Abstract: The slot die coating is widely utilized in fabric coating and advanced packaging as a predictive coating technology. The thickness distribution and stability of the liquid film formed during the slot die coating process affect the morphology and structure of the cured coating, ultimately influencing the properties of the product. However, due to the coupling of multiple operational parameters, the mechanism that influences film thickness distribution and stability remains unclear.
In this study, we conduct numerical simulations of the slot die coating process to investigate film formation. Firstly, relevant governing equations are established, the geometric model and boundary conditions are determined and meshed, the solution method is given and mesh-independence is verified. Subsequently, we verify the accuracy of our numerical simulations by comparing them with experimental data reported in the literature. Finally, we investigate the mechanisms through which operating conditions and fluid properties influence the thickness, uniformity, and stability of the liquid film.
The contour plots of liquid phase distribution shows that the thickness of the liquid film increases continuously with the elongation of fluid flow time. When the flow time is 0.1s, the liquid film thickness no longer changes with time, and the transient numerical calculation is completed. To investigate the coating mechanism and flow pattern of slot die coating, different substrate moving speeds, inlet velocities and fluids with different viscosities are set up for numerical calculation, and the role of each factor is analyzed in combination with the film thickness distribution and film-forming stability. It can be concluded from the film thickness distribution graph and velocity contour that: when the substrate moving speed is relatively low, the film-forming flow rate is less than the inlet flow rate, resulting in fluid accumulation at the die lip. Thus the film-forming flow rate and the liquid film thickness increase with the substrate moving speed; when the film-forming flow rate increases to the inlet flow rate, the liquid film thickness reaches the maximum. However, as the substrate moving speed further increases, the film-forming flow rate remains constant and equal to the inlet flow rate, leading to a decrease in liquid film thickness. Within the stable operating window, coating uniformity increases with the increase of the substrate moving speed. As the viscosity of the fluid increases, there is little noticeable change in the thickness of the coating, whereas the uniformity of the liquid film steadily decreases. This is attributed to the increased viscous force, which causes the substrate to entrain more fluid. Consequently, the film-forming flow rate equals the inlet flow rate, resulting in no further changes in film thickness. As the inlet velocity increases, the thickness of the liquid film keeps increasing and the uniformity of the liquid film does not change significantly. The simulation results show that the substrate velocity and inlet velocity are the main factors influencing the film thickness and its uniformity. A stable and uniform coating can only be achieved within a specific range of process parameters; otherwise, coating defects may arise. The analysis of the film formation mechanism of slot die coating provides theoretical guidance for the optimization of coating process parameters.

Key words: slot die coating, film thickness, film stability, CFD numerical simulation

摘要： 狭缝涂布过程形成液膜的厚度分布及稳定性会影响固化后涂层的形态和结构，进而影响产品性能。因为多操作参数耦合影响膜厚分布及稳定性的机理尚不明确，所以有必要对流体流动形态进行研究。通过CFD数值模拟狭缝涂布成膜过程，分析了入口速度、基材移动速度以及流体参数对液膜厚度、均匀性及稳定性的影响，并且使用无量纲法分析了各因素的交互作用。模拟结果表明：基材移动速度和入口速度为液膜厚度及其均匀性的主要影响因素；狭缝涂布成膜在一定的工艺参数范围内，能够确保获得稳定均匀的涂层，否则会产生涂层缺陷。研究结果可为涂布工艺参数的优化提供理论参考。

关键词: 狭缝涂布, 液膜厚度, 成膜稳定性, CFD数值模拟