Glass tempering furnaces
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 on the type of glass. The target is to introduce at the glass surface more important dimensions cations from the salt bath, to replace the smaller ones which are already inside the glass. This is possible thanks to the temperature increase (more than 400 degrees Celsius) which allows to 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/Oscillating horizontal tempering furnaces
There are 4 areas: loading, heating, tempering/cooling, unloading
In the loading area the operator prepares the batch and waits to introduce the glass inside the oven till the previous batch leaves the heating area.
In the heating area the glass moves continuously with oscillation on rolls to prevent deformations.
When the temperature required is reached the rolls exit quickly the glass from the heating area to the tempering area where under high pressure air is sent on the surface.
In the same time a new batch is introduced inside the heating area.
When the glass, in the tempering area, reaches the right temperature to be manually unloaded it is driven to the unloading area where operators unload the glass.
