Abstract: Extrusion of a tubular parison for blow molding is controlled according to a program of set point data defining parison wall thicknesses and extrusion rates. Progress of purge of plasticized material from an accumulator through a variable annular gap is measured. Parison profile set point signals defining wall section thickness at points correlated with progress of purge of the accumulator are produced in response to the measured progress. Purge velocity set point signals defining purge velocities correlated to selected profile points are produced in response to progress of purge of the accumulator. Variation of the annular gap is controlled in response to the parison profile set point signals and velocity of purge of the accumulator is controlled in response to the purge velocity set point signals.