多層膜共擠出成形過程數值模擬分析

Abstract

近年來塑膠膜為高分子產業極重要的一項產品，且朝向多功能性多層膜發展。複合型薄膜的鯊魚皮與層間厚度不均勻，為當前共擠出成形所遭遇的最主要困難。在模具內實際的共擠出過程，因模內的高溫高壓力環境，要直接地觀察其流變行為有其困難性。本研究以有限元素法模擬等溫與非等溫共擠出時，非結晶塑料PC與PMMA的流動現象，以及針對鯊魚皮等缺陷的產生原因進行探討，並嘗試提出解決對策。 模擬的結果得知，模內流動現象及鯊魚皮等缺陷，與匯合角度、材料黏度比、擠出速度比與薄膜厚度比有關。隨著匯合角度越大，支流路模壁位置剪應力會上升，而導致表面鯊魚皮發生，且會加劇出口界面包覆現象；增加支流與主流塑料黏度差異，界面位置會傾向主流路，且其界面剪應力呈不連續，而可能導致界面扭曲，包覆現象亦隨黏度差異增大而加劇；支流路擠出速度越快使得黏度差異降低可改善包覆現象，但可能會產生鯊魚皮與界面不穩定；薄膜厚度比方面，當支流路黏度高而厚度變薄，在擠出時會承受極大剪應力而產生界面不規則分佈情形；在非等溫系統時，支流路因黏度大所產生的黏滯熱較多，使溫度上升較多而導致黏度下降幅度比主流路多，使得包覆現象與等溫流動相比時較不明顯。

In recent years, plastic film has become a very important product in the polymer industry, with a particular trend toward the development of multifunctional composite films. The major difficulty in co-extrusion of composite films is sharkskin and non-uniformity in layer thickness. The actual extrusion process or rheological behavior within the mold is difficult to observe directly, due to the high temperature and the high pressure. In this study, we employed Finite Element Method to simulate the isothermal and non-isothermal co-extrusion process, to explore defects such as sharkskin and propose practical solutions. In this study, we showed how the flow behavior during extrusion and the defects were a result of the relationship between angle of convergence, viscosity of the material, extrusion velocity and layer thickness. When the angle of convergence increased, the shear stress increased at the die wall in the sub-channel, resulting in the occurrence of sharkskin, with the phenomenon of encapsulation becoming critical the interface of the die exit. The greater the viscosity, the more the interfacial position between the polymer layers shifted toward the main-channel. The interfacial shear stress was discontinuous, resulting in distortion at the interface. Encapsulation increased with an increase in viscosity. A higher extrusion velocity in the sub-channel was able to reduce the viscosity ratio to remedy the encapsulation phenomenon, but it produced sharkskin and interfacial instability. During the extrusion process, certain aspects of the film thickness were under tremendous shear stress, leading to irregular distribution at the interface, as the thickness within the high-viscosity sub-channel became thinner. In the non-isothermal system, viscous heating was greater in the high-viscosity sub-channel, which led to a significant rise in temperature, with a greater decrease in viscosity than in the main-channel. This resulted in a diminished degree of encapsulation compared with the isothermal system.