the passageway 14 by being designed to essentially respond to the following cubic equation r w is the wall radius measured perpendiclar to the axis 19 z is the distance along the axis 19 of the nozzle measured from the exit orifice 16 and once the criterion of constant extensional strain rate is selected the foregoing equation 1 may be derived by integrating the following equation u03b4v u03b4 z is the extensional strain rate represented by the partial derivative of velocity v with respect to distance measured from the exit orifice 16 along the nozzle axis 19 and r sub w is the wall radius as defined for equation 1 and and while also recalling the limit condition that r sub w is equal to r as defined for equation 1 at the exit orifice 16 preferably the constant a of equation 1 is determined by equating the constant extensional strain rate k of equation 2 with the maximum extension strain rate for a conical spinneret wherein the die swell is acceptable hereinafter referred to as 34 equivalent conical spinneret 34 then u03b4v u03b4 z max cone is the maximum extensional strain rate for the equivalent conical spinneret the well known equation for the wall profile of a conical spinneret is r w and z are as defined in equation 1 and u03b8 is the half angle of the equivalent conical spinneret preferably in the range of 2 to 71 2 degrees by combining equations 3 and 5 and recognizing that for a cone u03b4v u03b4 z is a maximum at the exit orifice 16 where z is zero and r sub w is r it will be determined that equ2 where u03b3 r is 4q.