## 3 Results
### Experiments
#### 3.1.2 Color Stability  
Figure 5 and Table 2 show significant differences in color stability between the groups. Post-hoc analysis demonstrated that significance was attributed to differences between the manual and FDM, two polyjets, and SLS methods. The color change (\(\Delta\)E) of the FDM, PJ-1, and PJ-2, and SLS groups was 32.58, 31.82, 24.67, and 60.95, respectively, while that of the manual group was 13.34. The color stability was, in the order of highest to lowest, milling, SLA, manual, DLP, PJ-2, PJ-1, FDM, and SLS. Only milling, which showed the lowest color change of 2.82, met the requirements of the clinical allowable range (\(\Delta\)E 3.3 or less).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & **Absorption** & **Solubility** & **Color Change** & **Flexural** & **Surface** \\  & & **(\(\upmu\)g/mm\({}^{3}\))** & (\(\upmu\)g/mm\({}^{3}\)) & (\(\Delta\)E) & **Strength (Mpa)** & **Hardness (HB)** \\ \hline
**Classification** & **Group** & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** \\ \hline Control group & Manual & 18.51 \(\pm\) 6.69\({}^{\text{a}}\) & 5.95 \(\pm\) 2.84\({}^{\text{ab}}\) & 13.34 \(\pm\) 16.12 \({}^{\text{a}}\) & 62.12 \(\pm\) 5.99 \({}^{\text{a}}\) & 83.37 \(\pm\) 1.5 \({}^{\text{a}}\) \\ \hline \multirow{6}{*}{Experimental groups} & FDM & 19.31 \(\pm\) 2.02 \({}^{\text{a}}\) & \(-\)1.43 \(\pm\) 0.49 \({}^{\text{a}}\) & 32.58 \(\pm\) 0.94 \({}^{\text{b}}\) & 58.84 \(\pm\) 0.39 \({}^{\text{a}}\) & 75.08 \(\pm\) 2.33 \({}^{\text{b}}\) \\ \cline{2-6}  & PJ-1 & 35 \(\pm\) 5.05 \({}^{\text{b}}\) & \(-\)1.06 \(\pm\) 1.19 \({}^{\text{a}}\) & 31.82 \(\pm\) 1.67 \({}^{\text{b}}\) & 83.71 \(\pm\) 1.24 \({}^{\text{b}}\) & 77.82 \(\pm\) 0.86 \({}^{\text{b}}\) \\ \cline{2-6}  & PJ-2 & 31.6 \(\pm\) 1.2 \({}^{\text{b}}\) & \(-\)1.55 \(\pm\) 0.38 \({}^{\text{a}}\) & 24.67 \(\pm\) 2.96 \({}^{\text{b}}\) & 94.95 \(\pm\) 2.94 \({}^{\text{b}}\) & 83.12 \(\pm\) 0.82 \({}^{\text{a}}\) \\ \cline{2-6}  & SLS & 19.04 \(\pm\) 4.12 \({}^{\text{a}}\) & \(-\)0.63 \(\pm\) 0.87 \({}^{\text{a}}\) & 60.95 \(\pm\) 4.94 \({}^{\text{c}}\) & 41.15 \(\pm\) 2.58 \({}^{\text{a}}\) & 73.8 \(\pm\) 5.58 \({}^{\text{b}}\) \\ \cline{2-6}  & SLA & 52.31 \(\pm\) 4.17 \({}^{\text{c}}\) & 15.6 \(\pm\) 6.29 \({}^{\text{c}}\) & 12.76 \(\pm\) 1.97 \({}^{\text{a}}\) & 69.37 \(\pm\) 2.1 \({}^{\text{a}}\) & 79.62 \(\pm\) 0.63 \({}^{\text{b}}\) \\ \cline{2-6}  & DLP & 17.51 \(\pm\) 5.39 \({}^{\text{a}}\) & 5.42 \(\pm\) 3.64 \({}^{\text{b}}\) & 14.26 \(\pm\) 4.61 \({}^{\text{a}}\) & 72.96 \(\pm\) 17.63 \({}^{\text{a}}\) & 86 \(\pm\) 0.58 \({}^{\text{c}}\) \\ \cline{2-6}  & Milling & 30.69 \(\pm\) 6.9 \({}^{\text{b}}\) & 5.11 \(\pm\) 4.25 \({}^{\text{b}}\) & 2.82 \(\pm\) 0.29 \({}^{\text{a}}\) & 120.36 \(\pm\) 6.63 \({}^{\text{c}}\) & 87.68 \(\pm\) 1.21 \({}^{\text{c}}\) \\ \hline \hline \end{tabular} SD, standard deviation; FDM, fusion deposition modeling; PJ, polyjet SLS, selective laser sintering; SLA, stereo-olithography apparatus; DLP, digital light processing; having the same characters (a–c) indicates no statistically significant difference.  
\end{table}
Table 2: Value of the water absorption, solubility, color stability, flexural strength, and surface hardness of the eight tested groups.  
Figure 4: Comparison of water solubility of each group. * denotes \(p<0.05/28\) for comparison between the manual and experimental groups; when 7.5 \(\upmu\)g/mm\({}^{3}\) or less, ISO10477 is satisfied (based on the red line).