JP5435829B2 - Fixing apparatus and image forming apparatus equipped with the same - Google Patents

Description

  The present invention relates to a fixing device that heats and melts unfixed toner on a sheet while passing the nip between a heated roller pair or a heating belt and a roller on a sheet carrying a toner image, and an image forming apparatus equipped with the fixing device It is about.

  In recent years, in this type of image forming apparatus, a belt system capable of setting a small heat capacity has attracted attention due to demands for shortening the warm-up time and energy saving in the fixing device (for example, see Patent Document 1). In recent years, the electromagnetic induction heating method (IH) that has the potential for rapid heating and high-efficiency heating has attracted attention. From the viewpoint of energy saving when fixing color images, electromagnetic induction heating is combined with the belt method. Many products have been commercialized. When combining the belt method and electromagnetic induction heating, many cases are adopted in which electromagnetic induction devices are arranged outside the belt because of the coil layout and ease of cooling, and the advantage that the belt can be directly heated (so-called). Envelope IH).

In the electromagnetic induction heating method described above, various technologies have been developed to prevent overheating in the non-sheet passing area according to the width of the sheet size (sheet passing width) that passes through the fixing device. In particular, there are the following prior arts as size switching means in the outer package IH (see, for example, Patent Documents 2 and 3).
In the first prior art (Patent Document 2), a magnetic member is divided into a plurality of pieces and arranged in the sheet passing width direction, and a part of the magnetic member is placed on the exciting coil in accordance with the sheet size (sheet passing width) to be passed. To be separated. In this case, it is considered that the heat generation efficiency is lowered by separating the magnetic member from the exciting coil in the non-sheet passing area, and the heat generation amount is smaller than the area corresponding to the sheet having the minimum sheet passing width.

  In the second prior art (Patent Document 3), another conductive member is disposed outside the minimum sheet passing width inside the heat generating roller, and the position of the conductive member is switched within or outside the magnetic field range. Is. In this prior art, first, the conductive member is positioned outside the magnetic field range and the heat generating roller is heated by electromagnetic induction. When the heat generating roller rises to near the Curie temperature due to the temperature rise, the conductive member is moved within the magnetic field range. By moving to, magnetic flux is leaked from the heat generating roller outside the minimum sheet passing width to prevent overheating.

JP-A-6-31801 JP 2003-107941 A Japanese Patent No. 3527442

  By the way, in the above-described prior art size switching means, in order to further improve the productivity, it is necessary to suppress the excessive temperature rise more than the current state. For example, in the second prior art (Patent Document 3), it is considered that the area of the conductive member that shields magnetism should be made larger than the current state in order to increase the effect of suppressing the excessive temperature rise beyond the current state.

  However, if the area of the conductive member is made too large, it becomes difficult to completely evacuate it from the magnetic field range. May affect the magnetic field. Therefore, although it is intended to increase the effect of suppressing excessive temperature rise, there is a limit to increasing the area of the conductive member.

In solving this problem, it is not preferable to separately arrange a plurality of conductive members that shield magnetism. This is because if a space is formed between the adjacent conductive members, the magnetic shielding effect cannot be exhibited in this space, and a flow path through which a magnetic field leaks can be obtained, and the fixing device becomes large.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a fixing device that can solve the above-described problems and further improve the magnetic shielding effect, and an image forming apparatus equipped with the fixing device.

  According to a first aspect of the invention for achieving the above object, a sheet on which a toner image is transferred in an image forming unit is sandwiched and conveyed between a heating member and a pressure member, and at least from the heating member in this conveyance process. A fixing device for fixing a toner image on a sheet by heat, which is disposed along an outer surface of the heating member and generates a magnetic field for induction heating of the heating member, and at least the opposite side of the heating member with the coil interposed therebetween And a magnetic core that guides the magnetic field generated by the coil toward the heating member by forming a magnetic path around the coil, and heats the magnetic path that is guided by the magnetic core toward the heating member. A path switching means for switching between a first path where induction heating of the member is promoted and a second path where induction heating of the heating member is suppressed; and both the first path and the second path. Including magnetism When it is arranged over the switching area of the path and when it is switched to the first path by the path switching means, it allows passage of the magnetic flux from the magnetic core to the heating member within the switching area, while it is switched to the second path Includes a magnetic adjustment member that shields the magnetic flux without passing through it, and the magnetic adjustment member is formed in a continuous endless shape as viewed in the axial direction of the wire material, and is formed in the longitudinal direction of the magnetic core. On the other hand, it is provided with a plurality of ring-shaped portions that are divided and arranged in accordance with a plurality of paper sizes to be conveyed.

  According to the first aspect of the present invention, the fixing device of the present invention suppresses the excessive heating of the heating member by basically switching the magnetic field path to the second path by the path switching means. Such a path switching means has a structural advantage in that it does not take up much space, but it is not possible to completely stop the inflow of magnetic flux just by switching the path. The suppression effect of temperature rise is not perfect. Therefore, it is not sufficient to switch the magnetic field path from the first path to the second path, and higher productivity cannot be realized as it is.

  Therefore, in the present invention, in addition to the path switching means that saves space but has a weak magnetic shielding effect, a magnetic adjustment member that is fixedly arranged is used. That is, when the magnetic adjustment member is switched to the first path, the magnetic adjustment member allows passage of magnetic flux over the entire switching area and maximizes the heating effect of the heating member, while being switched to the second path. Then, the passage of the magnetic flux is shut out over the entire switching area, and the overheating of the heating member is prevented.

  Thereby, at the time of switching between the first path and the second path, the heat generation contrast of the heating member can be increased. In addition, the magnetic adjustment member has a function (function like a magnetic filter) that allows a magnetic flux to pass through or is shielded on the contrary only by being in a fixed position without mechanically performing any operation. Even if it has the area of, when the temperature rise is necessary, the magnetic field is not affected. In addition, since the magnetic adjustment member only needs to be fixedly arranged, it is not necessary to provide a new movable member, and the space for the fixing device can be saved as much.

  In addition, the magnetic adjustment member of the present invention includes a plurality of ring-shaped portions made of a wire material, and is configured to be capable of dealing with a plurality of paper sizes. The entire material viewed in the axial direction is formed into a continuous endless shape. Therefore, it is possible to set a range in which one magnetic adjustment member can suppress heat generation stepwise in the longitudinal direction of the magnetic core. Further, as described above, since the plurality of ring-shaped portions can be eliminated by providing a space where the magnetic shielding effect is not exhibited and divided and arranged, the magnetic shielding effect is further improved. In addition, it contributes to space saving of the fixing device.

  According to a second aspect of the invention, in the configuration of the first aspect of the invention, the ring-shaped portion is formed of a highly conductive wire material in a ring shape, and the plurality of ring-shaped portions intersect with the traveling direction of the magnetic flux. By being connected to each other in a state adjacent to each other in the direction, the entire wire material as viewed in the axial direction is formed into a continuous endless shape, and when switched to the first path by the path switching means, a plurality of The induced current generated by the magnetic flux penetrating each inside the ring-shaped part has a structure in which the adjacent ring-shaped parts are opposite to each other, and the path switching means switches from the first path to the second path. In the state, the amount of magnetic flux penetrating a part of the plurality of ring-shaped portions is reduced.

  According to the second invention, in addition to the operation of the first invention, for example, when the magnetic adjustment member has at least two ring-shaped portions, these two ring-shaped portions are connected in a so-called “eight-shape”. It becomes the structure made. That is, in this case, when the magnetic flux penetrates two adjacent ring-shaped parts in the same direction, the direction of the induced current generated in one ring-shaped part is opposite to the direction of the induced current generated in the other ring-shaped part. Thus, the induced current is canceled as a whole inside the magnetic adjustment member. In this case, since the magnetic adjustment member hardly exerts influence on the magnetic field, the magnetic flux can be allowed to pass through without any problem.

  On the other hand, when switching to the second path, the amount of magnetic flux penetrating one of the ring-shaped portions is reduced (almost becomes 0), so that an induced current is generated in the other ring-shaped portion. This generates a magnetic flux (demagnetizing field) in the opposite direction to the penetrating magnetic flux. In this case, since the magnetic flux that attempts to pass through the second path is shielded, as a result, the magnetic adjustment member can exhibit the magnetic flux shielding effect as a whole.

A third invention is characterized in that, in the configurations of the first and second inventions, the surface portion of each ring-shaped portion or the surface portions adjacent to each other is subjected to insulation treatment.
According to the third invention, in addition to the effects of the first and second inventions, as described above, the ring-shaped portions without spaces are arranged close to each other. Since the surface portions of the portions or the surface portions close to each other are insulated, the ring-shaped portions can be reliably insulated from each other.

According to a fourth aspect of the present invention, in the configuration of the first to third aspects of the invention, the heating member is induction-heated by a coil over the maximum sheet passing area when viewed in the width direction of the conveyed paper. Is characterized in that it is arranged outside the sheet passing area of a sheet having a width smaller than the maximum width sheet corresponding to the maximum sheet passing area when viewed in the longitudinal direction of the magnetic core.
According to the fourth aspect of the invention, in addition to the effects of the first to third aspects, with such an arrangement, the end of the heating member that becomes a non-sheet passing area is excessively raised according to the size of the paper. Good protection from temperature.

  According to a fifth invention, in the configurations of the second to fourth inventions, the magnetic cores are arranged in pairs so as to form a magnetic path on both sides of the coil winding center. And a second core that is disposed between the paired first cores and forms a magnetic path that passes through the winding center of the coil and reaches the heating member, and the path switching means includes the first core When switched to the path, the magnetic flux is passed along the coil winding center from the second core to the heating member, whereas when switched to the second path, the positions are on both sides away from the coil winding center. The magnetic adjustment member is configured to pass the magnetic flux from the first core to the heating member, and the magnetic adjustment member arranges one ring-like portion on the first path passing through the winding center of the coil in the switching region, and on both sides thereof. Position two adjacent rings on the second path Characterized in that it is disposed, respectively Re.

  According to the fifth aspect of the invention, in addition to the effects of the second to fourth aspects of the invention, in the case of the above aspect, first, the magnetic adjustment member has one ring-shaped portion at the center, and both sides thereof. It has a structure corresponding to each paper size by having two adjacent ring-shaped portions. At this time, the center ring-shaped part is arranged on the extension line of the coil winding center, and the other ring-shaped parts are arranged on both sides thereof.

  And in the state switched to the first path by the path switching means, the first induced current generated in one central ring-shaped portion by the magnetic flux passing through the first path and the first path deviate. The magnetic fluxes that attempt to pass through the second paths located on both sides thereof pass through the other two adjacent ring-shaped portions, so that the second induced currents that are generated cancel each other. As a result, the magnetic adjustment member allows not only the first path but also the passage of the magnetic flux that deviates from both sides to pass the second path, and as a result, the magnetic flux passes through the entire switching area. Can be tolerated.

  On the other hand, when the path switching means switches to the second path, the magnetic flux hardly passes through one ring-shaped part in the center, and the magnetic flux passes through the other two adjacent ring-shaped parts, and the induced current Is generated. At this time, each magnetic flux generated in the other two ring-shaped portions cancels out the magnetic flux that is going to pass through the second path, and as a result, the magnetic adjustment member shields the magnetic flux in the entire switching region. Can do. In addition, since the magnetic flux shielding effect can be exerted by the two ring-shaped portions disposed on both sides of the ring-shaped portion disposed at the both sides only by suppressing the passage of the magnetic flux to one ring-shaped portion disposed at the center, the structure is simple. Thus, a shielding effect can be obtained efficiently.

According to a sixth aspect of the invention, there is provided an image forming apparatus in which the first to fifth fixing devices are mounted and the toner image formed by the image forming unit is fixed on the sheet by using the fixing devices.
According to the sixth invention, in addition to the effects of the first to fifth inventions, the magnetic shielding effect is further improved, so that a good toner image is formed. As a result, the image forming apparatus Reliability is improved.

  According to the present invention, the magnetic adjustment member capable of accommodating a plurality of paper sizes is formed in a continuous endless shape as viewed in the axial direction of the wire material, thereby further improving the magnetic shielding effect. In addition, it is possible to provide a fixing device that achieves space saving of the fixing device and an image forming apparatus equipped with the fixing device.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment. FIG. 2 is a longitudinal sectional view illustrating a first embodiment of a fixing unit. It is the disassembled perspective view which showed the arrangement | positioning relationship of a center core, a shielding member, an induction heating coil, and a magnetic adjustment member. It is a perspective view which shows the structural example (1) of a magnetic adjustment member. It is a model figure for demonstrating the function of a magnetic adjustment member. It is a top view explaining structural examples (1) and (2). It is a figure explaining arrangement | positioning of the structural example (2). It is a block diagram of a fixing unit. It is a figure which shows the operation example using structural example (1) and (2). It is a longitudinal cross-sectional view which shows 2nd Example of a fixing unit. FIG. 6 is a longitudinal sectional view showing a third embodiment of the fixing unit. FIG. 6 is a longitudinal sectional view showing a fourth embodiment of the fixing unit. FIG. 10 is a longitudinal sectional view showing a fifth embodiment of the fixing unit. It is a longitudinal cross-sectional view which shows 6th Example of a fixing unit. FIG. 10 is a longitudinal sectional view showing a seventh embodiment of the fixing unit. It is a longitudinal cross-sectional view which shows the 8th Example of a fixing unit. FIG. 10 is a longitudinal sectional view showing a ninth embodiment of the fixing unit. It is a figure which shows the state which attached the shielding member corresponding to the structural example (2) to the center core. It is a perspective view which shows the operation example at the time of shielding whole surface with the shielding member of FIG. FIG. 20 is a perspective view showing an operation example when the shielding member is rotated 60 ° clockwise from the state of FIG. 19. FIG. 20 is a perspective view illustrating an operation example when the shielding member is rotated 120 ° in the clockwise direction from the state of FIG. 19. FIG. 20 is a perspective view showing an operation example when the shielding member is rotated 180 degrees clockwise from the state of FIG. 19. FIG. 20 is a perspective view showing an operation example when the shielding member is rotated by 240 ° in the clockwise direction from the state of FIG. 19. FIG. 20 is a perspective view illustrating an operation example when the shielding member is rotated by 300 ° in the clockwise direction from the state of FIG. 19.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 includes a printer, a copier, a facsimile machine, and a multifunction machine having both functions of transferring a toner image onto the surface of a printing medium such as printing paper based on image information input from the outside. Or the like.

An image forming apparatus 1 shown in FIG. 1 is, for example, a tandem color printer. The image forming apparatus 1 includes a square box-shaped apparatus main body 2 that forms (prints) a color image on a sheet therein, and discharges the sheet on which the color image is printed on the upper surface of the apparatus main body 2. The discharge tray 3 is provided.
In the lower part of the apparatus main body 2, a paper feed cassette 5 for storing paper is disposed. In addition, a stack tray 6 for supplying manually fed sheets is disposed in the center of the apparatus main body 2. An image forming unit 7 is provided on the upper part of the apparatus main body 2. The image forming unit 7 forms an image on a sheet based on image data such as characters and designs transmitted from the outside of the apparatus.

  As shown in FIG. 1, a first transport path 9 for transporting paper fed from the paper feed cassette 5 to the image forming unit 7 is disposed on the left side of the apparatus main body 2. A second transport path 10 for transporting the sheet fed from the stack tray 6 to the image forming unit 7 is provided. A fixing unit (fixing device) 14 that performs a fixing process on a sheet on which an image is formed by the image forming unit 7 and a sheet on which the fixing process has been performed are disposed in the upper left part of the apparatus main body 2. And a third transport path 11 for transporting to the surface.

  The paper feed cassette 5 can be replenished by pulling it out of the apparatus main body 2 (for example, the front side in FIG. 1). The paper feed cassette 5 includes a storage unit 16 in which at least two types of paper having different sizes in the paper feed direction can be selectively stored. Note that the paper stored in the storage unit 16 is fed out to the first transport path 9 side by sheet by the paper feed roller 17 and the separating roller 18.

The stack tray 6 can be opened and closed on the outer surface of the apparatus main body 2, and manual sheets are loaded one by one or a plurality of sheets are stacked on the manual feed portion 19. Note that the sheets placed on the manual feed unit 19 are fed out one by one by the pickup roller 20 and the separating roller 21 to the second conveyance path 10 side.
The first transport path 9 and the second transport path 10 merge before the registration roller 22, and the paper supplied to the registration roller 22 waits here for a while, and after adjusting skew and timing, It is sent out toward the next transfer unit 23. The full color toner image on the intermediate transfer belt 40 is secondarily transferred to the sheet by the secondary transfer unit 23 on the sent sheet. Thereafter, the sheet on which the toner image is fixed by the fixing unit 14 is reversed in the fourth conveyance path 12 as necessary, and a full-color toner image is also formed on the surface opposite to the first by the secondary transfer unit 23. Secondary transferred. After the toner image on the opposite surface is fixed by the fixing unit 14, the toner image is discharged to the discharge tray 3 by the discharge roller 24 through the third conveyance path 11.

The image forming unit 7 includes four image forming units 26 to 29 that form black (B), yellow (Y), cyan (C), and magenta (M) toner images. The intermediate transfer unit 30 is configured to synthesize and carry the toner images of the respective colors formed in 29.
Each of the image forming units 26 to 29 includes a photosensitive drum 32, a charging unit 33 disposed so as to face the peripheral surface of the photosensitive drum 32, and a periphery of the photosensitive drum 32 on the downstream side of the charging unit 33. A laser scanning unit 34 for irradiating a laser beam to a specific position on the surface, and a developing unit disposed on the downstream side of the laser beam irradiation position from the laser scanning unit 34 and facing the peripheral surface of the photosensitive drum 32 35 and a cleaning unit 36 disposed on the downstream side of the developing unit 35 and facing the peripheral surface of the photosensitive drum 32.

The photosensitive drums 32 of the image forming units 26 to 29 are rotated counterclockwise in the drawing by a drive motor (not shown). Further, in the developing units 35 of the image forming units 26 to 29, black toner, yellow toner, cyan toner, and magenta toner are stored in the toner boxes 51, respectively.
The intermediate transfer unit 30 straddles the rear roller 38 disposed near the image forming unit 26, the front roller 39 disposed near the image forming unit 29, and the rear roller 38 and the front roller 39. The four intermediate transfer belts 40 and four transfer units arranged so as to be capable of being pressed against each other via the intermediate transfer belt 40 at positions downstream of the developing unit 35 on the photosensitive drums 32 of the image forming units 26 to 29. And a roller 41.

In the intermediate transfer unit 30, the toner images of the respective colors are superimposed and transferred onto the intermediate transfer belt 40 at the positions of the transfer rollers 41 of the image forming units 26 to 29. Become.
The first transport path 9 and the second transport path 10 are for transporting paper fed from the paper feed cassette 5 and the stack tray 6 to the intermediate transfer unit 30 side, and are located at predetermined positions in the apparatus main body 2. Are provided in front of the intermediate transfer unit 30 and a registration roller 22 for timing the image forming operation and the paper feeding operation in the image forming unit 7. .

  The fixing unit 14 performs processing for fixing the unfixed toner image on the paper by heating and pressurizing the paper on which the toner image has been transferred by the image forming unit 7. The fixing unit 14 includes a pair of rollers including, for example, a heating pressure roller (pressure member) 44 and a fixing roller 45, and the pressure roller 44 is made of, for example, metal, and the fixing roller 45 is made of a metal core. It has a surface layer (for example, silicon sponge) of a material and an elastic body and a release layer (for example, PFA). A heat roller 46 is provided adjacent to the fixing roller 45, and a heating belt (heating member) 48 is wound around the heat roller 46 and the fixing roller 45. The detailed structure of the fixing unit 14 will be described later.

  When viewed in the sheet conveyance direction, conveyance paths 47 are provided on the upstream side and the downstream side of the fixing unit 14, and the sheet conveyed through the intermediate transfer unit 30 is pressurized through the upstream conveyance path 47. It is introduced into the nip between the roller 44 and the fixing roller 45. The paper that has passed between the pressure roller 44 and the fixing roller 45 is guided to the third conveyance path 11 through the conveyance path 47 on the downstream side.

The third transport path 11 transports the paper on which the fixing process has been performed by the fixing unit 14 to the discharge tray 3. For this reason, a transport roller 49 is disposed at an appropriate position in the third transport path 11, and the discharge roller 24 is disposed at the outlet thereof.
[First embodiment]
FIG. 2 is a longitudinal sectional view showing a structural example of the fixing unit 14. In FIG. 2, the orientation is turned counterclockwise by about 90 ° from the state in which the image forming apparatus 1 is mounted. Accordingly, the sheet conveying direction from the bottom to the top as viewed in FIG. 1 is from right to left as viewed in FIG. In addition, when the apparatus main body 2 is larger (multifunction machine etc.), it may be mounted in the direction shown in FIG.

The fixing unit 14 of the present embodiment includes a pressure roller 44 having a diameter of 50 mm, for example, a fixing roller 45 having a diameter of 45 mm, a heat roller 46 having a diameter of 30 mm, and a heating having a thickness of 35 μm (1 μm = 1 × 10 ⁻⁶ m). A belt 48 is provided. In addition, the said belt 48 is adjusted to the range of 150-200 degreeC, for example.
As described above, since the fixing roller 45 has a silicon sponge elastic layer as a surface layer, a flat nip is formed between the heating belt 48 and the fixing roller 45.

  The heating belt 48 has a base material made of a ferromagnetic material (for example, Ni), a thin elastic layer (for example, silicon rubber) formed on the surface layer, and a release layer (for example, PFA) on the outer surface thereof. ) Is formed. In the case where the heating belt 48 does not have a heat generation function, a resin belt such as PI may be used. Further, the core of the heat roller 46 is a magnetic metal (for example, Fe, SUS), and a release layer (for example, PFA) is formed on the surface thereof.

More specifically, the pressure roller 44 uses, for example, Fe, Al or the like as a metal core material, forms a Si rubber layer on the core material, and further forms a fluororesin layer on the surface layer. It is what. In addition, the structure provided with the halogen heater 44a, for example inside the pressurizing roller 44 may be sufficient.
In addition, the fixing unit 14 includes an IH coil unit 50 outside the heat roller 46 and the heating belt 48 (not shown in FIG. 1). The IH coil unit 50 includes an induction heating coil 52, a pair of arch cores (magnetic core, first core) 54, a pair of side cores (magnetic core, first core) 56, and a center core (magnetic core, first core). 2 cores) 58.

〔coil〕
In the example of FIG. 2, the induction heating coil (coil) 52 is disposed on a virtual arc surface along the arc-shaped outer surface in order to perform induction heating in the arc-shaped portions of the heat roller 46 and the heating belt 48. . Actually, a bobbin (not shown) made of a heat-resistant resin such as PPS, PET, or LCP is disposed outside the heat roller 46 and the heating belt 48, and an induction heating coil 52 is wound around the bobbin. It is the structure arrange | positioned at linear form. The bobbin is formed in a semi-cylindrical shape along the outer surface of the heat roller 46, and the coil 52 is fixed to the bobbin using, for example, a silicon adhesive.

[Magnetic core, first core]
As shown in FIG. 2, the center core 58 is located in the center, and the arch core 54 and the side core 56 are arranged so as to make a pair on both sides thereof. Of these, the arch cores 54 on both sides are ferrite cores formed in a cross-sectional arch shape that is symmetrical to each other, and the overall length thereof is longer than the winding area of the induction heating coil 52. The side cores 56 on both sides are ferrite cores formed in a block shape. The side cores 56 on both sides are connected to one end (lower end in FIG. 2) of each arch core 54, and these side cores 56 cover the outside of the winding area of the induction heating coil 52.

  Of these, the arch cores 54 are disposed at a plurality of positions, for example, at intervals in the longitudinal direction of the heat roller 46. Further, the side cores 56 are continuously arranged in the longitudinal direction of the heat roller 46 without being spaced from each other, and the total length thereof corresponds to the length of the winding area of the induction heating coil 52. The arrangement of the cores 54 and 56 is determined in accordance with, for example, the magnetic flux density (magnetic field strength) distribution of the induction heating coil 52. The side core 56 compensates for the magnetic field focusing effect and leveles the magnetic flux density distribution (temperature difference) in the longitudinal direction.

For example, a resin core holder (not shown) is provided outside the arch core 54 and the side core 56, and the arch core 54 and the side core 56 are supported by the core holder. The material of the core holder is also preferably a heat resistant resin (for example, PPS, PET, LCP).
In the example of FIG. 2, the thermistor 62 (or the thermostat 75 of FIG. 8) is installed inside the heat roller 46. The thermistor 62 can be disposed inside a portion of the heat roller 46 that generates a large amount of heat, particularly due to induction heating. As shown in FIG. 8, the temperature of the heat roller 46 is output to the main body engine board 82. The substrate 82 is electrically connected to the inverter substrate 80, and the temperature of the heat roller 46 can be adjusted to be constant by a thermostat 75. The substrate 80 supplies power to the induction heating coil 52. On the other hand, the coil 52 is appropriately cooled by cooling air from the coil cooling fan 76, and a drive signal for the fan 76 is output from the engine board 82.

  The substrate 82 has a function of driving the rotation mechanism 64 of the center core 58 by outputting a drive signal to the stepping motor 66, for example. Specifically, the rotation angle of the center core 58 can be controlled by the number of drive pulses applied to the motor 66, and a control unit 83 is attached to the main body engine board 82. The control unit 83 can be configured by, for example, a control IC, an input / output driver, a semiconductor memory, and the like.

[Magnetic core, second core]
Returning to FIG. 2, the center core 58 is a ferrite core having a cylindrical cross section, for example. The center core 58 has a length corresponding to the maximum sheet passing width of 13 inches (for example, about 340 mm), similar to the heat roller 46. When the paper is used, an alternating current of 20 kHz or more (alternating frequency is 30 kHz, for example) is used to avoid an audible range. Further, although not shown in FIG. 2, the center core 58 is connected to a rotation mechanism (not shown), and can be rotated around the longitudinal axis by the rotation mechanism.

(Shielding member)
Further, a shielding member (path switching means) 60 is attached to the center core 58 along its outer surface. The shielding member 60 has a thin plate shape and is formed to be curved in an arc shape as a whole. For example, the shielding member 60 may be installed in a state where it is embedded in the thick portion of the center core 58 as shown in the figure, or may be installed in a state of being attached to the outer surface of the center core 58. The shielding member 60 can be attached using, for example, a silicon-based adhesive.

  The configuration of the shielding member 60 is preferably a non-magnetic and highly conductive member, such as oxygen-free copper. The shielding member 60 is shielded by generating a reverse magnetic field by an induced current caused by a perpendicular magnetic field penetrating through the ring and canceling the interlaced magnetic flux (perpendicular penetrating magnetic field). Further, by using a highly conductive member, Joule heat generation due to an induced current can be suppressed and a magnetic field can be efficiently shielded. In order to improve the conductivity, for example, methods such as (1) selecting a material with as low a specific resistance as possible and (2) increasing the thickness of the member are effective. Specifically, the plate thickness of the shielding member 60 is preferably 0.5 mm or more, and in this embodiment, for example, a thickness of 1 mm is used.

  As shown in FIG. 2, when the shielding member 60 is in a position (shielding position) close to the surface of the heating belt 48, the magnetic resistance increases around the induction heating coil 52 and the magnetic field strength decreases. On the other hand, when the center core 58 rotates 180 ° (the direction is not particularly limited) from the state shown in FIG. 2 and the shielding member 60 moves to a position (retracted position) farthest from the heating belt 48, the periphery of the induction heating coil 52 As a result, the magnetic resistance is reduced, a magnetic path is formed through the arch core 54 and the side core 56 on both sides centering on the center core 58, and a magnetic field acts on the heating belt 48 and the heat roller 46.

[Magnetic adjustment member]
In addition, the IH coil unit 50 extends between the induction heating coil 52 and the heating belt 48 (heat roller 46) on both sides of the center core 58 and the heating belt 48 (heat roller 46). A magnetic adjustment member 90 is fixedly disposed in the region. An appropriate clearance is secured between the center core 58 (the shielding member 60) and the magnetic adjustment member 90 so as not to inhibit the rotation of the center core 58.

  FIG. 3 is an exploded perspective view showing the positional relationship of the center core 58, the shielding member 60, the coil 52, and the magnetic adjustment member 90. As shown in FIG. As described above, the center core 58 has a total length longer than the maximum sheet passing width together with the heat roller 46, and accordingly, the winding area of the induction heating coil 52 is also viewed in the longitudinal direction of the center core 58. Has been expanded to cover the area.

  On the other hand, the shielding member 60 is disposed at both ends of the center core 58 as viewed in the longitudinal direction, and the magnetic adjustment member 90 is disposed at both ends of the center core 58 (or the heat roller 46) as viewed in the longitudinal direction. (Only one end is shown in FIG. 3). Note that the shielding member 60 and the magnetic adjustment member 90 are both disposed outside the minimum sheet passing width of the sheet size used in the image forming apparatus 1, for example.

[Structural example of magnetic adjustment member]
FIG. 4 is a perspective view showing a structural example (1) of the magnetic adjustment member 90. The magnetic adjustment member 90 of the present embodiment mainly has three ring-shaped portions 90A, 90B, and 90C, and these ring-shaped portions 90A, 90B, and 90C all have a square ring shape. Further, the three ring-shaped portions 90A, 90B, and 90C are not independent rings, but have an endless structure in which the entire magnetic adjustment member 90 is connected to each other by being connected to each other. doing. Hereinafter, the structure of the magnetic adjustment member 90 will be described.

  The magnetic adjustment member 90 has three short side portions 90a, 90e, and 90t at one end position when viewed in the longitudinal direction thereof, and also has three short side portions 90g, 90k, and 90q at the other end position. . The magnetic adjustment member 90 has two long side portions 90d and 90h extending in the longitudinal direction at the position of one side when viewed in the width direction (direction orthogonal to the longitudinal direction), and at the position of the other side. Also has two long side portions 90p and 90u extending in the longitudinal direction.

  Further, the magnetic adjustment member 90 has three long side portions 90b, 90f, 90j extending in the longitudinal direction between the central ring-shaped portion 90A and the adjacent ring-shaped portion 90B at a position closer to the center in the width direction. These long side portions 90b and 90f are arranged in the same straight line and face the long side portion 90j in the vertical direction. Furthermore, it also has three long side portions 90m, 90r, 90w extending in the longitudinal direction between the central ring-shaped portion 90A and another ring-shaped portion 90C adjacent thereto, and these long side portions 90m, 90w are arranged in the same straight line, and are opposed to the long side portion 90r vertically.

  Moreover, the magnetic adjustment member 90 has a short side portion 90s that connects the long side portion 90f and the long side portion 90r in the width direction in the range of the ring-shaped portion 90A, and the long side in the range of the ring-shaped portion 90B. A short side portion 90c that connects the portion 90b and the long side portion 90d in the width direction, and a short side portion 90i that connects the long side portion 90j and the long side portion 90h in the width direction, respectively. In addition, a short side 90v connecting the long side portion 90u and the long side portion 90w in the width direction, or a short side connecting the long side portion 90m and the long side portion 90p in the width direction in the range of the ring-shaped portion 90C. Each has a portion 90n.

[Center ring-shaped part]
More specifically, the center ring-shaped portion 90A includes three short side portions 90a, 90k, and 90s that are paired in the longitudinal direction, and the short side portions 90a, 90k, and 90s are ring-shaped portions. It is not directly connected within the range of 90A. That is, one end of each of the long side portions 90b and 90w is connected to both ends of the short side portion 90a of the ring-shaped portion 90A. Of these, the short side 90c of the adjacent ring-shaped portion 90B is connected to the other end of the long side portion 90b, and the short side of another adjacent ring-shaped portion 90C is connected to the other end of the long side portion 90w. The unit 90v is connected.

Similarly, one end of each of the long side portions 90j and 90m is connected to both ends of the short side portion 90k of the ring-like portion 90A, and the short side of the ring-like portion 90B is connected to the other end of the long side portion 90j. The short side portion 90n of the ring-shaped portion 90C is connected to the other end of the long side portion 90m of the portion 90i.
Further, both ends of the short side portion 90s of the ring-shaped portion 90A are connected to the middle of the long side portions 90f and 90r, respectively, and the short side portion 90e of the ring-shaped portion 90B is connected to one end of the long side portion 90f. The other side of the long side portion 90f is connected to the short side portion 90g of the ring-shaped portion 90B.

On the other hand, a short side 90t of the ring-shaped portion 90C is connected to one end of the long side 90r, and a short side 90q of the ring-shaped portion 90C is connected to the other end of the long side 90r.
Therefore, the short side portion 90a and the short side portion 90s paired in the ring-shaped portion 90A are not directly connected by the long side portions 90b and 90w that are also paired within the range, and the short side portion 90s. Also, the short side portion 90k is not directly connected by the long side portions 90j and 90m that are also paired within the range.

[Ring-shaped parts on both sides]
Next, of the two ring-shaped portions 90B and 90C adjacent to the central ring-shaped portion 90A in the width direction, one of the ring-shaped portions 90B is paired with two short side portions 90c and 90e in the longitudinal direction. Are connected via a long side portion 90d closer to the outer side, of which the short side portion 90e is connected to the short side portion 90s of the central ring-shaped portion 90A via the long side portion 90f as described above, and the short side portion 90c is described above. In this way, the long side portion 90b is connected to the short side portion 90a.

  Further, in the ring-shaped portion 90B, the two short side portions 90g, 90i that are paired in the longitudinal direction are connected via a long side portion 90h closer to the outer side, and the short side portion 90g is the long side described above. The short side portion 90s is connected to the short side portion 90s via the portion 90f, and the short side portion 90i is connected to the short side portion 90k of the central ring-shaped portion 90A via the long side portion 90j.

Similarly, for the other ring-shaped portion 90C, two short side portions 90t and 90v that are paired in the longitudinal direction are connected via a long side portion 90u closer to the outside, and among these, the short side portion 90t is the above-mentioned long side portion. The short side portion 90s is connected to the short side portion 90s via the side portion 90r, and the short side portion 90v is connected to the short side portion 90a via the long side portion 90w.
Further, in the ring-shaped portion 90C, the two short side portions 90n and 90q that are paired in the longitudinal direction are connected via the long side portion 90p on the outer side, and the short side portion 90n is the long side described above. The short side portion 90k is connected to the short side portion 90k via the portion 90m, and the short side portion 90q is connected to the short side portion 90s via the long side portion 90r.

  Then, the short side portion 90a of the ring-shaped portion 90A, the short side portion 90e of the ring-shaped portion 90B, and the short side portion 90t of the ring-shaped portion 90C are arranged at the position of the maximum sheet passing width W3 of the paper described later in FIG. The short side portion 90k of the ring-shaped portion 90A, the short side portion 90g of the ring-shaped portion 90B, and the short side portion 90q of the ring-shaped portion 90C are arranged inside the minimum sheet passing width W1.

  Further, the short side portion 90s of the ring-shaped portion 90A, the short side portions 90c and 90i of the ring-shaped portion 90B, and the short side portions 90n and 90v of the ring-shaped portion 90C are outside the intermediate sheet passing width W2, that is, intermediate Each of the magnetic adjustment members 90 is disposed between the paper width W2 and the maximum paper passing width W3. A portion of the magnetic adjustment member 90 divided by the short side portions 90a, 90e, and 90t to the short side portions 90s, 90c, and 90v is the middle. The magnetic adjustment unit M1 is responsible for the non-sheet passing area for the size paper, and the portion divided from the short side portions 90s, 90i, 90n to the short side portions 90k, 90g, 90q is the minimum size paper. It becomes the magnetic adjustment part M2 which bears a part of non-sheet passing area. This is because the non-sheet passing area for the minimum size sheet is borne by both the magnetic adjustment units M1 and M2.

[Overall structure]
From the above connection relationship, the entire magnetic adjustment member 90 has, for example, the short side portion 90a of the center ring-shaped portion 90A as a base point, from one end to the long side portion 90b, the short side portion 90c, the long side portion 90d, and the short side portion. 90e, long side 90f, short side 90g, long side 90h, short side 90i, long side 90j, short side 90k, long side 90m, short side 90n, long side 90p, short side 90q, the long side portion 90r, the short side portion 90t, the long side portion 90u, the short side portion 90v, the long side portion 90w, and the short side portion 90s are connected in sequence, thereby forming an endless shape as a whole. It has a structure.

  The short sides 90a, 90c, 90e, 90g, 90i, 90k, 90n, 90q, 90s, 90t, 90v and the long sides 90b, 90d, 90f, 90h, 90j, 90m, 90p, 90r, 90u, 90w are These are both made of a non-magnetic metal wire material (which may be a narrow plate material), and it is preferable that the surface portion, particularly the surface portions adjacent to each other, be provided with an insulating coating.

  Specifically, the surface portions adjacent to each other are preferably provided with a gap of about 0.5 to 1 mm, for example, and enamel coating or polyamideimide coating is applied. In addition to this coating, a heat-resistant insulating film may be interposed, or the surface portion may be covered with a PFA insulating tube or a Kapton film. This is because it is affected by heat generated by the coil 52 and radiant heat from the heating belt 48 and the like.

  Further, the three short side portions 90a, 90k, 90s included in the central ring-shaped portion 90A are formed in an arc shape along the outer surface shape of the center core 58, and the ring-shaped portions on both sides are further formed. The short sides 90c, 90e, 90i, 90g, 90n, 90q, 90t, and 90v included in 90B and 90C are formed in a circular arc shape along the inner peripheral surface shape of the induction heating coil 52. Thereby, in the state which attached the magnetic adjustment member 90, interference with the center core 58 and the induction heating coil 52 can be avoided favorably.

[Function of magnetic adjustment member]
FIG. 5 is a model diagram for explaining the function of the magnetic adjustment member 90. In FIG. 5, the magnetic adjustment member 90 is simply shown as a wire model, but the connection relationship between the ring-shaped portions 90A, 90B, and 90C is the same as in FIG. Further, in FIG. 5, for convenience, the short side portions 90a, 90c, 90e, 90g, 90i, 90k, 90n, 90q, 90s, 90t, and 90v are shown as straight lines.

[When magnetic flux passes]
FIG. 5 (A): Considering the magnetic adjustment member 90 as a wire model, the structure is such that one large ring (annular body) is twisted in a staggered direction at a plurality of locations, and the three ring-shaped portions 90A, 90B are as described above. , 90C, and two magnetic adjustment portions M1 and M2 formed in the longitudinal direction.

  In the magnetic adjustment member M1 of the magnetic adjustment member 90, for example, when the magnetic flux Φ1 enters the center ring-shaped portion 90A, the current i1 (magnetic flux Φ1 and the magnetic flux Φ1 and the magnetic flux Φ1) is applied to the ring-shaped portion 90A. An induced current that generates a canceling magnetic flux in the reverse direction is generated. Similarly, when the magnetic fluxes Φ2 and Φ2 ′ enter the two ring-shaped portions 90B and 90C adjacent to both sides (left and right), currents i2 and i2 ′ (magnetic flux Φ2) also enter the ring-shaped portions 90B and 90C, respectively. , .PHI.2 ', an induced current that generates a canceling magnetic flux in the opposite direction is generated.

At this time, the directions of the currents i2 and i2 ′ generated in the ring-shaped portions 90B and 90C on both sides are the same, but the current i1 generated in the central ring-shaped portion 90A is in the opposite direction. When 1) is satisfied, the current (total) flowing inside the magnetic adjustment member 90 is zero.
| I1 | = | i2 | + | i2 ′ | (1)
In addition, | i1 |, | i2 |, and | i2 ′ | indicate absolute values of current (magnetomotive force).

  Therefore, when the above conditional expression (1) is satisfied, all the magnetic fluxes Φ1, Φ2, and Φ2 ′ can pass through the ring-shaped portions 90A, 90B, and 90C without being canceled.

[When shielding magnetic flux]
FIG. 5B: Next, consider a case where only the magnetic flux Φ1 entering the central ring-shaped portion 90A is removed from the above state (Φ1 = 0). In this case, no current is generated in the central ring-shaped portion 90A (i1 = 0), and the current flowing inside the magnetic adjustment member 90 is only the right side (| i2 | + | i2 ′ |) of the conditional expression (1). Become.

Therefore, when the magnetic flux Φ1 of the central ring-shaped portion 90A is removed, the magnetic fluxes Φ2 and Φ2 ′ are canceled out by the currents i2 and i2 ′ in the ring-shaped portions 90B and 90C on both sides. Are blocked by the ring-shaped portions 90B and 90C, respectively.
From the above, the following conclusions about the magnetic adjustment member 90 are clear.
(1) When the relational expression of Φ1 = Φ2 + Φ2 ′ is satisfied, the current generated in the magnetic adjustment member 90 becomes 0, and the magnetic adjustment member 90 allows passage of all the magnetic fluxes Φ1, Φ2, and Φ2 ′. In this case, the magnetic adjustment member 90 does not have any influence on the magnetic field.

(2) When Φ1 = 0 from the state of (1) above, since the current of i2 + i2 ′ flows inside the magnetic adjustment member 90, the magnetic adjustment member 90 shields without passing the magnetic fluxes Φ2, Φ2 ′. . In this case, the magnetic adjustment member 90 exhibits a magnetic shielding effect within the range of the ring-shaped portions 90B and 90C.
Based on the above, in the fixing unit 14 of the first embodiment, the magnetic flux Φ1 (Wb) entering the central ring-shaped portion 90A of the magnetic adjustment member 90 and the magnetic flux entering the two ring-shaped portions 90B and 90C adjacent to both sides thereof. For Φ2 and Φ2 ′ (Wb), a structure and arrangement satisfying the following relational expression (2) are adopted.
Φ1 = Φ2 + Φ2 '(2)

By the way, the structural example (1) having the magnetic adjustment units M1 and M2 described above is not a simple connection of the two configurations shown in FIG.
That is, when two configurations in FIG. 2A are arranged in the longitudinal direction and adjacent portions are connected by a single line and shared, three closed ring-shaped portions appear due to this sharing, and the current i is The current (total) of the magnetic adjustment member 90 cannot be made zero by flowing only in the ring-shaped part. In other words, three closed circuits appear, no current flows through each connection portion, and each ring-shaped portion is always cut off so that the magnetic flux cannot be adjusted.

Here, in order to make all of these three ring-shaped parts function effectively, it is necessary to cut the closed circuit. As a method, the ring-shaped part and the ring-shaped part are connected by two lines. In addition, two connections with the two lines are formed at one place with one line.
In the structure example (1), the ring-shaped portion 90A and the ring-shaped portion 90B are connected by the two short side portions 90c and 90i, and the ring-shaped portion 90A and the ring-shaped portion 90C are two short-side portions 90n. , 90v. And the ring-shaped part 90B and the ring-shaped part 90C are connected by one short side part 90s (FIG. 5).

  As a result, the above structural example (1) is formed (FIG. 6B), which has two magnetic adjustment portions M1 and M2, and a space that becomes an ineffective portion between the magnetic adjustment portion M1 and the magnetic adjustment portion M2. The magnetic adjustment member 90 does not exist. If the range of the central ring-shaped portion 90A facing the center core 58 is connected by one line, the degree of freedom in designing the IH coil unit 50 can be increased.

Furthermore, the method described above can also be used when the corresponding size is increased as compared to the structural example (1).
Specifically, FIG. 6C shows a structural example (2). In this case, a magnetic adjustment unit M3 is disposed on the opposite side of the magnetic adjustment unit M1 with the magnetic adjustment unit M2 interposed therebetween. Magnetic adjustment portions M1, M2, and M3 are formed.

The magnetic adjustment unit M3 can take part of a non-sheet passing area for a medium-sized sheet. Thus, any number of magnetic adjustment units can be increased in the same manner.
Next, FIG. 7 is a view showing the arrangement of the magnetic adjustment member 90 in the structural example (2) of FIG. FIGS. 7A and 7B are a side view and a bottom view of the center core 58 and the magnetic adjustment member 90, respectively. In the figure, the outer surface of the center core 58 is given a halftone dot. Yes.

  FIG. 7A: When the paper size is the maximum, the fixing unit 14 retracts the shielding member 60 to the outside of the magnetic path as the center core 58 rotates (retreat position). By retracting the shielding member 60 in this way, the magnetic fluxes Φ1, Φ2, and Φ2 ′ that satisfy the conditional expression (2) can be passed through the magnetic adjustment units M1, M2, and M3. In this case, the heat roller 46 is induction-heated over the entire area of the maximum sheet passing width W3.

  FIG. 7B: When the paper size is smaller than the maximum sheet passing width W3, the fixing unit 14 causes the shielding member 60 to enter the magnetic path as the center core 58 rotates (shielding position). By placing the shielding member 60 in the shielding position in this way, the magnetic flux from the center core 58 to the heat roller 46 is shielded on both sides, and the magnetic shielding effect is exerted by the two magnetic adjustment units M1 on both sides, and the magnetic flux Φ1 = It can be set to zero. This prevents excessive temperature rise at both ends of the heat roller 46 outside the intermediate sheet passing width W2.

  The shielding member 60 is divided into two in the longitudinal direction of the center core 58, and these are symmetrical to each other. Each shielding member 60 has, for example, a right triangle shape in a plan view or a bottom view, and a portion corresponding to the apex of the triangle is positioned near the end surface of the center core 58 and the center of the core 58. That is, at the position closer to the center of the center core 58, the length of the shielding member 60 in the circumferential direction is the shortest, and from there, the shielding member 60 is gradually expanded in the circumferential direction toward both side ends of the center core 58. ing.

  Further, in the state of FIG. 7B, the shielding member 60 is provided on both outer sides of the minimum sheet passing width W1 orthogonal to the sheet passing direction, and the shielding member 60 is slightly within the range of the minimum sheet passing width W1. Is not provided. The shielding member 60 reaches slightly outside the maximum sheet passing width W3 of the sheet at both ends of the center core 58. The minimum sheet passing width W1 and the maximum sheet passing width W3 are determined by the minimum or maximum size sheet that can be printed by the image forming apparatus 1.

  If the center core 58 further rotates counterclockwise from the state of FIG. 7B, the shielding member 60 can shield the magnetic flux entering the central ring-shaped portion 90A for the four magnetic adjustment members M1 and M2 on both sides. (Φ1 = 0), and the magnetic flux entering the central ring-shaped portion 90A can also be shielded (Φ1 = 0) for the six magnetic adjustment members M1, M2, M3 on both sides. Depending on the desired magnetic flux shielding effect.

  In the first embodiment, the ratio of the length of the shielding member 60 to the outer peripheral length of the center core 58 in the rotational direction is different in the axial direction (longitudinal direction) of the center core 58. At this time, when the ratio of the length (Lc) of the shielding member 60 to the outer peripheral length (L) of the center core 58 is defined as the coverage (= Lc / L), the coverage is small inside the center core 58, and from there the axial direction It becomes larger toward the outside (both ends). Specifically, the coverage is minimum in the vicinity of the minimum sheet passing area (the range of the minimum sheet passing width W1), and conversely, is maximum at both ends of the center core 58.

  As described above, the correspondence to the paper size (sheet passing width) is to partially suppress the generated magnetic flux by moving the shielding member 60 to the retracted position and the shielding position and switching the magnetic path at each position (Φ1 = 0). To be realized. At this time, the rotation angle (rotational displacement amount) of the center core 58 is varied according to the paper size (paper passing width), and the magnetic shielding amount is reduced as the paper size increases, and conversely, the shielding amount increases as the paper size decreases. By doing so, it is possible to prevent both ends of the heat roller 46 and the heating belt 48 from overheating. In FIG. 7, only the rotation in the counterclockwise direction is indicated by an arrow, but the center core 58 may be rotated in the clockwise direction. Further, the paper passing direction may be opposite to the direction shown in FIG.

[Operation example]
FIG. 9 is a diagram showing an operation example accompanying the rotation of the center core 58 in the above structural examples (1) and (2). Each will be described below.
[First route]
FIG. 9A: When the shielding member 60 is moved to the retracted position along with the rotation of the center core 58, the main magnetic path of the magnetic field generated by the induction heating coil 52 is the side core 56, the arch core 54, and the center core 58. It is formed so as to pass through the heating belt 48 and the heat roller 46 through the first path (thick solid line in the figure). At this time, eddy currents are generated in the heating belt 48 and the heat roller 46, which are ferromagnetic materials, and Joule heat is generated due to the specific resistance of each material, and heating is performed. At this time, the magnetic flux Φ1 passes through the ring-shaped portion 90A at the center of the magnetic adjustment member 90.

  Further, on the inner side of the magnetic path passing through the heating belt 48 and the heat roller 46 through the side core 56, the arch core 54, and the center core 58, for example, a shortcut magnetic flux (thick one-dot chain line in the figure) that leaks from the arch core 54 is generated. It passes through the ring-shaped portions 90B and 90C on both sides of the magnetic adjustment member 90. At this time, the magnetic fluxes Φ2 and Φ2 'pass through the ring-shaped portions 90B and 90C. Therefore, not only the magnetic flux Φ1 passing through the main first path but also other leakage magnetic fluxes Φ2 and Φ2 can contribute to heat generation, and the heat generation efficiency during full width heating can be improved accordingly.

[Second route]
FIG. 9B: Next, when the shielding member 60 is moved to the shielding position, the shielding member 60 is located on the magnetic path outside the minimum sheet passing area, so that the magnetic path there is from the end face of the arch core 54. The second path (thick broken line in the figure) that reaches the heating belt 48 and the heat roller 46 without going through the center core 58 is switched. As a result, the amount of heat generation is suppressed outside the minimum sheet passing area, and excessive heating of the heating belt 48 and the heat roller 46 can be prevented.

[Function of magnetic adjustment member]
In the state switched to the second path, the magnetic flux passing through the central ring-shaped portion 90A of the magnetic adjustment member 90 is zero as shown in the figure (magnetic flux Φ1 = 0). At this time, a weak magnetic flux (a broken line that circulates around the inside of the arch core 54) that tends to leak from the arch core 54 is also generated in the second path. However, the magnetic adjustment member 90 is in the second path as described above. The shielding effect can be exerted on all the magnetic fluxes Φ2 and Φ2 ′ passing through. For this reason, the fixing unit 14 of the first embodiment can obtain a sufficient magnetic shielding effect in the non-sheet-passing area without excessively increasing the area of the shielding member 60, thereby heating the heating belt 48 and the heat. The excessive temperature rise of the roller 46 can be suppressed from the current level.

  Based on the fixing unit 14 of the first embodiment described above, the following fixing units 14 of the second to ninth embodiments can be further exemplified. Each example will be described below. In any case, the same reference numerals as those in the first embodiment denote the same components as those in the first embodiment, and a duplicate description thereof will be omitted. Even if the reference numerals are common, a description to that effect is added particularly when the materials are different.

[Second Embodiment]
FIG. 10 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the second embodiment. In the second embodiment, the arrangement and form of the magnetic adjustment member 90 are different from those in the first embodiment.
Specifically, the magnetic adjustment member 90 has a central ring-shaped portion 90A disposed between the center core 58 and the heating belt 48. The ring-shaped portions 90B and 90C on both sides of the magnetic adjustment member 90 are outside the induction heating coil 52. That is, it is arranged between the arch core 54 and the induction heating coil 52.

  Also in this example, as long as the conditional expression (1) is satisfied in the state switched to the first path as described above (the state in which the shielding member 60 is placed at the retracted position), the magnetism is the same as in the first embodiment. The adjusting member 90 can pass the magnetic flux satisfactorily. Further, if the magnetic flux Φ1 passing through the center ring-shaped portion 90A can be reduced to 0 in a state in which the second path is switched (the shielding member 60 is placed at the shielding position), the magnetic adjustment member 90 has a magnetic flux as a whole. The shielding effect can be exhibited.

[Third embodiment]
FIG. 11 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the third embodiment. In the third embodiment, the toner image is fixed by the fixing roller 45 and the pressure roller 44 without using the heating belt. For example, a magnetic material similar to that of the above-described heating belt is wound around the outer periphery of the fixing roller 45, and the magnetic material is induction-heated by the induction heating coil 52. In this case, the thermistor 62 is provided outside the fixing roller 45 at a position facing the magnetic layer.

The magnetic adjustment member 90 can also be applied to the fixing unit 14 of the third embodiment as shown in the figure.
[Fourth embodiment]
FIG. 12 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the fourth embodiment. The fourth embodiment is different from the first embodiment in that the heat roller 46 is made of a nonmagnetic metal (for example, SUS: stainless steel) and the center core 58 is disposed inside the heat roller 46. ing. In addition, an arch core 54 is connected at the center, and an intermediate core 55 is installed at the lower part thereof.

When the heat roller 46 is made of a nonmagnetic metal, the magnetic field generated by the induction heating coil 52 passes through the side core 56, the arch core 54 and the intermediate core 55, passes through the heat roller 46, and reaches the internal center core 58. The heating belt 48 is induction-heated by a penetrating magnetic field.
In the case of the fourth embodiment, when the shielding member 60 is separated from the intermediate core 55 as shown in FIG. 12, the state is switched to the first path (retracted position). The heating belt 48 is induction-heated in the maximum sheet passing area without the magnetic shielding effect. On the other hand, when the shielding member 60 is moved to a position (shielding position) facing the intermediate core 55, the magnetic path is switched to the second path, and excessive temperature rise is suppressed outside the sheet passing area.

Also in the fixing unit 14 of the third example as described above, the magnetic adjustment member 90 is disposed between the intermediate core 55 and the heating belt 48 and between the induction heating coil 52 and the heating belt 48, for example. The same function as the embodiment can be exhibited.
[Fifth embodiment]
FIG. 13 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the fifth embodiment. In the fifth embodiment, the IH coil unit 50 is a so-called inclusion IH type. Specifically, the heat roller 46 is made of a non-magnetic metal (for example, SUS) having a relatively large diameter (for example, 40 mm), and the induction heating coil 52 and the center core 58 are accommodated therein. Further, the arch core 54 and the side core 56 as in the first to fourth embodiments are not provided outside the heat roller 46. A release layer (PFA) is formed on the surface of the heat roller 46. The pressure roller 44 is the same as in the first to third examples.

  In the inclusion IH as in the fifth embodiment, the magnetic field generated by the induction heating coil 52 is guided by the center core 58 inside the heat roller 46 to inductively heat the heat roller 46. In the case of the fifth embodiment, when the shielding member 60 is separated from the induction heating coil 52 as shown in FIG. 13, the state is switched to the first path (retracted position). In this case, the magnetic shielding effect is obtained. The heating belt 48 is induction-heated in the maximum sheet passing area without working. On the other hand, when the shielding member 60 is moved to a position close to the induction heating coil 52 (shielding position), the magnetic path is switched to the second path, and excessive temperature rise is suppressed outside the sheet passing area.

Also in the fixing unit 14 of the fifth embodiment, for example, the magnetic adjustment member 90 can be fixedly disposed between the inner peripheral surface of the heat roller 46 and the induction heating coil 52 as illustrated.
[Sixth embodiment]
FIG. 14 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the sixth embodiment. In the sixth embodiment, the induction heating is performed not at the arcuate position of the heating belt 48 but at a planar position between the heat roller 46 and the fixing roller 45. In this case as well, the magnetic path can be switched by rotating the center core 58. The magnetic adjustment member 90 allows a magnetic flux to pass well when switched to the first path, and exhibits a magnetic flux shielding effect when switched to the second path.

Further, the magnetic adjustment member 90 has a shape in which only the central ring-shaped portion 90A is curved, and the ring-shaped portions 90B and 90C on both sides have a flat shape that is not curved. Such a magnetic adjustment member 90 is fixedly installed between the induction heating coil 52 and the heating belt 48, for example.
[Seventh embodiment]
Next, FIG. 15 is a longitudinal sectional view showing a structural example of the fixing unit 14 of the seventh embodiment. In the seventh embodiment, the magnetic path is switched by moving only the shielding member 60 without using the center core 58. The arch cores 54 are connected to each other on both sides, and the shielding member 60 moves along the inner surface of the arch core 54 in the direction of the arrow in the figure.

Although not particularly illustrated, the fixing unit 14 of the seventh embodiment includes a drive mechanism similar to that of the first embodiment, and the shielding member 60 is provided around the same center point as the rotation center of the heat roller 46 by this drive mechanism. Can be moved.
[First route]
As shown by a two-dot chain line in FIG. 15, the shielding member 60 is moved to a retracted position between the arch core 54 and the induction heating coil 52 at a position shifted from the winding center L of the induction heating coil 52. In the seventh embodiment, the state is switched to the first route. In this case, the magnetic flux reaches the heating belt 48 and the heat roller 46 along the winding center L from the center position of the arch core 54. At this time, the magnetic adjustment member 90 allows the magnetic flux to pass satisfactorily as in the first embodiment.

Further, as indicated by a two-dot chain line in FIG. 15, in the seventh embodiment, the magnetic adjustment member 90 may be disposed not only outside the induction heating coil 52 but also inside the induction heating coil 52.
[Second route]
On the other hand, as shown by the solid line in FIG. 15, when the shielding member 60 is positioned on the line of the winding center L of the induction heating coil 52, the state is switched from the first path to the second path. In this case, since the magnetic flux Φ1 entering the ring-shaped portion 90A at the center of the magnetic adjustment member 90 becomes 0, the magnetic adjustment member 90 can shield the magnetic flux as a whole.

[Eighth embodiment]
FIG. 16 is a partial longitudinal sectional view showing a configuration example of the fixing unit 14 of the eighth embodiment. In FIG. 16, the fixing unit 14 is shown by enlarging only the IH coil unit 50. Hereinafter, the difference from the first embodiment will be mainly described.
In the eighth embodiment, another connecting core 57 is disposed outside the center core 58, and this connecting core 57 connects the arch cores 54 on both sides. One of the arch cores 54 on the both sides (the right side in the figure) has one end bent at a substantially right angle on the side of the center core 58, and the bent part passes through the inside of the induction heating coil 52 and the heating belt 48 and It extends to the vicinity of the heat roller 46.

For this reason, in the eighth embodiment, the rotation center of the center core 58 is offset to one side (left side in the figure) with respect to the rotation center of the heat roller 46 by the amount of the bent portion provided in one arch core 54 (in the figure). F) It is in the position made. Further, the center core 58 is provided with a shielding member 60 in substantially half as viewed in the circumferential direction.
The magnetic adjustment member 90 is arranged not only outside the induction heating coil 52 but also inside as shown in the seventh embodiment. At this time, it is assumed that the bent portion of the arch core 54 is disposed at a position penetrating the ring-shaped portion 90C on one side of the magnetic adjustment member 90, for example.

  Also in the eighth embodiment, the first route and the second route can be switched by rotating the center core 58 as in the first embodiment. In particular, according to the structure of the eighth embodiment, the degree of magnetic coupling when switched to the second path can be improved by providing the arch core 54 with a bent portion. Further, for example, another shielding member 60 is bonded to the inner surface of the arch core 54, and such a shielding member 61 contributes to shielding leakage magnetic flux from the arch core 54.

Therefore, in the eighth embodiment, when switching from the first path to the second path, the magnetic fluxes Φ2 and Φ2 ′ can be reliably directed to the ring-shaped portions 90B and 90C on both sides of the magnetic adjustment member 90. Thus, the shielding effect can be reliably exhibited.
Although FIG. 16 shows an example in which one arch core 54 is provided with a bent portion, both arch cores 54 may be provided with a bent portion.

[Ninth embodiment]
FIG. 17 is a longitudinal sectional view showing a configuration example of the fixing unit 14 of the ninth embodiment. Hereinafter, the difference from the first embodiment will be mainly described.
The ninth embodiment is greatly different from the first embodiment in that the shielding member 60 has a ring shape. Further, the magnetic adjustment member 90 is different from the first embodiment in that the magnetic adjustment member 90 is installed outside and inside the induction heating coil 52 as in the eighth embodiment. Further, another shielding member 61 is bonded to the inner surface of the arch core 54, and the leakage magnetic flux from the arch core 54 is shielded by the shielding member 61. The shielding member 61 extends from the arch cores 54 on both sides to the outside of the center core 58 (upward in the drawing), and is connected to each other at this position.

[Ring-shaped shielding member]
FIG. 18 is a view showing a state where a ring-shaped shielding member 60 that can correspond to the structural example (2) is attached to the center core 58. 18A corresponds to a plan view and a side view of the center core 58, and FIGS. 18B, 18C, and 18D are a BB cross section, a CC cross section, and a DD cross section, respectively. It corresponds to.
The shielding member 60 has a reel-like shape as a whole, and has a pair of perforated annular portions 60A and 60B at both end positions as viewed in the longitudinal direction, and three linear portions between these two portions. It is the structure connected by 60a.

  The straight line portion 60a is disposed at intervals in the circumferential direction of the annular portions 60A and 60B, and the annular portion 60A is provided at an end portion of one end of the center core 58 (outside of the minimum sheet passing area: non-sheet passing area) The annular portion 60B is provided at the boundary between the non-sheet passing area and the sheet passing area. A shielding member 60 is also disposed at the other end of the center core 58 (not shown).

Following this annular portion 60B, there is a circular arc portion 60C having a hole shape in about 2/3 circles spaced in the longitudinal direction, and a circular arc shape having a hole shape in about 1/3 circle at the other end position. Part 60D.
Of the four annular portions 60A and 60B and the arc portions 60C and 60D, the three annular portions 60A and 60B and the arc portion 60C are connected to each other via three linear portions 60a. The remaining arc portion 60D at the other end position is connected to the adjacent arc portion 60C via two straight portions 60a.

  FIG. 18A: The shielding member 60 is also provided at the end of the center core 58 viewed in the longitudinal direction. At this time, the annular portion 60A farthest from the minimum sheet passing area is at a position corresponding to the maximum size P1 (for example, A3, A4R), and the next annular portion 60B is at a position corresponding to the medium size P2 (for example, B4R). The next arc portion 60C is located at a position corresponding to the medium-sized size P3 (for example, B4). The arc portion 60D in the vicinity of the minimum sheet passing area is at a position corresponding to the minimum size P4 (for example, A5R).

FIG. 18B: It can be seen that the annular portions 60A and 60B have a perforated shape as described above.
FIG. 18C: The arc portion 60C has a perforated shape of about two-thirds of a circle as described above. The lacked portion of the arc portion 60C is filled with the ferrite material of the center core 58.

FIG. 18D: The arc portion 60D has a perforated shape of about a third circle as described above. The arc portion 60D is also filled with the ferrite material of the center core 58 in the lacked portion.
[Operation example]
Next, an operation example when the shielding member 60 is applied will be described. FIGS. 19 to 24 are perspective views sequentially showing six examples of operation using the shielding member 60. Arrows indicated by bold lines in each figure indicate the generated induced current or the passing magnetic field. Each will be described below.

[Full screen (0 °)]
First, FIG. 19 is a perspective view showing an operation example when the entire surface is shielded by the shielding member 60. In each operation example, it is assumed that a magnetic field is generated in a direction penetrating from the upper side to the lower side with respect to the shielding member 60. Further, in the following description, it is assumed that the entire shielding state shown in FIG. 19 is 0 °, and the displacement amount of the shielding member 60 is represented by the rotation angle therefrom.

  When the shielding member 60 is moved to the rotation angle (0 °) at which the arc portion 60D is positioned below, the magnetic shielding effect can be exerted on the entire surface of the shielding member 60 in the longitudinal direction. That is, the annular portion 60A at the one end position, the arc portion 60D at the other end position, and the linear portion 60a connecting them form a ring portion having the maximum shape, so that the whole can be magnetically shielded. In this case, overheating of the heating belt 48 and the heat roller 46 can be prevented corresponding to the minimum size P4.

[No shielding (60 °)]
FIG. 20 is a perspective view showing an operation example when the shielding member 60 is rotated 60 ° clockwise from the state of FIG. In this case, since the straight portion 60a is located on the center line of the coil 52 (the state of FIG. 9A), the shielding member 60 is in the retracted position, and no magnetic shielding effect is generated.

[Small and medium size shielding (120 °)]
FIG. 21 is a perspective view showing an operation example when the shielding member 60 is rotated 120 degrees clockwise from the state of FIG. In this case, the magnetic shielding effect can be exhibited by one ring portion formed between the annular portion 60A and the arc portion 60C. In this operation example, overheating of the heating belt 48 and the heat roller 46 can be prevented in correspondence with, for example, the medium-sized size P3.

[No shielding (180 °)]
FIG. 22 is a perspective view showing an operation example when the shielding member 60 is rotated 180 degrees clockwise from the state of FIG. In this case, since the straight portion 60a is located on the center line of the coil 52 as in FIG. 20 (state of FIG. 9A), the shielding member 60 is in the retracted position and no magnetic shielding effect is generated.

[Medium size shielding (240 °)]
FIG. 23 is a perspective view showing an operation example when the shielding member 60 is rotated by 240 degrees in the clockwise direction from the state of FIG. In this case, the magnetic shielding effect can be exhibited by one ring portion formed between the annular portion 60A and the annular portion 60B. In this operation example, overheating of the heating belt 48 and the heat roller 46 can be prevented, for example, corresponding to the medium size P2.

[No shielding (300 °)]
FIG. 24 is a perspective view showing an operation example when the shielding member 60 is rotated 300 degrees clockwise from the state of FIG. In this case, since the linear portion 60a is located on the center line of the coil 52 as in FIGS. 20 and 22 (the state of FIG. 9A), the shielding member 60 is in the retracted position, and no magnetic shielding effect is generated. In the case of no shielding (60 °), (180 °), and (300 °), the heating belt 48 and the heat roller 46 can be induction-heated corresponding to the maximum size P1.

  As described above, according to the present embodiment, the overheating of the heating belt 48 is suppressed by basically switching the magnetic field path to the second path by the shielding member 60. Such a shielding member 60 has a structural advantage in that it does not take up much space, but it is not possible to completely stop the inflow of magnetic flux simply by switching the path, as described above. The suppression effect of overheating is not perfect. Therefore, it is not sufficient to simply switch the magnetic field path from the first path to the second path, and higher productivity cannot be realized as it is.

  Therefore, in this embodiment, in addition to the space-saving shielding member 60 having a weak magnetic shielding effect, the magnetic adjustment member 90 arranged in a fixed manner is used. In other words, the magnetic adjustment member 90 allows the magnetic flux to pass over the entire switching area in the state switched to the first path, and maximizes the temperature rising effect of the belt 48 while being switched to the second path. In this state, the passage of the magnetic flux is shut out throughout the switching area, and the belt 48 is prevented from being overheated.

  Thus, the heat generation contrast of the belt 48 can be increased when switching between the first route and the second route. In addition, the magnetic adjustment member 90 has a function (function like a magnetic filter) that allows the magnetic flux to pass through or is shielded on the contrary only by being in a fixed position without mechanically performing any operation. Even if it has a certain area, it does not affect the magnetic field when temperature rise is required. Further, since the magnetic adjustment member 90 only needs to be fixedly arranged, it is not necessary to provide a new movable member, and the space for the fixing device can be saved as much.

  In addition, the magnetic adjustment member 90 of the present embodiment includes a plurality of ring-shaped portions 90A, 90B, and 90C made of a wire material, and is configured so as to be compatible with a plurality of paper sizes. The ring-shaped portions 90A, 90B, and 90C are formed in a continuous endless shape as a whole as viewed in the axial direction of the wire material. Therefore, it is possible to set a range in which one magnetic adjustment member 90 can suppress heat generation stepwise in the longitudinal direction of the center core 58. Furthermore, since this space can be eliminated compared to the conventional configuration in which a space that can also be a flow path through which a magnetic field leaks is provided and divided, the magnetic shielding effect is further improved and fixing is performed. This also contributes to space saving of the unit 14.

  For example, when the magnetic adjustment member 90 has at least two ring-shaped portions, the two ring-shaped portions are connected in a so-called “eight-shape”. That is, in this case, when the magnetic flux penetrates the two adjacent ring-shaped portions in the same direction, the direction of the induced current generated in one ring-shaped portion 90A and the induced current generated in the other ring-shaped portions 90B and 90C. The direction of is reversed, and the induced current is canceled as a whole inside the magnetic adjustment member 90. In this case, since the magnetic adjustment member 90 hardly exerts influence on the magnetic field, the magnetic flux can be allowed to pass through without any problem.

  On the other hand, when switched to the second path, for example, the amount of magnetic flux penetrating through one ring-shaped portion 90A is reduced (almost becomes 0), so that the induced current is generated in the other ring-shaped portions 90B and 90C. This generates a magnetic flux (demagnetizing field) opposite to the penetrating magnetic flux. In this case, since the magnetic flux trying to pass through the second path is shielded, as a result, the magnetic adjustment member 90 can exert a magnetic flux shielding effect as a whole.

  Furthermore, as described above, the ring-shaped portions 90A, 90B, and 90C that eliminate the space by the magnetic adjustment portions M1 and M2 and the magnetic adjustment portions M1, M2, and M3 are arranged close to each other. Since the surface portions of the shape portions 90A, 90B, and 90C or the surface portions close to each other are subjected to insulation treatment, the ring-shaped portions 90A, 90B, and 90C can be reliably insulated from each other.

  Furthermore, in the case of the present embodiment, first, the magnetic adjustment member 90 has one ring-shaped portion 90A in the center and two ring-shaped portions 90B and 90C adjacent to both sides thereof. The structure corresponds to the paper size. At this time, the center ring-shaped portion 90A is disposed on an extension line of the coil winding center, and the other ring-shaped portions 90B and 90C are disposed on both sides thereof.

  And in the state switched to the 1st path | route by the shielding member 60, it deviates from the 1st induced current and the 1st path | route which generate | occur | produce in one center ring-shaped part 90A with the magnetic flux which passes a 1st path | route. Thus, the second induced currents generated by the magnetic fluxes that attempt to pass through the second paths located on both sides of each other pass through the other two adjacent ring-shaped portions 90B and 90C cancel each other. . As a result, the magnetic adjustment member 90 allows not only the first path but also the passage of magnetic flux that deviates from both sides to pass through the second path, and as a result, the entire magnetic flux within the switching area is changed. Passing can be allowed.

  On the other hand, when the second path is switched by the shielding member 60, almost no magnetic flux passes through the central ring-shaped portion 90A, and the magnetic flux passes through the other two adjacent ring-shaped portions 90B and 90C. To generate an induced current. At this time, each magnetic flux generated in the other two ring-shaped portions 90B and 90C cancels the magnetic flux that is going to pass through the second path, and as a result, the magnetic adjustment member 90 has a total magnetic flux within the switching region. Can be shielded. In addition, the magnetic flux shielding effect can be exerted by the two ring-shaped portions 90B and 90C disposed on both sides of the ring-shaped portion 90A disposed at the center only by suppressing the passage of the magnetic flux to the single ring-shaped portion 90A disposed at the center. The shielding effect can be obtained efficiently with a simple structure.

As described above, since the magnetic shielding effect is further improved, as a result of forming a good toner image, the reliability of the image forming apparatus 1 is improved.
The present invention can be implemented with various modifications without being limited to the above-described embodiments. For example, the cross-sectional shape of the center core 58 is not limited to a cylinder, and may be a cylinder or a polygon. Further, the shape of the shielding member 60 in plan view is not limited to a triangular shape, and may be a trapezoidal shape.

In addition, the shape and size of the ring of the magnetic adjustment member 90 and the number of parts to be divided are only examples, and are not particularly limited to one embodiment.
In addition, the specific form of each part including the arch core 54 and the side core 56 is not limited to the illustrated one, and can be appropriately modified.
In any of these cases, the magnetic shielding effect can be further improved as described above.

1 Printer (image forming device)
7 Image forming unit 14 Fixing unit (fixing device)
44 Pressure roller (Pressure member)
48 Heating belt (heating member)
50 IH coil unit 52 Induction heating coil (coil)
54 Arch core (magnetic core, first core)
56 Side core (magnetic core, first core)
58 Center core (magnetic core, second core)
60 Shielding member (route switching means)
90 Magnetic adjustment member 90A, 90B, 90C Ring-shaped part

Claims (4)

A fixing device that fixes a toner image onto a sheet by at least heat from the heating member in the conveyance process by conveying the sheet on which the toner image has been transferred in the image forming unit between the heating member and the pressure member. There,
A coil that is disposed along an outer surface of the heating member and generates a magnetic field for induction heating the heating member;
A magnetic core disposed at least on the opposite side of the heating member across the coil, and guiding a magnetic field generated by the coil toward the heating member by forming a magnetic path around the coil;
A magnetic path guided by the magnetic core toward the heating member includes a first path where induction heating of the heating member is promoted and a second path where induction heating of the heating member is suppressed. Route switching means for switching to either
The magnetic path is arranged over a switching area of a magnetic field path including both the first path and the second path, and when switched to the first path by the path switching unit, the magnetic core is moved from the magnetic core within the switching area. A magnetic adjustment member that allows passage of magnetic flux toward the heating member, while shielding the magnetic flux without passing through when the second path is switched;
The magnetic adjustment member is divided and arranged in accordance with a plurality of paper sizes to be conveyed with respect to the longitudinal direction of the magnetic core, and a plurality of rings in which a highly conductive wire material is formed in a ring shape. The plurality of ring-shaped portions are connected to each other in a state of being adjacent to each other in a direction intersecting the magnetic flux traveling direction, so that the whole of the wire material viewed in the axial direction is a continuous endless shape. And formed
When switched to the first path by the path switching means, the induced current generated by the magnetic flux penetrating each of the plurality of ring-shaped parts is opposite to each other when viewed from the adjacent ring-shaped parts. And
The path switching unit reduces the amount of magnetic flux penetrating through a part of the ring-shaped portion of the plurality of paths in a state where the path is switched from the first path to the second path. . The fixing device according to claim 1,
The fixing device according to claim 1, wherein a surface portion of each ring-shaped portion or a surface portion adjacent to each other is subjected to insulation treatment. The fixing device according to claim 1, wherein:
The heating member is
Seen in the width direction of the conveyed paper, it is induction-heated by the coil over the maximum paper passing area,
The magnetic adjustment member is
A fixing device, wherein the fixing device is disposed outside a sheet passing area of a sheet having a width smaller than a sheet having a maximum width corresponding to the maximum sheet passing area when viewed in the longitudinal direction of the magnetic core.   An image forming apparatus comprising: the fixing device according to claim 1 mounted on an image forming apparatus; and using this, the toner image formed by the image forming unit is fixed on a sheet.

Images