AN INVESTIGATION INTO THE DEFORMATION CHARACTERISTICS OF POLYMER PIPES USING FLOW-FORMING PROCESS

Abstract

Flow-forming is a cold-forming, rotary point extrusion process, well investigated and widely applied in the metal-forming industry. It is the intention of this research to apply this flow-forming process on polymeric material and to understand the deformation characteristics related to the process. Polypropylene pipes were chosen as the experimental specimens. For this research, a conventional lathe machine was modified and installed with a custom-designed twin motor driven rollers set-up for the flow-forming experiments. Various attempts were carried out in search of the best way to flow-form the pipes. Eventually, a particular way whereby the rollers and mandrel were made to rotate in such a manner that at the point of contact, the tangential forces of the rotating mandrel and rollers are pointing in the same direction was found to yield flow-formed pipes with remarkable improvement over the original material in term of mechanical properties. The operating conditions were observed to be optimum at feedrates of 55mm/min and below, and tangential speed ratios of mandrel to rollers ranging from 1 :6 to 1: 8. Tensile yield strength and ultimate tensile strength of the flow-formed pipes were found to increase significantly with percentage wall thickness reduction, especially along the direction of the helix lines, which are the twisted original extrusion markings on the outer pipe surface. Hydrostatic burst test, which is considered to be the overall test for the flow-formed pipes also shows that the hoop strength has increased markedly with percentage thickness reduction to as high as 72MPa at 70% pipe wall thickness reduction, which is almost two and a half times higher than the original strength. Unusual ballooning effect occurred during the hydrostatic burst test is considered an advantage in piping failure detection. Its occurrence has been explained by the scanning electron microscopic examination as well as anisotropic tensile behaviour of the flow-formed pipes. Generally, flow-formed pipes were found to be more ductile as demonstrated by the severe stretching of the pipe material before rupture failure, in contrast to the brittle failure of the unflow-formed pipes. On the other hand, finite element analysis in simulation of the hydrostatic burst test of pipes has produced results to complement the former explanation regarding the ballooning effect. The implication is that the power of the simulation tools could be further harnessed to assist in future investigation, largely to supplement the experimental findings and to cut down experimental trials and errors, saving time and cost eventually. Helix lines formation on the pipe wall has been observed to have adverse effect on the straightness of the pipe and may subsequently cause the pipe to twist under operating condition. With the application of anti-twist system, the twisting of pipe about the mandrel during the flow-forming process was successfully prevented. Subsequently, the flow-formed pipes were free from helix lines formation. However, a drawback was encountered in this particular system. The hoop strength of the flow-formed pipes started to decrease from 60% pipe wall thickness reduction onwards. The weakness was traced to the three slightly protruding splines on the inner pipe wall along the pipe axis, which were apparently caused by the three linear grooves on the anti-twist shaft used. The anti-twist system is still being improved on and looked into further as it has promised very good results. It is hoped that further research on the flow-forming of polymeric material can lead to industrial application in the relevant industry.