Image forming apparatus with induction heating type fixing unit

A fixing unit of an image forming apparatus includes a coil arranged along an outer surface of the heating member and generating a magnetic field, a first core arranged opposite the heating member with respect to the coil and forming a magnetic path, a second core so fixed between the first core and the heating member with respect to a direction in which the coil generates the magnetic field, as to form the magnetic path together with the first core, a shielding member positioned outward of the second core and shielding the magnetism in the magnetic path, and a magnetism adjusting unit moving the shielding member outward of the second core to switch the position of the shielding member between a shielding position where the shielding member shields the pass of the magnetism and a retracted position where the shielding member permits the pass of the magnetism.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus including a fixing unit which is configured to fix a toner image to a sheet by fusing the unfixed toner while the sheet is passed through a nip between a pair of heated rollers or between a heating belt and a roller.

2. Description of the Related Art

In the aforementioned kind of image forming apparatus, fixing belt systems attract attention due to growing demand for a reduction in warm-up time of a fixing unit and energy savings in recent years. This is because a fixing belt has a low heat capacity as mentioned in Japanese Unexamined Patent Publication No. 6-318001, for example. Also attracting attention recently is electromagnetic induction heating (IH) technology which offers a high-speed, high-efficiency heating capability. Today, products developed by using a combination of the IH technology and belt systems for achieving energy savings in a process of fusing color toner images are available in large quantities on the market. An arrangement widely used combining the IH technology and belt systems is to dispose an induction heating element on the outside of the heating belt (known as an external IH system). The external IH system is often used because this arrangement provides such advantages as ease of layout and cooling of an induction coil and a capability to directly heat the heating belt.

In practical applications of the IH technology, there exist various arrangements devised for preventing overheating of non-sheet passing areas of a fixing roller of a fixing unit according to the width (sheet passing width) of each sheet of paper passed through the fixing unit. For example, Japanese Unexamined Patent Publication No. 2003-107941 and Japanese Patent No. 3527442 introduce means for altering a heated area of a fixing roller according to the sheet passing width. These means of the prior art (hereinafter referred to as first and second prior art arrangements) intended particularly for external induction heating are configured as briefly described hereunder.

The first prior art arrangement shown in Japanese Unexamined Patent Publication No. 2003-107941 applied to a fixing unit includes a magnetic member, an exciting coil and a moving mechanism. The magnetic member is divided into a plurality of pieces which are arranged along a sheet passing width direction, and the moving mechanism moves part of the magnetic member toward and away from the exciting coil according to the width of each sheet passed through the fixing unit. It is supposed that an effect of this arrangement is to decrease heating efficiency in a non-sheet passing area by separating the magnetic member from the exciting coil, thus reducing the amount of heat generated in the non-sheet passing area than in an area corresponding to a minimum sheet passing width.

The second prior art arrangement shown in Japanese Patent No. 3527442 applied to a fixing unit is such that an additional electrically conductive member is disposed within a heating roller in an area outside a minimum sheet passing width, wherein this electrically conductive member is made movable between a position within a range of a magnetic field and a position outside the range of the magnetic field. In this prior art arrangement, the heating roller is preheated by induction heating with the electrically conductive member initially arranged outside the range of the magnetic field. When the heating roller is heated almost up to the Curie temperature, the electrically conductive member is moved to the outside of the range of the magnetic field, causing magnetic flux to leak from the heating roller outside the minimum sheet passing width to prevent overheating.

In the first prior art arrangement, the magnetic member should have a large movable range, so that this arrangement has a problem that the entirety of the fixing unit becomes unnecessarily large. On the other hand, the second prior art arrangement offers a space-saving capability because means for altering a heated area is provided in an internal space of the heating roller. The internal space of the heating roller is however a high-temperature environment. Therefore, if some kind of component is mounted inside the heating roller, it is necessary to increase the Curie temperature of the heating roller and, in addition, there arises a problem that the provision of a large-sized component having a large heat capacity within the heating roller causes an increase in warm-up time thereof.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a technique which makes it possible to reduce the number of components mounted within a heating element of a fixing unit of an image forming apparatus, thereby lowering total heat capacity and achieving a reduction in warm-up time of the fixing unit and space savings.

To accomplish the aforementioned object of the invention, an image forming apparatus includes an image forming section for forming a toner image and transferring the toner image onto a sheet, and a fixing unit including a heating member and a pressing member, and fixing the toner image onto the sheet while nipping and conveying the sheet between the heating member and the pressing member. The fixing unit further includes a coil arranged along an outer surface of the heating member and generating a magnetic field, a first core arranged opposite the heating member with respect to the coil and forming a magnetic path, a second core so fixed between the first core and the heating member with respect to a direction in which the coil generates the magnetic field, as to form the magnetic path together with the first core, a shielding member positioned outward of the second core and shielding the magnetism in the magnetic path, and a magnetism adjusting unit moving the shielding member outward of the second core to switch the position of the shielding member between a shielding position where the shielding member shields the pass of the magnetism and a retracted position where the shielding member permits the pass of the magnetism.

These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed description in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a preferred embodiment of the invention is described in detail with reference to the accompanying drawings.

FIG. 1is a schematic cross-sectional diagram showing the structure of an image forming apparatus1according to the preferred embodiment of the invention. The image forming apparatus1may be a printer, a copying machine, a facsimile machine or a hybrid apparatus thereof which are configured to perform printing operation by forming a toner image based on externally input image information, for instance, and transferring the toner image to a surface of a printing medium like a sheet of paper.

The image forming apparatus1shown inFIG. 1is a tandem-type color printer, for example. The image forming apparatus1includes a generally boxlike apparatus body2incorporating a print engine for forming (printing) a color image on a sheet and a sheet output portion (output tray)3arranged at the top of the apparatus body2where the sheet carrying the printed color image is output.

Referring toFIG. 1, provided at a lower part of the apparatus body2is a paper cassette5for holding a stack of sheets and provided on one side of the apparatus body2is a manual feed tray6on which a plurality of sheets can be placed for manually feeding one sheet after another. Incorporated in an upper part of an internal space of the apparatus body2is an image forming section7which forms an image on the sheet based on image data containing text and graphics data fed from an external source, for instance.

As illustrated inFIG. 1, there is provided a first paper path9on a left side of the apparatus body2for feeding each successive sheet supplied from the paper cassette5to the image forming section7. Also, there is provided a second paper path10extending from a right side of the apparatus body2to the left side thereof for manually feeding the sheet from the manual feed tray6to the image forming section7. Provided in an upper left part (as illustrated) of the internal space of the apparatus body2are a fixing unit14for performing fixing operation on the sheet carrying the image formed in the image forming section7and a third paper path11through which the sheet carrying the fixed image is conveyed to the sheet output portion3.

A user can replenish the stack of sheets in the paper cassette5by pulling the paper cassette5out of the apparatus body2(frontward as shown inFIG. 1). The paper cassette5has a boxlike compartment16for selectively storing at least two kinds of sheets having different sizes in a sheet passing direction. An uppermost one of the sheets stored in the paper cassette5is picked up and fed into the first paper path9one after another by a pickup roller17and a double feed preventing roller18.

The manual feed tray6is made swingable outward from a side surface of the apparatus body2and back to a vertical position. The manual feed tray6has a tray top19on which the user can place one or a plurality of sheets at a time for manual feeding one sheet after another. Each sheet placed on the tray top19is successively picked up and fed into the second paper path10by a pickup roller20and a double feed preventing roller21.

The first paper path9and the second paper path10join into a single path slightly upstream of a pair of registration rollers22. The sheet which has reached a position immediately upstream of the registration rollers22is kept standby for a while where adjustments for removing a skew (oblique feed) of the sheet and taking precise feed timing thereof are made. After these adjustments, the registration rollers22feed the sheet to a secondary image transfer portion23for transferring a full-color toner image formed on an intermediate image transfer belt40to the sheet. The sheet is then advanced to the fixing unit14to fix the toner image to the sheet. In the case of two-sided printing (or duplexing), the sheet carrying the full-color toner image fixed in the fixing unit14on one side is reversed in a switchback fashion and returned to the position upstream of the registration rollers22through a fourth paper path12for transferring a full-color toner image on the opposite side of the sheet. After the toner image on the opposite side of the sheet is fixed thereto in the fixing unit14, the sheet is conveyed through the third paper path11and ejected to the sheet output portion3by means of a pair of output rollers24.

The image forming section7includes four image forming units26-29for forming black (B), yellow (Y), cyan (C) and magenta (M) toner images, respectively, and an intermediate image transfer mechanism30for carrying the toner images in four colors (including black) formed by the individual image forming units26-29, wherein the four color toner images are superimposed one on top of another.

As shown inFIG. 1, the four image forming units26-29each include a photosensitive drum32, a charging unit33disposed face to face with a curved outer surface of the photosensitive drum32, a development unit35disposed face to face with the curved outer surface of the photosensitive drum32, a cleaning unit36disposed downstream of the development unit35face to face with the curved outer surface of the photosensitive drum32. Additionally, the four image forming units26-29is provided with a laser scanner34disposed downstream of each charging unit33for projecting a laser beam along specified positions on the curved outer surface of each photosensitive drum32. The development unit35of each of the image forming units26-29is arranged at a position downstream of the aforementioned positions scanned by the laser beam emitted from the laser scanner34.

Although not shown inFIG. 1, the photosensitive drums32of the four image forming units26-29are driven by individual driving motors to rotate in a counterclockwise direction as illustrated. The development units35of the image forming units26-29include toner boxes51containing black, yellow, cyan and magenta toners, respectively.

Referring toFIG. 1, the intermediate image transfer mechanism30includes a driving roller38arranged close to the black image forming unit26, a driven roller39arranged close to the magenta image forming unit29, the aforementioned intermediate image transfer belt40mounted between the driving roller38and the driven roller39, and four image transfer rollers41arranged at positions downstream of the development units35with respect to the counterclockwise turning direction of the respective image forming units26-29such that the image transfer rollers41can be pressed against the respective photosensitive drums32via the intermediate image transfer belt40.

The working of the intermediate image transfer mechanism30is such that the four color toner images (including the black toner image) are transferred to the intermediate image transfer belt40one on top of another at locations of the image transfer rollers41of the respective image forming units26-29to form a full-color toner image.

The first paper path9serves to convey the sheet supplied from the paper cassette5toward the intermediate image transfer mechanism30. The first paper path9is associated with a plurality of convey rollers43arranged at specific positions within the apparatus body2and the aforementioned registration rollers22which are provided upstream of the intermediate image transfer mechanism30for establishing correct timings of image forming and sheet convey operations performed by the image forming section7.

The fixing unit14performs the fixing operation to fix an unfixed toner image to the sheet by applying heat and pressure to the sheet carrying the toner image transferred thereto in the image forming section7. The fixing unit14is provided with a heatable roller pair including a pressing roller44and a fixing roller45, for example. The pressing roller44has a metallic core member and a surface layer made of elastic material (e.g., silicone rubber) whereas the fixing roller45has a metallic core member, a surface layer made of elastic material (e.g., silicone sponge) and a releasing layer made of perfluoroalkoxy (PFA), for instance. The fixing unit14is also provided with a heat roller46arranged adjacent to the fixing roller45as well as a heating belt48mounted between the fixing roller45and the heat roller46. The structure of the fixing unit14will be described later in greater detail.

There are provided upstream and downstream paper paths47on upstream and downstream sides of the fixing unit14with respect to a sheet feeding direction. The sheet conveyed through the intermediate image transfer mechanism30is introduced into a nip between the pressing roller44and the fixing roller45through the upstream paper path47. Then, the sheet which has passed between the pressing roller44and the fixing roller45is guided to the third paper path11through the downstream paper path47.

The third paper path11conveys the sheet carrying the toner image fixed thereto in the fixing unit14to the sheet output portion3. The third paper path11is provided with convey rollers49arranged at appropriate positions as well as the aforementioned output rollers24arranged at an output end of the third paper path11.

<Detailed Structure of Fixing Unit>

Now, the structure of the fixing unit14of the image forming apparatus1of the present embodiment is described in detail.

FIG. 2is a vertical cross-sectional diagram showing an example of the structure of the fixing unit14, in which the fixing unit14is shown in a position rotated counterclockwise by about 90 degrees from a position actually mounted in the image forming apparatus1. Therefore, the sheet feeding direction going upward from the lower part of the apparatus body2as illustrated inFIG. 1points from right to left inFIG. 2. It is to be noted that if the apparatus body2of the image forming apparatus1is large-sized (in the case of a hybrid machine, for example), there can be a case where the fixing unit14is installed in the position (direction) shown inFIG. 2.

The fixing unit14is provided with the pressing roller44, the fixing roller45, the heat roller46and the heating belt48as stated above. The sheet of paper having the toner image transferred thereon is nipped and conveyed between the pressing roller44and the heating belt48. At this time, the sheet of paper receives heat from the heating belt48and the toner image is fixed on the sheet of paper. The heating belt48has a sheet-conveyed region so set thereon that the sheet of paper of maximum size conveyable to the fixing unit14is brought into contact with the sheet-conveyed region. Since the fixing roller45has the surface layer made of the elastic material (e.g., silicone sponge) as mentioned above, there is formed a flat nip between the heating belt48and the fixing roller45.

The heating belt48employs a ferromagnetic substance (e.g., nickel) as a base material and has a surface layer made of elastic material (e.g., silicone rubber) of which outside is covered with a coating of releasing agent (e.g., PFA). If the heating belt48is not required to have a heating function, the heating belt48may be a simple resin belt made of polyimide (PI), for instance. The heat roller46has a metallic core member made of magnetic metal (e.g., iron or stainless steel) of which outer surface is covered with a coating of releasing agent (e.g., PFA).

More specifically, the pressing roller44employs such material as iron or aluminum as the metallic core member and has a silicone rubber layer covering the metallic core member as well as a fluoroplastic layer formed on an outer surface of the silicone rubber layer. The pressing roller44may be configured to incorporate a halogen heater44ain an internal space as illustrated, for instance.

Additionally, the fixing unit14is provided with an IH coil unit50(not shown inFIG. 1) arranged outside the heat roller46and the heating belt48. The IH coil unit50is configured with an induction heating coil52, a pair of arch cores54, a pair of side cores56and a center core58.

The fixing unit14shown in the example ofFIG. 2is configured such that induction heating is performed on the heat roller46and arc-shaped portions of the heating belt48over the substantially entire region of the heating belt48in the width direction thereof. Therefore, the induction heating coil52is arranged on an outer surface segment of an imaginary cylinder. In actuality, there is provided a plastic bobbin (not shown) outside the heat roller46and the heating belt48and the induction heating coil52is wound on this unillustrated bobbin which is formed into a semicylindrical shape disposed along a curved outer surface of the heat roller46. Preferably, the bobbin is made of a heat-resistant resin material, such as polyphenylene sulfide (PPS), polyethylene terephthalate (PET) or liquid crystal plastic (LCP).

As shown inFIG. 2, the center core58is arranged at a middle position while the aforementioned arch cores54and side cores56are arranged in pairs on both sides of the center core58. Among the arch cores54and the side cores56, the arch cores54on both sides of the center core58are ferrite cores (first cores) formed into a symmetrical arch-like shape in cross section, each of the arch cores54having an overall length longer than a winding area of the induction heating coil52. Also, the side cores56on both sides of the center core58are ferrite cores (first cores) formed into a block-like shape. The side cores56on both sides are connected to extreme ends (lower ends as shown inFIG. 2) of the respective arch cores54, covering the outside of the winding area of the induction heating coil52. The arch cores54are divided into plural core segments which are arranged at specific intervals along a longitudinal direction of the heat roller46, for example. On the other hand, each of the side cores56is a single (undivided) core segment arranged straight along the longitudinal direction of the heat roller46, the side cores56having an overall length corresponding to the length of the winding area of the induction heating coil52.

The arrangement of these cores54,56is determined in accordance with a distribution of magnetic flux density (magnetic field strength) produced by the induction heating coil52, for instance. As the core segments of the arch cores54are arranged at specific intervals as mentioned above, the side cores56make up for an effect of magnetic focusing in regions where no core segments of the arch cores54are present, thereby equalizing the magnetic flux density distribution along the longitudinal direction of the heat roller46. Outside the arch cores54and the side cores56, there is provided an unillustrated plastic core holder, for example, which supports the arch cores54and the side cores56. Preferably, the core holder is also made of a heat-resistant resin material, such as PPS, PET or LCP.

In the illustrated example ofFIG. 2, the heat roller46incorporates a thermistor62which may be arranged at a position where a large amount of heat is generated especially by induction heating within the heat roller46. Additionally, there may be provided a thermostat inside the heat roller46to achieve improved safety in the event of an abnormal temperature increase.

The aforementioned center core58is a ferrite core (second core) having a generally T-shape in cross section, for instance. Generally like the heat roller46, the center core58has a length corresponding to a maximum sheet passing width. The center core58is fixedly mounted between the arch cores54and the side cores56on both sides (or halfway in a magnetic path produced by the induction heating coil52). Although not illustrated inFIG. 2, the center core58is supported by the aforementioned plastic core holder.

A shielding member60is arranged outward of the center core58along an outer periphery of the center core58. The shielding member60is constituted by a thin-plate formed by bending in an arcuate shape. The shielding member60is supported by an unillustrated rotation mechanism out of contact with the center core58in a manner that the shielding member60can be rotated along the outer periphery of the center core58by the rotation mechanism in an arrow direction shown inFIG. 2. How the shielding member60is supported and how the aforementioned rotation mechanism is structured will be discussed later in further detail.

Preferably, the shielding member60is made of a nonmagnetic, good conductor like oxygen-free copper, for example. As a magnetic field penetrates the shielding member60at right angles to a surface thereof, an induction current is induced in the shielding member60. The induction current produces a magnetic field oriented in a direction opposite to the magnetic field applied to the shielding member60, canceling out interlinkage of magnetic flux (i.e., the perpendicularly penetrating magnetic field) and thus shielding the applied magnetic field. Also, as the good conductor is used in the shielding member60, it is possible to suppress generation of Joule heat by the induction current and efficiently shield the magnetic field. Electrical conductivity of the conductor used in the shielding member60can effectively be improved by (1) selecting a material having as low a resistivity as possible and/or (2) using a plate-like member having a large thickness, for instance. Specifically, the thickness of the shielding member60should preferably be equal to or larger than 0.5 mm. The shielding member60used in this embodiment is 1 mm thick.

If the shielding member60is at a position in the proximity of an outer surface of the heating belt48(i.e., at a shielding position) as shown inFIG. 2, magnetic reluctance increases in an area surrounding the induction heating coil52, causing a reduction in magnetic field strength. If the shielding member60is rotated by 180 degrees either clockwise or counterclockwise from the position shown inFIG. 2to a position farthest away from the heating belt48(i.e., at a retracted position), On the other hand, the magnetic reluctance decreases in the area surrounding the induction heating coil52and there is formed a magnetic path routed from the center core58through the arch cores54and the side cores56on both sides of the center core58. Consequently, the magnetic field acts on the heating belt48and the heat roller46.

FIGS. 3A and 3Bare perspective views showing exemplary structure (1) of a shielding member60,FIG. 3Ashowing the shielding member60at the retracted position as seen obliquely downward andFIG. 3Bshowing the same as seen obliquely upward. The shielding member60is configured chiefly with a shielding plate61forming a curved surface and a fan-shaped side plate63. The curvature of the shielding plate61is determined such that the shielding member60can be rotated around the outer periphery of the center core58. The side plate63is affixed to the inside of the shielding plate61at one end thereof and connected to a driving shaft70arranged at an apex of the fan-shaped side plate63. A central axis of the driving shaft70coincides with the center of curvature of the shielding plate61. When the driving shaft70is rotated by motive power produced by an unillustrated motor, the shielding member60is caused to turn about the central axis together with the driving shaft70. While the shielding member60of the embodiment shown inFIGS. 3A and 3Bhas a uniform width (in a portion of the shielding plate61) along a longitudinal direction, this structure may be modified such that the width of the shielding member60varies along the longitudinal direction as will be discussed in the following.

FIGS. 4A and 4Bare diagrams showing a shielding member60whose width is varied along the longitudinal direction as well as an example of an arrangement of this shielding member60,FIG. 4Ashowing the shielding member60arranged at the shielding position andFIG. 4Bshowing the shielding member60arranged at the retracted position.FIGS. 4A and 4Beach show a side view and a plan view of the center core58, respectively, in which outer surfaces of the center core58are shown by halftone dots.

The center core58has an overall length generally equal to or larger than the maximum sheet passing width W2as mentioned above. The shielding member60is divided into two portions along the longitudinal direction of the center core58, the two portions of the shielding member60being symmetrically shaped with respect to each other. The two divided portions of the shielding member60each have a trapezoidal shape in plan view as shown inFIGS. 4A and 4B. As can be seen from there Figures, the length of the shielding member60measured along a circumferential direction (or the width measured along the longitudinal direction) is the smallest in an area close to a mid-length part of the center core58and the length of the shielding member60measured along the circumferential direction thereof gradually increases toward both ends of the center core58.

Major parts of the two divided portions of the shielding member60are arranged on both outsides of a minimum sheet passing width W1which is perpendicular to a sheet passing direction, and only little parts of the two divided portions of the shielding member60extend into an area of the minimum sheet passing width W1. The two divided portions of the shielding member60reach slightly outward beyond the maximum sheet passing width W2at both ends of the center core58as illustrated. It is to be noted that the minimum sheet passing width W1and the maximum sheet passing width W2are determined according to minimum and maximum printable paper size of the image forming apparatus1.

As will be recognized from the foregoing discussion, the ratio of the length of the shielding member60measured along the circumferential direction to the entire length of the circumference along which the shielding member60is rotated varies along a sheet passing width direction in the present embodiment. The ratio of the length (Lc) of the shielding member60measured along the circumferential direction to the length (L) of one complete turn of the shielding member60is hereinafter referred to as a shielding ratio (=Lc/L). It is apparent from above that this shielding ratio (=Lc/L) is small in regions of the center core58closer to the mid-length part thereof and becomes gradually larger outward toward both ends of the center core58along the sheet passing width direction. Specifically, the shielding ratio is minimized in the proximity of outer ends of a minimum sheet-conveyed region (i.e., the range of minimum sheet passing width W1) and is maximized at both ends of the center core58.

The fixing unit14is adapted to different paper sizes (sheet passing widths) by varying the position of the shielding member60in a continuous or stepwise fashion to partly suppress the value of magnetic flux produced. As an example, the angular position (or the amount of angular displacement) of the shielding member60is varied according to the paper size. Specifically, the shielding member60is adjusted such that the larger the paper size, the smaller the amount of magnetic flux shielded by the shielding member60, and on the contrary, the smaller the paper size, the larger the amount of magnetic flux shielded by the shielding member60, in order to prevent overheating of both lateral end portions of the heat roller46and the heating belt48. WhileFIGS. 4A and 4Bshow counterclockwise and clockwise turning directions of the shielding member60by arrow, respectively, the fourth paper path12may be configured such that the shielding member60is allowed to turn in one direction only. Additionally, the sheet passing direction may be opposite to that shown inFIGS. 4A and 4B.

Described next with reference toFIGS. 5A and 5Bis how the aforementioned rotation mechanism for rotating the shielding member60on the outside of the center core58is structured.FIG. 5Ais a side view showing the structure of the rotation mechanism64for rotating the shielding member60andFIG. 5Bis a cross-sectional view taken along lines B-B ofFIG. 5Ashowing the working of the shielding member60. It is to be noted that the rotation mechanism64constitutes a magnetism adjusting unit.

As shown inFIG. 5A, the rotation mechanism64is structured to reduce rotation speed of a stepping motor66by means of a reducer mechanism68, for example, to drive the driving shaft70to rotate the shielding member60. While the reducer mechanism68of this embodiment employs a worm gear, for example, the reducer mechanism68may be otherwise structured as appropriate. The driving shaft70is fitted with a slit disk72at an extreme end as shown inFIG. 5Aas illustrated. The slit disk72is combined with a photointerrupter74for detecting the angular position (or the amount of angular displacement from a reference position) of the shielding member60.

Referring toFIG. 5A, the driving shaft70is connected to the side plate63of the shielding member60as previously mentioned and supports the entirety of the shielding member60including the shielding plate61via the side plate63. The angular position of the shielding member60can be controlled by the number of driving pulses applied to the stepping motor66, for instance. The rotation mechanism64is associated with a control circuit (not shown) for performing this control operation. The control circuit can be configured with such devices as a controller integrated circuit (IC), an input/output driver and a semiconductor memory. A sensing signal from the photointerrupter74is input into the controller IC through the input/output driver, and the controller IC detects the current angular position of the shielding member60based on this sensing signal. On the other hand, an unillustrated image forming control unit notifies the controller IC of information concerning a current paper size. On receiving this information, the controller IC reads out information about the angular position of the shielding member60suited to the current paper size from the semiconductor memory which is a read-only memory (ROM) and outputs a particular number of driving pulses required for the shielding member60to reach the aimed angular position at regular intervals. These driving pulses are applied to the stepping motor66via the input/output driver, causing the stepping motor66to operate accordingly.

FIGS. 6A and 6Bare diagrams showing examples of operation performed as a result of rotating action of the shielding member60. These examples of operation are individually described below.

FIG. 6Ashows the example of operation performed when the shielding member60is switched to the retracted position by the rotation mechanism64. In this case, the magnetic field produced by the induction heating coil52passes through the heating belt48and the heat roller46by way of the side cores56, the arch cores54and the center core58. Consequently, eddy currents flow in the heat roller46and the heating belt48made of the ferromagnetic substance, so that the heat roller46and the heating belt48are heated by Joule heat generated due to resistivities of the respective materials.

FIG. 6Bshows the example of operation performed when the shielding member60is switched to the shielding position by the rotation mechanism64. In this case, part of the shielding member60exists in the magnetic path outside the minimum sheet-conveyed region, so that generation of the magnetic field is partly suppressed. This serves to reduce the amount of heat generated outside the minimum sheet-conveyed region, thereby preventing overheating of the heat roller46and the heating belt48. Moreover, it is possible to adjust the amount of magnetic flux (magnetic field) shielded by the shielding member60by varying the angular position of the shielding member60little by little. If the angular position of the shielding member60is increased in small steps by rotating the shielding member60in the counterclockwise direction from the position shown inFIG. 6B, for example, the magnetic field becomes gradually not shielded on a left side of the fixing unit14but the shielding member60continues to shield the magnetic field on a right side of the fixing unit14. Compared to the example ofFIG. 6Ain which the shielding member60is in the retracted position, the magnetic field strength is decreased as a whole so that the amount of heat generated can be lowered.

FIG. 7is a perspective view showing exemplary structure (2) of a generally ring-shaped shielding member60which has four sides including a pair of straight segments60aarranged on opposite sides in a width direction and a pair of ring-shaped portions60barranged on opposite sides in the longitudinal direction. As in the shielding member60with exemplary structure (1) described above, the ring-shaped shielding member60is mounted such that portions of the shielding member60are arranged on the outside of the minimum sheet passing width at both ends of the center core58.

The shielding member60with this exemplary structure (2) is supported by a supporting member65at one longitudinal end, for instance. The supporting member65is configured with a fan-shaped side plate65aand an arc-shaped top plate65b, for example, the top plate65bbeing connected to one of the ring-shaped portions60balong a bottom side thereof. The side plate65aextends downward from the top plate65bas illustrated inFIG. 7and has an apex to which the aforementioned driving shaft70is connected. The shielding member60with this exemplary structure (2) is provided with a rotation mechanism64which is identical to the rotation mechanism64of the foregoing exemplary structure (1) of the shielding member60.

21Principle of Magnetic Shielding Effect>

FIGS. 8A,8B and8C are conceptual drawings explaining the principle of magnetic shielding effect produced by the ring-shaped shielding member60. In these Figures, the shielding member60is shown in a simplified form using a wire frame model.

Referring toFIG. 8A, if a magnetic field passes through or penetrates a ring surface of the ring-shaped shielding member60in a direction perpendicular to the ring surface (imaginary plane), producing interlinkage flux, an induction current flows within the shielding member60in a circumferential direction thereof. As a result, due to electromagnetic induction, a magnetic field directed opposite to the penetrating magnetic field is induced. The applied penetrating magnetic field and the induced oppositely directed magnetic field cancel each other out entirely. It will be appreciated from above that the ring-shaped shielding member60can shield the magnetic field (magnetic flux) by using the aforementioned magnetic field cancellation effect.

It is now assumed that magnetic fields directed in two opposite directions penetrate the ring surface of the ring-shaped shielding member60as shown in an upper part ofFIG. 8Band the sum of interlinkage flux is generally zero (±0). In this case, almost no induction current flows within the shielding member60so that the shielding member60does not produce any significant magnetic field cancellation effect and, thus, the magnetic fields directed in the two opposite directions pass through the shielding member60. The same situation also occurs when a magnetic field passes through the inside of the shielding member60in a U-shaped pattern as shown in a lower part ofFIG. 8B. When the shielding member60is in the retracted position, the magnetic field is allowed to pass through with the shielding member60arranged at a position where the magnetic field does not penetrate the shielding member60.

Shown inFIG. 8Cis a case where a magnetic field (interlinkage flux) is directed generally parallel to the ring surface of the ring-shaped shielding member60. In this case, almost no induction current flows within the shielding member60as in the case ofFIG. 8Bso that the shielding member60does not produce any significant magnetic field cancellation effect. Although this structure is not employed in the present embodiment, it is necessary to greatly displace the shielding member60in order to produce a magnetic field environment in a surrounding area of the induction heating coil52, thus requiring a large movable space for the shielding member60.

The aforementioned exemplary structure (2) employing the ring-shaped shielding member60produces the magnetic shielding effect due to the principle shown inFIG. 8A. Therefore, as is the case with the examples shown inFIGS. 6A and 6B, it is possible to shield the magnetic field (magnetic flux) in an optimal fashion as in exemplary structure (1) described above by displacing the ring-shaped shielding member60between the shielding position and the retracted position.

FIG. 9is a perspective view showing exemplary structure (3) of a shielding member60which is formed into a reel-like shape as a whole. Specifically, the shielding member60of this exemplary structure (3) has a pair of ring segments60cat both longitudinal ends and three straight segments60ainterconnecting the two ring segments60c. The three straight segments60aof the shielding member60are arranged at specific intervals in a circumferential direction of the ring segments60c. In this exemplary structure (3), a circular side plate67is affixed to the inside of one of the ring segments60cand the driving shaft70is connected to the side plate67at a central position thereof, whereby the entirety of the shielding member60is supported by the driving shaft70rotatably therewith. As in the aforementioned exemplary structures (1) and (2), portions of the shielding member60are arranged on the outside of the minimum sheet passing width at both ends of the center core58in this exemplary structure (3) as well.

In this exemplary structure (3) of the shielding member60, a ring-shaped portion (arch-like segment) is formed in three in a circumferential direction of the shielding member60with three ring surfaces defined by those ring-shaped portions. Specifically, the three straight segments60aadjoining in the circumferential direction are so connected to the pair of the ring segments60cthat the shielding member60has the three ring-shaped portions in the circumferential direction.

<Working of Exemplary Structure (3)>

FIGS. 10A and 10Bare diagrams showing examples of operation of the shielding member60in exemplary structure (3) discussed above.

FIG. 10Ashows the example of operation performed when the shielding member60is switched to the retracted position by the rotation mechanism64. In the case of exemplary structure (3), the principle shown in the lower part ofFIG. 8Ais applied under conditions where the shielding member60is set at the retracted position. Specifically, with one of the three straight segments60aof the shielding member60aligned with a center line of the induction heating coil52, the ring-shaped portion of the shielding member60arranged on an opposite side (upper side as illustrated) of the heat roller46is retracted to the outside of the magnetic field and the magnetic field is caused to pass through the inside of the other two ring-shaped portions in a U-shaped pattern, thereby creating a state in which the shielding member60does not produce the magnetic shielding effect. Therefore, the magnetic field passes through the heating belt48and the heat roller46by way of the side cores56, the arch cores54and the center core58. Consequently, eddy currents flow in the heat roller46and the heating belt48made of the ferromagnetic substance, so that the heat roller46and the heating belt48are heated by Joule heat generated due to resistivities of the respective materials.

FIG. 10Bshows the example of operation performed when the shielding member60is switched to the shielding position. In this case, one of the ring-shaped portions of the shielding member60exists in the magnetic path outside the minimum sheet-conveyed region and the magnetic field passes through the inside of the pertinent ring-shaped portion, so that generation of the magnetic field is partly suppressed due to the principle shown inFIG. 8A. This serves to reduce the amount of heat generated outside the minimum sheet-conveyed region, thereby preventing overheating of the heat roller46and the heating belt48.

FIG. 11is a perspective view showing exemplary structure (4) of a shielding member60which has a structure further developed from the above-described exemplary structure (3). Specifically, the shielding member60of this exemplary structure (4) has a ring-shaped plate60A at one longitudinal end and another ring-shaped plate60B at a particular distance from the shielding member60A in the longitudinal direction of the shielding member60. The shielding member60further has an approximately two-third ring-shaped plate60C at a particular distance from the shielding member60B in the longitudinal direction of the shielding member60and an approximately one-third ring-shaped plate60D at the opposite longitudinal end of the shielding member60. Although not illustrated inFIG. 11, a circular side plate67is affixed to the ring-shaped plate60A at one longitudinal end of the shielding member60and the driving shaft70is connected to the side plate67as in the aforementioned exemplary structure (3).

Among the aforementioned plates60A,60B,60C,60D, the first three plates60A,60B,60C are interconnected by three straight segments60aof the shielding member60, while the plate60D at the aforementioned opposite longitudinal end of the shielding member60is connected to the adjacent plate60C by two of the straight segments60a.

FIG. 12Ashows a side view and a plan view of the center core58, illustrating in particular a state in which the shielding member60of exemplary structure (4) is arranged such that portions of the shielding member60are arranged at opposite end portions of the center core58.FIGS. 12B,12C and12D are cross-sectional diagrams taken along lines B-B, C-C and D-D ofFIG. 12A, respectively.

As shown inFIG. 12A, the shielding member60of exemplary structure (4) also has portions arranged at both longitudinal ends of the center core58(although only one longitudinal end thereof is shown inFIG. 12A). Referring toFIG. 12A, the plate60A arranged farthest away from the minimum sheet-conveyed region is at a position corresponding to a maximum paper size P1 (e.g., A3 or A4R size). Similarly, the plate60B arranged next to the plate60A is at a position corresponding to a medium paper size P2 (e.g., B4R size), and the plate60C arranged next to the plate60B is at a position corresponding to a medium/small paper size P3 (e.g., B4 size). Finally, the plate60D arranged in the vicinity of the minimum sheet-conveyed region is at a position corresponding to a minimum paper size P4 (e.g., A5R size).

It is seen fromFIG. 12Bthat the plates60A and60B of the shielding member60are ring-shaped pieces, each having a vacant circular center. Also, it is seen fromFIG. 12Cthat the plate60C of the shielding member60is an approximately two-third ring-shaped member whose one-third ring-shaped empty part is a vacant space unoccupied by nonmagnetic material of the plate60C.

Additionally, it is seen fromFIG. 12Dthat the plate60D of the shielding member60is an approximately one-third ring-shaped member whose two-third ring-shaped empty part is a vacant space unoccupied by nonmagnetic material of the plate60D.

<Working of Exemplary Structure (4)>

Examples of operation of the shielding member60in exemplary structure (4) are described with reference toFIGS. 13 to 18which are perspective views showing six different situations which may occur when the shielding member60of exemplary structure (4) is used. Arrows shown in bold lines inFIGS. 13 to 18each represent an induction current produced or a magnetic field passing through the shielding member60. It is to be noted that members like the side plate67and the driving shaft70are not shown in these Figures. The individual examples of operation of the shielding member60are now described hereinbelow.

FIG. 13is a perspective view showing the example of operation performed when the magnetic field is entirely shielded by the shielding member60. It is assumed in the following discussion of the examples of operation that the magnetic field is produced in a direction penetrating the shielding member60from top to bottom. Also, in the following discussion, the angular position of the shielding member60shown inFIG. 13in which the magnetic field is entirely shielded is regarded as 0 degrees and the amount of angular displacement of the shielding member60is expressed in terms of the rotation angle of the shielding member60from the 0-degree position.

If the shielding member60is rotated to the angular position of 0 degrees at which the plate60D is at the bottom of the shielding member60, it is possible for the shielding member60to produce the magnetic shielding effect over an entire surface area along the longitudinal direction of the shielding member60. Specifically, the plate60A at one longitudinal end of the shielding member60, the plate60D at the opposite longitudinal end thereof and the straight segments60ainterconnecting the plates60A and60B together form an ring-shaped portion having a maximum size of which entirety can be used for shielding the magnetic field. In this case, it is possible to prevent overheating of the heat roller46and the heating belt48in a region corresponding to the minimum paper size P4.

FIG. 14is a perspective view showing the example of operation performed when the shielding member60is rotated clockwise by 60 degrees from the angular position shown inFIG. 13. In this case, one of the straight segments60aof the shielding member60is aligned with the center line of the induction heating coil52(as shown inFIG. 8A), so that the shielding member60is at the retracted position and does not produce any magnetic shielding effect.

FIG. 15is a perspective view showing the example of operation performed when the shielding member60is rotated clockwise by 120 degrees from the angular position shown inFIG. 13. In this case, it is possible for the shielding member60to produce the magnetic shielding effect by an ring-shaped portion formed between the plates60A and60B. This example of operation can prevent overheating of the heat roller46and the heating belt48in a region corresponding to the medium/small paper size P3, for example.

FIG. 16is a perspective view showing the example of operation performed when the shielding member60is rotated clockwise by 180 degrees from the angular position shown inFIG. 13. In this case, one of the straight segments60aof the shielding member60is aligned with the center line of the induction heating coil52(as shown inFIG. 8A) as in the example ofFIG. 14, so that the shielding member60is at the retracted position and does not produce any magnetic shielding effect.

<Magnetic Shielding for Medium Size at 240° Position>

FIG. 17is a perspective view showing the example of operation performed when the shielding member60is rotated clockwise by 240 degrees from the angular position shown inFIG. 13. In this case, it is possible for the shielding member60to produce the magnetic shielding effect by an ring-shaped portion formed between the plates60A and60B. This example of operation can prevent overheating of the heat roller46and the heating belt48in a region corresponding to the medium paper size P2, for example.

FIG. 18is a perspective view showing the example of operation performed when the shielding member60is rotated clockwise by 300 degrees from the angular position shown inFIG. 13. In this case, one of the straight segments60aof the shielding member60is aligned with the center line of the induction heating coil52(as shown inFIG. 8A) as in the example ofFIGS. 14 and 16, so that the shielding member60is at the retracted position and does not produce any magnetic shielding effect. It is to be noted that in the cases where no magnetic shielding is produced with the shielding member60set at the angular position of 60, 180 or 300 degrees from the angular position shown inFIG. 13, this example of operation can prevent overheating of the heat roller46and the heating belt48in a region corresponding to the maximum paper size P1.

FIG. 19is a diagram showing another exemplary structure of a fixing unit14configured to fix the toner image by a combination of a pressing roller44and a fixing roller45without using the earlier-described heating belt. This fixing unit14is configured such that the same magnetic material as used for forming the aforementioned heating belt48is wound around a curved outer surface of the fixing roller45and a layer of the magnetic material is heated by the induction heating coil52. In this exemplary structure, the thermistor62is mounted on the outside of the fixing roller45at a position facing the magnetic material layer. WhileFIG. 19shows the shielding member60of exemplary structures (3) and (4) described earlier, the shielding member60of exemplary structure (1) or (2) may be adopted instead. The fixing unit14of this exemplary structure is otherwise the same as previously described. It is possible to switch the shielding member60between the shielding position and the retracted position by rotating the shielding member60as thus far discussed.

FIG. 20is a vertical cross-sectional diagram showing another example of the structure of a fixing unit14which differs from the aforementioned structures in that a heat roller46is made of a nonmagnetic metallic material (such as stainless steel) and the center core58and the shielding member60are provided inside the heat roller46. In addition, the two arch cores54shown inFIG. 2are joined together at the middle into a single arch core54and an intermediate core55is provided below the arch core54as illustrated.

When the heat roller46is made of a nonmagnetic metallic material as mentioned above, a magnetic field generated by the induction heating coil52passes through the side cores56, the arch core54and the intermediate core55, penetrates the heat roller46and reaches the inside of the center core58. In the fixing unit14thus structured, the heating belt48is heated by induction heating due to the penetrating magnetic field.

If a ring-shaped portion of the shielding member60is switched to a position facing the intermediate core55(i.e., the shielding position) as shown inFIG. 20in this exemplary structure, the magnetic field is interrupted, making it possible to prevent overheating outside the minimum sheet-conveyed region. On the other hand, the shielding member60is at the retracted position when the shielding member60is in a state where the magnetic field does not pass through the ring-shaped portion of the shielding member60. In this case, the shielding member60does not produce any magnetic shielding effect and the heating belt48heated by induction heating within a maximum sheet-conveyed region. Here again, whileFIG. 20shows the shielding member60of exemplary structures (3) and (4) described earlier, the shielding member60of exemplary structure (1) or (2) may be adopted instead.

FIG. 21is a diagram showing another exemplary structure of an IH coil unit50. In this exemplary structure, induction heating is performed in a flat portion of the heating belt48between the fixing roller45and the heat roller46, and not in arc-shaped portions thereof. It is possible to shield the magnetic field by rotating the shielding member60in the same fashion as thus far discussed. WhileFIG. 21shows the shielding member60of exemplary structure (1) described earlier, the fixing unit14of this exemplary structure may employ a different arrangement, such as one of exemplary structures (1) through (4).

It is to be pointed out that the present invention is not limited to the above-described arrangements of the preferred embodiment but is applicable in variously varied forms. For example, the shielding member60is not limited to a trapezoidal or rectangular shape in plan view but may be formed into a triangular shape. Also, the ring-shaped shielding member60may be made of plural segments divided along the sheet passing width direction.

Additionally, while copper (oxygen-free copper) is used as the material for forming the shielding member60in the foregoing preferred embodiment, the shielding member60may be made of other kinds of nonmagnetic metallic material (such as stainless steel or aluminum).

Moreover, the above-described individual members including the arch cores54and the side cores56are not limited to those of the foregoing embodiment but may be modified as appropriate with respect to specific arrangements and structures.

While the image forming apparatus1of the preferred embodiments has thus far been described with reference to the drawings, the image forming apparatus1can be summarized as having the following preferable features.

The image forming apparatus preferably includes an image forming section for forming a toner image and transferring the toner image onto a sheet, and a fixing unit including a heating member and a pressing member, and fixing the toner image onto the sheet while nipping and conveying the sheet between the heating member and the pressing member. The fixing unit further includes a coil arranged along an outer surface of the heating member and generating a magnetic field, a first core arranged opposite the heating member with respect to the coil and forming a magnetic path, a second core so fixed between the first core and the heating member with respect to a direction in which the coil generates the magnetic field, as to form the magnetic path together with the first core, a shielding member positioned outward of the second core and shielding the magnetism in the magnetic path, and a magnetism adjusting unit moving the shielding member outward of the second core to switch the position of the shielding member between a shielding position where the shielding member shields the pass of the magnetism and a retracted position where the shielding member permits the pass of the magnetism.

The image forming apparatus structured as mentioned above employs an external IH system in which the heating member is heated by induction heating with the aid of the magnetic field produced by the coil to fuse the toner image, so that it is not necessary to provide any particular heating device within the heating member. Also, since the first core is arranged in an area surrounding the coil for forming the magnetic path along which the magnetic field produced by the coil is guided and the second core is arranged simply between the first core and the heating member, the aforementioned structure of the invention does not require an undesirably large space as a whole.

In the image forming apparatus thus structured, there is not provided a mechanism for magnetic shielding inside the heating member. It is therefore possible to lower total heat capacity and achieve a reduction in warm-up time of the fixing unit that much. Although the image forming apparatus employs the external IH system, the only movable component used in the external IH system is the aforementioned shielding member, so that it is possible to reduce the movable range of each member as a whole. Furthermore, as the movable component (shielding member) can be reduced in weight, it is possible to achieve a reduction in size of the fixing unit and eventually a reduction in overall size of the image forming apparatus. Moreover, even when a magnetic shielding mechanism is provided inside the heating member, it is still possible to reduce the total heat capacity because components like the coil are arranged outside the heating member.

Especially in the aforementioned image forming apparatus of the invention, it is possible to regulate the heat capacity of the heating member by simply moving the shielding member on the outside of the second core. Specifically, when the shielding member is shifted to the shielding position by the magnetism adjusting unit, the magnetic field produced by the coil and guided by the second core induces eddy currents which flow in the heating member, thereby performing the induction heating operation. On the other hand, when the shielding member is shifted to the retracted position by the magnetism adjusting unit, magnetic reluctance increases and magnetic field strength decreases within the magnetic path, thereby lowering the heat capacity of the heating member. Therefore, it is not necessary to move any of the cores toward and apart from the heating member for regulating the heat capacity of the heating member, making it possible to achieve space savings that much. Additionally, as it is not necessary to provide any core for magnetic shielding or any electrically conductive member for adjusting the magnetic field within the heating member, the aforementioned structure of the invention serves to avoid an increase in heat capacity and achieve a reduction in warm-up time of the fixing unit.

In the image forming apparatus structured as mentioned above, it is preferable that the magnetism adjusting unit rotates the second core along an outer periphery of the second core to switch the shielding member between the shielding position and the retracted position.

In the image forming apparatus thus structured, the movable range of the heating member is limited to the vicinity of the second core, making it possible to achieve space savings that much. Also, as the shielding member can be moved by rotary motion thereof, it is possible to simplify the structure that much.

In the image forming apparatus structured as mentioned above, it is preferable that the heating member has a sheet-conveyed region through which the sheet is conveyed, and is heatable in a width direction of the sheet over the entire sheet-conveyed region by induction heating by the coil, and the second core extends in the width direction of the sheet to form the magnetic path over the entire sheet-conveyed region, and the shielding member is positioned outward of the sheet-conveyed region set to a minimum with respect to the width direction of the sheet.

In the image forming apparatus thus structured, it is possible to prevent overheating of such members as the heating member when it is not necessary to heat the outside of the minimum sheet-conveyed region by switching the shielding member between the shielding position and the retracted position by means of the magnetism adjusting unit according to the paper size.

In the image forming apparatus structured as mentioned above, it is preferable that when the ratio of the length of the shielding member in the rotation direction of the shielding member relative to the length of the shielding member attained by one complete rotation thereof is defined as a shielding ratio, the shielding ratio varies in the width direction of the sheet. It is more preferable that the shielding ratio decreases in the width direction of the sheet from an end of the second core toward a central portion thereof.

In the image forming apparatus thus structured, when the shielding member is set at the shielding position, the amount of magnetic flux shielded by the shielding member decreases in areas where the shielding ratio is small. On the contrary, when the shielding member is set at the retracted position, the amount of magnetic flux shielded by the shielding member increases in areas where the shielding ratio is large. It is possible to vary the shielding ratio along the width direction (sheet passing width direction) of the sheet by varying the shielding ratio along the sheet passing width direction as mentioned above. In particular, if the shielding ratio is varied in a continuous or stepwise fashion, it is possible to alter a range where the heating member is heated by induction heating in a continuous or stepwise fashion by finely adjusting the angular position of the shielding member in discrete steps.

In the image forming apparatus structured as mentioned above, it is preferable that the shielding member is constituted by a pair of thin-plate members formed by bending in an arcuate shape along an outer periphery of the second core, and each of the thin-plate members extending in the width direction of the sheet from the corresponding one of ends of the second core toward a central portion thereof, and the length of each thin-plate member measured in a circumferential direction thereof decreases from the corresponding one of the ends of the second core toward the central portion thereof.

In the image forming apparatus structured as mentioned above, it is preferable that the shielding member includes a ring-shaped frame made of a nonmagnetic metallic material and a ring surface defined by the ring-shaped frame to face an outer periphery of the second core, and the magnetism adjusting unit adjusts the position of the ring surface relative to the outer periphery of the second core to switch the position of the shielding member between the shielding position and the retracted position. The ring surface of the shielding member may be employed in a plural number along the outer periphery of the second core. The ring surfaces may have different lengths in the width direction of the sheet.

In the image forming apparatus thus structured, if a magnetic field perpendicular to the ring surface passes through, or penetrates, the shielding member, eddy currents flow within the shielding member in a circumferential direction thereof. As a result, due to electromagnetic induction, a magnetic field directed opposite to the penetrating magnetic field is induced. The applied penetrating magnetic field and the induced oppositely directed magnetic field cancel each other out, whereby the shielding member can prohibit passage of the magnetic field. On the other hand, if magnetic fields directed in two opposite directions penetrate the ring surface of the ring-shaped shielding member or a magnetic field passes through the inside of the shielding member in a U-shaped pattern, the shielding member does not produce any magnetic shielding effect.

The inventors of the present invention have undertook an intensive study of the shielding member, focusing particularly on the above-described properties of the shielding member, and devised a fixing unit whose shielding member employs a space-saving mechanism, in which the shielding member produces the magnetic shielding effect when set at the shielding position where the magnetic field is allowed to pass through the ring-shaped frame, and the shielding member allows passage of the magnetic field when set at the retracted position where the magnetic field is not allowed to pass through the ring-shaped frame. Also, if the shielding member is ring-shaped, it is possible to achieve a reduction in weight of the shielding member and thus lower motive power (power consumption) required for moving the shielding member.

In the image forming apparatus structured as mentioned above, it is preferable that the coil is arranged to surround the heating member, and the first core are divided into core elements arranged on both sides of a central part of the coil, and the second core is arranged at a position where the magnetic path joins to the central part of the coil after passing the core elements of the first core on both sides thereof.

While the shielding member is arranged on the outside of the heating member in the aforementioned image forming apparatus, this structure may be modified such that the shielding member is arranged on the inside of the heating member. In this case, the heating member needs to be made of a nonmagnetic metallic material. The coil is arranged to surround the heating member in this case as well.

Even when the shielding member is arranged on the inside of the heating member, it is possible to cause the heating member to produce the magnetic shielding effect by shifting the shielding member between the shielding position and the retracted position within the heating member and create an environment suitable for successful warm-up operation.

Preferably, the shielding member is made of copper. Since copper has low electrical resistance and low permeability, it is possible to cause the heating member to produce the magnetic shielding effect by using copper in the shielding member.

Still preferably, the shielding member has a thickness within the range of 0.5 mm to 3 mm. Specifically, the shielding member efficiently shields the magnetic field while suppressing generation of Joule heat from the shielding member itself, the shielding member needs to be made of material having as low a resistivity (electrical resistance) as possible. If the shielding member has the thickness falling within the aforementioned range, it is possible to obtain good electrical conductivity and sufficient magnetic shielding effect by lowering the resistivity of the shielding member. This structure serves also to achieve a reduction in weight of the shielding member.

This application is based on Japanese patent application serial No. 2008-196801, filed in Japan Patent Office on Jul. 30, 2008, the contents of which is hereby incorporated by reference.