Electrophotography – Image formation – Charging
Reexamination Certificate
1998-09-11
2001-05-15
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Charging
C399S174000
Reexamination Certificate
active
06233419
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging device for electrically charging a member to be charged. More particularly, it relates to a charging device for charging a surface of the member to be charted by a flexible charging member which cooperates with the member to be charged to form a nip therebetween.
Furthermore, it relates to an image recording device (image forming apparatus) such as a copying machine, a printer or the like using such a charging device as charging means for an image bearing member.
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 disposes so that corona discharging opening thereof faces an image bearing member, that is, a member to be charted. 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 is applied between the corona discharging electrode and the shield electrode.
Recently, it has been proposed to employ a contact type charging apparatus as a charging apparatus for charging the image bearing member, that is, the member to be charged, in an image forming apparatus of low to medium speed. This is due to the fact that 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 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 eh 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 (charger 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) corona charging mechanism and (2) direct charging mechanism, both come into action. Thus, the characteristics of each of contact type charging apparatuses or methods are determined depending on which charging mechanism is the dominant.
(1) Electrical discharge based charging type or 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 through the amount thereof is remarkably small.
(2) direct charging system (direction injection charging mechanism)
This is a mechanim in which the surface of a member to be charged is charged by electrical charge directly injecting into the member 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 a member to be charged to directly inject electrical charge into the surface portion of the member 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 a contact type charging member is below the discharge starting voltage value, the member 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 a member to be charged greatly affects the manner in which the member is charged, since this charging mechanism is such a mechanism that directly charges a member. 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 given point on the surface of the object to be charged makes contact with a larger are of the charging member.
A) Charging apparatus with charger 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.
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 an object to be charged (hereinafter, “photosensitive member”), 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. 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. With conventional charging roller, the dominant charging mechanism through which a roller charging member charged a member to be charged was a corona charging mechanism.
FIG. 4
is a graph which shows an example of efficiency in contact type charging. In the graph, the abscissas represents the bias applied to a contact type charging member, and the axis of ordinate represents the potential levels correspondent to the voltage values of the bias applied to the contact type charging member.
The characteristics of the conventional charging by a roller are represented by a line designated by a character A. According to this line, when a charge roller is u
Chigono Yasunori
Hirabayashi Jun
Ishiyama Harumi
Brase Sandra
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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