Matthias Wesselmann

Abstract

Impact of the moulding conditions on the properties of short fibre reinforced high performance thermoplastic parts

Matthias Wesselmann - 14 february 2003

Composite parts moulded from polyetherimide reinforced with 30 weight percent of glass fibres are experimentally studied in terms of local moulding conditions, local microstructure and local mechanical properties. For the microstructural characterisation consequent use is made of the large angle sectioning technique. The effect of cavity length on the heterogeneity of microstructure and mechanical behaviour of these parts is studied using cavities with variable lengths. Fibres follow the movement of the matrix which conveys them to their final position in the composite part. Extensional flow, post shear flow and pre shear flow are factors which characterise the flow pattern of the melt surrounding the fibre. A strong positive extensional flow region on the flow path initiates a high degree of fibre alignment transverse to flow. Pre flow shear and post flow shear both attenuate the initial transverse alignment by aligning more and more fibres parallel to flow. Pre shear controls the alignment of the core region, post flow shear controls the thickness and alignment of the outer layers solidified under shear flow. An empirical correlation for a diverging strip bar relates the mean gap-wise alignment of a probe to post shear flow and pre shear flow.The microstructure and short-term behaviour of a diverging strip bar are mainly influenced by injection time and cavity geometry. Longer injection times promote anisotropy and heterogeneity of properties. Longer cavities increase alignment parallel to flow and mechanical properties parallel to flow. In the process window recommended for fibre reinforced polyetherimide mould temperature and melt temperature had little effect on microstructure and mechanical properties.The influence of temperature and frequency on fatigue behaviour is tested in pure tensile mode on double dumbbell specimen. The fatigue life of a specimen increases with its stiffness. Static and dynamic crack propagation behaviour is determined on compact tension specimen cut at several angles to the flow direction. The measured microstructure is compared to the microstructure predicted by simulation using the Hele-Shaw model. Large differences are seen in the region of strong extensional flow where the simulation overestimates the transverse alignment.

Keywords:
injection moulding process, moulding conditions, microstructure, mechanical behaviour, composites.

Last modified: 12/10/2006 04:20 PM