JPH035575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic copying method, and in particular, proposes an electrophotographic copying method capable of producing a plurality of copies with uniform image contrast.

In electrophotographic copying technology, for example, while a photoreceptor drum having a drum-like structure rotates at a constant speed, the surface of the photoreceptor is subjected to a series of uniform charging, image exposure, development, transfer, cleaning, and charge removal. A copying process is applied to copy the manuscript. As a result of uniform charging, for example, the surface potential of the photoreceptor is charged to a positive potential, and the charge in the non-image area is erased by image exposure, and an electrostatic latent image corresponding to the original image is formed. However, the potential difference between the background area (non-image area) and the image area in this electrostatic latent image has a large effect on the contrast of the transferred image. Incidentally, the surface potential of a photoreceptor is exposed to light and is attenuated by light and significantly lowered, and then left in the dark to undergo dark attenuation and gradually decrease. Therefore, the potentials of the image area and the background area constantly fluctuate, but in particular, the potential difference varies significantly over time immediately after exposure because light attenuation is not instantaneous. Due to such fluctuations in potential difference, when multiple copies are made from one original, there is a risk that the image contrast will fluctuate between each copy, especially when the mobility of charge carriers on the photoreceptor is small. Not only is this a problem, but it has become a bigger problem as copying machines have become faster in recent years. However, when designing a conventional electrophotographic process, the above-mentioned fluctuations in the surface potential of the photoreceptor have not been sufficiently taken into consideration.

The present invention has been made in view of the above points, and it is possible to prevent the development from occurring during the period when the potential of the surface of the photoreceptor is optically attenuated. An object of the present invention is to provide an electrophotographic copying method that suppresses fluctuations in image contrast between images. The electrophotographic copying method according to the present invention includes:
In an electrophotographic copying method in which the surface of a photoreceptor is uniformly charged, imagewise exposed to form an electrostatic latent image, and then developed to form a toner image, which is then transferred to a transfer material, from the imagewise exposure to the The present invention is characterized in that the time interval until development is set to be longer than the time required for completion of light attenuation of the photoreceptor due to the image exposure. This predetermined period is preferably longer than the time required for the light attenuation of the photoreceptor to end.

The method of the present invention will be specifically explained below.
First, the principle of the present invention will be explained. A photoreceptor, in which a photosensitive layer made of a photoconductive substance such as Se or As ₂ Se ₃ is formed on the surface of a conductive substrate such as Al, is uniformly charged to impart an electric charge to its surface, and then placed in a dark place. It decays in the dark when left alone, or it decays in light when exposed to light. FIG. 1 is a graph showing changes in the surface potential of a photoreceptor, with time plotted on the horizontal axis and surface potential plotted on the vertical axis. After the surface of the photosensitive member is uniformly charged to, for example, a positive potential, conduction of the surface charge within the photosensitive layer occurs in the dark, and the surface potential gradually decreases linearly, resulting in dark decay. Then, by image exposure, the background area (non-image area) on the surface of the photoreceptor is selectively exposed, the potential of the background area rapidly decreases, and light attenuation occurs. In this light attenuation, for example, positive and negative charge carriers are generated on the surface layer of the photosensitive layer due to exposure to light, and among these carriers, the negative carriers are neutralized with the charges on the surface of the photosensitive layer and disappear; , the positive carriers move within the photosensitive layer toward the conductive substrate, and some of them are captured by traps within the photosensitive layer, but most of them are neutralized with the negative charges induced in the conductive substrate and disappear. do. Incidentally, the decrease in the surface potential of the photoreceptor during this light attenuation process is determined by the mobility of positive charge carriers moving within the photosensitive layer. Therefore, the time t at which light attenuation in the background portion of the photoreceptor surface ends is approximately expressed by the following equation (1).

t=L/v=L/μE...(1) However, L: Thickness of photosensitive layer v: Speed of movement of positive charge carriers in the photosensitive layer μ: Mobility of positive charge carriers E: Strength of electric field End time of this light decay t is approximately 200 msec when the photoconductive material constituting the photosensitive layer is As ₂ Se ₃ , and after t time has passed after exposure, the skin potential enters a dark decay process and is not exposed to light and dark decays. The potential decreases approximately in parallel with the decrease in the potential of the non-exposed area of the photoreceptor, that is, the image area, which continues to decrease. The difference between the image area potential and the background area potential affects the contrast of the transferred image, and the larger the difference, the higher the contrast and the clearer the image. That is, the contrast CLT of the transferred image is expressed as the difference between the image portion potential V _d and the background portion potential V _g as shown in equation (2) below.

CLT=V _d −V _g (2) By the way, in the case of multiple copying, the electrostatic latent image formed by one uniform charging and image exposure is repeatedly developed and transferred to make multiple copies. A copy is made, but when the background potential is in the process of optical attenuation, when the first copy is developed, the image contrast of this copy is different from that of the second and subsequent copies. , uniform and good contrast cannot be obtained for all copies. This becomes a serious problem when a photoconductive material having a small charge carrier mobility .mu. is used in the photosensitive layer or when the speed of a copying machine is increased. In order to avoid such inconveniences, it is advisable to avoid developing during a transition period such as the light decay process.

The present invention has been made from this viewpoint, and one example of an apparatus used to carry out the method of the present invention will be described below with reference to FIG. The photosensitive drum 1 is formed by depositing a photoconductive substance As ₂ Se ₃ on the circumferential surface of a cylindrical conductive substrate to form a photosensitive layer. It is supported horizontally and can rotate at a constant speed. Near the circumferential surface of the photosensitive drum 1, a uniform charging device 2, an image exposure device 3, a developing device 4, a transfer roller 6, a cleaning blade 8, and a static eliminator 9 are arranged in this order in the rotation direction. . During one rotation of the photoreceptor drum 1, the surface of the photoreceptor is uniformly charged to a positive potential by the uniform charging device 2, and the background portion is selectively exposed by the image exposure device 3, corresponding to the original image. An electrostatic latent image is formed.
Next, negatively charged toner is supplied from the developing device 4 to the image area of the photoreceptor surface where the electrostatic latent image is formed, and the toner electrostatically adheres to the latent image charge to form a toner image. Ru. Then, in synchronization with the rotation of the image area on the circumferential surface of the photoreceptor drum 1 to the transfer roller 6 arrangement position, the transfer material 7 is transferred from the feeding device 5 between the transfer roller 6 and the photoreceptor drum 1. The toner image on the surface of the photoreceptor is transferred to a transfer material 7 whose back surface is pressed by a transfer roller 6 . Untransferred toner remaining on the photoreceptor surface without being transferred is removed by a cleaning blade 8, and the photoreceptor surface is neutralized by a static eliminator 9 such as an optical static eliminator.

The image exposing device 3 and the developing device 4 are spaced apart from each other so that the central angle θ with respect to the center of the photosensitive drum 1 satisfies the following equation (3).

θ≧ωt …(3) However, ω: rotational angular velocity of the photoconductor drum 1 t: as mentioned above, light attenuation end time For example, the rotation speed of the photoconductor drum 1 is 1 rotation/
sec, when the photosensitive layer is made of As ₂ Se ₃ , the light decay time t is about 200 msec, so θ≧
(1/5)×2π, and the image exposure device 3 and the developing device 4 are spaced apart from each other by at least 1/5 rotation of the photosensitive drum 1.

In this way, by setting the separation distance between the image exposure device 3 and the developing device 4 according to the rotational speed of the photosensitive drum 1 and the light decay time of the photosensitive layer, it is possible to control the distance when copying multiple sheets. Image contrast can be made uniform. That is, when obtaining a plurality of copies from one original, n copies are created through the following process.

Uniform charging → Image exposure → [Development → Transfer] ₁ → [Development →
Transfer] ₂ →...[Development → Transfer] _o → Cleaning → Static elimination.

That is, first, in the first rotation of the photoreceptor drum 1, an electrostatic latent image is formed by uniform charging and image exposure, then development and transfer are performed to obtain a first copy, and then cleaning and static electricity removal are performed. Then, while uniform charging and image exposure remain off, development and transfer are performed in the second rotation to obtain a second copy, and further,
Obtaining n copies while the photosensitive drum 1 rotates n times, such as obtaining the third copy at the third rotation,
Cleaning and neutralization are performed at the n-th rotation, and the series of multiple copying processes is completed.

In this multi-sheet copying process, the potentials of the image area and the background area of the photoreceptor surface change as shown in FIG. That is, the potential of the background area undergoes light attenuation after image exposure, and about 200 msec after exposure, it enters a dark attenuation process similar to that of the image area, and the potentials of the image area and the background area decrease at approximately the same speed. By the way, if the development for the first copy is performed within 100 msec after exposure (indicated by ' in the figure), the rotation speed of the photosensitive drum 1 is 1 rotation/sec, so two copies will be made. Development for copying the eyes is carried out at 1100 msec after exposure (indicated by ' in the figure). However, in this case, the first copy is developed during the light attenuation process in the background, so the image contrast is 2.
This is significantly different from that of the subsequent copies, and therefore, if the developing conditions, transfer conditions, etc. are set in accordance with the first copy, the image density of the second and subsequent copies will be extremely high.

In the present invention, since the image exposure device 3 and the development device 4 are set so that their central angle θ satisfies the above formula (3), the development for the first copy is easy. What is done is t (200msec) after exposure
After a period of time e.g. 300 minutes after exposure
Development is performed after msec has elapsed (indicated by in the figure). The second copy is developed 1300 msec after exposure after the photosensitive drum 1 rotates one more time (indicated by in the figure), and both the first and second copies are developed on the background. Since the image is developed when the partial potential is in the dark decay process, the image contrast is uniform. Further, since the potentials of the background portion and the image portion continue to darken attenuate after that, the image contrast of the third and subsequent copies becomes approximately the same as that of the first and second copies.

As described in detail above, in the present invention, the period from imagewise exposure of the surface of the photoreceptor to development is set to be longer than the time required for light attenuation of the photosensitive layer, so development during the light attenuation process is avoided. When multiple copies are made, the image contrast between each copy is uniform. It should be noted that the present invention should not be limited to the specific embodiments described above, and it goes without saying that various modifications can be made within the technical scope of the present invention. In addition, when As ₂ Se ₃ is not used as the photoconductive material constituting the photosensitive layer as in the above example, the light decay end time t is not necessarily 200 msec, and the charge of the photoconductive material used This is determined based on the carrier's mobility, etc.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the charging characteristics of the photoreceptor, FIG. 2 is a schematic diagram showing one embodiment of the method of the present invention, and FIG.
The figure is a graph diagram explaining the difference in image contrast. (Explanation of symbols), 1... Photosensitive drum, 3...
Image exposure device, 4...Developing device.

Claims (1)

[Scope of Claims] 1. A process of uniformly charging the surface of a photoconductor, then imagewise exposing it to form an electrostatic latent image, and developing the electrostatic latent image to form a toner image; In an electrophotographic copying method in which a process of transferring an image to a transfer material is repeated multiple times to obtain a plurality of copies with one electrostatic latent image, the time interval from the image exposure to the development is set to An electrophotographic method characterized in that the time is set to be longer than the time required for the end of light attenuation of the photoreceptor.