JPH04335386A - Fixing device - Google Patents

Abstract

PURPOSE:To prevent charge from flowing in a recording material, and to prevent an image from being faulty without lowering the heat conductivity of the surface layer of a fixing roller by connecting a resisting element between a power source which is connected to at least either one of core bars of the fixing roller or a pressure roller and the core bar. CONSTITUTION:A resistance R to control a current is connected between the fixing roller 1 and the power source 8. The surface layer of the fixing roller 1 ordinarily has enough voltage-proof, and a voltage required to prevent offset is impressed on the surface layer as it is. But when there is a part of the fixing roller in which insulation is not good such as a pin hole, the current flows from the part to the recording material, a tear occurs on the coating of the fixing roller, and dirt of the image caused by adherence of the toner occur. therefore, in this case, when the resistance R to control the current flowing in the recording material is connected, the current flowing through the pin hole is decreased, so that the tear of the coating can be prevented. For example, the tear of the coating can be prevented when the current flowing in the pin hole is made to <=5muA.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device for fixing an unfixed image to a latent image carrier in an electrophotographic or electrostatic recording printer.

0002

2. Description of the Related Art A heat roller fixing device used in a conventional image forming apparatus is shown in FIG.

The apparatus shown in FIG. 6 includes a fixing roller 1 in which a resin layer having heat resistance and mold releasability such as PFA or PTFE is attached on a core metal such as aluminum or iron, and a silicone rubber layer also on the core metal. Pressure roller 2 with an elastic layer of fluoro rubber
The recording material P on which the toner image Q is placed is passed between the pair of rollers and heated and pressurized to fix the toner image.

The fixing roller 1 is heated by an internal heater 3, and its surface temperature is detected by a temperature detection element 4.
Turns on/off the power to the heater 3 based on this detection output
By doing so, the predetermined temperature is maintained.

The recording material P is guided between the pair of rollers by an entrance guide 6, separated from the fixing roller 1 by a separation claw 5, and sent out of the apparatus by a paper discharge guide 7.

[0006] When fixing is carried out by conveying the recording material in this way, due to frictional charging between the roller and the recording material, charge accumulates on the surface of the fixing roller 1 and the toner is adsorbed, or the pressure roller 2 A problem has been that charges accumulate on the surface and repel the toner, causing electrostatic offset to the fixing roller 1. Therefore, it has been proposed in Japanese Patent Laid-Open No. 55-96970 to supply a bias voltage to the fixing roller 1 by a power source.

[0007] This is to repel the toner Q from the fixing roller 1 by applying a voltage of the same polarity as the toner Q to the core metal of the fixing roller 1.

[0008]

However, in the conventional example described above, since the coating layer on the surface of the fixing roller is not a completely uniform insulator, current flows to the recording material, resulting in the following problem.

(2) Charges of the same polarity as the toner are injected into the recording material, which cancels out the transfer current and causes transfer defects.

(2) Charges having the same polarity as the toner are injected into the recording material, thereby repelling the toner and causing an offset.

(2) Since an excessive current flows through the coating layer, the coating layer is destroyed and the mold releasability is impaired. As a result, toner adheres to the image, causing it to peel off or stain the white background.

[0012] As a countermeasure for the above problem, it is possible to improve the insulation of the coating layer on the surface of the fixing roller, but it is difficult to create a perfect insulation coating layer even if a PFA tube with highly uniform resin is used. It is. Increasing the thickness of the PFA tube will improve its insulation properties, but if it is too thick, the thermal conductivity between the core metal and the surface layer of the fixing roller will deteriorate, and the temperature of the core metal will rise too much, causing a decrease in the adhesive strength of the surface coating layer. There is a problem in that heat conduction from the core metal is delayed during paper passing, and the surface temperature of the fixing roller decreases, resulting in poor fixing performance.

[0013] Therefore, although the appropriate thickness of the PFA tube is 25 .mu.m to 60 .mu.m, it has not been possible to obtain a complete insulation coating with this thickness.

The present invention solves the above-mentioned problems and provides a fixing device that prevents charges from flowing into the recording material without reducing the thermal conductivity of the surface layer of the fixing roller and does not cause image defects. It is an object.

[0015]

[Means for Solving the Problems] According to the present invention, the above object is to provide a roller pair of a fixing roller and a pressure roller having an elastic layer or a resin layer on a core metal, and at least one of the roller pairs. This is achieved by connecting a resistive element between the power source and the core metal in a fixing device equipped with a power source for applying voltage to the core metal of the roller.

[0016]

[Operation] According to the present invention, since a voltage is applied to the core metal of the fixing roller or the pressure roller through the resistance element, when a recording material enters the pressure contact portion of the fixing roller and the pressure roller, Even if a portion of the surface layer of the fixing roller or pressure roller that has poor insulation comes into contact with the recording material, the resistive element prevents excessive current from flowing through the surface layer.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First to third embodiments of the present invention will be explained based on the drawings.

<First Embodiment> First, a first embodiment of the present invention will be described based on FIGS. 1 and 2.

FIG. 1 shows an apparatus according to a first embodiment of the present invention, in which a resistor R is connected between a fixing roller 1 and a power source 8 to limit the current.

Normally, the surface layer of the fixing roller 1 has sufficient voltage resistance, and the voltage V1 of the power source 8 is directly applied as the voltage necessary to prevent offset.

In the experiment, the surface layer of a fixing roller with an outer diameter of 30 mm was covered with a 50 μm thick PFA tube, and negative toner was used, and the fixing roller bias was set to V1.
When ≧-600V, offset could be prevented.

However, according to the conventional apparatus, when the bias is applied, a current flows to the recording material at a portion of the fixing roller with poor insulation (hereinafter referred to as a pinhole), causing a tear in the coating of the fixing roller. It happened. If the coating is torn in this way, toner will stick and stain the image.

Therefore, when a resistor R was connected to limit the current as in the present invention, it was found that the current flowing through the pinhole was reduced and the coating did not break. When the resistance R was actually varied, it was found that by setting R≧120 MΩ, the current flowing through the pinhole became 5 μA or less, and breakage of the coating was prevented.

Furthermore, it is also conceivable to apply an offset prevention voltage while flowing a current toward the resistor R' as shown in FIG. In this case, R'≪R and if a constant current power source is used as the power source 8, the current flowing from the fixing roller through the pinhole will be part of the current i flowing from the power source 8.

0025
]

[Equation 1] This is a value that can be sufficiently reduced to 5 μA or less. For example, if R' = 60MΩ and the output of the constant current power supply 8 is 10μA, then if R = 400MΩ, the pinhole will be 1.3
μA flows. With this level of current, the coating will not break at the pinhole area.

Another advantage is that load fluctuations applied to the power source 8 can be stabilized. In particular, in recent years, multiple loads are often connected to a power supply in order to reduce the size and cost of power supplies, but this method can stabilize the power supply and reduce the influence on other outputs.

<Second Embodiment> Next, a second embodiment of the present invention will be described based on FIG. 3. Note that the same reference numerals are given to the same parts as in the first embodiment, and the explanation thereof will be omitted.

FIG. 3 shows a second embodiment of the present invention, in which the output of the power supply 8 is divided by R1 and R2 to obtain a bias voltage. In this embodiment, by increasing R1 and R3, it is possible to reduce the load fluctuation that occurs when a pinhole exists, compared to the first embodiment.

In the apparatus of this embodiment shown in FIG. 3, the voltage V applied to the fixing roller 1 is equal to
, for R2

[0030]

It is obtained by [Equation 2]. This occurs when the surface layer of the fixing roller 1 has good insulation and is in contact with the recording material. At this time, no current flows through R3.

On the other hand, when the pinhole in the surface layer of the fixing roller 1 comes into contact with the recording material, a current tends to flow through R3. At this time, the current that is about to flow is caused by the combined resistance R of R2 and R3.

[0032]

[Math 3] Therefore, the current i flowing out from the power supply 8 is

[0033]

[Math 4] becomes.

From this, the current i flowing through R3 is

0035
]

[Math 5] It is.

[0036] If this leakage current is limited to below a certain level,
Destruction of the coating layer of the fixing roller 1 can be prevented.

Resistors R1 and R2 are determined so as to obtain the voltage necessary to prevent offset. From formulas (1) and (2), R1
When you erase

[0038]

[Math 6] becomes.

[0039] If i3≧5μA or more, the coating layer will be destroyed.

[0040]

V1, R1, R2, and R3 may be selected so that [Equation 7]. Regarding R3,

[0041]

[Math. 8] is obtained.

(Experiment example) The fixing roller 1 has an outer diameter of 30 mm.
The pressure roller 2 was coated with conductive silicone rubber so as to serve as a counter electrode to the bias voltage of the fixing roller 1, and the core metal was grounded. The temperature of this fixing roller 1 was 190° C., and the circumferential speed was 90 mm.

The toner used was charged to -5 to -15 μc/g. Under these conditions, the voltage that should be applied to the fixing roller core metal to prevent offset is at least -
It was 600V.

Table 1 shows the results of observing the presence or absence of toner fixation due to leakage current while changing the values of power supply voltage V1 and R1, R2, and R3.

[0045]

[Table 1]

As described above, even if there are pinholes in the surface coating of the fixing roller, by adding the resistor R3 that limits the current flowing to the recording material, it is possible to prevent toner from sticking and image staining. .

Note that since the actual recording material has a resistance Rp, R3 can be reduced by that amount, but in this embodiment, Rp is excluded from the calculation as a margin. Strictly speaking, R3 may be replaced with R3+Rp.

<Third Embodiment> Next, a third embodiment of the present invention will be described based on FIGS. 4 and 5. Note that the same reference numerals are given to the same parts as in the second embodiment, and the explanation thereof will be omitted.

FIG. 4 shows a third embodiment of the present invention. In this embodiment, a capacitor C is connected in parallel to the resistor R3 of the second embodiment. In this case, the voltage at point A is the offset prevention voltage since no current flows when the pinhole in the fixing roller 1 does not touch the recording material P.

00
50]

[Math. 9] is applied.

When the pinhole comes into contact with the recording material P, a current flows through point A and the voltage begins to drop.

The voltage V' at point A is

[0053]

[Math. 10] time constant from

[0054]

[Math. 11] in

[0055]

[Math. 12] attenuates to. That is,

[0056]

[Math. 13] becomes.

FIG. 5(b) shows the change in voltage at point A. In the second embodiment, as shown in FIG. 5(a), the voltage drops immediately when the pinhole comes into contact with the recording material, and when the resistance Rp of the recording material is low, the voltage drops as follows.

[0058]

[Equation 14] Therefore, offset cannot be prevented. On the other hand, by increasing V, it is possible to raise the voltage at point A to the offset prevention voltage, but in order to do so, it is necessary to increase the power supply voltage V1 and the voltage when there is no pinhole.

[0059]

The problem is that the equation (15) becomes high and further measures against discharge are required.

On the other hand, if a capacitor is connected as in this embodiment, even if current flows through the pinhole, the voltage V' at point A can be maintained at a level higher than the voltage V0 that can prevent offset during the time it passes through the nip. It is possible. The nip width, roller circumferential speed, recording material resistance, bias power supply voltage, dividing resistor, and capacitor that determine V' are selected as appropriate.

(Experimental Example 2) R1=400MΩ, R2=1
10MΩ, R3=500MΩ, Rp=50MΩ, C=1
If μF is used, τ = 107 sec, and the nip is 4.5
If mm, the time it takes for the pinhole to pass through the nip is 0.
．． It is 05 seconds. If V1=-2.8KV, the voltage at point A is calculated as V'=-582V. The voltage drop due to leakage from the pinhole is 18V, and if it is at this level, it will not cause an offset.

On the other hand, in the second embodiment, when the bias decreases stepwise, the voltage at point A decreases to V'=-26V, and in this case, offset cannot be prevented.

As described above, in the second embodiment, when the resistance of the recording material was lowered and a pinhole was present, an offset occurred, but this was solved by this embodiment.

In the above embodiments, a bias is applied to the fixing roller side, but it goes without saying that the present invention can also be applied to a structure in which a bias is applied to the pressure roller side.

[0065]

[Effects of the Invention] As explained above, according to the present invention,
Since we decided to connect a resistance element between the power supply connected to the core metal of at least one of the fixing roller or the pressure roller and the core metal, the surface layer of the fixing roller or the pressure roller should be made thicker than necessary. It is possible to prevent charges from flowing into the recording material without having to do so. Therefore, transfer defects do not occur and offset can be reliably prevented. Furthermore, since no excessive current is passed through the surface layer, good images can be obtained without impairing the releasability of the surface layer.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a schematic configuration of an apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of the device of the first embodiment in which another resistor is added to the device of FIG. 1;

FIG. 3 is a diagram showing a schematic configuration of a device according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a schematic configuration of a device according to a third embodiment of the present invention.

5(a) is a diagram showing a change in the voltage of the core metal in the device shown in FIG. 3, and (b) is a diagram showing a change in the voltage of the core metal in the device shown in FIG. 4. FIG.

FIG. 6 is a diagram showing a schematic configuration of a conventional device.

[Explanation of symbols]

1 Fixing roller 3 Pressure roller 8 Power supply R resistance

Claims (2)

[Claims] 1. A roller pair comprising a fixing roller and a pressure roller having an elastic layer or a resin layer on a cored metal, and a power source for applying a voltage to the cored metal of at least one of the rollers of the roller pair. A fixing device characterized in that a resistive element is connected between the power source and the core metal. 2. The fixing device according to claim 1, further comprising a capacitor connected in parallel with the resistive element.