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Black marking: From technical details to practical application

Black marking is a term used in the field of laser marking. The term refers to an effect that creates a deep black, non-reflective marking. This type of marking has many advantages for labeling sensitive products and is therefore predestined for applications in medical technology. We took a close look at this special deep black marking – from theory to practice.

What is black marking?

Black marking is an effect that is created by laser marking. Typically, black marking is used in medical technology: laser black marking is preferred for medical instruments and implants made of stainless steel, as the resulting marking with its outstanding qualities optimally meets the strict regulations of medical technology.

Which laser is best for black marking?

Ultrashort pulse lasers such as the F.0100-ir are used for black marking. With this laser, ultrashort pulses in the femtosecond and picosecond range are emitted onto the material with high pulse energy. As the high energy input is localized and minimally short, the high-precision structures known as black marking are created with virtually no heat input.

Even if ultrashort pulse lasers can be generally considered as the optimum solution for this application, nevertheless there are differences from laser to laser that can be decisive: Ultrashort pulse lasers with a pulse width that is continuously adjustable between the femto- and picosecond range offer exceptional precision and outstanding marking quality, as they can be optimally adjusted to the material and the marking requirements. This is particularly important in highly sensitive areas with strict regulations, such as medical technology, in order to achieve reliable markings.

How is the black marking effect created and what is so special about it?

For the black marking effect, an ultrashort pulse laser is used to create a nanostructure on the metal which traps the light. As a result, the marking appears deep black and matte regardless of the angle and incidence of light and does not reflect.

As there is virtually no heat input into the material during processing, black marking is also referred to as "cold laser marking". This provides another benefit: the marking is particularly gentle on the material, corrosion-free, and extremely durable.

Black marking is:

Deep black
Matte
Non-reflective
Corrosion-free
Durable
Resistant to external influences
Gentle on the material

For which applications can I use black marking?

Black marking is particularly widespread in the marking of medical stainless steel. The reason for the prevalence of black marking in medical technology is that black marking optimally meets the strict requirements in medical technology as well as the challenges that appear during the life cycle of medical stainless steel instruments and implants, as this type of black marking offers:

Gentle marking while providing high contrast
High-quality, non-reflective markings, even in minimal size
Reliable legibility throughout the entire product life cycle
Deep black, safe markings that do not fade and remain corrosion-free even after multiple reprocessing cycles

Due to its many advantages, black marking can also be used in other industries and for other materials. As for all other marking solutions, the following is important: Advice from a laser expert and marking tests help to find the optimum solution that is long-term convincing.

Why is black marking on stainless steel corrosion-resistant?

To answer this question, it is worth taking a look at the properties of (stainless) steel and the impact of lasers.

In general, stainless steel can be marked with different lasers – which laser is best suited depends on the marking requirements and the field of use of the product. The material reacts differently to the laser pulse depending on the laser source and laser parameters: For example, if a laser marking is created through more heat input, as with a fiber laser, an iron oxide layer forms, which becomes visible as a contrast. The stronger the heat input, the thicker the iron oxide layer. The thicker the iron oxide layer, the higher the contrast of the marking.

This sounds good at first, but the marked area can be susceptible to rust, as the corrosion resistance of "stainless" steel depends on the intact chromium oxide-rich passive layer. The formation of the iron-oxide layer changes the ratio of chromium to iron in stainless steel to the detriment of the chromium. If the chromium proportion is too low, the passive layer is irreversibly damaged, and the material is susceptible to corrosion. This can be problematic for use in the medical sector, for example.

Therefore, if it is important that the marked area remains corrosion-free, the material must be marked so gently that the passive layer is not damaged. Of course, it can also be renewed using chemical passivation processes. However, it should be noted that the marking could fade due to a chemical passivation unless all influencing factors are optimally adjusted.

Now, why is black marking with ultrashort pulse lasers considered corrosion-free? The key is the combination of high pulse peak power and short pulse duration: as described above, the material is exposed to almost no heat when marking with ultrashort pulse lasers so that the self-passivation mechanism is maintained. The marking is not caused by the formation of an iron oxide layer but by the optical nanostructural effect. As a further advantage, the markings are so resistant that they do not fade even after many cleaning and passivation cycles.

Black marking and UDI

Since the introduction of the UDI Directive (Unique Device Identification), direct part marking has been mandatory for medical devices intended for reprocessing in order to ensure traceability.

Due to the material properties and special conditions in the product life cycle (such as cleaning, sterilization, passivation), the marking of stainless steel instruments is challenging. The outstanding qualities of black marking have proven this type of marking to be the perfect solution for marking UDI codes on medical stainless steel.

Laser black marking provides safe markings that comply with medical regulations – even if there is only very limited space available for the marking, as is often with medical instruments and implants: even codes of minimal size are optimally machine-readable, since these markings are not only deep black and non-reflective but can also be applied particularly filigree.

Conclusion: Is black marking the right choice for my application?

Are you looking for the right solution for your marking application? Then the first step is to clarify your marking requirements. If the above-mentioned benefits of black marking meet your needs, black marking could be the perfect solution for your application. Further questions should be clarified with laser experts and marking tests on your material are crucial in order to determine the optimum laser parameters for your product and requirements.