Compositions – Magnetic – Free metal or alloy containing
Reexamination Certificate
1999-06-18
2001-04-24
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Free metal or alloy containing
C252S062540, C264S612000, C148S105000, C148S100000, C148S104000, C023S3130FB
Reexamination Certificate
active
06221270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a compound for rare earth metal resin-bonded magnet in powdery granular form for use as a resin-bonded magnet molded into a predetermined shape by mixing a ferromagnetic alloy powder containing a rare earth element such as R—Fe—B alloy, R—Co alloy, R—Fe—N alloy (where R represents a rare earth element such as Sm or Nd), together with a resin component as a binder.
2. Description of the Related Art
A permanent magnet molded into a predetermined shape is essential as a component of a motor or an actuator to be used in household electric appliances, automobiles, computers, and their peripheries. The scale reduction, energy saving, and weight reduction of these final products are always required, and also motors and actuators are required to have less dimension, less weight, and higher efficiency. In accordance with these needs, a smaller and stronger molded product of a permanent magnetic material is demanded as a component of a motor or an actuator.
Generally, such molded product of a permanent magnetic material can be classified into two kinds: a sintered magnet and a resin-bonded magnet. A sintered magnet is produced by molding a fine powder of a magnetic alloy into a predetermined shape with a high-molecule resin used as a binder, and heating the molded product to remove or burn the binder, followed by sintering. Therefore, the obtained magnet almost consists of a permanent magnetic alloy component alone, and is excellent in magnetic properties, so that it is preferable. However, this requires both the step of molding the magnetic powder and the sintering step. Further, the sintering step requires a comparatively long time, and moreover the production yield is low due to great shrinkage in the sintering step.
On the other hand, a resin-bonded magnet is obtained by using a resin, preferably a reaction-curing resin, as a binder and molding a ferromagnetic alloy powder and the resin into a predetermined shape. In the resin-bonded magnet, the relative ratio of the ferromagnetic alloy decreases due to the presence of the binder. As a result, the resin-bonded magnet is inferior to the sintered magnet in magnetic intensity. However, the resin-bonded magnet has characteristics such that the dimension change is small in the steps after the molding, resulting in a high yield, and it can conform to a variety of product shapes simply by selecting a suitable shape of the mold, thereby achieving an excellent freedom in the shape of the product to be obtained.
A resin-bonded magnet is completed by mixing and covering a ferromagnetic alloy powder with a resin, preferably a reactive resin material into a powdery granular compound for molding, compression-molding the compound in a mold having a predetermined shape, and preferably heating the molded product to reaction-cure the reaction-curing resin material. Conventionally, such a compound has been produced by the following steps.
(1) A ferromagnetic alloy powder of a rare earth metal alloy is ground to have a particle size of less than 250 &mgr;m, typically 30 to 250 &mgr;m. Then, a reaction-curing resin material and a solvent are added and the mixture is kneaded by means of a kneading apparatus such as a kneader.
(2) The obtained kneaded product composed of the magnetic powder and the reaction-curing resin sometimes contains a large agglomerate and is not suitable for molding as it is. Therefore, the kneaded product is ground and the particles are regulated.
(3) The ground kneaded product is then dried to remove the solvent.
(4) The kneaded product after drying, i.e. a compound, is sieved for classification to collect a compound of less than 250 &mgr;m composed of the magnetic powder and the reaction-curing resin.
(5) A lubricant is added and mixed with the compound to complete compound granules for compression molding.
However, the above conventional process involves the following problems.
a) A kneading apparatus such as a kneader having a large shear force is used in preparing the compound. Therefore, the powdery particles of the material alloy is further ground to generate a fine powder, causing change in the particle size distribution of the ferromagnetic alloy powder. As a result, the particle size distribution of the compound granules also changes, leading to decrease in the compression molding property.
b) A rare earth metal alloy, particularly an Nd alloy, especially the fine powder of the above exemplary magnetic alloy, is extremely liable to be oxidized, and the generated oxide decreases the magnetic properties of the resin-bonded magnet after molding.
c) The conventional process requires a long time of 20 to 25 hours for a step of preparing the compound, and moreover it involves a batch process, so that the production efficiency is poor and the cost is high.
The purpose of the present invention is to provide a process for producing a compound that solves the above-mentioned problem of the prior art. Namely, the present invention aims at continuously producing compound granules for a rare earth metal resin-bonded magnet in a short time in which little amount of fine powder is generated due to grinding of the material alloy in preparing the compound, resulting in excellent magnetic properties of the resin-bonded magnet.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a compound for a rare earth metal resin-bonded magnet, comprising: a slurry preparation step of mixing materials containing a magnetic alloy powder of a rare earth metal alloy, a resin binder, and an organic solvent into a slurry; and a drying step of spraying and instantly drying said slurry by means of a spray dryer apparatus to produce the compound.
The ferromagnetic alloy powder to be used in the rare earth metal resin-bonded magnet has a particle size of 30 to 250 &mgr;m, which is larger than the particle size of a magnetic alloy powder to be used in a sintered magnet, i.e. 1 to 10 &mgr;m. Therefore, even if the magnetic alloy powder is mixed with a resin component into a slurry, it deposits rapidly because its density is high. For this reason, it was considered almost impossible to produce a compound by such a process. The inventors of the present invention have found out that the purpose of the present invention can be achieved by supplying a slurry to a spray dryer in a short time after preparation of the slurry, thereby completing the present invention.
The preparation of the slurry does not need a kneading apparatus such as a kneader having a high shear force, further grinding of the particles of the magnetic alloy powder does not occur, thereby preventing generation of a fine particle magnetic alloy which is liable to be oxidized. In addition, the compound is completed in a short time.
The continuous production of the compound can be carried out by continuously supplying the materials to a slurry preparation apparatus and mixing them to continuously prepare a slurry, and then supplying the slurry to a spray dryer. Alternatively, two slurry preparation apparatus may be provided in which a slurry prepared in one preparation apparatus is supplied to a spray dryer while the materials are being measured and mixed in the other slurry preparation apparatus to prepare a slurry, whereby the slurry is continuously supplied to a spray dryer by switching the supplying passageway.
The organic solvent suitable for spray drying has, unlike water, a low concentration of dissolved oxygen and is effective in preventing deterioration of the magnetic alloy powder by oxidation. Also, by applying a supersonic wave to the organic solvent, it is possible to degas the organic solvent to remove the dissolved air and water, the concentration of the dissolved oxygen can be reduced easily before supplying the organic solvent for preparation of the slurry.
The above resin binder is preferably a reaction-curing resin because it increases the strength of the molded product.
In the present invention, the slurry preferably has a magnetic alloy powder
Ishigaki Naoyuki
Kaneko Yasunari
Mishima Shin
Nishino Yoshihiko
Otsuki Shinji
Knobbe Martens Olson & Bear LLP
Koslow C. Melissa
Sumitomo Special Metal Co. Ltd.
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