Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-06-15
2002-11-19
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C219S469000, C399S044000, C399S324000, C399S341000
Reexamination Certificate
active
06484009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophotographic printer in which toner is prevented from migrating from a print medium to a heat roller of a fixing unit during fixing operation.
2. Description of the Related Art
FIG. 11
illustrates a relevant portion of a conventional electrophotographic printer.
FIG. 12
illustrates a mechanism in which a toner image formed on a photoconductive drum is transferred onto a print medium.
Referring to
FIG. 11
, the conventional electrophotographic printer has a charging roller
12
, an LED head
13
, a developing unit
14
, a transfer roller
15
, and a cleaning roller
17
, all of which being disposed around a photoconductive drum
11
. These structural elements are disposed in the order in which a photographic process is carried out.
A print medium P travels in a direction shown by arrow A. The transfer roller
15
is disposed under the photoconductive drum
11
and rotates in contact with the photoconductive drum
11
, thereby defining a transfer section through which the print medium passes during printing. When the print medium P passes the transfer section, a toner image is transferred from the photoconductive drum
11
onto the print medium P. The charging roller
12
and cleaning roller
17
are disposed downstream of the transfer section with respect to the rotation of photoconductive drum
11
. The LED head
13
is disposed above the photoconductive drum
11
. The developing unit
14
is disposed upstream of the transfer section with respect to the rotation of the photoconductive drum.
There is provided an endless belt
10
that is entrained about rollers, not shown, and is sandwiched between the photoconductive drums
11
and transfer rollers
15
. The endless belt
10
runs in the direction shown by arrow A. The endless belt
10
attracts the print medium P that is fed from a paper cassette, not shown, and transports the print medium P along a transport path from an upstream end to a downstream end of the transport path.
The charging roller
12
uniformly charges the surface of the photoconductive drum
11
. An LED array, not shown, of the LED head
13
is selectively energized in accordance with print data to illuminate the surface of the charged photoconductive drum
11
to form an electrostatic latent image on the photoconductive drum
11
.
In the developing unit
14
, a sponge roller
14
b
supplies toner to a developing roller
14
a
. A developing blade
14
c
is in contact with the rotating developing roller
14
a
to form a uniform thin layer of toner on the surface of the developing roller
14
a
. The developing roller
14
a
rotates in contact with the photoconductive drum
11
, thereby depositing negatively charged toner on the latent image formed on the photoconductive drum
11
to form a toner image.
The transfer roller
15
is connected to a high voltage power supply
16
which applies a potential in the range of several hundred volts to several thousand volts across the photoconductive drum
11
and the transfer roller
15
, so that the toner T on the photoconductive drum
11
migrates from the photoconductive drum
11
to the print medium P that is transported by the endless belt
10
between the photoconductive drum
11
and the transfer roller
15
.
As shown in
FIG. 12
, dielectric polarization occurs such that toner image-receiving surface of the print medium P is positively charged. When the negatively charged toner T on the photoconductive drum
11
is brought into contact with the positively charged surface of the print medium P, the negatively charged toner T is attracted by the Coulomb force to the positively charged surface of the print medium P. In other words, the toner image is transferred onto the print medium P. A current flows through the print medium P between the print medium P and non-exposed areas, i.e., areas on the photoconductive drum
11
in which no toner exists, so that the residual charges are neutralized. Thus, the toner image-receiving surface of the print medium P is generally negatively charged. The toner transferred onto the print medium P remains attracted to the print medium by weak Coulomb force.
The cleaning roller
17
shown in
FIG. 11
removes the toner that remains on the photoconductive drum
11
after transferring.
A neutralizing brush
18
is provided in the transport path of print medium downstream of the transfer section. The free end of the neutralizing brush
18
is in contact with the non-printed surface of the print medium P in order to neutralize the charges deposited on the non-printed surface of the print medium P. The base portion of the neutralizing brush
18
is grounded.
Downstream of the neutralizing brush
18
, there is provided a fixing unit
19
that includes a fixing roller
19
a
and a pressure roller
19
b
. The fixing roller
19
a
has a heat source, not shown, that supplies heat to the surface of the print medium onto which the toner image has been transferred. The pressure roller
19
b
is disposed under the fixing roller
19
a
and presses the print medium P against the fixing roller
19
a.
The heat supplied from the fixing roller
19
a
fuses the toner deposited on the print medium and the pressure applied by the pressure roller
19
b
causes the fused tone to penetrate the print medium P. The fixing roller
19
a
and pressure roller
19
b
have insulating layers formed thereon such that the toner is easily peeled off therefrom.
With the aforementioned conventional electrophotographic printer, the surfaces of the fixing roller
19
a
and pressure roller
19
b
are covered with insulation that facilitates the peeling off of toner. However, the conventional printer suffers from the “offset” problem, i.e., an unintentional or faulty transfer of toner from a print medium to the fixing roller in contact with the print medium.
If the offset problem occurs, the toner deposited on the fixing roller
19
a
is then deposited to the pressure roller
19
b
and/or other parts of the print medium. The toner may also be deposited on the following page of print medium such that a ghost image may appear on the following page or the page is simply soiled.
The offset problem will occur if the negative potential on the toner image surface of the print medium P and the positive potential on the non-toner image surface are excessively high when the print medium P passes the fixing unit. For a tandem type color printer, the yellow, magenta, cyan, and black toner images are formed on the print medium attracted to the transport belt. Thus, the potential of the toner image surface of the print medium is apt to be higher for a tandem type color printer than for a monochrome printer. Especially, in a low temperature and low humidity environment, the impedance of the print medium is high and therefore the both opposed surfaces of the print medium tend to be high potential.
The surface of the print medium on which a toner image has not been transferred is easily neutralized by the neutralizing brush
18
, the frame or chassis of the printer body. However, the charges on the toner-image surface of the print medium and the non-toner image surface are opposite in polarity and are in equilibrium. Thus, it is very difficult to completely neutralize the charges on the both surfaces of the print medium.
If the negative potential on the toner image surface of the print medium and the positive potential on the non-toner image surface are excessively high, when the print medium P is brought into contact with the fixing roller
19
a
, a potential difference will develop between the fixing roller
19
a
and the print medium P, creating an electric field therebetween. The electric field causes the negatively charged toner on the toner image surface of the print medium to migrate to the fixing roller
19
a
, thereby resulting in the offset problem.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrophotographic printer in which the offset problem of a fixing unit is effectively prevented.
Another ob
Furuya Satoru
Ito Yukifumi
Chen Sophia S.
Oki Data Corporation
Rabin & Berdo P.C.
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