Microinjection moulding of miniaturised polymeric ortho-planar springs

Abstract

Microinjection moulding is an essential technology for mass production of components with micro-features. In addition, it can integrate multiple parts into a single piece, thus reducing the number of parts and removing assembly processes. An ortho-planar spring, a typical example of a micro-compliant mechanism, is a monolithic structure and contains a base body, multiple curved micro-beams as flexible members, and a moving platform in an out-of-plane direction as an end-effector. Owing to its complex layout, including micro-structures with a high-aspect-ratio, to the best of our knowledge, an injection moulding approach applied to the ortho-planar spring has never been previously reported. In this study, the ortho-planar spring usable in various micro-systems, is fabricated by the microinjection moulding of polyoxymethylene. Two springs, based on the same four-leg designs but with different leg dimensions, were arranged in a two-cavity mould that is equipped with a dynamic mould temperature control unit. A basic flow pattern in micro-cavities was observed from the partially filled moulded parts. In the cavity with the small leg dimension, a high-cavity pressure causes severe flashes at the parting plane, thus failing to fill the cavity completely. Mould temperature, holding pressure, and injection speed, were chosen as the process parameters for design of experiment (DOE) and analysis of variance (ANOVA) to allow identification of the significant parameters. Finally, the spring constant of the completely replicated ortho-planar micro-springs were measured and compared with the FEA results.