The effect of volume loading on the extrusion of bimodal glass bead mixtures

Additive manufacturing has provided new methods for generating complex geometries of composite energetic materials. Additive manufacturing of ammonium-perchlorate composite propellants through direct-ink-write experiences extrusion limitations due to the high viscosities of highly solids loaded propellants. Vibration-assisted printing (VAP) was developed as a method to extend the extrudability limits and extrusion speeds observed with direct-ink-write systems. This study compares the mass flowrates and extrudability limits for bimodal mixtures of glass beads and hydroxyl-terminated polybutadiene (HTPB) binder for both VAP and direct-ink-write printing as a function of volume percent solids loading. The VAP system was able to print higher volume loadings and significantly higher mass flowrate than the direct-ink-write system. The bimodal glass bead mixtures were also compared to a previous study that focused on the extrusion of monomodal glass beads/HTPB mixtures. Interestingly, bimodal mixtures were shown to extrude quicker than monomodal mixtures at all volume loadings and across both printing systems.