Rheological degradation of polyamide 12 powder during successive reuse in the powder bed fusion–laser beam of polymers process

During the powder bed fusion–laser beam of polymers (PBF–LB/P) process, only a small portion of the polymer powder is processed into parts. The remaining powder can be reused to some extent, but the high temperatures of the PBF–LP/process cause thermal ageing that eventually necessitates the unused powder be mixed with fresh powder or recycled. However, the changes that occur within the polymer powder during each cycle that eventually preclude its reuse have not been clarified. This study investigated the impact of successive PBF–LB/P cycles on the reusability of polyamide 12 (PA12) powder with a focus on the degradation in rheological properties. Thermal ageing was found to increase the zero-shear viscosity and storage modulus of the PA12 powder, attributed to an increase in molecular weight and modification of the polymer chains such as branching and crosslinking. These changes narrowed the thermal processing window by impairing the melt flow and reducing coalescence. Scanning electron microscopy revealed minor morphological reorganisation, but the powder flowability declined markedly owing to particle fragmentation and a depletion of flow-enhancing agents. X-ray computed tomography observed a direct relationship between powder degradation and part quality: only powders with the least thermal ageing could be used to fabricate a complete part. These findings indicate that degradation in rheological properties is a key mechanism that links the thermal ageing of polymer powders to their diminished processability and recyclability for PBF–LB/P.