Mechanical Properties and Reliability of Parametrically Designed Architected Materials Using Urethane Elastomers

Abstract

Achieving multiple physical properties from a single material through three-dimensional (3D) printing is important for manufacturing applications. In addition, industrial-level durability and reliability is necessary for realizing individualized manufacturing of devices using 3D printers. We investigated the properties of architected materials composed of ultraviolet (UV)-cured urethane elastomers for use as insoles. The durability and reliability of microlattice and metafoam architected materials were compared with those composed of various foamed materials currently used in medical insoles. The hardness of the architected materials was able to be continuously adjusted by controlling the design parameters, and the combination of the two materials was effective in controlling rebound resilience. In particular, the features of the architected materials were helpful for customizing the insole properties, such as hardness, propulsive force, and shock absorption, according to the user's needs. Further, using elastomer as a component led to better results in fatigue testing and UV resistance compared with the plastic foam currently used for medical purposes. Specifically, polyethylene and ethylene vinyl acetate were deformed in the fatigue test, and polyurethane was mechanically deteriorated by UV rays. Therefore, these architected materials are expected to be reliable for long-term use in insoles.

Keywords: 3D printing; Asker hardness; additive manufacturing; architected material; elastomer; foam; insole; lattice; metamaterial; reliability.

Figure A1

Cross-section images of each PU foam were observed by laser microscope. (a) PUfoam-1; (b) PUfoam-3; (c) PUfoam-5; (d) PUfoam-7.

Figure A2

Photographs of microlattices obtained by the method discussed in Section 2.1.1. (a) ML-1; (b) ML-2; (c) ML-3.

Figure A3

Photographs of metafoams obtained by the method discussed in Section 2.1.2. (a) MF-1; (b) MF-2; (c) MF-3; (d) MF-4; (e) MF-5.

Figure 1

3D structures of architected materials designed in OpenSCAD [25]: (a) unit cell structure; (b) 5 × 5 × 5 units lattice cube structure.

Figure 2

3D structures of metafoams designed in OpenSCAD [25]; (a) MF-1; (b); MF-2; (c) MF-3; (d) MF-4; (e) MF-5.

Figure 3

3D structures of architected materials designed in OpenSCAD [25]: (a) placement of ellipsoids; (b) metafoam structure obtained through the difference between the ellipsoidal array and 20 × 20 × 20 mm³ cube.

Figure 4

Comparative results for mechanical properties: (a) relationship between diameter of pillars in the unit cell and hardness (Asker C) for the microlattice structures; (b) relationship between hardness (Asker C) and rebound resilience.

Figure 5

Appearance of samples before and after carbon arc testing: (a) PUfoam-7 before carbon arc testing; (b) PUfoam-7 after carbon arc testing (discoloration occurred); (c) PEfoam before carbon arc testing; (d) PEfoam after carbon arc testing (shrinkage occurred); (e) microlattice before carbon arc testing; (f) microlattice after carbon arc testing (did not occur).

Figure 6

Appearance of samples after bending: (a) PUfoam-7 (cracking occurred); (b) PEfoam; (c) microlattice sample.

Figure 7

Stress–strain curve obtained by tensile test of each sample before (blue line) and after (red line) carbon arc testing. The solid (dashed) line indicates first (second) sample. (a) PUfoam-7; (b) PEfoam; (c) microlattice sample.

Figure 8

Comparative results of mechanical properties: (a) relationship between apparent density and hardness for microlattice structure (filled green square), metafoam structure (filled red triangle), and urethane foam (open blue circle); (b) relationship between rebound resilience and hysteresis loss rate for microlattice structure (filled green square), metafoam structure (filled red triangle), and urethane foam (open blue circle).

Figure 9

Ratio between sample thickness after a 10,000-cycle compression fatigue test and thickness before the test.