JP2014016445A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an appropriate temperature distribution in a width direction of a recording medium.SOLUTION: A fixing device 20 includes a heating roller 21 heated by induction heating means 30, a fixing roller 22, a fixing belt 24 stretched on the heating roller 21 and the fixing roller 22, and a pressurizing roller 23 forming a nip between itself and the fixing roller 22. The induction heating means 30 includes: a plurality of exciting coils 31a, 31b that are concentric to each other and have different lengths in an axial direction of the heating roller 21; a resonance circuit 35 for supplying an alternating current to the exciting coils 31a, 31b; and a switching circuit 34 for switching a connection state between the resonance circuit 35 and one exciting coil of the plurality of exciting coils 31a, 31b.

Description

  The present invention relates to a fixing device and an image forming apparatus. More specifically, the present invention relates to a fixing device mounted on an electrophotographic image forming apparatus such as a copying machine, a facsimile, or a printer.

  Various image forming apparatuses using an electrophotographic system have been devised as image forming apparatuses such as copying machines, facsimiles, and printers, and are well-known techniques. In the image forming process, an electrostatic latent image is formed on the surface of the photosensitive drum as an image carrier, and the electrostatic latent image on the photosensitive drum is developed with a toner as a developer to be visualized and developed. The image is formed by a process in which the transferred image is transferred to a recording medium (also referred to as paper, recording paper, or recording material) by a transfer device to carry the image, and the toner image on the recording paper is fixed by a fixing device using pressure, heat, or the like. Yes.

  Various types of fixing devices have been proposed. The fixing device includes a fixing roller (heating roller) maintained at a predetermined temperature and a pressure roller in pressure contact with the fixing roller. There is known a roller fixing system in which a transfer material carrying an unfixed toner image is heated and fixed while being nipped and conveyed by a nip portion formed by pressure contact.

  A nip formed by a pressure contact between the pressure roller and the fixing belt is provided with a fixing roller disposed opposite the pressure roller and an endless fixing belt stretched between the fixing roller and the heating roller. There is known a belt fixing system in which heat of a heating roller is applied to a transfer material through a fixing belt to press an unfixed toner image onto the transfer material and fix it.

  In recent years, from the viewpoint of energy saving, it has been required to shorten the rise time (warm-up time) by high-speed start-up until the fixing device can pass paper. A halogen heater is used as a heat source for fixing rollers and heating rollers. A fixing device has been proposed that uses electromagnetic induction heating, such as an IH heater, that has higher heat conversion efficiency than the conventional method used as a heat source.

  In this electromagnetic induction heating (also referred to as induction heating) type fixing device, it is known that it is difficult to control the temperature distribution in the longitudinal direction (axial direction) of the fixing member because the electromagnetic induction heat generating layer which is a heat generating member is thin. It has been. The fixing member refers to a fixing (heating) roller in the roller fixing method, and a fixing roller, a heating roller, and a fixing belt in the belt fixing method.

  A general image forming apparatus is configured to form an image on several types of recording media having different sizes in the width direction. Note that 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 width direction is different between when the longitudinal direction is the transport direction and when the short direction (the direction perpendicular to the longitudinal direction) is the transport direction. You are dealing with recording media of different sizes.

  When fixing a recording medium having a different width direction size with a fixing device, the heat distribution in the longitudinal direction of the fixing member varies depending on the width direction size of the recording medium, resulting in temperature unevenness. There is.

  For example, when fixing by passing a recording medium having a small width direction size, a lot of heat is taken away in the longitudinal direction (paper passing area) of the fixing member corresponding to the width direction size of the recording medium, The fixing temperature is lower than in other positions (non-sheet passing area). In other words, an excessive temperature rise occurs in the non-sheet passing region. This phenomenon becomes particularly remarkable when a recording medium having a small width size is continuously fed.

  In order to solve this problem, a method using a demagnetizing member in an induction heating type fixing device has been proposed. For example, in Patent Document 1, in addition to an exciting coil that performs electromagnetic induction heating, a degaussing secondary coil is installed at a position corresponding to a non-paper passing region, and a demagnetizing second coil generated by a change in magnetic flux generated by the exciting coil is disclosed. There has been disclosed a heating device that prevents excessive temperature rise by reducing the magnetic flux in the non-sheet passing region by the induced electromotive force and induced current of the secondary coil.

  Further, in Patent Document 2, a magnetic shunt alloy and a metal plate that change in magnetism and non-magnetism by changing to a Curie temperature or higher are arranged, the magnetic shunt alloy is interposed between an exciting coil and a metal plate, and the magnetic shunt alloy is curie temperature. When it becomes above, the heat roller apparatus which exhibits the self-temperature control function in the form which the repulsive magnetic flux by a metal plate cancels the induced magnetic flux by an exciting coil is disclosed.

  However, since both Patent Documents 1 and 2 are configured to heat the non-sheet passing area, the problem that the temperature of the non-sheet passing area becomes higher than the temperature of the sheet passing area cannot be solved.

  Further, Patent Document 3 discloses a heating roller that generates heat by electromagnetic induction, a plurality of excitation coils that are arranged along the width direction of the recording paper and that generates magnetic flux of electromagnetic induction, and power that is selectively supplied to the plurality of excitation coils. An electrical circuit including a plurality of power supply paths for supplying and an IH control unit for controlling power supply to the excitation coil, wherein at least one of the power supply paths is a predetermined excitation of the plurality of excitation coils In order to cut off or suppress the power supply to the coil, the IH control unit includes a bypass path that bypasses the predetermined excitation coil, and the IH control unit selects one of the plurality of power supply paths based on the width of the recording paper to be passed. An electromagnetic induction heating fixing device for selecting is disclosed.

  Further, Patent Document 4 discloses an exciting coil that is arranged along the heating member and generates an alternating magnetic flux to electromagnetically heat the heating member, and a part of the alternating magnetic flux generated by the exciting coil is circulated. A heating device is disclosed that includes a degaussing coil that generates an electromotive force in the direction of cancellation, and a degaussing current adjusting device that adjusts a current generated in the degaussing coil in a degaussing circuit including the degaussing coil.

  In Patent Documents 3 and 4 described above, an appropriate temperature distribution can be realized in the width direction of the recording medium. However, in the technique of Patent Document 3, it is necessary to perform control using a plurality of inverter circuits. In the invention of Patent Document 4, it is necessary to have a configuration of a degaussing coil as a demagnetizing unit and a demagnetizing current adjusting unit that adjusts a current generated in the degaussing coil, and the configuration and control for changing the heating width are complicated. It was.

  Therefore, the present invention adjusts the heating width in the axial direction of the fixing member with a simple configuration without using a plurality of inverter circuits and degaussing members such as a degaussing coil, and overheats in a non-sheet passing region. It is an object of the present invention to provide a fixing device capable of preventing the occurrence of the above problem and realizing an appropriate temperature distribution in the width direction of the recording medium, and an image forming apparatus including the fixing device.

  In order to achieve such an object, a fixing device according to the present invention is arranged so that a fixing member heated by induction heating means and at least a part of the fixing member can be pressed, and a nip portion is formed between the fixing member and the fixing member. In the fixing device, the induction heating unit includes a plurality of excitation coils that are concentric and have different lengths in the axial direction of the fixing member, and a resonance that supplies an alternating current to the excitation coil. The circuit includes a circuit, a resonance circuit, and a switching circuit that switches a connection state with any one of the plurality of exciting coils.

  According to the present invention, the heating width in the axial direction of the fixing member can be adjusted to prevent an excessive temperature rise in the non-sheet passing region, and an appropriate temperature distribution can be realized in the width direction of the recording medium. Can be.

1 is a cross-sectional view illustrating an embodiment of an image forming apparatus according to the present invention. 1 is a side cross-sectional view illustrating a configuration of an embodiment of a fixing device according to the present invention. It is a schematic diagram explaining arrangement | positioning of the exciting coil with respect to a heating roller. FIG. 6 is a heat distribution diagram showing the relationship between the position in the axial direction of the heating roller and the amount of heat generated when the excitation coil is switched. It is a temperature distribution diagram showing the relationship between the axial position of the heating roller and the heating roller temperature when passing small-size paper. 5 is a control flowchart of a switching circuit by a control unit of the fixing device.

  Hereinafter, a configuration according to the present invention will be described in detail based on the embodiment shown in FIGS.

(Image forming device)
FIG. 1 is a schematic configuration diagram illustrating the overall configuration of a tandem type color copier that is an embodiment of an image forming apparatus according to the present invention. The outline and operation of the internal configuration of the image forming apparatus will be described with reference to FIG.

  In FIG. 1, 1 is an image forming apparatus main body, 2 is a writing unit that emits laser light based on input image information, 3 is a document conveying unit that conveys a document D to a document reading unit 4, and 4 is image information of the document D. A document reading unit to be read, 7 is a paper feed unit that accommodates a recording medium P such as transfer paper, 9 is a registration roller that adjusts the conveyance timing of the recording medium P, and 11Y, 11M, 11C, and 11BK are each color (yellow, magenta, Cyan and black) on which the toner image is formed, 12 is a charging unit for charging the photosensitive drums 11Y, 11M, 11C, and 11BK, and 13 is the photosensitive drums 11Y, 11M, 11C, and 11BK. A developing unit 14 for developing the formed electrostatic latent image is a transfer-by unit that transfers the toner images formed on the photosensitive drums 11Y, 11M, 11C, and 11BK in an overlapping manner on the recording medium P. Surora (primary transfer bias roller), 15 denotes the photosensitive drums 11Y, 11M, 11C, a cleaning unit for collecting the untransferred toner on 11BK.

  Reference numeral 16 denotes an intermediate transfer belt cleaning unit that cleans the intermediate transfer belt 17, 17 an intermediate transfer belt on which toner images of a plurality of colors are transferred and superimposed, and 18 a color toner image on the intermediate transfer belt 17 on the recording medium P. A secondary transfer bias roller 20 for transferring the toner image onto the recording medium P indicates a fixing device for fixing the toner image on the recording medium P.

  Hereinafter, an operation during normal color image formation in the image forming apparatus will be described. First, the document D is transported from the document table in the direction of the arrow in the drawing by the transport rollers of the document transport unit 3 and placed on the contact glass 5 of the document reading unit 4. Then, the document reading unit 4 optically reads the image information of the document D placed on the contact glass 5. Specifically, the document reading unit 4 scans the image of the document D on the contact glass 5 while irradiating light emitted from an illumination lamp. Then, the light reflected by the document D is imaged on the color sensor via the mirror group and the lens. The color image information of the document D is read for each RGB (red, green, blue) color separation light by the color sensor, and then converted into an electrical image signal. Further, color conversion processing, color correction processing, spatial frequency correction processing, and the like are performed by the image processing unit based on the RGB color separation image signals to obtain yellow, magenta, cyan, and black color image information.

  Then, the image information of each color of yellow, magenta, cyan, and black is transmitted to the writing unit 2. The writing unit 2 emits laser light (exposure light) based on the image information of each color toward the corresponding photosensitive drums 11Y, 11M, 11C, and 11BK.

  On the other hand, the four photosensitive drums 11Y, 11M, 11C, and 11BK are rotated clockwise in FIG. First, the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK are uniformly charged at a portion facing the charging unit 12 (charging process). Thus, a charged potential is formed on the photosensitive drums 11Y, 11M, 11C, and 11BK. Thereafter, the charged surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK reach the irradiation positions of the respective laser beams.

  In the writing unit 2, laser light corresponding to the image signal is emitted from the four light sources corresponding to each color. Each laser beam passes through a different optical path for each color component of yellow, magenta, cyan, and black (exposure process).

  Laser light corresponding to the yellow component is irradiated on the surface of the first photosensitive drum 11Y from the left side of the drawing. At this time, the yellow component laser light is scanned in the rotation axis direction (main scanning direction) of the photosensitive drum 11Y by a polygon mirror that rotates at high speed. Thus, an electrostatic latent image corresponding to the yellow component is formed on the photosensitive drum 11Y charged by the charging unit 12.

  Similarly, the laser beam corresponding to the magenta component is irradiated onto the surface of the second photosensitive drum 11M from the left side of the paper, and an electrostatic latent image corresponding to the magenta component is formed. The cyan component laser light is applied to the surface of the third photosensitive drum 11C from the left side of the paper, and an electrostatic latent image of the cyan component is formed. The black component laser beam is applied to the surface of the fourth photosensitive drum 11BK from the left side of the paper, and an electrostatic latent image of the black component is formed.

  Thereafter, the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK on which the electrostatic latent images of the respective colors are formed reach the positions facing the developing unit 13, respectively. Then, the respective color toners are supplied from the developing units 13 onto the photosensitive drums 11Y, 11M, 11C, and 11BK, and the latent images on the photosensitive drums 11Y, 11M, 11C, and 11BK are developed (developing step).

  Thereafter, the surfaces of the photoconductive drums 11Y, 11M, 11C, and 11BK after the development process reach the opposed portions to the intermediate transfer belt 17, respectively. Here, a transfer bias roller 14 is installed at each facing portion so as to contact the inner peripheral surface of the intermediate transfer belt 17. Then, the toner images of the respective colors formed on the photosensitive drums 11Y, 11M, 11C, and 11BK are sequentially superimposed and transferred onto the intermediate transfer belt 17 at the position of the transfer bias roller 14 (primary transfer process). .

  Then, the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK after the transfer process reach positions facing the cleaning unit 15, respectively. Then, the untransferred toner remaining on the photosensitive drums 11Y, 11M, 11C, and 11BK is collected by the cleaning unit 15 (cleaning process).

  Thereafter, the surfaces of the photoconductive drums 11Y, 11M, 11C, and 11BK pass through a static elimination unit (not shown), and a series of image forming processes on the photoconductive drums 11Y, 11M, 11C, and 11BK is completed.

  On the other hand, the intermediate transfer belt 17 on which the toners of the respective colors on the photoconductive drums 11Y, 11M, 11C, and 11BK are transferred (carried) are run in the clockwise direction in the drawing and are connected to the secondary transfer bias roller 18. Reach the opposite position. The color toner image carried on the intermediate transfer belt 17 is transferred onto the recording medium P at a position facing the secondary transfer bias roller 18 (secondary transfer step).

  Thereafter, the surface of the intermediate transfer belt 17 reaches the position of the intermediate transfer belt cleaning unit 16. Then, the untransferred toner adhered on the intermediate transfer belt 17 is collected by the intermediate transfer belt cleaning unit 16, and a series of transfer processes in the intermediate transfer belt 17 is completed.

  Here, the recording medium P conveyed between the intermediate transfer belt 17 and the secondary transfer bias roller 18 (secondary transfer nip) is conveyed from the paper supply unit 7 via the registration roller 9 and the like. is there.

  Specifically, the recording medium P fed by the paper feeding roller 8 from the paper feeding unit 7 that stores the recording medium P is guided to the registration roller 9 after passing through the conveyance guide. The recording medium P that has reached the registration roller 9 is conveyed toward the secondary transfer nip at the same timing.

  Then, the recording medium P on which the full-color image is transferred is guided to the fixing device 20 by the transport belt. In the fixing device 20, a color image (toner) is fixed on the recording medium P at the nip between the fixing roller 22 and the pressure roller 23.

  Then, the recording medium P after the fixing step is discharged as an output image outside the apparatus main body 1 by a paper discharge roller, and a series of image forming processes is completed.

(Fixing device)
[Configuration of Fixing Device]
Next, the configuration and control of the fixing device 20 included in the image forming apparatus 1 will be described. FIG. 2 is a schematic cross-sectional view of the fixing roller 22 in the axial direction, showing an example of the induction heating type fixing device 20. FIG. 3 is a view for explaining the arrangement of the excitation coils 31a and 31b with respect to the heating roller 21 of the fixing device 20, and is a view of the excitation coil 31 as viewed from above in the cross-sectional view of FIG.

  As shown in FIGS. 2 and 3, the fixing device 20 according to the present embodiment is stretched around a heating roller 21 heated by induction heating means 30, a fixing roller 22, the heating roller 21, and the fixing roller 22. In the fixing device 20 that includes the fixing belt 24 and the pressure roller 23 that forms a nip portion between the fixing roller 22, the induction heating unit 30 is concentric and the axial direction of the heating roller 21. Excitation coils 31a and 31b having different lengths, resonance circuit 35 for supplying an alternating current to the excitation coils 31a and 31b, resonance circuit 35, and excitation of any one of the plurality of excitation coils 31a and 31b. And a switching circuit 34 that switches a connection state with the coil.

  As shown in FIG. 2, the induction heating type fixing device 20 includes a fixing roller 22 having an elastic layer 22b on the outer periphery of a metal core 22a, a heating roller 21 heated by exciting coils 31a and 31b, and a heating roller 21. An endless fixing belt 24 stretched between the fixing roller 22, a pressure roller 23 disposed on the outer peripheral side of the fixing belt 24 and forming a nip region with the fixing roller 22, and heating And induction heating means 30 for heating the roller 21.

  The fixing device 20 passes a recording medium P (paper) carrying an unfixed toner image through a nip portion between the fixing belt 24 and the pressure roller 23 formed by pressure contact between the fixing roller 22 and the pressure roller 23. Heat fixing. The heating roller 21, the fixing roller 22, and the pressure roller 23 are pivotally supported so as to be rotatable in the longitudinal direction of a housing (not shown) of the fixing device 20, and the driving means of each roller (not shown) are fixed to the housing. Is retained.

  The heating roller 21 has, for example, a length in the longitudinal direction (axial direction) of 320 mm and an outer diameter of 40 mm, and is composed of a conductive heat generating layer 21a, an elastic layer 21b, and a cored bar layer 21c that are heat generating member bodies. The positional relationship of each layer is in the order of the heat generating layer 21a, the elastic layer 21b, and the cored bar layer 21c.

  The heat generating layer 21a is formed of a metal member having good conductivity and good heat conductivity suitable for electromagnetic induction heating because eddy current is easily generated by an alternating magnetic field. As a metal suitable for electromagnetic induction heating, a metal having high resistance is generally known. However, even if the resistance is low, a metal member having good heat conductivity is thinned to make a substantial layer of the heat generating layer. The resistance value can be arbitrarily set, and the heat generation amount can be adjusted.

  As the heat generating layer 21a, for example, a structure in which copper having a thickness of 10 μm is plated on a nonmagnetic stainless steel layer having a thickness of 50 μm can be used. Since the heat generating layer 21a only needs to have good electrical conductivity and heat transfer, other metal layers such as silver, aluminum, magnesium, etc., or nickel or magnetic stainless steel, which are magnetic materials, may be used.

  For the elastic layer 21b, for example, an elastic body such as fluorine rubber, silicon rubber, or fluorosilicon rubber is used. The elastic layer 21b is made of sponge rubber to suppress heat transfer to the inside of the heating roller and to keep the heat of the heat generating layer 21a in a heat insulating state.

  The cored bar layer 21c is a support for the entire heating roller 21, and a metal such as iron or aluminum is used to ensure rigidity. Moreover, it is possible and preferable that the cored bar layer 21c is made of a nonmagnetic or insulating material such as nonmagnetic stainless steel or ceramic so as not to affect induction heating. For example, using SUS304, which is a nonmagnetic stainless steel having an outer diameter of 22 mm and a thickness of 2.0 mm, the energy of induction heating can be concentrated in the heat generating layer without loss.

  The fixing belt 24 is wound around the fixing roller 22 and the heating roller 21, and is in close contact with the heating roller 21 and the fixing roller 22. A fixing nip is formed by pressing the pressure roller 23 against the fixing belt 24 configured as described above at a position corresponding to the fixing roller 22.

  Further, the fixing belt 24 is composed of, for example, a PI belt which is an endless film of a heat-resistant resin having a thickness of 90 μm, and the surface layer has an anti-offset agent such as PFA (tetrafluoroethylene barfluoroalkyl vinyl ether copolymer resin). Is coated.

  The fixing roller 22 does not have a heat source, and is formed by covering a highly rigid core material (core metal 22b) such as metal (iron or aluminum) with a thick elastic layer 22a such as silicon rubber.

  The fixing roller 22 and the pressure roller 23 are rubber rollers arranged to face each other. When the pressure roller 23 is pressed toward the center of the fixing roller 22 via the fixing belt 24, the pressure roller 23. And a fixing belt 24 form a nip portion.

  Further, the driving unit drives the fixing roller 22 to rotate in the clockwise direction. As the fixing roller 22 rotates, the pressure roller 21 and the fixing belt 24 that are in pressure contact with the fixing roller 22 rotate at the same speed.

  The heating roller 21 is heated by the induction heating unit 30, and the toner image of the recording medium P is conveyed while being heated and melted in the nip portion by the reverse rotation of the pressure roller 23 and the fixing belt 24.

  Further, based on a temperature signal from a temperature detection means such as a thermistor (not shown), the induction heating means 30 is controlled so as to maintain a fixable temperature at which the fixing belt 24 can be fixed satisfactorily.

  According to the fixing device 20 having this configuration, the recording medium P on which the toner image is formed is moved from the pre-fixing guide (not shown) arranged on the upstream side of the fixing belt 24 in the sheet conveying direction to the arrow shown in FIG. The toner image on the recording medium P is heated and melted at the nip portion, whereby the toner image is fixed. Thereafter, the recording medium P is separated by a separation plate (not shown) or the like, and is discharged from the paper discharge unit on the downstream side in the paper transport direction.

  Here, the fixing device 20 has a sheet passing width capable of passing an A3 size recording medium P. When a recording medium P having a width narrower than the axial width of the heating roller 21 such as an A4 size recording medium P is heat-fixed, the central portion of the heating roller 21 is used.

  The induction heating means 30 of the fixing device 20 applies an alternating current to the two exciting coils (second exciting coil) 31a, the exciting coil (first exciting coil) 31b, and the exciting coils 31a and 31b as magnetic field generating means. A magnetic core 32 (32a, 32b, 32c) that prevents leakage of the magnetic field generated by the operation, and a coil holding member 33 provided at a position facing the magnetic core 32 with the exciting coils 31a, 31b interposed therebetween. The switching circuit 34 for switching the connection state between the excitation coils 31a and 31b and the resonance circuit 35, the resonance circuit (power supply circuit) 35 for supplying an alternating current for generating a magnetic field to the excitation coils 31a and 31b, and the heating roller 21 An end temperature detecting sensor (end temperature detecting means) 36 for detecting the temperature on the end side in the axial direction.

  In the induction heating means 30, the magnetic field generated by the exciting coil 31 induction heats the heat generating layer 21 a that is the surface layer of the heating roller 21. The exciting coils 31a and 31b are concentric and have different lengths in the axial direction of the heating roller 21. Each of the exciting coils 31a and 31b is composed of a bundle of bundles of copper wires whose surfaces are insulated, and is extended in the axial direction of the heating roller 21 and circulates along the heating roller 21 as shown in FIG. doing. Each excitation coil 31a, 31b is connected via a switching circuit 34 to a resonance circuit 35 that supplies an alternating current for generating a magnetic field.

  In the present embodiment, the exciting coil 31a has a length of 292 mm and a winding number of 11 in the axial direction of the heating roller 21, for example, and the exciting coil 31b has a length of 205 mm and a winding number in the axial direction of the heating roller 5, for example. 15 rolls. In this embodiment, an example in which two exciting coils 31 are used will be described. However, the number of exciting coils 31 is concentric, and three or more exciting coils 31 having different lengths in the axial direction of the heating roller 21 are used. You may comprise.

  As shown in FIG. 2, the magnetic core 32 includes a first magnetic core 32a disposed behind the exciting coil 31 so as to face approximately a half circumferential surface of the heating roller 21, and a coil hold-down from the first magnetic core 32a. The second magnetic core 32b extends to the central portion of the member 33, and the third magnetic core 32c extends to the end of the coil holding member 33 so as to surround the exciting coil 31. The magnetic core 32 is for efficiently causing the magnetic flux to reach the heat generating layer 21a of the heating roller 21, and is preferably made of a ferromagnetic material made of a material having high electrical resistivity. Specific examples of the material include ferrite and permalloy. Etc.

  The coil restraining member 33 is provided at a position facing the magnetic core 32 with the exciting coils 31a and 31b interposed therebetween, and plays a role of restraining the exciting coils 31a and 31b.

  The switching circuit 34 is a switch circuit provided between the excitation coils 31 a and 31 b and the resonance circuit 35, and is based on control from a control unit (not shown) of the fixing device 20 and the excitation coils 31 a and 31 b and the resonance circuit 35. Switch the connection status with.

  The resonance circuit 35 is a power supply circuit that supplies an alternating current for generating a magnetic field to the exciting coils 31a and 31b, and is configured by a resonance capacitor or the like.

  As shown in FIG. 3, the end temperature detection sensors 36 are provided at two locations outside the exciting coil 31 b and inside the exciting coil 31 a at both ends in the axial direction of the heating roller 21. Detect the temperature of The end temperature detection sensor 36 may be provided only on one end side.

[Control of fixing device]
In the fixing device 20 configured as described above, the AC roller supplied from the resonance circuit 35 flows through the exciting coil 31a or the exciting coil 31b, thereby induction heating the heating roller 21.

  Here, it is preferable that the plurality of exciting coils 31 having different lengths have the same inductance and electric resistance by changing the number of turns. In the present embodiment, the inductances of the exciting coils 31a and 31b are both 25 μH at 30 kHz, and the inductance is made uniform so that the circuit constant of the resonance circuit 35 is not changed when the exciting coils are switched. As a result, the resonance circuit 35 (resonance capacitor) of the two excitation coils 31a and 31b can be shared.

  In this way, by changing the number of turns of a plurality of exciting coils having different lengths and by aligning the inductance and electric resistance, it is possible to share a resonance circuit (power supply circuit) for the plurality of exciting coils.

  FIG. 4 is a heat distribution diagram showing the relationship between the axial position of the heating roller and the amount of heat generated when the exciting coil is switched, and FIG. 5 is the relationship between the axial position of the heating roller and the heating roller temperature when passing small-size paper. The temperature distribution figure which shows is shown. FIG. 6 is a flowchart of switching control of the switching circuit by the control unit of the fixing device.

  In the present embodiment, when large-size paper (A3) is passed, the resonance circuit 35 is connected to the excitation coil 31a by the switching circuit 34, and has a heat generation width shown in the heat generation distribution A shown in FIG. Here, when the small size paper (A4) is passed, the heat roller end portion side that is a non-sheet passing portion is not deprived of heat, so the temperature distribution of the heating roller 21 is as shown by the temperature distribution C in FIG. The temperature on the end side increases.

  Here, as shown in FIG. 6, the control means of the fixing device 20 performs temperature detection by the end temperature detection sensor 36 (S102) in a state where the exciting coil 31a is connected to the resonance circuit 35 (S101). Then, the detected temperature of the end temperature detection sensor 36 and a preset temperature set in advance are determined (S103). When the detected temperature is higher than the set temperature (S103: Yes), the switching circuit 34 is switched, Control to change the connection with the resonance circuit 35 from the exciting coil 31a to 31b is performed (S104). On the other hand, when the detected temperature is lower than the set temperature (S103: No), the exciting coil 31a remains connected to the resonance circuit 35 (S105).

  With this switching control, the heating width changes from the heat generation distribution A shown in FIG. That is, heating on the end side of the heating roller 21 is stopped, the temperature distribution changes from the temperature distribution C shown in FIG. 5 to the temperature distribution D, and the non-sheet passing area becomes higher than the sheet passing area. Can be prevented.

  According to the fixing device according to the present embodiment described above, the heating width in the axial direction of the fixing member is adjusted by a simple configuration without using a demagnetizing member such as a plurality of inverter circuits and degaussing coils. Only the paper region can be heated, and the non-sheet passing region can be prevented from being heated to a temperature higher than the sheet passing region, and an appropriate temperature distribution can be realized in the width direction of the recording medium.

  In addition, by using the image forming apparatus (FIG. 1) including the fixing device 20 having the above-described configuration, an image forming apparatus including a fixing device that can realize an appropriate temperature distribution in the width direction of the recording medium. Can be provided.

  The above-described embodiment is a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

  For example, in the above embodiment, the belt fixing type induction heating type fixing device has been described as an example. However, other methods using induction heating means, for example, the nip portion is formed by pressure contact between the pressure roller and the heating roller. Needless to say, the present invention may be applied to a fixing device of a film fixing method using a film fixing method using a film member stretched between a fixing roller and a heating roller instead of the fixing belt.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Writing part 3 Original conveyance part 4 Original reading part 5 Contact glass 7 Paper feed part 8 Paper feed roller 9 Registration roller 11Y, 11M, 11C, 11BK Photosensitive drum 12 Charging part 13 Developing part 14 Transfer bias roller ( Primary transfer bias roller)
DESCRIPTION OF SYMBOLS 15 Cleaning part 16 Intermediate transfer belt cleaning part 17 Intermediate transfer belt 18 Secondary transfer bias roller 20 Fixing device 21 Heating roller 21a Heat generating layer 21b Elastic layer 21c Core metal layer 22 Fixing roller 22a Metal core metal 22b Elastic layer 23 Pressure roller 24 Fixing belt 30 Induction heating means 31a Excitation coil 31b Excitation coils 32, 32a, 32b, 32c Magnetic core 33 Coil restraining member 34 Switching circuit 35 Resonance circuit 36 End temperature detection sensor D Document P Recording medium

JP 2001-135470 A JP 2000-30850 A JP 2009-276421 A JP 2008-185991 A

Claims (8)

A fixing member heated by induction heating means;
In a fixing device comprising: a pressure member that is disposed so as to be able to press at least a part of the fixing member and that forms a nip portion with the fixing member.
The induction heating means includes
A plurality of exciting coils that are concentric and have different lengths in the axial direction of the fixing member;
A resonance circuit for supplying an alternating current to the exciting coil;
A switching circuit for switching a connection state between the resonance circuit and any one of the plurality of excitation coils;
A fixing device. End temperature detecting means for detecting the temperature of the end side in the axial direction of the fixing member;
The connection state between the resonance circuit and the excitation coil is switched by controlling the switching circuit based on a comparison result between the temperature detected by the end temperature detection means and a predetermined set temperature. Item 4. The fixing device according to Item 1.   The fixing device according to claim 1, wherein the plurality of exciting coils have a common inductance and resistance value. The resonant capacitor included in the resonant circuit is:
The fixing device according to claim 1, wherein the fixing device is common to the plurality of exciting coils. The plurality of exciting coils are:
A first exciting coil having a length within a predetermined range from a central position in the axial direction of the fixing member;
5. The fixing device according to claim 1, further comprising: a second exciting coil having a length extending from a central position in the axial direction of the fixing member to a range wider than the predetermined range. . End temperature detecting means for detecting the temperature outside the first excitation coil and inside the second excitation coil in the axial direction of the fixing member;
The connection state between the resonance circuit and the excitation coil is switched by controlling the switching circuit based on a comparison result between the temperature detected by the end temperature detection means and a predetermined set temperature. Item 6. The fixing device according to Item 5. The fixing member includes the following (1) to (3)
(1) A heating roller heated by the induction heating means that forms a nip portion with the pressure member,
(2) a fixing roller that forms a nip portion with the pressure member, a heating roller that is heated by the induction heating means, a belt member that is stretched between the fixing roller and the heating roller,
(3) a fixing roller that forms a nip portion with the pressure member, a heating roller that is heated by the induction heating means, a film member that is stretched between the fixing roller and the heating roller,
The fixing device according to claim 1, wherein the fixing device is any one of the following.   An image forming apparatus comprising the fixing device according to claim 1.

Images