A model for thermal collapse of tubes: application to optical glass fibres

Abstract

The collapsing of a glass tube by repeated traverses of a moving burner is a part of the modified chemical vapour deposition (MCVD) process of producing optical glass fibres. The collapsing process has been modelled by taking into account the forces generated due to surface tension and the reduced viscosity caused by heating. The model indicates that a significant reduction in diameter can be achieved when the temperatures at both the inner and the outer surfaces of the tube are higher than the softening point of glass, and the amount of reduction increases with the width over which such conditions prevail. Accordingly larger heat input, smaller tube wall thickness and slower burner speeds are shown to favour reduction in the diameter of the tube. The number of passes required by the burner to collapse a tube completely into a rod was calculated and it was in reasonable agreement with observations.