Patent Publication Number: US-6336009-B1

Title: Image heating apparatus and heater for heating image

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image heating apparatus of a film heating system applied to image forming apparatuses such as a copying machine and a printer, particularly to a heater applied to an image heating apparatus. 
     2. Related Background Art 
     In conventional image forming apparatuses such as a printer, a copying machine and a facsimile apparatus, as a fixing apparatus (fixing device) for heating/fixing an unfixed image (toner image) formed and borne on a recording material (transfer material, photosensitive paper, electrostatic recording paper, printing sheet, and the like) by appropriate image forming means such as an electrophotographic system and an electrostatic recording system in a transfer (indirect) system or a direct system, an apparatus of a heat roller system is widely used. 
     The apparatus of the heat roller system has a fixing roller (thermal roller, heat roller) as a fixing member and a pressure roller as a pressurizing member which are pressed to contact each other and rotate. When a recording material with an unfixed image formed and borne thereon is introduced, nipped, conveyed, and passed via a fixing nip portion (heating nip portion) as a pressed portion of both rollers, the unfixed image can be heated/fixed as a permanent fixed image on a recording material surface by the heat of the fixing roller and the pressurizing force of the fixing nip portion. 
     In recent years, from the standpoint of promotion of energy saving, an apparatus of a film heating system has been placed for practical use as an on-demand image heating apparatus high in thermal conduction efficiency and fast in starting the apparatus. 
     As proposed in Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075 to 4-44083, and 4-204980 to 4-204984, this has a fixed/supported heating member, a heat resistant film which slides on the heating member, and a pressurizing member contacting the heating member via this film to form a fixing nip portion. The heating member is heated/adjusted to a predetermined temperature, and a recording material with an unfixed image formed/borne thereon is introduced between the film and the pressurizing member at the fixing nip portion, and nipped/conveyed with the film through the fixing nip portion, so that the unfixed image is heated/fixed as a permanent fixed image on a recording material surface by the heat from the heating member via the film and the pressurizing force of the fixing nip portion. 
     In the image heating apparatus of the film heating system, a linear heating member with a low thermal capacity such as a so-called ceramic heater as the heating member, and a thin heat resistant film with a low thermal capacity as the heat transfer member can be used. The temperature of the heating member is raised in a short time, and the rising of the temperature of the heating member or the fixing nip portion to a predetermined temperature can quickly be performed. No power is supplied to the apparatus (heating member) during standby, and the power consumption can be minimized. Therefore, as compared with the other image heating apparatus of the heat roller system or the like, power can be saved and wait time can be shortened (quick start property), so that the on-demand image heating apparatus can be constituted. 
     FIG. 10 is a schematic view showing a main part of one example of the image heating apparatus (heating/fixing apparatus) of the film heating system. 
     Specifically, the image heating apparatus has a heating member  11  (hereinafter referred to as the heater) fixed/supported on a stay holder (heater supporter)  12 , and an elastic pressure roller  20  held and pressed onto the heater  11  via a heat-resistant thin film  13  (hereinafter referred to as the fixing film) to form a fixing nip portion N with a predetermined nip width. 
     When electricity is supplied, the heater  11  is heated to a predetermined temperature, and the temperature is adjusted. 
     The fixing film  13  is a cylindrical member, an endless belt-like member, or a rolled web-like member having ends. The film is attached and slid onto the surface of the heater  11  in the fixing nip portion N, and conveyed/moved in a direction of arrow a. 
     When the heater  11  is heated to the predetermined temperature, the temperature is controlled, and the fixing film  13  is conveyed/moved in the direction of arrow a, a recording material P with an unfixed toner image t formed/borne thereon is introduced as a material to be heated between the fixing film  13  and the pressurizing roller  20  of the fixing nip portion N. Then, the recording material P is attached to the surface of the fixing film  13 , and held/conveyed with the fixing film  13  through the fixing nip portion N. 
     In the fixing nip portion N, the recording material P with the toner images t is heated by the heater  11  via the fixing film  13  so that the toner images t on the recording material P are heated/fixed. 
     The recording material portion passed through the fixing nip portion N is peeled off from the surface of the fixing film  13  and conveyed. 
     A ceramic heater is usually used in the heater  11 . FIG. 11A is a partially cut plan model view showing the front surface side (heating surface side) of the ceramic heater  11 , and FIG. 11B is a plan model view of the rear surface side (surface side opposite to the heating surface). 
     Specifically, for example, the front surface side (surface on the side facing the fixing film  13 ) of a ceramic substrate  11   a  of alumina having electric insulation properties, good thermal conductivity, and a low thermal capacity is provided with a energizing heating resistance layer (heating member)  11   b  of Ag/Pd (silver palladium), Ta 2 N, and the like formed along the longitudinal direction of the substrate by screen printing or the like. Furthermore, the surface with the energizing heating resistance layer formed thereon is covered with a thin glass protective layer  11   c . For the heater  11 , by supplying power via a power supplying electrode portion  11   d , the energizing heating resistance layer  11   b  is heated so that the temperature of the entire heater is rapidly raised. 
     The temperature rise of the heater  11  is detected by temperature detecting means  14  disposed on the heater rear surface, and fed back to a energizing controller (not shown) via electric path patterns  11   e , through holes  11   f , and electrode portions  11   g  for output to a temperature controller. 
     The energizing controller controls the energizing of the energizing heating resistance layers  11   b  so that the heater temperature detected by the temperature detecting means  14  is maintained at a substantially constant predetermined temperature (fixing temperature). Specifically, the heater  11  is heated and controlled or adjusted to the predetermined fixing temperature. 
     The fixing film  13  is formed to be remarkably thin as 20 to 70 μm in order to efficiently give the heat of the heater  11  to the recording material P as the material to be heated in the fixing nip portion N. This fixing film  13  is constituted of three layers, that is, a film base layer, a primer layer, and a mold release layer, the film base layer is on the side of the heater  11 , and the mold release layer is on the side of the pressurizing roller  20 . The film base layer is formed of polyimide, polyamide-imide, PEEK, or the like which is higher in insulation property than the glass protective layer  11   c  of the heater  11 , and has a heat resistance and a high elasticity. Moreover, the mechanical strengths such as tear strength of the entire fixing film  13  are kept by the film base layer. The primer layer is formed of a thin layer which has a thickness of about 2 to 6 μm. The mold release layer is a toner offset preventive layer to the fixing film  13 , and is formed by coating fluoroplastics such as PFA, PTFE and FEP in a thickness of about 10 μm. 
     Moreover, the stay holder  12  is formed, for example, of a heat-resistant plastic member to hold the heater  11 , and also serve as a conveyance guide of the fixing film  13 . 
     In the heating apparatus of the film heating system using such thin fixing film  13 , the pressurizing roller  20  having an elastic layer is flatted along the lower flat surface of the heater  11  pressurized via the film  13  by a high rigidity of the ceramic heater  11  in the pressurizing portion to form the fixing nip portion N with the predetermined width, and only the fixing nip portion N is heated to realize a quick start heating/fixing. 
     Character S denotes a recording material conveying standard (sheet passing standard), and in the apparatus of the example, the standard is disposed in the middle of the recording material conveying area of an image forming apparatus main body in the longitudinal direction. The apparatus has a “central standard”. 
     The width of the energizing heating resistance layer  11   b  of the heater  11  in the longitudinal direction, that is, an effective heat generating area W is formed to be slightly narrower as compared with a width D (pressurizing roller abutting area) of the elastic layer of the pressurizing roller  20  which abuts on the heater  11  via the fixing film  13 . This prevents a problem that the temperature locally rises and breakage is caused by thermal stress when the energizing heating resistance layer  11   b  is protruded from the pressurizing roller  20 . 
     Moreover, the effective heat generating area W of the energizing heating resistance layer  11   b  is formed in a sufficiently broader width than that of an area for conveying sheets with normal sizes such as A 4  and LTR, that is, a passing portion A (normal sized sheet passing portion, large sized sheet passing portion). This can eliminate the influence of end portion temperature sag (by heat leakage to electric contacts, connectors  31 ,  32 , and the like on the end portions of the heater  11 ), so that effective fixing properties can be obtained over the entire surface of the recording material P. 
     Furthermore, in some cases, the width of the energizing heating resistance layer  11   b  on the end portion of the sheet passing area is shortened, and the heating value of the end portion is increased to compensate for the fixing properties of the end portions. 
     Therefore, the heat generated by energizing the energizing heating resistance layer  11   b  of the heater  11  is given to the recording material P conveyed between the fixing film  13  and the pressurizing roller  20 , and acts to melt and fix the toner images t on the recording material P. 
     The temperature detecting element  14  such as a thermistor, and a thermo-protector  15  such as a temperature fuse and a thermo-switch for shutting down the energizing of the energizing heating resistance layer  11   b  of the heater  11  during runaway abut on the rear surface of the heater  11 . The temperature detecting element  14  and the thermo-protector  15  are disposed in an area for conveying small sized sheets such as envelopes, that is, a small sized sheet passing portion B (minimum width recording material conveying area). The thermo-protector  15  is interposed in series with the power supply path to the energizing heating resistance layer  11   b.    
     Here, the temperature detecting element  14  is disposed in the small sized sheet passing portion B, so that even when the recording material P having the minimum width that can be conveyed in the image forming apparatus main body is conveyed, the toner image t on the recording material P is heated/fixed at an appropriate fixing temperature without causing any fixing failure, high temperature offset, or other problems. 
     On the other hand, the thermo-protector  15  is disposed in the small sized sheet passing portion B, so that when the recording material P with the minimum width is conveyed, in a non-conveying area, that is, a small sized sheet non-passing portion C which has a smaller heat resistance than the small sized sheet passing portion B as the conveying area, a problem that the thermo-protector  15  is incorrectly operated by overheating in the small sized sheet non-passing portion C to shut out the energizing even during normal conveyance, or other problems are prevented from occurring. 
     Additionally, since the thermo-protector  15  abuts on the rear surface of the heater  11 , in some cases the heat amount generated in the energizing heating resistance layer  11   b  is taken by the thermo-protector  15 , a sufficient heat amount cannot be applied to the recording material P, and fixing failure occurs in the abutting position of the thermo-protector  15 . To prevent this, by slightly narrowing the energizing heating resistance layer  11   b  in the position corresponding to the abutting position of the thermo-protector  15  like  11   b ′ and by setting the resistance value of the energizing heating resistance layer  11   b ′ to be larger than the values of the other energizing heating resistance layer portions, the heat generating amount is secured. Thereby, the heat supply amount to the recording material P is set to be constant over the longitudinal direction of the heater  11 , and excellent heating/fixing is realized without any fixing nonuniformity. 
     Since the temperature detecting element  14  also abuts on the rear surface of the heater  11  in the same manner as the thermo-protector  15 , it is also feared that the heat generated by the energizing heating resistance layer  11   b  is taken by the temperature detecting element  14 . However, by using the temperature detecting element  14  with a small heat capacity such as a chip thermistor, the heat amount taken from the heater  11  can be minimized. Therefore, even if the above-described countermeasure is not taken like in the thermo-protector  15 , uniform fixing can be realized in the longitudinal direction of the heater without deteriorating the fixing uniformity of the recording material. 
     In the image heating apparatus of the film heating system as described above in the conventional example, when sheets (recording materials) different in size (sheet width) are passed, the heat amount taken from the heater differs in the sheet passing portion and the sheet non-passing portion. The temperature of the sheet non-passing portion in which heat is not taken by the sheet gradually rises as the sheets are passed (sheet non-passing portion temperature rise phenomenon), and finally exceeds the heat resistant temperatures of the heater, pressurizing roller, and heater holder. The problem is solved by enlarging the sheet passing interval. 
     However, in recent years, with the increase of adjustment temperature and input power for a higher speed printer, the temperature rise of the sheet non-passing portion has become more remarkable, which cannot be solved by the method of enlarging the sheet passing interval any more. 
     To solve this problem, zone heating is effective in which the heater is provided with a plurality of heating members (energizing heating resistance layers) different in heat generating area, and the heating/fixing is performed by changing the heating member to be heated in accordance with the sheet size. 
     FIGS. 12A,  12 B and  12 C are diagrams showing one example of the zone heating type heater  11  as the background art of the present invention. FIG. 12A is an enlarged transverse sectional model view of the heater  11 , FIG. 12B is a plan model view of the rear surface side, and FIG. 12C is a pattern model view of a normal sized sheet heating member and a small sized sheet heating member. 
     The heater  11  in this example is a rear surface (back surface) heating type ceramic heater. Specifically, in the constitution, the substrate rear surface side (non-heating surface side) facing away from the front surface side (heating surface side, surface of the side facing the fixing film) of the highly heat conductive ceramic substrate  11   a  such as Al 2 O 3  and AlN is provided with the heating member (energizing heating resistance layer such as Ag/Pb and Ta 2 N). 
     In the heater  11  of this example, a normal sized sheet heating member H 1 , and a small sized sheet heating member H 2  parallel with the member H 1  are formed along the longitudinal direction on the rear surface side of the ceramic substrate  11   a . Power supplying electrode portions  11   d   1 ,  11   d   1  are energized and formed on both end portions of the normal sized sheet heating member H 1 . Power supplying electrode portions  11   d   2 ,  11   d   2  are energized and formed on both end portions of the small sized sheet heating member H 2 . The thin glass protective layer  11   c  is formed to cover the surface on which the normal sized sheet and small sized sheet heating members are formed. The temperature detecting means (thermistor)  14  and the thermo-protector  15  are disposed to contact the surface of the glass protective layer  11   c  on the rear surface side of the heater. 
     Character S denotes a recording material conveying standard (sheet passing standard), and in the apparatus of the example, the standard is disposed in the middle of the recording material conveying area of the image forming apparatus main body in the longitudinal direction. The apparatus has a “central standard”. Character X denotes a sheet passing direction. 
     The normal sized sheet heating member H 1  is disposed for the recording materials of A4, LTR, LGL, and the like, its length L 1  is set to 222 mm (equal to effective heat generating area W), and its width W 1  is set to 3 mm. 
     The small sized recording material heating member H 2  is adapted to the small sized sheet passing portion B for envelopes such as com  10 , DL and monarch, its length L 2  is set to 116 mm, and width W 2  is set to 1.57 mm. 
     The temperature detecting element  14  and the thermo-protector  15  are disposed in the small sized sheet passing portion B. 
     When power is supplied between the power supplying electrode portions  11   d   1  and  11   d   1  during the passing of normal sized recording materials, the normal sized sheet heating member H 1  is heated and the temperature of the entire heater is rapidly raised. This temperature rise of the heater  11  is detected by the temperature detecting element  14  and fed back to the energizing controller (not shown). The energizing controller controls the energizing of the normal sized sheet heating member H 1  so that the heater temperature detected by the temperature detecting element  14  is maintained at a substantially constant predetermined temperature (fixing temperature). 
     When small sized recording materials are passed, power is supplied between the power supplying electrode portions  11   d   2  and  11   d   2 , and the small sized sheet heating member H 2  is heated. Subsequently, the temperature of the heater corresponding to the small sized sheet passing portion B is detected by the temperature detecting element  14  and fed back to the energizing controller. The energizing controller controls the energizing of the small sized sheet heating member H 2  so that the heater temperature detected by the temperature detecting element  14  is maintained at the substantially constant predetermined temperature (fixing temperature). 
     However, when the zone heating is performed by independently energizing the heating members H 1  and H 2  different in heat generating area and by passing the sheets, temperature distributions h 1  and h 2  are formed in the sheet passing direction of the heater substrate as shown in FIG.  13 . Specifically, when the heating member H 1  positioned on the upstream side of the sheet passing direction is energized in the fixing nip portion N, the temperature distribution h 1  is obtained in the sheet passing direction of the heater substrate, and the temperature in the fixing nip portion can be kept substantially uniformly. However, when the heating member H 2  positioned on the downstream side is energized, the temperature distribution h 2  is obtained in the sheet passing direction of the heater substrate, and a large temperature gradient is generated in the upstream/downstream direction in the fixing nip portion. This is because there is a large heat flux to the sheet from the heater on the upstream side on which sheet temperature is low, and there is a small heat flux on the downstream side on which the sheet temperature is high. 
     Therefore, when the temperatures of a plurality of heating members H 1  and H 2  are adjusted/controlled by one temperature detecting element (thermistor)  14 , and when the heating members H 1  and H 2  are independently energized as described above, a moderate temperature gradient (usually the vicinity of temperature peak) differs with each case. Even when the temperature detecting element is placed substantially between the temperature peaks, a problem occurs that the detected temperature of the member H 2  largely fluctuates within the attaching tolerance of the temperature detecting element. 
     Moreover, during the energizing of the heating member H 2  positioned on the downstream side, since the entire heater substrate cannot be kept at a high temperature, there is a problem that the excellent fixing properties cannot be obtained. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image heating apparatus and an image heater in which heater temperature can correctly be detected even when there is an attaching position error of a temperature detecting element. 
     Another object of the present invention is to provide an image heating apparatus and an image heater in which a small sized recording material can sufficiently be heated. 
     Further object of the present invention is to provide an image heating apparatus comprising a heater having a long base material, a temperature detecting element for detecting temperature of the heater, and a film having one surface which slides on the heater and the other surface which moves while contacts a recording material bearing an image. The heater is controlled by an output from the temperature detecting element to obtain a predetermined temperature, the image on the recording material is heated by heat from the heater via the film, the heater has a first heating member disposed along a longitudinal direction of the base material and heated by energizing and a second heating member shorter than the first heating member, and the first heating member is disposed on the upstream side of the second heating member with respect to a moving direction of the recording material. When a first size recording material is heated, the first heating member is energized and the second heating member fails to be energized. When a second size recording material smaller than the first size recording material is heated, the first heating member and the second heating member are energized. 
     Another object of the present invention is to provide a heater for heating image comprising a long base material, a temperature detecting element for detecting temperature, a first heating member disposed along a longitudinal direction of the base material and heated by energizing, and a second heating member shorter than the first heating member. The first heating member and the second heating member are arranged in a direction orthogonal to the longitudinal direction of the base material, the first heating member is disposed for a first size recording material and a second size recording material smaller than the first size recording material, and the second heating member is disposed for the second size recording material. 
     Still another object of the present invention is to provide a heater for heating image comprising a long base material, a first heating member disposed along a longitudinal direction of the base material and heated by energizing, a second heating member shorter than the first heating member, and a third heating member having substantially the same length as the length of the first heating member. The first heating member, the second heating member and the third heating member are arranged in a direction orthogonal to the longitudinal direction of the base material, and the second heating member is disposed between the first heating member and the third heating member. 
     Further objects of the present invention would be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of an example of an image forming apparatus to which the present invention is applied. 
     FIG. 2 is a transverse sectional model view of a fixing apparatus. 
     FIGS. 3A,  3 B and  3 C are explanatory views of a heating member. 
     FIG. 4 is a diagram showing temperature distributions in a width direction of a fixing nip portion when heating members H 1  and H 2  are both energized. 
     FIGS. 5A and 5B are diagrams showing the relations of power ratios between the heating members H 1  and H 2  and between a sheet passing portion and a sheet non-passing portion. 
     FIGS. 6A and 6B are diagrams showing the constitutions of heating members in other embodiments. 
     FIGS. 7A and 7B are diagrams showing the constitution of a heater in which a thermistor is disposed. 
     FIG. 8 is a diagram showing the constitution of the heating member in another embodiment. 
     FIG. 9 is a diagram showing an end portion temperature sag. 
     FIG. 10 is a diagrammatic view showing a main part of one example of an image heating apparatus (heating/fixing apparatus) of a film heating system. 
     FIGS. 11A and 11B are explanatory views showing the constitution of the heating member (surface heating type). 
     FIGS. 12A,  12 B and  12 C are explanatory views showing the constitution of a rear surface heating type heating member as the background art of the present invention. 
     FIG. 13 is a diagram showing the temperature distribution of the width direction of a fixing nip portion when the heating members H 1  and H 2  are independently energized. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described hereinafter with reference to the drawings. 
     &lt;First Embodiment&gt; (FIGS. 1 to  5 ) 
     (1) Image Forming Apparatus Example 
     FIG. 1 is a diagrammatic view showing the example of an image forming apparatus. The image forming apparatus of the example is a laser beam printer using a transfer type electrophotographic process. 
     Numeral  1  denotes a rotating drum type electrophotographic photosensitive unit as an image bearing unit (hereinafter referred to as the photosensitive drum). A photosensitive material layer such as OPC, amorphous Se and amorphous Si is formed on a cylindrical conductive substrate such as aluminum and nickel. The photosensitive drum  1  is rotated/driven with a predetermined peripheral speed (process speed) in a clockwise direction shown by an arrow. 
     First in the rotating process, the surface of the photosensitive drum  1  is uniformly charged to provide a predetermined polarity and potential by a charge roller  2  as a charge apparatus. 
     Subsequently, the surface is subjected to a laser beam scanning exposure  3  in accordance with a desired image information pattern by a laser scanner (not shown) as an exposure apparatus. Thereby, an electrostatic latent image is formed on the surface of the rotating photosensitive drum  1  in accordance with the desired image information pattern. 
     The laser scanner outputs a laser beam which is ON/OFF controlled in response to a time series electric digital pixel signal of the image information pattern transmitted from external apparatuses such as a host computer, and the surface of the photosensitive drum  1  to be uniformly charged/processed is scanned/exposed with this laser beam. 
     The electrostatic latent image formed on the surface of the photosensitive drum  1  is toner-developed and visualized by a developing apparatus  4 . As the developing method, a jumping developing method, two-component developing method, FEED developing method, and the like are used, and in many cases a combination of image exposing and reverse developing is used. 
     The toner image formed on the surface of the rotating photosensitive drum  1  is successively transferred to the recording material (transfer material) P which is supplied to a transfer nip portion T from a sheet supply portion (not shown) in a predetermined control timing in the transfer nip portion T formed by the photosensitive drum  1  and a transfer roller  5  contacting the photosensitive drum  1  with a constant pressurizing force as a transfer apparatus. 
     A predetermined transfer bias is applied to the transfer roller  5  from a power supply (not shown) in a predetermined control timing, and the toner image on the surface of the photosensitive drum  1  is successively transferred to the surface of the recording material P nipped/conveyed through the transfer nip portion T by the action of the transfer bias. 
     The recording material P receiving the transferred toner image in the transfer nip portion T and passing through the transfer nip portion T is separated from the surface of the rotating photosensitive drum  1 , and conveyed to a fixing apparatus  6  as an image heating apparatus, so that the toner image is fixed as a permanent image. 
     On the other hand, residual toner resulting from the transfer on the photosensitive drum  1  is removed from the surface of the photosensitive drum  1  by a cleaning device  7 . 
     In the embodiment, the process speed of the image forming apparatus is 94 mm/s, and throughput is 16 ppm (A4). 
     (2) Fixing Apparatus  6   
     The fixing apparatus  6  as the image heating apparatus in the embodiment is a heating/fixing apparatus of a film heating system using a cylindrical fixing film, of a pressurizing roller drive type and of a tensionless type. FIG. 2 is a transverse sectional model view of the apparatus  6 . 
     A fixing member  10  and a pressurizing member  20  abut on each other to form the fixing nip portion N. 
     The fixing member  10  is constituted of a heating member  11  (hereinafter referred to as the heater), an insulating stay holder  12 , a fixing film  13 , and the like. The pressurizing member  20  is an elastic pressurizing roller. 
     The heater  11  is a thin and horizontally long ceramic heater formed of a highly thermal conductive Al 2 O 3  or AIN substrate and extended long in a vertical direction to a sheet surface. In the embodiment, the ceramic heater of a zone heating type and-of a rear surface heating type is used. 
     There are provided a substrate  11   a  as a base material, a normal sized sheet heating member H 1  as a first heating member, a small sized sheet heating member H 2  as a second heating member, a thermistor  14  as a temperature detecting element, and energizing control means  16 . There is only one thermistor  14 . The energizing control means  16  controls the energizing to the heating members H 1 , H 2  based on an output from the thermistor  14  so that the heater  11  reaches a predetermined temperature. 
     The energizing control means  16  also controls the selection of the heating member to be energized from the heating members H 1 , H 2  in accordance with the size of the recording material. 
     This heater  11  will be detailed in the next paragraph (3). 
     The insulating stay holder  12  is a member for holding the heater  11 , and preventing heat radiation in the opposite direction to the fixing nip portion N, and is formed of liquid crystal polymer, phenol resin, PPS, PEEK, and the like. The insulating stay holder  12  of the embodiment has a transverse sectional form like a substantially semicircular arc trough, and is a horizontally long, heat resistant and electrically insulating member which can bear a heavy load. The heater  11  is engaged, fixed and supported in a groove portion disposed in the substantially central portion of the lower face of the insulating stay holder  12  along the longitudinal direction of the holder with its surface side facing or exposed downward. 
     The fixing film  13  is a cylindrical heat resistant film loosely attached to the insulating stay holder  12  including the heater  11  with an allowance placed along the peripheral length, and the insulating stay holder  12  supports the inner surface of the fixing film  13 . 
     The fixing film  13  with a small heat capacity is formed in a total thickness of 100 μm or less to realize quick start, and is formed of a base layer of polyimide, polyamide-imide, PEEK, PES, PPS, PFA, PTFE, FEP, or the like, which has heat resistance and thermoplasticity. Moreover, the total thickness of 20 μm or more is necessary for the film which has a sufficient strength to constitute a long life heating/fixing apparatus and which is superior in durability. Therefore, the optimum total thickness of the fixing film  13  is in the range of 20 μm to 100 μm. Furthermore, to prevent the offset and secure the separating property of the recording material, the surface layer is mixed with or covered singly with a heat resistant resin which is excellent in mold release property, such as PFA, PTFE, FEP, and silicone resin. 
     The elastic pressurizing roller  20  as the pressurizing member is constituted of-a core metal  21 , and an external elastic layer  22  formed by foaming heat resistant rubber, such as silicon rubber and fluororubber, or silicon rubber. Furthermore, a mold release layer  23  of PFA, PTFE, FEP or the like may be formed on the layer  22 . 
     This elastic pressurizing roller  20  is held by a bearing member (not shown), pressed onto the downward facing surface of the heater  11  fixed/supported on the lower surface side of the insulating stay holder  12  via the fixing film  13 , and sufficiently pressurized from both end portions of the longitudinal direction by pressurizing means (not shown) so as to form the fixing nip portion N necessary for the heating/fixing. 
     The pressurizing roller  20  is rotated/driven in a counterclockwise direction shown by an arrow by drive means (not shown). The pressurizing friction force generated between the outer surfaces of the roller  20  and the fixing film  13  in the fixing nip portion N by the rotation/drive of the pressurizing roller  20  exerts a rotating force to the fixing film  13 , so that while the inner surface of the fixing film  13  is attached and slid onto the downward facing surface of the heater  11  in the fixing nip portion N, the fixing film  13  is driven to rotate along the outer periphery of the insulating stay holder  12  with a peripheral speed substantially corresponding to the rotating peripheral speed of the pressurizing roller  20  in the clockwise direction shown by an arrow. 
     In this case, for the cylindrical fixing film  13  driven/rotated along the outer periphery of the insulating stay holder  12 , the fixing film portions other than the peripheral long fixing nip portion N and the fixing film portion in the vicinity of the portion N are in a tension free state (the state in which no tension is applied). 
     Since the inner surface of the fixing film  13  slidably contacts a part of each outer surface of the heater  11  and the insulating stay holder  12  and rotates, the friction resistance of the heater  11  and the insulating stay holder  12  with the fixing film  13  needs to be minimized. For this purpose, a small amount of lubricant such as heat resistant grease is applied to the surfaces of the heater  11  and the insulating stay holder  12 . Thereby, the fixing film  13  can smoothly rotate. 
     As described above, the pressurizing roller  20  is rotated/driven, the cylindrical fixing film  13  is accordingly driven/rotated along the outer periphery of the insulating stay holder  12 , the heater is energized to generate heat, and therefore the temperature of the fixing nip portion N rises to the predetermined temperature and is adjusted. In this state, the recording material P bearing a formed and unfixed toner image t is introduced to the fixing nip portion N, and the surface side bearing the unfixed toner image of the recording material P closely abuts on the outer surface of the fixing film  13  in the fixing nip portion N, and is nipped/conveyed together with the fixing film  13  through the fixing nip portion N. 
     In the nipping/conveying process of the recording material P, the heat of the heater  11  is transmitted to the recording material via the fixing film  13 , and the unfixed toner image t on the recording material P is thermally pressurized and fixed, 
     When the recording material P is passed through the fixing nip portion N, the material is separated from the outer surface of the fixing film  13  with a curvature and discharged onto a discharge tray (not shown). 
     (3) Heater  11  and Energizing Control 
     FIG. 3A is an enlarged transverse sectional view of the heater  11 , FIG. 3B is a plan model view of the rear surface side, and FIG. 3C is a pattern model view of the normal sized sheet heating member and the small sized sheet heating member. 
     The heater  11  is a zone heating and rear surface heating type of ceramic heater which has a constitution similar to the above-described constitution of FIG.  11 . 
     The normal sized sheet heating member H 1  for A4, LTR, and the like as a first size recording material is disposed on the upstream side of the sheet passing direction, and the small sized sheet heating member H 2  for envelopes, and the like as a second size recording material is disposed on the downstream side of the sheet passing direction. The length (sheet passing width) L 1  of the normal sized sheet heating member H 1  and the length L 2  of the small sized sheet heating member H 2  are 222 mm and 116 mm as described above. The normal sized sheet is also a maximum size sheet. The heating members H 1  and H 2  are disposed on the substrate along the longitudinal direction of the substrate  11   a.    
     The resistance value R 1  of the normal sized sheet heating member H 1  is set to 13.4Ω, so that even when input voltage fluctuates, power shortage is not caused, excellent fixing properties are obtained, and electric noise levels such as flicker and high harmonic wave distortion are suppressed. 
     The resistance value R 2  of the small sized sheet heating member H 2  is lowered because of a narrow sheet passing width, but is set to the same value as that of the normal sized sheet heating member H 1  by narrowing the heating member width W 2  because there are restrictions of electric noises such as flicker and high harmonic wave distortion. 
     Therefore, the width W 1  of the normal sized sheet heating member H 1  is 3 mm, the resistance value R 1  is 13.4Ω (about 746 W during 100 V input), and the width W 2  of the small sized sheet heating member H 2  is 1.57 mm. 
     Additionally, in the embodiment, to simplify the process of manufacturing the heater, the adjustment of the resistance value of the small sized sheet heating member H 2  is performed by changing the width, but can be performed also by changing the heating member material or thickness. 
     As described above, for the zone heating type heater having a plurality of heating members H 1  and H 2 , when the heating members H 1  and H 2  are independently energized, the temperature distributions during sheet passing are obtained as described above and shown by h 1  and h 2  of FIG.  13 . Specifically, when only the heating member H 1  is turned on, the temperature in the heater substrate is substantially uniformed. When only the heating member H 2  is turned on, however, a large temperature gradient is generated in the heater substrate. This is because the heat flux to the sheet from the heater during the sheet passing through the fixing nip portion N differs on the upstream side and the downstream side of the sheet passing direction. Specifically, while the sheet is passed through the fixing nip portion, temperature rises, the heat flux to the sheet from the heater increases on the upstream side on which sheet temperature is low, and the heat flux decreases on the downstream side on which the sheet temperature is high. Therefore, when the heating member H 2  is disposed on the downstream side, the temperature gradient increases particularly in the upstream and downstream directions of the fixing nip portion. 
     In the embodiment, for normal sized sheets, only the heating member H 1  is energized, and the heating member H 2  is not energized. The embodiment for small sized sheets will be described hereinafter. 
     FIG. 4 shows the temperature distributions when sheets are passed by changing the energizing duties of the normal sized sheet heating member H 1  and the small sized sheet heating member H 2  according to the present invention. The power ratio per unit longitudinal length of each heating member H 1  or H 2  is shown in Table 1, and the energizing duty and the power ratio per unit longitudinal length of the heating member H 1  or H 2  are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Power per Unit Longitudinal Length of each 
               
               
                 Heating Member 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Power P/W 
               
               
                   
                 Power P 
                 Length 
                 per Unit 
               
               
                 Heating Member 
                 (= V 2 /R) 
                 L 
                 Length 
               
               
                   
               
               
                 H1 for normal sized sheets 
                 746 W 
                 222 mm 
                 3.36 W/mm 
               
               
                 H2 for small sized sheets 
                 746 W 
                 116 mm 
                 6.43 W/mm 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Heating Mernber Lighting Duty and Power per 
               
               
                 Unit Longitudinal Length 
               
            
           
           
               
               
               
            
               
                   
                 Lighting Duty (H1/H2) 
                 Power Ratio per Unit Length (H1/H2) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 1/2 
                       0.26 
               
               
                   
                 1/1 
                 0.52 
               
               
                   
                 2/1 
                 1.05 
               
               
                   
                 3/1 
                 1.57 
               
               
                   
                   
               
               
                   
                 *The power ratio when the heating member having power P1 or P2 per unit longitudinal length is lit with a lighting duty a:b in calculated by P1 × a/(a + b):P2 × b/(a + b).  
               
            
           
         
       
     
     As seen from FIG. 4, when the small sized sheet heating member H 2  and the normal sized sheet heating member H 1  are simultaneously energized, the temperature on the upstream side of the fixing nip portion N rises. Furthermore, when the power ratios per unit lengths of the heating members H 1  and H 2  are brought close to each other, the temperature in the heater substrate is more uniformed. 
     Table 3 shows small sized sheet fixing properties and detected temperature errors generated by the attaching tolerance of the thermistor  14  as the temperature detecting member when only the small sized sheet heating member H 2  is energized and when the energizing is performed with the duties shown in the above Table 2. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Energizing Duty, Fixing Properties, Detected 
               
               
                 Temperature Error 
               
            
           
           
               
               
               
            
               
                   
                   
                 Detected 
               
               
                   
                   
                 Temperature 
               
               
                 Energizing Duty (H1/H2) 
                 Fixing Property 
                 Error 
               
               
                   
               
               
                 Only H2 for small sized 
                 Bad 
                 20 deg 
               
               
                 sheets 
               
               
                 1/2 
                 Good 
                 10 deg 
               
               
                 1/1 
                 Good 
                 10 deg 
               
               
                 2/1 
                 Good 
                  6 deg 
               
               
                 3/1 
                 Good 
                  6 deg 
               
               
                   
               
               
                 Fixing property; good, slightly bad, bad  
               
               
                 Temperature-controlled temperature: 190° C.  
               
            
           
         
       
     
     It is seen from Table 3 that when the normal sized sheet heating member H 1  is also energized during the passing of small sized sheets, the fixing property of the small sized sheet can be enhanced, and the temperature dispersion within the attaching tolerance of the thermistor  14  can also be reduced. 
     From this result, the power ratio per unit length is preferably set to H 1 /H 2 ≧about 0.5, so that an excellent small sized sheet fixing property can be obtained and the detected temperature error of the thermistor is suppressed down to about 10 deg. 
     As described above, in the embodiment, for the small sized sheet, the heating member H 1  is also energized together with the heating member H 2 . At this time, since the heating member H 2  is also energized, the energizing amount to the heating member H 1  becomes smaller than the energizing amount to the heating member H 1  for obtaining the predetermined fixing temperature only with the heating member H 1 . Therefore, the temperature rise of the sheet not-passing portion can be suppressed as compared with when the small sized sheet is fixed only with the heating member H 1 . 
     However, as shown in FIGS. 5A,  5 B, by changing the energizing duty of the normal sized sheet heating member H 1  and the small sized sheet heating member H 2 , the power ratio per unit length also changes, and similarly the power ratio of the sheet passing portion and the sheet non-passing portion also changes. Therefore, if the power of H 1  is excessively large, the temperature rise of the sheet non-passing portion is also influenced. Table 4 shows the relation between the power ratio of the sheet passing portion and the sheet non-passing portion with each energizing duty of Table 2 and the temperature rise of the sheet non-passing portion. Additionally, the end portion temperature rise indicates the temperature when 75 com envelopes are continuously passed with a throughput of 16 ppm. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Energizing Duty and Sheet Non-Passing Portion 
               
               
                 Temperature Rise 
               
            
           
           
               
               
               
               
            
               
                   
                 Power Ratio 
                 Power Ratio per 
                   
               
               
                 Energizing 
                 per 
                 Unit Length (Sheet 
                 End Portion 
               
               
                 Duty 
                 Unit Length 
                 Passing/Non-Passing 
                 Temperature 
               
               
                 (H1/H2) 
                 (H1/H2) 
                 Portion) 
                 Rise 
               
               
                   
               
               
                 Only H2 
                 — 
                 — 
                  100° C. 
               
               
                 1/2 
                 0.26 
                 4.85 
                 160° C. 
               
               
                 1/1 
                 0.52 
                 2.92 
                 230° C. 
               
               
                 2/1 
                 1.05 
                 1.95 
                 270° C. 
               
               
                 3/1 
                 1.57 
                 1.64 
                 270° C. or 
               
               
                   
                   
                   
                 more 
               
               
                   
               
               
                 *In the power ratio per unit length of H1:H2 = P1:P2, the power ratio of the sheet passing portion and the sheet non-passing portion is calculated by the sheet passing portion: the sheet noon-passing portion = P1:P1 + P2 (see FIG. 5).  
               
               
                 *The heat resistant temperature of the heater holder is 300 ° C. In this case, a margin of 10% is provided, and the design target value is set to 270° C. or less.  
               
            
           
         
       
     
     As seen from Table 4, to pass the envelope with a full throughput (16 ppm), the power ratio needs to be the sheet passing portion/the sheet non-passing portion≧about 1.95, but the sheet passing portion/the sheet non-passing portion≧about 1.4 is a level having no practical problem (the throughput reduction of about ⅔ of the normal sized sheet throughput is allowed). 
     Moreover, in the embodiment, the rear surface heating type heater  11  is used, but even when the conventional type (surface heating type provided with the heating layer on the pressurizing roller side of the substrate) of heater using the AlN substrate is used, the similar effect can be obtained. 
     As described above, in the-embodiment, since during the small sized sheet passing the normal sized sheet heating member H 1  and the small sized sheet heating member H 2  are lit with the power ratio of about 0.5≦H 1 /H 2 ≦2.5, the small sized sheet fixing property can be enhanced, and the temperature detection error by the attaching position deviation of the thermistor  14  can be reduced. 
     &lt;Second Embodiment&gt; (FIGS. 6,  7 ) 
     In the embodiment, the speed of the apparatus is increased to 24 ppm for A4 vertical, and the process speed of 151 mm/s. With the speedup, the power consumption during the normal sized sheet passing needs to be increased to 880 W from 746 W of the first embodiment, thereby causing a problem of an increase in flicker and high harmonic distortion. 
     To solve the problem, as shown in FIGS. 6A and 6B, the normal sized sheet heating member is divided into two members H 1  and H 1 ′. By allowing ON timing to deviate and independently driving the members, the electric noises such as flickers are reduced. In the constitution, the heating member H 1 ′ is disposed along the longitudinal direction of the substrate on the substrate in the same manner as the heating members H 1  and H 2 . An electrode  11   d   3  is common to the heating members H 1 , H 2 , H 1 ′, an electrode  10   d   4  is disposed for the heating member H 1 , an electrode  11   d   5  is for the heating member H 2 , and an electrode  11   d   6  is for the heating member H 1 ′. 
     As a method of arranging the normal sized sheet heating members H 1  and H 1 ′, there are two methods of arranging the normal sized sheet heating member H 1 , the small sized sheet heating member H 2 , and the normal sized sheet heating member H 1 ′ from the upstream side to the downstream side of the sheet passing direction as shown in FIG. 6A, and arranging the normal sized sheet heating member H 1 , the normal sized sheet heating member H 1 ′, and the small sized sheet heating member H 2  as shown in FIG.  6 B. Either method can provide the similar effect. In the arrangement of FIG. 6B, however, the distance d between wiring patterns needs to be longer than the distance between the wiring and the heating member or between the heating members (since there is no voltage drop by the heating member, a large potential difference is applied to the wiring pattern), the heater substrate width is enlarged, and the cost tends to increase. 
     Moreover, as shown in FIG. 6A, when the small sized sheet heating member H 2  is disposed between the heating members H 1  and H 1 ′, during the fixing of the small sized sheet, the mainly heated heating member H 2  can be disposed in the middle of the width direction of the substrate, which is advantageous for heating the entire substrate. 
     In the embodiment, the arrangement example of FIG. 6A will be described. Additionally, the other conditions are similar to those of the above-described first embodiment, and the description thereof is omitted. 
     Even in this constitution, when only the small sized sheet heating member H 2  is energized, the problems similar to those described above are generated. 
     To solve the problem, in the embodiment, the heating members H 1  and H 1 ′ are energized during the normal sized sheet passing, and the heating members H 1  and H 2  are energized during the small sized sheet passing. In this case, as described in the first embodiment, the heat flux to the sheet differs on the upstream side and the downstream side of the fixing nip portion N. Therefore, when the heating members H 1  and H 1 ′ are lit with the same power ratio, the temperature in the heater substrate is not uniformed. 
     Table 5 shows the detected temperature error generated by the attaching tolerance of the thermistor  14  when the heating members H 1  and H 1 ′ are energized with the changed power ratio. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Power Ratio of Heating Members H1, H1′ and 
               
               
                 Thermistor Detected Temperature Error 
               
            
           
           
               
               
               
            
               
                   
                   
                 Thermistor Detected Temperature 
               
               
                   
                 Power Ratio H1/H1′ 
                 Error 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 1 
                        15 deg 
               
               
                   
                 1.5 
                 10 deg 
               
               
                   
                 2 
                  8 deg 
               
               
                   
                 3 
                  8 deg 
               
               
                   
                   
               
            
           
         
       
     
     As seen from Table 5, when the power ratio H 1 /H 1 ′ is about 1.5 or more, the temperature in the fixing nip portion becomes substantially uniform, and the detection error by the position deviation of the thermistor  14  can be set to about 10° C. 
     Moreover, during the small sized sheet passing, when the heating members H 1  and H 2  are controlled with the energizing duty H 1 :H 2 =2:1 (for the power ratio per unit length, H 1 /H 2 =0.65, the sheet passing portion: the sheet non-passing portion=2.5:1), the excellent small sized sheet fixing property can be obtained in the same manner as in the above-described first embodiment, and the detected temperature deviation by the position deviation of the thermistor  14  can be reduced. 
     Additionally, the arrangement position of the thermistor  14  may be determined in consideration of a portion where the temperature distribution of the heater width direction generated when the heating members H 1  and H 1 ′ are energized is flatted, and a portion where the temperature distribution of the heater width direction generated when the heating members H 1  and H 2  are energized is flatted. In the embodiment of FIG. 6A, the thermistor  14  is disposed on the downstream side of the sheet passing direction slightly from the center of the-heating member H 2  as shown in FIG.  7 . Additionally, FIG. 7A is a front view as seen from the underside of the heater, and FIG. 7B is a side view. 
     As described above, even when the power consumption is increased by the increase of the print speed in the constitution, the detection error by the position deviation of the temperature detecting element  14  for temperature control can be reduced without increasing the electric noises such as flickers and high harmonic distortion, and the small sized sheet fixing property can be enhanced. 
     &lt;Third Embodiment&gt; (FIGS. 8,  9 ) 
     The third embodiment is constituted by providing both end portions of the heating member H 1 ′ not energized during the small sized sheet passing with shortening portions e, e as shown in FIG. 8 in the heater of the above-described second embodiment, so that the heat generating amount of both end portions of the heating member H 1 ′ is increased. Additionally, the other conditions are similar to those of the above-described second embodiment, and the description thereof is omitted. 
     The area capable of printing (area capable of fixing) is usually 5 mm inside each sheet end. In recent years, however, there has been a demand for printing in the vicinity of the endmost portion of the sheet. To satisfy the demand, the width of the fixing apparatus is preferably enlarged in consideration of temperature sag by heat radiation of the end portions of the fixing apparatus as shown in FIG.  9 . However, this results in an enlarged size of the printer. 
     Therefore, the third embodiment is constituted so that the heat generating amount of the heating member is increased at each end portion of the heating member H 1 ′ not energized during the small sized sheet passing so as to enhance the fixing property of the sheet endmost portion without enlarging the width of the fixing apparatus. Thereby, the fixing up to the sheet endmost portion can be realized without deteriorating the temperature rise of the sheet non-passing portion during the small sized sheet passing. 
     Table 6 shows the relation between the increase of the heat generating amount of the end portion of the heating member H 1 ′ and the fixing property of the sheet endmost portion. Additionally, the heat generating amount is controlled by changing the widths of the center and end portions of the heating member to change the resistance value, but may be controlled by changing the material and thickness of the heating member. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Heat Generating Amount UP of Heating Member 
               
               
                 H1′ and End Portion Fixing Property 
               
            
           
           
               
               
               
            
               
                   
                 Shortening Amount of Heating 
                 Fixing Property 5 mm outside 
               
               
                   
                 Member Hl′ 
                 End Portion 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 0% 
                               Slightly bad 
               
               
                   
                 4% 
                 Slightly bad 
               
               
                   
                 8% 
                 Good 
               
               
                   
                 12% 
                 Good 
               
               
                   
                   
               
               
                   
                 Fixing Property; Good, slightly bad, bad  
               
            
           
         
       
     
     As seen from Table 6, by increasing the heat generating amount of the end portion of the heating member H 1 ′ by about 8%, the fixing to the entire sheet surface can be realized. Moreover, for the small sized sheet fixing property and the thermistor detection temperature error, the effect similar to that of the above-described second embodiment can be obtained. 
     As described above, by increasing the heat generating amount of the end portion of the heating member H 1 ′ which is not lit at the same time as the small sized sheet heating member H 2 , among the normal sized sheet heating members H 1 , H 1 ′, the excellent end portion fixing property can be obtained. Additionally, the excellent small sized sheet fixing property can be obtained and the thermistor detection temperature deviation can be reduced. 
     Additionally, the image heating apparatus of the present invention is not limited to the heater  11  of the rear surface heating type in the embodiment, and may be of the surface heating type. 
     Moreover, the sheet passing standard of the recording material P may of course be a one-side standard. 
     Furthermore, the image heating apparatus of the present invention is not limited to the fixing apparatus of each embodiment, and can be used as means and apparatuses for extensively heating/processing the material to be heated, such as an apparatus for heating the image bearing recording material to enhance surface properties such as gloss, an image heating apparatus for a tentative fixing apparatus, an apparatus for heating/drying the material to be heated, and a heating laminate apparatus. 
     Additionally, the principle and process for forming the unfixed toner image t on the recording material P are not limited, and are arbitrary. 
     The embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and can variously be modified within the technical scope of the present invention.