JP5397647B2 - Fixing apparatus and image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine thereof, and a fixing device installed therein, and more particularly to a fixing device and an image forming apparatus using an electromagnetic induction heating method. It is.

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer, an electromagnetic induction heating type fixing device with a short start-up time and energy saving has been widely used (see, for example, Patent Document 1).
On the other hand, the electromagnetic induction heating type fixing device overheats both ends of the heat generating member (or fixing member) when a recording medium (small size paper) having a small width size is continuously fixed. It is known to be easy.

Details are as follows.
A general image forming apparatus is configured to form an image on a plurality of types of recording media having different sizes in the width direction (a direction perpendicular to the conveyance direction of the recording medium). 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. There is. 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.

  On the other hand, in Patent Document 1 and the like, in order to solve the above-described problem, a magnetic flux generated by the main induction coil in addition to the main induction coil (excitation coil) for induction heating the heat generating member (conductive member). A technique is disclosed in which auxiliary induction coils for generating magnetic flux are provided at both ends in the width direction so as to disappear. Specifically, when small-size paper is passed, current is supplied from the power source to the auxiliary induction coils installed at both ends in the width direction, and the magnetic flux due to the main induction coil acting on the non-sheet passing area of the heat generating member is reduced. I'll be caught. On the other hand, when large-size paper is passed, current is not supplied to the auxiliary induction coils installed at both ends in the width direction, and the magnetic flux generated by the main induction coil acts on the entire width direction of the heat generating member. Here, both the main induction coil and the sub induction coil are connected to one power source, and power is supplied from the power source.

  In the fixing device described in Patent Document 1 and the like described above, when small-size paper is passed, the magnetic flux acting on the non-sheet passing region of the heat generating member is reduced by the auxiliary induction coils installed at both ends in the width direction. For this reason, an effect of suppressing excessive temperature rise in the non-sheet passing region of the heat generating member (or fixing member) can be expected to some extent.

  However, since the main induction coil and the sub induction coil are supplied with electric power from a single power source, the fixing device disclosed in Patent Document 1 or the like is based on supply / non-supply (on / off) of current to the sub induction coil. The current supplied to the main induction coil may fluctuate greatly. When the current supplied to the main induction coil fluctuates, the heating amount of the heat generating member (or fixing member) heated by the main induction coil fluctuates greatly, and fixing of the output image due to variations in fixing temperature may occur. Will occur.

  The present invention has been made to solve the above-described problems, and even when the small-size paper is continuously passed, the overheating in the non-sheet passing region of the heat generating member is efficiently performed. It is another object of the present invention to provide an electromagnetic induction heating type fixing device and an image forming apparatus that can reliably suppress and have little variation in the heating amount of a heat generating member heated by an exciting coil.

According to a first aspect of the present invention, there is provided a fixing device including a heat generating member having a heat generating layer, an exciting coil that opposes the heat generating member, generates a magnetic flux, and induction-heats the heat generating layer with the magnetic flux, A demagnetizing coil that opposes the exciting coil and generates a magnetic flux in a direction to cancel the magnetic flux, and the degaussing coils are symmetrically arranged with the center in the width direction of the heat generating member as an axis. The plurality of pairs of degaussing coils are formed such that each pair has a different length in the width direction, and is electrically connected to each pair to constitute an electric circuit having one switch. .

According to a second aspect of the present invention, in the fixing device according to the first aspect, the plurality of pairs of demagnetizing coils are disposed at different positions in the width direction, and the cross-sectional areas are different for each pair. It is formed as follows.

The fixing device according to a third aspect of the present invention is the fixing device according to the first or second aspect, wherein the plurality of pairs of degaussing coils are arranged at different positions in the width direction , respectively. / And the loop area is different for each pair .

The fixing device according to a fourth aspect of the present invention is the fixing device according to any one of the first to third aspects, wherein the exciting coil is opposed to the heat generating member with the demagnetizing coil interposed therebetween. Are arranged.

According to a fifth aspect of the present invention, there is provided the fixing device according to any one of the first to fourth aspects , wherein the fixing device is divided into a plurality of portions in the width direction within the loop of the exciting coil. The center core is configured such that at least one of the divided center cores is disposed in a loop of the degaussing coil.

According to a sixth aspect of the present invention, there is provided a fixing device according to any one of the first to fifth aspects, wherein the heat generating member is melted with a toner image and a recording medium to be conveyed is added. The fixing roller is in contact with the pressure roller to be pressed.

The fixing device according to a seventh aspect of the present invention is the fixing device according to any one of the first to fifth aspects, wherein the heat generating member melts the toner image and includes a support roller and a fixing auxiliary roller. The fixing auxiliary roller is arranged to be in contact with the pressure roller that presses the conveyed recording medium via the fixing belt.

The fixing device according to an eighth aspect of the present invention is the fixing device according to any one of the first to fifth aspects, wherein the heat generating member heats the fixing belt that melts the toner image and also fixes the fixing. A support roller that stretches the fixing belt together with the roller, wherein the excitation coil is disposed so as to face the outer peripheral surface of the support roller.

An image forming apparatus according to a ninth aspect of the present invention includes the fixing device according to any one of the first to eighth aspects.

  In the present invention, in addition to the exciting coil for induction heating of the heat generating member, a demagnetizing coil that generates a magnetic flux in a direction that cancels out the magnetic flux that acts on the heat generating member when an induced current flows by the magnetic flux generated by the exciting coil. The demagnetizing coil is not electrically connected to a power supply unit that supplies an alternating current to the exciting coil. As a result, even when small-size paper is continuously passed, overheating in the non-sheet passing region of the heat generating member is efficiently and reliably suppressed, and the heat generating member heated by the exciting coil is prevented. It is possible to provide an electromagnetic induction heating type fixing device and an image forming apparatus with little variation in heating amount.

1 is an overall configuration diagram illustrating an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view showing a fixing device installed in the image forming apparatus of FIG. 1. It is the schematic diagram which looked at the exciting coil and the degaussing coil in the width direction. It is a circuit diagram which shows the electric circuit which has a degaussing coil. It is a figure which shows the state of the magnetic flux generated by the induction heating part. It is a figure which shows the state of magnetic flux when changing arrangement | positioning of a degaussing coil. 6 is a graph showing a temperature rise characteristic of a fixing roller in a non-sheet passing region. 6 is a graph showing a heat generation amount distribution and a temperature distribution of a conventional fixing roller. 7 is another graph showing a heat generation amount distribution and a temperature distribution of a conventional fixing roller. 3 is a graph showing a heat generation amount distribution and a temperature distribution of a fixing roller according to the first embodiment. It is a flowchart which shows the control at the time of continuous paper passing. It is sectional drawing which shows the fixing device in Embodiment 2 of this invention. It is the schematic diagram which looked at the exciting coil and the degaussing coil in the fixing device of FIG. 12 in the width direction. FIG. 6 is a distribution diagram showing a heat generation amount of the fixing roller when a plurality of degaussing coils are closed alone. FIG. 6 is a distribution diagram showing the temperature of the fixing roller when a postcard-size recording medium is continuously fed. It is a figure which shows the principal part of the fixing device in Embodiment 3 of this invention. FIG. 17 is a diagram illustrating a main part of a fixing device different from FIG. 16. It is the schematic diagram which looked at the exciting coil and degaussing coil of the fixing device in Embodiment 4 of this invention in the width direction. It is sectional drawing which shows the fixing device in Embodiment 5 of this invention.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

Embodiment 1 FIG.
A first embodiment of the present invention will be described in detail with reference to FIGS.
First, the configuration and operation of the entire image forming apparatus will be described with reference to FIG.
In FIG. 1, reference numeral 1 denotes an apparatus main body of a tandem color copier as an image forming apparatus, 2 a writing unit that emits laser light based on input image information, and 3 an original conveying unit that conveys an original D to an original reading unit 4. 4 is a document reading unit that reads image information of the document D, 7 is a paper feeding 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, and 11C. , 11BK are photosensitive drums on which toner images of respective colors (yellow, magenta, cyan, and black) are formed, 12 is a charging unit that charges the photosensitive drums 11Y, 11M, 11C, and 11BK, and 13 is each photosensitive drum. A developing unit that develops electrostatic latent images formed on 11Y, 11M, 11C, and 11BK, and a toner image formed on each of the photosensitive drums 11Y, 11M, 11C, and 11BK Transfer bias roller for transferring superimposed on the recording medium P, 15 denotes each of the photosensitive drums 11Y, 11M, 11C, cleaning unit for collecting the untransferred toner on 11BK, a.

  Reference numeral 16 denotes a transfer belt cleaning unit that cleans the transfer belt 17, reference numeral 17 denotes a transfer belt that conveys the recording medium P so that toner images of a plurality of colors are carried on the recording medium P, and reference numeral 19 denotes the recording medium P. 1 shows an electromagnetic induction heating type fixing device that fixes a toner image (unfixed image) of the toner.

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 photoconductor drums 11Y, 11M, 11C, and 11BK are uniformly charged at a portion facing the charging unit 12 (this is a 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 separate optical path for each of the yellow, magenta, cyan, and black color components (this is an 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 photoconductive drums 11Y, 11M, 11C, and 11BK on which the electrostatic latent images of the respective colors are formed reach 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 (development process). .)
Thereafter, the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK after the development process reach the facing portions of the 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 transfer belt 17. Then, the toner images of the respective colors formed on the photoconductive drums 11Y, 11M, 11C, and 11BK are sequentially superimposed and transferred onto the recording medium P on the transfer belt 17 at the position of the transfer bias roller 14 (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 (this is a cleaning process).
Thereafter, the surfaces of the photoconductive drums 11Y, 11M, 11C, and 11BK pass through a neutralization 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 recording medium P on which the toners of the respective colors on the photosensitive drums 11Y, 11M, 11C, and 11BK are transferred (carrying) is run in the direction of the arrow in the drawing and reaches a position facing the separation charger 18. . Then, the charge accumulated in the recording medium P is neutralized at a position facing the separation charger 18, and the recording medium P is separated from the transfer belt 17 without causing toner dust or the like.
Thereafter, the surface of the transfer belt 17 reaches the position of the transfer belt cleaning unit 16. Then, the deposit adhered on the transfer belt 17 is collected by the transfer belt cleaning unit 16.

Here, the recording medium P transported onto the transfer belt 17 is transported from the paper feeding unit 7 via the registration rollers 9 and the like.
Specifically, the recording medium P fed by the paper feeding roller 8 from the paper feeding unit 7 that stores the recording medium P passes through a conveyance guide (not shown) and is guided to the registration roller 9. The recording medium P that has reached the registration roller 9 is conveyed toward the position of the transfer belt 17 in time.

The recording medium P to which the full color image has been transferred is separated from the transfer belt 17 and then guided to the fixing device 19. In the fixing device 19, the color image (toner) is fixed on the recording medium P between the fixing roller and the pressure roller (a fixing nip portion).
Then, the recording medium P after the fixing step is discharged as an output image outside the apparatus main body 1 by a discharge roller (not shown), and a series of image forming processes is completed.

Next, the configuration and operation of the fixing device 19 installed in the image forming apparatus main body 1 will be described in detail.
As shown in FIG. 2, the fixing device 19 includes an induction heating unit 24 (magnetic flux generating means), a fixing roller 20 as a heat generating member, a pressure roller 30, a temperature sensor 55 as a temperature detecting means, and the like.
Here, the fixing roller 20 (fixing member) as a heat generating member is a multilayer structure in which an elastic layer 22, a heat generating layer 21 and the like are formed on the surface of a hollow core metal 23 made of a nonmagnetic material such as SUS304. .

Specifically, the fixing roller 20 has an outer diameter of about 40 mm, and has an elastic layer 22, a heat generating layer 21, an antioxidant layer (not shown), a release layer (not shown) on the core metal 23. Etc.), etc. are stacked.
The core metal 23 is made of nonmagnetic stainless steel such as SUS304, and has a thickness of 0.4 mm. As a result, the heat capacity is reduced, and the energy of electromagnetic induction heating is easily concentrated on the heat generating layer 21.
The elastic layer 22 is made of an elastic material such as silicone rubber and has a thickness of 50 to 500 μm. Thereby, the heat capacity is not so large, and a good fixed image can be obtained.

The heat generating layer 21 can have a two-layer structure of a first nonmagnetic layer and a second nonmagnetic layer.
As the first nonmagnetic material layer, SUS304, SUS301, SUS316 (all of which are nonmagnetic stainless steel) or the like as the nonmagnetic material layer can be used.
Copper (Cu) can be used as the second nonmagnetic material layer. The second nonmagnetic material layer is formed on the first nonmagnetic material layer by plating so that the layer thickness is in the range of 5 to 35 μm. The volume resistivity of the second nonmagnetic material layer is 1.7 × 10 ⁻⁸ Ω · m, which is smaller than the volume resistivity of the first nonmagnetic material layer. As the second nonmagnetic material layer, silver (Ag), aluminum (Al), or the like can be used.
The heat generating layer 21 composed of the first nonmagnetic material layer and the second nonmagnetic material layer is heated by electromagnetic induction by the magnetic flux emitted from the induction heating unit 24 (magnetic flux generating means).

The anti-oxidation layer of the fixing roller 20 is formed of nickel (Ni), and the thickness thereof is set to 5 μm or less. The antioxidant layer is for preventing oxidation of the second nonmagnetic material layer as the copper layer.
The release layer of the fixing roller 20 is made of a fluorine compound such as PFA and has a thickness of 30 μm. The release layer is for improving the toner release property on the surface of the fixing roller 20 with which the toner image (toner) T is in direct contact.
As described above, the fixing roller 20 according to the first exemplary embodiment functions as a fixing member that melts the toner image, and also functions as a heat generating member that is directly heated by the induction heating unit 24.

  In the first embodiment, the heat generating layer 21 has a two-layer structure of the first nonmagnetic layer and the second nonmagnetic layer. However, the heat generating layer 21 may have a single layer structure made of a magnetic metal material. . As the magnetic metal material for forming the heat generating layer 21, nickel (Ni) having a layer thickness of about 10 μm can be used. Moreover, iron, cobalt, nickel, copper, or alloys thereof can be used as the magnetic metal material for forming the heat generating layer 21.

  The pressure roller 30 is formed by forming an elastic layer 31 such as fluorine rubber or silicone rubber on a cylindrical member 32 made of aluminum, copper or the like. The elastic layer 31 of the pressure roller 30 is formed to have a thickness of 0.5 to 2 mm and an Asker hardness of 60 to 90 degrees. The pressure roller 30 is in pressure contact with the fixing roller 20. Then, the recording medium P is conveyed to a contact portion (a fixing nip portion) between the fixing roller 20 and the pressure roller 30.

  The induction heating unit 24 is disposed so as to face the outer peripheral surface of the fixing roller 20. The induction heating unit 24 includes an exciting coil 25, a degaussing coil 26 (coiled conductor), a first core 28, a second core 29 (center core), a coil guide 27 (insulating support member), and the like.

The exciting coil 25 is a wire bundle in which 90 copper wires having an outer diameter of 0.15 mm whose outer peripheral surface is insulated are bundled, and is arranged so as to face the outer peripheral surface of the fixing roller 20 with a demagnetizing coil 26 interposed therebetween. It is installed. Specifically, referring to FIG. 3, the exciting coil 25 is disposed in a spiral shape over the entire area on the coil guide 27 covering the surface of the fixing roller 20 so as to circulate around the second core 29. The length in the width direction of the exciting coil 25 is substantially equal to the length in the width direction (rotation axis direction) of the fixing roller 20 (slightly larger than the A3 size and about 310 mm). The exciting coil 25 has a straight portion and a curved portion, and is set so that the length of the straight portion is substantially equal to the length in the width direction of the fixing roller 20 so that the magnetic flux acting on the fixing roller 20 is uniform. ing.
As shown in FIG. 2, the exciting coil 25 is connected to the power supply unit 70 and generates a magnetic flux for inductively heating the heat generating layer 21 by supplying an alternating current.

The degaussing coil 26 is a wire bundle in which 90 copper wires having an outer diameter of 0.15 mm whose outer peripheral surface is covered with insulation are bundled, and is disposed so as to face the excitation coil 25. An induced current flows through the degaussing coil 26 due to the magnetic flux generated by the exciting coil 25, and the magnetic flux in the direction to cancel the magnetic flux of the exciting coil 25 from the degaussing coil 26 is a part in the width direction (non-paper passing region). Generated.
Specifically, the degaussing coil 26 includes a plurality of degaussing coils 26a to 26c arranged in parallel by changing the distance facing the exciting coil 25 for each of a plurality of variable adjustment ranges (non-sheet passing regions). Specifically, referring to FIG. 2 and FIG. 3, the position farthest from the exciting coil 25 and the position corresponding to the non-sheet passing area in the A4 size (210 mm) recording medium P (both ends in the width direction). Is provided with a first excitation coil 26a. A second excitation coil 26b is disposed at a position closer to the excitation coil 25 than the first excitation coil 26a and corresponding to the non-sheet passing region in the B5 size (182 mm) recording medium P. Yes. A third excitation coil 26c is disposed at a position closest to the excitation coil 25 and corresponding to a non-sheet passing area in the A5 size (148 mm) recording medium P. Each of the demagnetizing coils 26 a to 26 c is arranged in a spiral shape so as to go around a part (end in the width direction) of the second core 29. The end portions in the width direction of the degaussing coils 26a to 26c are arranged so as to coincide with the positions corresponding to the end portions in the width direction of the sheet passing area of each recording medium P.

Here, the degaussing coil 26 is not electrically connected to the power supply unit 70 that supplies an alternating current to the exciting coil 25. Specifically, as shown in FIG. 4, the degaussing coils 26 a to 26 c are installed in an electric circuit including changeover switches 51 a to 51 c as switching elements, respectively. The electric circuit including the degaussing coils 26 a to 26 c and the changeover switches 51 a to 51 c is not connected to the power supply unit 70 and is a closed circuit independent of the electric circuit having the power supply unit 70.
Then, the heating amount (heat generation amount) of the heat generating layer 21 in a desired adjustment range is adjusted and controlled by opening and closing the electric circuit by the changeover switches 51a to 51c. For example, when the recording medium P of A4 size is continuously passed and it is desired to adjust the heating amount in the adjustment range corresponding to the non-passage area, the first changeover switch 51a among the three changeover switches 51a to 51c. Only the first demagnetizing coil 26a is operated (magnetic flux generation) to demagnetize the magnetic flux at both ends in the width direction of the exciting coil 25 (A4 size non-sheet passing region). Thereby, an excessive temperature rise in the non-sheet passing area in the fixing roller 20 can be suppressed. Here, since the power supply unit 70 of the exciting coil 25 is not connected to the degaussing coil 26, the current flowing through the exciting coil 25 does not fluctuate. Therefore, the heating amount of the fixing roller 20 by the exciting coil 25 is also stabilized, and the problem that the fixing unevenness of the output image occurs due to the variation in the fixing temperature can be suppressed.

  In the first embodiment, power is not supplied to the electric circuit having the degaussing coil 26. However, the degaussing coil 26 is connected to a second power source unit different from the power source unit 70, and the demagnetizing coil 26 is connected. It is also possible to positively generate magnetic flux for demagnetization. Even in that case, an effect equivalent to the effect described above can be obtained.

Referring to FIG. 2, the coil guide 27 is made of an insulating resin material having high heat resistance and holds the exciting coil 25 and the degaussing coil 26 on the side facing the fixing roller 20.
The first core 28 is disposed so as to face the outer circumferential surface of the fixing roller 20 in the circumferential direction via the exciting coil 25 and the demagnetizing coil 26. The material of the first core 28 is preferably a ferromagnetic material such as ferrite or permalloy having a high electrical resistivity.
The second core 29 faces the outer peripheral surface of the fixing roller 20 closer to the outer surface than the first core 28 and extends in the width direction (the direction perpendicular to the paper surface of FIG. 2). The length in the width direction of the second core 29 is substantially equal to the length in the width direction (rotation axis direction) of the fixing roller 20. Referring to FIG. 3, the second core 29 is provided with a gap (or notch) in the width direction in order to avoid interference with the three sets of degaussing coils 26a to 26c. The material of the second core 29 is preferably a ferromagnetic material such as ferrite or permalloy having a high electrical resistivity. The second cores 29 </ b> A to 29 </ b> D do not have to be formed by integral molding, and can be configured by connecting short I-shaped cores so as to have a length substantially equal to that of the fixing roller 20.

2 and 3, a temperature detecting means for detecting the temperature of the fixing roller 20 at a position facing the outer peripheral surface of the fixing roller and at a width direction end corresponding to the non-sheet passing region. A temperature sensor 55 is provided. In the first embodiment, a thermopile that detects the temperature of the surface of the fixing roller 20 in a non-contact manner is used as the temperature sensor 55. Thermopile has a high reaction rate, so that fine temperature control is possible. Then, based on the detection result of the fixing temperature by the temperature sensor 55, opening / closing control of the electric circuits (demagnetization coils 26a to 26c) is performed by the changeover switches 51a to 51c. Thereby, it is possible to reliably suppress an excessive temperature rise in the adjustment range (non-sheet passing region) of the fixing roller 20. In addition to the thermopile, a contact type thermistor or the like can be used as the temperature sensor 55.
Here, in the first embodiment, a detection means for detecting the time or the number of sheets through which the small-sized recording medium P is continuously passed is provided, and an electric circuit by the changeover switches 51a to 51c based on the detection result. The opening / closing control of the (demagnetization coils 26a to 26c) can also be performed. Even in that case, it is possible to obtain substantially the same effect as described above.

The fixing device 19 configured as described above operates as follows.
When the fixing roller 20 is driven to rotate clockwise in FIG. 2 by a drive motor (not shown), the pressure roller 30 also rotates counterclockwise. The fixing roller 20 serving as a fixing member is heated by a magnetic flux generated from the induction heating unit 24 at a position (facing surface) facing the induction heating unit 24.

  Specifically, by supplying a high-frequency alternating current of 10 kHz to 1 MHz (preferably 20 kHz to 800 kHz) from the power supply unit (not shown) to the exciting coil 25, the magnetic lines of force are bidirectional from the exciting coil 25 toward the heat generating layer 21. It is formed so as to switch alternately. By forming an alternating magnetic field in this way, an eddy current is generated in the heat generating layer 21 of the fixing roller 20, and the heat generating layer 21 is inductively heated by generating Joule heat due to its electric resistance. Thus, the fixing roller 20 is heated by induction heating of the heat generating layer 21 of itself. At this time, the operation of the degaussing coil 26 is appropriately controlled to adjust the heating range in the width direction of the fixing roller 20.

Thereafter, the surface of the fixing roller 20 heated by the induction heating unit 24 reaches a contact portion with the pressure roller 30. Then, the toner image T (toner) on the conveyed recording medium P is heated and melted.
Specifically, the recording medium P carrying the toner image T through the image forming process described above is fed between the fixing roller 20 and the pressure roller 30 while being guided by a guide plate (not shown) (arrow). Y1 movement in the transport direction.) The toner image T is fixed to the recording medium P by the heat received from the fixing roller 20 and the pressure received from the pressure roller 30, and the recording medium P is sent from between the fixing roller 20 and the pressure roller 30.

The surface of the fixing roller 20 that has passed through the fixing position then reaches the position facing the induction heating unit 24 again.
Such a series of operations is continuously repeated to complete the fixing step in the image forming process.

  As described above, in the first embodiment, the demagnetizing coil 26 is provided at both ends in the width direction in addition to the exciting coil 26, and the degaussing coil 26 is not electrically connected to the power supply unit 70 of the exciting coil 25. It is configured as follows. As a result, even when small-size paper is continuously passed, overheating in the non-paper passing area of the fixing roller 20 is efficiently and reliably suppressed, and the fixing heated by the excitation coil 25 is performed. Variation in the heating amount of the roller 20 can be reduced.

Hereinafter, a mechanism for suppressing heating of the fixing roller 20 (heat generating member) by disposing the demagnetizing coil 26 in addition to the exciting coil 25 will be described with reference to FIG. FIG. 5 is a diagram illustrating a state of magnetic flux generated by the induction heating unit 24.
FIG. 5A shows the state of magnetic flux when the degaussing coil 26 is electrically open (in the first embodiment, the third changeover switch 51c is open). . As shown in FIG. 5A, when the changeover switch is opened (when the degaussing coil 26 does not operate), the magnetic flux B1 generated from the exciting coil 25 passes through the heat generating layer 21 through the cores 28 and 29 as a path. Return to the cores 28 and 29 again. At this time, the magnetic flux B <b> 1 forms a magnetic circuit that passes through the heat generating layer 21. Therefore, an induced current flows through the heat generating layer 21, and the heat generating layer 21 generates heat due to Joule heat.
At this time, since the degaussing coil 26 is electrically open, the demagnetizing coil 26 is not a coil but merely a bundle of insulated thin wires, and does not significantly affect the magnetic flux B1 of the exciting coil 25. This is due to the phenomenon that eddy current does not flow in the metal unless the area of the metal is larger than a certain size.

FIG. 5B shows the state of the magnetic flux when the degaussing coil 26 is electrically short-circuited (in the first embodiment, the third changeover switch 51c is closed). ing. As shown in FIG. 5B, the magnetic flux B1 generated from the exciting coil 25 is canceled out by the magnetic flux B2 generated from the degaussing coil 26 (26c), and becomes a very weak magnetic flux. Specifically, the magnetic flux B <b> 1 generated from the exciting coil 25 reaches the heat generating layer 21 after most of the magnetic flux B <b> 1 penetrates the degaussing coil 26. At that time, in the degaussing coil 26, a back electromotive force is generated in the direction to cancel the penetrating magnetic flux B1 due to the electromagnetic characteristics of the coil, and accordingly, a current flows through the degaussing coil 26 to generate a magnetic flux B2 that cancels the magnetic flux B1. Let In this way, the magnetic flux B1 for heating the heat generating layer 21 becomes very weak, and the heating amount of the fixing roller 20 can be reduced at the positions facing the degaussing coil 26 (both ends in the width direction).
In the first embodiment, the state of FIG. 5A and the state of FIG. 5B are switched by the control of the changeover switches 51a to 51c to optimize the heat amount distribution in the width direction of the fixing roller 20. Yes.

  In the above description, for the sake of simplicity, it is assumed that when the degaussing coil 26 is open, the magnetic flux of the exciting coil 25 is not greatly affected. However, the magnetic flux from the exciting coil 25 is actually consumed in a small amount. Therefore, depending on the thin wire constituting the degaussing coil 26, the efficiency of heating the heat generating layer 21 may be reduced. Therefore, it is desirable that the thin wire constituting the degaussing coil 26 has a certain cross-sectional area or less. Specifically, it is desirable that the cross-sectional area of the fine wire constituting the degaussing coil 26 is equal to or less than the cross-sectional area of the fine wire constituting the exciting coil 25.

Here, in the first embodiment, the demagnetizing coil 26 and the exciting coil 25 are arranged in this order from the outer peripheral surface side of the fixing roller 20. This is because more magnetic flux generated from the exciting coil 25 is passed through the degaussing coil 26.
In the first embodiment, since the exciting coil 25 and the degaussing coil 26 are not electrically connected, the electromotive force of the degaussing coil 26 is only the counter electromotive force induced by the magnetic flux generated from the exciting coil 25. Therefore, as compared with the configuration in which power is supplied from the power source to the demagnetizing coil 26, the positional relationship among the exciting coil 25, the demagnetizing coil 26, and the heat generating member 20 greatly affects the heat generation suppression rate of the demagnetizing coil 26.
FIG. 6 is a diagram showing the state of magnetic flux generated from the exciting coil 25 in more detail. As shown in FIG. 6, the magnetic flux generated from the exciting coil 25 passes through the cores 28 and 29 (shown in FIG. 5), and the leakage magnetic flux directly permeates the heating layer 21 and heats it. Exists.

Therefore, as shown in FIG. 6A, when the exciting coil 25, the degaussing coil 26, and the heat generating member 20 are arranged in this order (the configuration of the first embodiment), the magnetic flux from the exciting coil 25 is the core. Since both the magnetic flux passing through and the leakage magnetic flux penetrate through the degaussing coil 26, most of them are canceled by the magnetic flux of the degaussing coil 26. That is, the heat generation suppressing effect of the degaussing coil 26 is enhanced.
On the other hand, as shown in FIG. 6B, when the demagnetizing coil 26, the exciting coil 25, and the heat generating member 20 are arranged in this order, only the magnetic flux passing through the core among the magnetic flux generated from the exciting coil 25 is caused by the demagnetizing coil. The magnetic flux leakage is canceled and the heat flux 21 is heated without being affected by the degaussing coil. That is, the heat generation suppression effect of the degaussing coil 26 is insufficient.

When the inventor of the present application calculated the heat generation amount of the heat generation layer 21 in FIG. 6A and the heat generation amount of the heat generation layer 21 in FIG. 6B by induction heating simulation, the following results were obtained. . In the calculation, all the conditions other than the positional relationship between the degaussing coil 26 and the exciting coil 25 are assumed to be the same.
As a result, the amount of heat generated in the heat generating layer 21 in FIG. 6B was reduced only by about 70% when the degaussing coil 26 (changeover switch) was closed. In contrast, the amount of heat generated in the heat generating layer 21 in FIG. 6A was reduced to about 55% when the degaussing coil 26 (changeover switch) was closed. The calculation result of the heat generation amount is as follows. In each fixing device, the heat generation amount of the heat generation layer 21 when the demagnetization coil 26 is closed when the heat generation amount of the heat generation layer 21 when the demagnetization coil 26 is opened is 100. Is the ratio.
From the above results, by arranging the demagnetizing coil 26 and the exciting coil 25 in this order from the outer peripheral surface side of the fixing roller 20 (heat generating member), the leakage flux of the exciting coil 25 can be passed through the demagnetizing coil 26. It can be seen that the heat generation suppressing effect of the degaussing coil 26 can be enhanced.

Hereinafter, operations of the plurality of degaussing coils 26a to 26c when the recording medium P is passed will be described.
As described above, the first degaussing coil 26a is disposed at a position corresponding to the non-sheet passing area of the A4 size recording medium, and at a position corresponding to the non-sheet passing area of the B5 size recording medium. A second demagnetizing coil 26b is disposed, and a third demagnetizing coil 26c is disposed at a position corresponding to the non-sheet passing area of the A5 size recording medium.

In the image forming apparatus according to the first embodiment, the maximum width direction length of the recording medium P that can be passed is set to A3 size (297 mm). Therefore, when the A3 size recording medium is passed, all the demagnetizing coils 26a to 26c (changeover switches 51a to 51c) are opened to heat the fixing roller 20 over the entire width direction.
When the A4 size is passed, only the first degaussing coil 26a arranged from the end of the A4 size to the end of the fixing roller is closed, and the non-passage of the fixing roller 20 when the A4 size is passed. Reduce the amount of heat generated in the area.
When passing the B5 size paper, only the second degaussing coil 26b disposed from the end of the B5 size to the end of the fixing roller is closed, and the non-passage of the fixing roller 20 when the B5 size is passed. Reduce the amount of heat generated in the area.
When the A5 size is passed, only the third degaussing coil 26c arranged from the end of the A5 size to the end of the fixing roller is closed, and the non-passage of the fixing roller 20 when the A5 size is passed. Reduce the amount of heat generated in the area.

In this way, the degaussing coils 26a to 26c are respectively arranged at positions (adjustment ranges) facing the non-sheet passing regions of a plurality of types of recording media having different sizes in the width direction, and the corresponding degaussing coils are appropriately opened and closed. By doing so, the temperature distribution in the width direction of the fixing roller can be optimized regardless of which of the plurality of types of recording media having different width sizes.
The temperature sensor 55 (temperature detection means) described above may be disposed at a position corresponding to the end of the sheet passing area of each size A5, B5, A4, and A3 of the fixing roller 20. Based on the results detected by the plurality of temperature sensors 55, the opening / closing of the plurality of degaussing coils 26 a to 26 c and the application of power to the excitation coil 25 can be controlled.

FIG. 7 is a graph showing the temperature rise characteristics of the fixing roller in the non-sheet passing area when a B5 size recording medium is continuously fed.
In FIG. 7, a graph Q1 shows the temperature rise characteristic when the fixing device in the first embodiment is used, and a graph Q0 uses a conventional fixing device (without the demagnetizing coil 26). The temperature rise characteristic is shown. Also, the broken line in FIG. 7 indicates the fixing temperature of the paper passing area.
In the first embodiment, the demagnetizing coil 26c corresponding to the temperature at the end of the sheet passing area becomes equal to or higher than the first set temperature (180 ° C.) is closed, and the second set temperature is set. (The fixing set temperature is 170 ° C.) The demagnetizing coil 26c is controlled to be opened when the temperature becomes below.

As shown in FIG. 7, when continuous paper feeding is started, the temperature of the “paper feeding area” of the fixing roller 20 that contacts the recording medium is controlled so as to maintain a fixing set temperature of 170 ° C. For this reason, the temperature of the “non-sheet passing area” of the fixing roller where the amount of heat is not taken from the recording medium rises. Therefore, the end of the fixing device (graph Q0) without the degaussing coil continues to rise in temperature.
On the other hand, in the fixing roller 20 according to the first embodiment, the demagnetizing coil 26c is closed when the end temperature reaches the second set temperature (180 ° C.). As a result, as shown in the figure, the heat generation at the end of the fixing roller 20 is suppressed, and the temperature at the end is kept uniform.

Hereinafter, the merit of installing a plurality of degaussing coils 26a to 26c will be further described.
By installing a plurality of degaussing coils 26a to 26c, the temperature control of the fixing roller 20 when continuously passing recording media having different width direction sizes is simplified. Here, “continuous sheet feeding of recording media of different width direction sizes” means, for example, when a large-size (B5, A4, A3) recording medium is continuously fed after an A5 size recording medium is continuously fed Or, it is a case where a small-sized (A4, B5, A5) recording medium is continuously fed after an A3-sized recording medium is continuously fed.

When only a pair of degaussing coils are installed in the induction heating unit of the fixing device (for example, when only a demagnetizing coil 26a corresponding to the A4 size is installed), only by opening / closing the pair of demagnetizing coils, “recording media of different sizes” It is very difficult to uniformly control the temperature of the fixing roller with respect to “continuous paper feeding”.
FIG. 8A shows the heat generation amount distribution in the width direction of the fixing roller in the fixing device in which only the demagnetizing coil 26a corresponding to the A4 size is installed. In such a fixing device, the amount of heat generated in the non-heated area when the degaussing coil is closed is not increased when the small-size recording medium is continuously fed (or the temperature is not increased). Must be controlled). As shown in FIG. 8A, when the A4 size recording medium is continuously fed, the temperature of the non-sheet passing area of the fixing roller is not increased, and the same temperature as the sheet passing area is maintained. The degaussing coil is controlled.

  Under such control, when a recording medium (for example, A5 size recording medium) smaller than the size corresponding to the degaussing coil is continuously fed, the temperature distribution of the fixing roller 20 is as shown in FIG. turn into. In order to control the degaussing coil so that the temperature of the A4 size non-sheet passing area of the fixing roller becomes equal to the temperature of the sheet passing area, the end of the demagnetizing coil (A4 size sheet passing area) The temperature up to the end of the end becomes extremely high. For this reason, if a recording medium having a larger size (for example, A3 size) is continuously passed immediately after the A5 size recording medium is passed continuously, the first few sheets of the output image are between A5 and A4. Hot offset occurs in the area corresponding to.

  In order to solve such a problem, as shown in FIG. 9A, a demagnetizing coil is provided so that the temperature of the non-sheet passing area of the fixing roller is lowered when continuously passing an A4 size recording medium. When controlled, the temperature distribution after continuously passing an A5 size recording medium is as shown in FIG. As shown in FIG. 9B, the temperature of the A4 size non-sheet passing area and the area corresponding to the A3 size sheet passing area is lower than the set fixing temperature. When paper is used, the fixability in that range is reduced. In order to pass A3 size paper, it is necessary to raise the temperature of the A4 size non-sheet passing area and the area covering the A3 size paper passing area, and the degaussing coil must be opened. Then, since the entire width direction of the fixing roller is heated, the temperature of the center portion of the fixing roller is excessively increased, and the A3 size cannot be immediately passed.

That is, when only one pair of demagnetizing coils is installed in the induction heating unit of the fixing device, an excessive temperature increase occurs in a part of the fixing roller with any change in the size of the recording medium to be fed. It will occur.
On the other hand, in the first embodiment, since a plurality of degaussing coils 26a to 26c are installed corresponding to various size recording media, even if the size of the recording media to be passed is changed, It is possible to suppress a problem that an excessive temperature rise occurs in a part of the fixing roller.

  In order to suppress an excessive temperature rise at both ends in the width direction of the fixing roller when a small size paper is continuously passed, a plurality of papers divided in the width direction corresponding to various size recording media are used. A measure to install an exciting coil is also conceivable. However, in that case, interference of magnetic fluxes from multiple excitation coils occurs, resulting in non-uniform heat distribution of the fixing roller, complicated control of multiple excitation coils, Will be expensive. On the other hand, the configuration in which a plurality of degaussing coils 26a to 26c are installed corresponding to various size recording media as in the first embodiment does not cause the above-described problem, and the width direction of the landing roller. An excessive temperature rise at both ends can be suppressed.

FIG. 10A shows a calorific value distribution of the fixing roller 20 in the fixing device 19 of the first embodiment.
In the first embodiment, when the first degaussing coil 26a is closed, the temperature of the A4 size non-sheet passing area of the fixing roller 20 is controlled to be equal to the temperature of the A4 size sheet passing area. Similarly, when the second degaussing coil 26b is closed, the temperature of the B5 size non-sheet passing area of the fixing roller 20 is controlled to be equal to the temperature of the B5 size sheet passing area. Further, when the third degaussing coil 26c is closed, the temperature of the A5 size non-sheet passing area of the fixing roller 20 is controlled to be equal to the temperature of the A5 size sheet passing area. By providing a plurality of degaussing coils 26a to 26c as described above, good fixing is surely performed even in “continuous sheet feeding of recording media of different sizes”.

FIG. 10B shows the temperature distribution of the fixing roller immediately after the B5 size recording medium is continuously fed in the fixing device 19 of the first embodiment. As described above, the degaussing coil 26b is closed when the temperature of the B5 size non-sheet passing region reaches the second set temperature (180 ° C.) after the start of B5 size paper passing. Therefore, the temperature in the width direction of the fixing roller is uniform even immediately after the sheet is passed, and a recording medium of another size (A3, A4 size or A5 size) can be immediately passed (fixed). When the inventor of the present application continuously passes a B5 size recording medium using the fixing device of the first embodiment and then continuously passes a recording medium of another size (A3, A4 size or A5 size), In either case, the occurrence of hot offset or cold offset was not confirmed.
In the first embodiment, since the heat generation suppression rates of the degaussing coils 26a to 26c are equal, the temperature of the non-sheet passing region is kept constant regardless of the size of the recording medium that is continuously fed.

Hereinafter, the control at the time of continuous paper feeding will be described with reference to FIG.
When continuous feeding of the recording medium is started (step S1), first, it is determined whether the recording medium is a small size paper (a size smaller than A3 size) (step S2). As a result, when it is determined that the recording medium is not a small size paper, all of the plurality of degaussing coils 26a to 26c are opened (step S7), and this flow is ended (step S8).
On the other hand, if it is determined that the recording medium is a small size sheet, it is further determined whether the temperature t1 of the non-sheet passing area detected by the temperature sensor 55 is higher than a predetermined value t2 (step S3). .

As a result, if it is determined that the temperature t1 of the sheet passing area is higher than the predetermined value t2, it is assumed that the temperature of the non-sheet passing area needs to be lowered, and the corresponding degaussing coil is closed (step S4). ). On the other hand, if it is determined that the temperature t1 of the non-sheet passing area is equal to or lower than the predetermined value t2, it is assumed that the temperature of the non-sheet passing area needs to be raised, and the corresponding degaussing coil is opened. (Step S6).
Thereafter, it is determined whether or not continuous feeding of the recording medium of the same size has been completed (step S5). If it is determined that continuous feeding is continued, the flow after step S3 is repeated. Then, when it is determined that the continuous sheet passing has ended, this flow is ended (step S8).

  As described above, in the first embodiment, in addition to the exciting coil 25 that induction-heats the fixing roller 20 (heat generating member), an induced current flows by the magnetic flux generated by the exciting coil 25 and acts on the fixing roller 20. A demagnetizing coil 26 that generates a magnetic flux in a direction that cancels the magnetic flux to be generated is provided in a part in the width direction, and the demagnetizing coil 26 is not electrically connected to a power supply unit 70 that supplies an alternating current to the exciting coil 25. doing. As a result, even when small-size paper is continuously passed, overheating in the non-paper passing area of the fixing roller 20 is efficiently and reliably suppressed, and the fixing heated by the excitation coil 25 is performed. Variation in the heating amount of the roller 20 can be reduced.

  In the first embodiment, as the fixing roller 20, the elastic layer 22, the heat generating layer 21 (the first nonmagnetic material layer and the second nonmagnetic material layer), the antioxidant layer, and the release layer are formed on the core metal 23. , Was used. However, the configuration of the fixing roller 20 is not limited to this, and for example, a fixing roller 20 in which a heat insulating layer, a heat generating layer, an elastic layer, and a release layer are sequentially laminated on a cored bar may be used. it can.

Embodiment 2. FIG.
A second embodiment of the present invention will be described in detail with reference to FIGS.
FIG. 12 is a cross-sectional view showing fixing device 19 in the second embodiment. FIG. 13 is a schematic view of the exciting coil and the degaussing coil in the fixing device of FIG. 12 as viewed in the width direction. The fixing device 19 according to the second embodiment is variable in that a plurality of degaussing coils 26A to 26C are arranged in parallel in the width direction so as to fill between adjacent adjustment ranges among a plurality of variable adjustment ranges. This is different from the first embodiment in which a plurality of degaussing coils 26a to 26c are arranged in parallel by changing the distance facing the exciting coil 25 for each of a plurality of adjustment ranges.

  In the fixing device according to the second embodiment, as in the first embodiment, in addition to the exciting coil 25 that induction-heats the fixing roller 20 as a heat generating member, an induced current is generated by a magnetic flux generated by the exciting coil 25. Demagnetizing coils 26 </ b> A to 26 </ b> C that generate magnetic flux in a direction that cancels out the magnetic flux acting on the fixing roller 20 by flowing through the fixing roller 20 are provided at both ends in the width direction. The demagnetizing coils 26 </ b> A to 26 </ b> C are not electrically connected to a power supply unit (not shown) that supplies an alternating current to the exciting coil 25. The exciting coil 25 is opposed to the fixing roller 20 with the degaussing coils 26A to 26C interposed therebetween.

  Here, referring to FIGS. 12 and 13, the fixing device according to the second embodiment is different from the first embodiment in that three pairs of demagnetizing coils 26A to 26C are arranged in parallel in the width direction. Yes. Specifically, the first demagnetizing coil 26A is disposed between the A5 size end and the B5 size end, and the second demagnetizing coil 26B is disposed between the B5 size end and the A4 size end. A third demagnetizing coil 26C is disposed between the A4 size end and the A3 size end. Referring to FIG. 15, these degaussing coils 26 </ b> A to 26 </ b> C are connected to electric circuits having changeover switches 51 </ b> A to 51 </ b> C as switching elements, respectively.

Then, the changeover switches 51A to 51C of all the electric circuits having the degaussing coils arranged in the adjustment range among the three pairs of demagnetization coils 26A to 26C are simultaneously opened and closed according to the adjusted adjustment range.
Specifically, when an A5 size recording medium is passed, all three pairs of degaussing coils 26A to 26C (changeover switches 51A to 51C) arranged in the non-sheet passing area (adjustment range) are closed. Then, the amount of heating in the non-sheet passing area is adjusted. When a B5 size recording medium is passed, the two pairs of degaussing coils 26B and 26C disposed in the non-sheet passing area are closed. When an A4 size recording medium is passed, the pair of degaussing coils 26C arranged in the non-sheet passing area are closed. Further, when the A3 size recording medium is passed, all the degaussing coils 26A to 26C are opened.
With this configuration, as in the first embodiment, the temperature distribution in the width direction of the fixing roller 20 is made uniform regardless of the width-direction recording medium. In particular, in the second embodiment, a plurality of pairs of degaussing coils 26A to 26C are arranged not in the circumferential direction but in the width direction, so that the thickness in the circumferential direction of the induction heating unit 24 can be reduced. .

  In the second embodiment, referring to FIG. 13, temperature sensor 55 as temperature detecting means is disposed at a position corresponding to the position where each demagnetizing coil 26 </ b> A to 26 </ b> C is opposed and at the center in the width direction. Has been. Based on the detection results of these temperature sensors 55, the opening / closing of the degaussing coils 26 </ b> A to 26 </ b> C and the power supply to the excitation coil 25 are controlled.

  Also in the second embodiment, control equivalent to the control described in FIGS. 10A and 10B in the first embodiment is performed. Thereby, the calorific value distribution of the fixing roller 20 equivalent to that shown in FIG. 10A can be obtained, and the temperature distribution of the fixing roller equivalent to that shown in FIG. 10B can be obtained. That is, the temperature of the non-sheet passing area is kept constant regardless of the size of the recording medium that is continuously fed. When the inventor of the present application continuously passes a B5 size recording medium using the fixing device of the second embodiment and then continuously passes a recording medium of another size (A3, A4 size or A5 size), In either case, the occurrence of hot offset or cold offset was not confirmed.

Here, in the second embodiment, a recording medium in which a range longer than the longest adjustment range among the plurality of adjustment ranges is a non-sheet-passing area (for example, a recording medium with a postcard size smaller than A5 size). When the paper is passed, control is performed so that the changeover switch of the electric circuit having at least one degaussing coil among the plurality of degaussing coils 26A to 26C is opened and closed independently.
Specifically, the heat generation suppression rate of each of the degaussing coils 26A to 26C is set to be high so that the temperature of the small size non-sheet passing area is lower than the temperature of the sheet passing area. When the temperature of the non-sheet passing area is greatly reduced during continuous passage of small size paper, the width of the fixing roller 20 is reduced by opening the degaussing coil that suppresses heat generation in the non-sheet passing area. Always maintain the proper temperature distribution in the direction. Thereby, even if the recording medium having a size smaller than the size corresponding to the demagnetizing coils 26A to 26C is passed, a good fixing property can be obtained.

  FIG. 14 shows the calorific value distribution of the fixing roller when the three pairs of degaussing coils 26A to 26C are closed individually. Specifically, FIG. 14A shows only the first degaussing coil 26A closed, FIG. 14B shows only the second degaussing coil 26B, and FIG. 14C shows the third demagnetizing coil 26A. Only the degaussing coil 26C is closed. As shown in the figure, when the three pairs of degaussing coils 26A to 26C are closed individually, the heat generation of the fixing roller 20 is suppressed only in the region where the degaussing coils are closed. Therefore, when the temperature of the non-sheet passing region is greatly decreased during continuous passage of small size paper, the degaussing coil that suppresses heat generation in the non-sheet passing region may be opened.

FIG. 15A shows a temperature distribution in the width direction of the fixing roller 20 when a postcard size (100 mm) recording medium is continuously fed using the fixing device 19 according to the second embodiment.
All the degaussing coils 26A to 26C are closed when the paper is passed. However, since the degaussing coils 26A to 26C are set so that the temperature of the small size non-sheet passing area is lower than the temperature of the sheet passing area, the non-sheet passing area from the both ends of the postcard size to the both ends of the A5 size. The temperature rise of is suppressed. However, since the temperature from both ends of the A5 size to both ends of the fixing roller is very low, it is impossible to immediately pass the B5 size, A4 size, and A3 size after the postcard size.

However, in this state, as shown in FIG. 15B, the second degaussing coil 26B (second changeover switch 51C) and the third degaussing coil 26C (third changeover switch 51C) are opened. By doing so, it is possible to raise the temperature of the B5 size non-sheet passing area and the area corresponding to the A3 size sheet passing area.
At this time, if the first demagnetizing coil 26A (first changeover switch 51A) is closed, the heat generation from both ends of A5 to both ends of B5 remains suppressed. The amount of heat generated between the B5 size ends and the A4 size ends when the second demagnetizing coil 26B is opened is transferred to the area where the heat generation is suppressed. The temperature distribution in the width direction is made uniform.

  Furthermore, when the non-sheet passing region is excessively heated, the second degaussing coil 26B and the third degaussing coil 26C may be closed again. Specifically, the second demagnetizing coil 26B and the third demagnetizing coil 26C are based on the detection temperature of the temperature sensor 55 installed at a position corresponding to the position where each of the demagnetizing coils 26A to 26C of the fixing roller 20 is opposed. By performing the opening / closing control, the temperature distribution in the width direction of the fixing roller is optimized as shown in FIG.

  As described above, also in the second embodiment, in the same manner as in the first embodiment, in addition to the excitation coil 25 that induction-heats the fixing roller 20 (heat generating member), the induction is performed by the magnetic flux generated by the excitation coil 25. A demagnetizing coil 26 that generates a magnetic flux in a direction that cancels out the magnetic flux that acts on the fixing roller 20 when an electric current flows is provided in a part in the width direction, and the degaussing coil 26 is provided to a power supply unit that supplies an alternating current to the exciting coil 25. It is configured not to be electrically connected. As a result, even when small-size paper is continuously passed, overheating in the non-paper passing area of the fixing roller 20 is efficiently and reliably suppressed, and the fixing heated by the excitation coil 25 is performed. Variation in the heating amount of the roller 20 can be reduced.

Embodiment 3 FIG.
A third embodiment of the present invention will be described in detail with reference to FIGS.
FIG. 16 is a diagram illustrating a main part of the fixing device according to the third embodiment. FIG. 17 is a diagram illustrating a main part of a fixing device different from that in FIG. 16. The fixing device 19 according to the third embodiment is different from that according to the second embodiment in that the demagnetizing coil 26 is formed so that the force for canceling the magnetic flux generated by the exciting coil 25 differs depending on the position in the width direction. Is different.

In the demagnetizing coil 26 of the fixing device according to the third embodiment, a heat generation suppression rate is set according to the purpose. Here, the heat generation suppression rate of the degaussing coil 26 is determined by the shape of the degaussing coil 26. The shape of the degaussing coil is a cross-sectional area of the coil conductor portion on a plane orthogonal to the rotation axis of the fixing roller.
As shown in FIGS. 16A and 16B, when a difference is provided in the cross-sectional area of the coil conductor portion in the degaussing coil 26, the larger the cross-sectional area, the more magnetic flux from the exciting coil 25 increases. Therefore, the heat generation suppression rate of the degaussing coil 26 is increased. That is, the demagnetization coil in FIG. 16B has a higher heat generation suppression rate than the demagnetization coil in FIG.

Therefore, a demagnetizing coil having a small cross-sectional area may be installed in a region where the suppression effect of heat generation is to be reduced, and a demagnetizing coil having a large cross-sectional area may be installed in a region where the effect of suppressing heat generation is to be increased. Further, when it is desired to equalize the heat generation suppression effect of each degaussing coil, the cross-sectional area of each degaussing coil may be equalized.
As a method of controlling the cross-sectional area of the coil conductor portion, a method of controlling by the thickness of one wire bundle may be used, or a method of controlling the number of windings may be used. Specifically, in order to increase the cross-sectional area of the coil conductor portion, the thickness of one wire bundle is increased. That is, the number of wires to be bundled is increased or the number of windings is increased. Increasing the number of windings of the degaussing coil can provide the effect of reducing the current flowing per wire bundle in addition to the effect of increasing the heat generation suppression rate. By reducing the current flowing through the degaussing coil, the current resistance of the switching element of the degaussing coil can be reduced, so that the cost of components can be reduced.

FIG. 17 is a diagram showing a main part of another fixing device different from FIG. 17A is a view of the demagnetizing coil 26 and the exciting coil 25 as viewed in the width direction, FIG. 17B is a cross-sectional view showing the third demagnetizing coil 26C, and FIG. It is sectional drawing which shows the demagnetizing coil 26B.
As shown in FIG. 17, even if the cross-sectional area of the coil conductor portion in the plane perpendicular to the rotation axis of the fixing roller is equal, the loop area formed by the coil conductor portion (the area of the region formed in the loop shape). By making these different, the heat generation suppression rate of the degaussing coil can be adjusted. Specifically, the loop area of the second degaussing coil 26B is set larger than the loop area of the third degaussing coil 26C, and more magnetic flux generated from the exciting coil 25 can be interlinked. Increases heat generation suppression rate.

As shown in FIGS. 16 and 17, the heat generation suppression rate of the degaussing coil varies depending on the position in the width direction in the following cases.
For example, when comparing the heat distribution between the center in the width direction of the fixing roller and both ends in the width direction, the temperature tends to decrease because heat escapes to other members such as the fixing roller, journal, bearing, and side plate at both ends. There is. Therefore, by setting the heat generation suppression rate of the degaussing coil arranged at both ends in the width direction to be lower than the heat generation suppression rate of the demagnetization coil arranged near the center portion in the width direction, the temperature at both ends of the fixing roller is greatly reduced. It is possible to suppress a problem that a fixing failure occurs when A3 paper is passed.

  In addition, by setting the heat generation suppression rate of the degaussing coil disposed in the vicinity of the central portion in the width direction to be high, it is possible to cope with the temperature rise at the end of a recording medium having a smaller size. Specifically, when a recording medium having a postcard size smaller than the A5 size is passed, an excessive temperature rise occurs from both ends of the postcard size to both ends of the A5 size. At this time, overheating can be prevented by setting the heat generation suppression rate of the degaussing coil installed at the end of the A5 size high in advance. By closing this degaussing coil when passing a postcard, the fixing temperature in that area drops rapidly, and the amount of heat generated between both ends of the postcard size and the A5 size ends is released to the outside in the width direction of the fixing roller. Can do.

  As described above, it is preferable to increase the heat generation suppression rate of the degaussing coil facing the vicinity of the central portion in the width direction, rather than the degaussing coil facing both ends of the fixing roller 20 in the width direction. Specifically, it is preferable to increase the cross-sectional area of the coil conducting wire portion or increase the number of coil turns of the degaussing coil opposed to the vicinity of the central portion in the width direction, rather than the degaussing coil opposed to both ends in the width direction of the fixing member. . In addition, it is preferable to increase the loop area of the degaussing coil opposed to the vicinity of the central portion in the width direction, rather than the degaussing coil opposed to both ends in the width direction of the fixing roller.

  As described above, in the third embodiment, too, in the non-sheet passing region of the fixing roller 20, even in the case where small-size sheets are continuously fed, as in the above embodiments. The temperature can be efficiently and reliably suppressed, and fluctuations in the heating amount of the fixing roller 20 heated by the exciting coil 25 can be reduced.

Embodiment 4 FIG.
The fourth embodiment of the present invention will be described in detail with reference to FIG.
FIG. 18 is a schematic view of the exciting coil and the degaussing coil of the fixing device according to the fourth embodiment viewed in the width direction. The fixing device 19 according to the fourth embodiment is different from that according to the second embodiment in that a plurality of degaussing coils 26A to 26C constitute one electric circuit for each pair (for each size).

As shown in FIG. 18, in the fourth embodiment, three pairs of degaussing coils 26 </ b> A to 26 </ b> C respectively installed at both ends in the width direction are electrically connected, and one pair is included in each pair of electric circuits. Changeover switches 51A to 51C were installed.
In most cases, a pair of degaussing coils arranged symmetrically about the center in the width direction of the fixing roller are opened and closed simultaneously. Therefore, by configuring as shown in FIG. 18, the number of changeover switches and electric circuits can be reduced, so that the fixing device can be reduced in cost and size and switch control is facilitated.

  As described above, in the fourth embodiment, too, in the non-sheet passing area of the fixing roller 20, even in the case where small size sheets are continuously fed, as in the above embodiments. The temperature can be efficiently and reliably suppressed, and fluctuations in the heating amount of the fixing roller 20 heated by the exciting coil 25 can be reduced.

Embodiment 5 FIG.
A fifth embodiment of the present invention will be described in detail with reference to FIG.
FIG. 19 is a cross-sectional view showing fixing device 19 in the fifth embodiment. The fixing device 19 according to the fifth embodiment is different from the above-described embodiments in which the fixing belt 60 is used as a fixing member, and the fixing roller 20 is used as a fixing member.

  As shown in FIG. 19, the fixing device 19 according to the fourth embodiment includes an induction heating unit 24, a fixing belt 60 (fixing member) as a heating member, a support roller 41 as a heating member, a fixing auxiliary roller 50, and pressure. It is composed of rollers 30 and the like.

  Here, the fixing auxiliary roller 50 is formed by forming an elastic layer such as silicone rubber on the surface of a cored bar made of stainless steel or the like. The elastic layer of the auxiliary fixing roller 50 is formed so as to have a thickness of 1 to 5 mm and an Asker hardness of 30 to 60 degrees.

The support roller 41 as a heating member is composed of a first nonmagnetic material layer 41a made of SUS304 (nonmagnetic stainless steel) and a second nonmagnetic material layer 41b (copper layer) made of a copper plating layer. It has an exothermic layer. The configuration of the first nonmagnetic material layer 41a and the second nonmagnetic material layer 41b of the support roller 41 is substantially the same as the configuration of the first nonmagnetic material layer and the second nonmagnetic material layer of the fixing roller 20 in the first embodiment. It is equivalent.
The support roller 41 rotates clockwise in FIG. The heat generating layers 41 a and 41 b of the support roller 41 are induction heated by the magnetic flux generated from the induction heating unit 24.

The fixing belt 60 including the heat generating layer is stretched and supported by a support roller 41 and a fixing auxiliary roller 50 (two roller members).
The fixing belt 60 has, from the inner peripheral surface side, a heat generating layer (consisting of a first nonmagnetic material layer and a second nonmagnetic material layer), an antioxidant layer made of nickel, an elastic layer made of silicone rubber, etc. A release layer made of a fluorine compound is laminated. The configuration of each layer of the fixing belt 60 is substantially the same as the configuration of each layer of the fixing roller 20 in the first embodiment.
The fixing belt 60 circulates clockwise in FIG. The heat generating layer of the fixing belt 60 is directly induction heated by the magnetic flux emitted from the induction heating unit 24.
The induction heating unit 24 includes an exciting coil 25, demagnetizing coils 26A to 26C, a first core 28, a second core 29, a coil guide 27, and the like, as in the second embodiment. As in the above embodiments, the degaussing coils 26 </ b> A to 26 </ b> C are not connected to the power supply unit (not shown) of the excitation coil 25.

The fixing device 19 configured as described above operates as follows.
By the rotation driving of the auxiliary fixing roller 50, the fixing belt 60 rotates in the clockwise direction in FIG. 19, the support roller 41 also rotates in the clockwise direction, and the pressure roller 30 also rotates in the counterclockwise direction. The fixing belt 60 is heated at a position facing the induction heating unit 24.

  Specifically, a high-frequency alternating current of 10 kHz to 1 MHz (preferably 20 kHz to 800 kHz) is supplied from the power supply unit (not shown) to the excitation coil 25, so that the excitation coil 25 is directed toward the support roller 41 and the fixing belt 60. It is formed so that the magnetic field lines are alternately switched in both directions. By forming the alternating magnetic field in this way, eddy currents are generated on the surface of the support roller 41 and the heat generating layer of the fixing belt 60, and Joule heat is generated by the electric resistance of the support roller 41 and the heat generating layer. 41 and the heating layer are heated. Thus, the fixing belt 60 is heated by the heat received from the heat-generating support roller 41 and the heat generated by the heat generation layer. That is, the fixing belt 60 functions as a heat generating member that is directly heated by the induction heating unit 24 and is indirectly heated (heated via the support roller 41) by the induction heating unit 24. Will function as.

Thereafter, the surface of the fixing belt 60 heated by the induction heating unit 24 reaches a contact portion with the pressure roller 30. Then, the toner image T (toner) on the conveyed recording medium P is heated and melted.
The surface of the fixing belt 60 that has passed the fixing position then reaches the position facing the induction heating unit 24 again.
Such a series of operations is continuously repeated to complete the fixing step in the image forming process.

  As described above, in the fifth embodiment, in addition to the excitation coil 25 that induction-heats the fixing belt 60 and the support roller 41 (heating member), an induced current flows due to the magnetic flux generated by the excitation coil 25. The degaussing coils 26A to 26C that generate magnetic flux in the direction to cancel the magnetic flux acting on the fixing belt 60 and the support roller 41 are provided in a part in the width direction, and the degaussing coil is supplied to the power supply unit that supplies an alternating current to the excitation coil 25. 26A to 26C are configured not to be electrically connected. Accordingly, even when small-size paper is continuously passed, excessive temperature rise in the non-paper passing area of the fixing belt 60 and the support roller 41 is efficiently and reliably suppressed, and the exciting coil 25 is used. Variations in the heating amount of the fixing belt 60 and the support roller 41 to be heated can be reduced.

  In the fifth embodiment, the fixing belt 60 and the support roller 41 are both electromagnetic induction heated by the induction heating unit 24. On the other hand, only one of the fixing belt 60 and the support roller 41 may be configured to be electromagnetic induction heated by the induction heating unit 24. For example, when the heat generating layer is not provided on the fixing belt 60, only the support roller 41 functions as a heat generating member that is electromagnetic induction heated by the induction heating unit 24 and also functions as a heating member that heats the fixing belt 60. become. In that case, the same effect as in the fifth embodiment can be obtained.

  In the fifth embodiment, the induction heating unit 24 is disposed at a position facing the outer peripheral surface of the support roller 41 with the fixing belt 60 interposed therebetween. However, the induction heating unit 24 is directly opposed to the outer peripheral surface of the support roller 41. An induction heating unit 24 can also be provided. That is, the induction heating unit 24 can be directly opposed to the support roller 41 without using the fixing belt 60. Even in this case, the same effect as in the fifth embodiment can be obtained.

  It should be noted that the present invention is not limited to the above-described embodiments, and within the scope of the technical idea of the present invention, the embodiments can be modified as appropriate in addition to those suggested in the embodiments. Is clear. In addition, the number, position, shape, and the like of the constituent members are not limited to the above embodiments, and can be set to a number, position, shape, and the like that are suitable for carrying out the present invention.

1 image forming apparatus body (apparatus body),
19 fixing device,
20 fixing roller (fixing member, heating member),
21 heat generation layer, 22 elastic layer, 23 cored bar,
24 induction heating unit,
25 Excitation coil,
26, 26A-26C, 26a-26c degaussing coil,
27 Coil guide,
28 first core,
29 Second core (center core),
30 pressure roller, 41 support roller (heating member),
50 fixing auxiliary roller,
51A-51C, 51a-51c changeover switch (switching element),
55 Temperature sensor (temperature detection means),
60 fixing belt (fixing member, heating member),
70 Power supply.

JP 2001-34097 A

Claims (9)

A heating member having a heating layer;
An exciting coil that opposes the heat generating member and generates a magnetic flux to inductively heat the heat generating layer with the magnetic flux;
A demagnetizing coil that faces the excitation coil and generates a magnetic flux in a direction that cancels the magnetic flux,
With
The degaussing coils are a plurality of pairs of degaussing coils arranged symmetrically about the widthwise center of the heat generating member,
The plurality of pairs of degaussing coils are formed so that each pair has a different length in the width direction, and are electrically connected to each pair to constitute an electric circuit having one switch . The fixing device according to claim 1, wherein the plurality of pairs of degaussing coils are disposed at different positions in the width direction and have different cross-sectional areas for each pair . The plurality of pairs of degaussing coils are disposed at different positions in the width direction , respectively , and are formed so that the number of turns or / and the loop area are different for each pair. Fixing device.   The fixing device according to claim 1, wherein the exciting coil is disposed so as to face the heat generating member with the demagnetizing coil interposed therebetween. Comprising a center core extending so as to be divided into a plurality in the width direction within the loop of the exciting coil;
The fixing device according to claim 1, wherein at least one of the center cores divided into a plurality of parts is disposed in a loop of the degaussing coil.   The fixing device according to claim 1, wherein the heat generating member is a fixing roller that melts a toner image and abuts against a pressure roller that presses a conveyed recording medium. . The heat generating member is a fixing belt that melts a toner image and is stretched between a support roller and a fixing auxiliary roller,
6. The fixing auxiliary roller according to claim 1, wherein the fixing auxiliary roller is disposed so as to come into contact with a pressure roller that presses a conveyed recording medium via the fixing belt. The fixing device described. The heating member is a support roller that heats a fixing belt that melts a toner image and stretches the fixing belt together with a fixing auxiliary roller,
The fixing device according to claim 1, wherein the excitation coil is disposed so as to face an outer peripheral surface of the support roller.   An image forming apparatus comprising the fixing device according to claim 1.

Images