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

<p>Gaining a molecular understanding of material extrusion (MatEx) 3D printing is crucial to predicting and controllingpart properties. Here we report the direct observation of distinct birefringence localised to the weld regions betweenthe 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 foundto be detrimental to the weld strength measured by tensile testing perpendicular to the print direction. We employa molecularly-aware non-isothermal model of the MatEx flow and cooling process to predict the degree of alignmenttrapped 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 strengthis not limited by inter-diffusion, as commonly expected, but instead by the configuration of the entangled polymernetwork. We adapt the classic molecular interpretation of glassy polymer fracture to explain how the measured weldstrength decreases with increasing print speed and decreasing nozzle temperature.</p>