Final Report: Investigation of Diode Lasers for On-Aircraft Decoating ApplicationsEPA Contract Number: 68D00243
Title: Investigation of Diode Lasers for On-Aircraft Decoating Applications
Investigators: Lancaster, Frederick A.
Small Business: LANCORP Advanced Systems Inc.
Project Period: September 1, 2000 through March 1, 2001
Project Amount: $68,769
RFA: Small Business Innovation Research (SBIR) - Phase I (2000) RFA Text | Recipients Lists
Research Category: SBIR - Pollution Prevention , Pollution Prevention/Sustainable Development , Small Business Innovation Research (SBIR)
Description:The objective of this project was to demonstrate and determine the application of LANCORP's laser decoating system by combining it with an easy to use robot "crawler". The laser system is based upon a long wave pulsed semiconductor incoherent diode laser (diode laser) that enable used to remove coatings and soils from metallic and non-metallic substrates and is able to conduct selective coatings removal. The diode laser stripping system is powerful, efficient, robust, closed cycle, and affordable, easily maintainable by current field or maintenance personnel. The manipulation system is a commercially available robotic crawler that has been tested on aircraft fuselages for carrying non-destructive test equipment to automatically analyze the surface and is able to manipulate the laser over 90% of the aircraft. The system is comprised of mostly commercially available components for easy maintenance that are integrated in a proprietary manner and combined with proprietary software to provide a solution allowing for easy upgrading and modifications in order to be tailored to a wide range of specific needs.
Using high power incoherent laser diodes, LANCORP's laser stripping process allows for precision industrial applications such as the removal of paint, adhesives, powdercoat, electrocoat, and organic coatings from aluminum, steel, plastic, fiberglass, wood, and composite substrates. Parts to be stripped can be of various sizes and have substrate thickness from thick to extremely thin. There is minimal heating, averaging only approximately 150 degrees Fahrenheit at the surface, with no sub-surface heating. The process can also be refined by adjusting to lower powers for the precision cleaning of organic matter such as grease, carbon, rubber or plastic from surfaces such as mold dies, or for marking surfaces. The system includes specialized beam rastering of the laser and optical feedback to apply the laser energy uniformly and rapidly across the stripping path and controlling the depth of strip. The unique design of the laser beam rastering mated with an advanced high speed spectral feedback system optimizes the rate of coating removal while virtually eliminating the potential for surface/substrate damage.
Decoating tests were performed on aerospace specification approved coated test panels, and effluent and waste stream sampling was performed on an actual waste stream during testing. This testing built upon previous testing conducted with Naval Air Warfare Center Pax River SBIR for a hand held laser by expanding into large area removal. A commercially available robotic mobile crawler was adapted and tested to carry the LANCORP designed laser workhead. The result of this was a specific design for full-scale mobile system based upon a previous LANCORP's concept for a mobile laser depainting and cleaning system for aircraft fuselages and surfaces. LANCORP demonstrated and verified the unique results that diode lasers have to offer aerospace structures decoating such as the ability of total and partial stripping of organic coatings from metallic, composite and fiberglass surfaces. This technology did show the waste to be totally recoverable and overall operate in an efficient and cost-effective manner over other types of lasers and methods.
LANCORP Advanced Systems Incorporated (LANCORP ASI) performed evaluation testing for diode laser technology to evaluate the operational performance of this type of laser for aircraft coatings removal. This testing was performed on the same coatings as found on commercial and military aircraft currently in use in the fleets. Also, a robotic manipulation device, the commercially available AVRI Autocrawler was tested as the vehicle to manipulate the laser system across aircraft surfaces.
The screening testing and was performed using a 2-kW long pulsed semiconductor diode (diode) laser at 808nm in wavelength in a small robotic workcell. The panels used for the testing were the standard .035 inch and .025 inch aircraft aluminum blanks, 14 and 16 ply graphite epoxy composite panels, and fiberglass with the epoxy/polyurethane coating system applied in nominal thickness of 3 mils up to 8 mils of coating as well as panels coated with powdercoat and some applique. One of each color or type of panel was drilled in various spots to provide further deeper surface contact of the thermocouple in the rear of the plate. In addition to data on the laser set up, test data along the guidelines of SAE MA4872 were collected for each test panel, including: coating type, adhesion, thickness before, thickness after each pass, visual inspection/observations, temperature, hardness, and stripping rate. The testing performed for hardness did not show any degrading trends or changes to the substrate. The diode laser effectively removed all of the coatings from the test panels at high strip rates up to 200 square feet per hour removal for 1-1.5 mils of coating, with heat input of 150 degrees Fahrenheit on average. The process was also controllable to provide selective coatings removal.
Environmental testing was performed by an independent laboratory that determined the makeup of the gaseous and particle effluent, showing that the effluent could be filtered by conventional means for distribution of the air into a shop environment. The major finding was the reduction in waste to current methods is estimated to being greater than 90 percent.
Based on the evaluation testing pulsed high power diode laser stripping coupled with the AVRI Autocrawler shows that it has the ability to strip and clean sprayed organic coatings, powder coatings, appliques and soils in an efficient and cost-effective manner when compared to plastic media blasting and is comparable to other laser stripping methods for aircraft fuselages. This report will highlight the data collected from this round of testing.
The test matrix was designed around SAE MA 4872, which is a recognized aerospace specification. The test matrix was created by LANCORP technical staff and reviewed by an outside technical team consisting of US Airways personnel. The exception taken in this is that for artificial aging a one-week air cure and one week elevated cure at 150 degrees Fahrenheit was used instead of the cyclic aging specified in the SAE specification. This was in part to earlier observations that weathered coatings and fresh coatings behaved the same during removal, being "milled" off the surface. While there is an apparent difference to aged and unaged coatings when using chemicals or abrasive media, this is not evident with laser decoating. The process parameters for the laser decoating were based upon past efforts and derived parameters. The laser was set at 2 kilowatts, pulsing at both 6000 Hertz and in some cases 2000 Hertz, and the beam was placed at a focal distance of 3.5 inches focused to a flat spot 5/8- inches long and 1/8 -inches wide. The traverse rates for the testing ranged from 99 feet per minute (33m/min), 66 feet per minute (20 m/min), 22 feet per minute (10 m/min), and in a few instances 1l feet per minute (5m/min). Panels were checked for gross physical changes by measuring the conductivity and hardness before and after, as well as checking the surface roughness on parts stripped to primer to check the post-processing surface. The temperature of the panels were checked during processing using K-Type thermocouples imbedded in the surface as well as infrared pyrometers focused on the surface and the back of the panels.
Results: Removal of Epoxy Primer/Polyurethane Coating System from Aircraft
The diode laser removal process proved to be a quick and efficient in removing both commercial and military standard epoxy and polyurethane coating systems from aluminum composite and aircraft skin material. The removal rates could be varied based upon the traverse rate, removing approximately 1.5 to 2.0 mils of coating at 30 meters per minute (99ft/min) for a removal rate of approximately 210 square feet per hour, down to removing approximately 5 mils per pass at 5 meters per minute (33ft/min) for a removal rate of approximately 35 square feet per hour - for 5 mils. These rates along with keeping the power settings at 2 kilowatts and the pulse rates varying only slightly for dark or lighter colors, proved to be the same for composite or aluminum substrates. The only difference was that if the laser went past the primer on the composite, damage would result; concluding that only removing to the primer is acceptable on graphite epoxy composites. Fiberglass did not damage as graphite epoxy composite did when the primer was removed exposing the substrate.
The significant data achieved was the data on the thermal intrusion. Unlike other uses of lasers such as welding that induce heat into a part, the rapid pulsing only induces an instantaneous amount of heat into the surface, and from the thermocouple data, very low and very quickly. By using our specifically derived process parameters, consistent heat input below 160 degrees Fahrenheit for composite and aluminum surfaces and below 100 Fahrenheit for thicker metallic surfaces.
The composite panels that were stripped exhibited the same thermal characteristics as the aluminum panels. The general rule is to keep the temperature below 200 F in order to prevent damage to the resins (cooking the resin) in the composite. Being that the composite material is dark, black, it is a very good absorber of energy. The composite panels were processed and the temperature was well below 170 degrees F.
The effluent, both gaseous and particulate was tested over the duration of this testing using EPA approved methods for quantifying unknown sources. The effluent gasses were sampled in the vacuum hose and sent to the analyzer where carbon dioxide, carbon monoxide, and nitrous oxide (NO2) were given off during the ablation process, and were not significant. The particulate ash was also collected and tested then scanned for metals. This again only showed where some spikes, above OSHA and NIOSH limits for exposure, in chromium a small amount of cadmium and barium were detected, but all attributed to the metals and compounds of metals found in the original coating, with nothing contributed to the effluent waste stream by the process. As a result, no items were detected that could not be filtered by HEPA and conventional particulate filtration methods.
During this testing we calculated the average energy consumption of a 2-kilowatt laser workcell and found it to be approximately Four Dollars per hour to operate. Since most of the cell runs on electricity, with very little shop air usage, energy consumption is related to kilowatts used per hour. An average electricity price was used for this determination. The system is very efficient on energy, and when compared to other methods that employ large air exhausters and compressors or other types of lasers, diode is about 10 times less in cost to operate, and energy consumption. These calculations do not take into consideration the cost avoided by not having to dedicate a specific facility for depainting operations.
The mock up laser was placed on the robotic unit an maneuvered in various configuration with the following observations that this system meets the requirements of being;
Compact - Necessary to allow the laser head the space needed makes it accessible to up to 90% of the aircraft and under it without modification to the aircraft hanger.
Simple - The robotics are not complex, the programming is easy.
Capable - The Autocrawler can carry the weight of the laser and the control lines, even on the underside of the aircraft and has a +/-.005 inch accuracy for repeatability.
Expandable - As and option, the unit can be mated with a laser tracker to surface map the aircraft, then store that particular aircraft into a program library, where it can be recalled based upon the tail number, and retain a +/- .005 inch accuracy.
Overall the crawler proved to be the best option and a simple robotics solution to carry the laser and feedback system across an aircraft. The system was not a production unit, but little effort would have to be taken in order to make these into production units, including general aesthetics and refining the software and graphical user interface, for a low cost (<$35K) robot.