Black Spots on Laser Cutting/Welding Lens: Causes and Solutions Explored

What is a laser cutting head lens?

The laser cutting head lens is a critical precision component within the optical system of a laser cutting head. This specialized lens plays a pivotal role in focusing the laser beam to achieve optimal cutting performance. Typically made from high-quality materials such as zinc selenide (ZnSe) or gallium arsenide (GaAs), the lens is designed to withstand high-power laser radiation while maintaining its optical properties.

The primary function of the cutting head lens is to concentrate the collimated laser beam into a precise focal point, creating an intense energy density capable of melting, vaporizing, or ablating the target material. The focal length and diameter of the lens are carefully engineered to suit specific cutting applications, material thicknesses, and laser power levels.

The condition and proper installation of the laser cutting head lens directly influence several key cutting parameters, including:

1. Beam focus quality and spot size
2. Cutting speed and efficiency
3. Edge quality and kerf width
4. Depth of cut and penetration
5. Overall cutting precision and consistency

Why does the laser cutting head lens have black spots?

Black spots on the laser cutting head lens primarily result from slag and molten metal splatter adhering to the quartz lens surface during piercing and cutting operations. This phenomenon is particularly prevalent in high-power laser cutting of thick materials or when cutting reflective metals like aluminum or copper.

The formation of these black spots can significantly impair cutting performance and quality. As the laser continues to operate, these spots act as localized heat absorption centers, leading to several cascading issues:

1. Thermal lens effect: The uneven heating caused by black spots creates a thermal gradient across the lens, altering its refractive properties. This results in focal point drift, where the actual focal point shifts from its intended position, compromising cutting precision.
2. Reduced transmissivity: Black spots obstruct the laser beam, reducing the overall power delivered to the workpiece. This can lead to inconsistent cutting depth and quality across the material.
3. Lens degradation: The intense heat generated at these spots can cause micro-cracks or even thermal fracturing of the lens, significantly shortening its lifespan.
4. Focal length alteration: As the central area of the lens overheats due to the black spots, it can cause a localized change in the lens curvature, effectively reducing the focal length. This prevents normal cutting operations, especially for precision applications.
5. Beam distortion: The presence of black spots can introduce aberrations in the laser beam profile, leading to non-uniform energy distribution and potentially causing wider kerf or poor edge quality.

Why do black spots/burn spots appear on the lower protective lens of the laser cutter?

Reason 1: Excessive negative focus during piercing, leading to piercing slag returning to the lens of the laser cutting machine.

Solution: Raise the piercing focus.

Reason 2: Unsuitable piercing energy, piercing time too close to the limit, or explosive piercing.

Solution: Lower the duty cycle, increase piercing time.

Reason 3: Co-edge layout cutting of parts, deformation of sheet metal causing difficulty in connecting cuts.

Solution: Add piercing at the co-edge cutting position, or increase micro-connections in the layout of parts.

Reason 4: Metallic particle contamination in the airway pipeline.

Solution: Clean or replace the pipeline, identify and replace the source of the metal dust (e.g., pipe joint connections).

Why does oil/dust or moisture appear on the lower protective lens of the laser cutter?

Reason 1: The air compressor’s filter does not meet usage standards.

Solution: Check if the air compressor’s filter core is ineffective or upgrade the filter level.

Reason 2: The airway pipeline is dirty.

Solution: Disconnect the air inlet pipe of the cutting head, flush the machine tool’s air inlet with high-purity alcohol under atmospheric pressure, clean the pipeline 3-4 times, or replace the airway pipeline.

Reason 3: The air inlet seat filter of the fiber laser cutting machine is blocked by raw materials or other foreign objects.

Solution: Dismantle the air inlet seat, check if the air inlet seat filter is blocked by any foreign objects.

How can we avoid the occurrence of black spots?

The focusing lens of the optical fiber laser cutting machine, located at the cutting head, is quite expensive. Therefore, a protective lens is designed to safeguard it. Consequently, the replacement of the protective lens is routine. However, if the replacement frequency accelerates, it is necessary to identify the problem.

Black spots often occur due to the adhesion of molten slag splashed during the piercing and cutting processes onto the quartz lens. Because these black spots continuously grow and erode the protective lens during the light emission process, the center of the lens might experience focal drift due to the thermal lens effect of overheating expansion. This actually reduces the focal length, making cutting impossible, and cleaning these black spots is quite challenging.

There are two types of lenses in the laser cutting head: the focusing lens and the protective lens. Their primary function is to concentrate the laser beam; hence, it is essential to keep these lenses clean and translucent.

When black spots appear, what should we do?

1. The material of the laser cutting head lens is zinc oxide, which is brittle and not easy to fall off. Care should be taken during disassembly and assembly: avoid forceful pulling or collision with hard objects. The lens surface has an anti-reflective film, which is sensitive to pollution, humidity, grease, and scratches. Therefore, dry and degreased air should be provided during work. Never handle the lens directly with the laser cutting head to protect the lens, as skin oil can permanently damage the lens surface. Instead, you can operate it with gloves and special glove devices. For smaller lenses, optical tweezers or vacuum tweezers are more convenient. Regardless of the method used to hold the laser cutting machine to protect the lens, it should only be clamped along the non-optical surface, like the mirror edge.
2. Generally, the protective condition of the laser cutting head lens needs to be checked before and after use and cleaning. As most contaminants and surface defects are relatively small, we often need to use magnification equipment when inspecting the lens. Besides the magnification equipment, we sometimes also need to illuminate the optical surface with a brighter beam to enhance the mirror reflection of surface contaminants and defects, making them easier to detect.
3. When cleaning the laser cutting head to protect the lens, clean wiping paper and optical solvent are usually used to prevent damage from other contaminants. The wiping paper must be moistened with the appropriate solvent and should not be dry. Considering softness, available wipers include pure cotton gauze wiping paper or cotton balls, lens paper, cotton swabs, etc.

The lens of the laser cutting head is the core and needs good protection to effectively utilize the performance of the laser cutting head.

Suggestions for Dealing with Black Spots on Laser Welding Head Lenses

How to handle black spots on laser welding head lenses?

Cleaning these black spots can pose a challenge, primarily due to the following reasons:

1. Impure black spots have infiltrated the protective lens.
2. The protective lens has been damaged.

What to do if there are black spots on the lens of the laser cutting and welding head?

Since it’s difficult to deal with black spots on the lens of the laser cutting head, a direct replacement of the lens is recommended. To prevent black spots from appearing on the lens of the laser cutting head, it’s crucial to take preventative measures.

Finally, how should one choose the lens for the laser cutting head?

1. Size and shape compatibility, which is fairly straightforward to understand.
2. Material: Depending on the laser power and considering the cost, you could opt for K9 or quartz. In practical applications, professional manufacturers divide these two materials into different grades. Customers can choose the specifications they need based on their practical requirements.
3. Coating: Laser antireflection film. Depending on the operating environment, different coating technology parameters should be used.