Preliminary investigations on extrusion of high viscosity slurry using direct writing technique

Traditionally solid propellants are manufactured using casting and molding techniques. The effective burning rate of solid propellants is strongly depended on its cross section and geometry. The preparation of mold and mandrel for the manufacturability of various geometric profiles are tedious, time consuming, increases the cost and more human efforts are needed. In order to mitigate these issues, a disruptive technology called additive manufacturing (AM) is in the verge of development. Although the method is effective, additional study must be conducted to improve the flow characteristics of slurries for the high solid loading and there is a huge necessity to reduce the prolonged curing time. The present study focuses on preliminary investigations of extrusion of high viscosity slurry using a pneumatically driven extrusion system. The slurry was prepared with a 80 wt.% solid loading of NaCl having particle sizes of 45 µm and 150 µm, 15.6 wt.% HTPB, 2.2 wt.% TDI, 2.2 wt.% DOA and 0.03 wt.% of ferric acrylacetonate. The slurry was extruded with an aid of pneumatically controlled extruder and each layer was formed. Formed by extruding the slurry using 1.65 mm internal diameter nozzle. Infrared (IR) heater was utilized to transfer the radiational energy for partial curing of each layer and thereby adhesion of other layer was guaranteed. Simulation is performed to determine the temperature distribution using ANSYS platform for comparing the curing temperature of the printed part top surface. Preliminary experiments confirm that extrusion of slurry and heating of each layer can be effectively achieved with the proposed 3D printing technique. Three tensile specimens were produced in accordance with ASTMD 412-C and their corresponding mechanical properties are evaluated. The printed parts have the tensile strength of 0.7 MPa, elongation of 4.85 % and modulus of elasticity of 18.5 MPa which are comparable with the properties of conventional casted part.