The effect of moisture upon the mechanical properties of ceramic moulds during high pressure steam dewaxing

Investment casting research is being carried out by the University of Birmingham, sponsored by EPSRC and a consortium of industrial companies. The programme is aimed at developing a fundamental understanding of the process, with a view to routinely producing sound, net shape castings. A key stage within the investment process is that involving the removal of wax from the unfired ceramic shell. This important process is carried out within the confines of a sealed pressure vessel, more commonly referred to as a Boilerclave (trademark, Leeds and Bradford Boiler Co. Ltd, UK) with external pressure gauges as the only indication of what is actually happening inside the cavity. The dewax stage is a key stage in the process as wax needs to be removed from a weak ceramic shell system without cracking or dimensional alteration, which would be reflected in out of tolerance casting scrap. Owing to the nature of the process, wax removal is probably the least understood or controlled aspect of the whole investment sequence. The following paper is the third in a series which contain results obtained at the University of Birmingham using a specially instrumented stream autoclave which allows visual data capture, thermal and steam pressure profiles within the chamber and thermal instrumentation of waxes and shells to be obtained. The results of simple conductivity calculations and experimentation to determine the effect of moisture upon the strength of ceramic shells will be presented and the strengthening role of wax penetration into the primary coat of the ceramic shell during dewax will be investigated.