Image forming method for supplying electric power to a fixing heater

Abstract: A fixing device and an image forming apparatus having such a device is constructed by an unfixed image forming unit to form an unfixed image onto a recording material in accordance with start of an image formation, a heater in which a heat generating material which generates a heat by an energization is provided on a substrate, the unfixed image formed by the unfixed image forming unit being heated and fixed onto the recording material by a heat from the heater, and a power supplying unit for supplying an electric power to the heater in a first mode with a first maximum power by the start of the image formation and, subsequently, for supplying an electric power to the heater in a second mode with a second maximum power. The first maximum power is smaller than the second maximum power. In both of the first and second modes, the powers are controlled so that the heater is set to a predetermined temperature. When a predetermined time elapses from the start of the energization by the start of the image formation or when a temperature sensor detects a predetermined temperature of the heater, the operating mode is changed from the first mode to the second mode.

Description: BACKGROUND OF THE INVENTION Field of the Invention The invention relates to an image forming method such as is performed in a copying apparatus, printer, or the like. Related Background Art Hitherto, in an image forming apparatus such as copying apparatus, laser beam printer, or the like using an electrophotographic method or an electrostatic recording method, in case of forming an image, first, an electrostatic latent image corresponding to an original image or an image signal which was inputted is formed on the surface of a photosensitive material or a dielectric material as an image holding member. The electrostatic latent image is developed as a toner image by developing means by using a toner having predetermined charges. The toner image developed on the image holding member is electrostatically transferred onto a recording material by transfer means. The toner image on the recording material is fixed onto the recording material by fixing means. Hitherto, as a fixing method in such a kind of image forming apparatus, a heat roller method of conveying a recording material holding a toner image while sandwiching it by a heating roller and a pressurizing roller has been widely used. In such a heat roller method, since a heat capacity of the heating roller is large, there is a problem such that it takes a long time (what is called a warm-up time) which is required to heat the heating roller up to a predetermined temperature. Therefore, a film heating fixing method whereby a warm-up time is reduced by using a thermal head of a low heat capacity and a thin film which moves in slide contact with the thermal head has been proposed (refer to documents JP-A-63-313182, JP-A-2-157878, or the like). As such a thermal head of a low heat capacity, a ceramic heater constructed in a manner such that a heat generating layer of a resistor member is provided on a substrate made of ceramic or the like and a protecting layer is further formed on the heat generating layer is generally used. Since the ceramic heater has a low heat capacity, it is possible to rapidly raise a temperature in a short time. On the other hand, when the temperature is raised, if an electric power is applied to the ceramic heater, there is a case where the heater is deformed by a stress due to a temperature difference in the heater. When rapidly raising the temperature with a low heat capacity, it is desired that a thickness of the heater is reduced as thin as possible and a large electric power is applied to the ceramic heater. However, in this case, there is a problem such that a possibility that the heater is damaged by a stress due to the foregoing temperature difference in the heater rises. SUMMARY OF THE INVENTION It is an object of the invention to provide an image forming method which prevents that a heater is damaged by energization when the heater is made operative. An image forming method comprising the features summarized in the preamble of claim 1 is known from document EP-A-0 390 168. This document discloses image forming methods in which, after the start of the supply of electric power to the heater in accordance with the start of the image formation, a high first maximum power is supplied to the heater and, thereafter, a smaller second maximum power is supplied to the heater. The change from the first operating mode with the high first maximum power to the second operating mode with the smaller second maximum power is effected when the heater has reached a predetermined temperature. In these known methods, the highest maximum power is supplied to the heater right at the start of the energization in accordance with the start of the image formation, and, therefore, the temperature of the heater rises rapidly. This can result in the problem that the stresses due to temperature differences in the ceramic substrate damage the ceramic heater. Document EP-A-0 731 393 was published on September 11, 1996. This document is concerned with the supply of electric power to the heater on standby. As regards the supply of electric power during the copying process, i.e. after the start of the energization in accordance with the start of the image formation, this document teaches supplying electric power to the heater such that the heater has the fixing temperature. Document EP-A-0 762 234 was published on March 12, 1997 and teaches heating the heater to recover its operating temperature as quickly as possible after the start of the energization in accordance with the start of the image formation. Document JP-A-59 206 854 and Patent Abstracts of Japan, Vol. 009, No. 076 (P-346), 5 April 1985 disclose a control device for an electrophotographic copying machine. The heater of this known image forming apparatus is an infrared lamp and does not comprise a ceramic substrate. The power source of the image forming apparatus according to this document is an AC source. The phase of the source voltage is controlled such that the source voltage is suppressed to zero except for a conducting angle which continuously increases. This control is effected for preventing the detrimental effects of the rush of current at the time when the AC switch is turned on, i.e. for achieving a stable starting performance. Further background art is disclosed in documents JP-A-59-206 854, EP-A-0 523 638, EP-A-0 483 869, US-A-5 376 773 and US-A-5 367 369. SUMMARY OF THE INVENTION It is an object of the invention to provide an image forming method which prevents the heater having the ceramic substrate from being damaged by energization when the heater is made operative. This object is achieved by the method according to claim 1. In accordance with the invention, the power supplying means supplies the electric power with a first maximum power when the supplying of power to the heat generating material is started and switches to a second maximum power which is larger than the first maximum power after a temperature of said ceramic substrate has been raised. Therefore, when the temperature of the ceramic substrate rises, the maximum value of the electric power which is applied to the heater at the initial rising time is smaller than the maximum power applied in the subsequent rising time and during the fixing of the image. Thereby, the ceramic substrate is prevented from being damaged by stress generated by too rapid a temperature rise. The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side elevational view of a fixing device; Figs. 2A to 2D are side elevational views of a heater; Fig. 3 is a constructional diagram of an image forming apparatus in which the image forming method according to the invention can be performed; Fig. 4 is a plan view of a heater; Fig. 5 is an explanatory diagram of an electric power control according to a first embodiment; Fig. 6 is an explanatory diagram of an electric power control according to a second embodiment; Fig. 7 is a flowchart showing an electric power control according to a third embodiment; Fig. 8 is an explanatory diagram of an electric power control according to a fourth embodiment; Fig. 9 is an explanatory diagram of an electric power control according to a fifth embodiment; and Fig. 10 is an explanatory diagram of an electric power control according to a sixth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the invention will now be described on the basis of the drawings. [First embodiment] The first embodiment will be first explained with reference to Figs. 1 to 5. Fig. 3 is a diagram showing a schematic construction of an example of an image forming apparatus using a fixing device 60. As shown in Fig. 3, the image forming apparatus is an electrophotographic copying apparatus of a type of a fixed original supporting plate, an optical system moving type, a rotary drum type, and a transfer type. In the apparatus, as shown in Fig. 3, an original 19 is put on a fixed original supporting glass plate 20 as necessary and necessary copying conditions are set. After that, by pushing a copy start key 40 as image formation start signal input means, a signal based on the start of the image formation is generated and a photosensitive drum 39 is rotated clockwise shown by an arrow at a predetermined peripheral velocity. A light source 21 (reference numeral 22 denotes a reflecting cap) and a first mirror 23 are moved along the lower surface of the original supporting glass plate 20 from a home position on the left side of the glass to the right side of the glass at a predetermined speed V. A second mirror 24 and a third mirror 25 are moved in the same direction as the above direction at a speed of V/2. Thus, the downward image surface of the original 19 put on the original supporting glass plate 20 is illuminated and scanned from the left side to the right side. A reflected light of the illuminating and scanning light from the original surface passes through an image forming lens 29, is reflected by fixed fourth to sixth mirrors 26 to 28, and is exposed (slit exposure) so as to form an image onto the surface of the rotary photosensitive drum 39. The surface of the rotary photosensitive drum 39 has uniformly been charged to a positive or negative predetermined electric potential by a primary charging unit 30 before the exposure. By performing the above exposure to the charged surface, electrostatic latent images of a pattern corresponding to the original image are sequentially formed on the surface of the drum 39. The electrostatic latent image formed on the surface of the photosensitive drum 39 is visualized as a toner image by a developing roller 32 of a developing device 31. On the other hand, a recording material P is fed by a paper feed roller 51 and is introduced to a transfer portion between the drum 39 and a transfer charging unit 34 through a guide 33 at a predetermined timing and is come into contact with the drum 39 by receiving a transfer corona. Thus, the visual toner images on the surface of the drum 39 are sequentially transferred onto the surface of the recording material. The recording material P which passed through the image transfer portion is gradually separated from the surface of the drum 39 while being subjected to a discharging process to remove the charges on the rear surface by a charge removal needle 35. The recording material P is conveyed to the fixing device 60 by a conveying unit 38 and an inlet guide 8. The toner image is fixed as will be described hereinafter. The recording material is ejected to the outside of the apparatus as an image formed object. A fouling such as a remaining toner or the like on the surface of the drum 39 after the transfer is cleaned and eliminated by a cleaning blade 37 of a cleaning unit 36 and the clean surface is repetitively used for the subsequent image formation. The moving optical members 21 to 25 moved on the forward path as mentioned above are set so that when they reach a predetermined final point of the forward path, they are moved along the return path. Therefore, they are returned to the original home position and wait until the start of the next copying cycle (hereinbelow, this step is called a back step of the optical system). When a plurality of copies (for example, 100 sheets) is designated before the copy start key is pushed, after completion of the back step of the optical system, the above processing steps are repeated at a predetermined interval by a microcomputer (hereinbelow, referred to an MPU). The fixing device 60 which is installed to the apparatus will now be described in detail with reference to Fig. 1. Fig. 1 is a constructional diagram of the fixing device of the image forming apparatus according to the invention. In Fig. 1, reference numeral 3 denotes an endless belt-shaped fixing film. The fixing film 3 is stretched among a driving roller 1 on the left side, a driven roller 4 on the right side, and a linear heater 7 of a low heat capacity fixedly arranged under a portion between the rollers 1 and 4. The driven roller 4 is also used as a tension roller for applying a tension in the direction of stretching the fixing film 3 outward. In association with a clockwise rotation of the driving roller 1 in which a coefficient of friction is raised by coating a silicone rubber or the like onto the surface, the fixing film is rotated clockwise at a predetermined peripheral velocity without a wrinkle, a zigzag movement, and a velocity delay. Reference numeral 9 denotes a pressurizing roller serving as pressurizing means and having a rubber elastic layer such as a silicone rubber or the like of a good mold releasing performance. The roller 9 allows a film portion on the lower side of the endless belt-shaped fixing film 3 to be sandwiched between the roller 9 and heater 7, thereby allowing the film portion to face and to come into pressure contact with the lower surface of the heater 7 by urging means such as a spring or the like with a contacting force of, for example, 5 to 10 kg/cm. The pressurizing roller 9 rotates counterclockwise in the forward direction in the conveying direction of the recording material P. Reference numeral 10 denotes a cleaning roller which is arranged so as to be driven and rotated in association of rotation of the pressurizing roller 9. The cleaning roller 10 is made of metal. Since the endless fixing film 3 to be rotated is repetitively used to heat and fix the toner image, a thin film which is excellent in heat resistance, mold releasing performance, and durability and in which a thickness is generally equal to or less than 100 µm, preferably, 40 µm or less is used as a film 3. As an example, there is used an endless belt whose total thickness is equal to 30 µm obtained by a method whereby a resin of a low surface energy such as PTFE (tetrafluoroethylene resin), PFA (tetrafluoroethylene perfluoro alkylvinylether copolymer resin), or the like or a separating coating layer obtained by adding a conductive material such as carbon black or the like into any one of the above resins is coated by a thickness of 10 µm onto the outer circumferential surface of a thin endless belt having a thickness of 20 µm made of a high heat resisting resin such as polyimide, polyetherimide, polyethersulfone, polyether, etherketone, or the like or made of metal such as nickel, SUS, or the like. As shown in Fig. 4, the heater 7 of a low heat capacity is constructed as follows. A heat generating layer 13 is formed by coating a resistive material such as silver palladium, ruthenium oxide, or the like having a thickness of 10 µm and a width of 1.0 mm onto an alumina substrate 14 having a thickness of 1.0 mm, a width of 10 mm, and a length in the longitudinal direction of 340 mm and, further, a protecting layer 15 having a thickness of 10 µm made of glass or the like is formed onto the layer 13 in consideration of the sliding with the film 3. The heater 7 is attached to and is held by a heater supporting member 6, thereby fixedly supporting. As shown in Fig. 2A, in the heater 7, the substrate 14 on the paper ejecting side is chamfered and the heat generating layer 13 is deviated to the paper ejecting side with respect to the center in the width direction of the substrate 14. The heater supporting member 6 has a heat insulating performance, a high heat resistance, and a rigidity in order to insulate the heat and support the heater 7 for the fixing device and the image forming apparatus and is constructed by a high heat resisting resin such as PPS (polyphenylene sulfite), PEEK (polyether etherketon), liquid crystal polymer, or the like, a composite material of the above resin and ceramics, metal, or the like. The heat generating layer 13 of the heater is energized from both edges in the longitudinal direction. As shown in Fig. 4, electrodes 41 to energize are provided at both edges of the heat generating layer 13. The energization is performed by 100 ACV and is controlled in accordance with a detection temperature of a thermistor 5 as temperature detecting means such as an NTC (negative temperature-resistance coefficient) thermistor or the like which is adhered by a heat conductive silicone rubber adhesive agent or the like onto the back surface of the substrate 14, or is come into pressure contact therewith, or is formed integratedly therewith. The energizing control method is based on a phase control and an electric power supply to the heater can be controlled step by step. The operation of the fixing device of the apparatus according to the present embodiment will now be explained hereinbelow. When a power source of the main apparatus is turned on by a main switch 42, the driving roller 1 begins to rotate. In association with the rotation, the fixing film 3, tension roller 4, pressurizing roller 9, and cleaning roller 10 start rotating. The energization from a heater driving circuit 16 as control means and electric power supplying means to the heater 7 is started with a slight delay time after the start of the rotation. In the embodiment, an electric power of 500W is applied for five seconds while adjusting a temperature to 200°C. The temperature adjustment to 200°C is performed in a manner such that when the detection temperature of the thermistor 5 is lower than 195°C, a predetermined maximum electric power (500W during the preliminary energizing operation) is applied, when the detection temperature is equal to or higher than 195°C and is lower than 200°C, 500W is applied, when the detection temperature is equal to or higher than 200°C and is lower than 205°C, 200W is applied, and when the detection temperature is equal to or higher than 205°C, the energization is stopped. After the energizing time elapsed and the energization was finished, the rotation of the driving roller 1 is stopped with a slight delay time. The operations so far indicate the preliminary energizing operation. At the time of the image formation, when the image formation is started by depressing the copy start key 40 provided for the main body by the operator, the driving roller 1 starts rotating and the fixing film 3, tension roller 4, pressurizing roller 9, and cleaning roller 10 also begin to rotate in association with the rotation of the roller 1. The energization from the heater driving circuit 16 to the heater 7 is started by a slight delay time after the start of the rotation. An image formation start signal which is generated by the copy start key is a signal serving as a reference to start the energization. In this instance, a first mode to apply a first maximum applied electric power W1 is executed for a predetermined time after the start of the energization and a second mode to apply a second maximum applied electric power W2 which is larger than W1 is executed after the elapse of the predetermined time. In the embodiment, while adjusting the temperature to 200°C, 600W in maximum is applied for three seconds and, subsequently, 800W in maximum is applied. That is, in the first and second modes, the temperature adjustment is performed by selecting the electric powers of various magnitudes so that the temperature of the heater is set to 200°C as a predetermined fixing temperature. In this instance, the electric power is controlled in a manner such that the electric power of 600W in maximum is supplied in the first mode and 800W in maximum is supplied in the second mode. As mentioned above, the maximum electric power in the first mode is smaller than that in the second mode. Fig. 5 shows such a situation. When the recording material P enters a nip portion N, the thermistor detection temperature has reached a predetermined fixing temperature (200°C in the embodiment). The recording material P is overlapped with the fixing film 3 to enter the nip portion N while bearing an unfixed toner image T on the upper surface thereof. When passing through the nip portion N, the recording material P and unfixed toner image T receive the heat generated by the heat generating layer 13 via the fixing film 3 while receiving a pressuring force by the heater 7 and pressurizing roller 9. The toner image is melted at a high temperature and becomes an image which was softened and adhered to the recording material P. After a rear edge of the recording material P passed through the nip portion N, the energization to the heater 7 is stopped and, after that, the driving roller 1 is stopped after a slight delay time. The fixing device has a function (automatic shutoff function) whereby the power source of the main body is automatically turned off when five minutes elapses after the end of the energization. By controlling the fixing device as mentioned above, since a large electric power is not supplied to the heater at the initial stage of the activation of the heater, even if the device is used in an environment of a low temperature, no damage occurs in the fixing heater, particularly, in the substrate and a good fixing performance is obtained. As for the heater 7, as shown in Fig. 2B or 2C, the heat generating layer can be arranged symmetrically or asymmetrically with respect to the center line in the width direction of the substrate like a rectangle having a sectional surface which is symmetric with respect to the center line in the width direction of the substrate. Or, as shown in Fig. 2D, the heat generating layer can be also arranged symmetrically with respect to the center line in the width direction of the substrate having a sectional surface which is asymmetric with respect to the center line in the width direction of the substrate in a manner similar to the heater substrate 14. [Comparison example 1] In the embodiment 1, when the maximum applied electric power at the time of the preliminary energization is set to 800W in a manner similar to the case of the image formation or 800W is applied from the beginning without applying 600W for three seconds at the time of the image formation, there is an example in which the heater 7 is damaged during the energization of 800W in the environment of a low temperature. This is because by applying a large electric power to the heater 7 in a state of a low temperature, a temperature difference occurring between a portion near the heat generating layer 13 and the back surface of the heater 7 increases and the heater 7 cannot endure a stress due to the temperature difference. [Second embodiment] The second embodiment is fundamentally the same as the first embodiment and different points will be now explained hereinbelow. In the first embodiment, at the time of the image formation, the maximum applied electric power has been set to a low value for a predetermined time after the start of the energization. However, according to the second embodiment, the maximum applied electric power is set to a low value until the thermistor 5 detects a predetermined temperature after the start of the energization. In the embodiment, an electric power of 600W is applied until the detection temperature of the thermistor 5 reaches 70°C (first mode) and, after that, 800W is applied (second mode). Fig. 6 shows the above situation. By controlling the fixing device as mentioned above, it is possible to detect that the temperature of the heater has certainly risen by a low electric power, no damage occurs in the fixing heater even in the use under an environment of a low temperature, and a good fixing performance is obtained. [Third embodiment] According to the third embodiment, in addition to the first embodiment, when the copy start key is depressed during the preliminary energization, if the detection temperature of the thermistor 5 at that time is lower than a predetermined temperature, a predetermined preliminary energization is executed for a time which is shorter than the preliminary energizing time in the case where the copy start key is not depressed during the preliminary energization. After that, the operating mode advances to the image forming operation. That is, according to the third embodiment, when the copy start key is depressed before the preliminary energization has been performed for two seconds by applying 500W, so long as the detection temperature of the thermistor 5 is equal to or lower than 100°C, the preliminary energization is performed for two seconds and, after that, the operating mode advances to the image forming operation. When the detection temperature of the thermistor 5 is equal to or higher than 100°C or the copy start key is depressed after the elapse of two seconds or more after the start of the preliminary energization, even during the preliminary energization (five seconds in the embodiment), the operating mode advances to the image forming operation at that time. Fig. 7 is a flowchart showing such a situation. By controlling the fixing device as mentioned above, in the use under an environment of a low temperature, no damage occurs in the fixing heater and a good fixing performance is obtained. [Fourth embodiment] The fourth embodiment is fundamentally similar to the first embodiment and different points will now be described. Although the maximum applied power has been set to a small value for a predetermined time from the start of the energization at the time of the image formation in the first embodiment, the maximum applied power is changed in accordance with a detection temperature (T1) of the thermistor 5 when the copy start key 40 is depressed. In the fourth embodiment, when the detection temperature of the thermistor 5 is equal to or lower than 50°C, 600W is applied (first mode) for two seconds and, after that, 800W is applied (second mode). When the detection temperature of the thermistor 5 is higher than 50°C, the first mode is not performed but 800W is applied (second mode). Fig. 8 shows such a situation. By controlling the fixing device as mentioned above, even if the temperature of the heater decreases in the low temperature environment or the like, no damage occurs in the fixing heater and a good fixing performance is obtained. When the temperature of the heater is high, since the low power control is not performed, a rising timing of the heater can be made fast. In place of the detection temperature of the thermistor 5 when the copy start key is depressed, the maximum applied power can be also changed in accordance with the time from the end of the previous image formation (end of energization) to the start of the next energization. [Fifth embodiment] In addition to the first embodiment, in the fifth embodiment an environment temperature sensor 43 to measure a temperature near the fixing device is provided and, when a detection temperature (T2) of the environment temperature sensor 43 at a time point when the copy start key 40 is depressed is lower than a predetermined temperature, the predetermined time during which the maximum applied power was reduced from the start of the energization at the time of the image formation is extended. In the fifth embodiment, when the detection temperature of the environment temperature sensor is equal to or lower than 20°C, the time for applying 600W (first mode) is extended by two seconds and 600W is applied for total five seconds and, after that, 800W is applied (second mode). In this instance, the image forming operation other than the operation of the fixing device is started after the elapse of two seconds from the depression of the copy start key. When the detection temperature (T2) is higher than 20°C, the first mode is performed for ordinary three seconds. Fig. 9 shows such a situation By controlling the fixing device as mentioned above, even in the use under the low temperature environment, no damage occurs in the fixing heater and a good fixing performance is obtained. When the detection temperature of the environment temperature sensor is low, not only the applying time of the low electric power is extended but also the electric power can be further reduced. [Sixth embodiment] In addition to the first embodiment, in the sixth embodiment a time duration from the time point of the end of the preliminarily energizing operation or the copying operation is measured and the predetermined time during which the maximum applied power was reduced from the start of the energization at the time of the image formation is extended in accordance with a time (t1) when the copy start key is depressed. In the sixth embodiment, when ten minutes or more elapse after the end of the preliminary energizing operation or the copying operation, the time for applying 600W (first mode) is extended by two seconds and 600W is applied for total five seconds and, after that, 800W is applied (second mode). In this instance, the image forming operation other than the operation of the fixing device is started after the elapse of two seconds from the depression of the copy start key. When the measured time (t1) is shorter than 10 minutes, the first mode is performed for ordinary three seconds. By controlling the fixing device as mentioned above, even in the use under a low temperature environment, no damage occurs in the fixing heater and a good fixing performance is obtained. According to the invention as described above, in the fixing device, when the temperature of the heater in which the heat generating layer is provided on the substrate rises, the maximum value of the electric power which is applied to the heater at the initial rising time is set to be smaller than the maximum applied power in the subsequent rising time and during the image formation. Therefore, the heater is not damaged and the fixing device having a good fixing performance can be provided.

Claims: An image forming method comprising the steps of: forming a toner image (T) onto a recording material (P) in accordance with a start of an image formation by means of an image forming means (32, 34, 39); and heating the toner image (T) formed by said image forming means (32, 34, 39) and fixing it onto the recording material (P) by heat from a heater (7), wherein the heater (7) has a ceramic substrate (14) and a heat generating material (13) which is provided on said substrate, heats said ceramic substrate and generates the heat by energization, and wherein an electric power is supplied to the heat generating material (13) by means of a power supplying means (16), characterized in that said power supplying means (16) supplies the electric power with a first maximum power when the supplying of power to the heat generating material (13) is started and switches to a second maximum power which is larger than the first maximum power after a temperature of said ceramic substrate (14) has been raised. The method according to claim 1, wherein said power supplying means (16) switches to the second maximum power after the electric power has been supplied with the first maximum power for a predetermined time. The method according to claim 1, wherein a temperature of said ceramic substrate (14) is detected by a temperature detecting means (5), wherein the temperature of said ceramic substrate (14) is controlled by a temperature control means (16) so that the temperature detected by said temperature detecting means (5) is maintained at a fixing temperature during fixing, and wherein said power supplying means (16) switches from the first maximum power to the second maximum power when the temperature detected by said temperature detecting means (5) reaches a predetermined temperature lower than the fixing temperature. The method according to claim 1, wherein temperature at the time point of the start of the image formation is detected by a temperature detecting means (43), and wherein, when the temperature detected by said temperature detecting means (43) is lower than or equal to a predetermined temperature, an operating time during which said first maximum power is supplied is longer than when said detected temperature is higher than said predetermined temperature. The method according to claim 4, wherein said temperature detecting means (43) is a means for detection an environment temperature. The method according to claim 1, wherein, when an interval from end of a previous image formation to the start of a following image formation is longer than a predetermined time, an operating time during which said first maximum power is supplied is longer than when said interval is shorter than said predetermined time. The method according to claim 1, wherein a preliminary energization is performed to said heater (17) in association with the turn-on of a main switch (42) for turning an a off a power sauce of an image forming apparatus. The method according to claim 7, wherein a maximum power which is supplied to said heater (17) at the time of said preliminary energization is smaller than said first maximum power. The method according to claim 7, wherein, when a signal based on the start of the image formation is inputted during said preliminary energization, said preliminary energization is executed for a predetermined operating time and, after that, the image formation is started. The method according to claim 7, wherein a temperature of said heater (17) is detected by means of a temperature detecting means (5), wherein, when a signal based on the start of the image formation is inputted during said preliminary energization, if the temperature detected by said temperature detecting means (5) at the time of the input of said signal is lower than a predetermined temperature, the preliminary energization is performed for a predetermined operating time and, after that, the image formation is started, and, if said detected temperature is higher than said predetermined temperature, the image formation is started without further executing the preliminary energization. The method according to claim 9 or 10, wherein said predetermined operating time is shorter than the time required for the preliminary energization when the signal based on the start of the image formation is not inputted. The method according to claim 1, wherein said substrate (14) is made of alumina. The method according to claim 1, wherein said heat generating material (13) is made of a resistive material coated on said substrate (14). The method according to claim 1, wherein a film (3) is moved in slide contact with said heater (7) and a nip (N) is formed by a pressurizing roller (19) together with said heater (7) through said film (3) and wherein said recording material (P) bearing the toner image (T) is sandwiched and conveyed by said nip (N), and the toner image (T) is heated and fixed on said recording material (P) through said film (3).

Inventors: KAJI KEIGO, KUSAKA KENSAKU, KAJI, KEIGO, KUSAKA, KENSAKU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1996
Main classification: G03G15
IPC codes: G03G21, H05B3, G03G15