Patent Description:
In recent years, there have been provided fixing devices that achieve a reliable fixing property with a thin, film-like endless belt having a decreased thermal capacity and directly heated by a fixing heat source, even when the fixing devices are installed in image forming apparatuses having high productivity.

In such fixing devices, a rotation member disposed opposite an outer circumferential surface of the endless fixing belt is pressed against, via the fixing belt, a support member (or a nip forming member) fixed inside (or inside a loop formed by) the fixing belt, to form a fixing nip between the rotation member and the support member. The nip forming member may be provided with a heat equalizing member made of a metal material having an increased thermal conductivity, to uniformly heat the fixing belt and reduce a temperature rise at end portions of the fixing belt during continuous conveyance of recording media.

There has been known a configuration of such fixing devices in which a sheet material (or sliding sheet) made from fibers of, e.g., polytetrafluoroethylene (PTFE) impregnated with a lubricant such as silicone grease is disposed on the surface of the nip forming member to reduce the sliding resistance (or torque) between the fixing belt and the nip forming member. However, the sliding sheet may serve a heat insulation member, hampering the heat equalization of the fixing belt.

To address such a situation, there has been also known a configuration in which, instead of the sliding sheet, a slidable coating is directly applied to the surface of the heat equalizing member (for example, PTL1 and PTL <NUM>). Further, there has been proposed a configuration in which an appropriate surface roughness is formed on the surface of the coating layer to hold a grease. <CIT> discloses an image heating apparatus including a rotatable endless belt, and opposing member forming a nip together with an outer surface of the belt, and a non-rotatable pressure applying member that contacts an inner surface of the belt and is pressed toward the opposing member. <CIT> discloses an image heating apparatus for heating a toner image while conveying a recording material bearing the toner image at a nip portion. <CIT> discloses a fuser roller including a surface layer of anodized aluminium oxide impregnated with a fluorine containing sealant. <CIT> discloses a fixing device and a fixing roller having a releasing effect by the wear resistance and lubricating agents there.

When the surface of the heat equalizing member is coated with, e.g., a coating material containing PTFE, a firing process may be performed to enhance the adhesion and strength of a coating film. However, since the heat equalizing member is a thin plate, the heat equalizing member may be deformed due to the heat history during the firing process. To prevent such deformation of the heat equalizing member, the firing process is performed at a temperature lower than a temperature at which the original coating film performance is exerted.

Relatedly, when agglomerates are generated in the coating material, a minute convex shape is formed on the surface of the coating film. This convex shape promotes wear on an inner surface of the fixing belt (or an inner surface of a sleeve) and generates abrasion powder. As a consequence, the unit torque of the fixing device may increase at an early stage.

In light of the above-described problems, it is a general object of the present invention to provide a fixing device that performs a reliable fixing operation over time by keeping a reduced sliding resistance between the heat equalizing member and a fixing member without impairing the reliability and heat equalizing property of the heat equalizing member.

In order to solve the above-described problems and achieve the object, there is provided a fixing device as described in appended claims. Advantageous embodiments are defined by the dependent claims. Advantageously, the fixing device includes a rotatable and endless fixing member, a heat source, a pressure member, a nip forming member, and a heat equalizing member. The heat source is configured to heat the fixing member. The pressure member is disposed outside the fixing member to face the fixing member. The nip forming member is disposed inside the fixing member to form a nip between the fixing member and the pressure member. The heat equalizing member is configured to cover a face of the nip forming member, the face facing the fixing member, and transfer heat in an axial direction of the fixing member. The fixing member includes at least: a tubular base made of metal; and a sliding layer made of heat resistant resin on an inner circumferential surface of the base. The heat equalizing member is made of aluminum or an aluminum alloy. The heat equalizing member includes an alumite layer on a surface facing an inner circumferential surface of the fixing member. A plurality of micropores in the alumite layer is filled with a solid lubricant having a coefficient of friction lower than a coefficient of friction of the alumite layer. The alumite layer has a thickness smaller than a thickness of the sliding layer of the fixing member.

In a fixing device of an embodiment of the present disclosure, an alumite layer is formed on the surface of a heat equalizing member. A plurality of micropores in the alumite layer is filled with a solid lubricant. Accordingly, the fixing device maintains lubrication between the heat equalizing member and a fixing member. In addition, the alumite layer having a thickness smaller than the thickness of a sliding layer of the fixing member does not impair the heat equalizing property of the heat equalizing member. Further, the alumite treatment does not cause deformation of the member or slight convexity as compared to the coating treatment, thus enhancing the reliability of the heat equalizing member.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present disclosure are described in detail below. <FIG> is a schematic view of an image forming apparatus, according to an embodiment of the present disclosure. An image forming apparatus <NUM> is a color laser printer. In the center of a printer body of the image forming apparatus <NUM>, four image forming units 4Y, 4C, <NUM>, and <NUM> are arranged side by side along a direction in which an intermediate transfer belt <NUM> is stretched. The image forming units 4Y, 4C, <NUM>, and <NUM> have identical configurations while containing developers in different colors, that is, yellow (Y), cyan (C), magenta (M), and black (K) corresponding to color separation components of a color image.

Specifically, each of the image forming units 4Y, 4C, <NUM>, and <NUM> serving as an image station includes, e.g., a drum-shaped photoconductor <NUM> as a latent image bearer, a charging device <NUM> that charges the surface of the photoconductor <NUM>, a developing device <NUM> that supplies toner to the surface of the photoconductor <NUM>, and a cleaning device <NUM> that cleans the surface of the photoconductor <NUM>. Note that, in <FIG>, reference numerals are assigned to the photoconductor <NUM>, the charging device <NUM>, the developing device <NUM>, and the cleaning device <NUM> of the image forming unit <NUM> that forms a black toner image; whereas reference numerals are omitted for the other image forming units 4Y, 4C, and <NUM>.

An exposure device <NUM> is disposed below the image forming units 4Y, 4C, <NUM>, and <NUM> to expose the surface of the photoconductor <NUM>. The exposure device <NUM> includes, e.g., a light source, a polygon mirror, an f-θ lens, and a reflection mirror to irradiate the surface of each of the photoconductors <NUM> with a laser beam according to image data.

A transfer device <NUM> is disposed above the image forming units 4Y, 4C, <NUM>, and <NUM>. The transfer device <NUM> includes the intermediate transfer belt <NUM> as a transfer body, four primary transfer rollers <NUM> as primary transfer means, and a secondary transfer roller <NUM> as secondary transfer means. The transfer device <NUM> further includes a secondary transfer backup roller <NUM>, a cleaning backup roller <NUM>, a tension roller <NUM>, and a belt cleaning device <NUM>.

The intermediate transfer belt <NUM> is an endless belt entrained around the secondary transfer backup roller <NUM>, the cleaning backup roller <NUM>, and the tension roller <NUM>. Here, as the secondary transfer backup roller <NUM> is driven to rotate, the intermediate transfer belt <NUM> orbits (or rotates) in a direction indicated by arrow in <FIG>.

Each of the four primary transfer rollers <NUM> sandwiches the intermediate transfer belt <NUM> together with the corresponding photoconductors <NUM>, thereby forming a primary transfer nip between the intermediate transfer belt <NUM> and the corresponding photoconductor <NUM>. The primary transfer rollers <NUM> are coupled to a power supply of the printer body. The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage to the primary transfer rollers <NUM>.

The secondary transfer roller <NUM> sandwiches the intermediate transfer belt <NUM> together with the secondary transfer backup roller <NUM>, thereby forming a secondary transfer nip between the secondary transfer roller <NUM> and the intermediate transfer belt <NUM>. Similar to the primary transfer rollers <NUM>, the secondary transfer roller <NUM> is coupled to the power supply of the printer body. The power supply applies at least one of a predetermined DC voltage and a predetermined AC voltage to the secondary transfer roller <NUM>.

The belt cleaning device <NUM> includes a cleaning brush and a cleaning blade disposed to contact the intermediate transfer belt <NUM>. A bottle receptacle <NUM> is disposed in an upper portion of the printer body. Four toner bottles 2Y, 2C, <NUM>, and <NUM> containing fresh toner are removably attached to the bottle receptacle <NUM>. Toner supply tubes are interposed between the toner bottles 2Y, 2C, <NUM>, and <NUM> and the respective developing devices <NUM>. The fresh toner is supplied from each of the toner bottles 2Y, 2C, <NUM>, and <NUM> to the corresponding developing device <NUM> through the corresponding toner supply tube.

In a lower portion of the printer body are, e.g., an input tray <NUM> that accommodates a plurality of sheets P as recording media and a sheet feeding roller <NUM> that sends out the plurality of sheets P one at a time from the input tray <NUM>. Here, examples of the recording medium include, but are not limited to, plain paper, thick paper, a postcard, an envelope, thin paper, coated paper, art paper, tracing paper, and an overhead projector (OHP) transparency. Optionally, the image forming apparatus <NUM> may include a bypass feeder that imports such a recording medium placed on a bypass tray into the image forming apparatus <NUM>.

Inside the printer body, a conveyance passage R is defined by internal components of the image forming apparatus <NUM>. Along the conveyance passage R, the sheet P is conveyed from the input tray <NUM>, passing through the secondary transfer nip, and is ejected outside the image forming apparatus <NUM>. Along the conveyance passage R, a registration roller pair <NUM> is disposed upstream from the position of the secondary transfer roller <NUM> in a sheet conveyance direction in which the sheet P is conveyed. The registration roller pair <NUM> is conveying means that conveys the sheet P to the secondary transfer nip.

In addition, a fixing device <NUM> is disposed downstream from the position of the secondary transfer roller <NUM> in the sheet conveyance direction. The fixing device <NUM> fixes, onto the sheet P, an unfixed image that has been transferred onto the sheet P. Further, a sheet ejection roller pair <NUM> is disposed downstream from the fixing device <NUM> in the sheet conveyance direction along the conveyance passage R. The sheet ejection roller pair <NUM> ejects the sheet P outside the image forming apparatus <NUM>. An output tray <NUM> is disposed on an upper surface of the printer body. The plurality of sheets P ejected one at a time outside the image forming apparatus <NUM> lies stacked on the output tray <NUM>.

A description is now given of a basic operation of the printer according to the present embodiment. When an image forming operation starts, the photoconductor <NUM> is driven to rotate clockwise in <FIG> in each of the image forming units 4Y, 4C, <NUM>, and <NUM>. The charging device <NUM> uniformly charges the surface of the photoconductor <NUM> to a predetermined polarity. The exposure device <NUM> irradiates the charged surface of the photoconductor <NUM> with a laser beam to form an electrostatic latent image on the surface of the photoconductor <NUM>. Note that the image data according to which the photoconductor <NUM> is exposed is single-color image data obtained by separating a desired full-color image into individual color components of yellow, cyan, magenta, and black. The developing device <NUM> supplies toner to the electrostatic latent image thus formed on the photoconductor <NUM> to render the electrostatic latent image visible as a toner image.

Meanwhile, when the image forming operation starts, the secondary transfer backup roller <NUM> is driven to rotate counterclockwise in <FIG> and rotates the intermediate transfer belt <NUM> in a direction indicated by arrow in <FIG>. Each of the primary transfer rollers <NUM> is supplied with a constant voltage or constant current control voltage having a polarity opposite a polarity of the charged toner. Accordingly, a transfer electric field is generated at the primary transfer nip between each of the primary transfer rollers <NUM> and the corresponding photoconductor <NUM>.

When the toner images in different colors formed on the respective photoconductors <NUM> reach the respective primary transfer nips in accordance with rotation of the respective photoconductors <NUM>, the toner images are transferred, by the transfer electric fields generated at the respective primary transfer nips, from the respective photoconductor <NUM> onto the intermediate transfer belt <NUM> such that the toner images are sequentially superimposed one atop another on the intermediate transfer belt <NUM>. Thus, a full-color toner image is formed on the surface of the intermediate transfer belt <NUM>. The cleaning device <NUM> removes residual toner from the photoconductor <NUM>. In this case, the residual toner is toner that has failed to be transferred onto the intermediate transfer belt <NUM> and therefore remains on the photoconductor <NUM>. Then, a discharger discharges the surface of the photoconductor <NUM> to initialize the surface potential of the photoconductor <NUM>.

In the lower portion of the image forming apparatus <NUM>, the sheet feeding roller <NUM> starts rotation to feed the sheet P from the input tray <NUM> to the conveyance passage R. The registration roller pair <NUM> conveys the sheet P fed to the conveyance passage R to the secondary transfer nip between the secondary transfer roller <NUM> and the secondary transfer backup roller <NUM> at a proper time. At this time, the secondary transfer roller <NUM> is supplied with a transfer voltage having a polarity opposite a polarity of the charged toner contained in the full-color toner image formed on the intermediate transfer belt <NUM>, thereby generating a transfer electric field at the secondary transfer nip.

Thereafter, when the toner images on the intermediate transfer belt <NUM> reach the secondary transfer nip in accordance with rotation of the intermediate transfer belt <NUM>, the transfer electric field generated at the secondary transfer nip collectively transfers the toner images from the intermediate transfer belt <NUM> onto the sheet P. The belt cleaning device <NUM> removes residual toner from the intermediate transfer belt <NUM>. In this case, the residual toner is toner that has failed to be transferred onto the sheet P and therefore remains on the intermediate transfer belt <NUM>. The removed toner is conveyed and collected into a waste toner container disposed inside the printer body.

Thereafter, the sheet P is conveyed to the fixing device <NUM>. The fixing device <NUM> fixes the toner images resting on the sheet P onto the sheet P. Then, the sheet ejection roller pair <NUM> ejects the sheet P outside the image forming apparatus <NUM>. Thus, a plurality of sheets P lies stacked on the output tray <NUM>.

As described above, the image forming apparatus <NUM> performs an image forming operation to form a full-color image on the sheet P. Alternatively, the image forming apparatus <NUM> may use any one of the image forming units 4Y, 4C, <NUM>, and <NUM> to form a monochrome image. Alternatively, the image forming apparatus <NUM> may use two of the image forming units 4Y, 4C, <NUM>, and <NUM> to form a bicolor image, or may use three of the image forming units 4Y, 4C, <NUM>, and <NUM> to form a tricolor image.

<FIG> is a schematic cross-sectional view of a fixing device, according to an embodiment of the present disclosure. As illustrated in <FIG>, the fixing device <NUM> includes a fixing belt <NUM> formed into a loop, a pressure roller <NUM>, a temperature sensor <NUM>, a separating member <NUM>, and various components disposed inside the loop formed by the fixing belt <NUM>, such as heaters 23A and 23B, a nip forming member <NUM>, a stay member <NUM>, a heat equalizing member <NUM>, and reflecting members 28A and 28B. The fixing belt <NUM> and the components disposed inside the loop formed by the fixing belt <NUM> constitute a belt unit 21U, detachably coupled to the pressure roller <NUM>. The fixing belt <NUM> is an endless belt that is a thin, flexible, and tubular fixing member. The pressure roller <NUM> is a pressure member that contacts an outer circumferential surface of the fixing belt <NUM>. The fixing belt <NUM> is heated by radiation heat from the heaters 23A and 23B, serving as a plurality of heat sources (or fixing heat sources), disposed inside (or inside the loop formed by) the fixing belt <NUM>. A halogen heater is generally used as the heat source. Alternatively, the heat source may be, e.g., an induction heating device, a resistive heat generator, or a carbon heater.

Inside the fixing belt <NUM> are the nip forming member <NUM> and the stay member <NUM>. The nip forming member <NUM> forms a fixing nip N between the fixing belt <NUM> and the pressure roller <NUM>. The stay member <NUM> (serving as a support member) supports the nip forming member <NUM>. The stay member <NUM> secures and supports the nip forming member <NUM> disposed along an axial direction of the fixing belt <NUM>, thus preventing the nip forming member <NUM> from being bent by pressure that the nip forming member <NUM> receives from the pressure roller <NUM>. Accordingly, the fixing nip N is formed retaining an even width along an axial direction (i.e., longitudinal direction) of the pressure roller <NUM>.

<FIG> is a cross-sectional view of a fixing belt, according to an embodiment of the present disclosure. As illustrated in <FIG>, the fixing belt <NUM> includes at least a tubular base 21a made of metal or heat resistant resin, a release layer 21b made of heat resistant resin and provided on an outer circumferential surface of the base 21a, and a sliding layer 21c made of resin on an inner circumferential surface of the base 21a.

The base 21a has a thickness in a range of from <NUM> to <NUM>. The base 21a is made of a metal material such as nickel or steel use stainless (SUS), or a resin material such as polyimide (PI) or polyamide imide (PAI).

The release layer 21b has a layer thickness in a range of from <NUM> to <NUM>. The release layer 21b is made of a material such as tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) or polytetrafluoroethylene (PTFE). The release layer 21b ensures the releasability of the fixing belt <NUM> with respect to the toner image on the sheet P. Note that the "release" means to peel off an object from another object adhering to the object. The "releasability" means the ease with which the objects can be separated from each other.

Optionally, an elastic layer 21d made of, e.g., silicone rubber may be interposed between the base 21a and the release layer 21b. In a case in which the fixing belt <NUM> does not incorporate the elastic layer 21d, the fixing belt <NUM> has a decreased thermal capacity that improves fixing property. However, as the fixing belt <NUM> and the pressure roller <NUM> sandwich and press an unfixed image onto the sheet P at the fixing nip N, slight surface asperities in the fixing belt <NUM> may be transferred onto the toner image on the sheet P, resulting in appearance of an orange peel image having orange-peel-like variation in gloss in a solid image portion of the image. Here, the orange peel image means an image having slight surface asperities. To address such a situation, the elastic layer 21d made of silicone rubber preferably has a thickness not smaller than <NUM>. The deformation of the elastic layer 21d absorbs the slight surface asperities in the fixing belt <NUM>, thus preventing the appearance of the orange peel image.

As the sliding layer 21c, for example, PAI or fluororesin having heat resistance and slidability is preferable. As the fluororesin, PTFE or PFA is preferable. In a case in which the sliding layer 21c is made of a mixed coating material of fluororesin and PAI, the sliding layer 21c has a reduced coefficient of dynamic friction and enhances the adhesion to the base 21a.

The sliding layer 21c is applied to the inner circumferential surface of the base 21a of the fixing belt <NUM> by spray coating, for example, so as to have a thickness of about <NUM>. However, if the thickness is smaller than <NUM>, and particularly smaller than <NUM>, coating unevenness (i.e., partial color unevenness in the coating film) may occur.

The sliding layer 21c thus formed has a coefficient of dynamic friction not greater than <NUM> and a tensile elastic modulus not greater than <NUM> Mpa.

In order to reduce thermal capacity, the fixing belt <NUM> has a total thickness not greater than <NUM> and a loop diameter in a range of from <NUM> to <NUM>. In order to further reduce thermal capacity, preferably, the fixing belt <NUM> may have a total thickness not greater than <NUM>, and more preferably, not greater than <NUM>. Preferably, the loop diameter of the fixing belt <NUM> is not greater than <NUM>.

Referring back to <FIG>, a description is now resumed of other components. The nip forming member <NUM> is made of a heat resistant material having good mechanical strength and heatproof not less than <NUM>. In particular, the nip forming member <NUM> is made of heat resistant resin such as PI or polyether ether ketone (PEEK), or such a heat resistant resin reinforced with glass fibers. Thus, the nip forming member <NUM> is immune to thermal deformation at temperatures in a fixing temperature range desirable to fix a toner image onto a sheet P, thereby retaining a stable state of the fixing nip N and keeping the output image quality stable. Opposed longitudinal end portions of the stay member <NUM> and opposed longitudinal end portions of the heaters 23A and 23B are secured to and supported by a pair of side plates of the fixing device <NUM> or a pair of holders provided additionally.

The heat equalizing member <NUM> is a heat transfer aid member that facilitates heat transfer in the axial direction of the fixing belt <NUM>. The heat equalizing member <NUM> is disposed to cover a nip-side face of the nip forming member <NUM>. The nip-side face of the nip forming member <NUM> faces the inner circumferential surface of the fixing belt <NUM>. The heat equalizing member <NUM> proactively transfers heat in the axial direction of the fixing belt <NUM>, that is, in a longitudinal direction of the heat equalizing member <NUM>, thus preventing heat from staying at opposed axial end areas of the fixing belt <NUM> when small sheets P are conveyed over the fixing belt <NUM>. Thus, the heat equalizing member <NUM> eliminates unevenness in temperature in the axial direction of the fixing belt <NUM>. The heat equalizing member <NUM> of the present embodiment is made of aluminum or an aluminum alloy as a material having an increased thermal conductivity, thus enabling heat transfer in a short time.

The heat equalizing member <NUM> includes a belt sliding-contact face that faces in direct contact with the inner circumferential surface of the fixing belt <NUM>, thus serving as a nip forming face. In <FIG>, the belt sliding-contact face is flattened. Alternatively, the belt sliding-contact face may be given a concave shape or another suitable shape. For example, a concave nip forming face directs a leading edge of the sheet P toward the pressure roller <NUM> as the sheet P is ejected from the fixing nip N, thus facilitating separation of the sheet P from the fixing belt <NUM> and preventing a paper jam.

In order to reduce the wear of the fixing belt <NUM> and the heat equalizing member <NUM>, fluorine oil or fluorine grease containing a fluorine compound may be applied as a lubricant to the inner circumferential surface of the fixing belt <NUM>. The lubricant may be fluorine grease or silicone grease containing fluorine particles as a thickener.

The stay member <NUM>, having a T-shaped cross-section, includes an arm 25a extending away from the fixing nip N. The arm 25a is interposed between the heaters 23A and 23B as fixing heat sources, to separate the heaters 23A and 23B from each other. One of the heaters 23A and 23B includes a heat generating area at a longitudinal center portion of the one of the heaters 23A and 23B to heat toner images on small sheets P passing through the fixing nip N. The other one of the heaters 23A and 23B includes a heat generating area at each longitudinal end portion of the other one of the heaters 23A and 23B to heat toner images on large sheets P passing through the fixing nip N.

The power source situated inside the printer body supplies power to the heaters 23A and the 23B so that the heaters 23A and 23B generate heat. Specifically, a controller (e.g., a processor) is operatively connected to the power source and the temperature sensor <NUM> to control the power supply to the heaters 23A and 23B based on the temperature of the outer circumferential surface of the fixing belt <NUM> detected by the temperature sensor <NUM> disposed opposite the outer circumferential surface of the fixing belt <NUM>. Such heating control of the heaters 23A and 23B adjusts the temperature of the fixing belt <NUM> to a desired fixing temperature.

The reflecting member 28A is interposed between the heater 23A and the stay member <NUM>. The reflecting member 28B is interposed between the heater 23B and the stay member <NUM>. The reflecting members 28A and 28B reflect heat from the heaters 23A and 23B toward the fixing belt <NUM>, thus enhancing heating efficiency of the heaters 23A and 23B to heat the fixing belt <NUM>. In addition, the reflecting members 28A and 28B prevent radiation heat from the heaters 23A and 23B from heating the stay member <NUM>, thus reducing waste of energy. Alternatively, instead of the reflecting members 28A and 28B, the respective heater-side faces of the stay member <NUM> facing the heaters 23A and 23B may be insulated or given a mirror finish to enhance the heating efficiency of the heaters 23A and 23B and reduce the waste of energy.

The pressure roller <NUM> is constructed of a core, an elastic layer made of, e.g., silicone rubber foam or fluororubber and provided on the surface of the core, and a release layer made of, e.g., PFA or PTFE and provided on the surface of the elastic layer. As the pressure roller <NUM> is pressed against the fixing belt <NUM> by pressure means such as a spring, the elastic layer of the pressure roller <NUM> is deformed and thus forms the fixing nip N having a predetermined width at an area of pressure contact between the fixing belt <NUM> and the pressure roller <NUM>.

The pressure roller <NUM> is driven to rotate by a driving source such as a motor disposed inside the printer body. As the driving source drives and rotates the pressure roller <NUM>, a driving force of the driving source is transmitted from the pressure roller <NUM> to the fixing belt <NUM> at the fixing nip N, thus rotating the fixing belt <NUM>. While the fixing belt <NUM> rotates, a nip span of the fixing belt <NUM> located at the fixing nip N is sandwiched between the pressure roller <NUM> and the heat equalizing member <NUM>. On the other hand, a circumferential span of the fixing belt <NUM> other than the nip span is guided by flanges secured to the pair of side plates located at opposed axial end portions of the fixing belt <NUM>.

In the present embodiment, the pressure roller <NUM> is a solid roller. Alternatively, the pressure roller <NUM> may be a hollow roller, i.e., a tube. In a case in which the pressure roller <NUM> is a hollow roller, a heat source such as a halogen heater may be disposed inside the pressure roller <NUM>. The elastic layer of the pressure roller <NUM> may be made of solid rubber. Alternatively, in a case in which no heat source is situated inside the pressure roller <NUM>, the elastic layer may be made of sponge rubber. The sponge rubber is preferable to the solid rubber because the sponge rubber has an increased thermal insulation that draws less heat from the fixing belt <NUM>.

As described above, the temperature sensor <NUM> is disposed at an appropriate position opposite the outer circumferential surface of the fixing belt <NUM>, for example, upstream from the fixing nip N in a direction of rotation of the fixing belt <NUM>, to detect the temperature of the fixing belt <NUM>. The separating member <NUM> is disposed in a downstream position in the sheet conveyance direction in the fixing device <NUM> to separate the sheet P from the fixing belt <NUM>. The pressure means is also provided to releasably press the pressure roller <NUM> against the fixing belt <NUM>.

<FIG> is a schematic perspective view of an axial end portion of the fixing device <NUM> illustrated in <FIG>. Flanges <NUM> are disposed at the respective axial end portions of the fixing belt <NUM>. <FIG> illustrates one of the axial end portions of the fixing belt <NUM>.

The flange <NUM> is hollow and open on both axial sides of the flange <NUM>. The flange <NUM> includes a receiving portion <NUM> extending in an axial direction of the flange <NUM> and a flange portion <NUM> projecting from the receiving portion <NUM> in a radial direction. The receiving portion <NUM> is partially cylindrical or tubular, including a slit <NUM> in a partial circumferential span of the receiving portion <NUM>. The nip forming member <NUM> and the heat equalizing member <NUM> are inserted into the space defined by the slit <NUM>.

If the fixing belt <NUM> is moved or skewed in the axial direction of the fixing belt <NUM> in accordance with rotation of the fixing belt <NUM>, the axial end portion of the fixing belt <NUM> comes into contact with the receiving portion <NUM>, which restricts an axial motion of the fixing belt <NUM>. The flange portion <NUM> is secured to the side plate of the fixing device <NUM>. Optionally, a ring plate made of a material that provides the fixing belt <NUM> with good slidability may be interposed between the receiving portion <NUM> and the axial end portion of the fixing belt <NUM>.

<FIG> is an exploded perspective view of a nip forming member, a support member, and a heat equalizing member that construct a nip forming unit, according to an embodiment of the present disclosure.

As illustrated in <FIG>, the heat equalizing member <NUM> disposed on a fixing nip side of the nip forming member <NUM> engages the nip forming member <NUM>, which is given an approximately rectangular shape, such that the heat equalizing member <NUM> covers a nip-side face 24c, facing the inner circumferential surface of the fixing belt <NUM>, of the nip forming member <NUM>. Thus, the heat equalizing member <NUM> is coupled to the nip forming member <NUM>. The heat equalizing member <NUM> may engage the nip forming member <NUM> with, e.g., a projection to be coupled to the nip forming member <NUM>. Alternatively, the heat equalizing member <NUM> may be attached to the nip forming member <NUM> with, e.g., an adhesive to be coupled to the nip forming member <NUM>.

The heat equalizing member <NUM> includes a belt sliding-contact face 27a that faces the inner circumferential surface of the fixing belt <NUM>. The nip forming member <NUM> includes a stay-side face opposite the nip-side face 24c. The stay member <NUM> includes a nip-side face that faces the fixing nip N. The nip-side face of the stay member <NUM> supports the stay-side face of the nip forming member <NUM>. Preferably, the stay-side face of the nip forming member <NUM> and the nip-side face of the stay member <NUM> that contact each other may mount a recess and a projection (e.g., a boss and a pin), respectively, for example, to reduce an area of contact between the nip forming member <NUM> and the stay member <NUM>.

Subsequently, a description is given of a characteristic configuration according to an embodiment of the present disclosure.

<FIG> is a perspective view of a heat equalizing member according to an embodiment of the present disclosure. <FIG> is an enlarged cross-sectional view of the heat equalizing member illustrated in <FIG>. With reference to <FIG> and <FIG>, a detailed description is given of a configuration of the heat equalizing member <NUM>.

As illustrated in <FIG>, according to the present embodiment, the heat equalizing member <NUM> made of aluminum or an aluminum alloy includes an alumite film (hereinafter referred to as an alumite layer <NUM>) on a surface of the heat equalizing member <NUM> (specifically, a surface facing the inner circumferential surface of the fixing belt <NUM>). As illustrated in <FIG>, molybdenum disulfide (MoS<NUM>) <NUM> serving as a solid lubricant fills a plurality of micropores 54a regularly arranged in the alumite layer <NUM> (on an aluminum base material <NUM>).

The alumite layer <NUM> is very hard and has good wear resistance. In particular, the alumite layer <NUM> has a very strong property against abrasive wear. On the other hand, the molybdenum disulfide <NUM> is a solid lubricant having a coefficient of friction lower than the coefficient of friction of the alumite layer <NUM>. With such a configuration, the heat equalizing member <NUM> of the present embodiment serves a sliding member having both wear resistance and lubricity with respect to the inner circumferential surface of the fixing belt <NUM>.

<FIG> illustrate a way of manufacturing a heat equalizing member according to an embodiment of the present disclosure. As illustrated in <FIG>, the aluminum base material <NUM> is primarily electrolyzed by a typical anodic oxidation method to form the alumite layer <NUM> on the surface of the aluminum base material <NUM>. The innumerable (multiple) micropores 54a are generated as arranged regularly in the alumite layer <NUM>.

The thickness (t) of the alumite layer <NUM> is adjustable according to the amount of electric charge (= current × time) used for electrolysis. Since the thermal conductivity of the alumite layer <NUM> is lower than the thermal conductivity of the aluminum base material <NUM>, the aluminum base material <NUM> is desirably made as thin as possible. The alumite layer <NUM> has a property of being much harder than the sliding layer 21c (see <FIG>) of the fixing belt <NUM> that slides on the alumite layer <NUM>.

For example, when compared in terms of Martens hardness, the sliding layer 21c of the fixing belt <NUM> has a value in a range of from <NUM> (N/mm<NUM>) to <NUM> (N/mm<NUM>); whereas the alumite layer <NUM> has a value of about <NUM> (N/mm<NUM>).

Therefore, in the present embodiment, the alumite layer <NUM> has a thickness (t) smaller than at least the thickness of the sliding layer 21c of the fixing belt <NUM>. As described above, the sliding layer 21c of the fixing belt <NUM> has a thickness of about <NUM> so as not to cause coating unevenness. Therefore, in the present embodiment, the alumite layer <NUM> has a thickness (t) of about <NUM>, which is a size about one third of the thickness of the sliding layer 21c of the fixing belt <NUM>. The aforementioned thickness (t) of the alumite layer <NUM> is an example and is not limited thereto.

A pore diameter (d) of the micropores 54a is about <NUM>Å to <NUM>Å, though the pore diameter (d) varies depending on the treatment liquid used for the anodizing treatment. The number of the micropores 54a is such that the micropores 54a occupy from <NUM>% to <NUM>% of the surface area of the heat equalizing member <NUM>. The aforementioned pore diameter (d) and the number of the micropores 54a are examples and are not limited thereto.

Next, in an aqueous solution containing molybdenum thioate as a main ingredient, the aluminum base material <NUM> on which the alumite layer <NUM> is formed is made to the anode and thus secondarily electrolyzed. Then, as illustrated in <FIG>, the molybdenum sulfide (i.e., molybdenum disulfide <NUM>) is precipitated and fixed in the plurality of micropores 54a. This precipitation starts from a base portion 54b of the plurality of micropores 54a and proceeds toward an inlet (or an outermost surface layer) of the plurality of micropores 54a with the passage of the electrolysis time.

A description is now given of a reason why the molybdenum sulfide (i.e., molybdenum disulfide <NUM>) is precipitated from the base portion 54b. The molybdenum thioate in the secondary electrolyte dissociates into thiomolybdate ions. Since the ions are negatively charged, the ions are attracted to the anode and enter the micropores 54a by electrophoresis or diffusion. Since the size of the ions is much smaller than the size of the micropores 54a, the ions reach the depth of the micropores 54a. Thus, the molybdenum sulfide (i.e., molybdenum disulfide <NUM>) is precipitated from the base portion 54b of the plurality of micropores 54a.

When the molybdenum sulfide (i.e., molybdenum disulfide <NUM>) precipitated as described above is heat-treated after the secondary electrolysis, crystals having a graphite structure are formed. As a consequence, as illustrated in <FIG>, the plurality of micropores 54a in the alumite layer <NUM> is filled with the molybdenum disulfide <NUM> from the base portion 54b to the outermost surface layer of the plurality of micropores 54a.

As described above, the heat equalizing member <NUM> of the present embodiment includes the alumite layer <NUM> on the surface facing the inner circumferential surface of the fixing belt <NUM>. The plurality of micropores 54a in the alumite layer <NUM> is filled with the molybdenum disulfide <NUM> from the base portion 54b to the outermost surface layer of the plurality of micropores 54a. Since the alumite layer <NUM> is formed by altering the aluminum base material <NUM>, foreign matter is not mixed in during the formation of the alumite layer <NUM>. In addition, a minute convex shape due to coating unevenness does not occur. Accordingly, the inner surface of the fixing belt <NUM> is immune to local wear.

Further, the molybdenum disulfide <NUM> fills an entire area in a depth direction of the alumite layer <NUM>. Therefore, even if the alumite layer <NUM> is worn, the heat equalizing member <NUM> provides the fixing belt <NUM> with good slidability unchanged from the initial stage, as long as the alumite layer <NUM> is present.

Furthermore, according to the present embodiment, the heat equalizing member <NUM> attains a reduced surface roughness of the surface facing the inner circumferential surface of the fixing belt <NUM>. This is because the surface roughness for holding grease is not particularly needed. With such a reduced surface roughness, the heat equalizing member <NUM> prevents damage to the inner circumferential surface of the fixing belt <NUM> when the fixing belt <NUM> slides over the belt sliding-contact face 27a, thus attaining a further advantage. Note that, in the present embodiment, an arithmetic mean roughness (Ra) of the alumite layer <NUM> is about <NUM> to <NUM>.

In the embodiment described above, molybdenum disulfide is used as a solid lubricant. However, the solid lubricant is not limited to the molybdenum disulfide. Alternatively, the plurality of micropores 54a in the alumite layer <NUM> may be impregnated and filled with PTFE or fluorine grease.

The thickness (t) of the alumite layer <NUM> is preferably made as small as possible. From the viewpoint of rigidity, the hardness (e.g., Martens hardness) of the alumite layer <NUM> is preferably greater than the hardness of the sliding layer 21c of the fixing belt <NUM>. In particular, the alumite layer <NUM> is preferably harder than the sliding layer 21c of the fixing belt <NUM> about three times, and more preferably, in a range of from about <NUM> times to about <NUM> times.

Subsequently, a description is given of the heat equalizing performance of a heat equalizing member of the present embodiment.

Each of <FIG> is a cross-sectional view of a heat equalizing member and a fixing belt. Specifically, <FIG> illustrates, as a comparative configuration, a configuration of a heat equalizing member <NUM> and the fixing belt <NUM>. <FIG> illustrates a configuration of the heat equalizing member <NUM> and the fixing belt <NUM>, according to the present embodiment.

Table <NUM> presents a comparison of a comparative configuration and a configuration of the present embodiment of the fixing belt and the heat equalizing member.

As illustrated in <FIG>, in the comparative configuration, sliding layers 121c and <NUM> are interposed between an aluminum base material <NUM> of the heat equalizing member <NUM> and a base 121a of the fixing belt <NUM> to prevent mutual wear. The sliding layer <NUM> is a typical resin-based coating material such as polyimide resin or fluororesin. An interface <NUM> is interposed between the aluminum base material <NUM> of the heat equalizing member <NUM> and the sliding layer <NUM>. The total thickness of the sliding layers 121c and <NUM> is about <NUM>.

Note that the thickness of each of the sliding layers 121c and <NUM> is <NUM> for coating without causing coating unevenness, as described above.

By contrast, as illustrated in <FIG>, in the configuration of the present embodiment, the sliding layer 21c and the alumite layer <NUM> are interposed between the aluminum base material <NUM> of the heat equalizing member <NUM> and the base 21a of the fixing belt <NUM>. However, no interface is interposed between the aluminum base material <NUM> and the alumite layer <NUM> of the heat equalizing member <NUM>. The total thickness of the sliding layer 21c and the alumite layer <NUM> is about <NUM>.

Since the distance between the aluminum base material <NUM> of the heat equalizing member <NUM> and the base 21a of the fixing belt <NUM> is smaller than the distance between the aluminum base material <NUM> of the heat equalizing member <NUM> and the base 121a of the fixing belt <NUM>, the heat equalizing member <NUM> of the present embodiment enhances the thermal conductivity. In addition, since no clear interface is interposed between the alumite layer <NUM> and the aluminum base material <NUM>, the heat equalizing member <NUM> of the present embodiment is advantageous for heat conduction. Accordingly, the heat equalizing member <NUM> of the present embodiment enhances the heat equalizing property as compared to a comparative heat equalizing member.

With the configuration described above, a fixing device performs a reliable fixing operation over time. An image forming apparatus including the fixing device is providable as a product that prevents an increase in the fixing unit torque for a long period of time.

Some of the embodiments of the present disclosure have been described in detail. The embodiments have been described as examples and can be implemented with various modifications within the scope of the present invention. For example, in an embodiment described above, the nip forming member <NUM> and the heat equalizing member <NUM> are separate members. Alternatively, the heat equalizing member may be provided with a role (or function) as a nip forming member to be an integrated member.

The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.

Claim 1:
A fixing device (<NUM>) comprising:
a rotatable and endless fixing member (<NUM>);
a heat source (23A, 23B) configured to heat the fixing member (<NUM>);
a pressure member (<NUM>) disposed outside the fixing member (<NUM>) to face the fixing member (<NUM>);
a nip forming member (<NUM>) disposed inside the fixing member (<NUM>) to form a nip between the fixing member (<NUM>) and the pressure member (<NUM>); and
a heat equalizing member (<NUM>) configured to cover a face of the nip forming member (<NUM>), the face facing the fixing member (<NUM>), and transfer heat in an axial direction of the fixing member (<NUM>),
the fixing member (<NUM>) including at least: a tubular base (21a) made of metal; and a sliding layer (21c) made of heat resistant resin on an inner circumferential surface of the base (21a), the heat equalizing member (<NUM>) being made of aluminum or an aluminum alloy,
the heat equalizing member (<NUM>) including an alumite layer on a surface facing an inner circumferential surface of the fixing member (<NUM>),
a plurality of micropores in the alumite layer being filled with a solid lubricant having a coefficient of friction lower than a coefficient of friction of the alumite layer,
the alumite layer having a thickness smaller than a thickness of the sliding layer (21c) of the fixing member (<NUM>).