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

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. Seventy-two rectangular PMMA specimens were prepared under standardized milling conditions and subjected to three-point bending tests, 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. Static contact angle analysis was employed to assess surface wettability. Surface roughness parameters (Ra and Sa) were measured using two different instruments to ensure the accuracy and reliability of the results. Scanning electron microscope(SEM) was used to observe the surface morphology. The mean flexural strength was 118.017 ± 9.578 MPa in Group D and 123.822 ± 7.818 MPa in Group W. 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 < 0.05). 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 < 0.05). Furthermore, wet-milling specimens presented lower surface roughness. The arithmetic mean surface roughness (Ra) of wet-milling was 1.296 ± 0.346 μm versus 2.293 ± 0.907 μm in the dry-milling group ( p < 0.05), and the area-based surface roughness (Sa) was 1.081 ± 0.488 μm compared with 2.811 ± 1.580 μm in the dry-milling group ( p < 0.05). Wet milling improves the surface quality and wettability of CAD/CAM PMMA denture base resins without compromising flexural strength.