JPH09160394A - Electrophotographic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure both high transferring efficiency and excellent image quality for both toner in equalizing polarities of the toner having different polarity such as positive or negative on a photoreceptor by a pre-transfer electrification device. SOLUTION: On the photoreceptor 1, potential level of three steps such as a high potential region (VH), the medium potential region (VM) and the low potential region (VL) is severally formed by laser exposure, and then the toner having the polarity opposite to that of the photoreceptor is made positively developing on the high potential region (VH) and the toner having the same polarity as that of the photoreceptor 1 is made inversely developing on the low potential region (VL). Next, the polarity of the toner being developed on the high potential region (VH) is reversed, by impressing the electric charge whose polarity is the same as that of the photoreceptor 1 by the pre-transfer electrification device 6. The charge amount being impressed by the pre-transfer electrification device 6 at this time, is made the charge amount capable of raising the potential of the medium potential region (VM) to the approximately same potential as that of the high potential region (VH).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic apparatus capable of two-color printing, and more particularly to control of a pre-transfer charging device for reversing the polarity of toner developed on a photosensitive member.

[0002]

2. Description of the Related Art A tri-level method is known as one of methods for realizing two-color printing by using toner charged with positive and negative polarities (US Pat. No. 4,078,929).
issue). In this system, a high potential portion VH where the laser light is not irradiated onto the uniformly charged photosensitive member, an intermediate potential portion VM where a laser light amount PM of about the half exposure amount of the photosensitive member is irradiated, and 3 to 4 of the half exposure amount are used. A three-step potential of the low-potential portion VL irradiated with the laser light amount PH of about twice is formed.

Next, the first developing device normally develops the toner charged to the high potential portion VH with the opposite polarity to the photosensitive member, and the second developing device reverses the toner charged to the low potential portion VL with the same polarity as the photosensitive member. After being developed, the intermediate potential portion VM is not developed with toner and becomes a background portion (blank sheet portion). At this time, the bias voltage VBH between the high potential portion VH and the intermediate potential portion VM is applied to the first developing device that performs regular development, and the low potential portion VL is applied to the second developing device that performs reversal development. A bias voltage VBL between the intermediate potential portion VM and the intermediate potential portion VM is applied.

After development, the toners having different polarities on the photoconductor are transferred by being electrically attracted to the paper charged by the transfer device after the polarities are made uniform by the pre-transfer charging device and then by the fixing device. The toner is fixed on the paper by being heated and pressed.

[0005]

The efficiency with which the toner developed on the photosensitive member is transferred to the paper, the so-called transfer efficiency, is
There is a large correlation with the toner charge amount.

As the toner charge amount increases from zero, the electric attraction force to the paper charged with the opposite polarity to the toner by the transfer device increases, and the transfer efficiency increases, but the toner charge amount becomes too high. Then, the electric attraction between the toner and the photoconductor increases, and the transfer efficiency decreases.

Therefore, in order to ensure high transfer efficiency, it is necessary to control the charge amount of the toner so that it is neither too low nor too high.

However, when reversing the charge polarity of the toner by using the pre-transfer charging device, it is necessary to invert the charge polarity of the toner and apply the charge until the charge amount can ensure a high transfer efficiency. At this time, the toner whose charge polarity cannot be reversed is charged with the same polarity, which increases the charge amount of the toner, deviates from the optimum charge amount that can ensure high transfer efficiency, and the transfer efficiency may decrease. In addition to that, the electric repulsive force between the toner particles increases, the toner particles scatter, and the sharpness of the characters decreases.

Therefore, the pre-transfer charging device must have a minimum charge capable of reversing the toner polarity.

An object of the present invention is to provide an electrophotographic apparatus capable of ensuring high transfer efficiency and high image quality for both toners when the toners having different positive and negative polarities on the photoconductor are made uniform in polarity by the pre-transfer charging device. To provide.

[0011]

The object is to form a three-level potential level of a high potential portion, an intermediate potential portion, and a low potential portion by laser exposure on a photoconductor uniformly charged by a charging device, A first developing device that normally develops toner charged to the high potential portion in the opposite polarity to the photoconductor, a second developing device that reverse develops toner charged to the low potential portion in the same polarity as the photoconductor, and a second developing device on the photoconductor. In an electrophotographic apparatus provided with a pre-transfer charging device for aligning the polarities of toners having different positive and negative polarities, the pre-transfer charging device is provided with a function of giving a charge amount in which an intermediate potential part has substantially the same potential as a high potential part. To be achieved.

In the above structure, the amount of electric charge provided by the pre-transfer charging device so that the intermediate potential part has substantially the same potential as the high potential part is necessary for reversing the polarity of the toner developed in the high potential part. This is the minimum charge amount, and by setting this charge amount, it is possible to minimize the overcharge of the toner developed in the low potential portion.

To explain this by taking the case where the photosensitive member is negatively charged as an example, the toner developed in the high potential portion becomes positively charged, and is given a negative charge by the pre-transfer charging device, and the polarity is reversed to the negatively charged side. To be made.

Here, at the peripheral end portion of the high potential portion, the negative potential becomes the highest due to the edge effect, the negative charge given by the pre-transfer charging device repels, and the polarity of the toner cannot be reversed, so that the high potential is high. The peripheral edge portion of the toner image developed on the sheet is not transferred to the sheet.

Therefore, if the intermediate potential portion has substantially the same potential as the high potential portion, the edge effect of the peripheral edge portion of the high potential portion can be eliminated, and the peripheral edge portion of the toner image developed on the high potential portion can be eliminated. It becomes possible to reverse the charging polarity of the toner, and the toner can be transferred to the paper. That is, the amount of electric charge at which the intermediate potential portion has substantially the same potential as that of the high potential portion is the minimum amount of charge required to reverse the charging polarity of the toner developed in the high potential portion.

When comparing the black toner and the color toner, the black toner often contains conductive carbon, and the color toner generally has higher resistance. Therefore, the color toner can reverse the charging polarity with a smaller charge amount than the black toner, and the charge polarity can be easily reversed with the charge amount.

Further, generally, only black printing is performed, and the pre-transfer charging device is not required when only black printing is performed, and the cleaning period / replacement period of the pre-transfer charging device can be extended.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram schematically showing an embodiment of the present invention.

The photosensitive member 1 is arranged rotatably at a constant speed in the direction of the arrow, and the charging device 2, the optical device 3,
A first developing device 4, a second developing device 5, a pre-transfer charging device 6,
A transfer device 7 and a cleaning device 8 are arranged.

Further, high voltage is applied to the corotron wires of the charging device 2, the pre-transfer charging device 6, and the transfer device 7,
High voltage power supply 9 for charging device, high voltage power supply 10 for pre-transfer charging device, high voltage power supply 11 for transfer device for generating corona discharge
And a high voltage power source for bias 1 for applying a bias voltage to the first developing device 4 and the second developing device 5, respectively.
2 and 13 are installed.

A surface electrometer 14 is installed on the downstream side of the pre-transfer charging device 6, and the surface electrometer 14 is a surface potential controller 1.
5 is connected.

Next, the printing process of this embodiment will be described.

The photoconductor 1 will be described by taking a negative charging type as an example.

The photoconductor 1 is uniformly charged by the charging device 2, and the high potential portion VH (-900V) and the intermediate potential portion VM (-500) are irradiated by the laser light emitted from the optical device 3.
V) and the low potential portion VL (-100V) are formed.

Next, in the high potential portion VH, the first developing device 4 normally develops the positively charged color toner having the opposite polarity to the photoconductor 1. At this time, the bias voltage VBH (-600V) is applied to the developing roll of the first developing device 4, and the toner is not developed on the intermediate potential portion VM and the low potential portion VL.

Next, in the low potential portion VL, the second developing device 5 reversely develops the negatively charged black toner having the same polarity as that of the photoconductor 1.
At this time, the bias voltage VBL (-400V) is applied to the developing roll of the second developing device 5, and the intermediate potential portion V
M, the toner is not developed on the low potential portion VL.

After development, the positively charged color toners and the negatively charged black toners having different polarities on the photoreceptor 1 are given a negative charge by the pre-transfer charging device 6 and are aligned on the negatively charged side having the same polarity as the photoreceptor 1. . At this time, the pre-transfer charging device 6 causes the photosensitive member 1 to
The amount of charge given to the upper portion is such that the intermediate potential portion VM is high potential portion VH.
And the amount of electric charge at which the potential is almost the same.

The experimental results are shown in FIG. That is, FIG. 3 is a diagram showing the relationship between the charge amount of the pre-transfer charging device, the transfer efficiency, and the photoconductor potential.

From FIG. 3, it is understood that the point where the transfer efficiency of the positive and negative electrified color toner is almost saturated is the point where the intermediate potential portion VM becomes almost the same potential as the high potential portion VH.

After the polarities of the toners on the photoconductor 1 are made uniform, the toners are transferred to the paper by the transfer device 7 and heated and pressed by a fixing device (not shown) to fix the toners on the paper.

Further, the toner which is not transferred to the paper is cleaned by the cleaning device 8 and the above process is repeated to enable continuous printing.

The relationship between the electric potential of the photoconductor 1 and the amount of electric charge varies depending on the manufacturing lot of the photoconductor 1, the usage condition,
It is also possible that it will fluctuate depending on the environment. Therefore, the amount of electric charge given to the photoconductor 1 by the pre-transfer charging device 6 so that the intermediate potential portion VM becomes almost the same potential as the high potential portion VH,
Although it may be changed by the fluctuation, the amount of electric charge applied to the photoconductor 1 by the pre-transfer charging device 6 can be controlled by measuring the surface potential of the photoconductor 1. Then, this control sets the developers of the first developing device 4 and the second developing device 5 in a non-contact state with the photoconductor 1 at the time of power-on of the electrophotographic device, restart of the job or after replenishment of toner. This can be done by rotating the developing roll in the opposite direction to that during printing. By bringing the developer into a non-contact state, the photoconductor 1
To prevent toner adhesion to the.

Next, with the pre-transfer antistatic device 6 turned off and without the laser beam of the optical device 2 being emitted, the high potential portion VH is measured by the surface electrometer 14 and this value is controlled by the surface potential control. It is stored in the device 15.

Then, the laser light amount PM of about half the exposure amount is emitted, the intermediate potential portion VM is measured by the surface potential meter 14 and stored in the surface potential control device 15.

Next, the pre-transfer charging device 6 is turned on, and the high-voltage power supply 10 for the pre-transfer charging device is controlled so as to have the same value as the previously stored high potential portion VH.

Further, in the process of measuring the high potential portion VH, the set value of the high potential portion VH is set to the surface potential control device 15
By setting the value inside, the high voltage power source 9 for the charger is controlled so that the measured value and the set value are the same value.

Further, in the process of measuring the intermediate potential portion VM, the set value of the intermediate potential portion VM is set in the surface potential control device 15 so that the measured value and the set value are the same. Change the amount of laser light.

Furthermore, in the process of measuring the intermediate potential portion VM, the bias voltage applied to the first developing device and the second developing device with respect to the intermediate potential portion VM has a constant potential difference (1VM-VB1 = 100V). ),
The bias high-voltage power supplies 12 and 13 are controlled.

[0039]

According to the present invention, when the toners having different positive and negative polarities on the photoconductor are made uniform in polarity by the pre-transfer charging device, high transfer efficiency and high image quality can be secured for both toners.

In summary, according to the present invention, the amount of charge provided by the pre-transfer charging device can be set to the minimum amount of charge required for toner polarity reversal, and transfer efficiency and image quality can be improved. can do.

[Brief description of the drawings]

FIG. 1 is a schematic view of an electrophotographic apparatus showing an embodiment of the present invention.

FIG. 2 is a diagram showing a light decay curve of a photoconductor and a potential distribution on the photoconductor.

FIG. 3 is a diagram showing a relationship between a charge amount of a pre-transfer charging device, a transfer efficiency, and a photoconductor potential.

[Explanation of symbols]

1 ... Photosensitive member, 2 ... Charging device, 3 ... Optical device, 4 ... First developing device, 5 ... Second developing device, 6 ... Pre-transfer charging device, 14
... surface potential meter, 15 ... surface potential control device.

Claims (6)

[Claims] 1. A three-step potential level of a high potential part, an intermediate potential part, and a low potential part is formed by laser exposure on a photoconductor uniformly charged by a charging device, and the high potential part is opposite to the photoconductor. A first developing device that normally develops a toner charged with a polarity, a second developing device that reverse-develos a toner charged with the same polarity as the photoconductor on the low potential portion, and a toner having different positive and negative polarities on the photoconductor. In an electrophotographic apparatus provided with a pre-transfer charging device for aligning polarities, the pre-transfer charging device is provided with a function of giving a charge amount in which an intermediate potential part has substantially the same potential as a high potential part. apparatus. 2. The electrophotographic apparatus according to claim 1, wherein the toner having a polarity opposite to that of the photoconductor is a color toner. 3. A surface potential control device is provided on the downstream side of the pre-transfer charging device, and the charge amount given to the photoconductor by the pre-transfer charging device is adjusted so that the intermediate potential part has almost the same potential as the high potential part. The structure controlled by a surface potential control device.
An electrophotographic apparatus according to the above. 4. The potential of the high-potential portion is set to a set value,
The electrophotographic apparatus according to claim 3, wherein the surface potential control device controls the charging amount of the charging device. 5. The electrophotographic apparatus according to claim 3, wherein the surface potential control device controls the laser exposure amount so that the potential of the intermediate potential portion becomes a set value. 6. The surface potential control device controls the bias voltage applied to the first developing device and the second developing device so as to maintain a constant potential difference with respect to the potential of the intermediate potential portion. The described electrophotographic apparatus.