Process controlling a blow molding machine

A method for controlling a blow molding machine during a process of making an object, said machine including a die head, a mandrel, an extruder that rotates and drives a plastic material between the mandrel and the die head to form a plurality of elongated parisons and a controller that causes a material distribution function to change during the formation of the parisons in a preprogrammed manner so that parisons having desired relative thicknesses along their lengths are formed, said method comprising:

• setting the distance between the die head and the mandrel and the rotational speed of the extruder to accomplish predetermined target values of object weight and parison velocity;
• feeding said plastic material to said machine and operating said extruder for forming the parisons;
• predicting object weight and parison velocity when the distance between the die head and the mandrel or the extruder rotational speed is increased or reduced by predetermined amounts;
• detecting pressure in the plastic material in the extruder at a location near the die head while the plastic material is being driven by the extruder;
• selecting in response to said pressure an adjustment defined by the increase or reduction of the distance or extruder rotational speed that minimizes the differences between the predicted object weight and parison velocity and the predetermined object weight and parison velocity;
• and
• adjusting the distance between the die head and the mandrel or the speed of the extruder in the operation of the machine according to said selected adjustment so that object weight and velocity of the parisons made are maintained at the target values within predetermined tolerances, wherein said setting step sets and said adjusting step adjusts the function by setting and adjusting a reference distance between the die head and mandrel during the formation of the parisons.