JPH0744046A - Fixing device and image forming device - Google Patents

Abstract

PURPOSE:To provide a sufficient fixing strength irrespective of the thickness of recording sheet and without generating wrinkles or curls by supporting heating and pressing rollers on fixed positions by a supporting means so as to maintain a specified axle-to-axle distance. CONSTITUTION:Both ends of a heating roller 12 and both ends of the axis 26 of a pressing roller 20 are rotatably supported on fixed positions by a bearing 28 used for both rollers. At this time, an elastic body layer 24 is press-contacted with the heating and pressing rollers 12 and 20 in an elastically deformed condition and they are made rotatable. At this time, when a nip width is B, the diameter of the heating roller 12 is D1 and that of the pressuring roller is D2, an axle-to-axle distance L is decided by L=D1/2+D2/2-B<2>/4(1/D1+1/D2) and the heating and pressing rollers 12 and 20 are supported on the fixed positions by the bearing 28 so that this axle-to-axle distance L is maintained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device, and more particularly to a fixing device which includes a heating roller and a pressure roller and is used in an image forming apparatus such as an electrophotographic copying machine, a laser printer or an LED printer. .

[0002]

2. Description of the Related Art In a fixing device of this type, in many cases, a heating roller has good releasability from a cored bar made of aluminum or the like and a toner formed on the surface of the cored bar by coating with a fluororesin or the like. The pressure roller includes a core metal such as iron and an elastic layer formed of silicon rubber or the like on the surface of the core metal. In a certain prior art, an elastic layer is also formed on the surface of the core metal of the heating roller, and a release layer is formed thereon.

In the conventional fixing device 1 shown in FIG. 16, a heating roller 2 and a pressure roller 3 are rotatably supported by bearings 4 and 5, respectively. At least one bearing 4 and 5 is arranged at each end of the heating roller 2 and the pressure roller 3, respectively. The coil spring 6 presses the pressure roller 3 against the heating roller 2. The coil spring 6 is arranged between a bearing 5 supporting the pressure roller 3 and a main body frame (not shown),
Appropriate pressure contact force is applied between the heating roller 2 and the pressure roller 3.

[0004]

In the prior art of FIG. 16, since the coil spring 6 is used, it is difficult to assemble the fixing device and the image forming device. Further, in this conventional technique, the pressure contact force and the nip width between the heating roller 2 and the pressure roller 3 are always constant regardless of the thickness of the recording paper, so that wrinkles and curls occur on thin recording paper. ,on the other hand,
The fixing strength of the thick recording paper having a relatively rough surface is lowered.

Therefore, in another conventional technique, a special control device for variably setting the fixing temperature according to the type (size and thickness) of the recording paper is used. In this conventional technique, the above problem can be solved by lowering the fixing temperature for thin recording paper and raising the fixing temperature for thick recording paper. However, this conventional technique has a problem in that a special control device is required, and therefore the cost of the image forming apparatus is increased.

Therefore, the main object of the present invention is to satisfactorily fix a toner image on a recording sheet with a simple structure.
It is to provide a fixing device.

[0007]

According to the present invention, an elastic body is formed on at least one of the surfaces, and a pair of heating roller and pressure roller are pressed against each other in a state where the elastic body is elastically deformed, and a heating roller. Of the diameter D1, the diameter D2 of the pressure roller, the nip width B, and the axial distance L between the heating roller and the pressure roller,

[0008]

[Equation 3]

The fixing device is provided with supporting means for rotatably supporting the heating roller and the pressure roller at fixed positions so as to satisfy the above condition.

[0010]

First, the nip width B with which the toner image can be satisfactorily fixed on the recording sheet is determined by experiments, for example, regardless of the thickness of the recording sheet. From the nip width B, the diameter D1 of the heating roller and the diameter D2 of the pressure roller, the inter-axis distance L is determined according to equation 2. The heating roller and the pressure roller are supported at a fixed position by a supporting means such as a bearing so that the inter-axis distance L is maintained.

When the recording sheet is thin, the amount of deformation of the elastic body is small, so that heat and pressure are applied to the toner image and the recording sheet with a relatively narrow nip width. Therefore,
No wrinkles or curls on the recording sheet. At this time, since the recording sheet is thin, sufficient fixing strength can be obtained even if the nip width is narrow. On the other hand, when the recording sheet is thick, the amount of deformation of the elastic body is large, so that heat and pressure are applied to the toner image and the recording sheet with a relatively wide nip width. Therefore, an appropriate fixing strength can be obtained. At this time, a thick recording sheet does not generate wrinkles or curls even when passing through a wide nip width.

[0012]

According to the present invention, wrinkles and curls do not occur and sufficient fixing strength can be obtained regardless of the thickness of the recording sheet without using a coil spring or a special control device of the prior art. A fixing device can be obtained.
The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0013]

EXAMPLE Referring to FIG. 1, a fixing device 10 of this example.
Includes a heating roller 12. As shown in FIG. 4, the heating roller 12 includes a hollow cylindrical core metal 14 made of aluminum or the like. A release layer 16 is formed on the surface of the core metal 14. The release layer 16 is formed by coating with a fluororesin or the like in order to improve the releasability with the molten toner.
A heater 18 for heating the heating roller 12 is arranged in the hollow portion of the core metal 14.

A pressure roller 20 is pressed against the heating roller 12. The pressure roller 20 includes a hollow cylindrical cored bar 22. On the surface of the cored bar 22, an elastic layer 2 such as silicon rubber is provided.
4 is formed to have a thickness of 3 to 8 mm, for example. The elastic layer 24 preferably has a thickness of 5 mm. This is because if the elastic layer 24 is thicker than that, the cost becomes higher, while if it is thinner, the effect of correcting the nip width cannot be obtained. A shaft 26 is inserted or fixed to the cored bar 22. It should be noted that elastic bodies such as silicone rubber have been extended in service life and reduced in creep due to technological innovation, and the amount of contraction of the elastic bodies is about ten and several μm even after long-term use. Therefore, the state of the nip portion 48 (described later) can be maintained in a substantially desired state for a long period of time without using a spring or the like. Focusing on this point, in the present invention, by fixing the axial distance L between the heating roller and the pressure roller as follows,
The nip portion 48 is set to a desired state to prevent the harmful effects of the conventional technology.

Both ends of the heating roller 12 and the pressure roller 2
Both ends of the shaft 26 of 0 are rotatably supported at fixed positions by bearings 28 which also serve as both rollers. At this time, the heating roller 12 and the pressure roller 20 are separated by the elastic layer 2
4 is pressed into contact with and elastically deformable. The bearing 28 is formed, for example, as a pentagonal bearing plate as shown in FIG. A hole 30 for supporting the heating roller and a hole 32 for supporting the pressure roller shaft are formed by punching in the bearing 28. The distance H between the holes 30 and 32 is set so as to obtain the optimum nip width. That is, when the toner image is fixed on the recording sheet by the heating roller 12 and the pressure roller 20, the distance H is determined based on the nip width B such that there is no wrinkle or curl and an appropriate fixing strength is obtained. It

The bearing 28 has heat resistance and slidability in consideration of friction between the heating roller 12 and the pressure roller shaft 26 and the bearing 28. For example, PPS (polyphenylene sulfide) is added to PTFE (polytetrafluoroethylene). It is formed of a resin in which fluoroethylene) is mixed or a resin in which PTFE (polytetrafluoroethylene) is mixed with PI (polyimide). However, the bearing 28 may be formed of a heat resistant resin, and only the inner peripheral surfaces of the holes 30 and 32 may be coated with a slidable resin such as a fluororesin.

Here, the optimum nip width B is set according to the recording sheet conveying speed (process speed), and the relationship between the two is as shown in Table 1.

[0018]

[Table 1]

According to the nip width B, the axial distance L is determined. That is, the relationship between the diameter D1 of the heating roller 12, the diameter D2 of the pressure roller 20, the nip width B, and the inter-axis distance L is expressed as in Equation 3 with reference to FIG.

[0020]

[Equation 4]

For example, D1 = 25 mm, D2 = 30 mm, B
= 3 mm, the distance L between axes becomes L≈27.3 mm. The hole distance H between the holes 30 and 32 is set based on the shaft distance L. Returning to FIG. 1, protrusions 34 and 36 are formed on the upper and lower portions of the bearing 28, respectively. The protrusions 34 and 36 are formed in the holes 4 of the body frame 38 having a substantially L shape.
The bearing 28 is fixed to the main body frame 38 by being fitted in the 0 and 42. A thermistor 44 for controlling the surface temperature of the heating roller 12 is arranged on the heating roller 12.

As is well known (not shown), the toner image formed through the processes of charging, exposing and developing is transferred onto a recording sheet 46 such as paper. The recording sheet 46 onto which the toner image has been transferred is sent to a nip portion 48 formed by the pressure contact portion between the heating roller 12 and the pressure roller 20. Then, the toner image is fixed on the recording sheet 46 by receiving heat and pressure at the nip portion 48, and the recording sheet 46 is discharged through a sheet discharging path (not shown).

Here, as shown in FIG. 4, the relatively thin recording sheet 46a on which the toner image is transferred is the heating roller 12.
When the recording sheet 46a is sent between the pressure roller 20 and the pressure roller 20, the recording sheet 46a receives the heat quantity Q from the heating roller 12 and the pressure P1 due to the pressure contact while passing through the nip portion 48 having the nip width B1. The toner image is fixed. Regarding the fixing quality at that time, even if the recording sheet 46a is thin, since the time for receiving heat and pressure in the nip portion 48 is short, there is no wrinkle or curl and sufficient fixing strength can be obtained.

Further, as shown in FIG. 5, when a relatively thick recording sheet 46b on which a toner image is transferred is sent between the heating roller 12 and the pressure roller 20, only the thickness of the recording sheet 46b. The nip width B2 automatically becomes wider than the nip width B1, and the pressure P2 becomes higher than the pressure P1. Therefore, when the toner on the recording sheet 46b passes through the nip portion 48 having the nip width B2, it is fixed on the recording sheet 46b under a longer time and with a higher pressure than in the case of the recording sheet 46a (FIG. 4). Therefore, sufficient fixing strength can be obtained even if a recording sheet having a rough surface is used. Further, since the relatively thick recording sheet 46b is generally stiffer than the relatively thin recording sheet 46a, even if the recording sheet 4 is subjected to heat or pressure for a long time,
No wrinkles or curls occur on 6b.

As described above, according to the embodiment shown in FIG. 1, regardless of the thickness of the recording sheet 46, sufficient fixing strength can be obtained without wrinkles or curls. Further, the protrusions 34 and 36 of the bearing 28 are fitted into the holes 40 and 42 of the main body frame 38, respectively.
Since the bearing 28 is attached to the main body frame 38 by fitting it into the main body 38, it is not necessary to separately prepare a fastening member such as a screw for attaching the bearing 28 to the main body frame 38, and the assembly adjustment time can be shortened and maintenance can be performed. The working time can be shortened.

In order to fix the bearing 28 to the main body frame 50, as shown in FIG. 6, the projection 36 is fitted into the hole 52 formed in the plate-shaped main body frame 50, and the cover having a substantially V-shaped cross section. The protrusion 34 may be fitted in the hole 56 formed in the hole 54. The cover 54 is formed of a heat resistant resin such as metal or engineering plastic, and protects the peripheral members inside the main body and the user from the high temperature heating roller 12.

Further, as shown in FIG. 7, a substantially L-shaped holder 58 for supporting both ends of the heater 18 may be integrated with the bearing 28. In the above-described embodiment, one bearing plate is used as the bearing 28 to support both the heating roller 12 and the pressure roller 20, but as in the conventional case, the heating roller 12 and the pressure roller 20 are respectively supported. Needless to say, the bearings may be separately prepared to fix the axial distance L between the heating roller 12 and the pressure roller 20.

Next, a fixing device 60 of another embodiment of the present invention shown in FIG. 8 includes a heating roller 12 and a pressure roller 20 similar to the fixing device 10 of FIG. This fixing device 6
At 0, in order to insulate the high temperature heating roller 12 and the pressure roller 20 from the surrounding parts and to take a safety measure for the user, the heat roller 12 and the pressure roller 20 and their surroundings have a heat resistant upper cover 62 and a lower cover 64. Is covered with. The upper cover 62 and the lower cover 64 may be made of metal or resin.

In the embodiment, the upper cover 62 and the lower cover 64 are formed of a heat-resistant resin called engineering plastic, for example, like the cover 54 described above. At present, such heat-resistant resins are prevented from being deformed by heat of 200 ° C. or higher. On the other hand, a bearing 66 that supports the heating roller 12 and the pressure roller 20 at fixed positions and press-contacts both rollers.
And 68 are resins having both heat resistance and slidability, for example, a resin obtained by mixing PPS with PTFE, or P
It is formed of a resin in which I is mixed with PTFE, and its heat resistant temperature is 200 ° C. or higher, which is almost equal to the heat resistant temperature of the upper cover 62 and the lower cover 64.

In the embodiment shown in FIG. 8, paying attention to the fact that the upper cover 62, the lower cover 64, and the bearings 66 and 68 have approximately the same heat resistant temperature, the cover and the bearing are integrally molded to form the heating roller 12 and the pressure roller. The nip portion 48 is set in a desired state by fixing the axial distance L with respect to 20. The upper cover 62 and the integrated 2
Two bearings 66 and a lower cover 64 integrally molded with 2
The two bearings 68 respectively support both ends of the heating roller 12 and the pressure roller shaft 26.

When the fixing device 60 of FIG. 8 is assembled, the upper cover 62 and the lower cover 64 are joined or joined to each other with the heating roller 12 and the pressure roller 20 sandwiched therebetween, so that the heating roller 12 and Pressure roller 20
Are pressed against each other and elastically supported by bearings 66 and 68 in a state where the elastic layer 24 is elastically deformed.

As shown in FIG. 9, the bearing 66 has a semicircular bearing surface 70 (hatched portion) which comes into contact with the heating roller 12 and slides, and the bearing 68 comes into contact with the pressure roller 20 and slides therein. The semicircular bearing surface 72 (hatched portion) is
It is formed. Since the heating roller 12 and the pressure roller 20 are in contact with the bearing surfaces 70 and 72 while rotating, not only heat resistance but also slidability is required. Therefore, in this embodiment, the upper cover 62 and the lower cover 64 are integrally molded together with the bearings 66 and 68 with the above-mentioned resin having heat resistance and slidability. Since the parts other than the bearing surfaces 70 and 72 are not required to have slidability, the upper cover 62 and the lower cover 64 are molded together with the bearings 66 and 68 with a heat resistant resin such as PPS.
A slidable material such as fluorine may be coated or vapor-deposited only on the bearing surfaces 70 and 72.

The center-to-center distance H (FIG. 9) between the bearing surfaces 70 and 72 is sufficient without wrinkles or curls when the toner image is fixed on the recording sheet 46 by the heating roller 12 and the pressure roller 20. The nip width B (FIG. 3) is set so that the fixing strength can be obtained. The bearing surfaces 70 and 72 support both end portions of the heating roller 12 and both end portions of the pressure roller shaft 26, respectively.

Here, the optimum nip width B is set according to the recording sheet conveying speed (process speed) as in the previous embodiment, and in the case of this embodiment, the relationship between the two is as shown in Table 1. . Then, as in the previous embodiment, the axial distance L is determined by the equation 3 according to the nip width. Then, based on the distance L between the shafts, the distance H between the centers of the bearing surfaces 70 and 72
Is set.

Further, since the bearings 66 and 68 are integrally formed with the upper cover 62 and the lower cover 64, respectively, the upper cover 62 and the lower portion, which sandwich the heating roller 12 and the pressure roller 20, when the fixing device 60 is assembled. By simply joining or joining the cover 64, the heating roller 12 and the pressure roller 20 are supported in a fixed position in a state of being pressed against each other. As a result, it is not necessary to attach the bearing to the cover with high precision as in the conventional case, the assembly adjustment time can be shortened, and the work time during maintenance can be shortened. Also, the number of parts can be reduced.

In the above-described embodiment, as shown in FIG. 10, the heating roller 12 and the pressure roller 20 are in the vicinity of both ends thereof, and the heating roller 12 and the pressure roller 20 are in pressure contact with each other. , The heating roller 12 and the pressure roller 20 are bent. As a result, the pressure contact force between the heating roller 12 and the pressure roller 20 is not uniform in the axial direction,
The pressure contact force near the central portion in the axial direction becomes smaller than the pressure contact force near the end portions in the axial direction. Therefore, there is a problem that not only the fixing property of the central portion is lowered but also the conveying force of the recording sheet is lowered.

Therefore, in the embodiment shown in FIG. 11, the substantially central portion of the pressure roller 20 in the axial direction is elastically urged by the backup roller 8 connected to the spring 7 to correct the deflection of the pressure roller 20. That is, in the embodiment of FIG. 11, the central portion of the pressure roller 20 is pressed by the backup roller 8 to correct the distortion or deflection of the central portion of the pressure roller 20.

FIG. 12 shows another embodiment of the fixing device 80 which can correct the distortion or bending of the pressure roller 20. As shown in FIG. 13, the fixing device 80 includes the bearing 2
8 is the same as the embodiment of FIG. 1 except that an L-shaped correction member 82 is integrally formed. In this fixing device 80, both ends of the heating roller 12 are supported by the holes 30 of the bearing 28, and both ends of the pressure roller shaft 26 and slightly inside thereof are supported by the holes 84 of the correction member 82 and the holes 32 of the bearing 28, respectively. A force is applied to the shaft 26 by the bearing 28 in the direction in which the pressure roller 20 is pressed against the heating roller 12 as shown by the arrow a in FIG. A force is applied in the direction in which the pressure roller 20 is separated from the heating roller 12 as shown in (the opposite direction). As a result, as can be seen by comparing FIG. 10 and FIG. 14, the pressure roller 20 is brought into uniform close contact with the heating roller 12 in the axial direction, and the pressure contact force in the axial direction becomes uniform.
As described above, the pressure roller 20 generated when the heating roller 12 and the pressure roller 20 are supported by the bearing 28.
The distortion or flexure of the can be corrected by the correction member 82.

That is, in this embodiment, the heating roller 1
2 and the pressure roller 20 are supported by the same bearing 28 so as to satisfy the condition of Expression 4, thereby fixing the inter-axis distance L between the heating roller 12 and the pressure roller 20, and
The correction member 82 can correct the distortion or bending of the pressure roller 20. The number of parts is smaller than that in the embodiment of FIG. 11, and the cost can be reduced.

Further, since the spring 28 for pressure contact and the frames for holding the bearing 28 as shown in FIG. 16 can be replaced by the integrated bearing 28 and the correction member 82, a fastening member such as a screw can be greatly used. Can be omitted. as a result,
Assembly adjustment time can be greatly shortened, and maintenance work time can be greatly shortened. In addition, as shown in FIG.
When the correction member 82 is pulled in a direction opposite to the pressing direction of the pressing roller 20 with the spring 85 or the like, the distortion of the roller can be corrected more effectively.

In the fixing device 80 shown in FIGS. 12 and 13, the pressure contact length X between the heating roller 12 and the pressure roller 20 is 225 mm, the material of the heating roller 12 is aluminum,
When the material of the pressure roller 20 is iron and the pressure contact force P between the heating roller 12, the pressure roller 20 and the rollers is changed in Formula 5, the results shown in Table 2 are obtained.

[0042]

[Equation 5]

[0043]

[Table 2]

D _H : Heat roller outer diameter (mm) d _H : Heat roller inner diameter (mm) D _P : Pressure roller shaft diameter (mm) P: Roller pressure contact force (kg / mm) X: Both roller pressure contact Length (mm) E _H : longitudinal elastic modulus of heating roller (aluminum = 7300)
kgf / mm ² ) E _P : Pressure roller longitudinal elastic modulus (iron = 21000 kgf / mm
²⁾ Y _H: flexing a central portion of the heating roller (mm) Y _P: flexing a central portion of the pressure roller (mm), where the deflection of the central portion of the central portion of the deflection Y _H and the pressure roller 20 of the heating roller 12 Y _P is represented by the amount of deviation from the ideal position at the center of the roller in the axial direction when there is no bending.

From the experience so far, a combination smaller than the outer diameter D _H = 16 mm of the heating roller 12, the inner diameter d _H = 13 mm of the heating roller 12, and the diameter D _P = 7 mm of the shaft 26 of the pressure roller 20, that is, Table 2 It is considered that some kind of deflection preventing mechanism is required for a combination smaller than the third stage from the bottom. This is because the heating roller 1 becomes smaller as the fixing device becomes smaller.
This is because the smaller the pressure roller 20 and the pressure roller 20 are, the larger the bending becomes. Therefore, the fixing device 80 shown in FIG. 12 has the small-diameter heating roller 12 and the pressure roller 2.
It is particularly effective when 0 is used.

Further, as shown in FIG. 15, a repulsion roller 86 may be used as a correction means. That is, the repulsion rollers 86 are attached to both ends of the pressure roller shaft 26, and the surface of the repulsion roller 86 is brought into pressure contact with the heating roller 12. As a result, as shown by the arrow b in FIG.
A force is applied to the shaft 26 in the direction to separate the pressure roller 20 from the pressure roller 20 to correct the distortion or bending of the pressure roller 20. Also at this time, the bearing 28 applies a force in the direction in which the pressure roller 20 is brought into pressure contact with the heating roller 12 as shown by the arrow a. When the repulsion roller 86 shown in FIG. 15 is used, the same effect as that of the embodiment shown in FIG. 12 can be obtained.

In the embodiments shown in FIGS. 12 and 15, the case where the distortion or bending of the pressure roller 20 is corrected has been described. However, the distortion or bending of the heating roller may be corrected, or the heating roller may be heated. Distortion or bending of both the roller 12 and the pressure roller 20 may be corrected. Further, in each of the above-described embodiments, the elastic layer 24 is formed on the surface of the pressure roller 20, but the elastic layer may be formed on the surface of the heating roller 12 as well.
The elastic layer may be formed only on the surface of the.

1, FIG. 6, FIG. 7, FIG. 11 and FIG.
2, FIG. 15 and FIG. 16, since the bearing portion of the fixing device is symmetrical, only one bearing portion of the heating roller and the pressure roller is shown here in order to avoid complication of the drawings. Please note.

[Brief description of drawings]

FIG. 1 is a perspective view of an essential part showing an embodiment of the present invention.

FIG. 2 is a front view showing an example of the bearing of FIG. 1 embodiment.

FIG. 3 is an illustrative view showing a relationship among a diameter of a heating roller, a diameter of a pressure roller, a nip width and an axial distance.

FIG. 4 is an illustrative view showing a state of a nip portion when a toner image is fixed on a thin recording sheet.

FIG. 5 is an illustrative view showing a state of a nip portion when a toner image is fixed on a thick recording sheet.

FIG. 6 is a perspective view of a main part showing another embodiment of the present invention.

FIG. 7 is a perspective view of a main part showing another embodiment of the present invention.

FIG. 8A is a front view of a main portion of another embodiment of the present invention, and FIG. 8B is a side view of the main portion.

FIG. 9 is a front view showing an example of a bearing integrated with a cover.

FIG. 10 is an illustrative view showing a state where the pressure roller is bent.

FIG. 11 is a perspective view of an essential part showing another embodiment of the present invention capable of correcting the bending of the pressure roller.

FIG. 12 is a perspective view of an essential part showing another embodiment of the present invention capable of correcting the bending of the pressure roller.

13A is a front view showing a bearing and a correction member of the embodiment of FIG. 12, and FIG. 13B is a sectional view taken along line AA.

14 is an illustrative view for explaining distortion correction when the correction member in FIG. 13 or the repulsion roller in FIG. 15 is used.

FIG. 15 is a perspective view of a main part showing another embodiment of the present invention.

FIG. 16 is a perspective view of a main part showing a conventional technique.

[Explanation of symbols]

 10, 60, 80 ... Fixing device 12 ... Heating roller 20 ... Pressure roller 24 ... Elastic layer 28, 66, 68 ... Bearing 46 ... Recording sheet 48 ... Nip 62 ... Upper cover 64 ... Lower cover 70, 72 ... Bearing Surface 82 ... Correction member 86 ... Repulsion roller D1 ... Heating roller diameter D2 ... Pressure roller diameter B, B1, B2 ... Nip portion L ... Inter-axis distance

Claims (12)

[Claims] 1. A pair of a heating roller and a pressure roller, in which an elastic body is formed on at least one surface, and the elastic body is pressed against each other in an elastically deformed state, and a diameter D1 of the heating roller and the pressing roller. The relationship among the diameter D2 of the pressure roller, the nip width B, and the axial distance L between the heating roller and the pressure roller is as follows. The fixing device is provided with a supporting unit that rotatably supports the heating roller and the pressure roller at fixed positions so as to satisfy the above condition. 2. The support means comprises at least one bearing means formed with a first support for the heating roller and a second support for the pressure roller. Fixing device. 3. The fixing device according to claim 2, wherein the bearing means includes one bearing plate on which the first support portion and the second support portion are formed. 4. The bearing means according to claim 2, wherein the bearing means includes a first bearing plate on which the first support portion is formed and a second bearing plate on which the second support portion is formed. Fixing device. 5. A first cover for covering the heating roller, and a second cover for covering the pressure roller are further provided, and the first bearing plate and the second bearing plate are respectively the first cover. The fixing device according to claim 4, which is integrally formed with the second cover. 6. The first cover and the first bearing plate are integrally molded with a resin having at least heat resistance, and the second cover and the second bearing plate are integrally molded with a resin having at least heat resistance. The fixing device according to claim 5, which is molded. 7. The first cover and the first bearing plate are integrally molded with a resin having heat resistance and slidability, and the second cover and the second bearing plate are heat resistance and slidability. Molded integrally with a resin
The fixing device according to claim 5. 8. The first supporting portion of the bearing means supports both ends of the heating roller, and the second supporting portion supports both ends of a shaft of the pressure roller, and further comprises the heating roller and the heating roller. The fixing device according to claim 2, further comprising a correction unit that corrects a bend generated in at least one of the pressure rollers. 9. The flexure is generated when the pressure contact force between the heating roller and the pressure roller at the end portion in the axial direction is larger than that at the center portion, and the correction means uniformizes the pressure contact force in the axial direction. 9. The fixing device according to claim 8, further comprising: 10. The fixing device according to claim 9, wherein the means includes a pressing means for pressing an axial center portion of at least one of the heating roller and the pressure roller against the other roller. 11. The means includes another supporting means for supporting the shaft of the pressure roller outside the second supporting portion, and the second supporting means supports the central portion of the shaft by the other supporting means. The fixing device according to claim 9, wherein the fixing portion is biased toward the heating roller with the portion as a fulcrum. 12. An image forming apparatus comprising a fixing device for fixing a toner image on a recording sheet, wherein the fixing device has an elastic body formed on at least one surface thereof, and the elastic body is elastic. A pair of a heating roller and a pressing roller that are pressed against each other in a deformed state, a diameter D1 of the heating roller, a diameter D2 of the pressing roller, a nip width B, and an axial distance between the heating roller and the pressing roller. The relationship of L is An image forming apparatus including supporting means for rotatably supporting the heating roller and the pressure roller at fixed positions so as to satisfy the above condition.