Patent Publication Number: US-11644773-B2

Title: Heating device, fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2020-158612, filed on Sep. 23, 2020 in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a heating device, a fixing device, and an image forming apparatus, and more particularly to a heating device including a cooling device to cool a part of the heating device, and a fixing device and an image forming apparatus including the heating device. 
     Related Art 
     Various types of fixing devices used in an electrophotographic image forming apparatus are known, and one of the types is a surf fixing system that is excellent in energy saving performance and short in warm-up time. In the surf fixing method, a thin fixing belt having a low thermal capacity is contact-heated from the inside by a planar heater, a sheet passing through a fixing nip is heated by the fixing belt, and an unfixed toner image borne on the sheet is fixed under heat. 
     When small sheets are continuously printed by using such a fixing device, the temperature of the longitudinal end portions (non-sheet passing portions) of the fixing belt may rise excessively. The excessive rise in the temperatures of the end portions deteriorates the durability of the fixing belt. Additionally, when a large sheet is printed after the small sheets are printed, the excessive rise in the temperatures of the end portions may cause problems such as an offset and a jam due to sheet winding around the fixing belt. 
     SUMMARY 
     This specification describes an improved heating device that includes a heating rotator, a pressing rotator, a heater, a fan, a housing, and a pair of shutters. The pressing rotator is configured to press against the heating rotator to form a nip passing through a recording medium with the heating rotator. The heater is disposed inside the heating rotator. The housing houses the heating rotator and the pressing rotator. The housing has a pair of openings each configured to allow airflow generated by the fan to pass through each of the pair of openings. The pair of shutters are each configured to slide in corresponding one of the pair of openings in a longitudinal direction of the corresponding one of the pair of openings. Each of the pair of shutters is configured to slide between a position at which an outer end portion of the corresponding one of the pair of openings is opened in the longitudinal direction of the corresponding one of the pair of openings and a position at which an inner end portion of the corresponding one of the pair of openings is opened in the longitudinal direction of the corresponding one of the pair of openings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the 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 view of a configuration of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG.  2    is a schematic diagram illustrating the principle of how an image forming apparatus operates, according to the embodiment of the present disclosure; 
         FIG.  3    is a cross-sectional view illustrating a fixing device used in the image forming apparatus in  FIG.  1   ; 
         FIG.  4    is a plan view of a heater; 
         FIG.  5 A  is a perspective view of shutters and blower fans to be attached to a housing of the fixing device according to the embodiment of the present disclosure; 
         FIG.  5 B  is a schematic diagram illustrating an arrangement relationship among heaters, openings, and shutters of the fixing device of  FIG.  5 A ; 
         FIG.  6 A  is a schematic diagram illustrating a relationship between opening ranges of the openings and a sheet width of a small sheet; 
         FIG.  6 B  is a schematic diagram illustrating a relationship between opening ranges of the openings and a sheet width of a large sheet; 
         FIG.  7 A  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating an A3 sheet; 
         FIG.  7 B  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating a B4 sheet; 
         FIG.  7 C  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating an A4 sheet; 
         FIG.  7 D  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating a B5 sheet; 
         FIG.  7 E  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating a A5 sheet; 
         FIG.  7 F  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating a B6 sheet; 
         FIG.  7 G  is a schematic diagram illustrating a positional relationship between the shutters and the haters heating a A6 sheet; 
         FIG.  8    is an explanatory diagram illustrating temperature distributions of a fixing belt in a longitudinal direction of the fixing belt in the fixing device having the positional relationship between the shutters and the haters as illustrated in  FIG.  7 B ; 
         FIG.  9    is a table summarizing the positions of the shutters, an effective shielding width, and a blower fan control in  FIGS.  7 A to  7 G ; and 
         FIG.  10    is a cross-sectional view of a fixing device including two halogen heaters. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent 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 similar results. 
     Referring now to the drawings, embodiments of the present disclosure are described below. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Identical reference numerals are assigned to identical components or equivalents and a description of those components is simplified or omitted. 
     With reference to drawings, a description is given of a fixing device according to an embodiment of the present disclosure and an image forming apparatus such as a laser printer including the fixing device. The laser printer is just an example of the image forming apparatus, and thus the image forming apparatus is not limited to the laser printer. In other words, the image forming apparatus may be a copier, a facsimile machine, a printer, a plotter, an inkjet recording apparatus, or a multifunction peripheral having at least two of copying, printing, facsimile transmission, plotting, scanning, and inkjet recording capabilities. 
     The identical or similar parts in each drawing are designated by the same reference numerals, and the duplicate description thereof is appropriately simplified or omitted. Further, size (dimension), material, shape, and relative positions used to describe each of the components and units are examples, and the scope of the present disclosure is not limited thereto unless otherwise specified. 
     Although a “recording medium” is described as a “sheet” in the following embodiments, the “recording medium” is not limited to the sheet. Examples of the “recording medium” include not only the sheet of paper but also an overhead projector (OHP) transparency sheet, a fabric, a metallic sheet, a plastic film, and a prepreg sheet including carbon fibers previously impregnated with resin. 
     Examples of the “recording medium” include all media to which developer or ink can be adhered, and so-called recording paper and recording sheets. Examples of the “sheet” include thick paper, a postcard, an envelope, thin paper, coated paper (e.g., coat paper and art paper), and tracing paper, in addition to plain paper. 
     The term “image formation” used in the following description means not only giving an image such as a character or a figure to a medium but also giving an arbitrary image such as a pattern to the medium. 
     A configuration of the image forming apparatus according to an embodiment is described below. 
       FIG.  1    is a schematic view of a configuration of an image forming apparatus  100  (illustrated as a color laser printer) including a fixing device  300  according to the embodiment of the present disclosure.  FIG.  2    is a schematic diagram to simply describe principles of operations in the color laser printer. 
     The image forming apparatus  100  includes four process units  1 K,  1 Y,  1 M, and  1 C as image forming devices. Suffixes, which are K, Y, M, and C, are used to indicate respective colors of toners (black, yellow, magenta, and cyan toners in this example) for the process units. The process units  1 K,  1 Y,  1 M, and  1 C form images of color toners of black (K), yellow (Y), magenta (M), and cyan (C) corresponding to color separation components of a color image. 
     The process units  1 K,  1 Y,  1 M, and  1 C respectively include toner bottles  6 K,  6 Y,  6 M, and  6 C containing different color toners. The process units  1 K,  1 Y,  1 M, and  1 C have a similar structure except the color of toner. Thus, the configuration of the one process unit  1 K is described below, and the descriptions of the other process units  1 Y,  1 M, and  1 C are omitted. 
     The process unit  1 K includes an image bearer  2 K such as a photoconductor drum, a photoconductor cleaner  3 K, and a discharger. The process unit  1 K further includes a charging device  4 K as a charger that uniformly charges the surface of the image bearer and a developing device  5 K as a developing unit that renders visible an electrostatic latent image formed on the image bearer. The process unit  1 K is detachably attachable to a main body of the image forming apparatus  100 . Consumable parts of the process unit  1 K can be replaced at one time. 
     An exposure device  7  is disposed above the process units  1 K,  1 Y,  1 M, and  1 C in the image forming apparatus  100 . The exposure device  7  performs writing and scanning based on image data, in other words, irradiates the image bearer  2 K with laser light L emitted by a laser diode and reflected by mirrors  7   a  based on the image data. 
     A transfer device  15  is disposed below the process units  1 K,  1 Y,  1 M, and  1 C in the present embodiment. The transfer device  15  corresponds to the transfer unit TM in  FIG.  2   . Primary transfer rollers  19 K,  19 Y,  19 M, and  19 C are disposed opposite the image bearers  2 K,  2 Y,  2 M, and  2 C, respectively, in contact with an intermediate transfer belt  16 . 
     The intermediate transfer belt  16  is entrained around the primary transfer rollers  19 K,  19 Y,  19 M, and  19 C, a drive roller  18 , and a driven roller  17  and is rotated. A secondary transfer roller  20  is disposed opposite the drive roller  18  to contact the intermediate transfer belt  16 . Note that, when the image bearers  2 K,  2 Y,  2 M, and  2 C serve as primary image bearers to bear images of the respective colors, the intermediate transfer belt  16  serves as a secondary image bearer to bear a composite image in which the images on the respective image bearers  2 K,  2 Y,  2 M, and  2 C are superimposed one on another. 
     A belt cleaner  21  is disposed downstream from the secondary transfer roller  20  in a direction of rotation of the intermediate transfer belt  16 . A cleaning backup roller is disposed opposite the belt cleaner  21  via the intermediate transfer belt  16 . 
     A sheet feeder  200  including a tray loaded with sheets P is disposed in a lower portion of the image forming apparatus  100 . The sheet feeder  200  serves as a recording-medium supply device and can store a bundle of a large number of sheets P as recording media. The sheet feeder  200  is integrated as a single unit together with a sheet feed roller  60  and a roller pair  210  as a conveyor for the sheets P. 
     The sheet feeder  200  is detachably inserted in the main body of the image forming apparatus  100  to supply the sheet. The sheet feed roller  60  and the roller pair  210  are disposed at an upper portion of the sheet feeder  200  and convey the uppermost one of the sheets P in the sheet feeder  200  to a sheet feeding path  32 . 
     A registration roller pair  250  as a separation conveyor is disposed near the secondary transfer roller  20  and upstream from the secondary transfer roller  20  in a sheet conveyance direction and can temporarily stop the sheet P fed from the sheet feeder  200 . Temporarily stopping the sheet P causes slack on the leading-edge side of the sheet P and corrects a skew of the sheet P. 
     A registration sensor  31  is disposed immediately upstream from the registration roller pair  250  in the sheet conveyance direction and detects passage of a leading edge of the sheet. When a predetermined time passes after the registration sensor  31  detects the passage of the leading edge of the sheet, the sheet contacts the registration roller pair  250  and temporarily stops. 
     Conveyance rollers  240  are disposed downstream from the sheet feeder  200  to convey the sheet, which has been conveyed to the right side from the roller pair  210 , upward. As illustrated in  FIG.  1   , the conveyance rollers  240  conveys the sheet to the registration roller pair  250  upward. 
     The roller pair  210  includes a pair of an upper roller and a lower roller. The roller pair  210  can adopt a friction reverse roller (feed and reverse roller (FRR)) separation system or a friction roller (FR) separation system. 
     In the FRR separation system, a separation roller (a return roller) is applied with a certain amount of torque in a counter sheet feeding direction from a driving shaft via a torque limiter and pressed against a feed roller to separate sheets in a nip between the separation roller and the feed roller. In the FR separation system, the separation roller (a friction roller) is supported by a secured shaft via a torque limiter and pressed against a feed roller to separate sheets in a nip between the separation roller and the feed roller. 
     The roller pair  210  in the present embodiment has a configuration of the FRR separation system. That is, the roller pair  210  includes a feed roller  220  and a separation roller  230 . The feed roller  220  is an upper roller of the roller pair  210  and conveys a sheet toward an inner side of the image forming apparatus  100 . The separation roller  230  is a lower roller of the roller pair  210 . A driving force acting in a direction opposite a direction in which a driving force is given to the feed roller  220  is given to the separation roller  230  by a drive shaft through a torque limiter. 
     The separation roller  230  is pressed against the feed roller  220  by a pressing member such as a spring. A clutch transmits the driving force of the feed roller  220  to the sheet feed roller  60 , and the sheet feed roller  60  rotates left in  FIG.  1   . 
     The registration roller pair  250  sends the sheet P that contacts the registration roller pair  250  and has the slack on the leading-edge side of the sheet P toward the secondary transfer nip between the secondary transfer roller  20  and the drive roller  18 , which is illustrated as a transfer nip N in  FIG.  2   , at a suitable timing to transfer the toner image formed on the intermediate transfer belt  16  onto the sheet P. A bias applied at the secondary transfer nip electrostatically transfers the toner image formed on the intermediate transfer belt  16  onto the fed sheet P at a desired transfer position with high accuracy. 
     A post-transfer conveyance path  33  is disposed above the secondary transfer nip between the secondary transfer roller  20  and the drive roller  18 . The fixing device  300  is disposed near an upper end of the post-transfer conveyance path  33 . The fixing device  300  includes a fixing belt  310  as a heating rotator including a heat generator and a pressure roller  320  as a pressing rotator that rotates while contacting the fixing belt  310  with a predetermined pressure. 
     A post-fixing conveyance path  35  is disposed above the fixing device  300  and branches into a sheet ejection path  36  and a reverse conveyance path  41  at the upper end of the post-fixing conveyance path  35 . At this branching portion, the switching member  42  is disposed and pivots on a pivot shaft  42   a . At an opening end of the sheet ejection path  36 , a pair of sheet ejection rollers  37  is disposed. 
     The reverse conveyance path  41  begins from the branching portion and converges into the sheet feeding path  32 . Additionally, a reverse conveyance roller pair  43  is disposed midway in the reverse conveyance path  41 . An upper face of the image forming apparatus  100  is recessed to an inner side of the image forming apparatus  100  and serves as an output tray  44 . 
     A powder container  10  such as a toner container is disposed between the transfer device  15  and the sheet feeder  200 . The powder container  10  is removably installed in the main body of the image forming apparatus  100 . 
     The image forming apparatus  100  according to the present embodiment has a predetermined distance from the sheet feed roller  60  to the secondary transfer roller  20  in consideration of the conveyance of the sheet on which the toner image is to be transferred. The powder container  10  is disposed in a dead space caused by the predetermined distance to keep the entire image forming apparatus compact. 
     A transfer cover  8  is disposed above the sheet feeder  200  and on a front side to which the sheet feeder  200  is pulled out. The transfer cover  8  can be opened to check an interior of the image forming apparatus  100 . The transfer cover  8  includes a bypass feed roller  45  for bypass sheet feeding and a bypass feed tray  46  for the bypass sheet feeding. 
     Next, the principle of the above-described image forming apparatus  100  is described with reference to  FIG.  2   . 
     The image forming apparatus  100  includes an image bearer  2  such as a photoconductor drum and a photoconductor cleaner  3 . The image forming apparatus  100  further includes a charging device  4  as a charger that uniformly charges the surface of the image bearer, a developing device  5  that renders visible an electrostatic latent image on the image bearer, a transfer device TM disposed under the image bearer  2 , the discharger, and the like. 
     An exposure device  7  is disposed above the image bearer  2 . The exposure device  7  performs writing and scanning based on image data, that is to say, irradiates the image bearer  2  with laser light Lb emitted by a laser diode based on image data and reflected by a mirror  7   a.    
     A sheet feeder  200  including a tray loaded with sheets P is disposed in a lower portion of the image forming apparatus  100 . The sheet feeder  200  serves as a recording-medium supply device and can store a bundle of a large number of sheets P as recording media. The sheet feeder  200  is integrated as a single unit together with the sheet feed roller  60  as the conveyor for the sheets P. 
     Downstream from the sheet feed roller  60 , a registration roller pair  250  as a separation and conveyance means is disposed. The registration roller pair  250  temporarily stops the sheet P fed from the sheet feeder  200 . Temporarily stopping the sheet P causes slack on the leading-edge side of the sheet P and corrects a skew of the sheet P. 
     The registration roller pair  250  sends the sheet P contacting the registration roller pair  250  and having the slack on the leading-edge side toward a transfer nip N of the transfer device TM at a timing to suitably transfer a toner image on the image bearer  2  onto the sheet P. A bias applied at the transfer nip N electrostatically transfers the toner image formed on the image bearer  2  onto the sent sheet P at a desired transfer position. 
     The fixing device  300  is disposed downstream from the transfer nip N. The fixing device  300  includes the fixing belt  310  heated by a heater, and the pressure roller  320  that rotates while contacting the fixing belt  310  with the predetermined pressure. Note that the fixing belt  310  may be replaced by a fixing roller. 
     Referring to  FIG.  1   , operations of the image forming apparatus  100  according to the present embodiment are described below. 
     First, operations of a simplex or single-sided printing are described. 
     Referring to  FIG.  1   , the sheet feed roller  60  rotates in response to a sheet feeding signal from a controller  400  as circuitry of the image forming apparatus  100 . The sheet feed roller  60  separates the uppermost sheet from a bundle of sheets P (also referred to as sheet bundle) loaded in the sheet feeder  200  and feeds the uppermost sheet to the sheet feeding path  32 . 
     When the leading edge of the sheet P, which has been fed by the sheet feed roller  60  and the roller pair  210 , reaches a nip of the registration roller pair  250 , the sheet P is slackened and temporarily stopped by the registration roller pair  250 . The registration roller pair  250  corrects the skew on the leading-edge side of the sheet P and rotates in synchronization with an optimum timing so that a toner image formed on the intermediate transfer belt  16  is transferred onto the sheet P. 
     When the sheet P is fed from the bypass feed tray  46 , sheets P of the sheet bundle loaded on the bypass feed tray  46  are fed one by one from the uppermost sheet of the sheet bundle by the bypass feed roller  45 . Then, the sheet P passes a part of the reverse conveyance path  41  and is conveyed to the nip of the registration roller pair  250 . The subsequent operations are the same as the sheet feeding operations from the sheet feeder  200 . 
     As to image formation, operations of the process unit  1 K are described as representative, and descriptions of the other process units  1 Y,  1 M, and  1 C are omitted here. First, the charging device  4 K uniformly charges the surface of the image bearer  2 K to high potential. The exposure device  7  irradiates the surface of the image bearer  2 K with laser light L according to image data. 
     The surface of the image bearer  2 K irradiated with the laser light L has an electrostatic latent image due to a drop in the potential of the irradiated portion. The developing device  5 K includes a developer bearer to bear developer including toner and transfers unused black toner supplied from the toner bottle  6 K onto the irradiated portion of the surface of the image bearer  2 K having the electrostatic latent image through the developer bearer. 
     The image bearer  2 K to which the toner has been transferred forms (develops) a black toner image on the surface of the image bearer  2 K. The black toner image formed on the image bearer  2 K is transferred onto the intermediate transfer belt  16 . 
     The photoconductor cleaner  3 K removes residual toner remaining on the surface of the image bearer  2 K after an intermediate transfer operation. The removed residual toner is conveyed by a waste toner conveyor and collected to a waste toner container in the process unit  1 K. The discharger discharges the remaining charge on the image bearer  2 K from which the remaining toner is removed by the photoconductor cleaner  3 K. 
     Similarly, toner images are formed on the image bearers  2 Y,  2 M, and  2 C in the process units  1 Y,  1 M, and  1 C for the colors, and color toner images are transferred to the intermediate transfer belt  16  such that the color toner images are superimposed on one on another. 
     The intermediate transfer belt  16  on which the color toner images are transferred and superimposed travels such that the color toner images reach the secondary transfer nip between the secondary transfer roller  20  and the drive roller  18 . The registration roller pair  250  rotates to nip the sheet P contacting the registration roller pair  250  at a predetermined timing and conveys the sheet P to the secondary transfer nip of the secondary transfer roller  20  at a suitable timing such that a composite toner image formed by superimposing and transferring the toner images on the intermediate transfer belt  16  is transferred onto the sheet P. In this manner, the composite toner image on the intermediate transfer belt  16  is transferred to the sheet P sent out by the registration roller pair  250 . 
     The sheet P having the transferred composite toner image is conveyed to the fixing device  300  through the post-transfer conveyance path  33 . The sheet P conveyed to the fixing device  300  is nipped by the fixing belt  310  and the pressure roller  320 . The unfixed toner image is fixed onto the sheet P under heat and pressure in the fixing device  300 . The sheet P, on which the composite toner image has been fixed, is sent out from the fixing device  300  to the post-fixing conveyance path  35 . 
     When the fixing device  300  sends out the sheet P, the switching member  42  is at a position at which the upper end of the post-fixing conveyance path  35  is open, as indicated by the solid line of  FIG.  1   . The sheet P sent out from the fixing device  300  is sent to the sheet ejection path  36  via the post-fixing conveyance path  35 . The pair of sheet ejection rollers  37  nips the sheet P sent out to the sheet ejection path  36  and rotates to eject the sheet P to the output tray  44 . Then, the single-sided printing is finished. 
     Next, a description is given of operations of a duplex or double-sided printing. Similar to the single-sided printing described above, the fixing device  300  sends out the sheet P to the sheet ejection path  36 . In the duplex printing, the pair of sheet ejection rollers  37  rotates to convey a part of the sheet P outside the image forming apparatus  100 . 
     When the trailing edge of the sheet P passes through the sheet ejection path  36 , the switching member  42  pivots on the pivot shaft  42   a  as indicated with a dotted line in  FIG.  1 A  to close the upper end of the post-fixing conveyance path  35 . When the upper end of the post-fixing conveyance path  35  is closed, substantially simultaneously, each of the pair of sheet ejection rollers  37  rotates in reverse (in other words, in a direction opposite to the direction to convey a part of the sheet P outside the image forming apparatus  100 ) to convey the sheet P to an inner side of the image forming apparatus  100 , that is, to the reverse conveyance path  41 . 
     The sheet P sent out to the reverse conveyance path  41  reaches the registration roller pair  250  through the reverse conveyance roller pair  43 . The registration roller pair  250  sends out the sheet P to the secondary transfer nip at a suitable timing such that the toner image formed on the intermediate transfer belt  16  is transferred onto the other surface of the sheet P to which no toner image has been transferred. 
     When the sheet P passes through the secondary transfer nip, the secondary transfer roller  20  and the drive roller  18  transfer the toner image to the other surface (back side) of the sheet P to which no toner image has been transferred. The sheet P having the transferred toner image is conveyed to the fixing device  300  through the post-transfer conveyance path  33 . 
     In the fixing device  300 , the sheet P is nipped by the fixing belt  310  and the pressure roller  320 , and the unfixed toner image are fixed on the back side of the sheet P under heat and pressure. The sheet P having the toner images fixed to both front and back sides of the sheet P in this manner is sent out from the fixing device  300  to the post-fixing conveyance path  35 . 
     When the fixing device  300  sends out the sheet P, the switching member  42  is at a position at which the upper end of the post-fixing conveyance path  35  is open, as indicated by the solid line of  FIG.  1   . The sheet P sent out from the fixing device  300  is sent to the sheet ejection path  36  via the post-fixing conveyance path  35 . The pair of sheet ejection rollers  37  nips the sheet P sent out to the sheet ejection path  36  and rotates to eject the sheet P to the output tray  44 . Thus, the duplex printing is finished. 
     After the toner image on the intermediate transfer belt  16  is transferred onto the sheet P, residual toner remains on the intermediate transfer belt  16 . The belt cleaner  21  removes the residual toner from the intermediate transfer belt  16 . The waste toner conveyor conveys the toner removed from the intermediate transfer belt  16  to the powder container  10 , and the toner is collected inside the powder container  10 . 
     Next, a description is given of the fixing device  300  according to the present embodiment of the present disclosure. 
     As illustrated in  FIG.  3   , the fixing device  300  includes a thin fixing belt  310  having low thermal capacity and a pressure roller  320 . The fixing belt  310  includes, for example, a tubular base made of polyimide (PI). The tubular base has an outer diameter of 25 mm and a thickness of from 40 micrometers (μm) to 120 μm. 
     The fixing device  300  may be a roller fixing system or a belt fixing system in addition to the system using the fixing belt  310  as illustrated in  FIG.  3   . In any one of the fixing systems, heating small sheets P to fix the toner images onto the small sheets P may cause an excessive temperature rise in a non-sheet passage portion of the heating rotator through which the sheets P do not pass because the sheets P absorb heat of the heater from a sheet passage portion of the heating rotator through which the sheets P pass but do not absorb the heat from the non-sheet passage potion. 
     The fixing belt  310  includes a release layer serving as an outermost surface layer. The release layer is made of fluororesin, such as tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and polytetrafluoroethylene (PTFE), and has a thickness of from 5 μm to 50 μm to enhance durability of the fixing belt  310  and facilitate separation of the sheet P from the fixing belt  310 . An elastic layer made of rubber having a thickness of from 50 to 500 μm may be interposed between the base and the release layer. 
     The base of the fixing belt  310  may be made of heat-resistant resin such as polyetheretherketone (PEEK) or metal such as nickel (Ni) or stainless steel (Stainless Used Steel, SUS), instead of polyimide. An inner circumferential surface of the fixing belt  310  may be coated with polyimide, PTFE, or the like to produce a slide layer. 
     A detailed description is now given of a construction of the pressure roller  320 . The pressure roller  320  has an outer diameter of 25 mm, for example. The pressure roller  320  includes a cored bar  321 , an elastic layer  322 , and a release layer  323 . The cored bar  321  is solid and made of metal such as iron. The elastic layer  322  coats the cored bar  321 . The release layer  323  coats an outer surface of the elastic layer  322 . The elastic layer  322  is made of silicone rubber and has, for example, a thickness of 3.5 mm. 
     Preferably, the release layer  323  is formed by a fluororesin layer having, for example, a thickness of approximately 40 μm on the surface of the elastic layer  322  to enhance releasability. A biasing member presses the pressure roller  320  against the fixing belt  310 . 
     A stay  350  and a heater holder  340  are disposed inside the fixing belt  310  and extend in the axial direction of the fixing belt  310 . The stay  350  is made of a metal channel member, and both side plates of the fixing device  300  support both end portions of the stay  350 . The stay  350  reliably receives the pressing force of the pressure roller  320  to stably form a fixing nip SN. 
     The heater holder  340  holds a base  341  of the heater  330  of the fixing device  300  and is supported by the stay  350 . Preferably, the heater holder  340  is made of heat-resistant resin having low thermal conduction, such as a liquid crystal polymer (LCP). Such a configuration can reduce heat transfer to the heater holder  340  and effectively heat the fixing belt  310 . 
     The heater holder  340  has a shape that supports two portions of the base  341  near both end portions in a shorter-side direction of the base  341  to avoid contact with a high-temperature portion of the base  341 . Thus, the amount of heat flowing to the heater holder  340  can be further reduced to effectively heat the fixing belt  310 . 
     The fixing device  300  includes a thermistor TH 1  as a central temperature sensor disposed on the back side of the base  341  to detect temperature of heat generators  372  to  376  of the heater  330  described below. The thermistor TH 1  is pressed against the back side of the base  341  by a spring  387 . Due to the configuration, the accurate temperature of the heat generators  372  to  376  can be detected. 
     Additionally, the fixing device  300  includes a thermistor TH 2  as a central temperature sensor and a thermistor TH 3  as an end portion temperature sensor. The thermistors TH 2  and TH 3  detect temperatures of portions of the fixing belt  310  on an inner surface of the fixing belt  310  and the portions downstream from the fixing nip SN. The thermistor TH 2  faces a center portion of the fixing belt  310  in the longitudinal direction of the heater  330 , which is similar to the thermistor TH 1 . The thermistor TH 3  faces an end portion (that is a non-sheet passing portion) of the fixing belt in the longitudinal direction of the heater  330 . 
     The thermistors TH 1  and TH 2  face the center portion of the fixing belt corresponding to the center of the small sheet in a width direction of the small sheet. In addition, the thermistors TH 3  faces the non-sheet passing portion of the fixing belt that is an outside of a portion of the fixing belt corresponding to the small sheet in the width direction of the large sheet. The non-sheet passing portion of the fixing belt is a portion on which the sheet passing through the fixing device does not contact the fixing belt. Based on the temperature data from the thermistors TH 1  to TH 3 , the controller  400  as the circuitry controls electric power supplied to the heater  330  and a drive motor M 1  that drives shutters  307  described below to effectively prevent the temperature of the non-sheet passing portion of the fixing belt from rising. 
     A thermistor disposed opposite the outer peripheral surface of the pressure roller  320  may be substituted for the thermistors TH 1  to TH 3 . Since disposing the thermistor facing the pressure roller  320  means disposing the thermistor outside the fixing belt  310 , maintenance of the thermistor is easy. The fixing device  300  may be various types of fixing devices, and the fixing device  300  in  FIG.  3    described above is just one example. 
     Next, the heater  330  is described. 
       FIG.  4    illustrates an example of the heater  330  described above. The heater  330  includes three electrodes  370   h ,  370   i , and  370   j  disposed at both ends of the base  341 , and seven heat generators  371  to  377  disposed between the electrodes in the longitudinal direction of the base  341 . Changing connections between an alternating current (AC) power supply and the three electrodes  370   h ,  370   i ,  370   j  enables selecting the following three heat generation patterns. 
     Heat generation pattern 1: Central heat generators  372  to  376  generate heat (for example, when the fixing device heats the small sheet such as an A4 sheet, that is, a sheet having A4 size). 
     Heat generation pattern 2: All heat generators  371  to  377  generate heat (for example, when the fixing device heats the large sheet such as an A3 sheet, that is, a sheet having A3 size). 
     Heat generation pattern 3: Only end heat generators  371  and  377  generate heat (for example, to prevent the temperatures of end portions of the fixing belt from lowering when the fixing device heats the A3 sheet). 
     The central heat generators  372  to  376  are referred to as a central heater H 1 , and the heat generators  371  and  377  at both end portions of the heater  330  are referred to as end heaters H 2 . The central heater H 1  and the end heaters H 2  are arranged so as not to overlap each other when viewed from a short-side direction orthogonal to the longitudinal direction of the heater  330 . 
     A boundary position between the central heater H 1  and each of the end heaters H 2  in the longitudinal direction of the heater  330  may be arbitrarily set, but is generally set to a position corresponding to an edge of the small sheet that is frequently used. For example, the boundary position between the central heater H 1  and each of the end heaters H 2  described in the present embodiment corresponds to an edge of the A4 sheet that is fed in portrait orientation. 
     Five heat generators  372  to  376  at the center portion in the longitudinal direction of the heater  330  are coupled in parallel to a first electrode  370   h  and a second electrode  370   i  via conductors  370   e  and  370   f  having lower resistance than the heat generators. Two heat generators  371  and  377  at both end portions in the longitudinal direction are coupled in parallel to the second electrode  370   i  and a third electrode  370   j  via conductors  370   f  and  370   g  having lower resistance than the heat generators  371  and  377 . 
     A second electrode  370   i  on a right end portion of the heater  330  is always connected to the AC power supply, and a first electrode  370   h  and a third electrode  370   j  on a left end portion of the heater  330  are selectively connected to the AC power supply by switching of switches. Thus, the above-described three heat generation patterns 1 to 3 can be selected. 
     In the above-described heater  330  having the three heat generation patterns that can be selected, for example, a length in the longitudinal direction of the five heat generators  372  to  376  that are continuously arranged at the center portion in the longitudinal direction of the heater  330  is set to be a width of A4 sheet, and a length of all seven the heat generators  371  to  377  including the heat generators at both end portions is set to be a width of A3 sheet. Appropriately controlling heat generations of the heat generators  371  to  377  enables uniformly heating the A4 sheet and the A3 sheet. 
     A heat generation control method for the heat generators  371  to  377  is generally a duty cycle method in which the controller  400  changes a duty cycle that is a ratio of time turning on the heat generators to a predetermined time to get a suitable heat generation amount. The duty cycle is adjusted by controlling the phase of the AC power supply by a triac as a control means. The current is zero at a 0% duty cycle and is a maximum value at a 100% duty cycle. 
     The heat generators  371  to  377  may be, for example, positive temperature coefficient (PTC) elements. The PTC element is made of a material having a positive temperature resistance coefficient, and has a characteristic that the resistance value increases as the temperature T increases (the current I decreases and the heater output decreases). The temperature coefficient of resistance (TCR) may be, for example, 1500 parts per million (PPM). 
     The heat generators  371  to  377  are formed on the base  341 . The base  341  is an elongated thin metal plate member coated with an insulating material. Low-cost aluminum or stainless steel is preferable as the material of the base  341 . However, the material of the base  341  is not limited to metal and alternatively may be a ceramic, such as alumina or aluminum nitride, or a nonmetallic material having excellent thermal resistance and insulating properties, such as glass or mica. 
     To improve thermal uniformity of the heater  330  and image quality, the base  341  may be made of a material having high thermal conductivity, such as copper, graphite, or graphene. The heater  330  according to the present embodiment uses an alumina base having a lateral width of 8 mm, a longitudinal width of 270 mm, and a thickness of 1.0 mm. 
     Next, a cooling device of the fixing device  300  is described with reference to  FIGS.  1 ,  5 A, and  5 B . 
     As illustrated in  FIG.  1   , a housing (a cover)  301  houses the fixing belt  310  and the pressure roller  320  for thermal insulation and heat retention, and blower fans  520  are disposed on a side of the housing  301 . The sheet bearing the toner image enters at a lower inlet of the housing  301 , passes through the fixing nip SN, and exits from an upper outlet of the housing  301 . 
     As illustrated in  FIG.  5 B , the housing  301  has rectangular openings  304  formed on the side of housing  301 . The openings  304  are formed as a pair of openings at both end portions of the housing  301  in the longitudinal direction of the housing  301 . The openings  304  open toward both end portions of the fixing belt  310  in the longitudinal direction of the fixing belt  310  (that are non-sheet passing portions when the A4 sheet passes through the fixing device and portions heated by the end heaters H 2 ) and both end portions of the central portion of the fixing belt  310  heated by the central heater H 1  in the longitudinal direction of the fixing belt  310 . 
     The blower fans  520  disposed outside the openings  304  supply cooling airflow to the openings  304  through ducts. Combinations of rotation speeds of the blower fans  520  and opening degrees (that is, shielding widths) of the shutters  307  can efficiently prevent the temperature rise of the non-sheet passing portions of the fixing belt  310 . 
       FIG.  5 B  is a schematic diagram illustrating a positional relationship between the openings  304  and the shutters  307  with respect to the central heater H 1  (that is the heat generators  372  to  376 ) and the end heaters H 2  (that is the heat generators  371  and  377 ) in the heater  330  of  FIG.  4   . As can be seen from  FIG.  5 B , the openings  304  open to face the end heaters H 2  and both end portions of the central heater H 1 . In other words, the openings  304  cover the non-sheet passing portions of the fixing belt  310  in many cases when a plurality of sizes of sheets pass through the fixing device  300 . Note that  FIG.  5 B  illustrates heat generation areas of the end heaters H 2  shifted from a heat generation area of the central heater H 1  in the sheet conveyance direction inside the loop of the fixing belt  310 , but, as is clear from  FIG.  4   , the heat generation areas of the end heaters H 2  and the heat generation area of the central heater H 1  may be arranged in the longitudinal direction of the base  341 , which is the same in  FIGS.  7 A to  7 G  and  FIG.  8   . 
     As illustrated in  FIG.  5 B , a width of each of a pair of left and right shutters  307  in the longitudinal direction of the fixing belt  310  is shorter than an opening width of each of the openings  304  in the longitudinal directions of the openings  304 . The shutter  307  is configured to be freely movable in a shutter movement range from an inner end portion of the opening  304  to an outer end portion of the opening  304  in the longitudinal direction of the opening  304 , and can partially shield an opening area of the opening  304  within the shutter movement range. 
     Although a size of the non-sheet passing portion in which the temperature rise differs depending on the sheet size, freely changing a shield position of the shutter  307  in the shutter movement range from the inner end portion of the opening  304  to the outer end portion of the opening  304  can effectively prevent the temperature rise in the non-sheet passing portion of the fixing belt  310  regardless of the sheet size. 
     Next, a driving mechanism of the shutters  307  is described. 
     The pair of left and right shutters  307  are disposed so as to be slidable in the longitudinal direction of the fixing device  300 . As illustrated in  FIG.  5 A , each shutter  307  includes a rack  307   a  extending toward the center of the fixing device  300  in the longitudinal direction of the fixing device  300 , and the rack  307   a  meshes with a pinion  308  disposed at the center of the fixing device in the longitudinal direction of the fixing device  300 . A motor M 1  drives the pinion  308 , and the pinion  308  moves the left and right shutters  307  to approach or separate from each other, thereby opening or closing the opening  304 . 
     Excessive heat causes the excessive temperature rise at the end portion of the fixing belt  310 . Just moving the shutter  307  to open a half of the opening  304  or all of the opening  304  can discharge the excessive heat outside. In the present embodiment, in order to enhance the cooling effect cooling the end portion of the fixing belt  310 , each of the blower fans  520  is disposed outside the shutter  307 , that is, so as to face the opening  304  via the shutter  307 , as illustrated in  FIGS.  5 A and  5 B . 
     The blower fans  520  forcibly blow cooling air into the housing  301  toward the end portions of the fixing belt  310 , and the cooling airflow effectively cools the end portion of the fixing belt  310  and can prevent the excessive temperature rise in the end portions. The air blown into the housing  301  is warmed by the end portion of the fixing belt  310  and discharged upward. 
     The warm air discharged upward as described above can be effectively used to prevent condensation on the branching portion. As illustrated in  FIG.  1   , disposing the switching member  42  that guides the sheet to the reverse conveyance path  41  during the duplex printing above the fixing device  300  can prevent condensation on the switching member  42 . In order to effectively prevent the condensation, the switching member  42  is preferably disposed vertically above the opening  304 . 
     Next, positional relationships between the shutters  307  and sheets having different sheet widths are described. 
       FIGS.  6 A and  6 B  are schematic diagrams illustrating positions of the shutters  307  sliding according to the size of the sheet width. As illustrated in  FIG.  6 A , the shutters  307  are moved to the outer end portions of the openings  304  in the longitudinal directions of the openings  304  when a small sheet having a small sheet width passes through the fixing device  300 . Moving the shutters  307  as described above sets opening ranges of openings  304  close to (or adjacent to) both ends of the small sheet, respectively. As a result, the cooling airflow supplied from the blower fans  520  efficiently cools the non-sheet passing portions of the fixing belt  310  outside both ends of the small sheet. 
     In a fixing device according to a comparative embodiment, openings are fully opened to cool the non-sheet passing portions of the fixing belt when the small sheet having the width smaller than a width of the heat generation area of the central heater H 1  in the longitudinal direction of the central heater H 1  passes through the fixing device. However, in the fixing device  300  according to the present embodiment, the shutters  307  slide to change the positions of the shutters  307 , that is, the shield ranges of the shutters  307  when a range facing the central heater H 1  in the non-sheet passing portion is small as illustrated in  FIGS.  7 D and  7 E  and when the range facing the central heater H 1  in the non-sheet passing portion is large as illustrated in  FIGS.  7 F  and G. With reference to  FIGS.  7 D to  7 G , the present embodiment is described below. Changing the positions of the shutters  307 , that is, the shield ranges of the shutters  307  as described above restricts a preferred area of each opening  304  through which the cooling airflow from the blower fan  520  passes to improve cooling efficiency. 
     In addition, as illustrated in  FIG.  6 B , the shutters  307  are moved to the inner end portions of the openings  304  in the longitudinal directions of the openings  304  when the large sheet having the large sheet width passes through the fixing device  300 . Moving the shutters  307  as described above forms the opening ranges of the openings  304  close to (or adjacent to) both ends of the large sheet. As a result, the cooling airflow supplied from the blower fans  520  efficiently cools the non-sheet passing portions of the fixing belt  310  outside both ends of the large sheet. 
     With reference to  FIGS.  7 A to  7 G , the following describes control of the shutters  307 , the blower fans  520 , and the heaters H 1  and H 2  when sheets having various sizes (ex. A3 size to A6 size) pass through the fixing device. 
     Sheet sizes described in the present embodiments are A size and B size that are generally used in Japan, but a sheet size in the present disclosure is not limited to this. In the following descriptions, the sheet is fed in portrait orientation. 
     The blower fan  520  has three operation modes: high-speed rotation mode with high cooling capacity, low-speed rotation mode with low cooling capacity, and stop mode when cooling is unnecessary. In  FIGS.  7 A to  7 G , a dark halftone area is an effective shield range that is an effective shield width in which the shutter  307  covers the inside of the opening  304 . 
     When the A3 sheet (that is the largest sheet size in the image forming apparatus according to the present embodiment) passes through the fixing device  300 , the heaters H 1  and H 2  are energized.  FIG.  7 A  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the A3 sheet. 
     Hatching of the heater H 1  and the heater H 2  in  FIG.  7 A  means that each heater is energized (the same applies to  FIGS.  7 B to  7 G  described below). When the A3 sheet passes through the fixing device  300 , the controller  400  as the circuitry stops the blower fan  520  and does not move the shutters  307 . In other words, the shutters  307  are at arbitrary slide positions. Since the sheet width coincides with the heating region that the heaters H 1  and H 2  heats the fixing belt  310 , the local excessive temperature rise at the end portion of the fixing belt  310  does not occur. 
     Therefore, the controller  400  does not drive the blower fan  520 . Since the blower fan  520  is stopped, the cooling air does not flow through the opening  304 . 
     The controller  400  does not control driving the shutter  307 . The shutter  307  may be at any slide position, in other words, at an arbitrary position. In the embodiment illustrated in  FIG.  7 A , the shutter  307  is at a position closest to the center of the heater H 1 , but the controller  400  may freely determine the slide position of the shutter  307  in accordance with an operation of the fixing device. Note that the controller  400  may perform the same control for a sheet having the double-letter size as that for the A3 sheet described above. 
     When the B4 sheet passes through the fixing device  300 , the heaters H 1  and H 2  are energized.  FIG.  7 B  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the B4 sheet. 
     The controller  400  sets the blower fan  520  to the low-speed rotation mode at the start of sheet conveyance, and switches to the high-speed rotation mode in the middle when the number of B4 sheets passing through the fixing device  300  is large. The controller  400  sets the shutter  307  to be at the inner end portion of the opening  304  in the longitudinal direction of the opening  304  (that is, to be disposed toward the center of the fixing belt  310 ). 
     If the outer ends of the shutters  307  are positioned at both ends of the B4 sheet in a width direction of the B4 sheet in  FIG.  7 B , each of the shutters  307  covers an entire range of the opening  304  facing the B4 sheet. In this case, the controller  400  may set the blower fan  520  to the high-speed rotation mode. In the above, “the outer ends of the shutters  307  are positioned at both ends of the B4 sheet” means that the outer ends of the shutters  307  coincide with both ends of the B4 sheet in the width direction of the B4 sheet when viewed in a short-side direction of the heater H 1 , H 2 , or the fixing belt  310 . 
     However, when a pair of the shutters  307  as in the present embodiment is moved based on various sheet sizes in the width direction of the sheets, the outer ends of the shutters  307  are not positioned at both ends of a sheet having at least any one of sheet sizes in the width direction larger than the width of the heat generation area of the central heater H 1  in the longitudinal direction of the central heater H 1 . In the present embodiment, it is assumed that the outer ends of the shutters  307  do not coincide with both ends of the B4 sheet in the width direction as a result of optimization of the sizes of the shielding widths of the shutters  307  for the various sheet sizes. 
     In other words, the following description assumes a case in which the shielding width of the shutters  307  is slightly smaller than the width of the B4 sheet as illustrated in  FIG.  7 B . In this case, the outer ends of the shutters  307  disposed toward the center of the fixing belt  310  are positioned at positions inside and near the both ends of the B4 sheet in the width direction of the B4 sheet. Similar to the above description, “the outer ends of the shutters  307  are positioned at positions” means that the outer ends of the shutters  307  coincide with the positions when viewed in the short-side direction of the heater H 1 , H 2 , or the fixing belt  310 . 
     The length from one end of the B4 sheet in the width direction of the B4 sheet to one of the positions inside and near the both ends of the B4 sheet depends on the result of optimization of the sizes of the shielding widths of the shutters  307  for the various sheet sizes. In other words, the positions inside and near the both ends of the B4 sheet are determined so that the control of the blower fan  520  as a cooling fan can maintain temperatures of sheet passing and cooled areas R 1  within a fixing temperature tolerance range as described below. 
     Each of the sheet passing and cooled areas R 1  is indicated by a line sandwiched by two arrows facing each other in  FIG.  7 B . Because of a shortage of the shielding width of the shutters  307 , the cooling airflow from the blower fan  520  cools the sheet passing and cooled area R 1 . The sheet passing and cooled area is an area of the fixing belt  310  that contacts the sheet and is cooled by the cooling airflow. In this case, as illustrated in  FIG.  8 ( e ) , applying the cooling air to an end portion of the fixing belt  310  may cause a shortage of a fixing heat amount (in other words, a temperature drop) because the temperatures of the end portion of the fixing belt  310  and an end portion of the sheet is low immediately after the start of sheet conveyance. 
     Therefore, immediately after the start of sheet conveyance and before the temperature rise of the non-sheet passing portion, the controller  400  sets the blower fan  520  to the low-speed rotation mode or the stop mode. When the temperature of the non-sheet passing portion rises after a large number of sheets have passed through the fixing device  300 , the controller  400  switches the blower fan  520  to the high-speed rotation mode (see  FIG.  8 ( f ) ). 
     Since the temperature rise of the non-sheet passing portion affects the temperature of the end portion of a sheet passing portion that is the sheet passing and cooled area R 1  to be high, the high-speed rotation mode of the blower fan  520  cooling the sheet passing and cooled area R 1  does not cause the shortage of the fixing heat amount as illustrated in  FIG.  8 ( f ) . As described above, the controller  400  sets the shutter  307  to be disposed toward the center of the fixing belt  310  to cover the opening  304  and switches operation modes of the blower fan  520  (that is, the high-speed rotation mode, the low-speed rotation mode, and the stop mode) when the B4 sheets pass through the fixing device  300 . Such combinations of the shielding widths of the shutters  307  and the operation modes of the blower fan  520  can maintain the temperature of the sheet passing and cooled area R 1  within the fixing temperature tolerance range even when the shielding width of the shutter  307  in the opening  304  does not cover the sheet passing portion of the B4 sheet. 
     When the A4 sheet passes through the fixing device  300 , the heater H 1  is energized, and the heater H 2  is not energized.  FIG.  7 C  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the A4 sheet. 
     When the A4 sheet passes through the fixing device  300 , the controller  400  as the circuitry stops the blower fan  520  and does not move the shutters  307 . In other words, the shutters  307  are at arbitrary slide positions. Since the sheet width coincides with the heating region that the heater H 1  heats the fixing belt  310 , the local excessive temperature rise at the end portion of the fixing belt  310  does not occur. 
     Therefore, the controller  400  does not drive the blower fan  520 . Since the blower fan  520  is stopped, the cooling air does not flow through the opening  304 . The controller  400  does not control the shutter  307 . In other words, the shutter  307  may be at the arbitrary position. In the embodiment illustrated in  FIG.  7 C , the outer ends of shutters  307  are positioned at both ends of the A4 sheet, but the controller  400  may freely determine the slide position of the shutter  307  in accordance with an operation of the fixing device. Note that the controller  400  may perform the same control for a sheet having the letter size as that for the A4 sheet described above. 
     When the B5 sheet passes through the fixing device  300 , the heater H 1  is energized, and the heater H 2  is not energized.  FIG.  7 D  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the B5 sheet. 
     When the B5 sheet passes through the fixing device  300 , the controller  400  sets the blower fan  520  to the high-speed rotation mode and sets the shutters  307  at the inner end portions of the openings  304  in the longitudinal directions of the openings  304  (in other words, to be disposed toward the center of the fixing belt  310 ) so as to correspond to the width of the B5 sheet. 
     The width of the heat generation area of the central heater H 1  larger than the width of the B5 sheet in the longitudinal direction of the heat generation area of the central heater H 1  generates non-sheet passing and cooled areas R 2  of the fixing belt  310  on both end portions of the heat generation area of the central heater H 1  in the longitudinal direction of the heat generation area of the central heater H 1 . However, the high-speed rotation mode of the blower fan  520  can prevent the excessive temperature rise in the non-sheet passing and cooled areas R 2 . In addition, since the shutter  307  can perfectly cover the sheet passing portion of the fixing belt for the B5 sheet, the controller  400  can control the temperature of the sheet passing portion to maintain the target fixing temperature. 
     When the A5 sheet passes through the fixing device  300 , the heater H 1  is energized, and the heater H 2  is not energized.  FIG.  7 E  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the A5 sheet. 
     When the A5 sheet passes through the fixing device  300 , the controller  400  sets the blower fan  520  to the high-speed rotation mode and sets the shutters  307  at the inner end portions of the openings  304  in the longitudinal directions of the openings  304  (in other words, to be disposed toward the center of the fixing belt  310 ) so as to correspond to the width of the A5 sheet. 
     The width of the heat generation area of the central heater H 1  larger than the width of the A5 sheet in the longitudinal direction of the heat generation area of the central heater H 1  generates non-sheet passing and cooled areas R 3  of the fixing belt  310  on both end portions of the heat generation area of the central heater H 1  in the longitudinal direction of the heat generation area of the central heater H 1 . However, the high-speed rotation mode of the blower fan  520  can prevent the excessive temperature rise in the non-sheet passing and cooled areas R 3 . In addition, since the shutter  307  can perfectly cover the sheet passing portion of the fixing belt for the A5 sheet, the controller  400  can control the temperature of the sheet passing portion to maintain the target fixing temperature. 
     When the B6 sheet passes through the fixing device  300 , the heater H 1  is energized, and the heater H 2  is not energized.  FIG.  7 F  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the B6 sheet. 
     The controller  400  sets the blower fan  520  to the low-speed rotation mode at the start of sheet conveyance, and switches to the high-speed rotation mode in the middle when the number of B6 sheets passing through the fixing device  300  is large. The controller  400  sets the shutter  307  to be at the outer end portion of the opening  304  in the longitudinal direction of the opening  304  (that is, to be disposed toward an end of the fixing belt  310 ), and the shutters  307  cover both end portions of the fixing belt  310  outside both ends of the central heater H 1 . 
     In other words, the inner ends of the shutters  307  in the longitudinal direction of the shutters  307  are positioned at both ends of the central heater H 1 . In the above, “the inner ends of the shutters  307  are positioned at both ends of the central heater H 1 ” means that the inner ends of the shutters  307  coincide with both ends of the central heater H 1  when viewed in the short-side direction of the heater H 1 , H 2 , or the fixing belt  310 . 
     Disposing the shutter  307  at the outer end portion of the opening  304  in the longitudinal direction of the opening  304  is a shielding setting specific to the present embodiment. Both ends of the B6 sheet slightly protrude from the inner ends of the opening  304  in the longitudinal direction of the fixing belt  310 , and both end portions of the B6 sheet slightly overlap the opening  304  and form sheet passing and cooled areas R 4 . The size of the sheet passing and cooled area R 4 , that is, the length in the longitudinal direction of the fixing belt  310  from the inner end of the opening  304  to the end of the B6 sheet depends on the result of optimization of the sizes of the shielding widths of the shutters  307  for the various sheet sizes and the setting of the opening width of the opening  304 . In other words, the size of the sheet passing and cooled area R 4  is determined so that the control of the blower fan  520  as the cooling fan can maintain temperatures of sheet passing and cooled areas R 4  within the fixing temperature tolerance range. 
     The width of the heat generation area of the central heater H 1  larger than the width of the B6 sheet in the longitudinal direction of the heat generation area of the central heater H 1  generates non-sheet passing heated areas of the fixing belt  310  on both end portions of the heat generation area of the central heater H 1  in the longitudinal direction of the heat generation area of the central heater H 1  and outside the sheet passing and cooled areas R 4 . However, the high-speed rotation mode of the blower fan  520  and shielding the outer end portions of the openings  304  in the longitudinal directions of the openings  304  to the maximum by the shutters  307  can collect the cooling air and send the cooling air to the non-sheet passing heated areas to efficiently prevent the excessive temperature rise in the non-sheet passing heated areas. 
     When the shutter  307  in the present embodiment is moved based on all sheet sizes, the opening range of the opening  304  set by the shutter  307  does not match the non-sheet passing heated area outside a sheet having at least any one of the sheet sizes. In the present embodiment, it is assumed that the non-sheet passing heated area outside the B6 sheet does not match the opening range. That is, it is assumed that, as a result of overall optimization of the size of the width of the shutter  307  (a shutter width), the non-sheet passing heated area outside the B6 sheet does not match the opening range of the opening  304 . In the following description, it is assumed that the inner end of the opening range in the longitudinal direction of the opening  304  is positioned slightly inside the B6 sheet, in other words, positioned to slightly shift from the end of the B6 sheet toward the center of the B6 sheet. 
     The cooling airflow of the blower fan  520  cools the sheet passing and cooled area R 4  of the fixing belt  310  facing an end portion of the B6 sheet because the inner end in the longitudinal direction of the opening range overlaps the end portion of the B6 sheet. In this case, applying the cooling air to the end portion of the fixing belt  310  may cause a shortage of the fixing heat amount (in other words, the temperature drop) because the temperatures of the end portion of the fixing belt  310  and an end portion of the sheet is low immediately after the start of sheet conveyance. 
     Therefore, immediately after the start of sheet conveyance and before the temperature rise of the non-sheet passing portion, the controller  400  sets the blower fan  520  to the low-speed rotation mode or the stop mode. When the temperature of the non-sheet passing portion rises after a large number of sheets have passed through the fixing device  300 , the controller  400  switches the blower fan  520  to the high-speed rotation mode. 
     Since the temperature rise of the non-sheet passing portion affects the temperature of the end portion of the sheet passing portion that is the sheet passing and cooled area R 4  to be high, the high-speed rotation mode of the blower fan  520  cooling the sheet passing and cooled area R 4  does not cause the shortage of the fixing heat amount. As described above, the controller  400  sets the shielding width of the shutter  307  and switches operation modes of the blower fan  520  when the B6 sheets pass through the fixing device  300 . Such combinations of the shielding widths of the shutters  307  and the operation modes of the blower fan  520  can maintain the temperature of the sheet passing and cooled area R 4  within the fixing temperature tolerance range even when the non-sheet passing heated area outside the B6 sheet does not match the opening range of the opening  304 . 
     On the other hand, the controller  400  sets the shutter  307  to be at the outer end portion of the opening  304  in the longitudinal direction of the opening  304 . In other words, the inner ends of the shutters  307  in the longitudinal direction of the shutters  307  are positioned at both ends of the central heater H 1 . In the above, “the inner ends of the shutters  307  are positioned at both ends of the central heater H 1 ” means that the inner ends of the shutters  307  in the longitudinal direction of the shutters  307  coincide with both ends of the central heater H 1  when viewed in the short-side direction of the heater H 1 , H 2 , or the fixing belt  310 . 
     Since the end heaters H 2  are not energized and do not generate heat when the B6 sheet passes through the fixing device  300 , non-sheet passing portions of both end portions of the fixing belt  310  corresponding to the end heaters H 2  are not needed to be cooled. Therefore, the shutters  307  shield parts of the openings  304  facing the non-sheet passing portions. Shielding the parts of the openings  304  facing the non-sheet passing portions that are not needed to be cooled collects the cooling air and sends the cooling air to other non-sheet passing portions in which both end portions of the central heater H 1  cause local temperature rise. As a result, the cooling efficiency is improved. 
     When the A6 sheet passes through the fixing device  300 , the heater H 1  is energized, and the heater H 2  is not energized.  FIG.  7 G  is a schematic diagram illustrating a positional relationship between the shutters and the heaters heating the A6 sheet. 
     When the A6 sheet passes through the fixing device  300 , the controller  400  sets the blower fan  520  to the high-speed rotation mode and sets the shutter  307  to be at the outer end portion of the opening  304  in the longitudinal direction of the opening  304  (that is, to be disposed toward the end of the fixing belt  310 ), and the shutters  307  cover both end portions of the fixing belt  310  outside both ends of the central heater H 1 . Disposing the shutter  307  at the outer end portion of the opening  304  in the longitudinal direction of the opening  304  is the shielding setting specific to the present embodiment. 
     The setting of the opening range in each of the openings  304  for the A6 sheet is the same as that for the B6 sheet illustrated in  FIG.  7 F . Accordingly, the shutter  307  is not disposed toward the center of the fixing belt  310 . The above-described configuration can appropriately cool non-sheet passing portions of the fixing belt  310  outside the both ends of the A6 sheet in the longitudinal direction of the fixing belt  310 , the non-sheet passing portions in which the local temperature rise occurs. 
     On the other hand, the controller  400  sets the shutter  307  to be at the outer end portion of the opening  304  in the longitudinal direction of the opening  304 . In the present embodiment, the inner ends of the shutters  307  coincide with both ends of the central heater H 1 , which is similar to the shutters  307  illustrated in  FIG.  7 F . In addition, since the end heaters H 2  are not energized when the A6 sheet passes through the fixing device  300 , portions of the fixing belt  310  that are not heated by the end heaters H 2  are not needed to be cooled. 
     Since the openings  304  in the fixing device according to the comparative embodiment are fully opened to cool the non-sheet passing portions of the fixing belt, the cooling efficiency is bad in many cases. Since setting the shutters  307  as illustrated in  FIG.  7 G  can cool the non-sheet passing portions in which both end portions of the central heater H 1  cause local temperature rise, the cooling efficiency is improved. 
     The following describes temperature distributions of the fixing belt  310  when the B4 sheets pass through the fixing device  300 . 
       FIG.  8    schematically illustrates the temperature distributions from the center of the fixing belt to the right end of the fixing belt in the longitudinal direction of the fixing belt when the B4 sheets pass through the fixing device as described above with reference to  FIG.  7 B .  FIG.  8 ( a )  illustrates a temperature distribution before the B4 sheets pass through the fixing device  300 .  FIG.  8 ( b ) to ( d )  illustrate temperature distributions when the fixing belt is not cooled, and when the B4 sheets pass through the fixing device.  FIGS.  8 ( e ) and ( f )  illustrate temperature distributions when the fixing belt is cooled, and when the B4 sheets pass through the fixing device. 
     The vertical axis in each of (a) to (f) separately and independently indicates the temperature of surface of the fixing belt. Horizontal straight lines extending from the center of the fixing belt corresponding to the center of the sheet indicates that the temperature of the fixing belt is controlled to the target temperature (for example, 170° C.). The temperature distribution indicates whether the blower fan  520  is appropriately controlled. 
       FIG.  8 ( a )  illustrates the temperature distribution before the sheets start to pass through the fixing device, that is, under a standby state. The temperature of the entire fixing belt is uniformly maintained at the target temperature (for example, 170° C.).  FIGS.  8 ( b ) to  8 ( d )  illustrate the temperature distributions of the fixing belt arranged in chronological order when the B4 sheets continuously pass through the fixing device, and the blower fan is stopped. 
       FIG.  8 ( b )  illustrates the temperature distribution of the fixing belt in an initial stage when the B4 sheets pass through the fixing device, and the blower fan is stopped. The center heater H 1  and the end heaters H 2  are energized to generate heat and heat the fixing belt and the B4 sheets. The temperature starts to rise in the outer end portion of the fixing belt that is on the heat generation area of the end heater H 2  and does not contact the B4 sheets, that is, the non-sheet passing portion. In the initial stage when the B4 sheets pass through the fixing device, the temperature rise amount is small. The temperature of the fixing belt does not cause a problem of the fixing belt and other peripheral members (that is, cooling is unnecessary). 
       FIG.  8 ( c )  illustrates the temperature distribution of the fixing belt in a middle stage when the B4 sheets pass through the fixing device  300 , and the blower fan is stopped. The temperature of the non-sheet passing portion of the fixing belt further increases from the temperature illustrated in  FIG.  8 ( b )  (for example, higher than the high temperature threshold of 180° C. and lower than 200° C.). The large temperature rise may cause the problem of the fixing belt and other peripheral members. Cooling the non-sheet passing portion is preferable. 
       FIG.  8 ( d )  illustrates the temperature distribution of the fixing belt in a final stage when the B4 sheets pass through the fixing device  300 , and the blower fan is stopped. The temperature of the non-sheet passing portion of the fixing belt further increases from the temperature illustrated in  FIG.  8 ( c ) . The temperature rise affects the temperature of the end portion of the B4 sheet to be out of the fixing temperature tolerance range, which may cause an abnormal image in the end portion. The temperature out of the fixing temperature tolerance is, for example, higher than 200° C. and lower than 220° C., which causes a problem such as a reduction in heat resistance life of the fixing belt and other peripheral members. Cooling the non-sheet passing portion is preferable. 
       FIGS.  8 ( e ) and  8 ( f )  illustrate temperature distributions of the fixing belt when the blower fan is operated, and B4 sheets pass through the fixing device. The difference between  FIGS.  8 ( e ) and ( f )  is a switching timing of the operation modes of the blower fan. 
       FIG.  8 ( e )  illustrates the temperature distribution of the fixing belt in the initial stage when the B4 sheets pass through the fixing device  300 , and when the controller  400  sets the blower fan to the high-speed rotation mode immediately after the B4 sheets start to pass through the fixing device  300  to prevent the temperature rise in the non-sheet passing portion. The controller  400  performs the same control as the control when the B5, A5, or A6 sheets pass through the fixing device and are heated. The controller  400  controls the blower fan to rotate in the high-speed rotation mode in the initial stage when the B4 sheets pass through the fixing device. The portion indicated by the arrow in  FIG.  8 ( f )  is the sheet passing and cooled area R 1  in which the temperature drop is caused by cooling of the blower fan. 
     Since the temperature of the fixing belt is close to the target temperature inside the sheet passing and cooled area R 1 , operating the blower fan in the high-speed rotation mode in the initial stage when the B4 sheets pass through the fixing device similar to the operations when the B5, A5, or A6 sheets pass through the fixing device increases a heat mount removed from the sheet passing and cooled area R 1 , causes the temperature drop, and causes an abnormal image. For this reason, the above-described control is not possible when the B4 sheets pass through the fixing device. 
       FIG.  8 ( f )  illustrates the temperature distribution of the fixing belt when the controller  400  controls the blower fan to switch from the stop mode or the low-speed rotation mode to the high-speed rotation mode after the middle stage when the B4 sheets pass through the fixing device  300  to avoid the temperature drop as illustrated in  FIG.  8 ( e )  because the temperature rise of the non-sheet passing portion affects the temperature of the sheet passing and cooled area R 1  to be high after the middle stage. 
     Since the temperature rise of the non-sheet passing portion of the fixing belt is small from the initial stage to the middle stage when the B4 sheets pass through the fixing device as described above with reference to  FIGS.  8 ( b ) and ( c ) , the controller  400  does not set the blower fan to the high-speed rotation mode. As illustrated in  FIG.  8 ( d ) , the temperature rise of the non-sheet passing portion affects the temperature of the sheet passing and cooled area R 1  to be high in the final stage when the B4 sheets pass through the fixing device. 
     As illustrated in  FIG.  8 ( f ) , setting the blower fan to the high-speed rotation mode after the middle stage, not from the initial stage when the B4 sheets pass through the fixing device avoids the occurrence of the temperature drop as indicated by the arrow in  FIG.  8 ( e ) . In  FIG.  8 ( f ) , the temperature of the sheet passing and cooled area R 1  is within the fixing temperature tolerance range, and the temperature rise of the non-sheet passing portion can also be reduced. 
     When the B6 sheets pass through the fixing device, the controller  400  may control the blower fan to switch the operation modes of the blower fan in the same manner as the manner described above when the B4 sheets pass through the fixing device. As described above, the fixing device according to the present embodiment can effectively prevent the local temperature rise of the fixing belt with a simple configuration. 
     As described above, the temperature of the non-sheet passing portion of the fixing belt  310  on the end of the end heater H 2  is low before the sheets start to pass through the fixing device as illustrated in  FIG.  8 ( a ) , but the temperature of the non-sheet passing portion increases as illustrated in  FIGS.  8 ( b ) to ( d )  while the sheets pass through the fixing device. In the final stage when the sheets pass through the fixing device, the temperature of the non-sheet passing portion is out of the fixing temperature tolerance range as illustrated in  FIG.  8 ( d ) , which may cause a fixing failure. 
     To avoid the fixing failure, the controller  400  starts driving the blower fan  520  in the middle stage when the sheets pass through the fixing device to cool the non-sheet passing portion including the sheet passing and cooled area R 1  by the cooling air. As a result, the temperature of the sheet passing and cooled area R 1  is maintained within the fixing temperature tolerance range, which prevents the fixing failure. Starting driving the blower fan  520  from the initial stage when the sheets pass through the fixing device causes the temperature drop in the sheet passing and cooled area R 1  as illustrated in  FIG.  8 ( e ) , which may cause the fixing failure. 
       FIG.  9    is a table summarizing the above description. As illustrated in  FIG.  9   , controlling the positions of the shutters  307  and the operation modes of the blower fan  520  based on the sheet sizes can improve the cooling effect. 
     Although some embodiments of the present disclosure have been described above, embodiments of the present disclosure are not limited to the embodiments described above, and a variety of modifications can be made within the scope of the present disclosure. For example, the central heater H 1  and the end heaters H 2  are not necessarily planar heaters, but may be replaced with other types of heaters such as halogen heaters. 
       FIG.  10    is a schematic cross-sectional view of a fixing device  300  including two halogen heaters  390 - 1  and  390 - 2  instead of the central heater H 1  and the end heaters H 2 . The heat of the two halogen heaters  390 - 1  and  390 - 2  heats the inner surface of the fixing belt  310 . One halogen heater  390 - 1  corresponds to the central heater H 1 , and the other halogen heaters  390 - 2  corresponds to the end heaters H 2 . 
     A reflector  391  is disposed behind the halogen heaters  390 - 1  and  390 - 2 , and a shield  392  rotatable in the direction indicated by arrow is disposed in front of the halogen heaters  390 - 1  and  390 - 2 . In order to prevent the non-sheet passing portions at both ends of the fixing belt  310  from being overheated, the shield  392  covers the outer end portion of the halogen heater  390 - 2  in the longitudinal direction of the halogen heater  390 - 2  based on the sheet size. 
     A nip formation pad  393  is disposed so as to face the pressure roller  320  to form the fixing nip SN therebetween. A thermal equalizer  394  made of a high thermal conduction material is disposed on the back side of the nip formation pad  393  to equalize the temperature distribution in the longitudinal direction of the fixing belt  310 . 
     The heating rotator may be in any form such as a roller, a belt, or a sleeve, in addition to the form of the fixing belt. In the above embodiment, the blower fan  520  is disposed outside the shutter  307 . However, the blower fans  520  may be replaced by suction fans each disposed in an exhaust duct disposed on an upper side of the housing  301  and each connected to the opening  304 . When the suction fans suck and exhaust the air in the housing  301 , air sucked from the openings  304  in the horizontal lateral direction cools the non-sheet passing portions at both end portions of the fixing belt  310 . The shutter  307  adjusts an intake amount of air or an intake range of air. 
     Although the shutters  307  are disposed outside the openings  304  so as to be openable and closable, the shutters  307  may be disposed inside the openings  304  so as to be openable and closable. Although the heating rotator in the above-described embodiment is the fixing belt  310 , the heating rotator may be a fixing roller having a heater inside instead of the fixing belt  310 . 
     The heating device of the present disclosure may be used not only for the fixing device  300  described above, but also for a sheet drying device for an inkjet printer. In addition to the PTC element used in the heater  330 , other heat generator such as a ceramic heater may be used as the heat generator to heat the fixing belt  310 . 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the present disclosure, the present disclosure may be practiced otherwise than as specifically described herein. The number, position, and shape of the components described above are not limited to those embodiments described above. Desirable number, position, and shape can be determined to perform the present disclosure. Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions.