Re-use of polyamide-12 in powder bed fusion and its effect on process-relevant powder characteristics and final part properties

Powder bed fusion (PBF) is an additive manufacturing technique capable of fabricating highly complex, individualised, and lightweight polymer components. However, to maximise the potential of PBF, in terms of both economic efficiency and environmental sustainability, a successful powder re-use strategy is essential. During a build, ageing and degradation processes affect the re-usability of un-sintered powder, so used powder is usually refreshed with virgin material before re-use. This study considers the effectiveness of using a 70:30 refresh ratio in a specific PBF technique: laser sintering (LS). Across a total of seven printing cycles, polyamide-12 (PA-12) powder refreshed with 30 % virgin material after each build, revealed a 4.5 °C increase in melting temperature. There was also a 20 % reduction in particle flowability, which may be related to the presence of fine satellite particles and considerable particle cracking. This deterioration in powder quality resulted in a 5.8 % increase in total part porosity, and an 11 % reduction in the ultimate tensile strength of fabricated parts, over the seven build cycles. A Pearson correlation test indicated that the reduction in powder flowability was the most significant (p-value of 0.005) cause for the loss of part strength; emphasising that the revolution powder analyser could be a useful complimentary technique for determining the quality of used powder within laser sintering. Nonetheless, compared to previous studies which re-used 100 % aged PA-12 powder, without refreshing with any virgin material, the observed reduction in part strength is relatively modest. This suggests that a 70:30 refresh ratio offers a good compromise between maintaining part performance, particularly for non-critical applications, without having to add an unnecessary amount of virgin powder. Therefore, this study reveals the relationship between the deterioration of powder properties and reductions in part strength; yet highlights the benefits of operating with a 70:30 refresh ratio when re-using PA-12 powder across multiple build cycles.