Glass processing & toughening

Abrasive waterjet cutting utilizes a high velocity coherent stream of water and abrasive that can be used to cut almost all materials. Water at 40,000 to 55,000 psi accelerates through a sapphire, ruby or diamond orifice. The stream passes through a mixing region where the vacuum, induced by the stream, sucks in abrasive. Momentum of the water stream accelerates and entrains abrasive as it passes through the nozzle.

The stream exits the nozzle as a three phase mixture of air, water and abrasive particles with a cutting diameter of 0.020" to 0.060". The high velocity abrasive particles impact on the kerf face and do the actual cutting. Kerf material is removed as microchips, with negligible effects on the material.

The cutting stream carries 0.5 to 1.5 pounds per minute of abrasive. The quantity of abrasive is dependent on the cutting stream size, which is selected based on the material to be cut. Garnet is by far the most commonly used abrasive. It is environmentally clean, contains no free silica, and combines good cutting ability with reasonable wear on the consumables.

The main attributes of the cut are:

• No heat
• Narrow kerf
• Good edge finish
• High accuracy

Successful, cost effective, abrasive waterjet applications take advantage of these characteristics.

Abrasive waterjet offers many advantages not found in other cutting techniques.

1. No heat affected zone (HAZ)
2. Low contact force of cutting stream
3. No distortion and warping
4. Burr-free
5. Can cut any thickness
6. Near net shape cutting eliminates secondary operations
7. Can achieve high accuracy
8. Material thickness of 0.002 to 12" can be cut
9. Small kerf width allows for tight nesting and optimal material usage
10. Flexibility

For many applications such as buildings requiring large spans of glass, toughened glass is the only acceptable alternative. These glasses have exceptional strengths compared to standard annealed float glasses.

These improved properties are a result of the stress profile that is induced in the glass by the toughening heat treatment process. When performed correctly, the glass surface is in compression, while the centre is in tension. This stress profile in the glass is successful as most failures start at the surface from tensile loads. In toughened glass, the applied tensile load must overcome the compressive stress at the surface before the surface can go into tension and fail.

The heat treatment process involves heating annealed (stress free) glass up to a temperature between its glass transition temperature and its softening point and then rapidly cooling the surface. This is usually achieved using air jets. This process freezes the surface, while the interior may still be molten and consequently there is a temperature differential across the thickness of the glass. The hotter core section then contracts at a faster rate compared to the outside until an isothermal state is reached.

Initially the rapid cooling of the surface tends to induce a tensile stress in the surface. This is reversed in the latter stages of cooling, resulting in compressive stresses in the surface.

Key properties

• Superior strength compared to annealed glass
• When failure occurs, glass panels shatter into small pieces that rather than sharp shards as is the case with annealed glass.
• Good chemical resistance
• Attacked by hydrofluoric acid (HF)

Building and Construction Industry

Toughened glass finds many applications in the building industry from internal to external. Internally, toughened glass may be used in frameless shower screens and office partitions, while externally it can be used for large window spans and balustrading.

Toughened glass panels can be manufactured to almost any shape, with the limiting factor often being specific furnace dimensions (at both ends of the scale). The glass can also be machined, with holes, bevelled edges etc. However, all machining must be carried out prior to toughening and all edges must be polished. Unpolished edges may act as sites for crack propagation during the heat treatment process.