Electrophotography – Internal machine environment – Temperature
Reexamination Certificate
2001-03-19
2002-06-04
Grainger, Quana M. (Department: 2852)
Electrophotography
Internal machine environment
Temperature
C399S097000, C399S302000, C399S308000
Reexamination Certificate
active
06400915
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic apparatus and a method of forming an image on a recording medium, particularly, to a wet electrophotographic apparatus and a method of forming a image ion a recording medium using a liquid developer.
In a wet electrophotographic technology, a liquid developer prepared by dispersing toner in a petroleum solvent is used, and an electrophoresis of the toner within the petroleum solvent is utilized in the developing process. The wet electrophotographic technology produces various merits that cannot be achieved in the dry electrophotographic technology and, thus, attracts attentions in this technical field in recent years.
For example, the wet electrophotographic technology permits using a very fine toner having a particle diameter of sub-micron order so as to make it possible to achieve a high image quality. Also, since it is possible to obtain a sufficiently high image density with a small amount of the toner, the wet electrophotographic technology is advantageous in economy. In addition, the wet electrophotographic technology permits realizing a texture fully comparable with that of the printing, e.g., an offset printing. Further, since the toner can be fixed to a paper sheet under a relatively low temperature, the energy saving can be achieved in the wet electrophotographic technology.
FIG. 1
schematically shows a conventional wet electrophotographic apparatus. In the conventional wet electrophotographic apparatus shown in
FIG. 1
, the image formation is performed as follows. In the first step, the image holding surface of a photoconductor drum
101
, which is an electrophotographic photoconductor, is uniformly charged by a charger
102
-
1
while rotating the photoconductor drum
101
in the clockwise direction. Then, the charged image holding surface is exposed to a laser beam
103
-
1
modulated to conform with the image information of yellow. As a result, an electrostatic latent image conforming with the yellow image is formed on the image holding surface. Then, a yellow liquid developer is supplied from a developing device
104
-
1
onto the image holding surface having the electrostatic latent image formed thereon. As a result, a yellow developer image conforming with the electrostatic latent image is formed on the image holding surface.
The image holding surface having the yellow developer image formed thereon is charged by a charger
102
-
2
and, then, is exposed to a laser beam
103
-
2
. Further, a magenta liquid developer is supplied from a developing device
104
-
2
onto the image holding surface. As a result, a magenta developer image is formed on the image holding surface in addition to the yellow developer image. Further, the image holding surface having the yellow and magenta developer images formed thereon is charged by a charger
102
-
3
and, then, is exposed to a laser beam
103
-
3
. Still further, a cyan liquid developer is supplied from a developing device
104
-
3
onto the image holding surface. In the next step, the image holding surface is charged by a charger
102
-
4
and, then, exposed to a laser beam
103
-
4
. Further, a black liquid developer is supplied from a developing device
104
-
4
onto the image holding surface. As a result, yellow, magenta, cyan and black developer images are formed on the image holding surface of the photoconductor drum
101
.
The developer images of these four colors are transferred from the image holding surface onto an intermediate transfer roller
105
and, then, are further transferred from the intermediate transfer roller
105
onto a paper sheet
106
. A pressure transfer system in which pressure is applied from a press roller
107
onto the intermediate transfer roller
105
via the paper sheet
106
is employed in the transfer of the developer image from the intermediate transfer roller
105
onto the paper sheet
106
. The pressure transfer system is also employed for the transfer of the developer image from the photoconductor drum
101
onto the intermediate transfer roller
105
. In this case, however, a contact pressure is applied from the intermediate transfer roller
105
to the photoconductor drum
101
, and the intermediate transfer roller
105
is heated. Incidentally, the transfer system in which the intermediate transfer roller
105
is heated is disclosed in U.S. Pat. No. 5,570,173, the entire contents of which are incorporated herein by reference.
The present inventors have found that, in the wet electrophotographic apparatus shown in
FIG. 1
, a very high transfer efficiency can be achieved in the case where the relationship T
d
<T
g
<T
r
is satisfied among the temperature T
d
of the image holding surface of the photoconductor drum
101
, the surface temperature T
r
of the intermediate transfer roller
105
and the softening temperature (glass transition temperature) T
g
of the toner contained in the liquid developer. Note that the term “softening temperature” denotes the temperature dividing the degree of fluidity of a material. To be more specific, under temperatures lower than the softening temperature, the plastic viscosity of the material is markedly low. However, under temperatures equal to or higher than the softening temperature, the material exhibits a prominent fluidity and is soft.
For example, in the case of using a liquid developer containing a toner having a softening temperature T
g
of 40° C., a very high transfer efficiency can be achieved, if the temperature T
d
of the image holding surface of the photoconductor drum
101
is lower than 40° C. and the surface temperature T
r
of the intermediate transfer roller
105
is higher than 40° C. The particular relationship can be achieved in general by heating the intermediate transfer roller
105
by the heater in the intermediate transfer roller
105
.
However, in the conventional wet electrophotographic apparatus shown in
FIG. 1
, it is impossible to avoid the heat conduction from the intermediate transfer roller to the photoconductor drum
101
, with the result that the heat is accumulated in the photoconductor drum
101
. What should be noted is that, where the conventional wet electrophotographic apparatus having such a heater is used over a long period of time, it is possible for the temperature T
d
of the image holding surface of the photoconductor drum
101
to be elevated to a level substantially equal to the surface temperature T
r
of the intermediate transfer roller
105
. For example, where the temperature T
r
is about 50° C. to 80° C., the temperature T
d
is considered to be elevated to reach about 40° C. to 70° C.
Where the image holding surface of the photoconductor drum
101
is heated to a high temperature, it is impossible to satisfy the particular relationship denoted by the inequality noted above, leading to a problem that the transfer efficiency of the developer image from the photoconductor drum
101
to the intermediate transfer roller
105
is markedly impaired.
What should also be noted is that, where the image holding surface of the photoconductor drum
101
is heated to a high temperature, the liquid developers in the developing devices
104
-
1
to
104
-
4
are also heated so as to evaporate the carrier solvent contained in the liquid developer. Since it is undesirable in terms of the environmental problem for the evaporated solvent to leak to the outside of the case of the electrophotographic apparatus, it is necessary to further arrange a special means for preventing the evaporated solvent from leaking to the outside of the electrophotographic apparatus. Also, in this case, an additional problem is generated that the cycle for replenishing the solvent is shortened.
In order to cope with these problems, it is conceivable to arrange, for example, a water-cooling type or an air-cooling type cooling mechanism inside the photoconductor drum
101
so as to maintain constant the temperature of the image holding surface. However, in the case of arranging a cooling mechanism inside the photoconductor drum
101
Grainger Quana M.
Kabushiki Kaisha Toshiba
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