Thermoforming of unidirectional fiber-reinforced thermoplastics reveals a great potential for large volume production of structural components, due to low cycle times, recyclability, and material efficiency. Therefore, thermoforming plays an increasingly important role, especially for the automotive industry.

During thermoforming, a preconsolidated blank is heated initially well above the melting temperature of thermoplastic. Subsequently, the formable blank is transferred and positioned within the mold. Finally, the mold closes and transforms the 2D blank into a 3D part. Due to cooling of the part within the mold, the thermoplastic solidifies and the part can be ejected.

Schematic of the processing steps during thermoforming.

Thermoforming processes are influenced by several processing parameters, like stacking sequence, tool temperature, initial laminate temperature, forming velocity, or laminate gripping. Depending on these parameters, as well as on the formed geometry, manufacturing defects such as local wrinkling are possible. Moreover, a modification of the fiber structure, such as a changing of fiber orientation, is inevitable.

Local wrinkling  for a partly formed part

Local fiber orientation determined through a CT scan

SimuDrape comes as an add-on for ABAQUS with a fully automatized pre- and postprocessing through plugins for ABAQUS/CAE, as well as advanced modeling approaches through user-subroutines for ABAQUS/Explicit.

Discretization

For the thermoforming simulation with SimuDrape, the tools as rigid surfaces since the deformations within the tools are expected to be negligible. In contrast, the eight single plies of the orthotropic laminate layup are modeled with deformable shell elements, due to their high slenderness ratio. The single plies are connected via contact constitutive equations, to accurately capture inter-ply sliding between plies.

Boundary Conditions

The processing strategy of this use case includes pneumatic gripper elements at each vertice. These gripper elements are used for the transfer from the oven to the mold, as well as for applying tensile membrane forces during forming, to improve the formability of the laminate. To accurately account for the kinematics of these gripper elements, the ABAQUS built-in translator elements are adopted to model the rotation and elongation of the gripper elements, and the tensile forces are modeled through a force boundary condition in direction of the translator elements. Besides this, gravity loading is accounting for.

Regarding thermal effects, a homogeneous initial laminate temperature of 280 °C and a constant tool temperature of 110 °C is assigned.

Material modeling

Intra-ply material modeling is based on a physically decoupled approach, to accurately model the high fiber stiffness in combination with a low bending stiffness, which is not possible with conventional shell theories. Both membrane and bending behavior are modeled by nonlinear Voigt-Kelvin approach with a temperature-dependent elasticity and a temperature- and rate-dependent viscosity. Temperature-dependency is described by a WLF-approach, whereas rate-dependency is described by a Cross-approach.

Inter-ply material modeling takes into account temperature- and rate-dependent sliding in tangential direction for both the tool-ply and the ply-ply interfaces. For the ply-ply interfaces, additionally, tacking in the normal direction is accounted for.

The thermoforming simulation results obtained SimuDrape insights into formability throughout the whole forming step, from the positioning of the blank on the lower tool until the complete mold closure.

A semi-crystalline thermoplastic undergoes a phase transition during cooling, where the polymer chains are partially aligning. This phase transition is called recrystallization. SimuDrape predicts this process in terms of the state variable called relative crystallinity. Based on this, areas already solidifying during forming can be identified.

For this use case, significant cooling of the blank at the tool-ply contact areas is observed. This is especially valid for the part flanges, on which the blank is positioned at the beginning of the forming step, as well as around the corner bending to the end of the forming step. In these areas, local recrystallization is observed. Thus, the material has solidified in these areas already during forming.

The validation results of this use case prove that thermoforming simulation through SimuDrape is capable to accurately predict the local wrinkling behavior and the contour after forming. Based on this, SimuDrape becomes appicable to the virtual process design in terms of several aspects. First, the manufacturability of a specific geometry can be predicted. Second, suitable manufacturing strategies and suitable processing parameters to reduce or even prevent wrinkling behavior can be determined. Third, a blank tailoring to enable near-net shaped forming can be determined.

Moreover, it is observed that the prediction accuracy for the fully-coupled thermomechanical approach supplied with SimuDrape is higher compared to the related isothermal approach. Some local defects are predicted accurately only under consideration of temperature and crystallization kinetics. Therefore, a fully-coupled thermomechanical approach for thermforming simulation is to be preferred over an isothermal approach.