A novel approach for understanding laser sintering of polymers

Selective laser sintering (LS) of thermoplastic powders allows for the construction of complex parts with higher mechanical properties and durability compared to other additive manufacturing methods. According to the current model of isothermal laser sintering, semi-crystalline thermoplastics need to be processed within a certain temperature range, resulting in the simultaneous presence of the material both in a molten and solid state, which is present during part building. Based on this process model, high cycle times ranging from hours to days are a thought to be a necessity to avoid warpage. In this paper, the limited validity of the model of isothermal laser sintering is shown by various experiments, as ongoing solidification could be detected a few layers below the powder bed surface. The results indicate that crystallization and material solidification is initiated at high temperatures and further progresses throughout part build-up in z-direction. Therefore, a process-adapted material characterization was performed to identify the isothermal crystallization kinetics at processing temperature and to track changes of the material state over time. A dual approach on measuring surface temperatures by infrared thermography and additional thermocouple measurements in z-direction was performed to identify further influences on the material solidification. A model experiment revealed that a few millimeters below the surface, components produced by LS are already solidified. Based on these results, the authors present an enhanced process model of isothermal laser sintering, which considers material solidification in z-direction during part build-up. In addition, a new processing strategy is derived to increase the efficiency of LS processes significantly.