The Selection of Mould Design Variables in Direct Stereolithography Injection Mould Tooling 478

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Harris, Russ
Hopkinson, Neil
Dickens, Phill
Hague, Richard

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The introduction of rapid prototyping has allowed engineers and designers to generate physical models of required parts very early on in the design and development phase. Further to this the use of stereolithography (SL) cavities as a rapid tooling method has allowed plastic prototype parts to be produced in their most common production manner; by injection moulding. The process is best suited to small production runs where the high costs of conventionally machined tooling is prohibitive. One of the major drawbacks of the SL injection moulding process is the susceptibility of the tools to premature failure. SL tools may break under the force exerted by part ejection when the friction between a moulding and a core is greater than the tensile strength of the core resulting in tensile failure. Very few justified recommendations exist concerning the choice of mould design variables that can lower the part ejection force experienced and reduce the risk of SL tool failure. This research investigates the ejection forces resulting from injection moulding polypropylene (PP) and acrylonitrile-butadiene-styrene (ABS) parts from SL tools which are identical in all respects except for their build layer thickness and incorporated draft angles. This work attempts to identify appropriate evidence for recommendations with respect to these design variables and SL injection moulding. The results show that adjustment of draft angle results in a change of part ejection force as a reasonably linear relationship. An adjustment of the build layer thickness results in a change in part ejection force as a more non-linear relationship. The adjustment of build layer thickness had a greater effect on ejection force than the adjustment of draft angle. In both cases greater ejection forces were experienced by ABS parts as compared to PP parts. The results also show that the surface roughness of all tools remains unchanged after moulding a number of parts in both polymers.



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