Dimensional accuracy of injection-moulded components produced in an enhanced silicone rubber mould

The inherent technical limitations of rapid prototyping systems have led to the development of rapid tooling techniques. These techniques are capable of producing prototypes in final materials by the intended manufacturing process. One rapid tooling process that provides a low cost and rapid route to the manufacture of injection-moulded thermoplastic components is enhanced silicone moulding. During the moulding cycle the mould deforms under the pressures and temperatures experienced and components produced suffer a loss of dimensional accuracy. An analytical and experimental investigation was undertaken to quantify the dimensional accuracy of components produced by the process, and to identify the moulding parameters which contribute to the loss of dimensional accuracy. A mould was produced from a stereolithography master pattern and used to injection-mould components in high-impact polystyrene under various moulding conditions. Moulded components produced were typically oversized with a loss of dimensional accuracy of the order of 0.3-0.6mm. Two parameters, namely injection pressure and mould temperature, were found to have an influence on dimensional accuracy. The injection pressure causes the mould cavity to expand and results in manufacture of components which are oversized. Components produced at an injection pressure of 16 MPa were on average 0.34mm (1.3 per cent) oversized in width and 0.36mm (11.3 per cent) oversized in depth. Those produced at an injection pressure of 20 MPa were oversized by an average of 0.45 mm (1.8 per cent) in width and 0.59mm (18.6 per cent) in depth. An increase in mould temperature caused a reduction in the size of the mould cavity. This was because the rubber mould was restricted in a metal mould frame. The rubber mould expands into the free space of the cavity. Components produced at mould temperatures of 50, 70 and 90degreesC were oversized in width by an average of 0.37mm (1.5 per cent), 0.34mm (1.3 per cent) and 0.20mm (0.8 per cent) respectively.