Rheology and agglomeration behavior of semi-crystalline polyamide powders for selective laser sintering: A comparative study of PA11 and PA12 formulations

In this study, we investigated the rheological behavior of various polyamide powders as a function of temperature, shear rate, and relative humidity. Near the glass transition temperature (T_g), powders exhibited an abrupt increase in static flow threshold, indicating caking due to particle agglomeration. At higher temperatures, a transition from elastic to plastic flow was observed around 80^∘C, accompanied by changes in the plateau value of the shear modulus (G₀^′) for different powders. In the dense flow regime, a critical temperature value dictated the transition between caking (powder's tendency to form lumps or masses) and free-flowing behavior, with temperature promoting agglomeration and shear rate causing dispersion. The critical temperature increased with the strain rate, highlighting the delicate balance between agglomeration and dispersion. Also, it is found that Relative Humidity (RH) plays a crucial role in reducing the stress developed within the sample and counteracting the temperature effect. Additionally, the slow compaction dynamics of powders were studied, revealing an inverse logarithmic law with a characteristic relaxation time (τ) that strongly correlated with the angle of repose.