In this study, the force required to draw a polymer preform into optical fiber is predicted and measured, along with the resultant free surface shape of the polymer, as it is heated in an enclosed cylindrical furnace. The applied drawing force affects the degree of chain alignment within the polymer. Chain alignment causes orientational birefringence, an unwanted property that attenuates any propagating optical signal. The draw force is a function of the highly temperature dependent polymer viscosity. Therefore accurate prediction of the drawing force requires a detailed investigation of the heat transfer within the furnace. In this investigation, the full axi-symmetric conjugate problem (including both natural convection and thermal radiation) was solved using the commercial finite element package FIDAP. In addition, the location of the polymer/air interface was solved for as part of the problem and was not prescribed beforehand. Results show that thermal radiation accounts for approximately 70% of the total heating experienced by the deforming polymer, but only 15% of the cooling. The draw force is very sensitive to both the furnace wall temperature and to the feed rate of the polymer. Numerical results compared well with the experimentally measured draw tension and neck-down profiles for several preform diameters, draw speeds, and furnace temperatures. The predicted draw forces were typically within 20% of the experimentally measured values.

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