Impact of Flow Aid on the Flowability and Coalescence of Polymer Laser Sintering Powder

Small amounts of nanometer-sized flow aids are typically added to polymer powders for selective laser sintering to increase flowability. These additives reduce friction between particles by electrostatic repulsion, leading to better bed density and part properties. The same repulsion however can hinder particle coalescence in the melt, reducing part density. This study investigates the effect of different amounts of flow aid on flowability and coalescence. A polybutylene terephthalate (PBT) powder with spherical morphology, specially designed for selective laser sintering, is used as a base and ideal model material. The coalescence is monitored by hot-stage optical microscopy. It was found that with increasing amounts of flow aid, the flowability could be characterized by three regions; one of low flowability where the powder contained insufficient flow aid, followed by a sharp transition region towards a high flowability region, where the flow aid was most effective. Addition of flow aid impacted the coalescence, which was marked by an increase of the temperature at which the particles started to melt and flow. At the maximum concentration of 0.5 wt.-% flow aid, the melting of some particles was delayed, and they remained solid for longer time at temperatures beyond the melting temperature. The optimum amount of flow aid therefore lies in the plateau region of high flowability, but before the occurrence of delayed melting.Mechanical Engineerin