JPH08314208A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device capable of obtaining a desired printed image, without causing a failure in fixing and a high-temperature offset, even if environment and a voltage inputted to a heater are changed. CONSTITUTION: The device is provided with image forming parts 911... for forming a toner image on a recording material P, a heating/fixing device 919 provided with a heating roller 901 for heating/fixing the toner image and a temperature detecting member 905 detecting the temp. of the heating roller and a DC controller 923 controlling the temperature of the heating roller, based on the temperature detected by the temperature detecting member. The controller 923 sets a ready temp. for emitting a ready signal for showing that a signal for requesting the start of an image forming operation is acceptable or a standby temp. as a ready state, based on the temperature rising value per unit hour of the heating roller, after power is supplied. When the temperature rising value is low, the ready temp. or the standby temp. is raised and when the temperature rising value is high, the ready temp. or the standby temp. is lowered/set plus overshooting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a printer, a copying machine or a facsimile, and more particularly to an image forming apparatus such as an electrophotographic recording apparatus or an electrostatic recording apparatus having a heat fixing device.

[0002]

2. Description of the Related Art The outline of image forming operation in a conventional general electrophotographic laser printer is as follows.
A laser beam modulated according to image data is irradiated onto the image carrier uniformly charged by a charging unit such as a charging roller, and a desired electrostatic latent image is obtained. This latent image is subjected to reversal development at a developing portion by a developing device such as a developing device arranged so as to face the latent image and is visualized as a toner image. This toner image is electrostatically transferred by a transfer means such as a transfer roller to a sheet-shaped recording material (transfer paper, recording paper, etc.) that is fed and conveyed from a cassette or the like, and then is transferred to a heat fixing device or the like. The fixing means permanently fixes the recording material at a predetermined fixing temperature.

As the heating and fixing device, a heat roller type is often used because of its safety, high thermal efficiency, good fixing property and adaptability for speeding up. This heat roller type fixing device has a heat source such as a halogen heater inside and is composed of a fixing roller pair consisting of a heating roller rotating in a fixed direction and a pressure roller rotating in pressure contact with the heating roller. Then, the toner image is heated and pressed through the recording material carrying the toner image on the roller pair to fix the toner image on the recording material.

The surface temperature of the heating roller is detected by a temperature detecting member, and the heat source is adjusted via a control circuit in response to a signal from the temperature detecting member so that the surface temperature of the heating roller becomes a predetermined print temperature (fixing temperature). Temperature).
This is because if the surface temperature of the heating roller is too low, fixing failure of the toner image occurs, and if it is too high, offset transfer of the toner image to the heating roller (referred to as high temperature offset) occurs. Controlling to a predetermined constant value is extremely important.

Conventionally, in the above-described laser printer, the surface temperature of the heating roller is set at a standby temperature (standby temperature) in a ready state in which a start request signal of an image forming operation from a host such as a personal computer can be accepted at any time during standby. When printing after receiving the image data from the host, the controller of the laser printer confirms the ready state, then transmits a print signal, switches the standby temperature to a predetermined print temperature, and outputs the desired temperature. I'm getting a print image.

On the other hand, after the power is turned on, a ready signal indicating that the image forming (printing) operation is possible is made when the surface temperature of the heating roller reaches a predetermined ready temperature (ready temperature) by the heating source. I am emitting. These states are shown by the curve A in FIG. In the figure, the fixing roller pair is rotated from the room temperature T _R until the ready temperature T _r is reached in order to uniformly heat the pressure roller to improve the fixing property and reduce the fixing unevenness. In addition, the ready temperature T _r
And the standby temperature T _S are set to be the same, and the print temperature T _p is set higher than the standby temperature T _S. The standby temperature T _S is set to a temperature at which the print source can reach the print temperature T _p by the heating source before the recording material starts being fed after the print signal is received and the recording material enters the heat fixing device. Has been done. By setting in this way, it is devised to reduce the power consumption during standby.

[0007]

However, in the conventional example as described above, the ready temperature T _r (standby temperature T _S ) is fixed to a predetermined constant value. Variations in voltage and electric power of the heater, which is a heating source, also affect the print temperature T _p, and there is a drawback in that the above-described fixing failure and high temperature offset occur.

For example, when the upper limit voltage of the power supply voltage is input, particularly in the case of so-called morning print, which is a print immediately after the power is turned on, the overshoot of the heating body surface temperature becomes considerably large (see FIG. 8). (See curve B),
High temperature offset is likely to occur. On the other hand, when the lower limit voltage of the guaranteed range of the power supply voltage is input to the heater in a low temperature environment, it becomes difficult to increase the standby temperature to the print temperature (see the broken line C in FIG. 8), and the fixing failure occurs. Will occur.

Further, in order to reduce the cost of the apparatus, when the model of each destination, for example, the heat fixing apparatus of 100V machine for domestic use and 120V machine for North America is made common, The problem of becomes more prominent. In this regard, an example of a halogen heater, which is a commonly used tungsten incandescent light bulb, is shown. The rate of change in power consumption (W / W ₀ ) with respect to the rate of change in input voltage (V / V ₀ ) of the halogen heater approximately has the following relationship.

[0010]

[Number 1] _{W / W 0 = (V /} V 0) 1.54 ... (1) Consequently, the specified range of the supply voltage is 10% ±, in the case where entered the 90V and 132V to a heater, from the above equation to 90V On the other hand, when 132V is input, the power consumption is about 1.8 times, and it can be seen that it is very difficult to make the heating and fixing device of 100V and 120V common.

The present invention solves the problems of the prior art as described above, and even when the ambient environmental temperature or the input voltage to the heater changes, a desired print image can be formed without causing fixing failure or high temperature offset. 100V at the same time you get
It is an object of the present invention to provide an image forming apparatus in which a heating and fixing device for a 120V machine and a 240V machine for Europe and a 240V machine for Europe are also commonly used.

[0012]

In order to achieve the above object, the present invention provides an image forming means for forming a toner image on a recording material, and a heating body for heating and fixing the toner image formed on the recording material. And a control means for controlling the temperature of the heating element based on the temperature detected by the temperature detecting means, and a fixing means having a temperature detecting means for detecting the temperature of the heating element. Control means for setting a ready temperature for issuing a ready signal indicating that the start request signal for the image forming operation can be received, or a standby temperature in a ready state, based on the temperature rise value of the heating body per unit time thereafter. And is provided.

Further, as one form of the present invention, the control means raises the ready temperature or the standby temperature when the temperature rise value per unit time is low, and when the temperature rise value per unit time is high. The ready temperature or the standby temperature may be lowered and set in consideration of the amount of overshoot.

As another aspect of the present invention, there is provided storage means for pre-storing the relationship between the temperature rise value per unit time and the overshoot amount, and the control means refers to the storage data of the storage means and stores the relation. It is possible to set the ready temperature or the standby temperature.

As another aspect of the present invention, the control means starts the image forming operation after setting the ready temperature and before the detected temperature of the heating body reaches the set ready temperature. When there is a request signal, the ready temperature can be corrected in consideration of the current energization time, the fixing property due to the current energization time, the high temperature offset, and the like.

In still another form of the present invention, the heating body is a heating roller that is rotationally driven, and the control means sets a specific temperature before the detected temperature of the heating roller reaches the ready temperature. Stop the heating roller until
It is possible to control to start the rotation of the heating roller after reaching the specific temperature, and to variably set the value of the specific temperature according to the temperature increase value per unit time.

[0017]

According to the present invention, a ready signal indicating that the image forming operation start request signal can be received by an external device is issued based on the temperature rise value of the heating element per unit time after the power is turned on. Determine the temperature or standby temperature.

That is, when the surface temperature of the heating element of the heating and fixing device is raised to a predetermined ready temperature after the power is turned on, the degree of rise of the surface temperature (temperature rise per unit time) is detected, and the rise is slow. In this case, the ready temperature is raised, and when the rise is early, the ready temperature is lowered by taking overshoot into consideration. Similarly, the print temperature is controlled to a desired constant value by also determining the standby temperature, which is the standby state after issuing the ready signal, according to the rising degree.

[0019]

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

(First Embodiment) FIGS. 1 to 3 show a first embodiment of the present invention. FIG. 1 shows a schematic sectional structure of a laser printer (laser beam printer) which is an image forming apparatus according to a first embodiment of the present invention. The laser printer 910 of this embodiment is connected to a host 900 such as a personal computer or a workstation, and the host 9
When receiving the image data from 00, the controller 921
It is designed to be expanded into bitmap data. The image signal developed into the bitmap data is sent to the engine section of the laser printer 910 via the video interface 922 and the DC controller 923, and this engine section modulates the laser beam 913 by the scanner 924 based on the image signal. A desired image is formed by raster scanning.

At this time, the controller 921 and the engine portion of the laser printer 910 are connected to the video interface 92.
The following communication is carried out via 2. First, the engine section confirms the recording material size based on a signal from the controller 921, and when the paper can be fed and the printer can be operated, a ready signal is sent to the controller 92.
Send to 1.

Next, the controller 921 confirms that the ready signal from the engine unit is transmitted, and transmits a print signal, which is a paper feed command, to the engine unit.

Upon receipt of this print signal, the engine portion immediately feeds the recording material P from the recording material storage portion such as the cassette 920 by the paper feeding roller 915, and the registration roller 916.
Carry to. At the same time, in the heat fixing device 919, the halogen heater 9 disposed inside the heating roller 901 is used.
02 is controlled by the DC controller 923 to switch the surface temperature of the heating roller 901 from the standby temperature to the print temperature. This surface temperature is detected by a temperature detecting member 905 such as a thermistor (NTC), and the DC controller 92
A controlled temperature is controlled by 3.

Here, the temperature detecting circuit will be briefly described. A fixed resistor is connected in series to the temperature detecting member 905 which is in contact with the surface of the heating roller 901, and the voltage applied to these fixed resistors is connected in series. The partial pressure value is set to the heating roller 9
The temperature is detected by converting it into a voltage signal corresponding to the surface temperature of 01.

The recording material P is temporarily stopped by the registration roller 916, and after waiting for the rise of a scanner motor (not shown) in the engine section and the completion of preparatory rotation (so-called pre-rotation) such as potential stabilization of the photosensitive drum 911, It waits until the printer section becomes ready for image writing. Then, after exchanging a predetermined signal between the engine section and the controller 921, an image signal is sent to the engine section, and the recording material P is conveyed from the registration roller 916 to the transfer section.

Next, image formation in the laser printer section will be described. The photosensitive drum 911 having a photosensitive layer such as an organic semiconductor (OPC) is uniformly charged by a charging unit such as a charging roller 912, and then irradiated with the laser beam 913 modulated according to the above-described image signal to obtain a desired one. An electrostatic latent image is obtained. This electrostatic latent image has a developing device 9 having toner.
14 is developed and visualized as a toner image.
This toner image is electrostatically transferred to a recording material P by a transfer unit such as a transfer roller 917 in a transfer portion, and then the recording material P is conveyed to a heat fixing device 919 so that the toner image is permanently fixed and the paper discharge portion. The recording material P is discharged from the sheet, and a desired print image is obtained.

The toner left untransferred on the photosensitive drum 911 is cleaned by a cleaner 918 having a cleaning blade or the like, and the same image forming process as described above is repeated. Incidentally, the developing device 914, the charging roller 912, the photosensitive drum 9 of the image forming unit described above.
The cartridge 11 and the cleaner 918 may be integrally configured to be a cartridge that is detachable from the main body.

Next, the basic control of the heat fixing device 919 in the engine portion of this embodiment will be described with reference to the flowchart of FIG. First, when the engine is powered on (S100), the halogen heater 902 is energized to raise the heating roller surface temperature T toward the ready temperature. During this period, in the present embodiment, the output from the above-mentioned temperature detection circuit is detected, for example, every 5 ms while rotating the fixing roller pair 901 and 904 for the purpose of improving the fixing property and eliminating the fixing unevenness, and the unit thereof is The rising degree α of the temperature rise per unit time is calculated (S101). Due to the performance of the temperature detection circuit, it is difficult to detect an accurate temperature near the room temperature T _R.
Actually, the value α [° C / sec] is determined by the rising degree in the vicinity of the ready temperature T _r with high temperature detection accuracy.

Here, it is assumed that the time from the receipt of the print signal until the recording material P actually enters the heat fixing device 919 is, for example, 10 seconds in the device of this embodiment. It is assumed that the print temperature T _p of the main heating and fixing device 919 is set to 160 ° C. Assuming that the value α of the degree of rise α = 2 ° C./sec, which has been determined at that time, if the standby temperature T _S during standby is set to at least 140 ° C. or higher, an appropriate print image can be obtained. Therefore, in this embodiment, it is decided that T _S = 145 ° C. (S102).

The ready temperature T _r for issuing the ready signal may be set to 145 ° C., but is determined in consideration of the temperature corresponding to the relationship between the rising degree α and the overshoot amount stored in advance in the device. . In this embodiment, T _r =
It is decided to be 140 ° C. (S103).

Regarding the relationship between the degree of rising and the amount of overshoot stored in the apparatus in advance, the energization time to the heater 902 up to the ready temperature T _r is counted by a timer (not shown), and a predetermined time is set. If it is shorter, it is judged that there is less overshoot, and the ready temperature T
Needless to say, it is necessary to consider that _r is set to the same temperature as the standby temperature T _S.

After this, the surface temperature T of the heating roller 901 is
However, the same operation as described above is repeated until the T _r temperature determined at that time is reached (S104), and when the T _r temperature is reached, a ready signal is transmitted (S105). At this point, the print signal from the controller 921 is ready to be received.

If the print signal from the controller 921 due to the start request of the image forming operation from the external host 900 is not received by this time (S106), the surface temperature T of the heating roller 901 is directed to the standby temperature T _S. Control (S107), and a standby state for waiting a print signal is set.

Here, when a print signal is received from the controller 921 (S106), preparations such as paper feeding and pre-rotation are immediately started, and at the same time, the target temperature of the surface of the heating roller 901 is set to a preset print temperature T _p. The recording material P is temporarily stopped by the registration roller 916 (S108).

Next, the start-up of the scanner and the photosensitive drum 9
If the preparatory processing (so-called pre-rotation) performed before the normal image writing operation such as the potential stabilization of 11 is completed (S10)
9) After transmitting and receiving a predetermined signal, the registration roller 916 is rotated to convey the recording material to the transfer section (S1).
10).

Subsequently, when the recording material carrying the unfixed toner image is conveyed to the heating and fixing device 919 and the fixing operation is completed (S111), the heating roller surface temperature controlled to the print temperature T _p is set to standby. The temperature is controlled to T _S (S107), and the standby state for waiting for the print signal is resumed (S106).

The state of the surface temperature control of the heating roller in this embodiment described above is shown in FIG. 3 and will be described step by step. In the figure, the heat source from the state of room temperature T _R (halogen heater)
Energization to 902 is started, and the ready temperature T _r is reached at the rise of α = 2 ° C./sec. In the figure, since the print signal has not been transmitted up to this point, the ready temperature T
_At the time of _r , the power supply to the heating source 902 is once cut off.
By controlling in this way, overshoot is suppressed. Next, it is shown that the print signal is received and controlled to the print temperature T _p after being controlled to the standby temperature T _S as it is. Here, the first recording material P
It can be seen that the temperature is sufficient to fix the toner when it enters the heat fixing device 919. When this fixing is completed, the standby temperature T _S , which is the standby state, is controlled again.

In this embodiment, 600 W of electric power is consumed by the halogen heater 902 which is a heating source, and a rise of α = 2 ° C./sec is described. However, even if this rise speed changes, Since the ready temperature T _r and the standby temperature T _S are determined accordingly, a desired print image can be obtained without causing fixing failure even when the ambient environment or the input voltage to the heater changes.

(Second Embodiment) A second embodiment of the present invention will be described with reference to FIGS. 4 and 5. The description of the common points with the first embodiment will be omitted. FIG. 4 of the second embodiment shows an example in which printing is requested from the host 900 side before the room temperature T _R reaches the ready temperature T _r . Further, the halogen heater 902 is made to consume 800 W of electric power so that the rising speed becomes higher than that in the first embodiment.

As shown in the first embodiment, when the value of the rising degree α = 3 ° C./sec is established near the ready temperature T _r , the standby temperature T _S is set to 130 ° C. or higher, and the fixing failure occurs. because it does not occur, here, the T _S = 135 ℃.

Next, regarding the calculation of the ready temperature, the difference from the first embodiment will be described with reference to the flowchart of FIG. As shown in the flowchart of FIG. 5, after the standby temperature T _S is calculated, it is confirmed whether or not there is a request to start the image forming operation from the host 900 side (S).
120).

If there is no such request, the ready temperature T _r is calculated and determined by the same method as in the first embodiment (S10).
In 3), in the case of the present embodiment in which there is a request to start the image forming operation, the value of the ready temperature T _r is calculated and then corrected and confirmed (S121). That is, in addition to the relationship between the degree of rising and the amount of overshoot stored in advance described in the first embodiment, the relationship between the energizing time to the ready temperature, the fixing property in that case, and the high temperature offset is taken into consideration. The temperature of _r is corrected. In this device, T
It is established that _r = 130 ° C.

When T _r = 130 ° C. is reached, a ready signal is transmitted (S105) and a print signal is received (S105).
However, in this embodiment, unlike the first embodiment, the halogen heater 902, which is a heating source, is not energized and is controlled as it is toward the target temperature T _p (S108). ).

In this image forming apparatus, the recording material P enters the heating and fixing device 919 10 seconds after the print signal is received, and at the rising speed of α = 3 ° C./sec, the recording material P is calculated. At that point, it will be just 160 ° C. However, particularly when a high input voltage is applied to the normal rated voltage, an overshoot occurs due to the heat quantity of the heating roller 901, the thermal time constant of the NTC (902), etc., and the actual heating roller surface The temperature is higher than the printing temperature T _p = 160 ° C. This state is indicated by ΔT in FIG.

In this embodiment, the pair of fixing rollers 901 and 9 are used.
When the recording material enters the nip portion 04 (see the arrow portion of the first sheet in FIG. 4), the heat of the heating roller 901 is abruptly taken by the recording material to suppress the overshoot. That is, the ready temperature T _r for issuing the ready signal
It is possible to suppress the overshoot and prevent the occurrence of high temperature offset by correcting the value to a low level at which the fixing property can be secured and nipping and conveying the recording material to the nip portion at an appropriate timing.

(Third Embodiment) A third embodiment of the present invention will be described with reference to FIGS. 6 and 7. The description of the common points with the first embodiment will be omitted. In FIG. 6 of the third embodiment, T _r = T _S is set to simplify the description. In the apparatus of this embodiment, the room temperature T _R to the ready temperature T _R
The fixing roller pairs 901 and 904 are not rotated until the temperature reaches a preset temperature T _t until the temperature reaches _r, and the fixing roller pairs 901 and 9 from T _t to T _r .
Rotate 04. FIG. 6 shows how the surface temperature of the heating roller of the image forming apparatus is raised at this time. T _R
And each beta [° C. / sec] and gamma [° C. / sec] values of the rising degree of the temperature rise per unit time between through T _t and between T _t through T _r. It will be shown below that the present invention can be applied to an apparatus adopting the control as in the present embodiment.

The control procedure of the apparatus of this embodiment will be described with reference to the flowchart of FIG. Although β is determined as the rising value instead of α described in the first embodiment (S130), the difference from the first embodiment is that the value of γ is calculated before the standby temperature T _S is calculated. Calculate (S1
31) That is. To calculate γ, study the relationship between β and γ when conditions such as the ambient environment and the voltage within the guaranteed range of the power supply voltage are shaken in advance, and set the device so that γ can be calculated from β according to a predetermined decision. Just remember it.

Next, regarding the standby temperature T _S ,
It can be determined from the calculated γ in the same manner as in the above example (S103). Since the subsequent steps are similar to those of the first embodiment, the description thereof will be omitted.

On the other hand, the method of determining the temperature of T _t is fixed to a certain temperature in this embodiment, but β
Is large and it is estimated that the overshoot reaches an unacceptable range, the temperature of T _t is lowered and the rotation time of the fixing roller pair 901 and 904 is lengthened to suppress the overshoot. You may control to. On the contrary, if β is small and there is no concern about high temperature offset due to overshoot,
The weight time may be controlled to be shortened by increasing the temperature of T _t and shortening the rotation time of the fixing roller pair 901 and 904.

Further, in order to reduce the wait time, the fixing roller pair 901 and 904 are not rotated from the room temperature T _R until the ready temperature T _r is reached, that is, image formation without so-called multiple rotation before fixing. It is needless to say that it is also applicable to the control of the apparatus.

In the above-described first to third embodiments of the present invention, the heating roller surface temperature T when the power is turned on.
Has been described as a case where T ≦ T _r ,
In the case of T> T _r , for example, the control when the jam processing is performed immediately after printing will be described below.

If the power is not turned off during the jam processing,
After the jam processing, the ready signal is issued and the target temperature is controlled toward the standby temperature T _S until then, and the standby state is set. Further, since it is assumed that the power is turned off and the jam processing is performed, it is needless to say that the standby temperature T _S used until then must be held by a storage means such as a memory.

Although the embodiments of the present invention have been described above by the heat roller system, the present invention can be similarly applied to other heating devices such as an on-demand type film heating system.
Further, it goes without saying that the present invention can be applied not only to a laser printer but also to control of a heat fixing device such as a copying machine which is an electrophotographic device using LED exposure, a liquid crystal shutter and the like.

[0054]

As described above, according to the present invention,
The ready temperature and standby time are determined based on the temperature rise value of the heating element per unit time after the power is turned on, so printing is possible even when the ambient environment or the input voltage to the heater changes. Since the temperature is controlled to the desired constant value, it does not cause poor fixing or high temperature offset,
The desired printed image is obtained.

Further, according to the present invention, for the same reason, the heat fixing device of 100V machine and 120V machine is made common,
Furthermore, it is possible to provide an image forming apparatus that can be commonly used for 240V machines for Europe.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a laser printer according to a first embodiment of the present invention.

FIG. 2 is a flowchart showing a control procedure of the first embodiment of the present invention.

FIG. 3 is a characteristic diagram showing changes in the surface temperature of the heating roller in the device of the first embodiment of the present invention.

FIG. 4 is a characteristic diagram showing changes in the heating roller surface temperature in the apparatus of the second embodiment of the present invention.

FIG. 5 is a flowchart showing the control procedure of the second embodiment of the present invention.

FIG. 6 is a characteristic diagram showing changes in the heating roller surface temperature in the device of the third embodiment of the present invention.

FIG. 7 is a flowchart showing a control procedure of a third embodiment of the present invention.

FIG. 8 is a characteristic diagram showing a change in surface temperature of a heating roller in a conventional device.

[Explanation of symbols]

900 Host 901 Heating Roller 902 Heating Source (Halogen Heater) 904 Pressure Roller 905 Temperature Detection Member 910 Laser Printer 911 Photosensitive Drum 914 Developing Device 915 Paper Feeding Roller 916 Registration Roller 917 Transfer Roller 919 Heat Fixing Device 920 Cassette 921 Controller 923 DC Controller T _r Ready temperature T _S Standby temperature T _p Printing temperature α, β, γ Heating roller surface temperature rising speed

Claims (5)

[Claims] 1. A temperature detecting device, comprising: an image forming unit for forming a toner image on a recording material; and a heating body for heating and fixing the toner image formed on the recording material, and detecting the temperature of the heating body. Fixing means having means, and control means for controlling the temperature of the heating body based on the temperature detected by the temperature detecting means, based on a temperature rise value per unit time of the heating body after power is turned on. An image forming apparatus, comprising: a control unit that sets a ready temperature for issuing a ready signal indicating that the image forming operation start request signal can be received, or a standby temperature in a ready state. 2. The control means raises the ready temperature or the standby temperature when the temperature rise value per unit time is low, and takes the overshoot amount into consideration when the temperature rise value per unit time is high. The image forming apparatus according to claim 1, wherein the ready temperature or the standby temperature is set lower. 3. The relationship between the temperature rise value per unit time and the amount of overshoot is stored in advance, and the control means refers to the data stored in the storage means to obtain the ready temperature or the standby temperature. The image forming apparatus according to claim 2, wherein the image forming apparatus is set. 4. The control means, if there is a start request signal for the image forming operation after setting the ready temperature and before the detected temperature of the heating body reaches the set ready temperature, Energizing time of, fixing property by the energizing time,
The image forming apparatus according to claim 1, wherein the ready temperature is corrected in consideration of a high temperature offset and the like. 5. The heating element is a heating roller that is rotationally driven, and the control means stops the heating roller until the detected temperature of the heating roller reaches a specific temperature before reaching the ready temperature, The control for starting the rotation of the heating roller after reaching the specific temperature is performed, and the value of the specific temperature is variably set according to the temperature increase value per unit time. The image forming apparatus according to any one of claims.