The effect of inter- and intra-layer delay time on TPU parts fabricated by laser powder bed fusion.

Abstract

In polymer laser powder bed fusion (PBF-LB-P) techniques, such as laser sintering, the time between scanning a given point in one layer and the same x-y point in the next layer is known as the 'inter-layer delay time'. Multiple parts are normally fabricated in a PBF-LB-P build for efficiency; however, this leads to variation in the inter-layer delay time for individual parts; in this study, we present a specific investigation using a commercially available thermoplastic polyurethane (TPU). Multiple part layouts were used and the resulting parts were subject to tensile testing and fracture surface analysis. The results demonstrate that an increase in inter-layer delay time can lead to a significant reduction in mechanical properties. Fabricating specimens in groups of 5 led to a 10% reduction in ultimate tensile strength, 30% reduction in extension at break, and 15 $\%$ reduction in Young's modulus compared to specimens fabricated individually. Fractography suggests this is due to decreased inter-layer bonding and an increase in defects. This has significant implications for the production of multiple parts in a build where consistent mechanical properties are critical. Based on our understanding of this detrimental effect, we put forward a novel build packing approach for PBF-LB-P, based on scanning area equivalence rather than the conventional time minimisation, to mitigate against it.