Void formation and transport in manufacturing of polymer composites

Sammanfattning: In this thesis void formation and removal during the Resin Transfer Moulding process are considered. The problem is examined by doing real moulding as well as mouldings under microscope, model experiment and analytical computations. The materials utilised are in most cases glass fibre reinforcements and vinylester resins. Many of the results are, however, of general character and should be applicable to other material combinations. From micrographs of cured laminates, two types of voids are identified: cylindrically shaped voids within the fibre bundles, and spherically shaped voids between the bundles. It is established that the voids are formed at the resin flow front and that the preform geometry strongly influences both the formation and the transport of the voids. The formed voids move with the resin towards the resin flow front if the pressure gradient is sufficiently high to force the voids through constrictions in the reinforcement. It is shown that the cylindrical voids are more likely trapped in constrictions than the spherical voids. Voids that stay trapped in the reinforcement can, however, escape if their volume is reduced. This is the case if the pressure around the void increases or if the conditions are such that entrapped gas dissolves into the resin. In all injections presented, the voids are concentrated towards the outlet side of the mould. The model experiments, however, indicate that in more coInplicated mould geometries voids may also be located in other areas. These are, for instance, areas where the pressure gradient is low, where the resin flow is poor and where a dry spot has formed. The most efficient way to decrease the void contents is to apply a vacuum on the outlet side of the mould. This decreases the size of the formed voids and make them more mobile. The mould must be air tight and the pressure on the outlet side of the mould must be higher than the boiling pressure of the resin at the processing temperature. In the thesis other methods to eliminate voids are considered such as to degas the resin before the injection, to apply a pressure during cure, to use resins and reinforcements that are compatible, to use reinforcements in which voids are mobile and to continue to fill the mould after complete filling with a high driving pressure. In the thesis, also the influence on the permeability of the detailed fibre geometry is treated. The result is that minor alterations in the geometry greatly change the permeability. Finally, a method which measures the impregnation of fibre bundles is tested.