Our article “Residual alignment and its effect on weld strength in material-extrusion 3D printing of polylactic acid” has now been published in Additive Manufacturing. This work is a collaboration between the Universities of Lincoln, Genoa, Nottingham and the Basque Country, and was supported by a Royal Society International Exchange Scheme.

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

Andrea Costanzo, Roberto Spotorno, Mara Virginia Candal, Mara Mercedes Fernandez, Alejandro J. Muller, 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 laments, 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 foundcto 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.