Spring back analysis in incremental forming of polypropylene sheet: An experimental study

Cold working affects the strength and relaxation characteristics of polymer materials. This in turn can influence the spring back in formed components. In this study, the effect of cold Incremental Sheet Forming (ISF) on the spring back in the Polypropylene sheet is experimentally analyzed. The sheet is subjected to ISF by varying forming conditions. The relaxation, tensile and spring back tests are performed on the formed parts. The results reveal that the strength and relaxation properties (i.e., stress and strain reductions) of formed Polypropylene sheet are correlated in a way that reduction in the stress and strain decreases as the post-ISF yield strength increases. As a result, spring back in the produced parts reduces. This relation is found to be coherent with the applied stretching force as well. Simultaneous recording of strain (from relaxation tests) and spring back (from formed parts) for an extended time reveals that significant spring back occurs due to an-elastic recovery as well. These insights will prove helpful in understanding the spring back mechanism and thus to methodically minimize geometric deviation in the polymeric components.