Micro-hardness tests on thermoplastic polymers obtained by injection moulding

An experimental investigation was carried in order to verify whether micro-hardness tests could give information on the preferential orientation of polymer chains in a plastic piece obtained by injection moulding, and could be correlated to the process parameters. To this aim, shrinkage tests and Knoop micro-hardness tests were carried out on ABS specimens fabricated adopting two different melt temperatures. The shrinkage tests showed a strong dependence of the shrinkage on both the moulding temperature and distance from the injection gate. In particular, a higher dimensional change was associated with lower melt temperatures and lower distances from the gate. These results witnessed a change in polymer structure as a function of the previous parameters. Nevertheless, the Knoop hardness was insensitive to them; rather, it was strongly correlated to the orientation of the Knoop indentor with respect to the polymer flow during mould filling, revealing the material anisotropy. An explanation of the phenomena observed is given in terms of material structure, suggesting a different hardness test methodology to highlight the effect of process parameters.