JP5263356B2 - Fixing device and color image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device of a color image forming apparatus which can prevent unnecessary energy consumption by differentiating the startup setting to a fixing temperature for a black-and-white image from that for a color image when an image forming instruction is issued unlike the conventional way in which both settings are the same in spite of the fact that the fixing temperature for a black-and-white image can be set lower than the fixing temperature for a color image. <P>SOLUTION: Electric power which is supplied to a fixing device is differentiated depending on whether an image forming instruction is for a color image or a black-and-white image, and supplied electric power for a black-and-white image is smaller than the supplied electric power for a color image. The supplied electric power is classified into startup electric power to increase temperature at which the fixing device can start fixing, and paper passing time electric power which is supplied after paper passing starts. Each of the startup electric power and the paper passing time electric power is smaller supplied electric power in the case of the black-and-white image than in the case of the color image. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a fixing device used in an electrostatic recording image forming apparatus such as a color copying machine, a facsimile machine, or a printer.

In a color image forming apparatus, it is usually possible to form a black and white image. In such an apparatus, in general, in order to prevent an excessive power supply of fixing power at the time of paper passing, there is a control to change the power at the time of paper passing after a lapse of a predetermined time after the start of paper passing.
Color image formation in which the amount of toner transferred to the transfer material is twice that of black and white is relatively poor in fixability than black and white image formation. Conventionally, image forming apparatuses that ensure productivity equivalent to color image formation and black and white image formation are particularly suitable for color image formation with poor fixability immediately after startup from the viewpoint of fixability during image formation immediately after startup. Since the rising power, the paper feeding power, and the paper feeding power change time were combined, the power at the time of monochrome image formation was wasted.
Here, “rising” does not mean a temperature rise of the fixing device when the power of the image forming apparatus is turned on, but in a state where the image forming apparatus is already in a usable state. The time from when the image formation command is issued until the image can be fixed.
There is an example in which a fixing roller for a color image and a fixing roller for a black and white image are separately configured and each roller is heated to a necessary predetermined temperature (see, for example, Patent Document 1). However, there is a description that when power is turned on, power is supplied to both rollers within the range of commercial power supply capability, but there is no mention of the rise time from when the image formation command is issued.

There is an example in which a fixing roller is used in common, and a fixing lower limit temperature for a color image and a fixing lower limit temperature for a monochrome image are made different (for example, see Patent Document 2). In this configuration, an external heating roller is provided, and the fixing roller temperature when shifting to color image fixing after fixing a monochrome image is taken up as a problem. When shifting to color image fixing after monochrome image fixing, if the fixing roller temperature is lower than the color image fixing lower limit temperature, the power supplied to the external heating roller may be made larger than that for monochrome images. It is shown. However, there is no particular mention regarding the fixing rise time.
Various types of electromagnetic induction heating type fixing devices having quick start-up characteristics have been proposed (for example, see Patent Document 3). This document shows that the start-up can be performed in a short time, including the case where the fixing device is preheated and the case where it is not preheated, but there is no description of changing the power applied to the fixing device between a color image and a monochrome image. .

  Even when a black and white image is formed by a color image forming apparatus, if the fixing ability is matched to that for a color image, unnecessary energy consumption will be performed, and the object is to eliminate this waste.

The invention according to claim 1 includes a heating source, a heating member heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and the heating member and the pressure member In the fixing device of the color image forming apparatus that fixes the toner image on the recording medium by passing the recording medium on which the unfixed toner image is formed through a fixing unit formed between When the image forming command is issued, the image forming command is issued when the predetermined first voltage or the first current is supplied to the heating source from the time when the image forming command is issued to the time when the sheet passing start set temperature is reached. when a monochrome image formation instruction from the time when the said predetermined first voltage or first current supplied to the heating source set smaller than the case of a color image forming command, reaches the sheet passing the start set temperature Wherein the second voltage of a predetermined supplied to the heating source or the second current in after the temperature has begun to rise from the drop of the heating member is less certain than the predetermined first voltage or first current It is characterized by switching to voltage or current.

According to a second aspect of the present invention, in the fixing device according to the first aspect, the magnitude of the predetermined second voltage or second current is determined when the image formation command is a monochrome image formation command. It is characterized in that it is set smaller than in the case of a color image formation command.
According to a third aspect of the present invention, there is provided a heating source, a heating member heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and the heating member and the pressure member In the fixing device of the color image forming apparatus that fixes the toner image on the recording medium by passing the recording medium on which the unfixed toner image is formed through a fixing unit formed between As a result, a predetermined first voltage or first current is supplied to the heating source from the time when the image formation command is issued until the time when the temperature reaches a sheet start setting temperature at which fixing is possible. A predetermined second voltage or a second current smaller than the predetermined first voltage or the first current after the temperature of the heating member has changed from a drop to an increase after reaching the paper start set temperature. Switch to the predetermined The magnitude of the second voltage or a second current, when the image formation command is a black-and-white image formation instruction, and sets smaller than a color image forming command.

The invention according to claim 4 includes a heating source, a heating member heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and the heating member and the pressure member In the fixing device of the color image forming apparatus that fixes the toner image on the recording medium by passing the recording medium on which the unfixed toner image is formed through a fixing unit formed between As a result, when the predetermined first power is supplied to the heating source from the time when the image formation command is issued to the time when the sheet passing start set temperature is reached, the image formation command is a monochrome image formation command. If so, from the time when the said predetermined first power duty to be supplied to the heating source set smaller than the case of a color image forming command, it reaches the sheet passing start setting temperature, the temperature of the heating member Predetermined duty of the second power supply to the heating source in after turned upward from the drop, and switches to the duty of the predetermined first power small predetermined power than duty.
According to a fifth aspect of the present invention, in the fixing device according to the fourth aspect, the magnitude of the duty of the predetermined second power is a color image formation when the image formation command is a monochrome image formation command. It is characterized in that it is set smaller than in the case of a command.

The invention according to claim 6 includes a heating source, a heating member heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and the heating member and the pressure member In the fixing device of the color image forming apparatus that fixes the toner image on the recording medium by passing the recording medium on which the unfixed toner image is formed through a fixing unit formed between As a result, the predetermined first electric power is supplied to the heating source from the time when the image formation command is issued to the time when the sheet passing start set temperature at which fixing is possible is reached. When the temperature of the heating member has changed from a drop to an increase from the point of time reached, the duty is switched to a predetermined second power duty smaller than the predetermined first power duty, and the predetermined second power Duty It is come, when the image formation command is a black-and-white image formation instruction, and sets smaller than a color image forming command.

According to a seventh aspect of the invention, there is provided a color image forming apparatus using the fixing device according to any one of the first to sixth aspects .

  According to the present invention, even when a black and white image is formed using a color image forming apparatus, it is not necessary to supply unnecessary power to the fixing apparatus, so that energy consumption can be saved.

It is a figure for demonstrating the fixing device of the electromagnetic induction heating system to which this invention is applied. It is a figure for demonstrating an induction heating means. FIG. 6 is a diagram for explaining fixing rising characteristics in a color mode and a monochrome mode. It is a figure which shows the temperature change in a paper passing state after starting. It is a figure for demonstrating the other example of how to give the electric power after a standup. It is a conceptual diagram for demonstrating the belt which has a heat-emitting layer.

FIG. 1 is a view for explaining an electromagnetic induction heating type fixing apparatus to which the present invention is applied.
In the figure, reference numeral 1 is a heat generating roller, 2 is a fixing roller, 3 is a belt, 4 is a pressure roller, 5 is temperature detection means, 6 is induction heating means, 7 is an exciting coil, 8 is a coil guide plate, and 9 is a coil core. Reference numeral 10 denotes an exciting coil core support member, 11 denotes a recording medium, and T denotes toner.
The fixing device is stretched over the heat generating roller 1 that generates heat by electromagnetic induction of the induction heating unit 6, the fixing roller 2 arranged in parallel with the heat generating roller 1, and the heat generating roller 1 and the fixing roller 2. An endless belt-like heat-resistant belt (toner heating medium) 3 that is heated by 1 and rotated in the direction of arrow A by at least one of these rollers, and is pressed against the fixing roller 2 via the belt 3 and belt 3 and a pressure roller 4 that rotates in a forward direction with respect to 3.

The heat roller 1 is made of a hollow cylindrical magnetic metal member such as iron, cobalt, nickel, or an alloy of these metals, and has a low heat capacity and a high temperature rise.
The fixing roller 2 includes a metal core 2a made of, for example, stainless steel, and an elastic member 2b covered with a core metal 2a made of heat-resistant silicon rubber in a solid or foamed form. A contact portion having a predetermined width N is formed between the pressure roller 4 and the fixing roller 2 by the pressing force from the pressure roller 4. The belt 3 stretched between the heat generating roller 1 and the fixing roller 2 is heated at a contact portion W1 with the heat generating roller 1 that generates heat by the induction heating means 6. The inner surface of the belt 3 is continuously heated by the rotation of the rollers 1 and 2, and as a result, the entire belt is heated.
When the fixing device rises to a predetermined temperature, the recording medium 11 such as transfer paper carrying the unfixed toner T is inserted from the direction of arrow B into the contact portion of the width N formed between the fixing roller 2 and the pressure roller 4. Is done. The unfixed toner T on the recording medium is softened by heat received from the fixing roller side and pressure received from both rollers, and is fixed with increased adhesion to the recording medium.

FIG. 2 is a diagram for explaining the induction heating means. FIG. 3A is a cross-sectional view, and FIG.
As shown in FIG. 1 and FIGS. 2 (a) and 2 (b), the induction heating means 6 that generates heat from the heat generating roller 1 by electromagnetic induction is wound with an excitation coil 7 that is a magnetic field generating means and the excitation coil 7. And a coil guide plate 8. The coil guide plate 8 has a semi-cylindrical shape arranged close to the outer peripheral surface of the heating roller 1, and as shown in FIG. 2 (b), the excitation coil 7 is composed of a long excitation coil wire. 8 are alternately wound in the axial direction of the heat generating roller 1. The exciting coil 7 has an oscillation circuit connected to a drive power source (not shown) whose frequency is variable.
As shown in FIG. 1, a semi-cylindrical excitation coil core 9 made of a ferromagnetic material such as ferrite is fixed to the excitation coil core support member 10 and is disposed close to the excitation coil 7 outside the excitation coil 7. .
In the present embodiment, the exciting coil core 9 has a relative permeability of 2500.

A high frequency alternating current of 10 kHz to 1 MHz, preferably a high frequency alternating current of 20 kHz to 800 kHz is supplied to the exciting coil 7 from a driving power source, thereby generating an alternating magnetic field. The alternating magnetic field acts on the heat generating roller 1 in the contact area W1 between the heat generating roller 1 and the heat-resistant belt 3 and in the vicinity thereof, and an eddy current flows in a direction that prevents the change of the alternating magnetic field therein.
This eddy current generates Joule heat corresponding to the resistance of the heat generating roller 1, and the heat generating roller 1 is heated by electromagnetic induction mainly in the contact area between the heat generating roller 1 and the belt 3 and in the vicinity thereof.
In the fixing device of the present invention, the temperature of the surface of the fixing belt is detected by the temperature detecting means 5 as shown in FIG.
The image forming apparatus is configured in advance so that color image formation and monochrome image formation can be selected. The state of the apparatus when color image formation is designated is called a color mode, and the state of the apparatus when monochrome image formation is designated is called a monochrome mode.

FIG. 3 is a diagram for explaining the fixing rising characteristics in the color mode and the monochrome mode.
In the same figure, Gc1 is a rising curve in the color mode, Gm1 is a rising curve in the monochrome mode, PW is the power supply, TB is the reference temperature, Tc0 is the paper passing start temperature in the color mode, and Tm0 is in the monochrome mode. The paper passing start set temperature, tc1, tm1, and the like indicate rise times. In the figure, the thick line indicates the power supply.
In the figure, the horizontal axis represents time (t), and the vertical axis represents belt temperature (T) and input power (W).
As shown in the figure, when either color image formation or black-and-white image formation is selected before the start of fixing, the heating roller 1, fixing roller 2, belt 3 and pressure roller 4 shown in FIG. , The induction heating means 6 heats up, the detection means 5 detects the belt temperature, and the color image formation rises up to the paper passing start set temperature Tc0, and the black and white image formation rises until the paper passing start set temperature Tm0. . The reason why Tm0 is set to be smaller than Tc0 is that the monochrome image fixing consumes less energy than the color image fixing, and the temperature drop due to the paper passing is reduced accordingly. This is because there are fewer monochrome images than color images.
In the case of the electromagnetic induction heating method, the reference temperature TB is usually the ambient temperature itself. However, in the case of a fixing device having a large thermal inertia, there is a case where it is always heated even when an image is not being formed. In this case, the preheating temperature is meant. In that sense, the description in FIG. 10 can be applied to a fixing device using a heating unit such as a halogen lamp method instead of an electromagnetic induction method.

The rising curve in each mode differs depending on how the rising power is supplied.
A curve Gc1 shows a rising curve at the position of the detecting means 5 of the belt 3 when the maximum power PWM that can be supplied is given. At this time, the time until the sheet passing start set temperature Tc0 in the color mode is reached is tc1. At this point, recording paper may be introduced into the fixing device. Since the belt 3 rotates a plurality of times before reaching the time tc1, a stepwise temperature rise curve is shown microscopically, but is simply shown as a continuous temperature rise (the same applies to the subsequent drawings). .)
Assuming that the time for the same curve to pass the sheet passing start temperature Tm0 in the monochrome mode is tm1, in the monochrome mode, the recording sheet may be introduced into the fixing device at the time tm1 (Gm1). However, the time txm from when the monochrome mode is specified until the recording paper 11 reaches the fixing position may take longer than tm1. In this case, it is not necessary to force the rise time to fall within tm1, so in order to reduce the load applied to the apparatus as much as possible, supply power smaller than the maximum power PWM is applied so that the rise time becomes a desired time tm2. That's fine.

For example, if txm is equal to tc1, or if there is no significant problem even if the rise time in monochrome mode is set equal to tc1, the supply power in monochrome mode is lowered to PW2 and the rise curve is set to Gm1 ′. The time tm2 ′ = tc1 may be satisfied. However, in the monochrome mode, the time for the recording paper to reach the fixing position may be shorter than that in the color mode. If the rise time is equal to tc1, the waiting time becomes too long. It is preferable to set the supply power to PW3 larger than that and set the rise time tm2 equal to txm as shown by the curve Gm1 ″.
As another condition, in the color mode, when the time txc for the recording paper to reach the fixing position is larger than tc1, the power supply is lowered to PW1 to reduce the power load, and as shown by the curve Gc1 ′, The rise time tc2 may be set equal to txc.

FIG. 4 is a diagram showing a temperature change in the sheet passing state after the rising. FIG. 6A shows the case where the black and white mode rise curve is shared with the color mode, and FIG. 6B shows the case where the rise time of each mode is set independently.
In FIG. 5A, it is assumed that an image formation command is issued and the fixing device is started up. At this time, when the rising curve is not distinguished between the color mode and the black and white mode, the sheet passing start set temperature Tc0 in the figure is also applied to the black and white mode. In the present invention, the starting power PW0 is given as the initial supplied power, and when the belt temperature reaches the paper passing start set temperature Tc0, the power supplied to the fixing device (hereinafter referred to as paper passing power) is set to a predetermined power described later. Reduce to value. The rising power PW0 may be PWM in FIG. 3 or PW1.
When the recording medium reaches the fixing device at time tc 1, the paper passing stage is started, and the temperature of the belt 3 is deprived of heat by the recording medium 11 due to the entry of the recording medium 11, causing a temperature drop. When the number of the recording media 11 is limited to one, even if the temperature is lowered, once the recording medium 11 passes, there is no passage of the next recording medium, so that the belt temperature is not lowered. However, if continuous image formation is performed, since the recording medium 11 passes through the fixing device one after another, a continuous temperature decrease occurs. FIG. 4 shows such a state.

Here, power supply at the paper passing stage will be described. The minimum fixing temperature in the color mode is Tc1, and that in the monochrome mode is Tm1. When the amount of heat taken away by continuous paper feeding and the amount of heat supplied by electric power are substantially equal, the belt temperature becomes substantially constant. A predetermined value relating to the supplied power is determined so that the temperature at that time is Tc1 in the color mode and Tm1 in the monochrome mode. In the figure, the value is PW4 in the color mode and PW5 in the monochrome mode. Since this electric power value may change depending on the environmental temperature or the like, it may be changed according to those conditions.
The reason why Tm1 is set smaller than Tc1 is that the minimum fixing temperature for fixing a monochrome image is lower than that for fixing a color image.

In FIG. 5B, when the rising curves are determined independently for the color mode and the black and white mode, the sheet passing power is reduced to a predetermined value when the respective sheet passing start set temperatures are reached.
In the case of the figure, the color mode is the same as that of FIG. 10A. However, in the monochrome mode, when the rising power is PW3 and the paper passing start set temperature Tm0 is reached at time tm1, the paper is passed. At the same time, it lowers the paper feed power to PW5.
In any case, when the paper feeding is completed, the supplied power may be set to 0 at that time except in the case of the preheating method. Thereafter, due to natural cooling, the belt temperature gradually decreases, and when there is no next image formation, the belt temperature finally decreases to the reference temperature TB. In the case of the preheating method, it may be lowered to the supply power necessary for preheating.

Here, how to give power will be mentioned.
When the maximum power PWM is applied, there is no room for special contrivance, but there are several methods for supplying power smaller than the maximum power PWM. For example, if power is supplied continuously, there are methods of lowering the voltage and applying the current limit. When it is not necessary to be continuous, intermittent power supply may be performed. In other words, there are a method of giving desired power by changing the duty of pulsed power supply, and a method of maintaining a predetermined temperature by applying power whenever necessary by feeding back the output of the temperature detecting means. .
In either case, the effective power (average power) may be PW4 or PW5 in FIG.

FIG. 5 is a diagram for explaining another example of how to supply power after rising. FIG. 4A shows the case where the black and white mode rise curve is shared with that of the color mode, and FIG. 4B shows the case where the rise time of each mode is set independently.
In this power supply method, the paper passing power is switched to two stages. First, as the first stage, the supplied power is not changed immediately after the start of paper feeding, but the supplied power at the time of start-up is maintained as it is. In this case, if there is no sheet passing, the belt temperature overshoots and rises above the sheet passing start set temperature, but the temperature temporarily drops due to the sheet passing. In other words, the temperature is still rising at the time when the sheet passing start set temperature is reached, because the fixing roller 2 and the pressure roller 4 have not yet reached the stable temperature range. Therefore, if the power supply is continued even after the start of paper feeding, the temperature of each roller will continue to rise and enter the stable region. For this reason, if the paper is continuously fed, the temperature curve once lowered starts to rise and eventually approaches a certain value.
In this example, the supply power is lowered to a predetermined value as a second stage after a predetermined time from the start of paper feeding. The predetermined time is time tm2 in the monochrome mode and time tc2 in the color mode.

When the power supply is reduced simultaneously with the start of paper feeding, if the heat capacity of the fixing roller 2 or the pressure roller 4 is large to some extent, there is a risk that the belt temperature falls below the minimum fixing temperature due to a rapid temperature drop due to paper feeding. is there. For this reason, when the heat capacity is large, it is not possible to reduce the paper feeding power simultaneously with PW4 and PW5. As in this example, if the temperature is once increased and then the supplied power is lowered, the fixing roller 2 and the pressure roller 4 are sufficiently heated, so that the belt is not affected by the size of the heat capacity. The danger that the temperature falls below the minimum fixing temperature is reduced, and the paper feeding power can be set correspondingly to PW4 and PW5 as in the case of FIG.
The belt temperature drop due to the paper passing is less in the monochrome mode than in the color mode, so the timing for starting the temperature rise is earlier than in the color mode. Therefore, the time for switching the paper passing power to the second stage can be made smaller in tm2 in the monochrome mode than in tc2 in the color mode. That is, tm2 <tc2.

FIG. 6 is a conceptual diagram for explaining a belt having a heat generating layer.
In the figure, reference numeral 31 denotes a base layer, 32 denotes a heat generating layer, 33 denotes an elastic layer, and 34 denotes a release layer.
In the above description, a simple heat-resistant belt is assumed as the belt 3, but a heat generating layer may be formed on the belt itself.
This figure shows an example of a fixing belt having a heat generating layer. A heat generating layer 32 made of a conductive material such as silver is formed on the base layer 31, an elastic layer 33 made of silicon rubber or the like is formed thereon, and a release layer 34 made of fluorine resin or the like is formed thereon. .
The belt 3 is heated by the induction heating means 6 while passing through the contact portion W <b> 1 in the region wound around the heat roller 1, and the temperature rises together with the heat roller 1. The temperature rising portion of the belt 3 passes through a contact portion having a width N between the fixing roller 2 and the pressure roller 4 by the rotation of the belt 3, so that the release layer 34 becomes unfixed toner T on the recording medium 11. The toner is fixed on the recording medium by contacting with pressure.

DESCRIPTION OF SYMBOLS 1 Heating roller 2 Fixing roller 3 Belt 4 Pressure roller 5 Temperature detection means 6 Induction heating means 11 Recording medium

JP-A-2-191979 JP 2003-233274 A JP 2002-82549 A

Claims (7)

A fixing unit that includes a heating source, a heating member that is heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and is formed between the heating member and the pressure member In addition, in the fixing device of the color image forming apparatus, the toner image is fixed on the recording medium by passing the recording medium on which the unfixed toner image is formed.
In supplying the predetermined first voltage or the first current to the heating source from the time when the image formation command is issued until the time when the sheet passing start set temperature is reached. When the image formation command is a monochrome image formation command, the predetermined first voltage or first current supplied to the heating source is set to be smaller than that in the case of a color image formation command, and the sheet passing start set temperature is set. The predetermined second voltage or the second current supplied to the heating source after the temperature of the heating member has changed from a drop to an increase from the time when the temperature reaches the predetermined first voltage or the first current. A fixing device that switches to a predetermined voltage or current smaller than the predetermined voltage or current.   The fixing device according to claim 1, wherein the magnitude of the predetermined second voltage or second current is smaller when the image formation command is a monochrome image formation command than when the image formation command is a color image formation command. A fixing device characterized by setting. A fixing unit that includes a heating source, a heating member that is heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and is formed between the heating member and the pressure member In addition, in the fixing device of the color image forming apparatus, the toner image is fixed on the recording medium by passing the recording medium on which the unfixed toner image is formed.
When the image formation command is issued, a predetermined first voltage or first current is applied to the heating source from the time when the image formation command is issued until the time when the sheet passing start set temperature at which fixing is possible is reached. When the temperature of the heating member changes from a drop to an increase from the time when the sheet feeding start set temperature is reached, a predetermined second voltage smaller than the predetermined first voltage or first current or Switching to the second current, the predetermined second voltage or the magnitude of the second current is set smaller when the image formation command is a monochrome image formation command than when it is a color image formation command. A fixing device characterized by the above. A fixing unit that includes a heating source, a heating member that is heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and is formed between the heating member and the pressure member In addition, in the fixing device of the color image forming apparatus, the toner image is fixed on the recording medium by passing the recording medium on which the unfixed toner image is formed.
When the image forming command is issued, the image forming command is changed to black and white when the predetermined first power is supplied to the heating source from the time when the image forming command is issued to the time when the sheet passing start set temperature is reached. In the case of an image formation command, the duty of the predetermined first power supplied to the heating source is set to be smaller than that in the case of a color image formation command, and the heating member is reached from the time when the sheet passing start set temperature is reached. The duty of the predetermined second power supplied to the heating source after the temperature of the gas is changed from falling to rising is switched to a duty of a predetermined power smaller than the duty of the predetermined first power. Fixing device to do.   5. The fixing device according to claim 4, wherein the predetermined second power duty is set smaller when the image formation command is a monochrome image formation command than when it is a color image formation command. A fixing device characterized by the above. A fixing unit that includes a heating source, a heating member that is heated by the heating source, and a pressure member that contacts the heating member with a predetermined pressure, and is formed between the heating member and the pressure member In addition, in the fixing device of the color image forming apparatus, the toner image is fixed on the recording medium by passing the recording medium on which the unfixed toner image is formed.
When the image formation command is issued, a predetermined first electric power is supplied to the heating source from the time when the image formation command is issued until the time when the sheet passing start set temperature at which fixing is possible is reached. After the temperature of the heating member changes from a drop to an increase from the time when the start set temperature is reached, the duty is switched to a predetermined second power duty smaller than the predetermined first power duty. 2. The fixing device according to claim 1, wherein when the image formation command is a monochrome image formation command, the magnitude of the power duty is set to be smaller than that when the image formation command is a color image formation command.   A color image forming apparatus using the fixing device according to claim 1.

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