The core factors influencing the focusing effect in the beam control of laser cutting machines
1.Optical element characteristics and parameters
(1). Focus lens material and performance
Material selection:
Silicon glass: Suitable for ultraviolet lasers (resistant to high energy, low absorption).
Silicon/germanium crystal: Suitable for CO₂ lasers (10.6μm wavelength, high infrared transmittance).
Fused silica: Suitable for fiber lasers (1.06μm wavelength, good thermal stability).
Key parameters:
Focal length: The shorter the focal length, the smaller the spot diameter (spot diameter ≈ 1.22 × wavelength × focal length / lens diameter), but the focal depth (focusing clarity range) becomes shallower, suitable for thin material cutting; long focal length is suitable for thick materials, avoiding focusing shift caused by material surface undulations.
Lens diameter: The larger the diameter, the stronger the concentrating ability, the higher the energy density, but it will increase the volume of the optical system.
(2). Accuracy and coating of reflectors
Surface flatness: The surface roughness of the reflector needs to be <λ/10 (λ is the laser wavelength), otherwise it will cause the divergence angle of the light beam to increase and the focused spot to deform.
Coating process:
CO₂ laser reflector: Gold coating (reflection rate > 98%).
Fiber laser reflector: High reflection coating (reflection rate > 99.5%), reduces energy loss.
2.The beam quality of the laser source
(1). Laser wavelength
The shorter the wavelength, the smaller the diameter of the focused light spot (for example, for ultraviolet laser with λ = 0.355 μm, the focused light spot can be less than 10 μm; it is suitable for precision processing; for CO₂ laser with λ = 10.6 μm, the light spot diameter is usually > 50 μm, and the precision is lower).
(2). Beam mode (M² factor)
Basic mode (TEM₀₀): The beam energy is distributed in a Gaussian pattern, and the focused light spot has good roundness and concentrated energy (M² = 1), such as high-power fiber laser.
Higher-order mode (TEMmn): The energy distribution is dispersed, the focused light spot becomes larger, and the cutting efficiency decreases (for example, for low-power CO₂ laser, M² ≈ 10-20).
(3). Power stability
When the laser power fluctuates by more than ±5%, the focused energy density becomes unstable, resulting in inconsistent cutting depth (for example, for 10kW fiber laser power fluctuation of ±100W, it may affect the cutting quality of metals larger than 5mm).
3.Mechanical and motion control accuracy
(1). Stability of the optical path system
Mechanical structure rigidity: The vibration of the laser head bracket and worktable should be less than 50 μm; otherwise, the focusing position will shift (for example, when the cutting speed is 10 m/min, a 100 μm vibration will cause a cutting edge deviation).
Cooling system: When the temperature change of the lens/reflective mirror exceeds 2°C, thermal deformation will change the focal length (a water-cooled or air-cooled system needs to be equipped, with temperature control accuracy of ±0.5°C).
(2). Performance of the dynamic focusing system
Focusing response speed: When cutting thick materials or curved surfaces, the focusing mechanism should complete the focal length adjustment within 50 ms (for fiber laser cutting 20 mm carbon steel, the dynamic focusing error should be < ±0.1 mm).
Z-axis positioning accuracy: The precision of the Z-axis guide driven by servo motor needs to reach ±0.01 mm to avoid focusing height deviation.
4.Environment and Supporting Factors
(1). Environmental temperature and humidity
When the temperature fluctuates by more than 3℃/h, the change in air refractive index causes the beam to deviate (it is advisable to keep the workshop temperature at 25±2℃); when the humidity exceeds 70%, the lens is prone to fogging, and a dehumidifier needs to be equipped.
(2). Air cleanliness
When the dust concentration is greater than 0.5mg/m³, dust adheres to the lens surface, resulting in energy loss exceeding 10% (install a dust cover + air purifier, and wipe the lens with absolute ethanol regularly).
(3). Auxiliary gas interference
When the gas pressure fluctuates by more than 0.1MPa, the gas flow may disturb the beam path (for example, when cutting with oxygen, the gas pressure should be stable at 0.6-1.0MPa); when the gas contains oil and moisture content exceeds 50ppm, it will contaminate the lens.
5.Process parameter matching
(1). Cutting speed and focus position
When the cutting speed is too fast (e.g. the cutting speed for 10mm carbon steel > 1m/min), the focus needs to be adjusted (move the focus up by 0.5mm) to avoid slag accumulation; if the speed is too slow, the focus needs to be lowered to expand the heat affected zone.
(2). Material surface condition
When the thickness of the oxide layer or oil stain on the material surface is greater than 50μm, the laser reflectivity increases (for example, the reflectivity of rusty steel plate is 30% higher than that of a smooth steel plate), and it is necessary to clean in advance or increase the power compensation.
-----Rayther Laser Camila Wang