JP2008216390A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device that has a self-temperature control function for preventing excessive rise of the temperature of a paper non-passage part when small size sheets are consecutively passed. <P>SOLUTION: The fixing device has an excitation coil disposed in a fixing member and induction heats a heat generating member by electromagnetic induction. In the fixing device, the heat generating member is a magnetic member, and an excessive rise of temperature preventing member is disposed at a position opposite the excitation coil via the fixing member. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heating apparatus, and more particularly to an image forming apparatus, and more particularly, to a toner fixing apparatus used in an electrophotographic copying machine, a printer, a facsimile, and the like.

  Induction heating method: A copying machine, a printer, a FAX, and the like have a process of forming an image on a recording medium such as plain paper or OHP. Various image recording methods have been realized. Among them, the electrophotographic method is widely adopted in the above-mentioned devices because of high speed, image quality, cost, and the like.

  In the electrophotographic system, there is a fixing process in which an unfixed toner image is formed on a recording medium such as paper or OHP and fixed with heat and pressure. As the fixing method, the heat roller method is currently most frequently used from the viewpoint of high speed and safety.

  In the heat roller method, a fixing roller heated by a heat generating member such as a halogen heater and a pressure roller disposed opposite to the fixing roller are pressed to form a mutual pressure contact portion called a nip region, and a sheet is placed between both rollers. It is a method of passing and heating.

  The heat roller method has a problem that there is a limit to reducing the heat capacity (thinning) of the fixing roller and the start-up time is slow due to the slow start-up of the halogen heater itself.

  Therefore, in recent years, a fixing method using an induction heating method as a heating means has been studied. In this method, induction heating is performed by generating an eddy current by an alternating magnetic field generated by a magnetic field generating means on a heat generating member having a conductive layer. In this method, the rise of heating is faster than that of the halogen heater, and the object to be heated can be directly heated. Therefore, it is possible to start up in a short time compared to the heat roller method.

  Problem of induction heating method (end temperature increase): On the other hand, as a problem of induction heating, there is a problem that it is difficult to control the temperature distribution in the longitudinal direction of the heat generating member.

  The fixing device may cause a problem that part or all of the fixing member overheats when a small-sized recording medium is continuously fixed or when the device suddenly stops driving due to a paper jam or the like. there were.

  Details are as follows. A general image forming apparatus is configured to form an image on several types of recording media having different sizes in the width direction. Here, the recording media having different sizes in the width direction include recording media of irregular sizes in addition to recording media of various regular sizes in the A and B rows of JIS dimensions. Even in the case of recording media of the same size (for example, A4 size), the transport direction is the short direction (the direction perpendicular to the longitudinal direction) when the longitudinal direction is the transport direction. In some cases, recording media having different sizes in the width direction are handled.

  When fixing such a recording medium having a different size in the width direction with the fixing device, the heat distribution in the width direction of the fixing member varies depending on the size in the width direction of the recording medium, resulting in temperature unevenness. was there. For example, when fixing by passing a recording medium having a small width direction size, a large amount of heat is taken away at the position of the fixing member corresponding to the width direction size of the recording medium (the sheet passing area). The fixing temperature is lower than at other positions (non-sheet passing area). Such a phenomenon becomes particularly prominent when a recording medium having a small size in the width direction is continuously fed.

  Therefore, when trying to control the fixing temperature in the entire width direction of the fixing member with reference to the fixing temperature in the center portion in the width direction of the fixing member, the fixing temperature in the center portion in the width direction of the fixing member can be controlled to a desired temperature. The fixing temperature at both ends in the direction will rise (overheated). In this way, when a recording medium having a large width direction is fixed in a state where the fixing temperature at both ends in the width direction of the fixing member is increased, a hot offset occurs on the recording medium corresponding to the temperature increase position. Furthermore, when the fixing temperature at both ends in the width direction exceeds the heat resistance temperature of the fixing member, it may be considered that the fixing member is thermally damaged.

  On the other hand, if it is attempted to control the fixing temperature in the entire width direction of the fixing member with reference to the fixing temperature at both ends in the width direction of the fixing member, the fixing temperature at both ends in the width direction of the fixing member can be controlled to a desired temperature. However, the fixing temperature at the center in the width direction is lowered. As described above, when the recording medium is fixed in a state where the fixing temperature at the center portion in the width direction of the fixing member is lowered, a cold offset occurs on the recording medium corresponding to the temperature lowered position.

  To solve this problem, a conventional fixing device using a halogen heater as a heating source has controlled the temperature of the fixing member by controlling two heaters having a light distribution divided into a central portion and an end portion. Such control is very difficult in the induction heating in which the object is heated by the magnetic flux generated from the coil. There is a method of arranging two coils for heating the central part and two coils for heating the end part like a halogen heater, but there are many problems such as cost UP and interference between coils.

  Self-temperature control type IH fixing using a magnetic shunt alloy: Therefore, a self-temperature control type induction heating fixing method using a magnetic material in which the Curie point is adjusted near the fixing set temperature is proposed in Patent Document 1, etc. ing.

  These techniques use a magnetic body (magnetism control alloy) having an appropriate Curie point for the heat generating layer.

  When the temperature of the heat generating layer is lower than the Curie point, the heat generating layer is a magnetic material, so that the magnetic flux concentrates on the heat generating layer and has high heating efficiency, whereas the temperature of the heat generating layer is higher than the Curie point. Is a technique in which the heat generation layer becomes non-magnetic, magnetic flux is diverged, the heating efficiency is lowered, and the heat generation amount of the heat generation layer is reduced.

The above prior art is an invention in which when the temperature of the heat generating layer becomes equal to or higher than the Curie point, the heat generating layer becomes non-magnetic and heat generation is reduced, and it is a very useful invention for preventing temperature rise at the end, which is a problem of the IH fixing device.
JP 2006-208909 A

  However, when the magnetic shunt alloy exceeds the Curie point temperature, the amount of heat generation decreases, but the amount of decrease is small and is not sufficient to reliably prevent overheating.

  Further, when the magnetic shunt alloy exceeds the Curie point temperature, the magnetic shunt alloy becomes non-magnetic, and a large amount of magnetic flux generated from the coil is radiated outside the roller, so that the magnetic flux leaking outside the fixing device increases.

  In view of the above-described problems, the present invention is a fixing device that fixes a toner image to a recording medium. The fixing device is composed of a rotating body and sandwiches and conveys the recording medium to melt and fix the unfixed image on the recording material. In a fixing device having a fixing member, the fixing member having a heat generating member having conductivity, and disposed inside the fixing member, and having an exciting coil for induction heating the heat generating member by electromagnetic induction. Provides a fixing device with a self-temperature control function that prevents non-sheet-passing parts from over-heating when small-size paper is continuously fed by having an over-temperature prevention member at a position facing the exciting coil. The purpose is to do.

  According to the first aspect of the present invention, in the fixing device having an excitation coil that is disposed inside the fixing member and induction-heats the heating member by electromagnetic induction, the heating member is a magnetic member, and the excitation coil is connected to the excitation coil via the fixing member. In the fixing device, an excessive temperature rise prevention member is provided at an opposing position.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the thickness of the heat generating member corresponds to the frequency of the alternating current applied to the exciting coil, and the temperature of the heat generating member is equal to or lower than the Curie point. It is characterized by being formed so as to be thicker than the penetration depth.

  According to a third aspect of the present invention, in the fixing device according to the first or second aspect, the overheat prevention member is thicker than the penetration depth corresponding to the frequency of the alternating current applied to the exciting coil. It was formed as follows.

  According to a fourth aspect of the present invention, there is provided the fixing device according to the first aspect, wherein the overheating prevention member is made of a magnetic shunt alloy and is nonmagnetic and first overheating prevention. It consists of a second overheating prevention member having a volume resistivity lower than that of the members, and each member is a coil, a heating member, a first overheating prevention member, and a second overheating prevention member from the inside of the fixing member. It is arranged in the order of.

  According to a fifth aspect of the present invention, in the fixing device according to the fourth aspect, the thickness of the first overheating prevention member corresponds to the frequency of the alternating current applied to the exciting coil. It is characterized by being formed so as to be thicker than the penetration depth when the temperature of the excessive temperature rise prevention member is equal to or lower than the Curie point.

  According to a sixth aspect of the present invention, in the fixing device according to the fourth or fifth aspect, the second overheat prevention member has a penetration depth corresponding to the frequency of the alternating current applied to the exciting coil. It is formed to be thicker than the thickness.

  A seventh aspect of the present invention is the fixing device according to any one of the first to sixth aspects, wherein the fixing device has a casing that accommodates a fixing member, and a part of the casing is prevented from overheating. It is formed by a member.

  According to an eighth aspect of the present invention, in the fixing device according to any one of the first to sixth aspects of the present invention, the fixing device includes a pressure member made of a rotating body disposed in pressure contact with the fixing member. A fixing device in which a recording material is inserted into a fixing nip region formed therebetween, and the pressure member is pressed against the fixing member from the back surface of the pressure member within the range of the fixing nip region between the fixing member and the pressure member. A pressing means is provided, and the pressing means is formed by an excessive temperature rise prevention member.

  According to a ninth aspect of the present invention, there is provided an image forming apparatus comprising the fixing device according to any one of the first to eighth aspects.

  According to the present invention, it is possible to provide a fixing device having a self-temperature control function in which a non-sheet passing portion does not overheat when a small size sheet is continuously passed.

  Image Forming Apparatus Configuration Example: FIG. 22 shows a cross-sectional view of an image forming apparatus to which the present invention can be suitably applied. The entire image forming apparatus is composed of an upper part and a lower part.

  The upper part has an automatic document feeder (not shown) above it, an optical unit made up of optical elements not shown in detail below, and further each unit of the image forming system located below it, The lower part has a paper feed unit that houses a plurality of paper feed trays on which recording members P of a plurality of sizes are respectively placed.

  Reference numeral 41 is an example of an image carrier made of a rotating body, and shows a drum-shaped photoconductor. Around the photosensitive member 41, a charging device 42 including a charging roller, a mirror 43 constituting a part of an exposure unit, a developing unit 44 including a developing roller 44A, and a recording member P are arranged in the rotation direction indicated by an arrow. A transfer device 48 for transferring the developed image to the transfer paper as a transfer member, a cleaning means 46 provided with a blade 46A slidably contacting the peripheral surface of the photoreceptor 41, and the like are disposed. The exposure light LB is scanned on the photosensitive member 41 via the mirror 43 between the charging device 42 and the developing roller 44A. The irradiation position of the exposure light LB is called an exposure unit 150.

  The transfer device 48 faces the lower surface of the photoconductor 41. The facing portion is a transfer portion 47, and a transfer device 48 is provided in the transfer portion 47.

  A pair of registration rollers 49 are provided at a position further upstream of the transfer portion 47 in the conveyance path of the recording member P. A recording member P 2 guided by a conveyance guide and stored in the paper feed tray 40 is sent out from the paper feed roller 110 toward the registration roller 49. The fixing device 20 is disposed at a position further downstream of the transfer portion 47 in the conveyance path of the recording member P. An automatic duplex device 39 is disposed downstream of the conveyance path of the recording member P of the fixing device 20.

  In this image forming apparatus, image formation is performed as follows. The photosensitive member 41 starts to rotate, and during this rotation, the photosensitive member 41 is uniformly charged by the charging device 42 in the dark, and the exposure light LB is irradiated to the exposure unit 150 and scanned to form a latent image corresponding to the image to be created. It is formed. This latent image is moved to the developing device 44 by the rotation of the photosensitive member 41, where it is visualized with toner to form a toner image.

  On the other hand, feeding of the recording member P on the paper feed tray is started by the paper feed roller 110, temporarily stops at the position of the pair of registration rollers 49 through the transport path indicated by the broken line, and the toner image on the photoconductor 41 and the transfer are transferred. The timing of delivery is waited so as to match at the unit 47. When such a suitable timing arrives, the recording member P stopped on the registration roller 49 is sent out from the registration roller 49 and conveyed toward the transfer unit 47.

  The toner image on the photoconductor 41 and the recording member P coincide with each other at the transfer portion 47, and the toner image is transferred onto the recording member P by the electric field generated by the transfer member 48.

  In this way, the recording member P carrying the toner image in the image forming portion around the photoconductor 41 is sent out toward the fixing device 20. The toner image on the recording member P 1 is fixed to the recording member P while passing through the fixing device 20 and discharged to the paper discharge unit.

  The image forming apparatus 1 can form an image on both sides of the recording member P. The recording member P discharged to the automatic double-side device 39 by a branching claw (not shown) is switched back by the automatic double-side device 39 and is conveyed to the conveyance path before the registration roller 49.

  On the other hand, the residual toner remaining on the photosensitive member 41 without being transferred by the transfer unit 47 reaches the cleaning device 46 along with the rotation of the photosensitive member 41 and is cleaned while passing through the cleaning device 46 to prepare for the next image formation. It is done.

  Embodiment 1 (Claims 1 and 2): An embodiment will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view of a fixing device 20 in this embodiment. The fixing device 20 includes a fixing roller 2 as a fixing member, a pressure roller 4 that forms a nip region with the fixing roller 2, an excitation coil 3 as a magnetic field generating means, a bobbin 301 around which the excitation coil 3 is wound, an iron core of the coil, The ferrite core 5 and the excessive temperature rise prevention member 6 are configured.

  The fixing roller 2 has an outer diameter of 40 mm, and a magnetic field generated from the exciting coil 3 induction heats the fixing roller 2.

  The fixing roller 2 is rotationally driven in a clockwise direction in FIG. 1 by a driving unit (not shown).

  The recording member P is heated by being supplied with heat from the fixing roller 2 by passing through a nip region formed by the fixing roller 2 and the pressure roller 4.

  (Coil shape) The exciting coil 3 is formed by circling a wire bundle, which is a bundle of 90 copper wires having an outer diameter of 0.15 mm whose surface is insulated, around a bobbin 301 disposed inside the fixing roller 2. The wire is drawn in the direction of the rotation axis and circulated.

  The ferrite core 5 faces the circumferential surface of the fixing roller 2 at a position facing the circumferential surface of the fixing roller 2 and behind the coil, and the circumferential surface of the fixing roller 2 not through the coil. The second core 5B is disposed at a position closer to the fixing roller 2 than the core 5A, and the magnetic flux generated from the exciting coil 3 is concentrated on the portion of the fixing roller 2 where heat is desired to be generated. ing.

  (In this embodiment, the portion in front of the nip region of the fixing roller) The first core and the second core are preferably made of a ferromagnetic material having a high electrical resistivity. In addition to ferrite, materials such as permalloy are listed.

  (Configuration of Fixing Roller) As shown in FIG. 2, the fixing roller 2 includes a release layer 23, an elastic layer 22, and a heat generating layer 21 in this order from the outer periphery of the fixing roller toward the center of the roller.

  (Heat generation layer) The heat generation layer 21 is made of a magnetic conductive material. Here, as the material of the heat generating layer 21, a magnetic shunt alloy having a Curie point higher than the fixable temperature and lower than 300 degrees is used. Specifically, it is an alloy of nickel, iron, and chromium, and a desired Curie point can be obtained by adjusting the amount of each material added and the processing conditions. Thus, by forming the heat generating layer 21 with a magnetic conductive material having a Curie point near the fixing temperature of the fixing roller 2, the fixing roller 2 is heated without being excessively heated by electromagnetic induction. Become.

  In the first embodiment, a magnetic shunt alloy having a Curie point of 180 degrees is used for the heat generating layer 21 in order to reliably prevent overheating.

  Thus, by forming the heat generating layer 21 with a magnetic conductive material having a Curie point near the fixing temperature, the fixing roller 2 is heated without being excessively heated by electromagnetic induction.

  The thickness of the heat generating layer 21 was as thin as 100 μm to reduce the heat capacity. In this configuration, the thickness of the heat generating layer 21 is desirably equal to or greater than the current penetration depth when the temperature of the heat generating layer 21 is equal to or lower than the Curie point at the frequency of the alternating current applied. Thereby, it is possible to prevent an excessive temperature increase from being caused by electromagnetic induction.

  (Explanation of Penetration Depth) The eddy current induced in the metal by the alternating magnetic flux generated by the alternating current increases as it approaches the metal surface, and decreases exponentially as it enters the interior. When the metal is a magnetic material, the induced eddy current is further concentrated on the metal surface.

  The depth from the surface at the point where the eddy current is reduced to 0.368 times the current density at the surface is called the current penetration depth δ, and is expressed by Equation 1, and the eddy current flows from the metal surface to the inside from the penetration depth. Is very small compared to the surface and has little effect on induction heating.

  If the thickness of the metal is greater than or equal to the penetration depth, the magnetic flux that has entered from the metal surface loses energy in the metal layer, and hardly penetrates the metal plate.

  (Formula 1) ρ: Volume resistivity of metal (Ω · m) μ: Relative magnetic permeability of metal f: Frequency of alternating current (Hz)

  That is, if the thickness of the heat generating layer 21 is significantly thinner than the penetration depth, the magnetic flux of the coil passes through the magnetic shunt alloy even if the temperature of the magnetic shunt alloy is magnetic below the Curie point. Therefore, the energy of induction heating cannot be concentrated on the heat generating layer 21, and the self-temperature control capability is reduced.

  Therefore, it is desirable that the thickness of the heat generating layer 21 be equal to or greater than the current penetration depth when the temperature of the heat generating layer 21 is equal to or lower than the Curie point.

  However, if the thickness of the heat generating layer 21 is significantly increased, the heat capacity of the fixing roller 2 increases, so that the heating rate of the fixing roller 2 is slowed down.

  In this configuration, induction heating was performed with an alternating current of 25 kHz. The penetration depth when the temperature of the magnetic shunt alloy used in this example is 25 kHz at a Curie point or lower is preferably about 80 μm or more as the thickness of the magnetic shunt alloy, and the upper limit of the thickness is the heat capacity About 1.5 mm is desirable from the viewpoint.

  (Elastic layer) The elastic layer 22 of the fixing roller 2 is made of silicone rubber, fluorosilicone rubber or the like, and is formed to have a layer thickness of 50 to 500 μm and an Asker hardness of 5 to 50 degrees. Thereby, a uniform image quality without gloss unevenness can be obtained in the output image.

  (Release layer) The release layer 23 of the fixing roller 2 is made of tetrafluoroethylene resin (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin (PFA), tetrafluoroethylene / hexafluoropropylene. A fluororesin such as a copolymer (FEP), a mixture of these resins, or a dispersion of these resins in a heat resistant resin.

  The layer thickness of the release layer 23 is 5 to 50 μm (preferably 10 to 30 μm). Thereby, the toner releasability on the fixing roller 2 is ensured and the flexibility of the fixing roller 2 is ensured. A primer layer or the like can be provided between the layers 21 to 23 of the fixing roller 2.

  (Over-temperature rise prevention member) The over-temperature rise prevention member 6 is disposed outside the fixing roller and at a position facing the exciting coil. The excessive temperature rise prevention member may be disposed within the range of the magnetic field generated from the exciting coil 3 when the heat generating layer 21 reaches a temperature equal to or higher than the Curie point.

  The excessive temperature rise prevention member 6 is made of a conductive material that is nonmagnetic and has a volume resistivity lower than that of the heat generating layer 21.

  Since the volume resistivity of the heat generating layer 21 is 3.6 × 10 −7 Ω · m, the excessive temperature rise prevention member 6 is formed of a material having a volume resistivity lower than 3.6 × 10 −7 Ω · m. ing. Specifically, it is preferable to use copper, aluminum, gold, silver, or the like having a volume resistivity of 3.0 × 10 −8 Ω · m or less.

  In the first embodiment, the non-magnetic material aluminum is used as the material of the excessive temperature rise prevention member. By using a non-magnetic and electrically conductive member, it is possible to obtain an effect of shielding the magnetic flux while suppressing the heat generation of the excessive temperature rise prevention member 6 itself.

  More specifically, when the magnetic flux generated from the coil penetrates the metal, a counter electromotive force is generated in the metal in a direction to cancel the penetrated magnetic flux, and an eddy current flows in the metal layer. When an eddy current flows in the metal layer, the metal generates a magnetic flux (reaction magnetic field) in a direction to cancel the magnetic flux generated from the coil, and also generates Joule heat due to the effective resistance of the metal. Therefore, induction heating can be performed efficiently by using a metal having a large effective resistance as a heating element.

  However, since non-magnetic and low-resistance materials have low effective resistance, heat generation due to Joule heat is very small, and a large amount of reaction magnetic field is generated against the magnetic field generated from the coil. Therefore, the excessive temperature rise prevention member 6 is made of a conductive material that is nonmagnetic and has a volume resistivity lower than the volume resistivity of the heat generating layer 21.

  In addition, the thickness of the excessive temperature rise prevention member is desirably equal to or greater than the penetration depth. By increasing the thickness of the excessive temperature rise prevention member, the effective resistance of the excessive temperature rise prevention member can be reduced, so that the heat generation of the excessive temperature rise prevention member is reduced and the reaction magnetic field against the magnetic field generated from the coil is reduced. Can be generated.

  If the excessive temperature rise prevention member is made too thin, the effective resistance increases, and the amount of heat generated by the excess temperature rise prevention member itself increases, so that the self-temperature control capability decreases. If the thickness exceeds the penetration depth, the magnetic flux that has entered from the surface of the excessive temperature rise prevention member loses energy in the member and can hardly pass through the excessive temperature rise prevention member. However, it is not necessary to make it extremely thick. Thereby, the heat generation suppressing effect can be increased.

  In this configuration, since the induction heating is performed by an alternating current of 25 kHz, the thickness of the aluminum that is the excessive temperature rise prevention member is desirably 0.5 mm or more. In this configuration, the thickness of the excessive temperature rise prevention member 6 is 1 mm.

  (Operation of Fixing Device 20) The fixing device 20 configured as described above operates as follows. By the rotation driving of the pressure roller 4, the fixing roller 2 rotates in the direction of the arrow in FIG. The fixing roller 2 is heated by the heat generated by the heat generating layer 21.

  More specifically, when a high-frequency alternating current of 10 kHz to 1 MHz is passed through the excitation coil 3, the magnetic lines of force are alternately switched in both directions in the loop of the excitation coil 3. By forming an alternating magnetic field in this way, an eddy current is generated in the heat generating layer 21, Joule heat is generated, and the heat generating layer 21 is inductively heated. The toner image T on the transported recording medium P is heated and melted by the heat generated from the fixing roller 2 in this way.

  In such a fixing process, when the temperature of the heat generating layer 21 exceeds the Curie point, heat generation of the fixing roller 2 is limited.

  (Self-temperature control mechanism of magnetic shunt alloy) The self-temperature control mechanism by the magnetic shunt alloy will be described using the prior art as an example.

  FIG. 3 is a cross-sectional view of a conventional fixing device 20. The difference from this embodiment is that it does not have an excessive temperature rise prevention member, and other configurations are the same.

  FIG. 4 is a schematic view of a portion relating to induction heating of the conventional fixing device 20. FIG. 4A shows a state of the magnetic flux A generated from the coil 3 when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point.

  When the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, the magnetic flux A generated from the coil 3 passes through the heat generating layer 21 through the core 5 and returns to the core 5 again. At this time, since the magnetic shunt alloy as the heat generating layer 21 is a magnetic body, the magnetic flux concentrates on the heat generating body. An induced current flows through the heat generating layer 21 and heat is generated by Joule heat.

  On the other hand, the magnetic flux generated from the coil 3 when the temperature of the magnetic shunt alloy becomes equal to or higher than the Curie point is the magnetic flux B shown in FIG. The magnetic flux B generated from the coil 3 does not change where the basic path is the core 5, but the temperature of the magnetic shunt alloy as the heat generating layer 21 is equal to or higher than the Curie point, and the magnetic shunt alloy is a non-magnetic material. Therefore, the magnetic flux B does not concentrate on the heating element 21 but is dispersed in the space between the heating layer 21 and the coil 3 and the space outside the fixing roller.

  As a result, the magnetic flux that induces a current in the heat generating layer 21 is reduced, so that the amount of heat generation becomes small due to the difference between the magnetic fluxes A and B where the heat generation amount becomes small. The self-temperature control function works.

  (Problems of the prior art) However, the prior art has a problem that the self-temperature control capability is not sufficient to reliably prevent overheating.

  FIG. 5 shows the change over time of the temperature of the fixing roller when a continuous paper passing test is performed on a paper having a small size in the width direction using the conventional fixing device of FIG.

  FIG. 5 shows the temperatures of the non-sheet passing regions of “a fixing roller using Ni for the heat generation layer” and “fixing roller using a magnetic shunt alloy for the heat generation layer”. For reference, the temperature of the fixing roller in the paper passing area is also shown.

  When continuous paper feeding starts, the temperature of the “paper feeding area” where the paper on the fixing roller contacts is controlled so as to maintain the fixing setting temperature of 170 ° C. For this reason, the temperature at the end of the fixing roller in the “non-passage area” where heat is not taken from the paper rises. In the device using Ni for the heat generating layer, the temperature of the end continues to rise.

  It can be seen that in the fixing roller using the magnetic shunt alloy for the heat generating layer, the amount of heat generation is limited when the temperature of the magnetic shunt alloy exceeds the Curie point, and the temperature rise at the end is slowed down.

  However, the temperature rise at the end does not stop completely, and the temperature at the end gradually increases. In the fixing device using the magnetic shunt alloy for the heat generation layer of the prior art, the temperature of the end portion becomes 200 ° C. or more when 3 minutes have elapsed from the start of continuous sheet passing.

  Therefore, in the prior art, when continuously passing a small size of 3 minutes or more, excessive temperature rise at the end cannot be prevented only by the magnetic shunt alloy, and after passing a predetermined number of sheets, the passing of the paper is temporarily stopped. Therefore, temperature control such as lowering the temperature of the end portion is required, and the efficiency of image formation is significantly reduced.

  As a result of intensive studies, the inventor of the present application has found that the above problem can be solved by disposing the excessive temperature rise prevention member 6 at the position outside the fixing roller 2 and facing the exciting coil 3.

  (Explanation of Advantages of the Present Invention) FIG. 6 is a schematic diagram in which a portion relating to induction heating is extracted from the diagram of the fixing device 20 of FIG.

  FIG. 6A shows a state of the magnetic flux A generated from the coil 3 when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point.

When the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, the state of the magnetic flux A returns to the core 5 again through the heat generation layer 21 through the core 5 as a path as in the prior art of FIG.

  On the other hand, the magnetic flux generated from the coil 3 when the temperature of the magnetic shunt alloy becomes equal to or higher than the Curie point is the magnetic flux B shown in FIG.

  The magnetic flux leaking out of the fixing roller enters an excessive temperature rise prevention member installed within the range of the magnetic field generated by the coil. Therefore, the magnetic flux B forms a magnetic circuit including the heat generating layer 21 and the excessive temperature rise prevention member 6.

  Since the non-magnetic and low resistance aluminum is used for the excessive temperature rise prevention member 6, a large amount of induced current flows through the excessive temperature rise prevention member 6 having a low volume resistivity, and the heat generation of the heat generation layer 21 is extremely high. Get smaller.

  In addition, aluminum generates very little heat by induction heating, and the excessive temperature rise prevention member 6 does not become high temperature.

  FIG. 7 shows the heat generation amount of the heat generation layer 21 of the present example and the conventional example calculated by the alternating magnetic field analysis simulation.

  FIG. 7A shows the amount of heat generated when the temperature of the magnetic shunt alloy is below the Curie point (T <Tc) and the amount of heat generated when the temperature of the magnetic shunt alloy is above the Curie point (T> Tc). FIG. 7B shows the amount of heat generated when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point (T <Tc) and the temperature of the magnetic shunt alloy equal to or higher than the Curie point (T <Tc). The calorific value of each is shown.

  The amount of heat generated by the heat generating layer 21 is compared when an alternating voltage of 25 kHz is applied to the coil of the fixing unit similar to the present embodiment.

  In the conventional example, when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T> Tc), the calorific value is suppressed by 43%, whereas in this embodiment, the temperature of the magnetic shunt alloy is equal to or higher than the Curie point ( When T> Tc), the amount of heat generation is suppressed by 90% or more, and it can be seen that the heat generation suppression effect is greatly improved.

  FIG. 8 shows the change over time in the temperature of the non-sheet passing region of the fixing roller when the fixing device of the present embodiment is subjected to a continuous sheet passing test for paper having a small size in the width direction.

  Also shown is the temperature of the non-sheet passing area of the prior art fixing roller of FIG.

  As shown in FIG. 8, in the fixing device of the present embodiment, when the temperature of the magnetic shunt alloy exceeds the Curie point after the sheet passing is started, the amount of heat generation is limited, and the temperature rise at the end is almost stopped. I understand that. Therefore, in this embodiment, even if a small-size paper is continuously passed for a long time, the problem of edge temperature rise does not occur.

  (Regarding Leakage Magnetic Flux) Further, in this embodiment, the leakage magnetic flux released to the outside of the fixing roller when the temperature of the magnetic shunt alloy becomes equal to or higher than the Curie point (T> Tc) can be reduced.

  In the fixing device using induction heating, the magnetic flux that is not used for heating the fixing roller among the magnetic flux generated from the exciting coil may be released to the periphery of the fixing device as a leakage magnetic flux. Magnetic flux that leaks from the fixing device may affect peripheral devices, that is, it may cause malfunctions and noise problems in the surrounding electronic devices. The influence of electromagnetic waves on the human body as well as this problem cannot be ignored.

  In the prior art, when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T> Tc), as shown in FIG. 4B, heat generation is suppressed by not concentrating the magnetic flux on the fixing roller. There was a problem that the magnetic flux leakage was larger than that of the induction heating type fixing device which does not use a magnetic shunt alloy.

  Therefore, much cost and time have been spent on designing an electromagnetic shield for leaking radiation noise outside the fixing device or outside the image forming apparatus main body. However, according to the present embodiment, the leakage magnetic flux released to the outside of the fixing device can be reduced.

  The leakage magnetic flux emitted to the outside of the fixing roller was measured by the conventional technique and the present embodiment. FIG. 9 shows measurement points where the magnetic flux density is measured.

  The measurement was performed in a conventional fixing device (the conventional technology does not have the excessive temperature rise prevention member 6) at a point facing the coil, which is the direction in which the magnetic flux is most likely to leak, and at a point 40 mm away from the outer periphery of the roller.

  The fixing device measured is the fixing device of FIG. 3 whose heating layer is Ni, the fixing device of FIG. 3 whose heating layer is a magnetic shunt alloy, and the fixing device of this embodiment. The measurement results are shown below.

  In the prior art using Ni for the heat generating layer, even in the direction in which the magnetic flux is most likely to leak, it is 0.75 (G Gauss) at a point 40 mm away from the outer periphery of the roller, and the leakage magnetic flux is small. In contrast, in the prior art using a magnetic shunt alloy for the heat generating layer, the leakage flux is 4.5 (G) when the temperature of the magnetic shunt alloy is below the Curie point (T <Tc), and the temperature of the magnetic shunt alloy is further reduced. Is greater than 15 (G) when the value is above the Curie point (T> Tc).

  On the other hand, in the present embodiment, the leakage flux when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T> Tc) is reduced to 1G. This is almost the same value as the leakage flux of the prior art using Ni for the heat generation layer, and it is possible to prevent magnetic flux radiation noise with the electromagnetic shield design that is conventionally performed in the induction heating type fixing device. it can.

  Further, the magnetic flux leakage when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point (T <Tc) is as small as 0.2 (G Gauss).

  According to the present embodiment, the leakage magnetic flux emitted to the outside of the fixing roller can be reduced. Therefore, an electromagnetic shield design comparable to that of an induction heating type fixing device that does not use a magnetic shunt alloy can be used outside of the fixing device or image. Radiation noise can be prevented outside the main body of the forming apparatus.

  FIG. 10 shows the heating efficiency when the temperature of the magnetic shunt alloy is less than the Curie point (T> Tc) when the thickness of the magnetic shunt alloy which is the heat generating layer in Example 1 is changed. Yes.

  The heating efficiency on the vertical axis indicates the amount of heat generated by the heat generating layer when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, and the total amount of heat generated (heat generated by the heat generating layer + heat generated by the coil + heat generated by the excessive temperature rise prevention member) It shows the value divided by (quantity).

  The calorific value was calculated by AC magnetic field analysis simulation. It can be said that the higher the heating efficiency, the greater the amount of heat generated when the temperature is below the Curie point, and the better the temperature rise characteristics.

  It can be seen that the heating efficiency decreases when the thickness of the magnetic shunt alloy is less than 80 μm in terms of the penetration depth. This is because if the thickness of the heat generating layer 21 becomes significantly thinner than the penetration depth, the magnetic flux of the coil passes through the magnetic shunt alloy even if the temperature of the magnetic shunt alloy is below the Curie point and is magnetic. This is because the excessive temperature rise prevention member is reached.

  Therefore, it is desirable that the thickness of the heat generating layer 21 be equal to or greater than the current penetration depth when the temperature of the heat generating layer 21 is equal to or lower than the Curie point.

  (Thickness of excessive temperature rise prevention member) FIG. 11 shows each member when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point when the thickness of aluminum which is the excessive temperature rise prevention member in Example 1 is changed. It shows the heat generation amount and the heat generation suppression rate. The calorific value was calculated by AC magnetic field analysis simulation.

  When the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, a current is applied to the coil so that the overall heat generation amount (heat generation amount of the heat generation layer + heat generation amount of the coil + heat generation amount of the excessive temperature rise prevention member) is 1200 W. .

  That is, when the aluminum thickness is 0.1 mm, the total calorific value is 1200 W when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, whereas when the temperature is equal to or higher than the Curie point, it is reduced to about 540 W.

The heat generation inhibition rate was determined by the following formula.
Q1 = heat generation amount of the heat generation layer when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point Q2 = heat generation amount of the heat generation layer when the temperature is equal to or higher than the Curie point Heat generation suppression rate = Q1-Q2 / Q1 × 100

  It can be said that the larger the value of the heat generation suppression rate, the smaller the amount of heat generated when the temperature becomes equal to or higher than the Curie point, and the higher the self-temperature control capability.

  It can be seen that the self-temperature control ability decreases when the excessive temperature rise prevention member becomes thinner than the aluminum penetration depth of 0.5 mm.

  It can also be seen that when the overheat prevention member becomes thinner than 0.5 mm, the heat generation of the overheat prevention member itself increases remarkably.

  From the standpoint of the self-temperature control capability, there is no big problem in making the overheat prevention member thin. It is not preferable for safety of the device. Therefore, in order to improve the self-temperature control capability and to suppress the self-heating of the excessive temperature rise prevention member, it is preferable that the excessive temperature rise prevention member has a penetration depth or more.

  (Volume resistivity of overheat prevention member) In Embodiment 1, the overheat prevention member is formed of aluminum, but the overheat prevention member is formed of another nonmagnetic conductive material. You can also

  FIG. 12 shows the heat generation amount of each member and the heat generation suppression rate when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point when the volume resistivity of the excessive temperature rise prevention member is changed in Example 1. The thickness of the excessive temperature rise prevention member is 1 mm.

  The calorific value was calculated by AC magnetic field analysis simulation. When the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, a current is applied to the coil so that the overall heat generation amount (heat generation amount of the heat generation layer + heat generation amount of the coil + heat generation amount of the excessive temperature rise prevention member) is 1200 W. .

  The volume resistivity of the excessive temperature rise prevention member was analyzed in the range from 4.35 × 10 −7 Ω · m to 1.72 × 10 −8 Ω · m.

  The volume resistivity of the magnetic shunt alloy as the heat generating layer 21 is 3.6 × 10 −7 Ω · m. As can be seen from FIG. 12, as the volume resistivity of the excessive temperature rise prevention member decreases, the self-temperature control capability improves, and the heat generation of the excessive temperature rise prevention member itself decreases.

  In particular, the volume resistivity of the excessive temperature rise prevention member is 3.6 × 10 −8 Ω · m or less, which is 1/10 of the volume resistivity of 3.6 × 10 −7 Ω · m of the heat generating layer 21, It can be seen that the heat generation of the excessive temperature rise prevention member itself becomes very small and becomes 100 W or less.

  Thus, the self-temperature control capability can be improved by using a nonmagnetic conductive material having a volume resistivity lower than that of the heat generating layer 21 for the excessive temperature rise prevention member.

  Further, as can be seen from FIG. 12, the volume resistivity of the excessive temperature rise prevention member is desirably 1/10 or less of the volume resistivity of the heat generation layer 21, and specifically, the volume resistivity is 3.6 ×. It is preferable to use copper, aluminum, gold, silver, or the like, which is 10 −8 Ω · m or less, as the excessive temperature rise prevention member.

  (Distance between Over-Temperature Preventing Member and Heating Layer) FIG. 13 shows the heat generation suppression rate when the distance between the over-temperature preventing member and the heat generating layer is changed in the first embodiment.

  FIG. 13 shows that the heat generation suppression rate decreases as the distance between the excessive temperature rise prevention member and the heat generation layer increases. Therefore, it is preferable that the distance between the excessive temperature rise prevention member and the heat generating layer be as close as possible. In Example 1, the excessive temperature rise prevention member was disposed at a distance of 4 mm from the heat generating layer.

  (Other Configurations) (Excessive Temperature Raising Prevention Member is Integral with Housing of Fixing Device (Claim 4)) In this embodiment, the excessive temperature rise prevention member is disposed outside the fixing roller, but fixing is performed. You may comprise as a part of housing | casing which accommodates a member.

  The fixing device is usually configured by disposing a fixing member in a casing integrally formed of synthetic resin or the like. As illustrated in FIG. 14, the excessive temperature rise prevention member 6 may be disposed at a portion of the housing 201 that faces the excitation coil.

  With this configuration, it is not necessary to provide a space for disposing the excessive temperature rise prevention member 6 around the fixing roller, and space saving can be achieved.

  In addition, since the excessive temperature rise prevention member 6 serves as an electromagnetic shield for preventing leakage magnetic flux, it is not necessary to dispose the electromagnetic shield in the portion of the housing where the excessive temperature rise prevention member is attached. For example, as shown in FIG. 15, radiation noise to the outside of the fixing device can be reduced by arranging the electromagnetic shield 202 on the inner wall of the fixing device casing. However, in the portion where the excessive temperature rise prevention member is arranged. Does not require an electromagnetic shield. The electromagnetic shield 202 can be made of aluminum having a thickness of about 50 μm.

  (The excessive temperature rise prevention member is disposed only in the non-sheet passing region) The excessive temperature rise preventing member 6 may be disposed only in the non-sheet passing region. FIG. 16 shows a fixing device in which an excessive temperature rise prevention member is disposed only in the non-sheet passing region in the present embodiment.

  In this fixing device, the center in the width direction of the recording member through which the sheet passes coincides with the center in the rotation axis direction of the fixing roller. Are arranged symmetrically with respect to the axis.

  The end portions of the excessive temperature rise prevention member near the center of the fixing roller in the rotation axis direction are respectively disposed at positions corresponding to both ends of the smallest recording member through which the fixing device passes the fixing roller. .

  A thermopile is disposed as the temperature measuring means 7 at the center in the rotation axis direction of the fixing roller where the excessive temperature rise prevention member is not disposed.

  With this configuration, it is not necessary to secure a space for newly arranging the temperature detection means. Further, the total amount of the excessive temperature rise prevention member can be reduced and the weight can be reduced.

  (Other Coil Configuration) The exciting coil 3 may be other than the above-described coil configuration. It suffices if the coil is arranged inside the fixing member and the excessive temperature rise prevention member 6 is arranged outside the fixing member.

  The excessive temperature rise prevention member may be disposed within the range of the magnetic field generated by the coil when the temperature of the facing portion of the coil or the magnetic shunt alloy is equal to or higher than the Curie point.

  In FIG. 17, the shapes of the exciting coil 3 and the core 5 are different from those of the first embodiment. Since the exciting coil widely faces the inner peripheral surface of the fixing roller, a wide range of the fixing roller can be heated. As shown in FIG. 17, even if the coil shape changes, the excessive temperature rise prevention member is disposed within the range of the magnetic field generated by the coil when the temperature of the facing portion of the coil or the magnetic shunt alloy is equal to or higher than the Curie point. Accordingly, it is possible to provide a fixing device having a high heat generation suppression rate.

Second Embodiment (Claim 3) A second embodiment will be described below with reference to the drawings.
In Example 2, the overheat prevention member is a first overheat prevention member made of a magnetic shunt alloy, and the second is lower in volume resistivity than the first overheat prevention member. The point which consists of an excessive temperature rise prevention member differs greatly from Example 1.

  FIG. 18 is a cross-sectional view of the fixing device 20 in this embodiment. A fixing roller 2 that is a fixing member, a pressure belt 4 that forms a nip region with the fixing roller 2, and a pressure pad 60 that presses the pressure belt against the fixing roller from the back of the pressure belt are provided, and excitation that is a magnetic field generating unit The coil 3 includes a bobbin 301 around which the exciting coil 3 is wound, and a ferrite core 5 serving as an iron core of the coil.

  (Coil shape) The coil and core shapes are substantially the same as those in the first embodiment, except that the magnetic flux is concentrated in the nip region of the fixing roller 2. Specifically, the coil and core of Example 1 are rotated 90 degrees clockwise around the rotation axis of the fixing roller.

  (Heat generation layer) Although not shown, the fixing roller 2 has a release layer 23, an elastic layer 22, and a heat generation layer 21 formed in order on a substrate.

  The base material is made of an insulating heat-resistant resin material, and for example, polyimide, polyamideimide, PEEK, PES, PPS, fluorine resin, or the like can be used. The layer thickness of the base material is 30 to 200 μm from the viewpoint of heat capacity and strength. Further, it may be a high-resistance, non-magnetic metal that does not greatly affect induction heating by being thinned. For example, SUS304 etc. which are nonmagnetic stainless steel are mention | raise | lifted.

  The release layer and elastic layer are the same as in Example 1. The heat generating layer 21 is made of a highly conductive member. As a metal suitable for induction heating, a metal having high resistance is generally known. However, by reducing the thickness of a highly conductive member, the substantial resistance of the heat generating layer can be arbitrarily set. The calorific value can be improved.

  In this example, a silver layer having a thickness of 10 μm was used as the heat generating layer. Of course, since the heat generating layer only needs to have good conductivity, other metal layers such as copper, aluminum, magnesium, etc., or nickel, which is a magnetic substance, may be used.

  In the present configuration, the thickness of the heat generating layer 21 is desirably equal to or less than the penetration depth at the frequency of the alternating current applied. Thereby, it is possible to prevent an excessive temperature increase from being caused by electromagnetic induction.

  (Pressure Belt) The pressure belt 4 as a pressure member is a multi-layered endless belt in which an elastic layer and a release layer are sequentially formed on a base material (disposed on the inner peripheral surface side). It is. The base material is made of an insulating heat-resistant resin material, and for example, polyimide, polyamideimide, PEEK, PES, PPS, fluorine resin, or the like can be used. The layer thickness of the base material is 30 to 200 μm from the viewpoint of heat capacity and strength.

  The elastic layer of the pressure belt 4 is made of silicone rubber, fluorosilicone rubber or the like, and is formed to have a layer thickness of 50 to 500 μm and an Asker hardness of 5 to 50 degrees. Thereby, a uniform image quality without gloss unevenness can be obtained in the output image.

  The release layer of the pressure belt consists of tetrafluoroethylene resin (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin (PFA), and tetrafluoroethylene / hexafluoropropylene copolymer (FEP). Or the like, a mixture of these resins, or a dispersion of these resins in a heat resistant resin. The release layer has a thickness of 5 to 50 μm (preferably 10 to 30 μm).

  A primer layer or the like can be provided between the layers of the pressure belt 4.

  (Pressure pad) The pressure pad 60 as a pressing means is pressed against the back surface of the pressure belt 4 by a spring or the like (not shown).

  The pressure pad 60 includes an elastic layer 61, a first excessive temperature rise prevention member 62, and a second excess temperature rise prevention member 63. The elastic layer 61 is made of silicone rubber, fluorosilicone rubber, or the like, and is configured to obtain a uniform pressing force.

  (First excessive temperature rise prevention member) The first excessive temperature rise prevention member 62 uses a magnetic shunt alloy whose Curie point is not less than the fixable temperature and not more than 300 degrees.

  Specifically, it is an alloy of nickel, iron, and chromium, and a desired Curie point can be obtained by adjusting the amount of each material added and the processing conditions. As described above, the first excessive temperature rise prevention member 62 is formed of a magnetic conductive material having a Curie point near the fixing temperature of the fixing roller 2, so that the fixing roller 2 is excessively heated by electromagnetic induction. It will be heated without.

  In the second embodiment, a magnetic shunt alloy having a Curie point of 180 degrees is used for the first overheating prevention member 62 in order to reliably prevent overheating. The thickness of the first excessive temperature rise prevention member 62 is desirably equal to or greater than the current penetration depth when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point.

  (Second excessive temperature rise prevention member) The second excessive temperature rise prevention member 63 is made of a conductive material that is non-magnetic and has a volume resistivity lower than that of the first excess temperature rise prevention member 62. Has been.

  Since the volume resistivity of the first overheating prevention member 62 is 3.6 × 10 −7 Ω · m, the second overheating prevention member 63 has a volume resistivity of 3.6 × 10 −7 Ω · m. It is made of a material lower than m.

  Specifically, it is preferable to use copper, aluminum, gold, silver, or the like having a volume resistivity of 3.0 × 10 −8 Ω · m or less.

  In the first embodiment, the non-magnetic material aluminum is used as the material of the second overheating prevention member. By using a non-magnetic and highly conductive member, it is possible to obtain an effect of shielding the magnetic flux while suppressing the heat generation of the excessive temperature rise prevention member 6 itself. Further, it is desirable that the thickness of the second excessive temperature rise prevention member is not less than the penetration depth. With the configuration of the second embodiment, a fixing device having a high self-temperature control capability can be provided.

  (Self-Temperature Control Mechanism of Embodiment 2) FIG. 19 is a schematic view in which a portion relating to induction heating is extracted from the view of the fixing device 20 of FIG.

  FIG. 19A shows a state of the magnetic flux A generated from the coil 3 when the temperature of the first overheating prevention member, that is, the magnetic shunt alloy is equal to or lower than the Curie point.

When the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, the magnetic flux A generated from the coil 3 passes through the heat generating layer 21 through the core 5 and passes again through the heat generating layer through the magnetic shunt alloy as a magnetic material. Return to core 5. Since the magnetic shunt alloy is a magnetic substance and the penetration depth is very shallow, the magnetic flux does not pass through the magnetic shunt alloy and reach aluminum which is the second overheating prevention member.

  At this time, an induction current flows through the silver that is the heat generation layer 21 and the magnetic shunt alloy that is the first pressurizing temperature raising member, and heat is generated by Joule heat.

  On the other hand, the magnetic flux generated from the coil 3 when the temperature of the magnetic shunt alloy becomes equal to or higher than the Curie point is the magnetic flux B shown in FIG.

  The magnetic flux B transmitted through the heat generation layer 21 through the core 5 also passes through the magnetic shunt alloy that has become a non-magnetic material and reaches aluminum as the second overheating member.

  Therefore, the magnetic flux B forms a magnetic circuit including the heat generating layer 21, the first overheat prevention member 62, and the second overheat prevention member 63.

  Since the second overheat prevention member 63 is made of nonmagnetic and low resistance aluminum, a large amount of induced current flows through the second overheat prevention member 63 having a low volume resistivity, and the heat generation layer The exotherm of 21 becomes very small.

  In addition, aluminum generates very little heat due to induction heating, and the excessive temperature rise prevention member 63 does not become high temperature.

  The self-temperature control mechanism of Example 2 is such that when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point, the magnetic flux is passed through the heat generating layer and the magnetic shunt alloy, and when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point, When the heat generating layer, the magnetic shunt alloy and aluminum are passed through, the heat generating member, the first overheating prevention member, and the second overheating prevention member must be arranged in this order when viewed from the coil. Don't be.

  The self-temperature control mechanism of Example 2 is the same as that of Example 1 except that the heat generation layer is added to the magnetic circuit. For the same reason as in Example 1, the self-temperature control mechanism is the first overheating prevention member. It is preferable that the thickness of the magnetic alloy is equal to or greater than the current penetration depth when the magnetic alloy is below the Curie point. It is preferable that the second excessive temperature rise prevention member is not less than the penetration depth.

  It is preferable to use a nonmagnetic conductive material having a volume resistivity lower than that of the first overheating prevention member for the second overheating prevention member.

  The self-temperature control ability of Example 2 and the prior art of FIG. 3 was compared. In FIG. 20, “heat generation amount of the heat generation layer 21 + heat generation amount of the first excessive temperature rise prevention member 62 + heat generation amount of the second excessive temperature rise prevention member 62 of Example 2 calculated by the alternating magnetic field analysis simulation is shown. ”Indicates the heat generation amount of the heat generation layer 21 of the conventional example.

  In the present embodiment, the heat generated by the overheat prevention member that is a pressure pad is also transferred to the fixing roller and the paper through the nip region, so that “the amount of heat generated by the heat generation layer 21 + the first overheat prevention member 62. The self-temperature control capability is evaluated based on the change in the heat generation amount of “the amount of heat generated by + the heat generation amount of the second overheating prevention member 62”.

  FIG. 20A shows the amount of heat generated when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point (T <Tc) and the amount of heat generated when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T> Tc). FIG. 20B shows the amount of heat generated when the temperature of the magnetic shunt alloy is equal to or lower than the Curie point (T <Tc) and when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T <Tc). The calorific value of each is shown.

  In the conventional example, when the temperature of the magnetic shunt alloy is equal to or higher than the Curie point (T> Tc), the calorific value is suppressed by 43%, whereas in this embodiment, the temperature of the magnetic shunt alloy is equal to or higher than the Curie point ( Since T> Tc), the amount of heat generation is suppressed by about 60%, which indicates that the heat generation suppression effect is improved.

  FIG. 21 shows the change over time in the temperature of the non-sheet passing region of the fixing roller when the fixing device of the present embodiment is subjected to a continuous sheet passing test for small-sized paper.

  For reference, the temperature of the fixing roller in the sheet passing area and the temperature of the non-sheet passing area of the conventional fixing roller are also shown.

  As shown in FIG. 21, in the fixing device of the present embodiment, the heat generation amount is limited when the temperature of the magnetic shunt alloy exceeds the Curie point after the sheet passing is started, and the temperature rising rate at the end is higher than that of the conventional technique. It can be seen that the self-temperature control ability has been improved.

  As described above, the excessive temperature rise prevention member is divided into the first excess temperature rise prevention member made of a magnetic shunt alloy, the second excess temperature rise prevention member having a volume resistivity lower than that of the first excess temperature rise prevention member. By constituting the temperature rise prevention member, the fixing roller 2 is heated without being overheated by electromagnetic induction.

  Further, in this configuration, by arranging the excessive temperature rise prevention member as a pressure pad in the pressure belt, it is not necessary to provide a space for newly arranging the excessive temperature rise prevention member as in the first embodiment, and the fixing device Can be miniaturized.

  Further, the magnetic shunt alloy as the first overheating member is preferably thicker than the penetration depth, but in this embodiment, the thickness of the magnetic shunt alloy can be easily increased.

  In Example 1, since the magnetic shunt alloy was a heat generating member, increasing the thickness of the magnetic shunt alloy to achieve a smaller heat capacity led to a decrease in the startup performance of the fixing device. Even if the magnetic shunt alloy is thickened, the heat capacity of the heat generating member does not change, so that the start-up performance does not deteriorate.

  In general, a magnetic shunt alloy is more susceptible to fatigue failure due to stress than other metals such as iron, and using a shunt alloy as a heat-generating member and rotating it every time an image is formed has a problem in terms of durability. However, with this configuration, the stress applied to the magnetic shunt alloy can be reduced and the self-temperature control function can be improved.

  According to the present embodiment, in the induction heating type fixing device that uses a magnetic shunt alloy for the heat generating member, the overheating prevention member is provided at a position facing the exciting coil via the fixing member, thereby generating heat. The self-temperature control function is enhanced more than when the member is only a magnetic shunt alloy.

  The ability to control the self-temperature of the heat generating member is enhanced, and overheating is reliably suppressed even when the small-size recording medium is fixed in a short time, or when the device is suddenly stopped. The fixing device and the image forming apparatus can be provided. Further, it is possible to reduce the leakage magnetic flux released to the outside of the fixing device when the magnetic shunt alloy becomes nonmagnetic.

The effects of each claim will be described below.
[Claim 3] When the magnetic shunt alloy has magnetism, the heat generation efficiency is improved, and a fixing device having a high rise speed can be obtained.

  [Claim 4] It is possible to increase the rate of decrease in the amount of heat generated when the magnetic shunt alloy becomes non-magnetic, to increase the self-temperature control function and to reduce the heat generation / temperature increase of the excessive temperature rise prevention member itself. it can.

  [Claim 5] A fixing device having a self-temperature control function in which the temperature rise of the fixing roller stops near the Curie point of the magnetic shunt alloy can be provided.

  [Claim 6] When the magnetic shunt alloy has magnetism, the heat generation efficiency is improved, and a fixing device having a high rise speed can be obtained.

  [Claim 7] The rate of decrease in the amount of heat generated when the magnetic shunt alloy becomes non-magnetic can be increased, the self-temperature control function can be enhanced, and the heat generation / temperature increase of the excessive temperature rise prevention member itself can be reduced. it can.

  [Claim 8] There is no need to provide a new space for installing the excessive temperature rise prevention member, and a small-sized fixing device having a self-temperature control function can be provided.

  [Claim 9] There is no need to provide a new space for installing an excessive temperature rise prevention member, and a small fixing device having a self-temperature control function can be provided.

  [Claim 10] To provide an image forming apparatus in which excessive temperature rise is reliably suppressed even when a small-sized recording medium is continuously fixed or when the apparatus is suddenly stopped. it can.

  Each of the above-described embodiments is a preferred embodiment of the present invention, and various modifications can be made without departing from the scope of the present invention.

1 is a cross-sectional view of a fixing device according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a configuration of a fixing roller. It is sectional drawing of the fixing device of a prior art. It is the schematic which extracted the part regarding the induction heating of the fixing device of a prior art. FIG. 4 is a diagram illustrating a change with time of a temperature of a fixing roller when a continuous paper passing test is performed on a small paper in a width direction using the conventional fixing device of FIG. 3. FIG. 2 is a schematic diagram in which a portion related to induction heating is extracted from the view of the fixing device in FIG. 1. It is a figure which shows the emitted-heat amount of the heat generating layer of a present Example calculated by alternating current magnetic field analysis simulation, and a prior art example. This is a change over time in the temperature of a non-sheet passing region of the fixing roller when a continuous sheet passing test is performed on a small-sized paper in the fixing device of the present embodiment. It is a figure which shows the measurement point which measured magnetic flux density. It is a figure which shows the heating efficiency when the temperature of a magnetic shunt alloy is below a Curie point (T> Tc) when the thickness of the magnetic shunt alloy which is a heat generating layer in Example 1 is changed. It is a figure which shows the emitted-heat amount of each member when the temperature of a magnetic shunt alloy is more than a Curie point when the thickness of the aluminum which is an excessive temperature rise prevention member in Example 1 is changed, and a heat_generation | fever suppression rate. It is a figure which shows the emitted-heat amount of each member when the temperature of a magnetic shunt alloy is more than a Curie point when changing the volume resistivity of the excessive temperature rising prevention member in Example 1, and a heat_generation | fever suppression rate. It is a figure which shows the heat_generation | fever suppression rate when the distance of the excessive temperature rising prevention member and heat generating layer is changed in Example 1. FIG. FIG. 6 is a diagram in which an excessive temperature rise prevention member is disposed at a portion of the housing 201 that faces the excitation coil. FIG. 6 is a diagram in which an electromagnetic shield is disposed on the inner wall of the fixing device casing. In this embodiment, the fixing device has an excessive temperature rise prevention member arranged only in a non-sheet passing region. It is a figure which shows that the shape of the exciting coil 3 and the core 5 differs from this Embodiment 1. FIG. 1 is a cross-sectional view of a fixing device 20 in the present embodiment. It is the schematic which extracted the part regarding induction heating from the figure of the fixing device 20 of FIG. It is a figure which shows the emitted-heat amount of the present Example 2 calculated by alternating current magnetic field analysis simulation. This is a change over time in the temperature of a non-sheet passing region of the fixing roller when a continuous sheet passing test is performed on a small-sized paper in the fixing device according to the present embodiment. 1 is a cross-sectional view of an image forming apparatus to which the present invention can be preferably applied.

Explanation of symbols

2 Fixing Roller 3 Excitation Coil 4 Pressure Roller 5 Ferrite Core 6 Over Temperature Prevention Member 301 Bobbin

Claims (9)

In a fixing device having an exciting coil that is arranged inside a fixing member and induction-heats the heating member by electromagnetic induction,
A fixing device, wherein the heat generating member is a magnetic member, and an overheat prevention member is provided at a position facing the exciting coil via the fixing member.   The heat generating member is formed so that its thickness corresponds to the frequency of the alternating current applied to the exciting coil, and is thicker than the penetration depth when the temperature of the heat generating member is equal to or lower than the Curie point. The fixing device according to claim 1.   The overheat prevention member is formed so that the thickness thereof becomes thicker than the penetration depth corresponding to the frequency of the alternating current applied to the exciting coil. Fixing device.   A first overheating prevention member made of a magnetic shunt alloy; and a second overheating prevention member that is nonmagnetic and has a volume resistivity lower than that of the first overheating prevention member. The members are arranged in the order of a coil, a heat generating member, a first overheating prevention member, and a second overheating prevention member from the inside of the fixing member. Fixing device.   The first overheat prevention member has a thickness corresponding to the frequency of the alternating current applied to the exciting coil, and the penetration when the temperature of the first overheat prevention member is equal to or lower than the Curie point. The fixing device according to claim 4, wherein the fixing device is formed to be thicker than the depth.   The second overheat prevention member is formed so that its thickness is thicker than the penetration depth corresponding to the frequency of the alternating current applied to the exciting coil. The fixing device according to 1.   7. The fixing device according to claim 1, wherein the fixing device includes a housing that accommodates the fixing member, and a part of the housing is formed by the excessive temperature rise prevention member. Fixing device.   A fixing device having a pressure member made of a rotating body arranged in pressure contact with the fixing member, and a recording material being inserted into a fixing nip region formed between the fixing member and the pressure member. A pressing means that presses the pressure member against the fixing member from the back surface of the pressure member within a fixing nip region with the pressure member, and the pressing means is formed by the excessive temperature rise prevention member. The fixing device according to claim 1, wherein the fixing device is characterized in that:   An image forming apparatus comprising the fixing device according to claim 1.

Images