Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
2001-04-17
2003-03-25
Braun, Fred L (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C361S235000
Reexamination Certificate
active
06539184
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile, or the like, which employs an electrophotographic method to form an image. In particular, it relates to such an image forming apparatus that is provided with a charging member which charges an image bearing member by making contact with the image bearing member.
Conventionally, a corona based charging device has been used as a means for charging an image bearing member, such as an electrophotographic photosensitive member, an electrostatic recordable dielectric member, or the like, of an image forming apparatus, for example, a copying machine, a printer, and the like, which employs an electrophotographic, electrostatic, or the like recording method . When charging an image bearing member, a corona based charging device is placed close enough to the image bearing member to expose the peripheral surface of the image bearing member to the corona discharge from the corona based charging device so that the peripheral surface of the image bearing member is charged to the predetermined polarity and potential level.
In recent years, a contact type charging apparatus has been realized as a means for charging an image bearing member, because a contact type charging apparatus is advantageous over a corona based charging device in that a contact type charging apparatus produces a smaller amount of ozone, and consumes a smaller amount of electrical power, compared to a corona based charging device. When charging an image bearing member, a contact type charging apparatus, to which voltage is being applied, is placed in contact with the image bearing member, as an object to be charged, so that the peripheral surface of the image bearing member is charged to the predetermined polarity and potential level.
Among various contact type charging apparatuses, a roller based contact type charging apparatus, which employs an electrically conductive roller (hereinafter, “charge roller”) as a charging member, is preferably employed from the standpoint of charge stability. When charging an image bearing member with the use of a roller based contact type charging apparatus, the charge roller as a charging member is directly pressed upon the image bearing member, while applying voltage to the charge roller, so that the image bearing member is charged.
Referring to
FIG. 19
, a charge roller
101
, that is, the charging member of a conventional contact type charging apparatus, is kept directly pressed upon the peripheral surface of an electrophotographic photosensitive member
100
(hereinafter, “photosensitive drum”) in the form of a rotational drum, as an image bearing member, maintaining a predetermined amount of contact pressure between the charge roller
101
and photosensitive drum
100
. As a predetermined voltage is applied to the charge roller
101
from a charge bias power source
102
, the peripheral surface of the photosensitive drum
100
is charged to a predetermined potential level. Generally, the charge roller
101
is rotated by the rotation of the photosensitive drum
100
as the photosensitive drum
100
is rotationally driven.
When the photosensitive drum
100
is charged by the charge roller
101
placed in contact with the photosensitive drum
100
, the photosensitive drum
100
is charged by electrical discharge. Therefore, the photosensitive drum
100
begins to be charged as a voltage, the potential level of which is higher than a certain level, or the threshold level, is applied to the charge roller
101
from the charge bias power source
102
. For example, in order to charge the photosensitive drum
100
having a 25 &mgr;m thick photosensitive layer of OPC by directly pressing the charge roller
101
upon the photosensitive drum
100
, a voltage of approximately 600 V must be applied to the charge roller
101
. In other words, as the potential level of the voltage applied to the charge roller
101
is increased to approximately 600 V, the surface potential level of the photosensitive drum
100
begins to rise, and then, as the potential level of the voltage applied to the charge roller
101
is further increased, the surface potential level of the photosensitive drum
100
linearly increases in proportion to the increase in the level of the applied voltage.
Hereinafter, this threshold voltage value, that is, the value of the DC voltage at which an object to be charged (photosensitive drum) begins to be charged as the value of the DC voltage being applied to the charging member (charge roller) is increased, will be referred to as the “charge (discharge) start voltage value Vth” of the object.
There are different contact type charging methods: “DC based charging method” and “AC based charging method”, which will be described below. In a DC based charging method, only DC voltage is applied to a charging member to charge an object, whereas in an AC based charging method, such AC voltage (oscillating voltage: voltage, the value of which periodically changes with the elapsing of time) that has an AC component and a DC component is applied to a charging member to charge an object.
(1) DC Based Charging Method
When charging an object to be charged (photosensitive drum) to a predetermined surface potential level Vd using a DC based charging method, a DC voltage, the potential level of which is equal to the total of the potential level Vd and the charge start voltage value Vth of the object to be charged (photosensitive drum), is applied to a charging member. A DC based charging method is not satisfactory in terms of charge uniformity. Further, it lacks convergency regarding the potential level higher than Vd. Therefore, an object to be charged (photosensitive drum) needs to be pre-exposed.
(2) AC Based Charging Method
An AC based charging method is superior to a DC based charging method in terms of charge uniformity. When charging an object to a predetermined surface potential level of Vd using an AC based charging method, an oscillating voltage created by combining an offset DC voltage, the potential level of which is equivalent to a predetermined surface potential level to which the object is to be charged, with an AC voltage, the peak-to-peak voltage of which is no less than 2×Vth, is applied to a charge roller.
This charging method is employed to take advantage of the leveling effect of the AC component represented by the line a in
FIG. 20
, so that the surface potential level of the photosensitive drum, as an object to be charged, converges to the middle of the top and bottom peak voltage levels of the AC component, as represented by the line b in
FIG. 20
, according to the gaps between the charging member (charge roller) and the photosensitive drum.
In an image forming apparatus, the charging apparatus for charging the image bearing member (photosensitive drum) uniformly charges the peripheral surface of the image bearing member to a predetermined potential level to form an electrostatic latent image on the peripheral surface of the image bearing member. After the formation of the electrostatic latent image, the charging apparatus removes the electrical charge on the peripheral surface of the image bearing member in order to erase the potential level history on the image bearing member.
In other words, in an AC based charging method, when charging an image bearing member, the surface potential level of the image bearing member can be made to converge to the predetermined level of Vd uniformly across the entirety of the peripheral surface of the image bearing member by using a DC voltage, as an offset voltage, the potential level of which equals the potential level of the dark area (unexposed portion of charged peripheral surface of image bearing member), whereas when discharging the peripheral surface of the image bearing member, the surface potential level of the image bearing member can be made to converge to 0 V uniformly across the entirety of the peripheral surface of the image bearing member, by using 0 V as the offset
Nakamori Tomohiro
Saito Seiji
Sakai Hiroaki
Shimura Masaru
Takami Hiroshi
Braun Fred L
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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