# Effects of Post-Treatment to Improve the Surface Quality of 3D Printing Cement Mold Casting
## 3 Results and Discussion  
### Physical Property Evaluation  
The basic property of the molded body sample (G sample) after the powder bed 3D printing process was completed. The molded body sample was treated with colloidal silica (GC sample), followed by (GCH sample) subjecting it to heat treatment. Figure 3 shows the pressure compression strength of each sample with respect to the heating temperature. Each heating temperature was maintained for 30 min at 200 \({}^{\circ}\)C up to 1200 \({}^{\circ}\)C, excluding 100 \({}^{\circ}\)C, and the heating rate was 5 \({}^{\circ}\)C/min in an atmospheric environment. To keep the sample conditions for each step the same, the time was maintained in an oven at 100 \({}^{\circ}\)C. In the case of the G sample, in the heat treatment process at 200 \({}^{\circ}\)C, the sample obtained immediately after printing had the highest strength. The strength decreased as the temperature increased and increased again after heat treatment at 800 \({}^{\circ}\)C or higher. The increase in strength during the 200 \({}^{\circ}\)C heat treatment can be attributed to the increase in the role of the cement binder due to the evaporation of the remaining water, similar to the result of [PERSON] et al. [10]. The subsequent decrease in strength was due to the dehydration characteristics of alumina cement [9], and the increase in strength above 800 \({}^{\circ}\)C was confirmed in the complete dehydration reaction [11; 12]. Therefore, the printed sample (G sample) was obtained by drying at 200 \({}^{\circ}\)C for 30 min. The strength of the GC sample increased to 6.3 MPa at 4.4 MPa at 100 \({}^{\circ}\)C with colloidal silica impregnation, and higher intensity than that of the G sample was confirmed in most sections except 200 \({}^{\circ}\)C. When heat treatment was performed at 1000 \({}^{\circ}\)C or higher, it appears that the strength of the colloidal silica was significantly higher than 12.4 MPa due to the strengthening of the bonding force as a result of recrystallization [13; 14; 15; 16; 17; 18]. The colloidal silica-treated sample shows the same tendency as after printing, but the ethanol-based colloidal silica solvent evaporates sufficiently at 100 \({}^{\circ}\)C; thus, the same strength is expected to be displayed at 200 \({}^{\circ}\)C. It was confirmed that the strength was higher at heat treatment temperatures higher than 200 \({}^{\circ}\)C [18]. In the case of the GCH sample, the same strength as the G sample, which was heat treated at 1000 \({}^{\circ}\)C, was shown because it had already been heat treated at 1000 \({}^{\circ}\)C.  
The amount of gas generated in the mold for each process was confirmed by the change in mass of the sample stored at 100 \({}^{\circ}\)C during heat treatment at 1000 \({}^{\circ}\)C. The G sample showed a mass loss of 2.0 \(\pm\) 0.2%, and the GC sample also showed a similar mass loss of 2.2 \(\pm\) 0.4%. In addition, the GCH sample appeared to be almost unchanged at 1.0% \(\pm\) 0.3% (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multirow{2}{*}{**Mass Loss Rate (\%)**} & \multicolumn{3}{c}{**Bottom Area Shrinkage (\%)**} \\ \cline{3-5}  & & **5 mm** & **10 mm** & **15 mm** \\  & & **Thickness** & **Thickness** & **Thickness** \\ \hline G & 2.0 \(\pm\) 0.2 & 1.2 \(\pm\) 0.3 & 1.6 \(\pm\) 0.2 & 2.4 \(\pm\) 0.3 \\ Cast body & 2.2 \(\pm\) 0.4 & 1.5 \(\pm\) 0.2 & 2.4 \(\pm\) 0.5 & 4.4 \(\pm\) 0.5 \\ GCH & 1.0 \(\pm\) 0.3 & 0.9 \(\pm\) 0.3 & 1.3 \(\pm\) 0.1 & 1.5 \(\pm\) 0.2 \\ cast body & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mold and casting body shrinkage rate (%) for each process according to the thickness of the outer wall of the box mold. Figure 3: (**a**) Uniaxial compressive strength according to heat treatment temperature (**b**) Ra (\(\upmu\)m) of XY, XZ, YZ surfaces of the molded body by process (**c**) average porosity inside and outside the molded body (%) (**d**) dimensional error of molded object \(x,y,z\) axis.