Electrophotography – Image formation – Charging
Reexamination Certificate
1999-08-30
2002-05-14
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Charging
C399S174000
Reexamination Certificate
active
06389254
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a charging member and a charging device suitably used in an image forming apparatus of the electrophotographic type to charge a member to be charged such as a photosensitive member.
2. Related Background Art
In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a corona charger (corona discharger) has heretofore been often used as a charging device for uniformly charging (and removing charges) an image bearing member (a member to be charged) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member to a required polarity and potential.
The corona charger is a noncontact type charging device and is provided, for example, with a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode, and a discharge opening portion is disposed in an opposed noncontact relationship with the image bearing member which is a member to be charged, and a high voltage is applied to the discharge electrode and the shield electrode to thereby create a discharge current (corona shower), and the surface of the image bearing member is exposed to the discharge current to thereby charge the surface of the image bearing member to a predetermined polarity and potential.
[Contact Charging]
Recently, a charging device of the contact type (contact charging device) for charging a member to be charged by the charging member to which a voltage has been applied as previously described being brought into contact with the member to be charged has been put into practical use owing to the advantages of low ozone, low electric power, etc. as compared with the corona charger.
The contact charging device is such that an electrically conductive charging member of the roller type (charging roller), the fur brush type, the magnetic brush type, the blade type or the like is brought into contact with a member to be charged such as an image bearing member and a predetermined charging bias is applied to this charging member (a contact charging member or a contact charger, and hereinafter referred to as the contact charging member) to thereby charge the surface of the member to be charged to a predetermined polarity and potential.
The charging mechanism of contact charging (the mechanism of charging or the principle of charging) mixedly includes two kinds of charging mechanisms, i.e., (1) a discharge charging mechanism and (2) a direct injection charging mechanism, and depending on which mechanism is dominant, each characteristic presents itself.
(1) Discharge Charging Mechanism
This is a mechanism in which the surface of the member to be charged is charged by a discharge phenomenon occurring in the minute gap between the contact charging member and the member to be charged.
The discharge charging mechanism has a constant discharge threshold value for the contact charging member and the member to be charged and therefore, it is necessary to apply a voltage greater than the charging potential to the contact charging member. Also, as compared with a corona charger, it is unavoidable in principle for a discharge product to be produced though the quantity thereof produced is markedly small, and therefore the ill effects caused by active ions such as ozone is unavoidable.
For example, the charging system using an electrically conductive roller (charging roller) as the contact charging member is preferable in respect of the stability of charging and is widely used, but in this roller charging, the discharge charging mechanism is dominant as the charging mechanism thereof.
That is, the charging roller is produced by the use of an electrically conductive or medium-resistance rubber material or a foamed material. Further, there is also a charging roller of a laminated construction to thereby obtain a desired characteristic. The charging roller is given elasticity in order to obtain constant contact with the member to be charged, but therefore it is great in frictional resistance and in many cases, it is driven following the member to be charged or with some difference from the latter. Accordingly, a noncontact state is unavoidable due to the irregularity of the shape of the roller or the material adhering to the member to be charged and therefore, in the conventional roller charging, the discharge charging mechanism becomes dominant as the charging mechanism thereof.
More specifically, when a charging roller is pressed and made to abut against an OPC photosensitive member having a thickness of 25 &mgr;m as a member to be charged and a charging process is carried out, the surface potential of the photosensitive member begins to rise if a voltage of about 640 V or greater is applied to the charging roller, whereafter the surface potential of the photosensitive member linearly increases with an inclination 1 to the applied voltage. Hereinafter, this threshold value voltage is defined as a charging starting voltage Vth.
That is, to obtain the surface potential Vd of the photosensitive member required for electrophotography, a DC voltage of Vd+Vth greater than required becomes necessary for the charging roller. A system of applying only a DC voltage to the contact charging member in this manner to thereby effect the charging of an image bearing member is referred to as the “DC charging system”.
In the DC charging system, however, the resistance of the contact charging member is fluctuated by the fluctuation of the environment or the like and Vth is fluctuated if the film thickness is changed by the photosensitive member as the image bearing member being shaved and therefore, it has been difficult to render the potential of the photosensitive member into a desired value.
Therefore, in order to achieve the further uniformization of charging, as disclosed in U.S. Pat. No. 4,851,960, use is made of an “AC charging system” of applying to a contact charging member a vibration voltage comprising an AC component having a peak-to-peak voltage of 2×Vth or greater superposed on a DC voltage corresponding to desired Vd to thereby effect the charging of an image bearing member. This is directed to the level effect of potential by AC, and the potential of the image bearing member is converged to Vd which is the center of the peak of the AC voltage, and is not affected by the disturbance of the environment or the like.
(2) Direct Injection Charging Mechanism
This is a mechanism in which charges are directly injected from a contact charging member into a member to be charged to thereby charge the surface of the member to be charged. It is proposed in U.S. Pat. No. 5,809,379, etc.
A contact charging member of medium-resistance contacts with the surface of the member to be charged to thereby effect the direct injection of charges into the surface of the member to be charged without the intermediary of a discharge phenomenon, i.e., basically without using a discharging mechanism. Consequently, even if the applied voltage to the contact charging member is equal to or less than a discharge threshold value, the member to be charged can be charged to the potential corresponding to the applied voltage. This direct injection charging mechanism is not accompanied by the production of ions and therefore does not give rise to the ill effects caused by the production of.
More specifically, this is a mechanism in which a voltage is applied to a contact charging member such as a charging roller, a charging brush or a charging magnetic brush and charges are injected into a charge holding member for a trap order or electrically conductive particles or the like of a charge injection layer lying on the surface of a member to be charged (an image bearing member) to thereby effect direct injection charging. Since the discharge phenomenon is not dominant, the voltage required for charging is only on the desired surface of the image bearing member and there is no production of ozone.
FIG. 5
of the accompanying drawings shows an example
Chigono Yasunori
Hirabayashi Jun
Ishiyama Harumi
Brase Sandra
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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