Abstract:
A fixing unit includes: a support member; an endless-belt-like fixing member which is wound on the support member; a pressure member which rotates in a fixed rotation direction while being pressed onto the fixing member; a fixing portion in which the fixing member and the pressure member abut against each other to perform fixation on a recording medium; a separation unit which is provided in a space between the support member and the fixing member on a downstream side of the fixing member in a rotation direction of the fixing member and for separating the recording medium from the fixing member; a separation portion which is provided in the separation unit to press the fixing member onto the pressure member and guide the fixing member to leave the pressure member; and an adjustment unit as defined herein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-207896 filed on Sep. 16, 2010. 
       BACKGROUND 
     Technical Field 
       [0002]    The present invention relates to a fixing unit and an image forming apparatus. 
       SUMMARY 
       [0003]    According to an aspect of the invention, there is provided a fixing unit including: a support member; an endless-belt-like fixing member which is wound on the support member; a pressure member which rotates in a fixed rotation direction while being pressed onto the fixing member; a fixing portion in which the fixing member and the pressure member abut against each other to perform fixation on a recording medium; a separation unit which is provided in a space between the support member and the fixing member on a downstream side of the fixing member in a rotation direction of the fixing member and for separating the recording medium from the fixing member; a separation portion which is provided in the separation unit to press the fixing member onto the pressure member and guide the fixing member to leave the pressure member; and an adjustment unit which makes a separation position where the fixing member leaves from the pressure member, farther from the fixing portion in accordance with thickness of the recording medium, and which increases an angle between a tangent line direction of the pressure member and the fixing member in the separation position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0005]      FIG. 1  is a schematic view showing the whole of an image forming apparatus according to an exemplary embodiment of the invention; 
           [0006]      FIG. 2  is a schematic view showing the whole of a fixing unit according to the exemplary embodiment of the invention; 
           [0007]      FIG. 3  is a sectional view showing a main part of the fixing unit according to the exemplary embodiment of the invention; 
           [0008]      FIG. 4  is a sectional view showing the main part of the fixing unit according to the exemplary embodiment of the invention; 
           [0009]      FIG. 5  is a sectional view showing the main part of the fixing unit according to the exemplary embodiment of the invention; 
           [0010]      FIG. 6  is a sectional view showing a part of the fixing unit according to the exemplary embodiment of the invention; 
           [0011]      FIG. 7  is a sectional view showing a main part of a fixing unit according to another exemplary embodiment of the invention; 
           [0012]      FIG. 8  is a sectional view showing the main part of the fixing unit according to the other exemplary embodiment of the invention; 
           [0013]      FIG. 9  is a sectional view showing the main part of the fixing unit according to the other exemplary embodiment of the invention; and 
           [0014]      FIG. 10  is a plan view showing a sheet on which an image has been formed in Example of the invention. 
       
    
    
     DETAILED DESCRIPTION 
     (1) Fundamental Configuration and Operation of Image Forming Apparatus 
       [0015]    As shown in  FIG. 1 , this image forming apparatus  1  performs image processing based on color image information obtained by reading out a document or color image information sent from an image data input apparatus such as a personal computer, and forms a color image on a sheet P by electrophotography. The sheet P is an example of a member on which fixation is performed. Examples of recording media include resin sheets such as OHP sheets as well as paper sheets. 
         [0016]    The image forming apparatus  1  has image forming units  10 Y,  10 M,  10 C and  10 K as examples of image forming portions for forming toner images of respective colors, i.e. yellow (Y), magenta (M), cyan (C) and black (K). The image forming units  10 Y,  10 M,  10 C and  10 K are arrayed in series in the named order with respect to a traveling direction of an endless intermediate transfer belt  30  stretched among a backup roll  34  and a plurality of rolls  32 . In addition, the intermediate transfer belt  30  passes between a photoconductor drum  12  as an image retainer constituting each image forming unit  10 Y,  10 M,  10 C,  10 K and a primary transfer roll  16  disposed oppositely to the photoconductor drum  12 , and between the backup roll  34  and a secondary transfer roll  36  as an image transfer portion in contact with the backup roll  34 . 
         [0017]    Further, a charger  22 , an exposure unit  14 , a development unit  15  and a photoconductor cleaner  20  are provided around each photoconductor drum  12 . The charger  22  charges the surface of the photoconductor drum  12  uniformly. The exposure unit  14  serves as an exposure unit which irradiates the surface of the photoconductor drum  12  with light to form an electrostatic latent image on the surface of the photoconductor drum  12 . The development unit  15  visualizes the electrostatic latent image with toner to form a toner image. The photoconductor cleaner  20  removes transfer residual toner or the like from the surface of the photoconductor drum  12 . 
         [0018]    A sheet feed unit  41  is disposed in a lower portion of the image forming apparatus  1 . A pickup roll  42  for picking up the uppermost one of stacked sheets P is disposed above a left end portion of the sheet feed unit  41 . On a left side of the pickup roll  42 , a conveyance path is bent upward by a guide or the like. Paired conveyance rolls  43  are disposed on a downstream side of the upward bent portion of the conveyance path. The conveyance path is further bent right by a guide or the like, and paired conveyance rolls  43  are disposed on a downstream side of the right bent portion of the conveyance path. Paired registration rolls  44  are disposed on a downstream side of the paired conveyance rolls  43 . The aforementioned pair of the backup roll  34  and the secondary transfer roll  36  are disposed on a downstream side of the paired registration rolls  44 . On the right side of the pair of the backup roll  34  and the secondary transfer roll  36 , a fixing unit  60  is disposed with a conveyance belt  45  put therebetween. Paired output rolls  46  and an output guide  47  are disposed on a downstream side of the fixing unit  60 . The configuration of the fixing unit  60  will be described in detail later. 
         [0019]    Next, the operation of image formation performed by the image forming unit  10 Y for forming a yellow toner image will be described representatively. 
         [0020]    The surface of the photoconductor drum  12  provided in the image forming unit  10 Y is charged uniformly by the charger  22 . Further, the exposure unit  14  emits a light beam onto the photoconductor drum  12  based on image data to perform exposure corresponding to a yellow image. Thus, an electrostatic latent image corresponding to the yellow image is formed on the surface of the photoconductor drum  12 . 
         [0021]    The electrostatic latent image corresponding to the yellow image on the photoconductor drum  12  is developed with toner retained on a developing roll  18  of the development unit  15  so as to be formed as a yellow toner image. The yellow toner image is primarily transferred onto the intermediate transfer belt  30  by the pressure of the primary transfer roll  16  and an electrostatic attraction force of transfer bias applied to the primary transfer roll  16 . 
         [0022]    In this primary transfer, the yellow toner image is not wholly transferred onto the intermediate transfer belt  30  but partially remains on the photoconductor drum  12 . In addition, an external additive to the toner or the like also adheres to the surface of the photoconductor drum  12 . The photoconductor drum  12  after the primary transfer is passed through a position opposed to the photoconductor cleaner  20 , so that transfer residual toner etc. on the photoconductor drum  12  is removed. After that, the surface of the photoconductor drum  12  is charged again by the charger  22  for the next image formation cycle. 
         [0023]    In addition, in the image forming apparatus  1 , an image forming process the same as the aforementioned image forming process is performed in the respective color image forming units  10 Y,  10 M,  10 C and  10 K at timings in which a relative position gap between adjacent ones of the image forming units  10 Y,  10 M,  10 C and  10 K has been taken into consideration. Thus, toner images of the respective colors Y, M, C and K are superposed sequentially on the intermediate transfer belt  30  so as to form multiple toner images on the intermediate transfer belt  30 . 
         [0024]    Then, by the electrostatic attraction force of the secondary transfer roll  36  to which transfer bias is applied, the multiple toner images are collectively transferred from the intermediate transfer belt  30  to the sheet P which is, for example, conveyed from the sheet feed unit  41  through the pickup roll  42  and the paired conveyance rolls  43  to the secondary transfer roll  36  at a conveyance timing determined by the registration rolls  44 . Toner etc. which has not been transferred to the sheet P but remains on the intermediate transfer belt  30  is recovered by a belt cleaner  33 . 
         [0025]    Further, the sheet P to which the multiple toner images have been transferred from the intermediate transfer belt  30  is conveyed to the fixing unit  60  by which the multiple toner images are fixed to the sheet P by heat and pressure. Thus, a full color image is formed on the sheet P. After that, the sheet P is outputted through the output guide  47  and the paired output rolls  46 . 
       (2) Configuration of Fixing Unit 
       [0026]    Next, the configuration of the fixing unit  60  will be described with reference to  FIG. 2  et seq. In these drawings, the reference numeral  61  represents a fixing roll as an example of a support member. The fixing roll  61  is a cylindrical roll formed out of aluminum or the like, and a halogen lamp  61   a  is provided inside the fixing roll  61  to extend all over the length of the fixing roll  61 . The fixing roll  61  is rotated counterclockwise in  FIG. 2  by a not-shown drive mechanism. 
         [0027]    A fixing belt  62  as an example of a fixing member is wound on the fixing roll  61 . The fixing belt  62  is wound on tension rolls  63 ,  64 ,  65  and  66  so that a constant tension is applied to the fixing belt  62  by the tension rolls  63 ,  64 ,  65  and  66 . These tension rolls  63  and so on are formed out of aluminum or the like, and halogen lamps  64   a  and  65   a  each having a function equivalent to that of the halogen lamp  61   a  are provided inside the tension rolls  64  and  65  respectively. 
         [0028]    The fixing belt  62  is an endless belt, which is constituted by a multilayer structure consisting of a base layer, a silicone rubber layer and a release layer. The base layer is formed out of polyimide resin or the like. The silicone rubber layer is laminated on a surface side (outer circumferential surface side) of the base layer. The release layer made from tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) or the like is further formed on the silicone rubber layer. 
         [0029]    A pressure roll  70  as an example of a pressure member is brought into pressure contact with the fixing roll  61  on which the fixing belt  62  is wound. In the pressure roll  70 , a silicone rubber layer  72  is formed on the outer circumference of a cylindrical core  71  formed out of aluminum or the like, and a release layer  73  made from PFA is provided on the outer circumference of the silicone rubber layer  72 . The silicone rubber layer  72  pressed by the fixing roller  61  is elastically deformed to form a roll nip portion A as a fixing portion between the fixing roll  61  and the pressure roll  70 . The pressure roll  70  is rotated clockwise in  FIG. 2  with the movement of the fixing belt  62 . When the sheet P is put between the fixing belt  62  and the pressure roll  70 , the multiple toner images are fixed to the sheet P by heat and pressure of the fixing belt  62  so that a full color image is formed on the sheet P. In the following description, “rotation direction” will mean the rotation direction of the pressure roll  70 . 
         [0030]    As shown in  FIG. 3 , a separation unit  80  is provided in an exit of the roll nip portion A so as to extend all over the length thereof intersecting the moving direction of the fixing belt  62 . The separation unit  80  has a wedge member  81  whose front end is brought close to the roll nip portion A. The wedge member  81  is fixed to a not-shown body of the fixing unit  60 . The wedge member  81  pushes the fixing belt  62 , which is stretched between the roll nip portion A and the tension roll  63 , toward the pressure roll  70  so as to press the fixing belt  62  onto the pressure roll  70 . A concave curved surface  81   a  having an arc shape in side view from the rotation axis direction of the pressure roll  70  is formed in a rotation-direction-side end surface of the wedge member  81 . 
         [0031]    A separation pad  82  as an example of a separation portion is disposed on the rotation direction side of the wedge member  81 . The separation pad  82  has a support portion  84  which is rotatably supported by a shaft  83  as an example of an adjustment unit, and a pressing portion  85  whose vertical thickness increases gradually with increase of its distance from the support portion  84 . An end surface of the support portion  84  is formed into a semicircular convex curved surface  84   a . The convex curved surface  84   a  is fitted to the concave curved surface  81   a  of the wedge member  81 . In addition, the shaft  83  is rotated by a not-shown drive mechanism so that the pressing portion  85  can rotate clockwise and counterclockwise in  FIG. 3 . 
         [0032]    In the pressing portion  85 , a convex curved surface  85   a  having an arc shape in side view is formed in the portion where a pressing surface  85   b  intersects an end surface  85   c  which is on the opposite side to the support portion  84 . The curvature of the convex curved surface  85   a  is set to be larger than the curvature of the convex curved surface  84   a  of the support portion  84 . As shown in  FIG. 6 , a gap  81   b  is provided between the convex curved surface  84   a  and the concave curved surface  81   a  of the wedge member  81 . The gap  81   b  is expanded at the top and narrowed gradually toward the bottom. 
         [0033]    As shown in  FIG. 3 , between the fixing belt  62  and the pressure roll  70 , a roll nip portion A having a length A is formed by the fixing roll  61  pressing the fixing belt  62  onto the pressure roll  70 , and a pad nip portion B having a length B is formed by the wedge member  81  and the support portion  84  of the separation unit  80  pressing the fixing belt  62  onto the pressure roll  70 . Thus, fixation is performed on the sheet P in a total nip portion having a total length C. In the state shown in  FIG. 3 , the pad nip portion B has a minimum length. When the separation pad  82  is rotated clockwise in that state, the length of the pad nip portion B increases. 
       (3) Operation of Fixing Unit 
       [0034]    The sheet P to which the multiple toner images have been secondarily transferred enters the roll nip portion A and the pad nip portion B between the fixing belt  62  and the pressure roll  70  from the left side in  FIG. 2 . The multiple toner images are fixed to the sheet P by the heat and pressure of the fixing belt  62  so that a full color image is formed on the sheet P. Here, in this exemplary embodiment, the position of the separation pad  82  is changed in accordance with the thickness of the sheet P. In  FIG. 4 , fixation is performed on a sheet P which is thin paper having a basis weight smaller than 80 gsm. The separation pad  82  is rotated clockwise to the greatest extent possible. Thus, the fixing belt  62  is pressed onto the pressure roll  70  by the whole of the pressing surface  85   b  of the pressing portion  85 . In this case, the length of the pad nip portion B is the longest. 
         [0035]      FIG. 3  shows the state where fixation is performed on a sheet P of thick paper beyond a basis weight of 200 gsm. The separation pad  82  is rotated counterclockwise. Thus, of the separation pad  82 , only the support portion  84  presses the fixing belt  62  onto the pressure roll  70 . In this case, the length of the pad nip portion B is the shortest as described above. In  FIG. 5 , fixation is performed on a sheet P called regular paper having a basis weight of 80 gsm to 127 gsm, and a sheet P having a middle basis weight which is larger than 127 gsm but not larger than 200 gsm. The separation pad  82  is located in an intermediate state between the state shown in  FIG. 3  and the state shown in  FIG. 4 . In addition, in this case, the length of the pad nip portion B is an average length of the lengths shown in  FIGS. 3 and 4 . The length of the pad nip portion B may be set bit by bit in accordance with the thickness of the sheet P. The operation of fixation on the sheet P will be described sequentially in order from the case where the sheet P is thin paper. 
         [0036]    In the state shown in  FIG. 4 , the fixing belt  62  rises from the convex curved surface  85   a  of the separation pad  82  and leaves the pressure roll  70  to move toward the tension roll  63 . A tangent line L is drawn from a separation point Q where the fixing belt  62  leaves the pressure roll  70  to the outer circumferential surface of the pressure roll  70 . In this case, if an angle θ 1  between the tangent line L and the fixing belt  62  is large, the sheet P cannot follow the sudden change of the traveling direction of the fixing belt  62 , but will be separated from the fixing belt  62  by the elasticity of the sheet P. The angle θ 1  is set by the angle between the pressing surface  85   b  and the end surface  85   c  of the separation pad  82 . 
         [0037]    On the other hand, even if the angle θ 1  is large, the sheet P can move together with the fixing belt  62  without being separated from the fixing belt  62  when the convex curved surface  85   a  with which the fixing belt  62  should be bent in friction has a small curvature. Accordingly, the angle θ 1  and the curvature of the convex curved surface  85   a  obtained experimentally are set so that a thinnest sheet P which may be possibly used can be separated from the fixing belt  62 . 
         [0038]    In the state shown in  FIG. 4 , the length of the pad nip portion B is the largest. The temperature of the fixing belt is lowered for a sheet P of thin paper having a small heat capacity. Thus, the sheet P can be prevented from expanding due to vapor coming from moisture contained in the sheet P. When the top end of the sheet P reaches the convex curved surface  85   a  of the separation pad  82 , the traveling direction of the fixing belt  62  is changed to the angle θ 1  due to the curvature of the convex curved surface  85   a . Thus, the sheet P is separated from the fixing belt  62 . 
         [0039]    In addition, in this exemplary embodiment, the upper surface of the fixing belt  62  is pressed by the wedge member  81  and the separation pad  82  in the pad nip portion B. As a result, the sheet P can be prevented from expanding due to vapor coming from the moisture contained in the sheet P, and occurrence of an image defect due to the vapor blowing out from the upper surface of the sheet P can be prevented. Particularly in this exemplary embodiment, the gap  81   b  between the convex curved surface  84   a  of the support portion  84  and the concave curved surface  81   a  of the wedge member  81  in the separation pad  82  is formed to be expanded at the top and narrowed gradually toward the bottom. Thus, the change in fixing pressure between the convex curved surface  84   a  and the concave curved surface  81   a  can be reduced so as to suppress the expansion caused by the vapor. 
         [0040]    Next, description will be made on the operation in the case where fixation is performed on a sheet P of regular paper or middle-basis-weight paper.  FIG. 5  shows the state where the separation pad  82  has been rotated counterclockwise in the state shown in  FIG. 4 . The length of the pad nip portion B in  FIG. 5  is shorter than that in the case shown in  FIG. 4 . In addition, when a tangent line L is drawn from the separation point Q where the fixing belt  62  leaves the pressure roll  70  to the outer circumferential surface of the pressure roll  70 , an angle θ 2  between the tangent line L and the fixing belt  62  is smaller than the angle θ 1  shown in  FIG. 4 . Further, the traveling direction of the fixing belt  62  is changed in the convex curved surface  84   a  of the support portion  84  in the separation pad  82 . The curvature of the convex curved surface  84   a  is smaller than the curvature of the convex curved surface  85   a . Therefore, when fixation is performed on a sheet P of regular paper or middle-basis-weight paper shown in  FIG. 5 , the operation of separating the sheet P from the fixing belt  62  is not as conspicuous as that when fixation is performed on a sheet P of thin paper shown in  FIG. 4 . 
         [0041]    However, the sheet P of regular paper or middle-basis-weight paper has enough elasticity. Thus, the sheet P can be separated from the fixing belt  62  in spite of the angle θ 2  and the curvature of the convex curved surface  84   a  as shown in  FIG. 5 . That is, as soon as the top end of the sheet P reaches the convex curved surface  84   a  of the support portion  84  in the separation pad  82 , the traveling direction of the fixing belt  62  is changed to the angle θ 2  due to the curvature of the convex curved surface  84   a  so that the sheet P is separated from the fixing belt  62 . In addition, when excessive pressure and heat are applied to the sheet P of regular paper or middle-basis-weight paper, melting of toner may be uneven or a granular pattern may appear in an image . With respect to this point, in this exemplary embodiment, the length of the pad nip portion B is made shorter than that in the case where fixation is performed on the sheet P of thin paper shown in  FIG. 4 , so that occurrence of the aforementioned image defect can be suppressed. Since the applied pressure and heat can be suppressed, there is another effect that the level of gloss can be suppressed in the image. Further, the traveling direction of the fixing belt  62  is not changed with a large curvature as in the fixation on the sheet P of thin paper shown in  FIG. 4 . Accordingly, a shearing force acting on the sheet P is smaller than that in the case shown in  FIG. 4 . It is therefore possible to suppress occurrence of such a problem that the level of gloss differs from one place to another in the image. 
         [0042]    Next, description will be made on the operation in the case where fixation is performed on a sheet P of thick paper.  FIG. 3  shows the state where the separation pad  82  has been further rotated counterclockwise in the state shown in  FIG. 5 . The length of the pad nip portion B in  FIG. 3  is shorter than that in the case shown in  FIG. 5 . In addition, when a tangent line L is drawn from the separation point Q where the fixing belt  62  leaves the pressure roll  70  to the outer circumferential surface of the pressure roll  70 , an angle θ 3  between the tangent line L and the fixing belt  62  is smaller than the angle θ 1  shown in  FIG. 4 . Further, the traveling direction of the fixing belt  62  is changed in the convex curved surface  84   a  of the support portion  84  in the separation pad  82  in the same manner as in the case of the sheet P of regular paper or middle-basis-weight paper shown in  FIG. 5 . 
         [0043]    As soon as the top end of the sheet P reaches the convex curved surface  84   a  of the support portion  84  in the separation pad  82 , the traveling direction of the fixing belt  62  is changed to the angle θ 3  due to the curvature of the convex curved surface  84   a . Thus, the sheet P of hard thick paper is separated from the fixing belt  62 . In addition, when excessive pressure and heat are applied to the sheet P of thick paper, the phenomenon that melting of toner is uneven or a granular pattern appears in an image may occur more easily than the case of the sheet P of regular paper or middle-basis-weight paper. With respect to this point, in this exemplary embodiment, the length of the pad nip portion B is made shorter than that in the case where fixation is performed on the sheet P of regular paper or middle-basis-weight paper shown in  FIG. 5 , so that occurrence of the aforementioned image defect can be suppressed. Since the applied pressure and heat can be suppressed, it is therefore possible to suppress the level of gloss in the image while suppressing occurrence of such a problem that the level of gloss differs from one place to another in the image. 
         [0044]    In the fixing unit configured thus, the angle and the curvature based on which the fixing belt  62  changes its traveling direction become maximum when the length of the pad nip portion B is the largest. Thus, it is possible to separate the sheet P of thin paper from the fixing belt  62 . On the other hand, when the length of the pad nip portion B is shorter than that in the case where fixation is performed on a sheet P of thin paper, the angle and the curvature based on which the fixing belt  62  changes its traveling direction are also smaller that those in the case where fixation is performed on the sheet P of thin paper. Thus, occurrence of such an image defect that melting of toner is uneven or a granular pattern appears in an image can be suppressed to improve the characteristic of the image. 
       (4) Another Exemplary Embodiment 
       [0045]    Next, another exemplary embodiment of the invention will be described with reference to  FIGS. 7 to 9 . In the following description, constituent members equivalent to those in the aforementioned exemplary embodiment are referred to by the same numerals, and description thereof will be omitted. As shown in  FIGS. 7 to 9 , a shaft  91  as an example of a separation portion is provided adjacently to the wedge member  81 . The shaft  91  is pressed onto the pressure roll  70  through the fixing belt  62  by a spring while being allowed to move in the rotation direction of the pressure roll  70  and in the opposite direction thereto by a not-shown drive unit (adjustment unit). 
         [0046]    As shown in  FIGS. 7 to 9 , the wedge member  81  and the shaft  91  form a pad nip portion B.  FIG. 8  shows a state where fixation is performed on a sheet P of thin paper. In  FIG. 8 , the shaft  91  is located in the rotation-direction-side end portion of the pressure roll  70 . On this occasion, a portion D which is not backed up appears in the pad nip portion B. In  FIG. 8 , the fixing belt  62  rises from the outer circumferential surface of the shaft  91  and leaves the pressure roll  70  to move toward the tension roll  63  (see  FIG. 2 ). When a tangent line L is drawn from a separation point Q where the fixing belt  62  leaves the pressure roll  70  to the outer circumferential surface of the pressure roll  70 , due to the effect of an angle θ 1  between the tangent line L and the fixing belt  62  and the curvature of the outer circumferential surface of the shaft  91 , the sheet P cannot follow the sudden change of the traveling direction of the fixing belt  62 , but will be separated from the fixing belt  62  by the hardness of the sheet P. 
         [0047]      FIG. 9  shows a state where fixation is performed on a sheet P of regular paper or middle-basis-weight paper. In  FIG. 9 , the shaft  91  is closer to the wedge member  81  than that in the state shown in  FIG. 8 . Also in this case, a portion D which is not backed up appears in the pad nip portion B. The shaft  91  pushes the fixing belt  62  stretched between the roll nip portion A and the tension roll  63 , toward the pressure roll  70 . Accordingly, when the shaft  91  is brought closer to the wedge member  81 , an angle θ 2  between the tangent line L of the outer circumferential surface of the pressure roll  70  and the fixing belt  62  is smaller than the angle θ 1 . In  FIG. 9 , the sheet P is separated from the fixing belt  62  due to the hardness of the sheet P in spite of the angle θ 2  which is smaller than the angle θ 1 . In addition, the length of the pad nip portion B is shorter than that in the case where fixation is performed on the sheet P of thin paper shown in  FIG. 8 . Thus, pressure and heat applied to the sheet P can be suppressed to improve the characteristic of an image formed on the sheet P. 
         [0048]      FIG. 7  shows a state where fixation is performed on a sheet P of thick paper. In  FIG. 7 , the shaft  91  abuts against the wedge member  81  in the state shown in  FIG. 9 . In this case, an angle θ 3  between the tangent line L of the outer circumferential surface of the pressure roll  70  and the fixing belt  62  is smaller than the angle θ 2 . In  FIG. 7 , the sheet P is separated from the fixing belt  62  due to the hardness of the sheet P in spite of the angle θ 3  which is smaller than the angle θ 2 . In addition, the length of the pad nip portion B is shorter than that in the case where fixation is performed on the sheet P of regular paper or middle-basis-weight paper shown in  FIG. 9 . Thus, pressure and heat applied to the sheet P can be suppressed to improve the characteristic of an image formed on the sheet P. 
         [0049]    In the aforementioned exemplary embodiment, the portion D which is not backed up appears in the pad nip portion B as shown in  FIGS. 8 and 9 . Vapor rising can be coped with by a simple configuration in which a plate member is, for example, inserted from above to press the fixing belt  62  onto the pressure roll  70 . 
       Example 
       [0050]    Image formation shown in  FIG. 10  is performed on a sheet of thin paper having a basis weight of 64 gsm. The separation state of the sheet after fixation is checked on the conditions for thick paper shown in  FIG. 3  and on the conditions for thin paper shown in  FIG. 4 . As a result, on the conditions for thin paper, it is proved that the sheet can be separated if a margin of 0.5 mm is secured in the top end of the sheet. On the other hand, on the conditions for thick paper, it is proved that the sheet cannot be separated if the margin in the top end of the sheet is less than 4.0 mm. In the case where a sheet of paper having a middle basis weight of 127 gsm is used, it is proved that the sheet can be separated on the conditions for thin paper if a margin of 0.5 mm or less is secured in the top end of the sheet, and the sheet can be separated on the conditions for thick paper if a margin of 0.5 mm or less is secured in the top end of the sheet. 
         [0051]    Next, the width of the pad nip portion, the angle between the tangent line drawn on the outer circumferential surface of the pressure roll and the fixing belt, and the curvature of the fixing belt are checked. The same image is formed using the aforementioned sheet of thin paper and on the conditions for thin paper. As for values required for separating the sheet on the conditions for thin paper, the width of the pad nip portion is 7 mm or more, the angle is 41° or more, and the curvature is 6.5 or more. As a result of the same image which is formed using a sheet of middle-basis-weight paper and on the conditions for thick paper, it is proved that the required width of the pad nip portion is 4 mm or less. 
         [0052]    The fixing unit according to the invention is applicable to image forming apparatuses such as printing machines, copying machines, printers, facsimile machines, and complex machines having the same functions as those of these machines. 
         [0053]    The foregoing description of the 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 embodiments were 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 embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention defined by the following claims and their equivalents.