How does laser cleaning work?
Laser Cleaning Technology - WORKING PRINCIPLE
Laser cleaning technology works by sending millions of nanosecond-length pulses (0,00000001 sec) of laser light towards a surface. When it interacts with contaminants like rust, coating, paint,.. that absorb the pulses of laser light, the contaminants or coating particles will either turn into a gas or the pressure of the interaction will cause particles to free from the surface.
Laser cleaning - or ablation - is the process of clearing away undesired material from a solid surface by irradiating it with a laser beam. By absorbing the energy of the laser beam, the targeted material is heated very quickly, making it evaporate or sublimate. As the surface below does not absorb any energy, it stays untouched.
By manipulating the laser flux, its wavelength and its pulse length, the amount of material that is being removed by a single laser pulse can be controlled with extreme precision. Making laser cleaning equally suitable for rapid and deep rust removal as for removing only one thin layer of paint, without damaging the base coat.
Laser Cleaning parameters explained
Laser cleaning or surface cleaning by Flash laser heating using short‐pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm.
This is very useful because micron‐ and submicron‐sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal.
We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength (infrared-A or UV) that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laser irradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest laser power starting at ∼0.1 J/cm2.
Beam intensity
Beam intensity is the crucial parameter in the laser cleaning process. If it is not correctly selected, it may damage the substrate. The beam intensity and its effects on the substrate and the product to remove influence a lot on the speed of removing and the way of removing.
Beam size is one of the factors that influence cleaning efficiency and time of cleaning.
Beam Area
A smaller beam size is normally good for laser cleaning process, because of the high removal rate and the less cleaning time. However, if the laser beam area is too small, it may damage the surface. Thus the beam size should be selected carefully
Pulse duration
Pulse duration is the time it takes for the laser to emit one pulse
In the case of long-pulse lasers and continuous lasers (CW), the removal of surface material like pâint, dirt, oil, etc.. is done by vaporization and/or burning.
At the same time, ablation of the material occurs in the case of high-power short-pulse lasers, by acoustic shock waves created by the absorption of high-intensity laser energy. A higher removal rate can be achieved in the case of short pulses for a fixed amount of laser energy.
If the pulse duration is too short, the cleaning is not efficient; at the same time, if the pulse duration is too long, the substrate is affected by heat. Thus appropriate pulse duration should be selected. Therefor are needed laser sources with fine pulse adjusting.
Pulse repetition rate
Pulse repetition is the number of pulses per second.
If dwell time is too short, the dust from the graffiti, dirt, oil,.. and the laser pulse interfere with each other. Hence, the laser pulses do not reach the target. If the pulse repetition rate is too long, the cleaning takes more time. Therefore pulse repetition rate should be selected as appropriate.