Polyester moulding compounds with novel initial fibre orientation for use in injection moulding

Abstract

This paper describes a systematic study of the effect of initial fibre orientation on the flexural and impact strength of injection moulded thermosetting polyester moulding compounds. A range of special compounds was prepared, with three different types of initial fibre orientation: fully random (3-D), random-in-plane (2-D) and unidirectional (1-D). It was found possible to produce injection mouldings with low levels of fibre breakage and significantly improved properties by using starting compounds with either 2-D or 1-D fibre orientation. This does not appear to be possible with 3-D compounds, such as dough moulding compounds. In addition to using a starting material with preferred fibre orientation, it is also desirable that the matrix viscosity under processing conditions be as low as possible. In the case of sheet moulding compounds, ‘physically thickened’ compounds give better performance than MgO-thickened materials for this reason. Comparisons between the behaviour of dough moulding compounds and that of compounds made by the continuous impregnation method confirm the findings of other workers, that the latter technique is capable of producing compounds with a much lower level of initial fibre degradation.