JP2009198802A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device or the like that excels in temperature uniformity of a fixing belt 61, and has improved safety from a temperature increase in a fixing belt 61 in an abnormal state. <P>SOLUTION: The fixing device 60 includes: a fixing belt 61 containing a conductive layer that is heated by the action of a magnetic field; a driving section that rotates the fixing belt 61; an electromagnetic induction heating member 65 that generates a magnetic field; a pressure roller 62 that is firmly pressed against the circumferential face of the fixing belt 61, thereby forming a fixing nip N between the fixing belt 61 and itself, through which a recording material with an unfixed image held thereon is inserted; a pressure pad 63 that presses the fixing belt 61 against the pressure roll 62 from the inside of the fixing belt 61; and a temperature sensor 70 that detects the temperature of the fixing belt 61. The temperature detecting section contains a temperature sensitive magnetic material, and switches by virtue of magnet's movement at the Curie point of the temperature sensitive material or greater, thereby stopping the generation of the magnetic field in the heating section. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fixing device and the like, and more particularly, to a fixing device and the like that perform heating by an electromagnetic induction heating method.

2. Description of the Related Art Image forming apparatuses such as copying machines and printers using an electrophotographic system use a fixing device that simultaneously applies heat and pressure (nip pressure) to weld a resin component of toner transferred onto a recording material. .
As this fixing device, there is a fixing device that performs heating using electromagnetic induction. In this electromagnetic induction heating type fixing device, in order to ensure the temperature uniformity of the fixing belt, for example, the magnetic cores are arranged in a staggered manner at regular intervals, and the heat generation can be controlled by weakening the magnetic coupling. Has been proposed (see, for example, Patent Document 1).

JP 2006-267742 A

By the way, a temperature sensor that detects temperature during heating uses a non-magnetic metal, and a magnetic flux that performs electromagnetic induction acts on the temperature sensor, which may affect the heat distribution of the fixing belt.
In addition, the heating method using electromagnetic induction may be accompanied by a rapid temperature increase, and the sensor may not be able to detect the temperature sufficiently, and the temperature control may be insufficient.
An object of the present invention is to provide a fixing device and the like that can ensure temperature uniformity of the fixing belt and can accurately control the temperature.

  According to a first aspect of the present invention, there is provided a belt member including a conductive layer that is heated by the action of a magnetic field, a driving unit that rotates the belt member, a heating unit that generates the magnetic field, and a pressure contact with an outer peripheral surface of the belt member. A pressure member that forms a fixing nip portion for inserting a recording material holding an unfixed image between the belt member and the belt member from the inside of the belt member toward the pressure member. A fixing device comprising: a pressing member that presses a member; a temperature detection unit that detects a temperature of the belt member; and a structure that increases the strength of the magnetic field at a location where the temperature detection unit is installed. It is.

The invention according to claim 2 is the fixing device according to claim 1, wherein the structure is configured such that the temperature detection unit includes a temperature-sensitive magnetic material.
The invention according to claim 3 is the fixing device according to claim 2, wherein the temperature-sensitive magnetic material is a metal alloy material.
According to a fourth aspect of the present invention, the thickness of the temperature-sensitive magnetic material is k ₁ , the thickness of the conductive layer is k ₂ , the specific resistance of the temperature-sensitive magnetic material is ρ ₁ , and the specific resistance of the conductive layer is ρ formula is taken as ₂ that satisfy (1) is a fixing device according to claim 3, characterized.
2 × ρ ₁ / k ₁ <ρ ₂ / k ₂ (1)

The invention according to claim 5 is the fixing device according to claim 2, wherein the temperature-sensitive magnetic material has a thickness of 0.05 mm to 0.3 mm.
According to a sixth aspect of the present invention, the temperature detection unit further includes a magnet that attracts the temperature-sensitive magnetic material when the temperature-sensitive magnetic material has magnetism at a temperature lower than the Curie point, and the temperature-sensitive magnetic material 3. The fixing device according to claim 2, wherein when the magnetism disappears at a temperature equal to or higher than the Curie point, switching is performed by moving the magnet, and generation of the magnetic field in the heating unit is stopped. is there.

  The invention according to claim 7 comprises: a toner image forming unit that forms a toner image; a transfer unit that transfers the toner image to a recording medium; and a fixing unit that fixes the toner image to the recording medium. The fixing unit includes a belt member that is rotated by a driving unit, a heating unit that heats the belt member by the action of a magnetic field, and a temperature detection unit that uses a temperature-sensitive magnetic material for temperature detection. The image forming apparatus is characterized in that when the temperature of the temperature-sensitive magnetic material becomes equal to or higher than the Curie point of the temperature-sensitive magnetic material, generation of a magnetic field in the heating unit is stopped.

  According to an eighth aspect of the present invention, the temperature detection unit further includes a magnet that is installed opposite to the temperature-sensitive magnetic material, and the magnet has the sensitivity at a temperature that is less than the Curie point. The image forming apparatus according to claim 7, wherein attraction is performed on the hot magnetic material, switching is performed by moving away from the thermosensitive magnetic material at a temperature equal to or higher than the Curie point, and generation of a magnetic field in the heating unit is stopped. It is.

According to the first aspect of the present invention, it is possible to obtain a fixing device in which the temperature distribution of the fixing belt tends to be uniform as compared with the case where this configuration is not adopted.
According to the second aspect of the present invention, since the change in the magnetic field is less likely to occur at the temperature sensor installation portion than in the case where the present configuration is not adopted, the fixing device in which the temperature distribution of the fixing belt is likely to be uniform. Can be obtained.
According to the third aspect of the present invention, since the heat receiving portion of the temperature sensor that is not easily affected by the magnetic field can be created as compared with the case where this configuration is not employed, the fixing belt temperature distribution is likely to be more uniform. A device can be obtained.
According to the fourth aspect of the present invention, the self-heating at the heat receiving portion of the temperature sensor is less likely to occur as compared with the case where this configuration is not adopted, so that a fixing device with fewer malfunctions can be obtained.
According to the fifth aspect of the present invention, a self-heating at the heat receiving portion of the temperature sensor hardly occurs and a fixing device with few malfunctions can be easily obtained as compared with the case where this configuration is not adopted.
According to the sixth aspect of the present invention, since a temperature sensor can be created that has a quick response to a temperature rise at the time of abnormality as compared with the case where this configuration is not adopted, the fixing device further improves safety. Can be obtained.
According to the seventh aspect of the present invention, it is possible to use a fixing device in which the temperature distribution of the fixing belt is likely to be uniform as compared with the case where this configuration is not adopted, and thus a good image can be obtained. An image forming apparatus is obtained.
According to the eighth aspect of the present invention, a temperature sensor can be created that has a quick response to a temperature rise at the time of abnormality as compared with the case where this configuration is not adopted. An image forming apparatus with improved properties can be obtained.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus to which the exemplary embodiment is applied. The image forming apparatus shown in FIG. 1 is an intermediate transfer type image forming apparatus generally called a tandem type, and a plurality of image forming units 1Y, 1M, 1C, and 1K on which toner images of respective color components are formed by electrophotography. The primary transfer unit 10 that sequentially transfers (primary transfer) the color component toner images formed by the image forming units 1Y, 1M, 1C, and 1K to the intermediate transfer belt 15, and the superimposed toner image transferred onto the intermediate transfer belt 15. Are provided with a secondary transfer unit 20 that collectively transfers (secondary transfer) to a paper P that is a recording material (recording paper), and a fixing device 60 that fixes the secondary transferred image onto the paper P. Moreover, it has the control part 40 which controls operation | movement of each apparatus (each part).

  In the present embodiment, each of the image forming units 1Y, 1M, 1C, and 1K has a charger 12 that charges the photosensitive drum 11 around the photosensitive drum 11 that rotates in the direction of arrow A, and the photosensitive drum 11. A laser exposure device 13 for writing an electrostatic latent image thereon (exposure beam is indicated by a symbol Bm in the figure), each color component toner is accommodated, and the electrostatic latent image on the photosensitive drum 11 is visualized with toner. A developing unit 14, a primary transfer roll 16 that transfers each color component toner image formed on the photosensitive drum 11 to the intermediate transfer belt 15 in the primary transfer unit 10, and a drum cleaner that removes residual toner on the photosensitive drum 11. 17 and the like are sequentially arranged. These image forming units 1Y, 1M, 1C, and 1K are arranged substantially linearly in the order of yellow (Y), magenta (M), cyan (C), and black (K) from the upstream side of the intermediate transfer belt 15. Has been.

The intermediate transfer belt 15 as an intermediate transfer member is constituted by a film-like endless belt in which an appropriate amount of an antistatic agent such as carbon black is contained in a resin such as polyimide or polyamide. And the volume resistivity is formed so that it may become 10 < ⁶ > -10 < ¹⁴ > (omega | ohm) cm, The thickness is comprised by about 0.1 mm, for example. The intermediate transfer belt 15 is circulated and driven (rotated) at a predetermined speed in the direction of arrow B shown in FIG. 1 by various rolls. As these various rolls, a drive roll 31 that is driven by a motor (not shown) excellent in constant speed and rotates the intermediate transfer belt 15, and an intermediate that extends substantially linearly along the arrangement direction of the photosensitive drums 11. A support roll 32 that supports the transfer belt 15, a tension roll 33 that functions as a correction roll that applies a constant tension to the intermediate transfer belt 15 and prevents meandering of the intermediate transfer belt 15, and a backup provided in the secondary transfer unit 20. A cleaning backup roll 34 provided in a cleaning unit for scraping off residual toner on the roll 25 and the intermediate transfer belt 15 is provided.

The primary transfer unit 10 includes a primary transfer roll 16 that is disposed to face the photosensitive drum 11 with the intermediate transfer belt 15 interposed therebetween. The primary transfer roll 16 includes a shaft and a sponge layer as an elastic layer fixed around the shaft. The shaft is a cylindrical bar made of metal such as iron or SUS. The sponge layer is a sponge-like cylindrical roll formed of a blend rubber of NBR, SBR and EPDM containing a conductive agent such as carbon black and having a volume resistivity of 10 ^7.5 to 10 ^8.5 Ωcm. The primary transfer roll 16 is placed in pressure contact with the photosensitive drum 11 with the intermediate transfer belt 15 in between, and the primary transfer roll 16 has a voltage (with negative polarity; the same applies hereinafter) having a polarity opposite to that of the toner. (Primary transfer bias) is applied. As a result, the toner images on the respective photosensitive drums 11 are sequentially electrostatically attracted to the intermediate transfer belt 15 so as to form superimposed toner images on the intermediate transfer belt 15.

The secondary transfer unit 20 includes a secondary transfer roll 22 disposed on the toner image holding surface side of the intermediate transfer belt 15 and a backup roll 25. The backup roll 25 is composed of a tube of EPDM and NBR blend rubber with carbon dispersed on the surface, and EPDM rubber on the inside. And it forms so that the surface resistivity may be 10 < ⁷ > -10 < ¹⁰ > (omega | ohm) / square, and hardness is set to 70 degrees (Asker C), for example. The backup roll 25 is disposed on the back side of the intermediate transfer belt 15 to form a counter electrode of the secondary transfer roll 22, and a metal power supply roll 26 to which a secondary transfer bias is stably applied is disposed in contact with the backup roll 25. ing.

On the other hand, the secondary transfer roll 22 includes a shaft and a sponge layer as an elastic layer fixed around the shaft. The shaft is a cylindrical bar made of metal such as iron or SUS. The sponge layer is a sponge-like cylindrical roll formed of a blend rubber of NBR, SBR and EPDM containing a conductive agent such as carbon black and having a volume resistivity of 10 ^7.5 to 10 ^8.5 Ωcm. The secondary transfer roll 22 is disposed in pressure contact with the backup roll 25 with the intermediate transfer belt 15 interposed therebetween. Further, the secondary transfer roll 22 is grounded, and a secondary transfer bias is formed between the secondary transfer roll 22 and the backup roll 25. The toner image is secondarily transferred onto the paper P conveyed to the transfer unit 20.

  Further, on the downstream side of the secondary transfer portion 20 of the intermediate transfer belt 15, an intermediate transfer belt that removes residual toner and paper dust on the intermediate transfer belt 15 after the secondary transfer and cleans the surface of the intermediate transfer belt 15. A cleaner 35 is provided so as to be able to contact and separate. On the other hand, on the upstream side of the yellow image forming unit 1Y, a reference sensor (home position sensor) 42 that generates a reference signal serving as a reference for taking image forming timings in the image forming units 1Y, 1M, 1C, and 1K. It is arranged. Further, an image density sensor 43 for adjusting image quality is disposed on the downstream side of the black image forming unit 1K. The reference sensor 42 recognizes a predetermined mark provided on the back side of the intermediate transfer belt 15 and generates a reference signal. In response to an instruction from the control unit 40 based on the recognition of the reference signal, each image forming unit. 1Y, 1M, 1C, and 1K are configured to start image formation.

  Further, in the image forming apparatus according to the present embodiment, as a paper transport system, a paper tray 50 that stores paper P, a pickup roll 51 that picks up and transports the paper P accumulated in the paper tray 50 at a predetermined timing, and a pickup A transport roll 52 that transports the paper P fed by the roll 51, a transport chute 53 that feeds the paper P transported by the transport roll 52 to the secondary transfer unit 20, and transported after being secondarily transferred by the secondary transfer roll 22. A conveyance belt 55 that conveys the paper P to be fixed to the fixing device 60 and a fixing inlet guide 56 that guides the paper P to the fixing device 60 are provided.

  Next, a basic image forming process of the image forming apparatus according to the present embodiment will be described. In the image forming apparatus as shown in FIG. 1, image data output from an image reading device (IIT) not shown or a personal computer (PC) not shown is subjected to predetermined image processing by an image processing device (IPS) not shown. After being applied, the image forming operation is executed by the image forming units 1Y, 1M, 1C, and 1K. In IPS, the input reflectance data is subjected to predetermined image processing such as shading correction, position shift correction, brightness / color space conversion, gamma correction, frame deletion, color editing, moving editing, and other various image editing. Is done. The image data that has undergone image processing is converted into color material gradation data of four colors, Y, M, C, and K, and is output to the laser exposure unit 13.

  The laser exposure unit 13 irradiates the photosensitive drums 11 of the image forming units 1Y, 1M, 1C, and 1K, for example, with an exposure beam Bm emitted from a semiconductor laser in accordance with the input color material gradation data. Yes. In each of the photosensitive drums 11 of the image forming units 1Y, 1M, 1C, and 1K, the surface is charged by the charger 12, and then the surface is scanned and exposed by the laser exposure unit 13 to form an electrostatic latent image. The formed electrostatic latent images are developed as toner images of respective colors Y, M, C, and K by the respective image forming units 1Y, 1M, 1C, and 1K.

  The toner images formed on the photosensitive drums 11 of the image forming units 1Y, 1M, 1C, and 1K are transferred onto the intermediate transfer belt 15 in the primary transfer unit 10 where the photosensitive drums 11 and the intermediate transfer belt 15 come into contact with each other. Transcribed. More specifically, in the primary transfer portion 10, a voltage (primary transfer bias) having a polarity opposite to the toner charging polarity (minus polarity) is applied to the base material of the intermediate transfer belt 15 by the primary transfer roll 16, and the toner image. Are sequentially superimposed on the surface of the intermediate transfer belt 15 to perform primary transfer.

  After the toner images are sequentially primary transferred onto the surface of the intermediate transfer belt 15, the intermediate transfer belt 15 moves and the toner image is conveyed to the secondary transfer unit 20. When the toner image is transported to the secondary transfer unit 20, in the paper transport system, the pick-up roll 51 rotates in accordance with the timing at which the toner image is transported to the secondary transfer unit 20, and the paper of a predetermined size is transferred from the paper tray 50. P is supplied. The paper P supplied by the pickup roll 51 is transported by the transport roll 52 and reaches the secondary transfer unit 20 via the transport chute 53. Before reaching the secondary transfer unit 20, the paper P is temporarily stopped, and a registration roll (not shown) rotates in accordance with the movement timing of the intermediate transfer belt 15 on which the toner image is held. And the position of the toner image are aligned.

  In the secondary transfer unit 20, the secondary transfer roll 22 is pressed against the backup roll 25 via the intermediate transfer belt 15. At this time, the sheet P conveyed at the same timing is sandwiched between the intermediate transfer belt 15 and the secondary transfer roll 22. At this time, when a voltage (secondary transfer bias) having the same polarity as the toner charging polarity (negative polarity) is applied from the power supply roll 26, a transfer electric field is formed between the secondary transfer roll 22 and the backup roll 25. Is done. The unfixed toner image held on the intermediate transfer belt 15 is collectively electrostatically transferred onto the paper P in the secondary transfer unit 20 pressed by the secondary transfer roll 22 and the backup roll 25. .

Thereafter, the sheet P on which the toner image has been electrostatically transferred is transported as it is while being peeled off from the intermediate transfer belt 15 by the secondary transfer roll 22, and transported downstream of the secondary transfer roll 22 in the sheet transport direction. It is conveyed to the belt 55. The conveyance belt 55 conveys the paper P to the fixing device 60 in accordance with the optimum conveyance speed in the fixing device 60. The unfixed toner image on the paper P conveyed to the fixing device 60 is fixed on the paper P by being subjected to a fixing process by heat and pressure by the fixing device 60. Then, the paper P on which the fixed image is formed is conveyed to a paper discharge placement unit provided in a discharge unit of the image forming apparatus.
On the other hand, after the transfer onto the paper P is completed, the residual toner remaining on the intermediate transfer belt 15 is conveyed to the cleaning unit as the intermediate transfer belt 15 rotates, and the cleaning backup roll 34 and the intermediate transfer belt cleaner 35 are transferred. Is removed from the intermediate transfer belt 15.

Next, the fixing device 60 used in the image forming apparatus of the present embodiment will be described.
FIG. 2 is a schematic configuration diagram showing the configuration of the fixing device 60 of the present embodiment. As shown in FIG. 2, the fixing device 60 of the present embodiment is provided with a fixing belt 61 as an example of a belt member having an endless peripheral surface, and is disposed in pressure contact with the outer peripheral surface of the fixing belt 61. A pressure roll 62 as an example of a pressure member for drivingly rotating 61, a pressure pad 63 as an example of a pressure member disposed in pressure contact with the pressure roll 62 via the fixing belt 61 inside the fixing belt 61, and pressing The pad support member 64 that supports the pad 63 and the like is formed following the outer peripheral surface shape of the fixing belt 61, and is disposed with a predetermined gap from the fixing belt 61. The fixing belt 61 is electromagnetically extended in the longitudinal direction. An electromagnetic induction heating member 65 as an example of a heating unit that performs induction heating, a fixing belt that is disposed along the inner peripheral surface of the fixing belt 61 inside the fixing belt 61, and is formed by the electromagnetic induction heating member 65. Mainly by a temperature sensor 70 as an example of a temperature detection unit that is disposed in the vicinity of the surface of the fixing belt 61 and that measures the temperature of the fixing belt 61. The part is composed.

As shown in FIG. 3, the fixing belt 61 includes, in order from the inner peripheral surface side, a base layer 61a made of a sheet member having high heat resistance, a conductive layer 61b, an elastic layer 61c, and a surface release layer 61d serving as an outer peripheral surface. Are stacked in order. In addition, a primer layer for adhesion may be provided between the layers.
As the base layer 61a, a material having flexibility, excellent mechanical strength, and heat resistance, such as fluororesin, polyimide resin, polyamide resin, polyamideimide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, FEP resin, etc. Are preferably used. The thickness is 10 to 100 μm, preferably 50 to 100 μm. If the thickness is less than 10 μm, the strength of the fixing belt 61 cannot be obtained. If the thickness is greater than 100 μm, the flexibility is impaired, and the heat capacity increases and the temperature rise time becomes longer. is there. In the present embodiment, a sheet-like member made of a polyimide resin having an inner diameter of 30 mm, a width of 370 mm, and a thickness of 50 μm is used.
The conductive layer 61b is a layer (heat generation layer) that generates heat by induction by a magnetic field induced by the electromagnetic induction heating member 65, and a metal layer of iron, cobalt, nickel, copper, aluminum, chromium, or the like with a thickness of about 1 to 30 μm. What was formed is used. The material and thickness of the conductive layer 61b are appropriately selected so as to realize a specific resistance value that can generate sufficient heat by eddy current due to electromagnetic induction. In the present embodiment, copper having a thickness of about 10 μm is used.

  The elastic layer 61c has a thickness of 10 to 500 [mu] m, preferably 50 to 500 [mu] m, and silicone rubber, fluorine rubber, fluorosilicone rubber, etc. excellent in heat resistance and thermal conductivity are used. In the present embodiment, silicone rubber having a rubber hardness of 15 ° (JIS-A: JIS-KA type tester) and a thickness of 200 μm is used.

Further, since the surface release layer 61d is a layer that is in direct contact with the unfixed toner image transferred onto the paper P, it is necessary to use a material having excellent release properties and heat resistance. Therefore, as the material constituting the surface release layer 61d, for example, tetrafluoroethylene perfluoroalkyl vinyl ether polymer (PFA), polytetrafluoroethylene (PTFE), fluororesin, silicone resin, fluorosilicone rubber, fluororubber, silicone Rubber or the like is preferably used.
The thickness of the surface release layer 61d is preferably 5 to 50 μm. This is because, when the thickness of the surface release layer 61d is smaller than 5 μm, coating unevenness occurs at the time of coating and a problem occurs that a region having poor release properties is formed or durability is insufficient. Further, when the surface release layer 61d exceeds 50 μm, there arises a problem that heat conduction is deteriorated, and in particular, the surface release layer 61d formed of a resin material has too high hardness, and the elastic layer 61c. It is because the function which has is reduced. In this embodiment, PFA with a thickness of 30 μm is used.
Here, in order to improve toner releasability in the surface release layer 61d, an oil application mechanism for applying oil (release agent) for preventing toner offset to the surface release layer 61d is brought into contact with the fixing belt 61. It is also possible to arrange them. This is particularly effective when a toner containing no low softening substance is used.

  Next, as shown in FIG. 2, the pressure roll 62 includes a metal columnar member 62a as a core (core), and a silicone rubber, foamed silicone rubber, fluororubber on the surface of the columnar member 62a. It is composed of an elastic layer 62b having heat resistance such as a fluororesin and a surface release layer 62c on the outermost surface. The pressure roll 62 rotates in the direction of arrow C while both ends thereof are urged and supported by the fixing belt 61 by spring members (not shown). In the present embodiment, the pressure roll 62 is urged against the pressing pad 63 by a total load 294N (30 kgf) via the fixing belt 61. As a result, the pressure roll 62 holds the fixing belt 61 between the pressing pad 63 and forms a fixing nip portion N.

As shown in FIG. 2, the pressing pad 63 includes a pedestal 63a made of a metal such as SUS, iron, or a resin having heat resistance, and an elastic material such as silicone rubber or fluororubber coated on the surface of the pedestal 63a. And an elastic member 63b formed of The pressing pad 63 is disposed over a region that is slightly wider than the region (sheet passing region) through which the paper P passes in the width direction of the fixing belt 61, and extends substantially over the entire length of the pressing pad 63 in the longitudinal direction. Thus, the pressure roll 62 is configured to be pressed.
The contact surface of the pressing pad 63 with the fixing belt 61 is formed as a concave curved surface following the outer surface shape of the pressure roll 62. Therefore, a sufficiently wide nip width can be formed between the pressure roll 62 and the fixing belt 61.
Further, a lubricant such as heat-resistant grease is applied between the pressing pad 63 and the fixing belt 61 in order to reduce the sliding frictional force between the pressing pad 63 and the fixing belt 61 in the fixing nip portion N. Thereby, the fixing belt 61 can be smoothly rotated.

The pad support member 64 is a rod-shaped member having an axis in the width direction of the fixing belt 61 as shown in FIG. A pressing pad 63 is attached to a portion of the pad support member 64 facing the pressure roll 62, and the pressing force acting on the pressing pad 63 from the pressure roll 62 via the fixing belt 61 is applied to the pad support member 64. Is borne by. Therefore, it is necessary to use a pad support member 64 having such a rigidity that the amount of bending when receiving a pressing force from the pressure roll 62 is not more than a predetermined level, preferably not more than 1 mm. Furthermore, it is necessary to be difficult to be heated by the influence of magnetic flux by an electromagnetic induction heating member 65 described later. Therefore, as a material constituting the pad support member 64, for example, PPS (polyphenylene sulfide) with glass fiber, PET (polyethylene terephthalate) with glass fiber, liquid crystal polymer or other heat-resistant resin, heat-resistant glass, low specific resistance, induction heating. A metal such as aluminum which is not easily affected by the above is used. In the present embodiment, the pad support member 64 is made of aluminum having a cross-sectional shape formed in a rectangle having a long axis in the direction of the pressing force from the pressure roll 62.
The pad support member 64 is made of a material having a high magnetic permeability (for example, ferrite, permalloy, etc.), and an internal ferrite member 67 that is a ferromagnetic material for increasing the heating efficiency by the electromagnetic induction heating member 65 is disposed. Has been. In this embodiment, the ferromagnetic material disposed in the fixing belt 61 is a ferrite member. However, the present invention is not limited to this, and for example, a temperature-sensitive magnetic material member may be used. In this case, it is possible to suppress an excessive temperature rise of the fixing belt 61 by using a change in magnetic characteristics near the Curie point of the temperature-sensitive magnetic material.

As shown in FIG. 2, the electromagnetic induction heating member 65 is a ferromagnetic external ferrite member 65 a having a curved surface on the fixing belt 61 side along the outer peripheral surface shape of the fixing belt 61 along the width direction of the fixing belt 61. The excitation coil 65b supported by the external ferrite member 65a and the excitation circuit 65c for supplying a high-frequency current to the excitation coil 65b constitute a main part.
Since the external ferrite member 65a efficiently concentrates the magnetic flux generated by the exciting coil 65b and increases the heating efficiency for the fixing belt 61, a ferromagnetic material such as ferrite can be used.
In addition, as the exciting coil 65b, for example, an ellipse is formed of a litz wire in which a plurality of copper wires having a diameter of 0.5 mm that are insulated from each other by a heat-resistant insulating material (for example, polyimide resin, polyamideimide resin, etc.) are bundled. Or a closed loop shape such as an elliptical shape or a rectangular shape wound several times (for example, 11 turns). The excitation coil 65b is fixed to the external ferrite member 65a while maintaining its shape by being hardened by an adhesive.
Further, the distance between the exciting coil 65b and the internal ferrite member 67 and the conductive layer of the fixing belt 61 is preferably set as close as possible in order to increase the magnetic flux absorption efficiency. Is set within 5 mm, for example, about 2 mm.

In the electromagnetic induction heating member 65, when a high frequency current is supplied from the excitation circuit 65c to the excitation coil 65b, the magnetic flux repeatedly generates and disappears around the excitation coil 65b. Here, the frequency of the high-frequency current is set to 20 to 70 kHz, for example, but is set to approximately 30 kHz in the present embodiment. When the magnetic flux from the exciting coil 65b crosses the conductive layer of the fixing belt 61, a magnetic field is generated in the conductive layer of the fixing belt 61 so as to prevent the change of the magnetic field, thereby generating an eddy current in the conductive layer. In the conductive layer, Joule heat (W = I ² R) proportional to the skin resistance (R) of the conductive layer is generated by the eddy current (I), and the fixing belt 61 is heated to a predetermined temperature.

  As will be described in detail later, the temperature of the fixing belt 61 is measured in a non-contact state by a temperature sensor 70 disposed in the vicinity of the upstream side of the fixing nip N and close to the surface of the fixing belt 61 at that time. Is done. Then, based on the measured value of the temperature sensor 70, the control unit 40 (see FIG. 1) of the image forming apparatus controls the amount of power supplied to the exciting coil 65b or the supply time of the high-frequency current, etc., thereby fixing the fixing belt 61. Maintain a predetermined temperature.

  In the image forming apparatus according to the present embodiment configured as described above, the driving motor for driving the pressure roll 62 in the fixing device 60 is substantially the same as the operation for forming the toner image is started. Electric power is supplied to the electromagnetic induction heating member 65 (not shown) and the fixing device 60 is activated. Then, the fixing belt 61 rotates following the pressure roll 62. In addition, when the fixing belt 61 passes through a heating region facing the electromagnetic induction heating member 65, an eddy current is induced in the conductive layer of the fixing belt 61, and the fixing belt 61 generates heat. Then, in a state where the fixing belt 61 is uniformly heated to a predetermined temperature, the paper P holding the unfixed toner image is sent to the fixing nip portion N where the fixing belt 61 and the pressure roll 62 are pressed. In the fixing nip portion N in the sheet passing area, the sheet P and the toner image held on the sheet P are heated and pressurized, and the toner image is fixed on the sheet P. Thereafter, the paper P is peeled off from the fixing belt 61 due to a change in the curvature of the fixing belt 61 and is conveyed to a paper discharge mounting portion provided in a discharge portion of the image forming apparatus. At that time, as an auxiliary means for completely separating the paper P after fixing from the fixing belt 61, a peeling auxiliary member 75 can be disposed on the downstream side of the fixing nip portion N of the fixing belt 61. .

Next, the temperature sensor (temperature detection unit) 70 will be described in detail.
FIG. 4 is a diagram showing a schematic structure of the temperature sensor 70.
The temperature sensor 70 shown in FIG. 4 includes a head portion 71 provided on the temperature detection side and an attachment portion 72 for attaching the temperature sensor 70 to a predetermined location. Further, the head portion 71 is provided with a heat receiving portion 73 which is a location for detecting temperature.
As shown in FIG. 2, the temperature sensor 70 is disposed in the vicinity of the upstream side of the fixing nip portion N and close to the inner surface of the fixing belt 61. The heat receiving portion 73 is disposed excluding a part of the internal ferrite member 67 disposed in the axial direction, and is disposed in a non-contact manner with the fixing belt 61. In the contact state, the fixing belt 61 is deprived of heat, and the temperature distribution of the fixing belt 61 becomes non-uniform. The temperature sensor 70 may measure the temperature of the fixing belt 61 sequentially in order to maintain the fixing belt 61 at a predetermined temperature, or a function of a hard switch (thermostat) for cutting off power when the temperature rises abnormally. You may have.

However, when a conventional temperature sensor is used, the heat receiving portion is made of nonmagnetic metal aluminum or the like. And the thickness is about 0.5 mm. Since the magnetic flux from the exciting coil acts on the heat receiving portion, the heat distribution of the fixing belt 61 is likely to be affected.
FIG. 5 is a diagram illustrating the influence on the heat generation distribution of the fixing belt 61 when the conventional temperature sensor is used. The temperature distribution from the center to the end of the heat generation distribution of the fixing belt 61 is shown, and the horizontal axis is the position of the fixing belt 61 in the axial direction. The numerical value represents the distance from the central portion of the fixing belt 61, where the 0 mm position corresponds to the central portion, and the −150 mm and 150 mm positions correspond to the end portions. The vertical axis indicates the temperature corresponding to the axial position of the fixing belt 61.

A graph indicated by a dotted line in FIG. 5 is a temperature distribution in the axial direction of the fixing belt 61 when a conventional temperature sensor is used. A graph indicated by a solid line is a temperature distribution in the axial direction of the fixing belt 61 when no temperature sensor is installed.
As shown in FIG. 5, it can be seen that the graph indicated by the dotted line has a temperature drop in the portion from 0 mm to −50 mm near the center of the fixing belt 61 as compared to the graph indicated by the solid line. This portion is a portion where a conventional temperature sensor is installed.
That is, in the fixing device 60 shown in FIG. 2, the fixing belt 61 is arranged so as to be inserted into the electromagnetic induction heating member 65 and the internal ferrite member 67, and the main magnetic flux from the electromagnetic induction heating member 65 is the electromagnetic induction. It is distributed so as to loop between the heating member 65 and the internal ferrite member 67. Since the temperature sensor is present in the region through which the magnetic flux passes, it is electromagnetically induced to a nonmagnetic metal such as aluminum which is the metal of the heat receiving part. At this time, aluminum does not work to strengthen the magnetic flux. Further, the apparent heat resistance of the aluminum heat receiving portion is smaller than that of the conductive layer of the fixing belt 61. For this reason, an eddy current flows easily and the calorific value in a conductive layer falls. As a result, when the conventional temperature sensor is used, it is considered that the temperature distribution of the fixing belt 61 around the temperature sensor is lowered.

Therefore, in the present embodiment, the above-described problem is solved by configuring the temperature sensor 70 to include a temperature-sensitive magnetic material. In particular, since the heat receiving part is easily affected by magnetic flux, the heat receiving part is preferably made of a temperature-sensitive magnetic material.
The temperature-sensitive magnetic material refers to a magnetic material that changes its magnetic characteristics with respect to a temperature change of the magnetic material.

However, when a temperature-sensitive magnetic material is simply used, self-heating may occur due to eddy current flowing in the heat receiving portion. When this self-heating occurs, the heat receiving unit detects a temperature higher than the actual temperature of the fixing belt 61, which causes a malfunction.
Therefore, in this embodiment, the value of the ratio of the specific resistance of the material and the thickness of the region through which the eddy current flows is twice that of the conductive layer and that of the conductive layer. It is preferable to make the above. That is, it is preferable to satisfy the following formula (1).

Here, ρ ₁ and ρ ₂ are specific resistances of the conductive layer and the heat receiving part, respectively. K ₁ and k ₂ are the thicknesses of the conductive layer and the heat receiving part, respectively.

In general, the resistance R is represented by R = ρ (specific resistance) × l (current path length) / A (cross-sectional area through which current flows). When R = ρ × l / (thickness k × section coefficient m), l / m in the right equation can be erased because it is common to the conductive layer and the head as viewed from the section. Therefore, it can be considered that only ρ / k represents the resistance in Ohm's law. Therefore, the easiness of heat generation can be compared by comparing with the value of ρ / k. ρ ₁ / k ₁ represents the ease of heat generation of the conductive layer of the fixing belt 61, and ρ ₂ / k ₂ represents the ease of heat generation of the heat receiving portion of the temperature sensor 70.

The value of ρ ₁ / k ₁ for the conductive layer is, when copper is used as the material, the specific resistance ρ ₁ = 1.7 × 10 ⁻⁸ Ωm, so that the value of ρ ₁ / k ₁ is not too small. to needs _{k 1} is 20μm or more. If it is out of this range, it becomes difficult to obtain a sufficient calorific value (square of eddy current × resistance) as a conductive layer of the heating device.
Under this condition, the values of ρ ₂ and k ₂ of the heat receiving part are determined from the expression (1). At this time, from the viewpoint of suppressing the self-heating of the heat receiving part, an area where eddy current can flow is as much as possible. In order to reduce the thickness, it is preferable to make the thickness of the heat receiving portion as thin as possible. However, if the thickness is too thin, the magnetic flux easily penetrates and it is difficult to form a sufficient magnetic path. Therefore, the thickness of the heat receiving part is preferably 0.05 mm to 0.3 mm.

If an appropriate thickness and material are selected based on the above relationship, a safe fixing device that does not cause a nonuniform state of the temperature distribution of the fixing belt 61 and does not cause a malfunction of the temperature sensor 70 even if the temperature sensor 70 is introduced. realizable.
In order to make the eddy current difficult to flow, it is possible to select a material having a high magnetic permeability or increase the frequency.

  By the way, when the magnetic flux reaches a place other than the heat receiving portion, the heat receiving portion is more likely to be affected by heat generated by the conductive layer. Therefore, it is preferable to keep the magnetic flux within the range of the thickness of the heat receiving portion. The thickness at which the magnetic flux can penetrate can be expressed by the skin depth δ, which can be defined by the following equation (2).

Wherein (2), ρ is specific resistance (Ωm), f is the frequency (Hz), μ _r denotes the relative magnetic permeability.
Therefore, when the skin depth of the heat receiving portion and a [delta] _2, before the heat receiving portion reaches the Curie point,
It is preferable that k ₂ ≧ δ ₂ . Incidentally, when it becomes higher than the Curie, a state in which worked safety apparatus described later, the relationship is not necessarily true, it may be a k _{2 <δ} 2.

  Even when a conventional temperature sensor is used, a means for solving the problem of a decrease in the amount of heat generated in the conductive layer 61b of the fixing belt 61 is to reduce the temperature of the fixing belt 61 in the vicinity of the location where the temperature sensor is installed. Any means to supplement is sufficient. That is, the fixing device 60 only needs to have a structure that increases the strength of the magnetic field at the location where the temperature sensor (temperature detection unit) is installed.

  On the other hand, for example, the rotation of the fixing belt 61 may suddenly stop due to a failure or the like. In this case, the fixing belt 61 may be abnormally heated. This rate of temperature increase is very fast, for example, about 15 ° C./s. In such a case, it is necessary to immediately stop the supply of electric power to the electromagnetic induction heating member 65 and stop the generation of the magnetic field. However, when the temperature sensor of the conventional bimetal type thermostat is used, first, the heat of the fixing belt 61 is transferred to the head portion of the thermostat in a non-contact state, and further, heat transfer to the bimetal portion proceeds from there. The temperature rises to the set temperature, and the bimetal switch works to cut off the power supply signal line. Such a heat transfer mode cannot cope with a rapid temperature rise of the fixing belt 61 in terms of responsiveness and lacks safety.

Therefore, in the present embodiment, this problem is solved by making the structure of the temperature sensor 70 more appropriately use the properties of the temperature-sensitive magnetic material constituting the heat receiving portion.
In the present embodiment, attention is paid to the Curie point as a change in the magnetic characteristics of the temperature-sensitive magnetic material, which is used as a temperature sensor. That is, the temperature-sensitive magnetic material has a property that when the temperature is higher than the Curie point, the magnetism disappears and becomes a non-magnetic material (paramagnetic material). A temperature-sensitive magnetic material is disposed on the head portion of the sensor and this head portion is used as a heat receiving portion. For example, a sensor that switches off is created by using the Curie point as the operation set temperature and changing from magnetic to non-magnetic at this temperature.

  Hereinafter, a specific structure of a temperature sensor using such a temperature-sensitive magnetic material as a heat receiving portion will be described.

FIG. 6 is an example of a temperature sensor 70 using a temperature-sensitive magnetic material for the heat receiving portion, and is a diagram illustrating a head portion of the temperature sensor 70 including the heat receiving portion.
FIG. 6A is a diagram showing the first form. The main part of the head portion 710 shown in FIG. 6A is composed of a heat receiving portion 711 using a temperature-sensitive magnetic material, a magnet 712, a support member 713 that supports the magnet 712, and a spring member 714. Since the heat receiving portion 711 is made of a temperature-sensitive magnetic material, the heat receiving portion 711 is a ferromagnetic material at a temperature below the Curie point, attracts the magnet 712, and the heat receiving portion 711 and the magnet 712 are in contact with each other. However, when the heat receiving part 711 is equal to or higher than the Curie point, the heat receiving part 711 loses the attractive force with the magnet 712 because it changes to a paramagnetic material. As a result, the magnet 712 moves together with the support member 713 in the direction of arrow D due to the tensile force of the spring member 714 that has been stretched while the heat receiving portion 711 and the magnet 712 are attracted, and the heat receiving portion 711 and the magnet 712 are not in contact with each other. It becomes the state of. By using the contact / non-contact state between the heat receiving portion 711 and the magnet 712 as a switch, it can be used as a temperature sensor.

FIG. 6B is a diagram showing a second form of the head portion. The head portion 720 shown in FIG. 6B supports and fixes the heat receiving portion 721 using the temperature-sensitive magnetic material, the first magnet 722, the fixed second magnet 723, and the second magnet 723. The main part is composed of a support member 724 that keeps the state, and a protrusion 725 that obstructs direct contact between the heat receiving portion 721 and the first magnet 722.
The first magnet 722 is movable between the heat receiving part 721 and the second magnet 723. The first magnet 722 receives an attractive force from the second magnet 723. When the heat receiving part 721 is at a temperature equal to or lower than the Curie point, the heat receiving part 721 is a ferromagnetic body, and the heat receiving part 721 Since the suction force is strong, it moves toward the heat receiving part 721. However, since there is the protruding portion 725, the heat receiving portion 721 stops at the position shown in FIG. In this state, the first magnet 722 and the second magnet 723 are not in contact with each other. However, when the heat receiving portion 721 reaches a temperature equal to or higher than the Curie point, the heat receiving portion 721 becomes a paramagnetic material, and the attractive force between the heat receiving portion 721 and the first magnet 722 is lost. As a result, the first magnet 722 receives the attractive force from the second magnet 723, moves in the direction of arrow E, and comes into contact with the second magnet 723. By using the non-contact state and the contact state between the first magnet 722 and the second magnet 723 as a switch, it can be used as a temperature sensor. Note that the heat receiving portion 721 and the first magnet 722 are not contacted by the protrusion 725 below the Curie point. The heat receiving portion 721 is configured to have no contact object so that the first magnet 722 receives heat. This is because the heat of the portion 721 is not taken away and a more rapid operation is possible, which is a more preferable configuration.

  As described above, the heat receiving portions 711 and 721 have a thickness of 0.05 mm to 0.2 mm. The magnet 712, the first magnet 722, and the second magnet 723 are not particularly limited as long as they are magnets. For example, a normal hard ferrite magnet can be used.

  By using a temperature sensor having a structure as shown in FIG. 6, a sensor that operates at a higher speed than a bimetal temperature sensor can be realized.

Next, the specific composition of the temperature-sensitive magnetic material will be described.
Temperature-sensitive magnetic materials are roughly classified into metal materials and oxide materials, but oxide materials (for example, soft ferrite) are difficult to reduce in thickness and are easily broken and difficult to handle. In addition, since the heat capacity is large and the thermal conductivity is low, there is a problem that it is difficult to follow a rapid temperature change of the fixing belt with high sensitivity and heat generation control becomes difficult.
Therefore, a metal material that does not cause the above-described problems, is inexpensive, can be easily formed into a thin wall, has good workability, flexibility, and has high thermal conductivity is preferable. Of these, a magnetic shunt alloy and an amorphous alloy are preferable. More specifically, it is a metal alloy material made of Fe, Ni, Cr, Si, B, Nb, Cu, Zr, Co, V, Mn, Mo, or the like. Of these, it is particularly preferable to use an Fe—Ni binary magnetic shunt alloy or an Fe—Ni—Cr ternary magnetic shunt alloy.

The Curie point of the temperature-sensitive magnetic material, that is, the operating temperature of the temperature sensor, is preferably in the range from the set temperature of the fixing belt 61 to the heat resistance temperature of the fixing belt 61. For example, it is preferable that it is 140 to 240 degreeC, More preferably, it is 190 to 220 degreeC.
As the composition of the temperature-sensitive magnetic material, for example, in the Fe-Ni binary magnetic shunt steel, the Curie point can be set to around 230 ° C. by setting it to approximately 64% Fe and 36% Ni (atomic ratio). .

[Fixing device, temperature sensor]
The fixing device shown in FIG. 2 was used. The temperature sensor 70 as shown in FIG. 4 was used for the fixing device 60 shown in FIG. 2, and its temperature responsiveness was confirmed. As the temperature-sensitive magnetic material, an Fe—Ni binary magnetic shunt steel having a Curie point of 200 ° C. was used. At this temperature, the temperature sensor 70 performs switching, interrupts the power flowing into the electromagnetic induction heating member 65, and stops the generation of the magnetic field.
The distance between the fixing belt 61 and the heat receiving portion 73 (see FIG. 4) of the temperature sensor 70 was 3 mm.

〔test〕
The following tests were performed under such apparatus and conditions.
(Test 1)
The fixing device 60 was operated, and the temperature distribution in the axial direction of the fixing belt 61 was measured.
(Test 2)
Further, the fixing belt 61 is forcibly stopped during the operation of the fixing device 60, and the temperature sensor 70 is used as the temperature sensor 70 in the conventional thermostat method in accordance with the temperature rise of the fixing belt 61, and the present embodiment. The time until the power was cut off was compared with that of the form.

〔result〕
(Test 1)
When the temperature sensor 70 of the present embodiment is used, the temperature distribution in the axial direction of the fixing belt 61 is as shown by the solid line portion in FIG. 5, and the fixing belt 61 of the fixing belt 61 at the installation location due to the installation of the temperature sensor 70. There was no temperature drop.

(Test 2)
The experimental results of Test 2 are shown in FIG.
FIG. 7 is a diagram illustrating the elapsed time after the fixing belt 61 is stopped and the temperature of the inner peripheral surface of the fixing belt 61. The horizontal axis represents the elapsed time, and the vertical axis represents the temperature of the inner peripheral surface of the fixing belt 61.
When the conventional temperature sensor was used, it took 26 to 28 seconds after the fixing belt 61 was stopped until the power was cut off. At this time, the temperature of the fixing belt 61 rose to close to 700 ° C., which was in a dangerous state (see part G in FIG. 7).
On the other hand, when the temperature sensor 70 of the present embodiment is used, it takes 13 seconds to 15 seconds from when the fixing belt 61 is stopped to when the power is cut off (see portion F in FIG. 7). When the temperature sensor 70 according to the present embodiment is used in contrast to the case where the conventional temperature sensor is used, it has been confirmed that the operation time is about half and the safety is improved against an abnormality such as a failure.

1 is a schematic configuration diagram illustrating an image forming apparatus to which the exemplary embodiment is applied. 1 is a schematic configuration diagram illustrating a configuration of a fixing device according to an exemplary embodiment. FIG. 2 is a schematic diagram illustrating a configuration of a fixing belt. It is the figure which showed the schematic structure of the temperature sensor. FIG. 10 is a diagram illustrating an influence on a heat distribution of a fixing belt when a conventional temperature sensor is used. It is an example of the temperature sensor which used the temperature sensitive magnetic material for the heat receiving part, and is a figure explaining the head part of the temperature sensor including the heat receiving part. FIG. 6 is a diagram illustrating an elapsed time after the fixing belt is stopped and a temperature of an inner peripheral surface of the fixing belt.

Explanation of symbols

DESCRIPTION OF SYMBOLS 60 ... Fixing device, 61 ... Fixing belt, 61b ... Conductive layer, 62 ... Pressure roll, 63 ... Press pad, 64 ... Pad support member, 65 ... Electromagnetic induction heating member, 65a ... External ferrite member, 67 ... Internal ferrite member , 70 ... Temperature sensor, 71, 710, 720 ... Head part, 73, 711, 721 ... Heat receiving part, 712 ... Magnet, 722 ... First magnet, 723 ... Second magnet

Claims (8)

A belt member including a conductive layer heated by the action of a magnetic field;
A drive unit for rotating the belt member;
A heating unit for generating the magnetic field;
A pressure member that forms a fixing nip portion for inserting a recording material holding an unfixed image between the belt member and the outer peripheral surface of the belt member;
A pressing member that presses the belt member from the inside of the belt member toward the pressure member;
A temperature detector for detecting the temperature of the belt member;
A structure for increasing the strength of the magnetic field at the location where the temperature detection unit is installed;
A fixing device.   The fixing device according to claim 1, wherein the structure includes the temperature detection unit including a temperature-sensitive magnetic material.   The fixing device according to claim 2, wherein the temperature-sensitive magnetic material is a metal alloy material. When the thickness of the temperature-sensitive magnetic material is k ₁ , the thickness of the conductive layer is k ₂ , the specific resistance of the temperature-sensitive magnetic material is ρ ₁ , and the specific resistance of the conductive layer is ρ ₂ , the following formula ( The fixing device according to claim 3, wherein 1) is satisfied.
2 × ρ ₁ / k ₁ <ρ ₂ / k ₂ (1)   The fixing device according to claim 2, wherein the temperature-sensitive magnetic material has a thickness of 0.05 mm to 0.3 mm. The temperature detection unit further includes a magnet that attracts the temperature-sensitive magnetic material when the temperature-sensitive magnetic material has magnetism at a temperature lower than the Curie point,
3. The thermosensitive magnetic material according to claim 2, wherein when the magnetism disappears at a temperature equal to or higher than the Curie point, switching is performed by moving the magnet, and generation of the magnetic field in the heating unit is stopped. The fixing device described. Toner image forming means for forming a toner image;
Transfer means for transferring the toner image to a recording medium;
Fixing means for fixing the toner image on a recording medium;
Have
The fixing means is
A belt member that is rotated by a drive unit, a heating unit that heats the belt member by the action of a magnetic field, and a temperature detection unit that uses a temperature-sensitive magnetic material for temperature detection,
The image forming apparatus, wherein the temperature detection unit stops generating a magnetic field in the heating unit when the temperature of the temperature-sensitive magnetic material becomes equal to or higher than the Curie point of the temperature-sensitive magnetic material. The temperature detection unit further includes a magnet installed to face the temperature-sensitive magnetic material,
The magnet is switched by attracting the temperature-sensitive magnetic material to the temperature-sensitive magnetic material at a temperature below the Curie point, and leaving the temperature-sensitive magnetic material at a temperature above the Curie point, and The image forming apparatus according to claim 7, wherein the generation is stopped.

Images