Wiring Harness News May-Jun 2022 - Page 12

12 MAY / JUNE 2022 Wiring Harness News INDUSTRIAL INFO-TAINMENT

UV Laser Marking of Wire Insulation Materials

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Figure 1 . Laser-Surface Interactions .
changes color without any other visible surface modifications . E . All of the above .
Ablation is the cleanest way to alter the surface but provides low marking contrast because the affected area does not change color . Making deeper and wider marks improves legibility but reduces material integrity that is clearly unacceptable for aerospace applications . One possibility is applying additional layer of wire insulation and then selectively removing it exposing the undercoat of a different color , but this does not look very practical either .
Melting and burning marking processes are subject to long-term durability problems because the melted material and burned-out deposit may not stick well to the unaffected area . That resembles a 21st century hot stamping technique . Of course , this version is more advanced , flexible , and precise but it is still hot stamping with all its well-known deficiencies .
Color change can be an excellent solution under the conditions that it does not alter the material properties , provides sufficient contrast , good durability , and long-term stability . UV laser marking of aerospace wires and cables satisfies all these requirements .
Figure 2 . UV laser marking on ETFE ( top ) and PTFE ( bottom ) wires .

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Figure 3 . Marked wire cross sections for BMS13-48T10C01G022 ( left ) and BMS13-60T44C01G022 ( right ).
Fig . 2 shows ETFE , and PTFE insulated wires processed with Tri-Star Technologies M-100L-FG wire marking system . Well defined , legible prints stay intact even after extensive accelerated thermal aging .
Marking cross sections ( Fig . 3 ) confirm that darkening zone extends 10-20um under the surface ensuring that marking cannot be altered or removed without physically destroying the top layer of the insulation .
The question is how a light-colored polymer surface turns dark under laser exposure without burning or melting . The answer is magic substance called Titanium Dioxide ( TiO2 ). Luckily enough , this is a commonly used pigment that wire manufacturers use to make insulation look white or otherwise light-colored , such as gray , blue , green , yellow , pink , etc .
An optical band gap around 3.1 electron volts accounts for TiO2 ’ s intense absorption of UV radiation with wavelengths shorter than 380 nanometers . Irradiation with a UV laser permanently turns TiO2 particles from white to blue / black . The same effect occurs when those particles are embedded into a substrate . Ideally , laser radiation does not react with the base material and passes freely through the substrate surface . In contrast , the pigment particles within the substrate interact with the laser beam that modifies the particles ’ structure and appearance , including color . For example , thin PTFE films are practically transparent to the UV light while small (~ 0.3u ) TiO2 particles randomly distributed through the insulation layer strongly absorb the light and change color .
Fig . 4 illustrates the process in space and time . An incident laser beam penetrates freely through the first material layer losing a small fraction ( e . g ., 1 %) of its total energy on interaction with originally white TiO2 particles , turning them black . The same happens on the second layer and so on until most of the laser pulse energy is absorbed within the top 50-100 layers . In reality , the process is quite limited both in time and space as total pulse duration is normally below 30ns and marking depth does not exceed 50um or so .
Short nanosecond laser pulses prevent regular heat exchange between the additives and the surrounding material , limiting any structural and / or chemical modifications to the pigment particles themselves . Clearly this mark cannot be easily removed as most of it is distributed through the top layer but not on the very surface .
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