The method comprises setting the distance between the mandrel and bushing and the rotation speed of the extruder to accomplish predetermined target values of the weight of the package and parison velocity.

Plastic material is fed into the machine and the extruder is operated to form parisons. A parameter related to the viscosity or melt index of the plastic material in the extruder is detected and the material distribution function and the speed of the extruder are adjusted in response to the parameter, so that the package weight and velocity of the parisons made are maintained substantially at target values. The computer controls these functions with an algorithm driving the programmable logic controller (PLC).