Electrophotography – Supplemental electrophotographic process – Exposure or charging
Reexamination Certificate
2001-09-07
2003-04-22
Chen, Sophia S. (Department: 2852)
Electrophotography
Supplemental electrophotographic process
Exposure or charging
Reexamination Certificate
active
06553196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, and more particularly, to an image forming apparatus that irradiates light on a photosensitive member in order to remove charge from it.
2. Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotographic method is an apparatus employing a method and having a configuration in which a first image bearing member is caused to form and bear a transferable image by an appropriate image forming process; the transferable image is transferred from the first image bearing member to a second image bearing member; and the first image bearing member is repeatedly used for successive image formation.
For example, an image forming apparatus such as a copying machine and a laser beam printer is basically provided with an electrophotographic photosensitive member, which is generally a rotary drum, as the first image bearing member and charging means for uniformly charging the surface of the rotating photosensitive member to a predetermined polarity and potential.
Moreover, the image forming apparatus is provided with image exposing means for forming an electrostatic latent image on a charge processing surface of the rotating photosensitive member, developing means for developing the electrostatic latent image as a toner image and transferring means for transferring the toner image from the surface of the photosensitive member to a transfer paper as the second image bearing member (recording material).
Moreover, the image forming apparatus is provided with fixing means for fixing the toner image transferred to the transfer paper as a permanently fixed image and photosensitive member cleaning means (cleaner) for eliminating transferred residual toner on the surface of the rotating photosensitive member after the toner image has been transferred to the transfer paper so as to clean the surface of the photosensitive member.
The transfer paper to which image fixing processing is applied by the fixing means is discharged as an image formed product (copy or print).
The surface of the photosensitive member cleaned by the cleaning means is served for image formation repeatedly.
As a photosensitive member to be used for a conventional image forming apparatus of the electrophotographic method or the electrostatic recording method, a selenium series photosensitive member, an amorphous silicon (hereinafter referred to as a-Si) photosensitive member, an organic photosensitive member and the like are put to practical use. It is known that, among the photosensitive members, the a-Si photosensitive member is particularly excellent in terms of stability and durability.
The a-Si photosensitive member tends to absorb moisture because the surface of the photosensitive member becomes sensitive to humidity due to influence of a corona product caused by ozone that is generated from a back charger having a surface of high hardness This becomes a cause of drift of charge on the surface of the photosensitive member and brings about degradation of an image quality called a smeared image.
In order to prevent such a smeared image, various methods are used such as a method of heating with a heater described in Japanese Utility Model Examined Publication No. Hei 1-34205, a method of eliminating a corona product by rubbing a surface of a photosensitive drum by a brush formed of a magnet roller and magnetic toner described in Japanese Patent Examined Publication No. Hei 2-38956, and a method of eliminating a corona product by rubbing a surface of a photosensitive member by an elastic roller as described in Japanese Patent Application Laid-open No. Sho 61-100780.
On the other hand, contact charging is being put to practical use instead of corona charging as a charging method.
Contact charging is a method for charging the photosensitive member as a member to be charged to a predetermined polarity and potential of the surface of the photosensitive member by applying a predetermined charging bias to a conductive charging member such as a roller (charging roller), a fur brush, a magnetic brush or a blade and the like, and has advantages in that less ozone is generated and electric power consumption is small compared with the corona charging device.
Two types of charging methods, a corona charging method and a contact injection charging method, are mixed in a charging mechanism (charging principle) of the contact charging type, and respective properties emerge depending on which one is dominant.
The corona charging method uses a discharge phenomenon, such as corona discharge that occurs in a micro space between a contact charging member and a member to be charged, to charge the member to be charged by a discharge product. The corona charging method still generates a small amount of ozone, although it is markedly less than that in the case of the corona charger.
The contact injection charging method is a method in which charge is directly injected in a member to be charged from a contact charging member, whereby the surface of the member to be charged is charged.
This is also referred to as direct charging or injection charging. Japanese Patent Application Laid-open No. Hei 6-3921 and the like proposes a method of performing contact injection charging by injecting charge into a trap level existing on a surface of a photosensitive member or charge holding member, such as conductive particles and the like of a charge injection layer, using a contact charging member such as a charging roller, a fur brush or a magnetic brush and the like, in which conductive magnetic particles are magnetically constrained. Since contact injection charging does not use the discharge phenomenon, only a portion where the member to be charged and the charging member contact each other is charged.
Therefore, it is desirable to make peripheral speed differences or moving directions opposite each other in the charging member and the member to be charged so as to have a sufficiently high possibility of contact between the member to be charged and the charging member in order to perform fine charging without charging unevenly.
In addition, contact injection charging is a low power and ozoneless charging method in which there is no threshold voltage for starting charging and a voltage required for charging is only for a desired surface potential of a photosensitive member.
As a member to be charged for which the contact injection charging can be used, it is necessary to provide a charging injection layer in which conductive particulates as a charge holding member are dispersed on a surface of a photosensitive layer, for example, in the case of an organic photosensitive member. However, in the case of an inorganic photosensitive member represented by an a-Si photosensitive member, many trap levels based on a defect of a crystal exist on its surface even if a charge injection layer is not specifically provided, whereby injected charge is held by the trap levels and the injection charging can be applied to it.
Since contact injection charging does not generate a discharge product at all, if an a-Si photosensitive member is combined with the contact injection charging, there is a large advantage in that a smeared image that is a drawback of the a-Si photosensitive member can be fundamentally solved.
Therefore, it is unnecessary to provide means for always heating the a-Si photosensitive member with a heater or rubbing the surface of the photosensitive member with magnetic particles or an elastic member, whereby saving of power consumption and simplification of an apparatus can be realized.
However, the a-Si photosensitive member has such a characteristic that, if a light irradiated region and a dark region are simultaneously charged, attenuation (dark attenuation) of a potential is extremely large in the light irradiated region compared with the dark region, and a light memory (after image phenomenon) tends to occur.
That is, an a-Si series photosensitive member has many d
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