Abstract:
A control section controls the rotation of a registration roller that corrects the skew of a sheet and conveys the sheet, and the rotation of a loop forming roller which forms a loop of the sheet, as well as the nipping and releasing of the nip of the loop forming roller: This control section controls so that the sheet with a loop formed between the registration roller and loop forming roller is conveyed downstream in the sheet conveyance direction, and the sheet is released from the nipping of the loop forming roller during the conveyance. This control section further controls so that the total number of rotations of the registration roller from the start of sheet conveyance till releasing of the sheet from nipping of the loop forming roller is kept at a prescribed total number of rotations preset in conformance to the paper type.

Description:
[0001]    This application is based on Japanese Patent Application Nos. 2010-196453 and 2010-196454 filed on Sep. 2, 2010 with Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an image forming apparatus provided with the functions of a photocopier, printer, facsimile and multi-functions thereof. 
         [0003]    One of the techniques known in the conventional art is an image forming apparatus where a toner image is formed on the photoreceptor drum as an image carrier using electrophotographic process and the toner image is transferred onto a sheet of paper, and then the image is fixed by a fixing section. 
         [0004]    In this image forming apparatus, when an image forming operation has started, the sheets stored in a sheet accommodation section are sequentially fed to a sheet conveyance path by a sheet feeding unit. 
         [0005]    This sheet conveyance path is provided with a registration roller for nipping and conveying sheets and a loop forming roller arranged on the upstream side of this registration roller to nip and convey the sheets. 
         [0006]    Each of the aforementioned registration roller and loop forming roller is made up of a pair of rollers composed of a drive roller and a driven roller. 
         [0007]    The sheet fed out from a sheet feed section by the sheet feeding unit is conveyed by plural conveyance rollers including the loop forming roller arranged on the upstream side of the registration roller. The leading edge of the sheet is made to hit against the registration roller whose rotation has been suspended. After that, a loop is formed on the sheet by the rotation of the loop forming roller. This allows all area of the leading edge of the sheet to hit against the registration roller, whereby sheet skew is corrected. 
         [0008]    After this correction of sheet skew, the registration roller starts to rotate synchronously with formation of an image on the photoreceptor drum, and the sheet is fed again. In this sheet re-feed operation, the sheet is conveyed while the sheet is kept looped by the rotating loop forming roller and is nipped by the registration roller. 
         [0009]    In the aforementioned sheet re-feed process, width-wise bias of the sheet is corrected. 
         [0010]    With respect to the aforementioned skew correction and bias correction, a proposal has been made of a sheet conveyance apparatus (see, e.g., Japanese Unexamined Patent Application Publication No. 2008-265974). According to this proposal, the leading edge of the sheet being conveyed is made to hit against the registration roller, and then the sheet is fed out to the next process downstream from the conveyance path, with the start of rotation of the registration roller. A guide member having a loop shape and arranged to extend in the direction perpendicular to the sheet conveyance direction is rotatably provided close to the upstream side of the registration roller. 
         [0011]    In the Japanese Unexamined Patent Application Publication No. 2008-265974, the leading edge of the sheet having been conveyed by the conveyance rollers including the loop forming roller arranged upstream of the registration roller is made to hit against the registration roller whose rotation has been suspended. Further, a loop is formed on the sheet by the rotation of the loop forming roller, whereby skew of the sheet is corrected. 
         [0012]    After the sheet skew correction, synchronously with image formation on the photoreceptor drum, the registration roller and loop forming roller are rotated at approximately the same speeds and the sheet is re-fed with the loop kept formed. 
         [0013]    In the aforementioned sheet re-feeding step, bias of the sheet is corrected by width-wise traveling of the registration roller. In the aforementioned step of registration roller traveling, the aforementioned guide member is rotated in such a direction as to encourage displacement of the sheet that is displaced as a result of traveling of the registration roller, whereby distortion of sheets is minimized. 
         [0014]    When the leading edge of the skewed sheet has been hit against the registration roller to correct skew, a difference occurs in the size of the loop on both ends across the width perpendicular to the sheet conveyance direction. To be more specific, there is a difference in the shape of the loop. The difference in the shape of the loop is greater as the skew is increased. 
         [0015]    In the following description, the loop having a different shape is also called the uneven loop. 
         [0016]      FIGS. 2   a  and  2   b  are schematic diagrams illustrating the sheet skew and skew correction.  FIG. 2   a  shows that the sheet P is skewed and is conveyed by the loop forming roller  22 D.  FIG. 2   b  shows that, after the leading edge of the sheet P has been hit against the registration roller  23  whose rotation is suspended, a loop is formed on the sheet by the rotation of the loop forming roller  22 D, and the sheet skew is corrected. 
         [0017]    As shown in  FIG. 2   b , when the sheet P is skewed and the leading edge of the sheet P is made to hit against the registration roller  23 , and then a loop is formed to correct skew, the loop will have different sizes in the cross-wise direction perpendicular to the sheet conveyance direction, i.e., on the downside of the  FIG. 2   b  (also referred to as “on the near side” in the following description) and on the upside of the  FIG. 2   b  (also referred to as “on the far side” in the following description).  FIG. 2   b  shows an example wherein the loop L 1  on the near side is greater than the loop L 2  on the far side. 
         [0018]    After the leading edge of the sheet has been hit against the registration roller, and skew has been corrected, the registration roller starts rotation. Then the sheet is flipped out by the bias force in the sheet conveyance direction, which is generated by the loop due to the toughness of the sheet P, and the sheet is nipped by the registration roller. The sheet is further fed out downstream while the loop is maintained by the rotating loop forming roller. 
         [0019]    The aforementioned bias force differs according to the shape of a loop. The bias force is greater as the loop is smaller. Thus, the difference in the bias force is increased as the shape (size) of the loop is increased. 
         [0020]    Accordingly, when the rotation of the registration roller has started and the sheet is fed by the registration roller and loop forming roller, the amount of conveyance on the smaller loop side ( FIG. 2   b , loop L 2 ) is greater than that on the greater loop side ( FIG. 2   b , loop L 1 ) under the influence of the difference in the bias force of the uneven loop. Thus, the sheet is conveyed with the leading edge kept in the shape of a fan. 
         [0021]      FIG. 3  is a schematic diagram illustrating that a sheet is conveyed with the leading edge kept in a fan shape. A difference of distance d 1  occurs on the right and left of the leading edge of the sheet P, and the fan shape shown by oblique lines (hatching) is created. 
         [0022]    If the sheet is conveyed with such a fan shape, the positional accuracy of images on the front and rear surfaces will especially be deteriorated, in the case of duplex printing where the front surface is first printed and then the sheet P is reversed to perform printing on the rear surface. 
         [0023]      FIG. 4  is a drawing showing the positions of images when a fan shape such as one shown in  FIG. 3  has been produced on both surfaces in the duplex printing mode. In the drawing, the solid line indicates the front surface image position, and the broken line shows the rear surface image position. 
         [0024]    As described above, in the Japanese Unexamined Patent Application Publication No. 2008-265974, a rotatable guide member is used to correct skew and bias of the sheet, whereby distortion of the sheet is suppressed. However, action is not taken to remove a difference in the amount of conveyance due to a difference in loop shape, i.e., to suppress formation of the fan shape. This method cannot easily improve the positional accuracy in printing, in particular, that in the duplex printing. 
         [0025]    To minimize such image misalignment, it is preferred to release the sheet from the nipping of the loop forming roller  22 D as quickly as possible so that the sheet can be conveyed only by the registration roller  23 , without being adversely affected by the uneven loop. 
         [0026]    The position of the leading edge with respect to the registration roller is different in conformity to the type of a sheet. The aforementioned type of the sheet denotes the differences in paper thickness, and differences in whether or not a toner image has been formed on the first surface of the sheet. 
         [0027]    (1) In skew correction, when the leading edge of the sheet has been hit against the registration roller, the position of the leading edge with respect to the registration roller is different due to the difference in paper thickness. 
         [0028]      FIG. 5  is a diagram illustrating the contrast in the respective positions of the leading edges when the leading edges of the thin sheet and thick sheet have been hit against the registration roller  23 . It should be noted that  FIG. 5  does not represent the actual diameter of the registration roller or actual thicknesses of the thin sheet and thick sheet and the values in  FIG. 5  are exaggerated. In the drawing, a solid line indicates the leading edge of a thin sheet, while the broken line shows the leading edge of a thick sheet. 
         [0029]    As shown in  FIG. 5 , when the leading edge of the sheet has been hit against the registration roller  23 , the leading edge of the thin sheet comes closer to the nip section N formed by the drive roller  23   a  and driven roller  23   b,  than that of the thick paper. This produces a difference in distance d 21 . 
         [0030]    Thus, after the rotation of the registration roller  23  has started, the sheet is conveyed downstream with the loop kept formed, and if the sheet is released from the nipping of the loop forming roller  22 D after the lapse of a constant time, independently of the thickness of paper, the amount of conveyance before this release from the nipping is greater in the case of a thin sheet than that in the case of a thick sheet. To be more specific, the distance from the center of the registration roller nip to the leading edge of the sheet is greater in the case of a thin sheet than that in the case of a thick sheet. Thus, the fan-shaped portion is increased in size when a thin sheet is conveyed. To put it another way, the influence of the uneven loop is increased. 
         [0031]    To avoid this, if a step is taken to reduce the time before the sheet is released from the nipping of the loop forming roller  22 D, the amount of conveyance will be insufficient when the thick sheet is conveyed, and nipping at the registration roller  23  will be insufficient. Thus, failure of secure conveyance may be caused by nipping failure. 
         [0032]    (2) In skew correction, when the leading edge of the sheet has been hit against the registration roller, the position of the leading edge with respect to the registration roller differs according to whether or not a toner image is formed on the first surface of the sheet. 
         [0033]    As described above, each of the registration roller  23  and loop forming roller  22 D is composed of a pair of rollers consisting of a drive roller and driven roller. The drive roller is placed at the position in contact with the surface opposite to the surface where an image is formed, while the driven roller is located at the position in contact with the surface where an image is formed. 
         [0034]    In the following description, the term “opposite surface” indicates the surface of the sheet in contact with the drive roller, unless otherwise specified. 
         [0035]      FIG. 6  is a diagram showing the positional relationship between the drive roller  23   a  and the driven roller  23   b  of the registration roller  23 . 
         [0036]    In the duplex printing mode where images are formed on both sides of a sheet, after an image has been formed on the first surface of the sheet, the sheet is reversed to change the positions of the first and second surfaces. Then the sheet is again conveyed to the loop forming roller. Similarly to the case of forming an image on the first surface, sheet skew is corrected. 
         [0037]    Depending on whether or not an image is formed on the surface (opposite surface) of the sheet in contact with the drive roller  23   a,  namely, depending on the presence or absence of a toner image on the first surface, the friction coefficient on this opposite surface varies. Friction coefficient is smaller when there is a toner image than the case when there is no toner image. 
         [0038]    Accordingly, after the rotation of the registration roller  23  has started, the sheet is conveyed downstream with the loop kept formed. If independently of presence or absence of a toner image on the opposite surface, the sheet is released from the nipping of the loop forming roller  22 D after the lapse of a constant period of time, the amount of conveyance before the sheet is released from the nipping will be greater when there is no toner image on the opposite surface than when there is a toner image. The broken line of  FIG. 6  indicates the position of the leading edge when there is no toner image on the opposite surface. The solid line denotes the position of the leading edge when there is a toner image on the opposite surface. As described above, depending on the presence or absence of a toner image, there is a difference d 22  in the distance from the nip center of the registration roller  23  to the leading edge of the sheet. 
         [0039]    Thus, when there is no toner image, the size of the fan-shaped portion is increased. To put it another way, there is an increase in the influence of the uneven loop. 
         [0040]    To avoid this, if a step is taken to reduce the time before the sheet is released from the nipping of the loop forming roller  22 D, the amount of conveyance will be insufficient when there is a toner image on the sheet, and nipping at the registration roller  23  will be insufficient. Thus, failure of secure conveyance may be caused by nipping failure. 
         [0041]    In view of the problems described above, it is an object of the present invention to provide an image forming apparatus by which, in the step of conveying a sheet after skew thereof has been corrected by hitting the leading edge of the sheet against the registration roller and forming a loop, stable conveyance is ensured by preventing conveyance failure from occurring due to influence of the difference in the shape across the width of the loop formed at the time of skew correction. 
       SUMMARY 
       [0042]    To achieve at least one of the above mentioned objects, an image forming apparatus reflecting one aspect of the present invention includes the following. 
         [0043]    An image forming apparatus including: 
         [0044]    a registration roller having a pair of rollers for correcting a skew of a sheet and conveying the sheet; 
         [0045]    a loop forming roller arranged upstream of the registration roller in a sheet conveyance direction to hit a leading edge of the sheet against the registration roller and to form a loop of the sheet; and 
         [0046]    a control section for controlling rotation of the registration roller and rotation of the loop forming roller and for controlling nipping and releasing of a nip of the loop forming roller, 
         [0047]    wherein the control section controls to rotate the registration roller and the loop forming roller to conduct conveyance of the sheet with a loop formed thereon, downstream in the sheet conveyance direction, after forming the loop on the sheet by hitting the leading edge of the sheet against the registration roller, and controls the loop forming roller to release the sheet from nipping of the loop forming roller during the conveyance, and further controls to adjust a total number of rotations of the registration roller from a start of the conveyance of the sheet by the rotation of the registration roller and the rotation of the loop forming roller until the loop forming roller releases the sheet from the nipping, to a prescribed total number of rotations having been set according to a type of the sheet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]      FIG. 1  is a drawing showing an example of the image forming apparatus related to the present invention. 
           [0049]      FIGS. 2   a  and  2   b  are schematic diagrams illustrating the sheet skew and skew correction. 
           [0050]      FIG. 3  is a schematic diagram illustrating that sheet is conveyed with the leading edge keeping a fan-shape form. 
           [0051]      FIG. 4  is a drawing showing the position of an image when a fan shape has been produced on both surfaces in the duplex printing mode. 
           [0052]      FIG. 5  is a diagram illustrating that the leading edges of the thin sheet and thick sheet have been hit against on the registration roller. 
           [0053]      FIG. 6  is a diagram showing the positional relationship between the drive roller and the driven roller of the registration roller. 
           [0054]      FIG. 7  is a lateral cross sectional view showing the major components located in the vicinity of the registration roller and loop forming roller. 
           [0055]      FIGS. 8   a - 8   d  are diagrams showing the process of conveying a sheet by the registration roller and loop forming roller. 
           [0056]      FIG. 9  is a timing chart showing the process of conveying sheets given in  FIGS. 8   a - 8   d .    
           [0057]      FIG. 10  is a diagram showing a step of correcting the bias across the sheet width. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0058]    The following describes the embodiments of the present invention with reference to the drawings. However the present invention is not limited thereto. 
         [0059]    In the first place, an example of the image forming apparatus related to the present invention will be described with reference to the structural diagram of  FIG. 1 . 
         [0060]    This image forming apparatus includes an image forming apparatus main body GH and an image reading device YS. The image forming apparatus main body GH is called the tandem color image forming apparatus, and includes a plurality of image forming sections  10 Y,  10 M,  10 C and  10 K, belt-shaped intermediate transfer belt  5 , sheet feed and conveying unit and fixing device  8 , reverse sheet ejection section  600  and ADU (Auto Duplex Unit)  700  for conducting reverse conveyance, and others. 
         [0061]    Further, the image forming apparatus includes a control section AS for controlling each section. 
         [0062]    The top of the image forming apparatus main body GH is provided with an image reading device YS including an automatic document feed device  201  and scanning exposure device  202 . The document “d” placed on the document platen of the automatic document feed device  201  is conveyed by the conveying unit. An image on a surface or images on both surfaces of the document are subjected to scanning and exposure by the optical system of the scanning exposure device  202 , and is read into the line image sensor CCD. 
         [0063]    The signal formed by photoelectric conversion through the line image sensor CCD is subjected to analog processing, analog-to-digital conversion, shading correction and image compression in the image processing section, and is sent to the exposure units  3 Y,  3 M,  3 C and  3 K. 
         [0064]    The image forming sections  10 Y for forming a yellow (Y) image has a charging unit  2 Y, exposure unit  3 Y, development unit  4 Y and cleaning unit  7 Y arranged around the photoreceptor drum  1 Y. The image forming sections  10 M for forming a magenta (M) image has a charging unit  2 M, exposure unit  3 M, development unit  4 M and cleaning unit  7 M arranged around the photoreceptor drum  1 M. The image forming sections  10 C for forming a cyan (C) image has a charging unit  2 C, exposure unit  3 C, development unit  4 C and cleaning unit  7 C arranged around the photoreceptor drum  1 C. The image forming sections  10 K for forming a black (K) image has a charging unit  2 K, exposure unit  3 K, development unit  4 K and cleaning unit  7 K arranged around the photoreceptor drum  1 K. Latent image forming units are formed by a charging unit  2 Y and exposure unit  3 Y, a charging unit  2 M and exposure unit  3 M, a charging unit  2 C and exposure device  3 C, and a charging unit  2 K and exposure device  3 K 
         [0065]    The development units  4 Y,  4 M,  4 C and  4 K include the two-component developer made of yellow (Y), magenta (M), cyan (C) and black (K) toners having a small particle diameter, and carriers. The toner is made of pigment or dye serving as a coloring reagent, a wax helping separation of toner from the fixing member after fixing, and a binder resin for holding these together. 
         [0066]    The intermediate transfer belt  5  is wound around plural rollers and is supported rotatably. 
         [0067]    The fixing device  8  allows the toner image of a sheet P to be heated and pressed by a nip portion formed between the heated fixing belt  81  which is a fixing member and pressure roller  83  which is a pressing member, whereby the toner image is fixed. 
         [0068]    Thus, images of different colors formed by the image forming sections  10 Y,  10 M,  10 C and  10 K are sequentially transferred onto the rotating intermediate transfer belt  5  by the transfer units  6 Y,  6 M,  6 C and  6 K as a primary transfer, and a composite color toner image is created. 
         [0069]    The sheet P stored in the sheet feed cassette  20  is fed by the sheet feed unit  21 , and is conveyed to the transfer unit  6 A through the sheet feed rollers  22 A,  22 B and  22 C and loop forming roller  22 D, registration roller  23  and others. Then the color image is transferred onto the sheet P as a secondary transfer. 
         [0070]    At the loop forming roller  22 D and registration roller  23 , correction of skew and bias of the sheet P is conducted. Details of the correction of skew and bias will be described later. 
         [0071]    The sheet P with the color image transferred thereon is heated and pressed by the fixing device  8 , and the color toner image on the sheet P is fixed. After that, the sheet is sandwiched and conveyed by the sheet ejection roller  24  and is placed on the sheet ejection tray  25  provided outside the apparatus. 
         [0072]    The sheet P can be reversed to change the positions of the front and rear surfaces to be ejected by switching the position of the passage switching member  601  of the reversing sheet ejection section  600 . 
         [0073]    For example, in the step of reversing and ejecting the sheet P, the position of the passage switching member  601  is switched so that the sheet P is guided downward along the right side of the passage switching member  601 , and the sheet P is conveyed toward the roller pair  602 . After the trailing end of the sheet P has been sandwiched between the roller pair  602 , the roller pair  602  is rotated in the reverse direction so that the sheet P is raised. After that, the sheet P reaches the sheet ejection roller  24  through the left side of the passage switching member  601 . The sheet P is then sandwiched by the sheet ejection roller  24  and conveyed to be placed on the sheet ejection tray  25  outside the apparatus. 
         [0074]    In the duplex printing mode where the ADU  700  is employed, the sheet P with an image formed and fixed on one surface (the first surface) is led downward along the right side of the passage switching member  601 . With the trailing edge of the sheet P sandwiched by the roller pair  605 , conveyance is suspended. 
         [0075]    This is followed by the step of reverse rotation of the roller pair  605 . The sheet P is raised along the guide plate G 1  and is led to the ADU  700  provided with plural roller pairs  701 ,  703  and  705 , wherein the sheet P is reversed. 
         [0076]    In the meantime, after a color image has been transferred onto the sheet P by the transfer device  6 A, a cleaning device  7 A is used to remove the toner remaining on the intermediate transfer belt  5  from which the sheet P has been removed by curvature. 
         [0077]    The image forming apparatus for forming a color image has been described. The present invention is also applicable to an image forming apparatus for forming a monochromatic image. Further, use of an intermediate transfer belt is optional. 
         [0078]    The fixing device  8  can use a thermal roller fixing method where a roller equipped with a heating device is used as a fixing member. 
         [0079]    The following describes how the sheet P is conveyed by the registration roller  23  and loop forming roller  22 D. The drive of these rollers and contact/separation of the nip are controlled by the control section AS. 
         [0080]      FIG. 7  is a lateral cross sectional view showing the major components located in the vicinity of the registration roller  23  and loop forming roller  22 D. 
         [0081]    The registration roller  23  includes a drive roller  23   a  and a driven roller  23   b,  while the loop forming roller  22 D includes a drive roller  22 Da and driven roller  22 Db. A lower guide plate G 2  and an upper guide plate G 3  are also arranged. The upper guide plate G 3  is supported swingably toward the upper side (indicated by broken line) so that a loop of the sheet P can be formed on the upper side. The upper guide plate G 3  can be omitted if the guide of the leading edge of the sheet is not required because of the center distance between the registration roller  23  and loop forming roller  22 D or because of toughness of the sheet P to be used. Further, instead of the upper guide plate G 3  being swingably supported, it is possible to increase the clearance between the lower guide plate G 2  and the upper guide plate G 3  at a prescribed position, by giving consideration to the formation of a loop of the sheet P. 
         [0082]    There is no particular restriction to the material of the aforementioned rollers. For example, EPDM-made rubber rollers are preferably employed for the drive roller  23  a and drive roller  22 Da, while stainless steel rollers are preferably used for the driven roller  23   b  and driven roller  22 Db. 
         [0083]      FIGS. 8   a - 8   d  are diagrams showing the process of conveying sheets P by the registration roller  23  and loop forming roller  22 D. 
         [0084]    As described above, the sheet P accommodated in the sheet feed cassette  20  is fed by the sheet feed device  21 , and is conveyed to the loop forming roller  22 D through the sheet feed rollers  22 A,  22 B and  22 C. 
         [0085]    The loop forming roller  22 D conveys the sheet P to the registration roller  23  being suspended ( FIG. 8   a ). 
         [0086]    After having hit the leading edge of the sheet P against the registration roller  23 , the loop forming roller  22 D is rotated for a certain period of time, and is suspended after forming a loop L (FIG.  8   b ). This allows the entire side of the leading edge of the sheet to hit against the registration roller, whereby skew of the sheet P is corrected. 
         [0087]    After skew of this sheet P has been corrected, the registration roller  23  starts rotation synchronously with formation of an image on the photoreceptor drum, and the sheet P is fed again. In this step of sheet re-feeding the registration roller  23  and loop forming roller  22 D are also rotated, and the sheet P is conveyed while a loop is formed thereon ( FIG. 8   c ). 
         [0088]    After sheet re-feeding of the sheet P has started, the drive roller  22 Da and driven roller  22 Db of the loop forming roller  22 D are separated at a prescribed time and the sheet is released from the nipping (i.e., nipping is turned oft). This step removes the loop L of the sheet P ( FIG. 8   d ). What is meant by “a prescribed time” is not restricted to time and it can be the total number of rotations of the registration roller  23  subsequent to the start of sheet re-feeding, for example. 
         [0089]    Nipping condition is also called “nip on” state, while the cancellation condition of the nipping is also referred to as “nip off” state. 
         [0090]      FIG. 9  is a timing chart showing the process of conveying sheets given in  FIGS. 8   a  through  8   d . In  FIG. 9 , the time “t 1 ” corresponds to the time period before the loop forming roller  22 D is stopped, as shown in  FIGS. 8   a  and  8   b , and the time “t 2 ” corresponds to the time period where the loop forming roller  22 D in  FIG. 8   b  is suspended. The time “t 3 ” corresponds to  FIG. 8   c , and the time “t 4 ” corresponds to  FIG. 8   d . The time “t 3 ” of the chart indicates prescribed time duration from the start of sheet re-feeding of the sheet P, tilt the cancellation of the nipping in the loop forming roller  22 D. Thus, as described above, what is meant by “a prescribed time” is not restricted to time, it can be the total number of rotations of the registration roller  23  subsequent to the start of sheet re-feeding, for example. 
         [0091]    As described above, to prevent generation of a fan shape of  FIG. 3  and to minimize image misalignment, it is preferred to cancel the nipping of the loop forming roller  22 D as quickly as possible and to convey the sheet P by the registration roller  23  alone, thereby avoiding any influence of the uneven loop. To put it another way, it is preferred to shorten t 3  as much as possible. 
       Embodiment 1 
       [0092]    To shorten t 3  for avoidance of any influence of the uneven loop and to ensure nipping of the sheet P by the registration roller  23  for stabilization of conveyance, it is preferred to cancel the nipping of the loop forming roller  22 D at a constant distance after the leading edge of the sheet P has passed through the nip section N of the registration roller  23 , independently of paper thickness as a sheet type. To put it another way, the distance d 3  between the nip section N and the leading edge of the sheet P in  FIG. 8   d  is preferably kept at an approximately constant value. 
         [0093]    However, as shown in the aforementioned  FIG. 5 , when the leading edge of the sheet has been hit against the registration roller  23 , the leading edge of the thin sheet is placed closer to the nip section N formed by the drive roller  23   a  and driven roller  23   b,  than that of the thick sheet. This produces a difference of distance d 21 . 
         [0094]    Thus, the following problem occurs if the nipping of the loop forming roller  22 D is cancelled at a constant time subsequent to the start of sheet re-feeding independently of the thickness of paper as in the conventional art. That is, the distance d 3  between the nip section N and the leading edge of the sheet P when the nipping is cancelled is greater in the case of a thin sheet than in the case of a thick sheet. This problem makes it difficult to control the distance d 3  at a constant value. 
         [0095]    One possible way of controlling the distance d 3  at a constant value is to install a sensor to detect the leading edge of the sheet P, at a position immediately after the nip. Immediately after the nip, however, rollers and shafts are already installed and a limited space raises a problem difficult to solve. 
       Control Example 1 
       [0096]    In the present embodiment of the invention, the control section AS provides control in such a way that the total number of rotations of the registration roller  23  from the start of rotation of the registration roller  23  until the cancellation of the nipping of the loop forming roller  22 D is kept at a prescribed total number of rotations set in response to the thickness of the sheet P. 
         [0097]    To put it another way, control is provided in such a way as to cancel the nipping of the loop forming roller  22 D, when the total number of rotations of the registration roller  23  has reached a prescribed total number of rotations set in conformance to the thickness of the sheet P, subsequent to the start of rotation of the registration roller  23  (after start of sheet re-feeding). This means that the amount of conveyance from the start of sheet re-feeding until the cancellation of the nipping of the loop forming roller  22 D is equalized to the value in conformance to the thickness of paper. Immediately after the nipping has been cancelled, the rotation of the loop forming roller  22 D is suspended. 
         [0098]    The term “total number of rotations” indicates the total number of rotations of the registration roller  23 , subsequent to the start of rotation of the registration roller  23  (subsequent to the start of sheet re-feeding). 
         [0099]    The aforementioned total number of rotations can be detected by a rotation detecting mechanism (not illustrated) such as a rotary encoder for detecting the total number of rotations, which is connected to the registration roller  23  or a drive source as exemplified by a drive motor. By detecting the total number of rotations of the registration roller  23  or drive motor as described above, it is possible to remove influence of the fluctuation in the rotational speed of the drive motor or the fluctuation in the rotational speed at the time of initiating the rotation at the start-up. 
         [0100]    A stepping motor can be used to drive the registration roller  23 , and the control of the total number of rotations can be done by pulse control. 
         [0101]    As shown in  FIG. 5 , when the leading edge of the sheet has been hit against the registration roller  23 , the leading edge of the thin sheet is placed closer to the nip section N formed by the drive roller  23   a  and driven roller  23   b,  than that of the thick sheet. This produces a difference of distance d 21 . Accordingly, to keep the distance d 3  at approximately a constant level independently of paper thickness, a prescribed total number of rotations must be set at a higher value for thick sheets than for thin sheets. 
         [0102]    The aforementioned prescribed total number of rotations is set in advance in conformance to the paper thickness based on the design specifications or experiment It should be noted that the prescribed total number of rotations can be set for each of paper thicknesses. Alternatively, paper thicknesses is classified according to prescribed ranges to set up the groups of paper thicknesses, and the prescribed total number of rotations can be set for each of these groups. 
         [0103]    The paper thickness and prescribed total number of rotations set in conformance to the paper thickness are formulated into a data table, for example, and are stored into the storage section (not illustrated) of the image forming apparatus main body GH in advance. 
         [0104]    At the time of printing, an operator selects the type of paper (paper thickness) on an operation panel (not illustrated) of the image forming apparatus main body GH, or selects a sheet feed cassette  20 . Then the data on prescribed total number of rotations in conformance to paper thickness is selected from the aforementioned data table. The control section provides control based on the data. 
         [0105]    When the nipping of the loop forming roller  22 D has been cancelled, the loop disappears and the sheet P is conveyed after skew has been corrected. In the process of sheet conveyance, bias of the sheet P across the width is corrected. 
         [0106]      FIG. 10  is a diagram showing a step of correcting the bias. An end face detecting sensor S 101  for detecting the position of the end face of the sheet P across the width is arranged upstream of the registration roller  23 . Further, the registration roller  23  is supported movably across the width of the sheet P, and is moved in the direction of the arrow Y 11  by a traveling mechanism (not illustrated). 
         [0107]    When bias of the sheet P has been detected by the end face detecting sensor S 101 , the traveling mechanism moves the registration roller  23  in the direction of correcting the bias, whereby bias is corrected. 
         [0108]    As shown in  FIG. 10 , when the end face detecting sensor S 101  has found out that the sheet P is biased toward the near side (downward in the drawing) (portion marked by a solid line), the traveling mechanism moves the registration roller  23  toward the far side (upward in the drawing) so that the sheet P is moved (portion marked by a broken line), whereby bias is corrected. 
         [0109]    The aforementioned procedure allows the distance d 3  to be kept at the minimum distance which is approximately constant, independently of paper thickness. This prevents a fan shape from occurring due to loop unevenness (an uneven loop). This also ensures stable conveyance subsequent to registration, whereby image misalignment is suppressed. 
         [0110]    In the aforementioned description, the control for keeping the distance d 3  at an approximately constant level independently of paper thickness is achieved by detecting the total number of rotations of the registration roller  23  or drive motor (named Control Example 1). This can also be achieved by controlling the rotation time or rotational speed of the registration roller  23 . Examples are given below. 
       Control Example 2 
       [0111]    In the Control Example 2, the rotational speed of the registration roller  23  is kept constant independently of paper thickness, and the rotation time is controlled in conformance to paper thickness. Accordingly, a longer rotation time is set for a thick sheet than for a thin sheet. In the case of Control Example 2, if a motor of constant rotational speed is used to drive the registration roller  23 , only the rotation time can be placed under control. This makes it possible to omit the mechanism for detecting the total number of rotations of the registration roller  23 , and to simplify the structure of the mechanism. 
         [0112]    The rotation time in the Control Example 2 is set in advance according to the paper thickness based on the design specifications or experiment. It is preferably set with consideration given to fluctuations of the rotational speed at the time of initiating the rotation at the start-up of the drive motor. The rotation time can be set for each of the paper thicknesses. Alternatively, paper thicknesses is classified according to prescribed ranges to set up the groups of paper thicknesses, and the rotation time can be set for each of these groups. 
       Control Example 3 
       [0113]    In the Control Example 3, the rotation time of the registration roller  23  is set at a constant value, independently of paper thickness, and the rotational speed is controlled in conformance to paper thickness. Accordingly, the rotational speed is set at a higher value for a thick sheet than for a thin sheet. In the Control Example 3, the registration roller  23  is driven preferably by a drive motor whose rotational speed can be easily changed. Further, this makes it possible to omit the mechanism for detecting the total number of rotations of the registration roller  23 , and to simplify the structure of the mechanism. 
         [0114]    The rotational speed in the Control Example 3 is preset in conformance to paper thickness based on the design specifications or experiment. The rotational speed is preferably set with consideration given to fluctuations of the rotational speed at the time of initiating the rotation at the drive motor start-up. The rotation speed can be set for each of the paper thicknesses. Alternatively, paper thicknesses is classified according to prescribed ranges to set up the groups of paper thicknesses, and the rotational speed can be set for each of these groups. In the aforementioned Control Examples 2 and 3, in conformance to paper thickness, one of the rotational speed and rotation time is assumed to be constant, and the other is controlled. Without the present invention being restricted thereto, both the rotational speed and rotation time can be controlled by the control section according to paper thickness. 
       Embodiment 2 
       [0115]    To shorten t 3  for avoidance of any influence of the uneven loop and to ensure nipping of the sheet P by the registration roller  23  for stabilisation of conveyance, it is preferred to cancel the nipping of the loop forming roller  22 D at a constant distance after the leading edge of the sheet P has passed through the nip section N of the registration roller  23 , independently of the presence or absence of a toner image on the opposite surface as a sheet type. To put it another way, the distance d 3  between the nip section N and the leading edge of the sheet P in  FIG. 8   d  is preferably kept at an approximately constant value. 
         [0116]    However, as described above, the friction coefficient on the opposite surface of the sheet P varies depending on presence or absence of a toner image on the opposite surface. The friction coefficient is smaller when there is a toner image than when there is no toner image. Therefore, the slippage is greater when there is a toner image and the conveyance amount per one rotation of the registration roller  23  is smaller. 
         [0117]    Thus, the following problem occurs if the nipping of the loop forming roller  22 D is cancelled at a constant time subsequent to the start of sheet re-feeding independently of the presence or absence of a toner image on the opposite surface as in the conventional art. That is, the distance d 3  between the nip section N and the leading edge of the sheet P when the nipping is cancelled is greater in the case of absence of a toner image than in the case of presence of a toner image. This problem makes it difficult to control the distance d 3  at a constant value. 
         [0118]    One possible way of controlling the distance d 3  at a constant value is to install a sensor to detect the leading edge of the sheet P, at a position immediately after the nip. Immediately after the nip, however, rollers and shafts are already installed and a limited space raises a problem difficult to solve. 
       Control Example 4 
       [0119]    In the present embodiment of the invention, the control section AS provides control in such a way that the total number of rotations of the registration roller  23  from the start of rotation of the registration roller  23  until the cancellation of the nipping of the loop forming roller  22 D is kept at a prescribed total number of rotations set in response to presence or absence of a toner image on the opposite surface of the sheet P. 
         [0120]    To put it another way, control is provided in such a way as to cancel the nipping of the loop forming roller  22 D, when the total number of rotations of the registration roller  23  has reached a prescribed total number of rotations set in conformance to the presence or absence of a toner image, subsequent to the start of rotation of the registration roller  23  (after start of sheet re-feeding). 
         [0121]    This means that the amount of conveyance from the start of sheet re-feeding until the cancellation of the nipping of the loop forming roller  22 D is equalized to the approximately constant value independently of presence of absence of a toner image on the opposite surface of the sheet P. Immediately after the nipping has been cancelled, the rotation of the loop forming roller  22 D is suspended. 
         [0122]    The aforementioned total number of rotations can be detected by a rotation detecting mechanism (not illustrated) such as a rotary encoder for detecting rotations, which is connected to the registration roller  23  or a drive source as exemplified by a drive motor (not illustrated). By detecting the total number of rotations of the registration roller  23  or drive motor as described above, it is possible to remove influence of the fluctuation in the rotational speed of the drive motor or the fluctuation in the rotational speed at the time of initiating the rotation at the start-up. 
         [0123]    A stepping motor can be used to drive the registration roller  23 , and the control of the total number of rotations can be done by pulse control. 
         [0124]    As described above, the friction coefficient on the opposite surface varies depending on presence or absence of a toner image on the opposite surface of the sheet P. The friction coefficient is smaller when there is a toner image than when there is no toner image. Therefore, the slippage is greater when there is a toner image and the conveyance amount per one rotation of the registration roller  23  is smaller. 
         [0125]    Accordingly, to keep the distance d 3  at approximately a constant level independently of presence or absence of a toner image on the opposite surface of the sheet P, a prescribed total number of rotations must be set at a higher value when there is a toner image than when there is no toner image on the opposite surface. 
         [0126]    The aforementioned prescribed total number of rotations is set in advance in conformance to presence or absence of a toner image on the opposite surface based on the design specifications or experiment 
         [0127]    The presence or absence of a toner image and prescribed total number of rotations set in conformance to this are formulated into a data table, for example, and are stored into the storage section (not illustrated) of the image forming apparatus main body GH in advance. 
         [0128]    At the time of printing, based on information on whether an operator has selected the duplex printing or on whether a toner image has been formed on the first surface due to the duplex printing, the data on prescribed total number of rotations in conformance to whether a toner image has been formed on the opposite surface of the sheet P is selected from the aforementioned data table. The control section provides control based on the data. 
         [0129]    When the nipping of the loop forming roller  22 D has been cancelled, the loop disappears and the sheet P is conveyed after skew has been corrected. In the process of sheet conveyance, bias of the sheet P across the width is corrected. 
         [0130]      FIG. 10  is a diagram showing a step of correcting the bias. An end face detecting sensor S 101  for detecting the position of the end face of the sheet P across the width is arranged upstream of the registration roller  23 . Further, the registration roller  23  is supported movably across the width of the sheet P, and is moved in the direction of the arrow Y 11  by a traveling mechanism (not illustrated). 
         [0131]    When bias of the sheet P has been detected by the end face detecting sensor S 101 , the traveling mechanism moves the registration roller  23  in the direction of correcting the bias, whereby bias is corrected. 
         [0132]    As shown in  FIG. 10 , when the end face detecting sensor S 101  has found out that the sheet P is biased toward the near side (downward in the drawing) (portion marked by a solid line), the traveling mechanism moves the registration roller  23  toward the far side (upward in the drawing) so that the sheet P is moved (portion marked by a broken line), whereby bias is corrected. 
         [0133]    The aforementioned procedure allows the distance d 3  to be kept at the minimum distance which is approximately constant, independently of whether a toner image has been formed on the surface on the sheet P, which comes in contact with the drive roller  23   a  of the registration roller  23 . This prevents a fan shape from occurring due to loop unevenness (an uneven loop). This also ensures stable conveyance subsequent to registration, whereby image misalignment is suppressed. 
         [0134]    In the aforementioned description, the control for keeping the distance d 3  at an approximately constant level is achieved by detecting the number of rotations of the registration roller  23  or drive motor and by controlling the total number of rotations according to the presence or absence of a toner image (named Control Example 4). This can also be achieved by controlling the rotation time or rotational speed of the registration roller  23 . Examples are given below. 
       Control Example 5 
       [0135]    In the Control Example 5, the rotational speed of the registration roller  23  is kept constant independently of the presence or absence of a toner image on the opposite surface of the sheet P, and the rotation time is controlled in conformance to the presence or absence of a toner image, thereby controlling the total number of rotations of the registration roller  23 . Accordingly, a longer rotation time is set when there is a toner image than when there is no toner image. In the case of Control Example 5, if a motor of constant rotational speed is used to drive the registration roller  23 , only the rotation time can be placed under control. This makes it possible to omit the mechanism for detecting the number of rotations of the registration roller  23 , and to simplify the structure of the mechanism. 
         [0136]    The rotation time in the Control Example 5 is set in advance according to the presence or absence of a toner image based on the design specifications or experiment It is preferably set with consideration given to fluctuations of the rotational speed at the time of initiating the rotation at the start-up of the drive motor. 
       Control Example 6 
       [0137]    In the Control Example 6, the rotation time of the registration roller  23  is set at a constant value, independently of presence or absence of a toner image on the opposite surface of the sheet P, and the rotational speed is controlled in conformance to the presence or absence of a toner image, thereby controlling the total number of rotations of the registration roller  23 . Accordingly, the rotational speed is set at a higher value when there is a toner image than when there is no toner image. In the Control Example 6, the registration roller  23  is driven preferably by a drive motor whose rotational speed can be easily changed. Further, this makes it possible to omit the mechanism for detecting the total number of rotations of the registration roller  23 , and to simplify the structure of the mechanism. 
         [0138]    The rotational speed in the Control Example 6 is preset in conformance to whether a toner image has been formed on the opposite surface of the sheet P based on the design specifications or experiment. The rotational speed is preferably set with consideration given to fluctuations of the rotational speed at the time of initiating the rotation at the drive motor start-up. 
         [0139]    In the aforementioned Control Examples 5 and 6, in conformance whether a toner image has been formed on the opposite surface of the sheet P, one of the rotational speed and rotation time is assumed to be constant, and the other is controlled. Without the present invention being restricted thereto, both the rotational speed and rotation time can be controlled by the control section according to whether a toner image has been formed on the opposite surface of the sheet P. 
         [0140]    The aforementioned arrangement prevents generation of a fan shape due to loop unevenness (an uneven loop), independently of the type of the sheet, and ensures stable conveyance subsequent to registration, whereby image misalignment can be suppressed.