Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-09-01
2002-01-29
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S388000, C399S397000
Reexamination Certificate
active
06343203
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus having an improved structure in which a change in the speed of a photoreceptor web can be maintained to a minimum when an image formed on the photoreceptor web is indirectly transferred to a sheet of paper transferring between the fusing roller and the transfer roller.
2. Description of the Related Art
In general, an indirect transfer type image forming apparatus such as a color laser printer or a photocopier is for printing a desired image by developing an electrostatic latent image formed on a photoreceptor medium such as a photoreceptor web or a photoreceptor drum with a developer having a predetermined color and transferring the developed image to a sheet of paper.
Referring to
FIG. 1
, in the indirect transfer type image forming apparatus, an electrostatic latent image is formed on a surface of a photoreceptor web
10
circulating along an endless path, by an electrostatic latent image forming device such as a laser scanning unit (LSU)
32
. A toner image is formed on the surface of the photoreceptor web
10
by developing the electrostatic latent image with a development unit
20
. The image forming apparatus includes a transfer unit
30
having a transfer roller
22
and a fusing roller
24
to transfer the toner image formed on the photoreceptor web
10
to a sheet of paper P. However, when a leading edge E
l
of the paper enters a transfer nip N
t
between the transfer roller
22
and the fusing roller
24
, a pressing force applied to the transfer nip N
t
changes due to the thickness of the paper. As a result, the proceeding speed of the photoreceptor web
10
changes.
FIG. 2
is a graph showing changes in the speed of the photoreceptor web
10
at the moment when the leading edge E
l
of paper enters the transfer nip N
t
or a trailing edge E
t
thereof is exhausted from the transfer nip N
t
in a conventional indirect transfer type image forming apparatus, and a banding phenomenon of an image according to the above change, obtained through experiments. In the graph, the vertical axis indicates changes (&Dgr;v) in the speed of the photoreceptor web with respect to a regular speed (v) thereof which is calculated in percent. A positive value means acceleration and a negative value means deceleration in the speed of photoreceptor web
10
. The horizontal axis indicates time. Here, a portion A is a moment when the leading edge E
l
enters the transfer nip N
t
and a portion B is a moment when a trailing edge E
t
of the paper is exhausted from the transfer nip N
t
. That is, the speed of the photoreceptor web
10
changes due to impact generated when the paper P enters and is exhausted from the transfer nip N
t
. The factor possibly ill-affecting the constancy of the photoreceptor web driven by a driving roller is the thickness of the paper itself periodically entering between the transfer roller and the fusing roller, and this actually causes a considerable speed change of the photoreceptor web. The lower portion of the drawing shows an example of an image printed on a sheet of paper effected by changes in the speed of the photoreceptor web. Here, the image at a portion where the speed of the photoreceptor web increases excessively appears light while the image at the portion where the speed of the photoreceptor web decreases excessively appears dark, which is preferred to as image banding.
Here, the moment when the leading edge of the paper
26
enters between the transfer roller
22
and the fusing roller
24
is &Dgr;t
1
, the time for maintaining a constant speed after the paper enters is &Dgr;t
2
, and the moment when the trailing edge of the paper is exhausted between the transfer roller
22
and the fusing roller
24
is &Dgr;t
3
. The type of banding generated to the image, that is, the change in width &Dgr;L of banding or concentration of the image is closely related to the change in speed of the photoreceptor web
10
.
The change of the concentration of the image depends on the amount of the speed change of the photoreceptor web and the width of banding &Dgr;L depends on the duration of the change of the speed &Dgr;t. The width of the banding &Dgr;L can be expressed as shown in Equation 1.
&Dgr;
L=&ugr;×&Dgr;t
[Equation 1]
Here &ugr; is the speed of the photoreceptor web.
As the paper passing between the fusing roller and the transfer roller is always pressed by a constant pressing force, at the moment A when the leading edge of the paper is caught between the transfer nip or at the moment B when the trailing edge of the paper is exhausted from the transfer nip, the rotation speed of the transfer roller instantly increases or decreases so that the circulating speed of the photoreceptor web changes considerably. Then, when the electrostatic latent image is formed on the photoreceptor web by the laser scanning unit, a change in the scanning position is generated according to the change of speed of the photoreceptor web as shown in portions A and B of
FIG. 2
so that the quality of print is lowered.
SUMMARY OF THE INVENTION
To solve the above problems, it is an objective of the present invention to provide a method of reducing banding in an image due to changes in the speed of a photoreceptor medium of an indirect transfer type image forming apparatus, by which the change of speed of the photoreceptor web is minimized during the formation of an electrostatic latent image on the photoreceptor medium using an LSU, when the paper arrives at or is exhausted from the transfer nip, so that a superior print quality can be guaranteed.
Accordingly, to achieve the above objective, there is provided an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus for preventing banding occurring by a change in the speed of the photoreceptor medium generated when the leading edge and trailing edge of a sheet of paper pass through a transfer nip formed between a transfer roller and a fusing roller, which is achieved by (A) separating the transfer roller and the fusing roller a predetermined distance from each other before the leading edge of paper enters the transfer nip, (B) approximating the transfer roller and the fusing roller after the leading edge of paper has entered the transfer nip, and maintaining the transfer nip in a normal state, and (C) separating again the transfer roller and the fusing roller a predetermined distance from each other before the trailing edge of paper is exhausted from the transfer nip.
Also, to achieve the above objective, there is provided an image banding reduction method of a photoreceptor medium of an indirect transfer type image forming apparatus for preventing banding occurring by a change in the speed of the photoreceptor medium generated when the leading edge and trailing edge of a sheet of paper pass through a transfer nip formed between a transfer roller and a fusing roller, which is achieved by (A) reducing a pressing force applied between the transfer roller and the fusing roller to be less than the pressing force in a normal state, before the leading edge of paper enters the transfer nip, (B) restoring the pressing force applied between the transfer roller and the fusing roller to the normal state after the leading edge of paper has entered the transfer nip, and (C) reducing the pressing force applied between the transfer roller and the fusing roller less than the pressing force in the normal state, before the trailing edge of paper is exhausted from the transfer nip.
REFERENCES:
patent: 5956553 (1999-09-01), Park
Royer William J.
Samsung Electronics Co. LTD
Sughrue & Mion, PLLC
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