Electrophotography – Image formation – Development
Reexamination Certificate
2001-08-28
2004-03-09
Beatty, Robert (Department: 2852)
Electrophotography
Image formation
Development
C399S275000
Reexamination Certificate
active
06704534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a developing device for use in an image forming apparatus such as a copier, a printer or a facsimile apparatus.
2. Description of the Related Art
There is a well known method of bearing a dry developer on the surface of a developer bearing member, conveying and supplying the developer to the vicinity of the surface of an image bearing member on which an electrostatic latent image is formed, developing the electrostatic latent image with the developer while applying an alternating electric field between the developer bearing member and the image bearing member, and visualizing the electrostatic latent image.
Generally, a development sleeve is often used as the developer bearing member and therefore, the developer bearing member will hereinafter be referred to as the “development sleeve”, and a photosensitive drum is often used as the image bearing member and therefore, the image bearing member will hereinafter be referred to as the “photosensitive drum”.
The above-described developing method is classified roughly into four kinds of development, i.e., monocomponent noncontact development in which for a nonmagnetic toner, the development sleeve is coated with the toner by a blade or the like and for a magnetic toner, the development sleeve is coated with the toner by a magnetic force, the toner is carried to the photosensitive drum by the rotation of the development sleeve, and the electrostatic latent image on the photosensitive drum is developed with the toner being in noncontact with the photosensitive drum; monocomponent contact development in which the electrostatic latent image is developed with the toner being in contact with the photosensitive drum; two-component noncontact development in which for a developer consisting of a magnetic carrier mixed with a nonmagnetic toner is borne on the development sleeve by a magnetic force, the developer is carried to the photosensitive drum by the rotation of the development sleeve, and the electrostatic latent image is developed with the developer being in noncontact with the photosensitive drum; and two-component contact development in which the electrostatic latent image is developed with the developer being in contact with the photosensitive drum. From the two viewpoints of higher quality and higher stabilization of the image, two-component contact development is often used.
A toner image formed on the photosensitive drum by the development of the electrostatic latent image is transferred onto a transferring material such as paper, whereafter it is fixed by heat, pressure, etc., whereby a final copy image is obtained. On the other hand, the photosensitive drum after the transfer of the toner image has an adhering contaminant such as any untransferred toner residual on its surface removed by a cleaner, and thereafter is repeatedly used for image formation.
In recent years, in image forming apparatuses using a two-component developing device, development for still a higher quality of image, a longer life (higher stabilization) and downsizing has been put forward. To achieve the longer life of the developing device, it is important to present the deterioration of the toner and carrier due to the compression of the developer to thereby lengthen the life of the developer.
The place in the container (developing container) of the developing device where the developer is compressed is a developer layer thickness regulating portion, and in the construction of an ordinary developing device, the developer layer thickness regulating pole of a magnet in the development sleeve is located upstream of a developer layer thickness regulating blade with respect to the direction of rotation of the development sleeve, and in this area, the developer attracted to the layer thickness regulating pole is compressed between the development sleeve and the developing container.
To weaken the compression of the developer, it is effective to weaken the force with which the developer layer thickness regulating pole attracts the developer to the development sleeve (magnetic attraction acting in a direction perpendicular to the surface of the development sleeve).
As a method therefore, mention may be made of making the magnetization of the magnetic carrier in the developer small (the direction in which this magnetization of the carrier is decreased is a direction advantageous to a higher quality image in that the force rubbing against the developed toner image on the photosensitive drum in a developing portion becomes weak), on building up such a magnetic pattern that it is difficult for magnetic lines of force from the developer layer thickness regulating pole to go around to adjacent magnetic poles and the magnetic lines of force go out perpendicularly as far as possible from the surface of the development sleeve.
As one of the methods using the latter magnetic pattern, there has been proposed a developing method using one of the repulsive magnetic poles of the magnet in the development sleeve as the developer layer thickness regulating pole. When magnetic poles of the same polarity are adjacent to each other and form a repulsive magnetic field, the magnetic lines of force of each of those magnetic poles goes out perpendicularly to the surface of the development sleeve, and the rate of change of the magnetic flux density in the direction perpendicular to the surface of the development sleeve is small. As a result, the force attracting the developer to the development sleeve becomes small, and the compression of the developer weakens. Also, as a matter of course, the amount of developer itself collected at the developer layer thickness regulating pole becomes small and from that point as well, the compression of the developer weakens.
In such a case, if there is adopted a method of installing the developer layer thickness regulating pole and the regulating blade in the lower portion of the developing device with respect to the direction of gravity, magnetically attracting and drawing the developer onto the development sleeve, and stripping the developer after development in the portion above the layer thickness regulating pole with respect to the direction of gravity, the amount of developer itself collected in the layer thickness regulating portion can be made smaller and the compression of the developer can be made lighter in pressure, and it becomes easy to downsize the developing device by the simplification of the construction of the developing device.
Relative to the developing device in which the developer layer thickness regulating pole and regulating blade are installed in the lower portion of the developing device with respect to the direction of gravity, the exposed portion of the photosensitive drum is usually located on the upper portion of the photosensitive drum and therefore, the direction of rotation of the development sleeve and the direction of rotation of the photosensitive drum often become counter to each other.
On the other hand, for the downsizing of the developing device as a single body, it is the only method to downsize such parts as the development sleeve and a developer conveying screw constituting the developing device. Recently, regarding the downsizing of the main body of a copier, there has also been proposed a construction in which a drum cleaner is detached from the main body of the copier and cleaning simultaneous with developing is effected (a cleanerless system), but again in that case, the aim at the downsizing of the main body of the copier cannot be achieved unless each of the steps of charging, exposing, developing, transferring and fixing is made compact.
As described above, the development sleeve of the developing device is made small in diameter and one of the repulsive poles of the magnet in the development sleeve is used as the developer layer thickness regulating pole (cut pole), whereby it becomes possible to construct a developing apparatus achieving a longer life.
However, when one of the repulsive poles is used as the developer la
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