BIAXIAL ORIENTATION OF POLYMERS BY SOLID-STATE EXTRUSION THROUGH CONVERGENT-DIVERGENT DIES

Solid-state extrusion of crystalline thermoplastics is a well known method for the production of monoaxially oriented filaments exhibiting very high modulus and strength. This is achieved primarily by forcing the polymer through a converging conical die at temperatures below its melting point. In the present study the polymer is subjected to deformations simultaneously along the longitudinal and transverse directions by using dies featuring converging and diverging walls, perpendicular to each other, to produce flat extrudates exhibiting biaxial orientation. Two geometries are examined to determine the factors controlling the relative magnitude of the orientation in the extrusion and transverse directions respectively, which is being assessed by measuring the birefringence and tensile strength in various sections of the extrudates. Experiments have been carried out over a wide range of temperatures on billets of PTFE and UHMWPE, using dies mounted on a compression testing apparatus to measure the extrusion forces. The mechanics of the converging-diverging flow has been analysed to calculate the extrusion forces as function of the yield strength and coefficient of friction of the polymer and to establish the relationship between type of orientation in extruded products and die geometry.