September 25, 2020
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.