ORIENTATION EVOLUTION IN CONVERGENT-DIVERGENT DIES - SOLID-STATE EXTRUSION OF CRYSTALLINE POLYMERS

Solid-state extrusion experiments were carried out through constant ar ea convergent-divergent dies using two crystalline polymers, i.e. ultr a high molecular weight polyethylene (UHMWPE) and polytetrafluorethyle ne (PTFE), with the aim of producing extrudates exhibiting biaxial ori entation. The evolution of orientation was examined along the extrusio n axis by measuring the off-plane birefrigence values at various dista nces from the entry on specimens removed from the dies. Simple analyti cal expressions relating orientation factor to draw ratio in the three mutually perpendicular directions were derived for crystalline polyme rs. These generalised relationships were found applicable for all type s of deformations, i.e. monoaxial, biaxial and pure shear. On the basi s of the data obtained and the predictions from the theoretical analys is, the deformations taking place in dies with small converging angles , known as fish-tail dies, were found to be close to a state of pure s hear deformations, while dies with high converging angles initially, t hen changing to high divergencies in the second section, known as dual -taper dies, were found to give rise to a state of unbalanced biaxial deformations.