On the relevance of thermomechanics and crystallization kinetics for FE thermoforming simulation of semi-crystalline thermoplastic tapes

Thermoforming of continuously fiber-reinforced thermoplastics (CoFRTP) is currently of great interest for the automotive industry due to low cycle times, material efficiency and recyclability. Depending on material parameters and process conditions, manufacturing defects are possible. Finite Element (FE) forming simulation offers the possibility of a detailed analysis of the de-formation behavior of multi-layered thermoplastic blanks during forming, considering material behavior and process conditions by means of constitutive equations. Usually, thermoforming simulation is assumed to be isothermal, which is a reasonable assumption for semi-crystalline thermoplastics for temperatures above the onset of crystallization. However, it is not investigated so far, if the onset of recrystallization and its influence on formability is in general negligible for thermoforming simulation. In this study, characterization results for crystallization kinetics as well as for temperature-transient shear characterization for a unidirectional semi-crystalline thermoplastic tape (PA6-CF) are presented. A strong dependency on the cooling rate for recrystallization and shear behavior and a distinct increase in shear behavior with the onset of recrystallization is observed. To investigate temperature evolution and crystallization behavior on part level, a virtual forming study under variation of forming velocity is conducted. The results reveal that recrystallization during forming cannot be excluded for an unfavorable choice of process parameters. This reveals the necessity to take into account thermomechanics and crystallization kinetics in thermoforming simulation, especially in a process design phase.