Protective Lens for Laser Cutting Head: Why Quality Matters?

1. Introduction

The cutting head of a laser cutting machine operates in a harsh environment and requires protection for its internal components, particularly the expensive ones such as the collimator and focusing lens. To achieve this, upper and lower protective lenses are installed in the cutting head.

During the cutting process, dust and spatter produced on the workpiece surface can cause serious damage to the focus lens if they enter the cutting head. The lower protective lens helps to prevent this by blocking these particles.

The cleanliness of the protective lens is critical to the performance and quality of the laser cutting machine. If the lens becomes dirty, not only will the cutting effect and efficiency be impacted, but it may also cause damage to the internal components of the cutting head and laser output head.

For optimal results, it is important to use protective lenses made from materials with high light transmittance, low thermal expansion coefficients, and good durability. If the lens becomes dirty during processing, it must be cleaned or replaced.

Fig. 1 Laser cutting head

The processing of protective lenses has specific requirements in regards to selection of base materials, accuracy, surface defects, coating, stability, and mechanical properties such as specific gravity, brittleness, tensile strength, compressive strength, and hardness. These requirements are also evident in the processes of material preparation, waste folding, sand hanging, grinding, coring, and coating.

Only after strict quality inspection can the finished products be considered ready for the market. Choosing ordinary substrate materials and skipping the coating process in an effort to reduce costs can lead to poor quality lenses with low light transmittance, poor heat resistance, and a tendency to break easily. This can result in the lens bursting or breaking, releasing powder particles that can pollute the entire cutting head and cause damage to expensive components such as the focusing lens and sensor.

The maintenance cost of repairing a damaged cutting head can be as high as 2-50% of the cost of a new one, causing delays in production and processing, leading to greater economic losses. To avoid these losses, it is recommended to purchase laser accessories from a reputable manufacturer through regular channels.

2. Adverse effects of poor quality protective lenses

2.1 Thermal lens effect causes the focus shift of cutting head

A poor quality protective lens can increase the laser’s absorptivity. After sustained exposure to laser light, the temperature will rise, causing thermal deformation and the thermal lens effect. This, in turn, leads to the focus drift of the cutting head. The higher the power and the processing of high reflection materials, the faster the thermal expansion of the optical element, and the more pronounced the thermal lens effect will be.

• When cutting stainless steel, the thermal lens effect can cause inconsistent cutting sections, producing more and more slag, and potentially making it impossible to cut.
• When cutting carbon steel, the thermal lens effect can result in more and more slag at the bottom of the cut, making it difficult to cut accurately.

Fig. 2 Thermal lens effect

2.2 Protective lens burning point / burn-through

A lower protective lens with poor quality has limited ability to block dust and spatter during processing and is prone to causing a fire point. When processing with a 10000 watt ultra-high power head, it is crucial to carefully select the protective lens. A lens of poor quality can result in burn-through of the lens when exposed to the 10000 watt laser in severe situations.

Fig. 3 Protective lens burning point / burn-through

2.3 Burning point of the focusing lens

If the burn-through of the protective lens is not detected in a timely manner, the continuous laser processing will generate particulate dust, which can contaminate the installation components, sealing ring, and lower surface of the focusing lens.

This can result in the formation of a burn point on the surface of the focusing lens. In this case, it is necessary to thoroughly clean and replace the entire set of installation components and the focusing lens module.

Fig. 4 Burning point of focusing lens

2.4 Burnout of the laser output head

If the protective lens burns out, the collimator and focusing lens module in the cutting head are vulnerable to damage under the high power laser.

In severe cases, if the internal optical elements of the cutting head are damaged, the optical path will become abnormal and the reflected light will cause excessive heating of the window piece in the laser fiber output head, leading to the burnout of the QBH or LOE crystal. In this scenario, the entire laser output head must be replaced.

Fig. 5 Burnout of the laser output head

The use of low-quality protective lenses not only shortens the lifespan of the cutting head and leads to various accidents, but also negatively impacts the processing progress and quality of the factory, resulting in increased processing costs and reduced factory efficiency.

Therefore, it is advised that users only purchase protective lenses from reputable manufacturers and through reliable channels, in order to avoid significant losses and unsatisfactory gains.