Stan Ream
New polygon scanner doubles paint stripping efficiency
Paint is everywhere in our lives, and it has a multitude of uses. We paint surfaces for their protection and for their appearance. We paint objects to make them more obvious or less obvious. And, of course, paint can become art. But paint doesn’t last forever. It can peel, erode, flake, crack, or become unwanted for a variety of reasons. In many cases, the removal of old paint is a critical step in the long term preservation of infrastructure assets, such as bridges, storage tanks, rail cars, etc. Sometimes we just want a new color on our painted surfaces. Imagine, for instance, the paint removal challenge that an airline faces when it acquires a competitor’s fleet of aircraft.
FIGURE 1. The substantial range of peak power and interaction times over which laser paint stripping has been accomplished.
Paint removal can be accomplished in variety of ways, but the dominant two methods are media blasting and chemical removal. Sand is certainly the most common media for the blasting solution, and painted steel surfaces are the most common application for it. We’ve all seen the tents around bridges during repair and repainting. For softer substrates, such as aluminum, media blasting is carried out with plastic beads, walnut shells, starch, or even CO2 pellets. Alternately, chemical paint stripping is widely used on aircraft and off-aircraft components. The chemicals used for paint stripping have been improved over the years, but they remain toxic and hazardous in various degrees. Additionally, media blasting and chemical paint removal techniques both multiply the amount of hazardous waste that must (or should) be managed, and they present a variety of worker hazards. For these reasons, some aircraft may be flown out of the U.S. to conduct paint removal in more environmentally lenient countries, where worker safety may not be as high a priority.
Alternative paint removal techniques
The Air Force has been pursuing alternative paint removal techniques for decades. Their needs are strategic, and their aircraft present some unique paint removal “business cases”. Thus, the Air Force has been the driving force in the investigation and development of laser paint stripping technologies. A number of Air Force projects from the 1980s demonstrated the potential of laser paint stripping and identified the implementation challenges to come. As lasers became more “industrialized,” the development of laser paint stripping technologies continued, resulting in several installations of laser de-painting facilities at Air Force bases. The current Air Force laser paint stripping installations serve as demonstration and production facilities, even as the fundamental paint stripping technologies continue to evolve.
Most of the early, large area, laser paint stripping development was carried out with CO2 lasers of one type or another. Continuous wave, TEA laser, and e-beam pulsed lasers were among those evaluated. The far-infrared wavelength of these lasers is attractive from the standpoints of absorption by the paint and substrate damage resistance, but the beam delivery complexity of CO2 lasers encumbered some of the potential applications. Nevertheless, the multi-kilowatt power capability of these CO2 lasers established attractive benchmarks for paint removal rates and efficiencies. So, when robust, multi-kilowatt, fiber-delivered (1.06-1.07 micron), laser power became available in recent years, additional research was undertaken to evaluate this candidate wavelength regime.
The physical mechanisms of laser paint stripping have been described in a number of ways, including vaporization, ablation, combustion, multi-photo absorption, shock removal, etc. FIGURE 1 shows the substantial range of peak power and interaction times over which laser paint stripping has been accomplished. The “bottom line” here is that there are many physical mechanisms/interactions that can be applied, but some are easier to implement and more affordable than others. Regardless of the mechanism, one fundamentally important requirement is that the laser power be delivered in an intense, short period, in order that the delivered energy remains primarily contained in the removed paint and not transmitted or conducted to the substrate. This requirement can be fulfilled with a pulsed beam or with a rapidly scanning beam, both of which can successfully limit the local interaction time of the beam with the work.Considering that continuous wave laser power is usually more powerful, affordable, and robust than pulsed laser power, it is not surprising that the use of a beam scanner to produce the required, short interaction time with the work is an attractive solution. Indeed, this is the solution that the Air Force and others have been pursuing in the last few years. Galvo and servo-motor-driven scanners have both been evaluated for this purpose, but the former has achieved the greater success. Of course galvo scanners have achieved their greatest success in the low power marking applications, but galvos face some significant limitations in the multi-kilowatt regime, where laser paint stripping is most attractive. As the laser power increases, the galvo scanning mirrors become heavier, and the scanning speed and acceleration decrease. Typical maximum scanning speed for continuous, high-power, large area, galvo scanners is in the 10 m/s range, which results in a longer-than-optimal interaction time with the work surface. High power galvos also tend to be heavy and require long focal lengths to accomplish required scan widths. For these reasons and others, the Edison Welding Institute (EWI) and Craig Walters Associates undertook a joint project to develop a polygon scanner for high power laser paint stripping.
Polygon scanners
Polygon scanners had been investigated for laser paint stripping as early as 1986, but very little attention or improvement effort had been devoted to the technology until the recent EWI effort began. The fundamental elements of the patent-pending EWI scanner design are shown in FIGURE 2. The specific deployment shown here utilizes a fiber-delivered beam, but alternate solutions have been designed for CO2 laser input. The EWI scanner has only one moving part, the polygon itself, which rotates at a constant velocity and produces a unidirectional, essentially constant velocity path on the work surface. With only a modest rotational speed, the polygon scanner can produce a surface scanning velocity exceeding 50 m/s. This high scanning speed permits a short interaction time of the beam with the work surface and allows very high laser power to be utilized. EWI’s polygon scanner (FIGURE 3) has performed a multitude of paint stripping trials using 10kW of fiber laser power, and higher power testing is underway.
Large area paint stripping results with the EWI scanner have exceeded expectations. Comparisons of “normalized stripping rate” with previously reported, benchmark, laser paint stripping efforts (FIGURE 4) clearly illustrate this point. The metric here (normalized stripping rate) is essentially a measure of laser paint stripping process efficiency; specifically it is the volume of paint removed per amount of energy delivered. Considering further that the EWI scanner has applied the highest and most efficient laser power to date for paint stripping purposes, this advancement is indeed remarkable. The net result of this total applied laser power and the improved paint stripping process efficiency is that the EWI polygon laser scanner can remove paint nearly three times faster than any other reported laser paint stripping technology.
Additional serendipity was realized in the daunting area of effluent removal. As it turns out, at the high scanning speeds available with the polygon scanner, the incremental effluent evolution during each scan is able to be swept away with a modest, vacuum-induced air flow. This had been an area of considerable concern, since other programs using galvo scanners for high power paint stripping had reported significant effluent capture issues. Not only is the effluent removal highly manageable with the EWI scanner, but the resulting solids in the effluent appear to be completely “dry” rather than the sticky agglomeration that others have reported. Much study remains to be performed in the overall effluent management area, but it is reasonable to conclude that this version of laser paint stripping produces an absolute minimum of solid waste.
FIGURE 3. EWI’s polygon scanner has performed a multitude of paint stripping trials using 10kW of fiber laser power, and higher power testing is underway.
The benefits and advantages of the EWI polygon scanner technology for large area laser paint stripping are numerous and worth summarizing.
- Highest paint stripping power capability
- Can be used with multiple laser wavelengths
- Smaller, lighter, and more robust than other scanners
- Can be provided in a hand-held version
- Aero window eliminates need for consumable transmitting windows
- Highest reported laser paint stripping process efficiency
- Highest reported laser paint removal rates
- Facilitates efficient, complete, effluent removal
- Facilitates good sensor access for process control
In the overall scheme of a potential laser paint stripping facility, the scanner itself may be a small component. Still, the above advancements in the core laser paint stripping process technology are essential for the creation of stronger business cases for specific applications. For instance, the higher stripping efficiency and the lighter weight of the polygon scanner mean that the motion systems required for large paint stripping jobs (airplanes, ships, etc.) can be faster and lower cost. And, given the higher effective paint stripping rates of the polygon scanner, the overall productivity of a paint stripping facility can be higher. For these and other reasons, this enabling piece of laser paint stripping technology substantially enhances the business cases for a multitude of potential applications.
Additional development needed
As encouraging as these results are, several areas of required, additional development remain to be satisfied. Most important among them is the need for development of a process control technology. Specifically, it is essential that the laser scanner “system” be capable of monitoring and controlling the laser paint stripping process so that the right amount of paint is removed from the right location. Many solutions for this control requirement have been conceived; some have been patented; and some have been applied. Candidate control solutions for application to EWI’s polygon scanner technology are under investigation, and success in this area is considered to be attainable in the near future. Continuing advancements in sensors, cameras, and computing power make this essential task much simpler and more affordable than in the past.
FIGURE 4. Comparisons of “normalized stripping rate” with previously reported, benchmark, laser paint stripping efforts.
In summary, it is reasonable to conclude that all the required elements for successful, industrial, laser paint stripping application have been developed or are within our reach. Much work remains to be done, but the path forward is clearer today than ever before.
Acknowledgments
The Edison Welding Institute (EWI; www.ewi.org) gratefully acknowledges the contributions and collaboration with Dr. Craig Walters, co-developer of the EWI polygon scanner and widely-recognized leading expert in laser paint stripping technology, who can be reached at [email protected]. EWI also acknowledges the contributions and collaboration with Wayne Trail Technology (Bob Lewinski at [email protected]) and II-VI Incorporated in the production of EWI’s prototype scanner. Finally, EWI is grateful for the financial assistance from the Air Force (through CTC) in support of the initial demonstration of EWI’s polygon scanner technology.
Stan Ream is laser technology leader at the Edison Welding Institute, Columbus, OH, and can be reached at [email protected].