Patent Publication Number: US-8989641-B2

Title: Fixing device with mechanism capable of minimizing glossy streaks and stain on recording medium and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2011-143304, filed on Jun. 28, 2011, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium. 
       FIG. 1  illustrates a fixing device  110  installed in such image forming apparatuses, which includes a fixing roller  101  and a pressing roller  102  that apply heat and pressure to a recording medium P bearing a toner image. For example, the pressing roller  102  is pressed against the fixing roller  101  heated by a heater  106  disposed inside the fixing roller  101  to form a fixing nip N therebetween through which the recording medium P bearing the toner image is conveyed. As the fixing roller  101  and the pressing roller  102  rotate and convey the recording medium P through the fixing nip N in a recording medium conveyance direction D, the fixing roller  101  and the pressing roller  102  apply heat and pressure to the recording medium P, melting and fixing the toner image on the recording medium P. 
     A thermistor  112  contacts the outer circumferential surface of the fixing roller  101  to detect the temperature of the fixing roller  101  so that a controller controls the heater  106  to heat the fixing roller  101  to a desired temperature based on the temperature detected by the thermistor  112 . Additionally, a plurality of separation pawls  111  contacts the outer circumferential surface of the fixing roller  101  to separate the recording medium P discharged from the fixing nip N from the fixing roller  101 .  FIG. 2  illustrates the arrangement of the thermistor  112  and the separation pawls  111 . As shown in  FIG. 2 , the four separation pawls  111  and the single thermistor  112  are aligned in the axial direction of the fixing roller  101 . 
     As the recording medium P bearing the toner image is conveyed through the fixing nip N, toner of the toner image may adhere from the recording medium P to the fixing roller  101  due to heat from the fixing roller  101 . As the separation pawls  111  and the thermistor  112  slide over the rotating fixing roller  101 , they scratch toner adhered to the fixing roller  101 . Conversely, bands on the outer circumferential surface of the fixing roller  101  where the separation pawls  111  and the thermistor  112  do not slide over the fixing roller  101  continue carrying toner adhered from the toner image on the recording medium P, producing toner streaks on the outer circumferential surface of the fixing roller  101  as shown in  FIG. 2 . Accordingly, as the subsequent recording medium P is conveyed through the fixing nip N, the toner streaks produced on the fixing roller  101  are transferred onto the subsequent recording medium P, producing glossy streaks  113  on the subsequent recording medium P as shown in  FIG. 2 . Further, toner caught and accumulated by the separation pawls  111  and the thermistor  112  may fall onto the fixing roller  101  and may be further transferred from the fixing roller  101  onto the recording medium P, thus staining the recording medium P. 
       FIG. 3  is a graph illustrating a relation between the number of prints performed by the fixing device  110  shown in  FIG. 1  and a level of glossy streaks that appear on recording media P. As shown in  FIG. 3 , the greater the glossy streak level, the smaller the amount of glossy streaks that appear on the recording media P. By contrast, the smaller the glossy streak level, the greater the amount of glossy streaks that appear on the recording media P. The dotted line defines the acceptable limit of the gloss streak level of 3.5. As the number of prints increases, the glossy streak level deteriorates. Specifically, with the number of prints of 2,000 sheets or more, the glossy streak level is below the acceptable limit of 3.5. 
     To address the above-described drawbacks of the fixing device  110 , a fixing device that removes toner adhered from the recording medium P to the fixing roller  101  is proposed. For example, the fixing device includes a sliding roller that slides over the outer circumferential surface of a fixing roller to pick up toner therefrom and a cleaning roller that picks up toner from the sliding roller. However, the sliding roller sliding over the fixing roller may damage the outer circumferential surface of the fixing roller. 
     SUMMARY OF THE INVENTION 
     This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a fixing rotary body rotatable in a predetermined direction of rotation; an opposed rotary body contacting the fixing rotary body to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed; a plurality of separation pawls separatably contacting an outer circumferential surface of the fixing rotary body to separate the recording medium discharged from the fixing nip from the fixing rotary body; a temperature detector contacting the outer circumferential surface of the fixing rotary body to detect a temperature of the fixing rotary body; and a fixing rotary body cleaner interposed between the plurality of separation pawls and the temperature detector in the direction of rotation of the fixing rotary body and contacting the outer circumferential surface of the fixing rotary body to clean the outer circumferential surface of the fixing rotary body. 
     This specification further describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a fixing rotary body rotatable in a predetermined direction of rotation; an opposed rotary body contacting the fixing rotary body to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed; a plurality of separation pawls separatably contacting an outer circumferential surface of the fixing rotary body to separate the recording medium discharged from the fixing nip from the fixing rotary body; a temperature detector contacting the outer circumferential surface of the fixing rotary body to detect a temperature of the fixing rotary body; a plurality of first fixing rotary body cleaners interposed between the plurality of separation pawls and the temperature detector in the direction of rotation of the fixing rotary body and contacting the outer circumferential surface of the fixing rotary body to clean the outer circumferential surface of the fixing rotary body; and a plurality of second fixing rotary body cleaners interposed between the temperature detector and the plurality of separation pawls in the direction of rotation of the fixing rotary body and contacting the outer circumferential surface of the fixing rotary body to clean the outer circumferential surface of the fixing rotary body. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment of the present invention, the image forming apparatus includes the fixing device described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic vertical sectional view of a related-art fixing device; 
         FIG. 2  illustrates a horizontal side view of a fixing roller incorporated in the related-art fixing device shown in  FIG. 1  and a plan view of a recording medium bearing a toner image fixed by the related-art fixing device; 
         FIG. 3  is a graph illustrating a relation between the number of prints performed by the related-art fixing device shown in  FIG. 1  and the level of glossy streaks that appear on the recording medium shown in  FIG. 2 ; 
         FIG. 4  is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 5  is a vertical sectional view of a fixing device according to a first exemplary embodiment incorporated in the image forming apparatus shown in  FIG. 4 ; 
         FIG. 6  is a graph illustrating a relation between the number of prints performed by the fixing device shown in  FIG. 5  and the level of glossy streaks that appear on recording media; 
         FIG. 7A  is a horizontal side view of a fixing roller incorporated in the fixing device shown in  FIG. 5  and a comparative fixing roller cleaner; 
         FIG. 7B  is a horizontal side view of a fixing roller and a fixing roller cleaner incorporated in the fixing device shown in  FIG. 5 ; 
         FIG. 8  is a vertical sectional view of a fixing device according to a second exemplary embodiment; 
         FIG. 9  is a vertical sectional view of a fixing device according to a third exemplary embodiment; 
         FIG. 10  is a vertical sectional view of a fixing device according to a fourth exemplary embodiment; 
         FIG. 11  is a vertical sectional view of a fixing device according to a fifth exemplary embodiment; and 
         FIG. 12  is a schematic development view of the fixing device shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 4 , an image forming apparatus  100  according to an exemplary embodiment of the present invention is explained. 
       FIG. 4  is a schematic vertical sectional view of the image forming apparatus  100 . The image forming apparatus  100  may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment, the image forming apparatus  100  is a printer for forming color and monochrome toner images on a recording medium by electrophotography. 
     Referring to  FIG. 4 , the following describes the structure of the image forming apparatus  100 . 
     The image forming apparatus  100  includes four process units  1 Y,  1 C,  1 M, and  1 K detachably attached to the image forming apparatus  100 . Although the process units  1 Y,  1 C,  1 M, and  1 K contain yellow, cyan, magenta, and black toners that form yellow, cyan, magenta, and black toner images, respectively, resulting in a color toner image, they have an identical structure. Hence, the following describes the structure of one of them, that is, the process unit  1 Y that forms a yellow toner image. 
     For example, the process unit  1 Y includes a photoconductive drum  2 Y, that is, a photoconductor, serving as an image carrier that carries an electrostatic latent image and a resultant yellow toner image; a charging roller  3 Y serving as a charger that charges an outer circumferential surface of the photoconductive drum  2 Y; a development device  4 Y serving as a development unit that supplies a developer (e.g., yellow toner) to the electrostatic latent image formed on the outer circumferential surface of the photoconductive drum  2 Y, thus visualizing the electrostatic latent image into a yellow toner image with the yellow toner; and a cleaning blade  5 Y serving as a cleaner that cleans the outer circumferential surface of the photoconductive drum  2 Y. 
     Above the process units  1 Y,  1 C,  1 M, and  1 K is an exposure device  6  serving as an exposure unit that emits a laser beam L onto the outer circumferential surface of the respective photoconductive drums  2 Y,  2 C,  2 M, and  2 K to form an electrostatic latent image thereon. Below the process units  1 Y,  1 C,  1 M, and  1 K is a transfer unit  7  that accommodates an endless intermediate transfer belt  8  serving as a transferor, a driving roller  9 , a driven roller  10 , four primary transfer rollers  11 Y,  11 C,  11 M, and  11 K, a secondary transfer roller  12 , and a belt cleaner  13 . Specifically, the endless intermediate transfer belt  8  is stretched over the driving roller  9  and the driven roller  10  and rotatable in a rotation direction A. 
     Inside a loop formed by the intermediate transfer belt  8  and opposite the four photoconductive drums  2 Y,  2 C,  2 M, and  2 K are the four primary transfer rollers  11 Y,  11 C,  11 M, and  11 K serving as primary transferors that transfer the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  2 Y,  2 C,  2 M, and  2 K onto an outer circumferential surface of the intermediate transfer belt  8 . The primary transfer rollers  11 Y,  11 C,  11 M, and  11 K contact an inner circumferential surface of the intermediate transfer belt  8  and press the intermediate transfer belt  8  against the photoconductive drums  2 Y,  2 C,  2 M, and  2 K at opposed positions where the primary transfer rollers  11 Y,  11 C,  11 M, and  11 K are disposed opposite the photoconductive drums  2 Y,  2 C,  2 M, and  2 K, respectively, via the intermediate transfer belt  8 , thus forming primary transfer nips between the photoconductive drums  2 Y,  2 C,  2 M, and  2 K and the intermediate transfer belt  8  where the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  2 Y,  2 C,  2 M, and  2 K are primarily transferred onto the intermediate transfer belt  8  to form a color toner image thereon. Opposite the driving roller  9  is the secondary transfer roller  12  serving as a secondary transferor that transfers the color toner image formed on the intermediate transfer belt  8  onto a recording medium P. The secondary transfer roller  12  contacts the outer circumferential surface of the intermediate transfer belt  8  and presses the intermediate transfer belt  8  against the driving roller  9 , thus forming a secondary transfer nip between the secondary transfer roller  12  and the intermediate transfer belt  8  where the color toner image formed on the intermediate transfer belt  8  is transferred onto the recording medium P. 
     The belt cleaner  13 , disposed opposite the outer circumferential surface of the intermediate transfer belt  8  and in proximity to the secondary transfer nip, cleans the outer circumferential surface of the intermediate transfer belt  8 . Below the intermediate transfer unit  7  is a waste toner container  14  that collects waste toner conveyed from the belt cleaner  13  through a waste toner conveyance tube extending from the belt cleaner  13  to an inlet of the waste toner container  14 . 
     In a lower portion of the image forming apparatus  100  are a paper tray  15  that loads a plurality of recording media P (e.g., sheets) and a feed roller  16  that picks up and feeds a recording medium P from the paper tray  15  toward the secondary transfer nip formed between the secondary transfer roller  12  and the intermediate transfer belt  8 . In an upper portion of the image forming apparatus  100  are an output roller pair  17  that discharges the recording medium P onto an outside of the image forming apparatus  100  and an output tray  18  that receives and stocks the recording medium P discharged by the output roller pair  17 . 
     The recording medium P fed by the feed roller  16  is conveyed upward through a conveyance path R that extends from the paper tray  15  to the output roller pair  17 . The conveyance path R is provided with a registration roller pair  19  disposed below the secondary transfer nip formed between the secondary transfer roller  12  and the intermediate transfer belt  8 , that is, upstream from the secondary transfer nip in a recording medium conveyance direction. The conveyance path R is also provided with a fixing device  20  disposed downstream from the secondary transfer roller  12  and upstream from the output roller pair  17  in the recording medium conveyance direction. The fixing device  20  fixes the color toner image on the recording medium P. For example, the fixing device  20  includes a fixing roller  21  serving as a fixing rotary body; a pressing roller  22  serving as an opposed rotary body that contacts the fixing roller  21  to form a fixing nip N therebetween; and a plurality of separation pawls  33  serving as a separator that separates the recording medium P from the fixing roller  21 . 
     Referring to  FIG. 4 , the following describes the operation of the image forming apparatus  100  having the structure described above to form a color toner image on a recording medium P. 
     As a print job starts, a driver drives and rotates the photoconductive drums  2 Y,  2 C,  2 M, and  2 K of the process units  1 Y,  1 C,  1 M, and  1 K, respectively, clockwise in  FIG. 4  in a rotation direction B. The charging rollers  3 Y,  3 C,  3 M, and  3 K uniformly charge the outer circumferential surface of the respective photoconductive drums  2 Y,  2 C,  2 M, and  2 K at a predetermined polarity. The exposure device  6  emits laser beams L onto the charged outer circumferential surface of the respective photoconductive drums  2 Y,  2 C,  2 M, and  2 K according to yellow, cyan, magenta, and black image data contained in image data sent from an external device (e.g., a client computer), respectively, thus forming electrostatic latent images thereon. The development devices  4 Y,  4 C,  4 M, and  4 K supply yellow, cyan, magenta, and black toners to the electrostatic latent images formed on the photoconductive drums  2 Y,  2 C,  2 M, and  2 K, visualizing the electrostatic latent images into yellow, cyan, magenta, and black toner images, respectively. 
     As the driving roller  9  is driven and rotated counterclockwise in  FIG. 4 , the driving roller  9  drives and rotates the intermediate transfer belt  8  counterclockwise in  FIG. 4  in the rotation direction A. As a power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged yellow, cyan, magenta, and black toners to the primary transfer rollers  11 Y,  11 C,  11 M, and  11 K, a transfer electric field is created at the primary transfer nips formed between the primary transfer rollers  11 Y,  11 C,  11 M, and  11 K and the photoconductive drums  2 Y,  2 C,  2 M, and  2 K, respectively. Accordingly, the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  2 Y,  2 C,  2 M, and  2 K, respectively, are primarily transferred onto the intermediate transfer belt  8  successively by the transfer electric field created at the respective primary transfer nips, in such a manner that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt  8 . Consequently, a color toner image is formed on the intermediate transfer belt  8 . 
     After the primary transfer of the yellow, cyan, magenta, and black toner images from the photoconductive drums  2 Y,  2 C,  2 M, and  2 K, the cleaning blades  5 Y,  5 C,  5 M, and  5 K remove residual toner not transferred and therefore remaining on the photoconductive drums  2 Y,  2 C,  2 M, and  2 K therefrom. Then, dischargers discharge the outer circumferential surface of the respective photoconductive drums  2 Y,  2 C,  2 M, and  2 K, initializing the potential thereof so that the respective photoconductive drums  2 Y,  2 C,  2 M, and  2 K are ready for the next print job. 
     On the other hand, as the print job starts, the feed roller  16  is driven and rotated to feed a recording medium P from the paper tray  15  toward the registration roller pair  19  through the conveyance path R. The registration roller pair  19  feeds the recording medium P to the secondary transfer nip formed between the secondary transfer roller  12  and the driving roller  9  at a time when the color toner image formed on the intermediate transfer belt  8  reaches the secondary transfer nip. The secondary transfer roller  12  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, cyan, magenta, and black toners of the yellow, cyan, magenta, and black toner images constituting the color toner image formed on the intermediate transfer belt  8 , thus creating a transfer electric field at the secondary transfer nip. Accordingly, the yellow, cyan, magenta, and black toner images constituting the color toner image are secondarily transferred from the intermediate transfer belt  8  collectively onto the recording medium P by the transfer electric field created at the secondary transfer nip. 
     The recording medium P bearing the color toner image is conveyed to the fixing device  20  where the fixing roller  21  and the pressing roller  22  apply heat and pressure to the recording medium P, fixing the color toner image on the recording medium P. The separation pawls  33  separate the recording medium P bearing the fixed color toner image from the fixing roller  21 . Thereafter, the output roller pair  17  discharges the recording medium P onto the output tray  18 . After the secondary transfer of the color toner image from the intermediate transfer belt  8  onto the recording medium P, the belt cleaner  13  removes residual toner not transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  8  therefrom. The removed toner is conveyed and collected into the waste toner container  14 . 
     The above describes the image forming operation of the image forming apparatus  100  to form the color toner image on the recording medium P. Alternatively, the image forming apparatus  100  may form a monochrome toner image by using any one of the four process units  4 Y,  4 C,  4 M, and  4 K or may form a bicolor or tricolor toner image by using two or three of the process units  4 Y,  4 C,  4 M, and  4 K. 
     Referring to  FIGS. 5 to 7B , the following describes the configuration of the fixing device  20  according to a first exemplary embodiment that is installed in the image forming apparatus  100  described above. 
       FIG. 5  is a vertical sectional view of the fixing device  20 .  FIG. 6  is a graph illustrating a relation between the number of prints performed by the fixing device  20  shown in  FIG. 5  and the level of glossy streaks that appear on recording media P.  FIG. 7A  is a horizontal side view of the fixing roller  21  and a comparative fixing roller cleaner  34 C.  FIG. 7B  is a horizontal side view of the fixing roller  21  and a fixing roller cleaner  34  incorporated in the fixing device  20 . 
     As shown in  FIG. 5 , the fixing device  20  (e.g., a fuser unit) includes the fixing roller  21  serving as a fixing rotary body rotatable in a rotation direction R 1 ; the pressing roller  22  serving as an opposed rotary body rotatable in a rotation direction R 2  counter to the rotation direction R 1  of the fixing roller  21 ; and a resilient member (e.g., a compression spring) that biases the pressing roller  22  against the fixing roller  21  to press the pressing roller  22  against the fixing roller  21 , forming the fixing nip N therebetween. 
     Alternatively, at least one of the fixing rotary body and the opposed rotary body may be an endless belt formed into a loop inside which a roller or a pad is disposed in such a manner that the roller or the pad presses the endless belt against another one of the fixing rotary body and the opposed rotary body. Further, the opposed rotary body may not press against the fixing rotary body but may merely contact the fixing rotary body. 
     The fixing device  20  uses toner containing wax as a releasing agent that facilitates separation of toner of the toner image formed on the recording medium P from the fixing roller  21 . Accordingly, it is not necessary to coat the fixing roller  21  with oil. 
     The fixing device  20  further includes a lever switcher  51  that switches between an enhanced pressure state in which the pressing roller  22  presses against the fixing roller  21  with enhanced pressure and a reduced pressure state in which the pressing roller  22  presses against the fixing roller  21  with reduced pressure by moving a lever assembly  50  connected to the pressing roller  22  and the fixing roller  21 . The fixing roller  21  is supported by a fixing frame  25 ; the pressing roller  22  is supported by a pressing frame  26 . That is, the fixing roller  21  is rotatably mounted on the fixing frame  25 ; the pressing roller  22  is rotatably mounted on the pressing frame  26 . The pressing frame  26  mounted with the pressing roller  22  is rotatable about a shaft  31  mounted on the fixing frame  25 . The resilient member (e.g., a compression spring) attached to the fixing frame  25  and the pressing frame  26  exerts a resilient bias to the fixing frame  25  and the pressing frame  26 , thus pressing the pressing roller  22  supported by the pressing frame  26  against the fixing roller  21  supported by the fixing frame  25  to form the fixing nip N between the pressing roller  22  and the fixing roller  21 . The resilient member has a spring load of about 65 N; the fixing nip N has a nip load of about 340 N. 
     The fixing roller  21  includes a tubular body  65  constructed of a thermal conductive base layer  63  and an outer layer  64  coating the base layer  63 . A heater  24  (e.g., a halogen heater) is disposed inside the tubular body  65 . The outer layer  64  of the tubular body  65  is constructed of an elastic layer and a surface layer coating the elastic layer. 
     The thermal conductive base layer  63 , having a predetermined mechanical strength, is made of thermal conductive carbon steel or aluminum. The elastic layer of the outer layer  64  is made of synthetic rubber such as silicone rubber and fluoro rubber. The surface layer of the outer layer  64  is made of materials with high thermal conductivity and durability that facilitate separation of toner of a toner image on a recording medium P from the fixing roller  21  and enhance the durability of the elastic layer. For example, the surface layer of the outer layer  64  may be a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) tube, a layer coated with fluoroplastic such as PFA, or a layer cladded with silicone rubber or fluoro rubber. 
     The fixing roller  21  has an outer loop diameter in a range of from about 15 mm to about 40 mm. The elastic layer of the outer layer  64  has a thickness in a range of from about 0.5 mm to about 3.0 mm. The surface layer of the outer layer  64  has a thickness in a range of from about 10 micrometers to about 80 micrometers. According to this exemplary embodiment, the fixing roller  21  has an outer loop diameter of about 24 mm. The elastic layer of the outer layer  64  has a thickness of about 1 mm and the surface layer of the outer layer  64  has a thickness of about 43 micrometers. 
     The pressing roller  22  includes a metal core  67  and an outer layer  68  coating the metal core  67 . The outer layer  68  is constructed of an elastic layer and a surface layer coating the elastic layer. For example, the metal core  67  is made of carbon steel tubes for machine structural purposes (STKM). The elastic layer of the outer layer  68  is made of silicone rubber, fluoro rubber, silicone rubber foam, fluoro rubber foam, or the like. The surface layer of the outer layer  68  is made of a heat resistant fluoroplastic tube, such as PFA and polytetrafluoroethylene (PTFE), which facilitates separation of toner of the toner image on the recording medium P from the pressing roller  22 . 
     The pressing roller  22  has an outer loop diameter in a range of from about 20 mm to about 40 mm. The elastic layer of the outer layer  68  has a thickness in a range of from about 0.5 mm to about 10.0 mm. The surface layer of the outer layer  68  has a thickness in a range of from about 10 micrometers to about 80 micrometers. According to this exemplary embodiment, the pressing roller  22  has an outer loop diameter of about 30 mm. The elastic layer of the outer layer  68  has a thickness of about 8 mm and the surface layer of the outer layer  68  has a thickness of about 50 micrometers. 
     The lever assembly  50  of the lever switcher  51  includes a link assembly  71  constructed of a first linkage  50   a  and a second linkage  50   b . One end, that is, a left end in  FIG. 5 , of the second linkage  50   b  (e.g., a lever arm) in a longitudinal direction thereof is pivotally mounted on the fixing frame  25  by a pin  72 . Another end, that is, a right end in  FIG. 5 , of the second linkage  50   b  in the longitudinal direction thereof is pivotally mounted on the first linkage  50   a  by a pin  73 . One end of the first linkage  50   a  (e.g., a lever arm) in a longitudinal direction thereof is pivotally mounted on the pressing frame  26  by a pin  78 . 
     As the first linkage  50   a  of the lever assembly  50  swings about the pin  78  in a direction X 1 , since the first linkage  50   a  is coupled to the second linkage  50   b  through the pin  73 , the second linkage  50   b  swings about the pin  72  in a direction X 4 . Accordingly, the pressing frame  26  swings about the shaft  31  in a direction E 1 . Consequently, the pressing roller  22  presses against the fixing roller  21  with reduced pressure therebetween, producing the shorter fixing nip N in a recording medium conveyance direction D 1 . 
     Conversely, as the first linkage  50   a  of the lever assembly  50  swings about the pin  78  in a direction X 2 , the second linkage  50   b  of the lever assembly  50  swings about the pin  72  in a direction X 3 . Accordingly, the pressing frame  26  swings about the shaft  31  in a direction E 2 . Consequently, the pressing roller  22  presses against the fixing roller  21  with enhanced pressure therebetween, producing the longer fixing nip N in the recording medium conveyance direction D 1 . 
     The separation pawls  33 , a thermistor  32  serving as a temperature detector that detects the temperature of the fixing roller  21 , and a thermostat that prevents overheating of the fixing roller  21  are disposed opposite an outer circumferential surface of the fixing roller  21 . As a controller  99 , that is, a microprocessor, for example, operatively connected to the thermistor  32  and the heater  24  receives a detection signal from the thermistor  32 , the controller  99  controls the heater  24  based the detection signal sent from the thermistor  32 , maintaining the temperature of the fixing roller  21  at a predetermined temperature range. An entry guide, disposed upstream from the fixing nip N in the recording medium conveyance direction D 1 , guides the recording medium P to the fixing nip N. An exit guide, disposed downstream from the fixing nip N in the recording medium conveyance direction D 1 , guides the recording medium P discharged from the fixing nip N toward the output roller pair  17  depicted in  FIG. 4 . The separation pawls  33  disposed downstream from the fixing nip N in the recording medium conveyance direction D 1  separate the recording medium P bearing the fixed toner image discharged from the fixing nip N from the fixing roller  21 . 
     The separation pawls  33  serving as a separator are disposed opposite the outer circumferential surface of the fixing roller  21  at a position downstream from the fixing nip N in the recording medium conveyance direction D 1 . As shown in  FIG. 12  illustrating a horizontal side view of the fixing roller  21  and the separation pawls  33 , according to this exemplary embodiment, the four separation pawls  33  are aligned in an axial direction of the fixing roller  21 . However, the number of the separation pawls  33  is not limited to four as long as the plurality of separation pawls  33  is disposed opposite the outer circumferential surface of the fixing roller  21 . As shown in  FIG. 5 , each separation pawl  33  is supported by a shaft  38  in such a manner that each separation pawl  33  is rotatable about the shaft  38  independently from other separation pawls  33 . As the separation pawl  33  rotates about the shaft  38  clockwise in  FIG. 5 , a front end  33   a  of the separation pawl  33  is brought into contact with the outer circumferential surface of the fixing roller  21 . Conversely, as the separation pawl  33  rotates about the shaft  38  counterclockwise in  FIG. 5 , the front end  33   a  of the separation pawl  33  separates from the outer circumferential surface of the fixing roller  21 .  FIG. 5  illustrates the separation pawl  33  in contact with the fixing roller  21 . 
     The separation pawl  33  is made of PFA, polyetherketone (PEK), polyether ether ketone (PEEK), or the like that facilitates separation from and sliding over the fixing roller  21 . Alternatively, an outer circumferential surface of the separation pawl  33  may be coated with PFA or Teflon® that facilitates separation from and sliding over the fixing roller  21 . 
     A contact direction resilient member is attached to a base  33   b  of each separation pawl  33  disposed opposite the front end  33   a . The contact direction resilient member biases the separation pawl  33  against the fixing roller  21 , bringing the separation pawl  33  into contact with the fixing roller  21 . The base  33   b  of each separation pawl  33  is also attached with a separation pawl separator that separates the separation pawl  33  from the fixing roller  21 . 
     The fixing roller cleaner  34  serving as a fixing rotary body cleaner contacts the outer circumferential surface of the fixing roller  21  at a position where the separation pawl  33  and the thermistor  32  do not contact the outer circumferential surface of the fixing roller  21 . The fixing roller cleaner  34  is disposed downstream from the separation pawl  33  and upstream from the thermistor  32  in the rotation direction R 1  of the fixing roller  21 . Specifically, the fixing roller cleaner  34  contacts the outer circumferential surface of the fixing roller  21  at the position shifted by a predetermined angle in a circumferential direction, that is, the rotation direction R 1 , of the fixing roller  21  from a position where the separation pawl  33  contacts the outer circumferential surface of the fixing roller  21 . 
     The thermistor  32  contacts the outer circumferential surface of the fixing roller  21  at the position upstream from the fixing nip N in the rotation direction R 1  of the fixing roller  21 . By contrast, the separation pawl  33  contacts the outer circumferential surface of the fixing roller  21  at the position downstream from the fixing nip N in the rotation direction R 1  of the fixing roller  21 . The fixing roller cleaner  34  contacts the outer circumferential surface of the fixing roller  21  at the position interposed between the separation pawl  33  and the thermistor  32  in the rotation direction R 1  of the fixing roller  21 . 
     For example, the fixing roller cleaner  34  is a tube constructed of a core shaft  35  and a cleaner body  36  coating the core shaft  35  and contacting the outer circumferential surface of the fixing roller  21  to clean it. The fixing roller cleaner  34  is rotatable about an axis thereof in accordance with rotation of the fixing roller  21 . As shown in  FIG. 7B , the fixing roller cleaner  34  has a width equivalent to a width of the fixing roller  21  in the axial direction thereof and therefore contacts the outer circumferential surface of the fixing roller  21  throughout substantially the entire width of the fixing roller  21  in the axial direction thereof. Bearings biased toward the fixing roller  21  by compression springs  37  depicted in  FIG. 5  support both lateral ends of the fixing roller cleaner  34 , respectively. The compressing springs  37  press the fixing roller cleaner  34  against the fixing roller  21  with pressure in a range of from about 5 N to about 40 N, for example, about 12 N according to this exemplary embodiment. If the pressure is smaller than about 5 N, the fixing roller cleaner  34  may not remove an adherent adhered from the recording medium P onto the fixing roller  21 , such as a slight amount of offset toner and paper dust adhered from the recording medium P onto the fixing roller  21 , from the fixing roller  21 . Conversely, if the pressure is greater than about 40 N, the fixing roller cleaner  34  may damage the PFA tube, that is, the surface layer of the outer layer  64  of the fixing roller  21 . 
     The fixing roller cleaner  34  has a diameter in a range of from about 6 mm to about 20 mm with a thickness of the cleaner body  36  in a range of from about 0.1 mm to about 2.0 mm. According to this exemplary embodiment, the core shaft  35  has a diameter of about 10 mm and the cleaner body  36  has a thickness of about 1 mm. 
     The core shaft  35  may be made of free-cutting steel (SUM). According to this exemplary embodiment, the cleaner body  36  is made of artificial leather. For example, a sliced face of artificial leather uniformly contacts the outer circumferential surface of the fixing roller  21 . Weight per unit area of artificial leather is in a range of from about 200 g/m 2  to about 400 g/m 2 , for example, about 305 g/m 2  according to this exemplary embodiment. 
     Referring to  FIGS. 5 and 6 , the following describes glossy streaks that appear on the toner image on the recording medium P. As the recording medium P bearing the toner image passes through the fixing nip N, an adherent such as a slight amount of offset toner and paper dust may adhere from the recording medium P to the fixing roller  21 . As the separation pawls  33  and the thermistor  32  slide over the fixing roller  21 , they scrape toner adhered to the fixing roller  21  off the fixing roller  21 . However, toner adhered to the fixing roller  21  remains on the fixing roller  21  at positions where the separation pawls  33  and the thermistor  32  do not contact the fixing roller  21 , producing streaks of toner on the fixing roller  21 . As the fixing roller  21  rotates in the rotation direction R 1 , the streaks of toner on the fixing roller  21  are transferred onto a subsequent recording medium P passing through the fixing nip N, producing glossy streaks on the recording medium P. To address this problem, the fixing device  20  has the fixing roller cleaner  34  that cleans the fixing roller  21 .  FIG. 6  illustrates a graph showing the relation between the number of prints performed by the fixing device  20  and the level of glossy streaks that appear on recording media P. The greater the glossy streak level, the smaller the amount of glossy streaks that appear on the recording media P. By contrast, the smaller the glossy streak level, the greater the amount of glossy streaks that appear on the recording media P. The dotted line defines the acceptable limit of the glossy streak level of 3.5. As shown in  FIG. 6 , even when 100,000 sheets are printed, the glossy streak level is maintained at 4 greater than the acceptable limit of 3.5. That is, the fixing device  20  attains a lifespan of more than 100,000 prints. 
     Referring to  FIGS. 7A and 7B , the following describes a comparison between the comparative fixing roller cleaner  34 C with a comparative cleaner body  36 C made of felt and the fixing roller cleaner  34  with the cleaner body  36  made of artificial leather. Table below shows the specification and evaluation of the comparative cleaner body  36 C and the cleaner body  36  according to this exemplary embodiment. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Felt of comparative 
                 Artificial leather of 
               
               
                   
                 cleaner body 36C 
                 cleaner body 36 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Specifi- 
                 Material 
                 Aramid fiber 
                 polyester 65% 
               
               
                 cation 
                   
                   
                 polyurethane 35% 
               
               
                   
                 Fiber 
                 14 μm 
                 5 μm (ultrathin 
               
               
                   
                 diameter 
                   
                 polyester fiber) 
               
               
                   
                 Weight per 
                 400 g/m 2  to 
                 200 g/m 2  to 
               
               
                   
                 unit area 
                 1,000 g/m 2   
                 400 g/m 2   
               
               
                   
                 Surface 
                 Brushed 
                 Smoothly sliced 
               
            
           
           
               
               
               
            
               
                 Evaluation 
                 Producing fine 
                 No fine scratches on 
               
               
                   
                 scratches on the outer 
                 the outer circumfer- 
               
               
                   
                 circumferential surface 
                 ential surface of the 
               
               
                   
                 of the fixing roller 21 
                 fixing roller 21 
               
               
                   
                 Producing fine streaks 
               
               
                   
                 on the toner image on 
               
               
                   
                 the recording medium P 
               
               
                   
               
            
           
         
       
     
     Felt of the comparative cleaner body  36 C of the comparative fixing roller cleaner  34 C is made of aramid fibers constituting a brushed surface and has a greater weight per unit area in a range of from about 400 g/m 2  to about 1,000 g/m 2 . Accordingly, the brushed surface of the aramid fibers may produce fine scratches S on the outer circumferential surface of the fixing roller  21  that are transferred onto the toner image on the recording medium P as fine streaks. By contrast, artificial leather of the cleaner body  36  of the fixing roller cleaner  34  is made of ultrathin polyester fibers having a diameter of about 5 micrometers and has weight per unit area in a range of from about 200 g/m 2  to about 400 g/m 2  smaller than that of felt. A mechanically sliced face of artificial leather is not brushed and therefore contacts the outer circumferential surface of the fixing roller  21  uniformly without scratching it. Accordingly, the cleaner body  36  of the fixing roller cleaner  34  effectively removes an adherent such as a slight amount of offset toner and paper dust adhered from the recording medium P to the fixing roller  21 . 
     Consequently, the image forming apparatus  100  depicted in  FIG. 4  that incorporates the fixing device  20  attaining the advantages described above can form a high quality toner image on the recording medium P. 
     Referring to  FIG. 8 , the following describes a fixing device  20 S according to a second exemplary embodiment. 
       FIG. 8  is a vertical sectional view of the fixing device  20 S. Unlike the fixing device  20  depicted in  FIG. 5  that has the rotatable fixing roller cleaner  34 , the fixing device  20 S has a non-rotatable fixing roller cleaner  44 . For example, the fixing roller cleaner  44  includes a plate spring  45  and a cleaning pad  46  attached to a front end of the plate spring  45  and pressed against the outer circumferential surface of the fixing roller  21  by the plate spring  45 , thus contacting the outer circumferential surface of the fixing roller  21 . The cleaning pad  46  has a thickness in a range of from about 0.1 mm to about 2.0 mm. According to this exemplary embodiment, the plate spring  45  has a thickness of about 0.1 mm; the cleaning pad  46  has a thickness of about 1.0 mm. The fixing roller cleaner  44  has a width not smaller than a width of a maximum recording medium P in the axial direction of the fixing roller  21 . 
     The plate spring  45  is made of SUS stainless steel; the cleaning pad  46  is made of artificial leather. The cleaning pad  46  has weight per unit area of about 305 g/m 2 . A sliced face of artificial leather of the cleaning pad  46  uniformly contacts the outer circumferential surface of the fixing roller  21 . 
     The plate spring  45  biases the cleaning pad  46  against the fixing roller  21  with pressure in a range of from about 2 N to about 10 N. If the pressure is smaller than about 2 N, the fixing roller cleaner  44  may not remove an adherent, such as a slight amount of offset toner and paper dust adhered from a recording medium P to the fixing roller  21 , from the fixing roller  21 . Conversely, if the pressure is greater than about 10 N, the fixing roller cleaner  44  may damage the PFA tube, that is, the surface layer of the outer layer  64  of the fixing roller  21 . To address this circumstance, according to this exemplary embodiment, the pressure is about 3.5 N. 
     Referring to  FIG. 9 , the following describes a fixing device  20 T according to a third exemplary embodiment. 
       FIG. 9  is a vertical sectional view of the fixing device  20 T. Unlike the fixing device  20  depicted in  FIG. 5  that has the fixing roller cleaner  34  rotatable in accordance with rotation of the fixing roller  21 , the fixing device  20 T according to the third exemplary embodiment has a fixing roller cleaner  54  rotatable independently from the fixing roller  21 . For example, the fixing roller cleaner  54  is driven and rotated by a driver  98  (e.g., a motor) that rotates the fixing roller cleaner  54  exclusively in a rotation direction R 3  at a rotation speed Vb different from a rotation speed Va of the fixing roller  21  driven and rotated by a driver  97  (e.g., a motor), thus improving cleaning performance of the fixing roller cleaner  54  that cleans the outer circumferential surface of the fixing roller  21 . 
     Referring to  FIG. 10 , the following describes a fixing device  20 U according to a fourth exemplary embodiment. 
       FIG. 10  is a vertical sectional view of the fixing device  20 U. In addition to the fixing roller cleaner  34 , the fixing device  20 U includes a pressing roller cleaner  55  that contacts an outer circumferential surface of the pressing roller  22  to clean it. The pressing roller cleaner  55  has a configuration identical to that of the fixing roller cleaner  34  described above with reference to  FIGS. 5 and 7B . The pressing roller cleaner  55  may be rotatable in accordance with rotation of the pressing roller  22  that rotates in the rotation direction R 2 , may be rotatable independently from the pressing roller  22 , or may not be rotatable. Accordingly, the pressing roller cleaner  55  removes an adherent, such as a slight amount of offset toner and paper dust adhered from the recording medium P onto the pressing roller  22 , from the pressing roller  22 , thus reducing glossy streaks produced on the toner image on the subsequent recording medium P due to transfer of the adherent from the pressing roller  22  onto the subsequent recording medium P. 
     Referring to  FIGS. 11 and 12 , the following describes a fixing device  20 V according to a fifth exemplary embodiment. 
       FIG. 11  is a vertical sectional view of the fixing device  20 V.  FIG. 12  is a schematic development view of the fixing device  20 V. The fixing device  20 V includes two fixing roller cleaners, that is, a first fixing roller cleaner  56  and a second fixing roller cleaner  57  that contact the outer circumferential surface of the fixing roller  21  to clean it. For example, the first fixing roller cleaner  56  is disposed downstream from the separation pawl  33  and upstream from the thermistor  32  in the rotation direction R 1  of the fixing roller  21 . The second fixing roller cleaner  57  is disposed downstream from the thermistor  32  and upstream from the fixing nip N in the rotation direction R 1  of the fixing roller  21 . The first fixing roller cleaner  56  is a small tube that includes a core shaft  58  and a cleaner body  59  coating the core shaft  58  and contacting the outer circumferential surface of the fixing roller  21  to clean it. Unlike the fixing roller cleaner  34  depicted in  FIG. 5  that contacts the fixing roller  21  throughout substantially the entire width of the fixing roller  21  in the axial direction thereof, each first fixing roller cleaner  56  is disposed opposite a first band B 1  on the outer circumferential surface of the fixing roller  21  contacted by the separation pawl  33  as shown in  FIG. 12 . Hence, four first fixing roller cleaners  56  are aligned in the axial direction of the fixing roller  21 . 
     Each second fixing roller cleaner  57  includes a plate spring  60  and a cleaning pad  61  attached to a front end of the plate spring  60  and contacting the outer circumferential surface of the fixing roller  21 . As shown in  FIG. 12 , each second fixing roller cleaner  57  is disposed opposite a second band B 2  on the outer circumferential surface of the fixing roller  21  not contacted by the first fixing roller cleaner  56 . Accordingly, the second fixing roller cleaners  57  remove an adherent, such as a slight amount of offset toner and paper dust adhered from the recording medium P onto the fixing roller  21 , at a position on the outer circumferential surface of the fixing roller  21  where the separation pawls  33  are not disposed opposite the fixing roller  21 . Consequently, the adherent is not transferred to the subsequent recording medium P conveyed through the fixing nip N, reducing glossy streaks produced on a toner image on the subsequent recording medium P. 
     The present invention is not limited to the details of the exemplary embodiments described above, and various modifications and improvements are possible. For example, the image forming apparatus  100  may be a copier, a printer, a facsimile machine, a multifunction printer having at least one of copying, printing, facsimile, and scanning functions, or the like. According to the exemplary embodiments described above, the four separation pawls  33  are aligned in the axial direction of the fixing roller  21  as shown in  FIG. 12 . Alternatively, an arbitrary number of separation pawls  33  not smaller than two is available. 
     Referring to  FIGS. 5 ,  8 ,  9 ,  10 ,  11 , and  12 , the following describes advantages of the fixing devices  20 ,  20 S,  20 T,  20 U, and  20 V. 
     The fixing devices  20 ,  20 S,  20 T,  20 U, and  20 V include a fixing rotary body (e.g., the fixing roller  21 ) rotatable in the rotation direction R 1 ; an opposed rotary body (e.g., the pressing roller  22 ) to contact the fixing rotary body to form the fixing nip N therebetween through which a recording medium P bearing a toner image is conveyed; a plurality of separation pawls (e.g., the separation pawls  33 ) to separatably contact an outer circumferential surface of the fixing rotary body to separate the recording medium P discharged from the fixing nip N from the fixing rotary body; a temperature detector (e.g., the thermistor  32 ) to contact the outer circumferential surface of the fixing rotary body to detect the temperature of the fixing rotary body; and a fixing rotary body cleaner (e.g., the fixing roller cleaners  34 ,  44 ,  54 , and  56 ) interposed between the plurality of separation pawls and the temperature detector in the rotation direction R 1  of the fixing rotary body and contacting the outer circumferential surface of the fixing rotary body. For example, the fixing rotary body cleaner contacts the outer circumferential surface of the fixing rotary body at a position thereon shifted from a position where the plurality of separation pawls contacts the outer circumferential surface of the fixing rotary body by a predetermined angle in a circumferential direction, that is, the rotation direction R 1  of the fixing rotary body. 
     With this configuration, the fixing roller cleaner removes an adherent, such as a slight amount of offset toner and paper dust adhered from the recording medium P conveyed through the fixing nip N onto the fixing rotary body, from the fixing rotary body, thus minimizing glossy streaks that may appear on the toner image on the subsequent recording medium P due to transfer of the adherent from the fixing rotary body to the subsequent recording medium P. Further, even if toner caught and accumulated by the plurality of separation pawls and the temperature detector may fall onto the fixing rotary body, the fixing roller cleaner collects the fallen toner from the fixing rotary body, preventing the fallen toner from adhering to the subsequent recording medium P and therefore staining the subsequent recording medium P. 
     As shown in  FIG. 4 , the image forming apparatus  100  incorporates any one of the fixing devices  20 ,  20 S,  20 T,  20 U, and  20 V, attaining the advantages described above. 
     The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.