Extrusion of Fibre Reinforced Pipes with Controlled Orientation

Plastics pipes are used extensively by the gas and water industries. Fibre reinforced thermoplastics are widely used for the manufacturing of precision engineering components by injection moulding and to a lesser extent in pipe extrusion. The main reason for this is that the uniaxial flow of the fibre filled polymer melt in conventional pipe extrusion dies, aligns the fibres in the axial direction. The result of this orientation patiern is that stimless and strength are much higher in the axial direction than the tangential (hoop) and radial directions. The axial fibre orientation contradicts the stress system developed in a pipe under internal pressure or radial loading where the applied tangential (hoop) stress is much higher than the axial stress. The increased cost of composite materials is not justified by the marginal relative improvements of mechanical propenies of composite pipes produced by conventional (static) dies. Ideally, the higher strength values should be in the direction of higher stress values. In this paper, a new dynamic die design is presented which achieves a controllable degree of orientation in the radial, tangential andlor axial directions. The mechanics of the bi-axial flow governing the orientation processes are discussed and experimental results using filled Polypropylene and Polyethylene are reported.