JPH09297495A - Fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the rising of temp., without increasing the cost of a device by executing control so as to make the air quantity of a heat discharge fan larger, when the size of a recording paper is smaller than a previously set prescribed size. SOLUTION: A size detecting part 201 detects the size of the recording paper and informs a CPU 202. This CPU 202 compares the size of the recording paper detected by the size detecting part 201 with a reference size capable of making paper passage by a thermal fixing roller. When it is judged that the detected size of the recording paper is smaller, the CPU 202 controls the heat discharge fan 101, to make its air quantity larger. At this time, a special member is not required for discharging heat, so that the heat of a paper nonpassing part can be discharged without increasing the cost of the fixing device. Since the fan 101 discharges the heat of the paper nonpassing part of the thermal fixing roller, freedom in terms of the critical temp. of the thermal fixing roller can be obtained as well and the number of consecutive paper passing times can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device for fixing a toner image on a recording paper by passing the recording paper through a nip portion formed by a heat fixing roller and a pressure roller.
More specifically, the present invention relates to a fixing device that changes an air volume of an exhaust heat fan for exhausting heat radiated from a heat fixing roller to the outside of the device based on the size of a recording paper passing through a nip portion.

[0002]

2. Description of the Related Art As a conventional fixing device, for example, there is a fixing device for fixing a toner image on a recording paper by passing the recording paper through a nip portion formed by a heat fixing roller and a pressure roller. In such a fixing device, the temperature of the non-sheet-passing portion rises when the recording paper of a small size is inevitably passed except for one size of the recording paper which can be passed. In particular, the larger the difference between the size of the recording paper that can be passed and the size of the recording paper that has actually been passed, the more marked the temperature rise.

As a fixing device for solving this, Japanese Patent Laid-Open No. 5-119669, “Fixing device”, Japanese Patent Laid-Open No. 5-13
No. 4575, "Heat roller fixing device",
There is one disclosed in "Image forming apparatus" of Japanese Patent Publication No. 119667.

In the above-mentioned Japanese Patent Laid-Open No. 5-119669, a dedicated fan for cooling the non-sheet passing portion is provided to prevent the temperature rise of the non-sheet passing portion. In addition, JP-A-5-1345
In Japanese Patent No. 75, a plurality of heaters are used for the heat fixing roller to change the temperature distribution of the fixing device to prevent the temperature rise of the non-sheet passing portion.

Further, in Japanese Patent Laid-Open No. 5-119667, a plurality of temperature detecting elements are provided on a heat fixing roller, and the temperature of the non-sheet passing portion and the non-sheet passing portion are detected based on the size of the recording sheet, and the heat fixing is performed. The temperature of the roller heater is set to prevent the temperature rise in the non-sheet passing area.

[0006]

However, according to JP-A-5-119669, JP-A-5-134575 and JP-A-5-119667, a dedicated fan, a plurality of heaters or a temperature detecting element are provided. Although the temperature rise of the fixing device is prevented by providing the above, since a dedicated configuration is required for each,
There is a problem that the cost of the entire fixing device is increased.

The present invention has been made in view of the above, and it is an object of the present invention to prevent the temperature rise of a fixing device without increasing the cost of the device.

[0008]

In order to achieve the above object, a fixing device according to a first aspect of the invention passes a recording sheet through a nip portion formed by a heat fixing roller and a pressure roller,
In a fixing device for fixing a toner image on a recording paper, a heat discharging fan for discharging heat radiated from the heat fixing roller to the outside of the device, a recording paper size detecting means for detecting a size of the recording paper, and the recording paper. Based on the size of the recording paper detected by the paper size detecting means, when the size of the recording paper is smaller than a predetermined size set in advance,
An exhaust heat fan control means for controlling the exhaust heat fan so that the air volume of the exhaust heat fan increases.

A fixing device according to a second aspect of the present invention is the fixing device according to the first aspect, further comprising counting means for counting the number of sheets of recording paper that continuously pass through the nip portion. The heat fan control means controls the exhaust heat fan so that the air volume of the exhaust heat fan is increased when the number of consecutively passed sheets reaches a preset predetermined number based on the number of consecutively passed sheets counted by the counting means. is there.

A fixing device according to a third aspect of the present invention is the fixing device according to the first aspect, further comprising: temperature detecting means for detecting the temperature of the heat fixing roller; and temperature detected by the temperature detecting means. Heater control means for controlling ON / OFF of the heater built in the heat fixing roller, and ON / OF of the heater by the heater control means.
A duty detecting means for detecting the duty of F;
The exhaust heat fan control means varies the air volume of the exhaust heat fan based on the duty detected by the duty detection means.

According to a fourth aspect of the present invention, in the fixing device according to the third aspect, the exhaust heat fan control means compares the duty detected by the duty detection means with a preset ON duty. ，
The air volume is controlled so as to increase when the duty is lower than the ON duty, and to decrease when the duty is higher than the ON duty.

According to a fifth aspect of the present invention, there is provided a fixing device for fixing a toner image on a recording paper by passing the recording paper through a nip portion formed by a heat fixing roller and a pressure roller. A heat exhaust fan for exhausting heat radiated from the fixing roller to the outside of the apparatus, a recording paper size detecting means for detecting the size of the recording paper, and a recording paper size detected by the recording paper size detecting means Then, when the size of the recording paper is equal to or larger than a predetermined size set in advance, an exhaust heat fan control means for controlling the exhaust heat fan so as to reduce the air volume is provided.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A fixing device of the present invention will be described below in order of [First Embodiment], [Second Embodiment], [Third Embodiment] and [Fourth Embodiment] with reference to the drawings. Will be described in detail.

[First Embodiment] FIG. 1 is a configuration diagram of a fixing device according to a first embodiment. The fixing device 100 includes a heat exhaust fan 101, a fixing housing 102, a heat fixing roller 104 that fixes the toner image formed on the recording paper 103,
A pressure roller 105 for pressurizing the recording paper 103 to bring it into close contact with the heat fixing roller 104, and the heat fixing roller 10.
A heater 1 that is provided inside the heater 4 and heats the heat fixing roller 104.
06, and a separation claw 107 for separating the recording paper 103 having the toner image fixed thereon from the heat fixing roller 104. In the figure, reference numerals 108 to 112 represent a part of the structure of the electrostatic recording device in which the fixing device 100 is provided.
Reference numeral 08 is a photoconductor, 109 is a developing roller, 110 is a transfer roller, 111 is a cleaning blade, and 112 is a waste toner tank.

FIG. 2 shows the fixing device 1 according to the first embodiment.
It is a control block diagram of 00. The fixing device 100 according to the first embodiment includes a size detection unit 201 that detects the size of the recording paper 103, and a CPU 202 that serves as exhaust heat fan control means that controls the drive of the exhaust heat fan 101. The size detection unit 201 can be commonly used as a size detection sensor or the like equipped in the electrostatic recording device, and it is not necessary to add it as a special configuration.

Next, the operation of the above configuration will be described. The toner image formed on the photoconductor 108 is the recording paper 1
After being transferred onto the recording paper 103, the recording paper 103 is conveyed to the fixing device 100 and passes through a nip portion composed of a heat fixing roller 104 and a pressure roller 105 heated by a heater 106, so that a toner image is formed on the recording paper 103. To melt and fix.

After the recording paper 103 has passed, the photoconductor 108 is cleaned by the cleaning blade 111 of the unnecessary toner adhering to the photoconductor 108, and the cleaned unnecessary toner is stored in the waste toner tank 112. Retained.

Next, the operation of the fixing device 100 shown in FIG. 2 will be described. The CPU 202 uses the size detection unit 2
When the size of the recording paper 103 detected in 01 is input, it is compared with a preset reference size, and when the detected recording paper size is smaller than the preset reference size, the air volume of the exhaust heat fan 101 is large. Control to be.

Next, the heat fixing roller 104 of the first embodiment
The vicinity of will be described with reference to FIG. FIG. 3 is an explanatory diagram of the vicinity of the heat fixing roller 104 according to the first embodiment. Note that, here, the size (reference size) of the recording sheet through which the heat fixing roller 104 (nip portion) of the first embodiment can be passed is described as a B4 lateral size (hereinafter referred to as B4Y).

If the recording paper 103 has a B5 vertical size (hereinafter referred to as B5T), the diagonal line 301 in FIG.
As shown in, the non-sheet passing area appears. Further, the exhaust heat fan 101 is disposed on the right side in the figure, and allows air to flow from left to right to flow the heat generated by the heat fixing roller 104 and exhaust the heat.

Next, the size detector 201 and the CPU 2
The operation of No. 02 will be described. Size detection unit 20
1 detects the size of the recording paper 103 and notifies the CPU 202 of it. The CPU 202 compares the detected recording paper size with the reference size. Here, since the detected recording paper size (B5T) <reference size (B5Y) (that is, the detected recording paper size is smaller than the preset reference size), the heat exhaust fan 10
The exhaust heat fan 101 is controlled so as to increase the air volume of 1.

As a result of carrying out based on the above configuration and operation, as shown in FIG. 4, for example, the number Nt of consecutively-passed recording sheets 103 reaching the design limit temperature (Tmax) of the fixing device 100 is N1. To N3. Note that FIG. 4 is an explanatory diagram showing the number of continuously fed sheets and the temperature in the first embodiment.

As described above, according to the first embodiment, the size of the recording paper 103 detected by the size detection unit 201 is compared with the reference size that the heat fixing roller 104 can pass,
When it is judged that the size of the detected recording paper 103 is small,
Since the CPU 202 controls the exhaust heat fan 101 so as to increase the air volume of the exhaust heat fan 101, the shaded area 301 in FIG.
The heat of the non-sheet passing portion as shown in can be discharged.
In other words, since no special member is required for exhaust heat, the heat of the non-sheet passing portion can be exhausted without increasing the cost of the fixing device 100.

Further, the exhaust heat fan 101 is used as the heat fixing roller 1.
The heat fixing roller 10 is used to discharge heat from the non-sheet passing portion 04.
There is a margin for the limit temperature of 4 and the number of continuous sheets can be increased.

[Second Embodiment] In the second embodiment, a plurality of recording papers 1 are used in the fixing device 100 of the first embodiment.
A case of fixing No. 03 continuously will be described. Since the second embodiment is basically the same as the first embodiment,
Only the different points will be described with reference to the drawings.

FIG. 5 is a control block diagram of the fixing device 100 according to the second embodiment. The fixing device 100 according to the second embodiment
Is a size detection unit 20 that detects the size of the recording paper 103.
1, a CPU 202 as an exhaust heat fan control means for controlling the drive of the exhaust heat fan 101, and a counter 501 as a counting means for counting the number of sheets of the recording paper 103 continuously passing through the heat fixing roller 104. Has been done.

First, the principle of the second embodiment will be described with reference to the drawings. FIG. 6 is an explanatory diagram showing a case where a plurality of recording sheets 103 are continuously fixed. In the first embodiment, the size (reference size) through which the heat fixing roller 104 can pass is compared with the size of the recording paper 103, and the recording paper 10
When the size of 3 is smaller, the amount of air from the exhaust heat fan 101 is increased to exhaust heat. However, when a plurality of recording papers 103 are continuously fixed, as shown in the figure, the beginning of paper passing (hereinafter , Which is referred to as the initial stage of sheet passing), the heat exhausted from the non-sheet passing portion of the heat fixing roller 104 becomes poor and the temperature rises. Therefore, in the second embodiment, the counter 501 is provided as described above, and the heat fixing roller 104 is connected to the recording paper 1.
03 counts the number of sheets that have passed continuously, and detects the heat exhaustion state of the non-sheet passing portion based on this count value (that is, the number of sheets that have continuously passed).

Next, the operation of the configuration shown in FIG. 5 will be described. The counter 501 counts the number of recording sheets 103 from the start of the sheet passing operation. CPU 202
When the count value of the counter 501 (the number of continuously passing recording sheets 103) reaches a preset number, the air volume of the exhaust heat fan 101 is increased. The CPU 20
As shown in FIG. 4 described in the first embodiment, the number of sheets set to 2 is, for example, the sheet passing number N3, but the number of sheets is different as long as it does not exceed the design limit temperature Tmax. Of course, it is also good.

As described above, according to the second embodiment, the counter 501 counts the number of sheets of recording paper 103 that are continuously passed, and based on the count of the counter 501, the CPU 20
2 controls the air volume of the exhaust heat fan 101, so no special member is required for the exhaust heat, that is, the fixing device 1
It is possible to appropriately dissipate the heat of the non-sheet passing portion without increasing the cost of 00.

Further, since the CPU 202 controls the air volume of the exhaust heat fan 101 based on the count value of the counter 501 by the number N3 of continuously passing sheets, for example, the exhaust heat fan 101
Since the control by the switch element for turning ON / OFF the operation is not performed, the load on the switching element is not hardened, that is, the life of the switching element can be extended.

[Third Embodiment] In the third embodiment, the heat exhaust fan 10 is controlled based on the ON duty of the heater 106.
1 will be described below. The third embodiment
Since it is basically similar to the first embodiment, only different points will be described with reference to the drawings.

FIG. 7 is a control block diagram of the fixing device 100 according to the third embodiment. The fixing device 100 according to the third embodiment
Is a size detection unit 20 that detects the size of the recording paper 103.
1, a CPU 702 as an exhaust heat fan control unit that controls the drive of the exhaust heat fan 101, and a temperature detection unit 701 as a temperature detection unit that detects the temperature of the heat fixing roller 104.
And the CPU 702 has a temperature detecting hand portion 701.
On the basis of the detected temperature detected by the above, the role as a heater control means for controlling the ON / OFF of the heater 106 built in the heat fixing roller 104, and the ON / OFF of the heater 106
It serves as a duty detecting means for detecting the OFF duty, and further changes the air volume of the exhaust heat fan 101 based on the detected duty.

Next, the heat fixing roller 104 of the third embodiment
The vicinity of will be described with reference to FIG. FIG. 8 is an explanatory diagram of the vicinity of the heat fixing roller 104 according to the third embodiment. Here, for example, when the recording paper 103 is B5T, as shown by the diagonal line 801 in FIG.
The non-sheet-passing part where the paper does not pass appears. In addition, the exhaust heat fan 1
Numeral 01 is disposed on the right side in the drawing, and allows air to flow from the left to the right to flow the heat generated by the heat fixing roller 104 and exhaust the heat. At this time, the temperature of the heat fixing roller 104 is detected by the temperature detecting unit 701.

The ON / OFF duty of the heater 106 will be described here. The CPU 702 is the heater 1
When 06 is turned on and the heat fixing roller 104 reaches a preset temperature, the heater 106 is turned off. Now, for example, when there are a plurality of sheets of recording paper 103 fixed in succession and the current air volume of the exhaust heat fan 101 does not lower the temperature of the non-sheet passing portion, the temperature of the heat fixing roller 104 (that is, , The apparent temperature of the heater 106) is detected to be high, and therefore the CPU 702 turns off the heater 106. This turns on
The duty becomes low. In this way recording paper 1
There is a close relationship between the number 03 of sheets passed and the ON / OFF duty of the heater 106.

Therefore, in the third embodiment, a change in duty is detected, the detected duty is compared with a preset ON duty, and if the ON duty is lower than the preset ON duty, the air volume is increased and set. The air volume of the exhaust heat fan 101 is controlled so as to reduce the air volume when the duty is higher than the ON duty.

Next, referring to FIG. 9, the ON duty of the heater 106 at the time of continuous sheet feeding and at the beginning of sheet feeding will be described. FIG. 9 is an explanatory diagram showing the ON duty of the heater 106 during continuous sheet feeding and at the beginning of sheet feeding. As shown in FIG. 9A, in the initial sheet passing, that is, when the recording sheet 103 is passed through the heat fixing roller 104 one by one, the heater 106 is turned on for each sheet (O.
N) The state and the disconnected state (OFF) are repeated.

Further, as shown in FIG. 9B, in the case of continuous sheet feeding, the CPU 702 turns on the switch of the heater 106 based on the temperature detected by the temperature detecting section 701.
Control to turn off / off.

In this way, the temperature detecting unit 701 detects the temperature in the vicinity of the heat fixing roller 104, and controls the ON / OFF of the switch of the heater 106 based on the detected temperature in the vicinity of the heat fixing roller 104. . In other words, the ON duty is low (that is, the heater 106) by detecting the ON rate (that is, the ON duty) of the heater 106 at a predetermined time (constant time) T0.
Is small), the heat fixing roller 104
It is possible to detect that the temperature near the heat fixing roller 104 is low, and it is possible to detect that the temperature near the heat fixing roller 104 is low when the ON duty is high (that is, the ON ratio of the heater 106 is high).

Therefore, the CPU 702 detects the detected O
When the N duty is lower than the preset ON duty (reference value), the exhaust heat fan 101 is controlled to increase the air volume. As a result, as shown in FIG. 10, there is no unevenness in the temperature of the heat fixing roller 104 at the paper passing or non-paper passing positions, that is, at the positions on the heat exhaust fan 101 side and the opposite side. Note that FIG. 10 is an explanatory diagram showing the temperature of the heat fixing roller 104 and the temperature in the vicinity of the heat fixing roller 104 with respect to the air volume of the exhaust heat fan 101.

Next, the above operation will be described with reference to FIG. FIG. 11 is a flowchart showing the operation of the third embodiment. First, when the operation of the fixing device 100 is started, the CPU 702 starts the exhaust heat fan 101 with a small air volume (S1101), and the ON duty of the switch of the heater 106 is a preset value (reference value).
It is determined whether it is lower than A (S1102). The reference value A at this time corresponds to the previously set ON duty (reference value).

Then, in step S1102, C
When the ON duty of the switch of the heater 106 is higher than the reference value A, the PU 702 does not change the air flow rate of the exhaust heat fan 101 and operates it as it is (S1103),
On the other hand, when the ON duty of the switch of the heater 106 is lower than the reference value A, the air volume of the exhaust heat fan 101 is increased (S1104), and the process ends.

As described above, according to the third embodiment, a change in duty is detected, the detected duty is compared with a preset ON duty, and when the duty is lower than the preset ON duty, the air volume is increased. ，
Since the air volume of the exhaust heat fan 101 is controlled so as to reduce the air volume when the ON duty is higher than the set ON duty, the heat-fixing roller 104 is at the sheet passing and non-sheet passing positions, that is, on the heat exhaust fan 101 side and the opposite side. The temperature at the side position is uniform. Therefore, it is possible to maintain a constant temperature in the paper passing portion and the non-paper passing portion of the heat fixing roller 104.

Further, the switch of the heater 106 is turned ON / O.
Since the number of FFs can be reduced, the load on the switching element is not hardened, that is, the life of the switching element can be extended.

[Fourth Embodiment] In the fourth embodiment, a case will be described in which the heat-fixing roller 104 can pass the same size as the recording paper 103. Since the fourth embodiment has basically the same configuration as that of the first embodiment, only different points will be described.

The size detection unit 201 detects the size of the recording paper 103, and the CPU 202 compares the detected size of the recording paper 103 with a reference size through which the heat fixing roller 104 can pass (in the fourth embodiment, Standard size is B
4Y).

The CPU 202 controls the exhaust heat fan 101 so as to reduce the air volume of the exhaust heat fan 101 when the detected size of the recording paper 103 is equal to or larger than the reference size.

As described above, according to the fourth embodiment, when the size of the recording paper 103 detected by the size detection unit 201 is equal to or larger than the preset predetermined size (reference size), the air volume of the exhaust heat fan 101 is increased. Since the exhaust heat fan 101 prevents the temperature of the heat fixing roller 104 from being unnecessarily lowered, the fixing rate can be maintained.

[0048]

As described above, according to the fixing device (Claim 1) of the present invention, the recording paper is passed through the nip portion formed by the heat fixing roller and the pressure roller, and the recording paper on the recording paper is passed. In a fixing device for fixing a toner image, an exhaust fan for exhausting heat radiated from the heat fixing roller to the outside of the device, recording paper size detecting means for detecting the size of the recording paper, and recording paper size detection An exhaust heat fan control means for controlling, based on the size of the recording paper detected by the means, to increase the air volume of the exhaust heat fan when the size of the recording paper is smaller than a preset predetermined size; , The temperature rise of the fixing device can be prevented without increasing the cost of the device.

The fixing device of the present invention (claim 2) is
The fixing device according to claim 1, further comprising: a counting unit that counts the number of sheets of recording paper that continuously pass through the nip portion, wherein the exhaust heat fan control unit includes:
Based on the number of consecutively passed sheets counted by the counting means, when the number of consecutively passed sheets reaches a preset predetermined number, the air volume of the exhaust heat fan is controlled to increase, resulting in an increase in the cost of the apparatus. It is possible to prevent the temperature rise of the fixing device without increasing the number of sheets, and to set the number of sheets to pass the air volume from small to large so that the number of times of switching the actual air volume is reduced and the ON / OFF control element length is increased. Life can be extended.

The fixing device of the present invention (claim 3) is
The fixing device according to claim 1, further comprising: a temperature detecting means for detecting the temperature of the heat fixing roller, and turning on / off a heater built in the heat fixing roller based on the detected temperature detected by the temperature detecting means. Heater control means for controlling OFF, and ON of the heater by the heater control means
The exhaust heat fan control means varies the air volume of the exhaust heat fan based on the duty detected by the duty detection means, thereby increasing the cost of the device. It is possible to prevent the temperature of the fixing device from rising without causing any increase in the number of sheets, and to set the number of sheets to pass the air volume from small to large by setting the ON duty of the heater. The life of the ON / OFF control element can be extended.

The fixing device of the present invention (claim 4) is
4. The fixing device according to claim 3, wherein the exhaust heat fan control means compares the duty detected by the duty detection means with a preset ON duty, and increases the air volume when the duty is lower than the ON duty. Since the air volume is controlled to be smaller when the duty is higher than the ON duty, the temperature rise of the fixing device can be prevented without increasing the cost of the apparatus, and the number of sheets to be passed can be increased to decrease the air volume. By setting the ON duty of the above, the number of times of switching the actual use air volume is reduced, and the life of the ON / OFF control element can be extended.

The fixing device of the present invention (claim 5) is
In a fixing device for fixing a toner image on a recording paper by passing the recording paper through a nip portion formed by a heat fixing roller and a pressure roller, the heat radiation from the heat fixing roller is exhausted to the outside of the device. Based on the heat exhaust fan, the recording paper size detecting means for detecting the size of the recording paper, and the size of the recording paper detected by the recording paper size detecting means, a predetermined size of the recording paper is set in advance. When the size is equal to or larger than the size, the exhaust heat fan control means for controlling the exhaust heat fan so as to reduce the air volume is provided, so that the temperature rise of the fixing device can be prevented without increasing the cost of the device. At the same time, the fixing rate can be maintained by eliminating the temperature drop of the fixing device.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a fixing device according to a first embodiment.

FIG. 2 is a control block diagram of the fixing device according to the first embodiment.

FIG. 3 is an explanatory diagram around a heat fixing roller according to the first embodiment.

FIG. 4 is an explanatory diagram showing the number of continuously fed sheets and the temperature according to the first embodiment.

FIG. 5 is a control block diagram of the fixing device according to the second embodiment.

FIG. 6 is an explanatory diagram showing a case where a plurality of recording sheets are continuously fixed.

FIG. 7 is a control block diagram of the fixing device according to the third embodiment.

FIG. 8 is an explanatory diagram around the heat fixing roller according to the third embodiment.

FIG. 9 is an explanatory diagram showing the ON duty of the heater during continuous sheet feeding and at the beginning of sheet feeding.

FIG. 10 is an explanatory diagram showing the temperature of the vicinity of the heat fixing roller in the position of the heat fixing roller and the air volume of the exhaust heat fan.

FIG. 11 is a flowchart showing the operation of the third embodiment.

[Explanation of symbols]

 100 Fixing Device 101 Exhaust Heat Fan 102 Fixing Housing 103 Recording Paper 104 Thermal Fixing Roller 105 Pressure Roller 106 Heater 107 Separation Claw 108 Photoconductor 110 Transfer Roller 111 Cleaning Blade 112 Waste Toner Tank 201 Size Detection Unit 202, 702 CPU 501 Counter 701 Temperature detection part 801 Diagonal line (non-sheet passing part)

Claims (5)

[Claims] 1. A fixing device for fixing a toner image on a recording paper by passing the recording paper through a nip portion formed by a heat fixing roller and a pressure roller, and radiating heat from the heat fixing roller to the outside of the device. Based on the heat exhaust fan for discharging heat, the recording paper size detecting means for detecting the size of the recording paper, and the recording paper size detected by the recording paper size detecting means, the size of the recording paper is preset. A fixing device comprising: an exhaust heat fan control means for controlling the exhaust heat fan so that the air flow rate of the exhaust heat fan increases when the size is smaller than a predetermined size. 2. The fixing device according to claim 1, further comprising: counting means for counting the number of sheets of recording paper that continuously pass through the nip portion, wherein the exhaust heat fan control means has the counting means. The fixing device is characterized in that, when the number of continuously passed sheets reaches a predetermined number set in advance, the air volume of the exhaust heat fan is controlled to increase based on the number of continuously passed sheets counted in (4). 3. The fixing device according to claim 1, further comprising: a temperature detecting means for detecting the temperature of the heat fixing roller; and a detected temperature detected by the temperature detecting means,
The exhaust heat fan control is provided with: heater control means for controlling ON / OFF of a heater built in the heat fixing roller; and duty detection means for detecting a duty of ON / OFF of the heater by the heater control means. A fixing device, wherein the means varies the air volume of the exhaust heat fan based on the duty detected by the duty detecting means. 4. The fixing device according to claim 3, wherein the exhaust heat fan control means compares the duty detected by the duty detection means with a preset ON duty, and when the duty is lower than the ON duty, the air volume is reduced. The fixing device is characterized in that the air flow rate is increased and the air flow rate is controlled to be reduced when the duty ratio is higher than the ON duty. 5. A fixing device for fixing a toner image on a recording paper by allowing the recording paper to pass through a nip portion formed by a heat fixing roller and a pressure roller, and radiating heat from the heat fixing roller to the outside of the device. Based on the heat exhaust fan for discharging heat, the recording paper size detecting means for detecting the size of the recording paper, and the recording paper size detected by the recording paper size detecting means, the size of the recording paper is preset. A fixing device, comprising: an exhaust heat fan control means for controlling the exhaust heat fan so that the air flow rate of the exhaust heat fan is reduced when the size is equal to or larger than a predetermined size.