DESIGN OF STRAIN-RATE CONTROLLED DIES FOR THE SOLID-PHASE DRAWING OF POLYPROPYLENE ROD

The strength and stiffness of a polymer may be increased considerably by drawing the material when in the solid phase through a die and ther eby achieving orientation of the molecular chains in the drawn directi on. In many applications the commercial viability of the process is de pendent strongly on the speed of drawing achieved without fracture of the product. A method of designing dies is presented in which the stra in rate is controlled along the die length. It has been shown that whe n processing polypropylene using dies manufactured by this method, the processing speed can be increased and the possibility of fracture at the die exit reduced if the strain rate is either maintained constant or decreased progressively along the die. Processing with these dies i s preferable to using traditional conical dies or dies where the strai n rate increases along the die length. (C) 1997 Elsevier Science S.A.