In radiation heating furnace, the upper surface of glass receives the radiation heat transferred from the upper heating elements, and the lower surface receives the radiation, conduction and heat transfer of ceramic roller bed, and natural convective heat transfer. In forced convection heating furnace, the upper surface of glass mainly receives convective heat transfer of hot air and the radiative transfer from heating elements, and the lower surface receives radiation, conduction and heat transfer of ceramic roller bed and convective heat transfer of hot air.
For glass is poor conductor of heat, after it is put into heating furnace, temperature gradient will be formed between the internal and external layer, then, the external layer will expand with heat and will be restrained by the internal layer with pressure stress formed on glass surface and tension stress in the core layer. For glass has high compressive strength, it will not fracture in the initial stage of heating. The heat conductivity coefficient will increase under high temperature, and as the glass temperature increases gradually, the temperature gradient of the internal and external layer will decrease. When the surface temperature of glass is higher than fusing temperature, stress relaxation will occur on the surface along with the inner pressure stress decreasing gradually; when the internal and external temperature are the same, the stress in the glass plate will disappear, and then the glass can be treated with quenching.
After glass is put into radiation heating furnace, the upper surface of glass will contact high-temperature ceramic roller bed, and receive compound conduction of heat conduction, heat radiation and convection with the temperature increase rapidly. The upper surface only receives thermal radiation, so the temperature increases slowly. The dilatation of the lower surface of the glass is larger than that of the upper surface, so that the glass will bend upward. The weight of glass will be supported by the middle area of the glass and move on roller bed, which will cause optical damage," tempering white strip" in the middle area of the glass. This is a defect caused by uneven heating in the initial stage of heating, which needs operation personnel to adjust heating parameter.