Electrophotography – Image formation – Plural copies from same latent image
Reexamination Certificate
1999-12-16
2001-06-05
Moses, Richard (Department: 2852)
Electrophotography
Image formation
Plural copies from same latent image
C399S302000, C399S308000, C430S031000
Reexamination Certificate
active
06243551
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of electrophotography, and relates to a method and apparatus for electrophotographic copying enabling multiple-copy production from a single exposure procedure.
BACKGROUND OF THE INVENTION
Known electrophotographic processes, which are used in all modern electrophotographic copying devices, are typically cyclical processes, each cycle carrying out all sequential stages of the entire copy production process. In other words, to produce each copy from an original document, the entire cycle of the electrophotographic process is repeated. Generally, such a cycle includes the following stages:
(1) charging the photosensitive surface of an intermediate information carrier (photoreceptor);
(2) exposure of the charged surface, i.e., recording an image of the original document onto the charged surface of the photoreceptor, thereby forming a latent electrostatic image on this surface;
(3) developing the exposed surface of the photoreceptor;
(4) transfer and fixation of the obtained image on a final carrier (a copy), e.g. a paper; and also the following step
(5) neutralizing the residual charges and mechanically cleaning the photoreceptor surface to prepare it for the next cycle.
A copying apparatus based on the known electrophotographic processes has limited throughput. This is due to the inertness of an image scanning system applied for exposing the original document, and the so-called “fatigue” of a photosemiconductor layer under the cyclic stresses thereof in a high-speed charge/discharge mode. Additionally, mechanical wear of the photoreceptor occurs caused by the permanent operation of the cleaner.
In a classical electrophotographic process, storage of the latent electrostatic image for multiple-copy production utilizing single exposure is very difficult, due to the existence of so-called “dark potential decay” on the surface of the photoreceptor. The “dark potential decay” is a phenomenon consisting of time discharge of the photosemiconductor layer in darkness caused by the inherent dark conductivity of the photosemiconductor layer. This dark potential decay is characterized by the exponential nature of the discharge process dynamics, namely it is especially pronounced during the first few seconds after the charging of the photosemiconductor layer. For example, in the case of amorphous Se photosemiconductor layer, the surface potential decreases in darkness from the initial value of 700V to the value of 600V during the first 10 seconds. The more sensitivity of the photosemiconductor layers (e.g., SeTe or As
2
Se
3
), the more the dark potential drop, thus making the application of such photosemiconductors impossible in a classical electrophotographic process for production of a plurality of copies from a single exposure.
In addition to the fact that the dark discharge inside a photosemiconductor layer (i.e., passive discharge) reduces the potential level of a latent electrostatic image, the surface potential value is also affected by external factors, such as the conditions of the development and image transfer processes. The influence of external factors is essentially pronounced in a classical electrophotographic process, wherein surface charges are held solely by the compensating charges of the opposite polarity induced in a conductive substance, which charges have the tendency to flow. This is the reason that the external effect causes an even more intensive (active) discharge of the surface of the photoreceptor, and, consequently, reduction of the potential contrast, which negatively affects the quality of obtained copies.
Attempts have been made to produce a plurality of copies using the classical electrophotographic process. U.S. Pat. No. 4,286,865 discloses a copying apparatus in which a special system for controlling the copying process is used to perform the process in batches of ten copies from one image exposure. When more than 10 copies of the same original are required, the exposure is repeated after each ten copies.
The ability of this process is limited, because both the exposure speed and the process speed (i.e., speed of image reproduction) equal to the exposure speed, and, consequently, the throughput of the apparatus, directly depend on the sensitivity of a photosemiconductor layer, which cannot be sufficiently high in this case. Indeed, the increase in the photosemiconductor layer sensitivity causes acceleration of the dark potential decay, and makes it practically incapable of providing stable conditions for the potential relief of the latent electrostatic image at the level of its initial recording.
Techniques of electrophotographic copying are disclosed, for example, in U.S. Pat. Nos. 4,297,422, 4,297,423 and 4,442,191. According to these techniques, the charge storage on the surface of a photoreceptor is typically provided after the image transfer.
More specifically, in the electrophotographic process disclosed in U.S. Pat. No. '422, the image development stage is followed by initial frame illumination on the photoreceptor, and then, with the concurrent electrization during the image transfer, secondary illumination is performed through a material onto which the developed image is transferred. As specifically indicated in this patent, the charges are to be preserved within shadow sections of the photoreceptor, i.e., within the regions where the developer was present before the image transfer. It is assumed here that image sections are transparent for charge passage therethrough, but are opaque for light passage. This approach does not take into account the dark potential decay within the image sections. It is evident that such a technique is not capable of providing a stable and equal charge value for the production of a next copy.
U.S. Pat. No. '423 describes two examples of the charge storage technique on the surface of a multi-layer organic photoreceptor. According to one example, the charges of a latent electrostatic image on the photoreceptor are neutralized by the charges of the opposite polarity, by means of a preliminary chargeable dielectric layer-neutralizer, and contact recharge of the photoreceptor within the blanked regions of the image is simultaneously carried out. In other words, a negative latent electrostatic image is created, and, thereafter, reverse development and image transfer are performed. According to the other example, the development stage is followed by uniform recharge of the developed surface of the photoreceptor within the entire frame with the corona of the opposite polarity. Thereafter, the frame illumination with the charge neutralization within the blanked regions, and the cleaning of the photoreceptor surface from the developer are performed. Both of the described methods cannot be used for a high-quality high-speed copying process. Moreover, a low lifetime of the organic photoreceptor does not allow its operation in a high-speed copying mode.
Another technique, aimed at storing charges of a latent electrostatic image, utilizes a “locking” dielectric layer accommodated inside a multi-layer photoreceptor structure. For example, according to the disclosure in U.S. Pat. No. 4,407,918, such a dielectric layer is interposed between two photosemiconductor layers of different spectral sensitivity. The main drawback of this approach is that it requires a complicated multiple-operation process associated with the use of a special means of ultraviolet illumination. Moreover, a very thin (2 &mgr;m) surface photosemiconductor layer makes the photoreceptor not sufficiently strong, and consequently, unsuitable for operation under the conditions of the process of multiple-copies production.
U.S. Pat. No. 4,898,797 discloses the use of a multi-layer photoreceptor, in which a dielectric layer is interposed between a conductive substrate and a photosemiconductor layer. In this case, a latent electrostatic image is formed not on the photoreceptor surface, but rather at the inside of the photoreceptor structure, on the boundary dielectric-photosemiconductor. This doe
Elfotek Ltd.
Fitch Even Tabin & Flannery
Moses Richard
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