Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2001-04-20
2003-02-04
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C347S251000
Reexamination Certificate
active
06516162
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, printers and facsimiles, in which image formation is carried out in an electrophotographic process.
2. Description of the Related Art
Some image forming apparatuses based on the electrophotographic process use a two-component magnetic brush phenomenon method, the development method for making an electrostatic latent image on a photosensitive body visible, in which a two-component developer including an insulating toner and magnetic carriers is mixed and agitated and in which the magnetic carriers electrostatically attracting the insulating toner are magnetically attracted in the form of brush to a circumferential surface of a development roller by magnetic forces from magnetic poles in the development roller so that the developer carried on the development roller is transferred onto a surface of the photosensitive body as the development roller rotates. This method is widely employed particularly in a color image forming system that produces one color image through a plurality of electrophotographic processes using different color toners.
In the image forming based on the electrophotographic process using the two-component magnetic brush phenomenon method, however, when there are two continuous image areas in an image that have different densities, a phenomenon may occur in which an image density of one image area at the boundary with the other image area decreases.
For example, as shown in
FIG. 13A
, when an image changes from a half-tone area G
1
to a background area G
2
in a sub-scan direction Y (opposite the paper feed direction) perpendicular to the main scan direction X of an exposure beam for forming an electrostatic latent image on the surface of the photosensitive body, a sub-scan direction rear end part G
1
a
of the half-tone area G
1
which adjoins the half-tone area G
2
may decrease in density. Further, as shown in
FIG. 13B
, when an image changes from a low-density area G
3
to a high-density area G
4
in a sub-scan direction Y, a sub-scan direction rear end part G
3
a
of the low-density area G
3
adjoining the high-density area G
4
may decrease in density.
First, the density reduction in the rear end part of the half-tone area adjoining the background area is explained with reference to
FIGS. 14A and 14B
.
FIG. 14A
shows a front edge part of a latent image of the half-tone area formed on the photosensitive body in contact with a developer layer.
FIG. 14B
shows a rear end part of the latent image of the half-tone area in contact with the developer layer. To a development roller
102
is applied a development bias (e.g., −500V). The surface of a photosensitive drum
101
is charged by a charger
103
to a bias (e.g., −650V) higher in absolute value than the development bias. The potential of the latent image S
1
of the half-tone area is changed to a potential (e.g., −200V) lower in absolute value than the development bias by an exposure beam L.
As shown in
FIG. 14A
, when the front edge part S
1
a
of the latent image S
1
contacts the developer layer
104
formed over the circumferential surface of the development roller
102
, a forward development electric field acts on the toner tq present at a contact position Q between the surface of the photosensitive drum
101
and the developer layer
104
, which toner tq is attracted to the surface of the developer layer and then to the surface of the photosensitive drum
101
. When as shown in
FIG. 14B
the rear end part of the latent image S
1
contacts the developer layer
104
, a latent image S
2
of the background area comes near the developer layer
104
, with the result that a reverse development electric field which repels the toner tb away from the surface of the developer layer
104
down toward the circumferential surface of the development roller
102
acts on the toner tb present at a position in the developer layer
104
facing the rear edge part S
1
b
of the latent image S
1
.
The toner tb submerged toward the circumferential surface of the development roller
102
moves toward the surface of the developer layer
104
as the contact position Q approaches as a result of rotation of the development roller
102
, but there is a time delay before it reaches the surface of the developer layer
104
. Hence, the rear end part of the latent image S
1
of the half-ton area that adjoins the latent image S
2
of the background area is not attached with a sufficient amount of toner, resulting in a reduced image density at the rear end part of the half-tone area in the image.
In a case where there is a latent image S
2
of the background area in front of the latent image S
1
of the half-tone area as shown in
FIG. 14A
, when the front edge part S
1
a
of the latent image S
1
of the half-tone area is situated at the contact position Q, the toner tf that is repelled from the surface of the developer layer
104
by the latent image S
2
of the background area at the front exists in the developer layer
104
. However, as the development roller
102
rotates, the toner tf moves away from the contact position Q and the toner tq that is attracted to the surface of the developer layer
104
by the low potential of the latent image S
1
of the half-tone area immediately comes close to the contact position Q and adheres to the latent image S
1
. Therefore, the front end part of the half-tone area adjoining the background area in the image does not produce a reduction in the image density.
Next, the density reduction in the rear end part of a low-density area adjoining a high-density area will be explained by referring to
FIGS. 15A-15C
.
FIG. 15A
shows a front edge part of a latent image of the low-density area formed over the photosensitive body in contact with the developer layer.
FIG. 15B
shows a rear end part of the latent image of the low-density area in contact with the developer layer.
FIG. 15C
shows a latent image of a high-density area situated behind the latent image of the low-density area in contact with the developer layer. To the development roller
102
is applied a development bias (e.g., −500V). The surface of the photosensitive drum
101
is charged by the charger
103
to a potential (e.g., −650V) higher in absolute value than the development bias. The potential of a latent image S
3
of the low-density area is made lower in absolute value (e.g., −300V) than the development bias by an exposure beam L. The potential of a latent image S
4
of the high-density area is made lower in absolute value (e.g., −200V) than that of the latent image S
3
of the low-density area by the exposure beam L.
With the front edge part S
3
a
of the latent image S
3
of the low-density area in contact with the developer layer
104
of the development roller
102
as shown in
FIG. 15A
, the toner ta present at the contact position Q between the surface of the photosensitive drum
101
and a forward development electric field acts on the circumferential surface of the developer layer
104
, which attracts the toner ta toward the surface of the development roller
102
, allowing it to adhere to the surface of the development drum
101
. Thus, the toner tc adheres to the entire surface of the latent image S
3
of the low-density area formed over the surface of the photosensitive drum
101
as shown in
FIG. 15B
, until a rear edge part S
3
b
of the latent image S
3
of the low-density area reaches the contact position Q.
After this, when the latent image S
4
of the high-density area located behind the latent image S
3
of the low-density area comes to the contact position Q and begins to contact the developer layer
104
, as shown in
FIG. 15C
, a stronger development electric field is produced in the forward direction between the latent image S
4
and the developer layer
104
than that between the latent image S
3
and the developer layer
104
because the potential of the latent image S
4
is lower in absolute value than that o
Ino Toshiaki
Kawamoto Hiroshi
Kitagawa Takashi
Narimatsu Masayasu
Sakagami Yuka
Conlin David G.
Dike Bronstein Roberts & Cushman
Lee Susan S. Y.
Sharp Kabushiki Kaisha
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