Extruded amorphous thermoplastic pipe having reduced internal stress and method of making the same

摘要

Physical properties of extruded amorphous thermoplastic pipe are improved by a method of extrusion which allows for relaxation of internal stress introduced during the processing. The stress is relieved by using an extrusion die having an inner and outer diameter which is smaller than the final product inner and outer diameter. The extruded pipe is allowed to expand as a result of melt swell and thereby permit relaxation of internal stresses within the extruded pipe. The expansion brings the inner and outer diameter to approximately the size of the final pipe. The further introduction of internal stress is avoided by pulling the extruded pipe from the extruding apparatus at a rate which is approximately the same as the rate of extrusion. The pipe can undergo a final sizing step to bring the extruded pipe to the final product size.