Processability of SS316L powder - binder mixtures for vertical extrusion and deposition on table tests

Metal injection molding (MIM) feedstocks are mixtures made of a solid metal powder and a viscous polymeric binder. Vertical extrusion of MIM feedstock has seldom been investigated in the scientific literature, but it is the enabling technology for extrusion-based additive manufacturing processes (EAM). The EAM process adopts the movement of an extruder, relative to a build table, to deposit thin strands (roads) of the mixture and grow a 3D object, layer by layer. EAM may find applications for the consumer as well as industrial products. This paper addresses some of the challenges involved in achieving a consistent flow of molten feedstock mixture and discusses the consequences of the main process parameters on the extrusion-based manufacturing process. This work also relates the rheological behaviour of feedstock to the extrusion process, aiming at the dimensional stability of parts. An experimental study was performed on SS316L feedstock with a water-soluble binder system, by varying the percentage powder content of the mixtures, the extrusion temperature, the extrusion rate, the deposition table speed and the nozzle shape. The study evidences and explains why higher powder loading values and lower extrusion temperatures are the most useful conditions in order to obtain a stable flow with reduced swelling/shrinking phenomena.