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

Costanzo, Andrea; Spotorno, Roberto; Candal, María Virginia; Fernández, María Mercedes; Müller, Alejandro J; Graham, Richard; Cavallo, Dario; McIlroy, Claire

Authors

Andrea Costanzo

Roberto Spotorno

María Virginia Candal

María Mercedes Fernández

Alejandro J Müller

Richard Graham

Dario Cavallo

Claire McIlroy



Abstract

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, ∆n. 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.

Journal Article Type Article
Journal Additive Manufacturing
Print ISSN 2214-7810
Electronic ISSN 2214-8604
Publisher Elsevier
Peer Reviewed Peer Reviewed
APA6 Citation Costanzo, A., Spotorno, R., Candal, M. V., Fernández, M. M., Müller, A. J., Graham, R., …McIlroy, C. (in press). Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid. Additive Manufacturing,
Keywords Material Extrusion; Birefringence; Molecular Orientation; Weld Strength; Polylactic acid
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