# Development and Assessment of a 4D Printing Technique for Space Applications
## 4 Discussion  
### Influence of the Programming Device on the 4D Printing Performance  
The influence of the programming device on the performance of 4D printed hinges is a critical aspect that emerged from our experiments. A comprehensive analysis revealed distinct differences between the two iterations of the programming device, shedding light on their impact on the programming and recovery processes. Throughout all of the experiments conducted, a consistent observation was the notable inequality in the 4D printing performance between the first and second versions of the programming devices. This difference underscores the significance of the design variations in influencing the overall effectiveness of the process.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Specimen Type** & \(\mathbf{R_{\rm f}}\) & **SD(\(\mathbf{R_{\rm f}}\))** & \(\mathbf{R_{\rm r}}\) & **SD(\(\mathbf{R_{\rm r}}\))** \\ \hline M-i 3\(\pi\) & 69.4\% & 7.9\% & 94.5\% & 6.6\% \\ \hline M-i 3\(\pi\) & 62.5\% & 11.3\% & 95.2\% & 8.2\% \\ \hline M-i 3v2\(\pi\) & 75.8\% & 4.2\% & 97.9\% & 2.8\% \\ \hline M-i 3v2\(\pi\) & 73.4\% & 8.0\% & 97.6\% & 5.0\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: The mean fixity ratio \(R_{\rm f}\) and recovery ratio \(R_{\rm r}\) with their respective standard deviations for the third experiment on the influence of the cool-down force.  
Figure 13: The effect of forced cool down on the fixity/recovery ratio \(R_{\rm f}/R_{\rm r}\) of hinge specimens programmed and recovered at 175 \({}^{\circ}\)C with the standard deviation plotted as error bars.  
An evident trend in the data is the superior performance exhibited by the second version programming device in comparison with its predecessor. This improvement was consistent across all experiments, suggesting a positive correlation between the design enhancements in the second version and the overall efficiency of the 4D printing process.  
While the first version facilitated the ease of swapping hinges, a notable drawback was encountered during the programming process. Retracting the movable slider while simultaneously folding the hinges proved to be a laborious and time-consuming task. This operational complexity could have potentially contributed to the observed performance disparities.  
A crucial aspect influencing the 4D printing performance was the speed at which the programming device was closed after removal from the 3D printer. With the second iteration programming device, the closing process could be executed slightly faster. This heightened efficiency could account for the improved performance witnessed in experiments conducted with this version.  
For the first iteration programming device, thermal expansion of the hinge occasionally resulted in unintended expansion in the x direction. This expansion, occurring during bending, led to a few millimeters of slippage, causing off-center folds. Such deviations underscored the significance of hinge behavior under thermal conditions and emphasized the need for design modifications to mitigate these effects.  
The data presented in Figures 10-12 indicate that the programming device not only influences the programming process, but subsequently also significantly impacts the recovery process. The variations observed in recovery times further highlight the intricate interplay between the programming device design and the recovery of the hinges.  
This in-depth analysis of the programming devices provides valuable insights into their nuanced influence on 4D printing performance, laying the groundwork for a more comprehensive understanding of the experimental outcomes.