The formation of defects during the LCM processing of composite parts depends on various injection parameters. Industrial users need to realize pieces with good physical and mechanical properties and appearance. This requires to predict what is named a “processability window”. This term defines a range of parameters which will ensure a nearly absence of defects. Knowing that most of the defects created during an injection are voids, a bibliographical background about the formation and removal of these voids is presented. An original model of void fraction prediction is developed. This model is based on an empirical analysis of void formation and of the flow behaviour. An experimental qualitative validation of the model is presented.The proposed model can be used as an effective prediction tool at the design stage of a composite part.