Image heating apparatus with electrical connection between contact member and electroconductive layer of image heating member

An image heating apparatus includes magnetic flux generating means for generating a magnetic flux; an image heating member for generating heat by eddy current produced by the magnetic flux generated by the magnetic flux generating means and for heating an image on a recording material fed thereto; a diverting member for diverting the eddy current produced in the image heating member by contacting at least a part, with respect to a widthwise direction perpendicular to a feeding direction of the recording material, of the image heating member to establish electrical connection with the image heating member; and driving means for contacting and spacing the diverting member relative to the image heating member.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image heating apparatus which heats an image on recording medium, with the use of an inductive heating method. More specifically, it relates to an image heating apparatus which is excellent as a thermal fixing apparatus mounted in such an image forming apparatus as a copying machine or a printer, which forms an image using an electrophotographic image forming method or the like.

A heating apparatus of the heat roller type, which typically employs a halogen lamp as a heat source, has long been employed as a thermal fixing apparatus mounted in a printer, a copying machine, etc. In recent years, however, various fixing apparatuses which employ an inductive heating method have been realized as one of the answers to the recent trend of reducing office automation equipment in energy consumption, and some of them have been put to practical use, in place of a fixing apparatus of the heat roller type. An inductive heating apparatus can achieve both the object of reducing an image heating apparatus in energy consumption, and the object of reducing an image heating apparatus in the length of startup time.

A fixing apparatus employing an inductive heating method is made up of a magnetic flux generating means, and a member (inductive heating member) in which heat is generated through electromagnetic induction by the function of the magnetic flux generated by the magnetic flux generating means. In a fixing operation, a piece of recording medium as an object to heated is introduced into the heating portion of the fixing apparatus, and conveyed through the heating portion. While the recording medium is conveyed through the heating portion, the unfixed image on the recording medium is thermally fixed to the recording medium by the heat from the inductive heating member.

From the standpoint of reducing a fixing apparatus of the induction type in energy consumption as well as length of startup time, the inductive heating member should be reduced in thickness to reduce it in thermal capacity. Further, from the standpoint of reducing the inductive heating member in thermal capacity without reducing it in strength, it is formed of iron, nickel, SUS, or the like. However, this design suffers from the following problem. That is, as a substantial number of small sheets of recording paper, that is, sheets of recording paper which are narrower in track width than the largest sheet of recording paper that can be conveyed through a fixing apparatus (recording medium of full size), in terms of the direction perpendicular to the direction in which a recording medium is to be conveyed through the fixing apparatus, the portions of the heating member, which are outside the track of the small sheet of recording paper, excessively increase in temperature.

In the past, as one of the measures for preventing a fixing apparatus employing an inductive heating method from excessively increasing in temperature across the portions of its heating member, which are outside the sheet track, a magnetic flux blocking means has been employed. More specifically, according to Japanese Laid-open Patent Application 10-74009, that is, one of the various Japanese patent applications which propose the abovementioned measures, a means for adjusting a magnetic flux, more specifically, a magnetic flux blocking means, is disposed within a magnetic circuit to adjust in distribution the magnetic flux generated from the magnetic flux generating means.

With the employment of the above described structural arrangement, it was possible to change the magnetic flux in its distribution across a fixation roller, in terms of the lengthwise direction of the fixation roller. Therefore, it was possible to adjust the fixation roller in the distribution of the amount by which Joule heat is generated in the wall of a fixation roller by eddy current, in terms of the lengthwise direction of the fixation roller. In other words, it was possible to adjust the magnetic flux in the distribution of its density in the lengthwise direction of the fixation roller in order to adjust the fixation roller in the distribution of the amount by which Joule heat is generated in the fixation roller, in terms of the lengthwise direction of the fixation roller so that the temperature distribution of the fixation roller becomes optimal for the size of the recording medium being used for image formation.

Further, in the case of a structural arrangement in which the magnetic flux adjusting member is inserted into the abovementioned magnetic flux circuit from outside the magnetic circuit, in the direction parallel to the lengthwise direction of the fixation roller, a fixing apparatus has to be increased in size in the lengthwise direction of the fixation roller, by the amount equal to the size of the space into which the magnetic flux adjusting member is to be retracted. Therefore, in order to adjust the distribution of magnetic flux density in terms of the lengthwise direction of the fixation roller without increasing the fixing apparatus in size, a fixing apparatus is structured so that the magnetic flux adjusting member can be rotated in the direction parallel to the circumferential direction of the fixation roller, between a position in which it allows the magnetic flux to heat the portion of the fixation roller, which corresponds in position to the magnetic flux adjusting member, and a position in which it does not allow the magnetic flux to heat the portion of the fixation roller, which corresponds in position to the magnetic flux adjusting member.

However, in the case of the structural arrangement in which the magnetic flux adjusting member can be rotated in the circumferential direction of a fixation roller to adjust the distribution of the magnetic flux density in terms of the lengthwise direction of the fixation roller, the coil and core of the magnetic flux generating means must be disposed so that a fixation roller is heated across the portion in a specific space in terms of the circumferential direction of the fixation roller. Such a requirement restricts the magnetic circuit in structure. Moreover, in the case of the method which partially blocks the magnetic flux, the portions of the fixation roller, which are shielded from the magnetic flux by the magnetic flux blocking member, rapidly decreases in temperature, requiring therefore the magnetic flux adjusting member to be frequently shuttled between the magnetic flux blocking position and the retreat; it increases the number of times the magnetic flux adjusting member must be driven.

SUMMARY OF THE INVENTION

Thus, the primary object of the present invention is to provide an inductive image heating apparatus, the image heating member of which can be adjusted in the temperature distribution in terms of the direction perpendicular to the recording medium conveyance direction, without partially blocking the magnetic flux.

Another object of the present invention is to provide an image heating apparatus, the image heating member of which can be adjusted in the temperature distribution in terms of the direction perpendicular to the recording medium conveyance direction, by partially diverting the eddy current which is induced in the image heating member.

According to an aspect of the present invention, there is provided an image heating apparatus comprising magnetic flux generating means for generating a magnetic flux; an image heating member for generating heat by eddy current produced by the magnetic flux generated by said magnetic flux generating means and for heating an image on a recording material fed thereto; a diverting member for diverting the eddy current produced in said image heating member by contacting at least a part, with respect to a widthwise direction perpendicular to a feeding direction of the recording material, of said image heating member to establish electrical connection with said image heating member; and driving means for contacting and spacing said diverting member relative to said image heating member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) Example of Image Forming Apparatus

FIG. 2is a schematic drawing of the image forming apparatus in this embodiment, showing the general structure thereof. The image forming apparatus in this embodiment is a laser beam printer of the transfer type, which employs an electrophotographic process.

Designated by a referential symbol51is a photosensitive drum. The photosensitive drum51is made up of an electrically conductive cylindrical substrate formed of aluminum, nickel, or the like, and a photosensitive layer formed of a photosensitive substance such as OPC, amorphous selenium, amorphous silicon, or the like, around the peripheral surface of the cylindrical substrate.

The photosensitive drum51is rotationally driven in the clockwise direction indicated by an arrow mark at a preset peripheral velocity. As the photosensitive drum51is rotated, first, its peripheral surface is uniformly charged by a charge roller52as a charging apparatus to preset polarity and potential level.

Next, the uniformly charged peripheral surface of the photosensitive drum51is scanned (exposed) by a beam53aof laser light projected from a laser scanner53while being turned on or off in accordance with image formation data. As a result, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum51.

This electrostatic latent image is developed by a developing apparatus54into a visible image, that is, an image formed of toner (which hereinafter will be referred to as toner image). As the method for developing the latent image, the jumping developing method, two-component developing method, FEED developing method, or the like may be used. The above described exposing process is likely to be used in combination with a reversal developing method.

The toner image, that is, the visualized latent image, is transferred by a transfer roller55as a transferring apparatus from the peripheral surface of the photosensitive drum51onto a recording medium P, for example, a sheet of recording paper, OHP, etc., which is delivered with a preset timing.

As for the above described delivery of the recording medium P, the leading edge of the recording medium P is detected by a sensor58, and the timing of the delivery of the recording medium P is set so that as the recording medium P is delivered, the leading edge of the portion of the peripheral surface of the photosensitive drum51across which a toner image is formed, and the portion of the recording medium P, which will be the leading edge of the toner image after the transfer of the toner image from the photosensitive drum51onto the recording medium P, simultaneously arrive at the image transfer station. After being delivered with the preset timing to the transfer station, the recording medium P is conveyed through the transfer station while remaining pinched between the photosensitive drum51and transfer roller55, and while the recording medium P is conveyed through the transfer station, the toner image on the photosensitive drum51is transferred onto the recording medium P as if it were rolled out of the photosensitive drum51. After the transfer of the toner image onto the recording medium P, the recording medium P is separated from the peripheral surface of the photosensitive drum51, and conveyed to a fixing apparatus6as a heating apparatus. Then, the toner image on the recording medium P is thermally fixed as a permanent image to the recording medium P.

Meanwhile, the portion of the peripheral surface of the photosensitive drum51, from which the recording medium P has been separated, is cleaned by a cleaning apparatus57; the residual toner particles, that is, the toner particles remaining on the peripheral surface of the photosensitive drum51after the image transfer, are removed by the cleaning apparatus57. Then, the cleaned portion of the peripheral surface of the photosensitive drum51is used for the following image formation cycle. In other words, the peripheral surface of the photosensitive drum51is repeatedly used for image formation.

1) General Structure

Regarding the directions of the structural components, parts, etc., of the fixing apparatus in the following description of the fixing apparatus in this embodiment, the lengthwise direction means the direction perpendicular to the direction in which the recording medium P is conveyed through the recording medium conveyance passage of the fixing apparatus, whereas the width, or widthwise direction, means the direction parallel to the above described recording medium conveyance direction. The front side of the fixing apparatus means the side from which the recording medium P is fed into the fixing apparatus, and the rear side of the fixing apparatus means the opposite side from the front side (side from which recording medium P comes out of fixing apparatus). The left and right sides of the fixing apparatus means the left and right sides of the fixing apparatus as seen from the front side of the fixing apparatus. The upstream and downstream sides of the fixing apparatus means the upstream and downstream sides in terms of the recording medium conveyance direction.

FIG. 1is a diagrammatic drawing depicting the concept of the fixing apparatus, which is common to the first to seventh embodiments of the present invention.FIG. 3is an enlarged schematic sectional view of the essential portions of the fixing apparatus in this embodiment.FIG. 4is a schematic front view of the same essential portions of the fixing apparatus as those shown inFIG. 3.FIG. 5is a schematic vertical sectional view of the same essential portions of the fixing apparatus as those shown inFIG. 3, at the vertical plane coinciding with the axial line of the fixation roller.

This fixing apparatus56is a heating apparatus which employs an inductive heating method and a heat roller. Designated by a referential symbol1is a fixation roller as a heat generating member (which hereinafter may be referred to as inductive heat generating member, or simply as image heating member), and designated by a referential symbol2is an elastic roller as a pressure applying member. The two rollers1and2are vertically stacked in parallel, and are kept pressed upon each other by the application of a preset amount of pressure, forming a fixation nip N as a heating station.

The fixation roller1is made up of a cylindrical thin metallic roller, and an elastic layer covering the peripheral surface of the metallic roller. The cylindrical thin metallic roller is formed of such a substance as Ni, Fe, SUS, or the like (magnetic metal or the like substance) in which heat can be generated by electromagnetic induction. The thickness of its wall is in the range of 0.02 mm-3.0 mm. The elastic layer is formed of heat resistant fluorinated resin or the like coated on the peripheral surface of the metallic roller. It functions as a release layer. Referring toFIGS. 4 and 5, this fixation roller1is rotatably supported at both of its lengthwise ends, by the left and right lateral plates21and22of the fixing apparatus1, with a pair of bearings23disposed between the lengthwise ends of the fixation roller1and left and right lateral plates21and22, one for one.

In the hollow of the fixation roller1, an excitation coil unit3is non-rotationally and rigidly disposed; it is inserted into the hollow of the fixation roller1. The excitation coil unit3is a magnetic flux generating means for generating a high frequency magnetic field to generate Joule heat in the wall of the fixation roller1by inducing electric current (eddy current) in the wall.

The pressure roller2is made up of a metallic core2a, a heat resistant rubber layer2bformed around the peripheral surface of the metallic core2a, and a heat resistant release layer2cformed of fluorinated resin of the like, around the peripheral surface of the heat resistant rubber layer2b. The pressure roller2is rotatably supported at both of the lengthwise ends of the metallic core2a, by the abovementioned left and right lateral plates21and22, with a pair of bearings24disposed between the lengthwise ends of the metallic core2aand the lateral plates21and22, one for one. Further, the pressure roller2is kept pressed upon the downwardly facing portion of the peripheral surface of the fixation roller1by an unshown pressing means, which applies a preset amount of pressure against the elasticity of the heat resistant elastic layer2bso that the fixation nip N is formed.

Designated by a referential symbol G is a gear for driving the fixation roller1, which is rigidly fitted around the left end of the fixation roller1. The force for driving the fixation roller1is transmitted to the fixation roller1through a fixation roller driving mechanism inclusive of this gear G, so that the fixation roller1is rotationally driven in the clockwise direction ofFIG. 3, at a preset peripheral velocity. As the fixation roller1is rotationally driven, the torque is transmitted from the fixation roller1to the pressure roller2through the friction which is present between the fixation roller1and pressure roller2in the fixation nip N, causing the pressure roller2to follow the rotation of the fixation roller1. The fixation roller driving mechanism11is sequentially controlled by a control portion (CPU)104.

The excitation coil unit3is an assembly made up of a holder4, an excitation coil5, a magnetic core6, a cover plate4a, etc. The holder4is in the form of a trough which is roughly semicircular in cross section. The external diameter of holder4is smaller than the internal diameter of the fixation roller1. The excitation coil5is disposed within the hollow of the holder4. The magnetic core6has a T-shaped cross section.FIG. 6is a schematic exploded perspective view of this excitation coil unit3.

The holder4and cover plate4aare molded nonmagnetic members which are heat resistant and provided with a reasonable amount of mechanical strength. They are molded of heat resistant and electrically nonconductive engineering plastic, or the like, for example.

The excitation coil5must be capable of generating such an alternating magnetic flux that is strong enough in magnitude to generate heat by the amount sufficient for the image fixation. In order to achieve this objective, the excitation coil5needs to be low in electrical resistance and high in inductance. As the core wire of the excitation coil5, a piece of Litz wire, that is, a preset number of bundled pieces of fine wire with a preset diameter, is employed. As the fine wire for the Litz wire, fine electric wire coated with dielectric substance is employed. The Litz wire is wound multiple times around the center portion6aof the magnetic core6so that the contour of the resultant excitation coil5conforms to the internal surface of the holder4, and also, so that the resultant excitation coil5has the shape of a long and narrow boat, the lengthwise direction of which is parallel to the lengthwise direction of the fixation roller1. The positional relationship between the center portion6aof the magnetic core6and the excitation coil5is such that the center portion6ais located in the center of the excitation coil5.

Designated by a referential symbol4bis a groove of the cover plate4a, into which the bottom portion of the center portion6aof the magnetic core6is inserted to support the magnetic core6. The magnetic core supporting groove4bis located roughly at the center of the inward surface of the cover plate4a, in terms of the width direction of the cover plate4a, and extends from one widthwise end of cover plate4ato the other.

As the material for the magnetic core6, such a substance as ferrite, Permalloy, Sendust, amorphous silicon steel plate, or the like, which is large in permeability while being small in internal loss, is suitable. The holder4and cover plate4aalso function as an insulating member for insulating the magnetic core6and excitation coil5from each other.

The holder4is provided with a cylindrical hollow shaft portion4d, which constitutes the lengthwise left end portion of the holder4. The holder4is also provided with a solid shaft portion4e, which constitutes the lengthwise right end portion of the holder4. The solid shaft portion4eis provided with a D-cut portion4f, which constitutes the lengthwise rightmost end portion of the holder4. Referring toFIGS. 4 and 5, the excitation coil unit3is supported by the left and right auxiliary lateral plates25and26of the fixing apparatus, with the cylindrical shaft portion4dinserted in the circular hole cut through the left auxiliary lateral plate25, and the D-cut portion4fof the solid shaft portion4einserted in the D-shaped hole cut through the right auxiliary lateral plate26. Thus, the excitation coil unit3is non-rotationally supported between the left and right auxiliary lateral plates25and26, in such an attitude that the curved side of the excitation unit3faces downward. Further, referring toFIG. 3, the excitation coil unit3is disposed in the hollow of the fixation roller1so that a preset amount of gap is provided between the internal surface of the fixation roller1and the curved side of the excitation coil unit3. With the excitation coil unit3disposed as described above, the axial line of the fixation roller1, and the axial lines of the cylindrical hollow shaft portion4dand solid shaft portion4eof the holder4, roughly coincide.

In other words, the excitation coil unit3is disposed so that it opposes the internal surface of the fixation roller1, with such a uniform gap that extends from one lengthwise end of the excitation coil unit3to the other, being provided between the outward surface of the curved portion of the semicylindrical excitation coil unit3and the corresponding portion of the internal surface of the fixation roller1, in terms of the circumferential direction of the fixation roller1.

Through the cylindrical hollow shaft portion4d, a pair of lead wires5aand5bof the excitation coil5in the holder4are extended outward from within the holder4, and are connected to a high frequency driver power source (excitation circuit)13for supplying the excitation coil5with high frequency electric current.

Designated by a referential symbol8is an electrically conductive member (current diverting member) as a heat distribution adjusting means, which is placed in contact with, or separated from, the fixation roller1to divert the eddy current induced in the wall of the fixation roller1in order to adjust the fixation roller1in heat distribution, in terms of the lengthwise direction of the fixation roller1. Hereafter, this electrically conductive member8for adjusting the fixation roller1in heat distribution will be referred to as eddy current diverting member, or more simply as current diverting member. The eddy current diverting member8is placed between the excitation coil unit3as the magnetic flux generating means and the fixation roller1as the inductive heating member. It is placed in contact with the fixation roller1, across a specific range of the fixation roller1, in terms of the lengthwise direction of the fixation roller1, to establish electrical connection between the current diverting member8and the specific range of the fixation roller1. The eddy current diverting member8will be described in detail in the following section (Section 2)).

The high frequency driver power source13supplies the excitation coil5of the excitation coil unit3with high frequency electric current (alternating current) in response to the signals from the control portion104. The excitation coil5uses the high frequency electric current supplied from the driver power source13, to generate a high frequency magnetic field (alternating magnetic flux) which extends in the lengthwise direction of the fixation roller1. The alternating magnetic flux is guided by the magnetic core6, inducing eddy current in the wall of the fixation roller1. The eddy current interacts with the specific resistance of the fixation roller1, generating heat (Joule heat) in the wall of the fixation roller1. In other words, the fixation roller1is heated by electromagnetic induction. Since the fixation roller1is being rotationally driven, it becomes uniform in surface temperature. More specifically, the high frequency driver power source13is driven so that such electric current that is 10 kHz-100 kHz in frequency flows through the excitation coil5. The eddy current tends to converge to the skin portion of the fixation roller1, which opposes the excitation coil5(this phenomenon is referred as skin effect), increasing the surface portion (skin) of the fixation roller1in apparent resistance, enhancing the generation of heat (Joule heat) by the eddy current and the specific resistance of the fixation roller1.

FIG. 7is the combination of a schematic sectional view of the fixation roller and a graph of the heat distribution of the fixation roller1, showing the amount of heat generated in the wall of the fixation roller1, at a given point in terms of the circumferential direction of the fixation roller1. The drawing depicts the range in which the magnetic flux extends from the magnetic flux generating means3toward and beyond the fixation roller1, and the corresponding heat distribution of the fixation roller1in terms of the circumferential direction of the fixation roller1. As alternating electric current is flowed through the excitation coil5of the excitation coil unit3as the magnetic flux generating means, the excitation coil5generates alternating magnetic flux. The fixation roller1is formed of a magnetic substance, in particular, magnetic metal, as described above. Therefore, electric current (eddy current), which flows in the direction to neutralize the magnetic field, is induced in the wall of the fixation roller1. The fixation roller1is heated by the Joule heat generated by this current induced in the wall of the fixation roller1, increasing therefore in temperature. In the case of the above described structural arrangement for the fixing apparatus in this embodiment, the outward area of the semicylindrically bulging side of the excitation coil unit3, more specifically, the area outside the outwardly bulging side of the semicylindrical holder4in which the excitation coil5and magnetic core6are disposed, is the area in which the magnetic flux extending from the excitation coil unit3toward and beyond the fixation roller1is substantial in magnitude. In other words, the above described area constitutes the magnetic circuit (unshown) which the excitation coil5, fixation roller1, and magnetic core6establish. The fixation roller1is inductively heated in this area in which the magnetic flux is substantial in magnitude. In terms of the circumferential direction of the fixation roller1, the heat distribution of the fixation roller1in terms of the amount by which heat is generated in the fixation roller1in the area in which the magnetic flux is significant in magnitude has two areas H and H, that is, areas which are substantial in the amount of heat generated therein, as shown in the schematic drawing (FIG. 7).

Referring toFIG. 4, in the adjacencies of the peripheral surface of the fixation roller1, first and second temperature detecting means TH1and TH2for detecting the temperature of the fixation roller1are located. These temperature detecting means TH1and TH2are placed in contact, or virtually in contact with, the peripheral surface of the fixation roller1, being positioned so that they oppose the excitation coil5of the excitation coil unit3, with the presence of the wall of the fixation roller1between them and excitation coil5. Each of them is made up of an ordinary temperature detecting means, for example, a thermistor, a thermopile, a thermocouple, etc. The data regarding the temperature of the fixation roller1detected by the temperature detecting means TH1and TH2are inputted into the control portion104. The first temperature detecting means TH1is for controlling the temperature of the fixation roller1, and is positioned so that its position roughly coincides with the mid portion of the fixation roller1in terms of the lengthwise direction of the fixation roller1. The control portion104controls the high frequency driver power source13based on the temperature data inputted into the control portion104from the first temperature detecting means TH1. More specifically, the control portion104controls the amount by which electric power is supplied to the excitation coil5from the high frequency driver power source13so that the temperature level of the fixation roller1detected by the first temperature detecting means TH1and inputted into the control portion104is maintained at a preset fixation temperature level (target temperature level). The second temperature detecting means TH2is positioned so that it opposes one of the lengthwise end portions of the fixation roller1. The temperature data obtained by the second temperature detecting means TH2, that is, the temperature data of one of the lengthwise end portions of the fixation roller1(temperature data of portion of fixation roller outside recording paper track), are also inputted into the control portion104.

While the fixation roller1and pressure roller2are rotated, the fixation roller1is inductively heated by the excitation coil unit3as the magnetic flux generating means. As a result, the temperature of the fixation roller1reaches the preset fixation temperature level at which the temperature of the fixation roller1is maintained. While the temperature of the fixation roller1is maintained at the preset fixation temperature level, the recording medium P bearing the unfixed toner image t having just been transferred onto the recording medium P is introduced into the fixation nip N from the direction indicated by an arrow marka, and then, is conveyed through the fixation nip N while remaining pinched by the fixation roller1and pressure roller2. While the recording medium P is conveyed through the fixation nip N, the heat from the heated fixation roller1, and the pressure from the pressure roller2, are applied to the recording medium P and the toner image t thereon. As a result, the unfixed toner image t is fixed to the surface of the recording medium P; a permanent toner image is formed on the recording medium P. After being conveyed through the fixation nip N, the recording medium P is separated from the fixation roller1, and further conveyed leftward of the drawing.

In the case of the image forming apparatus and fixing apparatus in this embodiment, the recording medium P is conveyed through the apparatuses so that the center of the recording medium P coincides with the center of the recording medium passage of the apparatus in terms of the lengthwise direction of the fixation roller1. Referring toFIGS. 4 and 5, designated by a referential symbol O is the referential line (hypothetical line), that is, the center line of the recording medium passage (center line of fixing apparatus). Regarding the attributes of the recording medium P, “recording medium width” means the dimension of the recording medium in terms of the direction perpendicular to the recording medium conveyance direction a, provided that the recording medium remains flat. Designated by a referential symbol A is the track of a largest (in terms of “width”) recording medium (which hereinafter may be referred to as recording medium of the large size) conveyable through the apparatus (track A hereinafter may be referred to track of recording medium of the large size). Designated by a referential symbol B is the track of a smaller recording medium (which hereinafter may be referred to as recording medium of small size), that is, a recording medium narrower than the recording medium of the large size. The track B hereinafter may be referred to as track of recording medium of the small size. Designated by a referential symbol C is the out-of-track area, that is, the area of the recording medium passage, which will be outside the recording medium track when a recording medium the small size is conveyed through the apparatus (area resulting from difference in width between track of recording medium of large size and track of recording medium of small size. The second temperature detecting means TH2detects the temperature of the portion of the fixation roller1, which corresponds in position to the out-of-track area C.

FIG. 8is a diagrammatic drawing showing the positional relationship among the abovementioned recording medium track A, that is, the track of a recording medium of the large size, recording medium track B, that is, the track of a recording medium of the small size, and out-of-track area C, fixation roller1, excitation coil5, and eddy current diverting member8.

The eddy current diverting member8is a long and narrow electrically conductive member as a heat distribution adjusting means for changing the fixation roller1in the distribution of the amount by which heat is generated in the wall of the fixation roller1, in terms of the lengthwise direction of the fixation roller1. The fixing apparatus is provided with two eddy current diverting members8located so that they overlap with the left and right lengthwise end portions of the fixation roller1in terms of the direction perpendicular to the radius direction of the fixation roller1. There are positioned between the excitation coil unit3and the fixation roller1as the inductive heating member, extending in the lengthwise direction of the fixation roller1. The left and right eddy current diverting members8are given a length matching the width of the out-of-track area C, and are positioned so that they match in position the out-of-track areas C, one for one.

Designated by a referential symbol12is an eddy current diverting member driving means. There are two eddy current diverting member driving means12, which are attached to the left and right lateral plates21and22, respectively. The left and right eddy current diverting members8are supported by the left and right eddy current diverting member driving means12, respectively, in the cantilever fashion. The left and right eddy current diverting members8are the means for changing in position the left and right eddy current diverting members8, within the gap between the fixation roller1and the excitation coil unit3. Each of the left and right eddy current diverting member driving means12is made up of an electromagnetic solenoid-based electromagnetic mechanism, or a cam-based mechanism, for example.

The control portion104controls the left and right eddy current diverting member driving means12:

to move the left and right eddy current diverting members8into a first position in which they remain in contact with the internal surface of the fixation roller1as shown inFIG. 9(a); or to retract the left and right eddy current diverting members8into a second position, shown inFIG. 9(b), in which they have no contact with the internal surface of the fixation roller1.

When the size selection signal S (recording medium size signal from control panel, signal representing recording medium size detected while image is read, print signal, etc.) inputted into the control portion104indicates the selection of a recording medium of the large size, the control portion104controls the eddy current diverting member driving means12so that the left and right eddy current diverting members8are moved into, and held in, the second position in which the eddy current diverting members8do not contact the internal surface of the fixation roller1. In this case, the portions of the fixation roller1, which correspond in position to the track A, that is, the track of a recording medium of the large size, are efficiently and uniformly heated (heat is efficiently and uniformly generated), to the preset fixation temperature level. Therefore, their temperature is controlled so that the temperature level detected by the second temperature detecting means remains at the preset fixation temperature level.

On the other hand, when the size selection signal S having inputted into the control portion104indicates the selection of a recording medium of the small size, the control portion104controls the eddy current diverting member driving means12so that the left and right eddy current diverting members8are moved into, and held in, the first position in which the eddy current diverting members8remain in contact with the internal surface of the fixation roller1. Thus, the fixation roller1rotates, with the internal surfaces of its left and right lengthwise end portions sliding on the left and right eddy current diverting members8, respectively. In this case, the portion of the fixation roller1, which corresponds in position to the track B, that is, the track of a recording medium of the large size, is efficiently and uniformly heated (heat is efficiently and uniformly generated), to the preset fixation temperature level. Therefore, its temperature is controlled so that the temperature level thereof detected by the first temperature detecting means remains at the preset fixation temperature level. In this case, however, the portions of the fixation roller1, which correspond in position to the out-of-track areas C, are in contact with the electrically conductive eddy current diverting members8, one for one, by their internal surfaces. In other words, the lengthwise end portions of the fixation roller1, which correspond in position to the out-of-track areas C, have electrical connection to the eddy current diverting members8, one for one. Therefore, these portions of the fixation roller1are reduced in the efficiency with which heat is generated therein. Consequently, the fixation roller1is adjusted in the distribution of the amount by which heat is generated therein, in terms of the lengthwise direction of the fixation roller1, so that these portions of the fixation roller1are minimized in excessive temperature increase.

FIG. 1is a diagrammatic drawing depicting the concept on which the fixing apparatus in this embodiment is based. In the drawing, the eddy current diverting members8are in contact with the fixation roller1.FIG. 1(a) shows the equivalent resistance of the fixation roller1in this embodiment. As the eddy current diverting member8is placed in contact with a given portion of the fixation roller1, the portion with which the eddy current diverting member8is placed in contact reduces in resistance.FIG. 1(b) diagrammatically shows that placing the eddy current diverting member8in contact with a given portion of the fixation roller1causes the eddy current induced in the wall of the fixation roller1to be diverted to the eddy current diverting member8.

As the eddy current induced in the wall of the fixation roller1is partially diverted from a given portion of the fixation roller1, which is in contact with the eddy current diverting member8, this portion of the fixation roller1is reduced in the amount of the eddy current therein, being therefore reduced in the amount by which heat is generated therein, by the amount equivalent to the amount by which the eddy current is diverted therefrom. In other words, the given portion of the fixation roller1can be reduced in the amount of heat generated therein, by partially diverting the eddy current therefrom.

As for the direction in which the eddy current flows, the eddy current is generated so that it flows in the direction to neutralize the magnetic flux generated by the electric current flowed through the excitation coil5, as shown inFIG. 1(c); the eddy current flows in the opposite direction from the direction of the electric current in the excitation coil5. In reality, therefore, the eddy current diverting member8is placed electrically in contact with such a portion of the fixation roller1that corresponds in position to the out-of-track area C in terms of the lengthwise direction of the fixation roller1, and also, that is substantial in the amount by which eddy current is induced in the wall of the fixation roller1(portion of fixation roller1that opposes excitation coil5: portion of fixation roller1which corresponds to area H, inFIG. 7, which is significant in the amount by which heat in generated in wall of fixation roller1). Here, “the eddy current diverting member8is placed electrically in contact with the fixation roller1” means that as the eddy current diverting member8is placed in contact with the peripheral surface of the fixation roller1to allow electric current to flow from the fixation roller1to the eddy current diverting member8.

The density level at which eddy current is induced in the wall of the fixation roller1is higher across the portion of the fixation roller1, which opposes the excitation coil5. Therefore, placing the eddy current diverting member8in contact with this portion of the fixation roller1improves the efficiency with which the eddy current in diverted. As for the retraction of the eddy current diverting member8, it is desired that the eddy current diverting member8is retracted into a position in which it is roughly parallel to the magnetic flux generated by the excitation coil5. Even while the eddy current diverting member8is kept retracted, it has a slight effect upon the magnetic flux, and therefore, has a slight effect upon the heat distribution of the fixation roller1. However, such an effect is extremely small compared to the effect that the eddy current diverting member8has while the eddy current diverting member8is in contact with the fixation roller1.

The thicker the eddy current diverting member8, the more effective to divert the eddy current. In particular, if the eddy current diverting member8is thicker than the skin depth of the fixation roller1, the eddy current diverting member8is even more effective to divert the eddy current. As long as the eddy current diverting member8is no more than 2 mm in thickness, the effect it has on the magnetic flux while it remains retracted is insignificant. From the standpoint of the mechanical strength of the eddy current diverting member8, the thickness of the eddy current diverting member8is preferred to be no less than 0.05 mm.

The material for the eddy current diverting member8has only to be an electrically conductive metal. However, using, as the material for the eddy current diverting member8, such a metal as Cu, Al, Ag, Au, or the like, which is very high in electrical conductivity, yields the eddy current diverting member8which is superior in the eddy current diversion effect. As for the actual movement of the eddy current diverting member8, when a recording medium of the small size is conveyed, the eddy current diverting member8is placed in contact with the portion of the fixation roller1, which corresponds in position to the out-of-track area C, functioning thereby to divert the eddy current induced in the fixation roller1from this portion of the fixation roller1to itself to prevent this portion of the fixation roller1from excessively increasing in temperature.

Regarding the timing with which the eddy current diverting member8is moved into the first position in which the eddy current diverting member8remains in contact with the internal surface of the fixation roller1, it is also possible to move the eddy current diverting member8into the first position as the control portion104detects that the difference between the temperature level of the fixation roller detected by the first temperature detecting means and the temperature level of the fixation roller detected by the second temperature detecting means has become greater than a preset value (as the temperature of the portion of the fixation roller1, which corresponds to the out-of-track area C, exceeds a permissible level).

The portion of the eddy current diverting member8, which is rubbed by the fixation roller1, is subjected to mechanical stress. Therefore, in order to minimize the effect of the mechanical stress, the eddy current diverting member8should be rounded along the edges, and/or should be curved.

In this embodiment, the inductive heating member, that is, the fixing member, is in the form of a roller. However, as far as the present invention is concerned, the shape of the inductive heating member does not matter. For example, the effect of the present invention is the same as the above described one, even if the present invention is applied to an inductive heating member which is in the form of a belt.

Also in this embodiment, both the eddy current diverting member, and the portion of the fixation roller1with which the eddy current diverting member8is placed in contact, are rendered electrically conductive. However, as long as it is ensured that electrical current is allowed to flow between the eddy current diverting member and fixation roller1, the fixation roller1may be provided with a thin surface layer (electrically nonconductive layer) which is low in friction.

In the case of a magnetic flux blocking plate, it is limited in terms of the position in which it can effectively block the magnetic flux (it has to be placed directly below the center of coil). Thus, when it is unnecessary to block the magnetic flux, it has to be rotated in the direction parallel to the circumferential direction of the fixation roller1to be retracted away from the magnetic flux blocking position into the position in which its effect upon the magnetic flux is insignificant (for example, the opposite side of the excitation coil unit from the pressure roller, where its effect upon the magnetic flux is insignificant because of the positioning of magnetic core). This rotational retraction of the magnetic flux blocking plate sometimes creates such problems as that it comes into contact with the fixation roller; it hangs up on the coil holder; and/or the like problems. Further, a magnetic flux blocking plate almost entirely blocks the portion of the magnetic flux which would act on the fixation roller1, were it not for the magnetic flux blocking plate. Therefore, as soon as a given portion of the fixation roller1is shielded from the magnetic flux by the magnetic flux blocking plate, it reduces in temperature very quickly. Further, in order for a magnetic flux blocking plate to block the magnetic flux, its width (in terms of direction parallel to circumferential direction of fixation roller) must be greater than a certain value. In comparison, in the case of the eddy current diverting member8, simply separating the eddy current diverting member8from the designated area of the fixation roller1(area corresponding in position to coil) nullifies the current diverting effect of the eddy current diverting member8. Therefore, the eddy current diverting member8is operationally advantageous over a magnetic flux blocking plate in that it is shorter in the distance by which it has to be moved to control the inductive heating member in heat distribution, and also, that it is effective even if it is small in the abovementioned width. In addition, the eddy current diverting member8partially diverts the eddy current induced in the wall of the fixation roller1. Therefore, it does not occur that as soon as the eddy current diverting member8is placed in contact with a given area of the fixation roller1, this area of the fixation roller1very quickly reduces in temperature. Therefore, the eddy current diverting member8does not need to be driven as frequently as the magnetic flux blocking plate. Moreover, the eddy current diverting member8can be used even without the presence of the core. Therefore, it is thought to be superior to a magnetic flux blocking plate in terms of the requirements regarding the position into which they are to be retracted.

In this embodiment, the thickness of the inductive heating member of the fixing apparatus is no more than the skin depth of the fixation roller. The inductive heating member is disposed in the gap between the heat distribution adjusting means and magnetic flux generating means. The heat distribution adjusting means is an electrically conductive member, which is to be placed electrically in contact with, or moved away from, the area of the fixation roller1next to its lengthwise end.

FIG. 10(a) is a schematic sectional view of the essential portions of the fixing apparatus in this embodiment, andFIG. 10(b) is a schematic sectional plan view of the fixing apparatus, at Line (b)-(b) inFIG. 10(a).

In the fixing apparatus in this embodiment, an inductive heating member1as a heating member is in the form of a belt (which hereinafter will be referred to as fixation belt). Further, a pressing member2is also in the form of a belt (which hereinafter will be referred to as pressure belt). The fixation belt1is stretched around, being thereby suspended by, two rollers R1and R2, which are arranged in parallel. The pressure belt2is stretched around, being thereby suspended by, two rollers R3and R4, which are arranged in parallel. The two belts1and2are vertically stacked in parallel, and are pressed upon each other by the application of a preset amount of pressure, forming a fixation nip as a heating station. The fixation belt1and pressure belt2are circularly driven in the direction indicated by an arrow mark to convey a sheet of recording medium through the fixation nip while keeping the recording medium pinched between the two belts2. Above the fixation belt1, an excitation coil unit3as a magnetic flux generating means is disposed with the provision of a preset amount of gap between the excitation coil unit3and the portion of the fixation belt1, which is closest to the excitation coil unit3. The excitation coil unit3is an assembly made up of an excitation coil5, a magnetic core6, a holder4by which the coil5and core6are held, etc. The excitation coil5in this embodiment is flatly wound in the form of a “rectangular” volute, the lengthwise direction which is parallel to the width direction of the fixation belt1.

FIG. 10(c) shows an example of the laminar structure for the fixation belt1. The fixation belt1is made up of a metallic layer1ain which heat is inductively generated, a 100 μm-500 μm thick silicon rubber layer1bas an elastic layer layered on the metallic layer1a, and a release layer1cas a surface layer formed of polyimide or the like. When the recording medium P bearing a toner image t is conveyed through the fixation nip, it is conveyed on the surface layer side of the fixation belt1. In this embodiment, the thickness of the surface layer formed of dielectric nonmagnetic substance is unimportant.

When the fixation belt1is inductively heated, the eddy current concentrates to the portion of the skin of the inductive heating member (metallic layer)1a, which directly opposes the excitation coil unit3(excitation coil5). This phenomenon is called skin effect, which can principally expressed by the following mathematical equation:
skin depth δ=(2ρ/ωμ)1/2(m)

Given in the following Table 1 are skin depths of ordinary substances.

Reducing the inductive heating layer1aof the fixation belt1in thickness to a value equivalent to the skin depth of the material used for the inductive heating layer1aincreases the inductive heating layer1ain apparent resistance. This occurs because the reduction renders the inductive heating layer1aroughly uniform in the eddy current density in terms of its thickness direction.

In this embodiment, therefore, the thickness of the inductive heating layer1aof the fixation belt1is made to be no more than the value equivalent to the skin depth of the material used for the inductive heating layer1a. Further, the eddy current diverting member8is disposed so that the fixation belt1is sandwiched between the excitation coil5and eddy current diverting member8, with the eddy current diverting member8placed directly and electrically in contact with the opposite surface of the fixation belt1from the excitation coil5, that is, the inductive heating layer1aof the fixation belt1. This structural arrangement was proved to be effective to reduce the amount by which heat is generated, by diverting the eddy current. More specifically, the eddy current diverting member8is placed electrically in contact with the area of the fixation belt1, which corresponds in position to the out-of-track area C, and in which eddy current is induced (roughly opposite area of fixation belt from excitation coil5).FIG. 11is a diagrammatic drawing showing the heat distribution of the area of the fixation belt1which directly faces the excitation coil unit, in terms of the amount by which heat is generated. As will be evident fromFIG. 11, there are two areas H and H where heat is generated by a significant amount. The areas of the fixation belt1roughly facing the excitation coil5are higher in eddy current density. Therefore, placing the eddy current diverting member8in contact with these areas of the fixation belt1enhances the eddy current diverting effect of the eddy current diverting member8.

The material for the eddy current diverting member8has only to be an electrically conductive metal. However, using, as the material for the eddy current diverting member8, such a metal as Cu, Al, Ag, Au, or the like, which is very high in electrical conductivity, yields such an eddy current diverting member8that is superior in the eddy current diversion effect.

The thicker the eddy current diverting member8, the more effective to divert the eddy current. In particular, if the eddy current diverting member8is thicker than the skin depth of the fixation belt1, the eddy current diverting member8is even more effective to divert the eddy current.

Therefore, the eddy current induced in the fixation belt1is partially diverted from the fixation belt1to the eddy current diverting member8through the area of the fixation belt1, which is in contact with the eddy current diverting member8, reducing thereby the amount by which Joule heat is generated in this area of the fixation belt1through the interaction of the resistance of the fixation belt1and the eddy current. Consequently, the portion of the fixation belt1, which corresponds in position to the out-of-track area C, is prevented from excessively increasing in temperature.

The control for moving the eddy current diverting member8into the first position from the second position, or moving the second position to the first position is the same as that in the first embodiment.

In the first and second embodiment, the eddy current diverting member8is in the form of a piece of flat plate, and is rubbed by the fixation roller1and the metallic layers of the fixation belt1, respectively. Giving a flat shape to the eddy current diverting member8increases the eddy current diverting member8in the size of the area by which the eddy current diverting member8contacts the fixation belt1, rendering thereby the eddy current diverting member8more effective to reduce the fixation belt1in temperature. The portion of the eddy current diverting member8, which the fixation belt1first touches (begins to rub) as the eddy current diverting member8is placed in contact with the fixation belt1, is likely to be subjected to mechanical stress. Therefore, in order to prevent this portion of the eddy current diverting member8from being subjected to mechanical stress, this portion of the eddy current diverting member8should be rounded along the edges, or curved.

Referring toFIG. 12, in this embodiment, the eddy current diverting member8of the fixing apparatus is similar in concept to that in the second embodiment, except that the eddy current diverting member8in this embodiment is given the form of a roller. Therefore, while the eddy current diverting member8in the form of a roller is kept in contact with the metallic layer1aof the fixation belt1, it rotates so that the portion of the peripheral surface of the eddy current diverting member8, which is in contact with the fixation belt1, moves in the same direction as the moving direction of the fixation belt1. Giving the eddy current diverting member8the form of a roller prevents the eddy current diverting member8from being rubbed by the fixation belt1, making thereby the eddy current diverting member8more durable. The fixing apparatus may be structured so that the eddy current diverting member8in the form of a roller is rotated by the movement of the fixation belt1, or the eddy current diverting member8is driven independently from the fixation belt1. However, the arrangement that causes the movement of the fixation belt1to rotate the eddy current diverting member8is simpler in structure. On the other hand, the arrangement that drives the eddy current diverting member8independently from the fixation belt1is superior in terms of the ease with which the fixation belt1moves.

The eddy current diverting member8is provided with a position8′ into which it can be moved to break electrical contact between the eddy current diverting member8and fixation belt1. The recording papers are not uniform in size. Therefore, it is possible that as a recording paper of the small size is conveyed through the fixing apparatus, the portion of the fixation belt1, which is outside the track of the recording paper of the small size, will excessively increase in temperature. Thus, in order to prevent the occurrence of this phenomenon, the eddy current diverting member8is placed electrically in contact with the metallic layer1aof the fixation belt1to establish electrical connection between the eddy current diverting member8and fixation belt1, when the recording paper of the small size is conveyed through the fixing apparatus. However, when a recording paper of the large size is conveyed through the fixing apparatus, the eddy current diverting member8is moved into the abovementioned position8′ in which the eddy current diverting member8has no electrical contact with the fixation belt1, in order to prevent the fixation belt1from reducing in temperature. The eddy current diverting member8is disposed within the loop of the fixation belt1. Thus, the first position (contact position) for the eddy current diverting member8, in which the eddy current diverting member8is enabled to divert the eddy current induced in the fixation belt1, and the second position (noncontact position) for the eddy current diverting member8, in which the eddy current diverting member8has no electrical connection with the fixation belt1, are both in the loop of the fixation belt1.

The control for moving the eddy current diverting member8into the first position from the second position, or moving the second position to the first position is the same as that in the first embodiment. That is, the fixing apparatus is structured so that when a recording paper of the large size is conveyed through the fixing apparatus, the eddy current diverting member8is retracted into the noncontact position, whereas, when a recording paper of the small size is conveyed through the fixing apparatus, the eddy current diverting member8is moved into the contact position, in which the eddy current diverting member8remains electrically in contact with the fixation belt1.

Referring toFIG. 13, in this embodiment, the eddy current diverting member8of the fixing apparatus is the same in concept, but, it is given the form of a brush. The eddy current diverting member8in the form of a brush rubs the metallic layer1aof the fixation belt1, in terms of relative movement. The actual brush portion8aof the eddy current diverting member8is formed of a metal with a high level of electrical conductivity, and the base portion8bof the eddy current diverting member8is also formed of a metal with a high level of electrical conductivity. The metal as the material for the actual brush portion8amay be different from that for the base portion8b. Giving the eddy current diverting member8the form of a brush can increase in size the total contact area between the eddy current diverting member8and fixation belt1as an inductive heating member, improving thereby the fixing apparatus in the state of electrical contact between the eddy current diverting member8and fixation belt1.

Referring toFIG. 14, in this embodiment, the eddy current diverting member8is made up of an electrically conductive portion81aand an electrically nonconductive portion81b.

The eddy current diverting member8is made up of the electrically conductive portion81aand electrically nonconductive portion81b. The electrically conductive portion81ais formed of an electrically highly conductive metal, whereas the electrically nonconductive portion81bis formed of a resin or ceramic, which is highly dielectric. In the case of a fixing apparatus structured so that when a recording medium is conveyed through the fixing apparatus, the centerline of the recording medium is kept aligned with the centerline of the fixing apparatus, when a recording medium of the small size is conveyed through the fixing apparatus, an area C, which remains outside the track of the recording medium, results on both sides of the track of the recording medium of the small size, in the recording medium passage, in terms of the widthwise direction of the recording medium. In such a case, a pair of the electrically conductive portions81aare disposed so that they correspond in position to the portions of the fixation belt1, which are next to the lateral edges of the fixation belt1, one for one. Further, no electrical connection is provided between the two electrically conductive portions81alocated near the lateral edges of the fixation belt1, one for one, being therefore physically separated from each other. The reason for not providing electrical connection between the two electrically conductive portions81ais that if the two electrically conductive portions81aare electrically in connection to each other, the connection affects the two electrically conductive portions81in current diverting function, preventing them from providing the fixation belt1with a preset temperature distribution, because they are employed to utilize their ability to divert the eddy current. The electrically nonconductive portion81bplays the role of a substrate. As an example of the structure of the eddy current diverting member8, there is such a structure that the electrically nonconductive portion81bis given the form of a roller, and a pair of the electrically conductive portions81a, which are in the form of a hollow cylinder, are fitted around the portions of the electrically conductive portion81b,which are near the lengthwise ends of the electrically conductive portion81b. The two electrically conductive portions81afitted around the lengthwise end portions of the electrically nonconductive portion81b, as described above, have no electrical connection to each other. Rendering the electrically nonconductive portion81bthe same in diameter as the electrically conductive portions81aprevents the fixation belt1from becoming stepped as it comes into contact with the eddy current diverting member8; it prevents the eddy current diverting member8from affecting image quality.

The control for moving the eddy current diverting member8into the first position from the second position, or moving the second position to the first position is the same as that in the first embodiment.

In this embodiment, the eddy current diverting member8is in the form of a roller. However, the eddy current diverting member8may be given the form of a flat board or brush, with the same effects as those described above.

In this embodiment, the fixing apparatus is provided with two or more sets of eddy current diverting members8, which are different in dimension in terms of the direction perpendicular to the direction in which an object to be heated is conveyed.

FIG. 15(a) is a schematic sectional view of the essential portions of the fixing apparatus in this embodiment.FIG. 15(b) is a sectional plan view of the fixing apparatus at Line (b)-(b) inFIG. 15(a). The fixing apparatus in this embodiment is of the belt type as is the fixing apparatus in the second embodiment.

As high frequency electric current is flowed through the excitation coil5of the excitation coil unit3from a coil driving electric power source15, such eddy current that flows in a loop is induced in the portion of the fixation belt1, which roughly opposes the excitation coil5. Therefore, the eddy current diverting member8is effective to divert the eddy current, no matter where on the portion of the fixation belt1in which the eddy current that flows in a loop is induced, the eddy current diverting member8is placed in contact. As the fixing apparatus is seen from the recording medium conveyance direction, the fixation belt1has two sets of areas with which the eddy current diverting member8is to be placed in contact in order to effectively divert the eddy current (FIG. 11). Therefore, providing the fixing apparatus with an additional eddy current diverting member8A, which is different in lengthwise dimension from the eddy current diverting member8, per eddy current diverting member8, as shown inFIG. 15, makes it possible to change the fixation belt1in heat distribution according to the size of a recording paper conveyed through the fixing apparatus. For example, in the case of a fixing apparatus enabled to handle three types of recording medium different in size, that is, a paper P1, that is, the largest paper usable with the fixing apparatus, a paper P2, that is, the recording medium of the medium size (second smallest paper) usable with the fixing apparatus, and a paper P3, that is, the recording medium of the small size (smallest paper) usable with the fixing apparatus, two different out-of-track areas C and D, which are different in dimension in terms of the lengthwise direction of the fixing apparatus, are created. Therefore, providing the fixing apparatus with two sets of eddy current diverting members, that is, the eddy current diverting members8and eddy current diverting members8A, which are different in the dimension in terms of the lengthwise direction, makes it possible to prevent the out-of-track areas from excessively increasing in temperature, whether the out-of-track areas are the out-of-track area C or D.

The control for moving the eddy current diverting member8into the first position from the second position, or moving the second position to the first position is the same as that in the first embodiment.

This embodiment is characterized in that the fixing apparatus is provided with an inductive heating member backing member1(fixing belt backing member), which backs the fixing belt1as the eddy current diverting member8is placed in contact with the inductive heating member1, because the inductive heating member1(fixation belt) is pressed by the eddy current diverting member8as the eddy current diverting member8is placed in contact with the inductive heating member1.

The above described current diverting function of the eddy current diverting member8is an electrical function. Therefore, the contact resistance between the metallic layer1aof the fixation belt1and the eddy current diverting member8is desired to be as small as possible. In this embodiment, therefore, in order to minimize this contact resistance, the fixing apparatus is provided with a belt backing member34, so that the eddy current diverting member8can be pressed against the belt backing member34, with the fixation belt1sandwiched between the eddy current diverting member8and belt backing member34, as shown inFIG. 16, to ensure that the eddy current diverting member8remains perfectly in contact with the fixation belt1.

In order to prevent the belt backing member34from affecting the magnetic flux, the belt backing member34is desired to be formed of a dielectric substance. Further, the belt backing member34comes into contact with the surface of the fixation belt1, which comes into contact with the image bearing surface of a recording medium. Therefore, it is desired to be formed of such a resin, sponge, rubber, or the like, that is not likely to scar the fixation belt1.

Further, the belt backing member34may be integrated with the excitation coil unit3, because integrating the belt backing member34with the excitation coil unit3ensures that the eddy current diverting member8remains in contact with the fixation belt1, and also, eliminates the problematic possibility that the relationship, in terms of the magnetic flux strength, between the excitation coil unit3and fixation belt1will become unstable due to the changes in the distance between the excitation coil unit3and fixation belt1.

1) In the embodiments described above, the recording medium P is conveyed through the fixing apparatus so that the centerline of the recording medium P coincides with the centerline of the fixing apparatus. However, the present invention is also applicable to a fixing apparatus structured so that when the recording medium P is conveyed through the apparatus, one of the lateral edges of the recording medium P coincides with the referential line of the apparatus. The effects obtained by such an application are the same as those described above.

2) The application of the present invention is not limited to an image heating apparatus, such as the image heating apparatuses in the above described preferred embodiments of the present invention, that employs a heating method based on electromagnetic induction. For example, the present invention is also very effectively applicable to such an image heating apparatus as a fixing apparatus for temporarily fixing an unfixed image to a recording medium, a surface property improving apparatus for improving an image in surface properties such as glossiness by reheating a recording medium bearing a fixed image, and the like apparatuses.

This application claims priority from Japanese Patent Application No. 133237/2005 filed Apr. 28, 2005 which is hereby incorporated by reference.