JPH04274472A - Fixing device - Google Patents

Abstract

PURPOSE:To stabilize the potential of a transfer material, to reduce the electrification of the surface of a fixing roller, and to last an offset suppressing effect by providing a surface resistant layer on a second roller and an insulating layer over a low resistant layer. CONSTITUTION:As the fixing roller, a thing that the core metal 17 of aluminium, etc., is covered with a pure PFA tube 131 is used. It is desirable that the covering layer 131 as the insulating layer has >=10<14>OMEGAcm volume resistivity and >=1KV insulation breakdown strength. On the other hand, a conductive silicone rubber 51 as an elastic material is used for a surface layer on the pressure roller 3, and the elastic material is grounded. At such a time, the end A of the insulating layer 131 of the fixing roller 1 is outside compared with the end B of the low resistant layer 51 of the pressure roller 3, and a distance AB has a precribed value by a voiltage applied to the core metal 17 of the fixing roller 1.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device used in image forming apparatuses such as copying machines and electrophotographic printers for fixing unfixed images using pressure or heat and pressure.

[Background Art] FIG. 9 shows an example of a fixing device.

The upper fixing roller 1 has a hollow core made of aluminum, iron, etc., and a heater 2 such as a halogen lamp is placed inside the core, and the fixing roller 1 is heated by the heater 2. The lower pressure roller 3 is constructed by covering the outer periphery of a core metal 4 made of iron, stainless steel, etc. with an elastic body 5 having mold releasability such as silicone rubber. The above fixing roller 1
and pressure roller 3 are brought into contact with each other with a predetermined pressure by a biasing 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 that is brought into contact with the surface of the fixing roller 1, and detects the surface temperature of the fixing roller 1. According to the temperature detected by the temperature sensing element 6, the temperature control circuit controls the power supply to the heater 2, and the surface temperature of the fixing roller 1 is automatically controlled to a predetermined heat fixing temperature. A separating claw 7 separates the transfer material P supporting the toner image from the surface of the fixing roller 1, and is disposed so that its leading edge portion is in contact with the surface of the fixing roller 1 with an appropriate pressure. Reference numeral 8 denotes a cleaner made of felt or the like which is brought into pressure contact with the surface of the fixing roller 1, and wipes off toner tc, paper powder, etc. adhering to the surface of the fixing roller 1. 9 is the metal bottom plate of the fixing device, 10 and 11
A transfer material entrance guide and a transfer material exit guide are respectively attached and supported upwardly on the bent front wall and rear wall of the bottom plate 9. The transfer material P that has entered the fixing device through the entrance guide 10 enters the nip between the fixing roller 1 and the pressure roller 3, which are rotationally driven in pressure contact with each other, and passes through the nip between the rollers 1 and 3. going through the department. In this process of passing, the unfixed toner image t on the transfer material P
a is fixed as a permanent fixed image tb by heat and pressure.

The leading end of the transfer material P which has passed through the nip between the rollers 1 and 3 and has had its image fixed thereon is separated from the surface of the fixing roller 1 by a separating claw 7, and is discharged to the outside of the fixing device through an exit guide 11. .

In the above 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 surface of the fixing roller (hereinafter referred to as offset). ). Such offset toner tc may be fixed in an inappropriate place on the transfer material P as it rotates and stain the transfer material P, or it may transfer to the pressure roller 3 between sheets and stain the back side of the accumulated transfer material P. or make separation difficult. The two main causes of offset are the physical bonding force between the toner and the roller and the electrostatic force. For the offset caused by the former cause, it is effective to increase the smoothness of the roller surface or lower the surface energy. Specifically, P is a material with such properties.
Examples include fluororesins such as TFE and PFA, and these fluororesins also have excellent heat resistance, abrasion resistance, and corrosion resistance, which are necessary as coating materials for fixing rollers. On the other hand, the latter electrostatic force acts on the toner, which is a charged powder, in the electric field created by the charged fixing roller, pressure roller, transfer material, etc. due to friction or peeling, and there is an attraction between the fixing roller and the pressure roller. When the repulsive force between the transfer material and the transfer material is strong, a significant offset often occurs.

[0007] As a means to reduce the offset caused by such electrical effects, the surface coating layer of the fixing roller is made conductive to release the electrostatic charges, and an antistatic agent is applied to the surface to prevent electrostatic charges. There is a way to do this.

[0008] Making the surface of the fixing roller conductive has the effect of preventing the fixing roller from being charged, but cannot prevent the influence of the repulsive electric field caused by the pressure roller. Furthermore, in electrophotography, when the distance between transfer and fixation is shorter than the maximum size transfer material, the transfer current flows into the fixing roller especially in a high humidity environment, causing transfer failure.

[0009] Furthermore, if an antistatic agent is applied, when the transfer material that has been passed once is passed through again, the antistatic agent that has adhered to the surface of the transfer material will adhere to the photoreceptor, thereby interfering with the photosensitive phenomenon and the charging of the photoreceptor. become. In such a case, a good image cannot be expected. Furthermore, in the case of a system that is applied to a surface, a replenishing member is required to maintain its effect.

Therefore, in recent years, a method has been considered in which a voltage of the same polarity as that of the toner is applied to the fixing roller to suppress the toner by the action of an electric field.

An example of this is shown in FIG.

A bias voltage of -600 V having the same polarity as the toner is applied to the core metal of the fixing roller 1 from the power source 14.

13 is a 25 μm thick P plate provided on the core metal.
This is an FA resin coating layer.

However, in this fixing method, although a significant effect is achieved by applying a sufficient voltage to the fixing roller, a current of 5 to 400 μA flows during paper feeding. Originally pure PF
A is an insulator, but in the case of a single layer coating, there are many pinholes H in the structure of the surface coating material as shown in the figure.
exists, and the resistance value decreases.

In such a case, the current capacity of the power supply 14 is insufficient and the applied voltage drops, which not only reduces the effect on offset by half, but also causes current to flow through the transfer material, adversely affecting the transfer mechanism. . Furthermore, there is a risk that charges having the same polarity as the toner of the transfer material may be injected, and in this case, the toner holding power of the transfer material may be reduced, thereby promoting offset. To prevent this problem, the resistance value of the coating material can be increased by using a PFA tube as the coating material for the fixing roller, applying multiple PFA coatings, or using an insulating material as a primer for the coating layer.

However, as the insulation of the fixing roller coating layer is increased in this way, the surface of the coating layer becomes significantly charged, and the charges existing on the surface gradually accumulate and have a large effect on the electric field outside the roller. It becomes like this. The electric field generated by this charging tends to weaken the electric field generated by the voltage applied to the roller core metal for the purpose of preventing offset. The effect of such charging is sometimes observed to be so strong that it cancels out the effect of the voltage applied to the roller core metal, and offset often occurs, making it an insufficient countermeasure.

[0017] The state of charging is confirmed by measuring the surface potential of the roller. FIG. 7 shows how the surface potential of the fixing roller changes when -600 V is applied to the fixing roller coated with a PFA tube as a release layer to prevent offset of negatively charged toner and paper is continuously passed. In addition, FIG. 8 shows a detailed view of changes in the surface potential of the fixing roller during paper feeding over the time period of one sheet. Time t0 is the time when the leading edge of the transfer material enters the nip, and time t1 is the time when it exits from the nip. It is.

As is clear from FIG. 7, the voltage of -500V applied to the core metal of the fixing roller is directly observed as the surface potential before paper passes, but after passing 100 sheets, the voltage becomes -250V. It has changed to. This is because positive charges are accumulated on the surface of the fixing roller as the paper is repeatedly passed through the fixing roller, and the electric field formed thereby cancels out the electric field due to the potential of the core metal. In such a case, the effect of the core metal potential as an offset suppressing means was hardly seen, and significant offset was observed after 100 sheets had been passed. On the other hand, looking at FIG. 8, it can be seen that the main cause of such charging is not due to friction but due to peeling. From t0 to t1, the surface potential of the roller hardly changes even though there is friction between the transfer material and the roller surface, and at t1 the transfer material leaves the roller.
A large charge has been observed at . Such peeling electrification occurs because there is a large potential difference between the fixing roller core metal potential and the transfer material surface potential, but in the first place, applying a bias voltage to the core metal causes the electric field created by those potential differences to be generated. This is due to the effect of pressing the toner onto the transfer material by the toner, so the above-mentioned peel-off charging is in principle unavoidable.

In order to solve the above problems, we first developed a first roller having a core metal and a surface insulating layer provided on the outside of the core metal, a pressure roller that comes into pressure contact with the fixing roller, and a fixing roller. and a bias power supply for applying a bias voltage to the fixing device, the fixing device is configured to sandwich and transport a support material supporting an unfixed image between a fixing roller and a pressure roller to fix the image, the pressure roller being attached to the surface of the support material. We proposed one with a grounded low resistance layer.

[0020]

[Problems to be Solved by the Invention] However, if sufficient insulation is not provided between the fixing roller and the pressure roller in this configuration, the bias may leak and voltage cannot be applied to the first roller. Alternatively, electric discharge may occur between the first roller and the second roller, generating noise, which may interfere with the control of the apparatus itself or the operation of peripheral machines.

[0021]

[Means for Solving the Problems] The present invention, which solves the above problems, includes a first roller having a conductive core material and a surface insulating layer provided on the core material; A fixing device comprising a second roller that comes into pressure contact with the first roller and an electric field forming means having a bias power source that forms an electric field between the first roller and the second roller, the second roller having a surface low resistance layer, The above-mentioned insulating layer is characterized in that it is provided beyond this low-resistance layer.

[0022]

Embodiments Hereinafter, embodiments of the present invention will be explained based on the drawings.

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 made of aluminum or stainless steel is made of pure PFA.
A tube covered with a tube 131 is used. As the covering layer, in addition to such a fluororesin tube, a layer formed by multiple coatings of fluororesin, or a fluororesin adhesive having extremely high insulation and pressure resistance can be used.

The coating layer as an insulating layer has a volume resistance of 1014
The surface resistance is preferably Ωcm or more, the surface resistance is preferably 1012Ω or more, and the dielectric strength voltage is preferably 1 KV or more.

The fixing roller used in this example has a 50 μm PFA tube as a coating layer, and the actual resistance between the surface and the core metal was 1014 Ω or more (high resistance meter manufactured by Yokogawa Hewlett-Packard). 5 in Model 4329A
(Measured by applying 00V), the dielectric strength voltage was 2KV. By tube-molding products that exhibit such high insulation and pressure resistance, pinholes H as shown in Figure 6 are no longer included in the structure, ensuring the high insulation properties that PFA resin originally has. It's for a reason.

In this embodiment, which uses a fixing roller having such a PFA tube 131 as a coating layer, even if a high voltage is applied to the core metal, no current flows to the transfer material P, and the voltage source Even if the current capacity of 14 is small, the applied voltage does not drop, making it possible to supply a stable voltage. Further, although not shown in FIG. 1, there is no fear of leakage to the temperature sensing element disposed around the fixing roller, resulting in an excellent configuration from the standpoint of safety and protection of electrical elements.

Furthermore, even when this embodiment is used in an electrophotographic device in which the distance between the transfer position and the fixing position is shorter than the maximum transfer material, the leakage current is so small that it does not affect the transfer mechanism, resulting in good transfer. You can get the image.

On the other hand, the pressure roller 3 uses conductive silicone rubber 51 as a surface layer of an elastic material, and the elastic material is grounded. As described above, the pressure roller 3 made of a conductive elastic material is not charged even when it rubs against the transfer material, and does not exert electrostatic repulsion on the toner, which is effective in suppressing offset. The electrical resistance of the conductive elastic material 51 to exhibit such properties is 105Ω in terms of volume resistance.
cm or less, the surface as the actual resistance of the pressure roller (10c
m2) and the core metal is preferably 1011Ω or less (500V applied). Furthermore, in the case of a device configuration in which a part of the transfer material enters the main fixing device while being subjected to the transfer operation, if the pressure roller has extremely low resistance, the charge may be released from the transfer material indefinitely in a high humidity environment. This may cause problems such as poor transfer. Therefore, the volume resistance is preferably 102
It is preferable to use a conductive elastic material having a resistance of 10 5 Ωcm to 10 3 Ω to 10 11 Ω in actual resistance of the pressure roller (hereinafter, a pressure roller having this configuration will be referred to as a conductive pressure roller).

When suppressing offset by applying a voltage of the same polarity as the toner to the fixing roller core metal 1, by using this conductive pressure roller in combination, the potential of the transfer material is stabilized and the fixing roller surface is As a result, a high offset suppression effect can be maintained even when paper is continuously fed.

FIG. 10 shows the axial positional relationship between the fixing roller and the pressure roller.

The end of the insulating layer of the fixing roller 1 is located outward from the end B of the low resistance layer of the pressure roller 3, and the distance AB is AB= V
/300+αmm or more.

[0032] This value indicates that the dielectric strength of a normal space is 1K.
V/mm, whereas the voltage at which dielectric breakdown occurs along the surface of the insulator is lower, about 300 V/mm, which is the lower limit.

Further, α is a tolerance that occurs during assembly within the apparatus, and is the sum of the thrust backlash of the fixing roller 1 and the pressure roller 3, etc. It is usually 1 mm or less.

(Experimental Example) A PFA resin tube with a thickness of 50 μm was used as a surface covering material for the fixing roller, and a -
When applying 600V, compare and measure the surface potential of the fixing roller when paper is continuously passed between cases where a conductive material is used as the pressure roller's elastic material and it is grounded, and a case where an insulating material is used as the pressure roller's elastic material. did. The conductive elastic material used here has a relatively high volume resistivity of about 105 Ωcm, but in the device configuration used in this experiment, the fixing device is not far enough from the transfer mechanism, so it is difficult to prevent transfer failure. I chose something like this. Note that the actual resistance is approximately 109Ω. With this configuration, the distance between the end face of the low resistance layer of the pressure roller 3 and the end face of the insulating layer of the fixing roller 1 was set to 3 mm.

Tolerances such as thrust play of the pressure roller and fixing roller are 1 mm or less even if added, and V/300 = 2 mm.
Therefore, by keeping a distance of 3 mm, the core metal bias of the fixing roller 1 was not discharged to the low resistance layer of the pressure roller 3.

On the other hand, if the distance between the end of the insulating layer of the fixing roller and the end of the low resistance layer of the pressure roller is set to 1.5 mm, leakage will sometimes occur between the fixing roller and the pressure roller, causing noise to the surrounding devices. The action occurred.

FIG. 2 shows the results of measuring changes in the surface potential of the fixing roller with this configuration.

As is clear from the figure, when an insulating pressure roller is used, the fixing roller becomes charged after 1000 sheets have passed, and the surface potential drops by 400 V. , when using a conductive pressure roller, 1
Even after passing 1,000 sheets, the voltage dropped by only about 100V.

FIG. 3 shows the cumulative amount of offset toner measured with respect to the number of sheets passed in each of the above systems. Note that a negatively charged toner was used as the toner. When an insulating pressure roller is used, the amount of offset toner increases rapidly as the number of sheets passed increases, whereas when a conductive pressure roller is used, the amount of offset is kept very small.

In this way, in this embodiment, it was possible to prevent offset and noise from occurring due to bias leakage.

(Second Embodiment) FIG. 4 is a schematic diagram showing a second embodiment of the present invention. In this second embodiment, the conductive elastic material 51 of the pressure roller 3 is grounded via the rectifying element 15. The rectifying element 15 is connected in a direction that allows charges of opposite polarity to the toner to move from the ground side to the elastic material. As a result, the pressure roller is biased with a voltage of opposite polarity to that of the toner, which has the effect of suppressing offset (self-bias effect). The maximum value of the potential applied to the pressure roller core due to this self-bias effect is equal to the reverse withstand voltage of the rectifying element 15, and if a reverse withstand voltage that is too high is selected, charging of the fixing roller will be promoted in the event of charge-up. Moreover, there is a risk that the electric discharge may occur to the fixing roller core metal and surrounding members, causing damage to the members and radio wave interference. Therefore, it is preferable to select a rectifying element such that the maximum potential difference between the pressure roller core metal and the fixing roller core metal is 3 KV or less.

[0042] Furthermore, in the case where the resistance value of the conductive elastic material is extremely low in the first embodiment, in an apparatus configuration in which devices have become smaller in recent years and the conveyance distance from the transfer mechanism to the fixing device has become shorter. In this case, the transfer current leaks through the conductive pressure roller, causing problems such as transfer omission, but in this embodiment, even in such a case, the rectifying element 15 is effective in preventing transfer current leakage. have. Therefore, with the device configuration of this embodiment, a very low resistance layer can be used as the conductive elastic material.

In this case, the distance between the end B of the pressure roller low resistance layer and the end A of the fixing roller insulating layer is AB=(V+V')/3 with respect to the reverse breakdown voltage V' of the diode and the bias voltage V.
It is given by 00+α.

(Third Embodiment) FIG. 5 is a schematic diagram showing a third embodiment of the present invention. In this embodiment, the pressure roller 3 has a configuration in which the mold releasability is further improved, and a conductive PFA tube 16 is provided as a surface coating layer on the outside of the conductive elastic material 51 of the pressure roller. The electrically conductive PFA tube 16 generally has a higher volume resistivity than elastic materials such as electrically conductive silicone rubber, about 107 to 1010 Ωcm, but its thickness is several tens of micrometers or less, and the combination with the electrically conductive elastic material has an antistatic effect. Show enough. By providing a release layer on the surface of the pressure roller, compared to a single layer of silicone rubber, toner adhesion to the surface is dramatically reduced, preventing the transfer material from wrapping around the pressure roller and staining the back of the transfer material. will no longer occur.

Although a conductive PFA tube was used as the covering material in this embodiment, the same effect can be expected with a normal fluororesin tube or coating if the actual resistance is lowered by forming a thin covering layer. Furthermore, it is also possible to further reduce the resistance by using a conductive primer as a primer for these fluororesin coating layers.

[0046] If the pressure roller is made electrically conductive and the toner-holding charge flows out from the transfer material, the problem can be solved by incorporating the rectifying element 15 shown in the second embodiment.

In this case, the distance AB between the fixing roller insulating layer end A and the pressure roller low resistance layer end B is AB=V/
It is given by 300+α.

(Fourth Embodiment) Although the fixed end roller 1 described above has a pipe shape, it may be made thinner at both ends by drawing or press fitting as shown in FIG. It is also a good idea to add an insulating layer to the outside of this aperture.

As a result, the creeping insulation distance can be extended, and the space distance L can be made larger than the space distance l shown in FIG. 10 with respect to the pressure roller core metal, thereby preventing leakage and discharge through the space.

Furthermore, as shown in FIG. 12, the insulating layer may be made of a PFA tube, and the fixing roller may be left unadhered up to the squeezed portion at the end. As a result, the distance from the end of the low-resistance layer of the pressure roller 3 to the core metal of the fixing roller 1 can be set to approximately 2S, and an insulation distance from the pressure roller core metal can be provided, making the structure even more resistant to leaks. Can be done.

In all of the above embodiments, a bias is applied to the core metal of the fixing roller, but it is also possible to apply a voltage of opposite polarity to the toner to the core metal of the pressure roller, and to ground the core metal of the fixing roller side. .

[0052]

[Effects of the Invention] As explained above, according to the present invention, the potential of the transfer material is stabilized, the charge on the surface of the fixing roller is reduced,
Sufficient offset suppression effect is maintained.

[0053] The occurrence of noise and offset due to leakage of the fixing roller bias is also prevented.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram showing changes in fixing roller surface potential with respect to the number of sheets passed.

FIG. 3 is a diagram showing the offset toner weight relative to the number of sheets passed.

FIG. 4 is a schematic cross-sectional view showing another embodiment of the present invention.

FIG. 5 is a schematic cross-sectional view showing another embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view of a fixing device in which a bias voltage is applied to a fixing roller.

FIG. 7 is a diagram showing charging of the fixing roller in the apparatus of FIG. 6;

FIG. 8 is a diagram showing charging of the fixing roller in the apparatus of FIG. 6;

FIG. 9 is a longitudinal sectional view of a conventional fixing device.

FIG. 10 is a diagram showing the axial positional relationship between the fixing roller and the pressure roller in the embodiment of the present invention.

FIG. 11 is a cross-sectional view of another embodiment of the invention.

FIG. 12 is a cross-sectional view of another embodiment of the invention.

[Explanation of symbols]

1 Fixing roller 2 Heater 3 Pressure roller 4 Pressure roller core metal 14 Power supply 17 Fixing roller core metal 19 Insulating layer 51 Conductive elastic material for pressure roller 131 PFA tube layer P of fixing roller Transfer material ta Unfixed toner image H Pinhole

Claims (2)

[Claims] 1. A first roller having a conductive core material and a surface insulating layer provided on the core material, a second roller in pressure contact with the first roller, and a second roller having a conductive core material and a surface insulating layer provided on the core material; an electric field forming means having a bias power source for forming an electric field between two rollers, the second roller having a surface low resistance layer, and the insulating layer extending beyond the low resistance layer. A fixing device characterized by being provided. 2. The volume resistance value of the surface insulating layer is 101
4 Ωcm or more, and the volume resistivity value of the low resistance layer is 10
The fixing device according to claim 1, characterized in that the resistance is 10 Ωcm or less.