Polyamide 12 powder ageing in laser beam-based powder bed fusion and its combined effects on powder and component characteristics

Purpose: A key characteristic of powder bed fusion for polymers is that also the non-processed powder in the powder bed is exposed to elevated temperatures. This alters the properties of the remaining powder, which is compensated by refreshing the used powder with new powder. Nonetheless, it is discarded after a certain number of process iterations, which is economically and ecologically highly disadvantageous. Research works intensively to analyse and reduce the concurring effects responsible for powder ageing. This study aims to give a comprehensive overview of the cumulative changes in the powder and the printed parts when conducting several subsequent build cycles. Design/methodology/approach: New polyamide powder (PA12) was used in a total of nine subsequent build cycles with constant sintering parameters and without powder refreshing. After each iteration, the powder and parts were tested for their morphological, thermal and rheological properties. Findings: The results are related to three main changes in the powder during the build cycles: decreasing bulk density (through agglomeration), increasing melt viscosity (through polyamide post-condensation) and increasing melting peak and onset temperatures (through thermal annealing of the powder). Originality/value: Even though the ageing of PA12 powder in powder bed fusion is well-known, it is not yet fully understood. Studies are not complete and due to different ageing conditions only partially comparable. The detailed study aims to help understand the related effects of powder ageing for process-relevant properties and to show which factors require control to limit the powder ageing.