As a type of safety glass, hot bending laminated glass has a wide range of applications. This product is made of two or more layers of hot bending glass bonded with PVB film, which has high strength and toughness, strong collision resistance, safety and reliability, and high transparency. In addition, once broken, the strength of laminated glass under high-speed impact is higher than that of tempered glass. After the glass is broken, there is no debris splashing, and the glass fragments can still be adhered to the PVB sheet. Given this characteristic, hot bent laminated glass is almost applied to the front windshield glass of all automotive glass, and is also widely used in related fields such as building doors, windows, curtain walls, museums, exhibition halls, etc.
The production of hot bent laminated glass mainly involves the following processes: hot bending, lamination, vacuum preheating and pre pressing, high-temperature and high-pressure processes of glass.
1. Selection of hot bending molds
The forming molds used for hot bending glass play a crucial role in the process of forming hot bending glass. The types of hot bending molds are mainly divided into three types: solid mold, frame mold, and hollow mold. Based on this, many manufacturers have their own characteristics in mold processing. Solid mold, as the name suggests, is made of iron plate and has a solid center. The characteristic of this mold is that it is easy to ensure that the curvature of the glass is consistent with the spherical surface, and the glass does not bend too much. The requirements for operators are not high, but the disadvantage is that the mold has high production costs and a long production cycle. During the hot bending and firing process, the mold absorbs heat and causes slow heating, which can easily cause pits on the surface of the glass during the firing process; The production of hollow molds is made of angle steel and flat steel. This type of mold is relatively simple and uses fewer materials. During the hot bending and firing process, the mold absorbs less heat. During the firing process, the middle of the glass is supported by springs, and there will be no pitting on the surface of the product. The use of this type of mold requires high technical requirements for hot bending operations. Due to the thermal hysteresis phenomenon during the hot bending process of glass, the product is easily bent over; Frame mold is a type of mold that lies between solid mold and hollow mold. Its production is relatively simple compared to solid mold and has lower requirements for hot bending operations.
2. Operation process of hot bending
At present, most glass processing manufacturers use electric heating hot bending furnaces. This type of hot bending furnace has convenient temperature control, easy operation, and does not pollute glass. The product quality and consistency are high, and most have adopted computer integrated control. By setting various computer parameters, programmed control of the hot bending process has been achieved.
The hot bending operation process can be simply summarized as placing glass with matched large and small pieces evenly sprinkled with silicon powder between the two pieces on the concave mold, and then heating it to reach the softening point temperature. When the glass reaches the same curvature as the concave mold under its own gravity or external pressure, the heating is stopped, and annealing is slowly carried out until room temperature, thus completing the hot bending process. The control of the glass hot bending process mainly focuses on: during glass preheating, continuous heating or slow heating should be used to ensure consistent temperature in all parts of the furnace; Require that the curvature radii of two overlapping glass bends be consistent, otherwise it will cause optical distortion to the laminated glass; The glass must reach the temperature required for hot bending forming; When placing the mold on the load-bearing trolley, it must be ensured that the mold is placed horizontally; When the temperature inside the furnace reaches the required temperature of 640-710 ℃ for glass forming, the glass will begin to deform under its own gravity. In order to prevent sudden settlement of the glass near the softening temperature and the formation of thermal bending ripples on the glass surface, the operator must constantly observe the formation of the glass in the furnace and control the number, area, and time of heating lamp switches through observation; The annealing of glass should adopt a slow cooling method. The furnace temperature must be lowered to below 100 ℃ before taking out the glass. When the glass is hot bent and formed, the original stress has been eliminated. To prevent the generation of new stress due to temperature gradient during the cooling process, the cooling rate within the annealing temperature range should be strictly controlled, especially in the higher temperature stage. The glass should be slowly cooled until the glass structure is completely fixed to prevent the generation of permanent stress, The annealing curve should vary uniformly, and the glass that has been discharged from the furnace and dropped from the rack should not be placed at the workshop air vent or fan direct blowing location.
Analysis of common problems in the hot bending process
3.1 The problem of hot bent glass bursting in the furnace
In response to this issue, glass can be analyzed from the following aspects: a. Observe the glass before entering the furnace to see if there are any cracks or edge explosions. For this aspect, when cutting and grinding glass, it is necessary to require: 1) no exposed edges are allowed during edge grinding; For the phenomenon of glass corner drop, the defect is small and not obvious after repair, and it must be repaired; 2) Large angle loss and irreparable defects are not allowed to exist; 3) The edges of the cutting interface should be smooth and free from bumps and protrusions. b. The pressure roller used during glass forming; Some products with large curvature radii cannot achieve the required curvature and spherical surface by relying solely on gravity during glass forming. Therefore, a pressure roller must be used, and the pressure roller must be completely wrapped with glass wire cloth. If the exposed metal directly contacts the glass during contact with the glass, it will cause the glass to break in the furnace; In addition, when applying pressure to glass, if there are water stains or droplets adhering to the glass fiber cloth wrapped around the pressure roller, it will also cause the glass to break in the furnace during the pressure process; c. When using a hollow mold, the middle of the glass is unsupported before entering the furnace, especially when the glass size is large, which can easily cause the glass to break in the furnace. Therefore, when using a hollow mold, the middle of the glass must be supported. d. The heating rate of glass in the furnace is too fast, which can easily lead to uneven heating of the glass in the furnace and lead to glass cracking. e. During glass forming, excessive external forces during auxiliary molding can cause the glass to burst. For some products with larger curvature radii, glass must rely on auxiliary external forces to form a bonding mold. Every operator should understand that the molding of glass should mainly rely on temperature adjustment to meet the requirements.
