Let's talk about some little-known facts: what's the purpose of the black dots around car windows?

Fellow drivers drive every day, but I believe there are still many things in the car that we are not aware of. For example, every time we drive, we always have to look at the windshield. Do you know what the small black dots around the edge of the windshield are for? Do you know what technology is used in the windshield? Today, let's talk about this interesting topic.

Actually, our car's glass looks like it's just one piece, but as an important component of the car, the windshield has very high technical requirements. According to the Baidu Encyclopedia entry on 'Automobile Windshield': 'Today’s car windshields are generally made as a single large curved surface, with certain curvature on all sides—top, bottom, left, and right. From both the manufacturing process and the assembly fit, this type of curved glass is a product with very high technical requirements because it involves many issues such as vehicle model, strength, heat insulation, and assembly.'
 

In fact, the windshield of a car primarily faces the issue of strength. Simply put, in terms of transparency, the glass on home windows could also be used on a car. However, vehicles travel at high speeds, especially on highways, where speeds easily reach 80 km/h to 100 km/h. At this time, if something hits the glass, the force is very powerful. Speaking directly like this, drivers might not have much concept of it, so let's try applying the impact force formula to take a look.

Ft =Δ Mv

According to the Baidu Encyclopedia entry for "impact force": Ft = ΔMv, where F is the average force, t is the time, M is the mass of the object, v is the velocity, and Δ represents the change. The characteristic of impact force is that the action time is extremely short, but the magnitude can be very large. Because the collision interaction time is very short, often only a few hundredths or even thousandths of a second, the momentum changes greatly in this extremely short time, resulting in a large impact force.

In other words, even a small stone carries considerable energy due to its high speed, and this energy, released in an instant, will create a tremendous impact. The bullet, which can pierce anything, works on this principle. If the windshield does not have very high strength, problems will arise. Besides, it has to withstand wind, sun, summer temperatures of several tens of degrees, and winter temperatures of several tens of degrees below zero, without any issues. Most importantly, it must also overcome the problem of glass shattering from rapid temperature changes. Here, let's explain a small technical question: why does glass shatter when preheated after being cold?

In daily life, we can find that if we pour hot water on a cold piece of glass, the glass may crack. The same is true for cars. In cold regions, if a layer of ice forms on the glass and you use hot water to melt the ice, the windshield can also shatter. Why is this? We found an answer on Zhihu.

The answer stated that glass itself is actually a material with poor thermal conductivity and is also fragile. When cold glass meets hot water, because its thermal conductivity is relatively poor, the side of the glass in contact with the hot water will instantly become hot, while the other side of the glass remains cool. At this time, subtle changes occur inside the glass.

Due to the principle of thermal expansion and contraction, the glass on the hot side will expand slightly, while the cold glass does not change much. At this point, the hot and cold parts inside the glass begin to 'struggle' against each other. When the force exceeds the limit that the glass can withstand (exceeding its tensile strength), the glass will shatter. Therefore, we must never pour boiling water on glass in winter, and we should not rinse glass with ice water in summer.
 

And the small black dots around the edges of the truck windshield mentioned in our title have one function, which is for explosion-proof purposes. Simply put, our car glass is mounted on the metal frame of the cab, and metal is a good conductor of heat. Truck drivers are well aware of how hot a metal cab gets under the summer sun.
 

Glass and metal frames heat up at different rates under intense sunlight, so the glass may crack. These small dots are used to disperse heat to prevent the glass from heating unevenly and cracking. Of course, besides this, these small dots also have another use, which is decoration. 

Car owners who have had their car windows replaced all know that our car windows are glued to the car using black structural adhesive. During the installation process, no matter how good the glue application technique is, the adhesive will inevitably leave unsightly marks. By applying a coating around the car windows to cover them and improve appearance, it will certainly make our cars look much better.

In fact, besides a ring of small black dots, our car's glass itself also has extraordinary design. Baidu Encyclopedia's entry on 'Automobile Windshield' records: 'Laminated glass refers to a type of glass in which a transparent, adhesive plastic film is sandwiched between two or three layers of glass, combining the toughness of the plastic and the hardness of the glass, thereby increasing the glass's resistance to breaking. Tempered glass refers to ordinary glass that has been quenched to form certain internal stresses, which enhances the glass's strength, so that when it is impacted and broken, the glass shatters into small pieces with blunt edges, making it less likely to cause injury to passengers. Regional tempered glass is a new type of tempered glass that has undergone special treatment, so that when it is impacted and breaks, the cracks in the glass can still maintain a certain level of clarity, ensuring the driver’s field of vision is not affected. At present, automotive front windshields are mainly made of laminated tempered glass and laminated regional tempered glass, capable of withstanding considerable impact.'
 

In other words, besides being made of stronger tempered glass itself, our car windows may also have one or even multiple layers of reinforced plastic film inside. Take my own car as an example: I had an accident once that caused the car window to shatter, but surprisingly, my car window did not break completely; instead, it turned into a soft 'shattered glass cloth.' Clearly, this is the credit of laminated glass.

Actually, the glass on our car also has related specifications. In the corner of the car glass, we can find a marked area like the one shown in the picture below. In this area, we can see many things, such as the 3C certification mark, the code and logo of the glass manufacturer, and the production date of the glass, etc.

Here we need to pay attention to the production date, which might be useful when buying a used car. Generally speaking, the glass must be produced first and then transported to the car factory before the whole car can roll off the assembly line. So if the glass production date is after the car's production date, then the car's glass has definitely been replaced. And a car with replaced glass may have been involved in a major accident.

So how can we check the production date of the glass? Generally, the production date of glass is represented by a number and several small black dots. The number represents the last digit of the year. In the glass shown in the picture, the number is 4, which means it was produced in 2014. The small black dots represent the month of production. If the dots are before the number, it indicates the first half of the year. You subtract the number of black dots from 7 to get the month of production. If the dots are after the number, it indicates the second half of the year. The calculation method is the same as for the first half of the year, except you subtract from 13. In the picture, there are two dots, so 13 minus 2 equals November.