Using stereolithography tools for injection moulding: research into tensile tool failure and unexpected benefits of the process

The use of stereolithography (SL) parts as injection moulding tools offers many advantages over traditional tool making approaches. In particular, the time required to convert a computer aided design (CAD) file to the final tool is dramatically reduced as may be the costs in creating the tool. However, the process has been perceived to have a number of drawbacks which are, in the main, associated with the poor thermal and mechanical properties of the SL resins. Research was performed which involved the production and use of an SL tool to injection mould a series of parts in polypropylene. Measurements were made of the forces required to eject parts from the tool in order to assess the possibility of tensile tool failure during part ejection. The measured forces were compared with predicted values which had been calculated using previously devised equations. In addition, temperatures were recorded throughout the injection moulding cycle to assess the tool's strength at different times. Surface roughness measurements were also taken to characterize the ejection process more closely and observe any changes to surface roughness caused by moulding parts. A finite element analysis was made of the heat transfer in the tool to help with prediction of ejection forces. Measurements of ejection forces and heat transfer indicated that, contrary to existing recommendations, part ejection should be performed shortly after injection. Accurate predictions of ejection force were shown to be unattainable given the difficulty in finding accurate values for the variables used in the force prediction equations. Measurements of heat transfer coupled with surface roughness suggested that the low thermal properties of SL tools actually work in favour of the process rather than against it. This conceptual about-turn regarding the role of thermal properties in SL injection moulding tools indicates that further benefits may be derived from the new technology.