Electrophotography – Image formation – Development
Reexamination Certificate
2000-05-10
2002-09-10
Beatty, Robert (Department: 2852)
Electrophotography
Image formation
Development
C399S277000
Reexamination Certificate
active
06449452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a method and apparatus for image developing, and more particularly to a method and apparatus for image developing in which developer is caused to suitably form a magnetic brush in order to develop an image in a superior quality.
2. Discussion of the Background
In general, image forming apparatuses using an electrostatic recording method or an electrophotographic method, such as copiers, printers, facsimile machines, or the like, performs a common image forming operation. In such a common image forming operation, an electrostatic latent image is formed in accordance with original image information on a latent image carrying member including a photoconductive member such as a photoconductive drum, a photoconductive belt, or the like. Then, an image developing apparatus of the image forming apparatus performs an image developing operation relative to the latent image formed on the latent image carrying member so as to form a visible image.
Recently, a so-called magnetic brush image developing method using a two-component developer including toner and carriers has been mainstream in an image developing field, from viewpoints of image transferability, halftone reproducibility, stability of image development against varying temperature and humidity. With the magnetic brush image developing method, an image developing apparatus causes the two-component developer to form on a developer carrying member with a magnetic force thereof into a brush-like shape including a plurality of developer chain segments each made of chained developer particles. The developer thus formed in the brush-like shape is referred to as a magnetic brush. The magnetic brush formed on the developer carrying member supplies toner to a latent image formed on a latent image carrying member in a developing region which is formed between the developer carrying member and the latent image carrying member. The developing region is defined as a region in which the developer forms a magnetic brush on the developer carrying member and makes contact with the latent image carrying member.
The developer carrying member is generally composed of a hollow cylindrical sleeve (i.e., a developing sleeve) and a magnet member (i.e., a magnet roller) mounted inside the developing sleeve. The magnet roller forms magnetic fields for causing the developer deposited on the surface of the developing sleeve to rise in the form of a plurality of chain segments. More specifically, carrier particles contained in the developer rise along magnetic lines of force generated by the magnet roller and form developer chain segments. Onto such a developer chain segment, charged toner particles contained in the developer are deposited. The magnet roller includes a plurality of magnetic poles formed by the same plurality of magnets each of which has a rod-shape, for example. One of the magnets has a main magnetic pole (i.e., a developing magnetic pole) for especially causing the developer to form a magnetic brush relative to the developing region on the developing sleeve.
In the above configuration, when at least one of the developing sleeve and the magnet roller moves, it conveys the developer forming the rising developer chain segments towards the developing region. The developer brought to the developing region rises in the form of the magnetic brush along the magnetic lines of force generated by the main magnetic pole. As the head of the magnetic brush contacts the surface of the latent image carrying member, it yields itself. While the magnetic brush sequentially rubs against the latent image formed on the latent image carrying member at a speed determined on a basis of a difference of linear velocity between the developer carrying member and the developing sleeve, the toner is transferred from the developer carrying member to the latent image carrying member.
Conventionally, an analog image forming apparatus has been prone to cause a problem when a latent image is formed in a low contrast and has used an edge effect to compensate this problem. The edge effect is brought from relatively strong electrostatic fields which are generated around an image portion and a non-image portion of an electrostatic latent image formed on a photoconductive member. At an event that the edge effect is strong or great, or is produced, an electrostatic field is greater and an amount of toner used for image developing on an edge portion of an image is greater than that used for an inside portion of the image. As a result, the image will have a higher density. On the other hand, at an event that the edge effect is weak or small, or is not produced, an electrostatic field is smaller and an amount of toner used for image developing on an edge portion of an image is similar to that used for an inside portion of the image. As a result, the edge and inside portions of the image have even densities and the image will be produced in a superior quality.
However, a digital image forming apparatus which has recently come into widespread use has such a problem as described above. Accordingly, the digital image forming apparatus is required to develop an image in accordance with a latent image with as great a fidelity as possible so as to achieve an ideal image forming. To do this, the digital image forming apparatus is particularly required to perform a sophisticated image developing function capable of using a high image density. One known way for allowing an image developing to use a high image density is to make a developing gap narrow. The developing gap is specified as a distance between the latent image carrying member and the developer carrying member. Another known way is to make a developing nip wider. The developing nip is specified as a width of the developing region.
FIG. 1
shows variations of the edge effect when an image of one-dot-width vertical lines is developed with variations of a line density and the developing gap. In
FIG. 1
, the vertical axis represents a resultant edge effect ratio and the horizontal axis represents variations of the line density. When an image of one-dot-width vertical lines is developed with a line density of 150 lpi (lines per inch), the edge effect is defined as a value of 1. For example, as shown in
FIG. 1
, the edge effect with a developing gap (Gp) of 0.6 mm is flat at an edge effect level of 1 with the line density in a range of 150 lpi to 200 lpi but is increased to a value slightly below 1.4 with a line density of 100 lpi and to a value slightly below 1.7 with a line density of 50 lpi. For another example, the edge effect with a developing gap (Gp) of 400 &mgr;m is flat at the edge effect level of 1 with the line density in a range of 120 lpi to 200 lpi but is increased to a value about 1.05 with a line density of 100 lpi and to a value slightly above 1.2 with a line density of 50 lpi. With referring also to other examples having different Gp values shown in
FIG. 1
in this way, it is understood that the edge effect is prone to reduce as the developing gap is made narrower.
More specifically, it is understood that an image of one-dot-width lines with a fixed Gp is prone to receive a greater edge effect when the image has a smaller line density, or a smaller spatial frequency, that is, each line in the image is isolated. The reason is that when a line is isolated electric force lines are concentrated onto the isolated line, thereby increasing the intensity of the electric field around the isolated line by which more toner is attracted to the isolated line. As a result, the isolated line becomes thicker. It is also understood, on the contrary, that an image of one-dot-width lines with a fixed Gp is prone to receive a smaller edge effect when the image has a greater line density, or a greater spatial frequency, that is, the image is dense. The reason for this is that when a spatial frequency is great electric force lines are not concentrated onto the lines, thereby decreasing the intensity of the electric field around the lines by wh
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