ANALYSIS OF THE MECHANICS OF DIE DRAWING POLYPROPYLENE THROUGH STRAIN-RATE CONTROLLED DIES

The die drawing of polymers in the solid state is now an established p rocess for achieving molecular orientation in drawn products and an en hancement of their mechanical properties. A mechanics analysis is pres ented of the drawing of polypropylene through strain rate controlled d ies, where the strain rate is either constant, increasing at a uniform rate or decreasing through the die. The stress in an element of mater ial being drawn is determined from equilibrium considerations and appl ication of the Tresca yield criterion. Flow stress data are deduced fr om uniaxial stress-strain-strain rate tests and applied to the problem using equivalent stress-strain concepts. Results are presented of axi al stress, flow stress and die wall pressure throughout the dies and s how that polypropylene can prematurely leave contact with the die wall , particularly in decreasing strain rate dies at high drawing speeds. Increased friction of the die wall and increased drawing speed cause t his 'loss of contact' point to move progressively downstream in all di e cases. Theoretically predicted drawing loads show acceptable agreeme nt with experimental test results when low values of friction coeffici ent are assumed.