When the glass enters the glass tempering furnace, due to the physical contact with the rollers, the lower surface of the sheet will expand faster than the upper surface and cause the glass sheet bending up all around, leaving only the center part remaining in contact with the rollers. As a result, the temperature of part in contact with the rollers will rise faster than other suspending parts (due to heat conduction). The suspending parts will continue absorbing heat after they are softened and re-gain contact with the rollers.  However, the heat cumulated in the center has far exceeded the other areas by now. The excessive heat can easily lead to severe deformation in the center area and result in optical distortion.

To solve this problem, it is essential to properly turn up the speed of the upper convection blower to adjust the upper and lower temperatures in the chamber, allowing the glass sheet in the furnace to be flattened rapidly.