Printing Orientation Effects on Mechanical Properties of a 3D-Printed Composite.

Objectives: To evaluate the effect of printing-orientation (45° or 90°) and storage time (24hours or 8months) on flexural strength and microhardness of a composite-resin processed via 3D-printing.
Methods: 80 specimens of methacrylate-based composite-resin with zirconia filler particles (BioCrown, MakertechLabs) were produced in disc-shaped format (12x1mm) using a 3D-printer with Digital Light Processing technology (FlashForgeHunter, Flashforge) and following manufacturer's recommendations. Specimens were divided into two groups based on printing angle (45° or 90°). Half of the specimens in each group (n=20) were stored in an incubator (dry, 37°C, 24hours) while the other half was stored for 8months in same ambient condition. After storage periods, 10 discs from each group were tested in a biaxial flexural strength setup (piston-on-three-balls, 0.5mm/min). The remaining 10 specimens from each group were embedded in a plastic tube with acrylic resin and polished with sandpaper (grit 420, 600, 800, 1200, 1500 and 4000). Their Knoop microhardness was measured in a microhardness tester (50Kgf, 15s). Statistical analysis was performed using two-way-ANOVA and Tukey's test (alpha=0.05).
Results: Both printing-orientation and storage time significantly affected flexural strength of tested composite. Considering the same storage time, specimens printed at 90° resulted in significantly higher mean strength compared to those printed at 45°. Regardless of the printing-angle, groups stored for 8months exhibited significantly lower strength (approximately 25% decrease) compared to those measured for specimens stored for 24hours. As for Knoop microhardness, neither printing-angle nor storage time had any effect on mean values obtained for different experimental groups.
Conclusions: The angle orientation used for 3D-printing affected flexural strength of the studied composite. Specimens printed at 90° showed significantly higher flexural strength. However, no impact of the printing-orientation was observed on microhardness of composite. Storage for 8months significantly reduced flexural strength of the studied material compared to control group, nonetheless, its hardness remained unaffected by storage time.