Electrophotography – Image formation – Development
Reexamination Certificate
2000-03-23
2002-01-01
Braun, Fred L (Department: 2852)
Electrophotography
Image formation
Development
C015S256510
Reexamination Certificate
active
06336021
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic apparatus and a method of forming an electrophotographic image, particularly, to an electrophotographic apparatus and a method of forming an electrophotographic image for forming an image by using a liquid developer.
A wet electrophotographic apparatus using a liquid developer containing a toner and a solvent produces various merits that cannot be produced by a dry electrophotographic apparatus using a developing powder. For example, a very fine toner of sub-micron order can be used in a wet electrophotographic apparatus, making it possible to realize a high image quality. Also, since a sufficiently high image density can be obtained with a small amount of the toner, the wet electrophotographic apparatus is advantageous in economy. Also, the apparatus permits achieving a texture equivalent to that of an offset printing. Further, the toner can be fixed to a paper sheet at a relatively low temperature, leading to an energy saving.
However, some essential problems remain unsolved in the wet electrophotographic apparatus. A first problem inherent in the wet electrophotographic apparatus is derived from the use of a petroleum-based solvent, which exhibits a high resistivity or insulating properties, as a carrier solvent of the liquid developer. The problems derived from the use of the particular liquid developer will now be described with reference to
FIGS. 1 and 2
.
Specifically,
FIG. 1
schematically shows a conventional wet color electrophotographic apparatus.
FIG. 2
shows in a magnified fashion a part of the color electrophotographic apparatus shown in FIG.
1
.
In the wet color electrophotographic apparatus shown in
FIG. 1
, a photoconductor drum
1
is housed in an enclosure
100
. Four sets of chargers
2
-n, laser light exposure sections
3
-n, developing devices including developing rollers
10
-n and solvent recovery devices including solvent recovery rollers
20
-n, which correspond to four colors of yellow, magenta, cyan and black, are sequentially arranged about the outer surface of the photoconductor drum
1
. In each set of these four colors, the charger, the laser, the developing device, and the solvent recovery device are arranged in the order mentioned in the direction denoted by an arrow
17
. For simplification, the transfer section, etc. are omitted from the drawing of FIG.
1
. Also, the developing rollers
10
-n alone are depicted in the drawing concerning the developing devices. Further, the solvent recovery rollers
20
-n and cleaning blades
21
-n alone are depicted in the drawing concerning the solvent recovery devices.
As shown in
FIG. 2
, the developing rollers
10
-n are arranged only slightly apart from the photoconductor drum
1
. Also, the developing rollers
10
-n are partially dipped in a liquid developer housed in a developing agent reservoir (not shown).
The liquid developer is supplied to an image-holding surface
14
of the photoconductor drum
1
by rotating the developing rollers
10
-n in a direction opposite to the rotating direction of the photoconductor drum
1
, as denoted by arrows in FIG.
2
. Incidentally, the drum
1
is rotated in a clockwise direction; whereas, the developing rollers
10
-n are rotated in a counterclockwise direction. In short, drum
1
and the rollers
10
-n are opposite to each other in the rotating direction, as described above. Since the developing rollers
10
-n are rotated in a direction opposite to the rotating direction of the photoconductor drum
1
, the liquid developer
11
within the developing agent reservoir is taken up by the developing rollers
10
-n so as to be supplied onto the image-holding surface
14
of the photoconductor drum
1
.
In this step, a bias voltage of the polarity equal to that of the charged polarity of the toner
12
is applied to the developing rollers
10
-n. Also, an electrostatic latent image is formed in advance on the surface of the photoconductor drum
1
by the chargers
2
-n and the laser light exposure sections
3
-n. Therefore, an electric field is formed in the liquid developer positioned between the photoconductor drum
1
and the developing rollers
10
-n to permit a toner
12
to be moved toward the photoconductor drum
1
by electrophoresis. As a result, a developing agent image is formed on the image-holding surface
14
of the photoconductor drum
1
in a pattern corresponding to the electrostatic latent image.
It should be noted that a film of the solvent containing floating toners
12
that do not contribute to the formation of the developing agent image is formed on the image-holding surface
14
having the image of the developing agent formed thereon. The solvent recovery devices including the recovery rollers
20
-n and the blades
21
-n are mounted for removing the film of the solvent from the image-holding surface
14
.
Like the developing rollers
10
-n, the recovery rollers
20
-n are arranged only slightly apart from the photoconductor drum
1
, as shown in FIG.
2
. The solvent film is removed from the image-holding surface
14
of the photoconductor drum
1
by rotating the rollers
20
-n in a direction equal to the rotating direction of the drum
1
at a speed as high as possible while applying a bias voltage opposite in polarity to the charged polarity of the toner
12
to the recovery rollers
20
-n. In other words, the surface of the recovery roller
20
-n is moved as fast as possible in the reverse direction to the moving direction of the surface of the photoconductor drum
1
in a closest point to the photoconductor drum
1
. To be more specific, the solvent on the image-holding surface
14
and the toner
12
floating in the solvent are taken up by the recovery rollers
20
-n by a mechanism opposite to that described previously in conjunction with the developing rollers
10
-n so as to be removed from the image-holding surface
14
. The solvent and the toner
12
taken up by the recovery rollers
20
-n are scraped off by the blades
21
-n so as to be recovered in a recovery vessel (not shown).
It was customary in the past to remove the excess solvent on the image-holding surface
14
of the photoconductor drum
1
by the method described above. In the conventional method, however, it was difficult to remove sufficiently the excess solvent on the image-holding surface
14
. As a result, a difficulty was brought about that a film of the excess solvent accompanying formation of a developing agent image of a certain color impairs the formation of an electrostatic latent image by irradiation of the image-holding surface
14
with a laser light. In this case, formation of a developing agent image of another color is impaired. Also, in the conventional method, a film of the excess solvent accompanying formation of a developing agent image of a certain color remains on the image-holding surface
14
in forming an image of the developing agent of another color, giving rise to a color mixing. In short, it is difficult to achieve a satisfactory image quality by the conventional method.
In addition, where a large amount of the excess solvent remains on the image-holding surface
14
, the solvent excessively permeates into a paper sheet when the developing agent image is transferred onto the paper sheet. As a result, the image quality is lowered and, at the same time, a bad odor is generated from the paper sheet. It should be noted that, in view of environmental issues, release of the solvent from within the electrophotographic apparatus to the outside should be suppressed as much as possible. Under the circumstances, an additional problem is brought about that a mechanism for removing the solvent permeating the paper sheet or for decreasing the amount of the solvent permeating the paper sheet incurs an increased burden.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrophotographic apparatus and a method of forming an electrophotographic image capable achieving a satisfactory image quality.
Another object of the present inven
Braun Fred L
Kabushiki Kaisha Toshiba
Tran Hoan
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