Relation between the powder layer quality and the GranuDrum Dynamic Cohesive Index for polymeric powders at SLS process temperatures

Selective Laser Sintering (SLS) is an additive manufacturing process, characterized by the creation of 3D objects, obtained by alternating a powder distribution step with a spreading tool to get a thin layer and the sintering of the particles with a laser beam. To get acceptable 3D objects, the powder layers must be uniform, smooth, and with low porosity. The characteristics of the layers depend on the powder spreadability, which is the capacity of a powder to be spread across a plate. The spreadability depends on the characteristics of the powder, e.g. particle size and shape, interparticle forces, and process conditions like blade speed and process temperature. Therefore, it is important to assess the powder spreadability in the same process conditions to properly predict the quality of the powder layers. Two different approaches have been used to investigate the quality of the layer generated in the spreading process by a moving blade: a set-up from the University of Salerno (UNISA) and the GranuDrum (Granutools, Belgium). The UNISA set-up and the GranuDrum can work up to temperatures of about 150-200 °C, conditions similar to those occurring in a typical SLS machine. The UNISA setup mimics the distribution step of the SLS process, creating a layer of controlled thickness. Macroscopic pictures of the obtained layers are taken to do a qualitative analysis of the powder layer quality. In the GranuDrum, the cylindrical cell containing the powder rotates at different angular velocities. Snapshots of the powder are taken for each angular velocity and the air/powder interface is detected from each snapshot. For each velocity, the Dynamic Cohesive Index (DCI) is measured from the interface fluctuations, that depend on the cohesive forces acting between the particles. Tests with the GranuDrum have been performed at rotating speeds consistent with the scrolling velocities of the blade of the UNISA set-up to reproduce consistent shear conditions. Different polymeric powders have been tested at temperatures between the ambient and 140°C: two kinds of polyamide 6, thermoplastic polyurethane, and polypropylene powders. A good correlation has been found between the cohesiveness of the powder, evaluated by the Dynamic Cohesive Index metric, and the quality of the powder layer in the UNISA setup.