Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reissue Patent
1998-01-27
2002-09-10
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S251000, C492S018000
Reissue Patent
active
RE037834
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnet roll for use as a delivery means for a magnetic developer in an electrophotographic apparatus, an electrostatic recording apparatus etc., and a method of producing the magnet roll. Particularly, the present invention relates to a magnet roll which does not suffer from breakage or cracking, and a method of producing the magnet roll in which a cylindrical resin-bonded magnet is formed by injection molding integrally with one shaft and the other shaft is press-fitted into a bore of the cylindrical resin-bonded magnet at the opposite end, thereby reducing the number of manufacturing steps and production cost.
In conventional electrophotographic apparatus, electrostatic recording apparatus, etc., a magnet roll used as a developing roll or a cleaning roll generally has a structure as shown in FIG.
4
. The magnet roll
1
comprises a cylindrical permanent magnet
2
such as a resin-bonded magnet provided with a plurality of magnetic poles (not shown) on an outer surface thereof, and a shaft
3
coaxially secured into a central bore extending axially through the cylindrical permanent magnet
2
.
A pair of flanges
4
,
5
are rotatably mounted to the opposite ends of the shaft
3
via bearings
6
,
6
. A hollow cylindrical sleeve
7
is fixed to the flanges
4
,
5
to surround the cylindrical permanent magnet
2
. Incidentally, the flanges
4
,
5
and the sleeve
7
are made of a non-magnetic material such as aluminum alloys, stainless steel, etc.
In case the magnet roll
1
is employed in the developer roll
8
as shown in
FIG. 4
, the magnet roll
1
and the sleeve
7
rotate relative to each other. For instance, the magnet roll
1
is kept stationary while the sleeve
7
secured to the flanges
4
,
5
are allowed to rotate relative to the magnet roll
1
. By this construction, a magnetic developer is attracted onto an outer surface of the rotating sleeve
7
by a magnetic attraction force of the cylindrical permanent magnet
2
and conveyed into a developing region for carrying out the development of an electrostatic latent image on an image-bearing member (not shown) to obtain a developed toner image. Incidentally, the magnetic developer is usually of a one-component type including a magnetic toner, or of a two-component type including a toner and a magnetic carrier.
In the magnet roll
1
described above, the cylindrical permanent magnet
2
is generally made of a sintered magnet such as a ferrite magnet, or a resin-bonded magnet. The resin-bonded magnet is primarily used when the weight reduction of the developer roll
8
is desired. When the cylindrical permanent magnet
2
is made of a resin-bonded magnet, the magnet roll
1
is produced, for instance, by (1) blending a ferromagnetic powder (usually ferrite powder) and a polymer material (usually rubber or plastics) to prepare a mixture, (2) charging the mixture into a cavity of an injection-molding die, in which the shaft
3
as an insert is held in place, to shape a cylindrical resin-bonded magnet
2
fixed to the shaft
3
while applying a magnetic field thereto, (3) cooling and solidifying the cylindrical resin-bonded magnet
2
in the die and then removing an integral body composed of the cylindrical resin-bonded magnet
2
and the shaft
3
from the die, and (4) magnetizing the integral body in the anisotropic direction to form magnetic poles on the cylindrical resin-bonded magnet
2
, thereby obtaining the magnet roll
1
.
A variety of thermoplastic resin materials have been used to form the cylindrical resin-bonded magnet
2
of the magnet roll
1
. However, since the cylindrical resin-bonded magnet
2
is of such an elongated shape that a ratio of a length L to a diameter D
1
is 3 or more, a crystalline resin having a modulus of longitudinal elasticity of 1×10
5
kg/cm
2
or higher should be generally utilized to prevent the deformation of the cylindrical resin-bonded magnet
2
and reduce the molding time.
When the electrostatic latent image is developed, it is necessary to apply a bias voltage to the sleeve
7
of the developer roll
8
to prevent fogging on a recording medium or to achieve a reversal development. For this reason, the shaft
3
is generally made of a metal such as steel, stainless steel, etc. and is connected at one end thereof, for instance at an end adjacent to the flange
4
, to a voltage source so that a necessary bias voltage is applied via the shaft
3
and the bearings
6
,
6
, such as sintered bearings, roller bearings, etc. to the sleeve
7
.
However, in the magnet roll having such a structure, it is likely that the cylindrical resin-bonded magnet
2
breaks or cracks in its boundary region with the shaft
3
due to a large difference in coefficient of linear expansion therebetween. For instance, the shaft
3
made of steel has a coefficient of linear expansion of 1.1×10
−5
cm/cm/° C. and the cylindrical resin-bonded magnet
2
has a coefficient of linear expansion of about 3-4×10
−5
cm/cm/° C. (refer to Japanese Patent Publication No. 5-37331). When heated during the injection-molded process and thereafter cooled, the integral body is caused to shrink in the axial direction. In this case, a shrinkage of the cylindrical resin-bonded magnet
2
is larger than that of the shaft
3
so that breakage or cracking is likely to take place in a boundary region between the cylindrical resin-bonded magnet
2
and the shaft
3
. Particularly, when the size of the magnet roll
1
is reduced, the thickness of the cylindrical resin-bonded magnet
2
must be reduced correspondingly, for instance, to such an extent that a ratio of an inner diameter D
2
to an outer diameter D
1
is 0.4 or more. As a result, the tendency of breakage or cracking is further accelerated.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a magnet roll having an enhanced resistance to breakage or cracking with easiness of applying a bias voltage to a sleeve.
Another object of the present invention is to provide a method of producing such a magnet roll.
To achieve the above objects, in the first aspect of the present invention, there is provided a magnet roll comprising a cylindrical resin-bonded magnet made primarily of a ferromagnetic powder and a thermoplastic resin having a modulus of longitudinal elasticity of
2
×
10
4
kg/cm
2
or more, such as
1×10
5
kg/cm
2
or more
, a first shaft made of the same material as that of the cylindrical resin-bonded magnet and integrally formed at one end of the cylindrical resin-bonded magnet by injection molding, and a second shaft made of a rigid conductive material and press-fitted into a bore of the cylindrical resin-bonded magnet at the opposite end.
In the second aspect of the present invention, there is provided a method of producing a magnet roll composed of a cylindrical resin-bonded magnet made of a ferromagnetic powder and a thermoplastic resin having a modulus of longitudinal elasticity of
2
×
10
4
kg/cm
2
or more, such as
1×10
5
kg/cm
2
or more
, and first and second shafts concentrically projecting from opposite ends of the cylindrical resin-bonded magnet, comprising the steps of (a) mixing the ferromagnetic powder with the thermoplastic resin to prepare a mixture; (b) charging the mixture into an injection-molding die to produce an integrally shaped body of the cylindrical resin-bonded magnet having a first shaft concentrically projecting from one end of the cylindrical resin-bonded magnet and a bore at the other end thereof; (c) removing the integrally shaped body from the injection-molding die; (d) press-fitting a separately prepared second shaft made of a rigid conductive material into the bore of the cylindrical resin-bonded magnet; and (e) placing the cylindrical resin-bonded magnet in a magnetic field to form a plurality of magnetic poles on an outer surface of the cylindrical resin-bonded magnet.
Since the magnet roll according to the present invention has such a structure that th
Antonelli Terry Stout & Kraus LLP
Donovan Lincoln
Hitachi Metals Ltd.
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