Electrophotography – Image formation – Charging
Reexamination Certificate
2000-08-07
2003-06-17
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Charging
C399S176000
Reexamination Certificate
active
06580889
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging apparatus and a charging method, which use electrically conductive particles to charge an object, such as an image bearing member. It also relates to a process cartridge and an image forming apparatus, which are compatible with such a charging apparatus and a charging method.
Prior to the present invention, a corona type charger (corona discharging device) has been widely used as a charging apparatus for charging (inclusive of discharging) an image bearing member (object 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 (copying machine, printer, or the like) or an electrostatic recording apparatus.
The corona-type charging device is a noncontact-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 the corona discharging opening thereof faces an image bearing member, that is, an object 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 high voltage which is applied between the corona. discharging electrode and the shield electrode.
In recent years, it has been proposed to employ a contact-type charging apparatus as a charging apparatus for charging the image bearing member, that is, the object to be charged, in an image forming apparatus of low to medium speed. This is due to the fact that a contact-type charging apparatus has an advantage over a corona-type charging apparatus in terms of low ozone production, low power consumption, or the like. Also, such a contact-type charging apparatus has been put to practical use.
In order to charge an object, 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 object to be charged, and an electrical bias (charge bias) of a predetermined level is applied to this contact-type charging member so that the surface of the object 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 member (charge roller), a fur-brush-type member, a magnetic-brush type member, a blade-type member, and the like.
In reality, when an object 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 contact-type charging apparatuses or methods are determined by the charging mechanism which is the dominant one of the two in charging the object.
(1) Electrical Discharge Based Charging Mechanism
This charging mechanism is a charging mechanism in which the surface of an object to be charged is charged by electrical discharge, which occurs across a microscopic gap between a contact-type charging member and the object 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 an object to be charged, and therefore, in order for an object 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 object is to be charged. Thus, in principle, when the electrical discharge based charging mechanism is in action, it is impossible to avoid generating by-products of electrical discharge, that is, active ions, such as ozone ions. In reality, even a contact-type charging apparatus charges an object partially through the electrical-charge discharging mechanism as described above, and therefore, a contact-type charging apparatus cannot completely eliminate the problems caused by the active ions such as ionized ozone.
(2) Direct-charge Injection Mechanism
This is a mechanism in which the surface of an object to be charged is charged as electrical charge is directly injected into the object to be charged, with the use of a contact-type charging member. Thus, this mechanism is called “direct charging mechanism”, or “charge injection mechanism”. More specifically, a contact-type charging member with medium electrical resistance is placed in contact with the surface of an object to be charged to directly inject electrical charge into the surface portion of an object to be charged, without relying on electrical. discharge, in other words, without using electrical discharge in principle. Therefore, even if the value of the voltage applied to contact-type charging member is below the discharge starting voltage value, the object to be charged can be charged to a voltage level which is substantially the same as the level of the voltage applied to the contact-type charging member.
This direct injection charging mechanism does not suffer from the problems caused by the by-product of electrical discharge since it is not accompanied by ozone production. However, in the case of this charging mechanism, the state of the contact between a contact-type charging member and an object to be charged greatly affects the manner in which the object is charged, since this charging mechanism is such a mechanism that directly charges an object. Thus, this direct injection charging mechanism should comprise a contact-type charging member composed of high density material, and also should be given a structure which provides a large speed difference between the charging member and the object to be charged, so that a given point on the surface of the object to be charged makes contact with a larger area of the charging member.
A) Charging Apparatus with Charge Roller
In the case of a contact-type charging apparatus, a roller-charge system, that is, a charging system which employs an electrically conductive roller (charge roller) as a contact-type charging member, is widely used because of its desirability in terms of safety.
As for the charging mechanism in this roller charge system, the aforementioned (1) charging mechanism, which discharges electrical charge, is dominant.
Charge rollers are formed of rubber or foamed material with substantial electrical conductivity, or electrical resistance of a medium level. 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 an object to be charged (hereinafter, “photosensitive member”), a charge roller is given elasticity, which in turn increases the 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. As a result, problems occur: absolute charging performance declines, the state of the contact between the charge roller and the photosensitive drum becomes less desirable, and foreign matter adheres to the charge roller and/or the photosensitive member. Prior to the present invention, the dominant charging mechanism through which a roller charging member charged an object was a charging mechanism which discharged electrical charge, and therefore, even with the use of a contact-type charging apparatus, it was impossible to completely prevent the nonuniform charging of the photosensitive member.
FIG. 5
is a graph which shows
Chigono Yasunori
Hirabayashi Jun
Ishiyama Harumi
Nagase Yukio
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pendegrass Joan
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