YIELD STRESS BEHAVIOR OF METAL INJECTION-MOLDING SUSPENSIONS AT ELEVATED-TEMPERATURES

Yield behaviour of metal injection moulding (MIM) feedstocks was studied using a cone and plate controlled stress rheometer. Four feedstocks consisting of 58 vol % carbonyl iron (with 2 wt % nickel) in different ethylene vinyl acetate (EVA):beeswax ratio binder systems were studied at six temperatures ranging from 130 to 180 degrees C. The yield stress was found by extrapolating to zero shear rate of the measured data obtained using a controlled stress rheometer over low shear rate regime. The yield stress, tau(y), was found to increase with decreasing temperature. An Arrhenius equation was used to relate the dependence of yield stress on temperature and the corresponding activation energy for yield, E(y), can be determined. For a given temperture, feedstocks with higher EVA content exhibited higher tau(y) and E(y) Moreover, the effect of temperature on yield stress was found to be greater for feedstocks with higher EVA content.