Types of laser marking and engraving
The term "laser marking" refers to the permanent marking of all types of workpieces - regardless of the marking/engraving depth. Below is a summary of the different types. The individual marking processes that can be generated with CO2 and near-infrared lasers include:
- and color change
Depending on the material and area of application, one of the 4 processes may result in markings that have one or more of the following characteristics:
- durable - mechanically, thermally and chemically resistant
- small - hidden markings
- hard to imitate - protection against plagiarism
Data matrix marking on a cast aluminum workpiece, the aluminum was darkened by means of short pulses, in the second step the inverse matrix was polished, i.e. colored lighter to further increase the contrast.
By means of laser marking, abrasion-resistant texts, logos, serial numbers or QR codes can be permanently applied to materials.
Laser markings can also be functionally applied to a product - e.g. scaling marks on sliding calipers. Even when you are behind the wheel of your car, there are areas of application for engraving - namely the controls that are illuminated at night.
Laser engraving by ablation
The laser-induced increase in temperature leads to the vaporization of material in the laser focus. This happens within a period of much less than a millisecond. For short pulses (nanoseconds or shorter), this process is also referred to as ablation. However, the surrounding material is heated relatively little. The depth of the engraving essentially depends on the laser power, the marking speed and the material. The depth of the engraving is a few microns to about one millimeter (e.g. stamp engraving with a CO2 laser at high power and low feed rate). If materials with a high vaporization temperature (e.g. metals) are to be deeply engraved, the same marking can be lasered several times - the engraving depth increases with each pass. Removal of the engraving is only possible by means of grinding or milling.
Deep engraving: Time or quality optimized
The method is basically identical to laser engraving - but with the difference that deep engraving requires several passes. This method is often used on metal or ceramic workpieces and is either time-optimized or quality-optimized. With the time-optimized method, the engraving is produced as quickly as possible. This is used for e.g. serial numbers that only need to be readable but not aesthetically pleasing. With the quality-optimized method, however, laser parameters are chosen so that the desired engraving depth is achieved with the best quality. This method is often used for functional markings.
Laser engraving by removal
In laser marking by removal, a previously applied top layer, such as paint or anodic coating, is partially removed by the laser beam to expose the actual underlying, color of a component. Removal is suitable for coated metallic workpieces, but is also widely used for plastics. Particularly in the automotive industry, laser removal is often used to realize the so-called “day/night design” of switches or control dials. For this purpose, a translucent (transparent) plastic is first provided with a cover layer of paint. The desired marking - text or icon - is then exposed using a laser. In the dark, the backlight can illuminate the marking.
Annealing marking is a method that is exclusively limited to metals - more precisely, alloys with iron and at least low carbon content. Here, the action of heat and oxygen and diffusion of the alloy components changes the color of metal in the top layer. The laser-induced temperature is typically in the range of 220°C – 500°C, it remains below the melting temperature of the alloy. Annealing colors are found in everyday life e.g. in exhaust manifolds on motorbikes, which are often stained golden-violet. The process of staining is the same as the typical temperature of the exhaust gases of a four-stroke engine is around 300°C.
The big advantage of this method is that no material removal takes place. I.e. the mechanical integrity and, to a large extent, the chemical properties (e.g. rust-inhibiting) are retained. At the same time, however, the marking is also extremely durable, as long as the component does not reach the aforementioned temperatures in later use.
This method is mainly used for the laser marking of plastics. Due to the laser-induced increase in temperature (in any case below the vaporization temperature of the material), volatile components such as hydrogen, oxygen, nitrogen are emitted. This increases the concentration of carbon in the laser-processed zone. Pure carbon (as graphite) is dark gray/black - just like the produced marking. As virtually no material removal takes place, the mechanical properties are retained. As an analogy, it can be said that the color change in plastics produces similar markings to the annealing marking on metals. With the restriction that the color change markings are always dark.
Marking by means of color change on a plastic component, the marking is resistant to abrasion, withstands cleaning agents, oil, weak alkalies and acids.
One method of marking by means of laser, which often works with plastics, is so-called foaming. This method can be used with all materials that can melt. The laser heats the material above melting temperature but below vaporization temperature. The resulting molten mass begins to boil, which means that cavities (“bubbles”) form - similar to water in a saucepan. As re-solidification of the molten mass takes place very quickly after the end of the laser pulse, the bubbles can no longer escape from the molten mass. This creates an increase in the volume - foamed markings are raised, no impressions. The small inclusions create stray light, i.e. foamed markings are always light.
Summary and comparison
Different marking processes are shown here. They have something in common; a thermal effect. This is almost always true for all CO2 lasers and for all near-infrared lasers (Nd:YAG, Nd:YVO, fiber lasers) in the nanosecond range.
ABLATION: Vaporization of material with high power and short pulses for a precisely defined volume of interaction.
ANNEALING Metals: with moderate intensity - (increased) oxidation and diffusion due to temperature.
FOAMING of polymers by means of melting and boiling. Solidified molten mass has volume increase.
Color change in polymers by means of laser-induced temperature increase and outgassing of gaseous molecules or vapor (high peak power, low energy)
The other thing that all methods have in common is the possibility of durable, abrasion-resistant, chemically and thermally resistant marking. Direct marking of components (i.e. abandonment of labels) reduces running cots and solves the problems of poorly adherent labels. The threshold for product counterfeiting is (at least) significantly raised because the plagiarist requires the same laser (with the same optics) and an identical parameter set.