Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-07-03
2002-02-12
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06346565
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a synthetic resin composition for a resin magnet, which is produced by mixing and dispersing a magnetic powder in a resin binder, and which is suitably used as a molding material for forming a molded resin magnet adapted for a magnet roller used for an electrophotographic system or an electrostatic recording system, and also relates to a resin magnet molded from the synthetic resin composition into a desired shape.
A development method characterized by supplying toner with the aid of a so-called jumping phenomenon has been used for an electrophotographic system or electrostatic recording system such as a copying machine or printer. To be more specific, according to such a development method, as a developing roller for visualizing an electrostatic latent image on a latent image support such as a photosensitive drum, a magnet roller formed of a resin magnet is disposed in a rotating sleeve, whereby toner is supplied to the surface of the latent image support with the aid of the jumping phenomenon, which allows the toner supported on the surface of the sleeve to be jumped on the latent image support by the magnetic force characteristic of the magnet roller, to visualize the electrostatic latent image.
The above-described magnet roller has been produced in accordance with the process of preparing a synthetic resin composition for a resin magnet by mixing a magnetic powder in a binder composed of a thermoplastic resin, and injection-molding or extrusion-molding the resin composition by using a mold around which a magnetic field is formed, thereby forming the resin composition into a roller shape and simultaneously magnetizing it into a desired magnetic characteristic.
With the recent advance of electrophotographic systems and the like, the above magnet roller has been required to have a more complicated magnetic force pattern, and to meet such a requirement, there have been often adopted a magnet roller including a plurality of magnet pieces. The magnetic pieces, each of which is molded from the above synthetic resin composition and magnetized into one magnetic pole as one component of the desired magnetic pattern, are stuck around a shaft, to thereby achieve the desired magnetic force pattern as a whole.
The synthetic resin composition for a resin magnet adapted for the above magnet roller has been produced by mixing and dispersing a magnetic powder of ferrite or a rare earth magnet alloy in a resin binder composed of a polyamide resin such as polyamide 6 or polyamide 12, or polypropylene.
However, with the recent tendency toward higher quality, higher speed, and higher accuracy of OA equipment, the above magnet roller has been required to have a higher magnetic force. Additionally, in other fields, resin magnets have been usually required to have higher magnetic forces.
To meet such a requirement, it is required to increase the content of a magnetic powder in a synthetic resin composition for a resin magnet adapted for a magnet roller and the like; however, if the content of a magnetic powder is increased, the flowability of the molten resin composition Is lowered, so that the moldability of the molten resin composition is significantly reduced, to cause both the variation in magnetic force of a molded product and degradation of the dimensional accuracy thereof. For this reason, the content of a magnetic powder in a synthetic resin composition has been limited. In other words, it has been difficult to add, to a resin composition, a magnetic powder in an amount large enough to obtain a necessary high magnetic force.
A resin magnet has been used not only for the above-described magnet roller but also for various general-purpose parts, particularly, for precise parts of electric/electronic equipment. In this case, such a resin magnet has been produced by molding the above resin composition into a desired shape. The resin magnet thus produced, however, fails to sufficiently satisfy high dimensional accuracies required for the magnet roller and precise parts of electric/electronic equipment.
To be more specific, a resin magnet used for a magnet roller of an electrophotographic system or electrostatic recording system and precise parts of electric/electronic equipment requires a very severe quality in terms of dimensional accuracy and environmental change in dimension; however, the resin magnet produced by molding the above resin composition is insufficient in its dimensional accuracy because of contraction of the resin after molding, and even if the resin magnet exhibits a good dimensional accuracy at the time of molding, it cannot keep the dimensional accuracy for a long-period of time because of expansion and/or camber of the resin magnet caused by absorption of moisture in a service environment.
By the way, a resin magnet for a magnet roller has been produced in accordance with a process of preparing a resin composition by mixing and dispersing a magnetic powder in a resin binder and extruding the mixture into pellets, and injection-molding or extrusion-molding the pellets of resin composition into a roller shape and simultaneously magnetizing the roller-shaped product into a desired magnetic characteristic.
The above-described pellets of the resin composition to be injection-molded or extrusion-molded into a resin magnet have been produced by mixing a magnetic powder in a binder resin and extruding the mixture into pellets. In this case, as the above binder resin, a resin generally commercially available in the form of pellets has been used. The pellets of resin composition have been produced by pulverizing the pellets of resin, mixing the pellets thus pulverized with a magnetic powder, and extruding the mixture into pellets.
The reason why the pellets of resin are pulverized once before being mixed with a magnetic powder is as follows:
(1) If the pellets of binder resin are directly mixed with the magnetic powder, the binder resin is separated from the magnetic powder upon input of the pellets of binder resin and magnetic powder in a kneader or during the kneading operation thereof, with a result that the content of the magnetic powder in the final pellets of the resin composition varies or the mixing/kneading of the binder resin and the magnetic powder becomes insufficient; and at the worst case, the mixing/kneading of the binder resin and the magnetic powder becomes impossible.
(2) If the resin composition is produced by using pellets of a resin binder, the accuracy in supplying a magnetic powder and the resin is degraded, to vary the extruding rate, which may obstruct the stable production of the resin composition.
(3) If the content of a magnetic powder in the pellets of the resin composition varies, the magnetic characteristic of a product molded from the resin composition also varies, resulting in not only degradation of quality in terms of magnetic characteristic but also lowering of the production yield and frequent occurrence of defectives.
The method of producing the resin composition characterized by pulverizing pellets of a binder resin once, however, has the following problems: namely, since the method requires a pulverizer for pulverizing pellets of a binder resin, the cost of equipment is raised, and since the method additionally requires the pulverizing step, the cost required for preparing a raw material is raised, thereby increasing the production cost of a resin magnet produced from the resin composition.
SUMMARY OF THE INVENTION
A first object of the present Invention is to provide a synthetic resin composition for a resin magnet, which is good in flowability upon melting, and which is capable of keeping the good flowability upon melting even if the content of a magnetic powder is increased and obtaining a high magnetic force of a molded product without lowering the moldability, and to provide a resin magnet molded from the synthetic resin composition.
A second object of the present invention is to provide a synthetic resin composition for a resin magnet, which is capable of preventing a reduction i
Daifuku Hideharu
Kawano Kota
Bridgestone Corporation
Michl Paul R.
Sughrue & Mion, PLLC
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