A lot of experience is necessary to do hot forming of fused silica. Due to the high melting point and its steep viscosity, the temperature range in which it can be processed is rather narrow. If the temperature is too low, fused silica is solid, if it is too high, it gets very soft, almost liquid and it evaporates to form soot dust.
Additionally hot processing fused silica requires an annealing step (or multiple intermediate annealing steps) to relieve thermal stress. If not done correctly, fracturing of the glass can happen any time.
Two components of fused silica can be joined by welding. It is important to heat the two components, and keep the temperature high enough avoid a critical level of thermal stress. A small rod of fused silica (a
) is melted locally in order to fill the gap between the two glass components. Once the melt seam is done, or for large parts in multiple sections, the part needs to be annealed to relieve thermal stress.
- Glass blowing
- Elongation / compressing
Typically, a Tube is locally heated to the softening temperature and an under pressure is applied to the inside and the tube reduces its diameter. The process is frequently employed in
fiber optic production
to collapse the cladding onto the core rod.
In its extreme form, the process is able to yield rods, from a tube. This typically is the case in
A collapsed tube needs to be annealed to relieve thermal stress.