Vertical tempering furances
1/Introduction
There are to main reasons for glass tempering
To harden the glass surface
To prevent, in case of glass breaks, to have very dangerous big pieces of broken glasses
A/Different glass tempering
There are 2 different glass tempering but only one (the thermic one) reaches the 2 targets
Chemical tempering
Thermic tempering
1/The chemical tempering
The glass is dived inside heated salts bath. The bath chemical composition depends of the type of glass. The target is to introduce at the glass surface more important dimensions cations from the salts bath, to replace the smaller ones which are inside the glass. This is possible thank to the temperature increase ( more than 400 degrees Celsius) which allows the cations to move.
During the cooling the cations movement stops and the biggest are kept in a smaller space than the one they normally need. So they are going to take more space and to harden the glass surface.
Moreover the surface is going to be in compression and increase the glass resistance to break.
But if the glass breaks the big dangerous glass pieces remain.
The chemical tempering is used for glass less than 5 mm thickness.
2/The thermic tempering
The idea is the same, to put the glass surface in compression. But a thermic way is going to be used.
The glass is heated (more than 600 Celsius degrees) in an oven for a time depending the glass thickness (between 1 and 15 minutes). And quickly cooled down (with air under pressure).
The surface quickly cooled is going to be in compression and the internal part of the glass is going to be in extension
This way to temper has some advantages
The glass broke in small pieces (less than 1 cm²), so less dangerous for people;
It is less expensive
It is possible to temper all kind of thicknesses
B/The machines for thermic tempering
The tempering vertical ovens
The tempering oscillating horizontal ovens
The tempering continuous horizontal ovens
2/Vertical tempering
The glass is hold by crowbars and introduce inside the vertical furnace. When the required temperature is reached the glass is quickly moved to the tempering area where the cooling is done with under pressure air.
It is the cheaper way to temper glass especially glass with large dimensions. The problem comes from the crowbars with marks on the glass.
In industrial countries there are less and less vertical tempering furnaces
