Co-injection moulding: effects of process parameters on the skin/core distribution in a model mould

In co-injection moulding two polymers are injected sequentially into a mould, thus forming a skin/core structure which can be interesting for mechanical, aesthetic or economic reasons. In order to exploit this technology the materials distribution should be optimised, and breakthrough, i.e. the possibility that core breaks the skin and reaches the mould surface, must be avoided. In this work the effect of processing parameters on core breakthrough and material distribution in a simple mould was studied. PMMA was used for the skin and ABS for the core. In order to maximize the amount of core material while avoiding breakthrough, a series of computer simulations and experimental tests were performed according to the statistical approach of design of experiments (DOE), which allows to identify the influences of several parameters by means of a minimum number of experiments. The moulding parameters considered were the core volumetric fraction, its injection rate and temperature and the skin injection temperature. The comparison of experimental results and simulations shows that the latter overestimate the ABS content at which the breakthrough takes place. However the results of statistical analysis performed on data showed a significant influence both in simulation and experiments: an increase in core content is more likely to cause breakthrough, whereas a higher core material injection rate seems to have a beneficial effect. The core and skin material injection temperature seem to have a limited effect in the simulations, while showed no effect in the experiments, which is probably due to an inadequate temperature control during the long moulding cycle. The effects of other parameters are still to be assessed.