Fixing device and image forming apparatus including the same

A fixing device includes a rotating fixing body fixes a toner image on a recording medium by heating, the rotating fixing body having a cylindrical metal core formed of a magnetic shunt alloy and having a thickness less than a magnetic-field permeation depth at a temperature higher than or equal to a Curie temperature of the metal core, a coil provided along an outer surface of the rotating fixing body so as to generate magnetic flux that subjects the rotating fixing body to induction heating, and an electrically conductive arch-shaped saddle ring member disposed at a position opposing the coil with the metal core being disposed therebetween and in an orientation such that magnetic flux leakage associated with loss of ferromagnetism of the metal core penetrates the ring member.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent application No. 2010-050040, filed Mar. 8, 2010 and Japanese Patent application No. 2010-239519, filed Oct. 26, 2010, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to a fixing device that fixes a toner image on a recording medium by melting and fusing unfixed toner image with heat while passing the recording medium carrying the toner image through a nip between a pair of fixing rollers or a nip between a heating belt and a roller. The present disclosure also relates to an image forming apparatus including the fixing device.

BACKGROUND OF THE INVENTION

In recent image forming apparatuses, a belt fixing system that has a lower heat capacity has attracted attention in response to requests for a shorter warm-up time (a period from when an image forming apparatus is powered on to when fixing by a fixing device becomes ready) and energy saving in the fixing device. Also, in recent years, an electromagnetic induction heating (IH) technique capable of rapid heating and high-efficiency heating has attracted attention as a heating technique adopted in the fixing device. From the viewpoint of energy saving in fixing of color images, a large number of fixing devices utilizing the electromagnetic induction heating technique and the belt fixing system in combination have been commercialized. When the belt fixing system and the electromagnetic induction heating technique are used in combination, a device (coil) for generating magnetic flux for electromagnetic induction heating is often provided on the outside of a heating belt because this arrangement provides advantages such as ease of layout and cooling of the coil and direct heating of the belt (so-called external IH system).

In the above-described electromagnetic induction heating technique, there is known a technique of preventing an excessive temperature rise of the fixing device. More specifically, a fixing roller includes a magnetic shunt alloy layer and a nonmagnetic metal layer, and a coil and the nonmagnetic metal layer oppose each other with the magnetic shunt alloy layer being disposed therebetween. The thickness of the magnetic shunt alloy layer is set to be less than a magnetic-field permeation depth (a surface skin depth) at a temperature higher than or equal to the Curie temperature.

Thus, when the temperature of the magnetic shunt alloy layer is lower than the Curie temperature, a magnetic flux generated by the coil does not reach the nonmagnetic metal layer, and the magnetic shunt alloy layer generates heat. When this heat generation of the magnetic shunt alloy layer increases the temperature of the magnetic shunt alloy layer to be higher than or equal to the Curie temperature, the magnetic flux generated by the coil penetrates the magnetic shunt alloy layer, and reaches the nonmagnetic metal layer, so that an induced current is generated in the nonmagnetic metal layer. The magnetic flux penetrating the magnetic shunt alloy layer and a magnetic flux in a direction opposite the penetrating magnetic flux, which is generated by the induced current in the nonmagnetic metal layer, cancel each other. This suppresses heat generation in the nonmagnetic shunt alloy layer.

However, even when the above techniques of the related art are combined, there remain problems with further reduction of heat capacity of the fixing device. For example, it is conceivable that the heat capacity can be further reduced in the combination by decreasing the thicknesses of the layers. However, if the thicknesses of the layers are simply reduced, the heating efficiency or the magnetic flux reducing effect becomes more likely to decrease.

More specifically, if the thickness of the magnetic shunt alloy layer is reduced, the magnetic flux generated by the coil easily penetrates the magnetic shunt alloy layer. With this, this magnetic flux and the opposite-direction magnetic flux generated by the nonmagnetic metal layer cancel each other, so that the heat generation efficiency of the magnetic shunt alloy layer decreases. In contrast, if the thickness of the nonmagnetic metal layer is reduced, the sectional area where the induced current passes decreases, and the electric resistance of the nonmagnetic metal layer increases. As a result, the opposite magnetic flux is not easily generated.

SUMMARY OF THE INVENTION

Accordingly, according to some aspects of the present disclosure, the present disclosure is related to a fixing device that solves the above problems and that realizes further reduction of heat capacity, and an image forming apparatus including the fixing device.

A fixing device according to an aspect of the present disclosure includes a rotating fixing body configured to rotate on an axis extending in a width direction of a recording medium to be conveyed and to fix a toner image on the recording medium by heating, the rotating fixing body having a cylindrical metal core formed of a magnetic shunt alloy and having a thickness less than a magnetic-field permeation depth at a temperature higher than or equal to a Curie temperature; a coil provided along an outer surface of the rotating fixing body, the coil configured for generating magnetic flux that subjects the rotating fixing body to induction heating; and an electrically conductive arch-shaped saddle ring member, the ring member being disposed at a position opposing the coil with the metal core being disposed therebetween and in an orientation such that the magnetic flux leakage associated with loss of magnetism of the metal core penetrates the ring member.

The above and other objects, features, and advantages of various embodiments of the present disclosure will be more apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings. It will be appreciated by those skilled in the art that the foregoing brief description and the following detailed description are exemplary and explanatory of the present disclosure, but are not intended to be restrictive thereof or limiting of the advantages which can be achieved by this disclosure. Accordingly, the present disclosure serves to explain principles of embodiments of the disclosure, thus providing a better understanding of the disclosure, as well as operating advantages and specific objects that may be attained by some of its uses. Additionally, it is understood that the foregoing summary is representative of some embodiments of the disclosure, and is neither representative nor inclusive of all subject matter and embodiments within the scope of the present disclosure. Various features of novelty which characterize various aspects of the disclosure are pointed out in particularity in the claims annexed to and forming a part of this disclosure.

In this text, the terms “comprising”, “comprise”, “comprises” and other forms of “comprise” can have the meaning ascribed to these terms in U.S. Patent Law and can mean “including”, “include”, “includes” and other forms of “include”.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1schematically illustrates a configuration of an image forming apparatus1according to some embodiments of the present disclosure. The image forming apparatus1can be a printer that performs printing by transferring a toner image onto a surface of a sheet P, serving as an example of a recording medium, on the basis of externally input image information, a copying machine, a facsimile machine, or a multi-functional peripheral that has the above functions in combination. In the following embodiments, the recording medium is not limited to the sheet P, and may be other recording media (e.g., an OHP sheet).

The image forming apparatus1illustrated inFIG. 1is a tandem type color printer. The image forming apparatus1includes an apparatus main body2shaped like a rectangular box in which a color image is formed (printed) onto the sheets P. On an upper surface of the apparatus main body2, an output tray3is provided to receive output sheets P on which color images have been printed.

At the inner bottom of the apparatus main body2, a paper feed cassette5that stores sheets P is provided. Further, a stack tray6is provided at a right side surface of the apparatus main body2so as to supply sheets P, which are not stored in the paper feed cassette5, into the apparatus main body2. An image forming section7is provided in an upper part of the apparatus main body2, and forms a toner image on a sheet P on the basis of image information, such as characters and pictures, transmitted from a host apparatus (e.g., a personal computer (PC)) connected to the image forming apparatus1.

A first conveying path9through which a sheet P fed out from the paper feed cassette5is conveyed to a below-described secondary transfer unit23is provided in a left part of the apparatus main body2inFIG. 1. A second conveying path10through which a sheet P fed out from the stack tray6is conveyed to the secondary transfer unit23is provided from a right part to the left part of the apparatus main body2. Also, a fixing unit (fixing device)14and a third conveying path11are provided in an upper left part of the apparatus main body2. The fixing unit14conducts fixing on a sheet P of a toner image that has been transferred onto the sheet P in the secondary transfer unit23. After fixing, the sheet P is conveyed to the output tray3through the third conveying path11.

In a state in which the paper feed cassette5is pulled out of the apparatus main body2(for example, to the front side of the plane ofFIG. 1), sheets P can be replenished in the paper feed cassette5. The paper feed cassette5includes a storage portion16that can selectively store one of at least two types of sheets P having different sizes in the paper feed direction. When the image forming apparatus1performs image formation, sheets P stored in the storage portion16are fed into the first conveying path9one by one by a paper feed roller17and a pair of separating rollers18.

The stack tray6can open and close relative to an outer surface of the apparatus main body2. One or a plurality of sheets P is placed on a manual feed portion19of the stack tray6. Sheets P placed on the manual feed portion19are fed out into the second conveying path10one by one by a pickup roller20and a pair of separating rollers21.

The first conveying path9and the second conveying path10join together on the upstream side of a pair of registration rollers22. The conveyed sheet P is temporarily stopped at the registration rollers22, and is then conveyed toward the secondary transfer unit23after being subjected to skew correction and timing adjustment.

In the secondary transfer unit23, a full-color toner image is secondarily transferred from an intermediate transfer belt40onto the conveyed sheet P. After that, the toner image is fixed on the sheet P by the fixing unit14. As required, the sheet P is reversed in a fourth conveying path12, and a full-color toner image is also secondarily transferred on an opposite surface of the sheet P in the secondary transfer unit23. After the full-color toner image is fixed on the opposite surface by the fixing unit14, the sheet P bearing the color images on both surfaces passes through the third conveying path11, and is output to the output tray3by a pair of output rollers24.

The image forming section7includes four image forming units26,27,28, and29that respectively form toner images of black (B), yellow (Y), cyan (C), and magenta (M) colors, an intermediate transfer unit30that carries the color toner images formed by the image forming units26,27,28, and29in the superimposed manner, and a laser scanning unit34that is disposed below the image forming units26,27,28, and29and irradiates with laser beams photoconductor drums32(described below), in the image forming units26,27,28, and29, respectively, at a specific portion of the surface of the photoconductor drums32downstream of a charging unit33described below in the rotation direction of the photoconductor drums32.

Each of the image forming units26,27,28, and29includes the photoconductor drum32(image carrier), the charging unit33, a developing unit35, and a cleaning unit36. The charging unit33is provided to oppose a peripheral surface of the photoconductor drum32. The developing unit35opposes the peripheral surface of the photoconductor drum32on the downstream side of the laser beam applied position in the rotating direction of the photoconductor drum32. The cleaning unit36opposes the peripheral surface of the photoconductor drum32on the downstream side of the developing unit35in the rotating direction of the photoconductor drum32.

In each of the image forming units26,27,28, and29, each of the photoconductor drums32is rotated counterclockwise inFIG. 1by a driving motor (not shown). In each developing unit35, a developing device51stores two-component developer containing each of black toner, yellow toner, cyan toner, and magenta toner, respectively.

The intermediate transfer unit30includes a driving roller38disposed near the image forming unit26, a driven roller39disposed near the image forming unit29, a tension roller42disposed above the image forming unit28, an intermediate transfer belt40wound around the driving roller38, the driven roller39and the tension roller42, and four primary transfer rollers41. The primary transfer rollers41are in pressing contact with the photoconductor drums32in the image forming units26,27,28, and29at positions on the photoconductor drums32on the downstream sides of the developing units35in the rotating direction of the photoconductor drums32, respectively, in a manner such that the intermediate transfer belt40is located between the primary transfer rollers41and the photoconductor drums32.

In the intermediate transfer unit30, different color toner images on the photoconductor drums32in the image forming units26,27,28, and29are transferred and superimposed on the intermediate transfer belt40at the corresponding primary transfer rollers41, thereby finally forming a full-color toner image. The secondary transfer unit23and the intermediate transfer unit30constitute a transfer section8.

The first conveying path9and the second conveying path10convey sheets P sent from the paper feed cassette5and the stack tray6toward the secondary transfer unit23, and include a plurality of pairs of conveying rollers43at predetermined positions in the apparatus main body2, and a pair of registration rollers22disposed on the upstream side of the secondary transfer unit23. The registration rollers22serve to adjust timing between an image forming operation in the image fanning section7and a sheet feed operation.

The fixing unit14heats and pressurizes the sheet P on which a toner image is transferred in the secondary transfer unit23so as to fix the unfixed toner image on the sheet P. The fixing unit14includes a pair of fixing rollers, that is, a pressurizing roller44and a fixing roller45. The pressurizing roller44includes a metal core, an elastic surface layer (e.g., silicone rubber), and a release layer (e.g., tetrafluoroethylene-perfluoroalkylvinyl ether copolymer resin: PFA). The fixing roller45includes a metal core and an elastic surface layer (e.g., silicone sponge). Also, a cylindrical heat roller (rotating fixing body)46is provided adjacent to the fixing roller45. A heating belt48is wound around the heat roller46and the fixing roller45. A detailed structure of the fixing unit14, according to some embodiments, will be described below.

Conveying paths47are provided on the upstream and downstream sides of the fixing unit14in the sheet conveying direction, respectively. A sheet P conveyed through the secondary transfer unit23passes through the upstream conveying path47, and is led to a fixing nip between the pressurizing roller44and the fixing roller45(heating belt48). Then, the sheet P passing through the nip between the pressurizing roller44and the fixing roller45is led to the third conveying path11through the downstream conveying path47.

Through the third conveying path11, the sheet P subjected to fixing by the fixing unit14is output to the output tray3. For this reason, a pair of conveying rollers49is provided at an appropriate position in the third conveying path11, and the above-described pair of output rollers24is provided at the exit of the third conveying path11.

Next, a detailed description will be given of the fixing unit14applied to the image forming apparatus1according to some illustrative embodiments. It will be understood that various values of approximate dimensions and/or parameters are provided simply by way of example for purposes of clarity, and are not intended to be limiting of the present disclosure.

FIG. 2is a cross-sectional view illustrating an illustrative structure of the fixing unit14, in accordance with some embodiments. The fixing unit14illustrated inFIG. 2is in an orientation rotated about 90° counterclockwise from an orientation in which the fixing unit14is mounted in the apparatus main body2. Therefore, while the sheet conveying direction extends from the lower side toward the upper side inFIG. 1, the sheet conveying direction extends from the right side to the left side inFIG. 2. When the apparatus main body2is larger (for example, in the case of a multi-functional peripheral), the fixing unit14is sometimes mounted in the apparatus main body2in the orientation ofFIG. 2. Alternatively, the fixing unit14is sometimes mounted in the apparatus main body2while being inclined to the right or left from the orientation ofFIG. 2.

As described above, the fixing unit14of the illustrative embodiment includes the pressurizing roller44, the fixing roller45, the heat roller46, and the heating belt48. By way of example, the pressurizing roller44is a roller having a diameter of about 50 mm, in which a silicone rubber layer having a thickness of about 2 to about 5 mm is provided on a metal (e.g., SUS: stainless used steel) core and a release layer (e.g., PFA: tetrafluoroethylene perfluoroalkoxy vinyl ether copolymer resin) is further provided on a surface of the silicone rubber layer. The fixing roller45is a roller having a diameter of about 45 mm, in which a silicone rubber sponge layer having a thickness of about 5 to about 10 mm is provided on a metal (SUS) core.

The heat roller46includes a cylindrical metal core46amade of a magnetic shunt metal (e.g., a Fe—Ni alloy) and having a diameter of about 30 mm and a thickness of about 0.2 to about 1.0 mm. A release layer (e.g., PFA) is provided on a surface of the metal core46a. A magnetic shunt alloy, such as a Fe—Ni alloy, will be described below.

The heating belt48may be a resin belt that does not have a heating function and has a base layer of about 35 μm (1 μm=1×10−6m) in thickness that is made of a nonmagnetic material (e.g., PI: polyimide). An elastic layer (e.g., silicone rubber) having a thickness of about 200 to about 500 μm may be provided on a surface of the base layer, and a release layer (e.g., PFA) may be further provided on an outer surface of the elastic layer.

Since the fixing roller45has the elastic layer of silicone rubber sponge on the surface side, as described above, a flat fixing nip is formed between the heating belt48and the pressurizing roller44. A sheet P conveyed through the secondary transfer unit23is led into the fixing nip. Further, the pressurizing roller44is shaped like a hollow cylinder, and a halogen heater44ais provided in an inner space of the pressurizing roller44.

In addition, the fixing unit14includes an IH (Induction Heating) coil unit50on an outer side of the heat roller46and the heating belt48(not shown inFIG. 1). The IH coil unit50includes an induction heating coil52, a plurality of pairs of arch cores54, a pair of side cores56, and a center core58.

In accordance with some embodiments, such as that illustrated inFIG. 2, the induction heating coil (coil)52is provided on an imaginary arc in cross section extending along an arc-shaped outer surface portion of the heat roller46, in cross section, opposing the IH coil unit50so that induction heating is performed at the arc-shaped portion of the heat roller46in cross section. Also, a coil bobbin (not shown) is provided on the outer side of the heat roller46and the heating belt48, and the induction heating coil52is wound on the coil bobbin in a ring shaped manner. The coil bobbin may be formed of a heat resistant resin (e.g., PPS: polyphenylene sulfide resin, PET: polyethylene terephthalate resin, LCP: liquid crystal polymer resin). The coil52may be fixed to the coil bobbin with a silicone adhesive.

Referring toFIG. 2, the center core58is provided in the center of the IH coil unit50, and the arch cores54and the side cores56described above are arranged in pairs on both sides of the center core58. The arch cores54on both sides are formed of ferrite and are shaped symmetrically with respect to the center core58so as to have an arch-shaped cross section. The overall length of the arch cores54is larger than the length of an area where the induction heating coil52is provided. The side cores56on both sides are shaped like a block and formed of ferrite. Also, the side cores56are connected to ends (lower ends inFIG. 2) of the arch cores54, respectively. The side cores56cover the area where the induction heating coil52is provided.

The arch cores54are provided at a plurality of positions arranged spaced in the longitudinal direction of the heat roller46. In contrast, the side cores56are arranged in succession in the longitudinal direction of the heat roller46without any interval therebetween. The overall lengths of areas where the side cores56are arranged correspond to the length of the area where the induction heating coil52is provided. The positions of the arch cores54and the side cores56are determined in accordance with a magnetic flux density (magnetic field strength) of the induction heating coil52. Since the arch cores54are arranged at some intervals, the side cores56complement the magnetic flux concentrating effect in the intervals, so that the magnetic flux density distribution (temperature distribution) is averaged in the longitudinal direction of the heat roller46.

A core holder (not shown) made, for example, of a resin may be provided on the outer side of the arch cores54and the side cores56, and supports the arch cores54and the side cores56. The core holder is, for example, also formed of a heat resistant resin (e.g., PPS, PET, or LCP).

A thermistor may be provided in contact with a inner surface of the heat roller46in a portion of the heat roller46where amount of heat generation by induction heating is more than that of other parts of the heat roller46. More practically, the outer surface temperature of the heating belt48can be detected with a non-contact temperature sensor provided below the IH coil unit50and opposing the heating belt48.

In accordance with some embodiments, the center core58is formed of ferrite and has a rectangular cross section. Substantially similarly to the heat roller46, the center core58has a length corresponding to the maximum sheet passing width of, for example, 13 inches (about 340 mm). The center core58is fixed to the arch cores54and, in some implementations, may be provided integrally with the arch cores54.

The above-described metal core46aformed of a Fe—Ni alloy in the heat roller46can generate heat by magnetic flux from the induction heating coil52. More specifically, the metal core46ahas magnetic permeability and electric conductivity, and exhibits ferromagnetism at room temperature. However, when the temperature of the metal core46abecomes higher than or equal to a predetermined temperature (Curie temperature, which, for the Fe—NI alloy of the illustrative embodiment, is about 200° C.), the ferromagnetism disappears, and the magnetic permeability becomes about 1. That is, the metal core46abecomes nonmagnetic (paramagnetic) (magnetic shunting), and is capable of self temperature control.

The metal core46aof the embodiment is formed to have a thickness (e.g., 0.2 mm) less than a magnetic-field permeation (penetration) depth provided when the temperature of the metal core46abecomes higher than or equal to the Curie temperature of the metal core46a.

As depicted inFIG. 3in connection with an example using a Fe—Ni alloy, the field permeation depth of the metal core46acan be calculated using a resistance ρ of the magnetic shunt alloy, the magnetic permeability μ of the magnetic shunt alloy at the Curie temperature, and a power supply frequency f to be applied to the coil52. As shown by the illustrative calculation tabulated inFIG. 3, the magnetic-field permeation depth of the metal core46agreatly changes according to the Curie temperature (FIG. 3). In accordance with some embodiments, to set the Curie temperature of the metal core46ato be about 200° C., the Ni content of the Fe—Ni alloy is about 30% to about 40%. If the metal core has a thickness more than or equal to the magnetic-field permeation depth at a temperature higher than or equal to the Curie temperature of the metal core46a, an induced current is generated by magnetic flux generated by the coil52and a diamagnetic field is generated, so that the magnetic flux from the coil52can be blocked (flux reducing effect).

In contrast, when the thickness of the metal core46ais a thickness less than the magnetic-flux permeation depth at the temperature higher than or equal to the Curie temperature of the metal core46a, such as 0.2 mm, as in the embodiment, the magnetic flux generated by the coil52substantially passes through inside the metal core46aalong the surface of metal core46awhen the temperature of the metal core46ais less than the Curie temperature. In contrast, when the temperature of the metal core46abecomes higher than or equal to the Curie temperature, leakage flux passing through the metal core46ain the thickness direction of the metal core46aand traveling toward the side opposite the coil52, that is, toward a ring member60, which will be described below, is generated.

The ring member60of the illustrative embodiment is disposed at a position opposing the coil52with the metal core46abeing disposed therebetween, more specifically, to an inner portion of the heat roller46(refer toFIG. 2). For example, as illustrated inFIG. 4, the ring member60is curved in an arc form as a whole to be arch-shaped in side view and to be rectangular-shaped with a hollow inside thereof in top view. An upper surface61of the ring member60faces an inner surface of the metal core46a, and a lower surface62of the ring member60faces a rotation shaft of the heat roller46.

The ring member60includes two linear portions65parallel to the axial direction of the heat roller46, and two arch-shaped portions66that connect longitudinal ends of the linear portions65. A hollow space is defined in an inner side of the ring member60by the linear portions65and the arch-shaped portions66. Also, the ring member60is shaped like a hollow rectangle, as viewed from above.

It will be understood that the longitudinal ends of linear portions65, while depicted as intersecting the arch-shaped portions66at sharp corners (e.g., right angles), in various implementations they may intersect at tapered or rounded corners. Additionally, while the arch-shaped portions66may be circular arcs, in various implementations they may have a non-circular curvature; for example, they may be implemented as an elliptic arc or parabolic arc. For ease of reference and clarity of exposition, a ring member having the general shape as described is referred to herein as an arch-shaped saddle ring (or arch-shaped saddle ring member). As will thus be understood, as used herein, an arch-shaped saddle ring may have arch-shaped portions implemented as, for example, circular arcs, parabolic arcs, or elliptic arcs. Also for convenience, an arch-shaped saddle ring (member) having arch-shaped portions formed as circular arcs is referred to herein as a cylindrical saddle ring. The ring member60may be integrally formed, or may be formed by joining two or more formed portions (e.g., linear and arch-shaped portions) thereof.

Further, the ring member60is a nonmagnetic and highly electrically conductive material such as oxygen-free copper, and has a thickness ranging, for example, from about 0.1 mm to about 4 mm (e.g., 1 to 2 mm). The thickness of the ring member60may be uniform in the peripheral direction. The ring member60serves to suppress heat generation in the metal core46aeven if the temperature is higher than or equal to the Curie temperature.

Specifically, since the thickness of the metal core46ais less than the magnetic-field permeation depth at the temperature higher than or equal to the Curie temperature of the metal core46a, as described above, if the temperature of the metal core46ais lower than the Curie temperature, a magnetic flux generated by the induction heating coil52substantially travels in the metal core46avia the side cores56, the arch cores54, and the center core58, as shown by a solid arrow inFIG. 5A. More specifically, the magnetic flux generated by the induction heating coil52travels toward the side cores56after traveling through inside the metal core46a. In this case, eddy current is generated in the metal core46aof the heat roller46, and Joule heat is generated in the metal core46aowing to a specific resistance of the metal core46a, thereby heating the metal core46a.

In contrast, when the temperature of the metal core46abecomes higher than or equal to the Curie temperature, the ferromagnetism of the metal core46adisappears, and the magnetic permeability becomes about 1, so that the magnetic-field permeation depth greatly increases (seeFIG. 3). For this reason, a leakage flux passing through the metal core46ain the thickness direction and traveling toward the ring member60is generated, as shown by a dashed arrow inFIG. 5B.

This leakage flux is shown by a downward-pointing solid arrow inFIG. 6. The ring member60is fixed in an orientation such that the leakage flux penetrates an imaginary plane in the rectangle in a substantially perpendicular direction. For this reason, a diamagnetic field whose direction is opposite to the direction of the leakage flux (shown by an upward-pointing solid arrow inFIG. 6) is generated by the ring member60by an induced current generated in the ring member60by the leakage flux (shown by a broken arrow inFIG. 6). Since the diamagnetic field acts in a direction to cancel the leakage flux (vertical penetrating magnetic field), it blocks or reduces the magnetic flux (the leakage flux) from the induction heating coil52(flux reducing effect).

By forming the ring member60of a highly conductive member, generation of Joule heat by the induced current is suppressed and the flux from the induction heating coil52is efficiently blocked or reduced.

As illustrated inFIG. 7, the ring member60of the illustrative embodiment is divided into a plurality of parts. More specifically, the ring member60includes at least three types of rings60a,60b, and60c. The rings60a,60b, and60care separately arranged in the axial direction (longitudinal direction) of the heat roller46(FIG. 7), and are supported by a member such as a cover (not shown) of the fixing unit14.

The lengths of the rings60a,60b, and60cin the axial direction of the heat roller46are different corresponding to a plurality of widths of sheets P to be conveyed (lengths of the sheets P perpendicular to the sheet conveying direction). The rings60a,60b, and60cmay be separately formed like a ring member60illustrated on the left side ofFIG. 7or may be integrally formed like a ring member60illustrated on the right side ofFIG. 7as long as the rings60a,60b, and60chave the same thickness.

The three types of rings60a,60b, and60care arranged symmetrically with respect to a center of the axis of the heat roller46. The rings60aare provided corresponding to both ends of the heat roller46, and the rings60band60care arranged in order from the ends toward the center. The innermost rings60c(closest to the center) are provided outside a passing range of sheets P of the minimum size. The rings60bare provided outside a passing range of sheets P of the middle size, and the rings60aare provided outside a passing range of sheets P of the next larger size (maximum size).

For example, this arrangement can respond to four sheet sizes, that is, the maximum sheet size of 13 inches (340 mm) and three smaller sheet sizes. The three smaller sheet sizes are the A3-size (297 mm), the A4 portrait size (210 mm), and the A5 portrait size (149 mm). A ring60dillustrated on the left side ofFIG. 7prevents the temperature of the heat roller46from excessively becoming higher when temperature control with the above-described thermistor becomes difficult. However, the ring60dmay be omitted, as on the right side ofFIG. 7.

By way of example, in accordance with various embodiments, a slide belt (fixing belt member)68may be used instead of the heat roller46and the fixing roller45described above. More specifically, in various alternative implementations of fixing unit14, such as that illustrated inFIG. 8, components having the same functions as those of the above embodiment are denoted by the same reference numerals, and descriptions thereof are omitted. The fixing unit14includes a pressurizing roller44and the flexible slide belt68. A sheet P conveyed through a secondary transfer unit23is led into a fixing nip between the pressurizing roller44and the slide belt68.

More specifically, the pressurizing roller44is a roller having a diameter of about 25 mm, in which a silicone rubber layer having a thickness of about 2 to about 5 mm is provided on a metal (e.g., SUS) core and is covered with a PFA tube. The pressurizing roller44is shaped like a hollow cylinder, and a halogen heater may be provided in an inner space of the pressurizing roller44.

A base layer of the slide belt68is formed of a magnetic material (Ni). The thickness of the base layer is at least less than the magnetic-field permeation depth at a temperature higher than or equal to the Curie temperature of the base layer of the slide belt68, such as 40 μm. A thin elastic layer (e.g., silicone rubber) having a thickness of about 30 μm is provided on a surface of the base layer, and a release layer (e.g., PFA) having a thickness of about 30 μm is provided on an outer surface of the elastic layer. The slide belt68is an endless thin belt that has a diameter of about 30 mm and the temperature of the slide belt68is controlled to, for example, a range of about 150° C. to about 200° C. when the slide belt68is heated with the induction heating.

The surface temperature of the slide belt68can be measured with a non-contact temperature sensor provided outside an outer side of the slide belt68in the radial direction and at a predetermined distance from the slide belt68.

The pressurizing roller44is provided with a stepping motor (not shown), and is rotated by power from the stepping motor on an axis extending in the width direction of the conveyed sheet P. By the rotation of the pressurizing roller44, the slide belt68is driven to rotate, and a fixing nip is formed between the slide belt68and the pressurizing roller44.

More specifically, a slide member80is disposed on a portion of an inner surface of the slide belt68opposing the pressurizing roller44. The slide member80is shaped like a thin plate extending in the above-described axial direction, and is supported at both ends thereof by a cover (not shown) of the fixing unit14. A lower surface of the fixed slide member80is in sliding contact with the inner surface of the rotating slide belt68. The slide member80receives a pressing force from the pressurizing roller44over the axial direction, whereby a flat nip for fixing a toner image on the sheet P is formed between the slide belt68and the pressurizing roller44.

In various embodiments such as the embodiment illustrated inFIG. 8, a heat generating member70formed of a Fe—Ni alloy is provided at a position opposing a coil52with the slide belt68being disposed therebetween, that is, in the interior of the slide belt68. Moreover, the heat generating member70is disposed in contact with the inner surface of the slide belt68. The heat generating member70is shaped like a plate curved in an arc form as a whole in side view, and has a thickness less than the magnetic-field permeation depth at the temperature higher than or equal to the Curie temperature of the heat generating member70, such as 0.2 mm. Hence, when the temperature of the heat generating member70is lower than the Curie temperature of the heat generating member70, magnetic flux generated by the coil52substantially travels through inside the heat generating member70. In contrast, when the temperature of the heat generating member70is higher than or equal to the Curie temperature of the heat generating member70, a leakage flux that penetrates the heat generating member70in the thickness direction and travels toward an opposite side of the coil52, that is, toward the ring member60is generated.

Since the ring member60is fixed in the orientation such that the leakage flux, which penetrates the slide belt68and the heat generating member70, penetrates an imaginary plane in the rectangle thereof in the substantially perpendicular direction, a diamagnetic field, whose direction is opposite the leakage flux, is generated by an induced current due to the leakage flux in the ring member60, thereby cancelling the leakage flux for blocking or reducing the leakage flux.

The slide belt68may be a thin belt made of a nonmagnetic resin (e.g., PI, 90 μm), or a thin belt made of a nonmagnetic metal (e.g., copper, 5 μm). However, when a belt of 5 μm is made only of copper, it is difficult to maintain the shape of the belt. Hence, the belt made of copper may be supported on a base layer made of PI or the like.

An insulating sheet (insulating member) for heat insulation and electric insulation may be provided between the above-described magnetic shunt metal member, that is, the metal core46a(seeFIG. 2) or the heat generating member70(seeFIG. 8) and the upper surface61of the ring member60. Instead of using the insulating sheet, for example, the ring member60may be covered with an insulating tube or an insulating film. Further, a gap (e.g., about 0.5 to about 1 mm) may be provided between the metal core46aor the heat generating member70, and the upper surface61of the ring member60.

As described above, the heat roller46has the cylindrical metal core46ain the embodiment ofFIG. 2. The coil52of the IH coil unit50and the rectangular-shaped ring member60oppose each other with the metal core46abeing disposed therebetween. As described above, since the release layer is provided on the surface of the metal core46a, the heating belt48does not directly contact the metal core46athat generates heat. The metal core46ais formed of a magnetically permeable and electrically conductive magnetic shunt alloy, and has a thickness less than the magnetic-field permeation depth at the temperature higher than or equal to the Curie temperature of the metal core46a.

According to the embodiment ofFIG. 8, the flexible thin slide belt68is used, and the coil52of the IH coil unit50and the heat generating member70oppose each other with the slide belt68being disposed therebetween. Further, the coil52and the rectangular-shaped ring member60oppose each other with the heat generating member70being disposed therebetween. The heat generating member70is formed of a magnetically permeable and electrically conductive magnetic shunt alloy, and has a thickness less than the magnetic-field permeation depth at the temperature higher than or equal to the Curie temperature of the heat generating member70.

By thus combining the heat roller46formed of the magnetic shunt alloy or the heat generating member70formed of the magnetic shunt alloy with the nonmagnetic and electrically conductive ring member60, the decrease in heating efficiency can be prevented while maintaining the flux reducing effect, and the heat capacity can be reduced, in contrast to the case in which the heat generation efficiency is reduced or the flux suppressing effect is reduced when the thicknesses of the plate-shaped magnetic shunt alloy layer and the plate-shaped nonmagnetic metal layer, which are used in combination, are reduced. Therefore, the warm-up time and energy consumption can be reduced. As a result, it is possible to meet the request to further reduce the heat capacity.

In addition, since heat of the heat generating member70is always transmitted to the slide belt68in the embodiment ofFIG. 8, the heat capacity can be made much smaller than in the case in which rollers, such as the heat roller and the fixing roller, are used. This more reliably meets the request to further reduce the heat capacity.

Further, since the slide belt68is thin in the embodiment ofFIG. 8, magnetic flux generated by the coil52completely penetrates the slide belt68, and reaches the heat generating member70of the magnetic shunt alloy.

When the slide belt68is formed of copper, the slide belt68itself can generate heat, and therefore, effectively functions as a heat supply source. In contrast, the slide belt68formed of PI does not generate heat and does not function as a heat supply source. However, as described above, the PI slide belt68is in contact with the heat generating member70, and therefore, the above-described reduction of heat capacity is not hindered.

When the slide belt68is formed of Ni and has a thickness less than the magnetic-field permeation depth at the temperature higher than or equal to the Curie temperature of the slide belt68, the shape of the belt formed of only the magnetic metal can be maintained. In the case of the thickness that can maintain the belt shape, the thickness of the heat generating member70is added to the thickness of the slide belt68. Hence, the amount of leakage flux that can travels through inside the heat generating member70becomes smaller than when the slide belt68is formed by the thin copper belt as described above. Although this has an influence on the flux reducing effect when the temperature of the slide belt68reaches the Curie temperature, the slide belt68itself can generate heat, and therefore, effectively functions as a heat supply source.

When the sectional area of the ring member60is reduced, there is a fear that an induced current will not easily pass therethrough and that the ring member60itself will generate heat. However, this fear does not occur by making the above thickness setting. The induced current is efficiently generated when the leakage flux penetrates, and the magnetic flux can be blocked or reduced reliably with the ring member60.

By dividing the ring member60, the amount of leakage flux penetrating the rings60a,60b, and60cis reduced. In this case, heat generation of the ring member60itself can be prevented further.

By electrically disconnecting the ring member60, from the metal core46aor the heat generating member70, the induced current can be prevented from leaking from the ring member60to the metal core46aor the heat generating member70. Further, by avoiding heat conduction from the metal core46aor the heat generating member70to the ring member60, heat of the magnetic shunt alloy can be effectively utilized. This also contributes to reduction of heat capacity of the fixing device.

In addition, a small heat capacity is enough, and an excessive temperature rise in the area of the metal core46aor the heat generating member70where the sheet P is not conveyed is prevented. This improves the quality of toner images and also improves reliability of the image forming apparatus1.

The present disclosure is not limited to the above-described embodiments, and various modifications are possible. For example, the center core may be fixed or be rotatable.

While the metal core46aof the heat roller46is adopted in the above embodiments, the heat roller46may be omitted when, for example, the metal core of the fixing roller45is formed of a magnetic shunt alloy and the heating belt48can be wound around the fixing roller45. In other words, the rotating fixing body according to various embodiments of the present disclosure includes not only the heat roller but also the fixing roller.

In the above embodiments, the coil52of the IH coil unit50is arranged along the outer surface of the slide belt68. However, the coil52can be provided in the slide belt68and the heat generating member70and the ring member60can be provided on the outer side of the slide belt68as long as the coil52and the heat generating member70can oppose each other with the slide belt68being disposed therebetween and the coil52and the rectangular-shaped ring member60can oppose each other with the heat generating member70being disposed therebetween.

While the image forming apparatuses of the embodiments are embodied by the printer, the image forming apparatus of the present disclosure is also applicable to a multi-functional peripheral, a copying machine, and a facsimile machine. In any case, the heat capacity of the fixing device can be further reduced, similarly to the above embodiments. Also, as noted above, values, such as dimensions and/or dimensional ranges, resistivity, etc., provided in the foregoing embodiments are merely illustrative and are not intended to be limiting of the present disclosure.

Having thus described in detail embodiments of the present disclosure, it is to be understood that the disclosure disclosed by the foregoing paragraphs is not to be limited to particular details and/or embodiments set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope of the present invention.