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, $\overline{A}$, is linearly related to the extent of birefringence, Δn. 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.

对材料挤压（MatEx）3D打印有一个分子的了解对于预测和控制零件性能至关重要。在这里，我们报告了直接观察到的独特双折射位于印刷长丝之间的焊接区域的现象，这表明长丝主体中不存在分子取向。在较高的印刷速度和较低的喷嘴温度下，焊缝处的双折射值增加，并且发现这不利于通过垂直于印刷方向的拉伸试验测得的焊接强度。我们采用MatEx流动和冷却过程的分子感知非等温模型来预测玻璃化转变时困在焊缝中的对准程度。我们发现预测的残差对齐因子，$\overline{\mathrm{一种}}$，与双折射的程度Δn线性相关。因此，通过结合实验和分子建模，我们显示出焊接强度不受普遍预期的相互扩散限制，而是受缠结的聚合物网络构型的限制。我们采用玻璃状聚合物断裂的经典分子解释来解释所测量的焊接强度如何随着打印速度的增加和喷嘴温度的降低而降低。