A parison thickness control system includes an extruder die (1) and a mandrel (2) which are movable relative to each other to change wall thickness of a parison (4) extruded between the extruder die (1) and the mandrel (2) during an extrusion or forming cycle of the parison. A parison control pattern is derived by manually inputting master points each defined in terms of a target parison wall thickness and an elapsed time during the parison extrusion cycle. A further parison control pattern is derived by performing the spline interpolation to interpolate between the master points. A still further parison control pattern is derived by correcting the above-noted further parison control pattern to allow the maximum or minimum value of the master points to indicate a limit value for the target parison wall thickness during the parison extrusion cycle. The parison extrusion cycle is divided into a given number of equal time intervals to define cycle points. The target parison wall thickness is derived for each cycle point based on the above-noted still further parison control pattern. The relative movement between the extruder die (1) and the mandrel (2) is controlled based on the derived target parison wall thickness for each cycle point. At least one of the cycle points may be used for triggering operation of an associated device. <IMAGE>
