Patent Publication Number: US-2016246228-A1

Title: Fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2015-032685, filed on Feb. 23, 2015, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Exemplary aspects of the present disclosure relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device. 
     2. 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 a pressure rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium 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 pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. 
     SUMMARY 
     This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and a pressure rotator pressed against the fixing rotator to form a fixing nip therebetween, through which a recording medium bearing a toner image is conveyed. A first heater is disposed opposite an inner circumferential surface of the fixing rotator to heat the fixing rotator. The first heater includes a first heat generator having a first heating span in a longitudinal direction of the first heater to generate heat and a first support supporting the first heat generator. A second heater is disposed opposite the inner circumferential surface of the fixing rotator to heat the fixing rotator. The second heater includes a second heat generator having a second heating span in a longitudinal direction of the second heater to generate heat and a second support supporting the second heat generator. A screen is interposed between the first heater and the second heater to screen one of the first heater and the second heater from another one of the first heater and the second heater. At least one of the first support and the second support is disposed in proximity to a lateral end of at least one of the first heat generator and the second heat generator in the longitudinal direction of the first heater and the second heater. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image bearer to bear a toner image and a fixing device disposed downstream from the image bearer in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and a pressure rotator pressed against the fixing rotator to form a fixing nip therebetween, through which the recording medium bearing the toner image is conveyed. A first heater is disposed opposite an inner circumferential surface of the fixing rotator to heat the fixing rotator. The first heater includes a first heat generator having a first heating span in a longitudinal direction of the first heater to generate heat and a first support supporting the first heat generator. A second heater is disposed opposite the inner circumferential surface of the fixing rotator to heat the fixing rotator. The second heater includes a second heat generator having a second heating span in a longitudinal direction of the second heater to generate heat and a second support supporting the second heat generator. A screen is interposed between the first heater and the second heater to screen one of the first heater and the second heater from another one of the first heater and the second heater. At least one of the first support and the second support is disposed in proximity to a lateral end of at least one of the first heat generator and the second heat generator in the longitudinal direction of the first heater and the second 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 according to an exemplary embodiment of the present disclosure that is incorporated in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a plan view of a comparative halogen heater; 
         FIG. 4A  is a partial horizontal sectional view of the fixing device shown in  FIG. 2  illustrating a first halogen heater and a second halogen heater; 
         FIG. 4B  is a partial vertical sectional view of the fixing device shown in  FIG. 4A ; 
         FIG. 5  is a schematic vertical sectional view of a fixing device according to another exemplary embodiment of the present disclosure; 
         FIG. 6A  is a partial horizontal sectional view of the fixing device shown in  FIG. 5 ; 
         FIG. 6B  is a partial vertical sectional view of the fixing device shown in  FIG. 6A ; 
         FIG. 7A  is a partial horizontal sectional view of a fixing device as a modification example of the fixing devices shown in  FIGS. 2 and 5 ; 
         FIG. 7B  is a partial vertical sectional view of the fixing device shown in  FIG. 7A ; 
         FIG. 8A  is a partial horizontal sectional view of a fixing device as another modification example of the fixing devices shown in  FIGS. 2 and 5 ; and 
         FIG. 8B  is a partial vertical sectional view of the fixing device shown in  FIG. 8A . 
     
    
    
     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  1  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  1 . The image forming apparatus  1  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  1  is a monochrome printer that forms a monochrome toner image on a recording medium by electrophotography. Alternatively, the image forming apparatus  1  may be a color printer that forms a color toner image on a recording medium. 
     As shown in  FIG. 1 , the image forming apparatus  1  includes a sheet feeder  4 , a registration roller pair  6 , a photoconductive drum  8  serving as an image bearer, a transfer device  10 , and a fixing device  12 . 
     The sheet feeder  4  includes a paper tray  14  that loads a plurality of sheets P serving as recording media and a feed roller  16  that picks up an uppermost sheet P from the plurality of sheets P loaded on the paper tray  14  and feeds the uppermost sheet P to the registration roller pair  6 . The registration roller pair  6  temporarily halts the sheet P sent from the feed roller  16  to correct skew of the sheet P and conveys the sheet P to a transfer nip formed between the photoconductive drum  8  and the transfer device  10  at a time in synchronism with rotation of the photoconductive drum  8 , that is, at a time when a leading edge of a toner image formed on the photoconductive drum  8  corresponds to a predetermined position in a leading end of the sheet P in a sheet conveyance direction DP. 
     The photoconductive drum  8  is surrounded by a charging roller  18  serving as a charger, a mirror  20  constituting a part of an exposure device, a developing device  22  incorporating a developing roller  22   a , the transfer device  10 , and a cleaner  24  incorporating a cleaning blade  24   a , which are disposed in this order clockwise in  FIG. 1  in a rotation direction D 8  of the photoconductive drum  8 . A light beam Lb reflected by the mirror  20  irradiates and scans the photoconductive drum  8  at an exposure position  26  thereon interposed between the charging roller  18  and the developing device  22  in the rotation direction D 8  of the photoconductive drum  8 . 
     As the photoconductive drum  8  starts rotating, the charging roller  18  uniformly charges an outer circumferential surface of the photoconductive drum  8 . The exposure device emits a light beam Lb onto the charged outer circumferential surface of the photoconductive drum  8  at the exposure position  26  thereon according to image data sent from an external device such as a client computer, thus forming an electrostatic latent image on the photoconductive drum  8 . The electrostatic latent image formed on the photoconductive drum  8  moves in accordance with rotation of the photoconductive drum  8  to an opposed position thereon disposed opposite the developing device  22  where the developing device  22  supplies toner to the electrostatic latent image on the photoconductive drum  8 , visualizing the electrostatic latent image as a toner image. As the toner image formed on the photoconductive drum  8  reaches the transfer nip, the toner image is transferred onto a sheet P conveyed from the paper tray  14  and entering the transfer nip at a predetermined time by a transfer bias applied by the transfer device  10 . The sheet P bearing the toner image is conveyed to the fixing device  12  where a fixing belt  30  and a pressure roller  28  fix the toner image on the sheet P under heat and pressure. Thereafter, the sheet P bearing the fixed toner image is ejected onto an output tray disposed outside a body of the image forming apparatus  1  to stack the sheet P. 
     As residual toner failed to be transferred onto the sheet P at the transfer nip and therefore remaining on the photoconductive drum  8  moves under the cleaner  24  in accordance with rotation of the photoconductive drum  8 , the cleaning blade  24   a  scrapes the residual toner off the photoconductive drum  8 , thus cleaning the photoconductive drum  8 . Thereafter, a discharger disposed opposite the photoconductive drum  8  removes residual potential on the photoconductive drum  8 , rendering the photoconductive drum  8  to be ready for a next image forming operation. 
     With reference to  FIG. 2 , a description is provided of a construction of the fixing device  12  incorporated in the image forming apparatus  1  having the construction described above. 
       FIG. 2  is a schematic vertical sectional view of the fixing device  12 . As shown in  FIG. 2 , the fixing device  12  (e.g., a fuser or a fusing unit) includes the pressure roller  28  serving as a pressure rotator rotatable in a rotation direction D 28  and the flexible fixing belt  30  serving as a fixing rotator or an endless belt formed into a loop and rotatable in a rotation direction D 30 . Inside the loop formed by the fixing belt  30  are two halogen heaters, that is, a first halogen heater  34  and a second halogen heater  36  serving as a heater or a heat source to heat the fixing belt  30  with light irradiating an inner circumferential surface of the fixing belt  30 . A screen  32  is interposed between the first halogen heater  34  and the second halogen heater  36  to screen the first halogen heater  34  from the second halogen heater  36  and screen the second halogen heater  36  from the first halogen heater  34 . 
     The first halogen heater  34  and the second halogen heater  36  have different heating spans or different heat distributions, respectively, in a longitudinal direction of the first halogen heater  34  and the second halogen heater  36  parallel to an axial direction of the fixing belt  30 . For example, the first halogen heater  34  is a center heater having a center heating span of 217 mm in the longitudinal direction of the first halogen heater  34  and being transferred with electric power of 770 W. The second halogen heater  36  is a lateral end heater having a lateral end heating span of 63 mm at each lateral end of the second halogen heater  36  in the longitudinal direction thereof, thus having a combined heating span of 126 mm. The second halogen heater  36  is transferred with electric power of 440 W. Each of the first halogen heater  34  and the second halogen heater  36  includes a filament light emitter and a sealing. Since the screen  32  screens the first halogen heater  34  from the second halogen heater  36 , the screen  32  prevents the filament light emitter of one of the first halogen heater  34  and the second halogen heater  36  from heating the sealing of another one of the first halogen heater  34  and the second halogen heater  36 . 
     A nip formation pad  38  is disposed inside the loop formed by the fixing belt  30  and disposed opposite the inner circumferential surface of the fixing belt  30 . The nip formation pad  38  is disposed opposite the pressure roller  28  via the fixing belt  30  to form a fixing nip N between the fixing belt  30  and the pressure roller  28 . As the fixing belt  30  rotates in the rotation direction D 30 , the inner circumferential surface of the fixing belt  30  slides over the nip formation pad  38  directly or indirectly via a slide sheet (e.g., a low-friction sheet). As shown in  FIG. 2 , the fixing nip N is planar. Alternatively, the fixing nip N may be contoured into a recess or other shapes. If the fixing nip N defines a recess, the recessed fixing nip N directs a leading edge of the sheet P toward the pressure roller  28  as the sheet P is ejected from the fixing nip N, facilitating separation of the sheet P from the fixing belt  30  and suppressing jamming of the sheet P between the fixing belt  30  and the pressure roller  28 . 
     A detailed description is now given of a construction of the pressure roller  28 . 
     The pressure roller  28  separably contacts an outer circumferential surface of the fixing belt  30 . The pressure roller  28  is constructed of a cored bar  28   a , an elastic rubber layer  28   b  coating the cored bar  28   a , and a surface release layer coating the elastic rubber layer  28   b  and made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) or polytetrafluoroethylene (PTFE) to facilitate separation of the sheet P from the pressure roller  28 . As a driving force generated by a driver (e.g., a motor) situated inside the image forming apparatus  1  depicted in  FIG. 1  is transmitted to the pressure roller  28  through a gear train, the pressure roller  28  rotates in the rotation direction D 28  as shown in  FIG. 2 . Alternatively, the driver may also be connected to the fixing belt  30  to drive and rotate the fixing belt  30 . A spring or the like presses the pressure roller  28  against the nip formation pad  38  via the fixing belt  30 . As the spring presses and deforms the elastic rubber layer  28   b  of the pressure roller  28 , the pressure roller  28  produces the fixing nip N having a predetermined length in the sheet conveyance direction DP. A pressurization assembly (e.g., a spring) that presses the pressure roller  28  against the nip formation pad  38  via the fixing belt  30  releases pressure between the pressure roller  28  and the fixing belt  30  as needed to separate the pressure roller  28  from the fixing belt  30 . 
     The pressure roller  28  may be a hollow roller. If the pressure roller  28  is a hollow roller, a heater such as a halogen heater may be disposed inside the hollow roller. The elastic rubber layer  28   b  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  28 , the elastic rubber layer  28   b  may be made of sponge rubber. The sponge rubber has an increased insulation that draws less heat from the fixing belt  30  compared to the solid rubber. 
     A detailed description is now given of a construction of the fixing belt  30 . 
     The fixing belt  30  is a thin belt or film made of metal such as nickel and SUS stainless steel or resin such as polyimide. The fixing belt  30  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 T on the sheet P from the fixing belt  30 , thus preventing the toner of the toner image T from adhering to the fixing belt  30 . 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  30  does not incorporate the elastic layer, the fixing belt  30  has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image T is fixed on the sheet P properly. However, as the pressure roller  28  and the fixing belt  30  sandwich and press the unfixed toner image T on the sheet P passing through the fixing nip N, slight surface asperities of the fixing belt  30  may be transferred onto the toner image T on the sheet P, resulting in variation in gloss of the solid toner image T that may appear as an orange peel image on the sheet P. 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  30 , preventing formation of the faulty orange peel image. 
     As the pressure roller  28  rotates in the rotation direction D 28 , the fixing belt  30  rotates in the rotation direction D 30  in accordance with rotation of the pressure roller  28  by friction therebetween. At the fixing nip N, the fixing belt  30  rotates as it is sandwiched between the pressure roller  28  and the nip formation pad  38 ; at a circumferential span of the fixing belt  30  other than the fixing nip N, the fixing belt  30  rotates as it is guided by a flange serving as a holder mounted on a side plate of the fixing device  12  or located inside a frame of the fixing device  12  at each lateral end of the fixing belt  30  in the axial direction thereof. 
     A support  40  (e.g., a stay) that supports the nip formation pad  38  is situated inside the loop formed by the fixing belt  30  and disposed opposite the inner circumferential surface of the fixing belt  30 . As the nip formation pad  38  receives pressure from the pressure roller  28 , the support  40  supports the nip formation pad  38  to prevent bending of the nip formation pad  38  and produce an even nip length in the sheet conveyance direction DP throughout the entire width of the fixing belt  30  in the axial direction thereof. The support  40  is mounted on and held by the side plate of the fixing device  12  or the flange serving as the holder at each lateral end of the support  40  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  30 , thus being positioned inside the fixing device  12 . 
     A reflector  42  is interposed between the two heaters (e.g., the first halogen heater  34  and the second halogen heater  36 ) and the support  40  to reflect light radiated from the first halogen heater  34  and the second halogen heater  36  to the reflector  42  toward the fixing belt  30 , preventing the support  40  from being heated by the first halogen heater  34  and the second halogen heater  36  with radiation heat and thereby reducing waste of energy. Alternatively, instead of installation of the reflector  42 , an opposed face of the support  40  disposed opposite the first halogen heater  34  and the second halogen heater  36  may be treated with insulation or mirror finish to reflect light radiated from the first halogen heater  34  and the second halogen heater  36  to the support  40  toward the fixing belt  30 . 
     The screen  32  is mounted on and held by the side plate of the fixing device  12  or the flange serving as the holder at each lateral end of the screen  32  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  30 . A surface of the screen  32  is also treated with insulation or mirror finish to reflect light radiated from the first halogen heater  34  and the second halogen heater  36  to the screen  32  toward the fixing belt  30 . The fixing belt  30  and the components disposed inside the loop formed by the fixing belt  30 , that is, the screen  32 , the first halogen heater  34 , the second halogen heater  36 , the nip formation pad  38 , the support  40 , and the reflector  42 , may constitute a belt unit  30 U separably coupled with the pressure roller  28 . 
     A description is provided of a construction of a comparative fixing device. 
     The comparative fixing device is requested to shorten a warm-up time defining a time taken to warm up the comparative fixing device from an ambient temperature to a predetermined temperature (e.g., a reload temperature) at which printing is available after an image forming apparatus incorporating the comparative fixing device is powered on and reduce power consumption. To address those requests, the comparative fixing device shortens a waiting time for a user to wait until printing starts after the image forming apparatus is powered on and decreases power consumption during a standby time to wait for a print job. The comparative fixing device may include a heater, such as a ceramic heater and a halogen heater, to heat a thin fixing belt directly. The heater configured to heat the fixing belt directly increases the temperature of the fixing belt quickly. However, a non-conveyance span of the fixing belt where a sheet is not conveyed may overheat because the sheet does not draw heat from the non-conveyance span of the fixing belt. To address this circumstance, a plurality of heaters having different heating spans or different heat distributions, respectively, may be employed. The plurality of heaters is energized according to the size of a sheet, thus heating sheets of various sizes. 
     If the comparative fixing device includes a plurality of halogen heaters disposed inside a loop formed by the fixing belt, the halogen heaters may be screened from each other to prevent light emitted from one of the halogen heaters from heating a glass tube of another one of the halogen heaters. For example, the plurality of halogen heaters disposed inside the loop formed by an endless heating belt is disposed opposite each other via a reinforcement of a pressure pad that presses the heating belt against a pressure roller. The reinforcement is coupled with a reflector disposed opposite the halogen heaters. An inner circumferential surface of the heating belt slides over a guide of the pressure pad. The reinforcement projects from the guide beyond the halogen heaters. The reinforcement prevents the halogen heaters from heating the glass tubes thereof each other with light, reducing an amount of heat absorbed by the glass tubes to heat the heating belt effectively so as to shorten the warm-up time and prevent the shortened life of the halogen heaters due to overheating. 
     However, the glass tube has an increased thermal capacity compared to the thin fixing belt. Accordingly, the glass tube is requested to be shortened to eliminate redundancy. 
     A description is provided of a construction of a comparative halogen heater  33 C. 
       FIG. 3  is a plan view of the comparative halogen heater  33 C. As shown in  FIG. 3 , the comparative halogen heater  33 C includes a filament light emitter  44 ; a glass tube  46  accommodating the filament light emitter  44  and containing filler gas; and a sealing  48  sealing the glass tube  46 . The filament light emitter  44  serving as a heat generator includes a tungsten wire coiled at a predetermined interval. The number of coiling of the tungsten wire, the interval of coiling, and the like define the light emission intensity. The filler gas is produced by mixing a slight amount of a halogen substance such as iodine and bromine with inert gas such as nitrogen. The filler gas suppresses thermal vaporization of the tungsten wire, extending the life of the tungsten wire. The sealing  48  serving as a support that supports the heat generator includes a molybdenum foil  50  and a cement portion  52  that seal the glass tube  46 . 
     As the sealing  48  overheats, sealing of the sealing  48  is destroyed, causing faulty power supply and leakage of the filler gas that may shorten the life of the comparative halogen heater  33 C. To address this circumstance, the temperature of the sealing  48  is 35 degrees centigrade or lower. 
     In order to heat the fixing belt  30  evenly throughout the entire width of the fixing belt  30  in the axial direction thereof, the filament light emitter  44  of one comparative halogen heater  33 C is disposed opposite the filament light emitter  44  of another comparative halogen heater  33 C with a decreased interval therebetween. However, unless the screen  32  depicted in  FIG. 2  is interposed between the two comparative halogen heaters  33 C, the sealing  48  disposed outboard from the filament light emitter  44  in a longitudinal direction of the comparative halogen heater  33 C may be heated. That is, if the glass tube  46  of each comparative halogen heater  33 C is shortened without the screen  32 , the sealing  48  may suffer from heating. To address this circumstance, the glass tube  46  is elongated to each lateral end of the fixing belt  30  that is outboard from the filament light emitter  44  in the axial direction of the fixing belt  30  to prevent the filament light emitter  44  of one comparative halogen heater  33 C from heating the sealing  48  of another comparative halogen heater  33 C. However, the elongated glass tube  46  may increase waste of heat. 
     A description is provided of a construction of the first halogen heater  34  and the second halogen heater  36  of the fixing device  12  to address the above-described circumstances of the comparative halogen heater  33 C. 
       FIG. 4A  is a partial horizontal sectional view of the fixing device  12  illustrating the first halogen heater  34  and the second halogen heater  36 .  FIG. 4B  is a partial vertical sectional view of the fixing device  12  that is perpendicular to the horizontal sectional view shown in  FIG. 4A . 
     As shown in  FIG. 4A , the first halogen heater  34  serving as the center heater is disposed opposite a center span of the fixing belt  30  in the axial direction thereof. A heater holder  60  serving as a first heater holder situated inside the loop formed by the fixing belt  30  fixedly secures the first halogen heater  34  inside the loop formed by the fixing belt  30 . Conversely, the second halogen heater  36  serving as the lateral end heater is disposed opposite each lateral end span of the fixing belt  30  in the axial direction thereof. For example, two separate halogen heaters, that is, a first halogen heater and a second halogen heater, situated at both lateral ends of the fixing belt  30  in the axial direction thereof are electrically connected in series to constitute the second halogen heater  36 . The heater holder  60  serving as a second heater holder mounted on each side plate of the fixing device  12  or each flange serving as the holder and the heater holder  60  serving as a second heater holder situated inside the loop formed by the fixing belt  30  fixedly secure the second halogen heater  36  inside the loop formed by the fixing belt  30 . A surface of the heater holder  60  is treated with insulation or mirror finish to enhance heating efficiency for heating the fixing belt  30 . 
     As shown in  FIG. 4A , the first halogen heater  34  to heat the center span of the fixing belt  30  in the axial direction thereof includes a glass tube  70  not disposed opposite each lateral end span of the fixing belt  30  in the axial direction thereof. Conversely, the second halogen heater  36  to heat each lateral end span of the fixing belt  30  in the axial direction thereof includes a glass tube  71  not disposed opposite the center span of the fixing belt  30  in the axial direction thereof. 
     The first halogen heater  34  further includes the filament light emitter  44  serving as a heat generator coiled to generate heat and the sealing  48  serving as a support. The glass tube  70  spans substantially a heat generation span of the filament light emitter  44  in the longitudinal direction of the first halogen heater  34 . The sealing  48  is disposed in proximity to each lateral end of the filament light emitter  44  in the longitudinal direction of the first halogen heater  34 . Similarly, the second halogen heater  36  further includes the filament light emitter  44  serving as a heat generator coiled to generate heat and the sealing  48  serving as a support. The glass tube  71  spans substantially a heat generation span of the filament light emitter  44  in the longitudinal direction of the second halogen heater  36 . The sealing  48  is disposed in proximity to each lateral end of the filament light emitter  44  in the longitudinal direction of the second halogen heater  36 . 
     In order to shorten the warm-up time, the thin fixing belt  30  has a thickness in a range of from about 0.1 mm to about 0.3 mm. Contrarily, each of the glass tubes  70  and  71  has a thickness of 1 mm to enhance the mechanical strength and thus is thick compared to the fixing belt  30 . Accordingly, each of the glass tubes  70  and  71  has an increased thermal capacity compared to the fixing belt  30 . To address this circumstance, the volume of the glass tubes  70  and  71 , that is, the length of the glass tubes  70  and  71  in the longitudinal direction of the first halogen heater  34  and the second halogen heater  36 , respectively, is decreased to reduce an amount of heat absorbed and consumed by the glass tubes  70  and  71  so that the first halogen heater  34  and the second halogen heater  36  heat the fixing belt  30  effectively. 
     As shown in  FIGS. 4A and 4B , the glass tubes  70  and  71  are heated by light emitted from the filament light emitters  44  and reflected by the reflector  42  and the screen  32  to irradiate the glass tubes  70  and  71  and by heat conducted from the heated fixing belt  30  to the glass tubes  70  and  71  through air (e.g., convection). The shortened glass tubes  70  and  71  advantageously prevent themselves from being heated by such light and heat. The first halogen heater  34  and the second halogen heater  36  having the shortened glass tubes  70  and  71 , respectively, and being screened by the screen  32  prevent unnecessary absorption of heat. 
     A description is provided of a construction of a fixing device  62  according to another exemplary embodiment. 
       FIG. 5  is a schematic vertical sectional view of the fixing device  62 . Like the fixing device  12  depicted in  FIG. 2 , the fixing device  62  includes a pressure roller  78  serving as a pressure rotator; an endless fixing belt  80  serving as an endless belt or a fixing rotator formed into a loop; and a nip formation pad  88  disposed inside the loop formed by the fixing belt  80  and disposed opposite an inner circumferential surface of the fixing belt  80  to press against the pressure roller  78  via the fixing belt  80  to form a fixing nip N between the fixing belt  80  and the pressure roller  78 . 
     A support  90  (e.g., a stay) supporting the nip formation pad  88  is mounted on and held by a side plate of the fixing device  62  or a flange serving as a holder at each lateral end of the support  90  in a longitudinal direction thereof parallel to an axial direction of the fixing belt  80 . The support  90  is elongated vertically in  FIG. 5  and T-shaped in cross-section. The support  90  divides an interior inside the loop formed by the fixing belt  80  into a first compartment accommodating a first halogen heater  84  and a second compartment accommodating a second halogen heater  86 , preventing the first halogen heater  84  from heating a glass tube of the second halogen heater  86  and preventing the second halogen heater  86  from heating a glass tube of the first halogen heater  84 . Thus, the support  90  also serves as a screen that screens the first halogen heater  84  and the second halogen heater  86  from each other. 
     A stand of the support  90  mounts a reflector  92  at both opposed faces of the support  90  disposed opposite the first halogen heater  84  and the second halogen heater  86 , respectively. The reflector  92  shields the support  90  from radiation heat or light from the first halogen heater  84  and the second halogen heater  86 . Since the components of the fixing device  62  are equivalent in construction to the components of the fixing device  12  depicted in  FIG. 2 , a detailed description of the components of the fixing device  62  is omitted. 
     The first halogen heater  84  and the second halogen heater  86  have different heating spans or different heat distributions, respectively, in a longitudinal direction of the first halogen heater  84  and the second halogen heater  86  parallel to the axial direction of the fixing belt  80 . For example, the first halogen heater  84  is a center heater disposed opposite a center span of the fixing belt  80  in the axial direction thereof. The second halogen heater  86  is a lateral end heater disposed opposite each lateral end span of the fixing belt  80  in the axial direction thereof. 
       FIG. 6A  is a partial horizontal sectional view of the fixing device  62 .  FIG. 6B  is a partial vertical sectional view of the fixing device  62 . 
     As shown in  FIG. 6A , the first halogen heater  84  serving as the center heater is disposed opposite the center span of the fixing belt  80  in the axial direction thereof. A heater holder  61  serving as a first heater holder situated inside the loop formed by the fixing belt  80  fixedly secures the first halogen heater  84  inside the loop formed by the fixing belt  80 .  FIG. 6A  is a plan view of the fixing device  62  illustrating the first halogen heater  84  and the second halogen heater  86 . Conversely, the second halogen heater  86  serving as the lateral end heater is disposed opposite each lateral end span of the fixing belt  80  in the axial direction thereof. For example, two separate halogen heaters, that is, a first halogen heater and a second halogen heater, situated at both lateral ends of the fixing belt  80  in the axial direction thereof are electrically connected in series to constitute the second halogen heater  86 . The heater holder  61  serving as a second heater holder mounted on each side plate  91  of the fixing device  62  and the heater holder  61  serving as a second heater holder situated inside the loop formed by the fixing belt  80  fixedly secure the second halogen heater  86  inside the loop formed by the fixing belt  80 . 
     A description is provided of a construction of a fixing device  12 S as a modification example of the fixing devices  12  and  62  depicted in  FIGS. 2 and 5 , respectively. 
       FIG. 7A  is a partial horizontal sectional view of the fixing device  12 S.  FIG. 7B  is a partial vertical sectional view of the fixing device  12 S. 
     As shown in  FIG. 7A , in addition to the components shown in  FIG. 6A , the fixing device  12 S includes a sealing reflector  94  interposed between the filament light emitter  44  and the sealing  48  of each of the first halogen heater  84  and the second halogen heater  86  in the longitudinal direction of the first halogen heater  84  and the second halogen heater  86  to screen the sealing  48  from the filament light emitter  44 . The sealing reflector  94  reflects light emitted from the filament light emitter  44  to prevent the sealing  48  from being heated. The sealing reflector  94  includes an aluminum mirror plate or the like that has an increased reflectance. The sealing reflector  94  is interposed between the filament light emitter  44  and the sealing  48  in the longitudinal direction of the first halogen heater  84  and the second halogen heater  86  to screen the sealing  48  from the filament light emitter  44 , thus preventing light emitted from the filament light emitter  44  from heating the sealing  48 . The sealing reflector  94  suppresses temperature increase of the sealing  48 , extending the life of the first halogen heater  84  and the second halogen heater  86 . The sealing reflector  94  is also applicable to the first halogen heater  34  and the second halogen heater  36  depicted in  FIG. 4A . 
     A description is provided of a construction of a fixing device  12 T as another modification example of the fixing devices  12  and  62  depicted in  FIGS. 2 and 5 , respectively. 
       FIG. 8A  is a partial horizontal sectional view of the fixing device  12 T.  FIG. 8B  is a partial vertical sectional view of the fixing device  12 T. 
     As shown in  FIG. 8A , in addition to the components shown in  FIG. 7A , the fixing device  12 T includes a plurality of contact thermistors  96   a  and  96   b  that contacts the inner circumferential surface of the fixing belt  30  and does not overlap the first halogen heater  34  and the second halogen heater  36  in the longitudinal direction thereof. For example, the contact thermistor  96   a  serving as a first temperature detector is disposed outboard from the sealing  48  of the first halogen heater  34  serving as the center heater in the longitudinal direction thereof. The contact thermistor  96   b  serving as a second temperature detector is interposed between two pieces of the center sealing  48  of the second halogen heater  36  serving as the lateral end heater. 
     The two pieces of the center sealing  48  are disposed opposite the center span of the fixing belt  30  in the axial direction thereof. The heater holder  61  and the sealing reflector  94  screen the contact thermistors  96   a  and  96   b  from light emitted from the filament light emitter  44 , preventing the contact thermistors  96   a  and  96   b  from being heated by the first halogen heater  34  and the second halogen heater  36  directly, enhancing heat resistance of the fixing device  12 T. 
     Additionally, the contact thermistors  96   a  and  96   b  do not overlap the glass tubes  70  and  71 , respectively, in the longitudinal direction of the first halogen heater  34  and the second halogen heater  36 , enhancing flexibility in installation of the contact thermistors  96   a  and  96   b  and attaining stable contact of the contact thermistors  96   a  and  96   b  with the inner circumferential surface of the fixing belt  30 . 
     Contact thermistors manufactured at reduced costs are typically installed in fixing devices. However, if the contact thermistor contacts the outer circumferential surface of the fixing belt  30 , the contact thermistor may produce streaks on the outer circumferential surface of the fixing belt  30 . The streaks may be transferred to a toner image T on a sheet P conveyed over the fixing belt  30 , forming the faulty toner image T on the sheet P. To address this circumstance, the contact thermistor may contact the inner circumferential surface of the fixing belt  30 . 
     However, if the contact thermistor is installed in a fixing device employing a halogen heater, it is requested to locate the contact thermistor at a position where the contact thermistor is not heated by light emitted from the halogen heater, degrading flexibility in installation of the contact thermistor. For example, if the contact thermistor is installed in a fixing device employing a plurality of halogen heaters, it is difficult to locate the contact thermistor at the position where the contact thermistor is in contact with the inner circumferential surface of the fixing belt  30  and is not heated by light emitted from the halogen heaters. 
     Although the fixing device  12 T depicted in  FIG. 8A  includes the plurality of halogen heaters, that is, the first halogen heater  34  and the second halogen heater  36 , the glass tubes  70  and  71  are shortened in the longitudinal direction of the first halogen heater  34  and the second halogen heater  36 , sparing a space where the contact thermistors  96   a  and  96   b  contact the inner circumferential surface of the fixing belt  30 . 
     According to the exemplary embodiments described above, the fixing devices  12 ,  62 ,  12 S, and  12 T depicted in  FIGS. 2, 5, 7A, and 8A  include the two halogen heaters (e.g., the first halogen heater  34  and the second halogen heater  36  or the first halogen heater  84  and the second halogen heater  86 ). Alternatively, the fixing devices  12 ,  62 ,  12 S, and  12 T may include three or more halogen heaters having different heating spans or different heat distributions, respectively, in a longitudinal direction thereof as a plurality of heaters that heats the fixing belt  30  or  80 . The fixing devices  12 ,  62 ,  12 S, and  12 T may include a plurality of heaters of other types, such as ceramic heaters, which generates radiation heat. A screen (e.g., the screen  32  and the support  90 ) screens the plurality of heaters from each other so that the plurality of heaters does not thermally affect each other with heat radiated from the plurality of heaters. At least one of the glass tubes  70  and  71  of the plurality of heaters is shortened to suppress unnecessary absorption of heat. Hence, various modifications are possible in view of convenience in design and maintenance. 
     A description is provided of advantages of the fixing devices  12 ,  62 ,  12 S, and  12 T. 
     As shown in  FIGS. 2, 4A, 5, 6A, 7A, and 8A , a fixing device (e.g., the fixing devices  12 ,  62 ,  12 S, and  12 T) includes a fixing rotator or an endless belt (e.g., the fixing belts  30  and  80 ) rotatable in a predetermined direction of rotation (e.g., the rotation direction D 30 ); a pressure rotator (e.g., the pressure rollers  28  and  78 ) pressed against an outer circumferential surface of the fixing rotator; a plurality of heaters including a first heater (e.g., the first halogen heaters  34  and  84 ) and a second heater (e.g., the second halogen heaters  36  and  86 ) disposed opposite an inner circumferential surface of the fixing rotator; a nip formation pad (e.g., the nip formation pads  38  and  88 ) disposed opposite the inner circumferential surface of the fixing rotator to press against the pressure rotator via the fixing rotator to form the fixing nip N between the fixing rotator and the pressure rotator; and a screen (e.g., the screen  32  and the support  90 ) interposed between the first heater and the second heater to screen one of the first heater and the second heater from another one of the first heater and the second heater. 
     The first heater includes a first heat generator (e.g., the filament light emitter  44 ) having a first heating span in a longitudinal direction of the first heater to generate heat and a first support (e.g., the sealing  48 ) that supports the first heat generator. The second heater includes a second heat generator (e.g., the filament light emitter  44 ) having a second heating span in a longitudinal direction of the second heater to generate heat and a second support (e.g., the sealing  48 ) that supports the second heat generator. The first heating span of the first heat generator is different from the second heating span of the second heat generator in the longitudinal direction of the first heater and the second heater. At least one of the first support and the second support is disposed in proximity to a lateral end of at least one of the first heat generator and the second heat generator in the longitudinal direction of the first heater and the second heater. 
     Accordingly, the first heater and the second heater do not heat each other and do not heat a part of the first heater and the second heater unnecessarily, preventing the shortened life of the first heater and the second heater. 
     Additionally, the first heater and the second heater having the different heating spans or the different heat distributions, respectively, reduce unnecessary heat absorption, saving energy. 
     According to the exemplary embodiments described above, the fixing belts  30  and  80  serve as a fixing rotator. Alternatively, a fixing roller, a fixing film, a fixing sleeve, or the like may be used as a fixing rotator. Further, the pressure rollers  28  and  78  serve as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The present disclosure has been described above with reference to specific exemplary embodiments. Note that the present disclosure is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.