Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
1997-07-02
2002-05-21
RoDee, Christopher D. (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S108200, C430S124300
Reexamination Certificate
active
06391506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carrier used for developing a latent image created in electrostatic photography and electrostatic recording, a developer using this carrier, an image-forming method using this developer, and an image-forming method used in an image-forming apparatus such as a digital printer or a digital copier which processes images as digital signals.
2. Description of the Related Art
In a digital-image-forming apparatus based on letters or image data, on/off binary information is provided as two-dimensional information at a predetermined site on a photoreceptor. When a halftone image is recorded by the above-mentioned system, an area modulation method, which uses a mesh or line screen structure, was conventionally adopted in many printers and copiers based on digital photography because of the relative easy algorithm and low cost involved.
Meanwhile, a method in which image information is visualized via a latent image, e.g., electrostatic photography, is now widely used. Electrostatic photography comprises the steps of forming a latent image on a photoreceptor by electrostatic charging and exposure, developing the latent image using a developer containing toner, transferring the toner image, and fixing the transferred toner image to form a visible image on an image-receiving medium. There are two types of developer, i.e., a two-component developer composed of toner and carrier and a one-component developer composed of a single magnetic toner. The two-component developer, whose role is allotted to the carrier and toner, has superior control and is therefore widely used.
In the developing process used in an image-forming apparatus for reproducing a multiple gradation via electrostatic photography, the cascade method has been superseded by a magnetic brush method in which a magnetic roller is used as a developer carrying member. Particularly, in the case of a color image-forming apparatus, because of the stabilized charge of the developer, a magnetic brush method using a two-component developer, which comprises a carrier and toner, is more suitably employed.
Two types of magnetic brush method using a two-component developer are known: the conductive magnetic brush development (CMB), which uses a conductive carrier, and the insulated magnetic brush development (IMB), which uses an insulated carrier. In CMB development, because of the carrier's lower resistance level, the electrical charge is injected from a developing roller so that the carrier near the photoreceptor serves as a developing electrode to enhance a practical electric field for development. As a result, since toner is sufficiently transported, a superior solid image free of the edge effect can be reproduced. However, CMB development has the disadvantage that, since the relationship between latent image potential and image density of the photoreceptor changes abruptly as indicated by the steep slope of the curve, image defects such as white lines called brush marks, i.e., latent image destruction due electrical charge injection from the developing roller, and so-called carrier beads carry over, i.e., migration of the carrier to the photoreceptor, tend to occur. On the other hand, in IMB development, the relationship between latent image potential and image density of the photoreceptor is linear and has a gentle slope. The disadvantages of IMB development are that solid images are poorly filled and that the edge effect is significant.
If the degree of the above defects is insignificant, black-and-white images, which are formed by black toner alone, are not seriously influenced in sensory inspection. The above defects, however, present a fatal drawback to color images formed by the overlap of multiple toner colors. This is because, in a color image, these defects cause a slight change in color, causing “noise” due to different colors in a gradated range, even though these defects are only a slight change in density in black-and-white images. Accordingly, these defects extremely adversely affect the impression of color images in particular.
Because of this background, conductive magnetic brush methods, which exhibit superior performance in terms of filling of solid image, edge effect, carrier beads carry over and brush marks, have been disclosed.
For example, Japanese Patent Application Publication (JP-B) No. 7-120,086 discloses a method based on the electrical resistance of a carrier, which comprises a core having a relatively low electrical resistance coated with a resin having a high electrical resistance which abruptly changes in an electric field having a specific strength, causing the electrical resistance of the carrier to increase in a weak electric field, whereas the electrical resistance of the carrier decreases in a strong electric field. Based on this disclosure, since a stronger electric field exists in the latent image area and a weak electric field exists in the nonimage area, use of this carrier enables superior solid black image printing without carrier beads carry over to the nonimage area. However, based on the examples and description of the function of the invention in the above Japanese Patent Application Publication (JP-B) No. 7-120,086, the resin coating layer is so thin that a core having a lower electrical resistance is believed to be partly exposed, which leads to lower electrical resistance of the carrier under a strong electric field. To substantiate this assumption, comparative examples, which are described later, verify that the electrical resistance of a carrier, produced by completely coating a core with a thick layer of resin, exhibits a higher electrical resistance even in a strong electric field and does not provide a superior solid image. The partial coated carrier, which has a partly exposed core having lower electrical resistance, tends to cause brush marks in the latent image because the electrical charge moves easily via exposed surfaces.
Japanese Patent Application Laid-Open (JP-A) No. 61-107,257 and Japanese Patent Application Laid-Open (JP-A) No. 61-130,959 disclose a ferrite which has a relatively low electrical resistance and has surface roughness caused by primary particles. According to the disclosure, because of carrier particle roughness, leakage between oppositely polarized charges is inhibited so that brush mark formation is inhibited. The disadvantage, however, is that the presence of the roughness on the carrier surface increases the contact area between the carrier and toner so that toner adheres more to the carrier surface, which diminishes the charge-imparting capability of the carrier over time.
Japanese Patent Application Laid-Open (JP-A) No. 6-161,157 specifies the ratio of the electrical resistance of the core to the electrical resistance of the resin-coated carrier itself so that the carrier provides superior resolution, proper solid image density, and fine line reproduction. No remarked effect is seen, however, in preventing image defects, particularly in color images.
As stated above, none of the available carriers and image-forming methods perform satisfactorily in view of recent stringent requirements for high-quality images, including color images, because existing carriers and image-forming methods do not solve the problems of image defects associated with the conductive magnetic brush, namely, the problems of carrier beads carry over and brush marks caused by the destruction of the latent image due to bias leakage.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a carrier, which produces a superior solid image free of brush marks and carrier beads carry over, particularly in color images, and which is durable for a long time, and to provide a developer using this carrier and, further, to provide an image-forming method using this developer.
Another object of the present invention is to provide an image-forming method which have the advantage of stabilizing the amount of toner moving to a latent image, even if the photoreceptor is not u
Fukuhara Taku
Inaba Shigeru
Kim Suk
Oda Yasuhiro
Yanagida Kazuhiko
Fuji 'Xerox Co., Ltd.
Oliff & Berridge
RoDee Christopher D.
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