## 4 Discussion
Additionally, we employed two different urethane acrylate-based resin materials with a DLP 3D printer to fabricate the Merlon model, as listed in Table 1 (AL and AO groups). If the 3D-printing process parameters were the primary factors affecting the thickness reduction, both groups would exhibit similar results. However, only the AL group, which used a resin formulated for temporary crowns, exhibited a decrease in print thickness. In addition, using the dissolution of the printed resin in the cleaning solution, we could estimate this factor from the FE-SEM images of the AL, AO, and PO specimens (Figures 7 and 8). The 3D-printed lamination layer was clearly observed in the AO and PO specimens, whereas the lamination layer was not clearly visible in the AL specimen, suggesting dissolution of the printed materials. Thus, these findings suggest that the primary cause of the thickness reduction in the AL group was likely partial dissolution during the cleaning process. Several cleaning solutions for 3D-printed products have been introduced, including isopropyl alcohol (IPA), tripropylene glycol monomethyl ether, and water-based cleaning solutions [24]. Among these, IPA is commonly used because of its relatively low cost and short cleaning time. Excessive exposure to IPA or prolonged cleaning processes can degrade the mechanical properties and cause structural changes in polymeric materials owing to decreased chemical durability [25; 26]. To assess the degree of dissolution of the 3D-printed specimens in the IPA cleaning solution, a solubility test was conducted for the three experimental groups after immersion in IPA for 15 and 30 min (see Supplementary Materials). The results showed that the AL group exhibited higher solubility compared to the AO and PO groups at both time points. These findings suggest that the lower degree of polymerization in the AL group contributes to its increased solubility in cleaning solutions. Therefore, one of the main reasons for the thinning of the walls of the merlon in the AL group with materials for temporary crowns may be the degradation of the polymeric material during the cleaning process. Based on the bottom thickness measurements, the AL group recorded the lowest thickness value, while the ML group exhibited the highest thickness among all experimental groups. In addition, we observed similarities in the RMS results of the bottom thickness measurements. In models 3D printed using vat-photopolymerization technology, several factors affect the difference in thickness between the horizontal plane (XY direction) and vertical plane (Z direction) after all the processing, such as the light scattering of the 3D printer causing over-polymerization in the XY plane [20; 22], differences in the curing depth and shrinkage behavior of 3D-printing resin [23], and nonuniform shrinkage caused by cleaning and postcuring processes [25]. To minimize these deviation factors during the 3D-printing process, exposure settings should be optimized to prevent excessive overcuring in XY, the postcuring time should be controlled to achieve uniform shrinkage, and the IPA cleaning time should be minimized to avoid excessive resin swelling and shrinkage. Based on the results of all the tests conducted in this study, we confirmed that the Merlon fracture model developed for evaluating the machinability of SM models is a suitable method for assessing the manufacturing accuracy of models fabricated using vat-photopolymerization AM technologies (DLP and LCD). Additionally, we observed differences in the manufacturing accuracy between 3D-printing slurries designed for temporary and permanent dental products. Furthermore, it is anticipated that cleaning and post-processing procedures will be essential in reducing manufacturing tolerances in 3D-printed models.