Laser Cleaning: how it works

Why Clean Surfaces?

Industrial surface cleaning is performed for several reasons. It may be required to prepare the surfaces of parts prior to manufacturing processes such as welding or painting. The cleaning process is part of the production process and must be performed with precision to ensure optimal results.

Current Technologies

Traditional cleaning technologies include:

1. Abrasives: Grit blasting and abrasive water jetting. These methods are labor-intensive, pose personal safety concerns, and can result in uneven material removal, reducing the base metal’s thickness. Additionally, maintenance costs are high, with up to 50% downtime.
2. Acid Cleaning/Stripping: Slow processes that are environmentally unsafe and require masking. They can lead to stress corrosion cracking, pitting, and alloy depletion, limiting part repairs to one cycle.

Why Laser Cleaning?

Laser cleaning offers several advantages over traditional methods:

1. Precision: Selective removal of contaminants ensures minimal or no damage to the substrate.
2. Efficiency: Laser cleaning is faster and more efficient, reducing downtime in industrial applications.
3. Safety: Operators are exposed to fewer health risks compared to methods involving toxic chemicals or abrasive dust.
4. Environmentally Friendly: No need for abrasive materials or harsh chemicals, reducing waste and environmental impact.

General Applications

Laser cleaning can be used for various applications, including:

• Removal of oxides, rust, oil, and dirt.
• Paint and coating stripping from metal surfaces.
• Cleaning of metal molds and dies.
• Aluminum surface cleaning.
• Preparation of welding surfaces.

How It Works

Laser cleaning is performed using a scanned laser beam that vaporizes, sublimates, or burns away contaminants. Laser parameters can be optimized for specific process requirements, maximizing speed and avoiding substrate damage.

There are three different interactions between the laser beam and the surface:

• Photothermal Interaction: The laser beam’s energy is absorbed by the contaminants, causing them to vaporize, sublime, or break down into smaller particles. The rapid heating and expansion create a shockwave that helps detach the contaminants from the surface.
• Photomechanical Interaction: Rapid thermal expansion and contraction within the contaminated layer cause the contaminants to crack or peel off.
• Photochemical Interaction: Certain laser wavelengths induce photochemical reactions that weaken or break the chemical bonds within the contaminants, effectively removing organic materials.

Key Features for Source Choice

The limiting factor of the cleaning process is the ablation threshold of the contaminant. The laser cleaning system needs enough energy density to overcome this threshold. Key parameters linked to energy density include source power and beam quality (M²).

• Wavelength: The wavelength of the laser is selected based on the material being cleaned. Different materials respond differently to specific wavelengths, ensuring efficient and selective cleaning.
• Power and Energy: The laser source allows control over parameters such as power, pulse duration, and repetition rate. This control ensures adaptability to different cleaning requirements.
• Focus tolerance: Some surfaces may have irregularities or contours. A laser cleaning system with focus tolerance can accommodate variations in surface height, ensuring that the laser beam maintains the appropriate focus regardless of the surface geometry and increasing the working range. With high focus tolerance the system is more robust and less susceptible to minor disturbances, contributing to its reliability in various operational environments.

Beam quality of laser cleaning

Consider a Gaussian laser beam passing through a converging lens. As the beam converges, it reaches the “spot size” where the diameter is minimal. The waist position on the z-axis is influenced by the focal length, representing the lens’s converging or diverging strength. A smaller focal length results in a more converging lens, bringing the waist closer to the lens. Between all the characteristics of the laser beam, M² represents the quality factor, measuring how well the beam behaves compared to a theoretical TEM₀₀ Gaussian beam. A value of “1” is perfect, with deviations indicating a decrease in quality.

The depth of field is a specific distance around the beam waist with a small diameter compared to its spot size. A smaller focal length relative to the incident beam size yields a smaller depth of field, and vice versa.

M² AND DEPTH OF FIELD

At high M² values corresponds bigger spot size and elevated focus tolerance. This allow us to mark less lines to cover a specific area, thus speeding up the process.

At low M² values (1< M² <2) the laser spot size is smaller and the ability to precisely focus the laser is higher (low focus tolerance), allowing a more selective material removal. (i.e. the cleaning of “transparent” materials such us oil or grease).

As usual, there is a balance between quality and takt time and the possibility to use different laser source, with different characteristics, allow us to choose the correct source for the application.

Conclusion

Laser cleaning is a highly precise, efficient, and environmentally friendly alternative to traditional cleaning methods. With its numerous advantages and wide range of applications, it is revolutionizing the industrial cleaning process.