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

Costanzo, A, Spotorno, R, Candal, M , Fernandez, M, Muller, A, Graham, R, Cavallo, D and McIlroy, Claire (2020) Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid. Additive Manufacturing, 36 . p. 101415. ISSN 2214-8604

Full content URL: https://doi.org/10.1016/j.addma.2020.101415

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Residual alignment and its effect on weld strength in material-extrusion 3D-printing of polylactic acid
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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 is linearly related to the extent of birefringence. 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.

Keywords:birefringence, molecular orientation, weld strength, poly lactic acid
Subjects:F Physical Sciences > F200 Materials Science
H Engineering > H990 Engineering not elsewhere classified
G Mathematical and Computer Sciences > G120 Applied Mathematics
Divisions:College of Science > School of Mathematics and Physics
ID Code:41291
Deposited On:09 Jul 2020 13:23

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