Investigation of the spring-back phenomenon in two-dimensional bending of continuous glass fiber-reinforced polyamide composite sheets

Thermoplastic composite sheets have great potential for use in industrial applications owing to their fast-forming properties in the mold. One of the important matters of forming is the tendency of the part to recover elastically after it comes out of the mold. This behavior affects the dimensional accuracy of the final part. In this study, V-bending experiments were carried out to understand the spring-back phenomenon of continuous glass fiber-reinforced polyamide-6 composite sheets. Unidirectional and cross-ply composite sheets were used in the experiments and bending angle, die radius, pressure, and dwell-time were investigated systematically. After bending, the spring-back angles of the parts were measured and the effects of the inspected parameters were compared. Also, variations in part thickness caused by spring-back were evaluated. Finally, the deformations in the specimens resulting from the residual stress were visualized with a scanning electron microscope. When the results were compared, it was determined that the bending conditions where the spring-back defect could be minimized were 6 mm radius, 90° angle, 120 s dwell-time, and 3 MPa pressure. Under these situations and in sheets bent 90°, the amount of spring-back was determined as 22° for unidirectional composites and 15° for cross-ply composites.