Patent Publication Number: US-2020292972-A1

Title: Heating device, 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(a) to Japanese Patent Application No. 2019-045823, filed on Mar. 13, 2019, in the Japan 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 heating device, a fixing device, and an image forming apparatus, and more particularly, to a heating device incorporating a resistive heat generator, a fixing device incorporating the heating device, and an image forming apparatus incorporating the fixing device. 
     Discussion of the Background Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, and multifunction peripherals (MFP) having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data by electrophotography. 
     Such image forming apparatuses employ fixing devices of various types to fix the image on the recording medium. 
     For example, a fixing device includes a fixing belt that is thin and has a decreased thermal capacity. The fixing belt is looped over a plurality of rollers. A laminated heater constructed of a base and a resistive heat generator heats an inner circumferential surface of the fixing belt. A stay supports the laminated heater. The laminated heater directly heats the fixing belt at a fixing nip formed between the fixing belt and a pressure roller. 
     SUMMARY 
     This specification describes below an improved heating device. In one embodiment, the heating device includes an endless rotator that rotates in a rotation direction. A heater contacts an inner circumferential surface of the endless rotator and extends in an axial direction of the endless rotator. An elastic body is disposed opposite the inner circumferential surface of the endless rotator and disposed downstream from the heater in the rotation direction of the endless rotator. A holder holds the heater and the elastic body. A pressure rotator is disposed opposite the heater and the elastic body via the endless rotator to form a nip between the endless rotator and the pressure rotator. A slide sheet covers a surface of the elastic body. The slide sheet includes an upstream end in the rotation direction of the endless rotator. The upstream end is sandwiched between the heater and the elastic body and between the elastic body and the holder. The upstream end is secured to the holder. 
     This specification further describes an improved fixing device. In one embodiment, the fixing device includes an endless rotator that rotates in a rotation direction. A heater contacts an inner circumferential surface of the endless rotator and extends in an axial direction of the endless rotator. An elastic body is disposed opposite the inner circumferential surface of the endless rotator and disposed downstream from the heater in the rotation direction of the endless rotator. A holder holds the heater and the elastic body. A pressure rotator is disposed opposite the heater and the elastic body via the endless rotator to form a nip between the endless rotator and the pressure rotator, through which a recording medium bearing a developer is conveyed. A slide sheet covers a surface of the elastic body. The slide sheet includes an upstream end in the rotation direction of the endless rotator. The upstream end is sandwiched between the heater and the elastic body and between the elastic body and the holder. The upstream end is secured to the holder. 
     This specification further describes an improved image forming apparatus. In one embodiment, the image forming apparatus includes an image forming device that forms an image and the fixing device described above that fixes the image on a recording medium. 
    
    
     
       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. 1A  is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 1B  is a schematic cross-sectional view of the image forming apparatus depicted in  FIG. 1A , illustrating a principle thereof; 
         FIG. 2  is a cross-sectional view of a fixing device incorporated in the image forming apparatus depicted in  FIG. 1A ; 
         FIG. 3A  is a plan view of a heater incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 3B  is a cross-sectional view of the heater depicted in  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view of a heating device according to a first embodiment of the present disclosure, that is incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 4B  is a plan view of a slide sheet incorporated in the heating device depicted in  FIG. 4A ; 
         FIG. 4C  is a cross-sectional view of a heating device as a variation of the heating device depicted in  FIG. 4A ; 
         FIG. 5  is a cross-sectional view of a comparative fixing device; 
         FIG. 6A  is a cross-sectional view of a heating device according to a second embodiment of the present disclosure, that is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 6B  is a perspective view of the heating device depicted in  FIG. 6A ; 
         FIG. 6C  is a plan view of the slide sheet incorporated in the heating device depicted in  FIG. 6A , illustrating one end of the slide sheet; 
         FIG. 7  is a cross-sectional view of a heating device according to a third embodiment of the present disclosure, that is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 8  is a cross-sectional view of a heating device according to a fourth embodiment of the present disclosure, that is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 9  is a cross-sectional view of a heating device according to a fifth embodiment of the present disclosure, that is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 10A  is a plan view of the slide sheet depicted in  FIG. 4B  and an engagement that engages the slide sheet; and 
         FIG. 10B  is a plan view of a comparative slide sheet. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 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 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 have a similar function, operate in a similar manner, and achieve a similar result. 
     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. 
     Referring to drawings, a description is provided of a construction of a heating device, a fixing device incorporating the heating device, and an image forming apparatus (e.g., a laser printer) incorporating the fixing device according to embodiments of the present disclosure. 
     A laser printer is one example of the image forming apparatus. The image forming apparatus is not limited to the laser printer. For example, the image forming apparatus may be a copier, a facsimile machine, a printer, a printing machine, an inkjet recording apparatus, or a multifunction peripheral (MFP) having at least two of copying, facsimile, printing, scanning, and inkjet recording functions. 
     In the drawings, identical reference numerals are assigned to identical elements and equivalents and redundant descriptions of the identical elements and the equivalents are summarized or omitted properly. The dimension, material, shape, relative position, and the like of each of the elements are examples and do not limit the scope of this disclosure unless otherwise specified. 
     According to the embodiments below, a sheet is used as a recording medium. 
     However, the recording medium is not limited to paper as the sheet. In addition to paper as the sheet, the recording media include an overhead projector (OHP) transparency, cloth, a metal sheet, plastic film, and a prepreg sheet pre-impregnated with resin in carbon fiber. 
     The recording media also include a medium adhered with a developer or ink, recording paper, and a recording sheet. The sheets include, in addition to plain paper, thick paper, a postcard, an envelope, thin paper, coated paper, art paper, and tracing paper. 
     Image formation described below denotes forming an image having meaning such as characters and figures and an image not having meaning such as patterns on the medium. 
     A description is provided of a construction of a laser printer as an image forming apparatus  100 . 
       FIG. 1A  is a schematic cross-sectional view of the image forming apparatus  100  that incorporates the heating device or a fixing device  300  according to the embodiments of the present disclosure.  FIG. 1A  schematically illustrates a construction of a color laser printer as one embodiment of the image forming apparatus  100 .  FIG. 1B  is a schematic cross-sectional view of the image forming apparatus  100 , illustrating and simplifying a principle or a mechanism of the color laser printer. 
     As illustrated in  FIG. 1A , the image forming apparatus  100  includes four process units  1 K,  1 Y,  1 M, and  1 C serving as image forming devices, respectively. The process units  1 K,  1 Y,  1 M, and  1 C form black, yellow, magenta, and cyan toner images with developers in black (K), yellow (Y), magenta (M), and cyan (C), respectively, which correspond to color separation components for a color image. 
     The process units  1 K,  1 Y,  1 M, and  1 C have a common construction except that the process units  1 K,  1 Y,  1 M, and  1 C include toner bottles  6 K,  6 Y,  6 M, and  6 C containing fresh toners in different colors, respectively. Hence, the following describes a construction of a single process unit, that is, the process unit  1 K, and a description of a construction of each of other process units, that is, the process units  1 Y,  1 M, and  1 C, is omitted. 
     The process unit  1 K includes an image bearer  2 K (e.g., a photoconductive drum), a drum cleaner  3 K, and a discharger. The process unit  1 K further includes a charger  4 K and a developing device  5 K. The charger  4 K serves as a charging member or a charging device that uniformly charges a surface of the image bearer  2 K. The developing device  5 K serves as a developing member that develops an electrostatic latent image formed on the image bearer  2 K into a visible image. The process unit  1 K is detachably attached to a body of the image forming apparatus  100  to replace consumables of the process unit  1 K with new ones. 
     Similarly, the process units  1 Y,  1 M, and  1 C include image bearers  2 Y,  2 M, and  2 C, drum cleaners  3 Y,  3 M, and  3 C, chargers  4 Y,  4 M, and  4 C, and developing devices  5 Y,  5 M, and  5 C, respectively. In  FIG. 1B , the image bearers  2 K,  2 Y,  2 M, and  2 C, the drum cleaners  3 K,  3 Y,  3 M, and  3 C, the chargers  4 K,  4 Y,  4 M, and  4 C, and the developing devices  5 K,  5 Y,  5 M, and  5 C are indicated as an image bearer  2 , a drum cleaner  3 , a charger  4 , and a developing device  5 , respectively. 
     An exposure device  7  is disposed above the process units  1 K,  1 Y,  1 M, and  1 C disposed inside the image forming apparatus  100 . The exposure device  7  performs scanning and writing according to image data. For example, the exposure device  7  includes a laser diode that emits a laser beam Lb according to the image data and a mirror  7   a  that reflects the laser beam Lb to the image bearer  2 K so that the laser beam Lb irradiates the image bearer  2 K. 
     According to this embodiment, a transfer device  15  is disposed below the process units  1 K,  1 Y,  1 M, and  1 C. The transfer device  15  is equivalent to a transferor TM depicted in  FIG. 1B . Primary transfer rollers  19 K,  19 Y,  19 M, and  19 C are disposed opposite the image bearers  2 K,  2 Y,  2 M, and  2 C, respectively, and in contact with an intermediate transfer belt  16 . 
     The intermediate transfer belt  16  rotates in a state in which the intermediate transfer belt  16  is looped over the primary transfer rollers  19 K,  19 Y,  19 M, and  19 C, a driving roller  18 , and a driven roller  17 . A secondary transfer roller  20  is disposed opposite the driving roller  18  and in contact with the intermediate transfer belt  16 . The image bearers  2 K,  2 Y,  2 M, and  2 C serve as primary image bearers that bear black, yellow, magenta, and cyan toner images, respectively. The intermediate transfer belt  16  serves as a secondary image bearer that bears a composite toner image (e.g., a color toner image) formed with the black, yellow, magenta, and cyan toner images. 
     A belt cleaner  21  is disposed downstream from the secondary transfer roller  20  in a rotation direction of the intermediate transfer belt  16 . A cleaning backup roller is disposed opposite the belt cleaner  21  via the intermediate transfer belt  16 . 
     A sheet feeder  200  including a tray  50  depicted in  FIG. 1B  that loads sheets P is disposed in a lower portion of the image forming apparatus  100 . The sheet feeder  200  serves as a recording medium supply that contains a plurality of sheets P in a substantial number, that is, a sheaf of sheets P, serving as recording media. The sheet feeder  200  is combined with a sheet feeding roller  60  and a roller pair  210  into a unit. The sheet feeding roller  60  and the roller pair  210  serve as separation-conveyance members that separate an uppermost sheet P from other sheets P and convey the uppermost sheet P. 
     The sheet feeder  200  is inserted into and removed from the body of the image forming apparatus  100  for replenishment and the like of the sheets P. The sheet feeding roller  60  and the roller pair  210  are disposed above the sheet feeder  200  and convey the uppermost sheet P of the sheaf of sheets P placed in the sheet feeder  200  toward a sheet feeding path  32 . 
     A registration roller pair  250  serving as a conveyor is disposed immediately upstream from the secondary transfer roller  20  in a sheet conveyance direction. The registration roller pair  250  temporarily halts the sheet P sent from the sheet feeder  200 . As the registration roller pair  250  temporarily halts the sheet P, the registration roller pair  250  slacks a leading end of the sheet P, correcting skew of the sheet P. 
     A registration sensor  31  is disposed immediately upstream from the registration roller pair  250  in the sheet conveyance direction. The registration sensor  31  detects passage of the leading end of the sheet P. When a predetermined time period elapses after the registration sensor  31  detects passage of the leading end of the sheet P, the sheet P strikes the registration roller pair  250  and halts temporarily. 
     Downstream from the sheet feeder  200  in the sheet conveyance direction is a conveying roller  240  that conveys the sheet P conveyed rightward from the roller pair  210  upward. As illustrated in  FIG. 1A , the conveying roller  240  conveys the sheet P upward toward the registration roller pair  250 . 
     The roller pair  210  is constructed of a pair of rollers, that is, an upper roller and a lower roller. The roller pair  210  employs a friction reverse roller (FRR) separation system or a friction roller (FR) separation system. According to the FRR separation system, a separating roller (e.g., a reverse roller) is applied with a torque in a predetermined amount in an anti-feeding direction by a driving shaft through a torque limiter. The separating roller is pressed against a feeding roller to form a nip therebetween where the uppermost sheet P is separated from other sheets P. According to the FR separation system, a separating roller (e.g., a friction roller) is supported by a securing shaft via a torque limiter. The separating roller is pressed against a feeding roller to form a nip therebetween where the uppermost sheet P is separated from other sheets P. 
     According to this embodiment, the roller pair  210  employs the FRR separation system. For example, the roller pair  210  includes a feeding roller  220  and a separating roller  230 . The feeding roller  220  is an upper roller that conveys the sheet P to an inside of a machine. The separating roller  230  is a lower roller that is applied with a driving force in a direction opposite a rotation direction of the feeding roller  220  by a driving shaft through a torque limiter. 
     A biasing member such as a spring biases the separating roller  230  against the feeding roller  220 . The driving force applied to the feeding roller  220  is transmitted to the sheet feeding roller  60  through a clutch, thus rotating the sheet feeding roller  60  counterclockwise in  FIG. 1A . 
     After the leading end of the sheet P strikes the registration roller pair  250  and slacks, the registration roller pair  250  conveys the sheet P to a secondary transfer nip (e.g., a transfer nip N depicted in  FIG. 1B ) formed between the secondary transfer roller  20  and the intermediate transfer belt  16  pressed by the driving roller  18  at a proper time when the secondary transfer roller  20  transfers a color toner image formed on the intermediate transfer belt  16  onto the sheet P. A bias applied at the secondary transfer nip electrostatically transfers the color toner image formed on the intermediate transfer belt  16  onto a desired transfer position on the sheet P sent to the secondary transfer nip precisely. 
     A post-transfer conveyance path  33  is disposed above the secondary transfer nip formed between the secondary transfer roller  20  and the intermediate transfer belt  16  pressed by the driving roller  18 . The fixing device  300  is disposed in proximity to an upper end of the post-transfer conveyance path  33 . The fixing device  300  includes a fixing belt  310  and a pressure roller  320 . The fixing belt  310  is tubular and serves as an endless rotator or a rotator that accommodates a heater. The pressure roller  320  serves as a pressure rotator or a pressure member that rotates while the pressure roller  320  contacts an outer circumferential surface of the fixing belt  310  with predetermined pressure. 
     As illustrated in  FIG. 1A , a post-fixing conveyance path  35  is disposed above the fixing device  300 . At an upper end of the post-fixing conveyance path  35 , the post-fixing conveyance path  35  branches to a sheet ejection path  36  and a reverse conveyance path  41 . A switcher  42  is disposed at a bifurcation of the post-fixing conveyance path  35 . The switcher  42  pivots about a pivot shaft  42   a  as an axis. A sheet ejection roller pair  37  is disposed in proximity to an outlet edge of the sheet ejection path  36 . 
     One end of the reverse conveyance path  41  is at the bifurcation of the post-fixing conveyance path  35 . Another end of the reverse conveyance path  41  joins the sheet feeding path  32 . A reverse conveyance roller pair  43  is disposed in a middle of the reverse conveyance path  41 . A sheet ejection tray  44  is disposed in an upper portion of the image forming apparatus  100 . The sheet ejection tray  44  includes a recess directed inward in the image forming apparatus  100 . 
     A powder container  10  (e.g., a toner container) is interposed between the transfer device  15  and the sheet feeder  200 . The powder container  10  is detachably attached to the body of the image forming apparatus  100 . 
     The image forming apparatus  100  according to this embodiment secures a predetermined distance from the sheet feeding roller  60  to the secondary transfer roller  20  to convey the sheet P. Hence, the powder container  10  is situated in a dead space defined by the predetermined distance, downsizing the image forming apparatus  100  entirely. 
     A transfer cover  8  is disposed above the sheet feeder  200  at a front of the image forming apparatus  100  in a drawing direction of the sheet feeder  200 . As an operator (e.g., a user and a service engineer) opens the transfer cover  8 , the operator inspects an inside of the image forming apparatus  100 . The transfer cover  8  mounts a bypass tray  46  and a bypass sheet feeding roller  45  used for a sheet P manually placed on the bypass tray  46  by the operator. 
     A description is provided of operations of the image forming apparatus  100 , that is, the laser printer. 
     Referring to  FIG. 1A , the following describes basic operations of the image forming apparatus  100  according to the embodiments, which has the construction described above to perform image formation. 
     First, a description is provided of operations of the image forming apparatus  100  to print on one side of a sheet P. 
     As illustrated in  FIG. 1A , the sheet feeding roller  60  rotates according to a sheet feeding signal sent from a controller of the image forming apparatus  100 . The sheet feeding roller  60  separates an uppermost sheet P from other sheets P of a sheaf of sheets P loaded in the sheet feeder  200  and feeds the uppermost sheet P to the sheet feeding path  32 . 
     When the leading end of the sheet P sent by the sheet feeding roller  60  and the roller pair  210  reaches a nip of the registration roller pair  250 , the registration roller pair  250  slacks and halts the sheet P temporarily. The registration roller pair  250  conveys the sheet P to the secondary transfer nip at an optimal time in synchronism with a time when the secondary transfer roller  20  transfers a color toner image formed on the intermediate transfer belt  16  onto the sheet P while the registration roller pair  250  corrects skew of the leading end of the sheet P. 
     In order to feed a sheaf of sheets P placed on the bypass tray  46 , the bypass sheet feeding roller  45  conveys the sheaf of sheets P loaded on the bypass tray  46  one by one from an uppermost sheet P. The sheet P is conveyed through a part of the reverse conveyance path  41  to the nip of the registration roller pair  250 . Thereafter, the sheet P is conveyed similarly to the sheet P conveyed from the sheet feeder  200 . 
     The following describes processes for image formation with one process unit, that is, the process unit  1 K, and a description of processes for image formation with other process units, that is, the process units  1 Y,  1 M, and  1 C, is omitted. 
     First, the charger  4 K uniformly charges the surface of the image bearer  2 K at a high electric potential. The exposure device  7  emits a laser beam Lb that irradiates the surface of the image bearer  2 K according to image data. 
     The electric potential of an irradiated portion on the surface of the image bearer  2 K, which is irradiated with the laser beam Lb, decreases, forming an electrostatic latent image on the image bearer  2 K. The developing device  5 K includes a developer bearer  5   a  depicted in  FIG. 1B  that bears a developer containing toner. Fresh black toner supplied from the toner bottle  6 K is transferred onto a portion on the surface of the image bearer  2 K, which bears the electrostatic latent image, through the developer bearer  5   a.    
     The surface of the image bearer  2 K transferred with the black toner bears a black toner image developed with the black toner. The primary transfer roller  19 K transfers the black toner image formed on the image bearer  2 K onto the intermediate transfer belt  16 . 
     A cleaning blade  3   a  depicted in  FIG. 1B  of the drum cleaner  3 K removes residual toner failed to be transferred onto the intermediate transfer belt  16  and therefore adhered on the surface of the image bearer  2 K therefrom. The removed residual toner is conveyed by a waste toner conveyor and collected into a waste toner container disposed inside the process unit  1 K. The discharger removes residual electric charge from the image bearer  2 K from which the drum cleaner  3 K has removed the residual toner. 
     Similarly, in the process units  1 Y,  1 M, and  1 C, yellow, magenta, and cyan toner images are formed on the image bearers  2 Y,  2 M, and  2 C, respectively. The primary transfer rollers  19 Y,  19 M, and  19 C transfer the yellow, magenta, and cyan toner images formed on the image bearers  2 Y,  2 M, and  2 C, respectively, onto the intermediate transfer belt  16  such that the yellow, magenta, and cyan toner images are superimposed on the intermediate transfer belt  16 . 
     The black, yellow, magenta, and cyan toner images transferred and superimposed on the intermediate transfer belt  16  travel to the secondary transfer nip formed between the secondary transfer roller  20  and the intermediate transfer belt  16  pressed by the driving roller  18 . On the other hand, the registration roller pair  250  resumes rotation at a predetermined time while sandwiching a sheet P that strikes the registration roller pair  250 . The registration roller pair  250  conveys the sheet P to the secondary transfer nip formed between the secondary transfer roller  20  and the intermediate transfer belt  16  at a time when the secondary transfer roller  20  transfers the black, yellow, magenta, and cyan toner images superimposed on the intermediate transfer belt  16  properly. Thus, the secondary transfer roller  20  transfers the black, yellow, magenta, and cyan toner images superimposed on the intermediate transfer belt  16  onto the sheet P conveyed by the registration roller pair  250 , forming a color toner image on the sheet P. 
     The sheet P transferred with the color toner image is conveyed to the fixing device  300  through the post-transfer conveyance path  33 . The fixing belt  310  and the pressure roller  320  sandwich the sheet P conveyed to the fixing device  300  and fix the unfixed color toner image on the sheet P under heat and pressure. The sheet P bearing the fixed color toner image is conveyed from the fixing device  300  to the post-fixing conveyance path  35 . 
     When the sheet P is sent out of the fixing device  300 , the switcher  42  opens the upper end of the post-fixing conveyance path  35  and a vicinity thereof as illustrated with a solid line in  FIG. 1A . The sheet P sent out of the fixing device  300  is conveyed to the sheet ejection path  36  through the post-fixing conveyance path  35 . The sheet ejection roller pair  37  sandwiches the sheet P sent to the sheet ejection path  36  and is driven and rotated to eject the sheet P onto the sheet ejection tray  44 , thus finishing printing on one side of the sheet P. 
     Next, a description is provided of operations of the image forming apparatus  100  to perform duplex printing. 
     Similarly to printing on one side of the sheet P, the fixing device  300  sends out the sheet P to the sheet ejection path  36 . In order to perform duplex printing, the sheet ejection roller pair  37  is driven and rotated to convey a part of the sheet P to an outside of the image forming apparatus  100 . 
     When a trailing end of the sheet P has passed through the sheet ejection path  36 , the switcher  42  pivots about the pivot shaft  42   a  as illustrated with a dotted line in  FIG. 1A , closing the upper end of the post-fixing conveyance path  35 . Approximately simultaneously with closing of the upper end of the post-fixing conveyance path  35 , the sheet ejection roller pair  37  rotates in a direction opposite a direction in which the sheet ejection roller pair  37  conveys the sheet P onto the outside of the image forming apparatus  100 , thus conveying the sheet P to the reverse conveyance path  41 . 
     The sheet P conveyed to the reverse conveyance path  41  travels to the registration roller pair  250  through the reverse conveyance roller pair  43 . The registration roller pair  250  conveys the sheet P to the secondary transfer nip at a proper time when the secondary transfer roller  20  transfers black, yellow, magenta, and cyan toner images superimposed on the intermediate transfer belt  16  onto a back side of the sheet P, which is transferred with no toner image, that is, in synchronism with reaching of the black, yellow, magenta, and cyan toner images to the secondary transfer nip. 
     While the sheet P passes through the secondary transfer nip, the secondary transfer roller  20  and the driving roller  18  transfer the black, yellow, magenta, and cyan toner images onto the back side of the sheet P, which is transferred with no toner image, thus forming a color toner image on the sheet P. The sheet P transferred with the color toner image is conveyed to the fixing device  300  through the post-transfer conveyance path  33 . 
     In the fixing device  300 , the fixing belt  310  and the pressure roller  320  sandwich the sheet P conveyed to the fixing device  300  and fix the unfixed color toner image on the back side of the sheet P under heat and pressure. The sheet P bearing the color toner image fixed on both sides, that is, a front side and the back side, of the sheet P, is conveyed from the fixing device  300  to the post-fixing conveyance path  35 . 
     When the sheet P is sent out of the fixing device  300 , the switcher  42  opens the upper end of the post-fixing conveyance path  35  and the vicinity thereof as illustrated with the solid line in  FIG. 1A . The sheet P sent out of the fixing device  300  is conveyed to the sheet ejection path  36  through the post-fixing conveyance path  35 . The sheet ejection roller pair  37  sandwiches the sheet P sent to the sheet ejection path  36  and is driven and rotated to eject the sheet P onto the sheet ejection tray  44 , thus finishing duplex printing on the sheet P. 
     After the secondary transfer roller  20  transfers the black, yellow, magenta, and cyan toner images superimposed on the intermediate transfer belt  16  onto the sheet P, residual toner adheres to the intermediate transfer belt  16 . The belt cleaner  21  removes the residual toner from the intermediate transfer belt  16 . The residual toner removed from the intermediate transfer belt  16  is conveyed by the waste toner conveyor and collected into the powder container  10 . 
     Referring to  FIGS. 2, 3A, 3B, 4A, 4B, 4C, 6A, 6B, 6C, 7, 8, 9, 10A, and 10B , a description is provided of a construction of each of heating devices  90 ,  90 S,  90 T,  90 U,  90 V, and  90 W and the fixing device  300  according to first to sixth embodiments of the present disclosure, respectively. 
     Each of the heating devices  90 ,  90 S,  90 T,  90 U,  90 V, and  90 W is installable in the fixing device  300 . As illustrated in  FIG. 2 , the fixing device  300  includes a heater  350  that heats the fixing belt  310 . 
     A description is provided of the construction of the fixing device  300  including the heating device  90  according to the first embodiment of the present disclosure. 
     As illustrated in  FIG. 2 , the fixing device  300  includes the fixing belt  310  that is thin and tubular and has a decreased thermal capacity and the pressure roller  320 . As a sheet P that bears a toner image and is conveyed in a sheet conveyance direction DP passes through a fixing nip SN formed between the fixing belt  310  and the pressure roller  320 , the fixing belt  310  and the pressure roller  320  sandwich the sheet P and fix the toner image on the sheet P under heat. While the fixing belt  310  rotates in a rotation direction D 310  and slides over an insulating layer  370  covering heat generators  360  as illustrated in  FIG. 3A , the heat generators  360  heat the fixing belt  310 . 
     A detailed description is now given of a construction of the fixing belt  310 . 
     The fixing belt  310  includes a tubular base that is made of polyimide (PI) and has an outer diameter of 25 mm and a thickness in a range of from 50 micrometers to 70 micrometers, for example. The fixing belt  310  further includes a release layer serving as an outermost surface layer. The release layer is made of fluororesin, such as tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and polytetrafluoroethylene (PTFE), and has a thickness in a range of from 5 micrometers to 20 micrometers to enhance durability of the fixing belt  310  and facilitate separation of the sheet P and a foreign substance from the fixing belt  310 . Optionally, an elastic layer that is made of rubber or the like and has a thickness in a range of from 100 micrometers to 300 micrometers may be interposed between the base and the release layer. 
     The base of the fixing belt  310  may be made of heat resistant resin such as polyetheretherketone (PEEK) or metal such as nickel (Ni) and SUS stainless steel, instead of polyimide. An inner circumferential surface of the fixing belt  310  may be coated with polyimide, PTFE, or the like to produce a slide layer. The tubular base made of SUS stainless steel achieves sufficient strength even with a thickness in a range of from 20 micrometers to 40 micrometers. 
     A detailed description is now given of a construction of the pressure roller  320 . 
     The pressure roller  320  has an outer diameter of 25 mm, for example. The pressure roller  320  includes a cored bar  321 , an elastic layer  322 , and a release layer  323 . The cored bar  321  is solid and made of metal such as iron. The elastic layer  322  coats the cored bar  321 . The release layer  323  coats an outer surface of the elastic layer  322 . The elastic layer  322  is made of silicone rubber and has a thickness of 3.5 mm, for example. 
     In order to facilitate separation of the sheet P and the foreign substance from the pressure roller  320 , the release layer  323  that is made of fluororesin and has a thickness of about 40 micrometers, for example, is preferably disposed on the outer surface of the elastic layer  322 . A biasing member presses the pressure roller  320  against the fixing belt  310 . 
     A stay  330  serving as a support and a holder  340  serving as a holder are disposed inside a loop formed by the fixing belt  310  and extended in an axial direction of the fixing belt  310 . The stay  330  includes a channel made of metal. Both lateral ends of the stay  330  in a longitudinal direction thereof are supported by side plates of the heating device  90 , respectively. The stay  330  receives pressure from the pressure roller  320  precisely to form the fixing nip SN stably. 
     The holder  340  includes an upstream guide  341  disposed at an entry to the fixing nip SN and a downstream guide  342  disposed at an exit of the fixing nip SN. Each of the upstream guide  341  and the downstream guide  342  is an arc in cross section and contacts the inner circumferential surface of the fixing belt  310 . The upstream guide  341  and the downstream guide  342  guide the fixing belt  310  in a tangential direction tangential to the fixing nip SN. The holder  340  includes a recess  345  (e.g., a groove) that accommodates and holds a base  351  of the heater  350  depicted in  FIG. 3A . The stay  330  supports a rear face of the holder  340 . The holder  340  is preferably made of heat resistant resin having a decreased thermal conductivity, such as liquid crystal polymer (LCP). Accordingly, the holder  340  reduces conduction of heat thereto, improving heating of the fixing belt  310 . 
     In order to prevent contact with a high temperature portion of the heater  350 , the holder  340  has a shape that supports the heater  350  at two positions in proximity to both ends of the heater  350 , respectively, in a short direction thereof. Accordingly, the holder  340  reduces conduction of heat thereto further, improving heating of the fixing belt  310 . 
     The heater  350  (e.g., a laminated heater) includes the heat generators  360  (e.g., resistive heat generators). As illustrated in  FIGS. 3A and 3B , the heat generators  360  are mounted on the base  351 . The base  351  includes an elongate, thin metal plate and an insulator that coats the metal plate. 
     The base  351  is preferably made of aluminum, stainless steel, or the like that is available at reduced costs. Alternatively, instead of metal, the base  351  may be made of ceramic such as alumina and aluminum nitride or a nonmetallic material that has an increased heat resistance and an increased insulation such as glass and mica. 
     In order to improve evenness of heat generated by the heater  350  so as to enhance quality of an image formed on a sheet P, the base  351  may be made of a material that has an increased thermal conductivity such as copper, graphite, and graphene. According to this embodiment, the base  351  is made of alumina and has a short width of 8 mm, a longitudinal length of 270 mm, and a thickness of 1.0 mm. 
     As illustrated in  FIG. 3A , specifically, the heat generators  360  mounted on the base  351  are extended linearly in a longitudinal direction of the base  351  and are arranged in series and in two lines in parallel to each other. One lateral end of one of the heat generators  360  arranged in two lines is connected to an electrode  360   c  through a feeder  369   c.  One lateral end of another one of the heat generators  360  is connected to an electrode  360   d  through a feeder  369   a.  The feeders  369   a  and  369   c,  having a decreased resistance value, are disposed on one lateral end of the base  351  and extended in the longitudinal direction of the base  351 . The electrodes  360   c  and  360   d  supply power to the heat generators  360 , respectively. The electrodes  360   c  and  360   d  are coupled to a power supply including an alternating current power supply. 
     Another lateral end of one of the heat generators  360  is connected to another lateral end of another one of the heat generators  360  through a feeder  369   b  such that one of the heat generators  360 , that extends in the longitudinal direction of the base  351  and in a direction directed to the feeder  369   b,  is turned at the feeder  369   b  and another one of the heat generators  360  extends in the longitudinal direction of the base  351  and in an opposite direction. The feeder  369   b,  having a decreased resistance value, is disposed on another lateral end of the base  351  in the longitudinal direction thereof and extended in the short direction of the base  351 . Each of the heat generators  360 , the electrodes  360   c  and  360   d,  and the feeders  369   a,    369   b,  and  369   c  is produced by screen printing to have a predetermined line width and a predetermined thickness. 
     The heat generators  360  are produced as below. Silver (Ag) or silver-palladium (AgPd) and glass powder and the like are mixed into paste. The paste coats the base  351  by screen printing or the like. Thereafter, the base  351  is subject to firing. For example, each of the heat generators  360  has a resistance value of 10Ω at an ambient temperature. Alternatively, the heat generators  360  may be made of a resistive material such as a silver alloy (AgPt) and ruthenium oxide (RuO 2 ). 
     A thin overcoat layer or the insulating layer  370  covers a surface of each of the heat generators  360  and the feeders  369   a,    369   b,  and  369   c.  The insulating layer  370  attains insulation between the fixing belt  310  and the heat generators  360  and between the fixing belt  310  and the feeders  369   a,    369   b,  and  369   c  while facilitating sliding of the fixing belt  310  over the insulating layer  370 . 
     For example, the insulating layer  370  is made of heat resistant glass and has a thickness of 75 micrometers. The heat generators  360  heat the fixing belt  310  that contacts the insulating layer  370  by conduction of heat, increasing the temperature of the fixing belt  310  so that the fixing belt  310  heats and fixes the unfixed toner image on the sheet P conveyed through the fixing nip SN. 
     The inner circumferential surface of the fixing belt  310  is applied with a lubricant that facilitates sliding of the fixing belt  310  over the heater  350 . The lubricant is silicone oil having heat resistance and a predetermined kinetic viscosity. For example, the lubricant is preferably amino-modified silicone oil having an enhanced wettability or methylphenyl silicone oil having an enhanced heat resistance. In order to improve heat resistance, an antioxidant in a slight amount may be added to the silicone oil. 
     For example, the lubricant may be grease, dimethyl silicone oil, organometallic salt-added dimethyl silicone oil, hindered amine-added dimethyl silicone oil, dimethyl silicone oil added with organometallic salt and hindered amine, methylphenyl silicone oil, organometallic salt-added amino-modified silicone oil, hindered amine-added amino-modified silicone oil, perfluoro polyether oil, or the like. 
     As illustrated in  FIG. 2 , an elastic body  375  is disposed downstream from and abutted on the heater  350  (e.g., the laminated heater) that incorporates the heat generators  360  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310 . The elastic body  375  is rectangular in cross section in a short direction thereof and extended in a longitudinal direction of the heater  350 . 
     The elastic body  375  is made of silicone rubber having an Asker C hardness in a range of from 40 degrees to 50 degrees and a predetermined thickness in a range of from 2 mm to 3 mm, for example. In order to improve fitting to a height of the toner image on the sheet P, the elastic body  375  may be made of silicone sponge, heat resistant nonwoven fabric, felt, or the like that has the Asker C hardness in a range of from 20 degrees to 40 degrees. 
       FIG. 4A  is a cross-sectional view of the heating device  90 .  FIG. 4B  is a plan view of a slide sheet  380  incorporated in the heating device  90  depicted in  FIG. 4A .  FIG. 4C  is a cross-sectional view of a heating device as a variation of the heating device  90  depicted in  FIG. 4A . 
     As illustrated in  FIG. 4A , the recess  345  of the holder  340  accommodates the elastic body  375  in a downstream portion of the heater  350  in the rotation direction D 310  of the fixing belt  310 , thus positioning the elastic body  375 . The slide sheet  380  covers a surface of the elastic body  375 . As illustrated in  FIG. 4A , an upstream end  380   u  of the slide sheet  380  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310  is sandwiched between the elastic body  375  and the heater  350  and turned downstream in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310  along a bottom face of the elastic body  375 . For example, the upstream end  380   u  of the slide sheet  380  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310  is sandwiched between the elastic body  375  and the heater  350  and between the elastic body  375  and the holder  340 . 
     The slide sheet  380  is a non-porous sheet made of heat resistant resin. The non-porous sheet has no holes impregnated with a lubricant. The heat resistant resin has sufficient heat resistance against a fixing temperature at which a toner image is fixed on a sheet P. For example, the heat resistant resin includes thermosetting polyimide, thermoplastic polyimide, polyamide, polyamide imide, silicone resin, and fluororesin. 
     On the other hand, a downstream end  380   d  of the slide sheet  380  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310  is disposed opposite a downstream end of the holder  340  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310  in a state in which the downstream end  380   d  of the slide sheet  380  is sandwiched between the fixing belt  310  and the elastic body  375 . The downstream end  380   d  of the slide sheet  380  is not a fixed end but a free end, facilitating installation of the slide sheet  380 . The area of the slide sheet  380  is minimized, reducing manufacturing costs. 
     While the fixing belt  310  rotates in the rotation direction D 310  in a state in which the fixing belt  310  slides over the heater  350  frictionally, the fixing belt  310  exerts a downstream force in the rotation direction D 310  to the heat generators  360  of the heater  350  and the slide sheet  380 . 
     Referring to  FIG. 5 , a description is provided of a construction of a fixing device  300 C as a comparative example. 
     The fixing device  300 C includes a thin, fixing belt  310 C having a decreased thermal capacity. The fixing belt  310 C is looped over rollers  301 C and  302 C. A laminated heater  350 C constructed of a base  351 C and a resistive heat generator  360 C heats an inner circumferential surface of the fixing belt  310 C. A stay  500 C supports the laminated heater  350 C. The laminated heater  350 C directly heats the fixing belt  310 C at a fixing nip formed between the fixing belt  310 C and a pressure roller  320 C. 
     With the fixing device  300 C employing the laminated heater  350 C, since the laminated heater  350 C is platy, the laminated heater  350 C may not exert sufficient pressure to toner melted and softened at a downstream half part of the fixing nip in a rotation direction D 310 C of the fixing belt  310 C. When the fixing device  300 C fixes a color toner image on a sheet P, for example, insufficient pressure may cause faulty mixing of colors (e.g., black, yellow, magenta, and cyan) and faulty fixing. 
     To address this circumstance, an elastic body  375 C disposed opposite the downstream half part of the fixing nip retains sufficient pressure. A slide sheet  3   80 C covers a surface of the elastic body  375 C. The slide sheet  380 C reduces friction between the elastic body  375 C and the fixing belt  310 C while the fixing belt  310 C slides over the elastic body  375 C. Alternatively, a low friction agent may coat the surface of the elastic body  375 C to reduce friction between the elastic body  375 C and the fixing belt  310 C while the fixing belt  310 C slides over the elastic body  375 C. However, the slide sheet  380 C is often used to improve durability and reliability. However, if the slide sheet  380 C covers the elastic body  375 C, the slide sheet  380 C may be secured to the elastic body  375 C disadvantageously. 
     For example, an upstream end of the slide sheet  380 C in the rotation direction D 310 C of the fixing belt  310 C is sandwiched between the laminated heater  350 C and the elastic body  375 C. However, the upstream end of the slide sheet  380 C may not be secured to the elastic body  375 C precisely. Since a gap is produced between the laminated heater  350 C and the elastic body  375 C, the gap may vary pressure exerted to the sheet P at the fixing nip, causing faulty fixing and creasing the sheet P. 
     If the laminated heater  350 C shifts downstream in the rotation direction D 310 C of the fixing belt  310 C due to friction between the laminated heater  350 C and the fixing belt  310 C, the laminated heater  350 C may deform an upstream end of the elastic body  375 C in the rotation direction D 310 C of the fixing belt  310 C partially. The deformed upstream end of the elastic body  375 C may lift and crease the slide sheet  380 C partially. Accordingly, the slide sheet  380 C may increase friction between the elastic body  375 C and the fixing belt  310 C and driving torque of the fixing belt  310 C substantially. 
     If the slide sheet  380 C is secured to the elastic body  375 C insufficiently in the fixing device  300 C illustrated in  FIG. 5 , the slide sheet  380 C may shift downstream in the rotation direction D 310 C of the fixing belt  310 C. Accordingly, the upstream end of the elastic body  375 C in the rotation direction D 310 C of the fixing belt  310 C may be bulged and deformed. A part of the slide sheet  380 C may be lifted and deformed into creases. Consequently, the deformed upstream end of the elastic body  375 C and the deformed part of the slide sheet  380 C may exert pressure greater than predetermined pressure to a part of the fixing nip formed between the fixing belt  310 C and the pressure roller  320 C, thus increasing friction between the inner circumferential surface of the fixing belt  310 C and the elastic body  375 C and driving torque of the fixing belt  310 C which may result in faulty fixing. 
     To address this circumstance, according to the first embodiment of the present disclosure, as illustrated in  FIGS. 2 and 4A , an engagement  344  secures the upstream end  380   u  of the slide sheet  380  to the elastic body  375  precisely. Conversely, in the fixing device  300 C depicted in  FIG. 5 , the upstream end of the slide sheet  380 C in the rotation direction D 310 C of the fixing belt  310 C may not be secured to the elastic body  375 C precisely with pressure exerted by the heater  350 C and the elastic body  375 C that sandwich the slide sheet  380 C. As illustrated in  FIG. 4A , a notch  375   b  is disposed on a bottom of the elastic body  375  and is retracted from the engagement  344 . For example, the notch  375   b  accommodates the engagement  344 . Alternatively, if the elastic body  375  has a low rubber hardness degree that is smaller than 50 Hs, the engagement  344  may be pressed into the elastic body  375  without the notch  375   b.    
     The engagement  344  is molded with the holder  340  and mounted on a bottom face of a downstream portion of the recess  345  of the holder  340 . In order to prevent the slide sheet  380  from dropping from the holder  340 , the engagement  344  has an L-shape in cross section or is bent downstream in the rotation direction D 310  of the fixing belt  310  to have a hook shape. On the other hand, the upstream end  380   u  of the slide sheet  380  in the rotation direction D 310  of the fixing belt  310  is sandwiched between the heater  350  and the elastic body  375  and turned downstream in the rotation direction D 310  of the fixing belt  310  along the bottom face of the elastic body  375 . A turned end of the slide sheet  380  engages the engagement  344 . 
     As illustrated in  FIG. 4B , a plurality of rectangular holes  382  is disposed or arranged in the upstream end  380   u  of the slide sheet  380  in the rotation direction D 310  of the fixing belt  310  with an identical gap between adjacent ones of the rectangular holes  382  in the longitudinal direction of the heater  350 . The engagement  344  of the holder  340  is inserted into and engaged with the rectangular hole  382 . As illustrated in  FIG. 4A , the upstream end  380   u  of the slide sheet  380  is secured to the holder  340 , preventing the upstream end  380   u  of the slide sheet  380  from being lifted partially and deformed into creases precisely. Accordingly, while the fixing belt  310  rotates in the rotation direction D 310 , the fixing belt  310  slides from the heater  350  to the elastic body  375  smoothly. 
     Additionally, the upstream end  380   u  of the slide sheet  380  is sandwiched between the heater  350  and the elastic body  375  with no gap between the heater  350  and the slide sheet  380  and between the elastic body  375  and the slide sheet  380 , preventing a decreased load applied to the fixing nip SN, that might be caused by the gap between the heater  350  and the elastic body  375 . If the decreased load is applied to a part of the fixing nip SN, the fixing belt  310  is subject to increase in abrasion of the inner circumferential surface of the fixing belt  310  and increase in driving torque. 
     A method for securing the slide sheet  380  to the holder  340  is not limited to a method using the engagement  344 . Various methods or members may be used to secure the slide sheet  380  to the holder  340 . For example, a screw is screwed in the upstream end  380   u  of the slide sheet  380  and the holder  340  or the upstream end  380   u  of the slide sheet  380  is secured to the holder  340  with an adhesive. 
     A height of the elastic body  375  is preferably higher than a height of the insulating layer  370  of the heater  350  slightly. For example, a top face of the elastic body  375  protrudes toward the pressure roller  320  beyond a surface of the insulating layer  370 . Thus, the elastic body  375  exerts substantial pressure to the fixing belt  310 . 
     With the fixing device  300  employing the heater  350  as the laminated heater, the heater  350  may not exert sufficient pressure to toner melted and softened at a downstream half part of the fixing nip SN in the rotation direction D 310  of the fixing belt  310 . To address this circumstance, as described above, the elastic body  375  protrudes toward the pressure roller  320  beyond the heater  350 , preventing faulty mixing of colors (e.g., black, yellow, magenta, and cyan) and faulty fixing when the fixing device  300  fixes a color toner image on a sheet P, for example. 
     Conversely, the height of the insulating layer  370  of the heater  350  may be higher than the height of the elastic body  375 . Accordingly, the elastic body  375  does not dam the lubricant applied on the inner circumferential surface of the fixing belt  310  at a downstream end of the heater  350  in the sheet conveyance direction DP or the rotation direction D 310  of the fixing belt  310 , preventing the lubricant from moving and leaking outboard in the axial direction of the fixing belt  310 . 
     A first temperature sensor used to control the heater  350  is interposed between the heater  350  and the holder  340 . The first temperature sensor is disposed opposite a center of the heater  350  in the longitudinal direction thereof. A second temperature sensor may be provided separately from the first temperature sensor. The second temperature sensor detects the temperature of the elastic body  375  to measure an amount of heat stored in the fixing device  300 . 
     The fixing device  300  is cool when the fixing device  300  is warmed up at the opening time of an office where the image forming apparatus  100  is located, for example. The fixing device  300  is warmed sufficiently in the daytime while the fixing device  300  is used frequently. Thus, the amount of heat stored in the fixing device  300  varies depending on the time. To address this circumstance, the second temperature sensor detects the amount of heat stored in the fixing device  300  to control the temperature of the fixing belt  310  appropriately. The second temperature sensor is also preferably disposed opposite the center of the heater  350  in the longitudinal direction thereof. 
     If the second temperature sensor is installed into the fixing device  300 , as illustrated in  FIG. 4C , the holder  340  is provided with a first through hole  346  through which a second temperature sensor  390  is inserted. A second through hole  383  penetrates through the upstream end  380   u  of the slide sheet  380 . The second through hole  383  is adjacent to the rectangular hole  382 . A tip portion  390   a  of the second temperature sensor  390  is inserted into the second through hole  383  penetrating through the slide sheet  380  and brought into contact with the bottom face of the elastic body  375 . 
     A description is provided of the construction of the heating device  90 S according to the second embodiment of the present disclosure. 
       FIGS. 6A, 6B, and 6C  illustrate the heating device  90 S according to the second embodiment of the present disclosure. According to the second embodiment, engagements  344 S that secure the upstream end  380   u  of the slide sheet  380  to the holder  340  are disposed opposite the downstream guide  342  in a longitudinal direction of the slide sheet  380 . For example, a plurality of notches  343  is arranged in the downstream guide  342  with an identical gap between adjacent ones of the notches  343  in the longitudinal direction of the heater  350 . The engagements  344 S are mounted on bottoms of the notches  343 , respectively. For example, the notches  343  accommodate the engagements  344 S, respectively. 
     On the other hand, as illustrated in  FIG. 6C , a plurality of rectangular tongues  381  is disposed or arranged in the upstream end  380   u  of the slide sheet  380  with an identical gap between adjacent ones of the rectangular tongues  381  in the longitudinal direction of the slide sheet  380 . Each of the rectangular tongues  381  has a size accommodated in the notch  343 . The engagement  344 S is inserted into and engaged with a rectangular hole disposed in the rectangular tongue  381 . Other construction of the heating device  90 S is equivalent to that of the heating device  90  according to the first embodiment depicted in  FIGS. 4A, 4B, and 4C . 
       FIG. 6B  illustrates the engagement  344 S that is a prism as one example. Alternatively, like the engagement  344  depicted in  FIGS. 4A and 4C , the engagement  344 S may have an L-shape in cross section or may be bent downstream in the rotation direction D 310  of the fixing belt  310  to have a hook shape. The engagements  344 S are arranged with the downstream guide  342 , downsizing the holder  340  in a short direction thereof and facilitating downsizing of the heating device  90 S. 
     The heating device  90 S does not incorporate the notch  375   b  that is disposed on the bottom of the elastic body  375  and is retracted from the engagement  344  depicted in  FIGS. 4A and 4C . Thus, the heating device  90 S advantageously evens pressure exerted between the elastic body  375  and the fixing belt  310  at the fixing nip SN. 
     While the fixing belt  310  rotates in the rotation direction D 310  and slides over the heater  350  and the slide sheet  380  covering the elastic body  375  frictionally, the fixing belt  310  exerts a downstream force in the rotation direction D 310  to the heater  350 , the slide sheet  380 , and the elastic body  375 . An inner face of the downstream guide  342  contacts a downstream end of the elastic body  375  in the rotation direction D 310  of the fixing belt  310 . Accordingly, the downstream guide  342  suppresses shifting of the elastic body  375  and the heater  350  downstream in the rotation direction D 310  of the fixing belt  310 . 
     As illustrated in  FIG. 6B , an edge of the notch  343  disposed in the downstream guide  342  is preferably chamfered to define a C-shaped face or a round face. The notch  343  that is chamfered prevents the inner circumferential surface of the fixing belt  310  from being damaged by the edge of the notch  343  when the fixing belt  310  contacts the edge of the notch  343 . 
     A description is provided of the construction of the heating device  90 T according to the third embodiment of the present disclosure. 
       FIG. 7  illustrates the heating device  90 T according to the third embodiment of the present disclosure. The heating device  90 T includes a plurality of engagements  348  that is mounted on the inner face of the downstream guide  342  and arranged with an identical gap between adjacent ones of the engagements  348  in a longitudinal direction of the holder  340 . The rectangular holes  382  disposed in the upstream end  380   u  of the slide sheet  380  depicted in  FIG. 4B  engage the engagements  348 , respectively. According to the second embodiment depicted in  FIGS. 6A, 6B, and 6C , the notches  343  are disposed in the downstream guide  342 . Conversely, according to the third embodiment depicted in  FIG. 7 , the downstream guide  342  is continuous in the longitudinal direction of the holder  340 , improving stability of the fixing belt  310  guided by the downstream guide  342 . 
     The downstream end of the elastic body  375  in the rotation direction D 310  of the fixing belt  310  contacts the engagements  348 . However, pressure is not exerted to the downstream end of the elastic body  375  directly at the fixing nip SN. Hence, the heating device  90 T does not incorporate a notch that is disposed in the downstream end of the elastic body  375  and is retracted from the engagement  348 . The downstream end of the elastic body  375  contacts the plurality of engagements  348 , suppressing shifting of the elastic body  375  and the heater  350  downstream in the rotation direction D 310  of the fixing belt  310 , that might be caused by frictional sliding of the fixing belt  310  over the heater  350  and the slide sheet  380  covering the elastic body  375 . 
     A description is provided of the construction of the heating device  90 U according to the fourth embodiment of the present disclosure. 
       FIG. 8  illustrates the heating device  90 U according to the fourth embodiment of the present disclosure. The heating device  90 U includes a second holder  400  that holds the elastic body  375 . For example, unlike the heating devices  90 ,  90 S, and  90 T according to the first to third embodiments, respectively, that incorporate the elastic body  375  placed directly inside the recess  345  of the holder  340 , the elastic body  375  of the heating device  90 U is mounted on the second holder  400  and secured to the second holder  400  with an adhesive or the like. The second holder  400  mounting the elastic body  375  is placed inside the recess  345 . 
     The elastic body  375  mounted on the second holder  400  is placed inside the recess  345  of the holder  340 . Hence, compared to a configuration in which the elastic body  375  is solely placed inside the recess  345 , the elastic body  375  mounted on the second holder  400  is handled readily, improving installation and precision in positioning of the elastic body  375 . 
     Engaged holes  410  are disposed in a bottom face of the second holder  400 . Engagements  344 U mounted on the bottom face of the recess  345  engage the engaged holes  410 , respectively. The rectangular holes  382  disposed in the upstream end  380   u  of the slide sheet  380  engage the engagements  344 U, respectively, that engage the second holder  400 . Thereafter, the second holder  400  is placed on the bottom face of the recess  345 . Accordingly, the engagements  344 U mounted on the holder  340  engage the engaged holes  410  disposed in the second holder  400 , respectively. The upstream end  380   u  of the slide sheet  380  is sandwiched between the second holder  400  and the bottom face of the recess  345 . 
     As illustrated in  FIG. 8 , the heating device  90 U according to the fourth embodiment includes the engagements  344 U mounted on a bottom face of the holder  340 . The engagements  344 U are interposed between the holder  340  and the second holder  400 . Unlike the heating device  90  depicted in  FIG. 4A , the heating device  90 U depicted in  FIG. 8  does not incorporate the notches  375   b  disposed in the elastic body  375  and retracted from the engagements  344 U, respectively. The positions of the second holder  400  and the elastic body  375  are restricted at two positions, that is, the engagements  344 U mounted on the holder  340  and the downstream guide  342 . Accordingly, the engagements  344 U and the downstream guide  342  suppress shifting of the elastic body  375  and the heater  350  downstream in the rotation direction D 310  of the fixing belt  310 , that might be caused by frictional sliding of the fixing belt  310  over the heater  350  and the slide sheet  380  covering the elastic body  375 , more precisely. 
     If the second temperature sensor  390  that detects the temperature of the elastic body  375  is installed into the heating device  90 U, like the second temperature sensor  390  depicted in  FIG. 4C , the holder  340  is provided with the first through hole  346  through which the second temperature sensor  390  is inserted. The second through hole  383  penetrates through the upstream end  380   u  of the slide sheet  380 . The second through hole  383  is adjacent to the rectangular hole  382 . The second holder  400  is provided with a third through hole  430 . The tip portion  390   a  of the second temperature sensor  390  is inserted into the first through hole  346  penetrating through the holder  340 , the second through hole  383  penetrating through the slide sheet  380 , and the third through hole  430  penetrating through the second holder  400  and brought into contact with the bottom face of the elastic body  375 . 
     A description is provided of the construction of the heating device  90 V according to the fifth embodiment of the present disclosure. 
       FIG. 9  illustrates the heating device  90 V according to the fifth embodiment of the present disclosure. The heating device  90 V according to the fifth embodiment includes engagements that project in a direction opposite a direction in which the engagements  344 U of the heating device  90 U according to the fourth embodiment depicted in  FIG. 8  project. For example, an engaging projection  420  is mounted on the second holder  400 . An engaging recess  347  is disposed in the holder  340 . The engaging projection  420  mounted on the second holder  400  engages the engaging recess  347  through the second through hole  383  penetrating through the slide sheet  380 . Other construction of the heating device  90 V is equivalent to that of the heating device  90 U depicted in  FIG. 8 . 
     In order to install the second temperature sensor  390  into the heating device  90 V, like the second temperature sensor  390  depicted in  FIG. 8 , the holder  340  is provided with the first through hole  346  through which the second temperature sensor  390  is inserted. In addition to the rectangular hole  382 , the slide sheet  380  is provided with the second through hole  383  separately, through which the second temperature sensor  390  is inserted. Alternatively, the engaging recess  347  disposed in the holder  340  may be a through hole penetrating through to the bottom face of the holder  340 . The through hole may also be used as a through hole through which the second temperature sensor  390  is inserted. 
     A description is provided of the construction of the heating device  90 W according to the sixth embodiment of the present disclosure. 
       FIG. 10A  illustrates the heating device  90 W according to the sixth embodiment of the present disclosure. The heating device  90 W according to the sixth embodiment includes the slide sheet  380  having a fiber  380   a  oriented in a direction diagonal to a pull direction PD (e.g., the rotation direction D 310  of the fixing belt  310 ) in which the fixing belt  310  pulls the slide sheet  380  while the fixing belt  310  slides over the slide sheet  380  frictionally. A diagonal angle is not limited to 45 degrees and is an arbitrary angle of 30 degrees, 60 degrees, or the like. Since the fiber  380   a  is oriented in the direction diagonal to the pull direction PD, a force exerted to an inner peripheral edge of the rectangular hole  382  by the engagement  344  is dispersed to warp and weft of the fiber  380   a,  enhancing a strength with which the rectangular hole  382  engages the engagement  344  and durability of the slide sheet  380 . 
     Conversely, as illustrated in  FIG. 10B , a comparative slide sheet  380 C has a fiber  380   b.  Warp of the fiber  380   b  is oriented in a direction identical to the pull direction PD in which the fixing belt  310  pulls the comparative slide sheet  380 C while the fixing belt  310  slides over the comparative slide sheet  380 C frictionally. Weft of the fiber  380   b  is oriented in a direction perpendicular to the pull direction PD. Hence, if stress is concentrated on a corner of the rectangular hole  382 , cracks may generate from the corner of the rectangular hole  382  easily. 
     The above describes the embodiments of the present disclosure. However, the technology of the present disclosure is not limited to the embodiments described above and is modified within the scope of the present disclosure. For example, according to the embodiments described above, a heating device (e.g., the heating devices  90 ,  90 S,  90 T,  90 U,  90 V, and  90 W) is applied to a fixing device (e.g., the fixing device  300 ) for fixing a toner image on a sheet P, that is installed in an image forming apparatus (e.g., the image forming apparatus  100 ) for forming the toner image on the sheet P by electrophotography. However, the heating device according to the embodiments of the present disclosure is also applicable to devices other than the fixing device. For example, the heating devices  90 ,  90 S,  90 T,  90 U,  90 V, and  90 W are also applicable to a heating device that corrects curling of a recording medium used in an inkjet printer. 
     A description is provided of advantages of a heating device (e.g., the heating devices  90 ,  90 S,  90 T,  90 U,  90 V, and  90 W). 
     As illustrated in  FIGS. 2 and 4A , the heating device includes an endless rotator (e.g., the fixing belt  310 ), a heater (e.g., the heater  350 ), an elastic body (e.g., the elastic body  375 ), a holder (e.g., the holder  340 ), a pressure rotator (e.g., the pressure roller  320 ), and a slide sheet (e.g., the slide sheet  380 ). 
     The endless rotator rotates in a rotation direction (e.g., the rotation direction D 310 ). The heater contacts an inner circumferential surface of the endless rotator and extends in an axial direction (e.g., a longitudinal direction) of the endless rotator. The elastic body is disposed opposite the inner circumferential surface of the endless rotator and is disposed downstream from the heater in the rotation direction of the endless rotator. The holder holds the heater and the elastic body. The pressure rotator is disposed opposite the heater and the elastic body via the endless rotator to form a nip (e.g., the fixing nip SN) between the endless rotator and the pressure rotator, through which a recording medium (e.g., a sheet P) bearing an image is conveyed. The slide sheet is interposed between the endless rotator and the elastic body and covers a surface of the elastic body. For example, the slide sheet covers at least a surface of the elastic body, that is disposed opposite the endless rotator. The slide sheet includes an upstream end (e.g., the upstream end  380   u ) in the rotation direction of the endless rotator. The upstream end of the slide sheet is secured to the holder in a state in which the upstream end is inserted into a gap between the elastic body and the holder through a gap between the heater and the elastic body. For example, the upstream end is sandwiched between the heater and the elastic body and between the elastic body and the holder. 
     Accordingly, the upstream end of the slide sheet is secured to the holder precisely, preventing the slide sheet from being deformed into creases and preventing pressure exerted at the nip by a surface of the pressure rotator from destabilizing or fluctuating. 
     According to the embodiments described above, the fixing belt  310  serves as an endless rotator. Alternatively, a fixing film, a fixing sleeve, or the like may be used as an endless rotator. Further, the pressure roller  320  serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and features of different illustrative embodiments may be combined with each other and substituted for each other within the scope of the present disclosure. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.