Behaviour of a moulded composite part: Modelling of dilatometric curve (constant pressure) or pressure (constant volume) with temperature and conversion degree gradients

Exothermal reaction and rather low thermal conductivity are two important characteristics of thermosetting composite materials, leading to strong temperature and conversion degree gradients (especially in thick parts). This creates a heterogeneous volumetric behaviour in the part, which influences the pressure evolution (after the gelation of the injection gate) in an injection mould during the crosslinking. Some defects (warpage, poor surface aspect) can be then easily generated because of a high level of residual stresses and/or the unstuck of the part from the mould wall. We propose a model of the specific volume, which takes into account the temperature and the conversion degree gradients in a 5 mm thick part. This model is based on dilatometric experiments performed on a BMC (Bulk Moulding Compound) composite. After detailed the method and the assumptions leading to an expression of the pressure in a moulding cavity, we compare the experimental and calculated pressure curves for the injection of the BMC. This pressure model gives a good agreement with the experiment.