# Four Different Build Angles in 3D-Printed Complete Deture Bases: A Comparative In Vitro Study
## 4 Discussion  
Based on our findings, we rejected the null hypothesis that there would be no differences in the adaptation of tissue surfaces according to build angle and that there would be no differences in the distances between reference points on the polished surfaces. Our color difference map revealed a greater anterior-posterior variation than left-right variation across the different build angles. As the build angle increased, there was a tendency for increased displacement between the digital model and the tissue surface, particularly at the posterior margin and anterior rugae region of the denture base. Significant differences were observed in the inter-AP length based on build angle, whereas no significant differences were found in the inter-canine and inter-molar widths. As described in the Introduction, this may be related to the number of layers required for each build angle and variations in total printing time [18; 27]. In particular, the SLA 3D printer, which photopolymerizes resin by irradiating light along a straight line, was used in this study. SLA 3D printers require more printing time than DLP 3D printers, which polymerize the resin in layers [34]. In the present study, a 0\({}^{\circ}\) build angle required an output of 237 layers and took 2 h and 54 min. At 30\({}^{\circ}\), an output of 431 layers required 3 h and 46 min. At 60\({}^{\circ}\), an output of 600 layers took 4 h and 19 min. At 90\({}^{\circ}\), it took 4 h and 20 min to output 662 layers. This trend suggests that as the build angle increases, both the number of layers and the output time for the CD base tend to increase correspondingly. After the polymerization of each layer, the printed product is lifted. During this process, the product inside the resin solution might experience tensile forces due to the adhesive force generated between the product and the polyvinyl chloride film or glass plate at the bottom of the resin vat. [PERSON] et al. reported that lower viscosity resins achieve higher trueness in 3D-printed denture bases, which may be attributed to reduced tensile forces during the printing process [33]. Therefore, it is hypothesized that as the number of layers increases, the potential exposure to tensile stress may correspondingly increase. Gravity also has effects on the printed product as the printing time increases [29]. These two factors are thought to be the causes of discrepancies among denture bases with different build angles. In our study, as the build angle increased, both the number of layers and time for printing subsequently increased, leading to a greater risk of error. For denture bases with 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) build angles, the posterior region, which is printed before other parts of the denture base, experienced tensile forces from the increasing number of layers and the  
Figure 8: Boxplot of the inter-AP length for denture bases with 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) build angles. The true distance, according to the CAD model, is 48.48 mm (indicated by the orange dashed line). Outliers are marked with “o”, and significant differences are indicated by “**”. CAD, computer-aided manufacturing. effects of gravity during printing. This led to larger negative errors (misfits) compared to denture bases with a 0\({}^{\circ}\) build angle [28; 34]. Interestingly, the 90\({}^{\circ}\) build angle exhibited a significantly lower RMSE compared to the 0\({}^{\circ}\) print angle. Despite localized large discrepancies at the 90\({}^{\circ}\) build angle, the smaller cross-sectional area of the slices may have contributed to a more uniform distribution of overall errors, resulting in a lower RMSE value. Although the color difference map indicates notable deviations in specific regions, the reduced cross-sectional area at the 90\({}^{\circ}\) build angle likely led to less cumulative error across the entire surface.