Components may be contaminated with traces of drawing or cooling lubricant and preservative oils utilized during manufacturing. Such – often invisible – residues severely detract from the quality of subsequent energetic joining or adhesive bonding processes. Particularly in the highly automated manufacturing environment of the automotive industry and its suppliers, the standards imposed on the quality and reliability of components are constantly rising – to such an extent that testing for surface contaminants is now an indispensable part of the production process.
Traditional cleaning methods are time-consuming, cannot be automated, and are often harmful to the environment. And there is no reliable means of measuring their effectiveness. Uniform procedures for inspecting the particulate contamination of functionally relevant automotive parts have already been established in standards such as VDA 19 and the corresponding ISO guidelines. Greater interest in fluid contamination is a more recent development that has allowed laser cleaning to move into the spotlight. Pulsed lasers swiftly vaporize residual fluids within a targeted area, with measurable, reproducible results, in an environmentally friendly process that lends itself to automation.
Cleanliness improves reliability
In the production of gearwheel assemblies, laser welding is the established method of choice due to the low energy input required, thus limiting distortion. The numerous components in an automotive powertrain – all subject to the strictest quality standards – are made of different materials and alloys, which it is difficult or sometimes even impossible to weld using conventional techniques.
Strong laser – lower cycle
Carmakers need a rapid, automated way to completely rid joining surfaces of unwanted residues in order to obtain strong, pore-free welded and adhesive joints without micro-fissures. Laser cleaning systems meet all of these requirements, particularly those with pulsed solid-state lasers. The main factors determining the choice of laser are the desired degree of cleanliness and the cycle time available for cleaning the components.
Swift and to the point
In highly automated, high-volume production processes, time is of the essence. Unlike traditional cleaning methods that involve immersion of the entire component in a solvent, or the use of abrasive or thermal-shock techniques, laser cleaning focuses on defined functional surfaces such as weld joints and adhesive bonds – often measuring no more than a few square millimeters.
The time required for cleaning depends on the time needed for laser welding, which averages between four and ten seconds. Another time-saving advantage of selective cleaning is that it eliminates the intermediate step of transporting components from the cleaning station to the production line. The cleaning laser is integrated in the production line – preferably directly upstream of the welding or adhesive bonding station.
Cleaning around corners
To clean the defined surfaces, scanner optics move the laser beam rapidly back and forth across the workpiece. Different scanning patterns and rates can be employed, depending on the desired cleaning result. A linear scanning motion can be used to clean minor contamination; if a higher degree of cleanliness is required, the laser beam is applied in an overlapping circular pattern – similar to the motion of an electric toothbrush. In this case, the pulses of laser light pass over the same spot several times, thus intensifying the cleaning action.
The laser can also “clean around corners” without it being necessary to reposition the workpiece. Mirrors are positioned to direct the laser beam in such a way as to clean both the upper and lower surfaces of a tubular or profiled component simultaneously.
Only where necessary
Lasers are capable of cleaning even the most sensitive surfaces, such as aluminum or carbon-fiber composites, or coated parts, without damaging the underlying material. Alternatively, by choosing a different set of parameters, the same laser can roughen a surface to improve the strength of an adhesive bond. The flexibility of laser tools allows them to be adapted to each task as required. TRUMPF determines the laser power and pulse frequency suitable for each application, based on the principle of “no more than necessary and as little as possible.” Customers benefit from this approach, as laser cleaning is around five times cheaper than equivalent wet chemical cleaning processes.
Clean enough or not?
Standards and guidelines have already been established for particulate contamination inspection procedures, and sooner or later it can be expected that they will be extended to include fluid contamination. So it is not merely a question of which cleaning methods to apply but also of verifying and documenting the results. This is what prompted TRUMPF to team up with SITA Messtechnik GmbH
SITA Messtechnik GmbH
SITA Messtechnik GmbH develops, produces and sells devices for measuring the dynamic surface tension of liquids, fully automatic foam tester for analyzing the foaming behavior of liquids as well as fluorescence measuring technique for controlling the cleanliness of parts and for monitoring the contamination level in process liquids.
The company based in Dresden develops, produces and sells process measurement and control devices for applications including monitoring component cleanliness. SITA has developed a fluorescence measuring device that can be used to determine the operating parameters of laser cleaning systems according to the task in hand. By shining UV light on the component, it reveals traces of fluorescent organic substances such as lubricants, oils, cooling agents or solvents. A non-contact sensor measures the level of residual contamination – or provides confirmation that the cleaned surface is perfectly residue-free. Unlike conventional inspection methods based on the use of test inks, for example, the SITA device provides precise results for even the smallest test surfaces.
The clean alternative
“Clean” applies not only to the results of the laser process but also to the process itself. Unlike wet chemical processes, it doesn’t involve the use of toxic solvents, and it avoids the noise associated with abrasive methods. What’s more, laser cleaning is gentle, relatively economical, and distinctly faster than alternative methods. These advantages have already been recognized by the car industry, and other industries are now following suit, given that the laser is a cleaning tool with multiple uses.
While pulsed lasers can be used for the selective cleaning of joining surfaces or small objects such as ancient coins, UV lasers are ideal for cleaning even large-area components. They can be used to remove fluid residues from CFRP body components or, by setting other parameters, help to remove stubborn layers of paint or rust. As a high-tech cleaning tool, the laser is rich in potential.