JP2010224342A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress excessive temperature rise in a paper non-passing area and to suppress degradation of fixing performance in a paper passing area, in an induction heating type fixing device. <P>SOLUTION: A temperature sensitive magnetic member 64 that forms a magnetic path of AC magnetic field generated by an IH heater is arranged to face an IH heater across a fixing belt. A slit 64s for adjusting a self-calorific value is formed so that temperature in an area where paper passes may be maintained within a temperature range lower than a transition area where magnetic characteristics are changed between ferromagnetism and paramagnetism in accordance with the temperature. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

As a fixing device mounted on an image forming apparatus such as a copying machine or a printer using an electrophotographic system, an apparatus using an electromagnetic induction heating system is known.
For example, in Patent Document 1, an electromagnetic induction coil as a magnetic flux generating means is arranged inside a fixing roll made of a core metal cylinder made of magnetic metal, and an eddy current is induced in the fixing roll by an induced magnetic field generated by the electromagnetic induction coil. An induction heating type fixing device that directly heats the fixing roll is described.

JP 2003-186322 A

Here, in general, for example, the fixing member heated by an electromagnetic induction method is formed of a belt member having a small heat capacity, so that the time required to raise the fixing member to a fixable temperature (warm-up time) is shortened. However, for example, when a small-size sheet is continuously passed, the non-sheet passing area with low heat consumption may be excessively heated to damage the fixing member.
An object of the present invention is to suppress an excessive temperature rise in a non-sheet passing area in an induction heating type fixing device, and to suppress a decrease in fixing performance in the sheet passing area.

  According to the first aspect of the present invention, there is provided a fixing member that has a conductive layer, the toner is fixed to the recording material by electromagnetically heating the conductive layer, and an alternating magnetic field that intersects the conductive layer of the fixing member. The magnetic field generating member to be generated is disposed opposite the magnetic field generating member with the fixing member interposed therebetween, and the temperature in the region through which the recording material passes is between ferromagnetic and paramagnetic depending on the temperature. A heat generation adjusting unit that adjusts the amount of self-heating so as to be maintained in a temperature range lower than a transition region that transitions at a magnetic path forming member that forms a magnetic path of an alternating magnetic field generated by the magnetic field generating member; A fixing device characterized by comprising:

According to a second aspect of the present invention, the magnetic path forming member includes a plurality of cutout portions formed in an angular direction in which the heat generation adjusting portion intersects a direction orthogonal to the moving direction of the fixing member. 2. The fixing device according to claim 1, wherein an interval between the notch portions which are configured and adjacent to each other is set to be 3.5 mm or more and 6.8 mm or less along a direction orthogonal to the moving direction of the fixing member. Device.
The invention according to claim 3 is the fixing device according to claim 1, wherein the magnetic path forming member is configured such that a temperature width of the transition region relating to magnetic characteristics is 20 ° or less. .
According to a fourth aspect of the present invention, in the magnetic path forming member, the temperature at which the start temperature of the transition region is set in the fixing member as a temperature for melting and fixing the toner on the recording material to the recording material 4. The fixing device according to claim 3, wherein the fixing device is set higher than the fixing device.
According to a fifth aspect of the present invention, in the fixing device according to the first aspect, the magnetic path forming member is configured to have a thickness larger than a skin depth in a temperature region lower than the transition region. is there.

  According to a sixth aspect of the present invention, there is provided a toner image forming unit that forms a toner image, a transfer unit that transfers the toner image formed by the toner image forming unit onto a recording material, and a toner image that is transferred onto the recording material. A fixing member for fixing the toner image to the recording material, the fixing unit having a conductive layer, and fixing the toner to the recording material by electromagnetically heating the conductive layer. A magnetic field generating member that generates an alternating magnetic field that intersects with the conductive layer of the fixing member, and a magnetic field generating member that is opposed to the magnetic field generating member with the fixing member interposed therebetween. A magnetic path forming member that has a transition region that transitions between magnetism and forms a magnetic path of an alternating magnetic field generated by the magnetic field generating member, and is disposed on the magnetic path forming member, through which the recording material passes. The temperature of the magnetic path forming member in the region is Serial is an image forming apparatus characterized by comprising a heating adjusting portion for adjusting a self-heating value to be maintained in a temperature range lower than the transition region.

According to a seventh aspect of the present invention, the heat generation adjusting portion of the fixing unit includes a plurality of magnetic path forming members formed in an angle direction intersecting with a direction orthogonal to the moving direction of the fixing member. The interval between the notch portions adjacent to each other is set to 3.5 mm or more and 6.8 mm or less along a direction orthogonal to the moving direction of the fixing member. Item 7. The image forming apparatus according to Item 6.
The invention according to claim 8 is characterized in that the magnetic path forming member of the fixing means is configured such that a temperature width of the transition region relating to magnetic characteristics is 20 ° or less. An image forming apparatus.
The invention according to claim 9 is characterized in that the magnetic path forming member of the fixing unit is configured to have a thickness larger than a skin depth in a temperature region lower than the transition region. This is an image forming apparatus.

According to the first aspect of the present invention, as compared with the case where the present invention is not adopted, an excessive temperature rise in the non-sheet passing region in the induction heating type fixing device is suppressed, and the fixing performance in the sheet passing region is reduced. Can be suppressed.
According to the second aspect of the present invention, the self in the magnetic path forming member is maintained so that the temperature in the passage region of the recording material in the magnetic path forming member is maintained in a temperature range that is 10 ° or more lower than the transition region of the magnetic characteristics. The calorific value can be adjusted.
According to the third aspect of the present invention, it is possible to more reliably suppress the temperature rise of the non-sheet passing region of the fixing member beyond the heat resistance temperature of the fixing member, as compared with the case where the present invention is not adopted.
According to the fourth aspect of the present invention, compared to the case where the present invention is not adopted, it is possible to secure a sufficient amount of heat generation for fixing the toner on the recording material on the fixing member.
According to the fifth aspect of the present invention, eddy current generated in the magnetic path forming member can be reduced as compared with the case where the present invention is not adopted.

According to the sixth aspect of the present invention, as compared with the case where the present invention is not adopted, an excessive temperature rise in the non-sheet passing region in the induction heating type fixing device mounted on the image forming apparatus is suppressed, and in the sheet passing region. Decrease in fixing performance can be suppressed.
According to the seventh aspect of the present invention, the self in the magnetic path forming member is maintained so that the temperature in the recording material passage region in the magnetic path forming member is maintained in a temperature range lower than the magnetic property transition region by 10 ° or more. The calorific value can be adjusted.
According to the eighth aspect of the present invention, it is possible to more reliably suppress the temperature rise of the non-sheet passing region of the fixing member beyond the heat resistance temperature of the fixing member as compared with the case where the present invention is not adopted.
According to the ninth aspect of the present invention, eddy current generated in the magnetic path forming member can be reduced as compared with the case where the present invention is not adopted.

1 is a diagram illustrating a configuration example of an image forming apparatus to which a fixing device according to an exemplary embodiment is applied. FIG. 2 is a front view illustrating a configuration of a fixing unit of the present embodiment. FIG. 3 is a II-II sectional view of the fixing unit in FIG. 2. FIG. 3 is a cross-sectional layer configuration diagram of a fixing belt. (a) is a side view of an end cap member, (b) is a top view of the end cap member seen from the Z direction. It is sectional drawing explaining the structure of an IH heater. It is a figure explaining the state of a line of magnetic force in case the temperature of a fixing belt exists in the temperature range below the magnetic permeability change start temperature. FIG. 6 is a diagram illustrating an outline of a temperature distribution in a width direction of a fixing belt when small-size paper is continuously passed. FIG. 6 is a diagram for explaining a state of magnetic lines of force when the temperature of the fixing belt in a non-sheet passing region is in a temperature range exceeding the permeability change start temperature. It is the figure which showed an example of the temperature characteristic regarding the relative magnetic permeability of a temperature-sensitive magnetic member. It is the figure which showed the slit formed in a temperature sensitive magnetic member. It is the figure which showed the relationship between the space | interval of the mutually adjacent slit formed in the temperature-sensitive magnetic member, and the temperature of the temperature-sensitive magnetic member in a paper passing area | region.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
<Description of Image Forming Apparatus>
FIG. 1 is a diagram illustrating a configuration example of an image forming apparatus to which the fixing device of the present embodiment is applied. An image forming apparatus 1 shown in FIG. 1 is a so-called tandem color printer, and includes an image forming unit 10 that forms an image based on image data and a control unit 31 that controls the operation of the entire image forming apparatus 1. Further, for example, a communication unit 32 that receives image data by communicating with a personal computer (PC) 3 or an image reading device (scanner) 4, and a predetermined image for the image data received by the communication unit 32. An image processing unit 33 that performs processing is provided.

The image forming unit 10 includes four image forming units 11Y, 11M, 11C, and 11K (also collectively referred to as “image forming unit 11”), which are examples of toner image forming units arranged in parallel at a predetermined interval. I have. Each image forming unit 11 forms an electrostatic latent image and a photosensitive drum 12 as an example of an image holding body that holds a toner image, and charging that uniformly charges the surface of the photosensitive drum 12 with a predetermined potential. 13, an LED (Light Emitting Diode) print head 14 that exposes the photosensitive drum 12 charged by the charger 13 based on each color image data, and a developer that develops an electrostatic latent image formed on the photosensitive drum 12. 15. A drum cleaner 16 for cleaning the surface of the photosensitive drum 12 after transfer is provided.
Each of the image forming units 11 is configured in substantially the same manner except for the toner stored in the developing device 15, and each forms a toner image of yellow (Y), magenta (M), cyan (C), and black (K). To do.

  The image forming unit 10 also receives the intermediate transfer belt 20 onto which the color toner images formed on the photosensitive drums 12 of the image forming units 11 are transferred, and the color toner images formed on the image forming units 11. A primary transfer roll 21 that sequentially transfers (primary transfer) to the intermediate transfer belt 20 is provided. Further, a secondary transfer roll 22 that batch-transfers (secondary transfer) each color toner image transferred and superimposed on the intermediate transfer belt 20 onto a sheet P that is a recording material (recording paper), and each color toner that is secondarily transferred. A fixing unit 60 is provided as an example of a fixing unit (fixing device) that fixes the image on the paper P. In the image forming apparatus 1 of the present embodiment, the intermediate transfer belt 20, the primary transfer roll 21, and the secondary transfer roll 22 constitute a transfer unit.

  In the image forming apparatus 1 of the present embodiment, under the operation control by the control unit 31, image forming processing is performed by the following process. That is, the image data from the PC 3 or the scanner 4 is received by the communication unit 32, subjected to predetermined image processing by the image processing unit 33, and then sent to each image forming unit 11 as image data for each color. It is done. For example, in the image forming unit 11K that forms a black (K) toner image, the photosensitive drum 12 is uniformly charged at a predetermined potential by the charger 13 while rotating in the arrow A direction, and the image processing unit 33 is charged. The LED print head 14 scans and exposes the photosensitive drum 12 based on the K-color image data transmitted from. As a result, an electrostatic latent image relating to the K color image is formed on the photosensitive drum 12. The K-color electrostatic latent image formed on the photosensitive drum 12 is developed by the developing unit 15, and a K-color toner image is formed on the photosensitive drum 12. Similarly, yellow (Y), magenta (M), and cyan (C) color toner images are formed in the image forming units 11Y, 11M, and 11C, respectively.

  Each color toner image formed on the photosensitive drum 12 of each image forming unit 11 is sequentially electrostatically transferred (primary transfer) onto the intermediate transfer belt 20 that moves in the direction of arrow B by the primary transfer roll 21, and each color toner is superimposed. A superimposed toner image is formed. The superimposed toner image on the intermediate transfer belt 20 is conveyed to a region (secondary transfer portion T) where the secondary transfer roll 22 is disposed as the intermediate transfer belt 20 moves. When the superimposed toner image is conveyed to the secondary transfer unit T, the paper P is supplied from the paper holding unit 40 to the secondary transfer unit T in accordance with the timing. The superimposed toner image is collectively electrostatically transferred (secondary transfer) onto the conveyed paper P by the transfer electric field formed by the secondary transfer roll 22 in the secondary transfer portion T.

Thereafter, the sheet P on which the superimposed toner image is electrostatically transferred is conveyed to the fixing unit 60. The toner image on the paper P conveyed to the fixing unit 60 receives heat and pressure by the fixing unit 60 and is fixed on the paper P. Then, the paper P on which the fixed image is formed is conveyed to a paper stacking unit 45 provided in the discharge unit of the image forming apparatus 1.
On the other hand, the toner (primary transfer residual toner) adhering to the photosensitive drum 12 after the primary transfer and the toner (secondary transfer residual toner) adhering to the intermediate transfer belt 20 after the secondary transfer are respectively drum cleaner 16. , And the belt cleaner 25.
In this way, the image forming process in the image forming apparatus 1 is repeatedly executed for the number of printed sheets.

<Description of fixing unit configuration>
Next, the fixing unit 60 of this embodiment will be described.
2 and 3 are views showing the configuration of the fixing unit 60 of the present embodiment, FIG. 2 is a front view, and FIG. 3 is a sectional view taken along line II-II in FIG.
First, as shown in FIG. 3, which is a cross-sectional view, the fixing unit 60 is heated by electromagnetic induction by an IH (Induction Heating) heater 80 and an IH heater 80 as an example of a magnetic field generating member that generates an alternating magnetic field, thereby generating a toner image. A fixing belt 61 as an example of a fixing member to be fixed, a pressure roll 62 disposed so as to face the fixing belt 61, and a pressure pad 63 pressed from the pressure roll 62 via the fixing belt 61 are provided.
Further, the fixing unit 60 includes a holder 65 that supports constituent members such as the pressure pad 63, a temperature-sensitive magnetic member 64 that induces an alternating magnetic field generated by the IH heater 80 to form a magnetic path, and a temperature-sensitive magnetic member 64. A guide member 66 that guides the lines of magnetic force that have passed through the fixing belt 61, and a peeling auxiliary member 173 that assists in peeling the paper P from the fixing belt 61.

<Description of fixing belt>
The fixing belt 61 is formed of an endless belt member having an original cylindrical shape, and has a diameter of 30 mm and a length in the width direction of 370 mm in the original shape (cylindrical shape), for example. Further, as shown in FIG. 4 (cross-sectional layer configuration diagram of the fixing belt 61), the fixing belt 61 includes a base material layer 611, a conductive heat generating layer 612 laminated on the base material layer 611, and a toner image fixability. The belt member has a multilayer structure including an elastic layer 613 for improving the surface and a surface release layer 614 coated on the uppermost layer.

The base material layer 611 is composed of a heat-resistant sheet-like member that supports the thin conductive heat generating layer 612 and forms the mechanical strength of the fixing belt 61 as a whole. In addition, the base material layer 611 is made of a material having a physical property (relative magnetic permeability, specific resistance) that allows the magnetic field to pass therethrough so that the AC magnetic field generated by the IH heater 80 acts to the temperature-sensitive magnetic member 64, and the thickness. Formed with. On the other hand, the base material layer 611 itself is configured not to generate heat or hardly generate heat due to the action of a magnetic field.
Specifically, as the base material layer 611, for example, a nonmagnetic metal such as nonmagnetic stainless steel having a thickness of 30 to 200 μm (preferably 50 to 150 μm), a resin material having a thickness of 60 to 200 μm, or the like is used.

The conductive heating layer 612 is an example of a conductive layer, and is an electromagnetic induction heating element layer that is electromagnetically heated by an alternating magnetic field generated by the IH heater 80. That is, the conductive heat generating layer 612 is a layer that generates an eddy current when the AC magnetic field from the IH heater 80 passes in the thickness direction.
In general, a general-purpose power source that can be manufactured at low cost is used as a power source for an excitation circuit 88 (see also FIG. 6 below) that supplies an AC current to the IH heater 80. Therefore, the frequency of the alternating magnetic field generated by the IH heater 80 is generally 20 k to 100 kHz by a general-purpose power source. Thereby, the conductive heat generating layer 612 is configured such that an alternating magnetic field having a frequency of 20 k to 100 kHz enters and passes therethrough.

The region where the alternating magnetic field can enter the conductive heat generating layer 612 is defined as “skin depth (δ)”, which is a region where the alternating magnetic field attenuates to 1 / e, and is derived from the following equation (1). (1) In the equation, f is the AC magnetic field frequency (e.g., 20 kHz), [rho is resistivity (Omega · m), the mu _r is the relative permeability.
Therefore, the thickness of the conductive heat generating layer 612 is determined by the skin depth (δ) of the conductive heat generating layer 612 defined by the equation (1) such that an alternating magnetic field having a frequency of 20 k to 100 kHz penetrates and passes through the conductive heat generating layer 612. It is configured in a thinner layer. Further, as a material constituting the conductive heat generating layer 612, for example, a metal such as Au, Ag, Al, Cu, Zn, Sn, Pb, Bi, Be, Sb, or a metal alloy thereof is used.

Specifically, a nonmagnetic metal such as Cu having a thickness of 2 to 20 μm and a specific resistance of 2.7 × 10 ⁻⁸ Ω · m or less (relative magnetic permeability is approximately 1) is used as the conductive heating layer 612.
Further, from the viewpoint of shortening the time required for the fixing belt 61 to be heated to the fixing set temperature (hereinafter referred to as “warm-up time”), the conductive heat generating layer 612 is preferably formed as a thin layer.

Next, the elastic layer 613 is composed of a heat-resistant elastic body such as silicone rubber. The toner image held on the sheet P to be fixed is formed by laminating each color toner as powder. Therefore, in order to supply heat uniformly to the entire toner image at the nip portion N, it is preferable that the surface of the fixing belt 61 is deformed following the unevenness of the toner image on the paper P. Therefore, for example, silicone rubber having a thickness of 100 to 600 μm and a hardness of 10 ° to 30 ° (JIS-A) is suitable for the elastic layer 613.
Since the surface release layer 614 is in direct contact with the unfixed toner image held on the paper P, a material having a high release property is used. For example, PFA (tetrafluoroethylene perfluoroalkyl vinyl ether polymer), PTFE (polytetrafluoroethylene), silicone copolymer, or a composite layer thereof is used. If the thickness of the surface release layer 614 is too thin, it is not sufficient in terms of wear resistance, and the life of the fixing belt 61 is shortened. On the other hand, if it is too thick, the heat capacity of the fixing belt 61 becomes too large and the warm-up time becomes long. Therefore, the thickness of the surface release layer 614 is preferably 1 to 50 μm in consideration of the balance between wear resistance and heat capacity.

<Description of pressing pad>
The pressing pad 63 is made of an elastic body such as silicone rubber or fluorine rubber, and is supported by the holder 65 at a position facing the pressure roll 62. Then, it is arranged in a state of being pressed from the pressure roll 62 via the fixing belt 61, and a nip portion N is formed with the pressure roll 62.
The pressing pad 63 includes a pre-nip region 63a on the inlet side of the nip portion N (upstream side in the conveyance direction of the paper P) and a peeling nip region 63b on the outlet side of the nip portion N (downstream side in the conveyance direction of the paper P). Different nip pressures are set. That is, in the pre-nip region 63 a, the surface on the pressure roll 62 side is formed in an arc shape that substantially follows the outer peripheral surface of the pressure roll 62, thereby forming a uniform and wide nip portion N. Further, the peeling nip region 63b is formed so as to be locally pressed from the surface of the pressure roll 62 with a large nip pressure so that the radius of curvature of the fixing belt 61 passing through the peeling nip region 63b becomes small. As a result, a curl (down curl) in a direction away from the surface of the fixing belt 61 is formed on the paper P passing through the peeling nip region 63b to promote the peeling of the paper P from the surface of the fixing belt 61.

  In the present embodiment, a peeling assisting member 173 is arranged on the downstream side of the nip portion N as a peeling assisting means by the pressing pad 63. The peeling auxiliary member 173 is supported by the holder 172 in a state where the peeling baffle 171 is close to the fixing belt 61 in a direction opposite to the rotational movement direction of the fixing belt 61 (so-called counter direction). The curled portion formed on the paper P at the outlet of the pressing pad 63 is supported by the peeling baffle 171 to suppress the paper P from moving toward the fixing belt 61.

<Description of temperature-sensitive magnetic member>
Next, the temperature-sensitive magnetic member 64 is formed in an arc shape that follows the inner peripheral surface of the fixing belt 61, and a predetermined gap (for example, 0.5 to 1.5 mm) from the inner peripheral surface of the fixing belt 61. Are arranged close to each other but in a non-contact manner. The temperature-sensitive magnetic member 64 is disposed close to the fixing belt 61 because the temperature of the temperature-sensitive magnetic member 64 changes corresponding to the temperature of the fixing belt 61, that is, the temperature of the temperature-sensitive magnetic member 64 is changed. This is because the temperature is substantially the same as the temperature 61. Further, the temperature-sensitive magnetic member 64 is disposed in a non-contact manner with the fixing belt 61 because the heat of the fixing belt 61 is increased when the main switch of the image forming apparatus 1 is turned on and the fixing belt 61 is heated to the fixing set temperature. This is to prevent the temperature from flowing into the temperature-sensitive magnetic member 64 and shorten the warm-up time.

Further, the temperature-sensitive magnetic member 64 has a “permeability change start temperature”, which is a temperature at which the magnetic permeability of the magnetic characteristics changes suddenly, equal to or higher than a preset fixing temperature at which each color toner image is melted. Or a material set in a temperature range lower than the heat resistant temperature of the surface release layer 614. That is, the temperature-sensitive magnetic member 64 is made of a material having a characteristic (“temperature-sensitive magnetism”) that reversibly changes between ferromagnetic and non-magnetic (paramagnetic) in a temperature range including the fixing set temperature. . As a result, the temperature-sensitive magnetic member 64 functions as a magnetic path forming member in a temperature range that is equal to or lower than the permeability change start temperature exhibiting ferromagnetism, and induces magnetic field lines generated by the IH heater 80 and transmitted through the fixing belt 61 to the inside. Thus, a magnetic path passing through the inside of the temperature-sensitive magnetic member 64 is formed. The temperature-sensitive magnetic member 64 forms a closed magnetic path that encloses the fixing belt 61 and the exciting coil 82 of the IH heater 80 (see FIG. 6 at a later stage). On the other hand, in the temperature range exceeding the permeability change start temperature, the temperature-sensitive magnetic member 64 crosses the magnetic field lines generated by the IH heater 80 and transmitted through the fixing belt 61 in the thickness direction of the temperature-sensitive magnetic member 64. Make it transparent. Thereby, the magnetic lines of force generated by the IH heater 80 and transmitted through the fixing belt 61 form a magnetic path that passes through the temperature-sensitive magnetic member 64, passes through the inside of the guide member 66, and returns to the IH heater 80.
The temperature-sensitive magnetism of the temperature-sensitive magnetic member 64 will be described in detail later.

As a material used for the temperature-sensitive magnetic member 64, a binary system such as an Fe—Ni alloy (permalloy), for example, in which a magnetic permeability change start temperature is set in a range of 140 ° C. to 240 ° C. used as a fixing set temperature, for example. A ternary temperature-sensitive magnetic alloy such as a temperature-sensitive magnetic alloy or an Fe—Ni—Cr alloy is used. For example, in a Fe-Ni binary temperature-sensitive magnetic alloy, the magnetic permeability change start temperature can be set to around 225 ° C. by setting it to about Fe 64% and Ni 36% (atomic ratio). Such metal alloys such as permalloy and temperature-sensitive magnetic alloy are suitable for the temperature-sensitive magnetic member 64 because they are excellent in moldability and workability, have high heat conductivity, and are inexpensive. As other materials, a metal alloy made of Fe, Ni, Si, B, Nb, Cu, Zr, Co, Cr, V, Mn, Mo or the like is used.
Further, the temperature-sensitive magnetic member 64 is formed thicker than the skin depth δ (see the above formula (1)) with respect to the AC magnetic field (lines of magnetic force) generated by the IH heater 80. Specifically, for example, when an Fe—Ni alloy is used, the thickness is set to about 50 to 300 μm.

<Description of holder>
The holder 65 that supports the pressing pad 63 is made of a material having high rigidity so that the amount of bending in a state where the pressing pad 63 receives the pressing force from the pressing roll 62 becomes a certain amount or less. Thereby, the uniformity of the pressure in the longitudinal direction (nip pressure N) at the nip portion N is maintained. Furthermore, since the fixing unit 60 according to the present embodiment employs a configuration in which the fixing belt 61 is heated using electromagnetic induction, the holder 65 is made of a material that does not affect or hardly gives influence to the induced magnetic field. It is made of a material that is not affected or hardly affected by the induced magnetic field. For example, a heat-resistant resin such as glass-mixed PPS (polyphenylene sulfide) or a nonmagnetic metal material such as Al, Cu, or Ag is used.

<Description of induction member>
The induction member 66 is formed in an arc shape that follows the inner peripheral surface of the temperature-sensitive magnetic member 64, and has a predetermined gap (for example, 1.0 to 5.0 mm) from the inner peripheral surface of the temperature-sensitive magnetic member 64. Having a non-contact arrangement. The induction member 66 is made of a nonmagnetic metal having a relatively small specific resistance value, such as Ag, Cu, or Al. Then, when the temperature-sensitive magnetic member 64 rises to a temperature equal to or higher than the permeability change start temperature, an alternating magnetic field (line of magnetic force) generated by the IH heater 80 is induced, and the vortex is more vortexed than the conductive heating layer 612 of the fixing belt 61. A state in which the current I is easily generated is formed. Thereby, the thickness of the induction member 66 is formed with a thickness (for example, 1.0 mm) sufficiently thicker than the skin depth δ (see the above formula (1)) so that the eddy current I flows easily. .

<Description of Fixing Belt Drive Mechanism>
Next, a driving mechanism for the fixing belt 61 will be described.
As shown in FIG. 2 which is a front view, the fixing belt 61 is rotated in the circumferential direction while maintaining the cross-sectional shape of both ends of the fixing belt 61 in a circular shape at both axial ends of the holder 65 (see FIG. 3). An end cap member 67 to be driven is fixed. The fixing belt 61 directly receives the rotational driving force from both ends via the end cap member 67, and rotates and moves in the direction of arrow C in FIG. 3 at a process speed of 140 mm / s, for example.
5A is a side view of the end cap member 67, and FIG. 5B is a plan view of the end cap member 67 viewed from the Z direction. As shown in FIG. 5, the end cap member 67 is formed with a fixing portion 67 a fitted inside the both ends of the fixing belt 61 and has an outer diameter larger than that of the fixing portion 67 a, and when the end cap member 67 is attached to the fixing belt 61. Rotating through a flange portion 67d formed so as to project radially from the fixing belt 61, a gear portion 67b to which rotational driving force is transmitted, a support portion 65a formed at both ends of the holder 65, and a coupling member 166. A bearing bearing portion 67c that is freely coupled is provided. Then, as shown in FIG. 2, the support portions 65a at both ends of the holder 65 are fixed to both ends of the casing 69 of the fixing unit 60, whereby the end cap member 67 is coupled to the support portion 65a. It is rotatably supported via the bearing bearing portion 67c.
As a material constituting the end cap member 67, so-called engineering plastics having high mechanical strength and heat resistance are used. For example, phenol resin, polyimide resin, polyamide resin, polyamideimide resin, PEEK resin, PES resin, PPS resin, LCP resin and the like are suitable.

As shown in FIG. 2, in the fixing unit 60, the rotational driving force from the drive motor 90 is transmitted to the shaft 93 via the transmission gears 91 and 92, and both are transmitted from the transmission gears 94 and 95 coupled to the shaft 93. It is transmitted to the gear portion 67b (see FIG. 5) of the end cap member 67. As a result, a rotational driving force is transmitted from the end cap member 67 to the fixing belt 61, and the end cap member 67 and the fixing belt 61 are integrally rotated.
Thus, the fixing belt 61 rotates by receiving the driving force directly from both ends of the fixing belt 61, so that the fixing belt 61 rotates stably.

Here, when the fixing belt 61 rotates by receiving a driving force directly from the end cap members 67 at both ends, a torque of about 0.1 to 0.5 N · m is generally applied. However, in the fixing belt 61 of the present embodiment, the base material layer 611 is made of, for example, nonmagnetic stainless steel having high mechanical strength. For this reason, even when a torsional torque of about 0.1 to 0.5 N · m acts on the entire fixing belt 61, buckling or the like hardly occurs in the fixing belt 61.
Further, the flange portion 67d of the end cap member 67 suppresses the deviation of the fixing belt 61. In general, the fixing belt 61 at that time is generally 1 to 5 in the axial direction from the end portion (flange portion 67d) side. A compressive force of about 5N works. However, even when the fixing belt 61 receives such a compressive force, since the base material layer 611 of the fixing belt 61 is made of nonmagnetic stainless steel or the like, occurrence of buckling or the like is suppressed.
As described above, the fixing belt 61 according to the present embodiment rotates by receiving a driving force directly from both end portions of the fixing belt 61, and thus stable rotation is performed. At that time, the base material layer 611 of the fixing belt 61 is made of, for example, non-magnetic stainless steel having high mechanical strength, thereby realizing a structure in which buckling or the like hardly occurs against torsion torque or compression force. is doing. Furthermore, since the base material layer 611 and the conductive heat generating layer 612 are formed in a thin layer to ensure the flexibility / flexibility of the fixing belt 61 as a whole, deformation and shape restoration following the nip portion N are prevented. Done.

Returning to FIG. 3, the pressure roll 62 is arranged so as to face the fixing belt 61, and rotates in the direction of arrow D in FIG. 3 at a process speed of 140 mm / s, for example, following the fixing belt 61. Then, a nip portion N is formed in a state where the fixing belt 61 is sandwiched between the pressure roll 62 and the pressing pad 63, and the sheet P holding the unfixed toner image is passed through the nip portion N, so that the heat and pressure To fix the unfixed toner image on the paper P.
The pressure roll 62 includes, for example, a solid aluminum core (cylindrical metal core) 621 having a diameter of 18 mm, and a heat-resistant elastic body layer 622 such as a silicone sponge having a thickness of 5 mm, which is coated on the outer peripheral surface of the core 621. Further, for example, a release layer 623 made of a heat-resistant resin coating such as PFA containing carbon having a thickness of 50 μm or a heat-resistant rubber coating is laminated. Then, the pressing pad 63 is pressed through the fixing belt 61 with a load of 25 kgf, for example, by a pressing spring 68 (see FIG. 2).

<Description of IH heater>
Next, the IH heater 80 that performs electromagnetic induction heating by applying an AC magnetic field to the conductive heat generating layer 612 of the fixing belt 61 will be described.
FIG. 6 is a cross-sectional view illustrating the configuration of the IH heater 80 of the present embodiment. As shown in FIG. 6, the IH heater 80 includes a support 81 made of a nonmagnetic material such as a heat resistant resin, and an exciting coil 82 that generates an alternating magnetic field. Further, an elastic support member 83 made of an elastic body that fixes the excitation coil 82 on the support 81 and a magnetic core 84 that forms a magnetic path of an alternating magnetic field generated by the excitation coil 82 are provided. Furthermore, a shield 85 that shields the magnetic field, a pressure member 86 that pressurizes the magnetic core 84 toward the support 81, and an excitation circuit 88 that supplies an alternating current to the excitation coil 82 are provided.

The support 81 is formed in a shape whose cross section is curved along the surface shape of the fixing belt 61, and an upper surface (supporting surface) 81 a that supports the exciting coil 82 has a predetermined gap (for example, 0) from the surface of the fixing belt 61. 0.5 to 2 mm). Further, the support 81 has a pair of magnetic core support portions 81b disposed in parallel to the longitudinal direction at the center of the support surface 81a in the moving direction of the fixing belt 61, and both ends of the support surface 81a in the moving direction of the fixing belt 61. A magnetic core restricting portion 81 c that restricts the arrangement position of the magnetic core 84 in the moving direction of the fixing belt 61 is arranged. The pair of magnetic core support portions 81b support the magnetic core 84 so as to be movable forward and backward in the moving direction of the fixing belt 61 between the magnetic core restricting portions 81c provided at both ends of the support surface 81a. As a result, the gap between the magnetic core 84 and the support surface 81a, the shape of which is likely to vary in shape due to heat treatment during manufacturing, is substantially symmetric in the upstream region and the downstream region with the center in the moving direction of the fixing belt 61 as the center. The magnetic core 84 is supported.
Examples of the material constituting the support 81 include heat-resistant resins such as heat-resistant glass, polycarbonate, polyethersulfone, and PPS (polyphenylene sulfide), or heat-resistant resins obtained by mixing glass fibers with these materials. A non-magnetic material is used.

The exciting coil 82 is configured by winding, for example, 90 litz wires, which are bundled with, for example, 90 copper wires having a diameter of 0.17 mm and wound in a closed loop with a hollow shape such as an ellipse, an ellipse, or a rectangle. . Then, when an alternating current having a predetermined frequency is supplied to the exciting coil 82 from the exciting circuit 88, an alternating magnetic field centered around a litz wire wound in a closed loop is generated around the exciting coil 82. . Generally, the frequency of the alternating current supplied from the excitation circuit 88 to the excitation coil 82 is 20 k to 100 kHz generated by the general-purpose power source.
The elastic support member 83 is a sheet-like member made of an elastic body such as silicone rubber or fluorine rubber. The elastic support member 83 is set to press the excitation coil 82 against the support 81 so that the excitation coil 82 is fixed in close contact with the support surface 81a of the support 81.

The magnetic core 84 is made of, for example, a ferromagnetic material made of a high-permeability oxide or alloy material such as soft ferrite, ferrite resin, amorphous alloy (amorphous alloy), permalloy, or temperature-sensitive magnetic alloy. It functions as a path forming means. The magnetic core 84 induces a magnetic force line (magnetic flux) generated by the alternating magnetic field generated by the exciting coil 82, and crosses the fixing belt 61 from the magnetic core 84 toward the temperature-sensitive magnetic member 64. A path of magnetic lines of force (magnetic path) is formed so as to pass through and return to the magnetic core 84. That is, the AC magnetic field generated by the excitation coil 82 is configured to pass through the inside of the magnetic core 84 and the inside of the temperature-sensitive magnetic member 64 so that the magnetic lines of force wrap the fixing belt 61 and the excitation coil 82 inside. A closed magnetic circuit is formed. As a result, the magnetic field lines of the alternating magnetic field generated by the exciting coil 82 are concentrated in a region facing the magnetic core 84 of the fixing belt 61.
Here, the magnetic core 84 is preferably made of a material having a small loss due to magnetic path formation. Specifically, the magnetic core 84 is desirably used in a form that reduces the eddy current loss (current path interruption or division by slits, thin plate bundling, etc.), and is preferably formed of a material having a small hysteresis loss.
Further, the length of the magnetic core 84 along the rotation direction of the fixing belt 61 is configured to be smaller than the length of the temperature-sensitive magnetic member 64 along the rotation direction of the fixing belt 61. Thereby, the leakage of magnetic lines of force to the periphery of the IH heater 80 is reduced, and the power factor is improved. Furthermore, electromagnetic induction to the metal member constituting the fixing unit 60 is suppressed, and the heat generation efficiency of the fixing belt 61 (conductive heat generation layer 612) is increased.

<Description of the state in which the fixing belt generates heat>
Subsequently, a state in which the fixing belt 61 generates heat by the alternating magnetic field generated by the IH heater 80 will be described.
First, as described above, the permeability change start temperature of the temperature-sensitive magnetic member 64 is within a temperature range that is not less than the set fixing temperature for fixing each color toner image and not more than the heat resistance temperature of the fixing belt 61 (for example, 140 to 240 ° C.). When the temperature of the fixing belt 61 is equal to or lower than the magnetic permeability change start temperature, the temperature of the temperature-sensitive magnetic member 64 adjacent to the fixing belt 61 is also started corresponding to the temperature of the fixing belt 61. Below temperature. Therefore, since the temperature-sensitive magnetic member 64 exhibits ferromagnetism, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 pass through the fixing belt 61 and then pass through the inside of the temperature-sensitive magnetic member 64 along the spreading direction. To form a magnetic path. Here, the “spreading direction” means a direction orthogonal to the thickness direction of the temperature-sensitive magnetic member 64.

  FIG. 7 is a diagram for explaining the state of the lines of magnetic force (H) when the temperature of the fixing belt 61 is in the temperature range equal to or lower than the permeability change start temperature. As shown in FIG. 7, when the temperature of the fixing belt 61 is in a temperature range equal to or lower than the permeability change start temperature, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 are transmitted through the fixing belt 61. A magnetic path passing through the inside of the temperature-sensitive magnetic member 64 along the spreading direction (direction orthogonal to the thickness direction) is formed. Therefore, the number of magnetic field lines H (magnetic flux density) per unit area in the region crossing the conductive heat generating layer 612 of the fixing belt 61 increases.

  That is, after the magnetic field lines H are radiated from the magnetic core 84 of the IH heater 80 and pass through the regions R1 and R2 across the conductive heat generating layer 612 of the fixing belt 61, the magnetic field lines H enter the inside of the temperature-sensitive magnetic member 64 which is a ferromagnetic material. Be guided. Therefore, the magnetic field lines H crossing the conductive heat generating layer 612 of the fixing belt 61 in the thickness direction are concentrated so as to enter the inside of the temperature-sensitive magnetic member 64, and the magnetic flux density in the regions R1 and R2 increases. Further, even when the magnetic field lines H that have passed through the inside of the temperature-sensitive magnetic member 64 along the spreading direction return to the magnetic core 84 again, in the region R3 that crosses the conductive heating layer 612 in the thickness direction, the magnetic field in the temperature-sensitive magnetic member 64 is increased. It is concentrated toward the magnetic core 84 from the lower part. Therefore, the magnetic force lines H that cross the conductive heat generating layer 612 of the fixing belt 61 in the thickness direction are concentrated from the temperature-sensitive magnetic member 64 toward the magnetic core 84, and the magnetic flux density in the region R3 is also increased.

In the conductive heating layer 612 of the fixing belt 61 where the magnetic lines H cross in the thickness direction, an eddy current I proportional to the amount of change in the number of magnetic lines H per unit area (magnetic flux density) is generated. Thereby, as shown in FIG. 7, a large eddy current I is generated in the regions R1, R2 and R3 where the amount of change in magnetic flux density is large. The eddy current I generated in the conductive heat generation layer 612 generates Joule heat W (W = I ² R), which is the product of the specific resistance value R of the conductive heat generation layer 612 and the square of the eddy current I. Thereby, a large Joule heat W is generated in the conductive heat generating layer 612 where the large eddy current I is generated.
As described above, when the temperature of the fixing belt 61 is in the temperature range equal to or lower than the permeability change start temperature, large heat is generated in the regions R1 and R2 and the region R3 where the lines of magnetic force H cross the conductive heat generating layer 612. Thereby, the fixing belt 61 is heated.

  By the way, in the fixing unit 60 of the present embodiment, the temperature-sensitive magnetic member 64 is disposed in the vicinity of the fixing belt 61 on the inner peripheral surface side of the fixing belt 61. As a result, the magnetic core 84 that guides the magnetic force lines H generated by the exciting coil 82 to the inside and the temperature-sensitive magnetic member 64 that guides the magnetic force lines H transmitted through the fixing belt 61 in the thickness direction are close to each other. The configuration is realized. For this reason, the AC magnetic field generated by the IH heater 80 (excitation coil 82) forms a loop with a short magnetic path, so that the magnetic flux density and the magnetic coupling degree in the magnetic path increase. Accordingly, when the temperature of the fixing belt 61 is in a temperature range equal to or lower than the magnetic permeability change start temperature, heat is more efficiently generated in the fixing belt 61.

<Description of function for suppressing temperature rise in non-sheet passing area of fixing belt>
Next, the function of suppressing the temperature rise in the non-sheet passing area of the fixing belt 61 will be described.
First, a case where small-size paper P (small-size paper P1) is continuously passed through the fixing unit 60 will be described. FIG. 8 is a diagram showing an outline of the temperature distribution in the width direction of the fixing belt 61 when the small size paper P1 is continuously fed. In FIG. 8, the maximum sheet passing area which is the maximum size width (for example, A3 width) of the sheet P used in the image forming apparatus 1 is Ff, and the small size sheet P1 (for example, smaller than the maximum size sheet P) (for example, , A4 (vertical feed) passes through the area (small size paper passing area) as Fs, and the non-sheet passing area through which the small size paper P1 does not pass is Fb. In the image forming apparatus 1, it is assumed that the sheet is passed based on the center position.

  As shown in FIG. 8, when the small size paper P1 is continuously passed, heat for fixing is consumed in the small size paper passing area Fs through which the small size paper P1 passes. Therefore, the temperature adjustment control at the fixing set temperature is performed by the control unit 31 (see FIG. 1), and the temperature of the fixing belt 61 in the small size paper passing area Fs is maintained within the range near the fixing set temperature. On the other hand, temperature adjustment control similar to that of the small-size paper passing area Fs is performed also in the non-paper passing area Fb. However, heat for fixing is not consumed in the non-sheet passing area Fb. For this reason, the temperature of the non-sheet passing area Fb is likely to rise to a temperature higher than the fixing set temperature. Then, when the continuous passage of the small size paper P1 is continued in this state, the temperature of the non-sheet passing region Fb rises, for example, higher than the heat resistance temperature of the elastic layer 613 and the surface release layer 614 of the fixing belt 61, 61 may be damaged.

  Therefore, as described above, in the fixing unit 60 of the present embodiment, the temperature-sensitive magnetic member 64 is equal to or higher than the preset fixing temperature and is, for example, equal to or lower than the heat resistance temperature of the elastic layer 613 and the surface release layer 614 of the fixing belt 61. For example, an Fe—Ni alloy or the like having a magnetic permeability change start temperature set within the temperature range is used. That is, as shown in FIG. 8, the magnetic permeability change start temperature Tcu of the temperature-sensitive magnetic member 64 is equal to or higher than the fixing set temperature Tf, for example, a temperature equal to or lower than the heat resistance temperature Tlim of the elastic layer 613 and the surface release layer 614. It is set in the area.

Accordingly, when the small size paper P1 is continuously passed, the temperature in the non-sheet passing region Fb of the fixing belt 61 exceeds the magnetic permeability change start temperature of the temperature-sensitive magnetic member 64. Accordingly, the temperature in the non-sheet passing region Fb of the temperature-sensitive magnetic member 64 adjacent to the fixing belt 61 also exceeds the permeability change start temperature in the same manner as the fixing belt 61 corresponding to the temperature of the fixing belt 61. For this reason, the temperature-sensitive magnetic member 64 in the non-sheet-passing region Fb has a relative magnetic permeability close to 1, and the properties as a ferromagnetic material disappear. The relative magnetic permeability of the temperature-sensitive magnetic member 64 decreases and approaches 1 so that the magnetic field lines H in the non-sheet-passing region Fb are not guided into the temperature-sensitive magnetic member 64 but pass through the temperature-sensitive magnetic member 64. become. Therefore, in the non-sheet passing region Fb of the fixing belt 61, the magnetic field lines H after passing through the conductive heat generating layer 612 are diffused, and the magnetic flux density of the magnetic field lines H crossing the conductive heat generating layer 612 is reduced. Thereby, the eddy current I generated in the conductive heat generation layer 612 is reduced, and the heat generation amount (Joule heat W) in the fixing belt 61 is reduced. As a result, an excessive temperature rise in the non-sheet passing area Fb is suppressed, and damage to the fixing belt 61 is suppressed.
As described above, the temperature-sensitive magnetic member 64 functions as a detection unit that detects the temperature of the fixing belt 61 and suppresses an increase in temperature that suppresses an excessive temperature increase of the fixing belt 61 according to the detected temperature of the fixing belt 61. It also has a function as a department.

  The lines of magnetic force H after passing through the temperature-sensitive magnetic member 64 reach the guide member 66 (see FIG. 3) and are guided into this. When the magnetic flux reaches the induction member 66 and is induced therein, more eddy current I flows toward the induction member 66 where the eddy current I flows more easily than the conductive heat generation layer 612. Therefore, the amount of eddy current flowing in the conductive heat generating layer 612 is further suppressed, and the temperature rise in the non-sheet passing region Fb is suppressed.

  At this time, the thickness, material, and shape of the guiding member 66 are selected so that the guiding member 66 guides most of the magnetic force lines H from the exciting coil 82 and suppresses leakage of the magnetic force lines H from the fixing unit 60. . Specifically, the guide member 66 may be made of a material having a sufficiently thick skin depth δ. Thereby, even if the eddy current I flows through the induction member 66, the amount of heat generation is also minimized. In the present embodiment, the guide member 66 is made of Al (aluminum) having a substantially circular shape with a thickness of 1 mm along the temperature-sensitive magnetic member 64, and is not in contact with the temperature-sensitive magnetic member 64 (an average distance of, for example, 4 mm). ). As other materials, Ag and Cu are suitable.

  By the way, when the temperature in the non-sheet-passing region Fb of the fixing belt 61 becomes lower than the magnetic permeability change start temperature of the temperature-sensitive magnetic member 64, the temperature in the non-sheet-passing region Fb of the temperature-sensitive magnetic member 64 is also permeable. It becomes lower than the change start temperature. As a result, the temperature-sensitive magnetic member 64 changes to ferromagnetic again, and the magnetic field lines H are induced inside the temperature-sensitive magnetic member 64, so that a large amount of eddy current I flows through the conductive heating layer 612. Therefore, the fixing belt 61 is heated again.

  FIG. 9 is a diagram illustrating the state of the lines of magnetic force H when the temperature of the fixing belt 61 in the non-sheet passing region Fb is in the temperature range exceeding the permeability change start temperature. As shown in FIG. 9, when the temperature of the fixing belt 61 is in the temperature range exceeding the permeability change start temperature in the non-sheet passing area Fb, the temperature-sensitive magnetic member 64 in the non-sheet passing area Fb has a ratio. Magnetic permeability decreases. Therefore, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 change so as to easily pass through the temperature-sensitive magnetic member 64. Accordingly, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 (excitation coil 82) are radiated so as to diffuse from the magnetic core 84 toward the fixing belt 61 and reach the induction member 66.

That is, in the regions R1 and R2 where the magnetic force lines H are radiated from the magnetic core 84 of the IH heater 80 and cross the conductive heat generating layer 612 of the fixing belt 61, the magnetic force lines H are difficult to be guided to the temperature-sensitive magnetic member 64 and thus diffuse radially. As a result, the magnetic flux density (number of magnetic force lines H per unit area) of the magnetic field lines H crossing the conductive heat generating layer 612 of the fixing belt 61 in the thickness direction decreases. Further, even in the region R3 that crosses the conductive heat generating layer 612 in the thickness direction when the magnetic force line H returns to the magnetic core 84 again, the magnetic force line H returns to the magnetic core 84 from the diffused wide region. The magnetic flux density of the magnetic field lines H crossing 612 in the thickness direction decreases.
Therefore, when the temperature of the fixing belt 61 is in a temperature range exceeding the permeability change start temperature, the magnetic flux density of the magnetic field lines H that cross the conductive heating layer 612 in the thickness direction decreases in the regions R1, R2, and R3. It becomes. As a result, the eddy current I generated in the conductive heating layer 612 where the magnetic field lines H cross in the thickness direction is reduced, and the Joule heat W generated in the fixing belt 61 is reduced. As a result, the temperature of the fixing belt 61 decreases.

As described above, when the temperature of the fixing belt 61 in the non-sheet-passing area Fb is in the temperature range equal to or higher than the magnetic permeability change start temperature, the magnetic field lines H are induced inside the temperature-sensitive magnetic member 64 in the non-sheet-passing area Fb. The magnetic field lines H of the alternating magnetic field generated by the exciting coil 82 cross the conductive heat generating layer 612 of the fixing belt 61 while diffusing in the thickness direction. Therefore, the magnetic path of the alternating magnetic field generated by the exciting coil 82 forms a long loop, and the magnetic flux density in the magnetic path passing through the conductive heating layer 612 of the fixing belt 61 decreases.
Thereby, for example, in the non-sheet passing region Fb where the small size paper P1 is continuously passed and the temperature rises, the eddy current I generated in the conductive heat generating layer 612 of the fixing belt 61 is reduced and the fixing belt 61 is not passed. The amount of heat generation (joule heat W) in the paper region Fb is reduced. As a result, an excessive temperature rise in the non-sheet passing area Fb can be suppressed.

<Explanation on temperature-sensitive magnetism of temperature-sensitive magnetic member>
Here, the “temperature-sensitive magnetism” of the temperature-sensitive magnetic member 64 will be described.
The temperature-sensitive magnetic member 64 of the present embodiment starts a change from a temperature at which the magnetic permeability (for example, the magnetic permeability measured by JIS C2531) reaches the above-described “permeability change start temperature” to a decrease, Has a magnetic property that continues to decrease until reaching a Curie Point (“CP”), which is a temperature at which the magnetism disappears. The magnetic characteristic of the temperature-sensitive magnetic member 64 that reversibly changes between ferromagnetism and non-magnetism (paramagnetism) in a specific temperature range is referred to as “temperature-sensitive magnetism”.

  In the temperature-sensitive magnetic member 64 of the present embodiment, the “permeability change start temperature” is equal to or higher than the temperature (fixing set temperature) set in the fixing belt 61 for fixing each color toner image on the paper. It is comprised with the material set in the temperature range lower than the heat-resistant temperature of (the elastic layer 613 or the surface release layer 614). Accordingly, since the temperature-sensitive magnetic member 64 exhibits ferromagnetism in the fixing set temperature region, as shown in FIG. 7, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 (see FIG. 3) are After passing through the fixing belt 61, a magnetic path is formed that passes through the inside of the temperature-sensitive magnetic member 64 along the spreading direction (direction perpendicular to the thickness direction). As a result, the magnetic flux density of the magnetic field lines H crossing the fixing belt 61 is increased (regions R1, R2, and R3 in FIG. 7), and a large amount of heat is generated in the fixing belt 61.

  On the other hand, in the temperature range exceeding the magnetic permeability change start temperature, the magnetic permeability of the temperature-sensitive magnetic member 64 reaches the Curie point CP and decreases until the relative permeability becomes 1. As a result, when the temperature of the non-sheet-passing area (for example, the non-sheet-passing area Fb in FIG. 8) of the fixing belt 61 exceeds the fixing set temperature area, the temperature-sensitive magnetic member 64 in the area facing the non-sheet-passing area is not. Since it changes to magnetism (paramagnetism), the magnetic flux density of the magnetic field lines H crossing the fixing belt 61 decreases according to the temperature change (regions R1, R2, R3 in FIG. 9), and the amount of heat generation decreases. Thereby, the temperature rise in the non-sheet passing region of the fixing belt 61 is suppressed.

  In this case, in order to effectively suppress the temperature rise in the non-sheet passing region of the fixing belt 61, the magnetic permeability of the temperature-sensitive magnetic member 64 is directed toward the Curie point CP in the temperature region exceeding the magnetic permeability change start temperature. Therefore, it is preferable to have a characteristic of sharply decreasing. That is, the temperature region (“transition region”) in which the magnetic characteristics of the temperature-sensitive magnetic member 64 transition between ferromagnetic and non-magnetic (paramagnetic) is set within a certain narrow temperature range, for example, within 20 ° C. Thereby, the temperature rise suppression function in the non-sheet passing region by the temperature-sensitive magnetic member 64 is enhanced.

Here, FIG. 10 is a diagram showing an example of the temperature characteristic regarding the relative permeability μ _r of the temperature-sensitive magnetic member 64 of the present embodiment. As shown in FIG. 10, the relative permeability μ _r of the temperature-sensitive magnetic member 64 of the present embodiment is a linear function F _{1 as the} temperature T rises in the temperature range up to the permeability change start temperature TP1. The tendency which increases gradually according to (T) is shown. However, the relative magnetic permeability mu _r starts decreasing when the temperature T of the temperature-sensitive magnetic member 64 is greater than the permeability change start temperature TP1, then steeply according to the primary function F ₂ with increasing temperature T _(T) To decrease. Then, when the temperature T of the temperature-sensitive magnetic member 64 reaches the Curie point CP (TP4), the relative permeability mu _r of the temperature-sensitive magnetic member 64 1 (magnetic permeability of the temperature-sensitive magnetic member 64 mu = mu _0: μ ₀ = vacuum permeability).

In this embodiment, as quantitatively evaluating index transition region between the ferromagnetic and non-magnetic in the temperature characteristic of the relative permeability mu _r of such temperature-sensitive magnetic member 64, the index temperature TP2 and index temperature Let us define TP3. As shown in FIG. 10, the index temperature TP2 is an example of a starting temperature of the transition region, first representing the relationship between the temperature T and relative permeability mu _r in a temperature range up to the permeability change start temperature TP1 Is a linear function F ₁ (T) that is an example of the function of, and a linear function that is an example of a second function that represents the relationship between the temperature T in the temperature range exceeding the permeability change start temperature TP1 and the relative permeability μ _r. It is the temperature of the intersection with F ₂ (T).
The index temperature TP3 is an example of the end temperature of the transition region, and is the temperature at the intersection of the linear function F ₂ (T) and the relative permeability μ _r = 1.
Incidentally, as shown in FIG. 10, in a temperature range exceeding the permeability change start temperature TP1, although area relative permeability mu _r decreases with multidimensional function is present, which is an example of a second function The linear function F ₂ (T) represents the relationship between the temperature T and the relative permeability μ _r in the region where the linear function decreases in the temperature range exceeding the permeability change start temperature TP1.

The index temperature TP2 is a temperature at which the magnetic characteristic of the temperature-sensitive magnetic member 64 starts to change from ferromagnetic to non-magnetic (paramagnetic), and the temperature-sensitive magnetic member 64 is substantially equal to that of the fixing belt 61. It can be understood as a temperature at which the function of suppressing the temperature rise in the non-paper passing region starts to appear (function expression temperature). That is, the temperature of the temperature-sensitive magnetic member 64 be greater than the permeability change start temperature TP1, to a temperature region decreases as a linear function F _{2 (T),} the decrease in the relative permeability mu _r is not large. Therefore, the temperature-sensitive magnetic member 64 functions as a ferromagnetism in a temperature range that is equal to or higher than the magnetic permeability change start temperature TP1 and is equal to or lower than a temperature range that decreases with the linear function F ₂ (T). Therefore, the index temperature TP2 can be regarded as a function expression temperature at which the function of the temperature-sensitive magnetic member 64 starts to be substantially realized.
On the other hand, the relative permeability mu _r of temperature T and relative permeability mu _r and is a linear function F ₂ the temperature-sensitive magnetic member 64 in the hottest region part of the temperature range to be reduced by _(T) is almost 1. Therefore, the index temperature TP3 can be regarded as a substantial Curie point CP.
Therefore, the temperature region between the index temperature TP3 and the index temperature TP2 can be defined as a “transition region” in which the magnetic characteristics of the temperature-sensitive magnetic member 64 transition between ferromagnetic and nonmagnetic (paramagnetic). .

In the temperature-sensitive magnetic member 64 of the present embodiment, the temperature range between the index temperature TP3 and the index temperature TP2 (the difference temperature between the index temperature TP3 and the index temperature TP2) that is the transition region of the magnetic characteristics is 20 °. The magnetic characteristics in which the permeability (relative permeability μ _r ) sharply decreases in the temperature range exceeding the permeability change start temperature are realized. For example, the temperature-sensitive magnetic member 64 illustrated in FIG. 10 is configured such that the index temperature TP2 is, for example, 205 ° C. and the index temperature TP3 is, for example, 216 ° C., and the temperature width between the index temperature TP3 and the index temperature TP2 is 20 °. It is set to 11 ° which is within.
As described above, by configuring the transition region of the magnetic characteristics (temperature range between the index temperature TP3 and the index temperature TP2) within 20 °, the temperature of the non-sheet passing region of the fixing belt 61 is 20 ° (from the index temperature TP2). If the angle changes beyond 11 ° in the example of FIG. 10, the magnetic characteristics of the temperature-sensitive magnetic member 64 in the non-sheet-passing region change to non-magnetic (paramagnetic). As a result, even if the fixing set temperature is set to a high value of, for example, about 160 ° C. to 180 ° C., the occurrence of damage in the fixing belt 61 having a heat resistant temperature of about 220 ° C. is suppressed.

In other words, considering the function of the temperature-sensitive magnetic member 64, it is ideal that the temperature width (magnetic property transition region) between the index temperature TP3 and the index temperature TP2 is zero. However, in practice, the material constituting the temperature-sensitive magnetic member 64 has a transition region having a certain temperature range in which the magnetic characteristics transition between ferromagnetic and nonmagnetic (paramagnetic). If the temperature range of the transition region is large, even if the fixing belt 61 exceeds the fixing set temperature, the magnetic characteristics of the temperature-sensitive magnetic member 64 change slowly toward non-magnetism (paramagnetism). As a result, the time difference until the temperature of the fixing belt 61 in the non-sheet-passing area, where the temperature has risen above the fixing set temperature, decreases toward the fixing set temperature increases, and the temperature rise suppression in the non-sheet-passing area effectively works. hard. Also, for example, when the paper size is changed to a large size, the speed at which the temperature of the fixing belt 61 rises to the fixing set temperature also decreases in the area that has newly changed from the non-sheet passing area to the sheet passing area. It tends to occur.
Therefore, although it is ideal that the temperature range of the transition region between ferromagnetic and non-magnetic is 0, the permissible range in which the temperature rise in the non-sheet passing region of the fixing belt 61 can be effectively suppressed is considered. The transition region (temperature range between the index temperature TP3 and the index temperature TP2) of the magnetic characteristics in the warm magnetic member 64 is set within 20 °.

<Description of the configuration for suppressing the temperature rise of the temperature-sensitive magnetic member>
Next, a configuration for suppressing the temperature rise of the temperature-sensitive magnetic member 64 itself will be described.
In order for the temperature-sensitive magnetic member 64 to perform the function of suppressing an excessive temperature rise in the non-sheet-passing region (for example, the non-sheet-passing region Fb in FIG. 8) of the fixing belt 61, the longitudinal region of the temperature-sensitive magnetic member 64 is used. Each temperature changes corresponding to the temperature of each region in the longitudinal direction of the fixing belt 61 opposed to the temperature, and it is necessary to function as a detection unit that detects the temperature of the fixing belt 61 described above.
Therefore, regarding the temperature-sensitive magnetic member 64 itself, a configuration that is difficult to be induction-heated by the magnetic field lines H is adopted. That is, even if the temperature of the fixing belt 61 is equal to or lower than the permeability change start temperature and the temperature-sensitive magnetic member 64 exhibits ferromagnetism, the temperature-sensitive magnetic member 64 is included in the magnetic force lines H from the IH heater 80. There is a magnetic field line H that crosses in the thickness direction. As a result, a weak eddy current I is generated inside the temperature-sensitive magnetic member 64, and a slight amount of heat is generated in the temperature-sensitive magnetic member 64 itself. For this reason, for example, when a large amount of image formation is continuously performed, the self-heat generated heat is accumulated in the temperature-sensitive magnetic member 64, and the temperature of the temperature-sensitive magnetic member 64 is also in the paper passing area (see FIG. 8). Shows an upward trend. Thus, when the self-heating due to eddy current loss is large, the temperature of the temperature-sensitive magnetic member 64 rises and unintentionally reaches the temperature at which the permeability change starts, and the magnetic characteristics of the paper passing area and the non-paper passing area are improved. The difference is almost eliminated and the temperature rise suppression effect may not be obtained sufficiently. Therefore, the correspondence between the temperature of the temperature-sensitive magnetic member 64 and the temperature of the fixing belt 61 is maintained, and the temperature-sensitive magnetic member 64 functions as a detection unit that detects the temperature of the fixing belt 61 with high accuracy. It is necessary to suppress the Joule heat W generated in the member 64 itself.

Therefore, in order to reduce the eddy current loss and hysteresis loss in the temperature-sensitive magnetic member 64, first, the temperature-sensitive magnetic member 64 has physical properties (specific resistance value and magnetic permeability) that are difficult to be induction-heated by the lines of magnetic force H. Selected material is selected.
Second, the thickness of the temperature-sensitive magnetic member 64 exhibits ferromagnetism so that the magnetic field lines H are difficult to cross in the thickness direction of the temperature-sensitive magnetic member 64 at least in the temperature range below the permeability change start temperature. It is formed thicker (larger) than the skin depth δ in the state.

  Third, the temperature-sensitive magnetic member 64 intersects (orthogonally) at 90 ° with respect to a direction orthogonal to the moving direction of the fixing belt 61 so as to divide the flow of the eddy current I generated by the magnetic field lines H. A plurality of slits 64s (see FIG. 11 at a later stage) are formed as an example of the cutout portion in the angle direction. Even if the material and thickness of the temperature-sensitive magnetic member 64 are selected so that induction heating is difficult, it is difficult to set the eddy current I generated in the temperature-sensitive magnetic member 64 to zero. Therefore, by dividing the flow of the eddy current I generated in the temperature-sensitive magnetic member 64 by the plurality of slits 64s, the eddy current I is reduced and the Joule heat W generated in the temperature-sensitive magnetic member 64 is kept low. Yes.

  FIG. 11 is a view showing slits 64 s formed in the temperature-sensitive magnetic member 64. FIG. 11A is a side view showing a state in which the temperature-sensitive magnetic member 64 is installed in the holder 65, and FIG. 11B is a plan view seen from above (a direction). As shown in FIG. 11, the temperature-sensitive magnetic member 64 is orthogonal to the direction in which the eddy current I generated by the magnetic field lines H flows (direction perpendicular to the moving direction (x direction) of the fixing belt 61 (y direction)). A plurality of slits 64s are formed. Therefore, when there is no slit 64s, the eddy current I (broken line in FIG. 11B) that flows as a large eddy over the entire length of the temperature-sensitive magnetic member 64 is divided by the slit 64s. Accordingly, when the slit 64s is formed, the eddy current I flowing through the temperature-sensitive magnetic member 64 (solid line in FIG. 11B) is divided into the respective regions ("divided regions") divided by the slits 64s and 64s. ), And the amount of eddy current I as a whole is reduced. As a result, the amount of heat generated by the temperature-sensitive magnetic member 64 (Joule heat W) is reduced, and a configuration that hardly generates heat is realized. As described above, the plurality of slits 64 s utilize the fact that the self-heating amount is reduced by dividing the eddy current I generated in the temperature-sensitive magnetic member 64 for each divided region, and thus the self-heating of the temperature-sensitive magnetic member 64. It functions as a heat generation adjustment unit that adjusts the amount.

By the way, the amount of self-heating generated by the eddy current I generated in each divided region of the temperature-sensitive magnetic member 64 changes corresponding to the interval (mm) between the slits 64s adjacent to each other formed in the temperature-sensitive magnetic member 64. .
Here, FIG. 12 is a diagram showing the relationship between the interval between adjacent slits 64 s formed in the temperature-sensitive magnetic member 64 and the temperature of the temperature-sensitive magnetic member 64 in the paper passing region. FIG. 12 shows a case where the temperature setting magnetic member 64 having the temperature sensitivity shown in FIG. 10 is used and the fixing set temperature of the fixing belt 61 is set to 160 ° C.
As shown in FIG. 12, the temperature T of the temperature-sensitive magnetic member 64 in the sheet passing region has a characteristic that changes with a linear function G (s) with respect to the interval s between the slits 64s. When the interval s between adjacent slits 64s is 13 mm or more, the amount of self-heating increases, and the temperature T of the temperature-sensitive magnetic member 64 in the sheet passing region exceeds 205 ° C., which is the index temperature TP2. Thereby, as shown in FIG. 10, since the relative permeability mu _r is reduced even in a sheet passing area, the temperature of the fixing belt 61 in the paper passing area is reduced similarly to the non-paper passing area, fixing The set temperature is not maintained. Therefore, it is difficult to perform sufficient fixing even in the paper passing area.

On the other hand, since the amount of self-heating is reduced in the region where the interval s of the slits 64s is 9.75 mm or less, the temperature T of the temperature-sensitive magnetic member 64 in the paper passing region is 200 ° C. or less near the permeability change start temperature TP1. It becomes. That is, by setting the interval s between the slits 64 s to 9.75 mm or less, the amount of self-heating is suppressed so that the temperature T of the temperature-sensitive magnetic member 64 in the paper passing region is equal to or lower than the permeability change start temperature TP1. As a result, the temperature of the fixing belt 61 is maintained in a region near the fixing set temperature, and good fixing is performed in the paper passing region.
In this case, variations in the temperature of the fixing belt 61, overshooting during warm-up (a phenomenon in which the temperature of the fixing belt 61 temporarily exceeds the upper limit of the fixing set temperature), and the like occur. In view of this, it is possible to obtain a safety value (so-called margin) of about 5 ° from the magnetic permeability change start temperature TP1 with respect to the temperature T of the temperature-sensitive magnetic member 64 in the sheet passing region. It is preferable for stabilizing.

Therefore, in the present embodiment, the temperature T of the temperature-sensitive magnetic member 64 is 10 ° or more lower than the index temperature TP2 (205 ° C.) that is the start temperature of the transition region in the magnetic characteristics of the temperature-sensitive magnetic member 64. An interval s between adjacent slits 64s is set. That is, as shown in FIG. 12, from the linear function G (s) indicating the relationship between the interval s between the slits 64s and the temperature T of the temperature-sensitive magnetic member 64 in the paper passing region, and the index temperature TP2 (205 ° C.). Is also set to 6.8 mm or less, which is the interval between the slits 64s intersecting 195 ° C., which is 10 ° lower.
As a result, the temperature T of the temperature-sensitive magnetic member 64 is suppressed within a range having a margin of 10 ° or more until the transition region where the magnetic characteristics change suddenly, so that the temperature of the fixing belt 61 is fixed in the paper passing region. It is maintained in the vicinity of the set temperature. On the other hand, since the temperature rise due to self-heating is suppressed even in the non-sheet passing region, the temperature-sensitive magnetic member 64 functions as a detection unit that detects the temperature of the fixing belt 61 with high accuracy. Therefore, when the temperature of the non-sheet passing area of the fixing belt 61 exceeds the preset fixing temperature, the temperature increase of the fixing belt 61 is suppressed.

  At that time, the interval s between the slits 64s is set to 6.8 mm or less, and the temperature T of the temperature-sensitive magnetic member 64 is adjusted to be 10 ° or more lower than the start temperature (index temperature TP2) of the transition region. In the configuration in which the transition region (temperature range between the index temperature TP3 and the index temperature TP2) of the temperature-sensitive magnetic member 64 is set within 20 °, and the temperature rise suppression of the fixing belt 61 in the non-sheet passing region is enhanced. It is particularly effective.

  That is, by setting the transition region of the magnetic characteristics within 20 °, in the non-sheet passing region, when the temperature T of the temperature-sensitive magnetic member 64 reaches the transition region, the temperature of the fixing belt 61 is thereafter within 20 °. The temperature of the fixing belt 61 decreases rapidly only by increasing within the range. Therefore, the temperature rise of the fixing belt 61 in the non-sheet passing region is suppressed with high sensitivity. However, on the other hand, such a rapid temperature drop of the fixing belt 61 is likely to occur in the paper passing region as well. For example, when the self-heating amount of the temperature-sensitive magnetic member 64 is large, the temperature T of the temperature-sensitive magnetic member 64 becomes higher than the temperature of the fixing belt 61, and the temperature of the fixing belt 61 in the sheet passing region is the temperature-sensitive magnetic member. The temperature T of the temperature-sensitive magnetic member 64 reaches the transition region even though it does not reach the transition region of 64. In such a case, only a relatively small temperature fluctuation within 20 ° caused by a temperature change of the fixing belt 61 or self-heating of the temperature-sensitive magnetic member 64 itself occurs in the temperature-sensitive magnetic member 64, so In this case, the temperature of the fixing belt 61 rapidly decreases. Thereby, in the configuration in which the temperature rise suppression of the fixing belt 61 in the non-sheet passing region is enhanced, it is highly necessary to manage the self-heat generation amount of the temperature-sensitive magnetic member 64.

  On the other hand, in the present embodiment, the interval s between the slits 64s is set to 6.8 mm or less to reduce the self-heating amount of the temperature-sensitive magnetic member 64, whereby the magnetic characteristic of the temperature-sensitive magnetic member 64 is 20 °. The temperature of the temperature-sensitive magnetic member 64 is maintained at a temperature close to the temperature of the fixing belt 61 even if the transition region is within the range. That is, even in the configuration in which the magnetic characteristics of the temperature-sensitive magnetic member 64 have a transition region of 20 ° or less, the temperature-sensitive magnetic member 64 functions as a detection unit that detects the temperature of the fixing belt 61 with high accuracy. The temperature T of the temperature-sensitive magnetic member 64 is maintained in the vicinity of the fixing set temperature corresponding to the fixing belt 61. As a result, even in the configuration in which the temperature rise suppression of the fixing belt 61 in the non-sheet passing region is increased, the temperature of the temperature-sensitive magnetic member 64 is separated from the temperature of the fixing belt 61 and reaches the transition region in the sheet passing region. The rise is suppressed, and the fixing performance is stably maintained.

  By the way, when the interval s between the slits 64s is reduced, the rigidity of the temperature-sensitive magnetic member 64 is lowered, and it tends to be difficult to keep the gap between the temperature-sensitive magnetic member 64 and the fixing belt 61 constant. . Therefore, a lower limit exists in the interval s between the slits 64 s in terms of the rigidity of the temperature-sensitive magnetic member 64. In the experiments of the present inventor, from the viewpoint of the rigidity of the temperature-sensitive magnetic member 64, the gap between the temperature-sensitive magnetic member 64 and the fixing belt 61 is set by setting the interval s of the slits 64s to 3.5 mm or more. It has been confirmed that the rigidity that can be stably maintained can be secured. Therefore, in this embodiment, the interval s between the slits 64 s is set to 3.5 mm or more to ensure the rigidity for stably maintaining the gap with the fixing belt 61.

  In the temperature-sensitive magnetic member 64 illustrated in FIG. 11, the slit 64s is formed to be orthogonal (x direction) to the direction in which the eddy current I flows (y direction orthogonal to the moving direction of the fixing belt 61). As long as the flow of the eddy current I is divided, for example, a slit that intersects the direction of the eddy current I in an angular direction other than 90 ° (crossing angle with the y direction) may be formed. In addition to the configuration in which the slits 64 s as shown in FIG. 11 are formed over the entire region in the width direction of the temperature-sensitive magnetic member 64, the slit 64 s may be formed in a partial region in the width direction of the temperature-sensitive magnetic member 64. . Further, the number of slits, the position, the crossing angle, and the like may be set according to the amount of heat generated in the temperature-sensitive magnetic member 64.

  As described above, in the fixing unit 60 provided in the image forming apparatus 1 of the present embodiment, the temperature-sensitive magnetic member 64 is disposed in the vicinity of the inner peripheral surface of the fixing belt 61. The temperature-sensitive magnetic member 64 has a mutual interval of 3.5 mm or more and 6.8 mm or less as a heat generation adjustment unit that adjusts the eddy current I generated in the temperature-sensitive magnetic member 64 to divide and reduce self-heating. A set slit 64s is formed. As a result, the temperature of the fixing belt 61 is maintained near the fixing set temperature, and the stability of fixing is improved. Further, the temperature rise in the non-sheet passing area of the fixing belt 61 is suppressed.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 60 ... Fixing unit, 61 ... Fixing belt, 62 ... Pressure roll, 64 ... Temperature-sensitive magnetic member, 64s ... Slit, 66 ... Induction member, 80 ... IH heater, 82 ... Excitation coil, 84 ... Magnetic core, 611 ... base material layer, 612 ... conductive heating layer

Claims (9)

A fixing member having a conductive layer and fixing the toner to the recording material by electromagnetic induction heating of the conductive layer;
A magnetic field generating member that generates an alternating magnetic field that intersects the conductive layer of the fixing member;
The temperature in the region through which the recording material passes is arranged so as to face the magnetic field generating member with the fixing member interposed therebetween, than the transition region in which the magnetic characteristics transition between ferromagnetism and paramagnetism depending on the temperature. A heat generation adjustment unit that adjusts the amount of self-heating to be maintained in a low temperature range is formed, and includes a magnetic path forming member that forms a magnetic path of an alternating magnetic field generated by the magnetic field generation member. Fixing device.   The magnetic path forming member is composed of a plurality of cutout portions formed in an angle direction intersecting the direction orthogonal to the moving direction of the fixing member, and the cutout portions adjacent to each other. The fixing device according to claim 1, wherein an interval between the notch portions is set to 3.5 mm or more and 6.8 mm or less along a direction orthogonal to a moving direction of the fixing member.   The fixing device according to claim 1, wherein the magnetic path forming member is configured such that a temperature width of the transition region relating to magnetic characteristics is 20 ° or less.   In the magnetic path forming member, a start temperature of the transition region is set higher than a temperature set in the fixing member as a temperature for melting and fixing the toner on the recording material to the recording material. The fixing device according to claim 3.   The fixing device according to claim 1, wherein the magnetic path forming member is configured to have a thickness larger than a skin depth in a temperature region lower than the transition region. Toner image forming means for forming a toner image;
Transfer means for transferring the toner image formed by the toner image forming means onto a recording material;
Fixing means for fixing the toner image transferred onto the recording material to the recording material;
The fixing means is
A fixing member having a conductive layer and fixing the toner to the recording material by electromagnetic induction heating of the conductive layer;
A magnetic field generating member that generates an alternating magnetic field that intersects the conductive layer of the fixing member;
An alternating current generated by the magnetic field generation member that is disposed opposite to the magnetic field generation member with the fixing member interposed therebetween and has a transition region in which the magnetic characteristics transition between ferromagnetism and paramagnetism according to temperature. A magnetic path forming member for forming a magnetic path of the magnetic field;
A heat generation adjustment unit that is disposed on the magnetic path forming member and adjusts the amount of self-heating so that the temperature of the magnetic path forming member in a region through which the recording material passes is maintained in a temperature range lower than the transition region; An image forming apparatus comprising:   The heat generation adjusting unit of the fixing unit includes a plurality of cutout portions in the magnetic path forming member formed in an angle direction intersecting with a direction orthogonal to the moving direction of the fixing member, The image forming apparatus according to claim 6, wherein an interval between the notch portions adjacent to each other is set to 3.5 mm or more and 6.8 mm or less along a direction orthogonal to a moving direction of the fixing member.   The image forming apparatus according to claim 6, wherein the magnetic path forming member of the fixing unit is configured such that a temperature width of the transition region relating to magnetic characteristics is 20 ° or less.   The image forming apparatus according to claim 6, wherein the magnetic path forming member of the fixing unit is configured to have a thickness larger than a skin depth in a temperature region lower than the transition region.

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