Difference between powder bed density and green density for a free-flowing powder in binder jetting additive manufacturing

Abstract

In binder jetting additive manufacturing, the final part density is closely related to powder bed density and green density. Though powder bed density is considered one of the most critical factors to control and predict green density, their difference has not been fully understood. In this work, experiments and simulations were employed to investigate their difference for a free-flowing powder in a counter-rotating roller powder spreading system. Using an alumina powder with excellent flowability, both experiments and simulations show that powder bed density is higher than green density. Furthermore, simulations provide an in-depth understanding of the density difference. Specifically, in the powder bed density scenario where particles are able to move across different powder layers, a slower powder flow with a smooth velocity transition is observed. In the green density scenario where powder is spread on a printed powder layer, the confined layer space generates a powder flow with higher velocity and more turbulence. As a result, the powder packing structure in the green density scenario has more void space near layer interface than that in the powder bed density scenario. This paper reports a reason for the difference between powder bed density and green density for the first time.