Compression moulding of SMC experiments and simulation
Abstract: Due to excellent properties and relatively low material and manufacturing costs, the use of fibre reinforced polymer composites have increased during the last decades. One method that is suitable for large scale productions of e.g. lightweight vehicle components is compression moulding of sheet moulding compound (SMC). Although the technique has been considerably improved since it first was introduced, some further improvements need to be done. The main reason why it has not come in wider use in the vehicle industry is unsatisfactory conditions of the surface finish of parts manufactured due to voids. In this work, experiments and numerical simulations has been performed in order to increase the knowledge of the flow behaviour during the compression moulding process and how the flow affect the quality of the finished product. A process parameter experiment of the compression moulding phase, carried out with a design of experiment approach, was performed in order to investigate the effect of vacuum assistance,mould temperature and ram velocity on the void transport and flow behaviour for SMC. The relative amount of voids has been quantified with a high voltage insulation test and the flow behaviour has been quantified with image analysis of samples moulded with coloured SMC. In conclusion, the setting of high vacuum, low ram velocity and low mould temperature creates a homogeneous flow and minimises the amount of voids. In order to further increase the understanding of void removal during compression moulding, a model experiment was performed where a non-Newtonian fluid (grease) with added bubbles was compressed between two plates whereas the motion of the bubbles were tracked and evaluated using Particle Image Velocimetry. The bubble motion was furthermore analytically modelled and coupled to the experimental results. The experiments reveal an increase in bubble speed compared to the surrounding grease during the compression of the plates. During the latter stage of the compression, the particles change form from initially being approximately spherical, to have the characteristic form of a falling raindrop. The change in form coincides with the increase in speed of the bubble. The developed analytical model supports the shown development in the experiments. A full general solution comprising an arbitrary value of the Power Law exponent, for the velocity fraction coefficient representing the relative bubble speed, is however not covered at the present stage. Finally, the commercial software Ansys CFX were used to perform computational fluid dynamics (CFD) modelling of the flow during compression moulding with a two different multiphase models. The first model treats the flow of SMC as purely extensional and dependent on temperature, fibre volume fraction and strain rate. While the other one sees the flow as mainly extensional but also with thin shear layers near the surfaces of the moulding tool. Where the viscosity, in addition to temperature, fibre volume fraction and strain rate, also is dependant on shear strain rate. Of the two models, the latter seems to be more robust in modelling the pressure during moulding.
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