Electrophotography – Image formation – Development
Reexamination Certificate
2002-04-29
2004-03-09
Brase, Sandra L (Department: 2852)
Electrophotography
Image formation
Development
C399S223000
Reexamination Certificate
active
06704530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming an image in an electrophotographic printer, and more particularly, to a method for forming an image in an image forming system in which a developing unit is attached and detached from a photosensitive medium and developing proceeds. The present application is based on Korean Application No. 2002-1380, filed Jan. 10, 2002, which is incorporated herein by reference.
2. Description of the Related Art
In general, an electrophotographic printer such as a color laser printer, has an image forming system in which an electrostatic latent image is formed on a photosensitive medium and then, the electrostatic latent image is developed with a toner, and a developed image passes through a predetermined transferring medium and is transferred onto a paper, and then, the developed image is heated and pressed, and the heated and pressed image is finally fixed on the paper.
FIG. 1
illustrates an example of an image forming system of an electrophotographic printer.
The image forming system includes a photosensitive drum
10
as a photosensitive medium, a charger
11
for charging the photosensitive drum
10
, a laser scanning unit
12
as an exposure unit, for scanning light on the electrified photosensitive drum
10
and forming an electrostatic latent image, a developing unit
13
for developing the electrostatic latent image with a toner having four colors such as yellow (Y), magenta (M), cyan (C), and black (K), a transferring belt
14
for sequentially overlapping the four color images which are developed on the photosensitive drum
10
, a first transferring roller
14
a
for transferring the image which is developed on the photosensitive drum
10
to the transferring belt
14
, a second transferring roller
14
b
for transferring an image which overlaps four colors on the transferring belt
14
onto the paper, and a fixing unit
15
for heating and pressing the print and fixing the transferred image. Four-color developers
13
-Y,
13
-M,
13
-C, and
13
-K, which are included in the developing unit
13
, are elastically biased by a spring (not shown) in a direction where they are separated from the photosensitive drum
10
. The four color developers selectively move toward the photosensitive drum
10
through the rotation of a cam
13
b
, so that developing rollers
13
a
which are provided at the front end of the color developers contact the photosensitive drum
10
. Reference numerals
16
,
17
,
18
, and
19
denote a paper cassette, a blade for cleaning a photosensitive drum, an anti-electrostatic unit, and a transfer path through which the paper is discharged, respectively.
In the above structure, an image forming process is performed as described below. First, in a case where the photosensitive drum
10
is charged by the charger
11
, the laser scanning unit (LSU)
12
scans light and thus forms the electrostatic latent image of an image which is to be developed as a first color. For example, in a case where a yellow color is developed as the first color, as shown in
FIG. 2
, the developer
13
-Y with a yellow color is moved to the photosensitive drum
10
by the action of a corresponding cam
13
b
-Y and thus the electrostatic latent image formed on the photosensitive drum
10
is developed as a yellow toner.
FIG. 3
illustrates the state where the photosensitive drum
10
is in contact with a developing roller
13
a
-Y of the developer
13
-Y. In
FIG. 3
, a toner T which is stained at the outside circumferential side of the developing roller
13
a
-Y is moved onto the portion of the electrostatic latent image of the photosensitive drum
10
contacting the toner T. In such a case, the developing roller
13
a
-Y rotates at a linear velocity which is faster than the linear velocity of the photosensitive drum
10
so as to increase the amount of a toner per area with which the photosensitive drum
10
is coated. The yellow image developed thereby is firstly transferred to the transferring belt
14
through a first transferring nip N
1
.
Next, an electrostatic latent image for a second color is formed through charging and exposure of the photosensitive drum
10
. In a case where the second color is magenta, as shown in
FIG. 4
, the developer
13
-M is moved to the photosensitive drum
10
and thus the electrostatic latent image is formed. The magenta image developed thereby is overlapped and transferred to the transferring belt
14
to which the yellow image is transferred.
A cyan image for a third color and a black image for a fourth color are developed and transferred in the same way, and thereby an image having a desired color is finally made on the transferring belt
14
.
After that, the color image made thereby is transferred onto the paper which is supplied to a second transferring nip N
2
between the transferring belt
14
and the second transferring roller
14
b
. The transferred image passes through the fixing unit
15
, where it is heated and pressed and thus is completely fixed on the paper.
However, in the above configuration, since the four-color developers
13
-Y,
13
-M,
13
-C, and
13
-K of the developing unit
13
forms images while being alternately attached and detached from the photosensitive drum
10
, the photosensitive drum
10
may be shocked whenever each of the developing rollers
13
a
of each of the developers
13
-Y,
13
-M,
13
-C, and
13
-K collides with the photosensitive drum. In such a case, jitter errors caused by image tremble during collision may occur. However, a problem arises in that each of the developing rollers
13
a
of each of the developers
13
-Y,
13
-M,
13
-C, and
13
-K rotates at a linear velocity which is faster than that of the photosensitive drum
10
as mentioned above, and thus shock caused by a difference in the linear velocity occurs during contact, and thereby the shock applied to the photosensitive drum
10
is further increased.
That is, a photosensitive layer is usually coated on an aluminum material to form the photosensitive drum
10
, and the developing rollers
13
a
have a rubber material of predetermined hardness. Thus, in a case where the hardness of the rubber material is lowered, shock during contact may be slightly alleviated. However, the difference in the linear velocity described above is set so that the toner is sufficiently supplied to the photosensitive drum
10
. In a case where the developing roller
13
a
and the photosensitive drum
10
, which have different linear velocities, contact each other, the developers
13
-Y,
13
-M,
13
-C, and
13
-K are instantaneously pushed during contact, and thus registration defects in which the overlapping position between colors crosses each other, as well as the jitter errors occur, and thereby an image is not clear and abnormally developed.
In addition, due to shock during contact, the toner scatters and thus contaminates peripheral devices. The problem caused by the difference in linear velocities may occur during break off as well as during contact. In a case where the velocity of the developing roller is reduced continuously, the supply of the toner is insufficient during development, and thus image quality may be lowered.
Thus, for developing of a clear image, a method is needed in which the amount of the toner supplied to the photosensitive drum
10
during development is not reduced, and the above causes for the occurrence of defective images are eliminated.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a method for forming an image in an electrophotographic printer which is improved so that shock is alleviated when a photosensitive medium and a developer contact each other.
Accordingly, to achieve the above object, there is provided a method for forming an image in an electrophotographic printer, which is capable of removing a developing roller of a developer from a photosensitive medium on which an electrostatic latent image is formed, and of developing the electrostatic latent image
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