Role of elongational viscosity of feedstock in extrusion-based additive manufacturing of powder-binder mixtures

The 3D printing of metals and ceramics by the extrusion of a powder/thermoplastic binder feedstock is an extrusion-based additive manufacturing (EAM) technique and has received significant interest. EAM feedstocks are generally characterized by their shear viscosity. A quantitative comparison with the shear flow data, through an estimation of the Trouton ratio, indicates that the extensional viscosities are three orders of magnitude greater than their shear flow viscosity at a comparable shear rate obtained in three different high loaded polymers retained for this study. This experimental study addresses the unsolved issue of the role of elongational viscosity in the modelling of EAM of highly viscous melts. The study was conducted using three feedstocks with a water-soluble binder and high powder loading. The different powder materials used for this study are stainless steel, alumina and zirconia. Initially, the rheological properties of the feedstocks were assessed using capillary rheometers. A pressure drop model based on the shear and elongational components of the viscosity was proposed to predict the extrusion pressure during capillary tests. The model was adapted to develop a specific EAM machine, namely, an EFeSTO, equipped with a pellet extrusion unit. Experimental EAM tests were conducted, and the pressure drops were analytically predicted and experimentally measured. A total of 31 different combinations of extrusion velocities, nozzle diameters, 3D printed shapes and materials were tested through a total 184 experimental runs. The model predicts well the experimental pressures for the steel feedstock, whereas it underestimates the pressure for the two ceramic feedstocks owing to their different thermal properties. The results of this study clearly demonstrate that the pressure, and therefore the material flow during the EAM processes of viscous materials, cannot be modelled well without considering the elongational viscosity.