Electrophotography – Image formation – Development
Reexamination Certificate
2001-05-02
2004-06-29
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Development
C399S275000, C399S277000
Reexamination Certificate
active
06757509
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a printer, digital copier, facsimile apparatus or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing method for causing a developer to form a magnet brush on a developer carrier in a developing region for developing a latent image formed on an image carrier, and a device for practicing the same.
A developing device for an image forming apparatus is operable with either one of a one-ingredient type developer, or toner, and a two-ingredient type developer or toner and magnetic carrier mixture. The two-ingredient type developer allows the frictional charging of its toner to be easily controlled, causes the toner to cohere little, and therefore allows toner transfer to be effectively controlled by, e.g., a bias, compared to the one-ingredient type developer. Further, the toner of the two-ingredient type developer does not have to contain a magnetic material or needs only a small amount of magnetic material if necessary to obviate, e.g., fog. Particularly, the two-ingredient type developer insures images with clear colors. Moreover, when a developer layer contacts an image carrier in the form of a magnet brush, it sharply rises and contacts the image carrier in a desirable manner. This is why the two-ingredient type developer is predominant over the one-ingredient type toner although its toner must be controlled in amount relative to the carrier.
The two-ingredient type developer, however, brings about the following problems. A one-dot line formed in a direction perpendicular to a direction of sheet conveyance, i.e., a horizontal line is thinned, compared to a line formed in the direction of sheet conveyance (thinning of a horizontal line hereinafter). The trailing edge of, e.g., a halftone portion in the direction of sheet conveyance is lowered in density or practically lost (omission of a trailing edge hereinafter). In light of this, Japanese Patent Laid-Open Publication No.
7-140730
, for example, proposes to set the angle of the main pole of a magnet roller at the upstream side or to set up a preselected relation between a distance between a metering member and a developing sleeve and a distance between the developing sleeve and a photoconductive element. This kind of method should satisfy the following conditions (1) through (5):
(1) The main pole lies in a range of from 5° to 20° upstream of the closest position in a direction or developer conveyance;
(2) A doctor gap Hcut between the metering member and the developing sleeve or developer carrier is 0.25 mm to 0.75 mm;
(3) A development gap Dsd between the developing sleeve and the photoconductive element or image carrier is 0.30 mm to 0.80 mm;
(4) A ratio Dsd/Hcut is greater than 1.20, but smaller than 1.60; and
(5) A ratio of the moving speed Vs of the developing sleeve to the moving speed Vp of the photoconductive element is equal to or greater than 1.0, but equal to or smaller than 3.00.
The document mentioned above describes that when the conditions (1) through (5) are satisfied, a halftone portion or a solid portion is free from brush marks and the discontinuity of a fine line in a high copying speed range, achieving high, uniform density, and a clear-cut contour.
The method taught in the document, however, has the following problems left unsolved. As the ratio Dsd/Hcut shifts from 1, i.e., as the doctor gap Hcut becomes smaller than the development gap Dsd, the magnet brush between the developing sleeve and the photoconductive element becomes rough. The magnet brush therefore fails to uniformly contact the photoconductive element. Consequently, in a solitary dot image, in particular, in which dots with resolution of, e.g., 600 dpi (dots per inch) are recorded at the intervals of five to ten pixels, part of the dots is reduced in size or practically lost. This degrades the reproducibility and therefore tonality of a so-called high contrast portion. Further, as for a halftone image with image density ID ranging from 0.3 to 0.8, the irregular contact of the magnet brush aggravates granularity.
Japanese Patent Publication No. 2-59995 proposes to enhance a developing ability by bringing magnetic poles adjoining a main magnetic pole closer to the main magnetic pole. This document describes that although such a configuration lowers the density of a horizontal line (thinning of a horizontal line), this problem can be coped with by lowering the saturation magnetization of the carrier. However, when the saturation magnetization of the carrier is lowered, the deposition of the carrier is apt to occur. Should the amount of charge to deposit on the toner be reduced to avoid the deposition of the carrier, uncharged toner would increase and contaminate the background of an image.
Japanese Patent Laid-Open Publication No. 9-149063 teaches a non-contact type developing device using a two-ingredient type developer and arranging magnetic poles in such a manner as to prevent a magnet brush from contacting a photoconductive element. This developing device should satisfy the following conditions (1) through (3):
(1) The magnetic pole arrangement is set between a pair of N and S poles;
(2) The N and S poles make an angle of 40° to 70° therebetween, and each has a flux density of 500 mT or above; and
(3) A magnet angle between a position where an image carrier and, a magnet brush roll are closest to each other and the center between the poles is between 0° and one-tenth of the angle between the poles, and a developing position is located between the poles of the magnet.
The document describes that when the conditions (1) through (3) are satisfied, a high quality image is attainable that is free from fog ascribable to the deposition of a carrier on the background of the image carrier and local omission around the deposited carrier. However, an electric field for development available with non-contact type development using the two-ingredient type developer is too weak to enhance a developing ability.
Generally, the absolute value of a difference between the charge potential of a photoconductive element and a bias for development, i.e., so-called background potential is related to the thinning of a horizontal line and the omission of a trailing edge. In the conventional developing device, the above defects can be reduced to an acceptable level it the background potential is reduced to, e.g., about 100 V or about 50 V. Such a low background potential, however, brings about background contamination or fog. This is particularly true in a hot, humid environment.
On the other hand, Japanese Patent application No. 11-318490 discloses an image forming apparatus capable of obviating the thinning of a horizontal line and the omission of a trailing edge. Further, this apparatus prevents solitary dots from being lost due to the irregular contact of a magnet brush and frees a halftone image from granularity. In addition, the apparatus obviates the deposition of the carrier to thereby maintain a high developing ability. However, a problem with this apparatus is that the magnet brush actively moves in a small gap between an image carrier and a developer carrier, causing the carrier to fly about during development and deposit on the image carrier as well as an the other members. Consequently, the image carrier is apt to convey the carrier to an image transfer position. The carrier therefore prevents toner around the carrier from being transferred to a paper sheet or similar recording medium, resulting in a defective image. Moreover, if the carrier is transferred to the paper sheet, it simply constitutes an impurity in the resulting image because it is not fixed on the paper sheet.
Granularity often appears in images, particularly halftone images, output by the conventional image forming apparatuses. Granularity is one of major causes that lower image quality.
It is a common practice with an image forming apparatus to maintain the density of a toner image by forming a particular toner image (patch hereinafter) on a photoconductive e
Kai Tsukuru
Matsuura Nekka
Shoji Hisashi
Suzuki Hirokatsu
Takeuchi Nobutaka
Ngo Hoang
Oblon, Spivak, McCelland, Maier & Neustadt, P.C.
Ricoh & Company, Ltd.
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