Flow and void formation in powder injection moulding feedstocks made with PEG/PMMA binders Part 1 – Experimental observations

Abstract

Mixtures of 94 wt-% tungsten carbide (WC) and 6 wt-% cobalt (Co) powders with different particle sizes have been formed with binders composed of polyethyleneglycol (PEG) and polymethylmethacrylate (PMMA), in some cases with the addition of stearic acid (SA) as a lubricant. The presence of voids has had to be invoked to explain why, when the same moulding conditions were used, the as moulded and as leached densities varied with binder composition and in some cases, why the former were lower than expected from the proportions of binder components and solids used to prepare the feedstocks. For a given nominal binder content (vol.-%), calculated on the assumption of zero voidage, and using constant moulding conditions, it appears that the actual voidage tended to decrease as PMMA progressively replaced PEG, and to increase as SA was introduced to replace PEG. The former change to the binder composition should, with constant binder content and moulding conditions, increase the apparent viscosity of the feedstock, while the latter should reduce it. A slip band model is postulated which allows flow behaviour and void formation to be explained qualitatively. This is further developed in Part 2 of this paper. Data from previous studies using PEG/PMMA binders with stainless steel powders are included as these can now be given a qualitative explanation with the slip band model.