How the air pressure affects the cutting section

Air pressure is one of the key parameters that affect the quality of cutting (especially the quality of cutting sections). Its impact is not a single “high good” or “low good”, but an “optimal range” that needs to be precisely matched with parameters such as cutting speed, power/current, etc “. In general, the influence of air pressure on the cutting section is mainly reflected in the hanging slag situation, section roughness, slit width and bottom edge. The following two cases to explain in detail:

1. Laser cutting (oxygen assisted cutting metal as an example)

In laser cutting, the main role of gas (such as oxygen, nitrogen) is:

• Auxiliary combustion (oxygen): Exothermic reaction with metal, increasing energy.
• Blowing away the slag: Blowing away the molten material in the slit to form a clean cut.
• Protect the lens and gun body: prevent splashes from damaging the optical components.

Effects of low air pressure:

• Rough section, hanging slag serious: this is the most direct performance. Insufficient air pressure cannot effectively and timely blow away the molten metal at the bottom of the slit. These molten metals will adhere to the bottom of the plate, forming a slag (burr) that is difficult to remove. At the same time, the molten metal that cannot be blown away will re-solidify, making the cutting section rough, not smooth, and forming stripes.
• Cutting speed drop: In order to blow off the slag, it may be necessary to reduce the cutting speed, otherwise it will not be able to cut through and affect production efficiency.
• It may not cut through: in extreme cases, the air pressure is too low to blow through the molten layer, causing the cutting to be interrupted.

Effects of high air pressure:

• The cross-section produces whirling and roughness increases: too high a gas pressure will form a turbulent flow in the slit instead of a stable laminar flow. This turbulence will interfere with the normal flow of molten metal, and the formation of a regular lateral whirl on the side of the cut, so that the section looks “very flower”, but reduces the smoothness.
• The kerf widens: The high-pressure gas is equivalent to “blowing up” the laser focus, resulting in the upper and lower parts of the kerf being wider than normal, and the accuracy is reduced.
• The bottom of the cut is over-burned or wedge-shaped: high-pressure gas will bring too much heat to the bottom of the plate, resulting in excessive burning of the bottom material, forming a larger bottom edge (even rounded corners), and the cut presents a wedge-shaped with a wide top and a narrow bottom, instead of a vertical section.
• Waste gas, increase the cost: unnecessary high pressure will directly increase the cost of gas consumption.

Best air pressure:

• It needs to be comprehensively set according to the material, thickness, cutting speed and laser efficiency of the plate.
• For thick plates, a higher air pressure is usually required to ensure that there is enough energy to blow away the slag at the bottom.
• When oxygen is applied, the setting of air pressure also needs to consider the matching of combustion reaction.

2. Plasma cutting

• In plasma cutting, gas (and water) is the medium that generates the plasma arc, and it is also the energy that blows away the molten material and constrains the arc.

Effects of low air pressure:

• The arc is unstable and easy to break: the pressure is insufficient to form a stable and concentrated plasma arc.
• The cutting energy is reduced and the cutting cannot be cut through: the energy and speed of the plasma arc are insufficient to effectively melt and blow away the material, especially when cutting thicker plates.
• Serious slag hanging: similar to laser cutting, slag cannot be effectively removed, resulting in an increase in slag hanging at the bottom.
• The cut is “arched”: the cut will become a V-shape with a wide top and a narrow bottom, because the arc has insufficient energy at the bottom to cut vertically.

Effects of high air pressure:

• The arc is “blown” and the energy is not concentrated: too high a gas pressure will blow the plasma arc too divergent, resulting in a decrease in energy density. Although the airflow speed is very fast, the cutting energy may decrease, especially for thick plates.
• Increased electrode and nozzle wear: High pressure means higher working intensity, which significantly shortens the service life of wearing parts (electrodes, nozzles).
• The top of the cut melts, forming a “bell mouth”: the divergent arc will excessively melt the top of the cut, forming an inverted V-shaped cut with a wide top and a narrow bottom, which is a typical feature of plasma cutting with excessive air pressure.
• Rough section: unstable arc will lead to deeper grooves and corrugations in the cut section.

Best air pressure:

• The parameters recommended by the plasma power supply manufacturer for the specific power supply, nozzle aperture and sheet type/thickness must be strictly followed. Plasma cutting on the pressure matching requirements are more stringent.

Summary Comparison Table

Conclusions and Recommendations

1. There is no universal value: the optimal pressure value depends on the specific cutting process (laser/plasma), material type, thickness, cutting speed and other process parameters.

2. Determine by trial cutting: the most reliable method is to try cutting. Under the fixed power and speed, fine-tune the air pressure, observe the change of cutting section, and find the air pressure value with the least hanging slag, the smoothest section and the best verticality of cutting seam.

3. Pay attention to the overall matching: the air pressure must be closely matched with the cutting speed. Increasing the speed usually requires a corresponding increase in air pressure to ensure that the slag is blown away in time.

4. Ensure the quality of the gas source: In addition to the pressure, the purity and dryness of the gas are also very important, especially for laser cutting and plasma cutting of highly reflective materials.

Therefore, when the quality of the cut section is found to be poor, the air pressure is one of the key parameters that need to be checked and adjusted first.