Viscosity and melt rheology of metal injection moulding feedstocks

Four kinds of Fe-2Ni metal injection moulding (MIM) feedstocks with the same powder loading and with similar binder composition except for minor components were prepared and their viscosities at different temperatures and shear rates determined using a capillary rheometer. The flowability and the sensitivity of viscosity to shear rate and temperature were investigated. The viscosities of the binders and the general rheological indices of feedstocks were compared. The results show that the minor components of the binders play a very important role in the rheology of feedstocks. A feedstock based on a PW-PP binder can not be used for injection moulding owing to severe powder/binder separation. A feedstock based on a PW-EVA binder has the lowest viscosity and is the easiest to flow into the mould. The viscosity of a feedstock based on a PW-EVA-HDPE binder is higher than that of a feedstock based on the PW-EVA binder and is lower than that of a feedstock based on a PW-HDPE binder. The general rheological index, which includes the effect of flowability and the sensitivity of viscosity to shear rate and temperature, can be used to evaluate the general rheological properties of MIM feedstocks. At a reference temperature of 150 degrees C and reference shear rate of 1181 s(-1), the general rheological indices for the feedstocks based on the PW-EVA, PW-HDPE, and PW-EVA-HDPE binders are 7.8, 4.0, and 9.3 respectively. The feedstock based on the PW-EVA-HDPE binder has the best general rheological properties and is most suitable for injection moulding. The mechanical properties and shrinkage homogeneity of MIM tensile bars prepared with the feedstock based on the PW-EVA HDPE binder are also the most acceptable.