Laser Marking Methods


Laser Engraving

If marking involves visible material removal (approx. 50 um deep), this is so called “engraving”. In laser engraving, a groove is drawn into the material by the focused beam. The material is evaporated and/or melted and removed from the groove by the dynamic effect of the process. The material accumulates next to the mark which is acceptable for many components. Despite the considerably higher penetration depth up to a maximum of 100 um compared to marking by annealing, laser engraving is the quicker method. While the Q switch is not used for annealing, high pulse peak powers are used for engraving, thus reaching frequencies of between 2000 and 10000 Hz and ensuring a good interaction with the material. This method is used for all metals, for ceramics and some types of plastics. The typical line width measures approx. 100-150 um.


Marking by Annealing

If only the colour of the metal is changed by the effects of heat and oxygen without any material actually being removed, the process is called marking by “annealing”.


Marking by annealing is mainly used for stainless steel but also for all metals that change their colour if exposed to heat and oxygen. The creation of this colour, mainly black, is limited to a very small area because the temperature rise caused by the focused laser beam is local. The penetration depth of the colour is approx. 5 um. The line width used depends on the power and focal length and lies typically between 80 and 150 um. One big advantage of annealing is the fact that no material is accumulated next to the mark. Thus, this method is also suitable for marking finished components.


Colour Change

Laser beams can also be used to change the colours of many plastic surfaces. When exposed to high-intensity laser light, the plastic material changes its colour. This is caused either by pigments in the plastic material or the plastic material expands because of the heat and a colour change is perceived because of the different light reflection. If certain substances are added to the plastic material (mica, pigments), special and/or improved colour contrasts can be achieved. For a line width of 100 to 250 um, penetration depths of more than 200 um can be reached by colour change marking. Especially for marking plastics the diode pumped laser will be the tool of the future. Due to its very short pulses (colour change takes place within 10 - 15 ns) it is not heating up the material as much as lamp pumped lasers probably do. Keyboards, semiconductors, housing etc. are examples of products which are likely to be marked by diode lasers. Nevertheless, higher output powers of diode pumped lasers available in the very near future will bring access of this technology to all applications and substitute lamp pumped systems within the next years. Plastics manufacturers now offer materials specially pigmented for laser marking. In this way, suitable colour combinations can be selected for special applications. The high penetration depths ensure good abrasion resistance even for highly-stressed objects, such as computer keyboards. Especially for transparent, backlit controls, processes have been developed to remove an opaque layer from a transparent base material (day-night design).



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