# Effects of Post-Treatment to Improve the Surface Quality of 3D Printing Cement Mold Casting
## 3 Results and Discussion
### Physical Property Evaluation
Figure 2b shows the surface election lacquer ware of the sample for each process in terms of Ra value, and each surface of the 3D-printed cube sample was measured. The printed floor surface (X-Y plane) has a relatively better surface roughness than the side surface, which is thought to be due to the role of the rollers in recoating and solidifying the printed surface [19]. Therefore, there was almost no change in the surface roughness due to the post-treatment process. On the other hand, in the case of the X-Z and Y-Z planes, the surface roughness after colloidal silica treatment was improved, which was better than that after heat treatment. Owing to the 3D printing characteristics, this affects the surface quality of the printed product displayed on the 3D printing process, where bending occurs owing to the stacking error of the side surface. Thus, research to improve the resolution of the side surface of the printed product is actively progressing [20]. This is the same as the result demonstrating that nano-colloidal silica plays a role in smoothing the lateral bending [21]. Figure 3c shows the porosities of the samples for each process. The porosity of the surface after printing was smaller than the porosity of the inside because the activator saturation level of the molded end and the inside was different as set by the 3D printer. In addition, the surface impregnation after treatment with colloidal silica resulted in a surface porosity of 15.9%, and a decrease of nearly 10%. After the heat treatment process, the porosity increased, and the internal porosity appeared to be less than the pre-treatment level of conventional colloidal silica. However, the surface porosity was 36.18%, which is higher than the 25.69% of the G sample. Thus, an increase of more than 10% was seen. This is seen in the displayed changes as the heat treatment of colloidal silica promotes crystalline changes in the silica sand [18]. The thermal condensation of colloidal silica impregnated in the gaps between the aggregates increases the gap between the aggregates, which in turn results in an increase in the pores of the GCH sample [19]. Figure 3d shows the dimensional changes in the molded product for each process. Set to 10 mm on the 3D printing drawing, the lengths of the \(X\)-, \(Y\)-, and \(Z\)-axes show a dimensional error of 2% or less in the sample for each process, and the \(z\)-axis error was relatively smaller than the \(X\) and \(Y\) axes. This is because when the \(z\)-axis activator in the print direction penetrates, it was absorbed by the pre-printed part in the downward direction and was printed relatively accurately at the intended print height, while the lateral spread appeared to be a result that was not effectively controlled [20; 21]. To confirm the changes in the surface roughness and pore fraction of the molded product for each process, the SEM image (Figure 4), XRD crystal phase (Figure 5) analysis, and DSC (Figure 6) analyses were performed. The SEM image of the surface after printing (Figure 4) confirmed the bond between the sand and the cement binder; after treatment with colloidal silica, it was flattened by the nano-silica particles on the surface. Subsequent heat treatment supports an increase in the porosity of the surface when fine cracks occur in the treated portion of the colloidal silica [18]. In addition, after the treatment of colloidal silica, in order to confirm the change of the crystal phase due to heat treatment, the change of the XRD crystal after the heat treatment for 30 min at 700 and 1000 \({}^{\circ}\)C (Figure 5) was confirmed.