# Patent

## Title
NIP formation assembly, fixing device, and image forming apparatus

## Abstract

A nip formation assembly disposed opposite an opposed rotator via an endless belt includes a nip formation pad to press against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, through which a recording medium bearing a toner image is conveyed. A fixing heater is disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed. The fixing heater heats the endless belt. A lateral end heater is mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof. The lateral end heater heats the endless belt.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2014-264738, filed on Dec. 26, 2014, and 2015-220751, filed on Nov. 10, 2015, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.
BACKGROUND
Technical Field
Exemplary aspects of the present disclosure relate to a nip formation assembly, a fixing device, and an image forming apparatus, and more particularly, to a nip formation assembly for forming a fixing nip, a fixing device for fixing a toner image on a recording medium, and an image forming apparatus incorporating the fixing device.
Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing rotator, such as a fixing roller, a fixing belt, and a fixing film, heated by a heater and an opposed rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotator and the opposed rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
SUMMARY
This specification describes below an improved nip formation assembly. In one exemplary embodiment, the nip formation assembly is disposed opposite an opposed rotator via an endless belt. The nip formation assembly includes a nip formation pad to press against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, through which a recording medium bearing a toner image is conveyed. A fixing heater is disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed. The fixing heater heats the endless belt. A lateral end heater is mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof. The lateral end heater heats the endless belt.
This specification further describes an improved fixing device. In one exemplary embodiment, the fixing device includes an endless belt rotatable in a predetermined direction of rotation and an opposed rotator disposed opposite the endless belt. A nip formation pad presses against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, through which a recording medium bearing a toner image is conveyed. A fixing heater is disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed. The fixing heater heats the endless belt. A lateral end heater is mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof. The lateral end heater heats the endless belt.
This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image bearer to bear a toner image and a fixing device disposed downstream from the image bearer in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes an endless belt rotatable in a predetermined direction of rotation and an opposed rotator disposed opposite the endless belt. A nip formation pad presses against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, through which the recording medium bearing the toner image is conveyed. A fixing heater is disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed. The fixing heater heats the endless belt. A lateral end heater is mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof. The lateral end heater heats the endless belt. An electric circuit selectively energizes the fixing heater and the lateral end heater.



BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the disclosure and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
FIG. 1 is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present disclosure;
FIG. 2 is a schematic vertical sectional view of a fixing device incorporated in the image forming apparatus shown in FIG. 1;
FIG. 3 is a schematic vertical sectional view of the fixing device shown in FIG. 2 illustrating a low-friction sheet;
FIG. 4 is a partial vertical sectional view of the fixing device shown in FIG. 2;
FIG. 5 is a partial perspective view of the fixing device shown in FIG. 2;
FIG. 6 is a plan view of halogen heaters and lateral end heaters incorporated in the fixing device shown in FIG. 2;
FIG. 7 is an exploded perspective view of a nip formation assembly incorporated in the fixing device shown in FIG. 2;
FIG. 8 is a plan view of the lateral end heater shown in FIG. 6;
FIG. 9 is a schematic vertical sectional view of the fixing device illustrating the lateral end heater shown in FIG. 8;
FIG. 10A is a sectional view of a fixing belt, a nip formation pad, and the lateral end heaters incorporated in the fixing device shown in FIG. 2 according to a first exemplary embodiment of the present disclosure;
FIG. 10B is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters as a first variation of the first exemplary embodiment shown in FIG. 10A;
FIG. 11A is a sectional view of the nip formation pad and the lateral end heaters as a second variation of the first exemplary embodiment shown in FIG. 10A before a pressure roller presses the fixing belt against the lateral end heaters;
FIG. 11B is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters as the second variation of the first exemplary embodiment shown in FIG. 10A when the pressure roller presses the fixing belt against the lateral end heaters;
FIG. 12 is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters as a third variation of the first exemplary embodiment shown in FIG. 10A;
FIG. 13A is a graph showing temperature decrease in each lateral end of a comparative halogen heater;
FIG. 13B is a graph showing the position on the halogen heaters and the lateral end heaters shown in FIG. 6 in an axial direction of the fixing belt and the heat output of the halogen heaters and the lateral end heaters according to a second exemplary embodiment of the present disclosure;
FIG. 14 is a plan view of the halogen heaters and the lateral end heaters according to the second exemplary embodiment;
FIG. 15 is a plan view of the halogen heaters and the lateral end heaters as a first variation of the second exemplary embodiment shown in FIG. 14;
FIG. 16 is a schematic vertical sectional view of a fixing device incorporating a nip formation assembly as a variation of the nip formation assembly shown in FIG. 7;
FIG. 17 is a sectional view of the fixing belt incorporated in the fixing device shown in FIGS. 2, 3, 9, and 16;
FIG. 18 is a diagram of an electric circuit showing an electric connection between the halogen heaters and the lateral end heaters shown in FIGS. 10A, 10B, 11B, 12, 14, and 15;
FIG. 19A is a diagram of the electric circuit shown in FIG. 18 illustrating a first energization pattern;
FIG. 19B is a diagram of the electric circuit shown in FIG. 18 illustrating a second energization pattern;
FIG. 19C is a diagram of the electric circuit shown in FIG. 18 illustrating a third energization pattern;
FIG. 20 is a diagram of an electric circuit as a first variation of the electric circuit shown in FIG. 18;
FIG. 21A is a diagram of the electric circuit shown in FIG. 20 illustrating a first energization pattern;
FIG. 21B is a diagram of the electric circuit shown in FIG. 20 illustrating a second energization pattern;
FIG. 21C is a diagram of the electric circuit shown in FIG. 20 illustrating a third energization pattern; and
FIG. 22 is a diagram of an electric circuit as a second variation of the electric circuit shown in FIG. 18.



DETAILED DESCRIPTION OF THE DISCLOSURE
In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to FIG. 1, an image forming apparatus 100 according to an exemplary embodiment of the present disclosure is explained.
It is to be noted that, in the drawings for explaining exemplary embodiments of this disclosure, identical reference numerals are assigned, as long as discrimination is possible, to components such as members and component parts having an identical function or shape, thus omitting description thereof once it is provided.
FIG. 1 is a schematic vertical sectional view of the image forming apparatus 100. The image forming apparatus 100 may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this exemplary embodiment, the image forming apparatus 100 is a color printer that forms color and monochrome toner images on a recording medium by electrophotography. Alternatively, the image forming apparatus 100 may be a monochrome printer that forms a monochrome toner image on a recording medium.
A description is provided of a construction and an operation of the image forming apparatus 100.
The image forming apparatus 100 is a color printer employing a tandem system in which a plurality of image forming devices for forming toner images in a plurality of colors, respectively, is aligned in a rotation direction of an intermediate transfer belt.
The image forming apparatus 100 includes four photoconductive drums 20Y, 20C, 20M, and 20K serving as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively, that is, yellow, cyan, magenta, and black.
The yellow, cyan, magenta, and black toner images formed on the photoconductive drums 20Y, 20C, 20M, and 20K as visible images, respectively, are primarily transferred successively onto an intermediate transfer belt 11 serving as an intermediate transferor disposed opposite the photoconductive drums 20Y, 20C, 20M, and 20K as the intermediate transfer belt 11 rotates in a rotation direction A1 such that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt 11 in a primary transfer process. Thereafter, the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt 11 are secondarily transferred onto a sheet S serving as a recording medium collectively in a secondary transfer process. Each of the photoconductive drums 20Y, 20C, 20M, and 20K is surrounded by image forming components that form the yellow, cyan, magenta, and black toner images on the photoconductive drums 20Y, 20C, 20M, and 20K as they rotate clockwise in FIG. 1 in a rotation direction D20.
Taking the photoconductive drum 20K that forms the black toner image, the following describes a construction of components that form the black toner image. The photoconductive drum 20K is surrounded by a charger 30K, a developing device 40K, a primary transfer roller 12K, and a cleaner 50K in this order in the rotation direction D20 of the photoconductive drum 20K. The photoconductive drums 20Y, 20C, and 20M are also surrounded by chargers 30Y, 30C, and 30M, developing devices 40Y, 40C, and 40M, primary transfer rollers 12Y, 12C, and 12M, and cleaners 50Y, 50C, and 50M in this order in the rotation direction D20 of the photoconductive drums 20Y, 20C, and 20M, respectively. The charger 30K uniformly changes an outer circumferential surface of the photoconductive drum 20K. An optical writing device 8 optically writes an electrostatic latent image on the charged outer circumferential surface of the photoconductive drum 20K according to image data sent from an external device such as a client computer. The developing device 40K visualizes the electrostatic latent image as a black toner image.
As the intermediate transfer belt 11 rotates in the rotation direction A1, the yellow, cyan, magenta, and black toner images formed on the photoconductive drums 20Y, 20C, 20M, and 20K, respectively, are primarily transferred successively onto the intermediate transfer belt 11, thus being superimposed on the same position on the intermediate transfer belt 11 and formed into a color toner image. In the primary transfer process, the primary transfer rollers 12Y, 12C, 12M, and 12K disposed opposite the photoconductive drums 20Y, 20C, 20M, and 20K via the intermediate transfer belt 11, respectively, apply a primary transfer bias to the photoconductive drums 20Y, 20C, 20M, and 20K successively from the upstream photoconductive drum 20Y to the downstream photoconductive drum 20K in the rotation direction A1 of the intermediate transfer belt 11. The photoconductive drums 20Y, 20C, 20M, and 20K are aligned in this order in the rotation direction A1 of the intermediate transfer belt 11. The photoconductive drums 20Y, 20C, 20M, and 20K are located in four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively.
The image forming apparatus 100 includes the four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively, an intermediate transfer belt unit 10, a secondary transfer roller 5, an intermediate transfer belt cleaner 13, and the optical writing device 8. The intermediate transfer belt unit 10 is situated above and disposed opposite the photoconductive drums 20Y, 20C, 20M, and 20K. The intermediate transfer belt unit 10 incorporates the intermediate transfer belt 11 and the primary transfer rollers 12Y, 12C, 12M, and 12K. The secondary transfer roller 5 serves as a secondary transferor disposed opposite the intermediate transfer belt 11 and driven and rotated in accordance with rotation of the intermediate transfer belt 11. The intermediate transfer belt cleaner 13 is disposed opposite the intermediate transfer belt 11 to clean the intermediate transfer belt 11. The optical writing device 8 is situated below and disposed opposite the four image forming stations.
The optical writing device 8 includes a semiconductor laser serving as a light source, a coupling lens, an fθ lens, a troidal lens, a deflection mirror, and a rotatable polygon mirror serving as a deflector. The optical writing device 8 emits light beams Lb corresponding to the yellow, cyan, magenta, and black toner images to be formed on the photoconductive drums 20Y, 20C, 20M, and 20K thereto, forming electrostatic latent images on the photoconductive drums 20Y, 20C, 20M, and 20K, respectively. FIG. 1 illustrates the light beam Lb irradiating the photoconductive drum 20K. Similarly, light beams irradiate the photoconductive drums 20Y, 20C, and 20M, respectively.
The image forming apparatus 100 further includes a sheet feeder 61 and a registration roller pair 4. The sheet feeder 61, disposed in a lower portion of the image forming apparatus 100, incorporates a paper tray that loads a plurality of sheets S to be conveyed to a secondary transfer nip formed between the intermediate transfer belt 11 and the secondary transfer roller 5. The registration roller pair 4 serving as a conveyor conveys the sheet S conveyed from the sheet feeder 61 to the secondary transfer nip formed between the intermediate transfer belt 11 and the secondary transfer roller 5 at a predetermined time when the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt 11 reach the secondary transfer nip. The image forming apparatus 100 further includes a sensor for detecting that a leading edge of the sheet S reaches the registration roller pair 4.
The secondary transfer roller 5 secondarily transfers the color toner image formed on the intermediate transfer belt 11 onto the sheet S as the sheet S is conveyed through the secondary transfer nip. The sheet S bearing the color toner image is conveyed to a fixing device 150 where the color toner image is fixed on the sheet S under heat and pressure. An output roller pair 7 ejects the sheet S bearing the fixed color toner image onto an output tray disposed atop the image forming apparatus 100. In an upper portion of the image forming apparatus 100 and below the output tray are toner bottles 9Y, 9C, 9M, and 9K containing fresh yellow, cyan, magenta, and black toners, respectively.
The intermediate transfer belt unit 10 includes a driving roller 72 and a driven roller 73 over which the intermediate transfer belt 11 is looped, in addition to the intermediate transfer belt 11 and the primary transfer rollers 12Y, 12C, 12M, and 12K. Since the driven roller 73 also serves as a tension applicator that applies tension to the intermediate transfer belt 11, a biasing member (e.g., a spring) biases the driven roller 73 against the intermediate transfer belt 11. The intermediate transfer belt unit 10, the primary transfer rollers 12Y, 12C, 12M, and 12K, the secondary transfer roller 5, and the intermediate transfer belt cleaner 13 constitute a transfer device 71. The sheet feeder 61 includes a feed roller 3 that contacts an upper side of an uppermost sheet S of the plurality of sheets S loaded on the paper tray of the sheet feeder 61. As the feed roller 3 is driven and rotated counterclockwise in FIG. 1, the feed roller 3 feeds the uppermost sheet S to the registration roller pair 4.
The intermediate transfer belt cleaner 13 of the transfer device 71 includes a cleaning brush and a cleaning blade disposed opposite the intermediate transfer belt 11 to come into contact with the intermediate transfer belt 11. The cleaning brush and the cleaning blade scrape a foreign substance such as residual toner particles off the intermediate transfer belt 11, removing the foreign substance from the intermediate transfer belt 11 and thereby cleaning the intermediate transfer belt 11. The intermediate transfer belt cleaner 13 further includes a waste toner conveyer that conveys the residual toner particles removed from the intermediate transfer belt 11.
With reference to FIG. 2, a description is provided of a configuration of the fixing device 150 according to a first exemplary embodiment that is incorporated in the image forming apparatus 100 described above.
FIG. 2 is a schematic vertical sectional view of the fixing device 150. As shown in FIG. 2, the fixing device 150 (e.g., a fuser or a fusing unit) includes a thin, flexible, endless fixing belt 14, serving as an endless belt, a fixing rotator, or a fixing member, formed into a loop and rotatable in a rotation direction D14 and a pressure roller 16 serving as an opposed rotator disposed opposite the fixing belt 14 and rotatable in a rotation direction D16. Inside the loop formed by the fixing belt 14 is a nip formation assembly 18 (e.g., a nip formation unit) that forms a fixing nip N between the fixing belt 14 and the pressure roller 16, through which the sheet S is conveyed.
A detailed description is now given of a construction of the nip formation assembly 18.
The nip formation assembly 18 includes a nip formation pad 22, a lateral end heater 24, and a stay 26. The nip formation pad 22, disposed inside the loop formed by the fixing belt 14, presses against the pressure roller 16 via the fixing belt 14 to form the fixing nip N between the fixing belt 14 and the pressure roller 16. The lateral end heater 24 serving as a lateral end heater or a lateral end heat source is mounted on each lateral end of the nip formation pad 22 in a longitudinal direction thereof parallel to an axial direction of the fixing belt 14. The stay 26 supports the nip formation pad 22 against pressure from the pressure roller 16.
An inner circumferential surface of the fixing belt 14 slides over the nip formation pad 22 via a low-friction sheet 6 serving as a slide sheet as shown in FIG. 3. FIG. 3 is a schematic vertical sectional view of the fixing device 150 illustrating the low-friction sheet 6. The low-friction sheet 6 is applied with a lubricant such as fluorine grease and silicone oil to decrease a slide torque of the fixing belt 14. Alternatively, the nip formation pad 22 may contact the inner circumferential surface of the fixing belt 14 directly without the low-friction sheet 6 interposed between the nip formation pad 22 and the fixing belt 14.
The stay 26 has a box shape with an opening opposite the fixing nip N. Two halogen heaters 28a and 28b serving as a fixing heater or a fixing heat source are disposed inside the box of the stay 26. The halogen heaters 28a and 28b emit light that irradiates the inner circumferential surface of the fixing belt 14 directly through the opening of the stay 26, heating the fixing belt 14 with radiation heat.
A platy reflector 31 is mounted on an interior surface of the stay 26 to reflect light radiated from the halogen heaters 28a and 28b toward the fixing belt 14 so as to improve heating efficiency of the halogen heaters 28a and 28b to heat the fixing belt 14. The reflector 31 prevents light from the halogen heaters 28a and 28b from heating the stay 26, suppressing waste of energy. Alternatively, instead of the reflector 31, the interior surface of the stay 26 may be treated with insulation or mirror finish to reflect light radiated from the halogen heaters 28a and 28b toward the fixing belt 14.
A detailed description is now given of a construction of the pressure roller 16.
FIG. 4 is a partial vertical sectional view of the fixing device 150. As shown in FIG. 4, the pressure roller 16 is constructed of a hollow metal roller 16a, an elastic layer 16b coating an outer circumferential surface of the metal roller 16a and being made of silicone rubber, and a release layer 16c coating an outer circumferential surface of the elastic layer 16b. The release layer 16c, having a layer thickness in a range of from 5 micrometers to 50 micrometers, is made of perfluoroalkoxy fluoro resin (PFA) or polytetrafluoroethylene (PTFE) to facilitate separation of the sheet S from the pressure roller 16. As a driving force generated by a driver (e.g., a motor) situated inside the image forming apparatus 100 depicted in FIG. 1 is transmitted to the pressure roller 16 through a gear train, the pressure roller 16 rotates in the rotation direction D16 as shown in FIG. 2. Alternatively, the driver may also be connected to the fixing belt 14 to drive and rotate the fixing belt 14.
A spring or the like biases the pressure roller 16 against the fixing belt 14. As the elastic layer 16b of the pressure roller 16 is pressed and deformed, the pressure roller 16 produces the fixing nip N having a predetermined length Nw in a sheet conveyance direction DS as shown in FIG. 4.
Alternatively, the pressure roller 16 may be a solid roller. However, a hollow roller has a decreased thermal capacity. Further, a heater or a heat source such as a halogen heater may be disposed inside the pressure roller 16. The elastic layer 16b may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller 16, the elastic layer 16b may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because the sponge rubber has an increased insulation that draws less heat from the fixing belt 14.
A detailed description is now given of a construction of the fixing belt 14.
As shown in FIG. 2, the fixing belt 14 is an endless belt or film having a layer thickness in a range of from 30 micrometers to 50 micrometers and made of metal such as nickel and SUS stainless steel or resin such as polyimide. The fixing belt 14 is constructed of a base layer and a release layer. The release layer constituting an outer surface layer is made of PFA, PTFE, or the like to facilitate separation of toner of a toner image on the sheet S from the fixing belt 14, thus preventing the toner of the toner image from adhering to the fixing belt 14. Optionally, an elastic layer may be sandwiched between the base layer and the release layer and made of silicone rubber or the like. If the fixing belt 14 does not incorporate the elastic layer, the fixing belt 14 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 on the sheet S. However, as the pressure roller 16 and the fixing belt 14 sandwich and press the unfixed toner image on the sheet S passing through the fixing nip N, slight surface asperities of the fixing belt 14 may be transferred onto the toner image on the sheet S, resulting in variation in gloss of the solid toner image on the sheet S.
To address this circumstance, the elastic layer made of silicone rubber has a thickness not smaller than 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing belt 14, suppressing variation in gloss of the toner image on the sheet S. As the pressure roller 16 rotates in the rotation direction D16, the fixing belt 14 rotates in the rotation direction D14 in accordance with rotation of the pressure roller 16 by friction therebetween. At the fixing nip N, the fixing belt 14 rotates as it is sandwiched between the pressure roller 16 and the nip formation pad 22; at a circumferential span of the fixing belt 14 other than the fixing nip N, the fixing belt 14 rotates while the fixing belt 14 is supported at each lateral end in the axial direction thereof to retain a tubular shape. Thus, the fixing belt 14 is retained circular in cross-section stably.
As shown in FIG. 2, a separator 32 is disposed downstream from the fixing nip N in the sheet conveyance direction DS to separate the sheet S from the fixing belt 14.
According to this exemplary embodiment, as shown in FIGS. 2 to 4, the fixing nip N is planar. Alternatively, the fixing nip N may define a curve projecting toward the fixing belt 14 to produce a recess in the fixing belt 14 in cross-section or other shapes. If the fixing nip N defines the recess in the fixing belt 14, the recessed fixing nip N directs the leading edge of the sheet S toward the pressure roller 16 as the sheet S is ejected from the fixing nip N, facilitating separation of the sheet S from the fixing belt 14 and suppressing jamming of the sheet S. In this case, a nip formation face of the nip formation pad 22 is contoured into the recess. Similarly, a fixing belt side face of the lateral end heater 24 mounted on the nip formation pad 22 may be contoured along the recessed nip formation face of the nip formation pad 22.
A detailed description is now given of a configuration of the stay 26.
The stay 26 supports the nip formation pad 22 against pressure from the pressure roller 16 to prevent bending of the nip formation pad 22 and produce the even length Nw of the fixing nip N in the sheet conveyance direction DS throughout the entire width of the fixing belt 14 in the axial direction thereof. According to this exemplary embodiment, the pressure roller 16 is pressed against the fixing belt 14 to form the fixing nip N. Alternatively, the nip formation assembly 18 may be pressed against the pressure roller 16 to from the fixing nip N. The stay 26 has a mechanical strength great enough to support the nip formation pad 22 to prevent bending of the nip formation pad 22. The stay 26 is made of metal such as stainless steel and iron or metallic oxide such as ceramic. The fixing belt 14 and the components disposed inside the loop formed by the fixing belt 14, that is, the halogen heaters 28a and 28b, the nip formation pad 22, the lateral end heater 24, the stay 26, and the reflector 31, may constitute a belt unit 14U separably coupled with the pressure roller 16.
FIG. 5 is a partial perspective view of the fixing device 150. As shown in FIG. 5, both lateral ends of the fixing belt 14 in the axial direction thereof are rotatably supported by flanges 36, respectively. Each of the flanges 36 serves as a support projecting from a side plate 34 in the axial direction of the fixing belt 14. Although FIG. 5 illustrates the flange 36 and the side plate 34 situated at one lateral end of the fixing belt 14 in the axial direction thereof, the flange 36 and the side plate 34 are also situated at another lateral end of the fixing belt 14 in the axial direction thereof. The flange 36 that guides each lateral end of the fixing belt 14 in the axial direction thereof has an outer diameter substantially equivalent to an inner diameter of the fixing belt 14. The flange 36 projects inboard from a lateral edge of the fixing belt 14 by a length in a range of from 5 mm to 10 mm in the axial direction of the fixing belt 14. The flanges 36 guide the fixing belt 14 even when the fixing belt 14 rotates, retaining the fixing belt 14 to be circular in cross-section.
The flange 36 includes a slit 36a disposed opposite the fixing nip N to place the nip formation assembly 18 at a predetermined position. The stay 26 depicted in FIG. 2 has a width that spans the entire width of the fixing belt 14 in the axial direction thereof. Both lateral ends of the stay 26 in the axial direction of the fixing belt 14 are fixedly mounted on the side plates 34, respectively, thus being supported and positioned by the side plates 34.
FIG. 6 is a plan view of the halogen heaters 28a and 28b illustrating a light distribution of the halogen heaters 28a and 28b and a positional relation between the halogen heaters 28a and 28b and a plurality of lateral end heaters 24a and 24b. FIG. 6 illustrates a width W1 of an A3 size sheet in portrait orientation in the axial direction of the fixing belt 14 and a width W2 of an A3 extension size sheet and a 13-inch sheet in portrait orientation in the axial direction of the fixing belt 14.
As shown in FIG. 6, the halogen heater 28a is a center heater having a dense light distribution at a center span of the halogen heater 28a disposed opposite a center span of the fixing belt 14 in the axial direction thereof where a small sheet S having a decreased width in the axial direction of the fixing belt 14 is conveyed over the fixing belt 14. Conversely, the halogen heater 28b is a lateral end heater having a dense light distribution at a lateral end span of the halogen heater 28b disposed opposite a lateral end span of the fixing belt 14 in the axial direction thereof where a large sheet S having an increased width (e.g., an A3 size sheet) in the axial direction of the fixing belt 14 is conveyed over the fixing belt 14. As the small sheet S is conveyed over the fixing belt 14, the halogen heater 28a is powered on and the halogen heater 28b is not powered on, thus preventing the lateral end span of the fixing belt 14 where the small sheet S is not conveyed from being heated unnecessarily.
A description is provided of a configuration of a first comparative fixing device incorporating a center heater equivalent to the halogen heater 28a and a lateral end heater equivalent to the halogen heater 28b depicted in FIG. 6.
As a small sheet S is conveyed through the fixing nip N, the center heater having a center dense light distribution is powered on. As a large sheet S is conveyed through the fixing nip N, the lateral end heater having a lateral end dense light distribution is powered on together with the center heater. The center heater and the lateral end heater are powered on and off properly to heat sheets S of various sizes.
Taking the sizes of the sheets S and the frequency with which the sheets S are conveyed, sheets S up to the A3 size sheet are used frequently. The A3 size sheet is conveyed through the fixing nip N in portrait orientation. An A4 size sheet and a letter (LT) size sheet that are used with an increased frequency are generally conveyed in landscape orientation to enhance productivity. To address this circumstance, the center heater and the lateral end heater produce a heating span of about 300 mm in the axial direction of the fixing belt 14 that is great enough to heat 99 percent or more of the sizes of sheets S. On the other hand, the center heater and the lateral end heater are requested to heat large sheets S greater than the A3 size sheet in the axial direction of the fixing belt 14 such as an A3 extension size sheet and a 13-inch sheet although the large sheets S are used infrequently.
If a plurality of halogen heaters is used as the center heater and the lateral end heater, respectively, the plurality of halogen heaters used to heat the small sheet S is situated inside the loop formed by the fixing belt 14 or a fixing roller having a diameter of about 30 mm. Accordingly, the number of the halogen heaters is limited. To address this circumstance, the lateral end heater having the lateral end dense light distribution may be elongated to span a width of the large sheet S greater than a width of the A3 size sheet in the axial direction of the fixing belt 14.
As described above, the center heater and the lateral end heater heat the heating span of about 300 mm of the fixing belt 14 in the axial direction thereof frequently. However, if the elongated lateral end heater is employed, the elongated lateral end heater may heat an elongated heating span of about 330 mm of the fixing belt 14 in the axial direction thereof, wasting energy used to heat a differential between the heating span of about 300 mm and the elongated heating span of about 330 mm.
When the A3 size sheet in portrait orientation or the A4 size sheet in landscape orientation is conveyed through the fixing nip N, each lateral end of the elongated heating span of the fixing belt 14 in the axial direction thereof that corresponds to the differential between the heating span of about 300 mm and the elongated heating span of about 330 mm may overheat. In order to cool the overheated lateral end of the fixing belt 14, productivity defined by a conveyance speed of the sheets S may be degraded or a fan may be installed. If a reflection plate is interposed between the lateral end heater and the fixing belt 14, each lateral end of the lateral end heater in the axial direction of the fixing belt 14 may overheat.
To address this circumstance, a second comparative fixing device is proposed. The second comparative fixing device includes a thin, flexible endless belt to be heated quickly to a fixing temperature at which a toner image is fixed on a sheet S and a nip formation unit located inside a loop formed by the endless belt. The nip formation unit presses against a pressure roller via the endless belt to form a fixing nip between the endless belt and the pressure roller. A plurality of halogen heaters having different light distributions, respectively, is situated inside the loop formed by the endless belt. A plurality of lateral end heaters is disposed opposite both lateral end spans of the endless belt in an axial direction thereof, respectively, and upstream from the fixing nip in a rotation direction of the endless belt so as to heat an increased heating span of the endless belt corresponding to the width of the large sheet S in the axial direction of the endless belt. The lateral end heaters contact an inner circumferential surface or an outer circumferential surface of the endless belt. The lateral end heaters heat the increased heating span of the endless belt corresponding to the width of the large sheet S in the axial direction of the endless belt with a simple construction not incorporating an extra halogen heater directed to the large sheet S.
Since the lateral end heaters are disposed opposite both lateral end spans of the endless belt in the axial direction thereof and upstream from the fixing nip in the rotation direction of the endless belt to heat both lateral end spans of the endless belt in the axial direction of the endless belt, respectively, both lateral end spans of the endless belt may flap while the endless belt rotates and therefore may not contact the lateral end heaters precisely. To address this circumstance, the lateral end heaters may press against both lateral end spans of the endless belt with predetermined pressure. However, since the endless belt is exerted with pressure also at the fixing nip, rotation of the endless belt may degrade. Additionally, the lateral end heaters may melt residual toner failed to be fixed on the sheet S and therefore remaining on the endless belt again on both lateral end spans of the endless belt disposed opposite the lateral end heaters, respectively. Accordingly, the melted toner may adhere to the endless belt.
A width of the A3 size sheet in portrait orientation and a width of the A4 size sheet in landscape orientation are smaller than a width of the A3 extension size sheet in portrait orientation (e.g., 329 mm) and a width of the 13-inch sheet in portrait orientation (e.g., 330 mm) by a differential in a range of from 32 mm to 33 mm, respectively. Accordingly, if the fixing device 150 is configured to heat each lateral end span of the fixing belt 14 in the axial direction thereof, that is, if the fixing device 150 is configured to heat a half of the differential in range of from 32 mm to 33 mm, that is, a span in a range of from 16.0 mm to 16.5 mm, the maximum width of sheets S available in the fixing device 150 increases from the width W1 of the A3 size sheet to the width W2 of the A3 extension size sheet or the like as shown in FIG. 6. In other words, if the fixing device 150 is configured to heat each outboard span, that is, each lateral end span, of the fixing belt 14 disposed opposite each outboard span, that is, each lateral end span, of the halogen heater 28b that is outboard from each lateral end span of the halogen heater 28b in the axial direction of the fixing belt 14 and does not have the dense light distribution or has a light distribution of a decreased density, the large sheet S (e.g., the A3 extension size sheet) is available in the fixing device 150. Accordingly, the fixing device 150 includes the lateral end heater 24 constructed of small heaters, that is, the lateral end heaters 24a and 24b, having a decreased width of about 20 mm in the axial direction of the fixing belt 14.
As the large sheet S (e.g., the A3 extension size sheet and the 13-inch sheet) is conveyed through the fixing nip N, the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b are energized. Conversely, as the small sheet S (e.g., a sheet not greater than the A3 size sheet) is conveyed through the fixing nip N, the halogen heaters 28a and 28b are energized or the halogen heater 28a is energized. Hence, the lateral end heaters 24a and 24b are not energized.
If the halogen heater 28b is configured to have an increased heating span to heat the large sheet S such as the A3 extension size sheet, the halogen heater 28b may heat each lateral end span of the fixing belt 14 unnecessarily while the large sheet S is not conveyed through the fixing nip N, wasting energy. To address this circumstance, the fixing device 150 according to this exemplary embodiment incorporates a simple mechanism in addition to the halogen heaters 28a and 28b, that is, the lateral end heaters 24a and 24b being disposed opposite both lateral end spans G in the axial direction of the fixing belt 14 or in proximity to both lateral ends of the fixing belt 14 in the axial direction thereof, respectively.
FIG. 7 is an exploded perspective view of the nip formation assembly 18. As shown in FIG. 7, a side face 26a of the stay 26 that faces the pressure roller 16 mounts two ridges 26b and 26c extending in the axial direction of the fixing belt 14. The rectangular nip formation pad 22 is sandwiched and positioned between the two ridges 26b and 26c in the sheet conveyance direction DS and is attached to the side face 26a with an adhesive or the like. Thus, the side face 26a and the two ridges 26b and 26c accommodate the nip formation pad 22. Two recesses 22a and 22b that define a difference in thickness of the nip formation pad 22 are disposed at both lateral ends of the nip formation pad 22 in the longitudinal direction thereof. The lateral end heaters 24a and 24b are attached to the recesses 22a and 22b with an adhesive or the like or mounted on the recesses 22a and 22b, respectively, thus being accommodated by the recesses 22a and 22b. The nip formation pad 22 includes a nip formation face 22c that faces the pressure roller 16. The nip formation face 22c is sandwiched between the lateral end heaters 24a and 24b in the axial direction of the fixing belt 14.
A description is provided of a construction of the lateral end heaters 24a and 24b. 
FIG. 8 is a plan view of the lateral end heater 24a. Since the lateral end heaters 24a and 24b have an identical construction, FIG. 8 illustrates the lateral end heater 24a. The lateral end heater 24a includes a ceramic base 51, a resistive heat generator 52 layered on the base 51 with patterning, and an insulative layer 53 layered on the resistive heat generator 52. The base 51 has an outer size of about 10 mm×about 20 mm. The resistive heat generator 52 is a heat generator. The insulative layer 53 is a thin glass layer. Terminals 54, disposed at each lateral end of the lateral end heater 24a in the axial direction of the fixing belt 14, are connected to a power supply and a switching element.
As described above, the resistive heat generator 52 is mounted on a first face of the lateral end heater 24a so that the first face of the lateral end heater 24a that mounts the resistive heat generator 52 generates heat mainly while a second face of the lateral end heater 24a that does not mount the resistive heat generator 52 barely receives heat from the first face. According to this exemplary embodiment, the first face of the lateral end heater 24a that mounts the resistive heat generator 52 contacts the recess 22a depicted in FIG. 7. The terminals 54 are mounted on the first face of the lateral end heater 24a. 
FIG. 9 is a schematic vertical sectional view of the fixing device 150 illustrating the lateral end heater 24 (e.g., the lateral end heaters 24a and 24b). As shown in FIG. 9, the first face of the lateral end heater 24 that mounts the resistive heat generator 52 is isolated from the fixing belt 14. Accordingly, even if the insulative layer 53 depicted in FIG. 8 is broken, power supplied to the lateral end heater 24 is not transmitted to the fixing belt 14. If the fixing belt 14 is made of metal as described below, power may be transmitted to other components disposed inside the image forming apparatus 100 through metal of the fixing belt 14, for example, a thermistor contacting the fixing belt 14, thus adversely affecting the thermistor. To address this circumstance, the above-described configuration secures a predetermined interval between the inner circumferential surface of the fixing belt 14 and the resistive heat generator 52 extending along the inner circumferential surface of the fixing belt 14.
FIG. 10A is a sectional view of the fixing belt 14, the nip formation pad 22, and the lateral end heaters 24a and 24b. As shown in FIG. 10A, each of the lateral end heaters 24a and 24b includes a fixing belt side face 24c contacting the inner circumferential surface of the fixing belt 14. The fixing belt side face 24c of the respective lateral end heaters 24a and 24b is leveled with the nip formation face 22c of the nip formation pad 22 in a pressurization direction F (e.g., a direction of a reaction force against pressure from the pressure roller 16) in which the nip formation pad 22 presses against the inner circumferential surface of the fixing belt 14. In other words, the fixing belt side face 24c contacting the inner circumferential surface of the fixing belt 14 defines an extension of the nip formation face 22c in the longitudinal direction of the nip formation pad 22. According to this exemplary embodiment, the lateral end heaters 24a and 24b are coupled with the nip formation pad 22 to form the fixing nip N. Hence, the lateral end heaters 24a and 24b are situated in a limited space inside the loop formed by the fixing belt 14, saving space.
The fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 is leveled with the nip formation face 22c of the nip formation pad 22 in the pressurization direction F to define an identical plane. Accordingly, the pressure roller 16 is pressed against the lateral end heaters 24a and 24b via the fixing belt 14 sufficiently. Consequently, the fixing belt 14 rotates in a state in which the fixing belt 14 adheres to the lateral end heaters 24a and 24b, improving conduction of heat from the lateral end heaters 24a and 24b to the fixing belt 14 and thereby retaining improved heating efficiency of the lateral end heaters 24a and 24b. 
Since the lateral end heaters 24a and 24b are situated within the fixing nip N in the axial direction of the fixing belt 14 to heat the fixing belt 14, the lateral end heaters 24a and 24b do not heat a portion of the fixing belt 14 that is outboard from the fixing nip N in the axial direction of the fixing belt 14, preventing residual toner failed to be fixed on the sheet S and therefore remaining on the fixing belt 14 from being melted again and adhered to the fixing belt 14.
The pressure roller 16 also serves as a biasing member that presses the fixing belt 14 against the lateral end heaters 24a and 24b to adhere the fixing belt 14 to the lateral end heaters 24a and 24b so as to enhance conduction of heat from the lateral end heaters 24a and 24b to the fixing belt 14. Accordingly, a mechanism that presses the lateral end heaters 24a and 24b against the fixing belt 14 is not needed, simplifying the fixing device 150. In other words, pressure used to form the fixing nip N is also used to adhere the fixing belt 14 to the lateral end heaters 24a and 24b, improving conduction of heat from the lateral end heaters 24a and 24b to the fixing belt 14 without degrading rotation of the fixing belt 14.
As shown in FIG. 7, each of the recesses 22a and 22b is open at each lateral edge of the nip formation pad 22 in the longitudinal direction thereof. Alternatively, each of the recesses 22a and 22b may be closed and formed in a box defined by a bottom and four walls as shown in FIG. 10B. FIG. 10B is a sectional view of the fixing belt 14, the nip formation pad 22, and the lateral end heaters 24a and 24b illustrating the closed recesses 22a and 22b as a first variation. Alternatively, each of the recesses 22a and 22b may be closed at both ends in the axial direction of the fixing belt 14 and open at both ends in a direction perpendicular to the axial direction of the fixing belt 14.
FIG. 11A is a sectional view of the nip formation pad 22 and the lateral end heaters 24a and 24b illustrating the recesses 22a and 22b as a second variation. As shown in FIG. 11A, the recess 22a accommodates the lateral end heater 24a and an elastic member 38 supporting the lateral end heater 24a; the recess 22b accommodates the lateral end heater 24b and the elastic member 38 supporting the lateral end heater 24b. As shown in FIG. 11A, when the pressure roller 16 does not press the fixing belt 14 against the lateral end heaters 24a and 24b, the fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 is not leveled with the nip formation face 22c of the nip formation pad 22 in the pressurization direction F.
FIG. 11B is a sectional view of the fixing belt 14, the nip formation pad 22, and the lateral end heaters 24a and 24b when the pressure roller 16 presses the fixing belt 14 against the lateral end heaters 24a and 24b. As shown in FIG. 11B, when the pressure roller 16 presses the fixing belt 14 against the lateral end heaters 24a and 24b to form the fixing nip N, the elastic members 38 are deformed by pressure from the pressure roller 16 and the fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 is leveled with the nip formation face 22c of the nip formation pad 22 in the pressurization direction F. The elastic member 38 is made of rubber or includes a spring.
Since the lateral end heaters 24a and 24b are mounted and fixedly secured on the nip formation pad 22 as a separate component, the fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 may deviate from the nip formation face 22c of the nip formation pad 22 in height during assembly of the fixing device 150. To address this circumstance, the elastic members 38 support the lateral end heaters 24a and 24b to absorb a manufacturing error, thus leveling the fixing belt side face 24c of the respective lateral end heaters 24a and 24b with the nip formation face 22c of the nip formation pad 22 when the fixing nip N is formed.
According to this exemplary embodiment, the lateral end heaters 24a and 24b are mounted on or coupled with the nip formation pad 22 to constitute the nip formation assembly 18. Alternatively, the lateral end heaters 24a and 24b may not be mounted on or coupled with the nip formation pad 22 as shown in FIG. 12. FIG. 12 is a sectional view of the fixing belt 14, the nip formation pad 22, and the lateral end heaters 24a and 24b as a third variation. As shown in FIG. 12, the lateral end heaters 24a and 24b are disposed outboard from the nip formation pad 22 in the longitudinal direction thereof and within the fixing nip N in the axial direction of the fixing belt 14. Thus, the lateral end heaters 24a and 24b are separated from the nip formation pad 22 or the nip formation assembly 18. For example, the lateral end heaters 24a and 24b are mounted on supports 42a and 42b mounted on the side plates 34 depicted in FIG. 5, respectively. As shown in FIG. 12, the fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 is leveled with the nip formation face 22c of the nip formation pad 22 in the pressurization direction F. Alternatively, the elastic members 38 depicted in FIGS. 11A and 11B may support the lateral end heaters 24a and 24b shown in FIG. 12, respectively, to displace the lateral end heaters 24a and 24b. 
According to the exemplary embodiments described above, as shown in FIG. 2, the nip formation pad 22, the lateral end heaters 24a and 24b, the stay 26, and the halogen heaters 28a and 28b are assembled into a nip formation unit, that is, the nip formation assembly 18. Alternatively, the nip formation pad 22 and the lateral end heaters 24a and 24b may be assembled into a nip formation unit, that is, the nip formation assembly 18.
The lateral end heaters 24a and 24b may have a positive temperature coefficient (PTC) property. If the lateral end heaters 24a and 24b have the PTC property, a resistance value increases at a preset temperature or higher and therefore the lateral end heaters 24a and 24b do not generate heat at the preset temperature or higher. Hence, the lateral end heaters 24a and 24b do not burn or damage the fixing belt 14, achieving the safe fixing device 150.
Additionally, the lateral end heaters 24a and 24b situated inside the loop formed by the fixing belt 14 emit light that irradiates the inner circumferential surface of the fixing belt 14 to heat both lateral end spans of the fixing belt 14 in the axial direction thereof without degrading rotation of the fixing belt 14.
Further, if the fixing belt side face 24c of the respective lateral end heaters 24a and 24b that contacts the inner circumferential surface of the fixing belt 14 is made of a smooth material different from a material of a body of the respective lateral end heaters 24a and 24b, the smooth material suppresses the sliding friction of the fixing belt 14 as the fixing belt 14 slides over the lateral end heaters 24a and 24b, retaining stable rotation of the fixing belt 14.
According to the exemplary embodiments described above, as shown in FIG. 2, the nip formation assembly 18 situated inside the loop formed by the fixing belt 14 includes the nip formation pad 22 and the halogen heaters 28a and 28b being disposed opposite and heating at least the center span of the fixing belt 14 in the axial direction thereof where the sheet S is conveyed. The nip formation pad 22 mounts the lateral end heaters 24a and 24b being disposed opposite and heating both lateral end spans of the inner circumferential surface of the fixing belt 14 in the axial direction thereof, respectively.
The lateral end heaters 24a and 24b are disposed opposite the fixing nip N where the fixing belt 14 is exerted with pressure from the pressure roller 16. Accordingly, the fixing belt 14 is exerted with pressure at a single place, that is, the fixing nip N. Consequently, the nip formation assembly 18 reduces degradation in rotation of the fixing belt 14. As a result, the fixing device 150 incorporating the nip formation assembly 18 improves fixing performance and the image forming apparatus 100 incorporating the fixing device 150 performs improved image formation constantly.
With reference to FIGS. 13A, 13B, 14, and 15, a description is provided of a configuration of the fixing device 150 according to a second exemplary embodiment.
The components of the fixing device 150 according to the second exemplary embodiment that are identical to those of the fixing device 150 according to the first exemplary embodiment are assigned with the identical reference numerals and a description of the construction and the configuration mentioned above is omitted.
FIG. 13A is a graph showing a relation between the position on the halogen heaters 28a and 28b in a longitudinal direction thereof and the heat output of the halogen heaters 28a and 28b. FIG. 13B is a graph showing the position on the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b in the axial direction of the fixing belt 14 and the heat output thereof.
As shown in FIG. 13A, both lateral end spans of each of the halogen heaters 28a and 28b in the longitudinal direction thereof have a decreased heat output. Taking heat output of the halogen heater 28a that draws a trapezoid, each lateral end span of the halogen heater 28a in the longitudinal direction thereof has a decreased heat output. Each lateral end span of the halogen heater 28a in the longitudinal direction thereof has a boundary between a portion having a dense light distribution and a portion having a non-dense light distribution, which indicates a property peculiar to halogen heaters. Accordingly, the halogen heater 28a heats both lateral ends of the A3 size sheet in the axial direction of the fixing belt 14 insufficiently, resulting in faulty fixing.
In order to heat the entire sheet S including each lateral end in a width direction thereof parallel to the axial direction of the fixing belt 14 sufficiently, a width of a light emitter of the halogen heater 28b where the light distribution is dense is requested to be greater than a width of the sheet S in the width direction thereof. However, the light emitter having the increased width in the width direction of the sheet S causes overheating or temperature increase of a non-conveyance span of the fixing belt 14 in the axial direction thereof where small sheets S are not conveyed after the small sheets S are conveyed over the fixing belt 14 continuously. To address this circumstance, a light shield may shield the non-conveyance span of the fixing belt 14 from redundant light emitted from the light emitter having the increased width. However, while the small sheets S are conveyed over the fixing belt 14 continuously, the light shield may overheat. Further, as the halogen heater 28b heats the non-conveyance span of the fixing belt 14, the halogen heater 28b consumes energy unnecessarily, wasting energy.
To address this circumstance, the lateral end heaters 24a and 24b of the fixing device 150 according to the second exemplary embodiment are located relative to the halogen heater 28b as shown in FIG. 14. FIG. 14 is a plan view of the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b illustrating the light distribution of the halogen heaters 28a and 28b and the positional relation between the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b. As shown in FIG. 14, at least a part of a heating span of the lateral end heaters 24a and 24b in the axial direction of the fixing belt 14 overlaps an outboard part of a heating span of the halogen heater 28b in the axial direction of the fixing belt 14.
In other words, the lateral end heaters 24a and 24b supplement decrease in heat output of a part of the halogen heater 28b that suffers from a decreased heat output. Accordingly, as shown in hatched parts H in FIG. 13B, the lateral end heaters 24a and 24b supplement decrease in heat output of the part of the halogen heater 28b that suffers from the decreased heat output.
The lateral end heaters 24a and 24b are directed to supplement decrease in heat output of the part of the halogen heater 28b that is disposed at each lateral end span of the halogen heater 28b in the longitudinal direction thereof and suffers from the decreased heat output. Hence, each of the lateral end heaters 24a and 24b is a downsized heater having a width of about 20 mm in the axial direction of the fixing belt 14.
The fixing device 150 according to the second exemplary embodiment incorporates a simple mechanism in addition to the halogen heaters 28a and 28b, that is, the lateral end heaters 24a and 24b being disposed opposite both lateral end spans (e.g., the lateral end span G depicted in FIG. 6) of the fixing belt 14 or in proximity to both lateral ends of the fixing belt 14 in the axial direction thereof, respectively, thus addressing the circumstances described above.
The halogen heaters 28a and 28b and the lateral end heaters 24a and 24b are energized during an initial time of a print job of conveying sheets S continuously for fixing immediately after warming up the fixing device 150, for example, the initial time when the fixing belt 14 and the pressure roller 16 have not been heated sufficiently.
Conversely, when the fixing belt 14 and the pressure roller 16 have been heated sufficiently and temperature decrease at each lateral end of the fixing belt 14 in the axial direction thereof has been improved, the halogen heaters 28a and 28b are energized or the halogen heater 28a is energized. Hence, the lateral end heaters 24a and 24b are not energized.
Under such heating control, the fixing device 150 reduces overheating or temperature increase in the non-conveyance span on the fixing belt 14 where the sheet S is not conveyed. Additionally, the fixing belt 14 is not heated unnecessarily, improving heating efficiency and saving energy.
FIG. 15 is a plan view of the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b illustrating the light distribution of the halogen heaters 28a and 28b and the positional relation between the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b. As shown in FIG. 15, the lateral end heaters 24a and 24b have a width in the axial direction of the fixing belt 21 that is great enough to supplement decrease in heat output of the part of the halogen heater 28b that is disposed at each lateral end span of the halogen heater 28b in the longitudinal direction thereof and suffers from the decreased heat output. The width of each of the lateral end heaters 24a and 24b is also great enough to span the width of the A3 extension size sheet or the like.
According to the exemplary embodiments described above, the single lateral end heater 24a is disposed opposite one lateral end span of the fixing belt 14 in the axial direction thereof; the single lateral end heater 24b is disposed opposite another lateral end span of the fixing belt 14 in the axial direction thereof. Alternatively, a plurality of lateral end heaters 24a may be disposed opposite one lateral end span of the fixing belt 14 in the axial direction thereof and a plurality of lateral end heaters 24b may be disposed opposite another lateral end span of the fixing belt 14 in the axial direction thereof according to various sizes of sheets S, for example. The lateral end heaters 24a and 24b may be disposed outboard from the halogen heater 28b in the longitudinal direction thereof. Accordingly, the lateral end heaters 24a and 24b correspond to an increased number of sizes of sheets S and heat the fixing belt 14 precisely.
The lateral end heaters 24a and 24b having the PTC property may take an extended period of time to achieve a predetermined target temperature compared to the halogen heaters 28a and 28b. For example, if the lateral end heaters 24a and 24b and the halogen heaters 28a and 28b are energized simultaneously, the center span of the fixing belt 14 in the axial direction thereof is heated quickly, wasting energy. Further, as the sheets S conveyed over the fixing belt 14 draw heat from the fixing belt 14, the lateral end heaters 24a and 24b, due to their PTC property, take the extended period of time to retrieve the predetermined target temperature compared to the halogen heaters 28a and 28b. 
To address this circumstance, the fixing device 150 decreases productivity to correspond to a heating cycle of the lateral end heaters 24a and 24b, thus controlling heating of the fixing belt 14 to reduce variation in temperature of the fixing belt 14 in the axial direction thereof, that is, between the center span and each lateral end span of the fixing belt 14 in the axial direction thereof.
For example, while the lateral end heaters 24a and 24b that heat both lateral end spans of the fixing belt 14 in the axial direction thereof or the vicinity of both lateral ends of the fixing belt 14, respectively, where the A3 extension size sheet is conveyed are energized, actuation of the halogen heaters 28a and 28b that heat an inboard span inboard from both lateral end spans of the fixing belt 14 in the axial direction thereof where sheets smaller than the A3 extension size sheet are conveyed is controlled in accordance with temperature increase of both lateral end spans of the fixing belt 14 in the axial direction thereof.
As shown in FIG. 2, a temperature sensor 125 is disposed opposite the fixing belt 14 to detect the temperature of the fixing belt 14. A controller 99 is operatively connected to the temperature sensor 125, the halogen heaters 28a and 28b, and the lateral end heaters 24a and 24b. The controller 99 controls switches of an electric circuit described below to energize the halogen heaters 28a and 28b in accordance with increase in the temperature of the lateral end span of the fixing belt 14 that is detected by the temperature sensor 125. For example, the controller 99 (e.g., a processor) is a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM). The controller 99 may be disposed inside the fixing device 150 or the image forming apparatus 100.
Accordingly, the fixing device 150 prevents waste of energy caused by the halogen heaters 28a and 28b that heat the inboard span of the fixing belt 14 in the axial direction thereof where the sheets smaller than the large sheet S are conveyed quickly and unnecessarily while the lateral end heaters 24a and 24b generate a decreased amount of heat.
A conveyance speed at which the A3 extension size sheet heated by the lateral end heaters 24a and 24b is conveyed is smaller than a conveyance speed at which the sheets other than the A3 extension size sheet are conveyed. Thus, the fixing device 150 decreases productivity when the infrequently used, large sheet S (e.g., the A3 extension size sheet) is conveyed, simplifying the lateral end heaters 24a and 24b that heat both lateral end spans of the fixing belt 14 in the axial direction thereof, respectively, and reducing manufacturing costs. Consequently, the fixing belt 14 is heated effectively.
According to the exemplary embodiments described above, the fixing device 150 includes the two halogen heaters 28a and 28b serving as fixing heaters, respectively. Alternatively, the fixing device 150 may include three or more halogen heaters to correspond to various sizes of small sheets S.
A description is provided of a construction of a nip formation assembly 63 (e.g., a nip formation unit) as a variation of the nip formation assembly 18 depicted in FIG. 2.
FIG. 16 is a schematic vertical sectional view of a fixing device 150S (a fuser or a fusing unit) incorporating the nip formation assembly 63. As shown in FIG. 16, the nip formation assembly 63 includes the nip formation pad 22, the lateral end heaters 24a and 24b, and a stay 64 that supports the nip formation pad 22 against pressure from the pressure roller 16. The stay 64 includes a base 64a and a stand 64b coupled with the base 64a. The base 64a supports the nip formation pad 22 like the stay 26 depicted in FIG. 2. The stand 64b is substantially contoured into a triangle in cross-section. The halogen heaters 28a and 28b serving as a fixing heater or a fixing heat source are interposed between the stand 64b of the stay 64 and the fixing belt 14. The halogen heaters 28a and 28b heat the fixing belt 14 directly with light irradiating the inner circumferential surface of the fixing belt 14, thus heating the fixing belt 14 with radiation heat. An arcuate, platy reflector 65 is interposed between the halogen heaters 28a and 28b and the stand 64b of the stay 64 to reflect light radiated from the halogen heaters 28a and 28b toward the fixing belt 14 so as to improve heating efficiency of the halogen heaters 28a and 28b to heat the fixing belt 14.
The nip formation assembly 63 achieves advantages similar to those of the nip formation assembly 18 described above. Alternatively, instead of the reflector 65, an exterior surface of the stand 64b may be treated with insulation or mirror finish to reflect light radiated from the halogen heaters 28a and 28b toward the fixing belt 14. In this case, the halogen heaters 28a and 28b heat the fixing belt 14 with a slightly decreased heating efficiency compared to a heating efficiency with which the halogen heaters 28a and 28b heat the fixing belt 14 together with the reflector 65.
A detailed description is now given of a construction of the fixing belt 14 made of metal.
The fixing belt 14 shown in FIGS. 2, 3, 9, and 16 conducts heat received from the halogen heaters 28a and 28b to the fixing nip N as the fixing belt 14 rotates in accordance with rotation of the pressure roller 16 contacting the outer circumferential surface of the fixing belt 14. Since the fixing belt 14 is exerted with a substantial load, the fixing belt 14 may have an insufficient mechanical strength if the fixing belt 14 is made of resin such as polyimide. To address this circumstance, the fixing belt 14 includes the base layer made of metal that achieves a sufficient mechanical strength, such as stainless steel, nickel, aluminum, and copper.
FIG. 17 is a sectional view of the fixing belt 14. As shown in FIG. 17, the fixing belt 14 includes a base layer 46 made of metal, an elastic layer 47 coating an outer circumferential surface of the base layer 46, and a release layer 48 coating an outer circumferential surface of the elastic layer 47, for example. The base layer 46, the elastic layer 47, and the release layer 48 are layered by a typical method. The base layer 46 is requested to achieve durability, flexibility, and heat resistance to endure usage at the fixing temperature. The elastic layer 47 and the release layer 48 are also produced to achieve those durability, flexibility, and heat resistance. Nickel is more appropriate than stainless steel for the base layer 46 of the fixing belt 14 because nickel is superior to stainless steel in mechanical strength, durability, and readiness in manufacturing of the endless fixing belt 14 by an electroforming process.
A description is provided of an electric connection between the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b. 
FIG. 18 is a diagram of an electric circuit 91 showing the electric connection between the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b. Under a center conveyance system in which the sheet S is centered in the axial direction of the fixing belt 14 as the sheet S is conveyed over the fixing belt 14, the lateral end heaters 24a and 24b are energized simultaneously. Accordingly, the lateral end heaters 24a and 24b are electrically connected in series to a power supply 44 as shown in FIG. 18. Consequently, the lateral end heaters 24a and 24b are electrically controlled more simply compared to a control in which the lateral end heater 24a is powered on and off separately from the lateral end heater 24b. 
If one of the lateral end heaters 24a and 24b suffers from failure, the power supply 44 interrupts power supply to the lateral end heaters 24a and 24b simultaneously, achieving safety of the fixing device 150. The power supply 44 powers on and off the halogen heater 28a through a switch SW1, the halogen heater 28b through a switch SW2, and the lateral end heaters 24a and 24b through a switch SW3.
FIG. 19A is a diagram of the electric circuit 91 illustrating a first energization pattern. As shown in FIG. 19A, as a small sheet S smaller than the A3 size sheet is conveyed through the fixing nip N, the halogen heater 28a is energized.
FIG. 19B is a diagram of the electric circuit 91 illustrating a second energization pattern. As shown in FIG. 19B, as the A3 size sheet is conveyed through the fixing nip N, the halogen heaters 28a and 28b are energized.
FIG. 19C is a diagram of the electric circuit 91 illustrating a third energization pattern. As shown in FIG. 19C, as a large sheet S greater than the A3 size sheet is conveyed through the fixing nip N, the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b are energized simultaneously.
The halogen heaters 28a and 28b disposed opposite the fixing nip N via the stay 26 to heat a non-nip side portion of the fixing belt 14 that is opposite the fixing nip N are energized separately from the lateral end heaters 24a and 24b disposed opposite the fixing nip N to heat a nip side portion of the fixing belt 14 that constitutes the fixing nip N. Accordingly, the electric circuit 91 controls the lateral end heaters 24a and 24b to heat both lateral end spans of the fixing belt 14 in the axial direction thereof precisely. Additionally, the electric circuit 91 prevents overheating or temperature increase of the non-conveyance span of the fixing belt 14 in the axial direction thereof where the sheet S is not conveyed.
The lateral end heaters 24a and 24b are energized as the large sheet S greater than the A3 size sheet is conveyed through the fixing nip N. The halogen heater 28b directed to heat each lateral end span of the fixing belt 14 in the axial direction thereof is energized simultaneously. FIG. 20 is a diagram of an electric circuit 92 as a first variation of the electric circuit 91 shown in FIG. 18. As shown in FIG. 20, the halogen heater 28b is connected in series to the lateral end heaters 24a and 24b. Power supply to the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b is controlled by switching a path with the switch SW1 and a switch SW4 so as to attain advantages similar to the advantages described above. Accordingly, the controller 99 depicted in FIG. 2 to control power supply to the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b is simplified. Since a temperature property of the halogen heater 28b is different from that of the lateral end heaters 24a and 24b, the temperature of the halogen heater 28b and the lateral end heaters 24a and 24b is adjusted by switching the path.
FIG. 21A is a diagram of the electric circuit 92 illustrating a first energization pattern. As shown in FIG. 21A, as a small sheet S smaller than the A3 size sheet is conveyed through the fixing nip N, the halogen heater 28a is energized. The switch SW4 is isolated from a terminal of the halogen heater 28b and a terminal of the lateral end heaters 24a and 24b. 
FIG. 21B is a diagram of the electric circuit 92 illustrating a second energization pattern. As shown in FIG. 21B, as the A3 size sheet is conveyed through the fixing nip N, the halogen heaters 28a and 28b are energized.
FIG. 21C is a diagram of the electric circuit 92 illustrating a third energization pattern. As shown in FIG. 21C, as a large sheet S greater than the A3 size sheet is conveyed through the fixing nip N, the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b are energized simultaneously.
FIGS. 18 and 20 illustrate the electric circuits 91 and 92 having an electric wiring that connects the halogen heater 28b to the lateral end heaters 24a and 24b in series. However, when the large sheet S is conveyed through the fixing nip N, the halogen heater 28a is also energized simultaneously. Accordingly, the electric wiring that connects the halogen heater 28a to the lateral end heaters 24a and 24b in series attains the similar advantages.
FIG. 22 is a diagram of an electric circuit 93 as a second variation of the electric circuit 91 shown in FIG. 18 illustrating an electric connection between the halogen heaters 28a and 28b and the lateral end heaters 24a and 24b. As shown in FIG. 22, the lateral end heaters 24a and 24b are connected in parallel to the halogen heater 28b. 
When the switch SW3 is turned off, the lateral end heaters 24a and 24b are connected to a negative electrode. Hence, no electric current flows in the lateral end heaters 24a and 24b. Conversely, when the switch SW3 is turned on, the lateral end heaters 24a and 24b are connected to a positive electrode. Accordingly, an electric current from the power supply 44 flows in each of the lateral end heaters 24a and 24b, causing the lateral end heaters 24a and 24b to generate heat.
Accordingly, if one of the lateral end heaters 24a and 24b suffers from short circuit, a fuse prevents the lateral end heaters 24a and 24b from being out of control by overflow of the electric current, improving safety.
The present disclosure is not limited to the details of the exemplary embodiments described above and various modifications and improvements are possible. The advantages achieved by the fixing devices 150 and 150S are not limited to those described above.
A description is provided of advantages of the nip formation assemblies 18 and 63.
As shown in FIGS. 2 and 16, a nip formation assembly (e.g., the nip formation assemblies 18 and 63) serving as a nip formation unit includes the flexible, endless fixing belt 14 serving as an endless belt or an endless fixing rotator formed into a loop and rotatable in a predetermined direction of rotation (e.g., the rotation direction D14); the pressure roller 16 serving as an opposed rotator disposed opposite the fixing belt 14; and the nip formation pad 22 disposed inside the loop formed by the fixing belt 14 to press against the pressure roller 16 via the fixing belt 14 to form the fixing nip N between the fixing belt 14 and the pressure roller 16. As a sheet S serving as a recording medium bearing a toner image is conveyed through the fixing nip N, the fixing belt 14 and the pressure roller 16 fix the toner image on the sheet S. The nip formation assembly further includes a fixing heater (e.g., the halogen heaters 28a and 28b) and a lateral end heater (e.g., the lateral end heaters 24a and 24b). The fixing heater is disposed opposite at least the center span of the fixing belt 14 in the axial direction thereof where the sheet S is conveyed to heat the fixing belt 14. The lateral end heater is mounted on the nip formation pad 22 and disposed opposite the inner circumferential surface of the fixing belt 14 at the lateral end span of the fixing belt 14 in the axial direction thereof to heat the fixing belt 14.
Accordingly, the lateral end heater heats the fixing belt 14 effectively while retaining stable rotation of the fixing belt 14. Consequently, the lateral end heater prevents residual toner failed to be fixed on the sheet S and therefore remaining on the fixing belt 14 from being melted again and adhered to the fixing belt 14.
Since the lateral end heater is disposed opposite the fixing nip N where the fixing belt 14 is exerted with pressure, the fixing belt 14 is exerted with pressure at the single place, that is, the fixing nip N. Thus, the nip formation assembly reduces degradation in rotation of the fixing belt 14.
As shown in FIGS. 6, 10A, 10B, 11B, and 12, the lateral end heaters 24a and 24b are disposed opposite both lateral end spans of the fixing belt 14 in the axial direction thereof, respectively, because the fixing device 150 employs the center conveyance system in which the sheet S is centered on the fixing belt 14 in the axial direction thereof. Alternatively, one of the lateral end heaters 24a and 24b may be eliminated if the fixing device 150 employs a lateral edge conveyance system in which the sheet S is conveyed in the sheet conveyance direction DS along one lateral edge of the fixing belt 14 in the axial direction thereof. In this case, another one of the lateral end heaters 24a and 24b is distal from the lateral edge of the fixing belt 14 in the axial direction thereof.
According to the exemplary embodiments described above, the fixing belt 14 serves as an endless belt. Alternatively, a fixing film, a fixing sleeve, or the like may be used as an endless belt. Further, the pressure roller 16 serves as an opposed rotator. Alternatively, a pressure belt or the like may be used as an opposed rotator.
The present disclosure has been described above with reference to specific exemplary embodiments. Note that the present disclosure is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.

## Claims

1. A nip formation assembly disposed opposite an opposed rotator via an endless belt, the nip formation assembly comprising:
a nip formation pad to press against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, the fixing nip through which a recording medium bearing a toner image is conveyed;
at least one fixing heater disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed, the fixing heater to heat the endless belt; and
a lateral end heater mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof, the lateral end heater to heat the endless belt, wherein
the fixing heater and the lateral end heater are separated in a pressurization direction in which the nip formation pad presses against the opposed rotator.

2. The nip formation assembly according to claim 1,
wherein the nip formation pad includes a nip formation face contacting the inner circumferential surface of the endless belt, and
wherein the lateral end heater includes an endless belt side face disposed opposite the inner circumferential surface of the endless belt, the endless belt side face being leveled with the nip formation face of the nip formation pad in the pressurization direction in which the nip formation pad presses against the opposed rotator.

3. The nip formation assembly according to claim 2, further comprising another lateral end heater mounted on the nip formation pad and disposed opposite another lateral end span of the inner circumferential surface of the endless belt in the axial direction thereof, the another lateral end heater to heat the endless belt,
wherein the nip formation face is sandwiched between the lateral end heater and the another lateral end heater in the axial direction of the endless belt.

4. The nip formation assembly according to claim 2, further comprising a slide sheet sandwiched between the inner circumferential surface of the endless belt and the nip formation face of the nip formation pad.

5. The nip formation assembly according to claim 2, wherein the endless belt side face of the lateral end heater contacts the inner circumferential surface of the endless belt and defines an extension of the nip formation face of the nip formation pad in the axial direction of the endless belt.

6. The nip formation assembly according to claim 2, wherein the endless belt side face of the lateral end heater is leveled with the nip formation face of the nip formation pad in the pressurization direction while the nip formation pad forms the fixing nip.

7. The nip formation assembly according to claim 1, wherein the nip formation pad includes a recess mounting the lateral end heater.

8. The nip formation assembly according to claim 7, further comprising a deformable elastic member supporting the lateral end heater and disposed in the recess, the elastic member being deformed as the nip formation pad presses the elastic member against the endless belt via the lateral end heater.

9. The nip formation assembly according to claim 1, wherein the lateral end heater partially overlaps the fixing heater in the axial direction of the endless belt.

10. The nip formation assembly according to claim 1, wherein the lateral end heater partially overlaps an outboard span of the fixing heater in the axial direction of the endless belt, and the outboard span is where a light distribution radiated from the fixing heater has a decreased density.

11. A fixing device comprising:
an endless belt rotatable in a predetermined direction of rotation;
an opposed rotator disposed opposite the endless belt;
a nip formation pad to press against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, the fixing nip through which a recording medium bearing a toner image is conveyed;
a fixing heater disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed, the fixing heater to heat the endless belt; and
a lateral end heater mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof, the lateral end heater to heat the endless belt, wherein
the fixing heater and the lateral end heater are separated in a pressurization direction in which the nip formation pad presses against the opposed rotator.

12. The fixing device according to claim 11,
wherein the nip formation pad includes a nip formation face contacting the inner circumferential surface of the endless belt, and
wherein the lateral end heater includes an endless belt side face disposed opposite the inner circumferential surface of the endless belt, the endless belt side face being leveled with the nip formation face of the nip formation pad in the pressurization direction in which the nip formation pad presses against the opposed rotator.

13. The fixing device according to claim 12, wherein the endless belt side face of the lateral end heater is leveled with the nip formation face of the nip formation pad in the pressurization direction while the nip formation pad forms the fixing nip.

14. The fixing device according to claim 12, further comprising another lateral end heater mounted on the nip formation pad and disposed opposite another lateral end span of the inner circumferential surface of the endless belt in the axial direction thereof, the another lateral end heater to heat the endless belt,
wherein the nip formation face is sandwiched between the lateral end heater and the another lateral end heater in the axial direction of the endless belt.

15. The fixing device according to claim 12, further comprising a slide sheet sandwiched between the inner circumferential surface of the endless belt and the nip formation face of the nip formation pad.

16. The fixing device according to claim 12, wherein the endless belt side face of the lateral end heater contacts the inner circumferential surface of the endless belt and defines an extension of the nip formation face of the nip formation pad in the axial direction of the endless belt.

17. An image forming apparatus comprising:
an image bearer to bear a toner image; and
a fixing device disposed downstream from the image bearer in a recording medium conveyance direction to fix the toner image on a recording medium,
the fixing device including:
an endless belt rotatable in a predetermined direction of rotation;
an opposed rotator disposed opposite the endless belt;
a nip formation pad to press against the opposed rotator via the endless belt to form a fixing nip between the endless belt and the opposed rotator, the fixing nip through which the recording medium bearing the toner image is conveyed;
a fixing heater disposed opposite at least a center span of the endless belt in an axial direction thereof where the recording medium is conveyed, the fixing heater to heat the endless belt;
a lateral end heater mounted on the nip formation pad and disposed opposite a lateral end span of an inner circumferential surface of the endless belt in the axial direction thereof, the lateral end heater to heat the endless belt; and
an electric circuit to selectively energize the fixing heater and the lateral end heater, wherein
the fixing heater and the lateral end heater are separated in a pressurization direction in which the nip formation pad presses against the opposed rotator.

18. The image forming apparatus according to claim 17, further comprising a temperature sensor disposed opposite the endless belt to detect a temperature of the endless belt,
wherein the electric circuit energizes the fixing heater in accordance with increase in the temperature of the lateral end span of the endless belt that is detected by the temperature sensor while the electric circuit energizes the lateral end heater.

19. The image forming apparatus according to claim 17, wherein the endless belt and the opposed rotator convey the recording medium having an increased width in the axial direction of the endless belt at a decreased conveyance speed while the electric circuit energizes the lateral end heater.

20. The image forming apparatus according to claim 17, wherein the electric circuit energizes the fixing heater separately from the lateral end heater.

21. The nip formation assembly according to claim 1, wherein the fixing heater includes a radiation heater.

22. The nip formation assembly according to claim 21, wherein the fixing heater includes two heaters.

23. The nip formation assembly according to claim 1, wherein the lateral end heater includes a heat generator mounted on a face of the lateral end heater that faces away from the fixing nip.

24. The nip formation assembly according to claim 23, wherein the heat generator is isolated from the endless belt with an interval between the heat generator and the endless belt, which prevents power supplied to the lateral end heater from being transmitted to the endless belt.

25. The nip formation assembly according to claim 23, wherein the endless belt is made of metal.

26. The nip formation assembly according to claim 25, wherein the metal of the endless belt includes nickel.

27. The nip formation assembly according to claim 11, wherein the lateral end heater includes a heat generator mounted on a face of the lateral end heater that faces away from the fixing nip.

28. The nip formation assembly according to claim 27, wherein the heat generator is isolated from the endless belt with an interval between the heat generator and the endless belt, which prevents power supplied to the lateral end heater from being transmitted to the endless belt.

29. The nip formation assembly according to claim 27, wherein the endless belt is made of metal.

30. The nip formation assembly according to claim 29, wherein the metal of the endless belt includes nickel.

31. The nip formation assembly according to claim 17, wherein the lateral end heater includes a heat generator mounted on a face of the lateral end heater that faces away from the fixing nip.

32. The nip formation assembly according to claim 31, wherein the heat generator is isolated from the endless belt with an interval between the heat generator and the endless belt, which prevents power supplied to the lateral end heater from being transmitted to the endless belt.

33. The nip formation assembly according to claim 31, wherein the endless belt is made of metal.

34. The nip formation assembly according to claim 33, wherein the metal of the endless belt includes nickel.