JP2010197676A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly detect the abnormal temperature rise of a fixing member in a fixing device of an induction heating system. <P>SOLUTION: Thermistors 71 and 72 for detecting the temperature of a fixing belt 61 from the inside of the fixing belt 61 are disposed in an area which goes toward on one end part rather than a center position in the lengthwise direction of a thermosensitive magnetic member 64, and a thermoswitch 70 for cutting off power supplied to an IH heater by detecting that the temperature of the fixing belt 61 has exceeded a preliminarily determined temperature from inside the fixing belt 61 is disposed in an area opposite to the area where the thermistors 71 and 72 are disposed, with respect to the center position in the lengthwise direction of the thermosensitive magnetic member 64. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

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

JP 2003-186322 A

Here, in general, the fixing member heated by the electromagnetic induction method is configured by a belt member having a small heat capacity, so that the time required to raise the fixing member to the fixing set temperature (warm-up time) is shortened. On the other hand, since the fixing member constituted by the belt member has a small heat capacity, the temperature is likely to rise. Therefore, it is necessary to quickly detect an abnormal temperature rise exceeding the preset fixing temperature generated in the fixing member.
An object of the present invention is to quickly detect the occurrence of an abnormal temperature rise of a fixing member 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. The magnetic field generating member to be generated is disposed opposite to the magnetic field generating member across the fixing member, and is generated by the magnetic field generating member in a temperature range equal to or lower than the permeability change start temperature at which the magnetic permeability starts decreasing. A magnetic path forming member that forms a magnetic path of an alternating magnetic field and transmits the alternating magnetic field generated by the magnetic field generating member in a temperature range that exceeds the permeability change start temperature, and a longitudinal center position of the magnetic path forming member And a temperature detection member that is disposed in a region toward one end side and detects the temperature of the fixing member from the inside of the fixing member, and the temperature detection member with respect to the longitudinal center position of the magnetic path forming member Area where is placed Is disposed in the opposite region, and includes a blocking member that detects that the temperature of the fixing member has exceeded a predetermined temperature from the inside of the fixing member and blocks power supplied to the magnetic field generating member. This is a fixing device.

According to a second aspect of the present invention, recording is performed between the fixing member and the roll member by being arranged on the inner side of the fixing member and pressing the fixing member against a roll member arranged in contact with the fixing member. A pressing member that forms a nip portion through which the material passes, wherein the temperature detection member and the blocking member are downstream in the moving direction of the fixing member with respect to the arrangement position of the pressing member, and the magnetic path forming member 2. The fixing device according to claim 1, wherein the fixing device is disposed in a region upstream of a position where the magnetic path forming member is disposed, or a region where the magnetic path forming member is disposed.
According to a third aspect of the present invention, in the fixing device according to the first aspect, the temperature detecting member is disposed so as to be pressed toward an inner peripheral surface of the fixing member.
According to a fourth aspect of the present invention, in the blocking member, the whole or part of the portion facing the inner peripheral surface of the fixing member is more than the surface facing the inner peripheral surface of the fixing member of the magnetic path forming member. The fixing device according to claim 1, wherein the fixing device is disposed on the fixing member side.
The invention according to claim 5 is the fixing device according to claim 1, wherein the magnetic path forming member is disposed in non-contact with the fixing member.

  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. Fixing means for fixing the toner image to the recording material, the fixing means including a conductive layer, and a fixing member for 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 permeability change start temperature that is disposed opposite the magnetic field generating member with the fixing member interposed therebetween, and at which the magnetic permeability starts decreasing. A magnetic path that forms a magnetic path of an AC magnetic field generated by the magnetic field generation member in the following temperature range and transmits the AC magnetic field generated by the magnetic field generation member in a temperature range that exceeds the permeability change start temperature. A forming member; A temperature detecting member that is disposed in a region toward one end side from the longitudinal center of the path forming member and detects the temperature of the fixing member from the inside of the fixing member; and a longitudinal center position of the magnetic path forming member The magnetic field generating member is disposed in a region opposite to the region where the temperature detecting member is disposed, and detects that the temperature of the fixing member exceeds a predetermined temperature from the inside of the fixing member. An image forming apparatus comprising: a blocking member that blocks power supplied to the printer.

According to a seventh aspect of the present invention, the fixing unit is disposed inside the fixing member, and the fixing member and the roll member are pressed by pressing the fixing member against a roll member disposed in contact with the fixing member. A pressing member that forms a nip portion through which the recording material passes, and the temperature detecting member and the blocking member of the fixing unit are downstream of the fixing member in the moving direction with respect to the arrangement position of the pressing member. The image forming apparatus according to claim 6, wherein the image forming apparatus is disposed in a region upstream of a position where the magnetic path forming member is disposed, or in a region where the magnetic path forming member is disposed.
The invention according to claim 8 is the image forming apparatus according to claim 6, wherein the temperature detecting member of the fixing unit is disposed so as to be pressed toward an inner peripheral surface of the fixing member. is there.
According to a ninth aspect of the present invention, in the blocking member of the fixing unit, the whole or a part of the portion facing the inner peripheral surface of the fixing member is opposed to the inner peripheral surface of the fixing member of the magnetic path forming member. The image forming apparatus according to claim 6, wherein the image forming apparatus is disposed so as to be positioned closer to the fixing member than the surface to be fixed.
A tenth aspect of the present invention is the image forming apparatus according to the sixth aspect, wherein the magnetic path forming member of the fixing unit is disposed in a non-contact manner with the fixing member.

According to the first aspect of the present invention, it is possible to quickly detect the occurrence of an abnormal temperature rise of the fixing member 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, compared to the case where the present invention is not adopted, the adhesion between the temperature detection member and the fixing member is improved to improve the temperature detection accuracy of the fixing member, and the response performance of the blocking member is improved. Can do.
According to the third aspect of the present invention, it is possible to improve the temperature detection accuracy of the fixing member by improving the adhesion between the temperature detection member and the fixing member, as compared with the case where the present invention is not adopted.
According to the invention of claim 4, the response performance of the blocking member can be enhanced as compared with the case where the present invention is not adopted.
According to the fifth aspect of the present invention, as compared with the case where the present invention is not adopted, when the fixing member is heated to the fixing set temperature, the heat of the fixing member is suppressed from flowing into the magnetic path forming member, thereby fixing the fixing member. The time to reach the set temperature can be shortened.

According to the sixth aspect of the present invention, it is possible to quickly detect the occurrence of an abnormal temperature rise of the fixing member 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 invention of claim 7, compared with the case where the present invention is not adopted, the adhesion between the temperature detection member and the fixing member is improved to improve the temperature detection accuracy of the fixing member, and the response performance of the blocking member is improved. Can do.
According to the eighth aspect of the present invention, compared to the case where the present invention is not adopted, the adhesion between the temperature detection member and the fixing member can be improved and the temperature detection accuracy of the fixing member can be improved.
According to the ninth aspect of the present invention, the response performance of the blocking member can be improved as compared with the case where the present invention is not adopted.
According to the tenth aspect of the present invention, compared to the case where the present invention is not adopted, when the fixing member is heated to the fixing set temperature, the heat of the fixing member is suppressed from flowing into the magnetic path forming member, thereby fixing the fixing member. The time to reach the set temperature can be shortened.

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 an XX cross-sectional view of the fixing device 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 block diagram which shows an example of the circuit structure which controls the electric power supplied to an IH heater. FIG. 3 is a perspective view illustrating an inner configuration of a fixing belt. FIG. 3 is a cross-sectional configuration diagram of the inside of a fixing belt at a position where a thermistor is disposed. FIG. 6 is a cross-sectional configuration diagram of the inside of the fixing belt at a position where a thermo switch is disposed.

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 fixing belt 61, and a peeling auxiliary member 173 that assists in peeling the paper P from the fixing belt 61.

<Description of fixing belt>
The fixing belt 61 is formed of an endless belt member having an original cylindrical shape. For example, the fixing belt 61 has a diameter of 30 mm and a length in the width direction of 300 mm in the original shape (cylindrical shape). 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 that supplies an alternating current to the IH heater 80 (see also FIG. 6 below). 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, as the conductive heat generating layer 612, 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 (a paramagnetic material having a relative permeability of about 1). Is used.
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 an example of a pressing member, and is constituted by an elastic body such as silicone rubber or fluorine rubber, and is supported by the holder 65 at a position facing the pressure roll 62. Then, it is arranged in a state of being pressed from the pressure roll 62 via the fixing belt 61, and a nip portion N is formed with the pressure roll 62.
The pressing pad 63 includes a pre-nip region 63a on the inlet side of the nip portion N (upstream side in the conveyance direction of the paper P) and a peeling nip region 63b on the outlet side of the nip portion N (downstream side in the conveyance direction of the paper P). Different nip pressures are set. That is, in the pre-nip region 63 a, the surface on the pressure roll 62 side is formed in an arc shape that substantially follows the outer peripheral surface of the pressure roll 62, thereby forming a uniform and wide nip portion N. Further, the peeling nip region 63b is formed so as to be locally pressed from the surface of the pressure roll 62 with a large nip pressure so that the radius of curvature of the fixing belt 61 passing through the peeling nip region 63b becomes small. As a result, a curl (down curl) in a direction away from the surface of the fixing belt 61 is formed on the paper P passing through the peeling nip region 63b to promote the peeling of the paper P from the surface of the fixing belt 61.

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

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

Further, the temperature-sensitive magnetic member 64 has a “permeability change start temperature” (see later stage) equal to or higher than a fixing set temperature at which each color toner image melts, and heat resistance of the elastic layer 613 and the surface release layer 614 of the fixing belt 61. It is composed of a material set within a temperature range lower than the temperature. That is, the temperature-sensitive magnetic member 64 is made of a material having a characteristic (“temperature-sensitive magnetism”) that reversibly changes between ferromagnetism and paramagnetism in a temperature range including the fixing set temperature. The temperature-sensitive magnetic member 64 functions as a magnetic path forming member, and induces magnetic lines of force that are generated by the IH heater 80 and transmitted through the fixing belt 61 in a temperature range equal to or lower than the magnetic permeability change start temperature exhibiting ferromagnetism. Thus, a magnetic path passing through the inside of the temperature-sensitive magnetic member 64 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 exceeding the permeability change start temperature, the temperature-sensitive magnetic member 64 crosses the magnetic field lines generated by the IH heater 80 and transmitted through the fixing belt 61 in the thickness direction of the temperature-sensitive magnetic member 64. Make it transparent. Thereby, the magnetic lines of force generated by the IH heater 80 and transmitted through the fixing belt 61 form a magnetic path that passes through the temperature-sensitive magnetic member 64, passes through the inside of the guide member 66, and returns to the IH heater 80.
The “permeability change start temperature” here is a temperature at which the magnetic permeability (for example, the magnetic permeability measured by JIS C2531) starts to decrease continuously. For example, a member such as the temperature-sensitive magnetic member 64 This is the temperature point at which the amount of magnetic flux that passes through (the number of lines of magnetic force) starts to change. Therefore, the permeability change start temperature is close to the Curie point, which is the temperature at which the substance loses magnetism, but has a different concept from the Curie point.

As a material used for the temperature-sensitive magnetic member 64, a binary magnetic shunt steel such as an Fe—Ni alloy (permalloy) whose permeability change start temperature is set in a range of 140 (fixing set temperature) to 240 ° C., for example. And ternary shunt steels such as Fe—Ni—Cr alloy are used. For example, in the Fe-Ni binary magnetic shunt steel, the permeability change start temperature can be set around 225 ° C. by setting it to about Fe 64% and Ni 36% (atomic ratio). Such metal alloys such as permalloy and magnetic shunt steel are suitable for the temperature-sensitive magnetic member 64 because they are excellent in moldability and workability, have high thermal 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 smaller 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. The configuration and function of the temperature-sensitive magnetic member 64 will be described in further detail later.

<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 paramagnetic 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 a predetermined thickness (for example, 1.0 mm) sufficiently thicker than the skin depth δ (see the above formula (1)) so that the eddy current I can easily flow. It is formed.

<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 167. A bearing bearing portion 67c that is freely coupled is provided. Then, as shown in FIG. 2, the support portions 65a at both ends of the holder 65 are fixed to both ends of the casing 69 of the fixing unit 60, whereby the end cap member 67 is coupled to the support portion 65a. It is rotatably supported via the bearing bearing portion 67c.
As a material constituting the end cap member 67, so-called engineering plastics having high mechanical strength and heat resistance are used. For example, phenol resin, polyimide resin, polyamide resin, polyamideimide resin, PEEK resin, PES resin, PPS resin, LCP resin and the like are suitable.

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

Here, when the fixing belt 61 rotates by receiving a driving force directly from the end cap members 67 at both ends, a torque of about 0.1 to 0.5 N · m is generally applied. However, in the fixing belt 61 of the present embodiment, the base material layer 611 is made of, for example, nonmagnetic stainless steel having high mechanical strength. For this reason, even when a torsional torque of about 0.1 to 0.5 N · m acts on the entire fixing belt 61, buckling or the like hardly occurs in the fixing belt 61.
Further, the flange portion 67d of the end cap member 67 suppresses the deviation of the fixing belt 61. In general, the fixing belt 61 at that time is generally 1 to 5 in the axial direction from the end portion (flange portion 67d) side. A compressive force of about 5N works. However, even when the fixing belt 61 receives such a compressive force, since the base material layer 611 of the fixing belt 61 is made of nonmagnetic stainless steel or the like, occurrence of buckling or the like is suppressed.
As described above, the fixing belt 61 according to the present embodiment rotates by receiving a driving force directly from both end portions of the fixing belt 61, and thus stable rotation is performed. At that time, the base material layer 611 of the fixing belt 61 is made of, for example, non-magnetic stainless steel having high mechanical strength, thereby realizing a structure in which buckling or the like hardly occurs against torsion torque or compression force. is doing. Furthermore, since the base material layer 611 and the conductive heat generating layer 612 are formed as thin layers 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 via the fixing belt 61 with a load of 20 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). Moreover, as a material which comprises the support body 81, heat resistance, such as heat resistant resins, such as heat resistant glass, a polycarbonate, polyether sulfone, PPS (polyphenylene sulfide), or the glass fiber mixed with these, for example. Some non-magnetic materials are 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 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, and magnetic shunt steel. It functions as a 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 is suppressed, and the heat generation efficiency in the fixing belt 61 (conductive heat generation layer 612) is increased.

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

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

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

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

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

<Description of function for suppressing temperature rise 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, 61 may be damaged.

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

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

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

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

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

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

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

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

<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. That is, even if the temperature of the fixing belt 61 is equal to or lower than the permeability change start temperature and the temperature-sensitive magnetic member 64 exhibits ferromagnetism, the temperature-sensitive magnetic member 64 is included in the magnetic force lines H from the IH heater 80. There is a magnetic field line H that crosses in the thickness direction. As a result, a weak eddy current I is generated inside the temperature-sensitive magnetic member 64, and a slight amount of heat is generated in the temperature-sensitive magnetic member 64 itself. Therefore, for example, when a large amount of image formation is continuously performed, heat is accumulated in the temperature-sensitive magnetic member 64, and the temperature of the temperature-sensitive magnetic member 64 tends to increase even in the paper passing area (see FIG. 8). Presents. As a result, if the temperature-sensitive magnetic member 64 that has large eddy current loss and hysteresis loss and easily generates heat due to the passage of the magnetic field lines H is used, depending on the situation, even if the temperature of the fixing belt 61 does not exceed the magnetic permeability change start temperature, In some cases, the temperature-sensitive magnetic member 64 functions to suppress the temperature increase of the fixing belt 61 in the sheet passing region. Therefore, the correspondence between the temperature of the temperature-sensitive magnetic member 64 and the temperature of the fixing belt 61 is maintained, and the temperature-sensitive magnetic member 64 functions as a detection unit that detects the temperature of the fixing belt 61 with high accuracy. It is necessary to suppress the Joule heat W generated in the member 64 itself.

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

  Third, the temperature-sensitive magnetic member 64 is formed with a plurality of slits 64s that divide the flow of the eddy current I generated by the lines of magnetic force H (see FIG. 10). 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. .

  FIG. 10 is a view showing slits 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, in the temperature-sensitive magnetic member 64, a plurality of slits 64 s are formed orthogonal to the direction in which the eddy current I generated by the magnetic field lines H flows. Therefore, when there is no slit 64s, the eddy current I (broken line in FIG. 10B) that flows as a large eddy over the entire longitudinal direction of the temperature-sensitive magnetic member 64 is divided by the slit 64s. Thereby, when the slit 64s is formed, the eddy current I flowing through the temperature-sensitive magnetic member 64 (solid line in FIG. 10 (b)) becomes a small eddy in the region between the slit 64s and the slit 64s, The amount of eddy current I as a whole is reduced. As a result, the amount of heat generated by the temperature-sensitive magnetic member 64 (Joule heat W) is reduced, and a configuration that hardly generates heat is realized. 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 illustrated in FIG. 10, the slit 64s is formed perpendicular to the direction in which the eddy current I flows. However, if the flow of the eddy current I is divided, for example, the eddy current I flows. You may form the slit inclined with respect to the direction. 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.

<Description of fixing belt temperature control>
Next, temperature control of the fixing belt 61 will be described.
FIG. 11 is a block diagram illustrating an example of a circuit configuration for controlling the power supplied to the IH heater 80. As shown in FIG. 11, the power supplied to the excitation circuit 88 is controlled by the electromagnetic induction heating control unit 100 provided in the control unit 31 (see FIG. 1) and the IH heater 80 of the fixing unit 60. The excitation circuit 88.
The electromagnetic induction heating control unit 100 in the control unit 31 includes a CPU 160 that is a control circuit, an overheat detection circuit 162 that detects a temperature change of the fixing belt 61, OR circuits 164 and 165 that are logic elements, and an AND circuit 170. ing.
The excitation circuit 88 of the IH heater 80 transmits and receives signals to / from the control circuit CPU 158, the relay 153 for inputting (connecting) / cutting off power from the external commercial power supply 180, and the electromagnetic induction heating control unit 100. A photocoupler 156 is provided. Furthermore, an AND circuit 154 that is a logic element, a high-frequency switching circuit 152 that is a high-frequency generation circuit, an output port 150 that outputs power to the exciting coil 82, and an input port 151 that inputs power from the external commercial power supply 180 are provided.

First, the CPU 160 of the electromagnetic induction heating control unit 100 includes a temperature control circuit that controls the temperature of the fixing belt 61. That is, the CPU 160 outputs various control signals for controlling the temperature of the fixing belt 61 based on the temperature detection signals from the thermistors 71 and 72 that are examples of the temperature detecting member that detects the temperature of the fixing belt 61.
Specifically, the CPU 160 generates a high-frequency current from the high-frequency switching circuit 152 provided in the excitation circuit 88 to the excitation coil 82 in accordance with the presence / absence of an error signal from the excitation circuit 88 and the surface temperature of the fixing belt 61. A permission signal for permitting supply is output to the AND circuit 170. Then, the AND circuit 170 sends a signal (IH ON / OFF signal) for controlling ON / OFF of the IH heater 80 to the excitation circuit 88 based on the control signal from the overheat detection circuit 162 and the permission signal from the CPU 160. Output.
In addition, the CPU 160 outputs a power setting signal to the excitation circuit 88 based on the temperature detection signal from the thermistors 71 and 72 (mainly thermistor 71). Further, when the surface temperature of the fixing belt 61 rises above a specified value in light of the current operation state of the fixing unit 60, an abnormality signal indicating abnormality is output to the OR circuit 164.
Further, the CPU 160 outputs a signal (relay ON / OFF signal) for controlling ON / OFF of the relay 153 provided in the excitation circuit 88 to the OR circuit 165.

  The overheat detection circuit 162 of the electromagnetic induction heating control unit 100 detects a change in the surface temperature of the fixing belt 61 from the surface temperature of the fixing belt 61 detected by the thermistor 72 arranged on the end side of the fixing belt 61. . The overheat detection circuit 162 outputs a normal signal indicating that the surface temperature of the fixing belt 61 is normal when the amount of change in the surface temperature of the fixing belt 61 is within a predetermined range. It outputs to the circuit 170 and the OR circuit 164. On the other hand, when the amount of change in the surface temperature of the fixing belt 61 exceeds a predetermined range, an abnormal signal indicating that the surface temperature of the fixing belt 61 is in an abnormal state is sent to the CPU 160, the AND circuit 170, and the OR circuit 164. Output to.

Next, the AND circuit 170 is set to output an IH ON / OFF signal to the excitation circuit 88 when the permission signal from the CPU 160 and the normal signal from the overheat detection circuit 162 are supplied.
In addition, the OR circuit 164 generates a drive signal for driving the relay 153 of the excitation circuit 88 based on the abnormal signal from the CPU 160 and the abnormal signal from the overheat detection circuit 162. The OR circuit 164 includes a semiconductor switch element 166 provided in the electromagnetic induction heating control unit 100 with a relay 153 connected to a DC power line 181 (for example, 5 V) and a thermo switch 70 including a thermostat, a thermal fuse, or the like. It is opened and closed by controlling. Specifically, the OR circuit 164 shuts off the semiconductor switch element 166 via the OR circuit 165 when an abnormal signal from the CPU 160 and / or an abnormal signal from the overheat detection circuit 162 are input. Output a signal. In that case, the current flowing from the DC power line 181 to the electromagnetic coil 153a arranged in the relay 153 is cut off, and the relay 153 is cut off. Thereby, the power supply from the external commercial power supply 180 to the excitation circuit 88 is stopped. At the same time, the CPU 160 of the electromagnetic induction heating control unit 100 controls the high frequency switching circuit 152 for the excitation circuit 88 without using the CPU 158 to directly supply the high frequency current to the excitation coil 82. Stop.
Further, even when the temperature of the fixing belt 61 rises abnormally and the thermo switch 70 which is an example of a blocking member is cut off, the current flowing from the DC power line 181 to the electromagnetic coil 153a arranged in the relay 153 is cut off, Relay 153 is cut off.

A photocoupler 156 provided in the excitation circuit 88 transmits and receives signals to and from the electromagnetic induction heating control unit 100. That is, the power setting signal from the CPU 160 of the electromagnetic induction heating control unit 100 is supplied to the photocoupler 156 via the signal line. Further, an IH ON / OFF signal is supplied from an AND circuit 170 connected to the CPU 160. On the other hand, the photocoupler 156 outputs an error signal from the CPU 158 of the excitation circuit 88 to the CPU 160 of the electromagnetic induction heating control unit 100 via a signal line.
Then, the photocoupler 156 outputs the supplied power setting signal to the CPU 158 of the excitation circuit 88. The supplied IH ON / OFF signal is output to the CPU 158 and the AND circuit 154.

A CPU 158 provided in the excitation circuit 88 controls driving of the high-frequency switching circuit 152. That is, the high frequency switching circuit 152 is driven and controlled based on the power setting signal supplied from the CPU 160 of the electromagnetic induction heating control unit 100. Further, the CPU 158 determines various errors in the IH heater 80, generates an error signal, and outputs an error signal to the CPU 160 of the electromagnetic induction heating control unit 100.
In addition, the CPU 158 outputs an IH ON / OFF signal to the AND circuit 154 based on the IH ON / OFF signal supplied from the photocoupler 156 when no error or the like has occurred in the IH heater 80. The AND circuit 154 sends the IH ON / OFF signal to the high frequency switching circuit when the IH ON / OFF signal from the CPU 158 of the excitation circuit 88 and the IH ON / OFF signal from the photocoupler 156 are supplied simultaneously. It outputs to 152.

The high frequency switching circuit 152 provided in the excitation circuit 88 applies power set by the CPU 158 to the excitation coil 82 via the output port 150 when the IH ON / OFF signal from the AND circuit 154 is supplied. .
On the other hand, an AC voltage is supplied to the input port 151 to which power is input from the external commercial power supply 180 via a relay 153 and a noise filter (not shown). The AC voltage supplied via the input port 151 is supplied to each part of the excitation circuit 88.
Note that any one of the input ports 151 is provided with a fuse (not shown), and the power supply is cut off when an abnormality occurs. In addition, the excitation circuit 88 includes a rectifier circuit that rectifies the voltage of the external commercial power supply 180, a constant voltage circuit unit that adjusts the output voltage of the rectifier circuit to a certain level suitable for the operation of the CPU 158, and the like. Has been placed.

<Description of thermistor arrangement>
Next, the arrangement configuration of the thermistors 71 and 72 used for temperature control of the fixing belt 61 will be described.
FIG. 12 is a perspective view showing an inner configuration of the fixing belt 61. As shown in FIG. 12, both the thermistor 71 and the thermistor 72 are arranged in a region (M 1) from the center of the fixing belt 61 toward one end side with respect to the longitudinal direction of the fixing belt 61. . The thermo switch 70 is disposed in a region (M2) opposite to the region where the thermistors 71 and 72 are disposed from the center of the fixing belt 61 with respect to the longitudinal direction of the fixing belt 61.

  As described above, the fixing belt 61 rotates and moves in the circumferential direction while maintaining the cross-sectional shape of both ends circular by the end cap members 67 (see FIG. 5) provided at both ends. On the other hand, in a region other than both ends of the fixing belt 61, the circular cross-sectional shape set by the end cap member 67 is maintained by the rigidity of the fixing belt 61 itself. However, the fixing belt 61 passes through a peeling nip region 63b that locally forms a large nip pressure. Since a large nip pressure is locally formed in the peeling nip region 63 b, deformation is generated in a region other than both ends of the fixing belt 61 so that the radius of curvature of the surface of the fixing belt 61 becomes small. Therefore, on the downstream side after passing through the peeling nip region 63b, the fixing belt 61 receives a pulling force toward the peeling nip region 63b, and a tension toward the temperature-sensitive magnetic member 64 acts on the fixing belt 61.

For this reason, in the present embodiment, the thermistors 71 and 72 that detect the temperature of the fixing belt 61 are regions in which the tension toward the temperature-sensitive magnetic member 64 acts on the fixing belt 61 in the circumferential direction of the fixing belt 61. The fixing belt 61 is disposed downstream of the separation nip region 63b and upstream of the region where the fixing belt 61 is heated again. The thermistors 71 and 72 are arranged so as to press the fixing belt 61 outward (on the side opposite to the temperature-sensitive magnetic member 64) from the inner peripheral surface side of the fixing belt 61. Accordingly, the fixing belt 61 and the thermistor are caused by the tension toward the temperature-sensitive magnetic member 64 acting on the fixing belt 61 and the pressing force pressing the fixing belt 61 acting on the thermistors 71 and 72 from the temperature-sensitive magnetic member 64 side. 71 and 72 are set so as to enhance adhesion. By increasing the adhesion between the fixing belt 61 and the thermistors 71 and 72, the temperature accuracy of the fixing belt 61 detected by the thermistors 71 and 72 is improved.
Specifically, the thermistors 71 and 72 are fixed to the holder 65 by support members 71b and 72b, respectively, and are pressed to the inner peripheral surface side of the fixing belt 61 by the urging members 71a and 72a.

  FIG. 13 is a cross-sectional configuration diagram (plane D1 in FIG. 12) inside the fixing belt 61 at the position where the thermistor 71 is disposed. As shown in FIG. 13, a large nip pressure Np is locally set in the peeling nip region 63b. Therefore, in the region R that is downstream after the fixing belt 61 passes through the peeling nip region 63b and upstream of the region in which the fixing belt 61 is heated again, the fixing belt 61 faces the temperature-sensitive magnetic member 64 side. The heading tension Ft acts. On the other hand, the thermistor 71 is fixed to the holder 65 by the support member 71b while being given a pressing force Fq toward the inner peripheral surface of the fixing belt 61 by the urging member 71a. Thereby, the adhesion between the fixing belt 61 and the thermistor 71 is enhanced, and the accuracy of the temperature of the fixing belt 61 detected by the thermistor 71 is improved. The same applies to the other thermistor 72.

<Explanation of thermo switch layout>
Next, the arrangement configuration of the thermo switch 70 will be described.
FIG. 14 is a cross-sectional configuration diagram (plane D2 in FIG. 12) inside the fixing belt 61 at the position where the thermo switch 70 is disposed. As shown in FIG. 12, the thermo switch 70 is located in the region (the region M1 in FIG. 12) on the opposite side of the center of the fixing belt 61 with respect to the longitudinal direction of the fixing belt 61. Arranged in the region M2) of FIG. Then, as shown in FIG. 14, in the circumferential direction of the fixing belt 61, as with the thermistors 71 and 72, the fixing belt 61 is heated again on the downstream side after the fixing belt 61 has passed through the peeling nip region 63 b. It is arranged in the region R upstream of the region to be processed. Further, the thermo switch 70 is set so that all or a part of the surface of the thermo switch 70 is located closer to the fixing belt 61 than the surface position 64 a of the temperature-sensitive magnetic member 64.

As described above, in the region R downstream of the fixing belt 61 after passing through the peeling nip region 63b and upstream of the region where the fixing belt 61 is heated again, the temperature-sensitive magnetic member 64 is provided on the fixing belt 61. The tension Ft toward the side is acting. Therefore, except for the nip portion N, the fixing belt 61 passes through the closest position among the members arranged inside the fixing belt 61 is the temperature-sensitive magnetic member 64 positioned in the region R. Therefore, when an abnormal temperature rise occurs in the fixing belt 61 such that the thermoswitch 70 is disconnected and the fixing belt 61 contracts, the fixing belt 61 contacts the temperature-sensitive magnetic member 64 located in the region R at an early stage. To do. In particular, since the region where the temperature-sensitive magnetic member 64 is disposed is also a region where the fixing belt 61 is heated, a large temperature rise occurs compared to other regions. As a result, when the fixing belt 61 contracts, there is a high possibility that the temperature-sensitive magnetic member 64 located in the region R will first contact.
Therefore, in the present embodiment, the thermo switch 70 that is cut when the temperature of the fixing belt 61 rises abnormally is downstream of the fixing belt 61 after passing the peeling nip region 63b in the circumferential direction of the fixing belt 61. And the upstream side of the region where the fixing belt 61 is heated again. At this time, the surface of the thermo switch 70 is set so as to protrude from the surface position 64 a of the temperature-sensitive magnetic member 64 toward the fixing belt 61 so as to surely come into contact with the contracting fixing belt 61.

<Explanation of relationship between thermistor and thermo switch for longitudinal position>
As shown in FIG. 12, the thermo switch 70 has a region (region) opposite to the region (region M <b> 1) where the thermistors 71 and 72 are disposed from the center of the fixing belt 61 in the longitudinal direction of the fixing belt 61. M2).
As described above, when the fixing belt 61 is abnormally heated so that the thermoswitch 70 is disconnected and the fixing belt 61 contracts, the fixing belt 61 is positioned in the region R with respect to the circumferential direction of the fixing belt 61. It contacts the temperature-sensitive magnetic member 64 at an early stage. In this case, in the region M1 where the thermistors 71 and 72 are disposed, the thermistors 71 and 72 press the fixing belt 61 outward (on the opposite side to the temperature-sensitive magnetic member 64) from the inner peripheral surface side of the fixing belt 61. Are arranged as follows. Therefore, when the thermo switch 70 is arranged in the same region as the arrangement region M1 of the thermistors 71 and 72 with respect to the longitudinal direction of the fixing belt 61, the shrinking fixing belt 61 and the thermosensitive magnetism in the region R in the circumferential direction. Thermistors 71 and 72 are interposed between the member 64 and the member 64. Therefore, even if the surface of the thermo switch 70 is set so as to protrude from the surface position 64 a of the temperature-sensitive magnetic member 64 toward the fixing belt 61, the fixing belt 61 comes into contact with the surface of the thermo switch 70 through the thermistors 71 and 72. There is a possibility not to. In particular, in a configuration in which two thermistors such as a thermistor 71 that mainly detects the surface temperature of the fixing belt 61 and a thermistor 72 that mainly detects an abnormal state of the surface temperature of the fixing belt 61 are disposed, the contracted fixing belt 61 and The gap between the temperature-sensitive magnetic member 64 is easily formed over a wide region in the longitudinal direction including the position where the thermistors 71 and 72 are disposed. Therefore, in the region M1 where the thermistors 71 and 72 are arranged from the center of the fixing belt 61, the contracted fixing belt 61 and the thermo switch 70 are not in contact with each other, and the responsiveness of the thermo switch 70 may be lowered.

  Therefore, in the present embodiment, the thermo switch 70 is disposed in a region M2 on the opposite side of the center region of the fixing belt 61 from the region M1 of the thermistors 71 and 72 with respect to the longitudinal direction of the fixing belt 61. As a result, an abnormal temperature rise such that the thermo switch 70 is disconnected occurs in the fixing belt 61, and even when the fixing belt 61 contracts, the contracted fixing belt 61 and the thermosensitive magnetism generated by the thermistors 71 and 72 being interposed. The gap between the member 64 is difficult to reach the position where the thermo switch 70 is disposed. Even if the influence of the gap generated between the fixing belt 61 and the temperature-sensitive magnetic member 64 due to the thermistors 71 and 72 is extended to the position where the thermo switch 70 is disposed, the gap amount is small. Become. Therefore, by disposing the thermo switch 70 in the region M2, when the fixing belt 61 contracts, the reliability / immediateness of contact between the contracted fixing belt 61 and the thermo switch 70 increases, and the thermo switch 70 Responsiveness is improved. As a result, when the temperature of the fixing belt 61 rises abnormally, the thermo switch 70 is immediately cut off, and the current flowing from the DC power supply line 181 shown in FIG. 11 to the electromagnetic coil 153a arranged in the relay 153 is stopped. Thus, the relay 153 is cut off with good responsiveness.

  As described above, by arranging the thermo switch 70 in the region M2, when the fixing belt 61 contracts, the certainty and immediacy of contact between the contracting fixing belt 61 and the thermo switch 70 is increased. Therefore, when the response of the thermo switch 70 is improved and the temperature of the fixing belt 61 rises abnormally, the thermo switch 70 is immediately disconnected. Thereby, the certainty that the safety mechanism corresponding to the abnormal temperature rise of the fixing belt 61 operates is further improved.

As described above, in the fixing unit 60 provided in the image forming apparatus 1 of the present embodiment, the temperature-sensitive magnetic member 64 is disposed in the vicinity of the inner peripheral surface of the fixing belt 61. As a result, the temperature rise of the non-sheet passing region is suppressed excessively.
Furthermore, with respect to the longitudinal direction of the fixing belt 61, both the thermistors 71 and 72 are arranged in a region M 1 that is closer to one end side than the center of the fixing belt 61, and the thermistors 71 and 72 are arranged with respect to the center of the fixing belt 61. The thermo switch 70 is arranged in a region M2 opposite to the region M1. Thereby, an abnormal temperature rise in the fixing belt 61 is quickly detected.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 60 ... Fixing unit, 61 ... Fixing belt, 62 ... Pressure roll, 64 ... Temperature-sensitive magnetic member, 70 ... Thermo switch, 71, 72 ... Thermistor, 80 ... IH heater, 82 ... Excitation coil, 84 ... Magnetic core, 611 ... Base material layer, 612 ... Conductive heating layer

Claims (10)

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;
A magnetic path of an alternating magnetic field generated by the magnetic field generation member is formed in a temperature range below the magnetic permeability change starting temperature at which the magnetic permeability starts to decrease and is disposed opposite to the magnetic field generation member with the fixing member interposed therebetween. And a magnetic path forming member that transmits an alternating magnetic field generated by the magnetic field generating member in a temperature range that exceeds the permeability change start temperature,
A temperature detection member that is disposed in a region toward one end side from the longitudinal center position of the magnetic path forming member, and detects the temperature of the fixing member from the inside of the fixing member;
It is disposed in a region opposite to the region where the temperature detection member is disposed with respect to the longitudinal center position of the magnetic path forming member, and the temperature of the fixing member exceeds a predetermined temperature from the inside of the fixing member. And a blocking member for blocking the power supplied to the magnetic field generating member upon detecting the above. A nip portion that is disposed inside the fixing member and presses the fixing member in contact with the fixing member to form a nip portion through which the recording material passes is formed between the fixing member and the roll member. A pressing member,
The temperature detection member and the blocking member are a region on the downstream side in the movement direction of the fixing member with respect to the arrangement position of the pressing member and on the upstream side of the arrangement position of the magnetic path forming member, or the magnetic path formation. The fixing device according to claim 1, wherein the fixing device is disposed in a region where the member is disposed.   The fixing device according to claim 1, wherein the temperature detection member is disposed so as to be pressed toward an inner peripheral surface of the fixing member.   The blocking member is positioned such that all or part of the portion facing the inner peripheral surface of the fixing member is located closer to the fixing member than the surface of the magnetic path forming member facing the inner peripheral surface of the fixing member. The fixing device according to claim 1, wherein the fixing device is arranged.   The fixing device according to claim 1, wherein the magnetic path forming member is disposed in non-contact with the fixing member. 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;
A magnetic path of an alternating magnetic field generated by the magnetic field generation member is formed in a temperature range below the magnetic permeability change starting temperature at which the magnetic permeability starts to decrease and is disposed opposite to the magnetic field generation member with the fixing member interposed therebetween. And a magnetic path forming member that transmits an alternating magnetic field generated by the magnetic field generating member in a temperature range that exceeds the permeability change start temperature,
A temperature detection member that is disposed in a region toward one end side from the longitudinal center of the magnetic path forming member, and detects the temperature of the fixing member from the inside of the fixing member;
It is disposed in a region opposite to the region where the temperature detection member is disposed with respect to the longitudinal center position of the magnetic path forming member, and the temperature of the fixing member exceeds a predetermined temperature from the inside of the fixing member. An image forming apparatus comprising: a blocking member that detects that the power is supplied to the magnetic field generating member. The fixing unit is disposed inside the fixing member, and presses the fixing member against a roll member disposed in contact with the fixing member so that the recording material passes between the fixing member and the roll member. A pressing member for forming a nip portion;
The temperature detecting member and the blocking member of the fixing unit are regions downstream of the pressing member in the moving direction of the fixing member and upstream of the magnetic path forming member, or The image forming apparatus according to claim 6, wherein the magnetic path forming member is disposed in a region where the magnetic path forming member is disposed.   The image forming apparatus according to claim 6, wherein the temperature detection member of the fixing unit is disposed so as to be pressed toward an inner peripheral surface of the fixing member.   The blocking member of the fixing unit is such that all or part of the portion facing the inner peripheral surface of the fixing member is closer to the fixing member than the surface of the magnetic path forming member facing the inner peripheral surface of the fixing member. The image forming apparatus according to claim 6, wherein the image forming apparatus is disposed so as to be positioned.   The image forming apparatus according to claim 6, wherein the magnetic path forming member of the fixing unit is disposed in non-contact with the fixing member.

Images