Patent Publication Number: US-11022920-B2

Title: Image forming apparatus having a rubbing member that is moveable to abut a fixing belt

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to image forming apparatuses, such as printers, copying machines, facsimiles, and multifunction printers, which use the electrophotography. 
     Description of the Related Art 
     Image forming apparatuses include a fixing device. The fixing device applies heat and pressure to a recording material on which a toner image is formed, for fixing the toner image to the recording material. For example, Japanese Patent Application Publication No. 2017-161864 proposes a freebelt fixing device that includes an endless fixing belt that rotates, a heater that abuts against the inner circumferential surface of the fixing belt and heats the fixing belt, and a roller (referred to as a pressure roller) that abuts against the outer circumferential surface of the fixing belt. In such a fixing device, the fixing belt is pressed by the pressure roller so that a fixing nip portion is formed between the fixing belt and the pressure roller. The recording material is conveyed through the fixing nip portion, while pressurized, heated, and held between the fixing belt and the pressure roller, so that a toner image is fixed to the recording material. Here, while rotated, the fixing belt may move in a width direction that is orthogonal to a rotational direction of the fixing belt. For this reason, at both end portions of the fixing belt in the width direction, regulation members are disposed. Thus, when the fixing belt moves in the width direction, each of the regulation members receives a corresponding one of the end portions of the fixing belt and regulates the movement of the fixing belt in the width direction. 
     The surface layer of the fixing belt is made of soft resin for preventing toner from adhering to the fixing belt from the recording material. The soft resin may be PFA or PTFE, and has good mold releasability. Thus, the surface of the fixing belt easily has a flaw in the rotational direction (circumferential direction) of the fixing belt, caused by a cut end (so-called paper edge) of an edge portion of the recording material. Here, the cut end is formed when the recording material is cut. The flaw on the surface of the fixing belt causes a linear image defect on the recording material, to which a toner image is fixed. Here, for a case where a fixing roller is used, Japanese Patent Application Publication No. 2007-199596 proposes a technique that equalizes the roughness of the surface of the fixing roller for reducing the flaw caused by the paper edge. Specifically, this technique causes a rubbing rotatable member to abut against the surface of the fixing roller for rubbing the surface. In the above-described Japanese Patent Application Publication No. 2007-199596, when the fixing operation is not performed, the rubbing rotatable member is moved from a separation position, at which the rubbing rotatable member does not abut against the surface of the fixing roller, to an abutment position at which the rubbing rotatable member abuts against the surface of the fixing roller that is rotating. 
     As in the case where the fixing roller is used, when a fixing belt is used in a fixing device, it is preferable that the surface of the fixing belt is rubbed by a rubbing rotatable member. In addition, for increasing the life of the fixing belt, it is preferable that the rubbing rotatable member abuts against the surface of the fixing belt when the fixing operation is not performed, and is separated from the surface of the fixing belt when the fixing operation is performed. However, if the rubbing rotatable member of the freebelt fixing device abuts against the fixing belt that is rotating, the fixing belt may be damaged, possibly torn. This is because when the rubbing rotatable member abuts against the rotating fixing belt, strong force will be instantaneously applied to a portion of the fixing belt, against which the rubbing rotatable member abuts, and the fixing belt will be twisted. 
     SUMMARY OF THE INVENTION 
     According to a one aspect of the present invention, an image forming apparatus includes an image forming unit configured to form a toner image on a recording material, a fixing device configured to fix the toner image to the recording material, the fixing device including an endless fixing belt, a heating member configured to heat the fixing belt to a predetermined temperature, a driving rotary member configured to abut against the fixing belt to form a fixing nip portion between the fixing belt and the driving rotary member and configured to drive and rotate the fixing belt, the fixing nip portion being formed to fix the toner image to the recording material, a plurality of regulation members configured to regulate movement of the fixing belt in a width direction perpendicular to a rotational axis of the driving rotary member, each regulating member is disposed outside of each end of the fixing belt in the width direction, a rubbing rotatable member configured to rub the fixing belt, and a contact-and-separation mechanism configured to move the rubbing rotatable member between an abutment position at which the rubbing rotatable member abuts against the fixing belt and a separation position at which the rubbing rotatable member is separated from the fixing belt, and a control unit configured to perform a rubbing mode in which after the rubbing rotatable member moves from the separation position to the abutment position in a state where the fixing belt is in a stop state, the driving rotary member rotates the fixing belt in a state where the rubbing rotatable member is located at the abutment position. The control unit is configured to control the contact-and-separation mechanism in the rubbing mode such that if the predetermined temperature is a first temperature, the rubbing rotatable member moves from the abutment position to the separation position at a timing when a first time has elapsed since the rubbing rotatable member abutted against the surface of the fixing belt, and if the predetermined temperature is a second temperature higher than the first temperature, the rubbing rotatable member moves from the abutment position to the separation position at a timing when a second time shorter than the first time has elapsed since the rubbing rotatable member abutted against the surface of the fixing belt. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a configuration of an image forming apparatus of a first embodiment. 
         FIG. 2  is a cross-sectional view illustrating a fixing device of the first embodiment. 
         FIG. 3  is a perspective view illustrating a contact-and-separation mechanism and a rotation mechanism for a rubbing rotatable member. 
         FIG. 4  is a cross-sectional view illustrating the contact-and-separation mechanism. 
         FIG. 5A  is a diagram illustrating the contact-and-separation mechanism in a state where the rubbing rotatable member is located at an abutment position. 
         FIG. 5B  is a diagram illustrating the contact-and-separation mechanism in a state where the rubbing rotatable member is located at a separation position. 
         FIG. 6  is a flowchart illustrating a belt-surface rubbing process of the first embodiment. 
         FIG. 7  is a flowchart illustrating a belt-surface rubbing process of a second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Image Forming Apparatus 
     A first embodiment will be described. First, an image forming apparatus of the present embodiment will be described with reference to  FIG. 1 . An image forming apparatus  100  of  FIG. 1  is a color-image forming apparatus having a tandem intermediate-transfer system. Specifically, the image forming apparatus  100  includes an apparatus body  100   a  in which image forming portions PY, PM, PC, and PK for four colors (yellow, magenta, cyan, and black) are disposed facing an intermediate transfer belt  8 . In the present embodiment, the Image forming portions PY to PK described later, primary transfer rollers  5 Y to  5 K, the intermediate transfer belt  8 , a secondary transfer inner roller  66 , and a secondary transfer outer roller  67  constitute an image forming unit  500 , which forms a toner image on a recording material S. The recording material S may be a sheet material, such as a transfer sheet, an electrofax sheet, a dielectric-coated paper, an OHP sheet, a printing paper sheet, or a format paper sheet. 
     Next, a conveyance process of the image forming apparatus  100  for the recording material S will be described. The recording material S is stored in a cassette  62 , and stacked on a stacking portion of the cassette  62 . The recording material S is fed to a conveyance path  64 , one by one, by a sheet feeding roller  63  at an image forming timing. Here, the recording material S may be stacked on a manual feed tray or a recording-material stacking apparatus (both not illustrated), and fed to the conveyance path  64  one by one. When the recording material S is conveyed to a registration roller  65  disposed on the conveyance path  64 , the registration roller  65  performs skew correction and timing correction on the recording material S, and sends the recording material S to a secondary transfer portion T 2 . The secondary transfer portion T 2  is a transfer nip portion formed between the secondary transfer inner roller  66  and the secondary transfer outer roller  67 , which face each other. The recording material S is conveyed through the secondary transfer portion T 2 , while held between the secondary transfer inner roller  66  and the secondary transfer outer roller  67 . In addition, when a secondary transfer voltage is applied to the secondary transfer outer roller  67 , a toner image is secondary-transferred from the intermediate transfer belt  8  onto the recording material S. 
     In synchronization with the above-described conveyance process for the recording material S, which is performed between the cassette  62  and the secondary transfer portion T 2 , an image forming process is performed for sending an image to the secondary transfer portion T 2 . The image forming process will be described. First, the image forming portions PY to PK will be described. Here, the image forming portions PY to PK are substantially the same as each other in configuration, except that developing apparatuses  4 Y,  4 M,  4 C, and  4 K respectively use toner of yellow, magenta, cyan, and black. Thus, in the following description, the image forming portion PY for yellow will be described as an example, and the description for the other image forming portions PM, PC, and PK will be omitted. 
     Image Forming Portion 
     As illustrated in  FIG. 1 , the image forming portion PY includes a photosensitive drum  1 Y, a charging apparatus  2 Y, the developing apparatus  4 Y, the primary transfer roller  5 Y, and a drum cleaning apparatus  6 Y The charging apparatus  2 Y, the developing apparatus  4 Y, the primary transfer roller  5 Y, and the drum cleaning apparatus  6 Y are disposed around the photosensitive drum  1 Y The photosensitive drum  1 Y serving as an image bearing member is a drum-like photoreceptor in electrophotography, and is rotatably supported by the apparatus body  100   a  and rotated by a motor (not illustrated) clockwise in  FIG. 1  (in a direction indicated by an arrow R 1  in  FIG. 1 ). The surface of the rotary photosensitive drum  1 Y is uniformly charged in advance by the charging apparatus  2 Y, and then an electrostatic latent image is formed on the surface of the photosensitive drum  1 Y by an exposure apparatus  3 , which is driven in accordance with a corresponding image signal. The electrostatic latent image formed on the photosensitive drum  1 Y is then developed into a toner image and visualized by the developing apparatus  4 Y After that, a predetermined pressure and a primary transfer voltage are applied to the toner image formed on the photosensitive drum  1 Y, and the toner image is primary-transferred onto the intermediate transfer belt  8 . The primary transfer roller  5 Y faces the photosensitive drum  1 Y via the intermediate transfer belt  8 , so that a primary transfer portion T 1  for the toner image is formed between the photosensitive drum  1 Y and the intermediate transfer belt  8 . When the primary transfer voltage is applied to the primary transfer roller  5 Y, the toner image is primary-transferred from the photosensitive drum  1 Y onto the intermediate transfer belt  8  in the primary transfer portion T 1 . Remaining toner that is slightly left on the photosensitive drum  1 Y after the primary transfer, is removed by the drum cleaning apparatus  6 Y for the next image forming process. 
     As described above, each of the image forming portions PY to PK performs an image forming process for a corresponding color. In addition, the image forming process for each color is performed at a timing at which one toner image is transferred onto another toner image that was primary-transferred onto the intermediate transfer belt  8  at a position positioned upstream from the one toner image in the rotational direction of the intermediate transfer belt  8 . As a result, a full-color toner image is formed on the intermediate transfer belt  8 , and conveyed to the secondary transfer portion T 2 . Remaining toner on the intermediate transfer belt  8  left after the toner image has passed through the secondary transfer portion T 2  is removed from the intermediate transfer belt  8  by a transfer cleaner  11 . 
     Thus, the recording material S conveyed through the above-described conveyance process and the full-color toner image conveyed through the above-described image forming process are sent to the secondary transfer portion T 2  at the same timing, and thereby the toner image is secondary-transferred from the intermediate transfer belt  8  onto the recording material S. After that, the recording material S on which the toner image has been formed by the image forming unit  500  is conveyed to the fixing device  30 , and pressurized and heated by the fixing device  30  for fixing the toner image to the recording material S. The recording material S to which the toner image has been fixed by the fixing device  30  is discharged onto a sheet discharging tray  601  by a sheet discharging roller  69 . 
     Fixing Device 
     Next, the fixing device  30  of the present embodiment will be described with reference to  FIG. 2 . The fixing device  30  of the present embodiment is a so-called freebelt fixing device. As illustrated in  FIG. 2 , the fixing device  30  of the present embodiment includes a belt unit  101 , a pressure roller  102 , and a rubbing rotatable member  120 . The pressure roller  102  is rotatably supported by the apparatus body. The pressure roller  102  is disposed in parallel with the belt unit  101 . In addition, the pressure roller  102  is disposed so as to press a fixing belt  103  of the belt unit  101  when the pressure roller  102  abuts against the fixing belt  103 . In the present embodiment, the belt unit  101  can move between a pressure position and a non-pressure position. The pressure position is a position at which the fixing belt  103  abuts against the pressure roller  102  and is pressed by the pressure roller  102 , and the non-pressure position is a position at which the fixing belt  103  is separated from the pressure roller  102  and is not pressed by the pressure roller  102 . Thus, the belt unit  101  is movably supported by a pressure lever  112  that is swung by a pressure motor (not illustrated). 
     The pressure lever  112  is disposed so as to be able to swing on a rotation center shaft  111  via a pressure spring  113 . Thus, the belt unit  101  moves with respect to the pressure roller  102  in accordance with the swing of the pressure lever  112 . When the belt unit  101  moves close to and abuts against the pressure roller  102 , the fixing belt  103  is pressed by the pressure roller  102 . Thus, the fixing belt  103  and the pressure roller  102  are pressed by each other, so that a fixing nip portion N is formed between the fixing belt  103  and the pressure roller  102 . When the recording material S passes through the fixing nip portion N while pressed by the fixing belt  103  and the pressure roller  102 , the toner image is fixed to the recording material S. 
     The pressure roller  102  may have an elastic layer formed on the outer circumferential surface of a metal rotation shaft (core metal) and made of silicone rubber, fluororubber, or fluororesin. In addition, the pressure roller  102  may further have a release layer formed on the outer circumferential surface of the elastic layer and made of fluororesin, such as PTFE, PFA, or FEP. In the present embodiment, the pressure roller  102  has a rotation shaft, an elastic layer, and a release layer. The diameter φ of the rotation shaft is 15 mm, the thickness of the elastic layer is 5 mm, and the thickness of the release layer is 50 μm. Thus, the outer diameter φ of the pressure roller  102  is about 25 mm. The elastic layer is made of silicone rubber having ASKER hardness of 64°, and the release layer is a PFA tube. 
     The pressure roller  102  can be rotated by a driving motor M 1 , which is a driving source. When the fixing nip portion N is formed between the fixing belt  103  and the pressure roller  102 , the rotational force of the pressure roller  102  is transmitted to the fixing belt  103  by frictional force produced in the fixing nip portion N. In this manner, the fixing belt  103  is rotated by the pressure roller  102 , which is a driving rotary member (this driving is so-called driving by pressure roller). The recording material S is conveyed through the fixing nip portion N by the rotating pressure roller  102  and the rotating fixing belt  103 , while held between the pressure roller  102  and the fixing belt  103 . Here, when the fixing belt  103  and the pressure roller  102  are not pressed by each other, the fixing nip portion N is not formed between the fixing belt  103  and the pressure roller  102 , and the rotational force of the pressure roller  102  is not transmitted to the fixing belt  103 . 
     Belt Unit 
     As illustrated in  FIG. 2 , the belt unit  101  includes the fixing belt  103  and flanges  105 . The fixing belt  103  is endless, and has a cylindrical shape and flexibility. The flanges  105  is configured to abut against both ends, in the width direction orthogonal to the rotational direction of the fixing belt  103  (the width direction is equal to the rotation-axis direction of the pressure roller  102 ), of the fixing belt  103  and to regulate movement of the fixing belt  103  in the width direction. The fixing belt  103  is detachably attached to the belt unit  101 . The fixing belt  103  may be a resin belt made of resin and having an elastic layer with high thermal conductivity and low thermal capacity. Alternatively, the fixing belt  103  may be a composite-layer belt having a structure in which a base layer, an elastic layer, and a release layer are formed. In this case, the base layer is a metal belt made of stainless steel (SUS) or the like, the elastic layer is formed on the outer circumferential surface of the base layer, and the release layer is formed on the outer circumferential surface of the elastic layer. In the present embodiment, the fixing belt  103  has a base layer made of SUS, an elastic layer made of silicone rubber with a thermal conductivity of about 1.0 W/m·K and having a thickness of about 250 μm, and a release layer that is a PFA tube having a thickness of 30 μm. The release layer is preferably a sheet or a coated layer having high mold releasability, and may be made of fluororesin, such as PFA or PTFE. In another case, the fixing belt  103  may have a base layer, a conductive layer, and a release layer. In this case, the base layer may be a sheet-like member having high thermal resistance and made of material such as polyester, polyethylene terephthalate, or polyamide-imide; the conductive layer may be formed on the base layer; and the release layer may be formed on the conductive layer, as a front surface. In addition, the fixing belt  103  disclosed herein may be a film-like belt. 
     The flanges  105  are externally fit in the belt unit  101 , at both end portions of the fixing belt  103  in the width direction. When the fixing belt  103  moves in the width direction, each of the flanges  105 , which serves as a regulation member, receives a corresponding one of the end portions of the fixing belt and regulates the movement of the fixing belt in the width direction. In other words, when the fixing belt  103  moves in the width direction while rotated by the pressure roller  102 , one end portion of the fixing belt  103  in the width direction abuts against a corresponding one of the flanges  105 , so that the fixing belt  103  is prevented from further moving in the width direction. Here, there is a case in which the pressure roller  102  and the fixing belt  103  are not perfectly parallel with each other because of an error caused when the pressure roller  102  and the fixing belt  103  are mounted. In this case, the rotating fixing belt  103  may be moved in the width direction by the rotating pressure roller  102 . For this reason, the flanges  105  are externally fit in the belt unit  101  at both end portions of the fixing belt  103 , for preventing the movement of the fixing belt  103  caused by the pressure roller  102  in the width direction. 
     The belt unit  101  also includes a stay  106 , a pressing pad  107 , and a backup member  121 , which are disposed inside the fixing belt  103 . The stay  106  may be a rigid metal member that extends in the width direction along the fixing belt  103 , and supports the pressing pad  107  on a side opposite to the pressure roller  102  side. In the present embodiment, the pressing pad  107  supported by the stay  106  presses the inner circumferential surface of the fixing belt  103  against the pressure roller  102 , and thereby more reliably forms the fixing nip portion N. The pressing pad  107  is made of a material having good insulation property and thermal resistance. Examples of the material include phenol resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, and LCP resin. 
     The pressing pad  107  may be a molded product that extends in a longitudinal direction (i.e. width direction of the fixing belt  103 ), and is formed so as to be able to hold the heater  104 , which is a heating member. That is, the pressing pad  107  has a fitting groove (not illustrated) in which the heater  104  is fit, and by which the heater  104  is held. The fitting groove is formed in a surface of the pressing pad  107  opposite to the stay  106 , and extends along the longitudinal direction. The heater  104  held by the pressing pad  107  abuts against the inner circumferential surface of the fixing belt  103  and heats the fixing belt  103 . The heater  104  may be a ceramic heater that includes a ceramic substrate formed like a thin plate and having a resistance layer. The resistance layer generates heat when current flows in the resistance layer, and thus the temperature of the heater  104  is adjustable. The inner circumferential surface of the fixing belt  103  is applied with lubricant for smoothly sliding the fixing belt  103  on the heater  104 . Preferably, the lubricant is silicone oil. 
     In the present embodiment, temperature sensors  200  to detect the temperature of the heater  104  are disposed in the belt unit  101  for controlling the surface temperature of the fixing belt  103 . In the present embodiment, the temperature sensors  200  may be contact-type sensors such as thermistors. In the present embodiment, the temperature sensors  200 , which are detecting unit, are partly exposed from the pressing pad  107  toward the heater  104 , and disposed at a plurality of positions along the longitudinal direction of the pressing pad  107  (i.e. rotation-axis direction of the pressure roller  102 ). Thus, in the present embodiment, the surface temperature of the fixing belt  103  can be detected by the temperature sensors  200 . Preferably, the temperature sensors  200  are disposed at at least a center portion and end portions of the fixing belt  103  in the width direction. 
     The backup member  121  is a sliding member. When the later-described rubbing rotatable member  120  is positioned at the abutment position, the fixing belt  103  slides on the backup member  121 ; when the rubbing rotatable member  120  is positioned at the separation position, the fixing belt  103  does not slide on the backup member  121 . That is, the backup member  121  is firmly positioned inside the fixing belt  103  such that when the rubbing rotatable member  120 , which is moved by a later-described contact-and-separation mechanism  400  (see  FIG. 3 ), is positioned at the abutment position at which the rubbing rotatable member  120  abuts against the fixing belt  103 , the fixing belt  103  is held between the rubbing rotatable member  120  and the backup member  121 . When the rubbing rotatable member  120  abuts against the fixing belt  103 , the fixing belt  103  is pressed toward the backup member  121 , and the backup member  121  abuts against the inner circumferential surface of the fixing belt  103 . With this operation, the rubbing operation for the surface of the fixing belt  103  can be properly performed by the rubbing rotatable member  120 . In addition, the backup member  121  is disposed such that a clearance is formed between the backup member  121  and the inner circumferential surface of the fixing belt  103  when the rubbing rotatable member  120  is located at the separation position, at which the rubbing rotatable member  120  is separated from the fixing belt  103 . When the rubbing rotatable member  120  is separated from the fixing belt  103 , the fixing belt  103  having been pressed by the rubbing rotatable member  120  expands toward the rubbing rotatable member  120 , and the inner circumferential surface of the fixing belt  103  is separated from the backup member  121 . With this operation, when the rubbing rotatable member  120  is separated from the fixing belt  103 , the backup member  121  does not scrape off the lubricant applied to the inner circumferential surface of the fixing belt  103 . The backup member  121  may be made of heat-resistant resin such as liquid crystal polymer, or made of soft rubber material such as sponge or silicone rubber. 
     The rubbing rotatable member  120  is disposed to abut against the surface of the fixing belt  103  and rub the surface for equalizing the roughness of the surface of the fixing belt  103  and reducing flaw caused by the paper edge. The rubbing rotatable member  120  is a roller having a core metal, an adhesive layer, and a rubbing layer. The core metal has an outer diameter of 12 mm for example, and is made of SUS. The adhesive layer is formed on the core metal; and the rubbing layer is formed on the adhesive layer, as a surface layer. The rubbing layer is formed by causing densely-distributed abrasive grains, which serve as a rubbing material, to adhere to the adhesive layer. Thus, the rubbing layer is formed unevenly. In the present embodiment, the rubbing material is white alundum (WA) having an average particle diameter of about 12 μm. The average particle diameter of the rubbing material may be equal to or larger than 5 μm and equal to or smaller than 20 μm. Examples of the material of the abrasive grains include aluminum oxide, aluminum hydroxide oxide, silicon oxide, cerium oxide, titanium oxide, zirconia, lithium silicate, silicon nitride, silicon carbide, iron oxide, chromium oxide, antimony oxide, diamond, and a mixture thereof. Here, the particle diameter of the abrasive grains can be obtained by randomly picking up 100 or more abrasive grains by using a scanning electron microscope, S-4500 made by Hitachi, Ltd., and by calculating a number average particle diameter by using an image processing and analysis apparatus, Luzex3 made by NIRECO CORPORATION. 
     Contact-and-Separation Mechanism 
     The rubbing rotatable member  120  is disposed so as to be able to move between the abutment position at which the rubbing rotatable member  120  abuts against the surface of the fixing belt  103  and the separation position at which the rubbing rotatable member  120  is separated from the surface of the fixing belt  103 . When the rubbing rotatable member  120  is located at the abutment position, the rubbing rotatable member  120  slides on the fixing belt  103  and rubs the surface of the fixing belt  103 . For moving the rubbing rotatable member  120  between the abutment position and the separation position, the fixing device  30  of the present embodiment includes a contact-and-separation mechanism  400 . The contact-and-separation mechanism  400  will be described with reference to  FIGS. 3 to 5B . 
     As illustrated in  FIG. 3 , the contact-and-separation mechanism  400  includes a rubbing rotatable member bearing  126 , a supporting arm  129 , and a pressure arm  127 . The supporting arm  129  is rotatably supported by a fixed shaft  128 , which is disposed on the frame  115 . Similarly, the pressure arm  127  is rotatably supported by the fixed shaft  128 . The rubbing rotatable member  120  is rotatably supported by the supporting arm  129  via the rubbing rotatable member bearing  126  so as to be able to rotate with respect to the supporting arm  129 . In addition, as illustrated in  FIG. 4 , a pressure spring  130  is disposed between the supporting arm  129  and the pressure arm  127 . In addition, a separation spring  131  is disposed such that one end of the separation spring  131  is supported by the frame  115  (see  FIG. 3 ) and the other end is joined with the pressure arm  127 . The separation spring  131  may be a torsion coil spring that urges the pressure arm  127  toward a rubbing rotatable member cam  123 . 
     As illustrated in  FIG. 3 , a rubbing rotatable member cam shaft  122  is supported by the frame  115 , and the rubbing rotatable member cam  123  and a rubbing rotatable member cam gear  124  are fixed to the rubbing rotatable member cam shaft  122 . When a cam driving motor (not illustrated) is driven, the rubbing rotatable member cam shaft  122  is rotated, via the rubbing rotatable member cam gear  124 , by a driving-gear train coupled with the cam driving motor. As a result, the rubbing rotatable member cam  123  is rotated. The rubbing rotatable member cam  123  is stopped at a desired rotation angle by using a sensor  125 , and thus the rubbing rotatable member  120  is positioned at the abutment position or the separation position. 
     Hereinafter, an abutment operation to move the rubbing rotatable member  120  to the abutment position and a separation operation to move the rubbing rotatable member  120  to the separation position will be described with reference to  FIGS. 4 to 5B . First, the abutment operation to move the rubbing rotatable member  120  to the abutment position will be described. When the rubbing rotatable member cam  123  rotates counterclockwise, the pressure arm  127  is pushed by the rubbing rotatable member cam  123  and rotated toward a direction indicated by an arrow A in  FIG. 4 . When the pressure arm  127  is pushed, the supporting arm  129  is pushed by the pressure spring  130  and also rotated toward the direction indicated by the arrow A in  FIG. 4 . Then, the rubbing rotatable member  120  supported by the supporting arm  129  abuts against the backup member  121 , and stops while pressed by a predetermined amount of abutment force. Thus, as illustrated in  FIG. 5A , the rubbing rotatable member  120  moves to the abutment position, and is pressed against the fixing belt  103 . 
     Next, the separation operation to move the rubbing rotatable member  120  to the separation position will be described. When the rubbing rotatable member cam  123  rotates clockwise, the pressure arm  127  is rotated by the urging force of the separation spring  131 , toward a direction opposite to the direction indicated by the arrow A of  FIG. 4 . In this time, since the pressure arm  127  pushes the supporting arm  129  at a portion indicated by an arrow B of  FIG. 4 , the supporting arm  129  also rotates toward the same direction, together with the pressure arm  127 . With this operation, the rubbing rotatable member  120  moves to the separation position, as illustrated in  FIG. 5B , so as to retract from the fixing belt  103 . In this manner, the rubbing rotatable member  120  moves between the abutment position illustrated in  FIG. 5A  and the separation position illustrated in  FIG. 5B  by moving up and down, in accordance with the rotation of the rubbing rotatable member cam  123 . 
     In the present embodiment, when the rubbing rotatable member  120  is located at the abutment position, the rubbing rotatable member  120  rotates in the same rotational direction as that of the fixing belt  103 . Specifically, as illustrated in  FIG. 3 , the rubbing rotatable member gear  132  is coupled with the rubbing rotatable member  120  on the axis of the rubbing rotatable member  120 , and a pressure roller gear  108  is coupled with the pressure roller  102 . When the rubbing rotatable member  120  is moved from the separation position to the abutment position by the contact-and-separation mechanism  400 , the rubbing rotatable member gear  132  is moved together with the rubbing rotatable member  120 , and coupled with a gear  133  that is coupled with the pressure roller gear  108 . With this operation, the driving force of the driving motor M 1  (see  FIG. 2 ), which drives the pressure roller  102 , is transmitted to the rubbing rotatable member gear  132  via the pressure roller gear  108  and the gear  133 , so that the rubbing rotatable member  120  is rotated. In the present embodiment, the rubbing rotatable member  120  rotates at a rotational speed higher than that of the fixing belt  103 , which is rotated by the rotation of the pressure roller  102  (for example, the speed ratio is in a range from 105 to 115%). Thus, the gear ratio between the rubbing rotatable member gear  132 , the pressure roller gear  108 , and the gear  133  is set to achieve the speed ratio. The rubbing rotatable member gear  132  is a driving-force transmission portion (a driving-force transmission member), which can be linked with the driving motor M 1  and can rotate the rubbing rotatable member  120  when linked with the driving motor M 1 . On the other hand, when the rubbing rotatable member  120  is moved from the abutment position to the separation position by the contact-and-separation mechanism  400 , the rubbing rotatable member gear  132  and the pressure roller gear  108  are separated from each other. Thus, since the rubbing rotatable member gear  132  is separated from the driving motor M 1 , the driving force of the driving motor M 1  is not transmitted to the rubbing rotatable member gear  132  and the rubbing rotatable member  120  stops. 
     Control Unit 
     As illustrated in  FIG. 1 , the image forming apparatus  100  includes a control unit  300 . The control unit  300 , which is a control means, includes a central processing unit (CPU)  301  and a memory  302 . The CPU  301  performs various types of control, such as an image forming operation, on the image forming apparatus  100 . The memory  302  includes a read only memory (ROM) and a random access memory (RAM); and stores various types of programs and various types of data, such as table data, for controlling the image forming apparatus  100 . The CPU  301  can perform an image forming job (program) stored in the memory  302 , and operates the image forming apparatus  100  for forming images. In the present embodiment, the CPU  301  can perform a belt-surface rubbing process (program) (see  FIGS. 6 and 7  described later) stored in the memory  302 , and operate the image forming apparatus  100  for rubbing the surface of the fixing belt  103 . When the CPU  301  performs the belt-surface rubbing process, the control unit  300  can control the heater  104  illustrated in  FIG. 2 , the driving motor M 1 , the contact-and-separation mechanism  400  illustrated in  FIG. 3 , and the like. In addition, the CPU  301  has a counter function to count the cumulative number of recording materials S on which images have been formed, and a timer function to measure an elapsed time. The CPU  301  causes the memory  302  to store data on the cumulative number of recording materials S and data on the elapsed time. The memory  302  can temporarily store results of computation performed in each program. 
     As described above, the surface layer of the fixing belt  103  is made of soft resin, such as PFA or PTFE, having good mold releasability so that the melted toner, melted by heat when a toner image is fixed, does not adhere to the surface layer of the fixing belt  103 . Thus, as the cumulative number of image-formed recording materials S increases, the fixing belt  103  easily has a flaw on its surface due to the paper edge of the recording material S. The flaw on the surface of the fixing belt  103  causes a linear image defect. For this reason, the rubbing rotatable member  120  is disposed to abut against the surface of the fixing belt  103  and rubs the surface for equalizing the roughness of the surface of the fixing belt  103  and reducing the flaw caused by the paper edge. 
     However, as described above, if the rubbing rotatable member  120  of the freebelt fixing device  30  abuts against the rotating fixing belt  103 , the stress applied to an end portion of the fixing belt  103  will increase rapidly, possibly damaging the end portion of the fixing belt  103 . For this reason, in the present embodiment, the rubbing rotatable member  120  abuts against the fixing belt  103  that is not rotating. Hereinafter, specific descriptions will be made. 
     Belt-Surface Rubbing Process 
     A belt-surface rubbing process of the first embodiment will be described with reference to  FIGS. 1 to 3  and  FIG. 6 . The belt-surface rubbing process (rubbing mode) of the present embodiment is performed by the control unit  300  (specifically by the CPU  301 ) when the power for the apparatus body is turned on, or every time the cumulative number of image-formed recording materials S reaches a predetermined value (e.g. 1,000). 
     As illustrated in  FIG. 6 , when the fixing belt  103  is located at the non-pressure position at which the fixing belt is separated from the pressure roller  102  and is not pressed by the pressure roller  102 , the control unit  300  drives the pressure lever  112  by driving the pressure motor (not illustrated) and moves the fixing belt  103  from the non-pressure position to the pressure position (S 1 ). For example, when the power for the apparatus body is turned on, the fixing belt  103  is located at the non-pressure position and is in a stop state. Here, when the fixing belt  103  is already located at the pressure position, the control unit  300  keeps the position of the fixing belt  103 . The control unit  300  then obtains detection results from the temperature sensors  200 , and determines depending on the detection results whether the surface temperature of the fixing belt  103  is smaller than a predetermined temperature (e.g. target temperature in a range from 100 to 150° C.) (S 2 ). If the surface temperature of the fixing belt  103  is equal to or larger than the predetermined temperature (S 2 : NO), then the control unit  300  proceeds to Step S 5 . 
     If the surface temperature of the fixing belt  103  is smaller than the predetermined temperature (S 2 : YES), then the control unit  300  rotates the fixing belt  103  by driving the pressure roller  102  by driving the driving motor M 1 , and causes the heater  104  to heat the fixing belt  103  (S 3 ). The control unit  300  causes the heater  104  to keep heating the fixing belt  103  until the surface temperature of the fixing belt  103  reaches the predetermined temperature (e.g. temperature in a range from 100 to 150° C.) (S 4 : NO). If the surface temperature of the fixing belt  103  is equal to or larger than the predetermined temperature (S 4 : YES), the control unit  300  causes the heater  104  to stop heating the fixing belt  103 , and stops the rotation of the fixing belt  103  by stopping the pressure roller  102  by stopping the driving motor M 1  (S 5 ). 
     Then, the control unit  300  operates the contact-and-separation mechanism  400  to move the rubbing rotatable member  120  from the separation position to the abutment position for the fixing belt  103  whose surface temperature is equal to or larger than the predetermined temperature and which is in a stop state (S 6 ). After moving the rubbing rotatable member  120  to the abutment position, the control unit  300  rotates the fixing belt  103  by driving the pressure roller  102  by driving the driving motor M 1 , and causes the heater  104  to heat the fixing belt  103  (S 7 ). In this operation, since the driving force of the driving motor M 1  is transmitted to the rubbing rotatable member gear  132  via the pressure roller gear  108  and the gear  133  as described above, the rubbing rotatable member  120  located at the abutment position is rotated. The fixing belt  103  is heated by the heater  104  again to prevent the temperature fall of the fixing belt  103  caused by the rubbing rotatable member  120  and keep the surface temperature of the fixing belt  103  at the predetermined temperature. With this operation, the surface of the fixing belt  103  can be efficiently rubbed by the rubbing rotatable member  120 . 
     The control unit  300  then determines whether a predetermined time has elapsed since the rubbing rotatable member  120  moved to the abutment position (S 8 ). If the predetermined time has elapsed (S 8 : YES), then the control unit  300  operates the contact-and-separation mechanism  400  to move the rubbing rotatable member  120  from the abutment position to the separation position (S 9 ), and completes the belt-surface rubbing process. 
     As described above, in the present embodiment, when the fixing belt  103  is in a stop state, the control unit  300  moves the rubbing rotatable member  120 , which rubs the surface of the fixing belt  103 , from the separation position to the abutment position, and then rotates the fixing belt  103 . Thus, when the rubbing rotatable member  120  abuts against the fixing belt  103  that is in a stop state, the large amount of force is not applied to an end portion of the fixing belt  103 , unlike the case where the rubbing rotatable member  120  abuts against the fixing belt  103  that is rotating. Thus, the end portion of the fixing belt  103  cannot be damaged. That is, when the fixing belt  103  is rotated after the rubbing rotatable member  120  is moved to the abutment position, the abutment force of the rubbing rotatable member  120  is stably applied to the fixing belt  103  and the stress applied to an end portion of the fixing belt  103  does not rapidly increase. Therefore, in the present embodiment, the end portion of the fixing belt  103  can be prevented from being damaged. 
     In addition, in the present embodiment, the contact time (i.e. predetermined time in S 8 ) taken from the abutment of the rubbing rotatable member  120  against the fixing belt  103  to the separation of the rubbing rotatable member  120  from the fixing belt  103  differs depending on the target temperature (i.e. predetermined temperature in S 4 ) of the fixing belt  103 . Here, the higher the target temperature of the fixing belt  103 , the more easily the rubbing rotatable member  120  rubs the surface of the fixing belt  103 . Thus, when the target temperature of the fixing belt  103  is a first temperature, the rubbing rotatable member  120  is moved from the abutment position to the separation position at a timing when a first time has elapsed since the rubbing rotatable member  120  abutted against the fixing belt  103 . When the target temperature of the fixing belt  103  is a second temperature higher than the first temperature, the rubbing rotatable member  120  is preferably moved from the abutment position to the separation position at a timing when a second time shorter than the first time has elapsed since the rubbing rotatable member  120  abutted against the fixing belt  103 . In this manner, since the time of the rubbing mode is shortened, the downtime of the image forming apparatus can be reduced, and thus the image forming apparatus can be efficiently operated. 
     Second Embodiment 
     In the above-described belt-surface rubbing process of the first embodiment, the rubbing rotatable member  120  is moved from the separation position to the abutment position after the fixing belt  103  is heated to the target temperature. The present disclosure, however, is not limited to this. For example, the rubbing rotatable member  120  may be moved from the separation position to the abutment position before the fixing belt  103  is heated to the target temperature. Hereinafter, a belt-surface rubbing process of a second embodiment will be described with reference to  FIGS. 1 to 3  and  FIG. 7 . Here, in the second embodiment illustrated in  FIG. 7 , a process identical to a process of the belt-surface rubbing process of the first embodiment illustrated in  FIG. 6  is given an identical symbol, and the description thereof will be simplified. 
     As illustrated in  FIG. 7 , when the fixing belt  103  is located at the non-pressure position at which the fixing belt is separated from the pressure roller  102  and is not pressed by the pressure roller  102 , the control unit  300  moves the fixing belt  103  from the non-pressure position to the pressure position (S 1 ). In this time, if the pressure roller  102  is rotating, the control unit  300  stops the rotation of the pressure roller  102  by stopping the driving motor M 1 . That is, the control unit  300  causes the fixing belt  103  to be in a stop state. Then, the control unit  300  operates the contact-and-separation mechanism  400  to move the rubbing rotatable member  120  from the separation position to the abutment position for the fixing belt  103  that is in a stop state (S 6 ). 
     After moving the rubbing rotatable member  120  to the abutment position, the control unit  300  rotates the fixing belt  103  by driving the pressure roller  102  by driving the driving motor M 1 , and causes the heater  104  to heat the fixing belt  103  (S 7 ). In this operation, the rubbing rotatable member  120  located at the abutment position is rotated, as described above. In addition, the control unit  300  causes the heater  104  to keep heating the fixing belt  103  until the surface temperature of the fixing belt  103  reaches a predetermined temperature (e.g. temperature in a range from 100 to 150° C.) (S 4 : NO). On the other hand, if the surface temperature of the fixing belt  103  reaches the predetermined temperature (S 4 : YES), the control unit  300  determines whether a predetermined time has elapsed since the rubbing rotatable member  120  moved to the abutment position (S 8 ). If the predetermined time has elapsed (S 8 : YES), the control unit  300  operates the contact-and-separation mechanism  400  to move the rubbing rotatable member  120  from the abutment position to the separation position (S 9 ), and completes the belt-surface rubbing process. 
     As described above, also in the present embodiment, when the fixing belt  103  is in a stop state, the control unit  300  operates the contact-and-separation mechanism  400  to move the rubbing rotatable member  120 , which rubs the surface of the fixing belt  103 , from the separation position to the abutment position, and then rotates the fixing belt  103 . Therefore, also in the present embodiment, an end portion of the fixing belt  103  can be prevented from being damaged, as in the above-described first embodiment. 
     In addition, in the present embodiment, the contact time (i.e. predetermined time in S 8 ) taken from the abutment of the rubbing rotatable member  120  against the fixing belt  103  to the separation of the rubbing rotatable member  120  from the fixing belt  103  differs depending on the time that the surface temperature of the fixing belt  103  takes to reach the target temperature (i.e. predetermined temperature in S 4 ) of the fixing belt  103 . For example, when the predetermined temperature of the fixing belt  103  is a first temperature, the rubbing rotatable member  120  is moved from the abutment position to the separation position at a timing when a first time has elapsed since the surface temperature of the fixing belt  103  reached the first temperature. When the predetermined temperature of the fixing belt  103  is a second temperature higher than the first temperature, the rubbing rotatable member  120  is moved from the abutment position to the separation position at a timing when a second time shorter than the first time has elapsed since the surface temperature of the fixing belt  103  reached the second temperature. With this operation, the surface of the fixing belt  103  can be sufficiently rubbed by the rubbing rotatable member  120 . 
     Modifications 
     In the above-described embodiments, the predetermined temperature for the fixing belt  103  is reached. However, the present disclosure is not limited to this. For example, the rubbing rotatable member  120  may be in contact with the fixing belt  103  for a predetermined time, without the surface temperature of the fixing belt  103  reaching the predetermined temperature (that is, without controlling the surface temperature of the fixing belt  103 ). However, it is preferable that the rubbing rotatable member  120  rubs the surface of the fixing belt  103  whose surface temperature has reached the predetermined temperature, as described above. This is because the surface of the fixing belt  103  can be sufficiently rubbed. Here, whether the surface temperature of the fixing belt  103  is controlled may be selected by a user. 
     In addition, in the image forming apparatus of the above-described embodiments, toner images having different colors are primary-transferred from the photosensitive drums  1 Y to  1 K onto the intermediate transfer belt  8 , and then a composite toner image having the different colors is collectively secondary-transferred onto the recording material S. However, the present disclosure is not limited to this. For example, a direct-transfer image forming apparatus may be used. In this case, a transfer roller is disposed facing a photosensitive drum via a conveyance belt, and a nip portion is formed between the conveyance belt and the photosensitive drum. In addition, a recording material is conveyed by the conveyance belt, and a toner image formed on the photosensitive drum is directly transferred onto the recording material when a transfer voltage is applied to the transfer roller. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2019-000946, filed on Jan. 8, 2019, which is hereby incorporated by reference herein in its entirety.