## 4 Discussion  
This study aimed to assess the feasibility of utilizing the Merlon fracture model to evaluate the accuracy of two commonly employed vat-photopolymerization-based additive manufacturing (AM) techniques: DLP and LCD. In addition, we selected 3D-printing slurries for preparing temporary and permanent dental products to examine their influence on the dimensional precision of the two polymer-based 3D-printing materials. Comparative evaluation of the RMS results and colorization maps between the experimental groups showed that the RMS results of the AL and AO groups using the DLP 3D printer were significantly lower for all wall thicknesses; among them, the RMS results of  
Figure 8: FE-SEM micrographs of the merlon wall surfaces for PO and ML groups, illustrating surface morphology and structural characteristics. the AL group using the 3D-printing slurry for temporary dental product fabrication were significantly lower for the 0.4 and 0.5 mm thickness conditions (\(p<0.05\)). The colorization maps of the AL and AO groups showed a pattern similar to that of the RMS results, as shown in Figure 4. Interestingly, the RMS results of the PO group using the LCD 3D printer and ML control group based on SM increased equally as the wall thickness increased. In addition, the colorization map of the PO group showed an overall shrinkage of the Merlon fracture model, and the map of the ML group indicated a less ablative wall for all wall thicknesses. Estimating the overall structure of the experimental groups is difficult using only the colorization map image for one model in each group; however, we can speculate that the internal shrinkage (PO group) of the model using the LCD printer and inaccurate milling of the walls (ML group) using the milling machine were the main reasons for the high RMS values. The wall thickness results show that the thickness values of the AL, AO, and PO groups using 3D-printing material for permanent dental product fabrication were closer to the original thickness than those of the ML group (\(p<0.05\)). In particular, the thickness results of the AL group using 3D-printing material for the fabrication of temporary dental products exhibited thinner values than the original thickness. The decreased accuracy of the thickness of the final 3D-printed product can be attributed to multiple factors [19]. The intrinsic characteristics of the photopolymerization process and specific resin composition induce polymerization shrinkage [20]. In addition, variations in printing parameters such as layer thickness, light-source intensity, and exposure time further contribute to the discrepancy [21]. Finally, partial dissolution of the printed resin during postprocessing exacerbates the shrinkage [22; 23]. To investigate the underlying cause of the observed differences in results, we analyzed the degree of conversion (DC) of specimens printed using DLP and LCD technologies. FT-IR spectroscopy was employed to quantify the DC by comparing the absorption peaks of aliphatic C=C and aromatic C=C bonds--representative of monomeric structures--based on the intensity ratios before and after polymerization (see Supplementary Materials). The FT-IR results indicated that the DC values for the AL, AO, and PO groups were 36.60 \(\pm\) 0.42%, 53.20 \(\pm\) 1.41%, and 53.63 \(\pm\) 1.52%, respectively. These findings demonstrate that the AL group exhibited lower polymerization compared to the AO and PO groups. Regarding the polymerization shrinkage of the AL group, we could not determine any predominant effect on the polymer-shrinkage factor of the 3D-printed model based on the colorization map (Figure 4). In terms of the 3D-printing process parameters, the printed layer thickness of the DLP printer (0.05 mm) was half that of the LCD printer (0.10 mm), as noted in the footnote of Table 1; however, no difference in the printing accuracy was observed between the two printers despite this variation in layer thickness.