Solid state extrusion of polymers through convergent-divergent dies
Compared with metals, polymer materials have Iow strength and stiffness. However, molecular orientation can enhance many mechanical properties of polymer materials in the direction of orientation. Studies on solid state polymer extrusion (Le. extrusion carried out at temperatures below the melting point of the polymer) through convergent dies show that it is possible to produce extrudates exhibiting a high degree of monoaxial orientation in the extrusion direction. Although the strength of these extrudates has been greatly enhanced in the orientation direction, the strength decreases in the transverse direction to the axial orientation. Biaxially oriented polymer materials, on the other hand, show increased mechanical properties in more than one direction. But so far, extrusion processes that confer orientation in more than one direction have not received much attention. The present work is concerned with the development of biaxial orientation in thick thermoplastics extrudates by extrusion through dies exhibiting simultaneously converging and diverging walls perpendicular to each other and with a cross-section area at the die entry being the same as at the exit. Four die designs are examined, known respectively as the dual-taper die, the expansion fish-tail die, the constant fish-tail die and the cross die, using polytetrafluoroethylene and ultra-high molecular weight polyethylene. Measurements of birefringence and tensile strength on sections of the extrudates have shown that a preferential orientation along the transverse direction is normally achieved with the fish-tail dies and the dual-taper die, while the extrudates obtained with the cross die were found to exhibit a cross-ply orientation pattern with a bias in the extrusion direction for outerlayers and a preferential orientation in the transverse direction for the middle layers. The mechanics of the processes has been analysed by a plasticity approach for solid state extrusion, and by using variable wall boundaries for the melt extrusion analysis. The extrusion pressure predicted by the analysis compares very well with the values measured experimentally.