Wetting and wicking of liquid polymer degradation byproducts into the pattern coating during lost foam casting of Al alloys

Abstract

The behaviour of liquid polymer degradation byproducts in the lost foam casting of aluminium alloys, and the conditions under which it wicks into the permeable pattern coating, were examined using gel permeation chromatography. Both typical expanded polystyrene and brominated polystyrene pattern materials, with coatings of two different permeabilities, were examined. Heating reduced the molecular weight of the polymer leading to absorption of the liquid polymer degradation byproduct into the permeable pattern coating once a critical molecular weight was reached, the value of which was dependent upon coating permeability. Horizontally oriented lost foam plate castings, instrumented with thermocouples, were cast in an Al-Cu alloy, using a real time X-ray imaging technique to follow the advancing liquid metal front. This showed that the conditions for wicking were not likely to have occurred in the vicinity of the liquid metal/foam pattern interface, but absorption of the liquid polymer degradation byproducts into the permeable pattern coating would only occur after the metal front has passed.

Instability of the Liquid Metal–Pattern Interface in the Lost Foam Casting of Aluminum Alloys

“…Both explanations seem to be partly true, with a critical molecular weight needed for wetting of, and wicking into, a coating. Griffiths and Davies [12] determined the values of this critical molecular weight, and how it varied with coating permeability.…”