The Fraunhofer Institute for Laser Technology ILT has been developing a new cleaning technique utilizing laser radiation to avoid secondary waste. This new challenging technique is environmentally friendly and opens up advantages in precision and speed. Using the laser cleaning method, large objects such as bridges and aircraft can be cleaned and prepared for protective coating.
The cleaning or removal is based on locally confined interaction of pulsed or continuous laser radiation with the surface layer. Depending on the laser wavelength, the intensity and the interaction time of the laser radiation with the surface layer, as well as the material properties, a thermal decomposition (e.g. evaporation, sublimation) or chemical reaction occurs.
Fraunhofer ILT has a large range of laser sources, which operate in pulsed mode at pulse durations from fs to ns as well as in continuous wave (cw) mode. The output power of the sources ranges from some W to several kW. The wavelengths of the lasers used range from UV to IR. If there are no suitable commercial laser sources available, tailored laser sources with adapted characteristics are developed and qualified for the application by Fraunhofer ILT.
Depending on the geometry of the parts which have to be cleaned, the lasers are combined with standard processing and handling systems and/or very fast scanner systems, with processing speeds up to several m/s. Mobile systems have been developed specially for cleaning fixed objects (e.g. bridges, large steel constructions). If there are no standard beam deflecting and forming systems available, we can develop specially designed optics and beam deflecting systems, e.g. transformation optics for the generation of tailored beam shapes and high speed scanners with scanning speeds of several 10 m/s.
The resulting emissions are captured by an integrated suction system. If necessary, special-purpose suction nozzles can be developed to collect all emissions directly at the processing point.
Cleaning and removal can be applied to a wide range of combinations between surface layers and base materials. The technique can be used for large-scale cleaning as well as for cleaning locally well defined geometries down to the micro range.
If high cleaning rates are required, high-power Nd:YAG and CO2 lasers (in the kW range) are used. Examples of these applications include the removal of carbon-black-filled elastomer layers from a colored elastomer base material, the cleaning of printing screens and the removal of epoxy coating powder layers from conductor rails. With laser power in the kW range, typical cleaning rates for layer thicknesses of several 100 µm reach several cm2/s.
Examples of mobile applications include cleaning tools in assembled, hot condition and cleaning old weather-worn steel constructions (e.g. bridges, high-voltage pylons, large tanks and ships) before applying new corrosion-prevention coatings. The work is carried out with specially developed manually guided processing heads. Further mobile applications include cleaning airplanes and high-speed cleaning of rail heads, for which fiber-coupled laser sources working in pulsed mode with pulse durations of about 100 ns and average output powers of up to 1500 W are used.
Other applications include the removal of oil and grease as a pre-treatment before joining processes (e.g. welding, soldering and adhesive bonding), the descaling of metal sheets and the cleaning of printing rolls.
In the micro range the technique is used for the structured removal of lacquer layers before selective electroplating and oxide removal from copper as preparation for tin coating. Experience with laser cleaning in combination with new developments of laser sources and suitable system technology shows that high growth potential exists for this promising cleaning technique.
Fraunhofer Institute for Laser Technology ILT
