A brief introduction to laser cleaning, the technologies involved, and the advantages it offers over other cleaning methods
What are the advantages of laser cleaning?
Laser cleaning is a quiet, non-contact, non-abrasive, efficient, cost-effective, repeatable, high-precision and eco-friendly process that can be used to remove unwanted materials such as dirt, rust, paint, oil, oxides and other contaminants/impurities from surfaces.
The process, which can be used for micro-scale or large-scale cleaning of a number of materials, offers an attractive alternative to traditional industrial cleaning methods.
These can often be labour-intensive or require the use of consumables such as sand (sand-blasting techniques can also damage the underlying substrate) or hazardous chemical products/solvents (which then require costly disposal) depending on the material being removed. Laser cleaning on the other hand requires no consumables, only power, making it more environmentally friendly and cost-effective than such traditional methods.
How does laser cleaning work?
At its core laser cleaning involves an optical scanner being used to move a focused laser beam over a surface. Here the beam interacts with any unwanted material, which absorbs the energy and rises rapidly in temperature until its ablation threshold is reached – the point at which its molecular bonds break. This causes particulates of the material to be ejected away from the substrate, or for the material to be vapourised entirely.
The parameters of the laser beam – namely its power, wavelength, repetition rate, scanning speed and beam diameter – are carefully selected so that only the ablation threshold of the unwanted material is reached, rather than that of the substrate itself. This targeting of the right ablation threshold, combined with the fact that laser cleaning can be controlled with micrometre levels of precision, ensures that the underlying substrate is left completely free from damage.
The process can be conducted either in person, using a handheld system to move the scanning beam over the workpiece, or remotely, delivering the beam to a laser head via an optical fibre, which can then be moved over the workpiece automatically using robots within a safe enclosure.
To ensure user safety during laser cleaning, a suitable fume extraction system must be positioned next to the process to ensure that levels of harmful particulates that exceed the limits outlined by governments are not released into the air. Similarly, the user must also be protected in such a way that they are not being exposed to laser radiation levels that exceed the limits outlined by their government. The simplest, most effective way of ensuring user safety during laser cleaning is to have the application take place remotely, within a strictly controlled laser safety enclosure.
What substrates and contaminants can be cleaned using a laser?
Metals
Rust removal is one of the most common uses of laser cleaning, often done to ensure a clean, uncontaminted surface prior to welding or bonding applications. Rust removal is also performed to to restore metal products or structures, for example those with historic significance. The most common metals that can rust include iron, cast iron, wrought iron and steel.
For metals that don’t contain iron – and therefore do not rust – laser cleaning can be used on those that instead develop an unwanted oxide coating. Such coatings can influence the integrity of a product, and are a sign that its surface material is breaking down. However, removing the oxidation layer could expose it to further deterioration. This is where laser cleaning proves its advantage. Not only can it be used to remove the oxidation layer, but it can also remove any remaining contaminant oxides from the product, protecting it from further damage. The most common metals that form oxidation layers include common metals such as aluminium, bronze, brass and copper, as well as precious metals such as gold and platinum.
Similar to rust and oxides, the removal of grease, mould, dirt and paint is often done prior to metal welding or bonding to ensure a strong, contaminant-free bond. Thankfully laser cleaning is very capable of removing even the thickest layers of such unwanted coatings. With traditional cleaning methods, removing layers of chrome- or lead-based paints was previously a long, laborious and dangerous process. Lasers however can be used to safely remove such hazardous paints without creating additional waste.
Non-metals
As with metal cleaning, calibrated properly lasers can be used efficiently to ablate unwanted materials from ceramic surfaces without damaging the underlying substrate. Ceramic products frequently cleaned using lasers include print rollers, ceramic moulds, historical artefacts and baking moulds.
The flexibility of laser cleaning also makes it suited to removing grime and bacteria from natural stones such as marble, granite, limestone and concrete. In particular, the level of precision granted by laser cleaning makes it well-suited to cleaning extremely delicate stone objects, e.g. in the restoring of statues, figures, and ornaments, as well as larger surfaces such as building facades. Here, manual cleaning would have proven to be too abrasive, and could have damaged the substrates being cleaned.
While lasers are not as commonly used to clean plastics than the other-mentioned materials, they have been shown to be an effective solution for removing contaminants such as adhesion blockers from certain types of plastic. For example, polypropylene, acrylonitrile-butadiene-styrene copolymer, fibre-reinforced polyurethane and some carbon-fibre reinforced plastics (CFRPs) have all been cleaned successfully using lasers. For many other plastics, however, laser cleaning would likely not be suitable, as it would likely risk causing damage such as carbonisation or melting of the underlying substrate.
Similar to plastic, certain types of rubber have been cleaned successful using lasers. For example in the automotive industry lasers have been successfully used to clean the inside of tyres in order to bond sensors and other materials to them. However, also similar to plastic, the majority of rubber and elastomeric materials would likely not be as easily cleanable with a laser due the the ease at which they could be damaged. That’s not to say it isn’t impossible, but exceptional care (likely ensured via an automated, rather than handheld, laser cleaning solution) would be needed to achieve the extreme precision required to remove the surface contaminant without damaging the underlying substrate.
While not a common application of the technology, glass is another material that can be cleaned using lasers. Examples include the cleaning of sooted optical windows, the conservation and restoration of cultural heritage stained glass windows, and of course Tesla’s ambitious desire to create laser-based windscreen wipers (www.lasersystemseurope.com/news/tesla-laser-cleaning-patent).
What industries is laser cleaning used in?
Due to its flexibility in both how it can be deployed and the number of materials it can process, laser cleaning is already well-established across a wide range of industries in applications ranging in size from large aircraft down to tiny microchips.
In the automotive industry, for example, lasers are used to remove the phosphate layer from bevel gears and other parts prior to welding to ensure less spatter and few pores, or to clean tyre and tooling moulds of contamination. For the e-mobility sector, lasers can be used for the pre-treatment of aluminium components in the manufacture of battery housings, or for cleaning copper wires, hairpins and busbars for contacting.
In the aerospace and maritime industries, where the use of chemical solvents is restricted, the large panels and propellors on aircraft and ships can effectively be stripped of paint, vegetation or algae during regular maintenance using lasers.
In the oil & gas industry, pipelines can be affected by various kinds of build-ups that must be dealt with to ensure uninterrupted operation. Here laser cleaning could be performed remotely by automated robots. The technology poses a particular advantage here due to its lack of consumables, only power would need to be supplied to the robots over the long distances involved.
Laser cleaning can also be performed within an enclosure via automated robots. (Image: Shutterstock/Itsanan)
In the electronics industry, typical applications include cleaning the contact areas of plugs and pads, as well as the removal insulation layers in cables. In addition, before chips are soldered, the component pins must be completely de-oxidised to ensure optimal electrical contact.
In the medical industry, lasers can be used to clean steam steriliser systems, which themselves are used to clean medical tools at high tempartures. In cleaning the tools, contamination can embed itself into the internal surfaces of the steriliser, staining them and promoting corrosion. Lasers offer a quick and efficient way of removing such embeded contaminants.
In the pharmaceutical industry lasers can be used to clean objects such as reactor vessels, agitators, extrusion screws, air treatment, systems, storage tanks, tools, moulds, presses, rolls and conveyors.
Laser cleaning also plays an increasing role in restoration, being used to carefully treat not only stone buildings, statues, monuments and bridges, but also valuable artworks, each of which can be marred by air pollution, dust or soot particles.
What types of laser are used for cleaning?
Nanosecond-pulsed fibre lasers operating in the near-infrared wavelength are most commonly used for laser cleaning. Being able to deliver the beam down an optical fibre enables a wide range of integration possibilities from handheld systems to remote, robot-controlled solutions. The short pulse duration minimises the heat-impact on the underlying substrate being cleaned, while the repetition rate and power can be increased to achieve faster processing speeds. Laser systems offering powers from tens of watts to multiple kilowatts are now available, with handheld systems going all the way up to 3kW in power. Less frequently used for cleaning are direct-diode lasers, excimer lasers, CO2 lasers, continuous-wave lasers and ultrashort-pulse lasers.
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