Electrophotography – Control of electrophotography process – Control of transfer
Reexamination Certificate
1999-03-15
2001-07-24
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Control of transfer
C399S043000, C399S314000
Reexamination Certificate
active
06266495
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus using a transfer roller. Referring first to
FIG. 10
, an image forming apparatus not using the present invention is shown.
The image forming apparatus is a laser beam printer capable of automatic both sided printing on an A
3
sheet, and capable of automatic both sided print on A
4
sheet fed in a direction along a short side at speed of 24 sheets per minute (process speed of 100 mm/sec).
Referring to
FIG. 10
, the image formation strokes in the image forming apparatus will be described. First, a surface of a photosensitive drum
1
located substantially at a center of the apparatus is uniformly charged by a primary charger
29
, and is exposed to image light from light source
28
so that electrostatic latent image is formed on the surface of the photosensitive drum
1
. The electrostatic latent image is visualized into a toner image by a developing device
30
.
In synchronism with the image formation on the photosensitive drum
1
, a sheet of sheet
19
is picked out by a pick-up roller
25
from a sheet cassette
36
, and is fed to a pair of registration rollers by way of a feeding guide
34
, a feeding roller
33
and a feeding guide
32
. A pair of registration rollers
31
are rotated in synchronism with the image on the photosensitive drum
1
to feed the sheet
19
along a transfer guide
14
to a transfer nip formed by the photosensitive drum
1
and the transfer roller
4
. The multiple toner images formed on the photosensitive drum
1
are transferred onto the paper
19
by the transfer nip.
After the completion of the transfer, the paper
19
is separated from the photosensitive drum
1
by the curvature of the small diameter drum and the electrostatic separation using the discharging needle
8
. As shown in
FIG. 11
, the discharging needle
8
is supported by an insulation member
9
, and is supplied with a voltage having a polarity opposite from the polarity of the transfer bias, from the bias voltage source
51
. The paper
19
separated from the photosensitive drum
1
is fed along the feeding guide
10
to the fixing device
13
where the unfixed toner image is fixed into a fixed image.
The paper
19
after the fixing, is fed by a flapper
37
to a feeding guide
39
, and is once fed toward the feeding guide
41
by a feeding roller
40
, and then is fed toward the feeding guide
42
by the opposite rotation of the feeding roller
40
. In this process, the sheet
19
is inverted in its facing orientation.
The inverted sheet
19
is subjected to a transfer stroke after passing through the feeding roller
43
and the feeding guide
44
. The paper
19
having been subjected to the second transfer and fixing step, is fed upwardly by a flapper
37
, and is discharged onto a sheet discharge tray
46
by way of the feeding guide
38
and the sheet discharging roller
45
.
In
FIG. 10
, reference numeral
47
designates an outer casing which covers the entire apparatus, Designated by reference numeral
100
is a fan for discharging the air from the inside fo the apparatus. Designated by reference numeral
12
is a cleaner for cleaning the surface of the photosensitive drum
1
after the image transfer.
A description will be made as to transfer roller
4
. Referring to
FIG. 11
, there is shown a structure of the transfer portion in detail.
In this Figure, the photosensitive drum
1
is rotated at a peripheral speed of 100 mm/sec in the direction R
1
and the transfer roller
9
is rotated in the direction R
4
. The outer diameter of the photosensitive drum
1
is 30 mm, and the outer diameter of the transfer roller
4
is 20 mm.
The transfer roller
4
comprises a core metal
2
having an outer diameter of 8 mm and an elastic layer
3
having a thickness of 6 mm thereon. The elastic layer
3
comprises foam rubber such as EPDM or sponge, in which electronic conductive particles such as carbon or metal oxide is dispersed to provide a resistance of approx. 10
6
−10
9
&OHgr;cm, so that elastic layer
3
is electronic conductive.
In order to transfer the toner image from the photosensitive drum
1
onto the sheet
19
, the core metal
2
of the transfer roller
4
is supplied with a bias voltage having a polarity opposite from the charge polarity of the toner, from a transfer bias voltage source
50
.
The bias (transfer bias) applied to the transfer roller
4
is determined in the following manner.
In a prior process wherein the sheet
19
is not supplied (prerotation), the core metal
2
is supplied with a current ITO required for the image transfer, and the voltage VTO generated with the voltage is read. In the case of the transfer roller
4
having the structure described above, the proper current ITO is 8 &mgr;A. The time period in which the voltage is read, is generally an integer multiple of the time required for one full turn of the transfer roller in consideration of the resistance unevenness of the transfer roller
4
. When the outer diameter of the transfer roller
4
is 20 mm, the period is 630 mS. The read voltages are integrated and averaged to determine a representative value.
Calculation and discrimination are carried out on the basis of the read voltage VTO, the transfer voltage (transfer bias) is determined, fundamentally in the following manner.
Transfer voltage: Vt=ax VTO+b (a is a constant, and b is a constant voltage).
Depending on the value of the VTO, a or b is changed, but a=1, b=1.1 kV is used here as a most general case. With the transfer voltage thus determined, the current of 8 &mgr;A (current ITO) is assured which is enough for the image transfer, during the sheet passage.
However, the transfer roller
4
using electronic conductive material such as bubble generation EPDM for the elastic layer
3
, has a large surface area, with the result that amount of deposition of paper fibers and filler onto the surface is large. Such substance is water absorbent, and therefore, when they are deposited on the surface of the transfer roller
4
, the resistance of the transfer roller
4
is lowered.
Additionally, chains of the electroconductive particles in the material of the elastic layer
3
may be broken by stress resulting from bending, with the result of an increase of the resistance at the end portion, and therefore, the current shortage and the transfer defect occur.
Furthermore, due to the improper dispersion of the electroconductive particles and hysteresis of the pressure during handling, the resistance values may be different locally, with the result that a transfer defect occurs at the high resistance portions.
In order to avoid this defect, transfer voltage is set at a relatively high level, but this would result in a transferring electric field which is higher than necessary level at the transfer nip, so that toner scatters widely beyond the image (scattering).
As a counter measurement, a transfer roller using, for the elastic layer, an electroconductive polymer which has low unevenness of the resistance value against pressure, has recently been developed.
In the case of the transfer roller utilizing the electroconductivity of the polymer, the durability is good, but the resistance value is largely dependent on the temperature because of the property of the electroconductivity given to it.
FIG. 12
shows a temperature dependence of the resistance value of a transfer roller. A transfer roller using bubble generation EPDM having electronic conductivity exhibits a small influence against the resistance value from the temperature, whereas a transfer roller using NBR material or urethane material having polymer electronic conductivity exhibits a large variation of the resistance value thereof against temperature.
According to
FIG. 12
, when the temperature rises 1° C., the resistance value of the urethane material transfer roller rises up to approx. 0.95 time, and that of the NBR transfer roller rises approx. 0.93 time.
F
Shimura Masaru
Yuminamochi Takayasu
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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