In 1960, the world’s first laser was born. Professor Theodore Harold Maiman, an American scientist, created it using ruby. It has since paved the way for lasers to serve humanity.
In the time that followed, lasers for various fields were created. The promotion of laser technology has led to rapid scientific and technological development in medical care, equipment manufacturing, precision measurement, and remanufacturing engineering, accelerating the pace of social progress, and the application of lasers in the field of cleaning has also yielded important results.
Compared with mechanical friction, chemical corrosion, high-frequency ultrasonic, and other traditional cleaning methods, laser cleaning can achieve fully automatic operation.
Laser cleaning has the advantages of high efficiency, low cost, no pollution to the environment, no damage to the base material, and a wide range of applicable materials.
It is fully compatible with the green processing concept and is the most reliable and effective cleaning method available.
Cleaning is a prerequisite for detecting and processing used machine parts.
Laser cleaning technology can efficiently manage the substrate’s surface morphology and roughness and increase the substrate’s performance after cleaning.
It can also be applied to manufacturing, surface treatment, or remanufacturing large parts.
Although the current laser cleaning has not completely replaced the traditional cleaning methods, with the society’s awareness of energy saving and emission reduction in the manufacturing industry, laser cleaning will gradually enter people’s lives with its unique advantages.
Currently, people are becoming more and more aware of environmental protection and safety, and there will be fewer and fewer types of chemicals available for industrial production cleaning.
How to find a cleaner, the non-destructive cleaning method is a problem we have to consider.
Laser cleaning is non-abrasive, non-contact, non-thermal, and suitable for all objects. It can solve problems that traditional cleaning methods cannot be solved and are considered the most reliable and effective solution.
Introduction
For example, when sub-micron contamination particles adhere to the surface of the workpiece, these particles tend to stick very tightly and cannot be removed with conventional cleaning methods while cleaning the surface of the workpiece with nano-laser irradiation is very effective.
In addition, because the laser cleaning of the workpiece is non-contact, the cleaning of precision workpieces or they are fine parts is very safe and can ensure its accuracy.
As a result, laser cleaning has unique advantages in the cleaning industry.
Why can laser be used for cleaning? Why does it not damage the object being cleaned? First, you need to understand the nature of the laser.
In short, lasers are no different from the companion light around us (visible and invisible light). Still, lasers use resonators to focus light in the same direction, providing better performance than simple wavelength and coordination.
Therefore, in theory, all wavelengths of light can be used to form a laser.
However, very few media can be exciting, so the number of laser sources that can produce a stable, industrially suitable light source is quite limited.
Nd: YAG lasers, CO2 lasers, and excimer lasers are widely used.
Because Nd: YAG lasers can be transmitted through optical fibers and are more suitable for industrial applications, they are also widely used for laser cleaning.
In academic terms, laser ablation (scientific name laser cleaning) or photoablation is removing material from a solid (or sometimes liquid) surface by irradiation with a laser beam.
At low laser fluence, the material is heated by the absorbed laser energy and evaporates or sublimates.
At high laser fluxes, the material is usually converted into plasma. In general, laser ablation refers to removing material with a pulsed laser. Still, if the laser intensity is high enough, the material can be ablated with a continuous wave laser beam.
Optical ablation is the primary use for deep UV excimer lasers.
Optical ablation uses a laser wavelength of around 200 nm.
The amount of material removed by a single laser pulse is determined by the material’s optical characteristics, as well as the wavelength and pulse duration of the laser.
The ablation rate refers to the total mass ablated from the target per laser pulse.
The ablation process is influenced by laser radiation parameters such as laser beam scanning speed and scan line coverage.
The Principle of Laser Cleaning
Scientists like Beklemyshev and Alrn integrated laser technology with cleaning technology in the mid-1980s and conducted relevant research based on practical job requirements. Since then, the concept of laser cleaning technology has been born.
It is well known that the bonding forces between contaminants and substrates are divided into covalent bonds, double dipoles, capillary interactions, and van der Waals forces.
Overcoming or destroying this power can have a purifying effect.
Laser cleaning is the use of laser beam energy density, directional control, convergence ability, destroying the bonding force between the contaminant and the substrate or directly vaporizing the contaminant, thereby reducing the bonding strength between the contaminant and the substrate, to achieve the purpose of cleaning the surface of the workpiece.
The principle diagram of laser cleaning is shown in Figure 1.
When the workpiece surface of the contaminant absorbs the laser’s energy, after rapid vaporization or instantaneous thermal expansion, overcoming the force between the contaminant and the substrate surface.
Due to the increase in heating energy, the contaminant particles vibrate and fall off the surface of the substrate.
The entire laser cleaning process is roughly divided into four stages:
- Laser vaporization and decomposition
- Laser stripping
- Thermal expansion of the contaminant particles
- Substrate surface vibration and contaminant separation
Of course, in the application of laser cleaning technology, pay attention to the laser cleaning threshold of the object being cleaned, and choose the appropriate laser wavelength to achieve the best cleaning effect.
By modifying the grain structure and orientation of the substrate surface without destroying it and managing the roughness of the substrate surface, laser cleaning can improve the overall performance of the substrate surface.
The cleaning impact is primarily influenced by beam characteristics, substrate and dirt material physical factors, and the dirt’s ability to absorb beam energy.
At present, laser cleaning technology includes dry laser cleaning technology, wet laser cleaning technology, and laser plasma shock wave technology.
Dry Laser Cleaning
Direct irradiation with pulsed laser cleaning workpiece improves the substrate or surface contaminant energy absorption temperature, resulting in thermal expansion or thermal vibration of the substrate and then separating it.
This method can be broadly divided into two cases:
- One is the surface contaminants absorb the laser expansion.
- Another is the substrate absorbs the laser to produce thermal vibration.
Wet Laser Cleaning
Wet laser cleaning is in the pulse laser irradiation to be cleaned before the surface of the workpiece, pre-painted liquid film. Under the influence of the laser, the temperature of the liquid film rapidly rises and vaporizes.
In the moment of vaporization, shock waves act on the contaminant particles so that they fall off the substrate.
This method requires that the substrate not react with the liquid film, limiting the range of materials to which it can be applied.
Laser Plasma Shock Wave
A laser plasma shock wave is a spherical plasma shock wave formed by breaking through the air medium during laser irradiation.
The shock wave acts on the cleaning substrate’s surface, releasing energy to remove impurities.
Because the laser does not interact with the substrate, there is no risk of harm.
The laser plasma shock wave cleaning technology can remove particles as small as tens of nanometers, and the laser wavelength is not limited.
Multiple test techniques and related parameters should be selected as needed in actual production to get high-quality cleaning workpieces.
In the laser cleaning process, evaluating the surface cleaning efficiency and quality is an important criterion to determine the quality of laser cleaning technology.
Advantages
Laser cleaning than mechanical friction cleaning, chemical corrosion cleaning, strong impact cleaning of liquid solids, high-frequency ultrasonic cleaning, and other traditional cleaning methods have more advantages.
Laser cleaning is a “green” cleaning method that does not require any chemical agents and cleaning solutions.
The waste after cleaning is a solid powder, small in size, easy to store, can be recycled, and easy to solve the problem of environmental pollution caused by chemical cleaning.
Contact cleaning is common in traditional cleaning procedures; the surface of the object cannot be removed due to mechanical force, damage to the object to be cleaned, or cleaning media stuck to the object’s surface to be cleaned secondary pollution.
And laser cleaning of non-grinding and non-contact can solve these problems.
Lasers can be communicated via optical fiber, and robots, which are easy to operate over long distances, can clean portions that are difficult to reach with traditional methods and ensure people’s safety in some risky environments.
Laser cleaning can eliminate all types of pollutants from the surface of diverse materials, achieving a level of cleanliness that is impossible to reach with traditional cleaning.
It can also clean the surface of the material impurities selectively without causing damage to the material’s surface.
Laser cleaning is highly efficient and saves time.
Although a laser cleaning system is an expensive one-time investment, it may be used continuously for a long time, has low operating expenses and charges electricity per hour.
Working Principle
The photophysical reaction created by the interaction between the high-intensity beam and the short pulse laser and the contaminated layer is the basis for the pulsed Nd: YAG laser cleaning procedure, which is based on the properties of the light pulse generated by the laser.
The following is a summary of the physical principle:
- The laser’s beam is absorbed by the contaminated layer of the surface that needs to be treated.
- The absorption of the large energy forms a rapidly expanding plasma (highly ionized unstable gas) and generates a shock wave.
- The pollutant is broken up and eliminated by the shock wave.
- The light pulse width must be short enough to avoid the accumulation of heat that can damage the treated surface.
- Experiments have shown that plasma occurs on metal surfaces with oxides on the surface.
Depending on the contaminant or oxide layer removed, plasma is only created when the energy density exceeds a threshold value.
The substrate material will be damaged if the energy density reaches this threshold.
The laser settings must be changed according to the scene so that the energy density of the light pulse is tightly between the two thresholds to clean the substrate material effectively while maintaining substrate material safety.
Each laser pulse can remove a certain thickness of the contamination layer. If the contamination layer is very thick, multiple pulses are required for cleaning.
The level of contamination determines the number of pulses required to clean a surface.
Cleaning self-control is an important effect of these two thresholds.
The number of pulses required to clean a surface depends on surface contamination.
An important result of these two thresholds is the self-control of cleaning.
Light pulses with energy densities higher than the first threshold will remove contaminants until they reach the substrate material.
However, since their energy density is below the failure threshold of the substrate material, the substrate will not be damaged.
Practical Applications
Laser cleaning can be used to remove both organic and inorganic impurities, such as metal corrosion, metal particles, dust, etc.
Below are some examples of practical uses.
These techniques have been widely used for a long time.
Mold Cleaning
Tire factories worldwide create hundreds of millions of tires each year. To avoid downtime, tire molds must be cleaned swiftly and consistently during the manufacturing process.
Traditional cleaning methods include sandblasting and ultrasonic or carbon dioxide cleaning. Still, these methods must usually be moved to cleaning equipment for several hours after the highly heated molds have cooled.
Cleaning takes a long time and tends to damage the accuracy of the mold. Noise and chemical solvents can potentially cause issues with safety and environmental protection.
Because the laser may be sent by optical fiber, the laser cleaning method has a lot of flexibility.
It’s simple because the laser cleaning method can be linked to an optical fiber to direct light to a mold’s dead corner or a portion that can’t be easily removed for cleaning.
Because the rubber is not vaporized, no hazardous gas is formed, posing a risk to the working environment’s safety.
Laser cleaning tire molds is a commonly utilized technology in the tire business in Europe and America.
Although the initial investment is considerable, the benefits gained in reduced standby time, mold prevention, worker safety, and raw material conservation can be swiftly recovered.
According to the cleaning test of laser cleaning equipment on the production line of a tire company, it only takes 2 hours to clean a set of large truck tire molds online.
Compared with traditional cleaning methods, the economic benefits are obvious.
The anti-adhesive elastic film on molds must be renewed regularly to maintain hygiene in the food sector.
This application also lends itself to laser cleaning without chemical chemicals.
Weapons and Equipment Cleaning
Laser cleaning technology is widely used in weapons maintenance.
Laser cleaning systems allow efficient and fast removal of rust and contaminants, selection of removal sites, and automation of cleaning.
With laser cleaning, cleanliness is higher than in chemical cleaning processes, but there is almost no damage to the object’s surface.
By adjusting the conditions, a dense oxide protection coating or a metal melting layer can be formed on the surface of metal objects, improving surface strength and corrosion resistance.
Laser removal of scrap is non-polluting and can also be operated remotely, effectively reducing the health damage to the operator.
Old Aircraft Paint is Being Removed
Laser cleaning technologies have been used in the aviation industry in Europe for many years.
After a usage period, aircraft surfaces must be repainted, but the old paint must be entirely removed before painting.
The typical mechanical paint removal procedure is prone to damaging the aircraft’s metal surface, posing a hidden risk to safe flight.
The paint layer on the surface of the A320 Airbus can be entirely removed in two days without affecting the metal surface if several laser cleaning devices are utilized.
Cleaning of Building Facades
With more and more skyscrapers coming up, the problem of cleaning building facades is becoming more and more prominent. Chutian laser cleaning system provides a good solution for building facade cleaning using fiber optic up to 70 meters.
It can remove all types of impurities from various stones, metals, and glass, and its cleaning efficiency is many times greater than typical cleaning procedures.
It can also remove black spots and discoloration on various stones of buildings.
The experiment’s laser cleaning system in Songshan Shaolin Temple buildings and stone monuments shows that laser cleaning to protect the ancient buildings and restore their appearance has a very good effect.
Cleaning in The Electronics Industry
The use of laser decontamination in the electronics industry requires high-precision decontamination, which is particularly suitable for laser decontamination.
Before soldering a circuit board, component pins must be completely deoxidized to ensure optimal electrical contact and that the pins are not damaged during the decontamination process.
Laser cleaning can meet the requirements for use and is highly efficient. Only one laser irradiation is required for one pin.
Precision degreasing cleaning in the precision machinery industry
In the precision equipment industry, chemical procedures are frequently employed to remove the ester and mineral oil used for lubrication and corrosion prevention on the parts, and chemical cleaning typically leaves residues.
Without harming the surface of the parts, laser degreasing can eliminate esters and mineral oils.
Shock waves are used to remove pollutants from the environment.
Instead of mechanical interactions, the explosive vaporization of the thin oxide layer on the component surface provides a shock wave that removes impurities.
The material is completely de-esterified and used to clean mechanical parts in the aerospace industry.
Laser cleaning is also used to process mechanical parts to remove oil esters.
Cleaning of pipes in nuclear power plant reactors
In nuclear power plants, laser cleaning devices are also utilized to clean the pipelines.
It uses fiber optics to introduce a high power laser beam into the reactor to remove radioactive dust directly, and the material being cleaned is easy to clean.
In addition, since it is operated remotely, the safety of the staff can be guaranteed.
To summarize, laser cleaning is used in a variety of industries, including automotive manufacturing, semiconductor wafer cleaning, precision parts processing and manufacturing, military equipment cleaning, building facade cleaning, heritage preservation, circuit board cleaning, precision parts processing and manufacturing, LCD cleaning, chewing gum residue removal, and so on.
The Main Cleaning Methods
Laser cleaning can be done in four different ways:
- Laser dry cleaning method, direct radiation decontamination with a pulsed laser.
- The laser + liquid film method, that is, first deposited a liquid film on the surface of the substrate and then decontaminated it with laser radiation.
- laser plus the inert gas method, that is, while laser radiation, inert gas will be blown to the surface of the substrate. The dirt is blown away by the gas as soon as it is removed from the surface, preventing re-pollution and oxidation.
- After the laser loosens the dirt, it is cleaned by non-corrosive chemical methods.
The first three methods are the most commonly used. The fourth method is only used for cleaning stone artifacts. For more than ten years, laser cleaning technology has been used on stone materials.
