Fixing device, image forming apparatus including the fixing device, and fixing method

A fixing device includes an excitation coil, a heat-generating layer, a magnetic shunt layer, and a degaussing member. The excitation coil generates a magnetic flux. The heat-generating layer generates heat using the magnetic flux generated by the excitation coil. The magnetic shunt layer transmits heat generated by the heat-generating layer. The degaussing member sandwiches the magnetic shunt layer together with the excitation coil, and selectively performs degaussing by generating a repelling magnetic flux for canceling the magnetic flux generated by the excitation coil so as to activate a self-temperature-control function. The degaussing member selectively refrains from degaussing so as to deactivate the self-temperature-control function.

PRIORITY STATEMENT

The present patent application claims priority from Japanese Patent Application No. 2007-061764, filed on Mar. 12, 2007 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Example embodiments generally relate to a fixing device, an image forming apparatus including the fixing device, and a fixing method using, for example, electromagnetic induction heating, implemented by a fixing device incorporated in an image forming apparatus.

2. Description of the Related Art

A related-art image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a recording sheet). For example, an electrostatic latent image formed on an image carrier is made visible with toner into a toner image. The toner image is transferred from the image carrier onto a recording sheet. A fixing device applies heat and pressure to the recording sheet bearing the toner image to fix the toner image on the recording sheet by various methods. Such methods include, for example, a heating roller method, a film method, and an induction heating method.

In a fixing device using the heating roller method, a heat source (e.g., a halogen lamp) heats a heating roller. The heating roller opposes a pressing roller to form a fixing nip between the heating roller and the pressing roller so as to nip a recording sheet bearing a toner image therebetween. At the fixing nip, the heating roller and the pressing roller apply heat and pressure to the recording sheet bearing the toner image.

In a fixing device using the film method, a film having a thermal capacity smaller than a thermal capacity of the heating roller is used as a heating member for applying heat to a recording sheet bearing a toner image.

In one example of a fixing device using the induction heating method, an induction heating coil wound around a bobbin is provided inside a heating roller. When an electric current is applied to the induction heating coil, an eddy current is generated in the heating roller and the heating roller generates heat.

In the heating roller method, the heating roller is preheated so that the heating roller may be heated quickly. By contrast, in the induction heating method, the heating roller may be heated up to a desired temperature quickly, even when the heating roller is not preheated.

Another example of a fixing device using the induction heating method includes both an induction heater and a heating roller. The induction heater includes an induction heating coil to which a power source applies a high-frequency voltage. The heating roller includes a magnetic heat-generating layer that has a Curie point equivalent to a fixing temperature. When the power source applies a high-frequency voltage to the induction heater, the heat-generating layer generates heat.

Thus, for example, a temperature of a ferromagnet included in the heat-generating layer increases quickly until the temperature of the ferromagnet reaches the Curie point. When the temperature of the ferromagnet reaches the Curie point, the heat-generating layer loses its magnetism. Thus, the temperature of the ferromagnet does not exceed the Curie point and is maintained at a desired temperature. The Curie point of the ferromagnet is equivalent to the fixing temperature. Therefore, the temperature of the ferromagnet is maintained at the fixing temperature.

The advantage of such an arrangement is that the heating roller may be quickly and precisely heated to a desired temperature without a complex controller, while a surface of the heating roller provides a proper release property and heat resistance.

In order to self-control an amount of heat generation, such fixing devices using the induction heating method may include a magnetic shunt layer including a magnetic shunt alloy. The magnetic shunt layer is provided between the induction heating coil and a degaussing member. When a temperature of the magnetic shunt alloy increases to the Curie point or higher, a repelling magnetic flux generated by the degaussing member cancels an induction magnetic flux generated by the induction heating coil. For example, when the temperature of the magnetic shunt alloy is near the Curie point, a magnetic permeability of the magnetic shunt alloy sharply decreases. Accordingly, the induction magnetic flux permeates the degaussing member. The degaussing member generates a repelling magnetic flux to activate a self-temperature-control function to prevent the heating roller from being heated up to the Curie point or higher.

Currently, there is market demand for an image forming apparatus capable of providing gloss-mode imaging, in which a glossy toner image is formed. To cope with such demand, a higher Curie point may be applied to the magnetic shunt alloy so that the heating roller may melt and fix toner particles forming a toner image on a recording sheet at a higher fixing temperature. Accordingly, a higher temperature may be applied as an upper temperature limit for limiting temperature increase at both end portions of the heating roller in a direction perpendicular to a conveyance direction of the recording sheet. Consequently, when a large-size recording sheet is conveyed to the heating roller immediately after small-size recording sheets are conveyed to the heating roller, the heating roller may not apply heat of a uniform temperature uniformly to the large-size recording sheet because the small-size recording sheets contact a center portion of the heating roller and draw heat from the center portion. Therefore, a temperature of heat applied by the heating roller to both end portions on the large-size recording sheet in the direction perpendicular to the conveyance direction of the recording sheet differs from a temperature of heat applied by the heating roller to a center portion on the large-size recording sheet. As a result, a fixed toner image on the center portion on the large-size recording sheet may have a gloss level different from a gloss level of a fixed toner image on the both end portions on the large-size recording sheet.

Obviously, such a gloss level difference between the center and the periphery of the sheet is undesirable, and accordingly, there is a need for a technology to minimize or eliminate such gloss level difference.

SUMMARY

At least one embodiment may provide a fixing device that includes an excitation coil, a heat-generating layer, a magnetic shunt layer, and a degaussing member. The excitation coil generates a magnetic flux. The heat-generating layer generates heat using the magnetic flux generated by the excitation coil. The magnetic shunt layer transmits heat generated by the heat-generating layer. The degaussing member sandwiches the magnetic shunt layer together with the excitation coil, and selectively performs degaussing by generating a repelling magnetic flux for canceling the magnetic flux generated by the excitation coil so as to activate a self-temperature-control function. The degaussing member selectively does not perform degaussing so as to deactivate the self-temperature-control function.

At least one embodiment may provide an image forming apparatus that includes a fixing device to apply heat to a recording medium bearing a toner image to fix the toner image on the recording medium. The fixing device includes an excitation coil, a heat-generating layer, a magnetic shunt layer, and a degaussing member. The excitation coil generates a magnetic flux. The heat-generating layer generates heat using the magnetic flux generated by the excitation coil. The magnetic shunt layer transmits heat generated by the heat-generating layer. The degaussing member sandwiches the magnetic shunt layer together with the excitation coil, and selectively performs degaussing by generating a repelling magnetic flux for canceling the magnetic flux generated by the excitation coil so as to activate a self-temperature-control function. The degaussing member selectively does not perform degaussing so as to deactivate the self-temperature-control function.

At least one embodiment may provide a fixing method implemented by a fixing device incorporated in an image forming apparatus. The method includes generating a magnetic flux with an excitation coil, generating heat with a heat-generating layer using the magnetic flux generated with the excitation coil, and transmitting heat generated with the heat-generating layer with a magnetic shunt layer. The method further includes sandwiching the magnetic shunt layer with the excitation coil and a degaussing member, selectively degaussing with the degaussing member by generating a repelling magnetic flux for canceling the magnetic flux generated with the excitation coil so as to activate a self-temperature-control function, and selectively not degaussing so as to deactivate the self-temperature-control function.

Additional features and advantages of example embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims.

The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly toFIG. 1, an image forming apparatus1according to an example embodiment is explained.

As illustrated inFIG. 1, the image forming apparatus1includes a mirror43, an image forming device10, paper trays40, feed rollers110, a registration roller pair49, a transfer device48, a cleaner46, a fixing device20, and/or a duplex device39. The image forming device10includes a photoconductor41, a charger42, and/or a development device44. The development device44includes a development roller44A. The cleaner46includes a blade46A.

The image forming apparatus1may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, and facsimile functions, or the like. According to this non-limiting example embodiment, the image forming apparatus1functions as a monochrome printer for forming a monochrome image on a recording medium (e.g., a recording sheet). However, the image forming apparatus1is not limited to the monochrome printer and may form a color and/or monochrome image with other structure.

The photoconductor41is provided in an upper portion of the image forming apparatus1and serves as an image carrier. The photoconductor41may be an electrophotographic photoconductor having a drum shape and rotates in a direction of rotation A. The charger42, the mirror43, the development device44, the transfer device48, and the cleaner46are disposed around the photoconductor41in this order in the direction of rotation A. The charger42has a roller shape. The mirror43forms a part of an exposure device (not shown). The paper trays40and the feed rollers110are provided in a lower portion of the image forming apparatus1. The paper trays40load a recording medium (e.g., recording sheets P).

Referring toFIG. 1, the following describes an image forming operation performed by the image forming apparatus1. When the photoconductor41starts rotating, the charger42uniformly charges a surface of the photoconductor41in the dark. The exposure device emits a light beam LB toward the charged surface of the photoconductor41according to image data. For example, in the exposure device, a light source (not shown) emits a light beam LB toward the mirror43. The mirror43reflects the light beam LB toward an exposure position150between charger42and the development roller44A on the surface of the photoconductor41and the light beam LB scans on the surface of the photoconductor41. Accordingly, an electrostatic latent image is formed on the surface of the photoconductor41.

When the electrostatic latent image reaches a position near or contacting the development device44by the rotation of the photoconductor41, the development device44visualizes the electrostatic latent image with toner to form a toner image. The rotation of the photoconductor41moves the toner image to a transfer position47at which the transfer device48opposes a lower surface of the photoconductor41.

One of the feed rollers110feeds a recording sheet P from a corresponding paper tray40toward the registration roller pair49. For example, the recording sheet P is guided by a conveyance guide (not shown) and fed by conveyance rollers (not shown) toward the registration roller pair49via a conveyance path illustrated in a broken line. The registration roller pair49is provided upstream from the transfer position47in a conveyance direction of the recording sheet P. The registration roller pair49temporarily stops the recording sheet P and feeds the recording sheet P toward the transfer position47at a time when the toner image formed on the photoconductor41opposes a proper position on the recording sheet P at the transfer position47. Namely, the registration roller pair49feeds the recording sheet P stopped at the registration roller pair49toward the transfer position47at a proper time.

When the toner image formed on the photoconductor41opposes the proper position on the recording sheet P, to which the toner image is transferred, at the transfer position47, an electric field generated by the transfer device48attracts and transfers the toner image onto the recording sheet P.

The rotation of the photoconductor41conveys residual toner particles not transferred onto the recording sheet P at the transfer position47and thereby remaining on the surface of the photoconductor41to the cleaner46. While the residual toner particles pass the cleaner46, the blade46A slides on the surface of the photoconductor41to remove the residual toner particles from the surface of the photoconductor41. Thus, the photoconductor41becomes ready for a subsequent toner image forming operation.

The recording sheet P bearing the toner image is fed toward the fixing device20. The fixing device20is provided downstream from the transfer position47in the conveyance direction of the recording sheet P. When the recording sheet P passes through the fixing device20, the fixing device20applies heat and pressure to the recording sheet P to fix the toner image on the recording sheet P. The recording sheet P bearing the fixed toner image is output onto an output portion (not shown).

When a toner image is to be formed on another side (e.g., a back side) of the recording sheet P, a branch nail (not shown) guides the recording sheet P toward the duplex device39. The duplex device39is provided downstream from the fixing device20in the conveyance direction of the recording sheet P. The duplex device39reverses the recording sheet P to cause a front side of the recording sheet P, on which the toner image is formed, to face down, and feeds the reversed recording sheet P toward the transfer position47. For example, the duplex device39switches back and reverses the recording sheet P and feeds the reversed recording sheet P to the conveyance path provided upstream from the registration roller pair49in the conveyance direction of the recording sheet P.

Referring toFIG. 2, the following describes the fixing device20.FIG. 2is a sectional view of the fixing device20. The fixing device20includes a magnetic flux generator2, a fixing roller3, and/or a pressing roller4. The magnetic flux generator2includes a coil2A, side cores2B, a center core2C, and/or an arc core2D.

The fixing device20uses a roller method in which a pair of rollers (e.g., the fixing roller3and the pressing roller4) applies heat and pressure to a recording sheet P to fix a toner image T on the recording sheet P. The pressing roller4, serving as a rotating pressing member, pressingly contacts the fixing roller3, serving as a rotating heat generation member, to form a nip between the pressing roller4and the fixing roller3.

An inverter (not shown), serving as an induction heating circuit, drives the coil2A (e.g., an excitation coil or an induction coil) with a high-frequency current to generate a high-frequency magnetic field. The magnetic field generates an eddy current in the fixing roller3including metal and the eddy current generates heat. Thus, a temperature of the fixing roller3increases. The coil2A is provided between the fixing roller3and the arc core2D.

FIG. 3is a partially enlarged sectional view of the fixing roller3. The fixing roller3includes a degaussing layer3A, an insulating layer3B, a magnetic shunt layer3C, an antioxidant layer3D1, a heat-generating layer3E, an antioxidant layer3D2, an elastic layer3F, and/or a releasing layer3G.

The fixing roller3has a diameter of about 40 mm, for example. The degaussing layer3A (e.g., a core metal) is provided at an innermost portion of the fixing roller3. The insulating layer3B, the magnetic shunt layer3C, the antioxidant layer3D1, the heat-generating layer3E, the antioxidant layer3D2, the elastic layer3F, and the releasing layer3G are layered on the degaussing layer3A in this order in a direction B. Thus, the releasing layer3G is provided at an outermost portion of the fixing roller3and forms a surface layer contacting a toner image T on a recording sheet P.

The degaussing layer3A includes aluminum or an alloy of aluminum. The insulating layer3B includes air and forms a space having a thickness of about 5 mm. The magnetic shunt layer3C includes a known magnetic shunt alloy properly selected and has a thickness of about 50 μm. Each of the antioxidant layers3D1and3D2includes nickel strike plating and has a thickness of about 1 μm or smaller. The heat-generating layer3E includes copper plating and has a thickness of about 15 μm. The elastic layer3F includes a silicone rubber and has a thickness of about 150 μm. The releasing layer3G includes PFA (perfluoroalkoxy) and has a thickness of about 30 μm. Namely, the magnetic shunt layer3C, the antioxidant layer3D1, the heat-generating layer3E, the antioxidant layer3D2, the elastic layer3F, and the releasing layer3G have a thickness of from about 200 μm to about 250 μm in total, for example.

The magnetic shunt layer3C includes a magnetic body (e.g., a magnetic shunt alloy material including iron and/or nickel) having a Curie point of from about 100 degrees centigrade to about 300 degrees centigrade, for example. Pressure applied by the pressing roller4(depicted inFIG. 2) deforms the magnetic shunt layer3C to form a nip between the pressing roller4and the fixing roller3. The magnetic shunt layer3C prevents the heat-generating layer3E and/or the like from being overheated. The fixing roller3is deformed to have a concave shape to form the nip. Therefore, a recording sheet P may easily separate from the nip. According to this example embodiment, layers other than the degaussing layer3A, that is, the magnetic shunt layer3C, the antioxidant layer3D1, the heat-generating layer3E, the antioxidant layer3D2, the elastic layer3F, and the releasing layer3G, are deformed by the pressure applied by the pressing roller4.

FIG. 4Ais a sectional view of the fixing roller3. InFIG. 4A, thick solid arrows illustrate induction magnetic fluxes generated by the coils2A and thin solid arrows illustrate eddy currents. When a temperature Te of a magnetic shunt alloy included in the magnetic shunt layer3C is lower than a Curie point Tc, the magnetic shunt alloy included in the magnetic shunt layer3C has magnetism. Accordingly, the induction magnetic fluxes generated by the coils2A do not permeate the magnetic shunt layer3C or the insulating layer3B. Namely, when the temperature Te of the magnetic shunt layer3C is lower than the Curie point Tc, the induction magnetic fluxes do not permeate the magnetic shunt layer3C and thereby do not reach the degaussing layer3A, as illustrated inFIG. 4A. Thus, heat generation is not suppressed in the fixing device20.

FIG. 4Bis a sectional view of the fixing roller3.FIG. 4Billustrates induction magnetic fluxes permeating the magnetic shunt layer3C and the insulating layer3B and reaching the degaussing layer3A. Broken arrows illustrate induction magnetic fluxes generated by the degaussing layer3A including aluminum or an alloy of aluminum. When the temperature Te of the magnetic shunt alloy included in the magnetic shunt layer3C is higher than the Curie point Tc, the magnetic shunt alloy included in the magnetic shunt layer3C loses its magnetism and becomes a non-magnetic body. Accordingly, the induction magnetic fluxes reach the degaussing layer3A even if the insulating layer3B is provided. Thus, heat generation is suppressed in the fixing device20.

As illustrated inFIG. 3, the magnetic shunt layer3C, which includes a magnetic body and/or the function of the heat-generating layer3E, is heated instantly until the temperature Te of the magnetic shunt layer3C reaches the Curie point. When the temperature Te of the magnetic shunt layer3C reaches the Curie point, the magnetic shunt layer3C loses its magnetism and thereby is not heated further, maintaining a constant temperature. Therefore, when the magnetic shunt layer3C includes a magnetic body having a Curie point of from about 100 degrees centigrade to about 300 degrees centigrade, that is, a temperature used in roller type fixing devices like the fixing device20, the heat-generating layer3E and the degaussing layer3A may not be overheated and thereby may maintain a proper fixing temperature. Accordingly, a surface of the fixing roller3may provide a proper release property and heat resistance property without a complex control.

FIG. 5illustrates a magnetic permeability (e.g., an inductance permeability) varying depending on a temperature. InFIG. 5, Δ indicates a magnetic permeability at each temperature. The magnetic permeability sharply decreases at a reference temperature.

When the magnetic shunt layer3C (depicted inFIG. 3) includes a single layer, the magnetic shunt layer3C may deform when the magnetic shunt layer3C includes an alloy including iron and/or nickel and has a thickness of about 150 μm or smaller. Alternatively, the magnetic shunt layer3C may include a deformable base layer (not shown) and a magnetic layer (not shown) plated on the base layer, for example. Thus, the magnetic shunt layer3C may be properly deformed with reduced rupture of the magnetic shunt layer3C.

The insulating layer3B (depicted inFIG. 3), on which the magnetic shunt layer3C is formed, may preferably include a material having a thermal conductivity lower than a thermal conductivity of the magnetic shunt layer3C. Accordingly, the heat-generating layer3E (depicted inFIG. 3) may provide an increased thermal efficiency. The insulating layer3B may include a material having a thermal conductivity (e.g., about 0.1 W/mK) lower than the thermal conductivity of the magnetic shunt layer3C, such as a foamed silicone rubber. For example, when the magnetic shunt layer3C has a thermal conductivity of about 11 W/mK, the insulating layer3B may be an air layer as illustrated inFIG. 3or other layer. The insulating layer3B may or may not include an elastic body. When the insulating layer3B includes the elastic body, pressure (e.g., a nip pressure) applied by the pressing roller4(depicted inFIG. 2) may be increased to provide an improved fixing property.

The insulating layer3B may preferably have a thickness of about 10 mm or smaller or any other appropriate thickness calculated based on a strength of a magnetic flux and/or the like, so as to cause a magnetic flux permeating the magnetic shunt layer3C to reach a conductive material.

According to this example embodiment, the rotating heat generation member (e.g., the fixing roller3) has a roller shape. However, the rotating heat generation member may have a sleeve or a belt shape. When the magnetic shunt layer3C is provided separately from the heat-generating layer3E, the magnetic shunt layer3C may be fixed or may not be fixed to the heat-generating layer3E. When the magnetic shunt layer3C is not fixed to the heat-generating layer3E, a belt or a sleeve may include the heat-generating layer3E and a roller may include the magnetic shunt layer3C.

As illustrated inFIGS. 6A and 6B, the fixing device20further includes a magnetic core3H and/or a degaussing member3K. The magnetic core3H and the degaussing member3K are provided inside the magnetic shunt layer3C having an increased Curie point. The degaussing member3K has a plate shape forming a semi-cylindrical shape in cross section, and includes a material having a volume resistivity lower than a volume resistivity of a magnetic shunt alloy included in the magnetic shunt layer3C. The degaussing member3K is rotatable with the magnetic core3H inside the magnetic shunt layer3C. For example, the magnetic core3H rotates in a circular space formed by the magnetic shunt layer3C. A driver (not shown) supports and drives the magnetic core3H. Rotation of the magnetic core3H moves the degaussing member3K closer to and away from the magnetic flux generator2. Namely, rotation of the magnetic core3H selectively turns on and off a degaussing function of the degaussing member3K. The degaussing member3K may include a conductive material, such as aluminum or an alloy of aluminum. However, the degaussing member3K may include other material and may have a shape other than the shape illustrated inFIGS. 6A and 6B. The driver for driving the magnetic core3H may include various mechanisms for moving an element in a cylinder having a structure similar to the structure illustrated inFIGS. 6A and 6B.

The magnetic core3H includes a semicircular portion C illustrated in broken line inFIG. 6A. The semicircular portion C is provided opposite the degaussing member3K in a direction of rotation D of the degaussing member3K. The semicircular portion C includes a high-resistance magnetic body.

InFIGS. 6A and 6B, thick solid arrows indicate induction magnetic fluxes generated by the coils2A, thin solid arrows indicate eddy currents, and broken arrows indicate induction magnetic fluxes generated by the degaussing member3K including aluminum or an alloy of aluminum.

FIG. 6Ais a sectional view of the fixing roller3when the degaussing function of the degaussing member3K is turned on. The degaussing member3K is positioned close to the coil2A. When the temperature Te of the magnetic shunt alloy included in the magnetic shunt layer3C is not lower than the Curie point Tc, the magnetic shunt alloy included in the magnetic shunt layer3C loses its magnetism and becomes a non-magnetic body, providing an increased degaussing function.

FIG. 6Bis a sectional view of the fixing roller3when the degaussing function of the degaussing member3K is not turned on. The degaussing member3K is positioned away from and opposite to the coil2A. Therefore, although induction magnetic fluxes generated by the coil2A permeates the magnetic shunt layer3C, the temperature Te of the magnetic shunt alloy included in the magnetic shunt layer3C is higher than the Curie point Tc and thereby the degaussing member3K does not generate an induction magnetic flux. Accordingly, the degaussing member3K may not provide its degaussing function. A magnetic shunt alloy included in the magnetic shunt layer3C does not lose its magnetism and maintains to be a magnetic body.

The fixing device20according to this example embodiment may provide a control for suppressing heat generation by moving the degaussing member3K together with the magnetic core3H.FIG. 7illustrates a heat generation amount varying depending on a temperature. InFIG. 7, Δ indicates the heat generating amount when a degaussing member, such as the degaussing member3K depicted inFIGS. 6A and 6B, is turned on and ∘ indicates the heat generation amount when the degaussing member is turned off. Heat generated by the magnetic shunt layer3C (depicted inFIGS. 6A and 6B) may be controlled based on data illustrated inFIG. 7by changing a position of the degaussing member3K with respect to the coil2A (depicted inFIGS. 6A and 6B).

FIG. 8illustrates a gloss level varying depending on a temperature. InFIG. 8, □ indicates the gloss level when a degaussing member, such as the degaussing member3K depicted inFIGS. 6A and 6B, is turned on and x indicates the gloss level when the degaussing member is turned off. When the degaussing function is turned on and the magnetic shunt layer3C (depicted inFIG. 6A) may be heated up to a temperature not lower than the Curie point, the temperature is saturated at 180 degrees centigrade. When the degaussing function is turned off, the magnetic shunt layer3C (depicted inFIG. 6B) may be heated up to a temperature not lower than 200 degrees centigrade, providing a desired increased gloss level.

FIG. 9illustrates a gloss level varying depending on a temperature when the degaussing function suppresses heat generation. InFIG. 9, □ indicates the gloss level in a normal mode and Δ indicates the gloss level in a gloss mode for forming a toner image having a high gloss level. In the gloss mode providing a gloss level of from 30 to 45, the temperature of the magnetic shunt layer3C (depicted inFIG. 3), actually a temperature of the surface layer of the fixing roller3(depicted inFIG. 3), may be increased without temperature saturation when the degaussing function is not used. In the normal mode providing a gloss level of from 15 to 22 or 23, temperature increase of the magnetic shunt layer3C is saturated by using the degaussing function. A small-size recording sheet is conveyed on a center portion of the fixing roller3but is not conveyed on both end portions of the fixing roller3in a direction perpendicular to a conveyance direction of the recording sheet. A large-size recording sheet is conveyed on the center portion and the both end portions of the fixing roller3. Therefore, when a toner image on the large-size recording sheet is fixed after a toner image on the small-size recording sheet is fixed, the fixed toner image on the large-size recording sheet provides an increased difference in gloss level between the center portion and the both end portions.FIG. 9illustrates a comparison between the gloss mode and the normal mode at a low linear speed at which recording sheets are conveyed. When the temperature is 160 degrees centigrade, the difference in gloss level generated due to increase in temperature of both end portions of the fixing roller3after small-size recording sheets are continuously conveyed may be suppressed within about 10 percent. The gloss mode may provide a gloss level of up to about 45 percent.

Accordingly, in the normal mode, the degaussing member3K may be positioned as illustrated inFIG. 10Aand may operate as illustrated inFIG. 6A. In the gloss mode, the degaussing member3K may be positioned as illustrated inFIG. 10Band may operate as illustrated inFIG. 6B.

Referring toFIGS. 11A and 11B, the following describes a fixing device20A according to another example embodiment. The fixing device20A includes a pair of degaussing coils3L and/or a switch element5. The other elements of the fixing device20A are common to the fixing device20depicted inFIGS. 4A and 4B.

The pair of degaussing coils3L and the switch element5form a degaussing member. The pair of degaussing coils3L, serving as supplemental coils, is provided inside the magnetic shunt layer3C. The switch element5causes a short circuit (e.g., conduction) between the degaussing coils3L or opens to break conduction between the degaussing coils3L so as to suppress magnetic fluxes. The fixing device20A does not include a mechanism for moving the pair of degaussing coils3L, saving space.

InFIGS. 11A and 11B, thick solid arrows indicate induction magnetic fluxes generated by the coils2A. Thin solid arrows indicate eddy currents. Broken arrows indicate degaussing magnetic fluxes generated by the pair of degaussing coils3L and canceling the induction magnetic fluxes generated by the coils2A.

FIG. 11Ais a sectional view of the fixing roller3in which a degaussing function is activated. For example, the switch element5is turned on to cause a short circuit between the degaussing coils3L and to generate degaussing magnetic fluxes. Thus, the fixing device20A activates the degaussing function. Accordingly, magnetic fluxes affecting the heat-generating layer3E (depicted inFIG. 3) are reduced and heat generation is suppressed.

FIG. 11Bis a sectional view of the fixing roller3in which the degaussing function is not activated. For example, the switch element5is turned off to break conduction between the degaussing coils3L and not to generate degaussing magnetic fluxes. Thus, the fixing device20A does not activate the degaussing function. Accordingly, magnetic fluxes affecting the heat-generating layer3E (depicted inFIG. 3) are not reduced and heat generation is not suppressed.

The pair of degaussing coils3L is provided away from the coils2A and opposes the coils2A via the magnetic shunt layer3C. Therefore, the induction magnetic fluxes generated by the coils2A permeate the magnetic shunt layer3C. However, the pair of degaussing coils3L does not generate induction magnetic fluxes, because the temperature Te of the magnetic shunt alloy included in the magnetic shunt layer3C is higher than the Curie point Tc. Accordingly, the magnetic shunt alloy does not lose its magnetism and maintains to be a magnetic body.

As illustrated inFIG. 12, the fixing device20A further includes an inverter6. The inverter6serves as a driver or a power source, and forms a degaussing member together with the pair of degaussing coils3L and the switch element5.FIG. 12is a conceptual diagram illustrating a relation among the magnetic flux generator2including the coil2A (depicted inFIGS. 11A and 11B) serving as a main coil, the pair of degaussing coils3L, the switch element5, and the inverter6. The switch element5may include but is not limited to a switch or a variable resistive element. The pair of degaussing coils3L serves as sub coils or supplemental coils and is not directly connected to the driver.

The inverter6drives the pair of degaussing coils3L by applying a high-frequency current having a phase different from a phase applied to the coil2A (depicted inFIG. 11A) to cause the pair of degaussing coils3L to generate a magnetic flux canceling the magnetic flux generated by the coil2A.

As illustrated inFIGS. 11A and 11B, the center core2C divides the pair of degaussing coils3L. According to this example embodiment, one of the pair of degaussing coils3L provided in the left of the center core2C includes a plurality of coils. Another one of the pair of degaussing coils3L provided in the right of the center core2C also includes a plurality of coils. For example, about three coils may be preferably provided in the left and the right of the center core2C each. However, one or more coils may be provided and the number of coils is not limited.

The switch element5may perform control by changing a switch ratio per unit time. Various known controls may be applied to the switch element5.

FIG. 13Aillustrates the fixing device20A in the normal mode in which the switch element5is turned on to activate the degaussing function as illustrated inFIG. 11A.FIG. 13Billustrates the fixing device20A in the gloss mode in which the switch element5is turned off to deactivate the degaussing function as illustrated inFIG. 11B.

FIG. 14illustrates a fixing device20B according to yet another example embodiment. The fixing device20B includes a heating roller3M, a rotating fixing member3N, and/or a fixing belt3P. The other elements of the fixing device20B are common to the fixing device20depicted inFIGS. 10A and 10B.

The heating roller3M, serving as a rotating heat generation member, replaces the fixing roller3depicted inFIGS. 6A and 6Band includes a magnetic body. The rotating fixing member3N has elasticity and release property. The fixing belt3P is looped over the heating roller3M and the rotating fixing member3N.

The degaussing member3K is provided inside the heating roller3M. The fixing device20B has a basic structure similar to the structure of the fixing device20depicted inFIGS. 6A and 6Band may provide the degaussing function provided by the fixing device20.

In the fixing device20B, the heating roller3M generates heat using a magnetic flux generated by the magnetic flux generator2. Heat is transmitted from the heating roller3M to the fixing belt3P. The pressing roller4presses the rotating fixing member3N via the fixing belt3P to form a nip between the pressing roller4and the fixing belt3P. At the nip, the fixing belt3P and the pressing roller4apply heat and pressure to a recording sheet passing through the nip to fix a toner image on the recording sheet.

FIG. 15illustrates a fixing device20C according to yet another example embodiment. The fixing device20C includes the heating roller3M, the rotating fixing member3N, and/or the fixing belt3P (depicted inFIG. 14). The other elements of the fixing device20C are common to the fixing device20A depicted inFIGS. 13A and 13B. The pair of degaussing coils3L is provided inside the heating roller3M. The switch element5is connected to the pair of degaussing coils3L. The fixing device20C has a basic structure similar to the structure of the fixing device20A depicted inFIGS. 11A and 11Band may provide the degaussing function provided by the fixing device20A.

According to the above-described example embodiments, the degaussing function may be selectively used. Therefore, a fixing device (e.g., the fixing device20,20A,20B, or20C) may selectively perform its self-temperature-control function. Accordingly, even when the fixing device includes a magnetic shunt alloy, the fixing device may be heated up to a desired temperature.

According to the above-described example embodiments, an excitation coil (e.g., the coil2A depicted inFIGS. 6A,11A,14, and15) is provided outside a rotating magnetic shunt layer (e.g., the magnetic shunt layer3C depicted inFIGS. 6A and 11A), and a degaussing member (e.g., the degaussing member3K depicted inFIGS. 6A and 14) or a supplemental coil (e.g., the pair of degaussing coils3L depicted inFIGS. 11A and 15) is provided inside the rotating magnetic shunt layer3C. However, the excitation coil may be provided inside the rotating magnetic shunt layer, and the degaussing member or the supplemental coil may be provided outside the rotating magnetic shunt layer, as illustrated inFIGS. 16A,16B,17A,17B,18, and19.

According to the above-described example embodiments, an excitation coil (e.g., the coil2A depicted inFIGS. 6A,11A,14, and15) is provided outside a rotating magnetic shunt layer (e.g., the magnetic shunt layer3C depicted inFIGS. 6A and 11A), and a degaussing member (e.g., the degaussing member3K depicted inFIGS. 6A and 14) or a supplemental coil (e.g., the pair of degaussing coils3L depicted inFIGS. 11A and 15) is provided inside the rotating magnetic shunt layer3C. However, the excitation coil may be provided inside the rotating magnetic shunt layer, and the degaussing member or the supplemental coil may be provided outside the rotating magnetic shunt layer.

The present invention has been described above with reference to specific example embodiments. Nonetheless, the present invention is not limited to the details of example embodiments described above, but various modifications and improvements are possible without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the associated claims, the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative example embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.