Fixing device and image forming apparatus incorporating same

A fixing device for fixing a toner image on a recording medium includes a pressing member provided outside a loop formed by a fixing member to press the fixing member against a nip formation member provided inside the loop formed by the fixing member. A heat generator support is provided inside the loop formed by the fixing member to support a heat generator that generates heat to be transmitted to the fixing member. A temperature detector is provided downstream from the heat generator and upstream from the nip formation member in a direction of rotation of the fixing member to detect a temperature of the fixing member. A controller is connected to the temperature detector and the heat generator to control heat generation of the heat generator based on the temperature of the fixing member detected by the temperature detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to Japanese Patent Application No. 2010-058725, filed on Mar. 16, 2010, in the Japan Patent Office, which is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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 a toner image on a recording medium, and an image forming apparatus including the fixing device.

2. Description of the Related Art

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; 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 foaming the image on the recording medium.

The fixing device used in such image forming apparatuses may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image.FIG. 1is a vertical sectional view of a fixing device20R1including such a fixing belt204. The fixing belt204is looped around a heating roller202and a fixing roller203, in a state in which a tension roller206biases the fixing belt204. A pressing roller205presses against the fixing roller203via the fixing belt204to form a nip N between the pressing roller205and the fixing belt204. The fixing belt204is heated by a heater201provided inside the heating roller202. As a recording medium P bearing a toner image passes between the fixing roller203and the pressing roller205on the fixing belt204, the fixing belt204and the pressing roller205together apply heat and pressure to the recording medium P bearing the toner image to fix the toner image on the recording medium P.

One problem with such an arrangement, however, is that the heating roller202has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device20R1. To address this problem, instead of the fixing belt204the fixing device may include a fixing film having a relatively small heat capacity.FIG. 2is a vertical sectional view of such a fixing device20R2including a fixing film213. A ceramic heater211is provided inside a loop formed by the fixing film213. A pressing roller212presses against the ceramic heater211via the fixing film213to form a nip N between the pressing roller212and the fixing film213. As a recording medium bearing a toner image passes between the pressing roller212and the fixing film213, the fixing film213heated by the ceramic heater211and the pressing roller212together apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.

However, the fixing film213also has a drawback in that, over time, friction between the ceramic heater211and the fixing film213sliding over the ceramic heater211increases, resulting eventually in unstable movement of the fixing film213and increasing the required driving torque of the fixing device20R2.

Moreover, the temperature of the fixing device as the recording medium bearing the toner image enters the fixing device is critical to imaging outcome. In this respect, the fixing film213has another drawback in that the ceramic heater211heats the fixing film213at the nip N only, and therefore the rotating fixing film213is coolest when it reenters the nip N, resulting in formation of a faulty toner image on the recording medium due to the lower temperature of the fixing film213at that location.

To overcome these drawbacks, instead of the ceramic heater211the fixing device may include a heat generator provided inside the loop formed by the fixing film to heat the fixing film locally, and the temperature of the fixing film is detected by a temperature detector. However, there is a certain distance or a gap between the heat generator and the nip N in the direction of rotation of the fixing film, and the temperature detector is typically disposed in proximity to the heat generator. Accordingly, even if the temperature of the fixing film is controlled based on the temperature of the fixing film detected by the temperature detector disposed near the heat generator, the fixing film is still cooled when it enters the nip N. In other words, the temperature of the fixing film at the position where the heat generator faces and heats the fixing film directly may be different from the temperature of the fixing film at the nip N. As a result, a faulty toner image is formed on the recording medium due to the unstable fixing temperature of the fixing film at the nip N.

BRIEF SUMMARY OF THE INVENTION

This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device fixes a toner image on a recording medium and includes an endless belt-shaped fixing member, a nip formation member, a pressing member, a heat generator, a heat generator support, a temperature detector, and a controller. The fixing member is formed into a loop and rotates in a predetermined direction of rotation. The nip formation member is provided inside the loop formed by the fixing member. The pressing member is provided outside the loop formed by the fixing member and opposite the nip formation member to press the fixing member against the nip formation member to form a nip between the pressing member and the fixing member through which the recording medium bearing the toner image passes. The heat generator faces an inner circumferential surface of the fixing member to heat the fixing member. The heat generator support is provided inside the loop formed by the fixing member to support the heat generator at a predetermined position between the fixing member and the heat generator support. The temperature detector is provided downstream from the heat generator and upstream from the nip formation member in the direction of rotation of the fixing member to detect a temperature of the fixing member. The controller is connected to the temperature detector and the heat generator to control heat generation of the heat generator based on the temperature of the fixing member detected by the temperature detector.

This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes the fixing device described above.

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. 3, an image forming apparatus1according to an exemplary embodiment of the present invention is explained.

FIG. 3is a schematic view of the image forming apparatus1. As illustrated inFIG. 3, the image forming apparatus1may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment of the present invention, the image forming apparatus1is a tandem color printer for forming a color image on a recording medium.

As illustrated inFIG. 3, the image forming apparatus1includes image forming devices4Y,4M,4C, and4K disposed in a center portion of the image forming apparatus1, a toner bottle holder101disposed above the image forming devices4Y,4M,4C, and4K in an upper portion of the image forming apparatus1, an exposure device3disposed below the image forming devices4Y,4M,4C, and4K, a paper tray12disposed below the exposure device3in a lower portion of the image forming apparatus1, an intermediate transfer unit85disposed above the image forming devices4Y,4M,4C, and4K, a second transfer roller89disposed opposite the intermediate transfer unit85, a feed roller97and a registration roller pair98disposed between the paper tray12and the second transfer roller89in a recording medium conveyance direction, a fixing device20disposed above the second transfer roller89, an output roller pair99disposed above the fixing device20, a stack portion100disposed downstream from the output roller pair99in the recording medium conveyance direction on top of the image forming apparatus1, and a controller10disposed in the upper portion of the image forming apparatus1.

The toner bottle holder101includes toner bottles102Y,102M,102C, and102K. The four toner bottles102Y,102M,102C, and102K contain yellow, magenta, cyan, and black toners, respectively, and are detachably attached to the toner bottle holder101so that the toner bottles102Y,102M,102C, and102K are replaced with new ones, respectively.

The intermediate transfer unit85is disposed below the toner bottle holder101, and includes an intermediate transfer belt78formed into a loop, four first transfer bias rollers79Y,79M,79C, and79K, a second transfer backup roller82, a cleaning backup roller83, and a tension roller84disposed inside the loop formed by the intermediate transfer belt78, and an intermediate transfer cleaner80disposed outside the loop formed by the intermediate transfer belt78. Specifically, the intermediate transfer belt78is supported by and stretched over three rollers, which are the second transfer backup roller82, the cleaning backup roller83, and the tension roller84. A single roller, that is, the second transfer backup roller82, drives and endlessly moves (e.g., rotates) the intermediate transfer belt78in a direction D1.

The image forming devices4Y,4M,4C, and4K are arranged opposite the intermediate transfer belt78, and form yellow, magenta, cyan, and black toner images, respectively. The image forming devices4Y,4M,4C, and4K include photoconductive drums5Y,5M,5C, and5K which are surrounded by chargers75Y,75M,75C, and75K, development devices76Y,76M,76C, and76K, cleaners77Y,77M,77C, and77K, and dischargers, respectively. Image forming processes including a charging process, an exposure process, a development process, a primary transfer process, and a cleaning process are performed on the photoconductive drums5Y,5M,5C, and5K to form yellow, magenta, cyan, and black toner images on the photoconductive drums5Y,5M,5C, and5K, respectively, as a driving motor drives and rotates the photoconductive drums5Y,5M,5C, and5K clockwise inFIG. 3.

Specifically, in the charging process, the chargers75Y,75M,75C, and75K uniformly charge surfaces of the photoconductive drums5Y,5M,5C, and5K at charging positions at which the chargers75Y,75M,75C, and75K are disposed opposite the photoconductive drums5Y,5M,5C, and5K, respectively.

In the exposure process, the exposure device3emits laser beams L onto the charged surfaces of the respective photoconductive drums5Y,5M,5C, and5K according to image data sent from a client computer, for example. In other words, the exposure device3scans and exposes the charged surfaces of the photoconductive drums5Y,5M,5C, and5K at irradiation positions at which the exposure device3is disposed opposite the photoconductive drums5Y,5M,5C, and5K to irradiate the charged surfaces of the photoconductive drums5Y,5M,5C, and5K to form thereon electrostatic latent images corresponding to yellow, magenta, cyan, and black colors, respectively.

In the development process, the development devices76Y,76M,76C, and76K render the electrostatic latent images formed on the surfaces of the photoconductive drums5Y,5M,5C, and5K visible as yellow, magenta, cyan, and black toner images at development positions at which the development devices76Y,76M,76C, and76K are disposed opposite the photoconductive drums5Y,5M,5C, and5K, respectively.

In the primary transfer process, the first transfer bias rollers79Y,79M,79C, and79K transfer and superimpose the yellow, magenta, cyan, and black toner images formed on the photoconductive drums5Y,5M,5C, and5K onto the intermediate transfer belt78at first transfer positions at which the first transfer bias rollers79Y,79M,79C, and79K are disposed opposite the photoconductive drums5Y,5M,5C, and5K via the intermediate transfer belt78, respectively. Thus, a color toner image is formed on the intermediate transfer belt78. After the transfer of the yellow, magenta, cyan, and black toner images, a slight amount of residual toner, which has not been transferred onto the intermediate transfer belt78, remains on the photoconductive drums5Y,5M,5C, and5K.

In the cleaning process, cleaning blades included in the cleaners77Y,77M,77C, and77K mechanically collect the residual toner from the photoconductive drums5Y,5M,5C, and5K at cleaning positions at which the cleaners77Y,77M,77C, and77K are disposed opposite the photoconductive drums5Y,5M,5C, and5K, respectively.

Finally, dischargers remove residual potential on the photoconductive drums5Y,5M,5C, and5K at discharging positions at which the dischargers are disposed opposite the photoconductive drums5Y,5M,5C, and5K, respectively, thus completing a single sequence of image forming processes performed on the photoconductive drums5Y,5M,5C, and5K.

The following describes the transfer processes, that is, the primary transfer process described above and a secondary transfer process, performed on the intermediate transfer belt78. The four first transfer bias rollers79Y,79M,79C, and79K and the photoconductive drums5Y,5M,5C, and5K sandwich the intermediate transfer belt78to form first transfer nips, respectively. The first transfer bias rollers79Y,79M,79C, and79K are applied with a transfer bias having a polarity opposite a polarity of toner forming the yellow, magenta, cyan, and black toner images on the photoconductive drums5Y,5M,5C, and5K, respectively. Accordingly, in the primary transfer process, the yellow, magenta, cyan, and black toner images formed on the photoconductive drums5Y,5M,5C, and5K, respectively, are primarily transferred and superimposed onto the intermediate transfer belt78rotating in the direction D1successively at the first transfer nips formed between the photoconductive drums5Y,5M,5C, and5K and the intermediate transfer belt78as the intermediate transfer belt78moves through the first transfer nips. Thus, a color toner image is formed on the intermediate transfer belt78.

The second transfer roller89is pressed against the second transfer backup roller82via the intermediate transfer belt78in such a manner that the second transfer roller89and the second transfer backup roller82sandwich the intermediate transfer belt78to form a second transfer nip between the second transfer roller89and the intermediate transfer belt78. At the second transfer nip, the second transfer roller89secondarily transfers the color toner image formed on the intermediate transfer belt78onto a recording medium P sent from the paper tray12through the feed roller97and the registration roller pair98in the secondary transfer process. Thus, the desired color toner image is formed on the recording medium P. After the transfer of the color toner image, residual toner, which has not been transferred onto the recording medium P, remains on the intermediate transfer belt78.

Thereafter, the intermediate transfer cleaner80collects the residual toner from the intermediate transfer belt78at a cleaning position at which the intermediate transfer cleaner80is disposed opposite the cleaning backup roller83via the intermediate transfer belt78, thus completing a single sequence of transfer processes performed on the intermediate transfer belt78.

The recording medium P is supplied to the second transfer nip from the paper tray12which loads a plurality of recording media P (e.g., transfer sheets). Specifically, the feed roller97rotates counterclockwise inFIG. 3to feed an uppermost recording medium P of the plurality of recording media P loaded on the paper tray12toward a roller nip formed between two rollers of the registration roller pair98.

The registration roller pair98, which stops rotating temporarily, stops the uppermost recording medium P fed by the feed roller97and reaching the registration roller pair98. For example, the roller nip of the registration roller pair98contacts and stops a leading edge of the recording medium P. The registration roller pair98resumes rotating to feed the recording medium P to the second transfer nip, formed between the second transfer roller89and the intermediate transfer belt78, as the color toner image formed on the intermediate transfer belt78reaches the second transfer nip.

After the secondary transfer process described above, the recording medium P bearing the color toner image is sent to the fixing device20that includes a fixing sleeve21and a pressing roller31. The fixing sleeve21and the pressing roller31apply heat and pressure to the recording medium P to fix the color toner image on the recording medium P.

Thereafter, the fixing device20feeds the recording medium P bearing the fixed color toner image toward the output roller pair99. The output roller pair99discharges the recording medium P to an outside of the image foaming apparatus1, that is, the stack portion100. Thus, the recording media P discharged by the output roller pair99are stacked on the stack portion100successively to complete a single sequence of image forming processes performed by the image forming apparatus1.

Referring toFIG. 4, the following describes the structure of the fixing device20.FIG. 4is a vertical sectional view of the fixing device20. As illustrated inFIG. 4, the fixing device20includes the fixing sleeve21formed into a loop, a laminated heater22, a heater support23, a terminal stay24, power supply wiring25, a nip formation member26, a core holder28, and a thermistor33, which are disposed inside the loop formed by the fixing sleeve21, and the pressing roller31disposed outside the loop formed by the fixing sleeve21.

As illustrated inFIG. 4, the fixing sleeve21is a rotatable endless belt serving as a fixing member or a rotary fixing member that rotates in a rotation direction R1. The pressing roller31serves as a pressing member or a rotary pressing member that rotates in a rotation direction R2counter to the rotation direction R1, and contacts an outer circumferential surface of the fixing sleeve21to press the fixing sleeve21against the nip formation member26. The nip formation member26faces an inner circumferential surface of the fixing sleeve21, and is pressed against the pressing roller31via the fixing sleeve21to form a nip N between the pressing roller31and the fixing sleeve21through which the recording medium P bearing a toner image T passes. The laminated heater22also faces the inner circumferential surface of the fixing sleeve21in such a manner that the laminated heater22is capable of contacting or being disposed in close proximity to the inner circumferential surface of the fixing sleeve21, and serves as a heat generator that generates heat to be transmitted to the fixing sleeve21. The heater support23faces the inner circumferential surface of the fixing sleeve21and serves as a heat generator support that supports the laminated heater22serving as a heat generator at a predetermined position, in such a manner that the laminated heater22is provided between the heater support23and the fixing sleeve21. The thermistor33is provided downstream from the heater support23and upstream from the nip formation member26in the rotation direction R1of the fixing sleeve21, and serves as a temperature detector that detects a temperature of the fixing sleeve21so that the temperature of the fixing sleeve21is controlled based on a detection result of the thermistor33.

As noted above,FIG. 4illustrates a case in which the laminated heater22directly contacts the inner circumferential surface of the fixing sleeve21to heat the fixing sleeve21directly. Alternatively, the fixing device20may further include a fixing sleeve support (e.g., a pipe-shaped metal heat conductor) that supports and guides the fixing sleeve21rotating in the rotation direction R1.

Referring toFIGS. 5A and 5B, the following describes the fixing sleeve21.FIG. 5Ais a perspective view of the fixing sleeve21.FIG. 5Bis a vertical sectional view of the fixing sleeve21. As illustrated inFIG. 5A, the fixing sleeve21is a flexible, pipe-shaped or cylindrical endless belt having a predetermined width in an axial direction of the fixing sleeve21, which corresponds to a width of a recording medium P passing through the nip N formed between the fixing sleeve21and the pressing roller31depicted inFIG. 4. As illustrated inFIG. 5A, the axial direction of the pipe-shaped fixing sleeve21corresponds to a long axis, that is, a longitudinal direction, of the fixing sleeve21. By contrast, as illustrated inFIG. 5B, a circumferential direction of the pipe-shaped fixing sleeve21extends along a circumference of the fixing sleeve21or in the rotation direction R1of the fixing sleeve21, orthogonal to the long axis of the fixing sleeve21.

For example, the fixing sleeve21has an outer diameter of about 30 mm, and is constructed of a base layer made of a metal material and having a thickness in a range of from about 30 μm to about 50 μm, and at least a release layer provided on the base layer. The base layer of the fixing sleeve21is made of a conductive metal material such as iron, cobalt, nickel, an alloy of those, or the like. The release layer of the fixing sleeve21is a tube that covers the base layer. The release layer has a thickness of about 50 μm and is made of fluorine compound such as tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA). The release layer facilitates separation of toner of the toner image T on the recording medium P, which contacts the outer circumferential surface of the fixing sleeve21directly, from the fixing sleeve21.

On the other hand, the pressing roller31depicted inFIG. 4has an outer diameter of about 30 mm, and is constructed of a metal core made of a metal material such as aluminum or copper; a heat-resistant elastic layer provided on the metal core and made of silicon rubber (e.g., solid rubber); and a release layer provided on the elastic layer. The elastic layer has a thickness of about 2 mm. The release layer is a PFA tube covering the elastic layer and has a thickness of about 50 μm. Optionally, a heat generator, such as a halogen heater, may be provided inside the metal core as needed.

The pressing roller31is connected to a pressure apply-release mechanism that applies pressure to the pressing roller31to cause the pressing roller31to contact the outer circumferential surface of the fixing sleeve21and releases the pressure to separate the pressing roller31from the fixing sleeve21. Specifically, the pressure apply-release mechanism applies pressure to the pressing roller31to press the pressing roller31against the nip formation member26via the fixing sleeve21to form the nip N between the pressing roller31and the fixing sleeve21. For example, a portion of the pressing roller31contacting the fixing sleeve21causes a concave portion of the fixing sleeve21at the nip N. Thus, the recording medium P passing through the nip N moves along the concave portion of the fixing sleeve21.

A driving mechanism drives and rotates the pressing roller31, which presses the fixing sleeve21against the nip formation member26, clockwise inFIG. 4in the rotation direction R2. Accordingly, the fixing sleeve21rotates in accordance with rotation of the pressing roller31counterclockwise inFIG. 4in the rotation direction R1.

A longitudinal direction of the nip formation member26is parallel to the axial direction of the fixing sleeve21. At least a portion of the nip formation member26which is pressed against the pressing roller31via the fixing sleeve21is made of a heat-resistant elastic material such as fluorocarbon rubber. The core holder28supports and holds the nip formation member26at a predetermined position inside the loop formed by the fixing sleeve21. Preferably, a portion of the nip formation member26which contacts the inner circumferential surface of the fixing sleeve21may be made of a slidable and durable material such as Teflon® sheet. Alternatively, a lubricant (e.g., grease) may be applied to the inner circumferential surface of the fixing sleeve21to facilitate sliding of the fixing sleeve21over the nip formation member26.

The core holder28is made of sheet metal, and has a predetermined width in a longitudinal direction thereof, corresponding to a width of the fixing sleeve21in the axial direction of the fixing sleeve21. The core holder28is an H-shaped rigid member in cross-section, and is disposed at substantially a center position inside the loop formed by the fixing sleeve21.

The core holder28holds the respective components disposed inside the loop formed by the fixing sleeve21at predetermined positions. For example, the H-shaped core holder28includes a first concave portion facing the pressing roller31, which houses and holds the nip formation member26. In other words, the core holder28is disposed opposite the pressing roller31via the nip formation member26to support the nip formation member26at a back face of the nip formation member26disposed back-to-back to a front face of the nip formation member26facing the nip N. Accordingly, even when the pressing roller31presses the fixing sleeve21against the nip formation member26, the core holder28prevents substantial deformation of the nip formation member26. In addition, the nip formation member26held by the core holder28protrudes from the core holder28slightly toward the pressing roller31to isolate the core holder28from the fixing sleeve21without contacting the fixing sleeve21at the nip N.

The H-shaped core holder28further includes a second concave portion disposed back-to-back to the first concave portion, which houses and holds the terminal stay24and the power supply wiring25. The terminal stay24has a predetermined width in a longitudinal direction thereof, corresponding to the width of the fixing sleeve21in the axial direction of the fixing sleeve21, and is T-shaped in cross-section. The power supply wiring25extends on the terminal stay24, and transmits power supplied from an outside of the fixing device20. A part of an outer circumferential surface of the core holder28holds the heater support23that supports the laminated heater22. InFIG. 4, the core holder28holds the heater support23in a lower half region inside the loop formed by the fixing sleeve21, that is, in a semicircular region provided upstream from the nip N in the rotation direction R1of the fixing sleeve21. The heater support23can be adhered to the core holder28to facilitate assembly. Alternatively, the heater support23may not be adhered to the core holder28to suppress heat transmission from the heater support23to the core holder28.

The heater support23supports the laminated heater22in such a manner that the laminated heater22contacts the inner circumferential surface of the fixing sleeve21or the laminated heater22is disposed in close proximity to the inner circumferential surface of the fixing sleeve21across a predetermined gap therebetween. Accordingly, the heater support23includes an arc-shaped outer circumferential surface portion having a predetermined circumferential length and disposed along the inner circumferential surface of the circular fixing sleeve21in cross-section.

Preferably, the heater support23has a heat resistance that resists heat generated by the laminated heater22, a strength sufficient to support the laminated heater22without being deformed by the fixing sleeve21even when the rotating fixing sleeve21contacts the laminated heater22, and sufficient heat insulation so that heat generated by the laminated heater22is not transmitted to the core holder28but is transmitted to the fixing sleeve21. For example, the heater support23is molded foam made of polyimide resin. Specifically, when the laminated heater22is configured to contact the inner circumferential surface of the fixing sleeve21, the rotating fixing sleeve21applies tension to the laminated heater22, which pulls and stretches the laminated heater22toward the nip N. To resist this tension, the heater support23is required to have a strength sufficient to support the laminated heater22without being deformed. To address this requirement, the heater support23is molded foam made of polyimide resin. Alternatively, a supplemental solid resin member may be provided inside the molded foam made of polyimide resin to improve rigidity.

Referring toFIG. 6, the following describes the laminated heater22.FIG. 6is a horizontal sectional view of the laminated heater22. As illustrated inFIG. 6, the laminated heater22includes a heat generation sheet22sconstructed of a base layer22ahaving insulation; a resistant heat generation layer22bprovided on the base layer22aand including conductive particles dispersed in a heat-resistant resin; an electrode layer22cprovided on the base layer22ato supply power to the resistant heat generation layer22b; and an insulation layer22dprovided on the base layer22a. The heat generation sheet22sis flexible, and has a predetermined width in the axial direction of the fixing sleeve21depicted inFIG. 5Aand a predetermined length in the circumferential direction of the fixing sleeve21depicted inFIG. 5B. The insulation layer22dinsulates one resistant heat generation layer22bfrom the adjacent electrode layer22cof a different power supply system, and insulates an edge of the heat generation sheet22sfrom an outside of the heat generation sheet22s.

The heat generation sheet22shas a thickness in a range of from about 0.1 mm to about 1.0 mm, and has a flexibility sufficient to wrap around the heater support23depicted inFIG. 4at least along an outer circumferential surface of the heater support23.

The base layer22ais a thin, elastic film made of a resin having a certain level of heat resistance, such as polyethylene terephthalate (PET) or polyimide resin. For example, the base layer22amay be a film made of polyimide resin to provide heat resistance, insulation, and a certain level of flexibility.

The resistant heat generation layer22bis a thin, conductive film in which conductive particles, such as carbon particles and metal particles, are uniformly dispersed in a heat-resistant resin such as polyimide resin. When power is supplied to the resistant heat generation layer22b, internal resistance of the resistant heat generation layer22bgenerates Joule heat. The resistant heat generation layer22bis manufactured by coating the base layer22awith a coating compound in which conductive particles, such as carbon particles and metal particles, are dispersed in a precursor made of a heat-resistant resin such as polyimide resin.

Alternatively, the resistant heat generation layer22bmay be manufactured by providing a thin conductive layer made of carbon particles and/or metal particles on the base layer22aand then providing a thin insulation film made of a heat-resistant resin such as polyimide resin on the thin conductive layer. Thus, the thin insulation film is laminated on the thin conductive layer to integrate the thin insulation film with the thin conductive layer.

The carbon particles used in the resistant heat generation layer22bmay be known carbon black powder or carbon nanoparticles formed of at least one of carbon nanofiber, carbon nanotube, and carbon microcoil.

The metal particles used in the resistant heat generation layer22bmay be silver, aluminum, or nickel particles, and may be granular or filament-shaped.

The insulation layer22dis manufactured by coating the base layer22awith an insulation material including a heat-resistant resin identical to the heat-resistant resin of the base layer22a, such as polyimide resin.

The electrode layer22cis manufactured by coating the base layer22awith a conductive ink or a conductive paste such as silver. Alternatively, metal foil or a metal mesh may be adhered to the base layer22a.

The heat generation sheet22sof the laminated heater22is a thin sheet having a small heat capacity, and is heated quickly. An amount of heat generated by the heat generation sheet22sis arbitrarily set according to the volume resistivity of the resistant heat generation layer22b. In other words, the amount of heat generated by the heat generation sheet22scan be adjusted according to the material, shape, size, and dispersion of conductive particles of the resistant heat generation layer22b. For example, the laminated heater22providing heat generation per unit area of 35 W/cm2outputs a total power of about 1,200 W with the heat generation sheet22shaving a width of about 20 cm in the axial direction of the fixing sleeve21and a length of about 2 cm in the circumferential direction of the fixing sleeve21, for example.

If a metal filament, such as a stainless steel filament, is used as a laminated heater, the metal filament causes asperities to appear on a surface of the laminated heater. Consequently, when the inner circumferential surface of the fixing sleeve21slides over the laminated heater, the asperities of the laminated heater abrade the surface of the laminated heater easily. To address this problem, the heat generation sheet22shas a smooth surface without asperities as described above, improving durability in particular against wear due to sliding of the inner circumferential surface of the fixing sleeve21over the laminated heater22. Further, a surface of the resistant heat generation layer22bof the heat generation sheet22smay be coated with fluorocarbon resin to further improve durability.

InFIG. 4, the heat generation sheet22sof the laminated heater22faces the inner circumferential surface of the fixing sleeve21in a region in the circumferential direction of the fixing sleeve21between a position on the fixing sleeve21opposite the nip N via an axis of the fixing sleeve21and a position immediately upstream from the nip N in the rotation direction R1of the fixing sleeve21. Alternatively, the heat generation sheet22smay extend from the position on the fixing sleeve21opposite the nip N to the nip N or face the entire inner circumferential surface of the fixing sleeve21.

Referring toFIG. 4, the following describes the thermistor33used to control a fixing temperature of the fixing device20having the above-described structure at which the toner image T is fixed on the recording medium P.

The controller10, that is, a central processing unit (CPU) with associated memory components, controls the laminated heater22based on a detection result provided by the thermistor33serving as a temperature detector that detects the temperature of the fixing sleeve21so as to adjust the fixing temperature of the fixing device20, that is, a surface temperature of the fixing sleeve21at the nip N.

As illustrated inFIG. 4, the thermistor33is disposed downstream from the laminated heater22and upstream from the nip formation member26in the rotation direction R1of the fixing sleeve21. Preferably, the thermistor33is disposed near an entry to the nip N, that is, near the nip formation member26.

The surface temperature of the fixing sleeve21near the entry to the nip N detected by the thermistor33is substantially equivalent to the surface temperature of the fixing sleeve21at the nip N, that is, the fixing temperature of the fixing device20. Accordingly, with the configuration shown inFIG. 4, the controller10controls heat generation of the laminated heater22based on the surface temperature of the fixing sleeve21detected by the thermistor33near the entry to the nip N so that the laminated heater22adjusts the surface temperature of the fixing sleeve21, thus maintaining the fixing temperature of the fixing device20at a desired temperature and stabilizing fixing quality of the fixing device20.

The thermistor33faces the inner circumferential surface of the fixing sleeve21with or without contacting the inner circumferential surface of the fixing sleeve21. Accordingly, the thermistor33disposed inside the loop formed by the fixing sleeve21does not damage the outer circumferential surface of the fixing sleeve21, preventing the damaged fixing sleeve21from degrading the toner image T on the recording medium P. Further, the configuration shown inFIG. 4, in which the thermistor33is disposed inside the loop formed by the fixing sleeve21, allows further downsizing of the fixing device20compared to the configuration in which the thermistor33is disposed outside the loop formed by the fixing sleeve21.

Referring toFIGS. 7 and 8, the following describes an exemplary method of attaching the thermistor33to the fixing device20.FIG. 7is a partial sectional view of the fixing device20illustrating the core holder28and a flange35combined with the core holder28.FIG. 8is a partial perspective view of the core holder28and the flange35.

As illustrated inFIGS. 7 and 8, the flange35contacts and supports a lateral end of the core holder28in the longitudinal direction of the core holder28parallel to the axial direction of the fixing sleeve21. Although not shown inFIGS. 7 and 8, another flange35contacts and supports another lateral end of the core holder28in the longitudinal direction thereof.FIGS. 7 and 8illustrate an edge portion of the core holder28which is different from that illustrated inFIG. 4. However, the method of attaching the thermistor33to the fixing device20described below is also applicable to the core holder28having the shape illustrated inFIG. 4.

As illustrated inFIG. 8, the thermistor33includes a plurality of detection elements, that is, a center thermistor33aand a lateral-end thermistor33baligned in the longitudinal direction of the core holder28parallel to the axial direction of the fixing sleeve21. Although not shown inFIG. 8, another lateral-end thermistor33bis disposed at another lateral end of the core holder28in the longitudinal direction of the core holder28. For example, in the present embodiment, the center thermistor33ais disposed at a center portion of the core holder28and the lateral-end thermistors33bare disposed at lateral end portions of the core holder28in the longitudinal direction of the core holder28parallel to the axial direction of the fixing sleeve21. However, the number of detection elements and the positions thereof are not limited to those described above. Moreover,FIG. 8illustrates the center thermistor33aand one of the lateral-end thermistors33battached to the core holder28. Alternatively, the center thermistor33aand the lateral-end thermistors33bmay be attached to other components of the fixing device20. It is to be noted that the term “center portion” in the axial direction of the fixing sleeve21corresponds to a narrow conveyance region on the fixing sleeve21through which a recording medium P of any size is necessarily conveyed, and that the term “lateral end portions” in the axial direction of the fixing sleeve21correspond to a wide conveyance region on the fixing sleeve21through which only a large recording medium P having a larger width is conveyed. In other words, a small recording medium P is not conveyed through the lateral end portions in the axial direction of the fixing sleeve21.

The above-described configuration, in which the plurality of temperature detectors, that is, the center thermistor33aand the lateral-end thermistors33b, is aligned in the axial direction of the fixing sleeve21, can control heat generation of the laminated heater22according to the size of the recording medium P. For example, even when small recording media P pass over the fixing sleeve21continuously and therefore only the center portion on the fixing sleeve21is cooled by the small recording media P passing thereover, the plurality of temperature detectors detects the temperature differential of the fixing sleeve21between the center portion and the lateral end portions of the fixing sleeve21in the axial direction thereof, so that the controller10controls heat generation of the laminated heater22to eliminate the temperature differential of the fixing sleeve21in these different portions thereof.

The center thermistor33aand the lateral-end thermistors33bare connected to a drawer connector via a harness that connects the center thermistor33aand the lateral-end thermistors33bto the drawer connector. The harness extends inside the fixing sleeve21in the axial direction thereof and is clamped by the flange35disposed outside the fixing sleeve21and a chassis disposed inside the fixing device20.

Each of the lateral end portions of the core holder28in the longitudinal direction thereof contacts and engages a plurality of engagement portions35aand35bdisposed in an inner diametrical surface of the flange35mounted on the chassis inside the fixing device20so that the flange35supports the core holder28. For example, each of the lateral end portions of the core holder28includes slopes37and slits36disposed in the slopes37, respectively. The slits36of the core holder28engage the engagement portions35aand35bof the flange35, respectively, so that the flange35supports the core holder28.

A first distance between the nip N and one lateral-end thermistor33b, a second distance between the nip N and the center thermistor33a, and a third distance between the nip N and another lateral-end thermistor33bare substantially identical in the rotation direction R1of the fixing sleeve21, that is, in the circumferential direction of the fixing sleeve21. Thus, the center thermistor33aand the lateral-end thermistors33bdisposed with respect to the nip N with the identical distance therebetween can provide a uniform amount of heat radiation generated before the fixing sleeve21enters the nip N, thus preventing temperature variation in the axial direction of the fixing sleeve21due to variation in heat radiation amount.

Referring toFIGS. 9 to 12, the following describes assembly processes for assembling the fixing device20, that is, steps for putting together the components disposed inside the loop formed by the fixing sleeve21.FIG. 9is a perspective view of the laminated heater22and the heater support23.FIG. 10is a perspective view of the laminated heater22, the heater support23, and the terminal stay24.FIG. 11is a partial perspective view of the laminated heater22, the heater support23, the terminal stay24, and the power supply wiring25.FIG. 12is a vertical sectional view of the fixing device20illustrating the inner components disposed inside the fixing sleeve21.

As illustrated inFIG. 9, the laminated heater22further includes electrode terminal pairs22eand an attachment terminal22f. The heat generation sheet22sof the laminated heater22is adhered to the heater support23with an adhesive along the outer circumferential surface of the heater support23. The adhesive has a small heat conductivity to prevent heat transmission from the heat generation sheet22sto the heater support23.

The laminated heater22includes the electrode terminal pairs22e, each of which includes electrode terminals22e1and22e2. The electrode terminal pair22eis connected to the electrode layer22c(depicted inFIG. 6) at an edge of the heat generation sheet22sand sends power supplied from the power supply wiring25(depicted inFIG. 11) to the electrode layer22c. The plurality of electrode terminal pairs22eis disposed on one end of the heat generation sheet22sin the circumferential direction of the fixing sleeve21. InFIG. 9, the electrode terminal pairs22eare disposed on an edge of one end of the heat generation sheet22sdisposed opposite another end of the heat generation sheet22sdisposed closer to the nip N and the pressing roller31in the circumferential direction of the fixing sleeve21. The electrode terminal pair22eincluding the electrode terminals22e1and22e2is disposed on each of lateral ends of the heat generation sheet22sin the axial direction of the fixing sleeve21.

The following describes the rationales for the above-described arrangement of the electrode terminal pairs22e.

The laminated heater22includes at least two electrode terminal pairs22eto supply power to the resistant heat generation layer22bdepicted inFIG. 6. For example, when one electrode terminal pair22eis provided on each end of the heat generation sheet22sin the circumferential direction of the fixing sleeve21, a power source harness for power supply is connected to each electrode terminal pair22e. However, the heat generation sheet22sitself is a thin film with little rigidity. Accordingly, a terminal block that connects the harness to the electrode terminal pair22emust be provided on each end of the heat generation sheet22sin the circumferential direction of the fixing sleeve21, upsizing the fixing device20. To address this problem, according to this exemplary embodiment, both of the electrode terminal pairs22eare provided on one end of the heat generation sheet22sin the circumferential direction of the fixing sleeve21to downsize the fixing device20.

Alternatively, the electrode terminal pairs22emay be disposed on one end of the heat generation sheet22sin the axial direction of the fixing sleeve21. However, when the heat generation sheet22sis attached to the heater support23along the outer circumferential surface of the heater support23, the electrode terminal pairs22emay be bent, resulting in deformation of the electrode terminal pairs22ewhen the electrode terminal pairs22eare secured with screws, complication of the structure of the electrode terminals22e1and22e2, and complicated assembly. To address these problems, according to this exemplary embodiment, the plurality of electrode terminal pairs22eis disposed on one end of the heat generation sheet22sin the circumferential direction of the fixing sleeve21. Accordingly, even when the heat generation sheet22sis attached to the heater support23along the outer circumferential surface of the heater support23, the electrode terminal pairs22eare not bent, facilitating easy and precise assembly processes.

As illustrated inFIG. 9, the heat generation sheet22snear the electrode terminal pairs22eis bent along the edge of the heater support23in such a manner that the electrode terminal pairs22eare directed to a center of the circular loop formed by the fixing sleeve21depicted inFIG. 4. Then, each of the electrode terminals22e1and22e2is connected to the power supply wiring25on the terminal stay24, and secured to the terminal stay24as illustrated inFIGS. 10 and 11. For example, the electrode terminals22e1and22e2may be secured to the terminal stay24with screws, respectively, as illustrated inFIG. 11. As illustrated inFIG. 9, the attachment terminal22fis disposed on and protrudes from a center of the edge of the heat generation sheet22s, that is, the edge on which the electrode terminal pairs22eare disposed, in a longitudinal direction of the laminated heater22parallel to the axial direction of the fixing sleeve21. The attachment terminal22fprotrudes from the edge of the heat generation sheet22sand is also secured to the terminal stay24with a screw as illustrated inFIG. 10so as to support the heat generation sheet22s.

As illustrated inFIG. 12, the core holder28is attached to the terminal stay24in such a manner that the second concave portion of the H-shaped core holder28houses the terminal stay24. Further, the nip formation member26is attached to the core holder28in such a manner that the first concave portion of the H-shaped core holder28houses the nip formation member26, and the thermistor33is attached to the core holder28as described above, thus completing assembly of the inner components to be disposed inside the loop formed by the fixing sleeve21.

Finally, the assembled components are inserted into the loop formed by the fixing sleeve21at a position illustrated inFIG. 4, completing assembly of the fixing sleeve21and the inner components disposed inside the fixing sleeve21of the fixing device20.

When the heat generation sheet22sis not adhered to the heater support23with an adhesive, the electrode terminal pairs22eand the attachment terminal22f, which are disposed at a fixed end of the heat generation sheet22sopposite a free end of the heat generation sheet22sdisposed near the nip N in the circumferential direction of the fixing sleeve21, are secured to the terminal stay24with the screws, respectively. The rotating fixing sleeve21pulls the free end of the heat generation sheet22stoward the nip N to tension the heat generation sheet22s. Accordingly, the heat generation sheet22scontacts the inner circumferential surface of the fixing sleeve21stably in a state in which the heat generation sheet22sis sandwiched between the heater support23and the fixing sleeve21. Consequently, the heat generation sheet22sheats the fixing sleeve21effectively.

However, when the heat generation sheet22sis not adhered to the heater support23and therefore is levitated from the heater support23, the fixing sleeve21rotating back to allow removal of a jammed recording medium P may lift and shift the heat generation sheet22sfrom its proper position. Moreover, the moving heat generation sheet22smay twist and deform the electrode terminal pairs22e, breaking them. To address these problems, the heat generation sheet22sis preferably adhered to the heater support23to prevent the heat generation sheet22sfrom shifting from the proper position.

Conversely, when an entire inner surface of the heat generation sheet22sfacing the heater support23is adhered to the heater support23, heat generated by the heat generation sheet22smoves from the entire inner surface of the heat generation sheet22sto the heater support23easily. To address this problem, lateral end portions of the heat generation sheet22sin the axial direction of the fixing sleeve21, which correspond to non-conveyance regions on the fixing sleeve21through which the recording medium P is not conveyed, are adhered to the heater support23to prevent the heat generation sheet22sfrom shifting from the proper position. Further, a center portion of the heat generation sheet22sin the axial direction of the fixing sleeve21, which corresponds to a conveyance region on the fixing sleeve21through which the recording medium P is conveyed, that is, a maximum conveyance region corresponding to a width of the maximum recording medium P, is not adhered to the heater support23and therefore is isolated from the heater support23. Accordingly, heat is not transmitted from the center portion of the heat generation sheet22sin the axial direction of the fixing sleeve21to the heater support23. As a result, heat generated at the center portion of the heat generation sheet22sis used effectively to heat the fixing sleeve21.

The heat generation sheet22smay be adhered to the heater support23with a liquid adhesive for coating. Alternatively, a tape adhesive (e.g., a double-faced adhesive tape), which provides adhesion on both sides thereof and includes a heat-resistant acryl or silicon material, may be used. Accordingly, the laminated heater22(e.g., the heat generation sheet22s) is adhered to the heater support23easily. Further, if the laminated heater22malfunctions, the laminated heater22can be replaced easily by peeling off the double-faced adhesive tape, facilitating maintenance.

It is to be noted that, if the heat generation sheet22sand the heater support23merely sandwich the double-faced adhesive tape, the lateral end portions of the heat generation sheet22sin the axial direction of the fixing sleeve21, which are adhered to the heater support23, are lifted by a thickness of the double-faced adhesive tape. Accordingly, the center portion of the heat generation sheet22sin the axial direction of the fixing sleeve21, which is not adhered to the heater support23, does not contact the fixing sleeve21uniformly, decreasing heating efficiency for heating the fixing sleeve21and varying temperature distribution of the fixing sleeve21in the axial direction of the fixing sleeve21.

To address this problem, the lateral end portions of the heat generation sheet22sin the axial direction of the fixing sleeve21, which are adhered to the heater support23with the double-faced adhesive tape, have a thickness decreased by the thickness of the double-faced adhesive tape. Referring toFIG. 13, the following describes the configuration of the heat generation sheet22shaving the decreased thickness partially.

FIG. 13is a horizontal sectional view of the heater support23, the laminated heater22, and the fixing sleeve21. As illustrated inFIG. 13, the laminated heater22further includes edge grooves22gand double-faced adhesive tapes22t. The edge grooves22gare disposed at lateral edges, which correspond to the non-conveyance regions on the fixing sleeve21through which the recording medium P is not conveyed, of the heat generation sheet22sin the axial direction of the fixing sleeve21, respectively, on a surface of the base layer22a(depicted inFIG. 6) of the heat generation sheet22sthat faces the heater support23, and extend in the circumferential direction of the fixing sleeve21. Each of the edge grooves22ghas a depth equivalent to the thickness (e.g., about 0.1 mm) of the double-faced adhesive tape22t. The double-faced adhesive tapes22tare adhered to the edge grooves22gof the heat generation sheet22s, respectively, and then adhered to the heater support23. In other words, the heat generation sheet22sis adhered to the heater support23at predetermined positions on the heater support23via the double-faced adhesive tapes22t. Accordingly, when the heat generation sheet22sis adhered to the heater support23, a surface of the heat generation sheet22sthat faces the fixing sleeve21is planar in the axial direction of the fixing sleeve21. Consequently, the heat generation sheet22suniformly contacts the fixing sleeve21at the center portion of the heat generation sheet22scorresponding to the conveyance region on the fixing sleeve21over which the recording medium P is conveyed, providing improved heating efficiency for heating the fixing sleeve21and uniform temperature distribution of the fixing sleeve21in the axial direction thereof.

Alternatively, edge grooves may be provided in the heater support23instead of in the heat generation sheet22s.FIG. 14is a horizontal sectional view of the heater support23, the laminated heater22, and the fixing sleeve21. As illustrated inFIG. 14, the heater support23includes edge grooves23g.

The edge grooves23gare provided at lateral edges of the heater support23in the axial direction of the fixing sleeve21, which correspond to the non-conveyance regions on the fixing sleeve21through which the recording medium P is not conveyed, on a surface of the heater support23that faces the heat generation sheet22s, and extend in the circumferential direction of the fixing sleeve21. Each of the edge grooves23ghas a depth equivalent to the thickness of the double-faced adhesive tape22t. The double-faced adhesive tapes22tare adhered to the edge grooves23gof the heater support23, respectively, and then the heat generation sheet22sis adhered to the heater support23via the double-faced adhesive tapes22t. Accordingly, when the heat generation sheet22sis adhered to the heater support23, the surface of the heat generation sheet22sthat faces the fixing sleeve21is planar in the axial direction of the fixing sleeve21. Consequently, the heat generation sheet22suniformly contacts the fixing sleeve21at the center portion of the heat generation sheet22scorresponding to the conveyance region on the fixing sleeve21over which the recording medium P is conveyed, providing improved heating efficiency for heating the fixing sleeve21and uniform temperature distribution of the fixing sleeve21in the axial direction thereof.

Referring toFIGS. 3 and 4, the following describes operation of the fixing device20having the above-described structure.

When the image forming apparatus1receives an output signal, for example, when the image forming apparatus1receives a print request specified by a user by using a control panel or a print request sent from an external device, such as a client computer, the pressing roller31is pressed against the nip formation member26via the fixing sleeve21to form the nip N between the pressing roller31and the fixing sleeve21.

Thereafter, a driver drives and rotates the pressing roller31clockwise inFIG. 4in the rotation direction R2. Accordingly, the fixing sleeve21rotates counterclockwise inFIG. 4in the rotation direction R1in accordance with rotation of the pressing roller31. The laminated heater22supported by the heater support23contacts the inner circumferential surface of the fixing sleeve21, and the fixing sleeve21slides over the laminated heater22.

Simultaneously, an external power source or an internal capacitor supplies power to the laminated heater22via the power supply wiring25to cause the heat generation sheet22sto generate heat. The heat generated by the heat generation sheet22sis transmitted effectively to the fixing sleeve21contacting the heat generation sheet22s, so that the fixing sleeve21is heated quickly.

Alternatively, heating of the fixing sleeve21by the laminated heater22may not start simultaneously with driving of the pressing roller31by the driver. In other words, the laminated heater22may start heating the fixing sleeve21at a time different from a time at which the driver starts driving the pressing roller31. As described above, the controller10controls heat generation of the laminated heater22based on the temperature of the fixing sleeve21detected by the thermistor33so that the nip N is heated to a predetermined temperature desirable for fixing the toner image T on the recording medium P. After the fixing sleeve21is heated to the predetermined temperature, the recording medium P bearing the toner image T is conveyed to the nip N while the predetermined temperature is maintained.

In the fixing device20described above, the fixing sleeve21and the laminated heater22have a small heat capacity, shortening a warm-up time and a first print time of the fixing device20while saving energy. Further, the heat generation sheet22sis a resin sheet. Accordingly, even when rotation and vibration of the pressing roller31applies stress to the heat generation sheet22srepeatedly, and bends the heat generation sheet22srepeatedly, the heat generation sheet22sis not broken due to wear, and the fixing device20operates for a longer time.

When the image forming apparatus1does not receive an output signal, the pressing roller31and the fixing sleeve21do not rotate and power is not supplied to the laminated heater22to save energy. However, in order to restart the fixing device20immediately after the image forming apparatus1receives an output signal, power can be supplied to the laminated heater22while the pressing roller31and the fixing sleeve21do not rotate. For example, power in an amount sufficient to keep the entire fixing sleeve21warm is supplied to the laminated heater22.

Referring toFIGS. 15A,15B,16,17, and18, the following describes variations of the heat generation sheet22sof the laminated heater22.

In the heat generation sheet22sdepicted inFIG. 6, the resistant heat generation layer22bis provided on the entire surface or a part of the surface of the base layer22a. Alternatively, the resistant heat generation layer22bmay be divided among a plurality of regions zoned arbitrarily on the surface of the base layer22ain such a manner that each resistant heat generation layer22bgenerates heat independently.

FIG. 15Ais a plan view of a laminated heater22U as a first variation of the laminated heater22. As illustrated inFIG. 15A, the laminated heater22U, serving as a heat generator, includes a heat generation sheet22sU. The heat generation sheet22sU includes resistant heat generation layers22b1and22b2, the electrode layers22c, the insulation layers22d, which are disposed on the base layer22a(depicted inFIG. 6), and the electrode terminal pairs22edisposed on an edge of the heat generation sheet22sU.

FIG. 15Ais a plan view of the laminated heater22U spread on a flat surface before the laminated heater22U is adhered to the heater support23depicted inFIG. 4. A horizontal direction inFIG. 15Ais a width direction of the laminated heater22U parallel to the axial direction of the fixing sleeve21. A vertical direction inFIG. 15Ais a circumferential direction of the laminated heater22U parallel to the circumferential direction of the fixing sleeve21.

As illustrated inFIG. 15A, the heat generation sheet22sU is divided into three regions on a surface of the heat generation sheet22sU in a width direction of the heat generation sheet22sU parallel to the axial direction of the fixing sleeve21. Further, the heat generation sheet22sU is divided into two regions on the surface of the heat generation sheet22sU in a circumferential direction of the heat generation sheet22sU and the fixing sleeve21. Thus, in total, the heat generation sheet22sU is divided into six regions.

FIG. 15Bis a lookup table of a matrix with two rows in the circumferential direction of the fixing sleeve21and three columns in the axial direction of the fixing sleeve21, referred to as a 2-by-3 array of 6 elements corresponding to the six regions. The resistant heat generation layer22b1having a predetermined width and length is provided in the element (1,2) corresponding to the region provided at a lower center portion of the heat generation sheet22sU inFIG. 15Ain the axial direction of the fixing sleeve21. The resistant heat generation layers22b2having a predetermined width and length are provided in the elements (2,1) and (2,3) corresponding to the regions provided at upper lateral end portions of the heat generation sheet22sU inFIG. 15Ain the axial direction of the fixing sleeve21, respectively.

The electrode layers22cconnected to the resistant heat generation layer22b1are provided in the elements (1,1) and (1,3) corresponding to the regions provided at lower lateral end portions of the heat generation sheet22sU inFIG. 15Ain the axial direction of the fixing sleeve21, respectively. Each of the electrode layers22cis connected to the electrode terminal22e1that protrudes from one edge, that is, a lower edge inFIG. 15A, of the heat generation sheet22sU, forming a first heat generation circuit.

The electrode layer22cconnected to and sandwiched between the two resistant heat generation layers22b2is provided in the element (2,2) corresponding to the region provided at an upper center portion of the heat generation sheet22sU inFIG. 15Ain the axial direction of the fixing sleeve21. Each of the two resistant heat generation layers22b2is connected to the electrode layer22cthat extends to the lower edge of the heat generation sheet22sU inFIG. 15Ain the circumferential direction of the heat generation sheet22sU. Each of the electrode layers22cis connected to the electrode terminal22e2that protrudes from the lower edge of the heat generation sheet22sU, forming a second heat generation circuit.

The insulation layer22dis provided between the first heat generation circuit and the second heat generation circuit to prevent a short circuit of the first heat generation circuit and the second heat generation circuit.

In the laminated heater22U having the above-described configuration, when the electrode terminals22e1supply power to the heat generation sheet22sU, internal resistance of the resistant heat generation layer22b1generates Joule heat. By contrast, the electrode layers22cdo not generate heat due to their low resistance. Accordingly, only the region of the heat generation sheet22sU shown by the element (1,2) heats the center portion of the fixing sleeve21in the axial direction thereof.

On the other hand, when the electrode terminals22e2supply power to the heat generation sheet22sU, internal resistance of the resistant heat generation layers22b2generates Joule heat. By contrast, the electrode layers22cdo not generate heat due to their low resistance. Accordingly, only the regions of the heat generation sheet22sU shown by the elements (2,1) and (2,3), respectively, heat the lateral end portions of the fixing sleeve21in the axial direction thereof.

When a small size recording medium P having a small width passes through the fixing device20, power is supplied to the electrode terminals22e1to cause only a center portion of the heat generation sheet22sU to generate heat that is transmitted to the center portion of the fixing sleeve21in the axial direction thereof. By contrast, when a large size recording medium P having a large width passes through the fixing device20, power is supplied to the electrode terminals22e1and22e2to cause the heat generation sheet22sU to generate heat that is transmitted to the fixing sleeve21throughout the entire width thereof in the axial direction of the fixing sleeve21. Thus, the fixing device20provides desired fixing according to the width of the recording medium P with reduced energy consumption.

The controller10depicted inFIG. 4controls an amount of heat generated by the laminated heater22U according to the size of the recording medium P. Accordingly, even when the small size recording media P pass through the fixing device20continuously, the lateral end portions of the heat generation sheet22sU corresponding to the non-conveyance regions of the fixing sleeve21over which the recording medium P is not conveyed, respectively, are not overheated, thus preventing stoppage of the fixing device20to protect the components of the fixing device20and decrease of productivity of the fixing device20. The single, divided laminated heater22U provides varied regions of the heat generation sheet22sU, reducing temperature variation of the laminated heater22U in the axial direction of the fixing sleeve21compared to a plurality of separate, laminated heaters.

Edges of each of the resistant heat generation layers22b1and22b2contacting the insulation layers22dor the electrode layers22cwhich have a relatively high heat conductivity generate a smaller amount of heat due to heat transmission from the resistant heat generation layers22b1and22b2to the insulation layers22dor the electrode layers22c. Accordingly, in the configuration illustrated inFIG. 15Ain which a border between the center, resistant heat generation layer22b1and the adjacent electrode layer22cand a border between the lateral, resistant heat generation layer22b2and the adjacent electrode layer22care provided on an identical face, when power is supplied to the electrode terminals22e1and22e2, such borders have a decreased temperature, varying temperature distribution of the laminated heater22U in the axial direction of the fixing sleeve21. As a result, a faulty toner image is formed due to faulty fixing.

To address this problem, variations of the laminated heater22shown inFIGS. 16 and 17can be used in the fixing device20.FIG. 16illustrates a laminated heater22V as a second variation of the laminated heater22.FIG. 16is a plan view of the laminated heater22V. As illustrated inFIG. 16, the laminated heater22V, serving as a heat generator, includes a heat generation sheet22sV. The heat generation sheet22sV includes a resistant heat generation layer22b1V replacing the resistant heat generation layer22b1depicted inFIG. 15A.

The basic configuration of the laminated heater22V is identical to that of the laminated heater22U depicted inFIG. 15A. However, the laminated heater22V is different from the laminated heater22U in that the resistant heat generation layer22b1V has a longer width in the axial direction of the fixing sleeve21. Accordingly, the resistant heat generation layer22b1V partially overlaps each of the resistant heat generation layers22b2in a width direction of the heat generation sheet22sV parallel to the axial direction of the fixing sleeve21, to form an overlap region V. Accordingly, when power is supplied to the electrode terminals22e1and22e2, temperature decrease is prevented at a border between the resistant heat generation layer22b1V and the adjacent electrode layer22cand a border between the resistant heat generation layer22b2and the adjacent electrode layer22c.

FIG. 17is a plan view of a laminated heater22W as a third variation of the laminated heater22. As illustrated inFIG. 17, the laminated heater22W, serving as a heat generator, includes a heat generation sheet22sW. The heat generation sheet22sW includes resistant heat generation layers22b1W and22b2W replacing the resistant heat generation layers22b1V and22b2depicted inFIG. 16, respectively.

The basic structure of the laminated heater22W is identical to that of the laminated heater22V depicted inFIG. 16. However, the laminated heater22W is different from the laminated heater22V in that the resistant heat generation layer22b1W partially overlaps each of the resistant heat generation layers22b2W to form an overlap region W. In each overlap region W, a border between the resistant heat generation layer22b1W and the adjacent electrode layer22cis tapered with respect to a circumferential direction of the heat generation sheet22sW in a direction opposite a direction in which a border between the resistant heat generation layer22b2W and the adjacent electrode layer22cis tapered with respect to the circumferential direction of the heat generation sheet22sW. Thus, an amount of overlap of the resistant heat generation layer22b1W and the resistant heat generation layer22b2W is adjusted.

With the configuration shown inFIG. 16, a width of the overlap region V in which the resistant heat generation layer22b1V overlaps the resistant heat generation layer22b2in the width direction of the heat generation sheet22sV parallel to the axial direction of the fixing sleeve21, is unchanged. Accordingly, if the width of the overlap region V varies, an amount of heat generated by the heat generation sheet22sV varies. To address this problem, with the configuration shown inFIG. 17, the width of the overlap region W changes in the circumferential direction of the heat generation sheet22sW. For example, the width of the overlap region W of the resistant heat generation layer22b1W and the width of the overlap region W of the resistant heat generation layer22b2W decrease at a predetermined rate in a downward direction inFIG. 17. Accordingly, heat generation distribution is adjusted to reduce adverse effects of production errors of the laminated heater22W. As a result, the laminated heater22W provides uniform temperature throughout the axial direction of the fixing sleeve21.

Referring toFIGS. 15A,16, and17, the following describes a method of manufacturing the heat generation sheets22sU,22sV, and22sW. In the laminated heater22U depicted inFIG. 15A, portions on the surface of the base layer22aon which the resistant heat generation layers22b1and22b2are to be disposed are exposed and coated to form the resistant heat generation layers22b1and22b2. Then, portions on the surface of the base layer22aon which the insulation layers22dare to be disposed are exposed and coated to form the insulation layers22dmade of heat-resistant resin. Thereafter, portions on the surface of the base layer22aon which the electrode layers22care to be disposed are exposed and coated with a conductive paste to form the electrode layers22c. In other words, exposure of the portions on the surface of the base layer22aon which the resistant heat generation layers22b1and22b2are to be disposed is adjusted to form the resistant heat generation layers22b1and22b2having an arbitrary shape. Similarly, the resistant heat generation layers22b1V and22b2of the laminated heater22V depicted inFIG. 16and the resistant heat generation layers22b1W and22b2W of the laminated heater22W depicted inFIG. 17are formed.

The laminated heater (e.g., the laminated heater22,22U,22V, or22W) may include a plurality of layered heat generation sheets in each of which one or more resistant heat generation layers are provided on an arbitrary portion on the surface of the base layer22ain such a manner that the resistant heat generation layers generate heat independently from each other.FIG. 18illustrates a laminated heater22X including a plurality of heat generation sheets as a fourth variation of the laminated heater22.

FIG. 18is an exploded perspective view of the laminated heater22X. As illustrated inFIG. 18, the laminated heater22X, serving as a heat generator, includes a first heat generation sheet22s1, an insulation sheet22sd, and a second heat generation sheet22s2. The first heat generation sheet22s1includes the resistant heat generation layer22b1and the electrode layers22c. The insulation sheet22sdincludes the insulation layer22d. The second heat generation sheet22s2includes the resistant heat generation layers22b2and the electrode layers22c. The first heat generation sheet22s1is disposed on the insulation sheet22sddisposed on the second heat generation sheet22s2.

The first heat generation sheet22s1is divided into three regions on a surface thereof in a width direction of the first heat generation sheet22s1parallel to the axial direction of the fixing sleeve21. The resistant heat generation layer22b1is provided in a center region on the surface of the first heat generation sheet22s1. The electrode layers22c, which are connected to the adjacent resistant heat generation layer22b1, are provided in lateral end regions on the surface of the first heat generation sheet22s1, respectively.

The second heat generation sheet22s2is divided into five regions on a surface thereof in a width direction of the second heat generation sheet22s2parallel to the axial direction of the fixing sleeve21. The resistant heat generation layers22b2are provided in the second and fourth regions from left to right inFIG. 18, respectively. The electrode layers22c, which are connected to the adjacent resistant heat generation layers22b2, are provided in the first, third, and fifth regions from left to right inFIG. 18, respectively.

The first heat generation sheet22s1is provided on the second heat generation sheet22s2via the insulation sheet22sdin such a manner that the first heat generation sheet22s1and the second heat generation sheet22s2sandwich the insulation sheet22sd. Thus, an independent first heat generation circuit is provided in the first heat generation sheet22s1, and another independent second heat generation circuit is provided in the second heat generation sheet22s2.

When power is supplied to the first heat generation circuit, internal resistance of the resistant heat generation layer22b1generates Joule heat, and a center region on the surface of the first heat generation sheet22s1in the width direction of the first heat generation sheet22s1generates heat to be transmitted to the center portion of the fixing sleeve21in the axial direction of the fixing sleeve21. When power is supplied to the second heat generation circuit, internal resistance of the resistant heat generation layers22b2generates Joule heat, and lateral end regions on the surface of the second heat generation sheet22s2in the width direction of the second heat generation sheet22s2generate heat to be transmitted to the lateral end portions of the fixing sleeve21in the axial direction of the fixing sleeve21.

If the laminated heater22X is divided in a circumferential direction of the laminated heater22× as in the laminated heaters22U,22V, and22W depicted inFIGS. 15A,16, and17, respectively, the laminated heater22X needs to have an increased area to provide a desired heat generation amount, and therefore is not installed inside the small fixing sleeve21having a small diameter. To address this problem, the laminated heater22X includes the plurality of heat generation sheets layered in a thickness direction, that is, the second heat generation sheet22s2and the first heat generation sheet22s1provided on the second heat generation sheet22s2in such a manner that the resistant heat generation layer22b1of the first heat generation sheet22s1is shifted from the resistant heat generation layers22b2of the second heat generation sheet22s2in a width direction of the laminated heater22X as illustrated inFIG. 18. Accordingly, the laminated heater22X provides varied heat generation distribution in the axial direction of the fixing sleeve21like the laminated heaters22U,22V, and22W depicted inFIGS. 15A,16, and17, respectively, providing an increased output of heat while saving space and downsizing the fixing device20.

As illustrated inFIG. 4, when the fixing sleeve21rotates, the pressing roller31pulls the fixing sleeve21at the nip N. Accordingly, the pressing roller31applies tension to an upstream portion of the fixing sleeve21provided upstream from the nip N in the rotation direction R1of the fixing sleeve21. Consequently, the inner circumferential surface of the fixing sleeve21slides over the laminated heater22in a state in which the fixing sleeve21is pressed against the heater support23. By contrast, the pressing roller31does not apply tension to a downstream portion of the fixing sleeve21provided downstream from the nip N in the rotation direction R1of the fixing sleeve21. Accordingly, the downstream portion of the fixing sleeve21remains slack, a situation that is exacerbated if the fixing sleeve21rotates faster and destabilizing the rotation of the fixing sleeve21.

To address this problem, the fixing device20may include a fixing member support provided inside the loop formed by the fixing sleeve21to support at least the downstream portion of the fixing sleeve21.FIGS. 19A,19B,19C,19D, and19E illustrate such fixing member support.

FIG. 19Ais a vertical sectional view of a fixing sleeve support27A, the laminated heater22, and the nip formation member26. The fixing sleeve support27A is a metal member serving as a fixing member support that supports the fixing sleeve21depicted inFIG. 4serving as a fixing member, for example, a thin, stainless steel pipe. The laminated heater22is provided on an inner circumferential surface of the fixing sleeve support27A, and an outer circumferential surface of the fixing sleeve support27A supports the fixing sleeve21, providing stable rotation of the fixing sleeve21. Further, the rigid, metal fixing sleeve support27A supports the fixing sleeve21, facilitating assembly of the fixing device20. The fixing sleeve21does not slide over the laminated heater22by contacting the laminated heater22, preventing wear of a protective layer (e.g., a sliding layer) and an insulation layer disposed on a surface of the laminated heater22which may be caused by the fixing sleeve21sliding over the laminated heater22. Accordingly, electric conductors, such as the resistant heat generation layers22band the electrode layers22c, are not exposed, preventing short circuiting. However, the metal fixing sleeve support27A has a substantial heat capacity, providing a slower speed at which the temperature of the fixing sleeve21increases during warm-up than the structure shown inFIG. 4that does not include the fixing sleeve support27A.

FIG. 19Bis a vertical sectional view of the fixing sleeve support27A, the laminated heater22, and the nip formation member26as a variation of the structure shown inFIG. 19A. As illustrated inFIG. 19B, the laminated heater22is disposed on the outer circumferential surface of the fixing sleeve support27A to transmit heat to the fixing sleeve21more quickly than the laminated heater22provided on the inner circumferential surface of the fixing sleeve support27A shown inFIG. 19A. However, heat is adversely transmitted from an inner circumferential surface of the laminated heater22facing the fixing sleeve support27A to the fixing sleeve support27A.

To address this problem, the fixing device20may include a fixing sleeve support27B, instead of the fixing sleeve support27A, which has a heat conductivity smaller than that of the metal fixing sleeve support27A as inFIG. 19C.FIG. 19Cis a vertical sectional view of the fixing sleeve support27B, the laminated heater22, and the nip formation member26. The fixing sleeve support27B, serving as a fixing member support that supports the fixing sleeve21depicted inFIG. 4serving as a fixing member, includes solid resin having a heat conductivity smaller than that of the metal fixing sleeve support27A, suppressing heat transmission from the inner circumferential surface of the laminated heater22facing the fixing sleeve support27B to the fixing sleeve support27B. However, a heat resistance of resin is generally smaller than that of metal, and resin having a high heat resistance is expensive, resulting in increased manufacturing costs.

To address this problem, the fixing device20may include a fixing sleeve support27C instead of the fixing sleeve support27B. The fixing sleeve support27C is made of polyimide resin foam that provides heat insulation and rigidity.FIG. 19Dis a vertical sectional view of the fixing sleeve support27C, the laminated heater22, and the nip formation member26. The fixing sleeve support27C serves as a fixing member support that supports the fixing sleeve21depicted inFIG. 4serving as a fixing member.

FIG. 19Eis a vertical sectional view of the fixing sleeve support27C, the laminated heater22, the nip formation member26, and a resin member27D for enhanced rigidity. The resin member27D is made of polyimide foam, and is accessorily disposed inside the fixing sleeve support27C in such a manner that the resin member27D contacts an inner circumferential surface of the fixing sleeve support27C, providing an improved rigidity.

As described above, when the fixing device20is installed in the image forming apparatus1depicted inFIG. 3, the image forming apparatus1can stabilize the fixing temperature and improve fixing performance.

In the fixing device20according to the above-described exemplary embodiments, the pressing roller31is used as a pressing member. Alternatively, a pressing belt or the like may be used as a pressing member to provide the effects equivalent to those provided by the pressing roller31. Further, the fixing sleeve21is used as a fixing member. Alternatively, an endless fixing belt, an endless fixing film, or the like may be used as a fixing member.

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.