Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid

Costanzo, Andrea; Spotorno, Roberto; Candal, Mar�a Virginia; Fern�ndez, Mar�a Mercedes; M�ller, Alejandro J; Graham, Richard S.; Cavallo, Dario; McIlroy, Claire

Abstract

© 2020 Elsevier B.V. Gaining a molecular understanding of material extrusion (MatEx) 3D printing is crucial to predicting and controlling part properties. Here we report the direct observation of distinct birefringence localised to the weld regions between the printed filaments, indicating the presence of molecular orientation that is absent from the bulk of the filament. The value of birefringence at the weld increases at higher prints speeds and lower nozzle temperatures, and is found to be detrimental to the weld strength measured by tensile testing perpendicular to the print direction. We employ a molecularly-aware non-isothermal model of the MatEx flow and cooling process to predict the degree of alignment trapped in the weld at the glass transition. We find that the predicted residual alignment factor, A¯, is linearly related to the extent of birefringence, Δn. Thus, by combining experiments and molecular modelling, we show that weld strength is not limited by inter-diffusion, as commonly expected, but instead by the configuration of the entangled polymer network. We adapt the classic molecular interpretation of glassy polymer fracture to explain how the measured weld strength decreases with increasing print speed and decreasing nozzle temperature.

Citation

Costanzo, A., Spotorno, R., Candal, M. V., Fernández, M. M., Müller, A. J., Graham, R. S., …McIlroy, C. (2020). Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid. Additive Manufacturing, 36, Article 101415. https://doi.org/10.1016/j.addma.2020.101415

• Journal Article Type: Article
• Acceptance Date: Jun 22, 2020
• Online Publication Date: Jun 30, 2020
• Publication Date: Dec 1, 2020
• Deposit Date: Jun 29, 2020
• Publicly Available Date: Jul 1, 2021
• Journal: Additive Manufacturing
• Print ISSN: 2214-7810
• Electronic ISSN: 2214-8604
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 36
• Article Number: 101415
• DOI: https://doi.org/10.1016/j.addma.2020.101415
• Keywords: Material Extrusion; Birefringence; Molecular Orientation; Weld Strength; Polylactic acid
• Public URL: https://nottingham-repository.worktribe.com/output/4738436
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S2214860420307879
• Additional Information: This article is maintained by: Elsevier; Article Title: Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid; Journal Title: Additive Manufacturing; CrossRef DOI link to publisher maintained version: https://doi.org/10.1016/j.addma.2020.101415; Content Type: article; Copyright: © 2020 Elsevier B.V. All rights reserved.