Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
2002-07-19
2004-11-30
Brase, Sandra (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C399S076000
Reexamination Certificate
active
06826375
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus, in particular, a copying machine, a printer, a facsimile machine, or the like, which forms an image with the use of an electrophotographic image forming method.
An electrophotographic process makes it possible to instantly form an image with high quality and durability. Thus, its usage did not remain in the field of a copying machine; it has come to be widely used not only in the field of a copying machine, but also in the fields of various printers and facsimile machines.
In principle, an electrophotographic process comprises two distinctive processes: an actual image formation process, and an initialization process. The actual image formation process comprises: uniform charging of a photoconductive member; formation of an electrostatic latent image through the exposure of the charged photoconductive member to an optical image in accordance with an original; development of the latent image with the use of toner; transferring of the toner image onto recording medium such as a piece of paper (or sometimes intermediary transfer medium); and fixation of the toner image, whereas the initialization process is a process for removing the toner particles and electrical charge remaining on the peripheral surface of the photoconductive member, in order to repeatedly use the photoconductive member. Further, according to some reports, in order to stabilize the potential level of a photoconductive member at an early stage of the charging process, an auxiliary charging device is disposed on the upstream side of the charging device, in terms of the moving direction of the peripheral surface of the photoconductive member, more specifically, between the cleaning means and charging means.
The nucleus of an electrophotographic image forming method is a photoconductive member which uses photoconductive substance. In recent years, a photoconductive member which uses electrically conductive organic substance has been developed. Electrically conductive organic substance has some advantages over electrically conductive inorganic substance; for example, it is environmentally harmless, and easy to form into film.
In an electrophotographic process, a photoconductive member is gradually shaved or scratched due to the friction which occurs during the development, transfer, and/or cleaning. Thus, eventually, the thickness of the charge retaining capacity of the outermost layer (film) of the photoconductive member is reduced, reducing thereby the charge retaining capacity of the photoconductive member to a point at which the image forming apparatus employing this photoconductive member begins to form unsatisfactory images, that is, the images the quality of which does not meet a predetermined requirement; in other words, the photoconductive member reaches the end of its service life, and must be replaced with a new one at this point.
It is true that an organic photoconductive members of the current generation is at a highly advanced level due to the recent developments in the field of a photoconductive member. However, the materials for the charge transfer layer, or the outermost layer, of a photoconductive member are still polycarbonate, vinyl polymer, polyester, and the like, which cannot be said to be sufficiently resistant to shaving for the photoconductive member to be satisfactorily used within an electrophotographic image forming apparatus. Thus, the amount of the portion of the charge transfer layer shaved away by the friction, and the number of scars created in the surface of the charge transfer layer by the friction, relatively quickly increase, shortening the service life of a photoconductive member. In other words, the service life of an organic photoconductive member is relatively short, expiring after outputting approximately 50,000 copies.
In comparison, a photoconductive member, the main constituent of which is non-crystal silicon, and which is commonly called an amorphous photoconductive member, has come into use in recent years. The surface layer of this type of photoconductive member is hard, and therefore, is highly resistant to shaving, affording an amorphous photoconductive member an image output exceeding 50,000. Further, referring to
FIG. 9
, in terms of the relationship (E-V property) between the amount of the drop in the surface potential level of an amorphous photoconductive member and the amount of exposure light, an organic photoconductive member is nonlinear, whereas an amorphous photoconductive member is virtually linear, which in this case is superior. For this reason, an amorphous photoconductive member is characterized in that the difference in diameter among the discrete dots resulting from the use of an amorphous photoconductive is smaller relative to the difference in latent image contrast. Further, the specific inductive capacity of an organic photoconductive member is 2-3, whereas the specific inductive capacity of the amorphous photoconductive member is approximately 10, which is relatively large. Therefore, a toner image formed by developing an electrostatic latent image formed on an amorphous photoconductive member is superior in the development of the smallest picture elements of an image, which is common knowledge. Thus, an amorphous photoconductive member is widely used in the field of a high speed image forming apparatus capable of forming high quality images.
Also in recent years, in order to obtain images of higher quality, to store or freely edit the inputted image formation data, or the like purposes, digitization of an image formation process has been rapidly progressing. Thus, even in the field of an amorphous photoconductive member, the materials suitable for digitization have been developed, some of them having been already put to practical use.
An amorphous photoconductive member, however, is greater in specific inductive capacity and electrostatic capacity than an organic photoconductive member. Thus, in order to charge an amorphous photoconductive member to a potential level high enough to form a satisfactory image using a corona discharge type charging method, a large amount of current is necessary to trigger electrical discharge to the photoconductive member.
Thus, when a charging method based on electrical discharge is used as a method for charging an amorphous photoconductive drum, a large amount of the byproducts of electrical discharge, for example, ozone, NOx, and the like, is likely to adhere to the peripheral surface of the amorphous photoconductive drum, reducing the electrical resistance of its peripheral surface, which in turn disturbs a latent image formed on the peripheral surface of the amorphous photoconductive drum, in particular in a high temperature/high humidity environment in which the surface resistance reduces. This disturbance of a latent image has a blurring effect, resulting in the formation of a defective image; areas of an image made up of discrete dots become blurred, making the areas look like flowing water.
For the reason given above, an amorphous photoconductive member, which normally is chargeable to the positive polarity, is more desirable as a photoconductive member than an amorphous photoconductive member, which normally is chargeable to the negatively polarity and therefore, produces a larger amount of the byproducts of electrical discharge, such as ozone, than the former.
There are two types of developing methods for developing an electrostatic latent image formed by exposing the peripheral surface of a photoconductive member charged to its natural polarity and a predetermined potential level, to an optical image irradiated in response to electrical signals obtained by processing the image formation data into optional toner reproduction patterns. One is a reversal developing method, in which toner which is the same in polarity as the polarity to which a photoconductive member is charged is used, and the other is a normal developing method, in which a reversal image exposure process is used.
Based on the above desc
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