Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
1998-07-14
2001-07-10
Beatty, Robert (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C399S066000, C399S128000
Reexamination Certificate
active
06259869
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an image forming apparatus employed in a laser printer, a copying machine, a laser facsimile and the like.
BACKGROUND OF THE INVENTION
An image forming apparatus which develops an electrostatic image formed on a photosensitive drum by adhering toner and transfers the developed image onto a transfer paper wound around a transfer drum is known.
Such an image forming apparatus includes, for example, two corona chargers within a cylinder
501
having a dielectric layer
501
a
as shown in FIG.
69
: one is a corona charger
502
for attracting a transfer paper P, and the other is a corona charger
504
for transferring a toner image formed on the surface of a photosensitive drum
503
onto the transfer paper P. Including two corona chargers
502
•
504
makes it possible to attract the transfer paper P and transfer the toner image onto the transfer paper P independently.
Another image forming apparatus shown in
FIG. 70
includes a two-layer structure cylinder
601
made of a semi-conductive layer
601
a
serving as an outer layer and a base material
601
b
serving as an inner layer, and a grip mechanism
602
for holding the transported transfer paper P around the cylinder
601
. This image forming apparatus grips the end of the transported transfer paper P to hold the same around the surface of the cylinder
601
by means of the grip mechanism
602
first, then charges the surface of the cylinder
601
with electricity either by applying a voltage to the semi-conductive layer
601
a
serving as the outer layer of the cylinder
601
or triggering a discharge of a charger installed within the cylinder
601
, and then transfers a toner image formed on the photosensitive drum
503
onto the transfer paper P.
However, the cylinder
501
of the image forming apparatus shown in
FIG. 69
must have two corona charges
502
•
504
inside thereof, because the cylinder
501
, which serves as a transfer roller, is of a single layer structure using the dielectric layer
501
a
alone. This structure limits the size of the cylinder
501
and presents a problem that the image forming apparatus can not be downsized.
In contrast, the cylinder
601
in the image forming apparatus shown in
FIG. 70
, which serves as the transfer roller, is charged by exploiting its two-layer structure to transfer the toner image onto the transfer paper P, and thus the number of the chargers can be reduced. However, the grip mechanism
602
complicates the entire structure of the image forming apparatus. Moreover, the semi-conductive layer
601
a
serving as the outer layer and the base material
601
b
serving as the inner layer must be fixed with mounting hardware and secured to each other by small screws, a double-sided adhesive tape or the like to assemble the cylinder
601
. Accordingly, the image forming apparatus requires more components and presents a problem that the manufacturing costs increase.
To eliminate these problems, Japanese Laid-open Patent Application No. 2-74975/1990 discloses an image forming apparatus including a corona charger driven by a unipolar power source in the vicinity of a point where a transfer paper separates from a transfer drum made of a lamination of conductive rubber and a dielectric film on a grounded roll of metal.
With this image forming apparatus, a transfer paper is attracted to the transfer drum by inducing the charges on the dielectric film by means of the corona charger. Once the transfer paper is attracted, more charges are induced on the dielectric film, thereby enabling the transfer of a toner image onto the transfer paper.
Since this image forming apparatus uses a single charger to charge the surface of the transfer drum so as to attract the transfer paper and transfer the toner image onto the transfer paper, the transfer drum can be downsized. Also, the above image forming apparatus omits a mechanism such as the grip mechanism
602
, so that the transfer paper can be attracted to the transfer drum by a simple structure.
However, since the transfer paper adheres to the transfer drum electrostatically in this image forming apparatus, some charges remain on the transfer drum, which may cause the toner to adhere to the surface of the transfer drum. Thus, these residual charges present problems such as insufficient adhesion of the transfer paper to the transfer drum or back transfer on the transfer paper, thereby degrading the quality of a resulting image.
Accordingly, Japanese Laid-open Patent Application No. 6-51645/1994 discloses a transfer device provided in the vicinity of the transfer drum in an image forming apparatus, which includes cleaning means made of a conductive fur brush for scraping off the toner adhering to the transfer drum and charge removing means for removing the charges caused by the friction between the conductive fur brush and transfer drum. Note that the charge removing means applies a voltage to the conductive fur brush in a polarity reversed to that of the surface potential of the transfer drum, so that the residual charges on the transfer drum are removed. Since not only the charges remaining on the transfer drum are removed, but also the transfer drum is cleaned, the transfer paper can adhere to the transfer drum satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a good-quality image.
Also, Japanese Laid-open Patent Application No. 3-102385/1991 discloses a cleaning device for an image forming apparatus which attracts a transfer paper to the surface of the transfer drum electrostatically. The cleaning device removes post-transfer residual toner on the surface of a transferring body by applying a bias voltage to a brush cleaner in a polarity reversed to that of the toner. As shown in
FIG. 71
, the cleaning device includes a conductive brush
702
which makes contact with the inner side of a transfer drum
701
and a cleaning brush
703
which makes contact with the outer surface of the transfer drum
701
. According to this structure, the charges remaining on the transfer drum
701
are removed by the conductive brush
702
, while the surface of the transfer drum
701
is cleaned by the cleaning brush
703
. Thus, the transfer drum can attract the transfer paper satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a high-quality image.
However, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2-74975/1990 charges the surface of the transfer drum through an atmospheric discharge by a corona charger. For this reason, if a color image is formed by repeating a transfer process a number of times, the charges are replenished by the corona charger each time a toner image is transferred onto the transfer paper. Thus, the image forming apparatus demands a charging unit comprising a unipolar power source or the like to drive the corona charger under its control. As a result, the number of components of the image forming apparatus increases, thereby presenting a problem that the manufacturing costs increase.
In addition, a flaw on the surface of the transfer drum makes an electric field area developed by the atmospheric discharge smaller, and the electric field becomes out of balance over the flaw. Such off-balance of the electric field causes a defect in a transferred image such as a white spot (void), and hence degrades the quality of a resulting image.
Also, a considerably high voltage must be applied to charge the surface of the transfer roller through the atmospheric discharge, and the driving energy of the image forming apparatus increases accordingly. Further, since the atmospheric discharge is susceptible to the environments such as the temperature and humidity of air, the surface potential of the transfer roller varies easily, which causes insufficient adhesion of the transfer paper, disordered printing, etc.
The transfer device in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 6-51645
Abe Takuya
Iwakura Yoshie
Katoh Atsuyuki
Oikawa Tomohiro
Shimazu Fumio
Beatty Robert
Sharp Kabushiki Kaisha
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