Electrophotography – Image formation – Development
Reexamination Certificate
2001-07-30
2004-01-27
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Development
C399S277000
Reexamination Certificate
active
06684046
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention and Related Background Arts
The present invention relates to an image forming apparatus and method for forming an image by developing an electrostatic latent image formed on an image bearing member corresponding to an image to be recorded and recording it on a recording medium such as a plain paper. Furthermore, the present invention relates to a developing apparatus used for the image forming apparatus.
Heretofore, various types of electrophotographic developing apparatuses have been proposed and put into practical use. In general, they are classified into developing apparatuses using a one component developing system and developing apparatuses using a two component developing system. The one component system is a non-contact system in most cases, and as a representative example, there is a one component jumping developing system which uses a magnetic toner.
This developing system can provide a high quality image with a simple constitution. However, a clear color image cannot be provided because the toner contains a magnetic material. Further, in the case of the one component developing system using a nonmagnetic toner, although a color image can be provided, it is difficult to have the toner applied to a developing sleeve stably for a long period. Even when the non-magnetic toner is coated on the developing sleeve with an elastic blade, it is necessary to further improve its stability and durability.
On the other hand, in a two component developing method, development is carried out by conveying toner to a developing area using a magnetic carrier, and usually a developing process is carried out by bringing a developer into contact with a photosensitive drum. The developing process will be described with reference to FIG.
2
. In
FIG. 2
, reference numeral
30
denotes a developing sleeve, reference numeral
35
a magnetic roller fixedly disposed inside the developing sleeve, reference numerals
31
and
32
agitating screws, reference numeral
33
a control blade disposed for forming the developer in a thin layer on the developing sleeve surface and reference numeral
34
a developer container.
The developing process for visualizing the above-described electrostatic latent image using a two component magnetic brush method, by using the above-described developing apparatus and a circulating system of the developer, will be described below. By rotation of the developing sleeve
30
, developer drawn up at an N
3
pole is controlled by the control blade
33
during the process of being conveyed from an S
2
pole to an N
1
pole and formed in a thin layer on the developing sleeve
30
. When the developer formed in a thin layer is conveyed to a developing main pole S
1
, an ear-up is formed by a magnetic force. Using the developer formed in the ear-up shape, the above-described electrostatic latent image is developed, and, after that, the developer on the developing sleeve
30
is returned to the interior of the developing container
34
by a repulsive magnetic field of the N
2
pole and the N
3
pole. In the two component development method, as described above, magnetic poles generally having the same polarity are arranged side by side, and the unused developer, after the development process, is stripped off the developing sleeve so that no image hysteresis remains.
The developing sleeve is applied with a direct current bias and an alternating current bias from a power supply source (not shown). Generally, in the two component developing method, the application of an alternating current bias increases developing efficiency and enhances image quality.
As for latent image forming methods, there have been known such methods wherein an electrophotographic photosensitive member is scan-exposed by a laser beam, which is modulated in correspondence with the signal of an image to be recorded, and an electrostatic latent image is formed by distributing the latent image in a dot shape in correspondence with the image. Among those methods, however, the so-called pulse width modulation (PWM) method which modulates the width (that is, the duration) of a laser drive pulse current in correspondence with the shade of the image to be recorded can obtain a high recording density (that is, a high resolution) and also can obtain a high gradation.
In recent years, development has been made to attain miniaturization and longevity of the two component developing device. In order to achieve miniaturization from a development process side, it is necessary to miniaturize the container, the developing sleeve and the agitating conveyance screw. Making the space of a developer-reservoir portion smaller at a developer layer thickness control portion is also one of the effective methods. Execution of these steps of miniaturization and space reduction will severely reflect upon component accuracy and various latitudes and hence an elaboration of various ideas is necessitated.
In order to attain longevity, it is necessary to prevent deterioration of the toner and the carrier. For this purpose, there is required a constitution wherein the developer is not compressed. The location in which the developer is compressed is the developer layer thickness control portion and, in a usual constitution, the developer layer thickness control pole (the S
2
pole in
FIG. 2
) is situated further in the upper stream of the rotational direction of the developing sleeve than the control blade and the developer attracted to a developer layer thickness control pole in this area is compressed between the sleeve and the container. To reduce the compression of the developer, it is most effective to reduce a force (Fr: a magnetic attraction force which works in the vertical direction relative to the developing sleeve) by which the developer layer thickness control pole attracts the developer to the developing sleeve.
As a method for that purpose, there can be cited such methods as minimizing a magnetization of the magnetic carrier inside the developer and constructing a magnet pattern in which a magnetic line of force from the developer layer thickness control pole is difficult to go around adjacent magnetic poles, but goes out as vertically as possible from the developing sleeve. Minimizing the magnetization of the carrier can be said to be one of various means for achieving high quality from a point that a force rubbing the toner image, which has been developed on the photosensitive member, becomes weaker.
As one of the latter methods, there has been proposed a developing method, which uses one of the repulsive magnetic poles of the magnet roller as a developer layer thickness control pole. When a repulsive magnetic field is formed with magnetic poles of the same polarity being side-by-side, the magnetic lines of force of each magnetic pole go out almost vertically to the developing sleeve. In this case, a rate of change in a magnetic flux density in the vertical direction relative to the developing sleeve is small. As a result, the force, which attracts the developer to the developing sleeve, becomes smaller and a condensation of the developer is reduced. It has been considered that, by implementing both of the above-described methods, still greater longevity can be attained.
Regarding a constitution to form the repulsive magnetic field, if the developer layer thickness control pole, which is one of the repulsive magnetic poles, is arranged downstream of the gravitational direction relative to the stripping off pole, which is another magnetic pole forming the repulsive magnetic field, it is much easier to draw up the developer to be coated on the developing sleeve and the constitution becomes simpler. As described above, such a constitution may possibly reduce the reservoir amount of the developer in the developer layer thickness control portion and may also lead to miniaturization of the developer container.
However, in the case where a constitution using one of the repulsive magnetic poles as the developer layer thickness control pole is adapted to a small size developing sleev
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Grainger Quana M.
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