- Mar 12, 2018 -
In industrial laser cutting, there are two major technologies: fiber and CO2. Making the right choice is essential for high-speed, high productivity work, but the choice is more complex than it seems. For intermediate thicknesses of common ferrous metals there is often no clear winner between the two, or for plasma and water jet options either.
Laser cutting principle of operation. If oxygen is used as an assist gas, thicker materials can be cut.
How they work
“Laser” is an acronym for “light amplification by stimulated emission of radiation” and it’s an early example of a “quantum” electronic device. Co-created by several research teams in the late ‘50’s, in their original form, lasers were resonant cavities formed from ruby crystals and gas tubes with mirrored ends. When “pumped” by a high intensity light source or electric current, they amplified light as beams bounce back and forth through the cavity’s “gain medium”, until some of the light escaped through the partially silvered mirror at one end of the cavity. What made the laser useful was not just the power of the emitted light, but its unusual properties, namely high coherence and very narrow spectrum, make it valuable for industry. The coherence keeps the beam from diverging, allowing a lot of energy to be concentrated in a small spot, ideal for cutting and welding, while the narrow frequency has a major effect on cutting performance in some (typically highly reflective) materials.
The “gain medium” essential to laser operation can be solid, liquid or gas, and is a major determinant of the laser light frequency. Carbon dioxide lasers have two unique properties that make them ideal for industrial cutting: the first is the wavelength of light, emitting at 10.6um, near infrared, ideal for heating. The second useful property is high efficiency, over 30 percent, exceptional for gas lasers. The combination allows high power and good heating efficiency.
The resonators are housed in a stand alone cabinet with their associated gas handling, optical pumping and cooling equipment, transferring the laser beam through lenses and mirrors to deliver the energy to a single spot for clean cutting or welding. Typically, a movable table translates in two axes under the beam, defining the cut or weld in a material sheet. Metals are ideal candidates for CO2laser processing because of speed and fast, clean, nearly slag free cuts possible in thinner materials. 6kW power is possible with CO2 technology, although power is not the only important consideration in laser cutting.
Efficiency is good, but the energy not producing coherent laser light must be dissipated with a cooling system, placing a practical limit on the amount of power a commercial cutting of welding device can use. Fiber lasers use optical fibers, “pumped” with diodes to create solid state laser cutting machines with far fewer components and no gas consumables, lowering operating costs. They also convert pumped energy into laser light at up to twice the efficiency of gas lasers and typically use half the power. The wavelength of light emitted by fiber lasers is also shorter than CO2, typically 1 um, with a very small focal diameter, offering high intensity heating that can be an order of magnitude higher than CO2technology at similar power levels. They can also cut highly reflective materials such as brass or aluminum, which could damage the optics in a CO2 machine.
Which is better?
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 CO2lasers 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. In accuracy terms, lasers can achieve .005-.010 inch levels, while plasma is typically two to three times this figure.
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 laser machinery, 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.
What’s the best option? For thin sheet, fiber laser is the dominant technology for high rate cutting, while in thicker sections, CO2 is still preferred for high definition work with good cutting speeds. If accuracy isn’t as critical, plasma is a viable option for thick parts, but as fiber lasers grow in capability, look for high-volume partmakers to make the switch.