Determining cost-effective CO2 laser cutting

What is most important: more power, more automation, or choice of assist cutting gas for the best quality and price

David Bell

CO2 lasers with 5, 6, or 7 kW of power are more common today for the increased speed with which materials can be processed. Various levels of automation ranging from dual shuttle tables to automated lift systems or a combination of materials handling coupled with a variety of storage systems decreases the downtime and unit cost of the processed parts. Oxygen and nitrogen, normally accepted assist gases used for cutting, are in some cases being replaced by “air” supplied from either an in-house compressor or synthetically produced. Each of these items directly impacts the final process cost.

How do today’s metal fabricators optimize their process? The determination of the specific applications and requirements of the laser cutting shop will assist in making the proper choice for the total cutting system. Factors affecting both the quality of the desired cut and the characteristics of the cutting operations must be considered.

The critical points that will produce the “good” cut are the width of the kerf (material lost in the cut), oxidation and roughness of the cut surface, the geometry of cut parts, and their allowable dimensional tolerances. The critical points of the cutting operation are cutting speed, operating flexibility, accepted tolerances, and ease of production and initiation.

Laser power

It is not always true that the greater the laser power the faster the parts can be processed; other factors can affect the maximum cutting speed. The type of laser resonator can have an effect on the cut speed and cost effectiveness of the system. Laser powers have increased and improved beam quality has expanded the range of laser applications. Most systems have laser power higher than 2 kW, and today up to 7 kW of power is available. Higher power does not always increase the cutting speed. Increasing the power during thermal cutting beyond the 3- or 4kW value may cause increased heat-affected zones on the material and place higher demands on the motion system, thus limiting the cutting speed.

Beam quality can affect the cutting speed on certain materials as much as laser power does. Laser beams can be produced by a variety of CO2 resonator types: fast-axial-flow, transverse flow, or diffusion-cooled (sometimes referred to as slab lasers). Each type produces different beam quality and beam focus diameter as well as different laser power. Diffusion-cooled lasers may have good beam quality but might be limited in total power. Fast axial and transverse flow lasers can produce power in the 6-7 kW range but beam quality may degrade above 4 kW.

Metal considerations

Higher laser power is useful primarily in increasing cutting thickness in the materials to be processed. For carbon steel cutting with oxygen, feed rate gains are nominal as power is increased. Due to the exothermic reaction process there is a finite power/feed rate upper limit at which a given material thickness can be cut. Increasing the power into the material does not automatically increase the rate at which the material is processed.

High-pressure nitrogen assist cutting of thick stainless steel produces an oxide-free edge; for example a 6.0kW CO2 laser can process up to 1¼-inch stainless steel. In high-pressure or inert-gas cutting, the primary function of the assist gas is to shield the cut edge from oxide buildup and to blow the molten material quickly and cleanly through the kerf before it sticks to the edge and forms a burr.

Increasing cutting power increases cost, so doing so makes sense if there is an economic advantage. The majority of sheet metal processed today is up to 6mm thick, and processed using lasers with 3.5 kW and up power at high speeds, provided the beam quality is optimal.

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When thicker materials are to be processed, higher-powered beam sources can cut at higher speeds. The advantages of higher speed must be critically weighed in view of the higher investment costs and increased operating expenses associated with these lasers. Those applications that require them could hardly be done without the extra power (see Table).

Automation

The more on-time processing hours the greater the benefits of automation. Automation can be added to an existing system and/or upgraded as production demands increase. The basic levels of automation include automatic loading, automatic loading and unloading, and connection to a storage system. The automatic loading process normally includes a lift device to load material onto the cutting table. As the system is expanded to automatic load/unload functions the lift device loads new material and also unloads processed material. Adding a storage system with connection to the automatic load/unload cycle completes the fully automated system (see Figure 1).

Because each level of automation comes at additional capital investment cost, the return on investment will be realized more quickly when multiple work shifts are employed. Regardless of the hours the laser is processing, automation offers benefits in unmanned operation, increased productivity, safe working environment, optimal working conditions, and logistics (see Figure 2).

The final part of the equation is the selection of the cutting assist gas. A basic understanding of the laser cutting processes will help in the selection of the proper gas.

Assist gas cutting

Laser cutting with oxygen assist gas results from locally melting metal at the focal point of the focused laser beam and ejecting this molten material from the cut area with gas pressure. It is best used with low-alloy steels where the process receives additional energy from the exothermal reaction of the material when it is heated above its ignition point. The laser power is therefore lower than for laser inert gas cutting sometimes referred to as laser fusion cutting. Oxygen for carbon steel cutting is used for material up to 40mm thick.

Two-dimensional cutting of carbon steel is the domain of the CO2 laser because it yields the best cost-benefit ratio, with cutting speeds in the region of 10 m/min for 1mm metal thickness, 3 m/min for 6mm and, 1 m/min for 15mm thick material.

Oxygen is used to process plate in thick carbon steel processing where the primary function of the oxygen is to aid in burning the plate. It also helps eject the molten material. Typically, assist gas pressure and volume are very low. For instance, between 6 and 8 PSI of oxygen typically are used to process 1 5/8-inch-thick plate. Oxygen gas pressures that are too high tend to cause uncontrollable burning. Once the burning process is started, very little oxygen is required to sustain the combustion process. However, the molten material still must be cleared by a solid flow of the assist gas. If a standard nozzle is used, shock waves in the assist gas column will cause the cut edge to appear very striated and gouged. Annular flow nozzles can prevent this problem.

High-pressure inert gas cutting

For cutting high-alloy steels and aluminum an inert gas (nitrogen, argon) is typically used as the cutting gas so this process is affected solely by the energy in the laser beam. Laser power is therefore higher than for oxygen cutting. High pressure cutting does not oxidize the cut edges; important when welding is the next process step after cutting. Today, laser fusion cutting is used industrially for material up to 25mm thick. Typical cutting speeds are up to 8 m/min for 1mm thickness, 4.5 m/min for 3mm, and 1.5 m/min for 8mm thick material.

In these applications, high-pressure nitrogen is used as the assist gas to shield the cut edge from oxide buildup and to blow the molten material quickly and cleanly through the kerf. Assist gas pressures range between 300 and 400 PSI when cutting thick stainless steel. Thinner stainless can be cut with pressures in the lower ranges of 100 to 200 PSI.

Assist gas costs

Oxygen and nitrogen can be supplied in a variety of modes. The actual unit cost of oxygen and nitrogen is similar. Cutting with nitrogen costs more because more gas is consumed but offers the benefit of a “clean cut” that requires no secondary finishing process. The gas cost can be decreased based on the method of supply; unit cost in high-pressure cylinder is more expensive than gas supplied in the variety of available bulk gas systems. Rental fees also are required for the cylinder storage supply, the larger the volume capacity storage the greater the rental fee.

In some applications air (oxygen-nitrogen combination) can be used as the assist cutting gas. Compressed air is an existing component to the operation of a CO2 laser machine tool. Air can be used as a beam purge in the beam delivery system to prevent external contaminants from entering the enclosed beam path. An additional benefit of the beam purge air is to keep the beam delivery optics clean, extending the life of the optics. Air is also connected to valves, cylinders, and actuators to open and close doors, beam attenuators, and clamps. Certain materials may be cut faster. In these instances the air supply must be clean, dry, and filtered. With air cutting there is a thickness limit for the material to be cut; normally 3mm or less and air cutting will leave a slight oxidation on the cut surface. Although less expensive than oxygen or nitrogen, the air supply will still carry a cost requiring electrical power to generate the required pressure and volumes.

Regardless of gas selection the cost is normally less than 10 percent of the total operating cost. Air is least expensive but limited in applications, nitrogen most expensive but offers “clean” cuts requiring no secondary processing.

Operating cost

To determine operating cost consider the following:

• Capital expense-more laser power costs more, automation adds to cost

• Consumables

• Laser gas-low volume, high price, and resonator dependent, will require gas supply equipment/rental fees

• Assist gas-varies based on materials to be cut, equipment/rental fees will be required

• Electricity-more power and automation adds to operating cost

• Cooling requirements-may be included in electrical cost

• Maintenance and service-a scheduled maintenance program must be employed

• Personnel-do not look only at wages but determine a valid total charge. An experienced and trained operator is required.

The decision for investment can not be made until an economic analysis and study of the manufacturing process from design to completion of a finished quality part has been conducted. The success and cost effectiveness of the laser cutting process will depend on the knowledge offered by suppliers in addition to that of the laser operators.

At the time this article was written David Bell was manger special gas products for Lincoln Electric (www.lincolnelectric.com).