This work presents Computational Fluid Dynamics (CFD) simulations of the polymer flow inside the hot-end during material extrusion additive manufacturing. Two CFD models are investigated: a previously-published one-phase model, where the entire domain is filled with liquid, and a novel model, where the free surface of the polymer inside the channel is resolved. Both models predict a recirculation region between the nozzle wall and the incoming filament. With the free surface resolved, melting of the solid filament and filling of an empty liquefier channel are shown in detail. Moreover, the simulations predict the pressure and temperature distributions inside the channel. The molten polymer (ABS) is simulated as a Generalized Newtonian Fluid with shear- and temperature-dependent viscosity. The numerical results are compared with the experimental measurements of the filament feeding force, which relates to the pressure inside the flow channel. An inverse analysis of the heat transfer coefficient is performed to estimate the thermal resistance at the channel’s wall. It is shown that the model which resolves the free surface is able to predict the feeding force for typical working conditions with a reasonable accuracy. Moreover, it captures the change of the flow regime from stable to unstable extrusion at high feeding rates. A hypothesis that explains the pressure and melt zone oscillations that occur during unstable extrusion is given. The influence of the liquefier temperature, liquefier length and nozzle diameter on the flow are discussed. The predictions of the model become less accurate when different channel geometries are simulated, which is attributed to the simplified material model that does not capture viscoelastic effects and possible buckling of the solid filament.

这项工作提供了在材料挤出添加剂制造过程中聚合物在热端内部流动的计算流体动力学（CFD）模拟。研究了两个CFD模型：先前发布的单相模型（其中整个域充满液体）和新模型，其中解析了通道内聚合物的自由表面。两种模型都预测了喷嘴壁和进入的细丝之间的再循环区域。解析自由表面后，详细显示了实心丝的熔化和液化液槽的填充。此外，仿真预测了通道内的压力和温度分布。熔融聚合物（ABS）被模拟为具有取决于剪切和温度的粘度的广义牛顿流体。将数值结果与长丝进给力的实验测量值进行比较，后者与流道内部的压力有关。对传热系数进行逆分析，以估算通道壁处的热阻。结果表明，解析自由表面的模型能够以合理的精度预测典型工作条件下的进给力。而且，它捕获了高进料速率下流动状态从稳定挤出到不稳定挤出的变化。给出了一个假设，该假设解释了不稳定挤压过程中发生的压力和熔融区振荡。讨论了液化温度，液化长度和喷嘴直径对流动的影响。