Under laser inscription refers to the labeling or marking of objects by means of an intense laser beam . In contrast to laser printing , in which a weak laser beam only controls the application of pigment on the printed material, laser inscription changes the material itself. The process and the use of energy therefore depend on the material. Laser inscriptions are waterproof and smudge-proof and very durable. They can be generated quickly, automatically and individually, which is why the process is often used for numbering individual parts. It is also possible to attach very small, machine-readable labels such as the QR code or the DataMatrix code directly to products.
Laser markings are used for marking or sequential numbering of individual parts. In addition to the industrial application - machine-readable barcodes, printing of expiry 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 changed.
In the case of organic materials such as paper, cardboard, wood or leather, local heating triggers chemical conversion reactions that manifest themselves in a change in color. It's comparable to setting a brand . This can also be the case with the likewise 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 the heating, a color change takes place, whereby the lettering takes on a different color than the base material.
This expands the spectrum of achievable color variants. Computer keyboards are an example of this method.
With the carbon dioxide laser, engravings can also be made in PMMA . Its radiation is absorbed by almost all organic materials that are transparent to light. Laser engravings in PMMA are not discolored, they only create a contrast due to their light scattering (rough surface). They can be illuminated from the inside by illuminating the edge of the material (see floodlight ) so that only the engravings in an otherwise transparent plate shine. For particularly high-contrast laser marking of PMMA, only material in cast quality, PMMA “GS” or PMMA “Cast” for short, is suitable.
DGUV Information 213-729 of the German Social Accident Insurance can be used to assess the inhalation exposure to possible volatile decomposition products in the case of plastic lettering .
Another variant of laser marking is the targeted removal of color from coated objects so that the underlying color of the objects emerges.
The color removal is since the end of the 1980s u. a. used in the automotive industry to manufacture the symbols for operating elements that are illuminated from the inside. Mostly translucent plastics are used, which are painted in the desired color. The desired symbol is then "burned out" of the paint surface with the laser beam. The advantages of laser marking are the very short preparation time for changes to the symbols and the robustness of the marking. The laser is the most efficient and reliable way of labeling.
A lot of packaging is also laser-marked. So z. B. Batch numbers placed on metal-coated paper labels. The lettering is done in one shot using the mask projection process, 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. Here the organic dyes only have to be pyrolyzed by heating the anodized layer - the anodized layer is partially retained. Is reacted carbon dioxide laser , a, whose radiation is absorbed well in the mid-infrared of the anodized layer, regardless of the dye.
Start-up labeling is possible on metallic surfaces . Some metals, which are heated up to a certain temperature, develop a layer with a color effect on the surface through oxidation processes. In the case of steel, this is known as blueing ; the effect can be clearly observed on chrome-plated exhaust pipes of motorcycles in the various temperature-dependent stages. The same color change can also be achieved through the thermal effect of a laser.
Such tarnish colors also occur when the laser is heated to above the melting point (remelting). The prerequisite is always that the engraving is carried out with blowing gas containing oxygen (e.g. air). The font color is usually brown.
Laser-induced diffraction gratings
In addition to the start-up lettering, a color effect can also be generated by laser-induced periodic nanostructures. The nanostructures act as diffraction grids in the surface and thus produce a different color effect depending on the viewing angle.
These structures are generated with linearly polarized pulsed laser systems with pulse durations in the picosecond and femtosecond range. The grid spacing of the diffraction grating corresponds to the wavelength of the laser radiation. The grating is formed perpendicular to the direction of laser polarization. This labeling 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. With short, intense pulses, the material is discharged with almost no heat-affected zone or melt. Often the depth is achieved in several steps.
As with other engravings, both vector and grid-oriented processing is possible. To get clean, almost vertical edges, it is helpful to use several directions of movement of the laser beam.
Because of their depth, deep laser engravings can only be removed by grinding. Even after grinding, there remains a structural change (heat-affected zone), which can be made legible with crystallographic or criminological methods. You will therefore u. a. used for counterfeit-proof embossing of metal parts. They are often legible even after being painted over.
Inside glass engraving
When engraving the inside of transparent materials such as plexiglass or glass , the laser is focused inside the material. Field strengths that are so high that the material is no longer transparent and absorbs the laser energy are only achieved in the focus and in its immediate vicinity . As a result, the material briefly heats up locally to up to 20,000 ° C. After cooling, an opaque or light-scattering spot remains in the material. If you guide the focus of the laser through the material, three-dimensional images can be produced.
Laser engraving using a CO 2 laser source on solid-colored PMMA (GS quality)
Raster laser engraving of a vector file using a CO 2 laser source on standard cardboard
Process of a 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
- Contactless process - no wear and tear, difficult surfaces can be labeled
In the meantime, multi-colored motifs / engravings can also be created, for example through the different reactions during laser processing on metal.
- ↑ Laser marking - suitable materials ( Memento of the original from July 9, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . Laser knowledge base. Retrieved July 7, 2015.
- ↑ German statutory accident insurance e. V. (DGUV): DGUV Information 213-729: Recommendations for risk assessment by the accident insurance carriers (EGU) according to the Hazardous Substances Ordinance - laser marking of plastics. Retrieved July 4, 2019 .
- ↑ Guoqiang Li, Jiawen Li, Yanlei Hu, Chenchu Zhang, Xiaohong Li: Femtosecond laser color marking stainless steel surface with different wavelengths . In: Applied Physics A . tape 118 , no. 4 , November 9, 2014, ISSN 0947-8396 , p. 1189–1196 , doi : 10.1007 / s00339-014-8868-3 ( springer.com [accessed February 24, 2017]).
- About laser engraving , Leipzig University of Technology, Economics and Culture