JP3143468B2 - Fixing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device that is used in an image forming apparatus such as a copying machine and an electrophotographic printer and fixes an unfixed image by pressure or heat and pressure.

[Prior Art] An example of the fixing device of FIG. 9 is shown.

The upper fixing roller 1 is a heater in which a heater 2 such as a halogen lamp is placed inside a hollow metal core made of aluminum, iron, or the like, and the heater 2 heats the fixing roller 1. The lower pressure roller 3 is obtained by covering the outer periphery of a metal core 4 such as iron or stainless steel with an elastic body 5 having a releasable property such as silicon rubber. The fixing roller 1 and the pressure roller 3 are brought into contact with each other by a predetermined pressing force by urging means such as a spring (not shown), and are driven to rotate in the direction of the arrow.

Reference numeral 6 denotes a temperature-sensitive element such as a thermistor brought into contact with the surface of the fixing roller 1, and detects the surface temperature of the fixing roller 1. The power supply to the heater 2 is controlled by the temperature control circuit in accordance with the temperature detected by the temperature sensing element 6, and the surface temperature of the fixing roller 1 is automatically controlled to a predetermined heat fixing temperature. Reference numeral 7 denotes a separation claw that separates the transfer material P that supports the toner image from the surface of the fixing roller 1, and has a leading edge portion that is brought into contact with the surface of the fixing roller 1 with an appropriate pressing force. Reference numeral 8 denotes a cleaner such as felt that is brought into pressure contact with the surface of the fixing roller 1, and wipes and removes toner tc, paper dust, and the like attached to the surface of the fixing roller 1.
Reference numeral 9 denotes a bottom plate made of a metal material of the fixing device. Reference numerals 10 and 11 denote transfer material entrance guides and transfer guide exit guides, which are bent upwardly on the bottom plate 9 and mounted and supported on front and rear walls. The transfer material P that has entered the fixing device through the entrance guide 10 enters the nip portion between the fixing roller 1 and the pressure roller 3 that are rotationally driven while being in pressure contact with each other, and enters the nip between the rollers 1 and 3. Going through the department. In the course of this passage, the unfixed toner image ta on the transfer material P is fixed as a permanently fixed image tb by heat and pressure.

The transfer material P having passed through the nip portions of the rollers 1 and 3 and having undergone image fixing is separated from the surface of the fixing roller 1 by the separation claw 7 at the leading end, and is discharged to the outside of the fixing device through the outlet guide 11.

In the fixing process, the fixing roller 1 directly heats and presses the unfixed toner image ta to soften it and fix it to the transfer material P, but a part of the toner image adheres to the fixing roller surface (hereinafter referred to as offset). Sometimes. Such an offset toner tc is fixed to an improper place of the transfer material P with the rotation, and stains the transfer material P, or stains the back surface of the transfer material P accumulated by transferring to the pressure roller 3 between papers. Or make separation difficult. There are two main causes of the offset: the physical binding force between the toner and the roller and the electrostatic force. The offset caused by the former is effective by increasing the smoothness of the roller surface or decreasing the surface energy. Specific examples of materials having such properties include fluororesins such as PTFE and PFA.These fluororesins are also excellent in heat resistance, abrasion resistance, and corrosion resistance required as a coating material for the fixing roller. ing. On the other hand, the latter electrostatic force acts on the toner, which is a charged powder, in an electric field created by a fixing roller, a pressure roller, and a transfer material charged by friction or peeling. When the repulsion between the transfer material and the transfer material is strong,
Often causes significant offsets.

As a means for reducing the offset due to such an electric action, a method of imparting conductivity to the surface coating layer of the fixing roller to release charged charges or applying an antistatic agent to the surface to prevent charging is used. There is.

Making the surface of the fixing roller conductive has the effect of preventing charging of the fixing roller, but cannot prevent the influence of the repulsive electric field due to the pressure roller. Further, in the electrophotographic method, the distance between transfer and fixing is shorter than that of the transfer material having the maximum size, and the resistance is lowered, so that the transfer current flows into the fixing roller and causes transfer failure.

Further, when the antistatic agent is applied, when the transfer material once passed is re-passed, the surfactant adhered to the surface of the transfer material adheres to the photoreceptor and hinders the photosensitive phenomenon and the charging of the photoreceptor. In such a case, a good image cannot be expected.
In the case of a system that is applied to the surface, a replenishing member is required to maintain the effect.

Therefore, in recent years, a method of applying a voltage of the same polarity as the toner to the fixing roller and suppressing the toner by the action of an electric field has been considered.

 FIG. 6 shows an example of this.

From the power source 14, the same polarity as the toner is applied to the core of the fixing roller 1.
A bias voltage of 1.5 kV is applied.

 Reference numeral 13 denotes a 25 μm thick PFA resin provided on the cored bar.

[Problems to be Solved by the Invention] However, this fixing method has a remarkable effect of applying a sufficient voltage to the fixing roller.
A current of as much as 0 μA flows. Pure PFA is originally an insulator, but in the case of a single-layer coating, many pinholes H are present in the structure of the surface coating material as shown in the figure, and the resistance value is reduced.

In such a case, not only the current capacity of the power supply 14 becomes insufficient and the applied voltage drops, so that the effect on the offset is reduced by half, but also the current flows out through the transfer material and adversely affects the transfer mechanism. In addition, there is a possibility that charges having the same polarity as that of the transfer material toner may be injected, and in this case, the toner holding force of the transfer material may be reduced, thereby promoting the offset. In order to prevent this problem, the resistance of the coating material can be increased by using a PFA tube as a coating material for the fixing roller, applying multiple coatings of PFA, or using an insulating material as a primer for the coating layer.

However, as the insulating property of the fixing roller coating layer is increased in this way, the charging of the coating layer surface becomes remarkable,
The electric charge existing on the surface gradually accumulates and greatly affects the electric field outside the roller. The electric field generated by the charging tends to be generated in a direction to weaken the electric field generated by the voltage applied to the roller core for the purpose of preventing offset. Such an effect due to charging is sometimes observed to be strong enough to offset the effect of the voltage applied to the roller core, and often causes an offset, which is insufficient as a countermeasure.

The state of such charging is confirmed by measuring the surface potential of the roller. FIG. 7 shows how the surface potential of the fixing roller changes when −500 V is applied to the fixing roller coated with a PFA tube as a release layer for preventing offset of the negatively charged toner and continuous paper passing is performed. FIG. 8 shows how the surface potential of the fixing roller changes during paper passing.
Sheets of time for more information as viewed, time t ₀ is the time at which the leading edge of the transfer material enters the nip, the time t ₁ is the time that came out from the nip.

As is apparent from FIG. 7, the voltage of -500 V applied to the fixing roller core metal was observed as the surface potential as it was before the paper was passed, whereas -250 V was applied after 100 sheets were passed.
Has changed to This is because the positive electric charge accumulated on the surface of the fixing roller due to the charging while repeating the paper passing causes the electric field formed by the electric charge to cancel the electric field due to the metal core potential. In such a case, the effect of the core metal potential as the offset suppressing means was hardly observed, and a remarkable offset was confirmed after 100 sheets were passed. On the other hand, FIG. 8 shows that the main cause of such charging is not due to friction but to peeling. The surface potential of despite roller for friction is generated between the transfer material and the roller surface is between from t ₀ to t ₁ hardly changes,
A large charge is observed at t ₁ when the transfer material is separated from the roller. Such a peeling charge is generated due to a large potential difference between the fixing roller core metal potential and the transfer material surface potential. However, applying a bias voltage to the core metal lowers those potentials in the first place. Since the toner relies on the effect that the toner is pressed against the transfer material by the electric field, the above-described separation charging is in principle unavoidable.

[Means for Solving the Problems] The present invention for solving the above problems includes a fixing roller having a surface insulating layer provided outside a metal core, a pressure roller forming a nip with the fixing roller, and the fixing roller. Voltage applying means for applying a voltage to the metal core of the recording medium, and the recording material is nipped and conveyed so that the unfixed image on the recording material contacts the fixing roller at the nip, and the unfixed image is fixed on the recording material. In the fixing device, the pressure roller has a conductive elastic layer on or near the surface, and the volume resistance of the conductive elastic layer is 10 ²
~ 10 ⁵ Ωcm.

Embodiment FIG. 1 is a schematic sectional view of a fixing device according to an embodiment of the present invention. In this embodiment, as the fixing roller 1, a core metal 17 such as aluminum or stainless steel covered with a pure PFA tube 131 is used. As the coating layer, other than such a fluororesin tube, a fluororesin multi-coating treatment, or a fluororesin adhesive having extremely high insulation and pressure resistance can be used.

The coating layer as the insulating layer preferably has a volume resistance of 10 ¹⁴ Ωcm or more, and has a withstand voltage of 1 kV or more.

The fixing roller used in this example had a PFA tube of 30 μm in the coating layer, but showed a real resistance between the surface and the core metal of 10 ¹⁴ Ω or more (when 500 V was applied) and a dielectric strength of 2 KV. . For those exhibiting such high insulation and pressure resistance, the pinhole H as shown in FIG. 6 is not included in the structure by forming the tube, and the high insulation inherent to PFA resin is secured. That's because.

In this embodiment using such a fixing roller having the PFA tube 131 as a coating layer, even when a high voltage is applied to the core metal, no current flows out to the transfer material P, and the voltage source 14
However, even if the current capacity is small, the applied voltage does not drop, and a stable voltage can be supplied. Also,
Although not shown in FIG. 1, there is no fear of leaking to the temperature detecting element disposed around the fixing roller, and the configuration is excellent in terms of safety and protection of electric elements.

Also, when the present embodiment is used in an electrophotographic apparatus in which the distance between the transfer position and the fixing position is shorter than the maximum transfer material, a small leakage current does not affect the transfer mechanism and a good transfer image is obtained. be able to.

On the other hand, the pressure roller 3 uses a conductive silicone rubber 51 as an elastic material as a surface layer, and the elastic material is grounded. The pressure roller 3 made of the conductive elastic material does not become charged even when rubbed against the transfer material, and does not exert an electrostatic repulsive force on the toner. The conductive elastic material 51 for exhibiting such properties
The electrical resistance is 10 ⁵ Ωcm or less in volume resistance, and 10 ¹¹ Ω between the surface (10 cm ² ) and the core as the actual resistance of the pressure roller.
(500 V applied) or less. Further, in the case of an apparatus configuration in which a part of the transfer material enters the fixing device while undergoing the transfer operation, if the pressure roller has extremely low resistance, the charge may be unlimitedly discharged from the transfer material in a high humidity environment. As a result, adverse effects such as poor transfer may occur. Therefore, it is preferable to use a conductive elastic material having a volume resistance of 10 ² to 10 ⁵ Ωcm, and to have a configuration in which the actual resistance of the pressure roller is 10 ³ to 10 ¹¹ Ω (hereinafter, the pressure roller having this configuration is preferable). Is referred to as a conductive pressure roller).

When a voltage having the same polarity as that of the toner is applied to the fixing roller core 1 to suppress the offset, the potential of the transfer material is stabilized by using this conductive pressure roller in combination, and the potential of the fixing roller surface is reduced. As a result, a high offset suppression effect can be maintained even when continuous paper is passed.

[Experimental example] When a PFA resin layer with a thickness of 300 μm is used as the fixing roller surface coating material, and the fixing roller core is applied with -500 V as an offset suppression bias, it is conductive as an elastic material of the pressure roller. The surface potential of the fixing roller when the sheet was continuously passed was compared and measured when the sheet was grounded and when an insulating elastic material was used. The conductive elastic material used here was relatively high with a volume resistance of about 10 ⁵ Ωcm, but the device configuration used in this experiment was designed to prevent transfer omission because the fixing device was not sufficiently separated from the transfer mechanism. I chose something like this in a sense. The actual resistance is about 10 ⁹ Ω.

FIG. 2 shows the result. As is apparent from the figure, when the insulating pressure roller is used, the result is 10%.
When the fixing roller is charged after passing 00 sheets and the surface potential has dropped by 400 V, when using the conductive pressure roller, it is about 100 V even after passing 1000 sheets. Only descended.

FIG. 3 shows the results obtained by measuring the integrated amount of the offset toner with respect to the number of sheets passed in each system.
The toner used was negatively charged. When the insulating pressure roller is used, the number of sheets to be passed increases, and the amount of offset toner increases at an accelerated rate. On the other hand, when the conductive pressure roller is used, the offset amount is very small.

Second Embodiment FIG. 4 is a schematic view showing a second embodiment of the present invention.
In the second embodiment, the conductive elastic member 51 of the pressure roller 3 is grounded via the rectifying element 15. As a method of connecting the rectifying element 15, the rectifying element 15 is connected in such a direction that electric charges of the opposite polarity to the toner can move from the ground side to the elastic material. As a result, the pressure roller is biased to a voltage having a polarity opposite to that of the toner, and has an effect of suppressing offset (self-bias effect). Due to this self-bias effect, the maximum value of the potential applied to the core of the pressure roller is equal to the reverse breakdown voltage of the rectifying element 15. If a reverse breakdown voltage that is too high is selected, the charging of the fixing roller is promoted when charging up. Not only,
There is a possibility that the discharge of the fixing roller core metal and the surrounding members may cause damage to the members and radio interference. Therefore, it is preferable to select the rectifying element such that the maximum potential difference between the core of the pressure roller and the core of the fixing roller is 3 KV or less.

Further, in an apparatus configuration in which the apparatus is downsized as in recent years and the transport distance from the transfer mechanism to the fixing apparatus is short, if the resistance value of the conductive elastic material is extremely low in the first embodiment, The transfer current flows out through the conductive pressure roller to cause a problem such as transfer omission. However, in this embodiment, the rectifying element 15 has an effect of preventing the transfer current from leaking even in such a case. Therefore, according to the device configuration of the present embodiment, a very low-resistance conductive elastic material can be used.

Third Embodiment FIG. 5 is a schematic diagram showing a third embodiment of the present invention.
In this embodiment, the releasability of the pressure roller 3 is further enhanced, and a conductive PFA tube 16 is provided as a surface coating layer on the outside of the conductive elastic member 51 of the pressure roller. Conductive P
Although the volume resistance of the FA tube 16 is generally higher than that of an elastic material such as conductive silicone rubber, it is about 10 ⁷ to 10 ¹⁰ Ωcm,
Its thickness is several tens of μm or less, and the antistatic effect is sufficiently exhibited in combination with the conductive elastic material. By providing a release layer on the pressure roller surface, toner adhesion to the surface is dramatically reduced compared to a silicone rubber single layer, and the transfer material is wrapped around the pressure roller and the transfer material is stained on the back. Will not occur.

In the present embodiment, a conductive PFA tube is used as a coating material. However, a similar effect can be expected with a normal fluororesin tube or coating if a thin coating layer is used to reduce the actual resistance. Further, it is possible to further reduce the resistance by using a conductive primer as a primer of the fluororesin coating layer.

When the outflow of the toner holding charge from the transfer material becomes a problem due to the conductivity of the pressure roller, the rectifying element 15 shown in the second embodiment is completed to complete the problem.

[Effects of the Invention] As described above, according to the present invention, the potential of the transfer material is stabilized, the charging of the fixing roller surface is reduced, and a sufficient offset suppression effect is maintained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a fixing device according to an embodiment of the present invention. FIG. 2 is a diagram showing a change in the surface potential of a fixing roller with respect to the number of sheets passed. FIG. FIG. 5 is a schematic sectional view showing another embodiment of the present invention. FIG. 5 is a schematic sectional view showing another embodiment of the present invention. FIG. 6 is a schematic view of a fixing device in which a bias voltage is applied to a fixing roller. 7 and 8 are diagrams showing the charging of the fixing roller in the apparatus of FIG. 6. FIG. 9 is a longitudinal sectional view of a conventional fixing device. 1 is a fixing roller, 2 is a heater, and 3 is a pressure roller. Reference numeral 4 denotes a pressure roller core, 51 denotes a conductive elastic material of the pressure roller, 131 denotes a PFA tube layer of a fixing roller, 17 denotes a fixing roller core, and 14 denotes a fixing roller core.
Is power supply, P is transfer material, ta is unfixed toner image, H is pinhole

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Akihiko Takeuchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hiroto Hasegawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Referee Tetsuo Taki, Referee Chief Judge Takashi Fushimi, Referee Hiroshi Shirosho (56) References JP-A-59-180571 (JP, A) JP-A-55-96970 (JP, A) JP-A 63 -8677 (JP, A) JP-A-60-63575 (JP, A) JP-A-1-161278 (JP, A) JP-A-1-154287 (JP, A) JP-A-2-89079 (JP, A) JP-A-1-219879 (JP, A) JP-A-1-293376 (JP, A) JP-A-60-184075 (JP, U) JP-B-58-23626 (JP, B2)

Claims (1)

(57) [Claims] A fixing roller having a surface insulating layer provided outside the core, a pressure roller forming a nip with the fixing roller, and a voltage applying means for applying a voltage to the core of the fixing roller. A fixing device that nips and conveys the recording material so that the unfixed image on the recording material contacts the fixing roller at the nip and fixes the unfixed image on the recording material. A fixing device having a conductive elastic layer in the vicinity thereof, wherein the conductive elastic layer has a volume resistance of 10 ² to 10 ⁵ Ωcm.