John Vlachopoulos CAPPA-D, Chemical Engineering, McMaster University Hamilton, ON, Canada Corrugations add stiffness and increase the structural strength of pipes. Double-walled plastic pipes are extruded in such a way as to have an inside smooth surface and a corrugated outer wall. The corrugations are formed in the outside by extruding the outer layer between a sequence of moving molds and simultaneously applying an overpressure and vacuum. The two walls of the pipe must form an inseparable structure able to withstand significant loads. Large diameter pipes (usually over 300 mm and often exceeding 1000 mm in diameter) are used for municipal drainage and storm water management. In extrusion for such double-walled pipe, all the problems of pipe extrusion are encountered and in addition, several other problems related to the forming of the outer corrugations. HDPE, PP and PVC are the most frequently used materials. Polymer molecular and rheological characteristics determine extrudability, but the end pipe properties are also influenced by the shape and size and thickness of the corrugated outer wall. Uniform melt quality free from unmelts arriving at the extruder die lips is necessary to achieve the required pipe stiffness and flexibility. Reduction of structural weaknesses, due to weldlines forming behind “spiderlegs” in the die, is helpful for avoiding failures during subsequent testing or field performance. The formation of the inner smooth wall and outer corrugated one, requires careful control of the applied overpressure and external vacuum. The process is studied as a combination of pipe extrusion for the inside wall and extrusion blow molding for the outside. Keywords: melt quality, weldlines, forming, pipe stiffness.