HOPKINSON, N. and DICKENS, P.M., 2000. Predicting stereolithography injection mould tool behaviour using models to predict ejection force and tool strength. International journal of production research, 38(16), pp 3747-3757.
The work reported involved Finite Element Analysis (FEA) modelling of heat transfer
in a stereolithography (SL) tool and then performing a series of experiments 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 from the
modified FEA model were then used to predict the tensile strength of the tool at
various stages after injection of the thermoplastic melt.
Previously developed equations to predict ejection forces were used to estimate the
ejection forces required to push the moulding from the SL core. During the practical
experiments the true ejection forces 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 modifications 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 higher ejection forces.
The paper concludes that, due to the variations in required ejection forces, a reliable
tool to predict tensile failure will be difficult to produce however improved
performance may be gained by adopting processing conditions contrary to those
recommended in the current process guidelines.