Predicting stereolithography injection mould tool behaviour using models to predict ejection force and tool strength

The work reported involved Finite Element Analysis (FEA) modelling of heat transfer in a stereolithography (SL) tool and then performing a series of e xperiments to measure true heat transfer in the tool. The results from the practical measurement of heat transfer were used to validate and modify the FEA model. The results fi om the modified FEA model were then used to pred ict the tensile strength of the tool at various stages after injection of t he thermoplastic melt. Previously developed equations to predict ejection f orces were used to estimate the ejection Forces required to push the mouldi ng from the SL core. During the practical experiments the true ejection for ces were measured. The combination of predicted tool strength and ejection forces were intended to be used a basis for to determine whether certain SL tool designs will fail under tension during part ejection. This would help designers and manufacturers to decide whether SL tooling is suitable for a specific application. The initial FEA heat transfer model required some mo difications and the measured ejection forces were higher than the predicted values, possible reasons for these discrepancies are given. For any given processing conditions there was an inherent Variance in the ejection forces required however longer cooling periods prior to ejection resulted in high er ejection forces. The paper concludes that, due to the variations in requ ired ejection forces, a reliable tool to predict tensile failure will be di fficult to produce however improved performance may be gained by adopting p rocessing conditions contrary to those recommended in the current process g uidelines.