The Fabrication Technology Of Magnetic Powder For Injection Molding

Electronic devices and components developed towards the light, complicate and high performance direction since 20th century. Compared with sintering and molding press process,injection molding process can be used in manufacturing complicate and high size precision electronic devices and components, usually called all-in-one molding and net size shaping (do not need mechanical workout), in the fields such as micromachine rotor, magnetic ring, magnetic roller in duplicator, sensors and so on. Nowadays, domestic bonded magnetic powders can only be used in preparing some low value-added plastic magnetic components. This directly results in the low product competition. So investigating and preparing magnetic powder for injection molding which has proper morphology and size making use of domestic raw materials has very positive aspects of realistic significance.The strontium ferrites with Bi₂O₃ addition were prepared by conventional solid phase synthesis process. The effects of Bi₂O₃ additives on the phase compositions, microstructure and the intrinsic coercive force of strontium ferrites at different calcining temperatures were investigated. The experimental results indicate that the liquid phase containing Bi accelerates the reaction of SrO and Fe₂O₃ for forming Sr-ferrites and improves the growth of particles at the firing temperature of 1050°C, which increases the intrinsic coercive forces of Sr-ferrites. Under the circumstances of adding Bi₂O₃, the as-received Sr-ferrite particles have the size of about 1.2μm with good sphericity and the high intrinsic coercive force exceeding 300KA/m. This benefits manufacturing the bonding ferrites with high filler contents. In addition, a possible mechanism was proposed to interpret the influence of trace of Bi₂O₃ addition on the morphology of strontium ferrite particles: SrO and Fe₂O₃ dissolve in the Bi₂O₃ liquid, and thereafter form SrM phase in it. The liquid layers enwrap the SrM phase and become thinner as SrM phase grows outward. When all the SrO and Fe₂O₃ particles are consumed, Sr-ferrites stop growing and come into being homogeneous particles with near spherical shape.The results show that there is no obvious change on the magnetic properties of the fine NdFeB powders after ball-milling when ethanol and cyclohexane were used as media. However, the coercive of the powder decreases greatly in the condition of using water as medium. The possible reason was proposed to explain the decrease of the coercive that water easily enters into the NdFeB grain boundaries under the violent impulsion and vibration during the ball-milling, the bulky and netty Nd-rich phase comes into being a fast corrision channel, which results in the serious electrochemical corrosion on the grain boundaries. The fine powder with no oxidation can be used to prepare the bonded NdFeB magnets from injection molding.