Laser engraving is the process of marking of objects with the help of an intense laser beam. In contrast to laser printing, in which a weak laser beam is only used to control the pigment application on the printed material, laser marking changes the labeled material itself. The process and the energy input therefore depend on the material. Laser markings are waterproof and smudge-proof and very durable. They can be generated quickly, automatically and individually, which is why the process is often used to number individual parts and as a security element for personal data in debit cards. It is also possible to attach very small, machine-readable markings such as the QR code directly to products.
Laser markings are used to mark or sequentially number individual parts. In addition to industrial applications – machine-readable barcodes, printing of expiration dates, speedometer discs or markings on tablets – there are various art forms. Three-dimensional images inside glass is one of them. In contrast to laser printing, the labeled material itself is altered.
Advantages of Laser Engraving
- Computer-aided laser control – engraving of automated motifs
- Durability – results insensitive to abrasion, solvents and other environmental influences
- Material flexibility – acrylic glass, glass, wood, plastic, leather, metal, paper, cardboard
- Non-contact process – no wear, marking of difficult surfaces possible
- Security function for the unfalsifiable engraving of personal data in the production of debit cards[4] and ID cards (laser personalization)
In the meantime, multi-coloured motifs/engravings can also be created, for example through the different reactions of laser processing on metal.
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Usages of Laser Engraving
In the case of organic materials such as paper, cardboard, wood or leather, local heating triggers chemical conversion reactions, which manifest themselves in a change in colour. This can also be the case with organic plastics, but special plastics are often used. These often contain silicates or borides to better absorb the infrared of the laser beams. If pigments are deliberately destroyed by heating, a colour change takes place, which gives the lettering a different colour than the base material.
This expands the range of achievable colour variants. An example of this procedure is computer keyboards.
The carbon dioxide laser can also be used to incorporate engravings into PMMA. Its radiation is absorbed by almost all organic materials that are transparent to light. Laser engravings in PMMA are not discolored, they form a contrast only due to their light scattering (rough surface). They can be illuminated from the inside by illuminating the edge of the material (cf. floodlight), so that only the engravings in an otherwise transparent plate glow.
Paint Removal
Another variant of laser marking is the targeted color removal of coated objects so that the underlying color of the objects stands out.
Since the end of the 1980s, paint removal has been used, among other things, in automotive engineering to produce the symbols of internally illuminated control elements. In most cases, translucent plastics are used, which are painted in the desired color. The laser beam is then used to “burn out” the desired symbol from the paint surface. The advantages of laser marking lie in the very short preparation time for changes in the symbolism and in the robustness of the marking. The laser is the most efficient and reliable option for marking.
Many packages are also laser-marked. For example, batch numbers are printed on metal-coated paper labels. The marking is done in the mask projection process with one shot, so that the goods conveyed on a belt do not have to be stopped.
Another variant is the engraving of colored anodized layers on aluminum. In this case, the organic dyes only need to be pyrolyzed by heating the anodized layer – the anodized layer is partially retained. Carbon dioxide lasers are used, whose mid-infrared radiation is well absorbed by the anodized layer, regardless of the dye.
Tarnish
In the case of metallic surfaces, annealing is possible. Some metals, which are heated to a certain temperature, develop a layer with a color effect due to oxidation processes on the surface. In the case of steel, this is known as blueing, and on chrome-plated exhaust pipes of motorcycles, the effect can be easily observed in the various temperature-dependent stages. A similar color change can also be achieved by the thermal effect of a laser.
Such tarnish colours are also produced when heated with the laser to above the melting point (remelting). The prerequisite is always that the engrave is carried out with oxygenated blowing gas (e.g. air). The font color is mostly brown.
Laser-induced Diffraction Gratings
In addition to marking, a color effect can also be generated by laser-induced periodic nanostructures. The nanostructures act as diffraction grating in the surface and thus produce a different color effect depending on the viewing angle.
These structures are created using linearly polarized pulsed laser systems with pulse durations in the picosecond and femtosecond range. The grating distance of the diffraction grating corresponds to the wavelength of the laser radiation. The grating is perpendicular to the direction of laser polarization. This marking technology enables hologram effects on the labeled component surface. The process can be used on metals, glasses and semiconductors.
Laser Deep Engraving
Laser deep engraving or laser deep engraving is used, for example, in the production of stamps or embossing tools. Depending on the material, a liquid and/or gaseous phase is discharged. In the case of short intense pulses, the material is discharged with almost no heat-affected zone or melt. Depth is often achieved in several operations.
As with other engravings, both vector and raster-oriented editing is possible. In order to obtain clean, almost vertical edges, it is helpful to use several directions of movement of the laser beam.
Because of their depth, laser deep engravings can only be removed by sanding. Even after sanding, a structural change (heat-affected zone) remains, which can be made legible using crystallographic or criminological methods. They are therefore used, among other things, for forgery-proof embossing of metal parts. They are often legible even after they have been painted over.
Glass Interior Engraving
When engraving the inside of transparent materials such as plexiglass or glass, the laser is focused inside the material. Only in the focus and in its immediate vicinity are such high field strengths achieved that the material is no longer transparent and absorbs the laser energy. As a result, the material heats up locally to up to 20,000 °C for a short time. After cooling, an opaque or light-scattering spot remains in the material. If the focus of the laser is guided through the material, three-dimensional images can be produced.
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