INJECTION MOULDING OF WAX PATTERNS: USING COMPUTATIONAL FLUID DYNAMICS AND FLOW VISUALISATION TO UNDERSTAND FILLING BEHAVIOUR

摘要

"Fundamentals Of investment CASTing" (FOCAST), is a three-year project being undertaken at the University of Birmingham U.K., to study four processes: Wax injection moulding, ceramic shelling, de-waxing and casting. The filling stage of the wax injection moulding cycle can produce surface defects in the form of flow-lines, weld-lines and air bubbles. This paper considers the upward filling of a thick cylindrical pattern and the effect of entrance velocity on the amount of air entrapment and subsequent change to the pattern density. Experiments were performed on a Battenfeld screw-injection-moulding machine with closed-loop flow-rate control, using a typical commercial wax at a liquid temperature of 70°C (158°F) and at a paste temperature of 56°C (133°F); as well as a transparent wax at a liquid temperature of 70°C to visualise the formation of air bubbles. Results were compared with computational fluid dynamics (CFD) software simulations. In general the modelling and experimental results were found to be in good agreement, however several limitations of the methodology are discussed. The critical velocity of the transparent wax based on flow visualisation was found to be around 0.5 m·s{sup}(-1) (1.6 ft·s{sup}(-1)). The critical velocity for the commercial wax when liquid, based on the measurement of pattern density was found to be approximately 0.8 m·s{sup}(-1) (2.6 ft·s{sup}(-1)).