Electrophotography – Image formation – Charging
Reexamination Certificate
2000-07-31
2002-04-30
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Charging
Reexamination Certificate
active
06381431
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ARTS
The present invention relates to a charging apparatus for charging an image bearing member used in an image forming apparatus, such as a copying machine or a printer, which employs an electrophotographic system, an electrostatic recording system, or the like. In particular, it relates to a charging apparatus which employs a magnetic brush based charging system.
An image forming apparatus such as a copying machine, a page printer, or the like, which employs an electrophotographic system, an electrostatic recording system, or the like, also employs a charging apparatus for charging (inclusive of removing electrical charge) a latent image bearing member, for example, an electrophotographic photosensitive member, an electrostatically recordable dielectric member, or the like, or objects different from a latent image bearing member. In the past a corona based charging device, which is a noncontact device, has been used as the above described charging apparatus for an image forming apparatus.
In recent years, a contact charging apparatus has been put to practical use, and has been replacing a corona based charging device. This is due to the fact that a contact charging apparatus is low in ozone production and power consumption, compared to a noncontact charging apparatus. There are various contact charging apparatuses; those which employ a roller based charging system, that is, those which employ an electrically conductive roller as a contact charging member, have been preferably used because of their stability in charging performance. In a roller based charging system, an electrically conductive, elastic roller as a contact charging member is directly pressed upon an object (hereinafter, “photosensitive member”) which is to be charged, and the photosensitive member is charged by applying voltage to the roller.
However, even in the case of a contact charging apparatus, its fundamental charging mechanism is such that a photosensitive member is charged through electrical discharge from a contact charging member to the photosensitive member. Therefore, the value of the voltage applied to a contact charging member in order to charge a photosensitive member to a desired surface potential level must be greater than the value of the desired surface potential level. Also, ozone is generated although the amount is small. Further, when AC voltage is included as a component of the voltage applied to a contact charging member in order to improve the uniformity with which a photosensitive member is charged, unprecedented problems occur. For example, the amount by which ozone is generated increases; a contact charging member and a photosensitive member are caused to vibrate by the electrical field generated by the AC voltage, resulting in noises; and the rate at which the surface of a photosensitive member is deteriorated by electrical discharge is accelerated.
Thus, there has been desired a method for charging a photosensitive member by directly injecting electrical charge into the photosensitive member. U.S. Pat. No. 5,606,401 discloses such a charging method in which the peripheral surface of a photosensitive member is provided with a charge injection layer, and electrical charge is injected into this layer by a contact charging member. In this method, in other words, with the provision of a charge injection layer, a photosensitive member is sufficiently charged in a short time even if a contact charging member, the resistance value of which is no less than 1×40
4
&OHgr;, is used as a contact charging member, making it possible to solve the above described problems of a contact charging system, that is, the problems traceable to electrical discharge, at their roots.
More specifically, a magnetic brush type charging member which comprises a magnetic brush portion formed by magnetically confining electrically conductive magnetic particle in the form of a brush is employed. In charging a photosensitive member, this magnetic brush portion is placed in contact with the photosensitive member. A magnetic type charging member is preferably used because it makes it possible to create a larger contact nip between the charging member and a photosensitive member, and also because it can be placed evenly in contact with the peripheral surface of a photosensitive member, in terms of a microscopic level, to prevent the photosensitive member from failing to be properly charged.
The magnetic brush portion of the above described magnetic brush type charging member is formed by magnetically confining particles of magnetic material (hereinafter, they may be referred to as “charge carrier”), on the surface of a magnetic particle bearing member. More specifically, magnetic particles, such as ferrite particles, the resistance value of which is in a range of 1×10
4
-1×10
8
&OHgr;·cm, are magnetically confined in the form of a brush directly on a magnet, or on the peripheral surface of a sleeve in which the magnet is contained. In order to charge a photosensitive member, the magnetic brush type charging member is rotated, with its magnetic brush portion being placed in contact with the photosensitive member, and voltage is applied to the charging member.
One of the factors that affect the charging performance of a magnetic brush type charging apparatus which employs a magnetic brush, such as the one described above, as a contact charging member, is degree of uniformity with which the magnetic brush is placed in contact with the peripheral surface of a photosensitive member. In order to place the magnetic brush portion uniformly across the peripheral surface of a photosensitive member, a magnetic brush type charging apparatus is desired to be equipped with a regulating means for regulating the thickness of the magnetic brush, i.e., the layer of magnetic particles, on a magnetic particle bearing member, on the upstream side in terms of the rotational direction of the magnetic particle bearing member. With the provision of such a means, it is possible to accomplish uniformity in charging a photosensitive member. In other words, the provision of such a means makes it possible to provide a high degree of stability in charging a photosensitive member, and therefore, is preferable.
As for the material for the charge injection layer of a photosensitive member, compound material composed by dispersing microscopic electrically conductive particles in electrically insulative and transparent binder is preferably used. While the magnetic brush portion, to which voltage is being applied, is placed in contact with the charge injection layer, the electrically conductive particles behave as if they were numerous independent floating electrodes. As a result, numerous virtual condensers are formed by the electrically conductive substrate of the photosensitive member and these numerous floating electrodes, i.e., the electrically conductive particles, and electrical charge is taken up by these virtual condensers.
Therefore, the voltage applied to the contact charging member and the surface potential level of the photosensitive member converge to approximately the same value. In other words, the employment of a magnetic brush type charging member makes it possible to realize a low voltage charging method.
A charging method such as the one described above (method for charging an object by directly injecting electrical charge into the object) is called “charge injection”. With the use of a charge injection apparatus (charge injection device), it is possible to realize a cleanerless image forming apparatus, that is, a transfer type image forming apparatus, which does not require a cleaner dedicated for cleaning the toner particles left on the image forming apparatus after image transfer.
b) Cleanerless System.
Next, a cleanerless system will be described. A cleanerless system is a cleaning system employed by a transfer type image forming apparatus which employs the so-called reversal developing system. In the reversal developing system, a photosensitive member is negatively charged,
Kinoshita Masahide
Yamaguchi Seiji
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Royer William J.
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