Electrophotography – Machine operation
Reexamination Certificate
2002-04-26
2004-08-17
Ngo, Hoang (Department: 2852)
Electrophotography
Machine operation
C399S150000, C399S174000
Reexamination Certificate
active
06778791
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as an electrophotographic apparatus, electrostatic recording apparatus or the like using a contact charging type charging device for electrically charging an image bearing member such as an electrophotographic photosensitive member, dielectric member for electrostatic recording or the like, more particularly to a cleanerless type image forming apparatus which is not provided with a cleaner exclusively for cleaning the image bearing member. The image forming apparatus such as an electrophotographic apparatus requires an electric charging step of charging the image bearing member uniformly to a predetermined potential in order to form an electrostatic latent image on the image bearing member For this purpose, a non-contact type corona charger or the like has been used as a means for the charging.
However, the corona charger produces ozone and requires such a high voltage as approx. 10 KV has to be applied between the charging device and the image bearing member.
Recently, a charging means has been proposed to avoid these problems. In such a means, a charge member is directly contacted to the image bearing member and is supplied with a voltage by which the image bearing member is charged uniformly (so-called contact charging device).
Referring first to
FIG. 5
, there is shown a typical contact charging device is a charging roller
2
-X. S charging roller
2
-
x
comprises an electroconductive base roller, an intermediate resistance surface layer, and the roller is rotated by t image bearing member
1
in the codirectional peripheral movements in the direction indicated by arrow f (rotational direction a). Between the roller and the image bearing member
1
, a predetermined voltage is applied from a voltage source S
1
, so that said image bearing member
1
is electrically charged to a uniform potential.
Here, the voltage applied to the roller may be (1) a DC voltage only or (2) a DC voltage biased with an AC voltage.
(1) In the case of (1), in order to charge the image bearing member
1
to a potential of −600V, the applied voltage is approx. −1300V, and in the case of (2), the applied DC voltage is −600V and the AC voltage is not less than 1500 Vpp.
The charging mechanism in these cases is based on the Paschen's law, and an electric discharge phenomenon arises in a region satisfying the Paschen's law in which the distance between the charging roller
2
-X-a and the image bearing member
1
is within a predetermined range (region H in FIG.
5
).
However, as will be understood from the charging mechanism, the contact charging device of this type creates the discharge which is the same as with the corona charger within a fine space region H, and therefore, the ozone is produced although the amount of ozone production is remarkably smaller than with the corona charger. The ozone produces nitrogen oxide, and if it is deposited on the image bearing member
1
, an image defect is produced due to the low resistance of the deposited matter.
This injection charging process system is proposed in U.S. Pat. Nos. 6,134,407 or 6,081,681 and 6,128,456 in which is free of such a problem of ozone generation, and therefore, the voltage applied to the charging device can be further reduced.
The feature of the charging process is that surface potential of the charged image bearing member is substantially the same as the voltage applied to the charging device. This system does not use the electric discharge phenomenon, and charge injection occurs into the image bearing member by the transfer of electric charges between the surface of the image bearing member and the charge member contacted thereto.
FIG. 6
is a schematic view of a major part of the injection charging device
2
. A charging sponge roller
2
-A (charging roller) of an electroconductive sponge carries electroconductive particles Z deposited on its surface and rotates in such a direction that surface there is moved counterdirectionally (b) relative to the peripheral moving direction (a) of the image bearing member
1
at a nip C formed between t image bearing member
1
and the charging sponge roller, while injecting charge into the image bearing member
1
from the charging sponge roller
2
-A. By this, the image bearing member
1
is charged to a potential substantially equal to the potential of the charging sponge roller
2
-A.
The electroconductive particle are fine electroconductive particles (charging-promotion particle) for assisting the charging. The electroconductive particles are metal oxide fine particles of electroconductive zinc oxide or the like having a volume resistivity of not more than 1×10
12
&OHgr;.cm, preferably not more than 1×10
10
&OHgr;.cm, with or without electroconductive inorganic fine particles, organic material mixed therewith.
In this system, the charging sponge roller
2
-A is supplied with a DC voltage of −600V from a voltage source S
1
. Therefore, the surface potential of the image bearing member
1
tends to become the same potential at the portions where the charging sponge roller
2
-A and the electroconductive particle Z are contacted. At this time, if the charge is injected into the image bearing member
1
from the charging sponge roller
2
-A side beyond an energy barrier on the surface of the image bearing member
1
, the image bearing member
1
is electrically charged. If not, or if the charge moves back from the image bearing member
1
to the charging sponge roller
2
-A side at positions where the charging sponge roller
2
-A and the image bearing member
1
are apart from each other, the image bearing member
1
is not charged. This phenomenon is dependent on the energy barrier of the surface of the image bearing member
1
and/or the charge retention power. On the other hand, when it is taken as a competitive reaction, a frequency of chance of contact between the charging sponge roller
2
-A side and the image bearing member
1
is important. In order to increase the frequency, electroconductive particle Z having small particle sizes are deposited on the surface of the charging sponge roller
2
-A so as to increase the injection sites in the contact portion C between the image bearing member
1
and the charging sponge roller
2
-A, and in addition, the charging sponge roller
2
-A is rotated in the peripheral counterdirectional direction so as to increase the relative speed between the image bearing member
1
and the charging sponge roller
2
-A, thus increasing the number of contact to the image bearing member
1
in the injection sites per unit time.
In this manner, the charging sponge roller
2
-A and the electroconductive particle Z which establish injection sites of the charge to the image bearing member
1
are contacted to the image bearing member
1
at high opportunity, so that surface potential of the image bearing member
1
becomes substantially the same potential, that is, −600V applied to the charging sponge roller
2
-A. Microscopically, uniform charging is accomplished.
FIG. 7
is a schematic view of an example of a transfer type electrophotographic apparatus of a cleaner-less system type wherein the charging means for the image bearing member
1
is an injection charging device
2
using the electroconductive particles Z as described above, and no cleaner exclusively for cleaning the image bearing member
1
is used.
Designated by reference numeral
1
is an electrophotographic photosensitive member of a rotatable drum type (image bearing member), wherein rotated at a predetermined peripheral speed in the clockwise direction indicated by an arrow a. Designated by
2
-A is a charging sponge roller, which is contacted to the image bearing member
1
with a predetermined urging force to provide a contact portion (charging nip) C having a predetermined width. On the outer surface of the charging sponge roller
2
, charging sponge rollers
2
are deposited beforehand. The charging sponge roller
2
is rotated in the c
Kato Jun-ichi
Oba Hiroyuki
Satoh Hiroshi
Shimizu Yasushi
Yoshida Masahiro
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ngo Hoang
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