Patent Publication Number: US-10775724-B1

Title: Fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-094796 filed May 20, 2019. 
     BACKGROUND 
     (i) Technical Field 
     The present disclosure relates to a fixing device and an image forming apparatus. 
     (ii) Related Art 
     In the related art, a fixing device includes a cleaning section that performs cleaning of an adhering material such as a toner, which adheres to at least one of a heating section and a pressing section. In the fixing device, if the amount of the adhering material such as collected toner obtained by cleaning of the cleaning section exceeds an allowable holding amount, the collected toner and the like are transferred again from the cleaning section onto the heating section or the pressing section, and thus print stains are generated. Thus, as a technique related to such a technical problem, techniques disclosed in JP-A-2003-057985, JP-A-2011-232689, and JP-A-2011-232690 have already been proposed. 
     In JP-A-2003-057985, a predetermined number of transfer targets are caused to pass through a fixing nip portion, and then idling of a rotating body is performed at a temperature at which image formation is performed. A cleaning member is automatically caused to pass through the fixing nip portion, and thus a toner adhering to a cleaning rotating body is collected. 
     In JP-A-2011-232689, if a cleaning mode is set, plural types of cleaning operations are performed in a period in which one cleaning sheet passes through the nip portion. 
     In JP-A-2011-232690, a cleaning mode as follows is provided. In the cleaning mode, when a surface temperature of a fixing rotating body and a surface temperature of a pressing member, which are obtained based on a detected temperature of a temperature detection member are respectively set as Th and Tp, and a deformation end point and an outflow start point of a toner of an unfixed toner image, which are measured with a flow tester are respectively set as Tf 2  and Tf 3 , the fixing rotating body is rotated for a predetermined period in a state of Th≤Tf 3  and Tf 2 ≤Tp≤Tf 3 . After the fixing rotating body is rotated for the predetermined period, a recording material is nipped at the nip portion and transported. In this manner, stains of the pressing member are removed. 
     SUMMARY 
     Aspects of non-limiting embodiments of the present disclosure relate to improving the ability to remove collected toner from a cleaning unit as compared to a case where the temperature during idle cleaning is equal to or lower than the temperature during image fixing. 
     Aspects of certain non-limiting embodiments of the present disclosure address the above advantages and/or other advantages not described above. However, aspects of the non-limiting embodiments are not required to address the advantages described above, and aspects of the non-limiting embodiments of the present disclosure may not address advantages described above. 
     According to an aspect of the present disclosure, there is provided a fixing device including: a first pressing unit that rotates; a second pressing unit that comes into press-contact with the first pressing unit to form a nip portion at which a toner image is fixed on a recording medium; a heating unit that heats at least one of the first and second pressing units; and a first cleaning unit that comes into contact with a surface of at least one of the first and second pressing units to perform cleaning, the fixing device being configured to perform a high-temperature rotating operation that includes rotating the first or second pressing unit and heating a surface of the first or second pressing unit coming into contact with the first cleaning unit to a temperature higher than a temperature during fixing; and then to perform a cleaning operation that includes causing a cleaning material to pass through the nip portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic configuration diagram illustrating an image forming apparatus to which a fixing device according to a first exemplary embodiment of the present disclosure is applied; 
         FIG. 2  is a schematic diagram illustrating a toner; 
         FIG. 3  is a sectional configuration diagram illustrating the fixing device according to the first exemplary embodiment of the present disclosure; 
         FIG. 4  is a sectional configuration diagram illustrating a cleaning device; 
         FIG. 5  is a graph illustrating an index of wettability of a heating roll, a cleaning roll, and a collecting roll; 
         FIGS. 6A to 6C  are configuration diagrams illustrating an operation of a cleaning device in the related art; 
         FIG. 7  is a graph illustrating a relation between a temperature, and a toner cohesive force and an adhesive force of the toner to surfaces of the heating roll, the cleaning roll, and the collecting roll; 
         FIG. 8  is a schematic diagram illustrating a measurement principle for measuring the adhesive force of the toner; 
         FIG. 9  is a schematic diagram illustrating the measurement principle for measuring the adhesive force of the toner; 
         FIG. 10  is a schematic diagram illustrating a measuring device that measures viscosity of the toner; 
         FIG. 11  is a diagram illustrating a principle for collecting toner; 
         FIG. 12  is a diagram illustrating an adhesion state of the toner to the surfaces of the heating roll and the cleaning roll; 
         FIG. 13  is a diagram illustrating the adhesion state of the toner to the surfaces of the heating roll and the cleaning roll; 
         FIG. 14  is a diagram illustrating the adhesion state of the toner to the surfaces of the heating roll and the cleaning roll; 
         FIG. 15  is a configuration diagram illustrating an action of the fixing device according to the first exemplary embodiment of the present disclosure; 
         FIG. 16  is a configuration diagram illustrating a main part of a fixing device according to a second exemplary embodiment of the present disclosure; and 
         FIG. 17  is a schematic sectional diagram illustrating a fixing device according to a third exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present disclosure will be described with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  illustrates an outline of the entirety of an image forming apparatus to which a fixing device according to a first exemplary embodiment is applied. In the drawings, an arrow X indicates a width direction along a horizontal direction, an arrow Y indicates a depth direction along the horizontal direction, and an arrow Z indicates a vertical direction. 
     Overall Configuration of Image Forming Apparatus 
     An image forming apparatus  1  is configured as a color printer, for example. As illustrated in  FIG. 1 , the image forming apparatus  1  includes plural image forming devices  10 , an intermediate transfer device  20 , a paper feeding device  50 , a fixing device  40  according to the exemplary embodiment, and the like. The plural image forming devices  10  form a toner image developed with a toner constituting a developer. The intermediate transfer device  20  holds a toner image formed by each of the image forming devices  10  and finally transports the toner image to a secondary transfer position for secondarily transferring the toner image onto recording paper  5  as an example of a recording medium. The paper feeding device  50  accommodates and transports required recording paper  5  to be supplied to the secondary transfer position of the intermediate transfer device  20 . The fixing device  40  fixes the toner image on the recording paper  5 , which has been secondarily transferred by the intermediate transfer device  20 . The reference sign  1   a  indicates an apparatus main body including a support structure member, an exterior cover, and the like. 
     In the first exemplary embodiment, the plural image forming devices  10  and the intermediate transfer device  20  constitute an image forming unit that forms a toner image on a recording medium. The image forming unit may include a single image forming device and may be configured to directly form a toner image on a recording medium with the image forming device without passing through the intermediate transfer device. 
     The image forming devices  10  include four image forming devices  10 Y,  10 M,  10 C, and  10 K that exclusively form four toner images having four colors being yellow (Y), magenta (M), cyan (C), and black (K), respectively. The four image forming devices  10  (Y, M, C, K) are arranged in one row in an inclined state from the horizontal direction X in an internal space of the apparatus body  1   a.    
     As illustrated in  FIG. 1 , each of the image forming devices  10  (Y, M, C, K) includes a photosensitive drum  11  as an example of a rotating image holding section. Devices as follows are arranged around the photosensitive drum  11 . The basic devices include a charging device  12 , an exposure device  13  as an example of an electrostatic latent image forming section, a developing device  14  (Y, M, C, and K) as an example of a developing section, a primary transfer device  15  as an example of a primary transfer section, and a drum cleaning device  16  as an example of a cleaning section. The charging device  12  charges a circumferential surface (image holding surface) of the photosensitive drum  11 , on which image formation is possible, to a required potential. The exposure device  13  forms an electrostatic latent image (for each color) having a potential difference by irradiating the charged circumferential surface of the photosensitive drum  11  with light based on image information (signal). The developing device  14  (Y, M, C, and K) develops the electrostatic latent image with a toner of a developer having the corresponding color (Y, M, C, and K), so as to form a toner image. The primary transfer device  15  transfers each toner image to the intermediate transfer device  20 . The drum cleaning device  16  performs cleaning by removing an adhering material such as the toner, which remains on and adheres to the image holding surface of the photosensitive drum  11  after the primary transfer. The reference signs indicating the photosensitive drum  11 , the charging device  12 , and the like are attached only to the image forming device  10 Y of the yellow (Y) color, and will be omitted for the other image forming devices  10  (M, C, and K). 
     In the photosensitive drum  11 , an image holding surface having a photoconductive layer (photosensitive layer) made of a photosensitive material on a circumferential surface of a cylindrical or columnar base material to be grounded is formed. The photosensitive drum  11  is supported such that a driving force is transferred from a driving device (not illustrated) and thus the photosensitive drum rotates in a direction indicated by an arrow A. 
     The charging device  12  includes a contact type charging roll disposed to come into contact with the photosensitive drum  11 . A charging voltage is supplied to the charging device  12 . In a case where the developing device  14  performs reversal developing, a voltage or a current having the same polarity as the charged polarity of the toner supplied from the developing device  14  is supplied as the charging voltage. A cleaning roll  121  that performs cleans on the surface of the charging device  12  is disposed on the back surface side of the charging device  12  in a state of being in contact with the charging device. 
     The exposure device  13  includes an LED print head and the like that forms an electrostatic latent image in a manner that the photosensitive drum  11  is irradiated with light according to image information by light emitting diodes (LEDs) as plural light emitting elements arranged along a shaft direction of the photosensitive drum  11 . When a latent image is formed, information (signal) of an image input to the image forming apparatus  1  with any section is transmitted to the exposure device  13 . A device that forms an electrostatic latent image by irradiating the charged circumferential surface of the photosensitive drum  11  with a laser beam configured in accordance with information of an image input to the image forming apparatus  1  may be used as the exposure device  13 . 
     As illustrated in  FIG. 1 , any of the developing device  14  (Y, M, C, and K) is configured to arrange a developing roll  141 , agitation transporting members  142  and  143  such as a screw auger, a layer thickness regulating member  144 , and the like in a housing  140  in which an opening portion and a collection room of the developer are formed. The developing roll  141  holds a developer and transports the developer to a developing region facing the photosensitive drum  11 . The agitation transporting members  142  and  143  transport the developer to pass by the developing roll  141  while agitating the developer. The layer thickness regulating member  144  regulates the amount (layer thickness) of the developer held by the developing roll  141 . A developing voltage is supplied between the developing roll  141  and the photosensitive drum  11  in the developing device  14 , from a power source device (not illustrated). A driving force from the driving device (not illustrated) is transmitted to the developing roll  141  or the agitation transporting members  142  and  143 , and thus the developing roll  141  or the agitation transporting members  142  and  143  rotate in a required direction. A two-component developer containing a non-magnetic toner and a magnetic carrier is used as the four color developers (Y, M, C, and K). 
     For example, an emulsion aggregation (EA) toner is used as the toner T (Y, M, C, and K) having the colors of yellow (Y), magenta (M), cyan (C), and black (K). As illustrated in  FIG. 2 , the emulsion aggregation toner is obtained in a manner that a particulate color material  301 , a synthetic resin  302  having a low melting point, a general hot melt resin  303  in which a lubricant, and the like are dispersed are provided, and an outer circumference is coated with a general hot melt resin  304 , and an external additive  305  formed with functional fine particles for adjusting electrostatic property, cleanability, or the like is added to the outer circumferential surface. For example, a polyester resin is used as the synthetic resin  302  having a low melting point or the general hot melt resins  303  and  304 . The EA toner T has a number average particle diameter of about 3 to 5 μm and is formed in a substantially spherical shape. As the lubricant, for example, a fatty acid metal salt being a compound of a fatty acid such as stearic acid, lauric acid, ricinoleic acid or octylic acid and metal such as lithium, magnesium, calcium, barium or zinc is used. 
     As illustrated in  FIG. 1 , the primary transfer device  15  is a contact type transfer device that comes into contact with the circumferential surface of the photosensitive drum  11  at a primary transfer position through an intermediate transfer belt  21  and rotates, and includes a primary transfer roll to which a primary transfer voltage is supplied. As the primary transfer voltage, a DC voltage showing a polarity opposite to the charged polarity of the toner is supplied from the power source device (not illustrated). 
     The drum cleaning device  16  includes a cleaning blade  161 , a feeding member  162  such as a screw auger, and the like. The cleaning blade  161  is disposed in a container-like main body  160  and performs cleaning by removing an adhering material such as a residual toner. The feeding member  162  collects the adhering material such as the toner, which has been removed by the cleaning blade  161  and feeds the adhering material to a collection system (not illustrated). 
     The intermediate transfer device  20  is disposed to be provided at a position above each of the image forming devices  10  (Y, M, C, and K). The intermediate transfer device  20  basically includes an intermediate transfer belt  21  as an example of an intermediate transfer section, plural belt support rolls  22  to  25 , a secondary transfer device  30 , and a belt cleaning device  26 . The intermediate transfer belt  21  circulates in a direction indicated by an arrow B while passing through the primary transfer position between the photosensitive drum  11  and the primary transfer device  15  (primary transfer roll). The plural belt support rolls  22  to  25  hold the intermediate transfer belt  21  from the inner circumference to be in a desired state and support the intermediate transfer belt to be allowed to be circulated. The secondary transfer device  30  is disposed on the outer circumferential surface (image holding surface) side of the intermediate transfer belt  21  supported by the belt support roll  25  and secondarily transfers the toner image on the intermediate transfer belt  21  onto a recording paper  5 . The belt cleaning device  26  performs cleaning by removing an adhering material such as the toner or paper dust, which remains on and adheres to the outer circumferential surface of the intermediate transfer belt  21  after passing through the secondary transfer device  30 . 
     As the intermediate transfer belt  21 , for example, an endless belt produced with a material in which a resistance adjusting agent such as carbon black is dispersed in a synthetic resin such as a polyimide resin, a polyamide resin, or a polyamideimide resin is used. The belt support roll  22  is configured in a form of a driving roll. The belt support roll  23  is configured in a form of a chamfering roll that holds a traveling position of the intermediate transfer belt  21 . The belt support roll  24  is configured in a form of a tension applying roll. The belt support roll  25  is configured in a form of a secondary transfer backup roll. 
     The secondary transfer device  30  includes a secondary transfer roll  31  that rotates at a secondary transfer position corresponding to the outer circumferential surface portion of the intermediate transfer belt  21  supported by the belt support roll  25  in the intermediate transfer device  20 . A DC voltage showing a polarity which is opposite to or identical to the charged polarity of the toner is supplied to the secondary transfer roll  31  or the belt support roll  25  of the intermediate transfer device  20 , as a secondary transfer voltage. 
     The fixing device  40  is configured in a manner that a heating roll  41  as an example of a first pressing unit, a press belt  42  as an example of a second pressing unit, and the like are arranged. The heating roll  41  is heated by a heating section such that the surface temperature is held to a predetermined temperature. The press belt  42  comes into contact with the heating roll  41  at required pressure and rotates. In the fixing device  40 , a contact portion at which the heating roll  41  is brought into contact with the press belt  42  corresponds to a fixing nip portion N in which a required fixing process (heating and pressing) of fixing an unfixed toner image on the recording paper  5  is performed. The fixing device  40  will be described later in detail. 
     The paper feeding device  50  is disposed to be provided at a position below each of the image forming devices  10  (Y, M, C, and K). The paper feeding device  50  basically includes a single paper accommodation body or bodies  52  and a feeding device  53 . In the paper accommodation body  52 , recording paper  5  having a desired size, a desired type, and the like is accommodated in a state of being loaded on a loading board  51 . The feeding device  53  feeds the recording paper  5  from the paper accommodation body  52  one by one. For example, the paper accommodation body  52  is attached to be capable of being drawn to the front (surface on a side facing a user when the user operates) of the apparatus main body  1   a.    
     The image forming apparatus  1  includes a manual paper feeding device  70  that manually feeds the recording paper  5  to one side surface (right side surface in the example illustrated in  FIG. 1 ) of the apparatus main body  1   a . The manual paper feeding device  70  includes a manual feed tray  71 , a feeding device  72 , and the like. The manual feed tray  71  is mounted in the apparatus main body  1   a  to be openable and accommodates desired recording paper  5  in a state of being loaded thereon. The feeding device  72  feeds the recording paper  5  from the manual feed tray  71  one by one. The manual paper feeding device  70  is used, for example, when recording paper  5  as an example of a cleaning material as will be described later is fed in addition to the recording paper  5  desired to form an image. 
     Examples of the recording paper  5  include thin paper such as plain paper or tracing paper, which is used for an electrophotographic copier, a printer, and the like, and an OHP sheet formed with a transparent film-like medium made of a synthetic resin (PET and the like). In order to improve smoothness on the surface of an image after fixing, it is preferable that the surface of the recording paper  5  is as smooth as possible. For example, coated paper in which the surface of plain paper is coated with a resin or the like, so-called thick paper such as art paper for printing, which has a relatively large basis weight, and the like can be suitably used. As the recording paper  5 , embossed paper and the like in which unevenness is formed on a surface are used. 
     A single (or plural) paper transport roll pair  54  and a paper feeding transporting path  57  are provided between the paper feeding device  50  and the secondary transfer device  30 . The paper transport roll pair  54  transports the recording paper  5  fed from the paper feeding device  50  to the secondary transfer position. The paper feeding transporting path  57  includes transporting guide materials  55  and  56 . A transporting guide material  55   a  is provided between the manual paper feeding device  70  and the paper transport roll pair  54 . The paper transport roll pair  54  disposed at a position just before the secondary transfer position in the paper feeding transporting path  57  is configured as a roll (registration roll) for adjusting a transporting timing of the recording paper  5 , for example. 
     A paper transporting path  59  is provided between the secondary transfer device  30  and the fixing device  40 . The paper transporting path  59  includes, for example, a transporting guide member  58  that transports the recording paper  5  fed from the secondary transfer device  30  to the fixing device  40 . 
     An output transporting path  65  is provided on a downstream side of the fixing device  40 . The output transporting path  65  includes, for example, a paper transport roll pair  61  and a paper output roll pair  62 , or transporting guide materials  63  and  64 , which are used for outputting the recording paper  5  on which the toner image is fixed by the fixing device  40 , to a paper output unit  60  disposed at the upper portion of the apparatus main body  1   a.    
     In  FIG. 1 , the reference sign  100  indicates a control device as an example of a control section. The control device totally controls an operation of the image forming apparatus  1 . The control device  100  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), a bus connecting the CPU, the ROM, and the like to each other, a communication interface, and the like, which are not illustrated in  FIG. 1 . 
     Basic Operation of Image Forming Apparatus 
     A basic image forming operation by the image forming apparatus  1  will be described. 
     Here, an image forming operation when a full-color image constituted by combining toner images having four colors (Y, M, C, and K) is formed by using the four image forming devices  10  (Y, M, C, and K) will be described. An image forming operation when an image obtained by combining a single color toner image or a toner image having plural colors is formed by using any one or more of the four image forming devices  10  (Y, M, C, and K) is basically similar. 
     If the image forming apparatus  1  receives command information of a printing request, the image forming apparatus  1  initiates the four image forming devices  10  (Y, M, C, and K), the intermediate transfer device  20 , the secondary transfer device  30 , the fixing device  40 , and the like under control of the control device  100 . 
     In each of the image forming devices  10  (Y, M, C, and K), firstly, the photosensitive drum  11  rotates in the direction indicated by the arrow A, and the charging device  12  charges the surface of the photosensitive drum  11  to have a required polarity (negative polarity in the first exemplary embodiment) and a potential. The exposure device  13  irradiates the charged surface of the photosensitive drum  11  with light emitted based on a signal of an image obtained by converting information of an image input to the image forming apparatus  1  into color components (Y, M, C, and K). Thereby, an electrostatic latent image having each color component is formed on the surface of the photosensitive drum by a required potential difference. 
     Each of the developing devices  14  (Y, M, C, and K) supplies a toner which has been charged to a required polarity (negative polarity) and has the corresponding color (Y, M, C, or K), to the electrostatic latent image which has been formed on the photosensitive drum  11  and has each color component. Thus, developing is performed by the toner electrostatically adhering to the electrostatic latent image. By the development, the electrostatic latent images which have been formed on the photosensitive drum  11  and have color components are visualized as toner images of four colors (Y, M, C, K) respectively developed with the corresponding color toners. 
     If the color toner image formed on the photosensitive drum  11  in each of the image forming devices  10  (Y, M, C, and K) is transported to the primary transfer position, the primary transfer device  15  performs primary transfer in such a manner that the toner images of the respective colors are sequentially superimposed on the intermediate transfer belt  21  of the intermediate transfer device  20 , which rotates in the direction indicated by the arrow B. 
     In each of the image forming devices  10  (Y, M, C, and K) in which the primary transfer ends, the drum cleaning device  16  performs cleaning on the surface of the photosensitive drum  11  by removing the adhering material to scrape off the adhering material. Thus, each of the image forming devices  10  (Y, M, C, and K) becomes in a state where the next image formation operation is possible. 
     The intermediate transfer device  20  holds the toner image which has been primarily transferred by rotating the intermediate transfer belt  21  and transports the toner image to the secondary transfer position. The paper feeding device  50  feeds required recording paper  5  to the paper feeding transporting path  57  in accordance with the image formation operation. In the paper feeding transporting path  57 , the paper transport roll pair  54  as the registration roll feeds and supplies the recording paper  5  to the secondary transfer position in accordance with a transfer timing. 
     The secondary transfer roll  31  collectively transfers the toner image on the intermediate transfer belt  21  onto the recording paper  5  at the secondary transfer position. In the intermediate transfer device  20  after the end of the secondary transfer, cleaning is performed in a manner that the belt cleaning device  26  removes the adhering material such as the toner, which remains on the surface of the intermediate transfer belt  21  after the secondary transfer. 
     The recording paper  5  on which the toner image has been secondarily transferred is peeled from the intermediate transfer belt  21  and the secondary transfer roll  31  and then is transported to the fixing device  40  along the paper transporting path  59 . In the fixing device  40 , the recording paper  5  which holds an unfixed toner image after the secondary transfer enters into the fixing nip portion N between the heating roll  41  and the press belt  42  which rotate, and the recording paper  5  is caused to pass through the fixing nip portion N. Thus, the necessary fixing process (heating and pressing) is performed to fix the unfixed toner image onto the recording paper  5 . The recording paper  5  after the fixing ends is output to the paper output unit  60  provided at the upper portion of the apparatus main body  1   a  by the paper output roll pair  62  through the output transporting path  65 . 
     With the above operation, a full-color image formed by combining toner images formed with toners T of four colors (Y, M, C, and K) is output. 
     Configuration of Fixing Device 
     The fixing device  40  according to the first exemplary embodiment is a so-called free belt nip type fixing device. As illustrated in  FIG. 3 , roughly, the fixing device  40  includes a heating roll  41  as an example of the first pressing unit, a press belt  42  as an example of the second pressing unit, a press-contact member  44  as an example of a press-contact unit, a holding member  45  as an example of a holding unit, a sliding sheet  46  as an example of a sheet unit, and a felt member  47  as an example of a lubricant holding unit. The heating roll  41  rotates. The press belt  42  includes an endless belt and forms the fixing nip portion N which comes into press-contact with the heating roll  41  to fix an unfixed toner image Ti to the recording paper  5 . The press-contact member  44  is disposed in the press belt  42  and brings the press belt  42  in press-contact with the surface of the heating roll  41 . The holding member  45  holds the press-contact member  44 . The sliding sheet  46  is interposed between the press belt  42  and the press-contact member  44  to reduce sliding resistance. The felt member  47  holds a lubricant applied onto the inner circumferential surface of the press belt  42 . 
     The heating roll  41  includes a cylindrical core bar  411 , an elastic body layer  412 , and a release layer  413 . The core bar  411  is made of metal such as stainless steel, aluminum, or iron (thin-walled high-tensile steel pipe). The elastic body layer  412  covers the outer circumference of the core bar  411  and is made of an elastic material such as silicone rubber or fluororubber, which has heat resistance. The release layer  413  thinly covers the surface of the elastic body layer  412  and is made of perfluoroalkoxyalkane (PFA), polytetrafluoroethylene (PTFE), or the like. In the first exemplary embodiment, perfluoroalkoxyalkane (PFA) is used for the release layer  413  of the heating roll  41 . Two halogen lamps  414   a  and  414   b  as an example of a heating unit (heating source) are arranged in the heating roll  41 . The two halogen lamps  414   a  and  414   b  are appropriately used in accordance with the size in a direction intersecting with a transporting direction of the recording paper  5 , for example. 
     Both end portions of the heating roll  41  in the shaft direction are supported to a frame (not illustrated) of the fixing device  40  to be rotatable through a bearing member (not illustrated). The heating roll  41  is rotationally driven at a required speed in a direction indicated by an arrow C by a driving device (not illustrated) through a driving gear attached to one end portion along the shaft direction. A rotation speed of the heating roll  41 , that is, a fixing speed, can be set to plural speeds such as a high speed, medium speed, and a low speed, in accordance with the basis weight and the like of the recording paper  5 . In the first exemplary embodiment, only one type of fixing speed is set as the rotation speed of the heating roll  41 . The press belt  42  is driven to rotate at substantially the same speed with the rotation of the heating roll  41  in a state of press-contacting with the outer circumferential surface of the heating roll  41 . 
     The surface temperature of the heating roll  41  is detected by a non-contact type temperature sensor  431  as an example of a temperature detection section. Energization to the halogen lamps  414   a  and  414   b  is controlled by a temperature control circuit using a triac (not illustrated) based on a detection result of the temperature sensor  431 , and thereby the surface of the heating roll  41  is heated to a required fixing temperature (for example, 170° C. to 190° C.). 
     In the first exemplary embodiment, plural (three) temperatures which are 170° C. as a fixing temperature for thin paper, 180° C. as a fixing temperature for plain paper, and 190° C. as a fixing temperature for thick paper are set as the surface temperature of the heating roll  41  in fixing. Here, the plain paper refers to paper having a basis weight of 52 g/m 2  to 105 g/m 2 . The thick paper refers to paper having a basis weight which is greater than 105 g/m 2  and is equal to or smaller than 350 g/m 2 . The thin paper refers to a paper having a basis weight which is smaller than 52 g/m 2 . The classification of the thin paper, the plain paper, and the thick paper of the recording paper  5  is not limited to the above-described basis weight, and the recording paper  5  may be classified based on other basis weights. 
     As illustrated in  FIG. 3 , the press belt  42  is configured as a thin cylindrical flexible endless belt. The press belt  42  includes a base material layer, an elastic body layer with which the surface of the base material layer is coated, and a release layer with which the surface of the elastic body layer is coated. The press belt  42  may include the base material layer and the release layer with which the surface of the base material layer is directly coated. The base material layer is formed of a heat-resistant synthetic resin such as polyimide, polyamide, or polyamideimide, or of metal such as stainless steel, nickel, or copper. The elastic body layer is made of an elastic body such as silicone rubber or fluororubber having heat resistance. Similar to the heating roll  41 , the release layer is formed of perfluoroalkoxyalkane (PFA), polytetrafluoroethylene (PTFE), or the like. In the first exemplary embodiment, similar to the heating roll  41 , perfluoroalkoxyalkane (PFA) is used for the release layer (not illustrated) of the press belt  42 . The thickness of the press belt  42  may be set to about 50 to 200 μm, for example. 
     The press belt  42  is supported such that both end portions of the press belt in a longitudinal direction (shaft direction) is rotatable by guide member (not illustrated). 
     The press-contact member  44  is disposed in the press belt  42 . The press-contact member  44  includes a press pad that forms the fixing nip portion N between the heating roll  41  and the press belt  42  by bringing the press belt  42  into press-contact with the surface of the heating roll  41 . 
     The press-contact member  44  is formed from an elastic body having heat resistance, such as silicone rubber or fluororubber, in an elongated rectangular parallelepiped shape having a rectangular cross-section. The press-contact member  44  is disposed to bring the press belt  42  into press-contact with the outer circumferential surface of the heating roll  41  along a center line passing through the center O of the heating roll  41 . For example, an elongated rectangular plate material (not illustrated) made of a thin metal plate of stainless steel or the like is fixed on the back surface of the press-contact member  44  by a method such as adhesion. 
     The holding member  45  may be formed by using a heat-resistant and rigid synthetic resin such as polyphenylene sulfide (PPS), polyimide, polyester, or polyamide, or metal such as iron, aluminum, or stainless steel as a material. 
     As illustrated in  FIG. 3 , the holding member  45  is formed in an elongated rectangular parallelepiped shape having a substantially trapezoidal cross-section. A recess portion  451  for attaching the press-contact member  44  is provided on the surface of the holding member  45 , which faces the heating roll  41 . 
     It is not necessary that the entirety of the holding member  45  includes the same members. The holding member  45  may be formed by combining plural members, for example, a member that holds the press-contact member  44  and a member that presses the member that holds the press-contact member  44  against the outer circumferential surface of the heating roll  41 . 
     The cross-sectional shape of the holding member  45  is not limited to a substantially trapezoidal shape and may be formed to have any cross-sectional shape that holds the press belt  42  to be rotatable (circularly move). The holding member  45  is disposed so that a portion (lower end surface)  455  comes into contact with the inner circumferential surface of the press belt  42  via the sliding sheet  46 , in order to hold the press belt  42  to be rotatable. 
     The sliding sheet  46  includes an elongated planar rectangular sheet. As the sliding sheet  46 , for example, a sheet including a base layer made of a fluororesin such as polytetrafluoroethylene (PTFE) and a structure made of a woven fabric or a knitted fabric made of aramid fibers or the like laminated on the surface of the base layer or both front and back surfaces is used. As the sliding sheet  46 , a sheet including only a base layer made of a fluororesin such as polytetrafluoroethylene (PTFE) may be used. The thickness of the sliding sheet  46  may be set to about 100 to 200 μm. 
     The sliding sheet  46  is locked to plural convex portions  452  through plural engagement holes (not illustrated). The convex portions are provided at a step portion  453  of the holding member  45  toward the back surface side. The engagement holes are provided at the end portion of the sliding sheet  46  on an upstream side in the rotation direction of the press belt  42 . The end portion of the sliding sheet  46  on a downstream side in the rotation direction of the press belt  42  passes through the fixing nip portion N and extends to a position along the upper end portion of the holding member  45 . 
     As illustrated in  FIG. 3 , a mounting portion  454  is provided at the end portion of the holding member  45  on an opposite side of the fixing nip portion N. The mounting portion  454  is formed from a flat surface for attaching the felt member  47 . The felt member  47  is attached to the mounting portion  454  by a method such as sticking with a heat-resistant double-sided tape (not illustrated) or bonding with an adhesive. A required amount (for example, about 3 g) of a lubricant to be supplied in a state of being applied onto the inner circumferential surface of the press belt  42  is impregnated in the felt member  47  in advance. As the lubricant, amino-modified silicone oil or the like having a viscosity of 100 to 350 cs is used. Since the lubricant is impregnated in the felt member  47  in advance, the lubricant is supplied to be applied onto the inner circumferential surface of the press belt  42 . However, it is not limited thereto, and the lubricant may be supplied in a state of being initially applied onto the inner circumferential surface of the press belt  42 . 
     A cleaning device  80  that removes the toner, paper dust of the recording paper  5 , which adhere to the surface of the heating roll  41 , and the like is disposed on the surface of the heating roll  41 . The cleaning device  80  includes a cleaning roll  81  as an example of a first cleaning unit and a collecting roll  82  as an example of a second cleaning unit. The cleaning roll  81  comes into contact with the surface of the heating roll  41  at required pressing force. The collecting roll  82  comes into contact with the surface of the cleaning roll  81  at required pressing force. The cleaning roll  81  is disposed in the vicinity of the lower end portion in the figure, which is close to the fixing nip portion N, on the outer circumferential surface of the heating roll  41 . The position of the cleaning roll  81  is not limited to the lower end portion close to the fixing nip portion N, and may be disposed at another position on the outer circumferential surface of the heating roll  41 . 
     As illustrated in  FIG. 4 , the cleaning roll  81  is configured in a manner that an outer circumference of a metal core bar  811  which has a cylindrical shape and is made of stainless steel, iron, aluminum or the like is coated with a coating layer  812  made of a rubber material made of silicone rubber, fluororubber, or the like, to have a required thickness. In the first exemplary embodiment, silicone rubber is used for the coating layer  812  of the cleaning roll  81 . 
     The collecting roll  82  is formed of metal such as stainless steel, iron, or aluminum, a synthetic resin such as a phenol resin, and the like and is formed in a solid cylindrical shape. In order to improve the ability to collect and hold the toner, the outer circumferential surface  82   a  of the collecting roll  82  is subjected to a roughening process by blasting or the like, so as to have a predetermined surface roughness. In the first exemplary embodiment, a solid cylindrical member which is made of stainless steel and has a surface  82   a  subjected to blasting is used as the collecting roll  82 . The outer diameter of the collecting roll  82  is set to be smaller than that of the cleaning roll  81 . The outer diameter of the collecting roll  82  may be set to be equal to or larger than that of the cleaning roll  81 . 
     As illustrated in  FIG. 5 , the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  are set such that the wettability of the surfaces thereof decreases in order of the heating roll&gt;the cleaning roll&gt;the collecting roll. That is, the heating roll  41  has the worst wettability on the surface. The wettability of the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  is an index indicating the magnitude of the surface free energy. As the wettability becomes better (surface free energy increases), the adhesive force to the toner increases. Thus, among the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 , the collecting roll  82  has the largest adhesive force to the toner, the cleaning roll  81  is the next largest, and the heating roll  41  has the small adhesive force to the toner. Therefore, the toner adhering to the surface of the heating roll  41  is transferred to the cleaning roll  81  having a larger adhesive force. The toner transferred to the cleaning roll  81  is finally transferred to the collecting roll  82  having the highest adhesive force, and thus is collected and accumulated. 
     If the wettability of the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  is represented by a contact angle of water, perfluoroalkoxyalkane (PFA) forming the release layer  413  being a surface layer of the heating roll  41  has an angle of about 109 degrees. Silicone rubber forming the coating layer  812  of the cleaning roll  81  has an angle of about 80 to 90 degrees. Stainless steel forming the collecting roll  82  has an angle of about 80 degrees. If the collecting roll  82  is made of stainless steel and has a surface  82   a  subjected to blasting, the wettability is more improved (contact angle of the water is reduced). Aluminum usable as the material of the collecting roll  82  is excellent in wettability because the contact angle of the water is about 4.6 degrees, and this is very small. 
     As illustrated in  FIG. 3 , in the fixing device  40 , in a fixing operation, fixing processing is performed by causing the recording paper  5  holding an unfixed toner image Ti to pass through the fixing nip portion N. At this time, the unfixed toner image Ti held by the recording paper  5  comes into contact with the surface of the heating roll  41  at the fixing nip portion N. However, in the toner forming the unfixed toner image Ti, the amount of the toner transferred (offset) to the surface of the heating roll  41  is about zero or very small even though the toner is transferred, because the toner forming the unfixed toner image Ti has the very large adhesive force to the recording paper  5 . 
     Even if the toner is transferred to the surface of the heating roll  41 , the toner transferred to the surface of the heating roll  41  moves to a cleaning position coming into contact with the cleaning roll  81  by rotation of the heating roll  41 . The toner transferred to the surface of the heating roll  41  is subjected to cleaning (removed) by moving from the surface of the heating roll  41  to the cleaning roll  81  at the cleaning position. The toner moved to the surface of the cleaning roll  81  moves from the surface of the cleaning roll  81  to the collecting roll  82  at a collect position coming into contact with the collecting roll  82 , and then is collected. The toner collected by the collecting roll  82  is held in a state of adhering to the outer circumferential surface of the collecting roll  82 . 
     As described above, in the fixing device  40 , in the normal fixing operation, the amount of the toner transferred to the outer circumferential surface of the heating roll  41  is about zero or very small. Therefore, even in a case where the fixing device  40  is used for a long term, the total amount of the toner removed and collected by the cleaning roll  81  and the collecting roll  82  of the cleaning device  80  is not so much. Thus, the cleaning device  80  disposed on the heating roll  41  can be used until the fixing device  40  reaches the end of the life without replacing the cleaning roll  81  and the collecting roll  82 . 
     However, in the fixing device  40 , as illustrated in  FIG. 6A , for example, in a case where so-called borderless printing in which a full color image or the like is formed over the outer circumferential edge of the recording paper  5  is continuously performed, the toner tends to adhere to the surface of the heating roll  41  from the outer edge of the recording paper  5 . This is because, in a region other than the outer edge portion of the recording paper  5 , other toner images are provided around the toner image. Thus, an attraction force acts between the adjacent toner images, and the toner of the toner image has difficulty in adhering to the surface of the heating roll  41 . On the contrary, at the outer edge portion of the recording paper  5 , any other toner image is not provided outside the toner image at the outer edge portion. Thus, the attraction force does not act between the adjacent toner images, and the toner of the toner image adheres to the surface of the heating roll  41  easier than in the region other than the outer edge. The toner Tc adhering to the surface of the heating roll  41  is subjected to cleaning by the cleaning roll  81 , and then moves from the surface of the cleaning roll  81  to the collecting roll  82  to be collected. At this time, the toner Tc which is not subjected to cleaning at once by the cleaning roll  81  remains on the surface of the heating roll  41 . Here, the borderless printing is not limited to a case where the toner image is formed over the outer circumference of the recording paper  5 , and includes a case where the toner image is formed to provide a slight gap inside the outer circumference of the recording paper  5 . 
     Therefore, as illustrated in  FIG. 6B , the toner Tc removed from the surface of the heating roll  41  is gradually accumulated on the surface of the collecting roll  82  through the cleaning roll  81 . For example, in a case where a borderless print image is fixed on multiple sheets of the recording paper  5 , that is, if the amount of the collected toner Tc accumulated on the surface of the collecting roll  82  exceeds an allowable amount which can be held by the collecting roll  82 , as illustrated in  FIG. 6C , part of the collected toner Tc is reversely transferred to the cleaning roll  81 , and is accumulated on the outer circumferential surface of the cleaning roll  81 . 
     Further, if the number of recording paper  5  on which borderless printing is performed increases cumulatively, and thus the amount of the collected toner Tc accumulated on the outer circumferential surface of the cleaning roll  81  exceeds the allowable amount, as illustrated in  FIG. 6C , a portion of the collected toner Tc held on the outer circumferential surface of the cleaning roll  81  is reversely transferred to the surface of the heating roll  41 . 
     The collected toner Tc which has been reversely transferred to the surface of the heating roll  41  is transferred onto the recording paper  5  to be subjected to fixing processing thereafter, or is transferred to the press belt  42  and then transferred to the back surface of the recording paper  5  to cause image stain. As a result, the fixing device  40  has a lifespan which does not allow the cleaning device  80  to perform a cleaning function, and thus it is necessary to replace the entirety of the fixing device  40 . 
     In the fixing device  40 , for example, so-called jam (paper jam) which is a transport failure of the recording paper  5  may occur, that is, the toner T forming the unfixed toner image Ti on the recording paper  5  may adhere to the surface of the heating roll  41  in a large amount, in addition to the borderless printing described above. 
     More specifically, in a case where jam of the recording paper  5  that holds the unfixed toner image Ti occurs at the fixing nip portion N of the fixing device  40 , normally, a nip release mechanism (not illustrated) is operated to release the press-contact state between the heating roll  41  and the press belt  42 . In this state, the recording paper  5  of which the jam occurs at the fixing nip portion N is removed. In this case, the recording paper  5  that holds the unfixed toner image Ti does not come into contact with the surface of the heating roll  41  or comes into contact with the surface of the heating roll  41  by a weak force even though the contact occurs. Thus, a situation in which a large amount of the toner T of the unfixed toner image Ti held on the recording paper  5  adheres to the surface of the heating roll  41  does not occur. 
     However, depending on a user, when the jam of the recording paper  5  occurs at the fixing nip portion N of the fixing device  40 , the press-contact state between the heating roll  41  and the press belt  42  is not released, and the leading end of the recording paper  5  is grasped and pulled from the paper output roll pair  62  side of the apparatus main body  1   a . Thereby, the recording paper  5  of which the jam occurs may be removed. 
     In this case, when the recording paper  5  is pulled out, the heating roll  41  rotates while the recording paper  5  moves. Thus, the toner T forming the unfixed toner image Ti held on the recording paper  5  adheres to the surface of the heating roll  41  in a large amount. The toner Tc adhering to the surface of the heating roll  41  in the large amount is subjected to cleaning by the cleaning roll  81  at the cleaning position and is transferred to the surface of the cleaning roll  81 . The large amount of the toner transferred to the surface of the cleaning roll  81  is collected by the collecting roll  82  and is held on the surface of the collecting roll  82 . 
     As described above, if the recording paper  5  is removed without releasing the press-contact state between the heating roll  41  and the press belt  42 , the amount of the toner adhering to the surface of the heating roll  41  becomes large even once. If the amount of the collected toner Tc accumulated on the surface of the collecting roll  82  exceeds the allowable amount which can be held by the collecting roll  82 , a portion of the collected toner Tc starts to be reversely transferred to and accumulated on the outer circumferential surface of the cleaning roll  81 . If the amount of the collected toner Tc accumulated on the outer circumferential surface of the cleaning roll  81  exceeds the allowable amount, as illustrated in  FIG. 6C , a portion of the collected toner Tc held on the outer circumferential surface of the cleaning roll  81  starts to be reversely transferred to the surface of the heating roll  41 , and the fixing device  40  reaches the end of the life. 
     Further, in the fixing device  40 , embossed paper having an unevenness on a surface may be used as the recording paper  5 . The embossed paper is different from flat plain paper in that it is difficult to apply sufficient fixing pressure and heat to the unfixed toner image Ti at the recess portion of the surface when the embossed paper passes through the fixing nip portion N of the fixing device  40 . Therefore, in the embossed paper, it is easy to cause the toner T forming the unfixed toner image Ti to adhere to the surface of the heating roll  41 , and the amount of the toner collected by the cleaning roll  81  is much larger than that of plain paper. 
     As described above, in the fixing device  40 , if many sheets of embossed paper are used as the recording paper  5 , it may be easy to cause the toner T of the unfixed toner image Ti to adhere to the surface of the heating roll  41 , and a large amount of the collected toner Tc may be collected by the cleaning roll  81 . Thus, the amount of the collected toner Tc held by the cleaning roll  81  and the collecting roll  82  may exceed the allowable amount. 
     In any case, since a fixing device  40  in the related art does not include a section configured to remove the collected toner Tc from the cleaning roll  81  and the collecting roll  82 , if the cleaning roll  81  and the collecting roll  82  reach the end of the lives, the fixing device  40  is to be replaced with a new fixing device. 
     In the fixing device  40  according to the first exemplary embodiment, the removability of the collected toner Tc held by the cleaning roll  81  and the collecting roll  82  is improved. Even in a case where the amount of the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82  reaches the allowable amount, it is possible to continuously use the cleaning roll  81  and the collecting roll  82  by removing the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82 . 
     In order to clarify conditions in which the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82  can be removed, the present disclosers have considered how the force causing the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82  to aggregate each other, that is, a cohesive force which causes the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82  to maintain a state of aggregating each other and adhering to the cleaning roll  81  and the collecting roll  82  as it is, and an adhesive force with which the toner adheres to the surface of each of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  changes depending on the temperature. 
     As a result, the present disclosers have found the followings. That is, depending on the temperatures of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 , the cohesive force causing the collected toner Tc adhering to the outer circumferential surfaces of the cleaning roll  81  and the collecting roll  82  to aggregate each other causes the adhesive force of the collected toner Tc to the outer circumferential surface of the heating roll  41  to be increase. Thus, it is possible to remove the collected toner Tc adhering to the cleaning roll  81  and the collecting roll  82  by reversely transferring the collected toner Tc to the heating roll  41  from the cleaning roll  81  and the collecting roll  82 . 
       FIG. 7  is a graph in which a horizontal axis indicates a temperature, and a vertical axis indicates a result obtained by measuring a cohesive force Ftn of the collected toner, and adhesive forces Fhr, Fcln, and Fspt of the toner to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 , respectively. Regarding the collected toner and a new toner, paper dust or the like of the recording paper  5  may be mixed in the collected toner, and strictly speaking, the composition is different. However, here, the new toner was used as the collected toner. As the new toner, EA-Eco toner manufactured by Fuji Xerox Co., Ltd. was used. 
     The release layer  413  provided on the surface of the heating roll  41  was made of perfluoroalkoxyalkane (PFA). The coating layer  812  of the cleaning roll  81  was made of silicone rubber. The surface  82   a  of the collecting roll  82  was made of blasted stainless steel. 
     The adhesive force of the toner to the surface of each of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  was measured using a tacking tester. As the tacking tester, TAC-1000 manufactured by Resuka Co., Ltd. was used. As illustrated in  FIG. 8 , the tacking tester is used as follows. A predetermined amount of toner being a target for measuring the adhesive force is placed on the surface of a stage S. A probe P having a lower end surface made of the same material as that of the surface of each of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  is caused to come into press-contact with the surface of the toner placed on the stage S. Then, the maximum value of the adhesive force (kPa) required to pull up the probe P is measured. 
     In a case where the adhesive force of the toner to the surface of each of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  is larger than the cohesive force Ftn of the collected toner, as illustrated in  FIG. 9 , the toner is not peeled from the probe P, and the cohesive force Ftn of the aggregated toner reaches the limit. Thus, the toner is divided. Thus, if the temperature at which the state is moved from the state in  FIG. 8  to the state in  FIG. 9  is measured using the tacking tester, it is possible to obtain the temperature at which the adhesive force of the toner to the surface of each of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  is equal to the cohesive force Ftn of the collected toner in  FIG. 7 . 
     The adhesive force was measured by changing the temperatures of the probe P and the stage S over a range from room temperature to about 220° C. in a laboratory environment (temperature of 20° C., relative humidity of 50%). 
     The cohesive force of the collected toner was measured as follows. 
     The cohesive force of the toner corresponds to internal resistance when the toner flows. The cohesive force of the toner is a standard for fluidity of the toner. The cohesive force of the fluidized toner was obtained by measuring the viscosity of the toner. 
     As a measuring device for measuring the viscosity of the toner, a Koka type Flow Tester CFT-500C (manufactured by Shimadzu Corporation) was used. As illustrated in  FIG. 10 , the viscosity of the toner was measured in a manner as follows. Under conditions in which the diameter of pores of a die D, through which the fluidized toner passes was 0.5 mm, the length of the pore of the die D was 1.0 mm, a pressurization load of a cylinder Cy is 0.98 MPa (10 Kg/cm 2 ), a preheat time is 5 minutes, a heating rate was 1.0° C./min, a temperature measurement interval was 1.0° C., and a measurement start temperature was 65° C., 1.1 g of the toner was weighed, and the toner was set in the Koka type flow tester CFT-500C. The toner was melted out, and thereby the viscosity of the toner was measured. The melting point of a polystyrene resin mainly forming the toner is about 225 to 276° C. The unit of the viscosity of the toner is kPaS. 
     In order to allow the adhesive force of the toner, which was measured using the tacking tester to be compared with the viscosity of the toner, which was measured using the Koka type flow tester, as illustrated in  FIG. 7 , for example, as described above, the temperature at which the state is transferred from the state in  FIG. 8  to the state in  FIG. 9  may be measured using the tacking tester. 
     As is apparent from  FIG. 7 , the adhesive forces Fhr, Fcln, and Fspt of the collected toner Tc to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  reach a substantially predetermined value after rapidly increasing as the temperature rises from room temperature. The adhesive forces maintain the substantially predetermined values even though the temperature rises after that. 
     Regarding the adhesive forces Fhr, Fcln, and Fspt of the collected toner Tc to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 , considering the ability to collect the toner, the collecting roll  82  is set to have the largest adhesive force, the cleaning roll  81  is set to have the next largest adhesive force, and the heating roll  41  is set to have the smallest adhesive force (Fhr&lt;Fcln&lt;Fspt). 
     As is apparent from  FIG. 7 , the cohesive force Ftn of the collected toner Tc increases rapidly with an increase of the temperature, similar to the adhesive forces Fhr, Fcln, and Fspt of the collected toner Tc. After the cohesive force Ftn reaches the peak at a required temperature (the vicinity of a glass transition temperature), the cohesive force Ftn decreases in inverse proportion to the temperature. 
     If the adhesive forces Fhr, Fcln, and Fspt of the collected toner to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  are compared with the cohesive force Ftn of the collected toner Tc, as illustrated in  FIG. 7 , the adhesive forces Fhr, Fcln, and Fspt and the cohesive force Ftn simultaneously increase rapidly with the increase of the temperature. Then, the adhesive force Fhr of the toner to the surface of the heating roll  41  reaches a substantially predetermined value firstly, the adhesive force Fcln of the toner to the surface of the cleaning roll  81  reaches a substantially predetermined value secondly, and the adhesive force Fspt of the toner to the surface of the collecting roll  82  reaches a substantially predetermined value thirdly. The cohesive force Ftn of the collected toner Tc reaches the peak lastly. 
     The cohesive force Ftn of the collected toner Tc decreases rapidly with the increase of the temperature. The cohesive force Ftn falls below the adhesive force Fspt of the toner to the surface of the collecting roll  82  firstly, falls below the adhesive force Fcln of the toner to the surface of the cleaning roll  81  secondly, and falls below the adhesive force Fhr of the toner to the surface of the heating roll  41  lastly. 
     Here, in  FIG. 7 , a temperature Ti indicates a temperature at which a curve indicating the cohesive force Ftn of the collected toner Tc intersects with (is equivalent to) a line indicating the adhesive force Fhr of the toner to the surface of the heating roll  41 . A temperature T 2  indicates a temperature at which the curve indicating the cohesive force Ftn of the collected toner Tc intersects with a line indicating the adhesive force Fspt of the toner to the surface of the collecting roll  82 . A temperature T 3  indicates a temperature at which the curve indicating the cohesive force Ftn of the collected toner Tc intersects with the line indicating the adhesive force Fspt of the toner to the surface of the collecting roll  82 . A temperature T 4  indicates a temperature at which the curve indicating the cohesive force Ftn of the collected toner Tc intersects with a line indicating the adhesive force Fcln of the toner to the surface of the cleaning roll  81 . A temperature T 5  indicates a temperature at which the curve indicating the cohesive force Ftn of the collected toner Tc intersects with the line indicating the adhesive force Fhr of the toner to the surface of the heating roll  41 . 
     The temperature region from the temperature Ti to the temperature T 5  indicates a temperature region in which the cleaning function of removing the toner Tc adhering to the surface of the heating roll  41  by the cleaning roll  81  and the collecting roll  82  is established. 
     For example, the fixing temperature of the fixing device  40  is appropriately set in a range from a temperature T 6  located between the temperatures T 2  and T 3  to a temperature T 7  located between the temperatures T 4  and T 5 . 
     In  FIG. 7 , in a temperature region I which is lower than the temperature T 1 , the adhesive force Fhr of the toner to the surface of the heating roll  41  is substantially equal to the adhesive force Fcln of the toner to the surface of the cleaning roll  81 . Thus, as illustrated in  FIG. 11 , the toner Tc adhering to the surface of the heating roll  41  is not transferred to the surface of the cleaning roll  81 , but continuously adheres to the surface of the heating roll  41  at the cleaning position. Accordingly, it is not possible to perform cleaning of the toner. 
     Next, in a temperature region II from the temperature Ti to the temperature T 2 , the adhesive force Fcln of the toner to the surface of the cleaning roll  81  is larger than the adhesive force Fhr of the toner to the surface of the heating roll  41  (Fhr&lt;Fcln). Thus, the toner Tc adhering to the surface of the heating roll  41  is transferred to the surface of the cleaning roll  81  at the cleaning position, and thus it is possible to perform cleaning of the toner. 
     Similarly, in a temperature region III from the temperature T 2  to the temperature T 4 , the adhesive force Fspt of the toner to the surface of the collecting roll  82  is larger than the adhesive force Fcln of the toner to the surface of the cleaning roll  81  (Fcln&lt;Fspt), and the cohesive force Ftn of the collected toner is larger than the adhesive force Fcln of the toner to the surface of the cleaning roll  81 . Thus, the collected toner Tc adhering to the surface of the cleaning roll  81  moves to the surface of the collecting roll  82  and continuously remains on the surface of the collecting roll  82 . 
     Further, in a temperature region II from the temperature T 4  to the temperature T 5 , the adhesive force Fcln of the toner to the surface of the cleaning roll  81  is larger than the adhesive force Fhr of the toner to the surface of the heating roll  41  (Fhr&lt;Fcln), and the adhesive force Fcln of the toner to the surface of the cleaning roll  81  is larger than the cohesive force Ftn of the collected toner (Ftn&lt;Fcln). Thus, the toner Tc adhering to the surface of the heating roll  41  is transferred to the surface of the cleaning roll  81  at the cleaning position, and thus it is possible to perform cleaning of the toner. 
     On the contrary, in a temperature region IV which is higher than the temperature T 5 , the adhesive force Fcln of the toner to the surface of the cleaning roll  81  is larger than the cohesive force Ftn of the collected toner, and the adhesive force Fhr of the toner to the surface of the heating roll  41  is larger than the cohesive force Ftn of the collected toner (Ftn&lt;Fhr). Thus, the collected toner Tc coming into contact with the surface of the cleaning roll  81  in the collected toner Tc adhering to the surface of the collecting roll  82  is transferred to the surface of the cleaning roll  81  because the adhesive force Fcln of the toner to the surface of the cleaning roll  81  is stronger than the cohesive force Ftn of the collected toner Tc. 
     Similarly, in the temperature region IV which is higher than the temperature T 5 , the collected toner Tc coming into contact with the surface of the heating roll  41  in the collected toner Tc adhering to the surface of the cleaning roll  81  is transferred to the surface of the heating roll  41  because the adhesive force Fhr of the toner to the surface of the heating roll  41  is stronger than the cohesive force Ftn of the collected toner Tc (Ftn&lt;Fhr). 
     As a result, if the temperatures of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  are set to the temperature region IV which is higher than the temperature T 5 , it is possible to transfer the collected toner Tc held by the collecting roll  82  to the surface of the cleaning roll  81 , and to transfer the collected toner Tc transferred to the surface of the cleaning roll  81 , to the surface of the heating roll  41 . Here, the temperature in the temperature region IV which is higher than the temperature T 5  is set to 210° C., for example. 
     In  FIG. 11 , for convenience, gaps for clarifying the presence of the collected toner Tc adhering to each of the rolls are formed between the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 . However, the rolls may be disposed to come into contact with each other. 
     More specifically, as illustrated in  FIG. 12 , it is considered that the adhesive force Fcln to the surface of the cleaning roll  81 , the adhesive force Fhr to the surface of the heating roll  41 , and the cohesive force Ftn causing the collected toner to aggregate each other act on the collected toner Tc held on the surface of the cleaning roll  81 . Here, for convenience, a case where the collected toner Tc held on the surface of the cleaning roll  81  has two layers is illustrated. However, the above descriptions are similarly applied to a case where the collected toner Tc held on the surface of the cleaning roll  81  has three or more layers. 
     Next, in a case where the adhesive force Fcln of the collected toner Tc to the surface of the cleaning roll  81  is larger than the cohesive force Ftn of the collected toner Tc and is larger than the adhesive force Fhr of the collected toner Tc to the surface of the heating roll  41  (Fcln&gt;Ftn and Fcln&gt;Fhr), the collected toner Tc held on the surface of the cleaning roll  81  continuously adheres to the surface of the cleaning roll  81  even after coming into contact with the surface of the heating roll  41  at the cleaning position, as illustrated in  FIG. 12 . 
     In a case where the adhesive force Fhr of the collected toner Tc to the surface of the heating roll  41  is larger than the cohesive force Ftn of the collected toner Tc and is smaller than the adhesive force Fcln of the collected toner Tc to the surface of the cleaning roll  81  (Fhr&gt;Ftn and Fcln&gt;Fhr), when only the uppermost toner of the collected toner Tc held on the surface of the cleaning roll  81  comes into contact with the surface of the heating roll  41  at the cleaning position as illustrated in  FIG. 13 , the uppermost toner is reversely transferred to the surface of the heating roll  41 . 
     Further, in a case where the adhesive force Fhr of the collected toner Tc to the surface of the heating roll  41  is larger than the cohesive force Ftn of the collected toner Tc and is larger than the adhesive force Fcln of the collected toner Tc to the surface of the cleaning roll  81  (Fhr&gt;Ftn and Fhr&gt;Fcln), when the collected toner Tc held on the surface of the cleaning roll  81  comes into contact with the surface of the heating roll  41  at the cleaning position as illustrated in  FIG. 14 , the entirety of the collected toner Tc held on the surface of the cleaning roll  81  is reversely transferred to the surface of the heating roll  41 . 
     As described above, the heating roll  41  is heated by the halogen lamps  414   a  and  414   b  arranged in the heating roll  41 , and thus the surface temperatures of the cleaning roll  81  and the collecting roll  82 , further, the temperatures of the collected toner Tc adhering to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82  are changed by heat conduction from the heating roll  41 . Thus, if the collected toner Tc adhering to the surfaces of the cleaning roll  81  and the collecting roll  82  is transferred to the heating roll  41 , and then the cleaning operation of causing the recording paper  5  formed with plain paper as the cleaning material to pass through the fixing nip portion N, it is possible to remove the collected toner Tc adhering to the surfaces of the cleaning roll  81  and the collecting roll  82 . 
     Paper dust may be mixed in the collected toner Tc depending on the peeling of an external additive  305  or the material of the recording paper  5 , but the basic physical properties of the collected toner Tc are similar to those of a new toner T. In a case where the amount of paper dust contamination from the recording paper  5  is large, physical properties of the collected toner Tc, such as the cohesive force or the adhesive force, may be changed, and the cohesive force or the adhesive force tends to increase with increasing the amount of the paper dust contamination. This is apparent by the present discloser. 
     Although the cohesive force Ftn of the collected toner also depends on components such as a synthetic resin constituting the toner, even in a case where the components such as the synthetic resin constituting the toner are different, the same relation as that illustrated in  FIG. 7  is shown. 
     As a result, even in a case where the type of toner T used in the image forming apparatus  1  is different, and the configuration of the fixing device  40  is different, since the surface temperature of the heating roll  41  is set to a temperature higher than the temperature at which the cohesive force Ftn of the collected toner Tc is smaller than the adhesive force Fhr of the toner to the heating roll  41 , the collected toner Tc adhering to the outer circumferential surfaces of the cleaning roll  81  and the collecting roll  82  is reversely transferred (discharged) to the surface of the heating roll  41 . 
     In the first exemplary embodiment, for example, the surface temperature of the heating roll  41  as the temperature higher than the temperature at which the cohesive force Ftn of the collected toner Tc is smaller than the adhesive force Fhr of the toner to the heating roll  41  is set to 210° C. The surface temperature of the heating roll  41  is not limited to 210° C. The surface temperature of the heating roll  41  may be higher than the temperature at which the cohesive force Ftn of the collected toner is smaller than the adhesive force Fhr of the toner to the heating roll  41 , and may be set to 205° C., 215° C., 220° C., or the like. If the surface temperature of the heating roll  41  is equal to or higher than 230° C., the heating roll  41 , the press belt  42 , or other members constituting the fixing device  40  may be thermally damaged over time. Thus, the surface temperature of the heating roll  41  is desirably equal to or lower than 220° C. 
     Operation of Fixing Device 
     In the fixing device  40  according to the first exemplary embodiment, when a predetermined condition that it is determined that there is a possibility of exceeding the allowable amount of the collected toner which can be held by the cleaning device  80  is satisfied, a recovery (discharging) mode of discharging the collected toner obtained by cleaning of the cleaning device  80  and brining the cleaning device  80  back to a reusable state is performed. The recovery mode is performed when a predetermined condition is satisfied and after the previous image forming operation is completed or before the next image forming operation is started. 
     The control device  100  determines whether it is time to perform the recovery mode. As a timing for performing the recovery mode, for example, a time when jam of the recording paper  5  occurs in the fixing device  40 , a time when borderless printing is cumulatively performed on predetermined sheets or more of the recording paper  5 , a time when the fixing operation is cumulatively performed on predetermined sheets or more of embossed paper, and the like are set. The recovery mode is not limited thereto, and may be performed in a case where other conditions are satisfied, or may be performed by a user operation. The control device  100  has, for example, a function of determining and counting the type of the recording paper  5  and the cumulative number of sheets of the recording paper  5 . 
     When jam being transport failure of the recording paper  5  occurs, the control device  100  determines whether or not a region in which the jam of the recording paper  5  occurs is a region of the fixing device  40 . Here, whether or not the region in which the jam occurs is the region of the fixing device  40  is determined based on whether or not the recording paper  5  holding the unfixed toner image Ti is positioned at the fixing nip portion N of the fixing device  40 . 
     If the control device  100  determines that it is time to perform the recovery mode, the control device  100  performs the recovery operation as follows. 
     In the recovery mode, as illustrated in  FIG. 3 , the control device  100  energizes the halogen lamps  414   a  and  414   b  of the heating roll  41  to heat the surface temperature of the heating roll  41  to reach a required temperature (for example, 210° C.). At this time, the heating roll  41  is rotationally driven at a normal rotation speed. 
     As illustrated in (a) of  FIG. 15 , if the control device  100  determines that the surface temperature of the heating roll  41  has reached the required temperature (for example, 210° C.), the control device  100  controls the heating roll  41  to continuously rotate and drive at the normal rotation speed for a predetermined time (for example, about 1 to 2 minutes). In this high-temperature rotating operation, the surface temperature of the heating roll  41  is desirably set to a temperature higher than the temperature in fixing over the entire region through which the recording paper  5  of the maximum size passes in the direction intersecting with the rotation direction of the heating roll  41 . However, it is not necessary to cover the entire region, and a region portion in the direction intersecting with the rotation direction of the heating roll  41  may have a temperature higher than the temperature in fixing. 
     At this time, in the fixing device  40 , when the surface temperature of the heating roll  41  reaches the required temperature (for example, 210° C.), the cleaning roll  81  coming into contact with the surface of the heating roll  41  and the collecting roll  82  coming into contact with the cleaning roll  81  are also heated to a temperature (for example, 210° C.) which is the same as the surface temperature of the heating roll  41 , by heat conduction. 
     If the cleaning roll  81  and the collecting roll  82  are heated, as illustrated in  FIG. 7 , regarding the collected toner Tc held on the surfaces of the collecting roll  82  and the cleaning roll  81 , the cohesive force Ftn of the collected toner is smaller than the adhesive forces Fhr, Fcln, and Fspt of the collected toner Tc to the surfaces of the heating roll  41 , the cleaning roll  81 , and the collecting roll  82 . 
     As a result, the collected toner Tc held on the surface of the collecting roll  82  is transferred to the surface of the cleaning roll  81 , and the collected toner Tc transferred to the surface of the cleaning roll  81  is reversely transferred to the surface of the heating roll  41 . Thus, the collected toner Tc held on the surfaces of the collecting roll  82  and the cleaning roll  81  is reversely transferred to the surface of the heating roll  41  while the heating roll  41  is rotationally driven for a required time. 
     At this time, as illustrated in  FIG. 7 , the adhesive force Fcln of the collected toner Tc to the surface of the cleaning roll  81  is larger than the cohesive force Ftn of the collected toner. Therefore, as illustrated in (a) of  FIG. 15 , a thin layer (about one or two layers) of the collected toner Tc remains on the surface of the cleaning roll  81  by the adhesive force Fcln of the collected toner Tc. 
     Then, the control device  100  controls energization to the halogen lamps  414   a  and  414   b  of the heating roll  41  to lower the surface temperature of the heating roll  41  to a normal fixing temperature (for example, 170 to 190° C.). 
     As illustrated in (b) of  FIG. 15 , if the control device  100  determines that the surface temperature of the heating roll  41  has reached the normal fixing temperature (for example, 170 to 190° C.), the control device  100  performs the cleaning operation in which plural sheets (for example, about 5) of recording paper  5  are automatically fed from the paper feeding device  50 , or a user is urged to feed plural sheets (for example, about 5) of recording paper  5  from the manual paper feeding device  70  by displaying a message in an operation display unit (not illustrated), and thus the recording paper  5  is caused to pass through the fixing nip portion N of the fixing device  40 . 
     With the cleaning operation, as illustrated in (b) of  FIG. 15 , the collected toner Tc which has reversely transferred to the surface of the heating roll  41  adheres to the plural sheets of recording paper  5  at the fixing nip portion N. In this manner, cleaning of the surface of the heating roll  41  is performed. 
     Here, the reason that the surface temperature of the heating roll  41  is set to the normal fixing temperature (for example, 170 to 190° C.) is to prevent an occurrence of a situation in which, in a case where the surface temperature of the heating roll  41  is switched to a temperature lower than the normal fixing temperature, the adhesive force Fspt of the collected toner Tc to the surface of the collecting roll  82  is larger than the cohesive force Ftn of the collected toner, and thus the collected toner Tc adhering to the surface of the cleaning roll  81  to be a thin layer is transferred to the collecting roll  82 . 
     Then, the control device  100  controls energization to the halogen lamps  414   a  and  414   b  of the heating roll  41 , and lowers the surface temperature of the heating roll  41  to a temperature (for example, about 140° C.) lower than the normal fixing temperature. In  FIG. 7 , the low temperature is a temperature at which the cohesive force Ftn of the collected toner is larger than the adhesive force Fspt of the collected toner Tc to the surface of the collecting roll  82 . 
     As illustrated in (c) of  FIG. 15 , if the control device  100  determines that the surface temperature of the heating roll  41  has reached the temperature (for example, about 140° C.) lower than the normal fixing temperature, the control device  100  controls the heating roll  41  to continuously rotate and drive at the normal rotation speed for a predetermined time (for example, about two minutes) which is longer than that in rotation at a high temperature. In this low-temperature rotating operation, the surface temperature of the heating roll  41  is desirably set to a temperature lower than the temperature in fixing over the entire region through which the recording paper  5  of the maximum size passes in the direction intersecting with the rotation direction of the heating roll  41 . However, it is not necessary to cover the entire region, and a region portion in the direction intersecting with the rotation direction of the heating roll  41  may have a temperature lower than the temperature in fixing. 
     If the surface temperature of the heating roll is set to a low temperature, the collected toner Tc remaining on the surfaces of the heating roll  41  and the cleaning roll  81  is transferred from the heating roll  41  to the cleaning roll  81  and is further transferred from the cleaning roll  81  to the collecting roll  82 . The collected toner Tc transferred to the collecting roll  82  is held on the outer circumferential surface of the collecting roll  82 . 
     At this time, the collected toner Tc remaining on the surfaces of the heating roll  41  and the cleaning roll  81  is very small. Thus, the entirety of the remaining toner is transferred from the cleaning roll  81  to the collecting roll  82 , and then is held on the outer circumferential surface of the collecting roll  82  by the adhesive force Fcln and the cohesive force Ftn of the toner transferred to the outer circumferential surface of the collecting roll  82  and is not transferred to the cleaning roll  81 . 
     As described above, according to the fixing device  40  according to the first exemplary embodiment, it is possible to improve the removability of the collected toner Tc held by the cleaning roll  81  and the collecting roll  82  and to continuously use the fixing device  40 , in comparison to a case where the temperature when the recovery mode is idle is equal to or lower than the temperature when the image is fixed. 
     Second Exemplary Embodiment 
       FIG. 16  illustrates a fixing device according to a second exemplary embodiment. In the fixing device according to the second exemplary embodiment, the cleaning device is provided not only for the heating roll but also for the press belt. 
     That is, in the fixing device  40  according to the second exemplary embodiment, a second cleaning device  90  is provided on the outer circumferential surface of the press belt  42 , as illustrated in  FIG. 16 . The second cleaning device  90  includes a cleaning roll  91  as an example of the first cleaning unit and a collecting roll  92  as an example of the second cleaning unit. The cleaning roll  91  comes into contact with the surface of the press belt  42  at required pressing force. The collecting roll  92  comes into contact with the surface of the cleaning roll  91  at required pressing force. The cleaning roll  91  is disposed in the vicinity of the lower end portion in the figure, which is close to the fixing nip portion N, on the outer circumferential surface of the press belt  42 . 
     The cleaning roll  91  and the collecting roll  92  are configured similar to the cleaning roll  81  and the collecting roll  82 . 
     An elastic material  48  made of felt or the like is provided on the back surface of the press belt  42  with which the cleaning roll  91  comes into press-contact. 
     As described above, since the second cleaning device  90  is provided for the press belt  42 , the toner or the like adhering to the surface of the press belt  42  is transferred to the heating roll  41  once. Then, the toner can be immediately removed by the second cleaning device  90  of the press belt  42 , not be removed by the cleaning device  80  of the heating roll  41 . 
     Other configurations and operations are the same as those in the above-described exemplary embodiment, and thus description thereof will be omitted. 
     Third Exemplary Embodiment 
       FIG. 17  illustrates a fixing device according to a third exemplary embodiment. In the fixing device  40  according to the third exemplary embodiment, a heating belt is used as a first pressing unit, and a pressure roll is used as a second pressing unit. 
     That is, as illustrated in  FIG. 17 , the fixing device  40  according to the third exemplary embodiment includes a heating belt  201  as an example of an endless belt, and a pressure roll  202 . The cleaning device  80  is provided in the heating belt  201 . 
     A pressing member  204  including a heating unit  203  is disposed in the heating belt  201 . The heating belt  201  is pressed by the pressing member to form a fixing nip portion N between the heating belt  201  and the pressure roll. 
     A pressing member including a heating unit is disposed in the heating belt  201 . The heating belt  201  is pressed by the pressing member  204  to form a fixing nip portion N between the heating belt  201  and the pressure roll  202 . 
     Other configurations and operations are the same as those in the above-described exemplary embodiment, and thus description thereof will be omitted. 
     In the above exemplary embodiments, the full-color image forming apparatus in which the image forming unit includes the plural image forming devices and the intermediate transfer device are described. However, the present disclosure is not limited thereto, and the above exemplary embodiments can be applied to a monochrome image forming apparatus including a single image forming device. 
     In the above exemplary embodiments, a case where the roll or the belt is used as the first pressing unit, and the belt or the roll is used as the second pressing unit is described. However, belts may be used as the first and second pressing units, or rolls may be used as the first and second pressing units. 
     Furthermore, in the above exemplary embodiments, a case where the heating unit is provided in the first pressing unit is described. However, the heating unit may be provided in both the first and second pressing units. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.