3D printed polyurethane exhibits isotropic elastic behavior despite its anisotropic surface

Mechanical properties testing of the hyperelastic thermoplastic polyurethane (TPU) produced by the continuous digital light processing (CDLP) method of additive manufacturing. Primarily, this paper aims to verify that 3D printed TPU still satisfies commonly assumed volumetric incompressibility and material isotropy in elastic range. The secondary aim is to investigate the accuracy and reproducibility of the CDLP method.

Cylindrical samples were printed and subjected to a volumetric compression test to reveal their bulk modulus K and maximal theoretical porosity (MTP). Dog bone specimens were oriented along different axes and printed. Their dimensions were measured, and they were subjected to cyclic uniaxial tests up to 100% strain to reveal the level of stress softening and possible anisotropy. The hyperelastic Yeoh model was fitted to the mean response.

The authors measured the bulk modulus of K = 1851 ± 184 MPa. The mean MTP was 0.9 ± 0.5%. The mean response was identical in both directions and the data could be fitted by the isotropic third order Yeoh function with R^2 = 0.996. The dimensions measurement revealed the largest error (above 5%) in the direction perpendicular to the direction of the digital light projection while the dimensions in other two dimensions were much more accurate (0.75 and 1%, respectively).

The TPU printed by CDLP can be considered and modelled as isotropic and practically volumetrically incompressible. The parts in the printing chamber should be positioned in a way that the important dimensions are not parallel to the direction of the digital light projection.

The authors experimentally confirmed the volumetric incompressibility and mechanical isotropy of the TPU printed using the CDLP method.