Electrophotography – Control of electrophotography process
Reexamination Certificate
2002-12-27
2004-07-06
Ngo, Hoang (Department: 2852)
Electrophotography
Control of electrophotography process
C399S049000, C399S055000
Reexamination Certificate
active
06760553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cluster printing system for performing printing by using a plurality of electrophotographic recording apparatuses such as printers, facsimile machines or copying machines each capable of manifesting an image by using colored particles such as toner. Particularly, it relates to an image quality control method in an imaging and fixing process having electrification, exposure, development, transfer and fixation for forming a toner image on surfaces of a photoconductor and a sheet of recording paper, recording apparatuses using the image quality control method, and a method for operating the recording apparatuses.
2. Background Art
A conventional recording apparatus using electrophotography has an imaging process for manifesting an image of colored particles on a surface of a recording medium, and a fixing process for fixing the manifested image of colored particles on the recording medium. In this specification, a combination of the imaging process and the fixing process is referred to as imaging engine. Powder called “toner” exclusively used for electrophotography is used as the colored particles. In an electrifying step, the whole surface of a photoconductor is once electrically charged. Then, in an exposure step, the photoconductor irradiated with light is partially electrically discharged. On this occasion, potential contrast between a charged region and a discharged region is formed in the surface of the photoconductor. The potential contrast is referred to as “electrostatic latent image”.
In the next developing step, first, toner particles which are colored particles are electrically charged. As methods for electrically charging toner, there are a two-component developing method using carrier beads and a one-component developing method for electrically charging toner on the basis of friction between the toner and a member or the like. On the other hand, a method called “bias development” is often used as a method for manifesting the electrostatic latent image.
In the bias development, a bias voltage is applied to a developing roller so that electrostatically charged toner particles are separated from a developing agent on a surface of the developing roller and moved to the surface of the photoconductor by the action of electric field generated between latent image potential generated on a surface of a photoconductor and the potential of the developing roller to thereby form an image. Either the electrostatic charge potential or discharge potential may be used as the latent image potential, that is, as the potential of the image-forming portion of the photoconductor. Generally, the method using electrostatic charge potential as the latent image potential is referred to as “normal developing method” whereas the method using discharge potential as the latent image potential is referred to as “reversal developing method”.
Potential which is either of the electrostatic charge potential and the discharge potential but is not used as the latent image potential is referred to as “background potential”. The bias voltage of the developing roller is set to have potential middle between the electrostatic charge potential and the discharge potential. Similarly, the difference between the middle potential (bias voltage) and the latent image potential is referred to as “developing potential difference”. The difference between the middle potential (bias voltage) and the background potential is referred to as “background potential difference”. Generally, the developing potential difference having an influence on developing performance itself is set to be larger than the background potential difference. It is a matter of course that if the developing potential difference is large, developing performance becomes high because generated electric field (referred to as “developing electric field”) becomes intensive.
On the other hand, the background potential difference has an influence on the image quality of a background portion of an image. If the background potential difference is small, fogging of the background portion increases. If the background potential difference is too large, a rear end portion of the image in a direction of rotation of the developing roller is apt to be chipped. The direction of relative movement of the developing roller and the direction of relative movement of the photoconductor may be equal to each other or may be different from each other.
A plurality of developing rollers may be used in one developing device. A developing device having a plurality of developing rollers rotating in one direction may be provided or a developing device having a plurality of developing rollers rotating indifferent directions may be provided. In this case, there is also known a developing device in which the directions of rotation of adjacent developing rollers are made different to move the two developing rollers from their opposite positions toward the photoconductor so that the developing agent is carried toward the photoconductor while branching from the opposite positions of the developing rollers as if the developing agent was a fountain. The developing device is referred to as “fountain type developing device”. The formation of an electrostatic latent image and a toner image on a surface of the photoconductor has been described above.
Next, variation of the electrostatic latent image on the surface of the photoconductor with time will be described. When the photoconductor deteriorates as printing increases in quantity, the potential of an electrostatic charge region (charge potential) is so lowered that the electrostatic charge region can be hardly charged while the potential of a discharge region (discharge potential) is so heighten that the discharge region can be hardly discharged. The lowering of the discharging capacity is significant in the case where an intermediate potential region is provided so that the intermediate potential region is not perfectly discharged because a sufficient quantity of light is not given at exposure.
The intermediate potential region described here is often used for preventing thickening of an image region such as a thin line region or a halftone dot region in which the edge effect of electric field is so intensive that toner is developed excessively. The variation in potential operates to reduce developing electric field because it reduces the developing potential difference. On the other hand, in addition to this characteristic, the thickness of the photosensitive layer of the photoconductor is reduced by abrasion as printing increases in quantity. The reduction of the film thickness operates to increase the developing electric field. Which of the two antithetical tendencies is predominant varies in accordance with the printing apparatus.
That is, though image quality varies in accordance with variation with time in developing capacity, how the image quality varies depends on the printing apparatus. Reduction of variation in the developing electric field is required for keeping image quality constant with time. For this reason, it is necessary to consider variation in potential and electric field on the surface of the photoconductor.
There is known a method in which the potential on the surface of the photoconductor is detected by a potential sensor and the film thickness of the photoconductor is detected by some method to control the potential on the surface of the photoconductor to keep the developing electric field constant. For example, the related art concerning a method of controlling the surface potential of the photoconductor in consideration of the influence of the electric field has been described in JP-A-11-15214.
Variation in charge density of toner in the developing device is a main cause of variation in image quality as well as variation with time in potential and electric field of the electrostatic latent image on the surface of the photoconductor is a main cause thereof. Hence, there is also known a method for keeping image quality stabl
Ishii Masayoshi
Kubota Keisuke
Mitsuya Teruaki
Yamada Shintaro
Hitachi Printing Solutions, Ltd.
McGinn & Gibb PLLC
Ngo Hoang
LandOfFree
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