With the reduction of traditional fossil energy and the increase of pollution, the development of photovoltaic modules is getting faster and faster. As an important accessory of photovoltaic modules, photovoltaic glass has attracted the attention of various manufacturers for its quality. As a method to effectively improve the strength of glass, physical tempering has become a common glass processing technology. The quality of glass tempering will greatly affect the profit of each factory, so it is of great significance to study the physical tempering process of photovoltaic glass. This paper studies the common defects of tempering of 2.5 mm thick glass and proposes corresponding solutions. The tempering furnace used by our company is the MT-G series tempering furnace, and the glass is 2.5 mm photovoltaic rolled glass produced by our company. The bending test method is based on GB/T 17841-2008.

Common defects

(1) The waveform is large. The main reason for the large waveform is that the heating temperature is too high. In the glass heating stage, the glass will stay at the turning point of the swing for a short time. If the temperature is too high, the glass will deform due to the interval between the rollers, as shown in Figure 1. The reasons for the large glass waveform are: ①The temperature of the tempering furnace is too high; ②The tempering heating time is too long; ③The aramid rope in the quench section is worn, raised, and overlapped during winding; ④The ceramic rollers at the back of the tempering furnace are not on the same horizontal line; ⑤Coated film The layer is too thick; ⑥The original film has a large waveform. The corresponding solutions are: ① reduce the temperature of the tempering furnace; ② shorten the heating time of the tempering furnace; ③ replace the aramid rope in the quench section; ④ calibrate the ceramic roller level; ⑤ notify the coating to adjust the thickness of the coating film; ⑥ replace the original film.

Figure 1 Large waveform

(2) Cocked head. Head warping refers to the warping of the corners of the toughened glass head. The principle of warping head formation is that after the glass is heated to a softened state by the tempering furnace, the glass head will bend slightly downward under the action of gravity in the gap between the ceramic rollers, and will hit the ceramic roller or the aramid rope roller table when it continues to move forward. The reasons for sudden head warping when the tempering process is not adjusted are: ①The coating layer is too thick; ②The aramid rope roller table in the quench section is not level; ③The ceramic roller at the back of the tempering furnace is not level; ④The heating temperature is high or the heating time is long. The corresponding solutions are: ① notify the coating to adjust the thickness of the coating film; ② calibrate or replace the aramid rope roller table; ③ calibrate or replace the ceramic roller; ④ reduce the heating temperature or shorten the heating time.

(3) Bend up and down. Upward bending refers to the situation where the tempered glass sometimes deforms upwards to form a concave surface. If the concave glass is forced to a straight shape, when the external pressure is released, the glass will return to its original shape, as shown in Figure 2.

Figure 2 Glass bends up

After the heating is completed, the glass entering the tempering stage is flat. If the temperature of the upper and lower surfaces of the glass is the same, when the lower surface of the glass cools faster than the upper surface of the glass, the lower surface will solidify first, and the upper surface will continue to shrink, which will cause the glass an upward curve.

If the cooling rate is the same during tempering, when the temperature of the lower surface of the glass is lower than the temperature of the upper surface, the lower surface will solidify first and the upper surface will continue to shrink, which will also cause the glass to bend upward.

The reasons for the formation of upward bending in actual production are: ①The temperature of the upper part of the glass is higher than the temperature of the lower part; ②The wind pressure on the upper part of the glass is lower than the lower part of the glass. The corresponding solutions are: ① reduce the temperature of the lower part or increase the temperature of the upper part; ② increase the upper wind pressure or lower the lower wind pressure or adjust the height of the air grille of the quench section. Theoretically, adjusting the upward bend is to increase the wind pressure on the upper part of the glass, that is, to reduce the height of the upper air grille, but in the actual production process, some tempering furnaces increase the height of the upper air grille. The reason may be that after the air grille of some tempering furnaces are too close to the glass, causing the hot air cannot be discharged in time, and the cooling effect is reduced. In addition, the difference in the roughness of the upper and lower surfaces of the same glass results in different heat absorption. Therefore, when the placement direction of the upper and lower surface changes, the tempering process also needs to be adjusted. For example, the monofilament embossed glass produced by our company can be produced normally when the embossed side is facing down, and the lower part will have white fog and bend upwards when the embossed side of the same process is facing up. The reason for the upward bending is that the surface area of the embossed surface of the glass is larger than the surface area of the suede when the glass is at the back of the tempering furnace, which makes the embossing heat radiation heat transfer stronger than that of the suede, that is, the embossed surface at the back of the tempering furnace receives more heat radiation . The causes and solutions of downturns are the opposite.

(4) Bending. After a large piece of glass is tempered, the "bottom of the pot" is deformed. When external pressure is applied, the glass will bulge upward or downward, and when the external pressure is released, the glass will maintain its new shape, as shown in Figure 3. Commonly known as bullet bend. The reason is that the heating temperature around the glass is higher than the heating temperature in the middle of the glass. During the tempering stage, the lower part of the glass in the middle of the glass will solidify faster than the glass. When the glass is still continuously shrinking, it will force the middle of the glass to either bulge upward or bulge downward. The practical solutions in production are: ①Reduce the temperature of the side of the tempering furnace; ②Increase the speed of the tempering furnace; ③Reduce the height of the air grille.

Figure 3 Glass Bending

(5) Weakness. When the heating temperature in the middle of the glass is higher than the surrounding heating temperature, saddle-shaped deformation may also occur. The cause of saddle deformation is that the temperature in the middle of the glass is higher than the temperature around the glass. In the tempering stage, when the middle position of the glass still continues to shrink, the periphery of the glass will bend in different directions, resulting in saddle deformation. As shown in Figure 4. In production, it is commonly known as glass softening. The solution in actual production is to increase the temperature of the edge of the glass. In addition, there are common defects such as large and small heads, S-bends, and fragments at the wind grille.

Figure 4 Glass becomes soft

Concluding remarks

Common defects in photovoltaic glass tempering, such as upward bending, downward bending, and spring bending, are closely related to the process and equipment. As long as you understand the formation mechanism of various defects, you can find out the causes and corresponding solutions to reduce the occurrence of defects.