Many of the existing fabrication technologies for gun propellants utilize solvents such as volatile organic compounds (VOC), which can generate a large amount of hazardous waste. Solventless formulations that have been designed to circumvent these issues have significant manufacturing hazards, as well as performance, aging and safety concerns. This project aimed to reduce the impact of propellant fabrication using a novel direct-write additive manufacturing technology that enables the printing of extremely viscous mixtures, requiring less solvent for formulations, while benefiting from the ability to fabricate unique designs.

The approach taken was to apply the vibration assisted direct write printing system (VAP) that enables the rapid deposition of extremely viscous materials and yields fully dense three-dimensional (3D) printed structures at high resolution. This system was used to 3D print several formulations with low solvent content, including inert surrogate materials and energetic materials with different geometric features. The viscosity and feed limitations of these materials as a function of solids loading and solvent amount was investigated using rheometry. The properties of the printed structures were characterized to determine mechanical strength and porosity. 

The VAP approach was successfully used to extrude and deposit several formulations including a new double-base (DB) gun propellant surrogate that was developed in this work and an insensitive formulation with energetic particles at high solids loading. In addition, a number of inert and functional systems such as ceramics were 3D printed to determine the limits of solids loading. DB surrogates were printed at realistic formulations with no VOC, which typically require up to 50% by weight solvent. Several complex infill structures were produced at resolutions higher than other additive manufacturing methods for comparison. The prints had identical tensile strength and porosity to molded samples, showing no weak points between layers or tracks. New heterogeneous mixtures of particle-binder mixtures were formulated with optimal features. For formulations with a tendency to creep, a new dual-head system was used that combines VAP with polymer printing to retain the desired geometry, that can also add desired functions.

The results of this project provide a solid basis for transitioning the VAP approach to propellant fabrication with substantially reduced VOC solvent usage. The demonstrated capabilities also indicate that highly detailed propellant features can be produced that can be exploited to enhance lethality, while the parts have no porosity issues or suffer from reduced strength thereby maintaining required safety standards. Although the VAP approach solves the issues of extrusion and processing of highly viscous solids, this work points out to possible challenges upstream, where the materials are originally mixed before feeding in for extrusion and deposition. Further work looking at different combined dispensing-mixing systems based on the work here can potentially be integrated into existing process flows can address these challenges.