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.

Keywords:

• Lost foam casting
• Al alloys
• Molecular weight
• Pattern coating
• Wetting
• Wicking

The authors would like to gratefully acknowledge the assistance of Mr Adrian Caden of the University of Birmingham with the execution of the experiments, Mr Dennis Nolan of Foseco Morval Inc., Ontario, Canada, for provision of materials and EPSRC for the provision of a studentship for one of the authors (PJD). Rapra Technology is thanked for carrying out the gel permeation chromatography analyses.