Effect of Ball Milling Time on Strengths of Hybrid Fiber-reinforced Plaster Molds for Investment Casting

In order to prepare high strength plaster molds for investment casting, polypropylene and aluminum silicate hybrid fibers of contents 0.50 wt% were introduced into powder mixture of gypsum and mullite. The effects of different ball milling times on strengths of plaster molds were investigated in this work. To this end, modified mullite and gypsum powder were mixed and treated by ball milling for 2.0–8.0 min. The hybrid fibers were added to the solid mixture to prepare reinforced plaster molds with high strengths. The green, fired, and residual strengths of plaster mold specimens were all tested. The results showed a positive effect of ball milling on the strengths of plaster molds, in which strength could effectively be improved by raising the ball-milled time regardless of the presence or absence of the fiber. The strength increased by at least 29.7% when compared to un-milled specimen. However, the ball milling time should not be too long since exceeding 6.0 min resulted in a slow declining trend of the strength of the plaster molds until reaching a stable plateau. Shorter ball milling periods could refine the crystal size of refractory materials. For green specimens, the morphology of plaster crystal changed from short rods to fine needles and then to column crystals as milled time rose, promoting changes in strength. The microscopic morphologies of fired specimens were slightly impacted by the increase in milling time. Overall, ball milling periods of 4.0–6.0 min were identified as optimum for fiber-reinforced plaster molds. These findings look promising for future preparation of solid shell for investment casting.