Electrophotography – Image formation – Development
Reexamination Certificate
2000-11-30
2002-07-16
Braun, Fred L (Department: 2852)
Electrophotography
Image formation
Development
C335S296000, C335S306000
Reexamination Certificate
active
06421519
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnet roll used as a developing roll in electrophotography, electrostatic recording, etc.
PRIOR ART
In electrophotograph, electrostatic recording, etc., electrostatic image is formed on a surface of an image-bearing member (photo-sensitive body or dielectric body), and developed with a magnetic developer containing toner (one-component magnetic toner or two-component developer comprising toner and a magnetic carrier) conveyed to a developing region by a developing roll, and the resultant toner image is transferred to a transfer member (plain paper or the like) and fixed thereto is by heating and/or pressing.
Widely used as a developing roll is, for example, a magnet roll assembly having a structure as shown in FIG.
4
. Referring to
FIG. 4
, a magnet roll
1
comprises a cylindrical permanent magnet
11
having a plurality of magnetic poles extending on its surface along a longitudinal direction and a shaft
12
fixed concentrically to a center portion of the cylindrical permanent magnet
11
. As shown in
FIG. 3
, the magnet roll
1
is enclosed in a cylindrical sleeve
2
and supported by flanges
3
a
,
3
b
via bearings
4
a
,
4
b
at both ends of the shaft
12
. The sleeve
2
and the flanges
3
a
,
3
b
fixed to both ends thereof are made of non-magnetic materials such as aluminum alloys, austenitic stainless steel, etc. A numeral
5
denotes a seal member (oil seal). With the above structure, a magnetic developer is attracted onto a surface of the sleeve
2
and conveyed to a developing region (region in which the image-bearing member is positioned in opposite to the sleeve) by a relative rotation of the magnet roll
1
and the sleeve
2
(for instance, by rotating the flange
3
a
while keeping the magnet roll
1
stationary) to develop the electrostatic image.
The cylindrical permanent magnet constituting the above magnet roll is usually an elongated one having an outer diameter D of 10-60 mm and a length L of 200-300 mm, L/D≧5, and formed of an isotropic sintered ferrite magnet, or an anisotropic bonded magnet mainly composed of ferromagnetic particles (Sr ferrite or Ba ferrite) and a resin (polyamides, chlorinated polyethylene, etc.). The anisotropic bonded magnet is produced, for instance, by heat-blending a mixture of starting materials, extrusion-molding or injection-molding the molten blend in a magnetic field and then magnetizing the molded product according to a magnetization pattern.
Toner and carrier are made finer to satisfy the recent demand of higher image quality, and the magnet roll tends to increase its magnetic force to compensate for a decrease in attraction thereof to the magnet roll. The magnetic force required for the magnet roll is about 500-800 G on a sleeve surface, suitable for almost all developing processes. However, there have been provided a developing process requiring as high a magnetic force as about 1000-1300 G.
The isotropic sintered ferrite magnets can be provided with any magnetic force distribution by integral magnetization, so that they are materials good in stability and extremely easy to use. However, they are disadvantageous in that their residual magnetic flux density Br is about 2000 G. resulting in a magnetic flux density limited to about 900-1000 G on a sleeve surface, failing to meet the demand of higher magnetic field.
On the other hand, the anisotropic bonded magnets can easily be provided with a residual magnetic flux density Br of about 2600 G, for instance, generating a magnetic field stronger than that of the isotropic sintered ferrite magnets. However, magnetic poles can be formed only in predetermined directions (anisotropic directions), limiting a magnetic force distribution. Further, orientation should be carried out in a magnetic field during molding, resulting in an anisotropic bonded magnet suffering from unevenness in magnetic properties in a longitudinal direction and poor productivity.
Moreover, the sintered ferrite magnets are hard ceramics, which are brittle materials poor in impact resistance and difficult to be worked, so that working relies on grinding in most cases. On the other hand, the bonded magnets can overcome such disadvantages of the sintered ferrite magnets. However, because they are anisotropic magnets oriented in a magnetic field, they are provided with magnetic poles only in fixed directions even in a cylindrical shape. As a result, they do not have the degree of freedom that any desired arrangement of magnetic poles are formed by magnetization, unlike the isotropic sintered ferrite magnets. In addition, because the anisotropic bonded magnets are long in size, uniform magnetic field orientation is difficult, and their variation in the longitudinal direction of the magnet roll also increases several times that of the isotropic sintered ferrite magnets, which raises the problem that image quality is greatly influenced in a magnetic brush development system sensitive to the uniformity of magnetic force.
Also, because the temperature coefficient of Br is as high as 0.2 %/° C. in the sintered ferrite magnets, developing conditions may vary depending on the environment of use in high-image quality digital apparatuses having extremely high sensitivity in development, resulting in changes in developed images in some cases.
As described above, isotropic magnet materials having high magnetic properties have been desired as the solution to the problems (poor surface magnetic flux density, large variations in surface magnetic flux density, lack of the flexibility of magnetic pole formation, temperature changes in surface magnetic flux density, etc.) of the conventional magnet materials.
As magnets for meeting such a demand, isotropic bonded magnets comprising Nd—Fe—B magnet powder, which have (BH)
max
of about 3 MGOe, have been proposed. However, the Nd—Fe—B bonded magnets have a problem with regard to corrosion resistance, and are vulnerable to rusting, so that they should be coated with epoxy resins, fluororesins, etc. Long articles such as magnet rolls are likely to have coatings with defects, resulting in an increase in product cost. Also, in the case of magnet rolls, gaps between magnets and inner diameters of sleeves are generally set as small as about 0.5-1 mm to utilize surface magnetic flux densities of the magnets on the sleeves as efficiently as possible. Thus, if rust is generated on the magnets, the gaps between the magnets and the sleeves are clogged with rust, resulting in higher risk of locking accident.
Also, in long articles such as cylindrical permanent magnets for magnet rolls produced by extrusion molding, etc., magnet powder should be dispersed uniformly in the cylindrical permanent magnets to obtain uniform properties along the longitudinal direction, and magnet powder having a spherical shape is advantageous in uniform dispersion. Ferrite magnet powder is suitable for uniform dispersion because it has a particle size of about 1 &mgr;m. However, Nd—Fe—B magnet powder is in a shape of thin flake, so that it is difficult to be dispersed uniformly. If the Nd—Fe—B magnet powder is pulverized to 100 &mgr;m or less to achieve more uniform dispersion, it suffers from drastic deterioration of magnetic properties. Accordingly, although the bonded ferrite magnets are excellent in molding properties, they are low in surface magnetic flux densities and also poor in temperature stability. The Nd—Fe—B bonded magnets are high in surface magnetic flux densities, but have a problem in respect to molding properties. They are also low in corrosion resistance and temperature stability. As described above, both magnets have many problems to be solved for applying them to the magnet rolls.
In view of such circumstances, a magnet roll comprising a cylindrical permanent magnet concurrently satisfying demands for surface magnetic flux density, temperature stability, corrosion resistance and molding properties have been desired.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to solve the problems of the convent
Tobise Masahiro
Yamashita Keitaro
Braun Fred L
Hitachi Metals Ltd.
Morgan & Lewis & Bockius, LLP
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