Nitrogen needs for Laser Cutting

Nitrogen needs for Laser Cutting

September 25, 2020

Nitrogen is usually the assist gas of choice for cutting ferrous metals such as stainless steel and certain metal (aluminum and nickel) alloys, especially to attain a high finish quality. It is usually operated at a high pressure to blow away the melted material that builds up over the course of cutting. While the use of an inert gas results in a clean, oxide-free laser-cut edge, issues of low processing speed and burrs solidifying below the kerf arise.

These issues can be managed by adjusting certain specific parameters, some of which relate directly to the choice of assist gas.

Parameters

We’ve touched upon the broad parameters that affect the laser cutting process, categorizing them as material parameters, beam parameters and process parameters. When it comes to the assist gas in particular, its dynamics is affected by the pressure applied, the nozzle diameter and alignment and the stand-off distance.  

Pressure: The assist gas pressure needs to be set according to the properties of the material and its thickness (gauge). Cutting stainless steel usually calls for pressure settings in the range of 8–14 bar. Polymers require far less pressure. When it comes to thickness, the thicker the material, the higher the required pressure, and vice versa. The inverse is true in the case of oxygen. 

Focal position: The spot size and lens orientation impact laser cutting. Interestingly, the conventions for focusing the lens change depending on whether the assist gas used is inert or reactive. In nitrogen-assisted laser cutting, the focal point is required to be right at the bottom of the workpiece. This creates a V-shaped channel through which the molten material created is expelled. In oxygen-assisted laser cutting, the focal position is right above the workpiece or just below the surface, depending on the thickness of the material.

Nozzle diameter: In the formula to identify the assist gas flow rate, the nozzle diameter variable is squared. On the one hand, any change in the size of the nozzle has a significant effect on the flow rate. On the other hand, a change in pressure has a relatively lower impact on the flow rate. The nozzle diameter has a direct impact on the flow, and by adjusting it, you can avoid operating the assist gas at a high pressure, thereby cutting costs.  

Nozzle alignment: The alignment of the nozzle affects the alignment of the gas jet that shields the laser beam. A misalignment can lead to a laser-cut edge lacking finish. The gas jet emerging from the nozzle strikes the material at a gradient that is co-axial with the nozzle, resulting in a clean laser-cut edge. It may be noted that a specific type of off-axis nozzle arrangement has its advantages.

Stand-off distance: The distance between the tip of the nozzle and the work area influences the assist gas flow rate and, in turn, the cut quality. The stand-off distance should be kept as short as possible. In general, it should be less than the diameter of the nozzle. Long distances result in turbulence in the gap, resulting in a low-quality cutting edge. 

Nitrogen gas purity

The purity of nitrogen is a critical factor in rendering a clean, oxide-free laser-cut edge. The presence of even the smallest amount of oxygen can result in discoloration or the development of dross. Nitrogen of high purity (99.995%) results in a superior, impurity-free laser-cut edge.

Pneumatech provides multiple nitrogen solutions for laser cutting through the PPNG gas generator range that can attain a purity of up to 99.999%.

Pneumatech designs and manufactures both standard and engineered on-site gas generator products. Explore Pneumatech’s full range of nitrogen generators. Or get in touch with us right away. Call at +49 (0)2841 788 480 or e-mail to support-eu@pneumatech.com