Patent Publication Number: US-8532560-B2

Title: Recording sheet de-curling device and image forming apparatus using the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2009-045,472 filed on Feb. 27, 2009, with the Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a de-curling device which is configured to flatten a recording sheet, carrying images formed by image forming apparatuses, such as a copy machine, facsimile device, or a printing machine, and further relates to an image forming apparatus employing the same de-curling device. 
     BACKGROUND ART 
     In recent years, various technologies have been employed in order to flatten any curled recording sheets, which technologies are shown below. 
     (1) As general technologies, a de-curling device, including a paired-roller set formed of a soft roller and a hard roller, is well known, in which a curled recording sheet is conveyed through the soft roller and the hard roller so that the curled recording sheet is flattened. However, a curled recording sheet shown in  FIG. 8   a , which is curled perpendicular to the sheet conveyance direction, (hereinafter referred to as an “orthogonal curl”) can be effectively flattened, but a curled recording sheet shown in  FIG. 9   a , which is curled parallel to the sheet conveyance direction (hereinafter referred to as a “parallel curl”), cannot be flattened, and a curled recording sheet shown in  FIG. 10   a , which is obliquely curled, cannot be flattened (hereinafter referred to as an “oblique curl”). Further, in order to flatten various recording sheets, which are curled in the opposite direction as shown in  FIGS. 8   b ,  9   b , and  10   b , a switching operation to flatten said opposite curled sheet is not prepared in said de-curling device. 
     (2) Still further, Unexamined Japanese Patent Application Publication Number 2002-241,021 discloses a de-curler, in which a spiral roller and a normal roller are aligned in parallel to press to each other, so that a recording sheet sandwiched between them is flattened. In this de-curling device, spiral structures are symmetrically formed from the center to each end of the spiral roller to flatten the curled recording sheet. 
     However, the effect of the above technologies are adversely limited, depending on curling directions (such as a positive curl, and a negative curl), curling formations (being the orthogonal curl, the parallel curl, and the oblique curl), and curling amounts. That is, though the curl shown in  FIG. 8   a  is effectively flattened, it is very difficult to flatten the various curls shown in  FIGS. 8   b ,  9   a ,  9   b ,  10   a , and  10   b.    
     (3) Still further, Unexamined Japanese Patent Application Publication Number 5-341,600 discloses a de-curling device in which a nip section is formed of a belt and a nipping roller, and a curled recording sheet is conveyed through the nip section so that the curled recording sheet is flattened. However, the effect of the above technology is adversely limited, being the same way as the case of item (2). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to flatten the sheets curled in almost all directions, in an image forming apparatus, while being not limited to: the curling direction (being the convex curl or the concave curl); the curling formations (being the parallel curling, perpendicular curing and the oblique curling to the sheet conveyance direction); and curling amounts (being a major curl or a minor curl), in which the problems occurred in above described conventional technologies are effectively overcome, and to offer a de-curling device, wherein the de-curled sheets, which have been flattened by the de-curling device, are not curled again, and the de-curled sheets are stacked evenly on a tray, being convenient for the operator to handle the stacked sheets, and in particular to offer an image forming apparatus, employing the same de-curling device. 
     The above object is solved by the technical structure detailed below. 
     1. A sheet de-curling device, including (A), (B) and (C): 
     (A) a first de-curling roller, structured of a round axial shaft and a long bar-shaped member, wherein the long bar-shaped member is spiraled on the round axial shaft clockwise from a predetermined portion of a center of the round axial shaft toward a left end of the round axial shaft, and counterclockwise from the predetermined portion of the center of the round axial shaft toward a right end of the round axial shaft with the same spiraling pitch; and 
     (B) a second de-curling roller, having the same size as the first de-curling roller, to be paired with the first de-curling roller, structured of a round axial shaft and a long bar-shaped member, wherein the long bar-shaped member is spiraled on the round axial shaft counterclockwise from a predetermined portion of a center of the round axial shaft toward a left end of the round axial shaft, and clockwise from the predetermined portion of the center of the round axial shaft toward a right end of the round axial shaft, with the same spiraling pitch as the pitch of the long bar-shaped member of the first de-curling roller; and 
     (C) a driving mechanism which is configured to rotate the paired first and second de-curling rollers in such a way that, when a sheet is introduced to be nipped between the paired first and second de-curling rollers, and when the paired first and second de-curling rollers are rotated, plural contacting points, on which the sheet comes into contact with the long bar-shaped members of the first and second de-curling rollers, are configured to shift in a direction from a center to both edges of the sheet. 
     2. An image forming apparatus, including the above described de-curling device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiment will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in the several figures, in which: 
         FIG. 1  is a cross sectional view of an image forming apparatus, including a de-curling device, mounted near a sheet ejection tray at a downstream side of an image fixing device; 
         FIG. 2  shows a cross sectional view of the de-curling device, having an initial phase-angle changing section, and a clearance changing section to change the center distance between the first and second de-curling rollers; 
         FIG. 3  is a side cross sectional view of  FIG. 2 ; 
         FIG. 4  details the initial phase-angle changing section which defines relative positional relationships of the rotating directions of the first and second de-curling rollers; 
         FIG. 5   a  is a side view of the first and second de-curling rollers, wherein the initial phase-angle of the spirals of the first and second de-curling rollers is set to be 0°, and  FIG. 5   b  is the cross sectional view taken along line A-A of  FIG. 5   a;    
         FIG. 6   a  is a side view of the first and second de-curling rollers, wherein the initial phase-angle of the spirals of the first and second de-curling rollers is set to be 180°, and  FIG. 6   b  is the cross sectional view taken along line A-A of  FIG. 6   a;    
         FIG. 7   a  is a side view of the first and second de-curling rollers, wherein the initial phase-angle of the spirals of the first and second de-curling rollers is set to an intermediate angle between 0° and 180°, and  FIG. 7   b  is the cross sectional view taken along line A-A of  FIG. 7   a ; 
         FIG. 8   a  shows a positive curl, that is, a sheet is concave when viewed from above, and its generating line is perpendicular to the sheet feeding direction, and  FIG. 5   b  shows a negative curl, that is, a sheet is convex when viewed from above, and its generating line is perpendicular to the sheet feeding direction; 
         FIG. 9   a  shows another positive curl, that is, a sheet is concave when viewed from above, and its generating line is parallel to the sheet feeding direction, and  FIG. 9   b  shows another negative curl, that is, a sheet is convex when viewed from above, and its generating line is parallel to the sheet feeding direction, and  FIG. 9   c  shows a wave-curled sheet, and its generating line is parallel to the sheet feeding direction; 
         FIG. 10   a  shows yet another positive curl, that is, a sheet is concave when viewed from above, and its generating line is slant to the sheet feeding direction, and  FIG. 10   b  shows yet another negative curl, that is, a sheet is convex when viewed from above, and its generating line is slant to the sheet feeding direction; and 
         FIG. 11  shows a block diagram of a control system of the de-curling device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The image forming apparatus, incorporating the de-curling device of the present invention, will now be detailed, while referring to  FIG. 1 . 
       FIG. 1  is a central cross sectional view to show the interior structures of the image forming apparatus, incorporating the de-curling device. 
     Image forming apparatus  1  is a tandem-type color image forming apparatus, featuring intermediate transfer belt  50 . 
     Image forming apparatus  1  incorporates plural sheet accommodating sections  20 . Image forming section  40  and intermediate transfer belt  50  are provided above sheet accommodating sections  20 , and image reading section  30  is mounted on an upper area of image forming apparatus  1 . 
     An original document is placed on document supplying plate PL of double surfaces document feeding device  10 , and is conveyed to image reading section  30  by various rollers. 
     Plural sheet accommodating sections can be pulled out toward the front side (which is the depth direction of  FIG. 1 ). Plural sheets, including whites sheets, are accommodated based on their sizes within plural sheet accommodating sections  20 . Plural sheets, accommodated in each sheet accommodating section  20 , are picked up one by one by sheet supplying roller  21 . Further, special sheets, such as coated sheets, or OHP sheets, are set on inserting section  22 H. 
     Image forming section  40  includes image forming engines  400 Y,  400 M,  400 C and  400 K, which form toner images of Y, M, C and K, respectively. Image forming engines  400 Y,  400 M,  400 C and  400 K are mounted vertically in this order, and each has the same structure. 
     In order to detail the image forming engine, image forming engine  400 Y will be used as an example. Image forming engine includes photoconductive body  410 , which rotates counterclockwise, charging section  420 , exposure section  430 , development section  440 , and cleaning section  450 . Cleaning section  450  is positioned to include an area to face a lowermost part of photoconductive body  410 . 
     Endless intermediate transfer belt  50  is positioned at the center of image forming apparatus  1 , and has predetermined volume resistivity. Between primary transfer roller  510  (being a transfer section) and photoconductive body  410 , intermediate transfer belt  50  is sandwiched. 
     Color image forming operation in image forming apparatus  1  will now be detailed. 
     Photoconductive body  410  is driven by a drive motor (which is not illustrated), and charged to be negative polarity by charging section  420  (−800V for example). Next, exposure section  430  writes image information on photoconductive body  410  so that electrostatic latent images are formed. After said electrostatic latent images pass through developing section  440 , toner particles, having been charged to the negative polarity, adhere to the electrostatic latent images, due to negative polarity development bias in the developing section  440 , whereby toner images are formed on photoconductive body  410 . The formed toner images are transferred onto intermediate transfer belt  50 , which is pressure-contacted with photoconductive body  410 . After the transformation of the images, residual toner particles, remaining on photoconductive body  410 , are cleaned by cleaning section  450 . 
     The color toner images, which are formed by image forming engines  400 Y,  400 M,  400 C and  400 K, are superposed on intermediate transfer belt  50  by primary transfer roller  510  to be transferred, so that a full color image is formed on intermediate transfer belt  50 . The recording sheet, supplied one by one from one of sheet accommodating sections  20 , is conveyed to paired registration rollers  60 , serving as a registration conveyance section, through conveyance path  22 . After that, when the recording sheet impinges paired registration rollers  60 , the recording sheet temporarily stops, whereby the position of the recording sheet is corrected. When the superposed color toner images on intermediate transfer belt  50  come to a position to be transferred, the recording sheet is conveyed by paired registration rollers  60 . 
     After that, recording sheet is guided by a guide plate, and conveyed to a nipping portion to transfer the image, wherein the nipping portion is formed of intermediate transfer belt  50  and secondary transfer roller  70 . At said nipping portion, the recording sheet is pushed toward intermediate transfer belt  50 . Since the bias voltage (+500V for example), which exhibits an opposite polarity against the toner particles, is applied onto transfer section  70 , the toner image on intermediate transfer belt  50  is transferred to the recording sheet by the electrostatic forces. The recording sheet charges are then neutralized by a separation section (which is not illustrated), including a neutralization pointer, and the recording sheet is separated from intermediate transfer belt  50 , to be conveyed to image fixing section  80 , including a roller set of heating roller and a pressure applying roller, or including a heating belt and a pressure applying roller. Subsequently, the toner image is permanently fixed to the recording sheet, that is, the recording sheet, carrying the formed image, is flattened by de-curling device  700 , and ejected onto tray  25 . 
     The above description concerns a case in which the image is formed on a single surface of the recording sheet. For the case of image formations on both sides, switching member  26  opens sheet guide section  26 A, so that the recording sheet is conveyed in a direction shown by an arrow, illustrated by a dashed line. 
     Further, the recording sheet is conveyed to path  27 B by conveyance mechanism  27 A 1  and  27 A 2 , and the recording sheet is switched to go back by roller set  27 C of sheet conveyance device  27 . After that, the recording sheet is directed to another path by branching section  27 D, that is, a trailing edge of the recording sheet is turned to a leading edge, and the recording sheet is then conveyed to double-surface sheet supplying unit  130 . 
     Subsequently, the recording sheet is conveyed through conveyance guide  131  provided on double surface sheet supplying unit  130 , whereby the recording sheet is driven by roller  132  and again guided by conveyance path  22 . 
     After that, the recording sheet is conveyed toward secondary transfer roller  70 , and a toner image is transferred on a second surface. The toner image transferred on the reverse surface is then permanently fixed by image fixing device  80 . The recording sheet is conveyed to pass through de-curling device  700  to flatten any curling, and ejected onto tray  25 . 
     De-curling device  700  in the above explanation is provided just before tray  25 , or it can be provided between image fixing device  80  and switching member  26 . 
     Still further, after the superposed color images are transferred onto the recording sheet by secondary transfer roller  70 , the recording sheet is separated from intermediate transfer belt  50 , and said belt  50  is cleaned by cleaning section  190 A to remove the residual toner particles. 
     Image forming apparatus  1  of the present embodiment forms the full-color image on the recording sheet by an electro-photographic method. However, the image forming apparatus relating to the present invention is not limited to the present embodiment, that is, the image forming apparatus can be used as an image forming apparatus to form monochromatic images. 
     In the processes of the image formation, after the residual toner particles on each photoconductive body  410  and intermediate transfer belt  50  are removed by each cleaning section  450  and cleaning section  190 A, respectively, the removed toner particles are conveyed through pipe  605 , having a spiral member therein, and further conveyed to toner box  600 . 
     The structure of de-curling device  700  will now be detailed, while referring to  FIG. 2 , illustrating a cross-sectional view of its front surface, and  FIG. 3 , illustrating a cross-sectional view of its side surface. De-curling roller set  710  includes first de-curling roller  710 A, having rotating shaft  713 A, and second de-curling roller  710 B, having rotating shaft  713 B. Both ends of rotating shaft  713 A are rotatably supported by bearings  744 , mounted in bearing holders  743 , and both ends of rotating shaft  713 B are rotatably supported by bearings  744 , mounted in bearing holders  743 . Bearing holders  743  slide in guides, structured of guide grooves  742 G and guide plates  749 , provided on base frames  742 . Said base frames  742  are united to base member  741 , and are mounted on image forming apparatus  1  through base member  741 . 
     First de-curling roller  710 A and second de-curling roller  710 B exhibit spiral rollers, wherein the thread diameter, the root diameter, the lead angle, and the spiral pitch of first de-curling roller  710 A are equal to those of second de-curling roller  710 B. The spirals are formed clockwise and counterclockwise from the predetermined portion of the cylindrical shaft, which forms the root section, to both ends, that is, the spirals are formed to be symmetrical on the cylindrical shaft. Further, the directions of the lead angles of the spirals of first and second de-curling rollers  710 A and  710 B of de-curling roller set  710  are structured to be opposite to each other. The shapes and functions of first and second de-curling rollers  710 A and  710 B will be detailed later while referring to  FIGS. 5 ,  6  and  7 . 
     While using de-curling device  700 , in which first and second de-curling rollers  710 A and  710 B are included, the inventors of the present invention confirmed that the various curls as well as the positive or negative curls were effectively flattened. Further, the inventors have added a setting function and a changing function of the initial phase-angle and the center distance between the roller shafts, to regulate the relative positions of first and second de-curling rollers  710 A and  7108 , and finally the inventors have completed de-curling device  700 , which can easily flatten the almost curls, depending upon the curling directions, the curling shapes, and the curling amounts, of the positive and negative curls of the recording sheets. Further details of de-curling device  700  will continued below. 
     Initial phase-angle changing section  740 A is configured to set and change the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B, and center distance changing section  740 B is configured to change the center distance between first and second de-curling rollers  710 A and  710 B. 
     Center distance changing sections  740 B are structured of rack gears  745 , mounted in bearing holders  743 , motors  742 MA and  743 MB, mounted on base frames  742 , and pinion gears  746 , driven by said motors, engaging with rack gears  745 . Since bearing holders  743  of both shafts are vertically driven at a predetermined length to set or change the center distance between the shafts, a standard surface for the sheet conveying through path  747  is not vertically changed. 
     As shown in  FIG. 3 , shifting mechanisms, structured of rack gears  745 , pinion gears  746 , motors  742 MA and  742 MB, of center distance changing sections  740 B, are mounted at both ends of first de-curling roller  710 A, and at both ends of second de-curling roller  710 B. Accordingly, adjusting movements of the center distance can be easily conducted at both ends. 
     The initial phase-angle will now be detailed while referring to  FIG. 4 .  FIG. 4  is a front view of the ends of first and second de-curling rollers  710 A and  710 B, whereby the initial phase-angle changing section is detailed to define the relative positional relationships of both rollers  710 A and  710 B. Indication disk  715 A is concentrically mounted on the end of first de-curling roller  710 A, having thread section  711 A, root section  712 A, and rotating shaft  713 A. Indication disk  715 B is concentrically mounted on the end of second de-curling roller  710 B, having thread section  711 B, root section  712 B, and rotating shaft  713 B. Under a basic condition shown in  FIG. 4 , indicator “a” of indication disk  715 A meets point  1 A of stationary plate  731 A, while indicator “b” of indication disk  715 B meets point  1 B of stationary plate  731 B. Stationary plates  731 A and  731 E are adhered to the housings of motors  748 MA and  748 MB. When a job is started under this condition, indication disk  715 A, having indicator “a”, and indication disk  715 B, having indicator “b”, begin to rotate, as shown by formulas (1) and (2),
 
Φ A= 2 πt/T+αA   Formula (1)
 
Φ B= 2 πt/T+αB   Formula (2)
 
That is, based on formula (1), indication disk  715 A uniformly rotates clockwise, and returns to the basic position by one rotation (indicator “a” comes to meet point  1 A), which rotation will be continued, until the job will be stopped. In the same manner, based on formula (2), indication disk  715 E uniformly rotates counterclockwise, and returns to the basic position by one rotation (indicator “b” comes to meet point  1 B), which rotation will be continued, until the job will be stopped.
 
     In Formulas (1) and (2), “ΦA” and “ΦB” represent total phase-angles, “αA” and “αB” represent initial phase-angles, “t” represents time, and “T” represents cycle. Under the above described basic condition, “αA” and “αB”=0. “αA−αB” represents the difference between the initial phase-angles, which difference shows the relative positions of first and second de-curling rollers  710 A and  710 B. In the present invention, the adjustment is conducted under “αB=0”, so that the difference of the initial phase-angles is shown by “αA”. 
     In order to set the initial phase-angle between both indication disk  715 A and  715 B to be π radian (=180°), indicator “a” of indication disk  715 A is rotated clockwise from point  1 A to point  3 A, while indication disk  715 B is stayed at the initial position, that is, “αB” equals to zero. Further, in order to set the initial phase-angle difference between both indication disk  715 A and  7153  to be π/2 radian (=90°), indicator “a” of indication disk  715 A is rotated clockwise from point  1 A to point  2 A, while indication disk  715 B is stayed at the initial position, that is, “αB” equals to zero. Still further, in order to set the initial phase-angle difference between both indication disk  715 A and  715 B to be 3/2 π radian (=270°), indicator “a” of indication disk  715 A is rotated clockwise from point  1 A to point  4 A, while indication disk  715 B is stayed at the initial position, that is, “αB” equals to zero. By the above methods, the relative positional relationships between first and second de-curling rollers  710 A and  7108  can be determined. That is, after the initial phase-angle is set, and the job is started, de-curling rollers set  710  can rotate, while keeping the initial phase-angle difference. 
     Initial phase-angle changing section  740 A, to set and change the initial phase-angle, will be detailed while referring to  FIGS. 2 and 3 . 
     Changing operation of the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B should be conducted, after the center distance between said rollers is sufficiently increased, so that neither roller comes into contact with each other. As detailed before, motor  748 MA is connected to rotation shaft  713 A of first de-curling roller  710 A, and motor  748 MB is connected to rotation shaft  713 B of second de-curling roller  710 B. One of motors  748 MA and  748 MB, that is, motor  748 MA, for example, is activated to rotate at a predetermined angle, so that the initial phase-angle difference between de-curling rollers  710 A and  710 B can be changed to a predetermined angle. In addition, motors  748 MA and  748 MB are respectively mounted on brackets  748 , which are mounted on bearing holders  743 . 
     In order to change the initial phase-angle, a pulse motor, serving as motor  748 MA or motor  748 B, may be used, which motor rotates based on the number of pulses being added, so that the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B can be set to the predetermined value, or said angle difference can be changed to desired values. Further, encoder  752  is used, which is connected to the shaft of the above motor. On disk  752 A of encoder  752 , a small hole is provided at a point, corresponding to a predetermined initial phase-angle. When a beam sensor detects the small hole, motor  748 MA is controlled to stop, whereby the predetermined initial phase-angle is obtained corresponding to second de-curling roller  710 B. 
     After the initial phase-angles are determined as above, at least during the operation of the de-curling device, the initial phase-angles are kept without being changed for that operation. Said operation is easily attained by an electronic circuit, which supplies the same number of the pulses to both pulse motors  748 MA and  748 MB. 
     By the above methods, the initial phase-angle, and the center distance are correctively and desirably changed, whereby during the operation of de-curling device  700 , the initial phase-angle is not varied, and the de-curling operation can be effectively continued. 
     In the above explanations, after the initial phase-angle of second de-curling roller  710 B is set to 0, the initial phase-angle of first de-curling roller  710 A is obtained. Instead, after the initial phase-angle of first de-curling roller  710 A is set to 0, the initial phase-angle of second de-curling roller  710 B is also obtained. Both methods can be used. 
     The function of de-curling device  700  of the present invention will now be detailed while referring to  FIG. 5   a  showing a side view, and  FIG. 5   b  showing an A-A sectional view of  FIG. 5   a , wherein the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B of de-curling device  700  is set to zero as the standard. Further, the function of de-curling device  700  will also be detailed while referring to  FIG. 6   a  showing a side view, and  FIG. 6   b  showing an A-A sectional view of  FIG. 6   a , wherein the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B of de-curling device  700  is set to 180°. Still further, the function of de-curling device will be detailed while referring to  FIG. 7   a  showing a side view, and  FIG. 7   b  showing an A-A sectional view of  FIG. 7   a , wherein the initial phase-angle difference between first and second de-curling rollers  710 A and  710 E of de-curling device  700  is set to 90°, which is an intermediate value between 0° and 180°. 
     In  FIGS. 5   a  and  5   b , first de-curling roller  710 A and second de-curling roller  710 B, represent rollers having spiral sections, wherein diameter “D” of thread sections  711 A and  7113  is 38 mm, diameter “d” of root sections  712 A and  712 B is 18 mm, pitch “P” of spiral is 62 mm, length “m” of thread sections  711 A and  711 B in the axial directions is 24 mm, and A-A cross sectional views of thread sections  711 A and  711 B represent half circles. However, the invention is not limited to the above shapes and sizes. First de-curling roller  710 A has cylindrical shaft  712 A to form the root section. A spiral is formed clockwise at a uniform pitch from predetermined point “C” on the center of shaft  712 A to one end of said shaft  712 A, and another spiral is formed counterclockwise at a uniform pitch from predetermined point “C” on the center of shaft  712 A to the end of said shaft  712 A, wherein the pitch of the clockwise spiral is equal to the pitch of the counterclockwise spiral, and the clockwise spiral and the counterclockwise spiral have line symmetry with respect to a vertical line passing on point “C” on shaft  712 A. Further, a spiral is formed clockwise at a uniform pitch from predetermined point “C” on the center of shaft  712 B to one end of said shaft  712 B, and another spiral is formed counterclockwise at a uniform pitch from predetermined point “C” on the center of shaft  712 B to the end of said shaft  712 B, wherein the pitch of the clockwise spiral is equal to the pitch of the counterclockwise spiral, and the clockwise spiral and the counterclockwise spiral have line symmetry with respect to a vertical line passing on point “C” on shaft  712 B. Accordingly, the lead angle of the spiral of first de-curling roller  710 A is determined to be in opposite directions to the lead angle of the spiral of second de-curling roller  710 B, though the values of both lead angles are equal to each other. 
     In  FIGS. 5   a  and  5   b , showing the initial phase-angle difference of “0°”, thread section  711 A of first de-curling roller  710 A and thread section  711 B of second de-curling roller  710 B are formed of elastic members, wherein both elastic members are configured to sandwich, and come into pressure-contact with, the recording sheet. Accordingly, both de-curling rollers  710 A and  7103  press the recording sheets at plural contacting points of thread sections  711 A and  711 B, whereby when both de-curling rollers  710 A and  710 B are rotated, the plural contacting points shift from the predetermined point on the center of both  710 A and  710 B toward both ends of de-curling rollers  710 A and  710 B. By the shifts of the plural contacting points, the curled recording sheet is effectively flattened. Because, the curved surface, where the generating lines gather together, is pressed and extended in the direction perpendicular to the generating lines, whereby the positive curl and the negative curl, being the parallel curls, shown in  FIGS. 9   a  and  9   b , and the waved curl, shown in  FIG. 9   c  are effectively flattened. 
     In  FIGS. 6   a  and  6   b , showing the initial phase-angle difference of 180°, top portion T of thread sections  711 A and middle portion G of root section  712 B, of the spirals of first and second de-curling rollers  710 A and  710 B, are configured to face each other. When both de-curling rollers  710 A and  710 B are rotated, the recording sheet is alternately nipped at even intervals, and pulled by thread sections  711 A and  711 B. Therefore, the recording sheet is conveyed, while receiving tensions to function from the center of both de-curling rollers  710 A and  710 B toward both ends, whereby, the positive curl and the negative curl, being the orthogonal curls, shown in  FIGS. 8   a  and  8   b , are effectively flattened. Because, curved surfaces of the front and reverse, where the generating lines gather together, are pressed and evenly extended in the direction of the generating lines, whereby the positive curl and the negative curl may be flattened. 
     In  FIGS. 7   a  and  7   b , showing the initial phase-angle difference of 90°, being an intermediate angle between 0° and 180°, top portion T of thread section  711 A and middle portion G of root section  712 B are configured to face each other in a deflected state. Accordingly, both thread section  711 A and  711 B of de-curling rollers  710 A and  710 B exhibit uneven interval, and close to each other, whereby the recording sheet is extended toward both its edges (being in the width direction). Further, the positive or negative oblique curl, includes a curling axis, the generating line of which is θ° oblique to the width direction of the recording sheet. When de-curling rollers  710 A and  710 B are rotated, the oblique curled sheet is nipped alternately at an uneven interval under the deflected state, and thereby the recording sheet is conveyed, while receiving the tension directed toward the width direction. Subsequently, the oblique curl of the recording sheet is changed to be the orthogonal curl, and subsequently, the orthogonal curl is flattened. 
     Further, when the initial phase-angle is set to be 270°, being an intermediate angle between 180° and 360°, since the angle of the oblique curl is opposite to the case of angle 90°, the curled recording sheet is flattened in the same way as the case of 90°. 
     Still further, whichever way the curl may be formed, among the orthogonal curl, the parallel curl or the oblique curl, the center distance between first and second de-curling rollers  710 A and  710 B is adequately determined, based on the curling amounts. 
     Concerning the quality of material of first and second de-curling rollers  710 A and  7108 , at least thread sections  711 A and  711 B should be structured of elastic members, and urethane foam, exhibiting  7  on the ASCA C Scale, or silicon foam, exhibiting  13  on the ASCA C Scale, are more preferably used. Due to the elastic members, areas of the recording sheet, nipped by thread sections  711 A and  711 B, become larger, whereby frictional force and pulling force against the recording sheet become greater, a flattening effect for the curled sheet becomes stable. For the shaft sections, polyacetal resin is preferably used. However, metallic members can also be used for the shaft sections. Further, elastic thread sections and elastic root sections can be united on metallic shafts. 
     While de-curling device  700  is not activated, the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B can be changed, and the center distance between the above rollers  710 A and  710 B can be increased, whereby the durability of de-curling roller set  710 , and first and second de-curling rollers  710 A and  710 B, structuring the same set can be increased. 
     Further, while de-curling device  700  is not activated, without changing the center distance between first and second de-curling rollers  710 A and  710 B, the initial phase-angle can be changed from a condition on which thread sections  711 A and  711 B face to each other, to other condition on which thread sections  711 A face root sections  712 B. Due to this changing action, both de-curling rollers are prevented from pressing to each other, whereby the durability of the rollers can easily be increased. 
     As detailed above, according to the present invention, on whichever surfaces the recording sheet is curled, and however great the curl may be, and however great the recording sheet variety, since the spiral sections apply the friction force and the pulling force on the recording sheet, the curled sheet is desirably flattened. 
     De-curling device  700  of the present invention can flatten various curled sheets, exhibiting various directions of the curl, various shapes of curls, various amounts of curls, various types of recording sheets, various sizes of recording sheets, by the method as detailed above. In order to support the above cases, the experimental data will now be detailed. 
     De-curling device  700 , including first and second de-curling rollers  710 A and  710 B, detailed in  FIGS. 5   a  and  5   b ,  FIGS. 6   a  and  6   b , and  FIGS. 7   a  and  7   b , is used in the experiments, in which the initial phase-angle and the center distance between the rollers are changed, and various types of recording sheets are flattened, which will be detailed below. 
     [Preparation of the Recording Sheets as Samples] 
     
         
         Item (1) Types of the recording sheets, depending upon sheet quality: four qualities, being quality A, quality B, quality C and quality D; 
         Item (2) Sizes of the recording sheets: five sizes, being size A4, size A3, size A4R, size B4, and size B5; 
         Item (3) Shapes of the curl: five shapes, being no curl, orthogonal curl, oblique curl, parallel curl, and waved curl; 
         Item (4) Curling amount: five groups, being 3-8 mm, 9-14 mm, 15-20 mm, and 0-2 mm (exhibiting no curl); 
       
    
     As shown in Table 1, while items (1) to (4) are combined, the recording sheets as the samples are prepared for condition 1-1, condition 2-1, condition 2-2, condition 2-3, condition 3-1, condition 3-2, condition 3-3, condition 4-1, condition 4-2, condition 4-3, condition 5-1, condition 5-2, condition 5-3, wherein each condition includes 20 positively curled sheets and 20 negatively curled sheets. 
     [Experimental Results] 
     Each sample sheet under the above conditions is introduced into de-curling device  700  of the present invention, combined to image forming apparatus  1 . 
     The quality and the size of de-curling rollers  710 A and  710 B to receive the curled recording sheets have been detailed above. 
     When the initial phase-angle difference between first and second de-curling rollers  710 A and  710 E is 0°, and when the center distance between first and second de-curling rollers  710 A and  710 B is 36 mm, sample recording sheets under condition 1-1, (that is, recording sheets having no curl) are introduced into de-curling device  700 , and are normally ejected from said device  700 , whereby no adverse curl is generated on said recording sheets. 
     In this experiment, it is determined that the flattened recording sheet exhibits a maximum curl height of 0-2 mm, placed on the desk. 
     When the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B is 180°, and when the center distance between said rollers  710 A and  710 B is 33 mm, the sample sheets of condition 2-1 are introduced into de-curling device  700 , and are normally ejected from said device  700 . 
     When the center distance between said de-curling rollers is 30 mm, the sample sheets of condition 2-2 are introduced into de-curling device  700 , and are normally ejected from said device  700 . 
     When the center distance between said de-curling rollers is 27 mm, the sample sheets of condition 2-3 are introduced into de-curling device  700 , and are normally ejected from said device  700 . 
     When the initial phase-angle difference between first and second de-curling rollers  710 A and  710 B is 90°, and when the center distance between first and second de-curling rollers  710 A and  710 B is 33 mm, sample recording sheets under condition 3-1 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between the said rollers is 30 mm, sample recording sheets under condition 3-2 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between said de-curling rollers is 27 mm, sample recording sheets under condition 3-3 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the initial phase-angle difference between first and second de-curling rollers  710 A and  7103  is 0°, and when the center distance between first and second de-curling rollers  710 A and  710 B is 33 mm, sample recording sheets under condition 4-1 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between said de-curling rollers is 30 mm, sample recording sheets under condition 4-2 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between said de-curling rollers is 27 mm, sample recording sheets under condition 4-3 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between said de-curling rollers is 33 mm, sample recording sheets under condition 5-1 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the center distance between said de-curling rollers is 30 mm, sample recording sheets under condition 5-2 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     When the clearance between said de-curling rollers is 27 mm, sample recording sheets under condition 5-3 are introduced into de-curling device  700 , and are flattened by said device  700 , and normally ejected. 
     The above experimental results are shown on the right half of Table 1, while various conditions are shown on the left half of Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Actions and Results 
               
            
           
           
               
               
               
               
            
               
                   
                 Initial Phase-angle, and Facing 
                 Center 
                   
               
               
                 Formation of Sample Sheets Exhibiting 
                 Condition of Spirals of First and 
                 Distance 
               
               
                 Various Curls 
                 Second De-curling Rollers 
                 between First 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Sheet Introducing Conditions 
                 Initial 
                   
                 and Second 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Types 
                 Sizes 
                   
                 Amounts 
                 Phase- 
                 First De- 
                 Second De- 
                 De-curling 
                 Corrected 
               
               
                   
                 of 
                 of 
                 Shapes of 
                 of Curl 
                 angle 
                 curling 
                 curling 
                 Rollers 
                 Results 
               
               
                   
                 Sheets 
                 Sheet 
                 Curl 
                 (mm) 
                 (Degree) 
                 Roller 
                 Roller 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Condition 
                 Type A 
                 A4 
                 No Curl 
                 0-2 
                 0 
                 Top of Thread 
                 Top of Thread 
                 36 
                 0-1 
               
               
                 1-1 
                   
                   
                   
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type B 
                 A3 
                 Orthogonal 
                 3-8 
                 180 
                 Top of Thread 
                 Center of 
                 33 
                 0-2 
               
               
                 2-1 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Root Section 
               
               
                 Condition 
                 Type B 
                 A3 
                 Orthogonal 
                  9-14 
                 180 
                 Top of Thread 
                 Center of 
                 30 
                 0-2 
               
               
                 2-2 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Root Section 
               
               
                 Condition 
                 Type B 
                 A3 
                 Orthogonal 
                 15-20 
                 180 
                 Top of Thread 
                 Center of 
                 27 
                 0-2 
               
               
                 2-3 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Root Section 
               
               
                 Condition 
                 Type C 
                 A4R 
                 Oblique 
                 3-8 
                 90 
                 Top of Thread 
                 *1 
                 33 
                 0-2 
               
               
                 3-1 
                   
                   
                 Curl 
                   
                   
                 Section 
               
               
                 Condition 
                 Type C 
                 A4R 
                 Oblique 
                  9-14 
                 90 
                 Top of Thread 
                 *1 
                 30 
                 0-2 
               
               
                 3-2 
                   
                   
                 Curl 
                   
                   
                 Section 
               
               
                 Condition 
                 Type C 
                 A4R 
                 Oblique 
                 15-20 
                 90 
                 Top of Thread 
                 *1 
                 27 
                 0-2 
               
               
                 3-3 
                   
                   
                 Curl 
                   
                   
                 Section 
               
               
                 Condition 
                 Type D 
                 B4 
                 Parallel 
                 3-8 
                 0 
                 Top of Thread 
                 Top of Thread 
                 33 
                 0-1 
               
               
                 4-1 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type D 
                 B4 
                 Parallel 
                  9-14 
                 0 
                 Top of Thread 
                 Top of Thread 
                 30 
                 0-2 
               
               
                 4-2 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type D 
                 B4 
                 Parallel 
                 15-20 
                 0 
                 Top of Thread 
                 Top of Thread 
                 27 
                 0-2 
               
               
                 4-3 
                   
                   
                 Curl 
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type E 
                 B5 
                 Waved Curl 
                 3-8 
                 0 
                 Top of Thread 
                 Top of Thread 
                 33 
                 0-2 
               
               
                 5-1 
                   
                   
                   
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type E 
                 B5 
                 Waved Curl 
                  9-14 
                 0 
                 Top of Thread 
                 Top of Thread 
                 30 
                 0-2 
               
               
                 5-2 
                   
                   
                   
                   
                   
                 Section 
                 Section 
               
               
                 Condition 
                 Type E 
                 B5 
                 Waved Curl 
                 15-20 
                 0 
                 Top of Thread 
                 Top of Thread 
                 27 
                 0-2 
               
               
                 5-3 
                   
                   
                   
                   
                   
                 Section 
                 Section 
               
               
                   
               
               
                 *1: Portions other than Center of Root Section 
               
            
           
         
       
     
     By the above experimental results, independent of the positive or negative curl, the shape of curl, the curling amount, the types of recording sheets, and the sizes of recording sheets, the inventors understand that various kinds of curled sheets can be flattened by de-curling device  700 . 
     When de-curling device  700  is to be activated, the relative positional relationships between first and second de-curling rollers  710 A and  710 B, being suitable for flattening recording sheets, is set at the initial phase-angle by initial phase-angle changing section  740 A, based on information concerning the curled sheet, which information is inputted when the job is started. After that, the center distance between the first and second de-curling rollers  710 A and  710 B is inputted through center distance changing section  740 B. 
     The drive control of de-curling device  700  is conducted by control device  800 . The drive control will be detailed while referring to  FIG. 11 . 
     CPU (Central Processing Unit)  801  is electrically connected to ROM (Read Only Memory)  802 , RAM (Random Access Memory)  803 , and operation display section  805 , through system bus  807 . CPU  801  reads out the data table shown in Table 1, among various programs stored in ROM  802 , and expands it to RAM  803 . Subsequently, various items such as, a detected value concerning the size of sheet, sheet weight, and sheet type to be inputted into operation display section  805 , the shape of curl, and the curling amount, are compared with the data table, expanded in RAM  803 . Otherwise, the shape of curl and the curling amount can be detected by a curl shape detector or curling amount detector, each detector is mounted upstream of de-curling device  700 , shown by dashed lines in  FIG. 11 , and said detected shape and amount are compared to the data table. After that, the initial phase-angle of first de-curling roller  710 A, being equal to the data table, is selected to be set by initial phase-angle changing section  740 A, and the center distance between first and second de-curling rollers  710 A and  7103  is also selected to be equal to a value in Table 1. That is, various curls are effectively flattened, based on the selected initial phase-angle and the center distance between the rollers. 
     CPU  801  conducts various processes, using the programs in RAM  803 , and stores processed results in RAM  803 , and displays them on operation display section  805 . Further CPU  801  stores the processed results, stored in RAM  803 , in predetermined sections. In the present embodiment, CPU  801  works with ROM  802  and RAM  803 , so that a main section of control device  800  is established. 
     The data shown in Table 1 is an example of a model to flatten the curled sheet, which satisfies the sheet conveying conditions. Further, concerning the sheet conveying condition, the data of the initial phase-angle difference and the data of the center distance are the optimum data obtained by the experiments, to be used in de-curling device  700 . However, the sheet conveying conditions are not limited to the data shown in Table 1. That is, the total number of copied sheets in a day, and environmental conditions (being an installation area for the device, the temperature and humidity in the device) can be grouped to realize the most effective de-curling operation. Further, data of the initial phase-angles and data of the center distance between the first and second de-curling rollers  710 A and  710 B of de-curling device  700 , which satisfies the sheet conveying conditions, can be more precisely arranged by the same means as the above model. These data can be found by experiments, in the same way as the data of Table 1 was formed. 
     Such found data can be previously inputted in ROM  802  of control device  800  of de-curling device  700 , and when a sheet conveying condition, corresponding to data other than Table 1, is inputted, optimum initial phase-angle and center distance can be displayed. Further, based on the displayed information, de-curling device  700  can be more precisely adjusted, so that various curled sheets can be flattened in the same way as the case of Table 1. Still further, when a sheet conveying condition is inputted, the initial phase-angle and the center distance, to be used in de-curling device  700 , can be automatically changed so said device can easily perform the desired operation. 
     Concerning the effects of the present invention, based on the directions of curl (being the positive curl or the negative curl), the shape of curl (being parallel, orthogonal, or oblique, to the sheet conveyance direction), and the curling amount, most of various types of curls can be flattened. Due to this, when the printed sheets are ejected onto the tray, said sheets can be orderly stacked on the tray, so that the user can handle the stacked sheets with no difficulty. Further, while the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the appended claims.