Influence of mould thermal properties on the replication of micro parts via injection moulding

The surface quality that results when replicating micro features is one of the most important process characteristics in micro injection moulding, and it constitutes a manufacturing constraint in applying the technique to a wider range of micro engineering applications. Moulding micro features with a high aspect ratio is a critical task, in particular when the feature width is small, due to a faster temperature decrease than in macro/meso scale cavities. In order to investigate the influence of the thermal diffusivity of the mould material in micro structured surfaces replication, in this paper two moulds, made respectively of tool steel and zirconia ceramic composite, have been used to replicate a micro structured surface. Micro Electrical Discharge Machining (μEDM) was employed to manufacture both the steel and the ceramic mould. The thermal diffusivity of the mould materials was measured in order to relate it to the degree of replication. Then, micro features were replicated via micro injection moulding, at the same controlled process conditions, and the replication degree was measured by means of an optical coordinate measurement machine. The results of the experimental tests display a sharp improvement of the quality of the micro structure replicated with the ceramic mould, that is when using a mould material with low thermal diffusivity. This effect, which is related to the ability of the material to delay the polymer skin solidification when the cavity is filled in, can be effectively exploit to enhance the capabilities of the current micro injection moulding technologies in manufacturing components with features characterized by higher complexity and aspect ratio.