Laser Cutting Machine Nozzle Design

Nozzle design and airflow control technology: When laser cutting steel, oxygen and focused laser beam is shot through the nozzle to the material to be cut, thus forming a airflow beam. The basic requirement of the air flow is that the gas flow into the incision should be large and the speed should be high, so that enough oxidation can make the incision material fully exothermic reaction; At the same time, there is enough momentum to blow the molten material out. Therefore, in addition to the quality of the beam and its control directly affect the cutting quality, the design of the nozzle and the control of the airflow (such as the nozzle pressure, the position of the workpiece in the airflow, etc.) are also very important factors.
The nozzle used for laser cutting adopts a simple structure, that is, a conical hole with a small round hole at the end (Figure 4). It is usually designed by experiment and error. Because the nozzle is generally made of copper, small in size, it is a vulnerable part and needs to be replaced frequently, so the fluid dynamics calculation and analysis are not carried out. In use from the side of the nozzle through a certain pressure Pn (gauge pressure Pg) gas, called nozzle pressure, from the nozzle outlet, after a certain distance to the surface of the workpiece, the pressure is called cutting pressure Pc, and finally the gas expansion to the atmospheric pressure Pa. The research work shows that with the increase of Pn, the airflow velocity increases, and the Pc also increases.
It can be calculated by the following formula: V=8.2d2(Pg+1)
V- Gas flow rate L/min
d- Nozzle diameter mm
Pg- Nozzle pressure (gauge pressure) bar
There are different pressure thresholds for different gases, when the nozzle pressure exceeds this value, the gas flow is a normal oblique shock wave, and the gas flow rate transitions from subsonic to supersonic. This threshold is related to the ratio of Pn and Pa and the degree of freedom (n) of gas molecules: for example, n=5 of oxygen and air, so its threshold Pn=1bar×(1.2)3.5=1.89bar. When the nozzle pressure is higher Pn/Pa=(1+1/n)1+n/2 (Pn; 4bar), the normal inclined shock wave of the air flow becomes a positive shock wave, the cutting pressure Pc decreases, the air flow speed decreases, and the eddy current is formed on the surface of the workpiece, which weakens the effect of the air flow removing the molten material and affects the cutting speed. Therefore, the nozzle with a cone hole with a small round hole at the end is used, and the nozzle pressure of the oxygen is often below 3bar.
In order to further improve the laser cutting speed, a scale-type nozzle, Laval nozzle, can be designed and manufactured according to the principle of aerodynamics, without producing a positive shock wave under the premise of increasing the nozzle pressure. The structure as shown in Figure 4 can be used for the convenience of manufacturing. The Laser Center of the University of Hannover, Germany, used a 500WCO2 laser with a lens focal length of 2.5", and carried out tests with a pinhole nozzle and a Laval nozzle, respectively, as shown in Figure 4. The test results are shown in Figure 5, which respectively represent the functional relationship between incision surface roughness Rz and cutting speed Vc of NO2, NO4 and NO5 nozzles under different oxygen pressures. It can be seen from the figure that the cutting speed of NO2 small hole nozzle can only reach 2.75m/min when Pn is 400Kpa (or 4bar) (the thickness of carbon steel plate is 2mm). The cutting speed of NO4 and NO5 Laval nozzles can reach 3.5m/min and 5.5m/min when Pn is 500Kpa to 600Kpa. It should be noted that the cutting pressure Pc is a function of the distance between the workpiece and the nozzle. Because the oblique shock wave is reflected many times at the boundary of the gas flow, the cutting pressure changes periodically.
The first high cutting pressure area is close to the nozzle outlet, the distance between the workpiece surface and the nozzle outlet is about 0.5~1.5mm, and the cutting pressure Pc is large and stable, which is the process parameter commonly used in industrial production. The second highest cutting pressure area is about 3~3.5mm of the nozzle outlet, and the cutting pressure Pc is also large, which can also achieve good results, and is conducive to protecting the lens and improving its service life. Other high cutting pressure areas on the curve cannot be used because they are too far away from the nozzle outlet to match the focused beam.