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
● Less secondary waste
● No mechanical damage to the substrate using 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. These two mechanisms are shown in Figures 1 and 2.
Figure 1 Laser ablation steps
Figure 2 Thermal decomposition steps
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 important 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 diffusivity 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, with the ablation efficiency increasing as the repetition rate increases.
A study has been conducted investigating CW and pulsed operation of lasers using a 1.07 μm fiber laser [Ref 2]. In this study, the same continuous laser was turned 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 be dependent 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., CW laser turned on and off) on a stainless steel substrate.
Figure 3: The left plot shows the CW laser specific energy versus laser power, and the right plot shows the 1 kHz pulse specific energy versus laser duty cycle
The peak power of the pulsed laser (i.e., CW laser that switches on and off) is 1800 W, and its average power is almost the same as the CW laser, but as can be observed from the graph, the specific energy is almost 2 times lower. Pulsed mode compared to CW mode. The CW mode appears to have more losses compared to the pulsed mode because its laser power is always at peak.
However, the operating mode of the laser is not the only consideration in deciding whether to use a pulsed (i.e., continuous wave switched on and off) or a continuous wave laser for laser cleaning. The scanning mode is also another important consideration to consider. 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 operate 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 [Ref 3]. These polygon scanners are capable of handling higher laser powers and have been demonstrated 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 the interaction time of the beam with the work surface to be short and allows very high laser powers to be used. Figuygon 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 produces fast scanning speeds and is a promising option to consider when classical pulsed lasers are not available
