Patent Publication Number: US-10317828-B2

Title: Fixing device including a nip formation pad with a porous structure, 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(a) to Japanese Patent Application No. 2017-052669, filed on Mar. 17, 2017, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure generally relate to a fixing device and an image forming apparatus incorporating the fixing device, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus for forming an image on a recording medium with the fixing device. 
     Related Art 
     Various types of electrophotographic image forming apparatuses are known, including copiers, printers, facsimile machines, and multifunction machines having two or more of copying, printing, scanning, facsimile, plotter, and other capabilities. Such image forming apparatuses usually form an image on a recording medium according to image data. Specifically, in such image forming apparatuses, for example, a charger uniformly charges a surface of a photoconductor as an image bearer. An optical writer irradiates the surface of the photoconductor thus charged with a light beam to form an electrostatic latent image on the surface of the photoconductor according to the image data. A developing device supplies toner to the electrostatic latent image thus formed to render the electrostatic latent image visible as a toner image. The toner image is then transferred onto a recording medium either directly, or indirectly 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 onto the recording medium. Thus, an image is formed on the recording medium. 
     Such a fixing device typically includes a fixing rotator, such as a roller, a belt, and a film, and a pressure rotator, such as a roller and a belt, pressed against the fixing rotator. The fixing rotator and the pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image onto the recording medium while the recording medium is conveyed between the fixing rotator and the pressure rotator. 
     SUMMARY 
     In one embodiment of the present disclosure, a novel fixing device includes a rotator, an endless belt, and a nip formation pad. The rotator is rotatable in a direction of rotation. The endless belt contacts the rotator and rotates in a direction of rotation. The nip formation pad contacts an inner circumferential surface of the endless belt to form a fixing nip between the endless belt and the rotator that presses the endless belt against the nip formation pad. The nip formation pad includes a slide layer, a base layer, and a lubricant holding layer. The slide layer contacts the endless belt. The base layer is disposed away from the endless belt. The lubricant holding layer is interposed between the slide layer and the base layer to hold a lubricant inside the lubricant holding layer and supply the lubricant to the slide layer. The slide layer has a porous structure including a plurality of through holes in a thickness direction of the slide layer. 
     Also described is a novel image forming apparatus incorporating the fixing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the embodiments and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic sectional view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic sectional view of a fixing device incorporated in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic sectional view of a nip formation pad incorporated in the fixing device of  FIG. 2 ; 
         FIG. 4A  is a partial sectional view of the nip formation pad, illustrating a first molding stage; 
         FIG. 4B  is a partial sectional view of the nip formation pad, illustrating a second molding stage; 
         FIG. 4C  is a partial sectional view of the nip formation pad, illustrating a third molding stage; 
         FIG. 4D  is a partial sectional view of the nip formation pad, illustrating a fourth molding stage; 
         FIG. 4E  is a sectional view of the nip formation pad, illustrating a fifth molding stage; 
         FIG. 4F  is a sectional view of the nip formation pad, illustrating a final shape thereof; and 
         FIG. 5  is a schematic sectional view of a comparative nip formation pad. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of the present 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 have a similar function, operate in a similar manner, and achieve a similar result. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and not all of the components or elements described in the embodiments of the present disclosure are indispensable to the present disclosure. 
     In a later-described comparative example, embodiment, and exemplary variation, for the sake of simplicity like reference numerals are given to identical or corresponding constituent elements such as parts and materials having the same functions, and redundant descriptions thereof are omitted unless otherwise required. 
     As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     It is to be noted that, in the following description, suffixes Y, C, M, and K denote colors yellow, cyan, magenta, and black, respectively. To simplify the description, these suffixes are omitted unless necessary. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present disclosure are described below. 
     Initially with reference to  FIG. 1 , a description is given of an overall configuration of an image forming apparatus  1  according to an embodiment of the present disclosure. 
       FIG. 1  is a schematic sectional view of the image forming apparatus  1 . 
     The image forming apparatus  1  may be, e.g., a copier, a facsimile machine, a printer, a multifunction peripheral (MFP) having at least two of copying, printing, scanning, facsimile, and plotter functions. In the present embodiment, the image forming apparatus  1  is a color image forming apparatus that forms color and monochrome images on recording media by electrophotography. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes an image forming device  2  disposed in a center portion of the image forming apparatus  1 . The image forming device  2  includes four removable process units  9 Y,  9 C,  9 M, and  9 K. The process units  9 Y,  9 C,  9 M, and  9 K have identical configurations, except that the process units  9 Y,  9 C,  9 M, and  9 K contain developers in different colors, that is, yellow (Y), cyan (C), magenta (M), and black (K) corresponding to color-separation components of a color image. 
     Each of the process units  9 Y,  9 C,  9 M, and  9 K includes, e.g., a photoconductive drum  10 , a charging roller  11 , a developing device  12 , and a cleaner  13 . The photoconductive drum  10  is a drum-shaped photoconductor serving as an image bearer. The charging roller  11  serves as a charger. The photoconductive drum  10  is a drum-shaped rotator that bears toner as a developer of a toner image on an outer circumferential surface of the photoconductive drum  10 . The charging roller  11  uniformly charges the outer circumferential surface of the photoconductive drum  10 . The developing device  12  includes a drum-shaped developing roller that supplies toner to the outer circumferential surface of the photoconductive drum  10 . The cleaner  13  removes residual toner from the outer circumferential surface of the photoconductive drum  10 . In this case, the residual toner is toner that has failed to be transferred from the photoconductive drum  10  onto an intermediate transfer belt  16 , and therefore that remains on the photoconductive drum  10 . 
     Below the process units  9 Y,  9 C,  9 M, and  9 K is an exposure device  3 . The exposure device  3  emits a laser beam onto the photoconductive drum  10  according to image data. 
     Above the image forming device  2  is a transfer device  4 . The transfer device  4  includes, e.g., a drive roller  14 , a driven roller  15 , the intermediate transfer belt  16 , and four primary transfer rollers  17 . The intermediate transfer belt  16  is an endless belt rotatably entrained around the drive roller  14 , the driven roller  15 , and the like. Each of the four primary transfer rollers  17  is disposed opposite the corresponding photoconductive drum  10  via the intermediate transfer belt  16 . At the position opposite the photoconductive drum  10 , each of the four primary transfer rollers  17  presses an inner circumferential surface of the intermediate transfer belt  16  against the corresponding photoconductive drum  10  to form an area of contact, herein referred to as a primary transfer nip, between the intermediate transfer belt  16  and the photoconductive drum  10 . 
     A secondary transfer roller  18  is disposed opposite the drive roller  14  via the intermediate transfer belt  16 . The secondary transfer roller  18  is pressed against an outer circumferential surface of the intermediate transfer belt  16  to form an area of contact, herein referred to as a secondary transfer nip, between the secondary transfer roller  18  and the intermediate transfer belt  16 . 
     As described above, each of the four primary transfer rollers  17  sandwich the intermediate transfer belt  16  together with the corresponding photoconductive drum  10 , thereby forming the primary transfer nip between the intermediate transfer belt  16  and the photoconductive drum  10 . The primary transfer rollers  17  are coupled to a power supply. The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage to the primary transfer rollers  17 . 
     As described above, the secondary transfer roller  18  sandwiches the intermediate transfer belt  16  together with the drive roller  14 , thereby forming the secondary transfer nip between the secondary transfer roller  18  and the intermediate transfer belt  16 . Similar to the primary transfer rollers  17 , the secondary transfer roller  18  is coupled to the power supply. The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage to the secondary transfer roller  18 . 
     A belt cleaner  28  includes a cleaning brush and a cleaning blade disposed to contact the outer circumferential surface of the intermediate transfer belt  16 . The belt cleaner  28  removes residual toner from the intermediate transfer belt  16  as a waste toner. In this case, the residual toner is toner that has failed to be transferred from the intermediate transfer belt  16  onto a sheet P, and therefore that remains on the intermediate transfer belt  16 . In short, the belt cleaner  28  collects the waste toner. A waste toner conveyance tube extends from the belt cleaner  28  to an inlet of a waste toner container. The waste toner collected by the belt cleaner  28  passes through the waste toner conveyance tube and contained in the waste toner container. 
     The sheet feeder  5  is positioned in a lower portion of the image forming apparatus  1 . The sheet feeder  5  includes, e.g., a sheet tray  19  and a sheet feeding roller  20 . Sheets P, serving as recording media, can be loaded onto the sheet tray  19 . The sheet feeding roller  20  picks up and feeds the sheets P one by one from the sheet tray  19  to a conveyance passage  6 , which is defined by some internal components of the image forming apparatus  1 . 
     In the present embodiment, the sheets P are plain paper. Alternatively, the sheet P may be thick paper, postcards, envelopes, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, plastic films, prepreg, copper foil, and the like. 
     The sheets P are conveyed along the conveyance passage  6  from the sheet feeder  5  toward a sheet ejector  8 . Conveyance roller pairs including a registration roller pair  21  are disposed along the conveyance passage  6 . 
     The fixing device  7  includes, e.g., a fixing belt  22  serving as a fixing rotator and a pressure roller  23  serving as a pressure rotator. The fixing belt  22  is an endless belt heated by a heater. The pressure roller  23  presses against the fixing belt  22 . 
     The sheet ejector  8  is disposed in an extreme downstream part of the conveyance passage  6  in a direction of conveyance of the sheet P, hereinafter referred to as a sheet conveyance direction C. The sheet ejector  8  includes an ejection roller pair  24  and an output tray  25 . The ejection roller pair  24  ejects the sheets P onto the output tray  25  disposed atop a housing of the image forming apparatus  1 . Thus, the sheets P lie stacked on the output tray  25 . 
     In an upper portion of the image forming apparatus  1 , removable toner bottles  29 Y,  29 C,  29 M, and  29 K are disposed. The toner bottles  29 Y,  29 C,  29 M, and  29 K are filled with fresh toner of yellow, cyan, magenta, and black, respectively. A toner supply tube is interposed between each of the toner bottles  29 Y,  29 C,  29 M, and  29 K and the corresponding developing device  12 . The fresh toner is supplied from each of the toner bottles  29 Y,  29 C,  29 M, and  29 K to the corresponding developing device  12  through the toner supply tube. 
     To provide a fuller understanding of the embodiments of the present disclosure, a description is now given of an image forming operation of the image forming apparatus  1  with continued reference to  FIG. 1 . 
     As the image forming apparatus  1  starts the image forming operation in response to a print job assigned thereto, the exposure device  3  emits laser beams to the outer circumferential surface of the photoconductive drums  10  of the respective process units  9 Y,  9 C,  9 M, and  9 K according to image data, thus forming electrostatic latent images on the photoconductive drums  10 . The image data used to expose each of the photoconductive drums  10  is single color image data produced by decomposing a desired full color image into yellow, cyan, magenta, and black image data. For example, according to the yellow image data, the photoconductive drum  10  of the process unit  9 Y is irradiated with a laser beam. Thus, the electrostatic latent image is formed on the photoconductive drum  10 . Then, the developing device  12  supplies toner to the photoconductive drum  10 . Specifically, the drum-shaped developing roller supplies toner stored in the developing device  12  to the outer circumferential surface of the photoconductive drum  10 , rendering the electrostatic latent image visible as a toner image or developer image on the photoconductive drum  10 . In short, the developing device  12  develops the electrostatic latent image into a visible toner image. 
     In the transfer device  4 , a driver drives and rotates the drive roller  14 , thereby rotating the intermediate transfer belt  16  in a counterclockwise direction, herein referred to as a belt rotation direction A, in  FIG. 1 . As described above, the power supply applies voltage to the primary transfer rollers  17 . Specifically, the primary transfer rollers  17  are supplied with a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged toner. Accordingly, transfer electric fields are generated at the primary transfer nips. The transfer electric fields thus generated transfer yellow, cyan, magenta, and black toner images from the respective photoconductive drums  10  onto the intermediate transfer belt  16  such that the yellow, cyan, magenta, and black toner images are sequentially superimposed one atop another on the intermediate transfer belt  16 . Thus, a composite, full color toner image is formed on the intermediate transfer belt  16 . 
     In the meantime, in the lower portion of the image forming apparatus  1 , the sheet feeding roller  20  of the sheet feeder  5  is rotated to feed a sheet P from the sheet tray  19  toward the registration roller pair  21  along the conveyance passage  6 . Activation of the registration roller pair  21  is timed to send out the sheet P toward the secondary transfer nip between the secondary transfer roller  18  and the intermediate transfer belt  16  such that the full color toner image on the intermediate transfer belt  16  meets the sheet P at the secondary transfer nip. As described above, the power supply applies voltage to the secondary transfer roller  18 . Specifically, the secondary transfer roller  18  is supplied with a transfer voltage having a polarity opposite a polarity of charged toner of the full color toner image on the intermediate transfer belt  16 . Accordingly, a transfer electric field is generated at the secondary transfer nip. The transfer electric field thus generated transfers the full color toner image from the intermediate transfer belt  16  onto the sheet P at the secondary transfer nip. Specifically, the yellow, cyan, magenta, and black toner images constructing the composite, full color toner image are collectively transferred onto the sheet P. 
     The sheet P bearing the full color toner image is conveyed to the fixing device  7 , in which the fixing belt  22  and the pressure roller  23  fix the toner image onto the sheet P under heat and pressure. The sheet P bearing the fixed toner image is separated from the fixing belt  22  and conveyed by the conveyance roller pair to the sheet ejector  8 . The ejection roller pair  24  of the sheet ejector  8  ejects the sheet P onto the output tray  25 . 
     The above describes the image forming operation of the color image forming apparatus  1  to form the full color toner image on the sheet P serving as a recording medium. Alternatively, the image forming apparatus  1  may form a monochrome toner image by using any one of the four process units  9 Y,  9 C,  9 M, and  9 K, or may form a bicolor toner image or a tricolor toner image by using two or three of the process units  9 Y,  9 C,  9 M, and  9 K. 
     Referring now to  FIG. 2 , a detailed description is given of a configuration of the fixing device  7  incorporated in the image forming apparatus  1  described above. 
       FIG. 2  is a schematic sectional view of the fixing device  7 . 
     As illustrated in  FIG. 2 , the fixing device  7 , which may be referred to as a fuser or a fusing unit, includes, e.g., the fixing belt  22  serving as a fixing rotator and the pressure roller  23  serving as a pressure rotator pressed against the fixing rotator. In the present embodiment, the pressure roller  23  is a rotator that is rotatable in a direction of rotation, which is, in this case, a rotation direction R 2 . The fixing belt  22  is an endless belt that contacts the rotator and rotates in a direction of rotation, which is, in this case, a rotation direction R 1 . 
     Inside a loop formed by the fixing belt  22 , the fixing device  7  further includes, e.g., a nip formation pad  30 , a support  31 , a flange  32 , a heater  33 , and a reflector  34 . The support  31  supports the nip formation pad  30 . The flange  32  is disposed at each of opposed longitudinal or axial ends of the fixing belt  22 . The reflector  34  reflects heat radiating from the heater  33 . The fixing belt  22  and the components disposed inside the loop formed by the fixing belt  22 , that is, the nip formation pad  30 , the support  31 , the flange  32 , the heater  33 , and the reflector  34 , may constitute a belt unit  22 U detachably coupled to the pressure roller  23 . 
     The fixing belt  22  is an endless belt or film made of a metal material, such as nickel or stainless steel (e.g., steel use stainless or SUS), or a resin material such as polyimide. 
     The fixing belt  22  is constructed of a base layer and a release layer. The release layer, as an outer surface layer of the fixing belt  22 , is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like to facilitate separation of toner of the toner image on the sheet P from the fixing belt  22 . 
     Optionally, an elastic layer made of, e.g., silicone rubber may be interposed between the base layer and the release layer. 
     If the fixing belt  22  does not incorporate the elastic layer, the fixing belt  22  has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image is fixed onto the sheet P. However, as the fixing belt  22  and the pressure roller  23  sandwich and press the unfixed toner image onto the sheet P, slight surface asperities in the fixing belt  22  may be transferred onto the toner image on the sheet P, resulting in variation in gloss of the solid toner image that may appear as an orange peel image on the sheet P. To address this circumstance, the elastic layer, made of, e.g., silicone rubber, may be provided with a thickness not smaller than about 100 μm. As the elastic layer deforms, the elastic layer absorbs the slight surface asperities in the fixing belt  22 , thereby preventing formation of the orange peel image on the sheet P. 
     The nip formation pad  30  is disposed in contact with the inner circumferential surface of the fixing belt  22  to form a fixing nip N between the fixing belt  22  and the pressure roller  23 . In other words, the nip formation pad  30  contacts the inner circumferential surface of the fixing belt  22  to form the fixing nip N between the fixing belt  22  and the pressure roller  23  that presses the fixing belt  22  against the nip formation pad  30 . 
     The nip formation pad  30  has a nip formation surface  30 N on a front side facing the fixing nip N. An upstream side of the nip formation surface  30 N in the sheet conveyance direction C is curved, thereby being apart from the fixing belt  22 . In other words, the nip formation surface  30 N includes a curved face  30   r  upstream from the fixing nip N in the sheet conveyance direction C. The curved face  30   r  of nip formation surface  30 N separates the nip formation pad  30  from the fixing belt  22 , thereby preventing generation of a great friction force between the fixing belt  22  and the nip formation pad  30 , and further preventing damage to the fixing belt  22  by friction between the fixing belt  22  and the nip formation pad  30 . Except the curved face  30   r , the nip formation surface  30 N is plane parallel to the sheet conveyance direction C. In other words, the nip formation surface has a planar face  30   f  in addition to the curved face  30   r . In the present embodiment, the nip formation surface  30 N has the curved face  30   r  upstream and the planar face  30   f  downstream in the sheet conveyance direction C. The planar face  30   f  of the nip formation surface  30 N contacts the fixing belt  22 , thereby forming the fixing nip N between the fixing belt  22  and the pressure roller  23 . 
     The support  31  contacts a back side of the nip formation pad  30  to support the nip formation pad  30  from the back side of the nip formation pad  30 . Accordingly, when the nip formation pad  30  receives pressure from the pressure roller  23 , the support  31  prevents the nip formation pad  30  from being bent by such pressure, thereby maintaining a uniform width of the fixing nip N across the axial direction of the fixing belt  22 . 
     The flange  32  contacts the inner circumferential surface of the fixing belt  22  at each of the opposed axial ends of the fixing belt  22  to hold the fixing belt  22 . As illustrated in  FIG. 2 , the flange  32  guides each of the opposed axial ends of the fixing belt  22  in a circumferential span of the fixing belt  22 , other than a nip span thereof located at the fixing nip N. At the fixing nip N, the fixing belt  22  rotates while being sandwiched by the nip formation pad  30  and the pressure roller  23 . On the other hand, at a location other than the fixing nip N, the fixing belt  22  rotates while being guided by the flange  32 . The flange  32  also supports each of opposed longitudinal ends of the support  31 . Thus, the support  31  is positioned with respect to the flange  32 . 
     In the present embodiment, the heater  33  is a halogen heater. The heater  33 , disposed inside the loop formed by the fixing belt  22 , heats the inner circumferential surface of the fixing belt  22  by radiation heat. The heater  33  is not limited to a halogen heater. Alternatively, the heater  33  may be an induction heater (IH), a resistive heat generator, a carbon heater, or the like. 
     The reflector  34  is interposed between the heater  33  and the support  31  to reflect the radiation heat from the heater  33  toward the fixing belt  22 , thereby preventing transmission of the radiation heat to the support  31 , and enhancing heating efficiency of the heater  33  to heat the fixing belt  22 . Alternatively, instead of the reflector  34 , an interior surface of the support  31  facing the heater  33  may be insulated or given a mirror finish to reflect the radiation heat from the heater  33  toward the fixing belt  22 . 
     The pressure roller  23  includes a cored bar  23   a  and an elastic layer  23   b  provided on an outer circumferential surface side of the cored bar  23   a . The pressure roller  23  further includes a release layer as an outer surface of the pressure roller  23 . The release layer is made of, e.g., PFA or PTFE to facilitate separation of the sheet P from the pressure roller  23 . As a driver, such as a motor, drives and rotates the pressure roller  23 , a driving force of the driver is transmitted from the pressure roller  23  to the fixing belt  22  that is in pressure contact with the pressure roller  23  at the fixing nip N, thereby rotating the fixing belt  22 . 
     A biasing mechanism, such as a spring, presses the pressure roller  23  against the fixing belt  22 , thereby pressing and elastically deforming the elastic layer  23   b  of the pressure roller  23 . Thus, the fixing nip N is formed between the pressure roller  23  and the fixing belt  22 . 
     The pressure roller  23  may be either a solid roller or a hollow roller. If the pressure roller  23  is a hollow roller, optionally, a heater may be disposed inside the pressure roller  23 . The elastic layer  23   b  of the pressure roller  23  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  23 , the elastic layer  23   b  may be made of sponge rubber. The sponge rubber is preferable to the solid rubber because the sponge rubber has enhanced thermal insulation that draws less heat from the fixing belt  22 . 
     Typically, to reduce friction between a nip formation pad and a fixing belt, a fibroid slide aid holding a lubricant may be often disposed therebetween. The lubricant thus held passes through holes formed inside the fibroid slide aid and reaches a slide face of the fibroid slide aid over which the fixing belt slides. Thus, the lubricant is supplied to the slide face of the fibroid slide aid, thereby reducing a frictional force generated between the nip formation pad and the fixing belt. 
     However, as the fixing belt slides over the fibroid slide aid, the slide face of the fibroid slide aid is worn or deformed. Such wear or deformation of the slide face of the fibroid slide aid may crush and damage the holes on a slide face side, hampering the supply of the lubricant to the slide face of the fibroid slide aid. In short, the frictional force loaded on the slide face may be reduced hardly over a long period of time. 
     Hence, according to the present embodiment, the fixing device includes a nip formation pad that supplies a lubricant to a slide face of the nip formation pad over which the fixing belt slides over, while reducing friction between the nip formation pad and the fixing belt, over a long period of time. 
     Specifically, the nip formation pad includes a lubricant holding layer that holds a lubricant and an abrasion-resistant slide layer including through holes. The lubricant is supplied from the lubricant holding layer to the slide face via the through holes of the slide layer. The abrasion-resistant slide layer provided on a slide face side of the nip formation pad suppresses crushing of the through holes on the slide face side that may be caused by friction between the nip formation pad and the fixing belt. Accordingly, the nip formation pad can supply the lubricant to the slide face of the nip formation pad over which the fixing belt slides, while reducing friction between the nip formation pad and the fixing belt, over a long period of time. 
     Referring now to  FIG. 3 , a description is given of a layer structure of the nip formation pad  30  incorporated in the fixing device  7  described above. 
       FIG. 3  is a schematic sectional view of the nip formation pad  30 . 
     Note that  FIG. 3  and  FIGS. 4A through 4F , referred later, illustrate an example of the nip formation pad  30  having a planar nip formation surface that faces the fixing nip N. According to the embodiments of the present disclosure, the nip formation pad  30  may have, e.g., a partly curved nip formation surface (i.e., nip formation surface  30 N) as illustrated in  FIG. 2  or a planar nip formation surface as illustrated in  FIG. 3 . That is, the nip formation pad has a shape changeable as appropriate to a required performance of the nip formation pad. 
     As illustrated in  FIG. 3 , the nip formation pad  30  is constructed of three layers, namely, a base layer  30   b , a lubricant holding layer  30   c , and a slide layer  30   d , from the back side of the nip formation pad  30 . The base layer  30   b  is provided with a plurality of projections  30   a  projecting toward the support  31 . That is, the base layer  30   b  is disposed away from the fixing belt  22  and located on the back side of the nip formation pad  30  near the support  31  as illustrated in  FIG. 2 . In short, the slide layer  30   d  rests on the lubricant holding layer  30   c . The lubricant holding layer  30   c  rests on the base layer  30   b.    
     The base layer  30   b  is a layer that supports a load received from the pressure roller  23  illustrated in  FIG. 2 . The base layer  30   b  is made of an inorganic material or an organic material having sufficient pressure resistance and heat resistance to withstand ambient temperature around the nip formation pad  30  during operation of the fixing device  7 . For example, the base layer  30   b  may be made of an inorganic material such as ceramic, glass, or aluminum, rubber such as silicone rubber or fluororubber, fluororesin such as PTFE, PFA, ethylene tetrafluoroethylene (ETFE), or tetrafluoroethylene hexafluoropropylene (FEP), resin such as polyimide (PI), polyamide imide (PAI), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), liquid crystal plastic or liquid crystal polymer (LCP), phenolic resin, nylon, or aramid, or a combination thereof. 
     In particular, in the fixing device  7  in which the nip formation pad  30  is disposed near the fixing nip N, if the base layer  30   b  is made of a resin material, the resin material preferably has a noncombustibility conforming to a rating not lower than V-0 of UL-94 Standard released by Underwriters Laboratories Inc., a relative thermal index (RTI) not lower than about 100° C., and a heat deflection temperature not lower than about 260° C. Note that the RTI is a temperature index of thermotropic aging according to Underwriters Laboratories Inc. (UL) Standards, specifying the temperature at which electrical characteristics and mechanical characteristics can retain half or greater than original characteristics under exposure for a long period of time. The heat deflection temperature is measured according to Japanese Industrial Standards (JIS) K 7191. In the present embodiment, the base layer  30   b  is made of a liquid crystal polymer. 
     As illustrated in  FIG. 3 , the base layer  30   b  includes the projections  30   a  projecting toward the support  31 . The projections  30   a  are disposed at predetermined intervals in line in a longitudinal direction of the nip formation pad  30 . Note that the longitudinal direction of the nip formation pad  30  is parallel to the axial direction of the fixing belt  22  and perpendicular to the sheet conveyance direction C. In the present embodiment illustrated in  FIG. 3 , the projections  30   a  are aligned in two lines on an upstream side and a downstream side (i.e., left and right sides in  FIG. 3 ), respectively, in the sheet conveyance direction C. 
     The lubricant holding layer  30   c  is interposed between the base layer  30   b  and the slide layer  30   d . The lubricant holding layer  30   c  holds the lubricant inside the lubricant holding layer  30   c  and supplies the lubricant to the adjacent slide layer  30   d.    
     The lubricant holding layer  30   c  is, e.g., a woven fabric made of polyphenylene sulfide (PPS) resin fiber having good lubricant holding characteristics, aramid fiber, nylon fiber, or the like. Alternatively, the lubricant holding layer  30   c  may be a nonwoven fabric impregnated with a lubricant using, e.g., silicone oil or fluorine oil as a base oil. In the present embodiment, the lubricant holding layer  30   c  is made of the PPS resin fiber. 
     The slide layer  30   d  is an abrasion-resistant layer that contacts the fixing belt  22 . In other words, the fixing belt  22  slides over the slide layer  30   d  of the nip formation pad  30  that withstands abrasion caused by friction between the slide layer  30   d  and the fixing belt  22 . The slide layer  30   d  has a porous structure including a large number of through holes  30   h  that go all the way through the slide layer  30   d  from a lubricant holding layer  30   c  side toward a slide face  30   d   1  side. A slide face  30   d   1  of the slide layer  30   d  contacts the outer circumferential surface of the fixing belt  22 . In short, the slide layer  30   d  has a porous structure including a plurality of through holes  30   h  in a thickness direction TD of the slide layer  30   d . The lubricant supplied from the lubricant holding layer  30   c  to the slide layer  30   d  seeps into the slide face  30   d   1  via the through holes  30   h  of the slide layer  30   d . With the lubricant, the fixing belt  22  smoothly slides over the nip formation pad  30 . Thus, the lubricant supplied from the lubricant holding layer  30   c  to the slide face  30   d   1  enhances the wear resistance or abrasion resistance of the slide face  30   d   1  of the nip formation pad  30 . 
     To maintain a given abrasion resistance against sliding of the fixing belt  22 , the slide layer  30   d  preferably has a thickness not smaller than about 15 μm and a pencil hardness not lower than H. Note that the pencil hardness is a value measured by a pencil hardness test prescribed in JIS K 5600-5-4. In addition, to enhance smooth rotation of the fixing belt  22  and to maintain the abrasion resistance of the slide layer  30   d , a preferable coefficient of static friction is not greater than about 0.3 between the slide face  30   d   1  of the nip formation pad  30  and the outer circumferential surface of the fixing belt  22  while the slide face  30   d   1  is filled with the lubricant. Further, the slide layer  30   d  has a given heat resistance to withstand frictional heat generated when the fixing belt  22  slides over the slide layer  30   d  and a relatively high temperature of the fixing belt  22  when the fixing belt  22  reaches the fixing temperature. 
     In consideration of the above, the slide layer  30   d  may be made of, e.g., ceramic, PTFE, PFA, or a combination of these materials. In the present embodiment, the slide layer  30   d  is made of a coating material using PTFE. 
     Referring now to  FIGS. 4A through 4F , a description is given of how the nip formation pad  30  is molded. 
       FIG. 4A  is a partial sectional view of the nip formation pad  30 , illustrating a first molding stage.  FIG. 4B  is a partial sectional view of the nip formation pad  30 , illustrating a second molding stage.  FIG. 4C  is a partial sectional view of the nip formation pad  30 , illustrating a third molding stage.  FIG. 4D  is a partial sectional view of the nip formation pad  30 , illustrating a fourth molding stage.  FIG. 4E  is a sectional view of the nip formation pad  30 , illustrating a fifth molding stage.  FIG. 4F  is a sectional view of the nip formation pad  30 , illustrating a final shape thereof. 
     First, as illustrated in  FIG. 4A , a cloth material  41  forming the lubricant holding layer  30   c  is placed on a first mold  50 , which is a mold on a slide face side of the nip formation pad  30 . Then, as illustrated in  FIG. 4B , the first mold  50  and a second mold  51 , which is a mold on a back side of the nip formation pad  30 , are joined together, thereby forming a cavity  52  therebetween to mold the base layer  30   b.    
     In this state, as illustrated in  FIG. 4C , the cavity  52  is filled with a material for forming the base layer  30   b  and cooled down to be solidified. Thus, the base layer  30   b  is molded. Then, the joined first and second molds  50  and  51  are released to take out a molded article. That is, as illustrated in  FIG. 4D , an intermediate product is obtained including the base layer  30   b  and the lubricant holding layer  30   c  as an integral product. 
     Then, an opposite face  30   c   1  of the lubricant holding layer  30   c  facing away from the base layer  30   b  is spray coated, thereby forming the slide layer  30   d  on the opposite face  30   c   1  as illustrated in  FIG. 4E . As the slide layer  30   d  is formed by spray coating, the slide layer  30   d  includes gaps between particles of a coating material. That is, the slide layer  30   d  has a porous structure including a large number of through holes  30   h.    
     The slide layer  30   d  thus formed is then dried and solidified. Then, extra protruding portions of the solidified lubricant holding layer  30   c  are cut out to finally mold the nip formation pad  30 . Thus, the nip formation pad  30  is formed as illustrated in  FIG. 4F , with an integral three-layer structure constructed of the base layer  30   b , the lubricant holding layer  30   c , and the slide layer  30   d.    
     In the present embodiment, the slide layer  30   d  is formed by spray coating as described above. Alternatively, for example, the slide layer  30   d  may be formed by laminating the intermediate product illustrated in  FIG. 4D  with a sheet-like material having a large number of through holes  30   h  in advance and applying thermocompression bonding to bond the sheet-like material and the intermediate product together. 
     Unlike the configuration of the nip formation pad  30  according to the present embodiment in which the nip formation pad  30  includes the slide layer  30   d  as an integral part thereof, an abrasion-resistant slide aid is typically provided separately from a nip formation pad. That is, fixing or securing parts and a working process are added to secure the slide aid to the nip formation pad, resulting in an increase in the number of parts and an increase in the number of assembling steps. 
     Referring now to  FIG. 5 , a description is given of such a comparative nip formation pad having a configuration different from the configuration of the nip formation pad  30  of the present embodiment. 
       FIG. 5  is a schematic sectional view of a comparative nip formation pad  101 . 
     As illustrated in  FIG. 5 , the comparative nip formation pad  101  has a slide face  101   a  and an abrasion-resistant slide sheet  102  disposed on the slide face  101   a  to enhance the abrasion resistance. In the example of  FIG. 5 , the slide sheet  102  is wound around an entire circumference of the nip formation pad  101 . A double-sided tape  103  is interposed between the nip formation pad  101  and the slide sheet  102  to attach the slide sheet  102  to the nip formation pad  101 . The slide sheet  102  has one end overlapping the other end on a back side of the nip formation pad  101 , that is, an upper side of the nip formation pad  101  in  FIG. 5 . A screw  104  (i.e., external thread) passes through a plate  105  and an overlapped portion of the slide sheet  102  to engage with an internal thread of the nip formation pad  101 . Thus, the overlapped portion of the slide sheet  102  is secured to the nip formation pad  101 . 
     A comparative fixing device incorporating the comparative nip formation pad  101  includes, other than the nip formation pad  101 , the slide sheet  102  for enhancing the abrasion resistance of a slide face side of the nip formation pad  101 , securing members such as the double-sided tape  103  and the screw  104  for securing the slide sheet  102  to the nip formation pad  101 , and the like. That is, the comparative fixing device includes a larger number of parts than the number of parts of the fixing device  7  of the present embodiment. In addition, the comparative fixing device needs an increased number of working steps to wind the slide sheet  102  around the nip formation pad  101 , bond the nip formation pad  101  and the slide sheet  102  with the double-sided tape  103 , and thereafter secure the slide sheet  102  to the nip formation pad  101  with the screw  104 . 
     By contrast, in the present embodiment, the nip formation pad  30  has an integral, multilayer structure in which a plurality of layers is formed as an integral component. That is, the nip formation pad  30  obviates the need to provide securing parts or working processes, thereby reducing the number of parts and facilitating assembly. Note that “a plurality of layers is formed as an integral component” described above herein means that the slide layer  30   d , the lubricant holding layer  30   c , and the base layer  30   b  of the nip formation pad  30  are integrally molded, without being given additional securing members such as screws and tapes. 
     A description is given of advantages of the fixing device  7  according to the embodiments, examples, and variations described above. 
     As illustrated in  FIG. 2 , a fixing device (e.g., fixing device  7 ) includes a rotator (e.g., pressure roller  23 ), an endless belt (e.g., fixing belt  22 ), and a nip formation pad (e.g., nip formation pad  30 ). The rotator is rotatable in a direction of rotation (e.g., rotation direction R 2 ). The endless belt contacts the rotator and rotates in a direction of rotation (e.g., rotation direction R 1 ). The nip formation pad contacts an inner circumferential surface of the endless belt to form a fixing nip (e.g., fixing nip N) between the endless belt and the rotator that presses the endless belt against the nip formation pad. The nip formation pad includes a slide layer (e.g., slide layer  30   d ), a base layer (e.g., base layer  30   b ), and a lubricant holding layer (e.g., lubricant holding layer  30   c ). The slide layer contacts the endless belt. The base layer is disposed away from the endless belt. The lubricant holding layer is interposed between the slide layer and the base layer to hold a lubricant inside the lubricant holding layer and supply the lubricant to the slide layer. The slide layer has a porous structure including a plurality of through holes (e.g., through holes  30   h ) in a thickness direction (e.g., thickness direction TD) of the slide layer. 
     In short, the lubricant is supplied from the lubricant holding layer to a slide face of the nip formation pad via the through holes of the slide layer. The slide layer is an abrasion-resistant slide layer provided on a slide face side of the nip formation pad. The abrasion-resistant slide layer suppresses crushing of the through holes on the slide face side that may be caused by friction between the nip formation pad and the fixing belt. 
     Accordingly, the nip formation pad can supply the lubricant to the slide face of the nip formation pad over which the fixing belt slides, while reducing friction between the nip formation pad and the fixing belt, over a long period of time. 
     Although the present disclosure makes reference to specific embodiments, it is to be noted that the present disclosure is not limited to the details of the embodiments described above. Thus, various modifications and enhancements are possible in light of the above teachings, without departing from the scope of the present 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 embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure. The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings. 
     For example, the image forming apparatus according to the embodiments of the present disclosure is not limited to the color image forming apparatus  1  described above. Alternatively, the image forming apparatus may be a monochrome image forming apparatus that forms a monochrome image on a recording medium. In addition, the image forming apparatus according to the embodiments of the present disclosure may be, e.g., a copier, a facsimile machine, a printer, a multifunction peripheral (MFP) having at least two of copying, printing, scanning, facsimile, and plotter functions. 
     The fixing device according to the embodiments of the present disclosure is not limited to the fixing device  7  described above, which includes the fixing belt  22  as a fixing rotator and the pressure roller  23  as a pressure rotator. Alternatively, for example, the fixing device may include a fixing roller as a fixing rotator and an endless belt as a pressure rotator that presses against the fixing roller. That is, the fixing roller is a rotator that is rotatable in a direction of rotation. The pressure rotator is an endless belt that contacts the rotator and rotates in a direction of rotation. Alternatively, a fixing film, a fixing sleeve, or the like may be used as a fixing rotator. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from that described above. 
     Further, any of the above-described devices or units can be implemented as a hardware apparatus, such as a special-purpose circuit or device, or as a hardware/software combination, such as a processor executing a software program.