Shaping and forming of fused silica

You can process fused quartz and fused silica as any other glass. The processes can be grouped in the general sections:

• Mechanical processing
• Hot forming

In the following, we want to give you some general information regarding these processes and what is specific to fused silica.

Mechanical processing

Due to its hardness, fused silica requires diamond tools to process it mechanically. Because it is fragile, there is a limit to the force it can withstand before cracking and resulting from that the feed speed during processing needs to be chosen carefully.

The general processes incude:

• Cutting:
  Typically, band and wire saws as well as chop saws are used to cut fused silica. The cut loss and the surface quality depends on the machine and the feed speed.
  It is also possible to cut fused silica using CO2-Lasers or a water jet machine. While a laser produces flame-glazed surfaces, a water yet produces a rougher surface. If the thickness of the fused silica is too large for a single cut with a laser, it may be possible to do multiple consecutive cuts. After a laser cut it is necessary to anneal the glass piece to relieve thermally induced stress and keep the part from shattering.
  Both a laser and a water jet allow the manufacturing of fused silica plates of simple or complex contours
• Drilling:
  Holes in or small rods of fused silica can be produced with diamond drills. It is important to ensure proper cooling to prevent the tools from failing prematurely. For thin pieces of fused silica, e.g. in  plates , it may be easiest to use a laser to drill holes.
• Grinding
• Milling

Hot forming

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.

• Welding:
  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  welding rod ) 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
• Collapsing:
  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  CVD processing .

Other

• Etching