A gas-liquid two-phase model for a viscoelastic fluid is proposed and used to simulate and predict the behavior of melt welding in injection molding process, in which the extended pom-pom (XPP) model and cross-WLF viscosity model combined with Tait state equation are used to describe the constitutive relationship and viscosity change of the viscoelastic melt in this paper, respectively. Meanwhile, the coupled level-set and volume-of-fluid (CLSVOF) method is employed to capture the melt front, and the immersed boundary method is applied to the simulation of the polymer melt flows with the aid of a shaped level-set function to describe and treat the irregular mold cavities. A finite volume method on non-staggered grid is used to solve the mass, momentum, and energy conservation equations. Firstly, the benchmark problem of the single shear flow is simulated to verify the validity of the CLSVOF method. Then, the non-isothermal filling process of the viscoelastic fluid based on the XPP model in a mold with square inset is simulated, and the behavior of the weld line devolopment in the filling process is shown and compared with the experimental result. Finally, it is to simulate the evolution processes of the melt front interface and weld line in a mold with the circular notched inset;and the linear stress-optical rule is adopted to calculate the flow-induced birefringence. Numerical results show that the numerical model proposed in this paper can be employed to simulate the non-isothermal filling process in complex mold cavity and to capture the weld line automatically. Because of the complexity of polymer melt flows, the flow-induced stress increases quickly near the weld line region and then decreases gradually until reaching the mold cavity wall. The maximum value of the flow-induced stress appears at some point after the insert. The distributions of physical quantities, such as pressure and temperature in the mold, are given during the mold filling process. Moreover, it is also discussed the influence of melt and mold temperatures on the solidified layer thickness. The higher the melt or mold temperature, the thinner the solidified layer is. Thus, raising the melt or the mold temperature will improve or remove the weld line in melt filling process.%基于充模过程的两相黏弹性流体模型，采用同位网格有限体积法，结合浸入边界法和界面追踪的复合水平集流体体积方法实现了带嵌件型腔内充模过程的动态模拟。基于上述模型和算法模拟了熔体前沿界面及熔接线的动态演化过程，而且通过线性应力-光学定律得到了熔接线附近的流动诱导应力分布情况；讨论了熔体温度及模具温度对熔接线区域凝固层厚度的影响。数值结果表明：本文提出的方法可用于模拟复杂型腔内的充模过程以及熔接线的自动追踪；由于聚合物黏弹性熔体流动的复杂性，当两股熔体相遇后，熔接线不同位置的应力分布变化较大；熔体或模具温度越高，熔接线区域凝固层厚度越薄，提高熔体或模具温度能够改善甚至消除充模过程中的熔接线。
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