JP2000231296A - Fixing device and image forming device provided with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a recording material from wrinkling at the fixing time. SOLUTION: In this fixing device, a toner image is fixed while holding/ carrying the recording material S by a fixing nip part N between a fixing roller 31 and a pressure roller 32. At this time, a pulling-back force applied from the pressure roller 32 is imparted to the rear side of the recording material S. This force is liable to become smaller at the center part, and larger at the end parts. The device is allowed to impregnate oil in only end parts A and A of the cleaning web 35a held in contact with the pressure roller 32 over whole length in the lengthwise direction, excluding the center part B. In such a manner, the coefficient of friction (μ) at end parts on the pressure roller 32 surface are reduced, the back turning force allied from the pressure roller 32 to the recording material S are equalized, therefore the wrinkles are prevented from the occurrence.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device mounted on a copying machine, a laser printer, a facsimile or the like to fix a toner image on a recording material, and an image forming apparatus provided with the fixing device.

[0002]

2. Description of the Related Art A thermal fixing device using an upper roller (fixing roller) and a lower roller (pressing roller) is well known as a fixing device of a copying machine, a laser printer or the like. The lower roller is pressed against the upper roller from below to form a fixing nip portion (hereinafter simply referred to as “nip”) between the two rollers. The recording material carrying the unfixed toner image on the surface is heated while being nipped and conveyed at the nip, so that the toner image is thermally fixed on the surface.

When the above-described fixing device is used in, for example, a color copying machine, in order to prevent the recording material from winding around at the time of fixing, the upper roller on the side contacting the toner is constituted by a soft roller, and the lower roller is Is formed of a hard roller so that the recording material after fixing is curled toward the lower roller.

FIGS. 6 (a) and 6 (b) show an upper roller 31 and
The relationship between the lower roller 32 and the nip N is schematically shown. However, the figure is illustrated extremely. Since the upper roller 31 is softer than the lower roller 32, the nip N
As shown in (a), the upper roller 31 is configured to bite.

However, due to the nature of rubber, the upper roller 31 is actually deformed as shown in FIG. As shown in the drawing, the surface area of the upper roller 31 at the nip N increases, and, of course, the circumference also increases. On the other hand, since the angular velocity ω of the upper roller 31 is constant, the time required to rotate the constant angle θ is also constant. Within the predetermined time, the nip N moves a long distance by the increase in the circumferential length, and as a result, the circumferential speed increases. As a rough image, upper roller 3
The peripheral speed of 1 keeps a constant speed except for the nip N,
It is considered that the speed suddenly increases at the nip N, and then returns to the original speed after passing the nip N.

The increase in the speed at the nip N is due to the deformation of the rubber at the nip N and the corresponding increase in the circumferential length.
The increase in speed at is less.

For example, a case where the upper roller 31 is driven to rotate at a peripheral speed of V ₁ mm / sec and the lower roller 32 is driven to rotate with the rotation of the upper roller 31 will be described.

The difference between the peripheral speed at the nip N and the peripheral speed other than the nip N is large at the upper roller 31 and small at the lower roller 32. Therefore, assuming that there is no slip between the upper roller 31 and the lower roller 32 in the nip N and that the peripheral speeds of both rollers are equal, the speed relationship is: upper nip speed = lower nip speed> lower peripheral speed> upper peripheral speed. , Upper nip speed: the peripheral speed of the upper roller at the nip N Lower nip speed: the peripheral speed of the lower roller at the nip N Upper peripheral speed: the peripheral speed of the upper roller at positions other than the nip N Lower peripheral speed: the periphery of the lower roller at positions other than the nip N It will be fast.

FIG. 7 shows a case where the rubber thickness of the upper roller is fixed to 2 t and the rubber thickness of the lower roller is changed to 1 t, 1.4 t, and 1.5 t. It is an example of the measurement result of the speed and the peripheral speed of the lower roller. The peripheral speed of the lower roller is clearly higher than that of the upper roller. The upper roller is driven at a certain peripheral speed V ₁ , so the peripheral speed hardly changes.
The thinner the rubber thickness (1t), the faster. This is because the lower the rubber thickness of the lower roller becomes, the harder it becomes.
Is increased, the upper nip speed and the lower nip speed are increased, and the peripheral speed of the lower roller is increased.

In the above description, in the nip N, the upper roller 31
It is assumed that the upper nip speed and the lower nip speed are equal to each other and that the lower roller 32 is driven by rotation and the cleaning web 35a ( The lower nip speed is lower than the upper nip speed because a load such as that shown in FIG. That is, in the nip N, the upper roller 31 and the lower roller 3
2 has slippage.

Actually, the speed at the nip N was observed with a high-speed CCD camera, and the upper nip speed and the lower nip speed were measured; upper nip speed> lower nip speed.

As shown in FIG. 8, when the upper nip speed> the lower nip speed, a force F pulled back from the lower roller 32 acts on the recording material S conveyed by the upper roller 31. The pull-back force F acts on the back surface (the side in contact with the lower roller 32) of the conveyed recording material S, and acts more strongly as the difference between the upper nip speed and the lower nip speed is larger.

The pullback force F is such that the back surface of the recording material S
When the friction coefficient μ between the lower roller 32 and the lower roller 32 is uniform as in the case of a blank sheet of paper, the function is uniform. In a region with a large value, a strong pull-back force is obtained, and in a small region, a weak pull-back force is obtained.

FIG. 9 shows a case where toner is present on a recording material,
An example of the difference in the pulling-back force from the case where there is no case is shown. In the drawing, white / white indicates a recording material in which both the front and back surfaces are white, and white / solid indicates a recording material in which the front surface is white and the back surface is black. It can be seen from FIG. 4 that if the toner is present on the back surface that comes into contact with the lower roller 32, the friction coefficient μ becomes smaller and the pullback force becomes weaker. From this, for example, as shown in FIG.
A recording material S of an image (center band image) in which the toner is fixed only at the center in the left-right direction on the back surface of the recording material S
When performing the surface of the fixing, whereas larger pullback force F ₁ is larger left and right end portions of the friction coefficient mu, pullback force F ₂ is smaller central friction coefficient mu becomes smaller. In FIG. 10, actually the pullback force acting on the back surface of the recording material _{S F (F 1, F 2} ), for convenience of explanation, is shown on the surface of the recording material S.

As described above, the pulling-back force F is applied to the recording material S
When the recording material S is different between the both ends and the center, the center of the recording material S advances faster than both ends, and in the latter half of the recording material S, the left and right ends gradually move toward the center, and wrinkles occur.

Such wrinkles are likely to occur during double-sided copying (during double-sided image formation) when the first side to be copied first is a center band image. Conventionally, in order to prevent this wrinkle, the amount of oil applied to the upper roller as a release agent is increased, and the friction coefficient μ between the recording material and the roller is reduced.
Lower the pull-back force by the lower roller, weakly average it, increase the rubber thickness of the upper and lower rollers, and lower the pressure of the roller against the recording material to weaken the pull-back force of the lower roller. At least one of the rollers has an inverted crown shape.

[0017]

However, as described above, when the amount of oil is increased, the sliminess of the fixed image due to oil increases, and in the case of an OHP film, oil streaks become noticeable. I do. In addition, when the rubber of the roller is thickened as in the above, since the heat of the heater installed in the roller becomes difficult to be transmitted to the roller surface, the thermal responsiveness at the time of continuous copying is deteriorated, and energy consumption is accordingly reduced. More. In addition, like
When the upper roller or the lower roller has an inverted crown shape, adverse effects may occur on an image.

The present invention has been made in view of the above-mentioned circumstances, and has a problem of increasing the amount of oil, increasing the thickness of rubber, and forming an inverted crown shape, that is, slimy feeling, oil streaks, waste of energy, and the like. It is an object of the present invention to provide a fixing device that eliminates wrinkles of a recording material at the time of fixing, and an image forming apparatus provided with the fixing device.

[0019]

According to the first aspect of the present invention, there is provided a belt-shaped fixing nip portion in which a fixing roller and a pressure roller are brought into contact with each other. The recording material carrying the unfixed toner image is heated while the recording material carrying the unfixed toner image is pinched and conveyed at the fixing nip portion such that the surface thereof contacts the fixing roller by driving the pressure roller to rotate. In a fixing device that fixes a toner image on a surface by pressing, when a direction along the fixing nip portion is a longitudinal direction of the pressing roller, a friction coefficient between the pressing roller surface and the recording material is The pressure roller is different between a central portion and an end portion in the longitudinal direction.

According to a second aspect of the present invention, in the fixing device of the first aspect, the coefficient of friction between the surface of the pressure roller and the recording material is larger at the end than at the center in the longitudinal direction of the pressure roller. It is characterized in that it is made smaller in the part.

According to a third aspect of the present invention, in the fixing device of the second aspect, there is provided a cleaning web pressed against the surface of the pressure roller in a longitudinal direction, and an end of the cleaning web is impregnated with oil. It is characterized by the following.

According to a fourth aspect of the present invention, in the fixing device of the second aspect, there is provided a release agent application device for supplying a release agent to the surface of the fixing roller, and the fixing roller is provided by the release agent application device. The amount of release agent supplied to the surface is larger at the end than at the center in the longitudinal direction of the pressure roller.

According to a fifth aspect of the present invention, in the fixing device of the second aspect, the central portion in the longitudinal direction on the surface of the pressure roller is formed of a material having a large friction coefficient, and the end portion is formed of a material having a small friction coefficient. To be characterized.

According to a sixth aspect of the present invention, in the fixing device of the fifth aspect, a central portion in the longitudinal direction on the surface of the pressure roller is formed of silicone rubber, and an end portion is formed of fluoro rubber. I do.

According to a seventh aspect of the present invention, there is provided an image forming section for forming a toner image and carrying the toner image on a recording material surface, and transferring an unfixed toner image carried on the recording material surface by the image forming section to the recording material surface. And a fixing device for fixing the toner image to the image forming apparatus.
7. The fixing device according to 4, 5, or 6.

According to the present invention, by making the coefficient of friction between the surface of the pressure roller and the recording material different between the center and the end in the longitudinal direction of the pressure roller, the fixing nip portion Since the difference between the central portion and the end portion of the pull-back force applied from the pressure roller to the back surface of the recording material to be nipped and conveyed can be eliminated, wrinkles of the recording material due to the pull-back force can be eliminated.

[0027]

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

<Embodiment 1> FIG. 4 shows an example of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 is an electrophotographic four-color full-color laser printer, and FIG. 2 is a longitudinal sectional view showing a schematic configuration thereof.

The image forming apparatus shown in FIG. 1 includes a recording material transport system I provided from the right side of the main body 1 of the image forming apparatus in FIG. Latent image forming section II provided above
And a developing means provided near the left side of the latent image forming section II, that is, a rotary developing device III.

The recording material transport system I has the following configuration.

First, an opening is formed in the right wall of the main body 1 of the image forming apparatus, and detachable sheet cassettes 2 and 3 are mounted in this opening so that a part thereof can be projected outside. ing. Paper feed rollers 4 and 5 are disposed immediately above the paper feed cassettes 2 and 3. These paper feed rollers 4, 5
The transport roller 6 and the transport guides 7 and 8 are provided between the transfer roller 15 and the transfer drum 15 located on the left side. In the vicinity of the outer peripheral surface of the transfer drum 15, the rotation direction (arrow A)
Direction) A contact roller 9, a gripper 10, a recording material separating charger 11, and a separating claw 12 are disposed substantially in order from the upstream side to the downstream side. On the inner peripheral side of the transfer drum 15, a transfer charger 13 and a recording material separating charger 14 are provided. At the upper right side of the transfer drum 15, a transport belt 16 is provided in proximity to the separation claw 12, and a fixing device 30 is disposed at the end (right end) of the transport belt 16 in the recording material transport direction. The fixing device 30 will be described later in detail.
A detachable discharge tray 17 is mounted downstream of the fixing device 30 in the transport direction so as to extend outside the image forming apparatus main body 1.

The configuration of the latent image forming section II will be described.

The latent image forming section II includes a drum type electrophotographic photosensitive member (hereinafter, referred to as "photosensitive drum") 19 as an image carrier, which is disposed above the transfer drum 15.
The photosensitive drum 19 is in contact with the transfer drum 15,
It is supported rotatably in the direction of arrow B. Photosensitive drum 1
The charger 9 for static elimination, the cleaning device 21, and the primary charger 23 are arranged around the periphery of the charging roller 9 substantially in the rotation direction. Above the photosensitive drum 19, a laser scanner (exposure device) 24 and a reflection mirror 25 for forming an electrostatic latent image on the surface of the photosensitive drum 19 are provided.

The structure of the rotary developing device III will be described.

A rotatable rotary 26 is disposed on the left side of the photosensitive drum 19 so as to face the surface thereof. The rotary 26 has four color developing devices, that is, yellow, cyan, magenta, and cyan developing devices 27Y and 27.
M, 27C and 27BK are mounted. Of these four developing devices, one for developing the electrostatic latent image on the photosensitive drum 19 is arranged at a developing position facing the surface of the photosensitive drum 1 by the rotation of the rotary 26.

Next, the operation of the entire image forming apparatus having the above-described configuration will be described by taking as an example a case where a four-color full-color image is formed.

The surface of the photosensitive drum 1 rotated and driven in the direction of arrow B in FIG. 4 is uniformly charged by the primary charger 23 to a predetermined polarity and a predetermined potential. The surface of the charged photosensitive drum 19 is subjected to image exposure with the laser light E modulated by the yellow image signal of the document 28, so that an electrostatic latent image is formed. The electrostatic latent image is developed as a yellow toner image by attaching yellow toner by a yellow developing device 27Y arranged at a developing position by the rotation of the rotary 26.

On the other hand, the sheet cassette 2 or the sheet cassette 3
From the paper feed roller 4 or the paper feed roller 5,
The recording material conveyed via the conveyance roller 6 and the conveyance guides 7 and 8 is held by a gripper 10 at a predetermined timing, and is electrostatically transferred to the transfer drum by a contact roller 9 and an electrode opposed thereto. 15 wrapped around. The transfer drum 15 rotates in the direction of arrow A in synchronization with the photosensitive drum 19, and the yellow toner image on the photosensitive drum 19 is
The image is transferred by the transfer charger 13 at a transfer portion between the photosensitive drum 19 and the transfer drum 15. Transfer drum 15
Continues the rotation, and prepares for the transfer of the next color (magenta in FIG. 7).

On the other hand, the photosensitive drum 19 is provided with the charge removing charger 2.
0, and after being cleaned by the cleaning device 21, is uniformly charged again by the primary charger 23, and is exposed to the laser light E as described above by the next magenta image signal, thereby forming an electrostatic latent image for magenta. Is formed. The rotary developing device III rotates while the magenta electrostatic latent image is formed on the photosensitive drum 19, and arranges the magenta developing device 27M at the above-described developing position, and forms the magenta toner on the electrostatic latent image. And developed as a magenta toner image. This magenta toner image is
The image is transferred onto the recording material on the transfer drum 15 by the transfer charger 13. After the transfer of the toner image, the photosensitive drum 19 is neutralized by the neutralizing charger 20 and is cleaned by the cleaning device 21.

The same process as that for the above-described yellow and magenta is performed for the remaining cyan and black, and four color toner images are superimposed on the recording material on the transfer drum 15.

The recording material to which the four color toner images have been transferred is neutralized by the recording material separating charger 14, the gripping by the gripper 10 is released, and the recording material is separated from the transfer drum 15 by the separating claws 12. The sheet 16 is sent to the fixing device 30. The recording material is heated and pressed by the fixing device 30 so that the toner image is fixed on the surface thereof, and then discharged onto the paper discharge tray 17, whereby the four-color full-color image formation is completed.

Next, the fixing device mounted on the image forming apparatus having the above-described configuration and operation, that is, the fixing device 30 according to the present invention will be described in detail with reference to FIG. FIG. 3 is a longitudinal sectional view of the fixing device 30 in a direction along the conveying direction of the recording material S.

Reference numeral 31 in the figure denotes a fixing roller which is rotatably disposed. The fixing roller 31 has a structure in which a surface of a metal core is coated with silicone rubber and fluorine rubber to a predetermined thickness, and a heater 31 such as a halogen lamp is used.
a. The fixing roller 31 is driven to rotate in a direction indicated by an arrow R31 by a driving unit (not shown).

Below the fixing roller 31, a pressure roller 3
2 are provided. The pressure roller 32 has a structure in which a surface of a metal core is coated with a predetermined thickness of silicone rubber and fluorine rubber, and a heater 32 a such as a halogen lamp is disposed inside the pressure roller 32 similarly to the fixing roller 31. ing. The pressure roller 32 is in contact with the fixing roller 31 from below with a predetermined pressing force, and forms a belt-shaped nip (fixing nip) N with the fixing roller 31. Pressure roller 32
Rotates in the direction of the arrow R32 with the rotation of the fixing roller 31 in the direction of the arrow R31.

An oil supply device (release agent application device) 33 is disposed diagonally above and to the right of the fixing roller 31 in FIG. The oil supply device 33 includes an oil pan (release agent container) 33a containing silicone oil as a release agent.
And two pumping rollers 33b for pumping the release agent from the oil pan 33a, an application roller (application rotating body) 33c for applying the release agent to the fixing roller 31, and an application amount of the release agent. Regulating blade (regulating member) 33
d. Coating roller 3 rotatably supported
Reference numeral 3c denotes a sponge rubber surface coated with silicone rubber, which is disposed so as to be able to freely contact and separate from the surface of the fixing roller 31, and applies a release agent to the surface of the fixing roller 31 at the time of contact. The regulating blade 33d includes an application roller 33c.
Is fixed to a sheet metal at a fixed angle with respect to
c at a constant pressure. Therefore, the application amount of the release agent pumped up by the pumping roller 33b is adjusted to be constant by the regulating blade 33d, and is applied from the front end to the rear end of the recording material S.

A cleaning device 34 is provided obliquely above the fixing roller 31 and below the pressing roller 32 in FIG.
35 are provided. A cleaning web (nickel web) 34a is in contact with the surface of the fixing roller 31 by a pressing roller 34b, and a cleaning web (nomex felt) 35a is in contact with the surface of the pressing roller 32 by a pressing roller 35b. . In addition,
34a, 35a are web supply rollers, and 34c,
35c is a web take-up roller.

The fixing device 30 having the above-described configuration includes the fixing roller 3
1 and the pressure roller 32 rotate at a constant speed in a state of being in contact with the nip N. When the recording material S carrying the unfixed toner T is conveyed to the fixing nip N, the recording material S
When the toner passes through the nip N, the unfixed toner T carried on the surface is melted and fixed by being pressed and heated at a substantially constant pressure and temperature from both the front and back surfaces.

In this embodiment, the following configuration is employed in order to prevent the "wrinkling of the recording material" described above.

FIG. 1 shows an example of the fixing device 30A according to the present invention. FIG. 3 is a longitudinal sectional view showing a schematic configuration of the fixing device 30A. The fixing device 30A shown in the figure has the same configuration as the above-described fixing device 30 of FIG. 5 except that the configuration of an oil supply device (release agent application device) 33A is different. Hereinafter, different points will be mainly described.

The oil supply device 33 A is provided with the fixing roller 31.
Above, an oil pipe 38 disposed along the longitudinal direction of the fixing roller 31 is provided. A number of oil supply holes (not shown) are formed in the oil pipe 38, and oil as a release agent is dropped on the surface of the fixing roller 31 from these oil supply holes. The oil dripping on the surface penetrates into the felt 39 on the downstream side, and is applied uniformly over the entire length of the surface of the fixing roller 31 in the longitudinal direction. Excess oil is removed by the blade 40 further downstream of the felt 39 and returned to the oil pan 41.

In this embodiment, the pressure roller 32
In order to make the friction coefficient μ of the surface different between the central portion and the end portion in the longitudinal direction, the cleaning device 35 of the pressure roller 32
The oil impregnation amount of the cleaning web 35a is changed in the width direction. Specifically, as shown in FIG. 1A corresponding to the X-ray view of FIG.
Oil is impregnated into both ends A, A (hatched portion) of 5a,
The central part B is not impregnated. Since the cleaning web 35a contacts the surface of the pressure roller 32, the friction coefficient μ between the recording material S and the surface of the pressure roller 32 is small at the longitudinal end of the pressure roller 32, It gets bigger in the part. Therefore, when the toner image is fixed, the pullback force acting on the recording material S from the surface of the pressure roller 32 tends to increase at the end of the recording material S as described with reference to FIG. Since the friction coefficient μ of the surface becomes smaller at both ends, a substantially uniform pullback force acts over the entire width of the recording material S. Thereby, wrinkling of the recording material S at the time of fixing is prevented.

FIG. 2 shows an example of the experimental results. The figure shows
The difference in the amount of oil applied to the pressure roller 32 between the end portions A and A and the center portion B of the cleaning web 35a is 10 mg / A4.
FIG. 9 is a diagram showing the relationship between the oil impregnation amount of the cleaning web 35a and the friction coefficient μ between the cleaning web 35a and the paper (recording material) / pressure roller when the cleaning web 35a is used. From the figure, the friction coefficient μ
Is smaller as the oil impregnation amount is larger.
Therefore, the pressure roller 32 in which the cleaning web 35a in which only the ends A and A are impregnated with oil is in contact,
At the end, the friction coefficient μ between the recording material S and the recording material S decreases, and increases at the center. For this reason, for example, when fixing the toner image on the front surface side of the recording material S (see FIG. 10) having the center band image on the back surface, a blank portion (= end portion) having a large friction coefficient μ and a small toner portion (= end portion) were originally used. = Central part)
As described above, due to the effect of the cleaning web 35a impregnated with oil as described above, the friction coefficient μ on the surface of the pressure roller 32 in the longitudinal direction is made uniform, and the pulling force of the pressure roller 32 to the recording material S is also made uniform. . As a result, wrinkles at the time of fixing on the front surface side of the recording material S having the center band image on the back surface, which have conventionally been caused by the difference in the retraction force between the center portion and the end portion of the surface of the pressure roller 32, are reduced. Can be prevented.

Second Embodiment FIGS. 3A and 3B show a fixing device 30B according to a second embodiment. In the present embodiment, the amount of oil dripping from the oil pipe 38 to the surface of the fixing roller 31 is large at the ends A and A in the longitudinal direction of the fixing roller 31 and is small at the center B.

As the method, for example, the oil pipe 38
The number of oil supply holes (not shown) may be increased at the end of the oil pipe 38 and reduced at the center, or the oil supply hole may be larger at the end than at the center.

As described above, since the amount of oil supplied from the oil pipe 38 to the fixing roller 31 is larger at the end than at the center, the oil is soaked into the felt 39 and is fixed after being described by the blade 40. The amount of oil applied to the surface of the roller 31 is also large at the end portions and is small at the central portion.

Therefore, the amount of oil adhering to the surface of the pressure roller 32 which comes into contact with the fixing roller 31 via the fixing nip N is large at the longitudinal end of the pressure roller 32 and is large at the center. Less.

The operation and effect of this are the same as in the first embodiment. That is, the friction coefficient μ in the longitudinal direction on the surface of the pressure roller 32 is made uniform,
The force for pulling back to the recording material S is also uniformed. As a result, wrinkles at the time of fixing on the front surface side of the recording material S having the center band image on the back surface, which have conventionally been caused by the difference in the retraction force between the center portion and the end portion of the surface of the pressure roller 32, are reduced. Can be prevented.

<Third Embodiment> FIG. 11 is a front view of a pressure roller 32 in a fixing device according to a third embodiment.

In this embodiment, the pressure roller 32
The longitudinal ends 32a, 32a on the surface are formed of fluoro rubber, and the central part 32b is formed of silicone gum, so that the friction coefficient μ of the ends 32a, 32a is reduced by the central part 32a.
b is smaller than that of b.

Thus, for example, when fixing the toner image on the front surface side of the recording material S (see FIG. 10) having the center band image on the back surface, the white paper portion (=
The end portion) and the smaller toner portion (= center portion) have a uniform friction coefficient μ in the longitudinal direction on the surface of the pressure roller 32, and the force of the pressure roller 32 to return to the recording material S is also uniform. As a result, wrinkles at the time of fixing on the front surface side of the recording material S having the center band image on the back surface, which have conventionally been caused by the difference in the retraction force between the center portion and the end portion of the surface of the pressure roller 32, are reduced. Can be prevented.

Fourth Embodiment FIG. 12 is a front view of a pressure roller 32 in a fixing device according to a fourth embodiment.

In this embodiment, the pressure roller 32
By forming the longitudinal ends 32a, 32a of the surface with a rough surface and the central part 32b with a smooth surface, the friction coefficient μ of the ends 32a, 32a is made smaller than that of the central part 32b. I have.

Thus, for example, when fixing the toner image on the front surface side of the recording material S (see FIG. 10) having the central band image on the back surface, a blank paper portion (==
The end portion) and the smaller toner portion (= center portion) have a uniform friction coefficient μ in the longitudinal direction on the surface of the pressure roller 32, and the force of the pressure roller 32 to return to the recording material S is also uniform. As a result, wrinkles at the time of fixing on the front surface side of the recording material S having the center band image on the back surface, which have conventionally been caused by the difference in the retraction force between the center portion and the end portion of the surface of the pressure roller 32, are reduced. Can be prevented.

In the first, second, and third embodiments, the extreme case has been described by taking the recording material S having the center band image on the back surface as an example. However, in general, a toner image is formed at the center of the recording material S. Formed, and the edges remain as blank areas. Therefore, the present embodiment can achieve a corresponding effect even in a general case.

[0065]

As described above, according to the present invention,
By making the coefficient of friction between the surface of the pressure roller and the recording material different between the center and the end in the longitudinal direction of the pressure roller, pressure is applied to the back of the recording material nipped and transported at the fixing nip. Since the difference between the central portion and the end portion of the pull-back force acting from the roller can be eliminated, wrinkles of the recording material due to the pull-back force can be eliminated.

[Brief description of the drawings]

FIG. 1A is a longitudinal sectional view of a fixing device according to a first embodiment.
(B) is an X-ray arrow view of (a).

FIG. 2 is a diagram showing a relationship between an oil impregnation amount of a web and a friction coefficient μ.

FIG. 3A is a longitudinal sectional view of a fixing device according to a second embodiment.
(B) is a substantially front view of the fixing device according to the second embodiment.

FIG. 4 is a longitudinal sectional view illustrating a schematic configuration of the image forming apparatus.

FIG. 5 is a longitudinal sectional view illustrating a schematic configuration of a fixing device mounted on the image forming apparatus of FIG. 4;

FIGS. 6A and 6B are vertical cross-sectional views showing how a fixing roller and a pressure roller deform at a fixing nip portion.

FIG. 7 is a diagram showing a relationship between a rubber thickness of a lower roller (a pressure roller) and peripheral speeds of an upper roller (a fixing roller) and a lower roller in a fixing nip portion.

FIG. 8 is a diagram illustrating a pull-back force F acting on the back surface of a recording material nipped and conveyed from the pressing roller when the nip speed of the pressing roller is lower than the nip speed of the fixing roller.

FIG. 9 is a diagram illustrating a difference in the pullback force when the back surface is blank and solid black.

FIG. 10 is a diagram illustrating how a pull-back force acting on a recording material from a pressure roller differs between a central portion and an end portion when a central band image is formed on the back surface of the recording material.

FIG. 11 is a front view illustrating a configuration of a pressure roller according to a third embodiment.

FIG. 12 is a front view illustrating a configuration of a pressure roller according to a fourth embodiment.

[Explanation of symbols]

 30, 30A, 30B Fixing device 31 Fixing roller 32 Pressure roller 32a End of pressure roller 32b Central portion of pressure roller 35a Cleaning web N Fixing nip S Recording material μ Friction coefficient

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Mitsuhiro Ota 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 2H033 AA15 BA46 BA51 BB30

Claims (7)

[Claims] An unfixed toner image is formed by contacting a fixing roller and a pressure roller to form a belt-shaped fixing nip portion, and driving the fixing roller to rotate the pressure roller to rotate. In a fixing device for fixing a toner image on a surface by heating and pressing while holding and conveying the recording material held in the fixing nip portion such that the surface thereof is in contact with the fixing roller, the fixing device is disposed along the fixing nip portion. When the direction of the pressure roller is the longitudinal direction of the pressure roller, the coefficient of friction between the surface of the pressure roller and the recording material is different between a central portion and an end portion in the longitudinal direction of the pressure roller. A fixing device characterized by the above-mentioned. 2. The pressure roller according to claim 1, wherein a coefficient of friction between the surface of the pressure roller and the recording material is smaller at an end portion than at a central portion in a longitudinal direction of the pressure roller. 2. The fixing device according to claim 1. 3. The fixing device according to claim 2, further comprising a cleaning web pressed against the surface of the pressure roller in a longitudinal direction, and impregnating an end of the cleaning web with oil. And a release agent application device for supplying a release agent to the surface of the fixing roller, wherein a supply amount of the release agent to the surface of the fixing roller by the release agent application device is determined by a length of the pressing roller. The fixing device according to claim 2, wherein the number of the fixing devices is larger at an end than at a center in the direction. 5. The fixing device according to claim 2, wherein a central portion in the longitudinal direction of the surface of the pressure roller is formed of a material having a large friction coefficient, and an end portion is formed of a material having a small friction coefficient. . 6. The fixing device according to claim 5, wherein a central portion in the longitudinal direction of the surface of the pressure roller is formed of silicone rubber, and an end portion is formed of fluorine rubber. 7. An image forming section for forming a toner image and supporting it on the surface of a recording material, a fixing device for fixing an unfixed toner image carried on the surface of the recording material by the image forming section to the surface of the recording material, An image forming apparatus comprising: the fixing device according to claim 1, wherein the fixing device is the fixing device according to claim 1.