Charging apparatus employing charging particles, and image...

Electrophotography – Image formation – Charging

Reexamination Certificate

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Details

C361S225000, C399S176000

Reexamination Certificate

active

06549742

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging apparatus for charging an object. More precisely, it relates to a contact type charging apparatus (contact charging apparatus) which charges the surface of an object by placing a charging member in contact with the object.
The present invention also relates to an image recording apparatus (image forming apparatus), such as a copying machine, a printer, or the like, which employs a charging apparatus of the above described type as a means for charging an image bearing member of the image recording apparatus.
In a conventional contact charging apparatus, the surface of an object to be charged, for example, the surface of an image bearing member of an image forming apparatus, is charged to predetermined polarity and potential level by applying a predetermined charge bias to an electrically conductive member (contact type charging member, contact charging device) in the form of a roller (charge roller), a fur brush, a magnetic brush, a blade, or the like, while keeping the electrically conductive member in contact with the object to be charged.
All charging apparatuses which match the above description regarding a contact charging apparatus are roughly categorized as a contact charging apparatus. However, there are large differences in terms of charging mechanism (or charging principle) among the contact charging apparatuses. There are two types of contact charging mechanisms: (1) the charging mechanism based on electrical discharge, and (2) the charging mechanism based on direct charge injection. Which of the two types of charging mechanism is employed by a contact charging apparatus determines the characteristics of the contact charging apparatus. Thus, the charging principles and characteristics of the electrical discharge type charging mechanism and direction injection type charging mechanism will be described next.
(1) Charging Mechanism Based on Electrical Discharge
This is a charging mechanism which charges an object with the use of the products generated by the electrical discharge which occurs through the gap between a contact type charging member and the object to be charged.
An electrical discharge type charging system is characterized by an electrical discharge threshold value: there must be a certain amount of voltage difference between a contact type charging member and an object to be charged. More specifically, referring to
FIG. 7
, a voltage, the level of which is higher than the potential level of the object to be charged, needs to be applied to the contact type charging member, as is represented by Line A in FIG.
7
. Further, in principle, it generates by-products, admitting that the amount of the by-products is drastically smaller than that generated by a corona type charging apparatus.
From the standpoint of safety regarding electrical discharge, a roller based charging method (roller charging apparatus), which employs an electrically conductive roller (charge roller) as an electrical discharge based contact type charging member is preferable and is widely in use. An electrical discharge type charge roller comprises a base layer and a surface layer. The base layer is in the form of a roller and is formed of rubber or foamed substance, which is electrically conductive, or the electrical resistance of which is in the medium range. The surface layer is high in electrical resistance and covers the base layer. Electrical discharge occurs through the gaps between the roller and the object to be charged, immediately adjacent to the contact (interface) between the roller and the object, which are several tens of micrometers wide. Therefore, in order to stabilize the electrical discharge, the outward surface of the surface layer is rendered very smooth; its roughness in terms of Ra is less than one micrometer. Further, it is rendered high in hardness.
In order to charge an object with the use of an electrical discharge type roller, the voltage to be applied to the roller must be relatively high. Therefore, if there is a pinhole (exposure of the base layer of the object to be charged, caused by the damage to the surface layer of the object) in the surface of the object to be charged, not only does voltage drop occurs at the point of the pinhole, but also in the immediate areas of the pinhole, causing the object to be charged to be improperly charged. Thus, in order to prevent the voltage drop, the surface resistance of the surface layer is made to be no less than 10
11
ohm□.
(2) Direct Injection on Type Charging Mechanism
The direct injection type charging mechanism is a charging mechanism for charging the surface of an object to be charged, by directly injecting electrical charge into the object to be charged, from a contact type charging member, through the molecular level contact between the contact type charging member and the object to be charged. It is sometimes called a direct charging mechanism or an injection charging mechanism.
In this charging mechanism, that is, a direct injection type charging mechanism, the difference in potential level between a contact type charging member and an object to be charged is no more than several volts to several tens of volts. Line B in
FIG. 7
represents the charging performance characteristic of a direct injection type charging mechanism (magnetic brush type charging apparatus). In this case, the potential level of the voltage applied to a charging member is equal to the potential level to which an object will be charged. Therefore, there is no difference in potential level between the charging member and the object to be charged, which causes electrical discharge. Also in this case, the voltage necessary for charging an object can be kept low in potential level.
As described above, a direct charging system as a charging mechanism is not accompanied by ion production, and therefore, it does not cause problems related to the byproducts of electrical discharge. In other words, a direct charging system is a superior charging system in terms of environmental safety, component deterioration, and power consumption.
Next, a charging apparatus employing a direct injection type charging mechanism will be described.
In a direct injection type charging mechanism, one of the essential factors which determine the charging performance of a direct injection type charging mechanism is the state of contact between a contact type charging member and an object to be charged. This state of contact means the number of contacts, in microscopic terms, the contact type charging member makes contact with the object to be charged, while the object is passing through a charging apparatus. Thus, it is required that not only does the contact type charging member have an extremely fine surface structure, but also it must have a sufficient amount of elasticity for keeping its surface in contact with the surface of the object.
As for the configuration of a contact type charging member employed by a direct injection type charging apparatus, an electrical discharge based charge roller or the like has been widely tried. However, the attempts to use an electrical discharge based charge roller for direct charge injection have been unsuccessful. This is due to the following fact. That is, in the case of a charge roller which has a hard and smooth surface, its surface seems to be perfectly in contact with the surface of an object to be charged, but, at a molecular level, that is, at a microscopic level, there is virtually no contact between the surfaces of the charge roller and the object to be charged.
As a presently proposed direct injection type charging mechanism, there is a particle based charging method which employs a magnetic brush.
(3) Particle Based Charging Method
From the standpoint of improvement in contact density, a charging method which employs electrically conductive particles (particle based charging method) is advantageous. The electrically conductive particle used for a particle based charging method hereinafter will be called “charging particle”. A

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