New Technology｜UV Laser Marking of Cable Insulation

According to the IOP Laser Technology Handbook, in 1999, around 22,000 laser marking machines were used in various industries worldwide. Hexa Research expects the overall laser market to reach $3 billion by 2024. It seems that within a few years, everything that needs to be marked will be laser marked, including fruits and vegetables, and of course wires and cables.

This article describes the fundamentals of UV laser marking of wire and cable in the aerospace industry and its applicability to other markets.

Direct printing on wire and cable using ultraviolet (UV) lasers has been extensively tested and accepted by OEMs and end users in the aerospace industry. It is covered by several documents and standards published by SAE International (http://www.sae.org/AIR5558, AIR5468B, AS5649) and is reflected in the production specifications for large and small frame aircraft for commercial, industrial and military use . The OEM list includes Boeing, Airbus, Lockheed Martin, Sikorsky, Gulfstream, Bombardier, Pilatus and more. It is also used by government agencies such as the Department of Defense, NASA, FAA, etc. End users use UV laser markers in their regular maintenance and repair routines.

UV lasers leave permanent, indelible, high-resolution marks on substrate surfaces. To understand this phenomenon, we must consider how the laser beam interacts with the material. For example, a beam of light can be completely reflected from a surface, as sunlight from a mirror, or propagate unaffected as sunlight through a clear glass window. In these cases, no marks will be left on the surface. To mark a material, at least a portion of the laser radiation must be absorbed directly at or near the surface of the material.

Depending on the laser and material properties, there are several possible scenarios (Figure 1):

The irradiated material evaporates, leaving relatively sharp boundary grooves on the surface.

The irradiated material melts and spills from the inside out, creating hills and valleys in the middle of the plain.

The irradiated material heats and produces gaseous components that react with atmospheric oxygen, depositing combustion products (such as soot) on the surface.

Color changes. The material changes color without any other visible surface modification.

all of above.

Fig.1. Laser-surface interaction

Ablation is the cleanest method of altering the surface, but since the affected area does not change color, the marking contrast is low. Deeper, wider marks can improve legibility but reduce material integrity, which is clearly unacceptable for aerospace applications. One possibility would be to apply an extra layer of wire solarium and then selectively remove it to reveal a different colored primer, but that doesn't look very practical either.

Melt and burn marking processes have long-term durability issues, as melted material and burnt deposits may not adhere well to unaffected areas. This is similar to the 21st century hot stamping technique. Sure, this version is more advanced, more flexible, and more precise, but it's still hot stamping with all its well-known flaws.

Color change can be an excellent solution without changing material properties, providing adequate contrast, good durability and long-term stability. UV laser marking of aerospace wire and cable meets all these requirements.

Figure 2 shows ETFE and PTFE insulated wire processed using Samsung Technologies M-100L-FG wire marking system. Well-defined, clearly legible prints remain intact even after prolonged accelerated heat aging.

Figure 2. UV laser marking on ETFE (top) and PTFE (bottom) wires

Marking cross-sections (Figure 3) confirm that the darkened area extends 10-20um below the surface, ensuring that the marking cannot be altered or removed without physically damaging the top layer of the insulating layer.

Fig.3. Marked conductor cross sections for BMS13-48T10C01G022 (left) and BMS13-60T44C01G022 (right)

The question is how light-colored polymer surfaces can darken under laser light without burning or melting. The answer is a magic substance called titanium dioxide (TiO2). Fortunately, this is a commonly used pigment that wire manufacturers use to make insulation appear white or other light colors like gray, blue, green, yellow, pink, etc.

An optical band gap of around 3.1 eV accounts for the strong absorption of UV radiation by TiO2 with wavelengths shorter than 380 nm. Irradiation with UV laser permanently changes the TiO2 particles from white to blue/black. The same effect occurs when these particles are embedded in the substrate. Ideally, the laser radiation does not react with the substrate, but passes freely through the substrate surface. In contrast, pigment particles within the substrate interact with a laser beam, which changes the particle's structure and appearance, including color. For example, thin PTFE films are almost transparent to UV light, while small (~0.3u) particles of TiO2 randomly distributed in the insulating layer strongly absorb light and change color.

Figure 4 illustrates the process in space and time. The incident laser beam freely penetrates the first material layer and loses a small fraction (eg, 1%) of its total energy when interacting with the pristine white TiO2 particles, turning them black. The same thing happens in the second layer, and so on, until most of the laser pulse energy is absorbed in the top 50-100 layers. In practice, the process is very limited in time and space, as the total pulse duration is typically below 30ns and the marking depth does not exceed around 50um.

Fig.4. Schematic diagram of the path of a UV laser beam through a transparent medium doped with TiO2 particles

The short nanosecond laser pulses prevent periodic heat exchange between the additive and surrounding material, thereby limiting any structural and/or chemical modification of the pigment particles themselves. Obviously, this mark is not easy to remove because most of it is distributed on the top layer, not on the surface.

The question of what exactly happens to TiO2 particles under intense UV exposure is beyond the scope of this article, but the resulting color change is irreversible. For example, long-term stability of UV laser-marked TiO2-doped ETFE layers was studied in 1990 at the McDonnell Douglas Research Laboratory. The markers barely changed during both thermal aging (770 hours at 229°C) or simulated solar radiation (equivalent to 17 years of UV exposure in the Arizona desert).

Marking contrast is proportional to TiO2 concentration; however, excess may damage the insulating layer. Usually 2-4% is enough to achieve good contrast. Table 1 lists typical contrast levels achievable by direct UV laser printing on wire structures commonly used in the aerospace industry.

Table 1. UV Laser Marking Wire and Cable

Other wire types can also be marked as long as they contain magic pigments. The outermost layer of the sheath with TiO2 is the main limitation of the marking technique described above. In most applications, TiO2 is used as a white colorant, which turns black when exposed to UV laser light. Therefore, only light-colored wires can be clearly imprinted.

The remaining wires can also be laser marked by different mechanisms described in Figure 1. However, these marks will likely not meet strict aerospace standards unless we find another magic pigment that, for example, turns from black to white when exposed to a green laser.