Investigation of Tooling Geometry Using a Finite Element Simulation of UHMWPE Unidirectional Thermoforming

Abstract

Thermoforming is an economical and fast process for transforming a flat sheet of thermoplastic composite material into a complex-shaped preform. A binder ring is used to induce in-plane tension to mitigate wrinkling and to drive the dominant deformation mechanism to be in-plane. The degree of shear can vary over the part, creating variations in thickness and subsequent challenges with uniform consolidation for a stack of preforms. While material attributes and processing parameters are critical in successful preforming, variations in the tool geometry may also lead to significant changes in the preform outcome. The goal of this research was to develop a virtual process simulation that can guide design changes in the processing conditions, ply blank geometries and tooling to achieve a well consolidated part. A discrete mesoscopic modeling approach was implemented in LS-DYNA using thickness-change shell elements (ELFORM25) that can incorporate 3D material properties, including through-plane thickness changes. The current state of the simulation effectively incorporates material behavior, tool/ply friction and thickness changes that reflect variations in pressure during consolidation to radius. In the current study, the simulation is used to investigate the significance of tooling geometry, including the punch/die gap size. The die fillet results were verified through comparison to experimental results with two types of tooling geometry. A parametric study was performed to investigate tooling changes in relation to the number of layers, or thickness, of a unidirectional UHMWPE material. Observations show that the radius of the die fillet has significant impact on wrinkle onset as the material passes through that juncture.