In the past additive manufacturing processes relying on beam-based methods such as laser beam melting (LBM) of polymers were almost exclusively applied for the generation of prototypes. In recent years these methods are increasingly applied in the serial production (Wohlers 2014). With this increased relevance in industrial applications, the demands on the powder material increased as well. In consequence, there is a need for fine powders showing good powder flowability and high bulk density (Kruth et al. 2003). Currently, the commercial availability of polymer powders for LBM is very limited and the production processes are still not reliable enough due to poor powder characteristics (Goodridge et al. 2011). We have recently shown (Schmidt et al. 2012) for different materials (e.g. PS, PBT, POM, PEEK) that commercially available polymer granules can be ground down to the micron size range by a wet grinding process. Due to the grinding process the produced micron-sized particles are irregular in shape, which leads to a poor flowability of the powder. To overcome this problem the single particles are melted in a heated downer reactor and spherical particles are obtained by using the effect of the surface tension. The rounding of ground PBT micro particles was performed in a downer reactor with a length of 6000 mm and a diameter of 100 mm. The flow characteristics within the downer and influencing parameters are investigated. The mechanism of the rounding process in dependence of particle size, interfacial tension and melt viscosity will be shown using a sintering model (Kirchhof et al. 2009). The influence of residence time and solids density in the downer reactor on the particle shape and on the flowability of the bulk material is shown. Estimates from the sintering model allow for a reliable reactor design. Finally, scale-up rules are being derived. The obtained polymer material is processed in a LBM process and the effect of the change in morphology is shown.