JP2009003264A - Image heating device and image forming apparatus with image heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the temperature of a paper non-passing part from rising and also to shorten rising time by setting Curie temperature. <P>SOLUTION: The frequency of a current to be applied to a coil in a stage, in which the temperature of an image heating member is raised to a setup temperature near to the Curie temperature, is made higher than the frequency thereof in a stage in which an image on recording material is heated. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electromagnetic induction type image heating apparatus and an image forming apparatus such as a copying machine, a printer, and a facsimile including the electromagnetic induction type image heating apparatus.

  2. Description of the Related Art Image forming apparatuses such as electrophotographic systems and electrostatic recording systems include an image heating apparatus that fixes or heats a toner image formed on a sheet-like recording material (recording paper, transfer material, etc.) on the recording material. .

  The image heating apparatus includes an image heating unit that is a heating roller (fixing roller) or an endless heating belt, and a pressurizing unit that pressurizes the image heating unit to form a nip portion that sandwiches and conveys the recording material. .

  An image heating unit (hereinafter referred to as an image heating roller) is heated directly or indirectly from the inside or outside by a heating element, and the surface temperature is maintained at a predetermined fixing temperature. Examples of the heating element include a halogen heater and a resistance heating element.

  The recording material on which the unfixed toner image is formed is heated and pressurized at the nip portion, and the unfixed toner image is fixed on the recording material surface as a fixed image.

  In recent years, emphasis has been placed on achieving both energy saving of the image forming apparatus and improvement of user operability such as shortening the warm-up time. For this reason, an image heating apparatus (hereinafter referred to as an induction image heating apparatus) using an induction heating method in which an image heating roller generates heat directly and has high heat generation efficiency has been proposed (Patent Document 1).

  This induction image heating device generates an induction current (eddy current) in a hollow image heating roller made of a metal conductor by the action of magnetic flux generated by an exciting coil, and the heating roller itself generates Joule heat by the skin resistance of the heating roller itself. To do. Therefore, according to this induction image heating device, it is possible to shorten the warm-up time.

  Further, in such an induction image heating apparatus, electric power proportional to the skin resistance determined from the frequency of the applied high frequency current, the magnetic permeability of the heating roller and the specific resistance value is generated. Therefore, by controlling the frequency of the high-frequency current to be applied, the amount of heat generated by the heating roller can always be optimized, and energy saving of the apparatus can be achieved.

  On the other hand, in such an induction image heating apparatus, the skin depth generated by the magnetic flux is determined by the frequency at which the coil is energized and the specific resistance value of the image heating roller. Therefore, when the thickness of the heating roller is hotter than the skin depth, the amount of heat generated does not change, so the larger the thickness of the heating roller, the lower the heat generation efficiency, and the effect of shortening the warm-up time is obtained. It becomes difficult.

  On the other hand, if the thickness of the heating roller is smaller than the skin depth, the magnetic flux penetrates the heating roller, which not only reduces the amount of heat generation but also heats the metal members around the heating roller. Therefore, the thickness of the heating roller is desirably about 50 to 2000 μm.

  In such an induction image heating apparatus, similarly to the conventional image heating apparatus, when a small-sized recording material is continuously passed, the temperature rise in the non-sheet passing portion region where the recording material does not pass (temperature rise in the non-sheet passing portion). Occurs.

As this non-sheet passing portion temperature rise countermeasure, as disclosed in Patent Document 2, an induction image heating apparatus using a magnetic shunt alloy whose Curie temperature is adjusted to a predetermined fixing temperature is proposed for an image heating roller. Yes. In general, when a magnetic material is heated and exceeds its inherent Curie temperature, the spontaneous magnetization disappears. For this reason, the magnetic flux density passing through the magnetic material is reduced, and the eddy current induced in the magnetic material is reduced accordingly, so that the heat generation amount of the magnetic material is reduced. Therefore, by using a magnetic shunt alloy having a Curie temperature adjusted to a predetermined temperature as the material of the heating roller, the heating roller is not heated above the predetermined temperature. Therefore, it is possible to improve the above-described non-sheet passing portion temperature rise.
JP 59-33787 JP 2000-39797 A

  However, when an image heating roller having a Curie temperature in a range where the temperature rise of the non-sheet passing portion can be reduced as described above, the magnetic permeability decreases as the temperature of the image heating roller approaches the Curie temperature. Along with this, the amount of heat generation decreases, so the temperature rise slows down, and the time until the temperature of the image heating roller reaches the temperature at which the warm-up is completed becomes longer.

  On the other hand, the calorific value can be increased by reducing the skin depth in order to increase the calorific value by increasing the frequency.

  However, if a large frequency is used during image heating, the amount of heat generated is large, and thus the temperature rise of the non-sheet passing portion is increased.

  Accordingly, an object of the present invention is to shorten the warm-up time even when the Curie temperature is a temperature at which the temperature rise of the non-sheet passing portion can be reduced.

  A coil, a high-frequency circuit for supplying a high-frequency current to the coil, and a conductive layer that generates heat by magnetic flux generated by the high-frequency current passed through the coil, and heats the image on the recording material at a predetermined image heating temperature. An image heating apparatus having an image heating member and an energization control means for controlling energization to the coil, wherein the Curie temperature of the image heating member is equal to or higher than the image heating temperature and lower than a heat resistant temperature of the image heating apparatus. The frequency of the current applied to the coil during the process of raising the heating member to a set temperature close to the Curie temperature is set higher than the frequency during the process of heating the image on the recording material.

  According to the present invention, even when the Curie temperature is a temperature at which the temperature rise at the non-sheet passing portion can be reduced, the warm-up time can be shortened.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

(1) Example of Image Forming Apparatus FIG. 1 is a schematic configuration model diagram of an example of an image forming apparatus provided with an electromagnetic induction heating type image heating apparatus according to the present invention as a fixing device of the image forming apparatus.

  The image forming apparatus of this example is a digital image forming apparatus of a laser scanning exposure system using an electrophotographic process (copying machine, printer, facsimile, composite function machine thereof).

  Reference numeral 41 denotes a rotating drum type photosensitive member (photosensitive drum) as an image carrier, which rotates at a predetermined peripheral speed in the direction of an arrow. The primary charger 42 uniformly charges the photosensitive drum to a predetermined negative dark potential Vd.

  A laser beam scanner 43 is an image exposure unit. A laser beam L modulated in response to a digital image signal input from a host device such as an image reading device or a computer (not shown) is output, and the uniformly charged surface of the photosensitive drum 41 is scanned and exposed. By this scanning exposure, the exposed portion of the photosensitive drum 41 has a small potential absolute value and becomes a bright potential Vl, and an electrostatic latent image corresponding to the image signal is formed on the surface of the photosensitive drum 41. The electrostatic latent image is visualized as a toner image by the negatively charged toner adhering to the exposure light potential Vl portion of the photosensitive drum surface by the developing unit 44.

  On the other hand, the recording material P fed from a paper feed tray (not shown) is conveyed at an appropriate timing to a transfer portion where the transfer roller 45 as a transfer member to which a transfer bias is applied and the photosensitive drum 41 are in pressure contact with each other. Is done. Then, the toner images t on the photosensitive drum 41 are sequentially transferred onto the surface of the recording material P.

  The recording material P on which the toner image t is formed is separated from the photosensitive drum 41, introduced into the fixing device F, and the toner image t is fixed on the recording material by heat and pressure, and then discharged outside the apparatus. .

  After the recording material P has been separated, the surface of the photosensitive drum 41 is cleaned with the transfer residual toner remaining on the surface of the photosensitive drum by a cleaning device 46, and then repeatedly used for image formation.

(2) Fixing device F
2 is an enlarged cross-sectional model view of the main part of the fixing device F, which is an image heating apparatus, FIG. 3 is a front model view of the main part, and FIG. 4 is a longitudinal front model view thereof.

  The fixing device F is a heating roller type image heating device that generates heat by an electromagnetic induction heating method. It has a heating roller 1 (image heating member) having a conductive layer that generates heat by magnetic flux, and a pressure roller 2 as a pressure member that forms a nip portion that sandwiches and conveys the heating roller 1 and a recording material.

  The heating roller 1 has an outer diameter of 40 mm, a thickness of 0.8 mm, and a length of 340 mm. Further, in this embodiment, a cored bar 1a which is a conductive layer made of a magnetic shunt alloy having a specific resistance of about 5 Ω · m is blended with materials such as iron, nickel, chromium and manganese so that the Curie temperature becomes 210 ° C. Have This Curie temperature is equal to or higher than the image heating temperature for heating the image on the recording material during image formation (200 ° C. or higher in this embodiment) and is lower than the heat resistance temperature of the image heating apparatus (less than 230 ° C. in this embodiment). Set to temperature.

  In addition, a surface layer 1b having a thickness of 30 μm made of a fluororesin such as PFA or PTFE is provided on the core bar in order to improve the releasability with respect to the toner. In order to obtain a high-quality fixed image such as a color image, a heat-resistant elastic layer such as silicone rubber may be provided between the core metal 1a and the surface layer 1b.

  The heating roller 1 is disposed such that both end portions thereof are rotatably supported via bearings 23 between front and back side plates (fixing unit frames) 21 and 22 which are part of the frame of the fixing device. It is set up. Inside the heating roller 1, a coil assembly 3 is inserted as a magnetic field generating means having a coil for inducing an induction current (eddy current) in the heating roller 1 to generate a high frequency magnetic field for generating Joule heat. It is arranged.

  The pressure roller 2 has an outer diameter of 38 mm, a length of 330 mm, and a cored bar 2 a having an outer diameter of 28 mm and a wall thickness of 3 mm. Further, a heat-resistant elastic layer 2b having a thickness of 5 mm formed on the peripheral surface of the cored bar 2a and a surface layer 2c having a thickness of 30 μm made of fluororesin such as PFA and PTFE formed on the peripheral surface of the heat-resistant elastic layer 2b. Become.

  The pressure rollers 2 are arranged in parallel under the heating roller 1 and both end portions of the cored bar 2a are respectively bearings 26 between the front and rear side plates 21 and 22 of the fixing device frame. It is held rotatably through the.

  The heating roller 1 and the pressure roller 2 are fixed to each other by a pressure mechanism (not shown), and the recording material P is nipped and conveyed between the rollers 1 and 2 to heat and fix the toner image. Part N is formed.

  Here, in the present invention, the longitudinal direction (rotation axis direction) of the apparatus constituent member is a direction orthogonal to the conveyance direction of the recording material P on a plane including the fixing nip portion N. Moreover, a center part and an edge part are the center part and edge part of the longitudinal direction.

  The coil assembly 3 inserted into the heating roller 1 includes a bobbin 4, a core material (magnetic core) 5 (1, 2) made of a magnetic material, an excitation coil (coil) 6, a stay 7 made of an insulating member, and the like. Have. The magnetic core material 5 is held by the bobbin 4, and the exciting coil 6 is formed by winding an electric wire around the bobbin 4. The coil unit in which the bobbin 4, the magnetic core material 5, and the exciting coil 6 are integrated is fixedly supported by the stay 7.

  The coil assembly 3 holds a fixed gap between the inner surface of the heating roller 1 and the exciting coil 6 at both ends 7a and 7a of the stay 7, respectively, and holding members 24 and 25 on the front side and the back side of the fixing device, respectively. Non-rotatably fixedly supported. The units of the bobbin 4, the magnetic core material 5 and the exciting coil 6 are accommodated so as not to be exposed to the outside of the heating roller 1.

  The magnetic core material 5 is a material having a high magnetic permeability and a low residual magnetic flux density, such as ferrite and permalloy, and guides the magnetic flux generated by the exciting coil 6 to the heating roller 1. The magnetic core 5 in this embodiment has a T-shaped cross section, and two plate-like magnetic cores 5 (1) and 5 (2) constituting a T-shaped horizontal bar portion and vertical bar portion are provided. It is combined.

  As shown in FIG. 4, the exciting coil 6 extends in parallel with the longitudinal direction of the heating roller 1, and is wound a plurality of times in a horizontal boat shape in accordance with the shape of the bobbin 4 so as to go around the magnetic core material 5 and bends at both ends. It is a bundle of litz wires that are wound around. Further, the heating roller 1 is curved and arranged along the inner periphery. Reference numerals 6a and 6b denote two lead wires (coil supply wires) of the excitation coil 6, which are pulled out from the back side of the stay 7 to the high frequency inverter (high frequency circuit) 101 that supplies a high frequency current to the excitation coil 6. Connected. The high-frequency inverter has a switching element, and a current having a predetermined frequency can be passed through the coil by turning the switching element ON / OFF.

  Reference numeral 11 denotes a thermistor as a temperature detection member of the heating roller 1. This thermistor will be described later.

  A pre-fixing guide plate 12 guides the recording material P conveyed from the image forming mechanism side to the fixing device F to the entrance of the fixing nip N. Reference numeral 13 denotes a separation claw for suppressing the recording material P introduced into the fixing nip portion N and exiting the fixing nip portion N from being wound around the heating roller 1 and separating the recording material from the heating roller 1. . Reference numeral 14 denotes a post-fixing guide plate that guides the recording material P that has exited the exit of the fixing nip N to be discharged.

  The bobbin 4, the stay 7, and the separation claw 13 are made of heat-resistant and electrically insulating engineering plastic.

  G1 is a drive gear for driving the heating roller fixed to the inner end side of the heating roller 1. When the driving force is transmitted to the drive gear G1 from the driving source M1 through the transmission system, the heating roller 1 rotates in the clockwise direction indicated by the arrow A in FIG. 2 at a peripheral speed of 300 mm / sec in this embodiment. . The pressure roller 2 rotates in the counterclockwise direction B indicated by the arrow following the rotation of the heating roller 1 by the frictional force with the heating roller 1 at the fixing nip N.

  Reference numeral 15 denotes a heating roller cleaner as a cleaning member. A web feed shaft portion 15b holding the cleaning web 155a in roll form, a web winding shaft portion 15c, and a pressing roller 15d for pressing the web portion between the shaft portions 15b and 15c against the outer surface of the heating roller 1 are provided. The toner offset to the surface of the heating roller 1 is wiped by the web portion pressed against the heating roller 1 by the pressing roller 15d, and the surface of the heating roller is cleaned. The web portion pressed against the heating roller 1 is gradually updated by gradually feeding the web 15a from the feeding shaft portion 15b side to the winding shaft portion 15c side.

  In the present embodiment, the sheet passing is performed based on the central reference. S is the central reference. That is, for any recording material size, the central portion of the recording material passes through the central portion in the heating roller axial direction. In the image forming apparatus of this embodiment, the maximum size of the recording material that can be passed (hereinafter referred to as large size paper) is, for example, A3 (296 mm). Further, the minimum size of the recording material that can be passed (hereinafter referred to as small size paper) is, for example, B5 length (148 mm). P1 is the paper passing area width of the large size paper, and P2 is the paper passing area width of the small size paper.

  The thermistor 11 is provided at the center portion of the fixing roller corresponding to the substantially central portion of the sheet passing area width P2 for small size paper. Further, the thermistor 11 is arranged so as to be elastically pressed against the surface of the fixing roller 1 by an elastic member so as to face the exciting coil 6 across the heating roller 1. A detection signal of the heating roller temperature of the thermistor 11 is input to a control circuit unit (CPU) 100.

  The control circuit unit 100 of the image forming apparatus starts the apparatus by turning on the main power switch of the apparatus, and starts a predetermined start-up mode (step of increasing until the set temperature is reached). The rotation of the heating roller 1 starts when the drive source M1 is activated. The pressure roller 2 also rotates following the rotation of the heating roller 1. Further, the control circuit unit 100 activates the high frequency inverter 101 to flow a high frequency current through the exciting coil 6. In the present embodiment, the frequency f1 of the high-frequency current during the startup process is 60 kHz. As a result, a high-frequency alternating magnetic flux is generated around the exciting coil 6, and the heating roller 1 generates heat by electromagnetic induction and increases toward a fixing temperature, which is a predetermined image heating temperature, which is 200 ° C. in this embodiment. The fixing temperature has a temperature relationship close to the Curie temperature. The temperature rise of the heating roller 1 is detected by the thermistor 11, and the detected temperature information is input to the control circuit unit 100.

  When the temperature of the heating roller 1 reaches 200 ° C., it enters a standby state (standby mode) in which it waits for an image forming signal to be input. During standby (in standby mode), the control circuit unit 100 serving as an energization control unit controls the high-frequency current so that the substantially entire area of the large-size paper passing area width P1 of the heating roller 1 is maintained at a fixing temperature of 200 ° C. To do. In this embodiment, the standby temperature is 200 ° C. In this embodiment, the frequency f2 of the high frequency current is 20 kHz.

  When an image formation signal is input in this standby mode, a toner image is formed on the recording material. Then, the recording material P carrying the unfixed toner image t is nipped and conveyed by the fixing nip portion N, whereby the unfixed toner image t is recorded by the heat and pressure of the heating roller 1 maintained at a predetermined fixing temperature. Heat-fixed on the surface of the material P. During the image heating process (during the fixing operation), the control circuit unit 100 serving as an energization control unit maintains a high-frequency current so that the substantially entire area of the large-size paper passing area width P1 of the heating roller 1 is maintained at a fixing temperature of 200 ° C. To control. In this embodiment, the fixing temperature is 200 ° C. In this embodiment, the frequency f2 of the high-frequency current is 20 kHz, which is the same frequency as in the standby mode. During the image heating process, the power applied to the coil is changed according to the difference between the output of the thermistor and the fixing temperature.

  Here, the reason why the frequency is different between the start-up mode and the image heating process will be described. Increasing the frequency can increase the amount of heat generated at a temperature close to the Curie temperature. Therefore, in order to shorten the start-up time, it is preferable to increase the heat generation amount. However, there is a problem that the temperature rise rate of the non-sheet passing portion is increased when a small size recording material is continuously fed so as to increase the heat generation amount during image heating. In order to slow down the rising speed of the temperature rise of the non-sheet passing portion, it is preferable that the amount of heat generated during the image heating operation is not too large.

  In this embodiment, the reason why the frequency in the standby mode is different from the frequency in the start-up mode will be described. Increasing the frequency decreases the skin depth and increases the amount of heat generation. Increasing the amount of heat generated is advantageous in that the heated heating roller is heated in a short time. However, if a frequency that increases the amount of heat generation is selected in a state where the temperature of the heating roller reaches the fixing temperature, such as during standby, there is a drawback that the temperature ripple on the heating roller increases. When the image forming signal is input and the fixing operation is performed in a state where the temperature ripple of the heating roller is large, a problem such as gloss unevenness depending on temperature unevenness may occur depending on the image. Therefore, it is preferable that the temperature ripple is as small as possible during standby.

  Here, the principle of electromagnetic induction heat generation of the heating roller metal core 1a will be described with reference to FIG. An alternating current is applied to the exciting coil 6 from the high-frequency inverter 101, whereby the magnetic flux indicated by the arrow H repeats generation and disappearance around the exciting coil 6. The magnetic flux H is guided along a magnetic path formed by the magnetic core material 5 (1, 2) and the cored bar 1a. In response to the change in magnetic flux generated by the exciting coil 6, an eddy current is generated in the core bar 1a so as to generate the magnetic flux in a direction that prevents the change in magnetic flux. This eddy current is indicated by an arrow C.

  This eddy current C flows intensively on the surface of the cored bar 1a on the side of the exciting coil 6 due to the skin effect, and generates heat with power proportional to the skin resistance Rs (Ω) of the cored bar 1a.

  Here, the skin depth obtained from the frequency f (Hz) of the alternating current applied to the exciting coil 6, the magnetic permeability μ (H / m) of the conductor of the core metal 1a, and the specific resistance ρ (Ω · m) of the core metal 1a. The thickness δ (m) and the skin resistance Rs (Ω) are expressed by Equation 1 and Equation 2.

（式１）

(Formula 1)

（式２）

(Formula 2)

  Further, the electric power W generated in the cored bar 1a is expressed by Equation 3 with the eddy current induced in the cored bar 1a being If (A).

（式３）

(Formula 3)

  As described above, in order to increase the heat generation amount of the core metal 1a, the eddy current If may be increased or the skin resistance Rs may be increased.

  In order to increase the eddy current If, the magnetic flux generated by the exciting coil 6 may be increased, or the change in the magnetic flux may be increased. For example, it is preferable to increase the number of turns of the exciting coil 6 or use a magnetic core material 5 having a higher magnetic permeability and a lower residual magnetic flux density. Further, by reducing the gap d between the magnetic core material 5 and the core metal 1a, the magnetic flux guided into the core metal 1a increases, so that the eddy current If can be increased.

  On the other hand, in order to increase the skin resistance Rs, the frequency f of the alternating current applied to the exciting coil 6 is increased, or the metal core 1a is made of a material having a high magnetic permeability μ and a high specific resistance.

  Next, the Curie temperature will be described. In general, when a ferromagnetic material is heated to a Curie temperature unique to the material, it loses its spontaneous magnetization and the magnetic permeability μ decreases. Therefore, when the temperature of the cored bar 1a that is the conductive member of the heating roller 1 exceeds the Curie temperature, the skin resistance Rs decreases. As a result, the heating value W of the cored bar 1a is reduced.

  In general, the resistance value is determined by the magnetic permeability μ and the resistivity ρ when the frequency is constant as represented by the equation 2, and the resistivity generally increases gradually as the temperature rises.

  Here, the resistance value (skin resistance) Rs of the heating roller 1 corresponds to the apparent load resistance of the heating roller as viewed from the coil when a current is passed through the coil when the magnetic flux generating means is mounted on the heating roller.

  The apparent resistance value measurement method and the temperature dependence of the resistance value are measured as follows. Using an LCR meter (model number HP4194A) manufactured by Agilent, the resistance value of the heating roller when an alternating current with a frequency of 20 kHz was applied was measured. At this time, the heating roller 1, the exciting coil as the magnetic flux generating means, and the core are measured while mounted on the image heating apparatus. At this time, the temperature characteristic curve of the resistance value of the heating roller 1 can be obtained by changing the temperature of the heating roller and plotting the temperature and the resistance value of the heating roller simultaneously.

  Further, in order to change the temperature of the heating roller, the temperature of the heating roller is changed while maintaining the positional relationship in which the heating roller 1 and the magnetic flux generating means are mounted on the apparatus. And after saturating roller temperature to the temperature of a high temperature chamber, resistance value is measured by said measuring method.

  When measured in this way, the temperature dependence of the resistance value becomes a curve as shown in FIG.

  Moreover, the measuring method of a magnetic permeability is performed as follows. The measurement was performed using a BH analyzer (model number: SY-8232) manufactured by Iwatsu Measurement Co., Ltd. A predetermined primary coil and secondary coil of the apparatus are wound around the measurement sample, and measurement is performed at a frequency of 20 kHz. The measurement sample may be in a shape around which a coil is wound (the ratio between temperatures having different magnetic permeability is hardly changed).

  After setting the coil to the sample, put the sample in a constant temperature room to saturate the temperature, and plot the permeability at that temperature. The temperature dependence curve of permeability can be obtained by changing the temperature of the temperature-controlled room. The temperature of the temperature-controlled room is raised, and the permeability does not change at a certain temperature. The temperature at which the magnetic permeability does not change is regarded as the Curie temperature.

  When measured in this way, the temperature dependence of the magnetic permeability becomes a curve as shown in FIG.

  As shown in FIG. 7, when the thickness d (mm) of the cored bar 1a is narrower than δc (mm), an eddy current flows through almost all of the cross section of the cored bar 1a. . Actually, even when the temperature of the metal core 1a is lower than the Curie temperature, the magnetic permeability decreases as the temperature approaches the Curie temperature, and the heat generation amount decreases accordingly.

  On the other hand, in order to suppress a decrease in the heat generation amount, the frequency of the high-frequency current applied to the exciting coil during the start-up mode may be set larger than that during the image heating operation. Specifically, in the start-up mode, it is sufficient that the length of the skin depth at the temperature at which start-up is completed is smaller than the thickness d of the cored bar 1a. The length of the skin depth at least at the temperature at which the start-up is completed when the frequency f1 of the high-frequency current is applied should be smaller than the thickness d of the cored bar 1a. Furthermore, if the skin depth δc when the Curie temperature is exceeded when the frequency f1 of the high-frequency current is applied is less than the thickness d of the cored bar 1a, the amount of heat generated when the Curie temperature is exceeded is conventional. It becomes possible to increase more. As a result, even if the temperature of the cored bar 1a approaches the Curie temperature, the amount of heat generation is reduced less than before, so that the quick start performance is not impaired.

  On the contrary, when the skin depth δc when the Curie temperature is exceeded is smaller than the core metal 1a, the temperature rise at the non-sheet passing portion is deteriorated during the sheet passing.

  From the above, the frequency of the high-frequency current applied to the exciting coil 6 during the start-up mode and the image heating operation is set so as to satisfy Equation 1.

（式４）

(Formula 4)

  By adopting this configuration, it is possible to prevent or reduce the temperature rise of the non-sheet passing portion without impairing the quick start performance. In this embodiment, the frequency of the high frequency current can be switched between 60 kHz and 20 kHz. When the start-up mode was selected, the frequency f1 of the high-frequency current was 60 kHz, the frequency f2 of the high-frequency current was 20 kHz during the image heating operation, and the specific resistance ρ of the metal core 1a was 5 Ω · m. The frequency at standby is 20 kHz.

  In this embodiment, the frequency in the start-up mode is made constant as f1, and the frequency f2 of the high-frequency current in the standby mode is made constant.

  In this embodiment, the frequency is constant in the start-up mode and in the image heating operation. On the other hand, the power needs to be switched between the start-up mode and the image heating operation. In the present embodiment, a switching circuit for rectifying the voltage of the power source into a DC voltage and switching the DC voltage value is provided in the excitation circuit. By switching the voltage value in the DC voltage switching circuit, it is possible to switch the power applied to the coil while keeping the frequency constant.

  FIG. 6 shows an increase graph of the heating roller temperature in the start-up mode in the heat fixing apparatus of this embodiment. In this embodiment, high frequency power of 60 kHz is applied to the exciting coil in the start-up mode. When the heating roller temperature reaches 200 ° C., which is the fixing temperature, the mode is switched to the standby mode, the frequency is switched from 60 kHz to 20 kHz, and high frequency power of 20 KHz is applied to the exciting coil. The rise time (wait time) up to 200 ° C. in the heat fixing apparatus of this example was 29 seconds. As a comparative example, the Uwait time was 34 seconds when high frequency power of 20 kHz was applied to the exciting coil in both the startup mode and standby mode. The reason why there is a difference in the wait time is that, referring to FIG. 8, the point A at which the magnetic permeability starts to change corresponds to 170 ° C. in FIG. 6 in this embodiment. When the temperature exceeds 170 ° C., the permeability gradually begins to decrease, and the amount of heat generation is reduced, so that the wait time is lengthened.

  In this embodiment, the frequency during image heating is the same as that during standby.

  In the above embodiment, the frequency in the start-up mode is 60 kHz.

It may be fair to set a plurality of frequencies in the start-up mode that satisfy the above relationship and select a frequency from the plurality of frequencies according to the temperature of the image heating member.

  In standby mode,

A plurality of frequencies satisfying the above relationship may be provided, and the frequency may be selected appropriately.

  The image heating operation is when the recording material passes through the nip portion. In the above embodiment, the frequency at the time of the image heating operation is 20 kHz, but the frequency at the time of the image forming operation from the start of the pre-rotation by the input of the image forming signal to the end of the post-rotation after the end of the image formation is fixed to 20 kHz. It may be a configuration.

  In this embodiment, the Curie temperature of the magnetic shunt alloy is 210 ° C., the heating roller temperature and the fixing temperature in the standby mode are 200 ° C., but the present invention is not limited to this. That is, the present invention can be applied even if the Curie temperature is set to be optimum depending on the toner used and the configuration of the fixing device. Further, the present invention can be applied even if there are a plurality of fixing temperatures depending on the thickness of the paper to be conveyed and the state of heat of the heating roller. The high-frequency current applied to the exciting coil has been described as 60 kHz in the start-up mode and 20 kHz during the image heating operation. However, the optimum frequency is appropriately determined depending on the magnetic shunt alloy used, the high-frequency power source, the configuration of the image heating device, and the like. You can choose.

  In this embodiment, the heat roller image heating apparatus using a heating roller is used, but it is obvious that the present invention can be applied to a belt heating system using an endless belt. Further, when using a clad roller in which a plurality of different metals are laminated, the present invention can be applied if at least one layer of a magnetic shunt alloy is provided.

  Further, the present invention can obtain the same effect even when used in a color image forming apparatus that forms a color image by superposing different color toners on a recording material.

Schematic configuration diagram of an example of an image forming apparatus in the embodiment Enlarged cross-sectional model view of the main part of the fixing device (electromagnetic induction heating type image heating device) in the embodiment Similarly, front view of the main part The longitudinal front model view Diagram showing heat generation principle of heating roller The figure which shows the temperature rise of the heat fixing apparatus in an Example. The figure which shows the area | region where the eddy current is induced in an Example Diagram showing temperature dependence curve of permeability Figure showing temperature dependency curve of resistance value of pressure roller

Explanation of symbols

1 Heating roller (conductive member)
2 Pressure roller 3 Coil assembly (magnetic field generating means)
5 Magnetic core material (magnetic core)
6 Excitation coil 11 Thermistor (temperature detection means)
41 photoconductor (photosensitive drum)
42 Primary charger 43 Laser beam scanner 44 Developer 45 Transfer roller 46 Cleaning device F Fixing device P Recording material (heated material)
t Toner image

Claims (4)

A coil, a high-frequency circuit for supplying a high-frequency current to the coil, and a conductive layer that generates heat by magnetic flux generated by the high-frequency current passed through the coil, and heats the image on the recording material at a predetermined image heating temperature. In an image heating apparatus having an image heating member and energization control means for controlling energization to the coil,
The Curie temperature of the image heating member is equal to or higher than the image heating temperature and lower than the heat resistance temperature of the image heating device, and the current supplied to the coil during the process of raising the image heating member to a set temperature close to the Curie temperature. An image heating apparatus characterized in that the frequency is higher than the frequency during the process of heating the image on the recording material.   The image heating apparatus according to claim 1, wherein the set temperature is an image heating temperature.   The frequency of the current that is applied to the coil during the step and the frequency of the current that is applied to the coil during the image heating step are constant. Image heating device. The thickness of the conductive layer of the heating member is set to d (mm), the magnetic permeability of the heating member is μ (H / m), and the specific resistance of the conductive layer of the heating member is set to ρ (Ω · m). The frequency f1 (Hz) of the current supplied to the coil during the process of raising the temperature, and the frequency f2 of the current supplied to the coil to maintain the temperature of the image heating member at the image heating temperature during image heating. (Hz)

The image heating apparatus according to claim 3, wherein the relationship is satisfied.

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