Influence of polymerization protocols on the physical and mechanical properties of a 3D printed resin

Fast prototyped, or 3-dimensionally (3D) printed, materials enhance clinical efficiency when compared with other manufacturing methods. Nevertheless, standardization and information regarding the influence of different postprocessing protocols on the final physical and mechanical properties of 3D printed parts is lacking.

The purpose of this in vitro study was to evaluate the effect of different polymerization methods and times on the flexural strength, microhardness, and color stability of a 3D printed resin (OnX; SprintRay).

A total of 40 disks (Ø10×2 mm) and 40 bars (10×2×2 mm) were 3D printed, washed, and subdivided into 4 groups (n=10) according to the polymerization protocol: VALO Grand light polymerization unit for 40 and 120 seconds (VG40s andVG120s) and ProCure 2 polymerization chamber for 1 and 2 cycles (PC×1 and PC×2). The bars were stored in distilled water at 37 °C for 24 hours, and a 3-point bend test was performed with a universal testing machine with an 8-mm span and a downward movement at a rate of 0.5 mm/minute until fracture. The disks were polished with abrasive disks. Color stability was assessed after polymerization (baseline), after 1 and 7 days in dark, dry storage at 37 °C, and after 3 days of artificial aging in deionized water at 60 °C. Values of b* were used to calculate yellow shift/Δb* values after 3 days of artificial aging. Microhardness after 7 days in dark, dry storage was assessed with a Knoop indenter. The data were assessed for homogeneity using the Levene test and for normality using the Shapiro-Wilk test. Two-way ANOVA (flexural strength, microhardness, and Δb* tests) and 3-way repeated-measures ANOVA (color stability test) were followed by the Tukey HSD post hoc test (α=.05 for all tests).

For microhardness, the polymerization unit (P<.001), polymerization cycles (P=.003), and interaction between both factors (P=.005) were significantly different, with VG40s=VG120s>PC×1>PC×2. For flexural strength, the polymerization unit (P<.001), polymerization cycles (P<.001), and interaction between both factors (P<.001) were significantly different, with VG120s=PC×1=PC×2>VG40s. For color stability, the polymerization unit (P=.009), time (P<.001), and interaction between time and polymerization unit (P<.001) and time, polymerization unit, and cycle (P=.01) were significantly different. After 3 days of artificial aging, PC×1=PC×2>VG40s=VG120s. Significantly different Δb* was found for polymerization unit (P<.001) and polymerization cycles (P=.002), with VG120s

Resins polymerized using VG120s produced similar or better microhardness, flexural strength, and color stability results than PC while significantly decreasing the postpolymerization time. Specimens polymerized with PC×2 showed the lowest microhardness. Excessively increased polymerization time may jeopardize the properties of 3D printed parts.