An image fixing apparatus

Abstract: An image fixing apparatus for fixing an unfixed image (T) formed on an image carrying member (P), includes a sheet member (23) movable together with the image carrying member in contact with the unfixed image carried thereon, and a heat generating element (28) contacted to such a side of the sheet member as is opposite to a side thereof contactable to the unfixed image, the heat generating element being fixed in use.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image fixing apparatus for fixing an image on a recording medium by applying at least heat to an unfixed toner image formed on an image recording or carrying material with heat-fusible toner, more particularly to an image fixing apparatus of such a type wherein heat is applied to the unfixed toner image through a sheet moving together with the recording material. As for image fixing machines of the type wherein a toner image is fixed by heat, a heating roller type fixing system is widely used wherein an image recording material carrying an unfixed toner image is passed through a nip formed between a heating roller of a temperature maintained at a predetermined level and a pressing roller having an elastic layer for pressing the recording material to the heating roller. However, this system involves a problem that the heat capacity of the heating roller or a heating element has to be large, since the temperature of the heating roller has to be maintained at an optimum level in order to prevent toner offset, which is an unintended transfer of the toner to the heating roller. If the heat capacity of the heating roller is small, the heating roller temperature is easily shifted to a higher or lower temperature in response to reception of the recording material or other external disturbance in terms of heat supply from a heat generating element. If it is shifted to a lower temperature, the toner is softened or fused insufficiently with the result of insufficient image fixing and/or low temperature offset. If, on the other hand, it is shifted to a high temperature, the toner is completely fused with the result of lower toner coagulation force, and therefore, occurrence of a high temperature offset. When the heat capacity is large as required for the reasons described above, the warm-up period, that is, the time period required for the heating roller to reach a predetermined temperature, is long. Usually, the offset is not completely prevented even if the heat capacity is made large, and therefore, a parting agent such as a silicone oil is applied to the heating roller. As a proposal for preventing the offset, U.S. Patent No. 3,578,797 and Japanese Laid-Open Patent Application No. 94438/1973 (JP-A-48094 438) disclose that a web or a belt is interposed between an unfixed toner image and a heating roller for applying the heat, and the image fixing operation is performed through the following steps: (1) The toner image is heated by a heating element to a fusing temperature to fuse the toner; (2) After fusing, the toner is cooled to provide a relatively higher viscosity of the toner; and (3) The web is removed after the toner deposition tendency is lowered by the cooling. Since the web is removed from the toner after the toner is cooled in this method, the high temperature offset is eliminated, thus increasing the range for the fixing temperature. However, since the toner is heated by a heating roller having a heater therein, and therefore, having a large heat capacity, the problem of long warm-up period is still not solved. In addition, the heat radiation inside an image forming apparatus with which the fixing apparatus is used is large, with the result of a high temperature within the apparatus. As another problem with the fixing apparatus disclosed in U.S. Patent No. 3,578,797, the recording member is heated without being press-contacted to the heating roller, and therefore, the efficiency of the heat transfer from the heating roller to the toner is low, and in addition, the heat transfer tends to become non-uniform. In the above-mentioned Japanese Laid-Open Patent Application No. 94438/1973, the toner image is heated both from the upside and downside. In order to apply heat to the toner image from the side opposite to the side thereof carrying the toner image, it is required that the image carrying material is first heated to a sufficient extent, which requires large energy. In addition, in the cooling step, the image carrying material having been heated to a high temperature for the purpose of heating the toner image, has to be cooled sufficiently in order to allow the separation of the web, so that a forced cooling means is inevitable, with the result that the energy is consumed wastefully. As described, even though proposals have been made wherein the toner is heated and then cooled before the separation, so that the high temperature offset is prevented, they still involve the above-described problems, and therefore, they have not been put into practice. US-A-3811828 describes an image fixing apparatus having an endless sheet member in which an infrared radiation source is used as the primary source for providing heat to fuse the toner image. A resistive heating member, contacting the inside surface of the endless sheet member, is used for preheating and also to supplement the heat provided by the primary source during fixing. US-A-4566779 describes an image fixing apparatus having an endless sheet member and a resistive heating member, contacting the inside surface of the endless sheet member. The resistive heating member comprises a shoe of metal or other thermally conductive material, a backing layer of electrically insulating material, and electrical resistors attached to this layer. The present invention is intended as a remedy to the problems aforesaid. An image fixing apparatus in accordance with the present invention is defined in claim 1 of the claims appended. Embodiments of the present invention provide an image fixing apparatus wherein a high temperature offset is prevented, and the energy consumption is low. Embodiments of the present invention also provide an image fixing apparatus wherein after the toner is heated, it is immediately cooled. In the embodiments a temperature rise of an image carrying material or an image recording material is decreased, and the toner can still be fused efficiently. In the preferred embodiments, the image carrying member can be cooled so that it can be easily handled by an operator, even immediately after discharge from the apparatus. Features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of an electrophotographic copying apparatus incorporating an image fixing apparatus; Figure 2 is a sectional view of the image fixing apparatus shown in the preceding figure; Figure 3 is a sectional view of the image fixing apparatus of Figure 2 wherein a part thereof is opened; Figure 4 is a sectional view of an image fixing apparatus according to an embodiment of the present invention; Figure 5 is a sectional view of an image fixing apparatus according to a further embodiment of the present invention; Figure 6 is a cross-sectional view of a heat generating element which is used in embodiments of the present invention; Figures 7, 8 and 9 are graphs illustrating temperature control in the embodiments of the present invention; Figure 10 is a circuit diagram showing a control circuit for controlling energy supply to a heat generating element; Figures 11, 12 and 13 are graphs illustrating temperature changes; Figure 14 is a perspective view of a heat generating element which is applicable to an image fixing apparatus according to the embodiments of the present invention; Figures 15, 17, 28 and 29 are graphs illustrating temperature change; Figures 18 and 27 are sectional views of image fixing apparatus according to yet further embodiments of the present invention; and Figures 21, 22, 23, 24 and 25 are sectional views of a sheet material usable with an image fixing apparatus according to the embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described, referring to the drawings, in which like reference numerals have been used throughout to designate elements having corresponding functions. The description of the embodiments will be preceded by a discussion of other forms of image fixing apparatus, making reference to Figures 1 to 3 of the drawings. Referring now to Figure 1, there is shown an image fixing apparatus used with an electrophotographic copying apparatus. The electrophotographic copying apparatus comprises an original carriage having a transparent member such as glass or the like and reciprocally movable to scan an original when it is moved in a direction indicated by an arrow a . Directly below the original carriage, there is an array 2 of small diameter and short focus imaging elements. An original G to be copied placed on the original carriage 1 is illuminated by an illuminating lamp 7, and the reflected light image of the original is projected through a slit onto a photosensitive drum 3 by the array 2. The photosensitive drum 3 is rotatable in a direction b. The photosensitive member 3 is coated with a zinc oxide photosensitive layer or an organic semiconductive photosensitive layer or the like. The photosensitive layer is charged uniformly by a charger 4. The photosensitive drum 3 having been uniformly charged by the charger 4 is exposed to the image light through the lens array 2, so that an electrostatic latent image is formed. The electrostatic latent image is developed by a developing device 5 with a toner containing resin material or the like which has a property of being softened or fused if heated. On the other hand, recording sheets P are accommodated in a cassette S, and are fed one by one by a pick-up roller 6 and a pair of registration rollers 9 which are press-contacted to each other and are rotated in timed relation with an image formed on the photosensitive drum 3, to an image transfer station. In the image transfer station, the toner image formed on the photosensitive drum 3 is transferred onto the sheet P by a transfer discharger 8. Thereafter, the sheet P is separated from the photosensitive drum 3 by a known separating means, and is transported along a conveyance guide 10 to an image fixing apparatus 20, wherein the toner image is fixed on the sheet P, using heat. Subsequently, the sheet P is discharged onto a tray 11. After the toner image is transferred, the residual toner remaining on the photosensitive drum 3 is removed by a cleaner 12. After the cleaning, the photosensitive drum 3 is illuminated by a lamp 7, so that residual charge remaining thereon is removed, by which the photosensitive drum 3 is prepared for the next image formation. Referring to Figure 2, there is shown the image fixing apparatus 20 in an enlarged scale and in a cross-section. The fixing apparatus 20 comprises a heat generating element (heater) 21 which includes an electrically insulative and heat durable base member made of alumina or the like or a compound material containing it, and which includes a heat generating layer 28 which is mounted on the bottom surface of the base member and which has a width of 160 µm (microns) and a length (measured along a direction perpendicular to the sheet of the drawing) of 216 mm and which is made of, for example, Ta₂N or the like. The heat generating member 21 is disposed at a fixed position between a supply reel 24 and a take-up reel 27, particularly between the supply reel 24 and the separation roller 26. The heat generating layer 28 is in the form of a line or a stripe. The surface of the heat generating layer 28 is coated with a protection layer made of, for example, Ta₂O₅ functioning as a protection from sliding movement. A bottom surface of the heat generating member 21 is smooth, and the upstream and downstream ends are rounded to provide a smooth sliding contact with a heat resistive sheet 23. The heat resistive sheet 23 contains as a base material polyester. The sheet 23 has been treated to provide a heat resistive property. It has a thickness of approximately 9 µm (microns), for example. The sheet 23 is wound around the supply reel 24 for supply in a direction C. The heat resistive sheet 23 is brought into contact with the surface of the heat generating element 21 and is wound up on the take-up reel 27 by way of a separation roller 26 having a large curvature (small diameter). The fixing apparatus comprises a pressing roller 22 for providing press-contact between the heat generating element 28 and the heat resistive sheet 23 and between the heat resistive sheet 23 and the toner image. The pressing roller 22 comprises a core member made of metal or the like and an elastic layer made of a silicone rubber or the like. It is driven by a driving source (not shown) to press-contact the transfer material P carrying an unfixed toner image T and conveyed along a conveying guide 10, to the heat generating element 21 through a heat resistive sheet 23 moving in the same direction and at the same speed as the transfer material P. The conveying speed provided by the pressing roller 22 is preferably substantially equal to the conveying speed in the image forming apparatus, and the speed of the heat resistive sheet 23 is determined in accordance therewith. In the apparatus having the structure described above, the toner image formed by a heat fusible toner on the transfer sheet P is heated by the heat generating element 21 through the heat resistive sheet 23, by which at least the surface portion is completely softened and fused. After the toner image is moved away from the heat generating element 21 and before it reaches the separation roller 26, the heat of the toner image is spontaneously radiated so as to be cooled and solidified, and by passing between the separation rollers 26 having a large curvature, the heat resistive sheet 23 is separated from the transfer sheet P. Thus, since the toner T is once softened and fused, and then is solidified, the coagulation force of the toner is very large, whereby the toner particles behave as a mass. Also, since the toner is pressed by the pressing roller 22 while it is softened and fused by heat, the toner image T penetrates into the surface part of the transfer sheet P, and is cooled and solidified therein. Therefore, the toner is not offset to the heat resistive sheet 23, and is fixed on the transfer material P. The heat generating layer 28 and the heat generating element 21 may be small in size, and therefore, the heat capacity thereof may be small. For this reason, it is not required to generate the heat beforehand, so that the power consumption during non-image forming period, and also the temperature rise in the apparatus can be prevented. In this apparatus, it is possible to use as the heat resistive sheet 23 a polyester sheet which is thin and inexpensive and which has been treated for heat resistive property, so that the heat resistive sheet 23b may be stored in the form of a roll as shown in Figure 2, which is replaced with a fresh roll after it is used up. In this structure, a roll of a sheet having a predetermined length is set on a supply reel shaft 24, and is extended between the heat generating element 21 and a pressing roller 22 and between separation rollers 26, and then the leading edge of the sheet is fixed on the take-up reel shaft 27. Where this system is adopted, it is preferable that the remaining amount of the heat resistive sheet on the supply reel 24 is detected by a heat resistive sheet sensor arm 30 and an unshown sensor, and that when the remaining amount becomes small, a warning is produced by display or sound to the user to promote replenishment of the heat resistive sheet. Referring to Figure 3, it is preferable to make the fixing apparatus openable by rotation of the upper part thereof about a shaft 31, by which separation is made between the heat generating element 21 and the pressing roller 22 and between the separation rollers to facilitate the heat resistive sheet replenishing operation. In this arrangement when the heat resistive sheet is entirely taken up, a new roll of the sheet is used, the thickness of the sheet can be reduced without particular consideration to the loss of the durability of the heat resistive sheet, and for this reason, the heat capacity of the sheet itself can be reduced, and therefore, the power consumption can be reduced. As described hereinbefore, the high temperature offset to the heat resistive sheet does not occur in this arrangement the taken-up heat resistive sheet can be reused if the thermal deformation or deterioration of the sheet is not significant. In this case, the sheet can be rewound for reuse, or otherwise, the take-up reel and the supply reel may be exchanged, by which the roll of the sheet can be used a plurality of times. In this arrangement a pair of separation rollers 26 is used by which toner image cooling time to the separation rollers 26, while the toner image T is being pressed, can be made sufficiently large. In addition, the curvature of the separation rollers 26, particularly the separation roller contacted to the heat resistive sheet 23 is large enough to make easy the separation between the heat resistive sheet 23 and the transfer sheet P. By those effects, the toner offset at the separating position can be further prevented. However, in the case where the heat capacities of the heat generating layer 28 and the heat resistive sheet 23 are sufficiently small, and where the image fixing speed is small enough, the separation rollers 26 may be omitted since the toner image T is cooled in a short range after the transfer sheet P passes by the heat generating layer 28 so that the offset can be effectively prevented even without them. What is required is only to separate the heat resistive sheet and the transfer sheet after the toner image is once softened and fused and then cooled and solidified. The pressing roller 22 has a rubber layer in this arrangement so that the heat capacity is large, and therefore, it is difficult to raise the temperature thereof. Also, it has a sufficiently large diameter. Accordingly, the surface of the pressing roller 22 is not heated to higher than the toner fusing temperature. This provides a cooling effect to the back side of the transfer sheet, thus promoting toner cooling after the fusing thereof. Also, the transfer sheet discharged from the image fixing apparatus is not so hot as to allow comfortable handling of the sheet even immediately after it is discharged therefrom. The description will be made as to power supply to the heat generating element. The heat capacity of the heat generating layer 28 of the heat generating element 21 is energized intermittently, more particularly, pulse-wise. The heat capacity of the heat generating layer 28 is so small, it is instantaneously heated up to about 260 °C. The energization and de-energization of the heat generating surface 28 are timed on the basis of an output of a transfer sheet detecting sensor 29 interrelated with a transfer sheet detecting lever 25 which detects the leading and trailing edges of the transfer sheet P. Alternatively, the timing of energization and de-energization may be controlled on the basis of a transfer sheet detection by a sheet sensor provided on the image forming apparatus. Experiments using the image fixing apparatus according to this arrangement will be described. A toner image T was formed with a wax toner for an electrophotographic copying machine PPC PC-30 available from Canon Kabushiki Kaisha, Japan. The fixing speed was approximately 15 mm/sec. The heating layer 28 was energized for 2 ms for every 10 ms so as to provide heat of approximately 2000 Ws per one A4 size sheet. It was confirmed that the fixed image was practically without problem. By the energization, the heat generating layer 28 is heated up to approximately 260 °C. Since the heat capacity is small, the temperature lowers enough during de-energization period of 8 ms (= 10 ms - 2 ms). Therefore, the waiting period for heating up the heating element is eliminated. Since the thermal energy required for the image fixing is supplied intermittently, more particularly, pulsewise, the heat generating layer having a small heat capacity and, therefore, exhibiting a quick rise can be easily heated to substantially the same temperature level, periodically. When the image fixing is performed continuously, the pulse duration of energization may be gradually decreased, by which the temperature of the heat generating layer can be prevented from shifting to an extremely high temperature. The temperature of the toner image T exceeds the temperature which is conventionally said to be a limit for preventing the high temperature offset, even though it is for a very short period. However, since the heat resistive sheet 23 and the transfer sheet P are separated after the toner is sufficiently cooled down and solidified, the offset does not result. The wax of the toner which is a major component thereof has a fusing point of approximately 80 °C, and the viscosity thereof when it is fused is low enough. Therefore, when the toner is heated by a heating element having a temperature of approximately 260 °C, a conventional heat fixing apparatus has been such that the fused toner is penetrated into the transfer material too much so that the image is smeared, or the image is penetrated even to the backside of the sheet. This has been an obstruction to decreasing the fusing point of the toner. According to the above arrangement, the toner does not penetrate too much, because the heat capacity of the heat generating layer 28 is very small, and because the heating period is very short, by which only the surface part of the transfer sheet is heated for only a short period. This is further enhanced by the temperature of the surface of the pressing roller which is lower than the toner fusing temperature. Referring to Figure 4, a first embodiment of the present invention will be described. In the Figure the same reference numerals are assigned to the elements having corresponding functions, by which detailed description thereof is omitted for the sake of simplicity. In this embodiment a heat resistive sheet in the form of an endless web is used in place of the non-endless heat resistive sheet 23 of the foregoing arrangement. The heat resistive sheet 40 is repeatedly heated and is repeatedly brought into contact with the toner image T. In consideration of the repetitive use, the endless sheet is made of PFA resin (perfluoroalkoxy resin) having a thickness of 30µm (microns) which has a good parting property and heat resistivity. The heat resistive sheet 40 is driven by a sheet driving shaft 41 so as to provide a peripheral speed, which is the same as the conveying speed of the transfer material P. The heat resistive sheet 40 is stretched between the driving shaft 41 and an idler roller 42 which is urged to provide tension to the sheet, while allowing revolution of the endless sheet 23. The heat generating element 21 is provided with a temperature detecting element 43 for detecting the temperature of the base member. Further, it is provided with a temperature fuse or thermostat as a safety device 44 to prevent overheating. More particularly, when the base member is overheated, the safety device 44 is actuated to shut off the energy supply to the heat generating layer 28. The energy supply timing to the heat generating layer in this embodiment is controlled in accordance with a signal produced in an image forming apparatus. The image fixing speed, and the image forming speed is 50 mm/sec, which is higher than that of the foregoing arrangement . In view of this, the width of the heat generating layer 28 (heating width) is 300 µm (microns) which is larger than that of the foregoing arrangement. The energy supply period was 1.25 ms per 5 ms so as to provide approximately 2400 Ws per one A4 size sheet. The maximum temperature of the heat generating layer is about 300 °C. The temperature rise (heat accumulation) of the heat generating element 21 itself is larger than that in the foregoing arrangement, since the electric power density applied to the heat generating layer 28 is larger and also since the heat is applied for a shorter period. In consideration of this, the pulse width of energization is controlled in accordance with an output of the temperature detecting element 43 mounted to the heat generating layer 28. More particularly, when the temperature of the base member of the heat generating element 21 is high, the energization pulse width is decreased to prevent an extreme temperature rise of the heat generating element. The control of the energization pulse will be described hereinafter. Since the temperature of the heat generating layer 28 and the total thermal energy applied to one transfer sheet are increased to cope with the increased image fixing speed, the time period required for cooling the toner to a sufficient extent is increased, and therefore a longer distance is required to a position at which the sheet and the transfer sheet are separated. To solve this problem, a radiating plate 45 of aluminium is disposed in contact with the heat resistive sheet 40 between the heat generating element 21 and the separation roller 26. By the provision of the cooling means before the separation between the heat resistive sheet 40 and the transfer sheet P, the necessity for the long distance between the heat generating element 21 and the separating position can be eliminated without giving up the sufficient cooling of the toner before the separation. A separation pawl or pawls 46 are disposed as shown in Figure 4 to assure the separation of the transfer material P. Further, in order to remove foreign matters such as paper dust or the like deposited on the heat resistive sheet 40, a cleaning pad 47 made of felt is contacted to the heat resistive sheet 40. The felt pad 47 may be impregnated with a small amount of parting agent, such as silicone oil to improve the parting property of the heat resistive sheet 40. Since this embodiment uses the heat resistive sheet 40 made of PFA resin which is insulative, the heat resistive sheet tends to be electrostatically charged, by which the toner image can be disturbed. To obviate this problem, a discharge brush 48 which is grounded is used to discharge the heat resistive sheet 40. Here, it is possible that the brush is supplied with a bias voltage rather than being grounded to positively charge the heat resistive belt within the limit of not disturbing the toner image. It is preferable that conductive particles or fibres such as carbon black or the like are added in the PFA resin to prevent the electrostatic disturbance to the image. The same means for discharging or for providing conductivity may be used for the pressing roller. As another alternative, an anti-electrification agent may be applied or added thereto. As described hereinbefore, this embodiment uses an endless heat resistive sheet. The heat generating element 21 is disposed inside the endless sheet 40 and between the driving shaft 41 and idler roller 42. It is preferable that the heat generating element 21 is disposed upstream of the central position between the idler roller 42 and driving shaft 41 to assure the distance for cooling the fused toner. As for the position of the discharging brush 48, it is preferably disposed immediately upstream of the heat generating element 21, that is, between the heat generating element 21 and the roller 42. By doing so, the charge produced by separation of the sheet 40 from the roller 42 is also removed. It is further preferably positioned upstream of the position where the transfer material and the heat resistive sheet are contacted, since then the disturbance to the toner image by the electrostatic charge can be assuredly prevented. In this embodiment, the high processing speed results in the maximum power consumption of as large as approximately 1600 W. In consideration of this, the heat generating layer may be divided in the longitudinal direction into four elements which are sequentially energized, by which the maximum power consumption is reduced to 400 W. It has been described hereinbefore that the toner cooling effect from the backside of the transfer sheet can be provided by using a sufficiently large heat capacity and large diameter of the pressing roller to prevent the surface temperature of the pressing roller at the nip from becoming beyond the toner fusing temperature during the fixing operation. Referring to Figure 5, a further embodiment will be described in which the cooling effect by the pressing roller can be provided even if the heat capacity and the diameter of the pressing roller is small. In this embodiment, a cooling fan 49 is provided to apply air wind to the pressing roller so as to maintain the surface temperature of the pressing roller at a temperature lower than the toner fusing temperature. By the provision of such a fan, even if the surface temperature of the pressing roller temporarily rises at the nip, it is lowered during one rotation. It is preferable that the air flow by the cooling fan 49 is directed to the heat resistive sheet 40 to promote the cooling of the toner after the heat generating element 21. The fact that the surface temperature of the pressing roller is lower than the toner fusing temperature can be confirmed by applying a paint whose colour changes at the toner fusing temperature, on the pressing roller surface, or by coating the pressing roller with the toner and then checking the toner after the fixing operation performed. As described hereinbefore, the heat generating layer 28 is intermittently and pulse-wise energized. The description will be made as to the energization of the heat generating layer. Referring to Figure 6, there is shown a preferable heat generating element 21 provided with a temperature detecting element. The heat generating element 21 includes a base layer 54, a heat resistive layer 53 of a heat resistive and low thermal conductivity material on the base layer 54, a thermistor 55 functioning as a low heat capacity temperature sensor on the heat resistive layer 53, a thin insulative layer 52 thereon, and electrodes 50 and 50 thereon. Between the electrodes 50 and 50, a heat generating layer 28 having a width l is formed. The surface of the electrodes 50 and 50 and the heat generating layer 28 are coated with a protection layer 51. To the electrodes 50 and 50, a power source 61 for supplying power pulses is connected. The power source 61 is connected with a control circuit 60 including a microcomputer for controlling the pulses applied to the electrodes in response to a signal from the thermistor 55. The control circuit 60 is effective to control the amount of energy per one pulse of the power source by changing the pulse width so that the maximum temperature detected by the thermistor 55 is within the predetermined range. The thermistor 55 involves a response property including a rising delay and falling delay due to the presence of the insulating layer 52 between the heat generating layer 28 and the thermistor 55 (the insulative layer 52 provides the same thermal gradient as the protection layer 51). However, the situation is the same with the heating portion H, that is, the surface of the protection layer at the heat generating position 28. Therefore, the envelope covering the minimum values of the outputs of this thermistor 55 is substantially the same as the envelope covering the maximum values of the temperatures at the heating position H, and therefore, the thermistor 55 substantially detects the actual temperature. If constant power pulses are applied to the electrodes without controlling the applying power, the amount of heat generated exceeds significantly beyond the amount of radiation with the result that the heat generating layer 28 and the heating portion H is extremely heated to a high temperature by which the toner image can be non-uniformly fixed, or the heat generating layer 28 or the heat resistive sheet 40 can be damaged by heat. In Figures 4 and 5 embodiments, it should be noted that the temperature of the heat generating layer is detected through an insulative layer having a certain heat insulative property between the heat generating layer and the thermistor, rather than directly detecting the temperature of the heat generating layer. When the heat generating layer is energized pulse-wisely, the temperature change is very sharp because the heat capacity of the heat generating layer is very small. It is possible that the thermistor is not able to follow the sharp temperature change. In consideration of this, it is preferable that the temperature change is made more or less dull before the temperature detection, by the provision of the insulative layer 52. In the structure shown in Figure 6, the temperature is detected in the same condition as the surface of the protection layer 51, and therefore is preferable. It is more preferable, however, that the consideration is made also to the heat capacity of the heat resistive sheet 40 so that the detected temperature corresponds to the temperature of the outer surface of the heat resistive sheet 40 at the position where it is contacted to the toner. The thermal states are mainly determined by the heat capacity of the heat resistive sheet 40 rather than the protection layer, since the former has a larger heat capacity. The power control will be described. Since pulse heating is employed in these embodiments, the toner is heated only for a short period in the order of miliseconds. The temperature of the heating position H rather than the toner heating period is predominant as to the image fixing performance, and the temperature of the toner layer is increased in accordance with the maximum temperature of the heating position H. Therefore, by controlling the power supply to the electrodes 50 and 50 so that the maximum temperature of the heating portion H is maintained at a temperature T HO during the image fixing process, where T HO is a temperature of the heating position H by which the toner is softenedenough to be fixed, sufficient image fixing performance can be provided without consuming wasteful power. Among a starting temperature To of the heating position and a fixing temperature T HO of the heating position H to which it reaches, as shown in Figure 7, by supplying power to the electrode at a constant voltage level V for a period τ₀ there is the following relationship: T HO = To + A (1 - e -Bτ o ) where A and B are coefficients determined on the basis of power supplying conditions to the heat generating layer and heat radiation path from the heating portion H, and are substantially constant if those conditions are within the respective predetermined ranges. Then, if the temperature of the heating position H is T B , the following is satisfied: T HO = T B + A (1 - e -Bτ B) where τ B is a pulse supplying period required for increasing the temperature from T B to T HO . The equation (2) is expressed as: τ B = (1/B) x ln[1/{1-(T HO -T B )/A}] As will be understood from the foregoing the coefficients A and B can be determined beforehand by experiments. Therefore, if the temperature T HO is selected to a predetermined temperature, the temperature T B is measured, and the pulse energy having the pulse width τ B is applied, the temperature of the heating portion H can be raised to the fixing temperature T HO . In this embodiment, the energy is supplied to the electrodes 50 and 50 with a sufficiently small duty ratio as described, the temperature of the heating portion H is substantially equal to the temperature detected by the thermister 55 when the temperature of the heating portion H is minimum, that is, immediately before the start of the pulse energy supply. Therefore, next energy supply period is calculated in accordance with the above equation (3) by the control circuit 60 in accordance with the temperature detected by the thermister at this time. The power is supplied from the power source 61 to the electrodes 50 and 50 for the calculated period of time. Referring to Figure 8, the temperature change of the heating portion H with time is shown corresponding to the timing of the pulse energy supply to the electrode 50 and 50. In this embodiment, the voltage of the supply power to the electrodes is constant, and the frequency (1/τ) of the energy supply pulses is constant. In this Figure, the fixing operation is started at time t o when the temperature of the heating portion H is To. The temperature of the heating portion H increases by the energy supply having a pulse width τ o from the starting temperature To to the fixing temperature T HO , and then it decreases during the non-energy-supply period (τ - τ o ) which is sufficiently longer than the period τ o , down to a temperature T₁ which is higher than the temperature To. At time t₁ which is pulse period (τ) after the time t o , the second energy supply is effected with a pulse width τ₁ which is shorter than the period τ o and which is determined on the basis of the temperature T₁, by which the temperature of the heating portion H increases again up to the fixing temperature T HO . Similarly, the temperature decreases with the stoppage of the power supply. The subsequent operations are continued in the similar manner. More particularly, for each pulse period τ after the start of the power supply, the electrodes 50 and 50 are supplied with energy with the pulse width determined by the equation (3) on the basis of the temperature detected by the thermister 55, whereby the maximum temperature of the heating portion H can be maintained at the fixing temperature T HO . Accordingly, the power can be used effectively, and simultaneously therewith, the liability of the thermal deformation of the heat resistive sheet or of damage to the heat generating layer during a continuous image fixing operation can be minimized. Now, the description will be made as to the relationship between the pulse-wise energy supply and the conveying speed of the transfer material. As shown in Figure 27, the toner image T on the transfer sheet P which is being conveyed at a conveying speed of Vp (m/sec) is introduced into the effective fixing width l of the heating portion (heat generating layer 28) of the heat generating element 21 together with the image fixing film 23 which is being conveyed correspondingly to the movement of the transfer material. Figure 28 shows temperature change with time in this embodiment when a toner image having a thickness of 20 µm (microns) and formed with toner having a minimum fixing temperature of 125 °C is fixed on a transfer sheet having a thickness of 100µm (microns) with the use of a polyimide film having a thickness of 6 µm (microns) as the fixing film. The temperatures at the surface portion of the heating portion, at the inside part of the toner image and at the inside part of the transfer sheet are shown. The temperatures of Figure 28 are those when the energy supply pulse width to the heat generating layer is 2 ms, and was obtained by a well-known equation of one-dimensional heat conduction (This applies to the temperatures described hereinafter in conjunction with Graphs. As will be understood from this Figure, the inside part of the toner image layer is heated enough to be beyond the minimum fixing temperature so that image fixing is possible, whereas the inside part of the transfer material is hardly increased in temperature. It is understood from this that the energy consumption decreases with decrease of the width of the energy supplying pulse width. In the embodiment, the energy supplying pulse width τ (ms) applied to the heat generating layer satisfies τ < l /Vp. This means that it is preferable that the energy supplying pulse width τ is smaller than the time period ( l /Vp) required for the transfer material to pass through the effective heating width 1 µm (micron). Accordingly, in this embodiment, the heat generating layer is linear and integrally formed and is supplied with energy in the form of pulses, so that the temperature increase of the transfer material is constrained, while sufficient heat is assured to effectively and quickly heat and fuse the toner image within the effective width of the linear heat generating portion which is quickly heated in response to the temperature rise of the heating generating element; and further, unnecessary heating of the toner image is prevented to reduce the energy required for heating. The energy supplying pulse width is determined so as to accomplish those effects. If the energy supplying pulse width τ is larger than 1 /Vp, and the toner image is sufficiently heated, that portion of the toner image which receives superfluous heating becomes larger so that excessive energy is required. In this case, the temperature rise of the transfer material is large, thus increasing the consumption of unnecessary energy. Since in the present case the energy supplying pulse width τ is smaller than l /Vp, unnecessary heating of the toner image can be avoided, and furthermore, the temperature rise of the transfer material decreases with the decrease of the energy applying pulse width τ, whereby the energy consumption is reduced. The minimum value of the pulse width τ is determined in accordance with the durable temperature and durability to the thermal shock of the structural member of the image fixing apparatus such as the heat generating element or member, the fixing film and the like. The results of experiments will be described. A toner image T was formed with wax toner for a copying machine PPC PC-30 available from Canon Kabushiki Kaisha, Japan. The toner image was pulse-wisely heated for 2 ms for every 10 ms so that τ < l /Vp was satisfied and that the amount of heat per one A4 size sheet was approximately 2000 Ws. The image fixing speed was approximately 15 mm/sec. The resultant image does not practically involve any problem. By the energy supply, the heat generating layer was heated up to approximately about 260 °C. Since the heat capacity is so small that the temperature decreases during the de-energization period of 8 ms. Referring to Figure 29, the results are shown when the same operation was carried out with the apparatus of this embodiment under different conditions, as follows: Heating conditions: energy density of 32 W/mm² Heating duration: 2 ms Toner fixing temperature: 80 °C Fixing film: polyimide film having a thickness of 25 µm (microns) Thickness of the toner image: 20 µm (microns) Thickness of the transfer sheet: 100µm (microns) Ambient temperature: 20 °C In this test, the temperature of the heating portions was increased up to approximately 380 °C which is far higher than the toner fixing temperature which is 80 °C, and therefore, the toner is sufficiently heated above the toner fixing temperature by the very short heating duration (2 ms). Thus, the image is sufficiently fixed. On the other hand, the temperature rise of the transfer material is very small, and therefore, wasteful energy consumption is reduced as compared with conventional heat fixing rollers. The description will be made as to the frequency of the energy supplying pulses. In this embodiment, the frequency ν of the energy supplying pulses for the heat generating element is determined so as to satisfy: Vp/ l ≦ ν < 2Vp/ l This means that when the toner image T being conveyed at a speed Vp is periodically heated within the effective heating width l , each portion of the toner image T is heated at least once, but the same portion is not heated more than twice. Accordingly, in this embodiment, the heat generating layer is linear and integrally formed and is supplied with energy in the form of pulses, so that the temperature increase of the transfer material is constrained, while sufficient heat is assured to effectively and quickly heat and fuse the toner image within the effective width of the linear heat generating portion which is quickly heated in response to the temperature rise of the heating generating element without heating the same portion more than twice; and further, the unnecessary heating of the toner image is prevented to reduce the energy required for the heating. The energy supplying pulse width is determined so as to accomplish those effects. Results of experiments using an apparatus according to this embodiment will be described. A toner image T was formed with a toner which is softened and fixed at a room temperature which is 20 °C. The period (a reciprocal of the frequency) of the pulse energization was 10 ms, and the pulse width was controlled on the basis of the temperature detected by the thermister 55 so that the maximum temperature at the fixing portion (heating portion H) was 300 °C. The image fixing speed was approximately 15 mm/sec. The resultant image did not practically involve any problem. According to this embodiment, the heat capacity of the heating portion H is so small that the waiting period having been required to heat the heating portion H by supplying energy to the heat generating element beforehand is not required. In this embodiment, with the increased number of image fixing operations, the temperature of the heating portion H is more or less increased by the heat insulative effect of the insulating layer 53, with the result that the energy supplying pulse width decreases gradually, so that the average power consumption is small. The temperature rise in the apparatus was not a practical problem. Figure 9 is a graph showing test results of the temperature changes, with time, of the toner image and the transfer material, more particularly, the temperature at the centers of the thicknesses thereof when the image fixing apparatus according to this embodiment was operated to fix the toner image on the transfer sheet. The conditions were as follows: Heating condition: energy density of 25 W/mm² Heating duration: 2 ms Toner fixing temp.: 125 °C Fixing sheet: PET (polyethyleneterephthalate) film having a thickness of 6 µm (microns) Thickness of the toner image: 20 µm (microns) Thickness of the transfer sheet: 100 µm (microns) Ambient temperature: 20 °C In this test, the heating portion H was heated up to approximately 300 °C which was far-higher than the toner fixing temperature which was 125 °C, so that the toner was sufficiently heated beyond its fixing temperature, and the resultant fixed image was good. On the other hand, the temperature rise of the transfer material is very small, and the energy is not wastefully consumed as compared with conventional heat fixing rollers. The reason why the temperature rise of the transfer sheet is small is that the heat capacities of the heat generating layer, protection layer and the heat resistive sheet are very small. The heat generating layer, having a good thermal response property and having a sufficiently small heat capacity, preferably has 10⁻⁷ J/degree.cm - 10⁻² J/degree.cm. In this embodiment, 2 x 10⁻⁶ J/degree.cm. The thickness of the layers between the heat generating layer and the toner, that is, the thickness of the protection layer and the heat resistive sheet is not more than 50 µm (microns). From the results of the test, it is understood that even if excessive energy is applied by variation of the heating duration and a heating energy density, the high temperature offset does not occur, so that the tolerance of the heat control is wide. In this embodiment, the width of the energy supply pulse to the heat generating element is controlled. However, it is a possible alternative that the voltage of the power supply to the heat generating element is controlled with constant pulse width and the pulse frequency so as to maintain a constant maximum temperature of the heating portion H. When the temperature of the heating portion H is increased from a temperature T B to a temperature T HO by a pulse energy supply with the voltage of Vo for the period of τ o , the following relation is satisfied, as described hereinbefore: T HO = To + A (1 - e -Bτ o ) Here, A is generally expressed as A = kV² in those equations, B and k are constants independent from the voltage but determined by the structure and the material of the heat generating element. Then, the following results: T HO = T B + kV B 2 (1 - e -Bτ o ) V B = [(T HO - T B )/{k(1 - e -Bτ o )}] 1/2 where V B is a voltage of the power supply required for the temperature of the heating portion H to be increased from the temperature T B to the temperature T HO with the pulse energy supply during the period of τ o . Therefore, if the constants k and B are determined beforehand by experiments, and τ o and T HO are set to be certain values, and the temperature T B is measured, the heating portion H can be heated up to T HO by applying the voltage V B determined by equation (5). According to this embodiment, as contrasted to the foregoing embodiments, the ON/OFF timing of the power supply to the heat generating element is constant, and therefore, the processing by the microcomputer is easier. As for the position of the thermistor 55, it is not limited to the position described in the foregoing. For example, in a part of the protection layer, a heat releasing portion may be formed, where the thermistor may be disposed. What is preferable is that the thermistor is so positioned that the minimum temperature of the heating portion H can be detected. Further, it is not necessary to control the energy supplying pulse width for each period τ, but the control is effected at intervals which are longer than the period τ. In that case, the temperature of the heating portion H is not exactly maintained at the temperature T HO . However, as described hereinbefore, slight variation of the maximum temperature does not result in a satisfactory fixing performance. What is required is to maintain the temperature of the heating portion H within the temperature range in which practically good image fixing performance can be provided and which includes the temperature T HO . On the basis of this condition, the upper limit τ max of the control timing period, and the control interval is determined within the range between τ and τ max . Next, the description will be made as to the system wherein the pulse width is controlled. Referring to Figure 10 there is shown a control circuit in the above described embodiment. The control circuit includes a field effect transistor (FET) Q1 for controlling energization of the heater. The gate of the transistor Q1 is on-off-controlled by a transistor Q2, and the base of the transistor Q2 is controlled by a photocoupler Q3. A light emitting side of the transistor Q3 is on-off-controlled on the basis of a result of feed-back control by a pulse width controlling means U1. The pulse width control means will be further described. A resistance of the temperature detecting sensor 55 swings at the same frequency as the applied pulse voltage. The coefficient of the resistance change is positive as shown in Figure 11. As shown in Figure 10, voltage ratio V IN of the voltage across the resistor R6 and the voltage across the temperature sensor 55, and the relationship between a maximum input voltage Vp to non-reverse input to the operational amplifier Q4 in one pulse and a peak temperature Tp of the heat generating layer is determined beforehand on the basis of tests. Then, the input energy to the heat generating element, that is, the pulse width is controlled so that the voltage Vp is constant (reverse input voltage V F to an operational amplifier Q5 which will be described hereinafter), by which the peak temperature of the heat generating layer is controlled to be constant. In Figure 10, a capacitor C3 is effective to store the above described voltage Vp, and is discharged through a discharging circuit constituted by capacitor C3 and a resistor R10, the discharging circuit having a discharge time constant which is approximately 10 times the pulse period T of control pulses. Figure 12 shows the charging and discharging of the capacitor C3 by a curve B. A curve A indicates the actual temperature of the heat generating layer. As will be understood there is a time difference Δt between the actual temperature of the heat generating layer (A) and the output of the temperature sensor TH1. It is considered that this results from the heat transfer therebetween. The peak voltage Vp is compared with the reference voltage V F by a difference amplifier Q5, and the difference is multiplied by G = R13/(R11/R12), and is produced as an output Vout. The output Vout is compared with a reference triangle wave V1 by a comparator Q6, and as a result, a PWM output Vpwm is produced. When the peak temperature Tp of the heat generating layer increases so much that the non-reverse input voltage of the difference amplifier exceeds the reference voltage V F of the reverse input, the output Vout increases, so that the H-level of the PMW output Vpwm becomes shorter, by which the ON duration of the photocoupler 13 is shortened, and ultimately the ON duration of the power FET Q1 is shortened. Thus, the peak temperature Tp of the heat generating layer is corrected toward a lower temperature. On the other hand, when the peak temperature Tp decreases beyond a target temperature, the similar control is effected so as to increase the ON duration of the power field effect transistor Q1. Figure 13 shows this control. Referring to Figure 14 there is shown another example of the heat generating element 21, in which a thermistor is mounted on a heat resistive material layer 53. With repetition of the pulse energisations applied to the heat generating layer, the temperature of the heat generating element increases. If the temperature increase becomes large, the toner becomes influenced by the heat of the base layer of the heat generating element. As shown in Figure 15, it is preferable that if the temperature of the base layer reaches a certain level Ts, the power supply is stopped for a certain duration after the sheet which is being fixed, if any, is discharged, and the image fixing is resumed after the base plate is sufficiently cooled. Referring to Figure 18, the description will be made as to a further preferable embodiment wherein the heat generating layer is maintained at a constant temperature. In this embodiment, an endless heating resistive sheet 40 is used, which is repeatedly heated and contacted to the toner image layer T. In consideration of the repetitive use, the endless sheet 40 includes a base member made of polyimide resin having a thickness of 25 microns which is excellent in the heat resistivity and mechanical strength, and a parting layer made of fluorine resin or the like showing good parting property on the outer surface of the base member. The endless sheet 40 is driven by a driving shaft 41 to provide a peripheral speed which is the same as the speed of the transfer material. The endless sheet is stretched between the driving shaft 41 and a shaft 43 which is freely rotatable. An idler roller 42 is contacted to the outer surface of the endless sheet 40 to provide tension therein. In this embodiment, the heat generating layer of the heat generating element 21 is of PTC heat generating material such as barium titanate, which exhibits a positive coefficient of resistance-temperature (PTC). When the resistance layer is energised to produce heat up to about Curie temperature, the resistance rapidly increases with the result of lower heat produced, and therefore, it is self-controlled to a temperature inherent to the material of the resistance layer. By the heat generating element 21, the toner image T is effectively heated in the width N of the nip with the pressing roller 22. In order to obtain durability of the endless sheet 40, the thickness of the sheet is larger than in the embodiment wherein the sheet is not used repetitively. For this reason, the heat transfer from the heat generating element 21 to the toner image is slightly slower. In consideration of this, there is provided a portion M for pre-heating the endless heat resistive sheet 40 at an inlet side. Therefore, the heating portion of the heat generating element 21 is wider at the inlet side than at the outlet side. Since the PTC heat generating layer 60 in this embodiment has a little larger heat capacity, so that preferably it is preheated. However, it requires only a few seconds, and therefore, even if the preheating is started simultaneously with image formation, it is sufficiently heated by the time the image fixing operation starts after toner image formation on the transfer sheet. Accordingly, a waiting period is not necessary or can be minimal. As described, in this embodiment, the self-temperature control property of the PTC heat generating element eliminates the necessity of temperature detection and power supply control, and the temperature can still be maintained automatically at a constant level. The description will be made as to the heat resistive sheet. The sheet 40 is required to be strong and heat resistive enough. As for a material satisfying this, there is a polyimide film, for example. However, the polyimide film does not have good parting property with respect to toner with the result of a slight offset of the toner. A preferable heat resistive sheet will be described. Example 1 Figure 21 shows a sectional view of a first example of the heat resistive sheet wherein the heat resistive sheet includes a plurality of layers 231 and 232. The layer 231 is a base layer which is mechanically strong and heat resistive and which is made of a polyimide film having a thickness of 9 µm (microns). The upper surface of the polyimide film is contacted to the heat generating element 21. On the bottom surface of the heat resistive base layer made of polyimide, a parting layer 232 made of PTFE (polytetrafluoroethylene) having a thickness of 3.5 µm (microns) is provided, and the parting layer 232 is contacted to the toner. The sheet is produced in the following manner. A mixture of PTFE particles having an average particle size of 0.1 µm (micron) and a surface active agent for producing coagulation of the PTFE particles is uniformly applied on the surface of the heat resistive base layer 231, and is air-dried for one hour at 60 °C, and then sintered for 20 minutes at 350 °C. During the sintering, the parting layer of PTFE is heat-shrinked to curl the sheet. To reduce the influence of the curling, the thickness of the base layer 231 is preferably larger than the thickness of the parting layer 232. Thus, by employing a multi-layer structure rather than a single layer structure, more particularly, the multi-layer structure including at least a base layer having high strength and heat resistivity and a parting layer having good parting property, the sheet acquires sufficient durability and parting property. As for the material for the parting layer small surface energy materials are usable. Among them, fluorine resin such as PTFE and PFA (perfluoroalkoxy) resin, and silicone resin are preferable. As for the other material for the base layer 231, there are highly heat resistive resins such as polyether etherketone (PEEK), polyethersulfone (FES) and polyetherimide (PEI), and metal such as nickel, stainless steel and aluminium which are strong and heat resistive enough. The parting layer may be applied by electrostatic painting or the like, or may be formed by film forming technique such as evaporation or CVD. Comparison Example 1 When a sheet made only of polyimide was used, a slight amount of toner was offset to the sheet even if the recording material is separated after the toner was cooled. This is because the surface energy of the polyimide is large. Comparison Example 2 When a sheet made only of a fluorine resin such as PFA and PTFE was used, the sheet was heat-shrinked by the heating by the heat generating element. Also, the sheet was quickly worn, and therefore, was not durable enough. This is considered to be because the sheet is slit relatively to the heat generating element under a heated condition. Example 2 Where the sheet is multi-layer construction, the layers are liable to be peeled, if the adhesion between the layers is not enough. Referring to Figure 22, the sheet of this example includes a bonding layer 233 made of a fluorine resin between the base layer 231 and the parting layer 232. By the provision of the bonding layer, the adhesion between the base layer and the parting layer is enhanced, and therefore, the durability of the sheet is further improved. Example 3 As described, the provision of the bonding layer is effective to enhance the adhesion between the layers. From the standpoint of good thermal response, however, it is not desirable that the heat capacity of the sheet is increased. This is particularly so, when the heat generating element is pulse-wisely energized. Referring to Figure 23, this example is such that the adhesion between the base layer 231 and the parting layer 232 is improved without the provision of the bonding layer. The surface of the base layer 231 is roughened, and the roughened layer is coated with the parting layer 232. Because the sheet of this example is not provided with the bonding layer, the heat capacity of the sheet is not increased. This example is particularly preferable when the heat generating element is pulsewisely energized and heated. Example 4 In this example, the base polyimide film layer is provided with a laminated fluorine resin film as the parting layer 52. Between the polyimide film and the fluorine resin film a bonding layer 233 may be provided, as shown in Figure 23. Since the fluorine resin film has a good surface smoothness, and therefore, good offset preventing effect, and also since it provides the parting layer having good mechanical strength, it is preferable in the case where the fixing speed is low and/or where the amount of heat generated by the heat generating element is large. Example 5 Referring to Figure 24, the base layer 231 in this example is provided with a sliding layer 234 at its heat generating element side, the sliding layer 234 providing good slidability. By this structure, the frictional resistance between the sheet and the heat generating element can be reduced so that the driving force for the sheet can be decreased and that the movement of the sheet is stabilized. Therefore, this example is particularly preferable when the sheet slides on the heat generating element. Example 6 Referring to Figure 25, an example is shown wherein the frictional resistance between the sheet and the heat generating element is reduced without increasing the heat capacity of the sheet. In this example, that surface of the sheet which are contacted with the heat generating element is roughened to reduce the actual area of contact between the sheet and the heat generating element. Example 7 In this example, the parting layer 232 and/or the sliding layer 234 contains a high hardness material such as titanium oxide and titanium nitride. This is preferable when the parting layer 232 and/or the sliding layer 234 requires high hardness. According to the examples described above, the mechanical strength and the thermal durability of the entire sheet are assured by the base layer 231, and simultaneously, the parting property from the toner is assured by the provision of the parting layer 232, whereby the durability and the offset preventing effect can be provided. In the case where a highly heat resistive resin material is used such as polyimide for the base layer, the sheet tends to be electrically charged with the result of disturbance to the unfixed toner image upon image fixing operation, or electrostatic attraction of the toner image to the sheet, by which the above described good offset preventing effect can be disturbed. Examples of the sheet which can prevent the electrical charging thereof will be described. In those examples, the electric resistance of the surface layer except for the base layer, particularly, at least the parting layer 232 is reduced. Example 8 In this example, the parting layer 232 is made of PTFE in which carbon black is dispersed, by which the volume resistivity of the PTFE layer is reduced down to 10⁸ ohm.cm. By this reduction of the resistivity, the electric charging of the sheet is prevented, whereby the disturbance of the unfixed image due to the electrostatic force can be prevented. The electrostatic charging can result in attraction of dust by the sheet which reads to decrease of the parting property and damage to a pressing roller 22. These problems can be solved in this example. When the sheet is slid on the heat generating element, it is possible that the surface of the sheet contacted to the heat generating element is so charged that dust is present between the stationary heat generating element 21 and the sheet, which can result in damage of the heat generating element and the sheet. This example can solve this problem. Further, in order to ensure charge prevention on both sides of the sheet, it is preferable that the resistances of both of the surface layers of the sheet are low. More particularly, an additional layer is provided on the heat generating element side of the base layer of the sheet, as shown in Figure 24, and the resistivity of this layer is decreased. It is possible that a low resistance filler material such as carbon black is mixed directly into the base layer. However, it is preferable not to do so, since then heat resistivity and the strength of the base layer are reduced. A sufficient charge preventing effect was provided by reducing the volume resistivity of the low resistivity layer down to not more than 10¹¹ ohm.cm. Further preferably, the charge preventing effect was assured by reducing it down to more than 10⁹ ohm.cm. As another example of the low resistivity filler material, there are titanium nitride, potassium titanate, red iron oxide or the like. Comparison Example 3 The parting layer 232 and the sliding layer 234 of the sheet were made of PTFE coating layers without the low resistivity material such as carbon black and having the volume resistivity of not less than 10¹⁵ ohm/cm. When the image fixing operation was repeated using this sheet, dust sometimes was attached to the sheet, and the unfixed image on the recording material was sometimes disturbed. The reasons are considered to be as follows: (1) Electric discharging by the separation of the sheet from the recording or transfer material by the separation roller 26; and (2) Triboelectric discharging by the friction between the sheet and the heat generating layer 21. Example 9 In this example, as the low resistivity filler material, titanium oxide whiskers which are monocrystal fibres having electric conductivity (volume resistivity of 10⁴ ohm.cm) are preferred. Conductive whiskers are preferable because they have a charge preventing effect and are excellent in hardness, so that wear is further reduced, and durability of the sheet is further improved. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An image fixing apparatus (20) for fixing an unfixed heat-fusible toner image (T) formed on an image carrying member (P), which apparatus (20) comprises: an endless sheet member (40) movable in a conveyance direction (C) with its outside surface in contact with the unfixed toner image (T); a heater (21), extending in a direction transverse to said conveyance direction (C) for heating said unfixed toner image (T) via said endless sheet member (40), including an electrically resistive heating element (28) which alone is to generate heat to fix the unfixed heat-fusible toner image (T), and which heater (21) is located inside said endless sheet member (40); and a rotatable pressing contact member (22) cooperative with said heater (21) to form a nip with said endless sheet member (40) therebetween; which image fixing apparatus is characterised in that: said heater (21) includes a solid low thermal conductivity material (53) provided behind said electrically resistive heating element (28). Apparatus as claimed in claim 1 including electrical supply control means (60,61), co-operative with said heating element (28), to generate heat at a temperature exceeding, in use, the fusing temperature of the heat-fusible toner (T). Apparatus as claimed in claim 2 wherein said pressing contact member (22) is maintained at a temperature lower than said fusing temperature. Apparatus as claimed in any of claims 1 to 3 including cooling means (49) for cooling said pressing contact member (22) to maintain it at a temperature lower than the fusing temperature of the heat-fusible toner (T). Apparatus as claimed in claim 2, or any preceding claim depending from claim 2, wherein said electrical supply control means (60,61) includes power supply means (61) for supplying intermittently electric power to said heating element (28). Apparatus as claimed in claim 5 wherein said heater (21) incorporates a temperature detecting means (43;55), and said electrical supply control means (60,61) is co-operable with said power supply means (61) and is responsive to said temperature detecting means (55) to control power supply to said heating element (28). Apparatus as claimed in claim 6 wherein said temperature detecting means (43) is mounted behind said heater (21). Apparatus as claimed in claim 6 wherein said temperature detecting means (55) is provided in said heater (21) and is spaced to the rear of said heating element (28) by an insulative material (52). Apparatus as claimed in claim 8 wherein in use the insulative material (52) is such that temperature measured at said temperature detecting means (55) is substantially the same as the temperature at the front surface of a protective layer (51) disposed at the front of said heater (21). Apparatus as claimed in claim 8 wherein in use the insulative material (52) is such that temperature measured at said temperature detecting means (55) is substantially the same as the surface temperature of said endless sheet member (40) where it comes into contact with said toner image (T). Apparatus as claimed in claim 6 wherein, in use, said electrical supply control means (60,61) is responsive to said temperature detecting means (55) to vary the pulse width of pulses supplied to said heating element (28) so as to effect temperature control. Apparatus as claimed in claim 6 wherein, in use, said electrical supply control means (60,61) is responsive to a peak value output of said temperature detecting means (55). Apparatus as claimed in any preceding claim wherein said heater (21) comprises a temperature fuse or thermostat member (44) operable to shut off power supply to said heating element (28) in the event of an extreme temperature rise of said heater (21). Apparatus as claimed in claim 5 wherein said electrical supply control means (60,61) is operable to supply electric power pulses, a width τ of which pulses is related to a width ℓ of said heating element (28) in the conveyance direction (C), and a conveyance speed V p of said endless sheet member (40), by the following inequality: τ < ℓ / V p . Apparatus as claimed in claim 5 wherein said electrical supply control means (60,61) is operable to supply electric power pulses at a pulse repetition frequency ν which is related to a width ℓ of said heating element (28) in the conveyance direction (C) and a conveyance speed V p of said endless sheet member (40) by the following inequality: V p /ℓ ≦ ν < 2V p /ℓ Apparatus as claimed in any preceding claim including radiating cooling means (45) which is located downstream from said heater (21) in the direction of conveyance (C) and which is arranged to be disposed in contact with said endless sheet member (40). An apparatus according to any preceding claim wherein said endless sheet member (40) includes a heat resistive base layer (231) and a parting layer (232) provided at least on the opposite side of said endless sheet member (40) to said heater (21). An apparatus according to claim 17, wherein said parting layer (232) is of fluorine resin.

Inventors: ARAI ATSUSHI, HIRABAYASHI HIROMITSU, KUSAKA KENSAKU, TAKAYANAGAI YOSHIAKI, ARAI, ATSUSHI, HIRABAYASHI, HIROMITSU, KUSAKA, KENSAKU, TAKAYANAGAI, YOSHIAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G03G15
IPC codes: G03G15