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Materials Science in Additive Manufacturing Mouthguards: Disinfection versus properties changes

A B

Figure 8. Charpy impact properties of 3D-printed multi-material parts. (A) Impact strength and (B) absorbed energy for bi-layered and tri-layered
configurations. The pre-disinfected group includes specimens immersed in artificial saliva without further treatment. For reference, the impact strength and
absorbed energy of bulk poly(ethylene-vinyl acetate) (EVA; pink) demonstrated an impact strength of 5.7 ± 0.5 kJ·m and absorbed energy of 4.5 ± 0.4%
-2

Figure 9. Macrographs of 3D-printed specimens after the transverse impact test. Delamination at the material interface is clearly visible in bi-layered
configurations for both high-impact polystyrene (HIPS)- and poly(methyl methacrylate) (PMMA)-based parts across all treatment conditions (dry state,
pre-disinfected [immersion in artificial saliva with no further disinfection], post-UVC, and post-Polident)
intermediate TPU layer. This additional interface likely Since both disinfection methods (UVC radiation and
promotes higher cohesion between layers in the tri-layered Polident solution) directly affect the surface of 3D-printed
configuration, as compared to the direct TPU–platform parts, assessing surface mechanical properties is important
adhesion in bi-layered parts. Furthermore, delamination to understanding their impact on the overall mechanical
was more pronounced in PMMA-based specimens, likely behavior. A load of 50 g (HV , equivalent to 490 mN) was
0.05
due to the weaker interfacial bonding between PMMA and applied, and the results are presented in Figure 10.
TPU. This is consistent with the literature regarding poor
compatibility between these two polymers. 53,54 The results indicate that the bi-layer configuration
(represented by solid lines in Figure 10) exhibits higher
Ultimately, when comparing the tested multi-material average hardness values than the tri-layer configuration
configurations with conventional bulk EVA, it was found (dashed lines) for both materials. This outcome is likely
that EVA exhibits lower impact resilience and energy due to the greater thickness of the more rigid material
absorption capacity during transverse impact testing than (HIPS or PMMA) in the bi-layered specimens, as
all evaluated 3D-printed specimens. supported by literature, resulting in enhanced resistance
11
3.4. Surface characterization to plastic deformation compared to tri-layered specimens.
Furthermore, specimens comprising HIPS (black solid and
3.4.1. Vickers microhardness dashed lines) showed consistently higher hardness values
Microhardness experiments were conducted to assess the than those composed of PMMA (red solid and dashed lines).
Vickers hardness (HV) values of the printed specimens. This finding is in accordance with the flexural modulus

Volume 4 Issue 2 (2025) 10 doi: 10.36922/MSAM025130018