Image heating apparatus

The present invention provides an image heating apparatus including:

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image heating apparatus for heating an image borne by a recording material, and more particularly to an image heating apparatus adapted for use as a heat fixing device in an image forming apparatus such as a copying machine or a printer.

2. Related Background Art

In an image forming apparatus based for example on an electrophotographic process or an electrostatic recording process, a fixing apparatus as a heating apparatus for heat fixation of an unfixed toner image formed on a recording material (a transfer sheet, a printing sheet, a photosensitive sheet, an electrostatic recording sheet etc.) by a transfer process or a direct process is known in various types and configurations such as a heat roller type, a film heating type etc.

In a color image forming apparatus for forming a toner image of plural colors on a recording material, or in an image forming apparatus having a color image forming function for forming a toner image of plural colors on a recording material, a toner amount of an unfixed toner image formed on the recording material is several times larger in a full color mode than in a monochromatic mode.

As a fixing apparatus capable of satisfactory heat fixation even of an unfixed toner image of such large toner amount, there is known an apparatus employing an elastic surface roller, which is provided with an elastic layer on a fixing roller (heating rotary member) constituting a fixing member for heat fixing the toner image on the recording material.

By constructing the fixing roller as an elastic surface roller, the surface of the fixing roller undergoes elastic deformation, relative to the unfixed toner image on the recording material, corresponding to irregularities of such toner image and achieving a wrapping contact with the surface of the toner image, whereby an unfixed toner image having a large toner amount can also be heat fixed in satisfactory manner.

However, in a fixing roller equipped with an elastic layer, the elastic layer itself constitutes a heat insulating layer and there is encountered a drawback that the heat transfer efficiency deteriorates in cases where the heat source is provided inside the fixing roller.

Therefore, the present applicant already proposed, as disclosed in Japanese Patent Application Laid-Open No. 2002-236426, a fixing apparatus adopting an external heating configuration in which a heating rotary member serving as a fixing member is heated from the exterior of the fixing member, whereby the fixing member has a high heating efficiency even in case it is provided with an elastic surface, thereby ensuring a quick starting property and a fixing ability of the apparatus.

FIG. 6shows an embodiment of a fixing apparatus with such external heating configuration for the fixing member. A fixing roller201serves as a fixing member maintained in contact with an image bearing surface of a recording material P, thereby heat fixing an unfixed toner image T thereon. The fixing roller201is an elastic surface roller of a three-layered structure, having, in succession from the inside to the outside, a metal core211, an elastic layer212and a releasing layer213.

A pressurizing apparatus202is in a mutual pressed contact with the fixing roller201, thereby forming a fixing nip portion N4for nipping and conveying the recording material P. The pressurizing apparatus202also serves as a heating apparatus for heating the fixing roller201, and is provided with a rotatable cylindrical film221, and a film/heater holder224for supporting the film from the inside and maintaining it in contact across a plate-shaped heater222thereby forming the fixing nip portion N4.

In addition to the aforementioned pressurizing/heating apparatus202, there is provided an external heating apparatus203for heating the surface of the fixing roller201from the exterior. The external heating apparatus203is provided with a plate-shaped heater232and a heater holder234for supporting the heater so as to be in contact with the surface of the fixing roller201thereby forming a fixing roller heating nip portion N3.

The fixing roller201is rotated clockwise, as indicated by an arrow, by an unillustrated drive mechanism. By such rotating motion of the fixing roller201, the cylindrical film221of the pressurizing/heating apparatus202is driven counterclockwise, as indicated by an arrow, around the holder224, with an internal surface of the film in sliding contact with the surface of the plate-shaped heater222at the fixing nip portion N4.

The plate-shaped heater222in the pressurizing/heating apparatus202and the plate-shaped heater232in the external heating apparatus203are so-called ceramic heaters, of which temperature is rapidly elevated by current supplies from power supply circuits205,206to heat-generating resistor layers (not shown). Temperatures of the heaters222,232are detected by thermistors223,233constituting temperature detecting means provided on rear surfaces of the heaters, and temperature information thus detected are supplied to a control circuit (CPU)204.

The control circuit204executes a temperature control of the heater222by controlling the current supply from the power supply circuit205to the plate-shaped heater222in the pressurizing/heating apparatus202in such a manner that a temperature detected by the thermistor223is maintained at a predetermined heater temperature. It also executes a temperature control of the heater232by controlling the current supply from the power supply circuit206to the plate-shaped heater232in the external heating apparatus203in such a manner that a temperature detected by the thermistor233is maintained at a predetermined heater temperature.

In this manner the surface temperature of the fixing roller201is maintained at a predetermined surface temperature (fixing temperature) necessary for the heat fixation of the toner image.

As the fixing roller201is rotated to drive the film221of the pressurizing/heating apparatus202in a rotating motion, and as the plate-shaped heater222of the pressurizing/heating apparatus202and the plate-shaped heater232of the external heating apparatus203are powered to control the heaters222,232at the predetermined heater temperatures, the surface of the fixing201is heated by the heat of the plate-shaped heater222of the pressurizing/heating apparatus202across the film221at the fixing nip portion N4, and is also heated by the heat of the plate-shaped heater232of the external heating apparatus203at the heating nip portion N3, whereby the fixing roller201is heated to a predetermined surface temperature (fixing temperature) required for heat fixing the toner image.

In such state, a recording material P bearing an unfixed toner image T is introduced into the fixing nip portion N4, with a toner image side at the side of the fixing roller201, whereby the recording material P is nipped and conveyed in the fixing nip portion N4. In such conveying process, the unfixed toner image T is fixed, under a heat and a pressure, as a permanent image on the surface of the recording material P by the heat of the fixing roller201. The recording material P emerging from the fixing nip portion N4is separated from the surface of the fixing roller201and is conveyed for discharge.

Such apparatus, being provided with heating means for externally heating the surface of a fixing member such as a fixing roller, can rapidly elevate the temperature by rapid heating of only the surface, required for fixing, of the fixing member, and can improve the quick starting property and the thermal efficiency of the fixing apparatus even in case the fixing member is an elastic surface roller having an elastic layer.

In a fixing apparatus of an external heating configuration as explained in the foregoing, in order to maintain the surface temperature of the fixing roller heated by the external heating means as close as possible to a predetermined surface temperature necessary for heat fixation of the toner image thereby ensuring a fixing property for a first print immediately after the start of power supply or for all prints during a continuous sheet passing operation, it is desirable (1) to increase an electric power supplied to a heater of the external heating means, and (2) to increase a width of the heater of the external heating means.

However, an increase in the electric power supplied to the heater of the external heating means may result in a deterioration of components such as the fixing roller or the heater, or a runaway state of the heater, by a rapid temperature increase in the heater. Also a larger width of the heater of the external heating means may lead to a drawback of an increased cost of the heater.

SUMMARY OF THE INVENTION

The present invention, which has been made in consideration of the foregoing problems, has an object of providing an image heating apparatus having a fast start-up time to a temperature capable of sufficiently heating an image and also capable of ensuring a satisfactory fixing ability.

Another object of the present invention is to provide an image heating apparatus capable of ensuring a satisfactory fixing ability while suppressing a cost of a heater.

Still another object of the present invention is to provide an image heating apparatus including:

a conveying roller for conveying a recording material;

heat supply means which supplies heat to the conveying roller, the heat supply means and an external periphery of the conveying roller being in mutual contact to form a heating nip portion;

back-up means which forms a conveying nip portion in cooperation with the conveying roller for nipping and conveying the recording material;

wherein, in a rotating direction of the conveying roller, the heating nip portion has a width larger than a width of the conveying nip portion, and a total pressure applied to the conveying nip portion is larger than a total pressure applied to the heating nip portion.

Still other objects of the present invention will become fully apparent from the following detailed description, which is to be taken in conjunction with accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be explained with reference the to accompanying drawings.

(1) Example of Image Forming Apparatus

FIG. 5is a schematic view showing a configuration of an image forming apparatus employing an image heating apparatus of the present invention. The image forming apparatus of this example is a color laser printer utilizing an electrophotographic process of an intermediate transfer belt type.

a) Full Color Mode

A photosensitive drum101, constituting an image bearing member, is rotated counterclockwise, as indicated by an arrow, by unillustrated drive means, and is uniformly charged in a predetermined potential of a predetermined polarity by a primary charger102.

Then it is subjected to a laser scanning exposure L by an exposure apparatus (laser scanner)103whereby an electrostatic latent image is formed corresponding to an image pattern of a yellow component of a full-color image.

As the photosensitive drum101is rotated further, developing apparatuses104a,104b,104cand104dsupported by a rotary support member111are so rotated that the developing apparatus104a, containing a yellow toner, is opposed to the photosensitive drum101, and the aforementioned electrostatic latent image is rendered visible by the developing apparatus104a. The developing apparatuses104b,104cand104drespectively contain a magenta toner, a cyan toner and a black toner.

An intermediate transfer belt105is rotated clockwise, as indicated by an arrow, with a speed substantially same as that of the photosensitive drum101, and the toner image formed and borne on the photosensitive drum101is primary transferred onto an external peripheral surface of the intermediate transfer belt105, by a primary transfer bias applied to a primary transfer roller108a. On the other hand, a transfer residual toner, remaining on the photosensitive drum101, is removed by a cleaning apparatus107constituted of blade means.

A toner image forming step on the photosensitive drum101and a primary transfer step onto the intermediate transfer belt105are repeated in a similar manner also for an image pattern of a magenta component, an image pattern of a cyan component and an image pattern of a black component of the full-color image, whereby a synthesized color image, formed by superposed toner images of plural colors (yellow, magenta, cyan, and black), is formed on the intermediate transfer belt105.

Then, at a predetermined timing, a recording material P is supplied by a pickup roller113from a recording material cassette112, and is introduced through a sheet path116into a secondary transfer nip portion where a secondary transfer roller108band the intermediate transfer belt105are mutually pressed, and a secondary transfer bias is applied at the same time to the secondary transfer roller108b, whereby the aforementioned synthesized color toner image is collectively transferred from the intermediate transfer belt105onto the recording material P.

Then the recording material P is conveyed by a conveyor belt114to a heat fixing apparatus106for fixation by fusing, and is discharged through a sheet path117to an external sheet discharge unit118as a color print (color copy).

The fixing apparatus106is a heating apparatus of an external heating configuration according to the present invention. It will be explained in more detailed in following (2).

A transfer residual toner on the intermediate transfer belt105is given a charge by an intermediate transfer cleaning roller115, then is inversely transferred to the photosensitive drum101at a next primary transfer, and is eliminated from the photosensitive drum101by the cleaning apparatus107.

A latent image is formed on the photosensitive drum101in the same manner as in a color image formation, then a toner image is formed on the photosensitive drum101by the developing apparatus104dcontaining the black toner, and is primary transferred onto the intermediate transfer belt105. Then the monochromatic toner image thus formed alone is secondary transferred onto the recording material P and is fixed by the fixing apparatus106in a similar manner, whereby a black monochromatic image can be obtained. Thus, a time required for all the image formation on the intermediate transfer belt105is reduced to about ¼ in comparison with a case of a color image formation.

FIG. 1is a schematic view showing a configuration of the fixing apparatus106in the present embodiment. The fixing apparatus of the present embodiment is primarily constituted of three parts, namely a fixing roller (recording material conveying roller)10having an elastic layer, back-up means20maintained in contact with the fixing roller10thereby forming a fixing nip portion (conveying nip portion) N1, and heat supply means30maintain in contact with the fixing roller10in a position other than the fixing nip portion and serving to heat the surface of the fixing roller10. A contact between the heat supply means30and the external periphery of the fixing roller10forms a heating nip portion N2.

The fixing roller10is constituted of a metal core11of aluminum or iron, an elastic layer12provided on the outside thereof, and a releasing layer13covering the surface of the elastic layer12.

The elastic layer12is constituted, for example, of a solid rubber layer formed for example of silicone rubber, a sponge rubber layer formed by foaming silicone rubber in order to obtain a heat insulating effect, or a bubble dispersed rubber layer formed by dispersing a hollow filler in a silicone rubber layer in order to increase a heat insulating effect. In order that the fixing apparatus can be started up to a fixable state rapidly after the start of power supply, there is desired a configuration in which the heat supplied from the external periphery of the fixing roller is not easily transmitted to the interior thereof, so that the elastic layer12preferably has a high heat insulating property, such as in the sponge rubber layer or the bubble dispersed rubber layer.

The releasing layer13is preferably formed constituted of a fluorinated resin such as perfluoroalkoxy resin (PFA), polytetrafluoroethylene resin (PTFE), or tetrafluoroethyelene-hexafluoropropylene resin (FEP), or a fluorinated rubber such as GLS latex (trade name, manufactured by Daikin Co.). A covering on the elastic layer12can be formed for example by a method of forming these materials into a tube and fitting such tube, or a method of coating with a paint.

The back-up means20is provided with a cylindrical (endless belt-type) film (flexible rotary member)21, and a film holder22(hereinafter represented simply as a holder22) supporting the film from the interior thereof in contact with the fixing roller10thereby forming a fixing nip portion N1.

The cylindrical film21is loosely fitted on the external periphery of the holder22. A pressure is applied between longitudinal ends of the holder22and longitudinal ends of the fixing roller10, by unillustrated pressurizing means such as coil springs. Such configuration forms a fixing nip portion N1between the fixing roller10and the back-up means20and across the film21.

The film21is constituted of a resinous film, having a base layer of polyimide, polyamidimide, PEEK, PES, PPS, PFA, PTFE, FEP etc. having a heat resistance and a heat insulating property. It is surfacially covered singly with or by a mixture of a heat-resistant resin having a releasing property such as PFA, PTFE, FEP, silicone resin etc. It may also be constituted of a metal film.

The holder22is constituted of a heat-resistant resin with a heat resistance and a slidable property, such as a liquid crystal polymer, a phenolic resin, PPS, PEEK etc.

The heat supply means30is provided with a rotatable cylindrical (endless belt-type) heating film (flexible rotary member)31, a plate-shaped heater33, and a holder32supporting the film31from the inside thereof and also supporting the plate-shaped heater33as a heat source. A pressure is applied between longitudinal ends of the holder32and longitudinal ends of the fixing roller10, by unillustrated pressurizing means such as coil springs. Such configuration form a heating nip portion N2between the fixing roller10and the heat supply means30and across the film31. The cylindrical film31is loosely fitted on the external periphery of the holder32. The plate-shaped heater33is so-called ceramic heater, and is fixed by fitting in a fitting groove provided in the holder32, which is pressed to the fixing roller10by unillustrated pressurizing means thereby the heater forms a heating nip portion N2in cooperation with the fixing roller10across the heating film31.

The plate-shaped heater33constituting the heat source is not limited to a ceramic heater, but can also be a positive temperature coefficient (PTC) heater, an electromagnetic induction heat generating member, a nichrome heater etc.

Temperature detection means34detects a temperature on a rear surface of the heater33. It is provided, in case of a contact-type thermistor, with a temperature detecting surface thereof in contact with the rear surface of the heater33, but, in case of a non-contact infrared temperature sensor, opposed to the rear surface of the heater33in non-contact manner.

The film31is constituted of a resinous film, having a base layer of polyimide, polyamidimide, PEEK, PES, PPS, PFA, PTFE, FEP etc. having a heat resistance and a heat insulating property. It is surfacially covered singly with or by a mixture of a heat-resistant resin having a releasing property such as PFA, PTFE, FEP, silicone resin etc. It may also be constituted of a metal film.

The holder32is constituted of a heat-resistant resin with a heat resistance and a slidable property, such as a liquid crystal polymer, a phenolic resin, PPS, PEEK etc.

The ceramic heater33serving as a heater is provided with an insulating ceramic substrate such as of alumina or aluminum nitride, or a heat-resistant resinous substrate such as of polyimide, PPS or liquid crystal polymer, on a surface of which a heat-generating resistance layer for example of Ag/Pd (silver-palladium), RuO2or Ta2N is formed, by coating for example with screen printing in a linear or stripe form of a thickness of about 10 μm and a width of 1 to 5 mm followed by sintering. At an end of the surface of the heater33, there is provided a power supply electrode portion which is electrically connected with the heat-generating resistor layer and to which a voltage is supplied from a power supply circuit41through an unillustrated power supply connector.

The fixing roller10is rotated clockwise, as indicated by an arrow, by an unillustrated drive mechanism. By such rotating motion of the fixing roller10, the cylindrical film21of the back-up means20is driven counterclockwise, as indicated by an arrow, around the holder22, with an internal surface of the film in sliding contact with the surface, opposed to the fixing nip portion, of the holder22at the fixing nip portion N1.

Also the cylindrical film31of the heat supply means30is driven counterclockwise, as indicated by an arrow, around the holder32, with an internal surface of the film in sliding contact with the surface of the plate-shaped heater33at the heating nip portion N2.

The heater33of the heat supply means30causes a rapid temperature increase by a current supply from the power supply circuit41to the heat-generating resistor layer. The heat from the heater33heatgs the surface of the fixing roller10across the film31at the heating nip portion N2. The temperature of the rear surface of the heater33is detected by the contact or non-contact temperature detection means34. Temperature information of the rear surface of the heater33, detected by the temperature detection means34, is supplied to a control circuit (CPU)40. The control circuit40controls the power supply from the power supply circuit41to the heater33in the heat supply means30, in such a manner that the temperature detected by the temperature detection means34is maintained at a predetermined surface temperature (fixing temperature) of the fixing roller.

In the following, feature portions of the present invention will be explained. In the following, a fixing nip portion (conveying nip portion) N1and a heating nip portion N2respectively indicate a nip area formed between the back-up means20and the fixing roller10, and a nip area formed between the heat supply means30and the fixing roller10. In the present embodiment, as shown inFIG. 1, the heating nip portion N2is formed not only between the heater33and the fixing roller10but also between the holder32supporting the heater33and the fixing roller10. Therefore, the heating nip portion N2is so defined as to include also a nip area formed between the holder32and the fixing roller. The nip area is defined in a similar manner also in case the heater is provided at the side of the back-up means.

In the present embodiment, an area of the heating nip portion N2is made larger than an area of the fixing nip portion N1. Therefore, an arbitrary point on the surface of the fixing roller takes a longer time in passing the heating nip portion N2than in passing the fixing nip portion N1, whereby the heat supply means30can supply, in the heating nip portion N2, the fixing roller10with a heat amount sufficiently larger than a heat amount required in the fixing nip portion N1for fixing the toner image T on the recording material P.

As a result, it is rendered possible to supply the fixing roller10with a sufficient heat even without increasing the electric power supplied to the heater33and to suppress deterioration of the fixing roller10. In such situation, also the film31after passing the heating nip portion N2reaches a certain high temperature, so that a sufficient heat can be supplied to the fixing roller10even with a small width of the heater33.

Furthermore, the fixing nip portion N1formed with a small area allows to prevent a hot offset phenomenon resulting from an excessive heat supply at the fixing nip portion N1.

As a result, the surface temperature of the fixing roller10can be stabilized with an enlarged margin against the hot offsetting, so that it is rendered possible to improve the fixing property for a first print immediately after the start of power supply or for all the prints in a continuous sheet passing operation.

Furthermore, in the present embodiment, a total pressure in the fixing nip portion N1constituting a first nip portion is made larger than a total pressure in the heating nip portion constituting a second nip portion (a total pressure in the second nip portion being smaller than a total pressure in the first nip portion). Stated differently, the total pressure is so selected as to satisfy a relation N1>N2in spite of a fact that the width of the fixing nip portion N1is smaller than that of the heating nip portion N2. A local pressure therefore becomes large in the fixing nip portion N1, thereby further improving the fixing property for a first print immediately after the start of power supply or for all the prints in a continuous sheet passing operation.

The holder22of the present embodiment is so formed as illustrated inFIG. 1, but, by selecting, as shown inFIG. 7, the width in the conveying direction of the recording material, particularly the width of a nip forming surface of the holder22smaller than that shown inFIG. 1, it is rendered easier to select the total pressure applied in the fixing nip portion N1larger than that in the heating nip portion N2while maintaining the width of the fixing nip portion N1smaller than that of the heating nip portion N2. In any case, the width of a fixing nip forming surface of the holder22in the back-up means is to be made smaller than the width of a heating nip forming surface of the holder32in the heat supply means. In case the relations of the width and the total pressure are selected as explained above in the fixing nip portion N1and the heating nip portion N2, an intrusion amount of the back-up means20or the heat supply means30into the fixing roller10(namely a maximum recessed amount of the fixing roller10) becomes larger in the fixing nip portion N1than in the heating nip portion N2, as shown inFIGS. 8 and 9.FIGS. 8 and 9respectively show intrusion amounts in the fixing nip portion N1and in the heating nip portion N2. Thus, a peak value in a pressure distribution in the fixing nip portion N1also becomes larger than a peak value in a pressure distribution in the heating nip portion N2, thereby providing an improvement in the fixing property, such as an increase in the gloss of the image outputted from the image forming apparatus.

As a specific example, the fixing roller10in the present embodiment was formed by providing, outside an aluminum metal core11of an external diameter of 13 mm, a heat resistant elastic layer12of a thickness of 3.5 mm such as of silicone rubber or fluorinated rubber, and forming thereon a releasing layer13by coating or with a tube of a resin such as PFA or PTFE of a thickness of 50 μm.

Also the back-up means20was constituted of an endless film21of a cylindrical shape formed by coating a substrate of polyimide resin etc. of an external diameter of 20 mm and a thickness of 50 μm with a highly releasing material such as PFA resin with a thickness of 10 μm, and a holder22. The holder22had a width of the fixing nip forming surface in the film rotating direction, of 8 mm approximately same as that of the fixing nip portion N1.

Also the heat supply means30was constituted of an endless film31of a cylindrical shape formed by coating a substrate of polyimide resin etc. of an external diameter of 22 mm and a thickness of 40 μm with a highly releasing material such as PFA resin with a thickness of 10 μm, a holder32, a heater33, and temperature detection means34on a surface of the heater33, not in contact with the film31. The holder32had a width of the heating nip forming surface in the film rotating direction, of 9 mm approximately same as that of the heating nip portion N2.

Under conditions of a width of the heater33of 8 mm, an electric power of 600 W charged into the heater33and a process speed of 100 mm/sec, there were employed a total pressure of 196 N (20 kgf) in the fixing nip portion N1and a total pressure of 147 N (15 kgf) in the heating nip portion N2, thereby obtaining a width of 8 mm in the fixing nip portion N1and a width 9 mm in the heating nip portion N2, thus providing a satisfactory fixing performance. In this state, the fixing nip portion N1had an intrusion amount of 0.19 mm and the heating nip portion N2had an intrusion amount of 0.16 mm.

The present embodiment employed a configuration that the cylindrical film21of the back-up means20and the cylindrical film31of the heat supply means30are driven by the rotation of the fixing roller10, but a satisfactory fixing performance can naturally be obtained for example by a configuration in which a driving roller is provided inside the endless film and the film is rotated by driving such drive roller.

FIG. 2shows a second embodiment. A configuration of the image forming apparatus, including the heat fixing apparatus of the present embodiment, is same as that of the first embodiment, explained inFIG. 5, and will not, therefore, be explained. The present embodiment corresponds to the heat fixing apparatus106shown inFIG. 5, of which details will be explained with reference toFIG. 2. In the following, components same as or equivalent in function to those inFIG. 1are represented by same numbers and are omitted from explanation.

The fixing apparatus106of the present embodiment is characterized in that the heat supply means30is constituted of a heat roller (non-flexible rotary member). The fixing roller10and the back-up means20have a configuration same as that in the first embodiment.

The heat roller30constituting the heat supply means is provided with a heat-generating member36such as a halogen lamp inside a hollow metal core35of aluminum or stainless steel, and a releasing layer37of fluorinated resin etc. for preventing toner offsetting on the external surface of the metal core35.

The heat supply means30is maintained in pressure contact with the fixing roller10by unillustrated pressurizing means, whereby the heat roller30containing the heat-generating member36therein forms a heating nip portion N2in cooperation with the fixing roller10.

In the heat-generating member36of the heat supply means30, a current supply to the heat-generating member36from the power supply circuit41is controlled by the control circuit (CPU)40, based on a surface temperature information of the heat roller30detected by the temperature detection means38and taking, as a target temperature, a surface temperature of the fixing roller10required for fixing the toner image T on the recording material P at the fixing nip portion N1, in such a manner that the surface temperature of the fixing roller10is maintained at such target temperature.

In the fixing apparatus106of the aforementioned configuration, an area of the heating nip portion N2constituting a second nip portion is made larger than an area of the fixing nip portion N1constituting a first nip portion, and a total pressure applied at the fixing nip portion N1is made larger than a total pressure applied at the heating nip portion N2.

The use of the above-described heat roller system having a heat-generating member in the interior of a hollow metal core as the heat supply means allows, in addition to the effects of the first embodiment, to increase the pressure to the fixing roller in the fixing nip portion, thereby enabling a more efficient heat supply to the fixing roller. Also a reduction in the diameter can be realized.

FIG. 3shows a third embodiment. A configuration of the image forming apparatus, including the heat fixing apparatus of the present embodiment, is same as that of the first embodiment, explained inFIG. 5, and will not, therefore, be explained. The present embodiment corresponds to the heat fixing apparatus106shown inFIG. 5, of which details will be explained with reference toFIG. 3. In the following, components same as or equivalent in function to those inFIG. 1are represented by same numbers and are omitted from explanation.

The fixing apparatus106of the present embodiment is featured in employing heat supply means30of electromagnetic induction heating type. The heat supply means30is constituted in the fixing apparatus of the first embodiment, by modifying the ceramic heater constituting the heater33of the heat supply means30to an induction heat-generating member33asuch as an iron plate, and by providing an excitation coil42and a magnetic core43as magnetic field generating means for causing an induction heating in such member33a. The fixing roller10and the back-up means20have a configuration same as that in the first embodiment.

Under a high frequency magnetic field generated by a high frequency current supplied from an excitation circuit41to the excitation coil42, the member33agenerates an induction heat, and the external periphery of the fixing roller10is heated by such induced heat, across the film31at the heating nip portion N2.

A current supply to the excitation coil42of the heat supply means30from the power supply circuit41is controlled by the control circuit (CPU)40, based on temperature information detected by the temperature detection means34in the heat supply means30and taking, as a target temperature, a surface temperature of the fixing roller10required for fixing the toner image T on the recording material P at the fixing nip portion N1, in such a manner that the surface temperature of the fixing roller10is maintained at such target temperature.

In the fixing apparatus106of the aforementioned configuration, an area of the heating nip portion N2constituting a second nip portion is made larger than an area of the fixing nip portion N1constituting a first nip portion, and a total pressure applied at the fixing nip portion N1is made larger than a total pressure applied at the heating nip portion N2. In this manner, there can be obtained effects similar to those in the first embodiment.

Instead of employing the member33a, there may be adopted a configuration in which the film31itself has an induction heat-generating property. In such configuration in which the film31is rendered heat-generating by induction, since the film31directly generates heat, there can be obtained advantages of an extremely good heat response, and a satisfactory control on the surface temperature of the fixing roller10.

FIG. 4shows a fourth embodiment. A configuration of the image forming apparatus, including the heat fixing apparatus of the present embodiment, is same as that of the first embodiment, explained inFIG. 5, and will not, therefore, be explained. The present embodiment corresponds to the heat fixing apparatus106shown inFIG. 5, of which details will be explained with reference toFIG. 4. In the following, components same as or equivalent in function to those inFIG. 1are represented by same numbers and are omitted from explanation.

In the fixing apparatus106of the present embodiment the heat supply means30is constructed by maintaining a ceramic heater, constituting the heater33and fixed to the holder32, in direct contact with the surface of the fixing roller10, thereby forming a heating nip portion N2and heating the external periphery of the rotating fixing roller10. The fixing roller10and the back-up means20have a configuration same as that in the first embodiment.

A current supply to the heater33of the heat supply means30from the power supply circuit41is controlled by the control circuit (CPU)40, based on temperature information of the surface temperature of the heater33, detected by the temperature detection means34in the heat supply means30and taking, as a target temperature, a surface temperature of the fixing roller10required for fixing the toner image T on the recording material P at the fixing nip portion N1, in such a manner that the surface temperature of the fixing roller10is maintained at such target temperature.

In the fixing apparatus106of the aforementioned configuration, an area of the heating nip portion N2constituting a second nip portion is made larger than an area of the fixing nip portion N1constituting a first nip portion, and a total pressure applied at the fixing nip portion N1is made larger than a total pressure applied at the heating nip portion N2. In this manner, there can be obtained effects similar to those in the first embodiment.

Also the aforementioned configuration can reduce the cost because of a simple structure, and can achieve a very satisfactory heat transmission since the fixing roller10can be directly heated with the heater33without going through a film or the like.

It is also possible to replace the ceramic heater constituting the heater33by a induction heat-generating member as in the apparatus shown inFIG. 3and to provide an excitation coil and a magnetic core as magnetic field generating means for causing an induction heat in such member.

1) The recording material conveying roller10is not limited to a non-flexible roller member but can also be constituted of a flexible rotary member such as a rotary belt member.2) The heating apparatus of the present invention is applicable not only to an image heat-fixing apparatus in the foregoing embodiments, but also to various means and apparatus for a heat treatment of a heated material, such as an image heating apparatus for heating a recording material bearing an image thereby improving a surface property such as a surface gloss, an image heating apparatus for a temporary fixation, a heat drying apparatus for a heated material or a heat laminating apparatus.

The present invention has been explained by various examples and embodiments, but it will be understood, for those skilled in the art, that the concept and the scope of the present invention are not limited to specific explanations and drawings in the present specification but are subject to various modifications and alterations described in appended claims.