using means for gradually reducing the cross-section towards the outlet or around the preform draw end, e.g. tapered

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Fibre drawing or extruding details > Optical fibre draw furnaces > Manipulating the gas flow through the furnace other than by use of upper or lower seals, e.g. by modification of the core tube shape or by using baffles > using means for gradually reducing the cross-section towards the outlet or around the preform draw end, e.g. tapered