What are the two rows of black dots indicating the production date on car glass?
4 Answers
The two rows of black dots indicating the production date on car glass are integrated to show the manufacturing date. Here is an introduction about car glass: 1. Introduction: The production of car glass involves heating the glass to near its softening temperature in a heating furnace, then quickly transferring it into air grids with different cooling intensities for uneven cooling. 2. Function: This process creates different stress levels between the main viewing area and the peripheral areas of the glass. Generally, glass produced this way is zone-tempered glass. 3. Application: The peripheral areas are located in the strong cooling zones of the air grids, requiring full tempering. These areas have better fragmentation and higher tempering strength. The main viewing area is in the weak cooling zone, resulting in larger fragments and lower tempering strength.
I learned from a master at an old auto repair shop that the black dots in the corners of the windshield are like the ID date of the car glass. For example, if you see a symbol like 8•••, it means the glass was produced in the third quarter of 2018. Those two rows of small dots are actually auxiliary scales for numbers: the left side marks the year, and the number of dots on the right corresponds to the quarter (the four dots are divided into four parts, and the position of the solid dot indicates which quarter it is). This imprint is pressed onto the glass during high-temperature molding, serving both to record the production time and to balance temperature differences. Once, when helping a neighbor check out a used car, I noticed that the production date of the window glass was later than the car's manufacturing date, which clearly indicated the glass had been replaced. I managed to negotiate the price down by 3,000 yuan directly.
Last time during maintenance, I asked the technical director at the 4S dealership about the two rows of black dots on car windows. They are actually production date codes. For example, my car's window is marked with '2•••', which indicates it was manufactured in the second quarter of 2022. The specific decoding method is based on four dots to the right of the number—the position of the solid dot determines the quarter: the leftmost solid dot represents Q1, the next one to the right is Q2, and so on. These markings are formed simultaneously during the tempered glass's thermal fusion process, serving both to convey information and prevent edge thermal cracking. I recommend checking the accessory date when replacing glass—my car was found to have aftermarket glass that was 0.5mm thicker than the original, causing frequent fogging on rainy days.
Once when I got pulled over for modifying my headlights, I took the opportunity to ask the traffic police about car window markings. The numbers printed in the corner, like 7, represent the year 2017 or 2027. The four dots following it function like a month dial: a solid dot on the far left indicates January, moving to the right for the second quarter, and the far right for the fourth quarter. These raised dots are pressed by the mold during the glass cooling process and also help prevent edge cracking at high temperatures. Remember to check that all glass dates are close when buying a used car. My used Golf had the left window dated three months later, which later turned out to be evidence of a side collision.
