Sandwich Multi-Material 3D-Printed Polymers: Influence of Aging on the Impact and Flexure Resistances

Abstract

With the advances in new materials, equipment, and processes, additive manufacturing (AM) has gained increased importance for producing the final parts that are used in several industrial areas, such as automotive, aeronautics, and health. The constant development of 3D-printing equipment allows for printing multi-material systems as sandwich specimens using, for example, double-nozzle configurations. The present study aimed to compare the mechanical behavior of multi-material specimens that were produced using a double-nozzle 3D printer. The materials that were included in this study were the copolymer acrylonitrile-butadiene-styrene (ABS), high-impact polystyrene (HIPS), poly(methyl methacrylate) (PMMA), and thermoplastic polyurethane (TPU). The configuration of the sandwich structures consisted of a core of TPU and the outer skins made of one of the other three materials. The mechanical behavior was evaluated through three-point bending (3PB) and transverse impact tests and compared with mono-material printed specimens. The effect of aging in artificial saliva was evaluated for all the processed materials. The main conclusion of this study was that the aging process did not significantly alter the mechanical properties for mono-materials, except for PMMA, where the maximum flexural stress decreased. In the sandwich structures, the TPU core had a softening effect, inducing a significant increase in the resilience and resistance to transverse impact. The obtained results are quite promising for applications in biomedical devices, such as protective mouthguards or teeth aligners. In these specific applications, the changes in the mechanical properties with time and with the contact of saliva assume particular importance.

Keywords: additive manufacturing; artificial saliva; mechanical properties; multi-material; oral devices; sandwich structures.

Figure 1

Schematic representation of the printed sandwich structures with ABS, HIPS, or PMMA as skins, and TPU as the core material (left) and mono-material specimens (right).

Figure 2

Stress–displacement curves that were obtained from the 3PB tests of the mono-materials.

Figure 3

Bar plot representation of (a) σ_max and (b) E, which were obtained from the 3PB tests of the mono-materials for the dry and aged specimens.

Figure 4

(a) Resilience and (b) absorbed energy of the printed mono-material specimens, which were obtained using transverse impact tests.

Figure 5

Damage induced by the transverse impact tests on PMMA specimens: (a) as printed and (b) aged. Scale bar: 500 µm.

Figure 6

Representative curves of the sandwich structure materials that were obtained using 3PB tests.

Figure 7

Determined (a) σ_max and (b) E values of the dry and aged sandwich structure specimens that were obtained from the 3PB tests.

Figure 8

(a) Resilience and (b) absorbed energy, which were determined by transversal impact tests.

Figure 9

Surface damage of the as-printed sandwich-structured specimens after the transversal impact tests: (a) ABS_TPU_ABS, (b) PMMA_TPU_PMMA, and (c) HIPS_TPU_HIPS. Scale bar: 500 µm.