Patent Publication Number: US-9897950-B2

Title: Fixing device and image forming apparatus

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. 2015-208824, filed on Oct. 23, 2015, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Exemplary aspects of the present disclosure 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. 
     Description of the Background 
     Related-art image terming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on as recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred front the photoconductor onto a recording medium via an intermediate transfer belt; 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. 
     Such fixing device may include a fixing rotator, such as a fixing roller, a fixing belt, and a fixing film, heated by a heater and a pressure rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium hearing a toner image is conveyed. As the recording medium hearing the toner image is conveyed through the fixing nip, the fixing rotator and the pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. 
     SUMMARY 
     This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and a heater that heats the fixing rotator. A nip formation pad is disposed opposite an inner circumferential surface of the fixing rotator. A pressure rotator presses against the nip formation pad via the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. A low-friction member is sandwiched between the nip formation pad and the fixing rotator and bears a lubricant that flows in a flow direction. A thermal absorber is mounted on the inner circumferential surface of the fixing rotator. At least one irregular portion, mounted on the thermal absorber, blocks the lubricant flowing in the flow direction. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image bearer to bear a toner image and a fixing device disposed downstream from the image bearer in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and a heater that beats the fixing rotator. A nip formation pad is disposed opposite an inner circumferential surface of the fixing rotator. A pressure rotator presses against the nip formation pad via the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. A low-friction member is sandwiched between the nip formation pad and the fixing rotator and bears a lubricant that flows in as flow direction. A thermal absorber is mounted on the inner circumferential surface of the fixing rotator. At least one irregular portion, mounted on the thermal absorber, blocks the lubricant flowing in the flow direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure 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 cross-sectional view of an image forming apparatus according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a vertical cross-sectional view of a fixing device according to a first exemplary embodiment of the present disclosure that is incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a vertical cross-sectional view of a fixing device according to a second exemplary embodiment of the present disclosure that is installable in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 4  is a vertical cross-sectional view of a fixing device according to a third exemplary embodiment of the present disclosure that is installable in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 5  is a plan view of a light shield incorporated in the fixing device depicted in  FIG. 4 ; 
         FIG. 6A  is a partial perspective view of the fixing device depicted in  FIG. 4 , illustrating the light shield situated at a non-shield position; 
         FIG. 6B  is a partial vertical cross-sectional view of the fixing device depicted in  FIG. 6A ; 
         FIG. 6C  is a partial perspective view of the fixing device depicted in  FIG. 4 , illustrating the light shield situated at a shield position; 
         FIG. 6D  is a partial vertical cross-sectional view of the fixing device depicted in  FIG. 6C ; 
         FIG. 7  is an exploded perspective view of a nip formation pad incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 8A  is a plan view of a lubricant retainer as a first example incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 8B  is a front cross-sectional view of the lubricant retainer depicted in  FIG. 8A ; 
         FIG. 9A  is a plan view of a lubricant retainer as a second example incorporated in the fixing device depicted in FIG. and 
         FIG. 9B  is a front cross-sectional view of the lubricant retainer depicted in  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     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. 1 , an image forming apparatus  100  according to an exemplary embodiment of the present disclosure is explained. 
     It is to be noted that, in the drawings for explaining exemplary embodiments of this disclosure, identical reference numerals are assigned, as long as discrimination is possible, to components such as members and component parts having an identical function or shape, thus omitting description thereof once it is provided. 
       FIG. 1  is a schematic vertical cross-sectional view of the image forming apparatus  100 . The image forming apparatus  100  may be a copier, a facsimile machine, as printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this exemplary embodiment, the image forming apparatus  100  is a color printer that forms color and monochrome toner images on a recording medium by electrophotography. Alternatively, the image forming apparatus  100  may be a monochrome printer that forms a monochrome toner image on a recording medium. 
     Referring to  FIG. 1 , a description is provided of a construction of the image forming apparatus  100 . 
     The image forming apparatus  100  is a color printer employing a tandem system in which a plurality of image forming devices for forming toner images in a plurality of colors, respectively, is aligned in a stretch direction of a transfer belt. Alternatively, the image forming apparatus  100  may be a copier, a facsimile machine, a printer, an MFP, or the like. 
     The image forming apparatus  100  employs a tandem structure in which four photoconductive drums  20 Y,  20 C,  20 M, and  20 K serving as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively, are aligned. 
     The yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively, are primarily transferred successively onto an endless transfer belt  11  serving as an intermediate transferor disposed opposite the photoconductive drums  20 Y  20 C,  20 M, and  20 K as the transfer belt  11  rotates in a rotation direction A 1  such that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the transfer belt  11  in a primary transfer process. Thereafter, the yellow, cyan, magenta, and black toner images superimposed on the transfer belt  11  are secondarily transferred onto a recording medium S (e.g., a recording sheet and a transfer sheet) collectively in a secondary transfer process. 
     Each of the photoconductive drums  20 Y,  20 C,  20 M, and  20 K is surrounded by image forming components that form the yellow, cyan, magenta, and black toner images on the photoconductive drums  20 Y  20 C,  20 M, and  20 K as the photoconductive drums  20 Y,  20 C,  20 M, and  20 K rotate clockwise in  FIG. 1  in a rotation direction D 20 . 
     Taking the photoconductive drum  20 K that forms the black toner image, the following describes an image forming operation to form the black toner image. The photoconductive drum  20 K is surrounded by a charger  30 K, a developing device  40 K, a primary transfer roller  12 K, and a cleaner  50 K in this order in the rotation direction D 20  of the photoconductive drum  20 K. The photoconductive drums  20 Y,  20 C, and  20 M are also surrounded by chargers  30 Y,  30 C, and  30 M, developing devices  40 Y,  40 C, and  40 M, primary transfer rollers  12 Y,  12 C, and  12 M, and cleaners  50 Y,  50 C, and  50 M in this order in the rotation direction D 20  of the photoconductive drums  20 Y,  20 C, and  20 M, respectively. After the charger  30 K charges the photoconductive drum  20 K, an optical writing device writes an electrostatic latent image on the photoconductive drum  20 K. 
     As the transfer belt  11  rotates in the rotation direction A 1 , the yellow, cyan, magenta, and black toner images termed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively, are primarily transferred successively onto the transfer belt  11 , thus being superimposed on the same position on the transfer belt  11 . For example, the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K disposed opposite the photoconductive drams  20 Y,  20 C,  20 M, and  20 K via the transfer belt  11 , respectively, and applied with an electric voltage primarily transfer the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K at different times from the upstream photoconductive drum  20 Y to the downstream photoconductive drum  20 K in the rotation direction A 1  of the transfer belt  11 . 
     The photoconductive drums  20 Y,  20 C,  20 M, and  20 K are aligned in this order in the rotation direction A 1  of the transfer belt  11 . The photoconductive drums  20 Y,  20 C,  20 M and  20 K are located in four image forming stations that balm the yellow, cyan, magenta, and black toner images, respectively. 
     The image forming apparatus  100  includes the four image forming stations, as transfer belt unit  10 , a secondary transfer roller  5 , a belt cleaner  13 , and the optical writing device  8 . The transfer belt unit  10  is situated above and disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 K. The transfer belt unit  10  incorporates the transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K. The secondary transfer roller  5  serves as a transferor disposed opposite the transfer belt  11  and driven and rotated in accordance with rotation of the transfer belt  11 . The belt cleaner  13  is disposed opposite the transfer belt  11  to clean the transfer belt  11 . The optical writing device  8  is situated below and disposed opposite the four image forming stations. 
     The optical writing device  8  includes a semiconductor laser serving as at light source, a coupling lens, an fθ lens, a troidal lens, a deflection mirror, and a rotatable polygon mirror serving as a deflector. The optical writing device  8  emits light beams Lb corresponding to the yellow, cyan, magenta, and black toner images to be formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K thereto, forming electrostatic latent images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively.  FIG. 1  illustrates the light beam Lb irradiating the photoconductive drum  20 K. Similarly, light beams irradiate the photoconductive drums  20 Y,  20 C, and  20 M, respectively. 
     The image forming apparatus  100  further includes a sheet feeder  61  and a registration roller pair  4 . The sheet feeder  61  incorporates a paper tray that loads a plurality of recording media S to be conveyed one by one to a secondary transfer nip formed between the transfer belt  11  and the secondary transfer roller  5 . The registration roller pair  4  feeds a recording medium S conveyed from the sheet feeder  61  to the secondary transfer nip formed between the transfer belt  11  and the secondary transfer roller  5  at a predetermined time when the yellow, cyan, magenta, and black toner images superimposed on the transfer belt  11  reach the secondary transfer nip. The image forming apparatus  100  further includes a sensor for detecting that a leading edge of the recording medium S reaches the registration roller pair  4 . 
     The image forming apparatus  100  further includes a fixing device  200 , an output roller pair  7 , an output tray  17 , and toner bottles  9 Y,  9 C,  9 M, and  9 K. The fixing device  200  fixes a color toner image formed by the yellow, cyan, magenta, and black toner images secondarily transferred from the transfer belt  11  onto the recording medium S thereon. The output roller pair  7  ejects the recording medium S bearing the fixed toner image onto an outside of the image forming apparatus  100 , that is, the output tray  17 . The output tray  17  is disposed atop the image forming apparatus  100  and stacks the recording medium S elected by the output roller pair  7 . The toner bottles  9 Y  9 C,  9 M, and  9 K are situated below the output tray  17  and replenished with fresh yellow, cyan, magenta, and black toners, respectively. 
     The transfer belt unit  10  includes a driving roller  72  and a driven roller  73  over which the transfer belt  11  is looped, in addition to the transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K. 
     Since the driven roller  73  also serves as a tension applicator that applies tension to the transfer belt  11 , a biasing member (e.g., a spring) biases the driven roller  73  against the transfer belt  11 . The transfer belt unit  10 , the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, the secondary transfer roller  5 , and the belt cleaner  13  construct a transfer device  71 . 
     The sheet feeder  61  is situated in a lower portion of the image forming apparatus  100  and includes a feed roller  3  that contacts an upper side of an uppermost recording medium S of the plurality of recording media S loaded on the paper tray of the sheet feeder  61 . As the feed roller  3  is driven and rotated counterclockwise in  FIG. 1 , the feed roller  3  feeds the uppermost recording medium S to the registration roller pair  4 . 
     The belt cleaner  13  of the transfer device  71  includes a cleaning brush and a cleaning blade being disposed opposite and contacting the transfer belt  11 . The cleaning brush and the cleaning blade scrape a foreign substance such as residual toner particles off the transfer belt  11 , removing the foreign substance from the transfer belt  11  and thereby cleaning, the transfer belt  11 . The belt cleaner  13  further includes a waste toner conveyer that conveys the residual toner particles removed from the transfer belt  11 . 
     Referring to  FIG. 2 , a description is provided of a construction of the fixing device  200  incorporated in the image forming apparatus  100  having the construction described above. 
       FIG. 2  is a vertical cross-sectional view of the fixing device  200  according to a first exemplary embodiment. As illustrated in  FIG. 2 , the fixing device  200  (e.g., a fuser or a fusing unit) includes a fixing belt  201  formed into a loop and serving as a fixing rotator or a fixing member rotatable in a rotation direction D 201  and a pressure roller  203  serving as a pressure rotator disposed opposite the fixing belt  201  and rotatable in a rotation direction D 203 . A halogen beater  202  serving as a heater or a heat source is disposed inside the loop formed by the fixing belt  201 . The halogen heater  202  emits heat or light that irradiates an inner circumferential surface of the fixing belt  201  directly, heating the fixing belt  201  with radiant heat or light. A nip formation pad  206  disposed opposite the pressure roller  203  via the fixing belt  201  presses against the pressure roller  203  via the fixing belt  201  to form a fixing nip N between the fixing belt  201  and the pressure roller  203 . A low-friction sheet  210  serving as a low-friction member is sandwiched between the fixing belt  201  and the nip formation pad  206 . As the fixing belt  201  rotates in the rotation direction D 201 , the inner circumferential surface of the fixing belt  201  slides over the nip formation pad  206  indirectly via the low-friction sheet  210 . 
     The fixing device  200  further includes a support  207 , a holder  208 , and a reflector  209 . The fixing belt  201  and the components disposed inside the loop formed by the fixing belt  201 , that is, the halogen heater  202 , the nip formation pad  206 , the support  207 , the holder  208 , the reflector  209 , and the low-friction sheet  210 , may construct a belt unit  201 U separably coupled to the pressure roller  203 . As a recording medium S bearing an unfixed toner image T is conveyed through the fixing nip N, the fixing belt  201  and the pressure roller  203  melt and fix the toner image T on the recording medium S under heat and pressure. 
     As illustrated in  FIG. 2 , the fixing nip N is planar. Alternatively, the fixing nip N may be contoured into a recess, a curve, or other shapes, if the fixing nip N is recessed with respect to the pressure roller  203 , the recessed fixing nip N directs the leading edge of the recording medium S toward the pressure roller  203  as the recording medium S is discharged from the fixing nip N, facilitating separation of the recording medium S from the fixing belt  201  and suppressing jamming of the recording medium S. 
     A detailed description is now given of a construction of the fixing belt  201 . 
     The fixing belt  201  is an endless belt or endless film made of metal such as nickel and SUS stainless steel or resin such as polyimide. The fixing belt  201  is constructed of a base layer and a release layer. The release layer serving as an outer surface layer is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like to facilitate separation of toner of the toner image T on the recording medium S from the fixing belt  201  and prevent the toner from adhering to the fixing belt  201 . An elastic layer may be sandwiched between the base layer and the release layer and made of silicone rubber or the like. If the fixing belt  201  does not incorporate the elastic layer, the fixing belt  201  has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image T is fixed on the recording medium S. However, as the pressure roller  203  and the fixing belt  201  sandwich and press the unfixed toner image T on the recording medium S passing through the fixing hip N, slight surface asperities of the fixing belt  201  may be transferred onto the toner image T on the recording medium S, resulting in variation in gloss of the solid toner image T that may appear as an orange peel image on the recording medium S. To address this circumstance, the elastic layer made of silicone rubber has a thickness not smaller than 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing belt  201 , preventing formation of the faulty orange peel image. 
     A detailed description is now given of a configuration of the support  207 , the holder  208 , and the reflector  209 . 
     The support  207  (e.g., a stay) supports the nip formation pad  206  and is situated inside the loop formed by the fixing belt  201 . As the nip formation pad  206  receives pressure from the pressure roller  203 , the support  207  supports the nip formation pad  206  to prevent bending of the nip formation pad  206  and produce a predetermined nip length in a recording medium conveyance direction DS throughout the entire width of the fixing belt  201  in an axial direction thereof parallel to a longitudinal direction of the nip formation pad  206 . The support  207  is mounted on and secured to the holder  208  (e.g., a flange) at each lateral end of the support  207  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  201 , respectively, thus being positioned inside the fixing device  200 . The reflector  209  interposed between the halogen heater  202  and the support  207  reflects radiant light or heat radiated from the halogen heater  202  to the reflector  209  toward the fixing belt  201 , preventing the support  207  from being heated by the halogen heater  202  with radiant heat or the like and thereby reducing waste of energy. 
     Alternatively, instead of the reflector  209 , an opposed face of the support  207  disposed opposite the halogen heater  202  may be treated with insulation or mirror finish to reflect light radiated from the halogen heater  202  to the support  207  toward the fixing belt  201 . The halogen heater  202  serves as a heater for heating the fixing belt  201 . Alternatively, the heater for heating the fixing belt  201  may be an induction heater (IH), a resistive heat generator, a carbon heater, or the like. 
     A detailed description is now given of a construction of the pressure roller  203 . 
     The pressure roller  203  is constructed of a cored bar  205 , an elastic rubber layer  204  coating the cored bar  205 , and a surface release layer coating the elastic rubber layer  204  and being made of PFA or PTFE to facilitate separation of the recording medium S from the pressure roller  203 . As a driving force generated by a driver (e.g., a motor) situated inside the image forming apparatus  100  depicted in  FIG. 1  is transmitted to the pressure roller  203  through a gear train, the pressure roller  203  rotates in the rotation direction D 203 . A spring or the like presses the pressure roller  203  against the nip formation pad  206  via the fixing belt  201 . As the spring presses and deforms the elastic rubber layer  204  of the pressure roller  203 , the pressure roller  203  produces the fixing nip N having the predetermined nip length in the recording medium conveyance direction DS. 
     Alternatively, the pressure roller  203  may be a hollow roller that accommodates a heater such as a halogen beater. The elastic rubber layer  204  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  203 , the elastic rubber layer  204  may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because the sponge rubber has an increased insulation that draws less heat from the fixing belt  201 . 
     As the pressure roller  203  rotates in the rotation direction D 203 , the fixing belt  201  rotates in the rotation direction D 201  in accordance with rotation of the pressure roller  203  by friction therebetween. As the driver drives and rotates the pressure roller  203 , a driving force of the driver is transmitted from the pressure roller  203  to the fixing belt  201  at the fixing nip N, than rotating the fixing belt  201  by friction between the pressure roller  203  and the fixing belt  201 . At the fixing nip N, the fixing belt  201  rotates as the fixing belt  201  is sandwiched between the pressure roller  203  and the nip formation pad  206 ; at a circumferential span of the fixing belt  201  other than the fixing nip N, the fixing belt  201  rotates as the fixing belt  201  is guided by the holder  208  at each lateral end of the fixing belt  201  in the axial direction thereof. With the construction described above, the fixing device  200  attaining quick warm-up is manufactured at reduced costs. 
     Referring to  FIG. 3 , a description is provided of a construction of a fixing device  200 S according to a second exemplary embodiment. 
       FIG. 3  is a vertical cross-sectional view of the fixing device  200 S. Identical reference numerals are assigned to components identical or equivalent to the components incorporated in the fixing device  200  illustrated in  FIG. 2 . 
     The fixing device  200  according to the first exemplary embodiment depicted in  FIG. 2  includes the single halogen heater  202 . Conversely, the fixing device  200 S according to the second exemplary embodiment depicted in  FIG. 3  includes three halogen heaters  202   a ,  202   b , and  202   c . The halogen heaters  202   a ,  202   b , and  202   c  have different heat generation spans in the axial direction of the fixing belt  201  that correspond to different widths of the recording media S, respectively, suppressing redundant heating and saving energy. Other components of the fixing device  200 S according to the second exemplary embodiment are equivalent to the above-described components of the fixing device  200  according to the first exemplary embodiment depicted in  FIG. 2 . 
     Referring to  FIG. 4 , a description is provided of a construction of a fixing device  200 T according to a third exemplary embodiment. 
       FIG. 4  is a vertical cross-sectional view of the fixing device  200 T. Identical reference numerals are assigned to components identical or equivalent to the components incorporated in the fixing devices  200  and  200 S depicted in  FIGS. 2 and 3 , respectively. 
     As illustrated in  FIG. 4 , the fixing device  200 T includes two halogen heaters  202   d  and  202   e  instead of the three halogen heaters  202   a ,  202   b , and  202   c  depicted in  FIG. 3 . In addition to the components of the fixing device  200 S depicted in  FIG. 3 , the fixing device  200 T depicted in  FIG. 4  includes a light shield  211  interposed between the halogen heaters  202   d  and  202   e  and the fixing belt  201  to shield the fixing belt  201  from the halogen heaters  202   d  and  202   e.    
       FIG. 5  is a plan view of the light shield  211 . As illustrated in  FIG. 5 , the light shield  211  includes an aperture  211   a  serving as a non-shield portion that does not shield the fixing belt  201  from the halogen heaters  202   d  and  202   e . The aperture  211   a  has a plurality of widths in the axial direction of the fixing belt  201  that corresponds to a plurality of sizes of recording media S, that is, as width W 1  corresponding to a width of a postcard, a width W 2  corresponding to a width of a B4 size sheet, and a width W 3  corresponding to a width of an A3 size sheet. 
       FIG. 6A  is a partial perspective view of the fixing device  200 T illustrating the light shield  211  situated at a non-shield position where the light shield  211  does not shield the fixing belt  201  from the halogen heaters  202   d  and  202   e .  FIG. 6B  is a partial vertical cross-sectional view of the fixing device  200 T taken on a cross-section C 1  depicted in  FIG. 6A , illustrating the light shield  211  situated at the non-shield position.  FIG. 6C  is as partial perspective view of the fixing device  200 T illustrating the light shield  211  situated at a shield position where the light shield  211  shields the fixing belt  201  from the halogen heaters  202   d  and  202   e .  FIG. 6D  is a partial vertical cross-sectional view of the fixing device  200 T taken on a cross-section C 2  depicted in  FIG. 6C , illustrating the light shield  211  situated at the shield position. 
     As illustrated in  FIGS. 6A, 6B, 6C, and 6D , the light shield  211  is pivotable along the inner circumferential surface of the fixing belt  201  without contacting the fixing belt  201 . The light shield  211  is selectively pivoted to a plurality of shield positions according to the width of the recording medium S conveyed through the Fixing device  200 T, shielding the fixing belt  201  from the halogen heaters  202   d  and  202   e  in an axial span on the fixing belt  201  where heating of the fixing belt  201  is unnecessary.  FIGS. 6A and 6B  illustrate the light shield  211  situated at the non-shield position corresponding to the A3 size sheet.  FIGS. 6C and 6D  illustrate the light shield  211  situated at the shield position corresponding to the postcard. Even if a plurality of small recording media S having a width smaller than the width of the A3 size sheet is conveyed through the fixing device  200 T continuously, the light shield  211  shields the fixing belt  201  from the halogen heaters  202   d  and  202   e , preventing overheating of each lateral end of the fixing belt  201  in the axial direction, that is, a non-conveyance span where the small recording media S are not conveyed and removing a control to eliminate an overheated span on the fixing belt  201  in the axial direction thereof, which may degrade productivity of the fixing device  200 T. Hence, the fixing device  200 T incorporates the two halogen heaters  202   d  and  202   e  decreased compared to the three halogen heaters  202   a ,  202   b , and  202   c  of the fixing device  200 S depicted in  FIG. 3 . 
     A detailed description is now given of a construction of the nip formation pad  206 . 
       FIG. 7  is an exploded perspective view of the nip formation pad  206 .  FIG. 7  illustrates a light irradiation span S 202  where light emitted by the heater (e.g., the halogen heaters  202 ,  202   a ,  202   b ,  202   c ,  202   d , and  202   e ) irradiates the fixing belt  201 .  FIG. 7  further illustrates a recording medium S as an A6 size sheet. The nip formation pad  206  reduces overheating of the non-conveyance span of the fixing belt  201  on behalf of the light Shield  211 . Accordingly, the nip formation pad  206  allows the fixing devices  200 ,  200 S, and  200 T to eliminate the light shield  211  and a driver that drives the light shield  211  and to reduce the number of the halogen heaters  202   a ,  202   b ,  202   c ,  202   d , and  202   e , thus reducing manufacturing costs substantially. 
     As illustrated in  FIG. 7 , the nip formation pad  206  includes a thermal conductor  66  that conducts heat and mounts the low-friction sheet  210  depicted in  FIGS. 2 to 4 . As the fixing belt  201  rotates in the rotation direction D 201  as illustrated in  FIGS. 2 to 4 , the fixing belt  201  slides over the low-friction sheet  210  made of a low-friction material that reduces a frictional load imposed to the fixing belt  201  and decreases a driving torque developed between the fixing belt  201  and the nip formation pad  206 . The thermal conductor  66  is made of a material having an increased thermal conductivity, for example, copper. The thermal conductor  66  extends in a longitudinal direction thereof parallel to the axial direction of the fixing belt  201 . The thermal conductor  66  absorbs excessive heat stored in the non conveyance span on the fixing belt  21  and conducts the absorbed heat in the longitudinal direction of the thermal conductor  66 . Accordingly, the thermal conductor  66  equalizes heat in the axial direction of the fixing belt  201 . 
     The thermal conductor  66  includes arms  66   b  and  66   c  (e.g., bent portions). The arm  66   b  disposed upstream from the arm  66   c  in the recording medium conveyance direction DS has a sharp edge. While the fixing belt  201  rotates, the fixing belt  201  pulls the low-friction sheet  210  in the rotation direction D 201 . However, the sharp edge of the arm  66   b  catches or engages the low-friction sheet  210 , securing the low-friction sheet  210  to the nip formation pad  206  solidly. If the fixing belt  201  is configured to rotate in a reverse direction opposite the rotation direction D 201 , the arm  66   c  has a sharp edge. 
     The nip formation pad  206  further includes primary thermal insulators  83   a  and  83   b , secondary thermal insulators  83   c  and  83   d , a primary thermal absorber  81 , and secondary thermal absorbers  82 . The primary thermal insulators  83   a  are disposed at both lateral end spans of the nip formation pad  206  in the longitudinal direction thereof. The primary thermal insulator  83   b  is disposed at a center span of the nip formation pad  206  in the longitudinal direction thereof. The primary thermal insulators  83   a  and  83   b  are made of a material having a thermal conductivity smaller than a thermal conductivity of the thermal conductor  66 , for example, resin, thus preventing the primary thermal absorber  81  from absorbing heat from the fixing belt  201  excessively. Accordingly, the fixing belt  201  is immune from temperature decrease in a conveyance span of the fixing belt  201  where the recording medium S is conveyed over the fixing belt  201 , suppressing faulty fixing, shortening a warm-up time taken to warm up the fixing belt  201 , and reducing energy consumption. The warm-up time defines a time taken to warm up a fixing device (e.g., the fixing devices  200 ,  200 S, and  200 T) from an ambient temperature to a predetermined temperature (e.g., a reload temperature) at which printing is available after the image forming apparatus  100  is powered on. 
     Similarly, the secondary thermal insulator  83   c  is made of resin, for example. The secondary thermal insulator  83   c  adjusts an amount of heat conducted from the thermal conductor  66  to the primary thermal absorber  81  through the secondary thermal absorber  82 . The thickness and the width of the secondary thermal insulator  83   c  are adjusted based on the degree of overheating or temperature increase of the non-conveyance span of the fixing belt  201 . 
     Each of the primary thermal absorber  81  and the secondary thermal absorbers  82  is made of a material having an increased thermal conductivity. Each of the secondary thermal absorbers  82  is disposed opposite the non-conveyance span of the fixing belt  201  that is susceptible to overheating or temperature increase. Like the secondary thermal insulator  83   c , the thickness and the width of each of the secondary thermal absorbers  82  are adjusted based on the degree of overheating or temperature increase of the non-conveyance span of the fixing belt  201 . 
     Referring to  FIGS. 8A and 8B , a description is provided of a construction of a lubricant retainer  401  as a first example disposed opposite the inner circumferential surface of the fixing belt  201 . 
       FIG. 8A  is a plan view of the lubricant retainer  401 .  FIG. 8B  is a front cross-sectional view of the lubricant retainer  401 .  FIGS. 8A and 8B  illustrate a cross-section of an interior of the loop formed by the fixing bell  201 . 
     As described above, as a driving force generated by the driver (e.g., the motor) is transmitted to the pressure roller  203  through a driving gear  212 , the pressure roller  203  rotates in the rotation direction D 203 . Since a spring, or the like presses the pressure roller  203  against the nip formation pad  206  via the fixing belt  201 , the fixing belt  201  rotates in accordance with rotation of the pressure roller  203 . The driving gear  212  is coupled to one end of the pressure roller  203  in an axial direction thereof to reduce manufacturing costs. The nip formation pad  206  is disposed inside the loop formed by the fixing belt  201  and presses against the pressure roller  203  via the fixing belt  201  to form the fixing nip N between the fixing belt  201  and the pressure roller  203 . The low-friction sheet  210  is interposed between the nip formation pad  206  and the inner circumferential surface of the fixing belt  201 . 
     A description is provided of a construction of a comparative fixing device. 
     The comparative faxing device includes a low-friction sheet impregnated with a liquid lubricant to lubricate the low-friction sheet. The low-friction sheet impregnated with the lubricant is attached to an opposed face of a nip formation pad that is disposed opposite a fixing rotator (e.g., a fixing belt or fixing film) so that the low-friction sheet decreases a resistance between the nip formation pad and the fixing rotator that slides over the nip formation pad. 
     The lubricant applied or contained in the low-friction sheet moves in a particular direction varying depending on a weaving direction of the low-friction sheet, deviation in pressure in an axial direction of the fixing rotator at a fixing nip formed between the fixing rotator and a pressure rotator, or the like. When the lubricant dries up, the fixing rotator may slide over the low-friction sheet with an increased friction. Accordingly, the fixing rotator may rotate at varied linear velocities which may cause faulty conveyance of a recording medium. For example, the fixing rotator may crease the recording medium. As the fixing rotator rotates at varied linear velocities that vary in the axial direction of the fixing rotator, the fixing rotator may skew in the axial direction thereof at an increased linear velocity. Accordingly, a lateral edge face of the fixing rotator may suffer from an increased load, shortening the life of the fixing rotator. 
     The low-friction sheet is made of a low-friction material to enhance the durability of the fixing rotator and is applied with the lubricant. In order to decrease the resistance between the low-friction sheet and the fixing rotator that slides over the low-friction sheet, the lubricant is made of a material having a decreased coefficient of viscosity. However, the lubricant may have an increased flowability and ma flow out of the fixing rotator. The lubricant has a tendency to move in the particular direction varying depending on the weaving direction of the low-friction sheet, deviation in pressure in the axial direction of the fixing rotator at the fixing nip formed between the fixing rotator and the pressure rotator, or the like. Once the lubricant flown out of the fixing rotator produces a flow channel, the lubricant flows out of the fixing rotator continuously until the lubricant on the low-friction sheet dries up. 
     When the lubricant applied to or impregnated in the low-friction sheet dries up, the fixing rotator may slide over the low-friction sheet with an increased friction. Accordingly, the fixing rotator may rotate at varied linear velocities which may cause faulty conveyance of a recording medium. For example, the fixing rotator may crease the recording medium. As the fixing rotator rotates at varied linear velocities that vary in the axial direction of the fixing rotator, the fixing rotator may skew in the axial direction thereof at an increased linear velocity. Accordingly, the lateral edge face of the fixing rotator may suffer from an increased load, shortening the life of the fixing rotator. 
     As illustrated in  FIG. 8A , since the driving gear  212  is coupled to one end of the pressure roller  203  in the axial direction thereof, the fixing belt  201  is exerted with a driving force in addition to the load imposed at the fixing nip N. Accordingly, pressure exerted at the fixing nip N may vary in the axial direction of the fixing belt  201 . Consequently, as illustrated in  FIG. 8B , a lubricant L may flow from a driving side D 1  of the low-friction sheet  210  disposed opposite one lateral end of the fixing belt  201  in the axial direction thereof to a non-driving side D 2  of the low-friction sheet  210  disposed opposite another lateral end of the fixing belt  201  in the axial direction thereof. The driving side D 1  of the low-friction sheet  210  presses against the fixing belt  201  with increased pressure. Conversely, the non-driving side D 2  of the low-friction sheet  210  presses against the fixing belt  201  with decreased pressure smaller than the increased pressure. 
     A mechanism to vary the load exerted at the fixing nip N in advance may be employed by considering the driving force. However, since manufacturing error of parts or the like varies pressure exerted at the fixing nip N, such mechanism may not prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201 . 
     A weaving direction of the low-friction sheet  210  defines a flow direction DF of the lubricant L.  FIG. 8B  illustrates the flow direction DF corresponding to the weaving direction of the low-friction sheet  210  that is oblique relative to the axial direction of the fixing belt  201 . The lubricant L flows in the flow direction DF from the driving side D 1  to the non-driving side D 2  on the fixing belt  201 . To address this circumstance, the low-friction sheet  210  may attain a weaving direction different from the flow direction DF to cause the lubricant L to flow in a flow direction opposite the flow direction DF defined by variation in pressure exerted at the fixing nip N. However, it is difficult to achieve a balance between the weaving direction of the low-friction sheet  210  and the flow direction DF of the lubricant L defined by variation in pressure exerted at the fixing nip N due to the manufacturing error described above. 
     To address this circumstance, the fixing devices  200 ,  200 S, and  200 T installable in the image forming apparatus  100  include the lubricant retainer  401  that prevents the lubricant L from being dried up and thereby suppresses faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201  as described below. 
     As illustrated in  FIGS. 8A and 8B , the lubricant retainer  401  includes a thermal absorber  220  coating the inner circumferential surface of the fixing belt  201 . The thermal absorber  220  absorbs radiant heat radiated from the halogen heater  202  effectively. Coating of the fixing belt  201  by the thermal absorber  220  varies in the axial direction, that is, a longitudinal direction, of the fixing belt  201  to produce an irregular portion  221 . The irregular portion  221  includes a projection  221   a  projecting from the thermal absorber  220  and a depression  221   b  recessed into the thermal absorber  220 . Thus, the irregular portion  221  serves as a flow reducer that suppresses flow of the lubricant  1  from the fixing belt  201 . The thermal absorber  220  is made of a black material that absorbs radiant light radiated from the halogen heater  202  effectively. For example, the thermal absorber  220  is made of fluoroplastic that reduces friction between the low-friction sheet  210  and the thermal absorber  220 . 
     The irregular portion  221  is mounted on the thermal absorber  220  to block the lubricant L flowing on the thermal absorber  220  mounted on the fixing belt  201 , thus suppressing the lubricant L from being dried up and thereby preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . An amount of projection and depression, that is, a site of projection and depression (e.g., a height of the projection  221   a  and a depth of the depression  221   b ), of the irregular portion  221  is adjusted according to the flow direction DF and the amount of the lubricant L. According to the first example of the lubricant retainer  401 , the lubricant L flows in the flow direction DF toward the non-driving side D 2 . Accordingly, an amount of projection and depression of the irregular portion  221  disposed in the non-driving side D 2  is greater than an amount of projection and depression of the irregular portion  221  disposed at the driving side D 1 . 
     The lubricant L moves toward one lateral end of the fixing belt  201  in the axial direction thereof, while the lubricant L produces a flow channel, and reaches the holder  208  and other components situated inside the fixing device  200 . Hence, the irregular portion  221  is disposed opposite each lateral end of the fixing belt  201  in the axial direction thereof. If a plurality of irregular portions  221  is aligned in the axial direction of the fixing belt  201 , the plurality of irregular portions  221  suppresses flow of the lubricant L more effectively. However, the increased number of the irregular portions  221  increases manufacturing costs. To address this circumstance, according to the first example, the irregular portion  221  is disposed opposite each lateral end of the fixing belt  201  in the axial direction thereof. Since the holder  208  is disposed opposite each lateral end of the fixing belt  201  in the axial direction thereof, the irregular portion  221  is disposed inboard from the holder  208  in the axial direction of the fixing belt  201 . 
     Alternatively, the irregular portion  221  may be disposed opposite one lateral end of the fixing, belt  201  in the axial direction thereof. For example, the irregular portion  221  may be disposed opposite the non-driving side D 2  of the fixing belt  201  because the lubricant L flows toward the non-driving side D 2 . In other words, the irregular portion  221  may be disposed opposite one end of the pressure roller  203  in the axial direction thereof where the driving gear  212  is not provided. Thus, the irregular portion  221  suppresses the lubricant L from flowing out of at least one lateral end of the fixing belt  201  in the axial direction thereof and being dried up from the low-friction sheet  210 , preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     The projection  221   a  is disposed outboard from the depression  221   b  in the axial direction of the fixing belt  201 . In other words, the depression  221   b  is inboard from the projection  221   a  in the axial direction of the fixing belt  201  and is closer to as center of the fixing belt  201  in the axial direction thereof than the projection  221   a  is. The projection  221   a  is adjacent to or abuts on the depression  221   b . As illustrated in  FIG. 8A , the projection  221   a  blocks the lubricant L that flows. The depression  321   k  abutting on the projection  221   a  holds or stores the lubricant L. Thus, the projection  221   a  and the depression  221   b  suppress the lubricant L from being dried up front the low-friction sheet  210 , preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . The projection  221   a  and the depression  221   b  abutting on each other prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201  effectively. 
     The irregular portion  221  is disposed outboard from a nip formation span in the axial direction of the fixing belt  201  where the fixing belt  201  slides over the nip formation pad  206  via the low-friction sheet  210 . In other words, the irregular portion  221  is disposed outboard from the nip formation pad  206  in the axial direction of the fixing belt  201 . The irregular portion  221  is disposed inboard from a support span in the axial direction of the fixing belt  201  where the holder  208  contacts or supports the fixing belt  201 . In other words, the irregular portion  221  is disposed inboard from the holder  208  in the axial direction of the fixing belt  201  and disposed opposite a non-slide span of the fixing belt  201  where the fixing belt  201  slides over neither the nip formation pad  206  nor the holder  208 . If the irregular portion  221  is disposed opposite a slide span of the fixing belt  201  where the fixing belt  201  slides over the nip formation pad  206  or the holder  208 , the irregular portion  221  may suffer from abrasion over time and degradation in reduction of the lubricant L that flows out of the thermal absorber  220  mounted on the fixing belt  201 . To address this circumstance, the irregular portion  221  is disposed opposite the non-slide span interposed between the low-friction sheet  210  mounted on the nip formation pad  206  and the bolder  208  in the axial direction of the fixing belt  201 , thus preventing degradation in performance of the irregular portion  221  due to abrasion and thereby improving the life of the fixing belt  201 . 
     Referring to  FIGS. 9A and 9B , a description is provided of a construction of a lubricant retainer  402  as a second example disposed opposite the inner circumferential surface of the fixing belt  201 . 
       FIG. 9A  is a plan view of the lubricant retainer  402 .  FIG. 9B  is a front cross-sectional view of the lubricant retainer  402 .  FIGS. 9A and 9B  illustrate a cross-section of the interior of the loop formed by the fixing belt  201 . 
     As illustrated in  FIGS. 9A and 9B , the lubricant retainer  402  includes the thermal absorber  220  coating the inner circumferential surface of the fixing belt  201 . The thermal absorber  220  absorbs radiant heat radiated from the halogen heater  202  effectively. Coating of the fixing belt  201  by the thermal absorber  220  varies in the axial direction of the fixing belt  201  to produce a plurality of irregular portions  221 . The irregular portion  221  includes the projection  221   a  and the depression  221   b , serving as a flow reducer that suppresses flow of the lubricant L from the thermal absorber  220  mounted on the fixing belt  201 . The irregular portion  221  is perpendicular to the flow direction DF of the lubricant L flowing over the inner circumferential surface of the fixing belt  20  via the thermal absorber  220 , thus blocking the lubricant L flowing in the flow direction DF. The thermal absorber  220  is made of a black material that absorbs radiant light radiated from the halogen heater  202  effectively. For example, the thermal absorber  220  is made of fluoroplastic that reduces friction between the low-friction sheet  210  and the thermal absorber  220 . 
     The irregular portion  221  is mounted on the thermal absorber  220  such that the irregular portion  221  is perpendicular to the flow direction DF of the lubricant L so as to block the lubricant L being applied to the low-friction sheet  210  and flowing on the thermal absorber  220  mounted on the fixing belt  201 , thus suppressing the lubricant L from being dried up and thereby preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     The lubricant L moves toward one lateral end of the fixing belt  201  in the axial direction thereof while the lubricant L produces a flow channel, and reaches the holder  208  and other components situated inside the fixing device  200 . To address this circumstance, the plurality of irregular portions  221  is aligned more closely at an upstream position than at a downstream position in the flow direction DF of the lubricant L. In other words, a density of the upstream irregular portions  221  is greater than a density of the downstream irregular portions  221 . According to the second example of the lubricant retainer  402 , the lubricant L flows in the flow direction DF from the driving side D 1  to the non-driving side D 2 . Accordingly, a density of the irregular portions  221  disposed opposite the driving side D 1  is greater than a density of the irregular portions  221  disposed opposite the non-driving side D 2 . Thus, the irregular portions  221  prevent the lubricant L from being dried up from the low friction sheet  210  effectively, preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     A size of the upstream irregular portion  221  (e.g., the height of the projection  221   a  and the depth of the depression  221   b ) is greater than a size of the downstream irregular portion  221  disposed downstream from the upstream irregular portion  221  in the flow direction DF of the lubricant L. According to the second example of the lubricant retainer  402 , the lubricant L flows in the flow direction DF front the driving side D 1  to the non driving side D 2 . Accordingly, the size of the irregular portion  221  disposed opposite the driving side D 1  is greater than the size of the irregular portion  221  disposed opposite the non driving side D 2 . Thus, the irregular portions  221  prevent the lubricant L from being dried up from the low-friction sheet  210  effectively, preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     A width of each of the projection  221   a  and the depression  221   b  of the upstream irregular portion  221  is greater than a width of each of the projection  221   a  and the depression  221   b  of the downstream irregular portion  221  in the axial direction of the fixing belt  201 . According to the second example of the lubricant retainer  402 , the lubricant L flows in the flow direction DF from the driving side D 1  to the non-driving side D 2 . Accordingly, the width of each of the protection  221   a  and the depression  221   b  of the irregular portion  221  disposed opposite the driving side D 1  is greater than the width of each of the projection  221   a  and the depression  221   b  of the irregular portion  221  disposed opposite the non-driving side D 2 . Thus, the irregular portions  221  prevent the lubricant L from being dried up from the low-friction sheet  210  effectively, preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     A description is provided of advantages of the fixing devices  200 ,  200 S, and  200 T. 
     As illustrated in  FIGS. 2 to 4 , a fixing device (e.g., the fixing devices  200 ,  200 S, and  200 T) includes the fining belt  201  serving as a fixing rotator, the halogen heaters  202 ,  202   a ,  202   b ,  202   c ,  202   d , and  202   e  serving as a heater, the nip formation pad  206 , the pressure roller  203  serving as a pressure rotator, and the low-friction sheet  210  serving as a low-friction member. As illustrated in  FIGS. 8A, 8B, 9A, and 9B , the fixing device further includes the thermal absorber  220  and the irregular portion  221 . 
     As illustrated in  FIGS. 2 to 4 , the fixing belt  201  is rotatable in a predetermined direction of rotation (e.g., the rotation direction D 201 ). The heater is disposed opposite the fixing belt  201  and heats the fixing belt  201 . The nip formation pad  206  is disposed opposite the inner circumferential surface of the fixing belt  201 . The pressure roller  203  is pressed against the nip formation pad  206  via the fixing belt  201  to form the fixing nip N between the fixing belt  201  and the pressure roller  203 , through which a recording medium S bearing a toner image T is conveyed. The low-friction sheet  210  is sandwiched between the nip formation pad  206  and the fixing belt  201  and carries a lubricant L that flows in the flow direction DF. For example, the low-friction sheet  210  is applied or impregnated with the lubricant L. As illustrated in  FIGS. 8A, 8B, 9A, and 9B , the thermal absorber  220  is mounted on the inner circumferential surface of the fixing belt  201 . The irregular portion.  221  is mounted on the thermal absorber  220  and prevents the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201 . 
     As illustrated in  FIGS. 8A and 8B , the irregular portion  221  mounted on the thermal absorber  220  blocks the lubricant L leaking from the low-friction sheet  210 , thus suppressing the lubricant L from being dried up and thereby preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     Since the lubricant L flows out of at least one lateral end of the fixing belt  201  in the axial direction thereof, the irregular portion  221  is disposed opposite the lateral end of the fixing, belt  201  in the axial direction thereof. Thus, the irregular portion  221  prevents the lubricant L from flowing out of the lateral end of the fixing belt  201  in the axial direction thereof and being dried up from the low-friction sheet  210 , thus suppressing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     Since the lubricant L flows out of the lateral end of the fixing belt  201  in the axial direction thereof, the projection  221   a  of the irregular portion  221  is disposed opposite the lateral end of the fixing belt  201  in the axial direction thereof. The depression  221   b  abutting on the projection  221   a  is disposed inboard from the projection  221   a  in the axial direction of the fixing belt  201  such that the depression  221   b  is closer to the center of the fixing belt  201  in the axial direction thereof than the projection  221   a  is. Thus, the projection  221   a  blocks the lubricant L flowing in the flow direction DF. The depression  221   b  abutting on the projection  221   a  holds or stores the lubricant L. Thus, the projection  221   a  and the depression  221   b  prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201  and being dried up from the low-friction sheet  210 , preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     The irregular portion  221  is disposed outboard from the slide span in the axial direction of the fixing belt  201  where the fixing belt  201  slides over the nip formation pad  206  via the low-friction sheet  210 . The irregular portion  221  is disposed inboard from the support span in the axial direction of the fixing belt  201  where the holder  208  contacts or supports the fixing belt  201 . Thus, the irregular portion  221  is immune from abrasion and improves the life. 
     As illustrated in  FIGS. 9A and 9B , the plurality of irregular portions  221  holds or retains the lubricant L in an increased amount greater than a decreased amount retained by the single irregular portion, thus suppressing the lubricant L from being dried up and thereby preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201  effectively. The irregular portion  221  is perpendicular to the flow direction DF of the lubricant L that flows on the thermal absorber  220  mounted on the inner circumferential surface of the fixing belt  201 . The irregular portion  221  blocks the lubricant L flowing in the flow direction DF effectively, preventing the lubricant L from being dried up from the low-friction sheet  210  and preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201 . 
     The plurality of irregular portions  221  is aligned in the axial direction of the fixing belt  201  such that the density of the irregular portions  221  increases from the downstream position to the upstream position in the flow direction DF of the lubricant L that flows on the thermal absorber  220  mounted on the inner circumferential surface of the fixing belt  201 . Thus, the irregular portions  221  prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201  and being dried up from the low-friction sheet  210  effectively, preventing fruity conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201  effectively. 
     The size of the upstream irregular portion  221  that is defined by the height of the projection  221   a  and the depth of the depression  221   b  is greater than the size of the downstream irregular portion  221  disposed downstream from the upstream irregular portion  221  in the flow direction DF of the lubricant L flowing on the thermal absorber  220  mounted on the inner circumferential surface of the fixing belt  201 . Thus, the irregular portions  221  prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201  and being dried up from the low-friction sheet  210  effectively, preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201  effectively. 
     The width of the upstream irregular portion  221  is greater than the width of the downstream irregular portion  221  disposed downstream from the upstream irregular portion  221  in the flow direction DF of the lubricant L flowing on the thermal absorber  220  mounted on the inner circumferential surface of the fixing belt  201 . Thus, the irregular portions  221  prevent the lubricant L from flowing out of the thermal absorber  220  mounted on the fixing belt  201  and being dried up from the low-friction sheet  210  effectively, preventing faulty conveyance of the recording medium S by the fixing belt  201  and degradation in the life of the fixing belt  201  effectively. 
     The present disclosure is not limited to the details of the exemplary embodiments described above and various modifications and improvements are possible. For example, the thermal absorber  220  coating the inner circumferential surface of the fixing belt  201  may be made of a material other than the material described above. The size, the shape, and the like of the irregular portion  221  may be adjusted according to the flow direction DF of the lubricant L and the amount of the lubricant L. The basic construction of the fixing devices  200 ,  200 S, and  200 T may be modified properly. 
     Further, the construction of the image forming apparatus  100  may be modified arbitrarily. For example,  FIG. 1  illustrates the image forming apparatus  100  using toners in four colors. Alternatively, the image forming apparatus  100  may be a full color image forming apparatus using toners in three colors, a multicolor image forming apparatus using toners in two colors, or a monochrome image forming apparatus using toner in a single color. 
     According to the exemplary embodiments described above, the fixing belt  201  serves as a fixing rotator. Alternatively, a fixing film, a fixing sleeve, or the like may be used as a fixing rotator. Further, the pressure roller  203  serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The present disclosure has been described above with reference to specific exemplary embodiments. Note that the present disclosure 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 disclosure. It is therefore to be understood that the present disclosure 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 disclosure.