The structure of parts produced by stereolithography injection mould tools and the effect on part shrinkage

Abstract

Stereolithography (SL) tooling for plastic injection moulding provides a low cost and quick alternative to hard tooling methods when producing a small quantity of parts. However, work by the authors has shown that a different rate of polymer shrinkage was experienced in semi-crystalline parts when produced from SL moulds as compared to those from conventional metal tooling methods.Different shrinkage means the parts are not truly the same as those that would be produced by metal tooling and highlights a disadvantage to SL tooling. This work associates the increased shrinkage experienced to a greater percentage crystallinity developed in the parts due to their thermal history during processing. In these experiments the cooling rate, which is imparted due to the 2 heat transfer characteristics of the mould has been identified as the controlling factor of a parts % crystalline content and the cause of shrinkage anomalies.The morphology analysis results show that there is 30% more crystallinity developed in the nylon (PA66) parts produced in SL moulds than those produced from aluminium moulds. The results also reveal different characteristics during thermal analysis that may also be due to the thermal history imparted by the mould. The work utilises the thermal analysis technique Differential ScanningCalorimetry (DSC) to quantify the different levels of crystallinity in the parts. The thermal characteristics of the mould are demonstrated by real-time data acquisition.

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