Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-04-13
2003-04-29
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S132000, C347S140000, C399S051000, C399S232000
Reexamination Certificate
active
06556229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophotographic apparatus (recording apparatus of an electrophotographic method) which is used in a printer, a facsimile apparatus, a copier, or the like, and more particularly to an electrophotographic apparatus of a potential dividing development method which is a development method wherein an image is developed by using colored particles such as a toner, and in which, as in the case of two-color printing, electrostatic latent images are formed with dividing the surface potential of a photosensitive member into plural levels (so-called potential division) and the electrostatic latent images are developed by toners of different colors that are oppositely charged.
2. Description of the Related Art
A conventional development method and a potential dividing development method which are used in a conventional electrophotographic apparatus will be described.
A recording apparatus using the electrophotographic method implements the steps of: forming an electrostatic latent image on a recording member (photosensitive member); attaching colored particles to the electrostatic latent image in the surface of the recording member to develop the latent image as an image; transferring the developed colored particle image to another recording member; and fixing the transferred colored particle image. As the colored particles, powder dedicated to electrophotography and called a toner is used.
In the developing step, first, toner particles serving as colored particles are charged by using a developer in the following manner. The developer is a mixed powder of a toner and carrier beads which are magnetic particles. The developer is loaded into a developing unit, and then stirred therein to be charged by friction with the carrier beads.
In the developing process, the whole surface of the photosensitive member is once charged, and the photosensitive member is then irradiated (exposed) with light to cause partial discharging in accordance with image data. As a result, a potential contrast caused by a charged region and a discharged region is formed in the surface of the photosensitive member. The potential contrast is called an electrostatic latent image.
The image formation in the developing step is performed by moving the electrostatic latent image on the surface of the photosensitive member to a position (developing position) which is opposed to a magnet roller called a developing roller and transporting the developer. In the image forming process, a method called a bias development is usually used.
In the bias development, a bias voltage is applied to the developing roller to produce an electric field between the latent image potential formed in the surface of the photosensitive member and the developing roller, and the charged toner particles are separated from the developer on the surface of the developing roller to be moved to the surface of the photosensitive member by the function of the electric field, thereby forming an image. The difference between the bias voltage of the developing roller and the potential of the image forming portion of the photosensitive member is called the development potential difference.
It is a matter of course that, when the development potential difference is large, the formed electric field (called the development electric field) is enhanced and hence the developing performance is improved. Also when a method in which the distance between the developing roller and the photosensitive member is reduced, or that in which the electric resistance of the developer is lowered is employed, the same effect of enhancing the development electric field is attained so as to improve the developing performance.
As a modification of the above-described electrophotography bias development method, a development method has been proposed in, for example, JP-A-48-37148. In the proposed method, the potential of a charged region and a discharged region is divided by two, an intermediate potential region is disposed between the regions, a first developing unit which develops the discharged region (this development is called reversal development) is disposed to perform development using a first toner, and a second developing unit which develops the charged region (this development is called normal development) is disposed to perform development using a second toner, whereby toner developments of two kinds are conducted by one charging step and one light-irradiating step (exposing step).
In this development method, the toner is not attached to the intermediate potential region (referred to as the intermediate potential) on the photosensitive member and having a voltage which is between the bias voltage of the reversal developing unit (first developing unit) and that of the normal developing unit (second developing unit), to form the background portion as an image. Therefore, it is possible to form an image of two kinds of toners and composed of the background portion, a first image portion, and a second image portion. In the specification, this development method is referred to as the potential dividing development method.
In the potential dividing development method, generally, the two kinds of toners are configured by toners of different colors, respectively, and used for obtaining an image composed of two colors. Alternatively, the first development may be performed as the normal development, and the second as the reversal development. Also in the alternative, the potential dividing development is enabled.
In the bias development, when an electrostatic latent image is formed on a photosensitive member, an electric field emphasizing the development in end portions of the potentials (potential edges) of the latent image is produced, and at the same time an electric field of a polarity opposite to that of the latent image (reverse electric field) is produced in the periphery of the latent image. This reverse electric field is produced because the development is performed by an electric field which is formed by spatially differentiating the potential distribution, and hence a field emphasized region and an electric field the polarity of which is opposite to that of the emphasized region are produced in potential edges (the boundary between the potential of the charged or discharged region serving as the latent image region and the intermediate potential) of the image regions where the development potential is changed.
In the bias development in which the potential dividing development method is not used and only one kind of toner is used, the reverse electric field does not present a problem. By contrast, in the electric potential dividing development process in which positive and negative electrostatic latent images are formed on the photosensitive member, and development is performed by using two kinds of toners (hereinafter, description will be made with taking red toner and black toner as an example) that are charged in polarities respectively opposite to the polarities of the latent images, fringe development occurs in which the red toner is attached to the periphery of a black image and the black toner is attached to that of a red image by attraction due to the reverse electric fields.
The fringe development appears as erroneous printing in which toner is attached to an unexpected position. In order to solve this problem, a method may be employed in which an electric potential dividing development method without fringe development is realized by introducing an exposure control (hereinafter, often referred to as fringe control) wherein an expected position of occurrence of fringe development is predicted and auxiliary exposure is applied to the expected position to suppress a reverse electric field. In the auxiliary exposure, exposure is performed while controlling the auxiliary exposure amount so as to stepwise relax the development potential difference (the difference between the potential of the charged or discharged region and the intermediate potential) adjacent to the development
Akatsu Shinichi
Hoshi Nobuyoshi
Kobayashi Shin'ya
Mitsuya Teruaki
Okada Hisao
Hitachi Koki Co,. Ltd.
Pendegrass Joan
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