Glass cracking caused by heat bending

The stress conditions on the two surfaces of hot-bent glass are different. The convex surface is often in a tensile stress state, and the concave surface is often in a compressive stress state. The stress of some special-shaped hot-bent glass is more complicated. Although some residual stress can be eliminated after annealing, three-stage graded annealing is often used in continuous hot-bending furnaces, that is, there are 3 annealing furnaces, the temperature decreases step by step, and the glass is kept warm in each annealing furnace for some time, and finally annealing is completed. If the connection between the annealing furnaces is not good and the annealing system is unreasonable, it is easy to cause poor annealing, so that the residual stress of a part of the glass exceeds the strength of the glass, resulting in cracking.
The main reasons and treatment methods for the cracking of glass during hot bending are as follows.
① The mold is unqualified or the mold is not placed correctly. The mold should be checked, and the unqualified one should be sent for repair or replaced with a new mold; the mold should be placed firmly and flat in the furnace or on the car, and the position should be adapted to the heating center.
② The original glass has defects such as cracks and bruises. Check before entering the glass. Defective glass cannot enter the hot bending furnace. When placing the glass, be careful to prevent cracks.
③ The glass heating speed is uneven. It should be operated at the specified temperature. If the operation is normal, if the glass still cracks before hot bending, the glass heating speed can be adjusted or the reasonable temperature of each preheating chamber can be set.
④ The hot bending molding temperature is not enough or the molding speed is not properly controlled. The molding temperature can be adjusted appropriately or the molding speed can be reduced.

⑤ The glass is unevenly cooled and cooled too quickly. If it does not explode during hot bending, it explodes when it comes out of the furnace or explodes just after it comes out of the furnace. This is because the temperature is lowered too quickly during cooling, making the temperature uneven, or the temperature of the glass out of the furnace is too high. The cooling process should be adjusted to make the cooling uniform and the furnace temperature should be appropriately reduced. If it explodes after being placed for some time after it comes out of the furnace, it is caused by unreasonable placement. The glass just out of the furnace should be avoided from being placed at the cold air outlet, and hot glass should not be placed together with cold glass.