JPH0769652B2 - Fixing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is provided in an image forming apparatus such as an electrophotographic copying machine, and heat-fixes a heat-meltable toner image formed on a transfer material. It relates to the device.

[Conventional technology]

Conventionally, a fixing device used in this type of device is configured to generate an unfixed toner image by a heating roller that is maintained at a predetermined temperature and a pressure roller that has an elastic layer and is in pressure contact with the heating roller. A roller fixing method in which the formed transfer material is heated while being nipped and conveyed is often used. However, in this type of device, in order to prevent the so-called offset phenomenon in which the toner is transferred to the heating roller, it is necessary to maintain the heating roller at an optimum temperature, and the heat capacity of the heating roller or the heating body must be increased. There wasn't. That is,
When the heat capacity of the heating roller is small, the temperature of the heating roller is likely to fluctuate greatly toward the low temperature side or the high temperature side due to paper passing or other external factors due to the relationship with the amount of heat supplied by the heating element. When it fluctuates to the low temperature side, insufficient softening and melting of the toner causes fixing failure and low temperature offset, and when it fluctuates to the high temperature side, the toner is completely melted and the cohesive force of the toner decreases. , Causing high temperature offset.

If the heat capacity of the heating roller is increased in order to avoid such a problem, another problem occurs in that the time for raising the temperature of the heating roller to a predetermined temperature becomes long and the standby time becomes long when the device is used. .

As a measure for solving such a problem, US Pat.
As disclosed in No. 7, a toner image is heated to its melting point by a heating element to be melted, and after melting, the toner is cooled to have a relatively high viscosity, and the toner adhesion tendency is weakened. Peel from the heating element web.

There is known a method of fixing without causing offset by going through the process.

In addition to this, the above-mentioned known method employs a method of heating the toner image and the transfer material to the heating body without press-contacting them, so that it is not necessary to heat the transfer material, and compared with other methods. The toner can be melted with far less energy. However, as is well known, in the case of contact with a pressure body without press-contact, the heat transfer efficiency is lowered and it takes a relatively long time to heat and melt the toner.
Therefore, in Japanese Patent Application No. 47-25896, it is proposed that a known press-contacting technique be added to this to improve the heat transfer efficiency and to sufficiently heat and melt the toner in a short time.

[Problems to be Solved by the Invention]

However, in Japanese Patent Application No. 47-25896, in order to heat the toner in a relatively short time and sufficiently, the toner image and the transfer material are heated and sandwiched between a pair of heating bodies. Since the method of forcibly cooling after stopping the heating is adopted, there is a disadvantage that the energy required for fixing becomes large. That is, since the toner image is heated from above and below by being heated by the pair of heating members, it is considered to be efficient at first glance. It is necessary to do so, and for that reason a large amount of energy is needed. Further, in the cooling step, when the toner image is heated, the transfer material that has been heated and heated cannot be separated unless it is cooled, and a forced cooling means is required, resulting in a large waste of energy.

As described above, a method has been proposed in which the toner that has been heated once is cooled and then separated to fix the toner without causing high-temperature offset, but it has not been put into practical use because of the drawbacks described above.

In the above two proposals, the heating element is composed of a heating roller and a web fed by the heating roller and a heat source incorporated in the heating roller. Heating is performed through the web, and the heating roller functions as a web conveying roller. is doing. For this reason, the method of supplying power to the heat source and the form of contact and support of the temperature detecting element are complicated, and the temperature control accuracy tends to be poor. Furthermore, in the structure in which the temperature detecting element slides on the heating roller, there is a safety problem such as excessive temperature rise due to disconnection. In addition, in both of the above-mentioned two examples, since a heating body having a relatively large heat capacity is required, the heat radiation to the inside of the machine is increased, and the temperature rise inside the machine becomes remarkable.

The present invention solves the problems that the above-described conventional device has, and makes it possible to reduce the heat capacity of the heating element without causing defective fixing or offset, and as a result, standby time, power consumption, and further An object of the present invention is to provide a fixing device that can reduce the temperature rise inside the image forming apparatus.

[Means for Solving the Problems]

The gist to achieve the object of the present invention is to provide a heating element having a linear heating element extending along the width direction and a temperature detecting means for detecting the temperature in the vicinity of the heating portion, and the heating element pressed against the heating element. While the transfer member carrying the heat-meltable toner image is sandwiched between the rotating pressure roller and the toner image, the sheet surface member that moves at the same speed as the transfer member is brought into close contact with the transfer member. The heating element is energized by pulsed energization from the power feeding control means to generate heat, the toner image is melted through the sheet surface member in the heating section, and the sheet surface member is cooled and solidified on the downstream side in the moving direction from the heating section. The power supply control means changes the pulse width per cycle so that the maximum value of the heating portion temperature is within a predetermined range. In addition to controlling the amount, temporary energization is performed in a fixed cycle before starting the fixing operation, and the amount of energy per pulse to the heating element is determined from the detected temperature information, and the energization cycle during the fixing operation and during the temporary energization is different. The fixing device is characterized in that it is controlled as follows.

〔Example〕

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, the schematic structure of the image forming apparatus equipped with the fixing device of this embodiment will be described with reference to FIG. To scan. An array 2 of short focal length small image forming elements is arranged immediately below the document placing table, and a document image G placed on the document placing table 1 is illuminated by an illumination lamp 7, and a reflected light image thereof is the array 2 described above. Slit exposure is performed on the photosensitive drum 3. The photosensitive drum 3 rotates in the direction of arrow b. Reference numeral 4 denotes a charger, which uniformly charges the photosensitive drum 3 covered with, for example, a zinc oxide photosensitive layer or an organic semiconductor photosensitive layer. On the drum 3 uniformly charged by the charger 4, the element array 2 forms an electrostatic latent image which is image-exposed. This electrostatic latent image is visualized by the developing device 5 using a toner made of a resin or the like that is softened and melted by heating. On the other hand, the transfer material P stored in the cassette S is fed by the feeding roller 6 and a pair of conveying rollers 9 which are rotated in pressure contact with each other in the vertical direction in synchronization with the image on the photosensitive drum 3. , Sent onto the drum 3. Then, the transfer discharger 8 transfers the toner image formed on the photosensitive drum 3 onto the transfer material P. After that, the transfer material P separated from the drum 3 by the known separating means is guided to the fixing device 20 by the conveyance guide 10 and subjected to heat fixing processing, and then discharged onto the tray 11. After the toner image is transferred, the residual toner on the drum 3 is removed by the cleaner 12.

FIG. 2 is an enlarged view of the fixing device 20 of this embodiment. In the figure, reference numeral 21 is a heating element, which is made of, for example, Ta ₂ N on the lower surface of a heat-resistant and electrically insulating base material such as alumina or a base material made of a composite member containing the base material, and is conveyed to the heating portion. It has a linear or strip-shaped heating element 28 arranged with a component perpendicular to the direction, and Ta ₂ O ₅ , for example, is formed as a sliding protection layer on the surface thereof. The lower surface of the heating element 21 is smooth and the front and rear ends are rounded, and the heating portion (heating surface)
This makes it possible to slide on the fixing film 23 described later. The fixing film 23 is made of PET (polyester) as a base material and is heat-treated to have a thickness of, for example, about 6 μm.
It is wound around. The fixing film 23 comes into contact with the surface of the heating body 21 and is wound around the film winding shaft 27 via the separation roller 26 having a large curvature.

The heating element 28 of the heating element 21 has a small heat capacity, is energized in a pulse shape, and instantly raises the temperature to around 300 ° C. each time.
By detecting the front end and the rear end of the transfer material P moving on the transport guide 10 by the transfer paper detection lever 25 and the transfer paper detection sensor 29, the heating element 28 is energized at a necessary timing. At that time, the energization of the heating element may be controlled by detecting the position of the transfer sheet by a sheet feeding sensor of the image forming apparatus.

On the other hand, the pressure roller 22 has an elastic layer made of silicon rubber or the like on a core material made of metal or the like, and is driven by a drive source (not shown) and guided by the conveyance guide 10 to the unfixed state. The transfer material P having the toner image T is brought into close contact with the heating body via the fixing film 23 which moves at the same speed as the transfer material P. Here, the conveyance speed of the pressure roller 22 is preferably substantially the same as the conveyance speed at the time of image formation, and the movement speed of the fixing film 23 is set to a value corresponding to it.

In the present embodiment, since the heating element 28 instantly heats up, preheating is unnecessary, and heat transfer to the pressure roller during non-fixing is small. Further, even at the time of fixing, the fixing film, the toner image, and the transfer material are the heating element 28 and the pressure roller.
The temperature gradient can be made steep due to the fact that the pressure roller 22 is provided between the pressure roller 22 and the pressure roller 22 and the heat generation time is short. Even if the toner is formed, its temperature is maintained below the melting point of the toner.

In the fixing device of this embodiment having such a configuration, the toner image formed of the heat-fusible toner on the transfer material P is first heated and melted by the heating body 21 via the fixing film 23.
In particular, the surface layer portion greatly exceeds the melting point and is completely softened and melted. At this time, the heat roller, the fixing film, the toner image, and the transfer material are in good contact with each other by the pressure roller 22, and the heat is efficiently transferred.

After that, as the heat generation of the heat generating element 28 is stopped, the transfer material is conveyed, and as the transfer material moves away from the heat generating element position, the toner image radiates heat and cools and solidifies again, and passes through the separation roller 26 having a large curvature. After that, the fixing film 23 is separated from the transfer paper P. At that time, in this embodiment, the pressure roller 22
Since the temperature is maintained below the melting point of the toner,
It is possible to promote heat dissipation of the toner image. Therefore, the time required for cooling is short and the device can be downsized.

As described above, after the toner T is completely softened and melted,
Since the toner solidifies again, the cohesive force of the toner becomes very large, and the toner behaves as a group. Further, since it is pressed by the pressure roller 22 when it is heated and softened and melted, at least a part of the toner image T permeates the transfer material surface layer and is cooled and solidified as it is.
It is fixed on the transfer material P without being offset to 23.

A note will now be made regarding the toner state representations described herein.

The melting point of the toner and the temperature expressed for convenience means the minimum temperature required for the toner to be fixed, and at the fixing lower limit temperature, when the clay is lowered to the extent of melting,
There is a case where the clay is deteriorated such as softening. Therefore, even if it is expressed as melting when fixing, it may actually show a decrease in viscosity such as softening. Similarly, even when it is expressed for convenience that the toner is solidified by cooling, it may be considered that depending on the toner, it cannot be said that the toner is solidified and it is more appropriate to increase the viscosity.

FIG. 3 shows the structure of the heating element 21 of the fixing device of this embodiment. The heating body 21 forms a heat insulating layer 53 made of a low heat conductive heat resistant material such as bakelite on the lower surface of the base layer 54, and provides a thermistor 55 as a low heat capacity temperature detecting means on the lower surface thereof.
The electrodes 50,
A heating element 28 having a width 1 is formed as a heating layer between both electrodes 50, 50 in the vicinity of the thermistor 55, and a protective layer 51 is provided on the surface thereof. Then, the heating portion H is formed on the surface of the protective layer 51 of the heating element 28.

FIG. 4 shows the temperature of the heating portion H and the temperature detected by the thermistor 55 when the heating element 28 in the fixing device thus formed is energized in a pulsed manner via the electrodes 50, 50 to the heating element 28. The former temperature is based on a measurement value measured by an infrared radiation thermometer in a non-contact manner, and the latter temperature is based on a value obtained by converting the output power of the thermistor into temperature. When this graph is obtained, the pulse period is about 10 msec and the energization time is about 2 msec. The temperature of the heating part H rapidly rises during energization and then rapidly falls during rest, the non-energization time is sufficiently longer than the energization time, and the heat insulating layer 53 exists. , The temperature of the heating part H depends on the heating layer 28, the insulator 52, and the thermistor on the back surface thereof.
It becomes almost isothermal with 55. The thermistor 55 used in this embodiment is
It cannot follow a pulsed temperature change with a short cycle of 10 msec, and indicates a substantially minimum value of the pulse waveform. Therefore, the heating part H
The envelope curve of the minimum value of the surface temperature of is substantially coincident with the detection temperature curve of the thermistor 55.

By the way, when the energizing pulse width is fixed to a constant value τ ₀ and the fixing process is performed, the time change of the surface temperature of the heating portion H is as follows.
As shown in FIG. 8, the temperature of the heating portion H is initially the fixing temperature T.
_Although it is in the vicinity of _H0 , the amount of heat generated is constant despite the temperature rise around the heating element 28 and the minimum temperature rise. Therefore, the temperature of the heating section H greatly exceeds the fixing temperature T _H0 as the fixing operation proceeds. . Then, there is a problem that wasteful power is consumed and the temperature rise inside the machine becomes large. Further, when many fixing processing operations are continuously performed, the heating element may further remarkably rise in temperature and may be eventually damaged. Further, the fixing film brought into pressure contact with the heating portion H may also be thermally deformed.

In order to solve such a problem, the present invention changes the power supply pulse width to the heating element 28 to maintain the fixing temperature T _H0 at all times. In FIG. 5, the heating element 28 is supplied with power. An example of a power feeding circuit is shown. Reference numeral 60 denotes a control circuit including a microcomputer, which controls the switching means 73 such as a power FET according to the temperature detected by the thermistor 55 to generate heat by changing the pulse width of power supply from the power supply circuit 61 to the heating element 28. It controls the power supplied to the body 28.

The reason why the above power control is performed in the present embodiment will be described below. In this embodiment, a heat insulating layer 53 is provided in the heating element 21 in order to prevent heat radiation from the heating element 28 to the substrate 54. The purpose is to save energy by eliminating wasteful heat dissipation and improving energy efficiency. The two points are to reduce the temperature rise inside the machine due to heat radiation from the substrate 54.

By the way, if the heat is simply insulated without controlling the electric power supplied to the heating element 28, the amount of heat generation will significantly exceed the amount of heat radiation, and the heating element 28 and the heating portion H will be abnormally heated.
The heating element 28 and the fixing film 23 may be damaged by heat. Therefore, in order to prevent the abnormal temperature rise of the heating section H when the heat insulating layer 53 is provided, the control of the power supply to the heating element 28 is effective.

The power control method of this embodiment will be described below.

In the fixing system by pulse heating of the present embodiment, the toner is heated only for a short time of msec order as described above,
The temperature of the heating portion H rather than the heating time of the toner is dominant in the fixing performance, and the temperature of the toner layer rises according to the maximum temperature reached by the heating portion H. Then, when the temperature of the heating part H when the toner is softened to a sufficient state for fixing is T _H0 , the maximum temperature of the heating part H is almost T during the fixing process.
If the power supply to the heating element 28 is controlled so that it is maintained at _H0 ,
Sufficient fixing performance can be obtained without wasting power.

It is assumed that the temperature of the heating portion H reaches the fixing temperature T _H0 when the temperature of the heating portion H is the reference temperature T ₀ and the constant voltage V is supplied to the electrode 50 for the time t ₀ (see FIG. 6). ), The experiments by the inventors have revealed that there is a relationship of T _H0 = T ₀ + A (1-e ^−Bto ) (1) between T _H0 , T ₀ , and t ₀ .

A is a coefficient determined by the power supplied to the heating element 28,
B is a coefficient determined by the heat radiation path from the heating unit H.

And according to the experiments of the present inventors, It became clear that there is a relationship.

In addition, V is a power supply voltage to the heating element 28, R is an electric resistance of the heating element 28, and k is a constant.

When the temperature of the heating unit H is T _{B, and} the pulsed power supply time required to _{raise the} temperature to the fixing temperature T _H0 is τ _B , Becomes

B'is substantially constant if the room temperature and the temperature of the heating element are within a certain range. Therefore, it can be obtained by an experiment in advance by using the formula (1).

Therefore, B = B '.

A'is substantially constant if the power supply voltage V to the heating element 28 and the electric resistance R of the heating element 28 are constant. Therefore, in advance the standard voltage
If the value of A is found experimentally under the conditions of V ₀ and standard resistance R ₀ , then if VV ₀ and RR _0, then A ′ = A.

If A '= A and B' = B, the equation (3) becomes as follows.

From these things, A and B are obtained in advance by experiments,
When the fixing temperature T _H0 is set to a predetermined value, T _B is measured by the thermistor 55, and the pulse width τ _B obtained by the equation (4)
If only the pulse is applied, the maximum temperature of the heating element will be the fixing temperature.
It is possible to raise the temperature to T _H0 .

In this embodiment, as described above, when the heating element 28 is energized in a pulsed manner with a sufficiently small duty ratio, when the heating portion H that changes in a pulsed manner shows a minimum temperature, that is, immediately before the start of the next pulse energization, The temperature of the heating part H is almost thermistor
It becomes equal to the detected temperature of 55. Therefore, the thermistor at this time
Using the detected temperature of 55, the control circuit 60 of FIG. 5 calculates the next energization time according to the equation (4), and the power supply 61 supplies power to the heating element 28 for the calculated time τ _B.

FIG. 7 is a graph showing a time change of the temperature of the heating portion H during the fixing operation in the present embodiment, along with a timing chart of power supply to the heating element 28. In this embodiment, the power supply voltage V to the heating element 28 is constant, and the period τ of the energizing pulse is τ.
Is also constant. If the fixing operation is started at time t ₀ when the temperature of the heating portion H is T ₀ , the temperature of the heating portion H is fixed at the fixing temperature T ₀ by energization with a pulse width τ ₀ uniquely determined from the temperature T _0.
After reaching _H0, than tau ₀ sufficiently long non-power-supply time (tau-tau
_{During 0} ), it drops to T ₁ which is higher than T ₀ . Then at time t ₀
The pulse width τ that is uniquely determined by the temperature T ₁ shorter than τ ₀ at the second energization at time t ₁
By performing only _{one on the} heating element 28, the temperature of the heating section H rises to T _H0 again, and drops when the power supply is stopped. Similarly, after the start of energization, the thermistor will be activated every pulse period τ.
The maximum temperature of the heating section H can be maintained at the fixing temperature T _H0 by reading the temperature of 55 and supplying power to the heating element 28 with a pulse width τ _B obtained by the equation (4) according to the detected temperature.

When A <T _H0 −T ₀ , the temperature of the heating portion H does not reach the fixing temperature T _H0 by energizing with a pulse width τ _B , but several pulses,
That is, the fixing temperature T _H0 is reached within several 10 msec.

Even when A> T _H0 −T ₀ , if the rated maximum value of the pulse width derived from the power supply performance is smaller than the pulse width required to raise the temperature of the heating portion H to the fixing temperature T _H0 , A < T _H0 −T
_{As in} the case of ₀ , the temperature of the heating section H can be raised to the fixing temperature T _H0 in a very short time.

As described above, the heating element (heating element) of the present embodiment is sufficiently small in size, so that the heat capacity is small and it is not necessary to raise the temperature of the heating element in advance, so the power consumption during non-image formation is also small. Therefore, the temperature rise inside the machine can be prevented.

Further, in this embodiment, since the fixing film 23 may be heat-treated on the basis of a thin and inexpensive PET (polyester) film, the fixing film 23 can be wound up as shown in FIG. It is possible to take the form of exchanging after use. That is, a roll wound with a sheet of a predetermined length is set on the sheet delivery shaft 24, and the heating element 28
And a pressure roller 22 and a separation roller 26, and a winding shaft.
Secure the tip of the sheet to 27. When such a method is adopted, the fixing film sensor arm 30 and a sensor (not shown) detect the remaining amount of the fixing film, and when the sheet is near the end, a warning display or a warning sound is given to the user to notify the fixing film of the fixing film. It is good to encourage exchange. When the fixing film 23 is replaced, the heating element 28 and the pressure roller 22,
The rotary shaft 31 is separated from the separation roller 26.
It is desirable to be able to open and close as shown in FIG. In the present embodiment, the fixing film 23 can be thinned by the winding exchange method as described above regardless of the durability of the fixing film, and the power consumption can be reduced.

Further, in this embodiment, since the offset to the fixing film does not occur as described above, if the thermal deformation or deterioration of the fixing film is small, the wound fixing film can be reused and automatically rewound. Alternatively, it may be used a plurality of times by exchanging the winding side and the sending side.

Further, in this embodiment, by providing the separation roller 26,
The separation time between the fixing roller 23 and the transfer material P is facilitated by ensuring a cooling time of the toner image T under pressure up to the separating roller and increasing the curvature of the separating roller 26. It is possible to prevent the offset in the separation portion in synergy with the effect of (1).

However, in this embodiment, since the pressure roller 22 accelerates the cooling of the toner image, when the heat capacity of the heating portion H and the fixing film 23 is sufficiently small and the fixing speed is low, a special roller such as the separation roller 26 is used. Transfer material P without means
Since the toner image T is cooled in a short range after passing through the heating section H, the fixing process without offset can be performed even if the separation roller 26 shown in this embodiment is omitted. That is, it suffices that the fixing film and the transfer material can be separated after the toner image is once heated, softened and melted, and then thermally solidified again.

Next, the results of implementation by the apparatus of this embodiment will be shown with specific numerical values. A toner image T is formed on a transfer material P having a thickness of 100 μm using a toner that is softened and fixed at about 125 ° C. at a room temperature of 20 ° C., and the maximum temperature of the heating portion H is 300 ° C. with a pulse energization period of 10 msec. When a fixing test was conducted at a fixing processing speed of 50 mm / sec while controlling the pulse width τ _B using the temperature detected by the thermistor 55, an image having no problem in practical use was obtained. In this embodiment, since the heat capacity of the heating part H is extremely small, the waiting time for pre-energizing the heating element 28 and warming the heating part H is unnecessary. Further, in this embodiment, as the fixing process proceeds, the heating portion H warms to some extent due to the effect of the heat insulating layer 53, so that the pulsed energization time becomes gradually shorter, and the average power consumption can be reduced, and the temperature rise inside the machine can be reduced. There was no problem in practice.

FIG. 10 shows the time variation of the temperature of the toner and the transfer paper, specifically, the temperature at the central portion in each cross-sectional direction, when the transfer paper having the toner layer on the surface is subjected to the fixing process by using the fixing device of the present embodiment. In the graph of the test results obtained, the conditions are as follows.

Heating conditions: Energy density 25w / mm ² for 2ms heating, toner fixing temperature: 125 ° C, film: PET (thickness 6μm), toner thickness: 20μm, transfer paper thickness: 100μm, room temperature: 20 ° C main test Then, since the heating portion H is heated to about 300 ° C., which is much higher than the fixing temperature of 125 ° C. of the toner, the toner is sufficiently heated above the fixing temperature to obtain good fixing property. On the other hand, the temperature rise of the transfer paper is extremely small, and less energy is wasted as compared with the conventional heat roller fixing.

Further, in this test, even when the heating time or the heating energy density fluctuates and excessive energy is applied, high-temperature offset does not occur, and the allowable range of heating control is wide.

By the way, the coefficient A in the above equation (2) is determined by the heating element.
Since it depends on the applied voltage V to 28 and the electric resistance R of the heating element 28, if the applied voltage V and the electric resistance R deviate from the standard values V ₀ and R ₀ , the set fixing temperature T _H0 can be obtained. become unable.

According to the present invention, before the fixing operation is performed, a heating temperature rising characteristic detecting step (hereinafter referred to as a control heating step) for correcting the coefficient A to a correct value is performed, and the coefficient A is a standard value. Even when the value changes from, the fixing operation is performed by obtaining the correct value of the coefficient A.

That is, assuming that the amount of increase in T _B within the energization time when the heating element 28 is energized for a certain period of time with a constant cycle / pulse width is ΔT _B , ΔT _B is the amount of heat generated by the experiments by the present inventors. body
There was a positive correlation with the power applied to 28 and V ² / R.

Since (k is a constant), A has a positive correlation with ΔT _B , as shown in FIG.

Therefore, the relationship between ΔT _B and A is experimentally obtained in advance, and the relationship (see FIG. 14) is input to the control circuit 60 of the power feeding circuit, and the coefficient A is corrected by measuring ΔT _B with the thermistor 55. It can be carried out.

FIG. 12 is a sequence chart for explaining the control heating process for correcting the coefficient A described above, in which the main power switch (not shown) of the image forming apparatus equipped with the fixing device according to the present embodiment is turned on and off.
Turn on at t ₀ , turn on the copy button at time t ₁ ,
The above-described controlled heating step is performed from t ₂ to time t ₃ .

In the controlled heating step, the heating element 28 is energized under the same conditions as when the relationship between ΔT _B and A was obtained, and the amount ΔT _B of increase in T _B at that time is measured by the thermistor 55. Then, the control circuit 60 causes ΔT _B
The value of the coefficient A is determined according to the measured value of.

In the fixing step between times t _{4 and} t _5, the pulse width (τ _B ) is obtained according to the equation (4) using the value of the coefficient A obtained in the controlled heating step, and the maximum temperature of the heating element 28 is set. The fixing temperature T _H0 is controlled, and the image forming operation is completed at time t ₆ .

FIG. 11 shows details of the power supply circuit 61 in the power supply circuit shown in FIG.
DC voltage V that is rectified and smoothed by 72 and supplied to the heating element 28
To get

The variation of the DC voltage V due to the voltage fluctuation of the AC power source 70 is 0.85V ₀ ≤V≤1.15V ₀ , and the variation of the resistance value of the heating element 28 used in the present embodiment has the tolerance of the initial value and the variation with time. Including, 0.8R ₀ ≦ R ≦ 1.2R ₀ .

Therefore, the variation range of A is 0.60A ₀ ≦ A ≦ 1.65A ₀ (5).

As described above, in this embodiment, the control heating step is provided before the fixing step, and the value of the coefficient A is corrected there. Even if the value of A fluctuates as in Expression (5), the fixing step is always performed. Appropriate energization and heating control can be performed within the maximum temperature of the heating section H and within a certain temperature range.

Although the A value changes depending on the room temperature and the initial temperature around the heating body, it is possible to detect the fluctuation of the A value due to these factors in the controlled heating process.

[Comparative example]

In this embodiment, A = A '=
If the pulse width τ _B is determined from the equation (4) and the fixing process is performed with the constant value, the following inconvenience occurs.

Heating in one pulse from the equation _{(3 ') (T H0 -T} B) is, A
Proportional to.

Accordingly, the maximum temperature of the A <A ₀ Narahatsunetsutai_28nokyokudaiondowaT _H0 Yorihikukunari,A> A ₀ If the heating element 28 is higher than T _H0.

When T _H0 = 300 ° C and T _B = 50 ° C, T _H0- T _B = 250 ° C and (I): A = 0.60A ₀ (at the minimum power), the peak temperature of the heating part H is 200 ° C. Only heats up to. Therefore, the temperature of the toner heated by the heating unit H through the fixing film having a thickness of 0.6 μm does not reach the melting point of 125 ° C., and fixing failure occurs. (II): When A = 1.65A ₀ (at maximum electric power), the peak temperature of the heating part H reaches 400 ° C.

According to this embodiment, the pulse energization period τ ₁ of the controlled heating process
Then, in the period τ ₂ during the fixing step, by setting τ ₁ <τ ₂ , the pulse energization time during the controlled heating step is shortened. As a result, the instantaneous electric power applied to the heater can be reduced, and the temperature of the substrate can be sufficiently raised.

For example, when the above τ ₂ is 20 msec, in the controlled heating process,
When energization is performed at a cycle of 20 msec, the energization time for sufficiently raising the temperature of the substrate is 1 msec. However, it has been found from the experiments conducted by the present inventors that the temperature of the substrate can be sufficiently increased with the energization time of 0.2 msec when the energization period τ ₁ of the controlled heating step is 4 msec.

FIG. 15 shows the experimental result in this numerical example. The curve shown by the broken line in FIG. 15 is the case where the period is 20 msec and 1 msec is energized, and the solid line is the case where the period is 4 msec and 0.2 msec is energized. FIG. 15 shows the above-mentioned results, and similar results were obtained by experiments in other numerical examples.

In the case of this example, the instantaneous electric power applied to the heating element is 1/5 of that in the case where the present example is not used, and damage to the heating element can be prevented.

The present invention is important not only in the controlled heating step but also in the preheating step, in which abrupt and sufficient temperature rise of the heating portion is not important and the temperature of the substrate is sufficiently raised. FIG. 16 shows the heating step of the heating element when this example is used. τ
Pulse energization is performed so that ₁ <τ _2, and the temperature rise of the heat generating portion during the controlled heating step is reduced. This prevents damage to the heating element.

〔The invention's effect〕

As described above, according to the present invention, the heating element is provided with the linear heating portion integrally formed with the heating element, the energization and heating are repeated in a pulsed manner, and the maximum temperature of the heating portion becomes constant. In addition, it is possible to suppress the heating of the transfer member as much as possible and efficiently heat and melt the toner image, which brings about an effect of energy saving.

Further, it is possible to reduce the heat capacity of the heating body without causing offset due to poor fixing, and as a result,
It is possible to obtain an image forming apparatus that has a small standby time when the apparatus is used, power consumption, and a small temperature rise inside the apparatus, and further, it is possible to prevent damage to the heating element and the fixing film.

Further, in the temporary energization performed prior to the fixing step, by making the cycle during the temporary energization smaller than the cycle during the fixing process, it is possible to reduce the instantaneous power applied to the heating element.
It is possible to prevent damage to the heating element.

[Brief description of drawings]

FIG. 1 is a schematic view of an image forming apparatus equipped with an embodiment of a fixing device according to the present invention, FIG. 2 is an enlarged sectional view of the fixing device of FIG. 1, and FIG. 3 is an enlarged sectional view of a heating body. Fig. 4 is a diagram showing the principle of heating by pulsed energization, Fig. 5 is a block diagram of the power feeding circuit, Fig. 6 is a diagram showing temperature change in the heating portion during one pulse power feeding to the electrode, and Fig. 7 is FIG. 8 is a diagram showing a temperature change of the heating portion when the pulse width is changed, FIG. 8 is a diagram showing a temperature change of a comparative example, FIG. 9 is a cross-sectional view of the apparatus when the fixing film is replaced, and FIG. The figure which shows the temperature change of each section in the heating process under fixed conditions, Figure 11 is the circuit diagram which shows the details of the power source circuit in the feeder circuit in Figure 5, Figure 12 is the sequence chart, Figure 13, Figure 14 Is a diagram for explaining the controlled heating step, FIG. 15 is a diagram showing temperature changes due to periodic changes, and FIG.
FIG. 16 is a diagram showing a control heating step and a fixing step. 20 …… Fixing device, 21 …… Heating body 22 …… Pressing roller, 23 …… Fixing film 28 …… Heating body 55 …… Temperature detection element (thermistor) 60 …… Control circuit, P …… Transfer material T… …toner

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-1-263683 (JP, A) JP-A-1-279276 (JP, A) JP-A-59-68766 (JP, A) JP-A-59- 157678 (JP, A) JP-A-63-56662 (JP, A) JP-A-63-313182 (JP, A)

Claims (2)

[Claims] 1. A heating element having a linear heating element extending along the width direction and a temperature detecting means for detecting the temperature in the vicinity of the heating portion, and a pressure roller which rotates while being in pressure contact with the heating element. A transfer member carrying a heat-meltable toner image is sandwiched, and a sheet surface member that moves at the same speed as the transfer member is brought into close contact with the toner image, and the heating element of the heating element is energized in a pulse form from the power supply control means. Is a heating device that melts the toner image through the sheet surface member in the heating unit and separates the sheet surface member from the cooled and solidified toner image on the downstream side in the moving direction from the heating unit. The power supply control means controls the energy amount per pulse by changing the pulse width per cycle so that the maximum value of the heating portion temperature falls within a predetermined range, and before starting the fixing operation. The fixing is characterized in that temporary energization is performed for a fixed period, the energy amount per pulse for the heating element is determined from the detected temperature information, and the energization period during the fixing operation and the energization period during the temporary energization are controlled to be different. apparatus. 2. The fixing device according to claim 1, wherein the power supply control means sets a power supply cycle during temporary power supply to be shorter than a power supply cycle during fixing operation.