We investigated a method for forming a high-density near-net-shape dust core for a long, high-performance ignition coil center core. In the development, the causes of deterioration of dimensional accuracy were divided into the dispersion of the amount of powder fed into the mold and the dispersion of density that occurs in the mold during molding, and the factors were analyzed. First, by sufficiently reducing the charge amount of the raw material powder, the apparent density of the powder was stabilized, and variations in the powder feed amount were suppressed. In addition, the density unevenness inside the dust core could be reduced by restructuring the upper and lower punches of the die and optimizing the stroke during compaction. Based on these results, a dust core with little density unevenness, high density, and good dimensional accuracy was realized.

This technology was developed by surveying the characteristics of powder lubricants as raw material powders prepared for powder metallurgy from melting and mixing N, N’-ethylene-bis-stearamide (EBS) and lithium stearate. The author found that not only the particle size and composition of the EBS-containing wax composite agent but also the crystal polymorphism that originated from the EBS affected the powder characteristics, lubricant properties, and compact powder characteristics of the metal powder of the raw material and that the difference between the M-type and the S-type presented different trends. The M-type is preferable for the improvement of flow characteristics and the apparent density of the metal powder of the raw material, and the blending quantity of lithium stearate also affects the flow characteristics. The M-type is preferable for the effect of improving transverse rupture strength. The S-type is preferable for the realization of both the reduction of ejection pressure and improvement in the compact powder characteristics (green density and spring back).

The effect of porosity on coercive force was analyzed quantitatively about iron powder cores with different porosities. The coercive force of the iron powder cores decreased 11.0 A m^-1 with a decrease in porosity by 0.01. From the in-situ observation of Kerr effect microscopy, the nucleation of the reverse domain was observed at the local areas along narrow gaps such as a contact interface between particles / the fine pores among the particles, and there are no nucleation of the reverse domain at the coarse pores. It indicates that the local decrease in the diamagnetic field with a decrease in porosity might be reduced in these areas resulting in the coercive force decreases. From this result, densification of iron powder cores can be an effective measure to reduce coercive force.