# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 5 Conclusions  
This study provides a comprehensive evaluation of UV-curable solid rocket propellants designed for advanced additive manufacturing by means of the measurements of propellant properties and 3D printing tests carried out with a properly designed modular printer. The findings confirm that the propellants exhibit pseudoplastic behavior, which facilitates smoother extrusion during 3D printing. Although increasing the temperature effectively reduces the viscosity, it also shortens the propellant's pot-life, necessitating a careful balance between the print speed and thermal management. Based on the pot-life tests, the equilibrium temperature range for the optimal printability and pot-life should be between 60 \({}^{\circ}\)C and 70 \({}^{\circ}\)C. This range allows for a balance between reduced viscosity for easier extrusion and a manageable pot-life to ensure the material remains printable for a sufficient duration. Implementing a dynamic temperature control strategy that adjusts the temperature in real time based on the extrusion rate and viscosity measurements may help maintain the equilibrium temperature within the optimal range, enhancing the overall print quality and consistency. UV photopolymerization, particularly with UVA radiation, proved effective in curing the propellant and enhancing its mechanical properties. The preliminary results suggest that, under the tested conditions, UVC exposure did not significantly improve the polymer conversion rate and introduced additional challenges, such as heat dissipation and lower energy efficiency. However, given the potential of shorter wavelengths to influence curing at a microstructural level, further investigations--such as microtomography studies--will be conducted to explore the role of UVC in greater detail. For now, UVA irradiation remains the most reliable method for optimizing curing in this specific formulation. The 3D printing trials identified technical challenges, such as  
Figure 10: Deterioration in print quality with increase in moisture absorbed by AS salts, resulting in underextrusion and severe nozzle clogging. (**a**) Monolayer printing with dry AS salts. Samples were 50 mm \(\times\) 50 mm approx. (**b**) Multilayer printing with dry AS salts. (**c**) Printing with wet salts. material segregation within the extruder, highlighting the need for the further refinement of the extrusion system. To address these challenges, several technological improvements were proposed. Nozzle optimization, by utilizing nozzles with varying diameters (0.6 mm, 1.5 mm, and 3 mm), can help optimize the flow rate and reduce clogging. Smaller nozzles increase precision but may require higher extrusion pressures, while larger nozzles reduce the pressure but may affect the detail of the print. Implementing a redesigned shutter mechanism based on successful designs from the existing literature, such as the shutter valve-based extruder, can help control the flow of material and prevent unwanted extrusion at the end of each layer. Additionally, the moisture sensitivity of the propellant emerged as a significant issue affecting the print quality. Overall, these findings contribute to the ongoing development of UV-curable propellants for additive manufacturing, offering valuable insights into improving curing strategies, the printability, and the material performance. By addressing current limitations and exploring advanced characterization techniques, this research helps to advance the technology toward higher readiness levels, paving the way for its broader application in composite solid propellant manufacturing. Conceptualization, F.M., G.T., A.Z., G.P., L.S., A.F. and D.P. ; data curation, G.T. ; funding acquisition, D.P. ; investigation, G.T., A.Z. and C.N. ; methodology, F.M., G.T. and A.Z. ; project administration, F.M. and D.P. ; software, G.T. ; supervision, F.M., A.Z., C.N., A.F. and D.P. ; visualization, G.T. and A.Z. ; writing--original draft, G.T. and A.Z. ; writing--review and editing, F.M., A.Z. and D.P.