Material extrusion thermal model mapped across polyetheretherketone isothermal and continuous cooling transformation charts

Abstract

This study provides an insight into the polyetheretherketone (PEEK) crystallinity progression throughout the material extrusion (MEX) additive manufacturing process as a function of time and temperature, comparing it with the isothermal and continuous cooling transformation charts created over a wide range of isothermal crystallisation temperatures and constant cooling rates. The isothermal and non-isothermal crystallisation kinetics were explored using Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). The half-time, onset and ending of crystallisation were obtained for isothermal crystallisation temperatures between 150 °C and 330 °C, while the crystallisation under constant cooling was obtained using rates between − 0.5 K s−1 and − 45 K s−1. The results were used to draw the Continuous Cooling Transformation (CCT) and the Time-Temperature Transformation (TTT) diagrams and calculate the Avrami numbers using the parallel Avrami model. These results were then compared to the degree of crystallinity as a function of time and temperature for the MEX process. To evaluate the crystallisation within the MEX process a 1D transient transfer heat model was used to obtain the printing thermal profile, which was replicated using the FSC technique. The results showed that for the MEX printing process, the crystallisation usually is a product of a combination of rapid cooling and heating processes followed by periods of greater thermal stability which, depending on the nature of the process, can approach a quasi-isothermal crystallisation process. By superimposing the process thermal profile on the TTT and CCT diagrams and comparing the crystallinity values measured from each point in the thermal profile, it was possible to understand the crystallinity evolution and the remelting of the build surface promoted by the subsequent printed layers.