paint-stripping damage

Transcription

1 New C02 laser cuts paint-stripping damage HBy James D. Head p-fhft)cm z(c-74-j/ fw he use of laser energy to remove Samples of airframe materials, including graphsurface material has been handi- ite fiber composites (see Photo 1) and aluminum (see capped by control problems with cut- Photo 21, were subjected to repeated painting and ting lasers, which have difficulty laser-cleaning cycles, with testinn to determine the variations in pulse duration and pilse frequency as well as pulse energy density have been studied. One version produced pulses of ms duration. Another version had an output of 1.2 Jlpulse at a rate of 8 pulse& with a pulse duration of 25 ~ s. The preferred version produces an output of 1.2 J/pulse at a variable rate of pulsesh. This equipment can produce pulses of controlled duration varying ps. Much of the data in this article results from studies performed under contract with the Materials Laboratory of Air Force Wright Aeronautical Laboratories. Previous work done by other Air Force contractors established the existence of an operating window between low levels of energy per pulse, which promotes charring or burning of the paint, and high levels of energy per pulse, which might induce substrate damage. In the LTI studies, the optimum fluence for practical paint stripping has been determined and designs have been prepared for a 1-2-kW system capable of stripping aircraft at rates of ft2/hr. Waste-collection and beam-delivery problems have also been resolved in sufficient detail to ensure that a practical system can be readily built. PHOTO 1. This graphitehpoxy nose cone-subjected to repeated painting and laser cleaning cycles-shows spots of selective paint removal. to provide statistical data to support application of this methodology. In addition, characterizations of gas byproducts were included to develop data for design of a byproduct collection-and-handling system. Substrate heating The temperatures reached by the surface of the substrate during the laser-cleaning process are very moderate. Heat-sensitive strips exposed to pulsed laser energy impinging directly on the heat-sensitive material showed temperatures of less than 190" F with a single pulse. During practical paint removal, the energy density of each pulse, the pulse duration, and the pulse James D. Head is president of Laser Technology Inc., Colonial industrial Dr., South Lyon, MI

2 frequency contribute to heat buildup in the substrate material while stripping. The accumulated heat is also dependent on the heat conductivity of the substrate. Temperature measurements while cleaning were determined by a thermocouple on the reverse side of PHOTO 2. Laser energy was used to remove paint from this aluminum plate. the sample. Aluminum subjected to sustained pulses at a rate of 100 pulsesh with an energy concentration of 7 J/cm' and a pulse duration of 16 ps exhibited a maximum temperature of 200" F on the reverse side of the in. sample. In practical applications the applied energy is largely absorbed and utilized in paint decomposition, resulting in very small temperature rise in the substrate. Once the paint film is removed, a greater portion of the energy is directed to the substrate, and the reflectivity, conductivity, and mass of the substrate determine the temperature rise. The heat buildup in the less thermally conductive graphite epoxy composite materials is only marginally greater than in the case of aluminum. Maximum temperatures at the base of the first ply of a composite sample during total removal of paint did not exceed the recommended temperatures for the heat baking step of the original paint application. Other damage investigations Some evidence of potential ultrasonic or shock damage was detected with the use of very short pulse lasers (less than 2 ps). No damage was detected in the more practical operating range of ps pulses. No evidence of electronics damage could be found. The pulsed laser did not cause static on a battery-operated radio. Removal of paint from the radio case and removal of identifying numbers on micro chips within the radio by use of pulsed laser energy did not cause loss of function. A strip of plastic was removed from a card-type calculator with no effect on functionality. An acoustic shock wave is generated with each pulse of a laser cleaning system. However, measurement of the noise level produced during laser cleaning showed this to be of no significant concern. Noise levels exceeded 90 db at the surface being cleaned, but three feet away from the surface the noise level had decreased to less than 80 db. Adhesion tests confirming the quality of laser cleaning suggest the process may enhance adhesion, compared to current standard prepaint cleaning procedures. More failures were observed in control samples than in samples cleaned by laser stripping. Corrosion tests involving exposure in a 5% salt spray for 500 and 800 hours confirm the ultimate useful integrity of anodized surfaces after laser cleaning. The laser-cleaned samples significantly outperformed control samples in corrosion and humidity tests. This data supports the conclusion that materials should be painted directly after laser cleaning with no intermediate washing or other cleanup procedures. The cleaned samples showed no loss of integrity of the anodized coatings when such coatings were present. The corrosion tests confirmed this observation. Substrate mechanical properties effects Tests to determine the tensile yield strength at 0.2% offset, ultimate tensile strength, elongation, and tensile modulus of elasticity were conducted to compare samples of aircraft aluminum subjected to four cycles of laser cleaning and repainting. No evidence of change in any properties was detected. Fatigue crack growth tests on in. bare aluminum alloy 2024-T3 after four cycles of cleaning with a laser system using 100 pulsesh of 20-ps duration with energy density on a sample of 20- J/cm2 pulse showed no change from control samples. Residual stress tests of the type used for measuring the effects of abrasive media blasting on aluminum showed no effect with laser cleaning. Tensile strength and modulus were determined for graphite epoxy composite samples cleaned with 8 pulsesh of 25-ps duration with energy density on a sample of 20-J/cm2 pulse. The tests indicate that laser cleaning did no damage. Four point, flexure tests were carried out on unidirectional graphite epoxy composite after four cycles of cleaning and repainting. Cleaning in each case proceeded to the point where approximately 10% of the primer layer was removed. The flexural tests showed a small increase in strength of the samples tested. The test indicates that laser cleaning did no damage. Compression tests showed an increase in strength of the five samples of unidirec-

3 tional graphite epoxy composite tested. The test indicates that laser cleaning did no damage. The optimum cleaning rates and control of the degree of cleaning will involve removal of a number of thin layers of the paint film rather than complete removal in a single laser pulse of higher energy concentration. The optimum paint removal rates are obtained with fluence of greater than 6- DIAGUI E@ FIBERS E LASER DELIVERY SYSTEMS Dose rate = 104 R/H (Room temp ] \ -, Radiation resistance fiber E /' Doped core fiber,4=088pm & E /' 0,I Comparison of ; 10- DlAGUlDE 0 - c Irradiation Open RADIATION v) FIBER 03 I Time (minutes) CONNECTORS MITSUBISHI CABLE AMERICA, INC, Diaguide" Products Division 520 MADISON AVENUE, NEW YORK, NEW YORK PHONE Outside NY State CIRCLE 7 ON READER INQUIRY CARD J/cm2/pulse. Beam intensities of less than 3- Jlcm'lpulse can cause paint charring. Practical cleaning will involve surfaces with a wide variety of configurations. Complex shapes offered no problems that could not be solved by manual manipulation of the laser delivery head. Turbine blades, fastener heads, and composite structures were successfully cleaned. On an 18-in. Pure Silica Core Construction results in lower attenuation, better performance under radiation and higher power tolerance. glass epoxy composite nose cone painted with primer and top coat, lasers removed all the paint but left the clean primer. Waste products The paint is incinerated during removal by pulsed laser energy. The resulting waste-disposal problems are reduced to that of collecting the ash from such incineration. This ash results from the presence of inorganic pigments and fillers in the paint film. The ash was readily removed from the working surface and collected on simple, commercially available filters. The total material so collected conformed closely with the independently determined ash content of the paint film. These solids have only traces of organic material, indicating that the organic content of the paint film is largely converted to CO? and water. The composition of the byproducts is dependent on air flow rates in the incineration zone. At very low flow rates, under 1 ftls, the residual plume of ash interferes with the next incoming pulse of laser energy and gives low paint-removal efficiency. At flow rates of 1-3 ftls, the paint removed undergoes complete combustion, giving a waste stream containing COz, water, and inorganic pigment ash. At much higher air flow rates, the incineration of the paint binder material is prematurely quenched, and the waste stream contains major amounts of potentially toxic, partially consumed paint break-down products. The total organic material removed and burned during laser cleaning with a full-scale 2-kW system will be substantially less than that consumed in the same time period by the average car..

4 PULSE SYSTEMS, INC. PRELIMINARY DESIGN SPECIFICATIONS: LP-PDQ LASER THE LP-PDQ I S A PULSED, FLOWING GAS. LOW PRESSURE, TRANSVERSE DISCHARGE C02 LASER. THERE I S NO "STANDARD" REPETITION RATE OR PULSE WIDTH. RATHER, THE LP-PDQ'S MODULAR DESIGN CAN BE UTILI7Fn ~ ~ ~ _ TO ACHIEVE A CLIENT'S PARTICULAR REQUIREMENTS. REPETITION RATES FROM 1 TO 250 HZ CAN BE OBTAINED WITH PULSE WIDTHS RANGTNG FROM IO TO 100 MICROSECONDS. IN ORDER TO MEET A SPECIFIC APPLICATION. THE LP-PDQ I S ENGINEERED FOR FLEXIBILITY AND CAN BE CONFIGURED AS A ONE OR TWO BEAM SYSTEM. DESIGN SPECIFICATIONS: LASER HEAD DIMENSIONS: 15" x 15" CROSS-SECTION BY 11" PER MODULE, PLUS AN ADDITIONAL 12" OF LENGTH FOR THE OPTICAL CAVITY. ELECTRONICS MODULE/POWER SUPPLY: 1.5 CUBIC FEET PER MODULE. APERTURE: 6CM x 6CM. GAS CONSUMPTION: APPROXIMATELY ONE-THIRD STANDARD CUBIC FOOT PER HOUR. REPETITION RATE: 0 T O 250 HERTZ PULSE WIDTH: 10 TO 100 MICROSECONDS ENERGY PEE PULSE: UP TO 20 JOULES, DEPEND APERTURE. COOLING REQUIREMENTS: WATER RECIRCULATION (NO CHI NG UPON THE NUMBER OF MODULES AND LING I S REQUIRED). Pulse Systems, Inc., 422 Connie Avenue, Los Alamos, NM /FAX:

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6 Will Arthur Sales Manager SEPTEMBER 11, 1991 pulse Systems, /nc. 422 Connie Avenue Los Alamos, NM /6 72- I926 MR. C.P. MORSE RESEARCH ENGINEER R & M ELECTRONICS I c/o POST OFFICE BOX I RALEIGH, NC DEAR MR. MORSE: WITH RESPECT TO YOUR REQUEST FOR INFORMATION PERTAINING TO OUR PATENTED LP-PDQ LASER SYSTEM, ENCLOSED PLEASE FIND PRODUCT INFORMATION AND TECHNICAL SPECIFICATIONS FOR YOUR REVIEW AND EVALUATION. T H I S UNIQUE C02 LASER UTILIZES PULSE SYSTEMS' DISTINCTIVE LOd VOLTAGE AND LOW PRESSURE DESIGN, COUPLED WITH ADVANCED SOLID STATE ELECTRONICS TO INSURE HIGH AVERAGE & HIGH PEAK POWER SYSTEM PERFORMANCE. I WILL BE CALLING YOU WITHIN THE WEEK TO CONFIRM THAT YOU HAVE RECEIVED THESE MATERIALS AND TO DISCUSS YOUR SPECIFIC APPLICATION I N GREATER DETAIL. HOWEVER, I F YOU SHOULD HAVE ANY IMMEDIATE QUESTIONS, PLEASE FEEL FREE TO CONTACT ME DIRECTLY. WE CERTAINLY APPRECIATE YOUR INTEREST I N PULSE SYSTEMS AND I REMAIN.. RESPECTFULLY YOURS, LJL. /4$L$&k-.T WILLIAM L. ARTHUR SALES MANAGER WLAENH ENCL. Pulse Systems, Inc., 422 Connie Avenue, Los Alamos, NM /FAX: