Extrusion of metal powder-polymer mixtures: Melt rheology and process stability

The mixture of metal powder and a viscous polymeric binder (often referred to as feedstock) is commonly used for Metal Injection Moulding (MIM) applications. Recently, the interest towards the extrudability of metal/binder feedstocks is increasing, especially because of the growth of additive manufacturing techniques based on the vertical extrusion and layered deposition of filaments. In this experimental work, a feedstock prepared as a mixture of stainless steel 316 L powder with water-soluble binder was tested. The rheological behaviour of different mixtures (with powder loading between 50 and 63% in volume) was assessed using a capillary rheometer. The theoretical window of optimal extrudability was determined, in terms of temperature, shear rate and powder loading. Then, a specially designed CNC controlled extrusion system was used for performing vertical extrusion tests. The analysis of extrusion pressure profiles and the dimensional variability of the filaments was used to correlate the theoretical extrudability predicted by the rheological model with the actual extrusion tests. All results indicate that the conditions which yield better stability of the extrusion process are those that allow higher viscosity of the mixtures.