Thermal conditions in stereolithography injection mould tooling

The use of stereolithography (SL) as a rapid tooling technique for injection moulding provides a low cost and quick alternative to hard tooling methods when producing a small quantity of parts. However, previous work has shown that different characteristics are developed by crystalline plastic parts produced from SL moulds and those produced from conventional tooling methods. Differing characteristics means that the parts are not truly the same as those that would be produced by hard tooling and highlights a disadvantage to SL tooling. Such differences are due to the cooling rate experienced by the part. Parts produced from SL moulds are cooled more slowly than those from metal tools as a result of the differing thermal conductivity of the mould material itself. This work concerned establishing the extent of the difference in the heat transfer characteristics. The different cooling rates were demonstrated by real-time data acquisition. The results illustrated the very different thermal history imparted on the moulding that are likely to be the cause of characteristical differences in the parts. The work then describes how the thermal conditions experienced in stereolithography moulds can be used to an advantage. A case study details the use of SL moulds for the injection moulding of polyether-ether-ketone (PEEK) which has high process parameter demands. The results of the case study have shown that not only is the stereolithography rapid tooling method capable of producing a low volume of PEEK parts, but also under conditions that would not be possible using a metal mould. The thermal characteristics of stereolithography moulds allowed fully crystalline PEEK parts to be produced with the mould at room temperature; the equivalent steel mould would require a pre-moulding temperature of ~200 C and much higher injection pressures & speeds.