Why Clean the Surfaces?
Industrial surface cleaning is performed for several reasons. It may be necessary to prepare part surfaces 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:
- Abrasives: sandblasting and abrasive water jet. These methods are laborious, involve personnel safety concerns, and can result in uneven material removal, reducing the thickness of the base metal. In addition, maintenance costs are high, with downtime of up to 50 percent.
- Acid cleaning/decapping: slow, environmentally hazardous processes that require masking. They can cause stress corrosion cracking, pitting and alloy depletion, limiting part repairs to one cycle.
Why Laser Cleaning?
Laser cleaning has many advantages over traditional methods:
- Precision: selective removal of contaminants ensures little or no damage to the substrate.
- Efficiency: laser cleaning is faster and more efficient, reducing downtime in industrial applications.
- Safety: operators are exposed to fewer health risks than methods involving toxic chemicals or abrasive powders.
- Eco-sustainability: 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.
- Removal of paint and coatings from metal surfaces.
- Cleaning of metal molds and dies.
- Cleaning aluminum surfaces.
- Preparation of surfaces for welding.
How It Works
Laser cleaning is performed using a laser beam that vaporizes, sublimates, or burns contaminants. Laser parameters can be optimized for specific process requirements, maximizing speed while avoiding substrate damage.
There are three different interactions between the laser beam and the surface:
Photothermal Interaction: The energy of the laser beam is absorbed by the contaminants, causing them to vaporize, sublimate, or decompose into smaller particles. Rapid heating and expansion create a shock wave that helps dislodge contaminants from the surface.
Photomechanical Interaction: Rapid thermal expansion and contraction within the contaminated layer causes the contaminants to rupture or detach.
Photochemical Interaction: Some laser wavelengths induce photochemical reactions that weaken or break chemical bonds within contaminants, effectively removing organic materials.
Key Characteristics for Source Selection.
The limiting factor in the cleaning process is the contaminant ablation threshold. The laser cleaning system needs sufficient energy density to overcome this threshold. Key parameters related to energy density include source power and beam quality (M²).
Wavelength: The wavelength of the laser is selected according to the material being cleaned. Different materials respond differently to specific wavelengths, ensuring efficient and selective cleaning.
Power and Energy: The laser source allows parameters such as power, pulse duration, and repetition rate to be controlled. 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 adapt to variations in surface height, ensuring that the laser beam maintains the appropriate focus regardless of 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 operating environments.
Laser cleaning beam quality
Consider a Gaussian laser beam passing through a converging lens. When the beam converges, it reaches the “spot size” where the diameter is minimum. The position of the waist point on the z-axis is affected by the focal length, which represents the converging or diverging strength of the lens. A smaller focal length results in a more convergent lens, bringing the waist closer to the lens. Of all the laser beam characteristics, M² represents the quality factor, measuring how well the beam performs compared to a theoretical Gaussian TEM₀₀ beam. A value of “1” is perfect, with deviations indicating a decrease in quality.
Depth of field is a specific distance around the waist with a small diameter relative to the spot size. A focal length smaller than the size of the incident beam produces a smaller depth of field, and vice versa.
M² and Depth of Field
Higher values of M² correspond to a larger spot size and greater focus tolerance. This allows us to draw fewer lines to cover a specific area, thus speeding up the process.
At low values of M² (1 < M² < 2), the laser spot size is smaller and the laser’s ability to focus accurately is greater (low focus tolerance), allowing more selective material removal (e.g., cleaning of “transparent” materials such as oil or grease).
As usual, there is a balance between quality and cycle time, and the possibility of using different laser sources, with different characteristics, allows us to choose the correct source for the application.
Conclusions
Laser cleaning is a highly precise, efficient and environmentally friendly alternative to traditional cleaning methods. With its many advantages and wide range of applications, it is revolutionizing the industrial cleaning process.
