JPH08254917A - Fixing device - Google Patents

Abstract

PURPOSE: To provide a fixing device, used for an electrophotographic image forming device, capable of stably cope with fixing for a full color toner image or high speed fixing over a long term. CONSTITUTION: The peripheral surface of the fixing roller 1 of the fixing device is formed of a silicone rubber layer 8, which contains graphite powder having the particle size of 0.5 to 20μm and whose volume resistivity is about 10<10> Ωcm. A surface coated with a fluorine contained rubber layer may be adopted as the peripheral surface of the roller 1. By such constitution, the roller 1 can hold high heat conductivity and electrical conductivity while keeping proper elasticity. Therefore, heat loss on the surface of the roller caused by the high speed fixing is completely compensated, and even when charge is generated by the friction of the roller with paper 6 or a pressure member 2, it is discharged. Furthermore, excellent peeling property is maintained at the time of fixing the full color image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device which is used in an electrophotographic image forming apparatus such as a copying machine or a facsimile and which fixes a toner image formed on a recording sheet by heating and pressing.

[0002]

2. Description of the Related Art Generally, in an electrophotographic image forming apparatus, a toner image is formed on an image carrier, and the toner image is transferred to a P image.
Transfer to a recording sheet such as PC paper. Then, the recording sheet on which the toner image is transferred is guided to a fixing device, and the toner image is fixed on the recording sheet to form a permanent image. Conventionally, as a fixing device, a fixing roller having a heater inside a cylindrical cored bar and a heat-resistant resin film layer formed on the outer peripheral surface, and a fixing roller arranged in pressure contact with the fixing roller, the outer peripheral surface of the cylindrical cored bar. The one having a pressing member having a heat resistant elastic layer formed on it is generally used. In this fixing device, the fixing roller is rotationally driven, and the recording sheet on which the toner image is transferred is nipped and conveyed in the nip portion where the fixing roller and the pressing member are in pressure contact. The recording sheet onto which the toner image is transferred is heated and pressed while passing through the nip portion, and the toner melted by the heat is pressed against the recording sheet. At this time, the recording sheet is sent to the nip portion so that the surface on which the toner image is transferred faces the fixing roller, and the toner image is efficiently heated and melted. Further, in order to prevent the so-called offset in which the toner image adheres to the surface of the fixing roller, the surface of the fixing roller is coated with a fluororesin or the like, or the surface of the fixing roller is driven with a silicone oil or the like during rotation. The liquid release agent is applied and supplied little by little.

When such a fixing device is used in a full-color image forming apparatus, a higher performance is required for the releasability of the fixing roller from the toner. In a color image forming apparatus, fixing speed is increased and
This is because the low-viscosity color toner is often adhered over a large area, and an offset is likely to occur when fixing these. Based on such requirements, a fixing roller having an elastic layer made of silicone rubber or the like on the surface of a core bar has been proposed as a fixing roller having excellent releasability.

FIG. 4 shows an example of a fixing device having such a fixing roller 21, which is a cylindrical iron core 27 made of iron.
A silicone rubber layer 28 is provided on the outer peripheral surface of the core material, and an infrared lamp 23 disposed inside the core metal.
Then, the fixing roller is brought into pressure contact with the pressure roller 22, and the sheet 24 is nipped in the nip portion to be conveyed and heated at the same time.
The toner image melted by heat is pressed to the paper under pressure. In such a fixing device, due to the elasticity of the silicone rubber layer of the fixing roller, the adhesion to the paper is enhanced and the fixability is improved. Further, at this time, the hardness of the silicone rubber layer forming the fixing roller is made lower than that of the surface layer on the pressure roller 22 side, so that the fixing roller 21 side is depressed at the nip portion as shown in FIG. When the pressure contact is released to return to the original state, a force that causes a gap between the surface of the fixing roller and the toner layer acts momentarily. Therefore, the peeling property between the fixing roller and the toner is improved.

[0005]

However, such a fixing device has the following problems. In the fixing roller as described above, the resin material such as the silicone resin that constitutes the elastic layer has a low thermal conductivity, so if the image formation is continued at high speed, the heat source supplies the heat consumed by the surface of the fixing roller. The amount of heat applied cannot catch up, the surface temperature drops, and the fixability deteriorates. In addition, if the fixing roller has a low conductivity, it is significantly charged due to friction with the pressure member and the sheet at the time of fixing, toner is unnecessarily scattered, an image defect occurs, and the sheet is electrostatically adsorbed. A paper jam will occur.

To solve the problem that the surface temperature is lowered, Japanese Patent Publication No. 62-45993 discloses that a metal powder is mixed into an offset prevention layer on the surface to impart conductivity, and the temperature of the surface of the fixing roller is increased. How to prevent the decline,
Further, JP-A-3-80281 discloses a method of improving thermal conductivity by adding powder of a substance having high thermal conductivity such as carbon black or metal oxide to the elastic layer. However, even if these methods are used, it is not possible to obtain sufficient thermal conductivity to cope with a speeded-up color image forming apparatus. In addition, when carbon black is added as a filler to silicone rubber, the elastic modulus of the rubber decreases, and a metal oxide as a filler does not provide sufficient conductivity and the fixing roller is charged. Causes problems.

On the other hand, as a method for preventing the charging of the fixing roller, Japanese Patent Laid-Open No. 2-6985 and Japanese Patent Laid-Open No. 2-476 are known.
Japanese Patent Laid-Open No. 72-72 discloses a method in which the surface layer of a fixing roller composed of a cored bar and a surface layer is made electrically conductive, and this is grounded to remove electricity. In addition, Japanese Patent Laid-Open No. 4-1
Japanese Patent No. 47174 discloses a method of applying a bias voltage or canceling the charging of the fixing roller by discharging from the corotron to eliminate the charge.

However, in these techniques, there is a problem in that the size of the device becomes large and the stability of its effect is poor. No consideration is given to the decrease in the surface temperature of the fixing roller. For this reason, a satisfactory effect cannot be obtained with respect to the fixing of a full-color image and the demand for high speed. The present invention has been made in view of the above problems, and its purpose is to
It is an object of the present invention to provide a fixing device capable of preventing the surface temperature of the fixing roller from being lowered and charging, maintaining sufficient elasticity of the fixing roller, and being capable of sufficiently fixing a full-color image and increasing the speed.

[0009]

In order to solve the above problems, the present invention is used in an electrophotographic image forming apparatus and has a heating source inside and a silicone rubber layer on the outer periphery thereof. A recording medium having a fixing roller and a pressure portion that is in pressure contact with the fixing roller is pressed between the fixing roller and the pressure member and heated to record the toner image. In a fixing device for melting and fixing on a medium, the silicone rubber layer of the fixing roller contains graphite powder having a particle size of 0.5 to 20 μm and a volume resistivity of 10 ¹⁰ Ωcm or less.
Further, it is desirable that the peripheral surface of the fixing roller is covered with a fluororubber layer having releasability.

In the present invention, the particle size of the graphite powder contained is such that the elasticity of the silicone rubber is not impaired.
It is necessary to be 0.5 to 20 μm in that it can be mixed in a large amount in order to maintain sufficient thermal conductivity and conductivity. Further, when the volume resistivity of the silicone rubber layer exceeds 10 ¹⁰ Ωcm, the silicone rubber layer is easily charged, and electrostatically adsorbs paper or scatters toner to meet full-color fixing and speed-up requests. Therefore, the volume resistivity of the silicone rubber layer must be 10 ¹⁰ Ωcm or less.

[0011]

In the fixing device of the present invention, graphite powder is contained in the silicone rubber layer of the fixing roller, and graphite has higher thermal conductivity than carbon black, metal oxides, etc. The thermal conductivity can be increased. Further, the graphite powder imparts high conductivity. Therefore, heat can be smoothly supplied from the heat source even in high-speed fixing, and a decrease in the surface temperature of the fixing roller can be suppressed to a small level. On the other hand, since it has high conductivity, the electric charge generated by friction with the pressure member of the fixing roller and the recording paper can be released, and the adsorption of the recording paper and the unnecessary scattering of toner do not occur. Further, since the graphite powder contained in the silicone rubber layer of the fixing roller has a particle size of 0.5 to 20 μm, even if a large amount is blended with the silicone rubber layer in order to impart sufficient thermal conductivity and conductivity. Does not impair elasticity. Therefore, it is possible to meet high requirements such as fixing of a full-color image and speeding up, and it is possible to perform stable fixing for a long period of time. Further, if the peripheral surface of the fixing roller is covered with a fluororubber layer having releasability, the peelability of the recording paper after fixing the toner image is improved, and paper jam is prevented. Further, although this fluororubber layer has no conductivity, it is formed thin so that the surface of the fixing roller is not excessively charged.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an example of a fixing device according to the present invention. As shown in FIG. 1, this fixing device includes a fixing roller 1 for heating a recording sheet 6 having a toner image transferred thereon.
Then, the pressure roller 2 that is pressed against the fixing roller 1 and rotates following the fixing roller 1, and the recording paper 6 on which the toner image is transferred are guided to the pressure contact portion between the fixing roller 1 and the pressure roller 2. It has a paper guide 4 and a pair of conveyance rollers 5 that convey the recording paper 6 that has passed through the pressure contact portion.

The fixing roller 1 comprises a cored bar 7 and a silicone rubber layer 8 around the cored bar 7, and a heater 3 for heating the recording paper is provided inside the cored bar. The silicone rubber layer 8 of this fixing roller is formed as follows. Vinyl group-containing polydimethylsiloxane (DY32-663 manufactured by Toray Dow Corning Silicone Co., Ltd.)
U) 100 parts by weight of an organic peroxide of 2.5 dimethyl 2.5 di (tert-butylperoxy) hexane vulcanizing agent (RC-manufactured by Toray Dow Corning Silicone Co., Ltd.)
4 (50P) FD) was mixed in an amount of 0.6 part by weight, graphite (Mesocarbon microbeads of Osaka Gas Co., Ltd., average particle size: 6 μm) was added, and the mixture was uniformly kneaded with an oven roll. A core metal having an outer diameter of 44 mm is placed in a roller mold having an inner diameter of 50.5 mm, and the above silicone composition is vulcanized and molded by a rubber molding machine to form a cylindrical body composed of a silicone rubber layer having a hardness of 57 °. To do. This is further polished to an outer diameter of 50 mm for finishing.

The material constituting the silicone rubber layer of the fixing roller is not particularly limited as long as it is a polyorganosiloxane, but a vinyl group-containing polycondensate which is crosslinked with an organic peroxide or a hydroxyl group to form a rubber-like elastic body. Dimethylsiloxane is preferred. When an organic peroxide is used as a cross-linking material, it is not preferable that it contains an acyl group, since it may cause significant crosslinking inhibition when a large amount of graphite powder is added, such as dicumyl peroxide. Are preferred.

The amount of graphite to be blended is preferably 30 to 300 parts by weight with respect to 100 parts by weight of silicone rubber,
If it is less than 30 parts by weight, sufficient electrical conductivity cannot be imparted to the fixing roller, and if it exceeds 300 parts by weight, rubber elasticity may be impaired. In addition to the above, kneading of these silicone rubber compositions can be carried out by a conventionally known rubber kneading device such as a roll mill or an internal mixer. At this time, if necessary, a heat stabilizer of iron oxide or a rare earth compound is added. You may mix. Further, the method of molding these silicone rubber compositions on the cored bar can also be performed by conventionally known compression molding, injection molding, extrusion molding or the like.

It is desirable that the surface of the silicone rubber layer is further coated with vinylidene fluoride fluororubber as a release layer from the viewpoint of releasability from the toner. At this time, the release layer has a thickness of about 30 μm, which is slightly smaller than that of the silicone rubber layer, and the silicone rubber layer has sufficient conductivity. It is possible to reduce charging of the surface of the roller. When fluororubber is used as the release layer, the release property is further enhanced by using the modified silicone oil described in JP-B-64-6464 and JP-A-4-348377 on the surface thereof. You can

The cored bar 7 may be any one as long as it has conductivity, and the thickness of the cored bar is preferably 0.5 mm or more, preferably about 2 mm. The cored bar is electrically grounded, and even if an electric charge is generated by friction between the fixing roller and the paper or the pressing member, the electric charge is removed through the silicone rubber layer and the cored bar. The pressure roller 2 has a silicone rubber layer formed on the surface of a cylindrical support, and the hardness of the silicone rubber layer is set higher than that of the silicone rubber layer of the fixing roller. Therefore, when the fixing roller is pressed against the fixing roller, the fixing roller side is dented as shown in FIG. 5, and when the fixing roller and the pressure roller are separated from each other, the surface of the fixing roller is rapidly retracted to separate from the toner. Can be increased.

Using such a fixing device, A4 paper was continuously fixed at a speed of 36 sheets per minute, and the surface temperature change of the fixing roller was measured.
It became stable when the temperature dropped by 7 ° C. Further, the charging potential on the surface of the fixing roller at this time was measured to be −100 V, and in the conventional fixing device, it was possible to fix even using a thin paper which causes paper jam due to electrostatic attraction. Full-color high-density image (TMA = 2.0 mg /
When the peelability of the fixing roller in cm ² ) was examined,
The surface temperature of the fixing roller was 220.degree.

On the other hand, as a comparative example to the above embodiment,
A similar fixing test was conducted on a fixing device using a mixture of silica and alumina instead of graphite as the filler mixed in the silicone rubber layer of the fixing roller. With this fixing device, the hardness of the silicone rubber layer was 57 °, fixing was performed at a speed of 36 A4 sheets per minute in the same manner as in the above-mentioned embodiment, and the surface temperature change of the fixing roller was measured. A decrease in temperature was observed. Further, when the potential on the surface of the fixing roller at this time was measured, it was found to be charged to -1500V. Further, when the peeling property with respect to the toner in the full-color high-density image was evaluated in the same manner as in the above-mentioned examples, the peeling failure was observed when the surface temperature of the fixing roller was 200 ° C.

From the above, in the fixing device of this embodiment, the temperature of the surface of the fixing roller does not decrease much even when high-speed fixing is performed, and stable fixing can be performed. Further, toner scattering due to frictional charging and paper Also, thin paper can be used without causing the adsorption. Further, it is possible to maintain sufficient releasability for color toners.

Although the fixing roller and the pressure roller are pressed against each other in the above embodiment, an elastic member fixedly supported as the pressure member may be used as shown in FIG. This fixing device has a pressure member 12 including a silicone sponge impregnated with a liquid release agent and a surface layer covering the surface thereof, and the fixing roller 11 is pressed against the pressure member 12. . Also in such a fixing device, the same effect can be obtained by including graphite powder in the silicone rubber layer of the fixing roller.

Next, the results of comparing the characteristics of the silicone rubber used in the fixing device of the present invention with those of the conventional one will be shown.
The silicone rubber is the same as that used in the fixing roller of the above-mentioned embodiment, and the amount of graphite to be blended is 30 to 3.
With respect to each of the materials changed in the range of up to 00 parts by weight, each was uniformly kneaded with an oven roll and vulcanized and molded at 170 ° C. for 10 minutes by a press molding machine to prepare test pieces having different hardness. The hardness of the test piece is based on JIS K6301 A-type rubber hardness, and the results obtained by examining the thermal conductivity by the unsteady hot wire method and the repulsion elastic modulus by JIS K6301 for the test pieces of each hardness are shown in Table 1 and Table 2. . At this time, instead of graphite, a silica / alumina-based mixed filler was blended to prepare a test piece having the same hardness, and the same measurement was performed to compare the results.

[0023]

[Table 1] (Below margin)

[Table 2]

FIG. 2 is a graph showing the relationship between the rubber hardness and the thermal conductivity shown in Table 1. As is clear from Table 1 and FIG. 2, graphite is superior even if it has the same hardness. It can be seen that the thermal conductivity can be maintained. Furthermore, it is clear from Table 2 that even if the hardness is the same, the one in which graphite is blended can maintain higher impact resilience. In addition, when carbon black is mixed as the filler, the thermal conductivity is hardly improved.

Next Among these, the silicone rubber composition of the rubber hardness 57 °, was measured for volume resistivity, those blended with graphite, 10 ⁸ [Omega] cm
While it was a low value as below, in the case of the silica-alumina mixed filler, a high value of 10 ¹³ Ωcm or more was shown. From this, it is clear that the fixing roller containing graphite has a higher conductivity and an excellent antistatic effect. In this Example, the volume resistivity was measured by using a super insulation resistance meter, and each silicone rubber composition was used as a sheet of 2 mm to obtain DC10 by the current / voltage method.
It is assumed that 0 V is applied.

[0026]

As described above, in the fixing device of the present invention, by mixing a large amount of graphite powder in the silicone rubber layer of the fixing roller, high conductivity and thermal conductivity can be obtained without impairing elasticity. Can be kept. For this reason, the fixing roller is not charged even when fixing is performed at high speed, so that the toner image adheres to the fixing roller, the toner scatters to cause an image defect, or the paper is electrostatically attracted to the fixing roller. Paper jams are prevented from occurring. In addition, the decrease in the surface temperature of the fixing roller can be reduced,
The fixing can be stably performed, and a high quality image can be obtained. Furthermore, due to the elasticity of the silicone rubber layer,
When fixing a full-color image, good peelability can be maintained even for low-viscosity color toner. Further, if the peripheral surface of the fixing roller is covered with a fluororubber layer having releasability, the peelability of the recording paper after fixing the toner image is improved, and paper jam is prevented. Further, although this fluororubber layer has no conductivity, it is formed thin so that the surface of the fixing roller is not excessively charged. Therefore, according to the fixing device of the present invention, it is possible to sufficiently meet high requirements such as fixing of full-color images and speeding up, and it is possible to perform fixing stably for a long period of time.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a fixing device that is an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between rubber hardness and thermal conductivity.

FIG. 3 is a schematic configuration diagram illustrating a fixing device that is another embodiment of the present invention.

FIG. 4 is a diagram showing an example of a fixing roller and a pressure roller in a conventional fixing device.

5 is a diagram showing a contact state between a fixing roller and a pressure roller in the embodiment shown in FIG.

[Explanation of symbols]

 1, 11, 21 Fixing roller 2, 22 Pressing roller 3, 23 Heater 4 Paper guide 5 Conveying roller 6, 24 Recording paper 7, 27 Core metal 8, 28 Silicone rubber layer 12 Pressing member

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Claims (2)

[Claims] 1. A fixing roller used in an electrophotographic image forming apparatus, comprising a heat source inside and a silicone rubber layer on an outer periphery thereof, and a pressure section press-contacted with the fixing roller. A fixing device for pressing and heating a recording medium on which a toner image is transferred between the fixing roller and a pressure member to melt and fix the toner image on the recording medium, wherein the silicone rubber of the fixing roller is used. A fixing device, wherein the layer contains graphite powder having a particle size of 0.5 to 20 μm, and a volume resistivity of 10 ¹⁰ Ωcm or less. 2. The fixing device according to claim 1, wherein the peripheral surface of the fixing roller is covered with a fluororubber layer having releasability.