Effect of moulding conditions on the tensile strength of cyclic olefin copolymers (COC) injection moulded parts

Abstract

The effect of processing conditions including melt temperature, mould temperature, injection speed and packing pressure on the tensile strength of cyclic olefin copolymer (COC) parts has been investigated. Tensile test specimen of 2·5 mm (thick-wall) and 1 mm (thin-wall) thick were injection moulded under specified conditions. Both single gate and double gates were used to form parts with and without weldlines. Part tensile strengths were measured experimentally. The influence of part thickness on the tensile strength is also studied. Higher melt temperature and mould temperature as well as faster injection speed increased tensile strength whereas higher packing pressure decreased tensile strength. Melt temperature and packing pressure are two parameters that affect tensile strength most significantly within the current moulding window. Higher melt and mould temperatures not only lower the residual stress but also help the diffusion of molecular chains leading to a higher tensile strength. On the other hand, high packing pressure leads to higher residual stress formation and reduces the molecular bonding rate. Weldline plays a significant role in influencing part tensile strength. Part thickness also exhibits significant effect on tensile strength: 1·0-mm-thick specimens have higher tensile strength and higher Young's modulus than 2·5-mm-thick specimens. The present investigation provides a moulding guideline for injection-moulded COC parts to achieve a specific moulding objective in part tensile performance.