Mechanism of twist in incremental sheet forming of thermoplastic polymer

Single point incremental forming (SPIF) is one of the procedures with the most potential in aerospace development and human implants. However, twist phenomena occur frequently and affect the processing defects, which strongly influence the geometric accuracy of the formed parts. This work reveals the mechanism of twist phenomena in detail and figures out the optimal solution of the principal challenge during industrial application and artificial bone processing by SPIF. A new analytical model is proposed to calculate the twist angle based on the law of conservation of energy. Meanwhile, a novel alternate spiral tool trajectory is established according to the linear interpolation of the two adjacent contour lines. Based on the case studies including pyramid frustum shape with constant wall angle and truncated cone shape with varying wall angle, SPIF formed parts by using the alternate spiral trajectory are compared with that by traditional unidirectional spiral trajectory, to effectively understand the advantages of the developed tool trajectory strategy. It shows that the proposed alternate spiral trajectory provides an effective way to effectually restrain the twist, and finally improves the geometric accuracy of target part in practical application of SPIF.