A study of dual directional flow-formed polypropylene pipes

Abstract

Flow-forming, which is also known as tube spinning or shear forming, is a cold forming, rotary-point extrusion process. Some recent publications on single-pass flow-forming of polypropylene pipes have shown that the mechanical properties are improved with increasing percentage of thickness reduction. The present research investigates the possibility to further enhance the properties of the materials by Dual directional flow-forming. The Dual directional flow-forming was achieved by two-passes forward flow-forming process on a conventional lathe, using a twin-rollers system, with a change in the spindle direction for the second pass. The effects of varying the percentage thickness reduction on the hardness, tensile and fracture toughness properties were studied at three major orientations: along the pipe axis, along the helix angle and transverse to the helix angle. The hardness of the inner and outer surfaces were found to increase with the increase in percentage thickness reduction. Along the helix direction, the yield, tensile strength and secant modulus at yield were found to increase with increasing percentage thickness reduction whilst the elongation at break decreased. In the direction perpendicular to the helix, no significant tensile property change was observed. The critical stress intensity factor values were observed to increase to twice as high for tubes flow-formed to about 70% reduction in the helix direction. In the transverse direction, the critical stress intensity factor values dropped for 50% to 60% reduction pipes to values below the unflow-formed pipes, but increased slightly to above the values of the unflow-formed pipes at 65% to 70% reduction. The microfibrillar model of oriented polymer was found to provide a satisfactory explanation to the change in mechanical properties. © 1995.