Electrophotography – Image formation – Development
Reexamination Certificate
2001-01-29
2002-05-14
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Development
C399S299000
Reexamination Certificate
active
06389256
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid electrophotographic color image forming apparatus and a color image forming method and, more particularly, to a liquid electrophotographic image forming apparatus and method which prevents toner from a color image from being transferred to a developing roller.
2. Description of the Related Art
In a typical liquid electrophotographic image forming apparatus, an image is formed on a photoreceptor medium such as a photoreceptor web by using developer in which toner powder having a predetermined color and liquid carrier are mixed, and the image is printed on a sheet of print paper. To form and print an image, the image forming apparatus adopts the basic processes of discharging, charging, exposure, development, drying and transfer. Also, in the color image forming apparatus for forming a color image on a photoreceptor web, the exposure and development steps are usually repeated four times. With the trend toward high speed image forming apparatuses, four optical scanning units and four developing units are provided so that the exposure and development steps can be repeated four times during one turn of the photoreceptor web, that is, one cycle. The respective development units include developing rollers for sequentially developing a latent image formed on the photoreceptor web using developer for yellow (Y), cyan (C), magenta (M) and black (K) colors. A conventional liquid electrophotographic color image forming apparatus having the developing rollers is shown in FIG.
1
.
Referring to
FIG. 1
, a photoreceptor web
10
is installed to be capable of circulating around a plurality of rollers
11
. Devices for performing the above basic processes are sequentially installed around the photoreceptor web
10
in the direction that the photoreceptor web
10
circulates. These devices are a discharger
8
, a main charger
9
, four optical scanning units
12
a
-
12
d
and four developing units
13
a
-
13
d
alternately installed color by color, a drying unit
17
, and a transfer unit
19
. The units
13
a
-
13
d
are provided with developing rollers
15
a
-
15
d
and squeegee rollers
14
, respectively. The squeegee rollers
14
remove carrier from the developer on the photoreceptor web
10
. Each of the developing rollers
15
a
-
15
d
is installed to be separated from the photoreceptor web
10
by the same distance, i.e., developing gap (G). Also, an auxiliary charger such as a topping corona
16
is installed near the photoreceptor web
10
downstream from each of the developing units
13
a
-
13
d
. The auxiliary charger compensates for natural attenuation of the level of charging electric potential, by further charging the photoreceptor web
10
.
In the operation of the conventional liquid electrophotographic color image forming apparatus, first, while the photoreceptor web
10
circulates at a constant speed, the discharger
8
removes a remaining charge component. Next, the surface of the photoreceptor web
10
is charged to a charging electric potential of about 650-700V by the main charger
9
. The surface of the photoreceptor web
10
is exposed to light scanned by the optical scanning units
12
a
-
12
d
which are installed in order of color under the photoreceptor web
10
. An electrostatic latent image corresponding to image data for each color is formed on the sequentially exposed photoreceptor web
10
. The electrostatic latent image for each color is developed using developer which is supplied through a manifold
7
while passing each of the developing units
13
a
-
13
d
. About 60-70% of carrier in the developer used in the development is squeegeed by the squeegee rollers
14
and removed from the photoreceptor web
10
. The remaining carrier is vaporized by the drying unit
17
. Also, the toner powder in the developer used in the development is made filmy by the squeegee roller
14
and is used for forming a toner image. The toner image is finally printed on a sheet of print paper P via the transfer unit
19
.
The image forming method using the developing units
13
a
-
13
d
for each color is described in detail referring to an electric potential model related to the charging property.
That is, as shown in
FIG. 2A
, the photoreceptor web
10
is charged to a charging electric potential V
CY
of about 560-700V by the main charger
9
. Next, the photoreceptor web
10
is primarily exposed to light scanned by the optical scanning unit
12
a
for a yellow (Y) color and the electric potential of the surface of the exposed photoreceptor web
10
is lowered to an exposure electric potential V
e
of about 120V. An electrostatic latent image corresponding to the yellow image data is formed at a predetermined portion of the photoreceptor web
10
with electric potential lowered. Developer for the yellow color is supplied to an electrostatic latent image for the yellow color formed as above, through the developing roller
15
a
for the yellow color, and simultaneously, a development electric potential V
d
of about 450V is applied to the developing roller
15
a
. The charged toner component moves to the electrostatic latent image for the yellow color due to the difference in the electric potential between the exposure electric potential V
e
and the development electric potential V
d
, so that an image
10
a
for the yellow color is formed. When the electric potential of the toner component adhering to the yellow image
10
a
becomes almost the same as the development electric potential V
d
, the development does not continue any more, that is, balance in charge is achieved. The developed yellow image
10
a
becomes filmy by the squeegee roller
14
in a squeegeeing process. The filmy yellow image
10
a
, about 60-70% of its carrier being removed, remains on the photoreceptor web
10
.
The charging electric potential of the photoreceptor web
10
naturally attenuates while passing the yellow developing unit
13
a
prior to entering a cyan (C) image forming step. Thus, to compensate for the attenuation in the level of the charging electric potential of the photoreceptor web
10
, the topping corona
16
, an auxiliary charger, further charges the photoreceptor web
10
. Referring to
FIG. 2B
, the charging electric potential V
CC
of the photoreceptor web
10
which is further charged is higher than the charging electric potential V
CY
prior to the formation of the yellow image
10
a
. Also, even when the yellow image
10
a
is further charged, the electric potential thereof is lower than the charging electric potential V
CY
.
In this state, the optical scanning unit
12
b
for a cyan color scans light to the photoreceptor web
10
to form an electrostatic latent image for the cyan color. The development electric potential V
d
is applied to the developing roller
15
b
, and simultaneously, developer for the cyan color is supplied to the developing roller
15
b
. Then, the difference in the electric potential between the development electric potential V
d
and the exposure electric potential V
e
causes the charged toner of the cyan developer to move to the cyan electric potential due to the difference in the electric potential so that a cyan image
10
b
is formed. Here, a difference in the electric potential between the yellow image
10
a
formed in the previous step and the cyan developing roller
15
b
occurs. As a result, a wash-off phenomenon where some of toner of the yellow image
10
a
is transferred back to the cyan developing roller
15
b
due to an electric field generated by the different electric potentials occurs.
Also, when an image
10
b
for the cyan color is formed, the photoreceptor web
10
is further charged to form an image for a magenta color. Then, an electrostatic latent image for the magenta color is formed on the photoreceptor web
10
. As shown in
FIG. 2C
, the electric potential V
CM
of the photoreceptor web
10
is higher than the electric potential V
CC
in the previous step. Also, the electric potential levels of both the yellow image
10
a
and the cyan
Shin Kyu-cheol
Song In-yong
Grainger Quana M.
Samsung Electronics Co,. Ltd.
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