# Evaluation of UV-Curable Solid Rocket Propellants' Properties for Advanced 3D Printing Technologies
## 4 Results
### 3d Printing Test  
The prototype described in Section 2.4 was used to carry out a series of tests. The initial step that must be undertaken before printing is calibration. If this process is performed incorrectly, it is impossible to achieve a successful print [47]. The calibration process was particularly complex due to the binder's elasticity, which induced a non-linear response of the flow rate to the applied piston force and resulted in an unwanted extrusion through the nozzle at the end of each layer. The first issue caused a delay in the deposition of the material, which was addressed through a precompression of the propellant before each print. Regarding the second issue, a retraction mechanism was initially employed; however, it proved ineffective as it also eliminated the precompression, thereby reintroducing the first problem. Consequently, a shutter capable of mechanically blocking the flow was used. Preliminary printing tests performed with a shutter highlighted the interference between the printed part and the shutter blade. Therefore, a redesign was planned referring to the style of shutter valve-based extruder presented in [48]. After the initial calibration phase, preliminary printing tests were performed on simple single-layer and multilayer specimens, varying the geometric parameters of the extruder. It was observed that utilizing nozzles with a smaller output diameter increased both the precision and extrusion pressure, which in severe cases led to the separation of the solid component from its liquid counterpart. The remaining slurry in the extruder chamber after the printing process, shown in Figure 9, exhibited the separation of the solid phase of the slurry during extrusion. This phenomenon arose from the pressure necessary to move a high-viscosity biphasic material such as the slurry, resulting in the accumulation of AS powder within the extruder and the outflow of a high-PB-concentration slurry. To address this issue, limitations were primarily imposed on the nozzle geometry, printing speed, and slurry temperature. The main variables to consider in the design of the extruder are the following [49; 50]:  
* Viscosity: An increase in its value results in a non-linear reduction in the extrudable flow rate. The increase in temperature must be carefully evaluated to achieve a reduction in the viscosity of the slurry without drastically compromising the mechanical properties. * Nozzle diameter: An excessive value makes it impossible to form objects with a defined shape, while an insufficient value leads to nozzle clogging. The average flow velocity in relation to the nozzle diameter shows a complex trend in the case of biphasic materials such as solid propellants. To estimate the volumetric flow of the propellant, a simplified model can be used, which assumes a constant fluid density during the extrusion process, a steady flow within the nozzle, and that the nozzle exit diameter is significantly smaller than the syringe diameter. According to these assumptions, the pressure difference in the nozzle is defined by  
\[\Delta p=p-p_{ext} \tag{7}\]  
where (\(p\)) is the pressure applied by the syringe piston, while (\(p_{ext}\)) is the ambient pressure. If the slurry exhibits behavior describable by Equation (4), the theoretical flow rate can be obtained by [51]  
\[Q_{l}=\frac{\pi D_{i}^{3}D_{0}^{3}}{32}\bigg{[}\frac{3n\Delta p\tan(\theta_{0} )}{2K(D_{i}^{3n}-D_{0}^{3n})}\bigg{]}^{-\frac{1}{n}} \tag{8}\]  
where (\(\theta_{0}\)), (\(D_{i}\)), (\(D_{o}\)), and (\(L_{l}\)) represent the characteristic angle of the extruder nozzle, the inner inlet diameter of the nozzle, the outlet diameter, and the length between the two sections, respectively. Assuming a constant volumetric flow rate based on the design printing speeds, to reduce (\(\Delta p\)), it is necessary to increase the geometry of the nozzle's outlet diameter or reduce the viscosity by increasing the temperature (reducing the K factor) or vary the nozzle angle (\(\theta_{0}\)).