Method of controlling the thickness of an extruded plastic article

A method for monitoring and controlling the wall thickness of an extruded profile of heated polymeric material that issues from an extrusion die, the die including a plurality of discrete sectors each having a temperature controllable heat source and a temperature sensing device, said method comprising the following steps:

• (a) extruding a polymeric material in an extrusion direction through an extrusion die at an extrusion speed to provide an article of predetermined cross-sectional shape from the extrusion die, the article having a periphery and a wall thickness dimension and having a longitudinal axis extending in the extrusion direction;
• (b) measuring the wall thickness of the extruded article at a plurality of spaced measuring points on the periphery of the article, each of the measuring points spaced laterally outwardly of the longitudinal axis of the article, and calculating the overall average wall thickness of the extruded article from the measured wall thicknesses;
• (c) providing a predetermined desired average wall thickness, a predeterined wall thickness distribution, and a predetermined wall thickness tolerance;
• (d) providing data means indicative of a change in wall thickness of the article as a function of a change in the temperature of the extrusion die to permit a determination of a temperature change that is necessary to effect a desired wall thickness change;
• (e) determining from the wall thickness measurements the maximum wall thickness value, the position on the article corresponding with the maximum wall thickness value, the minimum wall thickness value, and the position on the article corresponding with the minimum wall thickness value;
• (f) determining from the data means the temperature change required at each sector of the die to change the measured wall thickness to correspond substantially with the calculated overall average wall thickness;
• and
• (g) changing the temperatures of the respective heat sources at the respective extrusion die sectors to provide the desired die sector temperatures and the desired article wall thicknesses at corresponding points on the extruded article.