JPH0711731B2 - Method for improving repetitive characteristics of photoconductor in reverse image formation - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for improving the repetitive characteristics of a photoreceptor in an electrophotographic apparatus of the type that forms a positive copy image from a negative original image.

[Prior Art] Generally, an electrophotographic apparatus uses a photoconductor that is activated by light. The photoconductor is charged in advance to a specific polarity and then a desired original image is exposed to form an electrostatic latent image corresponding to the image. Form. This latent image is developed with a developing powder to form a visible powder image, which is usually electrostatically transferred onto a suitable transfer material and then fixed to form a permanent copy image.

By the way, there is a tendency to use a drum-shaped photoconductor having a smaller diameter in response to a demand for miniaturization of the device. For example, in a device in which a part of the photoconductor is used in duplicate to complete one copy, The previously formed image appears as a history (optical memory) or appears as a fog on a portion of the body that is used repeatedly, thus contaminating the image and hindering repeated use of the photoconductor.

In order to erase the above history or fog and improve the repeating characteristics of the photoconductor, the most general method is to irradiate a strong eraser light before charging or is not a uniformly sufficient solution.

On the other hand, as disclosed in, for example, JP-A-61-27572, at the position after development and before transfer, a charge having the same polarity as the charge of the developing powder is applied to the photosensitive member, By illuminating the photoconductor at the same time or at a position after that, the specific charge of the developing powder is increased and the dark area potential of the photoconductor is lowered, thereby improving the transfer efficiency and the optical memory difference between the bright area and the dark area. There is known a method of preventing image deterioration due to the above and improving the repeating characteristics of the photoconductor.

[Problems to be Solved by the Invention] However, although such a conventional method is effective for an apparatus for forming a positive copy image from a positive original image, such a conventional method can be used for a microfilm printer or a laser printer. It has no effect on an electrophotographic apparatus of the type in which negative information such as transmitted light or laser beam is used as an original image and a positive copy image is formed from this negative original image. For example, after uniformly charging the surface of the photoconductor to the positive polarity, the negative original image is exposed to form an electrostatic latent image, and then the reversal development is performed with a pair of positively charged developing powder under an appropriate developing bias. In the apparatus for performing the above-mentioned method, when the pre-transfer charging having the same polarity as the charged electric charge of the developing powder is applied in accordance with the conventional method, and then the transfer discharge and the erase exposure by the opposite polarity, that is, the negative polarity are performed, the specific charge of the developing powder is However, due to the negative discharge and erase exposure at the time of transfer, a part of the negative charge is captured and remains in the photoconductor. This residual charge is gradually accumulated as the photoconductor is repeatedly used, which causes an increase in the residual potential of the photoconductor, which suppresses the positive corona charging at the time of forming the next latent image and causes a decrease in the charge potential of the photoconductor. This causes deterioration of the image or photosensitivity such as a decrease in density, a previous history, fog or induction development. As a result, improvement in repeatability of the photoreceptor cannot be expected.

The present invention has been made in view of the above-mentioned problems of the conventional method, and prevents the accumulation of reverse-polarity charges that cause deterioration of the image in an electrophotographic apparatus of the type that forms a positive copy image from a negative current image, and It is an object of the present invention to provide a method capable of effectively improving the repeatability of the body.

Another object of the present invention is to enhance the charge acceptability of the image-forming charge by previously filling the traps in the photoconductor with the charge of the specific polarity before the image-forming charge of the specific polarity, and to improve the image density and the contrast. The object is to provide a method of always obtaining an image of good image quality.

Another object of the present invention is that even when a developer having a relatively small specific charge such as a one-component developer is used, good image transfer is performed, and as a result, a reproduced image of good image quality can be formed. To provide a method.

[Means for Solving the Problems] Therefore, in the method according to the present invention, after the surface of the photoreceptor is charged to a specific polarity, the surface is negatively exposed to form an electrostatic latent image corresponding to the image, In an electrophotographic apparatus of a type in which the latent image is reversely developed with a developing powder having a charge of the same polarity as the specific polarity to form a positive powder image, and then the powder image is electrostatically transferred onto a transfer material. After the development, a charge having a polarity opposite to the charge polarity of the photoconductor and the powder image is applied to the photoconductor carrying the powder image to forcibly reverse the polarity of the powder image and the surface potential of the photoconductor To a zero, and then by applying an electric charge having the same polarity as the specific polarity, the surface of the photoconductor is charged to the specific polarity at the same time as the transfer of the powder image, and the potential difference between the dark area and the light area on the photoconductor. And then slowly erase the photoconductor surface by erase exposure. It is characterized by enhancing the effect and erasing the previous history of the photoconductor.

[Operation] By the above method, the surface of the photoconductor bearing the development powder image is charged with the opposite polarity, whereby the potential of the background area approaches zero or is reversed to the opposite polarity potential, and as a result, Attenuating near the potential of the image area to minimize the potential difference between the bright and dark areas. On the other hand, the powder image is forcibly inverted to the opposite polarity by this charging. Next, the transfer charge imparting the same polarity as the specific polarity acts so as to transfer the powder image on the surface of the photoconductor onto the transfer material, and is already attenuated to a substantially uniform potential by the reverse polarity charging or the reverse polarity. The entire surface of the photoreceptor, which has been somewhat reversed to the polar potential, is uniformly charged again to the specific polarity. By this transfer charging and subsequent erase exposure, charges of a specific polarity are captured in the traps in the photosensitive pair, and in this state, the photoconductor is provided for charging for the next latent image formation.

EXAMPLES Next, the present invention will be described with reference to the drawings.

The figure shows the essential parts of an electrophotographic apparatus embodying the present invention.
Is a photosensitive drum that is rotatably supported and that rotates in the direction of the arrow when activated. The photosensitive drum 1 has a photosensitive member 2 on its peripheral surface.
In this example, the photoconductor 2 is made of a copper phthalocyanine-based organic photoconductor, and in the following description, this photoconductor is described as a P type photoconductor, but other organic photoconductors or inorganic photoconductors may be used. Also, it may be an N type photoconductor. The photoconductor 2 follows the rotation of the drum.
The negative current image information (for example, transmitted light of microfilm information or laser beam) is exposed through the optical system 4 by being uniformly charged to a positive polarity by the corona charging device 3, and a dark potential region is formed on the surface of the photoconductor. And an electrostatic latent image composed of a bright portion potential region is formed. At this time, the light area and the dark area have surface potentials of about 30 V and about 500 V, respectively.

The electrostatic latent image is then provided to the developing device 5. Developing device 5
The zigzag example is a well-known magnetic brush type developing device, and conveys the positively charged toner powder to the surface of the photoconductor by the outer peripheral surface of the developing sleeve 7 which surrounds the magnet assembly 6 and rotates. A developing bias of, for example, about 250 V is applied to the developing sleeve 7, the toner powder is made to adhere to the area of the latent image at the bright part potential or less, and the reversal development is performed to form a positive powder image. The developing method is not limited to the magnetic brush method, and other methods such as a cascade method and a transfer method can be used, and the developer may be either a two-component developer or a one-component developer. good.

After development, the photoconductor on which the powder image has been formed is given a charge having a polarity opposite to that of the powder image, that is, a negative charge, by the pretransfer corona discharge device 8. This discharge is performed at a value sufficient to reverse the polarity of the powder image from positive to negative, for example, about 4500V. Due to this discharge, the powder image is forcibly inverted to the negative polarity, and the background region potential, that is, the dark region potential is attenuated or inverted to some extent negative to approach zero, and the potential difference with the bright region is minimized. To do. In this way, the photosensitive member is locally attenuated to a substantially uniform potential by the pre-transfer discharging device regardless of the light and dark regions, and then provided to the transfer position.

By reversing the polarity of the powder image as described above, the transfer of the powder image is performed by the transfer corona discharge device 9 which imparts the same polarity as the charging for forming the electrostatic latent image, that is, the positive charge.
Such transfer discharge is transferred by the transfer roller 11.
When the transfer material is not present in the transfer portion, the transfer corona discharge device 9 lowers the discharge potential to about the same value as the surface of the photoconductor receives when the transfer material is present. The influence on the temperature is controlled to be almost constant regardless of the presence of the transfer material. However, the transfer corona discharge may be always applied at a constant value regardless of the presence or absence of the transfer material. Transfer corona discharge is about 45
It is 00V. This discharge not only has the function of transferring the powder image from the photoconductor onto the transfer material, but also has the function of releasing and removing the negative charge remaining in the photoconductor in some cases due to the negative charging. The body surface is uniformly charged with a specific polarity. The transfer material carrying the powder image is separated from the photoconductor by the separating means 13 such as a separating claw and then sent to a fixing device (not shown) to form a permanent visible image. The separating means may be a separate corona discharge device using AC discharge.

On the other hand, the photoconductor is cleaned by the cleaning device 14 to remove the residual toner, and then uniformly exposed by the erase exposure device 15 with light having a relatively low illuminance of, for example, several tens Lx. By the erase exposure and the specific polarity charging in the transfer, the trap in the photoconductor is uniformly filled with the charge of the specific polarity. In the illustrated example, the erase exposure device 15 is located in the rear order of the cleaning device 14 with respect to the rotation of the photoconductor, but may be provided in the front position of the cleaning device. The photoconductor thus erase-exposed is again provided to the charging device 3 for the formation of the next latent image.

EFFECTS OF THE INVENTION With the above-described structure, the present invention is an electrophotographic apparatus of the type that forms a positive copy image from a negative original image. The potential difference between the dark area and the dark area is minimized to eliminate the previous history, and the traps in the photoconductor are made uniform by the charges of the specific polarity by sequentially receiving the actions of the transfer discharge device of the specific polarity and the erase exposure device. It is provided to be charged for the next latent image formation while being filled with the specific-polarity charge-accepting ability. This provides good repeatability with less history, fog or induction phenomena. Furthermore, according to the present invention, the developer is forcibly imparted with sufficient charge by the pre-transfer corona discharge device even when the specific charge of the powder itself is weak, such as a one-component developer. Good transfer can be performed.

[Brief description of drawings]

The figure is a schematic side view of the essential part of an electrophotographic apparatus embodying the method of the present invention. 2 ... Photoconductor, 3 ... Corona charging device, 5 ... Developing device, 6 ... Pre-transfer corona discharge device, 9 ... Transfer corona discharge device, 14 ... Cleaning device, 15 ... Erase exposure device.

Claims (1)

[Claims] 1. A surface of a photoconductor is charged to a specific polarity, and then the surface is exposed to a negative image to form an electrostatic latent image corresponding to the image, and the latent image has a charge of the same polarity as the specific polarity. In an electrophotographic apparatus of a type in which reversal development is performed with a developing powder to form a positive powder image, and then the powder image is electrostatically transferred onto a transfer material. A charge having a polarity opposite to the charge polarity of the photoconductor and the powder image is applied to forcibly reverse the polarity of the powder image and the surface potential of the photoconductor is decreased toward zero. By applying a charge of the same polarity, the surface of the photoconductor is charged to a specific polarity at the same time as the transfer of the powder image, and the potential difference between the dark area and the light area on the photoconductor is reduced. A special feature is to enhance the slow-electricity effect and erase the previous history of the photoconductor. Repetition characteristics improved method of the photosensitive member at the inverting imaging to.