Abstract:
A belt nip type fixing apparatus, which prevents a formation of an icicle-like uneven glossiness occurring on a surface of a fixed image due to pressure release in a fixing nip. In addition, an image forming apparatus provided with the fixing apparatus for preventing an occurrence of image failure even in a case where a coated paper sheet, which is most likely to cause the image failure, is used as a recording material and for enlarging a scope of application of the recording material while maintaining a productivity, to thereby attain an improved merchantability. The apparatus is configured such that a wedge-shaped space, which is formed between a belt, a separation roller, and a pressure pad, is adequately filled by a sliding sheet, thereby preventing a pressure in the fixing nip from being decreased.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image heating apparatus for heating an image on a recording material, which is used for an image forming apparatus such as a copying machine, a printer, or a facsimile machine. Examples of the image heating apparatus include a fixing apparatus for fixing an unfixed image on a recording material and a glossiness increasing apparatus for increasing a glossiness of an image by heating the image fixed on a recording material.  
         [0003]     2. Related Background Art  
         [0004]     Conventionally, there has been a belt fixing apparatus in which a fixing roller and a belt are used, as described in Japanese Patent Application Laid-Open No. H1-045025.  
         [0005]     In the belt fixing apparatus, it is possible to easily set a fixing nip width to be large along a direction of conveyance of the recording material by adjusting a contact width of the belt with respect to the fixing roller.  
         [0006]     Therefore, the fixing nip width can be secured without depending on a diameter of the fixing roller, making it possible to reduce a diameter and a heat capacity of the fixing roller, to thereby shorten rise time of the belt fixing apparatus. That is, the belt fixing apparatus is capable of melting a large amount of toner. Therefore, the belt fixing apparatus is suitable for the image forming apparatus, which uses a large amount of toner such as a color image forming apparatus.  
         [0007]      FIG. 9  is a schematic diagram of the belt fixing apparatus as described in Japanese Patent Application Laid-Open No. H1l-045025 mentioned above. A fixing roller  51  is disposed such that the fixing roller  51  can be freely rotated. On a lower side of the fixing roller  51 , a belt unit  53  is disposed.  
         [0008]     Inside the fixing roller  51 , a heater  58  such as a halogen lamp is disposed. In addition, a thermister (not shown) is disposed on the fixing roller  51 . The thermister may be in contact with or out of contact with the fixing roller  51 , and regulates a temperature of a surface of the fixing roller  51  by controlling a voltage supplied to the heater  58  via a temperature control circuit.  
         [0009]     The belt unit  53  is provided with a fixing belt  52  which is a flexible endless belt; three rollers of a first roller  55 , a second roller  56 , and a third roller  57 , which are plural winding and stretching members around which the fixing belt  52  is wound and stretched; a pressure pad  1000 ; and the like. The pressure pad  1000  is provided with a low friction sheet.  
         [0010]     The separation roller  56  and the pressure pad  1000  are brought into pressure contact with the fixing roller  51  via the fixing belt  52 , and thus a fixing nip portion N′ which is a contact portion of the fixing roller  51  and the fixing belt  52  are formed with a width large in the direction of conveyance of the recording material.  
         [0011]      FIG. 10  shows an example of a pressure distribution of the pressure of the fixing nip portion along the direction of conveyance of the recording material, and a curve A shows a case of a conventional belt fixing apparatus. A curve B shows an example of the pressure distribution of the fixing nip portion of the fixing apparatus using a conventional pair of rollers. A curve C shows a case in which in the conventional roller fixing type fixing apparatus, a thickness of an elastic member layer of the fixing roller is made to be thick and an amount of deformation of the elastic member layer is increased by increasing the pressurizing force with a pressure roller in order to increase a nip width without increasing an outer diameter of the fixing roller as less as possible.  
         [0012]     The belt fixing apparatus has a merit in making a nip width W 1  much wider than a nip width W 2  of the roller fixing type fixing apparatus. A nip width W 3  of the roller fixing type fixing apparatus is somewhat wider than the nip width W 2  as shown in the curve B. However, an effect obtained by enlarging the nip width is small, and in addition, the thick elastic member layer causes a heat capacity to be large, which is disadvantageous in view of energy conservation.  
         [0013]     In the belt fixing apparatus, the separation roller  56  and the pressure pad  1000  are separated from each other as shown in  FIG. 9 . The pressure between the separation roller  56  and the pressure pad  1000 , however, is not zero, as is understood from the pressure distribution of the fixing nip portion as shown in the curve A of  FIG. 10 , and the pressure of the separation roller  56  and the pressure of the pressure pad  1000  are continuous to each other through a tension of the fixing belt  52 .  
         [0014]     In the belt fixing apparatus, however, when a recording material, such as a coated paper sheet, having a low air permeability is used, there is another problem in that a portion in which the glossiness is low (haze: hereinafter referred to as uneven glossiness), as shown in  FIG. 11 , is formed in an icicle-like shape on a solid image.  
         [0015]     In other words, in a case where the recording material having a low air permeability such as a coated paper sheet is used, an image disturbance occurs due to an expansion of air or generation of steam inside the fixing nip. The expansion of air or the generation of water vapor is caused by the following reasons: that is, air existing in an unfixed toner layer stagnates in a portion A′ in which the pressure in the nip is low, namely, a boundary region of the pressure pad and the separation roller; and the stagnated air partly forms an air gap on a boundary surface of the recording material and the fixing roller, and disturbs a toner image which is not completely fixed yet.  
         [0016]     In a case where a plain paper sheet is used, the plain paper sheet has an air permeability higher than the air permeability of the coated paper sheet, so the air existing in the toner layer is retained within the plain paper sheet which is a porous member having a large capacity; or otherwise, the air escapes by passing the plain paper sheet. Therefore, it can be considered that the image disturbance does not often occur.  
         [0017]     Further, in the case of a recording material having an especially low air permeability such as a thick coated paper sheet, a larger amount of air is likely to be stagnated in the portion A′ in which the pressure is decreased. Therefore, the problem of occurrence of the uneven glossiness described above becomes more serious.  
         [0018]     Note that under an environment of high humidity in which the air contains high moisture content, the problem like this noticeably occurs.  
       SUMMARY OF THE INVENTION  
       [0019]     The present invention has an object to provide an image heating apparatus capable of controlling an image failure.  
         [0020]     The present invention has another object to provide an image heating apparatus in which less pressure release occurs in a heating nip.  
         [0021]     According to the present invention, there is provided an image heating apparatus including: a heat rotary member for heating an image on a recording material by a heating nip; an endless belt for forming the heating nip between the endless belt and the heat rotary member; a roller for pressing the endless belt in the heating nip toward the heat rotary member; a pad for pressing the endless belt in the heating nip toward the heat rotary member; and a sheet, which covers the pad and which is slidable on the endless belt, in which the image heating apparatus satisfies a relationship of 
 
0 &lt;S 1 /S 0 ≦ 0 . 25 ,  
 
 where a cross-section area of a space surrounded by the belt, the roller, and the pad is S 0 , and a cross-section area of a space surrounded by the belt, the roller, and the sheet is S 1 . 
 
         [0022]     Further features of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is an explanatory view of a first embodiment of the present invention;  
         [0024]      FIG. 2  is a view showing a pressure distribution of a fixing nip in each of fixing apparatuses according to the first embodiment of the present invention, a comparative example 1, and a comparative example 2;  
         [0025]      FIG. 3  is an explanatory view of a configuration of a fixing apparatus according to the first embodiment of the present invention;  
         [0026]      FIGS. 4A  and  FIG. 4B  are explanatory views of a configuration of a pressure applying member according to the first embodiment, a second embodiment, and a third embodiment of the present invention;  
         [0027]      FIG. 5  is an explanatory view of the comparative example 1;  
         [0028]      FIG. 6  is an explanatory view of the comparative example 2;  
         [0029]      FIGS. 7A  and  FIG. 7B  are views illustrating the second embodiment of the present invention;  
         [0030]      FIGS. 8A  and  FIG. 8B  are views illustrating the third embodiment of the present invention;  
         [0031]      FIG. 9  is an explanatory view of a configuration of a belt nip type fixing apparatus according to a conventional example;  
         [0032]      FIG. 10  is an explanatory view showing a comparison between the pressure distribution of the fixing nip of a roller type fixing apparatus and the pressure distribution of the fixing nip of the belt nip type fixing apparatus according to the conventional example;  
         [0033]      FIG. 11  is a view showing an icicle-like uneven glossiness in an image failure; and  
         [0034]      FIG. 12  is a view showing stagnation of air which causes to form the icicle-like uneven glossiness. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Hereinafter, the present invention is more specifically explained by referring to embodiments of the present invention. Note that the embodiments are examples of best modes for carrying out the present invention, but the present invention is not limited only to various kinds of structures explained in the embodiments. In other words, the various kinds of structures of the present invention explained in the embodiments can be substituted by other known structures within a scope of thought of the present invention.  
       First Embodiment  
       [0036]      FIG. 3  is an outline configuration diagram of a belt fixing apparatus, which is an image heating apparatus of this embodiment.  
         [0037]     A fixing roller  51  which is a heat rotary member (fixing rotary member) is configured to have a metal core  51   a  made of Fe which has an inner diameter of φ37.8 mm, an outer-diameter of φ38.4 mm, and a thickness of 0.3 mm. As an elastic layer  51   b , a silicone rubber layer having a thickness of 0.5 mm is formed on the metal core  51   a . A PFA tube having a thickness of 30 μm which is a releasing layer covers the elastic layer  51   b . As a result, the fixing roller has the outer diameter of 40 mm.  
         [0038]     A fixing belt  52 , which is an endless belt, has a base layer of a thickness of 100 μm made of polyimide. The silicone rubber layer having a thickness of 0.2 mm covers the base layer. As a result, the outer diameter of the fixing belt  52  is φ90 mm. Note that the fixing belt of this embodiment is a seamless belt.  
         [0039]     The fixing belt  52  is looped around three rollers of an entrance roller  55 , a separation roller  56 , and a tension roller  57 . The group of rollers rotates in accordance with the belt. More specifically, the fixing belt rotates by a driving force applied to the separation roller  56  from a driving source. The entrance roller  55  and the tension roller  57  are driven and rotated by the belt.  
         [0040]     The entrance roller  55  is a roller disposed on a side of the entrance through which a recording material advances into a fixing apparatus. The entrance roller  55  is fixed so that a conveyance path of the recording material is not changed. The tension roller  57  is biased by a spring so that a prescribed tension is applied to the fixing belt  52 .  
         [0041]     The separation roller  56  is a solid roller made from SUS. The fixing belt is stretched around the separation roller  56 , which supports the fixing belt at the inner surface of the fixing belt in a most downstream position of a fixing nip, which is a heating nip.  
         [0042]     The separation roller  56  pressurizes the fixing belt  52  toward the fixing roller  51  by a pressurizing mechanism (spring). At this time, the separation roller  56  is pressurized to the elastic layer  51   b  of the fixing roller  51  so that the fixing belt  52  inroads into the elastic layer  51   b . As a result, in a portion in which the elastic layer of the fixing roller  51  is deformed, the recording material P is separated from the surface of the fixing roller  51  by a self stripping. In this embodiment, the outer diameter of the separation roller  56  is φ15 mm, and a total pressure of 392 N (40 kgf) is applied.  
         [0043]     In addition, a pressure pad (pressurizing mechanism)  100  which is the pad (a pressure applying member) for applying pressure from the inner surface of the fixing belt toward the fixing roller is provided in order to form the fixing nip.  
         [0044]     As shown in  FIG. 4A , the pressure pad  100  is provided with a base plate  102  and an elastic layer  101  which is an elastic member laminated on the base plate  102 . Further, the pressure pad  100  is provided with a rigid rod (wire)  104  which is a rod-shaped member provided at an edge portion of the elastic layer  101  on a downstream side in the direction of conveyance of the recording material.  
         [0045]     In order to alleviate a sliding resistance between the pressure pad  100  and the inner surface of the fixing belt, that is, in order to reduce the sliding resistance between the elastic layer  101  and the inner surface of the fixing belt, a sliding sheet (low friction sheet)  103  is provided so as to cover the elastic layer  101  including the portion of the rod  104 .  
         [0046]     The rod  104  according to the first embodiment is a metal rod having a diameter of 1.2 mm which is circular in cross section. A glass cloth sheet coated with PTFE is used as the sliding sheet  103 , and the sliding sheet  103  is coated with silicone oil as a lubricant. In the first embodiment, the thickness of the sliding sheet  103  is 100 μm.  
         [0047]     The pressure pad  100  mentioned above is pressed toward the fixing roller  51  by a spring (not shown) disposed on the base plate  102  side via the fixing belt  52 .  
         [0048]     Because the pressure pad  100  is provided with the elastic layer  101 , a contacting surface of the sliding sheet  103  contacting a back surface of the fixing belt can be aligned with an outer peripheral surface of the fixing roller  51 . That is, when the pressure pad  100  is pressed toward the fixing roller  51  by applying a load of a specific value or more, the elastic layer  101  is deformed and the contacting surface of the sliding sheet  103  is deformed into a shape conforming to an outer peripheral surface of the fixing roller  51 . Therefore, when the pressure pad  100  is pressed to the fixing roller  51  by the spring (not shown) via the fixing belt  52 , the fixing belt  52  comes into pressure contact with the fixing roller  51  without a clearance. In the first embodiment, a total pressure of 588 N (60 kgf) is applied to the pressure pad  100 . Thus, the pressurizing force of 588 N of the pressure pad  100  and the pressurizing force of 392 N of the separation roller  56  add up to the total pressurizing force of 980 N ( 100  kgf). At this time, a maximum pressure at the pressure pad  100  in a fixing nip portion N is set to be lower than a maximum pressure at the separation roller  56 .  
         [0049]     The base plate  102  is a stainless steel member with a thickness of 5 mm. The base plate  102  is wedge-shaped toward a pressure contact portion between the fixing roller  51  and the separation roller  56  such that an edge portion  102   a  of the base plate  102  on a downstream side in the direction of conveyance of the recording material backs up an edge portion of the elastic layer  101  on a downstream side in the direction of conveyance of the recording material. As described above, the base plate edge portion  102   a  backs up the edge portion of the elastic layer  101 , to thereby reduce an amount of a drop in pressure between the pressure pad  100  and the separation roller  56  at the fixing nip portion N.  
         [0050]     The elastic layer  101  is made of a silicone rubber having a rubber hardness Hs of 30 degrees. The elastic layer  101  is configured such that the thickness thereof is gradually increased as compared to the thickness of the base plate  102  in a direction of movement of the fixing belt. In the first embodiment, the elastic layer  101  is shaped to have a curvature of a radius of 16 mm, and is in a shape contacting a line parallel to the base plate  102  at a nip entrance portion “a”. The thickness of the rubber at the nip entrance portion “a” is 3 mm, and a peripheral length at the nip entrance portion “a” is 20 mm.  
         [0051]     The rod  104 , which is provided at the edge portion of the elastic layer  101  on the downstream side in the direction of conveyance of the recording material, increases the pressurizing force of the edge portion of the elastic layer  101  on the downstream side in the direction of conveyance of the recording material. The sliding sheet  103  covers the rod  104  to cover the elastic layer  101 .  
         [0052]     As shown in  FIG. 4B , a side surface of the edge portion of the elastic layer  101  on the downstream side in the direction of conveyance of the recording material, on which the rod  104  is provided, forms a rubbing portion F with the separation roller  56  via the sliding sheet  103 . The edge portion of the elastic layer  101  on the downstream side in the direction of conveyance of the recording material, on which the rod  104  is provided, is held between the separation roller  56  and the fixing roller  51  to be elastically deformed, to thereby reduce an amount of a drop in pressure between the pressure pad  100  and the separation roller  56  at the fixing nip portion N.  
         [0053]     The fixing roller  51  is rotationally driven by a rotation mechanism (not shown). The fixing belt  52  is rotated in accordance with the rotation of the fixing roller  51 . The fixing roller  51  is heated by current supplied to a heater  58 , and a surface temperature of the fixing roller  51  is regulated to a predetermined fixing temperature in the rising operation. In this state, the recording material P bearing an unfixed toner image “t” is introduced into the nip entrance portion “a” of the fixing nip portion N from the entrance roller  55  side of a belt unit  53  with an image surface facing upward. The recording material P introduced into the fixing nip portion N is held between the fixing roller  51  and the fixing belt  52  to be conveyed. In the process of holding and conveying the recording material P in the fixing nip portion N, the unfixed toner image on the recording material P is brought into close contact with a surface of the fixing roller  51 , fused by heat, pressed to the recording material P by the pressure of the fixing nip portion N, to thereby be fixed as a permanent fixed image.  
         [0054]     After that, the recording material P, at a nip exit “b” of the fixing nip portion N, is naturally separated by the self stripping from the surface of the fixing roller  51  because of the inroad of the separation roller  56  into the elastic layer  51   b  of the fixing roller  51 , to be delivered and conveyed.  
         [0055]     In this embodiment, an occurrence of an excessive slippage phenomenon at the fixing nip portion N between the fixing roller  51  rotationally driven and the fixing belt  52  driven by the rotation of the fixing roller  51  is prevented. In other words, a unit  59  ( FIG. 3 ) configured to apply a driving force to the separation roller when a rotating speed of the separation roller  56  which forms a recording material separation portion becomes a rotating speed lower than the rotating speed of the fixing belt  52  by a predetermined amount is provided.  
         [0056]     More specifically, a one-way clutch gear is engaged with a fixing roller gear, and a first belt pulley is provided on the same axis as the one-way clutch gear. In addition, a second belt pulley is provided at an end portion of the separation roller, and the first belt pulley and the second belt pulley mentioned above are driven and connected with each other via a timing belt. The tension of the timing belt is maintained at an appropriate belt tension by a tensioner. Further, a drive train between the fixing roller and the separation roller is configured so that in a pressurized state, when the fixing belt is driven and rotated by the fixing roller, the drive is not inputted to the separation roller by a one-way clutch of the one-way clutch gear. A gear ratio is set so that the driving force from the fixing roller is inputted to the separation roller before occurrence of slippage of the fixing belt at the speed higher than the speed of the fixing roller by 4% or more, for example, when the fixing belt begins to slip on the fixing roller.  
         [0057]     Here, a method of covering with the sliding sheet (low friction sheet)  103  at the edge portion of the pressure pad  100  which is the pressure applying member in the direction of conveyance of the. recording material is as shown in  FIG. 1 .  
         [0058]     1) On the side of the pressure pad  100  opposing the fixing roller  51 , a position at which the rod  104  comes into contact with the sliding sheet  103  is A 1 , and a position at which the rod  104  comes into contact with the fixing belt  52  via the sliding sheet  103  is A 2 .  
         [0059]     2) On the side of the pressure pad  100  opposing the separation roller  56 , a position at which the rod  104  comes into contact with the sliding sheet  103  is B 1 , and a position at which the rod  104  comes into contact with the separation roller  56  via the sliding sheet  103  is B 2 .  
         [0060]     3) On the side of the separation roller  56  opposing the fixing roller  51 , a position at which the separation roller  56  comes into contact with the fixing belt  52  is C.  
         [0061]     4) A space (a cross section of the space) formed along an outer surface of the sliding sheet  103  protruding from the positions A 1  and A 2  and the positions B 1  and B 2  and along the outer peripheral surface of the rod  104  is S.  
         [0062]     5) A space (a cross section of the space) formed along the outer surfaces of the fixing belt  52 , the separation roller  56 , and the rod  104  with the positions A 1 , A 2 , B 1 , B 2 , and C being vertexes of the space is S 0 .  
         [0063]     In this case, covering with the sliding sheet  103  is carried out so that a relational expression 
 
0.75 ≦S/S 0&lt;1 
 
 is satisfied. 
 
         [0064]     Note that when a space surrounded by the fixing belt, the separation roller, and the sliding sheet is S 1  ( FIG. 1 ), a relational expression 
 
0 &lt;S 1 /S 0≦0.25 
 
 holds between the space S 0  and the space S 1 . 
 
         [0065]     Note that a configuration in which the rod  104  is not provided at the tip of the elastic layer  101  as shown in  FIG. 1  may be employed. In this case, it is preferable to make the shape of the elastic layer  101  itself is almost aligned with an edge line thereof when the rod  104  as shown in  FIG. 1  is provided. Therefore, a criterion for defining the positions A 1 , B 1 , and the like mentioned above is the elastic layer  101 , instead of the rod.  
         [0066]     Each of the positions mentioned above is measured under static state in which the pressure pad which the sliding sheet covers and the separation roller are pressed at a total pressurizing force of 980 N (100 kgf) just the same as in a fixing state. In other words, each of the points of contact A 1 , A 2 , B 1 , B 2 , and C viewed from a direction of a rotational axis (the direction of the cross section) of the separation roller are determined, distances between each of the points of contact are measured, and a filling ratio S/S 0  is obtained by calculating the cross sections of the spaces S and S 0 . The filling ratio S 1 /S 0  is obtained in the same way as the calculation of the filling ratio S/S 0 .  
         [0067]     The filling ratio S/S 0  is the filling ratio of the sliding sheet  103  in relation to a space portion S 0  on a back side of the belt corresponding to a portion of a boundary region of the pressure pad  100  and the separation roller  56  within a fixing nip region in which the pressure is decreased. By setting the filling ratio S/S 0  to 0.75 or more (by setting the filling ratio S 1 /S 0  to 0.25 or less), the decrease of the pressure at the portion in which the pressure is decreased is suppressed, and it is possible to prevent an image failure from occurring even when a coated paper sheet is used.  
         [0068]     In this embodiment, the filling ratio S/S 0  is set to 0.75 (the filling ratio S 1 /S 0  is set to 0.25).  
         [0069]     On the other hand, as a comparative examples cases as shown in  FIG. 5  and  FIG. 6  are verified.  FIG. 5  is a comparative example 1 when the filling ratio S/S 0  is set to 0.3 (the filling ratio S 1 /S 0  is set to 0.7), and  FIG. 6  is a comparative example 2 when the filling ratio S/S 0  is set to 0.5 (the filling ratio S 1 /S 0  is set to 0.5).  
         [0070]     Fixing nip pressure distribution of each of the first embodiment and the comparative examples 1 and 2 is measured by using a tactile sensor system manufactured by NITTA CORPORATION. In the tactile sensor system, a pressure sensor element is two-dimensionally arranged on a thin film, and it is possible to measure at a 1.0 mm pitch in the direction of conveyance of the recording material and at a pitch of 6.6 mm in a direction perpendicular to a direction of conveyance of the recording material. The fixing nip pressure distribution as described below is obtained by integrating the pressure of the sensor element perpendicular to the direction of conveyance of the recording material.  
         [0071]     By measuring the distributions of the first embodiment and the comparative examples 1 and 2 with the measurement system, the fixing nip pressure distribution as shown in  FIG. 2  is observed for each of the first embodiment and the comparative examples 1 and 2.  
         [0072]     That is, a substantial pressure distribution along the conveyance direction of the recording material of the fixing nip portion N is observed by taking an axis of a position x along the fixing nip, taking an axis of a pressure P along an axis perpendicular to the axis of the position x, and expressing the pressure P as a function P(x) of the position x; and  
         [0073]     a) a position at which the pressure at the nip portion formed by pressurized contact of the fixing roller  51  and the pressure pad  100  with each other via the fixing belt  52  becomes maximum is X 1 ;  
         [0074]     b) the pressure at the position X 1  is P(X 1 );  
         [0075]     c) a position at which the pressure at the nip portion formed by pressurized contact of the fixing roller  51  and the separation roller  56  with each other via the fixing belt  52  becomes maximum is X 2 ;  
         [0076]     d) the pressure at the position X 2  is P(X 2 ); and  
         [0077]     e) the pressure between the position X 1  and the position X 2  is P(X).  
         [0078]     In this case, minimum values Pmin of P(x) are observed as the pressure as shown in Table 1.  
                                     TABLE 1                                   Pmin(X)                                        Comparative Example 1   0.3*P(X1)           Comparative Example 2   0.5*P(X1)           First Embodiment   0.6*P(X1)                      
 
         [0079]     A comparative experiment is carried out with the fixing apparatuses of the first embodiment and the comparative examples 1 and 2 as mentioned above.  
         [0080]     A fixing condition is set so that a surface temperature of the fixing roller  51  is controlled to 170° C., a range of the surface temperature of the fixing belt  52  is set to be in the range of 100° C. to 150° C., and a process speed is set to 300 mm/sec.  
         [0081]     For the condition for the recording material P to be passed and the unfixed toner, a plain paper sheet having a basis weight of 64 g/m 2  and an air permeability of 45 sec, the coated paper sheet having a basis weight of 105 g/m 2  and an air permeability of 1,000 sec, and the coated paper sheet having a basis weight of 148 g/m 2  and an air permeability of 20,000 sec are used. When an unfixed solid image is formed and fixed on each of the coated paper sheets, a result of a situation of occurrence of uneven glossiness as shown in Table 2 is obtained. Note that the measurement of the air permeability is conducted by the measuring method of JIS P8111.  
         [0082]     Note that in this embodiment, the surface temperature of the fixing belt  52  is controlled to a prescribed temperature by installing a heater in the entrance roller  55  and by controlling an amount of power supplied to the heater. A unit for changing the surface temperature of the fixing belt  52  is not limited to the unit mentioned above. That is, the unit for changing the surface temperature of the fixing belt  52  may be a unit capable of raising the surface temperature in the range of 100° C. to 150° C. For example, a unit as described below may be used. That is, the fixing roller and the fixing belt are configured to abut against on and separate from each other, and the temperature of each of the fixing roller and the fixing belt is independently controlled. In this state, the fixing roller and the fixing belt are brought into contact with each other, and the paper sheet is immediately passed. Thus, it is possible to set the surface temperature to the desired temperature. In addition, for the temperature control of the fixing belt, the heater may be installed in the entrance roller  55  as described above, or otherwise it is possible to set the surface temperature to the desired temperature by controlling the fixing roller and the fixing belt to abut against and separate from each other and by receiving the heat from the fixing roller.  
                                                 TABLE 2                                   Plain paper   Coated paper   Coated paper           sheet   sheet 1   sheet 2           64 g/m 2     105 g/m 2     148 g/m 2                                      Air permeability [seconds]   45   1000   20000       Comparative Example 1   ∘   Δ   x       Comparative Example 2   ∘   ∘   Δ       First Embodiment   ∘   ∘   ∘                 ∘: uneven glossiness does not occur with the fixing belt temperature of 100 to 150° C.            Δ: uneven glossiness occurs with a part of a temperature region of the fixing belt temperature of 100 to 150° C.            x: uneven glossiness occurs with almost all regions of the fixing belt temperature of 100 to 150° C.             
 
         [0083]     As shown in Table 2, the result of the experiment is such that in the case of the plain paper sheet having the basis weight of 64 g/m 2  the uneven glossiness does not occur in all of the fixing apparatuses of the first embodiment and the comparative examples 1 and 2; and in the comparative example 1, in a case of a coated paper sheet  1 , a level indicated by a is obtained; and in the comparative example 2, in a case of a coated paper sheet  2 , a level indicated by x is obtained. As is known from Table 2, in the comparative example 2, the obtained level is improved compared to the comparative example 1, obtaining the level indicated by ∘ for the coated paper sheet  1  and the level indicated by Δ for the coated paper sheet  2 . In the first embodiment, the uneven glossiness does not occur both for the coated paper sheet  1  and the coated paper sheet  2 .  
         [0084]     From the results as mentioned above, it is comprehended that the level of occurrence of the uneven glossiness improves as the portion Pmin(X) at which the pressure between the separation roller  56  and the pressure pad  100  which is the pressure applying member is reduced becomes larger and that the uneven glossiness does not occur when 
 
 P  min( X )=0.6× P ( X 1). 
 
         [0085]     Next, an experimental result that suggests air stagnation in the fixing apparatuses of the comparative example 1 and the comparative example 2 observed by the inventor of the present invention is described.  
         [0086]      FIG. 12  is a view showing air stagnation remains. In this experiment, the fixing apparatus is turned off while the recording material on which a solid unfixed toner image is formed passes through the fixing apparatus. In  FIG. 12 , an upper side of  FIG. 12  shows a fixed portion, and a lower side of  FIG. 12  shows an unfixed portion. The air stagnation during the passing of the paper sheet through the fixing nip portion that is shown between the upper side and the lower side of  FIG. 12  is offset at a high temperature due to overheating of the fixing apparatus caused because the fixing apparatus is turned off while the recording material on which a solid unfixed toner image is formed passes through the fixing apparatus. In this high temperature offset portion, a portion in which the high temperature offset occurs in each of a pressure pad portion (pressure applying member portion), a separation roller portion, and a portion between the pressure pad portion and the separation roller portion can be observed. In a downstream of a portion in which an icicle-like uneven glossiness occurs, and in a low pressure portion between the elastic member portion and the separation roller portion, the air stagnation remains can be observed. In addition, it is recognized that the high temperature offset does not occur because the stagnated air exists between the fixing roller  51  and the recording material P and heat is not transferred from the fixing roller  51  to the recording material P.  
         [0087]     In accordance with the two experimental results as mentioned above, the covering with the sliding sheet  103  is carried out so that the filling ratio S/S 0  is expressed by an expression 
 
 S/S 0=0.75 ( S 1 /S 0=0.25). 
 
 Further, when the pressure between the position X 1  and the position X 2  is P(X), this embodiment is configured so that the pressure distribution in the fixing nip satisfies the expressions 
 
 P ( X 1)≦ P ( X 2) 
 
0.6 *P ( X 1)≦ P ( x )≦ P ( X 2). 
 
         [0088]     By using the fixing apparatus that satisfies those expressions, it is possible to prevent the stagnation of the air in an unfixed toner layer from occurring in the boundary region of the pressure pad  100  and the separation roller  56  even in a case of fixing the unfixed toner image on the paper sheet having low air permeability such as coated paper sheet.  
         [0089]     Therefore, it is possible to prevent the phenomenon from occurring such that the glossiness differs because of the difference in heat transfer between in the air stagnation portion and in the non-air stagnation portion caused by an air gap generated partly on a boundary surface of the recording material P and the fixing roller  51  and by the inhibition of the heat transfer from the fixing roller  51  to the recording material P.  
       Second Embodiment  
       [0090]     In a second embodiment of the present invention, a method of covering with the sliding sheet  103  is as shown in  FIG. 7A . That is, the covering with the sliding sheet  103  is carried out so that the filling ratio of S/S 0  of the sliding sheet  103  is expressed by an expression 
 
 S/S 0=0.80 ( S 1 /S 0=0.20). 
 
 Other configurations are the same as the configurations of the first embodiment. 
 
         [0091]     When the fixing nip pressure distribution as described in the first embodiment is measured with the above configuration, the fixing nip pressure distribution as shown in  FIG. 7B  is observed. In this case, when the pressure between the position X 1  and the position X 2  is P(X), the minimum value Pmin(X) of P(X) is expressed by the expression 
 
 P  min( X )=0.7 ×P ( X 1). 
 
         [0092]     A comparative experiment as described below is carried out with the above configuration. For the fixing conditions, the surface temperature of the fixing roller  51  is controlled to 170° C. and the range of the surface temperature of the fixing belt  52  is set to be in the range of 100° C. to 150° C. Further, the process speed is set to 300 mm/sec. For the recording material P, the coated paper sheet having a basis weight of 148 g/m 2  and an air permeability of 20,000 sec and the coated paper sheet having a basis weight of 300 g/m 2  and an air permeability of 30,000 sec are used. When an unfixed solid image is formed and fixed on each of the recording materials, a result of a situation of occurrence of uneven glossiness as shown in Table 3 is obtained.  
                                             TABLE 3                                   Coated paper   Coated paper           sheet 2   sheet 3           148 g/m 2     300 g/m 2                                          Air permeability [seconds]   20000   30000           First Embodiment   ∘   Δ           Second Embodiment   ∘   ∘                         ∘: uneven glossiness does not occur with the fixing belt temperature of 100 to 150° C.                Δ: uneven glossiness occurs with a part of a temperature region of the fixing belt temperature of 100 to 150° C.                x: uneven glossiness occurs with almost all regions of the fixing belt temperature of 100 to 150° C.             
 
         [0093]     As shown in Table 3, the result of the experiment is such that in the case of the coated paper sheet  2 , the uneven glossiness does not occur in the fixing apparatuses of the first and second embodiments, and that in the case of the coated paper sheet  3  having an especially high air permeability, the level indicated by Δ is obtained in the first embodiment, and in the second embodiment, the level indicated by ∘ is obtained; that is, the uneven glossiness does not occur in the second embodiment.  
         [0094]     In accordance with the experimental result as mentioned above, the covering with the sliding sheet  103  is carried out so that the filling ratio S/S 0  is expressed by an expression 
 
 S/S 0=0.80 ( S 1 /S 0=0.20). 
 
 Further, when the pressure between the position X 1  and the position X 2  is P(X), this embodiment is configured so that the pressure distribution in the fixing nip satisfies the expressions 
 
 P ( X 1)≦ P ( X 2) 
 
0.7 * P ( X 1)≦ P ( x )≦ P ( X 2). 
 
         [0095]     By using the fixing apparatus that satisfies those expressions, it is possible to prevent the stagnation of the air in an unfixed toner layer from occurring in the boundary region of the pressure pad  100  and the separation roller  56  even in a case of fixing the unfixed toner image on the paper sheet having particularly low air permeability such as thick coated paper sheet.  
         [0096]     Therefore, it is possible to prevent the phenomenon from occurring such that the glossiness differs because of the difference in the heat transfer between in the air stagnation portion and in the non-air stagnation portion caused by an air gap generated partly on a boundary surface of the recording material P and the fixing roller  51  and by the inhibition of the heat transfer from the fixing roller  51  to the recording material P.  
       Third Embodiment  
       [0097]     In a third embodiment of the present invention, a method of covering with the sliding sheet  103  is shown in  FIG. 8A . That is, the covering with the sliding sheet  103  is carried out so that the filling ratio of S/S 0  of the sliding sheet  103  is expressed by an expression 
 
 S/S 0=0.85 ( S 1 /S 0=0.15). 
 
 Other configurations are the same as the configurations of the first embodiment. 
 
         [0098]     When the fixing nip pressure distribution as described in the first embodiment is measured with the above configuration, the fixing nip pressure distribution as shown in  FIG. 8B  is observed. At this time, when the pressure between the position X 1  and the position X 2  is P(X), the minimum value Pmin(X) of P(X) is expressed by the expression 
 
 P  min( X )=0.8 ×x P ( X 1). 
 
         [0099]     A comparative experiment as described below is carried out with the above configuration. For the fixing conditions, the surface temperature of the fixing roller  51  is controlled to 170° C. and the range of the surface temperature of the fixing belt  52  is set to be in the range of 100° C. to 150° C. Further, the process speed is set to 300 mm/sec.  
         [0100]     The environmental conditions in this case are set to 23° C./50% RH and 23° C./80% RH. For the recording material to be passed, the coated paper sheet having an air permeability of 30,000 sec and a basis weight of 300 g/m 2  is used. When an unfixed solid image is formed and fixed on the coated paper sheet, a result of a situation of occurrence of uneven glossiness as shown in Table 4 is obtained.  
                                             TABLE 4                                   Environment 1   Environment 2           23° C./50% RH   23° C./80% RH                                        Second Embodiment   ∘   Δ           Third Embodiment   ∘   ∘                         ∘: uneven glossiness does not occur with the fixing belt temperature of 100 to 150° C.                Δ: uneven glossiness occurs with a part of a temperature region of the fixing belt temperature of 100 to 150° C.                x: uneven glossiness occurs with almost all regions of the fixing belt temperature of 100 to 150° C.             
 
         [0101]     As shown in Table 4, the result obtained through the experiment is as follows. In the case of the environment 1, the uneven glossiness does not occur in the fixing apparatuses according to the first embodiment and the second embodiment. On the other hand, under a high humidity condition of 80% RH, the level indicated by Δ is obtained in the second embodiment, while in the third embodiment, the level indicated by ∘ is obtained; that is, the uneven glossiness does not occur in the third embodiment.  
         [0102]     In accordance with the experimental result as mentioned above, the covering with the sliding sheet  103  is carried out so that the filling ratio S/S 0  is expressed by an expression 
 
 S/S 0=0.85 ( S 1 /S 0=0.15). 
 
 Further, when the pressure between the position X 1  and the position X 2  is P(X), this embodiment is configured so that the pressure distribution in the fixing nip satisfies the expressions 
 
 P ( X 1)≦ P ( X 2) 
 
0.8 ×P ( X 1)≦ P ( x )≦ P ( X 2). 
 
         [0103]     By using the fixing apparatus that satisfies those expressions, it is possible to prevent the image failure from occurring even in a case of fixing the unfixed toner image on the thick coated paper sheet having an especially low air permeability under a high humidity environment. Therefore, it is possible to prevent the phenomenon in which a heat transfer from the fixing roller  51  to the recording material P is inhibited, generating a difference in the heat transfer between in the air stagnation portion and in the non-air stagnation portion, leading to the different glossiness.  
         [0104]     According to each of the embodiments described above, the space (cross section) surrounded by the fixing belt, the pressure pad, and the separation roller in the fixing nip region is adequately filled by the sliding sheet, to thereby reduce an amount of a drop in the pressure in the space.  
         [0105]     Accordingly, the air is not stagnated in the portion of the space like this, and it is possible to suppress the occurrence of the image failure even in the case where the coated paper sheet or the like is used as the recording material.  
         [0106]     More specifically, in the case of fixing the unfixed toner image on the recording material having a low air permeability such as the coated paper sheet, it is possible to suppress the uneven glossiness of the image from being caused by an inhibition of the heat transfer from the fixing roller to the recording material in the portion of the space like this.  
         [0107]     This application claims priority from Japanese Patent Application Nos. 2004-305793 filed on Oct. 20, 2004 and 2005-233276 filed on Aug. 11, 2005, which are hereby incorporated by reference herein.