Abstract:
A fixing device includes: a fixing roll that is rotatable; a belt member that is stretched over the fixing roll; a pressurizing roll that pressurizes the fixing roll via the belt member; a belt pressing member that presses an outer surface of the belt member to the pressurizing roll from inside of the belt member on a downstream side at a pressure-contact position between the fixing roll and the pressurizing roll; and a controller that decrease a time difference between a passage time per unit area of a tip region of a recording medium and a passage time per unit area of a succeeding region of the recording medium to pass a pressure-contact portion between the fixing roll and belt pressing member and the pressurizing roll.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-212042 filed on Sep. 14, 2009. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This invention relates to a fixing device and an image forming apparatus. 
         [0004]    2. Related Art 
         [0005]    In an image forming apparatus such as a copier or a printer, for example, a recording medium with a toner image transferred is subjected to heating and pressurizing to carry out fixing processing of fixing the toner image on the recording medium. 
         [0006]    The fixing device employed for the fixing processing is composed of e.g. a belt member which is heated by a heating source such as a heater and also rotatably stretched by a fixing roll, and a pressurizing roll which pressurizes the fixing roll via the belt member. In such a fixing device, a nipping zone of heating and pressurizing the recording medium is formed at a pressure-contact position between the fixing roll and the pressurizing roll; the tone image transferred on the recording medium suffers from the heating and pressurizing operation while the recording medium passes the nipping zone so that it is fixed on the recording medium. 
       SUMMARY 
       [0007]    According to an aspect of the invention, a fixing roll that is rotatable; a belt member that is stretched over the fixing roll; a pressurizing roll that pressurizes the fixing roll across the belt member; a belt pressing member that presses an outer surface of the belt member to the pressurizing roll from inside of the belt member on a downstream side at a pressure-contact position between the fixing roll and the pressurizing roll; and a controller that decrease a time difference between a passage time per unit area of a tip region of a recording medium and a passage time per unit area of a succeeding region of the recording medium to pass a pressure-contact portion between the fixing roll and belt pressing member and the pressurizing roll. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Exemplary embodiments of the invention will be described in detail based on the following figures, wherein: 
           [0009]      FIG. 1  is a schematic diagram showing an image forming apparatus according to an exemplary embodiment of this invention; 
           [0010]      FIG. 2  is a side sectional view showing a schematic configuration of a fixing device according to an exemplary embodiment of this invention; 
           [0011]      FIG. 3  is a schematic sectional view showing a region in the vicinity of a nipping zone according to an exemplary embodiment of this invention; 
           [0012]      FIG. 4  is a view for explaining the mechanism of generating a gloss difference (hypothesis); 
           [0013]      FIG. 5  is a view for explaining the mechanism of generating a gloss difference (hypothesis); 
           [0014]      FIG. 6  is a view for explaining the mechanism of generating a gloss difference (hypothesis); 
           [0015]      FIG. 7  is a view illustrating gloss values at various positions of a sheet; 
           [0016]      FIG. 8  is a view illustrating the verifying result of a measure of dealing with a gloss difference based on the adjustment of a transporting speed; 
           [0017]      FIG. 9  is a side sectional view showing an exemplary configuration of a fixing device; and 
           [0018]      FIG. 10  is a view illustrating the verifying result of a measure of dealing with a gloss difference based on the adjustment of oil supply. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Now, referring to the attached drawings, a detailed explanation will be given of an exemplary embodiment of this invention. 
         [0020]    Additionally, the following description indicates, as examples, members constituting an image forming apparatus to which this exemplary embodiment is applied and the substance, thickness, hardness, etc. of each member; however, without being limited to such a construction, the construction according to a device designing condition such as a using object and an using condition may be adopted. 
         [0021]      FIG. 1  is a schematic diagram showing an image forming apparatus according to an exemplary embodiment of this invention. The image forming apparatus shown in  FIG. 1  is an intermediate transfer system of image forming apparatus generally called “a tandem type”. This apparatus includes plural of image forming units  1 Y,  1 M,  1 C,  1 K of forming toner images with respective color components through an electrophotographic system; a primary transfer unit  10  for successively transferring the toner images with respective color components formed by the respective image forming units  1 Y,  1 M,  1 C,  1 K to an intermediate transfer belt  15  (primary transfer); a secondary transfer unit  20  for collectively transferring a superposed toner image transferred on the intermediate transfer belt  15  onto a sheet P (secondary transfer) which is a recording medium; and a fixing device  60  for fixing the image secondary-transferred on the sheet P. The image forming apparatus also includes a control unit  40  for controlling the operation of each unit. 
         [0022]    In this exemplary embodiment, successively arranged around a photoconductor  11  rotating in the direction of arrow A in each of the image forming units  1 Y,  1 M,  1 C and  1 K are various devices for electrophotography such as a charger  12  which charges the photoconductor  11 , a laser exposure device  13  which writes an electrostatic latent image on the photoconductor  11  (in the figure, an exposure beam is indicated by symbol Bm), a developer  14  which incorporates the toner with each color component to visualize the electrostatic latent image on the photoconductor  11  using the toner; a primary transfer roll  16  which transfers the toner image with each color component formed on the photoconductor  11  to an intermediate transfer belt  15  through the primary transfer unit  10 ; and a photoconductor cleaner  17  which removes the toner remaining on the photoconductor  11 . These image forming units  1 Y,  1 M,  1 C and  1 K are nearly linearly arranged in the order of yellow (Y), magenta(M), cyan (C) and black (K) from the upstream side of the intermediate transfer belt  15 . 
         [0023]    The intermediate transfer belt  15  which is an intermediate transfer body is made of a film-like endless belt with polyimide or polyamide resin containing a suitable quantity of an anti-static agent such as carbon black. Its volume resistivity ranges from 10 6  to 10 14  Ωcm. Its thickness is e.g. about 0.1 mm. The intermediate transfer belt  15  is circulation-driven (rotation-run) at a predetermined speed in the direction of arrow B shown in  FIG. 1  by various rolls. These various rolls arranged include a driving roll  31  which is driven by a motor (not shown) with excellent capability of constant speed to rotation-run the intermediate transfer belt  15 ; a supporting roll  32  which supports the intermediate transfer belt  15  nearly linearly extending along the direction of arranging the photoconductors  11 ; a tension roll  33  which gives definite tension to the intermediate transfer belt  15  and also serves as a meandering preventing roll of preventing the meandering of the intermediate transfer belt  15 ; a backup roll  25  which is provided at the secondary transfer unit  20  so as to serve as a supporting roll of supporting the intermediate transfer belt  15  from behind; and a cleaning backup roll  34  which serves as a supporting roll provided at a cleaning unit scraping the remaining toner on the intermediate transfer belt  15 . 
         [0024]    The primary transfer unit  10  is constructed of primary transfer rolls  16  which are arranged oppositely to the photoconductors  11  across the intermediate transfer belt  15 . The primary transfer roll  16  is composed of a shaft and a sponge layer which is an elastic layer fixed to the periphery thereof. The shaft is a cylindrical rod made of a metal such as iron or SUS. The sponge layer is a sponge-like cylindrical roll made of a blended rubber of NBR, SBR and EPDM mixed with a conducting agent such as carbon black and having a volume resistivity of 10 7  to 10 9  Ωcm. The primary transfer roll  16  is arranged in pressure-contact with the photoconductor  11  across the intermediate transfer belt  15 . Further, the primary transfer roll  16  is supplied with a voltage (primary transfer bias) with a polarity opposite to the toner charging polarity (defined as a minus polarity, and ditto for the following description). Thus, the toner images on the respective photoconductors  11  are successively electro-statically sucked on the intermediate transfer belt  15  so that the superposed toner image is formed on the intermediate transfer belt  15 . 
         [0025]    The secondary transfer unit  20  includes a secondary transfer roll  22  arranged on the side of carrying the toner image of the intermediate transfer belt  15  and a backup roll  25 . The backup roll  25  is composed of a tube of blended rubber of EPDM and NBR dispersed with carbon in the surface and an EPDM rubber in the interior. The backup roll  25  is formed to have a surface resistivity of 10 7  to 10 10 Ω/□ and a hardness of 70° (asker C). The backup roll  25  is arranged on the back side of the intermediate transfer belt  15  and serves as an opposite electrode for the secondary transfer roll  22 ; a metallic power supply roll  26  to which a secondary transfer bias is stably applied is arranged adjacently thereto. 
         [0026]    On the other hand, the secondary transfer roll  22  is composed of a shaft and a sponge layer which is an elastic layer fixed to the periphery thereof. The shaft is a cylindrical rod made of a metal such as iron or SUS. The sponge layer is a sponge-like cylindrical roll made of a blended rubber of NBR, SBR and EPDM mixed with a conducting agent such as carbon black and having a volume resistivity of 10 7  to 10 9  Ωcm. The secondary transfer roll  22  is arranged in pressure-contact with the backup roll  25  across the intermediate transfer belt  15 . Further, the secondary transfer roll  22  is grounded to form a secondary transfer bias between itself and the backup roll  25  so that the toner image is secondary-transferred onto the sheet P transported to the secondary transfer unit  20 . 
         [0027]    Further, on the downstream side of the secondary transfer unit  20  of the intermediate transfer belt  15 , a belt cleaner  35  is separatably/contactably provided which removes the remaining toner or paper powder on the intermediate transfer belt  15  after secondary transfer thereby to clean the surface of the intermediate transfer belt  15 . On the other hand, on the upstream side of the yellow image forming unit  1 Y, a reference sensor (home position sensor)  42  is arranged which generates a reference signal of taking an image forming timing in each image forming unit  1 Y,  1 M,  1 C,  1 K. Further, on the downstream side of the black image forming unit  1 K, an image density sensor  43  is arranged which adjusts the image quality. The reference sensor  42 , when it recognizes a specific mark formed on the back side of the intermediate transfer belt  15 , generates the reference signal. By an instruction issued from the control unit  40  on the basis of recognition of the reference signal, each image forming unit  1 Y,  1 M,  1 C,  1 K starts image formation. 
         [0028]    Further, the image forming apparatus according to this exemplary embodiment includes, as a sheet transporting system, a sheet accommodating unit  50  which accommodates sheets P; a pickup roll  51  which takes out one of the sheets P stacked in the sheet accommodating unit  50  at a timing instructed by the control unit  40  and transport it; transporting rolls  52  which transport the sheet P taken out by the pickup roll  51 ; a sheet transporting path  53  which sends the sheet transported by the transporting roll  52  into the secondary transfer unit  20 ; a transporting belt  55  which transports the sheet P transported after secondary-transferred by the secondary transfer roll  22  to the fixing device  60 ; and a fixing entrance guide  56  which guides the sheet P to the fixing device  60 . 
         [0029]    Next, an explanation will be given of a basic imaging process in the image forming apparatus according to this exemplary embodiment. In the image forming apparatus as shown in  FIG. 1 , the image data produced from an image reading device not shown or a personal computer (PC) not shown, after image-processed by an image processing device not shown, are imaged by the image forming units  1 Y,  1 M,  1 C and  1 K. In the image processing device, the reflection coefficient data inputted are subjected to the image processing such as shading correction, displacement correction, brightness/color space conversion, gamma correction, frame canceling and various image editions such as color edition or shift edition. The image data image-processed are converted into color material tone data of four colors of Y, M, C and K, which are in turn sent to the laser exposure device  13 . 
         [0030]    In the laser exposure device  13 , according to the color material tone data inputted, the exposure beam Bm emitted from e.g. a semiconductor laser is applied to each photoconductor  11  of the image forming unit  1 Y,  1 M,  1 C,  1 K. In photoconductor  11  of the image forming unit  1 Y,  1 M,  1 C,  1 K, its surface is charged by the charger  12  and thereafter scanning-exposed by the laser exposure device  13  to form an electrostatic image. The electrostatic latent image formed is developed by the developer  14  of each image forming unit  1 Y,  1 M,  1 C,  1 K as the toner image of each color of Y, M, C, K. 
         [0031]    In the primary transfer unit  10  where each photoconductor  11  and intermediate transfer belt  15  are kept in contact, the toner image formed on the photoconductor  11  of the image forming unit  1 Y,  1 M,  1 C,  1 K is transferred onto the intermediate transfer belt  15 . More specifically, in the primary transfer unit  10 , a voltage having a polarity (plus polarity) opposite to the charging polarity of the toner (primary transferbias) is applied to the base material of the intermediate transfer belt  15  so that the toner images are successively superposed on the surface of the intermediate transfer belt  15  to carry out the primary transfer. 
         [0032]    After the toner images are successively primary-transferred onto the surface of the intermediate transfer belt  15 , the intermediate transfer belt  15  is moved so that the toner images are transported to the secondary transfer unit  20 . In synchronism with the timing when the toner images are transported to the secondary transfer unit  20 , in the sheet transporting system, the pickup roll  51  is rotated so that the sheet P having a size instructed by the control unit  40  is supplied from the sheet accommodating unit  50 . The sheet P supplied from the pickup roll  51  is transported by the transporting rolls  52  and reaches the secondary transfer unit  20  via the sheet transporting path  53 . Before reaching the secondary transfer unit  20 , the sheet P is temporarily stopped and the sheet P and the toner images are aligned with each other in such a manner that a resist roll (not shown) is rotated in synchronism with the moving timing of the intermediate transfer belt  15  holding the toner images. 
         [0033]    In the secondary transfer unit  20 , the secondary transfer roll  22  is pressed onto the backup roll  25  through the intermediate transfer belt  15 . At this time, the sheet P transported timely is sandwiched between the intermediate transfer belt  15  and the secondary transfer roll  22 ; in this case, if a voltage having the same polarity (minus polarity) as the charging polarity of the toner (secondary transfer vias) is applied to the intermediate transfer belt  15  from the power supply roll  26 , a transfer electric field is formed between the secondary transfer roll  22  and the backup roll  25 . The non-fixed toner images held on the intermediate transfer belt  15  are collectively electrostatically transferred onto the sheet P in the secondary transfer unit  20  where the secondary transfer roll  22  is pressed onto the backup roll  25 . 
         [0034]    Thereafter, the sheet P with the tone images electrostatically transferred is transported while being separated from the intermediate transfer belt  15  by the secondary transfer roll  22  and further transported to a transporting belt  55  provided on the downstream side of the sheet transporting direction of the secondary transfer roll  22 . In the transporting belt  55 , in synchronism with the transporting speed in the fixing device  60 , the sheet P is transported to the fixing device  60  at an optimum transporting speed. The non-fixed images on the sheet P transported to the fixing device  60  are subjected to the fixing processing by heat and pressure so that they are fixed on the sheet P. The sheet P with the fixed images is transported to an ejected sheet accommodating unit not shown provided in an ejecting unit of the image forming apparatus. 
         [0035]    On the other hand, the residual toners left on the intermediate transfer belt  15 , after the transfer onto the sheet P has been completed, are transported in tandem with the rotation-running of the intermediate transfer belt  15  and removed from the intermediate transfer belt  15  by a cleaning backup roll  34  and the intermediate transfer belt cleaner  35 . 
         [0036]    Next, an explanation will be given of the fixing device  60  employed in the image forming apparatus according to this exemplary embodiment. 
         [0037]      FIG. 2  is a side sectional view showing a schematic configuration of the fixing device  60  according to this exemplary embodiment. The fixing device  60  includes, as main components, a fixing belt module  61  and a pressurizing roll  62  arranged in pressure-contact therewith. 
         [0038]    The fixing belt module  61  includes, as a typical configuration, a fixing belt  610  which is an example of the belt member rotating in a direction of arrow D; a fixing roll  611  which is rotation-driven while stretching the fixing belt  610 ; a stretching roll  612  which stretches the fixing belt  610  from inside; a stretching roll  613  which stretches the fixing belt  610  from outside; a posture correcting roll  614  which corrects the posture of the fixing belt  610  between the fixing roll  611  and the stretching roll  612 ; a peeling pad  64  which is an exemplary belt pressing member, in a downstream region within a nipping zone N where the fixing belt module  61  and the pressurizing roll  62  are in pressure-contact with each other, arranged in the vicinity of the fixing roll  611 ; and a stretching roll  615  which stretches the fixing belt  610  on the downstream side of the nipping zone N. 
         [0039]    The fixing belt  610  is a flexible endless belt having a circumferential length of 314 mm and a width of 340 mm. The fixing belt  610  is structured in a multiple layer composed of a base layer of polyimide resin having a thickness of 80 μm, an elastic layer of silicone rubber having a thickness of 200 μm which is laminated on the surface side of the base layer (outer peripheral side) and a releasing layer of tetrafluoroethylene-perfluoroalkylvinylether copolymer resin (PFA) tube having a thickness of 30 μm which is formed on the elastic layer. Now, the elastic layer is provided to improve the image quality of particularly a color image. 
         [0040]    The fixing roll  611  is a cylindrical roll of aluminum having an outer diameter of 65 mm, a length of 360 mm and a thickness of 10 mm. The fixing roll  611  rotates in a direction of arrow C at a surface speed of 445 mm/s by driving force from a driving motor not shown. 
         [0041]    Inside the fixing roll  611 , a halogen heater  616   a  serving as a heating source having a rating of 900 W is arranged. A temperature sensor  617   a  is arranged in contact with the surface of the fixing roll  611 . On the basis of a measured value of the temperature sensor  617   a , the control unit  40  (see  FIG. 1 ) of the image forming apparatus controls the surface temperature of the fixing roll  611  at 150° C. 
         [0042]    The stretching roll  612  is a cylindrical roll of aluminum having an outer diameter of 30 mm, a wall thickness of 2 mm and a length of 360 mm. Inside the stretching roll  612 , a halogen heater  616   b  serving as a heating source having a rating of 1000 W is arranged. By means of a temperature sensor  617   b  and the control unit (see  FIG. 1 ), the surface temperature of the stretching roll  612  is controlled at 190° C. The stretching roll  612 , therefore, has a function of heating the fixing belt  610  as well as the function of stretching the fixing belt  610 . 
         [0043]    Further, at both ends of the stretching roll  612 , spring member (not shown) are arranged to press the fixing belt  610  outwardly to set the entire tension of the fixing belt  610  at 15 kgf. In this case, in order to make uniform the tension of the fixing belt  610  in the width direction and minimize a displacement in the axial direction of the fixing belt  610 , the stretching roll  612  is formed in a “crown shape” with the center being larger than the end by 100 μm in their outer diameter. 
         [0044]    The stretching roll  613  is a cylindrical roll of aluminum having an outer diameter of 25 mm, a wall thickness of 2 mm and a length of 360 mm. The surface of the stretching roll  613  is covered with PFA having a thickness of 20 μm as a releasing layer. The releasing layer is formed in order to prevent slight offset toner or paper powder produced from the outer surface of the fixing belt  610  from being deposited on the stretching roll  613 . Like the stretching roll  612 , the stretching roll  613  is also formed in a “crown shape” with the center being larger than the end by 100 μM in their outer diameter. Incidentally, both stretching roll  612  and stretching roll  613  may be formed in the crown shape, or either one thereof may be formed in the crown shape. 
         [0045]    Inside the stretching roll  613 , a halogen heater  616   c  serving as a heating source having a rating of 1000 W is arranged. By means of a temperature sensor  617   c  and the control unit  40  (see  FIG. 1 ), the surface temperature of the stretching roll  613  is controlled at 190° C. The stretching roll  613 , therefore, has a function of heating the fixing belt  610  from the outer surface as well as the function of stretching the fixing belt  610 . Thus, in this exemplary embodiment, the fixing belt  610  will be heated by the fixing roll  611 , stretching roll  612  and stretching roll  613 . 
         [0046]    The posture correcting roll  614  is a cylindrical roll of aluminum having an outer diameter of 15 mm and a length of 360 mm. In the vicinity of the posture correcting roll  614 , a belt edge position detecting mechanism (not shown) is arranged to detect the edge position of the fixing belt  610 . The posture correcting roll  614  is provided with an axial displacing mechanism of displacing the axial contact position of the fixing belt  610  according to the detection result of the belt edge position detecting mechanism thereby to control the meandering (belt-walk) of the fixing belt  610 . 
         [0047]    The peeling pad  64  is a block member of e.g. a rigid body of metal such as SUS or resin having a nearly arc shape in section. The peeling pad  64  is fixedly arranged over the axial entire region of the fixing roll  611  at the position in the vicinity on the downstream side of a region where the pressurizing roll  62  is in pressure-contact with the fixing roll  611  across the fixing belt  610  (referred to as a “roll-nipping zone N 1 ”, see  FIG. 3 ). Further, the peeling pad  64  is arranged to press the pressurizing roll  62  through the fixing belt  610  over a certain width region (e.g. width of 2 mm along the traveling direction of the fixing belt  610 ) with a predetermined load (e.g. an average of 10 kgf) thereby to form a “roll-nipping zone N 2 ” described below (see  FIG. 3 ). 
         [0048]    The stretching roll  615  is a cylindrical roll of aluminum having an outer diameter of 12 mm and a length of 360 mm. The stretching roll  615  is arranged in the vicinity on the downstream side in the traveling direction of the fixing belt  610  of the peeling pad  64  so that the fixing belt  610  having passed the peeling pad  64  smoothly rotation-runs toward the fixing roll  611 . 
         [0049]    The pressurizing roll  62  is composed of a base material of a cylindrical roll  621  of aluminum having a diameter of 45 mm and a length of 360 mm, and an elastic layer  622  of silicone rubber with rubber hardness of 30° (JIS-A) and a releasing layer  623  of a PFA tube having a film thickness of 100 μm which are laminated in order from the base material side. The pressurizing roll  62  is arranged to pressurize the fixing belt module  61  and rotates in a direction of arrow E to follow the fixing roll  611  in tandem with the rotation in arrow C of the fixing roll  611  of the fixing belt module  61 . The traveling speed thereof is 445 mm/s which is equal to the surface speed of the fixing roll  611 . 
         [0050]    Next, an explanation will be given of the nipping zone N where the fixing belt module  61  and pressurizing roll  62  are in pressure-contact with each other. 
         [0051]      FIG. 3  is a schematic sectional view showing a region in the vicinity of the nipping zone N. As seen from  FIG. 3 , in the nipping zone N where the fixing belt module  61  and the pressurizing roll  62  are in pressure-contact with each other, within the zone (wrapped zone) where the fixing belt  610  is wrapped around the fixing roll  611 , the pressurizing roll  62  is arranged in pressure contact with the outer peripheral surface of the fixing belt  610  thereby to form the roll-nipping zone (first nipping zone) N 1 . 
         [0052]    Now, in the fixing device  60  according to this exemplary embodiment, as described above, the fixing roll  611  is formed of a hard roll of aluminum whose surface is relatively hard and the pressuring roll  62  is formed of a soft roll, which is covered with the elastic layer  622  whose surface is relatively soft. For this reason, in the roll-nipping zone N 1 , the fixing roll  611  suffers from hardly any dent and only the surface of the pressurizing roll  62  is greatly dented (the amount of dent in the pressurizing roll  62 &gt;the amount of dent in the fixing roll  611 ). In this way, the nipping zone having a width in the traveling direction of the fixing belt  610  is formed. 
         [0053]    Thus, in the fixing device  60  according to this exemplary embodiment, the fixing roll  611  on the side where the fixing belt  610  is wrapped in the roll-nipping zone N 1  is not almost deformed to keep its cylindrical shape. The fixing belt  610 , therefore, rotation-runs along the circumferential surface of the fixing roll  611  without varying its running radius. Thus, the fixing belt  610  may pass the roll-nipping zone N 1  while keeping a constant running speed. Accordingly, also when the fixing belt  610  passes the roll-nipping zone N 1 , wrinkles or deformation in the fixing belt  610  is very hard to occur. As a result, it is possible to prevent disorder of the fixed image, thereby stably providing the fixed image with excellent quality. Incidentally, in the fixing device  60  according to this exemplary embodiment, the roll-nipping zone N 1  is set at a width of 15 mm along the traveling direction of the fixing belt  610 . 
         [0054]    In the vicinity on the downstream side of the roll-nipping zone N 1 , the peeling pad  64  is arranged which presses the fixing belt  610  onto the pressurizing roll  62  surface. Thus, in succession to the roll-nipping zone N 1 , a peeling-pad nipping zone (second nipping zone) N 2  is set where the fixing belt  610  is wrapped around the pressurizing roll  62  surface. 
         [0055]    As shown in  FIG. 3 , the peeling pad  64  providing the peeling-pad nipping zone N 2  is formed in a nearly arc-shape in section. For this reason, the fixing belt  610  having passed the peeling-pad nipping zone N 2  moves to follow the peeling pad  64  so that its traveling direction abruptly changes to warp toward the stretching roll  615 . Thus, the sheet P having passed the roll-nipping zone N 1  and the peeling-pad nipping zone N 2  is peeled from the fixing belt  610  at the time when it departs from the peeling-pad nipping zone N 2 , thereby stably executing curvature separation for the sheet P. Incidentally, in the fixing device  60  according to this exemplary embodiment, the peeling-pad nipping zone N 2  is set at a width of 2.5 mm along the traveling direction of the fixing belt  610 . 
         [0056]    Meanwhile, in the fixing processing using the fixing device  60  as described above, as the case may be, the tip region of the sheet P provides a higher gloss than the succeeding region to generate a gloss difference therebetween. The mechanism (hypothesis) of generating such a gloss difference will be explained referring to  FIGS. 4 to 6 .  FIGS. 4 to 6  represent a manner of deformation of the fixing belt  610  by connecting the corresponding positions of the outer surface of the fixing belt  610  and the rear surface thereof by connecting lines. 
         [0057]    Where the fixing belt  610  and the sheet P are placed in a free state (slippery for each other), it is assumed that the fixing belt  610  is deformed in the shape as shown in  FIG. 4 . Specifically, in the region along the fixing roll  611 , the outer surface of the fixing belt  610  is extended more greatly than the rear surface thereof by e.g. about 1%, whereas in the region along the peeling pad  64 , the outer surface of the fixing belt  610  is contracted more greatly than the rear surface thereof by several %. 
         [0058]    Additionally, in the nipping zone N, the sliding stress applied to the fixing belt  610  by the toner on the sheet P owing to the pressing from the pressurizing roll  62  is enough to sharing-deform the elastic layer of the fixing belt  610  (in this example, LSR (liquid-silicone rubber) layer). If the fixing belt  610  and the sheet P are in a free state (slippery for each other), it is assumed that the quantity of displacement (slippage) in the belt rotating direction (sheet traveling direction) due to slippage becomes 100 μm or more at the maximum, but because the toner image fixed on the sheet P generates no displacement, no slippage will be presumably generated in the region where the toner image is held on the sheet P. 
         [0059]    Now, the Young&#39;s modulus (longitudinal elastic coefficient) of the sheet P is approximately equal to that of polyimide resin employed in the base layer (rear side) of the fixing belt  610 ; so if the sheet P is kept in intimate contact with the fixing belt  610  through the toner, the outer surface of the fixing belt  610  travels in an extended state according to the sheet P. Thus, in an S-path of the nipping zone N, the corresponding positions of the outer surface of the fixing belt  610  and the rear surface thereof will be displaced. Accordingly, such shearing deformation as shown in  FIG. 5  will be presumably generated in the fixing belt  610 . In short, in the region facing the peeling pad  64  (peeling-pad nipping zone N 2 ), the outer surface of the fixing belt  610  precedes the rear surface in the rotating direction. 
         [0060]    Such a condition is prone to occur at the tip region of the sheet P and will be gradually cancelled with the traveling (transportation) of the sheet P, which eventually results in a contact balancing state as illustrated in  FIG. 6 . Namely, in contact balancing state, the outer surface of the fixing belt  610  is displaced onto the upstream side of the roll-nipping zone N 1 . 
         [0061]    Further, a difference in the way of deformation of the fixing belt  610  between the tip region of the sheet P and its succeeding region thereof as described above will presumably lead to a difference in the time taken for the sheet staying in the nipping zone N to be subjected to the fixing processing (nipping zone staying time). 
         [0062]    The mechanism as described above (hypothesis) is based on a series of actions of distortion and its absorption occurring in the fixing belt  610  in the S-shape nipping path formed by provision of the peeling pad  64 . 
         [0063]    It may be supposed that owing to the cause described above and various causes described below, the nipping zone staying time of a certain region of the sheet P becomes longer than the other region so that for example, as illustrated in  FIG. 7 , the tip region provides a higher gloss than the succeeding region to generate a gloss difference. 
         [0064]    With a horizontal axis representing the positions which partition the sheet P in the direction orthogonal to its traveling direction (lateral direction) and a vertical axis representing the gloss values (converted values of 60° gloss) which indicates the degrees of gloss,  FIG. 7  illustrates gloss values at the respective positions (lateral positions) of the sheet P for each of the tip region of the sheet P and the central region (an example of the succeeding region) thereof. As seen from  FIG. 7 , although there are slight variations, the tip region of the sheet P indicates a tendency of indicating a higher gloss value than the central region; thus, it may be seen that the tip region of the sheet P is visually darker to generate the gloss difference. This is attributable to the fact that as a result that the tip region of the sheet P has been fixing-processed for a longer time than the central region thereof, the surface of the toner image at the tip region becomes smooth thereby to suppress irregular reflection of light. 
         [0065]    This exemplary embodiment adopts techniques of first to fourth examples described below in order to decrease a time difference of the nipping zone staying time for a unit area between the respective regions of the sheet P (for example, between the tip region and its succeeding region) (to unify the times taken for the fixing processing), thereby suppress occurrence of the gloss difference. 
         [0066]    The first example intends to decrease the time difference in the nipping zone staying time which is generated by the following causes. 
         [0067]    Specifically, a thicker sheet P suffers from greater braking force at the entrance of the peeling-pad nipping zone N 2  so that the nipping staying time of the tip region of the sheet P is prone to become longer than the succeeding region. This is attributable to the fact that at the entrance of the peeling-pad nipping zone N 2 , the curvature of the nipping path varies in the direction opposite to that in the roll-nipping zone N 1  at the previous stage so that when the tip of the sheet P passes this varying zone, the braking force acts according to the strength of the sheet P (generally, it increases as the thickness of the sheet becomes thick). 
         [0068]    So, the first structural example is provided with a speed adjusting unit for adjusting the speed of transporting the sheet by the transporting belt  55  (in this example, realized by the control unit  40 ) which previously makes higher the speed of the tip region of the sheet P entering the nipping zone N (transporting speed before fixing) than a reference speed and thereafter returns it to the reference speed. Specifically, the speed of the tip region of the sheet P entering the nipping zone N is temporarily increased to prevent this speed from being decreased to the reference speed or lower (otherwise to reduce alienation from the reference speed) owing to the braking force at the entrance of the peeling-pad nipping zone N 2 , thereby decreasing a time difference in the nipping zone staying time between the tip region of the sheet P and its succeeding region. Considering the fact that the thicker the sheet P, the greater the braking force at the entrance of the peeling-pad nipping zone N 2 , for the sheet which is thicker, the transporting speed before fixing is set at a higher value. Incidentally, the speed adjustment may be also done according to other sheet attributes such as the kind of the sheet or its weight which influence the braking force; considering the degree of speed reduction at the entrance of the peeling-pad nipping zone N 2 , the speed adjustment has only to be done so that the nipping zone staying time of the tip region of the sheet P becomes approximately equal to that of the succeeding region. 
         [0069]      FIG. 8  is a graph illustrating the verified result of the measure for dealing with the gloss difference based on adjustment of the transporting speed. 
         [0070]    In this example, for the purpose of verification, using the fixing belt  610  having a thickness of 450 μm with the speed being fixed to 445 mm/s, for three kinds of sheets, i.e. an ordinary sheet with 82 gsm, a coated sheet with 210 gsm and a thick sheet with 350 gsm, their transporting speeds before fixing have been adjusted to 430 mm/s, 445 mm/s and 475 mm/s, respectively. The horizontal axis of the graph represents the transporting speed before fixing (mm/s) and the kind of the sheet; the vertical axis of the graph represents the grade (G) indicating the degree of the gloss difference between the tip region of a recording medium and its succeeding region. It may be evaluated that with G being smaller, the gloss difference is suppressed (good). In this example, the target value of G is set at 2 or less. 
         [0071]    According to  FIG. 8 , where the transporting speed before fixing is 430 mm/s, for the ordinary sheet, G=1; for the coated sheet, G=3; and for the thick sheet, G=4 so that only the ordinary sheet could attain the target. By increasing the transporting speed before fixing to 445 mm/s (equal to that of the fixing belt  610 ), G is improved (reduced) by 1 for the respective sheets so that the coated sheet as well as the ordinary sheet could attain the target. Incidentally, even when the transporting speed before fixing is further increased to 475 mm/s, G did not change. 
         [0072]    The second example intends to reduce the time difference in the nipping zone staying time generated by the following causes. Specifically, in the construction in which oil is supplied to the inner face of the fixing belt  610  in order to reduce the sliding load of the peeling pad  64 , if the oil is excessively supplied, the transmission of the driving force from the fixing roll  611  (driving roll) to the fixing belt  610  is attenuated so that the time difference in the nipping zone staying time is prone to occur. 
         [0073]    In order to obviated such inconvenience, as shown in  FIG. 9 , the second structural example is provided with an oil supply control unit (in this example, realized by the control unit  40 ) using an oil-impregnated wick  65  serving as an oil supplying member for supplying an oil to the inner face (rear face) of the fixing belt  610 . The oil supply control unit serves to control the oil quantity on the inner face of the fixing belt  610  so that it does not exceed a predetermined reference value. Namely, adjusting the oil quantity on the inner face of the fixing belt brings in balance the braking force at the entrance of the peeling-pad nipping zone N 2  and the belt driving force of the fixing belt  610  thereby to realize stabilized belt driving. Thus, even when the sheet P suffering from greater braking force such as the thick sheet has been transported, a time difference in the nipping zone staying time between the tip region of the sheet P and its succeeding region may be decreased. In place of the oil supply control unit or in addition thereto, an oil wiping pad  66  serving as an oil wiping member for wiping the oil on the inner face of the fixing belt  610  may be provided to adjust the oil quantity on the inner face of the fixing belt  610 . 
         [0074]      FIG. 10  is a graph illustrating the verified result of the measure for dealing with the gloss difference based on adjustment of the oil quantity. 
         [0075]    In this example, for the purpose of verification, using a thick sheet of 350 gsm as the sheet P, four color images with the toners of yellow (Y), magenta (M), cyan (C) and black (K) each having an area ratio of 60% are superposed. As the oil impregnated wick  65 , a “polyflon” (PTFE) paper having a hollow structure of an axial length of 330 mm, a width of 22 mm and a thickness of 1 mm is impregnated with an amino-metamorphosed oil. The horizontal axis of the graph represents an oil impregnated quantity (g) of the oil impregnated wick  65 ; and the vertical axis thereof represents a Grade (G) indicating the degree of the gloss difference between the tip region and its succeeding region. It may be evaluated that with G being smaller, the gloss difference is suppressed (good). In this example, the level giving a slight gloss difference hardly noticeable is set at G=2; the gloss difference with G=2 or less is made permissible. 
         [0076]      FIG. 10  uses the oil impregnation quantity in the oil impregnated wick  65  as a substitute of the oil supply quantity. It may be seen that the gloss difference of the image exceeds a permissible level from the vicinity of the region where the oil impregnation quantity exceeds about 3 g. So, it may be seen that the oil impregnated wick  65  is impregnated with a predetermined quantity of oil (about 3 g in this example). 
         [0077]    The third structural example intends to reduce the time difference in the nipping zone staying time generated by the following causes. 
         [0078]    Specifically, in the case of the sheet P having a certain thickness (e.g. thick sheet of 350 gsm), when its tip enters the roll-nipping zone N 1 , slippage occurs between the fixing belt  610  (driving roll) with the oil applied on the inner face to reduce a friction coefficient and the surface of the fixing roll  611  (driving roll) so that the speed of the fixing belt  610  is temporarily dropped. As a result, between the tip region passing the nipping zone N with the belt speed reduced and its succeeding region passing the nipping zone N with the belt speed returned to a normal speed, the time difference in the nipping zone staying time is prone to occur. 
         [0079]    So, the third structural example is provided with an angle adjusting unit for adjusting the transporting angle of the sheet P transported to the nipping zone N (in this example, realized by the control unit  40 ), which adjusts the transporting angle of the sheet P so that one of four corners of the sheet P which is thicker than a predetermined reference value first reaches the nipping zone N. Namely, the sheet P is not perpendicularly (from the side of the sheet P) entered the nipping zone N but entered obliquely (from the corner of the sheet P) so that the stress applied to the sheet P by the nipping zone N gradually changes from its entering end (corner). In this way, by reducing the load applied to belt driving to suppress the speed change in the fixing belt  610 , the time difference in the nipping zone staying time between the tip region of the sheet P and its succeeding region is decreased. Incidentally, the adjustment of the transporting angle of the sheet P by the angle adjusting unit may be done by controlling the transporting operation of the transporting belt  55  (for example, giving a difference in the transporting speed between the lateral positions in the transporting direction) or otherwise changing the axial angle of the nipping zone (for example, with the members including the fixing belt module  61  and pressurizing roll  62  being on a rotating stand, adjusting the rotating angle of the rotating stand). 
         [0080]    The fourth structural example intends to reduce the time difference in the nipping zone staying time generated by the following causes. 
         [0081]    Specifically, in the construction in which the oil is supplied to the inner face of the fixing belt  610  in order to reduce the sliding load of the peeling pad  64 , if the oil supply becomes excessive or insufficient, the transmission of the driving force from the fixing roll  611  (driving roll) to the fixing belt  610  become instable so that the time difference in the nipping zone staying time is prone to occur. 
         [0082]    In order to such inconvenience, the fourth structural example is provided with a driving member for rotation-running the fixing belt  610  as well as the fixing roll  611 . Specifically, by additively providing the driving member for the fixing belt  610  at a position not affected by the oil (or a position slightly affected by the oil compared with the fixing roll  611 ), the speed change of the fixing belt  610  is suppressed thereby to reduce the time difference in the nipping zone staying time between the respective regions of the sheet P (for example, between the tip region and its succeeding region). The driving member other than the fixing roll  611  may be realized in various manners such as using the pressurizing roll  62  which presses the fixing roll  611  through the fixing belt  610 ; using the stretching roll  615  provided inside the fixing belt  610  on the downstream side of the nipping zone N whose outer surface is made coarse to reduce the slippage due to the oil; or using the stretching roll  613  provided outside the fixing belt  610  on the downstream side of the nipping zone N. 
         [0083]    In the explanation hitherto made, the first to fourth structural examples have been proposed individually, but they may be combined with one another as long as control is made so that a time difference in the nipping zone staying time for the unit area between the respective regions of the sheet P (e.g. between the tip region of the sheet and its succeeding region) may be decreased. 
         [0084]    It should be noted that on the basis of the mechanism (hypothesis) described above, the tip region in the above explanation is presumably within the length in the transporting direction at a pressure-contact position between the fixing roll and belt pressing member, and the pressurizing roll. 
         [0085]    The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments are chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various exemplary embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.