Electrophotography – Image formation – Development
Reissue Patent
2001-09-27
2003-03-11
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Development
C399S274000
Reissue Patent
active
RE038026
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a developing device which is used in an image forming apparatus such as a copying machine, a printer, or the like, and which develops an electrostatic latent image on an image carrier.
2. Related Background Art
As conventional electrophotography methods, a large number of methods are known, as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748, and the like. In general, an electrical latent image is formed on a photosensitive body consisting of a photoconductive material, by one of various means, a toner image is formed using a toner, the toner image is transferred onto a transfer medium such as a paper sheet, as needed, and the toner image transferred onto the transfer medium is fixed by heat, a vapor of a solvent, or the like, thus obtaining a copy. Also, various methods of visualizing an electrical latent image using a toner are known.
As developing methods, a large number of developing methods, for example, a magnetic brush developing method described in U.S. Pat. No. 2,874,063, a powder cloud method described in U.S. Pat. No. 2,221,776, a fur brush method, a liquid developing method, and the like, are known.
Of these developing methods, a magnetic brush method, a cascade method, a liquid developing method, and the like, which use a developing agent mainly consisting of a toner and a carrier, have widely been put into practice. These methods are excellent in that they can relatively stably obtain satisfactory images, but have common drawbacks associated with a two-component developing agent, i.e., degradation of the carrier and a variation in mixing ratio of the toner and carrier.
In order to avoid such drawbacks, various developing methods using a one-component developing agent consisting of only a toner have been proposed. For example, U.S. Pat. No. 3,909,258 proposes a method of developing an image using a magnetic toner having conductivity. In this method, a conductive magnetic toner is supported on a cylindrical conductive sleeve having a magnetism therein, and an electrostatic latent image is developed by bringing the sleeve into contact with the latent image. In this case, a conductive path is formed between the surface of a recording medium and the surface of the sleeve by toner particles on a developing region, an electric charge is guided from the sleeve to the toner particles via the conductive path, and the toner particles become attached to an image portion by a Coulomb force with the electrostatic latent image portion, thereby developing the image. The developing method using a conductive magnetic toner is an excellent method since it can avoid conventional problems associated with a two-component toner. However, since the toner is conductive, it is difficult to electrostatically transfer the developed image from the recording medium to a final support member such as a normal paper sheet.
In order to solve this problem, as a developing method using a high-resistance toner which can be electrostatically transferred, a developing method utilizing dielectric polarization of toner particles is described in Japanese Laid-Open Patent Application No. 52-94140. However, this method has drawbacks such as a low developing speed at which the developed image cannot have a sufficient density, and it is difficult to put this method into practical use. As another method using a high-resistance toner, a method of developing an image by triboelectrification, i.e., electrification of toner particles by friction between toner particles themselves or friction between toner particles and a sleeve, and bringing the charged toner particles into contact with an electrostatic holding member, is known. However, this method has the following drawbacks. That is, the number of times of contact between the toner particles and the friction member is small, and the toner particles are often insufficiently triboelectrified. In addition, when the Coulomb force between the charged toner particles and the sleeve is strong, the toner particles easily cohere. Thus, it is pointed out that many practical problems remain unsolved.
Japanese Laid-Open Patent Application No. 54-43036 proposes a novel developing method which can eliminate the above-mentioned drawbacks. In this method, a very thin toner layer is coated on a developing sleeve, is triboelectrified, and is brought very close to an electrostatic latent image under the influence of a magnetic field to face the image without contacting it, thereby developing the image.
According to this method, since a very thin layer of a magnetic toner is coated to increase the number of times of contact between the magnetic toner and the developing sleeve, a triboelectrification electric charge amount required for development can be developed in the toner.
However, even in the above-mentioned method, it is known that the number of times of contact between the developing sleeve and the magnetic toner is smaller than that between the toner and carrier in a two-component type developing agent.
Also, it is well known that the number of times of contact between the magnetic toner and the developing sleeve required for the magnetic toner to acquire a triboelectrification electric charge amount required for development varies depending on the composition of the magnetic toner, and the like.
Therefore, in the above-mentioned method, in a system which requires a larger number of times of contact to acquire a required triboelectrification electric charge amount, a phenomenon caused by charging instability tends to occur.
Furthermore, it is well known that the number of times of contact between the magnetic toner and the developing sleeve required for the magnetic toner to acquire a triboelectrification electric charge amount required for development varies depending on the composition of the magnetic toner, and the like.
Therefore, in the above-mentioned method, in a system which requires a larger number of times of contact to acquire a required triboelectrification electric charge amount, a phenomenon caused by charging instability tends to occur.
The present inventors examined electric charges generated by the one-component developing method, and found that the toner behaved as follows in an electric charge developed portion of the one-component developing method.
In
FIG. 1
, a developing device
20
comprises a toner container
3
for storing a magnetic one-component toner, a developing sleeve
1
a which is arranged in the opening portion of the toner container
3
to be rotatable in the direction of an arrow in
FIG. 1
, and uses a non-magnetic member, a permanent magnet
1
b fixed in the interior of the developing sleeve
1
a, a magnetic blade
2
which is fixed to the toner container
3
and uses a magnetic member for regulating the thickness of a toner layer, and a toner convey member
4
arranged in the toner container
3
. The magnetic blade
2
is arranged to have a constant distance value W from the developing sleeve
1
a. In general, the distance is often set to fall within a range from 100 &mgr;m to 1 mm.
In the developing device shown in
FIG. 1
, a magnetic one-component toner is coated as a thin layer on the developing sleeve
1
a. The thickness of the toner layer is determined by the position of a cut line L shown in FIG.
2
.
According to the examinations of the present inventors, it was found that an electric charge was developed in a magnetic toner T when the magnetic toner T passed between the developing sleeve
1
a and the magnetic blade
2
. It was also found that the magnetic toner behaved as follows.
As shown in
FIG. 3
, planes perpendicular to a line connecting the developing sleeve
1
a and the magnetic blade
2
are assumed, the plane closer to the magnetic blade
2
is represented by S
1
, and the plane closer to the developing sleeve
1
a is represented by S
2
. In general, since the width of the magnetic blade
2
is set to be smaller than that of the permanent magnet
1
b, the magnetic flux densities on th
Itoh Isami
Tada Tatsuya
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