This marking method removes layers of material and makes depressions in the surface. Due to the very high power density of the laser radiation, local vaporization or melt expulsion occurs. Even very hard metallic materials or ceramics can be marked permanently with this marking method. The depression depth can be controlled via processing parameters. Engravings are not subject to wear and tear. For this reason, the method is commonly used for parts subject to abrasion or during anticorrosive treatments of coated parts.
Metals like stainless steels or titanium are locally heated by the laser, which results in changes to the microstructure and the creation of an oxide layer. These two factors generate a colored marking. Depending on the temperature and thickness of the oxide layer, this method creates different colors like yellow, green, red, blue and even black. Black marking achieves the highest contrast, is not subject to wear and tear, and the surface remains absolutely smooth since no material is removed. This marking method is always used where the surface must not be damaged, i. e. for surgical instruments, ball bearings, and hydraulic cylinders.
The marking method of foaming is only possible with certain synthetic materials. The laser beam locally melts the synthetic material and due to the vaporization of the carbons and additives, small gas bubbles are formed which foam up the synthetic material. The cooling rate is sufficiently high to firmly enclose the bubbles. This way, a raised marking is produced. The incident light is diffusely reflected in this "foam" and the marking appears bright and with high contrasts on dark resins.
This marking method acts as an indirect marking of coated materials, i. e. anodized aluminum, laser marking films, and painted base materials. The laser beam removes the already applied covering layers by vaporization and the base materials become visible again, which leads to the marking effect. This marking method is frequently used in applications of the automotive industry to produce a day/night design of dashboards : through the removal of the layers of varnish the user symbols are applied on backlit, translucent resins. Another application is the marking of laser marking films to create self-adhesive labels. During this process, the laser beam cannot only be used for marking, but also for cutting the film.