The Vello Process
In the Vello process the glass in the furnace flows in to a refractory bowl which has an orifice plate in its base (the “ring”). A vertical mandrel, the bottom end of which is flared (the “bell”), is suspended through the ring. Around the mandrel is a rotating sleeve which provides stirring for the glass in the bowl. Glass is drawn between ring and bell, initially vertically downwards, but then, as it cools, it is pulled through almost 90 o by the drawing machine on to the carbon support rollers. Dimensional stability is achieved by methods similar to those used in the Danner process, but the size range is limited and a change of tube size may require both ring and bell to be changed. Consequently, the Vello process is best used for long runs of a standard size, while the Danner process provides greater flexibility for shorter production runs over a wider range of sizes.