Influence of crystallization kinetics and flow behavior on structural inhomogeneities in 3D printed parts made from semi-crystalline polymers

We report the results of a study focusing on the influence of crystallization kinetics and the flow behavior on structural inhomogeneities in 3D printed parts made from polyamide 12 (PA12) and poly (lactic acid) (PLA) by Dynamic Mechanical Analysis (DMA), Differential Scanning Calorimetry (DSC), Fast Scanning Calorimetry (FSC) and Wide-Angle X-ray Diffraction (WAXD). Temperature-dependent WAXD measurements on the neat PLA filament reveal that PLA forms a single orthorhombic α phase during slow cooling and subsequent 2nd heating. The PA12 filament shows a well pronounced polymorphism with a reversible solid-solid phase transition between the (pseudo)hexagonal γ phase near room temperature and the monoclinic α′ phase above the Brill transition temperature TB = 140 °C. The influence of the print bed temperature Tb on structure formation, polymorphic state, and the degree of crystallinity χc of the 3D printed parts is investigated by height and depth dependent WAXD scans and compared with that of 3D printed single layers, used as a reference. It is found that the heat transferred from successive layers has a strong influence on the polymorphic state of PA12 since a superimposed mixture of γ and α phase is present in the 3D printed parts. In case of PLA a single α phase is formed. The print bed temperature has, in comparison to PA12, a major influence on the degree of crystallinity χc and thus the homogeneity of the 3D printed parts, especially close to the print bed. By comparing the obtained results from WAXD, DMA, DSC and FSC measurements with relevant printing times, guidelines for 3D printed parts with a homogeneous structure are derived.