Modeling casting dimensions: effect of wax rheology and interfacial heat transfer

As investment castings grow in size and complexity, control of their dimensions becomes increasingly important. The traditional method is to pour castings, measure them, and adjust the tooling to produce accurate castings. This is a costly, time-consuming task. It is much preferable to predict casting dimensions from first principles, and build accurate tooling to produce them. However, the prediction of accurate casting dimensions depends on predicting how the patterns, molds and castings distort in each of the manufacturing steps. Two contributing factors to successful modeling of final casting dimensions have been largely neglected - pattern wax rheology and interfacial heat transfer. The interaction of wax rheological properties with injection parameters has been ignored as investment founders generally use trial-and-error to adjust injection parameters. The large number of custom-blended and proprietary pattern waxes has contributed to discouraging development of wax models based on wax properties. This situation will be reviewed, The suggestion of Campbell that interfacial heat transfer coefficients would affect casting dimensions has been confirmed in work in which we are currently engaged. However, our work has also called into question current methods of measuring interfacial heat transfer coefficients, especially in the thin sections found in investment castings. The measurement problem will be reviewed and methods of achieving more accurate measurements discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.