Fixing device, image forming apparatus incorporating same, and fixing method

A fixing device includes a fixing rotary body to rotate in a predetermined direction of rotation and a pressing rotary body pressed against the fixing rotary body to rotate in a direction counter to the direction of rotation of the fixing rotary body and form a nip therebetween through which a recording medium bearing a toner image passes. A heat generator is disposed opposite the fixing rotary body at a section other than the nip to heat the fixing rotary body. A moving assembly is disposed opposite the heat generator to generate a magnetic force to move the heat generator with respect to the fixing rotary body so as to change one of a pressure and a distance between the heat generator and the fixing rotary body.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to Japanese Patent Application No. 2010-140508, filed on Jun. 21, 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, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus including the fixing device, and a fixing method for fixing a toner image on a recording medium.

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 forming the image on the recording medium.

The fixing device used in such image forming apparatuses may employ a fixing belt formed into a loop and a pressing roller pressed against the fixing belt to form a nip therebetween through which the recording medium bearing the toner image passes.

For example, Japanese patent publication no. JP-2002-251084-A proposes a configuration in which the fixing belt is stretched over and rotated around a rotatable fixing roller and a stationary heat generator (e.g., a resistance heat generator) and the pressing roller disposed outside the loop formed by the fixing belt is pressed against the fixing roller via the fixing belt to form the nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. With this configuration, the heat generator contacting the inner circumferential surface of the fixing belt heats the fixing belt; the fixing roller contacting the inner circumferential surface of the fixing belt rotates the fixing belt which in turn rotates the pressing roller by friction therebetween. As the fixing belt and the pressing roller rotate and convey the recording medium through the nip, they apply heat and pressure to the recording medium to fix the toner image on the recording medium. The fixing belt includes a ferromagnet that is attracted by a magnet of the heat generator, thus the fixing belt is adhered to the heat generator precisely with no gap therebetween, to improve heating efficiency of the fixing belt.

As another example, Japanese patent publication no. JP-2009-258453-A proposes a configuration in which the looped fixing belt is sandwiched between a heat generator (e.g., a temperature sensitive element) disposed inside the loop formed by the fixing belt and an exciting coil unit disposed outside the loop formed by the fixing belt. The heat generator contacts or is disposed opposite the inner circumferential surface of the fixing belt with a slight gap therebetween. As the heat generator generates heat by a magnetic flux from the exciting coil unit by electromagnetic induction, it heats the fixing belt.

However, the above-described configurations have a drawback in that the heat generator constantly contacting or disposed opposite the fixing belt may heat the fixing belt even in a standby mode in which the fixing belt is not rotated, resulting in localized overheating of the fixing belt. Accordingly, when a fixing process is started, the locally heated fixing belt, with a temperature not uniform and stable but instead varying in the direction of rotation of the fixing belt, may generate faulty fixing of the toner image on the recording medium.

BRIEF SUMMARY OF THE INVENTION

This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a fixing rotary body to rotate in a predetermined direction of rotation and a pressing rotary body pressed against the fixing rotary body to rotate in a direction counter to the direction of rotation of the fixing rotary body and form a nip therebetween through which a recording medium bearing a toner image passes. A heat generator is disposed opposite the fixing rotary body at a section other than the nip to heat the fixing rotary body. A moving assembly is disposed opposite the heat generator to generate a magnetic force to move the heat generator with respect to the fixing rotary body so as to change one of a pressure and a distance between the heat generator and the fixing rotary body.

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

This specification further describes an improved fixing method for fixing a toner image on a recording medium and including the steps of rotating a fixing rotary body in a predetermined direction of rotation; pressing a pressing rotary body against the fixing rotary body to rotate the pressing rotary body in a direction counter to the direction of rotation of the fixing rotary body and form a nip therebetween through which the recording medium bearing the toner image passes; heating the fixing rotary body with a heat generator disposed opposite the fixing rotary body at a section other than the nip; and moving the heat generator with respect to the fixing rotary body to change one of a pressure and a distance between the heat generator and the fixing rotary body with a moving assembly disposed opposite the heat generator and generating a magnetic force.

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 view of the image forming apparatus1. As illustrated inFIG. 1, 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 copier for forming an image on a recording medium.

Referring toFIG. 1, the following describes the structure of the image forming apparatus1.

As illustrated inFIG. 1, the image forming apparatus1includes an auto document feeder10, disposed atop the image forming apparatus1, which feeds an original document D bearing an original image placed thereon to an original document reader2disposed below the auto document feeder10. The original document reader2optically reads the original image on the original document D to generate image data and sends it to an exposure device3disposed below the original document reader2. The exposure device3emits light L onto a photoconductive drum5of an image forming device4disposed below the exposure device3according to the image data sent from the original document reader2to form an electrostatic latent image on the photoconductive drum5. Thereafter, the image forming device4renders the electrostatic latent image formed on the photoconductive drum5visible as a toner image with developer (e.g., toner).

Below the image forming device4is a transfer device7that transfers the toner image formed on the photoconductive drum5onto a recording medium P sent from one of paper trays12,13, and14, each of which loads a plurality of recording media P (e.g., transfer sheets), disposed in a lower portion of the image foiling apparatus1below the transfer device7. The recording medium P bearing the transferred toner image is sent to a fixing device20disposed downstream from the transfer device7in a recording medium conveyance direction, where a fixing belt21and a pressing roller31disposed opposite each other apply heat and pressure to the recording medium P, thus fixing the toner image on the recording medium P.

Referring toFIG. 1, the following describes the operation of the image forming apparatus1having the above-described structure.

An original document D bearing an original image, placed on an original document tray of the auto document feeder10by a user, is conveyed by a plurality of conveyance rollers of the auto document feeder10in a direction D1above the original document reader2. As the original document D passes over an exposure glass of the original document reader2, the original document reader2optically reads the original image on the original document D to generate image data.

The image data is converted into an electric signal and then sent to the exposure device3. The exposure device3, serving as an image writer, emits light L (e.g., a laser beam) onto the photoconductive drum5of the image forming device4according to the electric signal, thus writing an electrostatic latent image on the photoconductive drum5.

The image forming device4performs a plurality of image forming processes as the photoconductive drum5rotates clockwise inFIG. 1: a charging process, an exposure process, and a development process. In the charging process, a charger of the image forming device4charges an outer circumferential surface of the photoconductive drum5, accordingly the exposure device3emits light L onto the charged outer circumferential surface of the photoconductive drum5to form an electrostatic latent image thereon as described above in the exposure process. Thereafter, in the development process, a development device of the image forming device4develops the electrostatic latent image formed on the photoconductive drum5into a toner image with toner.

At the same time, a recording medium P is sent to a transfer nip formed between the photoconductive drum5and the transfer device7from one of the plurality of paper trays12,13, and14, which is selected manually by the user using a control panel disposed atop the image forming apparatus1or automatically by an electric signal of a print request sent from a client computer. If the paper tray12is selected, for example, an uppermost recording medium P of a plurality of recording media P loaded in the paper tray12is conveyed to a registration roller pair disposed in a conveyance path K extending from each of the paper trays12,13, and14to the transfer device7.

When the uppermost recording medium P reaches the registration roller pair, it is stopped by the registration roller pair temporarily and then conveyed to the transfer nip formed between the photoconductive drum5and the transfer device7at a time when the toner image formed on the photoconductive drum5is transferred onto the uppermost recording medium P by the transfer device7.

After the transfer of the toner image onto the recording medium P, the recording medium P bearing the toner image is sent to the fixing device20through a conveyance path extending from the transfer device7to the fixing device20. As the recording medium P passes through a fixing nip formed between the fixing belt21and the pressing roller31of the fixing device20, it receives heat from the fixing belt21and pressure from the fixing belt21and the pressing roller31, which fix the toner image on the recording medium P. Thereafter, the recording medium P bearing the fixed toner image is discharged from the fixing nip to an outside of the image forming apparatus1, thus completing a series of image forming processes.

Referring toFIGS. 2,3A, and3B, the following describes the structure and operation of the fixing device20installed in the image forming apparatus1described above.

FIG. 2is a vertical sectional view of the fixing device20.FIG. 3Ais a partially enlarged vertical sectional view of the fixing belt21of the fixing device20in a state in which the fixing belt21is rotated.FIG. 3Bis a partially enlarged vertical sectional view of the fixing belt21in a state in which the fixing belt21is not rotated.FIGS. 3A and 3Balso illustrate multiple layers of the fixing belt21and a heat generator23of the fixing device20.

As illustrated inFIG. 2, the fixing device20includes the fixing belt21formed into a loop; a nip formation pad22, the heat generator23, a magnetic member24, and a tension spring27, which are disposed inside the loop formed by the fixing belt21; and a permanent magnet26, the pressing roller31, guides35and37, a temperature sensor40, and a driver45, which are disposed outside the loop formed by the fixing belt21.

The fixing belt21is a flexible, thin, endless belt serving as a fixing member or a fixing rotary body that rotates or moves clockwise inFIG. 2in a rotation direction R1. As illustrated inFIG. 3A, the fixing belt21, having a thickness not greater than about 1 mm and a loop diameter of about 40 mm when assuming its operative looped shape, is constructed of a base layer21a; an elastic layer21bdisposed on the base layer21a; and a release layer21cdisposed on the elastic layer21b.

The base layer21aconstitutes an inner circumferential surface of the fixing belt21, that is, a contact face sliding over the nip formation pad22and the heat generator23disposed inside the loop formed by the fixing belt21. The base layer21ahas a thickness of about 200 μm and is made of polyimide (PI).

The elastic layer21b, made of a rubber material such as silicon rubber, silicon rubber form, and/or fluorocarbon rubber, has a thickness in a range of from about 100 μm to about 300 μm. The elastic layer21beliminates or reduces slight surface asperities of the fixing belt21at a nip NP formed between the fixing belt21and the pressing roller31. Accordingly, heat is uniformly transmitted from the fixing belt21to a toner image T on a recording medium P passing through the nip NP, minimizing formation of a rough image such as an orange peel image. According to this exemplary embodiment, silicon rubber with a thickness of about 150 μm is used as the elastic layer21b.

The release layer21chas a thickness in a range of from about 10 μm to about 50 μm, and is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), polyimide, polyetherimide, and/or polyether sulfide (PES). The release layer21creleases or separates the toner image T from the fixing belt21. According to this exemplary embodiment, the release layer21chas a thickness of about 30 μm and is made of PFA.

Inside the loop formed by the fixing belt21are disposed the nip formation pad22, the heat generator23, the magnetic member24, the tension spring27, and an insulator29depicted inFIGS. 2 and 3A. Outside the loop formed by the fixing belt21is the permanent magnet26disposed opposite the fixing belt21with a predetermined gap between the permanent magnet26and a part of an outer circumferential surface of the fixing belt21. A lubricant is applied to the inner circumferential surface of the fixing belt21to reduce friction between an outer circumferential surface of the nip formation pad22and the heat generator23and the inner circumferential surface of the fixing belt21sliding over the nip formation pad22and the heat generator23.

The nip formation pad22contacting the inner circumferential surface of the fixing belt21is a stationary member fixedly disposed inside the loop formed by the fixing belt21; thus, the rotating fixing belt21slides over the stationary, nip formation pad22. Further, the nip formation pad22presses against the pressing roller31via the fixing belt21to form the nip NP between the fixing belt21and the pressing roller31through which the recording medium P bearing the toner image T passes. Lateral ends of the nip formation pad22in a longitudinal direction thereof parallel to an axial direction of the fixing belt21are mounted on and supported by side plates of the fixing device20, respectively. The nip formation pad22is made of a rigid material that prevents substantial bending of the nip formation pad22by pressure applied from the pressing roller31.

The nip formation pad22is constituted by an opposed face (e.g., a contact face that contacts the inner circumferential surface of the fixing belt21sliding over the nip formation pad22) facing the pressing roller31and having a concave shape corresponding to the curvature of the pressing roller31. The recording medium P moves along the concave opposed face of the nip formation pad22in conformity with the curvature of the pressing roller31and is discharged from the nip NP in a direction Y11. Thus, the concave shape of the nip formation pad22prevents the recording medium P bearing the fixed toner image T from adhering to the fixing belt21, thereby facilitating separation of the recording medium P from the fixing belt21.

As described above, according to this exemplary embodiment, the nip formation pad22has a concave shape to form the concave nip NP. Alternatively, the nip formation pad22may have a flat, planar shape to form a planar nip NP. Specifically, the opposed face of the nip formation pad22disposed opposite the pressing roller31may have a flat, planar shape. Accordingly, the planar nip NP formed by the planar opposed face of the nip formation pad22is substantially parallel to an imaged side of the recording medium P. Consequently, the fixing belt21pressed by the planar opposed face of the nip formation pad22is precisely adhered to the recording medium P to improve fixing performance. Further, the increased curvature of the fixing belt21at an exit of the nip NP facilitates separation of the recording medium P discharged from the nip NP from the fixing belt21.

As illustrated inFIG. 2, the substantially semi-cylindrical heat generator23is disposed opposite the permanent magnet26via the fixing belt21at a section of the fixing belt21other than the nip NP. In the present embodiment, the heat generator23and the permanent magnet26are disposed directly opposite the nip NP, although their location is not limited thereto. In this case, the heat generator23separably contacts the inner circumferential surface of the fixing belt21. Shafts protruding from lateral ends of the heat generator23in a longitudinal direction thereof parallel to the axial direction of the fixing belt21, respectively, engage slots provided in the side plates of the fixing device20via bearings, respectively, to slidably support the heat generator23in a diametrical direction of the fixing belt21.

As noted above and illustrated inFIG. 3A, the heat generator23is constructed of multiple layers: a base layer23aconstituting an inner circumferential surface disposed opposite the insulator29; a heat generation layer23b, including a resistance heat generator, disposed on the base layer23a; and a protective layer23c, that is, an insulating layer disposed on the heat generation layer23b. According to this exemplary embodiment, the heat generator23has a length of about 320 mm in the longitudinal direction thereof and a length, that is, an arcuate length, of about 10 mm in a circumferential direction thereof. In the present embodiment, the base layer23ais made of aluminum oxide (alumina) and/or aluminum nitride. The heat generation layer23bis made of a resistance heat generator, that is, a laminated heat generator made of ceramic. Lateral ends of the heat generation layer23bin a longitudinal direction thereof parallel to the axial direction of the fixing belt21are connected to a power source. When the heat generation layer23bis supplied with an electric current, it is heated by its electric resistance, thus heating the fixing belt21that either contacts or is disposed opposite the heat generator23. It is to be noted that the heat generation layer23bmay be any device capable of generating heat, such as a metal dispersion resin with an adjusted resistance.

The protective layer23cis made of an insulating material, such as glass, that prevents the electric current applied to the heat generator23from flowing to the fixing belt21. The base layer23aof the heat generator23is mounted with the magnetic member24via the insulator29.

With the above-described configuration, the heat generator23generates heat by itself, conducting the heat therefrom to the fixing belt21. Then, the heat is applied from the outer circumferential surface of the heated fixing belt21to a toner image T on a recording medium P depicted inFIG. 2as the recording medium P passes through the nip NP formed between the fixing belt21and the pressing roller31.

The temperature sensor40, disposed opposite the outer circumferential surface of the fixing belt21, serves as a temperature detector that detects a temperature of the outer circumferential surface of the fixing belt21. The temperature sensor40may be for example, a thermistor, a thermopile, or the like. Based on the temperature detected by the temperature sensor40, a controller6depicted inFIG. 1, that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, controls output of the power source that applies the electric current to the heat generator23, thus adjusting the temperature of the fixing belt21to a desired fixing temperature.

As described above, according to this exemplary embodiment, the heat generator23has multiple layers including the heat generation layer23b. Alternatively, the heat generator23may have a single layer, that is, the heat generation layer23bonly.

As illustrated inFIGS. 2,3A, and3B, the permanent magnet26is disposed opposite the magnetic member24via the fixing belt21and the heat generator23. The permanent magnet26may be a ferromagnetic magnet, for example, a rare-earth magnet or a magnet made of a hard magnetic material such as neodymium-iron-boron alloy.

The permanent magnet26is slidably moved over a frame of the fixing device20, for example, by the driver45bidirectionally as indicated by the double-headed arrow A1inFIG. 2to change a distance between the permanent magnet26and the magnetic member24. As the driver45moves the permanent magnet26in the diametrical direction of the fixing belt21, the permanent magnet26alternately applies or ceases to apply a magnetic force to the magnetic member24or changes a magnitude of the magnetic force exerted on the magnetic member24, thus moving the heat generator23together with the magnetic member24in the diametrical direction of the fixing belt21, a detailed description of which is deferred.

The driver45that moves the permanent magnet26may be a mechanism that includes a cam contacting the permanent magnet26biased upward inFIG. 2by a spring.

Optionally, a fan that cools the permanent magnet26may be added to minimize the decrease in magnetic permeability due to the heated permanent magnet26.

As illustrated inFIG. 2, the substantially semi-cylindrical magnetic member24is attached to the heat generator23and is disposed opposite the fixing belt21via the heat generator23. The magnetic member24may be made of soft ferrite, but preferably is made of hard ferrite. The magnetic member24made of hard ferrite need to be disposed with respect to the permanent magnet26in such a manner that an attractive force is generated between the magnetic member24and the permanent magnet26. For example, the south pole of the magnetic member24is disposed opposite the north pole of the permanent magnet26, thus moving the heat generator23attached to the magnetic member24bidirectionally in the diametrical direction of the fixing belt21precisely by slidable movement of the permanent magnet26, a detailed description of which is deferred.

As illustrated inFIG. 3A, the insulator29is provided between the heat generator23and the magnetic member24. The insulator29, made of an insulating material such as sponge rubber or urethane rubber, minimizes the decrease in magnetic permeability due to the heated magnetic member24by heat conduction from the heat generator23to the magnetic member24.

With the above-described configuration of the insulator29combined with the heat generator23and the magnetic member24, in accordance with the bidirectional movement of the permanent magnet26as indicated by the double-headed arrow A1inFIG. 2, the insulator29also moves bidirectionally in the diametrical direction of the fixing belt21as indicated by the double-headed arrow A1together with the heat generator23and the magnetic member24.

As illustrated inFIG. 2, the tension spring27has one end in a longitudinal direction thereof which is attached to the heat generator23, the magnetic member24, and the insulator29and another end in the longitudinal direction thereof which is attached to a frame of the fixing device20. Thus, the tension spring27serves as a biasing member that biases the magnetic member24, the insulator29, and the heat generator23, against a magnetic force of the permanent magnet26to separate the heat generator23combined with the magnetic member24and the insulator29from the fixing belt21downward inFIG. 2in a direction D2.

As illustrated inFIG. 2, the pressing roller31serves as a pressing rotary body that presses against the nip formation pad22via the fixing belt21by contacting the outer circumferential surface of the fixing belt21at the nip NP. The pressing roller31is constructed of a hollow metal core32and an elastic layer33disposed on the metal core32. The elastic layer33, having a thickness of about 3 mm, is made of silicon rubber form, silicon rubber, and/or fluorocarbon rubber. Optionally, a thin surface release layer made of PFA and/or PTFE may be disposed on the elastic layer33. With the above-described configuration, the pressing roller31is pressed against the nip formation pad22via the fixing belt21to form the desired nip NP between the pressing roller31and the fixing belt21.

On the pressing roller31is mounted a gear engaging a driving gear of a driving mechanism that drives and rotates the pressing roller31counterclockwise inFIG. 2in a rotation direction R2counter to the rotation direction R1of the fixing belt21. Lateral ends of the pressing roller31in a longitudinal direction, that is, an axial direction thereof, are rotatably supported by the side plates of the fixing device20via bearings, respectively. Optionally, a heat source, such as a halogen heater, may be disposed inside the pressing roller31.

With the elastic layer33of the pressing roller31made of a sponge material such as silicon rubber form, the pressing roller31applies decreased pressure to the nip formation pad22via the fixing belt21at the nip NP to decrease bending of the nip formation pad22. Further, the pressing roller31provides increased heat insulation that minimizes heat conduction thereto from the fixing belt21, improving heating efficiency of the fixing belt21.

As a mechanism to convey the recording medium P bearing the toner image T to and from the nip NP formed between the fixing belt21and the pressing roller31, the fixing device20includes two guide plates, the guide35, that is, an entry guide plate, disposed at an entry to the nip NP and the guide37, that is, an exit guide plate, disposed at an exit of the nip NP. The guide35is directed to the entry to the nip NP to guide the recording medium P conveyed in a direction Y10from the transfer device7depicted inFIG. 1to the nip NP. The guide37is directed to a conveyance path downstream from the fixing device20in the recording medium conveyance direction to guide the recording medium P discharged from the nip NP in the direction Y11to the conveyance path. Both the guides35and37are mounted on the frame (e.g., a body) of the fixing device20.

Referring toFIGS. 1 and 2, the following describes the operation of the fixing device20having the above-described structure.

When the image forming apparatus1is powered on, the power source supplies an electric current to the heat generator23; at the same time, the pressing roller31starts rotating in the rotation direction R2. Accordingly, the fixing belt21rotates in accordance with rotation of the pressing roller31in the rotation direction R1counter to the rotation direction R2of the pressing roller31due to friction therebetween at the nip NP.

Thereafter, at the transfer nip formed between the photoconductive drum5and the transfer device7, the toner image T formed on the photoconductive drum5as described above is transferred onto a recording medium P sent from one of the paper trays12,13, and14. Being guided by the guide35, the recording medium P bearing the toner image T is conveyed from the transfer nip in the direction Y10toward the nip NP, entering the nip NP formed between the fixing belt21and the pressing roller31pressed against each other.

As the recording medium P bearing the toner image T passes through the nip NP, it receives heat from the heated fixing belt21and pressure from the fixing belt21, the nip formation pad22, and the pressing roller31that form the nip NP. Thus, the toner image T is fixed on the recording medium P by the heat and the pressure applied at the nip NP. Thereafter, the recording medium P bearing the fixed toner image T is discharged from the nip NP and conveyed in the direction Y11as guided by the guide37.

Referring toFIGS. 2,3A, and3B, the following describes the configuration of the fixing device20according to a first illustrative embodiment of the present invention.

As illustrated inFIG. 2, the fixing device20includes a moving assembly60, constructed of the magnetic member24, the permanent magnet26, the tension spring27, and the driver45, which moves the heat generator23combined with the magnetic member24and the insulator29to change the pressure with which the heat generator23presses against the fixing belt21or, if separated from the fixing belt21, a distance between the heat generator23and the fixing belt21disposed opposite the heat generator23. For example, the moving assembly60moves the heat generator23bidirectionally in a direction D3shown inFIG. 3Aand a direction D5shown inFIG. 3B.

As illustrated inFIG. 3A, the permanent magnet26is disposed opposite the magnetic member24via the fixing belt21, the heat generator23, and the insulator29, and is slidably moved by the driver45depicted inFIG. 2bidirectionally toward and away from the fixing belt21, changing a distance between the permanent magnet26and the magnetic member24. The magnetic member24, together with the insulator29, is attached to the heat generator23in such a manner that it is disposed opposite the fixing belt21via the insulator29and the heat generator23. As illustrated inFIG. 2, the magnetic member24and the heat generator23are biased by the tension spring27in the direction D2away from the fixing belt21.

As illustrated inFIG. 3A, as the driver45depicted inFIG. 2moves the permanent magnet26downward in a direction D4toward the fixing belt21and the magnetic member24, the permanent magnet26exerts an increased magnetic attractive force on the magnetic member24against a biasing force of the tension spring27depicted inFIG. 2, thus moving the heat generator23together with the magnetic member24upward in the direction D3. Simultaneously, the heat generator23presses against the fixing belt21with an increased pressure or, if separated from the fixing belt21, is disposed opposite the fixing belt21with a decreased distance between the heat generator23and the fixing belt21, thus improving heat conductivity from the heat generator23to the fixing belt21, that is, activating heat conduction from the heat generator23to the fixing belt21.

By contrast, as illustrated inFIG. 3B, as the driver45depicted inFIG. 2moves the permanent magnet26upward in a direction D6away from the fixing belt21and the magnetic member24, the permanent magnet26exerts a decreased magnetic attractive force on the magnetic member24against a biasing force of the tension spring27, thus moving the heat generator23together with the magnetic member24downward in the direction D5. Simultaneously, the heat generator23presses against the fixing belt21with a decreased pressure or is disposed opposite the fixing belt21with an increased distance between the heat generator23and the fixing belt21. That is, the heat generator23is isolated from the fixing belt21with no pressure therebetween, thus degrading heat conductivity from the heat generator23to the fixing belt21, that is, deactivating heat conduction from the heat generator23to the fixing belt21.

Accordingly, instead of a moving mechanism including a cam that contacts and moves the heat generator23, the fixing device20employs the permanent magnet26that moves the heat generator23by magnetic force without contacting the heat generator23, preventing elements of the fixing device20other than the fixing belt21from drawing heat generated by the heat generator23and thereby maintaining heating efficiency of the fixing belt21.

For example, even when the entire heat generator23does not contact the fixing belt21, with a gap therebetween of about 0.2 mm or smaller, preferably about 0.1 mm or smaller, an air layer of the gap degrades heat conductivity to an extent that can be ignored, maintaining high heat conductivity from the heat generator23to the fixing belt21. Accordingly, the driver45moves the permanent magnet26in such a manner that the position of the permanent magnet26is switchable between the two positions: a first position shown inFIG. 3A, where the permanent magnet26is disposed closer to the fixing belt21and the magnetic member24with a gap of about 0.2 mm or smaller, preferably about 0.1 mm or smaller, between the fixing belt21and the heat generator23; and a second position shown inFIG. 3B, where the permanent magnet26is disposed away from the fixing belt21and the magnetic member24with a greater gap of at least 0.2 mm between the fixing belt21and the heat generator23.

Optionally, the fixing device20may further include a stopper that restricts an amount of movement of the heat generator23moving upward in the direction D3and downward in the direction D5in accordance with movement of the permanent magnet26as described above, thus facilitating adjustment of the pressure with which the heat generator23presses against the fixing belt21or the distance between the heat generator23and the fixing belt21within a target range.

The moving assembly60that moves the heat generator23is controlled by the controller6depicted inFIG. 1according to rotation of the fixing belt21. For example, when the fixing belt21does not rotate, the moving assembly60moves the heat generator23to a position where the heat generator23presses against the fixing belt21with a pressure smaller than that when the fixing belt21rotates or to a position where the heat generator23is disposed opposite the fixing belt21with a distance greater than that when the fixing belt21rotates.

Specifically, when the fixing device20is warmed up or a recording medium P passes through the fixing device20and therefore the fixing belt21rotates clockwise inFIG. 2in the rotation direction R1, the driver45moves the permanent magnet26to the first position shown inFIG. 3Awhere the permanent magnet26is disposed closer to the fixing belt21, causing the heat generator23to contact the fixing belt21or causing the heat generator23to be disposed opposite the fixing belt21with a slight gap therebetween allowing heat conduction from the heat generator23to the fixing belt21. Simultaneously, as the fixing belt21rotates clockwise inFIG. 2in the rotation direction R1, a contact section on the inner circumferential surface of the fixing belt21where the fixing belt21contacts the heat generator23and is heated by the heat generator23moves in the circumferential direction of the fixing belt21, resulting in efficient and uniform heating of the fixing belt21over the circumferential direction thereof.

Conversely, in a standby mode in which the fixing belt21does not rotate, the driver45moves the permanent magnet26to the second position shown inFIG. 3Bwhere the permanent magnet26is disposed away from the fixing belt21, thus isolating the heat generator23from the fixing belt21or separating the heat generator23from the fixing belt21with a substantial gap therebetween that prohibits heat conduction from the heat generator23to the fixing belt21. Simultaneously, the fixing belt21, although it does not rotate, is not heated by the heat generator23locally, preventing temperature variation of the fixing belt21in the circumferential direction thereof, that is, the rotation direction R1. Moreover, heat radiated from the heat generator23isolated from the fixing belt21sufficiently reaches the fixing belt21substantially uniformly over the circumferential direction of the fixing belt21, thus heating the fixing belt21uniformly over the circumferential direction thereof although heating efficiency is degraded compared to when the heat generator23contacting the fixing belt21conducts heat to the fixing belt21. Accordingly, even when a recording medium P is conveyed to the nip NP for the fixing process immediately after the standby mode is finished, faulty fixing does not occur due to variation in the temperature of the fixing belt21in the circumferential direction thereof.

In addition to the above-described control, even when the fixing belt21rotates after conveyance of the recording medium P through the nip NP is finished, the controller6controls the moving assembly60to move the heat generator23to the position where the heat generator23presses against the fixing belt21with a decreased pressure or is disposed opposite the fixing belt21with a greater distance therebetween compared to when conveyance of the recording medium P through the nip NP is ongoing.

For example, when the fixing process is performed at the nip NP while a recording medium P is conveyed through the nip NP or until the fixing process is finished on the last recording medium P when a plurality of recording media P is conveyed through the nip NP continuously, the driver45moves the permanent magnet26to the first position shown inFIG. 3Awhere the permanent magnet26is disposed closer to the fixing belt21, causing the heat generator23to contact the fixing belt21or causing the heat generator23to be disposed opposite the fixing belt21with a slight gap therebetween allowing heat conduction from the heat generator23to the fixing belt21. Simultaneously, as the fixing belt21rotates clockwise inFIG. 2in the rotation direction R1, the contact section on the inner circumferential surface of the fixing belt21where the fixing belt21contacts the heat generator23and is heated by the heat generator23moves in the circumferential direction of the fixing belt21, resulting in efficient and uniform heating of the fixing belt21over the circumferential direction thereof.

Conversely, immediately after the fixing process is finished at the nip NP while a recording medium P is conveyed through the nip NP or immediately after the fixing process is finished on the last recording medium P when a plurality of recording media P is conveyed through the nip NP continuously, the driver45moves the permanent magnet26to the second position shown inFIG. 3Bwhere the permanent magnet26is disposed away from the fixing belt21, thus isolating the heat generator23from the fixing belt21or moving the heat generator23downward in the direction D5to the position where the heat generator23presses against the fixing belt21with a slight pressure of about 0.1 kgf/cm2or smaller. Simultaneously, the fixing belt21, although it rotates, does not contact the heat generator23or presses against it with the slight pressure therebetween, preventing deterioration or wear of the fixing belt21and the heat generator23and an increased torque of drivers installed in the fixing device20due to friction between the fixing belt21and the heat generator23that arises as the fixing belt21slides over the heat generator23.

As described above, the configuration according to the first illustrative embodiment changes the pressure with which the heat generator23presses against the fixing belt21or the distance between the heat generator23and the fixing belt21disposed opposite the heat generator23. Thus, even when the heat generator23presses against the fixing belt21or is disposed opposite the fixing belt21to heat the fixing belt21, the heat generator23can heat the fixing belt21efficiently. Further, even when the fixing belt21does not rotate, temperature variation of the fixing belt21does not arise in the rotation direction R1thereof.

Additionally, according to the first illustrative embodiment, the permanent magnet26generates an attractive force between the permanent magnet26and the magnetic member24and at the same time the tension spring27exerts a biasing force on the magnetic member24and the heat generator23downward inFIG. 2in the direction D2to separate the heat generator23from the fixing belt21. Alternatively, the permanent magnet26may generate a repulsive force between the permanent magnet26and the magnetic member24and at the same time a biasing member (e.g., a compression spring) may exert a biasing force (e.g., a compressive force) on the magnetic member24and the heat generator23upward inFIG. 2in a direction opposite the direction D2to move the heat generator23closer to the fixing belt21, thus attaining effects equivalent to the effects of the first illustrative embodiment.

Further, the configuration according to the first illustrative embodiment uses the permanent magnet26as a magnet that slidably moves over the frame of the fixing device20in the diametrical direction of the fixing belt21and exerts a magnetic force on the magnetic member24to cause the heat generator23to contact and separate from the fixing belt21or change pressure with which the heat generator23presses against the fixing belt21. Alternatively, an electromagnet or a superconducting magnet may be used as a magnet that exerts a magnetic force on the magnetic member24. Such magnets can also slidably move to cause the heat generator23to contact and separate from the fixing belt21or change pressure with which the heat generator23presses against the fixing belt21, thus attaining effects equivalent to the effects of the first illustrative embodiment.

Referring toFIGS. 4,5A, and5B, the following describes a fixing device20S according to a second illustrative embodiment.

FIG. 4is a vertical sectional view of the fixing device20S.FIG. 5Ais a partially enlarged vertical sectional view of the fixing belt21of the fixing device20in a state in which the fixing belt21is rotated.FIG. 5Bis a partially enlarged vertical sectional view of the fixing belt21in a state in which the fixing belt21is not rotated. Instead of the permanent magnet26depicted inFIG. 2of the fixing device20according to the first illustrative embodiment, which is slidably movable, the fixing device20S according to the second illustrative embodiment includes a permanent magnet26S that is rotatably movable.

As illustrated inFIGS. 4,5A, and5B, like the fixing device20according to the first illustrative embodiment shown inFIG. 2, the fixing device20S according to the second illustrative embodiment includes the fixing belt21formed into a loop; the nip formation pad22, the heat generator23, and the magnetic member24, which are disposed inside the loop formed by the fixing belt21; and the permanent magnet26S, the pressing roller31, the temperature sensor40, and a driver46, which are disposed outside the loop formed by the fixing belt21.

The fixing device20S further includes a moving assembly60S that moves the heat generator23combined with the magnetic member24and the insulator29to change pressure with which the heat generator23presses against the fixing belt21or a distance between the heat generator23and the fixing belt21disposed opposite the heat generator23.

For example, the moving assembly60S includes the permanent magnet26S, the magnetic member24, and the driver46that drives and rotates the permanent magnet26S.

The permanent magnet26S, disposed opposite the magnetic member24via the fixing belt21and the heat generator23, is rotated about a rotary shaft26aby the driver46to change the magnetic pole, that is, the north pole or the south pole, of the permanent magnet26S disposed opposite the magnetic member24. The magnetic member24, together with the insulator29depicted inFIG. 5A, is adhered to the heat generator23in such a manner that the magnetic member24is disposed opposite the fixing belt21via the insulator29and the heat generator23.

With this configuration, when the fixing belt21rotates, the driver46depicted inFIG. 4rotates the permanent magnet26S to a first position shown inFIG. 5Awhere the north pole of the permanent magnet26S is disposed opposite the fixing belt21and the magnetic member24; thus, the permanent magnet26S exerts a magnetic attractive force on the magnetic member24, which moves the heat generator23, together with the magnetic member24, upward in a direction D7as shown inFIG. 5A. Simultaneously, the heat generator23presses against the fixing belt21with an increased pressure or is disposed opposite the fixing belt21with a decreased distance therebetween, improving heat conducting efficiency from the heat generator23to the fixing belt21.

By contrast, when the fixing belt21does not rotate, the driver46rotates the permanent magnet26S to a second position shown inFIG. 5Bwhere the south pole of the permanent magnet26S is disposed opposite the fixing belt21and the magnetic member24; thus, the permanent magnet26S exerts a magnetic repulsive force on the magnetic member24, which moves the heat generator23, together with the magnetic member24, downward in a direction D8as shown inFIG. 5B. Simultaneously, the heat generator23presses against the fixing belt21with a decreased pressure or is disposed opposite the fixing belt21with an increased distance therebetween, that is, the heat generator23separates from the fixing belt21, rendering pressure between the heat generator23and the fixing belt21to zero. Accordingly, the fixing belt21, which is heated by heat conduction from the heat generator23, is now heated by heat radiation from the heat generator23, thus minimizing localized overheating of the fixing belt21while the fixing belt21does not rotate.

It is to be noted that, according to the second illustrative embodiment, the south pole of the magnetic member24is disposed opposite the permanent magnet26S.

According to the second illustrative embodiment, since the permanent magnet26S biases the magnetic member24and the heat generator23attached to the magnetic member24by its magnetic repulsive force to separate the heat generator23from the fixing belt21, the tension spring27of the fixing device20according to the first illustrative embodiment shown inFIG. 2is not attached to the magnetic member24. Alternatively, the tension spring27may be attached to the magnetic member24to add a supplementary biasing force that separates the heat generator23and the magnetic member24from the fixing belt21.

As described above, like the configuration according to the first illustrative embodiment, the configuration according to the second illustrative embodiment changes the pressure with which the heat generator23presses against the fixing belt21or the distance between the heat generator23and the fixing belt21disposed opposite the heat generator23. Thus, even when the heat generator23presses against the fixing belt21or is disposed opposite the fixing belt21to heat the fixing belt21, the heat generator23can heat the fixing belt21efficiently. Further, even when the fixing belt21does not rotate, temperature variation of the fixing belt21does not arise in the rotation direction R1thereof.

Referring toFIGS. 6 and 7, the following describes a fixing device20T according to a third illustrative embodiment and a fixing device20TV as a variation of the fixing device20T.

FIG. 6is a vertical sectional view of the fixing device20T.FIG. 7is a vertical sectional view of the fixing device20TV as a variation of the fixing device20T shown inFIG. 6. Instead of the permanent magnet26depicted inFIG. 2of the fixing device20according to the first illustrative embodiment, the fixing devices20T and20TV according to the third illustrative embodiment include an electromagnet28.

As illustrated inFIG. 6, like the fixing device20according to the first illustrative embodiment shown inFIG. 2, the fixing device20T according to the third illustrative embodiment includes the fixing belt21formed into a loop; the nip formation pad22, the heat generator23, the magnetic member24, and the tension spring27, which are disposed inside the loop formed by the fixing belt21; and the electromagnet28, the pressing roller31, the temperature sensor40, a power source50, and a variable resistor51, which are disposed outside the loop formed by the fixing belt21.

The fixing device20T further includes a moving assembly60T that moves the heat generator23combined with the magnetic member24and the insulator29depicted inFIG. 3Ato change pressure with which the heat generator23presses against the fixing belt21or a distance between the heat generator23and the fixing belt21disposed opposite the heat generator23.

For example, the moving assembly60T includes the electromagnet28, the magnetic member24, the tension spring27, the power source50, and the variable resistor51.

The electromagnet28is disposed opposite the magnetic member24via the fixing belt21and the heat generator23. The variable resistor51changes an amount of electric current applied to the electromagnet28(e.g., an electromagnetic coil) from the power source50to change a magnetic force exerted on the magnetic member24. The magnetic member24, together with the insulator29depicted inFIG. 3A, is adhered to the heat generator23in such a manner that the magnetic member24is disposed opposite the fixing belt21via the insulator29and the heat generator23.

With this configuration, when the fixing belt21rotates, the controller6depicted inFIG. 1controls the variable resistor51to supply an increased amount of electric current from the power source50to the electromagnet28; thus, the electromagnet28exerts an increased magnetic attractive force on the magnetic member24against a biasing force of the tension spring27, moving the heat generator23, together with the magnetic member24, upward inFIG. 6. Simultaneously, the heat generator23presses against the fixing belt21with an increased pressure or is disposed opposite the fixing belt21with a decreased distance therebetween, improving heat conducting efficiency from the heat generator23to the fixing belt21.

By contrast, when the fixing belt21does not rotate, the controller6controls the variable resistor51to supply a decreased amount of electric current from the power source50to the electromagnet28; thus, the electromagnet28exerts a decreased magnetic attractive force on the magnetic member24, moving the heat generator23, together with the magnetic member24, downward inFIG. 6with a biasing force of the tension spring27. Simultaneously, the heat generator23presses against the fixing belt21with a decreased pressure or is disposed opposite the fixing belt21with an increased distance therebetween, that is, the heat generator23separates from the fixing belt21, rendering pressure between the heat generator23and the fixing belt21to zero. Accordingly, the fixing belt21, which is heated by heat conduction from the heat generator23, is now heated by heat radiation from the heat generator23, thus minimizing localized overheating of the fixing belt21while the fixing belt21does not rotate.

According to the above-described fixing device20T according to the third illustrative embodiment, the controller6controls the variable resistor51to change the amount of electric current supplied from the power source50to the electromagnet28, thus causing the heat generator23to contact and separate from the fixing belt21. Alternatively, the controller6may change a direction in which the electric current is applied to the electromagnet28to change the magnetic pole thereof, that is, the north pole or the south pole, which exerts a magnetic force on the magnetic member24, thus causing the heat generator23to contact and separate from the fixing belt21.

For example, as illustrated inFIG. 7, the electromagnet28is disposed opposite the magnetic member24via the fixing belt21and the heat generator23. Instead of the variable resistor51shown inFIG. 6, the fixing device20TV includes a switching circuit52that changes the direction in which the power source50applies the electric current to the electromagnet28, thus changing the magnetic polarity of the electromagnet28that exerts a magnetic force on the magnetic member24.

As illustrated inFIG. 7, the fixing device20TV as a variation of the fixing device20T according to the third illustrative embodiment includes the fixing belt21formed into a loop; the nip formation pad22, the heat generator23, and the magnetic member24, which are disposed inside the loop formed by the fixing belt21; and the electromagnet28, the pressing roller31, the temperature sensor40, the power source50, and the switching circuit52, which are disposed outside the loop formed by the fixing belt21.

The fixing device20TV further includes a moving assembly60TV that moves the heat generator23combined with the magnetic member24and the insulator29depicted inFIG. 3Ato change pressure with which the heat generator23presses against the fixing belt21or a distance between the heat generator23and the fixing belt21disposed opposite the heat generator23.

For example, the moving assembly60TV includes the electromagnet28, the magnetic member24, the power source50, and the switching circuit52.

With this configuration, when the fixing belt21rotates, the controller6depicted inFIG. 1controls the switching circuit52to change the direction in which the power source50applies the electric current to the electromagnet28, causing the north pole of the electromagnet28to be disposed opposite the fixing belt21and the magnetic member24; thus, the electromagnet28exerts a magnetic attractive force on the magnetic member24, moving the heat generator23, together with the magnetic member24, upward inFIG. 7. Simultaneously, the heat generator23presses against the fixing belt21with an increased pressure or is disposed opposite the fixing belt21with a decreased distance therebetween, improving heat conducting efficiency from the heat generator23to the fixing belt21.

By contrast, when the fixing belt21does not rotate, the controller6controls the switching circuit52to change the direction in which the power source50applies the electric current to the electromagnet28, causing the south pole of the electromagnet28to be disposed opposite the fixing belt21and the magnetic member24; thus, the electromagnet28exerts a magnetic repulsive force on the magnetic member24, moving the heat generator23, together with the magnetic member24, downward inFIG. 7. Simultaneously, the heat generator23presses against the fixing belt21with a decreased pressure or is disposed opposite the fixing belt21with an increased distance therebetween, that is, the heat generator23separates from the fixing belt21, rendering pressure between the heat generator23and the fixing belt21to zero. Accordingly, the fixing belt21, which is heated by heat conduction from the heat generator23, is now heated by heat radiation from the heat generator23, thus minimizing localized overheating of the fixing belt21while the fixing belt21does not rotate.

It is to be noted that, in the fixing devices20T and20TV, the south pole of the magnetic member24is disposed opposite the electromagnet28.

As described above, like the configuration according to the above-described illustrative embodiments, the configurations according to the third illustrative embodiment and the variation thereof change the pressure with which the heat generator23presses against the fixing belt21or the distance between the heat generator23and the fixing belt21disposed opposite the heat generator23. Thus, even when the heat generator23presses against the fixing belt21or is disposed opposite the fixing belt21to heat the fixing belt21, the heat generator23can heat the fixing belt21efficiently. Further, even when the fixing belt21does not rotate, temperature variation of the fixing belt21does not arise in the rotation direction R1thereof.

Referring toFIGS. 8,9A, and9B, the following describes a fixing device20U according to a fourth illustrative embodiment.

FIG. 8is a vertical sectional view of the fixing device20U.FIG. 9Ais a partially enlarged vertical sectional view of a fixing belt21U installed in the fixing device20U in a state in which it is rotated.FIG. 9Bis a partially enlarged vertical sectional view of the fixing belt21U in a state in which it is not rotated. Unlike the fixing device20depicted inFIG. 2according to the first illustrative embodiment in which the heat generator23generates heat by its resistance, the fixing device20U according to the fourth illustrative embodiment has the configuration in which a heat generator23U is heated by an exciting coil unit25by electromagnetic induction.

As illustrated inFIG. 8, the fixing device20U includes the fixing belt21U faulted into a loop; the nip formation pad22, the heat generator23U, the magnetic member24, and the tension spring27, which are disposed inside the loop formed by the fixing belt21U; and the permanent magnet26, the driver45, the pressing roller31, the temperature sensor40, and the exciting coil unit25, which are disposed outside the loop formed by the fixing belt21U.

Like the fixing device20according to the first illustrative embodiment depicted inFIG. 2, the fixing device20U further includes the moving assembly60that moves the heat generator23U combined with the magnetic member24and the insulator29depicted inFIG. 3Ato change pressure with which the heat generator23U presses against the fixing belt21U or a distance between the heat generator23U and the fixing belt21U disposed opposite the heat generator23U. For example, the moving assembly60includes the permanent magnet26, the magnetic member24, the tension spring27, and the driver45.

The exciting coil unit25, serving as an induction heater, includes an exciting coil25aand an exciting coil core25b. The exciting coil25a, extending in a longitudinal direction of the exciting coil unit25parallel to the axial direction of the fixing belt21U, is constructed of litz wire formed by bundling thin wire and wound around the exciting coil core25bthat covers a part of an outer circumferential surface of the fixing belt21U. The exciting coil core25b, made of ferromagnet (e.g., ferrite) having a relative permeability of about 2,500, generates a magnetic flux toward a heat generation layer of the fixing belt21U and a heat generation layer of the heat generator23U efficiently.

Referring toFIG. 9A, a detailed description is now given of the fixing belt21U.

The fixing belt21U is constructed of three layers: a base layer21dconstituting an inner circumferential surface of the fixing belt21U, that is, a contact face that slides over the nip formation pad22and the heat generator23U; the elastic layer21bdisposed on the base layer21d; and the release layer21cdisposed on the elastic layer21b.

For example, the base layer21d, having a thickness of from about several microns to about several hundred microns, is made of a magnetic material, such as SUS420 stainless steel or Fe—Ni alloy, thus serving as a heat generation layer heated by the exciting coil unit25by electromagnetic induction. The configuration of the elastic layer21band the release layer21cof the fixing belt21U is identical to that of the fixing belt21depicted inFIG. 2installed in the fixing device20according to the first illustrative embodiment.

Referring toFIG. 9A, a detailed description is now given of the heat generator23U.

The heat generator23U is constructed of three layers like the heat generator23of the fixing device20shown inFIG. 3A, however, the configuration of the three layers is different from that of the heat generator23. For example, the heat generator23U includes an antioxidant layer23econstituting an inner circumferential surface of the heat generator23U, that is, an opposed face disposed opposite the magnetic member24; a heat generation layer23fdisposed on the antioxidant layer23e; and an antioxidant layer23gdisposed on the heat generation layer23f.

The heat generation layer23f, having a thickness of about 10 μm, is made of copper. As an exciting magnetic flux generated by the exciting coil unit25passes through the heat generation layer23f, it induces an eddy current that heats the heat generation layer23fby electromagnetic induction.

Each of the antioxidant layers23eand23g, having a thickness of about 30 μm, is made of nickel plate; the antioxidant layers23eand23gsandwich the heat generation layer23f, inhibiting oxidation of the heat generation layer23f.

With this configuration, the heat generator23U is heated by electromagnetic induction by an alternating magnetic field generated by the exciting coil unit25, thus heating the fixing belt21U contacting the heat generator23U. That is, the exciting coil unit25heats the heat generator23U directly by electromagnetic induction and heats the fixing belt21U indirectly via the heat generator23U by heat conduction from the heat generator23U to the fixing belt21U.

Further, since the fixing belt21U has the base layer21dthat functions as a heat generation layer, the fixing belt21U itself, that is, the base layer21d, is also heated directly by electromagnetic induction by the alternating magnetic field generated by the exciting coil unit25. That is, the fixing belt21U is heated directly by electromagnetic induction by the exciting coil unit25and at the same time is heated indirectly by the exciting coil unit25by heat conduction from the heat generator23U heated by electromagnetic induction by the exciting coil unit25, improving heating efficiency of the fixing belt21U.

Thereafter, the heated fixing belt21U heats a recording medium P bearing a toner image T.

The controller6depicted inFIG. 1controls output of the exciting coil unit25based on a detection result provided from the temperature sensor40disposed opposite the outer circumferential surface of the fixing belt21U to detect a temperature thereof, thus adjusting the temperature of the fixing belt21U to a desired fixing temperature.

Referring toFIGS. 1 and 8, the following describes the operation of the fixing device20U having the above-described configuration.

When the image forming apparatus1is powered on, a high-frequency power source supplies an alternating electric current to the exciting coil25aof the exciting coil unit25, and at the same time the pressing roller31starts rotating in the rotation direction R2. Accordingly, the fixing belt21U rotates in accordance with rotation of the pressing roller31in the rotation direction R1counter to the rotation direction R2of the pressing roller31due to friction therebetween at the nip NP.

Thereafter, at the transfer nip formed between the photoconductive drum5and the transfer device7, the toner image T formed on the photoconductive drum5as described above is transferred onto a recording medium P sent from one of the paper trays12,13, and14. The recording medium P bearing the toner image T is conveyed from the transfer nip in the direction Y10toward the nip NP, entering the nip NP formed between the fixing belt21U and the pressing roller31pressed against each other.

As the recording medium P bearing the toner image T passes through the nip NP, it receives heat from the heated fixing belt21U and pressure from the fixing belt21U, the nip formation pad22, and the pressing roller31that form the nip NP. Thus, the toner image T is fixed on the recording medium P by the heat and the pressure applied at the nip NP. Thereafter, the recording medium P bearing the fixed toner image T is discharged from the nip NP and conveyed in the direction Y11.

With the above-described configuration of the fixing device20U shown inFIGS. 8,9A, and9B, when the fixing belt21U rotates, the driver45moves the permanent magnet26to a first position shown inFIG. 9Awhere the permanent magnet26is disposed closer to the fixing belt21U, thus increasing a magnetic attractive force of the permanent magnet26exerted on the magnetic member24against a biasing force of the tension spring27, which moves the heat generator23U, together with the magnetic member24, upward in a direction D9. Simultaneously, the heat generator23U presses against the fixing belt21U with an increased pressure or is disposed opposite the fixing belt21U with a decreased distance therebetween, thus improving heat conductivity from the heat generator23U to the fixing belt21U.

By contrast, when the fixing belt21U does not rotate, the driver45moves the permanent magnet26to a second position shown inFIG. 9Bwhere the permanent magnet26is disposed away from the fixing belt21U, thus decreasing a magnetic attractive force of the permanent magnet26exerted on the magnetic member24and moving the heat generator23U, together with the magnetic member24, downward in a direction D10with a biasing force of the tension spring27. Simultaneously, the heat generator23U presses against the fixing belt21U with a decreased pressure or is disposed opposite the fixing belt21U with an increased distance therebetween, that is, the heat generator23U separates from the fixing belt21U, rendering pressure between the heat generator23U and the fixing belt21U to zero. Accordingly, the fixing belt21U, which is heated by heat conduction from the heat generator23U, is now heated by heat radiation from the heat generator23U, thus minimizing localized overheating of the fixing belt21U while the fixing belt21U does not rotate.

Even when the heat generator23U is isolated from the fixing belt21U, it is constantly disposed within a magnetic field indicated by the broken line inFIGS. 9A and 9B, which is generated by the exciting coil unit25. Accordingly, the fixing belt21U is heated precisely both during rotation and non-rotation. For example, while the fixing belt21U rotates, it is heated by heat conduction from the heat generator23U; while the fixing belt21U does not rotate, it is heated by heat radiation from the heat generator23U.

Preferably, the heat generation layer23fof the heat generator23U may be made of a magnetic shunt alloy.

For example, the base layer21d, that is, the heat generation layer, of the fixing belt21U is made of a ferromagnetic, magnetic shunt alloy such as iron, nickel, cobalt, or an alloy of these.

With such materials of the heat generation layer23fof the heat generator23U and the base layer21dof the fixing belt21U, the base layer21dof the fixing belt21U has a Curie temperature near an upper temperature limit of the fixing temperature with which the toner image T is fixed on the recording medium P, preventing overheating of the fixing belt21U with self temperature control of the magnetic shunt alloy and thereby minimizing thermal degradation of the fixing belt21U. Further, the base layer21dof the fixing belt21U has a Curie temperature equivalent to a temperature that maintains magnetic permeability against the heated magnetic member24, rendering the insulator29disposed between the heat generator23U and the magnetic member24unnecessary.

According to the fourth illustrative embodiment, the fixing belt21U includes the heat generation layer, that is, the base layer21d, heated by the exciting coil unit25by electromagnetic induction. Alternatively, the fixing belt21U may not include the heat generation layer. For example, the fixing belt21U is heated solely by the heat generator23U by heat conduction or heat radiation, which is heated by the exciting coil unit25by electromagnetic induction, thus further enhancing prevention of localized overheating of the fixing belt21U when the fixing belt21U does not rotate.

As described above, like the configuration according to the above-described illustrative embodiments, the configuration according to the fourth illustrative embodiment changes the pressure with which the heat generator23U presses against the fixing belt21U or the distance between the heat generator23U and the fixing belt21U disposed opposite the heat generator23U. Thus, even when the heat generator23U presses against the fixing belt21U or is disposed opposite the fixing belt21U to heat the fixing belt21U, the heat generator23U can heat the fixing belt21U efficiently. Further, even when the fixing belt21U does not rotate, temperature variation of the fixing belt21U does not arise in the rotation direction R1thereof.

According to the above-described exemplary embodiments, the fixing belts21and21U are used as a fixing rotary body that rotates in the predetermined direction of rotation; the pressing roller31is used as a pressing rotary body disposed opposite the fixing rotary body to form the nip NP therebetween and rotating in the direction counter to the direction of rotation of the fixing rotary body. Alternatively, a fixing film, a fixing roller, or the like may be used as a fixing rotary body; a pressing belt or the like may be used as a pressing rotary body, attaining effects equivalent to the effects of the fixing devices20,20S,20T,20TV, and20U according to the above-described exemplary embodiments.

Further, the fixing devices20,20S,20T,20TV, and20U according to the above-described exemplary embodiments are installed in the image forming apparatus1serving as a monochrome copier. Alternatively, they may be installed in color image forming apparatuses such as copiers, printers, facsimile machines, and multifunction printers having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like.

Further, according to the above-described exemplary embodiments, the fixing devices20,20S,20T, and20TV include the heat generator23that generates heat; the fixing device20U includes the heat generator23U heated by the exciting coil unit25by electromagnetic induction. Alternatively, the fixing devices20,20S,20T,20TV, and20U may include a heat generator heated by a heater (e.g., a halogen heater) by radiant heat, attaining effects equivalent to the effects of the fixing devices20,20S,20T,20TV, and20U according to the above-described exemplary embodiments.

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.