High shear flow of short glass fibre filled polybutadiene terephthalate - Part III: High rate transient start-up flow

Transient pressure traces as recorded with a prototype injection rheometer exhibit overshoots during the early period of injection shots of glass fibr e filled polybutylene terephthalate (GF-PBT) whereas the virgin polymer giv es steady pressure growth traces. A modelling is proposed that considers th e superposition of two events: A smooth pressure growth associated with the flow development of the virgin polymer, and a pulse pressure consumption a s fibres, initially randomly oriented, align themselves along the developin g flow paths, The model is applied to experimental data through non-linear regression techniques and a good agreement is observed, An interesting aspe ct of the approach is that it is not necessary to refer to non-linear visco elasticity to explain the behaviour of GF-PBT.