Patent Description:
Embodiments described herein relate generally to a fixing device and an image forming apparatus and methods related thereto.

A fixing device includes a fixing belt, a pressurizing roller, and a heating member. The pressurizing roller comes into press-contact with the fixing belt to form a nip. The heating member heats a sheet between the fixing belt and the pressurizing roller. The heating member includes a heating heater and a holding member that holds the heating heater.

A surface on the nip side of the heating member is sometimes formed in a curved convex shape. With this structure, the pressurizing roller presses the fixing belt in a bent state. Therefore, the pressurizing roller can equalize pressure in the nip in the axial direction of the fixing belt.

Since the surface on the nip side of the heating member is the curved convex shape, bending stress is generated if the heating member is pressed by the pressurizing roller. The bending stress decreases if the pressing by the pressurizing roller is released. Since the pressurization and the depressurization of the heating member by the pressurizing roller are repeated, the bending stress repeats an increase and a decrease. Therefore, durability of the heating member deteriorates over time. A fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device is known from <CIT>. <CIT> discloses a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus incorporating such a fixing device. A similar fixation device and image formation apparatus including the same is disclosed in <CIT>.

According to a first aspect of the present invention, it is provided a fixing device, comprising: an endless fixing belt configured to circularly move; a pressurizing roller configured to contact an outer circumferential surface of the fixing belt and form, between the pressurizing roller and the fixing belt, a nip for holding a sheet; a supporting member disposed on an inner side of the fixing belt and extending in an axial direction of the fixing belt; and a heating member having a first surface facing the pressurizing roller and a second surface opposite to the first surface, disposed on the inner side of the fixing belt and supported by the supporting member and extending in the axial direction of the fixing belt, configured so that when the pressurizing roller does not pressurize the fixing belt, end portions of the second surface are separated from the supporting member, wherein the heating member is positioned with respect to the supporting member by a positioning mechanism provided in a center in the axial direction. The positioning mechanism includes a positioning member and a locking claw, wherein the positioning member includes a locking hole and is provided in the center in the axial direction of the supporting member and the locking claw is provided in the center in the axial direction of the heating member and locked in the locking hole, whereby the heating member is positioned with respect to the supporting member.

Optionally, in the device according to the first aspect of the invention, the first surface of the heating member has a linear shape extending along the axial direction when viewed from a direction parallel to a tangential line in the nip, and the second surface of the heating member has a curved convex shape when viewed from the direction parallel to the tangential line in the nip.

Optionally, in the device according to the first aspect of the invention, the pressurizing roller is further configured to switch between a depressurizing form for not pressurizing the fixing belt and a pressurizing form for pressurizing the fixing belt, and a distance between end portions of the heating member and the supporting member in the pressurizing state is smaller than a distance between the end portions of the heating member and the supporting member in the depressurizing state.

Optionally, in the device according to the first aspect of the invention, the pressurizing roller is in the depressurizing form when not driven and in the pressurizing form when driven.

Optionally, in the device according to the first aspect of the invention, when a maximum thickness of the heating member is represented as Tmax, a minimum thickness of the heating member is represented as Tmin, and a maximum gap between the heating member and the supporting member is represented as G, following Expression (<NUM>) holds:<MAT>
G ≥ Tmax - Tmin (<NUM>).

Optionally, in the device according to the first aspect of the invention, the pressurizing roller comprises an elastic layer, and the elastic layer is compressed when the pressurizing roller contacts the outer circumferential surface of the fixing belt.

Optionally, in the device according to the first aspect of the invention, the heating member comprises a resistance film.

Optionally, in the device according to the first aspect of the inventio, resistance film comprises a plurality of resistance films in the axial direction.

Optionally, in the device according to the first aspect of the invention, the heating member has a middle portion and two end portions in the axial direction, and an uneven thickness in the axial directions.

Optionally, in the device according to the first aspect of the invention, a thickness of the middle portion is greater than a thickness of each of the two end portions.

According to a second aspect of the invention, it is provided a fixing method comprising: contacting an outer circumferential surface of an endless fixing belt with a pressurizing roller to form, between the pressurizing roller and the fixing belt, a nip for holding a sheet; and when the pressurizing roller does not pressurize the fixing belt, separating end portions of a second surface opposite to a first surface on a pressurizing roller side of a heating member from a supporting member, the supporting member disposed on an inner side of the fixing belt and extending in an axial direction of the fixing belt, the heating member disposed on the inner side of the fixing belt and supported by the supporting member and extending in the axial direction of the fixing belt, wherein the heating member is positioned with respect to the supporting member by a positioning mechanism provided in a center in the axial direction. The positioning mechanism includes a positioning member and a locking claw, wherein the positioning member includes a locking hole and is provided in the center in the axial direction of the supporting member and the locking claw is provided in the center in the axial direction of the heating member and locked in the locking hole, whereby the heating member is positioned with respect to the supporting member.

Optionally, in the method according to the second aspect of the invention, the first surface of the heating member has a linear shape extending along the axial direction when viewed from a direction parallel to a tangential line in the nip, and the second surface of the heating member has a curved convex shape when viewed from the direction parallel to the tangential line in the nip.

Optionally, the method according to the second aspect of the invention further comprises switching the pressurizing roller between a depressurizing form for not pressurizing the fixing belt and a pressurizing form for pressurizing the fixing belt, wherein a distance between end portions of the heating member and the supporting member in the pressurizing state is smaller than a distance between the end portions of the heating member and the supporting member in the depressurizing state.

Optionally, the method according to the second aspect of the invention further comprises not driving the pressurizing roller to obtain the depressurizing form and driving the pressurizing roller to obtain the pressurizing form.

Optionally, in the method according to the second aspect of the invention, when a maximum thickness of the heating member is represented as Tmax, a minimum thickness of the heating member is represented as Tmin, and a maximum gap between the heating member and the supporting member is represented as G, following Expression (<NUM>) holds:<MAT>
G ≥ Tmax - Tmin (<NUM>).

According to a third aspect of the invention, it is provided an image forming apparatus, comprising: a conveyance mechanism; a reader; a control section; and an image forming section comprising a fixing device according to the first aspect of the present invention.

Optionally, in the apparatus according to a third aspect of the invention, the first surface of the heating member has a linear shape extending along the axial direction when viewed from a direction parallel to a tangential line in the nip, and the second surface of the heating member has a curved convex shape when viewed from the direction parallel to the tangential line in the nip.

Optionally, in the apparatus according to a third aspect of the invention, the pressurizing roller is further configured to switch between a depressurizing form for not pressurizing the fixing belt and a pressurizing form for pressurizing the fixing belt, and a distance between end portions of the heating member and the supporting member in the pressurizing state is smaller than a distance between the end portions of the heating member and the supporting member in the depressurizing state.

Optionally, in the apparatus according to the third aspect of the invention, when a maximum thickness of the heating member is represented as Tmax, a minimum thickness of the heating member is represented as Tmin, and a maximum gap between the heating member and the supporting member is represented as G, following Expression (<NUM>) holds:<MAT>.

An object of embodiments is to provide a fixing device and an image forming apparatus that can improve durability of a heating section.

A fixing device according to an embodiment includes a fixing belt, a pressurizing roller, a supporting member and a heating member. The fixing belt is supported to be capable of circularly moving and is formed endless. The pressurizing roller comes into contact with an outer circumferential surface of the fixing belt and forms, between the pressurizing roller and the fixing belt, a nip for holding a sheet. The supporting member is disposed on an inner side of the fixing belt and extends in an axial direction of the fixing belt. The heating member is disposed on the inner side of the fixing belt and supported by the supporting member and extends in the axial direction of the fixing belt. If the pressurizing roller does not pressurize the fixing belt, end portions of a second surface opposite to a first surface on the pressurizing roller side of the heating member are separated from the supporting member.

A fixing device and an image forming apparatus according to an embodiment are explained below with reference to the drawings.

<FIG> is a diagram illustrating an overview of the image forming apparatus according to the embodiment. As illustrated in <FIG>, an image forming apparatus <NUM> includes a reading section R, an image forming section P, and a paper feeding cassette section C. In the following illustration and explanation, an XYZ coordinate system is used according to necessity. An X direction is the horizontal direction. The X direction is the lateral width direction of the image forming apparatus <NUM>. A Y direction is a direction orthogonal to the X direction in the horizontal plane. The Y direction is the front-rear direction of the image forming apparatus <NUM>. A Z direction is a direction orthogonal to the X direction and the Y direction. The Z direction is the height direction of the image forming apparatus <NUM>.

The reading section R reads, with a CCD (Charge-Coupled Device) image sensor or the like, a document sheet set on a document table and generates an optical signal. The reading section R converts the generated optical signal into digital data. The image forming section P acquires a document image read by the reading section R or printing data transmitted from an external personal computer. The image forming section P forms, on a sheet, a toner image based on the acquired document image or printing data. The image forming section P fixes the toner image formed on the sheet.

The image forming section P includes a laser scanning section <NUM> and photoconductive drums 201Y, <NUM>, 201C, and <NUM>. The laser scanning section <NUM> includes a polygon mirror <NUM> and an optical system <NUM>. The laser scanning section <NUM> irradiates, on the photoconductive drums 201Y to <NUM>, images to be formed on a sheet. The images on the sheet are images based on image signals of colors of yellow (Y), magenta (M), cyan (C), and black (K).

The photoconductive drums 201Y to <NUM> retain, according to irradiation positions on the sheet, color toner images supplied from a not-illustrated developing device. The photoconductive drums 201Y to <NUM> sequentially transfer the retained toner images onto a transfer belt <NUM>. The transfer belt <NUM> is an endless belt. A roller <NUM> is driven to rotate, whereby the transfer belt <NUM> conveys the toner images to a transfer position T.

A conveyance path <NUM> connects the paper feeding cassette section C, the transfer position Tr, a fixing device <NUM>, and a discharge tray <NUM>. A sheet stored in the paper feeding cassette section C is conveyed to the transfer position T along the conveyance path <NUM>. In the transfer position T, the transfer belt <NUM> transfers the toner images onto the sheet.

The sheet, onto which the toner images are transferred, is conveyed to the fixing device <NUM> along the conveyance path <NUM>. The fixing device <NUM> heats and melts the toner images to cause the toner image to permeate the sheet and fix the toner image. Consequently, the toner images on the sheet are prevented from being disturbed by an external force. The sheet, on which the toner images are fixed, is conveyed to the discharge tray <NUM> along the conveyance path <NUM>. The conveyed sheet is discharged to the outside of the image forming apparatus <NUM> from the discharge tray <NUM>.

A control section <NUM> is a unit that collectively controls devices and mechanisms in the image forming apparatus <NUM>. The control section <NUM> includes a central arithmetic unit such as a CPU (Central Processing Unit) and volatile and nonvolatile storage devices. The central arithmetic unit executes an arithmetic operation of computer programs stored in the storage devices, whereby the control section <NUM> controls the devices and the mechanisms in the image forming apparatus <NUM>. A part of functions may be implemented as a circuit.

A component including units for conveyance of a formation target image (toner image) to the transfer position Tr to transfer of the formation target image onto the sheet is a transfer section <NUM>.

The fixing device <NUM> is explained in detail. The fixing device <NUM> is a fixing section of a so-called direct heat type.

<FIG> is a front view of the fixing device <NUM> in a pressurizing form P2 (see <FIG>). The axial direction of a fixing belt <NUM> is sometimes simply referred to as "axial direction".

As illustrated in <FIG>, the fixing device <NUM> includes the fixing belt (a belt) <NUM>, a pressurizing roller (a roller) <NUM>, a heating member <NUM>, and a supporting member <NUM>. In <FIG>, "T" indicates a tangential line in a fixing nip N between the fixing belt <NUM> and the pressurizing roller <NUM>. The tangential line T is orthogonal to the axial direction (a Y direction).

The fixing belt <NUM> is formed of a flexible material in a tubular shape. The fixing belt <NUM> is an endless belt-like (film-like) member. Although not illustrated in <FIG>, the fixing belt <NUM> includes a base layer, an elastic layer, and a surface release layer. The base layer is configured by a sheet-like member having high heat resistance. The base layer is made of a metal material such as nickel or stainless steel, a resin material such as polyimide (PI), or the like. Surface coating may be applied to the inner surface of the base layer in order to improve frictional slidability with respect to the heating member <NUM>. The elastic layer is made of an elastic material such as silicone rubber. The surface release layer is made of tetrafluoroethylene-perfluoro alkylvinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. In order to prevent a warming-up time from being increased, the thicknesses of the elastic layer and the surface release layer are selected such that a heat capacity is not excessively large.

The fixing belt <NUM> is capable of circularly moving around the axis of the fixing belt <NUM> in a state in which the fixing belt <NUM> is supported by a not-illustrated supporting mechanism.

The pressurizing roller <NUM> is disposed side by side with the fixing belt <NUM>. The pressurizing roller <NUM> includes a core member 32a and an elastic layer 32b. The core member 32a is formed of metal or the like in a cylindrical shape. Both end portions of the core member 32a are supported by supporting bodies (not illustrated in <FIG>) in the fixing device <NUM> via bearings (not illustrated in <FIG>). The core member 32a is capable of rotating around the axis of the core member 32a. The elastic layer 32b is provided on the outer circumferential surface of the core member 32a. The elastic layer 32b is formed of foaming silicone rubber, silicone rubber, fluorocarbon rubber, or the like. A release layer (not illustrated in <FIG>) may be formed on the outer circumferential surface of the elastic layer 32b. PFA, PTFE, or the like is used as the release layer.

The pressurizing roller <NUM> is pressurized to the fixing belt <NUM> side by not-illustrated pressurizing means and comes into contact with the outer circumferential surface of the fixing belt <NUM>. In a portion where the pressurizing roller <NUM> and the fixing belt <NUM> come into press-contact, the elastic layer 32b of the pressurizing roller <NUM> is elastically compressed, whereby the fixing nip (a nip) N having a predetermined width in a conveying direction of a sheet S is formed. In the fixing nip N, the pressurizing roller <NUM> holds the sheet S between the pressurizing roller <NUM> and the fixing belt <NUM>.

The pressurizing roller <NUM> is driven to rotate by a driving source (not illustrated in <FIG>) such as a motor. For example, the pressurizing roller <NUM> can be driven to rotate by a driving mechanism including the driving source and a gear train (not illustrated in <FIG>). If the pressurizing roller <NUM> is driven to rotate, a driving force of the pressurizing roller <NUM> is transmitted to the fixing belt <NUM> in the fixing nip N. The fixing belt <NUM> rotates following the rotation of the pressurizing roller <NUM>. In conveying the sheet S, the fixing belt <NUM> rotates in a first direction (a delivering direction) D1 of the circumferential direction of the fixing belt <NUM>.

The pressurizing roller <NUM> can switch, with a not-illustrated switching mechanism, a depressurizing form P1 (explained below) and the pressurizing form P2 (explained below). The pressurizing roller <NUM> switches the fixing device <NUM> to the depressurizing form P1 if not being driven and switches the fixing device <NUM> to the pressurizing form P2 only if being driven to rotate. Consequently, it is possible to suppress a creep of the fixing belt <NUM> and the pressurizing roller <NUM>.

A surface of the pressurizing roller <NUM> opposed to the fixing nip N is formed in a straight shape (a linear shape) extending along the axial direction (the Y direction) (see <FIG>)when viewed from a direction parallel to the tangential line T (see <FIG>).

The heating member <NUM> includes a heating heater <NUM> and a holding member <NUM>. The heating member <NUM> is disposed on the inner side of the fixing belt <NUM>. The heating member <NUM> is formed in a long plate shape extending along the axial direction (the Y direction). The heating member <NUM> is generally disposed with the thickness direction of the heating member <NUM> directed to the pressurizing roller <NUM>. A direction in which the heating member <NUM> approaches the pressurizing roller <NUM> is referred to as front. A direction in which the heating member <NUM> separates from the pressurizing roller <NUM> is referred to as rear. A front surface 33a (a first principal plane) of the heating member <NUM> is a surface in contact with the fixing belt <NUM>. The front surface 33a is a surface on the fixing nip N side. A rear surface 33b (a second principal plane) is a surface opposite to the front surface 33a. The rear surface 33b is a surface opposite to the fixing nip N side.

The heating heater <NUM> is provided in a holding recessed section 36c of the holding member <NUM>. A front surface 35a of the heating heater <NUM> configures a part of the front surface 33a of the heating member <NUM>. The heating heater <NUM> includes a resistance film (not illustrated in <FIG>), a substrate (not illustrated in <FIG>), and a protection layer (not illustrated in <FIG>). The resistance film is laminated on the substrate. The resistance film generates heat with energization. The resistance film may be divided into a plurality of resistance films in the axial direction (the Y direction). The divided plurality of resistance films desirably can be independently energized. Consequently, temperatures can be independently decided concerning the plurality of resistance films. Therefore, only a region where the sheet S passes can be heated. The substrate is made of ceramic, stainless steel, or the like. The protection layer is provided on the surfaces of the resistance film and the substrate. For example, the protection layer is made of SiO<NUM>. The heating heater <NUM> is generally disposed with the thickness direction of the heating heater <NUM> directed to the pressurizing roller <NUM>.

The holding recessed section 36c, in which the heating heater <NUM> is provided, is formed on a front surface 36a of the holding member <NUM>. The holding recessed section 36c is formed in a groove shape extending along the axial direction (the Y direction). The holding member <NUM> holds the heating heater <NUM>.

The holding member <NUM> extends in the axial direction (the Y direction). The holding member <NUM> is formed in a long plate shape extending along the axial direction (the Y direction). The holding member <NUM> is made of an elastic material such as silicone rubber or fluoro rubber, heat resistant resin such as polyimide resin, polyphenylene sulfide (PPS), polyether sulfone (PES), or liquid crystal polymer (LCP), or the like.

A not-illustrated high heat conduction member may be disposed between the holding recessed section 36c and the heating heater <NUM>. For example, the high heat conduction member is formed in a sheet shape. The high heat conduction member has high thermal conductivity compared with the holding recessed section 36c and the heating heater <NUM>. For example, the high heat conduction member is made of metal having high thermal conductivity such as copper or aluminum. A graphite sheet may be used as the high heat conduction member. The high heat conduction member has an effect of reducing a temperature gradient in the longitudinal direction of the fixing belt <NUM> and the heating heater <NUM> and preventing a local temperature rise.

<FIG> is a sectional view of the heating member <NUM>. <FIG> is a diagram illustrating a cross section (a I-I cross section illustrated in <FIG>) orthogonal to the tangential line T (see <FIG>) in the fixing nip N.

As illustrated in <FIG>, the front surface 33a of the heating member <NUM> is formed in a straight shape (a linear shape) extending along the axial direction (the Y direction) at the position of the fixing nip N when viewed from the direction parallel to the tangential line T. The rear surface 33b (a surface opposite to the fixing nip N side) of the heating member <NUM> is formed in a curved convex shape when viewed from the direction parallel to the tangential line T. For example, the curved convex shape may be an arcuate shape or may be a higher-order curve shape (e.g., a quadratic curve shape) such as an elliptical arc shape, a parabolic shape, and a hyperbolic shape. Since the cross section of the front surface 33a is the straight shape and the cross section of the rear surface 33b is the curved convex shape, a center portion 33d of the heating member <NUM> is formed thick compared with end portions 33c in the axial direction (the Y direction) of the heating member <NUM>.

In detail, the bottom surface of the holding recessed section 36c of the holding member <NUM> is formed in a straight shape extending along the axial direction (the Y direction). A rear surface 36b (the rear surface 33b) of the holding member <NUM> is formed in a curved convex shape. Consequently, a center portion 36e of the holding member <NUM> is formed thick compared with end portions 36d in the axial direction (the Y direction) of the holding member <NUM>. The thickness of the heating heater <NUM> is fixed in the axial direction (the Y direction).

The heating member <NUM> illustrated in <FIG> is the thinnest at the end portions 33c. The thickness of the heating member <NUM> at the end portions 33c is represented as Tmin. For example, the thicknesses of both the end portions 33c of the heating member <NUM> are equal to each other. The heating member <NUM> is the thickest in the center portion 33d. The thickness of the heating member <NUM> in the center portion 33d is represented as Tmax.

As illustrated in <FIG>, the supporting member <NUM> supports the holding member <NUM>. The supporting member <NUM> includes an upper holding plate <NUM>, a coupling member <NUM>, and a lower holding plate <NUM>. For example, the upper holding plate <NUM> extends along an XY plane. At least a part in the axial direction (the Y direction) of a front end portion 37a of the upper holding plate <NUM> reaches an upper part of the holding member <NUM>. The coupling member <NUM> extends downward from a rear end portion 37b of the upper holding plate <NUM>. The lower holding plate <NUM> extends from the lower end portion of the coupling member <NUM> in a direction in which the lower holding plate <NUM> approaches the holding member <NUM>. The lower holding plate <NUM> is parallel to the upper holding plate <NUM>. At least a part in the axial direction (the Y direction) of a front end portion 39a of the lower holding plate <NUM> reaches a lower part of the holding member <NUM>.

<FIG> is a configuration diagram of the fixing device <NUM> at the time when the pressurizing roller <NUM> does not pressurize the fixing belt <NUM>. <FIG> is a diagram of the fixing device <NUM> viewed from the direction parallel to the tangential line T (see <FIG>).

As illustrated in <FIG>, a form of the fixing device <NUM> at the time when the pressurizing roller <NUM> does not press the fixing belt <NUM> is referred to as "depressurizing form P1". In the depressurizing form P1, a range including the center portion 33d in the rear surface 33b of the heating member <NUM> is in contact with the supporting member <NUM>. Since the rear surface 33b of the heating member <NUM> has a curved convex shape, portions including the end portions 33c in the rear surface 33b of the heating member <NUM> are separated from the supporting member <NUM>.

The heating member <NUM> can be positioned with respect to the supporting member <NUM> using a positioning mechanism <NUM>. The positioning mechanism <NUM> includes a positioning member <NUM> and a locking claw <NUM>. The positioning member <NUM> includes a locking hole <NUM> in which the locking claw <NUM> is locked. The positioning member <NUM> is provided in the center in the axial direction (the Y direction) of the supporting member <NUM> (e.g., the upper holding plate <NUM> and the lower holding plate <NUM>). The locking claw <NUM> is provided in the center in the axial direction (the Y direction) of the heating member <NUM>. The locking claw <NUM> is locked in the locking hole <NUM> of the positioning member <NUM>, whereby the heating member <NUM> is positioned with respect to the supporting member <NUM>.

The heating member <NUM> is positioned with respect to the supporting member <NUM> in the center of the axial direction (the Y direction) by the positioning mechanism <NUM>. Therefore, the heating member <NUM> can be positioned without being affected by displacement due to a bend of the supporting member <NUM>.

The thickness Tmin and the thickness Tmax of the heating member <NUM> are designed according to a bend amount of the supporting member <NUM> at the time when the fixing belt <NUM> is pressurized by the pressurizing roller <NUM>. For example, if a maximum gap between the heating member <NUM> and the supporting member <NUM> is represented as "G", the thickness Tmin and the thickness Tmax are set such that the following Expression (<NUM>) holds. For example, the maximum gap G is a gap between the end portions 33c of the heating member <NUM> and the supporting member <NUM>.

If Expression (<NUM>) holds, the front surface 33a of the heating member <NUM> can be kept in the straight shape. Therefore, it is possible to prevent unnecessary stress from being applied to the heating member <NUM>.

<FIG> is a configuration diagram of the fixing device <NUM> at the time when the pressurizing roller <NUM> pressurizes the fixing belt <NUM>. <FIG> is a diagram of the fixing device <NUM> viewed from the direction parallel to the tangential line T (see <FIG>).

As illustrated in <FIG>, a form of the fixing device <NUM> at the time when the pressurizing roller <NUM> pressurizes the fixing belt <NUM> is referred to as "pressurizing form P2". In the pressurizing form P2, the pressurizing roller <NUM> presses the supporting member <NUM> via the heating member <NUM>. The rear surface 33b formed in the curved convex shape of the heating member <NUM> presses a range including the center portion of the supporting member <NUM>. Therefore, a range including the center in the axial direction (the Y direction) of the supporting member <NUM> is pressed backward. The supporting member <NUM> is formed in a curved shape that is convex backward.

The distance between the end portions 33c of the heating member <NUM> and the supporting member <NUM> in the pressurizing form P2 is smaller than the distance between the end portions 33c of the heating member <NUM> and the supporting member <NUM> in the depressurizing form P1. If a pressing force by the heating member <NUM> is sufficiently high, the supportingmember <NUM> is in contact with the rear surface 33b of the heating member <NUM> over the entire length in the axial direction (the Y direction) of the supporting member <NUM>.

As illustrated in <FIG>, if the pressing by the pressurizing roller <NUM> is released, the fixing device <NUM> shifts to the depressurizing form P1.

Both of the surface of the pressurizing roller <NUM> opposed to the fixing nip N and the front surface 33a of the heating member <NUM> have the straight shapes. Therefore, the pressing force applied to the heating member <NUM> by the pressurizing roller <NUM> is equal in the axial direction (the Y direction). Therefore, if the fixing device <NUM> shifts from the depressurizing form P1 to the pressurizing form P2, a bending stress generated in the heating member <NUM> is small. Accordingly, bending deformation of the heating member <NUM> is small.

<FIG> is a diagram illustrating deformation amounts of the heating member <NUM> and the supporting member <NUM>. As illustrated in <FIG>, if the fixing device <NUM> shifts from the depressurizing form P1 to the pressurizing form P2, a shift amount of the front surface 33a of the heating member <NUM> (the surface on the nip side of the heating member) is substantially fixed in the axial direction (the Y direction). On the other hand, a shift amount of the supporting member <NUM> is large in the center portion and small at the end portions.

In the fixing device <NUM>, since the bending stress generated in the heating member <NUM> in the pressurizing form P2 is small, even if the depressurizing form P1 and the pressurizing form P2 are repeated, it is possible to improve durability of the heating member <NUM>.

In the fixing device <NUM>, since the front surface 33a of the heating member <NUM> has the straight shape, bending of the heating heater <NUM> is unnecessary. Accordingly, the fixing device <NUM> is excellent in manufacturability.

The thickest part in the axial direction of the heating member is not limited to the center portion and may be parts closer to the end portions than the center portion.

The front surface 33a of the heating member <NUM> in the embodiment has the straight shape and the rear surface 33b of the heating member <NUM> has the curved convex shape. However, the shape of the heating member is not limited to this shape. For example, the front surface of the heating member may have a curved convex shape.

Claim 1:
A fixing device (<NUM>), comprising:
an endless fixing belt (<NUM>) configured to circularly move;
a pressurizing roller (<NUM>) configured to contact an outer circumferential surface of the fixing belt (<NUM>) and form, between the pressurizing roller (<NUM>) and the fixing belt (<NUM>), a nip (N) for holding a sheet;
a supporting member (<NUM>) disposed on an inner side of the fixing belt (<NUM>) and extending in an axial direction of the fixing belt (<NUM>); and
a heating member (<NUM>) having a first surface (33a) facing the pressurizing roller (<NUM>) and a second surface (33b) opposite to the first surface (33a), disposed on the inner side of the fixing belt (<NUM>) and supported by the supporting member (<NUM>) and extending in the axial direction of the fixing belt (<NUM>), configured so that when the pressurizing roller (<NUM>) does not pressurize the fixing belt (<NUM>), end portions of the second surface (33b) are separated from the supporting member (<NUM>),
wherein the heating member (<NUM>) is positioned with respect to the supporting member (<NUM>) by a positioning mechanism (<NUM>) provided in a center in the axial direction,
characterized in that
the positioning mechanism (<NUM>) includes a positioning member (<NUM>) and a locking claw (<NUM>), wherein the positioning member (<NUM>) includes a locking hole (<NUM>), wherein the positioning member (<NUM>) is provided in the center in the axial direction of the supporting member (<NUM>), wherein the locking claw (<NUM>) is provided in the center in the axial direction of the heating member (<NUM>) and wherein the locking claw (<NUM>) is locked in the locking hole (<NUM>), whereby the heating member (<NUM>) is positioned with respect to the supporting member (<NUM>).