Experimental Investigation of a Backing Sheet Stiffener in Incremental Forming of Polycarbonate

Single point incremental forming (SPIF) is a dieless forming process for sheet materials. This process forms materials with a hemispherical forming tool which locally deforms the sheet at incremental depths. The freeform nature of this process promises significant efficiency improvements within small and medium volume industries where stamping is traditionally used. However, several drawbacks currently inhibit its widespread use. One of these drawbacks is springback or elastic recovery resulting in reduced geometrical accuracy. An existing approach to counter this involves using a dedicated backing die, increasing the cost of the forming apparatus and the overall energy input per part. Other springback reduction methods involve the direct addition of energy to the workpiece through electrical or heat input. This study investigates the use of sacrificial steel blanks as backing dies for incremental forming of polycarbonate sheets, to overcome the loss in geometrical accuracy affiliated with forming geometries with a relatively large distance between the geometry periphery and the clamped edge. The blanks were not bound to each other, but rather clamped along their edges. In this study, polycarbonate blanks were tested using a three-factorial design of experiments, with relative plate thicknesses of 0.4, 0.5, and 0.6, and wall angles of 15°, 30°, 45°, and 60° as independent factors. The test geometry used was a straight walled pyramid with a square base. Using the backing sheet, a reduction in the springback was observed, demonstrating the effectiveness of sacrificial backing blanks. Particularly, the ‘pillow effect’ at the base of the geometry was reduced. This is attributed to the higher stiffness of the steel plates, increasing the plastic strain on the polycarbonate. However, the formability is found to decrease for higher values of the backing plate thickness due to premature steel failure. In future studies, this work will be expanded to include additional thickness ratios, geometries, toolpath types, step sizes and materials to form a more complete trend.