Special attention shall be paid to the temperature rise for the tempering glass in the initial heating stage to avoid temporary excessive thermal stress. For the combination of temporary thermal stress and the original residual stress could cause glass fracture in the furnace, "destruction in furnace". Especially when glass temperature reaches above 350°C, the volume of glass will change and cause destruction of glass. When the temperature of thick glass increases too rapidly in the initial heating stage, temporary stress will be produced and cause the destruction of glass with bubble, calculus and other material defects.
In general, only 2/3 of the overall heating period can make the glass reach the tempering temperature or around the tempering temperature, after that, the temperature increases slowly, the latter period is mainly for homogenization of the internal temperature and eliminating stress of the glass. The period is necessary for improving the tempering quality and finished product rate.
For various Low-E coated glasses which can be tempered, it is required to perform the tempering with forced convection tempering furnace. The radiation wavelength of heating furnace is about 3μm and Low-E coated glass can reflect heat radiation of infrared band. If there is no forced convection system, glass will mainly absorb heat by the lower surface and cause uneven heating of both sides and bending upward of the glass; the bottom would be overheated for excessive contact period with ceramic roller bed and cause optical distortion and extended heating period of glass. Therefore, Low-E coated glasses which can be tempered shall be heated in the heating furnace of forced convection tempering furnace to guarantee the quality of tempered glass.