Bidirectional flow forming of polypropylene pipes

Abstract

The single directional flow forming process has been modified in the present study such that bidirectional flow forming of polypropylene pipes can be carried out. The process enables the polymer chains across the pipe thickness to be aligned in two different directions. Instead of having free turning rollers, flow forming was conducted using a roller driven by motor such that at the point of contact, the roller and the mandrel are turning in opposite directions. To assist the material to shear, the external surface of the pipe ahead of the roller was heated to the forming temperature of the material by hot air from a heating gun while the internal material remained unheated. Tensile tests of the bidirectionally flow formed pipes showed that although the elongation at break was somewhat lower than that from single directional flow forming, the ultimate tensile strength in the direction perpendicular to the helix was significantly increased. This resulted in pipes that were overall more balanced in mechanical properties than those from single directional flow forming.