Fixing device and image forming apparatus

A fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and an opposed rotator disposed opposite the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. A heater is disposed opposite the fixing rotator to heat the fixing rotator. A nip formation pad is disposed opposite an inner circumferential surface of the fixing rotator. The nip formation pad includes a base, a first thermal conductor sandwiched between the base and the fixing rotator and having a first thermal conductivity greater than a thermal conductivity of the base, and a bulge projecting from the first thermal conductor toward the opposed rotator at a downstream end of the first thermal conductor in a recording medium conveyance direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2013-174337, filed on Aug. 26, 2013, and 2014-144095, filed on Jul. 14, 2014, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.

BACKGROUND

1. Technical Field

Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an 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 development 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 belt, a fixing film, and a fixing roller heated by a heater and an opposed rotator such as a pressure roller and a pressure belt pressed against the fixing rotator to form a fixing nip therebetween. As a recording medium bearing a toner image is conveyed through the fixing nip, the fixing rotator and the opposed rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.

SUMMARY

This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a fixing rotator rotatable in a predetermined direction of rotation and an opposed rotator disposed opposite the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. A heater is disposed opposite the fixing rotator to heat the fixing rotator. A nip formation pad is disposed opposite an inner circumferential surface of the fixing rotator. The nip formation pad includes a base, a first thermal conductor sandwiched between the base and the fixing rotator and having a first thermal conductivity greater than a thermal conductivity of the base, and a bulge projecting from the first thermal conductor toward the opposed rotator at a downstream end of the first thermal conductor in a recording medium conveyance direction.

This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image forming device to form a toner image and a fixing device, disposed downstream from the image forming device 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 an opposed rotator disposed opposite the fixing rotator to form a fixing nip therebetween through which the recording medium bearing the toner image is conveyed. A heater is disposed opposite the fixing rotator to heat the fixing rotator. A nip formation pad is disposed opposite an inner circumferential surface of the fixing rotator. The nip formation pad includes a base, a first thermal conductor sandwiched between the base and the fixing rotator and having a first thermal conductivity greater than a thermal conductivity of the base, and a bulge projecting from the first thermal conductor toward the opposed rotator at a downstream end of the first thermal conductor in a recording medium conveyance direction.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular toFIG. 1, an image forming apparatus1according to an exemplary embodiment of the present invention is explained.

FIG. 1is a schematic vertical sectional view of the image forming apparatus1. The image forming apparatus1may 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 apparatus1is a color laser printer that forms color and monochrome toner images on recording media by electrophotography.

With reference toFIG. 1, a description is provided of a construction of the image forming apparatus1.

As shown inFIG. 1, the image forming apparatus1includes four image forming devices4Y,4M,4C, and4K situated in a center portion thereof. Although the image forming devices4Y,4M,4C, and4K contain yellow, magenta, cyan, and black developers (e.g., yellow, magenta, cyan, and black toners) that form yellow, magenta, cyan, and black toner images, respectively, resulting in a color toner image, they have an identical structure.

For example, each of the image forming devices4Y,4M,4C, and4K includes a drum-shaped photoconductor5serving as an image carrier that carries an electrostatic latent image and a resultant toner image; a charger6that charges an outer circumferential surface of the photoconductor5; a development device7that supplies toner to the electrostatic latent image formed on the outer circumferential surface of the photoconductor5, thus visualizing the electrostatic latent image as a toner image; and a cleaner8that cleans the outer circumferential surface of the photoconductor5. It is to be noted that, inFIG. 1, reference numerals are assigned to the photoconductor5, the charger6, the development device7, and the cleaner8of the image forming device4K that forms a black toner image. However, reference numerals for the image forming devices4Y,4M, and4C that form yellow, magenta, and cyan toner images, respectively, are omitted.

Below the image forming devices4Y,4M,4C, and4K is an exposure device9that exposes the outer circumferential surface of the respective photoconductors5with laser beams. For example, the exposure device9, constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam onto the outer circumferential surface of the respective photoconductors5according to image data sent from an external device such as a client computer.

Above the image forming devices4Y,4M,4C, and4K is a transfer device3. For example, the transfer device3includes an intermediate transfer belt30serving as an intermediate transferor, four primary transfer rollers31serving as primary transferors, a secondary transfer roller36serving as a secondary transferor, a secondary transfer backup roller32, a cleaning backup roller33, a tension roller34, and a belt cleaner35.

The intermediate transfer belt30is an endless belt stretched taut across the secondary transfer backup roller32, the cleaning backup roller33, and the tension roller34. As a driver drives and rotates the secondary transfer backup roller32counterclockwise inFIG. 1, the secondary transfer backup roller32rotates the intermediate transfer belt30counterclockwise inFIG. 1in a rotation direction R1 by friction therebetween.

The four primary transfer rollers31sandwich the intermediate transfer belt30together with the four photoconductors5, respectively, forming four primary transfer nips between the intermediate transfer belt30and the photoconductors5. The primary transfer rollers31are connected to a power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto.

The secondary transfer roller36sandwiches the intermediate transfer belt30together with the secondary transfer backup roller32, forming a secondary transfer nip between the secondary transfer roller36and the intermediate transfer belt30. Similar to the primary transfer rollers31, the secondary transfer roller36is connected to the power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto.

The belt cleaner35includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt30. A waste toner conveyance tube extending from the belt cleaner35to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt30by the belt cleaner35to the waste toner container.

A bottle holder2situated in an upper portion of the image forming apparatus1accommodates four toner bottles2Y,2M,2C, and2K detachably attached thereto to contain and supply fresh yellow, magenta, cyan, and black toners to the development devices7of the image forming devices4Y,4M,4C, and4K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles2Y,2M,2C, and2K to the development devices7through toner supply tubes interposed between the toner bottles2Y,2M,2C, and2K and the development devices7, respectively.

In a lower portion of the image forming apparatus1are a paper tray10that loads a plurality of sheets P serving as recording media and a feed roller11that picks up and feeds a sheet P from the paper tray10toward the secondary transfer nip formed between the secondary transfer roller36and the intermediate transfer belt30. The sheets P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, and the like. Optionally, a bypass tray that loads thick paper, postcards, envelopes, thin paper, coated paper, art paper, tracing paper, OHP transparencies, and the like may be attached to the image forming apparatus1.

A conveyance path R extends from the feed roller11to an output roller pair13to convey the sheet P picked up from the paper tray10onto an outside of the image forming apparatus1through the secondary transfer nip. The conveyance path R is provided with a registration roller pair12located below the secondary transfer nip formed between the secondary transfer roller36and the intermediate transfer belt30, that is, upstream from the secondary transfer nip in a sheet conveyance direction A1. The registration roller pair12serving as a conveyance roller pair or a timing roller pair feeds the sheet P conveyed from the feed roller11toward the secondary transfer nip at a proper time.

The conveyance path R is further provided with a fixing device20located above the secondary transfer nip, that is, downstream from the secondary transfer nip in the sheet conveyance direction A1. The fixing device20fixes a toner image transferred from the intermediate transfer belt30onto the sheet P conveyed from the secondary transfer nip. The conveyance path R is further provided with the output roller pair13located above the fixing device20, that is, downstream from the fixing device20in the sheet conveyance direction A1. The output roller pair13discharges the sheet P bearing the fixed toner image onto the outside of the image forming apparatus1, that is, an output tray14disposed atop the image forming apparatus1. The output tray14stocks the sheet P discharged by the output roller pair13.

With reference toFIG. 1, a description is provided of an image forming operation performed by the image forming apparatus1having the construction described above to form a color toner image on a sheet P.

As a print job starts, a driver drives and rotates the photoconductors5of the image forming devices4Y,4M,4C, and4K, respectively, clockwise inFIG. 1in a rotation direction R2. The chargers6uniformly charge the outer circumferential surface of the respective photoconductors5at a predetermined polarity. The exposure device9emits laser beams onto the charged outer circumferential surface of the respective photoconductors5according to yellow, magenta, cyan, and black image data constituting color image data sent from the external device, respectively, thus forming electrostatic latent images thereon. The development devices7supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors5, visualizing the electrostatic latent images into yellow, magenta, cyan, and black toner images, respectively.

Simultaneously, as the print job starts, the secondary transfer backup roller32is driven and rotated counterclockwise inFIG. 1, rotating the intermediate transfer belt30in the rotation direction R1 by friction therebetween. The power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged toner to the primary transfer rollers31, creating a transfer electric field at each primary transfer nip formed between the photoconductor5and the primary transfer roller31.

When the yellow, magenta, cyan, and black toner images formed on the photoconductors5reach the primary transfer nips, respectively, in accordance with rotation of the photoconductors5, the yellow, magenta, cyan, and black toner images are primarily transferred from the photoconductors5onto the intermediate transfer belt30by the transfer electric field created at the primary transfer nips such that the yellow, magenta, cyan, and black toner images are superimposed successively on a same position on the intermediate transfer belt30. Thus, a color toner image is formed on the outer circumferential surface of the intermediate transfer belt30. After the primary transfer of the yellow, magenta, cyan, and black toner images from the photoconductors5onto the intermediate transfer belt30, the cleaners8remove residual toner failed to be transferred onto the intermediate transfer belt30and therefore remaining on the photoconductors5therefrom, respectively. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductors5, initializing the surface potential thereof.

On the other hand, the feed roller11disposed in the lower portion of the image forming apparatus1is driven and rotated to feed a sheet P from the paper tray10toward the registration roller pair12in the conveyance path R. The registration roller pair12conveys the sheet P sent to the conveyance path R by the feed roller11to the secondary transfer nip formed between the secondary transfer roller36and the intermediate transfer belt30at a proper time. The secondary transfer roller36is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners constituting the color toner image formed on the intermediate transfer belt30, thus creating a transfer electric field at the secondary transfer nip.

As the yellow, magenta, cyan, and black toner images constituting the color toner image on the intermediate transfer belt30reach the secondary transfer nip in accordance with rotation of the intermediate transfer belt30, the transfer electric field created at the secondary transfer nip secondarily transfers the yellow, magenta, cyan, and black toner images from the intermediate transfer belt30onto the sheet P collectively. After the secondary transfer of the color toner image from the intermediate transfer belt30onto the sheet P, the belt cleaner35removes residual toner failed to be transferred onto the sheet P and therefore remaining on the intermediate transfer belt30therefrom. The removed toner is conveyed and collected into the waste toner container.

Thereafter, the sheet P bearing the color toner image is conveyed to the fixing device20that fixes the color toner image on the sheet P. Then, the sheet P bearing the fixed color toner image is discharged by the output roller pair13onto the outside of the image forming apparatus1, that is, the output tray14that stocks the sheet P.

The above describes the image forming operation of the image forming apparatus1to form the color toner image on the sheet P. Alternatively, the image forming apparatus1may form a monochrome toner image by using any one of the four image forming devices4Y,4M,4C, and4K or may form a bicolor or tricolor toner image by using two or three of the image forming devices4Y,4M,4C, and4K.

With reference toFIG. 2, a description is provided of a construction of the fixing device20incorporated in the image forming apparatus1described above.

FIG. 2is a vertical sectional view of the fixing device20. As shown inFIG. 2, the fixing device20(e.g., a fuser) includes a fixing belt21serving as a fixing rotator or an endless belt formed into a loop and rotatable in a rotation direction R3; a pressure roller22serving as an opposed rotator disposed opposite an outer circumferential surface of the fixing belt21to separably or unseparably contact the fixing belt21and rotatable in a rotation direction R4 counter to the rotation direction R3 of the fixing belt21; a single halogen heater23serving as a heater disposed inside the loop formed by the fixing belt21to heat the fixing belt21; a nip formation pad24disposed inside the loop formed by the fixing belt21and pressing against the pressure roller22via the fixing belt21to form a fixing nip N between the fixing belt21and the pressure roller22; a stay25serving as a support disposed inside the loop formed by the fixing belt21and contacting and supporting the nip formation pad24; a reflector26disposed inside the loop formed by the fixing belt21to reflect light radiated from the halogen heater23toward the fixing belt21; a temperature sensor27serving as a temperature detector disposed opposite the outer circumferential surface of the fixing belt21to detect the temperature of the fixing belt21; and a separator28disposed opposite the outer circumferential surface of the fixing belt21to separate a sheet P discharged from the fixing nip N from the fixing belt21.

The fixing device20further includes a pressurization assembly that presses the pressure roller22against the nip formation pad24via the fixing belt21. The fixing belt21and the components disposed inside the loop formed by the fixing belt21, that is, the halogen heater23, the nip formation pad24, the stay25, and the reflector26, may constitute a belt unit21U separably coupled with the pressure roller22.

A detailed description is now given of a construction of the fixing belt21.

The fixing belt21is a thin, flexible endless belt or film. For example, the fixing belt21is constructed of a base layer constituting an inner circumferential surface of the fixing belt21and a release layer constituting the outer circumferential surface of the fixing belt21. The base layer is made of metal such as nickel and SUS stainless steel or resin such as polyimide (PI). The release layer is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. Alternatively, an elastic layer made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber may be interposed between the base layer and the release layer.

A detailed description is now given of a construction of the pressure roller22.

The pressure roller22is constructed of a metal core22a; an elastic layer22bcoating the metal core22aand made of silicone rubber foam, silicone rubber, fluoro rubber, or the like; and a release layer22ccoating the elastic layer22band made of PFA, PTFE, or the like. The pressurization assembly presses the pressure roller22against the nip formation pad24via the fixing belt21. Thus, the pressure roller22pressingly contacting the fixing belt21deforms the elastic layer22bof the pressure roller22at the fixing nip N formed between the pressure roller22and the fixing belt21, thus creating the fixing nip N having a predetermined length in the sheet conveyance direction A1. A driver (e.g., a motor) disposed inside the image forming apparatus1depicted inFIG. 1drives and rotates the pressure roller22. As the driver drives and rotates the pressure roller22, a driving force of the driver is transmitted from the pressure roller22to the fixing belt21at the fixing nip N, thus rotating the fixing belt21by friction between the pressure roller22and the fixing belt21. Alternatively, the driver may also be connected to the fixing belt21to drive and rotate the fixing belt21.

As shown inFIG. 2, according to this exemplary embodiment, the pressure roller22is a solid roller. Alternatively, the pressure roller22may be a hollow roller. In this case, a heater such as a halogen heater may be disposed inside the hollow roller. If the hollow pressure roller does not incorporate the elastic layer, the pressure roller has a decreased thermal capacity that improves fixing property of being heated quickly to a predetermined fixing temperature at which a toner image T is fixed on a sheet P properly. However, as the pressure roller and the fixing belt21sandwich and press the toner image T on the sheet P passing through the fixing nip N, slight surface asperities of the fixing belt21may be transferred onto the toner image T on the sheet P, resulting in variation in gloss of the solid toner image T. To address this problem, it is preferable that the pressure roller incorporates the elastic layer having a thickness not smaller than about 100 micrometers. The elastic layer having the thickness not smaller than about 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt21, preventing variation in gloss of the toner image T on the sheet P. The elastic layer22bmay be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller22, the elastic layer22bmay be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because it has an increased insulation that draws less heat from the fixing belt21. According to this exemplary embodiment, the pressure roller22is pressed against the fixing belt21. Alternatively, the pressure roller22may merely contact the fixing belt21with no pressure therebetween.

A detailed description is now given of a configuration of the halogen heater23.

Both lateral ends of the halogen heater23in a longitudinal direction thereof parallel to an axial direction of the fixing belt21are mounted on side plates of the fixing device20, respectively. The power supply situated inside the image forming apparatus1supplies power to the halogen heater23so that the halogen heater23heats the fixing belt21. A controller (e.g., a processor), that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, operatively connected to the halogen heater23and the temperature sensor27controls the halogen heater23based on the temperature of the outer circumferential surface of the fixing belt21detected by the temperature sensor27so as to adjust the temperature of the fixing belt21to a desired fixing temperature. Alternatively, instead of the halogen heater23, an induction heater, a resistance heat generator, a carbon heater, or the like may be employed as a heater that heats the fixing belt21.

A detailed description is now given of a configuration of the nip formation pad24.

The nip formation pad24extends in the axial direction of the fixing belt21or the pressure roller22such that a longitudinal direction of the nip formation pad24is parallel to the axial direction of the fixing belt21or the pressure roller22. The nip formation pad24is mounted on and supported by the stay25. Accordingly, even if the nip formation pad24receives pressure from the pressure roller22, the nip formation pad24is not bent by the pressure and therefore produces a uniform nip width throughout the entire span of the pressure roller22in the axial direction thereof. The stay25is made of metal having an increased mechanical strength, such as stainless steel and iron, to prevent bending of the nip formation pad24. Alternatively, the stay25may be made of resin.

The nip formation pad24is made of a heat resistant material resistant against temperatures not lower than about 200 degrees centigrade. Thus, the nip formation pad24is immune from thermal deformation at temperatures in a fixing temperature range desirable to fix the toner image T on the sheet P, retaining the shape of the fixing nip N and quality of the toner image T formed on the sheet P. For example, the nip formation pad24is made of general heat resistant resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyamide imide (PAI), and polyether ether ketone (PEEK). According to this exemplary embodiment, the nip formation pad24is made of LCP TI-8000 available from Toray Industries, Inc.

The nip formation pad24is coated with a low-friction sheet serving as a slide aid. As the fixing belt21rotates in the rotation direction R3, the fixing belt21slides over the low-friction sheet that reduces a driving torque developed between the fixing belt21and the nip formation pad24, reducing load exerted to the fixing belt21by friction between the fixing belt21and the nip formation pad24. For example, the low-friction sheet is made of TOYOFLON® 401 available from Toray Industries, Inc.

A detailed description is now given of a configuration of the reflector26.

The reflector26is interposed between the stay25and the halogen heater23. According to this exemplary embodiment, the reflector26is mounted on the stay25. Since the reflector26is heated by the halogen heater23directly, the reflector26is made of metal having a high melting point. The reflector26reflects light radiated from the halogen heater23to the stay25toward the fixing belt21, increasing an amount of light that irradiates the fixing belt21and thereby heating the fixing belt21effectively. Additionally, the reflector26suppresses conduction of heat from the halogen heater23to the stay25or the like, saving energy.

Alternatively, instead of installation of the reflector26, an opposed face of the stay25disposed opposite the halogen heater23may be treated with polishing or mirror finishing such as coating to produce a reflection face that reflects light from the halogen heater23toward the fixing belt21. For example, the reflector26or the reflection face of the stay25has a reflection rate of about 90 percent or more.

Since the shape and the material of the stay25are not selectable flexibly to retain the mechanical strength, if the reflector26is installed in the fixing device20, the reflector26and the stay25provide flexibility in the shape and the material, attaining properties peculiar to them, respectively. The reflector26interposed between the halogen heater23and the stay25is situated in proximity to the halogen heater23, reflecting light from the halogen heater23toward the fixing belt21effectively.

In order to save energy and decrease a first print time taken to output the sheet P bearing the fixed toner image T upon receipt of a print job through preparation for a print operation and the subsequent print operation, the fixing device20is configured as below. For example, the fixing device20employs a direct heating method in which the halogen heater23heats the fixing belt21directly in a circumferential span of the fixing belt21other than the fixing nip N. As shown inFIG. 2, no component is interposed between the halogen heater23and the fixing belt21in a circumferential, direct heating span of the fixing belt21on the left of the halogen heater23where the halogen heater23heats the fixing belt21directly.

In order to decrease the thermal capacity of the fixing belt21, the fixing belt21is thin and has a decreased loop diameter. For example, the fixing belt21is constructed of the base layer having a thickness in a range of from about 20 micrometers to about 50 micrometers; the elastic layer having a thickness in a range of from about 100 micrometers to about 300 micrometers; and the release layer having a thickness in a range of from about 10 micrometers to about 50 micrometers. Thus, the fixing belt21has a total thickness not greater than about 1 mm. A loop diameter of the fixing belt21is in a range of from about 20 mm to about 40 mm. In order to decrease the thermal capacity of the fixing belt21further, the fixing belt21may have a total thickness not greater than about 0.20 mm and preferably not greater than about 0.16 mm. Additionally, the loop diameter of the fixing belt21may not be greater than about 30 mm.

According to this exemplary embodiment, the pressure roller22has a diameter in a range of from about 20 mm to about 40 mm. Hence, the loop diameter of the fixing belt21is equivalent to the diameter of the pressure roller22. However, the loop diameter of the fixing belt21and the diameter of the pressure roller22are not limited to the sizes described above. For example, the loop diameter of the fixing belt21may be smaller than the diameter of the pressure roller22. In this case, a curvature of the fixing belt21is greater than a curvature of the pressure roller22at the fixing nip N, facilitating separation of the sheet P from the fixing belt21as it is discharged from the fixing nip N.

As shown inFIG. 2, a bulge45′ projects from the nip formation pad24toward the pressure roller22at a downstream end of the nip formation pad24in the sheet conveyance direction A1 disposed opposite an exit of the fixing nip N. The bulge45′ does not press against the pressure roller22via the fixing belt21and therefore is not produced by indirect contact with the pressure roller22via the fixing belt21. The bulge45′ lifts the sheet P bearing the toner image T fixed at the fixing nip N from the fixing belt21, facilitating separation of the sheet P from the fixing belt21.

Since the fixing belt21has a decreased thermal capacity, it is susceptible to uneven temperature in the axial direction thereof as described below. As a small sheet P bearing a toner image T is conveyed through the fixing nip N, the small sheet P creates a conveyance span on the fixing belt21where the small sheet P is conveyed over the fixing belt21at a center of the fixing belt21in the axial direction thereof and a non-conveyance span on the fixing belt21where the small sheet P is not conveyed over the fixing belt21at each lateral end of the fixing belt21in the axial direction thereof. The sheet P and the toner image T thereon draw heat from the conveyance span of the fixing belt21but do not draw heat from the non-conveyance span of the fixing belt21. Accordingly, the non-conveyance span of the fixing belt21may store heat and overheat to a temperature higher than a predetermined temperature (e.g., the fixing temperature at which the toner image T is fixed on the sheet P properly). Such overheating may also occur on a fixing roller used as a fixing rotator instead of the fixing belt21.

To address this circumstance, a heat shield may surround the nip formation pad24to shield the nip formation pad24from the halogen heater23.

However, since the nip formation pad24is made of a material having an increased thermal conductivity, the nip formation pad24may absorb heat excessively. For example, when the heat shield is cool during warm-up of the fixing device20, the conductive nip formation pad24may absorb heat from the fixing belt21excessively, increasing energy consumption. Conversely, when the heat shield is heated, the heat shield may cause overheating of both lateral ends of the fixing belt21in the axial direction thereof.

With reference toFIG. 3, a description is provided of a configuration of a fixing device20A installable in the image forming apparatus1depicted inFIG. 1.

FIG. 3is a schematic vertical sectional view of the fixing device20A. As shown inFIG. 3, the fixing device20A includes two halogen heaters23serving as a heater situated inside the loop formed by the fixing belt21. The halogen heaters23generate light that irradiates the inner circumferential surface of the fixing belt21, heating the fixing belt21directly. Like the fixing device20depicted inFIG. 2, the fixing device20A includes the bulge45′ that projects from the nip formation pad24toward the pressure roller22at the downstream end of the nip formation pad24in the sheet conveyance direction A1 disposed opposite the exit of the fixing nip N. The bulge45′ does not press against the pressure roller22via the fixing belt21and therefore is not produced by indirect contact with the pressure roller22via the fixing belt21. The bulge45′ lifts a sheet P bearing a toner image T fixed at the fixing nip N from the fixing belt21, facilitating separation of the sheet P from the fixing belt21.

With reference toFIGS. 4,5A,5B, and5C, a description is provided of a configuration of a comparative fixing device20C that suffers from overheating of both lateral ends of the fixing belt21in the axial direction thereof.

FIG. 4is a partial schematic vertical sectional view of the comparative fixing device20C. In the comparative fixing device20C, heat conducted from the halogen heater23to the fixing belt21is further conducted from the fixing belt21to the medium and the components that contact the fixing belt21. For example, heat is conducted from the outer circumferential surface of the fixing belt21to the pressure roller22that contacts the outer circumferential surface of the fixing belt21and to the sheet P and toner of the toner image T on the sheet P as the sheet P is conveyed through the fixing nip N. Heat is conducted from the inner circumferential surface of the fixing belt21to a nip formation pad24C that contacts the inner circumferential surface of the fixing belt21. The nip formation pad24C is made of resin having a decreased thermal conductivity and therefore draws a decreased amount of heat from the fixing belt21. Accordingly, as a plurality of small sheets P having a decreased width in the axial direction of the fixing belt21is conveyed through the fixing nip N continuously, the fixing belt21stores heat at both lateral ends in the axial direction thereof, that is, a non-conveyance span, where the small sheets P are not conveyed over the fixing belt21and therefore do not draw heat from the fixing belt21. Consequently, the fixing belt21suffers from overheating in the non-conveyance span as the small sheets P having the decreased width that is smaller than a light emission span H of the halogen heater23spanning in the longitudinal direction thereof are conveyed through the fixing nip N continuously.

FIG. 5Ais a sectional view of the nip formation pad24C taken along line LA-LA inFIG. 4. It is to be noted thatFIG. 5Aillustrates a half of the nip formation pad24C in a longitudinal direction thereof parallel to the axial direction of the fixing belt21, from a center24A to a lateral edge24B of the nip formation pad24C in the longitudinal direction thereof.

FIG. 5Bis a diagram illustrating positional relations between the light emission span H of the halogen heater23and four conveyance spans A, B, C, and D of sheets P of four sizes in the longitudinal direction of the halogen heater23parallel to the axial direction of the fixing belt21. The halogen heater23of the comparative fixing device20C is constructed of a single heater extending in a longitudinal direction thereof parallel to the axial direction of the fixing belt21.

FIG. 5Cis a graph showing a relation between the distance from a center of the fixing belt21in the axial direction thereof and the temperature of the fixing belt21in a non-conveyance span outboard from the conveyance spans A, B, C, and D in the axial direction of the fixing belt21as sheets P of four sizes are conveyed over the fixing belt21.FIG. 5Cillustrates temperatures TA, TB, and TC in the non-conveyance span, that is, a lateral end of the fixing belt21in the axial direction thereof, where the sheet P is not conveyed over the fixing belt21.

For instance, when a plurality of sheets P having the smallest width is conveyed over the smallest conveyance span A of the fixing belt21continuously, the temperature TA of the fixing belt21increases in the greatest non-conveyance span outboard from the smallest conveyance span A in the axial direction of the fixing belt21. However, since the temperature of the halogen heater23increases to an increased temperature at a center in the longitudinal direction thereof whereas the temperature of the halogen heater23increases to a decreased temperature at a lateral end in the longitudinal direction thereof, the temperature TA of the fixing belt21marks a peak at a position outboard from the conveyance span A and decreases gently toward a lateral edge of the fixing belt21in the axial direction thereof. Contrarily, when a sheet P having the greatest width is conveyed over the greatest conveyance span D of the fixing belt21, the sheet P having the greatest width does not produce the non-conveyance span on the fixing belt21as it is conveyed over the fixing belt21. Hence, the temperature of the fixing belt21may barely increase in the non-conveyance span situated at the lateral end of the fixing belt21in the axial direction thereof.

If the diameter, the linear velocity, and the productivity of the fixing belt21and the pressure roller22are fixed, as the size of the non-conveyance span on the fixing belt21that defines a difference between the light emission span H of the halogen heater23and each of the conveyance spans A, B, C, and D increases, an amount of heat stored in the fixing belt21increases, thus increasing overheating of the lateral end of the fixing belt21and producing the temperature TA that is higher than the temperature TB higher than the temperature TC. As a result of overheating of the fixing belt21, the temperatures TA and TB may be above an upper limit of target temperature UT of the fixing belt21and the temperature TC may be below the upper limit of target temperature UT of the fixing belt21.

With reference toFIGS. 6,7A,7B,8A,8B, and8C, a description is provided of a configuration of the fixing device20according to an exemplary embodiment.

FIG. 6is a partial schematic vertical sectional view of the fixing device20. A typical fixing device, for example, the comparative fixing device20C depicted inFIG. 4, includes the nip formation pad24C made of resin as a base and contacting the fixing belt21. The nip formation pad24C is coated with a low-friction sheet serving as a slide aid. Contrarily, the fixing device20shown inFIG. 6includes the nip formation pad24including a base51and an equalizer41serving as a first thermal conductor sandwiched between the base51and the fixing belt21at the fixing nip N and extended in a longitudinal direction thereof parallel to the axial direction of the fixing belt21. The equalizer41is made of a material having a thermal conductivity greater than that of the base51to absorb excessive heat stored in the non-conveyance span of the fixing belt21and conduct the absorbed heat in the longitudinal direction of the equalizer41.

The nip formation pad24is not coated with the low-friction sheet so as to enhance heat absorption from the fixing belt21. However, if the equalizer41absorbs heat from the fixing belt21excessively or if friction between the equalizer41and the fixing belt21produces a torque that obstructs rotation of the fixing belt21, the low-friction sheet may coat the equalizer41. As the sheet P is conveyed over the fixing belt21, the sheet P draws heat from the equalizer41. Accordingly, heat conducts to a relatively cooler center of the equalizer41in the longitudinal direction thereof or a cooler portion of each lateral end of the equalizer41in the longitudinal direction thereof that is susceptible to overheating.

FIG. 7Ais a schematic sectional view of the nip formation pad24. As shown inFIG. 7A, the base51is mounted on the equalizer41in a thickness direction thereof perpendicular to the sheet conveyance direction A1 such that a length of the equalizer41is equivalent to a length of the base51in the sheet conveyance direction A1.

FIG. 7Bis a schematic sectional view of a nip formation pad24′ as a variation of the nip formation pad24shown inFIG. 7A. As shown inFIG. 7B, the base51is mounted on an equalizer41′ in a thickness direction thereof perpendicular to the sheet conveyance direction A1 such that a length of the equalizer41′ is greater than a length of the base51in the sheet conveyance direction A1. For example, an upstream arm and a downstream arm of the equalizer41′ in the sheet conveyance direction A1 sandwich the base51. The base51situated inward from each of the equalizers41and41′ inside the loop formed by the fixing belt21prevents excessive inward diffusion of heat from the equalizers41and41′, reducing waste of energy. Additionally, the base51extending in the axial direction of the fixing belt21facilitates conduction of heat in a longitudinal direction of the nip formation pad24′ parallel to the axial direction of the fixing belt21.

FIG. 8Ais a sectional view of the nip formation pad24taken along line LA-LA inFIG. 6.FIG. 8Aillustrates a half of the nip formation pad24in the longitudinal direction thereof parallel to the axial direction of the fixing belt21, from the center24A to the lateral edge24B of the nip formation pad24in the longitudinal direction thereof.FIG. 8Bis a diagram illustrating positional relations between the light emission span H of the halogen heater23and the four conveyance spans A, B, C, and D of sheets P of four sizes in the longitudinal direction of the halogen heater23parallel to the axial direction of the fixing belt21.

FIG. 8Cis a graph showing a relation between the distance from the center of the fixing belt21in the axial direction thereof and the temperature of the fixing belt21in the non-conveyance span outboard from the conveyance spans A, B, C, and D in the axial direction of the fixing belt21as sheets P of four sizes are conveyed over the fixing belt21.FIG. 8Cillustrates the temperatures TA, TB, and TC in the non-conveyance span, that is, the lateral end of the fixing belt21in the axial direction thereof, where the sheet P is not conveyed over the fixing belt21. The equalizer41contacting the inner circumferential surface of the fixing belt21at the fixing nip N extends in a span corresponding to the entire span of the halogen heater23in the longitudinal direction thereof parallel to the axial direction of the fixing belt21. Accordingly, regardless of the sizes of sheets P, the equalizer41suppresses overheating of both lateral ends of the fixing belt21in the axial direction thereof as shown inFIG. 8C.

Alternatively, the base51disposed opposite the fixing belt21via the equalizer41may be made of a material having an increased thermal conductivity to increase the thermal capacity of the equalizer41and thereby cause the equalizer41to suppress overheating of both lateral ends of the fixing belt21in the axial direction thereof effectively. The thermal capacity of the equalizer41in direct contact with the fixing belt21is adjusted to prevent the equalizer41from absorbing heat from the fixing belt21excessively.

For example, the thermal capacity of the equalizer41is optimized. In order to prevent overheating of both lateral ends of the fixing belt21in the axial direction thereof while saving energy, a heat flux from the fixing belt21to the base51is optimized. The thermal capacity of each of the equalizer41, the base51, and the low-friction sheet is optimized by considering the combined thermal resistance of the equalizer41, the base51, and the low-friction sheet. For example, with the combination of the equalizer41made of copper and the base51made of heat resistant resin, the thickness of the equalizer41is in a range of from about 9 micrometers to about 3 mm.

On the other hand, if the equalizer41is planar, the planar equalizer41may degrade separation of the sheet P bearing the fixed toner image T from the fixing belt21.

With reference toFIGS. 9A,9B,9C, and9D, a description is provided of configurations of the components that form the fixing nip N.FIG. 9Ais a partial sectional view of the nip formation pad24illustrating a downstream section thereof, that is, the exit of the fixing nip N, in the sheet conveyance direction A1.

As shown inFIG. 9A, a bulge45projects from the equalizer41toward the pressure roller22depicted inFIG. 6at a downstream end41aof the equalizer41in the sheet conveyance direction A1 disposed opposite the exit of the fixing nip N, that is, a downstream end of the fixing nip N in the sheet conveyance direction A1. The bulge45lifts the sheet P bearing the fixed toner image T that is conveyed through the exit of the fixing nip N from the fixing belt21, facilitating separation of the sheet P from the fixing belt21. A low-friction sheet59serving as a slide aid is wound around the nip formation pad24. For example, the low-friction sheet59coats the equalizer41, the bulge45, and the base51.

FIG. 9Bis a partial sectional view of a nip formation pad24S illustrating a downstream section thereof. As shown inFIG. 9B, the bulge45projects from the equalizer41toward the pressure roller22at the downstream end41aof the equalizer41. A stopper46projects from the equalizer41toward the stay25depicted inFIG. 6in a direction opposite a direction in which the bulge45projects from the equalizer41, that is, a thickness direction of the nip formation pad24S perpendicular to the sheet conveyance direction A1, at the downstream end41aof the equalizer41along a downstream face51aof the base51. The stopper46prevents the equalizer41from moving in a circumferential direction of the fixing belt21even when the equalizer41receives a predetermined force from the fixing belt21rotating in the rotation direction R3 and the sheet P conveyed in the sheet conveyance direction A1. The low-friction sheet59is wound around the nip formation pad24S. For example, the low-friction sheet59coats the equalizer41, the bulge45, and the stopper46. An end59aof the low-friction sheet59is nipped by and fixed between the base51and the stopper46.

FIG. 9Cis a partial sectional view of a nip formation pad24T illustrating a downstream section thereof. A single copper plate constituting the equalizer41is bent to produce a bulge45T that projects from the equalizer41toward the pressure roller22at the downstream end41aof the equalizer41. Thus, the bulge45T and the equalizer41are manufactured at reduced costs. The low-friction sheet59coats the equalizer41. The end59aof the low-friction sheet59is nipped by and fixed between the base51and the stopper46. However, a recess47is defined by the bulge45T, the equalizer41, and the base51. Accordingly, the base51, the equalizer41, and the bulge45T produce an air layer surrounded by them, which degrades heat conduction between the base51and the equalizer41.

To address this circumstance, the base51may be contoured as shown inFIG. 9D.FIG. 9Dis a partial sectional view of a nip formation pad24U illustrating a downstream section thereof. As shown inFIG. 9D, the base51serving as a resin layer includes a curved portion51bcurved along a curved portion41bof the equalizer41bent and curved to create a bulge45U. A curvature CB of the curved portion51bof the base51is smaller than a curvature CA of the curved portion41bof the equalizer41. Accordingly, as the pressure roller22is pressed against the base51via the fixing belt21and the equalizer41, the equalizer41is pressed against the base51precisely without being lifted from the base51. The low-friction sheet59coats the equalizer41, the bulge45U, and the stopper46. The end59aof the low-friction sheet59is nipped by and fixed between the base51and the stopper46.

With reference toFIGS. 10A and 10B, a description is provided of a first variation of the configurations of the components that form the fixing nip N described above.

FIG. 10Ais a schematic sectional view of the equalizer41and the base51constituting the nip formation pad24seen in a direction perpendicular to the longitudinal direction thereof parallel to the axial direction of the fixing belt21when a gap G is produced between the equalizer41and the base51. As shown inFIG. 10A, the gap G is produced between the equalizer41and the base51serving as the resin layer due to the shape of the base51and pressure between the pressure roller22and the base51. As shown inFIG. 6, the base51is supported by the stay25situated inward from the base51inside the loop formed by the fixing belt21. Both lateral ends of the stay25in a longitudinal direction thereof are mounted on the side plates of the fixing device20, respectively. If the pressure roller22has a hand drum shape or an hour glass shape in which the diameter of a center in the axial direction thereof is smaller than the diameter of each lateral end in the axial direction thereof, the diameter of a center51A of the base51in a longitudinal direction thereof is greater than the diameter of each lateral end51B of the base51in the longitudinal direction thereof so that the center51A of the base51projects toward the center of the pressure roller22. Thus, as the pressure roller22is pressed against the base51, the pressure roller22forms the fixing nip N even at the center of the pressure roller22. However, if the modulus of elasticity or the rigidity of the pressure roller22, the equalizer41, and base51is not considered, the gap G may be produced between the equalizer41and the base51.

FIG. 10Bis a schematic sectional view of the equalizer41and the base51seen in the direction perpendicular to the longitudinal direction thereof parallel to the axial direction of the fixing belt21when no gap is produced between the equalizer41and the base51. In order to adhere the equalizer41to the base51as the pressure roller22is pressed against the base51, a modulus of elasticity of the equalizer41is smaller than a modulus of elasticity of the base51as shown inFIG. 10B. Conversely, if a modulus of elasticity of the equalizer41is greater than a modulus of elasticity of the base51as shown inFIG. 10A, the equalizer41is not bent in conformity with bending of the base51, producing the gap G therebetween.

With reference toFIGS. 11A and 11B, a description is provided of a second variation of the configurations of the components that form the fixing nip N described above.

FIG. 11Ais a partial sectional view of the nip formation pad24illustrating the downstream section thereof, that is, the exit of the fixing nip N, in the sheet conveyance direction A1.

As shown inFIG. 11A, the low-friction sheet59serving as a slide aid is sandwiched between the equalizer41and the fixing belt21at the fixing nip N. The low-friction sheet59reduces abrasion of the inner circumferential surface of the fixing belt21, facilitating sliding of the fixing belt21over the equalizer41. The low-friction sheet59coats the bulge45mounted on the equalizer41and a downstream face41cof the equalizer41. That is, the low-friction sheet59is curved along the bulge45and extended along the downstream face41cof the equalizer41. The low-friction sheet59is manufactured separately from the equalizer41and inserted into a gap between the equalizer41and the fixing belt21. Alternatively, the low-friction sheet59may be produced by coating a nip face of the equalizer41disposed opposite the fixing nip N with a slide aid material. According to this exemplary embodiment, the low-friction sheet59serving as a slide aid is inserted into the gap between the equalizer41and the fixing belt21. In this case, surface asperities of the low-friction sheet59may reduce the area of the low-friction sheet59where the low-friction sheet59contacts the equalizer41, obstructing conduction of heat from the fixing belt21to the equalizer41. To address this circumstance, that is, to secure an increased area of the low-friction sheet59where the low-friction sheet59contacts the equalizer41and thereby facilitate conduction of heat from the fixing belt21to the equalizer41, an elastic layer57may be interposed or sandwiched between the equalizer41and the low-friction sheet59as shown inFIG. 11B.

FIG. 11Bis a partially enlarged sectional view of the equalizer41, the elastic layer57, and the low-friction sheet59illustrating a cutaway portion Q shown inFIG. 11A. The elastic layer57is conductive tape that prevents the low-friction sheet59from shifting relative to the equalizer41during continuous sliding of the fixing belt21over the low-friction sheet59while attaining enhanced heat conduction. For example, the conductive tape is metal tape. Alternatively, if grease is applied between the equalizer41and the low-friction sheet59to attain appropriate sliding of the low-friction sheet59over the equalizer41, conductive grease may be used to enhance heat conduction. For example, the conductive grease is silicone grease or grease added with conductive particles such as zinc oxide.

With reference toFIGS. 12,13A,13B, and13C, a description is provided of a construction of a fixing device20V according to another exemplary embodiment.

FIG. 12is a partial vertical sectional view of the fixing device20V.FIG. 13Ais a sectional view of a nip formation pad24V taken along line LA-LA inFIG. 12.FIG. 13Bis a diagram illustrating positional relations between the light emission span H of the halogen heater23and the four conveyance spans A, B, C, and D of sheets P of four sizes in the longitudinal direction of the halogen heater23.FIG. 13Cis a graph showing a relation between the distance from the center of the fixing belt21in the axial direction thereof and the temperature of the fixing belt21in the non-conveyance span outboard from the conveyance spans A, B, C, and D in the axial direction of the fixing belt21as sheets P of four sizes are conveyed over the fixing belt21.FIG. 13Cillustrates the temperatures TA, TB, and TC in the non-conveyance span, that is, the lateral end of the fixing belt21in the axial direction thereof, where the sheet P is not conveyed over the fixing belt21and temperatures tA, tB, tC, and tD in the conveyance span, that is, the center of the fixing belt21in the axial direction thereof, where the sheet P is conveyed over the fixing belt21.

As shown inFIG. 12, the fixing device20V includes the equalizer41sandwiched between the base51and the fixing belt21at the fixing nip N and extended in the longitudinal direction thereof parallel to the axial direction of the fixing belt21. The equalizer41serving as a first thermal conductor is made of a material having a thermal conductivity greater than that of the base51. The fixing device20V further includes an absorber42serving as a third thermal conductor sandwiched between the base51and the stay25and extended in a longitudinal direction thereof parallel to the axial direction of the fixing belt21. The absorber42is disposed opposite the fixing belt21via the base51and the equalizer41at the fixing nip N and in contact with the base51. The absorber42is made of a material having a thermal conductivity greater than that of the base51.

As shown inFIG. 13A, an absorber43serving as a second thermal conductor smaller than the equalizer41and the absorber42in the longitudinal direction of the equalizer41and the absorber42is sandwiched between the equalizer41and the absorber42. The absorber43is made of a material having a thermal conductivity greater than that of the base51. The absorber43is disposed opposite the fixing belt21via the equalizer41in the non-conveyance span on the fixing belt21outboard from the smallest conveyance span A in the axial direction of the fixing belt21where the fixing belt21is susceptible to overheating to the temperature TA. For example, the absorber43is disposed opposite an overheating span of the fixing belt21in the axial direction thereof where the fixing belt21is susceptible to overheating. The overheating span of the fixing belt21includes at least a part of the non-conveyance span on the fixing belt21and a contiguous span contiguous to the non-conveyance span in the axial direction of the fixing belt21, that is, a part of the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21.

Thus, the nip formation pad24V includes the base51, the equalizer41, and the absorbers42and43.

As shown inFIG. 13A, the nip formation pad24V includes an increased thermal conduction portion IP and a decreased thermal conduction portion DP. In the increased thermal conduction portion IP, the nip formation pad24V is constructed of a plurality of layers: the equalizer41and the absorbers43and42. Conversely, in each decreased thermal conduction portion DP, the nip formation pad24V is constructed of a plurality of layers: the equalizer41, the base51, and the absorber42. The thermal conductivity of the base51is different from that of the equalizer41and the absorbers42and43. For example, the thermal conductivity of the equalizer41and the absorbers42and43is greater than that of the base51. Thus, the nip formation pad24V is constructed of a plurality of layers made of a plurality of materials having different thermal conductivities, respectively, that are layered in a thickness direction of the nip formation pad24V.

The increased thermal conduction portion IP corresponding to the absorber43having an increased thermal conductivity provides a combined thermal conductivity combining thermal conductivities of the equalizer41and the absorbers42and43in the thickness direction of the nip formation pad24V that is greater than a combined thermal conductivity combining thermal conductivities of the equalizer41, the base51, and the absorber42in each decreased thermal conduction portion DP not corresponding to the absorber43. Accordingly, the increased thermal conduction portion IP of the nip formation pad24V absorbs heat from the fixing belt21readily. Consequently, even if the fixing belt21overheats substantially at an axial span thereof corresponding to the increased thermal conduction portion IP of the nip formation pad24V, the nip formation pad24V absorbs heat from the fixing belt21upward inFIG. 13Ain the thickness direction of the nip formation pad24V, thus suppressing overheating of the fixing belt21.

The equalizer41facilitates conduction of heat in the longitudinal direction thereof parallel to the axial direction of the fixing belt21, equalizing an amount of heat stored in the fixing belt21and thereby suppressing overheating of both lateral ends of the fixing belt21in the axial direction thereof. Conversely, the absorbers42and43facilitate conduction of heat in the thickness direction of the nip formation pad24V perpendicular to the longitudinal direction thereof and absorb heat from the base51and the equalizer41. As shown inFIGS. 13A and 13C, the absorber43is disposed opposite the greater non-conveyance span of the fixing belt21that is outboard from the smaller conveyance span A on the fixing belt21in the axial direction thereof and is susceptible to overheating to the temperature TA. The absorber43absorbs heat from the base51and the equalizer41and conducts the absorbed heat to the absorber42in contact with the absorber43. That is, the absorbers42and43supplement shortage of thermal capacity of the equalizer41. For example, the absorber42has an increased thermal capacity or an increased surface area to increase heat dissipation.

However, the equalizer41, as it has a predetermined thickness, absorbs heat in the thickness direction thereof. Each of the absorbers42and43, as it has an axial span in the axial direction of the fixing belt21, equalizes heat in the axial direction of the fixing belt21. Hence, the equalizer41achieves absorption as well as equalization. Similarly, the absorbers42and43achieve equalization as well as absorption.

With reference toFIGS. 14,15A,15B,15C, and16, a description is provided of a construction of a fixing device20W according to yet another exemplary embodiment.

FIG. 14is a partial vertical sectional view of the fixing device20W.FIG. 15Ais a sectional view of a nip formation pad24W taken along line LA-LA inFIG. 14.FIG. 15Bis a diagram illustrating positional relations between the light emission span H of the halogen heater23and the four conveyance spans A, B, C, and D of sheets P of four sizes in the longitudinal direction of the halogen heater23.FIG. 15Cis a graph showing a relation between the distance from the center of the fixing belt21in the axial direction thereof and the temperature of the fixing belt21in the non-conveyance span outboard from the conveyance spans A, B, C, and D in the axial direction of the fixing belt21as sheets P of four sizes are conveyed over the fixing belt21.FIG. 15Cillustrates the temperatures TA, TB, and TC in the non-conveyance span, that is, the lateral end of the fixing belt21in the axial direction thereof, where the sheet P is not conveyed over the fixing belt21and the temperatures tA, tB, tC, and tD in the conveyance span, that is, the center of the fixing belt21in the axial direction thereof, where the sheet P is conveyed over the fixing belt21.FIG. 16is a schematic exploded perspective view of the nip formation pad24W illustrating an A6 size sheet P conveyed in the sheet conveyance direction A1.

As shown inFIG. 14, the fixing device20W includes the equalizer41sandwiched between the base51and the fixing belt21at the fixing nip N and extended in the longitudinal direction thereof parallel to the axial direction of the fixing belt21. The equalizer41serving as a first thermal conductor is made of a material having a thermal conductivity greater than that of the base51. The fixing device20W further includes the absorber42serving as a third thermal conductor sandwiched between the base51and the stay25and extended in the longitudinal direction thereof parallel to the axial direction of the fixing belt21. The absorber42is disposed opposite the fixing belt21via the base51and the equalizer41at the fixing nip N and in contact with the base51. The absorber42is made of a material having a thermal conductivity greater than that of the base51.

As shown inFIG. 15A, the absorber43serving as a second thermal conductor smaller than the equalizer41and the absorber42in the longitudinal direction of the equalizer41and the absorber42is sandwiched between the equalizer41and the absorber42. The absorber43is made of a material having a thermal conductivity greater than that of the base51. For example, like the absorber43of the nip formation pad24V depicted inFIG. 13A, the absorber43of the nip formation pad24W depicted inFIG. 15Ais disposed opposite the overheating span of the fixing belt21in the axial direction thereof where the fixing belt21is susceptible to overheating. The overheating span of the fixing belt21includes at least a part of the non-conveyance span on the fixing belt21where the sheet P is not conveyed over the fixing belt21and the contiguous span contiguous to the non-conveyance span in the axial direction of the fixing belt21, that is, a part of the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21.

The equalizer41facilitates conduction of heat in the longitudinal direction thereof parallel to the axial direction of the fixing belt21, equalizing an amount of heat stored in the fixing belt21and thereby suppressing overheating of both lateral ends of the fixing belt21in the axial direction thereof. Conversely, the absorbers42and43facilitate conduction of heat in a thickness direction of the nip formation pad24W perpendicular to a longitudinal direction thereof and absorb heat from the base51and the equalizer41. As shown inFIGS. 15A and 15C, the absorber43disposed opposite the fixing belt21via the equalizer41is disposed opposite the greater non-conveyance span of the fixing belt21that is outboard from the smaller conveyance span A on the fixing belt21in the axial direction thereof and is susceptible to overheating to the temperature TA. The absorber43absorbs heat from the base51and the equalizer41and conducts the absorbed heat to the absorber42in contact with the absorber43. That is, the absorbers42and43supplement shortage of thermal capacity of the equalizer41. For example, the absorber42has an increased thermal capacity or an increased surface area to increase heat dissipation.

As shown inFIG. 14, since a space inside the loop formed by the fixing belt21is limited, the absorber42is interposed between the base51constituting the resin layer and the stay25and extended in the longitudinal direction thereof parallel to the axial direction of the fixing belt21. Alternatively, if a space is available, the absorber42may be upsized in the axial direction or the circumferential direction of the fixing belt21to increase the thermal capacity of the absorber42. Yet alternatively, the absorber42may contact the stay25to increase an apparent thermal capacity of the absorber42. In this case, the stay25needs to be cooler than the absorber42. Accordingly, in order to suppress conduction of heat from the reflector26heated by the halogen heater23to the stay25, an air layer or an insulation layer made of an insulation material is interposed between the reflector26and the stay25. Yet alternatively, instead of the absorber42, the stay25having a thermal capacity greater than that of the base51may contact the absorber43to absorb heat from the absorber43and the base51.

The absorbers42and43are made of metal such as copper. Alternatively, the absorbers42and43may be made of resin in view of temperature increase in the non-conveyance span produced at both lateral ends of the fixing belt21in the axial direction thereof.

Table 1 below shows the material and the thermal conductivity of the equalizer41and the absorbers42and43.

Table 2 below shows the material and the thermal conductivity of the base51.

As shown inFIGS. 15A and 16, the nip formation pad24W further includes a resin layer44sandwiched between the equalizer41and the absorber43. Hence, the nip formation pad24W includes the base51, the equalizer41, the absorbers42and43, and the resin layer44. The resin layer44is made of a material having a thermal conductivity smaller than that of the absorber43. The resin layer44interposed between the equalizer41and the absorber43in contact with the absorber42reduces an amount of heat conducted from the equalizer41to the absorber42through the absorber43. Accordingly, the temperature TA of the non-conveyance span outboard from the conveyance span A on the fixing belt21in the axial direction thereof is suppressed to a temperature lower than the upper limit of target temperature UT of the fixing belt21and at the same time shortage of heat in the conveyance span on the fixing belt21indicated by the temperatures tB, tC, and tD depicted inFIG. 15Cthat may lower the temperature of the fixing belt21below a fixing temperature FT is prevented while saving power.

If the resin layer44is thick excessively, the thick resin layer44may prohibit heat stored in the fixing belt21from being conducted to the absorber42, rendering the fixing belt21to be susceptible to overheating of the non-conveyance span produced at both lateral ends of the fixing belt21in the axial direction thereof. It is necessary to determine the thickness and the length of the resin layer44based on the degree of overheating of both lateral ends of the fixing belt21in the axial direction thereof. Overheating of both lateral ends of the fixing belt21in the axial direction thereof that may not be overcome by the equalizer41may occur at a plurality of spots spaced apart from each other. To address this circumstance, a plurality of absorbers43is disposed opposite the plurality of overheated spots on the fixing belt21, respectively. For example, as shown inFIG. 16, the plurality of absorbers43may be aligned in the longitudinal direction of the equalizer41. In this case, the thickness and the length of the resin layer44are determined based on the degree of overheating at the respective spots on both lateral ends of the fixing belt21in the axial direction thereof. The combined thickness of the absorber43and the resin layer44is equivalent to the thickness of the base51, allowing the absorber43to come into surface contact with the absorber42and thereby facilitating conduction of heat from the absorber43to the absorber42and vice versa.

Like the nip formation pad24V shown inFIG. 13A, the nip formation pad24W shown inFIG. 15Ais constructed of a plurality of layers: the equalizer41, the resin layer44, and the absorbers43and42in the increased thermal conduction portion IP. Conversely, the nip formation pad24W is constructed of a plurality of layers: the equalizer41, the base51, and the absorber42in each decreased thermal conduction portion DP. The thermal conductivity of the base51and the resin layer44is different from that of the equalizer41and the absorbers42and43. For example, the thermal conductivity of the equalizer41and the absorbers42and43is greater than that of the base51and the resin layer44. Thus, the nip formation pad24W is constructed of a plurality of layers made of a plurality of materials having different thermal conductivities, respectively, that are layered vertically inFIG. 15Ain the thickness direction of the nip formation pad24W.

The increased thermal conduction portion IP corresponding to the absorber43having an increased thermal conductivity provides a combined thermal conductivity combining thermal conductivities of the equalizer41, the resin layer44, and the absorbers42and43in the thickness direction of the nip formation pad24W that is greater than a combined thermal conductivity combining thermal conductivities of the equalizer41, the base51, and the absorber42in each decreased thermal conduction portion DP not corresponding to the absorber43. Accordingly, the increased thermal conduction portion IP of the nip formation pad24W absorbs heat from the fixing belt21readily. Consequently, even if the fixing belt21overheats substantially at an axial span thereof corresponding to the increased thermal conduction portion IP of the nip formation pad24W, the nip formation pad24W absorbs heat from the fixing belt21upward inFIG. 15Ain the thickness direction of the nip formation pad24W, thus suppressing overheating of the fixing belt21.

The equalizer41, the absorbers42and43, the resin layer44, and the base51that constitute the nip formation pad24W have the thickness for the length of about 10 mm of the fixing nip N in the sheet conveyance direction A1. For example, the equalizer41has a thickness in a range of from about 0.2 mm to about 0.6 mm. The absorber42has a thickness in a range of from about 1.8 mm to about 6.0 mm. The absorber43has a thickness in a range of from about 1.0 mm to about 2.0 mm. The resin layer44has a thickness in a range of from about 0.5 mm to about 1.5 mm. The base51has a thickness in a range of from about 1.5 mm to about 3.5 mm. However, the thickness of each of the equalizer41, the absorbers42and43, the resin layer44, and the base51is not limited to the above.

A rim projecting from each lateral end of the equalizer41in the sheet conveyance direction A1 toward the absorber42may extend throughout the entire span of the equalizer41in the longitudinal direction thereof. The equalizer41and the rim mounted thereon produce a U-like shape in cross-section that accommodates the base51, the resin layer44, and the absorbers42and43that are layered on the equalizer41. Alternatively, a projection may project from an inner face of the equalizer41to engage a through-hole penetrating through each of the base51, the resin layer44, the absorber43, and the like.

Each of the equalizer41and the absorber42is an independent part extending in a span corresponding to the light emission span H of the halogen heater23. Contrarily, the base51, the resin layer44, and the absorber43constitute multiple parts divided in the axial direction of the fixing belt21. As shown inFIG. 16, the length of the center base51in the axial direction of the fixing belt21is equivalent to the width, that is, a short side, of the A6 size sheet P in the axial direction of the fixing belt21.

AlthoughFIG. 16illustrates the absorber43constituting the increased thermal conduction portion IP that is disposed outboard from the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21in the axial direction thereof, the absorber43may extend to the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21so that the increased thermal conduction portion IP including the absorber43is disposed opposite the overheating span of the fixing belt21including at least a part of the non-conveyance span on the fixing belt21where the sheet P is not conveyed over the fixing belt21and the contiguous span contiguous to the non-conveyance span in the axial direction of the fixing belt21, that is, a part of the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21.

Alternatively, as shown inFIGS. 9A to 9D, the equalizer41of the fixing device20W may mount the bulge45,45T, or45U projecting from the downstream end41aof the equalizer41toward the pressure roller22and the low-friction sheet59coating the nip face of the equalizer41disposed opposite the fixing nip N. The configurations of the components that form the fixing nip N shown inFIGS. 9A to 9D,10B,11A, and11B are also applicable to the fixing device20W.

Typical Electricity Consumption (TEC) is an index of energy saving. The equalizer41may confront a trade-off between a TEC value and overheating of both lateral ends of the fixing belt21in the axial direction thereof. For example, if the equalizer41is excessively thin, it may not suppress overheating of both lateral ends of the fixing belt21in the axial direction thereof. Conversely, if the equalizer41is excessively thick, it may degrade the TEC value. To address this circumstance, the thickness of the equalizer41is in a range of from about 9 micrometers to about 3 mm.

With reference toFIG. 17, a description is provided of a construction of a nip formation pad24X according to yet another exemplary embodiment.

FIG. 17is a schematic exploded perspective view of the nip formation pad24X.FIG. 17illustrates an A6 size sheet P conveyed in the sheet conveyance direction A1. As shown inFIG. 17, like the nip formation pads24V and24W depicted inFIGS. 13A and 15A, respectively, the nip formation pad24X includes the absorber43sandwiched between the equalizer41and the absorber42and extended in the axial direction of the fixing belt21. The absorber43is embedded in a recess52produced in the base51. Hence, the nip formation pad24X includes the base51, the recess52, the equalizer41, and the absorbers42and43. The recess52does not penetrate through the base51. The recess52is thinner than a portion of the base51where the recess52is not produced. The thickness of the recess52is changed to adjust an amount of heat conducted from the equalizer41to the absorber42through the absorber43. Further, the length of the recess52in the sheet conveyance direction A1 is changed in accordance with an amount of heat to be absorbed by the absorber43. For example, as the amount of heat to be absorbed by the absorber43increases, the length of the recess52in the sheet conveyance direction A1 increases. Conversely, as the amount of heat to be absorbed by the absorber43decreases, the length of the recess52in the sheet conveyance direction A1 decreases.

A face of the absorber43disposed opposite the absorber42is leveled with a face of the base51disposed opposite the absorber42. Alternatively, the recess52may penetrate through the base51and may be equivalent in thickness to a portion of the base51where the recess52is not produced.

AlthoughFIG. 17illustrates the absorber43constituting the increased thermal conduction portion IP that is disposed outboard from the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21in the axial direction thereof, the absorber43may extend to the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21so that the increased thermal conduction portion IP including the absorber43is disposed opposite the overheating span of the fixing belt21including at least a part of the non-conveyance span on the fixing belt21where the sheet P is not conveyed over the fixing belt21and the contiguous span contiguous to the non-conveyance span in the axial direction of the fixing belt21, that is, a part of the conveyance span on the fixing belt21where the sheet P is conveyed over the fixing belt21.

With the construction of the nip formation pad24X described above, the temperature TA of the non-conveyance span outboard from the conveyance span A on the fixing belt21in the axial direction thereof is suppressed to a temperature lower than the upper limit of target temperature UT of the fixing belt21and at the same time shortage of heat in the fixing belt21is reduced while saving power.

With reference toFIGS. 18 and 19, a description is provided of a construction of a nip formation pad24Y according to yet another exemplary embodiment.

FIG. 18is a schematic exploded perspective view of the nip formation pad24Y seen from the fixing nip N shown inFIG. 14.FIG. 19is a schematic exploded perspective view of the nip formation pad24Y seen from the stay25shown inFIG. 14. The following describes mainly a construction of the nip formation pad24Y peculiar to it.

As shown inFIG. 18, each lateral end of the equalizer41in the sheet conveyance direction A1 is bent to produce a rim projecting toward the absorber42. Hence, the equalizer41is formed in a U-like shape in cross-section that accommodates the base51, the resin layer44, and the absorbers42and43that are layered on the equalizer41. The rim of the equalizer41includes teeth56. The teeth56are not continuously produced throughout the entire span of the equalizer41in the longitudinal direction thereof. For example, planar portions are aligned in the longitudinal direction of the equalizer41with a predetermined interval between the adjacent planar portions. The teeth56catch or engage the low-friction sheet59serving as a slide aid wound around an outer circumferential surface of the nip formation pad24Y, preventing the low-friction sheet59from being displaced in accordance with rotation of the fixing belt21. A jig used to attach the low-friction sheet59to the nip formation pad24Y comes into contact with the planar portion of the equalizer41.

As shown inFIG. 19, the teeth56are produced on the rim of the equalizer41at each lateral end thereof in the sheet conveyance direction A1. Alternatively, the teeth56may be produced at one lateral end of the equalizer41disposed opposite an entry to the fixing nip N in the sheet conveyance direction A1, that is, a lower end of the equalizer41inFIG. 19. Since the fixing belt21moves from the entry to the exit of the fixing nip N, if the teeth56situated at the entry to the fixing nip N catch the low-friction sheet59precisely, it may not be necessary to produce the teeth56at the exit of the fixing nip N.

As shown inFIG. 19, through-holes54serving as second through-holes and through-holes55serving as third through-holes penetrate through the absorber42. Through-holes53serving as first through-holes penetrate through the absorber43. Projections58serving as second projections projecting from an inner face of the base51toward the absorber42are inserted into the through-holes55. Projections57serving as third projections projecting from the inner face of the base51toward the absorber42are inserted into the through-holes54. Projections57serving as first projections projecting from an inner face of the resin layer44toward the absorbers43and42are inserted into the through-holes53and54. The projection57projecting from the resin layer44is inserted into the through-hole53produced through the absorber43to hold the absorber43. The projection58projecting from the base51is inserted into the through-hole55produced through the absorber42to hold the absorber42. The projection57projecting from the base51is inserted into the through-hole54produced through the absorber42to hold the absorber42. The projection58is longer than the projection57in a projection direction perpendicular to a longitudinal direction of the nip formation pad24Y. Accordingly, the projection58penetrating through the through-hole55produced through the absorber42engages an engagement hole of the stay25depicted inFIG. 14, thus mounting the nip formation pad24Y on the stay25.

As shown inFIG. 18, the bulge45projects from the equalizer41toward the pressure roller22at the downstream end41athereof disposed opposite the exit of the fixing nip N. The equalizer41is made of a single copper plate that is planar from the entry to the exit of the fixing nip N, that is, vertically upward inFIG. 18, and curved at the exit of the fixing nip N to project toward the pressure roller22depicted inFIG. 14, producing the bulge45.

According to the exemplary embodiments described above, the stationary equalizer41is mounted on the nip face of the base51pressing against the inner circumferential surface of the fixing belt21. Accordingly, the equalizer41prevents overheating of both lateral ends of the fixing belt21in the axial direction thereof without a driver or a holder that moves the equalizer41to both lateral ends of the fixing belt21in the axial direction thereof. Additionally, the absorbers42and43adjust an amount of heat absorbed therein in a thickness direction of a nip formation pad (e.g., the nip formation pads24,24′,24S,24T,24U,24V,24W,24X, and24Y). The equalizer41conducts heat in the axial direction of the fixing belt21and the absorbers42and43absorb heat conducted from the fixing belt21through the equalizer41, preventing overheating of the non-conveyance span produced at both lateral ends of the fixing belt21in the axial direction thereof and reducing energy consumption while preventing adverse effects such as an extended warm-up time to warm up the fixing belt21and shortage of heat in the fixing belt21. As shown inFIGS. 9A to 9D, the bulge45,45T, or45U projecting from the equalizer41at the downstream end41adisposed opposite the exit of the fixing nip N produces the nip face of the equalizer41that facilitates separation of the sheet P from the fixing belt21.

As shown inFIG. 11A, the low-friction sheet59interposed between the fixing belt21and the equalizer41facilitates sliding of the fixing belt21over the equalizer41. As shown inFIG. 11B, the elastic layer57or conductive grease interposed between the rough low-friction sheet59and the rough equalizer41having surface asperities SA eliminates the air layer produced between the low-friction sheet59and the equalizer41and increases the area of an interface between the equalizer41and the low-friction sheet59, facilitating conduction of heat from the low-friction sheet59to the equalizer41and thereby evening temperature distribution of the fixing belt21.

A description is provided of advantages of the fixing devices20,20V, and20W.

As shown inFIGS. 6,12, and14, a fixing device (e.g., the fixing devices20,20V, and20W) includes a fixing rotator (e.g., the fixing belt21) rotatable in the rotation direction R3; an opposed rotator (e.g., the pressure roller22) disposed opposite the fixing rotator; a heater (e.g., the halogen heater23) to heat the fixing rotator; a nip formation pad (e.g., the nip formation pads24,24′,24S,24T,24U,24V,24W,24X, and24Y) disposed opposite an inner circumferential surface of the fixing rotator; and a support (e.g., the stay25) to support the nip formation pad. The opposed rotator is pressed against the nip formation pad via the fixing rotator to form the fixing nip N between the opposed rotator and the fixing rotator, through which a recording medium (e.g., a sheet P) bearing a toner image is conveyed. The nip formation pad includes a base (e.g., the base51) and a first thermal conductor (e.g., the equalizer41) having a thermal capacity or a thermal conductivity greater than that of the base and being sandwiched between the fixing rotator and the base. As shown inFIGS. 9A to 9D, a bulge (e.g., the bulges45,45T, and45U) projects from the downstream end41aof the first thermal conductor in a recording medium conveyance direction (e.g., the sheet conveyance direction A1) toward the opposed rotator. The downstream end41aof the first thermal conductor is disposed opposite the exit of the fixing nip N in the recording medium conveyance direction.

The stationary first thermal conductor facilitates heat conduction. Accordingly, the fixing device prevents or suppresses overheating of both lateral ends of the fixing rotator in an axial direction thereof during a fixing operation to fix the toner image on the recording medium and reduces waste of energy while preventing adverse effects such as increased energy consumption, an extended warm-up time to warm up the fixing rotator, and shortage of heat in the fixing rotator. The first thermal conductor interposed between the fixing rotator and the base evens heat distribution of the fixing rotator and facilitates separation of the recording medium from the fixing rotator.

As shown inFIGS. 8B,13B, and15B, the conveyance spans A, B, C, and D where sheets P of various sizes are conveyed over the fixing belt21are centered in the axial direction of the fixing belt21. Hence, the non-conveyance span of the fixing belt21, outboard from each of the conveyance spans A, B, C, and D, where the sheets P are not conveyed over the fixing belt21is produced at each lateral end of the fixing belt21in the axial direction thereof. Alternatively, the conveyance spans A, B, C, and D may be defined along one lateral edge of the fixing belt21in the axial direction thereof and the non-conveyance span of the fixing belt21may be defined along another lateral edge of the fixing belt21in the axial direction thereof.

According to the exemplary embodiments described above, the fixing belt21serves as a fixing rotator. Alternatively, a fixing film, a fixing roller, or the like may be used as a fixing rotator. Further, the pressure roller22serves as an opposed rotator. Alternatively, a pressure belt or the like may be used as an opposed rotator.

The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention 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 invention. It is therefore to be understood that the present invention 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 invention.