Fixing apparatus and image forming apparatus

A fixing apparatus includes a magnetic-field generating part that generates a magnetic field, a fixing rotating body having a heat generating layer that generates heat by an electromagnetic induction action of the magnetic field, a pressurizing member that applies pressure to an outer circumferential surface of the fixing rotating body, a heating member that is arranged in contact with an inner side of the fixing rotating body so as be opposed to the magnetic-field generating part, and heats the fixing rotating body, and a temperature sensing part that is located within a region where the fixing rotating body is opposed to the magnetic-field generating part and where the fixing rotating body is in contact with the heating member, and senses a temperature of the fixing rotating body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-068201 filed Mar. 17, 2008.

BACKGROUND OF THE PRESENT INVENTION

1. Technical Field

The present invention relates to a fixing apparatus and an image forming apparatus.

2. Related Art

Among fixing apparatuses each provided in an image forming apparatus such as a printer and a copier, there is one by an electromagnetic induction heat-generating method using, as heat sources, a coil that generates a magnetic field by energization and a heat generating body that generates heat by generating eddy current by electromagnetic induction of the magnetic field.

SUMMARY

A fixing apparatus of a first aspect of the present invention includes a magnetic-field generating part that generates a magnetic field, a fixing rotating body having a heat generating layer that generates heat by an electromagnetic induction action of the magnetic field, a pressurizing member that applies pressure to an outer circumferential surface of the fixing rotating body, a heating member that is arranged in contact with an inner side of the fixing rotating body so as to be opposed to the magnetic-field generating part, and heats the fixing rotating body, and a temperature sensing part that is located within a region where the fixing rotating body is opposed to the magnetic-field generating part and where the fixing rotating body is in contact with the heating member and senses a temperature of the fixing rotating body.

“Within the region where the fixing rotating body is in contact with the heating member” means a region smaller than a maximum range in the circumferential direction where the fixing rotating body and the heating member are in contact, and a notched portion may be included in the region.

DESCRIPTION

An example of a first exemplary embodiment of a fixing apparatus and an image forming apparatus of the present invention is described based on the drawings.

FIG. 1shows a printer10as the image forming apparatus. In the printer10, optical scanning devices14Y,14M,14C and14K that emit optical beams corresponding to respective toners of yellow (Y), magenta (M), cyan (C), and black (K) are fixed inside of a housing12making up a body of the printer10. In a position adjacent to the optical scanning device14K, a controller70that controls operations of the respective parts of the printer10is provided.

The optical scanning devices14Y,14M,14C and14K scan optical beams emitted from light sources by rotating polygon mirrors (not shown), and the optical beams are reflected by a plurality of optical parts such as reflecting mirrors, so that optical beams16Y,16M,16C and16K corresponding to the respective toners are emitted.

The optical beams16Y,16M,16C and16K are guided to respective corresponding photoreceptors18Y,18M,18C and18K. The photoreceptors18Y,18M,18C and18K are each rotated in an arrow A direction by drive means made of a motor and a gear (not shown).

Chargers20Y,20M,20C and20K that charge surfaces of the photoreceptors18Y,18M,18C and18K are provided on the upstream side in a rotation direction of the photoreceptors18Y,18M,18C and18K. Also, developing units22Y,22M,22C and22K that develop the respective toners of Y, M, C and K on the photoreceptors18Y,18M,18C and18K are provided on the downstream side in the rotation direction of the photoreceptors18Y,18M,18C and18K.

An intermediate transfer belt28to which developed toner images are primarily transferred is arranged on the downstream side of the developing units22Y,22M,22C and22K in the rotation direction of the photoreceptors18Y,18M,18C and18K. The intermediate transfer belt28is made of a film-like endless belt in which an antistatic agent such as carbon black is contained in a proper amount in a resin such as polyimide and polyamide.

Inside of the intermediate transfer belt28and in positions where the photoreceptors18Y,18M,18C and18K and the intermediate transfer belt28are opposed, the primary transfer rolls24Y,24M,24C and24K that transfer the respective color toner images formed on the photoreceptors18Y,18M,18C and18K to the intermediate transfer belt28are arranged. These primary transfer rolls24Y,24M24C and24K make up a primary transfer part25that performs the primary transfer from the photoreceptors18Y,18M,118C and18K to the intermediate transfer belt18.

The primary transfer rolls24Y,24M,24C and24K each have a shaft, and a sponge layer as an elastic layer fixed around the shaft, which are not shown. The shaft is a columnar rod made of a metal such as iron and SUS. The sponge layer is a cylindrical roll formed of a blend rubber of NBR, SBR and EPDM into which a conducting agent such as carbon black is compounded.

Moreover, the primary transfer rolls24Y,24M,24C and24K are brought into pressure contact with the respective photoreceptors18Y,18M,18C and18K with the intermediate transfer belt28sandwiched therebetween. A voltage (primary transfer bias) having a polarity reverse to charging polarity of the respective toners (negative polarity in exemplary embodiment, the same applies to the following) is applied to the primary transfer rolls24Y,24M,24C and24K by voltage applying means (not shown).

The toner images on the respective photoreceptors18Y,18M,18C and18K are sequentially attracted electrostatically, so that the toner images superimposed on the intermediate transfer belt28are formed. Cleaners26Y,26M,26C and26K that remove remaining toners on the photoreceptors18Y,18M,18C and18K are provided on the downstream side in the rotation direction of the photoreceptors18M,18M,18C and18K.

Inside of the intermediate transfer belt28, a drive roll30that is driven by a motor (not shown) excellent in speed constancy to move the intermediate transfer belt28, and a supporting roll32that extends substantially linearly along an arrangement direction of the respective photoreceptors18Y,18M,18C and18K to support the intermediate transfer belt28are provided. This allows the intermediate transfer belt28to be driven circularly at a predetermined speed in an arrow B direction.

Moreover, inside of the intermediate transfer belt28, a tension roll34that provides a constant tension to the intermediate transfer belt28and prevents meandering of the intermediate transfer belt28. A secondary transfer part42that transfers the toner images on the intermediate transfer belt28onto recording paper P is provided on the downstream side in the moving direction of the intermediate transfer belt28.

The secondary transfer part42is made of a secondary transfer roll38arranged on the side of a toner image carrying surface of the intermediate transfer belt28and a back-up roll36.

The secondary transfer roll38is made of a shaft, and a sponge layer as an elastic layer fixed around the shaft, which are not shown. The shaft is a columnar rod made of a metal such as iron and SUS. The sponge layer is a cylindrical roll formed of a blend rubber of NBR, SBR and EPDM into which a conducting agent such as carbon black is compounded.

Moreover, the secondary transfer roll38is brought into pressure contact with the back-up roll36with the intermediate transfer belt28sandwiched therebetween. The secondary transfer roll38is earthed, and a secondary transfer bias is applied between the secondary transfer roll38and the back-up roll36, so that the toner images are secondarily transferred on the recording paper P conveyed to the secondary transfer part42.

In the back-up roll36, a surface thereof is made of a tube of a blend rubber of EPDM and NBR with carbon dispersed, and an interior portion thereof is made of EPDM rubber. Hardness is set to 70° (ASKER C), for example. Moreover, the back-up roll36is arranged on the side of a back surface of the intermediate transfer belt28, to for an opposite electrode of the secondary transfer roll38, and the secondary transfer bias is stably applied through a metal electric supply roll40arranged in contact with the back-up roll36.

On the downstream side of the secondary transfer part42in the moving direction of the intermediate transfer belt28, an intermediate transfer belt cleaner46that removes remaining toners or paper powders on the intermediate transfer belt28after the secondary transfer is provided in such a manner that it can be brought into contact with, and be separated from the intermediate transfer belt28. A cleaning back-up roll44is provided inside of the intermediate transfer belt28in the intermediate transfer belt cleaner46.

On the upstream side of the primary transfer roll24Y corresponding to the yellow toner and inside of the intermediate transfer belt28, a home position sensor48that generates a signal as a reference for timing of image formation corresponding to the respective toners is provided. The home position sensor48senses a predetermined mark provided on the back side of the intermediate transfer belt28to generate the reference signal. Based on this reference signal, the above-described controller70operates the respective parts of the printer10to start the image formation. Moreover, on the downstream side of the primary transfer roll24K corresponding to the black toner, an image density sensor43for performing image adjustment is provided.

On the other hand, on the lower side of the printer10, a paper tray50that contains the recording paper P is provided. On one end of the paper tray50, a pick-up roll52that takes out and conveys the recording paper P at predetermined timing is provided. Above the pick-up roll52, a plurality of conveyance rolls54,56that are driven rotationally by driving means made of a motor and a gear not shown to convey the recording paper P sent out by the pick-up roll52to the above-described secondary transfer part42are provided. On the downstream side of the conveyance rolls56in a conveyance direction of the recording paper P, a conveyance chute58that sends the recording paper P to the secondary transfer part42is provided.

A conveyance belt60that conveys the recording paper P to a fixing apparatus100after completing the secondary transfer of the toner images is provided in a sending-out direction of the recording paper P in the secondary transfer part42. The conveyance belt60is provided so as to be tensioned by tensioning rolls57and59and be movable by drive means made of a motor or a gear not shown.

A guide62that guides the recording paper P to the fixing apparatus100is provided on the inlet side of the fixing apparatus100. On the outlet side of the fixing apparatus100, a paper output tray64fixed to the housing12of the printer10is provided.

Now, the image formation of the printer10is described.

Image data outputted from an image reading apparatus, a personal computer or the like not shown is subjected to predetermined image processing by an image processing apparatus not shown. In the image processing apparatus, the predetermined image processing including shading correction, displacement correction, brightness/color space conversion, gamma correction, frame deletion, and various types of image edition such as color edition and movement edition is performed for inputted reflectivity data. The image data subjected to the image processing is converted to colorant gradation data of four colors of Y, M, C and K, and is outputted to the optical scanning devices14Y,14M,14C and14K.

The optical scanning devices14Y,14M,14C and14K irradiate the optical beams16Y,16M,16C and16K to the respective photoreceptors18Y,18M,18C and18K in accordance with the inputted colorant gradation data. The surfaces of the photoreceptors18Y,18M,18C and18K are charged in advance by the chargers20Y,20M,20C and20K, and are exposed by the optical beams16Y,16M,16C and16K so as to form electrostatic latent images. The formed electrostatic latent images are developed as toner images of the respective colors of Y, M, C and K by the developing units22Y,22M,22C and22K.

Subsequently, the toner images formed on the photoreceptors18Y,18M,18C and18K are transferred onto the intermediate transfer belt28in the primary transfer part25. This transfer is performed by adding the voltage (primary transfer bias) having the polarity reverse to the charging polarity of the toners (negative polarity) to the intermediate transfer belt28by the primary transfer rolls24Y,24M,24C and24K and sequentially superimposing the toner images on the surface of the intermediate transfer belt28. The intermediate transfer belt28to which the toner images are transferred is conveyed to the secondary transfer part42.

On the other hand, at the timing at which the toner images are conveyed to the secondary transfer part42, the pick-up roll52is rotated to send out the recording paper P of a predetermined size from the paper tray50. The recording paper P sent out by the pick-up roll52is conveyed by the conveyance rolls54,56, and reaches the secondary transfer part42via the conveyance chute58. Before reaching this secondary transfer part42, the recording paper P is once stopped to perform the alignment between the recording paper P and the toner images by rotating a resist roll (not shown) according to the movement timing of the intermediate transfer belt28carrying the toner images.

In the secondary transfer part42, the secondary transfer roll38is pressed by the back-up roll36with the intermediate transfer belt28sandwiched therebetween. At this time, the recording paper P, which has been timed to be conveyed, is sandwiched between the intermediate transfer belt28and the secondary transfer roll38. At this time, a voltage (secondary transfer bias) having the same polarity as the charging polarity (negative polarity) of the toners is applied from the electric supply roll40, so that a transfer electric field is formed between the secondary transfer roll38and the back-up roll36. The unfixed toner images carried on the intermediate transfer belt28is pressed by the secondary transfer roll38and the back-up roll36to be electrostatically transferred on the recording paper P collectively.

Subsequently, the recording paper P with the toner images transferred electrostatically is conveyed by the secondary transfer roll38in a state where the toner images are stripped off from the intermediate transfer belt28, and is conveyed to the conveyance belt60. In the conveyance belt60, the recording paper P is conveyed to the fixing apparatus100so that the conveyance speed conforms to an optimal conveyance speed in the fixing apparatus100. The unfixed toner images on the recording paper P conveyed to the fixing apparatus100are fixed on the recording paper P by the fixing apparatus100. The recording paper P after the fixing is discharged in an arrow C direction to be accumulated in the paper output tray64.

After the transfer to the recording paper P is completed, the remaining toners on the intermediate transfer belt28are conveyed to the intermediate transfer belt cleaner46with the rotation movement of the intermediate transfer belt28to be removed from the intermediated transfer belt28. In this manner, the image formation of the printer10is performed.

Next, a description of the fixing apparatus100is given.

As shown inFIG. 2, the fixing apparatus100includes a housing106in which openings for taking in and discharging the recording paper P are formed. Inside of the housing106, an endless fixing belt102, cap-like supporting members (illustration is omitted) are fitted in both side end portions thereof, is supported rotatably in an arrow D direction.

A bobbin108made of an insulating material is arranged in a position opposed to an outer circumferential surface of the fixing belt102. The bobbin108is formed into a substantially circular arc following the outer circumferential surface of the fixing belt102, and is provided with a projected portion108A toward the opposite side of the fixing belt102. A distance between the bobbin108and the fixing belt102is set to 1 to 3 mm.

In the bobbin108, an exciting coil110that generates a magnetic field H by energization is wound in a plurality of times in an axial direction of the bobbin108centering on the projected portion108A. In positions faced to the exciting coil110and at the opposite side of the fixing belt102, magnetic-path forming members112each made of a magnetic body such as ferrite and formed into a substantially circular arc following the circular arc of the bobbin108are arranged and supported by the bobbin108.

As shown inFIG. 4A, the plurality of magnetic-path forming members112are arranged along a width direction of the fixing belt102, and held by a holding member113made of a nonmagnetic body bridged in the width direction of the fixing belt102. The magnetic-path forming members112are arranged at even intervals in a central portion in a longitudinal direction of the holding member113while in both end portions in the longitudinal direction of holding member113, are arranged at narrower intervals or in contact with each other. This arrangement of the magnetic-path forming members112allows a distribution of the magnetic field H in the width direction of the fixing belt102to be adjusted.

As shown inFIG. 3A, the fixing belt102is made of a base layer130, a heat generating layer132, a protecting layer134, an elastic layer136, and a releasing layer138, in order from the inside to the outside, and these are layered to be integrated.

The base layer130is a base having a strength of the fixing belt102, for which polyimide is used with a thickness set to 50 to 200 μm. For the base layer130, besides a resin such as polyimide, a metal such as iron, nickel, silicon, boron, niobium, copper, zirconium, and cobalt, and a soft magnetic metal material made of an alloy composed of these metals may be used.

The heat generating layer132is made of a metal material that generates heat by electromagnetic induction in which eddy current flows so as to generate a magnetic field which cancels out the above-described magnetic field H. Moreover, the heat generating layer132needs to be formed so as to be thinner than a so-called skin depth in order to pass through a magnetic flux of the magnetic field H. For the heat generating layer132, for example, a metal of gold, silver, copper, aluminum, zinc, tin, lead, bismuth, beryllium, antimony or an alloy of these may be used. In the present exemplary embodiment, as the heat generating layer132copper having a thickness of 10 μm is used.

The skin depth δ (m) is represented by the following expression using a frequency f(Hz), a relative magnetic permeability μr and a specific resistance ρ (Ωm) of an exciting circuit.
δ=503(ρ/(f×μr))1/2

The skin depth δ (m) represented by the above-described expression indicates a depth of absorption of an electromagnetic wave used in the electromagnetic induction, and in a point deeper than this, the intensity of the electromagnetic wave is 1/e or less. In other words, it means that most energy is absorbed until a layer reaches this depth.

For the protecting layer134, a material that has mechanical strength higher than the heat generating layer132, has high repeated distortion resistance, and has high rust and corrosion resistance is preferable, and in exemplary embodiment, nonmagnetic stainless steel having a thickness of 30 μm is used.

For the elastic layer136, a silicone-based rubber or a fluorine-based rubber is used in view of excellent elasticity, heat resistance and the like. In the present exemplary embodiment, a silicone rubber having a thickness of 200 μm is used. The thickness of the elastic layer136is preferably 200 to 600 μm.

The releasing layer138is provided to easily stripe off the recording paper P from the fixing belt102by weakening adhesion force to a toner T (seeFIG. 2) fused on the recording paper P. In order to attain excellent surface releasability, a fluoride resin, a silicone resin, or a polyimide resin is used for the releasing layer138, and in the present exemplary embodiment, PFA (tetrafluorethylene/perfluoroalkoxyethylene copolymer resin) is used. The thickness of the releasing layer138is 30 μm.

As shown inFIG. 2, inside of the fixing belt102, a prismatic support114made of aluminum, which is a nonmagnetic material, is arranged in a noncontact state with respect to the fixing belt102with a longitudinal direction of the support114corresponding to the width direction of the fixing belt102. Both ends of the support114are fixed to the housing106of the fixing apparatus100. The support114is formed with a depressed portion114A along the longitudinal direction on the bottom side. A pressing pad116made of resin for pressing the fixing belt102outwards at a predetermined pressure is fixed to the depressed portion114A. The pressing pad116is made of a member having elasticity, and one end surface thereof is in contact with an inner circumferential surface of the fixing belt102to press the fixing belt outwards.

Moreover, inside of the fixing belt102and above the support114, a heating member is provided so as to be opposed to the exciting coil110, and in the present exemplary embodiment, a heat generating body118formed into a circular arc as shown inFIGS. 2 and 3Bis used.

As shown inFIGS. 2 and 3B, the heating generating body118is a substantially semicylindrical member whose longitudinal direction corresponds to the width direction of the fixing belt102, and is arranged so that a surface thereof is in contact with the inner surface of the fixing belt102. Moreover, the heat generating body118is made of an iron-based alloy, and forms a closed magnetic path by the above-described magnetic field H between the magnetic-path forming members112and the heat generating body118, and generates heat by the electromagnetic induction of the magnetic field H. The contact of the heat generating body118with the fixing belt102keeps temperature decrease of the fixing belt102smaller even if the heat of the fixing belt102is consumed by the recording paper P passing.

For the heat generating body118, a magnetic metal material having a thickness more than the skin depth noted above is preferably used. The thickness more than the skin depth brings about sufficient heat generation by the action of the magnetic field, and the heat is accumulated inside of the heat generating body118, which suppresses temperature decrease of the fixing belt102more. The magnetic metal material is desirably a ferromagnetic body having a relative magnetic permeability of 100 or more, more desirably a ferromagnetic body of a relative magnetic permeability of 500 or more, for example.

In the heat generating body118, a notched portion120as an opening portion is formed at a central portion in the longitudinal direction and at an end portion in a circumferential direction, and at the upstream side thereof with respect to the rotation direction of the fixing belt102. The notched portion120is formed in a position opposed to one of the magnetic-path forming members112which is provided at the central portion in the longitudinal direction of the holding member113(seeFIG. 4A). Since the notched portion120is formed only by cutting the end portion of the heat generating body118, the attachment of the temperature sensor124(described later) is easier as compared with a case where a through-hole is formed in the heat generating body118.

Moreover, although it is expected that the formation of the opening portion in the heat generating body118will decrease a quantity of heat value of the heat generating body118, the heating member such as the heat generating body118has larger heat release in a position closer to the end portion in a circumferential direction, and thus, the provision of the opening portion (notched portion) in the end portion keeps decrease of the quantity of heat value smaller in comparison with a case where the opening portion is provided at an intermediate part in a circumferential direction of the heat generating body118. Furthermore, when the heat generating body118generates heat by the electromagnetic induction of the magnetic field H as in the present exemplary embodiment, the quantity of heat value becomes larger in a position closer to a central portion in a winding width of the exciting coil110, and thus, the provision of the notched portion120in the end portion keeps decrease in the quantity of heat value smaller as compared with a case where a through-hole is provided at a position in the heating generating body118closer to the central portion in the winding width of the exciting coil110.

A supporting member122is provided at a predetermined position in the longitudinal direction of the heat generating body118and at each end portion in the circumferential direction at the inner circumferential side of the heat generating body118. At one-end of the supporting member122, a substantially L-shaped supporting portion is formed and is attached at the respective inner circumferential side end of the heat generating body118. The other end of the supporting member122is jointed to each side (right and left sides inFIG. 2) of the support114by screws123and128, by which the support114supports the heat generating body118.

In the notched portion120of the heat generating body118, the temperature sensor124that makes contacts with the inner circumferential surface of the fixing belt102to sense a temperature of the fixing belt102surface is disposed. The temperature sensor124measures the temperature of the fixing belt102surface by changing resistance value in accordance with an amount of heat given from the fixing belt102surface.

Moreover, the temperature sensor124is fixed in a terminal portion of a plate spring126made of a resin such as polyimide, and a base end portion of the plate spring126is jointed to the side of the support114(right side inFIG. 2) by the screw128. This allows the plate spring126to extend from the downstream side to the upstream side in the rotation direction of the fixing belt102along the inner circumferential surface of the fixing belt102, and the temperature sensor124is located in the notched portion120along the inner circumferential surface of the fixing belt102.

As shown inFIG. 4B, the temperature sensor124is connected to a control circuit142provided inside of the above-described controller70(seeFIG. 1) through wiring140. Moreover, the control circuit142is connected to an energization circuit146through wiring144, and the energization circuit146is connected to the above-described exciting coil110through wiring148and150.

The control circuit142measures a temperature on the inner circumferential side of the fixing belt102based on an amount of electricity sent from the temperature sensor124, and converts it to a temperature on the outer circumferential side of the fixing belt102, and then, compares this converted temperature with a fixing setting temperature stored in advance (170° C. in the present exemplary embodiment). When the converted temperature is lower than the fixing setting temperature, the energization circuit146is driven to energize the exciting coil110, and generate the magnetic field H as a magnetic circuit (seeFIG. 2). When the converted temperature is higher than the fixing setting temperature, the energization circuit146is stopped.

The energization circuit146is driven or the driving thereof is stopped based on the electric signal sent from the control circuit142, and an alternating current of a predetermined frequency is supplied to the exciting coil110through the wiring148,150, or the supply thereof is stopped.

As shown inFIG. 2, a pressurizing roll104that pressurizes the fixing belt102toward the pressing pad116, and is rotated in an arrow E direction by a driving mechanism made of a motor and a gear not shown is arranged in a position opposed to the outer circumferential surface of the fixing belt102.

The pressurizing roll104is constituted so that a silicone rubber and PFA are covered around a cored bar105made of a metal such as aluminum. The pressurizing roll104pressurizes the fixing belt102to the pressing pad116side, so that the fixing belt102is in a state depressed inwards at a contact portion (nip portion) formed between the fixing belt102and the pressurizing roll104.

A shape of this nip portion is curved in a direction where the recording paper P with the toner T placed thereon is stripped off from the fixing belt102when it passes this nip portion. Thereby, the recording paper P conveyed from an arrow IN direction is discharged in an arrow OUT direction while following the shape of the nip portion because of stiffness of the paper itself.

Next, operations of the first exemplary embodiment of the present invention are described. First, a fixing operation of the fixing apparatus100is described.

As shown inFIGS. 1 to 4B, the recording paper P (or envelope) to which the toner T is transferred via the image forming process of the above-described printer10is sent to the fixing apparatus100. In the fixing apparatus100, a drive motor not shown is driven by the controller70, so that the pressurizing roll104is rotated in an arrow E direction, and following this, the fixing belt102is rotated in the arrow D direction. At this time, the energization circuit146is driven based on an electric signal from the control circuit142, and an alternating current is supplied to the exciting coil110.

When the alternating current is supplied to the exciting coil110, the magnetic field H as a magnetic circuit is repeatedly generated and extinguished in the vicinity of the exciting coil110. When the magnetic field H crosses the heat generating layer132of the fixing belt102, an eddy current is generated in the heat generating layer132so as to generate a magnetic field preventing change of the magnetic field H. The heat generating layer132generates heat in proportion to the magnitudes of the skin resistance of the heat generating layer132and the eddy current flowing the heat generating layer132, resulting in the heating of the fixing belt102.

Similarly, the heat generating body118generates heat by the electromagnetic induction action of the magnetic field H to heat the fixing belt102. Thus, since the heat generating layer132and the heat generating body118are heated by the same exciting coil110, power consumption is lower as compared with a case where the heat generating layer132and the heat generating body118are heated by different heat sources.

The temperature of the fixing belt102surface is sensed by the temperature sensor124and when it does not reach the fixing setting temperature, the control circuit142controls the driving of the energization circuit146to apply an alternating current of a predetermined frequency to the exciting coil110. Moreover, when the temperature of the fixing belt102surface reaches the fixing setting temperature, the control circuit142stops the control of the energization circuit146.

The temperature sensor124is fixed to the terminal portion of the plate spring126, and the base portion of the plate spring126is jointed to the support114. Thereby, the plate spring126is extended out from the downstream side to the upstream side in the rotation direction of the fixing belt102along the inner circumferential surface of the fixing belt102. The temperature sensor124is arranged in the end portion of the heat generating body118at the upstream side in the rotation direction of the fixing belt102along the inner circumferential surface of the fixing belt102. Therefore, even if the temperature sensor124is pulled by the rotation of the fixing belt102, external force acts to the temperature sensor124in a direction where the temperature sensor124is contained in the notched portion120. This allows the temperature sensing position by the temperature sensor124to fall into the heating region opposed to the exciting coil110.

Subsequently, the recording paper P sent by the fixing apparatus100is heated and pressed by the fixing belt102and the pressurizing roll104at the predetermined fixing setting temperature so that the toner images are fixed on the recording paper P surface, and the fixed recording paper P is discharged into the paper output tray64.

Next, a temperature in the circumferential direction of the fixing belt102is described.

FIG. 5Ais a schematic diagram showing positions A to E that are sensing positions of the temperature of the fixing belt102in the circumferential direction and a sensing position of the temperature of the heat generating body118. The temperatures of the respective portions excluding B are sensed using temperature sensors not shown.

The position A is a sensing position at the inner circumferential surface of the fixing belt102. The position A is located at the upstream side of a region opposed to the exciting coil110in the circumferential direction of the fixing belt102. The position B is a sensing position where the temperature sensor124(seeFIG. 2) senses and located in a region opposed to the exciting coil110and the heat generating body118. The position B is located at the inner circumferential surface of the fixing belt102.

The position C is a sensing position corresponding to the projected portion108A of the bobbin108(seeFIG. 2), and located in a region not opposed to the exciting coil110at the inner circumferential surface of the fixing belt102. The position D is a sensing position symmetric to the position A centering on the position C, and located in the region not opposed to the exciting coil110at the inner circumferential surface of the fixing belt102.

The position E is a sensing position adjacent to the position B at an inner circumferential surface (opposite side of the fixing belt102) of the heat generating body118. The position E is set for comparing the temperatures at the inner circumferential surface of the heat generating body118and at the inner circumferential surface of the fixing belt102.

For example, a relation between the positions in the circumferential direction of the fixing belt102and the sensed temperature when the fixing belt102is heated in a state where the rotation is stopped is shown inFIG. 5B.

InFIG. 5B, in the position A, the sensed temperature of the fixing belt102is T1. In the position B, since the heat generating layer132(seeFIG. 3A) of the fixing belt102and the heat generating body118generate heat by the electromagnetic induction action of the magnetic field H created by the exciting coil110, the sensed temperature of the fixing belt102is T2, which is higher than T1.

In the position C, while the heat generating body118releases heat by heat conduction from a region heated by the magnetic field H to heat the fixing belt102, the exciting coil110does not exit thus, a heating amount is smaller and the sensed temperature of the fixing belt102is T3lower than T2. At the point D, since it is out of the region heated by the magnetic field H, the sensed temperature of the fixing belt102is the temperature T1equivalent to that of the position A.

At the point E, since heat capacity of the fixing belt102is smaller than heat capacity of the heat generating body118, temperature rising of the fixing belt102is faster. Therefore, the sensed temperature of the heat generating body118is T4lower than the sensed temperature T2at the point B of the fixing belt102.

Normally, since during fixing, the fixing belt102is rotated, the temperature of the heat generating body118is higher. However, when heating is performed in the state where the rotation of the fixing belt102is stopped as described above, a temperature rising rate of the fixing belt102, which has smaller heat capacity, is faster than that of the heat generating body118, and as a result, the temperature on the fixing belt102side becomes higher.

Since the magnetic-path forming members112collect the magnetic fields of the magnetic field H to form the closed magnetic path, the temperature on the inner circumferential surface of the fixing belt102in the heated region opposed to the exciting coil110becomes the highest, and the temperature in this region is sensed by the temperature sensor124(seeFIG. 2) to control the temperature by the control circuit142(seeFIG. 4B). This will suppress excessive temperature rising of the fixing belt102.

As another example of the heating generating body118of the fixing apparatus100, for example, heat generating bodies152and156shown inFIGS. 6A and 6Bmay be used. In the heat generating body152, a through-hole154is formed closer to the central portion in the circumferential direction of the heat generating body152and at the upstream side in the rotation direction of the fixing belt102than the notched portion120of the above-described heat generating body118. Moreover, in the heat generating body156, a notched portion158is formed in an end portion at the opposite side of the notched portion120(at the downstream side in the rotation direction of the fixing belt102). When the heating generating body156is used, the temperature sensor124is caused to be adhered, or a frame or the like for fixing is provided.

Next, a second exemplary embodiment of the fixing apparatus and the image forming apparatus of the invention is described based on the drawings. Basically the same parts as those in the above-described first exemplary embodiment are given the same numerals and signs as those of the first exemplary embodiment, and their descriptions are omitted.

A fixing apparatus160is shown inFIG. 7. The fixing apparatus160has a constitution using a heat generating body162in place of the heat generating body118of the fixing apparatus100of the first exemplary embodiment.

As shown inFIGS. 7,8A and8B, the heat generating body162is a substantially semicylindrical member whose longitudinal direction corresponds to the width direction of the fixing belt102, and is arranged so that a surface thereof is in contact with the inner surface of the fixing belt102. Moreover, the heat generating body162is made of an iron-based alloy and forms a closed magnetic path by the above-described magnetic field H between the magnetic-path forming members112and the heat generating body162, and at the same time, generates heat by the electromagnetic induction action of the magnetic field H1.

In the heat generating body162, a plurality of notched portions164,166and168are formed in an end portion in a circumferential direction and at the upstream side in the rotation direction of the fixing belt102. The notched portion166is located substantially in the center in the longitudinal direction of the heat generating body162, and is within a region of a width W2when recording paper of a small size passes on the fixing belt102. Moreover, the notched portions164and168are located in both end portions of the heat generating body162in the longitudinal direction, which are outside of the region of the width W2, and inside of a region of a width W1+W2+W3when recording paper of a large size passes on the fixing belt102.

In the respective notched portions164,166and168, the above-described temperature sensors124(124A,124B and124C) are disposed, respectively, and the respective temperature sensors124are fixed to the support114through the plate springs126(126A,126B and126C). Moreover, the respective sensors124A,124B and124C are arranged so as to be opposed to the magnetic-path forming members112.

Next, operations of the second exemplary embodiment of the invention are described.

As shown inFIGS. 7 and 8, in the fixing apparatus160, when fixing of the toners is continuously performed to the recording paper P of the small size, in the region of the width W2of the fixing belt102, an amount of heat is deprived by the recording paper P, so that the temperature of the fixing belt102becomes lower than the fixing setting temperature.

At this time, since the temperature sensed by the temperature sensor124B becomes the lowest within the temperature sensors124A to124C, the control circuit142(seeFIG. 4B) controls the energization circuit146(seeFIG. 4B) so as to bring the temperature of the fixing belt102closer to the fixing setting temperature, based on a difference between the sensed temperature by the temperature sensor124B and the fixing setting temperature, and the heat generating body162generates heat. Although this raises a temperature of the entire fixing belt102, in the regions of the widths W1and W3(non-paper-conveyance region) in the fixing belt102, the heat is not deprived by the recording paper P, and thus, an amount of heat is accumulated, which makes the temperature higher than that in the region of the width W2. As a result, as shown inFIG. 9, a graph exhibits a high temperature on the both end portion sides.

Since the temperature sensors124A and124C are located outside of the passage region (W2) of the recording paper P of the small size, and inside of the passage region (W1W3) of the recording paper P of the large size, a temperature of a highest temperature portion of the fixing belt102is sensed.

On the other hand, when the recording paper P of the large size is fixed, even if there is partially a position at a high temperature, any of the temperature sensors124A to124C senses the temperature of the high-temperature portion because the plurality of temperature sensors124A,124B and124C are arranged in the longitudinal direction of the heat generating body162.

As another example of the heat generating body162, for example, a heat generating body170shown inFIG. 10may be used. In the heat generating body170, through-holes172,173,174,175and176are formed in positions closer to the central portion in the circumferential direction than the notched portions164,166and168of the heat generating body162, and on the upstream side in the rotation direction of the fixing belt102. In the through-holes172to176, the temperature sensors124(124A to124E) are provided, respectively. In this manner, by arranging the plurality of temperature sensors124in the longitudinal direction of the fixing belt102, a temperature in a highest temperature portion of the fixing belt102in the longitudinal direction is sensed.

The present invention is not limited to the above-described exemplary embodiments.

The printer10may be not only of a dry electrophotographic system using a solid developer, but of a type using a liquid developer. Moreover, the heat generating bodies118and162may be sheet heat generating bodies that generate heat by supplying electricity. Furthermore, as sensing means of the temperature of the fixing belt102, a thermocouple may be used in place of the temperature sensor124.

A shape of the notched portion120may be not only rectangular but circular arc or multangular. Moreover, in place of the notched portion120, a depressed portion as an opening portion may be formed in an outer circumferential surface of each of the heat generating bodies118and162, where the temperature sensor124may be arranged. When the magnetic field H is the most intensive in a central portion in a width direction of the exciting coil110in a bundle, the notched portion120may be advantageously formed in a position opposed to the central portion of the exciting coil110to arrange the temperature sensor124.