Painting is an important step during a ship-building process because the corrosion and weather resistance of a ship’s body depends on it. The shipyards currently use mechanical methods such as sand blasting and grinding to clean the ship’s body before and after cleaning. Epoxy paint is difficult to remove from stainless steel substrates if its thickness is greater than 200 microns using laser cleaning or any mechanical cleaning technology. One way to overcome this problem is to use laser cleaning by controlling the process parameters to achieve effective cleaning results. Normally a pulse duration of 150 nsec is used for laser cleaning for this application. It is also useful to monitor the laser induced plume/plasma in order to determine if the cleaning was sufficient. By controlling the beam scanning patterns and the line overlap rate, it is possible to obtain effective cleaning results without introducing removal deviation. The observation from a study showed that a higher brightness of plume/plasma was generated when all the paint was removed and the evaporation of bare metal was observed (Ref 1).
Painting is used in ship industries for resistance against corrosion, weather resistance and also aesthetics. Blasting and grinding have been traditionally used before and after painting. However, there are concerns for these methods related to workers health and safety and also environmental pollution. Laser cleaning technology, on the other hand, enables environmentally friendly work and significantly reduces process cost when compared to mechanical and chemical cleaning methods. Figure 1 shows laser cleaning of the body of a ship.
Figure 1: Laser Cleaning of ship’s body
In one study researchers tried to remove a 200 micron thick epoxy paint from the steel surface of a ship using a fiber laser cleaning system with an average power of 100 watts without a beam scanning pattern function (Ref 1). This operation was not very successful. In another study, the optimal process parameters for the removal of epoxy based paint from the surface of metal substrates were derived and the ablation mechanism of the cleaning process was described using a Nd:YAG laser (Ref 2). Another research study explored the effectiveness of laser paint stripping for epoxy paint removal using a nanosecond fiber laser system (Ref 3).
In order to further expand the applicability of laser cleaning technology, it is necessary to control the process parameters that have not been addressed in the previous studies. In a new study, where small surfaces were cleaned, the test was conducted by selecting the beam scanning pattern and the laser beam overlap rates as the main process parameters for the laser cleaning (Ref 1). SS400 with a 320 microns epoxy coating were used. The topographic color distribution as well as roughness of base metal before painting were measured. The base metal had a roughness of 30-45 microns. As for the laser cleaning system it was a fiber laser with a 100 Watt power, pulse duration of 150 nsec, 50 mm scan line, beam diameter of 97 micron and energy density of 13.6 J/cm^2. Laser cleaning process parameters selected for this study were the laser beam scanning pattern and overlap rate. Researchers designed four beam scanning patterns which were line, circle, infinity and wave clean a shown in figure 2.
Figure 2: Different scanning patterns used for laser cleaning (Ref 1)
Installation of a high-speed camera in front of the laser cleaning equipment, made it possible to analyze the scan path of the laser beam and investigate the laser induced plume/plasma generated by the different beam scanning patterns. The laser cleaning performance was also studied as a function of different beam scanning types. It was found out that for line and circle patterns, the image clearly showed that the intensity of plume/plasma was greater at the edge. When the infinity and wave-clean patterns were used, the paint was removed uniformly without deviation for each area. Since the infinity and wave-clean patterns were more uniform, it was found out that not only the number of scans but also the scan path of the laser beam had a great influence on the laser cleaning results. It could be confirmed that when the component of the base material (Fe) was detected after the component of the paint was completely removed, the number of required scans was seven or higher. This showed that the epoxy paint on the steel surface can be effectively removed by laser cleaning technology.
The overlap rates of the pulse and the line could have an impact on the effectiveness of laser cleaning. Figure 3 shows the difference between the pulse and the line overlap rates.
Figure 3: Line and overlap rates for laser pulses (Ref1)
Several settings were used such as pulse overlap rates of 50% and 70 % and line overlap rates of 20%,50%, and 70%. For the minimum pulse overlap rate of 50% and line over lap rate of 20%, the 320 microns epoxy was removed during 13 scans (Ref 1).
One can divide the mechanism of laser cleaning process into a quantum process that does not involve heat and a thermal process that involves heat. As an example of the thermal process which is required for paint removal during ship-building, one study (Ref 1), showed that the laser cleaning performance can be significantly affected by the difference in the amount of heat input according to the laser beam overlap rates. Less number of line and pulse over lap rates, required higher number of scans to remove epoxy paint as shown in Figure 4.
Figure 4: Influence of line and pulse overlap
rate on the number of required scans to clean epoxy paint (Ref1)
The laser induced plume and plasma behaviour were also analyzed. The epoxy paint absorbed the laser energy that was irradiated according to the beam scanning pattern and excited it while undergoing evaporation. On the other hand, when the paint was completely removed and the surface of the metal was exposed, a high intensity plume was generated by the evaporation of the metal (Ref1).
In conclusion, controlling the process parameters such as beam scanning patterns and the laser beam overlap rates were proved to be very effective in laser cleaning of epoxy paint from stainless steel surface.
Allied Scientific Pro offers various laser cleaners of different wattages with the ability to provide linear, circular, infinity and other patterns. The 100 W. 200 W, 300 W and 500 W systems are available.
For further information, please refer to the following link:
https://www.alliedscientificpro.com/shop/category/lasers-1205?category=1205&search=laser+cleaning
References:
1- A study on the laser removal of Epoxy coatings on SS400 surface by beam scanning patterns, J.E. Kim et.al, Coatings 2021, 11, 1510.
2- Laser effects based optimal laser parameter identifications for paint removal from metal substrate at 1064 nm, J. Han et.al, J. Mod, 2017, 64.
3- Nanosecond fiber laser paint stripping with suppression of flames and sparks, Z. Kuang et, al, J. Mater. Process. Technol. 2019, 266.