Permanent magnets, which are formed of a rare earth, an iron family metal and boron, and have a rectangular, arcuate, semi-elliptical or crescent cross section and are given magnetic anisotropy by way of hot (or warm) plastic working, have been used industrially and commercially. These permanent magnets are produced as follows, for example.
A raw material prepared by mixing a rare earth, an iron family metal and boron is melted, and the molten magnet alloy so obtained is jetted out onto a rotational roll of copper, for example, to produce an extremely rapidly quenched flaky ribbon made up of crystalline grains of a nanoscale size. The magnet alloy powder obtained by the extremely rapid quenching process is crushed into particles of a required diameter, which are then cold pressed into a compact. The compact is then hot or warm pressed into a body having high density, which is then subjected to hot or warm plastic working to thereby form a magnet material of a desired shape having magnetic anisotropy. As a plastic working process to impart the magnetic anisotropy to the resulting magnet material, an extrusion process that is superior in material yield and acceptable product ratio is employed. The magnet material that has been subjected to plastic working is magnetized in a later step, whereby a practically useful permanent magnet having magnetic anisotropy is provided.
For example, JP-A-2001-15325 discloses and proposes a method for producing a plurality of permanent magnets having a radial anisotropy, for example, those having an arcuate cross section, by an extrusion process. In JP-A-2001-15325, a finned mandrel is inserted into a through hole formed in a die to define a plurality of divided holes, which coincide with a sectional shape of a permanent magnet to be obtained, between the mandrel and an inner wall of the die which defines the through hole. Then, a cylindrical blank filled in the through hole is pressed by a punch and the blank is extruded from the individual divided holes, whereby a plurality of permanent magnets which are anisotropic in a radial direction are produced.
In the production method disclosed in JP-A-2001-15325, there have been fears that, when the blank is extruded, the crystal orientation is not aligned at portions of the cylindrical blank where the cylindrical blank is divided by the fins of the mandrel, and hence the magnetic properties of the permanent magnets obtained are lowered at the divided portions. Additionally, since the blank is divided while being extruded, stress is increased at the divided portions, and the divided portions of the permanent magnets obtained may be cracked. Therefore, the cracked defective portions have to be ground out in a later step, leading to a problem that a grinding amount is increased to reduce the yield.