Laser cleaning and paint removal applications have received much attention in recent years, as traditional paint removal methods such as sandblasting and chemical paint stripping generate a lot of environmental pollution. It is time to take advantage of green paint removal solutions. By properly controlling parameters such as pulse width, energy density, repetition rate, and beam size, lasers can be used to perform high-quality work and remove coatings [Reference 1] The advantages of laser paint removal can be summarized as follows:
● Fewer consumables
● Reduced secondary waste
● No mechanical damage to the substrate due to the use of controlled laser parameters
● Better adhesion due to reduced surface roughness
● Faster than traditional methods
● More efficient than traditional methods
There are two ways to achieve laser cleaning. The first is laser ablation, where a high-energy pulse or an intense continuous wave beam will generate a plasma in the coating, and the shock wave generated by the plasma will blast the coating into particles. The second is thermal decomposition, where a lower-energy continuous wave beam or long pulse can heat the surface and eventually evaporate the coating.
Whatever the mechanism, uncontrolled laser parameters can damage the substrate and cause problems. Both continuous and pulsed lasers can be used for laser cleaning, but it is necessary to understand the different effects these lasers produce on different substrates. The absorption of a continuous laser by a substrate depends on its wavelength, with shorter wavelengths generally resulting in greater absorption. For a classical pulsed laser, on the other hand, the penetration depth LT into the substrate is independent of the wavelength and depends instead on the pulse width τp of the laser and the diffusion coefficient D of the substrate, as shown in Equation 1.
For a classical pulsed laser, an increase in pulse width increases the ablation threshold, which is defined as the minimum energy required to remove a unit volume of material according to the following equation:
where ρ is the density and Hv is the heat of vaporization (the amount of heat required to vaporize a unit mass of material in Joules per gram). Thus, longer pulses reduce the ablation efficiency. Classical pulsed lasers also depend on the pulse repetition rate, where the ablation efficiency increases with increasing repetition rate.
A study has been conducted to investigate the CW and pulsed operation modes of a laser using a 1.07 μm fiber laser [Ref 2]. In this study, the same CW laser was switched on and off to produce long width pulses. This study found that in CW mode, the specific energy (defined as the energy required to remove a unit volume of material (mm3) in Joules and inversely proportional to the ablation efficiency) decreases with increasing scan speed and laser power. For pulsed mode, the ablation efficiency was found to depend on the duty cycle (the ratio of the pulse width to the time interval between two pulses). Increasing the duty cycle, the ablation efficiency increased. This is in contrast to classical pulsed lasers, where, at a fixed repetition rate, increasing the pulse width (and thus the duty cycle) decreases the ablation efficiency. Figure 3 compares the specific energy versus power and scan speed for a 1 kHz CW laser and a pulsed laser (i.e., a CW laser switched on and off) on a stainless steel substrate.
The pulsed laser (i.e., a CW laser switched on and off) has a peak power of 1800 W and an average power almost the same as the CW laser, but as can be observed from the figure, the specific energy is almost 2 times lower. Pulsed mode versus CW mode. CW mode appears to have more losses than pulsed mode because the laser power is always at peak value.
However, the mode in which the laser is operated is not the only consideration in deciding whether to use a pulsed (i.e., continuous wave on and off) or a continuous wave laser for laser cleaning. The scanning pattern is another important consideration. It is important that the interaction time between the laser beam and the coating is short so that the effect of thermal damage is minimal. This can be achieved by using short pulses with high peak intensity or by using a continuous laser and fast scanning speeds.
Considering that continuous laser power is generally more powerful, cheaper, and more rugged than pulsed lasers, it is not a bad choice for laser cleaning. Unfortunately, the galvanometer scanners traditionally used for laser cleaning cannot handle multi-kilowatt lasers. Galvanometer scanners used for high-power lasers are also quite heavy and cannot run at high scanning speeds. Therefore, a new type of scanner called a polygon scanner has been proposed that has only one moving part, the polygon [Reference 3]. These polygon scanners are able to handle higher laser powers and have been shown to be three times faster than galvanometer scanners. Using modest rotational speeds, polygon scanners can produce surface scanning speeds in excess of 50 meters per second. This high scanning speed allows for short interaction times of the beam with the work surface and permits the use of very high laser powers. Figure 4 shows the design of a polygon scanner.
In summary, the choice of using a CW or pulsed laser (i.e., CW or classical short-pulse lasers that are switched on and off) for laser cleaning depends on several factors, such as the type of substrate, the absorptivity of the coating, and the cost of the laser. The combination of a polygon scanner and a continuous laser can produce fast scanning speeds and is a promising option that can be considered when classical pulsed lasers are not available.
