Generally speaking, there are three methods to control the type and number of cracks on the glass surface:
The first method is to use multiple laser radiation;
The second method is to use discrete points to form annular cracks;
The third method is to produce cracked surface cracks.
A clear-cut visible mark can be produced on the glass by one-time laser radiation, but the direction of crack and stress pattern will expand perpendicular to the movement of the laser. In a short time or even a few days after the mark is printed, these cracks perpendicular to the moving direction of the laser will form new cracks and expand to the nearby area outside the original mark to form fragments, which will affect the clarity of the mark. Using multiple laser radiation, the areas adjacent to the marked area are heated by heat conduction, so that these areas form a stress gradient and reduce the possibility of secondary fracture. This method is very effective in marking soda lime glass and borosilicate glass. Primary laser radiation is more effective in marking fused silica glass and quartz glass because of their low coefficient of expansion.
The second method is to use a series of annular cracks to form characters, bar codes, square or rectangular codes and other shape code patterns. Low density ring cracks are produced in glass through heating and cooling cycles. When the glass is heated, it expands and squeezes the surrounding materials. When the temperature rises to the glass softening point, the glass expands rapidly to form a dome of low-density materials protruding from the glass surface.
Because the spot energy is Gaussian distribution, the temperature at the center of the spot is higher. When the high temperature zone returns to the near starting position, the center of the annular crack is formed in this zone. A stable annular crack is formed at the joint between the low-density forming region and the standard density region. This method is suitable for marking on ordinary optical materials and tempered glass, chemically reinforced glass or ordinary sodium calcium float glass.
The third method also adopts the same heating and cooling process, which changes the surface of a specific volume of glass. However, the size of the spot used in the third method is relatively large, and the boundary at the junction of the two density regions is not as clear as that of the ring crack method. The marks produced by this method can not be seen immediately. It is required to pressurize slightly before lattice cracks begin to appear along the laser marked area. The generated fragment free cracked stripe filling pattern is used to form words, graphics and various codes. Because this method requires pure surface, clear marks can be printed with high-quality automotive glass.