There are various methods of laser cleaning, which means the removal of unwanted substances (e.g. dirt, paints, oxidized layers, contaminants) from surfaces by applying intense laser light. It may be seen as a special application of laser ablation in a wide range of industries, including like manufacturing and maintenance operations.
Basic Principle of Laser Cleaning
Essentially, laser cleaning is laser ablation applied to some kind of unwanted substances on some workpiece. Usually, the hit material exhibits substantial absorption of the incident intense laser light. The highly localized conversion of optical energy to heat leads to a very rapid temperature rise and consequently to evaporation of the material – possibly also involving partial or complete chemical destruction of the material. The strong temperature gradient resulting from the localized heating creates substantial mechanical stress which often supports the removal process.
The laser radiation often needs to be tightly focused to the surface of the target area, because the required ablation process works only above a certain intensity threshold. Cleaning may be possible with relatively low average power, but then requires tight focusing, which in turn leads to longer processing times. If a higher laser power is available, one may use a beam shape which is much elongated in one direction in order to clean a correspondingly wider stripe in one go. Often, the laser beam is systematically moved over the surface to be cleaned, covering the relevant area with a suitable pattern.
In many cases, one exploits substantially stronger absorption in the material to be removed, compared with the less absorbing substrate material which is supposed to remain. One may then exceed the intensity threshold for ablation of the dirt while not reaching it for the substrate material. It can further be helpful that the substrate material is often more robust in other ways, e.g. concerning hardness or thermal conductivity.
Sometimes, additional selectivity may be introduced by so tightly focusing a laser beam that the intensity rapidly decreases for longitudinal positions behind the beam focus. Shallow beam incidence can then also help.
Lasers for Cleaning
It is common to apply pulsed lasers with nanosecond or even shorter pulse durations for cleaning purposes, because compared with continuous-wave operation this leads to a lower heat load on the cleaned material (“cold ablation”). A high pulse repetition rate is usually wanted for a high enough processing speed. Pulsed fiber lasers and more conventional Q-switched solid-state lasers are often used. In some cases, one uses an excimer laser, because the ultraviolet light is particularly well absorbed in some materials.
Some continuous-wave high-power lasers are also used. For example, direct diode lasers are attractive because of their high wall-plug efficiency and relatively low installation cost, despite their very limited potential for pulse generation. CO2 lasers, partly in continuous-wave operation, are also still in use. One of their advantages is the lower risk for the eyes (e.g. if there is some stray light).
Features of Laser Cleaning Machines
Various kinds of laser cleaning machines are available for industrial applications. Some typical features:
- Some machines have a hand-held cleaning head, while others have automated (possibly robotic) machinery, e.g. with the processing head on computer-controlled positioning equipment.
- Usually, there is some suction device for removing ablated material and avoiding exposure of personnel to toxic fumes. For example, one may do this with a controlled air current in a suction tube. The laser beam itself may be applied through that tube.
- There are transportation mechanisms, or integration with conveyor belts for subsequently processing many pieces.
- Stationary machines often have a suitable enclosing, providing sufficient protection against the laser radiation for personnel standing nearby.
- Process diagnostics can be useful, e.g. based on a camera.
- The machines may be integrated with other production processes.
Typical Attractions and Limitations of Laser Cleaning
Although the advantages and limitations of laser cleaning can strongly depend on the circumstances, some typical advantages (e.g. in comparison to traditional techniques such as brushing, sandblasting, using chemical solvents, etc.) are the following:
- The above mentioned selective removal greatly reduces the risk of involuntarily removing some of the substrate material. There is no contamination with additional substances and no surface damage by abrasion. For such reasons, lasers allow the cleaning of even quite delicate surfaces.
- It may also be easier than with traditional cleaning methods to safely collect the ablated material through the extraction of fumes with the air. In other cleaning processes, removed material often contaminates the tools and may be deposited at other locations on the workpiece.
- The cleaning can be highly targeted, i.e., reliably applied to certain parts of workpieces while not affecting others. Also, the cleaning can be applied even to locations on workpieces which are difficult to access with other means.
- No consumables like fine sand, chemical solvents or brushes are required, and there is wear-off of used tools.
- Laser cleaning methods can often be well integrated into more complex automated fabrication processes.
- It is often emphasized that laser cleaning processes are more environmentally friendly compared with other cleaning methods. That depends substantially on the concrete circumstances, but various factors such as avoiding the use of consumables, e.g. toxic solvents, and the more limited production of waste as well as the avoidance of intense noise can indeed be very helpful.
On the other hand, there are also typical limitations:
- Stronger absorption of laser light by dirt compared with the substrate is not always given. While that often works well with metals, polymer parts, for example, are often more difficult to clean with lasers.
- The required laser cleaning equipment is relatively expensive. That disadvantage may be offset by lower operation cost, however.
- Lasers tend to be less robust and long-lived, compared with other cleaning tools. While the service intervals may be relatively long, occasionally required maintenance or repairs may be expensive.
- The often required high laser power leads to challenges of laser safety, which may need to be tackled with additional measures. For example, the personnel may need to wear laser protection goggles, which unfortunately may make it more difficult to see fine details while also not always being very comfortable.
- Also, there is some energy consumption – often higher than with mechanical cleaning methods, although usually not at a really problematic level.
In some disciplines, the results of comparisons with other cleaning technologies can vary a lot:
- The processing speed may be high or low, very much depending on the circumstances, including the available laser power. A single path of the cleaning head over the surface is normally sufficient, but the cleaning head may need to be moved quite slowly.
- The operation cost also depends too much on the circumstances to make general statements. While the cost of electricity consumption is normally moderate, there may be substantial maintenance cost for a laser system and the beam delivery system. Operation costs of other methods also vary a lot.
Applications of Laser Cleaning
Laser cleaning is used under quite different circumstances; the most important laser cleaning applications are explained in the following.
Cleaning of Industrial Workpieces in Factories
In numerous industrial fabrication processes, workpieces consisting of some metal (steel, cast iron, brass etc.), glass or ceramic material, for example, need to be freed of unwanted layers of substances like a paint or other coating, an oil film, an oxidized layer (e.g. rust) or a possibly not well defined kind of dirt. Such tasks occur in a wide range of industries, including automotive manufacturing, electronics, medical technology, aerospace and defense, plastics manufacturing and power plants. Depending on the circumstances, hand-held or highly automated cleaning machines are used.
Gas and Oil Pipelines
The inner sides of gas and oil pipelines can be affected by various kinds of depositions, which occasionally need to be removed. This is done with automated cleaning robots, which may apply different techniques, including laser cleaning. Here, a specific advantage of laser cleaning can be that no consumables are required, which would possibly need to be supplied at remote locations and transported over a long length of pipeline; only the required electrical energy needs to be provided.
Aerospace and Ships
The bodies and various special parts of ships, for example propellers, need to be regularly cleaned, because they are strongly affected by the deposition of various substances from the water. Here, powerful laser cleaning machines can help to get such work done effectively and quickly.
Similar tasks exist for airplanes, where large surface areas and various special parts need to be kept reasonably clean. Cleaning operations may be combined with the search for material defects.
Apart from maintenance, many production steps for airplane or ship parts require cleaning processes.
Preparation of Sensitive Processes
Various industrial processes, such as bonding processes, the application of coatings or semiconductor lithography, work reliably only with carefully cleaned surfaces. Here, laser cleaning can often provide a good solution.
Restoration of Artworks
Valuable ancient artworks (e.g. outdoor statues, monuments, stoneworks) are often substantially affected by air pollution, for example by deposition of dust or soot particles. It can be challenging to remove such dirt without damaging the work. Here, both the highly targeted application – e.g. only to specific points under careful visual control – and the selective absorption (e.g. of dark dirt particles on a lighter background) can be very helpful.
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