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