PATTERN TOOLING AND FINITE ELEMENT MODELLING OF INVESTMENT CASTING

Abstract

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 by trial-and-error procedures until casting dimensions are produced within acceptable dimensional tolerances. This is a costly, time-consuming task. Determining the pattern tooling dimensions is crucial to the dimensional control of the investment casting process. Recent surveys on dimensioning practices used in the investment casting industry found that the assignment of tooling dimensions for pattern die dimensioning is not consistent across the industry. To date, there are no industry-wide guidelines for tooling engineers that can be used for die dimensioning, and no comprehensive models are available for predicting dimensional changes in investment castings. In the present work, relevant properties of materials and process variables that have an important effect on tooling dimensions are analyzed. A review on waxes, shell materials, and investment casting alloys was conducted in order to identify constitutive models of those properties of the wax, shell, and alloy that need to be considered for accurate prediction of the final part dimensions. The literature survey indicates that dimensional changes between a pattern die and its casting part occur as a result of complex phenomena such as thermal expansion/contraction and hot deformation (elastic, plastic, and creep) during the processing of the pattern material (wax), mold material (shell), and solidifying alloy. The present work mainly deals with the estimation of wax pattern dimensions, which is one important factor in determining pattern tooling dimensions in investment casting. This work is an attempt to determine wax pattern dimensions using computer models that take into account the viscoelastic behavior of the wax. The wax pattern dimensions are determined using a two-dimensional finite element model for coupled thermal and mechanical analysis developed within the commercial software ANSYS 7.0. Cerita 29-51, an industrial wax is considered in this study. The results of numerical simulation are compared with experimental measurements on test patterns from the literature. Viscoelastic models exhibit good potential for accurately capturing the details of wax pattern deformation.