An insight into the tensile anisotropy of 3D-printed thermoplastic polyurethane

Abstract

The interface bonding strength between adjacent melt lines formed in the melt-extrusion-based 3D printing (ME3DP) process largely governs the mechanical properties of the as-printed parts. But this normally leads to mechanical anisotropy of which the mechanism is not very clear. In this work, based on the investigation of the temperature attenuation behavior of newly deposited thermoplastic polyurethane (TPU) melt in a ME3DP process, the mechanical dependence on the interface development was explored. First, the temperature attenuation of a newly deposited melt in an open environment was found to be well fitted by the Power-law function. Then the effect of envelope temperature on the tensile property of the as-printed TPU was studied using an electric-heating bed (EHB) which endowed the whole printed sheets with a temperature near EHB temperature. To explore the mechanism of mechanical anisotropy, tensile testing was then performed for the printed sheets in both longitudinal and transverse directions, respectively. Results showed that the ultimate tensile strength (UTS), as well as the elongation at break measured along the transverse direction, increased with the increase of EHB temperature, which was mainly attributed to the improved interface bonding. In comparison, the UTS along the longitudinal direction kept relatively stable, slightly increasing initially and then followed by a minor decrease. Of note is the fact that the mechanical properties of printed parts were governed simultaneously by both the wall-shear-induced orientation effect of molecular chains at the surface of the deposited melt line and the crystallization morphology inside the deposited molten filament was concluded in this work. A high envelope temperature not only promoted the molecular diffusion around the interface layer, enhanced the interface bonding, and overcome the mechanical anisotropy but also promoted the crystallization kinetics. The printed TPU with developed crystallized morphology exhibited improved yield stress in the longitudinal direction above a critical temperature. This work demonstrates an insight into the mechanical anisotropy of 3D-printed polymers, paving the way to print high-strength parts via ME3DP.