Research on rheological properties and extrusion behavior of aqueous alumina paste in paste-extrusion-based SFF processes

In this work, the rheological properties and extrusion behavior of aqueous alumina paste which can be used in paste-extrusion-based solid freeform fabrication (SFF) processes are presented. According to the analysis of dynamic viscoelastic properties, the aqueous alumina paste with solid loading of 50 vol% can be treated as a viscous–elastic–plastic body in the extrusion process. The paste shows the viscoelastic feature before yielding and viscoplastic feature after yielding. Although the paste possesses shear-thinning behavior, it can be extruded steadily due to relatively steady apparent viscosity within the range of shearing rate in the actual paste extrusion process. The ram extrusion experiments show that the variation of extrusion pressure is essentially determined by the rheological behavior of paste. The average extrusion pressure in steady extrusion stage can be estimated by Benbow–Bridgwater model. However, there exists a ram velocity threshold (0.1 mm/s in this study) below which the liquid phase migration will be significant and the extrusion pressure is hardly estimated. Due to the viscoplastic fluid feature of paste, the extrudate velocity cannot immediately reach to the expected value when paste extrusion starts and stops, and it could result in the imprecise deposition and “die drooling,” which can be improved by the operation of pulling back of ram.