Abstract:
An image heating apparatus includes a rotatable belt member for heating an image on a recording material; a rotatable member pressing against said belt member; a nip forming member, provided inside said belt member, for cooperating with said rotatable member to form a nip for nipping and feeding the recording material; a projection provided on a side of said nip forming member near the nip and projecting toward the nip; and an executing portion for executing a first image heating mode operation in which an image formed on the recording material having a first thickness with said projection projected into a nip region and a second image heating mode operation in which an image formed on the recording material having a second thickness which is smaller than the first thickness with said projection is outside the nip region.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating apparatus for heating the image on recording medium. 
         [0002]    There are various image heating apparatuses. For example, a fixing device for fixing an unfixed image on recording medium to the recording medium, an apparatus for heating a fixed image on recording medium to increase the image in glossiness, and the like can be listed as an image heating apparatus. 
         [0003]    There are a variety of image heating apparatuses, which have been known as a fixing device employed by an electrophotographic image forming apparatus to fix an unfixed toner image to recording medium. One of the fixing devices has been known as a fixing device of the heat roller type, the fixing member of which has an elastic roller. A fixing device of the heat roller type has two heating rollers, that is, a fixation roller and a pressure roller, and is structured so that the two heating rollers form a fixation nip by being pressed upon each other. In an operation for fixing an unfixed toner image on recording medium, the two heating rollers are kept at a preset level in temperature, and recording medium on which an unfixed toner image is present is moved through the fixation nip. As the recording medium on which an unfixed toner image is present is moved through the fixation nip of the fixing device, heat and pressure are applied to the unfixed toner image on the recording medium so that the toner image becomes fixed to the recording medium; the unfixed toner image is turned into a permanent image. There have also been known film (belt)-based fixing devices (Japanese Laid-open Patent Application H04-44075). In the case of some film (belt)-based fixing devices, the film (endless belt) is externally heated, whereas in the case of the other, which are referred to as a fixing device of the electromagnetic induction type (Japanese Laid-open Patent Application 2001-42670), the film (endless belt) is internally (electromagnetically) heated. Image fixing devices of the electromagnetic induction type have also been in practical use. 
         [0004]    In recent years, the wave of colorization has been spreading in the field of image forming apparatuses, such as printers and copying machines. A color image forming apparatus is used for outputting a photographic image. Therefore, it is required to be capable of outputting a glossy image. 
         [0005]    While the wave of colorization has been spreading in the field of image forming apparatuses, the following fixing device, has been disclosed in Japanese Laid-open Patent Application 2004-184518, which is excellent in terms of energy efficiency and can yield a permanent image which is high and uniform in glossiness. More specifically, this fixing device has a fixation film (endless belt) and a pressure roller. More concretely, the fixing device has also a pad on which the fixation film slides. The pad is placed within the loop which the fixation film forms. It is kept pressed against the pressure roller, with the presence of the fixation film between the pad and pressure roller, creating thereby a fixation nip between the fixation film and pressure roller. Further, the pad is provided with a ridge, the position of which relative to the pad is such that as the pad is pressed against the pressure roller (fixation film), it will be on the downstream side of the center of the fixation nip in terms of the direction in which recording medium conveyed through the fixation nip. Thus, the pressure peak of the pressure distribution in the fixation nip is on the downstream side of the center of the fixation nip. 
         [0006]    However, the fixing device disclosed in Japanese Laid-open Patent Application 2004-184518 has the following problems when thin paper or film, which is low in rigidity, is used as recording medium. That is, as a sheet of thin paper or film is sent into the fixation nip, it is possible that the sheet is adhered to the fixation film by the thermally melted toner. That is, it is possible that the so-called “wrapping jam” or the phenomenon that a sheet of recording medium wraps around the fixation film will occur. 
         [0007]    It seems to be reasonable to think that the primary cause of the above described jam is as follows: That is, as the toner softens between the fixation film and recording medium, it becomes adhesive, being therefore likely to cause the recording medium to adhere to the surface of the fixation film. The secondary cause of the above describe jam seems to be as follows: That is, the ridge of the pad is on the downstream side of the center of the fixation nip in terms of the recording medium conveyance direction, and presses the recording medium downward (toward pressure roller). Thus, as the recording medium comes out of the fixation nip, its leading end portion is made to bend upward (toward fixation film) by the ridge, impeding thereby the recording medium from separating from the fixation film. This is why the fixing device disclosed in the abovementioned patent application is likely to cause a sheet of thin paper or film to adhere to, and wrap around, the fixation film. 
       SUMMARY OF THE INVENTION 
       [0008]    Thus, the primary object of the present invention is to provide an image heating device which is superior to any of image heating devices in accordance with the prior art, not only in terms of recording medium separation, but also, in terms of glossiness level. 
         [0009]    According to an aspect of the present invention, there is provided an image heating apparatus comprising a rotatable belt member for heating an image on a recording material; a rotatable member pressing against said belt member; a nip forming member, provided inside said belt member, for cooperating with said rotatable member to form a nip for nipping and feeding the recording material; a projection provided on a side of said nip forming member near the nip and projecting toward the nip; and an executing portion for executing a first image heating mode operation in which an image formed on the recording material having a first thickness with said projection projected into a nip region and a second image heating mode operation in which an image formed on the recording material having a second thickness which is smaller than the first thickness with said projection is outside the nip region. 
         [0010]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic sectional view of the image forming apparatus which employs the fixing device in the first preferred embodiment of the present invention, and shows the general structure of the image forming apparatus. 
           [0012]      FIG. 2  is a schematic sectional view of the fixing device in the first preferred embodiment of the present invention. 
           [0013]      FIG. 3(   a ) is a schematic sectional view of an example of the pressure pad in the first preferred embodiment of the present invention, and  FIG. 3(   b ) is a perspective view of the pressure pad. 
           [0014]      FIG. 4  is a block diagram of the operational sequence for controlling the movement of the heating unit. 
           [0015]      FIG. 5  is an example of the flowchart of the image outputting (forming) operation of the image forming apparatus in accordance with the present invention. 
           [0016]      FIG. 6(   a ) is a schematic sectional view of the fixing device in the normal mode, and  FIG. 6(   b ) is a graph which shows the pressure and temperature distributions of the fixing nip in the normal mode. 
           [0017]      FIG. 7(   a ) is a schematic sectional view of the fixing device in the thin paper mode, and  FIG. 7(   b ) is a graph which shows the pressure and temperature distributions of the fixing nip in the thin paper mode. 
           [0018]      FIG. 8  is a schematic sectional view of the interface between the fixation roller and pressure roller, and its adjacencies, and shows the state of the leading edge portion of the sheet of recording medium when the sheet has just begun to come out of the fixation nip. 
           [0019]      FIG. 9  is a schematic side view of the fixing device, and is for describing the method for moving the heating unit. 
           [0020]      FIG. 10  is a schematic top plan view of the fixing device, and is for describing the method for moving the heating unit. 
           [0021]      FIG. 11  is an example of the timing chart of the image outputting operation in accordance with the present invention. 
           [0022]      FIG. 12  is a schematic sectional view of an example of the pressure pad which is different from the pressure pad shown in  FIG. 3 . 
           [0023]      FIG. 13  is a schematic sectional view of an example of the pressure pad which is different from the pressure pads shown in  FIGS. 3 and 12 . 
           [0024]      FIG. 14  is another example of the flowchart of the image outputting operation in accordance with the present invention. 
           [0025]      FIG. 15  is another timing of the image outputting operation in accordance with the present invention. 
           [0026]      FIG. 16  is a drawing for describing the various parameters which affect the performance of the fixing device in accordance with the present invention, and shows their relationship when the toner alignment on recording medium is idealistic. 
           [0027]      FIG. 17  is a schematic sectional view of yet another example of the pressure pad in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Hereinafter, the present invention is described in detail with reference to the image heating device in accordance with the present invention and the appended drawings. 
       Embodiment 1  
     1. Image Forming Apparatus 
       [0029]      FIG. 1  is a schematic sectional view of the image forming apparatus  100  which employs the fixing device in the first preferred embodiment of the present invention. This image forming apparatus  100  is a laser beam printer capable of forming a full-color image with the uses of an electrophotographic image forming method. It is of the intermediary transfer type, and is also of the tandem type. 
         [0030]    The image forming apparatus  100  has multiple image forming stations, more specifically, the first, second, third, and fourth image forming stations Sa, Sb, Sc, and Sd, respectively. In this embodiment, the image forming stations Sa, Sb, Sc, and Sd are practically the same in structure and operation, although they are different in the color of the toner they use. Therefore, they are described together. That is, the suffixes of the referential codes given to identify their structural components and the like will not be shown unless necessary to show the difference among the multiple image forming stations. 
         [0031]    The image forming station S has a photosensitive drum  101 , which is an electrophotographic photosensitive member (photosensitive member) as an image bearing member. It is in the form of a cylindrical drum. It is rotated in the direction (counterclockwise direction) indicated by an arrow mark R 1  in  FIG. 1 . The image forming station S has also the following means, which are in the adjacencies of the peripheral surface of the photosensitive drum  101 . That is, it has: a charge roller  102 , which is a charging member (charging device of contact type) as a charging means; a developing device  104  as a developing means; a first transfer roller  105  as the first transferring member (charging device of contact type) as a first transferring means; and a drum cleaner  106  as a means for cleaning the photosensitive member. 
         [0032]    The image forming apparatus  100  has also an exposing device  103  as an exposing means for exposing each of the photosensitive drums  101   a - 101   d.  The exposing device  103  is above the photosensitive drums  101   a - 101   d.  It has a light source, a polygonal mirror, etc. 
         [0033]    The image forming apparatus  100  has also an intermediary transfer belt  107  as an intermediary transfer member. The intermediary transfer belt  107  is an endless belt, and is positioned so that it opposes all of the photosensitive drums  101   a - 101   d  of the image forming stations Sa-Sd, respectively. The intermediary transfer belt  107  is suspended and kept stretched by a driver roller  171 , a tension roller  172 , and a second transfer roller  173 . As rotational driving force is transmitted to the driver roller  171 , the intermediary transfer belt  107  is rotated (circularly moved) by the rotation of the driver roller  171  in the direction (clockwise direction) indicated by an arrow mark R 2 . The first transfer rollers  105   a - 105   d  are on the inward side of the loop which the intermediary transfer belt  107  forms. They form first transfer stations T 1   a,  T 1   b,  T 1   c,  and T 1   d,  where the intermediary transfer belt  107  is placed in contact with the photosensitive drums  101   a - 101   d  by being pressed against the photosensitive drums  101   a - 101   d  by the first transfer rollers  105   a - 105   d,  respectively. The image forming apparatus  100  has also a second transfer roller  108  and a transfer belt backing roller  173 . The second transfer roller  108  is a second transferring member (charging device of contact type) which is a second transferring means. It is on the outward side of the loop which the intermediary transfer belt  107  forms. It is positioned in contact with the outward surface of the intermediary transfer belt  107  in such a manner that it is pressed against the transfer belt backing roller  173 , with the presence of the intermediary transfer belt  107  between the two rollers  108  and  173 , forming thereby the second transfer station T 2  where the intermediary transfer belt  107  is in contact with the second transfer roller  108 . 
         [0034]    Next, the image forming operation of this image forming apparatus  100  is described with reference to the formation of a full-color image, for example. First, the photosensitive drums  101   a - 101   d  are uniformly charged across their peripheral surface by the charging rollers  102   a - 102   d,  in the image forming stations Sa-Sd, respectively. Then, the charge portion of the peripheral surface of the photosensitive drum  101  is exposed by the exposing device  103 . More specifically, the exposing device  103  projects a beam of laser light while modulating the beam of laser light with the image formation signals obtained by separating the optical image of the image to be formed into four monochromatic images of primary colors, one for one. The beam of laser light is reflected by the rotating polygonal mirror of the exposing device  103 , in such a manner that it scans the peripheral surface of the photosensitive drum  101  while being focused on the generatrix of the photosensitive drum  101  by the f-θ lens of the exposing device  103 . As a result, an electrostatic latent image is formed on the peripheral surface of each of the photosensitive drums  101   a - 101   d.  Then, the electrostatic latent images on the photosensitive drums  101   a - 101   d  are developed by the developing devices  104   a - 104   d  into four visible images, that is, four monochromatic images formed of toners which correspond in color to the aforementioned primary colors, one for one. 
         [0035]    The developing devices  104   a - 104   d  contain yellow, magenta, cyan, and black developers, respectively, which are two-component developers. Basically, each developer is a mixture of nonmagnetic toner particles (toner) and magnetic carrier particles (carrier). The two-component developer is circulated in the developing devices  104   a - 104   d.  Incidentally, in order to compensate for the consumption of the toner in the developer by image formation, the developing devices  104   a  and  104   d  are supplied as necessary with a fresh supply of toner by toner supplying devices  141   a - 141   d,  respectively. 
         [0036]    As the image outputting operation is started, it is first confirmed whether or not the intermediary transfer belt  107  is in a preset position. As soon as it is confirmed that the intermediary transfer belt  107  is in the preset position, the driver roller  171  is rotated to circularly move the intermediary transfer belt  107 . At the same time as the driver roller  171  begins to be rotated, an image writing start signal is sent in, and then, a monochromatic image begins to be formed on the peripheral surface of the photosensitive drum  101   a,  with a timing set with reference to the image writing start signal, in the first image forming station Sa. 
         [0037]    The toner image formed on the peripheral surface of the photosensitive drum  101   a  in the first image forming station Sa is transferred (first transfer) onto the intermediary transfer belt  107  by providing the first transfer roller  105   a  with an electric field or electric charge, in the first transfer station  1   a.  This toner image, or the toner image of the first primary color, is conveyed to the first transfer station T 1   b  of the second image forming station Sb. 
         [0038]    Thereafter, the monochromatic toner images, different in color (primary color), formed in the second to fourth image forming stations Sb-Sd, one for one, as in the same manner as the monochromatic toner image of the first primary color is formed in the first image forming station Sa, are transferred (first transfer) in layers onto the intermediary transfer belt  107 . Then, the portion of the intermediary transfer belt  107 , onto which the four monochromatic toner images, different in color, have just been transferred in layers, is conveyed to the second transfer station T 2 . 
         [0039]    Meanwhile, one of the sheets P of recording medium in recording medium cassettes  109   a  or  109   b  is conveyed from the cassette  109   a  or  109   b  to the second transfer station T 2  by way of multiple pairs of recording medium conveyance rollers and a pair of registration rollers  110 , and then, is conveyed through the second transfer station T 2 . As the sheet P of recording medium is conveyed through the second transfer station T 2 , the toner images on the intermediary transfer belt  107  are transferred together (second transfer) by the provision of an electric field or an electric charge by the second transfer roller  108 . 
         [0040]    As the sheet P of recording medium, on which the unfixed monochromatic toner images are present, comes out of the second transfer station T 2 , it is conveyed to the fixing device  6  as an image heating device. In the fixing device  6 , heat and pressure are applied to the unfixed toner images on the sheet P of recording medium. Thus, the unfixed toner images become fixed to the sheet P of recording medium. The fixing device  6  is described later in detail. 
         [0041]    Thereafter, the sheet P of recording medium is discharged from the image forming apparatus  100 . As for the toner particles remaining on the peripheral surface of the photosensitive drum  101  after the first transfer, they are recovered by the drum cleaner  106 . The toner particles remaining on the intermediary transfer belt  107  after the second transfer are recovered by a belt cleaner  174  as an intermediary transfer belt cleaning means. 
         [0042]    In this embodiment, the image forming stations Sa-Sd, intermediary transfer belt  107 , second transfer roller  108 , etc., make up the image forming means for forming toner images on the sheet P of recording medium. 
         [0043]    Also in this embodiment, the toner is 5.5 μm in average particle diameter, and 1.1 g/cm 2  in specific gravity. Further, the theoretical amount by which toner is to be transferred (adhered) to the sheet P of recording medium to form a solid monochromatic portion of the image to be formed is 0.5 mg/cm 2 , whereas the maximum amount by which toner is transferred (adhered) to the sheet P of recording medium is 1.0 mg/cm 2 . The “theoretical amount by which toner is to be transferred (adhered) to the sheet P of recording medium to form a solid monochromatic portion of the image to be formed” means the amount by which toner is transferred (adhered) to the sheet P of recording medium (peripheral surface of photosensitive drum  101 ) per unit area when an electrostatic latent image, which is highest in terms of density, is developed by monochromatic toner. Further, the “average particle diameter of toner” means the weight average particle diameter measured with the use of the following method. First, 100-150 ml of water solution of electrolyte (roughly 1% water solution of NaCl, for example), which contains several milliliters of surfactant (preferably, alkyl benzene sodium sulfonate) is prepared. Then, 2-20 mg of toner is added to the water solution, and is dispersed several minutes with the use of an ultrasonic dispersing device. Then, the weight average particle diameter of the toner is obtained by measuring this solution with the use of a Coulter counter TA-11 (product of Beckman-Coulter Co., Ltd.). 
       2. Fixing Device 
       [0044]    Next, the fixing device  6  is described.  FIG. 2  is a schematic sectional view of the fixing device  6  in this embodiment. 
         [0045]    In this embodiment, the fixing device  6  is a fixing apparatus of the so-called film heating type. That is, the fixing device  6  has a fixation film  11  which is an endless belt and is circularly movable (circularly movable heating member). It has also a pressure roller  21  and a pressure pad  13 . The pressure roller  21  is a rotatable member (pressure applying rotatable member) and is kept pressed against the pressure pad  13  to form a nip N (fixation nip) through which the sheet P of recording medium is conveyed while remaining sandwiched by the pressure roller  21  and pressure pad  13 . The pressure pad  13  is one of the nip forming members for forming the fixation nip N, and is on the inward side of the loop which the fixation film  11  forms. Further, the fixing device  6  has an IH coil  31 , a side core  32 , a center core  33 , a pressure pad supporting member  12 , etc. The fixation film  11 , pressure pad supporting member  12 , pressure pad  13 , etc., make up the heating unit  10 . The pressure roller  21  makes up a pressure unit  20 . The means for heating the fixation film  11  is made up of the IH coil  31 , side core  32 , center core  33 , etc. 
         [0046]    The fixation film  11  has three layers, which are a substrate layer, an elastic layer, and a parting layer, listing from the inward side of the fixation film  11 . In this embodiment, the fixation film  11  is 30 mm in diameter. The substrate layer is a heat generating metallic layer, in which eddy current is generated by the alternating magnetic field generated by the IH coil  31 . It is formed of iron, stainless steel, nickel, or the like substance. It is desired to be no less than 10 μm and no more than 100 μm in thickness. If the substrate layer of the fixation film  11  is no more than 10 μm in thickness, the fixation film  11  is inferior in durability, and also, it can hardly absorb the electromagnetic energy, rendering therefore the fixation film  11  inferior in efficiency. On the other hand, if it is no less than 100 μm in thickness, it makes the fixation film  11  excessively rigid, that is, unlikely to easily bend. Thus, using a fixation film ( 11 ), the substrate layer of which is no less than 100 μm, is unrealistic, since the fixation film  11  has to be circularly moved. The elastic layer is formed of a substance which is heat resistant, excellent in thermal conductivity, and elastic. The elastic layer is desired to be no less than 10 μm and no more than 500 μm in thickness. As the material for the parting layer, a substance, such as fluorinated resin (PTFE, PFA, FEP, etc.) silicone resin, fluorinated rubber, silicone rubber, which is superior in parting properties and heat resistance, is desired. The thickness of the parting layer is desired to be no less than 1 μm and no more than 100 μm. If the parting layer is no less than 1 μm in thickness, it is likely to allow toner to offset from the sheet P of recording medium onto the fixation film  11 . On the other hand, if it is no less than 100 μm in thickness, it cannot fully transfer the heat generated in the heat generation layer, to the sheet P of recording medium and the toner thereon, and therefore, is likely to cause the fixing device  6  to fail to properly fix the toner images. 
         [0047]    The pressure roller  21  comprises a metallic core and an elastic layer. The elastic layer which is formed of silicone rubber or the like is for providing the pressure roller  21  with a certain amount of softness (elasticity). The elastic layer of pressure roller  21  may be coated with fluorinated resin such as PTFE, PFA, and FEP in order to improve the pressure roller  21  in surface properties. The pressure roller  21  is rotatably supported by its lengthwise ends, in terms of the direction parallel to the axial line of the metallic core, by the chassis (unshown) of the fixing device  6 , with a pair of bearings positioned between the lengthwise ends of the metallic core and the left and right walls (metallic plates) of the chassis. In this embodiment, the pressure roller  21  is 30 mm in diameter. 
         [0048]    The pressure roller  21  and pressure pad  13  are kept pressed against each other with the presence of the fixation film  11  between the roller  21  and pad  13 , forming thereby the fixation nip N (compression nip) between the fixation film  11  and pressure pad  13 . While the sheet P of recording medium, on which the unfixed toner images are present, is conveyed through the fixation nip N while remaining pinched by the pressure roller  21  and fixation film  11 , the toner images are heated and compressed. Consequently, the toner images become fixed to the sheet P. 
         [0049]    The pressure pad  13 , which is one of the nip forming members, is formed of a substance which is heat resistant and is rigid enough to compress the fixation film  11  against the pressure roller  21 . In this embodiment, the pressure pad  13  is formed of heat resistant engineering plastic. Further, the surface of the pressure pad  13 , which faces the metallic substrate of the fixation film, is covered with a slippery sheet, such as a glass sheet coated with PTFE, in order to make the surface slipperier. Incidentally, it may be simply coated with lubricant such as silicon oil. 
         [0050]      FIG. 3(   a ) is a schematic sectional view of the pressure pad  13  in this embodiment.  FIG. 13(   b ) is a perspective view of the pressure pad  13  in this embodiment. The pressure pad  13  comprises a base  13 B and a ridge  13 A. The base  13 B is the lengthwise direction of which is roughly perpendicular to the direction in which the sheet P of recording medium is conveyed. The ridge  13 A is on the surface  13 B 1  (which faces pressure roller  21 ) of the base  13 , on which the fixation film  11  slides as it is circularly moved. The ridge  13 A extends in the lengthwise direction of the base  13 B, across virtually entire range of the base  13 B. The surface  13 B 1  of the base  13 B, on which the fixation film  11  slides, is roughly parallel to the direction in which the sheet P of recording medium is conveyed through the fixation nip N. In this embodiment, it is roughly horizontal. The ridge  13 A is triangular in cross section, and is at the downstream edge of the base  13 B in terms of the recording medium conveyance direction. 
         [0051]    To describe in more detail, in this embodiment, the width of the pressure pad  13 , that is, the dimension of the pressure pad  13  in terms of the direction in which the sheet P of recording medium is conveyed, is 10 mm. As for the size and position of the ridge  13 A, the distance between the upstream edge of the ridge  13 A and the center of the pressure pad  13  in terms of the recording medium conveyance direction is 3.5 mm, whereas the distance between the tip of the ridge  13 A and the downstream edge of the base  13 B is 1.5 mm. Further, the height of the ridge  13 A is 0.75 mm. That is, in this embodiment, the tip of the ridge  13 A, which corresponds to the top of the triangular cross section of the ridge  13 A, is located 4.25 mm downstream of the center of the pressure pad  13  in terms of the recording medium conveyance direction. The angle a of the tip portion of the ridge  13 A is 90 degrees, and the angle β, which is the angle between the upstream lateral surface of the ridge  13 A and the surface  13 B 1  of the base  13 B is 45 degrees. 
         [0052]    In this embodiment, the ridge  13 A is an integral part of the pressure pad  13 . However, all that is necessary here is that the ridge  13 A is on the surface of the base  13 B, which faces the fixation nip N and protrudes toward the fixation nip N. In other words, the ridge  13 A may be such a section of the pressure pad  13  that is formed independently from the base  13 B of the pressure pad  13  and then, is attached to the surface  13 B 1  of the base  13 B with the use of an optional ridge attaching means such as adhesive. 
         [0053]    In this embodiment, the height of the ridge  13 A (distance between tip of triangular cross section of ridge  13 A and surface  13 B 1 ) is set to 0.75 mm to locally increase the fixation nip N in internal pressure. Also in this embodiment, the ridge  13 A of the pressure pad  13  is changed in its position relative to the fixation nip N, according to the type of recording medium. This feature of the fixing device  6  is described later in detail. 
         [0054]    The pressure pad supporting member  12  is formed of a metallic substance such as stainless steel, aluminum, or the like. It has the function of keeping the pressure pad  13  pressed against the pressure roller  21  with the presence of the fixation film  11  between the pressure pad  13  and pressure roller  21 . 
         [0055]    The IH coil  31  is in connection to an exciter circuit (unshown), which is capable of outputting high frequency waves which are 20 kHz-100 kHz in frequency with the use of a switching electric power source. 
         [0056]    The side core  32  and center cores  33  are formed of highly magnetic substance such as ferrite. They are in magnetical connection to each other because of the presence of the magnetic field generated by the IH coil  31 . Positioning the center core  33  and side cores  32  so that the center of the center core  33  coincides with the center of the IH coil, and also, so that the side cores  32  are in the adjacencies of the lengthwise edges of the HI coil  31 , one for one, strengthens the magnetic connection between the center core  33  and side cores  32 . 
         [0057]    Incidentally, in this embodiment, the IH coil  31  is used as the means to heat the fixation film  11 . That is, the fixation film  11  is internally heated. However, the fixation film  11  may be externally heated by placing an external heating member in contact with the fixation film  11 . Further, in this embodiment, the unfixed toner images become fixed to the sheet P of recording medium by being placed in contact with the fixation film  1  which is being heated by the IH coil  31  as a heat source. However, the present invention is also applicable to a fixing device, the fixation film of which is heated by a halogen heater as a heat source. The effects of such an application are the same as those obtained by the fixing device  6  in this embodiment. 
       3. Operation of Fixing Device 
       [0058]    The unfixed toner images on the sheet P of recording medium are made up of toner particles. Thus, as the sheet P on which the unfixed images are present is conveyed through the fixation nip N of the fixing device  6 , the unfixed toner images are heated and compressed in the fixation nip N, whereby they becomes solidly fixed to the sheet P as the sheet P is conveyed out of the fixing device  6  (fixation nip N). 
         [0059]    As alternating electrical current is flowed through the IH coil  31  of the fixing device  6 , an alternating magnetic field is generated, which in turn generates eddy current in the metallic substrate layer of the fixation film  11 . This eddy current generates heat in the metallic substrate layer. Consequently, the fixation film  11  becomes hot. As the temperature of the fixation film  11  becomes high enough for fixation, the pressure roller  21  is pressed against the pressure pad  13  with the presence of the fixation film  11  between the pressure roller  21  and pressure pad  13 , forming thereby the fixation nip N between the pressure roller  21  and fixation film  11 . As the pressure roller  21  is rotated, the fixation film  11  is circularly moved by the rotation of the pressure roller  21 . Then, as the sheet P of recording medium is conveyed through the fixation nip N of the fixing device  6  while remaining pinched by the fixation film  11  and pressure roller  21 , heat and pressure are applied to the toner particles, of which the toner images on the sheet P are formed. Thus, the toner particles, of which the unfixed toner images are formed, solidly adhere to the surface of the sheet P; the toner images become fixed to the sheet P. 
         [0060]    In this embodiment, an operator is allowed to select the type and size of the sheet P of recording medium with the use of the control panel  1  of the image forming apparatus  100 . Thus, as the operator makes a selection, the ridge  13 A of the pressure pad  13  is automatically moved to a preset position according to the selection, before the starting of the actual fixing operation, as will be described later in detail. 
         [0061]    Next, the method for changing the position of the heating unit  10  (that is, position of ridge  13 A of pressure pad  13 ) is described.  FIG. 4  depicts the control sequence for moving the heating unit  10  which comprises the fixation film  11 , pressure pad  13  (which are within fixation film loop), and pressure plate supporting member  12  (which also are within fixation film loop), etc. 
         [0062]    First, an operator is to select the type of the recording medium to be used for outputting images, using the control panel  1  of the image forming apparatus  100 . Then, the information regarding the selected recording medium type is transferred to a CPU  2  as the controlling means of the control portion  7 . The CPU  2  determines whether or not the selected sheet P of recording medium is no less than 80 g/m 2  in basis weight, with the reference to the information in a memory  5  as the storage means of the control portion  7 . The memory  5  stores the preset relationship between the type and basis weight of recording medium. 
         [0063]    Then, the information regarding the decision made by the CPU  2  is transferred to the control portion  4  of the control portion  7 , which is for controlling the motor  3  for moving the heating unit  10 . Then, the control portion  4  determines whether or not it is necessary for the heating unit  10  of the fixing device  6  to be moved. 
         [0064]    If the sheet P of recording medium selected to be used for outputting an image is no less than 80 g/m 2  in basis weight, the motor control portion  4  does not activate the motor  3  as the means for moving the heating unit  10 , and causes the fixation film  11  and pressure roller  21  to press upon each other. Then, it rotationally moves both the fixation film  11  and pressure roller  21  at 300 mm/s of peripheral velocity (normal mode). 
         [0065]    On the other hand, if the sheet P of recording medium selected to be used for outputting an image is no more than 80 g/m 2 , the motor control portion  4  moves the heating unit  10  by activating the motor  3 , in order to change the position of the ridge  13 A of the pressure pad  13  relative to the fixation nip N. As the heating unit  10  is moved, the fixation film  11 , pressure pad supporting member  12 , and pressure pad  13  move together relative to the pressure roller  21  which makes up the pressure unit  20 . Then, the CPU  2  causes the fixation film  11  and pressure roller  21  to press upon each other, and begins to rotationally move both the fixation film  11  and pressure roller  21  at 300 mm/s of peripheral velocity (thin paper mode). 
         [0066]    As the heating unit  10  is moved, the information regarding the movement of the heating unit  10  is stored in the memory  5 , so that the motor control portion  4  can know the current state of the fixing device  6 . 
         [0067]    When the motor control portion  4  activates the motor  3  after it activated the motor  3  to move the heating unit  10  of the fixing device  6  from the default position of the heating unit  10 , it controls the motor  3  in such a manner that the heating unit  10  is moved back into the default position. That is, the motor control portion  4  rotates in reverse by reversing the voltage to be applied to the motor  3 . 
         [0068]    In this embodiment, the “normal mode (image heating first mode)” is for outputting such an image that is highly glossy and brilliant in color, with the use of a sheet P of recording medium which is no less in basis weight than a referential value (which in this embodiment is 80 g/m 2 ). The “thin paper mode (image heating second mode)” is for reliably delivering a fixed image from the fixing device  6  reliably, that is, without allowing a sheet of recording medium to wrap around the fixation film  11 , even when the sheet of recording medium is no more in basis weight than a preset value, being therefore low in rigidity. 
         [0069]      FIG. 5  is a flowchart of the operational sequence for moving the heating unit  10 . 
         [0070]    As an operator sets the type for the sheet P of recording medium to be used for outputting an image, with the use of the control panel  1  of the image forming apparatus  100  (S 101 ), the information regarding the selected type is transmitted to the CPU  2  of the control portion  7  (S 102 ). Then, based on this information, the CPU  2  determines whether or not the selected sheet P of recording medium is no more than 80 g/cm 2  in basis weight (S 103 ). If the CPU  2  determines that the selected sheet P of recording medium is no less than 80 g/m 2  in basis weight, it places a flag  0  in the memory  5  (S 110 ). In this case, the heating unit  10  is kept in the default position, which corresponds to the normal mode in which the ridge  13 A of the pressure pad  13  is kept within the fixation nip N in terms of the recording medium conveyance direction. Then, image formation is started (S 106 ), and the fixation process is carried out (S 107 ). 
         [0071]    On the other hand, if the CPU determines that the selected sheet P of recording medium is no more than 80 g/m 2  in basis weight in S 103 , it places a flag  1  in the memory  5  (S 104 ). Then, the motor control portion  4  moves the heating unit  10  by 1.0 mm in the recording medium conveyance direction by activating the motor  3  in response to the information from the CPU  2  (S 105 ). With this movement of the heating unit  10 , the ridge  13 A of the pressure pad  13  is moved out of the fixation nip in terms of the recording medium conveyance direction, and is placed in the thin paper position. Then, image formation is started (S 106 ), and then, the resultant unfixed toner images are fixed (S 107 ). 
         [0072]    Therefore, if the job (operational sequence started in response to single start signal to form image on two or more sheets of recording medium) has not been completed (S 108 ), the CPU  2  returns to S 106 , in which images are formed and fixed. On the other hand, if the job has been completed (S 108 ), the CPU  2  determines whether or not the heating unit  10  is in the thin paper mode position, with reference to the flag in the memory  5  (S 109 ). If the CPU  2  determines that the heating unit  10  is not in the thin paper mode position, it ends the image outputting operation. On the other hand, if it determines that the heating unit  10  is in the thin paper mode position, it sends this information to the motor control portion  4 . Then, the motor control portion  4  controls the motor  3  according to the information. Thus, the motor  4  moves the heating unit  10  by 1.0 mm in the opposite direction to the recording medium conveyance direction (S 111 ). Thus, the ridge  13 A of the pressure pad  13  of the heating unit  10  is moved back into the normal mode position, which is in the fixation nip N in terms of the recording medium conveyance direction. Then, the CPU  2  ends the image outputting operation. 
         [0073]    Normally, a sheet P of recording medium (such as sheet of ordinary paper) which is no less than 80 g/m 2  in basis weight is thicker and more rigid than a sheet P of recording medium which is no more than 80 g/m 2  in basis weight, and therefore, it is unlikely to jam the fixing device  6  by wrapping around the fixation film  11 . Therefore, in this embodiment, the “normal mode (image heating first mode)” may be deemed as a mode for outputting a highly glossy and highly brilliant color image with the use of a sheet P of recording medium, the thickness of which is the first thickness, whereas the “thin paper mode (image heating second mode) may be deemed as the mode for ensuring that even if a sheet P paper, film, or the like, the thickness of which is the second thickness which is less than the first thickness, is used as recording medium, a fixed image is delivered from the fixing device  6  without allowing the sheet P to jam the fixing device  6  by wrapping around the fixation film  11 . More specifically, if an operator selects the recording medium type with the use of the control panel  1  of the image forming apparatus  100 , the CPU  2  determines whether or not the thickness of the sheet P of the selected recording medium is the first thickness, with reference to the information in the memory  5  as the storage means of the control section  7 . The memory  5  stores the preset relationship between the recording medium type and recording medium thickness. If the CPU  2  determines that the thickness of the selected sheet P of recording medium is the first one, it controls the fixing device  6  in the same manner as it does when it determines that the sheet P of selected recording medium is no less than 80 g/m 2 , whereas if it determines that the thickness is the second one, it controls the fixing device  6  in the same manner as it does when it determines that the basis weight of the selected sheet P of recording medium is no more than 80 g/m 2 , as described above. 
         [0074]    As described above, the fixing device  6  is provided with the image heating first mode (normal mode) in which the ridge  13 A is kept in the range of the fixation nip N in terms of the recording medium conveyance direction, to heat the unfixed image formed on a sheet P of recording medium which is no less in basis weight than a referential value (or on recording medium having first thickness). It is also provided with the image heating second mode (thin paper mode), in which the ridge  13 A is positioned downstream side of the downstream end of the range of the fixation nip N in terms of the recording medium conveyance direction, to heat an image formed on the sheet P of recording medium, which is no more in basis weight than a referential value (or recording medium having second thickness which is less than first thickness). In this embodiment, the control portion  7  plays the role of making the image heating device  6  to operate in the image heating first mode or image heating second mode. 
         [0075]    Incidentally, in this embodiment, the movement of the heating unit  10  is controlled based on the basis weight (or thickness) of a sheet P of recording medium. However, this embodiment is not intended to limit the present invention in terms of the parameter based on which the movement of the heating unit  10  is controlled. For example, the movement of the heating unit  10  may be controlled based on a table which shows the relationship between the recording medium type and how easily a sheet of recording medium of each type wraps around the fixation film  11 , which is predetermined based on the correlation between the recording medium type, such as “ordinary”, “photographic”, “OHP film”, and “thin” and the recording medium properties, such as basis weight, thickness, resiliency, rigidity, etc. That is, all that is necessary for the present invention to be applicable is that recording mediums are classified based on how easily each recording medium wraps around the fixation film  11 . 
       4. Function of Ridge 
       [0076]    In this embodiment, the ridge  13 A of the pressure pad  13  is changed in position relative to the fixation nip N, based on the recording medium type. That is, it is placed within or outside the range of the fixation nip N in terms of the recording medium conveyance direction, based on the recording medium type. Further, the ridge  13 A of the pressure pad  13  has two functions. 
         [0077]      FIG. 6(   a ) is a schematic sectional view of the fixing device  6  in the normal mode, and  FIG. 6(   b ) shows the distribution of the internal pressure (placed on sheet P of recording medium) in the fixation nip N, and the distribution of the internal temperature (applied to sheet P of recording medium) of the fixation nip N, when the image forming apparatus  100  is in the normal mode.  FIG. 7(   a ) is a schematic sectional view of the fixing device  6  when the image forming apparatus  100  is in the thin paper mode, and  FIG. 7(   b ) shows the distribution of the pressure placed on the sheet P of recording medium in the fixation nip N, and the temperature distribution in the fixation nip N, when the image forming apparatus  100  is in the thin paper mode.  FIG. 8  shows the manner in which the sheet P of recording medium P is discharged from the fixation nip N when the image forming apparatus  100  is in the thin paper mode. 
         [0078]    Referring to  FIG. 6(   a ), one of the aforementioned functions of the ridge  13 A of the pressure pad  13  is for when the ridge  13 A of the pressure pad  13  is in the range of the fixation nip N in terms of the recording medium conveyance direction (normal mode). In this case, the pressure distribution of the fixation nip N in terms of the recording medium conveyance direction is as shown in  FIG. 6(   b ). That is, in this case, the peak of the pressure distribution is in the fixation nip N, and on the downstream side of the center of the fixation nip N in terms of the recording medium conveyance direction. Thus, a large amount of pressure is applied to the sheet P while the surface temperature of the sheet P is high. Therefore, the toner particles are efficiently spread, raising thereby the level at which the glossiness of the fixed image will be as the fixed image comes out of the fixing device  6 . In the normal mode, the highest amount of pressure to which the sheet P of recording medium is subjected in the fixation nip N is 0.4 Mpa, and the surface temperature of the sheet P of ordinary paper with a basis weight of 80 g/m 2 , is 100° C. 
         [0079]    Next, referring to  FIG. 7(   a ), the second function of the ridge  13 A of the pressure pad  13  is for when the ridge  13 A of the pressure pad  13  is outside the fixation nip N in terms of the recording medium conveyance direction (thin paper mode). In this case, the fixation film  11  is deformed downward (toward pressure roller  21 ) by the ridge  13 A of the pressure pad  13  in a pattern which reflects the cross section of the ridge  13 A. Therefore, as the sheet P of recording medium is conveyed out of the fixation nip N, it is slightly downwardly angled, being thereby facilitated in terms of its separation from the fixation film  11 . 
         [0080]    Incidentally, it is not that a glossy image cannot be outputted in the thin paper mode. That is, a sheet P of thin paper is relatively small in basis weight, being therefore, smaller in thermal capacity, than a sheet P of ordinary paper which is thicker, being therefore greater in basis weight, than thin paper. Therefore, the amount by which heat is robbed from the fixation nip N by the sheet P of thin paper is smaller than that by the sheet P of ordinary paper. Thus, when the sheet P of thin paper is conveyed through the fixation nip N, the temperature of the center portion of the fixation nip N is higher than when the sheet P of ordinary paper is conveyed through the fixation nip N. Thus, the toner particles on the sheet P of thin paper are as well spread in the thin paper mode as the toner particles on the sheet P of ordinary paper are in the normal mode. This is why it is not that a glossy image cannot be outputted in the thin paper mode. In the thin paper mode, the amount of pressure placed on the sheet P of recording medium at the peak of the pressure distribution of the fixation nip N was 0.3 Mpa, and the surface temperature of the sheet P of thin paper which is 64 g/m 2  in basis weight was 110° C. Also in the thin paper mode, the heating unit  10  (ridge  13 A of pressure pad  13 ) is positioned downstream by a distance D of 1.0 mm in terms of the recording medium conveyance direction, from the position in which the heating unit  10  (ridge  13 A of pressure pad  13 ) is positioned in the normal mode. 
         [0081]    The temperature in the fixation nip N can be measured by conveying a sheet P of recording medium with a pasted thermocouple (micro thin film thermocouple KFST-10-100-200: product of Anbesmt Co., Ltd.). As for the pressure distribution in the fixation nip N, it can be measured with the use of a tactile sensor (Sealer: product of Nitta Co., Ltd). 
         [0082]    Referring to  FIG. 6(   a ), in this embodiment, in the normal mode, the ridge  13 A of pressure pad  13  is kept in the range of the fixation nip N in terms of the recording medium conveyance direction. Thus, an image outputted in the normal mode is higher in glossiness. Also in this embodiment, in the thin paper mode, the heating unit  10  is positioned 1.0 mm downstream of the position in which the heating unit  10  is positioned in the normal mode, in terms of the recording medium conveyance direction, so that the ridge  13 A of the pressure pad  13  is positioned on the downstream side of the downstream end of the fixation nip N in terms of the recording medium conveyance direction, as shown in  FIG. 7(   a ). With this placement of the heating unit  10  (ridge  13 A), the path of the fixation film  11  dips downward (toward pressure roller  21 ) on the downstream side of the downstream end of the fixation nip N, in terms of the recording medium conveyance direction, causing thereby the sheet P of recording medium to be discharged from the fixing device  6  at a slightly downward angle. Thus, it is easier for the sheet P to separate from the fixation film  11 . 
       5. Movement of Heating Unit 
       [0083]    Next, the method for moving the heating unit  10  is described.  FIG. 9  is a schematic side view of the fixing device  6 , and  FIG. 10  is a schematic top plan view of the fixing device  6 . 
         [0084]    First, referring to  FIG. 9 , the fixation film  11  is rotatably held by a fixation film flange  14 , which has a gear  15  which is an integral part of the flange  14 . The fixation film flange  14  is held by a flange supporting first metallic plate  40 , which has a gear  41  (rack gear), which meshes with a gear  15  (pinion gear), which is an integral part of the fixation film flange  14 . Further, the pressure roller  21  is rotatably supported by a second metallic plate  50 . 
         [0085]    Next, referring to  FIG. 10 , the fixation film flange  14 , first metallic plate  40 , second metallic plate  50 , gear  15 , and motor  3  are at each of the lengthwise edges (which is roughly perpendicular to circular movement of fixation film  11 ). Those components which are at one of the lengthwise edges of the fixation film  11  are synchronous in movement with the counterparts which are at the other lengthwise edge. 
         [0086]    As the motor  3  is driven, the gear  15  is rotated by the rotation of the motor  3 , whereby the heating unit  10  is moved along the first metallic plate  40  in the direction parallel to the recording medium conveyance direction. Consequently, the ridge  13 A of the pressure pad  13  is moved out of the fixation nip N in terms of the recording medium conveyance direction. In this embodiment, as the thin paper mode is selected, the heating unit  10  is moved by 1.0 mm in the recording medium conveyance direction by the rotation of the motor  3 , and kept there. In this embodiment, the heating unit moving means of the image heating device  6 , which is for moving the ridge  13 A of the pressure pad  13  relative to the fixation nip N, comprises: the fixation film flange  14 , gear  15 , first metallic plate  40  having teeth  41  (rack gear), motor  3 , motor control portion  4 , etc. 
         [0087]      FIGS. 11(   a ) and  11 ( b ) show the timings with which the motor  3  and fixing device  6  are turned on or off during an image forming operation carried out in the normal and thin paper modes, respectively. 
         [0088]    As the motor  3  is turned on for the first time in an image outputting job, the heating unit  10  is moved by 1.0 mm in the recording medium conveyance direction by the rotation of the motor  3 . Then, as the motor  3  is turned on for the second time, the heating unit  10  is moved by 1.0 mm in the opposite direction to the recording medium conveyance direction by the rotation of the motor  3 . On the other hand, as long as the motor  3  is kept turned off, the heating unit  10  remains where it is. During the period in which the “image forming operation” is ON, a toner image is formed through the charging process, exposing process, developing process, first transferring process, and second transferring process. During the period in which the “image forming operation” is OFF, no image is formed on a sheet P of recording medium. Further, during the period in which the “fixing operation” is ON, the fixation film  11  and pressure roller  21  are kept pressed against each other, and the fixation film  11  is circularly moved at 300 mm/s of peripheral velocity, to process the toner (particles) on the sheet P of recording medium to fix the toner image to the sheet P. On the other hand, during the period in which the “fixing operation” is OFF, the fixation film  11  and pressure roller  21  are kept separated from each other, and the fixation film  11  is circularly moved at 100 mm/s of peripheral velocity. Therefore, no image is fixed. A “post rotation period” means the period in which the image forming apparatus  100  (fixing device  6 ) is adjusted or prepared to end the on-going image forming operation. It is during this period that the operation for putting the heating unit  10  back into its default position (normal mode position) is carried out. 
         [0089]    Referring to  FIG. 11(   a ), in the normal mode, the heating unit  10  is positioned so that the ridge  13 A of the pressure pad  13  of the fixing device  6  is positioned in the downstream end portion of the fixation nip N in terms of the recording medium conveyance direction. This state of the fixing device  6  is the default state of the fixing device  6 . That is, in the normal mode, the motor  3  is not turned on. In other words, in the normal mode, images are formed and fixed without turning on the motor  3 . 
         [0090]    Next, referring to  FIG. 11(   b ), in the thin paper mode, first the motor  3  is turned on, whereby the heating unit  10  is moved by 1.0 mm in the recording medium conveyance direction. Then, images are formed and fixed. Then, the motor  3  is again turned on, whereby the heating unit  10  is moved by 1.0 mm in the opposite direction to the recording medium conveyance direction. 
         [0091]    Incidentally, all that is necessary is for the movement of the heating unit  10  to be completed before an image begins to be fixed. That is, the period in which the “image forming operation” is ON may overlap with the period in which the motor  3  is ON. 
         [0092]    In this embodiment, the target temperature level for the fixation film  11  in the normal mode, that is, the level at which the temperature of the fixation film  11  is kept in the normal mode, is set to 180° C., whereas that in the thin paper mode is set to 165° C. Although in this embodiment, the level at which the temperature of the fixation film  11  is kept in the normal mode is different from that in the thin paper mode, both modes may be the same in the target temperature level for the fixation film  11 . Further, in this embodiment, the dimension (width) of the fixation nip N in terms of the recording medium conveyance direction is set to 10 mm regardless of whether the image forming apparatus  100  (fixing device  6 ) is in the normal or thin paper mode. 
         [0093]    Also in this embodiment, in the normal mode, the peak of the pressure distribution of the fixation nip N is on the downstream side of the center of the fixation nip N in terms of the recording medium conveyance direction. Thus, the image forming apparatus  100  (fixing device  6 ) in this embodiment is superior to any of the conventional image forming apparatuses (fixing devices  6 ), in terms of the glossiness level at which an image is outputted. Let&#39;s assume, for comparison, that a sheet P of recording medium which is greater in basis weight (no less than 80 g/m 2 ) than a sheet of ordinary paper is used, and the target temperature level for the fixation film  11  is adjusted without adjusting the fixing device  6  in the pressure distribution of the fixation nip N to change the position of the peak, in order to ensure that images to be outputted will be as glossy as the images outputted on a sheet P of ordinary paper. In this case, it is possible that the toner particles on the fibers of the sheet P of recording medium excessively melt, and therefore, “hot offset”, that is, a problematic phenomenon that the toner particles on the sheet P of recording medium transfer onto the fixation film  11 , will occur. 
         6 . Modification of First Embodiment 
       [0094]    The shape of the pressure pad  13  does not need to be limited to the one in this embodiment. That is, it may be shaped so that its cross section looks as shown in  FIGS. 12 and 13 . 
         [0095]    The pressure pad  13  shown in  FIG. 12  has a ridge  13 A which is on the surface of the base  13 B of the pressure pad  13 , which faces the fixation film  11 . The ridge  13 A is shaped so that its peak is at the downstream end of the base  13 B in terms of the recording medium conveyance direction. That is, the downstream surface of the ridge  13 A is perpendicular to the base  13 B, whereas the upstream surface of the ridge  13 A gently declines from its downstream end toward its upstream end. Further, the shape of the ridge  13 A is such that in terms of cross section, its upstream surface has a curvature, which is equal to the curvature of a circle which is 17.5 mm in radius. That is, the ridge  13 A is zero in height at its upstream end, and very gradually increases in height toward the downstream end, being highest at the downstream end. Further, the ridge  13 A faces downward. In the case of the pressure pad  13  shown in  FIG. 12 , the height of the tip of its ridge  13 A from the base  13 B is 0.75 mm. Further, the dimension (width) of the pressure pad  13  in terms of the recording medium conveyance direction is 10 mm. 
         [0096]    The pressure pad  13  shown in  FIG. 13  has a ridge  13 A, which is rectangular in cross section. The ridge  13 A is on the surface of the base  13 B, on which the fixation film  11  slides. The ridge  13 A is positioned so that it is located 3.5 mm downstream from the center of the base  13 B. It is 0.5 mm in height and 1.0 mm in width (dimension in terms of recording medium conveyance direction). 
         [0097]    Further, in the first embodiment, the heating unit  10  is moved to move the ridge  13 A of the pressure pad  13  into the fixation nip N. However, the first embodiment is not intended to limit the present invention regarding the movement of the ridge  13 A. For example, it may be the pressure roller  21  that is moved to move the ridge  13 A of the pressure pad  13  into, or out of, the fixation nip N. 
         [0098]    Also in this embodiment, the type of the recording medium to be used for outputting an image is selected by an operator with the use of the control panel  1  of the image forming apparatus  100 . However, this embodiment is not intended to limit the present invention in terms of the method for setting the recording medium type. For example, the image forming apparatus  100  may be provided with an automatic recording medium type detecting means so that the recording medium type is automatically determined by detecting the thickness, surface properties, basis weight, and the like parameters of a sheet P of recording medium, with the use of the sensors with which the image forming apparatus  100  is provided. 
         [0099]    As described above, in this embodiment, the ridge  13 A of the pressure pad  13  is switched in the position relative to the fixation nip N according to the type of recording medium. Thus, it is possible to make the fixing device  6  to satisfactorily separate a sheet P of recording medium from the fixation film  11  even if the sheet P of recording medium is such a sheet of recording medium that tends to wrap around the fixation film  11 , without sacrificing the function of outputting a highly glossy image. That is, as is evident from the description of this embodiment given above, the present invention can provide a fixing device  6  capable of separating even a sheet P of recording medium, which is low in rigidity, from the fixation film  11 , without sacrificing glossiness. 
       Embodiment 2  
       [0100]    Next, the second preferred embodiment of the present invention is described. The image forming apparatus in this embodiment is the same in basic structure and operation as the image forming apparatus in the first preferred embodiment. Therefore, the components of the image forming apparatus in this embodiment, which are the same in function and structure as, or equivalent in function and structure to, the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, one for one, and are not described in detail. 
         [0101]    In the first embodiment, it was assumed that the sheets P of recording medium which are being used for an image forming job is not replaced with sheets P of recording medium of a different type during the same image forming job. In comparison, in this embodiment, however, the sheets P of recording medium which are being used for an image forming job are replaced with sheets P of recording medium of a different type during the same image forming job. 
         [0102]    Also in this embodiment, the “normal mode (image heating first mode)” is for outputting such an image that is highly glossy and brilliant in color, with the use of a sheet P of recording medium which is no less in basis weight than 80 g/m 2 , as it was in the first embodiment. Further, the “thin paper mode (image heating second mode)” is for delivering a fixed image from the fixing device  6  reliably, that is, while preventing the sheet P of recording medium from jamming the fixing device  6  by wrapping around the fixation film  11  even when a sheet P of thin paper, film, etc., which is no more in basis weight than 80 g/m 2 , being therefore very low in rigidity, is used as recording medium. In other words, the “normal mode (image heating first mode)” may be deemed as a mode in which a highly glossy and highly brilliant color image can be outputted with the use of a sheet P of recording medium, the thickness of which is the first thickness, whereas the “thin paper mode (image heating second mode)” may be deemed as the mode in which even if a sheet P recording medium, the thickness of which is the second thickness which is less than the first thickness, is used for image formation, a fixed image is delivered from the fixing device  6  reliably, that is, without causing the sheet P to jam the fixing device  6  by wrapping around the fixation film  11 . 
         [0103]    Referring to  FIG. 1 , in this embodiment, the first cassette  109   a  stores sheets P of ordinary paper, and the second cassette  109   b  stores sheets P of thin paper. The relationship between the cassette number and the type of the sheet P of recording medium in the cassette is registered in the memory  5  of the control portion  7  of the image forming apparatus  100  through the control panel  1  of the image forming apparatus  100 . 
         [0104]      FIG. 14  is a flowchart of the operational sequence, in this embodiment, for moving the heating unit  10 . 
         [0105]    As an operator sets the image forming apparatus  100  through the control panel  1  of the image forming apparatus  100  so that the image forming apparatus  100  automatically selects proper recording medium (S 201 ), the information regarding the recording medium in the first cassette  109 , which is the first to be used, is transmitted to the CPU  2  of the control portion  7  (S 202 ). Then, based on this information and the information stored in advance in the memory  5 , the CPU  2  determines whether or not the recording medium to be used for a given image formation job has to be no less than 80 g/m 2  in basis weight (S 203 ). If the CPU  2  determines in S 203  that the recording medium P is no less than 80 g/m 2  in basis weight, it places a flag  0  in the memory  5  (S 211 ). At this stage in the operation, the position of the heating unit  10  is the default position, that is, such a position that the ridge  13 A of the pressure pad  13  is within the range of the fixation nip N in terms of the recording medium conveyance direction. Then, the image forming operation is started (S 206 ), and the image fixing process is carried out (S 207 ). 
         [0106]    On the other hand, if the CPU  2  determines that the recording medium P is no more than 80 g/m 2  in basis weight, it places a flag  1  in the memory  5  (S 204 ). Then, it causes the motor control portion  4  to control the motor  3  according to the information from the CPU  2  so that the heating unit  10  is moved by 1.0 mm in the recording medium conveyance direction (S 205 ). In other words, the heating unit  10  is moved to the thin paper mode position, in which the ridge  13 A of the pressure pad  13  of the heating unit  10  is on downstream side of the downstream end of the fixation nip N in terms of the recording medium conveyance direction. Then, the image forming operation is started (S 206 ), and the image fixing process is carried out (S 207 ). 
         [0107]    During the image forming operation, the CPU  2  checks whether or not the first cassette  109   a  has run out of a sheet of recording medium P, that is, whether or not the recording medium delivery is to be switched from the first cassette  109   a  to the second cassette  109   b  (S 208 ). If it determines that recording medium delivery does not need to be switched from the first cassette  109   a  to the second cassette  109   b,  it checks whether or not the current job has been completed (S 209 ). On the other hand, if the CPU  2  determines that the recording medium delivery has to be switched from the first cassette  109   a  to the second cassette  109   b,  it switches the recording medium delivery from the first cassette  109   a  to the second cassette  109   b,  and returns to S 203 , in which it again determines the type of the recording medium P to start the subsequent control sequence. 
         [0108]    If the CPU  2  determines in S 209  that the current job has not been completed, it returns to S 206 , in which it makes the image forming apparatus  100  form images. Then, it makes the fixing device  6  fix images (S 207 ). On the other hand, if the CPU  2  determines in S 209  that the current job has been completed, it determines, with reference to the flag in the memory  5 , whether or not the heating unit  10  is in the thin paper mode position (S 210 ). If it determines that the heating unit  10  is not in the thin paper position, it ends the image outputting operation. On the other hand, if it determines that the heating unit  10  is in the thin paper mode position, it sends the information regarding the position of the heating unit  10  to the motor control portion  4 , causing thereby the motor control portion  4  to control the motor  3  according to the information. Thus, the heating unit  10  is moved by the motor  3  in the direction opposite to the recording medium conveyance direction by 1.0 mm (S 212 ). In other words, the heating unit  10  is returned to the normal mode position, in which the ridge  13 A of the pressure pad  13  of the heating unit  10  is within the range of the fixation nip N in terms of the recording medium conveyance direction. Then, the CPU  2  ends the on-going image outputting operation. 
         [0109]    Referring to  FIG. 6(   a ), in this embodiment, in the normal mode, the fixing device  6  is in the state in which the ridge  13 A of the pressure pad  13  is within the fixation nip N in terms of the recording medium conveyance direction. Therefore, the image forming apparatus  100  can output an image at a higher level of glossiness than when the fixing device  6  is in the state in which the ridge  13 A is out of the fixation nip N. Next, referring to  FIG. 7(   a ), also in this embodiment, in the thin paper mode, the fixing device  6  is in the state in which the heating unit  10  has been moved downstream by 1.0 mm in the recording medium conveyance direction, and therefore, the ridge  13 A of the pressure pad  13  is on the downstream side of the downstream end of the fixation nip N. Thus, the fixation film path sharply dips (bend toward pressure roller  21 ) on the downstream side of the downstream end of the fixation nip N, causing thereby the sheet of recording medium P to be discharged at a downward angle (toward pressure roller  21 ). In other words, in the thin paper mode, the sheet of recording medium P is better facilitated to separate from the fixation film  11  than in the normal mode. 
         [0110]      FIG. 15(   a ) is a timing chart of an image forming operation in which the image forming apparatus  100  (fixing device  6 ) is switched in operational mode from the normal mode to the thin paper mode during an image forming operation, whereas  FIG. 15(   b ) is a timing chart of an image forming operation in which the image forming apparatus  100  (fixing device  6 ) is switched in operational mode from the thin paper mode to the normal mode during an image forming operation.  FIGS. 15(   a ) and  15 ( b ) show the timing with which the motor  3 , image forming stations, and fixing device are turned on and off. The timing with which the abovementioned components of the image forming apparatus  100  in this embodiment are turned on or off in an operation in which switching is done between the two cassette  109   a  and  109   b  is the same as the timing with which the abovementioned components of the image forming apparatus  100  in the first embodiment are turned on or off, as shown in  FIGS. 11(   a ) and  11 ( b ). 
         [0111]    In the first period in which the motor  3  is ON, the heating unit  10  is moved by 1.0 mm in the recording medium conveyance direction. In the second period in which the motor is ON, the heating unit  10  is moved by 1.0 mm in the opposite direction to the recording medium conveyance direction. On the other hand, in the periods in which the motor  3  is OFF, the heating unit  10  is not moved. In the periods in which the image forming stations are ON, an image is being formed on a sheet of recording medium P through the charging, exposing, developing, first transferring, and second transferring processes. In the periods in which the image forming stations are OFF, no toner image is being formed on a sheet of recording medium P. In the periods in which fixing device  6  is ON, the toner (toner image) on a sheet of recording medium P is being processed (fixed) by keeping the fixation film  11  and pressure roller  21  pressed upon each other, and circularly moving the fixation film  11  at 300 mm/s of peripheral velocity. On the other hand, in the periods in which the fixing device  6  is OFF, the fixation film  11  and pressure roller  21  are kept separated from each other, and the fixation film  11  is circularly moved at 100 mm/s of peripheral velocity. In other words, no image is being processed (fixed) by the fixing device  6 . A “post-rotation period” is a period in which adjustments or preparations are made to end the on-going image formation operation. It is during the post-rotation period that the operation for moving the heating unit  10  back into the default position (normal mode position) is carried out. 
         [0112]    In this embodiment, the target temperature for the fixation film  11  in the normal mode is set to 180° C., and the target temperature for the fixation film  11  in the thin paper mode is set to 165° C. In this embodiment, the normal mode and thin paper mode are made different in the target temperature for the fixation film  11 . However, they do not need to be made different in the target temperature for the fixation film  11 . Further, the dimension (width) of the fixation nip N in terms of the recording medium conveyance direction is set to 10 mm regardless of whether the image forming apparatus  100  (fixing device  6 ) is in the normal mode or thin paper mode. 
         [0113]    Next, referring to  FIG. 15(   a ), the operational sequence for switching from the normal mode to the thin paper mode in the midst of an image formation job is described. Since the image forming apparatus  100  is in the normal mode, the heating unit  10  is in the default position, in which the ridge  13 A of the pressure pad  13  of the heating unit  10  is in the range of the fixation nip N in terms of the recording medium conveyance direction. In the normal mode, an image is fixed while the heating unit  10  is in the default position. Therefore, the motor  3  is not turned on to move the heating unit  10 . That is, in the normal mode, an image is formed while the fixing device  6  is in the above-described state. Thereafter, if the cassette  109   a  for the ordinary paper, that is, the cassette  109  from which sheets of recording medium P has been fed, becomes empty, the cassette  109  from which recording medium P is to be fed is switched to the cassette  109   b  for thin recording medium. Thus, as the cassette  109   a  becomes empty, an adjustment period is provided, during which the motor  3  is activated to move the heating unit  10  by 1.0 mm in the recording medium conveyance direction, and also, the target temperature for the fixation film  11  is reduced from 180° C. to 165° C. Then, the interrupted image forming operation is restarted, and the image fixing process is carried out. As soon as the on-going image formation job is completed, the motor  3  is activated again to return the heating unit  10  to the default position. 
         [0114]    Next, referring to  FIG. 15(   b ), the operational sequence for switching from the thin paper mode to the normal mode in the midst of an image formation job is described. At the beginning of a given image formation job, the heating unit  10  is in its default position, in which the ridge  13 A of the pressure pad  13  of the heating unit  10  is in the fixation nip N in terms of the recording medium conveyance direction. In the thin paper mode, however, the ridge  13 A of the pressure pad  13  has to be outside the fixation nip N, and on the downstream side of the downstream end of the fixation nip N in terms of the recording medium conveyance direction. Therefore, as the operational mode of the image forming apparatus  100  is switched from the normal mode to the thin paper mode, the motor  3  is activated to move the heating unit  10  by 1.0 mm in the recording medium conveyance direction. Then, the image forming processes are carried out, and then, the image fixing process is carried out. In such a situation that the cassette  109   b  from which sheets of thin recording medium P have been fed becomes empty, and therefore, the cassette  109  from which sheets of recording medium P is to be fed has to be switched from the cassette  109   b  to the cassette  109   a  which stores sheets of ordinary paper, an adjustment period is provided as soon as the cassette  109   b  becomes empty. In the adjustment period, the motor  3  is activated to move the heating unit  10  in the opposite direction to the recording medium conveyance direction by 1.0 mm. In addition, the target temperature for the fixation film  11  is raised from 165° C. to 180° C. Then, the interrupted image forming job is restarted, and then, the image fixing process is carried out. Thus, at the end of the job, the heating unit  10  is in its default position, and therefore, the motor  3  is not activated. 
         [0115]    As described above, in this embodiment, if recording medium is switched from one type to another in the midst of an image formation job, the fixing device  6  is switched in the position of the ridge  13 A of the pressure pad  13  according to recording medium type. Therefore, even if recording medium is switched from one type to another in the midst of an image formation job, the image forming apparatus  100  (fixing device  6 ) can output a glossy image while preventing recording medium from failing to properly separate from the fixation film  11 . 
       Embodiment 3  
       [0116]    Next, another preferred embodiment of the present invention is described. The image forming apparatus in this embodiment is the same in basic structure and operation as the image forming apparatus in the first preferred embodiment. Therefore, the components of the image forming apparatus in this embodiment, which are the same in function and structure as, or equivalent in function and structure to, the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, one for one, and are not described in detail. 
         [0117]    The first and second embodiments of the present invention were described with reference to a case in which the amount by which toner is transferred (deposited) on recording medium P to form a monochromatic image is relative large. In this embodiment, the present invention is described with reference to a case in which the amount by which toner is transferred onto a sheet of recording medium P to form a monochromatic image on the sheet is relatively small. 
         [0118]    In recent years, concern regarding environment has been increasing, and also, consumers are demanding further reduction in the cost of an image forming apparatus. Thus, the technologies for reducing an image forming apparatus in toner consumption have become very important. The technologies for reducing an image forming apparatus in toner consumption have come to play an important role from the standpoint of reducing an image forming apparatus in the amount of energy used to fix toner to recording medium. 
         [0119]    One of the methods which can be used to reduce an image forming apparatus in toner consumption is to increase the filler of toner in terms of its ratio to coloring agent of toner, in order to reduce an image forming apparatus in the overall amount of toner consumption per sheet of recording medium. This method, however, is problematic for the following reason. That is, reducing the amount by which toner is adhered to recording medium per unit area of recording medium means reducing the amount by which toner is adhered (transferred) onto a sheet of recording medium per unit area to form a solid monochromatic. Thus, if the amount by which toner is adhered to recording medium is reduced, it is possible that spaces may remain among the toner particles which make up the solid monochromatic image. Thus, the image bearing area of the surface of recording medium may fail to be fully covered with toner while the image is fixed, because the surface of recording medium is microscopically irregular in texture. 
         [0120]    In particular, if a sheet of recording medium P used for image formation is greater in irregularity in terms of texture and thermal capacity, toner is unlikely to fully melt. Therefore, the image forming apparatus  100  (fixing device  6 ) is likely to output an image which is low in reflection density. 
         [0121]    At this time, the amount by which toner is adhered to recording medium to form a monochromatic solid image is described. It is assumed here that a monochromatic solid image is formed under the condition in which toner particles idealistically align as they are transferred onto recording medium. 
         [0122]      FIG. 16  is a list which shows the parameters of toner, which are related to the idealistic toner particle alignment. Letters L [μm] and V [μm3] stand for the average particle size (diameter) of the toner and the average particle volume of the toner, respectively. A letter S [μm3] stands for the average projected area of the toner particles. Further, a letter Sb [μm2] stands for the average size of the recording medium surface per toner particle. There are following mathematical relationships among the abovementioned parameters. 
         [0000]    
       
         
           
             V 
             = 
             
               
                 4 
                 3 
               
                
               
                 
                   π 
                    
                   
                     ( 
                     
                       L 
                       2 
                     
                     ) 
                   
                 
                 3 
               
             
           
         
       
       
         
           
             Sa 
             = 
             
               
                 π 
                  
                 
                   ( 
                   
                     L 
                     2 
                   
                   ) 
                 
               
               2 
             
           
         
       
       
         
           
             Sb 
             = 
             
               
                 
                   3 
                 
                 2 
               
                
               
                 L 
                 2 
               
             
           
         
       
     
         [0123]    When a monochromatic toner image is formed on a sheet of recording medium under the condition in which toner particles align with the presence of virtually no gap between adjacent two toner particles, the amount H [μm] (volume per unit area=average height of toner layer) of toner on a portion of recording medium which corresponds to an actual image, which is formed of a single layer of toner particles, can be obtained from Mathematical Formula 4. 
         [0000]    
       
         
           
             H 
             = 
             
               
                 V 
                 Sb 
               
               = 
               
                 
                   
                     4 
                     3 
                   
                    
                   
                     
                       
                         π 
                          
                         
                           ( 
                           
                             L 
                             2 
                           
                           ) 
                         
                       
                       3 
                     
                     · 
                     
                       2 
                       
                         
                           3 
                         
                          
                         
                           L 
                           2 
                         
                       
                     
                   
                 
                 = 
                 
                   
                     π 
                      
                     
                         
                     
                      
                     L 
                   
                   
                     3 
                      
                     
                       3 
                     
                   
                 
               
             
           
         
       
     
         [0124]    In the case of Mathematical Formula 4 given above, in consideration of the state of toner particle alignment, it is assumed that “toner volume [μm] per unit area” equals “average height”. Normally, however, the weight per unit area [mg/m 2 ] of toner is used to control the amount by which toner is transferred onto recording medium. Therefore, Formula 4 which is for calculating the amount of toner on a portion of recording medium which corresponds to an actual image, which is formed of a single layer of toner particles, when the toner particle alignment is idealistic (toner particles are truly spherical and align with virtually no gap between adjacent to toner particles), is converted into the following Mathematical Formula (1) as the formula for obtaining the amount A [mg/m 2 ] of toner per unit area of the actual image portion of a monochromatic image on a sheet of recording medium. Incidentally, a term 1/10 in the following Mathematical Formula (1) is for the measurement unit normalization. 
         [0000]    
       
         
           
             
               
                 
                   A 
                   = 
                   
                     
                       ρ 
                       × 
                       H 
                     
                     = 
                     
                       
                         ρ 
                         × 
                         
                           1 
                           10 
                         
                         × 
                         
                           
                             π 
                              
                             
                                 
                             
                              
                             
                               L 
                               3 
                             
                           
                           
                             3 
                              
                             
                               3 
                             
                              
                             
                               L 
                               2 
                             
                           
                         
                       
                       = 
                       
                         
                           ρπ 
                            
                           
                               
                           
                            
                           L 
                         
                         
                           30 
                            
                           
                             3 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0125]    If the amount by which toner is transferred onto a sheet of recording medium per unit area (amount of toner, per unit area, of solid area of monochromatic image) to form a monochromatic image on the sheet of recording medium is less than the value calculated by Formula (1) given above, that is, if it satisfies Mathematic Formula (inequity) (2) given below, the resultant monochromatic image will be insufficient in reflection density. 
         [0000]        A&lt;ρ π L /(30√{square root over ( )}3)
 
         [0126]    This embodiment is described with reference to a case in which the amount by which toner is transferred onto a sheet of recording medium, per unit area of the sheet of recording medium, is less than the value calculated with the use of Mathematical Formula (1) given above. More concretely, the toner used in this embodiment is 5.5 μm in average particle diameter, and 1.1 g/cm 3  in specific gravity. The amount by which toner is transferred onto a sheet of recording medium to form a solid monochromatic portion of the image is 0.3 mg/m 2 . The maximum amount by which toner is transferred onto a sheet of recording medium is 0.6 mg/m 2 . 
         [0127]    As described above, if the amount by which toner is transferred onto a sheet of recording medium (paper in particular) to form a solid portion of the monochromatic image of the primary color is insufficient, the toner delivered to this portion of the sheet of recording medium fails to completely cover the surface of each of the fibers of which the sheet of recording medium is made of. Thus, the resultant image is insufficient in reflection density. Thus, in order to output an image which is satisfactory (sufficient) in reflection density, it is desired that each toner particle is spread wider by the fixing device  6  than if the aforementioned amount is sufficient. 
         [0128]    In this embodiment, therefore, in order to spread each toner particle wider, the fixing device  6  is provided with a pressure pad  13 , the cross sectional shape of which is shown in  FIG. 17 . The pressure pad  13  used in this embodiment is roughly the same in structure as that used in the first embodiment. That is, it has a base  13 B and a ridge  13 A. The ridge  13 A is on the surface  13 B 1  of the base  13 B, on which the fixation film  11  slides. 
         [0129]    To describe in more detail, in this embodiment, the dimension (width) of the pressure pad  13  in terms of the recording medium conveyance direction is 10 mm. The ridge  13 A is triangular in cross section. The dimension (width) of the ridge  13 A in terms of the recording medium conveyance direction is 1.8 mm. The downstream edge of the ridge  13 A coincides with the downstream edge of the base  13 B, whereas the upstream edge of the ridge  13 A is located 3.2 mm downstream from the center of the pressure pad  13 . Further, the ridge  13 A is 0.9 mm in height. That is, in this embodiment, the tip of the ridge  13 A in terms of the triangular cross section of the ridge  13 A is located 4.1 mm downstream from the center of the pressure pad  13 . Further, the apex angle α of the ridge  13 A is 90 degrees, and the angle β between the two lateral surfaces of the ridge  13 A is 45 degrees. 
         [0130]    In this embodiment, the ridge  13 A is made taller than in the first and second embodiments. Therefore, the fixation nip N in this embodiment is greater in internal pressure than those in the first and second embodiments, and therefore, the fixing device  6  in this embodiment can spread a toner particle wider, being therefore higher in the level at which the reflection density of the fixed image will be, than the fixing devices  6  in the first and second embodiments. 
         [0131]    Shown in Table 1 are the amount of the maximum pressure in the fixation nip N, that is, the pressure between the tip (peak) of the ridge  13 A of the pressure pad  13 , which is 0.9 mm in height, and fixation film  11 , in this embodiment, and the maximum internal pressure of the fixation nip N, that is, the pressure between the tip (peak) of the ridge  13 A of the pressure pad  13 , which is 0.75 mm in height, and the fixation film  11 , in the first embodiment. Table 1 shows also the reflection density of the solid portion of the image fixed with the use of the pressure pad  13  in this embodiment, and the reflection density of the solid portion of the image fixed with the use of the pressure pad  13  in the first embodiment. 
         [0132]    The pressure distribution was measured with the use of a tactile sensor (Sealer: product of Nitta Co., Ltd.). As for the reflection density, it was measured with the use of a spectral densitometer  503  (product of X-Rite Co., Ltd.). 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Ridge height 
                 Peak 
                 Reflection 
               
               
                 (mm) 
                 pressure (Mpa) 
                 density 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.9 
                 0.5 
                 1.45 
               
               
                 0.75 
                 0.4 
                 1.32 
               
               
                   
               
             
          
         
       
     
         [0133]    According to Table 1, the fixing device  6 , in this embodiment, which used the pressure pad  13 , the height of the ridge  13 A of which was 0.9 mm, was higher in the maximum internal pressure of the fixation nip N, that is, the pressure between the tip (peak) of the ridge  13 A and the fixation film  11  than the fixation device  6  in the preceding embodiments. Thus, the fixing device  6  in this embodiment can spread wider each toner particle than the fixing devices  6  in the preceding embodiments, and therefore, the fixing device  6  is compensated in glossiness, for the insufficiency in the amount by which toner is transferred onto a sheet of recording medium. 
         [0134]    Also in this embodiment, in a case where a sheet of recording medium P used for outputting an image is a sheet of ordinary paper or the like (which is no less than 80 g/m 2  in basis weight; first thickness), the heating unit  10  is positioned as follows: That is, the heating unit  10  is positioned so that the ridge  13 A of its pressure pad  13  is positioned in the downstream end portion of the fixation nip N in terms of the recording medium conveyance direction. In this embodiment, therefore, a larger amount of pressure than the amount of pressure applied to the toner particles on a sheet of recording medium in the fixation nip N in the preceding embodiment is applied to the toner particles on the sheet of recording medium, in the area of fixation nip N, which is higher in temperature. Therefore, the toner particles are spread wider, being enable to satisfactorily hide (cover) each of the fibers of which the recording medium P is made. 
         [0135]    On the other hand, in a case where the recording medium P used for image formation is a sheet of thin paper or the like (which is no more than 80 g/m 2  in basis weight, or the thickness of which is the second thickness which is less than the first thickness), the recording medium P is smaller in thermal capacity, and therefore, the toner particles on the recording medium P quickly increase in temperature. Therefore, even if the amount of the pressure applied to the toner particles in the downstream end portion of the fixation nip N is no larger than the normal one, the toner particles are spread wider anyway. In this case, the heating unit  10  is positioned so that the ridge  13 A of its pressure pad  13  is positioned on the downstream side of the downstream end of the fixation nip N in terms of the recording medium conveyance direction. This placement of the heating unit  10  improves the fixing device  6  in terms of the recording medium separation from the fixation film  11 . 
         [0136]    As for the method for changing the fixing device  6  in terms of the position of the ridge  13 A of the pressure pad  13  relative to the fixation nip N, the method similar to those used in the first and second embodiments can be used. 
         [0137]    As described above, according to this embodiment, even if an image forming apparatus ( 100 ) is designed to be smaller in the amount by which toner is adhered to a sheet of recording medium to form a monochromatic solid portion of an image than a conventional image forming apparatus, the image forming apparatus can be made to output a desirably glossy image without sacrificing the separation of recording medium from the fixation film, by structuring the image forming apparatus so that the ridge  13 A of the pressure pad  13  of the heating unit  10  of its fixing device can be switched in position relative to the fixation nip N of the fixing device  6  according to recording medium type. 
       Miscellaneous Embodiments  
       [0138]    The present invention has been described with reference to the preferred embodiments of the present invention. However, the preferred embodiments are not intended to limit the present invention in scope. 
         [0139]    For example, in each of the preferred embodiments of the present invention described above, the image heating device was provided with two rotationally movable members which are kept pressed upon each other. Further, one of the rotationally movable members was an endless film and the other was a roller. However, this setup is not intended to limit the present invention in scope. For example, the present invention is also applicable to an image heating device which employs a pair of endless belts suspended and stretched by multiple rollers, and which is structured so that the pair of belts are kept pressed upon each other by the rollers. 
         [0140]    Also in each of the preferred embodiments described above, the image forming apparatus  100  had only one image heating-and-pressing device. However, the present invention is also applicable to an image forming apparatus having multiple image heating-and-pressing devices, for example, two image heating-and-pressing devices. In the case of an image forming apparatus having two image heating-and-pressing devices, it is possible to use one of them as an ordinary fixing device, and the other as a glossiness enhancement device. In such a case, the present invention is applicable to each of the image heating-and-pressing devices. The resultant effects of such application are the same as those obtained by the image heating devices in the preceding embodiments. 
         [0141]    Also in each of the preferred embodiments described above, the image forming apparatuses had four image forming stations. However, this setup is not intended to limit the present invention in terms of the number of image forming stations with which an image forming apparatus is provided. That is, the applicability of the present invention to an image heating device has nothing to do with the number of the image forming stations of the image forming apparatus. 
         [0142]    Not only is the present invention applicable to an image forming apparatus such as those in the preceding preferred embodiments of the present invention, but also a printer, a copying machine, a facsimile machine, etc., and a multifunction image forming apparatus capable of performing two or more functions of the preceding image forming apparatuses. 
         [0143]    The measurements, materials, and shapes of the structural components of the image forming apparatus (image heating device), and the positional relationship among the structural components, in each of the preferred embodiments of the present invention described above, are not intended to limit the present invention in these attributes, unless specifically noted. That is, the present invention is applicable to image forming apparatuses which are different from those in the preferred embodiments, in terms of these attributes. 
         [0144]    As will be evident from the description of the preferred embodiments of the present invention given above, the present invention can provide an image heating device which is superior to any of the image heating devices in accordance with the prior art, not only in that its heating performance is high enough to reliably yield a highly glossy image, but also, in that it can reliably separate a sheet of recording medium from the image heating rotational members of the image heating device, even if the sheet of recording medium is such a sheet of recording medium that is likely to remain wrapped around the rotational heating members of the image heating device. 
         [0145]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0146]    This application claims priority from Japanese Patent Application No. 136540/2010 filed Jun. 15, 2010 which is hereby incorporated by reference.