Experiments and numerical modelling of the SMC and autoclave processes

Abstract: The work presented in this thesis focus on the flow taking place in two industrialmanufacturing methods for composite manufacturing. The first method underinvestigation is compression moulding of Sheet Moulding Compound (SMC).With this method, SMC prepreg are stacked and placed in a heated mould toolwhich then is closed and held under pressure until the part has cured. Thismethod can be used to manufacture large quantities since the cycle time is short.The drawbacks with the method include pores at the surface causing largervisual defects such as blowouts if the part is painted and relatively poormechanical properties as compared to composites made out of continuous fibres.The aim of this work has been to increase the understanding of the flow that isgenerated during the filling of the mould and how this affects defects such aspores. This has been done experimentally with a circular moulding toolequipped with pressure sensors and vacuum assistance capability. The utilisedpress allowed different temperatures and closure velocities to be used. Thequality of the moulded experimental SMC plates was quantified with differentmethods. Pressure sensors revealed the pressure inside the mould and the flowbehaviour could for example be analysed with image analysis of plates mouldedwith multi-coloured SMC. The relative void content was measured with a highvoltage insulation test. The experiments showed that SMC that had flowed alarger distance had less voids than SMC that still was in the centre.Investigations of the coloured SMC indicate that a high closing velocity (10 mms-1) of the mould gives a more homogenous flow and also that by applyingvacuum assistance, a more homogenous flow is achieved with low closingvelocity (2.5 mm s-1). Interestingly the settings that resulted in a morehomogenous flow also resulted in less voids, indicating that a more homogenous flow should be sought for if a low amount of voids is desired. The setting that gave overall best result was to use vacuum assistance (75% vacuum), the low mould temperature (144 °C vs. 154 °C) and low velocity. Vacuum assistance also seemed to prevent a back pressure inside the closed mould tool since the press was able to compress further with constant velocity with vacuum assistance. In addition to the experimental work, non-Newtonian viscosity models for SMC have been developed for Computational Fluid Dynamics simulations which could predict the pressure at different closing velocities. The models are complex, but the one thing that seemed to be most important in order to predict accurate pressure at different closing velocities was to allow the bulk material to behave shear thinning. Along with the models, a method of finding unknown constants is presented which allows the models to be used for various SMCs without knowledge about their real material properties. In addition to the experimental work with SMC, the behaviour of bubbles in a non-Newtonian fluid during compression was investigated experimentally with Particle Image Velocimetry and the bubble motion was furthermore also analytically modelledand the results were coupled to the experiment results.