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Vertical tempering furnaces |
Machines comparatives
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 heaten salts bath. The bath chemical composition depend 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 degre celsius) which allows the cations to move. During the cooling the cations mouvement 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 heaten( more than 600 Celsius degres) in an oven for a time depending the glass thickness ( between 1 and 15 minutes). And fastly cooled down ( with air under pressure). The surface fastly 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 fastly 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
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