Image heating apparatus

A fixing apparatus including a flexible belt having a heat generating layer, an outer ring provided on one edge side of the belt and along an outer peripheral surface of the belt, the outer ring being electrically connected to the heat generating layer, an inner ring provided on the one edge side of the belt and along an inner peripheral surface of the belt to face the outer ring through the belt, and a power supply unit that is electrically connected to the outer ring and that feeds power. A difference obtained by subtracting a diameter of the inner ring at a normal temperature from a diameter of the inner ring at a fixing temperature is equivalent to or larger than a difference obtained by subtracting a diameter of the outer ring at the normal temperature from a diameter of the outer ring at the fixing temperature.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to an image heating apparatus that heats and image on a sheet. The image heating apparatus is used in image forming apparatuses such as, for example, a copier, a printer, a fax machine, and a multi-functional apparatus provided with a plurality of the above functions.

Description of the Related Art

Hitherto, in image forming apparatuses such as an electrophotographic apparatus and an electrostatic recording apparatus, a toner image (a developer image) is formed on a sheet and is heated and compressed with a fixing apparatus serving as an image heating apparatus to fix the image on the sheet. In recent years, from the viewpoint of energy saving, a fixing apparatus of a type that uses a heat generating belt including a resistance layer has been proposed as a fixing apparatus with quick temperature rise (Japanese Patent Laid-Open No. 2014-232302). In the fixing apparatus of the above type, since the heat generating belt itself generates heat upon energization of the resistance layer, the heat of the belt can be efficiently transmitted to the sheet.

Furthermore, in the fixing apparatus described in Japanese Patent Laid-Open No. 2014-232302, a ring-shaped member is attached to an outer peripheral surface of the belt and power is fed to the resistance layer of the belt through the ring-shaped member. Such a configuration is capable of increasing the life of the belt.

However, such ring-shaped member goes through thermal expansion when heated upon execution of the fixing process and, accordingly, the adhesion with the belt decreases. Decrease in adhesion creates an unstable electrical connection between the ring-shaped member and brings about an occurrence of power feed failure.

SUMMARY OF THE INVENTION

An object of the present disclosure is to provide an image heating apparatus that suppresses occurrence of power feed failure.

Another object of the present disclosure is to provide an image heating apparatus, including a belt including a heat generating layer that generates heat, the belt being an endless and flexible belt that heats an image on a sheet; a drive unit that rotationally drives the belt; a first ring-shaped member that is provided on one edge side of the belt in a longitudinal direction of the belt and along an outer peripheral surface of the belt, the first ring-shaped member being electrically connected to the heat generating layer; a second ring-shaped member that is provided on the one edge side in the longitudinal direction and along an inner peripheral surface of the belt so as to face the first ring-shaped member through the belt; and a power feed unit that is electrically connected to the first ring-shaped member and that feeds power to the first ring-shaped member. In the image heating apparatus, a difference obtained by subtracting a diameter of the second ring-shaped member at a predetermined temperature from a diameter of the second ring-shaped member at a further higher temperature that is higher than the predetermined temperature is equivalent to or larger than a difference obtained by subtracting a diameter of the first ring-shaped member at the predetermined temperature from a diameter of the first ring-shaped member at the further higher temperature.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, modes for implementing the present disclosure will be described in detail while illustrating the exemplary embodiments. Note that in the following exemplary embodiments, description of an image forming apparatus will be given with a laser beam printer that employs an electrophotographic process as an example. In the description hereinafter, the laser beam printer will be referred to as a printer1.

Exemplary Embodiment

FIG. 1is a block diagram of the printer1serving as an image forming apparatus.FIG. 2is a front view of a fixing apparatus F. The printer1forms an image on a sheet P according to image information input to a control circuit100from an external host device200(FIG. 2). The control circuit100is a circuit including a CPU that performs an operation associated with various controls, and a non-volatile medium such as a ROM in which various programs are stored. Programs are stored in the ROM and the CPU reads out the programs and executes the programs so as to execute various controls. Note that the control circuit100may be an integrated circuit such as an ASIC as long as similar functions can be performed.

The sheet P is a medium on which an image can be formed with the image forming apparatus and, for example, includes fixed size or non-fixed size standard paper, thick paper, an envelope, a postal card, a sticker, a resin sheet, an OHP film, and glossy paper.

An image forming unit2includes four image forming stations3Y,3M,3C, and3K for forming images of toner on the sheet P. Each station includes a rotary drum type photoconductor4, a charging member5, a laser scanner6, a developer device7, a transfer member8, and a photosensitive drum cleaner9. The stations3Y,3M,3C, and3K form toner images of yellow, magenta, cyan, and black, respectively.

The toner images formed in the stations3Y,3M,3C, and3K are superimposed on an intermediate transfer belt11such that an image t serving as a synthetic toner image is primarily transferred.

Meanwhile, the sheets P that are placed on either of the sheet cassettes19and20, and the multi-feeding tray21are sent out sheet by sheet with a feeding mechanism (not shown) to a registration roller pair23. Then the registration roller pair23synchronizes with the synthetic toner image on the intermediate transfer belt11and sends the sheet P between the intermediate transfer belt11and a secondary transfer roller12. Accordingly, the image t on the intermediate transfer belt11is transferred onto the sheet P. Subsequently, the sheet P is sent towards the fixing apparatus (an image heating apparatus) F. Then, the fixing apparatus F applies heat and pressure to the image t on the sheet P so as to fix the image t on the sheet P.

A description of the fixing apparatus F will be given next.FIG. 3is diagram viewed from the direction of arrow III-III inFIG. 2.FIG. 4is an exploded perspective view of a belt unit according to the first exemplary embodiment.

The fixing apparatus F is an image heating apparatus that heats an image on a sheet with a belt unit (hereinafter, referred to as a unit30) and a pressure roller40(hereinafter, referred to as a roller40). The fixing apparatus F is a heat-generating fixing belt type fixing apparatus that employs, in the unit30, a fixing belt31(hereinafter, referred to as a belt31) that generates heat upon energization, and is capable of efficiently transmitting heat to the sheet P. Furthermore, the fixing apparatus F is a pressure roller driving type (a tensionless type) fixing apparatus that rotates the unit30with the roller40serving as a driving device. Accordingly, the unit30can be configured to have a low heat capacity such that the startup performance when starting fixing is excellent.

As illustrated inFIG. 2, the unit30and the roller40abutting against each other forms a nip portion N in between. Furthermore, as illustrated inFIG. 3, the roller40rotating in an arrow R40direction and the belt31rotating in an arrow R31direction convey the sheet P fed to the nip portion N in an arrow x direction. In the above, the heat of the unit30is added to the sheet P such that the image t on the sheet P is applied heat and pressure and is fixed to the sheet P in the nip portion N. The sheet P that has passed through the nip portion N is separated from the belt31and is ejected. In the present exemplary embodiment, the fixing process is performed in the manner described above. Hereinafter, the configuration of the fixing apparatus F will be described in detail with reference to the drawings.

Herein, as regards the fixing apparatus F of the present exemplary embodiment or the components of the fixing apparatus F, the front side is the surface of the apparatus viewed from the sheet entering side (FIG. 2) and the rear side is the surface on the other side (the sheet exiting side). Left and right refer to the left (the left side inFIG. 2and the front side of the drawing inFIG. 3) and the right (the right side inFIG. 2and the back side of the drawing inFIG. 3), respectively, when the apparatus is viewed from the front side. Upstream and downstream refers to the upstream side and the downstream side, respectively, with respect to a sheet conveying direction. Furthermore, a longitudinal direction (a width direction) and a sheet width direction refers to a direction that is substantially parallel to the direction (the left-right direction ofFIG. 2) orthogonal to the conveying direction of the sheet P in the surface of the sheet conveying passage. A short direction refers to a direction that is substantially parallel to the conveying direction (left-right direction ofFIG. 3) of the sheet P in the surface of the sheet conveying passage.

As illustrated inFIG. 3, the unit30includes the endless belt31that generates heat upon energization, a pressure pad32that is disposed inside the belt31, and a pressure stay33serving as a pad holding member. As illustrated inFIG. 4, a power feed ring38L serving as an electrode portion is attached on one edge side of the belt31in the longitudinal direction and a power feed ring38R is attached on the other edge side, and feed of power from the power feed rings38L and38R generates heat in the belt31. Details of the belt31and the power feed rings38L and38R would be described later. The pressure pad32(a nip pad: hereinafter referred to as a pad32) is a heat insulating member that is long in the left-right direction and that has a substantially rectangular cross section. The pressure stay33(hereinafter referred to as a stay33) is a rigid member that is long in the left-right direction and that has an inverted U-shaped cross section. It is desirable that the stay33is a member that does not easily warp even when high pressure is applied thereto and, for example, is a molded member of SUS304 (stainless steel). The pad32and the stay33are vertically arranged with respect to each other and are parallel to each other. The pad32is joined to leg portions of the stay33. The pad32is a pressing member that is in contact with and that slides against an inner surface of the belt31at the nip portion N and that presses the belt31towards the roller40from the inside. Since the pad32performs a role of a guide that regulates the rotation path of the belt31at a portion in the vicinity of the nip portion N, a heat resisting property and a sliding property with respect to the inner surface of the belt are required.

As illustrated inFIG. 4, as the material for the pad32, heat-resistance resin, such as a liquid crystal polymer, ceramics, and metal such as SUS may be used. A material such as SUS that has excellent sliding property may be used in the portion corresponding to the nip portion N, and heat-resistance resin, such as a liquid crystal polymer, that has excellent processing characteristics may be used in the guide portion. Furthermore, high-temperature grease may be coated on the surface that slides against the belt31.

As illustrated inFIG. 4, a thermistor TH serving as a temperature sensor is disposed at substantially the middle portion of the stay33in the longitudinal direction through an elastic member34such as a plate spring. The belt31is loosely fitted to the outside of the assembly including the pad32, the stay33, the elastic member34, and the thermistor TH. In the above case, the thermistor TH is in contact with the inner surface of the belt31in an elastic manner with a predetermined pressing force at substantially the middle portion of the belt31in the longitudinal direction with the elastic force of the elastic member34.

The unit30includes terminal members35L and35R that are mounted on the two end portions side of the assembly described above. The terminal members35L and35R perform a role of regulating the movement of the belt31in the width direction and guiding the inner peripheral surfaces of the two edge portions of the rotating belt31. The terminal members35L and35R are molded members of a heat resistant and electrically insulating resin and are disposed symmetrically in the left and right two edge portions of the belt31.

As illustrated inFIG. 4, the terminal members35L and35R have flange portions (flange seat portions)35afor receiving the edge faces of the power feed rings38L and38R with the abutting surfaces35bon the inner surface sides.

Furthermore, guide portions35cproject from the flange portions35atowards the center of the belt31in the longitudinal direction. The guide portions35cfit inside the power feed rings38L and38R and guide the inner peripheral surface of the power feed rings38L and38R into a circular shape. Furthermore, circular portions35dproject from the guide portions35ctowards the center of the belt31in the longitudinal direction. The circular portions35dare substantially coaxial with the guide portions35cand have outside diameters that are smaller than those of the guide portions35c.

Hole portions35eare provided in the guide portions35cof the terminal members35L and35R and the circular portions35dthat have small outside diameters and that continuously extend from the guide portions35c. Left and right end portions33aof the stay33are inserted in the hole portions35e. Furthermore, pressure-receiving block portions35fproject from the flange portions35atowards the outer side in the longitudinal direction of the belt31. Vertical groove portions35g(FIGS. 2 and 4) are provided in the pressure-receiving block portions35f, and the terminal members35L and35R are engaged with side plates51L and51R.

Since the left and right end portions33aof the stay33are inserted in the hole portions35eof the terminal members35L and35R, the leg portions of the stay33in the end portions33aare shorter than those in the longitudinal center portion. The pad32is joined to and held by the leg portions in the longitudinal center portion of the stay33.

By sufficiently fitting and engaging the end portions33aof the stay33to the hole portions35e, the terminal members35L and35R become mounted as a portion of the unit30. In the above state, the guide portions35care fitted inside the inner diameter portion of the power feed rings38L and38R of the unit30.

As illustrated inFIG. 4, power feed members60L and60R are disposed at the apexes of the flange portions35aof the terminal members35L and35R through the plate springs61formed of, for example, SUS. The power feed members60L and60R elastically abut against the outside diameter surfaces of the power feed rings38L and38R at the left and right two edge portions of the belt31with the spring forces of the plate springs61. In other words, the power feed members60L and60R are in contact and are in electrical communication with the outside diameter surfaces of the left and right power feed rings38L and38R of the belt31. Since an electrical current of 12 A flows between a power feed members60and the power feed rings38, the contact area between each power feed member60and the corresponding power feed ring38is desirably 10 mm2or more. In the present exemplary embodiment, the width of a power feed member60in the longitudinal direction is 6 mm and the width thereof in the short direction is 2 mm.

Note that in the present exemplary embodiment, a metal brush is used as each of the power feed members60L and60R. Furthermore, in place of the metal brush, an electro conductive member such as a metal block or a carbon chip may be used.

The roller40is a nip forming member that forms the nip portion N with the unit30(the belt31) while working together with the unit30(the belt31). As illustrated inFIG. 3, the roller40is an elastic roller that includes an elastic layer42that is formed in a shape of a roller coaxially and integrally on an outer periphery of a core metal41formed of a metal material, and an insulating layer43formed of fluoro plastic further formed on an outer peripheral surface of the elastic layer42. The material of the elastic layer42may be selected from heat-resistant rubber such as silicone rubber and fluoro rubber, and a foam body of silicone rubber. In order to convey the sheet P through the nip portion N in a stable manner without creating any crease in the sheet P, in the present exemplary embodiment, the shape of the outside diameter of the elastic layer42of the roller40is an inverted crown shape. Small diameter shaft portions41aare provided in a coaxial and an integral manner with respect to the core metal41at the two left and right end portions of the core metal41.

As illustrated inFIG. 2, the left and right small diameter shaft portions41aof the roller40are disposed so as to be rotatably held between the side plates51L and51R of an apparatus frame50through bearing members52L and52R. A drive gear G is disposed in a coaxial and integral manner at the end portion of the small diameter shaft portion41aon the right side. Driving force of a motor M (a drive source) that is controlled by the control circuit100is transmitted to the gear G through a motive power transmission mechanism (not shown). With the above, the roller40serving as a drive rotation member is rotationally driven in the direction (counterclockwise inFIG. 3) of the arrow R40at a predetermined circumferential speed.

Meanwhile, the belt unit30is disposed on the upper side of the roller40in a substantially parallel manner with respect to the roller40such that the pad32abuts against the roller40through the belt31. More specifically, the vertical groove portions35g(FIGS. 2 and 4) provided in the left and right terminal members35L and35R of the unit30engage with vertical edge portions of vertical guide slits51a(FIG. 3) provided in the left and right side plates51L and51R.

With the above, the left and right terminal members35L and35R are held by the left and right side plates51L and51R, respectively, while a slide motion in the vertical direction is allowed. In other words, the unit30is held so as to be able to perform a slide motion in the vertical direction with respect to the left and right side plates51L and51R.

As illustrated inFIG. 2, pressure springs (urging member)70L and70R are provided in a compressed manner between the pressure-receiving block portions35fof the left and right terminal members35L and35R, and spring receiver seats71that are fixed and are disposed above the pressure-receiving block portions35f. In a free state, the pressure springs70L and70R press the pressure-receiving block portions35fof the terminal members35L and35R on the left and right side in a uniform manner with a predetermined downward pressing force. In the present exemplary embodiment, the pressing force on one end side is 156.8 N (16 kgf) and the total pressing force is 313.6 N (32 kgf).

With the above, through the stay33and the pad32, the belt31counters the elasticity of the elastic layer42and comes in pressure contact (in a compressed state) against the upper surface of the roller40at a predetermined pressing force. Accordingly, a nip portion N having a predetermined width is formed between the unit30(the belt31) and the roller40in the short direction (the sheet conveying direction).

Furthermore, the fixing apparatus F includes pressure release mechanisms72L and72R that counter the pressing force of pressure springs70L and70R and that lift up the left and right terminal members35L and35R so as to release the compressed state with respect to the roller40of the belt31. Specifically, the pressure release mechanisms72L and72R move lifters73according to an instruction from the control circuit100so as to determine the held positions of the terminal members35L and35R.

By moving the terminal members35L and35R to predetermined lift positions, the overall unit30moves in a direction that separates itself from the roller40and the belt31is spaced apart from the roller40such that a pressure released state is maintained. Furthermore, upon descending of the lifters73from the pressure released state, the terminal members35L and35R are lifted down. Then, by moving the lifters73to predetermined descending positions, which are non-acting positions of the lifters73with respect to the terminal members35L and35R, the compressed state is reached again.

Although a specific configuration of the pressure release mechanisms72L and72R is not drawn, mechanisms each using an electromagnetic solenoid or mechanisms each using a cam and a motor, for example, may be adopted. Furthermore, the pressure release mechanisms72L and72R may adopt a configuration that uses a common mechanism for the left and right terminal members35L and35R.

InFIG. 2, the total width of the belt31is indicated by W31and the width of the roller40other than the small diameter shaft portions41ais indicated by W40. The width W40of the roller40is smaller than the total width W31of the belt31by a predetermined length. The length of the stay33excluding the left and right end portions33ais substantially the same as the width W40of the roller40. The length of the pad32is substantially the same as the width W40of the roller40. The width of the nip portion N in the longitudinal direction is the same as the width W40of the roller40. In the present exemplary embodiment, W31is 320 mm and W40is 340 mm.

The left and right power feed rings38L and38R of the belt31are positioned on the outside in the longitudinal direction with respect to the end portions of the roller40(end portions of the nip portion N). Wmax is a conveyance area width of the maximum sheet width (the maximum sheet passing width) that can be used in the fixing apparatus F and is small by a predetermined amount than the width W40of the nip portion N. The width of the resistance heat generating layer31bof the belt31(the width of the effective heating area of the belt31) in the present exemplary embodiment is larger than the sheet conveyance area width Wmax and is smaller than the width W40of the nip portion N.

As illustrated inFIG. 3, the fixing apparatus F includes an upper side cover plate53, a front side cover plate54, an entering side guide plate55, a rear side cover plate56, an exit side guide plate57, and a fixing discharge roller pair58. The fixing discharge roller pair58is rotationally driven in a predetermined direction at a predetermined circumferential speed by transmission of a driving force of the roller40through an interlocking mechanism (not shown). The electro conductive elastic support members61of the left and right power feed members60L and60R are electrically connected to a power supply circuit101(an AC power source) through wires102. Furthermore, the thermistor TH is electrically connected to the control circuit100through a wire (not shown).

In the fixing apparatus F, when a start signal of a print job is input, the control circuit100controls the power supply circuit101and starts energization of the resistance heat generating layer31b(hereinafter, referred to as a heat generating layer31b) of the belt31with a predetermined energization control pattern.

In other words, voltage is applied to the left and right power feed rings38L and38R through the left and right power feed members60L and60R. With the above, the heat generating layer31bis energized through an electrode layer31d(FIGS. 5A and 5B) described later that is in electrical communication with the power feed rings38L and38R. Furthermore, the belt31is heated all around in the width of the effective heating area by heat generation of the heat generating layer31bcaused by the energization.

When electrical information relating to the temperature of the belt31is input to the control circuit100from the thermistor TH, the control circuit100determines the energization control pattern on the basis of the detection temperature of the belt31. Then, on the basis of the determined energization control pattern, the power supply circuit101is controlled through phase control/frequency control or the like such that an appropriate electric power is supplied to the heat generating layer31b.

Furthermore, the control circuit100starts up the motor M and starts rotational driving of the roller40serving as a drive rotation member.

The roller40is rotationally driven in the counterclockwise direction of the arrow R40inFIG. 3at a predetermined circumferential speed. When the roller40is rotationally driven, running torque acts on the belt31with the frictional force in the nip portion N between the roller40and the outer surface of the belt31. With the above, the belt31is driven and is rotated in the arrow R31direction (clockwise direction inFIG. 3) at a circumferential speed that substantially corresponds to the rotational circumferential speed of the roller40.

The rotating belt31deviates and moves towards the left or the right along the longitudinal direction of the pad32; however, the deviation and movement are regulated to a predetermined range with the flange portions35aof the left and right terminal members35L and35R. In detail, the flange portions35aof the left and right terminal members35L and35R receive the motion of the power feed rings38L and38R that rotate together with the belt31. Furthermore, the guide portions35cguide the inner peripheral surfaces of the power feed rings38L and38R that rotate together with the belt31. Subsequently, the control circuit100starts the image forming operation of the image forming unit when the thermistor TH detects a further higher temperature (a job start temperature) that is higher than a first temperature (a standby temperature). Then, the sheet P on which the image t has been transferred is conveyed to the fixing apparatus F. On the other hand, when the thermistor TH detects a third predetermined temperature (a fusing temperature) that is higher than the second predetermined temperature, the control circuit100turns the energization of the heat generating layer31bof the belt31to a temperature adjustment control state. In the temperature adjustment control state, the power supply circuit101performs energization control on the heat generating layer31busing, for example, PI control so that the third predetermined temperature, in which the temperature of the belt31is a fusing temperature, is maintained at a substantially uniform temperature.

When the sheet P on which the image t has been transferred is conveyed to the fixing apparatus F, the sheet P is guided to the entering side guide plate55, enters the nip portion N, and is nipped and conveyed. With the above, the image t and the sheet P are heated and compressed such that the image t is fixed to the sheet P as a fixed image. In the present embodiment, while the sheet P is introduced into the fixing apparatus F based on a so-called center reference, which sets the center of the sheet width as a reference, not limited to the center reference, a so-called one-side reference may be set. The sheet P that has exited the nip portion N is separated from the belt31, is guided to the exit side guide plate57, enters the nip portion N of the fixing discharge roller pair58, and is ejected and conveyed.

After a predetermined print job of a single piece of sheet or a plurality of pieces of sheet in succession is completed, the control circuit100stops the energization of the heat generating layer31bof the belt31. Furthermore, the control circuit100stops driving of the motor M. In the above state, the control circuit100sets the fixing apparatus F in a waiting state until the next print job start signal is input.

FIG. 5Ais a schematic diagram of a cross section of the heat generating layer of the belt31.FIG. 5Bis a schematic diagram of a cross section illustrating a layer configuration of a left edge portion of the belt31. Since the belt31is symmetrical in the left and right in the longitudinal direction, a layer configuration of the right edge portion of the belt31is similar to that ofFIG. 5B.

The overall belt31is a flexible endless member (an endless belt). The belt31has a width that is larger than that of the roller40so that the power feed rings38can be attached to the two edge portions of the belt31in the longitudinal direction. As illustrated inFIGS. 5A and 5B, the belt31has a structure having at least three layers in which, in order from the outside to the inside, an insulating layer31a, the heat generating layer31bthat generates heat when power is fed thereto, and a cylindrical insulating base material31c(hereinafter, referred to as a base material31c) are stacked. In the present exemplary embodiment, in order to improve the fixing characteristics, an elastic layer31eis provided between the insulating layer31aand the heat generating layer31b. At the two edge portions of the heat generating layer31bin the longitudinal direction, electrode layers31dserving as conductive layers are provided all around the outer surfaces. In the present exemplary embodiment, the electrode layers31dare each provided in the 15 mm area of the corresponding one of the two edge portions of the belt31in the longitudinal direction.

The base material31cis a member that maintains the strength of the belt31and is flexible, being capable of deforming in the circumferential direction. The base material31cis a member having an insulation property. As the material of the base material31c, a resin belt formed of, for example, polyimide, polyimide-amide, PEEK, PTFE, PFA, and FEP, and furthermore, a metal belt formed of SUS, nickel, or the like may be used. Note that PEEK is polyetheretherketone, PTFE is polytetrafluoroethylene, PFA is perfluoroalkoxy alkane, FEP is perfluoroethylene-propene copolymer. However, when too thin, the base material31cis easily damaged, and when too thick, the base material31cdoes not easily deform itself; accordingly, it is desirable to use a heat-resistance resin material, such as polyimide, having a thickness of 20 μm or more to 100 μm or less. In the present exemplary embodiment, a cylindrical polyimide belt having a thickness of 50 μm and an inside diameter of 24 mm is used.

The elastic layer31eis a layer to facilitate the belt31follow the uneven surface of the sheet P and to improve the fixing characteristics. In order to reduce the heat capacity and improve the quick starting property, silicone rubber and fluoro rubber materials of high thermal conductivity that are 400 μm or under in thickness are used.

The heat generating layer31bis a layer that is formed on the outer peripheral surface of the base material31cand that generates heat upon energization. As for the material of the heat generating layer31b, a material in which electro conductive carbon and metal powders are distributed in a heat-resistance resin, such as polyimide, may be used. In the present exemplary embodiment, a coat layer of a resistance heating element that is formed of polyimide in which carbon is distributed and that has a thickness of 25 μm is used. The carbon distribution amount and the like is adjusted such that the resistance value in the portion between the left and right electrode layers31don the two edge portions side of the belt31is 10Ω at normal temperature. Accordingly, when 100 V is applied, the heat generating layer31bgenerates heat with an electric power of about 1000 W.

The insulating layer31ais formed on the entire heat generating layer31band a portion of the electrode layers31don the heat generating layer31bside. The insulating layer31aprevents electric current from flowing into portions other than the belt31and prevents the belt from becoming dirty due to adhesion of toner and the like. The insulating layer31ais required to have release characteristics with respect to the toner and, since the insulating layer31ais in contact with the electrode layers31dand the heat generating layer31b, is required to have an insulation property so that no electric current flows therein; accordingly, a fluoro plastic material having an insulation property such as PFA or PTFE may be used.

If the insulating layer31ais too thin, the life becomes short due to wear caused by friction with the sheet P and the roller40, and if too thick, the heat capacity will increase and the transmission of heat will be reduced and the energy saving property may disadvantageously be degraded; accordingly, it is desirable to use a fluoro plastic material of 10 to 50 μm. In the present exemplary embodiment, an insulating PFA resin tube of 20 μm in thickness is used.

The electrode layers31dare layers to evenly energize the whole circumference of the heat generating layer. It is desirable that the resistivity of the electrode layers31dare sufficiently lower than the resistivity of the heat generating layer31b, and in the present exemplary embodiment, a material having conductive characteristics including silver-palladium is used and the thickness thereof is 10 μm.

Note that as long as the adhesiveness between the power feed rings38and the belt31are satisfactory and unevenness in energization can be suppressed, the electrode layers31ddo not necessarily have to be provided.

Note that in the present exemplary embodiment, while the electrode layers31dare provided on the heat generating layer31b, the configuration of the belt31is not limited to the above. For example, the heat generating layer31bon the base material31cand the electrode layers31dmay be provided side by side.

A configuration of the power feed rings38L and38R will be described in detail next.FIGS. 6A to 6Care diagrams for describing a configuration of the power feed ring38L of the present exemplary embodiment.FIG. 7is a partial cross-section view of the fixing apparatus according to the present exemplary embodiment.

The fixing apparatus F of the present exemplary embodiment has a left and right symmetrical structure in which the power feed ring38L is provided on one end of the belt31in the longitudinal direction and the power feed ring38R is provided on the other end. Accordingly, the power feed ring38L will be used as an example in the following description. The power feed ring38L includes an outer ring46L serving as a first ring-shaped member, an inner ring47L serving as a second ring-shaped member, a fixing ring48L serving as a fixing member (a ring-shaped holding member). The power feed ring38R includes an outer ring46R serving as a third ring-shaped member, an inner ring47R serving as a fourth ring-shaped member, a fixing ring48R serving as another fixing member (a ring-shaped holding member). Hereinafter, the power feed rings38L and38R will be collectively referred to as power feed rings38. The outer rings46L and46R will be collectively referred to as outer rings46. The inner rings47L and47R will be collectively referred to as inner rings47. The fixing rings48L and48R will be collectively referred to as fixing rings48. The power feed members60L and60R will be collectively referred to as power feed members60.

By combining the members described above, the power feed rings38are joined to the edge portions of the belt31and function as electrode portions that are capable of rotating in an integrated manner with the belt31. The power feed rings38are rigid and do not easily become warped such that the rotation paths are each close to a perfect circle. Accordingly, the abutting state against the power feed members60can be maintained in a satisfactory manner. In other words, while conventional methods that does not use the power feed rings38and that directly abuts the power feed member against the electrode layer of the belt31encounter a problem in that a conduction failure occurs with the power feed member due to vibrations of the electrodes, the present exemplary embodiment has overcome the problem. Furthermore, different from conventional methods, in the present exemplary embodiment, the power feed members60do not directly come in contact with the electrode layers31d. Accordingly, peeling off of the electrode layer due to wear of the surface of the electrode layer can be prevented and increase in the life of the belt31can be achieved.

Furthermore, in the present exemplary embodiment, the power feed rings38are devised so that the adhesiveness between the power feed rings38and the belt do not become lower due to thermal expansion of the outer rings46. Specifically, the materials of the inner rings47and the outer rings46are selected so that a coefficient of linear expansion of the inner rings47is larger than a coefficient of linear expansion of the outer rings46. In such a configuration, since the amount of change in the outside diameter of the inner rings47caused by thermal expansion is larger than the amount of change in the inside diameter of the outer rings46caused by thermal expansion, the power feed rings38and the belt31can be adhered to each other regardless of the temperature of the power feed rings38. Detailed description will be given below using the drawings.

The outer rings46are electro conductive members that fit outside the annular electrode layers31don the outer peripheral surface sides of the edge portions of the belt31. Since the outer rings46are used as an energization path that electrically communicates the power supply circuit101and the belt31to each other, desirably, a metal with low electric resistance is used. Furthermore, in order for the power feed members60to be in contact in a stable manner, the outer rings46are, desirably, members with high rigidity that can maintain the shape close to a perfect circle. In the present exemplary embodiment, a ring-shaped member that is made by pressing a copper plate with a thickness of 1 mm is used as each of the outer rings46. Furthermore, protrusion portions46dare provided in one edge portion of each outer rings46. Note that the outer rings46abut against the power feed members60in a slidable manner and perform electrical connection.

The inner rings47are members that are disposed to face the outer rings46through the belt31. In the present exemplary embodiment, a ring-shaped member that is made by pressing an aluminum plate with a thickness of 1 mm is used as each of the inner rings47. The inner rings47each include an annular portion47athat is inserted in the inner peripheral surface side of the edge portion of the belt31, and a flange portion47bthat has a diameter that is larger than the belt diameter. Details of the inner rings47will be described later.

Fixing rings48are ring-shaped members that fix the inner rings47and the outer rings46to each other. The fixing rings48include annular portions48athat are inserted in the inner surfaces of the inner rings47, and hook portions48bat the edge portions. In the present exemplary embodiment, a PPS resin (a polyphenylene sulfide resin) that is excellent in resisting heat is used as the material of the fixing rings48. Note that the inside diameter and the outside diameter of each ring described later represents the dimension of the annular portion of each ring.

Furthermore, the outer rings46and the inner rings47are disposed so as to pinch the front and back surfaces of the edge portions of the belt31, and the fixing rings48fixing the above form the power feed rings38.

Specifically, by having protrusion portions46dbe caught in the hook portions48bof the fixing rings48, the positions of the outer rings46in the longitudinal direction of the belt31are fixed to the edge portions. Furthermore, by having the flange portions47bbe caught between the fixing rings48and the outer rings46, the positions of the inner rings47in the longitudinal direction of the belt31are fixed to the edge portions. In other words, the fixing rings48hold the outer edge portions of the outer rings46and hold the outer edge portions of the inner rings47to integrally fix the outer rings46and the inner rings47to each other. Furthermore, the inner rings47and the outer rings46being in close contact with the belt31enables the power feed rings38to rotate in an integrated manner with the belt31.

Note that while in the present exemplary embodiment, the inner rings47and the outer rings46are fixed with the fixing rings48, the fixing method is not limited to the above. For example, holes may be made at corresponding positions of the inner rings47and the outer rings46and a screw or the like may be fixed therein so as to fasten the inner rings47and the outer rings46to each other.

Incidentally, the power feed rings38attached in the above manner to the belt31that generates heat gradually becomes heated as the fixing process proceeds. Accordingly, the outer rings46, the material of which is metal (copper in the present exemplary embodiment), expand as the temperatures thereof rises and the diameters thereof becomes larger. Furthermore, when the diameters of the outer rings46become larger, the adhesiveness between the inner peripheral surfaces of the outer rings46and the outer peripheral surface of the belt31(the electrode layers31din the present exemplary embodiment) is lowered and the contact areas of the above are reduced. With the above, there may be cases in which feed of power to the belt31becomes disadvantageously unstable and the belt31disadvantageously causes heat generation failure. Furthermore, there may be cases in which the life of the belt31is disadvantageously shortened due to local heat generation caused by the electric current flowing locally in the belt31from the outer rings46and due to electric discharge between the outer rings46and the belt31.

Accordingly, in the present exemplary embodiment, in order to overcome the above problem, aluminum, as described above, is used as the material for the inner rings47. Specifically, investigation had been made on the material used for the inner rings47in the below manner.

When the diameter of the cylindrical member at normal temperature (25° C.) is D0, the linear expansion coefficient of the cylindrical member is α, and the temperature variation is ΔT, then, the diameter D of the cylindrical member that thermally expands is expressed by the following equation.
D=D0(1+αΔT)  (1)

Accordingly, a gap X that is the difference between the inside diameter of each outer ring46and the outside diameter of the corresponding inner ring47is expressed by the following equation.
X=D1(1+α1ΔT)−D2(1+α2ΔT),  (2)
where the inside diameter of each outside ring46at normal temperature is D1, the linear expansion coefficient of each outer ring46is α1, the outside diameter of each inner ring47at normal temperature is D2, and the linear expansion coefficient of each inner ring47is α2.

As can be understood from equation (1), the variation of the diameter of the cylinder is D0αΔT. In other words, when αΔT is constant, the larger the diameter D0, the variation in the diameter of the cylinder is large. Accordingly, it can be understood that, since the diameter of the inner rings47<the diameter of the outer rings46, when the outer rings46and the inner rings47using the same material are placed under the same temperature environment, then, the higher the environmental temperature, the more diameter difference between the outer rings46and the inner rings47occur. In other words, when the same material is used for the outer rings46and the inner rings47, as the temperature increases, the gap X between the outer rings46and the inner rings47become larger and the force pinching the belt31decreases. Accordingly, one can conceive of using a material, for the inner rings47, that is different with the material of the outer rings46. Table 1 illustrates linear expansion coefficients (coefficient of linear expansion [×10−6/° C.]) of representative metal materials.

As illustrated in table 1, the linear expansion coefficient of copper used in the outer rings46of the present exemplary embodiment is 16 [×10−6/° C.]. Accordingly, as the material of the inner rings47, aluminum (linear expansion coefficient: 23.9 [×106/° C.]) having a linear expansion coefficient that is larger than that of copper has been selected.

A configuration of the power feed rings38will be described next. In the present exemplary embodiment, the power feed rings38pinch areas of the belt31that have a thickness of 80 μm (the base material31cis 50 μm, the heat generating layer31bis 25 μm, the electrode layers31dare 5 μm). Furthermore, in consideration of the ease of assembling the power feed rings38to the belt31, the inside diameters of the outer rings46and the outside diameters of the inner rings47are provided with an intersection of 10 μm. Accordingly, the inside diameters of the outer rings46are designed so as to be 40.09 mm, the outside diameters of the inner rings47are designed so as to be 39.99 mm. Based on the above details, the size of the gap X is obtained by equation (2).

FIG. 9is a diagram illustrating the relationship between the temperature of the power feed ring38and the gap X. As illustrated inFIG. 9, it can be understood that as the temperature of the power feed ring38rises, the gap becomes smaller. Specifically, while the fixing process is continuously performed in the fixing apparatus F, when the power feed ring38reaches 150° C., the gap X is 77 μm. The above gap X is smaller than the thickness (80 μm) of the pinched portion (the base material31c+heat generating layer31b+the electrode layer31d) of the belt31. As described above, in the present exemplary embodiment, when the belt31is heated, the inner rings47are thermally expanded in a greater manner with respect to the thermal expansions of the outer rings46. In other words, the difference obtained by subtracting the diameter of the inner ring47at normal temperature from the diameter of the inner ring47at the temperature (150° C.) during fixing that is higher than the normal temperature (25° C.) is set equivalent to or larger than the difference obtained by subtracting the diameter of the outer ring46at normal temperature from the diameter of the outer ring46at the temperature during fixing. Accordingly, as illustrated inFIG. 9the size of the gap X becomes narrower as the temperature of the power feed ring38becomes higher. In other words, when the power feed ring38is at the temperature during fixing, the difference obtained by subtracting the outside diameter of the inner ring46from the inside diameter of the outer ring47is smaller than the difference obtained when the power feed ring38is at normal temperature. Accordingly, the power feed rings38are capable of firmly pinching the belt31.

(Effect of Present Exemplary Embodiment)

With the configuration described above, the fixing apparatus F of the present exemplary embodiment stabilizes the electric connection between the belt31and the power feed rings38. Specifically, when the belt31is heated, the inner rings47are thermally expanded in a greater manner with respect to the thermal expansion of the outer rings46so that the belt31is pressed towards the outer rings46with the inner rings47, thus, increasing the adhesiveness between the electrode layers31dand the outer rings46.FIGS. 8A and 8Bare diagrams for describing the effects of the present exemplary embodiment.

Since the outer rings46have quite some surface roughness, there are quite some surface unevenness on the inner peripheral surface of the outer rings46. Accordingly, as illustrated inFIG. 8A, when the outer rings46and the electrode layers31dare in contact with each other in a subtle manner, the contact areas between the outer rings46and the electrode layers31dare small. Accordingly, in the above state, the life of the belt31may disadvantageously become shortened due to local heat generation caused by a flow of electric current from the outer rings46into the belt31in a local manner and due to electric discharge created by minute gaps between the outer rings46and the belt31. In the present exemplary embodiment, since the inner rings47press the belt31towards the outer rings46, the relationship between the inner peripheral surfaces of the outer rings46and the electrode layers31dcan be brought to a state illustrated inFIG. 8B. In other words, the uneven surfaces of the inner peripheral surfaces of the outer rings46are pushed into the electrode layers31dand, accordingly, the outer rings46and the electrode layers31dcan be brought to a state with large contact surfaces. Accordingly, in the present exemplary embodiment, the problem illustrated inFIG. 8Ais suppressed from occurring.

An endurance test was conducted on the fixing apparatus F of the present exemplary embodiment and a fixing apparatus F of a comparative example in order to verify the effect of the present exemplary embodiment. Note that the fixing apparatus F of the comparative example uses copper as the material of the inner rings47; other configurations are similar to that of the present exemplary embodiment. A test of continuously performing processes on up to 200K sheets of A4-sized sheets P were conducted with the above-described two fixing apparatuses F. The results are illustrated in table 2. Table 2 is a table that compares the present exemplary embodiment and the comparative example with each other on the occurrence of burn outs of the electrode layers.

TABLE 2Occurrence of Burn out of Electrode Layer andNumber of Sheets P on Which Processes Have Been PerformedComparativeExemplaryExampleEmbodimentBurn Out ofOccurred on 150KthDid not occur onElectrode Layersheet200Kthsheet

As illustrated in table 2, in the comparative example, at around when the 150Kthsheet P was processed, a burn out on the surface of the electrode layer31dwas identified; however, no burn out was identified in the present exemplary embodiment. For verification, the endurance test was conducted on the fixing apparatus F of the present exemplary embodiment until the process was performed on the 200Kthsheet P, even so, no burn out was identified in the electrode layers31d. As described above, stable electrical connections between the inner peripheral surfaces of the outer rings46and the electrode layers31dwere verified in the present exemplary embodiment.

Note that while in the present exemplary embodiment, copper is used for the material of the outer rings46, and aluminum is used for the material of the inner rings47, the combination of the materials is not limited to the above. Any other combination of the materials in which the linear expansion coefficient of the outer rings46<the linear expansion coefficient of the inner rings47is satisfied may be adopted.

Furthermore, in the present exemplary embodiment, when the fixing process is performed, the power feed rings are firmly fixed to the belt31. Accordingly, when assembling the fixing apparatus F, the power feed rings38can be easily mounted on the belt31and sufficient amount of intersection can be provided to the outer rings46and the inner rings47.

OTHER EXEMPLARY EMBODIMENTS

A description of the exemplary embodiment to which the present disclosure can be applied has been given above; however, the numerical values such as the dimension and the like exemplified in the exemplary embodiment are examples and the present disclosure is not limited by the numerical values. The numerical values can be appropriately selected within the scope of the disclosure. Furthermore, the configuration described in the exemplary embodiment may be appropriately modified within the scope of the disclosure.

The belt31is not limited to the configuration in which the inner surface of the belt31is supported by the pad32and in which the belt31is driven by a roller40. For example, the belt31may be a belt of a belt unit type in which the belt is bridged across a plurality of rollers and that is driven by one of the plurality of rollers. However, from the viewpoint of lowering the heat capacity, a configuration such as that of the exemplary embodiment is desirable.

The member forming the nip forming member is not limited to a roller member such as the roller40. For example, a pressure belt unit that bridges a belt across a plurality of rollers may be used.

The image forming apparatus that has been described with the printer1as an example is not limited to the image forming apparatus that forms a full color image, and may be an image forming apparatus that forms a monochrome image. Furthermore, the image forming apparatus may be implemented for various purposes, such as for a copier, a fax machine, or a multi-functional apparatus that is provided with a plurality of the above functions.

The image heating apparatus that has been described above is not limited to the fixing apparatus that fixes an unfixed toner image on the sheet P. For example, the image heating apparatus may be an apparatus that fixes a half-fixed toner image on the sheet P, or may be an apparatus that performs a heat treatment on a fixed image. Accordingly, the image heating apparatus may be used as, for example, a surface heating apparatus that adjusts the glossiness and the surface properties of an image.

This application claims the benefit of Japanese Patent Application No. 2014-229324, filed Nov. 11, 2014, which is hereby incorporated by reference herein in its entirety.