Impact of part thickness and drying conditions on saturation limits in binder jet additive manufacturing

Abstract

Binder jetting (BJ) is a high build-rate additive manufacturing process with growing commercial interest. Growth in BJ applications is driven by the use of finer powders and improved post-processing methods that can produce dense, homogenous final parts. However, understanding of the basic droplet/powder interaction is relatively limited. This paper considers the impact of in-process drying, part geometry, and droplet size on a key printing parameter: binder saturation. Parts of varying thicknesses are printed with a range of saturation levels under various heating conditions. The ratio of the printed part mass to the theoretical part mass is used to detect bleeding. In unheated powder beds, part mass increases linearly with printing saturation levels across the range tested (30 %–130 %). However, when the powder is heated between layers, there is a wide range of print saturation levels (30–80 %) over which increasing binder saturation or droplet volume does not increase the part mass. This stable part mass corresponds to accurate part geometries without bleeding and is likely due to enhanced evaporation of the binder solvent between layers. Smaller droplet volume (42 pl) was also shown to decrease saturation levels in unheated powder bed and in single layer parts. The differences in part mass with print saturation and droplet volume are most pronounced in thin parts. These observations lead to a simple method for determining an appropriate print saturation parameter for a powder/binder combination in thick parts.