Since the operation of the first CO2 laser, there has been considerable confusionabout laser gases. This confusion is rooted in industry use of slang to explainlaser gas. Everything is called laser gas, leaving the impression that there is only one gas. The fact is, the CO2 laser uses three different and separate gas streams.
The accurate names of these lasers gases are:
- Resonator Gas
- Beam Pathway Purge Gas
- Assist Gas
This article will focus on use of the assist gas. Subsequent articles will review resonatorand beam pathway purge gases. Assist gas enters the cutting head of the laser below the focusing lens and above the cutting nozzle. The assist gas will chemically or mechanically cut the metal producinga kerf. The assist gas may be air, oxygen, nitrogen or, in rare cases, argon. The goal is to supply a gas clean enough to keep cutting and not shorten the life of the focusinglens while maintaining a dross-free cut. The final workpiece also needs to have a minimal amount of unexpected angularity.
The metals most commonly cut using lasers are aluminum, stainless steels, steelsand titanium. Each of these metals reacts differently to the chemistry of the assist gas.
A breakdown by gas and by metal will help to truly clarify your understanding of thechemistry and the process—and to improve your profits.
Air assist
The air used to cut any metal must have minimum purity. Of course, air is a mixtureof oxygen and nitrogen. In shops, it is frequently produced using an air compressor. Compressed air has excessive water and oil which can dirty a focusing lens.
This contamination limits the amount of energy passing through the lens, which contributes to lensoverheating and breakage. Untreated compressed air will cause this to occur within a few minutes, or up to a few hours, depending on the season, your plant location, andyour compressed air system.
A minimum treatment could involve installation of a refrigeration air dryer to reduce the water and oil content of the air.
Water content will drop from a high limit of 20,000 parts per million (ppm) to 2,000ppm. Oil, commonly called total hydrocarbon or THC, should be less than 2 ppm. Use of an air dryer will further reduce water content to less than a -40ºF pressure dewpoint(PDP) or less than 100 ppm of water. The most commonly used system is a D-13.
Users should avoid desiccant air dryers, because the desiccant will break down into very small (micron size) particles and contaminate the focusing lens, adding to lens replacement costs.
Oil should be managed by asking your air compressor serviceman to maintain less than 2 ppm of THC as a vapor and zero droplets. It is common for compressor service people to be unaware of your need for a very low THC content in your air supply.
Years ago it was an acceptable practice to let the air compressor add oil to the compressed air line to lubricate some air tools. Now that you have a clean air supply, look at the air lines. Avoid black pipe and grey pipe dope, as they add contamination to the air you just cleaned.
How can the air supply be tested? Shops can obtain test reports on the water and THC content of the air from their gas supplier or air compressor service team. This testing will not tell you if you have a particulate problem.
Particulates can and do scratch the focusing lens and shorten its life. You may need to install a particulate filter. A 10-μm filter is a common size, and an even smaller one will add more life to your focusing lens.
Air cuts aluminum and some thinner sections of steel well. It is frequently used to cut galvanized steel. Typically, air pressure from a shop air compressor is limited to about 100–125 PSI, which limits cutting thickness. Stainless steels cut poorly with air, because the oxygen attacks the chromium and nickel in the alloy to form oxides that might reduce the corrosion resistance of the metal near the cut.
Can thicker metal be cut with higher air pressure? Yes, you may be able to cut sheet 0.080 or even 0.125 in. thick, but the operating window becomes smaller. Dross will quickly form on parts, requiring a secondary operation to clean the parts and erasing your profits.
Edges will also start losing their square shape, which may add more indirect costs to the product.
Finally, later welding on the laser air-cut edge may not be suitable and paint may spall off, implying a low-quality product.
Oxygen versus nitrogen
Oxygen was used as an assist gas to cut aluminum, stainless steel and steel in the early days when CO2 lasers had limited power (1 to 1.5 kW). Today, lasers producing 2 to 6 kW
with oxygen are used to cut steel ¼ in. and thicker, while nitrogen is used to cut steel ¼ in. and thinner, and all thicknesses of aluminum, and stainless steel.
When oxygen is used as a laser assist gas, it forms a scale, or a black flakey surface, on the face of the cut. This scale is iron oxide, and it must be removed before welding or painting.
Removal means adding labor, handling, tools, and abrasive expenses to the cost of your product. The average laser these days has power of 4 kW, and, on steel 1/8-in. and thinner, nitrogen assist gas provides faster cutting speed than oxygen and eliminates indirect labor needed for cleaning edges.
Shop managers need to compare the cost of assist gas versus the cost of indirect labor in their laser cutting operation, keeping in mind production requirements.
A lower hourly assist gas expense does not automaticallymean the cost to make a product is lower. Managers should review all the factors to make their products price-competitive and maximize profit margins, but a general rule of thumb using the 4kW laser example is: when steel is thicker than ¼ in., oxygen is the preferred gas due to speed and overhead expenses.
Cutting steel 3/16-in. thick is not impacted by plant overhead as much as by labor, tools and abrasives required to prepare the parts for painting or welding. Cutting 3/8-in. thick steel at very low speeds with nitrogen adds $26 per hour to the gas expense and triples the time required to cut the parts. Plant managers must juggle labor costs, gas expense and overhead to determine the proper assist gas for the application.
The above gas costs are based on the use of liquid nitrogen. But what about nitrogen generation? It may be a viable option for you and will be reviewed in more detail in an upcoming
article, but here are some of the basics. Nitrogen should be used for cutting steel 3/16-in. thick and thinner. Pressure does not need to be 400 PSIG; it can be lower. Nitrogen generation will work provided the oxygen content in the gas does not grow iron oxide scale on the face of the cut. Generated nitrogen costs less than liquid nitrogen because there is no energy expense to deliver the product to you.
