Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
1999-07-02
2001-12-18
Braun, Fred L (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C399S128000
Reexamination Certificate
active
06332064
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrophotographic image forming apparatus.
DESCRIPTION OF THE PRIOR ART
An electrophotographic printer is a well-known prior art apparatus which performs the steps of charging, exposing, developing, transferring, and fixing.
The charging unit usually takes the form of a corona charger, which requires a high voltage power supply for outputting a high voltage of about 5-10 kV. A special care must be taken when handling the power supply. In addition, the high voltage power supply is very expensive. A corona charger suffers from a problem that the potential of the charged surface of a latent image bearing body is susceptible to environmental conditions such as humidity. The corona charger uses a corona discharge phenomenon which creates ozone. Ozone deteriorates the characteristics of the latent image bearing body and is harmful to human body as well. In order to prevent harmful effects to human body, a filter which absorbs and decomposes ozone is provided in the apparatus, thereby preventing escape of ozone. However, the lifetime of the filter is relatively short and therefore must be periodically replaced.
In order to solve the problems of corona charger, Japanese Patent Preliminary Publication (KOKAI) No. 63-208878 discloses a contact type charging device in which a charging roller having an electrical resistance ranging from 10
5
to 10
6
&OHgr; is brought into contact with the latent image bearing body and a direct current voltage is applied to the charging roller so as to charge the latent image bearing body.
FIG. 30
illustrates a general construction of a prior art electrophotographic printer having a contact type charging device.
The prior art image forming apparatus will be described with reference to FIG.
30
.
An electrostatic latent image bearing body in the form of a photoconductive drum
1
is rotatably supported. Disposed around the photoconductive drum
1
are a charging roller
2
, exposing unit having an LED head
3
, developing unit
4
, transfer roller
7
, cleaning device
9
, aligned in the order of respective stages of image-forming process. The charging roller
2
receives a negative voltage from a charging power supply
10
in the form of a constant voltage source.
FIG. 31
is a flowchart illustrating the outline of the photographic printing operation of the prior art apparatus.
The charging roller
2
rotates in contact with the photoconductive drum
1
to charge the surface of the photoconductive drum
1
(S
501
). The charging power supply
10
provides a voltage such that the surface of the photoconductive drum is charged to −800 V. The exposing unit
3
illuminates the charged surface of the photoconductive drum
1
to form an electrostatic latent image on the photoconductive drum
1
(S
502
). The electrostatic latent image is then developed with toner by a developing roller into a toner image (S
503
). Then, paper
6
passes a transfer point defined between the photoconductive drum
1
and a transfer roller
7
. The transfer roller
7
receives a voltage of a polarity opposite to that of the toner images formed on the photoconductive drum
1
. The voltage creates an electric field between the transfer roller
7
and the photoconductive drum
1
, thereby transferring the toner image to paper
6
(S
504
). Residual toner particles remaining on the photoconductive drum
1
after transferring operation are removed by the cleaning device
9
from the photoconductive drum
1
(S
505
).
With the aforementioned prior art apparatus, many small areas on the photoconductive drum of about 0.5 mm-diameter are extremely charged when the apparatus is operated in a high-temperature and high-humidity environment. Such overcharged areas cause local non-uniform charging of the surface of the photoconductive drum
1
.
Another problem is that the surface of the photoconductive drum
1
is not charged to a desired potential in an environment of low-temperature and low-humidity and therefore toner will adhere to non-latent image areas. This phenomenon is apt to occur when the latent image bearing body rotates at high speeds.
The prior art image forming apparatus which has been described with reference to
FIG. 30
suffers from the following disadvantages.
The potential of the photoconductive drum
1
is determined by the voltage that is applied to the charging roller
2
, the capacitance of the photoconductive drum
1
, and the impedance of the charging roller
2
.
The surface of the photoconductive drum
1
is charged to, for example, −800 V. We assume typical values of the parameters as follows:
Output voltage of charging power supply=Vch (volts)
Capacitance of the photoconductive drum=Cch (farads)
Impedance of the charging roller=Rch (ohms)
If the charging roller has an impedance higher than Rch, the surface of the photoconductive drum
1
is charged to a potential closer to zero volts than −800 V. If the photoconductive drum
1
has a capacitance larger than Cch, the surface of the photoconductive drum
1
is closer to zero volts than −800 V. The impedance of the charging roller
2
and the capacitance of the photoconductive drum
1
vary due to manufacturing variations. Moreover, the impedance of the charging roller
2
is susceptible to the temperature and humidity of an environment in which the printer is placed. Changes in the impedance of the charging roller
2
and capacitance of the photoconductive drum
1
greatly affect the surface potential of the photoconductive drum
1
of the printer.
In the specification, the term “image-forming device (IFD)” is used to cover a structure consisting of the charging roller
2
, photoconductive drum
1
, developing unit
4
, and cleaning device
9
. Image-forming devices IFD-
1
and IFD-
2
are subjected to a test in which the surface potential of the photoconductive drum
1
is measured when the charging power supply
10
provides a voltage of −1350 V to the charging roller
2
. The test was conducted under two different environmental conditions; 10° C. and 20% RH, and 28° C. and 80% RH.
FIG. 32
shows the test results.
As shown in
FIG. 32
, the surface potential of the photoconductive drum
1
deviates from a target value of −800 V depending on the image-forming devices and environmental conditions. For example, IFD-
1
was charged to −800 V in 10° C., 20% RH environment but to —about −920 V in a 28° C., 80%RH environment.
Deviation of the surface potential of the photoconductive drum
1
from the target voltage of −800 V results in poor image quality. For example, if the surface potential deviates upward from the target voltage (e.g., to −600 V), toner will adhere to non-image areas on the photoconductive drum
1
with the result that white part of the print paper will be soiled. Another problem is that the potential difference between illuminated areas and non-illuminated areas is small, loosing sharpness of images as well as resulting too dense print results. Conversely, if the surface potential deviates downward from the target voltage (e.g., to −1000 V), image areas are not sufficiently neutralized when illuminated by the exposing unit, attracting less toner during the developing process. Thus, print result is less dense.
As mentioned above, changes in the capacitance of the photoconductive drum
1
and changes in the impedance of the charging roller
2
result from environmental conditions (temperature and humidity) and change over time. Such changes cause changes in the surface potential of the photoconductive drum
1
, thereby resulting in adverse effects on the print results.
Printing speed is another factor that affects the range of variations of the electrical resistance of the charging roller
2
in which a reasonable image can be formed. At higher printing speeds, the range of electrical resistance in which good image is obtained becomes narrower. Thus, as the printing speed increases, it is increasingly difficult to accommodate changes in electrical resistanc
Ashida Kenichi
Ida Koji
Ishizaki Kimihide
Sato Hiroaki
Akin Gump Strauss Hauer & Feld L.L.P.
Braun Fred L
Oki Data Corporation
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