It is common knowledge that aircraft are frequently paint-stripped for inspection, maintenance, and non-destructive evaluation work. Moreover, military aircraft are paint-stripped and repainted for survivability (visual appearance, infrared signature), corrosion protection, appearance and durability. Aircraft maintainers have traditionally used hazardous chemicals and waste-generating processes to perform surface cleaning and coating removal. Currently, removing coatings requires not only the use of hazardous materials; it also generates large amounts of hazardous waste and poses a significant occupational health risk for workers.
Due to increasing restrictions and safety concerns regarding these processes the time is rapidly running out on the use of these conventional methods and environmentally acceptable alternative methods and approaches are urgently being investigated.
With regards to the decoating and decontamination of other surfaces, existing markets include the cleaning of high temperature vulcanized rubber molds, and the wire harness cleaning in the vehicle industry.
The use of CO2 lasers in the former application is especially favourable due to the excellent absorption characteristics of rubber in the CO2 lasers wavelength region. In addition a laser based system has several advantages over the current industry standard cleaning method in that the molds do not have to be cooled down and that the molds can be cleaned in place. This results in substantial savings due to decrease in down time.
Another application in this field is the cleaning of contaminated walls of buildings in the nuclear industry as well as the cleaning of contaminated components and storage containers used to store UF6 gas and other materials
Although the mechanism of paint removal is not known with absolute certainty at this time, it most likely occurs as discussed below.
- The laser pulse transforms the first few μm of the outermost layers of the paint into plasma.
- The plasma generates a shock wave that ejects the rest of the layers as fine particles and gaseous products
- The substrate is preserved by keeping energy density below the damage threshold.
Aircraft are increasingly being manufactured from composite materials, which must not be damaged during paint removal. Chemical stripping damages the composite substrate and industry is forced to remove the composite parts and clean them manually with abrasives. A laser based paint stripping process can be tailored to remove single layers of coating or paint leaving the underlying layers intact. This ability of the laser method allows it to de-coat composite materials and is a major distinguishing benefit and cost saver especially for the aircraft industry.
There are a number of salient properties of the CO2 laser that distinguish it from competitor laser systems in the field of laser based paint stripping:
- High paint absorption
- Low substrate absorption
- Temporal pulse shape
Most aircraft frames are manufactured from aluminum alloys for which the CO2 laser absorption is ~2%, Nd:YAG laser absorption is ~6%, and diode laser absorption is ~12%. The absorption figures for paint shows ~92% absorption for CO2 laser, ~11% for the Nd:YAG laser; and ~11% for the diode laser
The efficiency of the laser pulse to remove the paint layer is also determined by the ability of the laser to transfer its energy to the surface to optimize the removal process. This efficiency can be tailored by altering the energy in the pulse, the repetition rate, and most importantly the temporal profile. The CO2 laser amongst all other lasers lends itself to this tailoring and the PaR Systems sdilasersTM have been specifically designed to use these features.
The current competitive paint stripping processes employed in the industry is:
- Media Blast Systems
The fact that chemical stripping damages composite material substrates is a major drawback for the industry and limits its use to the removal of paint from metallic components. In addition it forces the industry to remove all composite parts before the stripping of an aircraft can commence. Never the less, it covers about 60% of the market.
As mentioned above, composite parts cannot be stripped chemically. Therefore the parts must be stripped and manually cleaned using abrasives. This leads to problems due to the human factor that causes damage to the composite parts during manual cleaning. Laser based paint stripping can save significant costs through not needing to remove the parts off the aircraft and eliminating the human factor. Abrasives are responsible for 30% of the paint stripping market.
The remaining 10% of the market has been cornered by media blast systems. However, these systems are expensive, create substantial amounts of waste, and are not suited for composite material cleaning.
The benefits of reducing waste and hazardous materials should be highlighted: reducing waste and finding an alternative to chemical and abrasive paint stripping is “smart business”. The following list highlights the benefits and cost advantage of cutting waste and switching to the laser method:
- Improved working environment
- Reduced regulatory compliance
- Savings on disposal costs
If the laser paint-stripping method is so “smart”, why then is it not currently used for industrial application? The answer is quite simple: lasers large enough to strip an entire aircraft have yet to be developed. What is needed for the rapid stripping of aircraft is >1,500Watt output, and this is what PaR Laser COE has developed.
At present our WH850 and WH1000 lasers are being used to strip smaller airframes and loose aircraft parts
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