On the mechanism of binder migration in furan binder jetting of sand molds and cores

Binder Jetting is a layer-based additive manufacturing process in which a printhead deposits droplets onto a pre-prepared layer of particles. Upon droplet impact, the binder begins to migrate and infiltrate adjacent areas away from its originally intended location. The purpose of this study is to investigate the mechanism of furan binder migration. Observing in-situ binder spreading is challenging, especially for the furan binder used in this work. The strong discoloration of the surrounding sand makes it difficult to distinguish between the printed pattern and the surrounding loose sand. For this reason, a fluorescent dye is added to the binder. A wavelength-matched laser in the binder jetting machine provides the excitation energy, and in-situ observation of binder migration becomes feasible. The results show an unexpected behavior where the actual microscopic fluid redistribution of the binder does not match the observed macroscopic measurements of other reports. It becomes clear that the migration mechanism of furan binder in sand binder jetting is strongly influenced by gaseous mass transport. To support this theory, a phenomenon called boundary zone is studied by micrographic and computed tomographic analysis. This outer shell region surrounds samples with higher binder contents and extends over a thickness of approximately 400 µm. The binder content here is significantly higher than that of the core of the specimen and its target value, demonstrating that liquid concentration equalization is not feasible. A plausible explanation is evaporation and condensation of binder, resulting in localized binder accumulation in areas of high catalyst to binder ratios. Since binder evaporation is an overlooked issue in furan binder jetting, additional experiments are performed to demonstrate the extent of evaporation. For this, particle layers are deposited on a scale, the printhead deposits binder, and the resulting mass loss is recorded. With a better understanding of binder migration, new strategies can be developed to reduce geometric deviation, improve geometric precision, and possibly allow for higher layer thicknesses in furan sand binder jetting.