Strengthening polypropylene pipes by flow-forming process with anti-twist system

Abstract

In the metal-forming industry, flow-forming process has been well investigated and widely applied. The intention of our research here is to apply this flow-forming process to polymeric material. In this research, a conventional lathe machine was modified and a custom-designed twin motor driven rollers set-up with an anti-twist system was fabricated for the flow-forming experiments. Successful outcome has been obtained by the application of anti-twist system which has totally prevented the twisting of pipe about the mandrel during the flow-forming process resulting in a straighter pipe. Experimental work conducted on polypropylene pipes have so far shown remarkable improvement over the original material in term of mechanical properties. Tensile yield strength and ultimate tensile strength of the flow-formed pipes increased significantly with percentage pipe wall thickness reduction, especially along the pipe axis direction. Hydrostatic burst test, which is considered to be the overall test for the flow-formed pipes also shows that the hoop strength increases markedly with percentage thickness reduction to as high as 55MPa at 60% pipe thickness reduction. Unusual ballooning effect observed during the hydrostatic burst test was found to be an added 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 pipe. However, a small drawback in our current anti-twist forming method is that three splines come into existence due to the anti-twist shaft used which posed as weak regions in the resulting flow-formed pipes. Nevertheless, the anti-twist system could still be improved on and looked into further in our subsequent investigation as it has promised very good results. It is hopeful that the current flow-forming method of polymeric material could be further researched and refined with subsequent application in the relevant industry in the near future.