Fabrication of Sr-Ferrite Plastic-Bonded Magnet Rotor by Injection Molding

A plastic-bonded strontium-ferrite rotor assembly for brushless dc motors is fabricated by injection molding to produce a four-pole arrangement of anisotropic SrO\cdot6Fe₂O₃ plastic-bonded magnets. The resultant rotor has superior magnetic flux density characteristics to a conventional four-pole plastic-bonded rotor. The degree of anisotropy and hence magnetic flux of the magnets is found to be dependent on the injection time, with faster injection times producing magnets with higher flux densities. The proposed fabrication method and rotor design eliminate the need for a magnetic path and dynamic balancing of the rotor. The resultant motor therefore operates at a significantly lower noise level compared to motors based on conventional rotors.