Very often laser marking and laser engraving are used as synonyms, but they actually refer to two different processes. They both imprint an indelible mark on a material by means of a laser beam, but while the engraving vaporizes it, the marking dissolves it
The melted surface expands, creating grooves up to 80 microns deep, altering the roughness of the material, and creating a black and white contrast, depending on the laser parameters used. Later we will look more specifically into what the black and white color of the marking depends on.
Let us specifically analyze the 3 steps by which marking takes effect.
The laser beam hits the surface of the material
All laser marking or engraving methods have one thing in common: that the laser beam is pulsed and releases energy at specific intervals. In one second, a 100W laser releases 100,000 pulses. Each pulse contains one millijoule of energy and can reach 10,000W peak power.
To control the amount of energy emitted by the laser, you need to adjust its parameters, which every technician knows. The most important parameters are speed and spacing, which determine the distance between pulses. The closer they are, the higher the concentration of energy.
With laser engraving, the pulses are much closer than with marking. This is because marking requires less energy than engraving, so it is faster. Speed is precisely one of the decision-making parameters when it comes to choosing between one process and another.
The material absorbs the energy of the laser beam
The surface of the material reflects most of the laser pulse energy, while the rest is absorbed and converted into heat. For marking to be carried out, the material needs as much energy as it requires for melting, and therefore less energy than it would need to evaporate as in engraving.
As soon as the energy is transformed into heat, the temperature of the material increases until it reaches its melting point. At that point, the surface heats up and becomes malleable, allowing its shape to change.
For a solid-state laser emitting a wavelength of 1064 nm, aluminum absorbs ≈5% and steel more than ≈30%. From this last figure, one would think that it is, therefore, easier to mark steel, but this is not the case. We have to consider other physical properties, including the melting point of the materials.
The localized surface expands and its roughness is altered
As the material melts and cools in milliseconds, localized changes occur on the surface. The surface roughness changes, creating the permanent marks that are the content of the marking: a code, a logo, a graphic.
Color changes appear as a result of different patterns on the surface. For high-quality laser markings, white and black offer the best contrasts.
Where surface roughness causes light rays to be reflected at different angles (i.e. diffused reflections), the surface appears white.
Where surface roughness causes light rays to become trapped (i.e. absorbed), the surface appears black.