JPH08179586A - Electrophotographic method - Google Patents

Abstract

PURPOSE: To prolong the life of a photoreceptor and developer and to realize a low running cost by providing this photoreceptor with a first set value and second set value as its electric charge potential and setting the photoreceptor at the first set value at the time off two-color printing and at the second set value at the time of printing of only the black, thereby assuring the potential contrast equal to the potential contrast of the conventional devices at the time of printing of only the black. CONSTITUTION: The electric charge potential of the photoreceptor 1 is set at the first set value by changing the grid impression voltage of an electrostatic charger 2 and three stages of potential are formed by laser beams at the time of two-color printing. The high-potential parts are then developed with color toners by a first developing device 4 and the low-potential parts are developed with the black toners by a second developing device 5. The electric charge potential of the photoreceptor 1 is set at the second set value by changing the grid impression voltage of the electrostatic charger 2 and the low-potential parts irradiated with the laser beam are developed with the black toners at the time of printing of only the black.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-color printable electrophotographic method, and more particularly to a long life of a photoreceptor and a developer when printing only black.

[0002]

2. Description of the Related Art As one of methods for realizing two-color printing by using toners charged with positive and negative polarities different from each other, a tri-level method as shown in US Pat. No. 4,078,929 is known. Has been. In this method, the high potential portion VH where the laser beam is not irradiated on the uniformly charged photoreceptor,
Three levels of potentials are formed: an intermediate potential portion VM irradiated with a laser light amount PM of about half the exposure amount of the photoconductor and a low potential portion VL irradiated with a laser light amount PH about 3 to 4 times the half exposure amount. Next, the first developing device normally develops the toner charged to the high potential portion VH with the opposite polarity to the photoreceptor, and the second developing device reversely develops the toner charged to the low potential portion VL with the same polarity as the photoreceptor. The intermediate potential portion VM becomes a background portion (blank sheet portion) without toner development. At this time, the first to perform regular development
A bias voltage VBH between the high potential portion VH and the intermediate potential portion VM is applied to the developing device, and a bias voltage between the low potential portion VL and the intermediate potential portion VM is applied to the second developing device for reversal development. VBL is being applied. After the development, the toners having different polarities on the photoconductor are made to have the same polarity by the pre-transfer charging device and transferred onto the paper at once, and the fixing device heats and presses the toner on the paper.

[0003]

The advantage of the tri-level system is that since one optical system can realize two colors in one pass, the device configuration is simple and high speed can be secured, and the printing position is accurate. . However, the drawback of the tri-level method is that
The difference between the potential of the developing portion and the potential of the background portion, that is, the so-called potential contrast is smaller than that of the conventional apparatus that uses the potential of two stages to form the potential of three stages. for that reason,
In order to secure the same print density as in the conventional apparatus using a two-step potential, the difference between the developing portion potential and the bias voltage must be almost equal, and the difference between the background portion and the bias voltage becomes small, causing fog. Easier to do. For this reason, surface potential control and bias voltage control are carried out to prevent the occurrence of fogging. However, even if this is done, the life of the photoconductor and the developer will be shorter and the running time will be shorter than in conventional devices that use a two-step potential. High cost.

This is allowed to some extent due to the advantage that two-color printing is possible. However, not all printing jobs require two-color printing, and there are quite a lot of black jobs. Therefore, when printing only in black, it is necessary to ensure the same life of the photosensitive member and the developer as in the conventional device using a two-step potential.

An object of the present invention is to realize a one-pass two-color system by the tri-level system, and to provide the same life of the photosensitive member and the developer as the conventional device which uses a two-step potential when printing only black. It is to be realized.

[0006]

SUMMARY OF THE INVENTION The above-mentioned object is to provide a first developing device and a photosensitive member which form a three-step potential on a charged photosensitive member by a laser beam to normally develop a first color toner having a polarity opposite to that of the photosensitive member. Second reversal development of the second color toner of the same polarity as the body
In an electrophotographic apparatus including a developing device and a pre-transfer charging device that aligns toners of different polarities on the photoconductor to the same polarity before transferring the toner to the paper, the charging potential of the photoconductor is set to a first set value. A charging device capable of changing to the second set value is provided, and when developing the first color toner and the second color toner, the charging potential of the photoconductor is set to the first set value to form a three-step potential by the laser beam, and the second When developing only the color toners, it is achieved by setting the charging potential of the photoconductor to a second set value and forming a two-step potential by laser light.

[0007]

In the electrophotographic method configured as described above, when the two-color printing is performed, the charging potential of the photoconductor is set as the first set value, and three levels of potential are formed by the laser light, and the high potential portion has a polarity opposite to that of the photoconductor. Toner (color) is subjected to normal development, toner (black) having the same polarity as the photoconductor is reversely developed on the low potential portion, and the intermediate potential portion becomes the background portion (blank paper portion). When printing only in black, the charging potential of the photoconductor is set to a second setting value to form a two-step potential by laser light, and the toner (black) of the same polarity as the photoconductor is reversely developed on the low potential part, and the high potential part is It becomes the background section (blank sheet section). Here, the second set value has a potential of 1 as compared with the intermediate potential portion.
The value is set to be higher by 00 to 200V. As a result, it is possible to secure the same potential contrast as that of the conventional device that uses a two-step potential. The charging potential can be controlled by the voltage applied to the grid of the charging device, and can be varied to the grid applied voltage corresponding to the first setting value and the second setting value, thereby realizing the first setting value and the second setting value. it can.

If the pre-transfer charging device for aligning the polarities of the toners is set so as to reverse the charging polarity of the color toners, the pre-transfer charging device becomes unnecessary during black printing, and the pre-transfer charging device need not be operated.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing the outline of an embodiment of the present invention. The photoconductor 1 is arranged so as to be rotatable at a constant speed in the direction of the arrow, and around it, the charging device 2, the optical device 3, the first developing device 4, the second developing device 5, the pre-transfer charging device 6, and the transfer device. The device 7 and the cleaning device 8 are arranged. Further, the charging device 2, the pre-transfer charging device 6, the charging device high-voltage power supply 9 for applying a high voltage to the corotron wire of the transfer device 7 to generate corona discharge, the pre-transfer charging device high-voltage power supply 10,
High voltage power supply 11 for transfer device, high voltage power supply 12 for grid for applying voltage to grid of charging device, high voltage power supply 1 for bias for applying bias voltage to first and second developing devices
It is composed of 3 and 14.

Next, the printing process of this apparatus will be described. The photoconductor 1 is uniformly charged by the charging device 2, and a laser beam emitted from the optical device 3 forms a three-step potential. The high potential portion VH is normally developed by the first developing device 4 with toner (color) having a polarity opposite to that of the photoconductor 1. At this time, the bias voltage VBH is applied to the developing roll of the first developing device 4.
Is applied, and toner is not developed on the intermediate potential portion VM and the low potential portion VL. Next, in the low potential portion VL, the toner having the same polarity as that of the photoconductor 1 is reversely developed by the second developing device 5. At this time, the bias voltage VBL is applied to the developing roll of the second developing device 5, and the toner is not developed on the intermediate potential portion VM and the high potential portion VH. After the development, the toners having different polarities on the photoconductor 1 are made to have the same polarity by the pre-transfer charging device 6 and transferred to the paper 15 by the transfer device 7. The untransferred toner remaining on the photoconductor 1 is cleaned by the cleaning device 8 and the above process is repeated. Further, the toner transferred onto the sheet 15 is heated and pressed by a fixing device (not shown) to be fixed on the sheet.

Next, the printing process according to the present invention will be specifically described by taking a negatively charged photoreceptor as an example. First, at the time of two-color printing, in order to set the charging potential of the photoconductor 1 to the first set value (−900V), the grid application voltage of the charging device 2 is set to be substantially the same as the first set value. The voltage applied to the grid is set several tens of volts higher in consideration of the dark decay characteristics of the photoconductor and the moving time between the charging device and the developing device. After charging to the first set value, the high voltage portion VH1 (-900V), the intermediate potential portion VM (-500V), and the low potential portion VL (-100V) by the laser light.
To form. A bias voltage VBH of -600V is applied to the developing roll of the first developing device 4 for developing the high voltage portion VH1.
Is applied to develop the low potential portion VL. A bias voltage VBL of -400V is applied to the developing roll of the second developing device 5. Therefore, in the case of the above tri-level method, the potential difference between the developing portion and the background portion, that is, the potential contrast is 40.
0 V, and the potential difference between the bias voltage and the background is 100
Only V can be secured. Next, at the time of printing only black, the charging potential of the photoconductor 1 is set to the second set value (−700 V), so that the grid application voltage of the charging device 2 is set to be substantially the same as the second set value. After charging to the second set value, a high potential portion VH2 (-700V) not irradiated with laser light and a low potential portion VL (-100V) irradiated with laser light are formed. Here, the developing roll of the second developing device 5 that develops the low potential portion VL is-
A bias voltage VBL of 400V is applied. Therefore,
In the case of two potential levels, the potential contrast is 6
00V, and the potential difference between the bias voltage and the background is 30
It is possible to secure 0 V and the same margin as the conventional device, and the life of the photoconductor and the developer can be made the same as that of the conventional device. At this time, the developing device is retracted by 2 to 5 mm so that the developer of the first developing device 4 is not in contact with the photoconductor 1.
Alternatively, the developing roll is rotated in the reverse direction to remove the developer on the developing roll. Further, the pre-transfer charging device 6 is necessary for reversing the charging polarity of the color toner developed by the first developing device 4, and is unnecessary when printing only black, and the high voltage power supply 10 for the pre-transfer charging device operates. do not do.

[0012]

As described above, according to the present invention, it is possible to realize one-pass two-color by the tri-level method, and when printing only black, by using a two-step potential, a photoreceptor and a developing device equivalent to those of the conventional apparatus can be obtained. The life of the agent can be secured.

[Brief description of drawings]

FIG. 1 is a schematic view of an electrophotographic apparatus according to the present invention.

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

FIG. 3 is an explanatory diagram showing a potential distribution on a photoconductor of a first set value and a second set value according to the present invention.

[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, 7 ... ..Transfer device, 9 ... High-voltage power supply for charging device, 12 ...
・ High voltage power supply for grid

Continued Front Page (72) Inventor Koichi Takahashi 1060 Takeda, Hitachinaka City, Ibaraki Prefecture Hitachi Koki Co., Ltd.

Claims (6)

[Claims] 1. A laser beam is applied onto a charged photoconductor to form 3
A first developing device for normally developing a first color toner having a polarity opposite to that of the photoconductor, a second developing device for reversal development of a second color toner having the same polarity as the photoconductor, and transferring the toner to the sheet. In an electrophotographic apparatus including a pre-transfer charging device that aligns toners of different polarities on the photoconductor to the same polarity before charging, a charging device that can change the charging potential of the photoconductor to a first set value and a second set value. Is provided, when the first color toner and the second color toner are developed, the charging potential of the photoconductor is set to a first set value to form a three-level potential by laser light, and when only the second color toner is developed, the photoconductor is provided. The electrophotographic method is characterized in that the charging potential of is set to a second set value and a two-step potential is formed by laser light. 2. The charging device comprises a scorotron having a grid and a high voltage power source for applying a constant voltage to the grid, and the voltage applied to the grid has a first set value and a second set value. The electrophotographic method according to claim 1. 3. The electrophotographic method according to claim 1, wherein the second color toner is black. 4. The electrophotographic method according to claim 1, wherein the developer of the first developing device is not in contact with the photoconductor when only the second color toner is developed. 5. The pre-transfer charging device reverses the polarity of the first color toner so that the first color toner and the second color toner have the same polarity as the photoconductor. Electrophotographic method. 6. The electrophotographic method according to claim 1, wherein the pre-transfer charging device does not generate an electric charge when developing only the second color toner.