Electrophotography – Image formation – Development
Reexamination Certificate
1997-12-23
2001-09-25
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Development
C399S055000, C399S174000, C430S122520
Reexamination Certificate
active
06295432
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as a copying machine or printer, more particularly to a developing device for developing an electrostatic image on an image bearing member through a two component contact developing method.
Referring first to
FIG. 6
, a conventional image forming apparatus will be described.
In this Figure, an original G is placed on an original carriage
10
with the side to be copied facing down. Upon depressing a copy key, the copying operation starts. A unit
9
integrally having a lamp for original projection, a short focus lens array and a CCD sensor, effects a scanning operation while illuminating the original, so that light reflected by the original surface is imaged on a CCD through a short focus lens array.
The sensor comprises a light receiving portion, a transfer portion and an output portion, wherein the light receiving CCD portion converts the light signal to a charge signal, the transfer portion sequentially transfers the charge signal to the output portion in synchronism with clock pulses, and the output portion changes the charge signal to a voltage signal, which is then amplified and is subjected to an impedance reduction treatment. The analog signal thus provided is subjected to a known image processing operation to be converted to digital signals.
The printer portion receives the image signal and forms an electrostatic latent image in the following manner. The photosensitive drum
1
is rotated in the direction indicated by the arrow at a predetermined peripheral speed about an axis thereof, and the surface thereof is uniformly charged by a charger
3
to approx. −650V during the rotation process. Then, the uniformly charged surface thereof is scanned by a beam deflected by a rotatable polygonal mirror rotating at a high speed, wherein the beam is emitted by a solid laser element and is rendered on and off in accordance with the image signal.
The electrostatic latent image is developed by toner by a developing device
4
into a toner image which is in turn electrostatically transferred onto a transfer material P by a transfer charger
7
. Thereafter, the transfer material P is electrostatically separated from the drum by a separation charger
8
, and is fed to a fixing device
6
in which the image is fixed, and then is outputted.
On the other hand, the surface of the photosensitive drum
1
after the image toner image transfer, is cleaned by a cleaner
5
so that deposited contamination such as residual toner is removed, and the photosensitive drum
1
is prepared for the next image formation operation.
Recently, a direct charging member has become used as a means for effecting a charging method not using a corona discharge with the increasing environment consciousness. Particularly, an injection charging type is highly desirable since the amount of discharge is extremely small when the photosensitive member is charged. The injection charging system includes a system in which the electric charge is injected to the trap potential of the photosensitive member surface material by a contact charging member to electrically charge it, and a system in which the electric charge is given to electroconductive particles of a charge injection layer in which the electroconductive particles are dispersed in the photosensitive member surface. It is known that in these cases, the charging efficiency is good when the volume resistivity of the surface layer of the photosensitive drum is approx. 10
9
-10
13
&OHgr;.cm.
When the injection charging system is used with the photosensitive drum having an adjusted volume resistivity of the surface layer of 10
9
-10
13
&OHgr;.cm approx., the charging efficiency is good, but the background fog is produced, and the image density of the output is low. It has been found that fog and image density decrease occurs when the development operation is carried out for the surface layer of the photosensitive member having the adjusted volume resistivity of 10
9
-10
13
&OHgr;.cm, under application of alternating electric field and using two component developer.
Various experiments and investigations have been made about the phenomenon of the fog production and the image density decrease, and it has been found that a charge is injected, during the developing operation, from the magnetic carrier into the photosensitive drum having the volume resistivity of the surface layer of approx. 10
9
-10
13
&OHgr;.cm.
The photosensitive drum having the surface layer volume resistivity of 10
9
-10
13
&OHgr;.cm approx. can be charged by rubbing the surface thereof with magnetic particles such as ferrite particles of a particle size of not more than 100 &mgr;m approx. preferably 15-50 &mgr;m carried on a charging sleeve containing therein a magnet, while a bias voltage is applied.
The developer carrier preferably has a volume resistivity of 10
6
-10
10
&OHgr;.cm since then an electrode effect is provided. It has been found that phenomenon similar to the injection charging unintentionally occurs during the development operation using the magnetic carrier having a volume resistivity of approx. 10
9
-10
10
&OHgr;.cm, particularly when the two component development operation is carried out wherein the magnetic carrier is contacted to the photosensitive drum.
It is also known in the injection charging that charging efficiency is good when an alternating electric field having a frequency of 100-6000 Hz preferably 500-2000 Hz is superimposed on DC. It is considered that same phenomenon occurs when the use is made with a developing method which is known per se if the two component development is carried out using the magnetic carrier and the developing bias containing an alternating electric field component of frequency 2000 Hz for the purpose of the development efficiency improvement and the image quality improvement.
When a reverse development operation is carried out for a photosensitive drum having an adjusted surface layer volume resistivity of approx. 10
9
-10
13
&OHgr;.cm with a two component developer including a magnetic carrier having a volume resistivity of approx. 10
6
-10
10
&OHgr;.cm and with an alternating electric field having a frequency of approx. 100-3000 Hz, the charge injection unintentionally occurs in the developing zone from the magnetic carrier for the development into the photosensitive drum with the result that the potentials of the white portion (the portion not exposed to the light after the uniform charging) and the black portion (the portion exposed to the light after the uniform charging) converges to the potential of the DC component of the voltage applied to the developing sleeve. Therefore, the potential difference between the white background portion and the developing sleeve reduces with the result of fog production and of the decrease of the image density because of the decrease of the potential difference between the black portion and the developing sleeve.
In the forgoing analysis, the reverse developing system is taken, but the problem is not peculiar to the reverse development system, and the same problem also arises with the regular developing system.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide an image forming apparatus wherein a toner image having a high image quality can be formed on an image bearing member having a surface layer surface resistivity of 10
9
-10
13
&OHgr;.cm.
It is another object of the present invention to provide an image forming apparatus wherein a bias voltage does not leak into the image bearing member through a carrier.
It is a further object of the present invention to provide an image forming apparatus wherein an injection charging system and a two component contact development can be used together with each other.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with th
Hibino Masaru
Inoue Masahiro
Kobayashi Yoshiaki
Waki Kenichiro
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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