Effects of incremental depth and tool rotation on failure modes and microstructural properties in Single Point Incremental Forming of polymers

Single Point Incremental Forming (SPIF) is a sheet forming process characterized by advantages that include low-cost and part-shape-independent tooling, higher formability and greater process flexibility as compared to conventional sheet forming. While recent work has demonstrated the possibility of SPIF of polymers the effects of incremental depth and tool rotation speed, key process parameters in SPIF, have rarely been examined. This work experimentally examines how incremental depth and tool rotation speed affect the failure mode during forming, forming forces as well as the void structure and crystallinity of the formed material in polymer SPIF. The dependence of both tearing and wrinkling on the incremental depth and tool rotation speed is uncovered. It is shown that contrary to SPIF of metals, greater incremental depths result in increased formability in polymer SPIF, but this advantage is limited by the occurrence of sheet wrinkling at excessively high incremental depths. Further, the occurrence of sheet wrinkling depends not just on the incremental depth but also on the part shape being formed. Microstructural examination of the formed material shows that greater incremental depth results in greater void densities and that the material formed with SPIF has greater crystallinity than the unformed material. Additionally, it is shown that higher tool rotation speed can cause earlier onset of wrinkling. The implications of these observations on SPIF of polymers are discussed.