Image forming apparatus and fixing apparatus

An image forming apparatus has image forming means for forming an unfixed toner image on a recording material, heating and fixing means for heating and fixing the unfixed toner image on the recording material, temperature sensing means for sensing the temperature of the heating and fixing means, and power controlling means for controlling power supplied to the heating and fixing means so that the heating and fixing means keeps a fixable temperature at least on fixing operation based on an output from the temperature sensing means. The power controlling means controls power supply to the heating and fixing means based on the output from the temperature sensing element during the time from receipt of a print signal by the image forming apparatus to performing a heating and fixing process on the recording material so that, in the case where the temperature of the heating and fixing means rises fast, a temperature control operation for keeping the fixable temperature should not be protracted before heating and fixing so as to control excessive rise in the temperature of the pressure member (pressure roller) and prevent a media slip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus and a fixing apparatus provided thereto and, in more particularly, to an apparatus for forming an unfixed toner image on a surface of a recording material by appropriate image forming processing means such as electrophotography, electrostatic recording and magnetic recording including a copier, a printer and a facsimile, using a toner made from a heat melting resin and so on by a direct or indirect method, and heating and fixing it on the surface of the recording material as a permanently fixed image by heating and fixing means.

2. Related Background Art

In an image forming apparatus, a fixing apparatus of a heat-roller method is widely used as a heating means for fixing an unfixed toner image formed on a recording material by an appropriate image forming processing means. The fixing apparatus of the heat-roller method keeps in contact a fixing roller as a heating member incorporating a heat generating means such as a halogen heater and a pressure roller as a pressure member so as to fix the unfixed toner image by applying heat and pressure while transporting the recording material.

In recent years, a fixing apparatus of a film heating method is rendered commercially practical from viewpoints of a quick start and energy conservation. The fixing apparatus of the film heating method is the one wherein a fixing nip is formed by having a heat-resistant thin film sandwiched between a ceramic heater as heat generating means and a pressure roller as a pressure member. It fixes the unfixed toner image by rotating the film and the pressure roller together to apply the heat and pressure while transporting the recording material. The film is heated by the ceramic heater at the fixing nip. The ceramic heater has its temperature sensed by a temperature sensing element provided on the back thereof, and energization to the ceramic heater is controlled and temperature control thereof is performed based on the results of the sensing.

As for the above fixing apparatus of the film heating method, heat capacity of the film as a heating member is very small compared to the heat-roller method, and so it is possible to efficiently use thermal energy from the heat generating means in a fixing process. For this reason, a temperature rising speed of the fixing apparatus is fast so that waiting time between power-up of the apparatus and a printable state thereof can be rendered shorter (quick start). In addition, there is no need to preheat the heating member during standby for printing so that power consumption of the image forming apparatus can be held low (energy conservation).

There is a proposal, as a fixing apparatus of a further high-efficiency film heating method, of the fixing apparatus of the electromagnetic induction heating method for causing a conductive film itself to generate heat. Japanese Utility Model Application Laid-Open No. 51-109739 discloses, as the fixing apparatus of the electromagnetic induction heating method, the fixing apparatus for having an eddy current induced to a metallic film by an alternating magnetic field to cause the metallic film to generate heat with Joule heat. As it is possible to cause the film itself to generate heat by the electromagnetic induction heating method, the thermal energy from the heat generating means can be used further efficiently in the fixing process.

Hereafter, the temperature control of the fixing apparatus on a start of printing will be described.

FIG. 23is a schematic view showing a fixing film temperature, a target temperature setting and timing of recording material reaching the fixing apparatus when starting the printing in the fixing apparatus of the past fixing apparatus (the fixing apparatus of the film heating method using the ceramic heater or the fixing apparatus of the electromagnetic induction heating method/film heating method).

Although the temperature control is off and no preheating is performed during standby for printing, preheating may also be performed. The image forming apparatus starts an image forming operation after receiving a print signal. The image forming apparatus starts power supply to the fixing apparatus at the same time, and increases the temperature of the fixing apparatus to a fixing temperature Tf. And the fixing apparatus keeps the fixing temperature Tfand prepares for fixing of the unfixed toner image on the recording material. The above steps will be collectively called a starting step of the fixing apparatus.

In the starting step of the fixing apparatus, the recording material are not put through paper so that most of the heat from the heat generating means is used to increase the temperature of the pressure roller via a film. In particular, in the case where the fixing apparatus is already warmed up, time twufor rising to the target temperature is short and time tp−twufor keeping the fixing temperature Tfis long, so that the temperature of the pressure roller further rises. For this reason, the temperature of the pressure roller is apt to rise excessively in the case where the starting step is repeated as with intermittent printing.

In the case of fixing the recording material requiring a lot of heat capacity for the fixing such as a cardboard or an OHT film in general, processing speed is reduced. In the stating step in such a case, time tpfrom the start of the image forming operation until the recording material reaches the fixing apparatus becomes longer, and so the time tp−twufor keeping the recording material at the fixing temperature Tfwithout putting it through paper becomes longer. For this reason, the temperature of the pressure roller is apt to rise excessively as with the intermittent printing.

As described above, there is a problem that, if the printing is performed in a state in which the temperature of the pressure roller has excessively risen, slipping of the recording material is apt to occur. It is because moisture in the recording material evaporates on the heating and fixing and frictional force between the pressure roller and the recording material is reduced. In particular, the higher the temperature of the pressure roller is, the more the amount of evaporated moisture becomes, and so the slipping of the recording material is more likely to occur. Furthermore, the slipping of the recording material occurs more conspicuously in the case of the fixing apparatus of the film heating method wherein a driving force is applied to the pressure roller and the film is rotated by being slaved to the pressure roller.

There was a problem that, if the slipping of the recording material occurs, the recording material does not move along a carriage guide member or winds itself around the film, resulting in occurrence of a jam. Furthermore, there was a problem that, as it is not possible to stably apply the heat and pressure to the unfixed toner image, quality of a fixed image is lowered.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above technological problems and control an excessive temperature rise of a pressure roller in a starting step of a fixing apparatus and thereby prevent a recording material from slipping in the fixing apparatus so as to stabilize carriage of the recording material and improve quality of a fixed image.

In order to attain the above object, a heating apparatus and an image forming apparatus according to the present invention are characterized by the following configuration.

(1) The image forming apparatus according to the present invention comprises image forming means for forming an unfixed toner image on the recording material, heating and fixing means for heating and fixing the above described unfixed toner image on the recording material, temperature sensing means for sensing the temperature of the above described heating and fixing means, and power controlling means for controlling power supplied to the above described heating and fixing means so that the above described heating and fixing means keeps a fixable temperature at least on fixing operation based on an output from the above described temperature sensing means, wherein the above described power controlling means controls power supply to the above described heating and fixing means based on the above described output from the temperature sensing element during the time from receipt of a print signal by the image forming apparatus to performing a heating and fixing process on the recording material so that, in the case where the temperature of the above described heating and fixing means rises fast, a temperature control operation for keeping a fixable temperature should not be protracted before heating and fixing.

(2) Preferably, the above described power controlling means performs a low temperature control step for controlling the heating and fixing means at a temperature lower than the fixable temperature or a non-heating step for heating no heating and fixing means during the time from after receipt of the print signal by the image forming apparatus to performing the heating and fixing process so as to control power supply to the heating and fixing means.

(3) Preferably, the temperature of the heating and fixing means is increased more than once by sandwiching the above described low temperature control step or the above described non-heating step during the time from after receipt of the print signal by the image forming apparatus to before performing the heating and fixing process, and at least by a temperature rise lastly performed thereof, it prepares for the heating and fixing process of the recording material by rendering the target temperature as a fixable temperature.

(4) Preferably, the temperature of the heating and fixing means is increased once after receipt of the print signal by the image forming apparatus so as to determine performance time of the above described low temperature control step or the above described non-heating step by this temperature rise behavior.

(5) Preferably, the temperature of the above described heating and fixing means is increased once to the fixable temperature or a lower temperature than that after receipt of the print signal by the image forming apparatus.

(6) Preferably, the above described heating and fixing means is comprised of a rotating heating member capable of rotation and heating the recording material, a rotating pressure member for forming a nip therewith to heat and pressurize the recording material, and heat generating means for increasing the temperature of the above described rotating heating member.

(7) Preferably, the above described rotating heating member is a cylindrical film.

(8) Preferably, the above described rotating heating member is driven by being slaved to the rotating pressure member.

(9) Preferably, the above described rotating heating member has a conductive member, and the heating means for heating the above described rotating heating member is magnetic field generating means including an exciting coil, which has an alternating magnetic field from the above described magnetic field generating means act upon the above described conductive member to generate an eddy current so as to cause the above described rotating heating member to generate heat.

(10) The fixing apparatus according to the present invention for heating and fixing the unfixed toner image on the recording material introduced from the image forming means comprises the temperature sensing means for sensing the temperature of the above described fixing apparatus and the power controlling means for controlling the power supplied to the above described fixing apparatus so that the above described fixing apparatus keeps the fixable temperature at least on fixing operation based on the output from the above described temperature sensing means, wherein the above described power controlling means controls the power supply to the fixing apparatus based on the above described output from the temperature sensing element during the time from after a print start to performing the heating and fixing process on the recording material so that, in the case where the temperature of the above described fixing apparatus rises fast, a temperature control operation for keeping a fixable temperature should not be protracted before the heating and fixing.

(11) Preferably, the above described power controlling means controls the power supply to the fixing apparatus by performing the low temperature control step for controlling the temperature of the fixing apparatus at a temperature lower than the fixable temperature or the non-heating step for heating no heating and fixing means during the time from after receipt of the print signal by the image forming apparatus to performing the heating and fixing process.

(12) Preferably, the temperature of the fixing apparatus is increased more than once by sandwiching the above described low temperature control step or the above described non-heating step during the time from after the receipt of the print signal by the image forming apparatus to before performing the heating and fixing process, and at least by the temperature rise lastly performed thereof, it prepares for the heating and fixing process of the recording material by rendering the target temperature as a fixable temperature.

(13) Preferably, the temperature of the fixing means is increased once after the receipt of the print signal by the image forming means so as to determine performance time of the above described low temperature control step or the above described non-heating step by this temperature rise behavior.

(14) Preferably, the temperature of the above described fixing apparatus is increased once to the fixable temperature or a lower temperature than that after the receipt of the print signal by the image forming apparatus.

(15) Preferably, the fixing apparatus is comprised of the rotating heating member capable of rotation and heating the recording material, the rotating pressure member for forming the nip therewith to heat and pressurize the recording material, and the heat generating means for increasing the temperature of the above described rotating heating member.

(16) Preferably, the above described rotating heating member is the cylindrical film.

(17) Preferably, the above described rotating heating member is driven by being slaved to the rotating pressure member.

(18) Preferably, the above described rotating heating member has the conductive member, and the above described heat generating means is the magnetic field generating means including the exciting coil, which has the alternating magnetic field from the above described magnetic field generating means act upon the above described conductive member to generate the eddy current so as to cause the above described rotating heating member to generate heat.

(19) Preferably, it has a first sequence group for sequentially operating at least following the receipt of the print signal by the image forming apparatus, and a second sequence group for determining timing of starting the operation according to a sensed temperature of the fixing apparatus after a predetermined time from the receipt of the print signal by the image forming apparatus.

(20) Preferably, the above described first sequence group at least includes control related to the temperature control of the heating and fixing means.

(21) Preferably, the above described second sequence group at least includes the control related to rotation of a development roller, the rotation of a photosensitive drum or application of a charging bias.

(22) Preferably, after the above described second sequence group starts the operation, the above described first sequence group operates by rendering criteria of the above described second sequence group as their new criteria.

(23) Preferably, the above described image forming means is a color image forming apparatus for forming an image by performing charging, exposure and development more than once.

According to the present invention, in a temperature starting step of the heating and fixing means (fixing apparatus) on the start of printing, the above described power controlling means controls the power supply to the above described heating and fixing means to control an excessive temperature rise of the pressure member based on the output from the temperature sensing element so that, in the case where the temperature of the above described heating and fixing means rises fast, a temperature control operation for keeping a fixable temperature should not be protracted before the heating and fixing, and the recording material is thereby prevented from slipping.

Accordingly, it is possible to stably carry the recording material on the fixing apparatus. In addition, it is also possible to have energy conservation effects such as reduction in power consumption and a decreased temperature rise in the machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, embodiments of the present invention will be described.

(1) Image Forming Apparatus

FIG. 1is a configuration schematic view of an example of an image forming apparatus. The image forming apparatus according to this embodiment is a color laser printer.

Reference numeral101denotes a photosensitive drum (image bearing member) made of an organic photosensitive member or an amorphous silicon photosensitive member, which is driven to rotate counterclockwise as indicated by an arrow at a predetermined carriage speed (peripheral velocity). The photosensitive drum101undergoes a uniform charging process of predetermined polarity and electric potential on a charging apparatus102such as a charging roller in the course of its rotation.

Next, a charging-processed surface thereof undergoes a scanning exposure process of target image information with a laser beam103outputted from a laser optical box (laser scanner)110. The laser optical box110outputs the laser beam103modulated (on/off) according to a time series electric digital pixel signal of the target image information from an unshown image signal generating apparatus such as an image reading apparatus, and an electrostatic latent image according to the target image information scanned and exposed on the photosensitive drum101surface is formed. Reference numeral109denotes a mirror for deflecting an output laser beam from the laser optical box110to an exposure position of the photosensitive drum101.

In the case of full color image formation, scanning exposure and latent image formation are performed as to a first color separation component image in a target full color image such as a yellow component image, and the latent image thereof is developed as a yellow toner image by the operation of an yellow developing device104Y of a four-color developing apparatus104. The yellow toner image is transferred to the surface of an intermediate transfer drum105in a primary transfer part T1which is a contact portion (or a proximity portion) of the photosensitive drum101and the intermediate transfer drum105. The surface of the photosensitive drum101after transferring the toner image to the intermediate transfer drum105is cleaned by a cleaner107by removing a sticking residue such as the toner remaining after transferring.

The above process cycle of charging, scanning exposure, development, primary transfer and cleaning is sequentially performed as to a second color separation component image (such as magenta component image, operation of a magenta developing device104M), a third color separation component image (such as cyan component image, operation of a cyan developing device104C), and a fourth color separation component image (such as black component image, operation of a black developing device104Bk) of the target full color image, and the four-color toner images of yellow, magenta, cyan and black toner images are sequentially transferred in superimposition to the surface of the intermediate transfer drum105so as to synthesize and form a color toner image in compliance with the target full color image.

The intermediate transfer drum105has a resilient layer of intermediate resistance and a surface layer of high resistance provided on a metallic drum, and is driven to rotate clockwise as indicated by an arrow at the same peripheral velocity as the photosensitive drum101while contacting or in proximity to the photosensitive drum101so that a bias potential is given to the metallic drum of the intermediate transfer drum105to transfer the toner image on the photosensitive drum101side to the above described intermediate transfer drum105side by means of a potential difference from the photosensitive drum101.

The color toner image formed on the surface of the above intermediate transfer drum105is transferred on the surface of a recording material P fed into a secondary transferring part T2from an unshown paper feed part in predetermined timing, the above described secondary transferring part T2being a contact nip portion of the above described intermediate transfer drum105and a transferring roller106. The transferring roller106sequentially transfers synthetic color toner images by one operation from the surface side of the intermediate transfer drum105to the recording material P side by supplying a charge of a polarity inverse to the toner from the back of the recording material P.

The recording material P having passed through the secondary transferring part T2is separated from the surface of the intermediate transfer drum105to be introduced to an image heating apparatus (fixing apparatus)100, where an unfixed toner image undergoes a heating and fixing process to become a fixed toner image, and is ejected to an unshown output tray outside the machine. The fixing apparatus100will be described in detail in the next section (2).

The intermediate transfer drum105after transferring the color toner images to the recording material P is cleaned by a cleaner108by having the sticking residue such as the toner remaining after transferring and paper powder removed. The cleaner108is ordinarily held in a non-contact state by the intermediate transfer drum105, and is held in a contact state by the intermediate transfer drum105in an implementation process of secondary transferring of the color toner images from the intermediate transfer drum105to the recording material P.

In addition, the transferring roller106is also ordinarily held in the non-contact state by the intermediate transfer drum105, and is held in the contact state by the intermediate transfer drum105via the recording material P in the implementation process of the secondary transferring of the color toner images from the intermediate transfer drum105to the recording material P.

Next, the fixing apparatus100provided to the above-mentioned image forming apparatus will be described.

The fixing apparatus100according to this embodiment adopts a film heating method using an electromagnetic induction heating method.FIGS. 2to5are the drawings showing a configuration of a major portion of the fixing apparatus100according to this embodiment, whereFIG. 2is a sectional model view of the side,FIG. 3is a front model view seen from direction A ofFIG. 2,FIG. 4is a sectional model view along a line IV—IV ofFIG. 2, andFIG. 5is a perspective model view showing the section along a line V—V ofFIG. 2(fixing film not shown) respectively. Hereafter, the fixing apparatus100according to this embodiment will be described by using the drawings.

InFIG. 2, film guides16aand16bhave a shape of approximately half-circular gutter in section, forming an approximate cylinder by mutually facing opening sides. A cylindrical fixing film10is loosely fitted to the rim surface side of the film guides16aand16b.

Magnetic field generating means is comprised of magnetic cores17a,17band17c, exciting coils18and an excitation circuit27(see FIG.7). The magnetic cores17a,17band17care placed like a letter T inside the film guide16a. The exciting coils18are held in a space surrounded by the magnetic cores17aand17cand the film guide16aand in a space surrounded by the magnetic cores17aand17band the film guide16a.

The magnetic cores17a,17band17care members of high permeability, desirably the materials used for the core of a transformer such as ferrite and permalloy, and the ferrite of which loss of magnetism over 100 kHz is little is preferably used.

As shown inFIG. 5, the exciting coils18have feeding parts18aand18b, and are connected to the excitation circuit27by the feeding parts18aand18b. The excitation circuit27is capable of generating high frequencies of 200 kHz to 500 kHz with a switching power supply. The exciting coils18generate an alternating magnetic flux with an alternating current (high frequency current) supplied from the excitation circuit27.

The fixing film temperature is controlled by a temperature control system including a temperature sensor26so as to keep a predetermined temperature by having current supply to the exciting coils18controlled. The temperature sensor26is a temperature sensing element such as a thermistor. To be more specific, fixing film sensing temperature information from the temperature sensor26is inputted to a control circuit200, and the control circuit200controls the power supplied from the excitation circuit27to the exciting coils18so as to have input temperature information from the temperature sensor26kept at a predetermined fixing temperature.

The film guides16aand16bpressurize a fixing nip part N, support the exciting coils18and the magnetic cores17as the magnetic field generating means, support the fixing film10, and stabilize carriage of the fixing film10when rotating. For the film guides16aand16b, a material capable of insulation not hindering passage of the magnetic flux and bearing a high load is used. As for such a material, a polyimide resin, a polyamide resin, a polyamide-imide resin, a polyether-ketone resin, a polyether-sulfon resin, a polyphenylene-sulfite resin, a liquid crystal polymer and so on can be named for instance.

As shown inFIG. 2, on the film guide16b, a slide member40longitudinal in a paper space vertical direction is placed inside the fixing film10on a surface side opposite a pressure roller30of the fixing nip part N. To be more specific, the slide member40is placed at a position opposite the above described pressure roller30via the fixing film10in the fixing nip part N. The slide member40is a member for supporting the fixing film10from its inner circular surface against pressurization of the pressure roller30in the fixing nip part N.

As for the slide member40, a member of good sliding ability is desirable in order to decrease slide resistance. For such a member, fluorine resin, glass, boron nitride, graphite and so on can be named. It is further desirable that the slide member40is a member of high thermal conductivity in addition to the sliding ability. Such a slide member40has an effect of rendering longitudinal temperature distribution even. For instance, in the case of putting a small-size sheet of paper through, an amount of heat of a non-paper-through part in the fixing film10is transmitted to the slide member40, and the amount is transmitted to a small-size paper-through part by longitudinal thermal transmission of the slide member40. It is also possible to thereby obtain an effect of reducing power consumption when putting the small-size sheet of paper through. For such a slide member40, a composite material such as a mirror-polished metal such as aluminum or a metal having fluorine resin particles, boron nitride particles, graphite particles or the like dispersed can be named. In addition, a member of two-layer configuration wherein a member of high thermal transmission is coated with a member of good sliding ability, such as aluminum nitride coated with glass may also be used. In this embodiment, an alumina substrate coated with glass is used.

In the case where the slide member40is conductive, it is desirable to place it outside a magnetic field generated from the exciting coils18and the magnetic cores17a,17band17cwhich are the magnetic field generating means in order not to be affected thereby. To be more specific, the slide member40should be placed at a position distant from the magnetic core17bagainst the exciting coils18so as to be placed outside a magnetic path made by the exciting coils18.

In order to further reduce a slide frictional force of the slide member40and the fixing film10in the fixing nip part N, it is also possible to place a lubricant such as a heat-resistant grease between the slide member40and the fixing film10. Application of the lubricant allows further reduction in slide resistance and longer life of the apparatus.

An internal plane part of the film guide16bhas in contact a rigid stay for pressurization22having a horizontally long horseshoe sectional shape. In addition, an insulating member19is provided between the rigid stay for pressurization22and each of the magnetic cores17for the purpose of insulating them.

Moreover, flange members23aand23b(seeFIG. 3) are fitted to the outside of both the right and left ends of assembly of the film guides16aand16b, and are rotatably mounted while fixing the above described right and left positions. The flange members23receive an end portion of the fixing film10when rotating and regulate a longitudinal approach motion of the film guides16.

The pressure roller30as the rotating pressure member is comprised of a core bar30aand a heat-resistant resilient material layer30bsuch as silicone rubber, fluorine rubber or fluorine resin, concentrically and integrally formed and coated around the above described core bar in a state of a roller. The pressure roller30is mounted by having both end portions of the core bar30aheld by bearings rotatably between chassis-side sheet metals (not shown) of the fixing apparatus.

InFIG. 3, pressure springs25aand25bare mounted in a pressed state between both the end portions of the rigid stay for pressurization22and spring bracket members29aand29bon the apparatus chassis (not shown) side respectively, so that a depressing force is applied to the rigid stay for pressurization22. Thus, the downside of the slide member40provided to the film guide16band the topside of the pressure roller30come into contact due to pressure, sandwiching the fixing film10so that the fixing nip part N of a predetermined width is formed.

The pressure roller30is driven by a driving means M to rotate counterclockwise as indicated by an arrow a in the drawing. The rotation drive of the pressure roller30generates frictional force between the pressure roller30and an outer surface of the fixing film10so that a torque acts upon the fixing film10. And the fixing film10rotates around the rims of the film guides16aand16bclockwise as indicated by the arrow b in the drawing at the peripheral velocity approximately equal to that of the pressure roller30while sliding with its internal circular face kept in intimate contact with the downside of the slide member40in the fixing nip part N. To be more specific, the fixing film10is rotated in synchronization with the pressure roller30by surface frictional force exerted with the pressure roller.

As shown inFIG. 5, on a rim surface of the film guide16a, a plurality of convex rib parts16eare formed longitudinally with predetermined intervals. A contact slide resistance between the rim surface of the film guide16aand an internal surface of the fixing film10is thereby reduced so as to decrease a rotation load of the fixing film10. Such convex rib parts can be formed and provided likewise to the film guide16b.

FIG. 6schematically represents how the alternating magnetic flux is generated by the magnetic field generating means.

A magnetic flux C represents a part of the generated alternating magnetic flux. The magnetic flux C led by the magnetic cores17a,17band17cgenerates the eddy current in a heat generating layer10aof the fixing film10between the magnetic cores17aand17band between the magnetic cores17aand17c. The eddy current has Joule heat (eddy current loss) generated in the heat generating layer10adue to specific resistance of the heat generating layer10a.

An amount of heat Q is determined by a density of the magnetic flux C passing through the heat generating layer10a, and shows distribution as in the graph in FIG.6. In the graph shown inFIG. 6, the vertical axis indicates the position of a circumferential direction in the fixing film10represented by an angle θ with the center of the magnetic core17aas 0, and the horizontal axis indicates the amount of generated heat Q in the heat generating layer10aof the fixing film10. Here, it is defined that a heat generating area H is the area of which maximum amount of the generated heat is Q, and amount of generated heat is Q/e or larger (e is a base of natural logarithm). This is the area capable of obtaining the amount of generated heat necessary for a fixing process.

As described above, the exciting coils18are fed by the excitation circuit27so that the fixing film10performs electromagnetic induction heating and rises to the predetermined temperature. And in a state of being controlled at the predetermined temperature, the recording material P having an unfixed toner tn image carried from the image forming means part formed thereon is introduced between the fixing film10and the pressure roller30so as to have an image surface opposite the fixing film surface. And in the process of having the recording material P supported and carried together with the fixing film10in the fixing nip part N, the unfixed toner tn on the recording material P is heated and fixed. After passing through the fixing nip part N, the unfixed toner tn is cooled to become a fixed toner tn′.

As the toner containing a low softening substance is used as the toner tn in this embodiment, an oil application mechanism for preventing an offset is not provided to the fixing apparatus100. In the case of using the toner containing no low softening substance, the oil application mechanism may be provided. In addition, oil application and cooling separation may be performed even in the case of using the toner containing the low softening substance.

A thermo switch50which is the temperature sensing element for interrupting feeding to the exciting coils18on a thermorunaway of the fixing apparatus is placed with no contact at a position opposite to the heat generating area H (seeFIG. 6) on an outer surface of the fixing film10. Distance between the thermo switch50and the fixing film10is approximately 2 mm. Thus, the fixing film10will not have a flaw due to contact with the thermo switch50, and so it is possible to prevent deterioration of the fixed image due to enduring use thereof.

FIG. 7is a circuit diagram of a thermorunaway preventing circuit used in this embodiment. The thermo switch50is built into this thermorunaway preventing circuit. The thermo switch50is serially connected to a 24V DC power supply and a relay switch70. If the thermo switch50is turned off, the feeding to the relay switch70is interrupted, and the relay switch70operates to interrupt the feeding to the excitation circuit27so as to interrupt the feeding to the exciting coils18.

According to this embodiment, on the thermorunaway of the fixing apparatus100due to a failure of the temperature control, the fixing apparatus100stops in a state of having the recording material P caught in the fixing nip part N, and even if the feeding to the exciting coils18is continued and the fixing film10keeps on generating heat, no heat is generated in the fixing nip part N with the recording material P caught, and so the recording material P will not be directly heated, which is different from the configuration wherein the heat is generated in the fixing nip part N. In addition, the thermo switch50is placed in the heat generating area H having a large amount of generated heat, so that the relay switch70operates to interrupt the feeding to the exciting coils18at a point in time when the thermo switch50senses an abnormal rise in temperature and becomes open. According to this embodiment, no paper gets ignited since ignition temperature of the paper is around 400 degrees, and thus heat generation of the fixing film10can be stopped. A thermal fuse may also be used in addition to the thermo switch.

Hereafter, each of the members used for the above-mentioned fixing apparatus (heating apparatus) will be described.

The exciting coils18constituting the magnetic field generating means use a bundle of a plurality of thin lines made of copper insulated and coated one by one as a conductor (electric wire) constituting a coil (line ring), which is wound more than once so as to form the exciting coils.

As for the coating member for performing insulating coating, it is desirable to use a heat-resistant coating in consideration of the heat transmission by the heat generation of the fixing film10. For instance, it is preferable to use the coating of amide-imide, polyimide or the like. It is also feasible to pressurize the exciting coils18from the outside so as to improve density.

As inFIG. 2, the shape of the exciting coils18is formed along a curved surface of the fixing film10. In addition, the distance between the heat generating layer of the fixing film10and the exciting coils18is set to be approximately 2 mm.

As for the material of the insulating member19, the one having good insulation performance and high heat resistance is desirable. For instance, it is preferable to select phenol resin, fluorine resin, polyimide resin, polyamide resin, polyamide-imide resin, polyether-ketone resin, polyether-sulfon resin, polyphenylene-sulfite resin, PFA resin, PTFE resin, FEP resin, LCP resin and so on.

The distances between the magnetic cores17a,17b,17c/exciting coils18and the heat generating layer of the fixing film10should be as close as possible to render absorption efficiency of the magnetic flux higher. It is desirable if the distance is 5 mm or less since the fixing film can absorb the magnetic flux with high efficiency. It is not desirable for the distance to be larger than the above range since the absorption efficiency of the magnetic flux is remarkably reduced thereby. In addition, as far as the distance between the heat generating layer of the fixing film10and the exciting coils18is 5 mm or less, it is not necessary for the distance to be fixed.

Moreover, as for18aand18bdrawn out of the exciting coils18inFIG. 5, the insulating coating is performed on the outside of the bundled lines.

FIG. 8is a layer constitution model view of the fixing film10as the heating member in this embodiment. The fixing film10according to this embodiment has a complex configuration of the heat generating layer10aas a base layer comprised of an electromagnetic induction heating metallic film or the like, a resilient layer10blaminated on the outer surface thereof, and a mold release layer10claminated on the outer surface thereof. It is also possible to provide primer layers (not shown) among the layers for the purpose of adhesion between the heat generating layer10aand resilient layer10band adhesion between the resilient layer10band mold release layer10c. Moreover, in the approximately cylinder-shaped fixing film10inFIG. 8, the heat generating layer10ais inside for contacting the slide member40, and the mold release layer10cis outside for contacting the pressure roller or the recording material (heating material).

As previously mentioned, the alternating magnetic flux acts upon the heat generating layer10ato generate the eddy current therein so that the heat generating layer10agenerates heat. The heat is transmitted to the resilient layer10band mold release layer10cto heat the entire fixing film so that the recording material P put through the fixing nip part N is heated and the toner image is heated and fixed.

While a magnetic or non-magnetic metal may be used for the heat generating layer10a, the magnetic metal is preferably used. As for such a magnetic metal, a ferromagnetic metal such as nickel, iron, ferromagnetic stainless, nickel-cobalt alloy or permalloy is preferably used. In addition, it is also desirable to use a member wherein manganese is added to the nickel in order to prevent metal fatigue caused by repeated curvature stress received on the rotation of the fixing film10.

As for thickness of the heat generating layer10a, it should preferably be thicker than a skin depth σ(m) represented by the following equation and 200 μm or less. If the thickness of the heat generating layer10ais in this range, the heat generating layer10acan efficiently absorb an electromagnetic wave so that the heat can be efficiently generated.
σ=(ρ/πfμ)1/2(1)

Here, f is a frequency (Hz) of the excitation circuit, μ is permeability of the heat generating layer10a, and ρ is a specific resistance (Ωm) of the heat generating layer10a.

The skin depth σ indicates the depth of the absorption of the electromagnetic wave used for electromagnetic induction, and the intensity of the electromagnetic wave at a location deeper than that is 1/e or less. To put it inversely, most of the energy is absorbed to this depth (see the relationship between the heat generating layer depth and the electromagnetic wave intensity shown in FIG.10).

The thickness of the heat generating layer10ashould more preferably be 1 to 100 μm. In the case where the thickness of the heat generating layer10ais thinner than the above range, it will be less efficient since most of the electromagnetic energy cannot be absorbed. In addition, in the case where the heat generating layer10ais thicker than the above range, rigidity of the heat generating layer10abecomes too high, and the curvature becomes deteriorated so that it will not be realistic to use it as a rotating member.

For the resilient layer10b, a material of high heat resistance and high thermal conductivity such as silicone rubber, fluorine rubber or fluoro-silicone rubber is preferably used.

The thickness of the resilient layer10bshould preferably be 10 to 500 μm in order to assure quality of the fixed image. In the case of printing the color image, and in particular a photographic image, a solid image is formed over large area on the recording material P. In this case, unevenness in heating arises if the heated surface (mold release layer10c) cannot follow projections and depressions on the recording material P or those on the unfixed toner tn, and unevenness in gloss arises between the portions of large and small amounts of transmitted heat. To be more specific, glossiness is high in the portion of large amount of transmitted heat, and it is low in the portion of small amount thereof. In the case where the thickness of the resilient layer10bis smaller than the above range, the above mold release layer10ccannot follow the projections and depressions of the recording material P or the unfixed toner tn so that image gloss unevenness arises. In addition, in the case where the resilient layer10bis excessively larger than the above range, the heat resistance of the resilient layer is too high such that it is difficult to implement a quick start. The thickness of the resilient layer10bshould more preferably be 50 to 500 μm.

If hardness of the resilient layer10bis too high, it cannot follow the projections and depressions of the recording material P or the unfixed toner tn so that image gloss unevenness arises. Thus, the hardness of the resilient layer10bshould be 60 degrees (JIS-A) or less, and more preferably 45 degrees (JIS-A) or less.

Thermal conductivity λ of the resilient layer10bshould preferably be 2.5×10−1to 8.4×10−1W/m·° C. In the case where the thermal conductivity λ is smaller than the above range, the heat resistance is too large such that the rise in temperature in the surface layer (mold release layer10c) of the fixing apparatus10becomes slow. In the case where the thermal conductivity λ is larger than the above range, the hardness of the resilient layer10bbecomes too high or a compression set is apt to arise. It should more preferably be 3.3×10−1to 6.3×10−1W/m·° C.

For the mold release layer10c, a material of good mold releasability and high heat resistance such as fluorine resin, silicone resin, fluoro-silicone rubber, fluorine rubber, silicone rubber, PFA, PTFE or FEP should preferably be used.

The thickness of the mold release layer10cshould preferably be 1 to 100 μm. In the case where the thickness of the mold release layer10cis thinner than the above range, unevenness in painting of a coating film arises so that problems such as occurrence of a portion of low mold releasability and lack in endurability arise. In addition, in the case where the mold release layer is thicker than the above range, the thermal conductivity deteriorates. In particular, in the case of using a resin material for the mold release layer10c, the hardness of the mold release layer10cbecomes so high that the resilient layer10bis no longer effective.

As shown inFIG. 9, it is also possible, in the fixing film10configuration, to provide an adiabatic layer10don the surface side of the heat generating layer10acontacting the slide member40. For the adiabatic layer10d, a 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 should preferably be used. In addition, the thickness of the adiabatic layer10dshould preferably be 10 to 1000 μm. In the case where the thickness of the adiabatic layer10dis thinner than 10 μm, no adiabatic effect is obtained and endurability is also insufficient. On the other hand, if it exceeds 1000 μm, the distance from the magnetic cores17a,17b,17c/exciting coils18to the heat generating layer10abecomes so large that the magnetic flux is no longer sufficiently absorbed by the heat generating layer10a. As the adiabatic layer10dcan insulate the heat generated in the heat generating layer10aso that the heat will not go inside the fixing film, efficiency of heat supply to the recording material P is better compared to the case of having no adiabatic layer10d. Thus, it is possible to control power consumption.

In addition, it is possible to alleviate the slide resistance between the slide member40and the fixing film10by constituting the adiabatic layer10dwith a material of good slidability.

(3) Starting Step

Hereafter, the temperature control in the temperature control starting step of the fixing apparatus100on the start of printing will be described. The control is implemented by a control circuit part200(FIG.2).

The control circuit200administers overall sequence of the image forming apparatus. And the control circuit200predicts the time required by the fixing apparatus100for the rise in temperature to the target temperature.

FIG. 11is a schematic view showing the fixing film temperature, setting of the target temperature of the temperature control, and timing of the recording material reaching the fixing apparatus in the starting step of the fixing apparatus according to this embodiment.FIG. 12is a flowchart of control sequence performed by the control circuit200.

Although the fixing apparatus according to this embodiment keeps the temperature control off to perform no preheating during standby for printings preheating may also be performed.

After receiving the print signal, the image forming apparatus starts the image forming operation. In a first temperature rising step, it starts the image forming operation and also starts power supply to the fixing apparatus at the same time. As for the timing of starting the first temperature rising step, it may be implemented after the receipt of the print signal, and is not limited to implementing it at the same time as the start of the image forming operation. The fixing apparatus starts to increase the temperature aiming at the target temperature, and in this embodiment, the target temperature of the first temperature rising step is the fixing temperature Tfto be used when fixing the toner on the recording material. And it measures time twurequired to increase the temperature to the fixing temperature Tffrom the start of the power supply to the fixing apparatus. Once it reaches the target temperature, the first temperature rising step is finished.

Next, it determines whether or not the non-heating step can be implemented and time for implementation thereof. This embodiment is characterized by predicting temperature rising time of the fixing apparatus in the second temperature rising step rather than that in the first temperature rising step.

According to this embodiment, the time twurequired to increase the temperature to the fixing temperature Tfin the first temperature rising step is measured by setting the target temperature in the first temperature rising step at the fixing temperature Tfas in the second temperature rising step. The temperature rising time twureflects elements related to the rise in the temperature of the fixing apparatus such as a surrounding ambient temperature, input voltage and a state of warming up of the fixing apparatus. The time required for the second temperature rising step is the temperature rising time twuin the first temperature rising step or less considering that the fixing apparatus is warmed up in the first temperature rising step. Thus, it is possible to assuredly increase the temperature to the fixing temperature Tfby securing the time twuas the time required for the second temperature rising step.

Whether or not the non-heating step to be performed after finishing the first temperature rising step can be implemented and the time for implementation toffare determined by the equation described hereafter.

If the time from the start of the first temperature rising step until fixing of the recording material is tp, the time required for the first temperature rising is twu, the time for performing the non-heating step is toff, the time allotted for the second temperature rising is twu, and spare time from starting the fixing temperature control step until entry of the recording material into the fixing nip is tα, the following relationship holds.
tp=twu+toff+twu+tα(1)

To implement the non-heating step, the following relationship must be fulfilled from equation (1).
toff=tp−(2twu+tα)>0  (2)

To be more specific, it is possible to implement the non-heating step if the temperature rising time twusatisfies the following.
twu<(tp−tα)/2  (3)

In the case where the temperature rising time twucannot satisfy equation (3), implementation of the non-heating step does not allow the second temperature rising step to be in time, and so it moves on to the fixing temperature control step without implementing the non-heating step.

On the other hand, in the case of implementing the non-heating step, the time for the non-heating step toffis the time calculated by the equation (2).

In the case where the starting step is repeated many times as with intermittent printing, the fixing apparatus is warmed up and the temperature rising time twubecomes shorter, it is possible to render the time for the non-heating step tofflonger. In addition, in the case where processing speed is slow and the time for carrying the recording material tpis long as when fixing an OHP film, it is also possible to render the time for the non-heating step tofflonger.

After finishing the first temperature rising step, the non-heating step is implemented. In this step, the power supply to the fixing apparatus is stopped, and the fixing apparatus is put in a non-heating state. The time for the non-heating step toffis the time from the timing of finishing the first temperature rising step until the timing of starting the second temperature rising step mentioned later. The longer the time for the non-heating step toffis, the more the temperature rising of the pressure roller can be controled. In addition, it is also possible to control the temperature rising inside the image forming apparatus and to reduce the power consumption.

After finishing the non-heating step, the second temperature rising step is implemented. The target temperature in the second temperature rising step is the fixing temperature Tf. The second temperature rising step has the previously measured temperature rising time tmvallotted thereto.

After finishing the second temperature rising step, the fixing temperature control step is implemented. In the fixing temperature control step, the spare time tα is provided as the time from starting the fixing temperature control step until entry of the recording material into the fixing nip. It is possible, during this time, to control overshooting of the temperature and control oscillation immediately after the rise in the temperature and also to fix the recording material after stabilizing the temperature of the fixing apparatus. Then, it keeps the fixing film at the fixing temperature Tfand fixes the unfixed toner image on the recording material after carrying the recording material to the fixing apparatus.

As it is possible, by the above-mentioned temperature control of the fixing apparatus, to control excessive temperature rising of the pressure roller in the starting step, it allows slipping of the recording material to be prevented, and it is also feasible to stabilize the carriage of the recording material and to render the fixed image of higher quality. In addition, it is also possible to have energy conservation effects such as reduction in power consumption and a decreased temperature rise in the machine.

Hereafter, the temperature control in the starting step of the fixing apparatus on the start of the printing according to a second embodiment will be described. The configurations of the image forming apparatus and the fixing apparatus are the same as those in the first embodiment.

FIG. 13is a schematic view showing the fixing film temperature, setting of the target temperature of the temperature control and the timing of the recording material reaching the fixing apparatus in the starting step of the fixing apparatus according to this embodiment.FIG. 14is a flowchart of a control sequence performed by the control circuit200.

As the temperature control in the first temperature rising step, the second temperature rising step and the fixing temperature control step is the same as those in the first embodiment during standby for printing, description thereof will be omitted.

This embodiment is characterized by providing the low temperature control step for controlling the target temperature at a temperature Tlowwhich is lower than the fixing temperature Tfinstead of providing the non-heating step for stopping the power supply to the fixing apparatus as in the first embodiment. It is the same as the non-heating step in the first embodiment as to whether or not the low temperature control step can be implemented and the method of calculating implementation time tlow. Thus, a minimum limit temperature of the fixing apparatus is assured even if the fixing apparatus is excessively cooled in the low temperature control step. Therefore, it is possible to securely complete the second temperature rising step within the predetermined time irrespective of fluctuation of the ambient temperature surrounding the image forming apparatus. To control the rise in the temperature of the pressure roller, it is preferable that the target temperature Tlowin the low temperature control step is low. In addition, although no preheating is performed during standby for printing in this embodiment, the target temperature Tlowin the low temperature control step may be the target temperature during the preheating in the case of the image forming apparatus and fixing apparatus for performing the preheating. It is possible, by the above-mentioned temperature control of the fixing apparatus, to control the excessive temperature rising of the pressure roller in the starting step.

Hereafter, the temperature control in the starting step of the fixing apparatus on the start of printing according to a third embodiment will be described. The configurations of the image forming apparatus and the fixing apparatus are the same as those in the first embodiment.

FIG. 15is a schematic view showing the fixing film temperature, setting of the target temperature of the temperature control and the timing of the recording material reaching the fixing apparatus in the starting step of the fixing apparatus according to this embodiment.FIG. 16is a flowchart of the control sequence performed by the control circuit200.

This embodiment is characterized by calculating the temperature rising time in the second temperature rising step by acquiring the temperature rising speed in the first temperature rising step. The temperature rising speed has the elements related to the temperature rise of the fixing apparatus such as the surrounding ambient temperature and input voltage reflected thereon.

First, after the receipt of the print signal, the first temperature rising step is implemented as in the first embodiment. In this embodiment, the target temperature in the first temperature rising step is set at a temperature Tprelower than the fixing temperature Tf. It is thereby possible to shorten the time for the first temperature rising step and to further control the temperature rise of the pressure roller.

After the fixing film temperature reaches Tpre, it measures time tprerequired for the temperature to rise from a temperature T1at the start of the first temperature rising step to Tpreand a temperature rising speed ΔT/Δt. And it determines whether or not the non-heating step can be implemented based on the temperature rising time tpreand the temperature rising speed ΔT/Δt according to the equation described below.

In addition to the temperature rising time tpre, if the time for implementing the non-heating step is toff, the temperature rising time until the fixing temperature calculated based on the temperature rising speed ΔT/Δt is tcalc, and the spare time from starting the fixing temperature control step until the entry of the recording material into the fixing nip is tα, the following relationship holds.
tp=tpre+toff+tcalc+tα(4)

To implement the non-heating step, the following must be fulfilled from equation (4).
toff=tp−(tpre+tcalc+tα)>0  (5)
Considering that the fixing film temperature Tprewhen finishing the first temperature rising step is equal to the fixing film temperature T2when starting the second temperature rising step, tcalcin the case of toff=0 is represented as follows.
tcalc=(Tf−Tpre)/(ΔT/Δt)  (6)

To be more specific, it is possible to implement the non-heating step if the temperature rising speed ΔT/Δt and the temperature rising time tpresatisfy the following from equations (5) and (6).
(Tf−Tpre)/(ΔT/Δt)+tpre<tp−tα(7)

In the case where the temperature rising speed ΔT/Δt and the temperature rising time tprecannot satisfy equation (7), the non-heating step is not implemented since there is no sufficient time before the entry of the recording material into the fixing nip. In this case, the target temperature is immediately switched from Tpreto the fixing temperature Tfto continue the rise in the temperature, and the fixing temperature control step is performed when it reaches the fixing temperature Tf.

In the case where the temperature rising speed ΔT/Δt and the temperature rising time tpresatisfy equation (7), the non-heating step is implemented after finishing the first temperature rising step. The time for the non-heating step toffis the time from the timing of finishing the first temperature rising step until the timing of starting the second temperature rising step mentioned later, and the length thereof is determined by the timing of starting the second temperature rising step. The timing of starting the second temperature rising step according to this embodiment is determined based on the temperature rising time tpreand the temperature rising speed ΔT/Δt in the first temperature rising step and the fixing film temperature T in the non-heating step.

The following relationship holds from equation (6) immediately before the non-heating step.
tcalc<tp−tpre−tα(8)

The following relationship holds immediately after starting the non-heating step considering that toffas a parameter to increase from 0 along with elapse of time for the non-heating step is added.
toff+tcalc<tp−tpre−tα(9)
However, toffis 0 at this point in time.

Next, the change of toffand tcalcduring the implementation of the non-heating step is considered. As toffon the left side of equation (9) is the time for the non-heating step, it increases from 0 along with the elapse of time. In addition, if the fixing film temperature when finishing the non-heating step is T, tcalcon the left side of equation (9) is calculated as follows.
tcalc=(Tf−T)/(ΔT/Δt)  (10)
As the fixing film temperature T becomes lower than Tprealong with the elapse of the time for the non-heating step, tcalcincreases from equation (10).

To be more specific, as the non-heating step proceeds, the left side of equation (9) comprised of the sum of the two terms of toffand tcalcincreases.

Thus, as for the timing of finishing the non-heating step, that is, the timing of starting the second temperature rising step, the change of the time for the non-heating step toffand the fixing film temperature T should be monitored, and it should be the timing wherein the two terms of toffand tcalcsatisfy the following equation for the first time.
toff+tcalc≦tp−tpre−tα(11)

If the fixing film temperature at this time is T2, tcalcmay be represented as follows.
tcalc=(Tf−T2)/(ΔT/Δt)  (12)

After finishing the non-heating step, the second temperature rising step is implemented. The target temperature in the second temperature rising step is the fixing temperature Tf. The temperature rising time tcalccalculated according to equation (12) is allotted to the second temperature rising step.

After finishing the second temperature rising step, the fixing temperature control step is implemented. In the fixing temperature control step, the spare time tα is provided from starting the fixing temperature control step until the entry of the recording material into the fixing nip. This time is utilized to have overshooting of the fixing film temperature after the rise in the temperature and so on converge so that the fixing film temperature is stabilized. And the fixing film is kept at the fixing temperature Tf, and after carrying the recording material to the fixing apparatus, the unfixed toner image on the recording material is fixed.

The above-mentioned temperature control of the fixing apparatus can control the excessive rise in the temperature of the pressure roller in the starting step. In addition, it is also possible to have the energy conservation effects such as the reduction in power consumption and the decreased temperature rise in the machine.

Hereafter, an embodiment of the present invention will be described along the drawings.

First, the overall configuration of the image forming apparatus will be described by referring to FIG.17.

FIG. 17is a longitudinal section showing the overall configuration of a laser beam printer A as an embodiment of the image forming apparatus. The photosensitive drum101is driven by an unshown driving means to rotate in the direction of the arrow in the drawing. Surrounding the photosensitive drum101, there are the devices placed such as the charging apparatus102for evenly charging the surface of the photosensitive drum101according to the direction of the rotation thereof, a scanner unit110for irradiating a laser beam based on image information to form the electrostatic latent image on the photosensitive drum101, the developing apparatus104for sticking the toner on the electrostatic latent image and developing it as the toner image, the transferring roller106for transferring the toner image on the photosensitive drum101to the recording material P, and the cleaner107for removing the toner remaining on the surface of the photosensitive drum101after transferring.

Here, the photosensitive drum101, charging apparatus102, developing apparatus104and cleaner107are integrally rendered as a cartridge to form a process cartridge207.

The scanner unit110is placed approximately in a horizontal direction of the photosensitive drum101, and image light corresponding to an image signal by a laser diode (not shown) is irradiated on a polygon mirror209rotated at high speed by a scanner motor (not shown). It has a configuration wherein the image light reflected on the polygon mirror209selectively exposes the surface of the charged photosensitive drum101via an image formation lens210so as to form the electrostatic latent image.

As for the transferring roller106placed opposite the photosensitive drum101, a metallic core covered with an elastic member such as EPDM (ethylene-propylene-diene ternary copolymer), urethane rubber or NBR (nitrile butadiene rubber) adjusted to volume resistivity of 107to 1011Ω·cm or so may be used for instance. The transferring roller106has a bias of straight polarity applied thereto from an unshown power supply, and the toner image of negative polarity on the photosensitive drum101is transferred by an electric field due to this bias to the recording material P in contact with the photosensitive drum101.

A paper feeding part8feeds and carries the recording material P to the image forming part, and has a plurality of sheets of the recording material P stored in a paper feeding cassette211. When forming the image, a paper feeding roller212(half moon roller) and a pair of registration rollers213are driven to rotate according to the image forming operation, where one sheet of the recording material P in the paper feeding cassette211is separated and fed, and a tip of the recording material P bumps into the pair of registration rollers213and stops once, forms a loop and then is fed to the nip formed by the transferring roller106and the photosensitive drum101. Reference numeral224denotes a registration sensor, and the image formation is performed with reference to the point in time when the recording material passes here.

The fixing apparatus100is a quick-start fixing apparatus of the electromagnetic induction heating method for fixing the toner image transferred to the recording material P, comprised of the cylindrical fixing film10as a rotating member having the heat generating layer (conductive magnetic member) and the pressure roller30in pressurized contact therewith for giving heat and pressure to the recording material P. To be more specific, the recording material P having the toner image on the photosensitive drum101transferred thereto is carried by the cylindrical fixing film10and the pressure roller30when passing through the fixing apparatus100, and is also given the heat and pressure. Thus, the toner image of a plurality of colors is fixed on the surface of the recording material P. The fixed recording material P is ejected face down from an ejection part216to the outside of the apparatus proper by a pair of ejection rollers215.

The control circuit200as control means controls the entire operation of the image forming apparatus A including the temperature control of the fixing apparatus, and has a CPU217, an RAM (Random Access Memory)218and an ROM (Read Only Memory)219. The ROM219has a program for controlling the image forming apparatus and various types of data written thereto, and the RAM218is used for purposes such as storing the data taken in for controlling the image forming apparatus.

A process cartridge will be described in detail by referring toFIGS. 18 and 19.FIGS. 18 and 19show a main section and a perspective view of a process cartridge207storing the toner. The process cartridge207is divided into the photosensitive drum101, a photosensitive drum unit250having charging means and cleaning means, and a developing unit104having developing means for developing the electrostatic latent image on the photosensitive drum101. The photosensitive drum101is constituted, for instance, by applying an organic photoconductive layer (OPC photosensitive member) on a rim surface of an aluminum cylinder of 30 mm diameter.

The photosensitive drum unit250has the photosensitive drum101rotatably mounted on a cleaning frame body251via bearings231(231a,231b). The photosensitive drum101has the charging apparatus102for uniformly charging the surface thereof and a cleaning blade260for removing the toner remaining thereon placed on the rim thereof, and furthermore, the remaining toner removed from the surface thereof by the cleaning blade260is sequentially sent by a toner feeding mechanism252to a waste toner room253provided behind the cleaning frame body. And the driving force of an unshown drive motor is conveyed to one end of the back shown in the drawing so as to rotate the photosensitive drum101counterclockwise as shown according to the image forming operation.

The developing unit104is comprised of a developing roller240for rotating in the direction of the arrow in contact with the photosensitive drum101, a toner container241accommodating the toner and a developing frame body245. The developing roller240is rotatably supported by the developing frame body245via a bearing member, and has a toner supplying roller243for rotating in the arrow Z direction in contact with the developing roller240and a developing blade244placed on the rim thereof respectively. Furthermore, the toner container241has a toner carriage mechanism242for stirring the accommodated toner and carrying it to the toner supplying roller243provided therein.

And the developing unit104has a hanging configuration wherein, centering on support axes249provided to bearing members247,248mounted on both ends of the developing unit104respectively, the entire developing unit104is reciprocatively supported against the photosensitive drum unit250by a pin249a, and when in a state of the process cartridge207alone (not mounted on the printer proper), the developing unit104is always energized by a pressure spring254so as to have the developing roller240contact the photosensitive drum101with angular moment centering on the support axes249. Furthermore, the toner container241of the developing unit104has a rib246for, when creating clearance between the developing roller240and the photosensitive drum101, being in contact with clearance means (described later) of the printer A proper integrally provided thereto.

Description of the fixing apparatus will be omitted since it has the same configuration as the fixing apparatus100used in the first embodiment.

Next, an operating mechanism when mounting the process cartridge207on the printer proper A will be described in detail.

As previously described, the process cartridge207always has the developing roller240in contact with the photosensitive drum101when in a state of the process cartridge207alone as in FIG.18.

On the other hand, a cam220is placed on the deeper side in the inserting direction of the process cartridge207of the printer proper A, for the purpose of creating clearance between the developing roller240and the photosensitive drum101against energization of the developing unit104. The cam220is rotated by an unshown driving means, and lifts the rib246so that the developing roller240creates clearance from the photosensitive drum101or releases the lifting of the rib246so that the developing roller240contacts the photosensitive drum101. Normally, if the process cartridge is mounted on the printer proper, the cam220lifts the rib246so that the developing roller240creates clearance from the photosensitive drum101. Accordingly, even in the case where it is not used for a long time with the process cartridge207mounted, the developing roller240always keeps the clearance from the photosensitive drum101, and so it is possible to securely prevent permanent deformation of a roller layer caused by keeping the developing roller240in contact with the photosensitive drum101for a long period of time. The photosensitive drum101and the developing roller240of the process cartridge207mounted on the image forming apparatus proper A can be separately driven by unshown motors.

The image forming operation according to this embodiment will be described by using the schematic view ofFIG. 17, the timing chart of FIG.20and the flowchart of FIG.21.

If the printing operation is started by inputting the print signal to the image forming apparatus proper (Start, S0), the CPU217first starts the temperature control of the fixing apparatus100, rotation of the photosensitive drum101and rotation of the scanner110(Heat-on, S1). The developing roller240remains stopped at this time. Next, it starts application of the charging bias when predetermined time t_ch elapses after the photosensitive drum101started the rotation (Ch-on, S2). It is because there is a possibility of creating a memory on the photosensitive drum if the rotation of the photosensitive drum and application of the charging bias are performed at the same time.

Next, the CPU217determines whether or not the temperature T of the fixing apparatus100has reached a predetermined temperature Ts (S3). The predetermined temperature Ts is the temperature wherein continuing the temperature control as-is is expected to allow the temperature of the fixing apparatus100to reach the fixing temperature Tfbefore the recording material P reaches the fixing apparatus100even when the image forming apparatus is under a low temperature environment or when supplied power supply voltage is a lower limit value. Hereafter, the predetermined temperature Ts is called an assured risen temperature. As a matter of course, the assured risen temperature Ts is set to be lower than the fixing temperature Tf.

If the temperature T of the fixing apparatus100reached the assured risen temperature Ts, it starts the rotation of the developing roller240and application of a development bias when the predetermined time t_dev elapses after the start of the application of the charging bias (Ch-on) (Dev-on, S4). At this time, if the temperature T of the fixing apparatus has not reached the assured risen temperature Ts, it continues to monitor the temperature of the fixing apparatus100, and if Ts has been reached within t_dev, it waits until reaching t_dev (S5), and then starts the rotation of the developing roller240and application of the development bias.

If the temperature reaches the assured risen temperature Ts past t_dev, it starts the rotation of the developing roller240and application of the development bias at the time of reaching Ts. To be more specific, it delays the timing of the rotation of the developing roller240and application of the development bias to be past t_dev so as to protract the temperature rising time of the fixing apparatus.

Normally, if there is a sufficient distance of clearance between the photosensitive drum101and the developing roller240so that the developing roller240keeps the clearance, there is no possibility of the toner flying from the developing roller240to the photosensitive drum101even if the surface of the photosensitive drum101is not properly charged. However, it starts the rotation of the developing roller240and application of the development bias after the time t_dev when the photosensitive drum101is charged and becomes a normal electric potential in order to prevent the toner from flying even in the case where the distance of clearance becomes shorter for some reason. Accordingly, even if the temperature T of the fixing apparatus has already reached the assured risen temperature Ts within t_dev, it does not perform the rotation of the developing roller240and application of the development bias until t_dev, so that the printing operation of starting the rotation of the developing roller240and application of the development bias at the time of t_dev is the shortest printing time.

After t_dev, the developing roller240is put in contact with the photosensitive drum101with reference to Dev_on after the predetermined time (D_R-on, S6), and then the recording material P is picked up (P-pick, S7) so as to form the image (Print, S8).

In the case where a temperature rising state of the fixing apparatus is determined after picking up the recording material, there is a possibility of lowering printing accuracy when only extension of the temperature rising time of the fixing apparatus is performed by stopping the image forming operation once based on a determination that the temperature rising state thereof is insufficient. Therefore, the temperature rising state must be determined before picking up the recording material. In the case where it is determined immediately before picking the recording material up, however, the rotations of the photosensitive drum and the developing roller have already started, and if the pickup operation is to be held on standby until the fixing apparatus reaches the predetermined temperature because the temperature rising state thereof is insufficient, it means that the photosensitive drum and the developing roller keep on rotating during that time. As the life of the developing device is significantly affected by the number of rotations of the developing roller, it is desirable to keep that number at a necessary minimum. On the other hand, a surface potential of the photosensitive drum once charged does not attenuate unless a transferring bias is applied or exposure is performed, and so a discharge for charging does not continue to occur if only the charging bias is applied and it is rotating. Accordingly, there is no fear that the surface of the photosensitive drum is cut away and its life becomes shorter due to the discharge.

Thus, it is possible, by controlling the timing of the start of rotation of the developing roller240and application of the development bias according to the temperature rising state of the fixing apparatus100as in this embodiment, to securely increase the temperature of the fixing apparatus without shortening the life of the developing device even when the image forming apparatus is under the low temperature environment or when the supplied power supply voltage is reduced to the lower limit value.

Hereafter, the temperature control in the starting step of the fixing apparatus on the start of the printing in the fourth embodiment will be described.FIG. 22is a flowchart of the control sequence performed by the control circuit200.

After the receipt of the print signal (S10), the image forming apparatus performs the power supply to the fixing apparatus (S11), and starts the first temperature rising step. As for the timing of starting the first temperature rising step, it may be implemented after the receipt of the print signal, and is not limited to implementing it at the same time as the start of the image forming operation. The fixing apparatus starts to increase the temperature aiming at the target temperature, and in this embodiment, the target temperature of the first temperature rising step is the fixing temperature Tfto be used when fixing the toner on the recording material.

Next, it is checked whether or not the fixing film temperature T has reached the assured risen temperature Ts(<fixing temperature Tf) described in the section “Printing Operation” (S12).

In the case where the fixing film temperature T is lower than the assured risen temperature Ts, it is checked whether or not temperature rising time t from the start of the power supply at the fixing film temperature T is shorter than t_ch+t_dev (S20). t_ch+t_dev is the shortest time from the start of the power supply to the fixing apparatus to the timing of the rotation of the developing roller and application of the development bias.

In the case where the temperature rising time t exceeds t_ch+t_dev, as described in the section “Printing Operation”, the temperature rising time of the fixing apparatus is extended until the fixing film temperature T reaches the assured risen temperature Tsby delaying operation timing of development-related sequences such as the rotation of the developing roller and application of the development bias (S21,22). To be more specific, the control exerted here extends the temperature rising time by delaying the sequences related to the image formation during the time until the fixing apparatus reaches the assured risen temperature Ts in the case where it is determined that the temperature rising of the fixing apparatus is slow. The case where the temperature rising time t exceeds t_ch+t_dev is a situation where the rise in the temperature of the fixing apparatus cannot be in time for the fixing process of the recording material unless the operation timing of the development-related sequences is delayed as described in the section “Printing Operation”, and so it is not possible, as a matter of course, to secure the time for performing the non-heating step as mentioned in the first embodiment. Thus, according to this embodiment, it does not proceed to the steps (S13to18) of determining the implementation of the non-heating step in this case, but it increases the fixing temperature Tfas the target temperature as-is so as to prepare for the fixing process of the recording material (S19).

In the case where the fixing film temperature T reaches Tsin a state where the temperature rising time t is t_ch+t_dev or lower, it is increased as-is targeting the fixing temperature Tf(S13). Thereafter, it proceeds to the step of determining whether or not the non-heating step can be implemented, but the operation thereafter (S13to18) including this step is the same as the starting step described in the first embodiment and so the description thereof will be omitted. In addition, the operation thereafter is not limited to the temperature control of the starting step described in the first embodiment, but it may also be the temperature control of the second or third embodiment.

Moreover, it was described that it does not proceed to the steps (S13to18) of determining the implementation of the non-heating step in the case where the temperature rising time t exceeds t_ch+t_dev. However, even if it proceeds to the steps of determining the implementation of the non-heating step, the non-heating step will hardly be implemented because the time twufor increasing the temperature to the fixing temperature Tfis longer than usual. Thus, in the case where the temperature rising time t exceeds t_ch+t_dev, it may proceed to the steps of determining the implementation of the non-heating step.

As described above, it is possible, by performing adequate printing operation and temperature control operation according to the temperature rising speed of the fixing apparatus, to constantly and stably supply the fixed image of high quality even if environmental conditions under which the image forming apparatus and the fixing apparatus are placed change and the temperature rising speed of the fixing apparatus changes.

Moreover, the image forming apparatus related to the present invention is not limited to the above-mentioned embodiments, but it is changeable in various ways within the outline thereof. To be more specific, while the photosensitive drum and the developing roller of the process cartridge were driven by separate motors in the above embodiments, it is also possible to use a method of dividing the drive by utilizing a gear and a clutch from one motor. In addition, another method such as using the cam instead of the clearance plate may also be used. Moreover, the timing of starting the fixing apparatus, photosensitive drum and scanner and the timing of contacting the developing roller and picking up the recording material may be different from the above order. While the scanner of an image scanning method was used in the above embodiments, it is of course possible to use an exposure apparatus employing an LED array. In that case, the starting operation as that of the scanner is not required, and so the timing of starting is different from that of the scanner. Furthermore, the present invention is also applicable to a color image forming apparatus having a plurality of photosensitive drums and development mechanisms.

1) Although the apparatus of the film heating method using the electromagnetic induction heating method is adopted as the fixing apparatus in the embodiments, the fixing apparatus according to the present invention is not limited thereto. It may also be the apparatus of the film heating method using a ceramic heater as the heat generating means. It may also be the apparatus of the heat roller method.

2) There is no restriction as to a formation principle/process of the unfixed toner image against the recording material of the image forming apparatus, and it is arbitrary. It may be either a transferring method or a direct method.