Electrophotography – Image formation – Photoconductive member
Reexamination Certificate
2002-06-18
2004-02-24
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Image formation
Photoconductive member
C399S175000, C430S056000
Reexamination Certificate
active
06697591
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an electrophotographic apparatus and a process cartridge, more particularly an electrophotographic apparatus and a process cartridge using a charging scheme wherein an electrophotographic photosensitive member is charged predominantly according to a charging mechanism whereby charges are directly injected into the photosensitive member surface from a charging member contacting the photosensitive member.
In an electrophotographic process, an electrophotographic photosensitive member comprising a photoconductor, such as selenium, cadmium sulfide, zinc oxide, amorphous silicon or an organic photoconductor is subjected to basic or unit processes, such as charging, exposure, development transfer and fixation, and in the charging process, a corona discharge phenomenon caused by applying a high voltage (on the order of DC 5-8 kV) to a metal wire has been conventionally used. According to the corona discharge scheme, however, corona discharge products, such as ozone and NO
X
, denature the photosensitive member to result in blurring or deterioration of images, or soil the wire to adversely affect the image quality, thus resulting in white dropout or black streaks in images.
Particularly, in the case of an electrophotographic photosensitive member having a photosensitive layer principally comprising an organic photoconductor, which has a lower chemical stability than other photosensitive members, such as a selenium photosensitive member and an amorphous silicon photosensitive member, the organic photosensitive member and amorphous silicon photosensitive member, the organic photosensitive member is liable deteriorate due to chemical reactions, principally oxidation, when exposed to such corona discharge products. Accordingly, when used repetitively in the corona discharge charging scheme, the organic photosensitive member is liable to show a lower printing or copying life, due to the deterioration thereof leading to difficulties, such as image blurring, a lowering in sensitivity and a lower image density due to an increase in residual potential.
Further, the corona discharge charging scheme exhibits a lower charging efficiency as only 5-30% of electricity is utilized as a current flowing toward the photosensitive member and a major portion thereof is directed to a shield plate. For alleviating these problems, contact charging methods not utilizing a corona discharger have been studied, as proposed in JP-A 57-178267, JP-A 56-104351, JP-A 58-40566, JP-A 58-139156, JP-A 58-150975, etc. More specifically, in such a contact charging scheme, a charging member, such as an electroconductive elastic roller, supplied with DC voltage of ca. 1-2 kV from an external supply is caused to contact an electrophotographic photosensitive member, thereby charging the photosensitive surface to a prescribed potential.
The contact charging scheme is disadvantageous compared with the corona charging scheme, in respects of the non-uniformity of charge and the occurrence of dielectric breakdown of the photosensitive member, which result in, e.g., a charging irregularity in a streak shape of ca. 2-200 mm in length and ca. 0.25 mm or below in a direction perpendicular to the moving direction of the photosensitive member, leading to an image defect of a white streak (in a solid black or halftone image) in the normal development scheme or a black streak in the reversal development scheme.
For providing an improved charging uniformity to solve the above-mentioned problem, a method of superposing an AC voltage on a DC voltage and applying the superposed voltage to a charging member has been proposed (JP-A 63-149668). According to the charging method, an AC voltage (Vac) is superposed on a DC voltage (Vdc) to form a pulsating voltage for application, thereby effecting uniform charging.
By ensuring a charging uniformity to obviate image defects, such as white spots in the normal development scheme, or black spots or fog in the reversal developing scheme, according to the superposed voltage charging scheme, the superposed AC voltage is required to have a peak-to-peak potential difference (Vpp) of at least twice a discharge initiation voltage (Vth) according to the Paschen's law.
However, as the superposed AC voltage is increased in order to obviate the image defects, the maximum applied voltage of the pulsating voltage is increased, and a dielectric breakdown due to discharge is liable to occur even at a slight defect in the photosensitive member. Particularly, in the case of a photosensitive member comprising an organic photoconductor having a lower dielectric strength, the dielectric breakdown is liable to be caused. Similarly as in the DC charging scheme, if such a dielectric breakdown is caused, a white image dropout is caused in the normal development scheme and a black streak image defect is caused in the reversal development scheme, in a longitudinal contact direction (i.e., a lateral direction of a recording material).
Further, also in the DC-AC superposed contact charging scheme, the charging mechanism still relies on a discharge phenomenon across a minute gap, and discharge products, such as NO
X
or ozone, deteriorate the photosensitive member surface and result in attachment of low-resistivity materials onto the surface, leading to problems, such as image blurring. Further, as the charging member contacts the photosensitive member and the photosensitive member is exposed to a much higher electric field intensity than in the corona charging scheme, a surface layer of the photosensitive member is liable to peel off to result in a shorter life of the photosensitive member.
In order to solve the above-mentioned problems, there has been proposed a charging process wherein charges are directly injected into a photosensitive member without being substantially accompanied by a discharge phenomenon.
The charging scheme wherein direct charge injection to a photosensitive member (which may also be called “injection charging”) is predominant is substantially different from the above-mentioned charging scheme wherein the discharge is predominant (which may also be called “discharge charging”). Some characteristics of the two charging schemes are described with reference to
FIG. 1
, which shows a relationship between DC applied voltages Vdc from a supply indicated on the abscissa and resultant surface potentials on an electrophotographic photosensitive member on the ordinate.
In the case of discharge charging, as shown in
FIG. 1
, discharge is initiated only after the voltage applied to the charging member has reached a discharge initiation voltage Vth, and an excess of the applied voltage over the discharge injection provides a surface potential on the photosensitive member. More specifically, in the case of discharge charging using only a DC voltage, a relationship according to the following formula (6) holds between the applied voltage Vdc and the resultant surface potential Vd on the electrophotographic photosensitive member:
|
Vd÷|Vdc|−|Vth|
(6).
In a typical case, Vth may be calculated according to the following formula based on the Paschen's law:
Vth=
(8837.7
×D
)
1/2
+312+6.2
×D,
wherein D=L/K, L is a thickness (&mgr;m) of a photosensitive layer, and K is a dielectric constant of the photosensitive layer.
On the other hand, in the case of injection charging, as shown in
FIG. 1
, a surface potential on an electrophotographic photosensitive member is nearly equal to a voltage applied to the charging member, and the absence of a threshold like the discharge initiation voltage in the case of discharge charging is a characteristic of this charging scheme. In other words, the satisfaction of a relationship according to the following formula (7) at least suggests the possibility of occurrence of injection charging:
|
Vdc|−|Vd|<|Vth|
(7).
However, this condition alone does not exclud
Morikawa Yosuke
Nakata Kouichi
Tanaka Daisuke
Yoshimura Kimihiro
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lee Susan S. Y.
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