Fixing device and image forming apparatus

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2013-072975, filed on Mar. 29, 2013, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

1. Technical Field

Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.

2. Description of the Background

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.

The fixing device may employ an induction heater to heat the recording medium quickly. For example, the induction heater heats a fixing rotary body, such as a fixing belt and a fixing roller, pressingly contacted by a pressure roller to form a fixing nip therebetween. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotary body and the pressure roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. Since the fixing rotary body incorporates a heat generation layer that generates heat by a magnetic flux generated by an exciting coil of the induction heater, the fixing rotary body is heated to a desired fixing temperature to fix the toner image on the recording medium quickly.

However, the heat generation layer is thin and therefore may cause temperature variation of the fixing rotary body in an axial direction thereof. For example, after a plurality of small recording media is conveyed over the fixing rotary body continuously, both lateral ends of the fixing rotary body in the axial direction thereof may overheat because the small recording media are not conveyed over both lateral ends of the fixing rotary body in the axial direction thereof and therefore do not draw heat therefrom. Accordingly, the temperature of the fixing rotary body varies in the axial direction thereof. Consequently, as a large recording medium is conveyed over the fixing rotary body immediately after conveyance of the small recording media, temperature variation of the fixing rotary body may vary gloss of a toner image on the large recording medium.

To address this problem, a self temperature control to offset a magnetic flux with a repulsive magnetic flux may be used. For example, a magnetic shunt alloy may be interposed between the heat generation layer and a metal plate serving as a degausser. When the temperature of the magnetic shunt alloy reaches a Curie temperature, a magnetic flux from the exciting coil penetrates the metal plate, allowing the metal plate to generate a repulsive magnetic flux that offsets the magnetic flux from the exciting coil.

In order to achieve the self temperature control, the exciting coil is situated in proximity to the magnetic shunt alloy. However, since the heat generation layer is disposed between the exciting coil and the magnetic shunt alloy, the degausser is situated in proximity to the heat generation layer. Accordingly, the degausser draws heat from the heated magnetic shunt alloy, elongating a warm-up time to warm up the heat generation layer to a target temperature.

To address this problem, two solutions are proposed. For example, as a first solution, as shown in JP-2013-003511-A, a part of the degausser that is requested to offset a decreased amount of the magnetic fluxes from the exciting coil is isolated from the heat generation layer with an increased interval therebetween, thus preventing the degausser from drawing heat from the heat generation layer. As a second solution, as shown in JP-2009-058829-A, the degausser rotates by 180 degrees with respect to the exciting coil, decreasing the repulsive magnetic fluxes generated by the degausser and thereby facilitating heat generation of the heat generation layer.

However, if the degausser is distanced from the exciting coil with an increased interval therebetween, the self temperature control of the degausser may degrade. Conversely, if the degausser is distanced from the exciting coil with a decreased interval therebetween, the degausser draws heat from the heat generation layer, degrading heat generation efficiency of the heat generation layer. For example, as the degausser is situated closer to the heat generation layer, the degausser is susceptible to magnetic fluxes leaked from the magnetic shunt alloy, which cause the degausser to generate repulsive magnetic fluxes that obstruct heat generation of the heat generation layer. Additionally, since the degausser is requested to generate an increased amount of repulsive magnetic fluxes to prevent temperature variation of the heat generation layer, it is necessary to locate the degausser close to the heat generation layer.

Accordingly, it is requested to locate the degausser at a position where the degausser enhances heat generation efficiency of the heat generation layer while generating a sufficient amount of repulsive magnetic fluxes to prevent temperature variation of the heat generation layer.

This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes an exciting coil to generate a magnetic flux and a heat generation layer disposed opposite the exciting coil to generate heat by the magnetic flux from the exciting coil. A temperature sensitive magnetic body, disposed opposite the exciting coil via the heat generation layer, obtains and loses magnetism at a temperature defined by a Curie temperature by composition adjustment to selectively create a heating region and a non-heating region of the heat generation layer. A degausser is made of a non-magnetic material having an electrical resistivity smaller than an electrical resistivity of the temperature sensitive magnetic body. The degausser is disposed opposite the exciting coil with an interval in a range of from about 4.2 mm to about 8.2 mm.

This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes the fixing device described above.

A more complete appreciation of the invention and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present invention;

FIG. 2 is a schematic vertical sectional view of a fixing device incorporated in the image forming apparatus shown in FIG. 1 ;

FIG. 3 is a sectional view of a heating roller incorporated in the fixing device shown in FIG. 2 ;

FIG. 4 is a partial perspective view of the fixing device shown in FIG. 2 ;

FIG. 5 is a graph showing a relation between an interval between an exciting coil and a degausser incorporated in the fixing device shown in FIG. 2 and heat generation efficiency of the heating roller and self temperature control of the degausser; and

FIG. 6 is a vertical sectional view of a fixing device according to another exemplary embodiment of the present invention.

In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to FIG. 1 , an image forming apparatus 100 according to an exemplary embodiment of the present invention is explained.

FIG. 1 is a schematic vertical sectional view of the image forming apparatus 100 . The image forming apparatus 100 may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this exemplary embodiment, the image forming apparatus 100 is a color copier that forms color and monochrome toner images on recording media by electrophotography.

As shown in FIG. 1 , the image forming apparatus 100 is a compact copier having an internal output tray accommodated inside a body of the image forming apparatus 100 . The image forming apparatus 100 includes an image forming device A, situated at substantially a center of the image forming apparatus 100 in a vertical direction, that forms a toner image on a recording medium. Below the image forming device A is a sheet feeder B that feeds the recording medium to the image forming device A. Optionally, another sheet feeder may be located below the sheet feeder B.

Above the image forming device A is an internal output tray D that receives the recording medium bearing the toner image. Above the internal output tray D is a scanner C that reads an image on an original. A recording medium conveyance path E indicated by the dotted line extends from the sheet feeder B to the internal output tray D.

A detailed description is now given of a construction of the image forming device A.

The image forming device A includes four drum-shaped photoconductors A 1 each of which is surrounded by components for forming a toner image. Taking the leftmost photoconductor A 1 as an example, the photoconductor A 1 rotatable in a rotation direction R 1 is surrounded by a charger A 2 that charges an outer circumferential surface of the photoconductor A 1 , an exposure device A 10 that emits a laser beam onto the charged outer circumferential surface of the photoconductor A 1 according to image data created by the scanner C, thus forming an electrostatic latent image on the photoconductor A 1 , and a development device A 3 that develops the electrostatic latent image formed on the photoconductor A 1 into a toner image.

In proximity to the four photoconductors A 1 are an intermediate transfer belt A 4 serving as an intermediate transferor and a secondary transferor A 5 . The toner images formed on the four photoconductors A 1 are primarily transferred onto the intermediate transfer belt A 4 such that the toner images are superimposed on a same position on the intermediate transfer belt A 4 to form a color toner image thereon. The color toner image is secondarily transferred onto a recording medium conveyed from the sheet feeder B by the secondary transferor A 5 .

A cleaner A 6 a is disposed opposite the photoconductor A 1 to remove residual toner failed to be transferred onto the intermediate transfer belt A 4 and therefore remaining on the photoconductor A 1 therefrom. A cleaner A 6 b is disposed opposite the intermediate transfer belt A 4 to remove residual toner failed to be transferred onto the recording medium and therefore remaining on the intermediate transfer belt A 4 therefrom. A cleaner A 6 c is disposed opposite the secondary transferor A 5 to clean the secondary transferor A 5 . In proximity to the cleaner A 6 a is a lubricant applicator A 1 a that applies a lubricant onto the photoconductor A 1 to decrease the friction coefficient of the outer circumferential surface of the photoconductor A 1 . Similarly, in proximity to the cleaner A 6 b is a lubricant applicator A 1 b that applies a lubricant onto the intermediate transfer belt A 4 to decrease the friction coefficient of an outer circumferential surface of the intermediate transfer belt A 4 . In proximity to the cleaner A 6 c is a lubricant applicator A 7 c that applies a lubricant onto the secondary transferor A 5 to decrease the frictional coefficient of an outer circumferential surface of the secondary transferor A 5 .

Downstream from the secondary transferor A 5 on the recording medium conveyance path E is a fixing device 1 that fixes the color toner image secondarily transferred from the intermediate transfer belt A 4 onto the recording medium thereon.

In order to facilitate maintenance, the photoconductor A 1 , the charger A 2 , the development device A 3 , the cleaner A 6 a , and the lubricant applicator A 1 a are integrated into a unit, that is, a process cartridge PC, detachably attached to the image forming apparatus 100 . Similarly, the cleaner A 6 b and the lubricant applicator A 1 b are integrated into a unit detachably attached to the intermediate transfer belt A 4 . The cleaner A 6 c , the lubricant applicator A 7 c , and a secondary transfer roller used as the secondary transferor A 5 are integrated into a unit detachably attached to the image forming apparatus 100 . The recording medium bearing the fixed color toner image discharged from the fixing device 1 is discharged by an output roller pair A 9 onto the internal output tray D which stocks the recording medium.

A detailed description is now given of conveyance of the recording medium to the image forming device A.

The sheet feeder B loads a plurality of new recording media and includes a feed roller B 1 and a paper tray. As the feed roller B 1 rotates, the feed roller B 1 picks up and feeds an uppermost recording medium from the plurality of recording media loaded on the paper tray toward a registration roller pair A 11 . The registration roller pair A 11 stops rotation temporarily to halt the recording medium conveyed from the feed roller B 1 and resumes rotation to feed the recording medium such that a leading edge of the recording medium reaches a secondary transfer nip formed between the intermediate transfer belt A 4 and the secondary transferor A 5 at a time when the color toner image formed on the intermediate transfer belt A 4 reaches the secondary transfer nip.

A description is provided of an image forming operation performed by the image forming apparatus 100 described above to form a color toner image on a recording medium.

The scanner C includes an exposure glass C 2 , a carriage C 1 , a lens C 3 , and a charge-coupled device (CCD) C 4 . As the carriage C 1 constructed of a light source and mirrors moves back and forth, the light source irradiates an original placed on the exposure glass C 2 with light. The light reflected by the original is deflected by the mirrors of the carriage C 1 into the lens C 3 and enters the CCD C 4 situated downstream from the lens C 3 in a light travel direction. Thus, an image on the original is read into an image signal by the CCD C 4 .

The image signal is digitalized and subject to image processing. Based on the processed signal, a laser diode of the exposure device A 10 emits light onto the outer circumferential surface of the photoconductor A 1 , forming an electrostatic latent image thereon. For example, the light emitted from the laser diode reaches the photoconductor A 1 through a polygon mirror and a lens.

The charger A 2 includes a charging member (e.g., a charging roller) and a biasing member that biases the charging member against the photoconductor A 1 with predetermined pressure. The charging member is constructed of a conductive shaft and a conductive elastic layer coating the conductive shaft. A voltage applicator applies a predetermined voltage to a gap between the conductive elastic layer of the charging member and the photoconductor A 1 through the conductive shaft, thus charging the outer circumferential surface of the photoconductor A 1 .

The development device A 3 includes an agitation screw, a development roller, and a development doctor. A developer containing toner, after being agitated sufficiently by the agitation screw, adheres to the development roller magnetically. The development doctor levels the developer on the development roller into a thin layer. The leveled developer moves to the electrostatic latent image formed on the photoconductor A 1 , visualizing the electrostatic latent image as a toner image.

A primary transfer roller electrically adheres the toner image onto the intermediate transfer belt A 4 . Residual developer, that is, residual toner, failed to be transferred onto the intermediate transfer belt A 4 and therefore remaining on the photoconductor A 1 is removed from the photoconductor A 1 by the cleaner A 6 a . The lubricant applicator A 1 a includes a lubricant application roller A 71 a constructed of a metal shaft and a brush wound around the metal shaft.

The lubricant application roller A 71 a biases against a solid lubricant A 72 a under its weight. A biasing member A 73 a biases the solid lubricant A 72 a against the lubricant application roller A 71 a . The lubricant application roller A 71 a , as it rotates, scrapes fine powder off the solid lubricant A 72 a and applies the fine powder on the outer circumferential surface of the photoconductor A 1 . For example, the fine powder of the solid lubricant A 72 a is applied on substantially the entire outer circumferential surface of the photoconductor A 1 that is greater than a cleaning area on the photoconductor A 1 where the cleaner A 6 a cleans the photoconductor A 1 . The cleaning area on the photoconductor A 1 is determined based on cleaning performance of the cleaner A 6 a or the like. Conversely, the solid lubricant A 72 a is applied to the entire area on the photoconductor A 1 where the cleaning blade contacts the photoconductor A 1 .

The lubricant applicator A 1 b and the cleaner A 6 b are integrated into a transfer cartridge detachably attached to the image forming apparatus 100 . A biasing member A 73 b biases a solid lubricant A 72 b against a lubricant application roller A 71 b (e.g., a brush roller) with predetermined pressure. The lubricant application roller A 71 b , as it rotates, scrapes fine powder off the solid lubricant A 72 b and applies the fine powder onto the outer circumferential surface of the intermediate transfer belt A 4 . Upstream from the lubricant applicator A 1 b in a rotation direction R 2 of the intermediate transfer belt A 4 is the cleaner A 6 b incorporating a brush roller and a cleaning blade that clean the intermediate transfer belt A 4 .

For example, the brush roller rotates in a direction identical to the rotation direction R 2 of the intermediate transfer belt A 4 to disperse a foreign substance from the outer circumferential surface of the intermediate transfer belt A 4 . The cleaning blade contacts the intermediate transfer belt A 4 with predetermined angle and pressure to remove residual toner failed to be transferred onto the recording medium and therefore remaining on the intermediate transfer belt A 4 therefrom.

Similarly, the cleaner A 6 c and the secondary transferor A 5 are integrated into a transfer cartridge detachably attached to the image forming apparatus 100 . The cleaner A 6 c removes residual toner remaining on the secondary transferor A 5 therefrom. The lubricant applicator A 7 c includes a biasing member A 73 c that biases a solid lubricant A 72 c against a lubricant application roller A 71 c so that the lubricant application roller A 71 c applies fine powder scraped off the solid lubricant A 72 c onto the secondary transferor A 5 .

With reference to FIG. 2 , a description is provided of a construction of the fixing device 1 incorporated in the image forming apparatus 100 described above.

FIG. 2 is a vertical sectional view of the fixing device 1 . As shown in FIG. 2 , the fixing device 1 (e.g., a fuser) employs a belt fixing method using a fixing belt 5 looped over a fixing roller 4 and a heating roller 2 A. The fixing roller 4 is disposed opposite a pressure roller 3 pressed against the fixing roller 4 . A heating assembly 2 includes the hollow heating roller 2 A rotatable in a rotation direction R 3 , a coil support 2 D disposed opposite the heating roller 2 A via the fixing belt 5 , an exciting coil 2 B supported by the coil support 2 D and disposed opposite the fixing belt 5 via the coil support 2 D, an arc core 2 C disposed opposite the fixing belt 5 via the exciting coil 2 B and the coil support 2 D, and a degausser 2 E disposed inside the hollow heating roller 2 A.

With reference to FIG. 3 , a detailed description is now given of a construction of the heating roller 2 A.

FIG. 3 is a sectional view of the heating roller 2 A. As shown in FIG. 3 , the heating roller 2 A includes a heat generation layer 2 A 1 and a temperature sensitive magnetic body 2 A 2 (e.g., a temperature sensitive magnetic layer) disposed opposite the exciting coil 2 B via the heat generation layer 2 A 1 . The heat generation layer 2 A 1 generates heat by induction heating as it receives a magnetic flux from the exciting coil 2 B. The heat generation layer 2 A 1 coats a surface of the temperature sensitive magnetic body 2 A 2 by conductive plating such as copper plating and has a thickness in a range of from about 3 micrometers to about 20 micrometers, facilitating production of an eddy current and heat generation of the heat generation layer 2 A 1 . For example, the heat generation layer 2 A 1 is made of copper, gold, or the like. Alternatively, the heat generation layer 2 A 1 may have a thickness of an upper limit of about 30 micrometers, that is, in a range of from about 3 micrometers to about 30 micrometers, thus facilitating generation of an eddy current that facilitates production of a repulsive magnetic flux.

The temperature sensitive magnetic body 2 A 2 is made of a magnetic shunt alloy. The magnetic shunt alloy is a magnetic material of which composition is adjusted such that the magnetic shunt alloy has a Curie temperature in a range of from about 100 degrees centigrade to about 300 degrees centigrade, for example, a magnetic shunt alloy material containing iron, nickel, or the like. The temperature sensitive magnetic body 2 A 2 obtains and loses magnetism below and above the Curie temperature. As the temperature sensitive magnetic body 2 A 2 obtains and loses magnetism, the temperature sensitive magnetic body 2 A 2 adjusts penetration of a magnetic flux through the heat generation layer 2 A 1 , selectively creating a heating region and a non-heating region of the heat generation layer 2 A 1 . For example, the heating region corresponds to a conveyance region of the fixing belt 5 where a recording medium P is conveyed, that is, a center span of the fixing belt 5 in an axial direction thereof. The non-heating region corresponds to a non-conveyance region of the fixing belt 5 where a recording medium P is not conveyed, that is, each lateral end span of the fixing belt 5 in the axial direction thereof.

According to this exemplary embodiment, the temperature sensitive magnetic body 2 A 2 is formed in a roller. Alternatively, the temperature sensitive magnetic body 2 A 2 may be formed in a film, an endless belt, or the like.

Accordingly, since the heating roller 2 A incorporates the heat generation layer 2 A 1 , the fixing belt 5 is constructed of a base layer made of polyimide resin. Although the fixing belt 5 does not incorporate a heat generation layer, the fixing belt 5 is heated to a predetermined temperature by the heating roller 2 A.

With reference to FIG. 2 , a detailed description is now given of a configuration of the exciting coil 2 B.

The exciting coil 2 B includes folded lateral ends in a longitudinal direction thereof parallel to an axial direction of the heating roller 2 A, that is, turn portions, and extensions contiguously extending from the turn portions, respectively. The length of the exciting coil 2 B including the extensions is equivalent to or greater than the width of a large recording medium (e.g., the width of an A 3 size recording medium of 297 mm) in the axial direction of the heating roller 2 A.

A detailed description is now given of a configuration of the arc core 2 C.

As shown in FIG. 2 , the arc core 2 C includes a center core 2 C 1 situated at a center of the arc core 2 C in a circumferential direction thereof and side cores 2 C 2 situated at both ends of the arc core 2 C in the circumferential direction thereof. The exciting coil 2 B is wound around the center core 2 C 1 .

A detailed description is now given of a configuration of the degausser 2 E.

As shown in FIG. 2 , the degausser 2 E is disposed opposite an outer circumferential surface of a shaft 6 rotatably mounting the heating roller 2 A. The degausser 2 E is made of a non-magnetic material and includes an arcuate opposed face 2 E 1 disposed opposite the exciting coil 2 B and contoured to correspond to an inner circumferential surface of the heating roller 2 A. The degausser 2 E is a non-magnetic conductor made of aluminum, an alloy of aluminum, or copper that has an electrical resistivity smaller than that of the temperature sensitive magnetic body 2 A 2 of the heating roller 2 A. The degausser 2 E is a pipe or a tube corresponding to the tubular heating roller 2 A. The degausser 2 E includes an arch 2 E 2 having a center angle θ greater than an angle defined by a circumferential span of the exciting coil 2 B disposed opposite the degausser 2 E.

According to this exemplary embodiment, the degausser 2 E is circular in cross-section as shown in FIG. 2 . Alternatively, the degausser 2 E may be arcuate in cross-section. When the temperature sensitive magnetic body 2 A 2 of the heating roller 2 A selectively creates the heating region and the non-heating region of the heat generation layer 2 A 1 , a magnetic flux reaching the degausser 2 E generates an eddy current in the degausser 2 E that generates a repulsive magnetic flux, preventing the magnetic flux penetrating through the heating roller 2 A from penetrating through the shaft 6 disposed opposite the heating roller 2 A via the degausser 2 E.

With reference to FIG. 4 , a description is provided of a configuration of a holder 7 that supports the degausser 2 E.

FIG. 4 is a partial perspective view of the fixing device 1 illustrating one lateral end of the degausser 2 E in a longitudinal direction thereof parallel to the axial direction of the heating roller 2 A. Although FIG. 4 does not illustrate another lateral end of the degausser 2 E in the longitudinal direction thereof, both lateral ends of the degausser 2 E in the longitudinal direction thereof are supported by the holder 7 situated inside the heating roller 2 A incorporating the heat generation layer 2 A 1 such that the holder 7 is disposed opposite the exciting coil 2 B via the heat generation layer 2 A 1 and the temperature sensitive magnetic body 2 A 2 of the heating roller 2 A.

An inverter connected to the exciting coil 2 B drives the exciting coil 2 B at high frequency, causing the exciting coil 2 B to produce a high frequency magnetic field, that is, a high frequency magnetic flux. The high frequency magnetic field moves an eddy current through the heat generation layer 2 A 1 of the heating roller 2 A, thus increasing the temperature of the heating roller 2 A. As shown in FIG. 2 , as a recording medium P bearing a toner image Tn is conveyed through a fixing nip N formed between the pressure roller 3 and the fixing belt 5 looped over the fixing roller 4 and the heating roller 2 A such that the toner image Tn faces the fixing belt 5 , the fixing belt 5 heated by the heating roller 2 A and the pressure roller 3 apply heat and pressure to the recording medium P, melting and fixing the toner image Tn on the recording medium P.

A detailed description is now given of a configuration of the pressure roller 3 .

The pressure roller 3 serves as a driving roller for driving the fixing belt 5 . The pressure roller 3 is pressed against the fixing roller 4 via the fixing belt 5 to form the fixing nip N between the pressure roller 3 and the fixing belt 5 . As the recording medium P bearing the toner image Tn is conveyed through the fixing nip N, the pressure roller 3 drives and rotates the fixing belt 5 by friction therebetween.

A description is provided of a configuration of the fixing device 1 having the construction described above, that enhances heat generation efficiency and degaussing efficiency.

As shown in FIGS. 2 and 3 , the fixing device 1 includes the heating roller 2 A that incorporates the heat generation layer 2 A 1 and the temperature sensitive magnetic body 2 A 2 and the fixing belt 5 that does not incorporate a heat generation layer. The fixing device 1 has a configuration that enhances heat generation efficiency and degaussing efficiency as described below. For example, the degausser 2 E is disposed opposite the exciting coil 2 B with an interval in a range of from about 4.2 mm to about 8.2 mm.

With reference to FIG. 5 , a description is provided of an experiment for examining an influence of the interval between the exciting coil 2 B and the degausser 2 E upon a heat generation efficiency of the heat generation layer 2 A 1 of the heating roller 2 A and a degaussing efficiency, that is, a self temperature control, of the degausser 2 E.

FIG. 5 is a graph showing a relation between the interval between the exciting coil 2 B and the degausser 2 E and the heat generation efficiency of the heating roller 2 A and the self temperature control of the degausser 2 E. The self temperature control shown in FIG. 5 defines 100 percent as the upper limit temperature of 210 degrees centigrade of both lateral ends of the heating roller 2 A in the axial direction thereof when both lateral ends of the heating roller 2 A in the axial direction thereof are heated. The self temperature control is calculated according to a formula (1) below.

S= 210/ T× 100  (1)

In the formula (1), S represents self temperature control as a percentage. T represents the upper limit temperature of both lateral ends of the heating roller 2 A in the axial direction thereof. For example, when the upper limit temperature T of both lateral ends of the heating roller 2 A in the axial direction thereof is 230 degrees centigrade, the self temperature control S is 91.3 percent.

An available range shown in FIG. 5 defines the upper limit temperature of both lateral ends of the heating roller 2 A in the axial direction thereof in a range of from 214.7 degrees centigrade to 230.0 degrees centigrade. Since the heat generation efficiency of a halogen heater is smaller than about 90 percent, the available range shown in FIG. 5 defines a range in which the heating roller 2 A demonstrates a desired fixing property, that is, the heating roller 2 A is heated quickly to a desired fixing temperature, when the heat generation efficiency is 90 percent. When the heat generation efficiency is 90 percent at which the heating roller 2 A demonstrates a desired fixing property in the available range, the temperature of both lateral ends of the heating roller 2 A in the axial direction thereof is 214.7 degrees centigrade.

Based on the results of the experiment shown in FIG. 5 , in order to enhance both the heat generation efficiency and the self temperature control, the interval between the exciting coil 2 B and the degausser 2 E is in a range of from about 4.2 mm to about 8.2 mm to employ the fixing belt 5 without a heat generation layer. Accordingly, even with the fixing belt 5 without a heat generation layer, the enhanced heat generation efficiency of the heating roller 2 A shortens a warm-up time to warm up the fixing belt 5 to a desired fixing temperature to fix the toner image Tn on the recording medium P and prevents overheating of the fixing belt 5 at both lateral ends in the axial direction thereof. For example, after a plurality of recording media P is conveyed through the fixing nip N continuously, both lateral ends of the fixing belt 5 in the axial direction thereof may overheat because the small recording media P are not conveyed over both lateral ends of the fixing belt 5 in the axial direction thereof and therefore do not draw heat therefrom. To address this circumstance, the degausser 2 E distanced from the exciting coil 2 B with the interval as defined above prevents overheating of both lateral ends of the fixing belt 5 in the axial direction thereof.

With reference to FIG. 6 , a description is provided of a variation of the degausser 2 E.

FIG. 6 is a vertical sectional view of a fixing device 1 S incorporating a degausser 2 E′ as a variation of the degausser 2 E shown in FIG. 2 . As shown in FIG. 6 , the degausser 2 E′ includes a linear, opposed face 2 E′ 1 disposed opposite the exciting coil 2 B. The linear, opposed face 2 E′ 1 is manufactured by bending the degausser 2 E′, not by curving the degausser 2 E′ along the inner circumferential surface of the heating roller 2 A into an arch, resulting in reduced manufacturing costs. As shown in FIG. 6 , the degausser 2 E′ is bent at a single place. Alternatively, the degausser 2 E′ may be bent at a plurality of places to contour the opposed face 2 E′ 1 into substantially an arch.

A description is provided of advantages of the fixing devices 1 and 1 S.

As shown in FIGS. 2, 3, and 6 , the fixing devices 1 and 1 S include the exciting coil 2 B, the heat generation layer 2 A 1 disposed opposite the exciting coil 2 B, the temperature sensitive magnetic body 2 A 2 disposed opposite the exciting coil 2 B via the heat generation layer 2 A 1 , and a degausser (e.g., the degaussers 2 E and 2 E′) disposed opposite the exciting coil 2 B via the heat generation layer 2 A 1 and the temperature sensitive magnetic body 2 A 2 . The exciting coil 2 B generates a magnetic flux. The heat generation layer 2 A 1 generates heat by the magnetic flux from the exciting coil 2 B. The temperature sensitive magnetic body 2 A 2 obtains and loses magnetism at a temperature defined by a Curie temperature by composition adjustment. The degausser is made of a non-magnetic material having an electrical resistivity smaller than that of the temperature sensitive magnetic body 2 A 2 . As the temperature sensitive magnetic body 2 A 2 obtains and loses magnetism, the temperature sensitive magnetic body 2 A 2 adjusts penetration of the magnetic flux through the heat generation layer 2 A 1 , selectively creating the heating region and the non-heating region of the heat generation layer 2 A 1 . The degausser is distanced from the exciting coil 2 B with an interval in a range of from about 4.2 mm to about 8.2 mm.

Accordingly, the interval between the exciting coil 2 B and the degausser defined as above satisfies heat generation efficiency of the heat generation layer 2 A 1 and degaussing efficiency of the degausser to achieve the self temperature control.

According to the exemplary embodiments described above, the fixing devices 1 and 1 S include the fixing belt 5 looped over the heating roller 2 A and the fixing roller 4 . Alternatively, the fixing devices 1 and 1 S may not include the fixing belt 5 and the fixing roller 4 . In this case, the pressure roller 3 is pressed against the heating roller 2 A to form the fixing nip N therebetween through which the recording medium P bearing the toner image Tn is conveyed.

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