Due to the contact between the glass and the rollers of the glass tempering machine, the lower surface of the glass expands faster than the upper surface after entering the furnace. This causes the glass to bend upwards, with the edges of the glass panel suspended in air and only the middle part in contact with the glass tempering furnace's rollers.

Under such conditions, the part of the glass in contact with the rollers heats up faster than the suspended edges due to heat conduction. The suspended parts will only touch the rollers and continue to absorb heat once the glass panel is fully softened. By this time, the temperature in the middle of the glass panel is much higher than at the edges. This can lead to deformation in the middle section of the glass due to the high temperature, resulting in optical distortion in the glass after tempering.

To address this issue, one should appropriately increase the speed of the convection fans at the top of the tempering furnace or adjust the temperature of the upper and lower parts of the furnace. This ensures that the glass lays flat quickly inside the furnace to rectify the problem.