The self-explosion of tempered glass has always troubled tempered glass manufacturers and manufacturers of kitchen utensils (stoves, range hoods, etc.) that use tempered glass.
According to incomplete statistics, the self-explosion rate of tempered glass produced by most tempered glass manufacturers in China is 0.3% to 0.5%, and the self-explosion rate of tempered glass produced by individual manufacturers is even higher.
1 Characteristics and production process of tempered glass
①Characteristics
Tempered glass has the characteristics of high bending strength, high impact strength, good thermal stability, smoothness and transparency. The bending strength of tempered glass is 3 to 5 times that of ordinary glass, and the impact strength is 5 to 10 times that of ordinary glass. When tempered glass is damaged by super impact, the fragments will be scattered into fine particles without sharp edges and corners, so it is also called safety glass. Tempered glass is 2 to 3 times more resistant to sudden cooling and heat than ordinary glass, generally can withstand temperature changes above 150°C, and has excellent thermal burst resistance.
②Production process
Tempered glass is made of ordinary flat glass or float glass (called original glass sheet). Ordinary flat glass requires selected or first-class products, while float glass requires superior or first-class products.
At present, the processes for producing tempered glass include physical tempering method and chemical tempering method. The physical tempering method is to heat the glass in the tempering furnace below the softening temperature, then quickly send it to the cooling device, and quench it with a certain pressure of normal temperature airflow. The outer layer of the glass shrinks and hardens first. Because the thermal conductivity of the glass is small, the inside of the glass is still at high temperature.
When the inside of the glass starts to harden, the hardened outer layer will prevent the inner layer from shrinking, so that the first hardened outer layer Compressive stress is produced, and the post-hardened inner layer produces tensile stress. It is precisely because of the existence of this compressive stress on the glass surface that when an external force acts on the surface, this part of the compressive stress must be counteracted first, which greatly improves the mechanical strength of the glass. Common building curtain wall glass and tempered glass on kitchen utensils are generally produced by physical tempering.
The chemical tempering method is to change the composition of the glass surface through ion exchange method of ordinary flat glass or float glass, so that a compressive stress layer is formed on the glass surface.
2 Reasons and solutions for self-exposure of tempered glass
The self-explosion of tempered glass is mainly caused by its own stress. The tempered glass produced by physical tempering process has compressive stress on the surface and tensile stress on the inner layer, which strengthens the glass. Any factor that breaks the balance of stress will cause the tempered glass to self-explode. The main reasons leading to self-explosion of tempered glass are: original glass factors, processing factors, and unreasonable structural design of tempered glass panels.
2.1 Factors of original glass
The self-explosion of tempered glass caused by the original glass sheet can be divided into two types: one is the self-explosion caused by visible defects in the original glass sheet, such as inclusions such as sand grains, bubbles, gaps, chipping, dark cracks, etc.; the other is It is a self-explosion caused by the expansion of nickel sulfide (NiS) impurities in the original glass sheet. These are two different types of self-destruction, which should be treated differently and dealt with in different ways. The former is generally visible visually, relatively easy to detect, and controllable in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the original glass sheet, which cannot be visually inspected and cannot be controlled in production. In actual production, the former can generally be eliminated before installation, while the latter continues to exist because it cannot be inspected, which has become the main reason for the self-explosion of tempered glass in use.
For the existing float glass manufacturing process, nickel sulfide inclusions are unavoidable. During the installation stage of the float glass production line, a large amount of nickel compounds will be produced by the welding of the steel structure of the furnace. During the manufacturing process of the original glass sheet, the nickel compound forms nickel sulfide with the sulfur in the fuel and the compound (raw material for the production of float glass). Nickel sulfide is a kind of crystal, and there are two crystal phases: high-temperature phase a-nickel sulfide, low-temperature phase b-nickel sulfide, and the crystal phase transition temperature is 379°C. When the float glass is heated in the tempering furnace, because the heating temperature is much higher than the crystal phase transition temperature, the nickel sulfide is completely transformed into the high-temperature phase a-nickel sulfide. In the subsequent quenching process, a-nickel sulfide has no time to transform into b-nickel sulfide and is frozen in the tempered glass. At room temperature, a-nickel sulfide is unstable and has a tendency to gradually phase into b-nickel sulfide, which lasts for several years at the longest and is accompanied by a volume expansion of 2% to 4%. When the volume expands enough to break the stress balance of the original tempered glass, it will cause the tempered glass to self-explode. The self-explosion of tempered glass caused by nickel sulfide impurities is shown in Figure 1. Since 2002, the tempered glass panel of kitchen utensils has widely adopted the anti-explosion film (net) process. Even after the tempered glass self-explosion, it is still attached to the anti-explosion film (net), which not only ensures the safety of users, but also provides a complete solution for identifying the cause of self-explosion. in kind.
The self-explosion initiation point of tempered glass caused by nickel sulfide impurities (tempered glass cracks are radial, and all have initiation points) is located in the middle of the tempered glass, and there are two small polygonal patterns at the initiation point similar to two butterfly wings (called butterfly spots ), carefully observe the common sides of two small polygons (the torso of the butterfly), there should be small black particles (nickel sulfide stones) visible to the naked eye. If the detonation point is located at the edge of the glass, it is generally caused by the stress concentration and the gradual development of the crack because the tempered glass has not been chamfered and edged, or there are gaps, chipping, and dark cracks on the edge.
The method to prevent self-explosion caused by nickel sulfide impurities is to perform homogenization treatment on tempered glass. Homogenization treatment is recognized as an effective method to solve the self-explosion problem of tempered glass. Homogenization treatment is to use a homogenization furnace to reheat the tempered glass and keep it warm for a certain period of time, so that the nickel sulfide completes the crystal phase transformation before the tempered glass leaves the factory, so that the tempered glass with self-explosion tendency is broken before leaving the factory.
2.2 Processing factors
① Cold working defects
Because glass is a brittle material, there are some small gaps, chipping and imperceptible dark cracks on the edges that have been cold-worked (opening holes, cutting, etc.). or round), switch knob hole position. In the subsequent tempering process, some original glass sheets with cold working defects will self-explode, while those that have not self-exploded during the tempering process are still prone to self-explosion during use. In order to reduce the self-explosion caused by cold processing defects, the cold processing accuracy of the original glass sheet should be improved, the tools that meet the accuracy requirements should be selected, the vibration during processing should be eliminated, and the opening and cutting edges should be chamfered (rounded). Water cooling is used in cutting, chamfering (rounding) and other processes to prevent local overheating.
②Hot processing
The main reasons for the self-explosion of tempered glass caused by thermal processing are: uneven heating and cooling of the tempering furnace; excessive tempering; insufficient stacking distance of tempered glass in the homogeneous furnace, resulting in uneven heating of single glass.
The main reasons for the uneven heating and cooling of the tempering furnace are: a. The local heating wire is blown, resulting in uneven local temperature; b. There is no heat balance device in the tempering furnace, so the temperature of the furnace body is not uniform; c. Due to poor maintenance, the wind gate section (used to transport quenched air) partially appears to block the air nozzle, resulting in uneven cooling of the glass; d. The original glass pieces are placed beyond the cooling range of the quenching air, resulting in uneven cooling of the glass.
Too high heating temperature or too long heating time of the tempering furnace and too high cooling intensity can easily cause excessive tempering of the original glass sheet. Over-tempered glass not only has low impact strength, but is also prone to self-explosion, and the particle size is very small after being broken.
From the above analysis, it can be seen that in order to avoid self-explosion of tempered glass caused by thermal processing, the process parameters of tempering furnace and homogenizing furnace should be controlled, equipment maintenance should be strengthened, and glass should be placed reasonably. In addition, the tempered glass should not be subjected to treatments such as opening holes and chamfering (rounding), otherwise the formed stress structure will be destroyed and the probability of self-explosion of the tempered glass will increase.
2.3 The structural design of the tempered glass panel is unreasonable
① Unreasonable hole design
Kitchenware designers do not have a good understanding of the characteristics of tempered glass, which often results in unreasonable design of tempered glass panel holes. For example: the position of the opening is too close to the corner, the chamfer is too small, too many openings, the radius of the square hole right-angle rounding is too small, etc., these are easy to cause the tempered glass to explode. A European-style range hood developed by a certain manufacturer has a square smoke pipe base, and the matching toughened glass panel is rounded at the right corners of the square opening, but the radius of the arc is only 3mm. During the transportation process, the glass panel self-explosion occurred many times. The drop test found that the explosion point of the tempered glass panel is located at the right angle of the square hole. Therefore, the design of the right-angle opening with a small radius should be avoided as much as possible during design.
② Thermal expansion and contraction of glass are not considered
The 4-burner and 5-burner gas cooker produced by a certain manufacturer has a tempered glass panel area and a large cooker bottom box, and users report a high self-explosion rate. After testing, when the surface temperature of the tempered glass panel reaches above 100°C, the two ends of the tempered glass panel are slightly tilted, bent towards the middle, and finally burst from the middle opening, causing self-explosion. Analysis of the reasons is mainly due to the expansion of the tempered glass after heating. Due to the different expansion rates of the tempered glass panel and the bottom box, and the two sides of the tempered glass panel are fixed, the tempered glass panel expands and squeezes toward the middle, which eventually leads to rupture in the middle opening. .
Therefore, in the product design stage, the difference in expansion rate between the tempered glass panel and the bottom box of the cooker should be fully considered, and the weight of the bracket and cooker should not directly or indirectly act on the tempered glass panel, and support should be added between the tempered glass panel and the bottom box .
③Arc structure
The stress distribution of the curved tempered glass panel is far less uniform than that of the flat tempered glass panel, and the direct load bearing of the curved tempered glass panel should be avoided during design.
④Tempered glass panel edge design
Some manufacturers design the edge of the tempered glass panel as a beveled structure in order to pursue the appearance effect, or to meet the matching between the edge of the tempered glass panel and the aluminum frame. For the original glass sheet with a thickness of 8 mm, the thickness of the edge is reduced by 4 to 6 mm after beveling. The stress at the edge after tempering is relatively concentrated, which increases the probability of self-explosion of tempered glass. For tempered glass panels with a thickness of 8mm, if the edge adopts a beveled edge structure, the thinning amount should not exceed 2mm.