Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Electrophoretic imaging – process – composition – or product
Reexamination Certificate
2002-10-01
2004-10-05
Chapman, Mark A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Electrophoretic imaging, process, composition, or product
C430S055000, C430S058050, C399S320000
Reexamination Certificate
active
06800410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as an electrostatic copier or a laser printer. More particularly, the present invention relates to an image forming apparatus involving a so-called fixing step for fixing a toner image onto a recording sheet such as a paper.
2. Description of the Related Art
A conventional image forming apparatus of this type has a structure as shown in FIG.
1
. An image forming apparatus shown in
FIG. 1
has a photoconductive drum
1
, charging means (charging unit)
2
, exposing means
3
, developing means
4
, transferring means
5
, cleaning means
7
, and fixing means
8
to form an image on a recording sheet
6
. A typical image forming process will be shown below.
(1) The charging means
2
charges a surface of the photoconductor
1
to a desired potential.
(2) The exposing means
3
exposes the photoconductor
1
to form a latent electrostatic image corresponding to a desired image on the photoconductor.
(3) The developing means
4
develops the latent electrostatic image formed by the exposing means
3
with a toner and thereby forms a toner image on the photoconductor
1
.
(4) The transferring means
5
transfers the toner image from the photoconductor to the recording sheet
6
carried by carrying means not shown.
(5) The cleaning means
7
cleans, from the photoconductor, the toner remaining thereon without being transferred to the recording sheet
6
.
(6) The recording sheet
6
is carried to the fixing means
8
.
(7) The fixing means
8
heats the toner (recording sheet
6
) to fix it onto the recording sheet.
The image forming apparatus forms a desired image on the recording sheet
6
by rotating the photoconductive drum in the direction indicated by the arrow in FIG.
1
and repeating the steps (1) to (7).
In general, the photoconductive drum
1
is formed by coating a photoconductor on a surface of a conductor. The mainstream of the photoconductor has been a so-called organic photoconductor. As the photoconductor, a multilayer type (multilayer organic photoconductor) composed of a so-called charge generating layer and a charge transporting layer stacked on a conductive base has been used in most cases because of the high durability of the charge transporting layer. There has also appeared a photoconductor having a protection layer in addition to the charge generating layer and the charge transporting layer, which is provided to enhance the durability of the charge transporting layer.
There has been known that, to provide such an image forming apparatus capable of image reproduction at even a spatial frequency at which a development field is high, the film thickness of a photoconductor should be reduced (thinned) (
Basic and Applied Electrophotographic Techniques
, pp.150-151, Corona Publishing Co., Ltd.).
However, a photoconductor small in film thickness is low in durability against cleaning-induced abrasion, flaws, or the like. If a charging step and an exposing step are performed repeatedly, the photoconductor small in film thickness deteriorates fast. Briefly, the durability of a photoconductor is reduced significantly if the film thickness thereof is reduced for the formation of a high-quality image. If the film thickness of the photoconductor is increased for the enhanced durability thereof, on the other hand, only a low-quality image is obtainable. To eliminate the tradeoff, it has been requested to satisfy the two requirements of enhanced durability of a photoconductor and high-quality image formation.
In a conventional multilayer organic photoconductor, polycarbonate has been used commonly as a binder resin in a charge transporting layer. In such a photoconductor, the film thickness of the charge transporting layer has normally been adjusted to a range of 20 &mgr;m to 30 &mgr;m. This indicates that a higher priority has been given to the high durability of the photoconductive than to image qualities.
SUMMARY OF THE INVENTION
When the present inventors formed an image at a resolution of 1200 dpi or more which had been known to be necessary for the discrimination of character images (font) by using an image forming apparatus having a multilayer photoconductor in which a charge transporting layer has a film thickness of 20 &mgr;m to 30 &mgr;m, the image forming apparatus could not reproduce an image at a so-called high spatial frequency such as a single isolated dot or a 1-dot line. This indicates that the conventional image forming apparatus cannot perform through outputting of input images such as so-called bit map images due to unsatisfactory reproduction of a single isolated dot or a 1-dot line. In other words, it is not until an input image has undergone complicated image processing steps that an image can be formed.
If the resolution is adjusted to 600 dpi or 400 dpi, a single isolated dot or a 1-dot line can be reproduced. However, since the single isolated dot or 1-dot line is increased in size, only a coarse image is formed. In an image including an oblique line, a reduction in resolution aggravates a so-called jaggy and degrades image qualities.
The present inventors also performed writing of image data processed with a halftoning operation using the number of lines of 200 lpi or more by using an image forming apparatus having a photoconductor including a charge transporting layer with a film thickness of about 20 &mgr;m to 30 &mgr;m. The resulting image was extremely low in tone. This proved that an image which needs tonic representation, such as a photographic image, could not be formed satisfactorily. This also proved that so-called banding was likely to occur and only an image with much noise was obtained.
When a halftoning operation was performed by using the number of lines less than 200 lpi, a sufficient tone was achieved. However, a dither texture was visually observed and a fine-texture image could not be obtained.
The present invention has been achieved in view of the aforementioned problems. It is therefore an object of the present invention to provide an image forming apparatus capable of providing a high-quality image by adjusting the diameter of a beam for forming a latent image which is applied to the photoconductor to 35 &mgr;m or less and combining a condition placed on the carrier mobility of the charge transporting layer of a photoconductor or a condition placed on the transmittance of a protection layer provided on the photoconductor with a condition placed on the film thicknesses of the protection layer and the charge transporting layer.
A first image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; and optical writing means for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon, the apparatus using an electrophotographic process in which a resolution of the optical write operation is 1200 dpi or more, the optical writing means performing the optical write operation by using a laser beam with a diameter of 35 &mgr;m or less, the photoconductor being provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, the charge transporting layer having a carrier mobility of 1×10
−5
cm
2
·V
−1
·sec
−1
or more under an electric field of 3×10
5
V·cm
−1
.
The image forming apparatus having such a structure is capable of reproducing an image at a high spatial frequency such as a single isolated dot or a 1-dot line and performing through outputting of bit map images or the like. Even in an image including an oblique line, therefore, a so-called jaggy does not occur. In character images also, various fonts can be discriminated.
A second image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; optical writing means for performi
Suzuki Yasuo
Tamoto Nozomu
Yasutomi Kei
Chapman Mark A.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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