The flat glass tempering process includes two important processes: heating and quenching. Here below are the matters to be paid attention to in the heating stage during glass tempering process.
The glass tempering process requires that the glass should be quickly and uniformly heated to the required temperature in the heating chamber of the glass tempering furnace, which is a principle requirement in the glass heating stage. ‘quickly’ means that the glass should be heated to the tempering temperature in the shortest time to avoid optical deformation, especially for the horizontal roller hearth glass tempering furnace; ‘uniformly’ means that during the glass heating process, the temperature of each area is uniform, the upper and lower surfaces are heated evenly, and the surface and middle temperatures are uniform when leaving the heating oven. After the glass is heated to the required temperature, it should be transported out of the heating furnace as soon as possible for rapid quenching.
1. Determination of heating temperature
1) The stress of the glass should be completely eliminated. If the residual stress in the glass cannot be completely eliminated, it is easy to cause the glass to break when the tensile stress is formed in the center of the glass in the early stage of quenching. Furnace temperature should be higher than Tg.
2) The glass temperature should not cause serious deformation of the glass, so the furnace temperature should not be set too high, and it is suitable to be 60-100°C higher than the glass exit temperature.
3) Since glass has the characteristics of selective absorption of infrared rays, the infrared wavelength of the heating furnace should be within the absorption range of glass. According to the absorption spectrum of the glass, the maximum radiation wavelength in the furnace is preferably about 3.0 μm, and the corresponding furnace temperature is set at about 700 °C.
2. Determination of heating time
For ordinary radiation heating glass tempering furnace and ordinary transparent silicate glass, when heating glass of different thickness, with the same furnace temperature setting, the basic heating time of glass is 40 s/mm. In consideration of actual production process, the furnace temperature for producing thin glass is set higher than that for producing thick glass. Therefore, the heating time of glass can generally be estimated according to the following formula:
T=40t+40(t-5)X0.1(s)
In the formula, t---the thickness of the glass, mm, the above formula is based on the glass with a thickness of 5mm.
For tinted glass, the heating time should be proportionally reduced according to the increase of its heating absorption coefficient. For ultra-clear glass, the heating time should be appropriately extended. For low-emissivity glass, the heating method of glass tempering machine is more closely related to the heating time, and the heating time needs to be specifically set according to the heating performance of the glass tempering machine.
