成形加工 18 巻 (2006) 9 号 p. 689-694
In order to clarify the relationship between the material properties of metal injection molding (MIM) parts and MIM processability, melt shear viscosity and Pressure-Volume-Temperature (PVT) measurements were performed for various MIM materials with different blend ratios of metal powder to polymer binder. The changes in internal cavity pressure for these blends were monitored and the appearance of molded parts were evaluated. The following results were obtained. The pellet density of MIM material increased monotonously with an increase in metal powder; however, above 70vol%, a reduction in density was observed as there was insufficient binder to enclose all the metal powder and voids appeared inside the pellets. The melt shear viscosity increased with increasing metal powder content, with an abrupt increase at 50vol%. PVT measurements showed a two-step decrease in specific volume, a result of the solidification of the polymeric binder (POM, PP, WAX, etc.). The internal cavity pressure for the MIM materials differed from the binder resins, and a two-step pressure decrease was also observed, showing similarities to the PVT data. Pressure energy, defined as the area under the internal cavity pressure vs time curve, was studied during the MIM cooling process. At 60vol% or less, the maximum internal cavity pressure and the holding pressure were nearly the same. The solidification rate increased with increasing metal powder content, with an abrupt increase at 65vol%. Furthermore, at 65vol% and over, cracks occurred in MIM products. These combined results indicated that the optimum MIM blend was 60vol% metal powder.