# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 2 Materials and Devices
### Cartesian 3D Printer  
The rheological and mechanical analyses conducted in this article, as well as in previous work [18; 19; 20; 21], aimed to pave the way for the development of a prototype 3D printer capable of producing scaled propellant samples. The construction of a prototype represents a crucial step towards conducting more accurate and reproducible tests and enhances the Technology Readiness Level. A modular prototype of a 3D printer was designed. This prototype can be modified if some of its components prove to be unsuitable for the propellant printing process, particularly the extruder, which is the most critical part. This section presents the developed 3D printer and the initial printing attempts, detailing the main issues encountered and the solutions implemented, while in Section 4.3 the printing test results will be presented. The 3D printing process of a photopolymerizable material is divided into three distinct phases. The first phase focuses on the transfer of the slurry from the reservoir to the extruder's nozzle tip. The subsequent phase involves the precise manipulation of the material on the print bed through the kinematic system. The final phase is the crosslinking process, accomplished via a specialized lighting system. Several approaches have been considered, and a downselection was carried out through an initial trade-off study. Stereolithography systems primarily use illumination from the bottom of a vessel containing non-crosslinked material, while other systems use a bed illuminated from above, where the material on the plane is managed by a spreading blade [37]. These systems appear to be unsuitable for the present purpose because the high viscosity of the material prevents the easy removal of the non-crosslinked material. The option of a coupled deposition bed-resin vat system does not seem feasible in the immediate future due to the complexity of implementation. Moreover, this option complicates the engineering of the illumination setup, as it requires the management of a large surface. The most viable solution appears to be a hybrid printing device that combines elements of stereolithography and Fused Deposition Modeling (FDM). This system employs an extruder to deposit the propellant onto the print bed. Subsequently, each layer is illuminated by an LED blade. This process is repeated for each layer until the desired grain geometry is achieved. The use of a worm screw mechanism to transfer the slurry from the reservoir to the print bed was found to be ineffective due to the separation  
Figure 4: On the left, the predicted irradiance; on the right, the measured irradiance of the UVA LEDs. of the binder and AS powders along its axis. Additionally, the use of such a component could produce significant risks, as the friction of the propellant between the screw and the inner wall of the extruder could cause unintentional ignition. Therefore, a volumetric extrusion system was chosen (Figure 5a), as it preserves the propellant and conveys it to a nozzle that deposits it on the print bed. However, handling the material is complex, as it involves moving a heterogeneous suspension through a pressurized duct. Due to the multiphase nature of the suspension, issues of segregation and separation between the binder and particles may arise. Consequently, the design of the extrusion system's geometry and the working pressure are critical factors. The 3D printer (Figure 5b), developed in collaboration with Microdigit, maneuvers a volumetric extruder on a Cartesian axis system. This extruder was developed by modifying the terminal part of a pneumatic piston (SMC CS96-32). The removed piston end section is replaced with a metal plate (also referred to as a _nozzle plate_), as depicted in Figure 5a. This plate is secured to the piston using four M5 screws. A centrally located threaded hole allows for the attachment of standard nozzles, which are typically used for ceramic 3D printing.