Electrophotography – Image formation – Charging
Reexamination Certificate
1998-09-14
2002-10-29
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Charging
C361S225000
Reexamination Certificate
active
06473582
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging device having a charging member for charging a member to be charged such as a photosensitive member or a dielectric member, and an image forming apparatus having the same.
Heretofore, a corona type charger (corona discharging device) has been widely used as a charging apparatus for charging (inclusive of discharging) an image bearing member (member to be charged) such as an electrophotographic photosensitive member or an electrostatic dielectric recording member to a predetermined polarity and a predetermined potential level in an image forming apparatus, for example, an electrophotographic apparatus or an electrostatic recording apparatus.
The corona type charging device is a non-contact type charging device, and comprises a corona discharging electrode such as a wire electrode, and a shield electrode which surrounds the corona discharging electrode. It is disposed so that corona discharging opening thereof faces an image bearing member, that is, a member to be charged. In usage, the surface of an image bearing member is charged to a predetermined potential level by being exposed to discharge current (corona shower) generated as a high voltage is applied between the corona discharging electrode and the shield electrode.
Recently, a charging device of a contact type (contact charging device) is put into practice wherein the member to be charged is charged by contacting the charging member supplied with a voltage to the member to be charged in view of its advantage of lower production of ozone and lower required electric power, as compared with the corona charger.
In order to charge a member such as an image bearing member with the use of a contact type charging apparatus, the electrically conductive charging member (contact type charging member, contact type charging device, or the like) of a contact type apparatus is placed in contact with the member to be charged, and an electrical bias (charge bias) of a predetermined level is applied to this contact type charging member so that surface of the member to be charged is charged to a predetermined polarity and a predetermined potential level. The charging member is available in various forms, for example, a roller type (charge roller), a fur brush type, a magnetic brush type, a blade type, and the like.
When a member is electrically charged by a contact type charging member, two types of charging mechanisms (charging mechanism or charging principle:
(1) mechanism which discharges electrical charge, and
(2) mechanism for injecting charge) come into action. Thus, the characteristics of each of contact type charging apparatuses or methods are determined by the charging mechanism which is the dominant one of the two in charging the member.
FIG. 11
shows a typical charging properties of them.
(1) Electrical Discharge Based Charging Mechanism
With this mechanism, the surface of the member to be charged is charged by the discharge development occurring in the small gap between the contact charging member and the member to be charged.
In the case of the electrical discharge based charging mechanism there is a threshold voltage which must be surpassed by the charge bias applied to a contact type charging member before electrical discharge occurs between a contact type charging member and a member to be charged, and therefore, in order for the member to be charged through the electrical discharge based charging mechanism, it is necessary to apply to the contact type charging member a voltage with a value greater than the value of the potential level to which the member is to be charged. Thus, in principle, when the electrical discharge based charging mechanism is in action, the discharge product is unavoidable, that is, active ions such as ozone ions are produced, even though the amount thereof is remarkably small.
For example, a charging type using the electroconductive roller(charging roller) as the contact charging member is widely used because of its charging stability, but the discharge-using charging mechanism is dominant in the roller charging.
The charging roller comprises a rubber material or foam member having an electroconductivity or an intermediate resistance. In some charge rollers, the rubber or foamed material is layered to obtain a specific characteristic.
In order to maintain stable contact between a charge roller and a member to be charged, a charge roller is given elasticity, which in turn increases frictional resistance between the charge roller and the photosensitive member. Also in many cases, a charge roller is rotated by the rotation of a photosensitive drum, or is individually driven at a speed slightly different from that of the photosensitive drum. Therefore, occurrence of local non-contact state is unavoidable due to the non-uniformity of the roller shape and the deposited matter on the member to be charged. For this reason, discharge-using charging mechanism is dominant in the conventional charging mechanism using the roller charging. More particularly, when a charging roller is press-contacted to an OPC photosensitive member having a thickness of 25 &mgr;m (member to be charged) to effect charging thereof, the charging roller is supplied with a voltage of not less than approx. 640 V, by which the surface potential of the photosensitive member starts to rise, and thereafter, with increase of the applied voltage, the surface potential of the photosensitive member rises linearly with inclination
1
. Such a starting threshold voltage is called charge starting voltage Vth (broken line in FIG.
11
).
In other words, in order to charge the surface of a photosensitive member to a potential level of V
d
which is necessary for electrophotography, a DC voltage of (V
d
+V
th
), which is higher than the voltage level to which the photosensitive member is to be charged, is necessary. Hereinafter, the above described charging method in which only DC voltage is applied to a contact type charging member to charge a member will be called “DC charging method”.
As for a counter measure for the above described problem, Japanese Laid-Open Patent Application No. 149, 669/1988 discloses an invention which deals with the above problem to effect more uniform charging of a photosensitive member. According to this invention, a “AC charging method” is employed, in which a compound voltage composed of a DC component equivalent to a desired potential level V
d
, and an AC component with a peak-to-peak Voltage which is twice the threshold voltage V
th
, is applied to a contact type charging member. This is intended to utilize the averaging effect of alternating current. That is, the potential of the image bearing member to be charged is caused to converge to the V
d
, that is, the center of the peaks of the AC voltage, without being affected by external factors such as operational ambience.
However, even in the case of the contact type charging apparatus in the above described invention, the principal charging mechanism is a charging mechanism which uses electrical discharge from a contact type charging member to the image bearing member. Therefore, as already described, the voltage applied to the contact type charging member needs to have a voltage level higher than the surface potential plus the discharge threshold. Thus, ozone is generated, although only in a small amount.
Further, when AC current is used so that member is uniformly charged due to the averaging effect of AC current, the problems related to AC voltage become more conspicuous. For example, more ozone is generated; noises traceable to the vibration of the contact type charging member and the photosensitive drum caused by the electric field of AC voltage increase; the deterioration of the photosensitive member surface caused by electrical discharge increases, which add to the prior problems.
(2) (2) Direct Charge Injection Mechanism
This is a mechanism in which the surface of a member to be charged is charged as electrical charge is directly injected into the member to be charged,
Chigono Yasunori
Hirabayashi Jun
Ishiyama Harumi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Grainger Quana M.
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