Image heating apparatus, heater and belt replacing method

An image heating apparatus includes: an endless belt; a heater for heating the belt, the heater forms a sliding portion between an inner surface of the belt and the heater; a rotatable member cooperative with the heater to sandwich the belt to form a nip between an outer peripheral surface of the belt and the rotatable member; a temperature detector provided on a surface of the heater relatively more remote from the sliding portion than the heater; a controller for controlling the timing of feeding the sheet to the nip on the basis of an output of the detector; and lubricant provided in at least a part of the sliding portion except for a range which is in an opposing relation with the detector. The sliding portion is free of the lubricant in the range.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a belt for heating an image on a sheet of a recording medium, a heater for heating a belt, an image heating apparatus having the preceding belt and heater, and a method for replacing a belt. An image heating apparatus is employed by an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction apparatus having two or more of the functions of the preceding apparatuses.

An image forming apparatus forms an image of toner, on a sheet of a recording medium, and fixes the image to the sheet by the application of heat and pressure with the use of its fixing device (image heating device). As fixing device used for the above described purpose, there has been proposed a fixing device which has a cylindrical fixation film and a heater, and gives the heat from the heater to the sheet through the fixation film (belt), by placing the heater in contact with the inward surface of the fixation film (Japanese Laid-open Patent Application 2004-47177). A fixing device which uses the above-described heating method is small in thermal capacity because of its structure. Therefore, it can be quickly started up in terms of temperature.

The fixing device disclosed in Japanese Laid-open Patent Application 2004-47177 is provided with a temperature detection element, which is disposed on the opposite surface of its heater from the surface of the heater, which contacts the fixation belt. This fixing device controls the temperature of its heater by controlling the electric power supply to the heater, based on the temperature detected by the temperature detection element. Further, the fixing device disclosed in Japanese Laid-open Patent Application 2004-47177 forms a nip between its fixation film and pressure roller, by placing the pressure roller in contact with the outward surface of the fixation film. Also in the case of this fixing device, as the pressure roller rotates by receiving a driving force from the driving section, friction occurs between the pressure roller and the fixation film, providing thereby the fixation film with rotational force. The fixation film is loosely fitted around the heater holder. Thus, as the fixation film receives the above described rotational force, it circularly moves while sliding on the bottom surface of the heater.

It is also disclosed in Japanese Laid-open Patent Application 2004-47177 that a layer of lubricant, such as heat resistant grease or the like, is placed between the fixation film and the heater to reduce the friction between the fixation film and the heater. There is no detailed description about how to place a layer of lubricant, in Japanese Laid-open Patent Application 2004-47177. However, if the placement of lubricant between the fixation film and the heater is intended to reduce the friction between the fixation film and the heater, it is reasonable to think that the entirety of the area of contact between the fixation film and the heater should be provided with lubricant.

However, if lubricant is evenly spread between the belt and the heater of a fixing device, across the entirety of area of contact between the belt and the heater, it becomes difficult to control the fixing device in temperature, based on the temperature information which a temperature detection element on the heater detects. To describe in greater detail, if lubricant is coated between the belt and the heater across the entirety of area of contact between the belt and the heater, the heat transmission between the belt and temperature detection element is impeded by the lubricant, and therefore, it is likely for the temperature detected by the temperature detection element to be different from the actual temperature of the belt. Therefore, in a case where the fixation temperature of a fixing device is controlled based on the temperature detected by the temperature detection element to heat an image on a sheet of a recording medium, it is possible that the toner image on the sheet is likely to be heated by the belt, the temperature of which is offset from the target level for the fixation temperature, and therefore, unsatisfactory images will be outputted.

SUMMARY OF THE INVENTION

Thus, the primary object of the present invention is to provide an image heating apparatus capable of preventing the outputting of unsatisfactory images.

Another object of the present invention is to provide a heater, a belt, and a belt replacing method, which can provide the same effects.

According to an aspect of the present invention, there is provided an image heating apparatus comprising: an endless belt configured to heat an image on a sheet in a nip; a heater configured to heat the belt, the heater being cooperative with the belt to form a sliding portion between an inner surface of the belt and the heater; a rotatable member cooperative with the heater to sandwich the belt to form the nip between an outer peripheral surface of the belt and the rotatable member; a detecting member provided on a surface of the heater relatively remoter from the sliding portion and configured to detect a temperature of the heater; a controller configured to control the timing of feeding the sheet to the nip on the basis of an output of the detecting member; and lubricant provided in at least a part of the sliding portion except for a range which is in opposing relation with the detecting member. The sliding portion is free of the lubricant in the range.

According to an aspect of the present invention, there is provided an image heating apparatus comprising: an endless belt configured to heat an image on a sheet in a nip; a heater configured to heat the belt, the heater being cooperative with the belt to form a sliding portion between an inner surface of the belt and the heater; a rotatable member cooperative with the heater to sandwich the belt to form the nip between an outer peripheral surface of the belt and the rotatable member; a detecting member provided on a surface of the heater relatively remoter from the sliding portion and configured to detect a temperature of the heater; a controller configured to control the timing of feeding the sheet to the nip on the basis of an output of the detecting member; and lubricant provided in at least a part of the sliding portion except for a range which is in opposing relation with the detecting member. The sliding portion is free of the lubricant in the range. These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention is concretely described with reference to some of the preferred embodiments of the present invention. By the way, these embodiments are nothing but examples of embodiments of the present invention. That is, they are not intended to limit the present invention in scope. In other words, the present invention can be embodied in various forms, which are different from the following embodiments, within the scope of the present invention.

Image Forming Apparatus

FIG. 2is a schematic vertical sectional view of the electrophotographic color printer1in this embodiment, which is an example of an image forming apparatus. It shows the structure of the printer1. There are disposed four image formation sections Y, M, C and Bk in the printer1. The four image formation sections are all electrophotographic processing system of the so-called laser exposure type, and are the same in structure, although they are different in the color of the developer (toner) they store in their developing device.

Each image formation section has an electrophotographic photosensitive drum2(which hereafter will be referred to as a drum), which is rotationally driven in the direction (counterclockwise) indicated by an arrow mark inFIG. 2. Further, each image formation section has a primary charging device3, a laser scanner4, a developing device5, a primary transfer blade6, and a cleaner7, which are processing means for processing the drum2and are disposed in the adjacencies of the peripheral surface of the photosensitive drum2. The four image formation sections form yellow (Y), magenta (M), cyan (c) and black (Bk) toner images on their drums2, respectively. The electrophotographic image formation principle and its process are well-known, and therefore, are not described here.

The toner image formed on the photosensitive drum2in each image formation section is transferred onto an intermediary transfer belt unit8of the image formation section. To describe this process in detail, four monochromatic toner images, different in color, are formed by the four image formation sections, one for one, and are sequentially transferred (primary transfer) onto a transfer belt9, as an intermediary transferring member, which is circularly moving in the direction (clockwise direction) indicated by an arrow mark inFIG. 2. Thus, a full-color toner image is formed of the four unfixed monochromatic yellow (M), magenta (M), cyan (C) and black (Bk) toner images, on the transfer belt9. The unit8has a driver roller10, a tension roller11, and a belt backing roller (which opposes secondary transfer roller12), by which the transfer belt9is suspended, and with which the transfer belt9is provided with a preset amount of tension. Against the roller12, the primary transfer roller13is pressed with the presence of the transfer belt9between the two rollers12and13.

Meanwhile, as the sheet feeding roller15of one of the pair of sheet cassettes14A and14B, or the sheet feeding roller19of a universal sheet feeding tray, is driven, the sheets P of a recording medium are fed into the main assembly of the printer1, one by one, while being separated from the rest, and are conveyed to a pair of registration rollers18through a recording medium conveyance passage16. The roller pair18sends each sheet P to the secondary transfer section, which is formed between the transfer belt9and roller13by the pressing of the roller13against the belt backing roller12, in synchronism with the arrival of the toner images on the belt9at the secondary transfer nip. Thus, the synthetic full-color image on the transfer belt9, which is made up of the four monochromatic toner images, different in color, is transferred onto the sheet, in the secondary transfer section; four monochromatic toner images are transferred together onto the sheet.

As the sheet P comes out of the secondary transfer section, it is separated from the surface of the transfer belt9, and is guided into a fixing device40as an image heating apparatus. The toner images, different in color, on the sheet P is subjected to heat and pressure by the fixing device40. Thus, they melt and mix. Then, they become fixed to the surface of the sheet P, thereby becoming a solid full-color image. After the toner images are separated from the transfer belt9in the second transfer section, the transfer belt9is cleaned by the belt cleaner22, and is repeatedly used for image formation.

When the printer1is in the one-sided print mode, as the sheet P is moved out of the fixing device40, it is discharged from the printer1through one of the recording medium conveyance passages preset for various printing jobs. To describe this process in detail, the sheet P is directed by a flapper23so that the sheet P will be conveyed to a tray25, which is such a tray that as the sheet S is discharged into the tray25, it faces upward, or a tray28, which is such a tray that as the sheet S is discharged into the tray28, it faces downward. Then, the sheet S is discharged onto one of the trays by a pair of discharge rollers27or24.

When the printer1is in the two-sided mode, as the sheet P comes out of the fixing device40, it is directed by the flapper23to a recording medium conveyance passage26. Then, it is guided upward by the passage26. As the trailing edge of the sheet P reaches the reversal point R, the recording medium conveyance passage26begins to convey the sheet P in the opposite direction from the direction in which the sheet P was conveyed to the reversal point. That is, the sheet P is conveyed into the recording medium conveyance passage29, which is for the two-sided mode. Then, the sheet P enters for the second time into the recording medium conveyance passage16from the recording medium conveyance passage29. When the sheet P is conveyed into the passage16after the first fixation, the sheet P has been turned over. Thus, while the sheet P is conveyed through the secondary transfer section for the second time, an unfixed toner image is transferred onto the second surface of the sheet P. As the sheet comes out of the secondary transfer section, it is guided into the fixing device40for the second time.

Thus, the sheet P having an image on both of its surfaces is discharged from the fixing device40. As the sheet P is discharged from the fixing device40, it is discharged from the printer1through one of the sheet passages preset for various jobs one for one.

FIG. 1is a schematic cross-sectional view of the essential portion of the fixing device40.FIG. 3is a vertical sectional view of the essential portion of the fixing device40, at the plane indicated by a pair of arrow marks (3) inFIG. 1, which does not show certain portions of the fixing device40.FIG. 4is an exploded perspective view of the essential portion of the fixing device40.

The fixing device40is a long and narrow device. That is, it is such a device that its lengthwise direction is parallel to the direction which is perpendicular to the direction X in which a sheet P of a recording medium is conveyed through the nip N (fixation nip). The fixing device40has a belt unit30, which is equipped with a fixation belt31(fixing member: it will be referred to simply as “belt” hereafter) for heating the image on a sheet P of a recording medium, in the nip. Further, the fixing device40has a pressure roller32, which forms the nip N between itself and the belt31in coordination with the belt31. The belt unit30and the pressure roller32are disposed in the casing50(50A×50B) of the printer1.

The belt unit30is an assembled combination of the belt31, a ceramic heater33, a guiding member34, a pressure application stay35, a pair of flanges36A and36B, etc. Here, the belt width direction (direction parallel to lengthwise direction of fixing device40) is the direction which intersects the belt movement direction.

The belt31is a cylindrical (endless) flexible member. It is heat resistant, and is capable of transmitting heat to a sheet P of a recording medium. The belt31is loosely fitted around the guiding member34of the belt unit30. The belt31heats the image on a sheet P of a recording medium, as will be described later. Referring to the enlarged portion ofFIG. 1, the belt31is made up of a heat resistant substrate31a, an elastic layer31b, and a release layer31c. The elastic layer31band release layer31care added as necessary. The substrate31ais no more than 100 μm, preferably, 50 μm, and no less than 20 μm, in thickness. It is endless and is formed of such material that is created by mixing thermally conductive filler in a resinous substance such as PTFE, PFA, FEP, polyamide, PEEK, PES, PPS, etc. The release layer31cis made of film, the surface of which is coated with a releasing agent. As the material for the releasing agent, PTE, PFA, FEP, and the like may be listed.

Incidentally, PTFE is an abbreviation for poly-tetrafluoroethylene, and PFA is an abbreviation for perfluoroalkoxyalkane, and FEP is an abbreviation for a copolymer of perfluoroethylene and propane. PEEK stands for polyether-ether-ketone, and PES stands for polyethersulfone. Further, PPS is an abbreviation for polyphenylsulfide. By the way, as the material for the substrate31a, a thin metallic belt, which is made of SUS or the like and which is no more than 50 μm, and no less than 20 μm, in thickness, may be used. Further, in order to obtain color images which are virtually free of the nonuniformity attributable to fixation, the elastic layer31bformed of such material that is created by adding thermally conductive filler in silicone rubber may be provided between the substrate31aand release layer31c.

The ceramic heater33(which hereafter will be referred to simply as the heater) is provided with a resistor which generates heat as electric current flows through it. It is a heating member which is small in thermal capacity. Thus, as electric current flows through it, it quickly increases in temperature. The heater33heats the belt31by being placed in contact with the inward surface of the belt31. Thus, an area of contact is formed between the belt31and the inward surface of the belt31. Here, the lengthwise direction of the heater33(lengthwise direction of heating member) is such a direction that intersects the belt movement direction, as will be described later.FIG. 5shows an example of structure for the heater33. To describe this structure in detail,FIG. 5(a)is a plan view of the outward surface side of the heater33(on which belt31slides), in which certain portions of the heater33are not shown.FIG. 5(b)is a plan view of the inward surface side of the heater33.FIG. 5(c)is an enlarged sectional view of the heater33, at the plane indicated by a pair of arrow marks (c) inFIG. 5(b).

The heater33has a ceramic substrate33awhich is long, narrow, and thin, and a resistor33bwhich was formed on the top surface (one of surfaces) of the ceramic substrate33ain such an attitude that its lengthwise direction became parallel to the lengthwise direction of the substrate33a, and which generates heat as electric current flows through it. The heater33is also provided with a pair of electrodes33cwhich are electrically connected to the lengthwise ends of the resistor33b, one for one. The outward surface of the substrate33ais covered with a protective layer33dwhich was formed in such a shape that it does not cover the portions of the substrate33a, which have the electrodes33c. That is, the resistor33bis covered with this protective layer33d, being thereby protected. In other words, it is on the outward surface of the protective layer33dof the heater33that the belt31slides.

In terms of the lengthwise direction of the heater33, the portion of the heater33, which corresponds in position to the resistor33bwhich is between the pair of electrodes33cwhich are at the lengthwise ends of the resistor33b, is the effective heat generation range of the heater33, which is 330 mm in length in this embodiment. In terms of the widthwise direction of the belt31, the dimension of the belt31is roughly the same as, or slightly larger than, this effective heat generation range of the heater33. Also, in terms of the widthwise direction of the belt31, the dimension of the largest sheet P of a recording medium, which can be introduced into the fixing device40, and which is conveyable through the fixing device40, is slightly smaller than the dimension of the effective heat generation range of the heater33.

Here, the width of a sheet P of a recording medium means the dimension of the sheet P in terms of the direction perpendicular to the sheet conveyance direction. The fixing device40in this embodiment is structured so that when the sheet P is conveyed through the fixing device40, the center of the sheet P coincides with the center of the recording medium conveyance passage in terms of the widthwise direction, regardless of the sheet dimension. A reference character O stands for the centerline, with which the centerline (theoretical line) of the sheet P coincides as the sheet P is conveyed through the fixing device40.

There is provided a thermistor TH, as a member (temperature detecting member) for detecting the temperature of the heater33, on the inward side (opposite side of substrate33afrom the belt31). To describe this structure in detail, the thermistor TH is disposed so that the temperature detecting surface of the thermistor TH contacts the inward surface of the heater33. That is, the thermistor TH is on the opposite surface of the heater33from the surface of the heater33on which the belt33slides. By the way, in terms of the lengthwise direction of the heater33, the position of the thermistor TH roughly coincides with the above described referential centerline for recording medium conveyance.

The downwardly facing surface of the guiding member34is provided with a heater accommodation groove34a(FIG. 1), which extends in the lengthwise direction. The heater33is fitted in this heater accommodation groove34ain such an attitude that the resistor bearing side (belt facing side: protection layer side) of the heater33faces outward. This is how the heater33is supported by the guiding member34.

The guiding member34has a function of playing the role of heater holder for holding the heater33as described above. Further, the guiding member34assists the heater33in pressing the belt31toward the pressure roller32. Moreover, the guiding member34has a function of a guide which stabilizes the circular movement of the belt31. As the materials for the guiding member34, substances which are heat resistant and thermally insulative are used. For example, they are phenol resin, polyamide resin, polyamide resin, polyamide-imide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, LCP resin, and the like. Incidentally, LCP stands for liquid polymer.

The pressure application stay35is a rigid member. It is long and narrow, and is U-shaped in cross section. As the materials for the pressure application stay35, metals such as iron are used. Since the guiding member34is formed of resin, being therefore relatively flexible, the pressure application stay35supports the guiding member34by being pressed upon the back surface of the guiding member34, to keep the guiding member34correct in shape.

The belt31is loosely fitted around the assembled combination of the heater33, the guiding member34, and the pressure application stay35.

The flanges36A and36B are members with which the lengthwise ends of the pressure application stay35are fitted one for one. The flanges36A and36B have a function of guiding the belt31by supporting the belt31by the inward surface of the belt31as the belt31is circularly moved, and also, a function of regulating the belt31in the widthwise deviation (in the lengthwise direction, the thrust direction). The flanges36A and36B are a pair of members which are the same in shape, and are symmetrically positioned. They are made of heat resistant resin, for example.

Referring toFIGS. 3 and 4, the flanges36A and36B have an actual flange36a, a rack portion36b, and a pressure bearing portion36c. The flange36ais a portion which regulates the belt31in the movement in the thrust direction, by catching the belt31by the corresponding edge of the belt31. Regarding the shape and size of the flanges36A and36B in terms of cross-sectional view, the flanges36A and36B are larger than the belt31. The rack portion36bis on the inward side of the pair of flanges36a, and is arc-shaped in cross-section. It helps the belt31remain cylindrical, by being placed in contact with the lateral edges of the inward surface of the belt31. The pressure bearing portion36cis on the outward surface of the flange36a, and bears the pressure applied by the pressure application mechanism37A and37B.

The pressure roller32has a metallic core32a, an elastic layer32b, and a release layer32c. The elastic layer32bis in the form of a hollow roller, and is coaxially formed around the metallic core32a. As the materials for the elastic layer32b, substances such as silicone rubber, fluorine rubber, fluorine resin, etc., which excels in heat resistance and elasticity, can be selected. As the material for the release layer32c, substances such as fluorine resin, silicone resin, fluorosilicone resin, fluorine rubber, silicone rubber, PFA, PTFE, FEP, etc., have an excellent releasing property and heat resistance can be selected.

Referring toFIG. 3, the pressure roller32is disposed between the pair of side plates50A and50B of the apparatus casing50. It is rotatably supported by the lengthwise ends of its metallic core33a, with the placement of a pair of bearings51between the metallic core32aand side plates50A and50B, one for one. The belt unit30is disposed between the pair of side plates50A and50B. It is disposed in such an attitude that its heater side faces the pressure roller32, and also, that it is practically parallel to the pressure roller32.

The flange36A is fitted in the flange guiding hole52of the side plate50A in such a manner that it is allowed to slide relative to the side plate36A. The flange36B is fitted in the flange guiding hole52of the side plate50B in such a manner that it is allowed to slide relative to the side plate36B. Further, the pair of flanges36A and36B are under a preset amount of pressure generated by the pressure application mechanisms37A and37B, respectively, in the direction to press the flanges36A and36B toward the pressure roller32.

As the belt unit30is moved toward the pressure roller32by the above described pressure, the heater31is pressed against the pressure roller32, with the presence of the belt31between the heater33and the pressure roller32. Thus, the elastic layer32bof the pressure roller32is compressed by the preset amount of pressure, against the resiliency of the elastic layer32b. Thus, a nip N, which has a preset width in terms of the sheet conveyance direction X, is formed between the belt31and the pressure roller32. That is, the pressure roller32functions as a nip forming member which works with the heater33to pinch the belt31to form the nip N between itself and belt31.

A control circuit45has a function of controlling the operation of the fixing device40. The control circuit45is electrically connected to an AC control circuit42to control the AC control circuit42. Further, the control circuit45is electrically connected to a motor M to control the motor M.

Moreover, the AC control circuit42is electrically connected to an AC power source41. It can provide the heater33with electrical current through the connectors48A and48B with which the lengthwise ends of the heater33are provided one for one.

The control circuit45is such a control section that controls the amount of electric current flowing though the heater33, with the use of the AC control circuit42, based on the temperature Theatdetected by the thermistor TH placed in contact with the inward surface of the heater33. That is, the control circuit45controls the heat generation of the heater33based on the output of the thermistor TH. Regarding the amount of electrical current flowing through the heater33, the control circuit45sets the amount P (%) of electric current provided to the heater33, between 0% where the heater33is provided with no electric current, and 100% where the heater33is continuously provided with electric current. As for the method for providing the heater33with a preset amount (%) of electric current, phase control or frequency control can be used. Further, the control circuit45controls the rotation of the pressure roller32by controlling the rotation of the motor M. To describe this structure in detail, the motor M is connected to a gear G, which is attached to the other end of the metallic core32aof the pressure roller32.

Next, the operation carried out by the fixing device40during a printing operation is described.FIG. 6is a flowchart of the fixation sequence (fixing process). As the control circuit45receives a print start signal (S1), it begins to supply the heater33with electric current by an electric current supply ratio (P1%) for start up (S2). During this period, the motor M is kept stationary. As the control circuit45detects that the temperature Theatdetected by the thermistor TH is no less than the motor driving temperature Tmotor(S3), it begins to drive the motor M to rotate the motor M at a preset speed (S4).

As the motor M is driven, the pressure roller32is rotationally driven in the direction indicated by an arrow mark R32(counterclockwise direction inFIG. 1). As the pressure roller32is rotationally driven, the belt31is rotated by the rotation of the pressure roller32in the direction indicated by an arrow mark R31(clockwise direction inFIG. 1). During this rotational movement of the belt31, the belt31slides on the heater33and the guiding member34, with its inward surface remaining in contact with the heater33and the guiding member34. That is, as the pressure roller32is rotationally driven, the belt31is given rotational torque by the friction which occurs between the belt31and the pressure roller32in the nip N. Thus, the belt31rotates around the combination of the guiding member34and the pressure application stay35at roughly the same speed as the speed of the pressure roller32, while sliding on the combination with its inward surface remaining airtightly in contact with the combination. That is, the pressure roller32functions as such a rotational member that rotates the belt31with its rotation.

As the control circuit45detects that the temperature Theatdetected by the thermistor TH is no less than the image formation start temperature Timage(S5), it makes the image forming sections start image forming operations (S6). Next, as the control circuit45detects that the temperature Theatdetected by the thermistor TH is no less than the target temperature level Ttarget(S7), it switches its method for controlling the electric current supply to the heater33, to the PID control to continue to control the heater33in temperature (S8).

The unfixed toner image formed through the image forming operation of the image forming section is transferred onto a sheet P of the recording medium, and is conveyed to the fixing device40while remaining in the state in which it was transferred onto the sheet P.

Then, the sheet P is guided into the nip N following the entrance guide34of the fixing device40. Then, the sheet P is moved through the nip N along with the belt31, with the toner image bearing surface of the sheet P remaining in contact with the outward surface of the belt31. That is, the control circuit45executes such control that conveys the sheet P to the above-described nip based on the temperature detected by the thermistor TH.

While the sheet P is conveyed, remaining pinched by the pressure roller32and the belt31, through the nip N, the heat generated by the heater33is given to the sheet P. Thus, the unfixed toner image T becomes welded to the surface of the sheet P. After being conveyed through the nip N, the sheet P is separated from the belt31by the curvature of the belt31. Then, it is discharged from the fixing device40by a pair of discharge rollers (unshown) with which the fixing device40is provided. A separation guide44is disposed in the adjacencies of the sheet exit of the nip N. It is positioned closer to the belt31than the pressure roller32. Further, the separation guide44is disposed so that a gap is provided between the separating edge of the separating guide44and the belt31to prevent the belt31from coming into contact with the separation guide44as the belt31is rotationally driven.

As soon as the control circuit45detects that the last sheet P of the recording medium in the on-going printing operation (printing job) has passed through the nip N (S9), it stops providing the heater33with electric current, and stops the motor M (S10).

In this embodiment, in order to smoothly rotate the belt31by reducing the friction between the belt31and the heater33, and also, the friction between the belt31and the guiding member34, lubricant is provided between the inward surface of the belt31and the heater33. Hereafter, the area in which the belt31slides on the heater33by its inward surface will be referred to as the slide section (slide area, slide portion). The slide section is the section of the inward surface of the belt31, which corresponds in position to the nip N which is on the outward side of the belt31. As lubricant, heat resistant oil or grease is desirable. For example, silicone oil, PFPE (perfluoropolyether), fluorine grease, and the like can be used as lubricant. In this embodiment, fluorine grease MOLYKOTE (registered commercial name) HP-300 (product of Toray-Dow-Coring Co., Ltd.) was used as lubricant.

Hereafter, an area33A of the outward surface (belt contacting surface) of the heater33, on which the belt31slides, and which corresponds in position to the temperature detecting surface of the thermistor TH, in terms of the lengthwise direction of the heater33, will be referred to as a thermistor area or thermistor placement area. Referring toFIG. 5, in terms of the lengthwise direction, the area33A is wider than the temperature detection surface of the thermistor TH. Further, in terms of the lengthwise direction of the heater33, the areas33B of the outward surface of the heater33, on which the belt slides, and which are the areas of the outward surface of the heater33, which are not the area33A (which are on the outward side of the area33A), will be referred to as areas33B.

In this embodiment, the belt contacting surface of the heater33of the fixing device40is coated in advance with lubricant to provide the area of contact between the belt31and the heater33, when the fixing device40is brand-new. To describe this in detail, referring toFIG. 5(d), the area33A of the belt contacting surface of the heater33is coated with virtually no grease130, whereas the areas33B are coated with a preset amount of grease130.

The expression “coated with virtually no grease” means both “coated with no grease130at all”, and “coated with a very small amount of lubricant”.

In this embodiment, the sum of the length of the areas33A and33B in terms of the lengthwise direction of the heater33is 330 mm, which corresponds to the length of the effective heat generation range of the resistor33b. Of the two areas, the area33A (preset area, preset range) is the center portion of the heater33in terms of the lengthwise direction of the heater33. It is 30 mm in length. The area33A is coated with 15 mg of grease130, in such a pattern that the width of the coated portion becomes 5 mm in dimension in terms of the widthwise direction of the heater33(sheet conveyance direction). The areas33B (remaining area, remaining range, portions which are on the outward side of preset area33A) are the portions of the belt contacting surface of the heater33, which remain as the area33A is excluded, the heater being 330 mm in dimension in terms of the lengthwise direction of the heater33. In terms of the lengthwise direction of the heater33, the sum of the dimensions of the two areas33B is 300 mm. The combination of the areas33B is coated with 750 mg of grease130in such a pattern that the dimension of the coated area becomes 5 mm in dimension in terms of the heater width direction (sheet conveyance direction). Incidentally, the amount of a given area of the belt contacting area of the heater33that is coated with grease per unit area can be obtained based on the total amount of grease coated on the given area, and the size of the given area.

That is, the amount by which the area33A is coated with grease130per unit area is 0.1 mm/mm2. Therefore, the total amount of grease130which is present on the heater33per 1 mm in terms of the heater length direction is 0.5 mg. That is, the amount of grease on the belt contacting area per unit length in terms of the heater length direction is 0.5 mg/mm. Hereafter, the grease130coated on the area33A will be referred to as the first lubricant.

The amount of the grease, per unit area, on the portion of the area33B coated with grease130is 0.5 mg/mm2. Therefore, the total amount of grease130which is present on the area33B per 1 mm in terms of the heater lengthwise direction is 2.5 mg/mm. That is, the amount of grease per unit length in terms of the heater length direction is 2.5 mg/mm. Hereafter, the grease coated on the areas33B will be referred to as the second lubricant.

That is, the amount of the grease, per unit area, on the grease-coated portion of the area33A of the belt contacting surface of the heater33is smaller than the amount of the grease, per unit area, on the grease-coated portion of the area33B.

By the way, before the heater31is attached to the heater holder after it is coated with the grease130, the amount of grease130per unit area on each area (portion) of the heater31can be confirmed through the following procedure. To begin with, the size of the grease-coated portion of a given area is measured. Then, the grease130on the given area is scraped away, and the total amount of the removed grease130is measured. Then, the value of the total amount of the removed grease130is divided by the value of the size of the given areas, to confirm the amount of the grease, per unit area, which was on the given area.

On the other hand, after the heater31is attached to the heater holder after it was coated with the grease130, the amount of the grease130, per unit area, on a given area of the heater131can be confirmed with the use of the following procedure. To begin with, the belt31is stopped at an optional point, and the belt33is cut to a preset depth following the contour of the heater33. Then, the heater31is removed from the guiding member34. During the removal of the heater31, the pieces of belt31which have resulted from the cutting of the belt31following the contour of the heater33, remain adhered to the heater33because of the viscosity of the grease130. Thus, it is possible to remove only the portion of the belt31, which was in contact with the belt contacting area of the heater33from the fixing device40.

Then, the grease130is scraped down from the target area of the belt contacting portion, and the total amount of the removed grease130is measured. To describe this in detail, the grease130is scraped down from both the heater33and belt31. Then, the amount of grease, per unit area, on the target area can be confirmed by dividing the value of the total amount of the thus collected grease by the value of the size of the target area.

The surface temperature of the belt31of the brand-new fixing device40having the heater33in this embodiment was measured while observing the temperature Theatdetected by the thermistor TH.FIG. 7shows the results of the measurement; it shows the temperature increase curve. The belt temperature shown inFIG. 7is the average temperature of the area of the belt31, which corresponds in position to the area33B.

FIG. 7includes the test results of a comparative example of heater33, which was uniformly coated in a width of 5 mm, with the grease130across the entirety of both the area33A and areas33B. More concretely, the amount of grease, per unit area, on the grease-coated area of the comparative example of heater33was 0.5 mg/mm2. That is, the total amount of grease per 1 mm in terms of the heater length direction was 2.5 mg (2.5 mg/mm). This comparative example of heater33was installed in the fixing device40, and the surface temperature of the belt31was measured while providing the heater33with electrical current and collecting the temperature

Theatmeasured by the thermistor TH.FIG. 7shows the results of the measurement; it shows the temperature increase curve.

Referring toFIG. 7, in the case of the heater33in this embodiment, the temperature Theatdetected by the thermistor TH reached 200° C. 14 seconds after the heater33began to be supplied with electric power. In comparison, in the case of the first comparative example of the heater33, the temperature Theatdetected by the thermistor TH reached to 200° C. 10 seconds after it began to be supplied with electric power.

The reason why the length of time it took for the temperature Theat detected by the thermistor TH to reach 200° C. is shorther than the length of time for the heater33in the first comparative example of heater33in this embodiment is as follows. That is, the amount of grease between the area33A of heater33, and belt31in the first comparative example of the heater33is greater, being therefore slower in the heat transfer from the heater33to the belt31, than the heater33in this embodiment. Therefore, the first comparative example of heater33increased faster in temperature than the heater33in this embodiment.

Referring toFIG. 7, in a case where the area33A is smaller in the amount of grease (this embodiment), when the temperature Theatdetected by the thermistor TH reached 200° C., the belt temperature had reached 170° C. In comparison, in the case where the entirety of both the area33A and areas33B are uniformly coated with the grease130(comparative example 1), when the temperature Theatdetected by the thermistor TH reached 200° C., the belt temperature had reached only 150° C. The reason for the occurrence of this phenomenon is as follows. That is, in the case of the comparative example of heater33, the amount of grease on the area33A was greater than that in the case of the heater in this embodiment. Therefore, the heater33alone had increased in temperature before heat transferred from the heater33to the belt31by a sufficient amount. That is, there had occurred a large amount of difference between the temperature Theatdetected by the thermistor TH and the surface temperature of the belt31.

In the case of the heater33in this embodiment, it was varied in the amount by which its area33A was coated in advance with grease, and the belt surface temperature was measured as the temperature Theatdetected by the thermistor TH reached 200° C. The relation between the amount of grease and the belt surface temperature is shown by the graph inFIG. 8. As will be evident fromFIG. 8, as long as the amount of grease is no more than roughly 1.0 mm/mm, the presence of grease did not significantly affect the belt surface temperature. However, as the area33A of the heater33was increased in the amount of grease beyond the 1.0 mg/mm, the presence of grease significantly affected the belt surface temperature; the greater the amount of grease on the area33A, the lower the belt surface temperature. Thus, the problem that the speed with which the belt31increases in surface temperature is slowed by the presence of grease on the heater33can be prevented by setting the amount of grease applied in advance to no more than 1.0 mm/mm (no more than 0.2 mg/mm2).

In this embodiment, the amount of grease130coating the areas per unit length was 2.5 mg/mm as described above. However, it is not mandatory that the amount is 2.5 mg/mm. It is desired that the amount of grease130coating the areas33B per unit length is no less than 1.5 mm/mm and no more than 4.5 mm/mm (no less than 0.3 mg/mm2and no more than 0.9 mg/mm2, per unit area). If the amount of grease130is no more than 1.5 mm/mm, the grease130cannot satisfactorily perform as a lubricant. Thus, a greater amount of torque is required to rotate the belt31, making it possible that the belt31will slip. On the other hand, if the amount of grease130is no less than 4.5 mg/mm, the excess amount of grease130travels in the lengthwise direction of the heater33, leaks at the ends of the belt31, and soils the apparatus. That is, if the amount of grease130coating the areas33B is increased beyond 4.5 mg/mm, the amount of wasted grease130is wasted increases.

That is, the amount, per unit area, of grease130coating the grease application area of the area33A is desired to be no more than ⅔ of the amount, per unit area, by which the grease application area of the area33B is coated with grease130.

FIG. 9is a modified version of the pattern in which the heater33is coated with grease130in this embodiment. The modified version is different from the original version in the areas of the heater33, which are coated with virtually no grease130. That is, in the case of the modified version, in terms of the heater length direction, the preset area33C (first area, preset area) of the belt contacting surface of the heater33(portion of surface of heater, on which belt31slides), which corresponds in position to the thermistor TH which is on the inward surface of the heater33, is coated with virtually no grease130. Also, in the case of the modified version, the areas33B (second area, outward areas), that is, the other grease application areas of the heater33other than this preset area33C, are coated with a preset amount of grease130. The amount of grease130coating the areas33B per unit length is desired to be no less than 1.5 mg/mm and no more than 4.5 mg/mm as it is in the above-described first embodiment.

In the modified version, the amount, per unit length, of grease130coating the area33C is 0.5 mg/mm. The area33C was varied in length (coating range). The modified versions different in the length of the area33C were put through endurance tests. Then, the modified versions were compared in belt slippage. The results of the tests are shown inFIG. 10.

The tests for confirming the performance of the above-described fixing device which was varied in the pattern of grease application were carried out under the following conditions. That is, the sheets P of the recording medium used for the tests were of size A, and were 80 g/m2in basis weight. After the fixing devices40were allowed to cool down to the room temperature, 20 sheets P of the recording medium (paper), on which an unfixed solid image (toner image) was present were continuously processed by the fixing devices40to fix the unfixed toner image. Then, it was examined whether or not toner peeled from the sheet P.

The conditions under which the tests for finding out whether the belt31slips or not were carried out are as follows: roughly 100,000 sheets of a recording medium (paper) which were of size A4 and 80 g/mm2in basis weight were processed for toner image fixation by the fixing device40. Then, a sheet P of a recording medium (paper) on which an unfixed solid image was present was processed for image fixation by the fixing device40. Then, the finished print was examined about image disturbance. It was also examined whether or not the fixing device40suffered from a paper jam.

The results of the tests are as follow. As long as the length L of the area33C which was coated with a smaller amount of grease130was no more than 10 mm, the fixing device40had no problem in fixation. However, in the case where the length L was 5 mm or less, and in the case where the amount of grease130coating the areas of the belt contacting surface (area) of the heater33was no more than 2.5 mg/mm (0.5 mg/mm2per unit area), toner peeled from the sheet P. That is, the fixing device40was not good in image fixation. That is, in the case where the width of the area33C was 5 mm, as the heater33was attached to the heater holder, a substantial amount of grease130spread onto the area33C from both edges of the area33C, and therefore, the area33C reduced in the thermal conductivity between the heater33and belt31.

On the other hand, as long as the length L of the area33C which was coated with a smaller amount of grease130is 70 mm or less, the belt slip problem did not occur. However, in the case where the length L of the area33C was no less than 80 mm, the belt31slipped enough to disturb the images. As for the reason for the occurrence of this problem, the amount of grease130that spreads onto the area33C from both ends (from areas33B) was relatively small compared to the size of the area33C. Therefore, the friction between the area33C of the heater33and the belt31was relatively high. That is, if the areas of the heater33, which are to be coated by a very small amount of grease130, is excessive in terms of the length L, the amount of grease130required to spread onto the area33C from the areas33B is insufficient. By the way, it has been confirmed that in a case where the entirety of both the area33C and the areas33B is uniformly coated with the grease130by only 0.5 mg/mm (0.1 mg/mm2per unit area), the belt31slips due to the insufficiency in the amount of grease130.

It is evident from the results described above that the following effects can be obtained by reducing the amount of grease130coating the area33C in advance, and setting the length L of the area33C which is coated with a very small amount of the grease130, to a value within a range of 10 mm-70 mm (10 mm≦L≦70) (setting ratio of L to length of effective heat generation range of heater33to be no less than 3% and no more than 21%). That is, according to this embodiment, it is possible to prevent the occurrence of the belt slip attributable to the increase in the amount of torque necessary to rotate the belt31. Further, according to this embodiment, even in a case where the belt contacting portion of the heater33is coated with the grease130to prevent the belt31from slipping, it is possible to prevent the difference between the belt temperature detected by the thermistor TH and the actual belt surface temperature from becoming substantial. Therefore, it is possible to prevent the occurrence of unsatisfactory fixation.

By the way, the method for replacing the belt of the fixing device40having the heater33in this embodiment is as follows.

To begin with, the belt unit30is removed from the casing50of the fixing device40. Then, either flange36A or36B is removed. With the flange36A or36B removed, it is possible to remove the used belt31fitted around the combination of the guiding member34by which the heater33is held, and the pressure application stay35.

Next, the used belt31is removed (process of removing used endless belt from image heating device). After the removal of the belt31, the grease130on the heater33is wiped away (process of removing lubricant from belt contacting surface of heater33). Therefore, the belt contacting surface of the heater33is coated with a fresh supply of lubricant. During this process, the area33A or33C is coated with virtually no grease130, whereas the areas B are coated with the grease130(process of coating heater33with grease130).

The amount of grease130coating the areas33B per unit area during this process is desired to be no less than 0.3 mg/mm2and no more than 0.9 mg/mm2. That is, the amount of grease130coating the areas33B per unit length in terms of the heater length direction is desired to be no less than 1.5 mg/mm and no more than 4.5 mg/mm. Further, the small amount of grease130coating the area33C per unit area is desired to be no more than 0.2 mg/mm2. That is, the small amount of grease130coating the area33C per unit length in terms of the heater length direction is desired to be no more than 1.0 mg/mm. Further, the length L (range across which heater is coated) of the area33C is desired to be no less than 10 mm and no more than 70 mm (10 mm L70) (ratio of L relative to length of effective heat generation range of heater33is desired to be no less than 3% and no more than 21%).

Then, a brand-new belt31is fitted around the combination of the guiding member34by which the heater33is held, and the pressure application stay35(process of attaching replacement belt31to image heating device).

Next, the removed flange36A or36B is reattached to reassemble the belt unit30. Then, this reassembled belt unit30is reattached to the casing of the fixing device40.

By the way, it is recommendable to put the printer1through a break-in operation after the belt replacement, in order to allow the brand-new belt31to be uniformly coated with the grease130.

By replacing the belt31as describing above, it is possible to make a used fixing device40as effective as a brand-new one.

That is, according to this embodiment, it is possible to prevent the occurrence of the belt slip attributable to the increase in the amount of torque necessary to rotate the belt31. Also according to this embodiment, it is possible to prevent the occurrence of a significant amount of difference between the temperature detected by the thermistor TH and the belt surface temperature. Therefore, it is possible to prevent the occurrence of unsatisfactory fixation.

The structure of the fixing device40in the second embodiment is roughly the same as that in the first embodiment. Therefore, the only significant differences between the two devices40are described. That is, the structural components of the fixing device40in the second embodiment, which are similar in structure to the counterparts in the first embodiment, are given the same reference characters as those given to the counterparts, and are not described in detail. In the first embodiment, the belt contacting surface of the heater33is coated with the grease130in advance. In comparison, in this embodiment, it is the inward surface of the belt31that is coated with the grease130in advance. By configuring the fixing device40as described above, this embodiment makes it possible to replace the belt31in the same manner as in the first embodiment.FIG. 11(a)is an external view of the belt31.FIG. 11(b)is a plan view of the belt31, extended at a line A-A inFIG. 11(a), and is for illustrating the areas of the inward surface of the belt31, which are to be coated with grease.

Referring toFIG. 11(b), in this embodiment, the areas31B, which correspond in position to the areas33B of the heater33are coated with the grease130in advance. The area31A of the inward surface of the belt31, which corresponds in position to the area33A of the heater33A, is not coated with the grease130. In this embodiment, the dimension of the area31A of the inward surface of the belt31is 30 mm in terms of the belt length direction. The areas31B of the inward surface of the belt31are entirely coated with the grease130; the inward surface of the belt31is coated with the grease130, except for the area31A. By the way, the belt31is 30 mm in internal diameter, and 330 mm in width (in lengthwise direction of fixing device40). The areas31B of the inward surface of the belt31are coated with the grease130by 2.0 mg/mm per unit length, in terms of the belt width direction. Since the diameter of the belt31is 30 mm, the belt31is roughly 94.2 mm in dimension in terms of its circumferential direction. Therefore, the amount, per unit area, of the grease130on the areas31B is 21 μm/mm2. Coating the inward surface of the belt31with the grease130in such a manner that a preset area of the inward surface is not coated at all makes it easier to control the belt coating process, and also, to manage the inventory, than coating the inward surface31with grease130in such a manner that the preset area is coated with a smaller amount of the grease130than the rest. However, even if the area31A of the inward surface of the belt31is coated with a very small amount of the grease130, the same effects as those obtainable by this embodiment can be obtained. Therefore, the area31A (or area31C, which will be described later) may be coated with a very small amount of grease130. In such a case, the amount of grease130coating the area31A (or area31C) is desired to be no more than 1.0 mg/mm in terms of the belt width direction (no more than 10 μg/mm2, per unit area), like the amount of grease130coating the area33A of the heater33in the first embodiment. The amount of grease130coating areas31B of the inward surface of the belt31per unit area is desired to be no less than 1.5 mm/mm and no more than 4.5 mg/mm (no less than 16 μg/mm2and no more than 47 μg/mm2, per unit area), as in the case of the above described first embodiment. Therefore, in a case where the area31A is coated with a very small amount of the grease130, the amount of grease130coating the area31A per unit area is desired to be no more than ⅔ the amount of grease130coating the areas31B per unit area.

The method for replacing the belt31of the fixing device40(method for replacing endless belt of image heating device) is as follows. First, the belt unit30is removed from the casing50of the fixing device40, and either the flange36A or36B is removed. With the removal of the flange36A or36B, it is possible to remove the used belt31fitted around the combination of the guiding member34and the pressure application stay35.

Then, the used belt31is removed (process of removing used endless belt from image heating device). After the removal of the belt31, the grease130on the heater33is wiped away (process of removing lubricant from belt contacting surface of heater33). Thereafter, a brand-new belt31, which has been partially coated with the grease130is fitted around the combination of the guiding member34by which the heater33is held, and pressure application stay35(process of attaching replacement endless belt to image heating device).

Then, the removed flange36A or36B is reattached to reassembled the belt unit30. Then, the reassembled belt unit30is reattached to the casing50of the fixing device40.

Then, electric power was supplied to the heater33of the fixing apparatus40, into which the belt31in this embodiment was installed, and which was therefore as good as a brand-new fixing device, as in the experiments in which the fixing device40in the first embodiment, was tested, and the belt temperature, and the temperature Theatdetected by the thermistor TH were obtained to confirm the temperature increase curve. Further, as the second example of comparative belt31, a brand-new belt31coated with the grease130across the entirety of its inward surface was installed in the fixing device40. Then, the belt temperature and the temperature Theatmeasured by the thermistor TH were obtained to confirm the temperature increase curve. In the case of the second example of comparative belt31, both the area31A and areas31B, that is, the entirety of the inward surface of the belt31, were uniformly coated with the grease130by 2.0 mg/mm per unit length (20 μg/mm2per unit area).

As a result, in the case where the area31A is small in the amount by which it is coated with the grease130(embodiment 2), when the temperature Theatdetected by the thermistor TH reached 200° C., the surface temperature of the belt31had reached 171° C. In comparison, in the case where the grease130was uniformly applied to both the area31A and areas31B, that is, the entirety of the inward surface of the belt31(comparative example 2), when the temperature Theatdetected by the thermistor TH reached 200° C., the surface temperature of the belt31had reached only 160° C. This result is attributable to the fact that in the case of the second example of the comparative belt31, the area33A is large in the amount of the grease130, and therefore, the heater33increases in temperature faster than the belt31; the heater33alone increased in temperature before the heat generated by the heater33was satisfactorily transmitted to the belt31. That is, there occurred a significant amount of difference between the temperature Theatdetected by the thermistor TH and the surface temperature of the belt31.

As described above, in this embodiment, in order to ensure that the area31A of the inward surface of the belt31, which corresponds to the area33A of the belt contacting surface of the heater33, remains free of the grease130, the inward surface of the belt31is coated with the grease130in the above-described manner. In this embodiment, therefore, it is possible to prevent the occurrence of the belt slip attributable to the increase in the amount of torque necessary to rotate the belt31. Further, in this embodiment, it is possible to prevent the problem that the belt does not increase in temperature as fast as desired when an undesirably large amount of grease130is between the heater33and the belt31, for example, when a fixing device is brand-new and/or immediately after the belt31was replaced. Therefore, it is possible to prevent the above-described unsatisfactory fixation.

FIG. 12is a modified version of the pattern in which the inward surface of the belt31is coated with the grease130in this embodiment. This modified version is different from this embodiment in the range of the inward surface of the belt31, which is coated with virtually no grease130. That is, in the case of the modified version, in terms of the belt width direction, the area31C of the inward surface of the belt31, which includes the area31A of the inward surface of the belt31, which corresponds in position to the thermistor TH, is coated with virtually no grease130. Also in the case of the modified version, the areas31B (second areas) of the inward surface of the belt31, that is, the areas of the inward surface of the belt31, which are not the preset area31C, are coated with a preset amount of grease130. The amount by which the area31B is coated with the grease130per unit length is desired to be no less than 1.5 mg/mm and no more than 4.5 mg/mm.

In the case of the modified version, the area31C was coated with the grease130by 0.5 mg/mm per unit length. This area31C was varied in length L (range). Then, each variation was tested for the initial performance in fixation. Then, each variation was tested for the occurrence of the belt slip after the fixing device was used for a substantial length of time. The results of the tests are given inFIG. 10.

Also in the case of the modified versions of this embodiment, the width L of the area31C was varied. Then, each variation was tested for the initial performance in fixation, and also for the occurrence of the belt slip after the fixing device was used for fixation for a substantial length of time, as the modified version of the first embodiment, shown inFIG. 9, was tested. The results of the tests were the same as those from the embodiment 1. That is, it became evident from the results of the tests that in terms of the width direction of the belt31, the width L of the area31C of the inward surface of the belt31is desired to be no less than 10 mm and no more than 70 mm (no less than 3% and no more than 21% in its ratio relative to dimension of belt31in width direction (10 mm≦L≦70 mm)).

As described above, according to this embodiment, it is possible to prevent the occurrence of the belt slip attributable to the increase in the amount of torque necessary to rotate the belt31. Also, according to this embodiment, it is possible to prevent the occurrence of a difference between the temperature detected by the thermistor TH and the surface temperature of the belt31, and it is possible to prevent the occurrence of unsatisfactory fixation.

The choice of the heater33as a heating member does not need to be limited to a ceramic heater. That is, any heater may be employed as long as it is of such a structure that it contacts the inward surface of the belt31. For example, the choice of heater33may be a magnetic member which can be heated by electromagnetic induction with the use of an excitation coil, or a Nichrome heater.

The fixing devices40in the first and second embodiments were structured so that when a sheet P of a recording medium is conveyed through the device40, the center of the sheet P coincides with the centerline of the recording medium conveyance passage of the fixing device40, in terms of the direction which is perpendicular to the recording medium conveyance direction, or one of the edges of the sheet P remains in contact with the corresponding edge of the recording medium conveyance passage.

Further, in the first and second embodiments, the fixing devices40were examples of an image heating device for heating an unfixed toner image on a sheet P of recording paper. However, these embodiments are not intended to limit the present invention in scope. For example, the present invention is also applicable to an image heating device for heating a fixed toner image on a sheet of recoding medium to increase the toner image in gloss.

As described above, according to the first and second embodiments, it is possible to prevent the problem that the belt31is caused to slip by the increase in the amount of torque necessary to rotate the belt31, which is attributable to the friction between the belt31and the heater33. Further, according to the first and second embodiments, it is possible to prevent the occurrence of a difference between the temperature detected by the thermistor TH and the surface temperature of the belt31, and it is possible to prevent the occurrence of the unsatisfactory fixation.

This application claims the benefit of Japanese Patent Applications Nos. 2014-095811 filed on May 7, 2014 and 2015-087912 filed on Apr. 22, 2015, which are hereby incorporated by reference herein in their entirety.