Modeling of the non-isothermal crystallization kinetics of polyamide 6 composites during thermoforming

The combination of thermoforming processes of continuous-fiber reinforced thermoplastics and injection molding offers a high potential for cost-effective use in automobile mass production. During manufacturing, the thermoplastic laminates are initially heated up to a temperature above the melting point. This is followed by continuous cooling of the material during the forming process, which leads to crystallization under non-isothermal conditions. To account for phase change effects in thermoforming simulation, an accurate modeling of the crystallization kinetics is required. In this context, it is important to consider the wide range of cooling rates, which are observed during processing. Consequently, this paper deals with the experimental investigation of the crystallization at cooling rates varying from 0.16 K/s to 100 K/s using standard differential scanning calorimetry (DSC) and fast scanning calorimetry (Flash DSC). Two different modeling approaches (Nakamura model, modified Nakamura-Ziabicki model) for predicting crystallization kinetics are parameterized according to DSC measurements. It turns out that only the modified Nakamura-Ziabicki model is capable of predicting crystallization kinetics for all investigated cooling rates. Finally, the modified Nakamura-Ziabicki model is validated by cooling experiments using PA6-CF laminates with embedded temperature sensors. It is shown that the modified Nakamura-Ziabicki model predicts crystallization at non-isothermal conditions and varying cooling rates with a good accuracy. Thus, the study contributes to a deeper understanding of the non-isothermal crystallization and presents an overall method for modeling crystallization under process conditions.