Thermoforming of continuously fibre-reinforced thermoplastics 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 like wrinkling, gapping or fibre fracture are possible. Finite Element forming simulation offers the possibility of a detailed analysis of the deformation behaviour of multi-layered thermoplastic blanks during forming, considering material behaviour and process conditions by means of constitutive equations. Usually, thermoforming simulation is assumed to be iso-thermal, which is a reasonable assumption for temperatures above the onset of crystallization for semi-crystalline thermoplastics. Especially in a process design phase, however, the onset of crystallization cannot be excluded for a specific process strategy and geometry.
In this study, a fully coupled thermomechanical approach for finite element forming simulation of thermoforming processes, predicting the evolution of temperature and crystallization of semi-crystalline thermoplastics, is presented. Based on this approach, the distinct increase of mechanical properties and thus the decreasing formability with the onset of crystallization is accurately predicted. The approach is parameterized for a thermoplastic UD-tape (PA6-CF) and successfully applied to forming simulation of a generic geometry with a very good agreement to experimental forming tests. Finally, the influence of forming velocity, initial laminate temperature and tool temperature on the obtained temperature distribution in the formed blank is analysed by means of a virtual sensitivity study.