The role of assist gases in a laser cutting machine's cut quality.

When watching a laser cutter, you can easily see the laser beam, the fast motion of the cutter, and the smooth edges the cutter leaves behind. There is a lot more going on than simply the laser beam doing all the work. Where the beam makes contact with the material, a stream of gas is blowing. This is the assist gas, and it is more critical to the quality of the cuts than most people understand.

When you are running a laser cutting machine, the assist gas is an integral part of the process and impacts cutting speed, the clarity of the edges, and whether there is dross hanging from the workpiece. Regardless of the material, different gases and different pressures and flow rates are required depending on the thickness of the material. Getting it right determines whether you have a quality part or one that requires additional work.

What Do Assist Gases Do? Why Are They Important?

Assist gas plays a multifaceted role. Firstly, they help clear molten material out of the cut kerf. When laser cutting, the molten metal created has to be cleared from the cut to avoid solidifying and ruining the edge of the kerf. The gas helps clear the molten metal from the bottom of the cut. Secondly, it protects the optics. The assist gas flow keeps the lens and nozzle clean from debris, sparks and metal droplets. Finally, in the case of some materials, the assist gas contributes to the cutting process. For instance, the exothermic reaction of oxygen gas helps accelerate the cutting process. The reactionless nitrogen helps clear the metal out of the cut too.

In order to select the appropriate gas for a specific application, it is important to understand the role of the assist gas. Also, different gases have different specifications, including flow, pressure, and purity.

Oxygen: A Gas That Generates Heat

Oxygen is one of the most commonly used gas for cutting mild steel. For industrial cutting applications, it is at least 99.5 percent pure. When high-purity oxygen is used, the heated metal at the cutting edge will start to burn. This combustion causes the metal to release energy and assists the cutting laser to cut thicker materials at a faster rate. The pouring of oxygen and metal precipitate reaction can increase the cutting speeds by 40 to 50 percent as compared to the use of gases that do not react.

Oxygen cutting is done at low pressure, typically in the range of 0.5 to 5 bar, or 7 to 70 PSI. For thick carbon steel that is 8 to 22 millimeters in thickness, a pressure of about 10 bar and gas consumption of about 20 to 22 cubic meters per hour, is used. For extremely thick plates, the pressure may remain in the range of 0.05 to 0.07 MPa when using specially designed dual-layer nozzles.

The downside to using pure oxygen is the result of a reaction known as the oxidized edge. The oxidized edge leaves a dark oxide layer that may have to be cleaned before it can be painted or welded. The edge may be a little bit coarser than using the other gases. The oxygen gas cutting ability for speed and thickness is usually the best option for mild steel for most fabrication shops.

Nitrogen: The Gas For Clean Edges

Compared to oxygen, nitrogen is inert and has a purity requirement of 99.95% or greater for laser cutting. Most practitioners prefer and use 99.99% purity. For most applications, this suffices and is economically more viable. It simply pushes the molten material out of the cut and clean surface.

As nitrogen is reliant on and uses mechanical force, it is significantly more demanding in terms of pressure. Typical working ranges are between 10 to 20 bar, or 150 to 300 PSI. In cutting less than 8mm, bar pressures ranging 15 is common coupled with a flow of 50 cubic meters per hour. For thicknesses ranging 12 to 15mm, bar pressures 15 to 22 are a must with flow rates of 120 cubic meters per hour. For the thickest section of 22mm, pressure 22 to 30 bar, or 319 to 435 PSI, and flow rates of 150 are typical.

When cutting stainless steel and aluminum using nitrogen, the edge is bright, and free from oxidation, discoloration, and scale. The parts that are cut are ready for welding and painting right off the machine. The downside is that there are slower cutting speeds with nitrogen and much higher gas consumption. For example, a 40-liter tank of nitrogen at 15 MPa pressure and 99.99 percent purity will last only 35 minutes when cutting with a 2.0 mm nozzle at a pressure of 1.2 MPa. This is significant with high-volume production.

Compressed Air: The Compromise Option

Compressed air is the cheaper alternative and is a literal compromise. The composition of compressed air is about 78 percent nitrogen and 21 percent oxygen, with a small percent of other gases. This explains why there is some oxidation from the oxygen component, but less than with pure oxygen. The cut edges will be cleaner than with oxygen but not as clean as with nitrogen. The cutting speeds using compressed air are better, but not as good as with the other two gases.

If you need to cut thin materials that are 3 to 4 mm thick and quality is not a major issue, using compressed air is a good way to save money. The only ongoing costs are the power needed to run the air compressor and the maintenance costs for the air compressor filtration system.

But, using compressed air for cutting jobs is a big risk. Air has a lot of contaminants like dust, water, and oil that will ruin the optics and ruin the quality of the cut. To avoid this kind of problem, you will need to add extra equipment that will protect your optics and cut quality. As for the air filters, industry standards advise that the compressed air used has no more than 0.01 ppm of oil and a dew point that is 10 degrees Celsius or lower. Because of this, you will have to use a high quality filtration multi-stage system, which is very expensive.

Specialty gases like argon are used to cut titanium as well as titanium alloys. They need to have a safe environment so argon creates that because it is an inert gas. An inert gas is one that does not react with most materials in the process. For titanium cutting jobs, the purity of argon should be 99.99%. Also, the gas pressure should be more than 1.2 MPa for thicker pieces of titanium.

There are certain custom gas blends designed for specific uses, but for most shops, oxygen, nitrogen, and air are the gas components that cover the majority of work.

Purity and Quality Standards of Gases

Gas purity is not just a specification, it is a key element that directly influences the quality of the cut. With regard to nitrogen, if a trace of oxygen is present, the stainless steel edges will discolour and this will completely ignore the desired purpose for using nitrogen. For oxygen, the impurities will reduce the efficiency of the exothermic reaction so that the cuts are made more slowly and the edges are of a poorer quality.

Below are the common requirements for different types of gases used in the industry:

The level of filtration and the variance in the quality of the air are both critical.

Practical Parameter Tables for Common Materials

While the type of compressed gas does matter when it comes to the quality of the cut, the pressure and flow rate also matter. If the pressure is too low, the molten material will not be removed and dross will be left at the bottom edge of the cut. If the pressure is too high, the cut could be disrupted, turbulence could be created, and excess cooling could occur to the material.

The flow rate within the cutting of nitrogen, in particular, is critical. The consumption of gas is recorded in liters per minute or cubic meters per hour, and it is influenced greatly by the size of the nozzle and the pressure used. For example, a nitrogen cylinder with a volume of 40 liters at a pressure of 15 MPa will last only about 13 minutes when a 4.0 millimeter nozzle is used at a pressure of 0.6 MPa, while it will last 35 minutes when a 2.0 millimeter nozzle is used at a pressure of 1.2 MPa. This is also the reason why nozzle selection is such an important factor in determining your operational costs.

There are several factors to take into consideration when determining how to set the focus or the optimal cut depth of the machine. The bottom of the cut will depend on the thickness of the material being cut. A bottom of a deeper kerf will require the ejection of the molten metal to be done at a higher rate, which will require higher pressure to be set. The higher the pressure to the system, the more the system will consume, which means finding the minimum pressure effective for the job will be the most effective means of controlling costs.

Gas Related Issues Troubleshooting

If the cut you are making is not right, the first issue you should look at is the gas. Dross on the bottom usually means not enough pressure or gas is not the right one for the material at hand. Increments of 0.5 bar should be adjusted to see if there is an improvement.

If there is discoloration on the cut of the stainless steel when using nitrogen gas, this means that there is some indication that the gas is allowed to enter the contaminated cut. The purity of the gas should be inspected for some flow leaks on the gas. Also, the nozzle should not be damaged. A yellow or light blue coloration will be the indication of the gas being contaminated.

Roughness of the edges of the cut can indicate that there is some turbulence being formed by a damaged nozzle or there is a pressure too high to the system. The nozzle orifice should be inspected to see if there is any distortion or damage to the nozzle. The edges of the cut should be uniform, which means that the flow of gas should also be uniform. If the cut is done towards the end of the metal plate, the gas flow must also be inspected to see if there are any blockages in the gas that is being supplied.

The Cost Side of Assist Gases

The assistance gases are another operational cost as they are purchased by the volume used during production. Nitrogen gases are the most expensive as an increase of 25 percent or more of operational cost can happen due to the consumption rates of nitrogen if the pressure is set just 4 bar higher than what is necessary. Meanwhile, oxygen is cheaper than nitrogen, and as long as the compressor and filtration system are available, air can be used free of charge.

However, the cost of the gas is only one part of the total cost. There is also the consideration of time. A faster cut with an oxygen assist gas could mean the savings of the labor cost could offset the loss of quality from the cut. On the other hand, a nitrogen assist gas may create a cleaner cut which would reduce the amount of post-processing grinding, sanding or other cut finishing operations that may be needed. Therefore, it is suggested to analyze the total cost of each part produced rather than focusing only on assistants gas consumption.

How DP Laser Helps You Get It Right

Each day, we help clients with their laser cutting systems to determine optimal configurations. Over our 20,000 systems in the field, we have gained insight into what works best for various materials and applications. When you get a machine from us, we do more than simply send it and consider it done. We have a hands-on approach to guide you in gas setting configurations, and we offer unparalleled support for genuine parts like nozzles and consumables, allowing you to maintain optimal flow rates through your machine.

Assist gases are a critical part of the cutting process, influencing speed, quality and cost. To choose the best gases for your cutting processes, you will need to consider the gas composition, gas purity, gas pressure, and gas flow in relation to the material you are cutting. Take time to analyze what your machine requires, and do a little experimenting while taking notes. Reach out if you need assistance. A good gas setup will have a significant impact on the materials produced from your machine.