In recent years, with the increasing demand for sheet metal cutting, the traditional cutting method is difficult to better meet the production requirements due to its low accuracy and large heat affected area.Fiber laser cutting has many advantages, such as small heat-affected area, high precision, fast speed, non-contact processing, and is gradually replacing the traditional cutting process.
 

The principle of laser cutting metal is to use the laser beam as a heat source to irradiate the metal material surface, making the metal material surface temperature rise to the melting (boiling) point, At the same time, the nozzle ejects cutting gas parallel to the laser beam irradiation direction to blow away the molten (gaseous) compounds (when the cutting gas is an active gas such as oxygen, the cutting gas will also react with metal materials to provide oxidation heat. Through the control of the motion device, the cutting head moves according to the predetermined line to cut various shapes of workpiece.

In the process of laser cutting metal, the power density of incident laser is different, and the changes of metal material surface are also different. Generally speaking, when the laser power density on the surface of metal material reaches 10 mW/cm2, the surface of metal material will rapidly rise to the boiling point of the material and strongly vaporize into metal vapor.
 

When the laser power density on the surface of metal materials exceeds 100 mW/cm2, the metal vapor that cannot be discharged in time will be reheated by the laser energy to form a plasma cloud.
 

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

1) Plasma clouds will stay on the surface of metal materials, hinder the transmission of laser energy and reduce 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 regulator, reduce the accuracy of the follow-up control, and affect the cutting effect.

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

In recent years, the advantages of single-mode lasers have been recognized by the market. With the continuous optimization of optical schemes, single-mode lasers power of the device is also gradually increasing. The higher beam quality (generally corresponding to the smaller core diameter of the pigtail) and the higher output power make the average power density of the light spot at the zero focal point higher and higher, and it is easier to generate plasma clouds when cutting thin metal plates.

To solve this problem, the following cutting processes can effectively reduce the impact of plasma cloud :

1. Pulse cutting is adopted. Adopting pulse cutting mode can ensure the peak power of laser on the one hand, and shorten the irradiation time of laser on metal materials on the other hand, reducing the generation of plasma cloud.

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

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

4. Reduce the cutting height appropriately. The cutting height not only directly determines the plasma cloud thickness between the nozzle and the metal material surface (the shorter the distance, the thinner the plasma cloud), but also the closer the cutting nozzle is, the higher the cutting gas pressure emitted from the nozzle center will be. The higher the cutting pressure will help to accelerate the dispersion of the plasma cloud below the nozzle and reduce the shielding of the plasma cloud to the incident laser. So on the premise of ensuring the safety of cutting head, the closer the follow-up distance is, the better.

5. Use suitable cutting nozzle. The appropriate nozzle can accelerate the dispersion of metal plasma cloud by faster gas flow without increasing the nozzle diameter.

6. Add side blowing device and nozzle cooling device on the cutting head. The side blowing device is used to blow away part of the plasma cloud and reduce the concentration of the 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 nozzle capacitance performance parameters.

7. High-sampling rate capacitive height adjuster is adopted. High-sampling rate capacitive height adjuster can not only ensure the follow-up accuracy, but also determine the change of plasma cloud below the nozzle by monitoring the change of capacitance value. The machine tool can take measures such as deceleration, pause, pulse cutting, etc. to reduce the impact of plasma cloud on cutting by monitoring the change of plasma cloud.

About HGTECH: HGTECH is the pioneer and leader of laser industrial application in China, and the authoritative provider of global laser processing solutions. We have comprehensively arranged laser intelligent machine, measurement and automation production lines, and smart factory construction to provide overall solutions for intelligent manufacturing.