sheet size and the number of sheets to be stapled together, so that accurate positioning and productivity are enhanced.

Second Embodiment

An alternative embodiment of the sheet finisher and image forming apparatus in accordance with the present invention will be described hereinafter. The illustrative embodiment is essentially similar to the previous embodiment except for the following.

In the center staple and fold mode, the illustrative embodiment also executes the procedure shown inFIGS. 22A through 22Cexcept for the steps S521and S522, FIG.22B. In the steps S527and S528,FIG. 22B, in which the pass sensor323monitors the passage of the center-folded sheet stack, the illustrative embodiment executes the following processing.

In the steps S527and S528, the illustrative embodiment makes the fold of a sheet stack more sharp, or more firm, with a sequence of steps shown in FIG.31. As shown, in the step S527, when the leading edge of a sheet stack moves away from the pass sensor323, the CPU360determines whether or not the leading edge of the sheet stack has arrived at the fold roller pair82(step S527-1) If the answer of the step S527-1is YES, then the CPU360causes the fold roller pairs81and82and lower outlet roller83to stop rotating (step S527-2). More specifically, in the step S527-1, the CPU360counts a period of time elapsed since the pass sensor323has sensed the leading edge of the sheet stack, and makes the decision on the basis of the time at which the leading edge is expected to reach the nip of the fold roller pair82.

In the step S527-2, after the fold roller pair82has nipped the leading edge of the sheet stack, the CPU360causes both of the fold roller pairs81and82to stop rotating with the roller pair81nipping the intermediate portion of the sheet stack, thereby sharpening the fold of the sheet stack (see FIG.34). Subsequently, on the elapse of a preselected period of time (YES, step S527-3), the CPU360causes the fold roller pairs81and82and lower outlet roller pair83to start rotating to thereby convey the sheet stack (step S527-4). This is followed by the step S528and successive steps.

FIG. 32shows another specific procedure for sharpening the fold of the sheet stack. In the procedure described above, the CPU360causes the rollers to stop rotating by counting the preselected period of time in the step S527-3. Considering the efficiency of folding operation, the preselected period of time should preferably be varied or set in accordance with the sheet size and the number of sheets, i.e., stack thickness. For this purpose, the CPU360executes the procedure ofFIG. 32instead of the step S527-3of FIG.31.

As shown inFIG. 32, after stopping the rotation of the fold roller pairs81and82and lower outlet roller pair83, the CPU360determines whether or not the size of the sheet stack is B4 or above (step S527-3-1). This decision is made on the basis of sheet size information received from the image forming apparatus PR and known beforehand. If the answer of the step S527-3-1is YES, then the CPU360determines whether or not the sheet stack has six or more sheets (step S527-3-2). If the answer of the step S527-3-2is YES, then the CPU360determines whether or not a preselected period of time T1(seconds) has elapsed (step S527-3-3). If the answer of the step S527-3-3is YES, then the CPU360again causes the fold roller pairs81and82and lower outlet roller pair83to rotate (step S527-4) If the answer of the step S527-3-2is NO, then the CPU360executes the step S527-4on the elapse of a preselected period of time T2(seconds). On the other hand, it the answer of the step S527-3-1is NO, then the CPU360determines whether or not the sheet stack has six or more sheets (step S527-3-5). Subsequently, the CPU360executes the step S527-4on the elapse of a preselected period of time T3(step S527-3-6) if the answer of the step S527-3-5is YES or executes it on the elapse of a preselected period of time T4if the answer of the step S527-3-5is NO.

While the periods of time T1through T4each are variable in accordance with the sheet size and the number of sheets, the larger the sheet size and the larger the number of sheets, the longer the period of time necessary for the next sheet stack to enter the folding section. Therefore, the period of time necessary for the next sheet stack to enter the folding section is used as a pressing time for thereby efficiency pressing the folded sheet stack without lowering productivity, i.e., without wasting time. The fold of the sheet stack is therefore sharpened and efficiently freed from a swell.

FIG. 33shows a further specific procedure for sharpening the fold of the sheet stack. InFIG. 33, steps S527-2-1through527-2-3are substituted for the step S527of FIG.31. As shown, the CPU360causes the fold roller pairs81and82and lower outlet roller pair83to stop rotating in the step S627-2-1and then causes them to rotate in the reverse direction by a preselected amount L in the step S627-2-2. Subsequently, the CPU360causes the fold roller pairs81and82and lower outlet roller pair83to again rotate forward by the amount L in the step S527-2-3. After the steps S527-2-2and S527-2-3have been repeated over the preselected period of time stated earlier, the CPU260causes the fold roller pairs81and82and lower outlet roller pair83to start rotating (step S527-4). This is followed by the step S528and successive steps.

As stated above, within the preselected period of time for pressing the fold of the sheet stack, the procedure ofFIG. 33causes the fold roller pair82to repeatedly rotate in opposite directions a plurality of times by an amount small enough to prevent the leading edge of the sheet stack from slipping out of the nip of the fold roller pair82, which is several millimeters wide. The above amount is represented by a nip length n in the direction parallel to the direction of conveyance in FIG.34. Such stroking is also successful to make the fold of the sheet stack more firm. Further, because the leading edge of the sheet stack does not slip out of the nip of the rollers82, part of the sheet stack around the fold is free from smears ascribable to sliding contact with the rollers82.

It is to be noted that the duration of the reciprocating motion described with reference toFIG. 33may also be varied in accordance with the sheet size and the number of sheets.

After the step S527-4, when the trailing edge of the sheet stack moves away from the pass sensor323(YES, step S528), the CPU360presses the lower rollers72against each other (step S529) and moves the fold plate74and guide plates54and55to their home positions (steps S530and S531).

In the above condition, the lower outlet sensor324monitors the passage of the sheet stack (steps S532and S533). When the trailing edge of the sheet stack moves away from the lower outlet sensor324(YES, step S533), the CPU360causes the fold roller pairs81and82and lower outlet roller pair83to further rotate over a preselected period of time and then stop rotating (step S534). Subsequently, the CPU360returns the belt52and jogger fence53to their stand-by positions (steps S535and S536) and then determines whether or not the sheet stack is the last stack to be dealt with by the job (step S537). If the answer of the step S537is NO, then the procedure returns to the step S506. If the answer of the step S537is YES, then the CPU360moves the belt52and jogger fence53to the home positions (steps S538and S539), stops rotating the inlet roller pair1, roller pairs2,7,9and10, staple outlet roller pair11, and knock roller12(step S540) Subsequently, the CPU360turns off the solenoid assigned to the path selector15(step S541), thereby restoring the initial condition.

The stapling and folding operations which the illustrative embodiment performs in the center staple and fold mode will be described more specifically hereinafter. As shown inFIG. 27, the rollers72are released from each other. Subsequently, as shown inFIG. 28, the fold plate74pushes the portion of the sheet stack around the staples toward the fold roller pair81substantially in the perpendicular direction. The fold roller pair81in rotation folds the sheet stack toward the center while conveying it.

As soon as the leading edge of the sheet stack enters the nip of the fold roller pair82, the fold roller pairs81and82stop rotating and again start rotating on the elapse of a preselected period of time (corresponding to the procedure of FIG.31). Again, the preselected period of time is variable in accordance with the sheet size and the number of sheets. More specifically, the larger the number of sheets, the longer the period of time necessary for the next sheet stack to enter the folding section; such a period of time is added to the preselected period of time (corresponding to the procedure of FIG.32). This is also successful to efficiently press the sheet stack and therefore to sharpen the fold more without lowering the productivity of the image forming apparatus PR.

Again, within the preselected period of time, the fold roller pair82may be caused to repeatedly rotate in opposite directions (solid arrow and phantom arrow,FIG. 34) by an amount small enough to prevent the leading edge of the sheet stack from slipping out of the nip of the fold roller pair82(corresponding to FIG.33).

As shown inFIG. 34, the sheet stack with the sharpened fold is driven out to the lower tray203via the lower outlet roller pair83. At this instant, when the pass sensor323senses the trailing edge of the sheet stack, the fold plate74and movable rear fence73return to their home positions while the lower rollers72are released from each other, preparing for the next sheet stack. If desired, the rear fence73may be held at the same position so long as the sheet size and the number of sheets to be dealt with by the next job are the same.

As stated above, the illustrative embodiment has various unprecedented advantages, as enumerated below.

(1) A fold roller pair stops the fold of a sheet stack at its nip over a preselected period of time to thereby sharpen the fold. This frees part of the sheet stack around the fold from smears ascribable to sliding contact with the roller pair, while efficiently obviating the swell of the sheets stack. This is in contrast to the conventional system in which a sheet stack is moved back and forth via the nip of a roller pair a plurality of times so as to have its fold intermittently pressed.

(2) Because the sheet stack is pressed while in a stop, it should only be nipped by the fold roller over a preselected period of time. Simple control therefore suffices for sharpening the fold.

(3) The fold of the sheet stack is pressed within the nip width of the fold roller pair parallel to the direction of conveyance. Therefore, simple control suffices for sharpening the fold if the fold roller pair is rotated in opposite directions within the above range.

(3) The duration of pressure to act on the fold of the sheet stack is variable in accordance with the sheet size and the number of sheets constituting a stack. Therefore, by using the fact that the period of time necessary for the next sheet stack to reach a folding section increases with an increase in sheet size or the number of sheets, such a period of time can be used to press the fold. This makes it needless to add a wasteful period of time that would lower the productivity of an image forming apparatus.

Third Embodiment

Another alternative embodiment of the sheet finisher and image forming apparatus in accordance with the present invention will be described hereinafter. This embodiment is also directed mainly toward the second object and similar to the second embodiment except for the configuration and operation of the fold plate74and those of the fold roller pair81. The following description will concentrate on differences between the second and third embodiments.

FIGS. 35 and 36show essential part of a pressure applying/canceling mechanism that allows the fold roller pair81(fold rollers81aand81b) to fold a sheet stack and is unique to the illustrative embodiment. As shown, the mechanism includes, in addition to the fold plate74and fold rollers81aand81b, angularly movable plates or first members511aand511b, swing arms or second members520aand520b, connecting members or third members524aand524b, first springs512aand512b, a second spring521, a cancel link (or third member)570, and a drive motor164assigned to the fold rollers81aand81b. The fold plate74is linearly movable back and forth, as shown inFIGS. 13 and 14. In the illustrative embodiment, the nip of the fold roller pair81(81aand81b) is positioned on the locus of movement501of the fold plate74.

InFIGS. 35 and 36, the various structural elements positioned above and below the locus of movement501are arranged substantially symmetrically to each other with respect to the locus501and are therefore simply distinguished from each other by suffixes a and b.

The plates511aand511bare angularly movably supported by fulcrums510aand510b, respectively, which are positioned on the front and rear side walls of the fold tray G. The swing arms520aand520bare respectively swingably supported by the plates511aand511bvia bearings515aand515bat one end thereof. The second springs512aand512brespectively exert on the plates511aand511bpressure necessary for conveying a sheet stack at the upstream end in the direction in which the fold rollers81aand81bconvey the sheet stack. The plates511aand511b, fulcrums510aand510b, swing arms520aand520band first and second springs512,512aand512beach are provided in pair on the inner surfaces of the front and rear side walls of the fold tray G, although not shown specifically. The fold rollers81aand81bare mounted on respective shafts expending perpendicularly to the direction of conveyance.FIGS. 35 and 36show only the members mounted on the front side wall of the fold tray G.

At the upstream side in the direction of sheet conveyance, the first springs512aand512bconstantly bias the plates511aand511b, respectively, such that their free ends tend to move toward each other. The fold rollers81aand81bare respectively supported by the free ends, or downstream ends, of the plates511aand511bvia the bearings515aand515b.

The swing arms520aand520b, like the plates511aand511b, are respectively supported by the fulcrums510aand510bat their upstream ends in the direction of conveyance. The second spring521is anchored to the downstream ends of the swing arms520aand520bin the direction of conveyance at opposite ends thereof, constantly biasing the ends of the swing arms520aand520btoward each other. As shown inFIG. 35, the swing arms520aand520bare positioned above and below, respectively, the fold rollers81aand81b.

In the above configuration, when the bearings515aand515bof the fold rollers81aand81bare moved away from each other by a preselected distance, the bearings515aand515brespectively abut against the inner edges of the swing arms520aand520bfacing each other and are therefore subject to the biasing force of the second spring521. Before the bearings515aand515babut against the above edges of the swing arms520aand520b, the fold rollers81aand81bare subject to the biasing forces of the first springs512aand512b.

More specifically, the bias of the second spring521is selected to be heavier than the bias of the first springs512aand512b. Therefore, when a sheet stack enters the nip between the fold rollers81aand81b, the comparatively light bias of the springs512aand512bacts on the sheet stack. Subsequently, when the bearings515aand515bof the fold rollers81aand81babut against the swing arms520aand520b, respectively, the comparatively heavy bias of the spring521acts on the sheet stack. In this configuration, the play between the position where the fold rollers81aand81bcontact each other and the position where the bearings515aand515brespectively contact the swing arms520aand520bplays an essential role in introducing a sheet stack to the nip between the fold rollers81aand81b.

The drive motor164assigned to the told rollers81aand81band a drive transmission mechanism associated therewith are used because the fold rollers81aand81bnot only fold a sheet stack, but also convey it. The drive transmission mechanism is implemented as a reduction gear train including gears552,551band551aheld in mesh with a gear mounted on the output shaft of the drive motor164. The gears551band551aare respectively held in mesh with gears550band550a, which are respectively coaxial with the fold rollers81aand81b, causing the fold rollers81aand81bto rotate at the same speed as each other.

The cancel links570, respectively positioned on the inner surfaces of the front and rear side walls, move back and forth along the locus501in interlocked relation to the fold plate74. The release links570cancel the pressure acting on the fold rollers81aand81bby regulating the positions of the swing arms520aand520b. More specifically, the connecting members524aand524brespectively connect the swing arms520aand520band a movable shaft523positioned downstream of the told rollers81aand81bin the direction of conveyance, thereby relating the position of the cancel links570and swing arms520aand520b. In this condition, the positions of the cancel links570determine the timing for exerting pressure on a sheet stack and the timing for canceling it.

The movable range of the shaft523is determined by the dimension of a guide slot530, which extends in parallel to the locus501, in the direction of the locus501. The movable range of the shaft523regulates the maximum gap between the fold rollers81aand81b. A path560along which a sheet stack is conveyed in a folded position is positioned such that the locus501is located at the center of the gap. The guide slot530that determines the movable range is only illustrative. Alternatively, the connecting members524aand524beach may be connected to the swing arm520aor520bby a single member, in which case the connecting portion will be implemented as a slot having a preselected dimension.

In the above configuration, the movement of the shaft520in the direction of sheet discharge is regulated by the dimension of the guide slot530, so that gaps or plays523aand523bare available between the swing arms520aand520band the bearings515aand515bat fold roller pressing portions522aand522b. In this condition, the transfer of the bias of the first spring521is regulated.

The second springs512aand512beach may be replaced with a compression spring inserted in the fold roller pressing portion522aor522bso as to exert the comparatively light bias. The dimension of each of the gaps523aand523bis determined by the position of the downstream end of the guide slot530in the direction of conveyance. It follows that the amount of play and the maximum gap between the fold rollers81aand81bare determined by the position of the slide guide530and the dimension of the cancel link570in the direction of movement.

The shaft523is connected to each cancel link570, as stated earlier. Therefore, when the cancel link570is moved in a direction indicated by an arrow U, the swing arms520aand520beach swing in a direction indicated by an arrow V with the result that a space is formed between each swing arm520aor520band the associated bearing515aor515bat the fold roller pressing portion522aor522b. Consequently, the transfer of the bias of the first spring521is canceled.

FIGS. 37 through 44show how the fold roller pair81is rotated in opposite directions to press the leading edge of a folded sheet stack a plurality of times, thereby sharpening the fold of the sheet stack. As for the operation itself,FIGS. 37 through 44correspond toFIGS. 28,34and30of the second embodiment. As shown inFIG. 37, the fold plate74pushes part of a center-folded sheet stack around staples into the nip of the fold roller pair81in the direction perpendicular to the sheet stack. As a result, as shown inFIG. 38, the sheet stack is conveyed by the fold roller pair81while being folded at its center thereby.

As shown inFIG. 39, when the pass sensor323senses the leading edge of the folded sheet stack, the fold plate74is retracted by a preselected distance. Subsequently, as shown inFIG. 40, the fold roller pair81and lower outlet roller pair83are caused to be rotated in the reverse direction and then stop at a position L mm spaced from the center of the nip. As shown inFIG. 41, the fold roller pair81and lower outlet roller pair83that have reached the above position are caused to rotate in the forward direction. As shown inFIG. 42, as soon as the pass sensor323senses the leading edge of the sheet stack, the fold roller pair81and lower outlet roller pair83are caused to stop. The fold roller pair81repeats the operation ofFIGS. 39 through 41in order to sharpen the fold of the sheet stack. The number of times and duration of the repetition may be manually input on an operation panel, not shown, mounted on the image forming apparatus PR or automatically set by the CPU360in accordance with the sheet size and the number of sheets.

The fold roller pair81and lower outlet roller pair83, once stopped in the positions shown inFIG. 42, are again caused to rotate in the forward direction to thereby discharge the folded sheet stack to the lower tray203. When the arrival sensor321senses the trailing edge of the sheet stack, the movable rear fence73is returned to the home position while the lower rollers72are pressed against each other, preparing for the next sheet stack. Again, the rear fence73may be held at the same position if the sheet size and the number of sheets to be dealt with by the next job are the same. As soon as the fold roller pair81and lower outlet roller pair83start rotating in the forward direction, the fold plate74is returned to the home position.

When the pass sensor323senses the leading edge of the folded sheet stack, the fold plate74is retracted by a preselected distance, as shown in FIG.39. As shown inFIG. 45, the preselected distance of retraction is such that the leading edge of the fold plate74is shitted from the center of the nip of the fold roller pair81toward the upstream side in the direction of conveyance by X mm. Assuming that the each fold roller81has a radius R, then the distance X should preferably be:

The above position is derived from the relative position between the sheet stack and the fold roller pair81and fold plate74and is not limited to X mm.

To effectively sharpen the fold of a sheet stack, the rotation of the fold roller pair81in opposite directions, as shown inFIGS. 39 through 42, should preferably be effected by a distance of 1 mm (FIG. 40) to 50 mm (FIG. 42) from the center of the nip of the fold roller pair81. Experiments showed that the fold a sheet stack was most effectively sharpened when the fold roller pair81pressed, at the center of its nip, the position of the sheet stack about 3 mm spaced from the leading edge of the fold of the innermost sheet. It is therefore preferable to move a sheet stack back and forth with its portion including the above position held at the nip. If desired, during the reciprocating movement, the fold roller pair81may be caused to temporarily stop rotating at the position 3 mm spaced from the leading edge of the fold and press the sheet stack over a preselected period of time. This preselected period of time may be suitably selected in accordance with the sheet size and the number of sheets.

FIG. 46Bshows a sheet stack moved back and forth over the particular range mentioned above and pressed while in a stop.FIG. 46Ashows a sheet stack not subjected to such a fold-sharpening procedure. It will be seen that the fold subjected to the sharpening procedure is lower in height than the fold not subjected to the same. Stated another way, the sharpening procedure makes the fold more firm and folds the highest portion of the sheet stack. This not only implements neat binding, but also allows more sheet stacks to be neatly stacked on the lower tray203.

FIGS. 47A through 47Dare flowcharts demonstrating the center staple and fold mode unique to the illustrative embodiment. As shown, when a sheet driven out of the image forming apparatus PR is about to enter the sheet finisher PD, the CPU360,FIG. 17, causes the inlet roller pair1, roller pair2, roller pairs7,9and10on the path D, staple outlet roller pair11and knock roller12to start rotating (step S601) The CPU360then turns on the solenoid assigned to the path selector15(step S602) for thereby causing it to rotate counterclockwise.

After the belt HP sensor311has sensed the belt52has reached its home position, the CPU360drives the discharge motor157so as to move the belt52to the stand-by position. Also, after the jogger fence HP sensor has sensed the jogger fence53has been brought to its home position, the CPU360moves the jogger fence53to the stand-by position. Further, the CPU360moves the guide plate54and movable guide55to their home positions (steps S603through S605). Subsequently, if the inlet sensor301has turned on and then turned off (steps S606and S607), if the staple outlet sensor305has turned on (step S608), and if the shift outlet sensor303has turned off (step S609), then the CPU360determines that a sheet is present on the staple tray F. The CPU360then turns on the knock solenoid170over a preselected period of time to bring the knock roller12into contact with the sheet and then urges it toward the rear fence51, thereby positioning the trailing edge of the sheet (step S610).

After the step S610, the CPU360drives the jogger motor158to move the jogger fence53inward by a preselected distance, thereby positioning the sheet in the widthwise direction. The CPU360then returns the jogger fence53to the stand-by position (step S611). As a result, the sheet on the tray F is positioned in both of the horizontal and vertical directions.

After the last sheet of a single set or copy has been positioned on the staple tray F (YES, step S612), the CPU360moves the jogger fence53inward by the preselected distance to thereby prevent the edge of the sheet stack from being dislocated (step S613) The CPU360then drives the discharge motor157in order to move the belt52by a preselected amount (step S614), so that the sheet stack is raised to the position where the center staplers S2are positioned. In this condition, the center staplers S2staple the sheet stack at the center (step S615).

Subsequently, the CPU360causes the belt52to move by a preselected amount (step S616) and moves the guide plate54and movable guide55by a preselected amount each, thereby clearing the path extending to the fold tray G (step S617). At the same time, the CPU360causes the upper and lower roller pairs71and72of the fold tray G to start rotating (step S618). After the movable rear fence73of the fold tray G has reached its home position, the CPU360causes it to move to the stand-by position (step S619).

After the fold tray G has been prepared for the entry of the sheet stack by the above steps, the CPU360causes the belt52to move by a preselected amount (step S520) until the sheet stack has been nipped by the discharge roller56and press roller57and conveyed toward the fold tray C thereby. After the leading edge of the sheet stack has reached the arrival sensor321(step S621) and then further conveyed by a preselected distance, the CPU360causes the upper and lower roller pairs71and72to stop rotating (step S622) and moves the guide plates51and52to their home positions (step S623). When the sheet stack is fully conveyed by the preselected distance, the CPU360causes the roller pairs71and72to stop rotating for thereby interrupting the conveyance of the sheet stack (step S624). The CPU360then releases the lower rollers72from each other (step S625).

After the step S625, the CPU360determines the number of sheets stapled together (step S625). If the number of sheets is five or less (YES, step. S626), then the CPU360causes the fold plate74to move forward to a position 3 mm short of the nip of the fold roller pair81while pushing the sheet stack (step S627). If the answer of the number of sheets is six or more (NO, step S626), then the CPU360causes the fold plate74to move to a position 1 mm short of the nip of the fold roller pair81while pressing the sheet stack (step S628). Further, the CPU360causes the fold roller pair81and lower roller pair83to start rotating forward (step S629) while stopping the movement of the fold plate74(step S630). In this condition, the CPU360causes the fold roller pair81and lower roller pair83to rotate forward by a preselected amount each (FIGS.37through39), causes the fold plate74to retract by a preselected distance (step S631; FIG.40), and then stops the movement of the fold plate74(step S632) with the edge of the plate74protruding into the path92.

When the pass sensor323turns on, thereby sensing the passage of the center-folded sheet stack (step S633; FIG.40), the CPU360causes the fold roller pair81and lower roller pair83to stop rotating (step S734) and then repeatedly executes the folding operation until the CPU360causes the fold roller pair81and lower roller pair83to start rotating forward (step S642). More specifically, the CPU360checks the preselected operation under way at the position upstream of the folding or the status of the arrival sensor321(step S635). If the preselected operation is not completed or if the arrival sensor321has not turned on, then the CPU360determines whether or not a counter, counting the reciprocating movement, has reached a preselected count. If the answer of this decision is negative, then the CPU360causes the fold roller pair81and lower roller pair83to rotate in the reverse direction by a preselected amount that brings the leading edge of the sheet stack to the position L mm spaced from the center of the nip shown inFIG. 40(steps S637and S638).

After the step S638, the CPU360causes the fold roller pair81and lower roller pair83to start rotating forward (step S639) and then causes them to stop rotating when the leading edge of the sheet stack moves away from the pass sensor323(YES, step S640). Thereafter, the steps S635through S641are repeated. When the preselected operation under way at the upstream side ends or the arrival sensor321turns on (YES, step S635) and if the counter reaches the preselected count (YES, step S636), the CPU360causes the fold roller pair81and lower roller pair83to rotate forward (step S642) and returns the fold plate74to the home position (step S643). As soon as the arrival sensor321turns off (YES, step S644), the CPU360presses the lower rollers72against each other to thereby prepare them for the entry of the sheet stack (step S645).

In the above condition, the pass sensor323monitors the passage of the sheet stack (steps S646and S647). When the trailing edge of the sheet stack moves away from the pass sensor323(YES, step S647), the CPU360causes the fold roller pair81and lower roller pair83to further rotate over a preselected period of time and then stop (step S648). The CPU360then moves the belt52and jogger fence63to their stand-by positions (steps S649and S650). Subsequently, the CPU360determines whether or not the sheet stack is the last set or copy to be dealt with by the job (step S651). It the answer of the step S651is NO, then the CPU360returns to the step S606. If the answer of the step S651is YES, then the CPU360returns the movable rear fence73, belt52and jogger fence53to their home positions (steps S652, S653and S654), causes the inlet roller pair1, roller pairs2,7,9and10, staple outlet roller pair11and knock roller12to stop rotating (step S655), and turns off the solenoid assigned to the path selector15(step S656). This is the end of the procedure shown inFIGS. 47A through 47D.

As stated above, the illustrative embodiment has various advantages, as enumerated below.

(1) A single fold roller pair81, which is rotated in opposite directions, suffices for sharpening the fold of a sheet stack. In addition, the rotation of the fold roller pair81occurs within the range of the nip to thereby prevent a sheet stack from moving away from the nip, so that the fold can be sharpened by simple control.

(2) The user can select a desired degree of fold sharpening in accordance with the sheet size and the number of sheets constituting a single stack. This insures an attractive bound sheet stack.

(3) Only the portion relating to fold sharpening is caused to move back and forth, allowing the fold to be most efficiently sharpened.

(4) The rotation of the fold roller pair81is controlled on the basis of the output of the pass sensor323, preventing errors in conveyance length from accumulating. This allows only the target range of the sheet stack to be accurately pressed and therefore promotes efficient sharpening.

(5) The fold roller pair81is rotated in the reverse direction at least once, so that the minimum degree of sharpening is achievable without regard to the number of sheets. It follows that the bound sheet stack is attractive without regard to the number of sheets constituting it.

(6) Even if the fold of the sheet stack slips out of the nip of the fold roller pair81when the roller pair81is reversed, the fold plate held at the stand-by position catches the sheet stack. Therefore, only if the fold roller pair81is again rotated forward, the fold of the sheet stack can again easily enter the nip of the roller pair81in a short period of time without jamming the path.