Rheology and fibre orientation in the injection moulding of long fibre reinforced nylon 66 composites

Abstract

‘Long fibre’ ( lf) injection moulding materials, which consists of parallel strands of reinforcement, well-impregnated with thermoplastic resin, enable products to be manufactured with a much higher residual fibre length than was previously possible. This paper compares the behaviour of lf nylon 66 moulding compounds with that of their short fibre ( sf) counterparts and with that of the unfilled ( uf) polymer. Despite their greater initial fibre length, lf materials are relatively easy to process by injection moulding, shear viscosity under moulding conditions being only a little higher than that of conventional sf thermoplastics. The extensional viscosity, which is important in injection moulding, is higher than with sf materials, but not has high as might be expected given the length of the fibres present. Fibres appear to be protected from breakage during flow because they remain locally parallel to one another, even in quite complex flow régimés. Flow appears to involve movement of domains of locally oriented fibre swirls, rather than of individual well-dispersed fibres. Clumps and bundles of oriented material even survive passage through the nozzle of a moulding machine, to appear in the core of the final product. LF injection mouldings have a layer structure, similar to that in sf mouldings, except that the central core is more disordered and much thicker.