JPH08171298A - Fixing device - Google Patents

Abstract

PURPOSE: To provide a durable fixing device capable of making the nip/pressure of a fixing roller and a pressure roller uniform and attaining excellent fixing. CONSTITUTION: A fixing device 10 for fixing a toner image in such a manner that a transfer paper on which the toner image is formed passes through between the fixing roller 1 composed of a thin cylindrical heated body and the pressure roller 3 which is inclined toward the shaft of the fixing roller 1 and in press-contact therewith is provided with straightening rollers 4a and 4b coming into contact with the outer periphery of the fixing roller 1 on both sides, in a direction perpendicular to the pressing direction of the pressure roller 3, to straighten collapse in the radial direction of the fixing roller 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a printer, a copying machine, an F
The present invention relates to a fixing device used in an image forming apparatus such as an AX.

[0002]

2. Description of the Related Art A fixing device of an image forming apparatus is mainly composed of a fixing roller having a heater built therein and a pressure roller pressed against the fixing roller. The sheet on which the toner image has been transferred is pressed against the fixing roller by a pressure roller, and the toner image is melted by the heat and pressure of the heater and fixed on the sheet.

[0003]

Since a high pressure is applied to the fixing roller described above, the fixing roller 51 is formed thick as shown in FIGS. 10a and 10b. However, if the fixing roller 51 is formed thick, the heat capacity of the heater 50 must be increased in order to heat the fixing roller to a predetermined temperature.
There is a problem that it consumes a lot of power and at the same time, it takes time for the fixing device to start up. Further, when the fixing roller 51 is pressed by the pressure roller 52 in the direction of the arrow F1 in the figure, the center portion of the fixing roller 51 is bent.

In the above-mentioned fixing device, a gap is formed between the central portion of the fixing roller 51 and the pressure roller 52 due to this bending of the fixing roller 51, and the width of the nip in the central portion of the fixing roller 51 is narrow. In addition, there is a problem that fixing pressure becomes insufficient and fixing failure occurs.

If the bending amount is within 0.1 mm, there is no problem as the nip distribution.

Further, as shown in FIGS. 11a and 11b,
It is considered that the fixing roller 61 is configured by a thin-walled cylinder (thickness of the core metal is 1 mm or less) having a small heat capacity to improve the start-up of the fixing device. However, the fixing roller 61 is a thin-walled cylindrical body. According to the structure, the pressure roller 62 causes the fixing roller 61 to be crushed in the radial direction thereof by the pressing force in the direction of the arrow F2 in FIG. In the thin fixing roller 61, due to the bending and crushing that occurs in the fixing roller, a gap is formed between the central portion of the fixing roller and the pressure roller, and the nip and pressure in the central portion of the fixing roller are insufficient, resulting in poor fixing. However, there is a problem in that the transfer paper is wrinkled or wrinkles are generated, which results in poor conveyance.
As shown in FIGS. 10a and 10b, the deformation amount d1 of the thick fixing roller 51 is smaller than the deformation amount d2 of the thin fixing roller 61.

In view of this, in Japanese Patent Laid-Open No. 59-38777, the auxiliary roller is pressed against the shaft of the fixing roller so as to prevent the bending of the fixing roller, thereby eliminating the insufficient pressure in the central portion of the fixing roller. The technology aimed at is introduced.

Further, Japanese Patent Application Laid-Open No. 61-90180 discloses a technique in which the fixing roller is elastically deformed by a guide member to eliminate the shortage of the nip between the fixing roller and the pressure roller.

The technique of providing the above-mentioned auxiliary roller has a problem that the pressure contact force of the auxiliary roller further causes the fixing roller to be crushed and the shortage between the nip and the pressure at the central portion of the fixing roller cannot be sufficiently solved. is there.

The technique of elastically deforming the fixing roller has a problem that when the pressure is increased, the central portion of the fixing roller becomes insufficient in pressure, resulting in defective fixing. Furthermore, if the fixing roller is rotated for a long time while being elastically deformed, there is concern about durability such as creep.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fixing device which can make the nip and pressure between the fixing roller and the pressure roller uniform, perform good fixing, and have durability.

[0012]

According to the first aspect of the present invention,
The toner image is fixed by passing the transfer paper on which the toner image is formed between a fixing roller composed of a heated thin cylindrical body and a pressure roller which is in pressure contact with the fixing roller at an angle. In the fixing device, a straightening roller which is in a direction orthogonal to the pressing direction of the pressure roller and which is in contact with both sides of the outer peripheral surface of the fixing roller to correct the radial collapse of the fixing roller is provided. To do.

According to a second aspect of the present invention, in the fixing device according to the first aspect, the swinging means for selectively swinging the correction roller between a position in contact with the fixing roller and a position separated from the fixing roller. And a control means for controlling the operation of the swinging means so that the correction roller is brought into contact with the fixing roller only when the transfer sheet passes.

According to a third aspect of the present invention, in the fixing device according to the first or second aspect, the straightening roller also serves as a cleaning roller for cleaning the fixing roller.

According to a fourth aspect of the present invention, in the fixing device according to the first aspect, the second aspect, or the third aspect, an elastic body having a JIS A hardness of 30 ° to 80 ° is provided on the outer peripheral surface of the straightening roller. It is characterized in that a layer is formed.

According to a fifth aspect of the present invention, in the fixing device according to the first aspect, the second aspect, the third aspect, or the fourth aspect, the pressure applied to the fixing roller by the pressure roller is changed to that of the transfer paper. It is characterized by having a pressing force changing means for changing it according to the thickness.

According to a sixth aspect of the present invention, in the fixing device according to the fifth aspect, when the pressing force changing means has a large thickness of the transfer sheet, the pressing force of the pressure roller to the fixing roller is increased. If the thickness of the transfer paper is small,
The pressure applied to the fixing roller by the pressure roller is reduced.

According to a seventh aspect of the present invention, in the fixing device according to the fifth or sixth aspect, the correction roller is responsive to a change in pressure applied to the fixing roller of the pressure roller by the pressure changing means. The pressure contact force changing means for changing the pressure contact force with respect to the fixing roller is provided.

According to an eighth aspect of the invention, in the fixing device according to the seventh aspect, the pressure contact force changing means fixes the fixing roller by the fixing roller when the pressure roller applies a high pressure to the fixing roller. When the pressure contact force against the roller is increased and the pressure applied to the fixing roller by the pressure roller is low,
The pressing force of the straightening roller against the fixing roller is reduced.

[0020]

According to the present invention, the fixing roller is crushed in the radial direction by the pressure of the pressure roller when the toner image is fixed, but the straightening rollers arranged in contact with both sides of the outer peripheral surface of the fixing roller prevent the crushing of the fixing roller. It is corrected so that the nip and pressure contact force between the fixing roller and the pressure roller are made uniform.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings. 1 and 2, reference numeral 10 represents a fixing device applied to the image forming apparatus. Fixing device 10
Is a thin wall (0.3-
(1.0 mm) cylindrical fixing roller 1, a pressure roller 3 in pressure contact with the fixing roller 1, a direction orthogonal to the pressure direction of the pressure roller 3, and both sides of the outer peripheral surface of the fixing roller 1 Pressing force changing means for changing the pressing force of the pressure roller 3 to the fixing rollers 1 and the correcting rollers 4a and 4b that come in contact with each other and correct the crushing of the fixing roller 1 in the radial direction according to the thickness of the transfer sheet. 20 and.

A resin layer of ethylene tetrafluoride or the like is formed on the outer peripheral surface of the fixing roller 1 in order to improve the releasability of the toner, and a heater 2 for heating the fixing roller 1 is provided inside the resin layer. Are arranged in the longitudinal direction of the fixing roller 1.

As shown in FIG. 2, the heater 2 has a stay 1 whose both ends are fixed to the casing 11 of the fixing device 10.
It is supported by 2a and 12b. A sensor 5 for controlling the temperature of the fixing roller 1 is provided on the outer peripheral surface of the fixing roller 1. The sensor 5 is connected to the control means 30, and the control means 30 controls energization of the heater 2 based on the information from the sensor 5.

As shown in FIG. 2, the fixing roller 1 is rotatably supported by bearings 6a and 6b fixed to a casing 11 via heat insulating bushes 7a and 7b.
A fixing roller gear 8 is press-fitted into one end of the fixing roller 1, and a driving gear 9 rotatably driven by a driving unit (not shown) is meshed with the fixing roller gear 8.

Generally, as the thickness of the transfer paper increases, the fixability deteriorates. This is because when the transfer paper enters the nip, the heat from the pressure roller 3 on the back surface is hard to be transferred, and the fiber density (tightness) of the transfer paper is high, so that the surface of the transfer paper is smooth. This is because the binding force between the melted toner and the transfer paper is weak due to the high temperature.

In this example, when the thickness of the transfer paper is large,
The pressure contact force of the pressure roller 3 to the fixing roller 1 is high, and the pressure contact force of the pressure roller 3 to the fixing roller 1 is low when the thickness of the transfer paper is small. Therefore, the pressure roller 3 will be described.

The pressure roller 3 includes a core metal 3a, a rubber layer 3b made of solid silicone rubber or foamed silicone rubber provided outside the core metal 3a, and a PFA tube layer wound around the rubber layer 3b. 3c and 3c. The pressure roller 3 has bearings 13 at both ends of its shaft 3d.
It is rotatably supported by a and 13b. A support plate 14 that supports the pressure roller 3 with respect to the fixing roller 1 so as to be able to come into contact with and separate from the fixing roller 1 is screwed to the bottom plate of the casing 11 with a screw 15.

The support plate 14 has a U-shaped cross section, and bearings 13a, 1 are provided on the rising surfaces of both ends thereof.
U grooves 16a and 16b for supporting 3b are provided, respectively. At the lower ends of the bearings 13a and 13b, the bearings 13a,
Pressure arms 17a and 17b for moving the 13b up and down along the U grooves 16a and 16b are arranged.

The pressurizing arms 17a and 17b have their base ends swingably supported by a shaft 18 provided on the support plate 14, and their free ends are biased by springs 19 in a direction in which they are pressed against the bearings 13a and 13b. Has been done. An eccentric cam 21 for controlling the swing amount of the pressure arms 17a, 17b and changing the pressure contact force of the pressure roller 3 to the fixing roller 1 is in contact with the free ends of the pressure arms 17a, 17b. There is. The shaft 21a of the eccentric cam 21 is rotated by the pulse motor M.

A paper thickness sensor 31 for detecting the thickness of the transfer paper and a power switch 32 are connected to the control means 30. When the paper thickness sensor 31 determines that the thickness of the transfer paper is normal, the control means 30 determines as shown in FIG.
The pulse motor M is driven so that the pressure roller 3 comes into contact with the fixing roller 1 with the normal pressing force FA, and the eccentric shaft 21a,
The eccentric cam 21 is rotated.

On the other hand, when the paper thickness sensor 31 determines that the thickness of the transfer paper is thicker than normal, the control means 30 operates as shown in FIG.
As shown in b, the pulse motor M is driven so that the pressure roller 3 comes into contact with the fixing roller 1 with the pressing force FB (higher than the normal pressing force FA) for thick paper, and the eccentric shaft 21a and the eccentric cam 21 are used. To rotate.

Further, the control means 30 includes a power switch 3
When 2 is off, in order to prevent the fixing roller 1 from creeping, as shown in FIG. 3B, the pulse motor M is driven to move the pressure roller 3 to a position separated from the fixing roller 1.

Here, the pressing arms 17a and 17b, the eccentric cam 21, the eccentric shaft 21a, the pulse motor M, and the paper thickness sensor 31 mainly constitute the pressing force changing means 20. Instead of the paper thickness sensor 31, a paper thickness key for selecting the thickness of the transfer paper may be provided on the operation panel of the image forming apparatus and manually input.

As shown in FIG. 4, the pressure roller 3 is arranged so as to be inclined with respect to the fixing roller 1 by a predetermined angle θ. This helps to even out the nip between the fuser roller 1 and the pressure roller 3.

As shown in FIG. 5, the correction rollers 4a and 4b are oscillating means 22 for selectively oscillating the rollers 4a and 4b between a position where they are pressed against the fixing roller 1 and a position where they are separated from the fixing roller 1. It is connected to the. The swinging means 22 includes a pair of L-shaped arms 23a and 23b that rotatably support the shafts 24a and 24b of the correction rollers 4a and 4b, and an arm shaft 25a that swingably supports the arms 23a and 23b, respectively. 25b and a solenoid 27 that swings the arms 23a and 23b about the arm shafts 25a and 25b via T-shaped links 26, respectively.

A long hole 26a is formed in the link 26, and pins 28a and 28b, which are respectively planted at one ends of the arms 23a and 23b, are slidably fitted in the long hole 26a. The solenoid 27 is connected to the control means 30 (see FIG. 1) and its operation is controlled.

On the other hand, in the control means 30, as shown in FIG. 6, a separation filler 33a and a separation sensor for detecting the front end of the transfer paper P on which the toner image on the image carrier 70 is transferred by the transfer separation charger 71. 33b is connected to a paper discharge filler 34a and a paper discharge sensor 33b for detecting the trailing edge of the transfer paper P that has passed through the nip between the fixing roller 1 and the pressure roller 3.

When the front end of the transfer paper P is detected by the separation filler 33a and the separation sensor 33b, the control means 30 excites the solenoid 27, pulls the plunger 27a downward in FIG. 5, and moves the arms 23a and 23b. The straightening roller 4a, which swings around the arm shafts 25a and 25b,
4b is swung to the position where it is pressed against the fixing roller 1, and when the trailing edge of the transfer sheet P is detected by the sheet discharge filler 34a and the sheet discharge sensor 33b, the solenoid 27 is demagnetized and the plunger 27a is moved to the position shown in FIG. Pull it upwards, arm 23a,
23b swings around the arm shafts 25a and 25b,
The straightening rollers 4a and 4b are swung to a position separated from the fixing roller 1.

Further, the pressing force of the straightening rollers 4a and 4b against the fixing roller 1 is determined by the plunger 27 of the solenoid 27.
It can be adjusted by changing the gap d between the ring 29 fitted in a and the body of the solenoid 27. Figure 5
By arranging the arms 23a and 23b symmetrically with respect to the fixing roller 1 as shown in FIG. 3, the pressure applied by the solenoid 27 can be evenly applied to both sides of the fixing roller 1.

The straightening roller 4a and the straightening roller 4b are
Since it is formed of the same material, the straightening roller 4a
The configuration will be described below. As shown in FIG. 7a, the straightening roller 4a includes a shaft 24a and a thick elastic layer 35a formed outside the shaft 24a. Further, as shown in FIG. 7b, the straightening roller 4a supports the cylindrical cored bar 37a on the shaft 24a via the flange 36a, and the thin elastic layer 35 is formed on the outer peripheral surface of the cylindrical cored bar 37a.
A'may be formed.

Further, in the present embodiment, the diameter of the shaft 24a is set to 1 so that the correction roller 4a does not bend when the correction roller 4a is pressed against the fixing roller 1 by the swinging means 22.
It is set to 0 mm or more.

The elastic layer 35a is preferably made of, for example, silicone rubber having a JISA hardness of 30 ° to 50 °. Fluorine rubber with JIS A hardness
The hardness may be 70 ° to 80 °. This is because the straightening roller 4a has sufficient hardness and rigidity (no bending occurs) to correct the crushing that occurs in the fixing roller 1, and does not cause the fixing roller 1 to be scratched or worn.

Offset toner and paper dust adhere to the surface of the fixing roller 1 when the toner image is transferred. Therefore, FIG. 8 shows an example in which the correction rollers 4a and 4b are made of a material having a lower releasability than the fixing roller 1 and are also used as a cleaning roller for cleaning the fixing roller 1.

Blades 38a and 38b are arranged in contact with the surfaces of the straightening rollers 4a and 4b, respectively.
The blades 38a and 38b are respectively provided in casings 39a and 39b provided so as to cover a part of the straightening rollers 4a and 4b.

A straightening roller 4a which is in pressure contact with the fixing roller 1,
Foreign matter T such as offset toner or paper dust on the fixing roller 1 adheres to the 4b. The adhered foreign matter T is scraped off by the blades 38a and 38b, and the casings 39a and 39a
It is stored in b and the fixing roller 1 is cleaned. Further, as shown in the figure, when the two fixing rollers 4a and 4b are arranged at symmetrical positions with respect to the fixing roller 1, the foreign matter T not adhered to the correcting roller 4b can be adhered to the correcting roller 4a, Sufficient cleaning performance can be maintained for a long time.

When the pressing force of the pressure roller 3 applied to the fixing roller 1 by the pressing force changing means 20 (see FIGS. 1 and 3) increases, the bending and crushing of the fixing roller 1 also increase. Therefore, it is necessary to increase the pressing force of the straightening rollers 4a and 4b against the fixing roller 1.

A concrete example thereof is shown in FIG. The straightening rollers 4a and 4b shown in this example include the pressure contact force changing means 40 that changes the pressure contact force with respect to the fixing roller 1 in accordance with the change in the pressure applied to the fixing roller 1 by the pressure applying force changing means 20. Have. Note that FIG. 9 shows a case where the thickness of the transfer paper P is normal.

The pressure contact force changing means 40 includes a straightening roller 4a,
4b, a pair of doglegged arms 41a and 41b that rotatably support the shafts 24a and 24b, and arms 41a and 41b.
arm shafts 42a, 42 for respectively swingably supporting b
b and the arms 41a and 41b are respectively connected to the arm shaft 42.
It is mainly composed of a pressure plate 43 that swings around a and 42b.

The pressure plate 43 is supported by the shaft 3d of the pressure roller 3 and moves in conjunction with the movement of the pressure roller 3 by the pressing force changing means 20. At both ends of the pressure plate 43,
The long holes 43a and 43b are provided respectively, and the long holes 4
Pins 44a and 44b, which are respectively implanted in the arms 41a and 41b, are slidably inserted into the arms 3a and 43b.

In this structure, when the pressure applied by the pressure roller 3 to the fixing roller 1 becomes high (see FIG. 3b, the transfer paper P).
Is thick), the pressure plate 43 also moves in conjunction with the movement of the pressure roller 3 to the fixing roller 1 side. With the movement of the pressure plate 43, the pins 44a and 44b move upward in FIG. 9, swing the arms 41a and 41b respectively, and press the correction rollers 4a and 4b against the fixing roller 1 more strongly.

On the other hand, when the pressure roller 3 is in a position separated from the fixing roller 1 (see FIG. 3c, power switch 32).
Is off), in the opposite direction to the above case, the pressure plate 43
Moves, the pins 44a and 44b move downward in FIG. 9, the arms 41a and 41b swing, and the correction rollers 4a and 4b move to positions separated from the fixing roller 1.

[0052]

According to the first aspect of the present invention, the fixing roller is crushed in the radial direction by the pressure of the pressure roller at the time of fixing the toner image, but the straightening roller is arranged in contact with both sides of the outer peripheral surface of the fixing roller. Corrects the crushing of the fixing roller and makes the nip and pressure contact force between the fixing roller and the pressure roller uniform, so that good fixing can be performed.

According to the second aspect of the invention, the control means controls the operation of the swinging means so that the correction roller is brought into contact with the fixing roller only when the transfer sheet passes, so that the stress on the fixing roller can be reduced and the durability can be improved. It is possible to provide a device with

In the third aspect of the invention, since the straightening roller also serves as the cleaning roller for cleaning the fixing roller, the number of parts can be reduced, the cost can be reduced, and the cleaning can be reliably performed for a long time.

According to the fourth aspect of the present invention, since the elastic layer having a JISA hardness of 30 ° to 80 ° is formed on the outer peripheral surface of the straightening roller, when the straightening roller is pressed against the fixing roller, the fixing roller is fixed. It is possible to prevent stress such as scratches and wear on the rollers.

According to the fifth aspect of the present invention, the pressing force changing means changes the pressing force applied to the fixing roller by the pressure roller according to the thickness of the transfer paper. Therefore, it is necessary even if the thickness of the transfer paper changes. Sufficient fixability can be obtained.

According to the sixth aspect of the invention, when the pressing force changing means increases the thickness of the transfer paper, the pressing force of the pressure roller to the fixing roller is increased, and when the thickness of the transfer paper is small,
Since the pressure applied to the fixing roller by the pressure roller is lowered, the durability of the apparatus is increased and necessary and sufficient fixing property can be obtained even if the thickness of the transfer paper changes.

According to the seventh aspect of the invention, since the pressure contact force changing means changes the pressure contact force of the correction roller with respect to the fixing roller in accordance with the change of the pressure applied to the fixing roller of the pressure roller by the pressure force changing means, the transfer is performed. Even if the paper thickness changes, the nip and pressure contact force between the fixing roller and the pressure roller are made uniform, and good fixing can be performed.

According to the eighth aspect of the present invention, the pressure contact force changing means increases the pressure contact force of the straightening roller against the fixing roller when the pressure of the pressure roller against the fixing roller is high. When the pressure applied to the fixing roller is low, the pressure contact force of the straightening roller with respect to the fixing roller is reduced, so that the nip and pressure contact force between the fixing roller and the pressure roller become uniform even if the thickness of the transfer paper changes. Good fixing can be performed.

[Brief description of drawings]

FIG. 1 is a schematic front view of a fixing device according to an embodiment of the present invention.

FIG. 2 is a vertical sectional view of the fixing device shown in FIG.

3A is a front view of a main part of a fixing device for explaining the operation of a pressing force changing unit when the thickness of a transfer sheet is normal, and FIG.
Is a front view of the main part of the fixing device for explaining the operation of the pressing force changing means when the thickness of the transfer paper is thicker than usual, and c is the operation of the pressing force changing means when the power switch is off. FIG. 3 is a front view of a main part of a fixing device for performing the above.

FIG. 4 is a perspective view showing a pressure roller that is pressed against the fixing roller while being inclined.

5 is a front view showing a swinging means of the correction roller shown in FIG. 1. FIG.

6 is a schematic front view of the image forming apparatus for explaining the operation of the swinging means of the correction roller shown in FIG.

7A is a sectional view of a straightening roller, and FIG. 7B is a sectional view of another straightening roller.

FIG. 8 is a front view of a fixing device showing an example in which a straightening roller is also used as a cleaning roller.

FIG. 9 is a front view of a main part of a fixing device according to another embodiment of the present invention.

10A is a front view of a fixing roller illustrating a problem of a conventional fixing device, and FIG. 10B is a side view of FIG. 10A.

11A is a front view of a fixing roller illustrating a problem of a conventional fixing device, and FIG. 11B is a side view of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fixing roller 2 Heater 3 Pressure roller 4a, 4b Straightening roller 5 Sensor 6a6b Bearing 10 Fixing device 20 Pressurizing force changing means 27 Solenoid 30 Control means 31 Paper thickness sensor 32 Power switch 40 Pressurizing force changing means

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Yura 1-3-3 Nakamagome, Ota-ku, Tokyo, Ricoh Co., Ltd.

Claims (8)

[Claims] 1. A transfer paper on which a toner image is formed is passed between a fixing roller composed of a heated thin cylindrical body and a pressure roller which is in pressure contact with the fixing roller in an inclined manner. In the fixing device for fixing the fixing roller, a correction roller is provided which is in a direction orthogonal to the pressing direction of the pressing roller and is in contact with both sides of the outer peripheral surface of the fixing roller to correct the radial collapse of the fixing roller. A fixing device characterized by the above. 2. An oscillating means for selectively oscillating the correction roller between a position in contact with the fixing roller and a position separated from the fixing roller, and the fixing roller only when the transfer paper passes through the correction roller. 2. The fixing device according to claim 1, further comprising a control unit that controls the operation of the swinging unit so as to contact the roller. 3. The fixing device according to claim 1, wherein the straightening roller also serves as a cleaning roller for cleaning the fixing roller. 4. The fixing according to claim 1, wherein an elastic body layer having a JIS A hardness of 30 ° to 80 ° is formed on the outer peripheral surface of the straightening roller. apparatus. 5. A pressing force changing means for changing the pressure contact force of the pressure roller against the fixing roller according to the thickness of the transfer paper. The fixing device according to claim 3 or 4. 6. The pressing force changing means increases the pressing force of the pressure roller to the fixing roller when the thickness of the transfer paper is large, and when the thickness of the transfer paper is small, The fixing device according to claim 5, wherein the pressure applied to the fixing roller by the pressure roller is lowered. 7. A pressure contact force changing means for changing a pressure contact force of the correction roller with respect to the fixing roller according to a change in pressure of the pressure roller against the fixing roller by the pressure changing means. The fixing device according to claim 5, wherein the fixing device is a fixing device. 8. The pressure contact force changing means increases the pressure contact force of the straightening roller against the fixing roller when the pressure of the pressure roller against the fixing roller is high, thereby fixing the pressure roller against the fixing roller. 8. The fixing device according to claim 7, wherein the pressing force of the straightening roller against the fixing roller is reduced when the pressing force against the roller is low.