Patent Publication Number: US-8532529-B2

Title: Fixing device and image forming apparatus using the same

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2010-023234 filed in Japan on 4 Feb. 2010, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device used for an image forming apparatus such as a laser beam printer, a multi-functional peripheral or the like and an image forming apparatus using the same, and particularly, relates to a fixing device including fixing roller that fuses an unfixed toner to fix to a recording medium and an image forming apparatus using the same. 
     2. Description of the Prior Art 
     Conventionally, as a fixing device used for an electrophotographic image forming apparatus such as a multi-functional peripheral, a printer or the like, a fixing device employing a so-called heat-roller fixing type is widely used. 
     The fixing device employing the heat-roller fixing type includes a pair of rollers (a fixing roller and a pressing roller) in press-contact with each other, and after the pair of the rollers is heated to a predetermined temperature (fixing temperature) by a heating portion composed of a halogen heater and the like arranged inside both rollers of the pair of the rollers or either one of the rollers, a recording medium such as a recording sheet on which an unfixed toner image is formed is fed to a press-contact portion (fixing nip portion) of the pair of the rollers to pass through the press-contact portion so that the toner image is fixed on the recording sheet by heat and pressure. 
     Incidentally, in the fixing device provided in a color image forming apparatus, an elastic roller provided with an elastic layer made of silicone rubber and the like is generally used on a fixing roller surface layer in contact with an unfixed toner image. 
     The elastic roller is used for the fixing roller, and the fixing roller surface is thereby elastically deformed corresponding to unevenness of the unfixed toner image to come in contact with the surface of the toner image so as to cover, so that it becomes possible to perform favorable heating and fixation for an unfixed color toner image having a larger amount of the toner compared to that of an unfixed monochrome toner image. 
     Further, the color image forming apparatus using the elastic roller as the fixing roller is able to improve a strain release properties with respect to a color toner which is easily subjected to offset compared to a monochrome toner due to a strain release effect of the elastic layer of a fixing nip portion. 
     Moreover, the color image forming apparatus has the fixing nip portion whose nip shape becomes a protruded shape toward the fixing roller side (what is called, an inverse nip shape), which makes it possible to improve peeling performance of a recording sheet, and to allow the recording sheet to be peeled off without using a peeling portion such as a peeling claw (self-stripping), so that it is possible to remove an image defect caused by the peeling portion. 
     Additionally, in order to form a color image forming apparatus corresponding to recent higher processing speed, the fixing device provided in the color image forming apparatus needs to be configured so as to form a fixing nip portion having a wide nip width. Methods of widening the nip width of the fixing nip portion include a method of thickening a layer thickness of the elastic layer of the fixing roller, a method of enlarging a fixing roller diameter and the like. 
     However, since the elastic layer provided in the fixing roller has very low heat conductivity, a problem arises such that a surface temperature of the fixing roller does not move on with increase in the processing speed in a case where the heating portion is inside the fixing roller. 
     On the other hand, a problem arises such that a larger fixing roller diameter requires a longer warming-up time until the surface temperature of the fixing roller reaches the fixing temperature and increased power consumption. 
     In order to solve the above-described conventional problems, a belt fixing type fixing device using a fixing belt is disclosed as a fixing device provided in a color image forming apparatus (refer to Patent Literature 1). 
     The belt fixing type fixing device has no heating portions provided in a fixing roller and a pressing roller; includes a heating roller separately from the fixing roller and the pressing roller; has a fixing belt suspended between the heating roller and the fixing roller; has the pressing roller facing the fixing roller via the fixing belt; and is configured such that the fixing belt is heated with a heat source arranged inside the heating roller. 
     In the belt fixing type fixing device, a fixing nip portion is formed in a press-contact part where the fixing roller comes into press-contact with the pressing roller via the fixing belt. Such a belt fixing type fixing device is configured so that the fixing belt whose heat capacity is small is heated, and it is thus possible to shorten the warming-up time until the surface temperature of the fixing belt reaches the fixing temperature. 
     Further, the belt fixing type fixing device does not need to arrange a heating portion such as a halogen heater inside the fixing roller, and the fixing roller provided with a thick elastic layer with low hardness made of sponge rubber or the like is able to be used, so that it is possible to form the fixing nip portion with a wide nip width. 
     In this manner, the belt fixing type fixing device is, since boot speed is fast, allowed to have no preheating during ready and waiting as well as to shorten a waiting time, and is provided in an image forming apparatus of not only a miniature low-speed machine but also a large machine and a high-speed machine. 
     Next, description will be given for cleaning of the fixing roller of the above-described fixing device. 
     First, description will be given in detail for an example of a conventional fixing device that does not use a fixing belt with reference to the drawings. 
       FIG. 1  is an illustrative diagram showing a configuration of the conventional fixing device that does not use the fixing belt. 
     A conventional fixing device  150  is provided with, as shown in  FIG. 1 , for example, a cleaning device  160  using a cleaning web  161  (hereinafter, simply referred to as “web”) in addition to a fixing roller  151  and a pressing roller  152 . 
     The fixing roller  151  is provided with heating portions  151   a  and  151   b . The pressing roller  152  is provided with a heating portion  152   a . The fixing roller  151  and the pressing roller  152  which are configured in this manner are brought into press-contact with each other so that a fixing nip  153  is formed. A recording sheet  32  on which an unfixed toner image T 0  is formed passes through the fixing nip  153  between the fixing roller  151  and the pressing roller  152 , thereby fusing an unfixed toner to be fixed onto the recording sheet  32 . 
     The cleaning device  160  is provided with a web  161 , a delivery roller  162 , a wind-up roller  163  and a press-contact roller  164 . 
     The web  161  is impregnated with anti-toner-adhesive oil, and comes into contact with the surface of the fixing roller  151 , thereby adsorbing and removing a toner that adheres to the surface of the fixing roller  151  as well as applying the anti-toner-adhesive oil to the surface of the fixing roller  151 . 
     The delivery roller  162  is rotatably provided to deliver the web  161 . The wind-up roller  163  is rotatably provided to be delivered from the delivery roller  162  and rewinds the cleaning web  161  after coming into contact with the fixing roller  151 . 
     The press-contact roller  164  is rotatably provided and brings the web  161  into press-contact with the surface of the fixing roller  151 . That is, the web  161  is held between the fixing roller  151  and the press-contact roller  164 . A press-contact portion between the fixing roller  151  and the web  161  thus becomes a cleaning nip portion  165 . 
     In the conventional fixing device  150 , the web  161  is delivered from the delivery roller  162 , passes through the cleaning nip portion  165 , and is rewound by the wind-up roller  163 . Subsequently, the web  161 , at the time of passing through the cleaning nip portion  165 , removes a toner that adheres to the surface of the fixing roller  151  as well as applies the oil to the surface of the fixing roller  151 . 
     In this manner, the fixing device  150  becomes a favorable state since the surface of the fixing roller  151  is cleaned up by the cleaning device  160 . 
     Next, description will be given for a conventional belt fixing type fixing device using a fixing belt. 
     In a fixing device using a fixing belt with low heat capacity, since heat storage performance from a heater to the fixing belt are smaller compared to a heat quantity that shifts from the fixing belt to the recording sheet, when the surface temperature of the fixing belt and the pressing roller is lowered and is not maintained in an appropriate temperature range, an offset amount (amount to be transferred to the fixing belt) of the toner is increased more than usual, and a problem that the fixing belt is contaminated easily occurs. Such a problem tends to occur notably when an image with a high halftone print rate is printed in large quantity. 
     Additionally, when image formation is performed using the above-described belt fixing method, the surface of the fixing belt is charged by combining the fixing belt and the pressing roller, and a phenomenon occurs such that the surface of the fixing belt attracts the toner from the recording sheet (what is called, electrostatic offset) in some cases. 
     When the electrostatic offset occurs, the toner is retransferred onto the recording sheet and comes up to the surface as a remained image after the fixing belt goes around, or a large amount of the toner adheres to a cleaning member (for example, cleaning roller) that is brought into contact with the fixing belt in order to prevent such a case, and the toner that once adheres to the cleaning member is thereby fused (melted) again to adhere to the fixing belt, then a phenomenon that contaminates the recording sheet occurs in some cases. 
     Moreover, there is a problem that since the temperature of the pressing roller is lower than the temperature of the fixing belt, the toner that adheres to the fixing belt (offset toner) moves to the surface of the pressing roller and adheres to the peeling claw to cause image contamination on the recording sheet. 
     In this manner, there is a case where a toner or an offset toner of an unfixed toner image is adhered to the surface of the fixing belt. The toner that adheres to the fixing belt is fixed to another recording sheet and recognized as image contamination or the like. Therefore, various measures are employed in order to prevent the toner from adhering to the surface of the fixing belt or to remove the toner that adheres to the surface of the fixing belt. 
     For example, a fixing device, in order to remove the offset toner, brings the pressing roller into contact with a roller applied with a slight amount of oil, and brings the roller applied with oil into contact with the cleaning roller that cleans up the surface thereof is disclosed (refer to Patent Literature 2). 
     As another example, a fixing device is disclosed that, in an image heating device that includes a heating member, a pressing member brought into press-contact therewith and a cleaning member arranged facing the pressing member, and performs heat-processing of a toner image by guiding a recorded material that carries the toner image to be held and conveyed by a press-contact nip portion formed with the heating member and the pressing member, includes a cleaning member contact/separation portion that, in order to clean up the offset toner caused by lowering of the temperature of the pressing roller, uses a metallic roller whose surface energy is higher than that of the pressing roller, and brings the cleaning member into contact with/separation from the pressing member corresponding to a type of the recorded member (refer to Patent Literature 3). 
     Further, a fixing device is disclosed that includes a fixing member arranged on the side of a toner image of a recording medium that carries the toner image and a pressing member arranged on the side of a non-toner image of the recording medium, and fixes the toner image to the recording medium by heating and pressing the toner of the toner image when the recording medium that carries the toner image passes through between the fixing member and the pressing member, and includes a variable mechanism that varies the distance between the fixing member and the pressing member, and a cleaning member that comes into contact with the pressing member when the fixing member is placed at a position where the fixing member separates from the pressing member to clean up the pressing member (refer to Patent Literature 4). 
     PRIOR ART LITERATURES 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open Hei 10 No. 307496 
         Patent Literature 2: Japanese Patent Application Laid-open No. 2006-201640 
         Patent Literature 3: Japanese Patent Application Laid-open No. 2001-75402 
         Patent Literature 4: Japanese Patent Application Laid-open No. 2008-129089 
       
    
     However, in the cleaning device using the roller applied with oil that is disclosed in Patent Literature 2, viscosity of oil that impregnates the cleaning roller is low viscosity (for example, 300 cs (centistoke) or less), and there is no adverse effect on peeling properties of the recording medium even though using a PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer) tube excellent in durability on the surface layer of the fixing belt, however, there has been a problem that the cleaning performance is retarded. 
     Additionally, there is an issue that since silicone oil with low viscosity is used, silicone oil contamination inside the device occurs due to dripping in the case of a high temperature. 
     Further, in the fixing device that brings the cleaning roller into contact with/separation from the pressing roller that is disclosed in Patent Literature 3, there is an issue that contamination of the cleaning roller is prevented from retransferring to the pressing roller, however, silicone oil with only low viscosity is able to be impregnated with the cleaning roller, thus the contamination inside the device occurs due to dripping as mentioned above. 
     On the other hand, in a cleaning device using a cleaning web, there is a need for use of silicone oil with relatively high viscosity (for example, 3,000 cs or more) as viscosity of the oil that impregnate the cleaning web in order to prevent the silicone oil from dripping from the cleaning web. 
     In this case, a large amount of silicone oil moves from the surface of the pressing roller to the fixing belt at the time of rotation before fixation of the fixing belt and the pressing roller before the recording medium passes through the fixing nip portion. There has been an issue that in a case where the surface layer of the fixing belt is formed of the PFA tube, there is great adhesive force between the silicone oil that adheres to the tube and the toner on the recording sheet so that peeling properties of the recording medium are extremely deteriorated. Here, the rotation before fixation means that the fixing belt and the pressing roller are rotated so that the fixing device becomes a fixable state before performing fixation operation at a fixing step (warming-up operation). 
     Furthermore, when a large amount of the silicone oil adheres to the surface of the fixing belt, the peeling properties of the recording medium are deteriorated due to adhesive strength of the silicone oil, a fixable temperature range narrows, and high-temperature offset occurs to cause image degradation. 
     Consequently, a technology that brings the cleaning member into contact with/separation from the pressing roller is considered in order to have an appropriate silicone oil amount on the surface of the fixing belt according to a type of the recording medium, however, it is difficult to manage whether an oil amount on the surfaces of the fixing belt and the pressing roller is an appropriate amount, and it is unadaptable to miniaturization since the mechanism is complicated. 
     Additionally, in Patent Literature 4, the pressing roller is brought into contact with the cleaning roller when the fixing belt separates from the pressing roller, thereby attempting prevention of image degradation and a longer operating life of the cleaning roller. However, even though the fixing belt comes into contact with/separation from the pressing roller employing a pressure release mechanism used as a pressure strain countermeasure of the fixing member, a contact/separation mechanism of the cleaning roller with/from the pressing roller becomes a complicated structure in order to correspond to position movement of the pressing roller, and has problems of high cost, a larger-size fixing device and the like. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the above-described conventional problems, and aims to provide a fixing device having a simple configuration that maintains a fixation range of a recording medium, allows the recording medium to have favorable peeling properties without contamination inside the device due to cleaning oil, and is able to form a uniform and high quality image, and an image forming apparatus using the same. 
     The fixing device and the image forming apparatus using the same according to the present invention in order to solve the above-described issues are provided as follows. 
     A first aspect of the present invention is that a fixing device that fixes an unfixed toner image to a recording medium while conveying the recording medium includes a fixing portion, a pressing roller, a cleaning portion and a contact/separation mechanism; wherein the fixing portion is provided with a fixing roller so as to rotate freely, a heating roller, and a fixing belt that is looped over the fixing roller and the heating roller and heats and fuses the unfixed toner image on the recording medium to fix onto the recording medium; the pressing roller is provided at a position facing the fixing roller via the fixing belt so as to rotate freely and conveys the recording medium while pressing to the fixing belt; the cleaning portion comprises a cleaning member impregnated with silicone oil that comes into contact with the pressing roller to clean up the pressing roller; the contact/separation mechanism brings the fixing portion into contact with/separation from the pressing roller; viscosity of the silicone oil is 3,000 cs (centistoke) or more; and the contact/separation mechanism separates the fixing belt and the pressing roller while preliminary rotation of the fixing belt and the pressing roller in preparatory operation before fixing (for example, at the time of warming-up). 
     Further, a second aspect of the present invention is that the fixing belt has a multilayered structure, and the surface layer of the fixing belt is formed of a fluororesin tube. 
     Further, a third aspect of the present invention is that the cleaning member is formed of a web material made of a heat-resistant nonwoven fabric. 
     Further, a fourth aspect of the present invention is that the cleaning portion is composed of a unit comprising a delivery portion that delivers the cleaning member, a wind-up portion that rewinds the cleaning member, and a press-contact roller that is rotatable so as to come into press-contact with the surface of the fixing roller via the cleaning member, and the cleaning portion is configured to be attachable to/detachable from the fixing device. 
     Further, a fifth aspect of the present invention is that an image forming apparatus that forms an image using a toner with electrophotography is provided with a photoreceptor drum on whose surface an electrostatic latent image is formed, a developing device that supplies a toner to the electrostatic latent image on the surface of the photoreceptor drum to form a toner image, a transfer device that transfers the toner image on the surface of the photoreceptor drum to a recording medium, and a fixing device that fixes the transferred toner image to the recording medium, wherein the fixing devices of any of the aspects 1 to 4 is used as the fixing device. 
     According to the first aspect of the present invention, viscosity of the silicone oil of the cleaning member is set to 3,000 cs or more, and cleaning performance of an offset toner on the surface of the fixing member thereby becomes favorable so that contamination inside the fixing device due to dripping or the like is able to be prevented in the case of a high temperature. 
     Moreover, silicone oil with high viscosity is able to suppress disaggregation of the toner once adhered in the cleaning member, and effective cleaning is able to be achieved. 
     Additionally, the contact/separation mechanism separates the fixing belt from the pressing roller at the time of the rotation operation before fixation (for example, at the time of warming-up), thereby securing a fixable range of the recording medium with a simple configuration to prevent the silicone oil from transferring onto the surface of the fixing belt as well as to prevent adhesion of a large amount of the silicone oil, so that it is possible to seek improvement of peeling properties of the recording medium to obtain a uniform and high quality image. 
     Note that, viscosity of the silicone oil of the cleaning member is preferably 30,000 cs or less. In the case of exceeding 30,000 cs, an exuding amount of the silicone oil lessens and there is no effect on the cleaning performance. When a coarse fiber mesh is used for the web in order to increase the exuding amount, toner collection performance as a web mechanical function is decreased. 
     Further, the peeling properties of the recording sheet is retarded when adhesive strength of the silicone oil is too great, the fixable temperature range narrows, and in the case of performing continuous printing, the temperature of the fixing roller is lowered so that the recording sheet is wound around the fixing roller, or a paper jam occurs, posing a problem. 
     Further, according to the second aspect of the present invention, the fixing belt has a multilayered structure, thereby allowing the fixing belt to have functions of strength, elasticity and release properties, and the surface layer of the fixing belt is formed of a fluororesin tube, thereby securing stable release properties and having favorable durability and abrasion resistance so that it is possible to have a long operating life. 
     Further, according to the third aspect of the present invention, the cleaning member is formed of a web material made of heat-resistant nonwoven fabric, so that it is possible to realize a cleaning member having high collection efficiency due to adhesion of a toner between fibers and excellent in cleaning performance. Additionally, since silicone oil with high viscosity is able to be used and scattering of the silicone oil is thus suppressed, the silicone oil contamination inside the device is able to be suppressed. 
     Further, according to the fourth aspect of the present invention, it is easy to change the cleaning member, and parts maintenance cost becomes decreased. Additionally, when the silicone oil contamination by the cleaning member occurs, the contamination is able to be kept inside the unit, and expansion of the contamination is thus able to be prevented. 
     Further, according to the fifth aspect of the present invention, a fixable range of a recording medium is secured with a simple configuration, the silicone oil is prevented from transferring onto the surface of the fixing belt constituting the fixing device, and adhesion of a large amount of the silicone oil is able to be prevented, so that it is possible to seek improvement of the peeling properties of the recording medium and obtain a uniform and high quality image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative diagram showing a configuration of a conventional fixing device; 
         FIG. 2  is an illustrative diagram showing a configuration of the entire image forming apparatus using the fixing device according to an embodiment of the present invention; 
         FIG. 3  is an illustrative diagram showing a state where a fixing belt and a pressing roller constituting the fixing device are in contact with each other; 
         FIG. 4  is an illustrative diagram showing a state where the fixing belt is separated from the pressing roller; and 
         FIG. 5  is a table comparing effects of examples and comparative examples of the fixing device of the present embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will be hereinafter given for a mode for carrying out the present invention with reference to the drawings. 
       FIG. 2  is an example of a mode for carrying out the present invention, and an illustrative diagram showing a configuration of the entire image forming apparatus using a fixing device according to the embodiment of the present invention. 
     An image forming apparatus  100  according to the present embodiment is provided with, as shown in  FIG. 2 , photoreceptor drums  101   a  to  101   d  (collectively referred to as “photoreceptor drum  101 ” in some cases) with an electrostatic latent image formed on the surface thereof, developing devices  102   a  to  102   d  (collectively referred to as “developing device  102 ” in some cases) that supplies a toner to the electrostatic latent image on the surface of the photoreceptor drum  101  to form a toner image, a secondary transfer roller (transfer device)  14  that transfers the toner image on the surface of the photoreceptor drum  101  via an intermediate transfer belt  11  to a sheet and a fixing device  15  that fixes the transferred toner image to the sheet, and forms an image using a toner with electrophotography. 
     The image forming apparatus  100  according to the present embodiment forms an image with multi-color and monochrome on a recording sheet  32  based on image data of a read document or image data transmitted via a network or the like. 
     The image forming apparatus  100  is provided with, as shown in  FIG. 2 , an exposure unit E, photoreceptor drums  101   a  to  101   d  with a latent image to be formed by the exposure unit E, developing devices  102   a  to  102   d , charging rollers  103   a  to  103   d  (collectively referred to as “charging roller  103 ” in some cases), cleaning units  104   a  to  104   d  (collectively referred to as “cleaning unit  104 ” in some cases), an intermediate transfer belt  11 , primary transfer rollers  13   a  to  13   d  (collectively referred to as “primary transfer roller  13 ” in some cases), a secondary transfer roller  14 , a fixing device  15 , paper feed paths P 1 , P 2  and P 3 , a paper cassette  16 , a manual paper feeding tray  17  and a paper discharge tray  18 . 
     The image forming apparatus  100  performs image formation at image forming portions Pa to Pd corresponding to each color phase using image data corresponding to each color phase of four colors of black (K), as well as cyan (C), magenta (M) and yellow (Y) as three primary colors of subtractive mixture obtained by color separation of color images. 
     Each of the image forming portions Pa to Pd has the same configuration, and for example, an image forming portion Pa of black (K) is composed of the photoreceptor drum  101   a , the developing device  102   a , the charging roller  103   a , the primary transfer roller  13   a , a cleaning unit  104   a  and the like. These image forming portions Pa to Pd are arranged in line in the direction in which the intermediate transfer belt  11  moves (in a vertical scanning direction). 
     The exposure unit E as an exposure device of the present embodiment is provided with an unillustrated semiconductor laser, a polygon mirror  4 , a first reflective mirror, a second reflective mirror  8  and the like, and irradiates each of light beams such as a laser beam modulated according to image data of each color phase of black (K), cyan (C), magenta (M) and yellow (Y) to each of the photoreceptor drums  101   a  to  101   d . Each of the photoreceptor drums  101   a  to  101   d  forms an electrostatic latent image by image data of each color phase of black (K), cyan (C), magenta (M) and yellow (Y). 
     In the present embodiment, for the exposure unit E, a method using a laser scanning unit (LSU) including a laser irradiation portion and a reflective mirror is used, however, it is allowed to use a method using, for example, an EL or an LED writing head in which light-emitting devices are arranged in an array. 
     The photoreceptor drums  101   a  to  101   d  disposed above the exposure unit E are image carriers having approximately cylindrical shapes, and controlled so as to be rotated in a predetermined direction by a driving portion and a control portion which are unillustrated. Additionally, each of the photoreceptor drums  101   a  to  101   d  is configured with a photoconductive layer formed on a base material thereof. For example, a metallic drum produced by aluminum or the like serves as the base material, and on the outer circumferential surface, photoconductive layers such as amorphous silicon (a-Si), selenium (Se), an organic photoconductor (OPC) and the like are formed in a thin film shape. Note that, a configuration of the photoreceptor drum  101  is not particularly limited to the above-described configuration. 
     The charging roller  103  is a contact-type charger that charges the surface of the photoreceptor drum  101  uniformly to a predetermined potential. In the present embodiment, as shown in  FIG. 2 , a contact-type roller-type charging roller  103  is used as a charger, however, it is also possible to use a contact-type charger using a charging brush, or a noncontact-type charger using a charged wire. 
     The developing device  102  supplies a toner as a developer to the surface of the photoreceptor drum  101  having an electrostatic latent image formed thereon to develop the electrostatic latent image to a toner image. Each of the developing devices  102   a  to  102   d  contains a toner of each color phase of black (K), cyan (C), magenta (M) and yellow (Y), and visualizes the electrostatic latent image of each color phase formed on each of the photoreceptor drums  101   a  to  101   d  to a toner image of each color phase. 
     The cleaning unit  104  removes and collects a toner remained on the surface of the photoreceptor drum  101  after developing and image transfer. 
     The intermediate transfer belt  11  is arranged above the photoreceptor drum  101 , and formed by an endless film with approximately 100 to 150 μm thickness. Then, the intermediate transfer belt  11  is stretched out between a driving roller  11   a  and a driven roller  11   b  to form a looped moving path. 
     An outer circumferential surface of the intermediate transfer belt  11  faces a photoreceptor drum  101   d , a photoreceptor drum  101   c , a photoreceptor drum  101   b  and a photoreceptor drum  101   a , in this order. 
     The primary transfer rollers  13   a  to  13   d  are arranged at positions facing each of the photoreceptor drums  101   a  to  101   d  holding the intermediate transfer belt  11  therebetween. The respective positions at which the intermediate transfer belt  11  faces the photoreceptor drums  101   a  to  101   d  are primary transfer positions. 
     To the primary transfer rollers  13   a  to  13   d , primary transfer bias having opposite polarity to charging polarity of the toner is applied by constant voltage control in order to transfer the toner images carried on the surfaces of the photoreceptor drums  101   a  to  101   d  onto the intermediate transfer belt  11 . Thereby, the toner images of the respective color phases formed on the photoreceptor drums  101   a  to  101   d  are overlapped and transferred onto the outer circumferential surface of the intermediate transfer belt  11  sequentially, and a full-color toner image is formed on the outer circumferential surface of the intermediate transfer belt  11 . 
     However, when image data for only a part of the color phases of yellow (Y), magenta (M), cyan (C) and black (K) is input, electrostatic latent images and toner images are formed at only a part of the photoreceptor drum  101  corresponding to the color phase of the input image data among the four photoreceptor drums  101   a  to  101   d . For example, during monochrome image formation, formation of an electrostatic latent image and formation of a toner image are performed only at the photoreceptor drum  101   a  corresponding to the color phase of black, and only a black toner image is transferred onto the outer circumferential surface of the intermediate transfer belt  11 . 
     Each of the primary transfer rollers  13   a  to  13   d  is configured by coating the surface of a shaft whose base material is metal such as stainless steel having a diameter of 8 to 10 mm with a conductive elastic material (for example, EPDM (ethylene-propylene diene rubber), urethane foam, etc.) and high voltage is applied uniformly to the intermediate transfer belt  11  by the conductive elastic material. In the present embodiment, although the primary transfer rollers  13   a  to  13   d  are used as transfer electrodes, other than those, a brush or the like is also usable. 
     The toner image transferred onto the outer circumferential surface of the intermediate transfer belt  11  at each primary transfer position is conveyed to a secondary transfer position which is a position facing the secondary transfer roller  14 , by the rotation of the intermediate transfer belt  11 . 
     The secondary transfer roller  14  is in press-contact, at a predetermined nip pressure, with the outer circumferential surface of the intermediate transfer belt  11  whose inner circumferential surface is in contact with a circumferential surface of the driving roller  11   a  at the time of image formation. 
     When the recording sheet  32  fed from the paper cassette  16  or the manual paper feeding tray  17  passes through between the secondary transfer roller  14  and the intermediate transfer belt  11 , high voltage with opposite polarity (+) to the charging polarity of the toner (−) is applied to the secondary transfer roller  14 . The toner images are thereby transferred onto the surface of the recording sheet  32  from the outer circumferential surface of the intermediate transfer belt  11 . 
     In this manner, the electrostatic latent images on respective photoreceptor drums  101   a  to  101   d  are visualized by the toner corresponding to each of the color phases to become toner images, respectively, and such toner images are layered on the intermediate transfer belt  11 . Thereafter, the layered toner images are moved to a contact position of the conveyed sheet with the intermediate transfer belt  11  by the rotation of the intermediate transfer belt  11 , and by the secondary transfer roller  14  arranged in this position, the toner images are transferred from the outer circumferential surface of the intermediate transfer belt  11  onto the recording sheet  32 . 
     Note that, toners remaining on the intermediate transfer belt  11  without being transferred onto the recording sheet  32  among toners adhered from the photoreceptor drum  101  to the intermediate transfer belt  11  are collected by an intermediate transfer belt cleaning unit  12  in order to prevent the colors from mixing at a next step. 
     In the intermediate transfer belt cleaning unit  12 , a cleaning blade as a cleaning member in contact with the intermediate transfer belt  11  is provided. A part of the intermediate transfer belt  11  with which the cleaning blade comes into contact is supported by an intermediate transfer belt driven roller  11   b  from a backside thereof. 
     The recording sheet  32  to which a toner image is transferred is guided by the fixing device  15  to pass through a fixing nip portion  15   e , and subjected to heating and pressing. Thereby, the toner image is fixed firmly on the surface of the recording sheet  32 . The recording sheet  32  on which the toner image has been fixed is discharged by a paper discharge roller  18   a  onto the paper discharge tray  18 . Note that, details of the fixing device  15  will be described below. 
     Further, the image forming apparatus  100  is provided with a paper conveyance path P 1  extending in an approximately vertical direction so that the recording sheet  32  contained in the paper cassette  16  is fed, through between the secondary transfer roller  14  and the intermediate transfer belt  11  and through the fixing device  15 , to the paper discharge tray  18 . 
     Arranged in the paper conveyance path P 1  are a pick-up roller  16   a  for feeding the recording sheet  32  in the paper cassette  16  into the paper conveyance path P 1  sheet by sheet, a conveying roller  16   b  for conveying the fed recording sheet  32  upward, a registration roller  19  for guiding the conveyed recording sheet  32  between the secondary transfer roller  14  and the intermediate transfer belt  11  at a predetermined timing, and the paper discharge roller  18   a  for discharging the recording sheet  32  to the paper discharge tray  18 . 
     In addition, inside the image forming apparatus  100 , a paper conveyance path P 2  on which a pick-up roller  17   a  and a feed roller  16   b  are arranged is formed between the manual paper feeding tray  17  and the registration roller  19 . Further, a paper conveyance path P 3  is formed between the paper discharge roller  18   a  and an upstream side of the registration roller  19  in the paper conveyance path P 1 . 
     The paper discharge roller  18   a  freely rotates in both forward and reverse directions, and is driven in the forward direction to discharge the recording sheet  32  to the paper discharge tray  18  at the time of single-sided image formation in which an image is formed on one side of the recording sheet  32 , and at the time of second side image formation in double-sided image formation in which an image is formed on both sides of the recording sheet  32 . 
     On the other hand, at the time of first side image formation of the double-sided image formation, the paper discharge roller  18   a  is driven in the forward direction until a rear edge of the sheet passes through the fixing device  15 , and is then driven in the reverse direction to guide the recording sheet  32  in the paper conveyance path P 3  in a state where the rear edge part of the recording sheet  32  is held. The recording sheet  32  on which an image has been formed only on one side at the time of double-sided image formation is thereby guided to the paper conveyance path P 1  in a state where the sheet is turned over and upside down. 
     The registration roller  19  guides the recording sheet  32  that has been fed from the paper cassette  16  or the manual paper feeding tray  17  or has been conveyed through the paper conveyance path P 3  between the secondary transfer roller  14  and the intermediate transfer belt  11  at a timing synchronized with the rotation of the intermediate transfer belt  11 . Thus, the rotation of the registration roller  19  is stopped when operating of the photoreceptor drum  101  or the intermediate transfer belt  11  is started, and the movement of the recording sheet  32  that has been fed or conveyed prior to the rotation of the intermediate transfer belt  11  is stopped in the paper conveyance path  21  in a state where a front edge thereof comes into contact with the registration roller  19 . 
     Thereafter, rotation of the registration roller  19  is started at a timing when the front edge part of the recording sheet  32  faces a front edge part of a toner image formed on the intermediate transfer belt  11  at a position where the secondary transfer roller  14  is in press-contact with the intermediate transfer belt  11 . 
     Note that, at the time of full-color image formation in which image formation is performed by all of the image forming portions Pa to Pd, the primary transfer rollers  13   a  to  13   d  bring the intermediate transfer belt  11  into press-contact with all of the photoreceptor drums  101   a  to  101   d.    
     On the other hand, at the time of monochrome image formation in which image formation is performed only by the image forming portion Pa, only the primary transfer roller  13   a  brings the intermediate transfer belt  11  into press-contact with the photoreceptor drum  101   a.    
     Next, description will be given in detail for a configuration of the fixing device  15  with reference to the drawings. 
       FIG. 3  and  FIG. 4  are illustrative diagrams showing the configuration of the fixing device  15 , in which  FIG. 3  is an illustrative diagram showing a state where a fixing belt and a pressing roller constituting the fixing device  15  are in contact with each other, and  FIG. 4  is an illustrative diagram showing a state where the fixing belt separates from the pressing roller. 
     The fixing device  15  is equipped in the image forming apparatus  100  as shown in  FIG. 2 . The fixing device  15  is provide with, as shown in  FIG. 3 , a fixing portion  15   a , a pressing roller  15   b , a cleaning unit (cleaning portion)  15   c  and a contact/separation mechanism (contact/separation means)  15   d , and fixes an unfixed toner image while conveying the recording sheet  32 . 
     The fixing portion  15   a  is a device for heating and pressing a toner image T 1  carried on the recording sheet  32  as a recording medium to fix onto the recording sheet  32 . The fixing portion  15   a  is provided with a fixing roller  15   a   1  rotating freely, a heating roller  15   a   2  rotating freely, a fixing belt  15   a   3 , looped over the fixing roller  15   a   1  and the heating roller  15   a   2 , that heats and fuses the unfixed toner image on the recording sheet  32  to fix on the recording sheet  32 . 
     The pressing roller  15   b  is provided at a position facing the fixing roller  15   a   1  via the fixing belt  15   a   3  so as to rotate freely and conveys the recording sheet  32  while pressing to the fixing belt  15   a   3 . 
     The cleaning unit  15   c  is provided with a cleaning web (hereinafter, simply referred to as “web”)  15   c   1  as a cleaning member impregnated with silicone oil that comes into contact with the pressing roller  15   b  to clean up the pressing roller  15   b.    
     The contact/separation mechanism  15   d  bring the fixing portion  15   a  into contact with/separation from the pressing roller  15   b.    
     That is, when the heating roller  15   a   2  comes into contact with the fixing belt  15   a   3  to heat the fixing belt  15   a   3 , and the recording sheet  32  passes through the fixing nip portion  15   e  formed by the fixing belt  15   a   3  and the pressing roller  15   b  at a predetermined fixing speed and a copy speed, the fixing device  15  heats and presses, then fixes the unfixed toner image T 1  carried on the recording sheet  32  onto the recording sheet  32 . 
     Subsequently, the fixing device  15  sets viscosity of the silicone oil that impregnates the web  15   c   1  to 3,000 cs or more, and causes the contact/separation mechanism  15   d  to separate the fixing belt  15   a   3  from the pressing roller  15   b  at the time of rotation operation before fixation of the fixing belt  15   a   3  and the pressing roller  15   b  in preliminary operation performed before fixation operation, as shown in  FIG. 4 . 
     Here, the rotation before fixation means that the fixing belt  15   a   3  and the pressing roller  15   b  are rotated so that the fixing device  15  becomes a fixable state before performing the fixation operation at a fixing step (warming-up operation). 
     The fixing roller  15   a   1  and the heating roller  15   a   2  are arranged to be approximately parallel to each other along an axis direction of the fixing roller  15   a   1 . That is, an axis of the fixing roller  15   a   1  and an axis of the heating roller  15   a   2  are disposed approximately parallel to each other. Thereby, the fixing belt  15   a   3  stretched out between the fixing roller  15   a   1  and the heating roller  15   a   2  is prevented from meandering at the time of rotation movement, and durability of the fixing belt  15   a   3  is able to be maintained over a long period of time. 
     Note that, the unfixed toner image T 1  ( FIG. 3 ) is formed of a developer (toner), for example, such as a nonmagnetic one-component developer (nonmagnetic toner), a nonmagnetic two-component developer (nonmagnetic toner and carrier) and a magnetic developer (magnetic toner). 
     Further, the fixing speed is, what is called, a process speed, and the copy speed is the number of copies per minute. Additionally, when the recording sheet  32  passes through the fixing nip portion  15   e , the fixing belt  15   a   3  comes into contact with the surface of the recording sheet  32  on which the tone image is carried. 
     The heating roller  15   a   2  is formed of a thin, hollow, cylindrical shaft-like body made of metal such as aluminum, iron or the like, and revolvably engaged with respect to a fixed portion (not shown) such as an device frame via an unillustrated bearing. Inside the heating roller  15   a   2 , halogen heaters (heat sources)  15   a   4  and  15   a   5  are provided in order to heat from the inside of the fixing belt  15   a   3 . The halogen heaters (heat sources)  15   a   4  and  15   a   5  are not revolved, however, may be revolvable with the heating roller  15   a   2 . The heating roller  15   a   2  may be configured so as not to revolve. 
     Moreover, near the outside of the heating roller  15   a   2  via the fixing belt  15   a   3 , a thermostat  15   a   6  and a temperature sensor  15   a   7  that detects a surface temperature of the fixing belt  15   a   3  are arranged. 
     The fixing roller  15   a   1  comes into press-contact with the pressing roller  15   b  via the fixing belt  15   a   3 , thereby forming the fixing nip portion  15   e  as well as conveying with an unillustrated driving motor (driving portion) the fixing belt  15   a   3  by rotationally driving in a rotation direction around a rotational axis. 
     The fixing roller  15   a   1  is formed of, for example, a columnar-shaped shaft-like body whose diameter is 30 mm, and composed of a double-layered structure formed of a metallic core and an elastic layer in the order from the inside to the outside. For the metallic core, for example, metal such as iron, stainless steel, aluminum, copper or alloys thereof is used. Further, for the elastic layer, a rubber material having heat-resistant such as silicone rubber or fluororubber is appropriate. 
     In the present embodiment, force when the fixing roller  15   a   1  comes into press-contact with the pressing roller  15   b  via the fixing belt  15   a   3  is approximately 216N. 
     The pressing roller  15   b  faces and comes into press-contact with the fixing roller  15   a   1  via the fixing belt  15   a   3 , and is provided around the rotational axis so as to rotate freely. The pressing roller  15   b  is driven by the rotation of the fixing roller  15   a   1  to rotate in a rotational direction. Additionally, the pressing roller  15   b  is composed of a three-layer structure formed of a metallic core, an elastic layer and a release layer in the order from the inside to the outside. For the metallic core, for example, metal such as iron, stainless steel, aluminum, copper or alloys thereof is used. Further, for the elastic layer, a rubber material having heat resistance such as silicone rubber or fluororubber is appropriate, and for the release layer, a fluororesin such as a PFA (tetrafluoroethylene perfuluoroalkyl vinyl ether copolymer) or PTFE (polytetrafluoroethylene) is appropriate. 
     As the pressing roller  15   b , for example, a roller whose roller diameter is 30 mm and in which an iron (STKM (carbon steel tubes for machine structural purposes) based on Japanese Industrial Standards: JIS G 3445) pipe whose diameter is 24 mm (thickness is 2 mm) is used for the metallic core, is able to be used, including silicone solid rubber whose thickness is 3 mm as an elastic layer in the outer circumferential portion, and covered with a PFA tube whose thickness is 30 μm as a release layer further outside. 
     Further, inside the pressing roller  15   b , a halogen heater (for example, rated power: 400 W)  15   b   1  that heats the pressing roller  15   b  is arranged. 
     The halogen heater  15   b   1  is illuminated by power supplied (energization) from a power circuit (not shown) with an unillustrated device control portion, and infrared rays are emitted. Thereby, an inner circumferential surface of the pressing roller  15   b  absorbs the infrared rays to be heated, and the entire pressing roller  15   b  is heated. Note that, the above-described halogen heater  15   b   1  heats from the inner surface of the pressing roller  15   b , however, other than that, a method of heating from the surface of the pressing roller  15   b  with a roller for heating the outer circumferential surface is able to be configured. 
     Near the outer circumferential portion of the pressing roller  15   b , a peeling claw  15   b   2 , a thermostat  15   b   3  and a temperature sensor  15   b   4  that detects a surface temperature of the pressing roller  15   b  are arranged. 
     The pressing roller  15   b  is provided with an arm  15   d   1  and a cam  15   d   2  as the contact/separation mechanism  15   d  that brings the fixing roller  15   a   1  into contact with/separation from the pressing roller  15   b . The pressing roller  15   b  is rotatably held at an approximately center part of the arm  15   d   1 . 
     The arm  15   d   1  has a one end side  15   d   11  pivotally supported as a fixing end, and has another end side  15   d   12  swingably provided by the cam  15   d   2  as a free end. The cam  15   d   2  is rotatably provided as well as in contact with the lower side of the another end side  15   d   12  of the arm  15   d   1  in  FIG. 3 . 
     As shown in  FIG. 3  and  FIG. 4 , the cam  15   d   2  rotates by an unillustrated pressing motor and the another end side  15   d   12  performs rocking motion with the one end side  15   d   11  of the arm  15   d   1  as a supporting point, so that the contact/separation mechanism  15   d  displaces the pressing roller  15   b  in a vertical direction in  FIG. 3 . 
     That is, when the another end side  15   d   12  of the arm  15   d   1  is displaced upward, the pressing roller  15   b  comes into contact with the fixing belt  15   a   3 , and when the another end side  15   d   12  is displaced downward, the pressing roller  15   b  separates from the fixing belt  15   a   3 . This makes it possible to perform contact/separation between the pressing roller  15   b  held by the arm  15   d   1  and the fixing belt  15   a   3 . 
     Note that, the contact/separation mechanism  15   d  that brings the fixing belt  15   a   3  into contact with/separation from the pressing roller  15   b  may be used in order to clear a pressure strain of the fixing roller  15   a   1  and the pressing roller  15   b . In this case, there is no need to separately provide a mechanism for clearing the pressure strain of the fixing roller  15   a   1  and the pressing roller  15   b , and it is possible to realize cost reduction of the device. 
     Moreover, near the pressing roller  15   b , as a cleaning member, the cleaning unit  15   c  including a web  15   c   1  made of heat-resistant nonwoven fabric impregnated with silicone oil whose viscosity is 3,000 cs or more at 25° C., a delivery roller  15   c   2 , a wind-up roller  15   c   3 , and a press-contact roller  15   c   4  that is rotatable so as to come into press-contact with the surface of the fixing roller  15   a   1  via the web  15   c   1  is provided. 
     The cleaning unit  15   c  is composed of a unit structure integrally provided with the above-described members, and configured so as to be attachable/detachable with/from the fixing device  15  (specifically, with respect to the pressing roller  15   b ). 
     The web  15   c   1 , insofar as the heat-resistant nonwoven fabric is used, is not particularly limited, and for example, includes nonwoven fabric including aromatic polyamide fibers and polyester fibers that soften at a high temperature and having both appropriate flexibility and mechanical strength. Such heat-resistant nonwoven fabric is commercially available, and for example, includes NOMEX (registered trademark), Himeron (registered trademark) and the like. 
     Further, the thickness of the web  15   c   1  is also not particularly limited, and is preferably 30 to 100 μm. In the present embodiment, a web material whose thickness is 40 μm is used. 
     Additionally, as silicone oil, silicone oil having 3,000 cs or more high viscosity at 25° C. is preferred to be used, and silicone oil used in this area includes, for example, dimethyl silicone oil, amino degeneration silicone oil, mercapto degeneration silicone oil, fluorine-modified silicone oil and the like. In the present embodiment, 3,000-cs silicone oil is impregnated at 25° C. 
     Note that, viscosity of the silicone oil is preferably 30,000 cs or less. In the case of exceeding 30,000 cs, an exuding amount of the silicone oil lessens, and there is no effect on the cleaning performance. When a coarse fiber mesh is used for the web in order to increase the exuding amount, toner collection performance as a web mechanical function is decreased. 
     Further, peeling properties of the recording sheet is inhibited when adhesive power of the silicone oil is too great, thus the fixable temperature range narrows, and in the case of performing continuous printing, the temperature of the fixing roller is lowered so that the recording sheet is wound on the fixing roller, or a paper jam is occurred, which poses a problem. 
     The delivery roller  15   c   2  is supported around the axis so as to be able to be driven to rotate. 
     The wind-up roller  15   c   3  is provided around the axis by an unillustrated driving portion so as to be driven to rotate. 
     The web  15   c   1  delivered from the delivery roller  15   c   2  is brought into contact with the surface of the pressing roller  15   b  by the press-contact roller  15   c   4 , and thereafter rewound by the wind-up roller  15   c   3 . 
     Next, description will be given for operation of the cleaning unit  15   c.    
     The cleaning unit  15   c  is subjected to operational control by an unillustrated CPU. 
     The CPU, after detecting by an unillustrated sensor, the number of rotations of the fixing roller  15   a   1  and the like that a predetermined number of recording sheets  32  has passed through the fixing nip portion  15   e , sends a control signal to an unillustrated driving portion that rotates the wind-up roller  15   c   3 . 
     The driving portion that receives the control signal rewinds a certain amount of the web  15   c   1  by rotating the wind-up roller  15   c   3 . Such rewinding operation allows the web  15   c   1  to be delivered from the delivery roller  15   c   2 , and the web  15   c   1  that reaches a cleaning nip portion (a press-contact portion of the pressing roller and the web)  15   c   5  adsorbs an offset toner that adheres to the surface of the pressing roller  15   b  as well as supplies oil to the pressing roller  15   b.    
     Note that, although the present embodiment shows an operation example of intermittent rewinding performed by the wind-up roller  15   c   3 , rewinding operation of the web  15   c   1  by the wind-up roller  15   c   3  is not limited thereto, and it may be configured to continuously rewind the web  15   c   1  in time with a timing when the recording sheet  32  passes through the fixing nip portion  15   e.    
     The press-contact roller  15   c   4  is a roller-shaped member whose both ends in a longitudinal direction (axial direction) are fitted in unillustrated bearings so as to be able to be driven to rotate and provided so as to be brought into press-contact with the surface of the pressing roller  15   b  via the web  15   c   1  by an unillustrated pressing portion. Then, the press-contact roller  15   c   4  is driven to rotate at the time of the rewinding operation of the web  15   c   1  by the wind-up roller  15   c   3 . The press-contact portion of the press-contact roller  15   c   4  and the pressing roller  15   b  is the cleaning nip portion  15   c   5 . 
     For the press-contact roller  15   c   4 , for example, a roller-shaped member containing a metallic core and an elastic layer formed on the surface of the metallic core is used. An elastic member constituting the elastic layer includes, for example, foam of heat-resistant rubber such as silicone rubber. 
     Note that, surface hardness of the elastic layer of the press-contact roller  15   c   4  is not particularly limited, however, is preferably 20° to 30° (Asker-C hardness of standards of The Society of Rubber Industry, Japan). 
     Further, pressing force of the press-contact roller  15   c   4  to the pressing roller  15   b  is preferably 3,793.6 Pa (0.039 kgf/cm 2 ) to 18,967.9 Pa (0.19 kgf/cm 2 ). 
     When the pressing force of the press-contact roller  15   c   4  to the pressing roller  15   b  is less than 3,793.6 Pa, there is a possibility that an offset toner leaks into the inside of the image forming apparatus  100 . On the other hand, when the pressing force of the press-contact roller  15   c   4  to the pressing roller  15   b  exceeds 18,967.9 Pa, a surface layer of the pressing roller is easily damaged, and there is a possibility that a fixation defect of an image occurs. 
     For the pressing portion that presses the press-contact roller  15   c   4  to the pressing roller  15   b , for example, a spring member or the like is used. It is preferable that a width of the press-contact roller  15   c   4  in a longitudinal direction (axial direction) is wider than a maximum width of an image forming area to be formed in the image forming apparatus  100 . 
     Moreover, the width of the cleaning nip portion  15   c   5  (cleaning nip width) is preferred to be designed in an appropriate range since a huge effect is given on the cleaning performance of the cleaning unit  15   c . Additionally, the cleaning nip width is determined mainly according to the pressing force of the press-contact roller  15   c   4  to the pressing roller  15   b , a roller diameter of the press-contact roller  15   c   4  and the like. 
     In the present embodiment, the image forming area is set to 290 mm, the width of the press-contact roller  15   c   4  in a longitudinal direction is set to 310 mm which is longer than that of the image forming area, the roller diameter is set to 20 mm, and the cleaning nip width is set to 5 mm. 
     In this case, as shown in  FIG. 2 , at the time of the fixation operation in which the fixing belt  15   a   3  comes into contact with the pressing roller  15   b , the silicone oil supplied from the web  15   c   1  is applied to the pressing roller  15   b  so as to be a silicone oil amount required for cleaning. The applied silicone oil is transferred to the recording sheet  32 , and thus keeps an appropriate amount. 
     On the other hand, even when the silicone oil is applied in the appropriate amount in transfer operation, in a case where the fixing belt  15   a   3  is in contact with the pressing roller  15   b  all the time, the silicone oil applied to the pressing roller  15   b  is transferred and moved to the fixing belt  15   a   3  at the time of a preceding rotation such as warming-up, so that a large amount of the silicone oil is applied to the fixing belt. As a result, viscosity properties of the high-viscosity silicone oil that is used with an aim to prevent contamination in the device causes decrease in the peeling properties of the recording sheet  32  to narrow the fixable range, and posing a problem that the degradation of image occurs. 
     However, like the present embodiment, the fixing belt  15   a   3  is separated from the pressing roller  15   b  at the time of the preceding rotation such as the time of warming-up, so that it is possible to prevent a large amount of the high-viscosity silicone oil from moving to the fixing belt  15   a   3 , and to maintain the peeling properties of the recording sheet  32 . 
     Further, the cleaning unit  15   c  is integrally provided as a unit so as to be attachable to/detachable from the fixing device  15 . Therefore, according to the present embodiment, the cleaning unit  15   c  containing the web  15   c   1  is designed so as to be attachable to/detachable from the fixing device  15  so that it becomes easy to change the web  15   c   1  and parts maintenance cost becomes decreased. 
     Additionally, in a case where the silicone oil contamination occurs, the leaked silicone oil is able to be kept inside the cleaning unit  15   c , and it is thus possible to prevent from expanding contamination into the fixing device. 
     The fixing belt  15   a   3  is heated to a predetermined temperature by the heating roller  15   a   2 , and heats the recording sheet  32  with an unfixed toner image formed thereon that passes through the fixing nip portion  15   e . The fixing belt  15   a   3  which is an endless belt is suspended by the heating roller  15   a   2  and the fixing roller  15   a   1 , and wound around the fixing roller  15   a   1  at a predetermined angle. Then, at the time of rotation of the fixing roller  15   a   1 , the fixing belt  15   a   3  is driven by the fixing roller  15   a   1  and rotates in a rotational direction. 
     The fixing belt  15   a   3  is configured to have a three-layer structure that an elastomer material excellent in heat resistance and elasticity as an elastic layer (for example, silicone rubber) is formed on the surface of a (flexible) hollow cylindrical base material made of heat-resistant resin such as polyimide or a metallic material such as stainless-steel, nickel or the like, and further on the surface thereof, a fluororesin material excellent in heat resistance and release properties (for example, fluororesin such as a PFA, PTFE or the like) is formed as a release layer. 
     The surface layer of the fixing belt  15   a   3  is preferred to be formed of a tube made of a fluororesin. A tube is used as a coating material, so that it is possible to secure stable release properties, and with favorable durability and abrasion resistance, it is possible to have a long operating life. Additionally, carbon black or the like may be added into polyimide of the base material. This makes it possible to prevent charging of the fixing belt  15   a   3  and reduce electrostatic offset. 
     The temperature sensors  15   a   7  and  15   b   4  are, for example, composed of thermistors, detect temperatures of the fixing belt  15   a   3  and the pressing roller  15   b , respectively, and an unillustrated control portion controls a heat source of the heating roller  15   a   2  (halogen heaters  15   a   4  and  15   a   5 ) and a heat source of the pressing roller  15   b  (halogen heater  15   b   1 ), respectively, based on the detection result. 
     Further, in the case of occurrence of a trouble in a control system, the heat sources of the heating roller  15   a   2  and the pressing roller  15   b  go out of control and are very dangerous. In order to prevent such a case, in the present embodiment, near the circumferential surfaces of the fixing belt  15   a   3  and the pressing roller  15   b , thermal protectors such as the thermostats  15   a   6  and  15   b   3  are disposed for detecting abnormal rising temperatures of the fixing belt  15   a   3  and the pressing roller  15   b  and disconnecting a power circuit in the case of exceeding a predetermined temperature. 
     Next, description will be given for electrical control in the fixing device  15 . 
     In the fixing device  15 , based on temperature data detected by the temperature sensor  15   a   7  on the side of a heating element and the temperature sensor  15   b   4  on the side of the pressing roller, and abnormal rising temperature data of the fixing belt  15   a   3  and the pressing roller  15   b  detected by the thermal protectors, a control circuit as a temperature control portion controls power supply to a heat generation member having a heating member as well as halogen heaters  15   a   4 ,  15   a   5  and  15   b   1 , via the power circuit, so as to set the surface temperatures of the fixing belt  15   a   3  and the pressing roller  15   b  to a predetermined temperature. 
     Furthermore, the control circuit controls a driving motor to rotate the fixing roller  15   a   1  around a rotation axis, then rotates the fixing belt  15   a   3 . Subsequently, the control circuit and the power circuit are comprehensively controlled by a device control portion that controls all operation of the fixing device  15 . 
     Specifically, the device control portion outputs a control signal that instructs power supply to the power circuit when receiving an input of an image formation instruction. Here, the image formation instruction is an instruction that is input from an operation panel for a user (not shown) provided on the upper surface of the image forming apparatus  100  in a vertical direction or an external device such as a computer connected to the image forming apparatus  100 , and the image formation instruction is input, and the fixing device  15  thereby starts fixation processing operation. 
     The power circuit in which the control signal is input from the device control portion supplies power to heat sources of the heating roller  15   a   2  and the pressing roller  15   b  (halogen heaters  15   a   4 ,  15   a   5  and  15   b   1 ) via a power control portion to heat the fixing belt  15   a   3  and the pressing roller  15   b.    
     The control circuit, when judging based on the input signal that surface temperatures of the fixing belt  15   a   3  and the pressing roller  15   b  become a predetermined fixing temperature, brings the fixing belt  15   a   3  into contact with the pressing roller  15   b  with a pressure release mechanism, further controls the driving motor, rotates the fixing roller  15   a   1  around a rotational axis, and rotates the fixing belt  15   a   3 . 
     When the fixing belt  15   a   3  is rotated in this manner, the recording sheet  32  on which an unfixed toner image is formed is conveyed to the fixing nip portion  15   e  formed between the fixing belt  15   a   3  and the pressing roller  15   b . At the time, the recording sheet  32  is conveyed so that the surface thereof on which the unfixed toner image is carried faces the side of the fixing belt  15   a   3 . Then, the unfixed toner image on the recording sheet  32  is conveyed by being held in close contact with the outer circumferential surface of the fixing belt  15   a   3 , so that heat is imparted from the fixing belt  15   a   3 , and is fixed to the surface of the recording sheet  32  by receiving applied pressure. 
     Next, description will be given for effects of examples and comparative examples of the fixing device  15  of the present embodiment with reference to a table.  FIG. 5  is a table that compares effects of the examples and the comparative examples of the fixing device of the present embodiment. 
     As shown in  FIG. 5 , in the fixing device  15 , by changing conditions of the configuration of the fixing belt  15   a   3 , the contact/separation state between the fixing belt  15   a   3  and the pressing roller  15   b , viscosity of the silicone oil that impregnates the web  15   c   1  and a fixation range (fixing temperature), effects and a determination result concerning cleaning properties and contamination inside the device were compared. 
     Example 1 
     Example 1 is to cover the surface layer of the fixing belt  15   a   3  with a tube of the PFA, separate the fixing belt  15   a   3  from the pressing roller  15   b , set the viscosity of the silicone oil to 3,000 cs and set the fixing temperature to 40° C. 
     According to the Example 1, even for 200 k (×1000) sheets, both image contamination and contamination inside the device did not occur. 
     Example 2 
     Example 2 is to coat the surface layer of the fixing belt  15   a   3  with the PFA, separate the fixing belt  15   a   3  from the pressing roller  15   b , set the viscosity of the silicone oil to 3,000 cs and set the fixing temperature to 50° C. 
     According to the Example 2, even for 200 k (×1000) sheets, both image contamination and contamination inside the device did not occur. 
     Comparative Example 1 
     Comparative Example 1 is to cover the surface layer of the fixing belt  15   a   3  with a tube of the PFA, separate the fixing belt  15   a   3  from the pressing roller  15   b , set the viscosity of the silicone oil to 300 cs and set the fixing temperature to 40° C. 
     According to the Comparative Example 1, even for 10 k (×1000) sheets, image contamination and contamination inside the device occurred. 
     Comparative Example 2 
     Comparative Example 2 is not to cover the surface layer of the fixing belt  15   a   3  with a tube of the PFA, in the state where the fixing belt  15   a   3  is in contact with the pressing roller  15   b , set the viscosity of the silicone oil to 3,000 cs and set the fixing temperature to 25° C. or less. 
     According to the Comparative Example 2, even for 5 k (×1000) sheets, image contamination occurred, and the image quality was deteriorated due to defects of the peeling properties. 
     According to the results of the Examples 1, 2 and the Comparative Examples 1, 2 described above, it was found that in the fixing device  15 , in a state where the fixing belt  15   a   3  is separated from the pressing roller  15   b , the viscosity of the silicone oil that impregnates the web  15   c   1  is set to 3,000 cs, so that a favorable image is able to be obtained without occurrence of image contamination and contamination inside the device. 
     As described above, the fixing device  15  of the image forming apparatus  100  is configured to be provided with the cleaning unit  15   c  including the web  15   c   1  impregnated with silicone oil that comes into contact with the pressing roller  15   b  to clean up the pressing roller  15   b , set the viscosity of the silicone oil of the web  15   c   1  to 3,000 cs or more, include the contact/separation mechanism  15   d  that brings the fixing belt  15   a   3  into contact with/separation from the pressing roller  15   b , and separate the fixing belt  15   a   3  from the pressing roller  15   b  at the time of rotation operation before fixation. The fixing device  15  prevents the high-viscosity silicone oil that impregnates the web  15   c   1  from moving to the fixing belt  15   a   3  with such a simple configuration to seek improvement of the cleaning properties of the pressing roller  15   b , and without image contamination, it is possible to suppress contamination caused by the silicone oil inside the device and favorably maintain the peeling properties of the recording sheet  32 . This makes it possible to form a uniform and high quality image on the recording sheet  32 . 
     Additionally, since the cleaning unit  15   c  does not retain extra heat capacity, the fixing device  15  is able to shorten the warming-up time until obtaining a predetermined fixing temperature, so that it is possible to realize a fixing device that lessens power consumption to seek energy conservation. 
     Note that, in the present embodiment, the contact/separation mechanism  15   d  provided with the arm  15   d   1  and the cam  15   d   2  for operating the pressing roller  15   b  is employed as contact/separation means that brings the fixing portion  15   a  into contact with/separation from the pressing roller  15   b , however, the present invention, insofar as the fixing portion  15   a  is able to be brought into contact with/separation from the pressing roller  15   b , is not limited to a configuration of the contact/separation means. 
     Hereinbefore, in the above-described embodiment, description has been given for an example that applies the configuration of an image forming apparatus according to the present invention to the image forming apparatus  100  as shown in  FIG. 2 , however, insofar as the image forming apparatus includes the fixing device using the heating portion and the pressing roller, the present invention is not limited to the image forming apparatus with the above-described configuration, therefore it is possible to develop to an image forming apparatus with the other configuration or the like. For example, it is applicable also to an image forming apparatus configured so as to press the heating roller as the heating portion directly to the pressing roller. 
     As described above, the present invention is not limited to the above-described embodiment, and is allowed to have various changes in the scope shown in the claims. That is, even embodiments obtained in combination with technical means to be changed as appropriate without departing from the spirit of the present invention are included in the technical range of the present invention.