Abstract:
A decurl device includes: a curl detecting section having a plurality of toothed wheels each which detects a curling amount and a curling direction of a sheet having a curl; and a plurality of uncurling sections each arranged downstream of the curl detecting section in a conveyance direction of the sheet having the curl, which uncurls the sheet by bending and transforming the sheet in a direction reverse to a curling direction thereof. One of the uncurling sections, which reduces the curl, is selected on the basis of the curling amount and the curling direction that have been detected by the curl detecting section, thereby uncurling the sheet.

Description:
This application is based on Japanese Patent Application No. 2006-149483 filed on May 30, 2006, the content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a decurl device which detects a curl of a sheet and uncurls the sheet, and an image forming apparatus provided with the decurl device. 
   An electrophotographic image forming apparatus, i.e., a copy machine, a printer, a facsimile or a multi-functional machine having the functions thereof, performs image formation by transferring a toner image, obtained by developing an electrostatic latent image formed on an image carrier, with a developing device, on a sheet directly or via an intermediate transfer member, then feeding the sheet through a pressuring position between a heat roller for fixing or a fixing belt and a pressure roller to fix the image. 
   In a fixing process, dehydration of a sheet causes so-called curling, a phenomenon such that the sheet after fixing warps upward or downward in a wavy form. Particularly, such curling of a sheet tends to noticeably occur due to increased amounts of toners supplied to the sheet at the time of forming a color image, the use of multifarious kinds of sheets, etc. 
   A curl of a sheet raises a problem particularly in a case of forming images on both sides of a sheet or a case of making an article bound by bookbinding or so in a finisher. 
   There is a case where a decurl member for correcting a curl of a sheet which occurs after fixing is used to avoid reduction in image quality or so originating from paper jamming, reduced sheet storability in a sheet ejecting section, a transfer failure at the time of double-side image formation, or the like all caused by such a curl. 
   A sheet curl control apparatus described in Unexamined Japanese Patent Application No. 05-238624 (FIG. 4) has a decurler and adjusting means which adjusts the decurling action of the decurler, detects the amount of curling by measuring a time interval between the first interruption of an infrared ray irradiated from one light source by passing of the leading edge of a sheet and the next interruption of the infrared ray using infrared sensors at two locations, and changes the decurling action according to the curling amount. 
   In above-mentioned sheet curl control apparatus, as the sensors are disposed downstream of a sheet in the sheet conveying direction, the sensors detect a curl at the leading edge of a sheet in the sheet conveying direction but cannot detect a curl which occurs on the entire surface of the sheet. Because a curl is detected by a horizontal sheet conveying section, the curling amount changes due to the dead weight of the sheet, disabling detection of the accurate curling amount. 
   A sheet conveying device described in Unexamined Japanese Patent Application No. 2002-60112 (FIG. 1) detects a curling amount with a reflection type displacement sensor, and performs such control as to change the sheet suction force generated by a suction fan according to the detected sheet curling amount. 
   Because the sheet conveying device also has the sensor disposed at an end portion of a sheet to detect the amount of displacement of the leading edge of the sheet in the sheet conveying direction at the horizontal sheet conveying section, it also cannot detect an accurate curling amount. 
   A decurl system described in Unexamined Japanese Patent Application No. 2002-80158 comprises decurl devices, curling amount measuring devices and correction amount modifying means, and detects a curling amount by the displacement of an actuator or a reflection type displacement sensor. 
   In this system, a curl is detected in the vicinity of the decurl device on the downstream side in the sheet conveying direction to detect the curling amount after the sheet is uncurled. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is a decurl device including a curl detecting section having a plurality of toothed wheels which detect a curling amount and a curling direction of a curled sheet, and a plurality of uncurling sections which are disposed downstream of the curl detecting section in a sheet conveying direction and uncurls the sheet by warping the curled sheet in the opposite direction to the direction of curling, whereby the uncurling section that reduces the curl is selected from the plurality of uncurling sections to uncurl the sheet based on the curling amount and the curling direction detected by the curl detecting section. 
   Another aspect of the invention is an image forming apparatus including an image forming apparatus main body which forms an image on a sheet, a decurl device as recited above, and a sheet finisher which performing a finishing process on the sheet ejected from the decurl device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general view of an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of a decurl device according to an embodiment of the present invention; 
       FIG. 3  is a plan view of a curl detecting section; 
       FIG. 4  is a front view of a detecting plate and a sensor; 
       FIG. 5(   a )- 5 ( c ) are perspective views showing various curls of a sheet; 
       FIG. 6  is a cross-sectional view showing layout positions of toothed wheels of the curl detecting section; and 
       FIG. 7  is a block diagram showing control of the curl detecting section, a first uncurling section and a second uncurling section. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While a preferred embodiment of the present invention will be described below, the invention is not limited to the embodiment. 
     FIG. 1  is a general view of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus includes an image forming apparatus main body A, a decurl device B and a sheet finisher FS. 
   [Image Forming Apparatus] 
   The image forming apparatus main body A has an automatic document feeder  1  and an image reading section  2  at an upper portion, and a printer section constituting a lower portion. 
   Reference numeral “3” denotes a sheet storing section which stores sheets S. A printer engine  5  forms an image on a photoreceptor  4  by an electrophotographic process of charging, exposing the photoreceptor  4  and developing. The printer engine  5  forms an image on a sheet S, and the image is fixed in a fixing unit  6 . The fixing unit  6  forms a nip with a heat roller  6   b  with a built-in heat source  6   a , and a pressure roller  6   c , and heats and presses the sheet S while conveying the sheet S to melt toners, and fixes the image on the sheet S. 
   The sheet S is fed by a first sheet feeder  3   a  and is temporarily stopped and then fed by a second sheet feeder  3   b  to have an image formed thereon. The image-formed sheet S is ejected from sheet ejection ports by ejection rollers  8 . 
   As conveyance paths for the sheet S, a sheet feeding path  7  from the sheet storing section  3  to the printer engine  5 , a conveyance path  9   a  which extends from the printer engine  5  to a sheet ejection port passing through the fixing unit  6  and the ejection rollers  8 , and a rear surface conveyance path  9   b  which effects a flipped-over conveyance. 
   Image forming modes include a one-side face-down ejection mode, a one-side face-up ejection mode and a two-side mode. In one-side face-down ejection mode, an image is formed on one side of a sheet S, and the sheet S having passed the fixing unit  6  is flipped over by the flip-over process, and then conveyed and ejected by the ejection rollers  8 . 
   In one-side face-up ejection mode, an image is formed on one side of a sheet S, and the sheet S conveyed to the conveyance path  9   a  is directly conveyed and ejected by the ejection rollers  8 . 
   In two-side mode, an image is formed on one side of a sheet S, and the sheet S having passed the fixing unit  6  is conveyed downward to the rear surface conveyance path  9   b , is flipped over, and then fed again to the sheet feeding path  7 . 
   The printer engine  5  forms a back image on the bottom side of the sheet S fed again, and the sheet S having the back image formed thereon passes through the fixing unit  6 , and is conveyed and ejected by the ejection rollers  8 . 
   Reference numeral “10” denotes an operation section, and various modes of the image forming apparatus main body A and the output mode using the sheet finisher FS can be set by operating the operation section  10 . 
   A controller C 1  provided in the image forming apparatus main body A is connected to a controller C 2  of the decurl device B and a controller C 3  of the sheet finisher FS via a communication section C 4 . 
   The sheet S ejected from the image forming apparatus main body A is conveyed to the sheet finisher FS through the decurl device B. The decurl device B will be described later. 
   [Sheet Finisher] 
   The sheet finisher FS, which performs various finishing processes on the sheet S ejected from the image forming apparatus main body A, is the generic term of a puncher, a folder, a flat binder, a center-binder, a pasting and bookbinding machine, a cutting machine and the like. 
   A pasting and bookbinding machine as a typified example will be explained as one embodiment of the sheet finisher FS. 
   The pasting and bookbinding machine is provided with a sheet take-in section  21 , an ejection section  22 , a sheet bundle storing section  23 , a sheet bundle conveying section  24 , a paste coating section  25 , a cover sheet feeding section  26 , a cover sheet cutting section  27 , a cover sheet wrapping section (wrapping and bookbinding section)  28 , and an aligning section  29 . Those sections are disposed in the pasting and bookbinding machine nearly in the vertical direction. 
   The sheet S supplied to the sheet take-in section  21  is conveyed to either the ejection section  22  or the sheet bundle storing section  23  by a switching gate  210 . 
   When sheet conveyance to the ejection section  22  is set, the switching gate  210  blocks a conveyance path  211  to the sheet bundle conveying section  24  and releases a conveyance path  212  to the ejection section  22 . The sheets S, which pass along the conveyance path  212  to the ejection section  22 , are conveyed upward to be stored on a fixed sheet ejection tray  221  located at the topmost portion of the sheet finisher FS. 
   The sheets S, which are branched leftward and conveyed in the illustration, at the downstream of the sheet conveying direction, by the switching gate  210 , are stored and stacked in the sheet bundle storing section  23  at a predetermined position in order, and are aligned widthwise and vertically to form a sheet bundle Sa containing a predetermined number of sheets S. 
   The sheet bundle Sa stacked on a sheet placing table  231  of the sheet bundle storing section  23  is conveyed obliquely downward, held by a holding member  241  of the sheet bundle conveying section  24 , and is rotated so that the side (spine portion) where a paste coating process is performed on the sheet bundle Sa while holding the sheet bundle Sa faces down, and is stopped at a predetermined position. 
   The paste coating section  25  includes a paste coating roller  251 , a paste container  252 , and a moving body  253  which can move to a paste coating position on the front side from the initial position of the spine side of the pasting and bookbinding machine while supporting the paste container  252 . 
   A cover sheet K stored in the cover sheet feeding section  26  is conveyed to the cover sheet wrapping section  28  along a sheet conveyance path  261  through the cover sheet cutting section  27 , and then has the trailing edge cut to have a predetermined length by the cover sheet cutting section  27 . The cut length of the cover sheet K is the length of two sheets S in the moving direction plus the length of the spine of the sheet bundle Sa. 
   The cover sheet wrapping section  28  receives and conveys the cover sheet K supplied from the cover sheet feeding section  26 , stops the cover sheet K at a predetermined position and performs widthwise positioning thereof by the aligning section  29 . The cover sheet wrapping section  28  moves a moving case  282  upward by a rise-and-fall section  281 , so that the center portion of the cover sheet K placed on a pressure member  283  is pressed against, and adhered to, a paste coated surface N of the sheet bundle Sa at the lift-up position. 
   The downward movement of the pressure member  283  facing the spine of the sheet bundle Sa and the movement of a pair of horizontally symmetrical folding members  284  disposed at the upper portion of the cover sheet wrapping section  28  causes the cover sheet K to be folded along the side edge of the paste coated surface N of the sheet bundle Sa, thus forming a sheet bundle Sa having the cover sheet K attached to the top and bottom sides of the sheet bundle Sa. 
   After the process of folding the cover sheet K, the cover sheet wrapping section  28  moves downward with the downward driving by the rise-and-fall section  281 , after which an ejection belt  285  which has been retracted outward in the widthwise direction of the cover sheet K with the retraction of the aligning section  29  moves into the widthwise internal space under the sheet bundle Sa and stops there. When holding by the holding member  241  is released thereafter, the sheet bundle Sa moves downward and stops at a position where the lower spine of the sheet bundle Sa abuts on the top surface of the ejection belt  285 . The rotating ejection belt  285  ejects a booklet wrapped and bookbound with the cover sheet K pasted adhered thereto outside the finisher. 
   [Decurl Device] 
     FIG. 2  is a cross-sectional view of the decurl device B according to an embodiment of the present invention. 
   The decurl device B is provided with a sheet introducing section  100 , a decurl and conveying section  110 , a curl detecting section  120 , an uncurling and conveying section  130 , a first uncurling section  140 , and a second uncurling section  150 . 
   The sheet S having an image formed thereon by the image forming section of the image forming apparatus main body A is ejected through the fixing unit  6  and the ejection rollers  8 , is introduced to the sheet introducing section  100  of the decurl device B, and is conveyed while being held by conveyance rollers  101 . A fan F disposed near the conveyance rollers  101  drops the temperature of the sheet S to be ejected from the image forming apparatus. 
   A switching gate  102  disposed downstream of the conveyance rollers  101  in the sheet conveying direction selectively switches the conveyance of the sheet S to a conveyance path  103  below or a bypass conveyance path  104  above. 
   The sheet S to be introduced to the conveyance path  103  is interposed and conveyed to the decurl and conveying section  110  by conveyance rollers  105 ,  107  along a conveyance path  106 . 
   The sheet S conveyed into the decurl and conveying section  110  passes through a guide plate  111 , is interposed by conveyance rollers  112 , is conveyed in U-turn by a guide plate  113 , is held vertically upward by conveyance rollers  114  and is conveyed to the curl detecting section  120  interposed by conveyance rollers  116 , passing through a guide plate  115 . A sensor PS 1  which detects the passing of the leading edge of the sheet S is disposed upstream of conveyance rollers  116  in the sheet conveying direction. 
   The curl detecting section  120  will be described later referring to  FIGS. 3 and 4 . 
   After the curling amount and the curling direction of the sheet S are detected by the curl detecting section  120 , the sheet S passes between a pair of guide plates  124 A,  124 B and is interposed and ejected from the decurl and conveying section  110  by conveyance rollers  117 . 
   The sheet S ejected from the decurl and conveying section  110  passes along a conveyance path  108 , and is interposed and supplied to conveyance rollers  131  of the uncurling and conveying section  130  by conveyance rollers  109 . 
   Either the sheet S which passes along the bypass conveyance path  104  or the sheet S which is ejected from the curl detecting section  120  through the decurl and conveying section  110  and passes along the conveyance path  108  is supplied to the conveyance rollers  131  of the uncurling and conveying section  130 . 
   The sheet S conveyed by the conveyance rollers  131  passes along a conveyance path  132 , and is branched to either one of an upper conveyance path  135 A and a lower conveyance path  135 B formed by a switching gate  134  which is actuated by an unillustrated drive member. 
   After a concave curl upward of the sheet S conveyed by the upper conveyance rollers  135 A is uncurled by the first uncurling section  140  to be described later, the sheet S is conveyed downward by conveyance rollers  136  along a vertically underlying conveyance path  135 C. The sheet S which does not need uncurling is conveyed downward by the conveyance rollers  136  along the lower conveyance path  135 B by path switching carried out by the switching gate  134 . 
   The sheet S conveyed by the conveyance rollers  136  is branched to either one of a rightward conveyance path  138 A and a leftward conveyance path  138 B formed by a switching gate  137  which is actuated by an unillustrated drive member. 
   After a convex curl upward of the sheet S conveyed by the rightward conveyance rollers  138 A is uncurled by the second uncurling section  150  to be described later, the sheet S is ejected out of the apparatus by ejection rollers  139  along a conveyance path  138 C. 
   The sheet S which does not need uncurling is ejected out of the apparatus by the ejection rollers  139  along the leftward conveyance path  138 B. 
   Selection of uncurling of the sheet S by the first uncurling section  140  and the second uncurling section  150  and whether uncurling is needed or not, are carried out based on the curling direction and the curling amount which are detected by the curl detecting section  120 , that will be described later. 
   [Uncurling Section] 
   Because the first uncurling section  140  and the second uncurling section  150  have substantially the same configuration, the first uncurling section  140  will be described as a typified example. 
   The first uncurling section  140  has a drive roller  141  which is rotated by an unillustrated drive member, a belt  143  wound around the drive roller  141  and drive roller  142 , a pressure roller  144  which is rotated pressed against the belt  143 , and a drive member which selectively presses the pressure roller  144  against the belt  143 . 
   As the sheet S conveyed along the conveyance path  135 A passes through the pressing position where the pressure roller  144  is pressed against the belt  143 , the sheet is uncurled. The pressure of the pressure roller  144  applied to the belt  143  is variably adjusted according to the curling amount of the sheet by an unillustrated drive member. 
   [Curl Detecting Section] 
     FIG. 3  is a plan view of the curl detecting section  120 . Because the curl detecting section  120  is arranged symmetrically with respect to a vertical sheet conveyance path “p” in  FIG. 2 , the curl detecting member shown on the left will be described. 
   Each of the conveyance rollers  116 ,  117  having a drive roller and a driven roller which convey the sheet S interposed therebetween are disposed upstream and downstream of the curl detecting section  120  has an integrated elastic layer in an axial direction longer by the maximum sheet width. The elastic layers of the drive roller and the driven roller of the conveyance rollers  116 ,  117  are made of the same material having the same rubber hardness. 
   For example, the elastic layers of the drive roller and the driven roller of the conveyance rollers  116 ,  117  are made of ethylene propylene rubber (EPDM) or urethane rubber or the like. 
   Toothed wheels  122 A,  123 A rotatably supported on a support plate  121 A are disposed at predetermined intervals, for example, 15 toothed wheels are disposed at equal intervals of about 15 mm, in the axial direction of a rotational shaft  127 A. 
   A disk-shaped detecting plate  125 A is fixed to one axial end of the rotational shaft  127 A which has a plurality of toothed wheels  122 A fixed thereto and is rotatably supported on the support plate  121 A. The rotational angle of the detecting plate  125 A is detected by a sensor PS 2 . 
   A disk-shaped detecting plate  126 A is fixed to one axial end of a rotational shaft  128 A which has a plurality of toothed wheels  123 A fixed thereto and is rotatably supported on the support plate  121 A. The rotational angle of the detecting plate  126 A is detected by a sensor PS 3 . 
   The curl detecting member having a support plate  121 B and toothed wheels  122 B,  123 B of the curl detecting section  120  on the right-hand side in  FIG. 2  has a configuration similar to that of the curl detecting member having the support plate  121 A and toothed wheels  122 A,  123 A on the left-hand side in the diagram, and both members are arranged line-symmetrical to the vertical sheet conveyance path “p”. 
   The toothed wheels  122 A,  123 A,  122 B,  123 B which detect a curl at the leading edge portion of the sheet are disposed across the entire sheet in the widthwise direction, and detect the curling amount and the curling direction with respect to various sizes of sheets to be conveyed. The toothed wheels can detect a curl at a corner portion of the sheet and curls generated at both widthwise edge portions of the sheet besides a curl at the leading edge portion of the sheet. 
     FIG. 4  is a front view of the detecting plate  125 A and the sensor PS 2 . 
   A plurality of elliptical holes (slits) are bored through the disk-shaped detecting plate  125 A fixed to the axial end of the rotational shaft  127 A, and the rotational angle of the detecting plate  125 A is detected by the translucent sensor PS 2 . 
   The detecting plate  126 A disposed at the axial end of the rotational shaft  128 A and the sensor PS 3 , and the unillustrated detecting plate and sensor of the toothed wheels  122 B,  123 B likewise have the same shapes as the detecting plate  125 A and the sensor PS 2 . 
     FIGS. 5(   a )- 5 ( c ) are perspective views showing various curls of the sheet S.  FIG. 5(   a ) shows a lead-edge curl and a trail-edge curl generated in the sheet conveying direction.  FIG. 5(   b ) shows a corner curl generated at a corner of the leading edge in the sheet conveying direction.  FIG. 5(   c ) shows a side curl generated on the side-edge portion in parallel to the sheet conveying direction. 
     FIG. 6  is a cross-sectional view showing layout positions of the toothed wheels  122 A,  123 A,  122 B,  123 B of the curl detecting section  120 .  FIG. 7  is a block diagram showing control of the curl detecting section  120 , the first uncurling section  140  and the second uncurling section  150 . 
   The toothed wheels  122 A,  122 B disposed horizontally symmetrically facing each other with the vertical sheet conveyance path “p” in between detect a large curl of 10 mm or larger on the sheet S. The toothed wheels  123 A,  123 B detect a small curl of 5 to 10 mm on the sheet S. 
   A distance L 1  at which the vertical sheet conveyance path “p” connecting the nip position of the conveyance rollers  116  to the nip position of the conveyance rollers  117  faces the outside diameter of the toothed wheels  122 A is set to 10 mm. The distance L 1  at which the vertical sheet conveyance path “p” faces the outside diameter of the toothed wheels  122 B is also set to 10 mm. 
   A distance L 2  at which the vertical sheet conveyance path “p” faces the outside diameter of the toothed wheels  123 A is set to 5 mm. The distance L 2  at which the vertical sheet conveyance path “p” faces the outside diameter of the toothed wheels  123 B is also set to 5 mm. 
   When the sensor PS 2  detects that the leading edge portion of the sheet S which is interposed and conveyed to the vertical sheet conveyance path “p” by the conveyance rollers  114 ,  116  abuts on any of the toothed wheels  122 A,  122 B and rotates, a large curl of 10 mm or larger on the sheet S and the curling direction are detected. 
   Likewise, when the sensor PS 3  detects that the leading edge portion of the sheet S abuts on any of the toothed wheels  123 A,  123 B and rotates, a small curl of 5 to 10 mm on the sheet S and the curling direction are detected. 
   Based on the curling amount and the curling direction detected by the curl detecting section  120 , one of the first uncurling section  140  and the second uncurling section  150  which reduces curling is selected to uncurl the sheet S. 
   The sensor PS 1  disposed upstream of the conveyance rollers  116  in the sheet conveying direction detects passing of the leading edge portion of the sheet S passing along the vertical sheet conveyance path “p”. A predetermined time from the arrival of the leading edge portion of the sheet at the curl detecting section  120  and completion of the passing is preset, and the control is executed in such a way that the rotation of the toothed wheels  122 A,  122 B,  123 A,  123 B originating from the abutment of the leading edge portion of the sheet S within the predetermined time is detected by the sensor PS 2 , PS 3 . 
   Although the foregoing description of the embodiment has been given of an image forming apparatus which has the decurl device B and the sheet finisher FS connected to a copy machine, the invention can be adapted to an image forming apparatus which has the decurl device B and the sheet finisher FS connected to the image forming apparatus main body A of a printer, a facsimile, a digital multi-functional machine or the like. The invention can be used as a single decurl device B. 
   The toothed wheels of the curl detecting section which detect the amount of curling at the leading edge portion of a curled sheet and the curling direction are disposed across the entire sheet in the widthwise direction, and detect the curling amount and the curling direction with respect to various sizes of sheets to be conveyed. The toothed wheels can detect a curl at a corner portion of the sheet and curls generated at both widthwise edge portions of the sheet as well as a curl at the leading edge portion of the sheet. 
   Based on the curling amount and the curling direction detected by the curl detecting section, one of a plurality of uncurling sections which reduces curling is selected to uncurl a curl of a sheet. 
   In the image forming apparatus having the image forming apparatus main body, the decurl device and the sheet finisher connected together, the decurl device cancels curling, waving or the like of a sheet to be ejected from the image forming apparatus main body, and sheets with a good flatness are fed to the sheet finisher, and a high-quality booklet is made after the finishing process.