JP5532646B2 - Fixing device and image forming apparatus - Google Patents

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 region in an induction heating type fixing device.

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. A magnetic field generating member to be generated; and a magnetic field generating member disposed opposite to the magnetic field generating member with the fixing member interposed therebetween, wherein the magnetic property transitions between ferromagnetism and paramagnetism depending on temperature. A magnetic path forming member that forms a magnetic path of an alternating magnetic field generated by the generating member, and the magnetic path forming member is disposed in a region through which a recording material having a minimum width among recording materials to be used passes. A first component and a second component disposed in a region other than a region through which a recording material having a minimum width in the recording material to be used passes, the first component and the second component Each of the transition regions has a permeability change at which the permeability begins to decrease. The first function that defines the correspondence between temperature and relative permeability in the temperature range up to the start temperature and the correspondence between temperature and relative permeability in the temperature range that exceeds the permeability change start temperature Defined in a region between a starting temperature that is an intersection with the second function and an ending temperature at which the relative permeability in the second function is 1, and the first component is the second component In comparison, the start temperature and the end temperature of the transition region are configured to be high , and the end temperature of the transition region in the first component is configured to be higher than the heat-resistant temperature of the fixing member, In the fixing device, the end temperature of the transition region in the second component is configured to be lower than a heat resistant temperature of the fixing member .

According to a second aspect of the present invention, in the fixing device according to the first aspect, the first function and the second function are linear functions.
The invention according to claim 3 is characterized in that the magnetic path forming member is configured such that the start temperature of the transition region of the first component is lower than the heat resistant temperature of the fixing member. The fixing device described.
The invention according to claim 4 further includes a temperature detection member for detecting the temperature of the fixing member, and the temperature detection member is provided in a region through which the recording material having the minimum width in the recording material to be used passes. The fixing device according to claim 1, wherein the fixing device is arranged.
According to a fifth aspect of the present invention, each of the first component portion and the second component portion of the magnetic path forming member is formed in an angular direction intersecting with the width direction of the fixing member. The fixing device according to claim 1, wherein a plurality of notches are formed.

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 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. And a magnetic path forming member that forms a magnetic path of an alternating magnetic field generated by the magnetic field generating member, and the magnetic path forming member is formed of a recording material to be used. In the area through which the smallest width recording material passes. The first component and the second component disposed in a region other than the region through which the recording material having the smallest width among the recording materials to be used passes, the first component and the second component Each of the transition regions of the portion includes a first function defining a correspondence relationship between the temperature and the relative permeability in a temperature range up to a permeability change start temperature at which the permeability starts to decrease, and the permeability change start temperature. Between the start temperature that is the intersection of the second function that defines the correspondence between the temperature and the relative permeability in a temperature range that exceeds the value, and the end temperature at which the relative permeability in the second function is 1 The first component portion is configured such that the start temperature and the end temperature of the transition region are higher than the second component portion, and the transition region in the first component portion is Is the heat resistance temperature of the fixing member. Remote higher configured, an image forming apparatus, wherein the finish temperature of the transition region in the second component is configured lower than the heat resistant temperature of the fixing member.

According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, the first function and the second function are linear functions.
The invention according to claim 8 is characterized in that the magnetic path forming member of the fixing unit is configured such that the start temperature of the transition region of the first component is lower than a heat resistant temperature of the fixing member. The image forming apparatus according to claim 6 .

According to the first aspect of the present invention, it is possible to suppress an excessive temperature rise in the non-sheet-passing region in the induction heating type fixing device as compared with the case where the present invention is not adopted.
According to the second aspect of the present invention, it is possible to determine the fixing set temperature set for the fixing member by a simple method as compared with the case where the present invention is not adopted.
According to the invention of claim 3, as compared with the case where the present invention is not adopted, the fixing performance can be improved within a temperature range in which the fixing member is not damaged in the sheet passing region.
According to the fourth aspect of the present invention, the temperature can be adjusted according to the temperature of the fixing member in the paper passing area, and the temperature of the paper passing area can be more accurately set to the fixing set temperature than when the present invention is not adopted. Can be set.
According to the fifth aspect of the present invention, it is possible to suppress an excessive temperature rise of the magnetic path forming member as compared with the case where the present invention is not adopted.

According to the sixth aspect of the present invention, it is possible to suppress an excessive temperature rise in the non-sheet passing region in the induction heating type fixing device mounted on the image forming apparatus, as compared with the case where the present invention is not adopted.
According to the seventh aspect of the present invention, it is possible to determine the fixing set temperature set for the fixing member by a simple method as compared with the case where the present invention is not adopted.
According to the eighth aspect of the present invention, it is possible to improve the fixing performance within a temperature range in which the fixing member is not damaged in the sheet passing region, 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 cross-sectional view taken along line XX 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 the slit formed in a temperature sensitive magnetic member. It is a perspective view explaining the structure of the longitudinal direction of a temperature-sensitive magnetic member. It is the figure which showed an example of the temperature characteristic regarding the relative magnetic permeability of the temperature-sensitive magnetic member arrange | positioned at a small size paper passage area | region. It is the figure which showed an example of the temperature characteristic regarding the relative magnetic permeability of the temperature-sensitive magnetic member arrange | positioned in the non-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 XX 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 sheet and a peeling assisting member 173 that assists in peeling the paper P from the fixing belt 61 are provided.

<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 2.5 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.

  The temperature-sensitive magnetic member 64 is formed of a so-called “temperature-sensitive magnetic material” having a transition region in which the magnetic characteristics reversibly transition between ferromagnetic and non-magnetic (paramagnetic) depending on temperature. Further, the transition region of the magnetic characteristics in the temperature-sensitive magnetic member 64 is within a temperature range (for example, 140 to 240 ° C.) that is not less than the fixing set temperature for fixing each color toner image and not more than the heat resistance temperature of the fixing belt 61. Is set. The temperature-sensitive magnetic member 64 functions as a magnetic path forming member in a temperature range exhibiting ferromagnetism, and induces a magnetic force line generated by the IH heater 80 and transmitted through the fixing belt 61 to the inside, so that the temperature-sensitive magnetic member 64 A magnetic path passing through the inside of the is formed. As a result, 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 exhibiting non-magnetism, the temperature-sensitive magnetic member 64 transmits the magnetic force lines generated by the IH heater 80 and transmitted through the fixing belt 61 so as to cross the thickness direction of the temperature-sensitive magnetic member 64. 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.

Temperature-sensitive magnetic materials are roughly classified into metal materials and oxide materials, but oxide materials (for example, soft ferrite) are difficult to be thinned (300 μm or less) and are cracked because they are ceramics. Very unwieldy. In particular, if it is thinned, it is easy to break, and if it is thinned because of a fired product, the dimensional accuracy is likely to be lowered and mass production is difficult. If the thickness cannot be reduced, the heat capacity becomes large, and the thermal conductivity of ceramics is low. Therefore, there are problems such as difficulty in following the temperature change of the fixing belt 61 and difficulty in controlling the heat generation at the target value. In order to solve this problem, in this embodiment, Fe-Ni binary thermosensitive magnetism that is inexpensive, can be easily thinned and molded, has good workability, flexibility, and high thermal conductivity. It is desirable to use an alloy or a Fe-Ni-Cr ternary thermosensitive magnetic alloy. Specifically, the temperature-sensitive magnetic alloy has a temperature at which the magnetic permeability starts to change (“permeability change start temperature”), which is used as a range from the fixing setting to the heat resistant temperature or less, for example, within a range of 140 ° C. to 240 ° C. Binary temperature-sensitive magnetic alloy such as Fe-Ni alloy (permalloy) or ternary temperature-sensitive magnetic alloy such as Fe-Ni-Cr alloy, etc. In a 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 with a thickness greater 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. If the thickness is equal to or greater than the skin depth δ, the magnetic path of the main magnetic flux can be formed. Therefore, in order for the temperature-sensitive magnetic member 64 to detect (sense) the temperature when the fixing belt 61 is heated with high sensitivity, the heat capacity From the viewpoint of, it is desirable that it is thinner.

<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 the driving force directly from both ends of the fixing belt 61, so that 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. 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 and 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 around the central portion in the moving direction of the fixing belt 61. 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 an oxide or alloy material having a high magnetic permeability 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 transition region where the magnetic characteristic of the temperature-sensitive magnetic member 64 transitions between ferromagnetic and non-magnetic (paramagnetic) is equal to or higher than the fixing set temperature for fixing each color toner image, and the fixing belt. It is set within a temperature range (for example, 140 to 240 ° C.) that is equal to or lower than the heat resistant temperature of 61. When the temperature of the fixing belt 61 is below the transition region, the temperature of the temperature-sensitive magnetic member 64 adjacent to the fixing belt 61 also corresponds to the temperature below the transition region corresponding to the temperature of the fixing belt 61. Become. 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 below the transition region. As shown in FIG. 7, when the temperature of the fixing belt 61 is in the temperature range equal to or lower than the transition region, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 pass through the fixing belt 61 and are temperature-sensitive magnetism. A magnetic path that passes through the inside of the 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 transition region, 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 of non-sheet passing portion of fixing belt>
Next, the function of suppressing the temperature rise at the non-sheet passing portion 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, and the fixing belt. 61 may be damaged.

  Therefore, as described above, in the fixing unit 60 of the present embodiment, the temperature-sensitive magnetic member 64 has a transition region in which the magnetic characteristics change between ferromagnetic and nonmagnetic (paramagnetic) at or above the fixing set temperature. For example, it is made of, for example, an Fe—Ni alloy or the like set within a temperature range 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. That is, as shown in FIG. 8, the transition region Tcu of the temperature-sensitive magnetic member 64 is set to a temperature region that is equal to or higher than the fixing set temperature Tf and is equal to or lower than the heat resistance temperature Tlim of the elastic layer 613 and the surface release layer 614, for example. Has been.

Thereby, when the small size paper P1 is continuously passed, the temperature in the non-sheet passing area Fb of the fixing belt 61 exceeds the transition area in 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 transition region 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 transition region of the temperature-sensitive magnetic member 64 thereafter, the temperature in the non-sheet-passing region Fb of the temperature-sensitive magnetic member 64 is also higher than the temperature in the transition region. Also lower. 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 for explaining the state of the lines of magnetic force H when the temperature of the fixing belt 61 in the non-sheet-passing area Fb is in the temperature range exceeding the transition area of the temperature-sensitive magnetic member 64. As shown in FIG. 9, when the temperature of the fixing belt 61 is in the temperature range exceeding the transition region of the temperature-sensitive magnetic member 64 in the non-sheet-passing region Fb, the temperature-sensitive magnetic member in the non-sheet-passing region Fb. In 64, the relative 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 a temperature range equal to or higher than the transition area of the temperature-sensitive magnetic member 64, the lines of magnetic force are generated 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.

<Description of the configuration for suppressing the temperature rise of the temperature-sensitive magnetic member>
In order for the temperature-sensitive magnetic member 64 to perform the function of suppressing the excessive temperature rise in the non-sheet passing region Fb described above, the temperature of each region in the longitudinal direction of the temperature-sensitive magnetic member 64 is the longitudinal direction of the fixing belt 61 facing it. It is necessary to fulfill a function as a detection unit that detects the temperature of the fixing belt 61 and changes according to the temperature of each region.
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. In other words, even if the temperature of the fixing belt 61 is equal to or lower than the transition region of the temperature-sensitive magnetic member 64 and the temperature-sensitive magnetic member 64 exhibits ferromagnetism, the line of magnetic force H from the IH heater 80 includes Magnetic field lines H that cross the magnetic member 64 in the thickness direction exist. 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 transition region, and there is almost no difference in the magnetic characteristics between the sheet passing region and the non-sheet passing region. The temperature rise suppression effect may not be sufficiently obtained. 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 is in a state of exhibiting 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 transition region. It is formed thicker than the skin depth δ.

  Third, the temperature-sensitive magnetic member 64 is formed with a plurality of slits 64s (see FIG. 10 in the subsequent stage) that divide the flow of the eddy current I generated by the lines of magnetic force H. 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. 10 is a view showing slits 64 s formed in the temperature-sensitive magnetic member 64. FIG. 10A is a side view showing a state in which the temperature-sensitive magnetic member 64 is installed on the holder 65, and FIG. 10B is a plan view seen from above (a direction). As shown in FIG. 10, the temperature-sensitive magnetic member 64 of the present embodiment will be described in detail later, but a plurality of (for example, three) temperature-sensitive magnetic members 64A, 64B, and 64C are arranged along the longitudinal direction. Each of the temperature-sensitive magnetic members 64A, 64B, and 64C is formed with a plurality of slits 64s orthogonal to the direction in which the eddy current I generated by the lines of magnetic force H flows. Therefore, when there is no slit 64s, the eddy current I (dashed line in FIG. 10B) that flows as a large vortex over the entire longitudinal direction of each of the temperature-sensitive magnetic members 64A, 64B, 64C is divided by the slit 64s. Is done. Accordingly, when the slit 64s is formed, the eddy current I (solid line in FIG. 10B) flowing through each of the temperature-sensitive magnetic members 64A, 64B, and 64C is in the region between the slit 64s and the slit 64s. A small eddy is formed, and the amount of eddy current I as a whole is reduced. As a result, the heat generation amount (Joule heat W) in the temperature-sensitive magnetic member 64 (64A, 64B, 64C) is reduced, and a configuration in which heat generation is difficult is realized. Therefore, the plurality of slits 64 s function as an eddy current dividing unit that divides the eddy current I.

In the temperature-sensitive magnetic member 64 (64A, 64B, 64C) illustrated in FIG. 10, the slit 64s is formed perpendicular to the direction in which the eddy current I flows. However, as long as the flow of the eddy current I is divided. For example, a slit inclined with respect to the direction in which the eddy current I flows may be formed. In addition to the configuration in which the slits 64 s as shown in FIG. 10 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 part in the width direction of the temperature-sensitive magnetic member 64. Further, the number, position, inclination angle, and the like of the slits may be set according to the amount of heat generated in the temperature-sensitive magnetic member 64.
Moreover, as a state where the inclination angle of the slit is maximized, the temperature-sensitive magnetic member 64 may be a small piece divided group in which the temperature-sensitive magnetic member 64 is divided into small pieces at the slit portion. The effect of is obtained similarly.

<Explanation regarding the configuration in the longitudinal direction of the temperature-sensitive magnetic member>
Next, the longitudinal configuration of the temperature-sensitive magnetic member 64 will be described.
FIG. 11 is a perspective view illustrating the configuration of the temperature-sensitive magnetic member 64 in the longitudinal direction. As shown in FIG. 11, the temperature-sensitive magnetic member 64 is divided into a plurality of (for example, three) components, that is, temperature-sensitive magnetic members 64A, 64B, and 64C along the longitudinal direction. A temperature-sensitive magnetic member 64A as an example of a component portion disposed in the center portion has a small-size paper P1 (for example, a width-wise arrangement region smaller than the maximum size paper P used in the image forming apparatus 1). The sheet is formed in correspondence with a small-size paper passing area Fs (see FIG. 8) through which A4 vertical feed (minimum width paper used in the image forming apparatus 1) passes. Further, the temperature-sensitive magnetic members 64B and 64C as an example of the components disposed at both ends of the temperature-sensitive magnetic member 64A have a non-sheet-passing region Fb in which the small-size paper P1 does not pass through the longitudinally arranged region. (See FIG. 8).

The temperature-sensitive magnetic member 64A disposed in the small-size paper passing area Fs has a temperature area (transition area) in which the magnetic characteristics change between ferromagnetic and nonmagnetic (paramagnetic) in the non-paper passing area Fb. The temperature sensitive magnetic members 64B and 64C arranged in the temperature region are set in a temperature region higher than the transition region.
Further, both the thermistor 71 and the thermistor 72, which are examples of temperature detecting members that detect the temperature of the fixing belt 61 in correspondence with the magnetic characteristics of each of the temperature-sensitive magnetic members 64 </ b> A, 64 </ b> B, and 64 </ b> C, Is disposed in the region where the temperature-sensitive magnetic member 64A is disposed (see FIG. 11). Furthermore, the thermoswitch 70, which is an example of a blocking member that cuts off the supply of power to the fixing unit 60 when the temperature of the fixing belt 61 abnormally rises, similarly applies temperature-sensitive magnetism in the longitudinal direction of the fixing belt 61. It arrange | positions in the arrangement | positioning area | region of the member 64A (refer FIG. 11). Thereby, the temperature control of the fixing belt 61 is temperature adjustment control at the fixing set temperature based on the temperature detected in the small size paper passing area Fs.

<Explanation about temperature-sensitive magnetism of each temperature-sensitive magnetic member>
Here, the temperature-sensitive magnetism of the temperature-sensitive magnetic member 64 (64A, 64B, 64C) will be described.
The temperature-sensitive magnetic member 64 of the present embodiment starts a change from “magnetic permeability change start temperature”, which is a temperature at which the magnetic permeability of the magnetic properties (for example, the magnetic permeability measured by JIS C2531) suddenly changes, to a decrease. However, it has a magnetic property that continues to decrease until it reaches a Curie Point (“CP”), which is the temperature at which the material loses magnetism. 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”.

Here, FIG. 12 is a diagram showing an example of the temperature characteristic related to the relative magnetic permeability μ _r of the temperature-sensitive magnetic member 64A arranged in the small size paper passing area Fs. As shown in FIG. 12, the relative magnetic permeability μ _r of the temperature-sensitive magnetic member 64A arranged in the small-size paper passing area Fs increases in the temperature T in the temperature area up to the magnetic permeability change starting temperature TP1. Along with this, a tendency of increasing gradually according to the linear function F ₁ (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).
Incidentally, the non-paper passing area Fb to arranged the temperature-sensitive magnetic member 64B, 64C also, the temperature characteristics related to the relative permeability mu _r denotes the same tendency as the temperature-sensitive magnetic member 64A.

In this embodiment, such a temperature-sensitive magnetic member 64 (64A, 64B, 64C) as quantitatively evaluating index transition region between the ferromagnetic and nonmagnetic in the temperature characteristic of the relative permeability mu _r of The index temperature TP2 and the index temperature TP3 are defined. For example, as shown in FIG. 12, the index temperature TP2 is an example of a starting temperature of the transition region, the representative of the relationship between the temperature T and relative permeability mu _r in a temperature range up to the permeability change start temperature TP1 A linear function F ₁ (T) that is an example of the function 1 and a linear function that is an example of a second function that represents the relationship between the temperature T and the relative permeability μ _r in the temperature range exceeding the permeability change start temperature TP1. It is the temperature of the intersection with the function 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. 12, 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-sheet passing portion 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 still functions strongly as a ferromagnetism in a temperature range that is equal to or higher than the permeability change start temperature TP1 and equal to or lower than the 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 transition region (index temperature TP3 and index temperature TP2) of the magnetic characteristics in the temperature-sensitive magnetic member 64A, which is a component disposed in the small-size paper passing area Fs. Is set to a temperature region that is higher than the transition region of the magnetic characteristics of the temperature-sensitive magnetic members 64B and 64C, which are components disposed in the non-sheet passing region Fb.

In the temperature-sensitive magnetic member 64A arranged in the small-size paper passing area Fs, the fixing setting by the control unit 31 (see FIG. 1) is performed based on the temperature of the fixing belt 61 detected by the thermistor 71 and / or the thermistor 72. Temperature adjustment control with temperature is performed. Therefore, the temperature of the fixing belt 61 in the small-size paper passing area Fs is maintained in the vicinity of the fixing set temperature, and exceeds the heat resistance temperature of the fixing belt 61 (heat resistance temperature of the elastic layer 613 and the surface release layer 614). Temperature rise is limited. As described above, the temperature adjustment control restricts the temperature rise exceeding the heat resistance temperature of the fixing belt 61 in the small-size paper passing area Fs. Therefore, the fixing setting temperature of the fixing belt 61 is set within the temperature range of the heat resistance temperature or less. The temperature can be set as close to the heat resistance temperature of the fixing belt 61 as possible. By setting the fixing set temperature in this way, the fixing performance is improved, and for example, more stable fixing is performed even on paper such as thick paper.
In that case, in the temperature-sensitive magnetic member 64, the index temperature TP <b> 2 at which the function of the temperature-sensitive magnetic member 64 starts to substantially appear is brought close to the heat-resistant temperature of the fixing belt 61 in accordance with the fixing set temperature of the fixing belt 61. To set. Thereby, the temperature-sensitive magnetic member 64 exhibits ferromagnetism at the fixing set temperature of the fixing belt 61. Then, as shown in FIG. 7, the magnetic field lines H of the alternating magnetic field generated by the IH heater 80 pass through the fixing belt 61 and then spread in the temperature-sensitive magnetic member 64 (perpendicular to the thickness direction). A magnetic path that passes along (direction). As a result, the magnetic flux density of the magnetic field lines H crossing the fixing belt 61 becomes high (regions R1, R2, and R3 in FIG. 7), and large heat is generated in the fixing belt 61.

On the other hand, since heat for fixing is not consumed in the non-sheet passing area Fb, the temperature of the non-sheet passing area Fb is likely to rise to a temperature higher than the fixing set temperature. Therefore, the index temperature TP2 at the temperature-sensitive magnetic members 64B and 64C arranged in the non-sheet passing area Fb is set to the heat resistant temperature of the fixing belt 61 in the same manner as the temperature-sensitive magnetic member 64A arranged in the small-size sheet passing area Fs. If the small size paper P1 is continuously passed, the temperature of the non-paper passing area Fb immediately rises above the heat resistance temperature of the fixing belt 61, and the fixing belt 61 can be damaged. Increases nature.
Therefore, in the temperature-sensitive magnetic members 64B and 64C arranged in the non-sheet passing region Fb, the index temperature TP3 that can be regarded as a substantial Curie point CP is set to be equal to or lower than the heat resistance temperature of the fixing belt 61. That is, the end temperature of the transition region of the magnetic characteristics in the temperature sensitive magnetic members 64B and 64C is set to be equal to or lower than the heat resistance temperature of the fixing belt 61.

As a result, the small-size paper P1 is continuously passed, and the temperature adjustment is performed within a range in the vicinity of the set fixing temperature in which the temperature of the fixing belt 61 in the small-size paper passing area Fs is set close to the heat resistance temperature of the fixing belt 61. Even if the control is performed, the relative magnetic permeability μ _r of the temperature-sensitive magnetic members 64B and 64C becomes approximately 1 when the temperature reaches the index temperature TP3 set to be equal to or lower than the heat resistance temperature of the fixing belt 61. As a result, the temperature of the non-sheet passing region Fb of the fixing belt 61 changes to nonmagnetic (paramagnetic) before reaching the heat resistance temperature of the fixing belt 61, and the magnetic flux density of the magnetic field lines H crossing the fixing belt 61 changes with temperature. Accordingly, it becomes smaller (regions R1, R2, and R3 in FIG. 9), and the amount of heat generation decreases. Thereby, the temperature rise in the non-sheet passing region Fb of the fixing belt 61 is suppressed.

Here, FIG. 13 is a diagram showing an example of temperature characteristics related to the relative magnetic permeability μ _r of the temperature-sensitive magnetic members 64B and 64C arranged in the non-sheet passing region Fb. In the temperature-sensitive magnetic members 64B and 64C arranged in the non-sheet passing region Fb illustrated in FIG. 13, the transition region of the magnetic characteristics is set to a temperature region where the index temperature TP2 is 231 ° C. and the index temperature TP3 is 240 ° C. ing.
On the other hand, in the temperature-sensitive magnetic member 64A arranged in the small-size paper passing area Fs illustrated in FIG. 12 above, the transition area of the magnetic characteristics is a temperature area where the index temperature TP2 is 240 ° C. and the index temperature TP3 is 249 ° C. Is set to
In this case, it is assumed that the heat resistance temperature of the fixing belt 61 is about 245 ° C. As a result, even if the fixing belt 61 is temperature-adjusted and controlled at a fixing temperature set at, for example, 235 ° C., which is close to the heat-resistant temperature, the temperature adjustment control is performed on the fixing belt 61 detected by the thermistor 71 and / or the thermistor 72. Since the process is performed based on the temperature, the temperature of the fixing belt 61 in the small-size paper passing area Fs is limited to a range that does not exceed the heat resistance temperature.
On the other hand, even if the fixing belt 61 is temperature adjusted and controlled at a fixing set temperature close to the heat resistance temperature, the index temperature TP3 (240 ° C.) below the heat resistance temperature (245 ° C.) of the fixing belt 61 in the non-sheet passing area Fb. in the temperature-sensitive magnetic member 64B, the relative magnetic permeability mu _r becomes 1 of 64C, the calorific value is lowered. Therefore, the non-sheet passing area Fb of the fixing belt 61 is suppressed from reaching the heat resistant temperature.

  As described above, the transition region of the magnetic characteristics in the temperature-sensitive magnetic member 64A arranged in the small-size paper passing area Fs is the magnetic characteristic transition in the temperature-sensitive magnetic members 64B and 64C arranged in the non-paper passing area Fb. By setting the temperature region higher than the transition region, the fixing performance of the fixing belt 61 is enhanced. At the same time, the temperature rise in the non-sheet passing area Fb of the fixing belt 61 is suppressed.

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), which is a transition region of the magnetic characteristics of the temperature-sensitive magnetic members 64A, 64B, and 64C, is within 20 °. With this configuration, a magnetic characteristic in which the permeability (relative permeability μ _r ) sharply decreases in the transition region is realized. For example, in the temperature-sensitive magnetic members 64A, 64B, and 64C illustrated in FIGS. 12 and 13, the temperature width between the index temperature TP3 and the index temperature TP2 is set to 9 ° that is within 20 °. Thus, by configuring the temperature range between the index temperature TP3 and the index temperature TP2 to be within 20 °, the magnetic characteristics of the temperature-sensitive magnetic member 64 particularly in the non-sheet passing region Fb are strong with a narrow temperature range of 20 °. Transition from magnetic to non-magnetic (paramagnetic). As a result, the fixing performance of the fixing belt 61 is further improved, and the temperature rise in the non-sheet passing region Fb of the fixing belt 61 is suppressed.

In the present embodiment, the fixing unit 60 is described in which the temperature-sensitive magnetic member 64 is arranged in a non-contact manner with the fixing belt 61 and the temperature-sensitive magnetic member 64 itself does not easily generate heat. The temperature-sensitive magnetic member 64 is disposed in contact with the fixing belt 61, and the temperature-sensitive magnetic member 64 itself is also applied to the fixing unit 60 configured to generate heat.
In the present embodiment, the configuration using the temperature-sensitive magnetic member 64 composed of two types of components having different magnetic property transition regions in the small-size paper passing region Fs and the non-paper passing region Fb has been described. However, in addition to such a configuration, for example, the small-size paper passing area Fs and the non-paper passing area Fb are each divided into a plurality of (for example, two) constituent parts along the longitudinal direction, so You may employ | adopt the structure which uses the temperature-sensitive magnetic member 64 which consists of a structure part from which the transition area | region of a magnetic characteristic differs in each of the paper area | region Fs and the non-paper passing area | region Fb. That is, you may use the temperature sensitive magnetic member 64 which consists of a 3 or more types of structural part from which the transition area | region of a magnetic characteristic differs.
Further, the temperature-sensitive magnetic member 64 of the present embodiment may be configured integrally with a plurality of components having different magnetic property transition regions, or may be configured separately.

  Further, in the case where the temperature-sensitive magnetic member 64 itself is brought into contact to function as a heating element in order to increase the speed of the fixing unit 60, if sufficient heat generation energy cannot be divided and supplied to the fixing belt 61 side, the temperature-sensitive magnetic member 64 is used. A heating source such as a heater may be separately disposed on the opposite side of the contact surface of the member 64 with the fixing belt 61. In this case, if the heat generating member can be maintained at a temperature about 5 ° C. to 25 ° C. higher than the set temperature of the fixing belt 61 at the time of fixing, the heat supply to the fixing belt 61 is sufficient, and the fixing belt 61 in the sheet passing region of the fixing belt 61 becomes sufficient. It is advantageous for speeding up the fixing unit 60 by suppressing a temperature drop during continuous fixing.

  As described above, in the fixing unit 60 provided in the image forming apparatus 1 of the present embodiment, the transition region of the magnetic characteristics in the temperature-sensitive magnetic member 64A arranged in the small-size paper passing region Fs is not passed. The temperature range is set higher than the transition region of the magnetic characteristics of the temperature-sensitive magnetic members 64B and 64C arranged in the paper region Fb. Thereby, the fixing performance by the fixing belt 61 is enhanced. At the same time, the temperature rise in the non-sheet passing area Fb of the fixing belt 61 is suppressed.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 60 ... Fixing unit, 61 ... Fixing belt, 62 ... Pressure roll, 64 (64A, 64B, 64C) ... Temperature-sensitive magnetic member, 66 ... Induction member, 80 ... IH heater, 82 ... Excitation coil , 84 ... Magnetic core, 611 ... Base material layer, 612 ... Conductive heating layer

Claims (8)

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 that forms a magnetic path of the magnetic field,
The magnetic path forming member has a first component portion disposed in a region through which a recording material having a minimum width in a recording material to be used passes and a recording material having a minimum width in the recording material to be used. A second component arranged in a region other than the region,
The transition regions of the first component and the second component each define a correspondence relationship between temperature and relative permeability in a temperature range up to a permeability change start temperature at which the permeability starts to decrease. A starting temperature that is the intersection of the function of 1 and the second function that defines the correspondence between the temperature and the relative permeability in a temperature range that exceeds the permeability change starting temperature, and the relative permeability of the second function Defined in the region between the end temperature where the magnetic susceptibility is 1,
The first component is configured such that the start temperature and the end temperature of the transition region are higher than the second component , and the end temperature of the transition region in the first component is It is configured to be higher than the heat resistance temperature of the fixing member,
The fixing device , wherein the end temperature of the transition region in the second component is configured to be lower than a heat resistant temperature of the fixing member .   The fixing device according to claim 1, wherein the first function and the second function are linear functions.   The fixing device according to claim 1, wherein the magnetic path forming member is configured such that the start temperature of the transition region of the first component is lower than a heat resistant temperature of the fixing member. A temperature detection member for detecting the temperature of the fixing member;
The fixing device according to claim 1, wherein the temperature detection member is disposed in a region through which the recording material having the minimum width among the recording materials to be used passes.   Each of the first component and the second component of the magnetic path forming member has a plurality of notches formed in an angular direction intersecting the width direction of the fixing member. The fixing device according to claim 1, wherein: 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 that forms a magnetic path of the magnetic field,
The magnetic path forming member has a first component portion disposed in a region through which a recording material having a minimum width in a recording material to be used passes and a recording material having a minimum width in the recording material to be used. A second component arranged in a region other than the region,
The transition regions of the first component and the second component each define a correspondence relationship between temperature and relative permeability in a temperature range up to a permeability change start temperature at which the permeability starts to decrease. A starting temperature that is the intersection of the function of 1 and the second function that defines the correspondence between the temperature and the relative permeability in a temperature range that exceeds the permeability change starting temperature, and the relative permeability of the second function Defined in the region between the end temperature where the magnetic susceptibility is 1,
The first component is configured such that the start temperature and the end temperature of the transition region are higher than the second component , and the end temperature of the transition region in the first component is It is configured to be higher than the heat resistance temperature of the fixing member,
The image forming apparatus , wherein the end temperature of the transition region in the second component is configured to be lower than a heat resistant temperature of the fixing member . The image forming apparatus according to claim 6, wherein the first function and the second function are linear functions. The image forming apparatus according to claim 6 , wherein the magnetic path forming member of the fixing unit is configured such that the start temperature of the transition region of the first component is lower than a heat resistant temperature of the fixing member. .

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