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

Abstract

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.

Experimental Investigation of In-Plane Shear Behaviour of Thermoplastic Fibre-Reinforced Composites under Thermoforming Process Conditions

“…Earlier works on the crystallisation kinetics of polymers focused on isothermal DSC tests, where the sample is rapidly cooled from its melting to the crystallisation temperature and the heat that has evolved when the sample is held isothermal is measured. Isothermal crystallisation kinetics of glass-fibre-filled PA6 composites were studied in [22], which concluded that the addition of filler materials, such as fibres, influence the thermal properties of the polymers The analysis of the thermoforming process requires thermal properties of the organosheet material at process-relevant cooling rates in a range greater than 10 K/s, which is not possible using conventional differential scanning calorimeters, which are limited to a maximum of approximately 2.5 K/s [23]. In addition, while isothermal crystallisation is suitable for theoretical analysis, a manufacturing process performed under dynamic conditions should be investigated with non-isothermal crystallisation [24].…”

“…In addition, while isothermal crystallisation is suitable for theoretical analysis, a manufacturing process performed under dynamic conditions should be investigated with non-isothermal crystallisation [24]. Non-isothermal crystallisation kinetics of fibre-reinforced PA6 composites at high cooling rates were studied in [23] using extremely fast-operating calorimeters discussed in [17]. The shifting of the crystallisation peak to lower temperatures with increasing cooling rates was observed in [23].…”

“…Non-isothermal crystallisation kinetics of fibre-reinforced PA6 composites at high cooling rates were studied in [23] using extremely fast-operating calorimeters discussed in [17]. The shifting of the crystallisation peak to lower temperatures with increasing cooling rates was observed in [23].…”

“…The DMA tests, especially, are conducted with reverse temperature sweeps in order to account for the change in recrystallisation temperatures with different cooling rates. In addition, the process-relevant cooling rates and their influence on the recrystallisation temperature are investigated using fast scanning calorimetry (Flash DSC) following a previous study in [23]. All of the individual experiments are analysed separately in the following sections and the correlations between results from the individual experiments are discussed.…”

“…This behaviour is important because the kinetics of crystallisation have a significant influence on the forming process. During thermoforming process, the transition of the matrix material from molten state to solid state is dictated by the crystallisation kinetics and their dependence on heating and cooling rates, which define a crucial limit for defect-free forming [23].…”