Theoretical prediction on double-layer coating in wet-on-wet optical fiber coating process

Abstract

A theoretical investigation is conducted on the wet-on-wet (WOW) process for double-layer resin coating of optical glass fiber. The coating process in the secondary coating die is modeled as a simple two-layer Couette flow of two immiscible fluids in an annulus with an assigned pressure gradient. The assumption of fully developed flow permits an exact solution to the Navier–Stokes equations. The solution discloses the relevant parameters in design and operation of a WOW coating applicator of optical fiber. For a given diameter of glass fiber and primary coating thickness, the secondary coating thickness is a function of the applied pressure gradient, the diameter of coating die, the viscosity ratio, and the fiber drawing speed. The influences of each of these control parameters on double-layer coating are scrutinized. A practical concept of thermal control of the coating process utilizing temperature-dependent viscosity of coating resins is explored.