Abstract:
An object of the present invention is to provide a fixing apparatus that has a fixing rotator in contact with a recording material supporting an unfixed image, a pressure rotator in pressure contact with the fixing rotator, a support member supporting the pressure rotator and swingable around a plurality of fulcrums, an elastic member for biasing the support member so that the pressure rotator is biased to the fixing rotator, and pressure contact force changing device for changing a pressure contact force by swinging the support member against the biasing force of the elastic member, wherein the pressure contact force changing device can change the pressure contact force while the pressure rotator is in pressure contact with the fixing rotator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a fixing apparatus for fixing an image on a recording material by applying pressure, which is suitably used in image forming apparatuses such as copiers and printers employing electrophotographic and electrostatic recording systems.  
           [0003]    2. Related Art  
           [0004]    An image forming apparatus employing electrophotographic system such as a copier, printer or facsimile forms an electrostatic latent image by charging the surface of an electrophotographic photosensitive body by means of a charging apparatus and exposing the surface of the charged electrophotographic photosensitive body by means of an exposing apparatus. This electrostatic latent image is developed by a developing apparatus to form a toner image which is transferred to a recording material such as paper by a transferring apparatus and then fixed on the recording material by a fixing apparatus as a permanent fixed image, and this fixed image is output.  
           [0005]    [0005]FIG. 8 is a schematic diagram showing an image forming apparatus according to an embodiment of the present invention. The above-mentioned fixing apparatus operates as follows. A recording material P such as paper to which a toner image has been transferred by a transfer nip between a photosensitive drum  2  and a transfer roller  5  as an image carrier is conveyed which being held by a fixing nip formed between the heating roller  12  and the pressure roller  13  of the fixing apparatus  7  through a conveyance guide  9  and the unfixed toner image is thermally fixed on the recording material P as a permanently fixed image by heat from a heater (halogen heater)  14  incorporated in the heating roller  12  and pressure from the above-mentioned fixing nip. Other constituent elements of the image forming apparatus  1  will be described with reference to embodiments of the present invention which will be described below.  
           [0006]    When an envelope is conveyed as a recording material in the image forming apparatus constituted as described above, if the envelope is nipped and conveyed while it is heated by the fixing apparatus, there arises a problem that it wrinkles. The envelope is formed by folding a single sheet of paper and pasting the overlapped portions. If there is a difference in shrinkage direction between the front side and the rear side of the sheet, there is no escaping for distortion at the time of shrinkage by heating, with the result that the sheet of paper is apt to wrinkle. To prevent wrinkling, a method making use of pressure means for reducing the contact pressure of the pressure roller is generally employed.  
           [0007]    For example, FIG. 6 shows an example of a pressure mechanism for the fixing apparatus  7 . The pressure mechanism comprises a heating roller  12  including a heater  14  (halogen heater), a pressure roller  13  contacting the heating roller  12 , and a frame  52  supporting these components. Both ends of the heating roller  12  are rotatably supported by the frame  52  through the intermediation of a bearing (not shown). One end of the heating roller  12  is provided with a gear (not shown) to be driven by drive force transmitted from a drive source. Both ends of the pressure roller  13  are rotatably supported by a support member  15  through the intermediation of a bearing (not shown). Reference numeral  16  denotes a rotatable lever which is a contact pressure switching means and movably supported by a shaft  52   a  provided on either side of the frame  52 . The rotatable lever  16  comprises an operation portion  16   a , a contacting portion  16   b  contacting the support member  15  and a stopping portion  16   c  for stopping the movement of the rotatable lever. Reference numeral  60  denotes an elastic member made of a coil spring for biasing the support member  15  and provided between the frame  52  and the support member  15 . An axis hole  15   a  which serves as a fulcrum is formed in the support member  15  so that the support member  15  is movably supported by a shaft  52   b  provided on either side of the frame  52 . On the free end side of the support member  15  is formed a bending portion  15   b  which serves as a contacting portion with the above-mentioned rotatable lever  16  and the support portion of the above-mentioned elastic member  60 .  
           [0008]    When an envelope is to be conveyed, the rotatable lever  16  is first rotated clockwise as shown in FIG. 7. Since the contacting portion  16   b  of the rotatable lever  16  contacts the contacting portion  15   b  of the support member  15 , and the rotatable lever  16  is biased in the direction indicated by an arrow in FIG. 7 (center direction of the shaft  52   a ) by the support member  15 , the moment generated by the biasing force of the elastic member  60  does not act on the rotatable lever  16 , and the rotatable lever  16  is stopped at a position shown in FIG. 7, and the support member  15  is kept down against the biasing force of the elastic member  60 .  
           [0009]    Since the rotatable lever  16  has a detent portion  16   c , it is prevented from moving more than required. The support member  15  is forced down by the operation of the rotatable lever  16  with the fulcrum  15   a  as the center. The pressure roller  13  supported by the support member  15  is kept at a position at a predetermined distance from the heating roller  12 .  
           [0010]    The pressure roller  13  comprises a rubber layer  13   b  formed on the surface of a core metal  13   a  which is in contact with the heating roller  12  to obtain a nip width. Since rubber has a large thermal expansion coefficient and a low heat conductivity, the temperature of the pressure roller  13  gradually rises and the outer diameter of the pressure roller  13  gradually increases (broken line portion in FIG. 7) according to the operation time of the image forming apparatus  1 .  
           [0011]    Since the biasing force of the elastic member  60  is extremely large, even when the outer diameter of the pressure roller  13  becomes large, the rubber layer  13   b  is deformed and the pressure roller  13  cannot move down easily. Owing to this constitution, the contact pressure to the heating roller  12  of the pressure roller  13  or the nip width increases, thereby causing the problem of the wrinkling of the envelope.  
           [0012]    When the amount by which the rotatable lever  16  is forced down (distance between the pressure roller  13  and the heating roller  12 ) is set taking into account the change in the outer diameter of the pressure roller  13  by thermal expansion to prevent wrinkling, the unfixed toner image will not be fully melt-fixed until the temperature of the pressure roller  13  is increased to a sufficient degree, a fixed outer diameter thereof is achieved and a predetermined pressure is obtained. In particular, when the thermal expansion of the pressure roller  13  is large, the heating roller  12  and the pressure roller  13  are set in a state that they do not contact each other, so that no heat is transmitted from the heating roller  12  to the pressure roller  13 , making it necessary to provide a heater  14  in the pressure roller  13  as well.  
           [0013]    That is, the wrinkling of the envelope and the melt-fixing of the unfixed toner image are contradictory and a long waiting time is required until the fixing apparatus  7  of the image forming apparatus  1  is fully warmed so as to prevent the wrinkling of the envelope and melt-fix the unfixed toner image.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the present invention to provide a fixing apparatus capable of fixing a toner image on a recording material without wrinkling the recording material.  
           [0015]    Another object of the present invention is to provide a fixing apparatus capable of fixing a toner image on an envelope.  
           [0016]    Still another object of the present invention is to provide a fixing apparatus capable of changing fixing pressure.  
           [0017]    Still another object of the present invention is to provide a fixing apparatus comprising:  
           [0018]    a fixing rotator in contact with a recording material supporting an unfixed image;  
           [0019]    a pressure rotator in pressure contact with the fixing rotator;  
           [0020]    a support member supporting the pressure rotator and swingable around a plurality of fulcrums as its center;  
           [0021]    an elastic member for biasing the support member so that the pressure rotator is biased by the fixing rotator; and  
           [0022]    pressure contact force changing means for changing pressure force by swinging the support member against the biasing force of the elastic member, wherein  
           [0023]    the pressure contact force changing means can change pressure contact force while the pressure rotator is in pressure contact with the fixing rotator.  
           [0024]    Other objects of the present invention will become apparent from the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a schematic longitudinal sectional view of a fixing apparatus according to Embodiment 1 of the present invention;  
         [0026]    [0026]FIG. 2 is a right side view of the fixing apparatus of FIG. 1;  
         [0027]    [0027]FIG. 3 is a diagram showing how force is applied in the state of FIG. 1;  
         [0028]    [0028]FIG. 4 is a diagram for explaining the operation of Embodiment 1 of the present invention;  
         [0029]    [0029]FIG. 5 is a diagram showing how force in applied in the state of FIG. 4;  
         [0030]    [0030]FIG. 6 is a schematic longitudinal sectional view showing an example of the fixing apparatus;  
         [0031]    [0031]FIG. 7 is a diagram for explaining the operation of the fixing apparatus of FIG. 6;  
         [0032]    [0032]FIG. 8 is a schematic diagram showing an image forming apparatus comprising the fixing apparatus in accordance with Embodiment 1 of the present invention;  
         [0033]    [0033]FIG. 9 is a partial view of a fixing apparatus according to Embodiment 2 of the present invention;  
         [0034]    [0034]FIG. 10 is a partial view of a fixing apparatus according to Embodiment 3 of the present invention;  
         [0035]    [0035]FIG. 11 is a schematic longitudinal sectional view of a fixing apparatus according to Embodiment 4 of the present invention; and  
         [0036]    [0036]FIG. 12 is a diagram for explaining the operation of the fixing apparatus of FIG. 11. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings.  
         [0038]    [0038]FIG. 8 is a schematic structural diagram showing an image forming apparatus (laser beam printer of electrophotographic system in this embodiment) comprising a fixing apparatus according to Embodiment 1 of the present invention.  
         [0039]    The image forming apparatus  1  comprises a photosensitive drum  2  having a photosensitive body on its surface as an image carrier. A charging roller  3 , developing apparatus  4  and transfer roller  5  are arranged around the photosensitive drum  2 , and an exposing apparatus  6  is arranged above the interface between the photosensitive drum  2  and the developing apparatus  4  to form an image forming section. On a down-stream side in the conveyance direction of the recording material of a transfer nip formed between the photosensitive drum  2  and the transfer roller  5 , a fixing apparatus  7  is provided through the intermediation of a conveyance guide  9 . The photosensitive drum  2 , charging roller  3  and developing apparatus  4  are integrated into a cartridge to form a process cartridge  8  which is detachably attached to an image forming apparatus body  40 .  
         [0040]    The photosensitive drum  2  is a negatively charged organic photosensitive body in this embodiment, has a photosensitive layer formed on an aluminum drum substrate, and is rotated in the direction indicated by an arrow (clockwise) by a driving means (not shown) at a predetermined peripheral speed (process speed) and negatively and uniformly charged by the charging roller  3  in contact with the drum  2  in the rotation process.  
         [0041]    The charging roller  3  as contact charging means is held in contact with the surface of the photosensitive drum  2  with a predetermined pressure to be rotated and charges the photosensitive drum  2  to a predetermined polarity or potential uniformly by charging bias applied from a charging bias power source (not shown).  
         [0042]    The developing apparatus  4  comprises a developing sleeve  4   a  which can rotate while it is substantially in contact with the surface of the photosensitive drum  2  at a developing portion and visualizes an electrostatic latent image on the photosensitive drum  2  as a toner image by adhering toner to the latent image at the developing portion.  
         [0043]    The transfer roller  5  as transfer means is held in contact with the surface of the photosensitive drum  2  with a predetermined pressure to form a transfer nip and transfers a toner image on the surface of the photosensitive drum  2  to a recording material P such as paper by transfer bias applied from a transfer bias power source (not shown) at a transfer nip between the photosensitive drum  2  and the transfer roller  5 .  
         [0044]    The exposing apparatus  6  forms an electrostatic latent image according to image information by outputting a laser beam (exposure beam) modulated according to the time sequence electric digital image signal of image information input from a personal computer (not shown) from a laser output section (not shown) and scanning and exposing the surface of the charged photosensitive drum  2  as indicated by symbol L.  
         [0045]    The fixing apparatus  7  has a heating roller  12  including a heater (halogen heater)  14  and a pressure roller  13 . The toner image transferred to the surface of the recording material P is thermally fixed by heat and pressure while the recording material P is conveyed and supported at a fixing nip between the heating roller  12  and the pressure roller  13 . The pressure roller  13  is rotated by the rotation of the heating roller  12  driven by driving means (not shown). Details of the fixing apparatus  7  will be given below.  
         [0046]    Next, the image forming operation of the above-mentioned image forming apparatus will be described.  
         [0047]    At the time of forming an image, the photosensitive drum  2  is driven to turn clockwise at a predetermined peripheral speed by drive means (not shown) and charged to a predetermined potential uniformly by the charging roller  3  to which charging bias is applied. A laser beam L is applied to the surface of the charged photosensitive drum  2  by the exposing apparatus  6  for scanning whereby the potential of the scanned and exposed portion of the photosensitive drum  2  is reduced to form an electrostatic latent image according to image information input from a personal computer (not shown) or the like.  
         [0048]    Toner is adhered to the electrostatic latent image formed on the photosensitive drum  2  by the developing sleeve  4   a  of the developing apparatus  4  to which development bias having the same polarity as the charging polarity (negative) of the photosensitive drum  2  is applied to the developing portion to visualize it as a toner image by reversal developing.  
         [0049]    Meanwhile, the recording material P such as paper in a cassette  21  is fed by a pick-up roller  22  and conveyed to a pair of registration rollers  26  in synchronism with the formation of a toner image on the photosensitive drum  2  by a feed roller  23 , retard roller  24  and feed and conveyance rollers  25 . When the toner image on the photosensitive drum  2  reaches the transfer nip between the photosensitive drum  2  and the transfer roller  5 , the recording material P is conveyed to the transfer nip by the pair of registration rollers  26  at this timing.  
         [0050]    Then, the toner image on the photosensitive drum  2  is transferred to the recording material P conveyed to the transfer nip by the transfer roller  5  to which transfer bias having opposite polarity (positive) as the above-mentioned toner is applied by electrostatic force generated between the photosensitive drum  2  and the transfer roller  5 . The recording material P to which the toner image has been transferred is conveyed to the fixing apparatus  7  through a conveyance guide  9  to thermally fix the toner image on the recording material P by heat and pressure at the fixing nip between the heating roller  12  including the heater  14  and the pressure roller  13 . The recording material P having the toner image fixed thereon is fed to the surface of a sheet discharge tray  29  through a pair of conveyance rollers  27  and a pair of sheet discharge roller  28  to complete a series of image formation operations.  
         [0051]    The residual transfer toner existent on the photosensitive drum  2  after the above transfer is removed and collected by a cleaning apparatus (not shown).  
         [0052]    Next, the constitution of pressure means provided on the above-mentioned fixing apparatus  7  will be described.  
         [0053]    As shown in FIG. 1 and FIG. 2, the fixing apparatus of this embodiment comprises the heating roller  12  including a heater  14  (halogen heater), the pressure roller  13  in contact with the heating roller  12  and a frame  52  for supporting these. Both ends of the heating roller  12  are rotatably supported by the frame  52  through a pair of bearings  51 .  
         [0054]    One end of the heating roller  12  is provided with a gear  53  to be driven to rotate by driving force transmitted from a drive source. Both ends of the heating roller  12  are engaged with a pair of rings  54 .  
         [0055]    Both ends of the core metal portion  13   a  of the pressure roller  13  are rotatably supported by a support member  55  through a pair of bearings  56  and engaged with a pair of rings  57 .  
         [0056]    Reference numeral  58  denotes a rotatable lever which is contact pressure switching means and rotatably supported by a shaft  52   a  provided on either side of the frame  52  and engaged with rings  59 . This rotatable lever  58  is provided such that it can be operated manually by the operator from the outside of the image forming apparatus.  
         [0057]    The rotatable lever  58  comprises an operation portion  58   a , a contacting portion  58   b  which contacts the above-mentioned support member  55  and a stopping portion  58   c  for stopping the movement of the rotatable lever  58 .  
         [0058]    Denoted by numeral  60  is an elastic member which is a coil spring for biasing the support member  55  and provided between the frame  52  and the support member  55  and whose position and pressure are set to enable the pressure roller  13  to bias the heating roller  12  with predetermined pressure.  
         [0059]    A round and long fulcrum hole  55   a  extending in substantially the same direction as the biasing direction of the above elastic member  60  is formed in the support member  55 , and swingably supported by a shaft  52   b  provided on either side of the frame  52 . On the free end side of the support member  55  is formed a bending portion  55   b  which serves as a support portion for the above-mentioned elastic member  60  and as a contacting portion with the rotatable lever  58 .  
         [0060]    Subsequently, the operation of the fixing apparatus will be described with reference to FIG. 1, FIG. 3, FIG. 4 and FIG. 5.  
         [0061]    Since the support member  55  is biased by the elastic member  60  in the direction indicated by an arrow in FIG. 1, the heating roller  12  is held in pressure contact with the pressure roller  13  and biased in the direction indicated by the arrow in FIG. 1. Therefore, reacting force in the direction indicated by the arrow in the figure is applied to the fulcrum  55   a  of the support member  55  to keep the upper portion of the fulcrum hole  55   a  of the support member  55  in contact with the shaft  52   b  of the frame  52 .  
         [0062]    The pressure contact force F 1  of the pressure roller  13  at this time will be schematically explained with reference to FIG. 3. The fulcrum P represents the rotational fulcrum  52   b  of the support member  55 , the function point Q represents a nip portion between the pressure roller  13  and the heating roller  12 , and the point of force R represents the contacting portion of the elastic member  60 .  
         [0063]    Assuming that the biasing force of the elastic member  60  is represented by P 1 , the distance from the contacting portion of the elastic member to the nip portion is represented by l 1  and the distance PQ from the rotational fulcrum  52   b  to the nip portion is represented by l 2 , the following equation (1) is obtained.  
           F   1   =P   1 ·( l   1   +l   2 )/ l   2   (1)  
         [0064]    Therefore, when the paper P to which the unfixed toner image has been transferred is fed to the fixing apparatus  7 , it is pressurized by this pressure contact force F 1  and the toner is molten by heating with the heating roller  12  so that the toner image is fixed on the paper P and conveyed.  
         [0065]    Next, the case where an envelope is conveyed will be described.  
         [0066]    The instruction manual of the fixing apparatus teaches that when an image is to be formed on the envelope, the operation portion  58   a  of the rotatable lever  58  must be rotated downward until it cannot be rotated any more.  
         [0067]    The operation portion  58   a  of the rotatable lever  58  is first grasped and rotated clockwise in FIG. 1 against the biasing force of the elastic member  60  with the fulcrum  52   a  as the center. Then, as shown in FIG. 4, the contacting portion  58   b  of the rotatable lever  58  contacts the contact surface  55   b  of the support member  55  and the rotatable lever  58  is biased in the direction indicated by an arrow in FIG. 4 (center direction of the shaft  52   a ) by the support member  55  whereby moment generated by the resistance of the elastic member  60  does not act on the rotatable lever  58 , the rotatable lever  58  is stopped at a position shown in FIG. 4, and the support member  55  is kept forced down against the biasing force of the elastic member  60 . Since the rotatable lever  58  has the detent portion  58   c , it is not rotated more than required.  
         [0068]    The support member  55  forced down moves such that the fulcrum hole  55   a  of the support member  55  moves along the shaft  52   b  of the frame  52  with the contacting portion  58   b  of the rotatable lever  58  as a fulcrum by the biasing force of the elastic member  60 . Therefore, when the fulcrum hole  55   a  of the support member  55  is an elongated arcuate hole with the contacting portion  58   b  as the center, the support member  55  can move smoothly.  
         [0069]    The pressure contact force F 2  of the pressure roller  13  at this time will be explained with reference to FIG. 5.  
         [0070]    Since the contact portion between the rotatable lever  58  and the support member  55  functions as a fulcrum S, assuming that the biasing force of the elastic member  60  is represented by P 2  and the distance RS from the contacting portion  58   b  of the rotatable lever  58  to the elastic member  60  is represented by l 3 , the following equation (2) holds true.  
           F   2   =P   2   l   3 /( l   1   +l   3 )  (2)  
         [0071]    If, for example, l 1 =l 2  and l 3 =l 1 /5, the equations (1) and (2) can be expressed as:  
           F   1   =P   1 ( l   1   +l   2 )/ l   2 =2 P   1   (1′)  
           F   2   =P   2     1     3 /( l   1   +l   3 )= P   2 /6  (2′).  
         [0072]    Since the elastic member  60  is forced down by the support member  55 , the amount of deflection increases, with the result that P 1 &lt;P 2 . Since the amount of deflection is extremely small, if P 1 =P 2 , the following equation is obtained from the equations (1′) and (2′).  
           F   1 =12 F   2    
         [0073]    Therefore, when the envelope is to be conveyed, it is pressurized by reduced pressure contact force F 2 =(F 1 /12) and the toner is melted by heating with the heating roller  12 , the toner image is fixed and conveyed without wrinkling the envelope.  
         [0074]    When printing is then carried out for a long period of time, the pressure roller  13  receives heat from the heating roller  12  and its temperature rises. Then, the rubber portion  13   b  of the pressure roller  13  expands, resulting in an increase in its outer diameter.  
         [0075]    However, according to the above-mentioned constitution, since contact pressure is small even when the pressure roller  13  thermally expands, the support member  55  can be easily turned clockwise with the fulcrum S as the center which is the contacting portion of the rotatable lever  58  as shown in FIG. 4 and FIG. 5. Therefore, pressure contact force F 2  scarcely changes. Also, according to the above-mentioned constitution, since the heating roller  12  and the pressure roller  13  are in contact with each other when the pressure is reduced, heat is easily transmitted to the pressure roller  13  from the heating roller  12 .  
         [0076]    That is, the time required for warming the pressure roller  13  to ensure fixability is short and the period from the time when the image forming apparatus  1  is turned on to the time when the image forming apparatus  1  can print is short.  
         [0077]    While the fixing apparatus  7  of this embodiment is a roller fixing unit having the heating roller  12 , the present invention is not limited to this and a surface fixing unit using a film or a fixing unit using an induction heater may be used. The surface fixing unit is composed of a pressure roller, a heater base provided with a heater opposing to the pressure roller and a cylindrical film which is rotatably fitted onto the above-mentioned heater base. In the surface fixing unit, an unfixed toner image on a recording material is brought into contact with the outer peripheral surface of the above-mentioned fixing film, the recording material is passed through a fixing nip between the above-mentioned fixing film and the above-mentioned pressure roller together with the above-mentioned fixing film, and the toner image is fixed by heat and pressure.  
         [0078]    The fixing apparatus may be used to remove the recording material stuck in the fixing nip portion by forcing down the pressure roller  13  with the lever  58 .  
         [0079]    (Embodiment 2)  
         [0080]    [0080]FIG. 9 shows Embodiment 2 of the present invention.  
         [0081]    In the above-mentioned Embodiment 1, the shaft  52   b  is inserted into the elongated fulcrum hole  55   a  extending vertically and formed in the support member  55 . In this embodiment, the fulcrum hole  55   a  of the support member  55  is formed as a U-shaped cut-away hole whose lower portion is open as shown in FIG. 9.  
         [0082]    In this case, the support member  55  can be easily assembled by placing it over the shaft  52   b  of the frame  52  from above.  
         [0083]    (Embodiment 3)  
         [0084]    FIG. 10 shows Embodiment 3 of the present invention.  
         [0085]    In Embodiment 1, the rotatable lever  58  is used as contact pressure switching means. In this embodiment, as shown in FIG. 10, a cam member  62  which is fixed to a shaft  61  rotatable by driving force transmitted from a controllable drive source M such as a stepping motor and detecting means (not shown) for detecting the position of the cam member  62  is used to automatically force down the support member  55 . Denoted by numeral  62   b  is a contacting portion and numeral  62   c  is a stopping portion.  
         [0086]    (Embodiment 4)  
         [0087]    [0087]FIG. 11 and FIG. 12 show Embodiment 4 of the present invention.  
         [0088]    In the figures, a round elongated fulcrum hole  71   a  extending in substantially the same direction as the biasing direction of an elastic member  72  is formed in a support member  71  for rotatably supporting the pressure roller  13  between the pressure roller  13  and the elastic member  72  and swingably supported by a shaft  73   b  provided on both sides of a frame  73 .  
         [0089]    On one end side of the support member  71  is formed a bending portion  71   b  which serves as a support portion for the above-mentioned elastic member  72  and as a contacting portion with a rotatable lever  74 . The rotatable lever  74  is rotatably supported by a shaft  73   a  provided on both sides of the frame  73 .  
         [0090]    The support member  71  is biased in a direction shown by an arrow in FIG. 11 by the elastic member  72  and the pressure roller  13  moves counterclockwise around the shaft  73   b  to pressurize the heating roller  12  with predetermined contact pressure. Therefore, reacting force in the direction indicated by an arrow in the figure acts on the fulcrum  73   b  of the support member  71  and the fulcrum  73   b  of the support member  71  is kept at a position where it is in contact with the upper portion of the fulcrum hole  71   a  of the support member  71 .  
         [0091]    Next, the case where an envelope is conveyed will be described. The operation portion  74   b  of the rotatable lever  74  is first grasped and rotated counterclockwise in FIG. 11 against the biasing force of the elastic member  72  with the fulcrum  73   a  as the center. As shown in FIG. 12, since the contacting portion  74   b  of the rotatable lever  74  contacts the contacting surface  71   b  of the support member  71  and the rotatable lever  74  is biased in the direction indicated by an arrow in FIG. 12 (center direction of the shaft  73   b ) by the support member  71 , moment generated by the resistance of the elastic member  72  does not act on the rotatable lever  74 , the rotatable lever  74  is stopped at the position shown in FIG. 12, and the support member  71  is kept down against the biasing force of the elastic member  72 . Since the rotatable lever  74  has a stopping portion  74   c , it is not rotated more than required.  
         [0092]    The support member  71  forced down moves such that the fulcrum hole  71   a  of the support member  71  moves along the shaft  73   b  of the frame  73  by the biasing force of the elastic member  72  using the contacting portion  74   b  of the rotatable lever  74  as a fulcrum.  
         [0093]    The same effect as that in the above-mentioned respective embodiments can be obtained by the above constitution, and a motor may be used to drive the fixing apparatus as in the above-mentioned Embodiment 3.  
         [0094]    As having been described above, according to the present invention, pressure contact force can be kept constant as the fixing apparatus is not influenced by the thermal expansion of a rotator such as a pressure roller at the time of reducing pressure, and it is possible to suppress the wrinkling of a material to be conveyed, such as an envelope.  
         [0095]    Since the heating roller and the pressure roller which are, for example, a pair of rotators are always in contact with each other at the time of reducing pressure, the elevation of the temperature of the heating roller is accelerated, thereby shortening the standby period of time for printing.  
         [0096]    While the present invention has been described in its preferred embodiments, it is to be understood that the present invention is not limited to these embodiments, and various changes and modifications may be made without departing from the scope and spirit of the invention.