CO2 Laser Cutting

The CO2 laser (carbon dioxide laser) is produced in a gas mixture, which mostly consists of carbon dioxide (CO2), helium and nitrogen. Such a laser is electrically pumped using an electric discharge.

CO2 lasers typically discharge at a wavelength of 10.6μm. Those utilized for material processing can generate beams of many kilowatts in power. The wall-plug efficiency of CO2 lasers is about 10%, which is higher than for most lamp-pumped solid-state lasers (eg ND:YAG lasers), but lower than for many diode-pumped lasers .

A CO2 laser can cut thicker materials (>5mm) faster than a fiber laser of the same power. It also generates a smoother surface finish when cutting thicker materials.

Laser cutting of sheet metals historically initiated with CO2 lasers. Most CO2 laser cutting machines are three-axis systems (X-Y, two-dimensional positioning control with a Z-axis height control).There are, however, a number of ways of achieving the X-Y movement: either moving the laser head, moving the work piece or a combination of both.

The most popular approach is known as a 'flying optics' system, where the work piece remains stationary and mirrors are moved in both X and Y axes. The advantages of this approach are that the motors are always moving a known, fixed mass. This can often be much heavier than the work piece, but it is easier to predict and control.

As the work piece is not moved, this also means that there is no real limit to sheet weight. The disadvantage of flying optics is the variation in beam size, as a laser beam is never perfectly parallel, but actually diverges slightly as it leaves the laser.

This means that without controlling the distinction, there may be some variation in cutting performance between different parts of the table, due to a change in raw beam size. This effect can be decreased by adding a re-collimating optic, or some systems even use adaptive mirror control.

The alternative is a 'fixed optic' system where the laser head remains stationary and the work piece is moved in both X and Y axes. This is the ideal situation optically, but the worse situation mechanically, especially for heavier sheets.

For relatively light sheet weights, a fixed optic system can be a viable option, but as the sheet weight increases, accurately positioning the material at high speed can be a problem.

The third option is known as a 'hybrid' system, where the laser head is moved in one axis and the material moved in the other axis. This is often an improvement over fixed optics, but still suffers from difficulties with heavier sheet weights.

Laser Cutting Work

The laser beam is a column of very high intensity light, of a single wavelength, or color. In the case of a typical CO2 laser, that wavelength is in the Infra-Red part of the light spectrum, so it is invisible to the human eye. The beam is only about 3/4 of an inch in diameter as it travels from the laser resonator, which generates the beam, through the machine’s beam path. It may be bounced in different directions by a number of mirrors, or “beam benders”, before at last focused onto the plate. The focused laser beam goes through the bore of a nozzle right before it hits the plate. Also flowing through that nozzle bore is a compressed gas, such as Oxygen or Nitrogen.

Focusing the laser beam can be done by a special lens, or by a curved mirror, and this takes place in the laser cutting head. The beam has to be precisely focused so that the shape of the focus spot and the density of the energy in that spot are perfectly round, consistent & centered in the nozzle. By focusing the large beam down to a single pinpoint, the heat density at that spot is extreme. Think about using a magnifying glass to focus the sun’s rays onto a leaf, and how that can start a fire. Now think about focusing 6 KWatts of energy into a single spot, and you can imagine how hot that spot will get.

The high power density results in rapid heating, melting and partial or complete vaporizing of the material. When cutting mild steel, the heat of the laser beam is enough to start a typical “oxy-fuel” burning process, and the laser cutting gas will be pure oxygen, just like an oxy-fuel torch. When cutting stainless steel or aluminium, the laser beam simply melts the material, and high pressure nitrogen is used to blow the molten metal out of the kerf.

On a CNC laser cutter, the laser cutting head is moved over the metal plate in the shape of the desired part, thus cutting the part out of the plate. A capacitive height control system maintains a very accurate distance between the end of the nozzle and the plate that is being cut. This distance is important, because it determines where the focal point is relative to the surface of the plate. Cut quality can be affected by raising or lowering the focal point from just above the surface of the plate, at the surface, or just below the surface.

There are many, many other parameters that affect cut quality as well, but when all are controlled properly, laser cutting is a stable, reliable, and very accurate cutting process. 

Focal Point Energy Comparison

From an efficiency and simplicity perspective, fiber would seem a clear winner but for thick section steel plate (typically 8-12mm) the power and speed of CO2 is still in front, although fiber units are approaching parity in cut quality and speed. Because of the differences in wavelength and beam path between the two types, power isn’t an accurate measure of relative performance. A 2kW fiber laser might outperform a 3kW CO2 unit in thin sheet steel, for example, while in ½-inch or thicker plate, CO2 may only be challenged by plasma or water jet technology. Fiber laser technology is scalable, however, and power levels are reaching performance parity with CO2 in all but the most demanding applications. Steel is still the most common material and cutting the most used laser process, but even with this well-understood material, cutting cost calculation can be complex.

Other cutting technologies competing with lasers include flame cutting, plasma, and abrasive waterjet. Each has benefits for specific applications. For a typical mild steel production cutting environment, plasma is often compared to lasers. Thinner sections (about 3/6 inch) are cut faster with lasers, but as plate thickness increases, plasma matches and then exceeds laser speed. Operating costs of CO2 lasers are also higher, although fiber technology is rapidly expanding to thicker materials. Where the laser shines, however, is in cut part accuracy, mainly due to the very small kerf width, and heat affected zone.

The assist gas is an important consideration, both for speed and cost. Inert gases such as nitrogen or air are used mainly to blow molten material out of the cut, but oxygen is a major contributor to the cutting process, burning away material as well as flushing the kerf. Oxygen assisted cutting is faster for both processes but trades cost for speed and heavy plate capability. It also levels the cutting speed between CO2 and fiber lasermachinery, which can make the added cost of a fiber machine a non-starter for thick section cutting. Nitrogen can present a similar issue where the very thin kerf of the fiber unit requires more gas to flush the cut. It’s a major expense in high-volume production, and illustrates why it’s important to choose based on specific applications and not on advertised machine specifications. Fiber lasers consume less energy than CO2 units, although most operators in North America enjoy lower rates low enough to make throughput a more important factor in machine choice. In Europe and Asia, power consumption is frequently more important, for cost and load reasons.