Feasibility analysis of hot incremental sheet forming process on thermoplastics

Incremental sheet forming has been proposed as a flexible manufacturing technique to process thermoplastic resins characterized by a glassy state at room temperature. Specifically, poly(methyl methacrylate) (PMMA) sheets were formed. A controlled room was designed, and the sheets were heated, before starting the forming phase, to a temperature above the PMMA glass transition, but avoiding onset of internal stresses that may lead to significant material springback at elevated temperature. Therefore, the process parameters, such as forming temperature and punch speed rate, have to be kept in well-defined ranges to be able to optimize the forming process of thermoplastic components by ISF. Furthermore, different sheet thicknesses were formed. Indeed, this size affects the flexural strength of the processed polymer as confirmed by reported tests. Experiments were planned out for the aim to take into account various process variables, i.e. spindle speed, step depth, punch diameter and feed rate. A plan based on a design of experiments (DoE) method was applied for a robust analysis. Macroscale observations were carried out to evaluate the product soundness, highlighting influences of the monitored process variables, on the process temperature and on the accuracy error of the formed parts. Furthermore, microscopic analyses evidenced the integrity grade of the surface on the side in contact with the punch for various combinations of process parameters. The results proved the process feasibility also for thermoplastics, which need to be heated before their forming phase.