The principle of laser cutting metal is to use the laser beam as a heat source to irradiate the surface of the metal material, causing the surface temperature of the metal material to rise to the melting (boiling) point. At the same time, the nozzle sprays cutting gas parallel to the direction of the laser beam irradiation to melt (vaporize) the material. Blow away (when the cutting gas is an active gas such as oxygen, the cutting gas will also react with the metal material to provide oxidation heat). By controlling the motion device, the cutting head moves along a predetermined route to cut workpieces of various shapes.

During the process of laser cutting machine cutting metal, the power density of the incident laser is different, and the changes on the surface of the metal material are also different. Generally speaking, when the laser power density on the surface of a metal material reaches the order of 10MW/cm², the surface of the metal material will quickly heat up to the boiling point of the material and vaporize strongly into metal vapor. When the laser power density on the surface of a metal material exceeds the order of 100MW/cm², the metal vapor that cannot be discharged in time will be reheated by the laser energy, forming a plasma cloud.

Most of the plasma cloud generated by laser cutting of metal materials will be blown away by the cutting gas, and the remaining small part will form a plasma cloud and affect metal cutting:

1) The plasma cloud will stay on the surface of the metal material, hindering the transmission of laser energy and reducing the cutting speed.

2) The plasma cloud trapped under the nozzle will not only change the capacitance medium between the nozzle and the metal material, but also heat the nozzle, affect its capacitance performance parameters, interfere with the detection results of the capacitive height controller, and reduce the follow-up The precision of control affects the cutting effect.

Taking the 2000W laser currently widely used on the market as an example, if used with a 100/125 (collimator lens focal length/focusing lens focal length) cutting head, when the core diameter of the pigtail is less than 40 μm, the average power density of the light spot at zero focus It will reach the order of 100MW/cm², especially when cutting thin metal plates, it is easier to generate plasma clouds.

To address this problem, the following cutting process can effectively reduce the impact of plasma cloud on the cutting process:

1. Adopt pulse cutting. The pulse cutting method can ensure the peak power of the laser on the one hand, and shorten the irradiation time of the laser on the metal material on the other hand, reducing the generation of plasma cloud.

2. Reduce the laser cutting power appropriately. Without changing other conditions, reducing the cutting power can reduce the average power density at the focus and reduce the generation of plasma clouds. For example, when using a single-mode 2000W laser to cut 1mm stainless steel at full power and zero focus, the cutting speed was not ideal due to the influence of the plasma cloud. When the cutting power was reduced to 1800W, the cutting speed increased by 50%.

3. Appropriately widen the cutting slit. Widening the cutting kerf not only provides a wider channel for the plasma cloud to disperse downward, reducing the impact of the plasma cloud on cutting, but also helps accelerate the discharge of slag in the kerf and enhances the cutting effect.

4. Appropriately shorten the cutting height. The cutting height not only directly determines the thickness of the plasma cloud between the nozzle and the surface of the metal material (the shorter the distance, the thinner the plasma cloud), but also the closer to the cutting nozzle, the higher the pressure of the cutting gas ejected from the center of the nozzle (see figure 2) The increase in cutting air pressure helps to accelerate the dispersion of the plasma cloud below the nozzle and reduces the shielding of the incident laser by the plasma cloud. Therefore, on the premise of ensuring the safety of the cutting head, the shorter the following distance, the better.

5. Use a suitable cutting nozzle. A suitable nozzle can increase the gas flow rate without increasing the diameter of the nozzle, and can accelerate the dispersion of metal plasma clouds.

6. Add a side blowing device and a nozzle cooling device to the cutting head. The side blowing device is used to blow away part of the plasma cloud and reduce the accumulation of plasma cloud below the nozzle. The nozzle cooling device can reduce the thermal impact of the plasma cloud on the nozzle and avoid affecting the capacitive performance parameters of the nozzle.

7. Use high sampling rate capacitive height adjuster. The high sampling rate capacitive height controller can not only ensure the following accuracy, but also determine the changes in the plasma cloud below the nozzle by monitoring the changes in capacitance value. By monitoring the changes in the plasma cloud, the machine tool can take measures such as deceleration, pause, and pulse cutting. To reduce the impact of plasma cloud on cutting.