Influence of different milling conditions on mechanical properties and surface characteristics of denture base resins

Objective This study aimed to investigate the effects of wet milling (water-based coolant) and dry milling (no active cooling) on the mechanical properties and surface characteristics of CAD/CAM-fabricated polymethyl methacrylate (PMMA) denture base resins. Methods 52 rectangular PMMA specimens were prepared under standardized milling conditions and subjected to three-point bending tests, scanning electron microscope observation, static contact angle analysis and surface roughness (Ra/Sa) measurements. All specimens were fabricated with strict control of milling parameters to eliminate potential confounding variables. The three-point bending test was conducted to evaluate the flexural strength and flexural stress at break. Scanning electron microscope(SEM) was used to observe the surface morphology at different magnifications to characterize the microstructural differences. Static contact angle analysis was employed to assess surface wettability, which is closely related to denture retention. Surface roughness parameters (Ra and Sa) were measured using two different instruments to ensure the accuracy and reliability of the results. Results No statistically significant difference in flexural strength was observed between the two groups ( p  > 0.05). However, wet-milling specimens exhibited significantly higher flexural stress at break, reaching 11.579 ± 0.983 MPa, in contrast to 6.878 ± 2.210 MPa for dry-milling specimens ( p  < 0.001). Wet-milling specimens demonstrated enhanced surface hydrophilicity, with a water contact angle of 68.8 ± 2.7°, which was notably lower than the 81.3 ± 3.2° observed in dry-milling specimens ( p  < 0.001). Furthermore,wet-milling specimens presented lower surface roughness. The arithmetic mean surface roughness (Ra) of wet-milling was 1.296 ± 0.346µm versus 2.293 ± 0.907µm in the dry-milling group ( p  < 0.001), and the area-based surface roughness (Sa) was 1.081 ± 0.488µm compared with 2.811 ± 1.580µm in the dry-milling group ( p  < 0.001). SEM images revealed that wet-milling specimens had uniform tool marks and fewer surface defects, while dry-milled specimens showed irregular protrusions and more pit-shaped defects. Conclusion These findings indicate that wet-milling improves the mechanical consistency, surface quality and wettability of CAD/CAM PMMA denture base resins, which may contribute to better clinical performance of removable complete dentures.