Characterization, modeling and validation of the crystallization kinetics of carbon fiber-reinforced polyamide-6 composites

Thermoplastic composite materials have the potential to meet the high-performance demands of the automotive and aviation industries in terms of cost and cycle time due to their rapid consolidation and forming capabilities. To take advantage of the rapid forming capabilities of thermoplastic materials, the processes may include external heating of the semi-finished products, followed by the forming process, including the compaction and solidification in isothermal molding tools. In the forming process, the laminate is continuously cooled whereby a non-isothermal crystallization process occurs in semi-crystalline polymers, which governs the phase transition from liquid to solid. As semi-crystalline thermoplastics can only be formed above their recrystallization temperature, the solidification phase sets the limits for the processing window.

In order to predict the manufacturing process limitations and boundaries, it is necessary to build up a holistic understanding of the solidification behavior. Therefore, this study aims to identify how temperature and crystallinity affect the formability of thermoplastic composites in industrial process conditions. By analyzing a CF/PA6 tape material as well as its neat PA6 polymer using differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC), the necessity and difficulty of scanning fiber–matrix composite samples is evaluated. Using a modified Nakamura-Ziabicki model, the measured relative degree of crystallinity (DoC) is fitted across a wide range of constant cooling rates for both the composite tape material and the neat matrix material. The presence of carbon fibers increases the crystallization growth rate, leading to faster crystallization kinetics at all measured cooling rates. The model is implemented into the commercial FE software Abaqus® using a HETVAL subroutine for numerical heat transfer simulations. To address the unsuitability of conventional mechanical methods for validating crystallization kinetics at the high cooling rates typical of industrial processes, a novel approach utilizing squeezing flow during compaction in a stamp forming experiment was developed to to validate the numerical simulations. The influence of various processing parameters on the forming process is studied by measuring the internal transient temperature of the laminate as well as the displacement of the testing machine during compaction. The analysis concludes that the compaction limit of the stamp-formed specimen due to recrystallization aligns well with both the measured local maximum in cooling rates and the numerically predicted DoC of 50 %. Consequently, implementing the modified Nakamura-Ziabicki model in the process simulation enables accurate prediction of crystallization kinetics. This enables virtual process studies to be integrated into the design phase, facilitating direct feedback into the composite mold development and resource-efficient process optimization.