Prediction of Fiber Die Coating Thickness
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0920-S04-07
Prediction of Fiber Die Coating Thickness An Yang, XiaoMing Tao, and XiaoYin Chen Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China, People's Republic of
ABSTRACT Fiber coating is an effective way to impart smartness to a fiber. Die coating is a process that utilizes a die to control the thickness and concentricity of the coating layer. In the present work the die coating mechanism is studied numerically. A mathematical model for the fiber coating thickness has been developed. Compared with the previous work, the proposed model considers the effect of gravity force to get the general solution. The shear rate acting on the fiber surface is proportional to the fiber draw speed in an unpressurized applicator and can be minimized in a pressurized applicator through the applied external pressure. A serials of experiments using open-cup and pressurized applicators have been designed and conducted to measure simultaneously the coating speed and coating thickness in the coating process. It was found that the gravity may be an important driving force for the coating flow when the drawing velocity is small and the viscous force decreases, but may be relatively insignificant for high speed coating process. The calculated results were compared with the experimental data and a good agreement was obtained. INTRODUCTION Fiber coating, as a popular post-metered coating method, is essential for the surface engineering of high quality fibers. It is important to understand the mechanisms of coating thin layers on a moving fiber and predict the film thickness as a function of the withdrawal speed of the fiber, and of the physical properties of the fluid. Landau&Levich[1],Derjaguin[2] studied the liquid film formed on a moving flat plate and derived the film thickness expression at small capillary numbers. Quéré and de Ryck[3] made a summary of theoretical treatments of this general problem and investigated the inertial effects at high fiber withdrawal velocities. Compared with free coating process, obstructed coating or die coating is widely used in industrial processes such as optical fiber manufacture. Several analyses of the die coating process appear in the published literature[4-5]. In these analyses, the gravity force was neglected in comparison with the viscous force. In recent years Electro-Active Polymers have attracted much attention because of their great potentials as actuators and sensors[6-7]. Fiber geometry may bring specific advantages to actuator design. For these polymer solutions, their viscosities are relatively low compared to the most widely used coating materials. Thus the gravity effect becomes unnegligible under this condition. In our development we will take the effect of gravity into account in order to obtain the general solution. In this article, we provide a detailed theoretical analysis of the fiber die coating process. Experimental results obtained from the obstructed coating process are presented for un- and pressurized applicators. T
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