Image heating apparatus with improved start of film driving

The present invention relates to an image heating apparatus in which a driving member drives a film at a first speed when driving is started and then drives film at a second speed, and the first speed is lower than the second speed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an image forming apparatus such as a
 copying machine or a printer, and particularly to an image heating
 apparatus applied to the apparatus.
 2. Related Background Art
 In an image forming apparatus, conventionally a thermal-roller type
 apparatus has been widely used as a fixing apparatus for heating and
 fixing an unfixed toner image indirectly or directly formed and borne on a
 recording material in appropriate image forming process means such as an
 electrophotography process on a surface of the recording material as a
 permanently fixed image.
 In recent years from a viewpoint of quick starting or energy saving, there
 are suggested a film-heating type apparatus and an electromagnetic
 induction heating type apparatus in which a metal film generates heat by
 itself.
 In Japanese Utility Model Application Laid-Open No. 51-109739, there is
 disclosed an induction heating fixing apparatus for inducing eddy-current
 on a metal layer (a heat generating layer) of a fixing film by using
 magnetic flux so as to make the film emit heat with the Joule heat. This
 apparatus allows the fixing film to emit heat directly by utilizing a
 generation of induced current, thereby achieving a fixing process
 efficiency higher than that of a thermal-roller type fixing apparatus
 having a heat source of a halogen lamp.
 In addition, to obtain energy which affects a fixing process very
 efficiently, an excitation coil is brought close to a fixing film which is
 a heat generating member or an alternating magnetic flux distribution of
 an excitation coil is focused in a vicinity of a fixing nip, by which a
 fixing apparatus in which the energy can be obtained very efficiently has
 been devised.
 In FIG. 19, there is provided an example of an outline configuration of an
 electromagnetic induction heating type fixing apparatus in which an
 alternating magnetic flux distribution of an excitation coil is focused on
 a fixing nip so as to improve an efficiency.
 A cylindrical fixing film 10 is a rotator having an electromagnetic
 induction heat generating layer (a conductive layer, a magnetic layer, or
 a resistive layer).
 The cylindrical fixing film 10 is loosely and externally coupled to a film
 guide member 16 having a cross-section almost semi-arc guttering shape.
 Magnetic field generating means 15 arranged inside the film guide member 16
 comprise an excitation coil 18 and E-shaped magnetic core (core material)
 17.
 An elastic pressurizing roller 30 is contacted with a predetermined contact
 pressure to a lower surface of the film guide member 16 with the fixing
 film 10 put therebetween so as to form a fixing nip portion N having a
 predetermined width.
 The magnetic core 17 of the magnetic field generating means 15 is arranged
 so as to be associated with the fixing nip portion N.
 The pressurizing roller 30 is driven by driving means M to rotate
 counterclockwise indicated by an arrow. The rotative driving of the
 pressurizing roller 30 affects the fixing film 10 with a rotary force due
 to a frictional force between the pressurizing roller 30 and an external
 surface of the fixing film 10, by which the fixing film 10, while sliding
 with its inner surface in closely contact with a lower surface of the film
 guide member 16 in the fixing nip portion N, rotates in a clockwise
 direction indicated by an arrow on an outer periphery of the film guide
 member 16 at a circumferential speed almost corresponding to a
 circumferential speed of the pressurizing roller 30 (in a pressurizing
 roller driving method).
 The film guide member 16 applies a pressure to the fixing nip portion N.
 supports the excitation coil 18 and the magnetic core 17 as the magnetic
 field generating means 15, supports the fixing film 10, and keeps a
 conveyance stability at a rotation of the film 10. This film guide member
 16 is made of insulating material which does not prevent a passage of
 magnetic flux and can sustain higher levels of load.
 The excitation coil 18 generates alternating magnetic flux by an
 alternating current supplied from an excitation circuit which is not
 shown. The alternate magnetic flux is distributed intensively in the
 fixing nip portion N by means of the E-shaped magnetic core 17
 corresponding to a position of the fixing nip portion N and the alternate
 magnetic flux generates an eddy-current on the electromagnetic induction
 heat generating layer of the fixing film 10. This eddy-current generates a
 Joule heat by means of a specific resistance of the electromagnetic
 induction heat generating layer. The electromagnetic induction heat
 generation of the fixing film 10 is intensively generated in the fixing
 nip portion N where the alternate magnetic flux is intensively
 distributed, by which the fixing nip portion N is heated very efficiently.
 A temperature of the fixing nip portion N is kept at a predetermined
 temperature by a control of a power supply to the excitation coil 17 with
 a temperature control system including temperature detecting means which
 is not shown.
 The pressurizing roller 30 is driven to rotate in this manner, with which
 the cylindrical fixing film 10 rotates on the outer periphery of the film
 guide member 16, and the electromagnetic induction heat generation occurs
 in the fixing film 10 by the power supply to the excitation coil 17 from
 the excitation circuit, by which a temperature in the fixing nip portion N
 rises to a predetermined level. In a temperature-controlled state, a
 recording material P bearing an unfixed toner image t conveyed from image
 forming means (not shown) is introduced with its image surface facing
 upward between the fixing film 10 of the fixing nip portion N and the
 pressurizing roller 30, in other words, so as to be opposite to the fixing
 film surface. In the fixing nip portion N, the recording material with the
 image surface put in closely contact with the outer surface of the fixing
 film 10 is pinched and conveyed therewith in the fixing nip portion N. In
 this process in which the recording material P is pinched and conveyed
 together with the fixing film 10 in the fixing nip portion N, the unfixed
 toner image t on the recording material P is heated and fixed by the
 electromagnetic induction heat generation of the fixing film 10. The
 recording material P is separated from the outer surface of the rotary
 fixing film 10 after passing the fixing nip portion N and then discharged.
 In the fixing apparatus having the above configuration in which a film is
 used as a rotary member, however, there are problems described below.
 Namely, a high driving load is applied since the inner surface of the film
 rubs against the supporting member during the rotation of the film. To
 reduce the driving load, it is very important to reduce a dynamic
 frictional resistance between the inner surface of the film and its
 supporting member. Therefore, as suggested in the Japanese Patent
 application Laid-Open No. 5-27619, for example, lubricant such as heat
 resistant grease is put between the inner surface of the film and its
 supporting member, by which slidability is secured. In addition, a rib is
 arranged on the film supporting member to reduce a contact area between
 the film and its supporting member, by which slidability is secured.
 When the film is driven to rotate from a resting condition, however, a
 static frictional force greater than a dynamic frictional force occurs,
 which causes a frictional resistance greater than that during the driving
 operation. At the first rising after the fixing apparatus is mounted on
 the image forming apparatus body, a very great torque may easily occur
 immediately after starting the rotative driving due to a backlash of a
 driving gear, in other words, a play between tooth faces of the gear. In
 addition, at rising in a condition in which the fixing apparatus is cooled
 down to a room temperature, a temperature of heat-resistant grease is low
 and its viscosity is high, thus causing a viscosity resistance greater
 than that under a temperature control with the film generating heat.
 Furthermore, at the rising, torque is caused by a necessity of
 accelerating a circumferential speed from the resting condition to a
 predetermined process speed.
 As described above, at the rising of the fixing apparatus, in other words,
 at starting a rotation drive, a very large driving torque is required in
 comparison with that at constant speed driving after the rising.
 Accordingly, there have been problems that the fixing film slips against a
 rotation of the pressurizing roller and that a driving motor for the
 fixing apparatus steps out. The latter problem can be solved by adopting a
 motor having a greater driving torque, but there is a problem that a
 product cost increases.
 Particularly in a fixing apparatus of a color image forming apparatus for
 fixing a full-color image having a large amount of mounted toner, however,
 a nip width need be elongated to improve a fixing performance. In
 addition, to improve a transmission of an OHT image, preferably a surface
 pressure of the nip portion is also increased. To satisfy these
 conditions, it is preferable to apply a pressure larger than that of a
 conventional fixing apparatus for mono-color images to the nip portion, by
 which a surface pressure between a rotator in the nip portion and its
 supporting member is further increased and a frictional resistance is
 particularly large. These problems described above are very serious in a
 color image forming apparatus.
 SUMMARY OF THE INVENTION
 Therefore it is an object of the present invention to provide an image
 heating apparatus in which a film is driven without a step-out of a
 driving member.
 It is another object of the present invention to provide an image heating
 apparatus in which a film is not slipped.
 It is still another object of the present invention to provide an image
 heating apparatus comprising a movable film, magnetic flux generating
 means for generating magnetic flux, a statically fixed supporting member
 for supporting said film, and a driving member for driving said film,
 wherein eddy-current occurs on said film due to magnetic flux generated by
 said magnetic flux generating means, said film generates heat due to the
 eddy-current, an image on a recording material is heated by heat on the
 film, said film slides over said supporting member, said driving member
 drives said film at a first speed at a start of the driving and then
 drives said film at a second speed, and said first speed is lower than the
 second speed.
 It is further object of the present invention to provide an image heating
 apparatus comprising a movable film, magnetic flux generating means for
 generating magnetic flux, a statically fixed supporting member for
 supporting said film, and a driving member for driving said film, wherein
 eddy-current occurs on said film due to magnetic flux generated by said
 magnetic flux generating means, said film generates heat due to the
 eddy-current, an image on a recording material is heated by heat on the
 film, said film slides over said supporting member via lubricant, said
 driving member drives said film after the film generates the heat.
 Other objects besides those discussed above shall be apparent from the
 description below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will described below by reference to
 accompanying drawings.
 Referring to FIG. 1, there is shown a schematic configuration diagram
 showing an example of an image forming apparatus. The image forming
 apparatus of this example is a color laser printer.
 A photosensitive drum (an image bearing member) 101, which is made of an
 organic photosensitive member or an amorphous silicon photosensitive
 member, is driven to rotate counterclockwise as indicated by an arrow at a
 predetermined process speed (circumferential speed).
 The photosensitive drum 101 is uniformly charged at predetermined polarity
 and potential by a charging device 102 such as a charging roller in its
 rotation process.
 Next, the charged surface is subjected to a scanning and exposure
 processing of target image information with a laser beam 103 output from a
 laser optical box (laser scanner) 110. The laser optical box 110 outputs a
 laser beam 103 which has been modulated (on/off) so as to be associated
 with a time-series electric digital pixel signal of the target image
 information from an image signal generating device such as an image reader
 which is not shown, by which a latent image is formed correspondingly to
 the target image information for which the scanning and exposure is
 performed on the surface of the photosensitive drum 101. A mirror 109 is
 used to deflect an output laser beam from the laser optical box 110 to an
 exposure position of the photosensitive drum 101.
 In forming a full color image, scanning and exposure and a latent image
 formation are performed for a first color separation component image of a
 target full color image such as, for example, an yellow component image
 and then its latent image is developed as an yellow toner image with an
 operation of an yellow developing device 104Y among four color developing
 devices 104. The yellow toner image is transferred to a surface of an
 intermediate transfer drum 105 in a primary transfer portion Tl which is a
 contact portion (or a vicinity portion) between the photosensitive drum
 101 and the intermediate transfer drum 105. The surface of the
 photosensitive drum 101 immediately after the toner image transfer opposed
 to the surface of the intermediate transfer drum 105 is cleaned by a
 cleaner 107 with removing adhering residue such as transfer residual toner
 (residual toner after transfer).
 The above process cycle of charging, scanning and exposure, developing,
 primary transfer, and cleaning is sequentially executed for respective
 color separation component images such as a second color separation
 component image (for example, a magenta component image, with an operation
 of a magenta developing unit 104M), a third color separation component
 image (for example, a cyan component image, with an operation of a cyan
 developing unit 104C), and a fourth color separation component image (for
 example, a black component image, with an operation of a black developing
 unit 104BK), and four color toner images of an yellow toner image, a
 magenta toner image, a cyan toner image, and a black toner image are
 sequentially superposed and transferred on the surface of the intermediate
 transfer drum 105, so that a color toner image is formed correspondingly
 to the target full color image.
 The intermediate transfer drum 105, which is made of an elastic layer
 having a middle resistance and a surface layer having a high resistance on
 a metal drum, is driven to rotate in a clockwise direction as indicated by
 an arrow at almost the same circumferential speed as that of the
 photosensitive drum 101 in contact with the photosensitive drum 101 or in
 the vicinity thereof so as to transfer a toner image on the photosensitive
 drum 101 to the surface of the intermediate transfer drum 105 with a
 potential difference from the photosensitive drum 101 by giving a bias
 potential to the metal drum of the intermediate transfer drum 105.
 The color toner image formed on the surface of the intermediate transfer
 drum 105 is transferred to a surface of a recording material P fed from a
 feeding portion which is not shown to a secondary transfer portion T2 at a
 predetermined timing in the secondary transfer portion T2 which is a
 contact nip portion between the intermediate transfer drum 105 and the
 transfer roller 106. The transfer roller 106 supplies electric charges
 having a polarity opposite to a polarity of toner from a back of the
 recording material P, by which the synthetic color toner image is
 gradually batch-transferred from the intermediate transfer drum 105 to the
 recording material P.
 The recording material P which has passed the secondary transfer portion T2
 is separated from the surface of the intermediate transfer drum 105 and
 introduced to a fixing apparatus (image heating apparatus) 100, where an
 unfixed toner image is subjected to heating and fixing processing and then
 the recording material P is discharged to a discharge (delivery) tray
 which is not shown outside the apparatus.
 The intermediate transfer drum 105 after transferring the color toner image
 to the recording material P is cleaned by the cleaner 108 with removing
 adhering residue such as transfer residual toner or paper lint. This
 cleaner 108 is ordinarily held so as not to be in contact with the
 intermediate transfer drum 105 and to be in contact with the intermediate
 transfer drum 105 in the secondary transfer execution process of the color
 toner image from the intermediate transfer drum 105 to the recording
 material P.
 The transfer roller 106 is also ordinarily held not to be in contact with
 the intermediate transfer drum 105 and to be in contact with the
 intermediate transfer drum 105 in the secondary transfer execution process
 of the color toner image from the intermediate transfer drum 105 to the
 recording material P via the recording material P.
 This apparatus is capable of executing printing in a white and black image
 or other mono-color image print mode. In addition, printing in a
 double-sided print mode can be also executed.
 In the double-sided print mode, the recording material P after first-side
 image printing output from the fixing apparatus 100 is reversed with
 upside down (a front surface and a rear surface) via a recycling conveying
 mechanism which is not shown, fed again to the secondary transfer portion
 T2 to be subjected to toner image transfer processing for the other
 surface, introduced to the fixing apparatus 100 again to be subjected to
 the toner image fixing processing for the other surface, and then a
 double-sided print is output.
 In this embodiment, the fixing apparatus 100 is of an electromagnetic
 induction heating type. Referring to FIGS. 2, 3, and 4, there are shown a
 sectional side elevation pattern view of a main portion of the fixing
 apparatus 100 according to this embodiment, a front pattern view of the
 main portion, and a front sectional pattern view of the main portion,
 respectively.
 The apparatus 100 in this embodiment is of a pressurizing roller driving
 type and of an electromagnetic induction heating type with a cylindrical
 electromagnetic induction heating film in the same manner as for the
 fixing apparatus shown in FIG. 19. The same reference characters designate
 constituent members or parts common to the apparatus in FIG. 19 to omit
 their description.
 Magnetic field generating means 15 comprises magnetic cores 17a, 17b, and
 17c and an excitation coil 18.
 The magnetic cores 17a, 17b, and 17c, which are members each having a high
 permeability, are preferably made of materials used for a core of a
 transformer such as ferrite or Permalloy, and further preferably ferrite
 whose loss is small even at 100 kHz or higher.
 An excitation coil 18 is connected to an excitation circuit 27 at power
 supply portions 18a and 18b (FIG. 5). This excitation circuit 27 is
 configured to generate high frequencies of 20 kHz to 500 kHz by means of a
 switching power supply.
 The excitation coil 18 generates alternating magnetic flux by means of
 alternating current (high-frequency current) supplied from the excitation
 circuit 27.
 Film guide members 16a and 16b each having a cross-section almost semi-arc
 guttering shape form an almost cylindrical body with their opening
 portions opposed to each other and with a fixing film 10 which is a
 cylindrical electromagnetic induction heat generating film loosely fitted
 (coupled) to the body externally.
 The film guide member 16a holds the magnetic cores 17a, 17b, and 17c as
 magnetic filed generating means 15 and the excitation coil 18 inside.
 The film guide member 16a also contains a good heat conducting member 40 on
 an opposite side to a press roller 30 in the nip portion N in the inside
 of the fixing film 10.
 In this example, aluminum is used for the good heat conducting member 40.
 The good heat conducting member 40 has a heat conductivity k of 240
 [w.multidot.m.sup.-1.multidot.K.sup.-1 ] and a thickness of 1 [mm].
 The good heat conducting member 40 is arranged in the outside of a magnetic
 field generated by the excitation coil 18 and the magnetic cores 17a, 17b,
 and 17c composing the magnetic field generating means 15 so as not to be
 affected by the magnetic field.
 Specifically, the good heat conducting member 40 is arranged in one side of
 the magnetic cores 17b and 17c opposite to the excitation coil 18 to be
 located in the outside of the magnetic path generated by the excitation
 coil 18 to prevent the good heat conducting member 40 from being affected
 by the magnetic path.
 An oblong pressurizing rigid stay 22 is arranged in contact with a rear
 side of a portion corresponding to the nip portion N of the good heat
 conducting member 40 and an inner plane portion of the film guide member
 16b.
 An insulating member 19 is used to insulate the magnetic cores 17a, 17b,
 and 17c and the excitation coil 18 from the pressurizing rigid stay 22.
 Flange members 23a and 23b are externally coupled to horizontal both ends
 of an assembly of the film guide members 16a and 16b and mounted rotatably
 with the both ends fixed to regulate a slippage in a longer direction of
 the film guide member of the fixing film by receiving the end portions of
 the fixing film 10 when the fixing film 10 is rotating.
 The pressurizing roller 30 as a pressurizing member comprises a core metal
 30a and a heat resistant elastic layer 30b such as a silicone rubber,
 fluorine rubber, and fluorine resin coating the core metal so as to be
 molded in a shape of a roller concentrically and integrally, with the both
 ends of the core metal 30a rotatably borne between chassis side plates
 which are not shown in the apparatus.
 A press-down force is applied to the pressurizing rigid stay 22 by
 arranging pressurizing springs 25a and 25b with being compressed between
 both ends of the pressurizing rigid stay 22 and spring bearing members 29a
 and 29b in the chassis side of the apparatus. With this application, a
 lower surface of a portion corresponding to the nip portion N of the good
 heat conducting member 40 and an upper surface of the pressurizing roller
 30 pinches the fixing film 10 to be contacted with pressure, by which a
 fixing nip portion N having a predetermined width is formed.
 The pressurizing roller 30 which is a driving member is driven rotatively
 in a counterclockwise direction indicated by an arrow by driving means M.
 The rotative driving of the pressurizing roller 30 applies a rotative
 force to the fixing film 10 with a frictional force between the
 pressurizing roller 30 and an external surface of the fixing film 10, by
 which the fixing film 10 rotates in an outer periphery of the film guide
 members 16a and 16b at a circumferential speed almost corresponding to
 that of the pressurizing roller 30 in a clockwise direction indicated by
 an arrow with sliding in contact with the lower surface of the good heat
 conducting member 40 where the fixing film 10 is a film supporting member
 in the fixing nip N in its inner surface.
 In this embodiment, lubricant such as heat-resistant grease is put between
 a lower surface of the good heat conducting member 40 of the fixing nip
 portion N and the inner surface of the fixing film 10 to reduce a mutual
 sliding frictional force between the lower surface of the good heat
 conducting member 40 and the inner surface of the fixing film 10 in the
 fixing nip portion N. In addition, as shown in FIGS. 20 and 21, the lower
 surface of the good heat conducting member 40 can be coated with a
 lubricating member 41 having a good slip property. It prevents the sliding
 fixing film 10 from being scratched with reducing its durability when
 material having a poor slip property such as aluminum is used for the good
 heat conducting member 40 or when a finishing process is simplified.
 The good heat conducting member 40 has an effect of unifying a temperature
 distribution in a longer direction. If a small-sized sheet is passed, for
 example, an amount of heat on a no recording material passing portion in
 the fixing film 10 conducts to the good heat conducting member 40 and the
 amount of heat on the no recording material passing portion is conducted
 to the recording material passing portion on the small-sized sheet by heat
 conduction in the longer direction in the good heat conducting member 40.
 Accordingly, there can be also achieved an effect of reducing a power
 consumption when the small-sized sheet is passed.
 As shown in FIG. 5, projecting rib portions 16e are formed at predetermined
 intervals in its longer direction on a circumferential surface of the film
 guide member 16a so as to reduce a contact sliding resistance between the
 circumferential surface of the film guide member 16a and the inner surface
 of the fixing film 10 to decrease a rotating load of the fixing film 10.
 This projecting rib portion 16e can also be formed on the film guide
 member 16b in the same manner.
 Referring to FIG. 6, there is shown a condition of generating alternating
 magnetic flux. Magnetic flux C represents a part of the alternating
 magnetic flux which has been generated.
 The alternating magnetic flux C guided by the magnetic cores 17a, 17b, and
 17c generates eddy-current in the electromagnetic induction heat
 generating layer 1 of the fixing film 10 between the magnetic core 17a and
 the magnetic core 17b and between the magnetic core 17a and the magnetic
 core 17c. This eddy-current generates Joule heat (eddy-current loss) in
 the electromagnetic induction heat generating layer 1 by a specific
 resistance thereof.
 A quantity of generated heat Q depends upon a density of magnetic flux
 passing the electromagnetic induction heat generating layer 1, having a
 distribution as shown in a graph in FIG. 6. In the graph of FIG. 6, an
 ordinate axis indicates a position in a circumferential direction in the
 fixing film 10 represented by an angle .theta. with a center of the
 magnetic core 17a as zero (0) and an abscissa axis indicates a quantity of
 generated heat Q of the fixing film 10 in the electromagnetic induction
 heat generating layer 1. If the heat generated region H is defined to be a
 region of Q/e or greater quantity of generated heat, where Q is the
 maximum quantity of generated heat. It is a region in which a quantity of
 generated heat required for fixing is obtained.
 A temperature of the fixing nip portion N is adjusted so as to maintain a
 predetermined temperature by controlling a power supply to the excitation
 coil 18 by means of a temperature control system including temperature
 detecting means 26 (FIG. 2).
 The temperature detecting means 26 is a temperature sensor such as a
 thermistor for detecting a temperature of the fixing film 10. In this
 embodiment, a temperature of the fixing nip portion N is controlled on the
 basis of temperature information of the fixing film 10 measured by the
 temperature sensor 26.
 In a condition that the fixing film 10 rotates by which electromagnetic
 induction heat is generated on the fixing film 10 as described in the
 above by a power supply from the excitation circuit 27 to the excitation
 coil 18 and the fixing nip portion N is risen to a predetermined
 temperature for a temperature control in this manner, a recording material
 P on which an unfixed toner image t conveyed from the image forming means
 is inserted between the fixing film 10 and the pressurizing roller 30 in
 the fixing nip portion N with its image surface directed upward, in other
 words, opposite to a fixing film surface and then pinched and conveyed in
 the fixing nip portion N together with the fixing film 10 with its image
 surface put in contact with the outer surface of the fixing film 10 in the
 fixing nip portion N.
 In this process of pinching and conveying the recording material P together
 with the fixing film 10 in the fixing nip portion N, the unfixed toner
 image t on the recording material P is heated and fixed by the
 electromagnetic induction heat generation on the fixing film 10.
 After passing through the fixing nip portion N, the recording material P
 separates from the outer surface of the fixing film 10 and conveyed to be
 discharged.
 A heated and fixed toner image on the recording material P is cooled to be
 a permanent fixed image after passing through the fixing nip portion.
 In this embodiment, as shown in FIG. 2, a thermo-switch 50 which is a
 temperature detecting element is arranged to turn off the power supply to
 the excitation coil 18 at runaway in an opposite position to the heat
 generated region H (FIG. 6) of the fixing film 10.
 Referring to FIG. 7, there is shown a circuit diagram of a safety circuit
 used in this embodiment. The thermo-switch 50 which is a temperature
 detecting element is connected to a DC power supply of 24 V and a relay
 switch 51 in series; if the thermo-switch 50 is turned off, the power
 supply to the relay switch 51 is immediately turned off, the relay switch
 51 operates, and a power supply to the excitation circuit 27 is
 immediately turned off, by which a power supply to the excitation coil 18
 is turned off. An OFF operating temperature of the thermo-switch 50 is set
 to 220.degree. C.
 The thermo-switch 50 is arranged not to be in contact with the outer
 surface of the fixing film 10 so as to be opposed to the heat generated
 region H of the fixing film 10. A distance between the thermo-switch 50
 and the fixing film 18 is determined to be about 2 mm. This prevents the
 fixing film 10 from being scratched by a contact with the thermo-switch 50
 so as not to deteriorate a fixed image with durability.
 According to this embodiment, at runaway of the fixing apparatus due to a
 trouble thereof, unlike a configuration in which heat is generated in the
 nip portion N as shown in FIG. 19, heat is not generated in the nip
 portion N where a sheet is pinched and therefore the sheet is not heated
 directly even if the fixing apparatus stops in a condition that a sheet is
 pinched in the fixing nip N and the fixing film 10 continues to generate
 heat with the excitation coil 18 continued to be powered. In addition, the
 thermo-switch 50 is arranged in the heat generated region H where a large
 quantity of heat is generated, and therefore the thermo-switch 50 detects
 the temperature of 220.degree. C. and powered off, when a power supply to
 the excitation coil 18 is immediately turned off by the relay switch 51.
 According to this embodiment, an ignition temperature of a sheet is about
 400.degree. C. and therefore it does not ignite, by which heat generation
 of the fixing film 10 can be stopped.
 A temperature fuse can be used as a temperature detecting element in
 addition to the thermo-switch 50.
 While toner made of toner t including low softening point material is used
 and therefore no oil coating mechanism for preventing an offset is
 arranged in the fixing apparatus in this embodiment, an oil coating
 mechanism can be arranged if toner not including the low softening point
 material is used. In addition, oil coating or cooling separation can be
 performed also when using toner including the low softening point
 material.
 Bundled small-gage wires made of copper each insulated with coating
 (bundled wires) are used as conductor wires (electrical wires) composing a
 coil (wire ring) for the excitation coil 18 and they are wound by a
 plurality of turns to form an excitation coil. In this embodiment, they
 are wound by ten turns to form the excitation coil 18.
 For the insulating coating, coating having a heat resistance is preferably
 used taking into consideration a heat conductivity caused by the heat
 generation of the fixing film 10. For example, it is preferable to use
 coating such as amide-imide or polyimide.
 A pressure can be applied to the excitation coil 18 from the outside to
 improve its density.
 A shape of the excitation coil 18 is configured to outline a curved surface
 of the heat generating layer of the fixing film 10 as shown in FIG. 2. In
 this embodiment, a distance between the heat generating layer 1 of the
 fixing film 10 and the excitation coil 18 is set to be about 2 mm.
 As material of the film guide members (excitation coil holding members) 16a
 and 16b, it is preferable to use material having superior insulating
 characteristics and heat resistance. For example, phenolic resin, fluorine
 resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK
 resin, PES resin, PPS resin, PFA resin, PTFE resin, FEP resin, LCP resin
 or the like is preferably used.
 While an absorption efficiency of magnetic flux is increased by narrowing a
 distance between the magnetic cores 17a, 17b, and 17c and the excitation
 coil 18 and the heat generating layer 1 of the fixing film 10 if possible,
 this efficiency is significantly decreased when the distance exceeds 5 mm
 and therefore it is preferable to restrain the distance to 5 mm or
 shorter. If it is 5 mm or shorter, the distance between the heat
 generating layer 1 of the fixing film 10 and the excitation coil need not
 be fixed.
 As for a lead-out wire from the film guide member 16a for the excitation
 coil 18, in other words, power supply portions 18a and 18b (FIG. 5), the
 bundled wires are externally insulated with coating in portions in the
 outside of the film guide member 16a.
 Referring to FIG. 8, there is shown a layer configuration typical diagram
 of the fixing film 10 in this embodiment. The fixing film 10 in this
 embodiment has a complex configuration formed by a heat generating layer 1
 made of a metal film or the like which is a base layer of the fixing film
 10 having an electromagnetic induction heat generating property, an
 elastic layer 2 laid on its outer surface, and a mold releasing layer 3
 laid on its outer surface.
 In order to bond the heat generating layer 1 to the elastic layer 2 and the
 elastic layer 2 to the mold releasing layer 3, a primer layer (not shown)
 can be arranged between respective layers.
 In the fixing film 10 having an almost cylindrical shape, the heat
 generating layer 1 is put in an inner surface side and the mold releasing
 layer 3 is put in an outer surface side. As described above, eddy-current
 is generated in the heat generating layer 1 by an action of the
 alternating magnetic flux on the heat generating layer 1, by which the
 heat generating layer 1 generates heat. A recording material P as a heated
 material passed through the fixing nip N is heated by the heat via the
 elastic layer 2 and the mold releasing layer 3, so that a toner image is
 fixed by heat.
 For the heat generating layer 1, ferromagnetic metal such as nickel, iron,
 ferromagnetic SUS, or nickel-cobalt alloy is preferably used.
 Although non-magnetic metal can also be used, metal having a superior
 magnetic flux absorption property such as nickel, iron, magnetic
 stainless, or cobalt-nickel alloy is more preferable.
 Its thickness is preferably greater than a depth of a skin expressed by the
 following formula and less than or equal to 200 .mu.m:
EQU .sigma.=503.times.(.rho./f.mu.).sup.1/2
 where .sigma.[mm] is the depth of the skin with a frequency f [Hz] of the
 excitation circuit 27, permeability .mu., and a specific resistance
 .rho.[.OMEGA.m].
 This represents a depth of an absorption of electromagnetic waves used for
 the electromagnetic induction. In portions having a depth greater than
 this, an intensity of the electromagnetic waves is less than or equal to
 1/e. Conversely, almost all the energy is absorbed up to this depth (FIG.
 9).
 The depth of the heat generating layer 1 is preferably 1 to 100 .mu.m. If
 the thickness of the heat generating layer 1 is smaller than 1 .mu.m,
 almost all the electromagnetic energy cannot be absorbed, thereby its
 efficiency is deteriorated. If the depth of the heat generating layer 1
 exceeds 100 .mu.m, rigidity is too high and a flexing property is
 deteriorated and therefore it is not practical to use it as a rotator.
 Accordingly, the thickness of the heat generating layer 1 is preferably 1
 to 100 .mu.m.
 The elastic layer 2 is made of material such as silicone rubber, fluorine
 rubber, or fluoro-silicone rubber having a superior heat-resistance and
 heat conductivity.
 The thickness of the elastic layer 2 is preferably 10 to 500 .mu.m. This
 thickness of the elastic layer 2 is necessary to ensure a fixing image
 quality.
 In printing a color image, particularly a photographic image, a solid image
 is formed in a large area on the recording material P. In this condition,
 unevenness of a heat generation occurs unless a heating surface (the mold
 releasing layer 3) can follow an uneven surface of a recording material or
 of a toner layer, by which an uneven gloss occurs on an image in portions
 having a large quantity of conducted heat and a small quantity thereof.
 The portions having a large quantity of conducted heat has a high gloss
 value and those having a small quantity of conducted heat has a low gloss
 value.
 The elastic layer 2 having a thickness less than or equal to 10 .mu.m
 cannot completely follow unevenness of the recording material or the toner
 layer, thereby causing uneven gloss on an image. If the elastic layer 2
 has a thickness of more than or equal to 1000 .mu.m, a heat resistance of
 the elastic layer becomes large, by which it is hard to perform a quick
 start. More preferably, the thickness of the elastic layer 2 is 50 to 500
 .mu.m.
 If a hardness of the elastic layer 2 is too high, the elastic layer 2
 cannot completely follow the unevenness of the recording material or the
 toner layer, thereby causing an uneven gloss of the image. Therefore, a
 hardness of the elastic layer 2 is preferably 60.degree. (JIS-A, i.e., a
 hardness determined by a JIS-K6301 A-type hardness tester) or lower, more
 preferably 45.degree. or lower.
 A heat conductivity .lambda. of the elastic layer 2 is preferably as
 follows:
 6.times.10.sup.-4
 [cal/cm.multidot.sec.multidot.deg.](6.times.10.sup.-4.times.4.
 186.congruent.25.1.times.10.sup.-4 [J/cm.multidot.sec.multidot.deg.]) to
 2.times.10.sup.-3
 [cal/cm.multidot.sec.multidot.deg.](2.times.10.sup.-3.times.4.
 186.congruent.8.4.times.10.sup.-3 [J/cm.multidot.sec.multidot.deg.])
 If the heat conductivity .lambda. is less than 6.times.10.sup.-4
 [cal/cm.multidot.sec.multidot.deg.](6.times.10.sup.-4.times.4.
 186.congruent.25.1.times.10.sup.-4 [J/cm.multidot.sec.multidot.deg.]) to
 2.times.10.sup.-3
 [cal/cm.multidot.sec.multidot.deg.](2.times.10.sup.-3.times.4.
 186.congruent.8.4.times.10.sup.-3 [J/cm.multidot.sec.multidot.deg.]), a
 heat resistance is large and a temperature on a surface layer (the mold
 releasing layer 3) of the fixing film increases more slowly.
 If the heat conductivity .lambda. is more than 2.times.10.sup.-3
 [cal/cm.multidot.sec.multidot.deg.](2.times.10.sup.-3.times.4.
 186.congruent.8.4.times.10.sup.-3 [J/cm.multidot.sec.multidot.deg.]), the
 hardness becomes too high or a permanent compression set is deteriorated.
 Accordingly, it is preferable that the heat conductivity .lambda. is within
 the range of 6.times.10.sup.-4
 [cal/cm.multidot.sec.multidot.deg.](6.times.10.sup.-4.times.4.
 186.congruent.25.1.times.10.sup.-4 [J/cm.multidot.sec.multidot.deg.]) to
 2.times.10.sup.-3
 [cal/cm.multidot.sec.multidot.deg.](2.times.10.sup.-3.times.4.
 186.congruent.8.4.times.10.sup.-3 [J/cm.multidot.sec.multidot.deg.]). More
 preferably, it is within a range of 8.times.10.sup.-4
 [cal/cm.multidot.sec.multidot.deg.](8.times.10.sup.-4.times.4.
 186.congruent.33.5.times.10.sup.-4 [J/cm.multidot.sec.multidot.deg.]) to
 1.5.times.10.sup.-3
 [cal/cm.multidot.sec.multidot.deg.](1.5.times.10.sup.-3.times.4.
 186.congruent.6.3.times.10.sup.-3 [J/cm.multidot.sec.multidot.deg.]).
 For the mold releasing layer 3, it is possible to select any of fluorine
 resin, silicone resin, fluoro-silicone rubber, fluorine rubber, silicone
 rubber, PFA, PTFE, FEP or other material having superior releasing
 property and heat resistance.
 A thickness of the mold releasing layer 3 is 1 to 100 .mu.m. If the
 thickness of the mold releasing layer 3 is less than 1 .mu.m, there occurs
 a problem that an uneven film of coating makes portions having a poor
 releasing property or that a durability is insufficient. Furthermore, if
 the mold releasing layer 3 exceeds 100 .mu.m, it causes a problem that the
 heat conductivity is deteriorated and, particularly for a resin mold
 releasing layer, the hardness becomes too high, which reduces an effect of
 the elastic layer 2.
 As shown in FIG. 10, in the configuration of the fixing film 10, an
 insulating layer 4 can be arranged in the film guide member side (in the
 opposite side to the elastic layer 2 of the heat generating layer 1) of
 the heat generating layer 1.
 For the insulating layer 4, it is preferable to use heat-resistant resin
 such as fluorine resin, polyimide resin, polyamide resin, polyamide-imide
 resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, or FEP
 resin.
 A thickness of the insulating layer 4 is preferably 10 to 1000 .mu.m. If
 the thickness of the insulating layer 4 is less than 10 .mu.m, a thermal
 insulating effect cannot be obtained and durability is insufficient. On
 the other hand, if the thickness exceeds 1000 .mu.m, a distance from the
 magnetic cores 17a, 17b, and 17c and the excitation coil 18 to the heat
 generating layer 1 is too long, by which the magnetic flux is not
 sufficiently absorbed by the heat generating layer 1.
 The insulating layer 4 works as a thermal barrier in such a way that heat
 generated in the heat generating layer 1 does not move to the inside of
 the fixing film 10, and therefore an efficiency of supplying heat to the
 recording material P side is improved in comparison with a condition in
 which the insulating layer 4 is not arranged. Accordingly, a power
 consumption can be restrained.
 A nip width of a fixing device in the full-color image forming apparatus is
 preferably at least 7.0 mm to ensure a sufficient fixing property of a
 full-color image having a relatively large amount of stacked toner. If the
 width is less than 7.0 mm, a sufficient quantity of heat cannot be given
 to unfixed toner and a recording material, thereby causing a fixing
 failure.
 Furthermore, to ensure a transmission property of an OHT full-color image
 sufficiently, a surface pressure of the nip portion is preferably 0.8
 kgf/cm.sup.2 or greater. If it is less than 0.8 kgf/cm.sup.2, a surface of
 the fixed toner layer cannot be smoothed enough, and therefore irregular
 reflected lights are increased, thus reducing a transmitted light volume
 of the OHT image portion.
 From this viewpoint, in the fixing apparatus in this embodiment, the
 pressurizing roller 30 and the fixing film 10 are pressed with a force of
 21 kgf with a nip width of approx. 8.0 mm and a surface pressure in the
 nip portion of 1.2 kgf/cm.sup.2 (a length 220 mm in the longer direction
 of the nip portion).
 Next, an explanation will be made for a rotative driving control at rising
 of the fixing apparatus which is a characteristic of the present
 invention.
 If the image forming apparatus is put in a standby condition, the fixing
 apparatus 100 is in a resting condition. The fixing apparatus 100 in this
 embodiment is of on-demand type, and therefore the fixing apparatus 100 is
 not powered in the standby condition.
 If a print signal is inputted to the image forming apparatus body, a
 driving force is transmitted from a driving motor of the image forming
 apparatus body to the fixing apparatus 100 via gears. Then the fixing film
 10 rotates in the outer periphery of the film guide members 16a and 16b
 while sliding over the lower surface of the good heat conducting member 40
 in the nip portion. Afterward, a power is supplied to the excitation
 circuit 27 of the fixing apparatus 100, the fixing film 10 is risen in
 temperature by an induction heat generation, and the fixing apparatus 100
 transits to a fixable condition.
 Although this specification of the present invention prescribes a control
 of a rotational speed of the film rotator 10 at a start of rotative
 driving, the fixing film 10 which is a film rotator rotates following the
 pressurizing roller 30 at a circumferential speed almost corresponding to
 the circumferential speed of the pressurizing roller 30. Accordingly, in
 this embodiment, a description is made for a speed control of the
 pressurizing roller 30 in which driving is transmitted from the driving
 motor practically.
 At a start of this fixing apparatus, a working torque necessary for
 rotating it from a resting condition can be classified into "an
 acceleration torque" and "a load torque".
 The former "acceleration torque" depends upon an acceleration from a
 resting condition to a process speed. By moderating a change of a driving
 speed of the pressurizing roller 30 immediately after the start of the
 rotative driving, in other words, by slowing it up, the acceleration
 torque can be reduced. In addition, if the driving speed is low, a
 viscosity torque is also reduced, and therefore it leads to a reduction of
 the load torque described later.
 The latter "load torque" depends upon a frictional load and an external
 load of a driven apparatus. Particularly immediately after a start of
 driving the fixing apparatus, a static frictional force greater than a
 dynamic frictional force is generated, and therefore the load torque is
 greater than that during a driving operation. In this embodiment, the load
 torque significantly changes due to a viscosity of heat-resistant grease
 as lubricant between the fixing film 10, the guide members 16a and 16b as
 its supporting members, and the good heat conducting member 40.
 Particularly, at a start of the fixing apparatus in a condition that it is
 cooled down to a room temperature, the viscosity of the heat-resistant
 grease is high and a viscosity load is large.
 Therefore, in the present invention, a slow-up rotative driving is
 performed to reduce the above "acceleration torque" and a temperature of
 the fixing apparatus is risen during the rotative driving, by which the
 viscosity of the heat-resistant grease which is lubricant is lowered to
 reduce the load torque together with it.
 Referring to FIG. 11, there is shown a conceptional diagram showing a
 change of a circumferential speed of the pressurizing roller at a start of
 rotative driving in a conventional fixing apparatus driving control. The
 abscissa axis indicates an elapsed time and an ordinate axis indicates a
 rotational speed of the pressurizing roller. As shown in FIG. 11,
 conventionally the pressurizing roller is generally driven at a
 predetermined process speed which is a constant speed from a moment at
 which a driving force is transmitted to the fixing apparatus in the
 resting condition. Normally, an acceleration time from the resting
 condition to a predetermined process speed is about 0.1 sec. Therefore, as
 well as a large acceleration torque, the pressurizing roller and the
 fixing film rotate at a predetermined process speed in a condition that
 the fixing film is not warmed up enough at the first run (start) in the
 morning, and therefore the viscosity of the heat-resistant grease is high
 and a very large torque is required at the start.
 Referring to FIGS. 12 to 14, there are shown conceptional diagrams showing
 changes of the circumferential speed of the pressurizing roller at the
 start of the rotative driving under the driving control of the fixing
 apparatus according to the present invention.
 As shown in FIG. 12, at a moment when the driving force is transmitted to
 the pressurizing roller, the pressurizing roller is driven at a very low
 speed (a first speed) and the circumferential speed is linearly slowed up
 gradually taking time so as to reach a predetermined process speed (a
 second speed) finally. The time required for the slow-up driving is set to
 an optimum value according to the fixing apparatus. Unlike rising in the
 conventional driving, however, it is not performed in a short time such as
 approx. 0.1 sec.
 Strictly speaking, if the driving roller (pressurizing roller) is risen
 from "0" to a certain speed, a low-speed region (acceleration region) for
 a very short time is inevitably generated, while the first speed region in
 the present invention does not include this low-speed region, but the
 first speed region is controlled so that the low-speed period is elongated
 by being set on the basis of the necessarily generated speed at rising.
 Time required for the slow-up driving is set to a period between an input
 of a print signal and an entry into the fixing nip of the recording
 material on which an image is formed, and the acceleration torque at the
 start of the rotative driving must be sufficiently reduced by the setting.
 This slow-up driving can be controlled to be a nonlinear driving in such a
 way that a circumferential speed is increased at a very small acceleration
 immediately after the start of the driving before the acceleration is
 gradually increased as shown in FIG. 13.
 In addition, as shown in FIG. 14, the slow-up driving can be controlled in
 such a way that the circumferential speed is increased in steps with
 dividing the circumferential speed into several grades as shown in FIG.
 14. In this case, the first speed includes a speed region in which the
 speed is constant for a predetermined time.
 Furthermore, the control patterns of the above three slow-up driving can be
 combined with each other.
 Any of the above driving control patterns is characterized by the
 circumferential speed immediately after the start of the driving of the
 pressurizing roller set to be lower than the process speed (fixing speed).
 Accordingly, the present invention prevents a step-out of the driving
 motor.
 In addition, a power supply to the fixing apparatus (a power supply to the
 excitation circuit 27) is started simultaneously with the start of the
 slow-up driving or during the slow-up driving. This heats the
 heat-resistant grease between the inner surface of the fixing film and its
 supporting member, by which the viscosity is lowered and the load torque
 is reduced. Particularly in the electromagnetic induction heat generating
 method as in this fixing apparatus, the heat-up speed is shorter than that
 in a thermal-roller type, so that the fixing film is heated up from a room
 temperature to 190.degree. C. which is a fixable temperature in 15 to 20
 sec or so. Therefore, if the slow-up driving time is set to around 15 sec,
 both of the acceleration torque and the load torque at the start-up can be
 reduced, by which torque required at the start-up can be significantly
 reduced.
 In the configuration of the electromagnetic induction heating type fixing
 apparatus in this embodiment, a heat generating portion is not located in
 the nip position, but in a slightly more upstream position as shown in
 FIGS. 2 and 6. While it is preferable to heat the heat-resistant grease on
 the rubbed nip portion directly in order to reduce the load torque, the
 nip portion cannot be heated up even if it is heated in the resting
 condition in the configuration of this embodiment. In this case, the
 slow-up rotation of the fixing film is very effective from the viewpoint
 of heating up the nip portion.
 As described above, by starting a temperature control together with the
 slow-up driving of the fixing apparatus, driving torque required at the
 start of the driving of the fixing apparatus can be significantly reduced.
 Next, another embodiment of the present invention will be described below.
 Configurations of an image forming apparatus and a fixing apparatus in this
 embodiment are the same as those of the above embodiment, and therefore a
 description of these configurations is omitted here.
 This embodiment is characterized by a change of a driving time for the
 fixing apparatus at slow-up driving according to a temperature of the
 fixing film 10 before starting the driving.
 If the fixing film 10 is not cooled down completely like the condition in
 which so much time is not elapsed after an end of printing, a viscosity of
 the grease coated inside the fixing film is low and the load torque is
 small. In this condition, more driving torque can be allocated to the
 acceleration torque in comparison with driving torque allocated at rising
 from a completely cooled condition of the fixing apparatus. Therefore, by
 using a control pattern in which time required for the slow-up driving
 immediately after a start of rotator driving is changed according to a
 temperature of the fixing film 10 before starting the rotative driving,
 the time required for the slow-up driving can be reduced to the minimum
 together with significantly reducing a driving torque required at the
 start of driving the fixing apparatus.
 First, FIG. 15 shows a driving control pattern of the fixing apparatus at
 the start of the rotative driving in this embodiment. This pattern is the
 same as that of the control shown in FIG. 14 in the above embodiment in
 that a circumferential speed increases in steps. In the driving control
 pattern of this embodiment, the circumferential speed of the pressurizing
 roller under the slow-up driving is divided into four grades for
 acceleration in steps. These steps are referred to as a first
 circumferential speed, a second circumferential speed, a third
 circumferential speed, and a fourth circumferential speed from a low speed
 side, respectively.
 While these four circumferential speeds are appropriately combined with
 each other according to a temperature of the fixing film 10 in this
 embodiment, it is also possible to accelerate the circumferential speeds
 linearly or nonlinearly relative to time. Furthermore, the above patterns
 can be combined for acceleration by a driving control including an
 acceleration in steps.
 Next, FIG. 16 shows a chart of a sequence of a driving control at the start
 of the rotative driving in this embodiment.
 First, when a print signal is inputted to the image forming apparatus body,
 the pressurizing roller 30 starts the rotative driving. At this time, the
 image forming apparatus body recognizes a temperature of the fixing film
 by means of a signal from a temperature detecting element. The recognized
 temperature of the fixing film is compared with preset temperatures
 (.degree. C.) T1, T2, and T3 (T1&gt;T2&gt;T3). Then it classifies the recognized
 temperature of the fixing film into four grades; lower than T1, more than
 or equal to T1 and less than T2, more than or equal to T2 and less than
 T3, and more than or equal to T3.
 Then, first of all, rotative driving is performed for t1 sec at the lowest
 first rotational speed independently of a temperature of the fixing film
 10. The first circumferential speed is used for reducing torque
 immediately after the rotative driving. Therefore, it is preferably the
 minimum speed.
 If the temperature of the fixing film at the start of the driving is lower
 than T1.degree. C., the circumferential speed is switched to the second
 circumferential speed for driving for t2 sec afterward, then it is
 switched to the third circumferential speed for driving for t3 sec and
 afterward to the fourth circumferential speed for driving for t4 sec.
 Finally after t1+t2+t3+t4 sec from the start of the driving, the speed is
 switched to a predetermined process speed to complete the slow-up driving.
 If the temperature of the fixing film at the start of the driving is more
 than or equal to T1.degree. C. and less than T2.degree. C., a step of
 driving at the second circumferential speed is skipped and the speed is
 switched to the third circumferential speed for driving for three sec and
 then switched to the fourth circumferential speed for driving for four
 sec. Finally after t1+t3+t4 sec from the start of the driving, the speed
 is switched to the predetermined process speed to complete the slow-up
 driving.
 If the temperature of the fixing film at the start of the driving is more
 than and equal to T2.degree. C. and less than T3.degree. C., steps of
 driving at the second and third circumferential speeds are skipped and the
 speed is switched to the fourth circumferential speed for driving for four
 sec. Finally after t1+t4 sec from the start of the driving, the speed is
 switched to the predetermined process speed to complete the slow-up
 driving.
 If the temperature of the fixing film at the start of the driving is equal
 to or more than T3.degree. C., steps of driving at the second, third, and
 fourth circumferential speeds are skipped and immediately the speed is
 switched to the predetermined process speed. In other words, after t1 sec
 from the start of the driving, the slow-up driving is completed.
 While the circumferential speeds under the slow-up driving and the film
 temperatures are previously classified into four grades in this
 embodiment, the number of these grades is appropriately determined
 according to a fixing apparatus. It is also possible to change each
 driving time for the preset circumferential speeds under the slow-up
 driving in such a way that it is associated with each temperature of the
 fixing film.
 In addition, the circumferential speed of the pressurizing roller can be
 continuously varied linearly or nonlinearly at realtime so as to be
 associated with a temperature of the fixing film.
 As described in the above, by changing a driving time for the fixing
 apparatus according to a temperature of the fixing film 10 previous to the
 start of the driving, the driving torque required at the start of the
 driving of the fixing apparatus can be lowered while reducing a time
 required for the slow-up driving to a minimum.
 Next, still another embodiment of the present invention will be described
 below.
 A configuration of an image forming apparatus in this embodiment is the
 same as that in the above embodiment, and therefore its description is
 omitted here.
 Referring to FIG. 17, there is shown a sectional side elevation typical
 view of a main portion of an electromagnetic induction heating type fixing
 apparatus in this embodiment. In this configuration of this embodiment, a
 heat generating portion is arranged in the nip portion N or in the
 vicinity thereof. In the same manner as for the above embodiment, a
 driving control for a fixing apparatus 200 at the slow-up driving can be
 performed in any driving control pattern shown in FIGS. 12 to 14 or in
 their combination pattern in the same manner as for the above embodiment.
 This embodiment is characterized by a temperature rise of the fixing film
 10 by previously starting the temperature control before the start of the
 rotative driving of the pressurizing roller 30 in the fixing apparatus
 having the above configuration. In the fixing apparatus 200 according to
 this embodiment, the heat generating portion is in the vicinity of the nip
 portion N, and therefore by starting the temperature control in a resting
 condition, it becomes possible to increase temperatures of members around
 the nip portion N rubbed by the fixing film 10 and the good heat
 conducting member 40.
 Accordingly, the viscosity of the heat-resistant grease applied to the
 inner surface of the fixing film of the nip portion N can be reduced and
 the load torque be lowered before the slow-up driving of the pressurizing
 roller 30 is started. Accordingly, a slip of the film can be reliably
 prevented.
 As described above, by starting the slow-up driving after heating up the
 fixing film 10 in the resting condition, the driving torque required at
 the start of driving the fixing apparatus can be significantly reduced
 even if the fixing apparatus 200 is cooled.
 In the induction heating type fixing apparatus as described in this
 embodiment, only the heat generating portion is heated up to the vicinity
 of 300.degree. C. in several sec or so and the fixing film 10 may be
 destroyed with heat if it is powered in the resting condition without
 lowering the power, for example, 1000 W or greater power is supplied.
 Therefore, more preferably an input power in the resting condition of the
 fixing apparatus is controlled to be reduced to about 500 W or lower.
 As set forth in the above, the fixing film 10 is previously heated up
 before the slow-up driving of the pressurizing roller 30, by which the
 driving torque required at the start of driving the fixing apparatus can
 be significantly reduced.
 It is possible to apply the above control method of driving the
 pressurizing roller after the heat-up of the film to the image heating
 apparatus having the configuration shown in FIG. 2.
 While the good heat conducting member is arranged in the above embodiment,
 it is also possible to arrange the guide members 16a and 16b over the
 circumference without an arrangement of the good heat conducting member as
 shown in FIGS. 22 and 23 and to arrange a sliding member (slipping member)
 41 which is a supporting member in a resting and fixed condition in a
 position corresponding to the nip N portion of the guide members 16a and
 16b, so that the film moves in contact with this sliding member 41.
 The electromagnetic induction heating type fixing belt 10 can also has a
 configuration not having the elastic layer 2 if it is used for heating and
 fixing a monochrome or single-path multi-color image. The electromagnetic
 induction heat generating layer 1 can be composed of resin including metal
 filler. It can also be a member of a single electromagnetic induction
 heating layer.
 The configuration of the fixing apparatus as a heating apparatus is not
 limited to the pressurizing roller driving type in this embodiment.
 For example, as shown in FIG. 18, the electromagnetic induction heating
 type fixing belt 10 in an endless belt form is suspended between the belt
 guide 16, the driving roller 31, and the tension roller 32, and the bottom
 portion of the belt guide 16 and the pressurizing roller 30 as a
 pressurizing member are contacted by a pressure with the fixing belt 10
 therebetween to form the fixing nip portion N, by which the fixing belt 10
 can be driven rotatively by the driving roller 31. In this case, the
 pressurizing roller 30 is a follower rotating roller.
 The pressurizing member 30 is not limited to a roller, but there can be a
 member having another form such as a rotary belt.
 In addition, to supply heat energy to the recording material from the
 pressurizing member 30 side, electromagnetic induction heating or other
 heating means can also be arranged in the pressurizing member 30 side for
 heat-up and temperature control to a predetermined temperature.
 A use of the heating apparatus of the present invention is not limited to
 the image heating and fixing apparatus of this embodiment, but it can be
 widely used as means or an apparatus for heating heated material such as
 an image heating apparatus for improving a quality of gloss or other
 surface properties by heating a recording material bearing an image, an
 image heating apparatus for temporary fixing, an apparatus for heating and
 drying heated material, or a heat laminating apparatus.
 As described above, in the fixing apparatus having a configuration in which
 the rotator is slid over its supporting member in the nip portion
 according to the present invention, the driving roller is rotated at a low
 speed when its driving is started and therefore a step-out of the driving
 motor of the fixing apparatus can be prevented, and the driving torque
 required at the start of driving the rotator can be reduced by heating up
 the film, by which a driving motor with a smaller driving torque can be
 used so as to reduce a product cost.
 Although the present invention has been described in its preferred
 embodiments, the present invention is not limited to the above embodiments
 and any changes can be resorted to within the scope of the technical idea
 of the present invention.