Electrophotography – Image formation – Charging
Reexamination Certificate
1999-09-01
2001-10-02
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Charging
C361S225000, C399S176000
Reexamination Certificate
active
06298205
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a desirable charging apparatus for an image forming apparatus which employs an electrophotographic system or an electrostatic recording system.
In the past, a corona discharging apparatus (corona type charging device) has been widely used as a charging apparatus for charging an image bearing member (member to be charged), for example, an electrophotographic photosensitive member, an electrostatically recordable dielectric member, or the like, in an image forming apparatus, for example, an electrophotographic apparatus, an electrostatic recording apparatus, or the like, to predetermined polarity and potential level.
A corona discharging apparatus is a noncontact type charging apparatus. It comprises an ion discharging electrode constituted of a piece of wire or the like, and an electrode in the form of a shield which surrounds the ion discharging electrode. The shield electrode is provided with an ion discharging opening directed toward the surface of an object to be charged, but, not in contact with the object. In operation, high voltage is applied to the ion discharging electrode and the shield electrode to generate discharge current (corona shower) to which the surface of the object is exposed to be charged to predetermined polarity and potential level.
In recent years, however, a substantial number of contact type charging apparatuses have been proposed, and some of them have been put to practical use as a charging apparatus for a low to medium speed image forming apparatus because of their advantages over a corona type charging apparatus; for example, they are smaller in the amount of ozone production and power consumption.
A contact type charging apparatus comprises an electrically conductive charging member in the form of, for example, a roller (charge roller), a fur brush, a magnetic brush, or a blade, which is placed in contact with a member to be charged, for example, an image bearing member or the like. In operation, charge bias, or electrical voltage with a predetermined potential level, is applied to the contact charging member, which is placed in contact with a member to be charged, for example, an image bearing member or the like, so that the peripheral surface of the object to be charged is charged to predetermined polarity and electrical potential.
The charging mechanism (charging principle) in a contact type charging apparatus comprises a mixture of two charging mechanisms: (1) a mechanism based on electrical discharge, and (2) a mechanism based on direct injection of electrical charge. Thus, the characteristics of a contact type charging apparatus vary depending on which of the two mechanisms is dominant.
(1) Charging Mechanism Based on Electrical Discharge
This is a charging mechanism which charges the peripheral surface of an object to be charged, with the use of the products generated by the electrical discharge which occurs between a contact type charging member and the object to be charged.
In a charging system based on electrical discharge, there is a threshold value. Thus, in order for an object to be charged to a predetermined potential level, voltage, the potential level of which is greater than the predetermined voltage level, must be applied to a contact type charging member. In addition, an electrical discharge based charging system inherently produces by-products, the amount of which, however, may be drastically small compared to those produced by a corona based charging device. Therefore, even if a contact type charging system is employed, it is impossible to completely avoid the problems caused by active ions such as ozone.
(2) Mechanism Based on Direct Electrical Charge Injection
This is a charging system which directly injects electrical charge into an object from a contact charging member so that the peripheral surface of the object is electrically charged. It is called the direct charging system or the injection charging system. More specifically, a contact type charging member, the electrical resistance of which is in a medium range, is placed in contact with the peripheral surface of an object to be charged, to charge the object without triggering the electrical discharge. In other words, this charging mechanism is a charging mechanism which directly injects electrical charge into the peripheral surface of an object to be charged. Principally, it does not rely on electrical discharge. Therefore, even if the potential level of the voltage applied to a contact type charging member is less than a threshold voltage level the object to be charged can be charged to a potential level substantially equal to the potential level of the applied voltage.
Since this direct injection charging system does not involve ion generation, it does not suffer from the ill effects associated with the by-products of electrical discharge. However, since a contact type charging system is a direct system, its performance is greatly affected by the state of contact between a contact type charging member and an object to be charged. Thus, it is very important that a contact type charging member is high in density, that there is provided a sufficient amount of difference in surface velocity between the charging member and the object charged, and that the contact type charging member makes contact with the object to be charged, with a sufficiently high frequency.
Next, the characteristics of the charging mechanism in each of various contact type charging members will be described.
A) Charging by Roller
A contact type charging apparatus which employs a roller type charging method, in other words, it employs an electrically conductive roller (charge roller) as a contact type charging member. It has been widely used because of its safety.
In the case of a charging roller, the charging mechanism based on electrical discharge (1) is the dominant charging mechanism.
A charge roller is formed of rubber or foamed material which is electrically conductive, or the electrical resistance of which is in the medium range. Sometimes, different materials are layered in order to obtain a predetermined characteristic.
A charge roller is provided with elasticity so that a predetermined state of contact can be kept between the charge roller and an object to be charged (hereinafter, photosensitive member). Therefore, a charger roller is given a large frictional resistance on its peripheral surface. Generally, it is enabled to follow the rotation of a photosensitive member, or is driven at a speed slightly different from that of the photosensitive member. Thus, when a charge roller is used to directly inject electrical charge into a photosensitive member, it cannot be avoided that the charge roller is deteriorated in its absolute performance and/or the state of contact between itself and the photosensitive drum by the contaminants adhered to the charge roller and/or the photosensitive member, and as a result, the photosensitive member is nonuniformly charged, in spite of the fact that a charge roller is a contact type charging member. In other words, in the case of a conventional charging roller, the charging mechanism based on electrical discharge is dominant in charging the photosensitive member.
FIG. 11
is a graph which shows the efficiencies of various contact type charging members. The abscissas represents the potential level of the bias applied to a contact type charging member, and the ordinate represents the correspondent potential level of a photosensitive member. The characteristic of a charge roller is depicted by a line A. In other words, the charging of the photosensitive drum begins when the potential level of the voltage applied to the charge roller passes the threshold value of approximately −500 V. Therefore, generally, in order to charge a photosensitive drum to a potential level of −500 V, either a DC voltage of −1,000 V is applied to the charge roller, or an AC voltage with a peak-to-peak voltage of 1,200 V is applied to the charge roller, in addition to a DC voltage of −500
Chigono Yasunori
Hirabayashi Jun
Ishiyama Harumi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pendegrass Joan
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