VOID FORMATION IN RTM

The problem of void formation in composites processing is very important since the physical and mechanical properties as well as the finish of the product are strongly influenced by the presence of voids. Examples of properties that are adversely affected by voids are the strength and the weather resistance of the composite. The knowledge about void formation in the RTM process is still very limited compared to what is known about autoclave production of composites, and it is not clear whether the voids are created by diffusion growth, mechanical entrapment or some other mechanism. Our research has been concentrated on the influence of vacuum assistance on the void formation in RTM. The experiments were made in a flat mould with edge injection (unidirectional flow from one side to the other) in which the vacuum level could be varied from atmospheric pressure to about 1 kPa (10 mbar). The total flow length was 0.55 m in all experiments. The void volume fraction was determined with optical microscopy and image analysis and it was found that most of the voids were concentrated to a small area close to the flow front. The influence of vacuum was that the maximum void content and the size of the region with voids decreased with increasing vacuum assistance. The qualitative behavior can be explained with a simple theory based on the ideal gas law and the assumption that the voids are created by mechanical entrapment.